Semi-alkaline sintered flux, used in conjunction with appropriate submerged arc welding wire (such as EH14, EA2, EM12K, etc.), is used for welding important structural steel. The weld hardly adds Si and Mn, and is metallurgically neutral. It has excellent welding process performance, stable arc, good surface quality, good slag removal performance, and high low-temperature impact toughness. When welding filler metal with direct current, the positive electrode should be connected. can be used in many fields such as boilers, pressure vessels, pipelines and low-alloy steel structures.