| The main emphasis of the present study has been placed on understanding the effects of heat treatment and Si on microstructures and properties of the low carbon cold-rolled TRIP aided steel sheets. Two kinds of 0.10%C-1.5%Mn-Si-0.5%Cu cold-rolled steel sheets were prepared, and were intercritically annealed following isothermal treatment at bainitic region. Microstructural observation and tensile tests were conducted, and also the volume fractions of retained austenite were measured. The volume fractions of retained austenite and the mechanical properties of the steel sheets were changed dramatically by heat-treatment conditions. The volume fractions of retained austenite increased with decreasing intercritical annealing temperature and isothermal treatment temperature, resulting in the improvement of tensile strength, elongation, and strengthductility balance(TS×Elong.). In case of the ECO-5 steel sheet (high Si content), the higher volume fractions of retained austenite and mechanical properties were obtained compared to the ECO-4 steel sheet (low Si content). From the results of the present study, it is reached a conclusion that the conditions (temperature and time) of intercritical annealing and isothermal treatment should be established in consideration of the stability of austenite with the solubility of alloying elements in the austenite formed during the intercritical annealing. |
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