Effect Of Heat Treatment On Microstructure And Mechanical Properties In Cu-containing HMS

High manganese high aluminum steel possesses excellent mechanical properties and low density.It can effectively decrease vehicle mass and reduce fuel consumption;HMS can also significantly improve the safety level due to the high absorption capacity during collision.In recent years,domestic and foreign research on high manganese high aluminum steel have obtained achievements,but how to further improve the strength and plasticity is also an urgent problem.There fore,we added copper into high manganese steel to obtain the Cu-enriched nano clusters.We took advantage of precipitation strengthening of nano clusters to further improve the strength and maintain the plasticity at the same time.We investigated the change rule of the micro-structure and mechanical properties at different heat treatment to illuminate the effect of Cu in high manganese steel.In this paper,three high manganese steel with different Cu concentration(Fe-10Al-15Mn-0.8C-5Ni-xCu,x=0,0.5,1.5)were prepared by Vacuum Non-consumed Arc Melting technology.Thermo-calc software was used to simulates the relationship between temperature and phase content in Fe-10Al-15Mn-0.8C-5Ni-xCu,(x=0,0.5,1.5).According to the model of thermodynamic SFE,we studied the effect of Cu concentration on the stacking fault energy of austenite,and then designed the heat treatment processing.The microstructures and mechanical properties of high manganese steel after solution treatment were investigated by means of OM,SEM,XRD and tensile test to understand the principle of the microstructure with time and temperature changes.The precipitation behavior of the nano phase during the aging process was studied by using the small angle neutron scattering.The effect of the aging treatment on the microstructure and mechanical properties of the copper-bearing high manganese steel was also studied.The densities of Fe-10Al-15Mn-0.8C-5Ni-xCu(x=0,0.5,1.5)high manganese steel were measured by drainage method(6.73g/cm3,6.75g/cm 3 6.79g/cm).The SFE of austenite in high manganese steel was 85.3mJ/m2,87.7mJ/m2,and 91.2mJ/m,respectively calculated by the thermodynamically model.The solution temperatures were determined to be 850?,900?and 950? respectively,and the aging temperature was 550?.The results show that the matrixes of Fe-10Al-15Mn-0.8C-5Ni-xCu(x=0,0.83,1.38)high manganese steel are all composed of ferrite and austenite mixed phase.In the process of solution,a large number of annealed twins emerge in the microstructure,and the annealing twin tends to increase with the increase of time and temperature.With the increase of the holding time,the tensile strength decreases gradually,and the elongation opposite trend.The elongation will improve with the increase of solution temperature,and the tensile strength is on the contrary.The change of the microstructure of the manganese steel with different Cu content in the solution treatment has the same trend.The grain size of the austenite increases with the increase of the solution temperature or the holding time.The volume fraction of the austenite increases,and the volume fraction of ferrite decreases slightly,which lead to the decrease of strength and the improvement of plasticity.After the specimen was pulled off,there was no obvious necking phenomenon,a large number of dimples,secondary cracks and quasi-cleavage patterns were found in the fracture.With the increase of copper content,the austenite stack fault energy shows an increasing trend.The increase of the SFE in the austenite matrix is caused by the decrease of annealing twin in the austenite matrix.The aging results of high manganese steel with different Cu contents after solid solution show that the aging results lead to the precipitation of nano copper-rich,and the size of nano copper-rich is about 1.5nm.The nano phase precipitates mainly in the ferrite,resulting in a significant increase on the hardness of the ferrite after aging treatment and there is no significant change on the hardness of the austenite.Three kinds of high manganese steel with high Cu content have high work-hardening rate.At the initial stage of true strain,the hardening rate of Cu-free high manganese steel is the highest,and the hardening rate decreases with the increase of copper content.At the second stage,the increase of Cu content results in a decrease of work-hardening rate.By calculating the volume fraction of austenite before and after stretching,it is found that the volume fraction of austenite in the high manganese steel of the three components is almost constant before and after deformation.It can be concluded that the transition of ??? did not occur during the process of plastic deformation in the three kinds of high manganese steel.By observing the different deformation of the micro structure,it was found that no twins were formed during the deformation.It is well known that the deformation mechanism of high manganese steel with this kind of component is mainly the plane slip of dislocation.After the solid solution aging of copper-bearing high-manganese steel,the tensile properties of high-manganese steel are greatly decreased due to the precipitation of a large amount of carbides on the grain boundary.The mechanical properties at room temperature are all brittle fracture and quasi-cleavage pattern and crystal sugar-like morphology can be observed in the fracture.