Investigation of Strengthening and Toughening Mechanisms in a V‐Microalloyed 07MnSiCuCrNi Weathering Steel and Its Fine‐Grained Heat‐Affected Zone

Under complex service conditions such as extreme plateau climate, frequent geological disasters, and marine environments characterized by high temperature and humidity, weathering steels must exhibit superior comprehensive properties. The present work studied the composition and process design of high‐strength and toughness matching in 550 MPa grade V‐containing weathering bridge steel through hot rolling, tempering, and welding thermal simulation testing. The results demonstrate that the microstructure of the experimental steels is composed of granular bainite and lath bainite, with dislocation strengthening and fine‐grain strengthening acting as the primary strengthening mechanisms. V‐microalloying (0.03–0.06 wt% V) combined with finish cooling temperature control (515°C–545°C) and tempering (550°C–600°C) can achieve an excellent combination of high strength (601–687 MPa), high toughness (240–270 J), and low yield ratio (0.78–0.84). Welding thermal simulation results indicated that under a low heat input of 20 kJ/cm, the fine‐grained heat‐affected zone (HAZ) of the 0.03 V steel exhibited higher hardness than the 0V steel. Its impact energy reached 197 J, only 21 J lower than that of the 0V steel. Whereas the impact toughness of the 0.06 V steel significantly decreased to 15 J. When the heat input was increased to 30 kJ/cm, the impact energy of the HAZ decreased to 78, 19, and 6 J for the 0, 0.03, and 0.06 V steels, respectively. The toughness reduction is attributed to grain coarsening, a lower density of high‐angle boundaries, and an increased content and size of M/A islands. Considering both base metal and welding performance, a V content of 0.03 wt% combined with a heat input of 20 kJ/cm represents the optimal processing window, achieving an excellent combination of high strength and high toughness.