Abstract: Aim at surface quality problem arose in the production process of hot galvanized dual phase steel which caused by adding alloying elements, such as small black spots and piping defect, the microstructure of the 800 MPa grade hot galvanized dual phase steel was analyzed and the effect of H₂ content in hot dip galvanization cooling section on mechanical properties and surface quality was studied. The results show that with the increase of H₂ content in cooling section, the yield strength and tensile strength of the specimen increase significantly, mainly because the increasing of H₂ content increases martensitic transformation amount in the process of cooling, so that improves the comprehensive mechanical properties. At the same time, the quantity of surface textured defect and small black spots decreases significantly, the specimen gets reduction in the cooling process so that the surface Mn and Si oxides are decreased, which ensures the formation of dense inhibition layer. However, the influence on surface quality is small and the elongation decreases significantly when the H₂ content is more than 10%, so considering the energy consumption, the H₂ input of cooling section should be controlled at 10%.

Key words: hot dip galvanized dual phase steel, cooling section, H₂ content, surface quality, mechanical properties