Non-Isothermal Melt Crystallization Kinetics for CaO–Al₂O₃–B₂O₃ F-Free Mould Fluxes

Abstract

Efforts have been made to develop fluorine-free mould fluxes for the continuous casting of steel process. In this work the crystallization behaviour of fluorine-free mould slags based on the CaO–Al₂O₃–B₂O₃ system was investigated by differential scanning calorimeter (DSC) and scanning electron microscopy equipped with energy dispersive spectroscopy(SEM-EDS). The crystallization kinetics for Ca₃Al₂O₆ primary crystals was analysed by combining modified Avrami analysis with Friedman isoconversional method. Avrami parameter n is close to 4 for samples with the ratios w(CaO)/w(Al₂O₃)=1 and w(CaO)/w(Al₂O₃)=1.2, indicating a crystallization mechanism of continuous bulk nucleation and 3D crystal growth. The Avrami parameter n for samples with w(CaO)/w(Al₂O₃)=0.9 is close to 3, indicating instantaneous bulk nucleation and 3D crystal growth. The crystallization rate constant is the highest and half crystallization time is the lowest for the samples with w(CaO)/w(Al₂O₃)=0.9, indicating the fastest crystallization. In the initial stage, effective activation energies were mainly determined by the undercooling values. In the final stage, kinetic barrier for crystallization could have some influence on crystallization; for the investigated mould fluxes crystallization in the final stage is retarded by increasing w(CaO)/w(Al₂O₃) ratio. Thus crystallization mechanisms were elucidated and effective activation energy of crystallization for the first crystal which precipitates from melt was determined.