Fe–(0.5–2.0)%Al–2.0%Mn 合金の一方向凝固におけるAlN の晶出挙動に及ぼす Al 濃度の影響

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 (Figure Presented).