Study On Austenite Reverse Transformation Behavior And Strengthening And Plasticizing Mechanism In 9 Manganese Steel

With the development of the automobile industry,the light weight of automobiles has gradually become the theme of the development of the new era.The third generation of advanced high strength steels with high strength-ductility balance has been widely concerned,and the medium manganese steel is one of the hot topics.The annealing microstructure of medium Mn steel is usually composed of ferrite and retained austenite.During the deformation process,the retained austenite alleviates local stress concentration and delays the initiation of cracks through martensite transformation,which refers to the process of strain-induced martensite transformation and transformation-induced plasticity increase.Medium Mn steel has become the focus of scholars for its excellent mechanical properties,hoping that it could be widely used in automotive body.Based on the optimization of the alloy composition of traditional TRIP steel,this article has studied the microstructure evolution of 9Mn steel during hot rolling annealing,warm rolling and subsequent annealing process,and revealed mechanical mechanism of 9Mn with its mechanical behavior.The main results of the article are as follows:(1)Based on the Fe-C-Mn-Si-Al composition system of traditional TRIP steel,the contribution of Mn element content to retained austenite content and low temperature annealing process is studied by Thermo-Calc software.After fully considering the content of retained austenite and the feasibility of heat treatment process,the composition of the experimental steel is determined to be Fe-0.11C-8.65Mn-1.02Si-0.3Al.(2)In order to shorten the conventional austenite reverse transformation annealing time,this paper have designed two-step annealing process,which is to add pre-quenching before conventional annealing.It is found that the two-step annealing process makes the C and Mn elements fully diffuse in a relatively short time,and the retained austenite exhibits lathy and block morphology.Comparing the two-step annealing with the conventional annealing,the tensile strength of the two-step annealed samples was increased by about 200 MPa,but the elongation is not reduced.The strength-ductility balance of the two-step annealed sample could exceed 40GPa·%.The main reason is that the retained austenite with different morphology makes the TRIP effect more effectively distributed in the whole strain stage.(3)In order to overcome the difficulty of large cold rolling reduction of 9Mn steel due to hot rolled plate with high strength,this paper have adopted the warm rolling process.It is found that a variety of microstructures could be obtained after rolling at different temperatures.During the warm rolling process,the elemental distribution behavior makes the sample rolled at 630℃obtain good structure and element ratio without annealing treatment,which well balances retained austenite content and stability.Through the effective and sustainable TRIP effect during the strain process,the sample rolled at 630℃ obtains good mechanical properties.The yield strength is 1093 MPa and the strength-ductility balance exceed 50GPa·%,which shows that it is feasible to increase the yield strength of medium manganese steel by introducing multiple strengthening mechanisms through the warm rolling process on the premise of ensuring high strength-ductility balance.(4)In order to further develop the potential of the material,this paper have carried out the subsequent annealing treatment on the warm rolled plate.The ferrite and retained austenite in the annealed microstructure show two kinds of microstructures:lathy and block.The ferrite with large distortion energy undergoes recrystallization during annealing to form a block microstructure.And the austenite growing at the boundary of the block ferrite also exhibits a block morphology.However,the ferrite with smaller distortion energy and the retained austenite of warm rolling haven’t enough recrystallization driving force,and only the recovery occurs during the annealing process,therefore lathy morphology have maintained.During the strain process,the retained austenite with various morphology and wide grain size width contribute to the sustainable occurrence of TRIP effect,and during the later stage of deformation,the coordinated deformation of martensite and ferrite providing supplement for the relatively weak TRIP effect and delaying the generation of cracks.Under the combined effect of these two aspects,the strength-ductility balance of the warm rolling and annealing plates could exceed 50GPa·%.