| 摘要 |
PROBLEM TO BE SOLVED: To provide a pipe material excellent in a durability for a repeated high pressure fatigue and the vibration of a car body by making an outer surface structure to a rolling structure and increasing a vibration fatigue strength by process-hardening the outer surface, in a fine diameter thick metal pipe consisting of a single pipe body dry drawn a final finish extending pipe by a surface reduction ratio specific value. SOLUTION: A fine diameter thick steel pipe consisting of a single pipe body is dry drawn at a surface reduction rate 1.2-6.4% in a final extending process after repeating an annealing and drawing process several times. Thereby, the outside of a pipe is made to a rolling structure by the effect of processing and process-hardening is carried out, but its inside is as being in the annealing state before final drawing process without the effect of processing and made to a pipe excellent in a vibration fatigue strength and inner pressure fatigue strength, even it is not annealed after drawing. When the surface reduction ratio is less that 1.2%, the hardness by the processing of the pipe outside is insufficient and when it is beyond 6.4%, the pipe outside is hardened sufficiently and the vibration fatigue strength is improved, by the pipe inside is hardened by the effect of processing and the hardness of the surface side in the pipe is increased and inner pressure fatigue strength is decreased. |
