Modeling transient slag-layer phenomena in the shell/mold gap in continuous casting of steel
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9/12/03
10:24 AM
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Modeling Transient Slag-Layer Phenomena in the Shell/mold Gap in Continuous Casting of Steel YA MENG and BRIAN G. THOMAS Mold-slag friction and fracture may cause heat-transfer variations in continuous casting, which leads to steel shell temperature and stress variations, resulting in surface cracks. Analytical transient models of liquid slag flow and solid slag stress have been coupled with a finite-difference model of heat transfer in the mold, gap, and steel shell to predict transient shear stress, friction, slip, and fracture of the slag layers. The models are validated by comparing with numerical models and plant measurements of mold friction. Using reported slag-fracture strength and time-temperature-transformation (TTT) diagrams, the models are applied to study the effect of casting speed and mold-powder viscosity properties on slag-layer behavior between the oscillating mold wall and the solidifying steel shell. The study finds that liquid-slag lubrication would produce negligible stresses. A lower mold-slag consumption rate leads to high solid friction and results in solid-slag-layer fracture and movement below a critical value. Crystalline slag tends to fracture near the meniscus and glassy slag tends to fracture near the mold exit. A medium casting speed may be the safest to avoid slag fracture, due to its having the lowest critical lubrication consumption rate. The high measured friction force in operating casters could be due to three sources: an intermittent moving solid slag layer, excessive mold taper, or mold misalignment.
I. INTRODUCTION
MANY phenomena in continuous casting, including the formation of surface defects, are greatly affected by heat transfer in the mold.[1–5] The interfacial slag layer(s) between the solidifying steel shell and the mold wall dominates the resistance to heat removal and, thus, controls mold heat transfer in powder casting.[6,7,8] Surface defects, such as longitudinal cracks and star cracks, have been attributed to variations of slag lubrication.[9,10] High meniscus heat transfer and variations in meniscus heat transfer correlate with increased surface or subsurface defects,[9,11,12] but the reasons are not understood. Thus, an improved understanding of slag-layer behavior is important for steel quality. In continuous casting, mold powder is added to the free surface of the liquid steel. It sinters and melts, spreading over the liquid steel surface according to the steel-surface contour and flow pattern.[13] During each oscillation stroke, liquid slag is pumped from the meniscus into the gap between the steel shell and the mold wall, where it acts as a lubricant,[14,15] so long as it remains liquid. A solid slag layer forms against the mold wall. Its thickness increases greatly just above the meniscus, where it is called the slag rim. Depending on the composition and cooling rate of the mold slag, the microstructure of the multiple layers that form may be glassy, crystalline, or mixtures of both.[16,17] Figure 1 shows a typical schematic
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