Effect of Nb Precipitates and Reversed Austenite Formed by QLT Process on Microstructure and Mechanical Properties of Nb-Bearing 7Ni Cryogenic Steel

The application of Ni-containing steels with excellent cryogenic toughness is limited by the high price of Ni resources. This study aims to explore the optimal process for quenching-intercritical annealing-tempering (QLT) treatment of newly designed low-cost Nb-bearing 7Ni steel. The effect of Nb microalloying on the strength-toughness combination and reversed austenite nucleation characteristics was investigated. The results show that the optimal comprehensive mechanical properties are obtained after intercritical annealing at 670 °C, and the cryogenic toughness is even better than that of 9Ni steel. The extra yield strength (~32 MPa) is attributed to the synergism of the refined martensite microstructure units for various strengthening mechanisms, caused by the addition of 0.04 pct Nb. The increased toughness is associated with the high proportion of high-angle grain boundaries (HAGBs) produced by the close-packeted plane (CP) group-dominated transformation. Such transformation provides more efficient nucleation sites, which preferentially locate at the lath boundaries and block boundaries, exhibiting film-like morphology. Although NbC precipitation attracts C atoms, which, on the other hand, induce nucleation and stabilize the reversed austenite. These results provide a novel design strategy for nickel-saving steel alloys towards liquefied natural gas (LNG) and hydrogen storage applications.