Transient fluid flow and superheat transport in continuous casting of steel slabs
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INTRODUCTION
CONTINUOUS casting of steel involves many complex phenomena including turbulent multiphase fluid flow, heat transfer, and solidification. The flow of molten steel and the associated transport of superheat in the upper portion of the liquid steel pool are critical to the quality of the final product. This flow starts downward from the bottom of the tundish and is driven by gravity through an submerged entry nozzle (SEN) at a rate controlled by a stopperrod restriction near the nozzle top. The flow issues from two or more submerged ports near the bottom of the nozzle, which direct steel jets into the mold cavity. The jets traverse across the molten pool in the confined space contained within the solidifying steel shell. For a classic “double-roll” flow pattern, they then impinge obliquely onto the narrow face, causing a locally high heat-transfer rate to the shell. The impingement point often coincides with the exit of the mold, where the solidified shell must be thick enough to withstand the ferrostatic pressure to prevent molten steel from bursting through the shell, causing an expensive “breakout.” Many casting operations restrict the casting speed according to the superheat in order to minimize the danger of such breakouts. From the impingement point, the jets split upward and downward, flowing to create an “upper roll” above each jet and a “lower roll” in the lower regions of the strand. The exact nature of the flow pattern depends on the nozzleport shape and angle, the submergence depth, the cast section size, the injected gas fraction, and the extent of electromagnetic stirring; these variables are studied elsewhere.[1–4] B. ZHAO, Graduate Student, and B.G. THOMAS and S.P. VANKA, Professors, are with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana–Champaign, Urbana, IL 61801. Contact e-mail: [email protected] R.J. O’MALLEY, Plant Metallurgist, is with Nucor Steel Decatur LLC, Decatur, AL 35609. Manuscript submitted May 23, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
At the top surface, the molten steel should retain sufficient superheat and speed to avoid solidification of the meniscus, which can lead to subsurface “hook” formation and associated defects in the solidified product.[5] An insufficient superheat, thus, leads to increased surface defects. Finally, the superheat in the mold controls the internal microstructure and the associated macrosegregation of the final product. Specifically, the degree of superheat controls the formation and remelting of crystal nuclei that grow into the equiaxed grains that eventually comprise the center of the strand. These grains are beneficial for avoiding centerline segregation. Excessive superheat in the mold is, thus, associated with larger columnar grains and increased segregation and internal cracking problems. To avoid these defects, the liquid flow pattern and superheat must be carefully optimized. Clearly, there is great incentive to quantify the turbulent heat transfer in the mold region of continuous c
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