Analysis of phase equilibria in liquid chromium-manganese steels in the presence of calcium. Report 1. Phase diagrams of oxide systems.

The metallurgical industry produces a wide range of chromium-manganese steels, deoxidized and modified with calcium. To determine the non-metallic inclusions (NI) that can form in the Fe–Ca–Cr–Mn–C–O system, it is necessary to study the double and triple phase diagrams included in the FeO–CaO–MnO–Cr2O3 system. Divalent chromium oxide, which is also included in the oxide melt in the Fe–Ca–Cr–Mn–C–O system, practically does not occur in the literature as a component of double or triple state diagrams; therefore, the activity of chromium(II) oxide was equated to its ion fraction and state diagrams with it were not considered in this work. The state diagrams of oxide systems FeO–CaO, FeO–MnO, FeO–Cr2O3, CaO–MnO, MnO–Cr2O3, and FeO–MnO–Cr2O3 were previously studied by the authors. In this work, the state diagrams of the CaO–Cr2O3, FeO–CaO–Cr2O3, FeO–CaO–MnO, and CaO–MnO–Cr2O3 systems are calculated and plotted. The mixing energies are selected to determine activities in solid (theory of regular ionic solutions) and liquid (theory of subregular ionic solutions) oxide solutions to calculate the coordinates of the liquidus and solidus lines of these state diagrams. The enthalpy (103.4 KJ/mol) and entropy (21.958 J/(mol•K)) of melt-ing of calcium chromite CaCr2O4 are specified. The invariant points of the state diagrams of the FeO–CaO–Cr2O3 and CaO–MnO–Cr2O3 systems have been determined. The study of double and triple phase diagrams belonging to the FeO–CaO–MnO–Cr2O3 system will make it possible to determine all possible NIs that can form in chromium-manganese steels in the presence of calcium under equilibrium conditions.