The Research Onannealing Organization And Mechanical Properties Of Cold-rolled Fe20Mn0.3C Steel With High Mn-content

Fe-Mn-C TRIP/TWIP steels with high Mn-content have an excellent strength and ductility and extraordinary strain hardening potential.The effect of annealing temperature on microstructure and mechanical properties of cold-rolled Fe20Mn0.3C steels was studied using scanning electron microscope,electron backscattering diffraction technique,X-ray diffraction,microhardness tester and MTS universal material specimen machine.The microstructure evolution and deformation mechanism of different annealed samples were investigated using quasi-situ tensile EBSD technique.The effect of microstructure of Fe20Mn0.3C steel on fatigue crack propagation was investigated on compact CT specimens using MTS fatigue machine.The results were shown as follows.The hardness and yield strength of cold rolled and annealed samples decreased with the annealing temperature increasing,while the total elongation first increased and then decreased.The specimens annealed at 800 ? had the best tensile properties.The microstructure of cold-rolled samples consisted of ?-martensite,a'-martensite,deformation twins and a large of dislocation.After annealing at 400 0C,all of ?-martensite and ?'-martensite have transformed into y-austenite.With the annealing temperature increasing,recrystallization occurred.Recrystallization starting temperature is 550 ? and complete recrystallization temperature of 650 ?.When the annealed temperature further increased,the austenitic grains were growing and the stability was decreasing,so the austenite with big grain sizes can transform into martensite during air-cooling.The specimens annealed at 950 ? or 1000 ?followed by air-cooling contained a lot of quenching ?th-martensite.The micro structure of samples annealed at 800 ? consisted of small equiaxed grains of austenite with good stability.The main deformation mechanisms of 800 ?annealed specimens exhibited twinning induced plasticity(TWIP)in the process of room-temperature tensile,which ensured the higher strength and better elongation of the materials.In the process of room-temperature tensile,the main deformation mechanisms of 850 ? annealed specimens containing a small amount of?th-martensite mainly exhibited twinning induced plasticity(TWIP),accompanying with that a small amount of y-austenite transformed into ?D-martensite.The?D-martensite prefered to nucleate and grow at the grain boundaries of ?/?th.The main deformation mechanisms of 1000 ? annealed specimens containing a lot of?th-martensite exhibited transformation inducing plasticity(TRIP),and the deformation induced a'-martensite usually nucleated and grew at the grain boundaries of ?/?.With the deformation increasing,the cracks easily nucleated and grew at the interface of the a'-martensite.So,the larger the ?th-martensite content is,the lower the elongation of the material will be.The fatigue crack propagation tests showed that when the fatigue-crack propagation rate was small,the 800 ? annealed specimen with small equiaxed austenitic grains had poor resistance of fatigue-crack propagation as compared with the 1000 ? annealed specimens with some sth-martensite.After the fatigue-crack propagation rate increased a certain value,the fatigue-crack propagation rate of the 800 ? annealed specimen was smaller than that of the 1000 ? annealed specimen.And this time,the fatigue-crack propagation rate of both samples first fluctuated around a small rate and then jumped a large rate to fluctuate.For 800 ? annealed specimen,the main deformation mechanisms of the area near cracks front exhibited twinning induced plasticity in the process of room-temperature fatigue-crack propagation.But the main deformation mechanisms of the area near cracks in the 1000 ? annealed specimen mainly exhibited transformation inducing plasticity in the process of room-temperature fatigue-crack propagation.