The sub-surface entrapment of alumina inclusions, slag and molten mold powder (or flux) particles in continuously cast steel slabs often leads to formation of sliver defects (refer to Figure 1) on the surface of final rolled products. This causes rejection and reworking of defective coils, particularly those produced from ultra low carbon (ULC) steel slabs, which are slated for surface-critical applications. Additionally, surface inspection systems have to be installed at the rolling mills to inspect every coil and detect any sliver defects, so that customer interests are always protected. Hence, from the standpoint of cost of poor quality, particle entrapment during the casting process is of primary concern to steelmakers. Figure 2(a) schematically summarizes various phenomena in a continuous caster mold that can lead to particle entrapment in the solidified steel shell. The metal jet exiting the submerged entry nozzle (SEN) transmits kinetic energy into the liquid pool and creates a turbulent double-roll type fluid flow pattern in the mold. Alumina particles, a by-product of the "killing" process that occurs during secondary steelmaking, are often present in the liquid steel entering the mold. Carry-over slag particles may also be transported into the mold from the ladle or tundish. The trajectory and residence time of these particles in the liquid pool will obviously be influenced by transient fluid flow patterns developed inside the mold. Additionally, interaction between the turbulent liquid steel level and molten flux layer at the top may cause entrainment of mold powder particles into the liquid pool.
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