Abstract: Microstructure and composition of a kind of spot-like defects on the surface of electrogalvanized steel sheet were analyzed by SEM, and its formation mechanism was discussed. The results show that the surface defects of the electrogaivanized steel sheet are locally rolled, and a large number of small black spots are adhered to the defect parts. The composition of the defects is mainly composed of C, O, Mg, S, Ca, Si, Mn, etc. Through production line investigation and further tracing of characteristic elements, it is found that the defects come from the base sheet. The root cause of the defects is the foreign matters adhering to the base sheet surface, which come from their accumulation in the water quenching tank and the inner wall of the water pipe connected with it. The surface quality of the base sheet can be controlled by cleaning the inner wall of water quenching tank and connected water pipe, so that the defects are eliminated.

Key words: electrogalvanized steel sheet, water quenching tank, spot-like defect