On the onset of air-gap formation in vertical continuous casting with superheat

The formation of an air gap at the mould-metal interface in continuous casting has long been known to have a detrimental effect on the efficiency of the process, and has therefore attracted many attempts at mathematical modelling. While many efforts consist of complex three-dimensional numerical simulations of the phenomenon, recent modelling work for a two-dimensional geometry and pure metals has used asymptotic techniques to derive a quasi-analytical model that captures the essential characteristics. The model allows for full two-way coupling between the thermal and mechanical problems: the formation of the air gap affects the heat transfer, whilst the heat transfer affects the stresses that lead to the formation and evolution of the air gap. In this contribution, numerical results obtained earlier for the case of superheat - when the molten metal temperature is greater than the metal's melting temperature - are complemented by an analysis of the criterion that predicts how the onset of air-gap formation depends on process parameters: the mould temperature, the casting speed and the superheat itself.