| 摘要 |
PURPOSE:To make stably a low-price steel sheet usable by providing a stepped part close to the cut part of a plastically-worked steel sheet and changing a residual stress in the circumference of a step-pushing part into a different kind of stress. CONSTITUTION:The inner diameter of a hole is secured accurately by using a boring tool 16 and a step-pushing die 17 in one body, for working a material in a state that the tool 16 is inserted in the hole before the die penetrates into the material. Because an inner surface part 15 receives a force, tending to move the material structure, by the step-pushing work, it produces a stress different in kind from a residual stress of drawing work, existing in the periphery of step- pushing part. Accordingly, a state almost free from the effect of residual stress of drawing work is produced in the inner-surface part of step-pushing part. As mentioned above, a low-price material can be used for a closed-vessel material without causing a crack at the time of pipe welding. |
