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- 저자 : Guanzhi Wu (검색결과 2 건)
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Bingge Zhao,Yuanfang Wang,Kai Ding,Guanzhi Wu,Tao Wei,Hua Pan,Yulai Gao 한국강구조학회 2021 International Journal of Steel Structures Vol.21 No.2
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.
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Bingge Zhao,Yuanfang Wang,Kai Ding,Guanzhi Wu,Tao Wei,Hua Pan,Yulai Gao 한국강구조학회 2023 International Journal of Steel Structures Vol.23 No.4
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.
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