Effect Of Strain Path Change On Mechanical Behavior Of TRIP Steels Based On Two-step Tension Experiment

TRIP(Transformation induced plasticity)steels,as one of the advanced high strength steels,shows great potential in lightweight vehicle manufacturing.The retained austenitic in TRIP steels transforms into martensite during plastic deformation.It not only brings about excellent combination of high strength and high plasticity,but also makes the constitutive models much more complex.Materials usually experience complicated strain path in industrial forming processes.The change of strain path strongly influences the mechanical behavior of TRIP steels.The constitutive model based on proportional loading experiments will usually bring distinct errors in calculating non-proportional loading cases.Therefore,it is necessary to carry out further study on the effect of strain path change on the mechanical behavior and obtain more precise constitutive model of TRIP steels.Two kinds of cold-rolled TRIP steels with 590 Mpa and 780 Mpa grade were investigated in this paper.Two-step tension experiment was carried out to investigate the effect of in-plane anisotropy,pre-strain and strain path change on the mechanical behavior of TRIP steel.The experimental results showed:(1)the tensile yield strength,ultimate tensile strength and total elongation of TRIP steels are almost equal in different orientation.No obvious anisotropy was observed for the tested TRIP steels;(2)the total elongation is increased with the increase of pre-strain,while the yield strength and ultimate tensile strength showed no significant differences.(3)TRIP steel is sensitive to the change of strain path.The strain path change has distinct influence on the hardening behavior and formability of the tested materials.The strain rate in the uniformly deformed zone and the necking zone were measured and calculated.The results showed that the strain path change promotes the development of the defects in the necking zone and leads to earlier fracture.A Voce-Hollomon mixed hardening model for TRIP steel is proposed according to the experimental investigation.This hardening model reshows the effect of strain path change on hardening behavior by adjusting the weight of Voce component and Hollomon component.A FEM model was created to simulate the two-step tensile tests and an ABAQUS/UMAT subroutine was developed to transfer internal variables between the two steps.The comparison of simulation results with the experiment result showed that the mixed hardening model is more accurate in the total elongation prediction than that of the Voce and Swift hardening models.