Plastic Instability And Microstructure Evolution Of Aluminum-containing Medium Manganese Steel During Uniaxial Tensile Progress

Medium manganese steel(MMS) has both high strength and high plasticity,which has a good application prospect in the field of automobile plate.The results show that the obvious plastic instability of aluminum-containing MMS during uniaxial tension may become a key problem limiting its mass production and wide application.The strain distribution between austenite and ferrite may be the reason for the strength and plasticity enhancement of medium manganese steel.Therefore,to understand deformation behavior and the microstructure evolution during tensile process is very crucial.In addition,obvious heterogeneous strain distribution was observed among phases of medium manganese steel.The strain distribution between austenite and ferrite may be the reason for the strength and plasticity enhancement of MMS.Therefore,it is of great significance to clarify the mechanism of plastic instability and microstructure evolution of aluminum-containing medium manganese steel during uniaxial tension.In this paper,the uniaxial tensile process of 5Mn hot rolled critical annealed medium manganese steel was studied.The temperature field,micro zone strain and microstructure were observed and analyzed by means of Infrared Thermography(IRT),Digital Image Correlation(DIC),Scanning Electron Microscopy(SEM)and Finite Element Method(FEM).The plastic instability of experimental steel was improved by pre-strain and secondary annealing,and the following conclusions were obtained:(1)With the increase of strain,the PLC(Portevin Le-Chatelier)band gradually transforms from A-type(continuous)to C-type(random),which is characterized by the randomness of nucleation position and the abnormal fluctuation of strain and temperature fields in each micro region;the evolution process of strain and temperature in PLC band is uniform and identical;the strain concentration in PLC band promotes the transformation of retained austenite into martensite,which leads to local strain hardening;SIMT(Strip Induced Martensite Transformation)in PLC band causes serrated stress fluctuation;(2)The microstructure of the critical annealed experimental steel(heat preservation at 750 ? for 60 min,referred to as IA750)consists of ferrite and austenite.The yield strength,tensile strength and total elongation are 629 MPa,930 MPa and 64.6%respectively.A small amount of pre-strain(3 %)can reduce the average grain size of retained austenite(from 0.52 to 0.41 ?m,decreased by 24.5%)and retard martensitic transformation.The yield strength,tensile strength and total elongation are 703 MPa,1058 MPa and 67.3 % respectively;(3)The recrystallization fraction of IA750 can be increased and dislocation density can be reduced by pre-strain and secondary annealing;the plastic instability can be effectively eliminated by pre-strain of 20% and annealing at 700 ? for 30 min,the yield strength,tensile strength and total elongation are 636 MPa,1197 MPa and 58 %respectively;(4)PLC band is closely related to austenite stability,blocky austenite is mostly located at multiple grain boundaries,which has a large crystal orientation difference with adjacent phases and low stability;equiaxed austenite is relatively stable,which is not easy to transform into martensite under the same strain condition,but rotates along the specific crystallization direction;The ?-DIC and simulation results show that the lath austenite with spatial arrangement perpendicular to the tensile direction has higher local strain and stress,and the strain concentration occurs preferentially in austenite with 45 °tensile direction,so martensite transformation is more likely to occur.