凝固過程における溶鋼中からAl₂O₃ 上への MnS の晶出挙動

Forming conditions and compositional changes of primary inclusions in molten steel have been studied due to the demand for high cleanliness of steels. MnS, a common inclusion in steel, does not form in molten steel, although it is observed in steel with oxide inclusions such as MnO, Al₂O₃ and SiO₂. On the other hand, Mn and S are enriched in molten steel due to the segregation phenomenon during the solidification process which suggests that MnS form in molten steel during solidification. However, the precipitation behavior of MnS inclusions in molten steel due to the enrichment of Mn and S and the interaction between the primary inclusion and the molten steel is still unclear. In this work, a new experimental technique was developed and the precipitation behavior of MnS from molten steel onto solid Al₂O₃ was studied. Solid MnS precipitates were observed on the Al₂O₃ rod immersed in the sample with adding Al whereas precipitates containing MnO, A₂O₃ and MnS were observed on the Al₂O₃ rod in the sample without adding Al. Thermodynamic analysis revealed that Mn enriched in molten steel is oxidized to form MnAl₂O₄ when Al content is low and the MnAl₂O₄ reacts with S in molten iron to form molten MnO–Al₂O₃–MnS. MnS can precipitate from the molten MnO–Al₂O₃–MnS. On the other hand, Mn enriched in molten steel does not react with Al₂O₃ when Al content is high. Therefore, MnS can precipitate at the final period of solidification where Mn and S are significantly enriched in molten steel.