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

A steel material for highly deformable line pipes that has superior strain aging resistance, superior HIC resistance, and a metallographic structure including ferrite, bainite, and martensite-austenite constituent. The area fraction of the martensite-austenite constituent is 0.5 to 5.0%, and the difference in hardness between the ferrite and the bainite is 60 or more in terms of Vickers hardness. Additionally, the steel material has a uniform elongation of 9% or more and a yield ratio of 90% or less both before a strain aging treatment at a temperature of 300° C. or lower and after the strain aging treatment.