EP4506481A1

EP4506481A1 - Martensite stainless steel material

Info

Publication number: EP4506481A1
Application number: EP23784651.4A
Authority: EP
Inventors: Toshiya Nishimura; Kyohei KANKI
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-04-08
Filing date: 2023-03-24
Publication date: 2025-02-12
Also published as: WO2023195361A1; US20250215546A1; JP7428952B1; JPWO2023195361A1; EP4506481A4

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 consists 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 14.00%, Ni: 5.00 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: 1.00 to 3.50%, and Co: 0.010 to 0.500%, with the balance being Fe and impurities, and has a yield strength of 758 MPa or more. An area fraction of δ-ferrite is 5.00% or less, and a length L (µm) of the δ-ferrite in the rolling direction and a distance D (µm) between particles of the δ-ferrite in the rolling direction satisfy Formula (1):

L / D \leq 10.5

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 sulfide 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 corrosion 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, Cu, 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 Literatures 1 and 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 14.00%,
Ni: 5.00 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: 1.00 to 3.50%,
Co: 0.010 to 0.500%,
Nb: 0 to 0.15%,
W: 0 to 1.50%,
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
- in a cross section parallel to a rolling direction of the martensitic stainless steel material,
- an area fraction of δ-ferrite is 5.00% or less, and
- a length L (µm) of the δ-ferrite in the rolling direction, and a distance D (µm) between a plurality of particles of the δ-ferrite in the rolling direction satisfy Formula (1) $L / D \leq 10.5$

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 view illustrating the relation between area fractions (%) of δ-ferrite in martensitic stainless steel materials in which the content of each element in the chemical composition is within the range of the present embodiment, and the absorbed energy (J) at -10°C.
[FIG. 2] FIG. 2 is a schematic diagram illustrating the shapes and the distribution state of δ-ferrite in a cross section parallel to the rolling direction of a martensitic stainless steel material.
[FIG. 3] FIG. 3 is a view illustrating the relation between a ratio (L/D) of a length L in the rolling direction of δ-ferrite to a distance D between particles of δ-ferrite in the rolling direction in a cross section parallel to the rolling direction of respective martensitic stainless steel materials, and the absorbed energy (J) at -10°C.
[FIG. 4] FIG. 4 is a schematic diagram illustrating a method for measuring a length L (µm) of δ-ferrite and a distance D (µm) between particles of δ-ferrite in a measurement visual field FV.
[FIG. 5] FIG. 5 is a view illustrating the relation between an initial substantial rolling reduction R1 in a blooming process, and the value of LID of δ-ferrite in martensitic stainless steel materials that are end products.

DESCRIPTION OF EMBODIMENTS

The present inventors conducted studies from the viewpoint of the chemical composition with respect to a martensitic stainless steel material having a yield strength of 758 MPa or more (110 ksi 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 14.00%, Ni: 5.00 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: 1.00 to 3.50%, Co: 0.010 to 0.500%, Nb: 0 to 0.15%, W: 0 to 1.50%, 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.
The present inventors then 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 found that when only the content of each element in the chemical composition is simply adjusted, it is difficult to obtain a yield strength of 758 MPa or more, excellent SSC resistance, and excellent low-temperature toughness.
Therefore, the present inventors attempted to improve the low-temperature toughness from the viewpoint of the microstructure, and not 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 present in a martensitic stainless steel material that contains Cr in an amount of 10.00 to 14.00%. The δ-ferrite embrittles the steel material and reduces the low-temperature toughness of the steel material. Accordingly, the present inventors considered that if the amount of δ-ferrite in a martensitic stainless steel material having the aforementioned chemical composition is reduced, excellent SSC resistance and excellent low-temperature toughness will be obtained even if the steel material has a high strength of 758 MPa or more.
Therefore, the present inventors investigated and studied the relation between the area fraction (%) of δ-ferrite in martensitic stainless steel materials having the aforementioned chemical composition, and the absorbed energy (J) at -10°C.
FIG. 1 is a view illustrating the relation between area fractions (%) of δ-ferrite in martensitic stainless steel materials having the aforementioned chemical composition, and the absorbed energy (J) at -10°C. The abscissa in FIG. 1 represents the area fraction (%) of δ-ferrite, and the ordinate represents the absorbed energy (J) at -10°C. Note that, FIG. 1 was prepared using values for area fractions (%) of δ-ferrite and absorbed energy (J) at -10°C that were obtained in examples that are described later.
As mentioned above, the present inventors had thought that if the δ-ferrite amount (δ-ferrite area fraction) is reduced, the low-temperature toughness will increase. However, in FIG. 1, even in cases where the area fraction of δ-ferrite was reduced to 5.00% or less, there were large variations in the absorbed energy at -10°C. In particular, even when the area fraction of δ-ferrite was suppressed to a low amount within the range of 1.00 to 2.00%, as illustrated by a region A1, the absorbed energy at -10°C varied significantly. In other words, FIG. 1 shows that even when the area fraction of δ-ferrite is reduced, low-temperature toughness does not necessarily increase.
Further, in a martensitic stainless steel material containing Cr in an amount of 10.00 to 14.00%, reducing the δ-ferrite amount (δ-ferrite area fraction) to the maximum requires strict production control in normal industrial production, and there is also the possibility that the production cost will increase.
Therefore, the present inventors changed their idea and instead of focusing on reducing the δ-ferrite amount as much as possible in a martensitic stainless steel material having the aforementioned chemical composition, the present inventors investigated means for obtaining a high strength of 758 MPa or more, excellent SSC resistance, and excellent low-temperature toughness even in a case where δ-ferrite is present to a certain extent in the martensitic stainless steel material.
Here, the present inventors focused their attention on the propagation direction of cracks attributable to δ-ferrite. The propagation direction of cracks attributable to δ-ferrite is mainly the rolling direction of the martensitic stainless steel material. Therefore, the present inventors considered that the shape and distribution state of δ-ferrite in a cross section parallel to the rolling direction of a martensitic stainless steel material influences low-temperature toughness.
Based on the above consideration, the present inventors had the idea that, in a cross section parallel to a rolling direction Z of a martensitic stainless steel material illustrated in FIG. 2, the length (µm) of particles of δ-ferrite 10 in the rolling direction Z can be used as an index of the shape of the particles of the δ-ferrite 10. Therefore, in a cross section parallel to the rolling direction Z of the martensitic stainless steel material, the present inventors defined a length L (µm) of δ-ferrite in the rolling direction by a method that is described later. The present inventors also had the idea that, in a cross section parallel to the rolling direction Z of the martensitic stainless steel material, a distance (µm) between the particles of the δ-ferrite 10 in the rolling direction Z can be used as an index of the distribution state of the δ-ferrite 10 in the rolling direction Z. Therefore, in a cross section parallel to the rolling direction Z of the martensitic stainless steel material, the present inventors defined a distance D (µm) between particles of δ-ferrite in the rolling direction by a method that is described later. Further, with respect to martensitic stainless steel materials having the aforementioned chemical composition, in cases where the δ-ferrite area fraction was reduced to 5.00% or less, the present inventors investigated the relation among the length L of δ-ferrite in the rolling direction Z, the distance D between particles of δ-ferrite in the rolling direction Z, and the low-temperature toughness. As a result, the present inventors discovered that in a martensitic stainless steel material in which the area fraction of δ-ferrite is reduced to 5.00% or less, an extremely high correlation is obtained between the low-temperature toughness and a ratio LID of the length L to the distance D.
FIG. 3 is a graph illustrating the relation between LID and absorbed energy at -10°C. FIG. 3 was prepared using results obtained in examples that are described later. Referring to FIG. 3, when LID is greater than 10.5, even if LID decreases, the absorbed energy at -10°C does not increase much. On the other hand, when LID is 10.5 or less, the absorbed energy at -10°C markedly increases as LID decreases. That is, in FIG. 3, there is an inflection point in the vicinity of LID = 10.5.
Based on the above findings, the present inventors discovered that in a martensitic stainless steel material having the aforementioned chemical composition, in a case where the area fraction of δ-ferrite is made 5.00% or less, if LID is made 10.5 or less, not only are a high strength of 758 MPa or more and excellent SSC resistance obtained, but excellent low-temperature toughness is also obtained.
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 14.00%,
- Ni: 5.00 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: 1.00 to 3.50%,
- Co: 0.010 to 0.500%,
- Nb: 0 to 0.15%,
- W: 0 to 1.50%,
- 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
  - in a cross section parallel to a rolling direction of the martensitic stainless steel material,
  - an area fraction of δ-ferrite is 5.00% or less, and
  - a length L (µm) of the δ-ferrite in the rolling direction, and a distance D (µm) between a plurality of particles of the δ-ferrite in the rolling direction satisfy Formula (1). $L / D \leq 10.5$
[2] The martensitic stainless steel material according to [1], wherein the chemical composition contains one or more elements selected from a group consisting of:
- Nb: 0.01 to 0.15%,
- W: 0.01 to 1.50%,
- 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 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 4.

(Feature 1)

The chemical composition consists 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 14.00%, Ni: 5.00 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: 1.00 to 3.50%, Co: 0.010 to 0.500%, Nb: 0 to 0.15%, W: 0 to 1.50%, 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)

In a cross section parallel to a rolling direction Z of the martensitic stainless steel material, an area fraction AR of δ-ferrite is 5.00% or less.

(Feature 4)

In a cross section parallel to the rolling direction Z of the martensitic stainless steel material, a length L (µm) of δ-ferrite in the rolling direction Z and a distance D (µm) between a plurality of particles of the δ-ferrite in the rolling direction Z satisfy Formula (1). $L / D \leq 10.5$
Hereunder, Feature 1 to Feature 4 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 an excessive amount of Cr carbides. As a result, even if the contents of other elements are within the range of the present embodiment, the low-temperature toughness 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 0.028%, more 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 0.05%, more 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 0.10%, more 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 excessively segregate to grain boundaries, which will cause the low-temperature toughness of the steel material to markedly decrease.
Therefore, the content of P is 0.030% or less.
The content of P is preferably as low as possible. However, excessively reducing the content of P will significantly increase the production cost. 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 an excessively large amount of MnS that is an inclusion will be formed. 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, excessively reducing the content of S will significantly increase the production cost. 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 14.00%

Chromium (Cr) forms a passivation film on the surface of the steel material in a sour environment and 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 14.00%, Cr carbides, intermetallic compounds containing Cr, and Cr oxides 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 Cr is 10.00 to 14.00%.
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 13.70%, more preferably is 13.50%, further preferably is 13.40%, and further preferably is 13.30%.

Ni: 5.00 to 7.50%

Nickel (Ni) forms sulfides on the passivation film in a sour 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 sour 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 5.00%, 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%, 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 5.00 to 7.50%.
A preferable lower limit of the content of Ni is 5.05%, more preferably is 5.10%, further preferably is 5.15%, and further preferably is 5.20%.
A preferable upper limit of the content of Ni is 7.30%, more preferably is 7.10%, and further preferably is 7.00%.

Mo: 1.10 to 3.50%

Molybdenum (Mo) forms sulfides on the passivation film in a sour environment. The Mo 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, Mo increases the SSC resistance of the steel material in a sour 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%, 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 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 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 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, by forming carbides or nitrides, Ti suppresses the occurrence of an excessive increase in strength caused by excessive formation of V precipitates (carbides, nitrides, and carbo-nitrides). 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: 1.00 to 3.50%

Copper (Cu) forms sulfides on the passivation film in a sour 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 sour environment. Cu also increases the strength of the steel material by precipitation strengthening. If the content of Cu is less than 1.00%, 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 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 1.00 to 3.50%.
A preferable lower limit of the content of Cu is 1.05%, more preferably is 1.20%, further preferably is 1.40%, further preferably is 1.50%, further preferably is 1.60%, further preferably is 1.80%, further preferably is 1.90%, and further preferably is 2.00%.
A preferable upper limit of the content of Cu is 3.30%, more preferably is 3.10%, and further preferably is 3.00%.

Co: 0.010 to 0.500%

Cobalt (Co) forms sulfides on the passivation film in a sour environment. The Co 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, Co increases the SSC resistance of the steel material in a sour 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 0.010%, 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 0.500%, 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 Co is 0.010 to 0.500%.
A preferable lower limit of the content of Co is 0.020%, more preferably is 0.050%, further preferably is 0.100%, and further preferably is 0.150%.
A preferable upper limit of the content of Co is 0.450%, more preferably is 0.400%, and further preferably is 0.350%.
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, in lieu of a part of Fe, one or more elements selected from the group consisting of:

Nb: 0 to 0.15%,
W: 0 to 1.50%,
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%.

These optional elements are described hereunder.

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

The chemical composition of the martensitic stainless steel material of the present embodiment may further contain one or more elements selected from the group consisting of Nb, W, 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, Nb, W, Sn, As, and Sb each increase the SSC resistance of the steel material.

Nb: 0 to 0.15%

Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%.
When contained, Nb forms fine precipitates (carbides, nitrides, and carbo-nitrides; hereunder, referred to as "Nb precipitates"). The Nb precipitates refine the substructure of the steel material by the 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 0.15%, Nb precipitates will excessively form, and the strength of the steel material will become 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 Nb is 0 to 0.15%.
A preferable lower limit of the content of Nb is 0.01%, more preferably is 0.02%, and further preferably is 0.03%.
A preferable upper limit of the content of Nb is 0.14%, more preferably is 0.13%, and further preferably is 0.10%.

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, W stabilizes the passivation film in a sour environment, and thus inhibits the 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%.
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%.

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, 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%, even if the contents of other elements are within the range of the present embodiment, Sn will segregate to grain boundaries. In such a case, the SSC resistance of the steel material will, on the contrary, decrease.
Therefore, the content of Sn is 0 to 0.0100%.
A preferable lower limit of the content of Sn is 0.0001%, more preferably is 0.0003%, further preferably is 0.0005%, and further preferably is 0.0007%.
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 even if the contents of other elements are within the range of the present embodiment. In such a case, the SSC resistance of the steel material will, on the contrary, decrease.
Therefore, the content of As is 0 to 0.0100%.
A preferable lower limit of the content of As is 0.0001%, more preferably is 0.0003%, further preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0015%.
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, 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 even if the contents of other elements are within the range of the present embodiment. In such a case, the SSC resistance of the steel material will, on the contrary, decrease.
Therefore, the content of Sb is 0 to 0.0100%.
A preferable lower limit of the content of Sb is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.
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 of the present embodiment may further contain one or more elements selected from the group consisting 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, B, Ca, Mg, and rare earth metal (REM) each increase hot workability of the steel material.

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, 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 even if the contents of other elements are within the range of the present embodiment. In such a case, the low-temperature toughness of the steel material will decrease.
Therefore, the content of B is 0 to 0.0050%.
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, 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%.
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, 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%.
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, 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%.
A preferable lower limit of the content of REM is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, further preferably is 0.0020%, and further preferably is 0.0025%.
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.

[Microstructure]

The microstructure of the martensitic stainless steel material according to the present embodiment is mainly composed of martensite. In the present description, the term "martensite" also includes tempered martensite and not just fresh martensite. Further, in the present description, the phrase "mainly composed of martensite" 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 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%.
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, as mentioned above, the volume ratio of retained austenite in the microstructure of the martensitic stainless steel material of the present embodiment is 0 to 20%. From the viewpoint of ensuring strength, a preferable upper limit of the volume ratio of retained austenite is 15%, and more preferably is 10%. As mentioned above, the microstructure of the martensitic stainless steel material of the present embodiment may be a martensite single-phase structure. Accordingly, the volume ratio of retained austenite may be 0%. On the other hand, in a case where even a small amount of retained austenite is present, the volume ratio of retained austenite is to be more than 0 to 20%, more preferably is more than 0 to 15%, and further preferably is more than 0 to 10%.

[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 AR of δ-ferrite that is determined by a method described in the section [Method for measuring area fraction AR 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. Here, the term "R/2 portion" means the central portion of a radius R in a cross section perpendicular to the axial direction of the round steel bar. If the martensitic stainless 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 average value of the volume ratios Vy of retained austenite of the six pairs is defined as the volume ratio (%) of retained austenite. $V γ = 100 / \{1 + (I α \times Rγ) / (I γ \times Rα)\}$
where, Iα is the integrated intensity of α phase. Rα is the crystallographic theoretical calculation value of α phase. Iγ 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.
The volume ratio of δ-ferrite is determined by the following method. Specifically, the area fraction AR (%) of δ-ferrite is determined by a method described in the section [Method for measuring area fraction AR of δ-ferrite] that is described later. The determined area fraction AR (%) 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 2) Regarding yield strength]

The yield strength of the martensitic stainless steel material of the present embodiment is 110 ksi 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.
A round bar specimen is taken from the martensitic stainless steel material. If the martensitic stainless steel material is a steel pipe, the 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, the round bar specimen is to be 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 to be taken from the central portion of the thickness of the steel plate.
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. Here, the term "rolling direction of the martensitic stainless steel material" means the pipe axis direction in a case where the martensitic stainless steel material is a steel pipe. If the martensitic stainless steel material is a round steel bar, the term "rolling direction of the martensitic stainless steel material" means the axial direction of the round steel bar.
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. 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 constituting the martensitic stainless steel material 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).

[(Feature 3) Regarding area fraction AR of δ-ferrite]

In the martensitic stainless steel material of the present embodiment, in addition, the area fraction AR of δ-ferrite in a cross section parallel to the rolling direction Z of the martensitic stainless steel material is 5.00% or less. When the area fraction AR of δ-ferrite is 5.00% or less, on the precondition that Feature 1, Feature 2, and Feature 4 are satisfied, excellent low-temperature toughness is obtained even when the martensitic stainless steel material has a high strength of 758 MPa or more.
A preferable upper limit of the area fraction AR of δ-ferrite is 4.80%, more preferably is 4.70%, further preferably is 4.60%, further preferably is 4.50%, further preferably is 4.30%, further preferably is 4.10%, and further preferably is 4.00%.
A lower limit of the area fraction AR of δ-ferrite is not particularly limited. However, excessively reducing the area fraction AR of δ-ferrite will raise the production cost. Therefore, when taking normal industrial production into consideration, a preferable lower limit of the area fraction AR of δ-ferrite is more than 0%, more preferably is 0.01%, further preferably is 0.02%, further preferably is 0.03%, further preferably is 0.04%, and further preferably is 0.05%.

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

The area fraction AR of δ-ferrite can be measured by the following method.
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 martensitic stainless steel material is a steel plate, the test specimen is to be taken from the central portion of the thickness of the steel plate.
The test specimen has an observation surface that is parallel to the rolling direction Z 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 rolling direction Z (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 rolling direction Z (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 Z and the thickness direction.
The test specimen is taken in a manner that makes it possible to identify the rolling direction Z of the martensitic stainless steel material. Specifically, the test specimen is taken in a manner so that, on the observation surface of the test specimen, the sides parallel to the rolling direction of the martensitic stainless steel material are long sides, and the sides perpendicular to the rolling direction are short sides. If the martensitic stainless steel material is a steel pipe, on the observation surface of the test specimen, the sides that are parallel to the rolling direction Z (pipe axis direction) are made long sides, and the sides that are parallel to the wall thickness direction are made short sides. If the martensitic stainless steel material is a round steel bar, on the observation surface of the test specimen, the sides that are parallel to the rolling direction Z (axial direction) are made long sides, and sides that are parallel to the radial direction are made short sides. If the martensitic stainless steel material is a steel plate, on the observation surface of the test specimen, the sides that are parallel to the rolling direction Z are made long sides, and sides that are parallel to the thickness direction are made short sides. By this means, in a method described in the section [Method for measuring LID of δ-ferrite] which is described later, the rolling direction Z can be identified in a measurement visual field FV.
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 FV is set to 30. Each measurement visual field FV is set as a rectangle with dimensions of 250 µm × 250 µm. 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 FV (30 visual fields) by the total number of lattice points is defined as the area fraction AR (%) of δ-ferrite.
Note that, with regard to determining whether or not a particle is δ-ferrite is performed by element concentration analysis (EDS analysis) using energy dispersive X-ray spectrometry (EDS). Specifically, particles in each measurement visual field are identified based on contrast. Each particle that is identified is subjected to 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.

[(Feature 4) Regarding LID of δ-ferrite]

In the martensitic stainless steel material of the present embodiment, in addition, in a cross section parallel to the rolling direction Z of the martensitic stainless steel material, a length L (µm) of δ-ferrite in the rolling direction Z, and a distance D (µm) between a plurality of particles of δ-ferrite in the rolling direction Z satisfy Formula (1). $L / D \leq 10.5$
As mentioned above, when the area fraction of δ-ferrite in a cross section parallel to the rolling direction of the martensitic stainless steel material is 5.00% or less, as illustrated in FIG. 3, the low-temperature toughness exhibits a negative correlation with L/D. Specifically, as illustrated in FIG. 3, when LID is greater than 10.5, even if LID decreases, the absorbed energy at -10°C (J) does not increase much. On the other hand, when LID is 10.5 or less, the absorbed energy at -10°C (J) markedly increases as LID decreases. That is, in the graph in FIG. 3, there is an inflection point in the vicinity of LID = 10.5.
In the present embodiment, LID is 10.5 or less, and thus satisfies Formula (1). Therefore, on the precondition that Feature 1 to Feature 3 are satisfied, even though the martensitic stainless steel material has a high strength of 758 MPa or more, the martensitic stainless steel material has excellent low-temperature toughness.
A preferable upper limit of LID is 10.3, more preferably is 10.0, further preferably is 9.5, further preferably is 9.3, further preferably is 9.0, further preferably is 8.7, further preferably is 8.5, further preferably is 7.5, further preferably is 5.5, and further preferably is 3.5.
A preferable lower limit of LID is not limited. The lower limit of LID is, for example, 0.1, or for example is 0.2, or for example is 0.3.

[Method for measuring LID of δ-ferrite]

The value of LID of the δ-ferrite can be measured by the following method.
In each of the 30 measurement visual fields FV described above in the section [Method for measuring area fraction AR of δ-ferrite], the length L (µm) of particles of δ-ferrite, and the distance D (µm) between the particles of δ-ferrite are measured. FIG. 4 is a schematic diagram illustrating a method for measuring the length L (µm) of δ-ferrite and the distance D (µm) between particles of δ-ferrite in each measurement visual field FV. Referring to FIG. 4, in each measurement visual field FV, a line segment SG having a length of 100 µm is set parallel to the rolling direction Z. At this time, the number of particles of the δ-ferrite 10 which partially or entirely overlap with the line segment SG is counted. The line segment SG is set at a plurality of locations, and the line segment SG that overlaps with the largest number of particles of the δ-ferrite 10 is selected. A value obtained by dividing the length (100 µm) of the selected line segment SG by the number of particles of the δ-ferrite 10 overlapping with the relevant line segment SG is taken as a ferrite distance D_FV (µm) in the relevant measurement visual field FV. Here, a value to the second decimal place which is obtained by rounding off the value at the third decimal place is taken as the distance D_FV.
In addition, in each measurement visual field FV, the particle of δ-ferrite which has the longest length in the rolling direction Z among the plurality of particles of δ-ferrite in the measurement visual field FV is selected. The length of the selected δ-ferrite in the rolling direction Z is taken as a δ-ferrite length L_FV (µm) of the relevant measurement visual field FV. Here, a value to the second decimal place that is obtained by rounding off the value at the third decimal place is taken as the length L_FV.
The distance D_FV and the length L_FV are determined in each of the 30 measurement visual fields FV. The arithmetic average value of the 30 distances D_FV is defined as the distance D (µm) in the martensitic stainless steel material. A value to the first decimal place that is obtained by rounding off the value at the second decimal place is taken as the distance D. Further, the arithmetic average value of the 30 lengths L_FV is defined as the length L (µm) in the martensitic stainless steel material. A value to the first decimal place that is obtained by rounding off the value at the second decimal place is taken as the length L. A value to the first decimal place that is obtained by rounding off the value at the second decimal place is taken as the value of L/D.
For example, in FIG. 4, line segments SG1 to SG3 that each have a length of 100 µm are set so as to overlap with particles of the δ-ferrite 10 in parallel to the rolling direction Z in the measurement visual field FV. At such time, the line segment SG1 overlaps with six particles of the δ-ferrite 10 which is the largest number among the plurality of line segments SG1 to SG3. Therefore, the line segment SG1 is selected, and 16.67 µm that is the value obtained by dividing 100 µm by 6 which is the number of particles of δ-ferrite is defined as the distance D_FV (µm) between the particles of δ-ferrite in the relevant measurement visual field FV.
Further, among the plurality of particles of the δ-ferrite 10 in the measurement visual field FV, δ-ferrite 10A which has the longest length in the rolling direction Z is selected. The length of the selected δ-ferrite 10A in the rolling direction Z is defined as the length L_FV (µm) of δ-ferrite in the relevant measurement visual field FV.

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

The martensitic stainless steel material of the present embodiment satisfies Feature 1 to Feature 4. Therefore, the martensitic stainless steel material of the present embodiment has high strength and has excellent SSC resistance. In addition, the martensitic stainless steel material of the present embodiment has excellent low-temperature toughness.

[Regarding SSC resistance]

Even though the martensitic stainless steel material according to the present embodiment has a high yield strength of 758 MPa or more, the martensitic stainless steel material has excellent SSC resistance in sour environments. In the present embodiment, whether the martensitic stainless steel material "has excellent SSC resistance" can be evaluated by the following method.

[SSC resistance evaluation method]

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. 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 martensitic stainless steel material. That is, 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.
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 0.10 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.
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.

[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 SPEC 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 SPEC 5CRA (2010) to convert the obtained absorbed energy to the absorbed energy for a full-size V-notch test specimen. Further, an integer value obtained by rounding off the first decimal place of the obtained numerical value is adopted as the absorbed energy (J) at -10°C. In the present embodiment, if the absorbed energy at -10°C is 40 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, the martensitic stainless steel material is a steel pipe for oil country tubular goods. The term " steel pipe for oil country tubular goods" means a steel pipe that is to be used in oil country tubular goods. Oil country tubular goods are, for example, casing pipes, tubing pipes, and drilling pipes which are 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 4. Therefore, the martensitic stainless steel material of the present embodiment has a high strength that is a yield strength of 110 ksi 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. The production method described hereunder is an example, and a method for producing the martensitic stainless steel material according to the present embodiment is not limited to the production method described hereunder. As long as a martensitic stainless steel material that satisfies Feature 1 to Feature 4 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) Cast material preparation process
(Process 2) Blooming process
(Process 3) Hot working process
(Process 4) Heat treatment process (quenching process and tempering process)

Each process is described hereunder.

[(Process 1) Cast material preparation process]

In the cast material preparation process, a cast material having the chemical composition that satisfies Feature 1 is prepared. Specifically, a molten steel whose chemical composition satisfies Feature 1 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. A cast material (slab, bloom, or ingot) is produced by the above production process.

[(Process 2) Blooming process]

In the blooming process, the cast material is rolled using a blooming mill to produce a starting material. Specifically, first the cast material is charged into a heating furnace and heated. The heated cast material is taken out from the heating furnace. The cast material taken out is then subjected to rolling using a blooming mill.
In the blooming mill, so-called "reverse rolling" is performed. In the case of reverse rolling, rolling is performed once when the cast material progresses from upstream to downstream to pass through the blooming mill, and rolling is performed once when the cast material progresses from downstream to upstream to pass through the blooming mill. That is, when rolling is performed in which the cast material passes forward and backward through the blooming mill, rolling of the cast material is performed twice. Note that, there are also cases where the cast material is not subjected to rolling when the cast material passes through the blooming mill. As described above, in the blooming process, rolling of the cast material is performed a plurality of times using a blooming mill to thereby produce a starting material. When the martensitic stainless steel material is a steel pipe or a round steel bar, the starting material is a billet. When the martensitic stainless steel material is a steel plate, the starting material is a sheet bar.
In the blooming process in which the above process is performed, the following conditions are satisfied.

(Condition 1) Heating temperature T1: 1250 to 1300°C
(Condition 2) Holding time t1 at heating temperature T1: 300 to 500 minutes
(Condition 3) Initial substantial rolling reduction R1 during blooming: 3.4% or more

Each condition is described hereunder.

[(Condition 1) Regarding heating temperature T1]

If the heating temperature T1 is more than 1300°C, δ-ferrite will excessively form. Consequently, the area fraction AR of δ-ferrite in the martensitic stainless steel material that is the end product will be more than 5.00%. On the other hand, if the heating temperature T1 is less than 1250°C, the load applied to the blooming mill during blooming will be excessively large. Therefore, the heating temperature T1 is 1250 to 1300°C.

[(Condition 2) Regarding holding time t1]

A large number of δ-ferrite particles are present in the cast material. Therefore, during the heating in the blooming process, the amount of δ-ferrite in the starting material is sufficiently reduced by holding the starting material at the heating temperature T1 for 300 minutes or more. As a result, the area fraction AR of δ-ferrite in the martensitic stainless steel material that is the end product becomes 5.00% or less. Note that, if the holding time t1 is more than 500 minutes, the production cost will increase. Therefore, the holding time t1 at the heating temperature T1 is 300 to 500 minutes.

[Regarding initial substantial rolling reduction R1 during blooming]

In the blooming using a blooming mill, as mentioned above, rolling is performed once when the cast material progresses from upstream to downstream to pass through the blooming mill, and rolling is performed once when the cast material progresses from downstream to upstream to pass through the blooming mill. Performing rolling once when the cast material passes through the blooming mill is referred to as a "one pass".
In the blooming, in some cases, in the first pass, the cast material is subjected to soft reduction at a low rolling reduction of less than 1.5% to remove scale that is formed on the surface of the cast material. In such cases, in the second and subsequent passes, substantial rolling in which the rolling reduction is 1.5% or more is performed on the cast material from which the scale was removed. The formation of flaws on the surface of the cast material is suppressed by removing the scale before performing the substantial rolling.
In the present embodiment, the rolling reduction in the initial pass in which substantial rolling in which the rolling reduction is 1.5% or more is performed during blooming is defined as an initial substantial rolling reduction R1 (%). In a case where soft reduction in which the rolling reduction is less than 1.5% is performed for the purpose of removing scale in the first pass, the rolling reduction in the initial pass in which the rolling reduction is 1.5% or more among the passes from the second pass onward is defined as the initial substantial rolling reduction R1. On the other hand, in a case where rolling with a rolling reduction of 1.5% or more is performed from the rolling in the first pass, the rolling reduction of the first pass is defined as the initial substantial rolling reduction R1.
Here, the initial substantial rolling reduction R1 (%) is defined by the following equation. Initial substantial rolling reduction R1 (%) = {1 - cross-sectional area perpendicular to rolling direction of starting material immediately after rolling with initial substantial rolling reduction/cross-sectional area perpendicular to rolling direction of starting material immediately before rolling with initial substantial rolling reduction)} × 100
For example, in the blooming, in a case where soft reduction with a rolling reduction of less than 1.5% is performed in the first pass, and thereafter rolling with a rolling reduction of 1.5% or more is performed in the second pass, the initial substantial rolling reduction R1 is the rolling reduction of the second pass. In this case, the initial substantial rolling reduction R1 is given by the following equation. Initial substantial rolling reduction R1 (%) = {1 - (cross-sectional area perpendicular to rolling direction of starting material after rolling of second pass/cross-sectional area perpendicular to rolling direction of starting material after rolling of first pass)} × 100
FIG. 5 is a view illustrating the relation between the initial substantial rolling reduction R1 in the blooming process and LID of δ-ferrite in the martensitic stainless steel material that is the end product. FIG. 5 was prepared based on the results of examples that are described later. Referring to FIG. 5, as the initial substantial rolling reduction R1 increases, LID rapidly decreases. Then, when the initial substantial rolling reduction R1 becomes 3.4% or more, the decrease in LID that accompanies an increase in the initial substantial rolling reduction R1 becomes somewhat gradual. When the initial substantial rolling reduction R1 becomes 9.0% or more, the decrease in LID that accompanies an increase in the initial substantial rolling reduction R1 becomes even more gradual. Accordingly, in the graph in FIG. 5, there is an inflection point in the vicinity of the initial substantial rolling reduction R1 = 3.4%, and in the vicinity of the initial substantial rolling reduction R1 = 9.0%, respectively.
When the initial substantial rolling reduction R1 is 3.4% or more, the rolling that is initially performed in the blooming is sufficiently strong. Therefore, δ-ferrite in the starting material can be made sufficiently fine by the rolling. Therefore, LID of the δ-ferrite in the martensitic stainless steel material that is the end product is 10.5 or less. Accordingly, the initial substantial rolling reduction R1 is to be 3.4% or more.
A preferable lower limit of the initial substantial rolling reduction R1 is 5.0%, more preferably is 6.0%, further preferably is 7.0%, and further preferably is 8.0%.
A further preferable lower limit of the initial substantial rolling reduction R1 is 9.0%. If the initial substantial rolling reduction R1 is 9.0% or more, LID will be 3.5 or less. In such a case, the low-temperature toughness will markedly increase.

[(Process 3) Hot working process]

In the hot working process, the starting material produced in the blooming process 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 hollow 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, the piercing-rolling can be performed by the following method. 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 cast 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 4) Heat treatment process (quenching process and tempering process)]

The heat treatment process includes a quenching process and a tempering process. Hereunder, the quenching process and tempering process are described.

[Quenching process]

In the quenching process, the intermediate steel material produced by the hot working process is subjected to quenching (quenching process). The quenching is performed by a well-known method. Specifically, the intermediate steel material after the hot working process is charged into a heat treatment furnace and held at a quenching temperature. The quenching temperature is equal to or higher than the A_c3 transformation point, and for example is 900 to 1000°C. After being held at the quenching temperature, the intermediate steel material is rapidly cooled (quenched). Although not particularly limited, the holding time at the quenching temperature is, for example, 10 to 60 minutes. 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 steel pipe 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.
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.

[Tempering process]

In the tempering process, the intermediate steel material after quenching is further 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 or more (758 MPa or more).
As mentioned above, in the tempering process, a tempering temperature T2 and a holding time t2 at the tempering temperature T2 can be appropriately adjusted according to the set yield strength and chemical composition. Although not particularly limited, the tempering temperature T2 and the holding time t2 are, for example, as follows.

(Condition 4) Tempering temperature T2: 500 to 650°C
(Condition 5) Holding time t2: 20 to 120 minutes

However, the tempering temperature T2 and the holding time t2 are not limited to the ranges described above. It suffices to appropriately adjust the holding time t2 at the tempering temperature T2 according to the set yield strength and chemical composition.
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. Even in such a case, as long as the martensitic stainless steel material satisfies Feature 1 to Feature 4, the martensitic stainless steel material will have a high yield strength of 758 MPa or more, excellent SSC resistance, and excellent low-temperature toughness.

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
1	0.009	0.20	0.44	0.015	0.0010	12.86	6.23	2.45	0.026	0.05	0.0111	0.134
2	0.013	0.36	0.38	0.020	0.0006	12.99	6.30	2.45	0.030	0.06	0.0149	0.105
3	0.010	0.25	0.39	0.018	0.0005	12.70	6.52	2.60	0.034	0.05	0.0090	0.113
4	0.029	0.28	0.42	0.017	0.0007	12.87	7.02	2.65	0.027	0.04	0.0207	0.090
5	0.014	0.94	0.35	0.014	0.0010	12.54	5.62	1.99	0.022	0.03	0.0211	0.122
6	0.008	0.08	0.48	0.016	0.0008	13.11	6.22	2.22	0.023	0.04	0.0236	0.142
7	0.007	0.24	0.95	0.016	0.0009	12.88	5.98	2.10	0.025	0.04	0.0050	0.133
8	0.010	0.20	0.08	0.018	0.0009	12.30	5.48	2.35	0.023	0.05	0.0089	0.128
9	0.009	0.22	0.36	0.029	0.0006	12.99	6.02	2.34	0.038	0.06	0.0152	0.129
10	0.013	0.28	0.40	0.010	0.0048	13.38	6.40	3.10	0.040	0.03	0.0189	0.137
11	0.011	0.35	0.40	0.023	0.0004	12.45	5.68	3.03	0.016	0.28	0.0253	0.136
12	0.015	0.25	0.35	0.022	0.0005	12.55	6.37	1.88	0.041	0.01	0.0221	0.130
13	0.007	0.25	0.38	0.019	0.0008	12.44	7.01	2.69	0.042	0.05	0.0115	0.023
14	0.012	0.25	0.44	0.014	0.0009	12.61	7.48	2.40	0.038	0.05	0.0152	0.091
15	0.015	0.28	0.44	0.018	0.0010	11.96	5.10	1.85	0.030	0.04	0.0180	0.112
16	0.011	0.24	0.36	0.015	0.0009	13.95	7.02	2.55	0.031	0.04	0.0305	0.108
17	0.007	0.28	0.38	0.009	0.0008	10.11	6.50	3.08	0.038	0.06	0.0159	0.099
18	0.007	0.23	0.38	0.010	0.0006	13.00	6.05	3.44	0.025	0.06	0.0336	0.118
19	0.015	0.24	0.37	0.017	0.0009	13.13	5.27	1.19	0.023	0.04	0.0077	0.102
20	0.011	0.23	0.41	0.009	0.0005	12.87	5.53	3.00	0.035	0.06	0.0389	0.107
21	0.012	0.25	0.42	0.017	0.0009	12.22	6.63	1.58	0.036	0.04	0.0038	0.105
22	0.009	0.27	0.42	0.012	0.0008	13.39	7.15	2.70	0.024	0.05	0.0188	0.110
23	0.011	0.24	0.38	0.010	0.0007	13.26	5.92	2.10	0.030	0.06	0.0375	0.091
24	0.011	0.26	0.39	0.018	0.0007	12.09	6.05	2.33	0.035	0.04	0.0285	0.119
25	0.012	0.27	0.38	0.011	0.0009	12.35	5.85	3.18	0.037	0.03	0.0213	0.110
26	0.013	0.20	0.34	0.015	0.0006	13.10	6.35	2.66	0.035	0.03	0.0080	0.122
27	0.016	0.25	0.38	0.016	0.0008	12.67	5.11	1.66	0.033	0.04	0.0095	0.123
28	0.010	0.24	0.44	0.018	0.0009	12.08	6.75	3.23	0.024	0.06	0.0135	0.128
29	0.031	0.24	0.42	0.013	0.0006	12.87	5.92	2.28	0.026	0.06	0.0324	0.106
30	0.008	0.29	0.39	0.009	0.0008	11.70	7.70	1.83	0.028	0.05	0.0204	0.094
31	0.008	0.26	0.40	0.014	0.0005	12.22	4.87	1.98	0.035	0.04	0.0303	0.103
32	0.010	0.28	0.40	0.013	0.0008	14.14	6.89	2.13	0.022	0.04	0.0167	0.094
33	0.015	0.24	0.41	0.013	0.0007	9.79	5.27	1.73	0.024	0.06	0.0177	0.119
34	0.015	0.26	0.39	0.009	0.0010	12.09	7.02	3.62	0.020	0.04	0.0168	0.112
35	0.013	0.25	0.37	0.017	0.0007	12.87	5.46	1.00	0.032	0.05	0.0311	0.102
36	0.014	0.26	0.39	0.011	0.0005	12.87	6.11	2.23	0.032	0.06	0.0125	0.107
37	0.008	0.24	0.38	0.014	0.0005	12.35	6.83	1.88	0.026	0.03	0.0326	0.108
38	0.011	0.25	0.44	0.017	0.0006	13.52	5.40	1.80	0.035	0.04	0.0096	0.098
39	0.013	0.28	0.36	0.011	0.0009	12.22	7.09	1.68	0.019	0.03	0.0183	0.116

[Table 1-2]

TABLE 1-2

Test Number	Chemical Composition (unit is mass%; balance is Fe and impurities)
	Cu	Co	Nb	W	Sn	As	Sb	B	Ca	Mg	REM
1	2.31	0.250	-	-	-	-	-	-	-	-	-
2	2.22	0.240	-	-	-	-	-	-	-	-	-
3	2.01	0.220	-	-	-	-	-	-	-	-	-
4	1.68	0.338	-	-	-	-	-	-	-	-	-
5	2.34	0.212	-	-	-	-	-	-	-	-	-
6	2.33	0.153	-	-	-	-	-	-	-	-	-
7	2.28	0.308	-	-	-	-	-	-	-	-	-
8	2.26	0.320	-	-	-	-	-	-	-	-	-
9	2.57	0.327	-	-	-	-	-	-	-	-	-
10	2.66	0.249	-	-	-	-	-	-	-	-	-
11	2.15	0.263	-	-	-	-	-	-	-	-	-
12	2.02	0.222	-	-	-	-	-	-	-	-	-
13	2.26	0.204	-	-	-	-	-	-	-	-	-
14	2.40	0.158	0.04	-	-	-	-	-	-	-	-
15	2.84	0.230	-	0.37	-	-	-	-	-	-	-
16	3.30	0.302	0.02	0.51	-	-	-	-	-	-	-
17	1.98	0.218	-	-	-	-	-	-	-	-	-
18	1.88	0.146	-	-	-	-	-	0.0016	0.0016	-	-
19	2.64	0.270	-	-	-	-	-	-	0.0049	0.0016	-
20	2.76	0.180	-	-	-	-	-	-	0.0028	-	0.0038
21	3.36	0.342	0.02	0.78	-	-	-	0.0004	0.0036	-	-
22	3.42	0.170	0.04	0.35	-	-	-	0.0037	0.0034	-	0.0026
23	1.07	0.252	0.05	0.65	-	-	-	0.0025	0.0028	0.0028	0.0056
24	2.78	0.470	0.03	0.94	0.0030	0.0020	0.0040	0.0039	0.0046	0.0021	0.0065
25	1.30	0.012	0.05	-	0.0010	0.0020	0.0050	-	-	-	-
26	2.08	0.310	-	-	-	-	-	-	-	-	-
27	2.18	0.110	-	-	-	-	-	-	-	-	-
28	1.48	0.240	0.03	0.89	0.0020	0.0020	0.0050	0.0035	0.0020	0.0011	0.0042
29	1.48	0.196	-	-	-	-	-	-	-	-	-
30	1.58	0.352	-	-	-	-	-	-	-	-	-
31	1.80	0.198	-	-	-	-	-	-	-	-	-
32	2.08	0.318	-	-	-	-	-	-	-	-	-
33	2.12	0.172	-	-	-	-	-	-	-	-	-
34	1.72	0.238	-	-	-	-	-	-	-	-	-
35	2.62	0.318	-	-	-	-	-	-	-	-	-
36	3.75	0.302	-	-	-	-	-	-	-	-	-
37	0.94	0.352	-	-	-	-	-	-	-	-	-
38	2.48	0.530	-	-	-	-	-	-	-	-	-
39	3.04	0.008	-	-	-	-	-	-	-	-	-

In Table 1-2, the symbol "-" means that the content of the corresponding element was 0% when a fraction of the numerical value described in Table 1-2 was rounded off. For example, "-" means that the content of Nb in Test No. 1 was 0% when rounded off to the second 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 B in Test No. 1 was 0% when rounded off to the fourth decimal place.
The molten steels were subjected to continuous casting as the cast material preparation process to produce blooms. Thereafter, the blooming process was performed. In the blooming process, first, each bloom was heated in a heating furnace. The heating temperature T1 (°C) is shown in the column "Heating Temperature T1 (°C)" of the column "Blooming Process" in Table 2, and the holding time t1 (min) at the heating temperature T1 is shown in the column "Holding Time t1 (min)" of the column "Blooming Process" in Table 2.

[Table 2]

TABLE 2

Test Number	Blooming Process			Tempering Process
Test Number	Heating Temperature T1 (°C)	Holding Time t1 (min)	Initial Substantial Rolling Reduction R1 (%)	Tempering Temperature T2 (°C)	Holding Time t2 (min)
1	1255	365	4.3	633	45
2	1255	432	4.6	640	66
3	1270	325	9.3	618	50
4	1255	442	6.5	616	53
5	1255	340	6.0	586	61
6	1260	315	5.6	597	52
7	1260	356	12.5	603	55
8	1270	462	5.0	565	48
9	1255	455	6.5	578	81
10	1280	402	7.5	623	43
11	1250	436	6.2	633	70
12	1260	311	7.6	641	41
13	1255	333	4.9	581	65
14	1260	418	13.9	647	40
15	1250	325	7.1	604	81
16	1270	456	6.7	592	47
17	1280	490	16.6	592	83
18	1255	335	7.7	616	62
19	1275	422	9.6	592	85
20	1250	305	23.2	649	54
21	1280	398	8.4	592	86
22	1290	475	5.0	623	32
23	1265	328	6.5	604	47
24	1270	400	7.0	634	87
25	1265	314	5.5	616	86
26	1255	333	2.8	620	35
27	1260	424	2.5	578	41
28	1275	321	2.0	605	33
29	1270	369	6.8	649	51
30	1250	310	22.2	643	77
31	1255	420	7.9	598	50
32	1285	336	3.4	647	71
33	1275	372	19.9	580	41
34	1275	341	3.4	628	55
35	1265	431	18.7	641	72
36	1270	470	6.2	604	90
37	1250	472	19.2	610	35
38	1285	343	3.6	604	71
39	1260	408	19.4	641	53

Each heated bloom was subjected to blooming to produce a cylindrical starting material (round billet) having a diameter of 310 mm. At such time, the initial substantial rolling reduction R1 (%) in the blooming was as shown in the column "Initial Substantial Rolling Reduction R1 (%)" of the column "Blooming Process" in Table 2.
The starting material produced in the blooming process 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 a hollow shell (seamless steel pipe) 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 the heat treatment process (quenching process and tempering process). In the quenching process, the quenching temperature was set to 910°C, and the holding time at the quenching temperature was set to 15 minutes. In the tempering process, the tempering temperature T2 (°C) was set as shown in the column "Tempering Temperature T2 (°C)" of the column "Tempering Process" in Table 2, and the holding time t2 (min) at the tempering temperature T2 was set as shown in the column "Holding Time t2 (min)" of the column "Tempering Process" in Table 2. The yield strength was adjusted by the heat treatment process. 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) SSC resistance evaluation test
(Test 4) Test to evaluate area fraction of δ-ferrite
(Test 5) Test to evaluate LID of δ-ferrite
(Test 6) 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 the rolling direction (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 (%)	δ-Ferrite				Absorbed Energy vE (-10°C) (J)	SSC Resistance	Remarks
Test Number	YS (MPa)	Martensite Volume Ratio (%)	Area Fraction AR (%)	Length L (µm)	Distance D (µm)	L/D	Absorbed Energy vE (-10°C) (J)	SSC Resistance	Remarks
1	763	87	3.67	76.1	7.4	10.3	95	E	Inventive Example
2	771	85	4.56	80.5	8.6	9.4	87	E	Inventive Example
3	799	95	2.30	57.5	19.8	2.9	205	E	Inventive Example
4	758	87	1.41	68.9	8.6	8.0	84	E	Inventive Example
5	802	95	4.44	73.8	7.7	9.6	77	E	Inventive Example
6	789	95	1.23	63.3	15.6	4.1	122	E	Inventive Example
7	768	88	0.87	23.5	38.1	0.6	233	E	Inventive Example
8	940	99	1.58	68.0	12.1	5.6	91	E	Inventive Example
9	854	98	3.87	82.0	10.9	7.5	64	E	Inventive Example
10	887	96	2.88	58.6	14.1	4.2	154	E	Inventive Example
11	935	95	4.55	101.2	13.4	7.6	80	E	Inventive Example
12	800	95	1.44	87.5	18.2	4.8	124	E	Inventive Example
13	859	97	1.30	68.9	8.7	7.9	115	E	Inventive Example
14	760	89	1.40	28.9	36.4	0.8	230	E	Inventive Example
15	828	97	1.47	72.3	8.7	8.3	72	E	Inventive Example
16	935	96	3.25	63.4	14.0	4.5	162	E	Inventive Example
17	876	97	0.94	17.7	50.6	0.3	266	E	Inventive Example
18	982	94	4.71	116.7	15.0	7.8	80	E	Inventive Example
19	972	97	1.60	30.8	25.7	1.2	201	E	Inventive Example
20	770	93	0.04	9.0	67.0	0.1	256	E	Inventive Example
21	780	94	3.99	97.6	14.0	7.0	132	E	Inventive Example
22	988	95	1.60	90.7	9.4	9.6	112	E	Inventive Example
23	771	94	3.51	67.4	13.6	5.0	155	E	Inventive Example
24	812	93	1.01	61.5	14.3	4.3	149	E	Inventive Example
25	941	91	4.18	83.6	12.0	7.0	168	E	Inventive Example
26	824	88	4.90	109.0	9.8	11.1	32	E	Comparative Example
27	861	96	4.10	123.0	9.4	13.1	18	E	Comparative Example
28	818	93	3.80	118.0	8.1	14.6	15	E	Comparative Example
29	848	88	1.16	65.0	14.3	4.5	33	E	Comparative Example
30	719	86	0.04	13.0	74.0	0.2	315	E	Comparative Example
31	786	-	-	-	-	-	51	NA	Comparative Example
32	892	94	4.53	68.0	12.3	5.5	30	E	Comparative Example
33	788	-	-	-	-	-	299	NA	Comparative Example
34	886	95	3.53	57.0	7.0	8.1	27	E	Comparative Example
35	861	-	-	-	-	-	324	NA	Comparative Example
36	1105	94	2.46	47.9	16.3	2.9	38	E	Comparative Example
37	780	-	-	-	-	-	287	NA	Comparative Example
38	875	94	4.49	103.9	7.1	14.6	21	E	Comparative Example
39	802	-	-	-	-	-	303	NA	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. 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) 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]. 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 6.35 mm and the length of the parallel portion was 25.4 mm. The longitudinal direction of the round bar specimen was parallel to the rolling direction (pipe axis direction) of the martensitic stainless steel material (seamless steel pipe).
After the test specimen was 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, and if cracking was not confirmed as a result of the observation, it was determined that the test specimen was excellent in SSC resistance (indicated by "E (Excellent)" in the column "SSC Resistance" in Table 3). On the other hand, if cracking was confirmed as a result of the observation, it was determined that excellent SSC resistance was not obtained (indicated by "NA (Not Accepted)" in the column "SSC Resistance" in Table 3). Note that, Test 2, Test 4, and Test 5 were not performed for the test numbers in which excellent SSC resistance was not obtained.

[(Test 4) Test to evaluate area fraction of δ-ferrite]

The area fraction AR of δ-ferrite in a cross section parallel to the rolling direction Z of the martensitic stainless steel material of each test number was determined based on the above-described [Method for measuring area fraction AR 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 Z (pipe axis direction) and the wall thickness direction of the martensitic stainless steel material (seamless steel pipe). Further, with respect to the observation surface of the test specimen, the sides parallel to the rolling direction Z (pipe axis direction) were made the long sides, and the sides parallel to the wall thickness direction were made the short sides. The determined area fraction AR (%) of δ-ferrite is shown in the column "Area Fraction AR (%)" of the column "δ-Ferrite" in Table 3.

[(Test 5) Test to evaluate LID of δ-ferrite]

The length L (µm), distance D (µm), and LID of δ-ferrite in a cross section parallel to the rolling direction Z (pipe axis direction) of the martensitic stainless steel material (seamless steel pipe) of each test number were determined based on the above-described [Method for measuring LID of δ-ferrite]. The determined length L (µm), distance D (µm), and LID of δ-ferrite are shown in the column "Length L (µm)", the column "Distance D (µm)", and the column "L/D", respectively, of the column "δ-Ferrite" in Table 3.

[(Test 6) 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]. 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 each of Test Nos. 1 to 25 the martensitic stainless steel material satisfied Feature 1 to Feature 4. Therefore, even though the yield strength was 758 MPa or more, the absorbed energy at -10°C was 40 J or more, and excellent low-temperature toughness was obtained. In addition, excellent SSC resistance was obtained.
On the other hand, in Test Nos. 26 to 28, although the content of each element in the chemical composition was appropriate, the initial substantial rolling reduction R1 in the blooming process was too low. Therefore, although Test Nos. 26 to 28 satisfied Feature 1, Feature 2, and Feature 3, LID of the δ-ferrite was more than 10.5 and thus Feature 4 was not satisfied. As a result, the absorbed energy at -10°C was less than 40 J and excellent low-temperature toughness was not obtained.
In Test No. 29, the content of C was too high. Therefore, the absorbed energy at -10°C was less than 40 J and excellent low-temperature toughness was not obtained.
In Test No. 30, the content of Ni was too high. Therefore, the yield strength was less than 758 MPa.
In Test No. 31, the content of Ni was too low. Therefore, excellent SSC resistance was not obtained.
In Test No. 32, the content of Cr was too high. Therefore, the absorbed energy at -10°C was less than 40 J and excellent low-temperature toughness was not obtained.
In Test No. 33, the content of Cr was too low. Therefore, excellent SSC resistance was not obtained.
In Test No. 34, the content of Mo was too high. Therefore, the absorbed energy at -10°C was less than 40 J and excellent low-temperature toughness was not obtained.
In Test No. 35, the content of Mo was too low. Therefore, excellent SSC resistance was not obtained.
In Test No. 36, the content of Cu was too high. Therefore, the absorbed energy at -10°C was less than 40 J and excellent low-temperature toughness was not obtained.
In Test No. 37, the content of Cu was too low. Therefore, excellent SSC resistance was not obtained.
In Test No. 38, the content of Co was too high, and LID of the δ-ferrite was more than 10.5. Therefore, the absorbed energy at -10°C was less than 40 J and excellent low-temperature toughness was not obtained.
In Test No. 39, 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 14.00%,

Ni: 5.00 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: 1.00 to 3.50%,

Co: 0.010 to 0.500%,

Nb: 0 to 0.15%,

W: 0 to 1.50%,

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

in a cross section parallel to a rolling direction of the martensitic stainless steel material,

an area fraction of δ-ferrite is 5.00% or less, and

a length L (µm) of the δ-ferrite in the rolling direction, and a distance D (µm) between a plurality of particles of the δ-ferrite in the rolling direction satisfy Formula (1): $L / D \leq 10.5$
The martensitic stainless steel material according to claim 1, wherein the chemical composition contains one or more elements selected from a group consisting of:
Nb: 0.01 to 0.15%,

W: 0.01 to 1.50%,

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%.