Research On Deformation Behavior Of TWIP950 Sheet Under Cyclic Loading

Because of its excellent strong plasticity,TWIP steel has great application potential in complex automobile structural parts and safety parts.However,TWIP steel has not been widely used in the actual production of auto parts due to serious rebound problems.Due to the fact that some structural parts of automobile chassis are subjected to cyclic loading in actual stamping and service,there is a lack of understanding of the deformation behavior of TWIP steel plate under cyclic loading at present.In order that TWIP steel is widely used in auto parts,it is very necessary to study the cyclic loading mechanical behavior of TWIP steel.In this paper,TWIP950 steel is taken as the main research object,and the research is carried out from three aspects: inelastic recovery behavior,large deformation repeated loading deformation behavior and small deformation cycle soft/hardening behavior.The influence of different strain rates on the basic mechanical properties and dynamic strain aging behavior of TWIP950 sheet was investigated through unidirectional tensile experiment.It is found that the increase of strain rate will lead to the decrease of basic mechanical properties and dynamic strain aging behavior of TWIP950 sheet.The effects of unloading interval and strain rate on the inelastic recovery behavior and dynamic strain aging behavior of TWIP950 sheet were investigated through cyclic tensile-unloading experiment.The variation of unloading chord modulus with real plastic strain at different unloading intervals and strain rates is obtained.It is revealed that the unloading interval and strain rate delay or weaken the dynamic strain aging behavior to different degrees.The influence of prestrain on the stress value and hardening behavior of TWIP950 sheet at characteristic points was investigated through large deformation repeated loading experiment.EBSD microscopic detection method was used to explore the change rules of grain size,small Angle grain boundary and texture strength of TWIP950 sheet under pre-strain and different loading modes.The results show that the stress at unloading point and yield point increases with the increase of pre-strain at the same loading stage.Under the same prestrain,with the increase of the number of backward loading,the stress at the yield point first decreases and then increases,and the stress at the unloading point gradually increases,presenting the phenomenon of cyclic hardening.Both the increase of prestrain and the increase of reverse loading lead to the increase of deformation twins and the decrease of grain boundary with small angles,but the maximum variation of texture strength is different between them.The effect of strain amplitude on the cyclic soft/hardening behavior of TWIP950 steel was studied by small deformation cyclic loading experiment.The results show that cyclic hardening occurs under different strain amplitudes,and the strain amplitude increases,the stress amplitude increases,and the degree of cyclic hardening increases.With the increase of strain amplitude,the number of deformation twins increases and the number of grain boundaries with small angles decreases.