Abstract: In order to study high heat input welding steels for industrial production processes, a high heat input welding steel for crude oil storage tank was developed in domestic steelworks by titanium nitride and oxide refinement dispersion technology. The process and condition of alloying addition were controlled. The impact toughness of a traditional steel and the trial steel was compared after welding thermal simulation and vertical position gas-electric welding. The effects of density, type and size of inclusion on heat affected zone were analyzed. The results showed that the mechanical properties of this steel meet the demand of national standard, and the yield ratio is below 0.9. The microstructure of fusion line with a heat input of 400 kJ/cm consists of proeutectoid polygonal ferrites and numerous acicular ferrites, therefore the welded joints have excellent low temperature toughness. TiOx-Al2O3-MnS complex inclusions with the size of 1~3 μm are the most favorable sites for the nucleation of IAF (intragranular acicular ferrites).