Research On Hot Deformation Behavior And Microstructure Property Of Hot Work Die Steel 5CrNiMoV(Nb)

5CrNiMoV steel,as a typical Cr-Mo-V martensitic hot work die steel,is widely used in the manufacture of various hot forging dies.However,the insufficient thermal strength of 5CrNiMoV steel affects its service life and limits application.A noval hot work die steel 5CrNiMoVNb,with high hardness and excellent toughness,was developed by optimizing the alloy composition of 5CrNiMoV steel based on thermodynamic calculations.The hot deformation behavior and service performance of this type of Cr-Mo-V hot work die steels were studied through the thermodilatometry,the electronic universal testing machine,Gleeble simulator,scanning electron microscope(SEM),transmission electron microscope(TEM),thermal fatigue testing machine,etc.The hot deformation mechanism and microstructure evolution of this type of Cr-Mo-V hot work die steels were revealed.The influence of the alloying elements of Cr,Mo and V on the thermal stability and thermal fatigue property was studied.The main results are as follows:(1)The volume fraction all kinds of carbides of 5CrNiMoVNb steel has increased significantly,especially the MC carbide.The volume fraction of MC,M23C6 and M7C3 below 670? is basically constant,which is beneficial to improve the tensile strength,toughness,thermal stability,thermal fatigue property,etc.Besides,the increase of Mo,V and Nb alloying elements increases the alloying elements solid solution temperature and the alloying elements solid solubility in the austenite phase,which is beneficial to inhibit the coarsening of austenite grains.Better mechanical properties of 5CrNiMoVNb steel can be acquired in a wider quenching and tempering temperature range than 5CrNiMoVNb steel.The best heat treatment process for 5CrNiMoVNb steel is quenching at 940? and tempering at 600? for 2h.(2)The hot deformation behavior of the two Cr-Mo-V hot work die steels was studied based on the single and double pass Gleeble hot compression experiments.The high temperature flow stress model,dynamic recrystallization model,sub-dynamic recrystallization model and grain growth model are established with high accuracy,which can be used in the the free forging process of large mould.During the hot deformation process,the austenite grain size of 5CrNiMoV steel increases with the increase of temperature and the decrease of strain rate.When complete dynamic recrystallization occurs,the higher strain rate and lower deformation temperature are beneficial to the improvement of strain storage energy,thereby promoting the refinement of recrystallized grains.Besides,the evolution of the recrystallized grain and grain boundary at different deformation conditions shows that discontinuous dynamic recrystallization(DDRX)is the main mechanism of recrystallization nucleation and grain growth during the hot deformation of 5CrNiMoV steel.(3)The orientation relationship between martensite phase and parent phase austenite in 5CrNiMoV steel is consistent with the N-W orientation relationship.The research on the hot deformation austenite microtexture shows that at the same strain rate,higher temperature contributes to greater MAD(random orientation distribution)value and stronger rotating Cube texture component.At the same deformation temperature,higher strain rate leads to smaller MAD value and weaker deformation texture component,which is due to the increase of the active slip system and the refinement of the austenite grains.The study of martensite transformation texture shows that the martensitic transformation texture composition depends on the selection of the variants during the martensitic transformation.Besides,the formation of martensitic transformation texture is always easy to be in a direction with a small orientation difference from the austenite texture.(4)The free forging numerical simulation model of a large-scale hot work module of 5CrNiMoV steel is established based on the obtained material model.The free forging simulation research shows that increasing the reduction rate and choosing a smaller anvil width not only can not only refine the grains,but also improve the uniformity of deformation of the large hot work die.Based on orthogonal simulation experiments,the optimized free forging process of large-scale hot work die of 5CrNiMoV steel is that the reduction rate is 40mm/s,the anvil width is 1000mm and the single pass reduction is 25%.(5)The comparative study of the thermal stability for 5CrNiMoV steel and 5CrNiMoVNb steel shows that the thermal stability of 5CrNiMoVNb steel is 35%and 45%higher than that of 5CrNiMoV steel at 600? and 650?,respectively.The initial tempered microstructure of the two Cr-Mo-V hot work die steels are composed of tempered martensite and carbides.Due to the high volume fraction of carbides and their dispersed distribution in the matrix,it acquires an excellent high-temperature thermal stability and tempering softening resistance.In addition,for the the reasonable content of medium and strong carbide forming elements such as Cr,Mo and V,the main precipitation strengthening phases MC,M7C3 and M23C6 have extremely low coarsening rate coefficients during the thermal stability of 5CrNiMoVNb steel.A certain amount of residual strain in 5CrNiMoV steel can increase the number of defects such as dislocation cells and martensite lath boundaries,which can inhibit the coarsening of martensite structure and carbides,thereby improving the thermal stability of 5CrNiMoV steel to a certain extent.(6)The thermal fatigue properties of 5CrNiMoV steel and 5CrNiMoVNb steel was analyzed based on the self-constrained fatigue test.After 2000 thermal fatigue cycles,thermal fatigue cracks appear in both steels,and the main crack lengths are 184.47?m and 104.06?m,respectively.It can also be found that the fatigue cack intiation and propagation rate of 5CrNiMoVNb steel are significantly lower than that of 5CrNiMoV steel.Due to the high content of small carbide particles and their strong inhibitory effect on dislocation movement and microstructure coarsening,5CrNiMoVNb steel has better microstructure stability and toughness.Therefore,the thermal fatigue property of 5CrNiMoVNb steel is better than that of 5CrNiMoV steel.