Anumber of innovative developments by VAI in the

FORMING PROCESSES Innovation trends in hot strip rolling The SmartCrown Technology Package, comprising a number of innovative plant design and process control developments, contributes significantly to improved dimensional tolerances, flatness, profile and mechanical properties of hot rolled strip. AUTHORS: Theo Nijhuis, Alois Seilinger, Thomas Kierner and Gerlinde Djumlija VOEST-ALPINE Industrieanlagenbau GmbH & Co (VAI) Anumber of innovative developments by VAI in the field of hot strip rolling technology substantially contribute to improved dimensional tolerances, flatness, profile and mechanical properties of the rolled product. These solutions are part of an advanced technology package referred to as the SmartCrown Technology Package, which includes the SmartCrown work roll contour, L-type bending and shifting block (mill stand), C-type bending block (mill stand), the Hot-Planicim roll for hot strip flatness measurements, dynamic work roll cooling, optimised shifting strategy, reduced stretch (nip) mill stands and an advanced profile and flatness control system. OVERVIEW The market is the driving force behind innovative developments in equipment and services for hot rolling. Key targets for steel producers include the production of zero-defect products, tighter dimensional tolerances, improved strip profile and flatness, good formability, higher strength steels and corresponding weight reductions. To meet these objectives, continuous process and equipment development is necessary. Tighter dimensional tolerances can be obtained only by decreasing the tolerances for centreline thickness and strip profile, by reducing the average deviation from the target value, and by reducing deviations from the average value. These improvements can be obtained through the use of equipment with greater control capabilities, tighter process control (both set-up and in-line control) as well as through a better knowledge of the processes themselves. Considerably improved strip profile and flatness in hot strip mills can be achieved through the installation of new technology packages offered by VAI. These include the new SmartCrown roll contour, the L-type bending and shifting block, an online roll-stack deflection model, a unique mill-stand design with reduced stretch, an optimised work roll shifting strategy, dynamic work roll cooling and a highly efficient profile and flatness control model. Key targets of these solution packages are to achieve a wide control range, easy-to-implement control r Fig.1 Mechanical properties of advanced high strength steels characteristics, a flexible pass schedule design, suppression of all types of strip buckles and the need for only one roll grind profile per mill stand. The market for rolled products not only demands improved dimensional tolerances, but also the development of new high strength steel grades with excellent formability properties. Figure 1 illustrates the mechanical properties of various advanced high-strength steels, the production of which requires tight control of the chemical composition and real-time temperature control during the rolling process. Innovative improvements in cooling equipment are, therefore, a major issue in hot rolling, and focus on aspects such as high cooling rates, as applied in plate cooling, and tighter temperature deviation tolerances along the length and width of the hot rolled strip. MILL DESIGN In the design of modern mill components investment cost, reliability and delivery times are key factors. VAI has introduced new development methods to identify the best solution for specific customer requirements. Application of parametric design techniques reduces the required number of engineering hours, investment costs and delivery times. A parameter list specifies the most important component data and standard parts, such as screws and bearings, can be chosen from the relevant tables of these lists. The parameters are transferred to the geometric model, which produces the design drawings. a 195

Quartic coefficient A 4 [-] Quadratic coefficient A 2 [-] installed in a new mill or in an existing rolling mill, the components and systems of this package can be individually compiled to achieve a maximum improvement in strip-profile and flatness performance. The package comprises the following elements: ` SmartCrown work roll contour ` L-type bending and shifting block ` C-type bending block ` Hot-Planicim roll for hot strip flatness measurement ` Dynamic work roll cooling ` Optimised shifting strategy ` Mill stand with reduced nip ` Advanced profile and flatness control system r Fig.2 Performance comparison between SmartCrown and third order roll contours SmartCrown work roll contour SmartCrown is usually installed in combination with work roll bending for profile and flatness control purposes and is a new type of roll contour. It employs lateral shifting of the work rolls to adjust the roll gap contour to modify the strip profile or to match the relative crown of the ingoing strip. The SmartCrown contour can be described as a sum of a sinusoidal and a linear function. Coefficients of this function are chosen so that at an arbitrary roll-shifting position the resulting unloaded roll gap profile is always cosine-shaped. Therefore, continuous shifting allows for continuous adjustment of the roll gap profile. The main benefits of SmartCrown are summarised as follows: r Fig.3 Deviation from a parabolic roll gap as a function of the contour angle Design Bending type Shifting type Conventional Positive Short stroke C-block Positive/Negative None T-block Positive Long stroke L-block Positive Short stroke r Table 1 Work roll bending and shifting systems Special material tests are performed to obtain precise information on fatigue properties of the materials, taking into consideration the manufacturing history and the operational boundary conditions of the component. A better knowledge of the material limits leads to design improvements; over-sizing of components can be reduced, thus decreasing investment costs. SMARTCROWN TECHNOLOGY PACKAGE The SmartCrown Technology Package is a decisive step for improved strip profile and flatness control in hot strip mills. Depending on whether the solutions are to be ` Employing work roll shifting in conjunction with specially profiled work rolls significantly increases the flatness control range compared with conventional work roll bending. ` Due to SmartCrown s large adjustment capability, a single grind profile per mill stand can replace all roll grind profiles of conventional rolls in a mill stand to comply with the profile and shape requirements of various rolling programmes. ` More flexible pass schedules and rolling programmes can be created and thus contribute to an optimum utilisation of mill capacity. ` SmartCrown is a simple, yet powerful profile and shape control system. Axial roll shifting is performed by means of standard hydraulic cylinders. In Figure 2 the control range performance is comparable to that of work rolls with a third-order-grinding contour with the same roll crown range. ` Enhanced shape control is achieved. The roll gap contour can be expressed as a cosine function with the unloaded roll gap contour corresponding to a certain portion of a cosine curve around its vertex. The position of the barrel edge corresponds to a 196

MS05-54 pp195-200 22/5/05 6:03 pm Page 197 FORMING PROCESSES r Fig.4 Mechanical principle of L-block r Fig.5 Cross-section of L-block and work roll chock r Fig.6 Longitudinal section of L-block with work rolls r Fig.7 Work roll chock for use with C-type bending block certain angle, the contour angle. By fine-tuning this contour angle, the transverse profile of the resultant roll gap can be adjusted such that quarter buckles can be avoided (see Figure 3). This avoidance is based on the fact that the local thickness reduction in the quarter buckle-sensitive area is decreased, since the unloaded roll gap height is somewhat larger in this region. A smaller local reduction results in a reduced tendency towards longitudinal compressive stresses in the strip, which are responsible for the occurrence of strip buckling. units, which are guided along the long L-side, into which a moment is introduced during roll shifting. The moment results from a non-symmetric load of the chocks on the L-pieces. The simple design is the result of the locally fixed bending block. The bending cylinders act between the two work roll chocks and there is no need to shift the bending block. Therefore, the clearances can be reduced in comparison with other design solutions. Another advantage of the system is easy handling during work-roll changes (see Figures 4 7). C-TYPE BENDING BLOCK Today s profile and flatness actuator systems require a combination of work-roll bending and work-roll shifting. In order to utilise the capability of axial shifting of work rolls, VAI offers a series of design options to meet the varying requirements of existing mill-stand designs (see Table 1). VAI recently developed a new system characterised by a simple design, low investment costs and low maintenance costs for positive bending in conjunction with the SmartCrown work roll contour. The solutions are L-shaped In addition to combined shifting and bending systems, VAI also offers a solution that allows positive and negative bending. PLANICIM ROLL FOR HOT STRIP FLATNESS MEASUREMENTS Flatness measurement systems currently available for hot strip are limited in many respects: either they measure strip head flatness only, or they do not show good performance in terms of maintainability or accuracy. a L-TYPE BENDING AND SHIFTING BLOCK 197

r Fig.8 Latest Hot- Planicim roll version Backed by extensive experience in steel and aluminum shape control, VAI has developed a design that overcomes these restrictions. The Planicim shapemeter roll allows the flatness of a hot rolled strip product to be measured with a high degree of reliability and accuracy. System development In partnership with Pechiney the system was first applied in an aluminum hot rolling mill in 1997. A cooling box was added to a shape meter roll to keep the roll temperature below 150 C. This product is now standard in the industry and commonly employed in cold rolling steel mills. VAI then decided to adapt the system for use in the harsh operating conditions of a hot rolling mill. The continuous measurement of the strip flatness at the exit of the rolling train or at an interstand position would allow quality and productivity improvements to be achieved. The development is currently being implemented according to the following schedule: ` Phase 1 Preliminary 3D finite-element calculation, integrating the feedback from a hot aluminum mill to determine the required loads to be applied for various steel applications. ` Phase 2 Design and manufacturing of an industrial prototype for hot strip mill rolling. ` Phase 3 Equipment testing in an industrial environment to: Quantify the measurement capacity Confirm the system design for hot rolling steel applications Testing was conducted at a strip casting and rolling facility in Italy, where the thermal conditions are the most difficult due to the high strip temperature, long contact time of roller and strip surfaces resulting from the low strip speed and continuous operations. Equipment description The roll (see Figure 8) is installed on an articulated arm bolted to the mill housing and moved by means of a hydraulic cylinder. This system allows the roll to be positioned when the strip is under tension. The strip is automatically pushed away from the roll when the mill is stopped. The roll is driven by a hydraulic motor and a free wheel assembly, allowing the rolling speed to be reached before the roll contacts the strip. When in contact, the roll is driven by the strip. The roll is partially enclosed in a cooling box that includes a spray system and which has wipers in contact with the roll that prevent water from dropping onto the strip. The roll temperature is kept below 100 C by direct cooling of the roll surface. The complete measuring and signal transmission system is re-used from the cold rolling application from sensors to signal processing hardware through integrated electronics and non-contact links. The roll design has been adapted to the specific operating conditions of a hot-strip mill while keeping the advantages of the basic equipment, namely: ` A continuous sleeve that is insensitive to the environment ` High resolution: 150 mm wide pitch with fast no-load and under-load measurements Results The results of the first runs confirm the suitability of the design: ` Cooling system validation; the roll temperature can be easily kept below 100 C with an acceptable cooling flow rate (150 300l/min). ` Roll design validation; all equipment is capable of withstanding the hot strip mill environment. ` The roll measures the strip tension distribution relative to the load applied during the contact with the strip DYNAMIC WORK ROLL COOLING VAI s solution for work roll cooling in hot strip mills focuses on the improvement in strip quality through enhanced shape control. The adjustment of specific flow rates along the barrel length, with the proper combination of cooling headers, enables the thermal expansion of the work roll and the profile of the loaded roll gap to be accurately controlled. Three headers per roll and per side are usually employed; one for basic cooling in conjunction with two additional headers for the work roll. By using dynamic flow rates in each header, it is possible to extend the adjustment range of the work roll shifting and bending system. If the thermal crown exceeds a certain limit, work roll bending may not be able to compensate for the expansion, and profile or flatness defects may occur. Dynamic work roll cooling ensures that the bending forces can stay within a certain range and that the target strip profile can always be reached (see Figure 9). OPTIMISED SHIFTING STRATEGY The application of an optimised work roll-shifting strategy 198

FORMING PROCESSES r Fig.9 Principle of thermal crown control has the objective of meeting the required profile and flatness performance while minimising roll wear, and thus extending rolling campaigns. The following goals were considered during the development of the shifting strategy: ` Improvement of strip contour through the reduction of high spots ` Extension of rolling schedules ` Increase in mill productivity ` Decrease in roll wear ` Schedule-free rolling These targets can be achieved by the pre-calculation of work roll shifting positions to minimise contour anomalies for all strips. This is done by the mathematical optimisation of a whole rolling schedule with consideration of wear and thermal crown. Comparisons of the optimised shifting strategy were prepared for a rolling schedule with equal strip width, for a rolling schedule with a totally free strip width distribution, and for a rolling schedule of a coffin shape. After rolling separate groups of 120 strips in different sequences, the maximum wear difference over the barrel length could be reduced from 60μm for rolling without shifting to about 40μm for cyclic shifting, and to 20μm with optimised r Fig.10 Results of the optimised shifting strategy for a coffin-shape rolling schedule shifting for strips all of the same width. For a coffin-shape schedule it is possible to reduce the roll wear after 120 strips from 50μm without shifting to 30μm with cyclic shifting, and to 10μm with optimised shifting. To find the optimised shifting positions, objective functions are used. An evaluation of the quality of work roll contours caused by wear and thermal crown is carried out in the strip contact zone by quality functions for each pass of the rolling schedule. Restrictions used are the maximum work roll shifting position and the maximum a 199

FORMING PROCESSES p Table 2 SmartCrown installations Application Company Country Date Aluminum cold-rolling mill, Bright Finishing Mill No.3 AMAG Austria 2001 Tandem cold rolling mill Wuhan China 2003 Reversing cold rolling mill Tangshan China 2004 6-high tandem cold rolling mill Tangshan China 2004 Steckel mill Swedish tool steel producer Sweden 2004 Steckel mill Jiuquan China 2004 Hot strip mill Zhangjiagang Hongchang SHAGANG China 2005 Reversing cold rolling mill Formosa Heavy Ind. China Ordered Minimill Maksi Group AST Russia Ordered r Fig.11 Finite element optimisation of mill stand housing shifting distance between two consecutive passes (see Figure 10). LOW STRETCH MILL HOUSING Due to the stretch or nip of the mill stand housing during rolling, clearances between the housing and chocks are required. Due to the required clearances, roll crossing can occur which can influence the roll gap contour. To reduce the chance of roll crossing, the clearances, and thus the nip of the mill stand housing, had to be reduced. Therefore, optimisation steps were carried out to reduce the mill stretch and necking of the housing. A reduction of about 60% of necking in comparison with conventional mill-stand designs could be reached (see Figure 11). INDUSTRIAL APPLICATIONS Several of the above technology packages, especially the control models, are currently being installed in the hot strip mill of voestalpine Stahl, Linz, Austria. The target is to improve the profile and flatness performance of the rolled strip by replacing the existing control system which dates back to 1995. The core of the new system is a highly accurate, on-line, 3D finite element roll stack deflection model, which is capable of executing calculations in 50ms. In addition to higher profile and flatness accuracy, the new control system provides other advantages such as greater flexibility in utilising the profile and flatness actuators of the mill, improved interstand flatness and fewer strip shape changes per stand for safer rolling conditions. In order to demonstrate the reliable design of the L-block and to check for possible improvements, the worst loading situations were simulated in an off-line long-term test. Bending forces up to 640kN were applied. During the test 52,000 tail end slaps were simulated, the block was then dismantled and checked. The L-block was in a very good condition; wear was almost negligible, and the design proved to be ready for application in a hot-strip mill. Positive and negative bending for hot strip mills was successfully implemented in 2002 at the Arcelor hot strip mill at Fos-sur-Mer in France, where it has performed very well. The first trials with the Hot-Planicim flatness measurement system were conducted at the Terni stripcasting and rolling plant in Italy where the suitability of this system for accurately measuring the gauge of hot strip has been confirmed. The reduced nip mill stand is in use in the cold rolling tandem mill of Bethlehem Steel, USA and will also be used to achieve similar improvements in the hot strip mill. The on-line roll stack deflection model is already in use in the hot strip mill of Ispat Annaba in Algeria, in the plate mill at voestalpine Grobblech in Linz, Austria, and at the EUROSTRIP line at TKN in Krefeld, Germany. The dynamic work-roll-cooling system is in operation in the hot strip mill of TISCO and at the Steckel mill of JISCO, both in China. SmartCrown installations are illustrated in Table 2. MS ACKNOWLEDGEMENT VAI is grateful to the Austrian Forschungsförderungsfond which partially funded these developments. Theo Nijhuis is Head of R&D for Rolling Mills, Alois Seilinger is Senior Expert for Rolling Mill Processes, Thomas Kierner is Technologist for Hot Rolling, Gerlinde Djumlija is Product Manager for Rolling Mills, all at VOEST-ALPINE Industrieanlagenbau GmbH & Co (VAI), Linz, Austria CONTACT: contact.hotrolling@vai.at 200