HIC STEEL MATERIAL FOR HIGHLY DEFORMABLE LINE PIPES HAVING SUPERIOR STRAIN AGING RESISTANCE AND SUPERIOR HIC RESISTANCE METHOD FOR MANUFACTURING SAME AND WELDED STEEL PIPE

摘要

for the purpose of providing an API 5L X60 to X70 grade high-deflection line pipe steel having excellent HIC characteristics in a humidified hydrogen sulfide environment of pH 5 or higher and having a lower yield ratio even after coating at 300 DEG C or lower, do. Wherein the ferrite and the bainite have a specific component composition and the metal structure has ferrite, bainite and island-shaped martensite, the island fraction of the island-shaped martensite is 0.5 to 5.0%, and the difference in hardness between the ferrite and the bainite is 60 or more And having a uniform elongation of 9% or more and a yield ratio of 90% or less with respect to each of the pre-strain aging treatment and the post-strain aging treatment at a temperature of 300 占 폚 or less.