Solidification of mould fluxes

It is well known that the physical and chemical properties of mould fluxes are important to the successful operation of continuous casting machines. The principal role of mould fluxes is to provide lubrication between the solidifying steel and the mould but mould fluxes also significantly influence heat transfer between the stand and the mould. Heat extraction from the steel being cast has a marked effect on the quality of the final product and several defects in cast products can be related to mould flux performance. Heat transfer in mould flux is complicated by the fact that the flux partially solidifies when in contact with the mould. Different rates of solidification of mould fluxes and the resultant structures are critical to the heat transfer characteristics of the mould. Mould fluxes are largely made up of oxides which can form crystalline or glassy solids on cooling from a melt, depending on composition and cooling rate. It is known that heat transfer in a crystalline material differs from heat transfer in the glassy or liquid form of the same material due to the differing heat transfer mechanisms operating in each state. In order to understand heat transfer through mould flux, it is essential to understand the contribution of liquid, glassy, and crystalline structures to the overall heat transfer. This in turn requires an understanding of the solidification behaviour of mould fluxes. Until recently, this knowledge has been based mostly on empirical studies, though more recent research has attempted to relate observed crystallization and glass forming phenomena to fundamental principles. Special techniques have been developed to study mould flux solidification, however, the thermal conditions and the mould flux structures' obtained during casting are difficult to reproduce in the laboratory. Methods such as differential thermal analysis, quench tests, high temperature microscopy, single and double hot thermocouple techniques, confocal microscopy and others have been applied, with varying degrees of success, to quantify crystallization and glass formation in mould fluxes. This paper summarises two studies carried on industrial mould fluxes at the University of Wollongong in cooperation with BHP Steel.