Giga-grade martensitic advanced high-strength steels are prone to sub-critical heat-affected zone (SCHAZ) softening during resistance spot welding. The article aims at understanding the role of HAZ softening on the fracture mode, load-bearing capacity, and energy absorption capability of MS1400 resistance spot welds during the cross-tension test. The highest load-bearing capacity was obtained when pullout failure was initiated from the martensitic coarse-grained HAZ. However, more severe HAZ softening and formation of a wider softened zone, promoted at high heat input conditions, encourages strain localization in SCHAZ, promoting transition in failure location to sub-critical HAZ. This change in pullout failure location is responsible for the observed reduction in the weld peak load at high welding currents. Therefore, control of martensite tempering in the HAZ is critical to obtain strong and reliable resistance spot welds in martensitic advanced high-strength steel sheets. To preclude the detrimental effect of the martensite tempering on the weld strength, the minimum welding current, which enables pullout failure mode, should be used for resistance spot welding of MS1400 advanced martensitic steel.

千兆级马氏体高级高强度钢在电阻点焊过程中易于发生亚临界热影响区（SCHAZ）的软化。本文旨在了解热影响区软化对在交叉拉伸试验期间MS1400电阻点焊的断裂模式，承载能力和能量吸收能力的作用。当从马氏体粗粒热影响区开始拉拔失效时，可获得最高的承载能力。然而，在高热量输入条件下促进了更严重的HAZ软化并形成了较宽的软化区，这促进了SCHAZ中的应变局部化，从而促进了失效位置向次临界HAZ的转变。拔出失效位置的这种变化是导致在高焊接电流下观察到的焊接峰值载荷降低的原因。因此，控制热影响区中的马氏体回火对于在马氏体高级高强度钢板中获得牢固而可靠的电阻点焊至关重要。为了避免马氏体回火对焊接强度的不利影响，应使用能够实现拉拔破坏模式的最小焊接电流进行MS1400高级马氏体钢的电阻点焊。