An Experimental Study On Silicon-Aluminum Complex Deoxidation And The Inclusions Of Titanium Bearing Ultra-Pure Ferritic Stainless Steel

Ultra-pure ferritic stainless steel has a number of excellent properties, such as high SCCresistance, localized corrosion resistance and resistance to high temperature oxidation. As itcontains less or none nickel, its cost is cheaper and more stable than austenitic stainless steel.Due to the dual advantage of cost and performance, ultra-pure ferritic stainless steel has a broadapplication prospects.On one hand some elements which have complicated interaction relationship are containedin the melt during the smelting process of ultra-pure ferritic stainless steel, such as aluminum,titanium, oxygen, nitrogen. On the other hand the aluminum and titanium in melt would reactwith the magnesium oxide in refractory materials and slag, the above reasons led to the complexcomposition of inclusions and difficulty to control them in ultra-pure ferritic stainless steel.In order to improve the deoxidizing process and the levels of controlling inclusions inultra-pure ferritic stainless steels, the experiments of the refining process of ultra-pure ferriticstainless steels were conducted in a MoSi₂high temperature resistance furnace at1873K. Theeffects of different Si/Al ratio（wSi/wAl=0、1、2.5、3.5、5）complex deoxidizer on deoxidationeffects and the composition, morphology, and size distribution of typical inclusions afterdeoxidation, titanium alloying and calcium treatment processes were studied, combined withrelative thermodynamic calculation. Results are as follows:The deoxidation effect of Fe-Si-Al complex deoxidizer is similar to that of pure aluminum,because of that the Fe-Si-Al complex deoxidizer stays for a longer time in melt and also the Al inFe-Si-Al alloy has high specific surface area than that of the pure Al. So using Fe-Si-Al complexdeoxidizer instead of pure Al could save a certain amount of aluminum. The oxygen contentdeoxidized by Fe-Si-Al alloy mainly depends on the dissolved aluminum content.The typical inclusions are MgO·Al₂O₃in samples deoxidized by pure Al. The aluminumwould immediately reacting with Mg dissolved in the melt. The typical inclusions areMgO·Al₂O₃、Al₂O₃-SiO₂、MgO·Al₂O₃-SiO₂in samples deoxidized by Fe-Si-Al alloy.After titanium alloying, the inclusions turned to be MgO·Al₂O₃-TiO_x（TiN）、Al₂O₃-TiO_x（TiN）、MgO·Al₂O₃-SiO₂-TiN（TiO_x） complex inclusions and TiN. The oxide and nitrideof titanium would precipitated on the former deoxidation products, at last forming core-shellstructure inclusions.After calcium treatment, the calcium would react with aluminate phase, and the typicalinclusions turn into Ca-Mg-Al-Ti-O-N complex inclusions. As the modification of aluminatewith calcium, the number and morphology of inclusions become less and little.Compare the low Al and Ti group with the high Al and Ti group, results can be found that:increasing the content of aluminum（0.01%⁰.04%）has little effect on deoxidizing inclusion types; with the increasing content of titanium（0.05%⁰.12%）, the number and size of TiNincreased.According to the experiments, the evolution laws of inclusions in different process aresummarized. The magnesium aluminum spine inclusions formation mechanism and calciummodification on inclusions mechanism are analyzed by thermodynamics theory, also comparedwith the experimental results. Aim at improving the deoxidizing process and the levels ofcontrolling inclusions in ultra-pure ferritic stainless steels.