| 摘要 |
PROBLEM TO BE SOLVED: To provide a manufacturing method and a manufacturing apparatus of a steel tube by which the steel tube having high dimensional accuracy and low YR is manufactured by suppressing the rise of YR on the side of the inner surface of the tube bottom part in the stage of roll forming of a sheet and making thickness reduction on the side of the inner surface of the tube or the generation of contact marks with the rolls preventable in the manufacture of the steel tube by a downhill forming system. SOLUTION: In a manufacturing method of the steel tube by the downhill forming system by which the pass line of the second rolls is situated in a lower position than that of the pass line of the first rolls after bending end parts of the width so that the upper surface becomes the side of the inner surface by pressing a pair of first upper and lower rolls against the upper and back surfaces of a sheet and when bending the middle part so that the upper surface becomes the side of the inner surface by pressing a pair of second upper and lower rolls against the sheet, a recessed part 9 is provided in an equivalent portion to the center part 100 of pressing of the upper roll 2T of the second rolls. COPYRIGHT: (C)2007,JPO&INPIT
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