<p>The mechanical properties and microstructural transformation of welded joints in the experimental steel (EH40 ship plate steel) were investigated following electro-gas vertical welding with a heat input of 300&#xa0;kJ/cm and flux-cored wire (OW-56). Tensile and Charpy impact toughness tests were conducted to assess the mechanical properties of the coarse-grained heat-affected zone, while microstructural analysis was performed using scanning electron microscopy. The results indicated that the experimental steel exhibited excellent mechanical properties and demonstrated a high tolerance for welding with high heat input. During tensile testing, fractures occurred away from the heat-affected zone (HAZ), suggesting that the mechanical properties were comparable to those of the base material. The HAZ exhibited a mixed microstructure with banded formations, including intragranular acicular ferrite (IAF), weld ferrite, polygonal ferrite, and alternating distributions of pearlite (P) and ferrite (F). A notable difference in impact toughness was observed between the Cap and Boot regions of the welded joint, with the impact energy in the coarse-grained HAZ of the Boot being 4.9% higher than that in the Cap. IAF nucleation was observed on composite inclusions of Ti₂O₃-Al₂O₃-MgO-MnS. Transmission electron microscopy, electron microprobe analysis, and electron backscatter diffraction were used to analyze the shear-type nucleation of IAF, which resulted from the combined effects of low misfit, linear expansion coefficient, manganese-deficient regions, and interfacial energy mechanisms. Additionally, dislocations on the IAF plates served as diffusion channels for elemental migration, promoting the secondary growth of IAF. The interlocking IAF and tangled dislocations formed a network-like microstructure that enhanced both the strength and toughness of the HAZ.</p>