Coupled thermo-mechanical model of solidifying steel shell applied to depression defects in continuous-cast slabs

A coupled finite-element model has been developed to simulate the thermal-mechanical behavior of a transverse slice through the solidifying steel shell as it moves down through the mold and upper support rolls in the spray chamber of a continuous slab-casting machine. The heat transfer model incorporates the effect on solidification of superheat delivered by turbulent flow in the liquid pool. The stress model evaluates the highly non-linear elastic-visco-plastic constitutive equations using an efficient time iteration algorithm which alternates between local and global levels. The stress model assumes a stress state of generalized plane strain and employs an efficient contact algorithm to achieve reasonable deformation of the shell, under the influence of internal ferrostatic pressure. The two models, which are coupled together through the size of the interfacial gap between the shell and the mold, have been validated with analytical solutions and plant measurements. Separate mathematical models have been applied to simulate fluid flow and heat transfer within the melting powder layer on the top surface of the liquid pool and thermal distortion of the mold. Together, these models are used to illustrate a multi-stage mechanism for the formation of longitudinal off-corner depression defects or `gutters' during continuous casting of steel slabs. Specifically, the depressions form during bulging below the mold, as a result of inadequate heat flow in the mold in the off-corner wide face region. The results suggest ways to avoid the problem that are consistent with industrial experience.