In the continuous casting of steel, unsteady bulging contributes to degradation of slab quality. It has been reported that unsteady bulging is promoted by uneven solidification in the mold, but the effect of uneven solidification on unsteady bulging has not been clarified. In this study, a Finite Element Method (FEM) simulation was conducted. Shell deformation was calculated by an elasto-plastic analysis assuming that the slab moves between the rolls, considering time dependency. The bulging value and mold level fluctuation, which change corresponding to the solidified shell thickness, ferrostatic pressure and roll pitch, were obtained.

In the simulation results, the shell is deformed by ferrostatic pressure. The bulging shell pushes out under the rolls in the thickness direction, and unsteady bulging occurs. While the shell is passing through rolls with the same pitch, unsteady bulging becomes larger. When the solidified shell is uneven, stress concentrates on the thinner portions, and this stress concentration accelerates unsteady bulging even at the same average shell thickness. Based on these results, an operational index for suppressing unsteady bulging by reducing unevenness of the solidified shell was proposed.