Enhanced Cross-Tension Property of the Resistance Spot Welded Medium-Mn Steel by In Situ Microstructure Tailoring

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 alpha 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.