Abstract: Shell defects of ultralow carbon galvanized sheet surface were detected and analyzed using SEM.The results showed that the large particles flawed Al2O3 inclusions were discovered between the surface shell and the substrate distinct layers,and the size of Al2O3 inclusions were 5-100 μm.Looked up the slab casting process of the defect coil,the SEN nozzle clogging was found and the stopper rised.Combined with a comprehensive analysis of the clogging substance of SEN,the clogging substance was caused of Al2O3 inclusions.Through process optimizing,improving the ladle slag effect to increase the w(CaO)/w(Al2O3) to 2.3, and reducing the steel oxygen at the end of RH decarburization from 367×10-6 to 315×10-6,the surface shell defects rate of ultralow carbon galvanized sheet was reduced from 0.10 % to 0.01 %,and controlled effectively.

Key words: surface defect, inclusion, nozzle clogging