<p>This study focuses on VL4-4, a type of low-temperature carbon–manganese steel. It examines the microstructure and mechanical properties of butt joints welded using the submerged arc welding technique under varying heat input conditions. The results reveal that with the increase in heat input, the WM microstructure shows a development of FSP in addition to the original AF and PF. The proportion of PF increases with its size while AF decreases considerably. In the CGHAZ, the LB and GB are gradually transformed into AF and PF. The slats of AF in WM gradually widen. LAGBs first increase and then decrease. In all regions of the welded joints, the physical phase is α-Fe, and an increase in heat input does not result in a change in this phase. The WM of welded joints contains inclusions of Al₂O₃ and FeO; Al₂O₃ can promote the generation of AF and grain refinement and improve the strength and toughness of the material, FeO will significantly reduce the low-temperature impact toughness of WM. The fracture mechanism of tensile fracture of welded joint is micropore aggregation type. With the increase in welding heat input, the impact absorption work of WM decreases gradually. The impact absorption work in the WM is the lowest, yet it still meets the required standards.The hardness values of the welded joints decrease, exhibiting a softening phenomenon in the heat-affected zone. The overall mechanical properties of the welded joints are optimal at a heat input of 35 kJ/cm. </p>