Two types of ultralow carbon steel weld metals (with and without added Cu-Nb) were prepared using gas metal arc welding (GMAW) to investigate the correlation between the microstructure and mechanical properties of weld metals. The results of microstructure characterization showed that the weld metal without Cu-Nb was mainly composed of acicular ferrite (AF), lath bainite (LB), and granular bainite (GB). In contrast, adding Cu-Nb to the weld metal caused an evident transformation of martensite and grain coarsening. Both weld metals had a high tensile strength (more than 950 MPa) and more than 17% elongation; however, their values of toughness deviated greatly, with a difference of approximately 40 J at −50°C. Analysis of the morphologies of the fracture surfaces and secondary cracks further revealed the correlation between the microstructure and mechanical properties. The effects of adding Cu and Nb on the microstructure and mechanical properties of the weld metal are discussed; the indication is that adding Cu-Nb increases the hardenability and grain size of the weld metal and thus deteriorates the toughness.

利用气体保护金属电弧焊（GMAW）制备了两种类型的超低碳钢焊接金属（添加和不添加Cu-Nb），以研究焊接金属的微观结构与力学性能之间的关系。显微组织表征结果表明，不含Cu-Nb的焊缝金属主要由针状铁素体（AF），板条贝氏体（LB）和粒状贝氏体（GB）组成。相反，在焊缝金属中添加Cu-Nb会导致马氏体的明显转变和晶粒粗化。两种焊缝金属均具有较高的抗拉强度（大于950 MPa）和大于17％的延伸率。但是，它们的韧性值相差很大，在-50°C时相差约40J。断裂表面和二次裂纹的形态分析进一步揭示了微观结构与力学性能之间的相关性。讨论了添加铜和铌对焊缝金属组织和力学性能的影响。这表明添加Cu-Nb会增加焊接金属的淬透性和晶粒度，从而降低韧性。