Evolution and deformability of inclusions in Al-killed steel with rare earth-alkali metals (Ca or Mg) combined treatment

The quality of stainless steel is closely related to the deformability of inclusions, which is significantly affected by their compositions. The present study first investigated the evolution of inclusion compositions in Al-killed steel with rare earth-alkali metals (Ca or Mg) combined treatment through four laboratory-scale experiments. The Ce contents in the final steel are 0.0080 wt%, 0.015 wt%, 0.016 wt% and 0.010 wt%, respectively. The Mg content is 0.0014 wt% in Ce–Mg combined treated steel, and the Ca content is 0.0015 wt% in Ce–Ca combined treated steel. The deformability of inclusions in both Ce₂O₃–Al₂O₃–CaO and Ce₂O₃–Al₂O₃–MgO systems was subsequently evaluated by calculating their Young's modulus at low temperature. The results show that irregular Al₂O₃ and MgAl₂O₄ with poor deformability are modified to CeAlO₃ and Ce₂O₃ by Ce treatment, resulting in the decrease of Young's modulus of inclusions. The deformability of inclusions is further improved due to the transformation from lumped-like CeAlO₃ to spherical CaO–Al₂O₃–Ce₂O₃ caused by Ca treatment, and some of these inclusions are the ones with low liquidus temperature. Thermodynamic analysis was used to discuss the control condition of the formation and evolution of inclusions. Accordingly, the appropriate addition amounts of Al, Mg, Ce, and Ca are expected to control inclusion compositions and properties, including deformability and liquidus temperature, thereby improving the steel performance.