Evolution Behavior And Formation Mechanism Of Non-metallic Inclusions In 316L Stainless Steel

This paper takes 316L stainless steel from a domestic steel plant as the research object,steel samples were taken at the AOD process,LF refining,calcium treatment and slab during the production of 316L stainless steel.The content of total oxygen and nitrogen,the number density and composition changes in inclusions in steel samples were analyzed,and combined with thermodynamic calculations,the formation mechanism and evolution law of inclusions in the smelting process were studied,so as to provide guidance for the control of inclusions and the improvement of product quality.The main conclusions are as follows:During the smelting process,the total oxygen content in the steel gradually decreases.After the LF standing is completion,The total oxygen content in the start casting of stove is 26 ppm,which is 153 ppm lower than that at the end of AOD reduction period,the total oxygen content is less and the content of(Fe O+Mn O)in the slag is less than 1.0%.The oxidability and basicity of the slag are well controlled.There is serious secondary oxidation during pouring,and the total oxygen content in the billet sample of start casting stove increases from 26 ppm to 74 ppm With the addition of Si-Ca block and Si-Ca line,the numerical density of inclusions increased greatly,and then decreased continuously due to the floating removal of inclusions.When the static placement was completed,the number density of start casting stove is 1.8 pcs/mm²,and that of normal stove is 1.42 pcs/mm².The number density of inclusions increased significantly during pouring,and then decreased with the pouring process.At the end of the AOD reduction period,the type of inclusion is mainly SiO₂-Mn O system;before AOD tapping,its composition is mainly CaO-Al₂O₃-SiO₂-Mg O,and this type of inclusion is maintained during LF refining period.According to the oxide content,the inclusions are divided into CaO-Al₂O₃-SiO₂ system and CaO-Al₂O₃-Mg O system.For CaO-Al₂O₃-SiO₂ system inclusions,the average composition of AOD deviates from the low melting point region before tapping.With the progress of refining,the content of Al₂O₃ in CaO-Al₂O₃-SiO₂ system inclusions in the start casting stove increased at first and then decreased,and the corresponding CaO content decreased at first and then increased.The change trend of normal stove is opposite to that of the start casting stove.After calcium treatment,the average composition of CaO-Al₂O₃-SiO₂system inclusions is near the low melting point region.For CaO-Al₂O₃-Mg O system inclusions,before AOD tapping,the content of Al₂O₃ is high and the content of Mg O is low in CaO-Al₂O₃-Mg O system inclusions,and the amount of this type of inclusions in the start casting stove is less than that in the normal stove.With the progress of refining,the content of Mg O in CaO-Al₂O₃-Mg O system inclusions and the number of Mg O·Al₂O₃ spinel increased,while after calcium treatment and static flotation,CaO-Al₂O₃-Mg O system inclusions decreased significantly.Due to the serious secondary oxidation in the process of continuous casting,the contents of Mn O and SiO₂ in the inclusions increase obviously.The inclusions can be divided into SiO₂-Mn O-Al₂O₃ system,CaO-Al₂O₃-SiO₂ system and CaO-Al₂O₃-Mg O system.For SiO₂-Mn O-Al₂O₃ system inclusions,the Mn O content and Al₂O₃ content are high,and the average composition of inclusions deviates from the low melting point region.With the pouring process,the content of SiO₂ and Mn O in SiO₂-Mn O-Al₂O₃system inclusions is decreasing,while the content of Al₂O₃ is increasing.Most of these inclusions are located in the corner of Al₂O₃,and there are no such inclusions at the end of pouring.For CaO-Al₂O₃-SiO₂ system inclusions,the Al₂O₃ content is higher and deviates from the low melting point region after 20 minutes of pouring.With the pouring process,With the process of pouring,the content of SiO₂ in CaO-Al₂O₃-SiO₂ system inclusions decreases and the content of CaO increases,and its average composition is close to the region of low melting point when the pouring is completed.For CaO-Al₂O₃-Mg O system inclusions,the Mg O content is lower after 20 minutes of pouring,and the Al₂O₃ content increases with the pouring process,and its average composition is located in the part of the Al₂O₃ angle in the ternary phase diagram when the pouring is completed.According to the analysis of Fact Sage software,it is easy to form magnesia-alumina spinel in molten steel.When the content of Mg O in the inclusion is less than20%and the CaO content is more than 30%,the magnesia-alumina spinel can be transformed into Mg O-Al₂O₃-CaO-SiO₂ composite inclusion with low melting point.CaO-Al₂O₃-SiO₂-(Mg O)inclusions come from AOD slag entrapment,and CaO-Al₂O₃-Mg O-(SiO₂)inclusions come from the reaction between inclusions and deoxidizer in steel.In the process of continuous casting,with the decrease of temperature,the content of Al₂O₃ in the inclusions increase rapidly,but the content of CaO and SiO₂ in the inclusions decrease,and the main composition of the inclusions are Al₂O₃-CaO,which is consistent with the experimental results.