Determination of thermal resistance at mould-strand interface due to shrinkage in billet continuous casting – Development and application of a novel integrated numerical model

Abstract

An industrial continuous casting process of steel billet is analysed using a novel, integrated numerical model for the heat transfer and solidification of the molten metal as it passes through the mould. One of the major difficulties related to determination of thermal resistance at the mould-strand interface is addressed in the present study. Contrary to empirical or semi-empirical methods commonly used in the industry for determination of the heat transfer at the interface, the present work deals with the determination of the interfacial thermal resistance by adopting a novel mathematical approach. A two-dimensional, transient, finite difference method based code is developed for a plane slice of molten steel moving with a given casting speed down the mould. The resultant solidification and thermal shrinkage are calculated from the temperature distribution and knowledge of the temperature dependent density of steel. The heat transfer from the cast strand to the cooling water is modelled by a network of thermal resistance. The model is validated against experimental data obtained from literature. The numerical results reveal a dominant role of advection of the gaseous mixture of air and other components from pyrolysis of casting oil, in the interfacial gap formed due to the shrinkage of steel, which has not been considered previously in the formulation of thermal resistance. The numerical model is also used to analyse the heat transfer in the mould and the growth of the solidified shell as the strand moves down the mould. In particular, the corner regions of the strand are found to be hotter than the off-corner regions, due to non-uniform heat transfer across the periphery. Also, the effect of casting speed in the mould is found to be dominant in comparison to other casting parameters, such as mould thickness and cooling water velocity.