Research on Microstructure and Mechanical Constitutive Model of TWIP Steel after Equal Channel Angular Pressing

In order to solve the problems of low yield strength of twinning induced plasticity（TWIP） steel, the grain refinement of TWIP steel were processed by equal channel angular pressing（ECAP）. Based on the microstructure analysis and mechanical properties testing, the true stress-true strain curves were fitted by Ludwik, Hollomon and swift constitutive models. The results show that after one pass and two pass of ECAP, the grains elongate in the shear direction, and a large number of sub grains are introduced. At the same time, with the progress of the deformation, the deformation twins and secondary twins with low thickness are produced. After four passes of ECAP, the dislocation and twinning activities lead to further refinement of microstructure. With the increase of ECAP passes, the yield strength and ultimate tensile strength of TWIP steel increase, while the plasticity and tensile toughness decrease. Three constitutive models, Ludwik, Hollomon and swift, were used to fit the true stress-true strain curve well, and the corresponding fitting parameters were obtained. The fitting parameters are helpful to the finite element simulation. After ECAP process, the strain hardening exponent n decreases significantly.