Study On Refining Process Optimization And Sulfide Morphology Control For Resulfurized Free-cutting Structural Steel

With excellent mechanical properties and good machinability,the resulfurized free-cutting structural steels have been widely applied in machinery manufacturing and automobile industries as one kind of important material.However,since the existence of aluminum and sulfurin the molten steel,inclusions with high melting temperature formed during refining process,which usually cause nozzle clogging and deteriorate castability.In addition,the stripe-shaped MnS inclusions along the rolling direction tend to reduce the transverse impact toughness,and impede the further enhancement of machinability of those steels.To solve the above questions,a free-cutting structural steel with 0.08⁰.13wt%[S]?MH-S steel?was taken as the object in the present study.The formation,evolution and controlling of inclusions in the molten steel were studied under the duplex refining process of LF-VD,and then the refining process was optimized.Meanwhile,in order to coordinately improve castability and performance of the steel,based on thermodynamic calculation,zirconium was chosen as the modifier for sulfide inclusions.Then a model,based on coupled microsegregation and inclusion precipitation upon solidification of steels,was established to explore the thermodynamic relation between sulfide inclusion precipitation and molten steel composition.Besides,the influences of Zr-treatment on the morphology of inclusions in steel were studied in laboratory,and the controlling method for simultaneously improving mechanical and cutting properties of steel was also discussed.Based on thermodynamic calculations and industrial investigations,it was found that in the refining process of MH-S steel,inclusions like CaS and MgO?Al₂O₃ with high melting point are likely to precipitate,and which lead to nozzle clogging.High desulfuration capacity of refining slag and inappropriate soft blowing can promoted the formation of CaS inclusions.The residual Al₂O₃ in molten steel at the end of LF treatment and the[Mg]from refining slag and refractory during VD treatment are all contributed to the formation of MgO?Al₂O₃ inclusions.To enhance Al₂O₃ absorption ability and decrease the desulfuration capacity,the target compositions of refining slag should be?FeO+Mn O?<1.5wt%,SiO₂=15²0wt%,CaO=45⁵0wt%,Al₂O₃=25³0wt%and MgO=6⁸wt%.Moreover,some measures were taken to reduce the CaS formation and strengthen the MgO?Al₂O₃ removement.As a result,it reached 6⁸ heats of a single tundish during the continuous casting.Through thermodynamic calculations of inclusions precipitation,it was found that to prevent the precipitating of Zr₃S₄ before the steel solidification which impedes the steel castability,the[Zr]in MH-S steel should be less than 0.17wt%.However,to increase the Zr₃S₄ precipitation ratio during the steel solidification which enhances the sulfide denaturation degree,the[Mn],T.O,and[N]content should be reduced,while the[Zr]should be increased.Therefore,it was proposed that the Zr treatment should be carried out after deep deoxidization and oxide inclusions removement,and meanwhile the[Mn]should be controlled at the medium and lower limit of the composition requirement of the steel and[N]content must be strictly controlled.Based on the experimental results about morphological transformation of inclusions in Zr-treated resulfurized steels,it was found that when the[Zr]in steel increases in the range of 0⁰.05wt%,there is no significant change of the total amount of inclusions,while the clustered MnS inclusions will be converted to granular?Mn_x,Zr_y?S,and the Zr content in?Mn_x,Zr_y?S inclusions gradually increases consequently,which causes the decrease of thermal deformation capacity of the inclusions.When the[Zr]in steel exceeds 0.05wt%,further increasing the[Zr]in steel will not affect the Zr content in?Mn_x,Zr_y?S inclusions and zirconium in steel will be surplus.The surplus of[Zr]will on one hand precipitate as hard and brittle zirconium sulfide particles,which increases the total amount of inclusions,and on the other hand,generates sheet-shaped Fe-Zr phase along the grain boundary.It was concluded that an appropriate sulfide denaturation degree can be obtained when the Zr₃S₄ precipitation ratio reaches 15wt%.Based on the results,the reasonable[Zr]content under the condition of typical molten steel composition was discussed using the coupled model.It was revealed that the Zr-treatment is more suitable for the molten steel with[S]<0.15wt%,and the reasonable[Zr]in steel should be[wt%Zr]_rea=0.28[wt%S]+0.017[wt%Mn].The mechanical properties measurement indicated that with the increase of[Zr]in steel,the plasticity and impact toughness of steel increase firstly and then decrease,while the toughness anisotropy of steel decreases firstly and then increases.When[Zr]=0.039wt%,the plasticity and toughness of the steel reach the peak,and the toughness anisotropy decreases obviously.In addition,when the[Zr]in steel is less than0.14wt%,there is no significant change of yield and tensile strength of steel,while,when[Zr]=0.25wt%,the measured yield and tensile strength of steel increases obviously.The measurement on machinability showed that the aggregated MnS inclusions can reduce the cutting force,and form a film on the tool face during the low-speed cutting process to protect the cemented carbide tool which increase the tool life.However,in the high-speed cutting,the protective film can not be generated from MnS,which leads to decrease the life of a cutting tool.?Mn_x,Zr_y?S inclusions can form a protective film on tool face in both low-speed and high-speed cutting.Therefore,compared with MnS inclusions,?Mn_x,Zr_y?S inclusions can extend tool's life in high-speed cutting.Zirconium sulfide inclusions with high hardness can not form a protective film in neither low-speed nor high-speed cutting,accelerating the tool wear rate.Consequently,it has a great significance in improving the mechanical properties and cutting performances of Zr-treated resulfuried steels by converting inclusions to non-aggregated and spindle-like?Mn_x,Zr_y?S inclusions,while the enhance of morphological control for chip is required.