Optimising the strength-ductility-toughness combination in ultra-high strength quenching and partitioning steels by tailoring martensite matrix and retained austenite

Undesirable fracture resistance has led to critical safety concerns in ultra-high strength quenching and partitioning (Q&P) steels. The present work proposes a new heat treatment method of tailoring simultaneously martensite matrix and retained austenite to enhance the fracture resistance of ultra-high strength Q&P steels, while keeping the good strength-ductility combination. To this end, both conventional and pre-cracked tensile tests are conducted on different Q&P steels with various microstructures consisting of different martensite matrix and retained austenite. Microstructure is analysed by neutron diffraction, atom probe tomography, transmission electron microscopy and dilatometry. For the optimum microstructure, the volume fraction of retained austenite play an important role in enhancing the strength-ductility combination, with an ultimate tensile strength reaching 1500 MPa and uniform elongation over 10%. In addition, the dislocation mobility in the martensite matrix of the optimum microstructure is enhanced due to the dislocation recovery, solute carbon depletion, and the transformation of transition carbides into cementite. The enhanced dislocation mobility reduces the flow stress and results in an improvement of intrinsic toughness of the martensite matrix. Furthermore, the size of cementite at grain boundaries of the optimum microstructure is so small (tens of nanometres) that brittle intergranular fracture is prevented. In summary, the new heat treatment method suggests that the optimal treatment should tailor simultaneously the martensite matrix and the retained austenite to optimise the strength, ductility and fracture resistance combination of ultra-high strength Q&P steels.