Research On Hot Deformation Behavior And Heat Treatment Process Of 30Cr4MoNiV Ultra High Strength Steel

Ultra-high-strength steel has excellent strength,toughness and good stress corrosion resistance,which is mainly used in aerospace,large ships,bridges,power plant equipment,high-pressure vessels,mining machinery and other large welded structures.In recent years,with the rapid rise of manufacturing industry in China,higher requirements have been put forward for the performance of bi-metal band saw blade.Therefore,ultra-high strength steel is used as the base metal of bi-metal band saw blade in the field of mechanical processing.This kind of bi-metal band saw blade has the advantages of high efficiency,long cutting life and can be used in complex working conditions,which has been widely studied and applied,and its hot processing and heat treatment have certain uniqueness,but there are few reports on the hot processing and heat treatment of the base metal of bi-metal band saw blade.In this paper,the hot deformation behavior of 30Cr4MoNiV ultra-high strength steel at high temperature is studied,the constitutive equation and processing maps suitable for the steel are constructed,the relationship between deformation temperature and strain rate,flow stress and microstructure are explored,and the heat treatment of 30Cr4MoNiV steel is systematically studied,the effects of different heat treatment parameters on microstructure and mechanical properties are discussed,which could provide theoretical reference and application guidance for hot processing of ultra-high strength steel and heat treatment of bi-metal band saw blade.The true stress-strain curves of 30Cr4MoNiV ultra-high strength steel were obtained by isothermal compression tests.It is found that the flow stress decreases with the increase of the deformation temperature,and increases with the increase of the strain rate.Constitutive analysis shows that the hot deformation behavior is consistent with the Arrhenius hyperbolic sine function,the activation energy is 520.815 k J·mol^-1,and the values of material constants(?,n and A)are 0.00581 mm²·N^-1,8.3,and1.951×10²⁰,respectively.According to the processing maps and microstructure,the optimal parameters of hot processing are the deformation temperature of 1263-1308 K and the strain rate of 0.01-0.1s^-1.Under these parameters,the dynamic recrystallization occurs,and the uniform equiaxed grains are obtained.In addition,the microstructure of 30Cr4MoNiV ultra-high strength steel has obvious dynamic recrystallization at both high temperature and low strain rate.The dynamic strain aging and the microstructure of the voids are the reasons for the plastic instability of the steel during the hot processing.Through a series of heat treatment tests,the effects of different heat treatment parameters on the microstructure and mechanical properties of 30Cr4MoNiV steel for bi-metal band saw blade have been proved.The suitable quenching temperature of30Cr4MoNiV steel for bi-metal band saw blade is about 1190?,the alloying elements are fully dissolved and the martensite structure is uniform and fine.With the increase of tempering temperature,the microstructure is gradually transformed from the tempering troostite to the tempering sorbite.When the tempering temperature is lower than 600?,the microstructure is dominated by the tempering troostite.When the tempering temperature is higher than 600?,the microstructure is dominated by the tempering sorbite,and the hardness,tensile strength and strength-ductility to decrease,while the elongation tended to increase.With the increase of tempering holding time,the microstructure was slightly coarsened,the hardness increases slightly,the tensile strength decreases,and the elongation and strength-ductility increases.Under the conditions of this experiment,tempering temperature is the most important parameter to determine the microstructure and properties of 30Cr4MoNiV steel for bi-metal band saw blade after tempering.The best heat treatment process parameters of 30Cr4MoNiV steel for bi-metal band are:Quenching(1190?+5 min)+tempering(600?+60min).Under this process condition,the strength-ductility of 30Cr4MoNiV steel is15.64 GPa%,the bending fatigue performance increases from 3745 to 5270 times,and the sawing fatigue performance is 25.6%higher than CDW steel.