This study elucidates the influence of rapid heating (300 °C/s) on martensitic transformation pathways, crystallographic variant selection, and the resulting mechanical performance in a medium-carbon steel. Compared with conventional heating, rapid heating markedly refines the prior austenite grain (PAG) and martensitic substructures, reducing the mean PAG size from 16.08 μm to 5.06 μm and the martensitic block size from 4.24 μm to 2.41 μm. The accelerated austenitizing and quenching promote a higher density of high-angle grain boundaries (HAGBs) and favor variant selection dominated by the closely packed (CP) group. Σ3 twin boundaries are also found to assist variant nucleation and contribute to microstructural complexity. Despite a marginal decrease in tensile strength, rapid-heated steels exhibit significantly enhanced ductility and a 28.3% increase in the product of strength and elongation (PSE) compared to their conventionally treated counterparts. These findings demonstrate that rapid heating not only enables effective refinement of martensitic substructures but also offers a powerful means of controlling variant evolution, thereby achieving a superior strength–ductility synergy in martensitic steels.

Achieving a reliable strength–toughness synergy in medium-carbon steels is crucial for lightweight and safety-critical components. Here, we compare a rapidheating route (RH) with a conventional furnace route (CH) to clarify how heating rate governs toughness in a Fe-0.3C-0.9Mn-0.8Si steel. Microstructural features were quantified by SEM and EBSD, and mechanical responses were evaluated through tensile testing, sub-zero Charpy impact, and J-based fracture toughness measurements. Despite nearly identical strength levels, RH steel exhibits a pronounced increase in both impact energy and fracture resistance. The improvement is associated with (i) a small fraction of ferrite retained after rapid austenitization, which introduces mechanical heterogeneity, and (ii) refined martensitic packets/grains accompanied by a higher population of high-angle boundaries. These microstructural changes jointly promote crack-tip plasticity, enhance crack deflection, and slow down crack advance. The present results highlight rapid heating as a viable, energy-efficient pathway to produce high-strength steels with superior toughness.