LECTURE 1: INTRODUCTION Contents: Attributes Types of Steels Effect of impurity elements Historical Perspectives Present Status of Steel Industry Key words: steel industry, steel plants, mini steel plants, integrated steel plants, steel production and consumption Attributes: Steel belongs to iron carbon system. This system has a unique feature to alloy with several elements of the periodic table to produce materials for diversified applications. Iron‐Carbon system is capable of creating any desired property by altering the microstructure through surface hardening, heat treatment and deformation processing. Steel is recyclable and hence is a “green material”. The above attributes make steel to be the most important engineering material. Around 2500 different grades are produced to cater the need of several industries ranging from structural to aero‐space. Types of steels: Below are given some applications. Details can be looked into references given at the end of the lecture. Broadly we have either plain carbon (carbon is the principle alloying element) or alloy (in addition to carbon there are other alloying elements like Nb, V, W, Cr, Ni etc) steel. Plain carbon steels are the following types: Properties Low carbon Medium carbon High carbon Carbon Lower than 0.25 weight In between 0.25 and 0.6 In between 0.6 and 1.4 Percent weight percent weight percent Some Excellent ductility and Low hardenabilty. Hardest, strongest and properties toughness. These steel grades can be Least ductile Weldable and machinable heat treated Not amenable to Martensite transformation Some Common products like For higher strength such Used where strength, applications Nuts, bolts, sheets etc. as in machinery, hardness and wear Automobiles and agric- resistance is required. cultural parts (gears, Cutting tools, cable, axels, connecting rods) Musical wires etc. etc. The alloy steels are classified as low (less than 5 weight% alloying elements), medium (in between 5 to 10 weight percent alloying elements) and high alloy steels (more than 10 weight percent alloying elements). Note: Whether plain carbon or alloyed ones, all steels contain impurities like sulphur, phosphorus, hydrogen, nitrogen, oxygen, silicon and manganese, tramp elements like copper, tin, antimony, and non‐metallic inclusions. These impurities are to be controlled during steelmaking Effect of impurity elements on steel properties (some effects are given; details can be seen in the references given at the end of this lecture) Carbon imparts strength to iron. It reduces ductility and impact strength. But presence of carbon allows heat treatment procedures. Sulphur segregates during solidification (segregation coefficient is 0.02). Sulphur causes hot shortness due to formation of FeS formed during solidification of steel. Sulphide inclusions lower weldability and corrosion resistance. Presence of sulphur may also lead to development of tear and cracks on reheating the steel. Phosphorus segregates during solidification (segregation coefficient is 0.02). Presence of phosphorus impairs plastic properties. Silicon and manganese: Silicon reduces the drawing capacity of steel. Manganese is beneficial; it increases strength without affecting ductility and sharply reduces hot shortness. Gases: Nitrogen impairs plastic properties and increases embrittlement at lower temperatures. Hydrogen causes defects such as flakes, fish‐scale fracture. Inclusions: Presence of inclusions at the grain boundary weakens intra‐granular bonds. Inclusions also act as stress concentrators. Some type of inclusions is brittle. Tramp elements: Tramp elements like copper, zinc, tin, antimony etc create problems during reheating of steels because their melting points are much lower than steel reheat temperature. Historical Perspectives: Year Developments 1856 Henry Bessemer developed a process for bulk steel production. He blew air in an acid lined pear shaped vessel. The process is termed Acid Bessemer Process. No heat was supplied from outside. It did not become possible for him to remove S and P. Moreover oxygen content of steel was high. Hot shortness was a problem during rolling. 1878 S.G.Thomas and Gilchrist developed basic Bessemer process. They lined the vessel with basic refractory. High nitrogen content of steel, no usage of scrap and plugging of bottom blown tuyeres were the problems. 1868 Siemens’s and Martins developed Open Hearth Process. In this process thermal energy was supplied through combustion of gaseous and liquid fuels thus enabling them to use steel scrap in addition to other charge materials. Open Hearth Process for steelmaking has dominated the steel production for over approximately a century. 1900 Paul Heroult showed use of electricity for steel production. The quality of steel was better than open hearth process. The process was mainly used to produce alloy and special steels from scrap. 1928 Bulk oxygen production technology was developed. 1950 Oxygen was used to produce steel at Linz and Donawitz and process was termed LD Converter steelmaking. Oxygen was supplied through a consumable single hole lance from top of a pear shaped vessel. 1960 Continuous casting was developed. Today most of the steel plants use continuous casting to produce billet/bloom/slab 1950 and till Major developments took place in the following areas date • Multi-hole lances for blowing of oxygen in LD Converter • Hot metal pre-treatment to control S and P • Simultaneous blowing of oxygen from top and inert gas/oxygen through the bottom. Industrially the process is known as combined blown steelmaking or hybrid blowing • Refractory lining materials and refractory maintenance and repairing procedures • Usage of ladles to perform refining, degassing, deoxidation and inclusion engineering • Process control and automation Present Status of Steel Industry: Plain carbon steels are produced principally by the following routes: 1) Blast furnace→ Basic oxygen furnace →Ladle treatments→continuous casting→Rolling →flat or long products. Adopted by Integrated Steel Plants 2) Electric Arc Furnace→ Ladle treatments→Continuous casting→Rolling →Mostly long products but occasionally flat products. Adopted by Mini Steel Plants Alloy and special steels are produced by route 2. Some plants employ Argon‐Oxygen‐decarburization process instead of Electric Arc Furnace Top steel producers in the world in the year 2010‐2011 Rank Plant Production (Million (million tons) 1 ArcelorMittal, Luxembourg 103.3 2 Nippon Steel, Japan 37.5 3 Baosteel Group china 35.4 4 Posco, South Korea 34.7 8 Tata Steel India 24.4 10 United States Steel Corporation 23.2 20 Sumitomo Steel Industries, Japan 14.1 21 SAIL, India 13.7 com /…/ largeststeel/TOP30‐Worlds‐ Largest‐ steel‐ Companies.steelads. • Indian iron and steel company (Reader may add more). • Visvesyary a iron and steel • RINKL.html . www. Ghosh and A chatterjee: Ironmaking and steelmaking B. Then more integrated steel plants were added. Private sector • Rourkela • TISCO Uttam steels Dispersed • Bhilai • ESSAR Kalyani steels In various • Durgapur • ISPAT Lloyd steel Parts of the country • Bokaro • JSW Usha martin • Salem Tata Metalics • Alloy steel plant Durgapur Mukand ltd.Steelmaking in India The first attempt to revive steel industry in India was made in 1874 when Bengal Iron Works cam into being at Kulti near Asansol in west Bengal. In 1907 Tata Iron and Steel Company was formed and produced steel in 1908‐1909. List of steel producers. In 1953 an integrated steel plant in public sector in Rourkela was signed with German Company. Vishakhapatnam A. wilkepedia C. Indian steel industry is organised in three sectors as shown in the following: Sectors Integrated steel plants Public sector Mini steel plants Induction furnaces. a) Types of converter steelmaking • In converter steelmaking pure oxygen is blown from top through a water cooled lance fitted with multi-hole nozzles. Tundish of a continuous caster is used to transfer molten steel to the continuous casting mould. This technology of refining of hot metal is called top blown steelmaking. This variant is not popular amongst steelmakers . removal of gases. Modern steelmaking comprises of hot metal / scrap to finished products through the following a) Primary steelmaking b) Secondary steelmaking c) Continuous casting d) Finishing operations Primary steelmaking Primary steelmaking consists of refining of hot metal or scrap +hot metal to steel in a) converter and b) Electric furnace. • In some converters. In all these vessels molten steel is handled for one or the other purpose. tundishes for some refining operations like deoxidation. control of S. These are called combined top blowing and bottom stirred processes. desulphurization etc. removal of inclusions etc. inclusion removal etc. continuous casting Concept The concept of modern steelmaking is to make use of the steelmaking vessels like converter. • In another version of converter steelmaking oxygen is blown from top and bath is gas stirred through the bottom. For examples ladles are used to transfer the molten steel either to ingot casting or continuous casting. In all these vessels the residence time of molten steel is sufficiently long so as to carry out some refining operations like composition adjustment. ingot casting. in ladle and tundish. inclusion modification. ladle metallurgy. Duplex blowing or hybrid blowing. ??2 is blown from top and bottom and these processes are called top and bottom blowing. The basic idea of employing ladles and tundishes for either refining or composition adjustment or for producing clean steels is to use the steelmaking units like converter and electric furnace for producing steels without much bothering for final chemistry. ladle and tundish of a continuous caster. The objective is to refine hot metal to the nearly desired chemistry. This has led into the development of ladles. • In some converters oxygen is blown through the bottom and the process is bottom blown converter.L 2 Modern steelmaking Contents: Concept Primary steelmaking Secondary steelmaking Continuous casting and thin strip casting Final finishing operations Key words: Primary steelmaking. and other operations like composition adjustment. Some amount of scrap is also used. Slag formation of desired chemistry and physico-chemical properties is vital for the successful operation of converter steelmaking technology. Principle chemical reactions Hot metal contains C ~ 3.6 to 1%. Oxygen is blown from top and the following reactions occur: [Fe] + [O] = (FeO) 1 [Si] + 2[O] = (SiO2 ) 3 2[P] + 5[O] = (P2 O5 ) 5 (Fe) + (MnO) = (FeO) + [Mn] 7 [C] + [O] = {CO} 2 [Mn] + [O] = (MnO) 4 [C] + (FeO) = {CO} + [Fe] 6 Note the following: • • No heat is supplied from outside.2%. Mn~ 0. . a pear shaped vessel is used and blast furnace hot metal is refined to plain carbon steel. 2. Si ~ 0.8% and P ~ 0.Fig.6 to 0.1 to 0. The heat produced due to chemical reactions is sufficient enough to raise the temperature of hot metal from around 1250℃ to 1300℃ to molten steel tapping temperature of 1600℃ to1650℃ .1 Types of converter steelmaking (a)Top blown steelmaking (b) Combined top and bottom blowing. and (c) Bottom blowing It is important to note that in all different types of converter steelmaking practices.5 to 4%. Except carbon which is removed as a gaseous phase rest all other elements form slag. Typically oxygen blowing time is independent of converter capacity i. In one of the recirculation degassing practice metal circulation is achieved by dipping the degassing vessel into the ladle. b) To inject slag forming powder either through a lance for further refining c) To produce clean steel either by removing inclusions or modify them by suitable injecting materials d) To carry-out deoxidation and degassing. with or without bottom stirring or post combustion. Secondary steelmaking in ladles has become an integral part of steelmaking. degassed and returned into the ladle. EAF generates a considerable noise. There are several practices adopted for degassing. stream degassing and recirculation degassing. Figure 2. EAF can be either normal power or ultra high power (UHP) with single or twin shell. thereby molten steel is raised into the vessel and recirculates back into the ladle through the other snorkel. Graphite electrodes are used to supply the current (see figure 2. the liquid steel is raised into the vessel. Now a days EAF has occupied a unique position in the steel industry: EAF can be switched over easily to produce plain C or alloy steel depending on the market requirements. Continuous casting and strip casting .2: Electric arc furnace Secondary steelmaking The objective of secondary steelmaking is to make the steel of desired chemistry and cleanliness by performing the following treatments in “Ladle”: a) To stir the molten steel by purging inert gas through the bottom of the ladle. In another practice a refractory lined vessel is equipped with two legs (called snorkels) for dipping into the ladle containing molten steel. O2 is blown for 15 to 20 minutes irrespective of the converter capacity. Pressure is reduced and argon gas is passed into one of the snorkel.2).e. Ladles have additional heating facility and are called Ladle furnaces (LF).• • Typically converter steelmaking technology allows to tap liquid steel in approximately every 50 to 60 minutes with specified steel chemistry and 500-1000ppm dissolved oxygen. b Electric Arc furnace (E A F) In electric arc furnace steelmaking scrap + hot metal + directly reduced iron is used to produce plain carbon steel Electric energy is the principle source of thermal energy. like vacuum tank degasser. The AC electric arc furnaces are very popular. In recirculation degassing steel is made to flow from the ladle into a separate degassing chamber and then returned after exposure to the vacuum. Tundish also acts as reservoir of molten steel during ladle change-over periods and sequence casting. blooms and slabs depending on the desired product i. Modern tundishes are equipped with furniture like dams. weirs. In the continuous casting. It feeds the molten steel into the molds placed beneath the tundish through a submerged nozzle. In continuous casting. Figure2. The following finishing operations are dealt with • • • Deformation processing technologies like forging rolling etc.3. mold and spray is shown in the figure 2. The original continuous casting machines were of vertical types. Final finishing operations: It has been considered appropriate to include final finishing operations in steelmaking course to appreciate integration between chemistry and cleanliness of steel and the final finishing operations.3b)also becoming popular in steel plants. Chakrabarti: Steel making . Strip casting is ( Figure 2.3a) or vertical mold with bending rolls. slotted dams etc.3: continuous casting process. Surface hardening treatment References: A. whether long or flat products. Modern developments include thin slab caster. Heat treatment to produce the finished product. liquid core reduction. mold and secondary cooling sprays are arranged such that steel is poured continuously from the tundish and the solidified cast product is withdrawn continuously. tundish is the important refractory lined vessel. It is thought that the reader can appreciate the role of steelmaking in the product development and failure. Here molten steel is cast directly into the strip. The arrangement of the tundish. tundish. Thin slab casters are connected to the strip mill. The objective is to integrate the casting and rolling in order to save reheating cost.e. to modify the molten steel flowing in the tundish during the process of continuous casting. Now most of the continuous casters have either curved mould (Figure 2. Heat treatment consists of heating the steel products to a temperature in the austenitic region and then cooling.Molten steel is being cast continuously in to billets. the impurities like carbon. steelmaking. phosphides. Hot metal is a multi-component solution in which impurities like carbon.Lecture 3: Science base of steelmaking Contents: Preamble Equilibrium between phases Activity of solution Raoult’s law Henry’s law Interaction parameter Key words: Solution. Equilibrium between the phases: The phases in steelmaking are hot metal. molten slag and gas. The mole fraction of the ith component in a solution of n components is . Henry’s law Preamble In steelmaking. gas/slag and gas/metal/slag reactions so as to produce steel of desired chemistry and cleanliness (cleanliness refers to the inclusions). manganese. manganese. silicates etc. silicon. Slag is a solution of predominantly oxides with small amounts of sulphides. Composition of the solutions in steelmaking is conveniently expressed either as weight% (Wt%) or mole fraction(N). phosphorus and sulphur are dissolved in very low amount (total concentration of all the impurities is approximately 5% to 6%) in iron. Raoult’s law. silicon. phosphorus and sulphur are removed from hot metal through a combination of gas/metal. Science of steelmaking involves equilibrium concentration of an impurity between the phases and the rate of transfer of an impurity from the hot metal. 1) where X i is the number of moles of ith component.P < 0. At chemical equilibrium the chemical potential of any component is identical in all phases. a process occurs spontaneously. where K is equilibrium constant. The equilibrium of a component between the liquid phases is expressed in terms of integral molar free energy. Knowledge of chemical potential is important in steelmaking because an impurity can transfer in the gaseous or slag phase only when its chemical potential is lower than in hot metal. where J is activity quotient and ∆Go is the standard free energy change.P = 0 for a finite process Where (dG)P is change in integral molar free energy At constant temperature and pressure when (dG)T. Activity of a component denotes its effective concentration. (∆G) = ∆Go + RT lnJ. The quantity Gim is the partial molar free energy of mixing of component i and represent the change of energy or work which a mole of pure component i can make available. At equilibrium ∆Go = − RT ln (J)eq = −RT ln K.P = 0 for an infinitesimal process and 3) (∆G)T. The criterion for equilibrium at constant temperature and pressure is the change in the integral molar free energy of the solution.e. 2) Gi Ni represents free energy of solution and Gio Ni is the free energy of pure components before entering into the solution. Integral molar free energy of solution Gm = ∑Gim Ni = ∑ Gi Ni − ∑Gio Ni = RT ∑ Ni ln ai.Ni = X i /∑Xi. (dG)T. (dG)T. It is related to fugacity as . 4) Activity of solution In dealing with chemical reactions in solution it is important to define the activity of a component. i. Chemical potential is a useful concept to describe chemical equilibrium between liquid phases.P. For an isothermal chemical reaction say A + B = C + D. which is stated as ai = γoi Ni . whereas most binary silicates i.ai = fi /fi0 5) fi is the fugacity of component i in solution and fi0 is the fugacity of a component in its standard state (standard state could be either pure element or compound at 1 atmospheric pressure) So at standard state activity equals 1. 7) The Fe-Mn forms an ideal solution. Henry’s law Liquid steel. FeO − SiO2 . Deviation from Raoult’ law is taken care by activity coefficient γi γi = ai / Ni. 8) where γoi is a constant (activity coefficient for the solute in dilute binary) and Ni is the mole fraction of the specie i. In binary liquid oxides. Raoults’s Law An ideal solution obeys Raoult’s law. Physically it implies that in Fe-Cu solution copper has a strong tendency to segregate. and in Fe-Si solution silicon has a strong tendency to form chemical compound with iron. and to a reasonable extent hot metal primarily fall in the category of dilute solution.e. whereas the Fe-Cu exhibits strong positive deviation and the Fe-Si strong negative from Raoult’s law. FeO-MnO behaves ideally. In an ideal gas activity of a component i is equal to its partial pressure. In a dilute binary solution activity of a solute obeys Henry’s law. Deviation from Henry’s law occurs when the solute concentration increases. in which activity of a component ai equals to its mole fraction Ni a i = Ni 6) Real solutions exhibit either positive or negative deviation from Raoult’s law for a binary solution. Solutes in all infinite dilute solutions obey Henry’s law. . MgO − SiO2 show negative deviation from Raoult’s law. CaO − SiO2 . eS Si = 0.028.In steelmaking the concentration of solute in molten steel is expressed in weight percent. For example. e1 Wt % J 11) j The term ei is known as interaction parameter describing the influence of solute j on the activity coefficient of solute i. This is defined as 9) hi /(Wt% i) = 1 when wt% i → 0 For weight percent i other than zero 10) hi = fi Wt% i Interaction parameter Molten steel contains several dissolved solutes in dilute scale.By assuming infinite dilute solution as the standard state.025. then J Log fi = e11 Wt %1 + e12 Wt %2 + e13 Wt %3 + e14 Wt %4 + … . S. 2….k are solutes in dilute state. Chatterjee: Ironmaking and steel making .43.Ghosh and A. The value of interaction parameter can be found in any book on thermodynamics. This steel is a multi-component solution. The concept of interaction parameter is very important in estimating the activity of a solute element in presence of other solute elements. we get fS = 10.5%. It is frequently most convenient to choose the infinitely dilute solution expressed in terms of weight percent as the standard state. and 1. molten steel contains C. Mn etc.24. In multi-component solution solutes interact with one another and thus influence activities of other solutes. Si. Mn =1% and S = 0. the activity of sulphur is given by hS = fS Wt%S Mn log fS = eSS Wt %S + eCS Wt %C + eSi S Wt %Si + es Wt %Mn Substituting the value of eSS = − 0. For example we want to calculate the activity of sulphur in hot metal of composition C = 4%.78 and activity of sulphur is 0. eCS = 0. P. If Fe is the solvent. Si = 1.066 and eS Mn = −0. References A.04% at 1600 ℃ . BOF steelmaking Preamble Slag plays a very important role in steelmaking to the extent that it is said that “make a slag and slag makes steel”. Mn to MnO. All these oxides float on the surface of the molten steel. synthetic slag. Slag is a separate phase because • It is lighter than molten steel and • It is immiscible in steel Slag is formed during refining of hot metal in which Si oxidizes to SiO2 . Fe to FeO. viscosity etc. The addition of oxides is done to obtain desired physico-chemical properties of slag like melting point. iron oxide. and others.Lecture 4: Slag in steelmaking Contents: Preamble The role of slag in steelmaking Structure of pure oxides Structure of pure silica Network former and breaker oxides Structure of slag Keywords: Steelmaking. The role of slag in steelmaking: • It acts as a sink for impurities during refining of steel • It controls oxidizing and reducing potential during refining through FeO content. MgO. and addition of oxides such as CaO. Electric steelmaking. basicity. Slag is a generic name and in steelmaking it is mostly a solution of oxides and sulphides in the molten state and the multi-crystalline phases in the solid state. Higher FeO makes the slag oxidizing and lower FeO reducing • It prevents passage of nitrogen and hydrogen from atmosphere to the molten steel • It absorbs oxide/sulphide inclusions . Synthetic slag is also used to absorb inclusions to produce clean steel for certain applications. and P to P2 O5 . 133 CN= Coordination number Examples .414 – 0.225 – 0. Both physical and chemical properties are controlled by composition and structure of slag. have radii smaller than that of cations of SiO2 . The oxides are either acidic or basic in nature. Fe2+ ) etc. In steelmaking slag is predominantly a mixture of oxides with small amounts of sulphides and phosphides. melting point.732 – 0.732 Octahedral 6 0.225 SiO2.414 CaO. Tetrahedral 4 0. Structure of pure oxides In pure oxides • Metallic cations are surrounded by oxygen ions in a three dimensional crystalline network • Each cation is surrounded by the maximum number of anions in a closed packed structure. We will first consider the structure of pure oxides and then we discuss what happens on addition of one type of oxide to the other. viscosity) and chemical properties (basicity. and this number is called coordination number • • Cations of basic oxides such as CaO. MgO. MgO. FeO etc. • In electric steelmaking slag prevents the radiation of heat of arc to the wails of the furnace and roof The above functions require that slag should possess certain physical (density. MnO. • It protects steel against re-oxidation • It emulsifies hot metal and promotes carbon oxidation. Al2 O3 Structure of an oxide depends on the ratio of radii of cations/anions as shown in the following table Structure CN Cation/anion Cubic 8 1 – 0. FeO (Ca2+ . Mg 2+. P2O5 Triangular 3 0. oxidation potential).• It acts as a thermal barrier to prevent heat transfer from molten steel to the surrounding. 1b. These oxides are called network breakers. each tetrahedron is joined at the vertex so as to obtain the three dimensional hexagonal network. Pure silica has very high viscosity at the melting point. During melting the crystalline network of silica is broken by thermal agitation as shown on figure 3. therefore. each atom of silicon is bonded with four oxygen atoms and each atom of oxygen is bonded with two silicon atoms. FeO dissociate and form simple ions like Ca2+ andO2−.1a. these tend to form hexagonal network. Basic oxides like CaO. MgO. Addition of basic oxides decreases the viscosity by breaking the hexagonal network of silica. called network formers or acids. In slags. These acidic oxides can accept one or several oxygen ions. Consider the addition of CaO to molten silica. Only at very high temperatures. and these simple ions group to form complex ions as (SiO4 )4− and(PO4 )3−. since they destroy the hexagonal network of silica by reacting with it.1: Structure of silica (a) solid and (b) molten As seen in the figure 3. All basic oxides are donors of oxygen ions. molten silica consists of equal number of SiO4− 4 and Si4+ ions. Na2 O. The elemental tetrahedral of silica are joined at the vertices to give the hexagonal network in three dimensions. The structure of pure solid and molten silica is shown in the figure Figure 3. Structure of slag Most slags are silicates.As can be seen in the table the basic oxides have octahedral and acidic oxides tetrahedral structure. Structure of pure silica In silica. Network former and breaker oxides It must also be understood that the bonding between cations and anions in acidic oxides like SiO2 and P2 O5 is strong. These oxides are. Calcium oxide dissociates to CaO = Ca2+ + O2− . the ratio of O/Si as shown in the table O:Si Formula Structure Equivalent silicate ions 2:1 SiO2 All corners of tetrahedron are shared (Si6O15)6− or (Si8O20)8− 5:2 MO.e. The resulting slag would consist of Ca2+ and SiO4− 4 ions References L.Each mole of CaO introduces one mole of oxygen ions in the hexagonal network of silica and can break two vertices of the hexagonal structure of silica.2SiO2 One broken link per tetrahedron (Si3O9)6− or (Si4O12)8− 3:2 MO.Hopkins and I. e. By adding 2 moles of O2− for every mole of silica all the four vertices are broken and we simply have Ca2+ and SiO4− as shown below Note that Ca2+ can combine with two tetrahedrons The reaction between alkaline base oxides.D. i.SiO2 All link are broken (SiO4)4− All the four vertices will be broken when 2 moles of CaO are added for each mole of silica.Coudurier.2SiO2 Three broken link per tetrahedron (Si2O7)6− 4:1 2MO.g. The number of vertices destroyed depends on the fraction of basic oxide. each tetrahedron of silica will have Na ion attached to oxygen ion.W. As a result one should expect more decrease in viscosity of silica on addition of alkaline base oxides as compared with basic oxides. Na2O and SiO2 is as follows: Na2 O + 2Na+ + O2− and Since Na has one charge.SiO2 Two broken link per tetrahedron (Si3O9)6− 7:2 3MO.wilkomirsky: Fundamentals of metallurgical processes . . steelmaking reactions Introduction: This lecture discusses the physico-chemical properties of slag which are relevant in steelmaking. MgO etc.Lecture 5: Physico-chemical properties of slag Contents Introduction Viscosity Basicity Oxidation and reduction potential of slag Slag foaming Operational advantages Quantification of slag foaming Keywords: Foaming. In some slags Al2 O3 is also present. Viscosity: Viscosity controls the fluidity of slag. The decrease in viscosity is greater with alkaline oxides like Na2 O and fluorides like CaF2 as compared with CaO and MgO for the reasons discussed in lecture 3. Slag is a multi-component system and in steelmaking it consists of acidic oxides such as SiO2 and P2 O5 . basicity of slag. In general viscosity of a slag is a function of temperature. and basic oxides such as FeO. addition of basic oxides decreases rapidly the viscosity of a slag which contains SiO2 and P2 O5 . . electric steelmaking. T is temperature and R is gas constant. CaO. whereas basic oxides are network breakers. composition and percent solid present in slag. A is an empirical constant. The slag should be fluid so that it can be removed easily during tapping of steel. In fact fluidity is inversely proportional to viscosity of slag. Viscosity of any slag composition decreases with the increase in temperature as given by the following expression: E RT ηO = A exp � � (1) ηO is viscosity. In the last lecture we noted that acidic oxides are network formers. E is activation energy. For a given temperature. For example. The ionic nature of slag assumes slag to consist of ions. slag will become neutral when CaO is 66. Thus SiO2 + 2O2− = SiO4− 4 � P2 O5 + 3O2− = 2(PO4 )3− (3) Amphoteric oxides behave as bases in presence of acid or as acids in presence of a base: Al2 O3 + O2− = Al2 O2− 4 � Al2 O3 = 2 Al3+ + 3O2− (4) Bases can supply O2− ions CaO = Ca2+ + O2− In a neutral slag enough oxygen ions will be present to ensure that each tetrahedron remains independent of each other.5 (2) Where ϵ is volume fraction of solids in slag If volume fraction of the solid is in between 5% to 10%. SiO2 . viscosity of slag increases by 114% to 130%. which corresponds to the formation of 2CaO. In 100 g of slag nO 2− = �nCaO + nMgO + nMnO + nFeO � + ⋯ − �2nSi O 2 + 3nP 2 O 5 + nAl 2O 3 � (5) . In binary CaO − SiO2 . whereas a basic slag is a donor of O2− ions.7%. Slag will be basic only when CaO content is more than 66.Alumina acts as a network breaker in an acidic slag and network former in a basic slag. Basicity: Basicity can be understood either from ionic or from molecular nature of slag.7%. Basicity can be expressed in terms O2− ions which are in excess than that required. thus satisfying the requirements of acidic oxides. Presence of solid particles in slag increases the viscosity of slag as shown in the following expression: η = ηO (1 − ϵ)−2. acidic oxides can accept one or several O2− ions. In slags. Similarly each mole of P2 O5 can accept 3 moles of O2− ions. 1 mole of SiO2 can accept 2 moles of O2− ions so that each tetrahedron in hexagonal structure becomes independent of each other. 2% P the calculation shows that free CaO in slag would be available when CaO content exceeds 4540Kg. In steelmaking. SiO2 and P2 O5 . MgO. 3CaO. SiO2 and P2 O5 slag is that amount which is available after the formation of neutral compound like 2CaO. P2 O5 Free lime (Kg) = Kg CaO + 112/60Kg(SiO2 ) + 168/142Kg (P2 O5 ) (8) For 100 ton hot metal with 1% silicon and 0.66 weight %MgO ) (weight % SiO 2 + weight % P 2 O 5 ) (7) In slag/metal reactions which involve desulphurization and dephosphorization. A liquid is said to be foaming when gas bubbles could not escape through the liquid and as a result height of the liquid increases. slag foaming can occur due to the following reactions: (FeO) + C = {CO} + [Fe] . The equilibrium between FeO of slag and oxygen of steel is (FeO) = [Fe] + [O] K = a (FeO ) a [O ] (9) The activity of oxygen in metal is proportional to the activity of FeO in slag. Free lime in CaO. The molecular approach assumes slag to consist of chemical compounds. Thus activity of FeO in slag is an important parameter. SiO2 . The basicity of slag is B = weight % CaO weight % SiO 2 (6) In presence of different basic oxides. the molecular approach is more useful. the concept of free lime in slag is useful. the different strength of the basic oxides should be considered.In industrial practice ionic definition of basicity is not useful. the basicity is B = (weight % CaO + 0. FeO content of slag determines the oxidation potential of slag. Oxidation and reduction potential of slag It refers to the capability of slag to transfer oxygen to and from the molten steel bath. In a slag which contains CaO. Slag foaming: Foam is a dispersion of gas bubbles in a liquid. The reaction 2 occurs within the slag Is slag foaming desirable? Yes to the extent that slag should not flow out of the reactor.5 (d b )0. The other reaction [C] + [O] = {CO} This reaction occurs at the gas/metal interface. In electric steelmaking foamy slag practice prevents the transfer of heat of the arch to the refractory lining. Presence of solid particles and surface active agents increases the foaming index. Foam life is directly proportional to foaming index Increase in slag viscosity increases foaming index. the slag is said to be foaming. Addition of calcium fluoride decreases the foaming index by decreasing the viscosity of slag. Operational advantages: A foaming slag • Shields molten steel against atmospheric oxidation • Acts as a thermal barrier to prevent heat losses • Shields the refractory lining particularly in electric arc furnace • Control heat transfer from the post combustion flame Quantification of slag foaming: Foaming index = Foam layer thickness/ average gas velocity Low foaming index means easy escape of gas bubbles which can be obtained either by smaller gas bubbles or higher gas velocities.e. Foaming index (FI) can be calculated from the physical properties of slag and size of the gas bubble: FI = 115 η o (1−ϵ)−2.5 (ργ )0.This reaction occurs within the slag. In both the cases when the CO gas bubbles are unable to escape through the slag. If the reaction between carbon and oxygen occurs deep into the bath i. Slag foaming enhances the reaction area.9 ϵ = Volume fraction of solids in slag ρ = Slag density kg/m3 (10) . reaction 2 then gas bubbles have enough time to grow in size and can easily escape through the slag layer as compared to when the gas bubbles are produced by reaction 2. 01m. 35% Al2 O3 and 1% Ca F2 at 1873 K whose ηo = 0.Chatterjee:: Ironmaking and steelmaking Zhang and Fruehan: Metallurgical and Materials Trans.005 m and 631 s for db = 0.γ = Surface tension of slag N/m db = Gas bubble diameter in m Calculate the foaming index slag of composition 60% CaO.005 m and 485 s for db = 0. Note that foaming index increases to 1. Substituting the value of the variables in eq. 35% Al2 O3 and 5% Si O2 at 1773 K slag from the following data: ηo = 3.005 m and 0. Foaming tendency decreases drastically due to production of CaF2 in slag. ρ = 2500 Kg m3.4 N/m and db = 0.Ghosh and A. ρ = 2700 Kg/m3 and γ = 1. 10 we get FI = 904s for db = 0. Reference to lectures 3 and 4 A. Consider a slag of composition 55% CaO. If the volume fraction of solid particles in slag is 0. This slag would have foaming index 9s. γ = 1.5 Kg ms .1 FI = 1175 s for db = 0.1 N/m and db = 0.7 Kg/ms.3 times due to presence of solid particles in slag.01 m.01m.01 m. 26(8). 1995 . B. iron oxidation.Lecture 6: Steel Making Reactions: Oxidation of Iron and Silicon Contents Introduction What are oxidation reactions? Iron oxidation Oxidation of silicon Key words: Steel making. round brackets () in slag and curly {} in gas. Raoult’s law is used to express activity of solutes in slag. Note the following • • • • Carbon can oxidize to CO and CO2 but at high temperature carbon oxidation to CO is highly probable. desiliconization Introduction In steelmaking the impurities in hot metal like carbon. For this purpose oxygen is supplied and slag of desired chemistry is formed. reactions between impurity and oxygen occur with dissolved oxygen. silicon. When oxygen is supplied. phosphorus and sulphur are removed through oxidation and slag formation so as to produce steel of desired chemistry and cleanliness. What are the oxidation reactions? The principle reactions in steelmaking comprise of oxidation of impurity elements by oxygen dissolved in hot metal or FeO content of slag. Since impurities are dissolved in molten metal. To understand the conditions favourable for the removal of an impurity. we will first consider oxidation of an individual impurity. manganese. In expressing activity of solutes in molten steel. oxidation of all impurities of hot metal including iron begins simultaneously. Henry’s law is used by using 1 weight % standard state. Square brackets [ ] in a reaction denote impurity in metal. deoxidation. We will consider oxidation of C to CO. [Fe] + [O] = (FeO) [Mn] + [O] = (MnO) (1) (2) . We will be using principles of thermodynamics to obtain the optimum conditions for the removal of an impurity. 17 [wt % O] = [aFeO {10( 6150 − 2. Consider the reaction [Fe] + [O] = (FeO) The equilibrium constant K Fe is K Fe = a FeO a Fe h O = a (FeO ) (6) a [Fe ] f O [wt % O] h is henrian and a is raoultian activity.e. Iron oxidation is unavoidable. hence its oxidation must be controlled. Oxidation of Fe is loss in productivity. all other impurities are removed as oxides and all these oxides float on the surface of the molten metal during refining of hot metal to steel. Since Fe in steel is pure. Iron Oxidation: Oxidation of iron i. C is removed as gas. Oxygen must be dissolved to remove an impurity from the hot metal. Except C. T is in K. 7 and 8 we get [wt% O] 10−0.17 [wt% O] log K Fe = 6150 T (8) − 2.604 In equation 8. We begin with considering oxidation of an individual element and evolve the optimum conditions using thermodynamic principles.[Si] + [O] = (SiO2 ) (3) 2[P] + 5[O] = (P2 O5 ) (5) (4) [C] + [O] = (CO) Note: • • • • • All reactions are exothermic. and (7) log fO = −0.604) T }−1 ] (9) . aFe = 1. reaction 1 is the most important since it controls • • • FeO content of slag and oxygen content of steel Loss of iron in slag and hence affects productivity Oxidation potential of slag In addition to the above FeO also helps in dissolution of lime in slag. By equations 6. In steelmaking FeO is present along with other oxides like calcium oxide.285 [wt% O]11 0.734 (12) + 2. We can determine [wt% O] at saturation for different temperatures: T (K) 1873 1923 [wt% O]sat .268 There is a slight difference in the values of dissolved oxygen content in steel.31.285 We note that increase in temperature increases oxygen dissolved in molten iron. SiO2 . manganese oxide etc.e. When pure FeO is in contact with Fe. magnesium oxide. But all equations suggest that increase in temperature increases dissolved oxygen in iron which is in contact with pure FeO. aFeO according to equation 10 is 0.The equation 9 can be used to determine wt% O in steel at any temperature T. In CaO.SiO2 -FeO system. T (K) 1873 1923 [wt% O]9 0. = − 6320 T 6400 T (11) + 2. γFeO depends on slag composition.280 [wt% O]12 0. when aFeO in slag is known.233 0.072 as calculated by equation 9. = − log[% O]sat .. 0. .5 .2665 (10) Consider a slag with NFeO = 0. The above values of dissolved oxygen correspond when pure FeO is in contact with Fe pure. Thus aFeO = γFeO NFeO where γFeO is activity coefficient and NFeO mole fraction of FeO in slag.233 0. log[% O]sat . aFeO = 1. Few other equations are available. as CaO/SiO2 ratio increases aFeO increases.220 0. i. This calculation indicates that control of temperature is important to limit the dissolution of oxygen in molten iron. 11 and 12 at different temperatures using aFeO = 1. physically it means that CaO replaces FeO from FeO. [wt% O] in steel would be 0.514 (NFeO )0. hence activity of FeO is influenced by other solute oxides.229 0. silica. The following expression is used to express aFeO : aFeO = 0.756 The calculations are made on [wt% O] by equations 9. silicon oxidation occurs practically to a very low value since SiO2 reacts with CaO and decreases activity of silica in slag. . Another important feature of silicon reaction is very high affinity of silicon with oxygen.Oxidation of Silicon: Consider reaction 2 K Si = [Si] + 2[O] = (SiO2 ) a (Si O 2 ) [wt % Si ][wt % O]2 [wt% Si] × [wt% O]2 = a (Si O 2 ) K Si Different sources give the following expression for K Si log K Si = log K Si = 30110 T 29700 T − 11. silicon can be used as a deoxidizing agent. Conditions favourable for silicon oxidation are • • Low temperature Low aSi O 2 in slag.308 × 10 −5 [wt % Si ] [wt% ] = � (18) Equation 18 shows drastic reduction in oxygen content of steel due to addition of silicon.25 (13) (14) (15) (16) Both equations predict that decrease in temperature increases K Si . K Si = 3.24 × 105 by equation 16 and using aSi O 2 = 1. we get 0. Normally silicon is used as ferrosilicon in steelmaking. There is a slight difference in values of K Si . A basic slag favours silicon oxidation.4 − 11. By equation 14 [wt% O] = � K a Si O 2 Si ×[wt % Si ] (17) At 1773K. In a basic slag. Equation 15 predicts 15-17% higher K Si than equation 16. This suggests that silicon is a very effective deoxidizer. Both reactions are exothermic. Oxidation of Manganese: Mn is oxidized readily at relatively low temperatures and can form oxides like MnO. whereas reaction 2 is a slag/metal reaction. decarburization. (1) [Mn] + [O] = (MnO) (2) [Mn] + (FeO) = (MnO) + [Fe] The reaction 1 occurs with dissolved oxygen in metal. Reduction of MnO in slag is important. whereas higher temperature favours reduction of MnO of slag and there occurs reversal of Mn. BOF steelmaking Behavoiur of manganese in iron-carbon melt: • • • Mn is soluble in iron in any proportion Mn forms ideal solutions in iron Carbon lowers the activity of Mn in Fe-Mn-C system by forming Mn3 C. Lower temperature favours oxidation of Mn from metal to slag. we get.Lecture 7: Oxidation of manganese and carbon Contents: Behaviour of manganese Oxidation of manganese Reduction of manganese Oxidation of carbon Rimming reaction Illustration Key words: Solidification of steel. MnO2 . we consider reaction 2 K= a (MnO ) a [Fe ] h [Mn ] a (FeO ) Replacing activity by mole fraction and using a[Fe ] = 1. Mn2 O3 etc. (3) . But MnO is stable at high temperature. ∆G° = −27800 + 11. that is reversal of reaction 2 is important. The reduction of MnO in slag transfers Mn from slag to metal and increases the concentration of manganese. 1873K and 1773K respectively) Condition for oxidation of Mn according to equation 6 • • High activity of FeO in slag which means an oxidizing slag Decrease in temperature increases K* according to equation 7. The following are the conditions for the reduction of MnO in slag • • Low activity of FeO in slag which means a reducing slag High temperature which decreases K ∗ Illustration: Consider a slag of basicity 1.N (MnO ) γ (MnO ) K= f Mn [wt % Mn ]× γ (FeO ) N (FeO ) (4) Grouping all activity coefficient terms and putting N(MnO ) ≈ (wt% Mn) We get. Determine the equilibrium content of Mn and O in steel at 1873K.33 at temperatures 1923K.8. Distribution of Mn between slag and metal can be written as φ= (wt % Mn ) [wt % Mn ] log K ∗ = 7940 T = K ∗ NFeO (6) (7) − 3. Reduction of Mn in slag Conditions for reduction of MnO.05 respectively.72 and 20. we can write ln[% Mn] = ∆G ° RT + ln a Fe γ MnO N MnO a FeO (8) .17 According to equation 7 K ∗increases with decrease in temperature (K ∗ = 9. 11. At this basicity the activity coefficient of MnO in slag is 1.25 and 0.6. The mole fraction of FeO and MnO in slag is 0.1. K∗ = K (γ FeO )f Mn γ MnO = (wt % Mn ) [wt % Mn ]× N FeO (5) Where K ∗ is an equilibrium quotient and it depends on composition of slag.8T Using equilibrium constant definition. Given [Mn] + (FeO) = (MnO) + [Fe] . 2665 We get aFeO = 0.e. Carbon oxidation is also known as “decarburizing” reaction (9) [C] + [O] = {CO} K CO = p CO hC hO = p CO [wt % C]×f C ×[wt % O]×f O [wt% C] × [wt% O] = p CO K CO [wt% C] × [wt% O] = p CO K CO ×f 1 C ×f O (10) (11) If we assume fC = fO = unity that is at low concentration of carbon and oxygen in molten metal then (12) . CO gas has a high calorific value and combustion of CO in steelmaking can contribute to energy efficiency. The reader may perform calculations at 1973K and interpret the calculations.032.514 (NFeO )0. we get at 1873K [% Mn] = 0. CO gas can cause slag to foam which leads to increase in surface area. oxidation of carbon is the reaction whose product is gas i. This means that decrease in temperature favours removal of manganese from metal to slag.36 Hence [wt% O] = [wt% O]sat × aFeO = 0.36 = 0.Substituting the values. Therefore this reaction is of very much significance during steelmaking because • • • CO gas during escape from the molten bath can induce stirring in metal and slag phases during steelmaking.048% Using equations ∆G° = −6880 aFeO = 0. CO. Oxidation of Carbon It is important to note that amongst all steelmaking reactions.084% Calculations performed at 1773K shows that [wt% Mn] is 0.233 × 0. 1873 K) / ton of hot metal.05 0.2 atm and 1.87 m3 CO (1 atm and 273K) which is equivalent to 12. 12 Kg C produces 22.0405 0. This volume of CO will evolve no doubt over a period of time but at any time large amount of CO will be escaping the system.2% carbon in steel CO production would be 488 m3 (1atm.5 atm The value of K CO is calculated from log K CO = 1056 T + 2.0048 0. Increase in pCO increases [wt% O] in steel Let us consider the evolution of CO gas. Also care must be taken for the easy and unhindered escape of CO gas from the vessel failing which foaming and eventually expulsion of slag may occur. the product [wt% C] × [wt% O] at a given temperature depends only on partial pressure of CO in equilibrium with melt.0242 0. 12.2 pCO = 1.83 m3 CO (1 atm and 1873 K) Now for 1000 Kg hot metal and 0. According to equation 9.0060 0.0024 0.13 [wt% C] 0. It is important to note that pCO depends on the location of nucleation of CO in steel melt.0040 0.0202 0.0030 From the table we note that • • Decrease in carbon content increases the oxygen dissolved in steel. If CO nucleates deep into the bath then pCO will be greater than atmospheric pressure.4 m3 CO (1 atm and 273K) 1 Kg C produces 1.According to eq.0 [wt% O ] pCO = 1 pCO = 1.0608 0. Let us calculate equilibrium content of carbon and oxygen at 1873K for pCO = 1 atm 1. Production of ultra low carbon steels will be accompanied with dissolved oxygen if precautions are not taken during steelmaking.5 0.1 0.0303 0. This is important in connection with production or ultra low carbon steel for certain applications. .0486 0. Escaping of this gas will agitate the bath and contribute to enhanced rates of mass transfer reactions.0020 0.5 1. This will lead to CO evolution during solidification and is called rimming reaction. As the temperature of molten steel decreases from 1873K to 1773K. . Rimming reaction induces stirring in the solidifying liquid steel and minimizes segregation of solutes.Rimming reaction Other aspect of carbon reaction is the evolution of CO during solidification of steel. K CO increases from 494 to 532 which results in decrease in [wt% C] × [wt% O] as steel cools. ∆ G° becomes positive which results in decomposition of P2 O5 to P and O.365T J/mol At T > 1382K. KP = Now aP 2O 5 [wt % P]2 [wt % O]5 [wt% O] = aFeO [wt% O]sat . and (2) (3) . But removal of phosphorus under reducing conditions is not practical since its removal is highly hazardous. Phosphorus has atomic number 15 and it can give up all 5 electrons from its outermost shell to become P5+ or accept 3 electrons to become P3− to attain stable configuration. Thus P removal is practised mostly under oxidizing conditions. Equilibrium Considerations: Phosphorus removal reaction 2 [P] + 5 [O] = (P2 O5 ) (1) ∆ G° = −740375 + 535. BOF steelmaking Preamble Phosphorus removal from hot metal is the most important refining reaction. This means that phosphorus can be removed both under oxidizing as well as reducing conditions. steelmaking reactions.Lecture 8: Dephosphorization Reaction Contents Preamble Equilibrium considerations How low γP 2 O 5 should be? Effect of FeO and CaO on dephosphorization Illustration Conditions for dephosphorization Conditions for simultaneous removal of C and P Key words: Dephosphorization. Thus removal of phosphrous requires that aP 2 O 5 must be reduced. 31 at 1773K. NFeO = 0.091NSi O 2 � − + 23.01. = − 6320 T + 2.1 andNSi O 2 = 0.591NMnO + 0. We get γP 2 O 5 = 4. .58 42000 T (7) + Alkaline oxides Na2 O and BaO are stronger than CaO but they are corrosive to the refractory lining and hence not used.12.log[wt% O]sat .1 and mole fraction of NP 2 O 5 in slag = 0. NMnO = 0.16 × 10−16 Now the question before us: how to attain such a low value of γP 2 O 5 in a slag of given composition? Such a low value of γP 2 O 5 can be attained when we use basic oxides which have a very strong tendency to form a stable chemical compound. NMgO = 0.734 (4) By equation 2 and 3 and replacing aP 2 O 5 by using Raoult’s Law and after rearrangement N (P 2 O 5 ) [wt % P]2 K P (a FeO )5 [wt % O]sat . log K P = 38668 T At 1773 K.96 (6) K P = 7.324 × 10−18 Decrease in temperature increases γP 2 O 5 which favours dephosphorization reaction. The different basic oxides have different ability to lower γP 2 O 5 .682NMgO + 0.06.64 �NCaO + 0.545NFeO − 0.6 We calculate γP 2 O 5 at different temperatures T (K) 1773 1823 1873 γP 2 O 5 1. The LHS of equation 5 is index of dephosphorization and denotes distribution of phosphorus between slag and metal.56. Consider a slag NCaO = 0. − 27.06 × 10−7 [wt% O] can be determined by equation 3 and 4. Let us calculate γP 2 O 5 which will allow dephosphorization. γ P 2O 5 = (5) γP 2 O 5 is activity coefficient of P2 O5 in slag. Initial %P in metal is 0.74 × 10−18 0. How low γP 2 O 5 should be? Consider dephosphorization in a slag of aFeO = 0. The following expression describes the relative effects of basic oxides on γP 2 O 5 . log γP 2 O 5 = −24.778 × 10−18 0. We substitute the values in equation 5. Higher value of LHS demands low γP 2 O 5 in a slag of a given composition. Optimum value of FeO is more or less independent of the basicity of slag. Conditions for dephosphorization: Dephosphorization requires oxidizing and basic slag: 2[P] + 5 (FeO) + 3 (CaO) = (3 CaO. Further increase in FeO beyond 15-16%.P 2 O 5 ) (a [P ] )2 • • • ≈ (wt % P) [wt % P] 3 = K 9 × �a(CaO ) � (a(FeO ) )5 (9) (10) (11) aCaO in slag should be high. Thus control of FeO in slag is important for efficient dephhosphorization. dephosphorization decreases. High basicity of slag is required. However for efficient dephosphorization the FeO content of slag should be in between 15 to 16%. P2 O5 ) K9 = a (3 CaO . The above behaviour is due to the dual role of FeO. FeO is the source of oxygen for oxidation of P according to the following reaction (8) 2[P] + (FeO) = (P2 O5 ) + [Fe] For a given basicity of slag. aFeO in slag should be high.1: increase in FeO content of slag and becomes maximum in between 15-16% FeO at all basicities.1 shows the variation of dephosphorization index (wt% P2 O5 )/[wt% P] as a function of wt%FeO for CaO-FeO-SiO2 slag at different basicities. The above behaviour can be observed at all basicities of slag. Any undissolved CaO will not be effective for dephosphorization. The maximum dephosphorization ratio increases with the increase in the basicity of slag as can be seen in the figure 7. Conditions for simultaneous removal of C and P . Higher basicity requires higher amount of CaO dissolved in slag. The dephosphorization ratio increases with Figure 7. as FeO content of slag increases oxidizing power of slag increases and phosphorus oxidation according to reaction 8 will be favoured because CaO of slag decreases the activity of P2 O5 by forming a stable compound. Beyond the optimum value of FeO in slag FeO replaces CaO and may either combine with CaO or with P2 O5 .Effect of FeO and CaO on dephosphorization Figure 7.P 2 O 5 ) 3 �a (CaO ) � (a (FeO ) )5 (a [P ] )2 a (3 CaO . FeO is a weak base compared with CaO as a result of which the dephosphorization ratio decreases with addition of FeO beyond an optimum value. slag should be oxidizing. Low temperature favours high K 9 .1. This means slag should have free dissolved lime. if carbon and phosphorus are to be removed simulataneously.6 × 10−21 1. Thermodynamically slag is required in which activity coefficient of P2 O5 is very low. Let us calculate the γP 2 O 5 when molten metal contains 2% C and 0.42 × 10−23 . K CO = 1305 T (17) + 1. The question is how low activity of P2 O5 should be?. Substituting all the values into equation 16 we get γP 2 O 5 . Thus. The results are given below T(K) 1673 1773 1873 The calculations show: γP 2 O 5 1.979 We can calculate K P and K CO from equations 16 and 17. This value can be determined by equations 14 and 15 γP 2 O 5 = K P [wt% P]2 K CO 5 [wt% C]5 NP 2 O 5 Replacing [wt% O] in equation 14 and 15. We can also calculate γP 2 O 5 at temperatures 1673K and 1873K. an important requirement is the availability of slag which acts as a sink for (P2 O5 ). Consider the following reactions occurring simultaneously (12) [C] + [O] = {CO} (13) 2[P] + 5[O] = (P2 O5 ) K CO = KP = p CO [wt % C][wt % O] (14) N P 2O 5 γP 2O 5 (15) [wt % P]2 [wt % O]5 It is assumed in eq. (16) From equation 16 one can determine the value of activity coefficient of P2 O5 which can lead to simultaneous removal of carbon and phosphorus. and after rearrangement. Both reactions 12 and 13 require oxygen but reaction 13 requires a slag which is basic in nature in addition to oxygen.15% P and temperature T=1773K.1. 14 and 15 that henrian activity is equal to (wt %).32 × 10−22 1.Removal of C and P both require oxidizing conditions but P removal is possible only when a basic and limy slag is formed. The mole fraction of P2 O5 in slag is 0. Ghosh and A.• • Both decarburization and dephosphorization are possible simultaneously in presence of slag in which γP 2 O 5 has extremely low value. Modern methods of steelmaking 3) A. Tupkary et.K. References for lectures 5 to 7 1) A. Thus low temperature is favourable.al. Chatterjee.Chakrabarti: Steelmaking . Low temperature requires γP 2 O 5 in slag to be higher than at high temperature. Ironmaking and steelmaking 2) R. The diversification on steel products and their cleanliness requirement in recent years have increased the demand for high quality refractory. High quality refractory at a cheaper cost is the main requirement because cost of refractory adds into the cost of product. metal and gases. furnaces Role of refractory Refractory materials have a crucial impact on the cost and quality of steel products. chemically and carbon bonded materials that are made in different combinations and shapes for diversified applications. refractory materials are required to produces steels. Steelmaking requires high temperatures of the order of 1600 degree centigrade. steelmaking. Why required? • To minimize heat losses from the reaction chamber . These phases are chemically reactive. slag and hot gases. It is necessary to produce range of refractory materials with different properties to meet range of processing conditions. What is a refractory? Refractories are inorganic nonmetallic material which can withstand high temperature without undergoing physico – chemical changes while remaining in contact with molten slag.LECTURE 9 Refractory Materials Contents: Role of refractory What is a refractory? Why required? Refractory requirements Melting point of some pure compounds used to manufacture refractory Properties required in a refractory Types of refractory materials Insulating materials Key words: Refractory. The refractory range incorporates fired. In addition steelmaking handles high temperature phases like molten steel. temperature 1600℃ Gases: CO. W. Fe3 O4 etc.• To allow thermal energy dependent conversion of chemically reactive reactants into products because metallic vessels are not suitable. In steelmaking. The above phases are continuously and constantly in contact with each other and are in turbulent motion. FeO. Ni. oxygen and hydrogen and different alloying elements like Cr. Melting point of some pure compounds used to Manufacture refractory MgO (pure sintered) Melting point (℃) CaO(limit) 2571 SiC pure 2248 MgO (90-95%) 2193 Cr2O3 2138 Al2O3(pure sintered) 2050 Fireclay 1871 SiO2 1715 Compounds 2800 . Ar containing solid particles of Fe2 O3 . temperature 1300℃ to 1600℃ . Mo etc. non metallic inclusions.. the physico.. N2 .chemical properties of the following phases are important: Slag: Mixture of acidic and basic inorganic oxides like SiO2 . Refractory requirements: The refractory materials should be able to withstand • • • • High temperature Sudden changes of temperature Load at service conditions Chemical and abrasive action of phases The refractory material should not contaminate the material with which it is in contact.etc. CaO. MgO. phosphorous.. temperature varies in between 1400℃ to 1600℃. tramp elements. manganese. dissolved gases like nitrogen. Molten steel: Iron containing carbon. Mo. silicon. Nb. CO2 . P2 O5 . e. Porosity and Slag permeability Porosity affects chemical attack by molten slag. But strength of bricks of higher specific gravity is greater than one with lower specific gravity. It should be higher than the application temperatures. The refractoriness is indicated by PCE (Pyrometric cone equivalent). hence strength under the combined effect of temperature and load. In the heat treating furnaces solid/reducing or oxidizing gases are simultaneously present. in steel-making vessels slag /metal /gases are simultaneously present in the vessel at high temperatures. i. metal and gases. SiO2) 1816 Chromite (FeO. Decrease in porosity increases strength and thermal Conductivity. Strength It is the resistance of the refractory to compressive loads. refractoriness under load is important. In some industrial units more than one phase are present e. In taller furnaces. Below are briefly described the properties of the refractory materials: Refractoriness Refractoriness is a property at which a refractory will deform under its own load. tension and shear stresses.g. .Kaolin (Al2O3. Cr2O3) 2182 Properties required in a refractory The diversified applications of refractory materials in several different types of industries require diversified properties to meet the physico-chemical and thermal requirements of different phases. Therefore more important is refractoriness under load (RUL) rather than refractoriness. the refractory has to support a heavy load. Spalling Spalling relates to fracture of refractory brick which may occur due to the following reasons: • A temperature gradient in the brick which is caused by sudden heating or cooling. Specific gravity Specific gravity of the refractory is important to consider the weight of a brick. Refractoriness decreases when refractory is under load. Cost of bricks of higher specific gravity is more that of lower specific gravity. of thermal expansion Spalling tendency ∝ max m shearing strain �thermal diffusivity Spalling tendency ∝ max m tensile strength �thermal diffusivity On sudden cooling coeff . Thermal conductivity Thermal conductivity of the bricks determines heat losses. heat capacity. Types of refractory materials . Bulk density: Decrease in bulk density increases volume stability.• • • Compression in a structure of refractory due to expansion Variation in coefficient of thermal expansion between the surface layer and the body of the brick Variation in coefficient of thermal expansion between the surface layer and the body of the brick is due to slag penetration or due to structural change. of thermal expansion Permanent Linear change (PLC) on reheating In materials certain permanent changes occur during heating and these changes may be due to • • • • Change in the allotropic form Chemical reaction Liquid phase formative Sintering reactions PLC(%)linear = Increase /decrease in length original length PLC%(volume) = × 100 Increase /decrease in volume original volume × 100 These changes determine the volume stability and expansion and shrinkage of the refractory at high temperatures. Increase in porosity decreases thermal conductivity but at the same time decreases strength also. On sudden heating coeff . A) Chemical composition Refractories are composed of either single or multi-component in organic compounds with non metallic elements.Magnesite refractory: used in inner lining of BOF and side walls of soaking pits. ZrO2 and alumino. Basic refractory Raw materials used are CaO.( basic refractory) d) Magnesite: Basic refractory in nature. Neutral refractory Neutral refractory is chemically stable to both acids and bases. They are manufactured from Al2 O3 . For details readers may see the references given at the end of lecture 10. b) Magnesite: Carbon refractory with varying amount of carbon has excellent resistance to chemical attack by steelmaking slags. These refractories are machine pressed and have uniform properties. . Wet ramming masses are used immediately on opening. They are mixed with water for use. quartz and silica. MgO. Special shapes with required dimensions are hand molded and are used for particular kilns and furnaces. Different types are: i.silicate. Typical refractories are fireclay. Below is given type of refractory depending on its chemical composition and physical shape.This can be discussed in several ways. B) Physical form Broadly speaking refractory materials are either bricks or monolithic. Magnesite bricks cannot resist thermal stock. Cr2 O3 and carbon. Basic refractories are produced from a composition of dead burnt magnesite. loose strength at high temperature and are not resistant to abrasion. Ramming refractory material is in loose dry form with graded particle size. dolomite and chrome-magnesite. for example chemical composition of refractory or use of refractory or method of manufacture or in terms of physical shape. They cannot be used under basic conditions. chrome ore. Acid refractory The main raw materials used are SiO2 . c) Chromite. They are used where slag and atmosphere are acidic. dolomite. Shaped refractories are in the form the bricks of some standard dimensions. a) Magnesite: Chrome combinations have good resistance to chemical action of basic slag and mechanical strength and volume stability at high temperatures. Choice of insulating materials would depend upon its effectiveness to resist heat conductivity and upon temperature. Plastic refractories have high resistance to corrosion. Ceramic fibres are important insulating materials and are produced from molten silica. used as insulating materials. High alumina with thermal conductivity 0. They are long weight.04 m℃ kcal . Monolithic refractories Monolithic refractories are replacing conventional brick refractories in steelmaking and other metal extraction industries.ii. short fibres and long fibres. titania. Insulating materials are porous in structure. Bricks are joined with mortars to provide a structure. excessive heat affects all insulating materials. Insulating materials The role of insulating materials is to minimize heat losses from the high temperature reactors. spraying etc. Castables refractory materials contain binder such as aluminate cement which imparts hydraulic setting properties when mixed with water.Gupta: Fuels. These materials are installed by casting and are also known as refractory concretes. These are used to fill the gap created by a deformed shell. Furnace and refractory . Ramming masses are used mostly in cold condition so that desired shapes can be obtained with accuracy. Mortars are finely ground refractory materials.P. Monolithic refractories are loose materials which can be used to form joint free lining. iii. which become plastic when mixed with water. and to make wall gas tight to prevent slag penetration. References: O. Zirconia etc in the form of wool. They have excellent insulation efficiency.028 kcal 0. These materials have low thermal conductivity while their heat capacity depends on the bulk density and specific heat. m℃ and silica with thermal conductivity etc are amongst others. iv. Plastic refractories are packed in moisture proof packing and pickings are opened at the time of use. The main advantages of monolithic linings are • • • • • Grater volume stability Better spalling tendency Elimination of joint compared with brick lining Can be installed in hot standby mode Transportion is easier Monolithic refractories can be installed by casting. . MgO − Carbon refractory materials with 15% high purity graphite have been found to provide increased corrosion resistance. The cylindrical portion of the converter (barrel) is lined with the ramming mass of tar dolomite and tar dolomite bricks. molten steel is in contact with slag. ladle and tundish.Lecture 10: Refractory in steelmaking Preamble BOF refractories Refractory for secondary steelmaking Refractory for continuous casting Refractory for circulation degassing Refractory for high temperature furnaces Emerging trends Refractory maintenance Future issues Assignment Key words: steelmaking. ladle. tundish. chrome-magnesite and Mag-chrome bricks. electric furnace. ladle metallurgy. BOF refractories Converter is lined with a permanent lining and above it there is a wear lining. In converter. fireclay. Electric arc furnace. In duplex blowing (hybrid blowing or combined blowing) MgO − C bricks are commonly employed for the bottom tuyeres and around them. and reheating furnaces. heat treatment and surface hardening methods. since these areas severely worn. The detachable bottom is constructed by using mica. refractory Preamble In steelmaking. electric furnace. refractory materials are used in converter. Permanent lining thickness may vary from 100mm to 120mm and is made of chrome-magnesite permanent lining which is given on the full height of the converter. wear lining is constructed. whereas in reheating furnaces steel in the solid form is reheated for deformation processing. Above the permanent lining. Fireclay bricks are used. Following condition may be noted: i) ii) iii) iv) v) High temperature and long holding times of steel in ladle. Molten steel agitation causes attack by motion of liquid steel. There are made of MgO boards or alumina bricks. Isostatic pressed submerged nozzle with alumina. and are excellent in oxidation resistance.graphite-fused silica are being used. mechanical erosion by molten steel movement contribute to the wear of the lining materials. . Refractory for continuous casting Tundish is a refractory lined vessel in continuous casting. Tundishes are equipped with dams and weirs. Wide variation in slag composition Many types of vacuum treatment. Refractory for secondary steelmaking There are many operation and process in secondary steelmaking like vacuum degassing. ladle refining etc. High alumina bricks are considered to be good for tundish hot rotation. MgO − C bricks with addition of a couple of metals provides high hot strength. MgO − C bricks are used at areas. chemical attack by slag. high alumina bricks are widely used. Some developments to counteract this lining wear are: i) ii) iii) iv) Dolomite (40% MgO) is added to create a slag of about 8% MgO which is close to saturation level of slag. Basic coating material is used over the lining. In certain cases MgO − C. Large thermal changes. Submerged nozzles must be resistant to corrosion and spalling. Molten steel from tundish to mold is fed by nozzle submerged into molten steel in mold. higher campaign life and cleanliness of steel. Critical wear zones (impact and top pads. Slag splashing in which the residual slag is splashed by high speed N2 has resulted into high lining life (refer lecture 14) Lowering FeO levels in slag and shorter oxygen-off to charge intervals have reduced refractory wear. Refractories are used in unique combinations of various bricks to meet diversified requirements. Selection of refractory is critical due to longer casting sequence. Typically MgO − SiO2 − Al2 O3 mixture is used as a coating material. Al2 O3 − C bricks and castables are used in impact areas. It contains molten steel with minimum heat losses.The slag and metal penetration between the refractory grains. where slag is in contact with steel. ASE-SKF. faster tundish turnaround. For general wall. slag tapping and trunion areas) are lined in furnaces with high quality bricks. For bottom zircon bricks are used to prevent molten steel penetration into brick joint. The coating installation method is gunning. nozzle clogging is also important. In all ladle refining processes such as ladle furnace. VAD process. slag coating. Fireclay and high alumina refractories are used. Al. Largest problem with use of monolithic refractories are: . bottom tuyere refractory in hybrid blowing. In this connection mention may be made of some refractory like MgO-C. immersion nozzles in continuous casting etc. gunning (flame gunning involves melting and spraying on hot surface). impact area of molten steel stream. Most of the continuous furnaces are lined with fireclay bricks.Mg and Al-Si alloy stabilized MgO – C brick. semi rebonded magnesia chrome bricks are used in the lower vessel and snorkels. Installation of the refractory is important. There are two basic types of monolithic lining. Direct bonded magnesia.In recirculation degassing steel is made to flow from the ladle into a separate degassing chamber. durability of monolithic lining depends on the design and installation of the anchors. These are materials which are installed in some form of suspension that ultimately hardens to form a solid mass. namely castable refractory and plastic refractory Castable refractory consists of mixtures of coarse and fine refractory grains together with a bonding agent which is normally based on high alumina cement. The refractory materials must have adequate spalling and abrasion resistance. and Al2 O3 – C b) Repairing methods like slag splashing. In RH process. Emerging trends Refractory has undergone many changes to meet the diversified requirements of the industry particularly steel industry. hot patching. The main objective is to increase the lining life at reduced cost by developing a) High quality refractory for critical applications in steel making at e. Extra high temperature burned magnesia –chrome bricks posses excellent corrosion and abrasion resistance and are preferred lining material. Due to relatively poor strength. a refractory lined vessel equipped with two legs (snorkels) is used. Refractory lining for high temperature furnaces Furnaces are used for heating steel within the temperature range 1000℃ to 1200℃ for heat treatment and deformation processing. Plastic chrome ore ramming mixture and hard burnt chrome magnesite bricks are used to line the hearth to provide resistance to scale. Many different types of furnaces are used namely soaking pits (batch type) and continuous furnaces. MgO – Ca O – C. zircon based refractory. Al2O3 – Si C – C. These snorkels are immersed into molten steel. Monolithic linings are installed by casting the refractory in a mould or by spraying the furnace shell.chrome bricks. c) Monolithic refractory Monolithic refractory Monolithic linings are a relatively recent development and consist of unshaped refractory products. slag line. volume stability and corrosion resistance at high temperature and in vacuum.g. Flame gunning involves simultaneous melting of a refractory powder and gunning at the hot surface. Durability of refractory for pairing nozzles and side dams determines the success of strip casting. i.e. Hot patching Self flowing refractory mixtures enable precise maintenance of the scrap impact zone. 3. Super fine powder processing technology to produce refractory. Furnace refractory maintenance: The following methods are commonly practiced. The splashed slag gets coated on the lining. maintenance of pre. Since the gunned repair material is dense and fused directly on the hot surface excellent results on life of lining is obtained in LD converter. Future issues of Refractory technology 1. vessel lining life can be extended. Slag coating and slag washing The small amount of liquid slag is retained in the vessel after tapping. 2. Slag is enriched with dolomite or raw dolomite to cool the slag and to increase its adhesive properties. some amount of slag is retained. In case of hybrid blowing practice formation of skull may result in a failure of the bottom stirring elements.worn areas with special gunning mixtures.high temperature refractories. Slag splashing Slag splashing is done in steelmaking vessels. Composition of slag with respect to FeO and MgO is adjusted. Gunning By gunning. Nitrogen is blown from top to splash the slag. excess slag is then poured before charging. tapping pad and bottom joint. Long drying time Steam explosion. To reduce excessive slag build up in the bottom. After steel tapping. Technology of mass melting of scrap in converter by using post combustion requires super. . FeO makes the slag adhesive on the lining and MgO makes the lining high temperature resistant. Vessel is rocked several times to coat the bottom and bottom joint with a slag. ISIJ Intern. PP 619-633 Y. Mills et.Mullinger and B. 5. Why? 7) Why is it necessary to add anti-shrinkage material for the manufacture of fireclay briskc from naturally ouuurring clay ores? 8) How are insulating bricks manufactured? . 5. 45(2005).4. Is it possible to use these bricks without any thermal treatment? 5) High alumina bricks are better than fireclay. No.: A review of slag splashing.al. Nano tech refractory is thermal shock and corrosion resistant The nano-particles act in two ways → They consist of mono spheres and improve properties like elasticity and strength → Control of molecular structure as the particles have many small pores of several hundred nanometers.Naruse: Trends of steelmaking refractories Assignments based on lecture 9 and 10 1) What do you understand by the spalling tendency of a refractory brick? Give reasons. 2) What is meant by refractoriness under load? What is its importance? 3) Explain the term inversions in relation to the behavior of silica brick on heating and cooling. Reference: P. 4) Silica bricks are manufactured from a naturally occurring quartzite. which contains 98% SiO2. Why? 6) High magnesite refractory show good resistance to attack by iron oxide. Use of monolithic refractory in steel making and refining furnaces require automating brick lying and intelligent repair. Jenkins: Industrial and process furnaces Kenneth C. . P and S from hot metal to the extent it is possible. These processes include top blown steelmaking and combined blown steelmaking processes. free from internal cracks. gas jet Preamble Converter steelmaking processes are also known as BOF (Basic oxygen Furnace) steelmaking. P and Si of hot metal to produce steel with good surface finish. Bonds between Fe and S are strong.0. oxygen is blown from top. pretreatment of hot metal. In all BOF processes. 0. In most of the steelmaking practices hot metal is pretreated to remove Si. These processes are based on hot metal. This lecture discusses pretreatment of hot metal and material balance. [S] + (O2− ) = (S 2−) + [O]. P and Si removal reactions are dealt in lectures 6.2 % P. lance. a S 2− [Wt % S ] = (Wt %S) [Wt %S] = K f s a O 2− [Wt % O] 1) 2) . Blast furnace hot metal contains 3-4 % C.8 to 1% Si.8 % Mn and 0. Si and P. 7 and 8. 0. Activity coefficient of S increases with increase in C. Desulphurization reaction is a slag /metal reaction. The objective is to reduce S.L 11 Converter steelmaking Contents: Preamble Pretreatment of hot metal Removal of sulphur Reagents for hot metal pretreatment Location of hot metal pretreatment Material balance Design of converter Lance Shop layout Feed materials Key words: BOF steelmaking.6 to 0. Sulphur exhibits negative deviation from Henry’s Law in molten iron. Here S removal is discussed Removal of Sulphur Removal of sulphur is called desulphurization.15 . material balance. Pretreatment of hot metal In recent years pretreatment of hot metal prior to charging in converter has become common practice. Hot metal from Blast furnace is refined to steel. Carbon. The best conditions for dephosphroization are discussed in lecture 8. From equation 2 the best conditions for desulphurization can be derived. Presence of silicon in hot metal impedes dephosphorization. Also soda ash generates dense fumes on addition to hot metal. Reaction 1 suggests that removal of S from hot metal is accompanied by oxygen transfer to metal. basic slag and oxidizing conditions. or in transfer ladle or in torpedo. (Wt % S)/[Wt % S] is called partition coefficient of sulphur or index of desulphurization.1a) has certain advantages: such as adequate mixing of the reagents due to flowing of hot metal. • In some situations mill scale and sinter fines are used to desiliconize hot metal. Low [Wt % O] in metal. Figure 11. silicon and phosphorus increases fs which means hot metal is better for desulphurization than molten steel. Location of hot metal pretreatment Hot metal pretreatment can be carried out either at blast furnace runner.K is equilibrium constant of reaction 1 and fs is activity coefficient of sulphur in metal. Reagents for hot metal pretreatment • Soda ash It is effective reagent for both desulphurization and desiliconization. • Calcium carbide and Magnesium granules Both are highly efficient desulphurizing agents and can decrease sulphur content to very low value. This oxygen must be removed for efficient desulphurization.e. Magnesium granules help to reduce injection time and slag volume. But disposal of soda bearing slag is a problem. • High fs i. Pretreatment in the blast furnace runner (Figure 11. This practice saves time and increases ladle availability compared with when treatment is carried out in ladles. High aO 2− in slag which means highly basic slag. Dephosphorization requires low temperature. high activity coefficient of sulphur in metal. • • • Silicon can be removed easily by oxygen and is discussed in detail in lecture6. Calcium carbide along with lime is injected into the bath.1 (a) Blast furnace runner Material balance (b) Torpedo car . High temperature which leads to high K. Mn and Fe balance. Calculate amount of steel. The composition of slag is CaO 54%. Basis: 1000 kg hot metal.Hot metal of composition 0. 4% C and rest iron is refined in a converter to produce steel of composition 0.2% P. oxygen and waste gases per ton hot metal. Carbon balance: C in hot metal + C in scrap = C in steel + C in waste gases (5) C in waste gases = 39. Design of converter From the metallurgical point of view an ideal converter keeps the liquid steel in space and allows all necessary metallurgical reactions to take place within the temperature range of1400 − 1600℃.21 kg Exit gas volume = 73. The volume of exit gas is6800m3 . X = 1080. FeO 18%.07 kg C in CO2 = 5.8% Si. During refining scrap is charged whose amount is 15% of hot metal.68 kg steel Y = 126.5. C.70 m3 (1atmospheric pressure and 273K) Material balance gives an idea of the charge materials required. These are the approximate values and are given to develop a feel of the converter design. and MnO 2.5%. 0.7 kg slag. O2 required can be calculated from Si. Pure oxygen is blown. .25% Mn.93 m3 (1 atmospheric pressure and 273K). For example 100 ton capacity would require approximately 93 tons hot metal and 1 ton scrap.86 kg C in CO = 33. 0. Fe balance: Let X kg is mass of steel and Y kg mass of slag Fe in hot metal + Fe in scrap = Fe in steel + Fe in slag (3) Mn balance: Mn in hot metal = Mn in slag (4) Putting the values in 3 and 4 we get. withCaO |Si O2 = 3.1% C and rest iron. Exit gases analyses 15% CO2 and 85% CO2 . slag. The amount of slag would be 12 tons and total oxygen required would be 1900m3 . Calculations give O2 = 52. V̇ min . A ratio of 3 m3 (internal volume/ ton) is typical in converter design. A modern LD-converter consists of a top cone with the lip ring a barrel section and a lower cone with a dished bottom. and stability of nose lining.386 . Lance .2 Figure 11. 273K). V̇ = 460 hw ≈ 3. which keeps the liquid steel in space. is steel shell lined with refractory material. (hb ) diameter of bath (db ) working height of the converter. (hw ) as shown in the figure 11. The inner volume enclosed by the refractory should be maximized so as to achieve an optimum metallurgical process without sloping of slag.328 (T)−0. skulling. The vessel is supported by a suspension system which transmits the load to the trunion ring.7 m excluding bottom refractory thickness.5 × bath height = 5. total converter height from conical to bottom becomes approximately 8m .48m.tons = 3. and db (m) = 0. For 150 ton converter capacity hb = 1. The nose diameter and angle are chosen with reference to problems of heat loss.704 (T)−0.0148 T is capacity in tons.07 Where V̇ is m3 of oxygen. Assuming bottom refractory thickness to be around 1 to 1.2 m.2 Nomenclature of the bath dimensions of a converter Some correlations are given below: hb db = 0. Total height of converter is 6. erosion.The mechanical part.5 m. m3 min (1atm. Converter design requires knowing height of molten steel bath.87m. db = 4. The lance is designed to operate at an upstream pressure of 10. Failures of the lance may be due to faulty cooling. fluxes. The shop is provided with separate cranes for handling hot metal. scrap and slag The hot metal mixer should be located on the ground floor. manufacturing defects and differential expansion between copper tip and steel tube. refined steel. To produce droplets Lance is designed to produce non-coalescing free oxygen get. Lance movement is controlled by electrically operated gear system. The most important part of the lance is the Laval nozzle. movement of oxygen lance and hot metal. . An efficient process control strategy using computers and automatic spectro-chemical analytical methods is required.12 bar. the tip of which fitted with multi-hole Laval nozzles made of copper Figure 11. Also layout should ensure smooth flow of ladles containing hot metal and steel. Functions of the nozzle are • • • • Supply and distribution of oxygen To produce a gaseous jet To induce bath agitation.3: Lance to blow oxygen Lance is nearly 8-10m long and its diameter varies between 20cm to 25cm depending on the furnace capacity. The refining of hot metal to steel is very fast and hence an efficient system of material transport and weighing is required. The number of vessels in a shop may be generally two or three and one out of two or two out of three operating at a time. Water requirements are around 50 − 70 m3 ⁄hr at a pressure of 5 − 7 kg⁄cm3 . Lance life is determined by the life of the nozzles.Oxygen gas is supplied through a water cooled lance. The refractory lining maintenance facilities must be adequate. Some essential considerations of layout A tall shop is required to raise and lower the lance in the vessel An elaborate gas cleaning facilities are required. Shop Layout Layout requires rational arrangement of equipments to ensure smooth handling of solid raw materials like scrap. Carbon is added to recarburize steel if required . Ni. Proportion of hot metal in the charge is 75-90% Fluxes Commonly used lime/limestone/ dolomite to bring down the softening point of the oxides. Refining reactions are exothermic Deoxidisers and alloying elements Elements like Al. Scrap and ore Used as coolants to best utilize the excess heat energy. Iron ore is sometimes used by some plant as a coolant and to promote slag formation. Source of heat: No external heat.8% Temperature of hot metal at charging is around 1250℃ to 1300℃. Nb etc are added as alloying elements. The elements like Cr. and also with single and multi-hole designs and is approximately 2. ferrosilicon and ferromanganese are added as deoxidisers. MnO tends to retard the dephosphorization of the bath. Mn could be 0.5 − 3 m3 ⁄min. to reduce the viscosity of slag and to decrease the activity of some components to make them stable in the slag phase.Feed Materials: • • • • • Hot metal Cold pig iron Steel scrap Fluxes Gaseous oxygen Hot Metal Sulphur in the hot metal should be close to final specification level Silicon content of hot metal determines amount of lime and slag. Oxygen Consumption of oxygen per tonne of steel varies with proportion of scrap and ore.5 to 0. Mn in hot metal produces MnO. V. p.Ghosh and A Chatterjee: Ironmaking and steelmaking . Vol.257-262.6. 59 (1988). No.References: S C Koria Dynamic variations of lance distance in impinging jet steelmaking processes Steel Research. A. Initially oxygen is blown soft by keeping lance distance higher to promote slag formation and to avoid ejection of small particles.87 m and X i = 2. Scrap and hot metal are charged. Oxygen is blown for nearly 15-20 minutes by progressively decreasing the lance distance such that slag foaming remains under control and oxidation reactions occur uninterruptedly.04 db is bath diameter in meter. Slag and metal samples are analyzed. lining is inspected. Converter is tilted into the vertical position and the lance is lowered in the vessel to start the blowing. db = 4. because hot metal is not covered by slag.Lecture 12 Converter Steelmaking Practice & combined blowing Contents: Refining of hot metal Composition and temperature during the blow Physico-chemical interactions Developments in Top blown steelmaking practice Concept of bottom stirring in top blowing Top blowing attributes Characteristic feature of converter steelmaking Environmental issues in oxygen steelmaking Causes of high turnover rates of BOF Key words: Top blown steelmaking. Lime may be added either at the beginning of the blow or in portion during the blow. For 150 Tons converter. bottom stirring. combined blowing. when oxygen flow rate in approximately 450 Nm3 /min. hot metal refining Refining of hot metal After the previous heat is tapped and slag is drained.8 m. Composition and temperature during the blow . The starting lance distance(X i ) for specific oxygen blowing rate 3 Nm 3 ton ×min can be calculated by X i = 0. Selection of the starting lance distance is such that the concentration of the force at the bath level should not cause ejection of tiny iron particles (sparking) and at the same time maximum bath surface area is covered by the oxygen jet.541(db )1. chemical interactions of molten bath with oxygen jet depends on the lance profile i. • Formation of slag begins with the oxidation of Si − SiO2 . hot metal composition. force of the oxygen jet creates metal droplets and as a . • Temperature of the bath increases continuously. Nevertheless. Oxygen jet penetrates into the bath and carbon reaction favours because oxygen is available now deep into the bath. The idea is to create a basic and limy slag at the early part of the blow to onset dephosphorization. Also Fe oxidizes to FeO. in all converters initial lance distance is such as to promote iron oxidation so that dissolution of CaO commences. • In the initial stages. carbon removal rate can be increased. Once phosphorus removal is complete. • Formation of basic and limy slag promotes removal of P. In the initial periods FeO helps lime dissolution. Mn → MnO and Fe → FeO • Dissolution of lime increases during the blow. • Note that Mn content of metal decreases initially but at later periods of blow Mn content of bath increases. Shallow jet penetration covers the larger bath surface and is favorable more for iron and silicon oxidation. Small amount of carbon may be removed. This is due to the onset of the following reaction: (MnO) + [Fe] = [Mn] + (FeO) In the later stages of the blow bath temperature increases due to decrease in carbon content and at the sametime FeO content of slag decreases. It may be noted that once slag is formed both C and P removal occur simultaneously. If carbon is removed at a faster rate in the beginning bath temperature would increase which impedes dephosphorization. oxygen flow rate.1 Figure 12. Si. carbon removal rate is kept lower than P removal since P removal is favoured at lower temperatures. sometimes iron ore additions are made to increase the FeO content of slag to adjust the Mn content of steel. Both conditions are responsible for increase in Mn content of the bath.Typical variation of composition of metal with the blowing time is shown in figure 12. Mn begin to oxidize simultaneously. At the same time. Rate of carbon removal is low in the beginning. Physico-chemical interactions Physico. The lance profile is specific to each converter and depends on converter profile. lance is lowered. Si and Mn oxidize faster relative to C. change of lance height during the blow. To overcome. Once slag is formed.1: Variation of composition of metal during the blow The following observations can be made: • Impurities like C.e. hot metal chemistry and steel of desired composition. carbon reaction commences. Evolution of CO is the principle cause of bath agitation. O2 jet could not produce adequate bath stirring. oxygen jet penetration into bath is shallow and slag formation occurs. typically observed in top blown steelmaking practice. Reaction between C and FeO of slag in slag will not allow CO bubbles to grow. both carbon and phosphorus removal occur at a faster rate. B1 There phase dispersion of slag/ gas bubbles / metal droplets We note that at higher lance distance. CO forms.2 shows the lance profile and the accompanying physico-chemical interactions A= hot metal. number and arrangement of bottom tuyeres and type of bottom injection elements i.e.3: Rate of carbon removal as a function of time of blow In the hatched regions. Foaming of slag has to be controlled to avoid expulsion of slag. jet penetrates deep into the bath.2 b and c).3 shows decarburization rate Vs time. These processes differ in terms of bottom gas rate. porous plugs or tuyeres and whether inert gas or oxidizing gas is used. Figure 12. In another type. Smaller size gas bubbles can be trapped easily in slag as compared to larger sizes. bath agitation is very weak particularly during the initial and final stages of the blow. which can be controlled by controlling C reaction with FeO in slag. Figure 12.2 a) As the lance distance is decreased. Both in initial periods (silicon oxidation period) and in final periods (where rate of carbon removal is mass . Slag may foam and may be expelled from the converter. C= oxygen jet. CO evolution in the bath is very low in pure top blown steelmaking.consequence three phase dispersion of gas/slag /metal droplets are formed which enhance the rate of decarburization. Developments in Top blown steelmaking practice The most important development in top blown steelmaking practice is the simultaneous gas stirring of the bath form the bottom of the converter. This has resulted in combination blowing processes. Three phase dispersion creates conditions for faster removal rates of C and P. oxygen is blown form top and bottom and is called top and bottom blowing processes. Figure 12. All processes which use top blowing of oxygen and bottom stirring by inert gas is known as bath stirred top blown processes. (See 12. Formation of three phase dispersion is a characteristic feature of the top blown steelmaking. droplets are produced which together leads to the formation of a three phase dispersion consisting of gas bubbles/slag/metal droplets (See 12. In the initial stages Si and Mn removal delays carbon removal whereas in the final stages carbon removal rate decreases. Concept of bottom stirring in top blowing In pure top blown steelmaking. B=slag. In this state of blow. oxygen blowing rate and bottom stirring. Characteristic feature of converter steelmaking Supply of oxygen in the form of free gas jet is an important feature of converter steelmaking both in pure and different versions of combined blown ones.Though this technology provides efficient bath stirring and enhanced carbon removal. The advantages of such a technology when compared with pure top blowing like reduced FeO content of slag and O content of steel etc. • Inadequate stirring of slag /metal phases.transfer controlled). This was reflected by evaluating dimensionless momentum flow rate vs. Several process technologies for combined blowing were developed under different names. The advantages of pure top and pure bottom are coupled and a new technology is developed under the name combined top blowing of oxygen and bottom stirring. all oxygen is injected through the bottom tuyeres . The reader may see the references given at the end of lecture to get technological details. (See reference at the end). but it is difficult to distribute oxygen within the bath and also to control slag formation. number and arrangement of bottom tuyeres and porous plugs. evolution of CO is low. In this form of oxygen supply. are obvious. These technologies differ in • Amount of inert gas • Type. Top blowing attributes • Energetic supply of oxygen • Control of slag formation • Control of oxygen distribution. the total time of blowing of oxygen is almost independent of converter capacity. In main part of the blow higher carbon removal rate produces higher amount of CO and produces enough bath stirring. Dimensionless momentum flow rate was correlated as . Slag analysis reveals higher rate of oxidation of Fe to FeO in both the periods which is due to weak stirring in the bath. It is considered appropriate to introduce bottom stirring gas in a top blowing converter to stir the bath. ratio of time of blowing (t) /total blowing time(t tot ) for different converter capacities ranging from 40-400 tons. • Simultaneous removal of C and P. In bottom blown steelmaking. • Emission of CO. Lance profile can be considered to generate soft blow initially and progressively harder blow with the progress of the blow. Causes of high turnover rates of BOF i) Energetic supply of oxygen: This method ensures. • Particulate matter exiting with the exit gas. . Thus soft and hard blow are essential requirement of refining of hot metal by impinging oxygen jet irrespective of the converter capacity and type of converter steelmaking practices (pure top blowing combined blowing) as a result the total oxygen blow time remains more or less same. • Availability of oxygen where it is needed during refining • Faster mechanism of mass transfer by producing droplets and slag/metal emulsion. ii) Bottom stirring iii) A basic and limy slag of required basicity is formed at the early stages of the blow. The first requirement is achieved by “soft blow” (shallow penetration of jet) and the other requirement is achieved by hardening the blow (deep penetration of jet into bath) progressively. The fundamental requirements of the lance profile in all converter steelmaking are formation of FeO rich slag in the initial stage and then removal of carbon and phosphorus by progressively increasing the availability of oxygen in the bath to avoid over oxidation of slag.25 × 10−3 (ϕ)0. • Disposition of slag • Capture and removal of contaminants in hot and dirty exit gas from the converter. for this purpose sufficient excess air must be used at the hood to burn CO. The dimensionless momentum flow rate number increases with the decrease in lance distance.ṁ ρ l g×L 3 = 7.78 Where ϕ = t�total It is illustrated in lecture 13 that dimensionless momentum flow rate describes the action of free oxygen jet produced by constant volume flow rate of oxygen at various lance distances. Environmental issues in oxygen steelmaking • Control of emissions during transfer of hot metal to desulphurization station. Decrease in lance distance makes the blow hard and increase in lance distance makes the blow soft. 9 p. tupkary: modern steel making S C Koria: Dynamic variations of lance distance in impinging jet steelmaking processes.15 (1988) p.421-426 S C Koria.236-240. S C Koria and K W Lange: Estimation of drop sizes in impinging jet steelmaking. Process. Vol.References: A.H.353356 S C Koria and K W Lange: Penetrability of impinging gas jets in molten steel bath.6.Ghosh and A.5 p. tupkary and V. Ironmaking & Steelmaking. 59 (1988).104109.219-224 S C Koria and K W Lange: Correlation between drop size distribution or total drop mass and oxygen top blowing parameter. S C Koria and K W Lange: Experimental investigation on selection of bottom injection parameters in combined blown steelmaking S C Koria and and A George: Experimental investigation on selection of bottom injection parameters in combined blown steelmaking.15 (1988) p.127-133 S C Koria: Nozzle design in impinging jet steelmaking processes . Technical Report (1987)p. Steel Research 58 (1987) No.15 (1988) p. No. Iron and Steel Congress Vo. K W Lange and R. Steel Research 58 (1987) No.3. Steel Research 59 (1988) No.127-133 . Steel Research. Ironmaking & Steelmaking.13 (1986) No. (1986) p. Process Techn. Techn.421-426 S C Koria and K W Lange: Experimental investigation on selection of bottom injection parameters in combined blown steelmaking. Institute for Ferrous Metallurgy. chatterjee: ironmaking and steelmaking A chakrabarti: steelmaking R. Vol. Vol.1-117. Vol. Development of blowing practice for combined top blowing and bottom stirred processes. S C Koria and K W Lange.9 p. S C Koria and A George Experimental investigation on selection of bottom injection parameters in combined blown steelmaking Ironmaking & Steelmaking./ Proceeding (5th Intern. combined blown Steelmaking converter. Proceedings 5th Intern.6 (1986) p.R. p.127-133 S C Koria and K W Lange: Penetrability of impinging gas jets in molten steel bath. Siemssen: Application of empirical correlations to develop blowing pattern for small scale. p. Ironmaking and steelmaking V.6. Iron and Steel Congress) Vol.257-262. Tokyo. . S C Koria and K W Lange Effect of Melting scrap on the mixing – time of bottom gas stirred melts Proceeding 6th Japan-Germany seminar. Eisenhuttenwes.91-101 S C Koria and K W Lange A new approach to investigate the drop size distribution of BOF steelmaking Met. Japan (1984) p.109-116. 97-100. 15B (1984) p. Trans. 55 (1984) p.S C Koria and K W Lange Mixing – time correlation in top gas stirred melts Arch. . A three phase dispersion consisting of slag/metal droplets/gas bubbles forms during the blow.1(B).LECTURE 13 Fundamentals of Converter Steelmaking Technology Contents: Preamble Availability of oxygen Behaviour of free gas jet Action of free gas jet Jet penetrability Key words: BOF steelmaking. lance profile and steel chemistry. manganese.1(B) Laval nozzle In BOF steelmaking oxygen is blown through a laval nozzle. hot metal composition. It takes around 15 to 20 minutes to blow the oxygen for refining. jet penetration. The tap to tap time varies in between 50 to 60 minutes depending upon oxygen flow rate. The minimum cross section area of the flow passage is called throat of the nozzle. A Laval nozzle also called a convergentdivergent nozzle and is characterized by a flow passage whose cross sectional area decreases in the direction of flow and attains a minimum cross section area and then increases further in the direction of flow as shown in the figure 13. phosphorus. The energetic availability of oxygen is obtained by passing a certain flow rate of oxygen through the nozzle as shown in figure 13. During the blow the lance distance is decreased to make the oxygen available into the bath for carbon removal. oxygen at supersonic speed is blown on hot metal to remove the impurities like carbon. silicon.1 Figure 13.1(A):Convergent nozzle 13. The laval nozzle can accelerate the gas to the supersonic velocity The details of the design can be seen in the references given at the end of the lecture . It is interesting to note that the oxygen blowing time and the tap to tap time do not depend significantly on the converter capacity. refining reaction. soft and hard jet Preamble In converter steelmaking. This lecture attempts to discuss the fundamentals of converter steelmaking practice Availability of oxygen Oxygen is available energetically during the refining. It is characterized by the Figure 13.4. Within the length LC gas velocity is above the supersonic value in both radial and axial direction. Beyond the potential core both radial and axial velocity begins to decrease due to entertainment of the surrounding. Thus radial spreading and axial velocity decay beyond the potential core are the main characteristics of a free gas jet Due to spreading mass of the jet increases which means that concentration of the gas at plane Z=0 decreases due to entrainment of the surrounding. A gas on exiting through a nozzle spreads in the surrounding and is called “free gas jet”. Behaviour of free gas jet First we will see the behaviour gas when it exits a single Laval nozzle in the surrounding which consists of air. Convergent nozzle can accelerate gas at ?? ≤ 1 Convergent – Divergent nozzle can accelerate gas at M>1 In converter steelmaking Laval nozzles are used. Beyond the supersonic core length the gas velocity is subsonic. ton at pressures 10-14 bar. Multi –hole nozzles are used. Number of nozzles varies from 3 to 6. Figure 13.Velocity of gas can be expressed in terms of Mach number M= velocity of gas velocity of sound = v a Thus at M= 1 gas exits the nozzle with the sonic velocity. Sonic velocity can be attained at the throat of the nozzle. In the potential core no entrainment of the surrounding occurs and hence velocity of gas in both axial and radial direction is that at the exit value. concentration of oxygen at plane Z2 is lower than at Z1 and at Z=0. Radial spreading of the jet can be seen in the figure. flow rate of oxygen is 3 Nm3 ⁄min. Gas velocity at the exit corresponds to ?? ≈ 2 to 2. If oxygen is flowing through the nozzle. because spreading is not confined.2 show a free jet in the surrounding.2: Discharge of pressurized fluid stream through a Laval nozzle potential (LP) and supersonic (LC) core length as shown in the figure. However a point Z1 is reached in the free gas jet at which the gas velocity attains a sonic value (M=1). M<1 gas exits the nozzle at subsonic velocity M>1 gas exits the nozzle at supersonic velocity. But mass of jet (jet consists of main fluid + . A coalescing jet is similar to that of a single jet. Number of nozzles in converter steelmaking varies with the converter capacity but in general lay between 3 to 6. ρj > ρsurr jet is denser than the surrounding. The axial velocity decay and radial spreading depend on the ratio ρ surr ρj If ρ surr ρj = density of surrounding density of jet (4) < 1 i. the multi-jets do not coalesce upto certain distance downstream the nozzle. This is an important property of the jet since it depends only on the upstream variables like pressure. The relationship between diameter of a single nozzle and the corresponding multi nozzles for the same flow rate of gas can be obtained from the area consideration in equation 2 A1 = N x An 2) From equation 2 it follows that (3) d1 = √N × dn where d1 is the diameter of a single nozzle and dn is the diameter of each nozzle in a multi-nozzle and N is the number of nozzles. This situation could be. Axial velocity of the jet is a function of axial distance measured from the nozzle exit. The multi-free gas jets downstream the nozzle can coalesce or not would depend on inclination angle and number of nozzles for a given upstream pressure and flow rate of gas. is 10 − 12° and for five holes is 15 to 16° with the axis of the lance. Momentum flow rate within the jet is conserved.e. It does not depend on the downstream conditions. The inclination angle of each nozzle for a three hole. (1) Multi hole nozzles are compared with a single. For details see the references given at the end of this lecture Behaviour of jets produced by multi-nozzles depends on number of nozzles and inclination angle (α) of each nozzle with the axis of the lance. A non-coalescing jet. how slow would depend on the value of ρ surr ρj . One of the important property of the free jet is that it carries with it momentum flow rate which on hitting the liquid is converted into force and penetrates into the liquid. number and diameter of the nozzle. When angle of inclination is 10 − 12° for a three hole lance. when impinged on the liquid will produce penetration equal to number of jets.surrounding) at Z1 < mass at Z2 . Accordingly jet velocity will decay slower at any distance downstream the surrounding. for example a cold jet is discharged . Such a jet will spread slowly in the surrounding. Axial velocity decreases as the distance downstream the nozzle increases due to entrainment of the surrounding. In the converter as the blow begins.in the hot surrounding. LP and length of the supersonic core LC will be longer than at If ρ surr ρj ρ surr ρj ρ surr ρj = 1.46 √N x 10 3 T 0.446 (7) For a 150 ton converter. Throat diameter is function of converter capacity and number of nozzles dn = 7. jet spreads faster which results in lower LP and LC than < 1. diameter of each nozzle of a 4 hole lance is calculated to be 0. Length of the potential core. but the momentum flow rate within the get is independent of the distance downstream the nozzle and can be calculated from: 0.104 (6) T= capacity in tons. for example cold jet is discharged into slag.035 m by equation 7. For most of the periods the jet is submerged into slag. The velocity of the jet depends on upstream pressure. downstream axial distance and the surrounding. the surrounding of oxygen jet is hot atmosphere.286 �� ???? ??̇ = 1.755(T)0. Po is calculated to be 11.5 ?? 0. < 1 jet is lighter than surrounding. . For more details the readers can look into the references given at the end of the lecture.029 × 105 ???? ??????2 ��1 − � � (5) Where ṁ = momentum flow rate in (Newton) Po = upstream stagnation pressure (bar) N= Number of nozzles P = Surrounding pressure ???? = Throat diameter of the nozzle(m) Upstream stagnation pressure can be calculated Po (bar) = 6.37 bar by equation 6 and total momentum flow rate produced by a four hole nozzle is 4057 N by equation 5. It is difficult to calculate the jet velocity when the surrounding is changing. As the blow continues the jet surrounding changes from carbon monoxide to slag. The surrounding in the converter is dynamic. Action of free oxygen jet Velocity of the free oxygen jet is important. Thus action of free jet can be described in terms of dimensionless flow rate number (φ) φ= ṁ ρl g x3 (8) ρl density of liquid (Kg⁄m3 ). g ( m⁄s 2 ) and X (m) is distance between the lance tip and liquid bath surface.Depth of penetration of an impinging jet (h) is h(m) = 4.Jet carries with it momentum flow rate which on hitting the bath is converted into force. We note that the dimensionless number increases with the decrease in the lance distance. when it hits the hot metal bath are given below.0175.66 9) At x = 3m. Shallow jet penetration as obtained at higher lance distance is a “soft jet” as compared to deep penetrating jet as obtained at lower distance and is termed “hard jet”.407 × (φ)0. This would mean that a constant volume flow rate of oxygen supplied at constant pressure when discharged through a nozzle can be made to hit the bath “soft “and can be made progressively harder. through “free jet” is very effective in terms of physico.5 m then φ = 2. Thus dimensionless flow rate number can be used to describe the dynamic variation of the lance distance.19 × 10−3 and= 0.5m. If X is 3m and 1.e. . h = 0.46m when the distance x is decreased to 1. The dimensionless momentum flow rate number signifies the action of the gas jet on the bath at a distance X against the gravity Jet penetrability We calculate now the depth of penetration when the jet hits the bath surface at a distance x . whereas at x=1.5m jet will penetrate deep into the bath. Thus method of oxygen supply in converter steelmaking practice i. This means that there will be shallow jet penetration in the bath at x=3 m. The effects induced by a reactive soft and hard jet impinging oxygen jet.chemical reactions. Dimensionless flow rate number describes the effect of lance distance on the penetrability of jet.23 m and increases to 0. Ind. Arch. 1991.55 (1984) p. S C Koria Fluid dynamic aspects of lance design for submerged gas injection practice Trans.104-109. 59 (1988). P removal is impaired CO evolution occurs deep into bath and its escape through the bath agitates the bath Droplets are produced which are then emulsified in the slag References: . Vol. 63-70 . S C Koria Dynamic variations of lance distance in impinging jet steelmaking processes Steel Research.257-262.421-426 S C Koria.427-432.1-117. K W Lange and R.3. Eisenhuttenwes. Siemssen Application of empirical correlations to develop blowing pattern for small scale.Soft Jet • • • • Oxidation of Fe Shallow penetration Slag/metal reaction Slag formation is promoted. S C Koria and K W Lange Penetrability of impinging gas jets in molten steel bath Steel Research 58 (1987) No. S C Koria Nozzle design in impinging jet steelmaking processes Steel Research 59 (1988) No.S C Koria and K W Lange An experimental study on the behaviour of an under expanded supersonic gas jet. p. Institute for Ferrous Metallurgy. Technical Report (1987)p.9 p. p. P removal is enhanced Too long duration of soft let will promote sloping of slag due to overoxidation Hard Jet • • • • O2 available deep in bath C oxidation is favoured. combined blown Steelmaking converter. Inst Metals 44 (2).6. No. (1) Thus post combustion of CO in BOF and transferring the heat of combustion to the slag and metal offers an additional amount of energy. slag splashing. Combustion of CO to CO2 produces large amount of thermal energy CO + 1 2 O2 = CO2 − ΔHfo = 283 × 103 = 12634 × 103 kJ m3 kJ kg mole . The amount of post combustion taking place in the furnace can be represented by post combustion ratio (P CR) Advantages: PCR = % CO 2 % CO +CO 2 • Higher melting rates can be achieved • Reduced green house gas emission /ton of steel because more scrap can be used (2) . slag free tapping Post Combustion The oxidation of carbon to CO in BOF steelmaking is the principle reaction. In converter steelmaking CO produced during refining exits the furnace in exhaust system where it is combusted with the ambient air. slag carry-over.Lecture 14 Modern trends in BOF steelmaking Contents: Post combustion Technology of post combustion Potential post combustion issues Slag splashing What is required for slag splashing Liquidus temperature Benefits of slag splashing Slag free tapping Mechanism of slag carry-over Key words: Post combustion. • Reduction in slopping. This design may provide better control of oxygen for post combustion without affecting the oxygen flow through the main lance. Still another method could be a lance with double flow for oxygen. number and diameter of ports for supply of oxygen needs to be established keeping in mind the refractory wear. One of main requirement of oxygen flow for post combustion is that velocity and angle of oxygen flow should be low to avoid the refractory wear. The supply of oxygen for the post combustion must be well distributed above the slag surface for an efficient combustion of CO. Technology of post combustion A technology is required which can inject oxygen in the converter just above the slag so that CO can be combusted to CO2 . angle of oxygen flow and location of nozzle are the principle design issues. Slag splashing Splashing of slag to coat the refractory lining has become a standard practice to increase the lining life. This is achieved by increase in slag temperature which helps dissolution of lime in slag and decrease in slag viscosity. One possible way is to add several small orifices around the main supersonic nozzle tip. a separate oxygen inlet and oxygen control system can be provided which is solely dedicated to post combustion. Potential post combustion issues • Repair costs of post combustion lance are higher • Post combustion lances are not rigid as standard oxygen lances. . Slag splashing is done as follows: • At the end of BOS process. Nozzle diameter. In the main oxygen lance. Transfer of the heat to the slag and metal phase is also to be considered. • Development of high quality refractory would be required because high temperature would be generated in the post combustion zone. steel is drained of and slag is retained in the vessel. Lance distance has to be adjusted so that oxygen through the orifice is available for combustion of CO to CO2 above the slag surface. • Optimization of angle. location. Lance skulling This is particularly severe if some metal is left in the vessel . SiO2 should be added Figure shows schematic diagram of slag splashing in a converter Figure 14. • The excess slag is poured out. Presence of MgO in the end slag should be greater than 13% to produce a high temperature phase and to increase slag viscosity. • Slag refractory provides a consumable refractory lining which protects the furnace lining. Too high liquidus temperature will lead to non uniform distribution of slag and there is tendency for the slag to build up at the bottom. What is required for slag splashing? a) Compositional adjustment of end slag against Fe Ox and MgO concentration. Low melting phase melts and thereby coating thickness decreases III. Problems with slag splashing A. Low melting phase ensures good adhesion between slag and refractory and high melting phase MgO.• The O2 lance is lowered and high pressure N2 is used to splash molten slag on the walls of BOS vessels for a period of 2 to 4 min. Slag layer should contain enough high temperature phases to prevent attack by slag during BOF process. II. Fe2 O3 imparts erosion resistance to the lining.1: schematic representation of slag splashing in converter Liquidus Temperature of splashing slag: The liquidus temperature of slag is important because I. Al2 O3 content of slag should be low CaO SiO 2 d) For ratio greater then 5. b) c) FeO reduces melting temperature of slag and increases the amount of low melting phase in slag. Mechanism of slag carry-over Slag can be carried-over from converter to ladle in the initial stage when converter is tilted. V. More recycling of BOF slags. • During synthetic slag practice in ladle. Decrease in sloping results in increase in yield. The slag coating interferes with the detection. drain velocity . part of the slag may be carried into ladle. II. III. Sensors are embedded in the refractories to detect the presence of slag during tapping. Rapid formation of slag occurs due to melting of low temperature phase of the coating. deoxidation etc. P2 O5 and CaO. FeO content varies form 15-18 %. During up-tilting. Longer life of furnace lining (over 60000 heats).B. C. During tapping. Benefits of slag splashing I. Slag free tapping Priamary steelmaking slag contains FeO. Vessel mouth skulling. bath level. carry-over of slag should be avoided for the following reasons: • During ladle treatment impurities can revert back from slag to metal. Blockage of tuyeres D. composition of slag will be altered which will affect secondary steelmaking refining operations like desulphurization. this will affect steel cleanliness. This will lead to increased consumption of deoxidizers • Carried-over slag can increase the refractory wear. The amount of drained slag depends on tilting speed of converter. Less CaO is needed in BOF process due to dissolution of basic slag coating. These slags are oxidizing in nature. SiO2 . IV. • FeO and MnO of slag react with Al and forms Al2 O3 which is a solid inclusion at the steelmaking temperature. As tapping proceeds converter is tilted further which leads to formation of vortex and top slag is carried into the ladle. Trans. Materilas reviews.1988. P53 . vol. 1994. Steel Research 65 (01). Tap-hole plug.Prevention of slag carry-over Several methods are developed and in use to minimize slag carry-over. Inst. Metals 47 (2-3). The interested reader may look into the references given at the end of the lecture References Kenneth. S C Koria and P Umakanth: Water model study of slag carry-over during molten steel transfer. Intrn. 45 (2005)PP 619-633. C Mills : A review of slag splashing. S C Koria and P Umakanth: Model studies of slag carry-over during drainage of metallurgical vessels .W. 121-130 R.J. slag cut ball pneumatic slag stopper and slag detection methods are commonly is use. 8-14. 1994.Lange: Thermodynamics and kinetics of secondary steelmaking processes. Ghosh : secondary steelmaking. |S|J International.Fruehan: Ladle metallurgy principles and practices K. Ind. the proportion of electric steel is around 40 to 45% in the total world steel production. It must be noted that EAF consumes lot of electric energy and hence the cost and availability of electrical power are important issues in electric steel development.LECTURE 15 Electric Furnace Steelmaking Contents: Introduction Type of Electric furnaces Construction of AC Electric Arc Furnace Transformer power Charging materials Plant layout Arc furnace operation Comparison with oxygen steelmaking Key words: Electric arc furnace. . Transformer power Introduction: Steelmaking in electric arc furnace has emerged as an important steelmaking process in recent years. Type of Electric furnaces: In principle an electric arc is formed between the electrode and the metallic charge and charge is heated from the arc radiation. Electric arc furnaces are of two type (a) alternating current and (b) direct current. Melt-down period. to an anode embedded in the bottom of the furnace. which acts as cathode. furnace operates by means of electric current flowing from one electrode of three to another through the metallic charge. In alternating current. The flexibility and easy adoptability of EAF steelmaking to accommodate the fluctuating market demand have evolved into the concept of mini steel plants to produce different grades of finished products (long or flat or mixed ) of plain carbon or alloy steels from scrap and other metallic charge materials. Although scrap is the preferred raw material but sponge iron and iron carbide are being used regularly in most plants because of shortage of steel scrap and to dilute the concentration of tramp elements. Several developments in the design and operation have made EAF steelmaking to contribute significantly to the overall total production of steel in the world. According to an estimate. In direct current. the current flows from carbon electrode. Heat is generated by the hot area formed between the electrodes and the charge. the charge at the furnace banks will be heated . The backing lining is few layers of high fired magnesite bricks on which working lining is rammed with either dolomite or magnesite mass. If the electrodes are placed close to each other and far from furnace walls. Permeable blocks or porous refractory elements are introduced through the bottom to inject inert gas for stirring. The EAF steel bath is shallow.2 to 0. The electrode current could vary from 12 to 16 A⁄cm2 for 400 to 600 m electrode diameter. The roof has three holes to allow insertion of the electrodes. The diameter of the circle passing through the centers of electrodes is called the diameter of the electrode spacing. electrodes will be heated and oxidized vigorously. and typically in segments with threaded coupling. The roof lining is water cooled which increases the life of refractory lining to at least 10-20 times more than without water cooling.Construction of AC Electric Arc Furnace The furnace consists of a steel shell. new segments can be added. Larger electrode diameter increases electric energy consumption. if current density is excessively high. The shell thickness is around 0. The electrodes are automatically raised and lowered by a positioning system. so that as the electrodes wear. Electrode consumption depends on • • • Oxidation of the surface of the electrode Mechanical losses due to fracture Dissolution in slag during carbon boil The diameter of the electrode should correspond to the current supplied. Three electrodes enter through the roof.22. the aspect ratio of the bath is around 0. The hearth lining consists of backing lining and working lining. The electrodes are positioned at apexes of an equilateral triangle. Graphite electrodes are preferred over carbon electrodes because of better electrical conductivity. Electrodes are round in section. Roof The roof is exposed to more heat than other furnace elements. The hood may be swung away for charging. Hearth The hearth contains metal and slag.005 times the shell diameter. Its lining is also subjected to radiant heat reflected from the walls and slag. High alumina bricks and magnesite – chromite bricks are used for roof lining. Electrode A typical alternating current operated EAF has three electrodes. lined with suitable refractory materials and is mounted on the tilting mechanism. sized and large furnaces. Side walls The side walls refractory materials should be able to withstand thermal shock and corrosive action of slag. sand. and quartzite are used to form a slag to refine the metal. In small furnaces.650 KVA per tonne of furnace capacity. reflected from bath surface during power input. 0. The melting process consists of two periods: meltdown and refining period.45 for small furnaces. the transformer capacity is in the range of 750-900 KVA per square meter Charging materials: Steel scrap is the principle raw material. Transformer power: Electric furnaces are powerful consumers of electric energy. The furnace transformer transforms high voltage energy into low voltage.35 for medium. It may constitute 60 to 80% of the charge. Additional 150 to 400 k Wh⁄ton power is required during refining depending on the practice. dolomite or chrome magnesite bricks up to the slag line.35 × 5560 = 1900mm.belatedly. Hot spot is formed on the side walls due to the radiation from arc flames. Large transformers are required to run electric arc furnaces.powerful furnaces. The side wall thickness is usually 450 to 500mm for 10 to 50 ton furnaces and 550 to 650mm for 100 to 200 ton furnaces. The electrode spacing diameter for the bath diameter could be 0. The variation is also due to whether the shop is provided with oxygen lancing and carbon injection facilities. Iron ore is also added. which will result in rapid wear of the lining. In terms of hearth area. Ferro-manganese. For decarburization oxygen lancing is used. Plant layout Layout of an electric arc furnace steelmaking shop varies from plant to plant due to difference in the quality of the product and the scale of production. ferrosilicon or aluminium are used for deoxidation. electric arcs will burn near the walls. In some practices sponge iron and or pig iron is also used for chemical balance. The transformer capacity is designed to suit melting requirements. To produce alloy steels. During melting more power is required than during refining. The capacity of the transformer is usually 470. In basic furnaces slag formers like limestone. Some plants have just one EAF while others have two. With large spacing diameter. and still lower for super. gas cleaning equipments and finished castings or ingots. the power consumption for melting is about 600k Wh⁄ton and it falls to 450k Wh⁄ton in big furnaces. The side wall is lined with magnesite. Broadly electric furnace steelmaking shop comprises of the following: . The operating voltage of a furnace is 100-800V and the current may reach several thousand amperes. In melt down period higher electric energy is required as compared with the refining period. alloying elements are added. fluorspar. For a bath diameter of 5560 mm of a 100 ton furnace the electrode spacing diameter would be 0. The roof is swung off the furnace.down time depends on • • Arc conditions: larger arc requires lower current and lower heat losses Deep or shallow bath: deep bath shortens the meltdown period. Burnt lime and spar are added to help early slag formation. Iron one or mill scale may also be added if refining is required during melt. Once the bath chemistry and its temperature are attained. melt down period and refining. and the furnace is charged. manganese and carbon oxidizes.down period. Once the arc is shielded by scrap. During meltdown period. The single oxidizing slag practice is employed when removal of sulphur is not required. Melt. electrodes are lowered and bored into the scrap. heat is deoxidized and finished for tapping. Also oxidizing and limy slag is produces which promotes dephosphorization as well. all the above facilities should be arranged so as to ensure smooth input and output of materials. .a) b) c) d) e) f) g) Electric furnace Transport facilities for ladle Scrap charging Auxiliary injection facilities Electrode movement mechanism Charging of raw materials and weighing system Slag disposal. voltage is increased to form molten metal pool to reduce the meltdown period. Some furnaces are equipped with continuous charging. In an ideal layout. Reducing slag helps to avoid loss of alloying elements. Lower voltages are selected in order to protect the roof and walls from excessive heat and damage from the arcs. In double slag practice. Removal of phosphorus must be complete before the rise in temperature and carbon boil. silicon. oxidizing slag is removed and reducing slag is formed after deoxidation with ferrosilicon or ferromanganese or aluminum. Arc Furnaces Operation It consists of charging. Hot metal is also charged as per the requirement. Refining continues even during melting. In the meltdown period. The large baskets containing heavy and light scrap are preheated through the exit gas. When both P and S are required to be removed double slag practice is used. May 1994.P. . May 1994.34 Manfred Haissig: : Iron and steel engineer. scrap melting and post combustion. P. Carbon injection is done to induce foamy slag practice References: F.25 Oxygen supply is continuously done to refine hot metal to steel. Autogeneous process Hot metal + 20 – 30% scrap Operating procedure Slag foaming is induced to shield refractory lining from the heat of arc.Edneral: Electrometallurgy of steel and ferro alloys AK chakrabarti: Steel Making Heinz G. P. Chemical energy. Muller: Iron and steel engineer. A three phase dispersion of slag/metal/gas forms to accelerate the refining rates.Comparison with oxygen steelmaking EAF Oxygen steelmaking Source of energy Electric + chemical energy Iron containing raw material Hot metal + directly reduced iron + scrap in the suitable proportion as per practice Oxygen lancing is to promote decarburization. Lecture 16 Developments in EAF steelmaking Contents: Introduction Furnace design Developments in EAF steelmaking a) b) c) d) Furnace design Process operating technologies Usage of chemical energy Carbon injection Requirements for usage of chemical energy Future of EAF steelmaking Key words: Electric steelmaking. Developments in EAF technologies are strongly supported by secondary steelmaking. Growth has been supported by updating installations and technologies to reduce the electric energy. electrode consumption and tap to tap time. One can note in the figure that the power consumption has decreased from 630 . foamy slag. 1 shows the developments in Figure 16. Figure16. chemical energy Introduction The growth of electric steel production around the world has been driven by lower investment. higher operational flexibility and easy adoptability to market demand on long or flat products of either plain carbon or alloy steels. scraps preheating.1: Trends in EAF steelmaking technologies developments in EAF steelmaking electric steelmaking technologies. bottom stirring. Eccentric bottom taping reduces tap times. and upper section containing side wall and roof. This leads to increase in shell volume which results in larger tonnage charge.Kwh/ton of steel to 290kWh/ton. Most modern furnaces operate at 500 k VA⁄ton and the trend is towards ultra high power ranging in between 700 k VA⁄ton to1000 k VA⁄ton. hot metal Furnace design: i) ii) iii) iv) v) Construction of hearth and lower side section of the shell of larger diameter than the top opening.2 kg/ton to as low as 1. Developments are in progress to install transformer with 1500 k VA⁄ton capacity. The two sections are coupled such that the upper section can be repaired easily. it is possible to increase capacity by up to 50%. shell structure is constructed in two sections: lower section which contains hearth and free board allowance for slag. a dc reactor. Lower maintenance costs are claimed and refractory costs are less for sidewall but more for the furnace bottom. Table shows the various developments Developments in EAF steelmaking Furnace design Process operating technologies Chemical energy Charge materials Split shell design Bottom stirring Oxidation reaction Transformer power Foamy slag practice DC arc furnace Scrap preheating Post combustion oxy fuel burner Carbon injection Directly reduced iron. Similarly tap tp tap time has decreased from 180 minutes to 40 minutes and electrode consumption has decreased from arounf 6. lower heat losses and improved thermal efficiency. The largest transformer in AC EAF corresponds to a rated power 0f 240 MVA for 300 ton furnace.2 kg/ton within the periods of representation in the figure. It is claimed that a 120 tons operating at 180 MVA transformer capacity and by using refining combined burner technology through oxygen gas and carbon injection. The electrode technology limits diameter to a . In the split shell design. DC (direct current) arc furnaces represent a different concept in arc furnace design. The strip producing plants are equipped with eccentric bottom tapping in electric arc furnaces. This reduces the downtime and increases furnace availability. and a thyristor all of which add cost to a dc furnace. Most DC furnaces are with single electrode where current flows down from the carbon electrode to an anode mounted in the bottom of the furnace. Noise levels for the dc furnaces are lower. This became possible with the several simultaneous developments in the secondary steel-making method. temperature losses and slag carry over into ladle. High powered transformers are the current trends. A dc arc furnace requires an addition of the bottom electrode (anode). Reduced electrode consumption of the order of 50 to 60 % is the major benefit of a dc furnace compared to a convectional threephase arc furnace. The foaming slag during this period is beneficial. Figure 16. progressive melting of scrap increases the irradiative heat transfer from arc to the side walls of the furnace. Figure 16. Due to absence of stirring large piece of scrap can take a long time to melt and may require oxygen lancing. whereas in indirect one the plug is embedded in a porous bottom refractory. In the indirect contact.2: Industrial bottom stirring systems in electric arc furnace ii) Foamy slag practice In EAF steelmaking. Note that in indirect contact large area of the bath is stirred as compared with direct contact plug. By covering the arc in a layer of slag.2 shows the direct contact and indirect contact plug for bottom stirring.2. Furnace size is limited to 200 tons. The gas enters the bath via the porous refractory hearth which results in stirring over a large area when compared with direct plug as shown in the figure 16. Process operating technologies Most of the developments in process operating technologies are in AC. i) Bottom stirring In convectional arc furnaces there is little natural electrical turbulence within the bath. the arc is shielded and more energy is transferred to the bath. In direct contact plug.maximum of 700 mm allowing a dc current of 100kA and 70 MVA power for single electrode furnace. electrode and power consumption and Improves yield of iron and alloys Industrial systems for bottom stirring are either with direct contact plug or with indirect contact plug. Argon or nitrogen stirring • • • • Eliminates temperature and concentration gradients Shortens tap-to-tap times Reduces refractory. the plug is not directly in contact the molten metal.electric arc furnaces as these furnaces are popular. . Further developments are in progress. the plug is in contact with molten metal. Moreover.4 kg/ton Tap to tap time reduces by 5 to 8 minutes.9 to 1. Gas producing reactions in steelmaking are: a) Reaction between FeO of slag with carbon (FeO)l + C = [Fe] + {CO} (1) b) Between carbon and oxygen dissolved in metal [C] + [CO] = {CO} c) Between chromium oxide and carbon: Cr2 O3 + 3C = 2Cr + 3CO (2) (3) Reactions 1 and 2 are important in carbon steelmaking whereas reaction 3 is important in stainless steel making. Preheating of scrap to 540℃ brings 81kwh/ton of additional energy.36 kg/ton Refractory consumption decreases by 0.5 to 5 kg/ ton of steel. In stainless steel making Cr2 O3 forms in preference to FeO due to higher affinity of Cr to oxygen.The effectiveness of slag foaming depends on slag basicity. Slag foams in steelmaking due to entrapment of gas bubbles. slag temperature and availability of carbon to react with either oxygen or FeO of slag. The novel technology utilizes the reduction of iron and chromium oxide by carbon as well as thermal dissociation of limestone contained in small briquette. The oxygen/carbon injection technique in the high chromium alloy steel production and to foam the slag is difficult. In high powered furnaces carbon injection is 5-10 kg/ton of steel. For this purpose it is important to know the energy balance of the electric furnace. Typically carbon injection rates for slag foaming are 2.3 to 0. Scrap preheating gives the following advantages: o o o o Reduction in energy consumption by 40-60 kwh/ton depending on the scrap preheat temperature Electrode consumption reduces by 0. when carbon content of the bath is insufficient. It is important to note that scrap preheating technology needs to be developed. (See the reference given at the end of the lecture) iii) Scrap preheating Preheating of scrap brings thermal energy into the furnace. as shown in the indicates that 20% of the total energy leaves the furnace in the . Thermal energy is required to preheat the scrap and is economical only when the waste heat from the furnace is utilized. Slag foaming is discussed in lectures 4 and 5. The energy balance of an EAF. additional chromium will also be lost due to oxygen injection. Injection of carbon and oxygen at several places in the bath assures slag foaming practice. FeO content of slag. The solubility of chromium oxide in the slag is considerably weaker in comparison to FeO for the same basicity and thermal conditions. In recent years the chemical energy supply amounts to 35% to 40% of the total energy in most of the modern EAFs.3%. C + 0.1%. The oxidation of iron though generates more energy than oxidation of carbon but iron oxidation results in loss in productivity. Greater penetrability of oxygen jet ensures the occurrence of oxidation reactions in the bath. scrap and exit gases move counter current to each other. In CONSTEEL. the efficiency of carbon oxidation to form CO drops and more and more FeO is generated in the slag. One such source is the chemical energy derived from chemical reactions. Heat content 6kW/m3 O2 Heat content 3. all oxygen reacts with carbon to produce CO. It is possible to preheat the scrap to ~320℃.5O2 = CO. Below 0. Usage of Chemical Energy The high electrical energy costs pushed EAF steelmakers to look for alternative energy sources. FeO levels in the slag can be quite high and represents an unavoidable yield loss.3) i) Oxidation reactions The main oxidation reactions are oxidation of iron and carbon besides oxidation of silicon and manganese. . Efficient utilization of thermal energy of exit gas is the key to realize the advantages of preheating of scrap.5kW/m3 O2 Hence oxygen injection must be controlled such that iron oxidation is kept minimum.5O2 = FeO.3% C. Increased carbon injection is necessary to control slag FeO levels and to prevent excessive refractory wear. Batch preheating and continuous preheating are the available technologies.Figure 16. Efficiency of heat transfer from oxidation reactions is extremely high due to the fact that these reactions are occurring in the bath. Fe + 0. For scrap carbon levels below 0.3 Energy balance of an electric furnace steelmaking waste gases and represents about 130 kWh/ton of steel produced. (See figure 16. For bath carbon levels above 0. For post combustion speed of oxygen injection must be low and also uniform distribution of oxygen is required.monoxide.5O2 = CO The reduction of FeO by carbon during slag foaming Hydrogen is generated by: • • The cracking of hydrocarbons (oil in scrap. Others recommend 32 k Wh/ ton of burner power to eliminate cold spots in a UHP furnace and 50 to 200 kWh/ ton of burner power for low powered furnaces. Carbon monoxide is generated in an EAF by • • • Partially combusted hydrocarbons entering the furnace with the scrap Combustion of charged and injected carbon via C + 0. It must be noted that oxygen flow should have low velocity to promote mixing with the furnace gases and to avoid scrap oxidation and rebound of oxygen from the scrap to the water cooled panels. iv) Carbon injection Injection of carbon brings following benefits: i. For 100 percent scrap practice or when carbon content of the bath is insufficient to produce CO for slag foaming. It is to be noted that carbon injection requires oxygen injection to onset carbon oxidation. whereas the foaming slag contains carbon. Oxy.133 MW of burner rating should be supplied per ton of furnace capacity.Post combustion ii) It is a practice of generating additional energy for melting steel by using the right amount of extra oxygen to combust CO and H2 which evolve within the EAF. On most modern UHP furnaces. Requirements for chemical energy usage The chemical energy usage requires to develop a device to inject oxygen in different modes: . together with pure oxygen to produce an extremely high flame temperature. Post combustion in the slag typically aims at combustion of 20 to 30% of the CO generated in slag and 70 – 80% at the free board. Typically industry practice indicates that 0. methane) The reduction of water: H2 O + CO = H2 + CO2 or H2 O + C = H2 + CO In EAF. Carbon oxidation produces CO which on post combustion generates thermal energy. Iii) Oxy – fuel burner Oxy-fuel burner uses natural gas or oil. carbon monoxide and hydrogen may be available at the freeboard. ii. carbon injection is beneficial. the primary function of burners is to provide heat to cold spots to ensure even scrap melting and to decrease the melting time necessary to reach a flat bath.fuel burners are used to melt unmelted scrap between the electrodes and to provide heat to cold spot. Secondary voltage up to 1. The next lecture deals with charge mix in EAF steelmaking.000kVA/t. 180MVA for 120 ton tapping weight. Submercible hand lances arc used through the slag door. CO-jet injectors are highly flexible in usage. latest oxygen and carbon injection technology and design features. It is interesting to compare a conventional 120ton EAF with the ultimate 120 ton EAF. Secondary voltage up to 1. An innovative design is CO-jet injectors which are fixed type and can be mounted on the furnace shell. Excessive repairs and down. The two main reasons for this are: • • The possibility of a higher electrical power input and A far higher efficiency of chemical energy. 120MVA for 120 ton tapping weight. Future of EAF steelmaking The EAF needs a metallurgical reactor that has the largest growth potential both in terms of production capacity and technology evolution. Hold mode (to prevent plugging) Burner mode (to heat and melt scrap) Soft lancing mode (for post combustion) Supersonic lancing mode (for decarburization and slag foaming) Carbon injection mode (when slag foaming is required) Injectors are either fixed type or moveable type. This combination leads to an Electric Arc Furnace where the tap to tap times can be extremely short and the corresponding productivity reaches the level of larger furnace sizes or converter plants.like Ultra high power input (up to1500 kVA/t).like ultra high shell design. Large opening in EAF shell is required. Slag and metal splashing restricts the device movement.time are associated with this technology. heavy mill type components.bucket charge Scrap bucket 130 m3 Furnace volume 145 m3 1-bucket charge Scrap bucket 185 m3 Furnace volume 210 m3 Transformer design upto 1. Future EAF will be equipped with all modern technologies.200V Transformer design upto 1. The reader may see the references given in this lecture.500kVA/t. decarburization and scrap preheating compared to the same size (tap weight) standard furnace.500V . Conventional 120 ton EAF Ultimate 120 ton EAF 2. P 191 .March 2009. It supplies chemical energy through chemical reactions of fuel and gas.T... EAF technology evolution by continuious charging. Of South African Institute of Mining and Metallurgy. Ironmaking and steelmaking. May 1994. and carbon injected into the furnace.386 M. References: S.P34 T. P. 1994.Ironmaking and steelmaking. Archives of Metallurgy and Materials. October 1988.G.al.Muller et.L.Utilization of chemical energy 3 oxygen gas burners 3 refined combined burners (RCB) 2 carbon injectors Utilization of chemical energy 3 oxygen gas burners 5 refined combined burners (RCB) 4 carbon injectors 4 post combustion injectors Refined Combined Burner (RCB) technology combines a conventional oxy/gas burner with a supersonic oxygen injection lance and is designed to optimize the injection of carbon and oxygen into EAF.27 G. 2005.al.Cantacuzene et.Hissig: The d-c shaft furnace. P 121 M. 2005. Electric Furnace Conference Proceedings. P 459 M. p203 d. Technol. 43(2004) 163 J. van Wijngaarden et.: Bottom stirring in an EAF: Performance results at Iscor Vereeniginh works. P 62 J. P.Gojic: Current state and Development of steelmaking processes: Metalurgija.Janke et.: Intern. Of Chem Tech. Mater. 1999.al. Jan.J. Volume 53.al.: D-c electric arc furnace. 34(6) 2000.: EAF foamy slag in stainless steel production…. 32. vol.Jiemin et.al. Jl.Reichel et. Iron and steel engineer. Mottahedi et. research vol 1 Jan-. 2008. oxygen.A trend-setting technology in steelmaking.al.A. . 25 H.Dressel: Use of DRI in EAFs: Iron and steelmaker.U. P.: Advanced EAF oxygen usage at Saint-Saulve steelworks. May 1994. P1 A. Iron and steel engineer. vol 32.al.: Scrap-based steel production and recycling of steel.Jones: Optimization of EAF operations through offgas system analysis. The Jl. The availability of scrap needed to meet the requirements of value added products is limited. The charge materials saturation is critical for several reasons: • The product mix served by EAFs is moving more towards value-added steels.8% Mn and 0.2% P. Pig iron contains 3 − 4 %C. DRI. Charging of pig iron means refining is to be done in EAF.6 − 0.6 − 0. We need to remove around 640kgC. To remove . electric arc furnace. The traditional EAF charge has been 100% cold scrap.1% carbon. Yield and energy consumption are both strongly dependent on the quality and physical characteristics of the charge materials availability. which are specified with low metallic residuals and low nitrogen levels (automotive flat rolled.5%C and 80% scrap with 0.1 − 0. 0.8% Si. cold heading.Lecture 17 Alternative Charge Materials in EAF Contents: Introduction Types of metallic charge materials Carbon content in DRI Charging methods Key words: Sponge iron. • • Type of metallic charge materials • • • Pig iron Pig iron/hot metal Directly reduce iron (DRI) Iron carbide (Fe3 C) Pig iron is a good charge material because of its • • • • High density Low melting point Carbon contribution No tramp elements It must be known that EAF is designed exclusively to use electrical energy. UHP furnaces Introduction Raw materials and operating practices affect EAF efficiency and yield.1%C to produce100 ton steel with 0. For example if we make a charge mix containing 20% hot metal with 3. The availability of scrap is decreasing as more and more near-net-shape metalworking operations appear. 0.rolled and wire). Consider 1000kg iron ore of composition 80% Fe2 O3 and 20% gangue minerals. The source of hot metal is either blast furnace or smelting reduction process. hence substituting DRI for scrap leads to dilution of tramp elements in steel. Al2 O3 . Quality of DRI can be judged in terms of metallization and oxygen remaining. otherwise one has to use pig iron and extra energy would be required to melt the pig iron Direct reduced iron Direct reduced iron (DRI) or hot briquetted iron (HBI) has emerged as an important substitute of the scrap. less oxygen would be required to remove oxygen from iron oxide of DRI during steelmaking. The product of gaseous reductant is HBI. Metallization (%) Free iron (kg) Fe-FeO (kg) Gangue (kg) Oxygen with FeO(kg) Total gangue (kg) 80 85 90 95 450 480 500 530 110 80 60 30 200 200 200 200 31. Metallization (M) is defined as M= Free Iron ×100 Total Iron (1) Higher is the metallization. Under practical condition 30%hot metal is suggested to give optimum results with regard to productivity.00 10. Reduction of iron ore produces DRI in which oxygen is present as FeO. Extra decarburization will increase tap to tap time and may decrease productivity. What we learn from this simple calculation is that one has to supply oxygen under EAF condition and has to handle large amount of CO as well. Mn → MnO and P → P2 O5 . More proportion of pig iron in charge mix will increase oxygen requirement. DRI does not contain any tramp element.00 3. P2 O5 etc). Under actual conditions we would be needing more oxygen to remove carbon if we take into account oxidation of Fe → FeO.this amount of carbon we would be needing ≈ 630 m3 oxygen on the assumption that oxygen utilization is 100% and is used for removal of carbon only. electric consumption etc. DRI contains: Free iron +oxygen combined with iron+ free carbon+ gangue minerals (SiO2 .00 18. DRI is produced by reduction of iron ore with carbon or gaseous reducing agent.00 310 280 260 230 We note from the table the following: . Hot metal can only be used when EAF plant is in close proximity of the blast furnace or smelting reduction units. Si → SiO2 . Quality of DRI is important. 2 36. Gangue content of the DRI increases with the increase in proportion of DRI. This would require extra amount of heat energy both chemical and thermal to reduce FeO to Fe and to raise the temperature of reactants to 1500 − 1600℃. Fe which is combined with oxygen as FeO increases with increase in the proportion of DRI. Energy is required to compensate for the endothermic reaction. Another important issue in case of DRI is the proportion of DRI in the charge. Lower amount of heat and reductant would be required to recover iron of FeO from DRI. What is the consequence? A) Tap to tap time may decrease with the increase in DRI but more proportion of DRI may increase tap to tap time for two reasons.4 29. We consider charge mix is scrap and DRI. The following table illustrates the influence of proportion of DRI on the free iron and other variables of DRI: DRI (%) Free iron (kg) Fe-FeO(kg) Gangue (kg) Total gangue (kg) 10 30 40 50 66 198 264 330 7 22.a) Increase in metallization increases free iron in DRI which is good. decrease in gangue minerals will decrease slag volume during production of steel using high metalized DRI as a charge material. . b) Increase in metallization decreases iron which is combined withFeO. the overall effect would be decrease in electric consumption and reduced electrode wear. Gangue minerals would be recovered as slag. Reduction of FeO with C is endothermic. For this purpose 1000 kg sponge iron of 90% metallization is to be used in different proportions. namely more time is required to reduce FeO → Fe and.0 27 81 108 135 36 110 146 181 We note the following: i) ii) iii) Increase in proportion of DRI increases the free iron in the charge. Overall economics of metalized DRI in electric steelmaking is important while deciding % metallization in the feed. d) Because of b) and c). The benefits arising due to the use of high metalized DRI in charge materials must be considered along with the production cost of metalized burden. c) Increase in metallization decreases the gangue minerals in DRI. Calculations show that 1300 kg iron ore would be needed to produce 1000 Kg DRI with 90% metallization. This would lead to increase in slag volume and heat load. to handle increase volume of slag. Heat losses resulting from delays are eliminated. . methane. However. (2) According to reaction 2 carbon requirement is 0. Iron carbide is lighter than steel and is introduced pneumatically below the slag layer through a lance. On dissolution carbon of iron carbide is released. Charging methods for DRI In small furnaces (lower than 5T) batch charging is preferred. Excess carbon aids chemical energy which reduces electrical energy requirement. In DRI produced from gaseous reductant. Carbon content in DRI Carbon content in DRI helps reducing FeO to Fe. However the benefits of iron carbide addition must be considered in relation to its cost of production and method of addition into electric arc furnace. While producing DRI from carbononaceous reductant. H2 and water vapour at temperatures between 550oC and 650oC and pressure 1. extra carbon can be mixed with DRI or otherwise carbon injection is to be done. Iron carbide has a melting point (2110 K) greater than molten steel and it dissolves in steel. Also CO produced foams the slag and brings advantages of slag foaming on the electric arc furnace operation. It is prepared by heating iron oxide fines in a mixture of CO. Oxygen in DRI is 0. Lower electrical losses. Iron carbide does not contain gangue and tramp elements. CO2. Reduction of FeO is FeO + C = Fe + CO. Reaction between carbon and FeO of DRI produces strong carbon boil during charging which improves heat transfer and slag/metal mixing. Charging and refining take place simultaneously which reduces tap to tap time. Continuous charging is preferred when the sponge iron or DRI is around 60%.17 kg/kg FeO. Iron carbide Other charging material is iron carbide which contains 6% carbon. reacts with oxygen and releases heat which leads to saving in electric energy. Continuous charging brings advantages like • • • • • Less power off-time. ultra high powered furnaces can operate with 100% sponge iron. excess carbon than reduction for FeO requirement is to be maintained.286 × FeO.8 bar.B) Increased proportion of DRI beyond a limit may increase electrode consumption and refractory wear. the limitation is due to the small furnace capacity. Excess carbon than required to reduce FeO would require oxygen injection. Use of iron carbide increases yield.Carbon content in excess of 0.17kg/ kg FeO would require oxygen injection and CO will be generated. Chakrabarti: Steelmaking G.References A.Dressel: Use of DRI in EAFs: Iron and steelmaker. P 121 . October 1988.L. 95%.Lecture 18: Stainless steel making Contents: Introduction Thermodynamics of decarburization of chromium melt Technology of stainless steel making Basis of development of a new technology AOD process Key words: Stainless steel. when both elements are in pure state. Mn. Ni. Certain grades contain C 0. V. Chromium imparts corrosion resistance to steel.12%C. . Aluminum. Under all practical conditions carbon oxidation can occur at temperatures above 1800oC in presence of chromium. S etc. Precipitation hardenable stainless steel: contain 18-20% Cr. It may be noted that all stainless steels contain chromium and carbon besides other elements. Titanium.25%. EAF Introduction Stainless steels contain typically 10-30 % chromium besides other elements like C. Mo. Ti.15% to 0. and C is in between 0. Si.15% Ferritic stainless steels: which contain 12% to 30% Cr and 0.6% to 0. Production of stainless steels requires controlling chromium and carbon. AOD process. 8 to 10 % Ni and copper. Thermodynamics of decarburization of chromium melt In stainless steel making both chromium and carbon oxidizes when decarburization of melt is done. Martensitic stainless steels: which contain around 13% Cr and C varying in between 0. Varying amounts of other alloying elements like Ni. etc may be added to obtain certain specific property. There are different types of stainless steels like Austenitic stainless steels: which contain 18% Cr. The Ellingham diagram for oxide formation indicates that carbon oxidation in preference to chromium oxidation can occur at temperatures greater than 1220oC. 8% Ni. Duplex stainless steels: in which Cr is around 25%.03-0.08% to 0. it follows ?? = [W cr ]3 [f cr w cr ]3 p 4co [f C w c ]4 [W c ]4 = 2) K×(f c )4 3) f cr 3 p 4co fc and fcr are activity coefficient of carbon and chromium in liquid iron at 1wt % standard state.21Wt %Ni + 8.25atm R1 129 295 619 1220 R 207 488 1077 2019 pco = 0. 2C + O2 = 2CO 2Cr + 1.76 − 0.Chromium oxidizes to Cr2 O3 or Cr3 O4 and C oxidizes to CO. Hilly and Kaveney proposed the following equation for distribution of chromium and carbon: log � wcr 13800 � = − T + 8. Table shows the influence of temperature and pCO on the distribution ratio (??) = � of 10Wt % Ni wcr w � without Ni and R1 = � wcr� in the presence wc c Table: chromium /carbon distribution ratio T(k) 1873 1973 2073 2173 R 25 55 128 240 pco = at1m.925 log pCO c 5) Equations 4and 5 describes distribution ratio of Cr and c in Fe-Ni –Cr-C melt.925 log pCO wc 4) The effect of Ni w 13800 log � wcr � = − T+4. R1 36 82 173 339 We can infer the following form the table: R 89 209 460 863 pco = 0.5 O2 = Cr2 O3 or 3Cr + 2O2 = Cr3 O4 Equilibrium distribution between Cr and C is represented by considering Cr3 O4 Cr3 O4 + 4C = 3Cr + 4 {CO} 1) Assuming pure Cr3 O4 and Cr and using henry’s law for carbon.76 − 0.1atm R1 301 690 1477 2852 . Technology of stainless steel making Electric arc furnace was used to produce stainless steel by melting scrap of the desired composition. Reduction in pressure of CO can be achieved either by vacuum or by using a mixture of Ar + O2 . The former one is vacuum oxygen decarburization (VOD) and the later is (AOD) AOD Process The process in carried out in a converter type of vessel. if stainless steel is produced at atmospheric pressure. . The charge is melted in EAF and after melt. Decrease in pco increases both R and R1 at all temperatures. Melt is charged in AOD vessel. When carbon reduces to 30% of the original value the ratio of O2 : Ar is changed to 2: 1 and blow is continued to attain 0. A mixture of argon +oxygen is injected through the tuyeres located on the side of the converter shell. Carbon oxidation occurs once the bath temperature rises to 1800℃ In the finishing stage. High carbon-ferrochrome is charged. charge consists of carbon steel scrap + stainless steel scrap +lime. First stage of blow generates sufficient amount of heat due to oxidation of Cr and hence coolants are added (5% of the change). Fe-Cr.Ni –C melt and basic Cr2 O3 slag forms.09 to 0. Nickel increases R and R1 The above observations suggest that high temperature is needed to remove carbon in presence of chromium. High temperature is required to suppress chromium oxidation in favor of carbon oxidation. all Nickel and carbon. Typically.down period. lower temperature can cause oxidation of carbon. Stainless steel scrap is used.12% C. Low carbon ferrochrome is required which in expensive. II. EAF was used only as a melting unit. The disadvantages with this technology are • • High temperature is required which cause damage to the refectory lining. low carbon ferrochrome is added to make the chromium content of steel to a desired value. III.chrome and to decarburize the bath at selectively lower temperatures would be required. Melt consists of Fe. If reduced pressures are used.I.Ni-C alloy melt is prepared in EAF.Cr. Basis of development of new technology Thus a technology which can use high carbon ferro. Initially chromium oxidizes until bath temperature rises to 1800℃. the melt contains around 10% Cr. a reduction in pressure of CO from 1 atmosphere to lower value would be required. In the initial stage a mixture of O2 : Ar in 3:1 ratio is blown through the side tuyeres. If carbon is to be oxidized in preference to Cr at low temperatures. Vessel is lined with magnesite brick.Cr –Ni –C alloy Oxygen is blown onto Fe. Increase in temperature increases R and R1 at all pco values. Cr3 O4 + 2Si = 3 Cr + 2SiO2 .5 to 2 by adding lime. Slag formation and slag metal reactions are facilitated by argon stirring of the bath. the ratio of O2 : Ar is changed 1:3 to bring C to the desired value.In the final stage.Ghosh and A.Si is added to recover Cr from slag and slag basicity is maintained at 1. Chatterjee: Ironmaling and steelmaking . A. Fe.015%. References: A. The bath is desulphurized to levels well below 0. Chakrabarti: steel making B. Converter steelmaking is based on hot metal. To develop a process which is flexible to use varying proportion of metallic materials depending on their availability. To develop technologies which are not electric energy dependent.1 is a schematic sketch of an energy optimizing furnace (EOF). namely furnace to produce steel. though developments are made to use chemical energy. reuse and recirculation within the process environment. This requires integrating the concept of recovery. EOF is a combination of three independent. In the above concept. . CONARC Basis: • • • To develop ecological balance technology. Such a technology would require processing the raw materials such that liquid products are discharged at the environmental temperature and waste products are discharged in their harmless states. two technologies are worthwhile to mention: Energy optimizing furnace (EOF) and CONARC (Converter + Arc furnace) Energy optimizing furnace (EOF) Figure 19. interconnected reactors. Electric steelmaking is dependent of availability of cheap electric power.Lecture 19: Emerging Steelmaking Technologies Contents: Basis Energy Optimizing Furnace CONARC Key words: EOF. preheater to heat the scrap and a recuperator to rcover waste heat and to reuse by heating oxygen. For a given Q. For scrap preheating residence time (R T) of the waste gas in the preheater RT= Diameter of the preheater flow velocity of exit gas = 0. Recuperator . Furnace The furnace is refractory lined and has a provision for injection of carbon and oxygen. Both D and Q are important.785 D3 Q (1) should be maximum. diameter of preheater must be such that furnace gases can cool within 700 ℃ to 800 ℃ from 1300 − 1350 ℃. scrap preheating chamber and a recuperator in an Energy optimizing Furnace Design of each reactor and their integration with each other is important such that furnace off gas exits the system at a low temperature to the extent possible and at the same time scrap is heated to the desired temperature and steel of required quality is tapped. Scrap size is also important to heat scrap to the desired temperature. Preheater A preheater is installed at the top of furnace to preheat the scrap by the waste gases flowing upward from the furnace.Figure19. Smaller size scrap can be preheated efficiently. Oxygen can be injected both at high speed to promote decarburization and at low speed to promote post combustion.1: Arrangement of furnace. The exit temperature of gas from the preheater is the inlet temperature of gas to the recuperator. D is diameter of preheater and Q is volume flow rate of furnace gases. The efficiency of the heat exchanger depends on the flow velocity of hot gas and oxygen. High % hot metal produces a product with very low tramp elements. . Higher the scrap proportion. The operation in this case starts with injection of carbon in the hot metal heel until 3% carbon is achieved. For a given cross section of the flow passage of the recuperator. To increase furnace life and effective preheating. This is followed by charging of the preheated scrap and refining commences immediately thereafter. special clean steels and steels for seamless piper etc. Due to continuous slag flushing. maximum and minimum size of scrap are controlled.Recuperator is a counter current heat exchanger in which hot gases at 700 ℃ − 800 ℃ from the scrap preheater enters and exit gases leave at 350 ℃ and oxygen enters at 25 ℃ and preheated oxygen enters the furnace. High CO partial pressure during the blow leads to very low N and H content in steel at turndown. Oxygen injection for refining and post combustion begins simultaneously. Important Features Oxygen is blown by a lance submerged in the melt for refining and simultaneously oxygen is also injected for post combustion. Advantages for die forging steels. With the above features EOF presents a low cost alternative for steelmaking combined with flexibility in terms of metallic charge mix. the length of the recuperator must be optimized so that heat transfer from the furnace gases is maximum to preheat the incoming oxygen. Utilization of sensible heat of furnace off-gases by preheating scrap and O2 Flexibility to use hot metal and scrap in any proportion Wet de-dusting system Liquid steel with low levels of phosphorus and sulphur Low noise level An average of 42 heat/ day is possible. turndown P can be obtained up to 0. Slag is allowed to form so that it can flow over the sill down the pit. The refractory consumption is kept within the limits by using water cooled panels and scrap supporting bass. 2. Steel quality aspects 1. higher is the amount of heel.025%. The efficiency of the recuperator can be defined as ?????????????? ?????????????????? ?? = ???????????????? ℎ?????? ???? ???????????? ???? ??ℎ?? ??????ℎ?????? ?????????????????????? ???????????????? ℎ?????? ???? ?????????????? ?????????? A liquid heel is left in the furnace from the previous heat in proportion which depends on scrap proportion. 3.008% and sulphur up to 0. 60/ 80 t and 100/ 120 t. Gas cleaning plant is wet system.1700℃ without ladle furnace and 1650℃ with ladle furnace. Hearth surface is 6. 2. 4. EOF has facilities for charging and weighing of bulk material and ferro-alloys. Consumption of raw materials/ ton of liquid steel Hot metal and scrap+ pig iron are charged in 70% :30% proportion Lime 45 kg/t (depending on P content of HM) O2 is consumed at 50-70 N cu. Tilting is allowed at high speed hydraulic cylinders to allow slag free tapping.5m Total height from working platform to top level =17 to 25 m No. Bottom car (2unit) is used for quick bottom exchange for a new campaign./t Fuel consumption is 5-10 M cal/t Tapping.Basic Characteristics Standard capacities of EOF are available in capacities 30/40 t. 3. Flexibility to in terms of metallic charge No need of electric energy High productivity Low tramp elements Low inclusions due to slag free toppings Energy saving is due to A.m. 2.3 to 7. Scrap preheating (850℃) C. Post combustion (95%) B. Advantages 1.6 to 22 square meters Shell diameter is around 5. One bottom car supports the EOF for operation and the other supports a second bottom for relining. . High operational of flexibility. 5. 3. of scrap-preheater staged 1to 2 Tilting tapping angle 80 Main equipments description 1. CONARC is the resultant of such discussions. pellets. Malmberg et. H Tupkary and V R Tupkary: Steel time International. scrap or hot metal This plant will be equipped with compact strip production technology to produce high grade finished steel like hot rolled coils.com. 2006. Vd 30 No. electricity or gas.CONARC In seventies there were discussions amongst scientists and technologies about the future of pure top blown and pure bottom blown steelmaking. sponge iron. It combines electrode arc melting with the oxygen steelmaking process. Vol 47(2007) P.e. The objective is to develop a technology utilizing benefits of both EAF and top blown steelmaking. ISIJ Intern. Worldwide there are around 12 units in operation. In recent years discussions are in progress about EAF and oxygen steelmaking with respect to use of energy and raw material. In addition. These discussions resulted into a combined top and bottom blown steelmaking technology which has almost replaced pure top blown and pure bottom blown steel making.al. Bhushan steel and strips limited have also installed CONARC. an overview. This allows a high flexibility with respect to use of raw materials as well as energy source.8 P28/31 Internet on EOF Site: www. This combination has given flexibility of alternative choices of energy resources to use for steelmaking-be it coal.minitechnologies. choice of alterative metallic charge material is also available i. be it iron are. Microwave technology in steel and metal industry. • In India Ispat industries in Dolvi. What is CONARC? The CONARC steelmaking process has been developed by Mannesman Demag.433 . Maharashtra has been integrated with Ispat’s converter and arc furnace. The process consists of two shells with one 3 phase electrode arm and one top blowing lance for hot metal treatment. References R.br D. Lecture 20: Process Control BOF Steelmaking Content Introduction How are processes controlled? Process control models Static model Semi-dynamic model Dynamic model Key words: Dynamic modeling, static modeling, Process controll Introduction Steelmaking in BOF is very fast. It takes approximately 15 to 20 minutes for oxygen blowing and 50 to 60 minutes to tap molten steel. Liquid steel at turndown results from several non linear interconnected complex processes like gas/ liquid metal reaction as dependent on oxygen availability, slag/metal reaction as dependent on physic-chemical properties of slag and faster reaction rate induced by three phase dispersion (CO+ slag +metal droplets). Control of the above processes is needed so that for a given input of hot metal, scrap, flux and oxygen flow rate, steel of the desired chemistry is produced with minimum loss of iron in slag How are the above processes controlled? Rates of gas/metal, slag/metal and gas/slag/metal droplet reaction are controlled by lance profile, oxygen flow rate and bottom stirring rate, it is required to raise or lower the lance distance for a given oxygen flow rate and bottom stirring rate so that steel of desired composition and tapping temperature can be obtained within the stipulated blowing time. For this purpose we need to develop process models which can describe the process quantitatively. These models must be supported by the data for accurate predictions for the future requirements. Process Control Models One of the objectives of the process control models is to predict turndown composition and temperature of steel so that unnecessary blowing of oxygen is not required. For a long time human expertise was the control tool. The operator used to deliver instructions to exercise the process control. These controls were human specific. Developments in computer has resulted into development of sophisticated models, like a. Static model b. Semi-dynamic model c. Dynamic model Static Model Static models are based on materials and heat balance by considering initial and final states of reactants. In the material balance, mass of all input and output elements is considered. Once mass balance is done then heat balance is done. Sensible heat of all inputs+ Heat produced or absorbed by oxidation reaction= Heat taken out by steel, slag and exit gases and fumes+ heat losses from the converter mouth and through the lining of the converter+ any other heat losses In making materials and heat balance some assumptions may be required for example iron loss in slag, carbon removal in the form of CO, complete dissolution of CaO in slag basicity of slag thermo-physical and thermo-chemical properties of slag and metal The above are some of the assumptions, further may be added. By coupling of mass and balance one can predict i) ii) iii) iv) v) Quantity of hot metal and scrap Amount of flux Total quantity of oxygen required to be blown. Amount of slag produced Volume of exit gases It is very much important that the prediction based upon the model is verified by the actual plant data. Tuning of the model is necessary because the predictions are based on equilibrium considerations and uncertainties due to simplified assumptions. Reliability of predictions increases when the predictions of the model are compared with the plant data for large number of heats. Statistical correlations can be developed and used to fine tune the model. For this purpose it is of utmost necessary to collect the reliable data from the reliable instruments BOF steelmaking is a stochastic process. Oxygen blowing produces lot of turbulent in the phases during hot metal refining. The amount of droplets emulsified in the slag, amount of lime dissolution, carbon removal rate, intensity of oxygen jet impinging the bath, lance distance, bottom stirring rate due to plugging of tuyeres etc. may vary from one heat to the other. The error in predictions may be due to i) ii) iii) iv) v) vi) Error in weighing Differences in lime dissolution from one heat to the other Effect of size of scrap on its dissolution. Large size will take more time to dissolve as compared with smaller ones. State of foaming of slag and entrained metal droplets in slag. This may vary from one heat to other due to behavior of oxygen jet in a dynamic surrounding as discussed in lecture 13. Surrounding of oxygen jet changes during the blow. As a result extent and magnitude of slag/metal reactions might change. Converter lining profile due to wear. Extent of mixing within the phases and between the phases. Semi – dynamic model The above features make static control models inadequate. Hence improvements are required. Static model predictions can be improved by measurements on temperature and composition of slag and metal by a sub lance during the blow. Immersion type sensors can also be used to measure carbon and oxygen concentration during the blow. All these data are collected and fed into the computer which compares the model predictions to suggest the action to be taken by the operator. These are semi-dynamic model. It must be noted that error in carbon measurement may need to over blow the heat when end carbon measured is greater than specified carbon. Also if end point carbon measured is lower than the specified, carburization has to be done in the ladle for carbon adjustment. Determination of carbon by a sub lance is indirectly done by measuring the temperature of steel through a sample collected by the sub lance during the flow. Dynamic Models Static models do not calculate the variation of blowing parameters as a function of time. For this purpose continuous measurement of some representative parameter is required during the blow. In a preprogrammed model, these values are fed continuously and corrections can be done during the blow. Dynamic models contain all features of static model; in addition it includes reaction kinetics and process dynamics. In steelmaking major reactions are oxidation of carbon and iron. In a dynamic model rate of decarburization and O2 consumed are determined from exit analysis. Computation of Fe is then done. Dynamic control requires measurements continuously. Exit gas temperature and its composition can be measured continuously and the data can be fed to computer. From the exit gas composition and temperature carbon, oxygen in bath can be determined as a function of time. From carbon balance, decarburization rate can be determined. Oxygen balance provides the following information: • • Total rate of oxidation Relative fraction of oxygen reacting with carbon and iron and other elements Enthalpy balance based on exit gas temperature and composition and amount gives information on energy leaving the system. Slag height can also be determined by measuring the acoustic sound produced by the slag during the blow. Sonic meter measures the intensity of sound. Intensity of sound is fed to computer which in turn adjusts the lance height, oxygen flow rate and bottom stirring rate to control the slag/metal height. It must be mentioned that large number of literature is available on the process control models in steelmaking. It is not possible to cover in one lecture. The interested reader may go through the references given at the end of the lecture. Some references are given. You may find more. References: 1. G. huidi et.al: A process model for BOF process based on bath mixing degree, intern. It. Of minerals, metallurgy and materials, vol17, No,6 dec.2010 page 715 2. J. Mailo et.al: BOF and point prediction: Metal producing and processing p14. www.metal production.com. Nov/Dec.2008 3. T. Oshima et.al: New process control for a steel plant, Fuji electric review. Vol. 53 P8. 4. A.Ghosh and A. Chatterjee: Ironmaking and steelmaking 5. A.Das, et.al. Process control strategies for a steel making using ANN with Bayesian regularization and ANFIS, Expert systems with applications, vol 37 March 2010 6. J.wendelstorf et.al.: A process model for EAF Steelmaking, AIS Tech, 2006, P 435 7. A. Mc Lean: Sensor aided process control in iron and steel making: Solid State ionics, volume 4041, Aug 1990 P 737 8. E.J.Longwells: Dynamic modeling for process control and operability, ISA Transactions, Vol 33, May 1994,P 3. and extra heating facility etc. This means that the bath is deep. One has to determine the amount of stirring gas and location of the injection of gas in the ladle. The gas can either be injected through the nozzle or porous plugs. Injection elements could be located either axis-symmetric or asymmetric to the center of the ladle. To deoxidize molten steel To improve cleanliness of steel by removing inclusions To engineer the inclusions so as to alleviate their harmful effects on mechanical properties of steel To add alloying elements To remove dissolved gases The effectiveness of each of the function requires modifying the ladle in terms of molten steel flow.9. ladles are employed to transfer molten steel from BOF/EAF to ingot casting or continuous casting. What modifications are required? Ladle is a cylindrical refractory lined vessel and aspect ratio of the bath varies between 0. Location of the injection elements is an important issue. bath can be agitated either by an inert gas or by induction. injection metallurgy. It has been realized that ladles can be used very effectively as a reactor which can perform any of the following functions: • • • • • • • To desulphurize molten steel tapped from BOF/EAF To homogenize molten steel to minimize gradients in concentration and temperature and to attain desired teeming temperature. .Lecture 21: Evolution of ladle Treatment and Requirements Contents: Preamble What modifications are required? Basics of gas stirring Mixing time Gas injection rates Choice of stirring energy Key words: Ladle metallurgy. Secondary steelmaking. synthetic slag practice Preamble In steelmaking. At high temperature. Bath agitation would be required to carry-out the functions effectively.8 and 0. 5m.Enough free board height in the ladle must be available to accommodate the quantity of slag required for refining and to absorb inclusions. At velocities greater than 0. s Where Q is gas flow rate� m g is�s 2 �. stirring action is small near the bottom of the ladle. Refractory materials for injection elements and their fixing must also be considered.14 m/s as the gas bubbling velocity.8) �ln �1 + 0. This can be achieved either by tapping steel at slightly higher temperature or to provide addition heating arrangement in the ladle itself. the most important would be the selection of refractory (see lecture 9 and 10) to meet the refining requirements. The liquid flow in the ladle occurs via bulk motion of the metal. In many situations it is required to inject the slag forming materials either for refining or for inclusion engineering. For gas injection at 50 l/min in a ladle of bath diameter 3. For gas stirring recirculation rate of molten steel is of interest.381 (2) Consider a ladle with bath height 3m and Q=650 l/min (1 atm and 273K). H is bath height (m) and V is bath volume (m3 ). Very roughly. Additional heating may be required to keep the molten steel to the teeming temperature. The stirring homogenizes bath composition and temperature.3 m/s at the slag/metal interface slag droplets may be entrained by the metal flowing along the interface and into the melt. Increasing the flow rate to 800 l/min increases Ṁ to 10.3 (H + 0.5m and bath height 3. With centrally placed nozzle at the bottom. Above all. . Basics of gas stirring Argon is usually bubbled into the molten steel covered with slag either through the top lance or through a porous plug fitted at the bottom. The mass flux in (tons/s) of entrained steel passing through the top section of the bubble plume can be calculated by the following semiempirical equation: Ṁ = 13. We can calculateṀ = 9.5 tons /s. An asymmetrically placed bubble plume gives velocities near the bottom which are greater than for symmetrically placed nozzle.3 tons/s.5 H �� 1. In this case suitable injection device must also be available. equation 1 calculates 0.48 × Q0. A plume of gas rises upwards when gas is injected through the bottom. characteristic velocity vc for gas bubbling is 1/3 Qg H 2 � V vc = � (1) m3 �. hb db b (4) expresses aspect ratio.006%VS )] × �W d b � where VS in %. Electromagnetic induction induces stirring energy about 100 W/m3 . t) High gas flow rats though supply large amounts of stirring energy. Gas injection rates The following are some typical values of specific gas injection rates: Stirring < 5Nl/ (min. Mixing time i. time to homogenize the bath indicates the conditions for stirring. H is bath height and P2 is atmospheric pressure). For evaluating the efficiency of gas stirring.00847 %V S −0. Scrap enhances the mixing time. large freeboards are needed. The energy input can be calculated by W = 6. At higher gas flow rates slag droplets may be entrained in molten steel which may increase the rates of refining reaction because of transitory phase contact (Readers may see the references at the end of this lecture).18 × 10−3 QT ��1 − 273 �+ T ln P1 � P2 (3) Here W is stirring energy in watt. Large freeboards decrease the molten steel holding capacity of ladle. Mixing time decreases with increasing the aspect ratio of bath. volume of scrap (VS ): h −0. The following results are to be noted: i. RH degassing gives about 800 W/m3 stirring energy at a circulating rate of 40 tons/minute. Lack of large freeboards and splashing often limits rate of gas injection. . t) Lime steel desulphurization ≈ 14Nl/ (min. Good mixing promotes the rate of slag/metal reaction as indicated by smaller mixing time.338 + 0.5564 τ = [exp(4. an alternative approach is to calculate energy input W in watt. Mixing time(??) in seconds can be correlated with the energy input in W/m3 . For gas injection rate 200 Nl/min into a bath of 2m height the stirring energy would be 390W/m3 when the bath diameter is 2.5 m. Homogenization is possible within a definite time only up to a certain maximum scrap ratio. iv. The mixing time decreases with increase in bottom gas rate iii. Q is flow rate in Nl/min. t is bath temperature in K.where ρl is density of molten steel. P1 is pressure at the bottom surface of the bath (P1 = P2 + ρl gH. t) Vacuum Arc Decarburization < 2Nl/ (min.The calculations show that the recirculation time of a 200 ton melt is 21 s for 600 Nl/min and 20 s for 800 Nl/min gas flow.e. ii. 60-66. Vol. leading to reduction of slag by deoxidizing elements. Inclusions removal would require weak stirring.2.42 (1989) No. Vigorous stirring in the neighborhood of slag/metal phase boundary activates interfacial mass transfer.1. Of Metals. Slag carry-over from BOF/EAF must be avoided. Less disturbance at the slag/metal interface can be obtained by induction stirring. Vol. Bath homogenization would also require weak stirring.21-27 S C Koria Modelling of submerged gas inecting lance design parameters Steel Research. Vo. Tokyo. p.91-101.484-491 S C Koria Thermodynamic considerations in designing gas injecting lances submerged in melt Ironmaking and Steelmaking 16 (1989) No.7.60. S C Koria and S Singh Experimental investigations on the design of gas injecting lances ISI Japan International. REFERENCES: S C Koria and S Singh Measurements on local properties of a heterogenerous air/water plume formed during uptward Injection of gas Steel Research. (1989) No.1.47-54 S C Koria and K W Lange Effect of Melting scrap on the mixing – time of bottom gas stirred melts Proceeding 6th Japan-Germany seminar. Vigorous mixing of metal and slag is achieved with gas stirring. p. and oxygen and nitrogen pick up from the atmosphere.60 (1989) No. Vo. p. p.8. p. and how vigorously. Inst.650-657 S C Koria and S Singh Measurements on local properties of a heterogenerous air/water plume formed during uptward Injection of gas Steel Research. Japan (1984) p. Vol. reversion of phosphorus. p.Choice of stirring energy Correct stirring is of utmost importance. S C Koria Model investigations on liquid velocity induced by submerged gas injection in steel bath Steel Research 59 (1988) No.11.7.60 (1989) No. Vigorous stirring would be required for slag/metal reaction such as desulphurization. It should be known when to stir what. Ind.29 (1989) No. p.301-307 S C Koria and C D Khai Model study on dissolution time of metallic materials in steelmelt Trans.301-307 . p.1. Ind. Inst. Of Metals.47-54 A. Vol.42 (1989) No.Chakrabarti: Steelmaking A.Ghosh: Secondary steelmaking .S C Koria and C D Khai Model study on dissolution time of metallic materials in steelmelt Trans. Fireclay ladles are not suitable if low sulphur steel is to be produced. Argon bubbling is done. Often Al is present to deoxidize the molten steel since transfer of sulphur from molten steel to slag is followed by transfer of oxygen from slag to steel. steel should be deoxidized and slag carry-over should be minimized. To avoid reoxidation of steel from atmospheric oxygen because the molten steel transfer operations are done under atmospheric condition. The following properties are desirable in synthetic slag: i) ii) iii) iv) Slag should have high sulphide capacity Basic slag is required Slag should be fluid to obtain faster reaction rates. Instead. Design of synthetic slag The synthetic slag contains CaO. Calcium fluoride increases the sulphide capacity of slag and helps fluidizing the slag.Lecture 22: Synthetic slag practice Contents: Introduction Desulphurization of steel Design of synthetic slag Alternative synthetic slag Characterization of synthetic slag Keywords: ladle metallurgy. desulphurization. Ca F2 . dolomite or other basic refractory lined materials should be used. Al2 O3 and with small amount ofSiO2 .005% Synthetic slag practice is attractive due to low capital cost on equipment. synthetic slag. Synthetic slag practice is adopted to meet the following objectives i) ii) iii) iv) v) To cover molten steel for cutting down heat losses. injection metallurgy Introduction Synthetic slag practice is employed to obtain clean steels and to desulphurize molten steel. Desulphurization of steel Synthetic slag practice can desulphurize steel up to 50% to 60% of original sulphur in steel. deoxidized steel can be desulphurized to as low as 0. For efficient desulphurization . Therefore deoxidation of steel is must for efficient . The principle component of synthetic slag is lime. To remove inclusions from molten steel. Slag should not cause excessive refractory wear. Using slag of desired basicity and sulphide capacity. Oxygen content of steel should be same for consistent results. On the basis of ionic theory. iv. better is for desulphurization. In terms of property of slag sulphide capacity Cs is pO 2 pS 2 Cs = (Ws ) × � (2) Where Ws is weight percent of sulphur in slag with a gas having partial pressure of oxygen ????2 and ????2 . Temperature drop could be of the order of 10℃ to 25℃ for 150 − 250 ton heat. Typically. 5 − 16% Al and 0 − 5%SiO2 . Composition of CaO and Al2 O3 can be selected so as to melt at 1400 − 1450℃. a modified sulphide capacity of slag is Cs1 = (W s )[h o ] (1) hs Where (Ws ) is % sulphur in slag and ho and hs are henrian activity of oxygen and sulphur in steel. The temperature drop is resulting from radiation heat loss from surface and heat transfer due to argon bubbling. Certain issues are: i. a neutral slag with CaO⁄SiO2 = 1 or 1. has been found to reduce the problems associated with pre fused slag. Synthetic slag practice appears to be simple and not much capital investment is needed. It is hygroscopic and leads to hydrogen pick up Argon bubbling is done to stir the bath. This slag is pre fused in solid state.375 (3) . when used for desulphurization. Alternative synthetic slag A pre melted slag based on CaO and Al2 O3 with small amount of Ca F2 can alleviate the problem of refractory wear and hydrogen pick. A synthetic slag consisting of 70%(50%CaO + 50% Al2 O3 ). Characterization of synthetic slag An important parameter to characterize synthetic slag for its suitability to desulphurize molten steel is sulphide capacity of slag. iii.slag contains 45 − 55%CaO. ii. Issues related to synthetic slag practice. Special synthetic slag can be designed for a specific purpose.over from BOF/EOF is not controlled. Small amount of Ca F2 may be added. 25% CaO and 5%Ca F2 could be used. The slag attacks the ladle refractory. when no desulphurization is needed.2 can be used. 10 − 20% Ca F2 . Excessive amount of Ca F2 results in refractory wear. For removal of oxide inclusions. Higher tap temperature increases refractory wear. Desulphurization may vary from one heat to other if slag carry.desulphurization. higher is the value of Cs . Cs and Cs1 are related with log Cs = log Cs1 + 936 T − 1. For a given slag. CaO is the main component. This remelted slag. 1 X MgO − 0.482 (9) Consider a slag with X CaO = 0.35 which is used to desulphurized steel at 1873 ??.01 wt % Al dissolved. By equation 5 log K s = log Csi − log h [o] log K s = log Cs − 936 T (6) + 1.8XAl 2 O 3 − X SiO 2 � − 9894 T + 2.65 and X Al 2 O 3 = 0. Alternatively Cs can be calculated by log Cs = 3. it is defined as Ks = (W s ) [W s ] Ci = [h s ] (5) o ho is activity of oxygen in steel and is determined by the amount of deoxidizer. Another important parameter is partition coefficient of S at equilibrium.375 − log h [o] (7) + 20.44�X CaO + 0. Cs C 1s = 0.6 and X SiO 2 = 0.38 in slag Let us calculate K s by equation 9.137 and 0.At T = 1823 K and 1873 K.4. also large value of K s requires low value of ho and high value of Cs as well.57 − 2 log Wal + log�aAl 2 O 3 �� (8) If aluminum is used to deoxidize steel. An aluminum killed steel can desulphurize steel much effectively that that when either FeMn or Fe Si is used to deoxidize steel Large value of K s ensures efficient desulphurization. Steel has 0.05 (4) For a slag with X CaO = 0.87 × 10−3 at 1873 K. log Cs by equation 4 = − 1.Ghosh: Secondary Steelmaking . Within the temperature range of desulphurization the ratio Cs⁄Cs1 does not depend significantly on temperature. ho in steel can be determined by 1 log ho = 3 �− 64000 T Combining equations 7 and 8 we get. Activity of Al2 O3 is 0.133 respectively. Cs according to equation 4 is 2. log K s = log Cs + 20397 T 2 3 1 3 + log[WAl ] + log�aAl 2 O 3 � − 5. Extent of desulphurization depends on extent of deoxidation.96 ∴ log K s by equation 9 = 97 References: A. Al. ladle furnace. transitory contact. NiO. Mo O2 Fe B. Ca. CaCN2. ladle refining. for example 90% CaO + 10% CaF2 or 70% CaO + 20% Al2 O3 + 10% CaF2 . Si. Table below shows the slag forming materials used for injection. permanent contact. CaO Alloying Fe Si. When .Al2O3. chloride slag) CaC2+ CaCO3. The type of powders used is governed by the purpose of injection. wire feeding Injection of solid powders Injection techniques have the advantages of dispersing the reactants in the steel bath and at the same time provide a large reaction surface area.Lecture 23: Injection ladle metallurgy Contents: Injection of solid powders Desulphurization Mechanism Illustration Alloying with gas injection Heating of steel Keywords: alloying. Fe Ti etc Deoxidation and inclusion shape control Al. Purpose Type of powder Dephosphorization CaO+CaF2+Fe2O3+ mill scale soda Desulphurization CaO +Al CaO + CaF2 +Al Ca C2 Mg+(MgO. Mn. Ca Si. The injection rate varies between 2 -4 kg/ton of melt. Ca Si and Ba Desulphurization mechanism Desulphurization can be carried out by injecting lime based powder. The slag forming materials are lighter than steel and deep injection would be required for the efficiency of the reaction.1 shows arrangement of ladle desulphurization carried out either by injecting cored wire or by pneumatic injection through a top lance.slag forming materials are injected into melt. In both argon is bubbled through a porous plug fitted at the bottom of the ladle.1 Ladle desulphurization carried out by powder injection technology (a) cored wire injection and (b) Pneumatic injection through a top lance . Residence time of the rising particles in the melt is also important. they melt and the molten slag particles begin to rise and accumulate at the top surface of the melt. Figure 23. which means that the gas powder injection velocity must be suitably selected. The desulphurization reaction occurs in two ways: • During contact between rising molten slag particles and the melt. Once all the powder is injected. • Contact between top slag and the melt. Powder melts and the rising gas imparts mixing in the melt. In this mechanism slag/metal interface area is important. The desulphurization efficiency can be estimated by the mass balance. reaction between top slag and sulphur of melt governs the final sulphur content of steel. As the molten slag particles rise they accumulate at the top surface of the melt and after a while top slag also takes part in the desulphurization. This mechanism is known as permanent contact It must be noted that methods for injection of powder must also be developed. Figure 23. Powder can be injected either through cored wire or pneumatic transport. In this mechanism of desulphurization it is important that the powder becomes molten on injection. This mechanism is known as transitory contact. Gas injection rate may be suitably selected to produce and entrain slag droplets into the melt for the faster rates of reaction. 33 8 0.018 0. integration of equation 1 and putting the limit t = 0.51 0.In the transitory contact mode. and (ms )o is initial S in slag and (ms ) is final sulphur in slag. Amount of slag injected is 4 kg/ton . [m s ] [m s ]?? = exp �− Ẇ s L s � 1000 (2) = exp(−Z) Ls is the partition coefficient of sulphur.38 0.73 and [ms ]⁄[ms ]?? Perm.76 Ls = 500 [ms ]⁄[ms ]?? trans. [ms ] = [ms ]o and t = t.53 0. ms is sulphur in steel at time t. Similarly in the permanent contact zone sulphur balance 1000[ms ]?? + W(ms )o = 1000 [ms ] + Ws (ms ) (3) Where W is amount of top slag. 8 kg/ton. [ms ]⁄[ms ]?? Perm.61 0.let use calculate [ms ]⁄[ms ]?? for each case: The following table gives the calculated values Ws kg/ton 4 Ls= 80 [ms ]⁄[ms ]?? trans.14 We note the following: . the desulphurization can be determined by performing sulphur balance: −1000 d[m s ] dt = Ẇ s Ls [ms ] (1) Ẇ s is rate of injection of slag powder.20 12 0. Consider two different slags whose partition coefficient is 80 and 500. 0. To illustrate the role of transitory and permanent contact on desulphurization .14 0. and 12 kg/ton. 0.0025 0. [ms ] = [ms ] we get. 0. Rearrangement of equation 3 gives [m ] Ls W s � 1000 +L s W s R = 1 − [m s] = � Illustration s ?? = ?? ??+?? (4) A slag with high value of Ls is desirable for efficient desulphurization. gas stirred by porous plugs and gas stirred using a tuyere. However. The dissolution and homogenization of the alloying additions are enhanced by stirring and small particle size. Heating of steel Synthetic slag practice with argon stirring or injection of solid powder requires higher tap temperatures to compensate for the heat losses during refining. For example for Ls = 80 . Both amount of slag and partition coefficient are important for desulphurization At lower amount of slag and lower partition coefficient there is not much difference in the ratio of [ms ]/[ms ]?? for both permanent and transitory contact modes. when the powder injection rate is increased to 8 and 12 kg/ton. . v. This will minimize reaction of steel exposed to air. also allows steelmakers to perform many metallurgical processes like i. ii. At higher value of Ls = 500. the said ratio is 0. or with a carrier gas.76 for permanent contact. In EAF increased power and electrode consumption and an increase in furnace time are the main issues to tap steel at higher temperature. Alloying with gas injection Alloying can be done during tapping by simply dropping the material on the surface.. Arc heated ladle processes have been developed. Ladle arc furnace in addition to allowing for lower temperatures. when the powder injection rate is 4 kg/ton.73 for transitory and 0. Bath homogenization by argon stirring Inclusion removal and inclusion engineering Desulphurization by synthetic slag or by injection metallurgy Holding of ladles for long periods if and when need arises for example in sequence casting. Stirring intermixes top slag with the bath which should be minimized to avoid oxidation. transitory contact gives much lower value of sulphur in metal than permanent contact Transitory contact mode requires designing powder injection systems which can inject powder at constant and uniform rate. transitory contact mode becomes more efficient than permanent contact. iv. The ability to make addition of alloying elements. This increased tap temperature causes problems in BOF such as poor phosphorus removal and increased lining wear. iii. The vigorous intermixing of top slag with the bath can be minimized by the lowering of a refractory lined cylinder into the liquid steel. There are three different types of ladle arc heated furnaces: Induction stirred. For the detailed description. 33 No. Induction stirring requires a stainless steel ladle or a stainless steel section of the ladle. Ghosh: Secondary steelmaking: principles and applications.SKF: The process uses induction stirring.2: Ladle furnace showing arrangement of electrodes and porous plug Figure 23.2 R.2 shows ladle arc furnace where a cover equipped with three graphite electrodes are shown. The ladle furnace is equipped with a hopper for additions of reactants.J.Mohr uses vacuum arc degassing (VAD) the system is under partial vacuum during heating. Fruehan : Ladle metallurgy principles and practices A. The processes are under the following names: ASEA. Diado ladle furnace: The process is gas stirred. readers may see the references. Fumes are taken out through the roof of the ladle. Argon bubbling is maintained through a porous plug. The ladle bottom has a porous refractory plug for argon stirring. . References: K W Lange : International materials Review. FineKl. 1988 vol. Electrodes are used for heating.Figure 23. The furnace uses three graphite electrodes with submerged are heating. No. Inst. Jl. 1998 453-459 S C Koria: Influence of injection metallurgy on mass transfer in steelmaking. Scand.SC Koria and K S Rao: Mathematical model for powder injection refining of steel melt.6 (2000) 259-270 . Metals 47 (10). 199. Trans. Of Metallurgy 29. 287 -299 S C Koria and R Dutta: Study on the effect of some process parameters on the powder injection refining by a mathematical model. Iron making and steelmaking 25 (6). Ind. This excess oxygen produces defects like blow holes and non. Semi –killed steel: Incompletely deoxidized steels containing some amount of oxygen which froms CO during solidification. These steels have good surface finish. De oxidation by single element is known as simple deoxidation. During solidification of molten steel. excess oxygen is rejected by the solidifying steel. During refining oxygen dissolves in steel. Ca − Si − Al etc.003% in solid steel during solidification. Rimming steel: Partially deoxidized or non.deoxidized low carbon steels evolving sufficient CO during solidification.23% which deceases to 0. . ii. carbon steels may be subdivided into three groups: i. Solubility of oxygen in steel is negligibly small.Lecture 24: Principles of Deoxidation Contents Introduction Sources of oxygen in steel Deoxidation of steel Kinetics of deoxidation Deoxidation practice Illustration Key Words: deoxidation. steelmaking Introduction Refining of hot metal to steel is done under oxidizing atmosphere. Defects have considerable effect on mechanical properties of steel.metallic oxide inclusion in solidified casting. Mn etc or by mixture of elements such as Si + Mn. According to the degree of deoxidation. Sources of oxygen in steel • • • • • Rust on steel Oxygen blowing Steelmaking slag Atmospheric oxygen dissolved in steel during teeming Oxidizing refractories At 1600℃ solubility of oxygen in liquid steel is 0. killed steel. removal of oxygen is called deoxidation. it is necessary to remove oxygen from steel. Al. iii. Deoxidation of steel Deoxidation can be carried out either by single element such as Si. Solidification of such steels does not give gas porosity (blow holes). Therefore. Killed steel: Oxygen is removed completely. Ca + Si. Increase in T increases K M . oxide is formed. Due to formation of liquid deoxidation product agglomeration of the product into large size can be obtained easily and can be floated easily.whereas deoxidation by a mixture of elements is known as complex deoxidation. which is called vacuum deoxidation. For larger WM . Deoxidation is also carried out by carbon under vacuum. interaction parameters need to be considered. If steel contains C. Here product is either solid MnO or liquid FeO. Elements are added in the form of Ferro-alloys Fe − Si. Using equations 2 and 3 one can calculate the variation of WO with WM when WM is in small quantity. Fe − Mn or Fe Si + Fe Mn etc. Ca + Si + Al is used. J (4) J (5) log fM = ∑J eM Wj log fO = ∑J eO Wj where J denotes all alloying elements. and Mn. MnO . (2) If deoxidation product is pure then activity of Ma Ob = 1 and if elements are in dilute solution 1 Where K M is deoxidation constant and equals to K 1 where K1M is equilibrium constant. log fO = eOO WO + eCO WC + eMn O WMn (6) Where e is interaction parameter All oxide products are definite compounds except compounds formed by Mn deoxidation. Simple deoxidation can be represented by a[M] + b[O] = (Ma Ob ) (1) [WM ]a [WO ]b = Km. In complex deoxidation where a mixture of Si + Mn. According to equation 2 K [WO ]b = M a [W ] M (7) . X T log K M = − + Y M (3) Where X and Y are constants and T is temperature. hence it is also termed precipitation deoxidation. the following advantages are reported as compared with simple one: • • The dissolved oxygen is lower. In both simple and complex deoxidation. Equation 7 indicates that weight percent oxygen in steel depends on value of K M for small concentration of deoxidizers. At 1600℃ the value of Km is 2.4 × 10−5 for the reaction Si + O2 = SiO2 and for the reaction 2 Al + 3O = Al2 O3 the value of K M = 3.32 × 10−14 . Similarly for the reaction Ca + O = CaO, K M = 9.84 × 10−11 . The value of K M indicates the deoxidizing ability of an element. For the above reaction, calcium is the most efficient deoxidizer and Si is not so efficient as compared to calcium. Aluminum is also a strong deoxidizing element when compared with silicon. Though calcium and aluminum are very efficient deoxidizers, but they oxidize very fast and moreover, their density is much lower than steel. Also Ca has a boiling point 1485℃ which means calcium is gaseous phase at the steelmaking temperature. Suitable injection methods or addition methods are to be developed. Kinetics of Deoxidation Total oxygen in steel equals to dissolved oxygen + oxygen present in deoxidation products (SiO2 , Al2 O3 , MnO etc). Even if the dissolved oxygen is low, deoxidation products (also called inclusions) have to be removed, otherwise steel is not clean (clean steel refers to number and size of inclusions in steel). Kinetics of inclusion is concerned with deoxidation reaction and separation of deoxidation products as well. The deoxidation process consists of the following steps: • Dissolution and homogenization of de oxidizer. Mechanism of dissolution depends on melting point. Ferro alloys melt at around 1500℃. Aluminium is expected to melt faster due to its much lower point. Intensity of agitation will govern the homogenization of deoxidizer in steel melt for faster kinetics of reaction between oxygen and deoxidizer. • Nucleation of solid product becomes easier if interface is present. Deoxidation by Al produces solid Al2 O3 and as such Al2 O3 /steel interface is useful for nucleation. • Growth of the de oxidation product: It depends on the state of the product. A liquid product can coalesce easily as compared with the solid product. Deoxidation with single elements like Al, Si etc. produce solid deoxidation product at the steelmaking temperature. Deoxidation with ferro silicon+ ferro manganese produces liquid deoxidation product. Boron, titanium zirconium are also quite effective deoxidizers. Manganese and silicon are used in the ratio 7:1 to 4:1 in order to obtain a thin liquid slag. • Removal of deoxidation product: Removal of de oxidation product is equally important. It is achieved by floatation and absorption into a slag. Following steps are important for removal of de oxidation products from steel: i. ii. Growth of de oxidation product Movement through molten steel to surface iii. Absorption of inclusion by a suitable designed synthetic slag. Floatation of an oxide product depends among physical properties of steel, on the size of the product. The rate of rise of a spherical particle in a quiet fluid or in a fluid of laminar flow (i.e. at very low Reynold’s number) can be described at steady state by Stoke’s law: Vt = gd 2 ∆ρ 18 η (8) Vt = terminal velocity (m⁄s)of the inclusion, g is acceleration due to gravity (m⁄s 2 ), ∆ρ = deiffernce in density of steel and deoxidation product and η is viscosity of steel (kg⁄m. s). Deoxidation products are lighter than steel; hence they move up. According to equation 8 the rising velocity is proportional to square of the size of the deoxidation product. Larger sizes move faster. Moreover different sizes of de oxidation product will move with different velocities. During their movement, they may collide with one another. Stirring of melt may help floating of de oxidation products. Degree of stirring in the melt is important. Vigorous stirring may not be of much help since deoxidation product may be circulated in the liquid. For the removal of deoxidation product, equally important is the design of synthetic slag to absorb the deoxidation product. Deoxidation practice Deoxidation can be carried out during tapping, in ladles runners and even in moulds. Bath stirring is important. During tapping, bath is stirred due to potential energy but this subsides towards the end. When deoxidation is carried out in ladle, it is called ladle deoxidation in industrial practice. Depending on the extent of deoxidation, killed, semi killed and rimming steels are produced. For carbon content less than 0.15% and enough oxygen in steel, rimming steel can be produced. Alloy steels are fully killed to obtain maximum recovery of alloying additions. Illustration Let us take an example of deoxidation of steel with ferromanganese. Manganese is a weak deoxidizer. We intend to reduce the dissolved oxygen in steel from 0.045 wt.% to 0.018 wt.%. How much manganese would be required? Consider the reaction (MnO) = [Mn] + [O]; ???????????? = − We can write ?????? ???????? = ?????? , ???? 11070 ?? +4.526 we have assumed hMn = WMn and hO = WO, where WO is weight percent Substituting the value of KMn = 0.0413 1t 1600℃ and WO = 0.018 we get ?????? = 2.29 ???????? For the deoxidation with manganese, the following reaction must also be considered (MnO) + [Fe] = [Mn] +(FeO); ????????????−???? = − 9370 ?? +4.330 By writing equilibrium constant and using the value of 2.061 wt. percent. ?????? ???????? = 2.29 we can find XFeO ≈ 0.9 and WMn = Total manganese required would be equal to manganese required to remove (0.045% ⎯ 0.018%) oxygen + Manganese required to raise the content of mn from 0.1% to 2.061% Calculation gives 20.53Kg Mn/ton steel. If the manganese content of ferromanganese is 60%, ferromanganese would be 34 kg/ton of steel Reference: A. Ghosh: Principles of secondary processing and casting of steel. K.W. Lange: Thermodynamics and kinetic aspects of secondary steelmaking. International Materials Reviews, 1988, vol. 33 p 53 Lecture 25: Principles of degassing Contents Introduction Principles Side reactions General considerations Fluid flow in degassing Material balance in de gassing Key words: Degassing, gases in steel, ladle steelmaking Introduction During steelmaking gases like oxygen, hydrogen and nitrogen dissolve in steel. The term degassing is employed to remove nitrogen and hydrogen from steel. Dissolved oxygen from steel melt cannot be removed as molecular oxygen. Removal of oxygen is termed deoxidation and is discussed in lecture 24. This lecture concerns with degassing. Both nitrogen and hydrogen impair the mechanical properties of steel. The maximum solubility of nitrogen in liquid iron is 450ppm and less than 10ppm at room temperature. During solidification excess nitrogen is rejected which may form either blow holes or nitrides. Excess nitrogen causes embrittlement of heat affected zone of welded steels and impair cold formability. Hydrogen in steel impairs steel properties. Solubility of hydrogen in steel is low at ambient temperature. Excess hydrogen is rejected during solidification and results in pinhole formation and porosity in steel. Few ppm of hydrogen causes blistering and loss of tensile ductility. Thus removal of nitrogen and hydrogen from steel is necessary. Principles Consider removal of hydrogen and nitrogen from liquid steel 1 2 [H] = {H2 } 1 2 [N] = {N2 } [Wt% N] = [Wt% H] = (1) (2) 1 P 2N ×K N 1 2 fN P 2H ×K H 2 fH (3) (4) fN and fH are activity coefficient of nitrogen and hydrogen in steel, PN 2 and PH 2 are equilibrium partial pressure of nitrogen and hydrogen in steel. J (5) J (6) log fN = eiN (% i) + eN (% J) + ⋯ log fH = eiH (% i) + eH (% H) + ⋯ Some values of interaction parameters are given below. Alloying elements i = eiH eiN log k N = − log k H = − 518 T C Cr Ti P Si 0.045 +0.005 -0.22 0.011 0.027 0.13 - 0.045 - 0.051 0.047 (8) + 2.937 1905 T (9) + 2.409 Let us calculate value of [Wt% H] when molten steel is degassed at 1850 K under vacuum. The pressure above the melt is reduced in one case to 1mm Hg and in other cases to 0.1 mm Hg and 10 mm Hg. Steel contains C = 0.05%, Cr 6% Ti 0.6%, Ni 2% rest iron. Assume eNi H = 0. log fH = 0.05 × 0.045 + 0.005 × 6 + 0.22 × 0.6 fH = 0.795 k H = 23.94 Pressure (mmHg) [ppm H] 0.1 0.218 1.0 1.092 10 3.450 We note that with the decreasing pressure above the melt dissolved hydrogen in steel decreases. At 10 mm Hg pressure above the melt, dissolved hydrogen is 3.450 ppm which decreases to 0.218 ppm at 0.1mm Hg pressure, which is 6% of hydrogen at 10mm Hg pressure. Higher degree of vacuum is beneficial with reference to degassing. But higher degree of vacuum requires proper selection of ladle refractory material to avoid decomposition of the refractory and vacuum equipment. Also side reactions may occur at higher degree of vacuum. General considerations 1) The desorption of gases is a gas/ metal interfacial reaction. Oxide inclusions can react with C SiO2 + C = {Si} + 2{CO} Lowering of CO pressure favours the forward reaction. 3) The degassing time must be kept to minimum. Mn and Fe have high vapor pressures and their losses occur during vacuum treatment. The increased surface area of molten steel exposed to vacuum e. Tapping of steel at a higher temperature. This requires increased heat load in BOF/EAF ii. To compensate for the loss of heat.Side reactions During vacuum degassing the following reaction may occur. i. Additional heating during vacuum treatment. the following alternative may be considered. The atomic nitrogen from the molten steel has to diffuse at the gas/metal interface. . Reaction between lining and carbon of liquid steel or decomposition of lining may occur: SiO2 + C = {SiO} + {CO} MgO = Mg(g) + [O] CaO = Ca(g) + [O] MgO + C = Mg(g) + {CO} Note that SiO is a highly reactive gas at the steelmaking temperature. Mg and Ca are stable gases at steelmaking temperatures. hence lowering of pressure promotes the occurrence of side reactions. in the form of a thin stream or gas induced stirring will accelerate the degassing process. 2) Temperature of molten steel drops during vacuum treatment. Loss of Al and Si is negligible. The effectiveness of vacuum treatment increases with increase in surface area of liquid exposed to vacuum. Volatilization of elements of high vapor pressure may occur. where it is converted to molecular nitrogen which can then be desorbed. More is the surface area of stream exposed to vacuum higher will be the temperature drop. All reactions generate one or more gaseous species. Nitride and oxide inclusions can decompose according to Al N = [Al] + [N] Application of vacuum decreases nitrogen which favors decomposition of Al N.g. In ladle degassing. P1 = Pressure at the base of downleg P2 = Pressure in vacuum chamber D= internal diameter of leg (m) Material balance in degassing Consider removal of hydrogen in recirculation degassing i. Fluid Flow in degassing Degassing can be carried out either by placing ladle containing molten steel under vacuum or by recirculation of molten steel in vacuum. Vacuum pumping capacity should be adequate.e. Circulation velocity increases with an increasing argon gas flow rate. Argon stirring is commonly employed during degassing. The speed of degassing increases with the increased rate of circulation (R) of liquid steel through the vacuum chamber.4) The degree of degassing increases with the degree of vacuum.( volume of gas becomes 6. Argon bubbling during degassing of molten steel leads to massive volumetric expansion of bubbles due to temperature. Radial expansion of gas bubble in vacuum processing impart to a radial motion to the surrounding fluid. Typically R ranges in between 10t/min to 100t/min.3 times at 1873K). In recirculation degassing argon is also bubbled through porous plugs located at the bottom of the ladle. Moreover rising gas bubbles absorb dissolved gases. Hence bath agitation would help exposing the entire content of molten steel to the vacuum. during circulation of molten steel in vacuum.42 × 103 Q1⁄3 d1⁄3 �ln � �� P2 (9) Here R= circulation rate Kg/s Q= argon injection rate ????3 /??????. Hydrogen balance Rate of hydrogen removal from steel �Ṁ 1 � = rate at which hydrogen is transferred in vacuum�Ṁ 2 � . The circulation rate (R) can be determined by P1 R = 7. Vacuum of the order of 1mm or even less than 1mm Hg (1mmHg=1torr) is employed in the practice. Bottom layers of steel are very much less affected by vacuum since these layers are under the influence of ferrostatic pressure due to column of liquid steel. the effectiveness of degassing decreases from top to bottom of the molten steel bath. and [PPmH]∗ is hydrogen concentration at equilibrium. Ghosh: Principles of secondary processing and casting of liquid steel. some problems related to degassing are discussed References: 1) A.If weight of steel is m in tones. Ghosh: Secondary steelmaking (13) (14) . t is time in minutes and ppmH is hydrogen concentration at any instant of time. By 10 and 11 R m d[PPmH ] dt = − [PPmH ]−[PPmH ]∗ (12) Integrating equation 12 and using boundary conditions [PPmH] = [PPmH]1 at t = 0 [PPmH] = [PPmH]2 at t = t R= m t [PPmH ] −[PP mH ]∗ ln [PPmH ]1 −[PPmH ]∗ 2 In lecture 30. 2) A. d[PPmH ]×10 �Ṁ 1 � = −m × 106 dt −6 = −m × d[PPmH ] dt �Ṁ 2 � = R × 106 {[PPmH] − [PPmH]∗ } × 10−6 (10) (11) R is circulation rate in tone /min. Stirring gas is introduced either from top through the roof by a submerged refractory tube or through the porous plug fitted at the bottom of the ladle. After a determined time ladle is removed from the chamber and is teemed for casting. Ladle degassing Ladle containing molten steel is placed in a chamber which is then evacuated. During degassing additions are made for deoxidation and alloying. Stirring the bath enhances rate of gas removal. . For effective degassing of fully killed steel. The vacuum chamber is equipped with a hopper so as to make additions of elements as and when it is needed. Pressure is maintained in between 1mmHg to 10mm Hg for effective degassing.Lecture 26 Degassing Practice Contents: Degassing processes Ladle degassing Stream degassing Recirculation degassing Key words: degassing. Therefore ladle is not filled completely and about 25% of its height is kept as freeboard to accommodate the splashed metal droplets. ladle degassing. Figure 26.1 Arrangement of ladle with porous plug and hopper for degassing Ladle is provided with a porous plug at its bottom to purge argon gas as shown in the figure. it is necessary that carry-over slag either from BOF or EAF should be as low as possible. Vigorous removal of gases causes metal splashing too. hydrogen and nitrogen in steel. recirculation degassing Degassing processes There are 3 methods of degassing which are in practice i) ii) iii) Ladle degassing Stream degassing Circulation degassing All these processes are carried out in ladles. Carry-over slag contains FeO and since oxygen content of steel is in equilibrium with FeO content of slag. In certain cases ladle is heated to compensate for the loss of heat during degassing. oxygen content of steel increases. it is necessary to purge argon through the bottom of the ladle. Figure 26.1 shows ladle degassing unit. For the effectiveness of degassing . In a vacuum chamber the ladle is placed. Height of the pouring stream is an important design parameter. a ladle with the stopper rod is placed in a vacuum chamber.2: Arrangement of ladle. nitrogen content of steel is also reduced. In some plants degassing is done during tapping. molten steel is teemed into another vessel which is under vacuum. Ladles are generally lined with high alumina bricks at upper part of the ladle while the lower portion is lined with fireclay. both induction stirring and arc heating are employed for degassing. Steel with 25℃ to 30℃ superheat is tapped into ladle. Stream degassing In stream degassing technology. Stream is allowed to fall in the ladle where molten steel is degassed. The major amount of degassing occurs during the fall of molten stream. From the pony ladle molten stream is allowed to fall into a ladle which is evacuated. which must be controlled. . Hydrogen is generally reduced to below 2ppm from 4 to 6ppm. For certain grades of alloy steels. Ladle containing molten steel from BOF or EAF is placed on top of the vacuum chamber and the gap is vacuum sealed. Ladle to ladle degassing In ladle to ladle degassing. Alloy additions are made under vacuum. Ladle is closed from top with a special cover which contains exhaust opening. Ladle to mould degassing Preheated mold with hot top is placed in vacuum chamber. In this arrangement molten steel from EAF is tapped into tundish or pony ladle. Above the chamber a tundish is placed. Alloy additions are made under vacuum. Stream degassing technology has following variants in the practice i. One ingot could weigh around as high as 400tons and several heats from different furnaces are used for casting.Electromagnetic stirring is employed for degassing. tundish and mold to degass molten steel ii. For this purpose ladle has to be made of non magnetic austenitic stainless steel or stainless window could be provided. Steel is bottom poured in the tundish. The final content of gas in steel depends on degree of vacuum and time of treatment.2 shows arrangement of vessels Figure 26. Figure 26. The pick-up of nitrogen from the atmospheric air may occur during open pouring of steel. Sudden exposure of molten stream in vacuum leads to very rapid degassing due to increased surface area created by breakup of stream into droplets. Steel tapped in the ladle at superheat equivalent to 30℃ is placed above the tundish. Circulation rate depends upon amount of lifter gas and the degree of vacuum.3 shows a schematic sketch of a RH degassing unit. and alumina bricks in the lower portion in order to sustain high temperature.075 m3 /ton. Germany). The operation of RH degasser is as follows: i) ii) iii) iv) v) vi) vii) Cylindrical chamber is heated to the desired temperature (varies in between 900℃ to 1500℃ in different plants). molten steel is allowed to circulate in the vacuum chamber continuously by special arrangement. Process has several advantages like • Heat losses are relatively low. amount of argon is around 0. Alloy additions can be made at the end of degassing depending on the superheat. The chamber is evacuated so that molten steel begins to rise in the chamber. This degassed steel is slightly cooler than steel in the ladle. Lifter gas is introduced. The legs are lined with alumina refractory. Buoyancy force created by density difference ( density of cooler liquid steel is > hot steel) stirs the bath Rate of circulation of molten steel in cylindrical chamber controls the degassing. Figure26. Top side of the cylindrical shell is provided with exhaust. This gas expands and creates a buoyant force to increase the speed of molten steel rising into the inlet snorkel. In RH degassing technology (developed by Rheinstahl Heinrich – Shutte at Hattingen. alloy additions. observation and control window. A lifter gas argon is injected at the inlet snorkel in order to increase the molten steel velocity entering into inlet snorkel. Molten steel in the chamber is degassed and flows back through the other snorkel into the ladle. Cylindrical Figure 26. . • Alloy additions can be adjusted more closely • Small vacuum pumping capacity is adequate since smaller volume is to be evacuated as compared with ladle to ladle or stream degassing.075 ???? 0.. A 110 T steel can be degassed in 20 minutes by circulating molten steel at 12 tons/min.Recirculation degassing In the recirculation degassing technology.a cylindrical refractory lined shell with two legs (also called snorkel) is designed such that steel is raised in one leg and falls back into the ladle after degassing through the second leg.3: Arrangement of cylindrical vessel and ladle in RH degassing technology shell is lined with fire bricks in the upper portion. The chamber is lowered into molten steel up to a desired level. The DH degassing unit can operate with lower superheats compared with RH since DH unit has heating facility References: 1. The DH chamber is equipped with heating facility.3 except the following: • • In DH unit. a small amount 10-15% of the total mass of steel is degassed at a time. The length of the snorkel is sufficiently large to realize the effect of atmospheric pressure on rise of steel in the snorkel. snorkel is lined with high quality alumina brick. A Chakrabarti: Steelmaking . Bottom of the cylindrical vessel is provided with a snorkel which can be dipped into molten steel. The process is repeated until required level of degassing is achieved.DH Gassing In DH degassing. The following steps may be noted for operation: i) ii) iii) iv) v) vi) DH vessel is preheated and lowered in the ladle so that snorkel tip dips below the molten steel surface The evacuated chamber is moved up and down so that steel enters the chamber The chamber is moved for 50-60 times with a cycle time of 20 seconds. Cylindrical vessel has heating facility. The arrangement of a vessel and the ladle is somewhat similar to figure 26. alloying addition arrangement and exhaust systems. The upper portion of the DH vessel is lined with the fireclay and the lower portion with the alumina bricks. R H Tupkary and VR Tupkary: An introduction to molten steel making 2. the cylindrical vessel has one snorkel . A layer of slag is kept in the ladle to minimize heat losses. Adequate degassing is possible in 20 -30 cycles. . N<50. TO<30 100 Bearings TO<10 15 Tire cord H<2N<40. TO<20 13 Wires N<40. Al2 O3 . clean steel What is clean steel? Clan steel refers to steel which is free from inclusions. SiO2 Sulphides: FeS. N<40. S. Therefore we can talk about cleaner steel. TO<15 20 Drawn and ironed cans C<30. N<30 100 S<30. inclusion engineering. Which steel is clean would depend on the applications. VN. Mn. SiO2 . Si. Ca etc) with non metals (O. FeO. Inclusions are non metallic particles embedded in the steel matrix. CaS. MnO. Practically it is not possible to produce steel without any inclusion. Table lists some applications which can tolerate some minimum inclusions size: Steel product Allowed impurity in ppm Allowed size(μm) Automotive and deep drawing sheet Line pipes C<30.Lecture 27 Clean Steel Contents What is clean steel Types of inclusions Morphology Properties of inclusions Inclusion assessment Key words: Inclusion in steel. Al2 O3 . BN etc. In this connection it is important to know that there is a limiting size below which inclusion does not affect mechanical property. MnS. TO<20 20 Types of inclusions: Inclusions are chemical compounds of metals like (Fe. C. N. Nitirides: TiN. AlN. MgS. . Al. Al2 O3 . MgO. Different types are: • • Oxides: FeO. Al2 O3 . TO<15 10 Heavy plate steels H<2 N+30 to 40. Ce2 S3 . H). MnO. SiO2 . . increase yield strength and hardness. MnO. CaS. Macro inclusions are harmful. manganese. iron aluminates and silicates are globular. Micro. On heating steels with these type of inclusions internal stresses of thermal origin can develop. chromium. etc. MnO. Fe2 P. which have thermal expansion greater than steel matrix. An inclusion is a mismatch with the steel matrix. etc inclusions have thermal expansion smaller than steel matrix. Morphology Globular shape is desirable. Unstable inclusions are iron and manganese sulfides and also some free oxides. chromites and aluminates. Al2 O3 . Micro inclusions can be used to enhance strengthening by dispersing them uniformly in the matrix. Micro inclusions are beneficial as they restrict grain growth. Mn5 P2 By mineralogical content. non –metallic inclusions are rather stable or unstable. aluminum and tungsten oxides and also crystalline silicates. oxysulphides. Certain inclusions like MnS. The void can act as cracks. There are inclusions like MnS. Polyhedral inclusions are not very harmful. On heating steel with these types of inclusions voids or parting of the matrix can occur.FeO. Platelet shape: undesirable. Spinels – Ferrites. On the other hand Al2 O3 .SiO2 with a mixture of iron. SiO2 (quartz)Al2 O3 (corundum) and other. Silicates. Ca S.inclusions act as nuclei for precipitation of carbides and nitrides. etc Phosphides: Fe3 P. Size of inclusions There are micro inclusions(size 1 − 100μm) and macro-inclusions (size greater than100μm) . oxygen inclusions are classified: • • • Free oxides . etc Carbonitrides: Titanium/ vanadium/Niobium carbonitirides. Cr2 O3 .• • • Oxysulphides: MnS. Macroinclusions must be removed. Properties of inclusions: i) Thermal expansion. Al2 O3 . By stability. SiO2 and CaO. Al deoxidized steels contain MnS in the form of thin films located along the grain boundaries. 2-2. rare earths.35 4. (ii) Spinel type double oxides AOB2 O3 (where A is Ca.(Cr etc) are deformable at temperatures greater than1200℃ .75 4. The following classification of inclusions according to kieslling is useful to the metallographers to determine type of inclusions: According to Kieslling (i) Calcium aluminates and Al2 O3 inclusions in steel are undeformable at temperatures of interest in steelmaking.6 4. V etc. Mg and Mn. MnO and (Fe. Al2 O3 .8 5.5 2. Mn)O are plastic at room temperature but gradually lose plasticity above 400℃. (iii) Silicates are deformable at higher temperature range. −Al2 O3 CaO − SiO2 − Cr2 O3 etc. MnS Magnesia. TiO2 are solid at steelmaking temperature. ZrO2 Iron silicate.0 4.04 3. The majority of inclusions belong to pseudo-ternary system: CaO − SiO2 − Al2 O3 . FeS Manganese sulphide. iii) Plastic deformability The plastic deformability of an inclusion will govern any change in its shape under the action of external forces and will determine the amplitude of stress concentration.6 4. Sulphide inclusions are mainly MnS. usually appear as solid solutions in existing inclusion phases. Other elements like Ti.and B is Al. Fe(l). . (iv) FeO. MgO − SiO2 . Brittle inclusions are dangerous as they may crack and cause fracture of the component under the application of external force.0 5. Zr. Silicates are not deformable at room temperature. (FeO)2SiO2 Iron sulphide. (vi) Pure silica is not deformable up to 1300℃ . (v) MnS which is highly deformable at 1000℃ temperature but becomes slightly less deformable above 1000℃ . Nb.58 Inclusions like MgO. MgO Melting point(0C) 1369 1785 1710 2050 2280 1825 2700 1205 988 1620 2800 Density at 200C(g/cm3) 5.Density and melting point ii) Density and melting point Composition of inclusions Ferrous oxides (FeO) Manganous oxides (MnO) Silica Alumina (Al2 O3) Chrome oxide Cr2 Titanium oxide. The extent of deformation depends on their chemical compositions. TiO2 Zirconium oxide.2 5. etc. etc. Total oxide inclusion content of steel can be determined from the analysis of oxygen by sampling and the use of vacuum /inert gas fusion apparatus. The energy dispersive x-ray analysis (EDX) attachment for SEM allows quantitative chemical analysis of inclusion as well as quantitative mapping of distribution of various elements in and around the inclusions. distribution.Inclusion assessment Inclusion counts are performed to assess their shape. Quantimet has an optical microscope fitted with video screen and associated microprocessor-based instrumentation. The readers can see the references given at the end of lecture 29 for the details . quantity and distribution to assess about the cleanliness of steel. it is inferred to whether it is silica/ silicate. From the shape of the inclusion and knowledge of the steelmaking process in a plant. aluminate or sulphide inclusion Electron probe micro analyzer enables to determine the chemical composition of individual inclusions. volume fraction. The routine plant procedure employes the microscopic method. Radioactive tracers can identify the origin of inclusion distributions. It can scan the specimen very quickly and provide a variety of information such as inclusion size. number. Oxygen pickup by teeming stream and consequent oxide formation. Chemical reactions. Reaction between rejected solute elements during solidification. reaction between sulphur and manganese. Brittle inclusions or inclusions that have low bond strength with the matrix break up during early straining and create voids at the inclusion/ matrix interface. iii. Sources of inclusion formation Inclusions can form either (a) during transfer of molten steel from one reactor to other or (b) during solidification of steel (lecture 27) or during solid state processing by any of the following mechanisms: i. iv. inclusion engineering. Inclusions can form during . Mechanical and chemical erosion of refractory and other materials. for example. control of inclusion in steel is necessary. whereas mechanisms ii and iii produce exogenous inclusions. and between oxygen and aluminium etc. macro. Hot fatigue strength of high strength steel is reduced by surface and subsurface inclusions those have lower expansion coefficient than steel The hot workability of steel is affected by the low deformability of inclusions Anisotropy of a property is caused by orientation of elongated inclusions along the direction of working. Inclusions produced by mechanism (i) are called endogenous. In view of the above. Macro inclusions of sulphides are desirable for better steel machining properties. ii. spherical inclusions are better. iv). clean steel Why inclusion control is necessary? Impact properties are adversely affected with an increase in volume fraction of inclusion as well as inclusion length.inclusions are harmful and they must be controlled.Lecture 28: inclusion sources and control Why inclusion control is necessary Sources of inclusion formation Control of inclusions Key words: Inclusion in steel. It must be mentioned and as pointed out earlier (see A. ii. BOF slags are highly oxidizing in nature and contain oxides like FeO. CaO. surface sulphurization. i. number of inclusions are 240/kg carry-over FeO of slag. During tapping of molten stream from BOF/EAF carry. Erosion of launder refractory is the possible source. SiO2 . which can form oxides. Here molten steel is in contact with the refractory. Thus carry-over of slag must be minimized by adopting slag free tapping technologies. Mg etc. electrode coatings are the possible source of inclusions in fusion welding processes. Another way for inclusion formations is reoxidation of tapping stream.etc. Pick up of oxygen from atmosphere and formation of FeO. Molten steel is in contact with stopper and nozzle refractory and elements like Ti. inner oxidation. e) Final finishing operations like heat treatment and deformation processing. MgO etc.a) Tapping of molten stream from BOF/EAF to ladle.. which in turn forms 0. b) Treatment of steel in ladle. Oxidation of weld pool. Air entrainment into molten steel stream brings oxygen and FeO formation is initiated. f) Fusion welding.286 kg. These oxides react with Al during ladle treatment and lead to inclusion formation.51kg Al2 O3 . Assuming spherical shape inclusion of 1mm dia (in practice different diameter of inclusions can form) and density kg of Al2 O3 = 4000 m 3 . Control of inclusions Inclusions can be controlled either at (a) during liquid steel processing stage or (b) during solid state processing.over of slag must be minimized if not prevented. Liquid state processing During liquid state processing inclusion control can be exercised at tapping and teeming of steel. . Tapping stream exposes very large surface area in the atmosphere and hence oxygen pick up leads to oxide inclusion formation. Here steel is heated to high temperature which may cause surface oxidation. Also during deoxidation and synthetic slag treatment oxide/sulphide inclusions may form c) Teeming of molten stream. Consider the reaction 3 FeO(l) + 2 Al = Al2 O3 + 3 Fe 3 MnO(l) + 2 Al = Al2 O3 + 3 Mn Carry over of 1 kg FeO in slag decreases Al by 0. MnO. Molten steel stream after treatment in the ladle is teemed into tundish and then from tundish to mould in the continuous casting. d) Solidification in mould due to precipitation of excess solute elements. iv. In this situation care must be exercised to ovoid slag entrainment into mold due to vortex formation. Segregation during solidification must be avoided. Since oxygen pick up occurs just before the solidification. Different technologies for shrouding molten stream are: refractory tube shroud. Use of shrouded and submerged nozzles will help control inclusion formation. . liquid pool is in contact with air and steel is prone to oxidation. Tundish operation Now a days sequence casting is commonly adopted in continuous casting. iii. The interested reader may see the references at the end.At the same time flux should also cover molten steel to prevent oxidation. As the steel solidifies the excess solute elements like oxygen. Here steel must be heated under inert atmosphere to avoid oxidation. etc. dissolved oxygen increases and forms inclusion during solidification. manganese etc are rejected and lead to inclusions formation. Selection of tundish flux Tundish flux should be selected such that it can easily absorb inclusions floating in the tundish . Kiessling an N. Lange: Non metallic inclusions in steel. Teeming of molten steel from tundish to continuous casting mould needs extra precaution in terms of protection of steel against atmosphere. Inclusion can also form in the mold during solidification. circular ring shroud. sulphur. Tundish should not be emptied completely.over tundish feeds the molten steel to different molds of the continuous casting machine. Here macro-inclusions rich in FeO and MnO can form. Moreover. References: A. resulting inclusions do not get sufficient time for separation owing to faster solidification in mold Teeming stream by inert gas or through the use of nozzles submerged in molten stream are the effective means to avoid inclusion formation. During ladle change. Here stirring of molten steel is effective to minimize the segregation and inclusion formation Solid state processing In the solid state processing steel is heated to a temperature ranging in between 800 − 1200℃ to perform heat treatment and hot working. During fusion welding. Inert shielding may avoid the inclusion formation.Teeming of steel form ladle to tundish requires shrouding of molten steel stream in order to avoid reoxidation. Also tundish lining material should be inert with Al v.Ghosh: secondary steelmaking R. Inclusion engineering also involves distribution of inclusion uniformly in the matrix. clean steel.Lecture 29 inclusion Engineering Preamble Requirements for inclusion engineering Inclusion engineering by calcium treatment Oxide shape control Sulphide shape control Effect of rare earth elements Key words: Inclusion engineering. • It should also be able to modify the shape i. Requirements for inclusion engineering One of the main requirements is to find an element which should be added to modify the inclusion. Macro-size inclusions must be removed. sharp edges ands corner of inclusions to spherical.Al with O. In some cases. and carbonitrides inclusions in hardening steel). Inclusion engineering does not refer to removal of inclusions but it refers to modify them either in terms of chemical composition or shape so that harmful effects of the inclusions can be converted to improve the steel properties. steelmaking Preamble Inclusions engineering technology is based on the design of inclusions so as to alleviate their harmful effects on the product properties. . So that composite properties can be generated in the product. depending on applications.g. Such inclusion can form by reaction between Ti. Element should meet the following • It should have high chemical affinity for the inclusion • It should be able to modify the composition so that it becomes liquid. nitrides. W. S or C.e. inclusion can be modified to minimize their harmful effects. N. deliberate attempts are made to form very fine inclusions (e. In all other cases. 37 It may be noted that in12 CaO. . Therefore suitable injection methods for calcium have to be developed. The different oxide compounds have different melting temperature as shown in the table. One method is to inject calcium deep into the molten bath such that the ferrostatic pressure overcomes vapor pressure of calcium.22 1. Addition of calcium in steel containing oxygen and sulphur can form two phases: oxide and sulphide. Similarly MnS inclusion in steel on deformation becomes stringer type. alumina inclusions clog the nozzle and consequently steel flow rate to the mold is affected. Al2 O3 compounds. The sulphide phase consists of CaS and MnS. CaO. Al2 O3 12 CaO. Alumina inclusions are solid at steelmaking temperature.27 (theoretically) in order to obtain liquid product. Al2 O3 inclusions are brittle and breaks on deformation. Wire containing Ca-Si powder is injected deep into bath to avoid a) oxidation of Ca and b) to keep calcium in the liquid form. During continuous casting. alumina inclusions are formed. The density of calcium is 1. 7 Al2 O3 compounds. The table also shows Ca/Al ratio in various CaO. Al2 O3 CaO. Al2 O3 has low melting point and it will remain in the liquid at steelmaking temperature.What types of inclusions need to be modified? Mainly Al2 O3 and MnS inclusions are modified. Calcium is used widely to modify inclusions in continuous casting of steel. Compound Melting point (K) Ca/Al 3 CaO. Oxide shape control It consists of modifying Al2 O3 inclusions. The Ca/Al ratio is 1.27 0. 2 Al2 O3 1800 1673 1868 2000 2.55 � 3 and is in vapour form cm at steelmaking temperature(1600℃).27 in12 CaO. In another method wire containing calcium components are injected at speeds of 80-300 m/min.74 0.7 Al2 O3 CaO. Al2 O3 is solid at steelmaking temperature and is brittle in nature. The melting point of CaS is 2000℃ and that of MnS 1610℃. Al2 O3 melt has some solubility for CaS. Inclusion engineering In aluminum killed steels. g The solubility of calcium in steel is 320 ppm. This suggests that calcium/aluminum ratio has to be adjusted at 1. alumina inclusions break up which is a serious surface defect. On rolling. The alumina may accumulate in the continuous casting nozzle causing clogging of the nozzle. Typically Ca-Si powder in which calcium content is 20-30% is used in the wire. Oxide phase consists of the compounds in CaO − Al2 O3 system. During solidification. The calcium will then desulphurize steel to very low levels. For sulphide shape control. It must be noted that rare earths have high atomic weights. This will result in precipitation of CaS which forms a duplex inclusion in which a CaS − MnS ring surrounds calcium aluminate core. Thus it is possible that calcium will react with sulphur to form CaS before there is oxide shape modification. The Mn and S enriches in the interdendritic region and MnS forms during the end of solidification. 2 Al2 O3 type of inclusions which will be converted to CaO. and their oxides. It must also be noted that 12 CaO. 7 Al2 O3 is liquid product at steelmaking temperature. sulphide or oxysulphides: 2 Ce + 3O = Ce2 O3 Ce + S = CeS 2Ce + 2O + S = Ce2 O2 S Correct addition of rare earth elements like cerium forms oxysulphide inclusion. Subramanian et al has designed tool steel to be self lubricating by . it has high sulphide capacity as well. As an example of inclusion engineering. If calcium aluminate inclusion has excess CaO content. Rare earths are strong deoxidizers and desulphrizers as well. Formation of MnS requires thermodynamically that the solubility product [%Mn] × [%S] > 2 at the solidification temperature. sulphides and oxysulphides have high density 5 to 6 g/????3 as compared with other oxides. These inclusions do not float easily. Mn and S are rejected from the solid steel causing an increase in their concentration in the remaining liquid. Thus it is important to control the concentration of Al. Sulphide shape control MnS has a low melting point and elongate in the direction of rolling and becomes stringer type inclusions. Effect of rare earth elements When rare earths (elements) like cerium. MnS also causes steel to be more susceptible to hydrogen induced cracking. This type of inclusion is spherical and does not elongate. Stringer type inclusions should be modified if not eliminated.For a normal aluminum killed steel calcium will first modify the oxide inclusions. Addition of calcium to aluminum killed steel will first form calcium aluminate inclusion. S and Ca in molten steel for engineering of oxide inclusions. oxide inclusions have been designed to make them useful to increase the machine tool life. Al2 O3 and finally liquid calcium aluminate rich isCaO. lanthanides are added to steel they can react to form an oxide. it is necessary to desulphurize steel to very low value say 0. The sulphur level where CaS is more stable will depend on Al content.006%. Thermodynamically it is easier to form CaO. These inclusions greatly reduce the transverse mechanical properties of steel. modifying the rheology of oxide inclusions.29 No. Fruehan: ladle metallurgy 2) R.2. lange: Nonmetallic inclusions in steel. vol. Kiessling and N.al: iron making and steelmaking 2004. Gaslick et. . References: 1) R. J. Ghosh: secondary steelmaking 4) Subramanian et. 3) A. Glassy inclusions are designed to soften at the tool/chip interface temperature so that a viscous layer of glass can lubricate the tool-chip interface. 31 p249 5) C. vol. The interested reader may go through the references.al: iron making and steelmaking 2002. 65 and X Al 2 O 3 = 0. We also increase the powder injection rate in one case to 4kg/ton and in other case to 6kg/ton.1. Ws can conclude that lower tempearure favours desulphurization.96.482 = 2. log Cs by equation 4 of lecture 22 = . The partition coefficient of sulphur is 50. log K s = log Cs + 20397 1773 2 3 1 3 + log 0. What will be the effect of a) sulphur partition coefficient and b) powder injection rate on desulphurization modes Note a Similar problem is solved in lecture 23.96 + 11.38.38 − 5.14 − 5.50 − 1. . In slag activity of alumina is 0. The slag is contact with molten steel at temperature T. Now we select a slag whose partition coefficient is 400.35.33 − 0. (B) Discuss the transitory and permanent contact mode of desulphurization based on the following calculations: Molten steel is desulphurised by injecting powder at the rate of 4 kg/ton.482 = −1.01 + log 0. with dissolved aluminum 0. use T = 1773K and 1850 K and interpret the results.01wt %.588 K s = 387 at T = 1773K K s = 130 at T = 1850K We note the partition coefficient is higher at 1773K as compared with that at 1850K.Lecture 30: Exercise in ladle metallurgy Contents Exercise on desulphurization Exercise on degassing Exercise on deoxidation Exercise on gas stirring Exercise on momentum flow rate 1) Problem of desulphurization (A) Calculate K s for a synthetic slag composed of X caO = 0. 045 × 0.5%.22 × 0. Ti Interaction parameter eCH = 0.11 fH = 0.837 .315 × 10−3 60 6−0.465 ln 15 3−0.12tonnes/min Ans.22 Calculate rate of circulation of molten steel.0027 + 0. Pressure inside vacuum chamber = 0.2) Problem on degassing (A) In RH degassing.1Tor r Discuss the results in terms of technology development and requirements (refer lecture 25.2 × fH 1. Ni 2% and rest iron. Given: temperature = 1600℃ Molten steel in ladle = 60 tons.2 tor r The molten steel analyzes C = 0.465 R= + 2.03 − 0. (B) Repeat the above calculations when T = 1650℃ and When pressure inside the chamber = 0. Ti = 0.06%.465 = 3.65 √0.5 = 0.65 [PPmH ∗ ] = = 0. [PPmH ∗ ] = k H �pH2 log k H = − 1905 T k H = 24. Solution R= m t [PPmH ] −[PPmH ]∗ ln [PPmH ]1 −[PPmH ]∗ 2 log fH = 0. Cr = 6%.005 and eH = −0. Molten steel attains equilibrium with to hydrogen.06 + 0.409 24. eCr H = 0.045.005 × 6 − 0.26) . liquid steel is circulated through the vacuum chamber in order to lower the hydrogen content from 6ppm to 3ppm in 15 minutes. Differentiate the expression 2 with respect ?????? to get For the minima dW O dW Al 1 Al 0.5eO . Substitute ℎ???? = ?????? ?????? and ℎ?? = ???? ???? and expression for ?????? and ???? in equation 2.30 + 2eOAl + 3eOO Wo e = zero and WAl = WAl we get e WAl = 0. Al2 O3 (S) = 2[Al] + 3[O] 1) logK Al = 2loghAl + 3loghO 2) Assume activity of alumina. since it is pure. titanium and zirconium are the strongest deoxidizers compared with manganese.868 + 2eAl Al + 3eO dWO WAl =− 1 dWAl 1. Determine the expression to calculate the value of Al in weight percent at minimum oxygen content in Fe-Al-O system O O Al Given: logfAl = eAl Al WAl + eAl WO and logfO = eO WAl + eO WO Let us consider the following reaction. aAl 2O3 = 1. (B) Aluminum. Deoxidation with either aluminium or zirconium or titanium shows a minimum solubility of oxygen in steel.3 × 103 kg/m3 Interpret the result and get a feel about the importance of size of de oxidation products on velocity.434 Al eAl Al + 1.3) Problems on deoxidation (Refer lecture 24) (A) Calculate the time required to float Al2 O3 deoxidation product through a 2 meters steel bath height from the following data: Density of liquid steel and Al2 O3 = 7 × 103 and 4 × 103 kg/m3 viscosity of liquid steel = 6 × 10−3 kg m−1 s−1 Size of deoxidation product = 10micron 50 micron and 100 micron Repeat the calculation when de oxidation product is silica and its density is 2. 013×10 5 +7000×981×1. (B) Calculate stirring energy produced in a bath by injection of gas through the bottom of the vessel for the following conditions: Argon flow rate: 500 Nl/min in 100 ton ladle.The readers may substitute the values of the interaction parameters to get a feel of the value of Al and to understand the role of interaction parameters in calculations.595] 1. (Volume is expressed at 1 atmospheric pressure and 273 K).013×10 5 = 8364W = 83.2m. Pressure above the surface =1bar.48W/m3 of steel melt. 5) Problem on momentum flow rate (reference lecture for the problem is 13) Calculate the momentum flow rate produced by passing oxygen gas through a lance fitted with four convergent-divergent nozzles in a 300 ton converter. Density of steel is 7000kg/m3 . Discuss the results of calculations.2 � 1. Temperature of argon 25℃. Stirring power (W) W = 6. The diameter of each nozzle is 45mm. Repeat the above calculations for gas flow rates 400Nl/min and 600Nl/min in a ladle of 150 ton capacity.64W/ton of steel = 585. 600m3 /s and 800m3 /s.6[0.5m.18 × 10−3 Q T ��1 − 273 �+ T P ln P 1 � 273 2 6. 4) Problems on gas stirring (Reference lectures 21. Bath height: 3 meter and temperature 1200℃ Discuss the result in terms of effect of gas rate on recirculation and the benefits accrued. Temperature of steel bath 1600℃.18 10−3 × 500 × 1873 ��1 − 1873 � − ln = 5787. Bath height is 1. Bath height is 1.22) (A) Calculate the recirculation rate of molten steel due to injection of gas in a ladle containing 250 tons of steel for the following conditions: Gas flow rate: 400m3 /s.85 + 0. . Calculate the depth of penetration of gas jet and discuss the nature of blow and the associated physico-chemical reactions as a function of lance distance. Use surrounding pressure 1.Hint: Calculate PO by equation 6 and use equation 5 to calculate momentum flow rate.5m.013 x 105 N/m2 The lance distance to start the blow is 3m upto 25% of the blow time which is then decreased to 2m for 25% to 75% of the blow time. Include in your calculation the depth of penetration of gas jet. Hint Calculate dimensionless momentum flow rate from equation 8 and depth of gas jet penetration from equation9 . Between 75% and upto the end the lance distance id decreased further to 1. Rapid solidification (faster movement of solid/liquid interface) minimizes the tendency of segregation of elements .1: Equilibrium phase diagram of a binary system (a) there is a complete solubility of the components in liquid and solid and (b) complete solubility of the components in the liquid phase and partial solubility in the solid phase. planar growth. Liquidus. In this connection it is important to distinguish between solidification of pure metals and alloys. Pure metals solidify at constant temperature. solidus and solvus lines are shown by blue. Vertical dotted line show the temperature changes that occurs during solidification of an alloy of composition CO Solidification variables Solidification behavior depends on parameters such as growth rate. The following binary phase diagram illustrates the solidification in alloys: (a) (b) Fig.Lecture 31: Principles of Solidification of Steel Contents: Preamble Solidification variables Equilibrium solidification Non equilibrium solidification Microstructure development Key words: Solidification. casting. temperature gradient.?? ?????? ?? are solid solutions. The rate of movement of solidification front determines solute redistribution during solidification. 31. under cooling and alloy constitution. dendritic growth. green and black lines. The temperature gradient (G) and growth rate (R) influence the solidification morphology and solidification substructure respectively. Growth rate or solidification rate is the rate of advance of the solid/liquid interface into the liquid. constitutional supercooling Preamble: Solidification is a phase transformation process in which liquid is transformed to solid when superheat and latent heat are removed. whereas alloys solidify over a range of temperature. scale of solidification substructure and the growth undercooling. at some temperature between TL (liquiudus temperature) and TS (solidus temperature) C S fS + C L f2 = C O . Thermal under cooling is required in solidification of pure metal where there is a significant nucleation barrier for liquid to solid transformation or when directional solidification is carried out at a lower rate where cellular structure forms. according to lever rule. Undercooling is the difference between the liquid temperature of the alloy of nominal composition and the actual temperature. If the liquid composition deviates from the bulk composition. then the melt temperature may differ from the liquidus temperature of the overall alloy. Solidification of an alloy of a composition begins when the temperature reaches to the liquidus line. . ???? and ???? are liquid and solid fraction respectively. Lowering of temperature changes the composition of elements both in liquid and solid phase. Temperature gradient in solid is diffusion dependent. ΔTC = Constitutional undercooling ΔTk = Kinetic undercooling. ΔTR = Undercooling due to curvature at solid/liquid interface.Temperature gradients both in solid and liquid are important. The total undercooling ΔT ΔT = ΔTTh + ΔTC + ΔTR + ΔTK . The tie line connects liquidus and solidus line at a temperature in the two phase region. the liquidus temperature of the melt is composition dependent. The change in transformation temperature due to compositional effect is called constitution undercooling . Thermal gradient in liquid is more critical as compared with solid and is strongly affected by mixing in liquid. ∆????ℎ = Thermal undercooling ΔTK and ΔTR are usually negligible in casting. Equilibrium solidification Equilibrium solidification of an alloy of any composition occurs when concentration of the liquid and that of solid follows the liquidus and solidus line of the phase diagram. Though it is an idealized concept. Composition of liquid and solid in the two phase region at any temperature is determined by a tie line. In the solidification of ingot and continuous casting where alloys are solidified. As equilibrium is assumed. In solidification of alloy. equilibrium solidification helps understanding the compositional changes that occur during solidification. thermal undercooling is not of interest. 2 at X=0. This means that in the actual solidification of alloy the concentration of solute in the liquid and solid rarely follows the liquidus and solidus lines of the respective phase diagram Consider solidification of an alloy of composition Co at temperature T (see figure 31. therefore we get CL = Co (1 − fs )K e −1 (4) Equation 4 is Sheil’s equation. which is shown in the figure 31. This can be illustrated by plotting concentration of solute Vs distance both in the solid and liquid Figure 31. ???? ???? ???? ???? = 1 at all fS. Diffusion in the solid is a slow process. The value of K e is given for some elements in iron in table.2: Concentration vs. As the solidification proceeds more and more solute will be rejected in the liquid but no gradient of the solute will be built-up in the liquid since complete mixing is assumed.Non equilibrium solidification Assumption of complete mixing does not prevail in actual solidification process. 1. otherwise concentration gradients will be present in the liquid also. In the figure Cs∗ is the interfacial concentration. As the solidification proceeds the composition of liquid follows the liquidus line (see figure 31. However. ??ℎ?????? K e is the partition coefficient. distance profile in solid and liquid in one dimensional solidification At the onset of solidification CL = CO and CS = K e CO . Mass balance of solute when a small amount of solid dfs forms and causes solute to increase in liquid dCL (CL − CS∗ ) dfs = fL dCL fs + fL = 1 and K e = C ∗s CL (1) (2) By 1 and 2 and integrating the equate on.1) but there will be gradient of solute since no mixing is assumed in solid. . Figure 31. The role of partition coefficient of an element on solidification becomes clear from equation 4. In other words liquid has a uniform composition at every stage of solidification. K e is segregation coefficient. The presence of gradients in the solid and liquid during solidification makes the solidification process to deviate from the equilibrium. Only faster mixing can minimize the concentration gradients. Fluid flow in liquid will govern the concentration gradient of the solute in the liquid. At Ke = 1.CL = K(1 − fs )K e −1 3) At begin of solidification fs = O and CL = CO we get K = CO .1) in which there is no mixing in the solid but mixing in liquid is sufficiently large so as to have no composition gradient in the liquid.2 shows the variation of concentration of solute in the solid by a green line. when Ke is < > 1for all values of fs. Figure 31.Element C S P O N H Mn Ti ??-iron 0.13 0.1b. In the figure ?? is boundary layer thickness in which gradient of concentration persists Microstructure development Nucleation and growth behavior. nucleation of solid is not an issue since nucleating sites are present.02 0. (It must be noted that the equilibrium liquidus temperature depends on the solute.) . a curve of equilibrium liquidus temperature versus distance from the S/L interface corresponding to figure 31.02 0. The solidification begins with the planar solid/liquid interface . (a) concentration profile of the solute enriched layer ahead of the planar solidification front.45 0.13 0.07 A more practical situation for solidification would be no mixing in either solid or in liquid.4b and c.54 0. Growth behavior is important.3a can be determined and such a curve is shown in figure 31.3: Concentration versus distance profile. As the solid/liquid interface advances into the liquid. In alloys.3. the liquidus temperature will be lower than when there is no segregation.4a the variation in composition as a function of distance from the planar solid/liquid interface is depicted. both determine development of microstructure in solidification of alloys.28 0. (b) condition for a stable planar front solidification and (c) condition for unstable planar solid/liquid front. In figure 31. solute concentration builds up near the interface since no mixing in the liquid is assumed. Growth behavior The solute redistribution at the solid/liquid interface governs the stability of the solidification front.During growth the morphological stability of the planer solid/liquid interface is governed by thermal and composition gradients at the interface Figure 31.02 0.32 0. as the solute segregates.36 0.95 0.4: Constitutional super cooling during plane front alloy solidification.iron 0.14 ??.84 0. By referring to the phase diagram such as one shown in figure 31. There will be gradient of concentration in the liquid as illustrated in the figure 31.06 0.02 0. Two conditions are possible based on the actual temperature distribution in the liquid imposed by temperature gradient arising from the heat flow conditions: Condition 1: Actual thermal gradient is steeper than equilibrium liquidus temperature at the interface due to the compositional gradient. the morphology of interface changes from planar to cellular and to dendrite References: 1. As G/R decreases.4c. Thus as the extent of constitutional super cooling ahead of the solid/liquid interface increases. plane front can no longer be stable. Condition 2: If. growth rate is also important. actual thermal gradient is smaller. then the liquid ahead of the interface is supercooled since the actual temperature ahead of the interface is lower than the equilibrium liquidus temperature. the growth rate (R) also plays an important role in determining solidification structure. this is due to constitutional super cooling and is responsible for cellular or dendrite solidification. Decrease in the imposed temperature gradient increases the tendency for planar solidification. as shown in figure 31. . Under these conditions the planar growth is no longer stable. ???? is the diffusion coefficient of solute in liquid.(G). 1989 vol 34 NO.P 213 2. Any perturbation of the plane front can no longer melt but instead will grow. Under this condition any perturbation developed at the solid/interface will remelt and the planar front will remain stable.5. The compositional gradient for steady state plane front growth can be determined by mass balance consideration at the planar interface to ???? ?? < ∆???? ???? 5) In the equation 5 ∆???? is the liquidus and solidus temperature difference at composition CO. ghosh : principles of secondary processing and casting of steel . Solutes with a smaller partition coefficient create conditions for instable plane front solidification. International materials Review. Several parameters affect the conmstitutional supercooling. Low G/R favors dendridic solidification. In addition to gradient. David and vitek: solidification and weld microstructure. GL is thermal gradient in the liquid. Higher temperature gradient and smaller growth rate I. however. For a given gradient. A. when G/R is large plane front solidification prevails. since more solute will be rejected in the liquid.e. The coated material decomposes during solidification which prevents sticking of solidified ingots with the inner walls of the mold. are used for rolling into flat products. decarburizes and alloying elements if required. In the modern steel plants. steel on solidification contracts more than cast iron which makes detachment of ingot easier from the mold. etc. wheels. Round ingots are used for tube making. Ingot casting is done in cast iron moulds having square. defects in casting. Deoxidizers. steel is cast continuously. Polygon ingots are used to produce tyres. whereas few hundred to 300 tons for forging. particularly those based on induction melting furnaces ingot casting is practiced. Ingot mould types Cast iron is used to fabricate the mould. Typically an ingot weighing 5-20 tons for rolling. Inner walls of the mould are coated by tar or fine carbon. Whereas. rails and other structural sections. Thermal coefficient of cast iron is lower than steel as a result. In several small scale plants. round or polygon cross section.1 (b) . The steel may be degassed either before or during casting. casting moulds Introduction Molten steel from BOF/EAF is tapped into a teeming ladle. Ingots with square cross section are used for rolling into billets.1 a) Narrow end up or big end down as shown in figure 32. Molds are essentially of two types: i) ii) Wide end up or narrow end down as shown in figure 32. are added for the final finishing with respect to oxygen content and other elements in steel. ingots with rectangular cross section (also known as slab).Lecture 32 Ingot Casting Contents Introduction Ingot mould types Mechanism of solidification Ingot defects Key words: Ingot casting. This surface has a fine equiaxed grains and the skin. The formation of skin results in decrease in rate of solidification. Due to expansion of mould through the heat transferred from the solidifying steel and contraction of solidified skin an air gap forms between the mould and the skin. CO. Killed steel solidifies in the ingot form as follows: i) ii) Metal near the mould walls and bottom is chilled by the cold surfaces and a thin shell or skin is formed on the ingot surface. Wide end up molds may have a solid bottom.Th is results in decrease in the heat transfer rate. Mechanism of solidification Molds are water cooled. Narrow-end-up molds facilitates easy escape of rimming reaction product. Insulating and exothermic materials are put on the top ingot which ensures availability of hot metal towards the end of solidification. Narrow end up molds are commonly used to produce rimming and semi-killed steel ingots. Molds are generally provided with hot top which acts as reservoir to feed the metal and to avoid formation of pipe.Figure 32. because air gap has a high thermal resistance to heat flow .1(a) wide end up moulds (b) Narrow end up moulds Wide end up moulds are used to produce forging ingots of killed plain carbon or alloy steels. Both bottom pouring and top pouring of steel are used in ingot casting. Fully deoxidized or killed steel used for high quality forgings shrink on solidification and may lead to formation of pipe. The columnar crystals rarely extend to the centre of the mould. In semi killed steels. Figure 32. The above zones of solidification depend on the evolution of CO gas due to carbon and oxygen reaction.2 (a) Narrow end up mould showing long pipe in killed steel Figure 32.2 (b) in wide end up mould while casting killed steel. not all oxygen removed from steel. Stirring circulates molten steel which brings hot metal to the surface and solidification of steel at top is delayed. The volumetric shrinkage leads to formation of pipe. This gives rise to rimming ingots in which gas is entrapped mechanically as blow holes. Columnar grain formation is prevented due to a more uniform temperature at interior of an ingot.iii) iv) The solidification front perpendicular to the mold faces moves inwards and towards the centre as a result columnar grains form next to the chill surface. In killed steels pipe formation occurs toward the end of solidification.2 (b) Wide end of mould showing pipe in killed steel . The necessary super saturation level of carbon and oxygen reaches towards the end of solidification. Ingot defects: Causes and remedies i) Pipe formation: Cause: Steel contracts on solidification. As a result the central zone of the equi. Solidification of rimming steels is controlled by evolution of CO during solidification. Oxygen content of steel is very low.2a shows primary and secondary pipe in narrow end up mould and 32. The central portion of the ingot solidifies as equi-axed grains of bigger size due to slow rate of solidification. Figure 32. Only primary pipe can be seen in wide end up mould. Rimming steels are not killed.axed crystal is disturbed by way of formation of blow holes in the top middle potion of the ingot. The gas is evolved at the solid/liquid interface which stirs the molten steel during solidification. produces blow holes. Remedy: soaking of ingots at high temperature can minimize segregation. Non metallic inclusions: . Segregation: It is the difference in composition of steel within the ingot than some average composition. All elements whose ?? < 1 tend to segregate. The initial chill layer of ingot has practically the same composition as that of liquid steel.2 (a) longer pipe can be seen). The portion of ingot containing pipe has to be discarded which affects yields. which when is unable to escape during solidification. Pipe formation is restricted in the hot top which can be discarded. Another method is to pour extra mass of metal. The rimming reaction produces CO.Rimming and semi-finished steel show very less tendency for pipe formation Wide end up moulds show smaller pipe as compared with narrow end up mould (in figure 32. partition coefficient of element in steel. Segregation is due to a) Difference in solubility of solute elements in liquid and solid steel i. Blow holes located inside the ingot can be welded during rolling. Rimming steels show blow holes due to rimming reaction between carbon and oxygen. Partition coefficient of solute (K) is defined as ?? = ?????????????????????????? ???? ???????????? ???? ?????????? ?????????????????????????? ???? ??ℎ?? ???????????? ???? ???????????? The value of K ≤ 1. Remedy: Control of gas evolution during solidification so that blow hole forms only within the ingot skin of adequate thickness.e. b) Rate of solidification: faster rate of solidification avoids the elements to segregate. Use of exothermic materials in the hot top keeps the metal hot in the top portion and pipe formation can be avoided. Decrease in rate of solidification causes elements to segregate. Semi-killed steels also show tendency to blow hole formation. ii) Blow holes Cause: Evolution of gas during solidification of steel. Remedy: use of hot top on the mold. c) Larger size ingots are prone to segregation than smaller size ones. The volume of the hot top is 10-15% higher than ingot volume. The solute elements whose K = 1 do not segregate. Entrapment of gas produces blow holes in the ingot. Larger size ingots require more time for solidification. Longitudinal cracks are formed due to lateral tension in the skin. Fine size inclusions when distributed uniformly are not harmful. Si. Mn. Al with non metallic elements like oxygen. Alloy steels are more prone to longitudinal cracks than mild steels. The cracks are formed due to thermal shocks. See lectures 27-30 for details about inclusions. Ti. As the aspect ratio of the ingot increases. Smooth corners of the mould and gradual curvature minimize restriction cracks. sulphides and nitrides formed by reaction between metal like Fe. sulphur etc. References: RH Tupkary. Sub. The improper design of mold taper and corner radius cause surface cracks. VR Tupkary: An introduction to modern steel making. nitrogen..Non metallic inclusions are inorganic oxides. Restriction cracks can be near the corner radius of the ingot.cutaneous cracks are internal fissures close to the surface. They are parallel to vertical axis of ingot. tendency to transverse crack formation increases. Inclusion modification is the remedy to alleviate the harmful effect of inclusions on properties of steel. Different types of cracks are: Transverse cracks: They are parallel to the base of ingot and are formed due to longitudinal tension in the ingot skin. An inclusion is a mismatch with the steel matrix. . Ingot cracks Surface cracks are formed due to friction between mold and ingot surface. Non deformable inclusions like ????2 ??3 are undesirable. Zr. mold and water spray is shown. secondary cooling.1. In figure 32. Higher extent of automation is possible Width of the slab can be adjusted with the downstream strip mill. water cooled mold. The required length of the strand is cut by torch cutter. the arrangement of tundish. .Lecture 33 continuous casting of steel Contents Introduction How casting is done continuously Tundish Mold secondary cooling Heat transfer in continuous casting Product and casting defect Keywords: continuous casting. The advantages of continuous casting over ingot casting are • • • • • • Quality of the cast product is better No need to have slabbing/blooming or billet mill as required when ingot casting is used. mold and water spray are arranged such that molten stream is poured from tundish to mold and solidified strand (billet/bloom/billet) is produced continuously. How casting is done continuously? The essential components of a continuous casting machine are tundish. tundish metallurgy. Tundish. molten steel is poured from the tundish in the water cooled mold and partially solidified bloom/billet or slab (hereafter called strand) is withdrawn from the bottom of the mold into water spray so that solidified bloom/billet or slab is produced constantly and continuously. water spray and torch cutters. Continuous casting is widely adopted by steelmakers. Hot direct charging of the cast product for rolling is possible which leads to energy saving. Continuously cast products show less segregation. defects in cast product Introduction In the continuous casting. tundish supplies molten steel to the molds. For single strand machines. In multi‐strand tundishes. number of molds are either 4 or 6 or 8. The number of mold is either one or more than one. . It supplies molten steel in presence of a slag cover to all continuous casting molds constantly and continuously at constant steel flow rate. During sequence casting and ladle change‐ over periods. Distributor Tundish distributes molten steel to different molds of the continuous casting machine at constant flow rant and superheat which is required for stand similarly with reference to solidification microstructure. Slab casters usually have either single or two molds. Normally bloom and billet casting machines are multi‐strand i. Control of superheat is required in all the moulds to reduce break‐out. The functions of the tundish are: Reservoir of molten steel Tundish acts as a reservoir for molten steel. mold and water spray in a curved mold machine (paste figure 2. The stream is shrouded as it enters from ladle to tundish. Liquid steel is usually tapped from ladle into tundish. It may be located symmetric or asymmetric to the centre of the tundish depending on the number of mold.3a) Tundish Tundish is a refractory lined vessel. Figure 33. The flow rate is maintained constant by maintaining a constant steel bath height in the tundish through teeming of molten steel from the ladle.1 Arrangement of tundish.e. molten stream enters from one side and exits the other side of the tundish. ladle stream is either at the centre of the tundish or displaced to the width side of the tundish. Location of ladles stream in the tundish is important. namely floatation and absorption by a flux added on the surface of the tundish. weirs. During this average residence time. The readers may see the references given at the end of this lecture for further reading.For this purpose flow of steel melt in the tundish has to be modified so as to accelerate the inclusion removal. . molten stream enters from the tundish into mold in presence of flux through the submerged nozzle immersed in the liquid steel.Inclusion removal Tundish helps to remove inclusions during the process of continuous casting. then the average residence time of molten steel in the tundish is 8 minutes.2: Tundish with flow control device. The whole idea is to utilize the residence time available before steel leaves the tundish.. Figure 33. inclusion removal can be exercised . For this purpose liquid steel flow in the tundish is modified by inserting dams. In the water cooled mold. The Inclusion removal is a two step step unit operation. if capacity of tundish is 40 tons and casting speed is 5 tons/min. slotted dams etc. or fly ash or some synthetic powder. For example. Flux is usually rice husk. namely weir and slotted dam Mold: Mold is the heart of continuous casting. It melts and penetrates between the surface of mold and the solidifying strand to minimize friction as shown in figure 33. Taper is typically 1% of the mold length. that is the meniscus for smooth caster operation.2.The oscillated frequency can be varied. Mold level sensors are used to control the meniscus level in the mould. Control of height of molten steel in the mould is crucial for the success of the continuous casting machine. flux melts and enters into the gap between mold surface and solidified strand.Solidification of steel begins in the mold. The casting powder is added onto the top of molten steel in the mold. Length of the mold is around 0. The solidification begins from the meniscus of steel level in the mould. The mold is oscillated up and down to withdraw the partially solidified strand (strand is either billet or bloom or slab). Figure 33. Mold cross section decreases gradually from top to bottom. Small amounts of alloying elements are added to increase the strength. Steel level in mould is controlled. The cross section of the mold is the cross section of the slab/bloom/billet.75 1. The functions of mold flux are. Mould extracts around 10% of the total heat. . Molds are made of copper alloys. Mold is tapered to reduce the air gap formation.4m and is more for large cross sections. For 100mm 100mm cross section of mold the taper is about 1mm for 1m long mold.2: Role of flux in continuous casting mold As seen in the figure. Sensors are used to control the meniscus level. At Tata steel slab caster frequency is varied in between 0 and 250cycles/min and the stroke length from 0 to 12mm. µ 3 10 . Mass flow rate of flux can be calculated by m ρ U m Powder feed rate kg/sm. High solidification temperature of flux reduces heat through mold. U casting speed m/s. For adequate lubrication low viscosity of the flux is required. For the above functions the flux should have the following properties. A lower viscosity helps the flux provide sufficient lubrication at higher casting speed. • • • Low viscosity Low liquidus temperature Melting rate of flux must match with the speed of the continuous casting.05ms m 0. ρ 3000kgm For a mold of length 1m. For example.18% are prone to cracking.08% C to 0. .low carbon aluminum killed steel requires flux which can absorb Al O inclusion without an adverse effect on viscosity. Minimization of heat losses. Flux on melting enters into the air gap and provides lubrication. m 6 kg/min Typically the range of composition for mold fluxes are. δ is boundary layer thickness. Prevention of oxidation.δ 0.1mm. Medium carbon grades 0. CaO SiO2 Al2O3 TiO2 C 25‐45% 20‐50% 0‐10% 0‐5% 1‐25% Na2O K2O FeO MgO MnO 1‐20% 1‐5% 0‐6% 0‐10% 0‐10% BaO Li2O B2O3 F 0‐10% 0‐4% 0‐10% 4‐10% Design of Mold flux There are specific requirements of mould flux for specific grade of steel. µ is viscosity of slag kg/ms .• • • • Inclusion absorption capability.1 . Consider slab casting speed 0. ρ is density of flux. High carbon grades too require flux of low viscosity and melting point. provided good lubrication. stable properties and minimal slag entrapment. Table gives effect of chemical composition on mold flux properties. . improve insulation.005 % requires flux which can absorb non metallic inclusions. • • • • Water drop flux Mean drop size Droplet velocity hitting the strand surface Wetting effects. Number of primary parameters which influence the rate of heat extraction are. A water vapour blanket forms on the strand surface which prevents direct contact of water droplets with the strand surface. Increase in Viscosity CaO SiO2 CaO/SiO2 Decrease Increase Decrease Increase Decrease Decrease Decrease Decrease Decrease Decrease Decrease Decrease No change Decrease Al2O3 Na2O F Fe2O3 MnO MgO B2O3 BaO Li2O TiO2 K2O solidification temperature Increase Decrease Increase Melting point Increase Decrease Increase Decrease Increase Decrease Decrease Increase Decrease Decrease Decrease Decrease Decrease Decrease Increase Decrease Decrease Decrease Decrease Decrease Decrease Decrease Decrease Increase Decrease The table can be read as for example increase in CaO decreases viscosity but increases solidification and melting temperature of the flux. Spray cooling essentially involves boiling heat transfer.Ultra low carbon steels C 0. Velocity of droplets should be such that droplet can penetrate the vapour layer so that droplets can wet the surface and cools the surface. Similarly the effects on other constituents on the viscosity and solidification/melting temperature can be understood. Secondary cooling Below the mold partially solidification strand is water sprayed to complete the solidification. In secondary cooling. The major requirements for secondary cooling . Mould taper.3 shows heat transfer in the mold and secondary cooling. Type of lubricant Type of mould straight or curve Casting speed. convection and radiation.3: Heat transfer in the mold and secondary cooling zone and the formation of solid shell. number of nozzles is distributed over the surface of the moving strand. Heat flux depends on. Overlapping of spray may occur. Figure 33. The higher heat flux in mould can lead to higher casting speeds. • • • • • Composition of steel. Heat transfer in continuous casting Heat transfer in continuous casting takes place in mold and in secondary cooling by a combination of conduction. Mushy zone and liquid core can also be seen In the mold air gap formation influences heat transfer. Figure 33. Distance between nozzles is important. Outer surface temperature should be greater than 850 to avoid volumetric expansion accompanying due to transformation of austenite to ferrite. Mist spray cooling i. s . phase transformation etc. a brief presentation is given on defect formation.• • • Partially solidified strand must have shell sufficiently strong at the exit of the mold to avoid breakout due to liquid pressure. mould flux. Slabs are cast within the speed ranging from 1. segregation coefficient of solute elements. Some of the issues are: • • Water spray must be distributed uniformly on the moving strand so that reheating of the strand does not occur.5m/min.e. Casting speed i.e. The intensity of heat extraction by water spray in secondary cooling is h f T T h is heat transfer coefficient W⁄m . In the following. mixture of air+ water provides more uniform cooling. T is surface and T water temperature. . Defects in continuous casting originate from several factors like mould oscillation. rate of linear movement of strand/ minute from the mould is important. Some advantages are: a) Uniform cooling b) Less water requirement c) Reduced surface cracking Products and casting defects Presently killed steels are cast continuously into slab for flat products and bloom and billet for structural products. Non‐ uniform cooling leads to generation of thermal stresses on the surface and surface cracks may appear. The liquid core should be bowl shaped Solidification must complete before the withdrawl roll. In secondary cooling solidification must be complete.5/min to 2. Here high pressure air+ water mixture is sprayed on the metal surfaces. Casting speed must match with the rate of solidification. The heat transfer coefficient h depends on water flow rate. Iron and steel soc. Readers may go through the references given in this lecture. Controlling water flux impinging the surface of the strand and minimizing reheating of strand can alleviate thermal stresses. Heat flow. 155 publication. Met. Friction. iv. 8B P 489 J. Bending and straightening operation. Defects Internal Surface • • • • • • • • Midway cracks Triple point cracks Center line cracks Diagonal cracks Center segregation and porosity Casting flux inclusion. Blow holes • • Longitudinal mid face and corner cracks Transverse mid face and corner cracks. AIME (1983) J. Roll pressure.al Crack formation in continuous casting of steel. Also air +water mist spray provides more uniform cooling. 1984 p 10 8. Mechanical stresses can be reduced by improving mold practices like • • • • Controlling powder feed rate Resonance in mold More accurate strand guidance Casting powder Thermal stresses are due to non‐uniform cooling in the secondary zone. Haris et. Material factors are related to δ γ transformation. Continuous casting of steel. High S and low Mn/s ratio cause mid face longitudinal cracks. References: D. vol.al.K. solidification and crack formation. 1977 vol. J. ii. iii. Transfer. Ferro static pressure. 1. Material factors are also responsible Mechanical stresses are created due to i.K. Deep oscillation masks shape Rhomboidity Longitudinal depression ovality Cracks are originated in the cast product due to mechanical and thermal stresses. Control of inclusion is also important. Brimacombe et. Brimacombe et. .al. j Moore: Review of axial segregation in continuously cast steel.J.F. Moore et. Sahai and Ahuja: Iron making and steelmaking 1983 (B) p 241 A. Schrave: Continuous casting of steel Y.J. 1 bid p 185 H.al: Overview for requirements of continuous casting mould fluxes . The mains frequency is usually used for stirring billets or small blooms beneath the mold . the superheat of the melt can be quickly dissipated which results in modification of structure of the solidifying strand from columnar to equiaxed and then to globular structure. In recent years considerable developments have taken place both in conventional continuous casting to improve the product quality and to develop new technology to produce nearly finished products. In continuous casting EMS can be applied in mold and secondary cooling zone to Decrease segregation and porosity Improve steel cleanliness by forcing inclusions to float on the surface where they can be absorbed by a slag Improve the steel quality in terms of reducing the defects Stirring induced by EMS modifies the flow pattern of molten steel of the solidifying strand. In addition continuous casting offers the possibility to integrate the hot strip or blooming mill by direct hot charging. EMS allows to increase the casting speed which increases productivity of the caster. A ladle containing 300 tons of molten steel at 1600 can be cast into approximately 60 minutes in a semi-finished product like bloom. B and J combine together to create an electromagnetic force which causes stirring in the bath. An inductive electromagnetic stirrer is the stator of an asynchronomous motor. Hot rolling. Improved stirring reduces segregation. Because of the imposed stirring. In secondary cooling zone low . direct charging Introduction Continuous casting is one of the most significant developments in the technology of steelmaking. material handling and as a result lead to increase in plant productivity. Electromagnetic Stirring(EMS) EMS is an electric method of inducing motion in liquid steel without using any mechanical device. which induces eddy current J. strip casting. It reduces energy consumption. Some of the developments are briefly described.Lecture 34 Advances in Continuous Casting of Steel Contents Introduction High speed slab casting Thin slab casting Key words: Thin slab casting. perpendicular to B and its velocity vector. the rotor of which is the liquid core of the solidifying strand. This stator produces either a rotating or travelling magnetic field B. billet and slab. Thus. Assuming density of liquid steel as 7 tons/m3 steel flow rate is 7. Thus. Volume of slab /minute = 1. High speed Casting High speed casting of slab increases the productivity.frequency (1 to 20Hz0 is required to electromagnetically stir the liquid steel of the solidifying strand because the thick solid steel shell shields the magnetic flux.6 3. the required steel flow rate would be 11. Now if we increase casting speed to 3 m/min. This is illustrated by the following example. Uniform cooling of the strand is the prerequisite for the success of the caster.2 tons/min/strand for 4 m/min casting speed.4 ton/min/strand. chances of inclusion floatation will be very low at higher casting speeds. Average casting speed in the conventional slab casters is on average 2 m/min. We would be requiring mold flux whose melting rate is relatively higher to keep pace with the casting speed. if tundish is to be used to remove inclusions during the process of continuous casting. .. The higher steel flow rate in the mold would require intense cooling in the mold. Similarly water spray in the secondary cooling zone has to be modified in view of the increase in the casting speed.6 11. In one modification tundish capacity needs to be increased.092 m3/minute.4 15. and which will increase to 15.3 We note that the average residence time of steel melt in the tundish decreases with the increase in the casting speed. Now let us calculate the average residence time of molten steel flowing in the tundish assuming 70 tons tundish will be used even for high speed casting Steel flow rate (tons/min/strand 7. Consider a conventional 2 strand caster casting slab of cross section 280mm x 1950mm at 2 m/min casting speed. a change in tundish design would be required.2 Residence time (minutes) 4. Now if want to increase the casting speed we have to consider design and operational features of tundish. mold and secondary cooling zone. the following modifications may be considered. One requires to consider the cross section of the submerged entry nozzle too.6 tons/minute/strand. The steel flow in the mold will be more turbulent. In other modification we improve the upstream steelmaking facility so that inclusion content in steel are within the tolerance limit.1 2. Molten steel is fed from a tundish of 70 tons capacity by submerged entry nozzles in both the molds of the continuous casting machine. One may consider other alternatives depending on the available resources. Thus. Also the mold powder consumption may increase. We have to consider further as to what will happen in the mold? Increase in steel flow rate will increase the steel velocity in the mould since the mold length is not being changed. Thin slab caster producing 20mm to 70mm thick slabs of cross section 20mm to70mm X 1000mm to 2000mm which could be fed directly into the finishing stands of the hot strip mill without any conditioning II. Strip casters producing strips less than 10mm thickness which is sent directly to the cold strip mill IV. III. Several unit operations like ingot soaking. to 500µm thickness and . An alternate to this route is the near net shape casting or direct strip casting. A slab of cross section 1500mm x 80mm would require melt flow rate 3. say around 60 to 80 mm. The near net shape casting units are classified into four categories. The thin strip or foil caster with a thickness of approximately 20 upto 300 mm wide. namely I. slabbing mill. finer grain size and higher strength Near Net shape casting The conventional methods of producing metallic strips and sheets require the casting of large ingots which are subsequently hot rolled and cold rolled to final thickness. The thick strp caster producing 10mm to 20mm thick strips which may need some limited rolling for metallurgical reasons. Since the mould size is smaller turbulence in the mould increases which results in entrapment of mould flux on surface causing surface defects.5 m/min casting speed.Thin Slab Casting Thin slab casting aims at to cast slabs of thicknesses less than 100 mm. This brings the following advantages. pickling are required to produce the strips. • Rolling can be eliminated • Improved internal quality in term of segregation Thin slab caster can be integrated with EAF units and hence mini steel plants are using thin slab casting to produce hot strips with lower segregation. The objective is to integrate caster. Development of liquid core reduction is one such technology that has helped thin slab casting development. Electromagnetic brakes are very effective in parallel moulds. Solid ingot with liquid core is subjected to on-line rolling. Use of funnel shaped mould with enlarged cross section at the meniscus is one remedy.78 tons/min at 4. intermediate annealing. reheating furnace and hot strip mill to increase the productivity. In this technology strips are cast close to the final desired thickness and thus many of the unit operations in the conventional technology can be eliminated. The condition of the mist stream is strongly affected by the design of the nozzle References: J. the water disperses on the casting section surface in the form of a mist. Trans.: Continuous strip and thin slab casting of steel. Details can be seen in the references given at the end of the lecture.Brimacombe et.1244 L.W. 27 S C Koria and R Datta: Design of air-water mist jet nozzle.: in continuous casting of steel vol. Dec. 394-401 .Teoh: Iron and steel Engr. control of volume flow rate of air to volume flow rate of water and upstream pressure must be suitably selected.Cramb: New developments in continuous casting of steel Part III:Thin slab casting.al. P. Iron and steelmaker. • A narrow submerged nozzle • Vertically curved mould with parallel broad faces that guide the strand vertically • Adjustable mould width • Dry casting technique In addition there are twin roll caster. P 31 see also July 1988 P. P.Several technologies are developed for the above casters. rheocaster and powder rolling caster.cygler et. March 1988. Air in which fine water droplets are distributed is blown on to the casting section.al. P. The features are. cooling is provided by a mist which is a two phase mixture of air and water. 1988.H. The mist spray provides uniform cooling. ISI Japan. 34 A. Mist Spray Cooling In mist spray .43 M. sinfle roll caster. water droplets are finely distributed in the air jet. August 1986. For the mist. Iron and steelmaker. drum and ring caster.K. In the technology developed by Mannesman demag Huettentechnik thin slab of 40mm to 70mm thickness and 1200 mm width can be produced.2 Mitsutsuka et.. 1992.al. Ironmaking and Steelmaking 19 (5). 25(1985). size and size distribution of phases and the impurities. The objectives of these operations are to generate the mechanical properties in the steel product required for a given application. Other types of inclusions must be suitably distributed within the matrix.Lecture 35 The Lecture Contains: Objectives Structure-property relation Property of the phases What are the final finishing operations? Surface hardening Steel types and hardening methods Objectives: The final finishing operations are performed to produce a product for a given application. . which ranges from structural to space applications. The important properties are: Strength : Measure of the resistance of material to permanent deformation Ductility : Measure of the degree of plastic deformation Hardness : Resistance to localized deformation Creep : Resistance to time dependent deformation under load Fatigue: Resistance of a material against fluctuating stresses. Impurities like sulphur and phosphrous are detrimental to most of the steel grades for all finishing operations. Fracture toughness: Resistance to brittle failure The above properties depend strongly on the number . In aluminum killed steels. In this connection it is important to note the role of steelmaking to produce steels of desired chemistry and cleanliness. alumina inclusions must be suitably modified prior to deformation processing. it acts as sinks as sinks for impurity atoms which tend to segregate to interfaces The equilibrium diagram of Fe-C system shows the following phases: • • • ?? ferrite: interstitial solution of carbon in bcc iron.) or nitrides (such as Al. Zr.008 % at 0℃. and grain boundary. W. It is stable at room temperature.08% at 1148℃ and decreases to 0. several other phases can be obtained by varying cooling rates. V. hard and brittle in nature.02% at 723℃ and decreases to 0. A polycrystalline cube 10mm on edge. Mo.Structure –property relation Properties at materials depend strongly on structure of metals we will be concerned with steel. Pure iron is highly ductile. Mo. Due to different atomic configurations.C system can alloy with several elements to promote either the formation of carbides (such as Ti. Mn etc. Grain boundary is the region of misfit between the grains. Ti etc)or to stabilize austenite (such as Ni. However.67 % carbon. would contain 1012 crystals with a grain boundary area of several square meters.) Steel is a polycrystalline material and its microstructure consists of grains (also called phases or crystals) oriented in different directions. Nb. with grains 0. Cr etc. Si etc. Maximum solid solubility of carbon is 0. It has 6. Addition carbon increases strength.8% at 723℃. In the table given on the next page the different phases are summarised which can be obtained during phases transformation of steel Property of the phases * Phase Structure Nature Spheroidite Small Fe3 C spheres in ?? − matrix Soft and ductile Alternative thick layers of ferrite and cementite Harder & stronger then pheroedite Fine pearlite Alter native this layers of ferrite and cementite Harder and strength than coarse pearlite Bainite Very fine and elongated particles of Fe3 C in ?? − ferrite matrix Hardness and strength is greater than fine pearlite. Fe.) or to stabilize ferrite(such as Cr. Grain boundaries are important.001 mm in diameter. W. Austenite: FCC crystal structure and solid solubility of carbon is 2. The above phases are obtained when steel from the austenitic region is cooled very slowly. Cementitie (Fe3 C): An intermetallic compound. V. Coarse pearlite Marten site Body centered tetragonal single phase Stronger & hander than . 2. For example fine grain size has better properties at room temperature. Very small Fe3 C sphere like particles in ?? − ferrite matrix Stranger and harder but ductility is greater than marten site Mechanical properties i. In ferrite type of steels both nitriding and nitro. Austenitic type of steels are hardened by carburizing. bainite. surface hardening 2. Thermo chemical surface hardening Thermal surface hardening Coating In thermo-chemical surface hardening composition of steel surface is altered and then steel is heat treated with or without quenching. The next lectures deal with other methods. The different methods are: 1. carbonitriding or cyaniding treatment. strength and plasticity depend on type. Carbon or Carbon + nitrogen are diffused in the ferrite phase. Deformation processing In this lecture we will deal salient features of surface hardening. 3. Coarse grain size improves creep properties. size and size distribution of phases. Inter-granular failure is always brittle and usually proceeds with particles of the brittle phase being separated out at the boundaries of the grain. Failure of metals consists of crack initiation and its growth. number. Crack can propagate either within the grain or at the grain boundary. In all these methods non-metallic elements C either singly or in combination with nitrogen are diffused into the austenitic phase. Steel can be heated either by induction of laser or by electron beam or by flame. Induction heating involves heating the component by induced eddy current to a temperature at which austenite forms rapidly.Tempered marten site needle shape. What are the final finishing operations? 1. In thermal surface hardening.e. Heat treatment 3. Metal surface is heated by using special inductors using an alternating current . Surface hardening Surface hardening methods are used to harden the surface.carburizing treatments are performed. heat alone is used to alter the microstructure without altering the composition. Steel is then quenched for martensite transformation. Depending on the required depth of hardening. see the references given at the end of 37 lecture Staislen steels Gas nitriding titanium carbide ton nitriding . Calliste r: Material Science and Engineering For more details.D.of frequencies between 50 hertz to 1000 hertz. In flame hardening. a high intercity flame is used to heat the metal as austenitic temperature and then following by quenching. Steel types and hardening methods Law carbon steels Carburizing Cyaniding Ferrite Nitro carburizing Carbonitriding Alloy steel Nitriding or ion nitriding Tool steels Titanium carbide gas nitriding ton nit riding Salt nit riding References: W. Martensite Martensite is a metastable phase. It consists of a supersaturated interstitial solid solution of carbon in body centered tetragonal iron. The hardness and strength of Fe-C martensite increase with increase in carbon content. coating. Isothermal decomposition of austenite Let us consider isothermal decomposition of austenite. below the transformation temperature. Depending on the quenching temperature. Steel in the austenitic condition is rapidly quenched to a particular temperature and then allowed to transform at that temperature. However such small cooling rates are not practically kept.e. By changing the rate of cooling. heat treatment Preamble Heat treatment is another finishing operation which is done on the finished or semi-finished product to create desired properties by altering number.1 shows isothermal transformation diagram for a eutectoid plain carbon steel showing formation of different phases. and the temperature at which transformation finishes is called martensite finish Ms temperature. Most martensitic plain carbon steels are tempered at723°C . different combinations of phases with different morphologies can be generated in all types of steel to obtain the desired property. Steels are heated to a single phase region to form austenite and then cooled to form a particular structure. Increase in weight percent carbon increases Ms temperature for Fe-C alloys. The temperature at which austenite to martensite transformation begins is called martensite start Ms . It must be emphasized that very slow cooling of steel from austenitic region will produce ferrite and cementite. The figure 36. Steel is heated to a temperature within the austenitic region and is then quenched. different phases can be formed. size and distribution of phases through heating and cooling. i. ductility and toughness decrease with increase in carbon content.1 Isothermal transformations for an eutectoid plain carbon steel . Figure 36. spray deposition.Lecture 36 Heat treatment Contents: Preamble Martensite Isothermal decomposition of austenite Continuous cooling Full and process annealing Key words: surface hardening. microstructure. However. vapour deposition. Line a denotes a very fast cooling rate which will transform all austentine into martensite. steel is not isothermally transformed at a temperature above the martensite start temperature but is continuously cooled from austenitic temperature to the room temperature.d.In the figure the lines a.3 shows different rates of cooling of eutectoid plain carbon steels cooled continuously from austenitic region to room temperature. In continuous cooling of a plain carbon steel. 2. Figure 36. Figure 36.3 Continuous cooling of eutectoid plain carbon steels. Figure 36.2: comparison of continuous cooling with isothermal cooling for heat treatment of steel a) In continuous cooling curve there are no transformation lines below about 450°C . Any type of mechanical property in most of the steels can be obtained by designing suitable cooling rates. MS and M90 are the temperatures to begin and 90% completion of martensitic transformation. Similar types of diagrams are available for hypo-and hypereutectoid steels.e. One notes from the figure that heat treatment of steels presents large opportunity to manipulate the number and proportion of different phases by predetermined cooling rates. Continuous cooling In industrial heat treating operations. austenite to peartite transformation occurs over a range of temperatures rather than at a single isothermal temperature.2 compares transformation during continuous cooling with that at isothermal cooling.c. Note the following Figure 36. The cooling rates are shown with different colors . for the austenite to pearlite transformation. It is assumed that there are no temperature gradients in the carbon steels in the austenitic region. This requires either a thin section or section has been soaked for a sufficient long time.b.f and g indicates the cooling rates. The cooling rate and the type of transformation are given in the table Line a b c d e f g Type of trnaformation from austenite All martensite All coarse pearlite All fine pearlite Approximately 50% fine pearlite and 50% martensite All upper bainite Approximately 50% lower bainite and 50% martensite All lower bainite. The start and finish transformation line is shifted to slightly longer times and to slightly lower temperatures in relation to isothermal diagrams. Improved ductility and impact resistance. The steel is first austenitized and then quenched in a molten salt bath kept at temperature above the Ms temperature.4 illustrates the full annealing and process annealing. Advantages of austempering: I. This will result in martensite and pearlite and is called split transformation Cooling curve K: Critical cooling rate at which a martensite is produced when steel is quenched in water. Austempering is an isothermal treatment aimed to produce a bainite structure in some plain carbon steels. Decrease in distortion II. and proportion of phases. This heat treatment procedure is called normalizing.Cooling curve x: very slow cooling and will result in coarse pearlite Cooling curve y: Slow cooling in air and fine pearlite will form. The martensite process consists of austenitizing steel and then quenching in hot oil or molten salt at a temperature just slightly above (or slightly below) the MS temperature. There are other heat treatment procedures like martempering and austempering.4: Temperature ranges for annealing of plain carbon steels . Steel is held at that temperature to allow austenite to transform to bainite. Fig 36. Full and process annealing Two most common types of annealing treatments that are applied to commercial plain carbon steels are a) full annealing and b) process annealing. Cooling curve Z: Steel is quenched in oil. What is important is to appreciate that the system possesses unique possibility to produce materials with different number. Figure 36. Martempering is a modified quenching procedure used for steels to minimize distortion and cracking that may develop during uneven cooling of the heat treated material. In the hot quenchent steel is soaked to attain the uniform temperature which is then followed by cooling at a moderate rate to room temperature to prevent temperature gradient. The hypereutectoid steels are heated between 40°C above the austenitic region Process annealing is used to relieve internal stresses induced due to cold working of metal. In this lecture a very brief account of heat treatment procedure is discussed with the aim to understand steelmaking from the product-process integration point of view. Purpose is • To refine grain structure • To increase strength of steel • To reduce segregation in castings or forgings Temperature regions are shown in the figure 36. Smith: Principles of materials science and engineering R.F.4. Detailed discussions on heat treatment procedures can be found in any heat treatment book References: W. It is normally applied to hypo eutectoid steels by heating to a temperature in between 550°C to 650°C . Normalising Steel is heated to austenitic temperature and then cooled in air. The steel is soaked and then cooled in the furnace.In full annealing hypo eutectoid and eutectoid steels are heated to a temperature 40°C above the austenitic-ferrite boundary as shown in the figure. Sharma: Phase transformation in steel .C. slabs. Hence no increase in either yield strength or hardness occurs. Hot working It is plastic deformation of metals above their recrystallization temperatures. These forms experience further deformation to produce the desired products formed by processes such as forging. The stresses could either be tensile or compressive or shear or combination of them. cold working.Lecture 37: Deformation Processing Contents Introduction Hot working Cold working Spring back phenomenon Annealing Key words: Hot working. In addition yield strength decreases as temperature increases and the ductility improves. some aspect of deformation processing is discussed. Hot working does not produce strain hardening. Deformation processing exploits the ability of steel to flow plastically without altering the other properties. Hot working of steel requires to heat steel near 1000oC for plastic deformation. Cast ingots. sheets. simple shearing. or any combination of these and other processes. extrusion and other sheet metal forming. Details can be obtained in any text book on deformation processing. One of the method is the deformation processing. The deformation may be bulk flow in three dimensions. . In this connection the steel chemistry and cleanliness are important factors for deformation processing. This is given to appreciate the efforts of steelmakers in producing quality steels. In the following general features of hot and cold working are described. The required forces are often very high. Deformation processing can be carried out either under hot or cold condition. Hot working of steel involves the deformation of fcc austenite. In the following. annealing Introduction The ultimate goal of a manufacturing engineer is to produce steel components with required geometrical shape and structurally optimized for a given application. blooms and billets are reduced in size and converted into plates. rods and others. simple bending. The readers should also understand the reverse engineering approach and to appreciate the steelmaking. Hot working can be used to drastically alter the shape of metals without fear of fracture and excessively high forces. the ductility and the yield point stress of steel are important. Some advantages are: • No heating is required • Better surface finish and superior dimensional control are achieved • Strength. The effect of ductility is shown below: . pores can be welded or reduced in size during deformation. The dendritic grain structure. Hot working results in reorientation of inclusions or impurity particles in the metal with the result that an impurity originally oriented so as to aid crack movement through the metal can be reoriented into a “crack arrestor” configuration. ductility and toughness. Elevated temperatures promote diffusion that can remove chemical inhomogeneties. hot drawing etc. small gas cavities and shrinkage porosity formed during solidification in large sections can be modified by hot working to produce a fine. forging. fatigue. and wear properties are improved • Directional properties can be imparted Disadvantages: • Heavier forces are required • Strain hardening occurs (may require intermediate annealing treatment to relieve internal stresses) • Residual stresses may be produced For cold working. Cold working Cold working is plastic deformation of metals below the recrystallization temperature and is generally performed at room temperature. The various hot working processes are rolling. randomly oriented. extrusion. spherical-shaped grain structure which results in a net increase in strength. bending. The various cold working processes are squeezing. the metal will fracture. Greater ductility would be available in the material and less force would be required to initiate and continue the deformation. At the other extreme if steel is strained to X4. But removal of load in the plastic region.1 for metal A i.1B is not suitable for cold deformation but may be suitable for shearing operations Cold working properties are also affected by the grain size and must be controlled during solidification of steel.1: Stress strain diagram for low carbon and high carbon steel to understand the suitability of steel for cold working In figure 37. the strained material returns to its original size and shape. High carbon steel which shows stress strain behaviour like figure 37. decreases the strain from x3 to x2 as shown in the figure 37. Springback Springback is also present in cold working operations. in cold working the deformation must be carried out beyond the desired point by an amount equal to the springback. x3 − x2 is elastic springback. (c) increase in dislocation density. shearing and drawing Annealing Plastic deformation of polycrystalline material in cold working produces microstructural and property changes that include (a) change in grain shape. which indicates the force required to initiate the permanent deformation and The extent of region of strain that is 0 to X4 which determines the extent of plastic deformation If considerable deformation is required then the tensile properties of steel should be that depicted in figure 37. From coldworking point of view the following is important: The magnitude of yield stress. (b) strain hardening. Appropriate heat treatment such as annealing reverts back to the pre-cold worked states.Figure37. Too large and too small grain size have undesirable effects. In the elastic region.1A. Permanent deformation can not occur until strain is greater than X1. The purpose of annealing may involve one or more of the following aims: To soften the steel and to improve machinability . The decrease in strain. Thus.1 variation of stress with strain is shown for (A) low carbon steel and (B) high carbon steel. which may be followed by grain growth. DeGarmo: Materials and processes in manufacturing . recrystallization. the strain-free grains will continue to grow. To relieve internal stresses induced by rolling. Recrystallization is the formation of a new set of strain-free and equixed grains (having approximately equal dimensions in all directions). forging etc. After recrystallization is complete. To remove coarseness of grains The annealing consists of Heating the steel to a certain temperature Soaking at this temperature Cooling at a predetermined rate Such restoration results from recovery. Even after recovery is complete. if the metal is left at the elevated temperature.P. but ductility increases. During recovery some of the stored internal strain energy is relieved by virtue of dislocation motion due to atomic diffusion. Strength and hardness decrease. References: E. the grains are still in a relatively high strain energy state. The value of scale factor indicates how big or small model would be. . water modeling. This is called scale factor λ. must be similar geometrically. a scale factor of 0. the model reactor and experiments are designed based on the similarity criteria between the prototype and model. The results of these studies can then be implemented for the desired objectives. steel market has become competitive both with respect to quality and cost of steel. there exists a corresponding point in the prototype. that is product. One of the research tools is to design the model of the actual process (here after we call proto type) so that specific studies can be made. continuous casting tundish and mold.2 means that diameter of the model cylindrical vessel is 1⁄5 of the diameter of the actual vessel.Lecture 38 Modeling of steelmaking processes Contents Introduction Physical model Design of a physical model for fluid flow in steel melt Key words: Physical modeling. model and prototype. if the actual vessel is cylindrical in shape. For example. In order to meet these objectives. Physical model In physical modeling. ladle. A model of the process can either be physical or mathematical. tundish metallurgic Introduction With the globalization. This can be achieved by maintaining a constant ratio between the linear dimensions of the systems. Steel industry is required to produce quality steel at a reasonable cost so that it remains competitive with the world market. Both. a sustainable research and development activities must be carried out in the plant to address the quality issues in the steel product and then to introduce changes in the steel processing line. chemically and thermally. The present lecture deals with some issue related to design of physical models of steelmaking processes. Two systems are said to be geometrically similar when for every point in the model. For a rectangular vessel all the linear dimensions of the model vessel are 1/5 of the actual ones. λ= Dm Dp = Lm Lp (1) The above relation suggests that two systems following the geometrical similarly should have the same aspect ratio of the vessel . dynamically.process integration approach. For this purpose constant and continuous efforts are required to introduce either new steelmaking technology or to improve the process technologies in the existing steel processing vessels like converter. Froude number similarity is very important to model the chemically active or inert gas injection in steelmaking processes. The ratio between inertia and gravity force is Froude number (Fr) (Fr) = u2 gL Modified Froude number ????1 is more relevant than simple Froude number Fr1 = ρg u2 �ρ l −ρ g �gL = aerodynamic force gravitational force (3) ρg is the density of gas and ???? is density of liquid. L is characteristic linear dimension and γ is kinematic viscosity. Reynold’s number characterize the type of flow.Dynamic similarity requires that the corresponding forces acting at corresponding time and location must bear the same ratio between the model and the prototype. In steelmaking the inertial.5 Rem = Rep and (6) The similarity in Reynold’s number requires that. (9) . The ratio between inertial and viscous force is called Reynold’s number inertial force force Re = viscous = ρ uL μ = uL γ (2) Where u is velocity. The dynamic similarity requires (5) 1 Frm = Frp or Frm = Frp1 Wem = Wep (7) u om u op = (8) u om u op = λ0. that is whether laminar or turbulent. Froude number determines the importance of aerodynamics force and gravitational force when gas jet either impinges the bath or submerged into the bath. γm γp × 1 λ The subscript m denotes model and p denotes prototype. viscous and surface tension forces are of relevance. And Weber number requires that. The similarity in Froude number requires that. Weber number (We) is the ratio of aerodynamic to surface tension force We = ρu 2 L σ (4) σ is surface tension of liquid. while the aspect ratio (bath height/bath diameter) for both vessels will be same. These optimum results can be verified in the prototype selectively.2for the purpose of illustration. We select scale factor λ = 0. ii. similarity in kinematic viscosity in both the fluids ensures similar fluid flow behavior. If the industrial ladle has a diameter of 4m. In this connection water is the fluid whose kinematic viscosity is 10−6 m2 /s which is very close to that of molten steel melt.9. Selection of model vessel The prototype vessels in steelmaking are converter. In fact water model has been very widely used to investigate the behavior of steel melt. Fluid flow in the tundish of a continuous caster is also important to evaluate the performance of the tundish with reference to its ability to distribute molten steel in all molds at constant superheand to remove inclusions during the process of continuous casting. Model vessel is designed by geometric similarity. similarly we can design model converter and model tundish by selecting a suitable scale factor.5 (10) Weber number is relevant when droplet formation occurs in the actual system. Full scale models may become difficult to handle since the dimensions involved would be large. Though absolute value of viscosity and density of steel melt may differ from the model liquid. ladle and tundish. Therefore model bath height is 72cm. Selection of model steel melt phase In order to compare the results of two geometrically similar systems it is essential that transport mechanisms should be similar in both the systems. Some references are given at the end of the lecture. model vessel diameter would be 80cm. Converters and ladles are more or less cylindrical in shape. fluid flow in steel melt controls mixing and mass transfer reactions in converter and ladle. Whereas tundish is a rectangular with side walls inclined. A scale factor =1 represents full scale model. that is γ= μ ρ (11) is called kinematic viscosity of fluid. . The ratio of density to viscosity. Experiment in full scale size of the steelmaking vessel with molten steel is very difficult and pose practical difficulties.u om u op = ρp σm ρm σp λ0. Kinematic viscosity represents the diffusion of momentum flux into the liquid and governs the fluid flow behavior. Suitably designed models are very helpful to conduct large number of experiments to arrive at optimum results. Thus water can be selected as model liquid. Density represents inertia of fluid against an applied force and viscosity is internal friction of fluid. The aspect ratio of industrial ladle is 0. i. Design of a physical model for fluid flow in steel melt In steelmaking. For example if flow is turbulent in the prototype then turbulent flow should also prevail in model liquid. Density and viscosity of the fluid are the two important fluid properties that govern fluid flow behavior. ρuL � μ m ρuL � μ p � =� u2 gL m ρu 2 L � σ � m u2 gL p ⎬ ρu 2 L ⎪ � σ � p⎭ � � =� � = ⎫ ⎪ 12) From equations 12 we get for Reynold’s number similarity um up = γm γp 1 λ × (13) For Froude number similarity um up σ 0.5 = � σm � p (14) For Weber number similarity um up = � ρ p 0. iv) Selection of model velocity of gas In steelmaking processes. transitory contact mode is difficult to model. gas is used to stirr the molten phases Dynamic similarity must be observed between model and prototype.iii) Selection for model slag phase It is in fact very difficult to find a low temperature model slag which is similar to actual slag. Transitory and permanent contact of slag phase with molten steel are the principle refining mechanisms. In transitory contact.5 λ−0. Permanent contact mode can be modeled. Due to large difference in the density of molten slag and steel.5 (15) . we get. Density of slag in prototype is around 0.5 ρm � σm � σp � 0. that (Re)m = (Re)p (Fr)m = (Fr)m and (We)m = (We)p Substituting the quantitative value of the dimensionless numbers. the refining occurs by rising molten slag droplets. Slag floats on steel.4 times that of molten steel. mineral oil can be used as a model slag to study the physics of slag/ metal interface. Several organic oils like paraffin. Froude and Reynold’s number similarity can be obtained in a aqueous model only when λ = 1. This lecture highlights some of the important aspects of physical modeling of steelmaking processes. In the past several years physical modeling or more precisely water modeling has become a very important tool to investigate the physical effects in steelmaking caused by either impinging or submerged gas jets.The Weber number similarity can be neglected since the inertial forces in the prototype are very large as compared to surface tension forces. . The references are given at the end of the lecture 39. water modelling Tapping of molten steel Let us design a 0.5 0. water will be used to simulate the steel.12 m3 ⁄s. Molten steel is plunged from a height of 4.5 p .5 Lm � Lp 7000 ×10×4.2 × 4. Dynamic similarity is required.5 = 0.Lecture 39: illustration of physical modeling Contents Tapping of molten steel Gas stirred ladle Tundish model Key words: Physical modeling.2)3 × 40 = 0.s and density is 7000 kg m3 Diameter of the model ladle dm = 0.scale physical model to simulate the fluid flow behavior for tapping of steel in the ladle.447 ∴ Um = 5.12 = 2. Molten steel enters at velocity 12m/s velocity.9m Volume Vm = λ3 Vp = (0.2.5 = 2.81×4. Viscosity of steel = 7 × 10−3 kg m.27 A very large value of Reynold’s number in the prototype indicates that flow of steel melt is dominated by the inertial forces rather than viscous forces. We have to find velocity of the water in the model to perform experiments.5 = 0.5 × 107 = 9. The time required to fill the ladle of volume 40m3 is 8 minutes.34m/s The time of tapping in the model can be derived from Lm tm tp × L = λ0.47 × 10−3 m3 s In the physical model. =� = (0.2)2.2)0. Rep = ρ u Lp μ = Frp = u2 g Lp Um Up 0.5m.007 10×10 = 4. Inertial force is also embedded in the Froude number hence Froude number similarity will be sufficient to find velocity of water in the model. The volume flow rate is 0.32 m3 Volumetric flow rate Q m = λ5⁄2 Q p = (0.5 × 0. Solution: water will be used as an analogue of steel. Several investigators including the present author have studied behavior of steel melt flowing in a single and multi strand tundish. width 1100mm and height 1320mm. Now we have to design a tundish. The tundish is rectangular in cross section with slopping side walls. Physical model of the tundish is designed to study the fluid flow behavior in terms of flow pattern and residence time distribution. The tundish may be provided with the grooves to insert dams and weirs etc. Gas is injected at 70 Nl/min through the porous plug fitted at the bottom of the ladle. Scale of physical model = 0. Tundish is rectangular shaped with slopping walls. To full fill the above objective it became necessary to modify the existing tundish design. Physical model of a tundish of single and multi strand casters has been designed by several investigators to study the fliuid flow behavior in the tundish Design a physical model of a single strand slab caster tundish. Volumetric flow rate of steel in the tundish is 224 l/min. The dimensions of the model tundish are 1725mm × 550mm × 600mm. Bath temperature is 1600℃. In the original installation of the continuous casting machine.6 minutes Gas stirred ladle Design a physical model of an industrial gas stirred ladle with an elliptical shape. The base length of the tundish is 3450mm.082m minimum. This is for your exercise.2)0.∴ tm tp = Lm Lp × λ −0.5. .5 = 3.5 ∴ t m = t p × (0. It has soon been realized that tundish can be used to float inclusions or to add alloying elements during the process of continuous casting. The scale of the physical model should be 1/3. One must keep in mind the future modifications that would be required.282m maximum and 2. A submerged stream is poured from the model ladle to the tundish. role of tundish was to act as a distributor of molten steel to different molds at constant speed.4m3 . We have to make previsions to insert flow modifiers like dam and weir etc. Tundish model Fluid flow in the tundish has been investigated thoroughly. The equations given in lecture 38 could be used to design the model.5 = λ0. We may require to modify the flow of fluid in the tundish. Molten steel height is 2. Internal diameter of the ladle is 2. Submerged ladle shroud diameter is 78mm.46m and volume is 9. In the present model scale factor is 0. both numbers can be made similar for scale factor 1. As pointed out in lecture 38. Based on the analysis of different lengths of the eddies in molten steel and water system.5. we get = 6 × 104 Similarly one can calculate Froude number of the prototype u 2p Frp = g D = 16 Q̇ 2 π2 D5 g 2 224 � 1000 ×60 = 16 × � Frp = 0.078)5 ×9.s and Froude.14×0.14)2 (0.5 × 1100 = 550mm Height of tundish = 0. Therefore Froude number similarity is considered . irrespective of the scale of the model and type of fluid.5 × 224 = 40 l/min Dynamic similarity requires similarity in Reynold’s and Froude number Rem = Rep Frm = Rep Rep = D up γ μ up is velocity of steel.5 × 1320 = 660mm Volumetric flow rate = λ2.s number should be same.79 1 × (3.s number as long as flow is turbulent. It is shown (see the references given at the end of this lecture) that in turbulent flow. hence either Reyonld’s number similarity can be observed or Froude number similarity. In the prototype tundish velocity of steel melt is not known. it is shown that the lengths of the eddies produced at the exit of the submerged shroud for water and steel are of the same order of magnitude. Putting value of up = Rep = = 4Q πDγ 4×224×10 6 1000 ×60×3.5 × 3450 = 1725mm Width of the tundish = 0.078 Q̇ π D2 in the Reynolds number.Base length of model tundish = 0. This suggests that within the turbulent flow regime the mechanism of flow of steel melt in the prototype and water in the model may not depend significantly on the absolute value of the Reynold. There is a wide spectrum of eddies of different lengths in a fully developed turbulent flow. γ = ρ kinematic viscosity of steel melt = 1 × 10−6 m2 ⁄s.81 For the dynamic similarity both Reylold. the transport of the momentum to the neighboring fluid layers occurs via eddies. Following Froude number similarity model submerged ladle shroud diameter can be determined. 1995. 294-300. 784-793 S Singh and S C Koria Study of fluid flow in tundishes due to different types of inlet streams Steel Research 66 (70). 255-263 S C Srivastava and S C Koria Effect of argon shrouded stream ouring on the behaviour of fluid flowing in a tundish Scand. Jl. 1994. 1996. 1993. 1997. 221-230 S Singh and S C Koria Model study of the dynamics of flow of steel melt in the tundish ISIJ International 33 (12). 123-132 . S Singh and S C Koria Tundish steel melt dynamics with and without flow modifiers through physical modelling Ironmaking and Steelmaking 23(3). Of Metallurgy. 1⁄5 16×Q̇ 2 � 2 π ×g Dm = � ≈ 38mm References: RIL Guthrie: engineering in process metallurgy S Singh and S C Koria Physical modelling of steel flow in continuous casting tundish Ironmaking and Steelmaking 20. 1993. 1228-1237 S Singh and S C Koria Physical modelling of the effects of the flow modifier on the dynamics of molten steel flowing In a tundish ISIJ International 34 (10). 26(3). Lecture 40: Status of Steelmaking in India Steel belongs to iron-carbon system and this system has the following unique features: • • Fe-carbon system possesses some solubility for several elements of the periodic table whic results into production of diversified range of materials for application to all industries including aero. Steel is one of the infrastructural materials that are needed for the economic growth and hence for the industrial growth of any nation. Integrated steel plant sector is the biggest producer of steel. These projections may look to be optimistic. To sustain the steel requirement. but one thing is certain and that is steel requirement will increase in India in the near future. The per capita consumption of steel in India is 40 kg as compared with an average 150 kg across the globe and 250 kg in China. The steel production is based entirely on hot metal. and induction furnace. Low per capita consumption of steel indicates that no meaningful infrastructural development has occurred in the country as a whole. It is estimated that per capita steel consumption will rise to 110 kg by the year 2020-21 and 300 kg by the year 2030-31. Why Steel consumption in India should rise? India is a developing nation.and auto industries Steel is recyclable and hence is a green material. Indian Steel Industry Indian steel industry is organised in three sectors. namely integrated plant. The route consists of Blast furnace-converter-secondary steelmaking-continuous . It requires industrial growth for the prosperity. mini steel plant based on EAF. Indian steel industry has to grow fast. Steel is a material which can be made available for most of the engineering applications. Also the main element in steel is iron and iron is the fourth abundant element in the earth crust and corresponds to 5% of the weight of the earth crust. India needs large consumption of steel for the growth of the following infrastructures: Modernization of air ports Expansion of railway tracks Auto industry Road projects Real estates Safety and other development programmes of Indian railways Growth in the above sectors will automatically raise the per capita steel consumption. is well below the world average (150kg) and that of developed countries (400 kg). This route is dependent entirely on the availability of hot metal through blast furnace. Durgapur. particularly the induction furnace route. Integrated steel plants are both in public and private sector. Rourkela. Induction furnace based steel producers have modernised by using refining equipments as well as continuous casting units. Several mini steel plants are dispersed in the country to produce mild steel and alloy steel both for long and flat products.02 million tonnes in 2008-09 as per the report given by the Joint Plant Committee.casting-and rolling. iron ore and sponge iron are. pig iron and sponge iron.5 million tonnes during April-December 2010 in the country as per the report of the joint Plant Committee. The crude steel performance accounted for 31% of the total crude steel production in the country during 2009-10. In many of these units. RINL has plant at Vishakhapattnem. This sector depends entirely on availability of scrap and sponge iron. Refining by ladle arc furnace is also in use. contributed mainly by the strong trends in the growth of the electric route of steelmaking. The combined EAF + IF sector had contributed 50% in the total steel production in the year 2005-06 Steel production in India India produced 55. In the private sector Essar. Going forward. The production is expected to be nearly 110 million tonnes by 2012-13. Bokaro.4% during the period 2005-06 to 2009-10. at around 40 kg. Raw materials situation in India To meet the future demand of steel. Metallurgical grade coking coal Blast furnace-BOF route will continue to play a significant role to meet the increased demand of steel. (RINL) are the main producers of steel. Burnpur. Induction furnaces consume less power. Steel production in India has increased by a compounded annual growth rate (CAGR) of 8. one has to ensure the supply of basic raw materials to the steel industry. Limitation is only that induction furnace based plants are not suitable for bulk steel production. In the public sector Steel Authority of India Ltd. mini blast furnace hot metal is also used in the charge. and there is no expenditure on electrode. SAIL has plants located at Bhilai.4 million tonnes of finished carbon steel in 2006-07 which has marginally increased to 59. The total capacity of steel production is 18-19 million tons per annum. as per capita consumption of steel in India. Salem and Bhadravati. Blast . ISPAT. among others the important raw materials for steel production through different routes. scrap. Crude steel production was registered at 51. Metallurgical grade coking coal. Many producers have installed high frequency induction furnaces (IF) to produce mild steel and alloy and special steels. (SAIL) and Rashtriya Ispat Nigam Ltd. Jindal and TISCO are the main producers with the total capacity of 12 million tons per annum. Mini steel plant sector mainly produces steel through electric arc furnace using scrap. growth in India is projected to be higher than the world average. The EAF holders have also installed induction melting furnaces. furnace cannot work without coke and coke is produced by carbonization of metallurgical grade coking coal. Shortage of metallurgical grade coking coal reserves within the country is of serious concern. For the long time India is dependent on the imported high grade, low ash coking coal from Australia amounting to 30 to50% of its total requirement. In the future import of coking coal may become expensive. Thus, integrated steel plants producers must search ways and means to operate the blast furnace with the bare minimum coke consumption (the bare minimum coke is that which is just required to maintain the permeability of the bed in the blast furnace). Other technologies like pulverised coal and tar injection in the blast furnace must also be explored. Additionally, smelting reduction processes like COREX, ROMELT must also be developed to supplement the hot metal. Iron ore The situation with respect to iron ore is good as long as appropriate export policies are put in place. With the growth of steel production to 110 million tons per annum by 2019-20, iron ore requirement would be around 200 million tons. Iron ore reserves may not create much problem to sustain the steel production at least for the next two or three decades. Scrap Scrap is an important raw material for the growth of EAF and IF. The shortage of scrap necessitates the search for alternative raw materials. In this connection sponge iron (also known as directly reduced iron or DRI) has proved to be a promising alternative to scrap. In fact there are electric furnaces which are operating with large percent of sponge iron in the feed. Sponge iron is produced by reducing iron ore either by using coal or natural gas. Under Indian conditions coal based sponge iron processes are more attractive as compared with gas based ones. Several coal based sponge iron plants are operating in the states of Chattisgarh, Orrisa, West Bengal and Jharkhand doe to availability of high grade iron ore in these states. DRI plants are installing various capacity induction furnaces to produce mild steels for long product applications. Secondary producers of steel are located in all parts of India to meet the local specialised demand of steel. Their share in India’s total steel production may reach up to 50% Outlook The outlook for Indian steel industry is very bright. India’s lower wages and favourable energy prices will continue to promise steel production at lower cost as compared with western part of the world. In the future, Indian steel industry has to grow either by adding capacity in the exisating steel olanta or by installing new integrated steel plants. Following information is reproduced from the internet search: Category: Steel companies in India-Wilkepedia: Bhushan Steel limited will be setting up an intergrated steel plant in west Bengal with facilities including slab plant, coke ovens and captive power plant. They have also proposed to set up a 6 million tonne per annum integrated steel plant as an expansion of its existing plant being set up at Meramandali (Distt dhenkanal) in Orissa. Bokaro steel plant is undergoing a mass modernisation drive after which its output capacity is expected to cross 10 million tonnes Jindal Vijayanagar steel (JVSL) will be adding 3.2 million tons per annum to achieve 11 million tonnes per annum by 2011. POSCO signed a memorandum of understanding with the government of Orrisa to set up a 12 million tonnes per annum green field steel plant near paradip, Jagatsinghpur district, Orrisa. Tata steel has set an ambitious target to achieve a capacity of 100 million tonnes by 2015 through a series of Greenfield projects in India and outside which includes 6 million tonnes plant in Orissa, 12 million tonnes in Jharkhand, 5 million tonnes in Chattisgarh, 5 million tonnes capacity expansion at Jamshedpur and in few other countries like Iran, Vietnam and Bangladash Vizag steel plant is the onl;y Indian shore based steel plant and is poised to become up to 20 million tonnes in a single campus. Recently it has gone expansion from 3.1 million tonnes to 6.3 million tonnes. Another public sector company, NMDC is to set up a 3 million ton per annum integrated plant at Nagarnar, Chhattisgarh. The plant is likely to be commissioned in 2014. In addition to the above the steel plants at Bhilai, Rourkela, Durgapur and others are in the process of modernisation and capacity addition. India is poised to be world’s 2nd largest producer of steel before 2016. Indian’s steel production will be nearly 124 million tonnes by 2012 and that the country could achieve an annual capacity of around 275 million tonnes by 2019-20 Refernce: Category: Steel companies in India-Wilkepedia, Internet Chandra Bhushan: Challenge of new balance Internet Joint Plant Committee report as available on the internet Lecture 41:Exercises on steelmaking Contents Lectures 3 to 5 Lectures 6 to 8 Lectures 11 to 20 Lectures 21 to40 Lectures 3 to 5 I. What do you understand by 1 weight percent standard state? Discuss with reference to thermodynamic calculations on refining of hot metal to steel. II. In a binary solution of Fe with an element x, what is the physical significance of ideal and nonideal behavior of the element X. Give example to illustrate the answer III. IV. V. VI. VII. VIII. What is the importance of slag in steelmaking? Why is there a difference in the structure of CaO and SiO2? Discuss the effect of addition of CaO to molten silica. At what percent addition of CaO the hexagonal structure of silica will be broken to independent ??????44− ? Obtain an expression to calculate the basicity of slag which contains ??????, ??????, ??????, ??????2, ????2 ??3 , ??2 ??5 from ionic approach What is the role of slag foaming in BOF and EAF steelmaking Calculate the foaming index slag of composition 60% CaO, 35% Al2 O3 and 5% Si O2 at 1773 K slag from the following data: IX. ηo = 3.5 Lectures 6 to 8 I. II. III. IV. V. Kg ms , ρ = 2500 Kg m3, γ = 1.1 N/m and db = 0.005 m and 0.01m. Discuss the conditions to remove simultaneously carbon and phosphorus from hot metal. What may be the cause of reversal of Mn from slag to metal and how can it be rectified? What is the importance of carbon removal in steelmaking? What is rimming reaction? Derive the conditions for removal of sulphur in iron melt containing C, Si, Mn,S, and P. Based on the conditions, discuss whether sulphur removal can be achieved efficiently in ironmaking or in steelmaking. Lectures 11 to 20 1) Discuss the functions of a nozzle in converter steelmaking 2) Given the expressions in lecture 13, calculate for a 200 ton converter the following; a. Diameter of a nozzle for 4 hole lance b. Oxygen supply pressure c. Momentum flow rate produced by the four hole nozzle d. Jet penetration depth when oxygen is blown at (a) 3m lance distance, and (b)1.5 m lance distance e. The effects produced by the jet on impinging the molten steel bath at distances mentioned in d 3) What is a foamy slag practice and what advantages this practice offers in electric steelmaking. How foamy slag is practiced (a) in plain carbon steelmaking and (b) in stainless steel making? 4) Increasing cost of electric energy demands to use chemical energy in electric steelmaking. In what different ways chemical energy can be used? Discuss 5) Consider 1000kg iron ore of composition 80% Fe2 O3 and 20% gangue minerals. Reduction of iron ore produces DRI in which oxygen is present as FeO.Calculate amount of free iron, amount of FeO and total gangue minerals for metallization ranging in between 85% to 95%free iron. Discuss the results with reference to usage of directly reduced iron in electric steelmaking. 6) In stainless steel making using high carbon ferrochrome, decarburization of the melt is required. Calculate the ??ℎ???????????? ???????????? ?????????? as a function of temperature and partial pressure of CO. 7) What is a foaming slag? Explain the formation of foamy slag in EAF. What are its advantages in EAF steelmaking practice. 8) What is the role of oxygen lancing in EAF steelmaking? 9) What is an oxy-fuel burner? Describe its functions in EAF Steelmaking. 10)Explain how the metallurgical quality of DRI/HBI affects the electric power consumption in EAF 11) A EAF is operating with 100% scrap. What modifications would it be required to use chemical energy? 12) What is the equivalent carbon in DR and what is its significance? 13) Name the sources og CO and H2 generation in EAF steelmaking. 14) What are the requirements of injection of oxygen for decarburization and post combustion in EAF steelmaking? 15) Calculate the amount of CO and Oxygen required generating chemical energy 30 kWh/ton of steel. 16) Discuss the mechanism of foaming in EAF stainless steelmaking. 17) Calculate the supply of thermal energy in each case when 1000 kg scrap from 25oC is heated to 250oC, 300oC and 400oC width 1100mm and height 1320mm. The time required to fill the ladle of volume 45m3 is approximately 8 minutes. 6) Design a physical model of an industrial gas stirred ladle with an elliptical shape.082m minimum.s and density is 7000 kg m3 .5. Internal diameter of the ladle is 2. Molten steel height is 2. 4) Discuss the possibility of obtaining different microstructures when a plain carbon eutectoid steel is cooled from the austenitic region just above the eutectoid temperature 5) What does the bainite microstructure consist of? What is the microstructural difference between the upper and lower bainite. Below are some addition problems 1) What do you understand by the constitutional supercooling? Explain by taking a suitable example. Viscosity of steel = 7 × 10−3 kg m.282m maximum and 2. if required may be chosen.1 m3 ⁄s. The volume flow rate is 0. 7) Design a physical model of a single strand slab caster tundish. Scale of physical model = 0. Bath temperature is 1600℃. Molten steel is plunged from a height of 4m.46m and volume is 9. The scale of the physical model should be 1/3.4m3 .scale physical model to simulate the fluid flow behaviour for tapping of steel in the ladle. The base length of the tundish is 3450mm. 8) Let us design a 0. The tundish capacity may be chosen to 70 tons. Tundish is rectangular shaped with slopping walls.Llectures 21 to 40 Some problems are discussed in lecture 30. Gas is injected at 70 Nl/min through the porous plug fitted at the bottom of the ladle. Volumetric flow rate of steel in the tundish is 224 l/ min.5. Molten steel enters at velocity 15m/s velocity. 2) What are the necessary conditions for plane front solidification and dendritic solidification? Discuss with the help of a binary phase diagram by drawing concentration vs distance profile 3) What technological modifications would be required to convert a conventional slab caster of cross section 2000mm x 250mm to a high speed slab caster casting at 4m/minute. Submerged ladle shroud diameter is 78mm. Other dimensions. (06) 4a)Explain the cause of pipe formation during solidification. 05) 4c)What are the principle objectives of oxidizing and reducing periods in electric arc furnace steelmaking? 08) 4d)Enumerate the factors that determine the consumption of electrode in EAF steelmaking.Lecture 42: Self Assessment questions 1a) Explain the role of a basic oxidising slag in steelmaking. What are the operational benefits of this technology? (06) . (08) 2a) Explain the procedure for charging sponge iron in an electric arc furnace. (06) 3b) What are the metallurgical controls adopted in the tundish. (04) 1b) Carry-over of the basic oxidising slag from the converter is undesirable to the transfer ladle. (06) 2b) Discuss the basis of technological development in the electric arc steelmaking? Explain in detail the contribution of chemical energy in electric steelmaking in reduction of electric consumption. Discuss the advantages and limitations of using sponge iron as a charge material. Why? How is it possible to minimise the carry-over of slag? (08) 1c) Explain the influence of lance height and the number of nozzles in the lance on the decarburization and dephosphorization reactions in the LD Converter. 02) 5a)What do you understand by complex deoxidation of steel melt? What are the advantages of complex deoxidation over simple deoxidation (08) 5b Give the evolution philosophy of combined blown steelmaking. How do you avoid dead zones in the tundish? (08) 3c) Explain the effects of melt superheat and the rate of withdrawl on the quality of the concast billet. How can it be prevented? 05) 4b)Describe the mechanism of solidification of killed steel. (08) 2c) What are the advantages of external desiliconization of hot metal prior to steelmaking (06) 3a) Draw a neat sketch of the setup for the continuous casting of steel. (06) 6.5c) Explain the sequence of elimination of impurities between top blown and bottom blown converter steelmaking. Choose the correct answer i) The most powerful dexodizer for steel is (a) (b) (c) (d) Silicon Magnesium Aluminium Chromium ii) Calcium carbide injection is done (a) (b) (c) (d) To remove hydrogen gas To increase calcium content of steel To modify inclusions To remove sulphur from hot metal iii) Calcium silicide injection into liquid steel is done mainly for (a) (b) (c) (d) Deoxidation Inclusion modification Dehydrogenation Dephosphorizarion iv) Hydrogen in liquid steel is dissolved (a) (b) (c) (d) In the form of tiny gas bubbles In atomic form In molecular form In ionic form v) In a good rimming steel (a) (b) (c) (d) Carbon and silicon content should be low Silicon content should be low but carbon cintent should be high Both Carbon and silicon content should be high Silicon content should be high but carbon cintent should be low vi) Activity of P2 O5 in the steelmaking slag is lowered by (a) (b) (c) (d) Increase in slag basicity Decrease in slag basicity Increase in bath temperature None of the above . vii) Sulphide inclusion is steel may be modified by (a) (b) (c) (d) Ca-Si injection CaF2 injection Iron ore fines injection Injection of passivated Mg granules viii) Oxygen lance nozzles are made of (a) (b) (c) (d) Copper Ceramic materials Same type of steel as that of lance Graphite ix) Circulation degassing of liquid steel is carried in (a) (b) (c) (d) VAD refining process VOD refining process RH process Stream degassing process x) Complete deoxidation of liquid steel in necessary for the production of (a) (b) (c) (d) Killed steel Semi-killed steel rimming steels Capped steels xi) In a Fe-Cr-C system. the temperature for preferential decarburization during oxygen lancing is lowered by (a) (b) (c) (d) A reduction in the partial pressure of CO On increasing the partial pressure of CO An increase in the Cr content in the bath Decrease in the Cr content in the bath Xii) Minimum segregation occurs in an ingot of (a) (b) (c) (d) Rimming steel Semikilled steel Killed steel None of the above xiii) Dephophorization is favoured by (a) (b) (c) (d) High temperature and a basic oxidizing slag Low temperature and a basic oxidizing slag Low temperature and an acidic oxidizing slag Low temperature and a reducing basic slag . X. 8) Give specific answer to the following I. IV. IX. cast iron and steel Why big ingots with circular cross section are not cast? What is the current level of steel production in India Name the integrated steel plants in India in public sector Name five mini steel plants in India Why are Laval nozzles used in steelmaking/ What are the functions of argon gas in R-H degassing process Is electric steelmaking autogeneous? Explain in brief What are conditions of producing rimming steel ingots Give parameters to judge quality of sponge iron for its usage in electric steelmaking How does preheating of scrap decrease electric consumption in electric steelmaking . VII. XI. VI. V. II. Differentiate between pig iron. VIII.xiv) Austenitic stainless steel is prepared in (a) (b) (c) (d) AOD Converter OBM converter Ladle furnace Open hearth furnace xv) Mould oscillation is used in continuous casting of steel (a) (b) (c) (d) To heal cracks formed on the surface of the casting To obtain good mixing of liquid inside the mould To float out inclusions To avoid rhomboidity of steel 7) State true or false (a) The viscosity of slag increase with decreasing basicity of slag (b) The rail steel is produced in Bhilai steel plant c) Complete solidification of the slag occur in the continuous casting mould d) Fireclay bricks are not resistant to attack by iron oxide e) Calcium is a weak alumina inclusion modifier f) Inclusion engineering involves removal of inclusions from steels g) It is desirable to have martensite in steel for toughness h) Springback phenomenon is associated with the hot working of steel i) In electric steelmaking foaming of slag is desirable j) Rimming reaction in steel is due to evolution of hydrogen during solidification. III. XVIII. XIII. XVI. XX. Why is it necessary to inject carbon in electric steelmaking? What are the future prospects of steel industry in India? What is the cause of Mn reversion in LD steelmaking Why is it necessary to modify alumina inclusions in steel? Is excessive addition of aluminium for deoxidation of steel harmful from metallurgical point of view? What is teelmaking? What is the mechanism of decrease in viscosity of pure liquid silica on addition of CaO Define basicity of slag from ionic slag model Why is it necessary to have excess lime in slag in converter steelmaking? . XIX. XIV.XII. XVII. XV. Assignments based on lecture 9 1) What do you understand by the spalling tendency of a refractory brick? Give reasons. Is it possible to use these bricks without any thermal treatment? 5) High alumina bricks are better than fireclay. which contains 98% SiO2. Why? 7) Why is it necessary to add anti‐shrinkage material for the manufacture of fireclay briskc from naturally ouuurring clay ores? 8) How are insulating bricks manufactured? . 4) Silica bricks are manufactured from a naturally occurring quartzite. Why? 6) High magnesite refractory show good resistance to attack by iron oxide. 2) What is meant by refractoriness under load? What is its importance? 3) Explain the term inversions in relation to the behavior of silica brick on heating and cooling. Why? 7) Why is it necessary to add anti‐shrinkage material for the manufacture of fireclay briskc from naturally ouuurring clay ores? 8) How are insulating bricks manufactured? . Is it possible to use these bricks without any thermal treatment? 5) High alumina bricks are better than fireclay. which contains 98% SiO2. 2) What is meant by refractoriness under load? What is its importance? 3) Explain the term inversions in relation to the behavior of silica brick on heating and cooling. Why? 6) High magnesite refractory show good resistance to attack by iron oxide. 4) Silica bricks are manufactured from a naturally occurring quartzite.Assignments based on lecture 10 1) What do you understand by the spalling tendency of a refractory brick? Give reasons. (06) 2b) Discuss the basis of technological development in the electric arc steelmaking? Explain in detail the contribution of chemical energy in electric steelmaking in reduction of electric consumption. (04) 1b) Carry-over of the basic oxidising slag from the converter is undesirable to the transfer ladle. (08) 2a) Explain the procedure for charging sponge iron in an electric arc furnace. (06) 3b) What are the metallurgical controls adopted in the tundish. 02) 5a)What do you understand by complex deoxidation of steel melt? What are the advantages of complex deoxidation over simple deoxidation (08) 5b Give the evolution philosophy of combined blown steelmaking.Lecture 42: Self Assessment questions 1a) Explain the role of a basic oxidising slag in steelmaking. 05) 4c)What are the principle objectives of oxidizing and reducing periods in electric arc furnace steelmaking? 08) 4d)Enumerate the factors that determine the consumption of electrode in EAF steelmaking. How do you avoid dead zones in the tundish? (08) 3c) Explain the effects of melt superheat and the rate of withdrawl on the quality of the concast billet. (06) 4a)Explain the cause of pipe formation during solidification. Why? How is it possible to minimise the carry-over of slag? (08) 1c) Explain the influence of lance height and the number of nozzles in the lance on the decarburization and dephosphorization reactions in the LD Converter. (08) 2c) What are the advantages of external desiliconization of hot metal prior to steelmaking (06) 3a) Draw a neat sketch of the setup for the continuous casting of steel. How can it be prevented? 05) 4b)Describe the mechanism of solidification of killed steel. Discuss the advantages and limitations of using sponge iron as a charge material. What are the operational benefits of this technology? (06) . Choose the correct answer i) The most powerful dexodizer for steel is (a) (b) (c) (d) Silicon Magnesium Aluminium Chromium ii) Calcium carbide injection is done (a) (b) (c) (d) To remove hydrogen gas To increase calcium content of steel To modify inclusions To remove sulphur from hot metal iii) Calcium silicide injection into liquid steel is done mainly for (a) (b) (c) (d) Deoxidation Inclusion modification Dehydrogenation Dephosphorizarion iv) Hydrogen in liquid steel is dissolved (a) (b) (c) (d) In the form of tiny gas bubbles In atomic form In molecular form In ionic form v) In a good rimming steel (a) (b) (c) (d) Carbon and silicon content should be low Silicon content should be low but carbon cintent should be high Both Carbon and silicon content should be high Silicon content should be high but carbon cintent should be low vi) Activity of P2 O5 in the steelmaking slag is lowered by (a) (b) (c) (d) Increase in slag basicity Decrease in slag basicity Increase in bath temperature None of the above .5c) Explain the sequence of elimination of impurities between top blown and bottom blown converter steelmaking. (06) 6. vii) Sulphide inclusion is steel may be modified by (a) (b) (c) (d) Ca-Si injection CaF2 injection Iron ore fines injection Injection of passivated Mg granules viii) Oxygen lance nozzles are made of (a) (b) (c) (d) Copper Ceramic materials Same type of steel as that of lance Graphite ix) Circulation degassing of liquid steel is carried in (a) (b) (c) (d) VAD refining process VOD refining process RH process Stream degassing process x) Complete deoxidation of liquid steel in necessary for the production of (a) (b) (c) (d) Killed steel Semi-killed steel rimming steels Capped steels xi) In a Fe-Cr-C system. the temperature for preferential decarburization during oxygen lancing is lowered by (a) (b) (c) (d) A reduction in the partial pressure of CO On increasing the partial pressure of CO An increase in the Cr content in the bath Decrease in the Cr content in the bath Xii) Minimum segregation occurs in an ingot of (a) (b) (c) (d) Rimming steel Semikilled steel Killed steel None of the above xiii) Dephophorization is favoured by (a) (b) (c) (d) High temperature and a basic oxidizing slag Low temperature and a basic oxidizing slag Low temperature and an acidic oxidizing slag Low temperature and a reducing basic slag . cast iron and steel Why big ingots with circular cross section are not cast? What is the current level of steel production in India Name the integrated steel plants in India in public sector Name five mini steel plants in India Why are Laval nozzles used in steelmaking/ What are the functions of argon gas in R-H degassing process Is electric steelmaking autogeneous? Explain in brief What are conditions of producing rimming steel ingots Give parameters to judge quality of sponge iron for its usage in electric steelmaking How does preheating of scrap decrease electric consumption in electric steelmaking . VIII.xiv) Austenitic stainless steel is prepared in (a) (b) (c) (d) AOD Converter OBM converter Ladle furnace Open hearth furnace xv) Mould oscillation is used in continuous casting of steel (a) (b) (c) (d) To heal cracks formed on the surface of the casting To obtain good mixing of liquid inside the mould To float out inclusions To avoid rhomboidity of steel 7) State true or false (a) The viscosity of slag increase with decreasing basicity of slag (b) The rail steel is produced in Bhilai steel plant c) Complete solidification of the slag occur in the continuous casting mould d) Fireclay bricks are not resistant to attack by iron oxide e) Calcium is a weak alumina inclusion modifier f) Inclusion engineering involves removal of inclusions from steels g) It is desirable to have martensite in steel for toughness h) Springback phenomenon is associated with the hot working of steel i) In electric steelmaking foaming of slag is desirable j) Rimming reaction in steel is due to evolution of hydrogen during solidification. IV. III. VII. V. 8) Give specific answer to the following I. II. XI. X. IX. VI. Differentiate between pig iron. XX. XIV. XVI. XVIII. XV.XII. Why is it necessary to inject carbon in electric steelmaking? What are the future prospects of steel industry in India? What is the cause of Mn reversion in LD steelmaking Why is it necessary to modify alumina inclusions in steel? Is excessive addition of aluminium for deoxidation of steel harmful from metallurgical point of view? What is teelmaking? What is the mechanism of decrease in viscosity of pure liquid silica on addition of CaO Define basicity of slag from ionic slag model Why is it necessary to have excess lime in slag in converter steelmaking? . XIII. XIX. XVII.