Steel pipe for fuel injection pipe and fuel injection pipe using the same

A steel pipe for fuel injection pipe having a chemical composition consisting, by mass percent, of

• C:
• 0.12 to 0.27%,
• Si:
• 0.05 to 0.40%,
• Mn:
• 0.3 to 2.0%,
• Al:
• 0.005 to 0.060%,
• N:
• 0.0020 to 0.0080%,
• Ti:
• 0.005 to 0.015%,
• Nb:
• 0.015 to 0.045%,
• Cr:
• 0 to 1.0%,
• Mo:
• 0 to 1.0%,
• Cu:
• 0 to 0.5%,
• Ni:
• 0 to 0.5%,
• V:
• 0 to 0.15%, and
• B:
• 0 to 0.005%,
• the balance being Fe and impurities, and
• contents of Ca, P, S, and O in the impurities being
• Ca:
• 0.0001 to 0.0007%,
• P:
• 0.02% or less,
• S:
• 0.01% or less, and
• O:
• 0.0040% or less,
• and having a metal micro-structure consisting of a tempered martensitic structure, or a mixed structure of tempered martensite and tempered bainite, in which a prior-austenite grain size number is 10.0 or more, wherein
• the steel pipe has an outer diameter of 20 mm or less, an inner diameter 2.5 mm or more, a wall thickness of 1.5 mm or more, a tensile strength of 800 MPa or higher, and a critical internal pressure satisfying a following formula (i):
• IP≥0.3×TS×α (i)
• α=[(D/d)2−1]/[0.776×(D/d)2] (ii)
• where, in the above formula (i), IP denotes a critical internal pressure (MPa), TS denotes a tensile strength (MPa), and α is a value represented by the above formula (ii), and where, in the above formula (ii), D denotes an outer diameter (mm) of the steel pipe for fuel injection pipe, and d denotes an inner diameter (mm) of the steel pipe for fuel injection pipe.