Influence of Deoxidation and VOD Slag on the Inclusions' Behavior of Ti-Nb Stabilized Fe-21Cr Ferrite stainless steel

Ultra pure ferritic stainless steels are Fe-Cr or Fe-Cr-Mo alloys bearing 11%~30% Cr and total C and N contents are limited to less than 0.015%. Ultra pure ferritic stainless steels are appreciated for their excellent uniform and localized corrosion resistance. Unfortunately, large inclusions such as Al2O3, TiOx, TiN, MgO and their complex in the steel are harmful to the surface quality, corrosion resistance and mechanical performance of the products. It is necessary to optimize the composition of VOD refining slag to absorb the inclusions in the practical production process. The influence of deoxidation process and VOD slag compositions on the behavior of inclusions during refining of Ti-Nb stabilized Fe-21%Cr ferrite stainless steels were studied in this work. The results indicated that the main inclusions in the Al-Si deoxidized samples were MgO·Al2O3. And the main inclusions after Ti addition were MgO·Al2O3-TiOx and Al2O3-TiN. After calcium treatment, the main inclusions became spherical MgO·Al2O3-CaO and Al2O3-SiO2-CaO combined with TiOx and TiN. As the basicity of refining slag was increased from 2.5 to 4, the number density of inclusions in the sample reduced, while the average particle size of inclusions increased. When the basicity of refining slag was 3.5, reducing Al2O3 content in the refining slag was conducive to decrease the number of larger inclusions. There were fewer inclusions and the size of most inclusions was distributed within 1 to 2m when the Ti-Nb stabilized Fe-21%Cr ferrite stainless steel was refined with the optimized refining slag with the composition of CaO-SiO2-20%Al2O3-10%MgO-5%CaF2 and the basicity of 3.5. The experiment about the influence of refining slag on the inclusions in Nb stabilized Fe-18Cr ferrite stainless steel without titanium was carried out, too. The refining slag which the composition of CaO-SiO2-10%Al2O3-10%MgO-5%CaF2 and R=3.5 had a good refining effect for Nb stabilized Fe-18Cr ferrite stainless steel.