Evolution of size, composition, and morphology of primary and secondary inclusions in Si/Mn and Si/Mn/Ti deoxidized steels

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 600degreesC 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 mum 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 SiO2 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.