Abstract: Of a fused type flux for submerged arc welding, the main factors affecting the diffusible hydrogen content, the toughness of weld metal and the weldability——the basicity of flux, ratio between CaF₂ and SiO₂, transition of microalloy elements such as titanium, boron and rare earth——have been studied. The authors have succeeded in the development of a new flux of the CaO-SiO₂-TiO₂-CaF₂ neutral slag system, the YD504 flux for low hydrogen and high toughness welds. This type of flux has been successfully applied to the welding of steel offshore structures and pressure vessels.