Thermodynamics of yttrium, calcium, magnesium and aluminium interaction with oxygen in liquid steel

Yttrium belongs to the same group as lanthanum, and its chemical properties are close to those of the latter. Therefore, yttrium is referred to as a rare earth metal. When added to a heavily deoxidized metal, yttrium refines a primary cast grain, reduces the area of treeing, purifies grain boundaries of non-ferrous inter-metallic compounds, reduces the size of non-metallic inclusions, and enhances heat resistance and high temperature strength of special steels. Optimal yttrium additive in the metal does not exceed 0.3 to 0.5 wt. %. Complex interaction of yttrium with oxygen in the presence of residual concentrations of aluminum, calcium and magnesium in the metal was not studied at all. Thermodynamic modelling of steel deoxidation using yttrium in the presence of low concentrations of aluminum, calcium or magnesium is developed. The surfaces of component solubility in liquid steel are created for Fe-Y-Al-O-C, Fe-Y-Ca-O-C, and Fe-Y-Mg-O-C systems. To make calculations related to oxide systems, all possible phase equilibria are established, coordination of equilibrium constants dependency on temperature is carried out, and energy parameters of the subregular ionic solution theory are specified. The surfaces of component solubility show that deoxidation by yttrium is a complex process due to the presence of calcium and aluminum in steel. Deoxidation products will contain complex phase compositions of oxides based on CaO, Al2O3, and Y2O3. In case of steel deoxidation by yttrium and magnesium, the process is performed alternately. Either yttrium or magnesium acts as a deoxidizer.