Morphology and composition of inclusions in Si-Mn deoxidized steel at the solid-liquid equilibrium temperature

Morphology and composition of inclusions change with temperature. However, besides the temperature conditions during steelmaking or continuous casting, other factors contributing to changes in the morphology and composition of inclusions during solidification are still unknown. In this study, the formation of complex inclusions in Si-Mn deoxidized steel after isothermal holding at the solid-liquid equilibrium temperature (T_S) was investigated. The typical inclusions found in the alloy were MnO-SiO₂ based, spherically shaped and homogeneously distributed. With isothermal holding at the solid-liquid equilibrium temperature, formation of a secondary SiO₂-rich inclusion phase occurred. The changes in the composition of the inclusions depended on the manganese and silicon contents in the metal. The general mechanism of inclusion formation observed in this study can be divided into three steps: 1) the formation of primary MnO-SiO₂ inclusions above the liquidus temperature when the steel is in a completely molten state as a result of the deoxidation process; 2) the nucleation of secondary inclusions as the molten steel becomes supersaturated with the solute elements while holding at the solid-liquid equilibrium temperature; and 3) the growth and coalescence of inclusions due to natural convection in the molten alloy. From this, the inclusions formed in Si-Mn deoxidized alloys held isothermally at the solid-liquid equilibrium temperature were of three types: primary MnO-SiO₂ inclusions, secondary SiO₂ inclusions and complex inclusions with both primary MnO-SiO₂ inclusions and precipitated secondary SiO₂ inclusions.