To investigate the macro and micro behaviors of TRIP （transformation induced plasticity） steel under biaxial loading, experiment and finite element simulation were carried out for TRIP780 steel under proportional biaxial tension with displacement ratio of 1 ： 1, 2 ： 1, 3 ： 1 and 4 ： 1, respectively. The results show that cruciform specimens of TRIP780 steel fractured under proportional biaxial stretching when effective strain was about L 5 %, and fracture was always generated on the cross arm or cross links. During biaxial tension, stress and strain components in x and y directions of the center of the samples have the same nonlinear developing tendency, decreasing in one direction and increasing in another direction. Equal biaxial stretching stress state was helpful for retained austenite-martensite transformation than the other biaxial stress state. With increasing displacement ratio （DR） from 1 ： 1 to 4 ： 1, corresponding stress distributed unevenly on the yield ellipse from 30° to 60° in the first quadrant of stress space and corresponding retained austenite volume fraction distributed symmetrically in bow tie format.
Hong-yang LIYan-jing ZHANGWen-jun ZHAOZhi-fei GUXian-chao LIJin-long MA
To investigate the transformation behavior of TRIP steel retained austenite under cyclic load, cyclic V-bending deformation of low carbon Si-Mn TRIP600 was studied by experiment and finite element in this paper. The results showed that, under cyclic V-bending deformation, retained austenite in TRIP steel transformed into martensite gradually with the increasing of bending times, and for the symmetrical characteristic, upper surface and lower surface presented the same transformation tendency. From the first to the fourth V-bending deformatiort, retained austenite volume fraction decreased nearly linearly and then attained saturation step by step. Compressive stress state was helpful for martensite transformation than tension stress state with V-bending deformation, and strain magnitude was the determining factor for retaining anstenite martensitic transformation. With the increasing of bending times effective stress increased and the relationship between maximum effective stress and bending times was nearly linear. Effective stress and effective strain distribution were non-uniform, the maximum effective stress and effective strain were present in the center of the samples. The relationships between retained austenite and V-bending times, and retained austenite with effective strain were set up as Eqs.（1）-（5）. The relationship was typical quadric function, decreased linearly for the initial deformation and attained saturation finally.
