| 摘要 |
PROBLEM TO BE SOLVED: To provide a cold rolling-formed ring at low cost and in high quality by preventing generation of a big crack during cold rolling, realizing a cold rolling of a high pipe expanding ratio and a high working ratio, and having no necessity of containing expensive alloy elements and preliminary machining before cold rolling or others. SOLUTION: The cold rolling of a medium carbon steel containing, by wt., 0.37-0.61% C, 0.15-0.35% Si, 0.60-0.90% Mn, and impurities of <=0.03% P, <=0.035% S, <=0.30% Cu, <=0.20% Ni, <=0.20% Cr, <=0.35% (Ni+Cr), and the balance Fe with inevitable impurities, is divided into multiple stages: (1) in the cold-forming by rolling, pipe expanding is executed up to a limit of generating a crack, and then (2) annealing is executed for the purpose of removing internal stress, dislocation and others, and by repeating the means (1) and (2) to obtain a product having a hardness of <=90 HRB, finish worked to an aimed size and free from cracks such as indicated in Fig. (a) is obtained. Fig. (b) indicates a conventional product having cracks.
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