Microstructures and Properties Investigation on DP980 Dual-Phase Steel CMT + P Welded Joints

The increasing demand for the lightweight production of advanced high-strength steel puts forward higher requirements for the quality of welded joint forming. The common CMT welding process has certain limitations and is difficult to meet the needs of lightweight manufacturing. In this study, the CMT + P welding technology was used to weld the DP980 dual-phase steel plate with 1.2 mm thickness. The ER120S-G welding wire was used as the filling material to conduct a 70° angle lap welding experiment. The effects of wire feeding speed (3 m/min~5 m/min) and welding speed (350 mm/min~600 mm/min) on the forming, microstructures, and mechanical properties of DP980 dual-phase steel welded joints were analyzed. The results show that the CMT + P welding process can produce lap weldments with good formability and properties. The welded joints can be divided into the weld zone, the HAZ, and the base metal zone, where the HAZ can be divided into the coarse-grained zone and the softened zone. The role of the elements Ni and Si is to promote the production of martensite and to increase the ferrite strength in welded joints. As the wire feeding speed increases, the grain size of the coarse grain zone in the HAZ increases from 31.90 μm to 50.93 μm; while the welding speed increases, the grain size of the coarse grain zone decreases from 45.48 μm to 35.73 μm. The average microhardness of the weld zone is 420 HV. In contrast, the average microhardness of the softening zone in HAZ is reduced to 250 HV. When the wire feeding speed is 4 m/min and the welding speed is 550 mm/min, the tensile properties of the weldment are optimal, its tensile strength can reach 973 MPa, and the tensile fracture is ductile fracture.