Steel material for high-strength line pipe with high fracture toughness in hydrogen and method for producing the same, and steel pipe for high-strength line pipe and method for producing the same

A steel material for a high-strength line pipe with high fracture toughness in hydrogen, the steel material comprising:

• a chemical composition containing:
• on a mass percent basis,
• C:
• 0.02% to 0.15%,
• Si:
• 0.01% to 2.0%,
• Mn:
• 0.5% to 1.5%,
• P:
• 0.0001% to 0.015%,
• S:
• 0.0002% to 0.0015%,
• Al:
• 0.005% to 0.15%,
• O:
• 0.01% or less,
• N:
• 0.010% or less,
• Nb:
• 0.10% or less, and
• H:
• 0.02 ppm or less, and
• at least one selected from
• Ca:
• 0% to 0.005%,
• Ni:
• 0% to 2.0%,
• Ti:
• 0% to 0.1%,
• Cu:
• 0% to 1.0%,
• Cr:
• 0% to 1.0%,
• Mo:
• 0% to 0.60%,
• W:
• 0% to 1.0%,
• V:
• 0% to 0.10%,
• Zr:
• 0% to 0.050%,
• Mg:
• 0% to 0.01%,
• rare earth metals (REM):
• 0% to 0.01%,
• B:
• 0% to 0.0020%,
• Ta:
• 0% to 0.2%,
• Hf:
• 0% to 0.2%,
• Re:
• 0% to 0.005%,
• Sn:
• 0% to 0.3%, and
• Sb:
• 0% to 0.3%,
• the remainder being Fe and an incidental impurity element,
• a metallic microstructure containing bainite and 15 pieces/100 mm2 or less of inclusions having an aspect ratio of 2.0 or more and a length of 10 m or more, the bainite in a range from a surface to a middle of a thickness of the steel material having a maximum grain size of 25 m or less,
• tensile strength of 520 MPa or more, and
• a hydrogen-induced crack growth threshold KIH in a high-pressure hydrogen gas environment of 1 MPa or more of 80 MPa·m1/2 or more.