At the present time, rare-earth elements in metallurgy are used in the form of mischmetal – a rare-earth elements natural mixture (with atomic numbers from 57 to 71). It contains about 50 wt. % of cerium. The remaining elements are mainly lanthanum and niobium. The specific composition is determined by the ore deposit. Inconstant composition of the modifier containing rare-earth metals (REM) can significantly reduce its efficiency. Experimentally, for every branded steels composition the ratio of various REMs can’t be selected because of the high costs of obtaining technically pure rare-earth metals. The task of determining the each rare earth element optimum concentrations and complex ligature composition can be solved by thermodynamic modeling. In the framework of thermodynamic modeling, the interaction between magnesium, aluminum and lanthanum with oxygen in liquid iron is presented. And the thermodynamic model of steel deoxidation by these active metals composition is considered. On the basis of available literature data on the phase diagrams of the systems MgO – Al2O3 , MgO – La2O3 and La2O3 – Al2O3 , the coordinates of the invariant equilibria points in the system MgO – La2O3 – Al2O3 were determined. The phase diagram of the system MgO – La2O3 – Al2O3 was constructed. It made possible to establish all phase equilibria realized in the process of deoxidation of steel with magnesium, lanthanum and aluminum and to describe these phase equilibria by chemical reactions equations. The activity of the components in liquid oxide melts was determined using the theory of subregular ionic solutions, which takes into account the dependence of the coordination number of cations on the composition of the oxide melt. The activity of components in metal melts conjugated with oxide systems were determined by Wagner’s theory using the parameters of the first order interaction. Equilibrium constants values for the steel deoxidation reactions are installed indirectly by thermodynamic calculations. On the basis of the obtained data the components solubility surface in the metal melts of Fe – Mg – Al – La – O system was constructed, which allowed to determine the liquid metal composition regions associated with the corresponding oxide phase.
Phase equilibria in systems involving the rare-earth oxides. Part II. Solid state reactions in trivalent rare-earth oxide systems