| 摘要 |
PROBLEM TO BE SOLVED: To provide a producing method of a round cast billet for a seamless steel pipe, by which the development of flaw on the inner surface of the pipe is restrained at the time of pierce-rolling into the steel pipe and the cross sectional shape has accurate roundness. SOLUTION: After casting the cast billet having the round shape (diameter is D (mm)) in the cross section by a continuous casting method, to the cast billet in which unsolidified part exists, (the thickness is L (mm) of the unsolidified part at the unsolidified rolling reduction position), the unsolidified rolling reduction is applied at the draft rate P (mm) satisfying the formula I: R<=D-d+C, the formula II: r=(D+Y)-d and the formula III: 0.8<=R/L<=1.8 (the width spreading rate is Y (mm) at this time, Y=4.52×103×R2.04). Further, a form-rolling is applied at the draft rate (r) (mm) from the orthogonal direction to the unsolidified rolling reduction direction with caliver rolls. Then, C in the formula I is the max. deformed rate (mm) of the cast billet in the orthogonal direction to the form-rolling direction and C=(r-12)×(0.06 Vc-1) and wherein, Vc is casting velocity (mm/s).
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