Yield strength development from high strength steel plate to UOE pipe

Abstract The yield strength of the UOE pipe is different from raw steel plate and inhomogeneously distributed on pipe body, because of the Bauschinger effect and work hardening during UOE forming process. The yield strength development of high-strength pipeline steels from raw plate to UOE pipe is investigated by using a modified Chaboche hardening model that incorporates the Bauschinger effect and yield plateau of X70, X80 and X90 steels. The final yield strength of pipe is evaluated by finite element simulation of uniaxial tension of a pipe specimen and an empirical equation. The influences of UOE process parameters on yield strength development are discussed. The results show the modified hardening model can predict the yield stress with good accuracy. The circumferential variation of yield stresses for X80 and X90 pipes are about two times more than that for X70 pipe due to their stronger Bauschinger effect. In UOE forming, C-, U- and O-forming reduce tensile yield stress, and expansion increases and homogenizes yield stress. Fattening operation reduces the measured yield stress, which should be considered in expansion ratio design especially for high strength steels. The yield strength increases logarithmically with t / D ratio and linearly with expansion ratio.