2001 Volume 87 Issue 6 Pages 433-440
"In-situ" observation of MnS precipitation in Fe-Si alloys was made on cooling using a confocal scanning laser microscope. The phase transformation during cooling in low Si alloys (less than 2.0 mass% Si) was from δ phase via γ phase to α phase, but that in high Si alloys (more than 3.0 mass% Si) was from δ phase to δ+γ phase. The MnS precipitation in low Si alloys occurred in two steps but in high Si alloys in one step. In low Si alloys some large MnS precipitates appeared in high temperature by the solute pile up, and many small MnS precipitates appeared in low temperature by the change of the solubility due to phase transformation. In high Si alloys, in which δ phase and γ phase were present at the same time, most MnS precipitate in δ phase according to the high supersaturation of Mn and S in comparison with γ phase. The precipitates in δ phase grew fast, but those in γ phase grew very slowly, since the diffusion coefficients of Mn and S in γ phase are much smaller than those in δ phase.
