Influence Of VOD Refining Slag On The Inclusions In Ultra Pure Ferritic Stainless Steel

Ultra pure ferritic stainless steel is Fe-Cr or Fe-Cr-Mo alloy，which contains11%³0%ofCr，and less than150ppm of total [C+N]. The organization structure of ultra pure ferriticstainless steel is ferrite in using condition. The steel has a good uniform corrosion and localizedcorrosion resistance performance. The main inclusions in ultra pure ferritic stainless steel areAl₂O₃, TiO_x, TiN, and the compound inclusions centering on MgO Al₂O₃and TiO_x. Largeparticle inclusions and MgO Al₂O₃spinel inclusions in the steel have harm to the surface qualityof the products, and reduce the usability. The inclusions in steel can be absorbed in refining slagduring VOD refining process. In this case, purity of steel will be improved. Therefore, with thepurpose of developing refining slag with high quality and improving the effect of VOD refiningprocess and stabilizing the production technology, the physicochemical properties of the refiningslag were studied, and the influence of the refining slag on the behavior of inclusions in theferritic stainless steel was discussed.In this paper, the physicochemical properties of CaO-SiO₂-A1₂O₃-MgO-CaF₂slag werestudied. The initial melting temperature, fusion temperature and melting enthalpy were measuredby using differential scanning calorimetry （DSC）. The refining slag which the composition is w（CaO）/w（SiO₂）=3.5,20%Al₂O₃,10%MgO and5%CaF₂has a suitable physicochemicalproperties. The desulfurization capacity of refining slag and its effect on the Al deoxidation wereobtained by thermodynamic calculation. The oxygen content in liquid steel decreased with thedecreasing of the activity of Al₂O₃in refining slag while the desulfurization capacity of refiningslag increased and the effect of Al deoxidation in ultra pure ferritic stainless steel was better withthe decreasing of the activity of Al₂O₃.Based on studying of the physicochemical properties of the refining slag, the compositionof refining slag was adjusted. The equilibrium experiment during deoxidation alloying process offerritic stainless steel was simulated in the MoSi2tubular resistance furnace. And the influence ofthe refining slag composition on the number, the composition, morphology and size of inclusionswere analysed.The main inclusions in the deoxidation finishing sample of445J1M ferritic stainless steelwhich was stabilized by Titanium were MgO·Al₂O₃spinel inclusions. And the main inclusionsafter Ti alloying were MgO·Al₂O₃-TiO_xand Al₂O₃-TiN inclusions. After calcium treatment, themain inclusions were MgO·Al₂O₃-CaO and Al₂O₃-SiO₂-CaO compounding with TiO_xand TiN,which were spherical.As the basicity of refining slag increased from2.5to4, the quantity of per unit area of theinclusions in the sample reduced, while the average particle size of inclusions increased. Whenthe basicity of refining slag is3.5, with the Al₂O₃decreasing from20%to15%, the averageparticle size of inclusions decreased from1.356μm to1.317μm. To reduce the Al₂O₃in therefining slag was conducive to decrease the larger inclusions granules.In the equilibrium experiment between the445J1M ferritic stainless steel and the refiningslag which the composition is R=3.5,20%Al₂O₃,10%MgO and5%CaF₂, the quantity per unit area of the inclusions in the sample was fewer, and the size of most inclusions were distributedwithin1²μm. We can conclude that the optimized refining slag was helpful to reduce thenumber and the size of the inclusions in the445J1M ferritic stainless steel. And the optimizedrefining slag could absorb the inclusion of larger particles well.The experiment about the influence of refining slag on the inclusions in444ferritestainless steel without titanium was researched in the study. Total oxygen content in liquid steelgreatly reduced after Si-Al deoxidization alloy was added to the molten steel. T.[O] content inthe deoxidation finishing sample ranged from63ppm to74ppm. T.[O] content in the steel wasreduced with improving the basicity of refining slag and reducing the activity of Al₂O₃inrefining slag. And the result is consistent with the thermodynamic calculation.After adding the deoxidizing alloy, the inclusions in the sample during deoxidizationprocess were Al₂O₃and MgO Al₂O₃and compound inclusions containing a small amount ofSiO₂and MnO. After calcium treatment, the main inclusions were MgO·Al₂O₃-CaO inclusionswhich were spherical.With the increasing of the basicity of refining slag, the quantity of per unit area of theinclusions in the sample as well as the inclusion content in the average area were decreasedduring the refining process. The mean particle size of inclusions reduced with the decreasing ofAl₂O₃content in slag. The refining slag with lower initial Al₂O₃content was conducive toabsorb the inclusions of larger particles. The mole ratio of MgO to Al₂O₃in the MgO Al₂O₃inclusions, log[α MgO/αAl₂O₃, increased by increasing the activity ratio of MgO to Al₂O₃of therefining slag, log[X MgO/XAl₂O₃.Compared the changes of the particle sizes of inclusions in the sample and the quantity ofper unit area of the inclusions, the inclusions content in average area, we can conclude that therefining slag which the composition was R=3.5,10%Al₂O₃and10%MgO and5%CaF₂had agood refining effect.