Abstract: Regarded as one of the major defects of mandrel mill-made seamless steel pipes as developed during the rolling process, the defective inner surface scar is directly related to the product primary qualification rate and the yield. The deoxidant-purging process acts as an effective measure for removing inner surface scale of the shell, whereas it could at the same time lead to development of the said defective inner surface scar. Analyzed here in the paper are the causes for the scar. Accordingly, preventative actions are proposed mainly based on the deoxidant itself and the technological parameters of the purging process