Study On The Silicon Aluminum Composite Deoxidation Mechanism And Inclusions In Ultra Pure Ferritic Stainless Steels

Ultra-pure ferritic stainless steel is one of important development directions in the modern ferritic stainless steel process, which requires the content of carbon plus nitrogen that less than150×10-6, with titanium, niobium, copper, aluminum, vanadium, etc. for improving the property. Ultra-pure ferritic stainless steel is the ideal resource-saving environmentally friendly materials and has no nickel which allergic to the human body. The corrosion resistance, weldability, toughness are better than traditional ferritic stainless steel.Based on the theory of thermodynamic, the relationship of the content of aluminum, silicon and dexidation product was analyzed. From the on-site sample analysis, the large quantity of Al2O3-MgO-TiO2complex inclusions was found which mostly has a high melting point region in the Al2O3-MgO-TiO2phase diagram.Based on the thermodynamics analysis, deoxidation and wire feeding experiments were simulated as AOD position in ultra-pure ferritic stainless steel process. The effect of deoxidizer with different silicon and aluminum ratio and wire feeding process on oxygen content and inclusion, mechanical properties of rolled steel of experimental heats were analyzed.From the total oxygen content’s measurement of439stainless steel’s deoxidation experiments, the total oxygen content is0.0040%while3#(Si/Al=0.71) deoxidizer was selected and the effect is best, the effect of deoxidition of the other heats is1#,4#,2#. From the total oxygen content’s measure of443NT stainless steel’s deoxidation, total oxygen content is0.0047%while3#(Si/Al=0.71) deoxidizer is selected, the effect of deoxidition of the other heats is1#,2#,4#.From the process of every refining experiment, the average size, the total number, the total area of inclusion in the439and443NT stainless steel decrease gradually. From the terminal samples’results, the ratio of diameter less than2μm is maximum while3#(Si/Al=0.71) deoxidizer is selected, there is not obvious difference between the others. The inclusions in the terminal sample are almost spherical shape, inclusions’composition lies on the low melting point region in the Al2O3-CaO-SiO2and Al2O3-CaO-SiO2-TiO2phase diagram. This indicates that silicon aluminum deoxidation and calcium treatment have good effect on the inclusions control.Tested the mechanical properties of steel samples, it could be found that the data all comply with ASTM standard and is better than those reported in literature except in heat2sample for rolled defect. A better effect on the tensile strength and yield strength is obtained while3#(Si/Al=0.71)deoxidizer is selected. Considering the total oxygen content, the shape and the content of inclusion, the shape of inclusions, the author advices the silica-alumina deoxidizer with the calcium treated process can be the best candidate for on-site ultra-pure ferritic stainless steel during the refining process.