“STUDY OF THE NON METALLIC INCLUSIONS ANDTHEIR EFFECT ON THE PROPERTIES OF STEEL” A Thesis submitted to CHHATTISGARH SWAMI VIVEKANAND TECHNICAL UNIVERSITY Bhilai (C.G.), India For the Award of Degree of Master of Technology In Metallurgy Engineering (Specialization in Steel Technology) By DEEPAK PATEL (QUROOPHQW1R $' Under the Guidance of Dr. VARSHA CHAURASIA Sr. Associate Professor H.O.D. METALLURGY U.P.U.G.P.D. University Teaching Department Chhattisgarh Swami Vivekananda Technical Unversity …………………Bhilai Session 2015 - 2016 DECLARATION BY THE CANDIDATE I the undersigned solemnly declare that the report of the thesis work entitled “Study of the nonmetallic inclusions and their effect on the properties of steel” is based on my own work carried out during the course of my study under the supervision of Dr. Varsha Chauraisa, Sr. Associate Professor and H.O.D., Department of Metallurgy Engineering, U.P.U. Govt. Polytechnic, Durg, (C.G.). I assert that the statement made and conclusions drawn are an outcome of the project work. I further declare that to the best of my knowledge and belief that the report does not contain any part of any work which has been submitted for the award of any other degree/diploma/certificate in this university/deemed university of India or any other country. All help received and citations used for the preparation of the thesis have been duly acknowledged. _____________________ (CANDIDATE) Deepak Patel Roll No. 5005612005 Enroll. No. AD2437 ___________________ (SUPERVISOR) Dr.V. Chaurasia Sr. Associate Professor H.O.D. Department of Metallurgy Engineering, U.P.U. Govt. Poly. Durg C.G. II CERTIFICATE BY THE SUPERVISOR This is to certify that the report of the thesis entitled “Study of the non-metallic inclusions and their effect on the properties of steel” is a record of research work carried out by Deepak Patel bearing Roll No.: 5005612005 & Enrollment No.: AD2437 under my guidance and supervision for the award of Degree of Master of Technology in the faculty of Metallurgy Engineering with specialization in Steel Technology of Chhattisgarh Swami Vivekanand Technical University, Bhilai (C.G.), India. To the best of my knowledge and belief the thesis i) Embodies the work of the candidate herself/himself ii) Has duly been completed iii) Fulfills the requirement of the Ordinance relating to the M. Tech Degree of the University and iv) Is up-to the standard in respect of both contents and language for being referred to the examiners. ___________________ (SUPERVISOR) Dr.V. Chaurasia Sr. Associate Professor H.O.D. Department of Metallurgy Engineering, U.P.U. Govt. Poly. Durg C.G. Forwarded to Chhattisgarh Swami Vivekanand Technical University, Bhilai C.G. _____________________________________________ REGISTRAR CHHATTISGARH SWAMI VIVEKANAND TECHNICAL UNIVERSITY NORTH AVENUE SEC – 8, BHILAI, CHHATTISGARH III ____________________ ____________________ Internal Examiner External Examiner Date: Date: IV .: AD2437 has been examined by the undersigned as a part of the examination and is hereby recommended for the award of the degree of Master of Technology in the faculty of Metallurgy Engineering with specialization in Steel Technology of Chhattisgarh Swami Vivekanand Technical University. Bhilai (C.: 5005612005 & Enrollment No. Roll No.CERTIFICATE BY THE EXAMINERS The Thesis entitled “Study of the non-metallic inclusions and their effect on the properties of steel” submitted by Deepak Patel.). G. I specially would like to thank Professor A. I would like to express my special gratitude to my main supervisor. Lax minarsimham and his team including one of my college friend Ms. They have given me a great insight in both research and production process of world-class quality steel. Thanks to all my friends and c olleagues at the Department of Meta llurgy Engineering U. Her excellent guidance to see the mind of a researcher will always be in my heart. for his encouraging advice and comments. for her advices and encouragements during these years of studies.P.D. Finally.O.D. I am truly grateful to Dr.K. OPJU. for their valuable help throughout all industrial studies. Neelam Sharma. Met.G. fo r his constant support and valuable discussions throughout this work. H. Varsha Chaurasia. Deepak Patel June 2015 V . @ RAIGARH C. regarding help with the industrial visits. I am thankful for the support from JINDAL STEEL & POWER LIMITED. His boundless energy and positive attitude were very impressive to me for completing my work. I sincerely respect his passion for the study and research.D. Verma. for their friendship and kindness. Ashok Srivastava. of Technical Services Department & Qua lity Control Shri B. I would like to express my respect and gratitude to my parents for their continuous trust and love.P.U.G. Dr.O. I also thanks to the VP & H.ACKNOWLEDGEMENT First. as they influence the microstructure and structural properties effectively. reviewing the main methods used for the characterization of inclusions in clean steels. physical appearance or morphology.ABSTRACT Non-metallic inclusions are a major issue during the production clean steels. shape. This is the reason for. analysing and assessment of non-metallic inclusions is important for quality control. the key issue is to control the characteristics of the inclusion population in the liquid steel. size and distribution. therefore many industrial efforts are directed towards improving inclusion removal. experimental reports for information on how steel cleanness is evaluated today. They are often considered as harmful to the final product quality and to the steel processing productivity. and machinability and corrosion resistan ce. In general. composition. The influences of inclusions on the p roperties of steels are dis cussed. toughness. such as qu antity/limit. such as formability. Another way is to use non-metallic inclusions to produce steels with enhanced properties. The main part of this work has been a literature survey. The application of new secondary refining techniques and non-metallic inclusion reduction techniques in steel production processes has greatly reduced the size and amount of nonmetallic inclusions remaining in molten steels and steel products due to which inspection of inclusions is very difficult. In both cases. the higher quality of steel. As inclusions have influence on several properties of steel. and how the steel cleanness is related to the performance of clean steels as a product. the less severe the inclusions. VI . LIST OF ABBREVIATIONS Symbols Units d Maximum particle size ΔG° Free energy of formation K Equilibrium constant T Temperature K T[O] Total oxygen ppm λ Wavelength μm α Coefficient of thermal expansion μm kCal K-1 Element Abbreviations Compound Abbreviations Al C Ca Cu Cr Fe O P Pt Mg Mn N Ni S Si Al2O3 CaO CaO•Al2O3 Alumina Calcia Calcium aluminate CaO •SiO2 Calcium silicate CaS FeO FeO•Al2O3 FeS MgO MnO MgO•Al2O3 MnO•Al2O3 Calcium sulphide Wüstite Hercynite Troilite Periclase Manganosite Spinel MnO•SiO2 Rhodonite MnS SiO2 Manganese sulphide Silica Aluminum Carbon Calcium Copper Chromium Iron Oxygen Phosphorus Platinum Magnesium Manganese Nitrogen Nickel Sulphur Silicon Galaxite Abbreviations ASTM American Society for Testing and Materials BSE Backscattered Electron DIC Differential Interference Contrast EAF Electric Arc Furnace VII . JSPL Jindal Steel and Power Limited IA Image Analysis LCM Laser Confocal Microscope OES Optical Emission Spectrometry OM Light-Optical Microscope ppm parts per million SE Secondary Electrons SEN Submerged Entry Nozzle SEM Scanning Electron Microscope wt% weight percentage IS Indian Standards NMI Non Metallic Inclusion VIII . Figure 2.7: The effect of manganese content on stability of oxide phases resulting from steel deoxidation at 1550ºC (m: mullite.9: Schematic representation of MnO-SiO2-Al2O3 ternary phase diagram Figure 2.2: Free energy of formation for various sulphides. l: liquid manganese silicate) Figure 2.1.12. Figure 2. Figure 2. Schematic drawing of Slab caster tundish furniture Figure 3. Dash-dot line indicates equal sulphur pressure in unit of atmosphere.1: Sources of inclusions in liquid steel Figure 2.4: Deoxidizing power of various elements at 1600 0C Figure 2.10: Morphology of NMI’s occurred in steel Figure 2.11: Schematic representation of mold powder entrapment Figure 2. A schematic diagram of the process route in SMS at JSPL Figure 2.6: Equilibrium relations for manganese-silicon deoxidation of steel at various temperatures Figure 2.LIST OF FIGURES Figure 1. Dash-dot line indicates equal oxygen pressure in unit of atm.3: Free energy of formation for various oxides.5: a) As-polished (2-dimensional) steel sample showing Al2O3 dendrite b) Partial slime extracted (3-dimensional) steel sample showing the same Al2O3 dendrite Figure 2.1: Flow chart of scheme of experiments IX .8: CaO-Al2O3 equilibrium phase diagram. Raigarh Figure 3.3: Scanning Electron Microscope @ JSPL.5: Images acquired using (a) optical microscopy.3: (a) SEM image of inclusion in Heat ID 1 at 799X magnification (b)EDS spectrum of point 3 shown in image Figure 4. (b) laser confocal microscopy.JSPL for inclusion rating Figure 4.1: Force applied by a Wheel on Rail Figure 4.6: Photograph processed by image analysis showing detected area as inclusions (a) Laser confocal microscopy.4: Image analyser attached with optical microscope Figure 3. (c) SEM (secondary electron mode) and (d) SEM (backscattered electron mode) Figure 3.Figure 3.2: Sample images taken @ TSD.2: Light Optical Microscope @ JSPL Figure 3.4: Spectral imaging of inclusion in Heat ID 2 at 3210X magnification X . (b) SEM (backscattered electron mode) Figure 4. LIST OF TABLES Table 2-1: Possible Sources of Inclusion Table 2-2: Stoichiometric composition of reported inclusion phases Table 2-3: Inclusion distribution characteristics in solidified slab samples collected from original and modified design tundish operations Table 4-1: The importance of clean steel with respect to mechanical properties of the product Table 4-2: Chemical composition of Grade 880 rails as per IRS T-12 2009 specifications Table 4-3 Inclusion Rating Results XI . 5 Role of continuous casting CHAPTER 3 EXPERIMENTAL ASPECTS AND METHODOLOGY 34 35 3.1) Introduction 43 4.1 Image Acquisition 38 3. Non-Metallic Inclusions.1.2.1. definition & role 4 CHAPTER 2 2.4 Influence of inclusions on the properties of steel 25 2.2 ) LITERATURE REVIEW 5 Non-metallic inclusions in steel 2.3.2.1 ) 2. Clean steel 3 1.5 Non-metallic inclusions during industrial practice and their control 27 Clean steel 31 2.1.3) Result 48 XII .2) Quantitative Assessment 37 3.1 Classification & Sources of nonmetallic inclusions 6-9 2.1.4 Salient steps adopted during Vacuum Degassing for steel cleanliness 33 2.2.1.2 Formation of nonmetallic inclusions 10-23 2.2.1) Overview 36 3.1.TABLE OF CONTENT: CHAPTER 1 INTRODUCTION 1 1. Need for the Work 2 1.2.2) Experimental procedure 46 4.1 Role of secondary refining on steel cleanliness 31 2.2.2.2.2 Role of Tapping addition on steel cleanliness 32 2.2 Image Analysis 40 CHAPTER 4 RESULT AND DISCUSSION 42 4.3 Morphology of nonmetallic inclusions 24 2.3 Salient steps adopted during secondary refining for Steel Cleanliness 32 2. CHAPTER 5 CONCLUSION AND SCOPE OF FURTHER WORK REFRENCES 50 53 ANNEXURE ATTACHED EXTENDED SUMMARY IS 4163 2004 XIII . CHAPTER – 1 INTRODUCTION 1 . [19] In this thesis. causing fatigue of the steel [S. Chapter 2 deals with the literature survey. 2000]. Mainly the presence of aluminium oxide inclusions is considered as detrimental both for the production process itself and for the steel properties [A. [A.. For example. 2000][19].1 NEED FOR THE WORK In the global steel scenario aimed at superior properties. though precipitates that are formed at stage of solidification.. inclusions have been found to be harmful to the mechanical properties and corrosion resistance of steel. However. The number of inclusions has been variously estimated to range between 1010 and 1015 per ton of steel. In the course of rolling. a microscopic examination cannot faithfully assess cleanliness. Nozzle clogging lead to a declined production. These inclusions take shape during deoxidation of the steel. Partial elimination of the non-metallic inclusions during secondary metallurgy and reoxidation of the steel melt stimulates nozzle clogging at the SEN in continuous casting. [16] As a generalization. at last chapter 5 deals with conclusion and future scope. As a result.K. sources. which is basic for continuous casting. followed by chapter 3 dealing with the experimental aspects containing Quantitative analysis on NMI through SEM. 2 . EDS and Microscope. there is a move to produce clean steel. 2002]. dendrites and aggregates fractures. Challenges like tweaking the chemical composition and the homogeneity have been replaced by troubles triggered by the presence of non-metallic inclusions. At high strains often voids are detected amongst these fragmented particles. due to slower casting rate (since the decreasing diameter) and due certain simultaneous casting disruptions [R. Choudhary. including. chapter 4 is about results and discussion. GHOSH et al. Again. most of the inclusions are submicroscopic. 2011]. no steel can be totally free from inclusions. [21]. control and cleanliness of steel turn out to be more and more dynamic. the yardstick for cleanliness depends on how one assesses it. This is more so for high-strength steels for critical applications. morphology and formation. Its thickness is linked to the volume of steel cast along with the cleanliness of the steel.1. frequently next to the necks and subgrains by virtue of which elongated strings of fragmented particles forms. Dekker’s et al. The accretion of clogged material constitutes significant clusters of NMI. Therefore. inclusion classification. GHOSH et al. Thus non-metallic inclusions vary from the precipitates that are already present in the liquid steel. tundish geometry and casting methods. In some steels this is the most important criteria in judging their quality. has often been considered prima facie evidence of its undesirability. 2. 2012][12] The inclusions are the source of many defects. sulfur.H. Steel cleanliness is optimized by an extensive choice of operating practices right through the steelmaking practices. therefore. nitrogen.1. Tupkary. The steel making process route of JSPL is schematically shown in Fig. Fig. and hydrogen and non-metallic inclusions. incongruent with the metal lattice. covering systems.2 CLEAN STEEL The word “clean steels” is uncertain in class and commonly indicates steel with very low contents of phosphorus. A schematic diagram of the process route in SMS at JSPL 3 . Several applications limits the maximum size of inclusions therefore size distribution of the inclusions is significant. oxygen. These consist of the phase and position of deoxidation and alloy additions. 1. stirring and transfer means. the pros and cons of secondary metallurgy refining. The fact that it is nonmetallic and.1. [R. Steel cleanliness is used to refer relative freedom from the entrapped nonmetallic particles of solid ingot. GHOSH et al. Lange et al. Sulphur.3 NONMETALLIC INCLUSIONS DEFINITION AND ROLE WHAT ARE NON-METALLIC INCLUSIONS? “Compounds of metals (Fe.[A.. anisotropy.. [Kiessling & N. Only 1 ppm each of oxygen and sulphide will still contains 109 -1012 non-metallic inclusions per ton. are termed non-metallic inclusions. The harmful effects of non-metallic inclusions on fatigue properties of steel parts are because they can act as potential sites of stress concentration that can initiate cracks under cyclic loadings. which may be present in steel. since they can initiate ductile and brittle facture. It is widely believed that due to the presence of sulphide and oxide inclusions some of the mechanical properties of steels like ductility. 2000][19] Apart from some applications where inclusions are supposed to be demanded.” ROLE IN STEEL MAKING Non-metallic inclusions are naturally occurring and typically undesired products that are formed into various types depending on their favorable thermodynamic conditions in almost all treatment practices involving molten steels. A beneficial effect on steels properties by nucleating acicular ferrite during the austenite to ferrite phase transformation especially in low carbon steels.1. toughness. they usually deteriorate mechanical properties and surface quality of steel products and could cause nozzle clogging and disruption of steelmaking and forming processes. and formability might be negatively affected. 1978] [14] COMMENTS ON NMI’S: Non-metallic inclusions in steel normally have a negative contribution to the mechanical properties of steel. 4 . Mn. Hydrogen. secondary metallurgy treatments and casting of steel. Phosphorus). Nitrogen. like sulphides for improving machinability (that could be argued with recently available cutting machines and tools). Si) with nonmetals (Oxygen. melting process. The type and appearance of these non-metallic inclusions depends on factors such as grade of steel. CHAPTER – 2 LITERATURE REVIEW 5 . Exogenous inclusions come mainly from reoxidation. Deoxidation products cause the majority of indigenous inclusions in steel. 2012][12] They generally have the most deleterious effect on machinability. and excessive tool wear. Turkdogan. They easily float out. type B: Aluminates.1 CLASSIFICATIONS OF NON-METALLIC INCLUSIONS:Traditionally non-metallic inclusions have been divided into four types (type A: Sulphides. entrained slag. they produce chatter. such as alumina inclusions in low-carbon Aluminium killed steel and silica inclusions in Silicon killed steel.2. and type D: globular Oxides) [18] [WD CALLISTER et al. Indigenous inclusions are deoxidation products or inclusions that precipitate during cooling and solidification. causing pits and gouges on the surface of machined sections.H. Based on the sources of inclusion they can be either indigenous or exogenous. and chemical reactions. 1996][3] 6 . Because they are usually entrapped accidently during teeming and solidification. Exogenous inclusions arise from unintended chemical and mechanical interaction of liquid steel with its surroundings. Consequently. [R. They are generated by the reaction between the dissolved oxygen and the added deoxidant. In machining.1 NON METALLIC INCLUSIONS IN STEEL 2.1: Sources of inclusions in liquid steel [E. and mechanical properties because of their large size and location near the surface.1. such as aluminium and silicon. lining erosion. 2003]. exogenous inclusions are sporadic. frequent breakage. Tupkary. surface quality. so they only concentrate in regions of the steel that solidify rapidly or where their escape by fluid transport and flotation is hampered. they are often found near the ingot surface Fig 2.T. type C: Silicates. [A. 2000][19] 7 .. GHOSH et al.Table 2-1 Possible Sources of Inclusion. 02% while having sulphur content at around 0. Calcium aluminate (CaO•Al2O3) type inclusions are also considered complex oxide inclusions. due to manganese’s stronger affinity for sulphur. where metal A has +2 oxidation number and metal B has +3 oxidation number. which effectively lower the melting temperature of inclusions from 2293K to around 1700K. Cr2O3. usually higher than steelmaking temperature of 1873K. As liquid steel cools. Sulphides Sulphide inclusions are important to consider since it is common to have steel with oxygen content less than 0. TiO2 • Complex oxides. Complex oxide inclusions are sometimes known as spinel type (MgO•Al2O3) inclusions for their similarity in structures. to form MnS (Tm = 1870K). MnO•Al2O3.03%.CLASSIFICATION BASED ON INCLUSION CHEMISTRY AND COMPOSITION:Oxides In general. but do not form spinel structures due to their relatively large ionic radius. sulphur segregates and forms FeS with melting point of 1460K. Spinel type inclusions are characterized by faceted structure and high melting temperature. FeO•Cr2O3. SiO2. Spinel inclusions are especially harmful during steel processing as they do not deform during hot rolling and often cause poor surface finish. MgO•Al2O3. Calcium and barium. Al2O3. oxide inclusions can be classified into: • Single oxides. The Sulphur affinity of various elements can be compared with 8 . MnO. some common examples: FeO. Examples of common sulphide inclusions include MnS. (Mn. MnO•Cr2O3 [Kiessling and Lange et al. Fe2O3. 1978][6]. Some common examples are FeO•Al2O3. Fe)S and CaS. FeS often causes embrittlement of steel during heat treatment. FeS. Liquid steel has a high solubility of sulphur where solid steel usually has significantly lower sulphur solubility. the usual Al2O3 inclusions are modified to calcium aluminates. Therefore it has become a common practice to add sufficient amount of Mn. have +2 oxidation number. With common calcium treatment practice. often takes the general form of AO•B2O3. Types of sulphide inclusions will a l s o d e p e n d o n m a n g a n e s e t o S u l p h u r r a t i o . H.2 gives a plot of curves for common elements found in steelmaking. Figure 2.1. 9 . etc. Dash-dot line indicates equal sulphur pressure in unit of atmosphere [4]. Two morphologies are frequently observed: • Globular: Both simple sulphides and oxysulphides. titanium. BN. or calcium.2: Free energy of formation for various sulphides. VN. [R. nitride inclusion contents in steel are significantly less than that of oxides and sulphides. • Faceted: Often appears in steel heavily deoxidized with aluminum. Tupkary .free energy of sulphide formation. Nitrides In the presence of elements having high affinity for nitrogen. Like carbides. where the latter consists of sulphides and oxides coexisting in one inclusion. 2012][12] can form as a result of molten steel contacting with air atmosphere during unprotected vessel transfer. TiN. Figure 2. ZrN. nitrides such as AlN. This type of morphology is generally present in silicon killed or semi-killed steel using aluminum. Oxides. Solid metals and alloys have lower solubilities for solutes as compared to those for liquids. The decrease in the temperature of liquid steel in the mold during freezing shifts the reaction equilibria in favor of the formation of oxides and sulfides. As a consequence. The driving force is supersaturation of solutes leading to precipitation of reaction products. the supersaturation arises for the following reasons: 1. This can be generally understood from the Ellingham diagrams. Even nitrides and carbides have been found to form. Some oxygen is invariably picked up during teeming. and subsequent reactions during solidification occur on them. viz. 2 Al + 3 O = Al2O3 (s) 2. Here. However. the occasional addition of deoxidizers. The phenomenon is known as segregation. nucleation is not required. and the growth of inclusions occurs without the need for appreciable supersaturation. most experimental observations indicate that an abundance of nonmetallic particles are always present. The cause of supersaturation in a ladle is the addition of deoxidizers to the bath. Also. We may consider the specific case of deoxidation of steel by aluminum. and some oxysulfides are typical products. This assumption constitutes the basis for thermodynamic analysis of inclusion formation. such as aluminum shots. into the mold is practiced.1.2. that is not the situation in the mold. sulfides.2 FORMATION OF INCLUSIONS DURING SOLIDIFICATION Inclusions form during solidification by chemical reactions. As far as the kinetics of inclusion formation is concerned. 10 . which is one of the casting defects. 3. This causes rejection of solutes by the solidifying material into the melt at the solidliquid interface and leads to nonuniform chemical composition in the cast material. Deoxidation is commonly carried out by additions of elements having greater affinity for oxygen than iron. at temperature slightly below its melting point. [wt%C] • [wt%O] = ~0. approaches zero. this method is also known as precipitation deoxidation [17]. The oxygen affinity of various elements can be compared with free energy of oxide formation. Equation 2-1 describes carbon-oxygen relationship in iron up to 0. 1996] [3]. The oxygen solubility in solid iron.1. majority of dissolved oxygen will precipitate as FeO inclusions.4 depicts the deoxidizing power of various elements at 1600 ‘C 11 . Figure: 2. it is also important to consider that activity of these elements in solution with liquid steel deviates from that of the pure elements. While elements having free energy of oxide formation lower than FeO are potential candidates as deoxidizers.0023 [2-1] In order to prevent blowhole (carbon monoxide gas) formation.STEEL DEOXIDATION Maximum solubility of oxygen in liquid iron at the eutectic of 1527ºC is about 0.1.16% [E. porous cast product.T.6% carbon. Figure 2. In steel. Turkdogan. the presence of alloying elements such as carbon can influence the dissolved oxygen content. THERMODYNAMICS OF DEOXIDATION The role of deoxidation process is to lower the oxygen content in liquid steel.2 gives a plot of curves for common elements found in steelmaking. Upon solidification. or precipitation of FeO inclusions in sizeable quantities. liquid steel must be deoxidized prior to casting [12]. 4: Deoxidizing power of various elements at 1600 ‘C [5] 12 . Figure 2.Figure 2.3: Free energy of formation for various oxides. Dash-dot line indicates equal oxygen pressure in unit of atm [4]. and therefore are characterized by a dendritic structure. Carbon deoxidation does not generate inclusions and therefore will not be discussed further. 1978][6]. having higher melting temperature than steel. On the other hand. during the casting process. M is the dissolved deoxidizer. however. would solidify before steel. for steel containing more than 0. O is oxygen. The manganese deoxidation reaction. Mn is often introduced to steel in the form of low C or high C ferroalloy. [Mn] + [O] = (MnO) [2-3] 13 . For inclusions with MnO content of up to 30%. A detailed study by [Lismer and Pickering] [7] has revealed that Mn deoxidation products are typically small and homogeneously distributed in the steel and the morphology of this inclusion type is mostly influenced by the MnO-FeO ratio. These inclusions rich in FeO had solidified after the matrix steel was solid. Mn and Fe will both participate in the deoxidation reaction forming MnO-FeO product in liquid or solid solutions. carbon in liquid steel may reduce oxide inclusions resulting in gas formation and pinhole porosity [Kiessling and Lange et al..7%Mn. x[M]steel + y[O]steel = (MxOy) [2-2] MANGANESE DEOXIDATION Manganese. and aluminum. f o r m i n g g a s e o u s d e o x i d a t i o n products. Nearly pure MnO inclusions. is rarely utilized as a deoxidizer. the morphology was globular single-phase or sometimes dual-phase spheres. where x and y are stoichiometric terms. manganese. A general deoxidation reaction can be described using Equation 2-2. Carbon is often c o n s i d e r e d a n e f f e c t i v e d e o x i d a t i o n e l e m e n t . in pure form. it was found that the deoxidation products are mostly pure MnO. silicon.SINGLE COMPONENT DEOXIDATION Four cost-effective deoxidizers are carbon. 1978] [6] . Low quartz. Deoxidation with pure silicon will yield either liquid iron silicates or solid silicon oxide as reaction products at steelmaking temperature.88 x 10-2 at 1600ºC [2-6] SILICON DEOXIDATION It can be seen from Figure 2. Iron silicate inclusions. silicon has a much-improved deoxidizing power compared with manganese.where tridymite and cristobalite are high temperature modifications of silica. and cristobalite are among the common modifications [Kiessling and Lange et al. The given reaction time and temperature during ladle treatment are inadequate for the transformation of quartz to tridymite or cristobalite to reach completion. like many other silicates. do not transform to quartz within the time-frame of subsequent cooling and casting of steel. high-quartz. Silicon oxides within steel exist in several modifications as a result of various possible spatial arrangements of the SiO2 tetrahedral molecules.1. On the contrary.and corresponding equilibrium constant equation. %Mn %O 12440 For 5. often formed as deoxidation product. tridymite. tridymite and cristobalite. are usually glassy in appearance and globular in morphology. 14 . the value of the equilibrium constant for manganese deoxidation is = %Mn %O = 4.4. However. low quartz-high quartz transformation as well as tridymite-cristobalite transformation are fast and can be easily reversed. the transformation between quartz and tridymite or cristobalite is a much slower process as the energy associated with breaking the tetrahedral bonds are greater. Therefore. the type of modification and composition can be utilized as indicators for assessing silica inclusion’s origin.33 [2-4] [2-5] = 1. Due to similar structures. [2-8] % % 30000 T For 2 11. [8] that Al2O3. [Kiessling and Lange et al.4. follows dendritic growth pattern as shown in Figure 2-4. usually having the particle size of 1 to 5 Pm.1. Imodification). Corundum inclusions. It has been reported by [ Rege et al]. there are generally two species of deoxidation products: solid hercynite (FeO-Al2O3 spinel) and solid corundum (Al2O3. clusters of these particles tend to remain as inclusions in steel.The silicon deoxidation reaction.5 [2-9] = 1. Corundum phase is characterized by having unique faceted shapes and relative smaller diameter as single particles. have a tendency to agglomerate upon colliding with one another in order to lower the overall contact area with molten steel and therefore effectively stabilize the entire unit by minimizing the surface energy. [2-7] [Si] + 2[O] = SiO2 (s) and corresponding equilibrium constant equation. For steels deoxidized solely with aluminum. In aluminum deoxidized steel. 1978] [6] 15 . Among the two deoxidation products. corundum is the dominant species found in steel. during deoxidation.26 x 10-5 at 1600ºC [2-10] ALUMINUM DEOXIDATION From Figure 2. the value of the equilibrium constant for silicon deoxidation is = % % = 2. Į-Al2O3 products are formed. it is clear that Aluminum is one of the most effective deoxidizers used for steel deoxidation. Manganese and silicon deoxidation products as well as emulsified furnace slag and eroded refractories can serve as low-energy sites for Al2O3 inclusions to nucleate without reaching super-saturation in the bath. therefore.Figure 2. Nucleation and growth on existing nuclei: The existing nuclei can be both indigenous and exogenous in nature. The resulting inclusions are finely dispersed corundum clusters [Kiessling and Lange et al. This phenomenon is mainly caused by solid alumina inclusions having high contact angles with liquid steel. 1978] [6] II. Nucleation by super-saturation: Al2O3 inclusions nucleate homogeneously in the steel bath as a result of super.5: a) As-polished (2-dimensional) steel sample showing Al2O3 dendrite b) Partial slime extracted (3-dimensional) steel sample showing the same Al2O3 dendrite [1] Solid deoxidation products are often associated with nozzle clogging during casting of liquid steel. 16 .saturation. Indigenous inclusions from aluminum deoxidation may take on different morphology depending on the generation mechanism. alumina inclusions will readily anchor onto refractory surfaces followed by subsequent agglomeration of inclusions. There are generally three Al2O3 inclusion generation processes: I. the value of the equilibrium constant for aluminum deoxidation is = % % = 9. % 62780 T For 2 3 [2-12] % 20.III.5 [2-13] = 1. therefore the products are partly molten Al2O3 inclusions sometimes having glassy appearance. The aluminum deoxidation reaction.58 x 10-14 at 1600ºC [2-14] MULTI-COMPONENT DEOXIDATION In conventional ladle deoxidation. This practice has many advantages: (1) promotes the formation of low-melting-point deoxidation products with ease of removal from the melt. Reactions that occur under localized superheat may reach the melting point of Al2O3. giving much lower residual oxygen in the bath. (3) minimizes nitrogen pick-up during furnace tapping[4]. (2) improves the solubility of elements having relative high vapor pressure such as calcium and magnesium. It is a common practice to perform partial deoxidation while filling the tap ladle followed by final killing of steel with aluminum at the ladle furnace station. R e a c t i o n between aluminum metal and oxygen: Excess aluminum addition or poor homogenization of the bath can lead to local high concentration of aluminum metal reacting with dissolved oxygen. a combination of deoxidizers are utilized to achieve improved deoxidation result. 2[Al] + 3[O] = Al2O3 (s) [2-11] and corresponding equilibrium constant equation. 17 . SILICON-MANGANESE PARTIAL DEOXIDATION Figure 2. Two general types of deoxidation products may result from Si-Mn deoxidation: solid silica and liquid manganese silicate at the steelmaking temperature.1. which govern the type of deoxidation products formed. Under the influence of increasing manganese content. It was suggested by [ Turkdogan. deoxidation products deviate from pure silica to molten manganese silicate.6. As shown in Figure 2. for liquid steel containing higher manganese content (right of the curve) the primary deoxidation products are likely to be liquid manganese silicate. On the other hand. the activity of silica is lowered.6: Equilibrium relations for manganese-silicon deoxidation of steel at various temperatures [3] The practice of tap ladle deoxidation can effectively improve the extent of deoxidation and at the same time minimize aluminum deoxidizer additions. for steel compositions left of the curve. 1996][3] that there exist critical ratios of [%Si]/[%Mn]2 at a given temperature. the deoxidation products will be solid silica which indicates the absence of manganese participation in the reaction. As the activity of silica decreases. 18 . On the other hand. [Si] + 2MnO = 2[Mn] + SiO2 [2-15] and corresponding equilibrium constant equation.1. The p h a s e s o f r e s u l t i n g d e o x i d a t i o n p r o d u c t s d e p e n d h e a v i l y o n s t e e l chemistry and reaction temperature as illustrated in Figure 2. manganese is added as an inclusion modifier yield liquid manganese silicates for improved coalescence and flotation to the slag layer. In the absence of manganese. only solid phases such as silica. as well as aluminum. [2-16] 1. the stability range of liquid manganese silicate also increases with increasing manganese content. alumina and mullite are possible. To facilitate the removal of deoxidation products.liquid manganese silicate becomes stable. with manganese participating in steel deoxidation.7. 19 . the fourth phase . % % 1510 T . or silicomanganese. The charge deoxidizers often consist of all three deoxidizers. it is common to charge deoxidizers into the tapping ladle during ladle filling. manganese and silicon in the form of ferromanganese.The equilibrium reaction governing Mn/Si deoxidation. MANGANESE-SILICON-ALUMINUM DEOXIDATION In modern practice.27 [2-17] The Mn/Si deoxidation products are typically found to be globular and glassy in appearance along with silica or rhodonite precipitation within the matrix of manganese silicate. ferrosilicon. however. the primary 20 . The outer glassy MnO-Al2O3-SiO2 matrix. These precipitates can nucleate easily on small steel particles or solidified slag droplets within the inclusion. limited solubility in steel (0. The primary deoxidation products are therefore calcium silicates. The challenge. was often found to precipitate phases such as mullite. and it is usually added as various iron-containing Ca-Si alloys. it can be seen that calcium has a strong affinity to oxygen and could potentially be utilized as steel deoxidizer. l: liquid manganese silicate) [9] Liquid silicates. are characterized by an aluminum-rich core and a shell of gradual increase in MnO-SiO2 content towards steel-inclusion interface. and high vapor pressure at 1600ºC (1. galaxite.032% Ca at 1600ºC). CALCIUM MODIFICATION From Figure 2. in this deoxidation process.Figure 2.81atm) [OTOTANI et al. and corundum lathes upon cooling in solid state. which may also contain other oxides. lies in the following properties of calcium: low boiling point (1439ºC).1. When combinations of Ca and Al or Mn/Si deoxidation are carried out. 1986] [10].7: The effect of manganese content on stability of oxide phases resulting from steel deoxidation at 1550ºC (m: mullite. it is rather difficult to introduce calcium to molten steel in its metallic form. Due to these reasons.3. in metastable condition. 3CaOxAl2O3 and CaOxAl2O3 are liquid. Hence. By converting the solid alumina inclusions to liquid calcium aluminates. in alumina inclusions.8) and therefore exists in the liquid state at steelmaking temperatures. the extent of deoxidation can be improved from 8-10ppm O to 1ppm O in Alkilled steel (0. Figure 2.deoxidation products can be modified to oxides with lower activity and hence improve the removal of dissolved oxygen. liquid calcium aluminates will coalesce upon contact due to better wetting with liquid steel and will not easily attach onto refractory surfaces.8: CaO-Al2O3 equilibrium phase diagram.1. calcium treated Al2O3 can reach a melting point of 1360ºC at the CaO-Al2O3 eutectic (Figure 2. With a CaO:Al2O3 ratio of 12:7. solid deoxidation products can also be calcium treated so that the steel casting process is clogging-free. 12CaOx7Al2O3.05% Al)[S MILLMAN.8. Moreover. while CaOx2Al2O3 and CaOx6Al2O3 are solid at steelmaking temperatures. there exist five modifications of calcium aluminates as indicated in Figure 2. 2004] [9]. 21 . [19] Instead of agglomerating.1. MnO. Si. Figure 2. Figure 2. with the exception of FeO-SiO2 (counterpart to MnO22 . Considerable numbers of MnO-SiO2Al2O3 inclusion phases exist with complete or part substitution of MnO with FeO due to wide range of solid solubility.9: Schematic representation of MnO-SiO2-Al2O3 ternary phase diagram[6] Other inclusion systems such as FeO-SiO2-Al2O3 and MnO-SiO2-Cr2O3 share many similarities with the MnO-SiO2-Al2O3 system.MANGANESE OXIDE – SILICON OXIDE – ALUMINUM OXIDE SYSTEM The MnO-SiO2-Al2O3 system effectively covers most of relevant inclusion phases that result from combination of Mn. and Al deoxidation.1.9 summarizes many complex inclusions having compositions made up of various SiO2. and Al2O3 primary oxide contents. It is important to note that each inclusion species will have its own homogeneity range in addition to stoichiometric compositions listed in Table 2-1. SiO2). manganese has a stronger affinity for oxygen than iron and therefore it is also common to find MnO among inclusions belonging to the FeO-SiO2-Al2O3 system.SiO2 54 46 -- Tephroite 2MnO. Al2O3 and Cr2O3 are interchangeable at elevated temperatures due to their structural resemblance. [Kiessling and Lange et al.SiO2 70 30 -- 23 .Al2O3 41 -- 59 Mullite 3Al2O3. Corresponding inclusion phases were often reported in both MnOSiO2-Al2O3 and MnO-SiO2-Cr2O3 with notable difference in the absence of ternary phases in the MnO-SiO2-Cr2O3 system[SOLMAN AND EVANS. and MnO-SiO2-Cr2O3. On the other hand. According to Figure 2. which has yet to be reported as an inclusion phase in the literature. SiO2 -- 28 72 Rhodonite MnO. FeO-SiO2-Al2O3.3.1. Corresponding phases relating to MnO-SiO2-Al2O3. 1951][5]. Table 2-2: Stoichiometric composition of reported inclusion phases. 1978][6] Mineral classification Chemical formula Stoichiometric composition (wt%) MnO SiO2 Al2O3 Corundum Al2O3 -- -- 100 Cristobalite SiO2 -- 100 -- Tridymite SiO2 -- 100 -- Quartz SiO2 -- 100 -- Manganosite MnO 100 -- -- Galaxite MnO. Morphology of dendrite shaped inclusions may be improved by addition (after deep deoxidation by aluminum) of small amounts of rare earth (Ce. Steels deoxidized by aluminum contain manganese sulfides and oxysulfides in form of thin films (platelets) located along the steel grain boundaries.2.3 MORPHOLOGY OF NON-METALLIC INCLUSIONS:- Globular shape of inclusions is preferable since their effect on the mechanical properties of steel is moderate. iron aluminates and silicates.1. toughness and fatigue strength of the steel part. Such inclusions are formed as a result of eutectic transformation during solidification. Mg) elements. Platelet shaped inclusions. Spherical shape of globular inclusions is a result of their formation in liquid state at low content of aluminum. These inclusions have melting point higher than that of steel. They considerably weaken the grain boundaries and exert adverse effect on the mechanical properties particularly in hot state (hot shortness). Due to their more globular shape polyhedral inclusions exert less effect on the steel properties than dendrite shape inclusions. Dendrite shaped inclusions. Platelet shaped inclusions are most undesirable.La) or alkaline earth (Ca. 24 . Sharp edges and corners of the dendrite shaped inclusions may cause local concentration of internal stress. which considerably decrease of ductility. Excessive amount of strong deoxidizer (aluminum) results in formation of dendrite shaped oxide and sulfide inclusions (separate and aggregated). Polyhedral inclusions. Examples of globular inclusions are manganese sulfides and oxysulfides formed during solidification in the spaces between the dendrite arms. then microcracks develop. 2. fatigue strength. 2. Property factors: deformability and modulus of elasticity at various temperatures. an inclusion/matrix interface has a mismatch. and this led Kiessling [6] to develop the idea of critical size. The factors responsible for these may be classified as follows: 1.4 INFLUENCE OF INCLUSIONS ON THE PROPERTIES OF STEEL The properties that are adversely affected are fracture toughness. shape (may be designated as the ratio of major axis to minor axis). Application of external forces during working or service can augment it. Geometrical factors: size. 1978][6].Fig.10: Morphology of NMI’s occurred in steel [Kiessling and Lange et al. it is customary to divide inclusions by size into macroinclusions and 25 . Investigations have established that only large inclusions are capable of doing this kind of damage. impact properties. 2. and total volume fraction of inclusions. This causes local stress concentration around it.1. coefficient of thermal expansion From a fundamental point of view. In practice. The propagation of microcracks leads to fracture. and hot workability. If the local stress becomes high. size distribution. The hot workability of steel is affected by the low deformability of inclusions (i.. Macroinclusions of sulfides are desirable for better steel machining properties.5 mm). and properties. especially those that have lower coefficients of thermal expansion than steel.. we first have to know how various factors connected with inclusions affect the properties of steel. To sum up the effects. including service requirements. its size will be very small. This is known as inclusion modification. for example. restrict grain growth. Technologically. Broadly speaking. Brittle inclusions or inclusions that have low bond strength with the matrix break up early during straining. Therefore. spherical inclusions are better. we have to put up with some macroinclusions. 3. Impact properties are adversely affected with an increase in volume fraction as well as inclusion length. should be to produce steel that does not contain any macroinclusion (i. this is difficult to achieve without escalating the cost to a high level. Macroinclusions ought to be eliminated because of their harmful effects. nitrides. The fatigue strength of high-strength steel is reduced by surface and subsurface inclusions. These set up stresses in the matrix and are primarily responsible for fatigue failure. They can. 5.microinclusions. However. 26 . more brittleness at hot working temperatures). [19] It decreases with an increase in yield stress. it is in the range of 5 to 500 μm (5 × 10–3 to 0. and in this context we have to determine how to reduce their harmful effects by controlling their size. the following statements may be made: 1. etc. shape. Anisotropy of a property is caused by orientation of elongated inclusions along the direction of working or the elongation of inclusions during working. The critical inclusion size is not fixed but depends on many factors. the presence of microinclusions can be tolerated. 4. therefore. and act as nuclei for the precipitation of carbides. above the critical size).e. Kiessling advocated the use of fracture mechanics concepts for theoretical estimation of the critical size for a specific situation. since they do not necessarily have a harmful effect on the properties of steel and can even be beneficial. 2. The objective. increase yield strength and hardness.e. and to carry it out. with the initiation of voids at the inclusion/matrix interface. In high-strength steels. 1. Quaternary inclusions: generated during solid state phase transformation.2. are results of reoxidation. such as deoxidation products. In addition. simple detection methods (due to larger size) as well as fewer occurrences have reduced the concern for exogenous inclusions significantly [12]. the amount of exogenous inclusions can be minimized. Secondary inclusions: generated due to equilibrium shift as temperature decreases during vessel transfer. Although exogenous inclusions are generally more harmful than indigenous inclusions. usually characterized by rapid cooling IV. are generated by chemical reactions between dissolved species in the steel bath and are generally smaller in size [12]. as summarized below: I. Tertiary inclusions: generated during the process of solidification. Indigenous inclusions often go through a series of transformations as the steel cools from 1600°C to room temperature. slag entrainment and refractory erosion.[19] While trying to maintain equilibrium with the surroundings. Indigenous inclusions can therefore be categorized into formation steps. or solidify and take the form of supersaturated solid solution. with careful control of stirring and flowrate monitoring. which causes changes in solubility limits of various constituent. inclusions may be undercooled during some steps of the treatment and result in amorphous phases. Primary inclusions: generated during deoxidation reaction II. The presence of a few large indigenous inclusions has a strong effect on the properties of steel products. • Indigenous inclusions.5 NON-METALLIC INCLUSIONS DURING INDUSTRIAL PRACTICE AND THEIR CONTROL @ JSPL There are generally two sources of inclusions in steel: exogenous. indigenous. 27 . Deoxidation products originate from the reaction between dissolved oxygen and added deoxidant and can be both solid and liquid at steelmaking temperatures. • Exogenous inclusions. usually larger in size. such as tapping and teeming operations III. the following factors affect slag entrainment into the molten steel: Vortexing effect in tundish during end of casting results slag entrainment into the solidified strand. The process of mould Slag entrapment due to level fluctuation illustrated in Fig 2. 1996][3]. 2.Exogenous inclusions are the real cause of concern during continuous casting. including well block sand. is a very common source of large exogenous inclusions which are typically solid and heavier in nature. broken refractory brickwork and ceramic lining particles. Emulsification and slag entrainment at the top surface especially under gas stirring above a critical gas flow rate. Fig.1. These particles flushed out with liquid metal and got entrapped into the solidified strands. Air is the most common source of re-oxidation. arise primarily from the incidental chemical (re-oxidation) and mechanical interaction of liquid steel with its surroundings (slag entrainment and erosion of lining refractory) [TURKDOGAN. which comes into contact with molten metal during casting when it is poured from ladle to tundish and tundish to mold. Severe mould level fluctuation also leads to mould powder entrapment into solidified strands.11: Schematic representation of mold powder entrapment [3] 28 . SOURCES OF EXOGENOUS INCLUSIONS For continuous casting process. Turbulence at the meniscus in the mold.11 Erosion of refractories. loose dirt. [16] Shroud straightness and Argon flow rate are important parameters. Free opening of the ladles are also monitored regularly to avoid such occurrences. Special type of pre-heated Zirconia based Nozzlex powders are used to ensure Free Opening of the ladle [16]. Any abnormal conditions results excessive re-oxidation followed by formation of large indigenous inclusions and nitrogen pick up in final steel. Even at the end of the casting and during sequencing the efforts are made to keep the tundish level constant. Tundish levels are also maintained at a constant level throughout the casting duration to avoid vortexing of slag. Starting of the casting is considered to be the most unsteady state of casting.0 ppm nitrogen from ladle to final steel. Special gaskets are also being used at the joints of shrouding to avoid air ingression. Shroud submergence depth is ensured >150mm to avoid opening of eye during Ar shrouding. Auto Mould Level Controllers are in place in all the casters to take care of mould level fluctuations during casting operation to avoid mould slag entrainment. which are monitored continuously to avoid air ingression from joints and slag eye formation. Hydraulic shroud manipulator assembly is installed in shrouding system for tight sealing of the shrouds and it helps to minimize nitrogen pick up during casting. The study reveals average pick up of 4. Without Free opening cases are the most vital sources of inclusion due to re-oxidation of steel due to use of oxygen to open the ladle nozzles and casting without ladle shroud. which is an indicator of minimal re-oxidation of steel. 29 . Tundish level also gone down.To avoid such occurrences following steps are adopted during continuous casting: Metal in ladle is fully covered with ladle covering compound like Radex and the ladle is also covered with lid during casting to minimize heat loss and gaseous entrapment. Al2O3 base ladle shroud is used with Argon shrouding between ladles to tundish. if the next ladle in sequence could not open without free opening.05. A rigorous Water Modeling study and mathematical modeling was conducted for slab caster and Combination caster tundish to improve yield and cleanliness of steel.12. Fig 2.12 illustrated the modified design of the slab caster tundish with use of different type of furnitures for flow modification.1. Flow control in the tundish is the key to the production of clean steel. Different combinations of pouring boxes and permanent dams are used for different tundish at JSPL. Schematic drawing of Slab caster tundish furniture 30 . Pouring boxes helps in upward directional flow supports inclusion floatation and assimilation into tundish slag.Expendables and permanent dams c. Weirs and d. dampen turbulence in the shrouding areas and to provide directional flow of metal in order to provide nearly identical residence time to all strands in multi-strand tundish. Different types of flow modifiers are used in the tundish after doing mathematical modeling and water modeling of the tundish a.Slotted dams. Fig 2. These flow modifiers are invariably employed to protect excessive weir of tundish refractory. Pouring box b. 1 Clean Steel: Role of Secondary Refining The cleanliness of steel depends right from selection of charge mix. Steel cleanliness is a widely spread area and secondary refining only plays a part of the entire process for production of clean steel. It is a combination of all the processes with stringent quality standards and SOP’s at every stages of steel making. killing practices and subsequently on secondary refining process [19].2 CLEAN STEEL 2. Table 2-2 illustrated the inclusion level before and after modification of slab caster tundish.The inclusion rating of the collected samples from tundish before and after modification clearly shows improvement in steel cleanliness after incorporation of the pouring box in the slab caster tundish. Secondary refining alone cannot be the process. Table 2-3: Inclusion distribution characteristics in solidified slab samples collected from original and modified design tundish operations 2. right from selection of input raw material to end of casting decide the final quality of the steel. 31 . which can helps in producing Clean Steel. primary refining process.2. pre-conditioned Synthetic slag and lime is added.2. In addition to this freshly prepared lime also added during tapping for initial slag formation and for effective desulphurization. During tapping Si-Mn. Al ingots.2. first the SOP’s are made for all the areas from EAF to caster for finalization of the procedure to be followed for the production of steel. It is planned for 80% additions of the major ferroalloys must be completed during tapping itself.2. The basic objective of controlled tapping addition is to lower down oxygen potential at opening of secondary refining for ensuring effective desulphurization and to reduce total processing time. The quantity of coal based DRI is reduced by design and % of Hot metal and HBI is increased proportionately. 2. selective charge mix are designed for the Electric Arc Furnace.3 SALIENT STEPS ADOPTED DURING SECONDARY REFINING FOR STEEL CLEANLINESS The tapping additions are designed in such a fashion that during secondary refining only trimming additions are required to achieve the aim chemistry.2 ROLE OF TAPPING ADDITION ON STEEL CLEANLINESS At JSPL. The grade specific tapping additions are designed for initial killing of the bath. The opening Aluminum is maintained around 32 . The in-built Eccentric Bottom tapping facility in EAF helps in 100% slag free tapping. Mild purging with Argon after tapping carried out to ensure minimum air entrapment. To give a preferential Carbon boil 100 kg of CPC also added at the bottom of the ladle just before tapping. For clean steel. Trimming additions were carried out in the initial period of processing along with vigorous purging for effective desulphurization Addition of lime is restricted to 2-3 kg/ton during secondary refining to avoid unwanted Hydrogen pick up in steel. 4 SALIENT STEPS ADOPTED DURING VACUUM DEGASSING FOR STEEL CLEANLINESS The steel cleanliness is largely depends on inclusion level in final steel and final gaseous content in final steel is considered to be an indirect measure of steel cleanliness. For Vacuum degassed heats after degassing Calcium silicide treatment is carried out followed by mild rinsing for three minutes for effective floatation of the inclusion [19].4. Boiler grades. During mild rinsing it is ensured that slag eye should not be opened. The steel is hold under vacuum level at < 1. To increase the inclusion absorption capacity of slag. Mild Argon rinsing without opening of slag eye for minimum three minutes at the end of processing is ensured after Calcium silicide treatment. (FeO + MnO) % is monitored in slag and it is maintained below 1.0%. For critical applications like wire drawing. Line pipes. This helps in effective slag metal interaction for removal of inclusion from steel.0-4. Seamless pipes.08 which ensure formation and subsequent floatation of Calcium aluminates. Calcium Silicide treatment is carried out at the end of processing to achieve a minimum Ca/Al ratio of 0.0 at end of secondary refining. Depending on customer requirements and based on the end application of the steel process route is decided.0.0 mbar for min 10 minutes to achieve the favorable gaseous level and inclusion level in steel [17]. 2.06% during start of secondary processing to avoid further additions Aluminum in subsequent process.0 and (FeO+MnO) % 33 . Forging. For effective desulphurization the slag basicity also is maintained at 2.5 . JSPL is having the facilities of Vacuum Tank degasser and RH degasser both in steel manufacturing units. Oxygen potential in final steel is considered to be an indirect measure of steel cleanliness.2. Slag basicity is maintained 3. Fasteners grades and Automobile grades are routed through vacuum degassing.04-0. size distribution.0%. The Celox reading for dissolved oxygen for vacuum degassed heats aimed at 4. because large macro-inclusions are the most harmful to mechanical properties though the large inclusions are far outnumbered by the small ones.in slag positively maintained below 1.2. The inclusion size distribution is particularly important. their total volume fraction may be larger [19]. their morphology. composition and morphology of inclusions and precipitates. The source of most fatigue problems in bearing steel are hard and brittle oxides. especially large alumina particles over 30μm [18] . Ductility & impact toughness is appreciably decreased by increasing amounts of oxide or sulphide inclusions.0 ppm max. This also describes in detail about various measures adopted during Continuous Casting to avoid the occurrences of the above problems. The rest of this report is an extensive review on sources of inclusions during continuous casting.5 CLEAN STEEL: ROLE OF CONTINUOUS CASTING Non-metallic inclusions are the most significant cause of concern in cast steels which can lead to field failures. Inclusions also lower resistance to Hydrogen Induced Cracks. 34 . 2. and sources of gaseous ingression in steel during casting. Mechanical behavior of steel is controlled to a large extent by the volume fraction. which act as stress raisers. CHAPTER – 3 EXPERIMENTAL ASPECTS AND METHODOLOGY 35 . which ultimately leads to the construction of inclusion particle size distribution by image analysis method. the experimental approach was divided into two parts: qualitative and quantitative aspects. The experimental approaches are summarized in fig 3-1 36 . Quantitative assessment involves the inclusion detection and size determination. To do this.3. Qualitative assessment involves inclusion morphology examination and inclusion type determination by sample preparation and analytical techniques such as scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS).1 OVERVIEW The main purpose of this study was to characterize the non-metallic inclusions found in high strength low alloy steel for structural applications and to track the development of inclusions throughout the melting and casting operations. 3. Scanning electron microscope Figure 3.2: Light Optical Microscope @ JSPL 37 . inclusion width. The ideal technique for providing images of the sample surface must offer accurate representation of inclusion distribution. number of inclusion per unit area. quantitative assessment involves a combination of a microscopic technique to provide images of the sample surface (image acquisition) and an image analysis system to accurately measure the inclusion size. 3. mean free path. Using as-polished metal samples. etc.1 IMAGE ACQUISITION Image acquisition is a crucial part in the process of quantitative analysis. Light optical microscope 2.2.2 QUANTITATIVE ASSESSMENT A complete assessment of steel cleanliness not only consists of qualitative information. volume fraction. Laser confocal microscope 3. but also quantitative information such as inclusion length. Analytical instruments involved in this research project consist of the following: 1. Once the height information is obtained. As the magnification increases. the light intensity decreases.3μm. which has a wavelength of 473nm. • With DIC (differential interference contrast). is limited by the fixed wavelength of light (λ ≈ 0. LCM utilizes blue laser as the transmitting medium. Therefore it becomes rather difficult to utilize the best-possible resolution of light in a conventional light-optical microscope. high sensitivity. quantitative surface area and volume measurement can then be calculated using the operating software. but without the issues of charging in non-metallic areas of interest such as inclusions. 38 . minimal sample preparation and ease of operation. Therefore. which is approximately 0. The secondary electron mode of a SEM provides an improved spatial resolution of 5~20 nm [15]. which results in darker image. The best-possible spatial resolution of a light-optical microscope. 2003][13]. when compared to light optical microscope. Laser confocal microscope: The laser confocal microscope (LCM) distinguishes itself from conventional optical microscope and SEM in the following way: • Laser confocal microscope is able to provide height information accurate to 0.Light optical microscope: Prior to the advent of electron microscopy. etc. Scanning electron microscope: SEM and EDS are among the most employed methods of inclusion investigation mainly due to the following advantages: high resolution. This technique is especially important for particle analysis of metallurgical samples such as isolated inclusions. quantifiability.5μm) [ASTM. laser confocal microscope provides dimensional images comparable to that of SEM. LCM offers a slightly improved lateral spatial resolution at approximately 200nm.01 μm. light-optical microscopy was used to quantify and characterize inclusions based on morphology. the images formed are topographical representations of the specimen. will be rather difficult when inclusion size is small. However. 39 . Raigarh The three modes used are secondary electron (SE). backscattered electron (BSE) and EDS modes.Fig 3-3 Scanning Electron Microscope @ JSPL. given the topography of the specimen is flat. SE mode was used to image inclusions on polished and SPEED etched surfaces for inclusion morphology study. the BSE mode was used in conjunction with image analysis software. The BSE mode. the signals received will reflect the surface structures of the specimen. utilizes backscattered electrons to create images showing elemental contrast. thereby revealing the locations of non-ferrous inclusions in the ferrous matrix. BSE images are also able to provide information on the homogeneity of inclusions. on the other hand. using SE mode to locate inclusions in a polished sample. Since secondary electrons have a very small escape depth. Using the SE mode. For inclusion quantification. Inclusion type determination was performed by EDS mode simultaneously. In the current investigation. Figure 3-4 (a)-(b) are examples where surface defects such as voids and gas holes due to solidification shrinkage. 40 . voids and deep scratches. or limited hot ductility may be detected as oxide inclusions in optical microscopy and LCM images.2 IMAGE ANALYSIS Figure 3.2.4 Image analyser attached with optical microscope Detection and discrimination of inclusions utilize the difference in gray level intensity between each inclusion species and the unetched matrix steel. Other surface defects may also result from improper polishing techniques. Measurements are made based on counting the number of picture point elements (termed pixels) that satisfy the user-defined gray level threshold. creating excessive relief pits. using four image acquisition techniques: optical microscopy. Figure 3-4 (SE mode).742 μm/pixel Figure 3-4 shows images taken of the same sample area. The dimension of each image pixel is dependent on both microscope magnification setting and image resolution. The images for the purpose of quantitative analysis in this study are taken with the following parameters [13]: Magnification: 100X Image resolution: 512 X 676 pixel Dimension of each pixel: 1.3. although reduced in number of surface defects. SEM (SE mode) and SEM (BSE mode). laser confocal microscopy. proved to be difficult in image analysis processing due to lack of contrast between inclusion and matrix steel. because their gray level range is comparable to that of oxides. Figure 3-5: Images acquired using (a) optical microscopy, (b) laser confocal microscopy, (c) SEM (secondary electron mode) and (d) SEM (backscattered electron mode) The presence of defects in acquired images shown in Figure 3-5 (a) and (b) can greatly affect the reliability of subsequent inclusion detection and measurement represented in Figure 3-6 (a), where the voids and scratches were identified as inclusions by the image analysis software. However, complete elimination or minimization of these defects at the image acquisition stage can be achieved using SEM under BSE imaging mode as shown in Figure 3-5 (d) and its respective image analysis result in Figure 3-6 (b). Thus, SEM- BSE is chosen as the most suitable image acquisition technique for the quantitative analysis of inclusions. Figure 3-6: Photograph processed by image analysis showing detected area as inclusions (a) Laser confocal microscopy, (b) SEM (backscattered electron mode) 41 CHAPTER – 4 RESULT AND DISSCUSSION 42 4.1 INTRODUCTION STEEL CLEANLINESS OF RAILS: In order to obtain the satisfactory cleanliness of steel it is necessary to control and improve a wide range of operating practices throughout the steelmaking processes like deoxidant- and alloy additions, secondary metallurgy treatments, shrouding systems and casting practice. Table 4-1: The importance of clean steel with respect to mechanical properties of the product [12] Element Form S, O Sulfide and oxide inclusions Mechanical Properties Affected Ductility, Charpy impact value, anisotropy Formability (elongation, reduction of area and bendability) Cold forgeability, Drawability Low temperature toughness C, N Solid solution Solid solubility (enhanced), hardenability Settled dislocation Strain aging (enhanced), ductility and toughness (lowered) Pearlite and cementite Dispersion (enhanced), ductility and toughness (lowered) Carbide and nitride precipitates Precipitation, grain refining (enhanced), toughness (enhanced) P Fatigue strength Solid solution Embrittlement by intergranular precipitation Solid solubility (enhanced), hardenability (enhanced) Temper brittleness Separation, secondary work embrittlement Rail steel needs to conform to stringent quality standards described in the standards owing to its critical nature of its application. Chemical composition range of Grade 880, which is a common rail grade as per IRS-T12, is shown in Table 4-2. 43 03 0. The classic explanation is that. so that the initiation & propagation of crack gets driven. IRS T12 2009 specifies that the inclusion rating level of rails. & up to as long as sizeable fraction of a metre). governed by cyclic shear stresses.5 A. In these circumstances. shall not be worse than 2. D thin or 2. Later. which determines fatigue life. are enhanced by lateral (curving) longitudinal (traction & braking) loads. stretching occurs normal to the crack tip. when a flat crack is open by tensile stresses.0 A. The extremely high contact stresses & the enormous power density (i. the initiation of crack is almost inevitable [21].6 max Hydrogen in rail is restricted to a maximum of 1. C. Fatigue is the result of progressive initiation & subsequent propagation of crack. B.60- 0. under the influence of bulk bending stresses in the body of rail. As far as inclusions are concerned.e the power passing through per unit) concentrated at the contact under the vertical loads. Initiation is dependent on slip processes. 44 . thereby advancing its position.Table 4-2: Chemical composition of Grade 880 rails as per IRS T-12 2009 specifications Grade %C %Mn %Si %S %P %Al Grade 0. when examined as per IS: 4163. In a generally compressive field. early growth by shear is the only possible mechanism available to advance the crack.03 0. Propagation is generally governed by cyclic tensile stresses & is caused by repeated plastic stretches & blunting at the crack tip. Initiation is typically accepted to involve crack development. The really important crack dimension. is penetration into the load bearing area.80- 0. the crack grows by tensile opening & closing. EFFECT OF INCLUSIONS TO THE PHYSICAL CONTINUITY OF RAILS: Inclusions act as the barrier to the physical continuity of metal.50 max max max %Nb - H in ppm 1.10- 0.30 0. The area in the vicinity of inclusion develops a local residual stress field. D thick. B.microcracks (size ranging from micrometer to millimetre) transforming into macro cracks (greater than millimetre.015 880 0. it is well known that they are detrimental to rails.80 1. such as that under a wheel contact. C.6 ppm which makes degassing necessary. Large inelastic inclusions. Rail industry has been constantly working in this regard to lower down the size & amount of inclusion prevailing. The most common of which includes those of MnS. These inclusions which are themselves brittle in nature. subsequently resulting in failure. present in the material are considerably elongated by the loading of the rail in service and contribute to spontaneous cracking. thus leading to loss of serviceability.Fig. Al. such as those comprising of Ca. MnS inclusions can become crack initiators as they deform in a non-uniform manner to produce long thin inclusions. 4.1 Force applied by a Wheel on Rail A wide variety of inclusion always exists in the rail steels of the composition shown in Table 2. 45 . [14] This study assesses the level and type of inclusions in rail steels produced at JSPL and tries to minimise the inclusion level by carrying out appropriate modifications in steel making & simultaneously carrying out the comparative study between VD & RH processed heat. Si and O tends to act as a nucleation site for crack growth below the surface of the rail head. under the influence of stresses can shear in a brittle manner. Al2O3 and SiO2. Studies reveal that MnS inclusions. Fig 4-2 Sample images taken @ TSD.2 Rough filing is done on the surface to be polished by using stone grinder to remove the cut marks.1 A 20mmX20mmX10mm sample is cut from the standard location of the 60-100mm long rail sample. 320. 4.3 The specimen is polished by using coarse emery papers of size 240.2.2 EXPERIMENTAL PROCEDURE SAMPLE PREPARATION 4. 4. 4. It shall be located halfway between the outer surface and the center. 400 to get the surface free from scratches.2. 4.2.4. It shall be parallel to the longitudinal axis of the product.4 Again it is polished by using fine emery papers of size 1/0. as per IS: 4163 by using Abrasive Cutter Machine.2. JSPL for inclusion rating 46 . The polished area of the specimen shall be approximately 200mm2.2. Then it is washed with water and dried by using blower.5 Fine polishing of the rail sample is done by using Cloth Polishing Machine where the polishing media is Alumina powder to get mirror surface. 3/0 and 4/0 to get further smooth and scratch free surface. 2/0. 4.highly malleable. 4. for each type of inclusion.Numerous and non-deformable.2. individual grey particles and generally rounded ends. black or bluish particles (at least 3) aligned in the deformation direction. Randomly any ten numbers of worst fields are chosen and each field is compared with the standard chart for each type of inclusion. 4. Group A (Sulphide Type) – highly malleable. Group C Silicate . 4. individual black or dark grey particles and generally sharp ends.7 The following types of inclusion are determined in this method.2. Group B Alumina .6 Inclusion content determination is done by using Optical Microscope at 100 magnification. 4.10 The entire polished surface is examined.2. Group D Globular Oxide – non deformable. angular. angular or circular.DETERMINATION OF CONTENT OF INCLUSION 4.8 The image is projected on the ground glass and a clear plastic overlay is placed over the ground glass projection screen.9 The image within the test square is compared with the standard chart diagrams of IS: 4163 Specification.2.11 In each worst field.2. total length of the inclusion is measured and corresponding severity number is noted down from the comparison chart of IS: 4163 specification 47 . black or bluish randomly distributed particle.2. 4.0 2 1.5 Group C (SILICATE) (Thin) - Thick Group D (OXIDE) (Thin) 1.0 To confirm that the inclusions are of sulphide type.5 Group A (SULPHIDE) (Thin) 1.5 1. SEM-EDS analysis was also carried out. (a) (b) Fig.2 RESULT Table 4-3 Inclusion Rating Results Heat ID A type Thin Thick B type Thin Thick C type Thin D type Thick Thin 1 1.3: (a) SEM image of inclusion in Heat ID 1 at 799X magnification (b)EDS spectrum of point 3 shown in image 48 .4.5 0.0 - 0.5 Group B (ALUMINA) (Thin) 0. The control of sulphur and its associated level of sulphide inclusions in rail steel is a challenge in spite of RH-degassing. This can be attributed to the silicon killing practice adopted in rail steels and RH-degasser’s limitations for desulphurization understanding the effect of secondary refining parameters on desulphurization and inclusion removal. 4.3: Spectral imaging of inclusion in Heat ID 2 at 3210X magnification SEM-EDS analysis confirms the results of inclusion rating and reveals that the inclusions are Manganese Sulphide (MnS) stringers.Fig. 49 . CONCLUSION AND FUTURE WORK 50 . Calcium Treatment is major tool for inclusion modification and flotation Argon stirring improves floatation of inclusion. Tundish metallurgy has big importance in steel cleanliness. The effect of an inclusion on the fatigue properties depends on its size. Mold is the last refining step where inclusions can be safely removed. thermal and elastic properties and its adhesion to the matrix.. Inclusion size has the major effect on the fatigue properties.CONCLUSION Presence of non-metallic inclusion can negatively affect both properties of product and subsequent processing. Shape & their chemistry. Differences in the thermal expansion coefficients of the inclusion and the matrix can generate internal stresses around inclusions. Inclusions can come into steel from various sources main are deoxidation and refractory. For Evaluation of steel Cleanliness it is necessary to combine several methods together. In case of Al killed steel Al2O3 is major headache. Four different image acquisition techniques were evaluated for the quantitative analysis of inclusions and it was found that SEM-backscattered electron imaging mode is the most suitable choice 51 . shape. Oxides and sulphide are more detrimental for steel. Inclusions can be classified depending on Source. inclusion species tend to develop from simple primary oxides to complex binary and ternary oxides. Aluminium oxide precipitates are formed during fast cooling of the liquid steel. FUTURE WORK Correlate the development of inclusion composition and count in the furnace. ladle. Development of automatic/online inclusion behavior and assessment technology during processing and production of steel 52 . tundish and mold slags with inclusions found at each respective steelmaking vessel. steel cleanliness improvements were achieved. 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