Abstract: Based on the identification of the grain zones of the C.C.steel billet for tube rolling,the unique phenomenon of "walling" of the billet as being pierced is analyzed.Discussed are the main elements of the C.C.billet responsible for the common defect——internal folding of the hot-rolled tube,i.e.,the macrostructure and chemical composition,and the relationship between different grain zones and the size of columnar crystal and the said internal folding.Moreover the regression method is used to deduce the formula of the relationship between carbon equivalent and the internal folding rate.