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 1600°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 µm 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.