ISIJ International, Vol. 46 (2006), No. 11, pp. 1635–1644

In the conventional continuous casting of steel, either mold powder or oil can be added to lubricate the mold from sticking to the solidifying steel shell. The mold powder melts to form a molten slag or flux layer, and also acts to protect the molten steel from oxidation, to insulate the molten steel from heat loss, to absorb inclusions from the molten steel, to regulate heat transfer to mold wall, and to minimize the formation of surface defects. Thus, most of world’s steel production is continuous-cast using slag lubrication, which leads to more uniform and usually lower heat transfer between the strand and the mold wall compared with oil lubrication. The slag infiltrates into the narrow gap between the surface of the shell and mold, and creates an “interfacial layer,” as shown in Fig. 1(a), which controls both mold heat transfer and lubrication. A thicker interfacial layer is associated with a higher rate of flux infiltration into the gap. This provides better lubrication and lowers heat flux. It also improves heat flux uniformity, which decreases surface defects. To maintain stable continuous casting, enough mold slag must infiltrate between the solidifying shell and the mold wall to lower friction, prevent sticking, and avoid defects and sticker breakouts. Ideally, the slag consumption should be large enough to maintain a continuous molten slag layer over the entire surface of the strand in the mold. The mold powder consumption decreases as the casting speed (vs in mm s ) increases. This restricts operation with higher casting speed, because breakouts are more likely to occur due to the thinner shell thickness and higher frictional force during high speed casting. Consumption is readily measured by counting the mass of mold powder added per unit time (kg/s). Normalized consumption per unit area of strand (Qarea in kg m ) is a better way to represent lubrication, because it provides a measure of the mean flux thickness between the solidifying shell and the mold wall that is independent of mold perimeter, so long as consumption is uniform. Liquid layer thickness is sometimes estimated from the density of liquid slag (r slag in kg m ), i.e. dgap Qarea/r slag. However, this relation is inaccurate because it neglects the velocity gradients which exist across the liquid flux layer, the flux transported by the moving solid layer, and the flux consumed in the oscillation marks. The interfacial gap consists of layers of liquid and solid