Developing advanced high strength Ni-Cr-Mo-V steels with a superlative strength-elongation-toughness synergy through different processing routes

This article addresses advanced high-strength Ni-Cr-Mo-V steels developed by different processing routes. First, three preliminary microstructures containing ferrite-pearlite, ferrite-martensite, and fully martensitic were made. These microstructures were also subjected to the cold-rolling with a 25 % thickness reduction. Ultimately, intercritical treatment (IT) was performed on non-deformed and deformed preliminary microstructures at a given temperature and time. The phase characterization through optical microscopy (OM), field emission scanning electron microscopy (FE-SEM), and electron backscattered diffraction (EBSD) revealed a fine-tuned microstructure in the intercritically treated steel with non-deformed fully martensitic preliminary microstructure. There were lath-like ferrite with a finer size, lath-like new martensite, tempered martensite, higher geometrically necessary dislocations (GNDs) density, and a higher fraction of $\sum 3$ coincident site lattice boundaries (CSLBs) in this steel. Such formed microstructure could well tailor mechanical performance and lead to achieving the best strength-elongation-toughness synergy. At this condition, yield strength, ultimate tensile strength, elongation, tensile toughness, and Charpy impact energy were 1205±31 MPa, 1366±33 MPa, 13.1±1.0 %, 168.4±11.0 MJ/m³, and 83±5 J, respectively. It was found that the steel with a cold-rolled fully martensitic microstructure consisted of a complex microstructure after IT among the intercritically treated steels with deformed preliminary microstructures. Ultra-fine ferrite with lath and polygonal morphologies, lath-like martensite, blocky-like martensite, higher GNDs density, and more fraction of $\sum 3$ CSLBs were detected in this steel. Such a complex microstructure attained extraordinary strength-elongation-toughness synergy. Yield strength, ultimate tensile strength, elongation, tensile toughness, and Charpy impact energy equaled 1175±19 MPa, 1402±25 MPa, 11.7±0.4 %, 150.8±13.0 MJ/m³, and 102±2 J, respectively. Dimple and cleavage features were observed in the fracture surfaces of all intercritically treated steels after performing uniaxial tensile and Charpy impact tests at the room temperature. Failure and toughening mechanisms were comprehensively discussed, as well.