Robust control design to prevent unsteady bulging in continuous slab casters

In continuous casting of steel slabs, the ferrostatic pressure in the liquid core of the strand causes bending (also called bulging) of the strand shell between the guiding rolls. Unsteady bulging refers to a time-varying bending of the strand shell, leading to unwanted mold level fluctuations that can degrade the quality of the cast strand. Most mold level controller designs presented in the literature do not explicitly account for this disturbance due to the absence of suitable unsteady bulging models. As a result, these controllers often fail to sufficiently suppress unsteady bulging or even provoke it. This work presents a novel robust model-based controller design that systematically considers unsteady bulging by incorporating a control-oriented unsteady bulging model. Unlike previous approaches, the proposed method also accounts for variations in plant parameters to ensure robust suppression of unsteady bulging and consistent control performance across the entire range of operating conditions. The proposed controller is validated on industrial continuous slab casters, where it is now permanently used and achieves a significant reduction in mold level fluctuations.