Modeling superheat removal during continuous casting of steel slabs
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I.
INTRODUCTION
S T E E L is poured into the mold at a temperature above the liquidus. The sensible heat contained in the liquid steel represented by this temperature difference is known as the superheat. The average rate of removal of superheat, Q,h, (kW) can be calculated from* *Symbols are defined in Table I.
Q,h = (To -
Zliq)
pCpVzWN
[ 1]
The temperature difference between To and Tl,q is referred to as the superheat temperature, AT,, and should not be confused with the superheat itself. Superheat is important because (1) it must be removed before the steel can solidify; (2) it has a great effect on the solidified microstructure; and (3) it affects the formation of defects, such as breakouts, oscillation marks, and cracks, through its influence on the formation of the growing shell. The superheat can be advected to the solidifying steel shell while in the mold and conducted through the shell to the copper mold walls, and it can travel below the mold region and dissipate lower in the caster. Assuming 60 pct of the superheat in Eq. [1] is removed by the mold, approximate calculations show that this superheat removal rate (501 kW) represents about 20 pct of the total heat extracted by the mold (2481 kW). These calculations are given in Appendix I for typical slab casting conditions (case B in Table I). m In a slab caster, the steel jet usually impinges first on the solidifying shell near the narrow face. This produces X. HUANG, Research Associate, B.G. THOMAS, Associate Professor, and F.M. NAJJAR, Graduate Student, are with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL, 61801. Manuscript submitted September 10, 1990. METALLURGICAL TRANSACTIONS B
a local maximum in superheat extraction at this location, so a disproportionately large amount of the superheat is delivered to the narrow face. m In addition, the rate of heat extraction from the solidified shell by the narrow faces of the mold is usually less than from the wide faces ~2~ because of poorer contact and larger interfacial gaps. Moreover, Appendix I shows that the total superheat extraction rate (835 kW) is more than twice the power needed to continuously form a uniform, 10-mm-thick shell on the narrow faces (347 kW). These numbers indicate that the manner of superheat dissipation in the mold is very important. If too much superheat is delivered to the narrow faces, then shell growth there may significantly slow down or even reverse locally. This is likely to increase the incidence of breakouts near the narrow faces and could have an important effect on other quality problems as well. Superheat also has an important effect on surface defect formation. By increasing heat input locally to the inside of the solidifying shell, it can slow down or even stop shell growth and produce local hot spots on the outer surface of the shellJ 31 Upon exiting the mold, this hotter and thinner shell is more susceptible to deformation, bulging, and crack formation. Equally important is the temperature of the steel near the m
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