Role of Intercritical Annealing in Enhancing the Cross-Tension Property of Resistance Spot-Welded Medium Mn Steel

Among the third-generation advanced high-strength steels, medium Mn steels are one of the most promising candidates in the modern automotive industry. However, their application is challenged by the weldability and corresponding poor mechanical property. In this study, the resistance spot welding (RSW) technique was used to join 7Mn steels. A post-weld heat treatment containing intercritical annealing was followed to tune the nugget microstructure. In this case, the trapped C and Mn during RSW can fully diffuse, diminishing the grain boundary embrittlement. Meanwhile, the austenite reversion transformation occurs in the joint, facilitating the formation of a mixed microstructure consisting of ferrite and austenite. With these efforts, the cross-tension strength (CTS) of the RSW 7Mn steel can reach as high as 11.6 kN with outstanding plastic deformation, giving rise to a button pullout fracture mode. This study provides an accessible approach to tailor the microstructure and final CTS of RSW joints of the medium Mn steel, which can guide the optimization of the RSW process.