In-situ observation of the precipitation of manganese sulfide in low-carbon magnesium-killed steel

The in-situ observation of MnS precipitation was made on cooling using a confocal laser microscope. It was found that two types of precipitates appeared at the different temperatures. The first type precipitated at the nucleation sites of MgO or MnO-TiOx inclusions in gamma-Fe in the temperature range between 1500 and 1200 K. The shape was triangular or rodlike, and the size was between 1 and 5 mum. The second type of MnS drastically precipitated in alpha-Fe at 1100 K just after the Ar-3 transformation. The shape was triangular or polygonal, and the size was between 0.1 and 0.5 mum, much smaller than that of the first type. No significant precipitation was observed in the intermediate temperature range between 1200 and 1100 K. The reasons why MnS precipitation took place twice at the different temperature ranges and why no precipitation occurred between the two temperature ranges were discussed from the thermodynamic and kinetic viewpoints. The diffusion coefficients of Mn in gamma- and alpha-Fe and the supercooling are the key factors which explain the reason.