| 摘要 |
In cold-rolling a seamless metal pipe by means of a cold pilger mill, too great an outer-diameter reduction as compared with wall thickness reduction creates too great a circumferential strain on the flange sides to result in too great a circumferential compressive stress, whereby wrinkles occur on the inner surface of the pipe. Further, the wrinkles are folded on the groove bottom sides to create cold shut folds. Repetition of these phenomena causes flaws on the inner surface. Unlike a tube stock produced by an extrusion process, a tube stock produced by a Mannesmann-mandrel process has, on the inner surface, wrinkles which have occurred during the sizing (using a stretch reducer or a sizer), and the wrinkles are further encouraged by cold-rolling. Therefore, attention must be paid to this problem. In view of these facts, the ratio of outer-diameter reduction to wall thickness reduction in cold-rolling using a cold pilger mill is adjusted to 1/2 or below, whereby the occurrence of flaws on the inner surface is prevented in the cold rolling, and a high-quality seamless metal pipe having few flaws on the inner surface can be obtained.
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