Fluid–structure interaction modeling of unsteady bulging on the solidifying shell during slab continuous casting

Under high casting speed, unsteady bulging of the solidifying shell is a primary cause of abnormal mold level fluctuations, posing significant threats to slab quality and production efficiency. Suppressing unsteady bulging is therefore a critical challenge in the development of next-generation continuous casting technologies. In this study, an advanced fluid–structure interaction (FSI) approach is adopted to achieve a comprehensive mathematical description of unsteady bulging and to explore its underlying mechanisms. The proposed model couples a Computational Fluid Dynamics (CFD) code based on the Large Eddy Simulation (LES)–Volume of Fluid (VOF) method with a Lagrangian Finite Element Method (FEM) code via a sequential coupling scheme. This framework enables simultaneous prediction of flow, solidification, heat transfer, and deformation behavior during the casting process. The model demonstrates excellent consistency at the fluid–solid interface, with relative errors of only 0.3% for displacement and 2.2% for temperature. Compared with fluid-only simulation (FOS) and solid-only simulation (SOS), the FSI model removes the decoupling between flow and deformation, allowing mutual interaction between the two fields and achieving enhanced multi-physics coupling. The simulation results also show improved agreement with practical observations. Based on this model, the mechanism of unsteady bulging is revealed: under high casting speed, the deformation of the strand is alternately governed by promoting factors (ferrostatic pressure of molten steel) and restraining factors (shell thickness and stiffness). Moreover, the non-uniform temperature distribution induced by the impinging high-temperature jet flow significantly amplifies bulging instability. • A fluid–solid interaction (FSI) framework for the simulation of slab continuous casting is developed. • The FSI model demonstrates significant advantages over traditional fluid-only and solid-only simulations. • The mechanism of unsteady bulging during slab continuous casting is revealed through FSI simulation.