Mechanical Properties And Microscopic Mechanism Of TRIP-Assisted Duplex Stainless Steel Under Asymmetric Strain Cycling

In recent years,TRIP(Transformation Induced Plasticity)-assisted duplex stainless steel with Mn-N instead of Ni reduces the cost and can realize the TRIP effect through the martensitic phase transformation during the deformation process,which improves the plastic deformation ability of the material.It has excellent potential in rail transit car body production.Due to the complexity of the forming process,the sheet metal is often subject to obvious asymmetric cycling deformation during the entire forming process.At the same time,TRIP-assisted duplex stainless steel will produce deformation-induced martensitic transformation during the deformation process,making its cyclic deformation characteristics more complex.The cyclic deformation characteristics,the cyclic martensitic phase transition rule and the microscopic mechanism of this new high performance stainless steel under asymmetric cycle condition were revealed,and the comparison with the condition under symmetric cycle condition could provide reference and basis for the industrial application of the test steel and the further development of its properties.In this paper,a new type of TRIP-assisted duplex stainless steel alloyed by Mn-N,was taken as the research object.Its cyclic softening / hardening characteristics were studied by cyclic loading test and microstructure observation experiment under asymmetric strain control(strain ratio 0).The macroscopic law of α’martensitic transformation under cyclic loading at different strain levels was measured by the experimental platform composed of an INSTRON fatigue testing machine and ferrite measuring instrument,the microscopic rules of martensitic transformation and the changes in dislocation structure and density were analyzed using XRD(X-ray diffraction),EBSD(Electron Backscattered Diffraction),and TEM(Transmission Electron Microscope).Combined with the results of macroscopic mechanical properties and microstructure observation,the microscopic mechanism of macroscopic mechanical properties of test steel under asymmetric cyclic loading was analyzed and revealed.The hysteresis loop under asymmetric cyclic loading is statistically analyzed,and the mechanism of the effect of microstructure on macroscopic mechanical properties is further analyzed and verified.Comparing the test results of the test steel under an asymmetric cycle with those under symmetrical cyclic loading,it is found that the asymmetric cyclic loading condition is more likely to produce secondary hardening,and the secondary hardening rate is higher.It is mainly due to the significant influence of the average tensile strain under asymmetric cyclic loading.