Interfacial Friction-Related Phenomena in Continuous Casting with Mold Slags

Many phenomena in continuous casting including the formation of surface defects are greatly affected by heat transfer in the mold. The interfacial slag layers between the solidifying steel shell and the mold wall dominates the resistance to heat removal and thus controls mold heat transfer. Surface defects, such as longitudinal cracks and star cracks have been associated with variation of slag lubrication . High meniscus heat transfer and variation in meniscus heat transfer mechanisms correlate with increased sliver defects, but the reasons are not understood. Thus, 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. 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, 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. Figure 1 shows a typical schematic of this region of the continuous casting process. A substantial fraction of the slag consumed in the mold is entrapped in oscillation marks moving down at the casting speed. When a solid layer stably attaches to the mold wall, the remaining slag consumed is from the flowing liquid layer and is here called “lubrication consumption”. The hydrostatic or “ferrostatic” pressure of the molten steel pushes the unsupported steel shell against the mold walls, causing friction between the steel shell and the oscillating mold wall. At the corners, the shell shrinks away to form a gap, so friction is negligible. Another significant source of friction is at the bottom of the narrow faces, if excessive taper generates friction by squeezing the wide face shell. Finally, misalignment of the mold and strand can cause friction, especially if the stroke is large. It has been proposed that friction may impede increased casting speed. We believe that friction may also cause fracturing of the solidified slag layer that produces local heat flux variation. The accompanying temperature and stress variations in the steel shell