Austenite formation and mechanical behavior of a novel TRIP-assisted steel with ferrite/martensite initial structure

A newly developed TRIP-assisted steel with ferrite/martensite initial structure was designed to investigate the microstructural evolution during overall annealing cycle. The results were compared with the traditional cold-rolled TRIP steel. The characteristic of austenite formation from different initial microstructure was clarified based on combining the experiments and modeling. Results indicated that the new austenite obtained during intercritical annealing was fine-grained and uniformly-distributed. It comprised of acicular structure formed in pre-existing martensite (M-pre) matrix and blocky structure occurred at phase interfaces or prior austenite grain boundaries. The kinetics of austenite formation in specimens involving M-pre are similar in the early heating stage, and then decreased with decreasing M-pre content at temperatures greater than 770 degrees C. Both experimental and DICTRA results indicated that the austenite fractions of specimens with 90% and 100% M-pre fraction attained almost the maximum at the end of heating, and decreased with isothermal holding time during the intercritical annealing. Moreover, the retained austenite at interfaces of ferrite and pre-existing phase followed the Kurdjumov-Sachs (K-S) relationship with adjacent tempered martensite, while only part of the retained austenite had identical relationship with neighboring ferrite. In addition, the amount of retained austenite increased with Mpre content, although their stability against martensite transformation during tensile testing was similar and greater than that of cold-rolled specimen. As a consequence, the yield strength was increased from 590 MPa to 753 MPa and the tensile strength increased slightly from 995 MPa to 1046 MPa with M-pre fraction increasing from 50% to 100%, and the total elongation was similar to 30%, similar to the third generation advanced high strength steels.