Seamless steel pipe for line pipe and method for producing the same
A seamless steel pipe for line pipe has a chemical composition consisting, by mass percent, of C: 0.03-0.15%, Si: ≤0.50%, Mn: 1.0-2.0%, P: ≤0.050%, S: ≤0.005%, Cr: 0.1-1.0%, Al: 0.001-0.10%, N: ≤0.01%, Ni: 0.05-2.0%, B: 0.0003-0.0015%, Ca: 0.0002-0.0050%, Mo: 0.10-0.50%, Ti: 0.001-0.05%, Cu: 0-2.0%, Nb: 0-0.05%, V: 0-0.10%, the balance: Fe and impurities, and satisfying the conditions of 2Nb+4V+Mo≤0.50, wherein a metal micro-structure of the steel pipe contains 50% or more of bainite, in an area fraction, a wall thickness of the steel pipe is 25 mm or larger, and in a scale formed on the surface of the steel pipe, metal particles consisting mainly of Ni or Cu having an average circle-equivalent diameter of 0.1-5 mm exist. A distance from a boundary between the base metal of the steel pipe and the scale to a region in which the metal particles do not exist is 20 mm or longer.
1. A seamless steel pipe for line pipe having a chemical composition consisting, by mass percent, of
C: 0.03 to 0.15%,
Si: 0.50% or less,
Mn: 1.0 to 2.0%,
P: 0.050% or less,
S: 0.005% or less,
Cr: 0.1 to 1.0%,
Al: 0.001 to 0.10%,
N: 0.01% or less,
Ni: 0.05 to 2.0%,
B: 0.0003 to 0.0015%,
Ca: 0.0002 to 0.0050%,
Mo: 0.10 to 0.50%,
Ti: 0.001 to 0.05%,
Cu: 0 to 2.0%,
Nb: 0 to 0.05%,
V: 0 to 0.10%,
the balance: Fe and impurities, and
satisfying the following formula (i),
wherein a metal micro-structure of the steel pipe contains 50% or more of bainite, in an area fraction;
a wall thickness of the steel pipe is 25 mm or larger; and
in a scale formed on the surface of the steel pipe, metal particles consisting mainly of Ni or Cu having an average circle-equivalent diameter of 0.1 to 5 μm exist, and a distance from a boundary between the base metal of the steel pipe and the scale to a region in which the metal particles do not exist is 20 μm or longer:
2Nb+4V+Mo≤0.50 (i)
where each symbol of element in formula (i) represents the content (mass %) of each element.
2. The seamless steel pipe for line pipe according to claim 1 , wherein
the chemical composition contains, by mass percent,
one or more elements selected from
Cu: 0.01 to 2.0%,
Nb: 0.01 to 0.05%, and
V: 0.01 to 0.10%.
3. The seamless steel pipe for line pipe according to claim 2 , wherein at a position 10 μm distant from the boundary to the scale side, the number density of the metal particles observed per unit area is 5×10 3 pieces/mm 2 or higher.
4. The seamless steel pipe for line pipe according to claim 1 , wherein at a position 10 μm distant from the boundary to the scale side, the number density of the metal particles observed per unit area is 5×10 3 pieces/mm 2 or higher.
5. A method for producing the seamless steel pipe for line pipe according to claim 4 , wherein the steel pipe having the chemical composition consisting, by mass percent, of
C: 0.03 to 0.15%,
Si: 0.50% or less,
Mn: 1.0 to 2.0%,
P: 0.050% or less,
S: 0.005% or less,
Cr: 0.1 to 1.0%,
Al: 0.001 to 0.10%,
N: 0.01% or less,
Ni: 0.05 to 2.0%,
B: 0.0003 to 0.0015%,
Ca: 0.0002 to 0.0050%,
Mo: 0.10 to 0.50%,
Ti: 0.001 to 0.05%,
Cu: 0 to 2.0%,
Nb: 0 to 0.05%,
V: 0 to 0.10%,
the balance: Fe and impurities, and
satisfying the following formula (i),
is conveyed into a furnace after the finish of hot rolling, being heated in an atmosphere with the temperature of Ac 3 +50° C. or higher and the water vapor concentration of 5% or higher, and thereafter is subjected to quenching treatment in which accelerated cooling is performed at a rate of 10° C./s or higher, subsequently being tempered at a temperature of Ac 1 −50° C. or lower:
2Nb+4V+Mo≤0.50 (i)
where each symbol of element in formula (i) represents the content (mass %) of each element.
6. The method for producing the seamless steel pipe for line pipe according to claim 5 , wherein
the chemical composition contains, by mass percent,
one or more elements selected from
Cu: 0.01 to 2.0%,
Nb: 0.01 to 0.05%, and
V: 0.01 to 0.10%.
7. A method for producing the seamless steel pipe for line pipe according to claim 1 , wherein the steel pipe having the chemical composition consisting, by mass percent, of
C: 0.03 to 0.15%,
Si: 0.50% or less,
Mn: 1.0 to 2.0%,
P: 0.050% or less,
S: 0.005% or less,
Cr: 0.1 to 1.0%,
Al: 0.001 to 0.10%,
N: 0.01% or less,
Ni: 0.05 to 2.0%,
B: 0.0003 to 0.0015%,
Ca: 0.0002 to 0.0050%,
Mo: 0.10 to 0.50%,
Ti: 0.001 to 0.05%,
Cu: 0 to 2.0%,
Nb: 0 to 0.05%,
V: 0 to 0.10%,
the balance: Fe and impurities, and
satisfying the following formula (i),
is conveyed into a furnace after the finish of hot rolling, being heated in an atmosphere with the temperature of Ac 3 +50° C. or higher and the water vapor concentration of 5% or higher, and thereafter is subjected to quenching treatment in which accelerated cooling is performed at a rate of 10° C./s or higher, subsequently being tempered at a temperature of Ac 1 −50° C. or lower:
2Nb+4V+Mo≤0.50 (i)
where each symbol of element in formula (i) represents the content (mass %) of each element.
8. The method for producing the seamless steel pipe for line pipe according to claim 7 , wherein
the chemical composition contains, by mass percent,
one or more elements selected from
Cu: 0.01 to 2.0%,
Nb: 0.01 to 0.05%, and
V: 0.01 to 0.10%.