Properties of Mould Fluxes and Slag Films

Abstract

In this chapter, the properties and the structure of the slags used in continuous casting are collated, analysed and discussed. The required property values of mould slags for successful casting are determined by the mould dimensions, the casting conditions and the steel grade being cast; these can be calculated using empirical rules. Required values of the powder consumption (Q s req ) viscosity (ηreq), break temperature (T br req ) and the fraction crystal in the slag film (fcrys) are the key properties. However, other properties such as the interfacial tension are important when dealing with slag entrapment, thus a large amount of property data for mould slags are needed. Mould slags have been classified here into two groups (i) Conventional slags used in continuous casting and (ii) Specialised slags, which includes F-free slags, non-Newtonian slags, calcium aluminate (CA) type slags, etc. Some properties are very dependent on the structure of the slags. Thus, published data on slag structures for all types of slag have been collated and analysed. Routines, developed to estimate the various properties, are outlined and reviewed. The data for the following properties of the various mould slag families have been reviewed (i) viscosity (ii) Liquidus (Tliq), Break (Tbr) and Glass transition (Tg) temperatures (iii) Thermal conductivities (iv) Surface and Interfacial tensions (v) Densities and thermal expansion coefficients (vi) Heat capacity and enthalpy and (vii) Optical properties. In addition, the crystallisation process in mould slags and the ability of the liquid slag to dissolve inclusions are reviewed and discussed. The requirements and the properties of slags used in continuous casting and ingot casting are compared and discussed.