Behaviour of Sulphide and Non-alumina-Based Oxide Inclusions in Ca-Treated High-Carbon Steel

Although alumina-based inclusions in Al-killed steel have been widely investigated, few studies focus on non-metallic inclusions in Si-killed steel. Most high-carbon steel, which is expected to have higher potential for the formation of alumina, is deoxidised by silicon to avoid alumina-based inclusions. Many types of high-carbon steel such as free-cutting steel and grinding media for the mining industry include sulphur and calcium; therefore, non-alumina-based oxide and CaS-based inclusions co-exist in molten steel. In this study, the behaviour of sulphide and non-alumina-based oxide inclusions in Ca-treated liquid steel was investigated via transition of the composition and morphology in the liquid state. Using a confocal scanning laser microscope (CSLM), sulphide particles being pulled toward oxide inclusions were directly observed and changes in the composition of inclusions investigated using different analytical techniques. We identified two types of sulphide phase after remelting and cooling, polygonal particles and a ring-shaped layer around the oxide phase. When the sulphide capacity Cs multiplied liquid fraction of oxide had a higher value, the sulphide layer tended to become wider.