Effect of oxides on MnS precipitation in aluminum-deoxidized steel

High strength steel with slightly higher S content, deoxidized by aluminum, was smelted in a vacuum induction furnace of 10 kg capacity. Total oxygen in the steel was 0.0010%, and the S content reached 0.0190%. An ASPEX explorer auto-scanning electron microscope (SEM) was used to detect nonmetallic inclusions in the steel. It is found that 98% of the inclusions are dispersed MnS and MnS+Al₂O₃ complex inclusions. The shape of MnS inclusions is angular, belonging to type III. Al₂O₃ inclusions, with an average diameter of 1.5 μm, acting as nucleation for MnS precipitation, generate wrapped inclusions of MnS+Al₂O₃, and they rank 9% to 32% of the total inclusions. The formation process of MnS+Al₂O₃ can be described as follows:small size Al₂O₃ inclusions are pushed to liquid phase at melt steel solidification, and then Al₂O₃ inclusions work as the sites of MnS inclusion precipitation due to Mn and S microsegregation. However, related calculations about the pushing and engulfment behavior of Al₂O₃ inclusions were carried out. The calculated results indicate that Al₂O₃ inclusions with the size smaller than 4 μm are pushed in liquid steel, acting as heterogeneous nucleation for the formation of MnS inclusions.