MnS Precipitation Behavior in MnO–SiO2 Inclusion in Fe–Mn–Si–O–S Alloy System at Solid-Liquid Coexistence Temperature

Abstract

With the emerging significance of creating an acicular ferrite microstructure to provide an optimum set of properties in steel, MnS precipitation behavior on a MnO–SiO2 inclusion at the solid-liquid coexistence temperature was experimentally investigated and thermodynamically elucidated in this study. Using a direct method of forming inclusions, alloy samples with varying sulfur concentrations [Fe-1.1Mn-0.10Si-0.05O-S (initial mass%); 0.005 to 0.031 initial mass% S] were prepared by holding at the solid-liquid coexistence temperature for 1 hour.

In samples with less than 0.011 mass% sulfur, the formation of a MnO–SiO2 inclusion with a SiO2-rich precipitate was observed. Formation of SiO2 was described as a consequence of silicon enrichment in the liquid phase, which, under appropriate thermodynamic conditions, homogeneously precipitated and later on coalesced with the primary MnO–SiO2 phase. On the other hand, in samples with more than 0.022 mass% sulfur, heterogeneous precipitation of MnS along the boundary of the primary MnO–SiO2 inclusion and the alloy matrix was observed. Also, the SiO2-rich phase was found to disappear with increasing sulfur addition. Since the likelihood of heterogeneous nucleation is higher than homogeneous nucleation, it was assumed that MnS precipitation on the surface of the primary MnO–SiO2 prevented the secondary SiO2-rich inclusion from coalescing with the existing MnO–SiO2 inclusion. This was also further validated for solute enrichment in the liquid phase, wherein MnS precipitation temperature was found to shift to higher temperatures in alloys with higher sulfur content.