Microstructures And Mechanical Properties Of Cold-rolled Fe-29Mn-3Al-3Si Steel With High Mn-content

Fe-Mn-Al-Si TRIP/TWIP steels with high Mn-content is ideal material for vehicle body structure owing to its excellent strength and strain-hardening rate and extraordinary ductility.In this paper,the influence of annealing temperature on microstructure relating to mechanical properties of cold-rolled Fe-29Mn-3Al-3Si TWIP steels was studied by scanning electron microscope,X-ray diffraction,electron backscattering diffraction technique,transmission electron microscope,MTS electro-hydraulic-servo material fatigue testing machine and microhardness meter.The dynamic compression test was carried by Split Hopkinson Compression Bar to investigate the strain rate sensitivity and the microstructure evolution of the samples at various strain rates at room temperature,the deformation mechanism and orientation transformation was also discussed.At the same time,the fatigue crack propagation test of the compact tension samples was carried to investigate the effect of grain size on the property of the material.The results showed that the Fe-29Mn-3Al-3Si TWIP steel with cold rolled reduction of 65%contains dislocation pile-up,deformation twins,nanocrystalline,deformation band companied with little?-martensite.Which indicate the main deformation mechanism by cold roll was dislocation slip,shear deformation,twinning and?-martensite transformation.After the annealing thermal treatment,no martensite transformation was occured at the annealing range of 400¹050?C.The recrystallization start at 550?C and finish at 750?C,then the recrystallization grain keep growing as the annealing temperature increases.As annealing temperature increase,the hardness,yield strength and ultimate strength decrease,while the elongation of the material increase substantially.The dynamic compression test of the cold-rolled and annealed Fe-29Mn-3Al-3Si TWIP steel was conducted by Split-Hopkinson Pressure Bar?SHPB?at high strain rates ranging from 650s^-1 to 3800s^-1.The results showed that under high-speed impact condition,the materials exhibit positive strain rate sensitivity and the main deformation mechanism is the twinning induced plasticity?TWIP?.With the increase of strain rat e,the strength of the material increase.During compression deformation,the<001>oriented grains is soft owing to be easy for slipping and twinning,while the<110>oriented grains is hard owing to be difficult for slipping and twinning.With the increase of strain,the fiber texture gradually forms.The adiabatic heating induced by high strain-rate can make dislocation to dynamic recovery but has not obvious influence on DTs.The TWIP effect,work hardening,dynamic recovery and dynamic Hall-Petch induced by DTs improve the strength of materials greatly and keep the high ductility of material.The fatigue crack propagation test was carried on compact tension samples cold-rolled and annealed at 750oC,900oC and 1000oC respectively.The result showed that all the fatigue crack propagation rate vs stress intensity factor?SIF?of crack tip curves of three samples show ladder-like feature as SIF reach to a extend.The 750oC annealed sample shows the smallest throshold valve and performs worst in resisting the crack propagation among three samples,as annealing temperature increase,the 900oC annealed and 1000oC annealed samples get a better property.When SIF reach to the ladder-like stage,the microstructure of the region close to the crack tip contain a lot of deformation product such as deformation twins,these organization resist the crack propagation and relieve local stress concentration,while the annealing twins can also resist the propagation of fatigue crack.