Abstract: Analyzed in the article are the causes for the external fold defect as occurring during the piercing of the CC round to be used for making the automobile axle tube.It is regarded via the morphology of the external fold as observed that the fold is originated from the cracks as existing on the surface and sub-surface of the billet.Also checked are the inclusions of the steel tube,the chemical composition of the CC billet,the Als andNcontents of the steel,and the temperature of the liquid steel in the tundish.The metallographic morphology of the CC billet is observed,and the particle inclusions on the fracture of the specimen of sub-surface cracking part of the billet is analyzed.Thanks to the above mentioned inspections,such data as follows are obtained:in the steel:Als∧0.025%,N≥0.007 5%;the superheat degree of the liquid steel:∧33℃,which means that both the Als andNcontents,and the said superheat degree are too high,and this is just the major cause for the cracking of the billet.The main inclusions as identified at the location of the crack defect include MnS+TiN+BN+AlN.The conclusion is that in case that both very high Als andNcontents and high super heat degree of the CC liquid steel occur simultaneously in one heat of the steel,it will become very possible to lead the said CC billet crack.