Warm Forming Process Of Medium-Mn Automobile Steel And The Effect On The Microstructure And Mechanical Properties After Forming

The key development direction of modern automobile is energy saving,consumption reduction,environmental protection and safety.It is an important guarantee to develop light weight and high performance automobile steel and to study its forming mechanism and mechanical properties of forming parts.The third generation advanced high strength steel,represented by medium manganese steel(Mn:4%-12%),not only has high strength but also has strong plastic forming ability.Due to the double effects of low atomic weight of Al and lattice expansion,the density of steel can be reduced by 1.3%for every 1%increase of Al element in steel.In recent years,light medium manganese automobile steel and its forming process have become the focus of the research and development of the third generation advanced high performance steel.In order to further study the influence of warm forming process on the microstructure and mechanical properties of medium manganese automobile steel after forming,this paper aims at Fe-0.2C-(6/8.5)Mn-(1.5/3)Al series light medium manganese steel,from the perspective of warm forming process,microstructure evolution,mechanical property control,deformation mechanism and high-temperature deformation strengthening mechanism,through reducing the heating temperature in the process of warm forming,developing low heating temperature By means of microstructure evolution,mechanical property control and analysis of deformation mechanism in the forming process,the relationship between microstructure evolution,mechanical property control mechanism and deformation mechanism is established,and the relationship between warm forming process and high temperature deformation strengthening mechanism is clarified,so as to reveal the effect of warm forming mechanism on the part force after forming of medium manganese steel The mechanism of action.The main research contents and results are as follows:(1)Light medium manganese steel has ultra-fine multiphase structure after hot stamping.When Fe-0.2C-8.5Mn-3Al experimental steel is heated at 710? and kept warm for 5min,the volume fraction of austenite in the structure can reach 53.1vol%after stamping at 500?,the stamping parts obtain excellent mechanical properties:tensile strength is 982 MPa,elongation is 53.6%,and strong plastic product is 52.6 GPa%.(2)During warm forming process,the heating temperature has a significant effect on the tensile deformation and fracture characteristics of the formed steel:for Fe-0.2C-8.5Mn-3Al light medium manganese steel,at 710?,the tensile curve shows discontinuous TRIP effect and Luders band expansion behavior.The plasticity of the sample heated at 690? is mainly related to the deformation of ? ferrite.The fracture mode of the samples formed at 690,710 and 730? is ductile fracture,and that of the samples formed at 750 and 770? is brittle fracture.(3)The volume of austenite grain in Fe-0.2C-(6/8.5)Mn-(1.5/3)Al system light medium manganese steel increases with the increase of heating temperature,and the volume fraction of austenite increases first and then decreases.When the volume fraction of austenite reaches the maximum,the austenite grain size increases gradually with the increase of temperature,which leads to the decrease of austenite stability.At the time of quenching,the austenite begins to increase Transformation to quenched martensite.(4)Compared with the experimental results of Fe-0.2C-(6/8.5)Mn-(1.5/3)Al,it is found that the increase of Mn and Al content can improve the mechanical properties of the experimental steel after warm forming.The enrichment of Mn in austenite can improve the stability of austenite and increase the volume fraction of retained austenite.The addition of A1 can inhibit the formation of cementite,promote the diffusion of C element to austenite,improve the stability of austenite,and increase the volume fraction of retained austenite after quenching.(5)The fracture of Fe-0.2C-(6/8.5)Mn-(1.5/3)Al series light medium manganese steel mainly occurs ductile fracture under high temperature tensile test.The elongation first increases with the increase of temperature,then decreases gradually when it reaches the maximum value,and then increases again.The reason why the elongation of experimental steel decreases and then increases is that austenite recovery occurs at high temperature,which makes the experimental steel obtain better plasticity again.(6)The change of tensile strength of Fe-0.2C-(6/8.5)Mn-(1.5/3)Al light medium manganese steel at high temperature is related to strain rate and tensile temperature.When the tensile temperature is constant but the strain rate changes,the tensile strength of the steel increases with the increase of strain rate.Under the condition of constant strain rate and change of tensile temperature,with the increase of tensile temperature,the tensile strength of experimental steel decreases gradually.