Investigating Dynamic Thermal Behavior of Continuous Casting of Steel with CONOFFLINE
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RODUCTION
THE process of continuous casting of steel slabs, shown in Figure 1, is best operated in steady state. The metallurgical length, which defines the region of final solidification, should occur within the roll containment region. Changing casting conditions sometimes extends the liquid core beyond containment, which causes a serious ‘‘whale’’ defect, where the internal ferrostatic pressure causes expensive and dangerous bulging.[1] To lessen centerline defects such as macrosegregation and porosity, the final solidification region should fall within the range of soft-reduction equipment, which is available in many casters to provide extra squeezing of the strand on segregation-sensitive steel grades during this critical time.[2–4] Changing casting speed requires changing the location down the casting machine where soft reduction is required. Spray cooling systems are designed to control the surface temperature of the strand, in order to avoid surface cracks, especially during unbending, where crack-sensitive grades suffer metallurgical embrittlement and transverse cracks, if the surface temperature is in a detrimental temperature range.[5,6] Many operations use a simple spray table to accomplish this task, which works well during steady-state conditions. During a speed change, however, spray
BRYAN PETRUS is with the Nucor Steel Decatur, 4301 Iverson Blvd, Trinity, AL 35673. ZHELIN CHEN and JOSEPH BENTSMAN are with the University of Illinois at Urbana-Champaign, 1206 W Green St, Urbana, IL, 61801. BRIAN G. THOMAS is with the Colorado School of Mines, 1610 Illinois St, Golden, CO, 80401. Contact e-mail: [email protected] Manuscript submitted October 14, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
cooling should be adjusted dynamically in each spray zone at different times,[5,7] which is more challenging. Sudden drops in casting speed, which often occur due to automated alarm systems designed to detect and avoid sticker breakouts in the mold,[8,9] can lead to internal cracks due to increased thermal stress,[10] and other problems, such as centerline bridging and severe centerline segregation.[8] In fact, most defects in the process arise during transient conditions, especially involving changes in casting speed.[7,8,10–12] Although steady-state conditions are preferred, casting speed changes arise during operation for many reasons. In addition to the examples just mentioned, casting speed is usually lowered during startups, tailouts, ladle changes, tundish changes, and other operational conditions. Speed changes are also required to accommodate schedule changes, upstream delays in steelmaking, or downstream delays in rolling (for operations with hot charging or thin-slab casting). Because so many defects arise due to these changes, it is important to understand the dynamic thermal behavior of the continuous-casting process during transient conditions. This includes how the strand surface temperature, shell thickness profile, and especially the metallurgical length vary during changes in casting speed, for differ
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