Synergistic control of microstructure and mechanical properties in low-carbon alloyed bulb flat steel through multistage induction quenching

High-strength, tough bulb flat steel is critical for hull stiffness regulation and lightweight design. Conventional furnace quenching of bulb flat steel suffers from inherent limitations including significant deformation, challenging straightening, and considerable fluctuations in mechanical properties. This study introduces a multi-stage induction heating technique that effectively addresses these issues, enabling successful induction quenching of large-scale bulb flat steel while incorporating embedded thermocouples for real-time monitoring of the temperature field. Experimental results demonstrate that three-stage high-temperature quenching (3IH) reduces the peak temperature variation across the bulb section by 55 % and decreases the yield strength differential between the bulb and flat by 73 %. The superior performance of 3IH is attributed to enhanced microstructural homogeneity under elevated temperatures, which improves strength uniformity through dislocation-mediated mechanisms. In contrast, two-stage low-temperature quenching (2IH) exhibits marginally lower strength but achieves exceptional ultra-low temperature toughness (−100 °C, KV 2 ≥ 190 J) via synergistic effects of multiphase microstructure and grain refinement. The formation of multiphase microstructure in 2IH is governed by carbon diffusion coefficient, diffusion duration, and undissolved ferrite content. Furthermore, the flat region in 2IH undergoes a distinctive two-stage austenitization process that introduces high-density dislocations, resulting in greater dislocation strengthening contributions compared to 3IH.