FINISHING PROCESSESRecent advances in temper and skin-pass rolling technology Improvements have been made in the surface quality, flatness and surface finish of skin-passed or temper rolled strip. The use of the drytemper mill, contoured work rolls, 2-stand skin-pass mills and improved mill set-up and control, aided by advances in modelling, provide a powerful ● Figure 1 Dry-temper equipment package of available tools. millennium steel2k3 Gerhard Finstermann, Gregor Nopp, Norman Eisenköck and Georg Keintzel VOEST-ALPINE Industrieanlagenbau GmbH & Co (VAI) 202 The skin passing and temper rolling of sheet products is the final forming step of the production route in which material properties, flatness and surface morphology of cold-rolled flat products are tailored to meet stringent customer requirements. Prior to temper or skin-pass rolling, the sheet is subject to a long chain of process steps that involve considerable conversion costs. For this reason it is worthwhile to carefully treat the substrate and final product and not waste valuable material in the form of scrap. The potential for yield losses are manifold. It is a common occurrence for large portions of sheet to remain on the pup coils as a result of problems in handling and often the required consistency of material properties or surface morphology is not met throughout a coil. Another issue is incorrect strip shape or surface deterioration as a result of wet temper agent remaining on the strip. Surface aspects The key criteria in skin-pass rolling are surface cleanliness and roughness, which are important for subsequent painting, welding and deformation processes. Rolling influences include the type of roll, roll-surface roughness, rolling forces, position in rolling schedule, elongation (degree of deformation), roll surface cleaning and the agent applied to the roll and strip surfaces. Dry-temper process The development of the dry-temper process has been marked by substantial improvements in recent years. The staining problem has been resolved to a great extent, although modern mills must still offer parallel wet tempering, mostly to safeguard the skin passing of coated material. In any case, the issue is to keep the surface of the rolls clean in order to prevent particles being imprinted into the strip surface in the roll gap. With the dry-temper process (Figure 1), this is achieved by a combination of brushes, which rotate and/or oscillate while being lightly pressed against the roll surfaces, thereby cleaning all particles that stick to the roll surface. The dirt is collected by a system of exhaust nozzles. The conventional wet system (Figure 2), applies fluid (a water-agent mixture), either into the entry of the mill bite or onto the surface of the backup rolls. A blowoff system at the exit of the mill ensures a dry strip surface. Both systems are designed so that application of both is possible. Even with such an effective blow-off system, residuals of the agent remain on the surface which can cause the well-known spot rust or ‘brown streaks’. Nitrite residuals cause problems during coating processes, which make it necessary to clean the strip prior to coating. Additionally, the agent is very costly higher bearing temperature).700 mm Max. Technological effects: ■ Increase of rolling force (10–15 per cent) ■ Temperature increase of roll surface (change in work-roll crown. Linz/Austria is shown in Figure 3. with plant details given in Table 1.405 mm/1. ● Figure 3 Skin-pass mill at voestalpine Stahl. Service life of rolls: ■ No change compared to the wet process ■ Backup rolls – 40. 203 .620 mm Max.000 t ■ For backup rolls – 160. 3% ● Table 1 Technical data Operational results Excellent product quality: ■ Nitrite-free surface ■ 50 per cent less spot rust ■ No brown streaks ■ No stains from liquid drops. Benefits ■ Suitable for highest quality demands (ie.000 mm 2 x 400 kW Max.700 mm 1520–1.3–3 mm 600–1.000 t ■ Work rolls – 800 t. Linz Lifetime of brushes: ■ For work rolls – 160. 30 t 1100–2. 900 m/min 14 MN 602–587 mm/1. The first application at voestalpine Stahl.000 t (cleaning during back up roll change). exposed automotive sheets) millennium steel2k3 and the waste has to be disposed of at high cost due to its aggressive nature.FINISHING PROCESSES ● Figure 2 Wettemper equipment Attribute Type of mill Capacity Strip thickness Strip width Coil weight Coil diameter Main-drive power Mill speed Rolling force Work roll diameter/ barrel length Backup roll diameter/ barrel length Elongation Data 4-high 900 kt/a 0. The position of the barrel edge corresponds to a certain angle. Saudi Arabia ■ BETHLEHEM STEEL. Sparrows Point. The SmartCrown contour (Figure 5) can be described as a sum of a sinusoidal and a linear function. almost all skin-pass and temper-mill setups are created using offline calculations and trialand-error procedures during operation. The unloaded roll gap contour corresponds to a certain portion of a cosine curve around its vertex. in addition to other factors such as acceleration/deceleration and slippage. The contour of the roll gap can be expressed by a cosine function. width and yield stress. is an effective solution for enlarging the adjustment range of the flatness controller. The benefits of SmartCrown are shown in Figure 7. even rapid and accurate controls are not able to immediately compensate for errors from setup deviations. the ‘contour angle’. Dunaùjvárosz. By fine-tuning this angle. Linz. Application of axial shifting of work rolls and conventional work-roll bending in combination with the recently developed SmartCrown system by VAI. Salzgitter. Powerful actuators to apply rolling forces up to 20. and to match the relative crown of the incoming strip. ● Figure 4 Low friction mill ● Figure 5 SmartCrown contour ■ Cost savings (less expenditures for agents and disposal costs) ■ Environmental improvement (no aggressive agent) ■ Reliable technology – voestalpine Stahl has used dry skin-passing exclusively since January 1999 on its Sheet Skin Pass mill ■ No restrictions regarding roughness transfer. continuous shifting allows for continuous adjustment of the roll-gap profile. MD. Flatness A wide spectrum of parameter variations such as thickness. resulting mainly from the incoming material. millennium steel2k3 (Note: additional space is required at exit of mill for dry skin-pass and exhaust equipment.) 204 Other reference plants are: ■ voestalpine Stahl. Therefore. Even today. This is a result of the fact that the local thickness reduction in the quarter-buckle-sensitive area is decreased. Coefficients of this function are chosen such that at an arbitrary roll-shifting position. The more frequently changes in sheet sizes are made. leads to a high number of work-roll crowns. Al Jubail. Sophisticated . USA ■ SALZGITTER Flachstahl. the more important this becomes. Modelling approaches for skin pass and temper rolling Respective improvements in the technological controls for elongation and flatness counteract inconsistencies born of a variety of process disturbances. and results in costly logistic measures and a large number of required SmartCrown SmartCrown is a new type of roll contour that applies lateral shifting of the work rolls to adjust the unloaded and loaded roll-gap contourm to adjust the working range of roll bending. A smaller local reduction results in a reduced tendency towards longitudinal compressive stresses in the strip. Austria ■ HADEED. Hungary. since the unloaded roll gap height is somewhat larger in this region. the transverse profile of the resultant roll gap can be adjusted in such a way that quarter buckles can be avoided (Figure 6). which are responsible for the occurrence of strip buckling.FINISHING PROCESSES work rolls. the resulting unloaded roll-gap profile is always cosine-shaped. Germany ■ DUNAFERR.000 kN per mill stand together with the wear plates to guide work and backup rolls within the mill stand create a total force of up to 150 kN. However. A solution to this problem is to introduce several strategies that together make a ‘low friction mill’ to keep the actuators for flatness control as precise as possible (Figure 4). rollflattening behaviour and roughness transfer in order to improve the setup of skin-pass and temper mills. [J Hitchcock. was developed along the same lines as that of Orowan but uses some additional suggestions and is a simplified variant of Orowan’s model. Int. Vol. The fact that these assumptions are not always true had been found in experiments about 60 years ago by Orowan [E Orowan. While conventional models assume the shape of the roll gap to be circular. The most influencial work was done in the late 1980s by Fleck and Johnson [NA Fleck. this assumption no longer holds true for temper rolling. pp. KL Johnson. VAI has developed and implemented sophisticated algorithms in cooperation with the University of Linz. Sci. (1948)]. can be used only when the roll deformation is small.FINISHING PROCESSES ● Figure 6 SmartCrown deviation from a parabolic roll gap as a function of the contour angle ● Figure 7 SmartCrown benefits circular arc of the enlarged radius. 159. The Hitchcock formula: 2 16•(l-v )•FW R’ = l+ π•E•b•∆h R where R’ is the radius of the deformed work roll. Vol. To calculate the precise roll force.. The elastic deformation of the rolls is neglected or it is assumed that the deformed profile of the rolls remained a 205 . 150. To further reduce unacceptable strip dimensions it is vital to improve setup procedures by adding appropriate models to the automation structure of such mills. Vol. pp148–153. Classical models (before Fleck). J. but the equations developed there do not allow analytical solution for pressure distribution. pp140–167. such as the model developed by Bland and Ford [ DR Bland. (1948)]. temper rolling and foil rolling can be divided in three parts. Therefore. pp. Mech. FW is the roll force. H Ford. The existing well-known roll-gap models developed for hot and cold rolling. Classical models One of the most comprehensive one-dimensional studies that covers hot and cold rolling was developed by Orowan. 150. The calculation of roll pressure in hot and cold flat rolling. Mech. the steel producer voestalpine Stahl and the Industrial Competence Center for Mechatronic & Automation (IKMA). Appendix 1. In all these models the equation of equilibrium in the longitudinal direction for plastically deformed material in the roll gap is derived. R is the nominal work roll radius. It is also assumed that relative slip between rolls and the strip exist throughout the roll gap with the exception of neutral point where the slip changes direction. Inst. The calculation of roll pressure in hot and cold flat rolling. b the width and ∆h the reduction.139–141. as given by Hitchcock.140–167. Proceedings of the Institution of Mechanical Engineers. ASME Research Publication. The model of Karman [TH v Karman. No. 7. These algorithms are the basis for setup models that allow online calculation within a short period of time (Figure 8). thin-gauge models (Fleck’s model) and improvements of thin-gauge models. In the following 20 years numerous works have been devoted to include an influence function millennium steel2k3 online process modelling techniques are not frequently employed. Proc. (1987)]. VAI has investigated new offline and online modelling approaches to predict rolling force. Roll Neck Bearings. either because they are not available or are considered unnecessary. The calculation of roll force and torque in cold strip rolling with tensions. (1935)]. the pressure distribution in the roll gap is required. ZAMM 2. there have been attempts to make additional suggestions in order to simplify the Orowan equation and to get some approximate solutions. 29. Eng. [E Orowan. (1948)]. E the Youngs modulus of the rolls. skin-pass. (1925)]. Proceedings of the Institution of Mechanical Engineers. pp507–524. Towards a new theory of cold rolling thin foil. the deformed roll shape is calculated by a Green (or influence) function of type: 2 ξ+a0 a20 . as an upper limit.x2 + const ∫ p(ξ)ln dξ 2R ξ–X -a0 a b(x)= b0 - ● Figure 9 Deformation zones in the roll gap (Fleck neglected the elastic zones) millennium steel2k3 describing the roll deformation into standard equilibrium equations of Orowan. the usual iteration to find a deformed roll shape associated with a certain contact pressure Since this complex type of function includes a variable lower limit. but is now independent of the strip material behaviour in that part of the roll bite. Since the flattened roll geometry is now prescribed. which is documented by various excellent results in the literature obtained by offline calculations. Cold rolling of foil. and as such is only dependent on the elastic properties of the roll material. the solution of the overall equilibrium equation can suffer considerable convergence problems. a key element was to also introduce an intermediate zone called ‘contained plastic flow zone’ (Figure 9). as well as the contact pressure. [NA Fleck.FINISHING PROCESSES ● Figure 8 New materials need new models applied becomes redundant. mostly due to a convergence problem in the vicinity of neutral point where roll pressure peak occurs. Whether five regions along the contact length actually exist will depend on the rolling case under consideration. The main structure of such a model is as follows: the contact length between strip and roll is divided into five zones in which the classical Orowan equilibrium equation and the Treca yield criterion are applied to the plastic reduction zones on entry and exit (before and after the contained plastic flow zone). which for older models can be considered as the main reason for failure when applied to thin-gauge rolling. Elastic compression (entry) and elastic recovery (exit) zones are taken into account by the equations of plane strain elasticity theory. includes an infinite deformed roll radius. Vol. The quite stringent demands on solution time and stability. The associated surface friction is described by the Coulomb (sliding) criteria. These iterations also include the adjustment of the transition positions from one zone to the other. LC Zhang. except the central flat region. however. the length of this super-flattened region is also unknown and must be found iteratively by quite complex procedures. Fleck and Johnson published a theory for cold rolling of thin foils that was an entirely new approach to this subject. which have to be somehow estimated at the start. yielding a flat roll surface parallel to the strip middle plane. 206 Thin-gauge models In 1987. ME Mear. expressed by the parameter a0 which is also part of the integrand. Improvements on thin-gauge models A large number of papers published during the past ten years dealt with the subject of how to improve . but without success. The contact pressure remains as the basic unknown. still remain an obstacle for using this model without further simplifications (and also for online models). They were able to present a theory that basically omitted the use of simplifying presumptions on roll-flattening geometry. which can add significantly to the solution time required. The apparent feature of the model is that the physical behaviour inside the roll gap is very close to reality. In all zones. (1992)]. This means the existence and physical length of the central flat region must be found during the equilibrium iteration. 206. This approach takes into account the fact that the work rolls can suffer extreme flattening in an intermediate part of the contact region which. Proceedings of the Institution of Mechanical Engineers. pp119–131. KL Johnson. Unfortunately. In addition to the elastic and plastic zones upon entry and exit to the roll bite. Further compatibility conditions have to be imposed on the interfaces of these regions to ensure continuity of the contact pressure from one region to the other. European J. Viewed from a cross-section of the strip after stamping with the textured roll surface ● Figure 11 Hodograph example case. the use of other types of influence functions was investigated. the finite element (FE) method based with strong relaxation laws [P Gratacos. advantage was taken from this perception in the online version. However. [SA Domanti. pp79–106. or slab method-based. J. Red = high value). 43. This can be regarded as a quite promising approach with respect to an online use in temper mills. was able to reduce computation time significantly by using a different friction law for the strip-roll interface. the offline version enabled the release of some of the time-consuming assumptions for an online model. JL Chenot. within the required accuracy values. Int. Vol. On more refined FE models [A Hacquin. (1998)]. CF Zorowski. computation times of between 1 and 5 hours have been reported. It could be shown that very similar results to those obtained by Le and Sutcliffe. pp195–210. (1992)]. without losing too much of the accuracy required to describe the thin-gauge rolling 207 . Since convergence is strongly influenced by how the roll flattening is described. By combination with more refined calculation procedures. 17. 2nd International Conference on Modelling of Metal Rolling Processes (1996)]. 2. Journal of Mechanical Sciences. A quite extended offline version of this model includes a) the contained plastic flow region and b) the roll flattening contribution caused by roll-strip interface shear stresses. [D Jortner. JP Guillerault. (2001)]. the convergence stability was improved significantly. in that the omission of the flat region is possible. Int. An analysis of cold strip rolling. of Mechanics. JF Osterle. The model described is based on the Fleck theory.3. The computation times achieved so far. pp283–286. Vol. as for instance reported in the work of Domanti et al. for more than 95 per cent of the temper rolling cases of steel sheet material down to 0. No. Vol. Different attempts have been made to improve the convergence. were obtained.2 mm of exit thickness. Nevertheless. pp. Southampton: AA Bakema Publishers. This model. A plane-strain elestoplastic finite-element model for cold rolling of thin strip. Sci. C Fromholz. Interactions between roll surface and cold rolling parameters. (1993)]. 34. P Montmitonnet. WJ Edwards. A recent paper by Le and Sutcliffe [HR Le. depending on the degrees of freedom used in such models. Also Zhang [LC Zhang. however. Vol. J. Interesting results have been obtained. P Montmitonnet. New VAI model The models developed by VAI are based on the Fleck theory but also take advantage of the approach described by Le and Sutcliffe by omitting the contained plastic flow region on the roll gap. ie. A robust model for rolling of thin strip and foil. MPF Sutcliffe. (1960)]. as well as to provide smart re-formulations of some of the governing equations. Mech. including between 100 and 2000 iterations very much dependent on the strip material and applied pass reduction used in the simulation (the harder the material and smaller the reduction.. A/Solids. made the assumption that the flat (contained plastic flow) region in the roll gap was superfluous. but introduces a number of enhancements that improved the convergence stability and greatly reduced the computation time. Int. Consequently.FINISHING PROCESSES ● Figure 10 Equivalent plastic strain plotted with colours (Blue = low value.. computational mechanics from concepts to computations. pp179–194. A threedimensional semi-analytical model of rolling stand deformation with finite element validation. which drastically millennium steel2k3 the solution stability and shorten the required computation time of the basic thin-gauge model outlined above.1405–1419. however. the longer the computation time). would not allow the model to be used online. the computation time can still be considerable. A simple approach for cold rolling foil. did not reproduce the results obtained by Fleck et al. Sci. Mech. A new effect taken into account is the tangentialinduced deformation of the roll. Figure 10 shows an example obtained by FE calculations and the equivalent plastic strain distribution in a strip specimen close to the strip surface. (2001)]. can be plotted as a so-called Hodograph (Figure 11). a proper roughness evolution model for setup purposes is the main objective behind this work. reduction. Further work on this topic has to be done in order to fully understand the mechanisms and governing laws of roughness transfer.und Walzvorgang. This effect becomes more important as the strip gets thinner because the area (or volume) of influence inside the total strip crosssection seen in the rolling direction considerably increases. Beside a closed-loop control strategy. In reality there is also a deformation caused by the interface shear stresses. In common non-circular arc models the roll is allowed to displace in the radial direction only. Rz and PC values are used). The amount of deformation work required to generate the impressions (craters) in the strip surface has to considered in the roll-force calculation in order to obtain reliable dependence of the roll force on roll roughness (which usually is done by modifying the coefficient of friction to meet an observed roll force). Important for temper rolling (a non-circular arc formulation here is indispensable) is that the model can reproduce. The shape of this path depends on strip thickness. roll radius and friction conditions and allows investigation and classification of the process of roughness transfer. Faculty of Mechatronics. [F Rechberger. When knowing the roughness distribution of the textured work roll (Ra. When omitting the assumption of a predefined flat region in the roll gap. This has considerable impact on the accuracy of the predicted rolling forces and hence is vital for a proper mill setup. this was implemented by some basic analytical work without adding too much computational burden. the shear stressinduced roll deformation mainly derives the existence of neutral zone instead of having only one single neutral point. one can predict the total amount of work and hence make respective correction in the roll force calculation. The implementation of the new temper rolling model in conjunction with a roll-stack model into a level-2 . In temper rolling the work rolls are roughened to provide a desired roughness on the strip surface for various purposes. data from the strip surface. Dressieren als kombinierter Präge. the useful implementation of which very much depends on the accuracy of online measured roughness. rolling speed. and an elastic recovery zone on exit to the roll bite. It has also been seen that the effect of interface shear stress on roll deformation cannot be neglected completely. Doctoral Thesis at the Johannes Kepler University Linz. The formulae describing this phenomenon in general were derived semianalytically. The online model hence includes only three zones along the contact length: an elastic compression zone on entry. which becomes more important as the strip becomes thinner and harder. The equivalent plastic strain in conjunction with the corresponding stresses can be used as measure for the deformation work required to stamp the strip surface by a roughness peak from the workroll surface.FINISHING PROCESSES millennium steel2k3 ● Figure 12 2-stand skin-pass mill 208 reduced the computation time. However. A second effect taken into account for temper rolling is related to the deformation work balance in and on the interface planes of the roll gap. whenever necessary. the extension of a neutral point into a neutral region. a plastic compression and reduction zone containing the neutral region of contact where the strip sticks to the roll surface. The path of the tool stamping into the strip surface (a roughness peak on the roll surface). which has the advantage of having respective displacement derivatives available without a numerical burden. Automation. including material handling. enables this mill to process the entire product range from micro-alloyed. Concluding remarks On the basis of VAI R&D and implementation activities in the field of temper and skin-pass rolling. In single-stand configurations maintaining constant material properties and uniform surface roughness is difficult. coupled with an advanced control strategy. operates fully automatically. interstand tension measurement roll and shapemeter roll with tension measurement cells for strip tension measurement ■ X-ray gauge at mill exit ■ Online surface inspection. or electro-discharge textured) work rolls can be used on one or both stands ■ Rewinding operation with installed or removed work rolls on one or on both mill stands in forward and reverse direction. The authors wish to express their grateful acknowledgement for this support to IKMA. also allows the application of powerful process controls for setup on temper mills. electro-beam. Acknowledgements Part of the development work was sponsored by the joint venture IKMA (Industrial Competence Center for Mechatronics & Automation). and will substantially contribute to reduce yield losses on high value added material. An optimised mechanical setup. this is becoming increasingly important for highest-quality sheet applications in the automotive and appliance industry. Gerhard Finstermann is Vice President Cold Band. all at VOEST-ALPINE Industrieanlagenbau GmbH & Co (VAI). Key design features include: ■ Shape-meter roll at mill exit for flatness control ■ Laser speedmeter at mill entry. the following conclusions can be made: ■ Wet-skin passing can be fully substituted by dry-skin passing for uncoated material ■ Ever increasing production demands require a lowfriction mill with high technological standards and the need of a wide adjustment range for elongation and flatness control ■ SmartCrown represents an ideal solution for the stringent flatness requirements of a modern skinpass mill ■ New models for new materials must consider the roll deformation more accurately and the stamping effect for roughness transfer. This new mill concept is being realised by VAI together with SALZGITTER AG in Germany (Figure 12). Key operational aspects include: ■ Rolling operation with very small rolling forces (300 kN per mill stand) for interstitial-free (IF) steel grades ■ Rolling operation with very large rolling forces (16.000 kN per mill stand) for IF and micro-alloyed steel grades ■ Rolling with only one mill stand is possible. threading and the rolling process. ■ Mixed operation modes: > Dry skin-pass rolling on one mill stand and wet skin-passing on the other mill stand > Smooth or textured (sand blasted. Linz. Gregor Nopp is Head of Proposals/Engineering Cold Rolling Mills. Austria. as both are mainly influenced by the roll force. This is a major step forward to achieve highest quality demands on tempered and skin-passed sheet material in a reproducible manner. Technology Cold Rolling and Georg Keintzel is Project Manager Technological Controls. highstrength to extra soft. 209 . millennium steel2k3 environment. This technology enables independent control of elongation and roughness transfer. In conjunction with advanced surface texturing patterns. Norman Eisenköck is Project Manager. extra deep drawing quality (EDDQ) steel grades. This high-performance mill. the local government of Upper Austria and the Republic of Austria.FINISHING PROCESSES 2-stand skin-pass technology A great deal of emphasis has been placed on 2stand skin-pass technology for sheet material. interstand and mill exit for elongation control ■ Online roughness measurement at mill exit ■ Entry bridle with tension measurement cells.
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