Study On Mechanical Behaviors And Microstructure Evolution Of Medium Manganese Steel With High Product Of Strength And Plasticity During Uniaxial Tensile Progress

The 3rd generation advanced high strength steel with high product of strength and plasticity,typically medium manganese transformation induced plasticity steel,became the key development direction of automotive steel.Through the optimization of composition design and heat treatment process,the Fe-C-Mn-Al experimental steel can obtain excellent comprehensive mechanical properties,and the product of strength and plasticity can reach 30~60GPa·%.In order to ensure service performance and improve application space,it is necessary to master the mechanical behavior,structure evolution and strength-toughness mechanism during plastic deformation.In this paper,Fe-0.4C-5Mn-3.2Al(wt.%)hot-rolled experimental steel was taken as the research object,and the effects of intercritical annealing,pre-strain and baking on the microstructures and mechanical properties were discussed.The in-situ observation and systematic analysis of the microstructures,microstrains and temperature fields of experimental steel during uniaxial tension were carried out by means of Digital Image Correlation(DIC)and Infrared Thermography(IRT)method.And the following conclusions were obtained.(1)As the critical annealing temperature rised from 700 to 800?,the volume fraction of retained asutenite were 16.8~33.3vol.%,the grain size increased from 0.58 to 0.95?m,the mechanical stability was decreased,and the martensite transformation rate was increased to 100%.Excellent comprehensive mechanical properties were obtained by holding at 750? for 1 hour,in which the tensile strength,total elongation and product of strength and plasticity could reach 930 MPa,64.6% and 60.1GPa·% respectively,and the resistance to crack-propagation was good.(2)The intercritical annealing steel at 750? yielded obviously after predeformation treatment.The yield strength of 40% pre-strain experimental steel could be increased to 956 MPa.With the increase of pre-strain,the secondary hardening ability of experimental steel was decreased.As pre-strain were more than 10%,the tensile strength was increased up to 1019 MPa,and with the increase of pre-strain,the total elongation of experimental steel was reduced to 25.4%.(3)Baking would strengthen the above results,increase the width of yield platform,increase the yield strength up to 1110 MPa,and further decrease the hardening ability.After baking,the tensile strength of the experimental steel was further increased to 1119 MPa.The total elongation of experimental steel of 20% pre-strain below was barely influenced by baking.After baking,the total elongation of intercritical annealing experimental steel was increased up to 77.8%,and of 40% pre-strain experimental steel was reduced to 16.9%.(4)Continuous martensitic transformation of retained austenite with appropriate stability gradient at right strain resulted in the continuous TRIP effect.As a result,the local strain was increased successively and alternately in a “ladder” shape,and the local temperature fluctuated in a continuous "wave" shape alternately.Which initiated the continuous nucleation and propagation of Portevin Lechatelier(PLC)bands and contributed to the improvement of mechanical properties.