The Performance and Properties of Lime-Alumina Based Mould Powders during Continuous Casting of High-Aluminium Steels

High-aluminium steels contain a significant amount of aluminium. The reaction between Al in the liquid steel and SiO2 in lime-silica-based mould powders during the continuous casting process of high Al steel causes chemical compositional changes in the mould powders, subsequently affecting the surface quality of slabs. In order to solve the aforementioned problem, lime-alumina based mould powders have been developed which can lead to an increase in the surface quality of cast slabs by inhibiting molten steel/slag interaction. However, the mould slag tends to crystallise easily, which leads to a deterioration of the mould lubrication. In view of this aspect, it is important to develop and optimize lime-alumina based mould powders to meet the requirements of continuous casting of high-aluminium steels. In this study, the changes in crystallinity, viscosity and melting temperature of lime-alumina-based mould powders with the addition of B2O3, TiO2 and ZrO2 and the effects of increasing the CaO/Al2O3 ratio have been observed through STA (Simultaneous Thermal Analysis), HSM (Hot Stage Microscopy), XRD (X-ray Diffraction), IPT (Inclined Plate Test) and rotational viscometry. The crystallisation behaviour of these mould powders were evaluated by generating CCT (continuous cooling transformation) diagrams. The changes in slag chemistry over time have also been analysed using XRF (X-ray fluorescence). Additionally, FactSage software and empirical models were used to calculate relevant thermo-physical properties of the mould powders. The results of these analyses demonstrated that crystallinity of lime-alumina-based mould powder is suppressed while the melting temperature and viscosity are decreased by B2O3 additions. However, the addition of TiO2 and CaO/Al2O3 tends to inhibit liquid steel/slag interaction and enhance the crystallisation of mould powders. Finally, it has been reported that 5%wt. ZrO2 addition does not affect the thermodynamics of lime-alumina based slag system.