Study On Formation Mechanism And Control Model Of Inclusion In Stainless Steel

The inclusions in stainless steel have a large effect on surface qualities of stainless steel products. The rejection rate of products induced by inclusions is up to20%in some stainless steel production companies at present. Therefore the mechanism of typical inclusion formation and its control technology are the hot spots in stainless steel field.In present work, the formation mechanism and control conditions of MgO·Al2O3inclusion are firstly studied in Si-killed430ferrite stainless steel, which are based on thermodynamic analysis method and coherent investigation of product quality and thermal simulation experiments in laboratory. Further, the productive process of430stainless steel by Al deoxidization is preliminary researched, in which the mechanism and physical chemistry character of Ca-MgO·Al2O3reduction are also being exploratory development. Moreover, this paper not only extracts the mechanism of Ca reducing MgO·AlO3spinel and the evolution of spinel in reduction process but also analyzes the control conditions and influencing factors of MgO·Al2O3inclusion with Ca treatment process. In the end, some thermal simulation experiments of430stainless steel with Al deoxidization has been done to verify the compatibility between experimental results and theoretical analysis of Ca reducing MgO·Al2O3each other.It is indicated from coherent investigation and thermodynamic analysis that the MgOAl2O3spinel inclusion in molten steel is an important reason to induce linear-scale deficiency on thickness cold rolled plate surface of stainless steel. The phase predominance area diagrams of MgO·Al2O3were drawn in Mg-Al-O system and Mg-Al-Si-O system of430stainless steel melts, from which it was found that the formation of MgO·Al2O3is very easy. The number of MgO-Al2O3existence in molten steel will decrease with FeSil.O alloy being utilized in simulation experiments with basicity of slag and Al2O3content being controlled at2.5and10%respectively.The research results of430ferrite stainless steel production with Al deoxidization was show that MgO Al2O3spinel is propitious to translate into low melting point plastic inclusion with a certain temperature and molten steel composition. The result of exploratory development of mechanism of Ca-Mg·OAl203reduction indicates that Ca in molten steel would selective reduce component of MgO·Al2O3inclusion. In a certain temperature and molten steel composition, Ca would reduce Al2O3component of MgO·Al2O3inclusion firstly. and the reduction process is layer-by-layer order from external to internal of spinel inclusion. In this reduction process, CaO·xAlO3·y MgO ternary complex inclusions would be formed together with some dissociative MgO and dissolved Al. The molten steel temperature and Ca, Al and S content are important influencing factors in Ca-MgO·AlO3reduction process. The thermal simulation experiments of Al-killed430stainless steel show when dissolved Al and S content being at0.03%and0.0035%respectively at1853K, MgO-Al2O3would change to CaO-MgO-Al2O3inclusion gradually in case of Ca content being at29×10-6. The experimental results and theoretical analysis of Ca-MgO·AlO3reduction mechanism is coincident basically.Based on metallurgical thermodynamic calculative method and law of conservation of mass, a control model of CaO-Al2O3-SiO2inclusion content has been established in present work, and the model is applied to inclusion control of304austenite stainless steel at the same time. In condition of Si deoxidation only and Ca treatment behind Si deoxidation of304stainless steel, the requisite thermodynamic factors which control CaO-Al2O3-SiO2inclusion content change has been found out through model calculation. Further, some thermal simulation experiments have been done to verify the model. At last, the improved suggestion for steelmaking operation would be extracted. It is expected that this work will provides credible theory evidence for improving CaO-Al2O3-SiO2inclusion plastic deformability and increasing the cold rolled products percent of pass.The calculative results of control model of CaO-Al2O3-SiO2inclusion content in stainless steel show that it is practical to control each component contents of inclusion by way of controlling Ca, Al and Si contents and then impacting the chemical reaction between steel melts and inclusions consequently. When dissolve oxygen and Si content being at0.001%and0.5%respectively in304stainless melts at1873K, to make this type of inclusion translating into CaO-Al2O3-SiO2system plastic deformability region, the model calculative results show that Al content would be controlled less than5×10-6with Si deoxidation only, but if it contains some dissolved Ca content about3×l0-6～10×10-6in molten steel, the suitable dissolved Al content would be up to20”10-6. The effect of temperature to inclusion content change is minor in either case.The research results of simulation experiments of CaO-Al2O3-SiO2inclusion content control in304austenite stainless steel indicate that there are two methods could making the final CaO-Al2O3-SiO2inclusion into CaO-Al2O3-SiO2system plastic deformability region in laboratory conditions. One is1kg304stainless steel using8g Si deoxidation then feeding1.0g Ca metal, and anther is using lOg Si deoxidation then feeding0.5g Ca metal. The statistical analysis method is utilized in CaO-Al2O3-SiO2inclusions content in final sample. The statistical results show that the measured value of CaO, Al2O3and SiO2content in CaO-Al2O3-SiO2inclusion fluctuate around model calculative value, and the two values converge by and large. So thermodynamics model established is credible. In practice production process of304austenite stainless steel, the FeSi75A11.0alloy should be applied, and the molten steel must be feed some Ca. When Si content being at0.4%-0.5%in stainless melts at1873K, the suitable Ca content should be controlled at5×10-6～8×10-6, and while Si content is at superior limit, the dissolving Ca content should be at lower limit, whereas the dissolving Ca content should be at superior limit.