Steel material and oil-well steel pipe
The steel material according to the present invention contains, in mass %, C: 0.15 to 0.45%, Si: 0.10 to 1.0%, Mn: 0.10 to 0.8%, P: 0.050% or less, S: 0.010% or less, Al: 0.01 to 0.1%, N: 0.010% or less, Cr: 0.1 to 2.5%, Mo: 0.35 to 3.0%, Co: 0.05 to 2.0%, Ti: 0.003 to 0.040%, Nb: 0.003 to 0.050%, Cu: 0.01 to 0.50%, and Ni: 0.01 to 0.50%, and satisfies the following Formulae. A prior-austenite grain diameter of its microstructure is less than 5 μm, and a block diameter of its microstructure is less than 2 μm. The microstructure contains a total of 90% by volume or more of tempered martensite and tempered bainite. C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15−Co/6+α≥0.70 (1) (3C+Mo+3Co)/(3Mn+Cr)≥1.0 (2).
1. A steel material comprising a chemical composition consisting of, in mass %:
C: 0.15 to 0.45%,
Si: 0.10 to 1.0%,
Mn: 0.10 to 0.8%,
P: 0.050% or less,
S: 0.010% or less,
Al: 0.01 to 0.1%,
N: 0.010% or less,
Cr: 0.1 to 2.5%,
Mo: 0.35 to 3.0%,
Co: 0.05 to 2.0%,
Ti: 0.003 to 0.040%,
Nb: 0.003 to 0.050%,
Cu: 0.01 to 0.50%,
Ni: 0.01 to 0.50%,
V: 0 to 0.5%,
B: 0 to 0.003%,
W: 0 to 1.0%,
Ca: 0 to 0.004%,
Mg: 0 to 0.004%, and
rare earth metal: 0 to 0.004%,
with the balance being Fe and impurities, and satisfying Formulae (1) and (2), wherein
a prior-austenite grain diameter of a microstructure is less than 5 μm,
a block diameter of the microstructure is less than 2 μm, and
the microstructure contains a total of 90% by volume or more of tempered martensite and tempered bainite:
C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15−Co/6+α≥0.70 (1)
(3C+Mo+3Co)/(3Mn+Cr)≥1.0 (2)
Effective B=B−11(N−Ti/3.4)/14 (3)
where α in Formula (1) is 0.250 when the effective B defined by Formula (3) (mass %) is 0.0003% or more, or zero when the effective B is less than 0.0003%, and symbols of elements in Formula (1) to Formula (3) are to be substituted by contents of corresponding elements (in mass %).
2. An oil-well steel pipe comprising,
the steel material according to claim 1 , and
a wall thickness of 15 mm or more.
3. The steel material according to claim 1 , wherein the chemical composition contains one or more types of element selected from the group consisting of:
Ca: 0.0003 to 0.004%;
Mg: 0.0003 to 0.004%; and
rare earth metal: 0.0003 to 0.004%.
4. An oil-well steel pipe comprising,
the steel material according to claim 3 , and
a wall thickness of 15 mm or more.
5. The steel material according to claim 1 , wherein the chemical composition contains
V: 0.015 to 0.5%.
6. An oil-well steel pipe comprising,
the steel material according to claim 5 , and
a wall thickness of 15 mm or more.
7. The steel material according to claim 5 , wherein the chemical composition contains one or more types of element selected from the group consisting of:
B: 0.0003 to 0.003%; and
W: 0.05 to 1.0%.
8. An oil-well steel pipe comprising,
the steel material according to claim 7 , and
a wall thickness of 15 mm or more.
9. The steel material according to claim 7 , wherein the chemical composition contains one or more types of element selected from the group consisting of:
Ca: 0.0003 to 0.004%;
Mg: 0.0003 to 0.004%; and
rare earth metal: 0.0003 to 0.004%.
10. An oil-well steel pipe comprising,
the steel material according to claim 9 , and
a wall thickness of 15 mm or more.
11. The steel material according to claim 5 , wherein the chemical composition contains one or more types of element selected from the group consisting of:
Ca: 0.0003 to 0.004%;
Mg: 0.0003 to 0.004%; and
rare earth metal: 0.0003 to 0.004%.
12. An oil-well steel pipe comprising,
the steel material according to claim 11 , and
a wall thickness of 15 mm or more.
13. The steel material according to claim 1 , wherein the chemical composition contains one or more types of element selected from the group consisting of:
B: 0.0003 to 0.003%; and
W: 0.05 to 1.0%.
14. An oil-well steel pipe comprising,
the steel material according to claim 13 , and
a wall thickness of 15 mm or more.
15. The steel material according to claim 13 , wherein the chemical composition contains one or more types of element selected from the group consisting of:
Ca: 0.0003 to 0.004%;
Mg: 0.0003 to 0.004%; and
rare earth metal: 0.0003 to 0.004%.
16. An oil-well steel pipe comprising,
the steel material according to claim 15 , and
a wall thickness of 15 mm or more.