Experimental Study On Microstructure And Properties Of 3Cr2MnNiMo Mould Steel During Hot Working Process

The plastic mold industry in recent years are higher than the national growth rate of GDP growth.With the development of automobile industry and large household appliances,the quality of plastic mold steel requirements are increasingly high.However,there are still some differences between the domestic mould steel and the imported mould steel in the aspects of microstructure uniformity and hardness uniformity of the section,especially for the production of high-end products,most of the mold materials also rely on imports,large cross-section,highquality plastic mould steel gap is still great.3Cr2MnNiMo plastic mould steel is the first choice of high quality plastic products because of its high hardenability and good polishing performance.It has a good market prospect.The thesis was based on the "13th Five-Year" national key project "research and application of high performance steel mold",taking 3Cr2MnNiMo mould steel as the object of study,the behavior of hot deformation at elevated temperature and the behavior of phase change under continuous cooling were investigated.Through the actual forging experiment combined with SEM,EDS,EPMA and other laboratory detection methods,the influence of different diffusion time on the dendrite segregation of solidification microstructure and the influence of forging process on the microstructure and properties of 3Cr2MnNiMo mould steel were studied.The main original works of this thesis are presented as follows:(1)By high temperature uniaxial compression tests,the deformation behavior of 3Cr2MnNiMo mould steel during high temperature compression was investigated,the flow stress curve of 3 Cr2MnNiMo die steel was measured,the deformation structure characteristics and microstructure evolution law were observed,and the high temperature constitutive equation of 3Cr2MnNiMo die steel was established.The results show that when the deformation temperature is higher,the strain rate is lower and the deformation degree is larger,the dynamic recrystallization of 3Cr2MnNiMo mould steel is beneficial.The dynamic recrystallization activation energy is determined,and a mathematical model of deformation resistance with higher accuracy is regressed,which provides a theoretical reference for the actual forging process.(2)Using the expansion method and metallography-hardness method,the continuous cooling transformation curve of 3Cr2MnNiMo mould steel under the condition of deformation and non deformation was plotted,and the influence law of deformation and cooling rate on the phase transformation structure of 3Cr2MnNiMo mould steel was obtained.The results show that due to the combined effects of alloying elements,3Cr2MnNiMo mould steel has good hardenability,continuous cooling of pearlite transformation does not appear in the picture,while comparing the deformation and deformation curves of austenite continuous cooling transformation,found hot plastic deformation slightly delayed the occurrence of bainite transformation.(3)To observe the solidification structure before forging,it is found that the cast structure of 3Cr2MnNiMo die steel has obvious dendrite segregation from the edge to the core,and the composition between the first solidified branch and the later solidified branch is not uniform.High temperature diffusion experiments of different incubation time on solidification structure,with the increase of holding time,dendrite segregation can be improved,dendrite arm and dendrite gradually become blurred,the element distribution is gradually uniform,which for the experimental steel,diffusion velocity of Mn and Ni elements to be significantly higher than Cr elements.(4)The forging experiment of 3Cr2MnNiMo mould steel,through a pull forging process after upsetting,forging microstructure of proeutectoid ferrite and bainite and martensite,dendrite segregation phenomenon has been eliminated,but due to tissue inhomogeneity caused by micro hardness fluctuation is bigger.After quenching and tempering,the obtained structures are tempered bainite.At the same time,the hardness fluctuation of the same section is very little,and the maximum hardness difference is only 28HV;The impact test was carried out,the longitudinal impact toughness is better than the lateral,and the impact toughness and isotropic properties of the edge are better than the heart...