METHOD FOR MANUFACTURING SEAMLESS STEEL PIPE

摘要

A method for manufacturing a seamless steel pipe which is excellent in properties of its inner surface and its wall. A round billet having a center segregation zone parting degree W (%) of 20 % or more, defined by the following formula, is used as its material. W (%) = ( DELTA w/D) x 100 (wherein W is the center segregation band parting degree (%), DELTA w is the center segregation zone parting width (mm) at the thickness center of the round billet, and D is the external diameter (mm) of the round billet). The round billet is pierced into a hollow blank pipe while maintaining the temperature at a predetermined temperature, and is then formed into a steel pipe of a predetermined size by elongating or by elongating and drawing the blank pipe. The continuously cast slab is preferably a slab cast by using a molten steel the overheat temperature of which is 25 to 65 DEG C, using secondary cooling water is such a way that the amount of secondary cooling water at the central portion of the slab width is greater than that at the peripheral portion in the slab width direction, and that the ratio of the secondary cooling water till the complete solidification is 1.2 to 2.0 l/kg-steel.