Prediction of oxide phases formed upon internal oxidation of advanced high-strength steels

The effect of Cr on the oxidation of Fe–Mn-based steels during isothermal annealing at different dew points was investigated. The Fe–Mn–Cr–(Si) phase diagrams for oxidizing environments were computed to predict the oxide phases. Various Fe–Mn steels with different concentrations of Cr and Si were annealed at 950 °C in a gas mixture of Ar or N₂ with 5 vol% H₂ and dew points ranging from − 45 to 10 °C. The identified oxide species after annealing match with those predicted based on the phase diagrams. (Mn,Fe)O is the only oxide phase formed during annealing of Fe–Mn binary steel alloys. Adding Cr leads to the formation of (Mn,Cr,Fe)₃O₄ spinel. The dissociation oxygen partial pressure of (Mn,Cr,Fe)₃O₄ in the Fe–Mn–Cr steels is lower than that of (Mn,Fe)O. The Si in the steels results in the formation (Mn,Fe)₂SiO₄, and increasing the Si concentration suppresses the formation of (Mn,Cr,Fe)₃O₄ and (Mn,Fe)O during annealing.