Study Of Hot Deformation And Continuous Cooling Transformation Behaviors Of 42CrMo Steel

42CrMo steel has excellent comprehensive mechanical properties and process properties,and is widely used in automobile manufacturing,engineering machinery,petroleum industry and other fields.However,there are still problems in the production and use of 42CrMo steel such as uneven organization,substandard properties and quenching fracture.In order to solve the production problems and improve the product performance,this study conducted hot compression experiments on 42CrMo steel with deformation temperature of 850-1100℃and strain rate of 0.1-20s^-1;and dynamic continuous cooling transformation experiments with deformation temperature of 800 and950℃and cooling rate of 0.1-30℃/s.The results of the hot compression experiments showed that the flow curves exhibited significant peaks at 0.1s^-1/900～1100℃and 1s^-1/900～1100℃.The flow stress reached a maximum value of 276 MPa at 20s^-1/850℃.The effect of temperature on the stress value was greatest at a strain rate of 0.1s^-1,and the effect of strain rate on the flow stress was greatest at a deformation temperature of 1100°C.The strain-compensated Arrhenius constitutive model and the Jihua Zhou-Keizhi model were developed,respectively.The average relative error（AARE）between the predicted and tested values of the Zhou Jihua-Keizhi model is 4.987%,which can predict the heat deformation rheological stress of 42CrMo steel more accurately.By analyzing the numerical simulation results,it was found that the deformation conditions affect the equivalent stress field,the equivalent strain field and the temperature field,but the strain rate has the greatest effect.The maximum strain decreases by 0.3 as the strain rate increases from0.1s^-1 to 20s^-1,while the size of the deformation zone increases significantly with the increase of the strain rate.The maximum stress increases by 150 MPa when the strain rate increases from 0.1s^-1 to 20s^-1.The temperature rise caused by the deformation process increases with the increase of strain rate,and the temperature rise is 49℃,75℃and 90℃for strain rates of 0.1s^-1,1s^-1 and 20s^-1,respectively.By analyzing the high-temperature austenite organization of 42CrMo steel after thermal deformation,it is found that all austenite organizations are completely dynamic recrystallization（DRX）when the deformation temperature is above 950℃.The critical strain point of DRX decreases with increasing deformation temperature and decreasing strain rate,and the relationship between the critical strain and peak strain of DRX isε_c=0.65ε_p.When the deformation temperature is 850～900℃,the softening mechanism is DRV and DRX act together;when the deformation temperature is 950～1100℃,the softening mechanism is mainly DRX.It is confirmed by room temperature tissue analysis and calculation results that the substructure introduced by deformation affects the martensite phase transformation,and the multilevel structure of martensite is destroyed when DRX is incomplete,while the martensite tissue shows an ordered multilevel organization when complete DRX occurs.The dynamic continuous cooling transformation（CCT）curves of 42CrMo steel were established by analyzing the microstructure and thermal expansion curves at different cooling rates.The CCT curves at deformation temperatures of 800 and 950℃both showed high-temperature ferrite/pearlite（F/P）,medium-temperature bainite（B）and low-temperature martensite（M）transformations.By comparing the CCT,it was found that the F/P transformation zone shifted to the right and the B transformation zone became larger when the deformation temperature was 950℃.The hardness values for both deformation temperatures of 800℃and 950℃increased with the cooling rate.The hardness value at 800℃is slightly lower than that at 950℃when the cooling rate is less than 5℃/s.This is because the ferrite content in the organization at 800℃is higher,which reduces the hardness value.The hardness value at 800°C is higher than that at 900℃when the cooling rate is greater than 5℃/s,which is due to the finer organization of the slat M at 800℃and thus increases the hardness value.The JAM model for pearlite and bainite phase transformation and the K-M model for martensite phase transformation were developed respectively.The JAM model for the bainite phase change was better fitted.The JAM model for the bainite phase transition at 950°C-1°C/s is f=1-exp(-3.96×10-7*t^3.099),and R² reaches 0.998.The K-M model for the martensitic phase transition at 800°C-10°C/s is f=1-exp（-0.036*（293-T））with R² of 0.994.