Study On Element Migration Behavior And Inclusion Control In High Stress Spring Steel Refining System

The element migration behavior and inclusion control of55SiCr high stress spring steel during refining were studied.It mainly involves the modification of inclusions in spring steel,the wettability of inclusions at steel slag interface,the dissolution of inclusions in refining slag and the simulation of four-phase reaction of molten steel,refining slag,inclusions and refractories.The morphology,compositions and size distribution of the inclusions in the Fe-Si-Mn-Cr-Al-Ti-Ca-O system were characterized by field emission scanning electron microscopy and energy dispersive spectroscopy（FE-SEM&EDS）.The evolution mechanism of the inclusions in the Fe-Si-Mn-Cr-Al-Ti-Ca-O system was clarified by combining the interaction coefficient of elements in the molten steel.It is found that the inclusions in this system are mainly Si O₂-Al₂O₃-Ti₃O₅-Ca O-Mn O composite inclusions,and Ti/Al ratio and calcium treatment intensity need to be controlled to make the high melting point inclusions transfer into liquid phase.The addition of Ti element to the melt can re-duce（Al₂O₃）and（Si O₂）in the inclusion,and the reduction sequence is closely related to the dissolved aluminum[Al]in the melt.At the same time,the Ca treatment in molten steel can reduce the titanium oxide（Ti₃O₅）in the inclusions,and the reduction ability depends on the dis-solved oxygen[O]content in the melt.In addition,FACTSAGE thermo-dynamic software was used to calculate the predominance-area of inclu-sion,and the results are in good agreement with the experimental results.In the study of the wettability of inclusions at steel slag interface,the wettability angle of molten steel on alumina substrate was measured by the horizontal drop method.The influence of O,S,Ti and Ca contents on the wettability of alumina inclusions in molten steel was studied.It is found that there is a positive correlation between the O,S contents in molten steel and the wettability of alumina.Therefore,it is very important to reduce the O and S contents in liquid steel in the early stage.For Ti element,the higher the content of Ti element in molten steel,the lower the wettability of alumina inclusion.Therefore,proper Ti content in mol-ten steel is beneficial to the removal of alumina inclusions.The influence of Ca element in molten steel on the wettability of alumina inclusions is the same as that of Ti element,but the contribution of high Ca element to wettability is small,and the optimum Ca content is about 25 ppm.In ad-dition,the surface tension of liquid steel with different S content is pre-dicted by using the formula of K.C.Mills et al.The dissolution of inclusions in refining was studied by using sin-gle-filament thermocouple（SHTT）and high temperature confocal mi-croscope（CLSM）.The results show that the dissolution kinetics of alu-mina particles is controlled by liquid film diffusion（external diffusion）,and the dissolution rate depends on the characteristics of refining slag.Therefore,for the comparison of dissolution capacity of Ca O-Al₂O₃-Si O₂-Mg O refining slag on the isotherm,it is found that the best dissolution rate is when the content of Al₂O₃ in slag is about 27.5wt.%.In addition,through the calculation of the kinetic diffusion equa-tion,the diffusion coefficients of five different Al₂O₃ content refining slag are obtained,which are 9.55×10^-5cm²/s（21.11%）,7.77×10^-5cm²/s（24.69%）,1.31×10^-4cm²/s（27.52%）,1.19×10^-4cm²/s（30%）and 1.75×10^-4cm²/s（35%）,respectively.In a four-phase high-temperature simulation experiment of molten steel,refining slag,inclusions and refractory material,the evolution of inclusions with time after adding refining slag was investigated by high-frequency induction furnace.The composition,morphology and size distribution of the samples were characterized by inductively coupled plasma emission spectrometer（ICP-OES）,oxygen,nitrogen and hydro-gen analyzer and FE-SEM&EDS.The change of elements in molten steel was calculated by the interaction coefficient of molten steel,and the mechanism of element migration between molten steel and refining slag was explained.The results show that[Si]in molten steel is very easy to reduce（Al₂O₃）in slag at the interface of steel slag when there is higher dissolved oxygen[O]in molten steel at the initial stage of smelting.At the same time,[Ti]in liquid steel also reduces（Ca O）in slag interface,which keeps a high level of[Ca]content in steel.Under the action of in-terfacial reaction,on the one hand,the dissolved[O]content in molten steel decreases continuously,on the other hand,it also promotes the mod-ification of Al₂O₃ in molten steel into larger calcium aluminate inclusions by[Ca],which enhances the removal of inclusions in molten steel.In ad-dition,through the analysis of the evolution law of the inclusions,it is found that the element migration behavior is basically consistent with the morphology,composition and size distribution of the inclusions,which well explains the removal mechanism of the inclusions in the system.Furthermore,a four-phase reaction dynamic model was established by using FACTSAGE macro processing,and the evolution law of inclusions in the model was basically consistent with the average composition of in-clusions in this experiment.