| 摘要 |
PROBLEM TO BE SOLVED: To provide an extremely thick-wall seamless tube which is available for parts for machines or parts for mechanical structure which have an inside diameter equal to a greasing hole in the middle part of a cross section while a seamless hollow tube stock is rolled without performing macining, and to provide a stretch-reduction method for preventing the generation of harmful flaws by bulging to products and the break of a roll. SOLUTION: Using a stretch reducer 10 in which plural grooved roll stands 12, 13 are arranged, thickening is performed by imparting compressive stress to the seamless hollow tube stock 15 which is worked with a piercer and an elongation mill and the ratio of the thickness to the outside diameter of this thickened seamless tube is set to >=25%. In the stretch-reduction method, the state of stress between plural respective grooved-roll stands 12, 13 which his added to the seamless hollow tube stock 15 is defined as compression and the wall thickness of the tubular body part of the seamless tube to which compressive stress is added is thickened than that of the tube end parts of the seamless tube to which the compressive stress is not added.
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