EP4592408A1

EP4592408A1 - Martensitic stainless steel material

Info

Publication number: EP4592408A1
Application number: EP23868229.8A
Authority: EP
Inventors: Toshiya Nishimura; Kyohei KANKI
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-09-21
Filing date: 2023-09-20
Publication date: 2025-07-30
Also published as: JPWO2024063108A1; JP7488503B1; WO2024063108A1; EP4592408A4; MX2025003234A; CN119895068A

Abstract

A martensitic stainless steel material that has high strength and excellent SSC resistance, and also has excellent low-temperature toughness is provided. A martensitic stainless steel material of the present disclosure contains, by mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cr: 10.00 to 16.00%, Ni: 4.00 to 7.50%, Mo: 1.10 to 3.50%, Al: 0.005 to 0.050%, V: 0.01 to 0.30%, N: 0.0030 to 0.0500%, Ti: 0.020 to 0.150%, Cu: 0.01 to 3.50%, and Co: 0.01 to 0.50%, and has a yield strength of 758 MPa or more. An area fraction Sd (%) of δ-ferrite and an area fraction Sc (%) of intermetallic compounds in a cross section parallel to the rolling direction satisfy Formula (1) and Formula (2).

0 < Sd \leq 10.00

Sc / Sd \leq 5.00

Description

TECHNICAL FIELD

The present disclosure relates to a steel material, and more particularly relates to a martensitic 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 a corrosive environment containing a corrosive gas. Here, the term "corrosive gas" means carbon dioxide gas and/or hydrogen sulfide gas. Steel materials for use in oil wells are required to have excellent stress corrosion cracking resistance (SSC resistance) in corrosive environments.
It is known that chromium (Cr) is effective for improving the SSC resistance of a steel material in corrosive environments. Therefore, in corrosive environments, martensitic stainless steel materials containing about 13% by mass of Cr, which are typified by API L80 13Cr steel material (normal 13Cr steel material) and Super 13Cr steel material in which the C content is reduced, are used.
In addition, in recent years, due to the deepening of oil wells, steel materials are required to have not only SSC resistance but to also have enhanced strength. For example, steel materials with strengths of 110 ksi (758 MPa) or more are being requested.
Martensitic stainless steel materials which have a high strength of 110 ksi or more and also have excellent SSC resistance are proposed in Patent Literature 1 (International Application Publication No. 2019/065115 ) and Patent Literature 2 (International Application Publication No. 2020/095559 ).
In Patent Literature 1 and Patent Literature 2, it is attempted to achieve both high strength and SSC resistance from the viewpoint of the chemical composition with respect to a martensitic stainless steel material containing Cr in an amount of 10.0 to 14.0%. Specifically, the chemical composition is adjusted so that the contents of C, Mn, Cr, Ni, Mo, W, Nb, N, and Ti in the chemical composition satisfy a specific parametric equation.

CITATION LIST

PATENT LITERATURE

Patent Literature 1: International Application Publication No. 2019/065115
Patent Literature 2: International Application Publication No. 2020/095559

SUMMARY OF INVENTION

TECHNICAL PROBLEM

Recently, oil wells are also being developed in cold regions. Oil-well steel pipes to be used in deep wells in such cold regions are required to not only have high strength and excellent SSC resistance, but to also have excellent low-temperature toughness. In Patent Literature 1 and Patent Literature 2, low-temperature toughness is not investigated.
An objective of the present disclosure is to provide a martensitic stainless steel material that has high strength and excellent SSC resistance, and also has excellent low-temperature toughness.

SOLUTION TO PROBLEM

A martensitic stainless steel material according to the present disclosure has a chemical composition consisting of, by mass%,

C: 0.030% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.030% or less,
S: 0.0050% or less,
Cr: 10.00 to 16.00%,
Ni: 4.00 to 7.50%,
Mo: 1.10 to 3.50%,
Al: 0.005 to 0.050%,
V: 0.01 to 0.30%,
N: 0.0030 to 0.0500%,
Ti: 0.020 to 0.150%,
Cu: 0.01 to 3.50%,
Co: 0.01 to 0.50%,
Nb: 0 to 0.150%,
W: 0 to 1.50%,
Zr: 0 to 0.0100%,
Sn: 0 to 0.0100%,
As: 0 to 0.0100%,
Sb: 0 to 0.0100%,
B: 0 to 0.0050%,
Ca: 0 to 0.0050%,
Mg: 0 to 0.0050%, and
rare earth metal (REM): 0 to 0.0100%,
with the balance being Fe and impurities,
wherein:
- a yield strength is 758 MPa or more; and
- an area fraction Sd (%) of δ-ferrite and an area fraction Sc (%) of intermetallic compounds which have an equivalent circular diameter of 1.0 µm or more in a cross section parallel to a rolling direction of the martensitic stainless steel material satisfy Formula (1) and Formula (2). $0 < Sd \leq 10.00$ $Sc / Sd \leq 5.00$

ADVANTAGEOUS EFFECTS OF INVENTION

The martensitic stainless steel material according to the present disclosure has a high strength that is a yield strength of 758 MPa or more (110 ksi or more) and excellent SSC resistance, and also has excellent low-temperature toughness.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a graph illustrating the relation between a δ-ferrite area fraction Sd, a coarse intermetallic compounds area fraction Sc, and low-temperature toughness.
[FIG. 2A] FIG. 2A is a schematic diagram for describing a method for identifying intermetallic compounds in respective visual fields that are divided into 512 × 512 pixels when measuring the coarse intermetallic compounds area fraction Sc.
[FIG. 2B] FIG. 2B is a schematic diagram for describing a process that follows the process in FIG. 2A.
[FIG. 2C] FIG. 2C is a schematic diagram for describing a process that follows the process in FIG. 2B.
[FIG. 2D] FIG. 2D is a schematic diagram for describing a process that follows the process in FIG. 2C.
[FIG. 2E] FIG. 2E is a schematic diagram for describing a process that follows the process in FIG. 2D.
[FIG. 3] FIG. 3 is a graph that, with respect to a steel material satisfying the chemical composition of the present embodiment, illustrates the relation among a time t1 until an intermediate temperature reaches 900°C from the A_c3 point and a holding time t2 at a quenching temperature T1 in a quenching process, the content of Cu, and the coarse intermetallic compounds area fraction Sc.

DESCRIPTION OF EMBODIMENTS

The present inventors conducted studies from the viewpoint of the chemical composition with respect to a steel material having a yield strength of 758 MPa or more (110 ksi grade or more) and excellent SSC resistance. As a result, the present inventors considered that if a martensitic stainless steel material has a chemical composition consisting of, by mass%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cr: 10.00 to 16.00%, Ni: 4.00 to 7.50%, Mo: 1.10 to 3.50%, Al: 0.005 to 0.050%, V: 0.01 to 0.30%, N: 0.0030 to 0.0500%, Ti: 0.020 to 0.150%, Cu: 0.01 to 3.50%, Co: 0.01 to 0.50%, Nb: 0 to 0.150%, W: 0 to 1.50%, Zr: 0 to 0.0100%, Sn: 0 to 0.0100%, As: 0 to 0.0100%, Sb: 0 to 0.0100%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, and rare earth metal (REM): 0 to 0.0100%, with the balance being Fe and impurities, a yield strength of 758 MPa or more and excellent SSC resistance will be obtained.
Therefore, the present inventors also conducted studies regarding means for increasing low-temperature toughness in a martensitic stainless steel material in which the content of each element in the chemical composition is within the range described above. As a result, the present inventors have found that when only the contents of the elements in the chemical composition are simply adjusted, although excellent SSC resistance is obtained in a case where the yield strength is adjusted to 758 MPa or more, in some cases excellent low-temperature toughness cannot be obtained.
Therefore, the present inventors attempted to improve the low-temperature toughness from the viewpoint of the microstructure, and not from the viewpoint of the chemical composition. Here, the present inventors focused their attention on δ-ferrite in the martensitic stainless steel material. There is δ-ferrite in a martensitic stainless steel material that contains Cr in an amount of 10.00 to 16.00%. The δ-ferrite present in the steel material embrittles the steel material. Therefore, the present inventors have considered that if the amount of δ-ferrite formed in a steel material is large, the δ-ferrite will affect the low-temperature toughness of the steel material.
Therefore, with respect to a martensitic stainless steel material having the chemical composition described above and having a yield strength of 758 MPa or more, the present inventors investigated the relation among the amount of δ-ferrite that is formed, the hot workability of the steel material, and the absorbed energy at - 10°C that is an index of low-temperature toughness. As a result, although it has been found that hot workability of the steel material decreases in a case where the area fraction Sd of δ-ferrite that serves as an index of the amount of δ-ferrite formed is more than 10.00%, no clear correlation was recognized between the δ-ferrite area fraction and the absorbed energy at -10°C.
Therefore, the present inventors conducted further studies regarding the microstructure of the aforementioned martensitic stainless steel material. As a result, the present inventors obtained the following new finding.
In a region where δ-ferrite forms in the steel material, coarse intermetallic compounds easily form in the δ-ferrite or at the interface between δ-ferrite and the parent phase. Here, intermetallic compounds formed in a martensitic stainless steel material having the chemical composition described above are Laves phase and/or chi phase (χ phase). The Laves phase is Fe₂Mo, and the χ phase is Fe₃₆Cr₁₂Mo₁₀. The term "coarse intermetallic compounds" means intermetallic compounds having an equivalent circular diameter of 1.0 µm or more.
The present inventors focused their attention on the aforementioned coarse intermetallic compounds formed accompanying formation of δ-ferrite. The present inventors have thought that the ratio of the amount of coarse intermetallic compounds to the amount of δ-ferrite might be related to the low-temperature toughness. Therefore, the present inventors investigated the relation among the δ-ferrite area fraction Sd, the coarse intermetallic compounds area fraction Sc, and low-temperature toughness.
FIG. 1 is a graph illustrating the relation among the δ-ferrite area fraction Sd, the coarse intermetallic compounds area fraction Sc, and low-temperature toughness. Referring to FIG. 1, the abscissa represents Sc/Sd. The ordinate represents the absorbed energy (J) at -10°C. Referring to FIG. 1, the lower Sc/Sd was, in other words, the lower the coarse intermetallic compounds area fraction Sc was relative to the δ-ferrite area fraction Sd, the higher the absorbed energy became. Further, when Sc/Sd became 5.00 or less, the slope of the graph became more gradual compared to when Sc/Sd was more than 5.00. That is, there was an inflection point in the vicinity of the location where the value of Sc/Sd was 5.00.
Based on the results of the above investigations, the present inventors have discovered that in a martensitic stainless steel material having the chemical composition described above, by making Sc/Sd 5.00 or less while also ensuring sufficient hot workability by making the δ-ferrite area fraction Sd 10.00% or less, in addition to excellent SSC resistance, excellent low-temperature toughness is also obtained.
Although the reason why excellent low-temperature toughness is obtained by making the value of Sc/Sd 5.00 or less is not certain, the present inventors consider that the reason may be as follows. As described above, coarse intermetallic compounds form in δ-ferrite or at the interface between δ-ferrite and the parent phase. Coarse intermetallic compounds promote the propagation of cracks. When Sc/Sd is higher than 5.00, the proportion of coarse intermetallic compounds that are present in the δ-ferrite or at the interface between the δ-ferrite and the parent phase is high. Consequently, the coarse intermetallic compounds promote the propagation of cracks, and it becomes easy for a crack to propagate within the δ-ferrite or at the interface between the δ-ferrite and the parent phase. It is considered that, as a result, the low-temperature toughness decreases.
The mechanism described above is the presumed mechanism, and there is also a possibility that excellent low-temperature toughness is obtained because of a different mechanism. However, the fact that excellent low-temperature toughness is obtained by making Sc/Sd 5.00 or less has been demonstrated by FIG. 1 and also by examples that are described later.
The martensitic stainless steel material according to the present embodiment was completed based on the technical idea described above, and is as follows.

[1] A martensitic stainless steel material having a chemical composition consisting of, by mass%,
- C: 0.030% or less,
- Si: 1.00% or less,
- Mn: 1.00% or less,
- P: 0.030% or less,
- S: 0.0050% or less,
- Cr: 10.00 to 16.00%,
- Ni: 4.00 to 7.50%,
- Mo: 1.10 to 3.50%,
- Al: 0.005 to 0.050%,
- V: 0.01 to 0.30%,
- N: 0.0030 to 0.0500%,
- Ti: 0.020 to 0.150%,
- Cu: 0.01 to 3.50%,
- Co: 0.01 to 0.50%,
- Nb: 0 to 0.150%,
- W: 0 to 1.50%,
- Zr: 0 to 0.0100%,
- Sn: 0 to 0.0100%,
- As: 0 to 0.0100%,
- Sb: 0 to 0.0100%,
- B: 0 to 0.0050%,
- Ca: 0 to 0.0050%,
- Mg: 0 to 0.0050%, and
- rare earth metal (REM): 0 to 0.0100%,
- with the balance being Fe and impurities,
- wherein:
  - a yield strength is 758 MPa or more; and
  - an area fraction Sd (%) of δ-ferrite and an area fraction Sc (%) of intermetallic compounds which have an equivalent circular diameter of 1.0 µm or more in a cross section parallel to a rolling direction of the martensitic stainless steel material satisfy Formula (1) and Formula (2). $0 < Sd \leq 10.00$ $Sc / Sd \leq 5.00$
[2] The martensitic stainless steel material according to [1], wherein the chemical composition contains one or more kinds of element selected from a group consisting of, by mass%,
- Nb: 0.001 to 0.150%,
- W: 0.01 to 1.50%,
- Zr: 0.0001 to 0.0100%,
- Sn: 0.0001 to 0.0100%,
- As: 0.0001 to 0.0100%,
- Sb: 0.0001 to 0.0100%,
- B: 0.0001 to 0.0050%,
- Ca: 0.0001 to 0.0050%,
- Mg: 0.0001 to 0.0050%, and
- rare earth metal (REM): 0.0001 to 0.0100%.
[3] The martensitic stainless steel material according to [1] or [2], wherein the chemical composition contains, by mass%,
Cu: 1.00 to 3.50%.

The martensitic stainless steel material of the present embodiment is described in detail hereunder. The symbol "%" in relation to elements means "mass percent" unless otherwise noted.
The martensitic stainless steel material of the present embodiment satisfies Feature 1 to Feature 3.

(Feature 1)

The chemical composition consists of C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cr: 10.00 to 16.00%, Ni: 4.00 to 7.50%, Mo: 1.10 to 3.50%, Al: 0.005 to 0.050%, V: 0.01 to 0.30%, N: 0.0030 to 0.0500%, Ti: 0.020 to 0.150%, Cu: 0.01 to 3.50%, Co: 0.01 to 0.50%, Nb: 0 to 0.150%, W: 0 to 1.50%, Zr: 0 to 0.0100%, Sn: 0 to 0.0100%, As: 0 to 0.0100%, Sb: 0 to 0.0100%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, and rare earth metal (REM): 0 to 0.0100%, with the balance being Fe and impurities.

(Feature 2)

The yield strength is 758 MPa or more.

(Feature 3)

An area fraction Sd (%) of δ-ferrite and an area fraction Sc (%) of intermetallic compounds which have an equivalent circular diameter of 1.0 µm or more in a cross section parallel to the rolling direction of the martensitic stainless steel material satisfy Formula (1) and Formula (2). $0 < Sd \leq 10.00$ $Sc / Sd \leq 5.00$
Hereunder, Feature 1 to Feature 3 are described.

[(Feature 1) Regarding chemical composition]

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

C: 0.030% or less

Carbon (C) is unavoidably contained. That is, the content of C is more than 0%.
C increases hardenability of the steel material, and increases the strength of the steel material. However, if the content of C is more than 0.030%, C will combine with Cr and will form Cr carbides. As a result, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will decrease.
Therefore, the content of C is 0.030% or less.
A preferable lower limit of the content of C is 0.001%, more preferably is 0.003%, further preferably is 0.005%, and further preferably is 0.006%.
A preferable upper limit of the content of C is 0.029%, more preferably is 0.028%, further preferably is 0.025%, further preferably is 0.020%, and further preferably is 0.015%.

Si: 1.00% or less

Silicon (Si) is unavoidably contained. That is, the content of Si is more than 0%.
Si deoxidizes the steel. However, if the content of Si is more than 1.00%, 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 1.00% or less.
A preferable lower limit of the content of Si is 0.01%, more preferably 0.05%, further preferably is 0.10%, further preferably is 0.15%, and further preferably is 0.20%.
A preferable upper limit of the content of Si is 0.70%, more preferably is 0.50%, further preferably is 0.45%, and further preferably is 0.40%.

Mn: 1.00% or less

Manganese (Mn) is unavoidably contained. That is, the content of Mn is more than 0%.
Mn increases hardenability of the steel material, and increases the strength of the steel material. However, if the content of Mn is more than 1.00%, even if the contents of other elements are within the range of the present embodiment, Mn will form coarse inclusions. The coarse inclusions will reduce the low-temperature toughness of the steel material.
Therefore, the content of Mn is 1.00% or less.
A preferable lower limit of the content of Mn is 0.01%, more preferably 0.10%, further preferably is 0.20%, and further preferably is 0.25%.
A preferable upper limit of the content of Mn is 0.90%, more preferably is 0.80%, further preferably is 0.70%, further preferably is 0.60%, and further preferably is 0.50%.

P: 0.030% or less

Phosphorus (P) is an impurity that is unavoidably contained. That is, the content of P is more than 0%.
If the content of P is more than 0.030%, even if the contents of other elements are within the range of the present embodiment, P will segregate to grain boundaries and the low-temperature toughness of the steel material will markedly decrease.
Therefore, the content of P is 0.030% or less.
The content of P is preferably as low as possible. However, if the content of P is excessively reduced, the production cost will significantly increase. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, further preferably is 0.005%, and further preferably is 0.007%.
A preferable upper limit of the content of P is 0.028%, more preferably is 0.025%, further preferably is 0.023%, and further preferably is 0.020%.

S: 0.0050% or less

Sulfur (S) is an impurity that is unavoidably contained. That is, the content of S is more than 0%.
If the content of S is more than 0.0050%, S will excessively segregate to grain boundaries, and S will combine with Mn and form an excessively large amount of MnS that is an inclusion. In such a case, the low-temperature toughness and hot workability of the steel material will markedly decrease even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of S is 0.0050% or less.
The content of S is preferably as low as possible. However, if the content of S is excessively reduced, the production cost will significantly increase. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0002%, further preferably is 0.0003%, and further preferably is 0.0004%.
A preferable upper limit of the content of S is 0.0040%, more preferably is 0.0030%, further preferably is 0.0020%, and further preferably is 0.0015%.

Cr: 10.00 to 16.00%

Chromium (Cr) forms a passivation film on the surface of the steel material in a corrosive environment, and thereby improves the SSC resistance of the steel material. If the content of Cr is less than 10.00%, 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 Cr is more than 16.00%, δ-ferrite will excessively form. In such a case, the hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Cr is 10.00 to 16.00%.
A preferable lower limit of the content of Cr is 10.05%, more preferably is 10.10%, further preferably is 10.50%, and further preferably is 11.00%.
A preferable upper limit of the content of Cr is 15.90%, more preferably is 15.80%, further preferably is 15.50%, further preferably is 15.00%, further preferably is 14.50%, and further preferably is 14.00%.

Ni: 4.00 to 7.50%

Nickel (Ni) forms sulfides on the passivation film in a corrosive environment. The Ni sulfides inhibit chloride ions (Cl^-) and hydrogen sulfide ions (HS^-) from coming into contact with the passivation film, and thus inhibit the chloride ions and hydrogen sulfide ions from breaking the passivation film. Therefore, Ni increases the SSC resistance of the steel material in a corrosive environment. Ni is also an austenite forming element, and martensitizes the microstructure of the steel material after quenching. If the content of Ni is less than 4.00%, 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 more than 7.50%, austenite will excessively stabilize, and sufficient martensite will not be obtained after quenching. Consequently, the yield strength will decrease even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of Ni is 4.00 to 7.50%.
A preferable lower limit of the content of Ni is 4.05%, more preferably is 4.10%, further preferably is 4.50%, further preferably is 5.00%, and further preferably is 5.50%.
A preferable upper limit of the content of Ni is 7.35%, more preferably is 7.10%, and further preferably is 6.90%.

Mo: 1.10 to 3.50%

Molybdenum (Mo) forms sulfides on the passivation film in a corrosive environment. The Mo sulfides inhibit chloride ions (Cl^-) and hydrogen sulfide ions (HS^-) from coming into contact with the passivation film, and thus inhibit the chloride ions and hydrogen sulfide ions from breaking the passivation film. Therefore, Mo increases the SSC resistance of the steel material in a corrosive environment. If the content of Mo is less than 1.10%, 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 Mo is more than 3.50%, δ-ferrite will excessively form. Consequently, the 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 1.10 to 3.50%.
A preferable lower limit of the content of Mo is 1.15%, more preferably is 1.30%, further preferably is 1.50%, further preferably is 1.70%, and further preferably is 2.00%.
A preferable upper limit of the content of Mo is 3.40%, more preferably is 3.30%, further preferably is 3.20%, further preferably is 3.00%, and further preferably is 2.80%.

Al: 0.005 to 0.050%

Aluminum (Al) deoxidizes the steel. If the content of Al is less than 0.005%, the aforementioned 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 Al is more than 0.050%, even if the contents of other elements are within the range of the present embodiment, coarse Al oxides will form and the low-temperature toughness of the steel material will decrease.
Therefore, the content of Al is 0.005 to 0.050%.
A preferable lower limit of the content of Al is 0.007%, more preferably is 0.010%, and further preferably is 0.015%.
A preferable upper limit of the content of Al is 0.047%, more preferably is 0.043%, and further preferably is 0.040%.
Note that, in the present description, the term "content of Al" means the content of sol. Al (acid-soluble Al).

V: 0.01 to 0.30%

Vanadium (V) forms V precipitates such as carbides, nitrides, and carbo-nitrides in the steel material, and thereby increases the strength of the steel material. If the content of V is less than 0.01%, 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 more than 0.30%, V precipitates will excessively form and the strength of the steel material will be too high. In such a case, the low-temperature toughness 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 V is 0.01 to 0.30%.
A preferable lower limit of the content of V is 0.02%, and more preferably is 0.03%.
A preferable upper limit of the content of V is 0.25%, more preferably is 0.20%, further preferably is 0.15%, further preferably is 0.10%, and further preferably is 0.08%.

N: 0.0030 to 0.0500%

Nitrogen (N) increases the pitting resistance of the steel material and, as a result, increases the SSC resistance of the steel material. If the content of N is less than 0.0030%, 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 N is more than 0.0500%, coarse TiN will form. In such a case, the low-temperature toughness 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 0.0030 to 0.0500%.
A preferable lower limit of the content of N is 0.0033%, more preferably is 0.0035%, and further preferably is 0.0038%.
A preferable upper limit of the content of N is 0.0450%, more preferably is 0.0420%, further preferably is 0.0400%, further preferably is 0.0350%, further preferably is 0.0300%, further preferably is 0.0250%, and further preferably is 0.0200%.

Ti: 0.020 to 0.150%

Titanium (Ti) combines with C or N to form carbides or nitrides. In this case, coarsening of grains is suppressed by the pinning effect, and the strength of the steel material increases. In addition, Ti forms carbides or nitrides. Thus, an excessive increase in strength caused by excessive formation of V precipitates (carbides, nitrides, and carbo-nitrides) is suppressed. As a result, the low-temperature toughness of the steel material increases. If the content of Ti is less than 0.020%, 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 Ti is more than 0.150%, an excessive amount of Ti carbides or Ti nitrides will form. In such a case, the low-temperature toughness 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 Ti is 0.020 to 0.150%.
A preferable lower limit of the content of Ti is 0.030%, more preferably is 0.040%, further preferably is 0.050%, further preferably is 0.060%, further preferably is 0.070%, and further preferably is 0.080%.
A preferable upper limit of the content of Ti is 0.140%, and more preferably is 0.130%.

Cu: 0.01 to 3.50%

Copper (Cu) forms sulfides on the passivation film in a corrosive environment. The Cu sulfides inhibit chloride ions and hydrogen sulfide ions from coming into contact with the passivation film, and thus inhibit the chloride ions and hydrogen sulfide ions from breaking the passivation film. Therefore, Cu increases the SSC resistance of the steel material in a corrosive environment. If the content of Cu is less than 0.01%, 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 more than 3.50%, hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. In addition, if the content of Cu is more than 3.50%, the strength of the steel material will be too high. In such a case, the low-temperature toughness of the steel material will decrease.
Therefore, the content of Cu is 0.01 to 3.50%.
A preferable lower limit of the content of Cu is 0.10%, more preferably is 0.50%, further preferably is 0.80%, further preferably is 1.00%, further preferably is 1.30%, and further preferably is 1.50%.
A preferable upper limit of the content of Cu is 3.30%, more preferably is 3.10%, and further preferably is 3.00%.

Co: 0.01 to 0.50%

Cobalt (Co) forms sulfides on the passivation film in a corrosive environment. The Co sulfides inhibit chloride ions (Cl^-) and hydrogen sulfide ions (HS^-) from coming into contact with the passivation film, and thus inhibit the chloride ions and hydrogen sulfide ions from breaking the passivation film. Therefore, Co increases the SSC resistance of the steel material in a corrosive environment. Co also suppresses formation of retained austenite, and thereby suppresses variations in the strength of the steel material. If the content of Co is less than 0.01%, 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 more than 0.50%, δ-ferrite will excessively form. Consequently, the 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 Co is 0.01 to 0.50%.
A preferable lower limit of the content of Co is 0.02%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.15%.
A preferable upper limit of the content of Co is 0.49%, more preferably is 0.45%, further preferably is 0.40%, and further preferably is 0.35%.
The balance of the chemical composition of the martensitic stainless steel material according to the present embodiment is Fe and impurities. Here, the term "impurities" refers to substances which, during industrial production of the martensitic stainless steel material, are mixed in from ore or scrap that is used as the raw material, or from the production environment or the like, and which are not intentionally contained but are allowed within a range that does not adversely affect the advantageous effects of the martensitic stainless steel material of the present embodiment.

[Regarding Optional Elements]

The chemical composition of the martensitic stainless steel material according to the present embodiment may further contain one or more kinds selected from a group consisting of a first group and a second group in lieu of a part of Fe.

[First group]

Nb: 0 to 0.150%,
W: 0 to 1.50%,
Zr: 0 to 0.0100%,
Sn: 0 to 0.0100%,
As: 0 to 0.0100%, and
Sb: 0 to 0.0100%.

[Second group]

B: 0 to 0.0050%,
Ca: 0 to 0.0050%,
Mg: 0 to 0.0050%, and
rare earth metal (REM): 0 to 0.0100%.

Hereunder, these optional elements are described.

[First group: Nb, W, Zr, Sn, As, and Sb]

The chemical composition of the martensitic stainless steel material according to the present embodiment may further contain one or more kinds of element selected from the first group that consists of Nb, W, Zr, Sn, As, and Sb in lieu of a part of Fe. Each of these elements is an optional element, and does not have to be contained. When contained, these elements increase the SSC resistance of the steel material. Hereunder, each element of the first group is described.

Nb: 0 to 0.150%

Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%.
When contained, that is, when the content of Nb is more than 0%, Nb forms fine precipitates (carbides, nitrides, and carbo-nitrides; hereunder, these precipitates are referred to as "Nb precipitates"). The Nb precipitates refine the substructure of the steel material by a pinning effect. As a result, the SSC resistance of the steel material increases. 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 more than 0.150%, Nb precipitates will excessively form. In such a case, the low-temperature toughness 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 Nb is 0 to 0.150%, and when contained, the content of Nb is 0.150% or less.
A preferable lower limit of the content of Nb is 0.001%, and more preferably is 0.002%.
A preferable upper limit of the content of Nb is 0.100%, more preferably is 0.050%, and further preferably is 0.030%.

W: 0 to 1.50%

Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%.
When contained, that is, when the content of W is more than 0%, W stabilizes the passivation film in a corrosive environment, and thus inhibits chloride ions and hydrogen sulfide ions from breaking the passivation film. Therefore, the SSC resistance of the steel material increases. 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 more than 1.50%, W will combine with C and form coarse carbides. In such a case, the low-temperature toughness 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 W is 0 to 1.50%, and when contained, the content of W is 1.50% or less.
A preferable lower limit of the content of W is 0.01%, more preferably is 0.03%, and further preferably is 0.05%.
A preferable upper limit of the content of W is 1.20%, and more preferably is 1.00%.

Zr: 0 to 0.0100%

Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%.
When contained, that is, when the content of Zr is more than 0%, Zr forms fine precipitates (carbides, nitrides, and carbo-nitrides; hereunder, these precipitates are referred to as "Zr precipitates"). The Zr precipitates refine the substructure of the steel material by a pinning effect. As a result, the SSC resistance of the steel material increases. 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 more than 0.0100%, Zr precipitates will excessively form. In such a case, the low-temperature toughness 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 Zr is 0 to 0.0100%, and when contained, the content of Zr is 0.0100% or less.
A preferable lower limit of the content of Zr is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.
A preferable upper limit of the content of Zr is 0.0070%, more preferably is 0.0050%, and further preferably is 0.0030%.

Sn: 0 to 0.0100%

Tin (Sn) is an optional element, and does not have to be contained. That is, the content of Sn may be 0%.
When contained, that is, when the content of Sn is more than 0%, Sn increases the SSC resistance of the steel material. If even a small amount of Sn is contained, the aforementioned advantageous effect will be obtained to a certain extent.
On the other hand, if the content of Sn is more than 0.0100%, Sn will segregate to grain boundaries. In such a case, the toughness 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 Sn is 0 to 0.0100%, and when contained, the content of Sn is 0.0100% or less.
A preferable lower limit of the content of Sn is 0.0001%, more preferably is 0.0003%, further preferably is 0.0005%, further preferably is 0.0007%, and further preferably is 0.0010%.
A preferable upper limit of the content of Sn is 0.0090%, more preferably is 0.0080%, further preferably is 0.0070%, further preferably is 0.0060%, and further preferably is 0.0050%.

As: 0 to 0.0100%

Arsenic (As) is an optional element, and does not have to be contained. That is, the content of As may be 0%.
When contained, As increases the SSC resistance of the steel material. If even a small amount of As is contained, the aforementioned advantageous effect will be obtained to a certain extent.
On the other hand, if the content of As is more than 0.0100%, As will segregate to grain boundaries. In such a case, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will, on the contrary, decrease.
Therefore, the content of As is 0 to 0.0100%, and when contained, the content of As is 0.0100% or less.
A preferable lower limit of the content of As is 0.0001%, more preferably is 0.0003%, further preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of As is 0.0090%, more preferably is 0.0080%, further preferably is 0.0060%, further preferably is 0.0040%, and further preferably is 0.0030%.

Sb: 0 to 0.0100%

Antimony (Sb) is an optional element, and does not have to be contained. That is, the content of Sb may be 0%.
When contained, that is, when the content of Sb is more than 0%, Sb increases the SSC 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.
On the other hand, if the content of Sb is more than 0.0100%, Sb will segregate to grain boundaries. In such a case, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will, on the contrary, decrease.
Therefore, the content of Sb is 0 to 0.0100%, and when contained, the content of Sb is 0.0100% or less.
A preferable lower limit of the content of Sb is 0.0001%, more preferably is 0.0003%, further preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of Sb is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0060%.

[Second group: B, Ca, Mg, and rare earth metal (REM)]

The chemical composition of the martensitic stainless steel material according to the present embodiment may further contain one or more kinds of element selected from the second group that consists of B, Ca, Mg, and rare earth metal (REM) in lieu of a part of Fe. Each of these elements is an optional element, and does not have to be contained. When contained, these elements increase hot workability of the steel material. Hereunder, each element of the second group is described.

B: 0 to 0.0050%

Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%.
When contained, that is, when the content of B is more than 0%, B segregates to austenite grain boundaries and strengthens the grain boundaries. As a result, hot workability of the steel material increases. 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 more than 0.0050%, Cr carbo-borides will form. In such a case, the low-temperature toughness 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 B is 0 to 0.0050%, and when contained, the content of B is 0.0050% or less.
A preferable lower limit of the content of B is 0.0001%, and more preferably is 0.0002%.
A preferable upper limit of the content of B is 0.0045%, and more preferably is 0.0040%.

Ca: 0 to 0.0050%

Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%.
When contained, that is, when the content of Ca is more than 0%, Ca controls the morphology of inclusions and increases hot workability of the steel material. The phrase "controls the morphology of inclusions" refers to, for example, spheroidizing inclusions and refining inclusions and the like. 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 more than 0.0050%, coarse oxides will form. In such a case, the low-temperature toughness 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 Ca is 0 to 0.0050%, and when contained, the content of Ca is 0.0050% or less.
A preferable lower limit of the content of Ca is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0015%.
A preferable upper limit of the content of Ca is 0.0045%, and more preferably is 0.0040%.

Mg: 0 to 0.0050%

Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%.
When contained, that is, when the content of Mg is more than 0%, similarly to Ca, Mg controls the morphology of inclusions and 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 more than 0.0050%, coarse oxides will form. In such a case, the low-temperature toughness 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 Mg is 0 to 0.0050%, and when contained, the content of Mg is 0.0050% or less.
A preferable lower limit of the content of Mg is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of Mg is 0.0045%, more preferably is 0.0035%, and further preferably is 0.0030%.

Rare earth metal (REM): 0 to 0.0100%

Rare earth metal (REM) is an optional element, and does not have to be contained. That is, the content of REM may be 0%.
When contained, that is, when the content of REM is more than 0%, similarly to Ca, REM controls the morphology of inclusions and 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 more than 0.0100%, coarse oxides will form. In such a case, the low-temperature toughness 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 REM is 0 to 0.0100%, and when contained, the content of REM is 0.0100% or less.
A preferable lower limit of the content of REM is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.
A preferable upper limit of the content of REM is 0.0080%, and more preferably is 0.0070%.
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.

[(Feature 2) Regarding yield strength]

The yield strength of the martensitic stainless steel material of the present embodiment is 110 ksi grade or more, that is, 758 MPa or more.
In the present description, the term "yield strength" means 0.2% offset yield stress (MPa) that is determined by a tensile test carried out at normal temperature (24 ± 3°C) in accordance with ASTM E8/E8M (2021).

[Method for measuring yield strength]

The yield strength is determined by the following method.
If the martensitic stainless steel material is a steel pipe, a round bar specimen is to be taken from the central portion of the wall thickness of the steel pipe. If the martensitic stainless steel material is a round steel bar, a round bar specimen is to be taken from an R/2 portion, that is, from the central portion of a radius R in a cross section perpendicular to the axial direction of the round steel bar. If the steel material is a steel plate, a round bar specimen is to be taken from the central portion of the thickness.
Regarding the size of the round bar specimen, for example, the diameter of the parallel portion is 4.0 mm and the gage length is 16.0 mm. The longitudinal direction of the round bar specimen is to be made parallel to the rolling direction of the martensitic stainless steel material. Specifically, if the martensitic stainless steel material is a steel pipe, the longitudinal direction of the round bar specimen is to be made parallel to the pipe axis direction of the steel pipe. If the martensitic stainless steel material is a round steel bar, the longitudinal direction of the round bar specimen is to be made parallel to the axial direction of the round steel bar. If the martensitic stainless steel material is a steel plate, the longitudinal direction of the round bar specimen is to be made parallel to the rolling direction of the steel plate.
A tensile test is carried out at normal temperature (24 ± 3°C) in accordance with ASTM E8/E8M (2021) using the round bar specimen, and the 0.2% offset yield stress (MPa) is determined. The determined 0.2% offset yield stress is defined as the yield strength (MPa).
A preferable lower limit of the yield strength of the martensitic stainless steel material of the present embodiment is 760 MPa, more preferably is 770 MPa, and further preferably is 780 MPa. Although an upper limit of the yield strength of the martensitic stainless steel material of the present embodiment is not particularly limited, when the contents of the elements are within the range of the chemical composition described above, the upper limit of the yield strength is, for example, less than 1069 MPa (155 ksi), and preferably is less than 1000 MPa (145 ksi).

[Microstructure]

In the microstructure of the martensitic stainless steel material according to the present embodiment, martensite is the main constituent. In the present description, the term "martensite" also includes tempered martensite and not just fresh martensite. Further, in the present description, the phrase "martensite is the main constituent" means that the volume ratio of martensite is 80% or more in the microstructure. The balance of the microstructure is retained austenite and δ-ferrite. That is, in the martensitic stainless steel material of the present embodiment, the total volume ratio of retained austenite and δ-ferrite is within the range of more than 0 to 20%. The volume ratio of retained austenite is preferably as low as possible. A preferable lower limit of the volume ratio of martensite in the microstructure of the martensitic stainless steel material of the present embodiment is 85%, and more preferably is 90%.
Note that, in a case where a martensitic stainless steel material having a chemical composition that satisfies Feature 1 also satisfies Feature 2, that is, has a yield strength of 758 MPa or more, the volume ratio of martensite in the microstructure will be 80% or more.
A small amount of retained austenite in the microstructure does not result in a significant decrease in strength, and markedly increases the low-temperature toughness of the steel material. However, if the volume ratio of retained austenite is too high, the strength of the steel material will markedly decrease. Accordingly, in the microstructure of the martensitic stainless steel material of the present embodiment, from the viewpoint of ensuring strength, a preferable upper limit of the volume ratio of retained austenite is 15%, and more preferably is 10%. A preferable lower limit of the volume ratio of retained austenite is 0%.

[Method for measuring volume ratio of martensite]

The volume ratio (%) of martensite in the microstructure of the martensitic stainless steel material of the present embodiment is determined by the following method. The volume ratio (%) of retained austenite is determined by a method described hereunder. Further, an area fraction Sd of δ-ferrite that is determined by a method described in the section [Method for measuring area fraction Sd of δ-ferrite] that is described later is regarded as the volume ratio (%) of δ-ferrite. The volume ratio of martensite is determined by subtracting the total of the determined volume ratio of retained austenite and volume ratio of δ-ferrite from 100%.
The volume ratio of retained austenite is determined by X-ray diffractometry. Specifically, a test specimen is taken from the martensitic stainless steel material.
If the martensitic stainless steel material is a steel pipe, the test specimen is to be taken from the central portion of the wall thickness of the steel pipe. If the martensitic stainless steel material is a round steel bar, the test specimen is to be taken from an R/2 portion of the round steel bar. If the steel material is a steel plate, the test specimen is to be taken from the central portion of the thickness of the steel plate. Although not particularly limited, the size of the test specimen is, for example, 15 mm × 15 mm × 2 mm in thickness. In this case, the thickness direction is the wall thickness direction in a case where the test specimen is a steel pipe, is the radial direction in a case where the test specimen is a round steel bar, and is the thickness direction in a case where the test specimen is a steel plate.
Using the obtained test specimen, the X-ray diffraction intensity of each of the (200) plane of α phase, the (211) plane of α phase, the (200) plane of γ phase, the (220) plane of γ phase, and the (311) plane of γ phase is measured to calculate an integrated intensity of each plane. In the measurement of the X-ray diffraction intensity, the target of the X-ray diffraction apparatus is Mo (Mo Kα ray), and the output thereof is 50 kV - 40 mA. After calculation, the volume ratio Vy (%) of retained austenite is calculated using Formula (I) for combinations (2 × 3 = 6 pairs) of each plane of the α phase and each plane of the γ phase. Then, an arithmetic average value of the volume ratios Vy of retained austenite of the six pairs is defined as the volume ratio (%) of retained austenite. $Vγ = 100 / \{1 + (Iα \times Rγ) / (Iγ \times Rα)\}$ where, Iα is the integrated intensity of α phase. Rα is the crystallographic theoretical calculation value of α phase. Iy is the integrated intensity of γ phase. Ry is the crystallographic theoretical calculation value of γ phase. Note that, in the present description, Rα in the (200) plane of α phase is 15.9, Rα in the (211) plane of α phase is 29.2, Ry in the (200) plane of γ phase is 35.5, Ry in the (220) plane of γ phase is 20.8, and Ry in the (311) plane of γ phase is 21.8. Note that, a value obtained by rounding off decimals of the obtained numerical value is adopted as the volume ratio of retained austenite.
In addition, the area fraction Sd (%) of δ-ferrite is determined by a method described in the section [Method for measuring area fraction Sd of δ-ferrite] that is described later. The determined area fraction Sd (%) of δ-ferrite is regarded as the volume ratio (%) of δ-ferrite. Note that, a value obtained by rounding off to the second decimal place of the obtained numerical value is adopted as the volume ratio of δ-ferrite.
The volume ratio (%) of martensite in the microstructure of the martensitic stainless steel material is determined by the following equation using the determined volume ratio of retained austenite (%) and the determined volume ratio (%) of δ-ferrite. Volume ratio (%) of martensite = 100 - (volume ratio (%) of retained austenite + volume ratio (%) of δ-ferrite)

[(Feature 3) Regarding Formula (1) and Formula (2)]

In the martensitic stainless steel material of the present embodiment, in addition, in a cross section parallel to the rolling direction, an area fraction Sd (%) of δ-ferrite and an area fraction Sc (%) of intermetallic compounds which have an equivalent circular diameter of 1.0 µm or more satisfy Formula (1) and Formula (2). $0 < Sd \leq 10.00$ $Sc / Sd \leq 5.00$

[Regarding Formula (1)]

In the martensitic stainless steel material of the present embodiment, formation of a certain amount of δ-ferrite is allowed. Therefore, the δ-ferrite area fraction Sd is more than 0%. However, if the area fraction Sd of δ-ferrite is more than 10.00%, hot workability of the steel material during the process of producing the martensitic stainless steel material will decrease. If the δ-ferrite area fraction Sd is 10.00% or less, in other words, if the δ-ferrite area fraction Sd satisfies Formula (1), sufficient hot workability will be obtained during the process of producing the martensitic stainless steel material.
A preferable lower limit of the δ-ferrite area fraction Sd is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.15%, further preferably is 0.20%, and further preferably is 0.25%.
A preferable upper limit of the δ-ferrite area fraction Sd is 9.00%, more preferably is 8.80%, further preferably is 8.00%, further preferably is 7.50%, and further preferably is 7.00%.

[Regarding Formula (2)]

In the martensitic stainless steel material of the present embodiment, the intermetallic compounds are Laves phase and/or chi phase (χ phase). The Laves phase is Fe₂Mo, and the χ phase is Fe₃₆Cr₁₂Mo₁₀. Intermetallic compounds having an equivalent circular diameter of 1.0 µm or more are referred to as "coarse intermetallic compounds". Here, the term "equivalent circular diameter" means the diameter of a circle having an area equivalent to the area of the intermetallic compound.
When δ-ferrite is present in the steel material, coarse intermetallic compounds easily form in the δ-ferrite or at the interface between the δ-ferrite and the parent phase accompanying the formation of the δ-ferrite. The area fraction of the coarse intermetallic compounds is defined as "Sc". A ratio Fn of the coarse intermetallic compounds area fraction Sc to the δ-ferrite area fraction Sd is defined by the following equation. $Fn = Sc / Sd$
Referring to FIG. 1, as described above, the lower Fn (= Sc/Sd) is, that is, the smaller the coarse intermetallic compounds area fraction Sc is relative to the δ-ferrite area fraction Sd, the higher the absorbed energy at -10°C becomes. Further, when Fn is 5.00 or less, the slope of the graph becomes gradual. That is, there is an inflection point in the vicinity of the location where Fn is 5.00.
Therefore, if Fn is 5.00 or less, excellent low-temperature toughness will be obtained. Specifically, the absorbed energy at -10°C obtained by a method described later in the section [Low-temperature toughness evaluation method] will be 120 J or more.
A preferable upper limit of Fn is 4.90, more preferably is 4.80, further preferably is 4.50, further preferably is 4.00, further preferably is 3.80, further preferably is 3.40, further preferably is 3.00, further preferably is 2.50, and further preferably is 2.00.
Referring to FIG. 1, when Fn is 2.00 or less, the slope of the graph becomes steep once again. That is, in FIG. 1, there is also an inflection point in the vicinity of the location where Fn is 2.00. If Fn is 2.00 or less, the absorbed energy at -10°C will be 160 J or more, and even more excellent low-temperature toughness will be obtained.
A lower limit of Fn is not particularly limited. As described above, the δ-ferrite area fraction Sd is more than 0%, and the coarse intermetallic compounds area fraction Sc is also more than 0%. Therefore, the lower limit of Fn is, for example, 0.01, more preferably is 0.02, and further preferably is 0.05.

[Method for measuring area fraction Sd of δ-ferrite]

The area fraction Sd of δ-ferrite can be measured by the following method.
A test specimen is taken from the martensitic stainless steel material according to the present embodiment. If the martensitic stainless steel material is a steel pipe, the test specimen is to be taken from the central portion of the wall thickness. If the martensitic stainless steel material is a round steel bar, the test specimen is to be taken from an R/2 portion. If the martensitic stainless steel material is a steel plate, the test specimen is to be taken from the central portion of the thickness.
The test specimen has an observation surface that is parallel to the rolling direction of the martensitic stainless steel material. If the martensitic stainless steel material is a steel pipe, the observation surface of the test specimen includes the pipe axis direction and the wall thickness direction. If the martensitic stainless steel material is a round steel bar, the observation surface of the test specimen includes the axial direction and the radial direction. If the martensitic stainless steel material is a steel plate, the observation surface of the test specimen includes the rolling direction and the thickness direction.
The observation surface is mirror polished. The mirror-polished observation surface is electrolytically etched using a 30% by mass NaOH aqueous solution to reveal the microstructure on the observation surface. On the observation surface on which the microstructure has been revealed, the area fraction of δ-ferrite is determined by a point counting method in accordance with ASTM E562 (2019). At such time, the measurement magnification is set to ×400, the number of lattice points is set to 400, and the number of measurement visual fields is set to 30. Each measurement visual field is set as a rectangle with dimensions of 250 µm × 250 µm. The rectangle is divided into equal sections, and the number of lattice points in the rectangle is set to 400. If δ-ferrite overlaps with a lattice point, it is counted as "1". If an interface between the parent phase and δ-ferrite overlaps with a lattice point, it is counted as "0.5". A value (%) obtained by dividing the count with respect to the lattice points (400 points) in all of the measurement visual fields (30 visual fields) by the total number of lattice points is defined as the area fraction Sd (%) of δ-ferrite.
Note that, a determination as to whether or not a particle is δ-ferrite is carried out by performing element concentration analysis (EDS analysis). Specifically, particles in each measurement visual field are identified based on contrast. Each particle that is identified is subjected to element concentration analysis (EDS analysis). In the EDS analysis, the acceleration voltage is set to 20 kV, and the EDS analysis is conducted for quantification of N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, and Nb as elements to be analyzed. Based on the EDS analysis result for each particle, if the content of Cr in the particle is 14.00% by mass or more and the content of Ni in the particle is a multiple of 0.8 times or less the content of Ni in the parent phase (that is, the content of Ni in the martensitic stainless steel material), it is determined that the relevant particle is δ-ferrite.

[Method for measuring coarse intermetallic compounds area fraction Sc]

The coarse intermetallic compounds area fraction Sc can be measured by the following method.
A test specimen is taken from the martensitic stainless steel material according to the present embodiment. If the martensitic stainless steel material is a steel pipe, the test specimen is to be taken from the central portion of the wall thickness. If the martensitic stainless steel material is a round steel bar, the test specimen is to be taken from an R/2 portion. If the martensitic stainless steel material is a steel plate, the test specimen is to be taken from the central portion of the thickness.
The test specimen has an observation surface that is parallel to the rolling direction of the martensitic stainless steel material. If the martensitic stainless steel material is a steel pipe, the observation surface of the test specimen includes the pipe axis direction and the wall thickness direction. If the martensitic stainless steel material is a round steel bar, the observation surface of the test specimen includes the axial direction and the radial direction. If the martensitic stainless steel material is a steel plate, the observation surface of the test specimen includes the rolling direction and the thickness direction.
The observation surface of the test specimen is mirror polished. On the observation surface after mirror polishing, visual fields that include δ-ferrite are arbitrarily selected at 20 locations. These visual fields are subjected to element mapping analysis (area analysis) using an electron probe micro analyzer (EPMA). The measurement magnification is set to 500×, and the visual field area is set to 100 µm × 100 µm. In the element mapping analysis using the EPMA, the acceleration voltage is set to 15 kV. Further, the elements to be measured are Fe, Cr, Mo, Ni, and C. Element mapping in each visual field is obtained by the above measurement.
As described above, the intermetallic compounds are Laves phase (Fe₂Mo) and/or χ phase (Fe₃₆Cf₁₂Mo₁₀). In these intermetallic compounds, the Mo concentration is higher than in the parent phase. Therefore, the intermetallic compounds are identified based on the Mo concentration of each pixel.
Specifically, the element mapping region of each visual field is divided equally into 512 × 512 pixels. Element amounts included in each pixel are represented by a count of characteristic X-rays. An arithmetic average value CMo_ave of the Mo counts for the 512 × 512 pixels is determined. Among the 512 × 512 pixels, pixels for which the Mo count is 10 times or more the CMo_ave value are excluded as abnormal value pixels. Among the Mo counts of the pixels that remain excluding the abnormal value pixels from the 512 × 512 pixels, a maximum value CMo_max is identified. Among the multiple pixels excluding the abnormal value pixels, pixels that exhibit an Mo count that is a multiple of 0.600 times or more the maximum value CMo_max are recognized as intermetallic compound region pixels.
Intermetallic compounds are identified by the following method based on the obtained intermetallic compound region pixels. FIG. 2A to FIG. 2E are schematic diagrams for describing a method for identifying intermetallic compounds in each visual field divided into 512 × 512 pixels. Referring to FIG. 2A, a case is assumed in which a plurality of intermetallic compound region pixels PX1 to PX4 are present in a visual field VA. In this case, among the plurality of intermetallic compound region pixels PX1 to PX4, attention will be focused on an arbitrary intermetallic compound region pixel PX1. As illustrated in FIG. 2B, a determination region A1 of 5 pixels × 5 pixels that centers on the intermetallic compound region pixel PX1 is drawn. When another intermetallic compound region pixel is present within the determination region A1 of the intermetallic compound region pixel PX1, the other intermetallic compound region pixel is recognized as a region that is within the same intermetallic compound as the intermetallic compound region pixel PX1. In FIG. 2B, the intermetallic compound region pixel PX2 is included in the determination region A1. Therefore, the intermetallic compound region pixel PX2 is recognized as a region that is within the same intermetallic compound as the intermetallic compound region pixel PX1.
Next, as illustrated in FIG. 2C, a determination region A2 of 5 pixels × 5 pixels that centers on the intermetallic compound region pixel PX2 is drawn. In FIG. 2C, the intermetallic compound region pixel PX3 is included in the determination region A2. Therefore, the intermetallic compound region pixel PX3 is recognized as a region that is within the same intermetallic compound as the intermetallic compound region pixel PX2. Accordingly, in this case, the intermetallic compound region pixels PX1 to PX3 are recognized as regions within the same intermetallic compound.
Similarly, as illustrated in FIG. 2D, a determination region A3 of 5 pixels × 5 pixels that centers on the intermetallic compound region pixel PX3 is drawn. In FIG. 2D, apart from the intermetallic compound region pixel PX3 and also the intermetallic compound region pixel PX2 which was already recognized as being within the same intermetallic compound, there is no new intermetallic compound region pixel included in the determination region A3. In a case where there is no other new intermetallic compound region pixel in the determination region, the intermetallic compound region pixels included in the same intermetallic compound are confirmed. Specifically, in the case illustrated in FIG. 2D, it is confirmed that the intermetallic compound region pixels PX1 to PX3 are included in the same intermetallic compound, and that the intermetallic compound region pixel PX4 is not included in the intermetallic compound that includes the intermetallic compound region pixels PX1 to PX3.
When the intermetallic compound region pixels included in the same intermetallic compound have been confirmed, as illustrated in FIG. 2E, a circumscribing rectangle IM that includes the intermetallic compound region pixels PX1 to PX3 included in the same intermetallic compound is identified as one intermetallic compound. Note that, in the visual field divided into 512 × 512 pixels, each side of the circumscribing rectangle IM is to be parallel to a row direction X or a column direction Y of the pixels.
The area of the intermetallic compound identified by the above method is the area of the circumscribing rectangle IM. The equivalent circular diameter of the intermetallic compound is calculated based on the obtained area of the intermetallic compound. Note that, the term "equivalent circular diameter" refers to the diameter of a circle having an area that is equivalent to the area of the intermetallic compound. Note that, each visual field is 100 µm × 100 µm, and each visual field is divided equally into 512 × 512 pixels. Therefore, the size of each pixel is 0.195 µm × 0.195 µm.
Among the identified intermetallic compounds, intermetallic compounds having an equivalent circular diameter of 1.0 µm or more (coarse intermetallic compounds) are identified. In each visual field, the area fraction of the identified coarse intermetallic compounds is determined by the following formula. Area fraction of coarse intermetallic compounds = (total of areas of identified coarse intermetallic compounds/area of visual field) × 100
The arithmetic average value of the area fractions of coarse intermetallic compounds obtained in the respective visual fields at the 20 locations is defined as the coarse intermetallic compounds area fraction Sc (%).

[Advantageous effects of martensitic stainless steel material of present embodiment]

The martensitic stainless steel material of the present embodiment satisfies Feature 1 to Feature 3. Therefore, even though the martensitic stainless steel material of the present embodiment has a high yield strength of 758 MPa or more, excellent SSC resistance is obtained, and in addition, excellent low-temperature toughness is obtained.

[Regarding SSC resistance of steel material]

In the martensitic stainless steel material according to the present embodiment, even though the martensitic stainless steel material has a high yield strength of 758 MPa or more, excellent SSC resistance is obtained. In the present embodiment, the phrase "has excellent SSC resistance" means that cracking is not confirmed in [SSC resistance evaluation method 1] described hereunder.

[SSC resistance evaluation method 1]

The SSC resistance of the martensitic stainless steel material of the present embodiment can be evaluated by a SSC resistance evaluation test conducted at normal temperature. The SSC resistance evaluation test is carried out by a method in accordance with NACE TM0177-2016 Method A.
Specifically, a round bar specimen is taken from the martensitic stainless steel material according to the present embodiment. If the martensitic stainless steel material is a steel pipe, the round bar specimen is taken from the central portion of the wall thickness of the steel pipe. If the martensitic stainless steel material is a round steel bar, the round bar specimen is taken from an R/2 portion of the round steel bar. If the martensitic stainless steel material is a steel plate, the round bar specimen is taken from the central portion of the thickness of the steel plate. The size of the round bar specimen is not particularly limited. For example, the round bar specimen has a size in which the diameter of the parallel portion is 6.35 mm and the length of the parallel portion is 25.4 mm. Note that, the longitudinal direction of the round bar specimen is to be made parallel to the rolling direction of the steel material. Specifically, if the martensitic stainless steel material is a steel pipe, the longitudinal direction of the round bar specimen is to be parallel to the axial direction of the steel pipe. If the martensitic stainless steel material is a round steel bar, the longitudinal direction of the round bar specimen is to be parallel to the axial direction of the round steel bar. If the martensitic stainless steel material is a steel plate, the longitudinal direction of the round bar specimen is to be parallel to the rolling direction of the steel plate.
A 20% by mass sodium chloride aqueous solution having a pH of 4.0 is employed as the test solution. The test solution is prepared by adding acetic acid to an aqueous solution containing 20% by mass sodium chloride and 0.41 g/L of sodium acetate to adjust the pH to 4.0. A stress equivalent to 90% of the actual yield stress is applied to the prepared round bar specimen. The test solution at 24°C is poured into a test vessel so that the round bar specimen to which the stress has been applied is immersed therein to form a test bath. After degassing the test bath, H₂S gas at 0.07 bar and CO₂ gas at 0.93 bar are blown into the test bath to saturate the test bath with H₂S gas. The test bath in which the H₂S gas is saturated is held at 24°C for 720 hours.
After the test specimen has been held for 720 hours, the surface of the parallel portion of the test specimen is observed with a magnifying glass having a magnification of ×10 to confirm the presence or absence of cracking. In the martensitic stainless steel material according to the present embodiment, the presence of cracking is not confirmed after 720 hours elapse in an SSC resistance evaluation test conducted by the method described above.
Preferably, the martensitic stainless steel material of the present embodiment satisfies Feature 1 to Feature 3, and in addition, the content of Cu is 1.00 to 3.50%. In such a case, even more excellent SSC resistance will be obtained. Specifically, cracking is not confirmed in [SSC resistance evaluation method 2] described hereunder.

[SSC resistance evaluation method 2]

In SSC resistance evaluation method 2, the only difference compared to SSC resistance evaluation method 1 is the test bath. Specifically, similarly to SSC resistance evaluation method 1, a 20% by mass sodium chloride aqueous solution having a pH of 4.0 is employed as the test solution. A stress equivalent to 90% of the actual yield stress is applied to the prepared round bar specimen. The test solution at 24°C is poured into a test vessel so that the round bar specimen to which the stress has been applied is immersed therein to form a test bath. After degassing the test bath, H₂S gas at 0.10 bar and CO₂ gas at 0.90 bar are blown into the test bath to saturate the test bath with H₂S gas. The test bath in which the H₂S gas is saturated is held at 24°C for 720 hours. That is, in SSC resistance evaluation method 2, the SSC resistance is evaluated under stricter conditions than in SSC resistance evaluation method 1.
After the test specimen has been held for 720 hours, the surface of the parallel portion of the test specimen is observed with a magnifying glass having a magnification of 10× to confirm the presence or absence of cracking. In the martensitic stainless steel material according to the present embodiment, the presence of cracking is also not confirmed after 720 hours elapse in an SSC resistance evaluation test conducted according to the method described above.

[Regarding low-temperature toughness]

As described above, the martensitic stainless steel material according to the present embodiment has excellent low-temperature toughness. In the present embodiment, whether the martensitic stainless steel material "has excellent low-temperature toughness" can be evaluated by the following method.

[Low-temperature toughness evaluation method]

The low-temperature toughness of the martensitic stainless steel material according to the present embodiment is evaluated by a Charpy impact test carried out in accordance with ASTM E23 (2018).
Specifically, a full-size or sub-size V-notch test specimen is prepared from the martensitic stainless steel material in accordance with API 5CRA (2010). A Charpy impact test in accordance with ASTM E23 (2018) is carried out on the prepared V-notch test specimen to determine the absorbed energy (J) at -10°C. Note that, in a case where a sub-size V-notch test specimen is used, the absorbed energy that is obtained is divided by a reduction factor described in API 5CRA (2010) to convert the obtained absorbed energy to the absorbed energy for a full-size V-notch test specimen. Further, the decimals of the obtained numerical value is rounded off and the resultant value is defined as the absorbed energy (J) at -10°C. In the present embodiment, if the absorbed energy at -10°C is 120 J or more, the martensitic stainless steel material is evaluated as having excellent low-temperature toughness.

[Shape and uses of martensitic stainless steel material]

The martensitic stainless steel material according to the present embodiment is a steel pipe, a round steel bar (solid material), or a steel plate. In a case where the martensitic stainless steel material is a steel pipe, preferably the martensitic stainless steel material is an oil-well steel pipe. An oil-well steel pipe is, for example, a casing pipe, a tubing pipe, a drilling pipe or the like that is used for drilling an oil well or a gas well, extracting crude oil or natural gas, and the like. In a case where the martensitic stainless steel material is a steel pipe, preferably the martensitic stainless steel material is a seamless steel pipe.
In a case where the martensitic stainless steel material is a round steel bar, the martensitic stainless steel material is, for example, a steel material for use in a downhole member.
As described above, the martensitic stainless steel material of the present embodiment satisfies Feature 1 to Feature 3. Therefore, the martensitic stainless steel material of the present embodiment has a high strength that is a yield strength of 110 ksi grade or more (758 MPa or more) and has excellent SSC resistance, and also has excellent low-temperature toughness.

[Production method]

One example of a method for producing the martensitic stainless steel material according to the present embodiment will now be described. Note that, as long as a martensitic stainless steel material that satisfies Feature 1 to Feature 3 described above can be produced, the method for producing the martensitic stainless steel material is not limited to the production method described hereunder, and the martensitic stainless steel material of the present embodiment may be produced by another production method. However, the production method described hereunder is a favorable example of the method for producing the martensitic stainless steel material according to the present embodiment.
One example of the method for producing the martensitic stainless steel material according to the present embodiment includes the following processes.

(Process 1) Starting material preparation process
(Process 2) Hot working process
(Process 3) Quenching process
(Process 4) Tempering process

Each process is described hereunder.

[(Process 1) Starting material preparation process]

In the starting material preparation process, a cast material having a chemical composition that satisfies Feature 1 is prepared. Specifically, a molten steel having the chemical composition described above is produced by a well-known method. The produced molten steel is used to produce a cast piece by a continuous casting process. Here, the cast piece is a slab or a bloom. Instead of a cast piece, an ingot may be produced by an ingot-making process using the aforementioned molten steel. The bloom or ingot may be subjected to blooming to form a billet. A starting material (slab, bloom, billet, or ingot) is produced by the above production process.

[(Process 2) Hot working process]

In the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. The method of hot working for producing the intermediate steel material is not particularly limited. That is, in the present embodiment the hot working may be hot forging, may be hot extrusion, or may be hot rolling.
If the martensitic stainless steel material is a steel pipe (seamless steel pipe), the starting material is subjected to hot working to produce a hollow shell (seamless steel shell). In this case, as hot working, for example, the Ugine-Sejournet process or the Ehrhardt push bench process (that is, hot extrusion) may be performed. In a case where the intermediate steel material is a seamless steel pipe, furthermore, as hot working, for example, piercing-rolling (that is, hot rolling) according to the Mannesmann process may be performed.
For example, in the case of performing piercing-rolling according to the Mannesmann process in the hot working, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1250°C. After being taken out from the heating furnace, the starting material is subjected to piercing-rolling to produce an intermediate steel material (hollow shell). When performing the piercing-rolling, although not particularly limited, the piercing ratio is, for example, 1.0 to 4.0. The billet after piercing-rolling is subjected to elongating using a mandrel mill. In addition, as necessary, the billet after elongating is subjected to diameter adjusting rolling using a stretch reducing mill or a sizing mill. The hollow shell is produced by the above processes. Although not particularly limited, the cumulative reduction of area in the hot working process is, for example, 20 to 70%.
If the martensitic stainless steel material is a round steel bar, the starting material is subjected to hot working to produce an intermediate steel material (round steel bar). In this case, hot rolling is performed as hot working. Although not particularly limited, the heating temperature before the hot rolling is, for example, 1100 to 1250°C. When performing the hot rolling, preferably the hot rolling is performed using a continuous mill. In a continuous mill, a horizontal stand having a pair of grooved rolls arranged one on the other in the vertical direction, and a vertical stand having a pair of grooved rolls arranged side by side in the horizontal direction are alternately arranged.
If the martensitic stainless steel material is a steel plate, the starting material is subjected to hot working to produce an intermediate steel material (plate-shaped steel material). In this case, hot rolling is performed as hot working. Although not particularly limited, the heating temperature before the hot rolling is, for example, 1100 to 1250°C. After being taken out from the heating furnace, the starting material is subjected to hot rolling using a continuous mill to produce an intermediate steel material (plate-shaped steel material).
As described above, an intermediate steel material having a desired shape is produced by the hot working 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. Furthermore, for example, after subjecting the starting material to the aforementioned blooming, hot rolling using the aforementioned continuous mill may be performed.
The intermediate steel material produced by the hot working may be aircooled. The intermediate steel material produced by the hot working may also be subjected to direct quenching after the hot working without being cooled to normal temperature, or may be subjected to quenching after undergoing supplementary heating (reheating) after the hot working. In a case where direct quenching is performed after hot working, or quenching is performed after performing supplementary heating after hot working, in order to eliminate residual stress, stress relief annealing (SR treatment) may be performed before the heat treatment process (quenching and tempering) that is the next process.

[(Process 3) Quenching process]

In the quenching process, the intermediate steel material produced by the hot working process is subjected to quenching. In the quenching process, quenching is performed in a manner that satisfies the following conditions:

Condition 1: Quenching temperature T1 is to be set within the range of 900 to 1090°C;
Condition 2: FA defined by Formula (A) is to be 1.71 or less; $FA = \{(t 1 / t 2) - 1\} / Cu$

_c3

FA is an index relating to the amount of intermetallic compounds formed in the intermediate steel material after quenching. The temperature range from the A_c3 point to 900°C is a temperature range in which formation of intermetallic compounds is promoted. Therefore, during heating in the quenching process, a shorter time t1 can suppress the formation of coarse intermetallic compounds. On the other hand, in the quenching temperature range of 900°C or more, coarse intermetallic compounds dissolve. Therefore, even if the time t1 in the temperature range from the A_c3 point to 900°C is long, so long as the holding time t2 at the quenching temperature T1 is longer than the time t1 to the extent that condition 2 is satisfied, coarse intermetallic compounds that are formed during heating will dissolve.
In this connection, as described above, in a martensitic stainless steel material having a chemical composition that satisfies Feature 1, when δ-ferrite is formed, coarse intermetallic compounds are likely to form in the δ-ferrite or at the interface between the δ-ferrite and the parent phase. In other words, the greater the amount of δ-ferrite formed is, the greater the amount of coarse intermetallic compounds also tends to be formed. In a chemical composition that satisfies Feature 1, Cu is an austenite forming element. That is, the lower the content of Cu is, the more likely it is for δ-ferrite to form. Therefore, the lower the content of Cu is, the more likely it is for coarse intermetallic compounds to form. Thus, the content of Cu in the intermediate steel material also affects the amount of coarse intermetallic compounds that are formed.
FIG. 3 is a graph illustrating the relation among the time t1 and time t2, the content of Cu, and the area fraction Sc of coarse intermetallic compounds in a steel material having a chemical composition that satisfies Feature 1. The ordinate in FIG. 3 represents t1/t2. The abscissa represents the content of Cu (%). A straight line FA in FIG. 3 represents the aforementioned Formula (A). The "○ " marks in FIG. 3 represent test results in which Fn (= Sc/Sd) was 5.00 or less. The "•" marks represent test results in which Fn was more than 5.00. FIG. 3 was prepared based on test results obtained in examples that are described later.
Referring to FIG. 3, in the region above the straight line FA, FA is more than 1.71. In this case, the amount of coarse intermetallic compounds that are formed is large relative to the amount of δ-ferrite that is formed, and therefore Fn is more than 5.00. On the other hand, in the region below the straight line FA, FA is 1.71 or less. In this case, the amount of coarse intermetallic compounds that are formed is suppressed to within an appropriate range relative to the amount of δ-ferrite that is formed, and therefore Fn is 5.00 or less.
As described above, in the quenching process, the quenching temperature T1 is set within the range of 900 to 1090°C, and furthermore, FA is made 1.71 or less. By this means, the amount of coarse intermetallic compounds that are formed can be sufficiently suppressed relative to the amount of δ-ferrite that is formed.
After being held at the quenching temperature T1 for the holding time t2, the intermediate steel material is quenched. The quenching method is, for example, water cooling or oil cooling. The quenching method is not particularly limited. For example, the intermediate steel material may be rapidly cooled by immersing the intermediate steel material in a water bath or an oil bath. In a case where the intermediate steel material is a steel pipe, the intermediate steel material may be rapidly cooled by pouring or jetting cooling water onto the outer surface and/or the inner surface of the steel pipe by shower cooling or mist cooling.
Note that, in a case where the intermediate steel material is a hollow shell (seamless steel pipe), quenching (direct quenching) may be performed immediately after the hot working, without cooling the hollow shell to normal temperature after the hot working process. Further, quenching may be performed after the hollow shell has been held at the quenching temperature after being charged into a holding furnace before the temperature of the hollow shell decreased after the hot working.

[(Process 4) Tempering process]

In the tempering process, the intermediate steel material after quenching is subjected to tempering. In the tempering process, the yield strength of the martensitic stainless steel material can be adjusted by appropriately adjusting the tempering temperature according to the chemical composition. Specifically, the tempering conditions are adjusted so that the yield strength of the martensitic stainless steel material becomes 110 ksi grade or more (758 MPa or more).
In the tempering process, a tempering temperature and a holding time at the tempering temperature can be appropriately adjusted according to the set yield strength and chemical composition. The tempering temperature is not particularly limited. The tempering temperature is, for example, 500 to 650°C.
The martensitic stainless steel material of the present embodiment can be produced by the process described above. Note that, the production method described above is a description of one example of the method for producing the martensitic stainless steel material according to the present embodiment. The martensitic stainless steel material according to the present embodiment may be produced by a production method other than the production method described above.

EXAMPLES

The advantageous effects of the martensitic stainless steel material of the present embodiment will now be described more specifically by way of examples. The conditions adopted in the following examples are one example of conditions adopted for confirming the feasibility and advantageous effects of the martensitic stainless steel material of the present embodiment. Accordingly, the martensitic stainless steel material of the present embodiment is not limited to this one example of conditions.
Martensitic stainless steel materials (seamless steel pipes) having the chemical compositions shown in Table 1-1 and Table 1-2 were produced.

[Table 1-1]

TABLE 1-1

Test number	Chemical composition (unit is mass%; balance is Fe and impurities)
Test number	C	Si	Mn	P	S	Cr	Ni	Mo	Al	v	N	Ti	Cu	Co
1	0.009	0.25	0.35	0.013	0.0010	13.60	7.22	2.91	0.044	0.05	0.0485	0.100	3.25	0.11
2	0.029	0.19	0.48	0.015	0.0006	11.20	5.66	2.01	0.006	0.01	0.0099	0.022	1.89	0.23
3	0.008	0.92	0.22	0.026	0.0012	12.67	6.24	1.68	0.033	0.04	0.0111	0.123	1.08	0.27
4	0.008	0.25	0.22	0.014	0.0009	11.24	5.33	3.00	0.035	0.01	0.0080	0.138	0.01	0.25
5	0.009	0.24	0.50	0.013	0.0005	10.37	5.35	2.14	0.031	0.02	0.0120	0.135	0.65	0.38
6	0.010	0.24	0.44	0.012	0.0007	12.65	6.67	2.60	0.030	0.03	0.0075	0.125	1.95	0.34
7	0.011	0.23	0.35	0.016	0.0008	13.56	7.10	3.29	0.045	0.04	0.0110	0.110	2.75	0.12
8	0.012	0.22	0.41	0.010	0.0006	11.25	6.80	2.94	0.022	0.05	0.0135	0.105	3.22	0.15
9	0.008	0.26	0.35	0.012	0.0005	13.01	6.85	2.78	0.046	0.03	0.0171	0.075	0.87	0.13
10	0.010	0.25	0.89	0.018	0.0009	13.15	6.60	2.41	0.026	0.03	0.0080	0.132	2.35	0.35
11	0.010	0.20	0.33	0.011	0.0005	15.78	7.33	1.33	0.038	0.02	0.0154	0.136	3.33	0.16
12	0.008	0.22	0.38	0.015	0.0006	12.40	4.13	1.41	0.022	0.01	0.0166	0.135	2.85	0.28
13	0.007	0.30	0.44	0.009	0.0007	10.22	5.47	1.56	0.030	0.02	0.0133	0.105	1.42	0.13
14	0.011	0.24	0.40	0.016	0.0006	13.40	7.46	3.23	0.033	0.04	0.0075	0.126	1.34	0.19
15	0.010	0.18	0.35	0.012	0.0008	12.02	5.09	1.85	0.034	0.02	0.0068	0.149	1.67	0.25
16	0.009	0.22	0.41	0.013	0.0008	11.13	7.18	3.45	0.026	0.03	0.0080	0.129	2.23	0.36
17	0.015	0.31	0.34	0.012	0.0005	13.88	6.10	1.13	0.039	0.05	0.0137	0.140	3.24	0.13
18	0.010	0.23	0.31	0.011	0.0008	11.55	6.80	2.57	0.040	0.05	0.0092	0.140	3.44	0.15
19	0.009	0.33	0.41	0.009	0.0006	12.21	6.45	2.57	0.036	0.04	0.0128	0.121	1.08	0.10
20	0.010	0.20	0.39	0.015	0.0008	12.51	6.53	2.53	0.031	0.04	0.0098	0.127	1.96	0.47
21	0.008	0.22	0.42	0.017	0.0008	12.99	6.65	2.61	0.035	0.03	0.0084	0.126	2.08	0.02
22	0.008	0.26	0.45	0.011	0.0007	13.05	6.67	2.55	0.031	0.04	0.0128	0.134	2.07	0.14
23	0.010	0.23	0.38	0.010	0.0010	11.55	6.99	2.98	0.026	0.03	0.0101	0.099	1.64	0.21
24	0.007	0.21	0.43	0.018	0.0009	12.56	6.58	2.74	0.040	0.04	0.0076	0.122	1.97	0.20
25	0.010	0.25	0.36	0.015	0.0008	12.96	6.70	2.52	0.035	0.05	0.0094	0.133	1.95	0.32
26	0.009	0.23	0.40	0.017	0.0009	12.67	6.51	2.59	0.038	0.05	0.0068	0.141	2.03	0.11
27	0.010	0.22	0.37	0.012	0.0010	10.55	7.40	3.20	0.025	0.03	0.0188	0.121	2.71	0.17
28	0.010	0.20	0.35	0.010	0.0008	13.85	6.51	2.54	0.041	0.03	0.0155	0.140	0.28	0.11
29	0.009	0.30	0.45	0.015	0.0008	12.51	6.58	2.54	0.031	0.03	0.0085	0.129	2.05	0.20
30	0.010	0.25	0.42	0.014	0.0008	13.74	6.25	1.34	0.028	0.04	0.0235	0.133	3.31	0.18
31	0.010	0.26	0.45	0.014	0.0008	11.68	6.29	1.84	0.029	0.05	0.0201	0.134	1.64	0.25
32	0.007	0.25	0.45	0.016	0.0007	10.38	5.24	1.37	0.030	0.02	0.0085	0.135	1.24	0.25
33	0.031	0.54	0.68	0.025	0.0015	14.20	5.41	1.88	0.043	0.06	0.0254	0.149	1.26	0.33
34	0.012	0.28	0.48	0.016	0.0010	16.10	5.51	2.05	0.025	0.02	0.0227	0.106	1.63	0.41
35	0.011	0.18	0.21	0.010	0.0005	9.85	5.16	3.14	0.035	0.01	0.0140	0.129	2.44	0.12
36	0.008	0.15	0.26	0.014	0.0006	14.84	7.68	1.27	0.040	0.02	0.0099	0.130	1.70	0.22
37	0.009	0.35	0.58	0.011	0.0010	10.63	3.93	2.06	0.021	0.06	0.0238	0.085	2.65	0.30
38	0.008	0.44	0.25	0.018	0.0008	10.89	7.11	3.62	0.030	0.05	0.0384	0.065	3.14	0.10
39	0.010	0.30	0.40	0.014	0.0006	13.89	7.25	1.00	0.035	0.04	0.0156	0.121	2.73	0.14
40	0.011	0.28	0.39	0.013	0.0007	12.69	6.30	3.14	0.033	0.05	0.0082	0.097	3.64	0.12
41	0.007	0.18	0.30	0.015	0.0005	11.99	6.95	3.02	0.034	0.04	0.0079	0.101	-	0.12
42	0.006	0.29	0.30	0.010	0.0010	13.52	7.32	3.37	0.032	0.05	0.0266	0.120	3.11	0.53
43	0.008	0.21	0.50	0.018	0.0005	12.18	6.28	2.57	0.032	0.04	0.0184	0.128	1.58	-

[Table 1-2]

TABLE1-2

Test number	Chemical composition (unit is mass%; balance is Fe and impurities)
Test number	Nb	W	Zr	Sn	As	Sb	B	Ca	Mg	REM
1	-	-	-	-	-	-	-	-	-	-
2	-	-	-	-	-	-	-	-	-	-
3	-	-	-	-	-	-	-	-	-	-
4	-	-	-	-	-	-	-	-	-	-
5	-	-	-	-	-	-	-	-	-	-
6	-	-	-	-	-	-	-	-	-	-
7	-	-	-	-	-	-	-	-	-	-
8	-	-	-	-	-	-	-	-	-	-
9	-	-	-	-	-	-	-	-	-	-
10	0.140	-	-	-	-	-	-	-	-	-
11	-	0.41	-	-	-	-	-	-	-	-
12	-	-	0.0080	-	-	-	-	-	-	-
13	-	-	-	-	-	-	0.0046	-	-	-
14	-	-	-	-	-	-	-	0.0045	-	-
15	-	-	-	-	-	-	-	-	0.0045	-
16	-	-	-	-	-	-	-	-	-	0.0090
17	-	-	-	0.0023	-	-	-	-	-	-
18	-	-	-	-	0.0020	-	-	-	-	-
19	-	-	-	-	-	0.0021	-	-	-	-
20	0.002	0.85	0.0005	-	-	-	0.0003	0.0031	-	-
21	-	-	-	-	-	-	-	-	0.0002	0.0002
22	0.002	0.20	0.0005	0.0018	0.0022	0.0019	0.0003	0.0034	0.0002	0.0002
23	0.002	1.38	0.0005	0.0019	0.0017	0.0024	0.0003	0.0025	0.0002	0.0002
24	0.002	0.02	0.0005	0.0078	0.0075	0.0084	0.0003	0.0026	0.0002	0.0002
25	-	-	-	-	-	-	-	-	-	-
26	-	-	-	-	-	-	-	-	-	-
27	0.002	0.02	-	0.0013	0.0009	0.0007	0.0003	0.0026	0.0002	0.0002
28	-	-	-	-	-	-	-	-	-	-
29	-	-	-	-	-	-	-	-	-	-
30	-	-	-	-	-	-	-	-	-	-
31	-	-	-	-	-	-	-	-	-	-
32	0.002	0.02	-	0.0008	0.0014	0.0012	0.0003	0.0032	0.0002	0.0002
33	-	-	-	-	-	-	-	-	-	-
34	-	-	-	-	-	-	-	-	-	-
35	-	-	-	-	-	-	-	-	-	-
36	-	-	-	-	-	-	-	-	-	-
37	-	-	-	-	-	-	-	-	-	-
38	-	-	-	-	-	-	-	-	-	-
39	-	-	-	-	-	-	-	-	-	-
40	-	-	-	-	-	-	-	-	-	-
41	-	-	-	-	-	-	-	-	-	-
42	-	-	-	-	-	-	-	-	-	-
43	-	-	-	-	-	-	-	-	-	-

In Table 1-1 and Table 1-2, the symbol "-" means that the content of the corresponding element was 0% when a fraction of the relevant numerical value was rounded off. For example, "-" means that the content of Nb in Test No. 1 was 0% when rounded off to the third decimal place. Likewise, "-" means that the content of W in Test No. 1 was 0% when rounded off to the second decimal place. Further, "-" means that the content of Zr in Test No. 1 was 0% when rounded off to the fourth decimal place.
As the starting material preparation process, blooms were produced by continuous casting using the molten steels. The prepared blooms were each subjected to blooming to produce a starting material (round billet). The starting material was subjected to the hot working process. Specifically, the starting material was charged into a heating furnace and heated to 1100 to 1250°C. After being taken out from the heating furnace, the starting material was subjected to hot rolling (hot working) according to the Mannesmann-mandrel process to thereby produce an intermediate steel material (hollow shell) of each test number. At such time, the piercing ratio was within the range of 1.0 to 4.0, and the cumulative reduction of area in the hot working process was within the range of 20 to 70%.
The hollow shell after the hot working was subjected to a quenching process. The A_c3 point of the hollow shell of each test number was as shown in the column "A_c3 point (°C)" in Table 2. The quenching temperature T1 (°C), t1/t2, and FA in the quenching process were as shown in the column "Quenching temperature T1 (°C)", the column "t1/t2", and the column "FA" in Table 2.

[Table 2]

TABLE 2

Test number	A_c3 point (°C)	Quenching process
Test number	A_c3 point (°C)	Quenching temperature T1 (°C)	t1/t2	FA
1	752	1010	1.1	0.03
2	782	960	2.6	0.85
3	784	910	2.6	1.48
4	802	940	1.0	0.00
5	797	950	1.6	0.92
6	772	980	4.0	1.54
7	760	920	5.5	1.64
8	760	920	6.3	1.65
9	778	960	1.9	1.03
10	769	920	1.6	0.26
11	753	980	1.3	0.09
12	793	930	2.0	0.35
13	790	910	3.2	1.55
14	767	900	1.0	0.00
15	792	900	3.5	1.50
16	763	1090	3.3	1.03
17	768	900	1.9	0.28
18	759	1000	1.0	0.00
19	781	990	2.4	1.30
20	773	950	1.1	0.05
21	771	970	1.5	0.24
22	771	930	2.1	0.53
23	770	900	1.1	0.06
24	773	960	1.9	0.46
25	771	890	2.7	0.87
26	773	870	1.3	0.15
27	757	880	1.1	0.04
28	786	920	1.6	2.14
29	772	980	4.6	1.76
30	765	950	7.0	1.81
31	778	940	4.0	1.83
32	795	930	3.5	2.02
33	789	900	2.6	1.27
34	786	1010	3.1	1.29
35	784	980	1.3	0.12
36	763	940	1.7	0.41
37	796	930	3.3	0.87
38	755	1020	1.4	0.13
39	759	970	2.5	0.55
40	762	940	1.0	0.00
41	784	950	1.0	-
42	754	970	1.6	0.19
43	779	900	2.8	1.14

After undergoing the quenching process, each hollow shell was subjected to tempering to thereby adjust the yield strength. The tempering temperature was 500 to 650°C. Martensitic stainless steel materials (seamless steel pipes) were produced by the above production process.

[Evaluation tests]

The martensitic stainless steel material (seamless steel pipe) of each test number was subjected to the following evaluation tests.

(Test 1) Yield strength evaluation test
(Test 2) Martensite volume ratio evaluation test
(Test 3) Test to evaluate δ-ferrite area fraction Sd
(Test 4) Test to evaluate coarse intermetallic compounds area fraction Sc
(Test 5) Hot workability evaluation test
(Test 6) SSC resistance evaluation test
(Test 7) Low-temperature toughness evaluation test

Each test is described hereunder.

[(Test 1) Yield strength evaluation test]

The yield strength (MPa) of the martensitic stainless steel material of each test number was determined based on the above-described [Method for measuring yield strength]. Note that, a round bar specimen was taken from the central portion of the wall thickness of the martensitic stainless steel material (seamless steel pipe) of each test number. Regarding the size of the round bar specimen, the diameter of the parallel portion was 4.0 mm and the gage length was 16.0 mm. The longitudinal direction of the round bar specimen was parallel to pipe axis direction of the martensitic stainless steel material (seamless steel pipe). The determined yield strength (MPa) is shown in the column "YS (MPa)" in Table 3.

[Table 3]

TABLE 3

Test number	YS (MPa)	Martensite volume ratio (%)	Area fraction Sd of δ-ferrite (%)	Area fraction Sc of coarse intermetallic compounds (%)	Fn	Hot workability	SSC resistance		Absorbed energy vE (-10°C) (J)	Remarks
Test number	YS (MPa)	Martensite volume ratio (%)	Area fraction Sd of δ-ferrite (%)	Area fraction Sc of coarse intermetallic compounds (%)	Fn	Hot workability	Evaluation method 1	Evaluation method 2	Absorbed energy vE (-10°C) (J)	Remarks
1	923	94	1.85	0.10	0.05	EX	EX	EX	214	Inventive Example
2	902	93	1.76	2.40	1.36	EX	EX	EX	173	Inventive Example
3	947	96	1.99	4.60	2.31	EX	EX	EX	152	Inventive Example
4	881	93	1.56	2.50	1.60	EX	EX	NA	165	Inventive Example
5	891	89	0.28	1.30	4.64	EX	EX	NA	128	Inventive Example
6	865	88	0.22	0.10	0.45	EX	EX	EX	205	Inventive Example
7	915	93	3.50	3.80	1.09	EX	EX	EX	177	Inventive Example
8	897	94	1.06	2.90	2.74	EX	EX	EX	149	Inventive Example
9	894	90	1.61	3.00	1.86	EX	EX	NA	162	Inventive Example
10	1011	100	0.36	1.50	4.17	EX	EX	EX	130	Inventive Example
11	783	80	8.76	0.60	0.07	EX	EX	EX	218	Inventive Example
12	799	85	0.67	2.90	4.33	EX	EX	EX	130	Inventive Example
13	871	93	1.34	4.20	3.13	EX	EX	EX	143	Inventive Example
14	999	97	1.52	5.50	3.62	EX	EX	EX	138	Inventive Example
15	905	92	3.10	5.20	1.68	EX	EX	EX	165	Inventive Example
16	885	87	6.70	0.10	0.01	EX	EX	EX	223	Inventive Example
17	864	92	0.99	4.80	4.85	EX	EX	EX	122	Inventive Example
18	885	94	0.01	0.04	4.00	EX	EX	EX	135	Inventive Example
19	760	80	4.76	0.10	0.02	EX	EX	EX	216	Inventive Example
20	937	90	6.60	1.50	0.23	EX	EX	EX	211	Inventive Example
21	1048	99	1.46	1.20	0.82	EX	EX	EX	184	Inventive Example
22	944	97	1.70	3.50	2.06	EX	EX	EX	155	Inventive Example
23	1033	98	2.20	4.00	1.82	EX	EX	EX	164	Inventive Example
24	936	96	2.47	1.50	0.61	EX	EX	EX	196	Inventive Example
25	830	89	2.25	22.00	9.78	EX	EX	EX	20	Comparative Example
26	780	81	6.01	31.00	5.16	EX	EX	EX	105	Comparative Example
27	925	94	3.47	25.00	7.20	EX	EX	EX	60	Comparative Example
28	938	92	6.50	35.00	5.38	EX	EX	NA	97	Comparative Example
29	903	94	1.94	20.10	10.36	EX	EX	EX	11	Comparative Example
30	897	92	3.42	29.00	8.48	EX	EX	EX	42	Comparative Example
31	865	88	5.10	30.00	5.88	EX	EX	EX	82	Comparative Example
32	864	88	2.45	25.00	10.20	EX	EX	EX	13	Comparative Example
33	762	83	2.05	4.30	2.10	EX	NA	NA	153	Comparative Example
34	802	74	13.80	0.30	0.02	NA	EX	EX	210	Comparative Example
35	1066	99	0.66	1.20	1.82	EX	NA	NA	167	Comparative Example
36	732	77	0.44	2.20	5.00	EX	EX	EX	128	Comparative Example
37	947	94	3.80	2.80	0.74	EX	NA	NA	195	Comparative Example
38	819	75	12.50	0.10	0.01	NA	EX	EX	230	Comparative Example
39	824	86	2.70	2.00	0.74	EX	NA	NA	184	Comparative Example
40	798	80	4.80	2.30	0.48	NA	EX	EX	201	Comparative Example
41	935	97	0.41	1.60	3.90	EX	NA	NA	135	Comparative Example
42	786	70	15.20	1.20	0.08	NA	EX	EX	208	Comparative Example
43	868	88	2.43	4.10	1.69	EX	NA	NA	174	Comparative Example

[(Test 2) Martensite volume ratio evaluation test]

The martensite volume ratio (%) of the martensitic stainless steel material of each test number was determined based on the above-described [Method for measuring volume ratio of martensite]. Note that, a test specimen was taken from the central portion of the wall thickness of the martensitic stainless steel material (seamless steel pipe) of each test number. The size of the test specimen was 15 mm × 15 mm × 2 mm in thickness, and the thickness direction of the test specimen was the wall thickness direction of the seamless steel pipe. The determined martensite volume ratio (%) is shown in the column "Martensite volume ratio (%)" in Table 3.

[(Test 3) Test to evaluate δ-ferrite area fraction Sd]

The area fraction Sd of δ-ferrite in a cross section parallel to the rolling direction of the martensitic stainless steel material of each test number was determined based on the above-described [Method for measuring area fraction Sd of δ-ferrite]. Note that, a test specimen was taken from the central portion of the wall thickness of the martensitic stainless steel material (seamless steel pipe) of each test number. The observation surface of the test specimen was a surface including the rolling direction and the wall thickness direction of the martensitic stainless steel material. The determined area fraction Sd (%) of δ-ferrite is shown in the column "δ-ferrite area fraction Sd (%)" in Table 3.

[(Test 4) Test to evaluate coarse intermetallic compounds area fraction Sc]

The area fraction Sc of intermetallic compounds in a cross section parallel to the rolling direction of the martensitic stainless steel material of each test number was determined based on the above-described [Method for measuring coarse intermetallic compounds area fraction Sc]. Note that, a test specimen was taken from the central portion of the wall thickness of the martensitic stainless steel material (seamless steel pipe) of each test number. The observation surface of the test specimen was a surface including the pipe axis direction and the wall thickness direction of the martensitic stainless steel material. The determined area fraction Sc (%) of intermetallic compounds is shown in the column "Coarse intermetallic compounds area fraction Sc (%)" in Table 3. Further, Fn (= Sc/Sd) is shown in the column "Fn" in Table 3.

[(Test 5) Hot workability evaluation test]

The surface of the hollow shell of each test number was observed by visual observation at a stage that was after the hot working process and was before the quenching process, and the presence or absence of cracking was confirmed. The result of confirming the presence or absence of cracking is shown in the column "Hot workability" in Table 3. If "NA (Not Accepted)" is shown in the "Hot workability" column, it indicates that a crack with a length of 1.0 mm or more in the pipe axis direction was confirmed, and sufficient hot workability was not obtained. If "EX (Excellent)" is shown in the "Hot workability" column, it indicates that a crack with a length of 1.0 mm or more in the pipe axis direction was not confirmed, and sufficient hot workability was obtained.

[(Test 6) SSC resistance evaluation test]

The SSC resistance of the martensitic stainless steel material of each test number was evaluated based on the above-described [SSC resistance evaluation method 1] and [SSC resistance evaluation method 2].
Specifically, a round bar specimen was taken from the central portion of the wall thickness of the martensitic stainless steel material (seamless steel pipe) of each test number. Regarding the size of the round bar specimen, the diameter of the parallel portion was 6.35 mm and the length of the parallel portion was 25.4 mm. The longitudinal direction of the round bar specimen was made parallel to the rolling direction of the steel material (pipe axis direction of seamless steel pipe).
A 20% by mass sodium chloride aqueous solution having a pH of 4.0 was employed as the test solution. The test solution was prepared by adding acetic acid to an aqueous solution containing 20% by mass sodium chloride and 0.41 g/L of sodium acetate to adjust the pH to 4.0. A stress equivalent to 90% of the actual yield stress was applied to the prepared round bar specimen. The test solution at 24°C was poured into a test vessel so that the round bar specimen to which the stress had been applied was immersed therein to form a test bath. For each test number, tests were conducted in accordance with the following evaluation method 1 and evaluation method 2, respectively.
Evaluation method 1: After degassing the test bath, H₂S gas at 0.07 bar and CO₂ gas at 0.93 bar were blown into the test bath to saturate the test bath with H₂S gas. The test bath in which the H₂S gas was saturated was held at 24°C for 720 hours.
Evaluation method 2: After degassing the test bath, H₂S gas at 0.10 bar and CO₂ gas at 0.90 bar were blown into the test bath to saturate the test bath with H₂S gas. The test bath in which the H₂S gas was saturated was held at 24°C for 720 hours.
In each of evaluation method 1 and evaluation method 2, after the test specimen had been held for 720 hours, the surface of the parallel portion of the test specimen was observed with a magnifying glass having a magnification of 10× to confirm the presence or absence of cracking.
For each of the test specimen tested according to evaluation method 1 and the test specimen tested according to evaluation method 2, the surface of the parallel portion of the test specimen was observed with a magnifying glass having a magnification of 10×. The observation results are shown in the columns "Evaluation method 1" and "Evaluation method 2" of the column "SSC resistance" in Table 3. If "EX (Excellent)" is described in the "Evaluation method 1" or "Evaluation method 2" column of the "SSC resistance" column, it indicates that a crack was not confirmed in the test specimen and excellent SSC resistance was obtained. On the other hand, if "NA (Not Accepted)" is described in the "Evaluation method 1" or "Evaluation method 2" column of the "SSC resistance" column, it indicates that a crack was confirmed in the test specimen and excellent SSC resistance was not obtained.

[(Test 7) Low-temperature toughness evaluation test]

The low-temperature toughness of the martensitic stainless steel material of each test number was evaluated based on the above-described [Low-temperature toughness evaluation method]. Note that, a full-size V-notch test specimen was used for the evaluation. The determined absorbed energy vE (-10°C) at -10°C is shown in the column "Absorbed energy vE (-10°C) (J)" in Table 3.

[Evaluation results]

Referring to Table 1-1, Table 1-2, Table 2, and Table 3, in Test Nos. 1 to 24, the martensitic stainless steel material satisfied Feature 1 to Feature 3. Therefore, sufficient hot workability was obtained. Furthermore, even though the yield strength was 758 MPa or more, excellent SSC resistance was obtained in the test conducted according to evaluation method 1. In addition, the absorbed energy at - 10°C was 120 J or more and excellent low-temperature toughness was obtained.
Further, among Test Nos. 1 to 24, the content of Cu was 1.00% or more in Test Nos. 1 to 3, 6 to 8, and 10 to 24. Therefore, not only was excellent SSC resistance obtained in the test conducted according to evaluation method 1, but excellent SSC resistance was also obtained in the test conducted according to evaluation method 2 that was stricter than evaluation method 1.
Furthermore, among Test Nos. 1 to 24, Fn was 2.00 or less in Test Nos. 1, 2, 4, 6, 7, 9, 11, 15, 16, 19 to 21, 23 and 24. Therefore, the absorbed energy at -10°C was 160 J or more, and even more excellent low-temperature toughness was obtained.
On the other hand, in Test Nos. 25 to 27, the quenching temperature T1 in the quenching process was too low. Therefore, although Test Nos. 25 to 27 satisfied Feature 1 and Feature 2, Fn was more than 5.00 and thus Feature 3 was not satisfied. As a result, the absorbed energy at -10°C was less than 120 J, and excellent low-temperature toughness was not obtained.
In Test Nos. 28 to 32, although the content of each element in the chemical composition was appropriate, FA in the quenching process was more than 1.71. Therefore, although Test Nos. 28 to 32 satisfied Feature 1 and Feature 2, Fn was more than 5.00 and thus Feature 3 was not satisfied. As a result, the absorbed energy at -10°C was less than 120 J, and excellent low-temperature toughness was not obtained.
In Test No. 33, the content of C was too high. Therefore, excellent SSC resistance was not obtained.
In Test No. 34, the content of Cr was too high. Therefore, the area fraction Sd of δ-ferrite was more than 10.00%. As a result, sufficient hot workability was not obtained.
In Test No. 35, the content of Cr was too low. Therefore, excellent SSC resistance was not obtained.
In Test No. 36, the content of Ni was too high. Therefore, sufficient yield strength was not obtained, and the martensite volume ratio was less than 80%.
In Test No. 37, the content of Ni was too low. Therefore, excellent SSC resistance was not obtained.
In Test No. 38, the content of Mo was too high. Therefore, the area fraction Sd of δ-ferrite was more than 10.00%. Therefore, sufficient hot workability was not obtained.
In Test No. 39, the content of Mo was too low. Therefore, excellent SSC resistance was not obtained.
In Test No. 40, the content of Cu was too high. Therefore, sufficient hot workability was not obtained.
In Test No. 41, the content of Cu was too low. Therefore, excellent SSC resistance was not obtained.
In Test No. 42, the content of Co was too high. Consequently, the area fraction Sd of δ-ferrite was more than 10.00%. Therefore, sufficient hot workability was not obtained.
In Test No. 43, the content of Co was too low. Therefore, excellent SSC resistance was not obtained.
An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range that does not depart from the gist of the present disclosure.

Claims

A martensitic stainless steel material having a chemical composition consisting of, by mass%,
C: 0.030% or less,

Si: 1.00% or less,

Mn: 1.00% or less,

P: 0.030% or less,

S: 0.0050% or less,

Cr: 10.00 to 16.00%,

Ni: 4.00 to 7.50%,

Mo: 1.10 to 3.50%,

Al: 0.005 to 0.050%,

V: 0.01 to 0.30%,

N: 0.0030 to 0.0500%,

Ti: 0.020 to 0.150%,

Cu: 0.01 to 3.50%,

Co: 0.01 to 0.50%,

Nb: 0 to 0.150%,

W: 0 to 1.50%,

Zr: 0 to 0.0100%,

Sn: 0 to 0.0100%,

As: 0 to 0.0100%,

Sb: 0 to 0.0100%,

B: 0 to 0.0050%,

Ca: 0 to 0.0050%,

Mg: 0 to 0.0050%, and

rare earth metal (REM): 0 to 0.0100%,

with the balance being Fe and impurities,

wherein:
a yield strength is 758 MPa or more; and

an area fraction Sd (%) of δ-ferrite and an area fraction Sc (%) of intermetallic compounds which have an equivalent circular diameter of 1.0 µm or more in a cross section parallel to a rolling direction of the martensitic stainless steel material satisfy Formula (1) and Formula (2). $0 < Sd \leq 10.00$ $Sc / Sd \leq 5.00$
The martensitic stainless steel material according to claim 1, wherein the chemical composition contains one or more kinds selected from a group consisting of, by mass%,
Nb: 0.001 to 0.150%,

W: 0.01 to 1.50%,

Zr: 0.0001 to 0.0100%,

Sn: 0.0001 to 0.0100%,

As: 0.0001 to 0.0100%,

Sb: 0.0001 to 0.0100%,

B: 0.0001 to 0.0050%,

Ca: 0.0001 to 0.0050%,

Mg: 0.0001 to 0.0050%, and

rare earth metal (REM): 0.0001 to 0.0100%.
The martensitic stainless steel material according to claim 1 or claim 2, wherein the chemical composition contains, by mass%,
Cu: 1.00 to 3.50%.