To suppress the appearance of Lu¨ders strain and to decrease yield to tensile strength ratio in the L-direction (longitudinal direction), as well as the C-direction (circumferential direction), have been more important for strain-based design. In this study, conventional UOE and ERW pipes were examined in terms of tensile properties in both directions. In the case of UOE pipes, yield point was clearly observed on the stress-strain curve in the C-direction. However, stress-strain curves in the L-direction showed the round-house type. This difference became prominent after heat treatment for the anti-corrosion. Namely, clear Lu¨ders strain appeared in the C-direction at a lower aging temperature compared with that in the L-direction. On the other hand, contrasting results were obtained in the case for ERW pipes. Thus far, it’s been thought that the difference between UOE and ERW pipe was caused by the direction of final strain during the pipe forming process. There are also differences in the occurrence of Lu¨ders strain between each grade. A stress-strain curve maintained the round-house type in X100 grade pipe after the heat treatment at 240°C for five minutes; however, X70 grade pipe showed the stress-strain curve in the L-direction with Lu¨ders strain after the heat treatment at the same temperature.

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