Intermixing model of continuous casting during a grade transition
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INTRODUCTION
INCREASING the production efficiency of the continuous casting process requires casting longer sequences of ladles without stopping and restarting the caster. As the demand for wider ranges of steel products increases at the same time, the intermixing of dissimilar grades is becoming a problem of growing concern. Steel producers need to know exactly where the mixed region exists in order to cut out and downgrade the minimum amount of steel necessary to satisfy customer product specifications. In addition, casting conditions should be optimized to minimize the costs associated with intermixing. Several different procedures exist to handle the casting of dissimilar grades. The easiest method is to continue casting the different grades as a single sequence. This method involves only a simple "ladle change." The process schematic in Figure 1 shows that " n e w " steel flowing from the next ladle will first mix with " o l d " steel left in the tundish. Steel then flows into the mold, where it undergoes further mixing in the long liquid pool in the strand while it solidifies. This method completely avoids losses in productivity but produces the maximum amount of intermixed steel which needs to be downgraded. Eu Casting conditions should be chosen to minimize the amount of intermixed steel, and/or a secondary market must be found. The most extreme altemative is to stop the caster when the first grade is finished and to restart it with the next grade as a new sequence. This method avoids producing any in-
XIAOQING HUANG, tbrmerly Postdoctoral Research Associate, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, is Process Modeling Engineer, Concurrent Technologies Corporation, Johnstown, PA 15904. BRIAN G. THOMAS, Associate Professor, is with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana. IL 61801. Manuscript submitted January 31, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
termixed steel to be downgraded. However, production time is lost to restart the caster. In addition, yield losses are incurred due to quality problems at the end and start of the cast strands. 121 Another method, the "flying tundish change," avoids stopping the caster while preventing mixing in the tundish. The tundish is changed at the same time the ladle containing the new grade is opened, so mixing occurs only in the strand. This method is demanding on the plant operation and incurs a yield loss of the old steel remaining in the tundish, in addition to limiting the tundish life. Thus, its benefit depends on the amount of intermixed steel saved. To further minimize intermixing, a "grade-separator" plate can be inserted into the mold. This method is capable of completely preventing mixing in the strand.f 31 However, physical insertion of the "perfect grade separator" requires significant slowdown or even stoppage of strand, which incurs the risks of excessive bulging and cracks in the strand, breakouts, and even damage
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