Martensitic Cr-containing steel and oil country tubular goods
A martensitic Cr-containing steel having excellent corrosion resistance, SSC resistance, and IGHIC resistance is provided. A martensitic Cr-containing steel according to the present invention includes: a chemical composition consisting of, by mass %, Si: 0.05 to 1.0%, Mn: 0.1 to 1.0%, Cr: 8 to 12%, V: 0.01 to 1.0%, sol. Al: 0.005 to 0.10%, with the balance being Fe and impurities, wherein an effective Cr amount defined by “Cr−16.6×C” is not less than 8%, and an Mo equivalent defined by “Mo+0.5×W” is 0.03 to 2%; a micro-structure wherein a grain size number of prior-austenite crystal grain is not less than 8.0; and a yield strength of less than 379 to 551 MPa, wherein a grain-boundary segregation ratio of Mo and W is not less than 1.5.
1. A martensitic Cr-containing steel, comprising:
a chemical composition consisting of, by mass %,
Si: 0.05 to 1.00%,
Mn: 0.1 to 1.0%,
Cr: 8 to 12%,
V: 0.01 to 1.0%,
sol. Al: 0.005 to 0.10%,
N: not more than 0.100%,
Nb: 0 to 1%,
Ti: 0 to 1%,
Zr: 0 to 1%,
B: 0 to 0.01%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%, and
rare earth metal (REM): 0 to 0.50%, and further consisting of one or two selected from the group consisting of
Mo: 0 to 2% and
W: 0 to 4%, with the balance being Fe and impurities, wherein the impurities include
C: not more than 0.10%,
P: not more than 0.03%,
S: not more than 0.01%,
Ni: not more than 0.5%, and
O: not more than 0.01%, and wherein
an effective Cr amount defined by Formula (1) is not less than 8%, and
an Mo equivalent defined by Formula (2) is 0.03 to 2%;
a micro-structure wherein a grain size number (ASTM E112) of prior-austenite crystal grain is not less than 8.0, and which consists of, in volume fraction, 0 to 5% of ferrite and 0 to 5% of austenite, with the balance being tempered martensite; and
a yield strength of 379 to less than 551 MPa, wherein
a grain-boundary segregation ratio, which is defined, when either one of Mo and W is contained, as a ratio of a maximum content at grain boundaries to an average content within grains of the contained element, and when Mo and W are contained, as an average of ratios of a maximum content at grain boundaries to an average content within grains of each element, is not less than 1.5:
Effective Cr amount=Cr−16.6×C (1)
Mo equivalent=Mo+0.5×W (2)
where, symbols of elements in Formulae (1) and (2) are substituted by corresponding contents (by mass %) of the elements.
2. The martensitic Cr-containing steel according to claim 1 , wherein:
the chemical composition contains one or more selected from the group consisting of
Nb: 0.01 to 1%,
Ti: 0.01 to 1%, and
Zr: 0.01 to 1%.
3. The martensitic Cr-containing steel according to claim 1 , wherein:
the chemical composition contains B: 0.0003 to 0.01%.
4. The martensitic Cr-containing steel according to claim 2 , wherein:
the chemical composition contains B: 0.0003 to 0.01%.
5. The martensitic Cr-containing steel according to claims 1 , wherein:
the chemical composition contains one or more selected from the group consisting of
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%, and
REM: 0.0001 to 0.50%.
6. The martensitic Cr-containing steel according to claim 2 , wherein:
the chemical composition contains one or more selected from the group consisting of
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%, and
REM: 0.0001 to 0.50%.
7. The martensitic Cr-containing steel according to claim 3 , wherein:
the chemical composition contains one or more selected from the group consisting of
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%, and
REM: 0.0001 to 0.50%.
8. The martensitic Cr-containing steel according to claim 4 , wherein:
the chemical composition contains one or more selected from the group consisting of
Ca: 0.0001 to 0.01%,
Mg: 0.0001 to 0.01%, and
REM: 0.0001 to 0.50%.
9. Oil country tubular goods, wherein:
the oil country tubular goods are produced using the martensitic Cr-containing steel according to claim 1 .
10. Oil country tubular goods, wherein:
the oil country tubular goods are produced using the martensitic Cr-containing steel according to claim 2 .
11. Oil country tubular goods, wherein:
the oil country tubular goods are produced using the martensitic Cr-containing steel according to claim 3 .
12. Oil country tubular goods, wherein:
the oil country tubular goods are produced using the martensitic Cr-containing steel according to claim 4 .
13. Oil country tubular goods, wherein:
the oil country tubular goods are produced using the martensitic Cr-containing steel according to claim 5 .
14. Oil country tubular goods, wherein:
the oil country tubular goods are produced using the martensitic Cr-containing steel according to claim 6 .
15. Oil country tubular goods, wherein:
the oil country tubular goods are produced using the martensitic Cr-containing steel according to claim 7 .
16. Oil country tubular goods, wherein:
the oil country tubular goods are produced using the martensitic Cr-containing steel according to claim 8 .