Transient Thermo-fluid Model of Meniscus Behavior and Slag Consumption in Steel Continuous Casting
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ous casting of steel, initial solidification in the mold near the meniscus is very important to ultimate quality of the steel. Defects[1,2] arising near the meniscus remain at the surface of the eventual steel products, and are expensive or impossible to remove. To prevent oxidation of the molten steel by exposure to air, a layer of mold powder is maintained on the top surface by periodic additions of this carefully selected, proportioned, and mixed combination of metal oxide powders and graphite. This powder provides lubrication, maintains uniform heat transfer between the mold and steel shell, and removes inclusions that rise up from the molten steel.[3] Figure 1(a) shows a schematic of the continuous casting process. Liquid steel flows from the tundish (not shown in figure) into the mold, through the submerged entry nozzle’s (SEN) bifurcated ports that direct the flow of the molten liquid jets toward the narrow face mold walls and eventually upwards to the meniscus region at the top surface around the mold perimeter. Cooling water flows through the channels of the mold, and extracts heat, ASM JONAYAT, Graduate Research Assistant, and BRIAN G. THOMAS, C.J. Gauthier Professor, are with the Department of Mechanical Science and Engineering, University of Illinois at UrbanaChampaign, 1206 West Green Street, Urbana, IL-61801. Contact e-mail: [email protected] Manuscript submitted April 26, 2014. Article published online July 9, 2014. 1842—VOLUME 45B, OCTOBER 2014
causing the superheated liquid steel to solidify against the mold walls as a shell or steel strand, which is pulled downward at the casting speed. To prevent sticking, the mold oscillates with a given frequency, stroke (2 9 amplitude) and sometimes a modification ratio for nonsinusoidal oscillation.[4] During the casting process, the mold powder gets heated, sinters, and melts to form a molten slag layer that floats on top of the molten steel.[5] The shape of the interface between the slag and steel curves in the meniscus region, according to the surface tension, buoyancy, and momentum forces, and changes with time according to the mold oscillation and turbulent flow.[6,7] The liquid slag is eventually consumed into the thin gap between the mold and the solidifying steel shell by the downward movement of the strand. The amount of slag that has to be added to the mold surface over time is termed as slag consumption. Figure 1(b) shows a close-up schematic of the phenomena near the meniscus at the mold hot face, where the molten slag transforms to solid slag as it cools due to heat removal into the water-cooled mold. As a result, the gap between the mold hot face and the steel shell contains slag in two phases—solid and liquid. A thicker layer of solid slag termed the ‘‘Slag Rim’’ solidifies against the mold hot face above the liquid slag layer.[8] The slag rim sticks to the mold and oscillates with it. In addition to affecting the heat flux, the oscillating slag rim periodically pushes on the liquid or partially solidified meniscus[9] which may form depressions on the s
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