Real-Time, Model-Based Spray-Cooling Control System for Steel Continuous Casting
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IN continuous casting of steel, robust and accurate control of secondary cooling is vital to the production of high-quality slabs.[1] Defects such as transverse surface cracks form unless the temperature profile down the caster is optimized to avoid stress, such as unbending, during temperature regions of low ductility.[2] This is especially important in thin-slab casters because high casting speed and a tight machine radius exacerbate cracking problems and because surface inspection to detect defects is difficult. Thus, great incentive exists to implement control systems to optimize spray cooling to maintain desired temperature profiles. Secondary cooling presents several control challenges. Conventional feedback control systems based on hardware sensors have not been successful because emissivity variations from intermittent surface scale and the harsh environment of the steam-filled spray chamber make optical pyrometers unreliable. Thin-slab casting is particularly difficult because the high casting speed requires faster response. Modern air-mist cooling nozzles offer the potential advantages of faster and more uniform cooling but introduce the extra challenge of air flow rate as another process variable to control. Most casters control spray-water flow rates using a simple look-up table with casting speed. This produces undesirable temperature transients during process changes, so recent BRYAN PETRUS, KAI ZHENG, X. ZHOU, Graduate Research Assistants, BRIAN G. THOMAS, Gauthier Professor, and JOSEPH BENTSMAN, Professor, are with the Department of Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801. Contact e-mail: [email protected] Manuscript submitted July 14, 2009. Article published online December 7, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
dynamic control systems have been developed based on real-time computational models. However, their application to thin-slab casting has been prevented by the short response times needed and the increased relative importance of solidification in the mold, which is not easy to predict accurately. Several previous attempts have been made to implement real-time dynamic control of cooling of continuous casters. It has been recognized for a long time that the spray-water flow should be adjusted so that each portion of the strand surface experiences the same desired thermal history. This is especially important, and not always intuitive, during and after transients such as casting slowdowns during ladle exchanges. Okuno et al.[3] and Spitzer et al.[4] each proposed real-time model-based systems to track the temperature in horizontal slices through the strand to maintain surface temperature at four to five set points. Computations were performed every 20 seconds and online feedbackcontrol sensors calibrated the system. In practice, these systems have been problematic, owing to the unreliability of temperature sensors such as optical pyrometers. Barozzi et al. developed a system to control both spray cooling and casting speed dynamically at the same time.[5]
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