New strategy to simultaneously improve strength-toughness balance for low-carbon ultrastrong steel by multi-step heat treatment process

The quenching-partitioning-tempering (QPT) multi-step heat treatment process was employed to enhance the strength-toughness balance of low-carbon ultrastrong steel based on the controllable transformation-induced plasticity (TRIP) effect and co-precipitation strengthening. The aging embrittlement of ultrastrong steel, with an impact toughness of only 5.8 J at peak strength, has been effectively addressed through the QPT process. The sufficient reversed austenite (RA) can be obtained by controlling the partition temperature to 650 °C, followed by aging at 550 °C to fully re-precipitate nanoparticles. The volume fraction of RA in QPT650 steel reached 16.4 %, and the number density of nanoparticles increased dramatically, resulting in a high yield strength of 1233 MPa and excellent impact toughness of 60.4 J. The coarsening and re-precipitation of nanoparticles were clarified, and the strengthening increments of QPT650 steel reaches 584.6 MPa, and the substantial enhancement of Orowan strengthening is responsible for the high yield strength. The excellent impact toughness of QPT650 steel is attributed to the obvious TRIP effect of RA that consumes a large amount of crack propagation energy, and a small number of sheared nanoparticles provide limited micro-cracks, resulting in a strong crack resistance. •The strength-toughness strategy of the controllable co-precipitation strengthening and TRIP effect was proposed.•The sufficient reversed austenite of 16.4 % and high-density nanoparticles were obtained through the QPT process.•The QPT650 steel simultaneously achieves a high yield strength of 1233 MPa and excellent impact toughness of 60.4 J.•The enhancement of Orowan strengthening and TRIP effect are responsible for excellent strength-toughness.