Evolution of nonmetallic inclusions in high Mn–high Al steel during heat treatment

Abstract

In order to reveal the evolution behaviours of the inclusions in a high Mn–high Al steel ( w(Mn) = 18 wt% and w(Al) = 1.5 wt%) at 1200 °C, heat treatment experiments were carried out in laboratory for different hours (0–10 h). It was found that, CaO-Al2O3-MnO-MgO system oxides (Type I), CaO-Al2O3-MnO-MgO + (Ca, Mn)S complex oxysulphides (Type II) and (Ca, Mn)S sulphides (Type III, w(MnS) > 95 wt%) were detected before and after heat treatment. During heat treatment, the dominant inclusions changed from Type I (oxides) into Type II (oxysulphides), and the size of the inclusions increased slightly. With heating time, the Al2O3 content increased in the oxide part of Type I and Type II inclusions, while the content of CaO decreased. MnS was quite stable in the steel, and it can precipitate on the oxides, thus forming a (Ca, Mn)S transition layer in between the MnS outer layer and the oxides. The MnS outer layer might be helpful to reduce the stress concentration during rolling.