Modelling solidification and slag infiltration during the continuous casting of slabs

Quality requirements for continuously cast slabs are stringent since any defects introduced in the product during solidification within the mould may be detrimental to either further processing or the final properties. For instance, deep oscillation marks on the slab surface can cause cracking during hot-rolling. The need to predict the formation of these defects has driven the development of numerical models to simulate slab solidification inside the mould. Although almost all major steelmaking companies are using some form of modelling, most models have the same limitation; the rate of heat removal is empirically derived from plant data. As expected, these models have proven reliable when the conditions analysed are close to those of the data used in the derivation. However, this approach is not accurate when the conditions are extrapolated. This is the case with significant increases in casting speed or new steel grades such as TRIP or TWIP steels, which are especially difficult to cast. Nor do existing models accurately describe more complex phenomena such as the formation of oscillation marks or the relation between slag consumption and mould oscillation frequency. In this paper we present a model which overcomes many of these limitations by directly simulating slag infiltration, metal flow, shell solidification, and heat transfer through the mould. This has allowed the first direct predictions of liquid slag film development and the resultant heat flux fluctuations, showing an excellent correlation to both of water models and high temperature mould simulators. Predicted shell thicknesses and heat fluxes as a function of casting speed are also in good agreement with plant measurements. These predictions provide new insights into the interaction between the initial solidification stages, slag infiltration, and oscillation mark formation.