Calcium treatment is a well-established method, nowadays, to transform oxide and sulphide inclusions in steel to less harmful inclusions which can even have beneficial effects to steel properties. Transformation of solid alumina clusters to liquid calcium aluminates by calcium is a common technique to avoid nozzle blocking problems in continuous casting of Al-deoxidized steels. Nowadays, calcium treatment is also one of the key methods to improve steel machinability. Further improvements in machinability can be attained by increased sulphur content. Resulphurized steels, however, tend to have casting problems due to deficient modification of oxides or formation of solid CaS inclusions which have a clogging tendency too. In this study formation and transformation of liquid and solid inclusions containing oxide and sulphide 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 in a wide temperature range in order to study both the formation of species in the melt corresponding to ladle treatment conditions as well as casting. In order to examine inclusion behaviour during casting and solidification the IDS solidification model was coupled with the thermodynamic equilibrium calculations. The results show the conditions in which liquid inclusions can be formed with Ca-treatment. The calculations were performed at different temperatures and varying calcium, aluminium and sulphur contents. The effect of total oxygen content was studied as well. The main components in liquid Ca-aluminates were CaO and Al_2O_3 plus some SiO_2 and CaS. The stability relations of different compounds alter as a function of the temperature. An abrupt change happens during solidification when the partition of the inclusion forming elements between the liquid and solid iron phases are taken into account.
展开▼
