New steel manufacturing technology has been developed to attain excellent HAZ toughness of high-strength steels. In this steel, the HAZ microstructure near a weld fusion line is refined markedly by utilizing the strong retardation of austenite grain growth as well as the formation of intragranular ferrite. A dual phase microstructure is very effective for obtaining both high uniform elongation and low Yield / Tensile ratio in the high strength UOE pipe body. High strength, low Y/T ratio and high uniform elongation of the steel pipe can be attained by optimizing the area fraction of ferrite and the grain size by applying accelerated cooling technology. New high strength UOE pipes called “Tough-Ace” possessing both excellent HAZ toughness and deformability have been developed, and the X60 UOE pipe has been mass-produced for the Sakhalin Project.

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