Role of martensite/austenite constituents in novel ultra-high strength TRIP-assisted steels subjected to non-isothermal annealing

A novel ultra-high strength transformation-induced plasticity (TRIP) steels with submicron-size martensite/austenite (M/A) constituents was processed by non-isothermal annealing process and compared with quenching and partitioning (Q&P) steels. The microstructure was characterized by means of SEM equipped with EPMA, XRD, EBSD and TEM and its effects on tensile strength and ductility were studied. The non-isothermal annealing significantly refined the parent austenite grains and enhanced inhomogeneous distribution of C atoms within the grains. Moreover, a large fraction of M/A constituents were separated and refined to hundreds of nanometers through bainite transformation in TRIP steels, while blocky retained austenite mainly existed near the carbon-rich prior grain boundaries in Q&P steel. TRIP steels had a higher content of less stable retained austenite by isothermal bainite transformation than Q&P steel, thereby favoring for improving the ductility. The contribution of fresh martensite to the ultimate tensile strength increased gradually from 49.7% to 67.3% with increase in the volume fraction of fresh martensite in TRIP steels. Optimizing the volume fraction and size of M/A constituents was important to obtain a high ultimate strength with desired ductility. The excellent mechanical properties were obtained in TRIP-350 (austempering at 350 degrees C) with ultimate tensile strength of 1011 MPa, total elongation of 28.80% and the product of strength and elongation (PSE) of 29.1 GPa%, exceeded the properties of Q&P steel.