Electric resistance welded steel pipe
View Patent ↗An electric resistance welded steel pipe which has sufficient strength and low-temperature toughness and a low yield ratio and which is suitable as a line pipe to be laid in depths of the sea, characterized in that: the composition of the base material contains, in mass %, 0.05 to 0.10% of C, 1.00 to 1.60% of Mn, and 0.005 to less than 0.035% of Nb, and has a Ceq value of 0.23 to 0.38; and the metal microstructure of the base material contains 3 to 13% of martensite in area fraction with the balance being ferrite.
1. An electric resistance welded pipe comprising a base material having a chemical composition comprising, by mass %,
C: 0.05 to 0.10%,
Mn: 1.00 to 1.60%,
Ti: 0.005 to 0.030%,
Nb: 0.005% to less than 0.035%, and
N: 0.001 to 0.008%,
further comprising one or both of
Si: 0.01 to 0.60% and
Al: 0.001 to 0.10%, and
a balance of iron and unavoidable impurities, wherein contents are restricted to
P: 0.02% or less and
S: 0.005% or less, and
a Ceq, represented by the following formula 1, satisfies 0.23≦Ceq≦0.38, and having a metal microstructure comprising, by area ratio, 3 to 13% of martensite, and a balance of ferrite, an average value of an equivalent circle diameter of the martensite being 0.5 to 1.5 μm:
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (formula 1)
where, in formula 1, C, M, Cr, Mo, V, Ni, and Cu are values which represent the contents of the elements by mass %.
2. The electric resistance welded pipe according to claim 1 wherein the chemical composition of the base material further comprises, by mass %, one or more of
Ni: 1.0% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 0.5% or less,
V: 0.2% or less,
Ca: 0.006% or less, and
REM: 0.006% or less.
3. The electric resistance welded pipe according to claim 1 wherein the chemical composition of the base material satisfies
Mn: 1.00 to 1.50% and
Si: 0.40% or less and
further satisfies
0.23≦Ceq≦0.30,
and
a tensile strength of the steel pipe is 490 to 760 MPa.
4. The electric resistance welded pipe according to claim 3 wherein the chemical composition of the base material satisfies
Nb: 0.005 to 0.020%.
5. The electric resistance welded pipe according to claim 3 wherein the chemical composition of the base material further comprises, by mass %, one more of
Ni: 0.5% or less,
Cu: 0.5% or less,
Cr: 0.5% or less,
Mo: 0.2% or less,
V: 0.1% or less,
Ca: 0.006% or less, and
REM: 0.006% or less.
6. The electric resistance welded pipe according to claim 4 wherein the chemical composition of the base material further comprises, by mass %, one more of
Ni: 0.5% or less,
Cu: 0.5% or less,
Cr: 0.5% or less,
Mo: 0.2% or less,
V: 0.1% or less,
Ca: 0.006% or less, and
REM: 0.006% or less.
7. The electric resistance welded pipe according to claim 1 , wherein the electric resistance weld pipe has a Charpy absorption energy at −30° C. of 190 J or more.
8. The electric resistance welded pipe according to claim 2 , wherein the electric resistance weld pipe has a Charpy absorption energy at −30° C. of 190 J or more.
9. The electric resistance welded pipe according to claim 3 , wherein the electric resistance weld pipe has a Charpy absorption energy at −30° C. of 190 J or more.
10. The electric resistance welded pipe according to claim 4 , wherein the electric resistance weld pipe has a Charpy absorption energy at −30° C. of 190 J or more.
11. The electric resistance welded pipe according to claim 5 , wherein the electric resistance weld pipe has a Charpy absorption energy at −30° C. of 190 J or more.
12. The electric resistance welded pipe according to claim 6 , wherein the electric resistance weld pipe has a Charpy absorption energy at −30° C. of 190 J or more.