Effect of heat treatment on microstructure and mechanical properties of cold rolled C–Mn–Si TRIP-aided steel

In the present study, a 0.27C–1.60Mn–2.0Si wt.% TRIP-aided cold-rolled steel sheet was fabricated and optimal heat-treatment conditions (intercritical annealing “IA” and isothermal bainite treatment “IBT”) were investigated to maximize the volume fraction and stability of retained austenite. The effects of temperature and time for IA and IBT were studied via optical microscopy, SEM, TEM and XRD. Its tensile properties and formability were also evaluated. Two-stage heat-treatment carried out in a salt-bath furnace revealed that the volume fraction of retained austenite (RA) increased with increasing intercritical annealing time for a particular set of IA temperature, IBT temperature and IBT holding time. When the volume fraction of the retained austenite was 9% or higher, the stability of RA in terms of carbon concentration was same, and better formability was obtained. It was also observed that high tensile strength of 652 MPa, tensile elongation of 31% or above, and n-value 0.2 could be successfully produced in this cold-rolled steel sheet at IA temperature of 800 °C, holding for 5 min, and IBT of 400 °C for 3 min holding time.