| During pipe forming, the pipe-wall fibres undergo different deformation patterns, resulting in a stress gradient through the wall. The cold flattening and subsequent tensile-testing operation leads to additional tension-tension and compression-tension cycles on the inner and outermost fibres of the pipe wall, respectively. The compressive strains on outer surface reduce the yield strength due to the Bauschinger effect, while the additional cold work in the other surface increase the yield strength. Therefore, the yield strength of the ERW pipe becomes much different from that of the original material. Such phenomenon should be considered in designing forming condition for high strength pipes. In this study, the relation between work hardening and the Bauschinger effect is experimentally obtained and discussed in order to evaluate final yield strength of pipes. For the ferrite-pearlite Line pipe steels used, the following conclusions are reached. The Nb+Ti+V and Nb+Mo steels(API-X70 grade) were predicted a significant reduction in yield strength, averaging about 4.2 ㎏/㎟ in 4% prestrain. Much less difference in the reduction of yield strength was predicted in the Nb+Ti+V and Nb+Mo steels. API-X65 and X52 grade steels showed 2.3 ㎏/㎟ yield strength loss in 4% prestrain. |
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