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 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. 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.