Development of Low-Alloy Steels with High Strength and Good Ductility with the Aid of Nanoscale Troostite

We aimed to design nanoscale troositic microstructures in medium-carbon low-alloyed steel by using annealing plus quenching–partitioning–tempering processes to simultaneously obtain high strength and good ductility. The troositic microstructure and the related mechanical properties of the studied steel were studied systematically. After quenching to 200 °C in 5 s and then holding at various partitioning temperatures for 60 s and subsequently tempering at 400 °C for 1800 s, the steel held at the partitioning temperature of 200 °C for 60 s exhibited a high tensile strength (σ_TS) of 1620 ± 10 MPa and a yield strength (σ_y) of 1220 ± 10 MPa, together with a good ɛ_f of 25% at a strain rate of 1 × 10⁻³ s⁻¹. Both σ_TS and σ_y values were two times higher than the values for the coarse-grained counterpart with identical chemical composition. The key strengthening factor is associated with the nanoscale troostite phase. The mechanical properties of the steel with an average thickness of troositic lamellae at 200 ± 15 nm are consistent with the line predicted by the H–P relationship.