Microstructural evolution during tempering process and mechanical properties of Cr–Ni–Mo–V/Nb high strength steel

High strength steels exhibit superior mechanical properties due to the unique microstructure, which successfully solves the drawback of the inevitable strength-toughness trade-off that occurs in traditional alloys. Here we investigated the effect of matrix and precipitates on mechanical properties of Cr–Ni–Mo–V/Nb steel after water quenching and tempering (150–500 °C). The results showed that the microstructure of the present steel is noticeably tuned by changing the tempering temperature. An excellent combination of strength (a yield strength of 1308 MPa with a total elongation of 8.2%) and toughness (Charpy V-notch impact toughness of 40.5 J/cm²) is obtained upon tempering at 200 °C. This is attributed to the lath martensite containing high dislocation density, the martensite-twin substructure, and the strengthening effects of the precipitated needle-like ε-carbides and spherical VC particles. The acicular ε-carbides are replaced by the rod-shaped Fe₃C at the tempering temperature of 350 °C, resulting in the remarkable deterioration in strength, hardness, and elongation. Spheroidized carbides formed at a tempering temperature of 500 °C are beneficial to the enhancement of the elongation and toughness, but the strength decreases due to the matrix softening caused by the recovery of dislocation.