When CaO–Al2O3 based mould fluxes for casting high aluminium transformation induced plasticity steel are used, some problems such as thick slag rims and low slag consumption resulted in the failure of sequence casting. The main reason for those problems is the strong crystallisation tendency of mould fluxes. In this paper, an HF-200 mould slag film simulator, hot thermocouple technique, scanning electron microscope and X-ray diffraction were applied to investigate the crystallisation tendency of mould fluxes at different CaO/Al2O3 ratios. The results showed that the thickness of slag film varied with CaO/Al2O3 ratio, being a minimum at 1·5 where the crystallisation tendency was weakest. With the increase in CaO/Al2O3 ratio, the precipitated crystal changed from CaF2 and LiAlO2 to Ca3Al2O6 and CaO. In order to reduce the yield of thick slag rim and improve consumption, the optimum CaO/Al2O3 ratio of mould fluxes is 0·9–2·0.
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