Dissolution And Precipitation Behavior Of Composite Second Phase In Titanium Microalloyed Die Steel

Hot-working die steel not only bears the cycle of high temperature and low temperature,but also bears the influence of stress in different directions,which results in the change of surface and internal structure,thus deteriorating its properties,so its service life is reduced.Therefore,how to improve the comprehensive performance of hot-working die steel is extensive focued.To this end,Cr-Mo-V hot-working die steel was taken as the research object in this paper,and a trace amount of Ti element was added to the steel.Through the thermodynamics and dynamics calculation of the dissolution and precipitation process of titanium-containing die steel,combined with simulation of dissolution and precipitation process using the thermodynamics software,and the solid solution test and stress relaxation test of titanium-containing die steel were conduced to study comprehensively the strengthening mechanism of die steel by adding trace element Ti.The dissolution thermodynamic model of five-element second phase for die steel was established.The absolute solid solution temperatures of Cr₂₃C₆,Cr₇C₃,VC,MoC,Mo₂C and TiC were calculated to be 766.7°C,976.5°C,1040.8°C,1197.3°C,698.5°C and 1029.3°C,respectively,considering the interaction of various elements in die steel.The amount of solid solution[Ti]in steel increases with the increase of Ti content in steel,and decreases with the increase of Cr,Mo and V content.The hardness and impact toughness increase with the increase of dissolution temperature.The toughness of Ti-containing die steel is better than that of Ti-free die steel.With the increase of dissolution temperature,the grain size of Ti-containing die steel increases from fine to coarse and martensite increases.The average grain size of Ti-containing die steel is 21.4?m smaller than that of Ti-free die steel.The size of undissolved phase in titanium-containing steel is about 0.2?m smaller than that in titanium-free steel.Through the thermodynamics and dynamic calculation of the carbide dissolution and precipitation process in die steel and their software simulation,it is found that the solid solution carbides in the steel are mainly CrC,MoC,VC,TiC,Cr₂C,Mo₂C and V₂C.As the temperature increases,Cr₂C,Mo₂C,and V₂C disappear and convert into CrC,MoC,and VC.Ti element can effectively increase the precipitation and precipitation temperature of MC phase in the die steel,and reduce the precipitation temperature range of Cr₂₃C₆ and the content of M₂₃C₆ precipitated carbide.During precipitating of?Ti,V?C in 0.4C-0.02Ti-0.9V steel,the NrT curve shows obviously anti-“C”curve,the PTT curve shows obvious“C”curve.The maximum nucleation rate and precipitation temperature of homogeneous nucleation are 850°C and 860°C respectively,and the maximum nucleation rate and precipitation temperature of dislocation line are 900°C and 880°C respectively.The maximum nucleation rate temperature of the NrT curve of TiC and the fastest precipitation temperature of the PTT curve decrease with the increase of Mo,Cr and V contents,and increase with the increase of Ti content.The stress relaxation experiments of titanium-containing steel and titanium-free steel were carried out by Gleeble-3800 thermal simulation tester to analyze the influence of Ti element on the precipitation process of the second phase particles in die steel.The results show that with the increase of temperature and deformation,the flow stress decreases.The higher the temperature of stress relaxation process is,the lower the relaxation limit is.The stress platform of titanium-containin steel is more obvious than Ti-free steel.Increase of deformation can increase the stress relaxation rate.The nose point temperature of Ti-containing steel is about 930°C.The precipitation starting and end time of the 15%deformation amount lag behind the deformation amount of 30%.The precipitation phase mainly precipitates along the vicinity of the grain boundary,and the increase of the deformation amount is favorable for the precipitation of carbides.Grain size of die steel and distribution,morphology and size of second phase during stress relaxation experiment were studied,three types of nano-precipitates were found in Ti-containing steel:square TiC precipitates,spherical VC precipitates and composite rounded square precipitates with Ti and V.The precipitated phase of titanium-containing steel precipitates uniformly and orderly in the crystal,and the precipitated phase of titanium-free steel agglomerates and precipitates disorderly.The precipitation phase of titanium-containing steel is 1.05 nm smaller than that of titanium-free steel.The precipitation phase of titanium-containing steel under 10 nm accounts for 92.67%,and the precipitation strengthening effect is obvious.