Development of a low-carbon carbide-free nanostructured bainitic steel with extremely high strength and toughness

High-carbon carbide-free nano-bainitic steels show extremely high strength but relatively poor fracture toughness and welding prospects. Therefore, an attempt has been made to develop a strong and damage tolerant low-carbon bainitic steel in the current study by taking recourse to two-step austempering at 350 °C for 20 min and 250 °C for 7 h (20min-TS) and the properties have been compared with another block prepared using a single step transformation for 3 h at 350 °C (3hr-SS). Two stage heat-treatment leads to lath thickness refinement in addition to block refinement as compared to a single step transformation. Moreover, a NW misorientation developed in 20min-TS as compared to KS in a single-stage transformation. An average bainitic lath thickness below 100 nm in a steel having 0.26 wt% carbon has been obtained in 20min-TS which in turn results in an ultimate tensile strength of 1.5 GPa, percentage elongation of 18.5, impact energy of 31 J and plane-strain fracture toughness of 82 MPam−1/2. This is an around two and a half times increase in toughness without significantly compromising on the strength when compared to nanostructured bainitic steels produced in a similar duration of austempering for high carbon steels.