Abstract: Researched and developed is theФ559 mm×31.8 mm X70 hi-strain SAWL pipe for offshore service by means of using the polygonal hi-strain steel plate with ferrite+bainite dual-phase structure,together with the pre-bending formation,expanding dies,and the process optimization,as well as the welding material matching test research.Nevertheless the said pipe as made via the single furnace trial production still shows low qualification rate of uniform elongation,and its strain hardening exponent(n value)has not met what is expected.In such a case,it is necessary to conduct an in-deep analysis of the influencing factors for the uniform elongation and strain hardening exponent so as to further optimize the chemical composition,microstructure and properties of the steel plate.Due to the fact that the impact energy of the wall central fusion line and outer weld are relatively low,a further research is needed to optimize the welding materials and processes.