Enhanced Cross-Tension Property of the Resistance Spot Welded Medium-Mn Steel by In Situ Microstructure Tailoring
Enhanced Cross-Tension Property of the Resistance Spot Welded Medium-Mn Steel by In Situ Microstructure Tailoring
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DOI：
10.1007/s13296-021-00464-3
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发表时间：
2021-02
影响因子：
1.5
通讯作者：
Bing Zhao;Yuanfang Wang;Kai Ding;Guanzhi Wu;Tao Wei;H. Pan;Yulai Gao
中科院分区：
文献类型：
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作者：
Bing Zhao;Yuanfang Wang;Kai Ding;Guanzhi Wu;Tao Wei;H. Pan;Yulai Gao
中锰钢是最有发展前途的第三代先进高强度钢之一，它在优异的力学性能和生产成本之间取得了良好的平衡。作为一种典型的中锰钢，7Mn钢具有优异的力学性能，但其较差的交叉拉伸性能成为制约其在汽车工业中应用的致命弱点。目前的研究主要集中在7Mn钢电阻点焊的交叉拉伸性能。一般情况下，在点焊过程中，熔核中会产生马氏体。但是，通过直接优化焊接参数，可以相应地调整焊核中的显微组织。在焊后脉冲作用下，可进行原位回火，减少了马氏体板条晶界上的Mn偏析，促进了组织由马氏体向回火马氏体的转变。熔核组织的调整有利于交叉拉伸强度(CTS)从1.5 kN提高到3.7 kN。虽然这两种情况都以界面断裂模式失败，但在焊后处理的样品中显示了部分延性断裂，这归因于低碳α相和第二相粒子的出现。研究表明，熔核中偏析的减少和显微组织的转变是影响CTS的主要因素。因此，减少锰偏析和形成回火马氏体能提高中锰钢RSW接头的焊接性。
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-carbonαphase 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.