EP4600380A1

EP4600380A1 - Duplex stainless steel material

Info

Publication number: EP4600380A1
Application number: EP23874887.5A
Authority: EP
Inventors: Akiko Tomio; Yusaku TOMIO
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-10-06
Filing date: 2023-10-04
Publication date: 2025-08-13
Also published as: EP4600380A4; AR130673A1; JPWO2024075761A1; MX2025003521A; WO2024075761A1; CN119998479A; JP7486013B1

Abstract

A duplex stainless steel material that has excellent general corrosion resistance in a high-temperature and high-pressure strongly acidic corrosive environment, and has excellent pitting resistance in a high-temperature and high-pressure chloride corrosive environment is provided. The duplex stainless steel material of the present disclosure has a chemical composition consisting of, in mass%, C: 0.050%) or less, Si: 0.2 to 1.2%, Mn: 0.5 to 7.0%, P: 0.040% or less, S: 0.010% or less, Cr: 20.0 to 27.0%, Ni: 4.0 to 9.0%, Mo: 0.5 to 5.0%, As: 0.0005 to 0.0100%, one or more types of element among Ca and Mg: 0.0005 to 0.0100% in total, sol. Al: 0.001 to 0.050%, N: 0.40% or less, and O: 0.100% or less, with the balance being Fe and impurities, and satisfies Formulae (1) and (2).

0.70 < 10000 \times As / (Ni + Cu) < 16.00

(Ca + Mg) / O < 1.50

Description

TECHNICAL FIELD

The present disclosure relates to a duplex stainless steel material.

BACKGROUND ART

Geothermal power generation is attracting attention as one type of low carbon energy. In the geothermal power generation, steam is generated using a geothermal fluid extracted from a geothermal well in which high-temperature and high-pressure hot water has been accumulated. The term "geothermal fluid" means high-temperature and high-pressure hot water and steam. The steam is supplied to a steam turbine to generate electric power.
Recently, deep geothermal wells at deeper layers than in the past is being developed. Steam that is obtained from such deep geothermal wells contains hydrogen sulfide (H₂S) and carbon dioxide (CO₂), and also contains a reducing acid such as sulfuric acid (H₂SO₄) and/or hydrochloric acid (HCl). In some cases, a pipe used to extract such kind of steam as well as a pipe which conveys such kind of steam becomes an environment with a high temperature of 180°C and a high pressure of 5 bar. Therefore, a strongly acidic aqueous solution containing sulfuric acid and/or chloride ions is generated under the high-temperature and high-pressure environment within the pipe. As mentioned above, the steam in the pipe contains highly-corrosive hydrogen sulfide (H₂S). Therefore, the inside of the pipe that is used for geothermal power generation becomes an extremely severe corrosive environment.
In an environment containing hydrogen sulfide and a reducing acid, general corrosion is the dominant cause of corrosion. Further, in an environment containing hydrogen sulfide and chloride ions, pitting is the dominant cause of corrosion. Therefore, in order to have excellent corrosion resistance in the aforementioned high-temperature and high-pressure corrosive environment, it is necessary to have excellent general corrosion resistance in a high-temperature and high-pressure strongly acidic corrosive environment which is an environment at a high temperature of 180°C and a high pressure of 5 bar and which contains hydrogen sulfide and sulfuric acid, and to have excellent pitting resistance in a high-temperature and high-pressure chloride corrosive environment which is an environment at a high temperature of 180°C and a high pressure of 5 bar and which contains hydrogen sulfide and chloride ions.
An alloy with which excellent corrosion resistance can be obtained in a strongly acidic environment containing a reducing acid is proposed in International Application Publication No. WO2009/119630 (Patent Literature 1). The alloy disclosed in Patent Literature 1 is an Ni alloy containing, in mass%, C: 0.03% or less, Si: 0.01 to 0.5%, Mn: 0.01 to 1.0%, P: 0.03% or less, S: 0.01% or less, Cr: 20% or more to less than 30%, Ni: more than 40% to 60% or less, Cu: more than 2.0% to 5.0% or less, Mo: 4.0 to 10%, Al: 0.005 to 0.5%, and N: more than 0.02% to 0.3% or less, and which satisfies the formula 0.5Cu + Mo ≥ 6.5.
Further, a duplex stainless steel material with which excellent corrosion resistance can be obtained in a corrosive environment at a temperature of about 150°C that contains hydrogen sulfide and chloride ions is proposed in International Application Publication No. WO2013/035588 (Patent Literature 2). The duplex stainless steel material disclosed in Patent Literature 2 consists of, in mass%, C: 0.03% or less, Si: 0.2 to 1%, Mn: more than 50% to 10% or less, P: 0.040% or less, S: 0.010% or less, Ni: 4.5 to 8%, sol. Al: 0.040% or less, N: more than 0.2% to 0.4% or less, Cr: 24 to 29%, Mo: 0.5 to less than 1.5%, Cu: 1.5 to 3.5%, and W: 0.05 to 0.2%, with the balance being Fe and impurities, and which satisfies the formula Cr + 8Ni + Cu + Mo + W/2 ≥ 65.

CITATION LIST

PATENT LITERATURE

Patent Literature 1: International Application Publication No. WO2009/119630
Patent Literature 2: International Application Publication No. WO2013/035588

SUMMARY OF INVENTION

TECHNICAL PROBLEM

However, in Patent Literature 1 and Patent Literature 2, the aforementioned general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment, and the aforementioned pitting resistance in the high-temperature and high-pressure chloride corrosive environment are not discussed.
An objective of the present disclosure is to provide a duplex stainless steel material with which excellent general corrosion resistance in a high-temperature and high-pressure strongly acidic corrosive environment can be obtained, and excellent pitting resistance in a high-temperature and high-pressure chloride corrosive environment can be obtained.

SOLUTION TO PROBLEM

A duplex stainless steel material of the present disclosure has a chemical composition consisting of, in mass%,

C: 0.050% or less,
Si: 0.2 to 1.2%,
Mn: 0.5 to 7.0%,
P: 0.040% or less,
S: 0.010% or less,
Cr: 20.0 to 27.0%,
Ni: 4.0 to 9.0%,
Mo: 0.5 to 5.0%,
As: 0.0005 to 0.0100%,
one or more types of element among Ca and Mg: 0.0005 to 0.0100% in total,
sol. Al: 0.001 to 0.050%,
N: 0.40% or less,
O: 0.100% or less,
Cu: 0 to 4.0%,
V: 0 to 1.50%,
Co: 0 to 2.00%,
Ta: 0 to 2.00%,
W: 0 to 4.00%,
Nb: 0 to 2.00%,
Ti: 0 to 2.00%,
Zn: 0 to 0.0100%,
Pb: 0 to 0.0100%,
Sb: 0 to 0.0100%,
Sn: 0 to 0.0100%,
Bi: 0 to 0.0100%,
B: 0 to 0.0100%,
rare earth metal: 0 to 0.050%,
Zr: 0 to 2.00%, and
Hf: 0 to 2.00%, with
the balance being Fe and impurities,
and satisfies Formulae (1) and (2): $0.70 < 10000 \times As / (Ni + Cu) < 16.00$ $(Ca + Mg) / O < 1.50$
where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formulae, and if an element is not contained, "0" is substituted for a corresponding symbol of an element.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the duplex stainless steel material of the present disclosure, excellent general corrosion resistance in a high-temperature and high-pressure strongly acidic corrosive environment can be obtained, and excellent pitting resistance in a high-temperature and high-pressure chloride corrosive environment can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a view illustrating the relation between Fnl and a corrosion rate (g•cm^-2•h^-1) in a high-temperature and high-pressure strongly acidic corrosive environment with respect to duplex stainless steel materials.
[FIG. 2] FIG. 2 is a view illustrating the relation between Fn2 and a corrosion rate (g•cm^-2•h^-1) in a high-temperature and high-pressure chloride corrosive environment with respect to duplex stainless steel materials.

DESCRIPTION OF EMBODIMENT

In the present description, a high-temperature and high-pressure strongly acidic corrosive environment, and a high-temperature and high-pressure chloride corrosive environment are defined as follows.
High-temperature and high-pressure strongly acidic corrosive environment: an environment at a high temperature of 180°C and a high pressure of 5 bar which is an environment containing hydrogen sulfide and sulfuric acid
High-temperature and high-pressure chloride corrosive environment: an environment at a high temperature of 180°C and a high pressure of 5 bar which is an environment containing hydrogen sulfide and chloride ions
The present inventors conducted studies from a viewpoint of a chemical composition with respect to a duplex stainless steel material with which excellent general corrosion resistance can be obtained in the high-temperature and high-pressure strongly acidic corrosive environment, and excellent pitting resistance can be obtained in the high-temperature and high-pressure chloride corrosive environment. As a result, the present inventors considered that if a duplex stainless steel material has a chemical composition consisting of, in mass%, C: 0.050% or less, Si: 0.2 to 1.2%, Mn: 0.5 to 7.0%, P: 0.040% or less, S: 0.010% or less, Cr: 20.0 to 27.0%, Ni: 4.0 to 9.0%, Mo: 0.5 to 5.0%, one or more types of element among Ca and Mg: 0.0005 to 0.0100% in total, sol. Al: 0.001 to 0.050%, N: 0.40% or less, O: 0.100% or less, Cu: 0 to 4.0%, V: 0 to 1.50%, Co: 0 to 2.00%, Ta: 0 to 2.00%, W: 0 to 4.00%, Nb: 0 to 2.00%, Ti: 0 to 2.00%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, Sb: 0 to 0.0100%, Sn: 0 to 0.0100%, Bi: 0 to 0.0100%, B: 0 to 0.0100%, rare earth metal: 0 to 0.050%, Zr: 0 to 2.00%, and Hf: 0 to 2.00%, with the balance being Fe and impurities, there is a possibility that excellent general corrosion resistance can be obtained in the high-temperature and high-pressure strongly acidic corrosive environment and excellent pitting resistance can be obtained in the high-temperature and high-pressure chloride corrosive environment.
However, when the duplex stainless steel material having the aforementioned chemical composition was applied in the high-temperature and high-pressure strongly acidic corrosive environment, in some cases excellent general corrosion resistance could not be obtained. Therefore the present inventors conducted further studies. As a result, the present inventors discovered that arsenic (As) increases general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. Therefore, as a result of conducting further studies, the present inventors considered that if the duplex stainless steel material satisfies Feature 1 which is that the duplex stainless steel material contains 0.0005 to 0.0100% of As in lieu of a part of Fe of the aforementioned chemical composition, there is a possibility that excellent general corrosion resistance will be obtained in the high-temperature and high-pressure strongly acidic corrosive environment.

(Feature 1)

The chemical composition consists of, in mass%, C: 0.050% or less, Si: 0.2 to 1.2%, Mn: 0.5 to 7.0%, P: 0.040% or less, S: 0.010% or less, Cr: 20.0 to 27.0%, Ni: 4.0 to 9.0%, Mo: 0.5 to 5.0%, As: 0.0005 to 0.0100%, one or more types of element among Ca and Mg: 0.0005 to 0.0100% in total, sol. Al: 0.001 to 0.050%, N: 0.40% or less, O: 0.100% or less, Cu: 0 to 4.0%, V: 0 to 1.50%, Co: 0 to 2.00%, Ta: 0 to 2.00%, W: 0 to 4.00%, Nb: 0 to 2.00%, Ti: 0 to 2.00%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, Sb: 0 to 0.0100%, Sn: 0 to 0.0100%, Bi: 0 to 0.0100%, B: 0 to 0.0100%, rare earth metal: 0 to 0.050%, Zr: 0 to 2.00%, and Hf: 0 to 2.00%, with the balance being Fe and impurities.
However, even when using the duplex stainless steel material which satisfied Feature 1, there were still some cases where excellent general corrosion resistance could not be obtained in the high-temperature and high-pressure strongly acidic corrosive environment. Further, even when using the duplex stainless steel material which satisfied Feature 1, in some cases excellent pitting resistance could not be obtained in the high-temperature and high-pressure chloride corrosive environment.
Therefore, the present inventors conducted further studies regarding means for increasing general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment and for increasing pitting resistance in the high-temperature and high-pressure chloride corrosive environment with respect to the duplex stainless steel material satisfying Feature 1. As a result, the present inventors discovered that if the duplex stainless steel material which satisfies Feature 1 also satisfies Feature 2 and Feature 3, excellent general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment can be obtained and excellent pitting resistance in the high-temperature and high-pressure chloride corrosive environment can be also obtained.

(Feature 2)

The chemical composition satisfies Formula (1). $0.70 < 10000 \times As / (Ni + Cu) < 16.00$
Where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula. If an element is not contained, "0" is substituted for the corresponding symbol of an element.

(Feature 3)

The chemical composition satisfies Formula (2). $(Ca + Mg) / O < 1.50$
Where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula. If an element is not contained, "0" is substituted for the corresponding symbol of an element.

[Regarding Feature 2]

With respect to Feature 2, Fn1 is defined as follows. $Fn 1 = 10000 \times As / (Ni + Cu)$
Where, in a case where the chemical composition of the duplex stainless steel material of the present embodiment consists of essential elements, "0" is substituted for Cu in Fn1, and Fn1 = 10000 × As/Ni.
Fn1 is an index relating to general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. Each of As, Ni, and Cu increases the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. In addition, the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment is markedly increased by adjusting a ratio of the content of As to a total content of Ni and Cu.
FIG. 1 is a view illustrating the relation between Fn1 and general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment with respect to duplex stainless steel materials satisfying Feature 1 and Feature 3. FIG. 1 was created based on results obtained by a test to evaluate general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment conducted in Examples that are described later.
Referring to FIG. 1, when Fn1 is 0.70 or less, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment becomes markedly faster, and excellent general corrosion resistance cannot be obtained. On the other hand, when Fn1 is higher than 0.70, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment becomes markedly slower, and the general corrosion resistance markedly increases. Therefore, if Fn1 satisfies Formula (1), on the precondition that the duplex stainless steel material satisfies Feature 1 and Feature 3, excellent general corrosion resistance can be obtained in the high-temperature and high-pressure strongly acidic corrosive environment. Note that, if Fn1 is too high, the hot workability of the duplex stainless steel material will decrease. Therefore, an upper limit of Fn1 is set to less than 16.00.

[Regarding Feature 3]

With respect to Feature 3, Fn2 is defined as follows. $Fn 2 = (Ca + Mg) / O$
Fn2 is an index relating to pitting resistance in the high-temperature and high-pressure chloride corrosive environment. Ca and Mg suppress formation of coarse Mn sulfides by combining with S to form sulfides. When coarse Mn sulfides are present in an outer layer of the steel material, in the high-temperature and high-pressure chloride corrosive environment the coarse Mn sulfides in the outer layer dissolve and pitting easily occurs. Therefore, Ca and Mg increase the pitting resistance in the high-temperature and high-pressure chloride corrosive environment.
However, in a case where a content of S in the steel material is within the range (0.010% or less) described in Feature 1, if a total content of Ca and Mg with respect to a content of O is too high, Ca and Mg will combine not only with S but also with O, and consequently coarse Ca oxysulfides and Mg oxides will be excessively formed. Similarly to coarse Mn sulfides, coarse Ca oxysulfides and coarse Mg oxides are liable to dissolve in the high-temperature and high-pressure chloride corrosive environment, and are liable to become starting points for pitting.
FIG. 2 is a view illustrating the relation between Fn2 and pitting resistance in the high-temperature and high-pressure chloride corrosive environment with respect to duplex stainless steel materials satisfying Feature 1 and Feature 2. FIG. 2 was created based on results obtained by a test to evaluate pitting resistance in the high-temperature and high-pressure chloride corrosive environment conducted in Examples that are described later.
Referring to FIG. 2, when Fn2 is 1.50 or more, even if the duplex stainless steel material satisfies Feature 1 and Feature 2, the corrosion rate is high and the pitting resistance in the high-temperature and high-pressure chloride corrosive environment is low. On the other hand, when Fn2 is less than 1.50, on the precondition that the duplex stainless steel material satisfies Feature 1 and Feature 2, in the high-temperature and high-pressure chloride corrosive environment, the corrosion rate becomes markedly slower and excellent pitting resistance can be obtained.
The duplex stainless steel material of the present embodiment was completed based on the technical ideas described above, and is as follows.
A duplex stainless steel material according to a first configuration has a chemical composition consisting of, in mass%,

A duplex stainless steel material according to a second configuration is the duplex stainless steel material according to the first configuration, wherein
when:

a particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a total of a content of Ca and a content of S is higher than 5.0%, a content of O is 1.0% or more, and a content of Ca is higher than a content of S is defined as a fine Ca oxysulfide;
a particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a content of Mg is 5.0% or more, a content of O is 1.0% or more, and a content of S is 15.0% or less is defined as a fine Mg oxide;
a particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a content of Al is 20.0% or more and a content of N is 20.0% or more is defined as a fine Al nitride; and
a particle which has an equivalent circular diameter of 1.0 to 2.0 µm, and in which, in mass%, a content of Ti is 30.0% or more and a content of N is 20.0% or more is defined as a fine Ti nitride,
a total number density of the fine Ca oxysulfides, the fine Mg oxides, the fine Al nitrides, and the fine Ti nitrides is 2.00 pieces/mm² or more.

A duplex stainless steel material according to a third configuration is the duplex stainless steel material according to the first or second configuration, wherein
the chemical composition contains one or more types of element selected from a group consisting of:

Cu: 0.1 to 4.0%,
V: 0.01 to 1.50%,
Co: 0.01 to 2.00%,
Ta: 0.01 to 2.00%,
W: 0.01 to 4.00%,
Nb: 0.01 to 2.00%,
Ti: 0.01 to 2.00%,
Zn: 0.0001 to 0.0100%,
Pb: 0.0001 to 0.0100%,
Sb: 0.0001 to 0.0100%,
Sn: 0.0001 to 0.0100%,
Bi: 0.0001 to 0.0100%,
B: 0.0001 to 0.0100%,
rare earth metal: 0.001 to 0.050%,
Zr: 0.01 to 2.00%, and
Hf: 0.01 to 2.00%.

Hereunder, the duplex stainless steel material according to the present embodiment is described in detail. Note that, the symbol "%" in relation to an element means mass percent unless otherwise stated. Further, in the following description, the duplex stainless steel material is also referred to as simply a "steel material".

[Features of duplex stainless steel material according to present embodiment]

The duplex stainless steel material according to the present embodiment satisfies the following Feature 1 to Feature 3.

(Feature 1)

The chemical composition consists of, in mass%, C: 0.050% or less, Si: 0.2 to 1.2%, Mn: 0.5 to 7.0%, P: 0.040% or less, S: 0.010% or less, Cr: 20.0 to 27.0%, Ni: 4.0 to 9.0%, Mo: 0.5 to 5.0%, As: 0.0005 to 0.0100%, one or more types of element among Ca and Mg: 0.0005 to 0.0100% in total, sol. Al: 0.001 to 0.050%, N: 0.40% or less, O: 0.100% or less, Cu: 0 to 4.0%, V: 0 to 1.50%, Co: 0 to 2.00%, Ta: 0 to 2.00%, W: 0 to 4.00%, Nb: 0 to 2.00%, Ti: 0 to 2.00%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, Sb: 0 to 0.0100%, Sn: 0 to 0.0100%, Bi: 0 to 0.0100%, B: 0 to 0.0100%, rare earth metal: 0 to 0.050%, Zr: 0 to 2.00%, and Hf: 0 to 2.00%, with the balance being Fe and impurities.

(Feature 2)

(Feature 3)

The chemical composition satisfies Formula (2). $(Ca + Mg) / O < 1.50$
Where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula. If an element is not contained, "0" is substituted for the corresponding symbol of an element.
Feature 1 to Feature 3 are described hereunder.

[(Feature 1) Regarding chemical composition]

The chemical composition of the duplex stainless steel material of the present embodiment contains the following elements.

C: 0.050% or less

Carbon (C) is unavoidably contained. That is, the content of C is more than 0%.
C forms Cr carbides at crystal grain boundaries and increases corrosion susceptibility at the grain boundaries. Therefore, if the content of C is more than 0.050%, even if the contents of other elements are within the range of the present embodiment, excellent general corrosion resistance will not be obtained in the high-temperature and high-pressure strongly acidic corrosive environment.
Accordingly, the content of C is to be 0.050% or less.
The content of C is preferably as low as possible. However, if the content of C is lowered excessively, the production cost will significantly increase. Therefore, when industrial production is taken into consideration, a preferable lower limit of the content of C is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.
A preferable upper limit of the content of C is 0.048%, more preferably is 0.046%, further preferably is 0.044%, and further preferably is 0.042%.

Si: 0.2 to 1.2%

Silicon (Si) deoxidizes the steel in the process for producing the steel material. If the content of Si is less than 0.2%, the aforementioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of Si is more than 1.2%, the toughness and hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Si is to be 0.2 to 1.2%.
A preferable lower limit of the content of Si is 0.3%, more preferably is 0.4%, and further preferably is 0.5%.
A preferable upper limit of the content of Si is 1.1%, more preferably is 1.0%, and further preferably is 0.9%.

Mn: 0.5 to 7.0%

Manganese (Mn) increases the hardenability of the steel material, thereby increases the strength of the steel material. If the content of Mn is less than 0.5%, the aforementioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of Mn is more than 7.0%, Mn will form a large number of coarse Mn sulfides. In the high-temperature and high-pressure chloride corrosive environment, coarse Mn sulfides that are present in the vicinity of the surface of the steel material will dissolve. Hollows will be formed at parts where the coarse Mn sulfides dissolved. The hollows will act as starting points for pitting. Consequently, even if the contents of other elements are within the range of the present embodiment, excellent pitting resistance will not be obtained in the high-temperature and high-pressure chloride corrosive environment.
Accordingly, the content of Mn is to be 0.5 to 7.0%.
A preferable lower limit of the content of Mn is 0.6%, more preferably is 0.7%, and further preferably is 0.8%.
A preferable upper limit of the content of Mn is 6.8%, more preferably is 6.0%, further preferably is 5.5%, further preferably is 4.5%, further preferably is 3.5%, further preferably is 2.5%, and further preferably is 2.0%.

P: 0.040% or less

Phosphorus (P) is an impurity that is unavoidably contained. That is, the content of P is more than 0%.
If the content of P is more than 0.040%, P will segregate excessively at grain boundaries. Consequently, even if the contents of other elements are within the range of the present embodiment, the toughness of the steel material will decrease.
Accordingly, the content of P is to be 0.040% or less.
The content of P is preferably as low as possible. However, if the content of P is lowered excessively, the production cost will significantly increase. Therefore, when industrial production is taken into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%.
A preferable upper limit of the content of P is 0.035%, more preferably is 0.030%, further preferably is 0.026%, and further preferably is 0.022%.

S: 0.010% or less

Sulfur (S) is an impurity that is unavoidably contained. That is, the content of S is more than 0%.
If the content of S is more than 0.010%, S will segregate excessively at grain boundaries. Consequently, even if the contents of other elements are within the range of the present embodiment, the toughness and hot workability of the steel material will decrease.
Accordingly, the content of S is to be 0.010% or less.
The content of S is preferably as low as possible. However, if the content of S is lowered excessively, the production cost will significantly increase. Therefore, when industrial production is taken into consideration, a preferable lower limit of the content of S is 0.001%, and more preferably is 0.002%.
A preferable upper limit of the content of S is 0.009%, more preferably is 0.008%, and further preferably is 0.007%.

Cr: 20.0 to 27.0%

Chromium (Cr) forms a passive film that is an oxide on the surface of the steel material. As a result, the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment is increased. In addition, the pitting resistance in the high-temperature and high-pressure chloride corrosive environment is increased. If the content of Cr is less than 20.0%, the aforementioned effects cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of Cr is more than 27.0%, intermetallic compounds typified by sigma phase (σ phase) will be liable to form. Consequently, even if the contents of other elements are within the range of the present embodiment, the toughness of the steel material will decrease.
Accordingly, the content of Cr is to be 20.0 to 27.0%.
A preferable lower limit of the content of Cr is 20.2%, more preferably is 20.5%, further preferably is 21.0%, and further preferably is 21.5%.
A preferable upper limit of the content of Cr is 26.8%, more preferably is 26.6%, further preferably is 26.4%, and further preferably is 26.2%.

Ni: 4.0 to 9.0%)

Nickel (Ni) increases the general corrosion resistance of the steel material in the high-temperature and high-pressure strongly acidic corrosive environment. If the content of Ni is less than 4.0%, the aforementioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of Ni is more than 9.0%, a volume ratio of austenite will be too high. In such a case, the strength of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Ni is to be 4.0 to 9.0%.
A preferable lower limit of the content of Ni is 4.2%, more preferably is 4.4%, further preferably is 4.6%, and further preferably is 4.8%.
A preferable upper limit of the content of Ni is 8.8%, more preferably is 8.6%, further preferably is 8.2%, further preferably is 7.9%, further preferably is 7.8%, further preferably is 7.7%, and further preferably is 7.6%.

Mo: 0.5 to 5.0%

Molybdenum (Mo) increases the pitting resistance of the steel material in the high-temperature and high-pressure chloride corrosive environment. If the content of Mo is less than 0.5%, the aforementioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of Mo is more than 5.0%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Mo is to be 0.5 to 5.0%.
A preferable lower limit of the content of Mo is 0.7%, more preferably is 1.0%, further preferably is 1.5%, further preferably is 2.0%, further preferably is 2.4%, further preferably is 2.6%, and further preferably is 2.8%).
A preferable upper limit of the content of Mo is 4.8%, more preferably is 4.6%, further preferably is 4.4%, and further preferably is 4.2%.

As: 0.0005 to 0.0100%

Arsenic (As) increases the general corrosion resistance of the steel material in the high-temperature and high-pressure strongly acidic corrosive environment. If the content of As is less than 0.0005%, the aforementioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of As is more than 0.0100%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of As is to be 0.0005 to 0.0100%.
A preferable lower limit of the content of As is 0.0010%, more preferably is 0.0015%, further preferably is 0.0020%, and further preferably is 0.0025%.
A preferable upper limit of the content of As is 0.0090%, more preferably is 0.0080%, further preferably is 0.0070%, and further preferably is 0.0060%.
One or more types of element among Ca and Mg: 0.0005 to 0.0100% in total
Calcium (Ca) and magnesium (Mg) form fine Ca oxysulfides or fine Mg oxides. The fine Ca oxysulfides and fine Mg oxides function as segregation sites for As at the interface with a parent phase. As a result of the fine Ca oxysulfides and fine Mg oxides dispersing in the steel material, As that segregates on the surfaces of these fine particles also disperses in the steel material. As a result, the general corrosion resistance of the steel material in the high-temperature and high-pressure strongly acidic corrosive environment is increased. If the total content of Ca and Mg is less than 0.0005%, the aforementioned effect cannot be sufficiently obtained.
On the other hand, if the total content of Ca and Mg is more than 0.0100%, coarse Ca oxysulfides or coarse Mg oxides will form. In the high-temperature and high-pressure chloride corrosive environment, coarse Ca oxysulfides and coarse Mg oxides that formed in an outer layer of the steel material will easily dissolve. Therefore, even if the contents of other elements are within the range of the present embodiment, the pitting resistance of the steel material in the high-temperature and high-pressure chloride corrosive environment will decrease.
Accordingly, the total content of Ca and Mg is to be 0.0005 to 0.0100%.
A preferable lower limit of the total content of Ca and Mg is 0.0010%, more preferably is 0.0015%, further preferably is 0.0020%, further preferably is 0.0025%, and further preferably is 0.0030%.
A preferable upper limit of the total content of Ca and Mg is 0.0095%, more preferably is 0.0090%, further preferably is 0.0085%, further preferably is 0.0080%, and further preferably is 0.0075%.

Sol. A1: 0.001 to 0.050%

Aluminum (Al) deoxidizes the steel in the process for producing the steel material. Al also combines with N to form fine Al nitrides. The fine Al nitrides function as segregation sites for As. Therefore, if a large number of fine Al nitrides are formed and are dispersed in the steel material, it will be even easier for As to disperse in the steel material. As a result, the general corrosion resistance of the steel material in the high-temperature and high-pressure strongly acidic corrosive environment will increase. If the content of sol. Al is less than 0.001%, the aforementioned effect cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.
On the other hand, if the content of sol. Al is more than 0.050%, coarse oxides will excessively form. Therefore, even if the contents of other elements are within the range of the present embodiment, the toughness of the steel material will decrease.
Accordingly, the content of sol. Al is to be 0.001 to 0.050%.
A preferable lower limit of the content of sol. Al is 0.002%, more preferably is 0.005%, and further preferably is 0.010%.
A preferable upper limit of the content of sol. Al is 0.045%, more preferably is 0.040%, further preferably is 0.035%, and further preferably is 0.030%. Note that, as used in the present description, the term "content of sol. Al" means the content of acid-soluble Al.

N: 0.40% or less

Nitrogen (N) is unavoidably contained. That is, the content of N is more than 0%.
N stabilizes austenite in the steel material. N also increases the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment, and the pitting resistance of the steel material in the high-temperature and high-pressure chloride corrosive environment. In addition, N combines with Al and Ti to form fine Al nitrides and fine Ti nitrides. The fine Al nitrides and fine Ti nitrides function as segregation sites for As. Therefore, if a large number of fine Al nitrides and fine Ti nitrides are formed and are dispersed in the steel material, it will be even easier for As to disperse in the steel material. As a result, the general corrosion resistance of the steel material in the high-temperature and high-pressure strongly acidic corrosive environment will increase. If even a small amount of N is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of N is more than 0.40%, even if the contents of other elements are within the range of the present embodiment, the toughness and hot workability of the steel material will decrease.
Accordingly, the content of N is to be 0.40% or less.
A preferable lower limit of the content of N is 0.01%, more preferably is 0.02%, further preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.15%.
A preferable upper limit of the content of N is 0.38%, more preferably is 0.36%, further preferably is 0.34%, further preferably is 0.32%, and further preferably is 0.30%.

O: 0.100% or less

Oxygen (O) is an impurity that is unavoidably contained. That is, the content of O is more than 0%.
If the content of O is more than 0.100%, coarse Ca oxysulfides and coarse Mg oxides will excessively form. Therefore, excellent pitting resistance in the high-temperature and high-pressure chloride corrosive environment will not be obtained even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of O is to be 0.100% or less.
The content of O is preferably as low as possible. However, if the content of O is lowered excessively, the production cost will increase. Therefore, when industrial production is taken into consideration, a preferable lower limit of the content of O is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.
A preferable upper limit of the content of O is 0.090%, more preferably is 0.085%, further preferably is 0.080%, and further preferably is 0.075%.
The balance of the chemical composition of the duplex stainless steel material according to the present embodiment is Fe and impurities. Here, impurities in the chemical composition means elements which are mixed in from ore and scrap as the raw material or from the production environment or the like when industrially producing the duplex stainless steel material, and which are not intentionally contained but are permitted within a range not adversely affecting the duplex stainless steel material according to the present embodiment.

[Regarding optional elements]

The chemical composition of the duplex stainless steel material of the present embodiment may further contain, in lieu of a part of Fe, one or more types of element selected from the group consisting of:

Cu: 0 to 4.0%,
V: 0 to 1.50%,
Co: 0 to 2.00%,
Ta: 0 to 2.00%,
W: 0 to 4.00%,
Nb: 0 to 2.00%,
Ti: 0 to 2.00%,
Zn: 0 to 0.0100%,
Pb: 0 to 0.0100%,
Sb: 0 to 0.0100%,
Sn: 0 to 0.0100%,
Bi: 0 to 0.0100%,
B: 0 to 0.0100%,
rare earth metal: 0 to 0.050%,
Zr: 0 to 2.00%, and
Hf: 0 to 2.00%.

Each of these optional elements is described hereunder.

[First group: Cu, V, Co, Ta, W, Nb, Ti, Zn, Pb, Sb, Sn, and Bi]

The chemical composition of the duplex stainless steel material of the present embodiment may contain one or more types of element selected from the group consisting of Cu, V, Co, Ta, W, Nb, Ti, Zn, Pb, Sb, Sn, and Bi, in lieu of a part of Fe. Each of these elements is an optional element, and each element increases the general corrosion resistance of the steel material in the high-temperature and high-pressure strongly acidic corrosive environment. Each of these elements is described hereunder.

Cu: 0 to 4.0%

Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%.
When contained, that is, when the content of Cu is more than 0%, in the high-temperature and high-pressure strongly acidic corrosive environment, Cu forms sulfides on a passive film. Active dissolution of the steel material is suppressed by the sulfides. Therefore, the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment is increased. If even a small amount of Cu is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Cu is more than 4.0%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Cu is to be 0 to 4.0%, and when contained, is to be 4.0% or less.
A preferable lower limit of the content of Cu is 0.1%, more preferably is 0.2%, further preferably is 0.5%, and further preferably is 1.0%.
A preferable upper limit of the content of Cu is 3.8%, more preferably is 3.5%, further preferably is 2.5%, and further preferably is 2.0%.

V: 0 to 1.50%

Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%.
When contained, that is, when the content of V is more than 0%, V suppresses active dissolution of the steel material in the high-temperature and high-pressure strongly acidic corrosive environment, thereby increases the general corrosion resistance. If even a small amount of V is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of V is more than 1.50%, the strength of the steel material will be excessively high. In such a case, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of V is to be 0 to 1.50%, and when contained, is to be 1.50% or less.
A preferable lower limit of the content of V is 0.01%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.20%.
A preferable upper limit of the content of V is 1.40%, more preferably is 1.30%, further preferably is 1.20%, and further preferably is 1.00%.

Co: 0 to 2.00%

Cobalt (Co) is an optional element, and does not have to be contained. That is, the content of Co may be 0%.
When contained, that is, when the content of Co is more than 0%, Co increases the general corrosion resistance of the steel material in the high-temperature and high-pressure strongly acidic corrosive environment. If even a small amount of Co is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Co is more than 2.00%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Co is to be 0 to 2.00%, and when contained, is to be 2.00% or less.
A preferable lower limit of the content of Co is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.20%, and further preferably is 0.30%.
A preferable upper limit of the content of Co is 1.90%, more preferably is 1.80%, further preferably is 1.70%, further preferably is 1.60%, further preferably is 1.50%, and further preferably is 1.00%.

Ta: 0 to 2.00%

Tantalum (Ta) is an optional element, and does not have to be contained. That is, the content of Ta may be 0%.
When contained, that is, when the content of Ta is more than 0%, Ta increases the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. If even a small amount of Ta is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Ta is more than 2.00%, the strength of the steel material will be excessively high. In such a case, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Ta is to be 0 to 2.00%, and when contained, is to be 2.00% or less.
A preferable lower limit of the content of Ta is 0.01%, more preferably is 0.05%, and further preferably is 0.08%.
A preferable upper limit of the content of Ta is 1.50%, more preferably is 1.00%, further preferably is 0.70%, and further preferably is 0.50%.

W: 0 to 4.00%

Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%.
When contained, that is, when the content of W is more than 0%, W suppresses active dissolution of the steel material in the high-temperature and high-pressure strongly acidic corrosive environment, thereby increases the general corrosion resistance. If even a small amount of W is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of W is more than 4.00%, the strength of the steel material will be excessively high. In such a case, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of W is to be 0 to 4.00%, and when contained, is to be 4.00% or less.
A preferable lower limit of the content of W is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.20%, further preferably is 0.30%, and further preferably is 0.50%.
A preferable upper limit of the content of W is 3.90%, more preferably is 3.80%, further preferably is 3.70%, further preferably is 3.50%, further preferably is 3.00%, further preferably is 2.50%, further preferably is 2.00%, and further preferably is 1.80%.

Nb: 0 to 2.00%

Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%.
When contained, that is, when the content of Nb is more than 0%, Nb forms carbides or nitrides and thereby suppresses the formation of Cr carbides. Thus, the formation of Cr-depleted zones at grain boundaries is suppressed. As a result, the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment is increased. If even a small amount of Nb is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Nb is more than 2.00%, the strength of the steel material will be excessively high. In such a case, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Nb is to be 0 to 2.00%, and when contained, is to be 2.00% or less.
A preferable lower limit of the content of Nb is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.20%, further preferably is 0.30%, and further preferably is 0.40%.
A preferable upper limit of the content of Nb is 1.50%), more preferably is 1.00%, further preferably is 0.70%, and further preferably is 0.50%.

Ti: 0 to 2.00%

Titanium (Ti) is an optional element, and does not have to be contained. That is, the content of Ti may be 0%.
When contained, that is, when the content of Ti is more than 0%, Ti forms carbides or nitrides and thereby suppresses the formation of Cr carbides. Thus, the formation of the Cr-depleted zones at grain boundaries is suppressed. As a result, the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment is increased. In addition, in a case where Ti forms a large number of fine Ti nitrides, the fine Ti nitrides dispersed in the steel material function as segregation sites for As. Therefore, it becomes even easier for As to disperse in the steel material. As a result, the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment is further increased. If even a small amount of Ti is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Ti is more than 2.00%, the strength of the steel material will be excessively high. In such a case, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Ti is to be 0 to 2.00%, and when contained, is to be 2.00% or less.
A preferable lower limit of the content of Ti is 0.01%, and more preferably is 0.05%.
A preferable upper limit of the content of Ti is 1.50%, more preferably is 1.00%, further preferably is 0.70%), and further preferably is 0.50%.

Zn: 0 to 0.0100%

Zinc (Zn) is an optional element, and does not have to be contained. That is, the content of Zn may be 0%.
When contained, that is, when the content of Zn is more than 0%, Zn forms stable sulfides, and thereby increases the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. If even a small amount of Zn is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Zn is more than 0.0100%, mechanical properties of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Zn is to be 0 to 0.0100%, and when contained, is to be 0.0100% or less.
A preferable lower limit of the content of Zn is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of Zn is 0.0050%, more preferably is 0.0030%, and further preferably is 0.0025%.

Pb: 0 to 0.0100%

Lead (Pb) is an optional element, and does not have to be contained. That is, the content of Pb may be 0%.
When contained, that is, when the content of Pb is more than 0%, Pb forms stable sulfides, and thereby increases the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. If even a small amount of Pb is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Pb is more than 0.0100%, mechanical properties of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Pb is to be 0 to 0.0100%, and when contained, is to be 0.0100% or less.
A preferable lower limit of the content of Pb is 0.0001%, more preferably is 0.0003%, further preferably is 0.0005%, further preferably is 0.0008%, and further preferably is 0.0010%.
A preferable upper limit of the content of Pb is 0.0070%, more preferably is 0.0050%, further preferably is 0.0030%, and further preferably is 0.0020%.

Sb: 0 to 0.0100%

Antimony (Sb) is an optional element, and does not have to be contained. That is, the content of Sb may be 0%.
When contained, that is, when the content of Sb is more than 0%, Sb increases the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. If even a small amount of Sb is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Sb is more than 0.0100%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Sb is to be 0 to 0.0100%, and when contained, is to be 0.0100% or less.
A preferable lower limit of the content of Sb is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0005%.
A preferable upper limit of the content of Sb is 0.0070%, more preferably is 0.0050%, further preferably is 0.0030%, further preferably is 0.0020%, and further preferably is 0.0015%.

Sn: 0 to 0.01 00%

Tin (Sn) is an optional element, and does not have to be contained. That is, the content of Sn may be 0%.
When contained, that is, when the content of Sn is more than 0%, Sn increases the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. If even a small amount of Sn is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Sn is more than 0.0100%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Sn is to be 0 to 0.0100%, and when contained, is to be 0.0100% or less.
A preferable lower limit of the content of Sn is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0005%.
A preferable upper limit of the content of Sn is 0.0070%, more preferably is 0.0050%, further preferably is 0.0030%, further preferably is 0.0020%, and further preferably is 0.0015%,

Bi: 0 to 0.0100%

Bismuth (Bi) is an optional element, and does not have to be contained. That is, the content of Bi may be 0%.
When contained, that is, when the content of Bi is more than 0%, Bi increases the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. If even a small amount of Bi is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Bi is more than 0.0100%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Bi is to be 0 to 0.0100%, and when contained, is to be 0.0100% or less.
A preferable lower limit of the content of Bi is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0005%.
A preferable upper limit of the content of Bi is 0.0070%, more preferably is 0.0050%, further preferably is 0.0030%, further preferably is 0.0020%, and further preferably is 0.0015%.

[Second group: B, rare earth metal, Zr, and Hf]

The chemical composition of the duplex stainless steel material of the present embodiment may contain one or more types of element selected from the group consisting of B, rare earth metal (REM), Zr, and Hf, in lieu of a part of Fe. Each of these elements is an optional element, and each element increases the hot workability of the steel material. Each of these elements is described hereunder.

B: 0 to 0.0100%

Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%.
When contained, that is, when the content of B is more than 0%, B suppresses segregation of P and S in the steel material to grain boundaries, thereby increases the hot workability of the steel material. If even a small amount of B is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of B is more than 0.0100%, B nitrides will excessively form. Consequently, the toughness of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of B is to be 0 to 0.0100%, and when contained, is to be 0.0100% or less.
A preferable lower limit of the content of B is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0020%.
A preferable upper limit of the content of B is 0.0090%, more preferably is 0.0080%, further preferably is 0.0070%, and further preferably is 0.0050%,

Rare earth metal: 0 to 0.050%

Rare earth metal (REM) is an optional element, and does not have to be contained. That is, the content of REM may be 0%.
When contained, that is, when the content of REM is more than 0%, REM controls morphology of inclusions, and thereby increases the hot workability of the steel material. If even a small amount of REM is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of REM is more than 0.050%, oxides in the steel material will coarsen. Consequently, the toughness of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of REM is to be 0 to 0.050%, and when contained, is to be 0.050% or less.
A preferable lower limit of the content of REM is 0.001%, more preferably is 0.003%, further preferably is 0.005%, further preferably is 0.008%, and further preferably is 0.010%.
A preferable upper limit of the content of REM is 0.045%, more preferably is 0.040%, further preferably is 0.035%, and further preferably is 0.030%.
In the present description the term "REM" means one or more types of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, that are lanthanoids. In the present description the term "content of REM" refers to the total content of these elements.

Zr: 0 to 2.00%

Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%.
When contained, that is, when the content of Zr is more than 0%, Zr forms carbo-nitrides and thereby increases the strength and hot workability of the steel material. If even a small amount of Zr is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Zr is more than 2.00%, the strength of the steel material will be excessively high. Consequently, the toughness of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Zr is to be 0 to 2.00%, and when contained, is to be 2.00% or less.
A preferable lower limit of the content of Zr is 0.01%, more preferably is 0.02%, and further preferably is 0.05%.
A preferable upper limit of the content of Zr is 1.50%, more preferably is 1.00%, further preferably is 0.50%, and further preferably is 0.30%.

Hf: 0 to 2.00%

Hafnium (Hf) is an optional element, and does not have to be contained. That is, the content of Hf may be 0%.
When contained, that is, when the content of Hf is more than 0%, Hf forms carbo-nitrides and thereby increases the strength and hot workability of the steel material. If even a small amount of Hf is contained, the aforementioned effect will be obtained to a certain extent.
However, if the content of Hf is more than 2.00%, the strength of the steel material will be excessively high. Consequently, the toughness of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Accordingly, the content of Hf is to be 0 to 2.00%, and when contained, is to be 2.00% or less.
A preferable lower limit of the content of Hf is 0.01%, more preferably is 0.10%, further preferably is 0.15%, and further preferably is 0.20%.
A preferable upper limit of the content of Hf is 1.50%, more preferably is 1.00%, further preferably is 0.80%, and further preferably is 0.75%.

[(Feature 2) Regarding Formula (1)]

The chemical composition of the duplex stainless steel material of the present embodiment also satisfies Formula (1). $0.70 < 10000 \times As / (Ni + Cu) < 16.00$
Where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula. If an element is not contained, "0" is substituted for the corresponding symbol of an element.
Fn1 (= 10000 × As/(Ni + Cu)) is an index relating to the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. The general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment markedly increases by adjusting the ratio of the content of As to the total content of Ni and Cu. Specifically, as illustrated in FIG. 1, if Fn1 is higher than 0.70, on the precondition that the duplex stainless steel material satisfies Feature 1 and Feature 3, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment becomes markedly slower. Therefore, excellent general corrosion resistance is obtained in the high-temperature and high-pressure strongly acidic corrosive environment.
On the other hand, if Fn1 is too high, although the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment increases, the hot workability of the steel material decreases. If Fn1 is less than 16.00, sufficient hot workability can be obtained in the steel material. Therefore, Fn1 is to be higher than 0.70 and less than 16.00.
A preferable lower limit of Fn1 is 0.71, more preferably is 1.00, further preferably is 2.00, further preferably is 3.00, and further preferably is 4.00. Note that, referring to FIG. 1, in a case where Fn1 is 5.50 or more, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment markedly decreases in comparison to a case where Fn1 is more than 0.70 and less than 5.50. Therefore, a more preferable lower limit of Fn1 is 5.50, and further preferably is 6.00.
A preferable upper limit of Fn1 is 15.50, more preferably is 15.00, and further preferably is 14.50,
Note that, in the present embodiment, Fn1 is a numerical value to the second decimal place obtained by rounding off the third decimal place of the obtained numerical value.

[(Feature 3) Regarding Formula (2)]

The chemical composition of the duplex stainless steel material of the present embodiment also satisfies Formula (2). $(Ca + Mg) / O < 1.50$
Where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula. If an element is not contained, "0" is substituted for the corresponding symbol of an element.
Fn2 (= (Ca + Mg)/O) is an index relating to pitting resistance in the high-temperature and high-pressure chloride corrosive environment. As mentioned above, Ca and Mg combine with S to form sulfides. By this means, the formation of coarse Mn sulfides is suppressed. As a result, the pitting resistance in the high-temperature and high-pressure chloride corrosive environment increases.
However, in a case where the content of S in the steel material is within the aforementioned range (0.010% or less), if the total content of Ca and Mg with respect to the content of O is too high, Ca and Mg will combine not only with S but also with O, and will form coarse Ca oxysulfides and Mg oxides. Similarly to coarse Mn sulfides, the coarse Ca oxysulfides and coarse Mg oxides will easily dissolve in the high-temperature and high-pressure chloride corrosive environment, and are liable to become starting points for pitting. Therefore, if the total content of Ca and Mg with respect to the content of O is too high, the pitting resistance in the high-temperature and high-pressure chloride corrosive environment will decrease.
As illustrated in FIG. 2, if Fn2 is less than 1.50, on the precondition that the duplex stainless steel material satisfies Feature 1 and Feature 2, the corrosion rate in the high-temperature and high-pressure chloride corrosive environment becomes markedly slower. As a result, excellent pitting resistance is obtained in the high-temperature and high-pressure chloride corrosive environment.
A preferable upper limit of Fn2 is 1.45, more preferably is 1.43, further preferably is 1.40, further preferably is 1.35, and further preferably is 1.30.
The lower limit of Fn2 is not particularly limited. A preferable lower limit of Fn2 is 0.01, and more preferably is 0.02.
Note that, in the present embodiment, Fn2 is a numerical value to the second decimal place obtained by rounding off the third decimal place of the obtained numerical value.

[Advantageous effect of duplex stainless steel material according to present embodiment]

The duplex stainless steel material according to the present embodiment satisfies Feature 1 to Feature 3. Therefore, with the duplex stainless steel material according to the present embodiment, excellent general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment can be obtained, and in addition, excellent pitting resistance in the high-temperature and high-pressure chloride corrosive environment can be obtained.
Here, excellent general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment, and excellent pitting resistance in the high-temperature and high-pressure chloride corrosive environment are defined as follows by a test to evaluate general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment and a test to evaluate pitting resistance in the high-temperature and high-pressure chloride corrosive environment which are described hereunder.

[Test to evaluate general corrosion resistance in high-temperature and high-pressure strongly acidic corrosive environment]

The test to evaluate general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment is conducted by a following method.
A test specimen is taken from the duplex stainless steel material. If the duplex stainless steel material is a steel pipe, the test specimen is taken from a center position of the wall thickness. In this case, a longitudinal direction of the test specimen is to be made parallel to an axial direction of the steel pipe. If the duplex stainless steel material is a round steel bar, the test specimen is taken from an R/2 position. Here, the term "R/2 position" means a center position of a radius R in a cross section perpendicular to an axial direction of the round steel bar. In this case, the longitudinal direction of the test specimen is to be made parallel to the axial direction of the round steel bar. If the duplex stainless steel material is a steel plate, the test specimen is taken from a center position of a thickness. In this case, the longitudinal direction of the test specimen is to be made parallel to a rolling direction of the steel plate. A size of the test specimen is to be made, for example, 40 mm in length, 10 mm in width, and 3 mm in thickness. The mass of the test specimen is measured before starting the test.
Sulfuric acid (H₂SO₄) aqueous solution with a concentration of 0.01 mol/L is prepared as a test solution. The test solution is placed in an autoclave. The test specimen is immersed in the test solution, and a gaseous mixture of H₂S gas at 0.05 bar and CO₂ gas at 5.00 bar is sealed under pressure in the autoclave and a corrosion test is started. The test duration is set to 336 hours. The temperature inside the autoclave during the test is maintained at 180°C.
After the test duration has elapsed, the corrosion products are removed from the test specimen. Removal of the corrosion products from the test specimen is performed, for example, according to the method specified in ASTM G31-21. The mass of the test specimen from which the corrosion products have been removed is measured. The corrosion rate (g•cm^-2•h^-1) is determined by dividing the difference between the mass of the test specimen before starting the test and the mass of the test specimen after the test duration has elapsed and the corrosion products have been removed by the surface area of the test specimen and the test duration. If the corrosion rate is 0.100 g•cm^-2•h^-1 or less, it is determined that the relevant duplex stainless steel material is excellent in general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment.

[Test to evaluate pitting resistance in high-temperature and high-pressure chloride corrosive environment]

The test to evaluate pitting resistance in the high-temperature and high-pressure chloride corrosive environment is conducted by a following method.
A test specimen is taken from the duplex stainless steel material. If the duplex stainless steel material is a steel pipe, the test specimen is taken from a center position of the wall thickness. In this case, the longitudinal direction of the test specimen is to be made parallel to the axial direction of the steel pipe. If the duplex stainless steel material is a round steel bar, the test specimen is taken from an R/2 position. In this case, the longitudinal direction of the test specimen is to be made parallel to the axial direction of the round steel bar. If the duplex stainless steel material is a steel plate, the test specimen is taken from a center position of the thickness. In this case, the longitudinal direction of the test specimen is to be made parallel to the rolling direction of the steel plate. The size of the test specimen is to be made, for example, 40 mm in length, 10 mm in width, and 3 mm in thickness. The mass of the test specimen is measured before starting the test.
A 25% by mass aqueous sodium chloride (NaCl) solution is prepared as a test solution. The test solution is placed in an autoclave. The test specimen is immersed in the test solution, and a gaseous mixture of H₂S gas at 0.05 bar and CO₂ gas at 5.00 bar is sealed under pressure in the autoclave and a corrosion test is started. The test duration is set to 336 hours. The temperature inside the autoclave during the test is maintained at 180°C.
After the test duration has elapsed, the corrosion products are removed from the test specimen. Removal of the corrosion products from the test specimen is performed, for example, according to the method specified in ASTM G31-21. The mass of the test specimen from which the corrosion products have been removed is measured. The corrosion rate (g•cm^-2•h^-1) is determined by dividing the difference between the mass of the test specimen before starting the test and the mass of the test specimen after the test duration has elapsed and the corrosion products have been removed by the surface area of the test specimen and the test duration.
In addition, the surface of the test specimen after the test ends is observed using a magnifying glass with a magnification of ×10 to check for the presence or absence of pitting. If a location where pitting is suspected is found by the observation with the magnifying glass, a cross section of the location where pitting is suspected is observed using an optical microscope with a magnification of ×100 to confirm the presence or absence of pitting.
In a where the corrosion rate is 0.005 g•cm²•h^-1 or less, and pitting is not confirmed over the entire surface of the test specimen, it is determined that the relevant duplex stainless steel material is excellent in pitting resistance in the high-temperature and high-pressure chloride corrosive environment.
As mentioned above, the duplex stainless steel material according to the present embodiment satisfies Feature 1 to Feature 3. Therefore, excellent general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment, and excellent pitting resistance in the high-temperature and high-pressure chloride corrosive environment can be obtained.

[Microstructure]

Note that, a microstructure of the duplex stainless steel material according to the present embodiment is composed of ferrite and austenite. Here, the phrase "composed of ferrite and austenite" means that the microstructure contains, for example, in volume ratio, 30 to 80% of ferrite, with the balance being austenite. Note that, microstructures other than ferrite and austenite are negligibly small. For example, in the duplex stainless steel material according to the present embodiment, the volume ratio of precipitates and inclusions is negligibly small compared to the volume ratio of ferrite and austenite. In other words, apart from ferrite and austenite, the microstructure of the duplex stainless steel material according to the present embodiment may also contain precipitates and/or inclusions or the like.

[Microstructure observation method]

The volume ratio of ferrite in the duplex stainless steel material can be determined by a method in accordance with JIS G 0555 (2020). Specifically, a test specimen for microstructure observation is prepared from the duplex stainless steel material. If the steel material is a steel pipe, a test specimen having, for example, an observation surface with dimensions of 5 mm in a steel pipe axis direction and 5 mm in a steel pipe diameter direction is prepared from a center position of the wall thickness. If the steel material is a round steel bar, a test specimen having, for example, an observation surface with dimensions of 5 mm in an axial direction and 5 mm in a radial direction is prepared from an R/2 position. If the steel material is a steel plate, a test specimen having, for example, an observation surface with dimensions of 5 mm in a rolling direction and 5 mm in a thickness direction is prepared from a center position of the thickness. Note that, a size of the test specimen is not particularly limited as long as the aforementioned observation surface can be obtained.
The observation surface of the prepared test specimen is mirror-polished. The mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etching solution to reveal the microstructure. The observation surface on which the microstructure has been revealed is observed in 10 visual fields using an optical microscope. The area of each visual field is not particularly limited, and for example is 1.00 mm² (magnification of ×100). In each visual field, ferrite and austenite are identified based on contrast. The area fraction of the identified ferrite is measured by a point counting method in accordance with JIS G 0555 (2020). The arithmetic average value of the area fractions of ferrite obtained in the 10 visual fields is defined as the volume ratio (%) of ferrite. Note that, a value obtained by rounding off the first decimal place of the obtained value is adopted as the volume ratio (%) of ferrite. A value obtained by subtracting the obtained volume ratio of ferrite from 100% is defined as the volume ratio (%) of austenite.

[Shape and uses of duplex stainless steel material]

The shape of the duplex stainless steel material according to the present embodiment is not particularly limited. The duplex stainless steel material according to the present embodiment may be a steel pipe, may be a round steel bar (solid material), or may be a steel plate. Further, the steel pipe may be a seamless steel pipe or may be a welded steel pipe.
The duplex stainless steel material according to the present embodiment can be widely applied for use in the high-temperature and high-pressure strongly acidic corrosive environment or for use in the high-temperature and high-pressure chloride corrosive environment. The duplex stainless steel material according to the present embodiment, for example, may be applied for geothermal well use, or may be used for oil well use.

[Preferable embodiment of duplex stainless steel material according to present embodiment]

Preferably the duplex stainless steel material according to the present embodiment satisfies the aforementioned Feature 1 to Feature 3, and in addition, satisfies the following Feature 4.

(Feature 4)

When:

a particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a total of a content of Ca and a content of S is higher than 5.0%, a content of O is 1.0% or more, and a content of Ca is higher than a content of S is defined as a fine Ca oxysulfide;
a particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a content of Mg is 5.0% or more, a content of O is 1.0% or more, and a content of S is 15.0% or less is defined as a fine Mg oxide;
a particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a content of Al is 20.0% or more and a content of N is 20.0% or more is defined as a fine Al nitride; and
a particle which has an equivalent circular diameter of 1.0 to 2.0 µm, and in which, in mass%, a content of Ti is 30.0% or more and a content of N is 20.0% or more is defined as a fine Ti nitride,
a total number density ND of the fine Ca oxysulfides, the fine Mg oxides, the fine A1 nitrides, and the fine Ti nitrides is 2.00 pieces/mm² or more.

In a case where the duplex stainless steel material according to the present embodiment satisfies Feature 1 to Feature 3, and also satisfies Feature 4, even more excellent general corrosion resistance can be obtained in the high-temperature and high-pressure strongly acidic corrosive environment. Hereunder, Feature 4 is described.

[(Feature 4) Regarding total number density ND]

As mentioned above, As increases the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment. If As is present in a dispersed state in the steel material, the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment will be further increased. Here, among particles that are inclusions and precipitates in the steel material, fine particles that have an equivalent circular diameter of 1.0 to 2.0 µm are liable to segregate As at the interface with the parent phase. That is, the surfaces of such fine particles function as segregation sites for As. If the segregation sites are dispersed in the steel material, As will also easily disperse in the steel material. Therefore, even in a case where the content of As is small, As can be dispersed in the steel material. As a result, the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment markedly increases.
In the duplex stainless steel material that satisfies Feature 1 to Feature 3, a numerical proportion accounted for by fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides among all fine particles in the duplex stainless steel material is high. Therefore, if the total number density ND of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides can be increased, segregation sites for As can be sufficiently dispersed in the steel material. As a result, the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment can be further increased.
The total number density ND (pieces/mm²) is the total number density of the principal fine particles (fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides) which act as segregation sites for As. If the total number density ND is 2.00 pieces/mm² or more, a sufficient amount of As segregation sites will be dispersedly present in the steel material. Consequently, it will be easy for As to disperse sufficiently in the steel material. As a result, the general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment will further increase.
In particular, in a case where the duplex stainless steel material satisfies not only Feature 1 to Feature 3, but also satisfies Feature 4, even if Fn1 is more than 0.70 and less than 5.50, the corrosion rate obtained in the aforementioned [Test to evaluate general corrosion resistance in high-temperature and high-pressure strongly acidic corrosive environment] will be 0.080 g•cm^-2•h^-1 or less, and further excellent general corrosion resistance will be obtained.
A preferable lower limit of the total number density ND is 2.01 pieces/mm², more preferably is 2.05 pieces/mm², further preferably is 2.07 pieces/mm², and further preferably is 2.10 pieces/mm².
The higher the total number density ND, the more As segregation sites there are, which tends to increase general corrosion resistance. Therefore, the upper limit of total number density ND is not particularly limited. For duplex stainless steels that satisfy Feature 1, the upper limit of total number density ND, for example, is 30.00 pieces/mm², preferably is 28.50 pieces/mm², more preferably is 25.00 pieces/mm², further preferably is 20.00 pieces/mm², further preferably is 17.00 pieces/mm², further preferably is 15.00 pieces/mm² further preferably is 10.00 pieces/mm², further preferably is 5.00 pieces/mm², and further preferably is 3.00 pieces/mm².

[Method for measuring total number density ND]

The total number density ND (pieces/mm²) of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides can be determined by a following method.
Specifically, a test specimen is prepared from the duplex stainless steel material. If the steel material is a steel pipe, a test specimen having an observation surface that includes a steel pipe axis direction and a steel pipe diameter direction (wall thickness direction) is prepared from a center position of the wall thickness. If the steel material is a steel plate, a test specimen having an observation surface that includes a rolling direction and a thickness direction is prepared from a center position of the thickness. If the steel material is a round steel bar, a test specimen having an observation surface that includes an axial direction and a radial direction is prepared from an R/2 position in a cross section that is perpendicular to the axial direction of the round steel bar.
The observation surface of the prepared test specimen is mirror-polished using a diamond paste abrasive. An observation visual field at a thickness center position of the mirror-polished observation surface is observed at a magnification of ×500 using a scanning electron microscope (SEM). If the steel material is a steel pipe, the thickness center position of the observation surface means the center position in the wall thickness direction of the steel pipe on the observation surface. If the steel material is a steel plate, the thickness center position of the observation surface means the center position in the thickness direction of the steel plate on the observation surface. If the steel material is a round steel bar, the thickness center position of the observed surface means the center position in a radial direction of the round steel bar on the observation surface. A number of observation visual field is not limited as long as the total area of the observation visual field is 1125 mm². When selecting multiple rectangular observation visual fields so that the total area of the observation visual field is 1125 mm², the multiple observation visual fields are selected so that the multiple observation visual fields are arranged in a row on the observation surface and one edge of adjacent observation visual fields is tangent to each other.
For example, if the size of each observation visual field is a rectangle of 15 mm ×15 mm, the number of observation visual fields is 5 (15 mm × 15 mm × 5 = 1125 mm²). The five observation visual fields are selected so that the five observation visual fields are arranged in a row on the observation surface and one edge (15 mm) of the adjacent observation visual fields is tangent to each other.
Particles in the observation visual field are identified based on contrast. The equivalent circular diameter (µm) of each identified particle is determined. Here, the term "equivalent circular diameter" means the diameter (µm) of a circle having an area which is the same as the area of the relevant particle. In addition, each identified particle is subjected to an element concentration analysis (EDS analysis). The element concentration analysis can be performed using an apparatus in which a scanning electron microscope is provided with an element concentration analysis function (SEM-EDS apparatus). In the element concentration analysis, an accelerating voltage is set to 20 kV, and element concentrations are determined for N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb as elements to be analyzed. Based on an EDS analysis result for each particle, the total content in percent by mass of N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr, and Nb is taken as 100%, and fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides are identified as described hereunder.
A particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%), a total of a content of Ca and a content of S is higher than 5.0%, a content of O is 1.0% or more, and a content of Ca is higher than a content of S is identified as a "fine Ca oxysulfide".
A particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a content of Mg is 5.0% or more, a content of O is 1.0% or more, and a content of S is 15.0% or less is identified as a "fine Mg oxide".
A particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a content of Al is 20.0% or more and a content of N is 20.0% or more is identified as a "fine Al nitride".
A particle which has an equivalent circular diameter of 1.0 to 2.0 µm, and in which, in mass%, a content of Ti is 30.0% or more and a content of N is 20.0% or more is identified as a "fine Ti nitride".
The fine Ca oxysulfides, the fine Mg oxides, the fine Al nitrides, and the fine Ti nitrides identified by the above method in the observation visual field are counted.
The total number density ND (pieces/mm²) of the fine Ca oxysulfides, the fine Mg oxides, the fine Al nitrides, and the fine Ti nitrides is determined based on the total number of the fine Ca oxysulfides, the fine Mg oxides, the fine Al nitrides, and the fine Ti nitrides in all the observation visual fields and the total area of the observation visual fields. The total number density ND is a numerical value to the second decimal place that is obtained by rounding off the third decimal place of the determined numerical value.

[Production method]

An example of a method for producing the duplex stainless steel material according to the present embodiment will now be described. One example of a method for producing the duplex stainless steel material according to the present embodiment includes a starting material production process, a hot working process, and a solution treatment process. Each process will be described in detail.

[Starting material production process]

In the starting material production process, a starting material that satisfies Feature 1 to Feature 3 is prepared. Specifically, molten steel that satisfies Feature 1 to Feature 3 is produced. The method for producing the molten steel is not particularly limited. The molten steel may be produced using a converter, may be produced using an electric furnace, or may be produced by another method.
A starting material is produced using the produced molten steel. The starting material is, for example, a cast piece or an ingot. Specifically, the cast piece is produced by a continuous casting process using the molten steel. The cast piece may be a slab, may be a bloom, or may be a billet. Alternatively, the ingot may be produced by an ingot-making process using the molten steel. The cast piece or the ingot may further be subjected to hot forging or blooming or the like to produce a billet. The starting material is produced by the above process.

[Hot working process]

In the hot working process, the produced starting material is subjected to well-known hot working to produce an intermediate steel material. If an end product will be a steel pipe, the intermediate steel material is a hollow shell. If the end product will be a round steel bar, the intermediate steel material is a bar-shaped steel material. If the end product will be a steel plate, the intermediate steel material is a plate-shaped steel material. The hot working may be hot forging, may be hot extrusion, or may be hot rolling. The method of hot working is not particularly limited, and it suffices to use a well-known method.
An example of the hot working process in a case where the end product will be a seamless steel pipe is as follows. First, a billet that is the starting material is heated in a reheating furnace. Although not particularly limited, the heating temperature is, for example, 1000 to 1300°C. The billet extracted from the reheating furnace is subjected to hot working to produce a hollow shell (seamless steel pipe) that is the intermediate steel material. The method of hot working is not particularly limited, and it suffices to use a well-known method. For example, piercing-rolling according to the Mannesmann process may be performed as hot working to produce a hollow shell. In this case, a round billet is subjected to piercing-rolling by a piercing machine. When performing piercing-rolling, although not particularly limited, for example, the piercing ratio is 1.0 to 4.0. The round billet subjected to piercing-rolling is further subjected to hot rolling with a mandrel mill, a reducer, a sizing mill or the like to produce a hollow shell. The cumulative reduction of area in the hot working process is, for example, 20 to 70%. A hollow shell may be produced from the billet by performing another hot working method. For example, in a case where the steel material is a thick-wall steel pipe of a short length such as a coupling, a hollow shell may be produced by forging by the Ehrhardt process or the like. A hollow shell is produced by the above process.
An example of the hot working process in a case where the end product will be the round steel bar is as follows. First, the starting material is heated in a reheating furnace. Although not particularly limited, the heating temperature is, for example, 1000 to 1300°C. The starting material extracted from the reheating furnace is subjected to hot working to produce an intermediate steel material in which a cross section perpendicular to the axial direction is a circular shape. The hot working is, for example, blooming performed using a blooming mill or hot rolling performed using a continuous mill. In a continuous mill, a horizontal stand having a pair of grooved rolls arranged one on the other in a vertical direction, and a vertical stand having a pair of grooved rolls arranged side by side in a horizontal direction are alternately arranged.
An example of the hot working process in a case where the end product will be the steel plate is as follows. First, the starting material is heated in a reheating furnace. Although not particularly limited, the heating temperature is, for example, 1000 to 1300°C. The starting material extracted from the reheating furnace is subjected to hot rolling using a reverse mill and a tandem mill to produce a plate-shaped intermediate steel material. Note that, the plate-shaped intermediate steel material may also be produced by performing hot forging, thereafter reheating the starting material after hot forging to 1000 to 1300°C, and then subjecting the starting material after reheating to further hot rolling.

[Solution treatment process]

In the solution treatment process, the intermediate steel material produced by the hot working process is subjected to a well-known solution treatment. For example, the intermediate steel material may be charged into a heat treatment furnace, held at a desired temperature, and thereafter rapidly cooled. Note that, in the case of performing the solution treatment by charging the intermediate steel material into the heat treatment furnace, holding the intermediate steel material at the desired temperature, and thereafter rapidly cooling the intermediate steel material, the term "solution treatment temperature" means the temperature (°C) of the heat treatment furnace for performing the solution treatment. The term "solution treatment time" means the time period for which the intermediate steel material is held at the solution treatment temperature. The solution treatment temperature is, for example, 900 to 1100°C. The solution treatment time is, for example, 5 to 180 minutes. A rapid cooling method used in the solution treatment is, for example, water cooling.
The duplex stainless steel material according to the present embodiment is produced by the production method described above.

[Preferable production conditions]

The method for producing the duplex stainless steel material according to the present embodiment preferably satisfies following condition 1 and condition 2.

(Condition 1)

In the starting material production process, an average cooling rate CR1 until the surface temperature of the starting material reaches 1100°C from 1350°C when casting the molten steel is to be 8 to 25°C/min.

(Condition 2)

In the solution treatment process, after being held at the solution treatment temperature for the solution treatment time, an average cooling rate CR2 until the surface temperature of the intermediate starting material reaches 850°C from the solution treatment temperature is to be 200°C/min or less, and an average cooling rate CR3 until the surface temperature of the intermediate starting material reaches 300°C from 850°C is to be 1000°C/min or more.
If condition 1 and condition 2 are satisfied, the produced duplex stainless steel material will satisfy Feature 1 to Feature 3 and will also satisfy Feature 4. Hereunder, condition 1 and condition 2 are described.

[(Regarding condition 1)]

In the duplex stainless steel material satisfying Feature 1 to Feature 3, during casting of the molten steel, Ca oxysulfides and Mg oxides form when the surface temperature of the starting material is in a temperature region from 1350°C to 1100°C. If the average cooling rate CR1 is too slow, Ca oxysulfides and Mg oxides will coarsen. In such a case, the number density of the fine Ca oxysulfides and the fine Mg oxides will be low. As a result, the total number density ND will be low. On the other hand, if the average cooling rate CR1 is too fast, the amount of the fine Ca oxysulfides and the fine Mg oxides that form will be insufficient.
If the average cooling rate CR1 is 8 to 25°C/min, on the precondition that condition 2 is satisfied, the total number density ND will be 2.00 pieces/mm² or more.
Note that, a method for controlling the average cooling rate CR1 is not particularly limited, and it suffices to use a well-known method. For example, in the case of producing the starting material by continuous casting, the cooling rate can be controlled by adjusting an amount (specific water volume) of cooling water that cools the cast piece. For example, in addition, in the case of producing the starting material by an ingot-making process, the cooling rate can be controlled by a material of a mold or by water-cooling of the mold.
Note that, the surface temperature of the starting material can be measured by a non-contact infrared radiation thermometer. The average cooling rate CR1 (°C/min) can be determined by measuring the time until the surface temperature of the starting material reaches 1100°C from 1350°C.

[(Regarding condition 2)]

In the solution treatment process, a temperature region T2 from the surface temperature of the intermediate steel material when the intermediate steel material is extracted from the heat treatment furnace until the surface temperature reaches 850°C is a temperature region in which the fine Al nitrides and the fine Ti nitrides form. If the average cooling rate CR2 in the temperature region T2 is more than 200°C/min, an amount of the fine Al nitrides and the fine Ti nitrides that form in the temperature region T2 will be insufficient. If the average cooling rate CR2 is 200°C/min or less, a sufficient amount of the fine Al nitrides and the fine Ti nitrides will form. As a result, the total number density ND will be 2.00 pieces/mm² or more.
Note that, the average cooling rate CR3 until the surface temperature of the intermediate steel material reaches 300°C from 850°C is to be 1000°C/min or more. If the intermediate steel material is water-cooled, the average cooling rate CR3 will be 1000°C/min or more,
The surface temperature of the intermediate steel material can be measured using a non-contact infrared radiation thermometer. The average cooling rate CR2 (°C/min) can be determined by measuring the time until the surface temperature of the intermediate steel material reaches 850°C from the solution treatment temperature. Similarly, the average cooling rate CR3 (°C/min) can be determined by measuring the time until the surface temperature of the intermediate steel material reaches 300°C from 850°C. Note that, as mentioned above, the average cooling rate CR3 can be made 1000°C/min or more by subjecting the intermediate steel material to water cooling.

[Regarding other processes]

In the method for producing the duplex stainless steel material according to the present embodiment, processes that are other than the processes described above may also be performed. For example, the intermediate steel material after the solution treatment process may be subjected to a cold working process. That is, the cold working process is an optional process.
In the cold working process, the intermediate steel material is subjected to well-known cold working. The cold working, for example, may be cold drawing, or may be cold rolling. The strength of the duplex stainless steel material can be increased by performing cold working on the intermediate steel material after the solution treatment.
Note that, the production method described above is one example. Therefore, the method for producing the duplex stainless steel material according to the present embodiment is not limited to the one example described above.

EXAMPLES

The advantageous effects of the duplex stainless steel material of the present embodiment will now be described more specifically by way of examples. The conditions adopted in the following examples are one example of conditions employed for continuing the workability and advantageous effects of the duplex stainless steel material of the present embodiment. Accordingly, the duplex stainless steel material of the present embodiment is not limited to this one example of conditions.
Duplex stainless steel materials having the chemical compositions shown in Table 1-1 and Table 1-2 were produced.

[Table 1-1]

TABLE 1-1

Test No.	Chemical Composition (unit is mass%; balance is Fe and impurities)
Test No.	C	Si	Mn	P	S	Cr	Ni	Mo	As	Ca	Mg	Ca+Mg	Sol.Al	N	O
1	0.020	0.5	1.1	0.019	0.004	26.7	4.7	2.8	0.0005	0.0025	0.0061	0.0086	0.035	0.36	0.081
2	0.038	0.4	1.4	0.017	0.002	22.1	4.6	3.9	0.0071	-	0.0006	0.0006	0.039	0.33	0.057
3	0.032	0.3	0.7	0.017	0.001	25.1	7.4	2.2	0.0094	0.0015	0.0057	0.0072	0.041	0.08	0.021
4	0.020	0.6	1.2	0.014	0.006	24.3	5.0	3.6	0.0026	-	0.0088	0.0088	0.009	0.36	0.071
5	0.015	1.1	0.5	0.018	0.001	23.7	6.9	3.8	0.0036	0.0044	0.0039	0.0083	0.025	0.07	0.064
6	0.036	0.6	1.1	0.010	0.006	22.3	7.9	3.9	0.0021	-	0.0091	0.0091	0.007	0.12	0.042
7	0.009	0.5	1.5	0.015	0.001	23.3	4.3	2.5	0.0056	0.0052	0.0033	0.0085	0.009	0.33	0.007
8	0.044	0.9	1.8	0.011	0.004	21.5	4.4	2.5	0.0087	-	0.0085	0.0085	0.023	0.13	0.098
9	0.035	0.5	0.9	0.013	0.002	21.7	7.8	4.5	0.0011	0.0023	-	0.0023	0.049	0.32	0.008
10	0.014	0.5	1.3	0.011	0.008	25.2	5.5	4.8	0.0055	0.0056	0.0036	0.0092	0.026	0.28	0.012
11	0.003	0.4	1.7	0.017	0.006	20.1	7.9	3.2	0.0007	0.0039	0.0037	0.0076	0.033	0.34	0.041
12	0.001	0.7	1.7	0.019	0.003	22.8	7.2	4.6	0.0026	0.0056	-	0.0056	0.041	0.27	0.084
13	0.013	0.3	1.2	0.016	0.001	24.3	6.9	3.1	0.0008	0.0043	0.0034	0.0077	0.046	0.07	0.061
14	0.045	0.7	1.6	0.017	0.005	24.7	6.9	2.7	0.0006	-	0.0033	0.0033	0.018	0.13	0.031
15	0.022	0.6	1.3	0.016	0.003	26.1	4.5	2.6	0.0007	-	0.0079	0.0079	0.033	0.34	0.077
16	0.025	0.7	0.8	0.012	0.009	22.6	7.4	2.2	0.0033	0.0055	0.0018	0.0073	0.036	0.18	0.008
17	0.030	0.4	1.7	0.019	0.005	21.4	6.8	3.2	0.0013	-	0.0045	0.0045	0.008	0.39	0.076
18	0.023	0.2	1.8	0.018	0.007	25.4	4.9	2.9	0.0043	0.0044	0.0022	0.0066	0.017	0.18	0.068
19	0.002	0.6	1.5	0.018	0.006	21.4	7.3	3.2	0.0046	0.0027	0.0034	0.0061	0.039	0.39	0.075
20	0.028	1.2	1.8	0.020	0.006	20.9	7.9	3.0	0.0044	0.0043	0.0046	0.0089	0.017	0.38	0.008
21	0.030	0.9	1.5	0.017	0.008	21.2	4.6	2.3	0.0065	0.0081	-	0.0081	0.043	0.22	0.024
22	0.040	0.8	0.6	0.022	0.006	25.4	5.7	2.6	0.0013	0.0066	0.0010	0.0076	0.005	0.15	0.035
23	0.009	0.5	0.7	0.013	0.009	23.9	4.9	3.1	0.0045	0.0023	0.0036	0.0059	0.019	0.17	0.008
24	0.021	0.3	1.1	0.015	0.005	20.3	5.3	2.1	0.0006	0.0091	-	0.0091	0.002	0.22	0.041
25	0.021	0.3	1.1	0.015	0.005	26.7	6.8	2.1	0.0006	0.0051	-	0.0051	0.003	0.07	0.041
26	0.047	0.8	1.3	0.017	0.003	23.1	7.1	2.0	0.0024	0.0037	0.0026	0.0063	0.028	0.33	0.007
27	0.042	1.1	0.7	0.016	0.003	21.4	4.5	3.5	0.0048	0.0026	0.0058	0.0084	0.007	0.12	0.006
28	0.008	0.2	0.5	0.016	0.007	21.6	5.9	3.7	0.0071	0.0021	0.0067	0.0088	0.018	0.32	0.036
29	0.016	0.4	0.9	0.011	0.002	23.1	5.7	3.5	0.0068	-	0.0072	0.0072	0.044	0.24	0.034
30	0.039	0.4	0.8	0.019	0.004	20.9	7.7	2.5	0.0079	0.0012	0.0031	0.0043	0.029	0.24	0.014
31	0.027	0.8	1.1	0.016	0.008	26.5	4.8	2.8	0.0006	-	0.0088	0.0088	0.033	0.18	0.083
32	0.016	0.3	0.8	0.014	0.004	25.9	7.6	2.3	0.0021	0.0033	-	0.0033	0.025	0.11	0.011
33	0.046	0.6	1.2	0.013	0.003	22.5	5.8	2.4	0.0042	0.0014	0.0054	0.0068	0.025	0.11	0.028
34	0.011	0.5	0.6	0.015	0.001	24.5	7.6	4.2	0.0062	0.0055	-	0.0055	0.022	0.01	0.017
35	0.008	0.5	0.7	0.016	0.002	21.4	7.0	2.9	0.0035	-	0.0015	0.0015	0.027	0.02	0.034
36	0.016	0.5	6.7	0.015	0.003	25.3	8.2	0.7	0.0014	0.0024	0.0022	0.0046	0.025	0.15	0.027
37	0.021	0.7	5.4	0.017	0.002	25.7	8.6	1.2	0.0017	0.0068	-	0.0068	0.019	0.18	0.025
38	0.024	0.5	1.1	0.018	0.002	24.3	4.7	2.5	0.0014	0.0062	0.0027	0.0089	0.037	0.34	0.021
39	0.026	0.7	0.9	0.015	0.003	25.1	5.3	2.4	0.0008	0.0059	-	0.0059	0.042	0.39	0.042
40	0.005	0.3	1.6	0.013	0.003	18.8	6.5	2.3	0.0060	-	0.0092	0.0092	0.041	0.18	0.017
41	0.042	1.1	2.0	0.019	0.003	25.1	5.1	3.3	-	0.0033	0.0041	0.0074	0.021	0.30	0.043
42	0.025	0.8	1.1	0.014	0.005	26.7	4.1	2.1	0.0003	0.0071	-	0.0071	0.026	0.35	0.033
43	0.022	0.3	2.0	0.018	0.009	21.2	5.0	2.2	0.0076	-	0.0001	0.0001	0.043	0.28	0.005
44	0.009	0.4	2.0	0.020	0.006	23.1	4.1	3.3	0.0098	0.0005	0.0050	0.0055	0.008	0.29	0.029
45	0.023	0.7	0.8	0.017	0.008	21.5	4.5	2.1	0.0082	0.0021	0.0047	0.0068	0.022	0.15	0.021
46	0.038	0.7	1.2	0.016	0.004	22.5	4.3	2.7	0.0079	0.0041	-	0.0041	0.021	0.18	0.015
47	0.013	1.2	0.8	0.011	0.007	26.4	7.7	3.1	0.0005	0.0070	-	0.0070	0.034	0.35	0.066
48	0.025	0.7	0.7	0.015	0.006	22.1	7.5	2.7	0.0005	0.0084	-	0.0084	0.027	0.11	0.047
49	0.022	0.6	0.5	0.016	0.007	21.8	7.7	2.6	0.0006	-	0.0072	0.0072	0.018	0.12	0.033
50	0.038	0.4	1.7	0.016	0.008	23.1	7.8	3.7	0.0066	0.0030	0.0064	0.0094	0.026	0.37	0.002
51	0.042	0.5	0.9	0.017	0.004	21.2	5.7	2.5	0.0044	0.0067	0.0012	0.0079	0.019	0.17	0.005
52	0.039	0.8	1.1	0.014	0.005	22.3	5.5	2.4	0.0027	0.0091	-	0.0091	0.032	0.14	0.006

[Table 1-2]

TABLE 1-2

Test No.	Chemical Composition (unit is mass%; balance is Fe and impurities)
Test No.	Cu	V	Co	Ta	W	Nb	Ti	Zn	Pb	Sb	Sn	Bi	B	REM	Zr	Hf
1	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
2	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
3	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
4	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
5	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
6	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
7	-	1.15	-	-	-	-	-	-	-	-	-	-	-	-	-	-
8	1.8	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
9	-	-	0.08	-	-	-	-	-	-	-	-	-	-	-	-	-
10	-	-	-	0.46	-	-	-	-	-	-	-	-	-	-	-	-
11	-	-	-	-	3.60	-	-	-	-	-	-	-	-	-	-	-
12	-	-	-	-	-	0.45	-	-	-	-	-	-	-	-	-	-
13	-	-	-	-	-	-	0.33	-	-	-	-	-	-	-	-	-
14	-	-	-	-	-	-	-	0.0012	-	-	-	-	-	-	-	-
15	-	-	-	-	-	-	-	-	0.0011	-	-	-	-	-	-	-
16	-	-	-	-	-	-	-	-	-	0.0006
17	-	-	-	-	-	-	-	-	-	-	0.0012	-	-	-	-	-
18	-	-	-	-	-	-	-	-	-	-	-	0.0009	-	-	-	-
19	-	-	-	-	-	-	-	-	-	-	-	-	0.0040		-	-
20	-	-	-	-	-	-	-	-	-	-	-	-	-	0.041	-	-
21	-	-	-	-	-	-	-	-	-	-	-	-	-	-	0.26	-
22	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	0.69
23	0.5	0.60	-	-	1.70	-	-	-	-	-	-	-	-	-	-	-
24	-	-	-	-	0.90	-	-	-	-	-	-	-	0.0030	-	-	-
25	1.7	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
26	0.2	-	-	-	-	-	-	-	-	-	-	-	-	0.016	-	-
27	-	0.70	-	-	0.10	-	-	-	-	-	-	-	0.0010
28	0.6	0.09	-	-	3.80	-	-	-	-	-	-	-	-	0.019	-	-
29	0.3	-	-	-	-	-	-	-	-	-	-	-	0.0080	-	-	-
30	-	-	0.55	-	-	0.07	-	-	-	-	-	-	-	-	-	-
31	-	-	1.60	-	-	-	-	-	-	-	-	-	-	-	0.06	-
32	-	-	-	0.09	-	-	0.08	-	-	-	-	-	0.0070	-	-	-
33	-	-	-	-	-	-	-	-	-	0.0008	0.0007	0.0011	-	0.011	-	-
34	1.8	-	-	-	-	-	-	-	0.0015	-	-	-	-	-	-	-
35	-	-	-	-	0.50	-	-	-	-	-	-	-	-	-	-	-
36	3.8	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
37	3.1	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
38	1.6	-	-	-	-	-	-					-	-	-	-	-
39	-	-	-	-	-	-	0.09	-	-	-	-	-	-	-	-	-
40	-	-	-	-	-	-	-	-	-	-	-	-	0.0020		-	-
41	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
42	-	-	-	-	0.50	-	-	-	-	-	-	-	-	-	-	-
43	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
44	-	-	-	-	0.90	-	-	-	-	-	-	-	-	-	-	-
45	0.3	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
46	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
47	1.9	1.30	-	-	-	-	-	-	-	-	-	-	-	0.039	-	-
48	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
49	1.1	-	-	-	-	-	-	0.0022	-	-	-	-	-	-	-	-
50	-		-	-	1.60	-	-	-	-	-	-	-	-	-	-	-
51	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
52	-	-	0.02	-	-	0.17	-	-	-	-	-	-	-	-	-	-

The symbol "-" in Table 1-1 and Table 1-2 means that the content of the corresponding element was at an impurity level. For example, it means that the content of V of Test No. 1 was 0%, when rounded off to two decimal places. It means that the content of Ca of Test No. 2 was 0% when rounded off to four decimal places.
For each test number, 30 kilograms of molten steel was melted using a highfrequency vacuum furnace. The molten steel was used to produce an ingot by an ingot-making process. During casting, the average cooling rate CR1 (°C/min) until the surface temperature of the ingot reached 1100°C from 1350°C was as shown in the column "CR1 (°C/min)" in Table 2.

[Table 2]

TABLE 2

Test No.	Fn1	Fn2	CR1 (°C/min)	CR2 (°C/min)	ND (pieces/mm²)	High-temperature and High-pressure Strongly Acidic Corrosive Environment	High-temperature and High-pressure Chloride Corrosive Environment		Remarks
Test No.	Fn1	Fn2	CR1 (°C/min)	CR2 (°C/min)	ND (pieces/mm²)	Corrosion Rate (g•cm^-2•h^-1)	Corrosion Rate (g•cm^-2•h^-1)	Pitting	Remarks
1	1.06	0.11	22	120	2.16	0.077	0.002	None	Inventive Example
2	15.43	0.01	22	120	2.06	0.037	0.001	None	Inventive Example
3	12.70	0.34	18	180	2.16	0.021	0.004	None	Inventive Example
4	5.20	0.12	22	180	2.16	0.069	0.001	None	Inventive Example
5	5.22	0.13	18	180	2.28	0.063	0.001	None	Inventive Example
6	2.66	0.22	18	180	2.55	0.052	0.001	None	Inventive Example
7	13.02	1.21	22	180	2.21	0.024	0.001	None	Inventive Example
8	14.03	0.09	22	180	2.12	0.026	0.004	None	Inventive Example
9	1.41	0.29	18	180	2.34	0.068	0.002	None	Inventive Example
10	10.00	0.77	22	180	2.14	0.015	0.002	None	Inventive Example
11	0.89	0.19	35	120	1.18	0.088	0.003	None	Inventive Example
12	3.61	0.07	22	120	2.17	0.044	0.001	None	Inventive Example
13	1.16	0.13	18	180	2.28	0.077	0.002	None	Inventive Example
14	0.87	0.11	22	180	2.14	0.071	0.002	None	Inventive Example
15	1.56	0.10	22	120	2.32	0.078	0.001	None	Inventive Example
16	4.46	0.91	22	180	2.07	0.052	0.001	None	Inventive Example
17	1.91	0.06	22	180	2.02	0.068	0.004	None	Inventive Example
18	8.78	0.10	18	180	2.21	0.025	0.002	None	Inventive Example
19	6.30	0.08	22	120	2.26	0.014	0.001	None	Inventive Example
20	5.57	1.11	18	180	2.57	0.018	0.002	None	Inventive Example
21	14.13	0.34	22	180	2.17	0.017	0.004	None	Inventive Example
22	2.28	0.22	18	180	2.53	0.069	0.004	None	Inventive Example
23	8.33	0.74	6	180	1.07	0.038	0.002	None	Inventive Example
24	1.13	0.22	22	180	2.03	0.077	0.003	None	Inventive Example
25	0.71	0.12	18	180	2.14	0.072	0.002	None	Inventive Example
26	3.29	0.90	22	120	2.24	0.063	0.004	None	Inventive Example
27	10.67	1.40	18	180	2.11	0.024	0.004	None	Inventive Example
28	10.92	0.24	22	180	2.15	0.018	0.001	None	Inventive Example
29	11.33	0.21	22	180	2.11	0.023	0.001	None	Inventive Example
30	10.26	0.31	22	120	2.05	0.012	0.002	None	Inventive Example
31	1.25	0.11	22	120	2.21	0.073	0.001	None	Inventive Example
32	2.76	0.30	22	120	2.03	0.063	0.001	None	Inventive Example
33	7.24	0.24	22	120	2.09	0.026	0.002	None	Inventive Example
34	6.60	0.32	10	120	2.09	0.028	0.001	None	Inventive Example
35	5.00	0.04	22	180	2.03	0.065	0.002	None	Inventive Example
36	1.17	0.17	22	180	2.10	0.075	0.001	None	Inventive Example
37	1.45	0.27	22	1000	1.79	0.087	0.002	None	Inventive Example
38	2.22	0.42	9	100	28.23	0.042	0.001	None	Inventive Example
39	1.51	0.14	9	100	16.38	0.043	0.002	None	Inventive Example
40	9.23	0.54	22	180	2.05	0.170	0.008	Present	Comparative Example
41	0.00	0.17	22	180	2.07	0.220	0.002	None	Comparative Example
42	0.73	0.22	22	180	2.01	0.180	0.003	None	Comparative Example
43	15.20	0.02	22	180	0.25	0.110	0.001	None	Comparative Example
44	23.90	0.19	22	180	-	-	-	-	Comparative Example
45	17.08	0.32	22	180	-	-	-	-	Comparative Example
46	18.37	0.27	22	180	-	-	-	-	Comparative Example
47	0.52	0.11	22	180	2.02	0.250	0.001	None	Comparative Example
48	0.67	0.18	22	180	2.08	0.370	0.001	None	Comparative Example
49	0.68	0.22	22	180	2.15	0.360	0.001	None	Comparative Example
50	8.46	4.70	22	180	2.03	0.018	0.007	Present	Comparative Example
51	7.72	1.58	22	180	2.13	0.025	0.007	Present	Comparative Example
52	4.91	1.52	22	180	2.13	0.068	0.011	Present	Comparative Example

The ingot of each test number was heated for three hours at 1200°C. After being heated, each ingot was subjected to hot forging to produce an intermediate steel material in which a cross section perpendicular to the longitudinal direction was 70 mm × 100 mm. The intermediate steel material was heated for one hour at 1250°C. After being heated, the intermediate steel material was subjected hot rolling to produce an intermediate steel material having the shape of a steel plate with a thickness of 17 mm.
The intermediate steel material after hot rolling was subjected to a solution treatment. The solution treatment temperature was set to 950°C, and the holding time at the solution treatment temperature was set to 15 minutes. After the holding time elapsed, the intermediate steel material was cooled. Specifically, the average cooling rate CR2 until the surface temperature of the intermediate starting material reached 850°C from the solution treatment temperature (950°C) was as shown in the column "CR2 (°C/min)" in Table 2. Further, for each test number, the cooling performed thereafter was water cooling. Therefore, the average cooling rate CR3 until the surface temperature of the intermediate starting material reached 300°C from 850°C was 1000°C/min or more.
The duplex stainless steel material (steel plate) of each test number was produced by the production processes described above.
Note that, the microstructure of the duplex stainless steel material of each test number was observed using the method described in "Microstructure observation method" that is described above. For the microstructure observation, a test specimen having an observation surface with dimensions of 5 mm in the rolling direction and 5 mm in the thickness direction was prepared from the center position of the thickness of the steel plate. As a result, in each test number, the microstructure of the duplex stainless steel material was composed of ferrite and austenite, and the volume ratio of ferrite was 30 to 80%.

[Evaluation tests]

The duplex stainless steel material of each test number was subjected to the following evaluation tests.

(Test 1) Total number density ND measurement test
(Test 2) Test to evaluate general corrosion resistance in high-temperature and high-pressure strongly acidic corrosive environment
(Test 3) Test to evaluate pitting resistance in high-temperature and high-pressure chloride corrosive environment

Test 1 to Test 3 are described hereunder.

[(Test 1) Total number density ND measurement test]

The total number density ND (pieces/mm²) of the fine Ca oxysulfides, the fine Mg oxides, the fine Al nitrides, and the fine Ti nitrides in the duplex stainless steel material of each test number was determined by the method described in [ Method for measuring total number density ND] that is described above. Note that, the size of each observation visual field was set to a rectangle of 15 mm × 15 mm, the number of the observation visual fields was set to five, and the observation surface was observed at a magnification of ×500 using an SEM. The obtained total number density ND is shown in the column "ND (pieces/mm²)" in Table 2.

[(Test 2) Test to evaluate general corrosion resistance in high-temperature and high-pressure strongly acidic corrosive environment]

The corrosion rate (g•cm^-2•h^-1) in the high-temperature and high-pressure strongly acidic corrosive environment of the duplex stainless steel material of each test number was determined by the method described in [Test to evaluate general corrosion resistance in high-temperature and high-pressure strongly acidic corrosive environment] that is described above. Removal of corrosive products from the test specimens was performed according to the method specified in ASTM G31-21. The size of the test specimen was set to 40 mm in length, 10 mm in width, and 3 mm in thickness. The obtained corrosion rate is shown in the column "Corrosion Rate (g•cm^-2•h^-1)" of the column "High-temperature and High-pressure Strongly Acidic Corrosive Environment" in Table 2.

[(Test 3) Test to evaluate pitting resistance in high-temperature and high-pressure chloride corrosive environment]

The corrosion rate (g•cm^-2•h^-1) in the high-temperature and high-pressure chloride corrosive environment of the duplex stainless steel material of each test number was determined and the presence or absence of pitting was confirmed by the method described in [Test to evaluate pitting resistance in high-temperature and high-pressure chloride corrosive environment] that is described above. Removal of corrosive products from the test specimens was performed according to the method specified in ASTM G31-21. The size of the test specimen was set to 40 mm in length, 10 mm in width, and 3 mm in thickness. The obtained corrosion rate is shown in the column "Corrosion Rate (g•cm^-2•h^-1)" of the column "High-temperature and High-pressure Chloride Corrosive Environment", and whether or not pitting was present is shown in the column "Pitting" in Table 2.

[Evaluation results]

The evaluation results are shown in Table 2. Note that, in Table 2, Fn1 of each test number is shown in the column "Fn1", and Fn2 of each test number is shown in the column "Fn2".
Referring to Table 1-1, Table 1-2, and Table 2, the duplex stainless steel materials of Test Nos. 1 to 39 satisfied Feature 1 to Feature 3. Therefore, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was 0.100 g•cm^-2•h^-1 or less. In addition, the corrosion rate in the high-temperature and high-pressure chloride corrosive environment was 0.005 g•cm^-2•h^-1 or less, and pitting was also not confirmed. Thus, in the duplex stainless steel materials of these test numbers, excellent general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment was obtained, and excellent pitting resistance in the high-temperature and high-pressure chloride corrosive environment was obtained.
In addition, among Test Nos. 1 to 39, in Test Nos. 1 to 10, 12 to 22, 24 to 36, 38 and 39 aforementioned condition 1 and condition 2 were satisfied in the production process. Therefore, the duplex stainless steel materials of these test numbers satisfied Feature 1 to Feature 3, and also satisfied Feature 4. As a result, with respect to the duplex stainless steel materials of these test numbers, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was even more excellent.
Specifically, with respect to Test Nos. 1 to 39, in a case where the Fnl values of the duplex stainless steel materials were approximately equal, more excellent general corrosion resistance was obtained in the duplex stainless steel material that satisfied Feature 1 to Feature 4 in comparison to the duplex stainless steel material that satisfied Feature 1 to Feature 3 but did not satisfy Feature 4.
For example, looking at Test No. 11 and Test No. 14, Fn1 of Test No. 11 was 0.89, which was a value close to Fn1 (= 0.87) of Test No. 14. However, although the duplex stainless steel material of Test No. 11 satisfied Feature 1 to Feature 3, it did not satisfy Feature 4, while on the other hand the duplex stainless steel material of Test No. 14 satisfied Feature 1 to Feature 4. As a result, with respect to Test No. 14, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was slower than in Test No. 11, and more excellent general corrosion resistance was obtained in the high-temperature and high-pressure strongly acidic corrosive environment.
Similarly, looking at Test No. 18 and Test No. 23, Fn1 of Test No. 18 was 8.78, which was a value close to Fn1 (= 8.33) of Test No. 23. However, the duplex stainless steel material of Test No. 18 satisfied Feature 1 to Feature 4, while on the other hand, although the duplex stainless steel material of Test No. 23 satisfied Feature 1 to Feature 3, it did not satisfy Feature 4. As a result, with respect to Test No. 18, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was slower than in Test No. 23, and more excellent general corrosion resistance was obtained in the high-temperature and high-pressure strongly acidic corrosive environment.
Looking at Test No. 9 and Test No. 37, Fnl of Test No. 9 was 1.41, which was a value close to Fn1 (= 1.45) of Test No. 37. However, the duplex stainless steel material of Test No. 9 satisfied Feature 1 to Feature 4, while on the other hand, although the duplex stainless steel material of Test No. 37 satisfied Feature 1 to Feature 3, it did not satisfy Feature 4. As a result, with respect to Test No. 9, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was slower than in Test No. 37, and more excellent general corrosion resistance was obtained in the high-temperature and high-pressure strongly acidic corrosive environment.
In particular, in a case where Fn1 was more than 0.70 to less than 5.50, in each of the duplex stainless steel materials that satisfied Feature 1 to Feature 4 (Test Nos. 1, 4 to 6, 9, 12 to 17, 22, 24 to 26, 31, 32, 35, 36, 38 and 39), the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was 0.080 g•cm^-2•h^-1 or less. On the other hand, in a case where Fnl was more than 0.70 to less than 5.50, in the duplex stainless steel materials (Test Nos. 11 and 37) which satisfied Feature 1 to Feature 3 but did not satisfy Feature 4, although the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was 0.100 g•cm²•h^-1 or less, the corrosion rate was more than 0.080 g•cm^-2•h^-1.
Note that, in Test Nos. 2, 3, 7, 8, 10, 18 to 21, 23, 27 to 30, 33 and 34, Fn1 was 5.50 or more. Therefore, with regard to the duplex stainless steel materials of these test numbers, irrespective of whether or not Feature 4 was satisfied, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was 0.040 g•cm^-2•h^-1 or less, and thus even more excellent general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment was obtained.
On the other hand, in Test No. 40, the content of Cr was too low. Therefore, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was more than 0.100 g•cm^-2•h^-1, and excellent general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment could not be obtained. In addition, the corrosion rate in the high-temperature and high-pressure chloride corrosive environment was more than 0.005 g•cm^-2•h^-1, and pitting was also confirmed, and thus excellent pitting resistance in the high-temperature and high-pressure chloride corrosive environment could not be obtained.
In Test Nos. 41 and 42, the content of As was too low. Therefore, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was more than 0.100 g•cm^-2•h^-1, and excellent general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment could not be obtained.
In Test No. 43, the total content of Ca and Mg was too low. Therefore, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was more than 0.100 g•cm^-2•h^-1, and excellent general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment could not be obtained.
In Test Nos. 44 to 46, although the duplex stainless steel materials satisfied Feature 1, Fn1 was too high. Therefore, in the process of subjecting the ingot to hot forging during the production process, a crack occurred. Therefore, for these test numbers, the production processes after hot forging were not performed and tests were also not performed.
In Test Nos. 47 to 49, although the duplex stainless steel materials satisfied Feature 1, Fn1 was too low. Therefore, the corrosion rate in the high-temperature and high-pressure strongly acidic corrosive environment was more than 0.100 g•cm^-2•h^-1, and excellent general corrosion resistance in the high-temperature and high-pressure strongly acidic corrosive environment could not be obtained.
In Test Nos. 50 to 52, although the duplex stainless steel materials satisfied Feature 1, Fn2 was too high. The corrosion rate in the high-temperature and high-pressure chloride corrosive environment was more than 0.005 g•cm^-2•h^-1 and pitting was also confirmed, and thus excellent pitting resistance in the high-temperature and high-pressure chloride corrosive environment could not be obtained.
An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range not departing from the gist thereof.

Claims

A duplex stainless steel material having a chemical composition consisting of, in mass%,
C: 0.050% or less,

Si: 0.2 to 1.2%,

Mn: 0.5 to 7.01%,

P: 0.0401% or less,

S: 0.010% or less,

Cr: 20.0 to 27.0%,

Ni: 4.0 to 9.0%,

Mo: 0.5 to 5.0%,

As: 0.0005 to 0.0100%,

One or more types of element among Ca and Mg: 0.0005 to 0.0100% in total,

sol. Al: 0.001 to 0.050%,

N: 0.40% or less,

O: 0.100% or less,

Cu: 0 to 4.0%,

V: 0 to 1.50%,

Co: 0 to 2.00%,

Ta: 0 to 2.00%,

W: 0 to 4.00%,

Nb: 0 to 2.00%,

Ti: 0 to 2.00%,

Zn: 0 to 0.0100%,

Pb: 0 to 0.0100%,

Sb: 0 to 0.0100%,

Sn: 0 to 0.0100%,

Bi: 0 to 0.0100%,

B: 0 to 0.0100%,

rare earth metal: 0 to 0.050%,

Zr: 0 to 2.00%, and

Hf: 0 to 2.00%, with

the balance being Fe and impurities,

and satisfying Formulae (1) and (2): $0.70 < 10000 \times As / (Ni + Cu) < 16.00$ $(Ca + Mg) / O < 1.50$

where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formulae, and if an element is not contained, "0" is substituted for a corresponding symbol of an element.
The duplex stainless steel material according to claim 1, wherein
when:
a particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a total of a content of Ca and a content of S is higher than 5.0%, a content of O is 1.0% or more, and a content of Ca is higher than a content of S is defined as a fine Ca oxysulfide;

a particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a content of Mg is 5.0% or more, a content of O is 1.0% or more, and a content of S is 15.0% or less is defined as a fine Mg oxide;

a particle which has an equivalent circular diameter of 1.0 to 2.0 µm and in which, in mass%, a content of Al is 20.0%, or more and a content of N is 20.0% or more is defined as a fine Al nitride; and

a particle which has an equivalent circular diameter of 1.0 to 2.0 µm, and in which, in mass%, a content of Ti is 30.0% or more and a content of N is 20.0% or more is defined as a fine Ti nitride,

a total number density of the fine Ca oxysulfides, the fine Mg oxides, the fine Al nitrides, and the fine Ti nitrides is 2.00 pieces/mm² or more.
The duplex stainless steel material according to claim 1 or claim 2, wherein the chemical composition contains one or more types of element selected from a group consisting of:
Cu: 0.1 to 4.0%,

V: 0.01 to 1.50%,

Co: 0.01 to 2.00%,

Ta: 0.01 to 2.00%,

W: 0.01 to 4.00%,

Nb: 0.01 to 2.00%,

Ti: 0.01 to 2.00%,

Zn: 0.0001 to 0.0100%,

Pb: 0.0001 to 0.0100%,

Sb: 0.0001 to 0.0100%,

Sn: 0.0001 to 0.0100%,

Bi: 0.0001 to 0.0100%,

B: 0.0001 to 0.0100%,

rare earth metal: 0.001 to 0.050%,

Zr: 0.01 to 2.00%, and

Hf: 0.01 to 2.00%.