Analysis The Precipitation Behavior Of Inclusions In Ultra Clean Ferritic Stainless Steel

Ultra clean ferritic stainless steels is to control the carbon and nitrogen content which intraditional ferritic stainless steel at lower levels （[C+N]<150ppm）, containing11%³0%Cr. Thesteel has good uniform corrosion and localized corrosion resistance performance.Ultra clean ferritic stainless steels are mainly deoxidized by silicon-aluminum complexdeoxidizer. In the titanium alloying process, titanium can combined with C and N to avoid theformation of chromium carbide and chromium nitride, and then to avoid the depletion of Cr onthe grain boundary. The inclusion in ultra clean ferritic stainless steels is mainly includedeoxidizing products Al₂O₃, SiO₂, MgO·Al₂O₃；Al₂O₃-MgO-SiO₂-TiO_x which after titaniumalloying and then small-size TiN/Ti （CN）, NbC,（Nb,Ti）（C,N） precipitated in solidificationprocess; spherical Al₂O₃-CaO-MgO-SiO₂-TiO_x composites in calcium treatment. Theseinclusions have very important effects on surface quality, corrosion resistance and property of theproducts. Study the composition, morphology, size distribution of the non-metallic inclusionswhich in ultra clean ferritic stainless steels has important significance to improve the refiningprocess, control the number and the size of inclusions, surface quality of ultra pure ferriticstainless steel.In this paper, extracted the inclusions and precipitates which in ultra pure ferritic stainlesssteel without any damage by the electrolytic method in a non-aqueous solution to avoid thelimitation of two-dimensional metallographic specimen observing and the impact of base metalcomposition for inclusions. The paper using10%AA electrolyte（10%Acetyl acetone+1%tetramethyl ammonium chloride+methyl alcohol）extracted four different types of ultra cleanferritic stainless steels（445,443,445J1M,439） from a iron and steel company. Based on the designof electrolytic device and analysis of the influencing factors in experiment, extract the inclusions（precipitation） in the ultra pure ferritic stainless steel effectively. Observing the inclusion in thescanning electron microscopy （SEM） and analysis of the composition of inclusions with EDS.The experiments of deoxidization process, titanium alloying process and calcium treatmentprocess in the refining processes of ultra clean ferritic stainless steels were conducted in theMoSi2high temperature resistance furnace, the mainly mission is to investigatethe effects of slagcomposition on morphology, composition, quantity and size of the inclusions in refiningprocesses of ultra clean ferritic stainless steel. Specific research results are as follows:（1） There are mainly3types inclusions in ultra clean ferritic stainless steels: oxideinclusions and carbonitride inclusion, and the complex precipitates. Oxide inclusions are mainly for deoxidizing products, such as Al₂O₃, SiO₂· Al₂O₃,SiO₂ordeoxidizing products composite with other metal forming composite inclusions which size largerthan10μm, such as MgO·Al₂O₃-TiO_x, MgO·Al₂O₃-TiO_x·CaO.Carbonitride inclusion are mainly for TiN/Ti （CN）, NbC which existed in single type oftitanium or niobium stability of stainless steel, the size of such inclusions in1³μm.There aretwo types TiN/Ti （CN）, one is separate, another is precipitate on oxide inclusions and theenwrapped complex inclusions are then formed. NbC in shape of cross, few in globular ordendritic, NbC effectively fixed C elements in steel, refine the grain, improve the equiaxialcrystal. It should be control the content of Nb, avoid precipitation dendritic inclusions insolidification process which harmful for steel surface quality.Complex precipitates are mainly for （Nb,Ti）（C,N） which existed in in Ti-Nb double stabledferritic stainless steel, the size of such inclusions in2⁵μm. Nbc is overgrowth on TiN, Nb （CN）,Ti （CN） solid solution together, which dual fixed C, N elements in the solidification process. Itcan improve the surface properties and comprehensive performance of product.（2） The inclusions in the steel mainly for spherical MgO·Al₂O₃, SiO₂and few complexoxides with CaO, SiO₂after adding deoxidizer in the liquid steel. The content of CaO in theinclusions is increase, the content of SiO₂decreases with the increase of refining slag basicity.At the same time, the content of MgO in magnesia-alumina spinel increase too.（3） Compare the effect of different basicity refining slag deoxidizing in inclusion quantities,sizes, changes in the composition, it’s found that should choose refining slag which the basicityR=w（CaO）/w（SiO₂）=3.5,20%Al₂O₃,10%MgO,5%CaF2.