Strengthening,Plasticizing And Low-cycle Fatigue Behavior Of High Manganese Austenitic Twinning-induced Plasticity Steel

TWIP(Twinning-induced plasticity)steels,which possess the high strength,excellent ductility,good forming property and high energy absorption capacity,have received extensive attentions as a promising candidate for next generation of automotive steel.However,the researches about TWIP steels are just beginning,and there are many technical problems to be solved.Increasing the strength of TWIP steels is an effective way to decrease the weight of cars and thus improve fuel economy,reduce greenhouse gas emissions and improve service safety.The investigations on the fatigue behavior of TWIP steels can provide a reliable theoretical basis for the fatigue resistant design and safety usage of automobile components.In the present thesis,Fe-Mn-C grade high-manganese austenitic TWIP steels as objects were studied.Several approaches,such as increasing the C content,Cr/Mo alloying and grain refinement,have been adopted to further enhance the tensile properties of TWIP steels.Meanwhile,microstructural evolution and deformation twins were investigated by optical microscopy(OM),scanning electron microscopy(SEM),electron back scattered diffraction(EBSD)and transmission electron microscopy(TEM).The stress-control low-cycle fatigue behaviors of 0Al and 3Al TWIP steels were investigated.And the strain distribution and crack growth behavior were analyzed by digital image correlation(DIC)technique.The main results are as follow:The tensile deformation behavior of a high-carbon(Fe-18Mn-1.0C)TWIP steel was investigated and compared with that of a medium-carbon(Fe-22Mn-0.6C)TWIP steel.The results indicated that the high-carbon TWIP steel showed higher yield and tensile strengths without losing ductility,as compared with the medium-carbon TWIP steel.At low strains,deformation twins were suppressed in the high-carbon TWIP steel due to its higher SFE with the increased C content;however,the dynamic strain aging was enhanced and the formation of thinner twins was more rapidly and persistently at high strains in the high-carbon TWIP steel than that in the medium-carbon TWIP steel.The unique twinning behavior and the enhanced dynamic strain aging sustain the high strain-hardening rate of high-carbon TWIP steel up to higher strains and postpone the plastic instability,which are the main reasons for its superior tensile properties.The tensile deformation behavior of a Cr/Mo-alloyed(Fe-18Mn-1.0C-2Cr-1Mo)steel was investigated and compared with that of a Cr/Mo-free(Fe-18Mn-1.0C)steel.The results showed that the yield strength,tensile strength and elongation increased simultaneously with Cr/Mo alloying.Deformation twins were suppressed with Cr/Mo addition,which is supposed to be related with the weakened dynamic strain aging in Cr/Mo-alloyed steel;at high strains,deformation twins in Cr/Mo-free steel were prematurely saturated,however,Cr/Mo-alloyed steel was still able to release deformation twins.Meanwhile,the thickness of deformation twins decreased with Cr/Mo addition.As compared with the Cr/Mo-free steel,more persistent formation of thinner twins in Cr/Mo-alloyed steel sustains the high strain-hardening rate up to higher strains,which delays the plastic instability and increases the both strength and ductility simultaneously.Grain refinement increases the strength but decreases the ductility of common low-medium carbon or carbon-free TWIP steels.The effect of grain size on the tensile deformation behavior of a high-carbon,Fe-18Mn-1.0C was investigated,it is demonstrated that the strength and ductility of the steel increase simultaneously with decreasing grain size,which is distinct from previous results.As compared with the coarse-grained sample,deformation twinning was suppressed in the fine-grained sample at the early stages of straining;however,the formation of thinner deformation twins became more prevalent in the fine-grained samples at high strain levels.Grain refinement prevented the premature saturation of deformation twinning and the thicknesses of deformation twins decreased with decreasing grain size.The dynamic strain aging was enhanced with the increased C content,thereby increasing the strain-hardening capacity of high-carbon TWIP steel.Furthermore,the formation of twins was more prevalent at high strains in the high-carbon TWIP steel.A smaller grain size,in combination with higher carbon content,favors gradual and persistent release of thinner twins in fine-grained samples at high strains,which sustains the high strain-hardening rate up to higher strains,thereby leading to simultaneously enhanced strength and ductility.The stress-control low-cycle fatigue behaviors of 0Al(Fe-22Mn-0.6C)and 3Al(Fe-22Mn-0.6C-3Al)steels were studied.The results showed that when both steels fatigued at stress amplitude of 450 MPa,cyclic strain response curves of two steels can be divide into three similar stages,i.e.,cyclic hardening,cyclic saturation and cyclic softening,and the fatigue life of 3Al steel was much shorter than that of 0Al steel.The cyclic hardening capacity of TWIP steel was weakened by Al,the strain amplitude of 3Al steel was much higher than 0Al steel at the same stress amplitude of 450 MPa,which is the main reason for the decreased fatigue life of 3Al steel.Crack growth behavior analysis of 0Al steel showed that at the early stage,the crack growth rate was relatively slow and the crack deflection was obvious;with increasing the number of cycles,the crack growth rate increased and the deflections on crack reduced.Non-uniform strain distribution was observed by digital image correlation technique and the crack tended to pass through the transition area connecting the region of larger strain and the region of smaller strain.