Formation mechanism of non-metallic inclusions in 202 stainless steel

Non-metallic inclusions generally deteriorate the quality of stainless steel products, such as skin laminations or line defects on the rolled strip in stainless steel. Thus, the formation mechanism of non-metallic inclusions in 202 stainless steel was investigated with industrial trials and thermodynamic calculation. Steel samples were analyzed by scanning electron microscopy and energy dispersive spectroscopy. Compositions of the steel samples were determined by inductively coupled plasma-optical emission spectrometer. After Si−Mn deoxidation, the main inclusions were spherical Ca−Si−Mn−O inclusions during LF refining process. The liquid phase region of the Mn−Si−O phase diagram was affected by the residual aluminum content in Si−Mn deoxidized stainless steel. 1×10⁻⁵ mass fraction of aluminum in steel enlarged the liquid phase region of the Mn−Si−O phase diagram. However, more than 3×10⁻⁵ mass fraction of aluminum led to the formation of alumina inclusions and the reduction of the liquid phase region of the Mn−Si−O phase diagram. After the continuous casting process, the main inclusions in the steel were changed from Ca−Si−Mn−O to Mn−Al−O. Compared with the steel sample taken during the LF refining process, the MnO and Al₂O₃ content of inclusions in the continuous casting samples increased significantly, while the content of CaO and SiO₂ decreased significantly. At the same time, the amount of inclusions increased from 5.5 mm⁻² to 11.3 mm⁻² after continuous casting. Combined with thermodynamic calculations, it was found that Mn−Al−O inclusions were formed during solidification, which became the main type of inclusion after continuous casting. In addition, the effect of aluminum content on the formation of oxide inclusions during continuous casting was discussed. Thermodynamic calculation indicated that the alumina inclusions were formed in the steel containing more than 3×10⁻⁵ mass fraction of aluminum during continuous casting. High aluminum content promoted the formation of alumina and inhibited the formation of Mn−Al−O inclusions during solidification.