| TRIP steels exhibits low formability under multi-axial stress condition. The purpose of the present study is to suggest a way of improving the formability of TRIP steels under plane strain mode. For this purpose, the formability under different stress states and their relationships with the microstructure of the secondary phases have been investigated in the cold rolled 0.2C-1.5Si-1.5Mn TRIP steels experienced with various bainitic isothermal transformation at 300, 350, 400 and 450℃. Two kinds of mechanical tests were conducted to evaluate the formability: the tensile test under uni-axial stress condition, and the Limiting Dome Height(LDH) test for the plane strain mode under multi-axial stress condition. The formability of the present TRIP steels strongly depends on the isothermal bainitic transformation temperatures. The discrepancy between EL and LDH at 350℃ could be explained by the stability of the retained austenite and the distribution of the secondary phases. Consequently, this study revealed that effects of the microstructure on the formability were different depending on the deformation modes in TRIP steels. In order to improve the EL property, the stability of RA should be controlled, and in order to improve the LDH property, the continuously networked RA and the fine bainite should be formed. |
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