Modeling of steel grade transition in continuous slab casting processes
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I.
INTRODUCTION
A s the efficiency and flexibility of the continuous casting process improves, it is becoming increasingly common to cast different grades of steel as successive heats in a single casting sequence. Unfortunately, mixing tends to occur during the transition between steel grades. Slabs cast during the transition vary in composition between the grade of the first heat, or "old grade," and that of the second heat, or "new grade," in the sequence. The final product is often referred to as "intermixed steel" and is naturally undesirable since it must be downgraded. The cost can be significant, since each meter of intermixed slab length contains roughly 2 tonnes of steel. The continuous casting process is illustrated in Figure 1. In this work, the term "strand" refers to the solidifying shell containing the liquid steel while it travels down through the continuous casting machine. The "slab," which is the final solid product, does not exist until the strand is completely solid and has been cut into pieces at the torch cutoff point. Mixing in the mold and the strand is governed by two phenomena. The first is mass transport or convection due to the mean flow velocities, which depend directly on the casting speed. The second is diffusion due to turbulent eddy motion and random molecular motion. Generally, the turbulent component is much more important than the molecular one due to the high turbulence in most regions of the liquid pool within the shell. Several different methods have been developed to
X. H U A N G , Research Associate, and B.G. T H O M A S , Associate Professor, are with the Department of Mechanical and Industrial Engineering, University of Illinois at U r b a n a - C h a m p a i g n , Urbana, IL 61801. Manuscript submitted September 2, 1992. METALLURGICAL TRANSACTIONS B
minimize the extent of the intermixing region during grade transitions. The most effective way is to insert a physical barrier into the mold between the two heats, tl~ Steel freezes around the steel barrier, which is often shaped like an "1" to help hold the two strands together. This essentially prevents any mixing of the two grades. However, this method requires the casting process to be slowed down or even stopped for a significant time, which risks damaging the strand or even the casting machine. There is also an increased risk of breakouts compared with other methods. The most common method is simply to change ladles and carefully track and downgrade the intermixed slabs, according to the severity of the composition change due to the mixing. Sequence casting is then completely unaffected by composition changes. The disadvantage of this method is that significant mixing occurs in the tundish, in addition to the strand. Thus, several slabs containing many tonnes of intermixed steel are created. Another common method, the "flying tundish change," is used to reduce the amount of intermixed steel generated. 12.3"~1By changing the tundish simultaneously when changing ladles to a new grade, mixing occurs only in the strand.
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