Deoxidization Refining And Inclusion Control Of Ti Containing Ultra Clean Ferritic Stainless Steel

Ultra clean ferritic stainless steels, which are Fe-Cr or Fe-Cr-Mo alloys, containing11%³0%Cr, the content of [C+N] is less than150ppm, and the structure under using state isferritie. In many fields, ultra clean ferritic stainless steels are gradually replacing the austeniticstainless steel, due to its well resistance to high temperature oxidation and localized corrosion,also the lower price.Ultra clean ferritic stainless steels are mainly deoxidized by silicon-aluminum complexdeoxidizer. In the following titanium alloying process, titanium can combined with C and N toavoid the formation of chromium carbide and chromium nitride, and then to avoid the depletionof Cr on the grain boundary. The typical non-metallic inclusions in the ultra clean ferriticstainless steels are mainly Al₂O₃, TiO_x, TiN, MgO·Al₂O₃and their composites. These inclusionshave very detrimental effects on surface quality and property of the products.In this paper, the formation conditions of inclusions in Ti containing ultra clean ferriticstainless steel were analyzed by thermodynamic calculation. The experiments of deoxidizationprocess, titanium alloying process and calcium treatment process in the refining processes of Ticontaing ultra clean ferritic stainless steels were conducted in the MoSi2high temperatureresistance furnace. The effects of different Si/Al ratio complex deoxidizer on deoxidizing effectsand behavior of the deoxidization products were studied. The influence of titanium alloying andcalcium treatment on morphology, composition, quantity and size of the inclusions wasinvestigated. Under the same conditions, the effects of slag composition on morphology,composition, quantity and size of the inclusions in refining processes of Ti containing ultra cleanferritic stainless steel were discussed. Specific research results are as follows:Total oxygen content in the steel after deoxidizing showed that total oxygen content waslower when using Al deoxidizer, but the total oxygen content was slightly increased byincreasing Si/Al raito of the complex deoxidizer. When only considering the oxygen level insteel after deoxidization, the deoxidizing effects of Al and Si-Al complex deoxidizer were almostequivalent, however, the quantity and size of deoxidization products were both smaller. Andwhen the complex deoxidizer contained a suitable amount of Al, the amphodelite with lowmelting temperature could be formed, this kind of inclusions could be easily absorbed by slag. Inaddition, the aluminum resources could be saved by using Si-Al complex deoxidizer. Byconsidering deoxidizing effects, the quantity and size of inclusions in steel, the complexdeoxidizer with Si/Al=2.5was proposed.After titanium alloying, the titanium oxide often combined with MgO·Al₂O₃orAl₂O₃-MgO-SiO₂（-CaO） into composite inclusions. Small-size TiN mostly precipitated with small deoxidization products as the nucleation site. Comparing with inclusions in the steel afterdeoxidizing, the quantity of inclusions was increased and the average size of inclusion wasdecreased by adding titanium alloy. The inclusions after calcium treatment were mainly sphericalAl₂O₃-CaO-MgO-SiO₂-TiO_xcomposites, most of which were amphodelite with low meltingtemperature. The quantity of inclusions after calcium treatment was decreased than after titaniumalloying. The low melting temperature inclusions formed after calcium treatment was easilyabsorbed by slag.The results showed that pure Al₂O₃would immediately be formed after adding deoxidizer inthe liquid steel, which was corresponded with the calculated results. As time goes on, the contentof Al in steel was decreased and the content of Mg was gradually increased, which resulted in thecontent of MgO in spinel inclusions was also increased. MgO-Al₂O₃spinel inclusion easilyformed with the existence of a little amount of Mg in the liquid steel.By investigating the effects of slag composition on deoxidizing effects and inclusionbehavior, the results showed that high basicity helped to reduce total oxygen content in steel. Thequantity, total area and average size of inclusions were all decreased with increasing the basicityof slag, the proportion of inclusions size within2μm was increased. Thus，improving the basicityof slag had good effects on reducing inclusion size and removing larger inclusions out of thesteel, furthermore, high basicity slag had weaker erosion on refractories.