Microstructural design of retained austenite and bainitic variants for improved ductility–toughness synergy in a low-carbon railway steel

Pearlitic steels are widely employed in high-speed railway rails but suffer from an inadequate strength-toughness balance and fatigue-related defects. Conventional bainitic steels provide higher strength but are still limited by modest ductility and toughness, high yield ratios and poor work-hardening capacity. To overcome these limitations, a low-carbon bainitic (LCB) rail steel was developed and treated by a two-stage continuous controlled cooling process. By optimizing the cooling rate and final temperature of forced cooling stage, a microstructure consisting of nanoscale film-like retained austenite (RA) and dense twin-related intervariant boundaries (TBs) was obtained, resulting in an ultimate tensile strength of 1088 MPa, a reduced yield ratio of 0.70, a total elongation of 18.0 % and an impact toughness of 173 J/cm2, i.e. almost ten times higher than that of pearlitic rail steels (10–20 J/cm2). Using a combination of SEM, TEM, TKD and in-situ EBSD, the evolution of TBs and film-like RA during plastic deformation was revealed, demonstrating their beneficial role in the ductility-toughness synergy of the LCB steel. The nanoscale RA was shown to enhance the strength–toughness combination through an activated TRIP effect. At large plastic strain, dislocations pile up at TBs and are transmitted into adjacent variants, with bainitic variant pairs V5/V6 transforming into V1/V2, accompanied by an increase in geometrically necessary dislocation (GND) density to 1.139 × 1015 m−2. These findings establish a microstructural design strategy, rooted in advanced electron microscopy characterization, that enables the concurrent enhancement of ductility and toughness in low-carbon bainitic railway steels and underscores their potential for high-speed railway applications.