| 摘要 |
PROBLEM TO BE SOLVED: To provide a manufacturing method for a seamless steel pipe excellent in an inner surface and inner quality property. SOLUTION: A round billet, which is formed into a round billet by hot rolling a slab made by continuous casting, and whose segregation parting degree W(%) is 20% or more defined by the formula W(%)=(Δw/D)×100 (where, P: parting degree of segregation (%),Δw: parting width of segregation at the center of round billet thickness (distance between final solidified positions) (mm), D: total thickness (outer diameter) (mm) of round billet)), is used, and heated to the predetermined temperature to form a hollow raw pipe by boring and rolling, then drawn/rolled or further restricted/rolled to form a predetermined sized steel pipe. The slab made by the continuous casting is preferably cast by using a molten steel whose degree of superheat of the molten steel is made to be 25-65 deg.C, the secondary cooling water amount in the central part of the slab width during the casting is made larger compared to the secondary cooling water amount in the vicinity in the direction of the slab width, and the secondary cooling water ratio up to complete solidification is made to be 1.2-2.0 l/kg-steel to obtain the cast slab.
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