Upsizing high-strength fail-safe steel through warm tempforming

Abstract Fail-safe (FS) steel is designed with enhanced fracture toughness through the crack deflection mechanism in an anisotropic microstructure. In this study, commercial boron (B)-bearing medium-carbon low-alloy steel bars with a cross-sectional area of 1390 mm2 were water-quenched and tempered at temperatures between 400 °C and 500 °C, followed by multi-pass caliber-rolling (i.e., warm tempforming). Tensile, Charpy V-notch (CVN) impact, and hydrogen embrittlement (HE) properties of the warm tempformed samples were investigated and compared with the rolling reductions of 45–92% to optimize the rolling conditions for upsizing the FS steel. Warm tempforming at 400 °C with a rolling reduction of 66% successfully produced an FS steel bar with a large cross-sectional area of 470 mm2. The evolution of a heterogeneous and anisotropic ultrafine-grained structure through the warm deformation of tempered martensite and pearlite generated an excellent combination of CVN toughness and HE resistance at the ultimate tensile strength of 1400 MPa. Delayed fracture resistance of the FS steels was also evaluated to be far superior to those of conventionally quenched and tempered steels, based on the balance between the HE susceptibility and hydrogen entry. Furthermore, it was observed that excessive warm tempforming to a rolling reduction of 92% tended to degrade the tensile elongation and HE resistance. The mechanism of the variations in mechanical properties during warm tempforming is discussed in association with the microstructural evolution.