Regulation Of Microstructure And Properties And Damage Mechanism Study For 4Cr5MoSiV1(Ti)

With the development of the mold industry to large,complex,precise,high-efficiency and fast-paced,its service environment is becoming more and more demanding.Higher requirements are put forward for the safety,reliability and service life of the mold and its materials.Especially when the hot work mold is in service,it is subject to high temperature,large stress and mutual coupling,which leads to a serious reduction in service life.Therefore,the development of new high-strength,toughness and long-life hot work die steel is imminent.How to regulate the type,size,distribution and interface relationship of carbides in steel through microalloying and heat treatment is the prerequisite for the development of high strength and toughness hot work die steel.However,there are many types and high contents of alloy elements in hot work die steels.It is difficult to realize the ideal distribution of carbide types and characteristic parameters in order to fully exploit the performance potential of die steels during service.Meanwhile,the service conditions of hot work die steel are often high temperature,high stress and interaction,which leads to complex and variable damage factors.Therefore,it is of great significance to carry out research on the regulation of the microstructure and properties of hot work die steel and the damage mechanism under the coupling of thermo-mechanics.In this paper,4Cr5MoSiV1 was first taken as the research object,the internal relationship between microstructure and properties of 4Cr5MoSiV1 hot work die steel and its high temperature fracture mechanism were constructed;the intrinsic correlation between the characteristic parameters such as carbide type,size,morphology and distribution and crack initiation and propagation were analyzed;the evolution of the microstructure near the crack was observed;which all the previous results can provide a certain theoretical basis for the development of high strength and toughness hot work die steel.Based on this,a high strength and toughness titanium microalloyed4Cr5MoSiV1Ti hot work die steel was prepared.Microstructure and property of4Cr5MoSiV1Ti hot work die steel was controlled by changing alloy composition and heat treatment process.A dynamic damage physical test platform for molds under thermal-mechanical coupling conditions was designed and developed.The damage behavior of 4Cr5MoSiV1Ti die steel during hot extrusion was researched.The damage mechanism of titanium microalloyed 4Cr5MoSiV1Ti hot work die steel under the condition of thermal-mechanical coupling was investigated,which laid a theoretical foundation for the development of high-end die steel.Obtained results show that:It mainly consists of V-type MC-type carbides and Cr-containing M₇C₃ and M₂₃C₆type carbides in 4Cr5MoSiV1 steel;MC-type carbides are precipitated in the tempered martensite bundle and at the bundle boundary,which has a semi-coherent interface with the matrix and can enhance the thermal stability and tempering softening ability.The M₇C₃ and M₂₃C₆ type carbides are mainly precipitated at the prior grain boundary or martensite bundle boundary,and have a non-coherent interface relationship with the matrix.With the increase of aging temperature and time,the hardness decrease of4Cr5MoSiV1 steel is mainly caused by the recovery of high-density dislocation tempered martensite,local recrystallization,precipitation of alloy carbide and growth of recrystallized grains.Due to the non-coherent interface relationship and the irregular shape of the carbide(including M₇C₃ and M₂₃C₆),the growth activation energy is lower and the carbides is easier to grow.Hence,it is easier to grow and there are sharp corners that form stress concentration,and it is easier to promote crack initiation under stress during the stretching process.MC-type carbides with coherent/semi-coherent interface relationship can inhibit the dislocations from accumulating at the grain boundaries,and can effectively transfer loads during the crack propagation process,and have a certain inhibitory effect on crack initiation and propagation.During low-temperature stretching,because a large amount of nano-scale second phase particles and high-density dislocations are distributed on tempered martensite in 4Cr5MoSiV1 hot-working die steel,dislocation slip can only be performed within a limited distance and the die steel has high strength and low toughness.After increasing the stretching temperature,higher elongation and cross-section shrinkage can be obtained because of the recovery of tempered martensite,local recrystallization,and the reduction of dislocation density causing the matrix to soften,slip and cross-slip.Under the uniaxial stretching condition of 580°C and 1 mm/min,the Ferrite nanocrystal deformation band of about 100 nm is formed on both sides of the crack with transgranular fracture in 4Cr5MoSiV1 steel,which is mainly due to the dynamic equilibrium of recrystallization about tempering martensite and plastic deformation in the process of high temperature tensile.Due to the strong stress on the crack tip and the obvious grain rotation/sliding phenomenon,the concentric slip ring with a size of about 200 nm was found in the crack tip of the4Cr5MoSiV1 hot work die steel for the first time.Compared with 4Cr5MoSiV1 hot work die steel,the strength and elongation of4Cr5MoSiV1 with 0.13%Ti increased by 13.5%and 17.7%,respectively.Meanwhile,adding Ti content of 0.13%in 4Cr5MoSiV1Ti steel can significantly improve the peak hardness of secondary hardening point（increased by 3.9 HRC）,secondary hardening peak temperature（increased by 20°C）and the diffusion activation energy（improved23k J/mol）.In other words,it has better high temperature stability and temper softening resistance,which is related to the addition of a trace amount of Ti to form MC type carbides having a size of about 50 nm and rich in Ti and V.The softening process of4Cr5MoSiV1Ti during tempering is caused by high-density dislocation interaction on tempered martensite,offset,further forming dislocation walls or dislocation columns,the wavy change of the tempered martensite boundary,the accelerated diffusion of alloying elements at high temperature and growth of local subgrains.The dynamic damage physical test platform of mold steel under the thermal-force coupling condition mainly includes hydraulic control system,medium frequency induction heating system,pressure displacement acquisition system,convex surface temperature measurement system and blank transfer system.Based on the physical test platform,the macroscopic morphology,microstructure,element distribution and mechanical properties of the convex life cycle of 4Cr5MoSiV1Ti hot work die steel were tested at 1000℃during hot-force coupling.Under the thermal-force coupling,the damage form of 4Cr5MoSiV1Ti hot work die steel back-extruded 45#steel is mainly manifested by the increase of the fillet size of the punch working strip caused by plastic rheology,the oxidation of the punch surface and the high temperature wear.After hot extrusion,the microstructure of the punching strip at the fillet can be divided into three parts:fine grain zone,plastic rheological zone and original topography zone;and the width of the plastic rheological zone of the surface metal gradually increases with the number of extrusions.The surface softening of 4Cr5MoSiV1Ti under thermo-mechanical coupling is mainly caused by the over-tempering phenomenon under the thermo-mechanical coupling,the interaction between carbide and dislocation,and the coarsening behavior of carbide.The Ti-V composite carbide with core-shell structure（The core Ti C has a completely coherent interface relationship with the spherical VC）was first discovered in the 4Cr5MoSiV1Ti hot die steel,which can effectively improve the high temperature performance stability of the material during the service.