Effects of microstructure and pipe forming strain on yield strength before and after spiral pipe forming of API X70 and X80 linepipe steel sheets

API X70 and X80 linepipe steel sheets were shaped in the form of pipe with different strains (thickness/diameter ratio) by spiral pipe forming. Tension specimens taken from steel sheets or pipes at an interval of 2.5 mm were tested, and their yielding behavior, yield strength, and yield ratio before and after forming were analyzed. In the pipes, the continuous yielding and low yield ratios were shown in the inner side, whereas the discontinuous yielding and high yield ratios were shown in the outer side. This was because the Bauschinger effect and the strain hardening effect were mainly dominant in the inner and outer sides, respectively. The overall yield strength after spiral piping was defined by the competing effect of the strain hardening and the Bauschinger effect. The competing effects depended on the microstructure and the pipe forming strains. The low-temperature transformation microstructures were preferred for achieving the larger increase of overall yield strength after pipe forming. For a specific microstructure an optimization of the yield strength can also be achieved by controlling the pipe forming strain in order to maximize the strain hardening effect and to minimize the Bauschinger effect.