Mechanistic study of unsteady bulging formation in slab continuous casting based on fluid–structure interaction

Abnormal mold level fluctuations induced by unsteady bulging is a critical constraint in high-efficiency slab continuous casting. This study systematically investigates the formation mechanism of unsteady bulging using a fluid-structure interaction (FSI) framework. Results demonstrate that shell bulging is predominantly governed by high-temperature creep, contributing over 55% of total deformation and exceeding 70% in localized high-temperature zones. A pivotal finding is that intrinsic creep unsteadiness is not the direct driver of severe mold level fluctuations under normal conditions. Instead, abnormal level oscillations originate from the rapid dynamic transition between positive bulging and negative displacement. This transition generates a significant instantaneous volumetric change rate (VC) of the molten pool, which is positively correlated with level fluctuation amplitudes. Furthermore, negative displacement is spatially predictable and coupled with secondary cooling configurations. These regions consistently emerge near the downstream ends of cooling zones (e.g., rolls 5# and 11#), where local surface temperature troughs significantly enhance deformation resistance, leading to rigid squeezing under roll support. Conversely, temperature recovery at subsequent zone entrances softens the shell, restoring stable bulging behavior. Based on these insights, strategies to enhance mold level stability are proposed, including the optimization of cooling gradients at zone boundaries to suppress temperature troughs and targeted roll-gap management in high-instability regions. This research provides a theoretical foundation for improving high-precision mold level control in modern steel production.