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Mn-TRIP steels of which composition is mainly Fe-(0.5-3mass%)Al-(2-10mass%)Mn are expected to be new advanced high-strength sheet steels. During the solidification process of Fe-Al-Mn alloy, AlN inclusions precipitate at the grain boundary, which leads to the severe deterioration of hot ductility. However, the precipitation behavior of AlN inclusion is not known enough. In this work, a unidirectional solidification experiment of Fe-(0.5-2.0)mass%Al-2.0mass%Mn alloys and numerical analysis on the forming condition of AlN were carried out, and the precipitation behavior of AlN inclusions was studied. Al2O3 inclusions were observed in the alloy with 0.5 mass%Al. On the other hand, AlN inclusions were observed in alloys with 1.0, 1.5, and 2.0 mass%Al. The volume fraction of AlN inclusions increased with increasing Al content of the alloy. The thermodynamic analysis revealed that AlN is thermodynamically unstable at temperatures above the liquidus of the alloy. When Al content of molten steel is increased, AlN becomes thermodynamically stable. Accordingly, the forming amounts of AlN in the alloys during the solidification were analyzed considering the segregation. The results show that the precipitation of AlN inclusions increases significantly during solidification due to the enrichment of Al in the liquid phase. In the Fe-(1.0-2.0)mass%Al-2.0 mass%Mn alloy, Al2O3-AlN inclusions were also observed, where AlN is present around Al2O3. These inclusions are considered to be formed by the precipitation of AlN, which becomes stable as the Al concentration increases due to solidification segregation, on Al2O3, which is stable and precipitated in the early stage of solidification. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]