Selection of Mould Fluxes and Special Mould Fluxes for Continuous Casting

Abstract

The various factors influencing the selection of mould powder compositions are discussed in this chapter. Mould powders contain two types of component (i) the minerals which form the slag film and (ii) Carbon which controls the melting rate. Both are vital to the successful performance of the mould powder. The composition of the mould powder is determined by the mould dimensions, the casting conditions and the steel grade being cast. Empirical rules have been developed to provide the required values of the powder consumption (Q s req ), the viscosity (ηreq) and the break temperature (T br req ) for satisfactory casting (i.e. free from longitudinal cracking and sticker break-outs). More than 85% of mould powders conform to these empirical rules. However, other factors can also influence the powder selection process. These include (i) Increasing the slag viscosity or the interfacial tension to reduce slag entrapment. (ii) Increasing the interfacial tension to minimise scaling of the steel surface. (iii) Minimising carbon pick-up (especially for LC and ULC steels). (iv) Providing both support in the mould and optimal heat transfer when casting round billets. (v) Providing reasonable heat transfer in mould configurations with corners acting as heat sinks. (vi) Handling the large amounts of Al2O3 formed when casting high-Al steels whilst maintaining good lubrication throughout the casting. (vii) Handling Ti-stabilised steels which form either TiN or Ti(CN) in the slag pool (which has low solubility in mould slag) or CaO·TiO2 (which has a high Tliq). (viii) Reducing the fluoride content of slag to minimise environmental and health concerns. All of these problems are discussed in detail and possible solutions are outlined.