Study On Control Of Inclusion In Quenched And Tempered Steel 31CrMoV9 Containing Sulfur

Tempering and nitrizing may significantly improve the mechanical strength and corrosion resistance as well as the tenacity of 31CrMoV9 steel,consequently it is widely utilized for the production of axle and gear parts.Currently,many domestic iron and steel enterprises have been trying to produce 31CrMoV9 steel,while the quality of domestic product is still inadequate comparing to imported products.But the influence of quantity and distribution as well as the control of inclusion on ultimate property of 31CrMoV9 steel were still not clarified.Therefore,the analysis and control of inclusion in sulfurated tempered steel was carried out in this study.Inclusion quantity and type in various 31CrMoV9 steel samples from different manufacturers was analyzed in this research.It is indicated in the results that the precise control of composition and quantity as well as the size of inclusion are the major factors affecting the quality of steel.The inclusion in imported steel are mainly multiple compounds of Al₂O₃,CaO and MnS,the quantity and formation of which are ideal while the distribution of inclusion in steel is also relatively homogeneous.Consequently,the ductility and toughness of imported steel is superior in comparison to domestic products.It is reported that MnS inclusion in strip shapes may lead to the formation of cracks during stretching process.Observation with confocal laser scanning microscope（CSLM）showed that the formation of MnS in steel is mainly at the end of solidification.The number of strip MnS inclusion are increased with the rising of temperature due to the increased of solid solubility of Mn and S elements.The nodulizing of MnS inclusion is noticed in the temperature zone between 600℃ and 1000℃,while solid solution of MnS will occur with the further increasing of temperature.FactSage was applied to theoretically analyze the low melting point area and desirable composition of CaO-Al₂O₃-SiO₂-MnO and CaO-Al203-SiO₂-MgO-MnO multiple compound.It is shown in the results that when Al₂O₃ content is between 25%and 30%,CaO content is between 35%and 40%,simultaneously CaO/SiO₂ is between 3.5 and 4.5,the smelting point of CaO-SiO₂-Al₂O₃-MnO compound is observed to be decreased.Simultaneously,the aforesaid conditions are also conducive for the pentary slag system of CaO-25%Al₂O₃-SiO₂-4.8%MgO-MnO to obtain a superior plastic deformation performance.The content ranges of S and P are strictly controlled during the production of 31CrMoV9 steel,while the value of T.O is fixed at 0.0010%.Meanwhile,superior thermoplasticity is achieved when the value of Mn/S is above 20 and the shape of MnS inclusion is likely to be fusiform.The content of Al₂O₃ is restricted between 25%and 30%while the basicity of（CaO+MgO+MnO）/SiO₂ is about 5.0 in actual production,by which the inclusions in steel are apt to be complex.Moreover,the contents of MgO,Al₂O₃ and CaF2 in slag are supposed to be respectively about 8%,15%,5%with the slag basicity of 3.0 in order to optimize the quality of steel by regulating the quantity and morphology of inclusion.FeS and Ca-Si line alloying method was adopted in actual liquid alloying process of 31CrMoV9 steel,which is helpful for the formation of strip-type sulfide impurities in steel.The Ca/S is supposed to be limited between 0.045 and 0.060 in order to minish the influence of inclusion on quality of steel.Usually,low casting speed and slow cooling schedule are adopted in actual casting process.Rolling process should be carried out under 1000℃ with the deformation of casting billet is over 50%in order to improve the shape and plasticity as well as distribution of inclusion.Besides,low deformation rate in high temperature zone and high deformation rate in low temperature zone are effective operations to remit the adverse impact of inclusion.