DUPLEX STAINLESS STEEL MATERIAL

Description

TECHNICAL FIELD

The present disclosure relates to a steel material, and more particularly relates to a duplex stainless steel material.

BACKGROUND ART

Oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to simply as “oil wells”) may be in a corrosive environment containing a corrosive gas. Here, the term “corrosive gas” means carbon dioxide gas and/or hydrogen sulfide gas. That is, steel materials for use in oil wells are required to have excellent corrosion resistance in a corrosive environment.

To date, as a method for increasing the corrosion resistance of a steel material, there has been a known method that increases the content of chromium (Cr) and forms a passive film mainly composed of Cr oxides on the surface of the steel material. Therefore, a duplex stainless steel material in which the content of Cr has been made high may in some cases be used in an environment where excellent corrosion resistance is required.

In recent years, furthermore, deep wells below sea level are being actively developed. Therefore, there is a need to increase the strength of duplex stainless steel materials. That is, there is a growing need for duplex stainless steel materials that achieve both high strength and excellent corrosion resistance.

Japanese Patent Application Publication No. 2014-043616 (Patent Literature 1) and International Application Publication No. WO2021/246118 (Patent Literature 2) each proposes a duplex stainless steel material that has high strength and excellent corrosion resistance.

The duplex stainless steel material disclosed in Patent Literature 1 has a chemical composition consisting of, 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 balance being Fe and impurities, in which a σ phase susceptibility index X (=2.2Si+0.5Cu+2.0Ni+Cr+4.2Mo+0.2W) is 52.0 or less, a strength index Y (=Cr+1.5Mo+10N+3.5W) is 40.5 or more, and a pitting resistance equivalent PREW (=Cr+3.3 (Mo+0.5W)+16N) is 40 or more. In the structure of the steel, in a cross section in the thickness direction which is parallel to the rolling elongation direction, when a straight line is drawn to be parallel to the thickness direction from the outer layer to a depth of 1 mm, the number of boundaries between a ferrite phase and an austenite phase which intersect with the straight line is 160 or more. It is described in Patent Literature 1 that the strength of this duplex stainless steel can be increased without loss of corrosion resistance, and that by combining the use of cold working with a high reduction rate, this duplex stainless steel exhibits excellent hydrogen embrittlement resistance characteristics.

The duplex stainless steel material disclosed in Patent Literature 2 consists of, in mass %, C: 0.002 to 0.03%, Si: 0.05 to 1.0%, Mn: 0.10 to 1.5%, P: 0.040% or less, S: 0.0005 to 0.02%, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.05%, and N: 0.06 to 0.35%, with the balance being Fe and impurities. In addition, this duplex stainless steel has a microstructure containing, by volume ratio, austenite phase: 20 to 70% and ferrite phase: 30 to 80%, and in the duplex stainless steel, a yield strength is 448 MPa or more, a number density of oxide-based inclusions with an average grain size of 1 μm or more is 15 pieces/mm² or less, and among the oxide-based inclusions, the proportion of oxide-based inclusions containing Al is 50% by mass or less. It is described in Patent Literature 2 that this duplex stainless steel has high strength, high toughness, and excellent corrosion resistance.

CITATION LIST

Patent Literature

• • Patent Literature 1: Japanese Patent Application Publication No. 2014-043616
  • Patent Literature 2: International Application Publication No. WO2021/246118

SUMMARY OF INVENTION

Technical Problem

According to Patent Literatures 1 and 2 described above, a duplex stainless steel material having high strength and excellent corrosion resistance can be obtained. However, a duplex stainless steel material that achieves both high strength and excellent corrosion resistance may also be obtained by a technique other than the techniques disclosed in the aforementioned Patent Literatures 1 and 2.

An objective of the present disclosure is to provide a duplex stainless steel material that achieves both high strength and excellent corrosion resistance.

Solution to Problem

A duplex stainless steel material according to the present disclosure consists of, in mass %,

• • C: 0.030% or less,
  • Si: 0.20 to 1.00%,
  • Mn: 0.5 to 7.0%,
  • P: 0.040% or less,
  • S: 0.0200% or less,
  • Al: 0.100% or less,
  • Ni: 4.0 to 9.0%,
  • Cr: 20.0 to 30.0%,
  • Mo: 0.5 to 2.0%,
  • Cu: 1.5 to 3.0%,
  • N: 0.15 to 0.30%,
  • V: 0.01 to 0.50%,
  • Co: 0.05 to 1.00%,
  • Sn: 0.001 to 0.050%,
  • Nb: 0 to 0.300%,
  • Ta: 0 to 0.100%,
  • Ti: 0 to 0.100%,
  • Zr: 0 to 0.100%,
  • Hf: 0 to 0.100%,
  • W: 0 to 0.200%,
  • Sb: 0 to 0.100%,
  • Ca: 0 to 0.020%,
  • Mg: 0 to 0.020%,
  • B: 0 to 0.020%, and
  • rare earth metal: 0 to 0.200%,
  • with the balance being Fe and impurities,
  • wherein:
  • a yield strength is 758 MPa or more;
  • the microstructure is composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite; and
  • a dislocation density ρ(α) in the ferrite and a dislocation density ρ(γ) in the austenite satisfy Formula (1):

0.3 < ρ ⁡ ( γ ) / ρ ⁡ ( α ) < 4. ( 1 )

• • where, in Formula (1), a dislocation density in the austenite in m⁻² is substituted for ρ(γ), and a dislocation density in the ferrite in m⁻² is substituted for ρ(α).

Advantageous Effects of Invention

The duplex stainless steel material according to the present disclosure achieves both high strength and excellent corrosion resistance.

DESCRIPTION OF EMBODIMENTS

The present inventors attempted to obtain a duplex stainless steel material having, specifically, a yield strength of 758 MPa or more as high strength. Therefore, first, the present inventors conducted studies from the viewpoint of the chemical composition with regard to a duplex stainless steel material in which both a high yield strength of 758 MPa or more and excellent corrosion resistance are achieved. As a result, the present inventors considered that if a duplex stainless steel material consists of, in mass %, C: 0.030% or less, Si: 0.20 to 1.00%, Mn: 0.5 to 7.0%, P: 0.040% or less, S: 0.0200% or less, Al: 0.100% or less, Ni: 4.0 to 9.0%, Cr: 20.0 to 30.0%, Mo: 0.5 to 2.0%, Cu: 1.5 to 3.0%, N: 0.15 to 0.30%, V: 0.01 to 0.50%, Co: 0.05 to 1.00%, Sn: 0.001 to 0.050%, Nb: 0 to 0.300%, Ta: 0 to 0.100%, Ti: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, W: 0 to 0.200%, Sb: 0 to 0.100%, Ca: 0 to 0.020%, Mg: 0 to 0.020%, B: 0 to 0.020%, and rare earth metal: 0 to 0.200%, with the balance being Fe and impurities, there is a possibility that both a high yield strength of 758 MPa or more and excellent corrosion resistance can be achieved.

Here, the microstructure of a duplex stainless steel material having the chemical composition described above is composed of ferrite and austenite. The present inventors have found that in a duplex stainless steel material having the chemical composition described above, if the microstructure is composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite, the strength and corrosion resistance are stably increased. That is, in a duplex stainless steel material according to the present embodiment, the microstructure is composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite. Note that, in the present description, the phrase “composed of ferrite and austenite” means that the amount of any phase other than ferrite and austenite is negligibly small.

In addition, the present inventors conducted detailed studies regarding a technique for increasing corrosion resistance while maintaining the yield strength with respect to a duplex stainless steel material having the chemical composition and microstructure described above and having a yield strength of 758 MPa or more. Specifically, the present inventors focused their attention on dislocations in the duplex stainless steel material. When the dislocation density in a duplex stainless steel material is increased, the yield strength of the steel material increases. That is, in the duplex stainless steel material according to the present embodiment in which the yield strength is increased to 758 MPa or more, there is a possibility that the dislocation density will be increased to a certain level or more.

On the other hand, it is considered that a region where the dislocation density is high in a steel material is liable to act as a starting point for corrosion. In other words, if a region where the dislocation density is locally high is present in the duplex stainless steel material, there is a risk that the corrosion resistance of the duplex stainless steel material will decrease. That is, the present inventors have considered that there is a possibility that the distribution of the dislocation density in the microstructure influences the corrosion resistance of the steel material.

As a result of further detailed studies conducted by the present inventors taking into consideration the above findings, it has been revealed that in a duplex stainless steel material that has the chemical composition described above and a microstructure composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite, and also has a yield strength of 758 MPa or more, when a dislocation density ρ(α) in the ferrite and a dislocation density ρ(γ) in the austenite satisfy the following Formula (1), a yield strength of 758 MPa or more and excellent corrosion resistance can both be achieved:

0.3 < ρ ⁡ ( γ ) / ρ ⁡ ( α ) < 4. ( 1 )

where, in Formula (1), the dislocation density in the austenite in m⁻² is substituted for ρ(γ), and the dislocation density in the ferrite in m⁻² is substituted for ρ(α).

The reason why, in a duplex stainless steel material that has the chemical composition described above and a microstructure composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite, and also has a yield strength of 758 MPa or more, both a yield strength of 758 MPa or more and excellent corrosion resistance can be achieved if the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the above Formula (1) has not been clarified in detail. However, the present inventors surmise that the reason is as follows.

As mentioned above, it is considered that in a duplex stainless steel material having the chemical composition and microstructure described above, the dislocation density is increased to a certain level or more as a result of increasing the yield strength to 758 MPa or more. Further, when the dislocation density in the duplex stainless steel material is increased by work hardening or the like, dislocations are introduced locally in some cases, and thus the dislocation density is liable to increase locally. On the other hand, there is a possibility that when a ratio between the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite is controlled within a certain range, localization of the dislocation density in the duplex stainless steel material is lessened. The present inventors surmise that it is likely that, as a result, while maintaining the yield strength, an increase in the local dislocation density is lessened and the corrosion resistance of the duplex stainless steel material increases.

Note that, there may be also a possibility that in a duplex stainless steel material having the chemical composition and the microstructure described above, as a result of the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfying the aforementioned Formula (1), the duplex stainless steel material has a yield strength of 758 MPa or more and excellent corrosion resistance because of a mechanism that is different from the mechanism described above. However, the fact that in a duplex stainless steel material having the chemical composition and the microstructure described above, as a result of the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfying the aforementioned Formula (1), both a yield strength of 758 MPa or more and excellent corrosion resistance can be achieved has been proven by Examples that will be described later.

The gist of the duplex stainless steel material according to the present embodiment, which has been completed based on the above findings, is as follows.

[1]

A duplex stainless steel material consisting of, in mass %,

• • C: 0.030% or less,
  • Si: 0.20 to 1.00%,
  • Mn: 0.5 to 7.0%,
  • P: 0.040% or less,
  • S: 0.0200% or less,
  • Al: 0.100% or less,
  • Ni: 4.0 to 9.0%,
  • Cr: 20.0 to 30.0%,
  • Mo: 0.5 to 2.0%,
  • Cu: 1.5 to 3.0%,
  • N: 0.15 to 0.30%,
  • V: 0.01 to 0.50%,
  • Co: 0.05 to 1.00%,
  • Sn: 0.001 to 0.050%,
  • Nb: 0 to 0.300%,
  • Ta: 0 to 0.100%,
  • Ti: 0 to 0.100%,
  • Zr: 0 to 0.100%,
  • Hf: 0 to 0.100%,
  • W: 0 to 0.200%,
  • Sb: 0 to 0.100%,
  • Ca: 0 to 0.020%,
  • Mg: 0 to 0.020%,
  • B: 0 to 0.020%, and
  • rare earth metal: 0 to 0.200%,
  • with balance being Fe and impurities,
  • wherein:
  • a yield strength is 758 MPa or more;
  • the microstructure is composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite; and
  • a dislocation density ρ(α) in the ferrite and a dislocation density ρ(γ) in the austenite satisfy Formula (1):

0.3 < ρ ⁡ ( γ ) / ρ ⁡ ( α ) < 4. ( 1 )

• • where, in Formula (1), a dislocation density in the austenite in m⁻² is substituted for ρ(γ), and a dislocation density in the ferrite in m⁻² is substituted for ρ(α).
    [2]

The duplex stainless steel material according to [1], containing one or more elements selected from a group consisting of:

• • Nb: 0.001 to 0.300%,
  • Ta: 0.001 to 0.100%,
  • Ti: 0.001 to 0.100%,
  • Zr: 0.001 to 0.100%,
  • Hf: 0.001 to 0.100%,
  • W: 0.001 to 0.200%,
  • Sb: 0.001 to 0.100%,
  • Ca: 0.001 to 0.020%,
  • Mg: 0.001 to 0.020%,
  • B: 0.001 to 0.020%, and
  • rare earth metal: 0.001 to 0.200%.

Note that, 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. Note that, the term “round steel bar” refers to a steel bar in which a cross section perpendicular to the axial direction is a circular shape. Further, the steel pipe may be a seamless steel pipe or may be a welded steel pipe.

Hereunder, the duplex stainless steel material according to the present embodiment is described in detail. Note that, in the following description, the duplex stainless steel material is also referred to as simply “steel material”.

[Chemical Composition]

The chemical composition of the duplex stainless steel material according to the present embodiment contains the following elements. The symbol “%” relating to an element means “mass percent” unless otherwise noted.

C: 0.030% or Less

Carbon (C) is unavoidably contained. That is, the lower limit of the content of C is more than 0%. C forms Cr carbides at grain boundaries and increases corrosion susceptibility at the grain boundaries. Therefore, if the content of C is too high, the corrosion resistance of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of C is to be 0.030% or less. A preferable upper limit of the content of C is 0.028%, and more preferably is 0.025%. The content of C is preferably as low as possible. However, extremely reducing the content of C will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of C is 0.001%, and more preferably is 0.005%.

Si: 0.20 to 1.00%

Silicon (Si) deoxidizes the steel. If the content of Si is too low, the aforementioned advantageous effect will not 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 too high, 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. Therefore, the content of Si is to be 0.20 to 1.00%. A preferable lower limit of the content of Si is 0.25%, and more preferably is 0.30%. A preferable upper limit of the content of Si is 0.95%, and more preferably is 0.90%.

Mn: 0.5 to 7.0%

Manganese (Mn) deoxidizes the steel and desulfurizes the steel. Mn also improves hot workability of the steel material. If the content of Mn is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, Mn segregates to grain boundaries together with impurities such as P and S. Therefore, if the content of Mn is too high, corrosion resistance of the steel material in a high temperature environment will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, 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 0.8%, and further preferably is 1.0%. A preferable upper limit of the content of Mn is 6.5%, and more preferably is 6.2%.

P: 0.040% or Less

Phosphorus (P) is unavoidably contained. That is, the lower limit of the content of P is more than 0%. P segregates to grain boundaries. Therefore, if the content of P is too high, the corrosion resistance of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of P is to be 0.040% or less. A preferable upper limit of the content of P is 0.035%, and more preferably is 0.030%. The content of P is preferably as low as possible. However, extremely reducing the content of P will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of P is 0.001%, and more preferably is 0.003%.

S: 0.0200% or Less

Sulfur(S) is unavoidably contained. That is, the lower limit of the content of S is more than 0%. S segregates to grain boundaries. Therefore, if the content of S is too high, 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. Therefore, the content of S is to be 0.0200% or less. A preferable upper limit of the content of S is 0.0180%, and more preferably is 0.0160%. The content of S is preferably as low as possible. However, extremely reducing the content of S will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0015%.

Al: 0.100% or Less

Aluminum (Al) is unavoidably contained. That is, the lower limit of the content of Al is more than 0%. Al deoxidizes the steel. On the other hand, if the content of Al is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxide-based inclusions will be formed and toughness of the steel material will decrease. Therefore, the content of Al is to be 0.100% or less. A preferable lower limit of the content of Al is 0.001%, more preferably is 0.005%, and further preferably is 0.010%. A preferable upper limit of the content of Al is 0.090%, and more preferably is 0.085%. Note that, as used in the present description, the term “content of Al” means the content of “acid-soluble Al,” that is, the content of sol. Al.

Ni: 4.0 to 9.0%

Nickel (Ni) stabilizes the austenitic microstructure of the steel material. That is, Ni is an element necessary for obtaining a stable duplex microstructure of ferrite and austenite. Ni also increases corrosion resistance of the steel material. If the content of Ni is too low, the aforementioned advantageous effects will not 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 too high, even if the contents of other elements are within the range of the present embodiment, the volume ratio of austenite will be too high and the yield strength of the steel material will decrease. Therefore, the content of Ni is to be 4.0 to 9.0%. A preferable lower limit of the content of Ni is 4.1%, more preferably is 4.3%, and further preferably is 4.5%. A preferable upper limit of the content of Ni is 8.8%, more preferably is 8.5%, and further preferably is 8.0%.

Cr: 20.0 to 30.0%

Chromium (Cr) forms a passive film as an oxide on the surface of the steel material and thereby increases corrosion resistance of the steel material. Cr also increases the volume ratio of the ferritic microstructure of the steel material. By obtaining a sufficient ferritic microstructure, corrosion resistance of the steel material is stabilized. If the content of Cr is too low, the aforementioned advantageous effects will not 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 too high, hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cr is to be 20.0 to 30.0%. A preferable lower limit of the content of Cr is 20.5%, more preferably is 21.0%, and further preferably is 21.5%. A preferable upper limit of the content of Cr is 29.5%, more preferably is 29.0%, and further preferably is 28.5%.

Mo: 0.5 to 2.0%

Molybdenum (Mo) increases corrosion resistance of the steel material. Mo also dissolves in the steel and increases the yield strength of the steel material. In addition, Mo forms fine carbides in the steel and thereby increases the yield strength of the steel material. If the content of Mo is too low, the aforementioned advantageous effects will not 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 too high, hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Mo is to be 0.5 to 2.0%. A preferable lower limit of the content of Mo is 0.6%, more preferably is 0.7%, and further preferably is 0.8%. A preferable upper limit of the content of Mo is 1.9%, more preferably is 1.7%, and further preferably is 1.5%.

Cu: 1.5 to 3.0%

Copper (Cu) precipitates in the steel material and thereby increases the yield strength of the steel material. If the content of Cu is too low, the aforementioned advantageous effect will not 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 Cu is too high, hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cu is to be 1.5 to 3.0%. A preferable lower limit of the content of Cu is 1.6%, more preferably is 1.8%, and further preferably is 2.0%. A preferable upper limit of the content of Cu is 2.9%, more preferably is 2.8%, and further preferably is 2.7%.

N: 0.15 to 0.30%

Nitrogen (N) stabilizes the austenitic microstructure of the steel material. That is, N is an element necessary for obtaining a stable duplex microstructure of ferrite and austenite. N also increases the corrosion resistance of the steel material. If the content of N is too low, the aforementioned advantageous effects will not 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 N is too high, 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. Therefore, the content of N is to be 0.15 to 0.30%. A preferable lower limit of the content of N is 0.16%, more preferably is 0.18%, and further preferably is 0.20%. A preferable upper limit of the content of N is 0.29%, and more preferably is 0.27%.

V: 0.01 to 0.50%

Vanadium (V) increases the yield strength of the steel material. If the content of V is too low, the aforementioned advantageous effect will not 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 V is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high, and toughness and hot workability of the steel material will decrease. Therefore, the content of V is to be 0.01 to 0.50%. A preferable lower limit of the content of V is 0.02%, more preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the content of V is 0.45%, and more preferably is 0.40%.

Co: 0.05 to 1.00%

Cobalt (Co) forms a coating on the surface of the steel material and thereby increases the corrosion resistance of the steel material. Co also increases hardenability of the steel material and stabilizes strength of the steel material. If the content of Co is too low, the aforementioned advantageous effects will not 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 Co is too high, the production cost will increase extremely even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Co is to be 0.05 to 1.00%. A preferable lower limit of the content of Co is 0.06%, more preferably is 0.08%, and further preferably is 0.10%. A preferable upper limit of the content of Co is 0.95%, more preferably is 0.90%, and further preferably is 0.85%.

Sn: 0.001 to 0.050%

Tin (Sn) increases the corrosion resistance of the steel material. If the content of Sn is too low, the aforementioned advantageous effect will not 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 Sn is too high, even if the contents of other elements are within the range of the present embodiment, liquation cracking will occur at grain boundaries, which will cause hot workability of the steel material to decrease. Therefore, the content of Sn is to be 0.001 to 0.050%. A preferable lower limit of the content of Sn is 0.002%, more preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Sn is 0.045%, and more preferably is 0.040%.

The balance of the chemical composition of the duplex stainless steel material according to the present embodiment is Fe and impurities. Here, the term “impurities” in the chemical composition refers to substances 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 permitted within a range that does not adversely affect the duplex stainless steel material according to the present embodiment.

[Optional Elements]

The chemical composition of the duplex stainless steel material described above may further contain one or more elements selected from the group consisting of Nb, Ta, Ti, Zr, Hf, and W in lieu of a part of Fe. Each of these elements is an optional element, and each of these elements increases strength of the steel material.

Nb: 0 to 0.300%

Niobium (Nb) is an optional element, and does not have to be contained.

That is, the content of Nb may be 0%. When contained, Nb forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Nb is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of Nb is to be 0 to 0.300%. A preferable lower limit of the content of Nb is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Nb is 0.280%, and more preferably is 0.250%.

Ta: 0 to 0.100%

Tantalum (Ta) is an optional element, and does not have to be contained. That is, the content of Ta may be 0%. When contained, Ta forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of Ta is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ta is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of Ta is to be 0 to 0.100%. A preferable lower limit of the content of Ta is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Ta is 0.080%, and more preferably is 0.070%.

Ti: 0 to 0.100%

Titanium (Ti) is an optional element, and does not have to be contained. That is, the content of Ti may be 0%. When contained, Ti forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of Ti is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ti is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of Ti is to be 0 to 0.100%. A preferable lower limit of the content of Ti is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Ti is 0.080%, and more preferably is 0.070%.

Zr: 0 to 0.100%

Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%. When contained, Zr forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Zr is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of Zr is to be 0 to 0.100%. A preferable lower limit of the content of Zr is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Zr is 0.080%, more preferably is 0.070%, further preferably is 0.060%, further preferably is 0.050%, and further preferably is 0.045%.

Hf: 0 to 0.100%

Hafnium (Hf) is an optional element, and does not have to be contained. That is, the content of Hf may be 0%. When contained, Hf forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of Hf is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Hf is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of Hf is to be 0 to 0.100%. A preferable lower limit of the content of Hf is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Hf is 0.080%, and more preferably is 0.070%.

W: 0 to 0.200%

Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%. When contained, W forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of W is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of W is to be 0 to 0.200%. A preferable lower limit of the content of W is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of W is 0.180%, and more preferably is 0.150%.

The chemical composition of the duplex stainless steel material described above may further contain Sb in lieu of a part of Fe.

Sb: 0 to 0.100%

Antimony (Sb) is an optional element, and does not have to be contained. That is, the content of Sb may be 0%. When contained, Sb increases corrosion resistance of the steel material. If even a small amount of Sb is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Sb is too high, even if the contents of other elements are within the range of the present embodiment, high-temperature ductility of the steel material will decrease, and hot workability of the steel material will decrease. Therefore, the content of Sb is to be 0 to 0.100%. A preferable lower limit of the content of Sb is more than 0%, more preferably is 0.001%, further preferably is 0.002%, and further preferably is 0.003%. A preferable upper limit of the content of Sb is 0.080%, and more preferably is 0.070%.

The chemical composition of the duplex stainless steel material described above may further contain one or more elements selected from the group consisting of Ca, Mg, B, and rare earth metal in lieu of a part of Fe. Each of these elements is an optional element, and increases hot workability of the steel material.

Ca: 0 to 0.020%

Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%. When contained, Ca fixes S in the steel material as a sulfide to make it harmless, and thereby increases hot workability of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ca is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the toughness of the steel material will decrease. Therefore, the content of Ca is to be 0 to 0.020%. A preferable lower limit of the content of Ca is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Ca is 0.018%, more preferably is 0.015%.

Mg: 0 to 0.020%

Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%. When contained, Mg fixes S in the steel material as a sulfide to make it harmless, and thereby increases hot workability of the steel material. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Mg is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the toughness of the steel material will decrease. Therefore, the content of Mg is to be 0 to 0.020%. A preferable lower limit of the content of Mg is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Mg is 0.018%, and more preferably is 0.015%.

B: 0 to 0.020%

Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%. When contained, B suppresses segregation of S in the steel material to grain boundaries, and thereby increases hot workability of the steel material. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of B is too high, even if the contents of other elements are within the range of the present embodiment, boron nitride (BN) will be formed and will cause the toughness of the steel material to decrease. Therefore, the content of B is to be 0 to 0.020%. A preferable lower limit of the content of B is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of B is 0.018%, and more preferably is 0.015%.

Rare Earth Metal: 0 to 0.200%

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, REM fixes S in the steel material as a sulfide to make it harmless, and thereby increases hot workability of the steel material. If even a small amount of REM is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of REM is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and toughness of the steel material will decrease. Therefore, the content of REM is to be 0 to 0.200%. A preferable lower limit of the content of REM is more than 0%, more preferably is 0.001%, further preferably is 0.005%, further preferably is 0.010%, and further preferably is 0.020%. A preferable upper limit of the content of REM is 0.180%, and more preferably is 0.160%.

Note that, in the present description the term “REM” means one or more elements 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. Further, in the present description the term “content of REM” refers to the total content of these elements.

[Yield Strength]

The yield strength of the duplex stainless steel material according to the present embodiment is 758 MPa or more. The duplex stainless steel material according to the present embodiment has the chemical composition described above and has a microstructure composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite, and in the duplex stainless steel material, a dislocation density ratio ρ(γ)/ρ(α) to be described later is more than 0.3 to less than 4.0. As a result, the duplex stainless steel material according to the present embodiment has excellent corrosion resistance even though the yield strength is 758 MPa or more.

A preferable lower limit of the yield strength of the duplex stainless steel material according to the present embodiment is 760 MPa, and more preferably is 765 MPa. Although the upper limit of the yield strength of the duplex stainless steel material according to the present embodiment is not particularly limited, for example the upper limit is 1000 MPa.

The yield strength of the duplex stainless steel material according to the present embodiment can be determined by the following method. Specifically, a tensile test is carried out by a method in accordance with ASTM E8/E8M (2022). A test specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, a tensile test specimen is prepared from a center portion of the thickness. In this case, the longitudinal direction of the tensile test specimen is to be made parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, an arc-shaped test specimen having a thickness which is the same as the wall thickness of the steel pipe and having a width of 25.4 mm and a gage length of 50.8 mm is prepared. In this case, the longitudinal direction of the arc-shaped test specimen is to be made parallel to the pipe axis direction of the steel pipe. If the steel material is a round steel bar, a tensile test specimen is prepared from an R/2 position. In this case, the longitudinal direction of the tensile test specimen is to be made parallel to the axial direction of the round steel bar. In the present description, the term “R/2 position” of a round steel bar means the center position of a radius R in a cross section perpendicular to the axial direction of the round steel bar. When preparing the tensile test specimen, the tensile test specimen is prepared so as to be a size with, for example, a parallel portion diameter of 6 mm and a gage length of 24 mm. A tensile test is carried out at normal temperature (25° C.) in air using the test specimen. In the present embodiment, the 0.2% offset proof stress obtained by the tensile test is defined as the yield strength (MPa). In the present embodiment, the decimals of the obtained numerical value is rounded off, and the resultant value is adopted as the yield strength (MPa).

[Microstructure]

The duplex stainless steel material according to the present embodiment has the chemical composition described above and has a microstructure composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite, and in the duplex stainless steel material, a dislocation density ratio ρ(γ)/ρ(α) to be described later is more than 0.3 to less than 4.0. As a result, the duplex stainless steel material according to the present embodiment has excellent corrosion resistance even though the yield strength is 758 MPa or more. In the present description, the phrase that the microstructure is “composed of ferrite and austenite” means that the amount of any phase other than ferrite and austenite in the microstructure is negligibly small. For example, in the chemical composition of the duplex stainless steel material according to the present embodiment, the volume ratios of precipitates and inclusions are negligibly small as compared with the volume ratios 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 and the like, in addition to ferrite and austenite.

In the microstructure of the duplex stainless steel material according to the present embodiment, the volume ratio of ferrite is 35 to 65%. If the volume ratio of ferrite is too low, in some cases the yield strength and/or corrosion resistance of the steel material may decrease. On the other hand, if the volume ratio of ferrite is too high, in some cases the toughness or hot workability of the steel material may decrease. Therefore, in the microstructure of the duplex stainless steel material according to the present embodiment, the volume ratio of ferrite is 35 to 65%. A preferable lower limit of the volume ratio of ferrite is 36%, and more preferably is 37%. A preferable upper limit of the volume ratio of ferrite is 64%, and more preferably is 63%.

In the present embodiment, the volume ratio of ferrite in the duplex stainless steel material can be determined by a method in accordance with ASTM E562 (2019). A test specimen for microstructure observation is prepared from the duplex stainless steel material according to the present embodiment. If the steel material is a steel plate, a test specimen having an observation surface with dimensions of 5 mm in the rolling elongation direction and 5 mm in the width direction is prepared from a center portion of the thickness. If the steel material is a steel pipe, a test specimen having an observation surface with dimensions of 5 mm in the pipe axis direction and 5 mm in the pipe circumferential direction is prepared from a center portion of the wall thickness. In the present description, the pipe circumferential direction of a steel pipe means the direction that is perpendicular to the pipe axis direction and the pipe diameter direction. If the steel material is a round steel bar, a test specimen having an observation surface with dimensions of 5 mm in the axial direction and 5 mm in the circumferential direction is prepared from an R/2 position. In the present description, the term “circumferential direction” of a round steel bar means the direction that is perpendicular to the axial direction and the radial direction. Note that, the size of the test specimen is not particularly limited as long as the aforementioned observation surface is 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 reagent 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, for example, 1.00 mm² (magnification of ×100). In each visual field, ferrite is identified based on contrast. The area fraction of the identified ferrite is measured by a point counting method in accordance with ASTM E562 (2019). In the present embodiment, the arithmetic average value of the area fractions of ferrite obtained in the 10 visual fields is defined as the volume ratio (%) of ferrite. In the present embodiment, a value obtained by rounding off decimals of the obtained numerical value is adopted as the volume ratio (%) of ferrite.

[Dislocation Density Ratio]

The duplex stainless steel material according to the present embodiment has the chemical composition and the microstructure described above, has a yield strength of 758 MPa or more, and a dislocation density ρ(α) in the ferrite and a dislocation density ρ(γ) in the austenite of the duplex stainless steel material satisfy the following Formula (1):

0.3 < ρ ⁡ ( γ ) / ρ ⁡ ( α ) < 4. ( 1 )

where, in Formula (1), the dislocation density in the austenite in m⁻² is substituted for ρ(γ), and the dislocation density in the ferrite in m⁻² is substituted for ρ(α).

Let Fn1 be defined as Fn1=ρ(γ)/ρ(α). Fn1 means the distribution ratio of the dislocation density in austenite with respect to the dislocation density in ferrite in a duplex stainless steel material having the chemical composition and microstructure described above. The larger Fn1 is, the greater the proportion of dislocations that are localized in the austenite. The smaller Fn1 is, the greater the proportion of dislocations that are localized in the ferrite. That is, if Fn1 is too high, the dislocation density in austenite will locally increase, and the corrosion resistance of the steel material will markedly decrease. On the other hand, if Fn1 is too low, the dislocation density in ferrite will locally increase, and the corrosion resistance of the steel material will decrease. Therefore, in the duplex stainless steel material according to the present embodiment, Fn1 is more than 0.3 to less than 4.0. A preferable lower limit of Fn1 is 0.4, and more preferably is 0.5. A preferable upper limit of Fn1 is 3.9, and more preferably is 3.8.

The dislocation density ratio Fn1 in the present embodiment can be determined by the following method. A thin film sample for dislocation density measurement is prepared from the duplex stainless steel material according to the present embodiment. Specifically, a test specimen is cut out from the duplex stainless steel material. Further, electropolishing using a twin-jet method is performed to prepare a thin film sample from the test specimen that was cut out. Note that, if the steel material is a steel plate, a thin film sample having an observation surface perpendicular to the rolling elongation direction is prepared from a test specimen that was cut out from a center portion of the thickness. If the steel material is a steel pipe, a thin film sample having an observation surface perpendicular to the pipe axis direction is prepared from a test specimen that was cut out from a center portion of the wall thickness. If the steel material is a round steel bar, a thin film sample having an observation surface perpendicular to the axial direction is prepared from a test specimen that was cut out from an R/2 position. Further, the size of the test specimen and the size of the thin film sample are not particularly limited as long as an observation visual field to be described later is obtained.

Ferrite and austenite are each identified in the observation surface of the obtained thin film sample. The ferrite and the austenite in the observation surface can be identified by identification of the crystal structure by electron diffraction. The specified visual fields are subjected to microstructure observation using a transmission electron microscope (hereinafter, also referred to as “TEM”). The area of each observation visual field is not particularly limited, and it suffices that the area is an area obtained at a magnification at which dislocations can be easily observed. The area of the observation visual field is, for example, within the range of 100 nm×100 nm to 800 nm×800 nm. In addition, the volume (m³) of each observation visual field is determined based on the area of the observation visual field and the thickness of the observation visual field. Note that, the thickness of the observation region is determined based on the total integrated intensity of an electron energy loss spectrum (EELS) and the integrated intensity of a zero-loss spectrum with respect to the thin film sample.

The microstructure observation for each observation visual field is conducted using an accelerating voltage of 300 kV and diffraction conditions set to conditions suitable for observing dislocations. The phrase “diffraction condition suitable for observing dislocations” means conditions under which a two-wave approximation in which a transmitted wave and one diffracted wave are excited is possible. Specifically, with respect to austenite, the phrase refers to conditions under which a reciprocal lattice vector g=40-2 is excited, and with respect to ferrite the phrase refers to conditions under which a reciprocal lattice vector g=200 or 30-1 is excited. In the present embodiment, in order to obtain diffraction conditions suitable for observing dislocations, the thin film sample is tilted and the observation region of the thin film sample is subjected to bright field observation. Note that, instead of bright field observation, dislocations may be observed by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). In observation by HAADF-STEM, dislocations can be observed more easily in comparison to bright field observation.

In addition, each observation visual field is photographed by performing exposure for an appropriate time. In the generated photographic images, dislocations are identified based on contrast, and the length of each dislocation is measured. The lengths of the dislocations can be measured by a well-known method. For example, the lengths of dislocations that were identified based on contrast may be determined by image analysis. The dislocation density ρ(α) in the ferrite (m⁻²) is determined based on the total (m) of the obtained lengths of the dislocations in ferrite in five visual fields, and the total volume (m³) of ferrite in the five visual fields. Similarly, the dislocation density ρ(γ) in the austenite (m⁻²) is determined based on the total (m) of the obtained lengths of the dislocations in austenite in five visual fields, and the total volume (m³) of austenite in the five visual fields.

A ratio Fn1 (=ρ(γ)/ρ(α)) of the dislocation density ρ(γ) in the austenite (m⁻²) to the dislocation density ρ(α) in the ferrite (m⁻²) which were determined by the above method is determined. In the present embodiment, a value obtained by rounding off to the first decimal place of the obtained numerical value is adopted as the dislocation density ratio Fn1.

Note that, in the present embodiment, the dislocation density ρ(α) in the ferrite (m⁻²) and the dislocation density ρ(γ) in the austenite (m⁻²) are not particularly limited as long as the yield strength is 758 MPa or more and Fn1 satisfies a condition of being within the range of more than 0.3 to less than 4.0. In the duplex stainless steel material according to the present embodiment, the dislocation density ρ(α) in the ferrite (m⁻²) is, for example, 1.0×10¹⁴ to 8.0×10¹⁵ (m⁻²). In the duplex stainless steel material according to the present embodiment, the dislocation density ρ(γ) in the austenite (m⁻²) is, for example, 1.0×10¹⁴ to 8.0×10¹⁵ (m⁻²). If the dislocation density ρ(α) in the ferrite (m⁻²) is 1.0×10¹⁴ to 8.0×10¹⁵ (m⁻²) and the dislocation density ρ(γ) in the austenite (m⁻²) is 1.0×10¹⁴ to 8.0×10¹⁵ (m⁻²), on the condition that the other requirements of the present embodiment are satisfied, a duplex stainless steel material that has a consistent yield strength of 758 MPa or more and also has excellent corrosion resistance can be obtained.

[Corrosion Resistance]

The yield strength of the duplex stainless steel material according to the present embodiment is 758 MPa or more. The duplex stainless steel material according to the present embodiment has the chemical composition described above and has a microstructure composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite, and in the duplex stainless steel material, a dislocation density ratio Fn1 (=ρ(α)/ρ(α)) is more than 0.3 to less than 4.0. As a result, the duplex stainless steel material according to the present embodiment has excellent corrosion resistance even though the yield strength thereof is 758 MPa or more. In the present embodiment, whether or not the duplex stainless steel material has excellent corrosion resistance is evaluated as follows.

A test specimen for a four-point bending test is prepared from the duplex stainless steel material according to the present embodiment. Regarding the size of the test specimen, for example, the test specimen has a thickness of 2 mm, a width of 10 mm, and a length of 75 mm. If the steel material is a steel plate, the test specimen is prepared from a center portion of the thickness. In this case, the longitudinal direction of the test specimen is to be made parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, the test specimen is prepared from a center portion of the wall thickness. In this case, the longitudinal direction of the test specimen is to be made parallel to the pipe axis direction of the steel pipe. If the steel material is a round steel bar, the test specimen is prepared from the 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.

A 20% by mass sodium chloride aqueous solution adjusted to pH=4.0 is used as the test solution. In accordance with ASTM G39-99 (2021), stress corresponding to 90% of the actual yield stress is applied to the test specimen by four-point bending. The test specimen to which stress has been applied is enclosed in an autoclave together with the test jig. The test solution is poured into the autoclave so as to leave a vapor phase portion, and this is adopted as the test bath. After the test bath is degassed, a gaseous mixture of H₂S gas at 0.1 bar and CO₂ gas at 10 bar is charged under pressurization into the autoclave, and the test bath is stirred to cause the gaseous mixture to saturate. After sealing the autoclave, the test bath is stirred for 720 hours at 90° C.

In the present embodiment, if cracking is not confirmed after 720 hours elapses in the test environment described above, it is evaluated that the duplex stainless steel material “has excellent corrosion resistance”. Note that, in the present description, the phrase “cracking is not confirmed” means that cracking is not confirmed in a case where the test specimen after the test is observed by the naked eye.

[Shape of Duplex Stainless Steel Material]

As described above, the shape of the duplex stainless steel material according to the present embodiment is not particularly limited. Preferably, the duplex stainless steel material according to the present embodiment is a seamless steel pipe. In a case where the duplex stainless steel material according to the present embodiment is a seamless steel pipe, even if the wall thickness is 5 mm or more, the duplex stainless steel material has a yield strength of 758 MPa or more and excellent corrosion resistance.

[Production Method]

One example of a method for producing the duplex stainless steel material according to the present embodiment which is composed as described above will now be described. Note that, a method for producing the duplex stainless steel material according to the present embodiment is not limited to the production method described hereunder. One example of a method for producing the duplex stainless steel material according to the present embodiment includes a starting material preparation process, a hot working process, a first cold working process, a solution treatment process, and a second cold working process. Hereunder, each production process is described in detail.

[Starting Material Preparation Process]

In the starting material preparation process according to the present embodiment, a starting material having the chemical composition described above is prepared. The starting material may be prepared by producing the starting material, or may be prepared by purchasing the starting material from a third party. That is, the method for preparing the starting material is not particularly limited.

In the case of producing the starting material, for example, the starting material is produced by the following method. A molten steel having the chemical composition described above is produced. A cast piece (a slab, a bloom, or a billet) is produced by a continuous casting process using the molten steel. An ingot may also be produced by an ingot-making process using the molten steel. As required, a slab, a bloom, or an ingot may be subjected to blooming to produce a billet. The starting material is produced by the above process.

[Hot Working Process]

In the hot working process according to the present embodiment, the starting material prepared in the aforementioned starting material preparation process is subjected to hot working to produce an intermediate steel material. In the present description, the term “intermediate steel material” refers to a plate-shaped steel material in a case where the end product will be a steel plate, refers to a hollow shell in a case where the end product will be a steel pipe, refers to a bar-shaped steel material in which a cross section perpendicular to the axial direction is a circular shape in a case where the end product will be a round steel bar, and refers to a wire-shaped steel material in a case where the end product will be a wire rod. 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.

If the intermediate steel material is a hollow shell (seamless steel pipe), in the hot working process, for example, the Ugine-Sejournet process or the Ehrhardt push bench process (that is, hot extrusion) may be performed, or the intermediate steel material may be subjected to piercing-rolling (that is, hot rolling) according to the Mannesmann process. Note that, hot working may be performed only one time or may be performed multiple times. For example, after performing the aforementioned piercing-rolling on the starting material, the aforementioned hot extrusion may be performed. For example, in addition, after performing the aforementioned piercing-rolling on the starting material, elongation rolling may be performed. That is, in the hot working process, hot working is performed by a well-known method to produce an intermediate steel material having the desired shape.

[First Cold Working Process]

In the first cold working process according to the present embodiment, cold working is performed on the intermediate steel material subjected to the aforementioned hot working process. The cold working may be cold rolling or may be cold drawing. That is, in the first cold working process, it suffices to perform well-known cold working under well-known conditions. For example, the temperature of the intermediate steel material during cold working may be within the range of room temperature to less than 150° C.

Here, in the first cold working process, an area reduction ratio Rd1(%) of the intermediate steel material is defined as follows. Note that, although not particularly limited, the area reduction ratio Rd1(%) in the first cold working process is, for example, 2 to 30%.

Rd1(%)={1−(cross-sectional area perpendicular to working direction of intermediate steel material after first cold working process/cross-sectional area perpendicular to working direction of intermediate steel material before first cold working process)}×100

[Solution Treatment Process]

In the solution treatment process according to the present embodiment, a solution treatment is performed on the intermediate steel material subjected to the aforementioned first cold working process. A method for performing the solution treatment is not particularly limited, and it suffices to perform a well-known method. For example, the intermediate steel material is loaded into a heat treatment furnace, and after being held at a desired temperature, is rapidly cooled. In this case, the temperature at which the solution treatment is performed (heat treatment temperature) means the temperature (° C.) of the heat treatment furnace for performing the solution treatment. The time for holding at the solution treatment temperature (holding time) means the time (mins) for which the intermediate steel material is held at the heat treatment temperature.

Preferably, the heat treatment temperature in the solution treatment process of the present embodiment is set within the range of 950 to 1150° C. If the heat treatment temperature is too low, in some cases the ferrite volume ratio in the duplex stainless steel material after the solution treatment will be less than 35%, and the strength and/or corrosion resistance of the produced duplex stainless steel material will decrease. On the other hand, if the heat treatment temperature is too high, in some cases the volume ratio of ferrite in the duplex stainless steel material after the solution treatment will be more than 65%, and the corrosion resistance of the steel material will, on the contrary, decrease.

Therefore, when performing a solution treatment by loading an intermediate steel material into a heat treatment furnace, holding the intermediate steel material at a desired temperature, and thereafter performing rapid cooling, preferably the solution treatment temperature is set within the range of 950 to 1150° C. A more preferable lower limit of the solution treatment temperature is 960° C., and further preferably is 970° C. A more preferable upper limit of the solution treatment temperature is 1140° C., and further preferably is 1120° C.

When performing a solution treatment by loading an intermediate steel material into a heat treatment furnace, holding the intermediate steel material at a desired temperature, and thereafter performing rapid cooling, the solution treatment time is not particularly limited, and it suffices that the solution treatment time is in accordance with a well-known condition. The solution treatment time is, for example, 5 to 180 minutes. The rapid cooling method is, for example, water cooling.

[Second Cold Working Process]

In the second cold working process according to the present embodiment, cold working is performed on the intermediate steel material subjected to the aforementioned solution treatment process. The cold working may be cold rolling or may be cold drawing. That is, in the second cold working process, similarly to the first cold working process, it suffices to perform well-known cold working under well-known conditions. For example, the temperature of the intermediate steel material during cold working may be within the range of room temperature to less than 150° C.

Here, in the second cold working process, an area reduction ratio Rd2(%) of the intermediate steel material is defined as follows.

Rd2(%)={1−(cross-sectional area perpendicular to working direction of intermediate steel material after second cold working process/cross-sectional area perpendicular to working direction of intermediate steel material before second cold working process)}×100

The area reduction ratio Rd2(%) in the second cold working process significantly influences the strength of the duplex stainless steel material to be produced. Therefore, if the area reduction ratio Rd2 is too small, in some cases the yield strength of the produced duplex stainless steel material will not consistently be 758 MPa or more. On the other hand, if the area reduction ratio Rd2 is too large, in some cases the dislocation density in austenite will increase and the dislocation density ratio Fn1 will become 4.0 or more. Therefore, in the present embodiment, the area reduction ratio Rd2 is to be 4 to 20%.

As described above, in a preferable method for producing the duplex stainless steel material according to the present embodiment, a starting material preparation process, a hot working process, a first cold working process, a solution treatment process, and a second cold working process are performed. Here, the ratio Fn1 (=ρ(γ)/ρ(α)) of the dislocation density ρ(γ) in the austenite to the dislocation density ρ(α) in the ferrite is strongly influenced by the cold working, and the value of the ratio Fn1 varies accordingly. That is, in the preferable production method described above, the value of the dislocation density ratio Fn1 varies according to the balance between the first cold working process and the second cold working process.

Therefore, in the preferable production method according to the present embodiment, the area reduction ratio Rd1(%) in the first cold working process and the area reduction ratio Rd2(%) in the second cold working process satisfy the following Formula (A). As a result, a duplex stainless steel material that has the chemical composition and microstructure described above and also has a yield strength of 758 MPa or more, and in which the dislocation density ratio Fn1 satisfies a condition of being within the range of more than 0.3 to less than 4.0 can be stably produced:

Rd ⁢ 1 / Rd ⁢ 2 > ( Ni + 20 ⁢ N + 10 ⁢ Sn + 4 ⁢ Co + 0.5 Mn + 0.5 Cu ) / ( Cr + 3 ⁢ Mo + 2 ⁢ Si ) ( A )

where, in Formula (A), the area reduction ratio in percentage in the first cold working process is substituted for Rd1, the area reduction ratio in percentage in the second cold working process is substituted for Rd2, and the content of the corresponding element in percent by mass is substituted for each symbol of an element.

Here, by performing cold working before the solution treatment, recrystallization in the solution treatment is promoted, and thus it becomes easier to reduce a variation in the sizes of the grains. That is, the area reduction ratio Rd1(%) in the first cold working process influences the variation in the sizes of the grains after the solution treatment. If the variation in the sizes of the grains after the solution treatment is small, dislocations are more likely to be uniformly distributed between ferrite and austenite by the cold working in the second cold working process. In such case, the dislocation density ratio Fn1 is more likely to be small.

On the other hand, as described above, if the area reduction ratio Rd2(%) in the second cold working process is too large, the dislocation density in the austenite will tend to increase, and consequently Fn1 will be likely to increase. Therefore, in the preferable production method according to the present embodiment, Rd1 is defined relative to Rd2. That is, by increasing Rd1 to a certain level or more in accordance with Rd2, it is possible to regulate in advance the variation in the size of the grains of the intermediate steel material in the second cold working process. In other words, the occurrence of a local increase in the dislocation density ρ(γ) in the austenite in the second cold working process can be suppressed. As a result, the dislocation density ratio Fn1 can be reduced.

In addition, let FnA be defined as FnA=(Ni+20N+10Sn+4Co+0.5Mn+0.5Cu)/(Cr+3Mo+2Si). FnA is an index that indicates the degree to which the variation in the size of the grains is regulated in the microstructure of a duplex stainless steel material having the chemical composition described above. The larger FnA is, the more likely it is for a variation in the grain size to be large. Therefore, even in a case where FnA is large, if Rd1 is increased in accordance with Rd2, the effect of regulating the variation in the size of the grains will increase.

Therefore, in the preferable production method according to the present embodiment, the ratio of Rd1 to Rd2 is made greater than FnA. In this case, in the second cold working process, the occurrence of a local increase in the dislocation density ρ(γ) in the austenite can be suppressed. As a result, the dislocation density ratio Fn1 can be reduced. Thus, according to the preferable production method of the present embodiment, a duplex stainless steel material that has a yield strength of 758 MPa or more and in which the dislocation density ratio Fn1 satisfies a condition of being within the range of more than 0.3 to less than 4.0 can be stably produced.

[Other Processes]

The production method according to the present embodiment may also include production processes other than the production processes described above. For example, the duplex stainless steel material according to the present embodiment may be subjected to an aging heat treatment. The term “aging heat treatment” means a treatment in which the produced duplex stainless steel material is held at a desired temperature. In this case, the aging heat treatment is not particularly limited, and it suffices to perform the aging heat treatment using a well-known method. Furthermore, for example, the duplex stainless steel material according to the present embodiment may be subjected to a pickling treatment. In this case, the pickling treatment is not particularly limited, and it suffices to perform the pickling treatment using a well-known method. In addition, the duplex stainless steel material which was subjected to the second cold working process may also be subjected to other well-known post treatments.

The duplex stainless steel material according to the present embodiment can be produced by the processes described above. Note that, the method for producing the duplex stainless steel material that is described above is one example, and the duplex stainless steel material according to the present embodiment may be produced by other methods. Hereunder, the present invention is described in further detail by way of examples.

EXAMPLE

Molten steels having the chemical compositions shown in Table 1-1 and Table 1-2 were melted using a 50 kg vacuum furnace, and ingots were produced by an ingot-making process. Note that, the symbol “-” in Table 1-2 means that the content of the corresponding element was at an impurity level. For example, the symbol “-” means that the content of Nb, the content of Ta, the content of Ti, the content of Zr, the content of Hf, the content of W, the content of Sb, the content of Ca, the content of Mg, the content of B, and the content of REM of steel A were each 0% when rounded off to the third decimal place. In addition, for each test number, FnA (=(Ni+20N+10Sn+4Co+0.5Mn+0.5Cu)/(Cr+3Mo+2Si)) that was determined based on the chemical composition described in Table 1-1 and the above definition is shown in Table 2.

	TABLE 1-1
	Chemical Composition (unit is mass %; balance is Fe and impurities)

Steel

C

Si

Mn

P

S

Al

Ni

Cr

Mo

Cu

N

V

Co

Sn

A	0.020	0.48	3.1	0.020	0.0106	0.010	4.8	25.2	1.1	2.5	0.23	0.36	0.12	0.008
B	0.020	0.95	3.7	0.021	0.0193	0.058	4.4	20.1	1.8	2.9	0.20	0.24	0.88	0.030
C	0.026	0.91	1.1	0.028	0.0195	0.025	5.1	29.4	1.6	2.6	0.20	0.01	0.44	0.024
D	0.010	0.76	3.6	0.003	0.0132	0.043	6.9	28.4	0.7	2.7	0.21	0.38	0.15	0.001
E	0.019	0.65	5.0	0.032	0.0083	0.039	6.2	27.3	2.0	2.7	0.24	0.22	0.81	0.030
F	0.022	0.78	0.7	0.031	0.0130	0.077	4.0	24.4	1.1	2.6	0.21	0.50	0.33	0.003
G	0.011	0.75	4.0	0.018	0.0151	0.011	4.7	26.6	1.2	1.8	0.27	0.41	0.07	0.039
H	0.020	0.82	2.5	0.027	0.0072	0.078	5.2	29.9	1.0	1.8	0.16	0.02	0.21	0.019
I	0.011	0.32	2.6	0.018	0.0033	0.054	5.7	22.9	0.6	3.0	0.16	0.03	0.88	0.008
J	0.022	1.00	1.7	0.027	0.0172	0.036	4.3	23.9	1.3	2.4	0.19	0.46	0.45	0.004
K	0.013	1.00	2.4	0.004	0.0153	0.023	7.2	23.7	1.9	2.2	0.24	0.41	0.74	0.033
L	0.023	0.38	5.3	0.013	0.0199	0.013	5.8	25.7	1.6	2.1	0.28	0.49	0.13	0.047
M	0.027	0.92	5.2	0.001	0.0149	0.029	4.7	26.8	1.9	1.6	0.20	0.36	0.22	0.015
N	0.013	0.29	1.3	0.040	0.0162	0.027	6.0	24.6	1.1	2.6	0.23	0.01	0.94	0.005
O	0.020	0.82	6.0	0.022	0.0095	0.045	6.7	20.2	0.6	2.9	0.26	0.06	0.78	0.015
P	0.013	0.27	6.6	0.006	0.0035	0.092	6.9	25.7	1.5	2.2	0.23	0.23	0.38	0.003
Q	0.023	0.53	2.0	0.036	0.0022	0.090	8.9	21.5	1.1	1.5	0.24	0.35	0.86	0.012
R	0.007	0.46	2.8	0.005	0.0176	0.095	5.5	25.8	1.3	1.7	0.22	0.23	0.88	0.004
S	0.020	0.29	4.4	0.033	0.0066	0.026	8.7	25.4	0.7	2.7	0.16	0.03	0.68	0.040

	TABLE 1-2
	Chemical Composition (unit is mass %; balance is Fe and impurities)

Steel	Nb	Ta	Ti	Zr	Hf	W	Sb	Ca	Mg	B	REM
A	—	—	—	—	—	—	—	—	—	—	—
B	0.259	—	—	—	—	—	—	—	—	—	—
C	—	0.045	—	—	—	—	—	—	—	—	—
D	—	—	0.042	—	—	—	—	—	—	—	—
E	—	—	—	0.038	—	—	—	—	—	—	—
F	—	—	—	—	0.056	—	—	—	—	—	—
G	—	—	—	—	—	0.139	—	—	—	—	—
H	—	—	—	—	—	—	0.098	—	—	—	—
I	—	—	—	—	—	—	—	0.016	—	—	—
J	—	—	—	—	—	—	—	—	0.012	—	—
K	—	—	—	—	—	—	—	—	—	0.008	—
L	—	—	—	—	—	—	—	—	—	—	0.159
M	0.229	—	—	—	—	—	0.022	—	—	—	—
N	—	0.049	—	—	—	—	—	0.012	—	—	—
O	—	—	0.067	—	—	—	—	—	0.016	—	—
P	—	—	—	0.009	—	—	—	—	—	0.019	—
Q	—	—	—	—	0.018	—	—	—	—	—	0.085
R	—	—	—	—	—	0.040	0.030	0.008	—	—	—
S	0.123	0.072	—	—	—	—	—	—	0.004	—	—

	TABLE 2
	First Cold	Solution Treatment	Second Cold

			Working	Heat Treatment	Holding	Working
Test			Rd1	Temperature	Time	Rd2
Number	Steel	FnA	(%)	(° C.)	(mins)	(%)	Rd1/Rd2

1	A	0.43	6	1040	20	7	0.86
2	B	0.57	10	1020	30	13	0.77
3	C	0.36	11	990	40	10	1.10
4	D	0.46	8	1090	160	15	0.53
5	E	0.53	7	1040	30	12	0.58
6	F	0.38	12	990	50	9	1.33
7	G	0.43	13	1060	50	11	1.18
8	H	0.34	7	1020	120	19	0.37
9	I	0.60	4	1040	20	6	0.67
10	J	0.40	8	1010	70	5	1.60
11	K	0.56	14	1080	120	11	1.27
12	L	0.51	14	1030	100	13	1.08
13	M	0.38	12	1050	170	15	0.80
14	N	0.57	6	1080	40	9	0.67
15	O	0.83	12	990	80	13	0.92
16	P	0.57	9	1070	50	13	0.69
17	Q	0.74	15	990	80	15	1.00
18	R	0.51	14	1070	40	10	1.40
19	S	0.66	10	980	80	11	0.91
20	A	0.43	5	1040	20	3	1.67
21	B	0.57	6	1020	30	2	3.00
22	A	0.43	11	1040	20	23	0.48
23	N	0.57	15	1040	20	25	0.60
24	A	0.43	5	1120	20	15	0.33
25	Q	0.74	7	1000	30	13	0.54
26	R	0.51	6	1020	30	14	0.43

The ingot of each steel type was subjected to hot working to produce a hollow shell (seamless steel pipe). The hollow shell of each test number on which the hot working had been performed was subjected to a first cold working with an area reduction ratio Rd1(%) that is described in Table 2. Further, the hollow shell of each test number was subjected to a solution treatment at a heat treatment temperature (° C.) for a holding time (mins) which are each described in Table 2. In addition, the hollow shell of each test number on which the solution treatment had been performed was subjected to a second cold working with an area reduction ratio Rd2(%) that is described in Table 2. The ratio of the area reduction ratio Rd1(%) in the first cold working to the area reduction ratio Rd2(%) in the second cold working in each test number is shown in the column “Rd1/Rd2” in Table 2. Note that, cold drawing was performed for each of the first cold working and the second cold working.

[Evaluation Tests]

A seamless steel pipe of each test number was obtained by the above process. The obtained seamless steel pipe of each test number was subjected to a tensile test, a microstructure observation test, a dislocation density ratio measurement test, and a corrosion resistance test.

[Tensile Test]

The seamless steel pipe of each test number was subjected to a tensile test in accordance with ASTM E8/E8M (2022), and the yield strength was determined. Specifically, an arc-shaped test specimen for a tensile test was prepared from the seamless steel pipe of each test number. The thickness of the arc-shaped test specimen was made the same as the wall thickness of the steel pipe, and the width thereof was set to 25.4 mm and the gage length was set to 50.8 mm. The arc-shaped test specimen of each test number was used to carry out a tensile test at normal temperature (25° C.) in air, and the 0.2% offset proof stress (MPa) was determined. The determined 0.2% offset proof stress was defined as the yield strength (MPa). The obtained yield strength of each test number is shown in the column “YS (MPa)” in Table 3.

TABLE 3
		Ferrite	Dislocation
Test	YS	Volume Ratio	Density Ratio	Corrosion
Number	(MPa)	(%)	ρ(γ)/ρ(α)	Resistance

1	848	56	1.2	EX
2	841	46	3.8	EX
3	793	45	2.5	EX
4	800	57	3.1	EX
5	827	45	1.6	EX
6	772	60	2.8	EX
7	848	41	1.9	EX
8	931	61	3.5	EX
9	772	37	0.7	EX
10	772	52	0.5	EX
11	800	47	2.2	EX
12	841	43	2.5	EX
13	855	52	1.8	EX
14	786	41	2.1	EX
15	848	57	2.6	EX
16	772	64	1.9	EX
17	793	45	2.2	EX
18	855	52	0.9	EX
19	862	48	1.6	EX
20	724	56	0.4	EX
21	703	46	0.6	EX
22	972	56	4.5	NA
23	993	44	5.5	NA
24	917	51	5.1	NA
25	896	47	4.7	NA
26	889	55	4.4	NA

[Microstructure Observation Test]

The seamless steel pipe of each test number was subjected to microstructure observation, and the volume ratio of ferrite was determined. Specifically, a test specimen for microstructure observation having an observation surface with dimensions of 5 mm in the pipe axis direction x 5 mm in the pipe circumferential direction was prepared from a central portion of the wall thickness of the seamless steel pipe of each test number. The observation surface of the test specimen of each test number was polished to obtain a mirror surface, and was then electrolytically etched in a 7% potassium hydroxide etching reagent. The observation surface on which the microstructure had been revealed by the electrolytic etching was observed in 10 visual fields using an optical microscope. The area of each visual field was 1.00 mm² (magnification of ×100).

In each visual field of each test number, phases other than ferrite and austenite in the microstructure were negligibly small. That is, the seamless steel pipe of each test number had a microstructure composed of ferrite and austenite. In each visual field of each test number, ferrite and austenite were each identified based on contrast. The area fraction (%) of the identified ferrite was determined by image analysis in accordance with ASTM E562 (2019). The arithmetic average value of the area fractions of ferrite in the 10 visual fields was defined as the ferrite volume ratio (%). The determined ferrite volume ratio (%) of each test number is shown in Table 3.

[Dislocation Density Ratio Measurement Test]

The seamless steel pipe of each test number was subjected to a dislocation density ratio measurement test, and the dislocation density ratio Fn1 (=ρ(γ)/ρ(α)) was determined. Specifically, a thin film sample was prepared from the seamless steel pipe of each test number by the method described above. In addition, the thin film sample of each test number was used to determine the dislocation density ρ(α) in the ferrite (m⁻²) and the dislocation density ρ(γ) in the austenite (m⁻²) by the method described above. Note that, in the present embodiment, the dislocations were observed by bright field observation. In each test number, the dislocation density ρ(α) in the ferrite was 1.0×10¹⁴ to 8.0×10¹⁵ (m⁻²), and the dislocation density ρ(γ) in the austenite was 1.0×10¹⁴ to 8.0×10¹⁵ (m⁻²). The dislocation density ratio Fn1 (=ρ(γ)/ρ(α)) was determined based on the obtained ρ(α) (m⁻²) and ρ(γ) (m⁻²). The determined dislocation density ratio Fn1 is shown in the column “Dislocation Density Ratio ρ(γ)/ρ(α)” in Table 3.

[Corrosion Resistance Test]

The seamless steel pipe of each test number was subjected to a corrosion resistance test, and the corrosion resistance was evaluated. Specifically, a test specimen was prepared from the seamless steel pipe of each test number by the method described above. A 20% by mass sodium chloride aqueous solution adjusted to pH=4.0 was used as the test solution. In accordance with ASTM G39-99 (2021), stress corresponding to 90% of the actual yield stress was applied to the test specimen by four-point bending. The test specimen to which stress had been applied was enclosed in an autoclave together with the test jig. The test solution was poured into the autoclave so as to leave a vapor phase portion, and this was adopted as the test bath. After the test bath was degassed, a gaseous mixture of H₂S gas at 0.1 bar and CO₂ gas at 10 bar was charged under pressurization into the autoclave, and the test bath was stirred to cause the gaseous mixture to saturate. After sealing the autoclave, the test bath was stirred at 90° C. for 720 hours.

Test specimens in which cracking was not confirmed after 720 hours elapsed were determined as “having excellent corrosion resistance” (“EX” (Excellent) in Table 3). On the other hand, test specimens in which cracking was confirmed after 720 hours elapsed were determined as “not having excellent corrosion resistance” (“NA” (Not Acceptable) in Table 3). The evaluation result for the seamless steel pipe of each test number is shown in Table 3.

Referring to Table 1-1, Table 1-2, Table 2, and Table 3, in the seamless steel pipes of Test Nos. 1 to 19, the chemical composition was appropriate. In addition, the production method used to produce these seamless steel pipes was the preferable production method described herein. As a result, in these seamless steel pipes, the yield strength was 758 MPa or more, the volume ratio of ferrite was 35 to 65%, and the dislocation density ratio Fn1 satisfied a condition of being within the range of more than 0.3 to less than 4.0. As a result, in the corrosion resistance test, it was determined that these seamless steel pipes had excellent corrosion resistance. That is, in the seamless steel pipes of Test Nos. 1 to 19, a high yield strength of 758 MPa or more and excellent corrosion resistance were both achieved.

On the other hand, for the seamless steel pipes of Test Nos. 20 and 21, the area reduction ratio Rd2 in the second cold working process was too small. As a result, in these seamless steel pipes the yield strength was less than 758 MPa.

For the seamless steel pipes of Test Nos. 22 and 23, the area reduction ratio Rd2 in the second cold working process was too large. Consequently, in these seamless steel pipes the dislocation density ratio Fn1 was 4.0 or more. As a result, in the corrosion resistance test, it was determined that these seamless steel pipes did not have excellent corrosion resistance.

For the seamless steel pipes of Test Nos. 24 to 26, the area reduction ratio Rd1 in the first cold working process, the area reduction ratio Rd2 in the second cold working process, and FnA did not satisfy Formula (A). Consequently, in these seamless steel pipes the dislocation density ratio Fn1 was 4.0 or more. As a result, in the corrosion resistance test, it was determined that these seamless steel pipes did not have excellent corrosion resistance.

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 embodiment within a range that does not depart from the gist of the present disclosure.