Medium Mn steels, one of the most promising 3rd generation advanced high strength steels (AHSS), achieve an encouraging trade-off between the outstanding mechanical property and the production cost. As a typical medium Mn steel, 7Mn steels have superior mechanical property but their poor cross-tension property becomes the Achilles' heel, hindering the application in the automotive industry. The current study focuses on the cross-tension property of the resistance spot weld of 7Mn steel. Generally, martensite is produced in the nugget during the resistance spot welding (RSW). However, the microstructure in the weld nugget can be correspondingly tuned by directly optimizing the welding parameters. With post-weld pulses, in situ tempering occurs, which can decrease the segregation of Mn existing along martensite lath boundaries and facilitate the microstructure transition from martensite to tempered martensite. The tuning on the nugget microstructure facilitates the increase of cross-tension strength (CTS) from 1.5 to 3.7 kN. Although both cases fail in an interfacial fracture mode, a partial ductile fracture is demonstrated in the specimen with post-weld treatment, which is attributed to the occurrence of low-carbon α phase and second phase particles. This study elucidates that the decrease of segregation and the microstructure transition in the nugget are the dominant factor determining the CTS. It is therefore demonstrated that the reduction of Mn segregation and the formation of tempered martensite can increase the weldability of RSW joints of the medium Mn steels.

中锰钢是最有前途的第三代高级高强度钢（AHSS）之一，在出色的机械性能和生产成本之间取得了令人鼓舞的折衷。作为一种典型的中锰钢，7Mn钢具有优越的机械性能，但其差的交叉拉伸性能成为致命弱点，阻碍了其在汽车工业中的应用。目前的研究集中在7Mn钢电阻点焊的交叉拉伸性能上。通常，在电阻点焊（RSW）期间在熔核中产生马氏体。然而，可以通过直接优化焊接参数来相应地调节熔核中的微观结构。利用焊后脉冲，发生原位回火，这可以减少沿马氏体板条边界存在的Mn的偏析，并促进从马氏体向回火马氏体的显微组织转变。熔核微观结构的调整有助于将横向拉伸强度（CTS）从1.5 kN增加到3.7 kN。尽管两种情况均以界面断裂方式失败，但在焊后处理的样品中发现了部分韧性断裂，这归因于低碳的发生。α相和第二相颗粒。这项研究阐明了金块中偏析的减少和微观结构的转变是决定CTS的主要因素。因此证明，减少Mn偏析和形成回火马氏体可以提高中锰钢RSW接头的可焊性。