Thermodynamics and Kinetics of the Modification of Al₂O₃ Inclusions

Abstract

The use of thermodynamic data in describing calcium modification of aluminium oxide inclusions has been summarised and reviewed. The majority of the published Al-S equilibrium diagrams, based on the following reaction:
3CaO+3S+2Al=3CaS+Al₂O₃,
vary significantly due to different sources for thermodynamic data, especially for the activity of CaO and Al₂O₃. Using ThermoCalc, the activities of CaO and Al₂O₃ have been calculated and compared to the published data. Calculations in the present work pertaining to the molten range of the system are in good agreement with the most recently published experimental data of Fujisawa et al.
Based on the assessed thermodynamic data and observed phenomena during calcium modification of inclusions in steel melts of moderate sulphur content, a model for alumina modification by calcium treatment has been developed. According to the model, the reactions progress by the following sequence until the activity of Al₂O₃ becomes so low that precipitation of CaS occurs:
Al₂O₃⇒CA₆⇒CA₂⇒CA⇒CA_x(liquid)