Coupled Precipitation Behavior of MnS–Nb(C, N) Composite Inclusion in Medium Carbon Micro-alloyed Steel

High-temperature smelting experiments and thermodynamic calculations were performed to investigate the coupled precipitation behavior of MnS–Nb(C, N) composite inclusion in MCMA steel. The size distribution of inclusions suggests that the addition of sulfur contributed to forming large-sized inclusions, while the addition of niobium was in favor of small-sized inclusions. However, the composite addition of sulfur and niobium increased the ratio of large-sized inclusions that is more than the single addition of sulfur. There are only near-spherical inclusions that are composed of Al₂O₃, MnS, and V(C, N) in steel without niobium, while including near-spherical, dendritic, and irregular with niobium addition. The near-spherical inclusions are composed of Al₂O₃, MnS, and (Nb, V) (C, N), while the large-sized dendritic and irregular inclusions are composed of MnS and (Nb, V)(C, N). According to thermodynamic calculation, the segregation of manganese becomes weak after niobium addition. After sulfur addition in niobium-containing steel, the segregation of manganese becomes more intense, while the segregation of niobium becomes weak. The precipitation process was divided into four parts for niobium-containing steel. Therefore, there are three stages in the inclusion precipitate process for steel A1 and A3 and four stages for steel A2 and A4. After PNCN precipitated, the niobium content decreased, and the manganese segregation became intense. After MnS precipitated, the sulfur content increased, and the segregation of niobium became weak. And then, the SNCN formed that nucleated on MnS.