Insights Into Paint‐Bake Toughening of Resistance Spot Welds of a 2.0 GPa Warm‐Stamped Medium‐Mn Steel

Warm‐stamped medium‐Mn steels have emerged as promising alternatives to conventional hot‐stamped boron steels for automotive structural applications due to their high strength and formability. This study investigates the resistance spot welded joints of a 2.0 GPa grade warm‐stamped medium‐Mn steel, focusing on microstructural evolution and mechanical property changes induced by paint baking (PB). Microstructural analysis revealed the coexistence of lath martensite and twinned martensite, along with dense dislocation tangles, in the fusion zone (FZ) of the as‐welded (AW) joint. Under cross‐tension testing, cracks initiated and partially propagated through the FZ of the AW joint, resulting in partial interfacial failure (PIF) with a peak load of approximately 3.65 kN and energy absorption of about 11.93 J. Following PB treatment, low‐temperature tempering was activated, as evidenced by the precipitation of fine cementite particles within the FZ, which enhanced toughness. Consequently, crack propagation resistance increased significantly, shifting the failure mode to complete pullout failure (PF). This transition was accompanied by substantial increases in peak load of approximately 101% and energy absorption of about 291% relative to the AW joint. In contrast, RSW joints of an equivalent‐strength 2.0 GPa hot‐stamped boron steel exhibited much smaller PB‐induced improvements. The superior PB response of the medium‐Mn steel is attributed to its lower martensite‐start ( Ms ) temperature (~295°C), which suppresses auto‐tempering during welding and preserves effective low‐temperature tempering capacity.