Microstructure and mechanical properties of low carbon Mn-Si steel by different processes

The microstructure and mechanical properties of a low-carbon Mn-Si multiphase steel treated by two different thermal-mechanical processes, the hot-rolling process for TRIP steel based on dynamic transformation of undercooled austenite and the process based on bainitic isothermal treatment, were investigated by hot uniaxial compression tests on a Gleeble-1500 hot simulator, in combination with optical microscopy, scanning electron microscopy, X-ray diffraction and tensile testing. A comparison was carried out to analyze the influence of multiple-phase microstructure states by the two processes on the work-hardening behaviors of the steel. The multiple-phase microstructure consisting of fine-grained ferrite matrix, bainite, and retained austenite is formed by the hot-rolling process for TRIP steel based on dynamic transformation of undercooled austenite, but the multiple-phase microstructure which is composed of lath bainite matrix and retained austenite is formed by the process based on bainitic isothermal treatment. The volume fraction of retained austenite and the mass fraction of carbon in retained austenite in the former are higher than those in the latter. The yield strength and tensile strength of the steel with the multiple-phase microstructure consisting of lath bainite matrix and retained austenite are higher;but the steel has lower uniform elongation and total elongation, which should be attributed to its lower work-hardening capability resulting from the lower stability of retained austenite.