Extensive studies to develop high deformability linepipe have been conducted. In the case of linepipes laid in seismic region or permafrost field, higher resistance to buckling against large strain induced by ground movement is required. In order to improve the deformability of pipes, two different types of microstructural control technologies were proposed, based on theoretical and analytical studies on the effect of microstructural characteristics on stress-strain behavior. Grade X65 to X100 linepipes with ferrite-bainite microstructure were manufactured by optimizing the microstructural characteristics. Grade X80 linepipe with bainitic microstructure containing dispersed fine MA constituents was also developed by applying new conceptual TMCP process. Deformability of developed linepipes with two different types of microstructure was evaluated by axial compression and bending tests, and all the developed linepipes showed superior resistance to buckling comparing with conventional pipes. Plate manufacturing technologies for producing recent high strength linepipe steel and the concept for microstructure control for improving deformability were also introduced in this paper.

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