MnS precipitation in association with manganese silicate inclusions in Si/Mn deoxidized steel

When manganese silicate inclusions were formed during cooling from 1600 degreesC, manganese and sulfur contents in the manganese silicate inclusions were much lower than their equilibrium values within the steel matrix, i.e., the steel matrix was supersaturated with Mn and S against the inclusions. The formation of a Mn-depleted zone around an inclusion and the precipitation of a MnS phase on the inclusion were greatly affected by the thermal history of the steel. Slow cooling helped the formation of both the Mn-depleted zone and the MnS phase on the inclusion, but fast cooling suppressed it. Subsequent isothermal holding at 1200 degreesC diminished the existing Mn-depleted zone in slow-cooled steel, but created a Mn-depleted zone for fast-cooled steel. The mass transfer within an inclusion was sluggish, and the formation of a MnS phase is due to the local saturation of Mn and S at the outer part of an inclusion. It was suggested that the major factors affecting the formation of the Mn-depleted zone and the MnS phase are the cooling rate, isothermal holding, and the sulfide capacity of the inclusion.