This study aims to investigate and compare the evolution of microstructure and mechanical behavior in API X70 steels welded using SMAW techniques. A comprehensive analysis of the microstructure revealed that the weldability of low-carbon X70 steels is significantly influenced by the carbon content and microalloying elements. The microstructural evolution demonstrated that higher carbon content in X70 steel results in increased levels of martensite-austenite (M-A) compounds in the heat-affected zone (HAZ). Additionally, higher carbon content also leads to enhanced growth of ferritic grains in the HAZ. Furthermore, the rate of M-A compound formation in the HAZ increases with higher carbon content, but this rate is limited when a high proportion of niobium is introduced. Optimal mechanical properties, appropriate grain coarsening, and an acceptable ratio of M-A compounds throughout the HAZ were observed at a carbon content of 0.082%. These findings suggest that the weldability of HSLA-X70 steels can be improved by incorporating microalloying elements, without the need for complex metallurgical processes to enhance the microstructure. Fractographic examinations revealed a shift in the failure mode from low to high carbon ratios in X70 steels.