Microstructure and Retained Austenite Characteristics of Ultra High-strength TRIP-aided Martensitic Steels

A new type of 0.2%C–1.5%Si–1.5%Mn ultra high-strength low alloy TRIP-aided steel consisting of lath martensite structure matrix and metastable retained austenite films, “TRIP-aided martensitic steel; TM steel”, was developed by means of quenching and partitioning process. In addition, effects of partitioning temperature and time on the microstructure and retained austenite characteristics were investigated. The matrix structure was composed of two kinds of lath martensite structures, or wide and narrow lath martensite structures. Most of the retained austenite of about 3 vol% was located along the narrow martensite lath boundary. On the other hand, a small amount of fine and needle-like carbides precipitated only in wider lath martensite structure. Partitioning at temperatures lower than 250°C for 1000 s after quenching in oil or ice brine considerably increased carbon concentration of the retained austenite phase to about 1.0 mass%, maintaining volume fractions of retained austenite and carbide. Also, the carbon-enrichment mechanism in the retained austenite was proposed through TEM observation, as well as the carbide precipitation and coarsening mechanisms.