Thermodynamic examination of inclusion modification and precipitation from calcium treatment to solidified steel

Abstract

Calcium treatment is a well established method to transform oxide and sulphide inclusions in steel to less harmful inclusions which can have beneficial effects on properties. Transformation of solid alumina clusters to liquid calcium aluminates by calcium is a much used technique to avoid nozzle blocking in continuous casting of aluminium deoxidised steels. Calcium treatment is also a key method to improve machinability, which can be further improved by increased sulphur content. Resulphurised steels, however, tend to have casting problems due to deficient modification of oxides or formation of solid CaS inclusions which also have a tendency to clog the nozzle. In this study, the formation and transformation of liquid and solid inclusions containing oxidic and sulphidic components were examined by thermodynamic calculations. A quasichemical slag model was applied to calculate equilibrium oxide and sulphide inclusions in steel. Calculations were carried out over a wide temperature range to study the formation of species in the melt corresponding to ladle treatment conditions as well as casting. To examine inclusion behaviour during casting and solidification, the interdendritic solidification model, IDS, was coupled with thermodynamic equilibrium calculations. The results show the conditions in which liquid inclusions can be formed with calcium treatment. The calculations were performed at different temperatures and varying calcium, aluminium and sulphur contents. The effect of total oxygen content was also studied. The main components in liquid calcium aluminates were CaO and Al2O3 plus some SiO2 and CaS. The stability relations of different compounds alter as a function of temperature. An abrupt change occurs during solidification when the partition of the inclusion forming elements between the liquid and solid iron phases are taken into account.