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Primary and secondary inclusions in Si/Mn and Si/Mn/Ti deoxidized structural steels subjected to different thermal histories were investigated in view of evolution of size, composition, and morphology. Primary inclusions quenched from 1 600°C contained very low levels of sulfur, and hence MnS precipitation on them was hardly found. The mean diameter of secondary inclusions lied in the range of 1-3 mm depending on the cooling rate and chemical compositions of steels. Both MnO and MnS content were higher in smaller secondary inclusions. MnS which precipitated on manganese silicate inclusions in Si/Mn deoxidized steels mostly grew into the inclusions. As inclusion size increased, the number of MnS precipitates on each inclusion was also increased. Titanium in steel had a tendency to reduce SiO 2 content in inclusions and to associate with MnO in the inclusions to form a stoichiometric relationship of Mn/Ti ratio in the inclusions. If Ti content in Si/Mn/Ti deoxidized steels was low, the secondary inclusions were found to form with multiple phases; viz., manganese silicate phase, Mn-Ti oxide phase, and MnS phase. The MnS phase always precipitated in the manganese silicate phase. The proportion of manganese silicate phase in each inclusion decreased with a corresponding increase in Ti content in the steel, and eventually disappeared completely when the Ti content exceeded a certain level (70 ppm in the present steel compositions). In this case MnS was found to precipitate outside Mn-Ti oxide inclusions and grew into the steel matrix. In order to interpret and predict the behavior of inclusion precipitation and growth, a model has been developed which incorporates both thermodynamic and kinetic considerations.

When manganese silicate inclusions were formed during cooling from 1600 ЊC, 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 ЊC 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 Mndepleted zone and the MnS phase are the cooling rate, isothermal holding, and the sulfide capacity of the inclusion.

The effect of Al on the evolution of non-metallic inclusions in the Mn-Si-Ti-Mg deoxidized steels during solidification were investigated based on the experiments and thermodynamic calculations. The inclusions belonged to the MgO-TiO 2 -Ti 2 O 3 -Al 2 O 3 ϩMnSϩTiN system. In particular, the major oxide inclusion was the Mg-Ti-Al-O spinel phase of which composition was continuously changed from the Mg-Ti-O to MgAl 2 O 4 with the concentration Al in steels. The spinel compositions calculated from thermodynamic databases are in good agreement with experimental results. TiN was only observed on the surface of MnS. MgAl 2 O 4 aggregates were also observed at high Al concentration. In general, the evolution of inclusions is well explained by thermodynamic calculations.KEY WORDS: oxide metallurgy; evolution of inclusion; Mn-Si-Ti-Mg deoxidized steel; MgO-TiO 2 -Ti 2 O 3 -Al 2 O 3 ; thermodynamic calculation.