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

When manganese silicate inclusions were formed during cooling from 1600 deg reesC, manganese and sulfur contents in the manganese silicate inclusions w ere 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 precipitati on of a MnS phase on the inclusion were greatly affected by the thermal his tory of the steel. Slow cooling helped the formation of both the Mn-deplete d zone and the MnS phase on the inclusion, but fast cooling suppressed it. Subsequent isothermal holding at 1200 degreesC diminished the existing Mn-d epleted 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 f ormation of a MnS phase is due to the local saturation of Mn and S at the o uter part of an inclusion. It was suggested that the major factors affectin g the formation of the Mn-depleted zone and the MnS phase are the cooling r ate, isothermal holding, and the sulfide capacity of the inclusion.