STEEL MATERIAL FOR HIGHLY-DEFORMABLE LINE PIPES HAVING SUPERIOR STRAIN AGING CHARACTERISTICS AND ANTI-HIC CHARACTERISTICS, METHOD FOR MANUFACTURING SAME, AND WELDED STEEL PIPE

摘要

The purpose of the present invention is to provide: a steel material for API 5L X60 to X70 grade highly-deformable line pipes that exhibits excellent anti-HIC characteristics in a wet hydrogen sulfide environment having a pH of 5 or more, and that has a low yield ratio even after undergoing a coating process at 300°C or less; a method for manufacturing the steel material; and a welded steel pipe.　The steel material for highly-deformable line pipes that exhibits strain aging characteristics and anti-HIC characteristics has: a specific composition; has a metal structure primarily comprising ferrite and bainite, the area fraction of the ferrite and bainite totaling at least 90%, and the difference in hardness between the ferrite and the bainite being at least 70 in terms of Vickers hardness; and a uniform elongation of at least 9% and a yield ratio of not more than 90% with regard to pre- and post-strain aging treatment at 300°C or less.