Effect of modifying matrix microstructures and nanosized precipitates on strengthening mechanisms and ductile-to-brittle-transition-temperature in a 1000 MPa Ni–Cr–Mo–Cu steel

A superior combination of yield strength (1001 MPa) and − 20 °C impact toughness (166 J) was obtained in Nb–V–Ti-microalloyed Ni–Cr–Mo–Cu steel treated by direct quenching and tempering route (DQT). The tested steels treated by DQT route and re-austenitization and tempering route (QT) were compared with each other in terms of mechanical properties and microstructures characterized by optical microscopy, transmission electron microscopy, X-ray diffraction, electron back-scattered diffraction method and so on. Strength and Vickers hardness of the tested steel treated by the above two routes vary with isothermal aging temperature (400–600 °C), shown as under-aged state, peak-aged state and over-aged state. All DQT specimens show higher strength and Vickers hardness than QT specimens with the same aging condition. Furthermore, the largest difference of yield strength between DQT and QT specimens was shown in DQT₆₀₀ and QT₆₀₀ specimens. DQT₆₀₀ or QT₆₀₀ specimens refers to direct quenched (DQ) or quenched (Q) specimens isothermally aged at 600 °C. The main disparities in quenched microstructure between DQ and Q specimens are mainly in morphology of prior austenite grains, dislocation density of martensite matrix and solution amount of Nb and Mo elements dissolving in martensite matrix, which play key roles in affecting microstructure and mechanical properties of DQT and QT specimens. Higher dislocation density of matrix and finer average diameter of both MC (M is any combination of Nb, Mo and V) and Cu-rich particles were shown in DQT₆₀₀ specimens than in QT₆₀₀ specimens. Strengthening from dislocations and nanosized MC and Cu-rich particles mainly leads to the largest difference of yield strength between DQT₆₀₀ and QT₆₀₀ specimens. In addition, strong dislocation strengthening and precipitation strengthening in DQT₆₀₀ specimen also elevated its ductile-to-brittle-transition-temperature, compared with QT₆₀₀ specimen.