Growth Behavior of MnS on CaO-MgO-Al₂O₃ Oxide in Al-killed Ca-treated Resulfurized Steel

Abstract

The addition of Ca is a useful method to control the shape of sulfides in steel. In this paper, in order to identify growth behavior of MnS on CaO-MgO-Al₂O₃ oxides in steel, based on two heats of Al-killed Ca-treated resulfurized steel, the characteristics of duplex (Ca,Mn)S inclusions in bars and blooms were observed and analyzed. It is found that there are three types of duplex (Ca,Mn)S inclusions with CaO-MgO-Al₂O₃ core oxides, named as Type-C, Type-MC and Type-M, respectively. In bar, Type-C behaves circular shape, Type-MC behaves spindle shape, and Type-M behaves long strip shape. From Type-C to Type-M, CaO or MgO content in core oxides decreases, Al₂O₃ content in core oxides increases, sizes of core oxides decrease, and the area ratios of wrapping sulfides and core oxides increase. Growth of MnS on CaO-MgO-Al₂O₃ oxides are influenced by the sizes and compositions of oxides. MnS inclusions are easier to grow on CaO-MgO-Al₂O₃ oxides with smaller sizes, lower CaO content, lower MgO content or higher Al₂O₃ content. In order to obtain more spindle-shaped duplex (Ca,Mn)S inclusions, appropriate compositions of core oxides are 5~20% CaO, 5% MgO and 75~90% Al₂O₃, and appropriate sizes of core oxides are 1~3μm.