Effect Of Thermomechanical Treatment On Microstructure And Properties Of Low Carbon Martensitic Steel

In this paper,27Si2Mn2Cr2AlMoNi low-carbon martensitic steel is taken as the research object,and the hot working range with good formability is obtained through thermal mechanical simulation test.By means of microstructure characterization and mechanical properties test,the effect of different deformation temperature and deformation amount on Microstructure and properties of low-carbon martensitic steel was revealed.On this basis,a composite thermomechanical treatment process was designed,and the strengthening and toughening mechanisms of different thermomechanical treatment processes for low-carbon martensitic steel were summarized.The results of thermomechanical simulation test show that the strain rate and deformation temperature have significant influence on the change trend of flow stress,which is the result of dynamic recovery recrystallization and work hardening during hot deformation.According to the results of peak stress,the hot deformation constitutive equation of the test steel was established under the processing parameters of 900～1100℃and 0.01～10/s strain rate.Based on dynamic material model,the hot working diagram of test steel under true strain of 0.1～0.9 is established.Through the analysis of power dissipation coefficient and instability region,the hot working range with good formability is obtained as follows:true strain of 0.3～0.5,deformation temperature of 850～975℃,strain rate of 0.1～1/s.Under different heat treatment conditions,the microstructure of the steel is lath martensite and thin film retained austenite.Compared with direct quenching,different heat treatment processes reduce the M_S point of the test steel,refine the original austenite size and martensite strip thickness,and improve the dislocation density of ferrite.With the increase of deformation,the martensite strip shows a more obvious rule of optimal arrangement,and the number of martensite variants decreases.The mechanical properties of the steel show that the high temperature deformation heat treatment improves the yield strength and plasticity of the steel,but has little influence on the tensile strength.The low temperature deformation heat treatment technology significantly improves the hardness and strength of the steel,but reduces the plasticity.The impact toughness of the steel was improved under different heat treatment processes.The change rule of grain size,microstructure and phase parameters of the steel is similar to that of the two sub processes,and the strength and plastic toughness of the steel are improved obviously.This shows that the change of the microstructure and properties of the composite deformation heat treatment is the result of the joint action of fine grain strengthening and dislocation strengthening in the thermal deformation.Under the condition of 50%of the total deformation,the tensile strength of the composite deformation heat treatment process sample reaches 2GPa,the elongation and the strong plastic product are16%and 32.4GPa%respectively,and the impact toughness is 91.4J/cm~2.The results show that compared with the two sub processes,the composite deformation heat treatment can achieve high strength and toughness improvement,and also achieve a better strong plastic matching.