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The designed steel of Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt pct) treated by a novel quenching-partitioning-tempering (Q-P-T) process demonstrates an excellent product of strength and elongation (PSE) at deformed temperatures from 298 K to 573 K (25°C to 300°C) and shows a maximum value of PSE (over 27,000 MPa pct) at 473 K (200°C). The results fitted by the exponent decay law indicate that the retained austenite fraction with strain at a deformed temperature of 473 K (200°C) decreases slower than that at 298 K (25°C); namely, the transformation induced plasticity (TRIP) effect occurs in a larger strain range at 473 K (200°C) than at 298 K (25°C), showing better mechanical stability. The work-hardening exponent curves of Q-P-T steel further indicate that the largest plateau before necking appears at the deformed temperature of 473 K (200°C), showing the maximum TRIP effect, which is due to the mechanical stability of considerable retained austenite. The microstructural characterization reveals that the high strength of Q-P-T steels results from dislocation-type martensite laths and dispersively distributed fcc NbC or hcp e-carbides in martensite matrix, while excellent ductility is attributed to the TRIP effect produced by considerable retained austenite.

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