Stainless steel and stainless steel product for oil well
View Patent ↗A stainless steel is provided having good corrosion resistance and good low-temperature toughness. A stainless steel contains, in mass %, Cr: 15.5 to 18.0%. The stainless steel has a matrix structure having, by volume ratio, 40 to 80% tempered martensite, 10 to 50% ferrite and 1 to 15% austenite. When a microstructure image obtained by photographing the matrix structure at a magnification of 100 times is positioned in an x-y coordinate system and each of 1024×1024 pixels is represented by a gray scale level, β defined by Equation (2) is not smaller than 1.55: 1.0≤Mo+0.5W≤3.5 (1). Here, Mo and W are the Mo and W contents in mass %. [ Formula 1 ] β = Su Sv ( 2 ) In Equation (2), Su is defined by Equation (3), and Sv is defined by Equation (4): [ Formula 2 ] Su = ∑ u = 1 1023 F ( u , 0 ) ( 3 ) Sv = ∑ v = 1 1023 F ( 0 , v ) ( 4 ) In Equations (3) and (4), F(u,v) is defined by Equation (5): [ Formula 3 ] F ( u , v ) = ∑ x = 0 1023 ∑ y = 0 1023 f ( x , y ) exp { - 2 π i ( ux 1024 + vy 1024 ) } ( 5 ) In Equation (5), f(x,y) represents the gray level of the pixel at coordinates (x,y).
1. A stainless steel having a chemical composition including, in mass %:
C: 0.001 to 0.06%;
Si: 0.05 to 0.5%;
Mn: 0.01 to 2.0%;
P: up to 0.03%;
S: less than 0.005%;
Cr: 15.5 to 18.0%;
Ni: 2.5 to 6.0%;
V: 0.005 to 0.25%;
Al: up to 0.05%;
N: up to 0.06%;
O: up to 0.01%;
Cu: 0 to 3.5%;
Co: 0 to 1.5%;
Nb: 0 to 0.25%;
Ti: 0 to 0.25%;
Zr: 0 to 0.25%;
Ta: 0 to 0.25%;
B: 0 to 0.005%;
Ca: 0 to 0.01%;
Mg: 0 to 0.01%; and
REM: 0 to 0.05%,
further including one or two selected from the group consisting of:
Mo: 0 to 3.5%; and
W: 0 to 3.5%
in an amount that satisfies Equation (1),
the balance being Fe and impurities,
wherein the stainless steel has a matrix structure having, by volume ratio, 40 to 80% tempered martensite, 10 to 50% ferrite and 1 to 15% austenite,
when a microstructure image with dimensions of 1 mm×1 mm obtained by photographing the matrix structure at a magnification of 100 times is positioned in an x-y coordinate system with an x-axis extending in a wall-thickness direction and a y-axis extending in a length direction and each of 1024×1024 pixels is represented by a gray scale level, β defined by Equation (2) is not smaller than 1.55:
1.0≤Mo+0.5 W ≤3.5 (1),
where Mo and W are the Mo and W contents in mass %,
[
Formula
1
]
β
=
Su
Sv
(
2
)
in Equation (2), Su is defined by Equation (3), and Sv is defined by Equation (4):
[
Formula
2
]
Su
=
∑
u
=
1
1023
F
(
u
,
0
)
(
3
)
Sv
=
∑
v
=
1
1023
F
(
0
,
v
)
(
4
)
in Equations (3) and (4), F(u,v) is defined by Equation (5):
[
Formula
3
]
F
(
u
,
v
)
=
∑
x
=
0
1023
∑
y
=
0
1023
f
(
x
,
y
)
exp
{
-
2
π
i
(
ux
1024
+
vy
1024
)
}
(
5
)
in Equation (5), f(x,y) represents the gray level of the pixel at coordinates (x,y).
2. The stainless steel according to claim 1 , wherein the chemical composition includes one or two selected from the group consisting of, in mass %:
Cu: 0.2 to 3.5%; and
Co: 0.05 to 1.5%.
3. The stainless steel according to claim 1 , wherein the chemical composition includes one or more selected from the group consisting of, in mass %:
Nb: 0.01 to 0.25%;
Ti: 0.01 to 0.25%;
Zr: 0.01 to 0.25%; and
Ta: 0.01 to 0.25%.
4. The stainless steel according to claim 1 , wherein the chemical composition includes one or more selected from the group consisting of, in mass %:
B: 0.0003 to 0.005%;
Ca: 0.0005 to 0.01%;
Mg: 0.0005 to 0.01%; and
REM: 0.0005 to 0.05%.
5. A stainless steel product for an oil well made of the stainless steel according to claim 1 .
6. The stainless steel according to claim 2 , wherein the chemical composition includes one or more selected from the group consisting of, in mass %:
Nb: 0.01 to 0.25%;
Ti: 0.01 to 0.25%;
Zr: 0.01 to 0.25%; and
Ta: 0.01 to 0.25%.
7. The stainless steel according to claim 2 , wherein the chemical composition includes one or more selected from the group consisting of, in mass %:
B: 0.0003 to 0.005%;
Ca: 0.0005 to 0.01%;
Mg: 0.0005 to 0.01%; and
REM: 0.0005 to 0.05%.
8. . A stainless steel product for an oil well made of the stainless steel according to claim 2 .
9. . The stainless steel according to claim 3 , wherein the chemical composition includes one or more selected from the group consisting of, in mass %:
B: 0.0003 to 0.005%;
Ca: 0.0005 to 0.01%;
Mg: 0.0005 to 0.01%; and
REM: 0.0005 to 0.05%.
10. . A stainless steel product for an oil well made of the stainless steel according to claim 3 .
11. . A stainless steel product for an oil well made of the stainless steel according to claim 4 .
12. . The stainless steel according to claim 6 , wherein the chemical composition includes one or more selected from the group consisting of, in mass %:
B: 0.0003 to 0.005%;
Ca: 0.0005 to 0.01%;
Mg: 0.0005 to 0.01%; and
REM: 0.0005 to 0.05%.
13. . A stainless steel product for an oil well made of the stainless steel according to claim 6 .
14. . A stainless steel product for an oil well made of the stainless steel according to claim 7 .
15. A stainless steel product for an oil well made of the stainless steel according to claim 9 .
16. A stainless steel product for an oil well made of the stainless steel according to claim 12 .