| Ultra-pure ferritic stainless steel has excellent corrosion resistance,processability and weldability due to its extremely low carbon and nitrogen content(w[C+N]?150×10-6),and the addition of stabilizing elements such as titanium and niobium.As a nickel-saving economic stainless steel,it has been widely used in various applications including light industry,household appliances,building decoration,and automobile.The steelmaking process of ultra-pure ferritic stainless steel is blast furnace-molten iron pretreatment-AOD-VOD-LF-CC in Jiuquan Iron and Steel Company.During the steelmaking process,there is a problem of the clogging of submerged entry nozzle(SEN).At the same time,the ultra-pure ferritic stainless steel products sometimes have surface defects Therefore,based on the actual production process of ultra-pure ferritic stainless steel in Jiugang,this work investigated the metallurgical process and product quality problems encountered above.The clogging of submerged entry nozzle and the surface line defects of steel was dissected by scanning electron microscopy.It was found that inclusions led to the nozzle clogging and surface defects.Combining industrial experiments,laboratory experiments and thermodynamic calculation,the effect of aluminum,calcium and titanium content on the formation of inclusions in steel was analyzed.The control metallurgy for aluminum content and calcium content during the refining process was proposed.Further,combining the slag-steel experiment and the ion and molecule coexistence theory(IMCT),the effect of the CaF2 content in the slag on the steel composition and inclusion formation was studied.The metallurgical control range of the refining slag composition was clarified.According to the results of industrial experiments and laboratory research,the key metallurgy for controlling inclusions in steel was proposed,which has been successfully applied in industrial production.The following research results were obtained:(1)The deposition sample of SEN can be divided into four layers:nozzle refractory,initial layer,deposits,and final layer,respectively.In the initial layer and the final layer,a large number of aggregated inclusions mainly containing alumina were observed.A large amount of(MgO-Al2O3)satd-CaO-TiOx inclusions were found in the deposits layer.Chain-shaped Mg-Al-Ca-Ti-O inclusions with size larger than tens or even hundreds of microns were observed around the surface defects.The main causes of the clogging of submerged entry nozzle and the surface line defects of steel were the MgO-Al2O3-CaO-TiOx inclusions formed in the molten steel during steelmaking process.(2)Laboratory research has shown that:the content of aluminum and titanium in steel has a great influence on the formation of inclusions.The main type of inclusions in the steel samples located in the liquid oxide phase region of Al-Ti-O phase diagram was spherical Al-Ti-O complex inclusion.The proportion of liquid inclusions was high.The number density of inclusions was low.The steel cleanliness in these samples was obviously better than the steel samples located out of the liquid oxide phase region of Al-Ti-O phase diagram.Combined with the thermodynamic calculation results,the reasonable aluminum and titanium content in stainless steel was obtained:0.0627[%Ti]+0.0024?[%Al]?0.1488[%Ti]+0.0028.(3)Combined with industrial experiments,it was found that the calcium content has a great effect on the formation of inclusions in stainless steel.When the calcium content in the steel was too low,(MgO-Al2O3)satd-CaO high-melting inclusions were formed.Most of the inclusions were located in the spinel phase.When the calcium content in the steel was too high,(CaO-TiOx)satd-MgO-Al2O3 inclusions were formed after titanium addition,which were mostly located in the perovskite phase.Combined with the industrial experiments results and thermodynamic calculations,the reasonable calcium content after A1 deoxidation was obtained:[ppm Ca]=6.621n[ppm Al]-20.32.(4)The slag/steel reaction results have shown that:The CaF2 content in LF refining slag has a great effect on the stability of titanium content and the formation of inclusions in stainless steel.When the slag basicity(CaO/SiO2)remained unchanged,MgO content was around 5%,the yield of titanium in steel increased from 3.9%to 6.7%with the increase of the CaF2/Al2O3 mass ratio from 0 to 0.47.Meanwhile,the magnesium content in the molten steel also increased from 12 ppm to 23 ppm,and the average magnesium oxide content of inclusions increased from 11%to 30%.The thermodynamic results based on the ion and molecule coexistence theory also indicated:increasing the CaF2 content would increase the activity of titanium and magnesium in the molten steel.The reasonable LF refining slag composition was obtained:CaO 48%?55%,SiO28%?14%,Al2O3 15%?25%,MgO 5%?10%,CaF2 5%?10%,TiO2?5%.(5)Combined with the steelmaking process of ultra-pure ferritic stainless steel,the effect of aluminum,calcium,titanium content and slag composition on the formation of inclusions in steel during the refining process was clarified.The key metallurgy technology was proposed,which was successfully applied in actual production.The incidence of nozzle clogging and surface defects on the steel was significantly reduced.The trouble free heat sequence casting of ultra-pure ferritic stainless steel increased from 1 heat in 2016 to 4 heat in 2019.At the same time,the proportion of surface defects on stainless steel product was also reduced from 18%to 0.9%. |
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