The causes of outer fold defects of 0. 20% C steel pipe are studied by means of pickling

morphology observation and metallographic examination. The results show that the subsurface blowholes and pinholes defects of the continuous casting billet lead to the longitudinal cracks on the surface of the round steel

which are the direct causes of the outer fold defects of the steel pipe. By increasing the acid-soluble aluminum content from 0.006% ~ 0.010% to 0.010% ~ 0. 020%

controlling casting speed (1.8 ± 0.03 ) m/min and superheates extent 15 ~30℃

and increasing the temperature of Rotary hearth furnace from 1250 ℃ to 1270 ℃ etc improvement measures

the first qualified steel rate of the 0.20% C steel piercing pipe is increased from 47. 01% to above 96%.