Pipeline and manufacturing method thereof

A pipeline comprising a plurality of weld zones where ends of a plurality of steel pipes are joined by welding, wherein:

• the plurality of the steel pipes are produced from a plurality of charges of molten steel having one or more steel compositions, by mass %,
• C:
• 0.04% or more and 0.15% or less,
• Mn:
• 1.0% or more and 1.75% or less,
• Nb:
• 0.005% or more and 0.10% or less,
• Ti:
• 0.005% or more and 0.02% or less,
• Al:
• 0.001% or more and 0.06% or less,
• Si:
• limited to 0.4% or less,
• P:
• limited to 0.015% or less,
• S:
• limited to 0.005% or less,
• N:
• limited to 0.007% or less, and
• Cu:
• 0.1% or more and 1.0% or less,
• Ni:
• 0.1% or more and 1.0% or less,
• Cr:
• 0.1% or more and 1.0% or less,
• Mo:
• 0.05% or more and 0.3% or less, and
• V:
• 0.01% or more and 0.1% or less, and
• a balance consisting of Fe and unavoidable impurities,
• a yield point elongation is present on a stress-strain curve that is obtained when stress is applied in a longitudinal direction of each of the steel pipes;
• t which is a thickness of each of the steel pipes in units of mm and D which is an average pipe outside-diameter in units of mm satisfy a following Expression 5;
• ΔYS which is a yield strength difference between one steel pipe and the other steel pipe that are welded to each other in units of MPa, and YR which represents a yield ratio that is a ratio of the yield strength to a tensile strength of a pipe having a low yield strength out of the one steel pipe and the other steel pipe, satisfy a following Expression 6:
• t/D×100≦6 (5)
• ΔYS≦−0.69×YR+125 (6),
• wherein a ΔCeq value which is an absolute value of a difference of Ceq values expressed by a following Expression 8 between the one steel pipe and the other steel pipe is 0.045% or less, and
• the tensile strength of the plurality of the steel pipes is 450 MPa to 850 MPa or less,
• Ceq=[C]+[Mn]/6+([Ni]+[Cu])/15+([Cr]+[Mo]+[V])/5 (8)
• wherein [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] represent amounts of C, Mn, Ni, Cu, Cr, Mo, and V by mass %.