Influence Mechanism Of Deformation Conditions On Mechanical Properties Of C-Mn Steel

In industrial production and application,the mechanical properties of steel are the key to ensure the service safety.However,the mechanical properties of steel are different under different use and test conditions,which causes some trouble to the selection of steel.The influence law and mechanism of deformation conditions on the mechanical properties of steel is one of the important basic researches to improve the properties of steel materials and enhance its application.In this paper,a kind of C-Mn steel was taken as the research object.The tensile tests at high temperature and room temperature were carried out on Gleeble 3500 thermal simulation testing machine and Instron 8801 testing machine.The microstructure of the tested steel was observed by metallographic microscope,scanning electron microscope and transmission electron microscope.The grain orientation and element distribution in the tested steel were detected by electron backscatter diffraction and electron probe microanalysis.The effects of deformation temperature,strain rate and pretreatment process on microstructure were studied,and the mechanism of the effects on mechanical properties was revealed.With the increase of deformation temperature,the yield strength(YS)and ultimate tensile strength(UTS)of the tested steel show a downward trend;the reduction of area(RA)first decreases and then increases,and the minimum value is 55%at 720℃.The fracture surface of the tested steel in the two-phase zone(720℃)presents intergranular fracture morphology,and the ferrite in the room temperature structure presents network distribution along the original austenite grain boundary;the fracture surfaces of the tested steel in the austenite single-phase zone(900℃)and the ferrite phase zone(600℃)present dimple morphology,and the microstructures near the fracture surfaces at room temperature are martensite and bainite respectively.The accumulation of S atoms at the original austenite grain boundary is detected near the fracture surface after stretching at 720℃,which is one of the reasons for the decrease of plasticity of the tested steel.With the increase of strain rate,the yield strength and tensile strength of the tested steel show an upward trend,and the difference increases gradually.The reduction of area shows an upward trend with the increase of strain rate,but the minimum values appear at strain rate of 10-3s-1-deformation temperature of 600℃ and strain rate of 10-2s-1-deformation temperature of 880℃,which are 84%and 93%respectively.At low strain rate,equiaxed ferrite is found near the fracture surface(600℃).With the increase of strain rate,the grain size decreases gradually.When the strain rate reaches 10-1s-1,ferrite is mainly elongated along the tensile direction near the fracture surface.With the increase of strain rate,the strain hardening index of the tested steel increases from 0.04(10-4s-1-720℃)to 0.13(100s-1-720℃);the dynamic recovery and recrystallization degree of the tested steel decrease,the grain diameter decreases from 3.7μm(10-4s-1-600℃)to 1.7μm(100s-1-600℃),and the plasticity increases.The tensile strength of group A2 samples treated with high temperature solution treatment(process A)before stretching is higher,and the yield strength of group B2 samples treated with constant temperature treatment at deformation temperature(process B)before stretching is higher;the samples treated with process B have better plasticity.The tensile fractures of the samples treated by process A are intergranular fracture(720℃),and the microstructures near the fracture are martensite and proeutectoid ferrite.The original austenite grain diameter is more than 100μm,and no recrystallization is found.After treatment by process B,the tensile fracture surfaces of the samples present dimple morphology,and the ferrite near the fracture surfaces is small in size and dynamic recrystallization occurs.Compared with the process B without high temperature solution treatment,the non-equilibrium grain boundary segregation of solute atoms caused by thermal cycle and tensile stress may occur in the samples treated process A.This is another reason for the worse plasticity of the samples treated by process A.