Effect Of Deformation And Heat Treatment On Microstructure Evolution And Mechanical Characteristics Of 20Mn7 Medium Mn Steel

As a typical 3^rd-generation AHSS,medium Mn steels have attracted more and more attention due to their excellent combination of strength and ductility after intercritical annealing（IA）treatment.At present,many researchers attempted to enhance the mechanical properties and eliminate the plastic instability caused by Lüders band of the medium Mn steel.However,most reported methods to overcome Lüders strain causes additional negative influences on mechanical properties or raised cost.Therefore,this work focuses on studying the effect of deformation and heat treatment on microstructure evolution and mechanical characteristics of 20Mn7 medium Mn steel,and proposing a novel process to improve mechanical properties and avoid Lüders strain by designing the thermal deformation conditions to control the prior microstructure before IA,and combining with subsequent IA.Meanwhile,this novel process proposed is simple and with relatively low industrial cost.The main work includes the following parts:Regarding the cracking problem of medium Mn steel during thermal deformation,the thermal deformation behavior of the test steel was studied,as well as the preferred thermal deformation conditions are obtained;The micro-mechanism of mechanical properties strengthening and avoiding Lüders band by the novel process are systematically studied.The main results and conclusions are as follows:（1）The dynamic/static recrystallization behavior of the test steel were studied by single-pass and two-pass hot compression test with Gleeble-3500 thermal-mechanical simulator.Flow stress constitutive relation and static/dynamic recrystallization kinetic model of the test steel were established.In addition,the influence of hot working process on test steel was studied through the establishment of 3D hot working diagram.And the results shown that the instability region appears at high deformation temperature with high strain rate(1100～1150°C&0.05～1s^-1)under all true-strain conditions,which should be avoided during the hot deformation process.（2）The hot-rolling and annealed 20Mn7 steel is composed of film-like of ferrite and reversed austenite,while the cold-rolling test steel is composed of block-like morphology.Besides,the product of tensile strength and elongation of the hot-rolling test steel reached the highest value after IA at 645°C for 1h,namely 40 GPa·%.However,as the cold-rolling and annealing,the product of tensile strength and elongation of the hot-rolling test steel reached the highest value after IA at 630°C for2h,namely 42 GPa·%.Besides,the yield behavior of the hot-rolling and cold-rolling steel are obvious different.The hot-rolling and annealed test steel shows continuous yield behavior,while the cold-rolling and annealed test steel shows obvious Lüders band with higher yield strength（YS）.This is due to the lower total dislocation density of the cold-rolling test steel,the specimens cannot rely on dislocation slip to achieve plasticity and the mobile dislocations are easily locked by solute atoms and form Cottrell atmosphere.Thus,a higher applied stress is necessary to start the Cottrell atmosphere,which produce Lüders band to coordinate the deformation.（3）The experimental results show that the samples after different thermal deformation and IA treatment exhibit excellent mechanical properties comparing with cold-rolling and hot-rolling test steel.Especially the sample after deformed at low temperature and high strain rate(850°C&0.1s^-1)and annealed at 645°C for 1h exhibits superior comprehensive mechanical properties with a UTS×TE value of 53 GPa·%.The main reason of the novel process combining thermal deformation and IA improving the mechanical properties of the test steel is that the samples after the novel process shows two different austenite morphology after IA treatment,say,film-like and block-like morphology,resulting in a special double-stage TRIP effects occur in the samples.Due to the different austenite stability,block-like austenite is activated first and dominant in 1^st-stage of double-stage TRIP effects,and then the film-like austenite plays a major role in 2^nd-stage.（4）The main reason for the elimination of Lüders band of the novel process is that the nucleation and growth mechanisms of block-like and film-like grains are different.The film-like ferrite contains high density dislocations so that the nucleation of dislocation is not required to achieve plasticity.However,the lower dislocation density of block-like ferrite is directly related to the yield point phenomenon.Besides,the maximum shear stress value in film-like ferrite is smaller than those in block-like ferrite,concluding that the film-like ferrite initiate dislocation plasticity earlier than the block-like ferrite.Thus,the deformation of film-like ferrite grains dominates the plastic strain at yield stage of tensile curves,resulting in the elimination of Lüders band.（5）Due to the cracking and plastic instability of Lüders band in the industrial production and forming process,the application of the medium Mn steel is limited.Thus,this work proposed a novel process combining thermal deformation and IA to enhance mechanical properties and avoid Lüders strain of medium Mn steel.The process is simple and with relatively low industrial cost,which provides a novel low-cost routine for fabricating Lüders strain free medium Mn TRIP steel with excellent mechanical performance.