HIGH-STRENGTH STEEL PIPE FOR LINE PIPE HAVING EXCELLENT HYDROGEN-INDUCED CRACKING RESISTANCE, HIGH-STRENGTH STEEL PIPE FOR LINE PIPE USING SAME, AND METHOD FOR MANUFACTURING SAME

摘要

Steel pipe for high strength line pipe use excellent in hydrogen induced crack resistance which can prevent cracking at the surface layer of steel pipe even if the ratio of thickness and outside diameter is 0.035 or more, characterized in that it has a predetermined chemical composition, has a maximum hardness of a surface layer region from the topmost surface of two front and back plate surfaces down to depth of 5 mm of 300Hv or less, and has a total fraction of polygonal ferrite and deformed ferrite with an aspect ratio of 3 or more at the surface layer region from the topmost surface of the two front and back plate surfaces down to depth of 5 mm of 0.1 to 20%.