Duplex stainless steel and manufacturing method thereof
View Patent ↗A duplex stainless steel containing C, Si, Mn, P, S, Al, Ni, Cr, Mo, N (nitrogen, O (oxygen), Ca, Mg, Cu, B, and W, and the balance Fe and impurities, where a number of oxide-based inclusions, which have a total content of Ca and Mg of 20 to 40% by mass and also have a long diameter of not less than 7 μm, is not more than a 10 per 1 mm 2 of the cross section perpendicular to the working direction, or further, the number of oxide-based inclusions, which have a content of S of not less than 15% by mass and also have a long diameter of not less than 1 μm, is not more than 10 per 0.1 mm 2 of the cross section perpendicular to the working direction. Particularly, the contents of Cu, B and W are desirably 0.2 to 2%, 0.001 to 0.01%, and 0.1 to 4% by mass, respectively.
1. A duplex stainless steel containing, by mass %, C: not more than 0.03%, Si: 0.01 to 2%, Mn: 0.1 to 2%, P: not more than 0.05%, S: not more than 0.001%, Al: 0.003 to 0.05%, Ni: 4 to 12%, Cr: 18 to 32%, Mo: 0.2 to 5%, N (nitrogen): 0.05 to 0.4%, O (oxygen): not more than 0.01%, Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%, Cu: 0 to 2%, B: 0 to 0.01%, and W: 0 to 4%, and the balance of Fe impurities, where a number of oxide-based inclusions, which have a total content of Ca and Mg of 20 to 40% by mass and also have a long diameter of not less than 7 μm, is not more than a 10 per 1 mm 2 of the cross section perpendicular to the working direction, wherein a pitting resistance index PREW represented by the following equation (1) is not less than 40:
PREW=Cr+3.3(Mo+0.5W)+16N (1)
wherein each chemical symbol represents the content of each element (% by mass).
2. A duplex stainless steel containing, by mass %, C: not more than 0.03%, Si: 0.01 to 2%, Mn: 0.1 to 2%, P: not more than 0.05%, S: not more than 0.001%, Al: 0.003 to 0.05%, Ni: 4 to 12%, Cr: 18 to 32%, Mo: 0.2 to 5%, N (nitrogen): 0.05 to 0.4%, O (oxygen): not more than 0.01%, Ca: 0.0005 to 0.005%, Mg: 0.0001 to 0.005%, Cu: 0 to 2%, B: 0 to 0.01%, and W: 0 to 4%, and the balance of Fe and impurities, where a number of oxide-based inclusions, which have a total content of Ca and Mg of 20 to 40% by mass and also have a long diameter of not less than 7 μm, is not more than a 10 per 1 mm 2 of the cross section perpendicular to the working direction, and a number of oxide-based inclusions, which have a content of S of not less than 15% by mass and also have a long diameter of not less than 1 μm, is not more than 10 per 0.1 mm 2 of the cross section perpendicular to the working direction,
wherein a pitting resistance index PREW represented by the followina equation (1) is not less than 40;
PREW=Cr+3.3(Mo+0.5W)+16N (1)
wherein each chemical symbol represents the content of each element (% by mass).
3. The duplex stainless steel according to claim 1 , further containing 0.2 to 2% of Cu by mass.
4. The duplex stainless steel according to claim 1 , further containing 0,001 to 0.01% of B by mass.
5. The duplex stainless steel according to claim 1 , further containing 0.1 to 4% of W by mass.
6. A method for producing a duplex stainless steel, according to claim 1 , characterized by treating stainless steel alloy in molten state to a reductive treatment carried out in a condition providing a slag basicity represented by the following equation (2) is 0.5 to 3.0, killing to tapped molten steel at the temperature not lower than 1500° C. for not less than 5 minutes followed by casting, and fanning the resulting bloom on the condition that the total working ratio R, represented by the following equation (3), is not less than 10;
[
Slag
Basicity
]
=
(
Ca
O
+
Mg
O
)
/
(
Al
2
O
3
+
Si
O
2
)
(
2
)
[
Total
working
ratio
R
]
=
∏
n
=
1
i
(
A
0
n
A
n
)
(
3
)
wherein each compound in the equation (2) represents the concentration in slag of each compound (% by mass), A0 n and A n in the equation (3) represent a cross-sectional area before deformation in a plastic deformation process and a cross-sectional area after deformation in the plastic deformation process, respectively, and each subscript n (1, 2, . . . i) represents each stand order in the plastic deformation process.
7. The duplex stainless steel according to claim 2 , further containing 0.2 to 2% of Cu by mass.
8. The duplex stainless steel according to claim 2 , further containing 0.001 to 0.0 1% of B by mass.
9. The duplex stainless steel according to claim 3 , further containing 0.001 to 0.01% of B by mass.
10. The duplex stainless steel according to claim 2 , further containing 0.1 to 4% of W by mass.
11. The duplex stainless steel according to claim 3 , further containing 0.1 to 4% of W by mass.
12. The duplex stainless steel according to claim 4 , further containing 0.1 to 4% of W by mass.
13. A method for producing a duplex stainless steel, according to claim 2 , characterized by treating stainless steel alloy in molten state to a reductive treatment carried out in a condition providing a slag basicity represented by the following equation (2) is 0.5 to 3.0, killing to tapped molten steel at the temperature not lower than 1500° C. for not less than 5 minutes followed by casting, and forming the resulting bloom on the condition that the total working ratio R, represented by the following equation (3), is not less than 10;
[
Slag
Basicity
]
=
(
Ca
O
+
Mg
O
)
/
(
Al
2
O
3
+
Si
O
2
)
(
2
)
[
Total
working
ratio
R
]
=
∏
n
=
1
i
(
A
0
n
A
n
)
(
3
)
wherein each compound in the equation (2) represents the concentration in slag of each compound (% by mass), A0 n and A n in the equation (3) represent a cross-sectional area before deformation in a plastic deformation process and a cross-sectional area after deformation in the plastic deformation process, respectively, and each subscript n (1, 2, . . . i) represents each stand order in the plastic deformation process.
14. A method for producing a duplex stainless steel, according to claim 3 , characterized by treating stainless steel alloy in molten state to a reductive treatment carried out in a condition providing a slag basicity represented by the following equation (2) is 0.5 to 3.0, killing to tapped molten steel at the temperature not lower than 1500° C. for not less than 5 minutes followed by casting, and forming the resulting bloom on the condition that the total working ratio R, represented by the following equation (3), is not less than 10;
[
Slag
Basicity
]
=
(
Ca
O
+
Mg
O
)
/
(
Al
2
O
3
+
Si
O
2
)
(
2
)
[
Total
working
ratio
R
]
=
∏
n
=
1
i
(
A
0
n
A
n
)
(
3
)
wherein each compound in the equation (2) represents the concentration in slag of each compound (% by mass), A0 n and A n in the equation (3) represent a cross-sectional area before deformation in a plastic deformation process and a cross-sectional area after deformation in the plastic deformation process, respectively, and each subscript n (1, 2, . . . i) represents each stand order in the plastic deformation process.
15. A method for producing a duplex stainless steel, according to claim 4 , characterized by treating stainless steel alloy in molten state to a reductive treatment carried out in a condition providing a slag basicity represented by the following equation (2) is 0.5 to 3.0, killing to tapped molten steel at the temperature not lower than 1500° C. for not less than 5 minutes followed by casting, and forming the resuldng bloom on the condition that the total working ratio R, represented by the following equation (3), is not less than 10;
[
Slag
Basicity
]
=
(
Ca
O
+
Mg
O
)
/
(
Al
2
O
3
+
Si
O
2
)
(
2
)
[
Total
working
ratio
R
]
=
∏
n
=
1
i
(
A
0
n
A
n
)
(
3
)
wherein each compound in the equation (2) represents the concentration in slag of each compound (% by mass), A0 n and A n in, the equation (3) represent a cross-sectional area before deformation in a plastic deformation process and a cross-sectional area after deformation in the plastic deformation process, respectively, and each subscript n (1, 2, . . . i) represents each stand order in the plastic defonnation process.
16. A method for producing a duplex stainless steel, according to claim 5 , characterized by treating stainless steel alloy in molten state to a reductive treatment carried out in a condition providing a slag basicity represented by the following equation (2) is 0.5 to 3.0, killing to tapped molten steel at the temperature not lower than 1500° C. for not less than 5 minutes followed by casting, and forming the resulting bloom on the condition that the total working ratio R, represented by the following equation (3), is not less than 10;
[
Slag
Basicity
]
=
(
Ca
O
+
Mg
O
)
/
(
Al
2
O
3
+
Si
O
2
)
(
2
)
[
Total
working
ratio
R
]
=
∏
n
=
1
i
(
A
0
n
A
n
)
(
3
)
wherein each compound in the equation (2) represents the concentration in slag of each compound (% by mass), A0 n and A n in the equation (3) represent a cross-sectional area before deformation in a plastic deformation process and a cross-sectional area after deformation in the plastic deformation process. respectively, and each subscript n (1, 2, . . . i) represents each stand order in the plastic deformation process.
17. The duplex stainless steel according to claim 9 , further containing 0.1 to 4% of W by mass.