Electric-resistance welded steel pipe
The present invention provides electric-resistance welded steel pipe optimal for applications such as production of shale gas which is comprised of steel having a chemical composition which contains, by mass %, respectively, C: 0.08 to 0.18%, Si: 0.01% to 0.50%, Mn: 1.30 to 2.1%, Al: 0.001 to 0.10%, Nb: 0.005 to 0.08%, and Ti: 0.005 to 0.03%, is limited to N 0.008% or less, P: 0.020% or less, and S: 0.010% or less, and has a balance of Fe and unavoidable impurities, wherein a structure at a center part of thickness is 40% to 70% by area of ferrite phase having a circle-equivalent diameter of 1.0 μm to 10.0 μm and a balance of a low temperature transformation phase comprising a bainite phase, and Ceq satisfies 0.32≤Ceq≤0.60.
1. Electric-resistance welded steel pipe comprised of steel having a chemical composition which contains, by mass %, respectively
C: 0.08 to 0.18%,
Si: 0.01% to 0.50%,
Mn: 1.30 to 2.1%,
Al: 0.001 to 0.10%,
Nb: 0.005 to 0.08%, and
Ti: 0.005 to 0.03%,
is limited to
N: 0.008% or less,
P: 0.020% or less, and
S: 0.010% or less, and
has a balance of Fe and unavoidable impurities, wherein
a structure at a center part of thickness is 40% to 70% by area of ferrite phase having a circle-equivalent diameter of 1.0 μm to 10.0 μm and a balance of a low temperature transformation phase comprising a bainite phase, and
a Ceq expressed by the following formula (1) satisfies 0.32≤Ceq≤0.60:
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15 (1)
where the “C”, “Mn”, “Cr”, “Mo”, “V”, “Cu”, and “Ni” in formula (1) are values expressing the contents of the respective elements by mass % and where when these elements are not included, the elements are calculated as 0,
wherein an area rate of the bainite phase is 90% or more of the balance other than the ferrite phase,
wherein an area rate of a residual austenite phase is not more than 0.5%,
wherein an area rate of a martensite phase is not more than 0.5%,
wherein a yield strength according to a tensile test in a longitudinal direction using a tensile test piece is 758 MPa to 822 MPa in strength,
wherein a yield ratio according to a tensile test in a longitudinal direction using the tensile test piece is 85 to 95%, and
wherein the tensile test piece is an arc shaped tensile test piece taken from the electric-resistance welded steel pipe in the longitudinal direction thereof on the basis of JIS Z 2241.
2. Electric-resistance welded steel pipe according to claim 1 wherein said chemical composition further contains, by mass %, one or more of
V: 0.08% or less,
Cu: 0.5% or less,
Ni: 0.5% or less,
Cr: 0.5% or less,
Mo: 0.5% or less,
Ca: 0.005% or less, and
REM: 0.005% or less.
3. Electric-resistance welded steel pipe according to claim 2 wherein said chemical composition further is limited to, by mass %,
B: 0.0004% or less.
4. Electric-resistance welded steel pipe according to claim 2 wherein there is no yield elongation in a stress-strain curve of a tensile test.
5. Electric-resistance welded steel pipe according to claim 2 wherein the thickness is 7 to 12.7 mm.
6. Electric-resistance welded steel pipe according to claim 1 wherein said chemical composition further is limited to, by mass %,
B: 0.0004% or less.
7. Electric-resistance welded steel pipe according to claim 1 wherein there is no yield elongation in a stress-strain curve of a tensile test.
8. Electric-resistance welded steel pipe according to claim 1 wherein the thickness is 7 to 12.7 mm.