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

It is an object to provide an API 5L X60 to X70 grade steel material for highly deformable line pipes that exhibits superior HIC resistance in a wet hydrogen sulfide environment with a pH of 5 or more and has a low yield ratio even after coating treatment at 300°C or lower and to provide a method for manufacturing the steel material and a welded steel pipe. The steel material for highly deformable line pipes that has superior strain aging resistance and superior HIC resistance has a specific chemical composition and has a metallographic structure composed mainly of ferrite and bainite. The total area fraction of the ferrite and the bainite is 90% or more, and the difference in hardness between the ferrite and the bainite is 70 or more in terms of Vickers hardness. Both before strain aging treatment at a temperature of 300°C or lower and after the strain aging treatment, the steel material has a uniform elongation of 9% or more and a yield ratio of 90% or less.