关键词: Q420B钢, 激光焊接, 焊接接头, 退火, 组织, 力学性能

Abstract: Q420B high-strength low-alloy steel hot rolled plate with 5 mm thickness was laser welded and post-weld annealed, and its microstructural evolution and mechanical properties of the welded joints before and after annealing were thoroughly investigated. The results indicate that prior to annealing, from the weld zone to the base metal zone, the grain morphology changes gradually from coarse columnar to fine equiaxed grains, where the weld zone width is approximately 800 μm, the heat-affected zone (HAZ) is about 300 μm, and the weld zone microstructure is composed of lath bainite (LB) and ferrite (F). After annealing, the LB degenerates, and the carbides tend to precipitate along grain boundaries. The kernel average misorientation (KAM) value in the weld zone decreases with increasing annealing temperature, indicating a reduction in geometrically necessary dislocation density and residual stress. The average microhardness of the weld zone decreases from 359 HV0.5 before annealing to 303 HV0.5 and 257 HV0.5 respectively after annealing at 820 ℃ and 720 ℃, no softened zone is observed in the welded joint, and the annealing temperature has no significant effect on the tensile properties. The impact absorbed energy reaches its maximum (approximately 40 J) after 720 ℃ annealing, primarily attributed to LB degeneration and residual stress relief. The laser welding +720 ℃ annealing process employed in this study results in a narrow welded joint (approximately 1.4 mm) with superior mechanical and impact properties.

Key words: Q420B steel, laser welding, welded joint, annealing, microstructure, mechanical properties