Abstract: In this study, the metallurgical cause of battery shell pinhole defect during the cold stamping was analyzed and determined firstly. The pinhole defects were detected by ASPEX scanning electron microscopy. A lot of Al₂O₃ inclusion particles were found in the defect portion. It was considered that the large-size Al₂O₃ inclusions were broken and resulted in pinhole defects during the cold stamping process. Therefore, a systematic sampling analysis was performed on the smelting process and slab of battery shell steel. The results showed that the mass fraction of FeO in the ladle slag decreased to 11.12 % after slag modification during RH refining, and further decreased to 5.85 % at the end of the casting, which indicated that the oxidizing slag continuously transferred oxygen to the molten steel during the casting. The number density of inclusions in the molten steel was reduced from 28.1 pcs/mm² to 10.5 pcs/mm² by holding 10 minutes after RH treatment. The T.O and [N] mass fraction in the molten steel were reduced from 43.4×10^-6 and 32×10^-6 to 29.6×10^-6 and 28.5×10^-6. It was found that the number density of inclusions in the molten steel increased from 10.5 pcs/mm²  to 45.2 pcs/mm² at the beginning of casting, and the mass frction of T.O and [N] increased from 29.6×10^-6 and 28.5×10^-6 to 57.8×10^-6 and 56.1×10^-6 respectively, while the [Al_s] mass fraction in the molten steel reduced by 220×10^-6. It is necessary to improve the slag adjustment effect to reduce the incidence of pinhole defects in the battery shell, and the mass fraction of FeO in the slag should be reduced to less than 5 %.

Key words: battery shell steel, pinhole defect, cleanliness, inclusion, reoxidation