Study On Behavior And Control Of Non-metallic Inclusions In Low Carbon And Low Silicon Steel

Low carbon and low silicon （LCLS） steel is widely used in automobile, appliancesand construction and other industries due to its low intensity, high plasticity andcold-rolled ductility. The main technical difficulties of LCLS steel are the control ofAl₂O₃inclusions and the chemical elements of silicon during the deoxidization process.Improper control of Al₂O₃inclusions not only influence continuous castability but alsocause resilication, so it is very important significant to study the deoxidization andinclusions of LCLS steel.The basic theory of LCLS steel in the deoxidizing process is analysised in thispaper. Different amount of aluminum content matches with different dissolved oxygenin the thermodynamic equilibrium conditions, which can be produced different types ofinclusions in the liquid steel. After the analysis, it is determined that the deoxidizingproducts are FeO·Al₂O₃inclusions when ω[Al]＜6×10^-5%in the molten steel, while thedeoxidizing products are Al₂O₃inclusions when ω[Al]＞6×10^-5%. It is necessary tochange Al₂O₃inclusions to low melting point of calcium aluminate inclusions. Thethemodynamic calculation concludes as follows. The Al₂O₃inclusions transform into12CaO·7Al₂O₃when the Ca content reaches to0.029%, while to3CaO·Al₂O₃when theCa content expandes to0.040%. Meanwhile, the deoxidizing model is eatablishedduring the vacuum decarburization and aluminium deoxidization of LD-RH-CC process.The model can predict the total oxygen reaches to20ppm in steel after30min ofrefining time.The number, morphology and composition of inclusions in LCLS steel byLD-RH-CC process are analysed using metallographic microscope, scanning electronmicroscopy. The total oxygen content in steel is tested by ON-900. It is indicated thatthe type of inclusions is FeO while the dissolved oxygen in steel is controlled to400ppm before RH refinement. The content of total oxygen is16ppm after RH refinementand the type of inclusions mainly is Al₂O₃which failed to rise from steel to slag, andcontains some small amount of MgO and MnO. Tundish metallurgy precipitates the T[O]increases to22.5ppm which air is drawn into the steel, Al₂O₃inclusions are still themain object including small amount of CaO, MgO and SiO₂compound.The inclusions of LCLS steel in different process （LD→RH→CC and LD→LF （calcium treatment）→CC） are compared by this study. The results indicate that thetypes of inclusions using LF refinement （calcium treatment） are CaO-Al₂O₃-MgO andCaO-Al₂O₃-SiO₂composite oxides whose size can be spread about5μm. And the typeof inclusions using RH refinement is Al₂O₃whose size is about3μm, in addition, thecleanliness of the slab is improved. It is turned out that it is reasonable and feasible withRH dry vacuum treatment to process LCLS steel.