JP7364955B1 - Duplex stainless steel material - Google Patents

Abstract

The present invention provides a duplex stainless steel material having high strength and excellent hot workability. [Solution] The duplex stainless steel material according to the present disclosure includes, in mass %, C: 0.030% or less, Si: 1.00% or less, Mn: 0.10 to 9.00%, and P: 0.040%. Below, S: 0.0010% or less, Cr: 20.0-32.0%, Ni: 3.5-10.0%, Mo: 0.5-5.0%, Cu: 0.5-6 .0%, V: 0.01% or more and less than 0.10%, B: 0.0010 to 0.0050%, N: less than 0.150%, and O: 0.0001 to 0.0070%. However, the balance consists of Fe and impurities, and the chemical composition satisfies formulas (1A) and (2), the microstructure consists of 30 to 80% ferrite in terms of volume fraction, and the balance consists of austenite, and a yield strength of 655 MPa or more. and has. 24.0<Cr+3.3×Mo+16×N<35.0 (1A)(N×S/B)×103<11.5 (2) [Selection diagram] Figure 2

Description

TECHNICAL FIELD The present disclosure relates to steel materials, and more particularly to duplex stainless steel materials.

Oil wells and gas wells (hereinafter referred to collectively as "oil wells") are located in corrosive environments containing corrosive hydrogen sulfide gas (H ₂ S) and carbon dioxide gas (CO ₂ ). be. It has been known that chromium (Cr) is effective in improving the corrosion resistance of steel materials in such corrosive environments. Therefore, in oil wells, which are corrosive environments, duplex stainless steel materials with increased Cr content are sometimes used.

In recent years, development of deep wells beneath the sea surface has become more active. Therefore, there has been a demand for higher strength duplex stainless steel materials. Therefore, Japanese Patent Application Publication No. 5-132741 (Patent Document 1), International Publication No. 2012/111536 (Patent Document 2), Japanese Patent Application Publication No. 2014-043616 (Patent Document 3), and Japanese Patent Application Publication No. 2016-3377 (Patent Document 4) proposes a duplex stainless steel material having high strength.

The duplex stainless steel material disclosed in Patent Document 1 is a duplex stainless steel, and in weight percent, C: 0.03% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.040% or less, S: 0.008% or less, sol. Al: 0.040% or less, Ni: 5.0 to 9.0%, Cr: 23.0 to 27.0%, Mo: 2.0 to 4.0%, W: more than 1.5 to 5. 0%, N: 0.24 to 0.32%, the balance being Fe and inevitable impurities, and PREW (=Cr+3.3(Mo+0.5W)+16N) is 40 or more. Patent Document 1 describes that this duplex stainless steel exhibits excellent corrosion resistance and high strength.

The duplex stainless steel material disclosed in Patent Document 2 is a duplex stainless steel, and in mass %, C: 0.030% or less, Si: 0.20 to 1.00%, Mn: 8.00% or less. , P: 0.040% or less, S: 0.0100% or less, Cu: more than 2.00 to 4.00% or less, Ni: 4.00 to 8.00%, Cr: 20.0 to 30.0 %, Mo: 0.50 to less than 2.00%, N: 0.100 to 0.350%, and Al: 0.040% or less, with the remainder consisting of Fe and impurities, and ferrite. The hardness of the ferrite is 30 to 70% and the hardness of ferrite is 300Hv _10gf or more. Patent Document 2 describes that this duplex stainless steel material has high strength and high toughness.

The duplex stainless steel material disclosed in Patent Document 3 is a duplex stainless steel, and in mass %, C: 0.03% or less, Si: 0.3% or less, Mn: 3.0% or less, P: 0.040% or less, S: 0.008% or less, Cu: 0.2 to 2.0%, Ni: 5.0 to 6.5%, Cr: 23.0 to 27.0%, Mo: 2 .5 to 3.5%, W: 1.5 to 4.0%, N: 0.24 to 0.40%, and Al: 0.03% or less, with the remainder consisting of Fe and impurities. , σ phase susceptibility index X (=2.2Si+0.5Cu+2.0Ni+Cr+4.2Mo+0.2W) is 52.0 or less, strength index Y (=Cr+1.5Mo+10N+3.5W) is 40.5 or more, and pitting corrosion resistance is achieved. It has a chemical composition with an index PREW (=Cr+3.3(Mo+0.5W)+16N) of 40 or more. The structure of steel is such that when a straight line parallel to the thickness direction is drawn from the surface layer to a depth of 1 mm in a thickness direction cross section parallel to the rolling direction, the number of boundaries between the ferrite phase and the austenite phase that intersect with the straight line is 160. That's all. Patent Document 3 describes that this duplex stainless steel material can be made to have high strength without impairing its corrosion resistance, and exhibits excellent hydrogen embrittlement resistance when combined with a high degree of cold working.

The duplex stainless steel material disclosed in Patent Document 4 is a duplex stainless steel pipe, and in mass %, C: 0.03% or less, Si: 0.2 to 1%, Mn: 0.5 to 2.0. %, P: 0.040% or less, S: 0.010% or less, Al: 0.040% or less, Ni: 4 to less than 6%, Cr: 20 to less than 25%, Mo: 2.0 to 4. 0%, N: 0.1 to 0.35%, O: 0.003% or less, V: 0.05 to 1.5%, Ca: 0.0005 to 0.02%, and B: 0. 0005 to 0.02%, with the remainder being Fe and impurities, the metal structure is composed of a two-phase structure of a ferrite phase and an austenite phase, and there is no precipitation of a sigma phase, and In terms of area ratio, the proportion of the ferrite phase in the metal structure is 50% or less, and the number of oxides with a grain size of 30 μm or more existing in a 300 mm ² field of view is 15 or less. Patent Document 4 describes that this duplex stainless steel material has excellent strength, pitting corrosion resistance, and low-temperature toughness.

Japanese Patent Application Publication No. 5-132741 International Publication No. 2012/111536 Japanese Patent Application Publication No. 2014-043616 JP 2016-3377 Publication

By the way, duplex stainless steel materials are sometimes subjected to hot working such as hot rolling or hot extrusion during manufacture. Therefore, in addition to high strength, duplex stainless steel materials are required to have excellent hot workability. However, in Patent Documents 1 to 4 mentioned above, hot workability is not studied.

An object of the present disclosure is to provide a duplex stainless steel material that has high strength and excellent hot workability.

The duplex stainless steel material according to the present disclosure includes:
In mass%,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 0.10-9.00%,
P: 0.040% or less,
S: 0.0010% or less,
Cr: 20.0-32.0%,
Ni: 3.5-10.0%,
Mo: 0.5-5.0%,
Cu: 0.5-6.0%,
V: 0.01% or more and less than 0.10%,
B: 0.0010-0.0050%,
N: less than 0.150%, and
Contains O: 0.0001 to 0.0070%,
The remainder consists of Fe and impurities,
A chemical composition that satisfies formulas (1A) and (2),
30 to 80% ferrite by volume, and
A microstructure in which the remainder consists of austenite,
It has a yield strength of 655 MPa or more.
24.0<Cr+3.3×Mo+16×N<35.0 (1A)
(N×S/B)×10 ³ <11.5 (2)
Here, each element symbol in formulas (1A) and (2) is substituted with the content of the corresponding element in mass %.

The duplex stainless steel material according to the present disclosure includes:
In mass%,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 0.10-9.00%,
P: 0.040% or less,
S: 0.0010% or less,
Cr: 20.0-32.0%,
Ni: 3.5-10.0%,
Mo: 0.5-5.0%,
Cu: 0.5-6.0%,
V: 0.01% or more and less than 0.10%,
B: 0.0010-0.0050%,
N: less than 0.150%, and
Contains O: 0.0001 to 0.0070%, and further,
Nb: 0.100% or less,
Al: 0.100% or less,
Ta: 0.100% or less,
Ti: 0.100% or less,
Zr: 0.100% or less,
Hf: 0.100% or less,
W: 0.50% or less,
Co: 0.50% or less,
Sn: 0.1000% or less,
Sb: 0.1000% or less,
Bi: 0.1000% or less,
Pb: 0.0030% or less,
Ca: 0.0050% or less,
Mg: 0.1000% or less, and
Contains one or more selected from the group consisting of rare earth elements: 0.1000% or less,
The remainder consists of Fe and impurities,
A chemical composition that satisfies formulas (1B) and (2),
30 to 80% ferrite by volume, and
A microstructure in which the remainder consists of austenite,
It has a yield strength of 655 MPa or more.
24.0<Cr+3.3×(Mo+0.5×W)+16×N<35.0 (1B)
(N×S/B)×10 ³ <11.5 (2)
Here, each element symbol in formulas (1B) and (2) is substituted with the content of the corresponding element in mass %. If an element is not contained, "0" is assigned to the corresponding element symbol.

Duplex stainless steel materials according to the present disclosure have high strength and excellent hot workability.

Figure 1 shows the relationship between the value of Fn1 (=Cr+3.3×(Mo+0.5×W)+16×N) in this example and the reduction of area (%) which is an index of hot workability of duplex stainless steel material. It is a figure showing a relationship. FIG. 2 is a diagram showing the relationship between the value of Fn2 (=(N×S/B)×10 ³ ) and the reduction of area (%), which is an index of hot workability of duplex stainless steel material, in this example. It is.

First, the present inventors considered obtaining a duplex stainless steel material having a yield strength of 655 MPa or more as high strength. In other words, the present inventors investigated and studied a method for obtaining a duplex stainless steel material that has both a yield strength of 655 MPa or more and excellent hot workability. As a result, the present inventors obtained the following findings.

The present inventors first studied a duplex stainless steel material that has both a yield strength of 655 MPa or more and excellent hot workability from the viewpoint of chemical composition. As a result of detailed studies by the present inventors, it has been revealed that the hot workability of steel materials is significantly improved by reducing the nitrogen (N) content. Until now, in duplex stainless steel materials, N has been known to have the effect of stabilizing austenite, increasing the corrosion resistance of steel materials, and increasing the strength of steel materials by solid solution in the steel materials. . Therefore, in duplex stainless steel materials, the N content has been increased to a certain level. However, as a result of detailed studies by the present inventors, it has become clear that if the N content is reduced to less than 0.150%, the hot workability of the steel material may be significantly improved.

Therefore, the present inventors investigated the chemical composition of a duplex stainless steel material that can achieve both a yield strength of 655 MPa or more and excellent hot workability on the premise of reducing the N content to less than 0.150%. were examined in detail. As a result, the present inventors found that in mass %, C: 0.030% or less, Si: 1.00% or less, Mn: 0.10 to 9.00%, P: 0.040% or less, S: 0.0010% or less, Cr: 20.0 to 32.0%, Ni: 3.5 to 10.0%, Mo: 0.5 to 5.0%, Cu: 0.5 to 6.0%, Contains V: 0.01% or more and less than 0.10%, B: 0.0010 to 0.0050%, N: less than 0.150%, and O: 0.0001 to 0.0070%, with the remainder being Consists of Fe and impurities, and when containing arbitrary elements, in place of a part of Fe, Nb: 0.100% or less, Al: 0.100% or less, Ta: 0.100% or less, Ti: 0 .100% or less, Zr: 0.100% or less, Hf: 0.100% or less, W: 0.50% or less, Co: 0.50% or less, Sn: 0.1000% or less, Sb: 0.1000 % or less, Bi: 0.1000% or less, Pb: 0.0030% or less, Ca: 0.0050% or less, Mg: 0.1000% or less, and rare earth elements: 0.1000% or less. It was considered that a duplex stainless steel material containing one or more of the selected types has the possibility of achieving both a yield strength of 655 MPa or more and excellent hot workability.

Here, the microstructure of the duplex stainless steel material having the above-mentioned chemical composition consists of ferrite and austenite. Specifically, the microstructure of the duplex stainless steel material having the above-mentioned chemical composition consists of ferrite with a volume fraction of 30 to 80%, and the remainder being austenite. In this specification, "consisting of ferrite and austenite" means that the amount of phases other than ferrite and austenite is negligible.

On the other hand, even with a duplex stainless steel material having the above chemical composition, the above microstructure, and a yield strength of 655 MPa or more, excellent hot workability may not be stably obtained in some cases. Therefore, the present inventors conducted a detailed investigation and study on a method to improve hot workability for duplex stainless steel materials having the above-mentioned chemical composition, above-mentioned microstructure, and yield strength of 655 MPa or more. . As a result, the present inventors obtained the following findings.

[Regarding formulas (1A) and (1B)]
As a result of detailed study by the present inventors, the chromium (Cr) content, molybdenum (Mo) content, It has become clear that when the N content is contained, the hot workability of the steel material can be improved by adjusting the tungsten (W) content. Specifically, if the chemical composition consists of essential elements, formula (1A) is satisfied, and if the chemical composition consists of essential elements and optional elements, formula (1B) is satisfied, then the other configurations of this embodiment are satisfied. It has become clear that it is possible to achieve both a yield strength of 655 MPa or more and excellent hot workability under the following conditions.
24.0<Cr+3.3×Mo+16×N<35.0 (1A)
24.0<Cr+3.3×(Mo+0.5×W)+16×N<35.0 (1B)
Here, each element symbol in formulas (1A) and (1B) is substituted with the content of the corresponding element in mass %. If an element is not contained, "0" is assigned to the corresponding element symbol.

It is defined as Fn1=Cr+3.3×(Mo+0.5×W)+16×N. As described above, when W is not contained, "0" is substituted for "W" in Fn1. Therefore, in this specification, in the following description, Formulas (1A) and (1B) will be explained using Fn1 regardless of whether W is contained or not. Fn1 is known as an index of pitting corrosion resistance of duplex stainless steel materials. Specifically, in the above Patent Documents 1 and 3, Fn1 is described as "PREW", and it is disclosed that the pitting corrosion resistance of the steel material is increased by setting Fn1 to 40 or more (paragraph [ of Patent Document 1] 0005] and paragraph [0038] of Patent Document 3). Note that pitting corrosion resistance means corrosion resistance against pitting corrosion and/or crevice corrosion.

As disclosed in Patent Documents 1 and 3 above, it is considered that the larger Fn1 is, the better the pitting corrosion resistance is. On the other hand, as a result of detailed study by the present inventors, if Fn1 is reduced to less than 35.0, the hot workability of the duplex stainless steel material increases, provided that the other configurations of this embodiment are satisfied. It became clear. Specifically, FIG. 1 is a diagram showing the relationship between the value of Fn1 and the reduction of area (%), which is an index of hot workability of a duplex stainless steel material, in this example. FIG. 1 was created using the value of Fn1 and the aperture value (%) for an example that satisfies the configuration other than Fn1 of this embodiment among the examples described later. Note that the aperture value (%) was determined by the method described below.

Referring to FIG. 1, in a duplex stainless steel material that satisfies the other configurations of this embodiment, if Fn1 is less than 35.0, the reduction of area will be 80% or more, and it will have excellent hot workability. Can be confirmed. Moreover, if Fn1 is too low, the pitting corrosion resistance of the steel material may decrease. Therefore, the duplex stainless steel material according to this embodiment has Fn1 of more than 24.0 and less than 35.0, provided that the other configurations of this embodiment are satisfied.

[About formula (2)]
As a result of detailed study by the present inventors, regarding a duplex stainless steel material having the above chemical composition satisfying Fn1 of more than 24.0 to less than 35.0, the above-mentioned microstructure, and a yield strength of 655 MPa or more, It has been revealed that the hot workability of steel materials can be stably improved by adjusting the N content, sulfur (S) content, and boron (B) content. Specifically, in a duplex stainless steel material having the above-mentioned chemical composition with Fn1 of more than 24.0 and less than 35.0 and the above-mentioned microstructure, if the following formula (2) is satisfied, the yield is 655 MPa or more. It has become clear that both strength and excellent hot workability can be stably achieved.
(N×S/B)×10 ³ <11.5 (2)
Here, each element symbol in formula (2) is substituted with the content of the corresponding element in mass %.

Define Fn2=(N×S/B)×10 ³ . Fn2 is an index of hot workability of a duplex stainless steel material having the above-mentioned chemical composition. As a result of detailed study by the present inventors, the hot workability of duplex stainless steel material increases significantly if Fn2 is reduced to less than 11.5, provided that the other configurations of this embodiment are satisfied. It became clear. Specifically, FIG. 2 is a diagram showing the relationship between the value of Fn2 and the reduction of area (%), which is an index of hot workability of the duplex stainless steel material, in this example. FIG. 2 shows, among the examples described below, examples having the above-mentioned chemical composition, the above-mentioned microstructure, and a yield strength of 655 MPa or more, and satisfying Fn1 of more than 24.0 and less than 35.0. It was created using the Fn2 value and the aperture value (%). Note that the aperture value (%) was determined by the method described below.

Referring to FIG. 2, in a duplex stainless steel material having the above chemical composition, the above microstructure, and a yield strength of 655 MPa or more, and satisfying Fn1 of more than 24.0 and less than 35.0, Fn2 is If it is less than 11.5, the reduction of area will be 80% or more, and it can be confirmed that it has excellent hot workability. Therefore, the duplex stainless steel material according to the present embodiment has the above-mentioned chemical composition, the above-mentioned microstructure, and a yield strength of 655 MPa or more, and satisfies Fn1 of more than 24.0 and less than 35.0. Furthermore, Fn2 is made less than 11.5. As a result, the duplex stainless steel material according to this embodiment can have both high strength of 655 MPa or more and excellent hot workability.

In addition, by setting Fn2 to less than 11.5, a two-phase structure having the above-mentioned chemical composition, the above-mentioned microstructure, and a yield strength of 655 MPa or more, and satisfying Fn1 of more than 24.0 and less than 35.0. The details of why the hot workability of stainless steel materials increases significantly are not clear. However, the present inventors speculate as follows. When the hot workability of a duplex stainless steel material is low, cracks occur at the interface between ferrite and austenite (phase interface) and/or at the grain boundaries of each phase (ferrite and austenite), and the cracks propagate. It is likely that it has become easier. In other words, if the phase interfaces and grain boundaries of each phase can be strengthened, the hot workability of the duplex stainless steel material may be improved. Here, S segregates at phase interfaces and grain boundaries of each phase, embrittles these, and may promote the generation and propagation of cracks. In addition, B segregates at phase interfaces and grain boundaries of each phase, strengthens these, and has the possibility of suppressing the occurrence and propagation of cracks. Note that, as described above, if the N content is reduced, the hot workability of the duplex stainless steel material may be improved. Therefore, the present inventors speculate that the hot workability of duplex stainless steel materials may be improved by controlling the balance of N content, S content, and B content. . Note that hereinafter, the phase interface (the interface between ferrite and austenite) and the grain boundaries of each phase (ferrite and austenite) are also collectively referred to as "grain boundaries."

Note that it is possible that the hot workability of the duplex stainless steel material is enhanced by a mechanism other than the above-mentioned mechanism. However, if the duplex stainless steel material has the above-mentioned chemical composition, the above-mentioned microstructure, and a yield strength of 655 MPa or more, and satisfies Fn1 of more than 24.0 and less than 35.0, Fn2 is less than 11.5. The fact that this significantly improves hot workability is proven by the Examples described below.

The gist of the duplex stainless steel material according to this embodiment, which was completed based on the above knowledge, is as follows.

[1]
In mass%,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 0.10-9.00%,
P: 0.040% or less,
S: 0.0010% or less,
Cr: 20.0-32.0%,
Ni: 3.5-10.0%,
Mo: 0.5-5.0%,
Cu: 0.5-6.0%,
V: 0.01% or more and less than 0.10%,
B: 0.0010-0.0050%,
N: less than 0.150%, and
Contains O: 0.0001 to 0.0070%,
The remainder consists of Fe and impurities,
A chemical composition that satisfies formulas (1A) and (2),
30 to 80% ferrite by volume, and
A microstructure in which the remainder consists of austenite,
having a yield strength of 655 MPa or more,
Duplex stainless steel material.
24.0<Cr+3.3×Mo+16×N<35.0 (1A)
(N×S/B)×10 ³ <11.5 (2)
Here, each element symbol in formulas (1A) and (2) is substituted with the content of the corresponding element in mass %.

[2]
In mass%,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 0.10-9.00%,
P: 0.040% or less,
S: 0.0010% or less,
Cr: 20.0-32.0%,
Ni: 3.5-10.0%,
Mo: 0.5-5.0%,
Cu: 0.5-6.0%,
V: 0.01% or more and less than 0.10%,
B: 0.0010-0.0050%,
N: less than 0.150%, and
Contains O: 0.0001 to 0.0070%, and further,
Nb: 0.100% or less,
Al: 0.100% or less,
Ta: 0.100% or less,
Ti: 0.100% or less,
Zr: 0.100% or less,
Hf: 0.100% or less,
W: 0.50% or less,
Co: 0.50% or less,
Sn: 0.1000% or less,
Sb: 0.1000% or less,
Bi: 0.1000% or less,
Pb: 0.0030% or less,
Ca: 0.0050% or less,
Mg: 0.1000% or less, and
Contains one or more selected from the group consisting of rare earth elements: 0.1000% or less,
The remainder consists of Fe and impurities,
A chemical composition that satisfies formulas (1B) and (2),
30 to 80% ferrite by volume, and
A microstructure in which the remainder consists of austenite,
having a yield strength of 655 MPa or more,
Duplex stainless steel material.
24.0<Cr+3.3×(Mo+0.5×W)+16×N<35.0 (1B)
(N×S/B)×10 ³ <11.5 (2)
Here, each element symbol in formulas (1B) and (2) is substituted with the content of the corresponding element in mass %. If an element is not contained, "0" is assigned to the corresponding element symbol.

[3]
The duplex stainless steel material according to [2],
The chemical composition is
Nb: 0.100% or less,
Al: 0.100% or less,
Ta: 0.100% or less,
Ti: 0.100% or less,
Zr: 0.100% or less,
Hf: 0.100% or less,
W: 0.50% or less, and
Co: 0.50% or less, containing one or more selected from the group consisting of;
Duplex stainless steel material.

[4]
The duplex stainless steel material according to [2] or [3],
The chemical composition is
Sn: 0.1000% or less,
Sb: 0.1000% or less,
Bi: 0.1000% or less, and
Contains one or more selected from the group consisting of Pb: 0.0030% or less,
Duplex stainless steel material.

[5]
The duplex stainless steel material according to any one of [2] to [4],
The chemical composition is
Ca: 0.0050% or less,
Mg: 0.1000% or less, and
Rare earth elements: 0.1000% or less, containing one or more selected from the group consisting of;
Duplex stainless steel material.

The shape of the duplex stainless steel material according to this embodiment is not particularly limited. The duplex stainless steel material according to this embodiment may be a steel pipe, a round steel (solid material), or a steel plate. Note that the round steel means a steel bar whose cross section perpendicular to the axial direction is circular. Further, the steel pipe may be a seamless steel pipe or a welded steel pipe.

Hereinafter, the duplex stainless steel material according to this embodiment will be explained in detail. "%" with respect to elements means mass % unless otherwise specified. Further, in the following description, the duplex stainless steel material is also simply referred to as "steel material".

[Chemical composition]
The chemical composition of the duplex stainless steel according to this embodiment contains the following elements.

C: 0.030% or less Carbon (C) is unavoidably contained. In other words, the lower limit of the C content is over 0%. C forms Cr carbides at grain boundaries, increasing corrosion susceptibility at grain boundaries. Therefore, if the C content is too high, the corrosion resistance of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.030% or less. A preferable upper limit of the C content is 0.028%, more preferably 0.025%. It is preferable that the C content is as low as possible. However, extreme reduction in C content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the C content is 0.001%, more preferably 0.002%.

Si: 1.00% or less Silicon (Si) is unavoidably contained. That is, the lower limit of the Si content is over 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, the hot workability of the steel material will decrease even if the other element contents are within the ranges of this embodiment. In this case, the toughness of the steel material may further deteriorate. Therefore, the Si content is 1.00% or less. A preferable upper limit of the Si content is 0.90%, more preferably 0.80%. The lower limit of the Si content is preferably 0.12%, more preferably 0.15% in order to more effectively obtain the above effects.

Mn: 0.10-9.00%
Manganese (Mn) deoxidizes steel and desulfurizes steel. Mn further improves the hot workability of the steel material. If the Mn content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, Mn will segregate at grain boundaries together with impurities such as P and S. In this case, even if the contents of other elements are within the ranges of this embodiment, the corrosion resistance of the steel material decreases. Therefore, the Mn content is between 0.10 and 9.00%. The preferable lower limit of the Mn content is 0.30%, more preferably 0.50%, even more preferably 0.70%, even more preferably 0.90%, and still more preferably 1.00%. %. A preferable upper limit of the Mn content is 8.50%, more preferably 7.50%, 6.50%, and even more preferably 6.20%.

P: 0.040% or less Phosphorus (P) is an impurity that is inevitably contained. That is, the lower limit of the P content is over 0%. If the P content is too high, even if the contents of other elements are within the ranges of this embodiment, P will segregate at grain boundaries and the toughness of the steel material will decrease. Therefore, the P content is 0.040% or less. A preferable upper limit of the P content is 0.035%, more preferably 0.030%. It is preferable that the P content is as low as possible. However, extreme reduction in P content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.003%.

S: 0.0010% or less Sulfur (S) is an impurity that is inevitably contained. That is, the lower limit of the S content is more than 0%. If the S content is too high, even if the contents of other elements are within the ranges of this embodiment, S will segregate at grain boundaries and the hot workability of the steel material will deteriorate. In this case, the toughness of the steel material may further deteriorate. Therefore, the S content is 0.0010% or less. A preferable upper limit of the S content is 0.0009%, more preferably 0.0008%, and still more preferably 0.0007%. It is preferable that the S content is as low as possible. However, extreme reduction in S content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the S content is 0.0001%, more preferably 0.0002%.

Cr:20.0~32.0%
Chromium (Cr) increases the corrosion resistance of steel materials. Cr further increases the volume fraction of ferrite in the steel material. If the Cr content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, even if the contents of other elements are within the ranges of this embodiment, intermetallic compounds such as the σ phase are likely to be formed, and the toughness of the steel material will be reduced. Harmful phases such as sigma phase tend to precipitate, leading to a decrease in toughness. Therefore, the Cr content is between 20.0 and 32.0%. The lower limit of the Cr content is preferably 21.0%, more preferably 21.5%, and still more preferably 22.0%. A preferable upper limit of the Cr content is 31.0%, more preferably 30.0%, and still more preferably 29.0%.

Ni: 3.5-10.0%
Nickel (Ni) is an element that stabilizes austenite in steel materials. That is, Ni is an element necessary to obtain a stable two-phase structure of ferrite and austenite. Ni further improves the corrosion resistance of the steel material. If the Ni content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. Furthermore, in this case, the volume fraction of ferrite may become too high. On the other hand, if the Ni content is too high, manufacturing costs will increase. Therefore, the Ni content is 3.5-10.0%. The preferable lower limit of the Ni content is 4.0%, more preferably 4.5%, and still more preferably 5.0%. A preferable upper limit of the Ni content is 9.0%, more preferably 8.5%, still more preferably 8.0%, and still more preferably 7.5%.

Mo: 0.5-5.0%
Molybdenum (Mo) improves the corrosion resistance of steel materials. If the Mo content is too low, even if the contents of other elements are within the ranges of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Mo content is too high, the hot workability of the steel material will decrease even if the other element contents are within the ranges of this embodiment. Therefore, the Mo content is 0.5-5.0%. The lower limit of the Mo content is preferably 1.0%, more preferably 1.2%, and still more preferably 1.5%. A preferable upper limit of the Mo content is 4.8%, more preferably 4.6%, still more preferably 4.3%, and still more preferably 4.0%.

Cu: 0.5-6.0%
Copper (Cu) increases the corrosion resistance of steel materials. Cu further precipitates into the steel material and increases the strength of the steel material. If the Cu content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Cu content is too high, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.5-6.0%. Cu further precipitates into the steel material and improves the machinability of the steel material. The preferable lower limit of the Cu content to effectively obtain the above effect is 1.3%, more preferably 1.5%, still more preferably 1.7%, and even more preferably 2.0%. be. A preferable upper limit of the Cu content is 5.5%, more preferably 5.0%, and still more preferably 4.0%.

V: 0.01% or more and less than 0.10% Vanadium (V) forms carbonitrides and increases the strength of steel materials. Furthermore, V dissolves in the steel material in a high-temperature environment and improves the hot workability of the steel material. If the V content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will become too high and the hot workability of the steel material will decrease. Therefore, the V content is 0.01% or more and less than 0.10%. The lower limit of the V content is preferably 0.02%, more preferably 0.03%, and even more preferably 0.04%. A preferable upper limit of the V content is 0.09%, more preferably 0.08%, and still more preferably 0.07%.

B: 0.0010-0.0050%
Boron (B) segregates at grain boundaries in steel materials and strengthens the grain boundaries. As a result, the propagation of minute cracks at grain boundaries is suppressed and the hot workability of the steel material is improved. If the B content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content is too high, even if the contents of other elements are within the ranges of this embodiment, nitrides (BN) will be generated, reducing the toughness of the steel material. Therefore, the B content is 0.0010 to 0.0050%. The lower limit of the B content is preferably 0.0012%, more preferably 0.0015%, even more preferably 0.0020%, and even more preferably 0.0034%. A preferable upper limit of the B content is 0.0045%, more preferably 0.0040%.

N: less than 0.150% Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is over 0%. N is an element that stabilizes austenite in steel materials. N further increases the corrosion resistance of steel materials. Furthermore, N is dissolved in the steel material and increases the strength of the steel material. On the other hand, if the N content is too high, the hot workability of the steel material will decrease even if the other element contents are within the ranges of this embodiment. In this case, the toughness of the steel material may further deteriorate. Therefore, the N content is less than 0.150%. A preferable upper limit of the N content is 0.130%, more preferably 0.110%, even more preferably less than 0.100%, still more preferably 0.090%, and still more preferably 0.100%. 0.070%, more preferably 0.050%. Extreme reduction in N content significantly increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the N content is 0.001%, more preferably 0.005%, and still more preferably 0.010%.

O: 0.0001 to 0.0070%,
Oxygen (O) segregates to grain boundaries and strengthens the grain boundaries. If the O content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the O content is too high, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the steel material will deteriorate. In this case, the toughness of the steel material may further deteriorate. Therefore, the O content is 0.0001 to 0.0070%. The upper limit of the O content is preferably 0.0060%, more preferably 0.0055%. The preferable lower limit of the O content is 0.0002%, more preferably 0.0003%, even more preferably 0.0005%, still more preferably 0.0010%, even more preferably 0.0015%. %, more preferably 0.0020%.

The remainder of the chemical composition of the duplex stainless steel material according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition are those that are mixed in from ores used as raw materials, scrap, or the manufacturing environment when duplex stainless steel materials are manufactured industrially, and are unintentionally contained. This means what is permissible within a range that does not adversely affect the duplex stainless steel material according to this embodiment.

[Optional element]
The chemical composition of the duplex stainless steel material according to the present embodiment further includes one or more selected from the group consisting of Nb, Al, Ta, Ti, Zr, Hf, W, and Co in place of a part of Fe. May be contained. All of these elements are optional elements and increase the strength of the steel material.

Nb: 0.100% or less Niobium (Nb) is an optional element and does not need to be contained. That is, the Nb content may be 0%. When contained, the lower limit of the Nb content is over 0%. Nb forms carbonitrides and increases the strength of steel materials. If even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will become too high and the toughness of the steel material will decrease. Therefore, the Nb content is 0 to 0.100%, and if it is contained, it is 0.100% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.003%, and still more preferably 0.005%. A preferable upper limit of the Nb content is 0.080%, more preferably 0.070%.

Al: 0.100% or less Aluminum (Al) is an optional element and may not be contained. That is, the Al content may be 0%. When contained, the lower limit of the Al content is over 0%. Al forms nitrides and increases the strength of steel materials. If even a small amount of Al is contained, the above effects can be obtained to some extent. However, if the Al content is too high, coarse oxide-based inclusions and/or nitride-based inclusions will be formed even if the content of other elements is within the range of this embodiment, resulting in the formation of coarse oxide inclusions and/or nitride-based inclusions. Processability decreases. Therefore, the Al content is 0 to 0.100%, and if it is contained, it is 0.100% or less. The preferable lower limit of the Al content is 0.001%, more preferably 0.003%, still more preferably 0.005%, and even more preferably 0.010%. A preferable upper limit of the Al content is 0.090%, more preferably 0.085%. Note that the Al content referred to in this specification refers to "acid-soluble Al", that is, sol. It means the content of Al.

Ta: 0.100% or less Tantalum (Ta) is an optional element and does not need to be contained. That is, the Ta content may be 0%. When contained, the lower limit of the Ta content is more than 0%. Ta forms carbonitrides and increases the strength of steel materials. If even a small amount of Ta is contained, the above effects can be obtained to some extent. However, if the Ta content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will become too high and the toughness of the steel material will decrease. Therefore, the Ta content is 0 to 0.100%, and if it is contained, it is 0.100% or less. The lower limit of the Ta content is preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%. A preferable upper limit of the Ta content is 0.095%, more preferably 0.090%.

Ti: 0.100% or less Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, the lower limit of the Ti content is more than 0%. Ti forms carbonitrides and increases the strength of steel materials. If even a small amount of Ti is contained, the above effects can be obtained to some extent. However, if the Ti content is too high, coarse Ti-based carbonitrides will be formed even if the contents of other elements are within the ranges of this embodiment, and the hot workability of the steel material will deteriorate. Therefore, the Ti content is 0 to 0.100%, and if contained, it is 0.100% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%. A preferable upper limit of the Ti content is 0.080%, more preferably 0.070%.

Zr: 0.100% or less Zirconium (Zr) is an optional element and does not need to be contained. That is, the Zr content may be 0%. When contained, the lower limit of the Zr content is more than 0%. Zr forms carbonitrides and increases the strength of steel materials. If even a small amount of Zr is contained, the above effects can be obtained to some extent. However, if the Zr content is too high, coarse Zr-based carbonitrides will be formed even if the contents of other elements are within the ranges of this embodiment, and the hot workability of the steel material will deteriorate. Therefore, the Zr content is 0 to 0.100%, and if contained, it is 0.100% or less. The lower limit of the Zr content is preferably 0.001%, more preferably 0.002%, and still more preferably 0.003%. A preferable upper limit of the Zr content is 0.090%, more preferably 0.080%.

Hf: 0.100% or less Hafnium (Hf) is an optional element and does not need to be contained. That is, the Hf content may be 0%. When contained, the lower limit of the Hf content is over 0%. Hf forms carbonitrides and increases the strength of steel materials. If even a small amount of Hf is contained, the above effects can be obtained to some extent. However, if the Hf content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will become too high and the toughness of the steel material will decrease. Therefore, the Hf content is 0 to 0.100%, and if it is contained, it is 0.100% or less. The lower limit of the Hf content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. A preferable upper limit of the Hf content is 0.080%, more preferably 0.070%.

W: 0.50% or less Tungsten (W) is an optional element and does not need to be contained. That is, the W content may be 0%. When contained, the lower limit of the W content is over 0%. W solidly dissolves in steel and increases the strength of the steel material. If even a small amount of W is contained, the above effects can be obtained to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will become too high and the toughness of the steel material will decrease. Therefore, the W content is 0 to 0.50%, and if contained, it is 0.50% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.02%, even more preferably 0.03%, and still more preferably 0.05%. The upper limit of the W content is preferably 0.40%, more preferably 0.30%, even more preferably 0.20%, and still more preferably 0.15%.

Co: 0.50% or less Cobalt (Co) is an optional element and does not need to be contained. That is, the Co content may be 0%. When contained, the lower limit of the Co content is more than 0%. Co forms a film on the surface of steel and improves the corrosion resistance of the steel. Co further improves the hardenability of the steel material and stabilizes the strength of the steel material. If even a small amount of Co is contained, the above effects can be obtained to some extent. However, if the Co content is too high, even if the other element contents are within the range of this embodiment, the manufacturing cost will increase extremely. Therefore, the Co content is 0 to 0.50%, and if it is contained, it is 0.50% or less. The preferable lower limit of the Co content is 0.01%, more preferably 0.03%, even more preferably 0.05%, and still more preferably 0.10%. A preferable upper limit of the Co content is 0.45%, more preferably 0.43%, and still more preferably 0.40%.

The chemical composition of the duplex stainless steel material according to the present embodiment may further contain one or more selected from the group consisting of Sn, Sb, Bi, and Pb in place of a part of Fe. All of these elements are optional elements and improve the corrosion resistance of steel materials.

Sn: 0.1000% or less Tin (Sn) is an optional element and does not need to be contained. That is, the Sn content may be 0%. When contained, the lower limit of the Sn content is over 0%. Sn increases the corrosion resistance of steel materials. If even a small amount of Sn is contained, the above effects can be obtained to some extent. However, if the Sn content is too high, even if the other element contents are within the ranges of this embodiment, the manufacturing cost will increase extremely. Therefore, the Sn content is 0 to 0.1000%, and if it is contained, it is 0.1000% or less. The preferable lower limit of the Sn content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%. A preferable upper limit of the Sn content is 0.0500%, more preferably 0.0300%.

Sb: 0.1000% or less Antimony (Sb) is an optional element and does not need to be contained. That is, the Sb content may be 0%. When Sb is contained, the lower limit of the Sb content is more than 0%. Sb increases the corrosion resistance of steel materials. If even a small amount of Sb is contained, the above effects can be obtained to some extent. However, if the Sb content is too high, even if the contents of other elements are within the range of this embodiment, the manufacturing cost will increase extremely. Therefore, the Sb content is 0 to 0.1000%, and if contained, it is 0.1000% or less. The preferable lower limit of the Sb content is 0.0001%, more preferably 0.0002%, and still more preferably 0.0003%. A preferable upper limit of the Sb content is 0.0500%, more preferably 0.0300%.

Bi: 0.1000% or less Bismuth (Bi) is an optional element and does not need to be contained. That is, the Bi content may be 0%. When contained, the lower limit of Bi content is more than 0%. Bi increases the corrosion resistance of steel materials. If even a small amount of Bi is contained, the above effects can be obtained to some extent. However, if the Bi content is too high, even if the other element contents are within the range of this embodiment, the manufacturing cost will increase extremely. Therefore, the Bi content is 0 to 0.1000%, and if it is contained, it is 0.1000% or less. The lower limit of the Bi content is preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. A preferable upper limit of the Bi content is 0.0500%, more preferably 0.0300%.

Pb: 0.0030% or less Lead (Pb) is an optional element and does not need to be contained. That is, the Pb content may be 0%. When contained, the lower limit of the Pb content is more than 0%. Pb increases the corrosion resistance of steel materials. If even a small amount of Pb is contained, the above effects can be obtained to some extent. However, if the Pb content is too high, the manufacturing cost will increase even if the other element contents are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.0030%, and if contained, it is 0.0030% or less. The lower limit of the Pb content is preferably 0.0001%, more preferably 0.0003%, and still more preferably 0.0005%. A preferable upper limit of the Pb content is 0.0025%, more preferably 0.0020%.

The chemical composition of the duplex stainless steel material according to the present embodiment may further include one or more selected from the group consisting of Ca, Mg, and rare earth elements (REM) in place of a part of Fe. All of these elements are optional elements and improve the hot workability of the steel material.

Ca: 0.0050% or less Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, the lower limit of the Ca content is more than 0%. Ca fixes O and S in steel materials, rendering them harmless and strengthening grain boundaries. As a result, the hot workability of the steel material increases. If the Ca content is too low, even if the contents of other elements are within the ranges of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarser, and the hot workability of the steel material will deteriorate. In this case, the toughness of the steel material further decreases. Therefore, the Ca content is 0 to 0.0050%, and if it is contained, it is 0.0050% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0003%, even more preferably 0.0005%, and still more preferably 0.0010%. A preferable upper limit of the Ca content is 0.0045%, more preferably 0.0040%.

Mg: 0.1000% or less Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, the lower limit of the Mg content is over 0%. Mg fixes S in the steel material as sulfide, rendering it harmless and strengthening grain boundaries. As a result, the hot workability of the steel material increases. If even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarser, and the hot workability of the steel material will deteriorate. In this case, the toughness of the steel material further decreases. Therefore, when contained, the Mg content is 0 to 0.1000%, and when contained, it is 0.1000% or less. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0003%, even more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Mg content is preferably 0.0500%, more preferably 0.0300%.

Rare earth element: 0.1000% or less Rare earth element (REM) is an optional element and does not need to be contained. That is, the REM content may be 0%. When contained, the lower limit of the REM content is more than 0%. REM fixes S in steel materials as sulfide, rendering it harmless and strengthening grain boundaries. As a result, the hot workability of the steel material increases. If even a small amount of REM is contained, the above effects can be obtained to some extent. However, if the REM content is too high, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarser, and the hot workability of the steel material will deteriorate. In this case, the toughness of the steel material further decreases. Therefore, the REM content is between 0 and 0.1000%, and when included, it is 0.1000%. The preferable lower limit of the REM content is 0.0001%, more preferably 0.0003%, still more preferably 0.0005%, and still more preferably 0.0010%. A preferable upper limit of the REM content is 0.0900%, more preferably 0.0500%, and still more preferably 0.0300%.

In this specification, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids such as lanthanum (La) with atomic number 57 to atomic number 71. One or more elements selected from the group consisting of lutetium (Lu). Moreover, the REM content in this specification is the total content of these elements.

[Regarding formulas (1A) and (1B)]
The chemical composition of the duplex stainless steel material according to the present embodiment further satisfies formula (1A) when it consists of essential elements, and satisfies formula (1B) when it consists of essential elements and optional elements.
24.0<Cr+3.3×Mo+16×N<35.0 (1A)
24.0<Cr+3.3×(Mo+0.5×W)+16×N<35.0 (1B)
Here, each element symbol in formulas (1A) and (1B) is substituted with the content of the corresponding element in mass %. If an element is not contained, "0" is assigned to the corresponding element symbol.

Fn1 (=Cr+3.3×(Mo+0.5×W)+16×N) is known as an index of pitting corrosion resistance of duplex stainless steel material. On the other hand, in a duplex stainless steel material that satisfies the other configurations of the present embodiment, if Fn1 is less than 35.0, the reduction of area becomes 80% or more and has excellent hot workability. Moreover, if Fn1 is too low, the pitting corrosion resistance of the steel material may decrease. Therefore, the duplex stainless steel material according to this embodiment has Fn1 of more than 24.0 and less than 35.0, provided that the other configurations of this embodiment are satisfied.

In short, the duplex stainless steel material according to this embodiment has the above-mentioned chemical composition, the above-mentioned microstructure, and a yield strength of 655 MPa or more, satisfies Fn2 of less than 11.5, and further has Fn1 of 24 More than .0 to less than 35.0. As a result, the duplex stainless steel material according to this embodiment can have both high strength of 655 MPa or more and excellent hot workability. The lower limit of Fn1 is preferably 24.5, more preferably 25.0, even more preferably 25.5, and still more preferably 26.0. A preferable upper limit of Fn1 is 34.5, more preferably 34.0, still more preferably 33.5, and still more preferably 33.0. In this embodiment, Fn1 is used by rounding off the obtained value to the second decimal place.

[About formula (2)]
The chemical composition of the duplex stainless steel material according to this embodiment further satisfies the following formula (2).
(N×S/B)×10 ³ <11.5 (2)
Here, each element symbol in formula (2) is substituted with the content of the corresponding element in mass %.

Fn2 (=(N×S/B)×10 ³ ) is an index of hot workability of a duplex stainless steel material having the above-mentioned chemical composition. In a duplex stainless steel material that satisfies the other configurations of this embodiment, if Fn2 is less than 11.5, the reduction of area becomes 80% or more and has excellent hot workability. Therefore, the duplex stainless steel material according to the present embodiment has the above-mentioned chemical composition, the above-mentioned microstructure, and a yield strength of 655 MPa or more, and satisfies Fn1 of more than 24.0 and less than 35.0. Furthermore, Fn2 is less than 11.5. As a result, the duplex stainless steel material according to this embodiment can have both high strength of 655 MPa or more and excellent hot workability.

The upper limit of Fn2 is preferably 11.4, more preferably 11.0, even more preferably 10.5, and still more preferably 10.0. The lower limit of Fn2 is not particularly limited, but is substantially 0.0. The lower limit of Fn2 may be 0.1 or 0.2. In this embodiment, Fn2 is used by rounding off the obtained value to the second decimal place.

[Microstructure]
The microstructure of the duplex stainless steel material according to this embodiment consists of 30 to 80% ferrite in terms of volume fraction, and the remainder austenite. In this specification, "consisting of ferrite and austenite" means that the amount of phases other than ferrite and austenite is negligible. For example, in the chemical composition of the duplex stainless steel material according to the present embodiment, the volume fraction of precipitates and inclusions is negligibly small compared to the volume fraction of ferrite and austenite. That is, the microstructure of the duplex stainless steel material according to the present embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to ferrite and austenite.

In the microstructure of the duplex stainless steel material according to this embodiment, the volume fraction of ferrite is 30 to 80%. If the volume fraction of ferrite is too low, austenite may become coarse and the corrosion resistance of the steel material may decrease. On the other hand, if the volume fraction of ferrite is too high, desired mechanical properties may not be obtained. Therefore, in the microstructure of the duplex stainless steel material according to this embodiment, the volume fraction of ferrite is 30 to 80%. The lower limit of the volume fraction of ferrite is preferably 32%, more preferably 35%. A preferable upper limit of the volume fraction of ferrite is 75%, more preferably 70%.

In the present embodiment, the volume fraction of ferrite in the duplex stainless steel material can be determined by a method based on JIS G 0555 (2020). A test piece for microstructure observation is prepared from the duplex stainless steel material according to this embodiment. When the steel material is a steel pipe, a test piece is prepared that has an observation surface extending 5 mm in the pipe axis direction and 5 mm in the pipe diameter direction from the center of the wall thickness. When the steel material is a steel plate, a test piece having an observation surface extending 5 mm in the rolling direction and 5 mm in the plate thickness direction from the center position of the plate thickness is prepared. When the steel material is a round steel, a test piece having an observation surface of 5 mm in the axial direction and 5 mm in the radial direction from the R/2 position is prepared. In addition, in this specification, the R/2 position means the center position of the radius R in a cross section perpendicular to the axial direction of the round steel. Note that the size of the test piece is not particularly limited as long as the above observation surface is obtained.

Mirror-polish the observation surface of the prepared test piece. The mirror-polished observation surface is subjected to electrolytic corrosion in a 7% potassium hydroxide etchant to reveal the structure. The observation surface on which the tissue is exposed is observed in 10 fields using an optical microscope. The field of view area is not particularly limited, but is, for example, 1.00 mm ² (100x magnification). In each field of view, identify ferrite from the contrast. The area ratio of the identified ferrite is measured by a point counting method based on JIS G 0555 (2020). In this embodiment, the arithmetic mean value of the area ratio of the obtained ferrite in 10 fields of view is defined as the volume ratio (%) of the ferrite. Note that the volume fraction (%) of ferrite is used by rounding the obtained value to the first decimal place.

[Yield strength]
The yield strength of the duplex stainless steel material according to this embodiment is 655 MPa or more. The duplex stainless steel material according to the present embodiment contains the above-mentioned element content, has a chemical composition that satisfies formula (1A) or (1B), and satisfies formula (2), and has a volume fraction of 30 to 80. It has a microstructure consisting of % ferrite and the balance austenite, and has a yield strength of 655 MPa or more. As a result, the duplex stainless steel material according to this embodiment can have both high strength and excellent hot workability.

A preferable lower limit of the yield strength of the duplex stainless steel material according to the present embodiment is 660 MPa, more preferably 665 MPa, and still more preferably 670 MPa. Although the upper limit of the yield strength of the duplex stainless steel material according to the present embodiment is not particularly limited, it is, for example, 862 MPa.

The yield strength of the duplex stainless steel material according to this embodiment can be determined by the following method. First, a tensile test piece is prepared from the duplex stainless steel material according to this embodiment. The size of the tensile test piece is not particularly limited. The tensile test piece is, for example, a round bar tensile test piece with a parallel portion diameter of 8.9 mm and a gage length of 35.6 mm. If the steel material is a steel pipe, prepare a tensile test piece from the center of the wall thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the steel pipe. When the steel material is a round steel, a tensile test piece is prepared from the R/2 position. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the round steel. If the steel material is a steel plate, prepare a tensile test piece from the center of the plate thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the rolling direction of the steel plate.

Using the produced tensile test piece, a tensile test is performed at room temperature (24±3° C.) in accordance with ASTM E8/E8M (2021) to determine the 0.2% offset yield strength (MPa). The obtained 0.2% offset yield strength is defined as yield strength (MPa). Note that the yield strength (MPa) is used by rounding the obtained value to the first decimal place.

[Hot workability]
The duplex stainless steel material according to the present embodiment contains the above-mentioned element content, has a chemical composition that satisfies formula (1A) or (1B), and satisfies formula (2), and has a volume fraction of 30 to 80. It has a microstructure consisting of % ferrite and the balance austenite, and has a yield strength of 655 MPa or more. As a result, the duplex stainless steel material according to this embodiment can have both high strength and excellent hot workability. In this embodiment, excellent hot workability is defined as follows.

Specifically, a hot workability test (Greeble test) is conducted on the duplex stainless steel material according to this embodiment. Specifically, a test piece for the Greeble test is prepared from a material obtained in the process of manufacturing the duplex stainless steel material according to this embodiment. The material may be a slab or an ingot, and the slab may be a billet, a bloom, or a slab. Preferably, a slab or ingot that has been hot forged or bloomed is used.

In the present embodiment, the location at which the Greeble test specimen is prepared from the material is not particularly limited, but the test piece is prepared avoiding the center of the material where segregation and defects are likely to occur during solidification. The test piece is, for example, a round bar test piece with a diameter of 10 mm and a length of 130 mm. The longitudinal direction of the test piece is parallel to the direction in which hot working is performed on the material. For example, when the material is a round billet and piercing rolling is performed as hot working, the longitudinal direction of the test piece is parallel to the axial direction (rolling direction) of the round billet.

A test piece for the Greeble test is heated to 1250°C and held for 3 minutes. Thereafter, the specimen is cooled to 1100°C at a rate of 100°C/min. A tensile test is performed on a test piece at 1100° C. at a strain rate of 5/s ⁻¹ . The tensile test is performed until the specimen breaks. Determine the aperture value (%) from the fracture surface of the fractured round bar test piece. When the obtained reduction of area value is 80% or more, it is determined that the duplex stainless steel material exhibits excellent hot workability. Note that the aperture value (%) is used by rounding the obtained value to the first decimal place.

[Shape and usage]
As described above, the shape of the duplex stainless steel material according to this embodiment is not particularly limited. Specifically, the duplex stainless steel material according to this embodiment may be a steel pipe, a round steel (solid material), or a steel plate. Further, the steel pipe may be a seamless steel pipe or a welded steel pipe. The duplex stainless steel material according to this embodiment is suitable for use as a steel material for oil wells, for example. The steel material for oil wells is, for example, oil country tubular goods. Oil country tubular goods are, for example, casings, tubing, drill pipes, etc. used for drilling oil or gas wells, extracting crude oil or natural gas, and the like.

[Production method]
An example of a method for manufacturing a duplex stainless steel material according to this embodiment having the above-described configuration will be described. Note that the method for manufacturing the duplex stainless steel material according to this embodiment is not limited to the manufacturing method described below. An example of the method for manufacturing a duplex stainless steel material according to the present embodiment includes a material preparation step, a hot working step, a solution treatment step, and a cold working step. Each manufacturing process will be explained in detail below.

[Material preparation process]
In the material preparation step according to this embodiment, a material having the above-mentioned chemical composition is prepared. The materials may be manufactured and prepared or purchased from a third party. That is, the method of preparing the material is not particularly limited.

When manufacturing the material, for example, it is manufactured by the following method. Molten steel having the above chemical composition is produced. The method for producing molten steel is not particularly limited, and may be produced using a converter, an electric furnace, or other methods. Slabs (slabs, blooms, or billets) are manufactured by continuous casting using molten steel. A steel ingot may be manufactured by an ingot method using molten steel. If necessary, the slab, bloom, or ingot may be bloomed and rolled to produce a billet. The material is manufactured through the above steps.

[Hot processing process]
In the hot working step according to the present embodiment, the material prepared in the material preparation step is hot worked to produce an intermediate steel material. In this specification, intermediate steel materials are plate-shaped steel materials when the final product is a steel plate, raw pipes when the final product is a steel pipe, bar-shaped steel materials when the final product is a steel bar, and intermediate steel materials when the final product is a steel plate. If it is a wire rod, it is a wire-shaped steel material. The hot working may be hot forging, hot extrusion, or hot rolling. The hot working method is not particularly limited, and may be any known method.

When the final product is a seamless steel pipe, the material (billet) is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1000 to 1300°C. Hot working is performed on the billet extracted from the heating furnace to produce a raw pipe (seamless steel pipe). The hot working method is not particularly limited, and may be any known method. For example, the raw pipe may be manufactured by performing Mannesmann piercing rolling as hot working. In this case, the round billet is pierced and rolled using a piercer. In the case of piercing rolling, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The hole-rolled round billet is further hot-rolled using a mandrel mill, reducer, sizing mill, etc. to form a blank tube. The cumulative area reduction rate in the hot working step is, for example, 20 to 70%. Other hot working methods may be used to produce blank tubes from billets. For example, if the steel material is a short thick-walled steel pipe such as a coupling, the raw pipe may be manufactured by forging such as the Erhard method. A raw pipe is manufactured through the above steps.

If the final product is round steel, first the material is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1000 to 1300°C. Hot working is performed on the material extracted from the heating furnace to produce an intermediate steel material having a circular cross section perpendicular to the axial direction. The hot working is, for example, blooming rolling using a blooming mill or hot rolling using a continuous rolling mill. A continuous rolling mill has a horizontal stand having a pair of grooved rolls arranged in parallel in the vertical direction and a vertical stand having a pair of grooved rolls arranged in parallel in the horizontal direction, which are arranged alternately.

When the final product is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1000 to 1300°C. The raw material extracted from the heating furnace is hot-rolled using a blooming mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate. In this manner, in the hot working process, hot working is performed by a well-known method to produce an intermediate steel material having a desired shape.

[Solution treatment process]
In the solution treatment step, the intermediate steel material manufactured by the hot working step is subjected to solution treatment. The solution treatment method is not particularly limited, and any known method may be used. For example, the intermediate steel material may be charged into a heat treatment furnace, maintained at a desired temperature, and then rapidly cooled. In addition, when intermediate steel materials are charged into a heat treatment furnace, held at a desired temperature, and then rapidly cooled and subjected to solution treatment, the solution temperature refers to the temperature of the heat treatment furnace for performing the solution treatment ( ℃). Furthermore, in this case, the solution time means the time during which the intermediate steel material is held at the solution temperature.

Preferably, the solution temperature in the solution treatment step of this embodiment is 900 to 1100°C. If the solution treatment temperature is too low, precipitates (for example, σ phase, etc., which is an intermetallic compound) may remain in the intermediate steel material after the solution treatment. In this case, the corrosion resistance of the manufactured duplex stainless steel material decreases. Furthermore, if the solution temperature is too low, the volume fraction of ferrite may become too low in the produced duplex stainless steel material. On the other hand, if the solution temperature is too high, the volume fraction of ferrite may become too high in the produced duplex stainless steel material. In this case, desired mechanical properties cannot be obtained in the manufactured duplex stainless steel material.

When the intermediate steel material is charged into a heat treatment furnace, maintained at a desired temperature, and then rapidly cooled and subjected to solution treatment, the solution treatment time is not particularly limited, and the solution treatment may be performed under known conditions. The solution time is, for example, 5 to 180 minutes. The rapid cooling method is, for example, water cooling.

[Cold working process]
In the cold working step, cold working is performed on the intermediate steel material that has been subjected to solution treatment in the solution treatment step. The cold working method is not particularly limited, and any known method may be used. In this specification, "cold working" may be cold drawing or cold rolling. By cold working the intermediate steel material after solution treatment, the yield strength of the intermediate steel material is increased.

In the cold working process, the area reduction rate R defined by formula (A) is set to 5 to 20%.
R={(S ₀ - S _f )/S ₀ }×100 (A)
Here, S ₀ is the cross-sectional area perpendicular to the rolling direction of the intermediate steel material before cold working, and S _f means the cross-sectional area perpendicular to the rolling direction of the intermediate steel material after cold working. That is, the cross-sectional area reduction rate R (%) of the cold working process defined by the formula (A) means the percentage of the ratio of the cross-sectional area perpendicular to the rolling direction of the intermediate steel material changed by the cold working.

In the cold working step, if the area reduction rate R is too small, a yield strength of 655 MPa or more may not be obtained. On the other hand, if the area reduction rate R is too large, restrictions may arise on equipment and product size. Therefore, in the cold working process according to this embodiment, the area reduction rate R is set to 5 to 20%. A preferable upper limit of the area reduction rate R is 18%. In addition, in the cold working process according to this embodiment, the number of times of cold working is not particularly limited. That is, the area reduction rate R may be set to 5 to 20% by one cold working process, and the area reduction rate R may be set to 5 to 20% by multiple cold workings. When cold working is performed a plurality of times, preferably, the cold working is performed continuously without performing heat treatment or the like between the cold workings. Through the above steps, the duplex stainless steel material according to this embodiment is manufactured.

[Other processes]
In the method for manufacturing a duplex stainless steel material according to this embodiment, steps other than the above steps may be performed. For example, the aging heat treatment may be performed on the intermediate steel material that has been subjected to the solution treatment step. For example, the duplex stainless steel material that has been subjected to the cold working step may be further subjected to aging heat treatment and then further cold worked. When carrying out aging heat treatment, the method of aging heat treatment is not particularly limited, and any well-known method may be used. For example, a steel material to be subjected to aging heat treatment may be charged into a heat treatment furnace and held at a desired temperature.

When performing aging heat treatment, the preferable heat treatment temperature is 340 to 660°C. When performing aging heat treatment, the preferable heat treatment time is 20 to 80 minutes. In addition, in this case, the heat treatment temperature means the temperature (° C.) of the heat treatment furnace for carrying out the aging heat treatment. Furthermore, in this case, the heat treatment time means the time during which the steel material is maintained at the heat treatment temperature.

When performing aging heat treatment, the lower limit of the heat treatment temperature is more preferably 350°C, and even more preferably 380°C. In this case, the upper limit of the heat treatment temperature is more preferably 650°C, and even more preferably 630°C. When performing aging heat treatment, the more preferable lower limit of the heat treatment time is 25 minutes, and still more preferably 30 minutes. In this case, the upper limit of the heat treatment time is more preferably 70 minutes, and even more preferably 60 minutes.

In the method for manufacturing a duplex stainless steel material according to the present embodiment, the duplex stainless steel material that has been subjected to the cold working step may further be subjected to pickling treatment. In this case, the pickling treatment may be carried out by a known method and is not particularly limited. When the pickling treatment is carried out, a passive film is formed on the surface of the produced duplex stainless steel material, further increasing the corrosion resistance of the duplex stainless steel material.

Through the above steps, the duplex stainless steel material according to this embodiment can be manufactured. Note that the method for manufacturing the duplex stainless steel material described above is an example, and the duplex stainless steel material may be manufactured by other methods. Hereinafter, the present invention will be explained in more detail with reference to Examples.

Molten steel having the chemical composition shown in Tables 1-1 and 1-2 was melted using a 50 kg high-frequency vacuum melting furnace, and steel ingots were produced by the ingot forming method. The outer diameter of the ingot was 150 mm. Note that "-" in Tables 1-1 and 1-2 means that the content of the corresponding element was at the impurity level. For example, the Nb content, Al content, Ta content, Ti content, Zr content, and Hf content of steel code A are rounded to the fourth decimal place and mean 0%. do. Furthermore, the W content and Co content of steel symbol A were rounded to the second decimal place, meaning that they were 0%. Furthermore, the Sn content, Sb content, Bi content, Pb content, Ca content, Mg content, and REM content of steel code A are rounded to the fifth decimal place and are 0%. It means something.

Furthermore, Table 2 shows the element content of each steel symbol listed in Tables 1-1 and 1-2, and Fn1 and Fn2 determined from the above definitions.

Ingots of each steel code were hot forged to have an outer diameter of 75 mm, and then machined to produce billets with an outer diameter of 70 mm. After the manufactured billets of each test number were heated at 1250° C., they were subjected to Mannesmann piercing rolling as hot working. The raw tubes of each test number thus manufactured were subjected to solution treatment in which the tubes were held at the solution temperature (° C.) listed in Table 2 for the solution treatment time (minutes) and then rapidly cooled. Cold working was performed on the raw tubes of each test number that had been subjected to solution treatment at the cross-sectional reduction rate R shown in Table 2. Through the above steps, duplex stainless steel materials (seamless steel pipes) of each test number were obtained.

A microstructural observation test, a tensile test, and a hot workability test were conducted on the obtained seamless steel pipes of each test number.

[Microstructure observation test]
Microstructure observation was performed on the seamless steel pipes of each test number using a method based on JIS G 0555 (2020), and the volume fraction (%) of ferrite was determined. Specifically, a test piece for microstructural observation having an observation surface of 5 mm in the tube axis direction and 5 mm in the tube diameter direction was prepared from the center of the wall thickness of the seamless steel tube of each test number. After mirror-polishing the observation surface of the prepared test piece, the observation surface was subjected to electrolytic corrosion in a 7% potassium hydroxide etchant to reveal the structure. The observation surface on which the tissue was exposed was observed in 10 fields using an optical microscope at a magnification of 100 times. The area ratio of ferrite identified from the contrast was measured using a point counting method based on JIS G 0555 (2020). The arithmetic mean value of the area ratio of the obtained ferrite in 10 fields of view was defined as the volume ratio (%) of ferrite. In addition, the volume fraction (%) of ferrite was used by rounding the obtained value to the first decimal place. Table 2 shows the volume fraction (%) of the obtained ferrite.

[Tensile test]
A tensile test based on ASTM E8/E8M (2021) was performed on the seamless steel pipes of each test number to determine the yield strength (MPa). Specifically, a round bar tensile test piece with a parallel portion diameter of 8.9 mm and a gauge length of 35.6 mm was prepared from the wall thickness center position of the seamless steel pipe of each test number. The axial direction of the round bar tensile test piece was parallel to the axial direction of the steel pipe. Using the produced round bar tensile test piece, a tensile test was conducted at room temperature (24±3° C.) in accordance with ASTM E8/E8M (2021), and the 0.2% offset yield strength (MPa) was determined. The obtained 0.2% offset yield strength was defined as yield strength (MPa). Note that the yield strength (MPa) was used by rounding the obtained value to the first decimal place. The yield strength obtained is shown in the "YS (MPa)" column of Table 2.

[Hot workability test]
A hot workability test (Greeble test) was conducted on the billets of each test number using the method described above. As mentioned above, the billet of each test number means a billet that was machined to have an outer diameter of 70 mm after hot forging an ingot manufactured from the molten steel of each test number. Round bar test pieces were prepared from the billets of each test number. Specifically, for the billets of each test number, the direction in which hot rolling was performed (corresponding to the rolling direction of the manufactured seamless steel pipe) was specified. Furthermore, a round bar test piece was prepared from the R/2 position of the billet of each test number. Note that the round bar test piece had a diameter of 10 mm and a length of 130 mm. The longitudinal direction of the round bar test piece was parallel to the direction in which the billet was hot rolled (the rolling direction of the seamless steel pipe).

The produced round bar test piece was heated to 1250°C and held for 3 minutes, and then the test piece was cooled to 1100°C at a rate of 100°C/min. A tensile test was performed on the test piece at 1100° C. at a strain rate of 5/s ⁻¹ to break the round bar test piece. The aperture value (%) was determined from the fracture surface of the fractured round bar test piece. Note that the aperture value (%) was used by rounding the obtained value to the first decimal place. The obtained aperture values (%) are shown in Table 2.

[Evaluation results]
Referring to Table 1-1, Table 1-2, and Table 2, the seamless steel pipes of test numbers 1 to 22 have appropriate element contents and Fn1 of more than 24.0 and less than 35.0. It had a chemical composition satisfying Fn2 of less than 11.5. Furthermore, the manufacturing method of these seamless steel pipes also satisfied the preferred manufacturing method described in the specification. As a result, the volume fraction of ferrite was 30 to 80%, and the yield strength was 655 MPa or more. As a result, the aperture value was 80% or more. That is, the seamless steel pipes of test numbers 1 to 22 had both a high yield strength of 655 MPa or more and excellent hot workability.

On the other hand, seamless steel pipes with test numbers 23 to 25 had Fn2 of 11.5 or more. As a result, the reduction of area was less than 80%, and it did not have excellent hot workability.

The seamless steel pipes with test numbers 26 to 28 had Fn1 of 35.0 or more. As a result, the reduction of area was less than 80%, and it did not have excellent hot workability.

Seamless steel pipes of test numbers 29 and 30 had too high S content. As a result, the reduction of area was less than 80%, and it did not have excellent hot workability.

The seamless steel pipe of test number 31 had too low V content. As a result, the reduction of area was less than 80%, and it did not have excellent hot workability.

The seamless steel pipe of test number 32 had too high a V content. As a result, the reduction of area was less than 80%, and it did not have excellent hot workability.

The B content of the seamless steel pipe of test number 33 was too low. As a result, the reduction of area was less than 80%, and it did not have excellent hot workability.

The seamless steel pipes of test numbers 34 and 35 had too high a N content. As a result, the reduction of area was less than 80%, and it did not have excellent hot workability.

The seamless steel pipe of test number 36 had too high an Al content. As a result, the reduction of area was less than 80%, and it did not have excellent hot workability.

The seamless steel pipe of test number 37 had too high a Ti content. As a result, the reduction of area was less than 80%, and it did not have excellent hot workability.

The seamless steel pipe of test number 38 had too high a Zr content. As a result, the reduction of area was less than 80%, and it did not have excellent hot workability.

The seamless steel pipes of test numbers 39 and 40 had too low cross-sectional area reduction ratio R during cold working. As a result, the yield strength was less than 655 MPa, which did not have the desired high strength.

The embodiments of the present disclosure have been described above. However, the embodiments described above are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the embodiments described above, and the embodiments described above can be modified and implemented as appropriate without departing from the spirit thereof.

Claims (5)

In mass%,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 0.10-9.00%,
P: 0.040% or less,
S: 0.0010% or less,
Cr: 20.0-32.0%,
Ni: 3.5-10.0%,
Mo: 0.5-5.0%,
Cu: 0.5-6.0%,
V: 0.01% or more and less than 0.10%,
B: 0.0010-0.0050%,
N: less than 0.150%, and
Contains O: 0.0001 to 0.0070%,
The remainder consists of Fe and impurities,
A chemical composition that satisfies formulas (1A) and (2),
30 to 80% ferrite by volume, and
A microstructure in which the remainder consists of austenite,
having a yield strength of 655 MPa or more,
Duplex stainless steel material.
24.0<Cr+3.3×Mo+16×N<35.0 (1A)
(N×S/B)×10 ³ <11.5 (2)
Here, each element symbol in formulas (1A) and (2) is substituted with the content of the corresponding element in mass %. In mass%,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 0.10-9.00%,
P: 0.040% or less,
S: 0.0010% or less,
Cr: 20.0-32.0%,
Ni: 3.5-10.0%,
Mo: 0.5-5.0%,
Cu: 0.5-6.0%,
V: 0.01% or more and less than 0.10%,
B: 0.0010-0.0050%,
N: less than 0.150%, and
Contains O: 0.0001 to 0.0070%, and further,
Nb: 0.100% or less,
Al: 0.100% or less,
Ta: 0.100% or less,
Ti: 0.100% or less,
Zr: 0.100% or less,
Hf: 0.100% or less,
W: 0.50% or less,
Co: 0.50% or less,
Sn: 0.1000% or less,
Sb: 0.1000% or less,
Bi: 0.1000% or less,
Pb: 0.0030% or less,
Ca: 0.0050% or less,
Mg: 0.1000% or less, and
Contains one or more selected from the group consisting of rare earth elements: 0.1000% or less,
The remainder consists of Fe and impurities,
A chemical composition that satisfies formulas (1B) and (2),
30 to 80% ferrite by volume, and
A microstructure in which the remainder consists of austenite,
having a yield strength of 655 MPa or more,
Duplex stainless steel material.
24.0<Cr+3.3×(Mo+0.5×W)+16×N<35.0 (1B)
(N×S/B)×10 ³ <11.5 (2)
Here, each element symbol in formulas (1B) and (2) is substituted with the content of the corresponding element in mass %. If an element is not contained, "0" is assigned to the corresponding element symbol. The duplex stainless steel material according to claim 2,
The chemical composition is
Nb: 0.100% or less,
Al: 0.100% or less,
Ta: 0.100% or less,
Ti: 0.100% or less,
Zr: 0.100% or less,
Hf: 0.100% or less,
W: 0.50% or less, and
Co: 0.50% or less, containing one or more selected from the group consisting of;
Duplex stainless steel material. The duplex stainless steel material according to claim 2,
The chemical composition is
Sn: 0.1000% or less,
Sb: 0.1000% or less,
Bi: 0.1000% or less, and
Contains one or more selected from the group consisting of Pb: 0.0030% or less,
Duplex stainless steel material. The duplex stainless steel material according to claim 2,
The chemical composition is
Ca: 0.0050% or less,
Mg: 0.1000% or less, and
Rare earth elements: 0.1000% or less, containing one or more selected from the group consisting of;
Duplex stainless steel material.

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