Research On Cyclic Softing/Hardening Characteristics And Microscopic Mechanism Of TRIP Duplex Stainless Steel Under Symmetric Strain

In recent years,the substitution of Mn-N for Ni in TRIP duplex stainless steel not only reduces the cost,but also realizes the trip effect through martensitic transformation in the deformation process,which improves the plastic deformation ability of the material,it has great application potential in the field of rail transit car body production.Dual phase stainless steel will bear cyclic load in forming complex structural parts or formed structural parts in service,and will undergo cyclic elastic-plastic deformation at local po-sition.The cyclic deformation characteristics of materials under cyclic load are obviously different from those under monotonic load,while TRIP dual phase stainless steel will produce martensitic transformation during deformation,which makes its cyclic defor-mation characteristics more complex.The cyclic deformation characteristics,cyclic mar-tensitic transformation law and micro mechanism of this new type of high performance stainless steel are revealed,which can provide a useful reference for the industrial appli-cation and further development of its properties.In this study,a new type of Mn-N alloyed TRIP duplex stainless steel was taken as the research object.By symmetry strain controlled cyclic loading test and microstructure observation experiment,studied its circulation soft and hardening characteristics and mechanism of martensite phase transformation regularity of macroscopic and microscop-ic,martensite phase transformation and dislocation structure and density effect on the cy-clic softing and hardening rule,the relationship between cyclic deformation,cyclic prop-erties and microstructure evolution of the TRIP duplex stainless steel was preliminarily revealed.A series of strain amplitudes are selected,and the cyclic loading tests of step-up and single-stage symmetrical strain are carried out.The curves of hysteresis loop and stress amplitude with cycle cycle are obtained.The cyclic softening and hardening characteris-tics are analyzed,and the cyclic response characteristics of different strain amplitude are revealed.The hysteresis loop curve is analyzed statistically,and the M-?_r curve is ob-tained.The change law of the position and quantity of the peak in the curve is analyzed,and the corresponding relationship between the cyclic soft and hardening characteristics and the peak position and quantity is revealed.The in-situ measurement platform of cyclic martensitic transformation was built by INSTRON 8801 testing machine and FMP30 ferrite measuring instrument.Based on this experimental platform,the martensitic transformation under different strain amplitude and different cycle times was measured,the transformation law of cyclic martensitic transformation was analyzed,and the transformation dynamic model for predicting the variation of cyclic martensitic transformation with cycle times was established.The ex-pressions for predicting the starting point of martensitic transformation under different strain amplitudes are derived theoretically.The effect of crystal orientation on martensitic transformation and the nucleation mechanism of martensitic phase deformation were analyzed by microstructure character-ization.The cyclic deformation characteristics of two phases,the variation of dislocation structure and dislocation density,and the effect of martensite content on cyclic softening and hardening were analyzed by combining macroscopic mechanical properties with mi-crostructure observation.The microscopic mechanism of cyclic softening and hardening characteristics of trip duplex stainless steel was revealed.