Austenite/martensite structure and corresponding ultrahigh strength and high ductility of steels processed by Q&P techniques

The microstructure of steels treated by QP the fresh martensite formed at the final quenching step looks like ‘blocky’ type phase, and the retained austenite is mainly located on the packet boundary and initial austenite grain boundary. The inhomogeneous microstructure causes the experimental optimum quenching temperature corresponding to maximum retained austenite fraction to be higher than the calculation based on CPE (constrained paraequilibrium) model. The product of tensile strength and total tensile elongation is 47. 5 GPa%, and tensile strength of 1760 MPa was obtained for the steel with carbon content of 0.51 wt%. The TRIP (transformation induced plasticity) effects of the large fractioned metastable austenite make a main contribution to the high ductility improvement, and the martensitic matrix provides high strength.