Exceptional Cross-Tension Property in Resistance Spot Welded 7Mn Steels by Combining Cyclic Heat Treatment and Intercritical Annealing

Among the 3rd generation advanced high strength steels, medium Mn steels receive extensive attention sed in modern automotive industry attributing to their encouraging mechanical performance and lower production cost. Unfortunately, the poor cross-tension property of 7Mn steel, one of the typical medium Mn steels, remains a key challenge to hinder its application in automotive industry. In this study, multiple post-weld heat treatment (MPWHT) consisting of cyclic heat treatment and intercritical annealing was employed to improve the cross-tension property of the resistance spot weld of 7Mn steel. With this method, the diffusion of Mn was significantly enhanced, eliminating their severe segregation along the martensite lath boundaries. Additionally, MPWHT facilitated the structure tailoring: the cyclic heat treatment between room and austenization temperature produced the so-called martensitic transformation induced grain refinement and austenite stability while the intercritical annealing triggered austenite reversion transformation, leading to a mixed structure of refined ferrite and austenite across the weld. With the optimization in composition homogeneity and structure tuning, the cross-tension strength (CTS) of a resistance spot welded 7Mn steel stood at 12.7 kN, which is so far the ultrahigh level to our best knowledge. Moreover, this effort brought on button failure mode, suggesting the dominant ductility deformation during the cross-tension test. In a word, the current study achieved an exceptional trade-off between the outstanding CTS and ductility of the resistance spot weld of 7Mn steel by the MPWHT consisting of cyclic heat treatment and intercritical annealing.