Investigation on the material behaviors of TRIP steels under uniaxial and biaxial stress state

The martensitic transformation of retained austenite in TRIP steels is the fundamental of TRIP steels phase transformation plasticity, which depend on the macroscopic stress and strain condition greatly. To investigate the relationship between microscopic material behaviors and macroscopic deformation conditions, low-carbon Si-Mn TRIP600 steel material behaviors of top and transitional section in cupping test was studied. It is shown that during cupping test, the stress state of the top section of the deformation area is equal biaxial stretching plane stress, meanwhile, the transitional section is uniaxial stretching plane stress. The martensitic transformation in TRIP steels was sensitive to strain condition, the retained austenite volume fraction in both top and transitional section shows a nearly linear decrease trend with the increasing of strain. But the trend of gradual saturation can also be found with the increasing of strain. Compared with the bottom, the equal biaxial stretching plane stress condition in the top section is beneficial to the martensitic transformation, and under a nearly same strain condition, the retained austenite volume fraction under biaxial stretching plane stress condition is lower than uniaxial stretching plane stress condition. For the the Si-Mn TRIP600 steel of this paper, under 6% strain condition, there is 7% difference between the retained austenite volume fraction of equal biaxial stretching plane stress condition and uniaxial stretching plane stress condition.