Pipeline and manufacturing method thereof

1. 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

optionally one or more of,

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 not 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 1;

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 2:

t/D ×100≦6  (1)

ΔYS≦−1.75×YR+230  (2);

wherein a ΔCeq value which is an absolute value of a difference of Ceq values expressed by a following Expression 4 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  (4)

wherein [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V]represent amounts of C, Mn, Ni, Cu, Cr, Mo, and V by mass %.

2. A method of producing the pipeline according to claim 1 , comprising:

a first yield strength measuring process in which the yield strength of the plurality of the steel pipes is measured; and

a first welding process in which steel pipes in which the ΔYS satisfies the Expression 2 are welded.

3. A method of producing the pipeline according to claim 1 , comprising:

a first assigning process in which production numbers capable of specifying each of the steel pipes are assigned to the plurality of the steel pipes in order of rolling; and

a second welding process in which steel pipes produced from the same molten steel and the same hot-rolled coil and having a difference between the production numbers of 1 or more and 5 or less are welded.

4. 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

optionally one or more of,

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 comprising Fe and unavoidable impurities,

a yield point elongation is not 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 1;

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 2:

t/D ×100≦6  (1)

ΔYS ≦−1.75×YR+230  (2);

wherein a ΔCeq value which is an absolute value of a difference of Ceq values expressed by a following Expression 4 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  (4)

wherein [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] represent amounts of C, Mn, Ni, Cu, Cr, Mo, and V by mass %.