Abstract The main emphasis of the present study has been placed on investigating and understanding the effects of retained austenite on the formability of TRIP-aided cold-rolled steel sheets. The steel sheets were intercritically annealed and followed by isothermal treatment at bainitic region. 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 results are summarized as follows:
(1) The tensile property and the formability of the TRIP-aided steel was 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 TRIP-aided steel 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.
(2) The effects of retained austenite on the formability of a 0.1C-1.5Si-1.5Mn-0.5Cu TRIP-aided cold rolled steel sheet were investigated after various heat treatments. The results showed plausible relationships between the formability and retained austenite parameters such as stability and initial volume fraction of retained austenite. The formability was improved with the increase of initial volume fraction and stability of retained austenite. Thus, the conditions of intercritical annealing and isothermal treatment in TRIP-aided cold rolled steel sheets should be established in consideration of volume fraction and stability of retained austenite.
(3) Relationships between retained austenite parameters(volume fraction and stability) and amounts of alloying elements on the formability of C-Si-1.5Mn-(0.5Cu) TRIP-aided cold-rolled steel sheets were investigated. When the carbon and silicon contents were increased, the volume fraction of retained austenite was markedly increased. In particular, C increased the stability of retained austenite with increasing the carbon concentration of retained austenite, but the stability of retained austenite seemed to be less sensitive to the silicon contents of 1.0∼1.5%. In the case of the 0.1C TRIP-aided steels containing Cu, formabilities were excellent than the 0.15C-1.5Si-1.5Mn TRIP-aided steel by decreasing strength difference between second phase and ferrite matrix due to Cu solid solution strengthening effect
It is reached a conclusion that the stability of retained austenite must be suitably minimized with maintaining the largest volume fraction of retained austenite for using merits of TRIP-aided cold-rolled steel sheets.