Flow and thermal behavior of the top surface flux/powder layers in continuous casting molds
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INTRODUCTION
IN the continuous casting process, mold flux is added as a powder on to the surface of the liquid steel. The flux absorbs heat from the molten steel, sinters, and melts into a pool of liquid, beneath the floating powder. Below the liquid flux layer, steel enters the mold cavity through the submerged entry nozzle (SEN), recirculates up the narrowface wall and along the top surface, and flows back toward the SEN (Figure 1). The top-surface flux layers provide thermal and chemical insulation of the top surface of the liquid steel and aid in the removal of nonmetallic inclusions. The liquid flux then infiltrates the gap between the mold wall and the solidifying steel strand, where it acts as a lubricant and promotes slow, uniform heat transfer in the mold-strand gap. Collectively, these functions work to reduce such problems as surface and subsurface inclusions, nonuniform shell growth and breakouts, surface depressions, and cracks. Given the impact that the mold flux has on steel slab quality, many previous studies have investigated various aspects of powder/flux behavior. Most have been experimental or industrial. For example, studies performed at Nippon Steel (Kimitsu, Japan) I~,21 and Inland Steel (East Chicago, IN) ~31have confirmed an inverse relationship between optimum liquid flux viscosity and casting speed to achieve uniform heat flow and effective lubrication in the mold-stand gap. Other work by Sardemann and Schrewe~41
R.M. McDAVID, Graduate Research Assistant, and B.G. THOMAS, Associate Professor of Mechanical Engineering, are with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana~hampaign, Urbana, IL 61801. Manuscript submitted July 25, 1995. 672--VOLUME 27B, AUGUST 1996
has shown the importance of maintaining a sufficiently thick liquid slag layer. Specifically, they found that the formation of cracks in the steel slab is reduced by a thick liquid flux layer. Sufficiently thick liquid slag layers are also important to prevent carbon "pickup" by the steel from the flux, as reported by Nakato e t altS] Several researchers have characterized the formation of the liquid flux layer using the empirical property of "melting rate."t6.v.sj For example, BranionI71 comments that the melting rate determines the ability of the flux to maintain a stable liquid layer. Using an apparatus developed by Lindefelt and Hasselstrom,tgl several investigators have measured the rate at which molten flux is produced. This melting rate decreases with increasing carbon present in the flux, as demonstrated by Xie. E6J Flux behavior has also been investigated using mathematical models. Most previous models attempt to characterize flux behavior in the thin mold-strand gap.t~o-~n Bommaraju and Saad ['~ modeled flux flow as Couette flow between the stationary copper mold and the steel shell moving at the casting speed. Anzai e t aL[~41 go a step further by solving the 1-D Navier-Stokes equation which includes the pressure term neglected in the work by Bommaraju. Anzai predicted
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