| The main emphasis of the present study has been placed on investigating the formability and understanding the effects of retained austenite on the formability of a 0.15C-1.5Si-1.5 Mn multi-phase cold-rolled steel sheet (ECO-1 steel sheet). The steel sheet was intercritically annealed at 800℃, and isothermally treated at 400℃ and 430℃. Microstructural observation, tensile tests, and limiting dome height (LDH) tests were conducted, and the change of retained austenite volume fractions as a function of tensile strain was measured. The tensile properties and the formability of the ECO-1 steel sheets were superior to the conventional low carbon cold-rolled steel sheets, due to maintaining high strain hardening rate in the high strain region by the strain induced transformation of retained austenite to martensite. The formability of the ECO-1 steel sheets was dependent on the stability of retained austenite. If the stability of retained austenite was high, the strain induced transformation of retained austenite to martensite can be stably progressed, resulting in the delay of necking to high strain region and the improvement of formability. The effects of retained austenite on formability of the multi-phase cold-rolled steel sheet could be clearly understood in terms of the stability of retained austenite. |
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