WO2026013963A1 - Martensitic stainless steel material - Google Patents

Abstract

Provided is a martensitic stainless steel material having excellent SSC resistance. A martensitic stainless steel material according to the present disclosure has the chemical composition described in the specification. In the steel material, the number density ND1 of fine and low S inclusions, which contain S and have an Fn1 defined by equation (1) of at least 5.0 and an equivalent circle diameter of 2.0-10.0 μm, is at least 3.0/mm², the number density ND2 of coarse and low S inclusions, which contain S and have an Fn1 of at least 5.0 and an equivalent circle diameter of greater than 10.0 μm, is at least 2.0/mm², and the number density ND3 of high S inclusions, which contain S and have an Fn1 of less than 5.0 and an equivalent circle diameter of at least 2.0 μm, is 2.0/mm² or less.　(1): Fn1=(Mg+Al+Ca+Ti+V+Mn)/S, where the element symbols in equation (1) are substituted with the contents of the corresponding elements in the inclusions in mass%.

Description

Martensitic stainless steel

This disclosure relates to steel materials, and more specifically to martensitic stainless steel materials.

Oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") may contain large amounts of corrosive substances. Examples of corrosive substances are corrosive gases such as hydrogen sulfide and carbon dioxide. In this specification, an environment containing hydrogen sulfide and carbon dioxide is referred to as a "sour environment." When steel is used in a sour environment, an electrochemical reaction occurs when the steel surface comes into contact with the corrosive substance, generating hydrogen on the steel surface. This hydrogen makes the steel susceptible to sulfide stress corrosion cracking (SSC). Therefore, steel used in a sour environment requires excellent SSC resistance.

It is known that chromium (Cr) is effective in improving the carbon dioxide corrosion resistance of steel. Therefore, in oil wells in environments containing a large amount of carbon dioxide, martensitic stainless steels containing about 13 mass% Cr, such as API L80 13Cr steel (normal 13Cr steel) or Super 13Cr steel with a reduced C content, are used depending on the partial pressure and temperature of the carbon dioxide. 13Cr steel and Super 13Cr steel are mainly used in oil wells in sour environments where the _H2S partial pressure is 0.03 bar or less.

Japanese Patent Publication No. 2000-192196 (Patent Document 1) and Japanese Patent Publication No. 2012-136742 (Patent Document 2) propose steel materials with excellent SSC resistance.

The martensitic stainless steel material in Patent Document 1 contains, by weight, the following: C: 0.001-0.05%, Si: 0.05-1%, Mn: 0.05-2%, P: 0.025% or less, S: 0.01% or less, Cr: 9-14%, Mo: 3.1-7%, Ni: 1-8%, Co: 0.5-7%, sol. Al: 0.001-0.1%, N: 0.05% or less, O (oxygen): 0.01% or less, Cu: 0-5%, W: 0-5%, with the balance consisting of Fe and unavoidable impurities. The inclusion of Mo lowers the Ms point. Therefore, by including Co along with Mo, the decrease in Ms point is suppressed and the microstructure becomes a martensitic single-phase structure. Patent Document 1 states that this improves SSC resistance.

The martensitic stainless steel material in Patent Document 2 contains, by mass%, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1-2.0%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0-15.5%, Ni: 5.5-7.0%, Mo: 2.0-3.5%, Cu: 0.3-3.5%, V: 0.20% or less, Al: 0.05% or less, N: 0.06% or less, with the balance being Fe and unavoidable impurities. In the martensitic stainless steel material in this document, the C content is set to 0.01% or less, and Cr, Ni, and Mo are adjusted to appropriate ranges. Furthermore, appropriate amounts of Cu and V or appropriate amounts of W are also included. Patent Document 2 states that this results in excellent SSC resistance.

Japanese Patent Application Laid-Open No. 2000-192196 JP 2012-136742 A

Patent Documents 1 and 2 mentioned above propose a means of improving SSC resistance in sour environments by adjusting the element content in the chemical composition. However, the SSC resistance of steel materials in sour environments may also be improved by means other than those proposed in the above patent documents.

The purpose of this disclosure is to provide a martensitic stainless steel material with excellent SSC resistance.

The martensitic stainless steel material of the present disclosure has a chemical composition, in mass%, of C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.10 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 10.0 to 14.0%, Ni: 5.00 to 7.50%, Mo: 1.50 to 3.50%, Cu: 0.01 to 2.50%, Ti: 0.02 to 0.40%, V: 0.01 to 0.050%, and 0.50%, Co: 0.01 to 0.50%, Al: 0.010 to 0.100%, Ca: 0.0001 to 0.0040%, N: 0.001 to 0.020%, O: 0.010% or less, W: 0 to 1.50%, Sn: 0 to 0.010%, Nb: 0 to 0.50%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.100%, and the balance being Fe and impurities. In a martensitic stainless steel material, the number density ND1 of fine low-S inclusions, which contain S and have an Fn1 defined by formula (1) of 5.0 or more and an equivalent circle diameter of 2.0 to 10.0 μm, is 3.0 pieces/ ^mm2 or more, the number density ND2 of coarse low-S inclusions, which contain S and have an Fn1 of 5.0 or more and an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces/ ^mm2 or less, and the number density ND3 of high-S inclusions, which contain S and have an Fn1 of less than 5.0 and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces/ ^mm2 or less.
Fn1=(Mg+Al+Ca+Ti+V+Mn)/S (1)
Here, the element symbols in formula (1) are substituted with the content of the corresponding element in the inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol.

The martensitic stainless steel material disclosed herein has excellent SSC resistance.

FIG. 1 is a schematic diagram for explaining the positional relationship between molten steel in a mold and a submerged entry nozzle in continuous casting.

The inventors conducted research into martensitic stainless steel materials with excellent SSC resistance.

The inventors first investigated martensitic stainless steel materials with excellent SSC resistance from the perspective of chemical composition. As a result, the following composition was found to be desirable: C: 0.030% or less, Si: 0.10-1.00%, Mn: 0.10-2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 10.0-14.0%, Ni: 5.00-7.50%, Mo: 1.50-3.50%, Cu: 0.01-2.50%, Ti: 0.02-0.40%, V: 0.01-0.50%, Co: 0.01-0.50%, Al: 0.010-0.100%, Ca: The inventors believed that a martensitic stainless steel material with a chemical composition consisting of 0.0001-0.0040%, N: 0.001-0.020%, O: 0.010% or less, W: 0-1.50%, Sn: 0-0.010%, Nb: 0-0.50%, B: 0-0.0100%, Mg: 0-0.0100%, rare earth elements: 0-0.100%, and the balance being Fe and impurities, could potentially achieve excellent SSC resistance.

The inventors further investigated means for achieving excellent SSC resistance from the perspective of microstructure.

The inventors first investigated the mechanism by which SSC occurs in martensitic stainless steel materials in sour environments. As a result, the following was discovered:

In sour environments, some of the inclusions present on the surface of martensitic stainless steel materials may dissolve, forming dents on the steel surface. These dents are thought to promote the occurrence of SSC in sour environments.

In a sour environment, among the inclusions in the surface layer of martensitic stainless steel materials, inclusions with a high S content, such as Mn sulfides or Ca sulfides, tend to dissolve preferentially. In the following explanation, inclusions with a high S content will be referred to as "high S inclusions." A specific definition of high S inclusions will be provided later.

High-S inclusions in the surface layer of martensitic stainless steel dissolve in a sour environment, forming pits. The pits formed by high-S inclusions promote the occurrence of SSC in a sour environment. Therefore, if the number density of high-S inclusions in martensitic stainless steel can be reduced, the occurrence of SSC in a sour environment can be suppressed.

In order to reduce the number density of high-S inclusions, it is effective to reduce the S content in the steel as much as possible. Therefore, the inventors attempted to improve SSC resistance by reducing the S content in the steel as much as possible. However, not only is there a limit to how much S content can be reduced in steel, but excessive reduction of the S content significantly increases manufacturing costs.

Therefore, rather than reducing the number density of high-S inclusions by minimizing the S content, the inventors considered reducing the number density of high-S inclusions by other means while allowing a certain amount of S content in the steel.

Here, the inventors have focused on composite inclusions, a viewpoint different from conventional ones. Composite inclusions are inclusions formed by the aggregation of multiple types of inclusions. Examples of multiple types of inclusions include oxides such as _Al2O3 and _MgO , sulfides such as MnS and CaS, and nitrides such as TiN. Note that TiN may also form a solid solution with V. Inclusions other than oxides, sulfides, and nitrides may combine with oxides, sulfides, nitrides, etc. to form composite inclusions.

The S content in composite inclusions is lower than in high-S inclusions. Therefore, in a sour environment, composite inclusions are less likely to dissolve than high-S inclusions. Even if composite inclusions do dissolve, the parts of the composite inclusions containing S dissolve locally. As a result, the depressions that form are very small. These very tiny depressions are easily repassivated and do not become the starting point for SSC. Therefore, if S is incorporated into the composite inclusions, the depressions can be kept small even after dissolution, improving SSC resistance. Furthermore, if the number density of composite inclusions is increased, S will be incorporated into the composite inclusions. This reduces the amount of S available for generating high-S inclusions. As a result, the number density of high-S inclusions decreases.

Inclusions have a sufficiently low S content when Fn1, as defined by formula (1), is 5.0 or more. Such inclusions are considered to be composite inclusions. In the following description, inclusions containing S and having an Fn1 of 5.0 or more will be referred to as "low-S inclusions."
Fn1=(Mg+Al+Ca+Ti+V+Mn)/S (1)
Here, the element symbols in formula (1) are substituted with the content of the corresponding element in the inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol.

As described above, it is believed that if the number density of low-S inclusions in martensitic stainless steel is increased, the depressions caused by dissolution can be kept to a minimum, improving SSC resistance. Increasing the number density of low-S inclusions also causes more S to be incorporated into the low-S inclusions. This reduces the amount of S available for forming high-S inclusions. As a result, the number density of high-S inclusions can be sufficiently reduced, further improving SSC resistance.

Based on the above considerations, the inventors attempted to increase the number density of low-S inclusions in martensitic stainless steel materials. As a result, they discovered that increasing the number density of low-S inclusions in martensitic stainless steel materials can sufficiently reduce the number density of high-S inclusions.

However, even when the number density of low-S inclusions was increased and the number density of high-S inclusions was reduced, there were still cases where sufficient SSC resistance was not achieved in sour environments. Therefore, the inventors conducted further research, and as a result, the following was discovered.

In sour environments, even low-S inclusions, coarse low-S inclusions with an equivalent circle diameter of more than 10.0 μm, are easily dissolved. Therefore, by increasing the number density of fine low-S inclusions with an equivalent circle diameter of 10.0 μm or less and reducing the number density of high-S inclusions, while also reducing the number density of coarse low-S inclusions with an equivalent circle diameter of more than 10.0 μm, the SSC resistance of martensitic stainless steel materials in sour environments can be improved.

Based on the above findings, the present inventors conducted further studies and found that excellent SSC resistance can be obtained in a martensitic stainless steel material having the above-mentioned chemical composition if the number density ND1 of fine low-S inclusions, which contain S and have an Fn1 defined by formula (1) of 5.0 or more and have an equivalent circle diameter of 2.0 to 10.0 μm, is 3.0/ ^mm2 or more, the number density ND2 of coarse low-S inclusions, which contain S and have an Fn1 of 5.0 or more and have an equivalent circle diameter of more than 10.0 μm, is 2.0/ ^mm2 or less, and the number density ND3 of high-S inclusions, which contain S and have an Fn1 of less than 5.0 and have an equivalent circle diameter of 2.0 μm or more, is 2.0/ ^mm2 or less.

The martensitic stainless steel material of this embodiment, developed based on the above findings, has the following configuration.

The martensitic stainless steel material of the first configuration has a chemical composition, in mass %, of C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.10 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 10.0 to 14.0%, Ni: 5.00 to 7.50%, Mo: 1.50 to 3.50%, Cu: 0.01 to 2.50%, Ti: 0.02 to 0.40%, V: 0.01 The martensitic stainless steel material contains S, has an Fn1 value defined by formula (1) of 5.0 or more, and has a number density ND1 of 3.0 pieces/ ^mm2 or more of fine low-S inclusions having an equivalent circle diameter of 2.0 to 10.0 μm. Furthermore, the number density ND2 of coarse low-S inclusions, which contain S, have an Fn1 of 5.0 or more, and are inclusions with an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces/ ^mm2 or less. Furthermore, the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and are inclusions with an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces/ ^mm2 or less.
Fn1=(Mg+Al+Ca+Ti+V+Mn)/S (1)
Here, the element symbols in formula (1) are substituted with the content of the corresponding element in the inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol.

The martensitic stainless steel material of the second configuration is the martensitic stainless steel material of the first configuration, and has a chemical composition containing one or more elements selected from the group consisting of W: 0.01-1.50%, Sn: 0.001-0.010%, Nb: 0.01-0.50%, B: 0.0001-0.0100%, Mg: 0.0001-0.0100%, and REM: 0.001-0.100%.

The martensitic stainless steel material of the third configuration is a martensitic stainless steel material of the first or second configuration, and is a steel pipe.

The martensitic stainless steel material of this embodiment is described in detail below.

[Features of the martensitic stainless steel material according to this embodiment]
The martensitic stainless steel material of this embodiment satisfies the following features 1 and 2.
(Feature 1)
The chemical composition is, in mass%, C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.10 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 10.0 to 14.0%, Ni: 5.00 to 7.50%, Mo: 1.50 to 3.50%, Cu: 0.01 to 2.50%, Ti: 0.02 to 0.40%, V: 0.01 to 0.50%, Co: 0 .01 to 0.50%, Al: 0.010 to 0.100%, Ca: 0.0001 to 0.0040%, N: 0.001 to 0.020%, O: 0.010% or less, W: 0 to 1.50%, Sn: 0 to 0.010%, Nb: 0 to 0.50%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.100%, and the balance being Fe and impurities.

(Feature 2)
In a martensitic stainless steel material, the number density ND1 of fine low-S inclusions, which contain S and have an Fn1 defined by formula (1) of 5.0 or more and an equivalent circle diameter of 2.0 to 10.0 μm, is 3.0 pieces/ ^mm2 or more, the number density ND2 of coarse low-S inclusions, which contain S and have an Fn1 of 5.0 or more and an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces/ ^mm2 or less, and the number density ND3 of high-S inclusions, which contain S and have an Fn1 of less than 5.0 and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces/ ^mm2 or less.
Fn1=(Mg+Al+Ca+Ti+V+Mn)/S (1)
Here, the element symbols in formula (1) are substituted with the content of the corresponding element in the inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol.
The following describes Feature 1 and Feature 2. Note that, hereinafter, the martensitic stainless steel material will also be simply referred to as "steel material."

[(Feature 1) Chemical composition]
The chemical composition of the martensitic stainless steel material of this embodiment contains the following elements: "%" relating to elements means mass % unless otherwise specified.

C: 0.030% or less Carbon (C) is inevitably contained. That is, the C content is more than 0%. C improves the hardenability of the steel material and increases the strength of the steel material. However, if the C content exceeds 0.030%, the strength of the steel material becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the SSC resistance of the steel material decreases.
Therefore, the C content is 0.030% or less.
The lower limit of the C content is preferably 0.001%, more preferably 0.003%, even more preferably 0.005%, and still more preferably 0.007%.
The upper limit of the C content is preferably 0.027%, more preferably 0.025%, and even more preferably 0.020%.

Si: 0.10-1.00%
Silicon (Si) deoxidizes steel. If the Si content is less than 0.10%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Si content exceeds 1.00%, the hot workability of the steel material will be reduced even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Si content is 0.10 to 1.00%.
The lower limit of the Si content is preferably 0.11%, more preferably 0.12%, even more preferably 0.15%, and still more preferably 0.20%.
The upper limit of the Si content is preferably 0.95%, more preferably 0.90%, and even more preferably 0.85%.

Mn: 0.10-2.00%
Manganese (Mn) improves the hardenability of steel and increases its strength. If the Mn content is less than 0.10%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Mn content is excessive, a large number of coarse Mn sulfides are generated. In this case, the Mn sulfides dissolve in a sour environment, forming pits. These pits may become the starting points for SSC, causing SSC. If the Mn content exceeds 2.00%, the above-mentioned pits are generated, and SSC resistance decreases, even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Mn content is 0.10 to 2.00%.
The lower limit of the Mn content is preferably 0.15%, more preferably 0.20%, and even more preferably 0.30%.
The upper limit of the Mn content is preferably 1.90%, more preferably 1.80%, and even more preferably 1.70%.

P: 0.040% or less Phosphorus (P) is inevitably contained. In other words, the lower limit of the P content is more than 0%. P segregates at grain boundaries. Therefore, if the P content exceeds 0.040%, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the P content is 0.040% or less.
The P content is preferably as low as possible. However, excessive reduction in the P content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
The upper limit of the P content is preferably 0.035%, more preferably 0.030%, and even more preferably 0.025%.

S: 0.0050% or less Sulfur (S) is inevitably contained. In other words, the lower limit of the S content is more than 0%. S generates high-S inclusions, typified by sulfides. As described above, high-S inclusions dissolve in a sour environment and form pits. The pits of high-S inclusions cause SSC. If the S content exceeds 0.0050%, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the S content is 0.0050% or less.
The lower limit of the S content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.
The upper limit of the S content is preferably 0.0048%, more preferably 0.0045%, even more preferably 0.0040%, and still more preferably 0.0035%.

Cr:10.0~14.0%
Chromium (Cr) forms a passive film on the surface of the steel material, thereby improving the SSC resistance of the steel material. If the Cr content is less than 10.0%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Cr content exceeds 14.0%, δ (delta) ferrite is likely to form in the steel material, and in this case, the toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Cr content is 10.0 to 14.0%.
The lower limit of the Cr content is preferably 10.2%, more preferably 10.4%, and even more preferably 10.6%.
The upper limit of the Cr content is preferably 13.8%, more preferably 13.5%, even more preferably 13.2%, and still more preferably 13.0%.

Ni: 5.00-7.50%
Nickel (Ni) is an austenite-forming element and transforms the structure after quenching into martensite, thereby increasing the strength of the steel material. Ni also forms sulfides on the passive film in sour environments. Ni sulfides prevent chloride ions (Cl ⁻ ) and hydrogen sulfide ions (HS ⁻ ) from coming into contact with the passive film. Therefore, the passive film is less likely to be destroyed by chloride ions and hydrogen sulfide ions. As a result, the SSC resistance of the steel material is improved. If the Ni content is less than 5.00%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Ni content exceeds 7.50%, the hydrogen diffusion coefficient in the steel material decreases, and in this case, even if the contents of other elements are within the ranges of this embodiment, the SSC resistance of the steel material decreases.
Therefore, the Ni content is 5.00 to 7.50%.
The lower limit of the Ni content is preferably 5.10%, more preferably 5.20%, and even more preferably 5.30%.
The upper limit of the Ni content is preferably 7.30%, more preferably 7.20%, even more preferably 7.10%, and still more preferably 6.90%.

Mo: 1.50-3.50%
Molybdenum (Mo) forms sulfides on the passive film in sour environments. Mo sulfides prevent chloride ions (Cl ⁻ ) and hydrogen sulfide ions (HS ⁻ ) from coming into contact with the passive film, preventing the passive film from being destroyed by chloride ions and hydrogen sulfide ions. This improves the SSC resistance of the steel. Mo also dissolves in the steel to increase its strength. If the Mo content is less than 1.50%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Mo content exceeds 3.50%, it becomes difficult to stabilize austenite, and as a result, even if the contents of other elements are within the ranges of this embodiment, it becomes difficult to stably obtain a microstructure mainly composed of martensite.
Therefore, the Mo content is 1.50 to 3.50%.
The lower limit of the Mo content is preferably 1.55%, more preferably 1.60%, and even more preferably 1.80%.
The upper limit of the Mo content is preferably 3.45%, more preferably 3.40%, even more preferably 3.30%, and still more preferably 3.20%.

Cu: 0.01-2.50%
Copper (Cu) dissolves in steel to enhance the SSC resistance of the steel. Furthermore, Cu forms sulfides on the passive film in sour environments. Cu sulfides prevent chloride ions (Cl ⁻ ) and hydrogen sulfide ions (HS ⁻ ) from coming into contact with the passive film, thereby preventing the passive film from being destroyed by chloride ions and hydrogen sulfide ions. Therefore, the SSC resistance of the steel is enhanced. If the Cu content is less than 0.01%, the above-mentioned effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Cu content exceeds 2.50%, the hot workability of the steel material will be reduced even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Cu content is 0.01 to 2.50%.
The lower limit of the Cu content is preferably 0.02%, more preferably 0.05%, even more preferably 0.10%, and still more preferably 0.50%.
The upper limit of the Cu content is preferably 2.00%, more preferably 1.50%, and even more preferably 1.35%.

Ti: 0.02-0.40%
Titanium (Ti) combines with C or N to form precipitates. In this case, the pinning effect suppresses grain coarsening, increasing the strength of the steel. If the Ti content is less than 0.02%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Ti content exceeds 0.40%, δ-ferrite is likely to form, and in this case, the toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Ti content is 0.02 to 0.40%.
The lower limit of the Ti content is preferably 0.04%, more preferably 0.06%, and even more preferably 0.08%.
The upper limit of the Ti content is preferably 0.35%, more preferably 0.30%, and even more preferably 0.28%.

V:0.01~0.50%
Vanadium (V) improves the hardenability of steel and increases its strength. If the V content is less than 0.01%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the V content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material becomes excessively high, and the SSC resistance of the steel material decreases.
Therefore, the V content is 0.01 to 0.50%.
The lower limit of the V content is preferably 0.02%, more preferably 0.04%, and even more preferably 0.06%.
The upper limit of the V content is preferably 0.48%, more preferably 0.45%, and even more preferably 0.40%.

Co:0.01~0.50%
Cobalt (Co) forms sulfides on the passive film in sour environments. Co sulfides prevent chloride ions (Cl ⁻ ) and hydrogen sulfide ions (HS ⁻ ) from coming into contact with the passive film, preventing the passive film from being destroyed by chloride ions and hydrogen sulfide ions. This improves the SSC resistance of the steel. Co also improves the hardenability of the steel, ensuring stable high strength. Specifically, Co suppresses the formation of retained austenite and reduces the variation in strength of the steel. If the Co content is less than 0.01%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Co content exceeds 0.50%, the toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Co content is 0.01 to 0.50%.
The lower limit of the Co content is preferably 0.02%, more preferably 0.04%, and even more preferably 0.06%.
The upper limit of the Co content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.36%.

Al: 0.010-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.010%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Al content exceeds 0.100%, coarse oxides are formed, and in this case, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Al content is 0.010 to 0.100%.
The lower limit of the Al content is preferably 0.012%, more preferably 0.015%, and even more preferably 0.020%.
The upper limit of the Al content is preferably 0.095%, more preferably 0.090%, and even more preferably 0.085%.
The Al content in this specification means the content of sol. Al (acid-soluble Al).

Ca: 0.0001-0.0040%
Calcium (Ca) combines with S in the steel material to form Ca sulfides, which suppress the formation of coarse Mn sulfides. In this case, the SSC resistance of the steel material is improved. If the Ca content is less than 0.0001%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Ca content exceeds 0.0040%, Ca sulfides are formed in excess. In this case, even if the contents of other elements are within the ranges of this embodiment, the SSC resistance of the steel material decreases. Therefore, the Ca content is 0.0001 to 0.0040%.
The lower limit of the Ca content is preferably 0.0002%, more preferably 0.0005%, and still more preferably 0.0010%.
The upper limit of the Ca content is preferably 0.0035%, more preferably 0.0030%, and still more preferably 0.0025%.

N: 0.001-0.020%
Nitrogen (N) combines with Ti to form fine Ti nitrides. The fine TiN has a pinning effect, which suppresses grain coarsening. As a result, the strength of the steel material is increased. If the N content is less than 0.001%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the N content exceeds 0.020%, coarse nitrides are formed, and in this case, the toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the N content is 0.001 to 0.020%.
The lower limit of the N content is preferably 0.002%, more preferably 0.004%, and even more preferably 0.006%.
The upper limit of the N content is preferably 0.019%, more preferably 0.018%, and even more preferably 0.017%.

O: 0.010% or less Oxygen (O) is inevitably contained. That is, the lower limit of the O content is more than 0%. O forms oxides and reduces the toughness of the steel material. If the O content exceeds 0.010%, the toughness of the steel material will be significantly reduced even if the contents of other elements are within the ranges of this embodiment.
Therefore, the O content is 0.010% or less.
The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the O content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.
The upper limit of the O content is preferably 0.009%, more preferably 0.008%, and even more preferably 0.007%.

The balance of the chemical composition of the martensitic stainless steel material according to this embodiment consists of Fe and impurities. Here, the term "impurities" in the chemical composition refers to substances that are mixed in from raw materials such as ore or scrap, or the manufacturing environment, when industrially manufacturing martensitic stainless steel material, and are acceptable within a range that does not adversely affect the martensitic stainless steel material according to this embodiment.

[Optional Elements]
The chemical composition of the martensitic stainless steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of W: 0-1.50%, Sn: 0-0.010%, Nb: 0-0.50%, B: 0-0.0100%, Mg: 0-0.0100%, and rare earth elements (REM): 0-0.100%. All of these elements are optional elements. These elements will be described below.

[Regarding W and Sn]
The chemical composition of the martensitic stainless steel material according to this embodiment may further contain one or more elements selected from the group consisting of W and Sn, instead of a portion of Fe. Both of these elements improve the SSC resistance of the steel material. Each element will be described below.

W: 0 to 1.50%
Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%.
When W is contained, that is, when the W content exceeds 0%, W stabilizes the passive film and prevents the passive film from being destroyed by chloride ions and hydrogen sulfide ions. This improves the SSC resistance of the steel. Even if even a small amount of W is contained, the above effect can be obtained to some extent.
However, if the W content exceeds 1.50%, W combines with C to form coarse carbides, which reduces the toughness of the steel material even if the contents of other elements are within the ranges of this embodiment.
Therefore, the W content is 0 to 1.50%.
The lower limit of the W content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.10%.
The upper limit of the W content is preferably 1.40%, more preferably 1.30%, and even more preferably 1.20%.

Sn: 0-0.010%
Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%.
When Sn is contained, that is, when the Sn content is more than 0%, Sn enhances the SSC resistance of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent.
However, if the Sn content exceeds 0.010%, Sn segregates at grain boundaries, and in this case, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Sn content is 0 to 0.010%.
The lower limit of the Sn content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.
The upper limit of the Sn content is preferably 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[Regarding Nb]
The chemical composition of the martensitic stainless steel material according to this embodiment may further contain Nb in place of a portion of Fe.
Nb: 0-0.50%
Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%.
When Nb is contained, that is, when the Nb content exceeds 0%, Nb bonds with C and/or N to form precipitates. In this case, the pinning effect suppresses the coarsening of crystal grains, thereby increasing the strength of the steel. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent.
However, if the Nb content exceeds 0.50%, excessive precipitates are formed, and in this case, the toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Nb content is 0 to 0.50%.
The lower limit of the Nb content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.08%.
The upper limit of the Nb content is preferably 0.48%, more preferably 0.40%, and even more preferably 0.20%.

[Regarding B, Mg and rare earth elements (REM)]
The chemical composition of the martensitic stainless steel material according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of B, Mg, and rare earth elements (REM). All of these elements improve the hot workability of the steel material. Each element will be described below.

B: 0-0.0100%
Boron (B) is an optional element and may not be contained, that is, the B content may be 0%.
When B is contained, that is, when the B content exceeds 0%, B suppresses the segregation of S to grain boundaries in the steel material and improves the hot workability of the steel material. Even if even a small amount of B is contained, the above effects can be obtained to some extent.
However, if the B content exceeds 0.0100%, coarse B nitrides are formed, and in this case, the toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the B content is 0 to 0.0100%.
The lower limit of the B content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.
The upper limit of the B content is preferably 0.0095%, more preferably 0.0090%, and even more preferably 0.0080%.

Mg: 0-0.0100%
Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%.
When Mg is contained, that is, when the Mg content exceeds 0%, Mg fixes S in the steel material as sulfides, rendering it harmless and improving the hot workability of the steel material. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent.
However, if the Mg content exceeds 0.0100%, coarse oxides are formed, and in this case, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Mg content is 0 to 0.0100%.
The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.
The upper limit of the Mg content is preferably 0.0095%, more preferably 0.0091%, and even more preferably 0.0085%.

Rare earth elements: 0-0.100%
Rare earth elements (REM) are optional elements and may not be contained, i.e., the REM content may be 0%.
When REM is contained, that is, when the REM content is more than 0%, REM fixes the S in the steel as sulfides, rendering it harmless and improving the hot workability of the steel. Even if even a small amount of REM is contained, the above effect can be obtained to some extent.
However, if the REM content exceeds 0.100%, coarse oxides are formed, and in this case, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the REM content is 0 to 0.100%.
The lower limit of the REM content is preferably 0.001%, and more preferably 0.003%.
The upper limit of the REM content is preferably 0.095%, and more preferably 0.090%.

In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and the lanthanides lanthanum (La), atomic number 57, to lutetium (Lu), atomic number 71. Furthermore, the REM content in this specification refers to the total content of these elements.

[(Feature 2) Number density of fine low-S inclusions, coarse low-S inclusions, and high-S inclusions]
For inclusions containing S in the martensitic stainless steel material of this embodiment, Fn1 is defined by formula (1).
Fn1=(Mg+Al+Ca+Ti+V+Mn)/S (1)
Here, the element symbols in formula (1) are substituted with the content of the corresponding element in the inclusion in mass %. If an element is not contained, "0" is substituted for the corresponding element symbol. Fn1 is the value obtained by rounding the obtained value to one decimal place.

Furthermore, inclusions in martensitic stainless steel materials are defined as follows.
(A) Inclusions containing S, having an Fn1 of 5.0 or more, and an equivalent circle diameter of 2.0 to 10.0 μm are defined as "fine low-S inclusions."
(B) Inclusions containing S, having an Fn1 of 5.0 or more, and an equivalent circle diameter of more than 10.0 μm are defined as "coarse low-S inclusions."
(C) Inclusions containing S, having an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more are defined as "high S inclusions."

In this case, the number density ND1 of fine low-S inclusions in the martensitic stainless steel material is 3.0 pieces/ ^mm2 or more, the number density ND2 of coarse low-S inclusions is 2.0 pieces/ ^mm2 or less, and the number density ND3 of high-S inclusions is 2.0 pieces/ ^mm2 or less.

Furthermore, even if inclusions containing S and with an equivalent circle diameter of less than 2.0 μm dissolve and form dents, the dents will quickly re-passivate. Therefore, in martensitic stainless steel materials with the chemical composition of Feature 1, inclusions containing S and with an equivalent circle diameter of less than 2.0 μm do not affect SSC resistance.

As described above, in the martensitic stainless steel material of this embodiment, rather than drastically reducing the S content in the chemical composition, a certain amount of S content is tolerated while increasing the number density ND1 of fine low-S inclusions, which are composite inclusions, thereby reducing the S content available for forming high-S inclusions. This makes it possible to reduce the number density ND3 of high-S inclusions, which are easily dissolved in sour environments.

Furthermore, among low-S inclusions with an Fn1 of 5.0 or more, coarse low-S inclusions are coarse even when the S content is low. Therefore, coarse low-S inclusions are likely to dissolve in a sour environment and form coarse pits. These coarse pits are difficult to repassivate, promoting the occurrence of SSC. Therefore, the number density ND1 of fine low-S inclusions is increased and the number density ND3 of high-S inclusions is reduced, while the number density ND2 of coarse low-S inclusions is also reduced.

[Regarding the number density ND1 of fine low-S inclusions]
If the number density ND1 of the fine low-S inclusions is less than 3.0 pieces/ ^mm2 , the amount of fine low-S inclusions in the steel material is insufficient. In this case, the steel material contains a large amount of S that can be used to form high-S inclusions, so the number density ND3 of the high-S inclusions exceeds 2.0 pieces/ ^mm2 . As a result, sufficient SSC resistance in a sour environment cannot be obtained.
Therefore, the number density ND1 of fine low-S inclusions is set to 3.0 pieces/ ^mm2 or more.
The preferred lower limit of the number density ND1 is 3.5 pieces/mm ² , more preferably 4.0 pieces/mm ² , and even more preferably 4.5 pieces/mm ² .
However, in a martensitic stainless steel material that satisfies Feature 1, the upper limit of the number density ND1 is, for example, 45.0 pieces/mm ² , for example, 40.0 pieces/mm ² .

[Number density ND2 of coarse low-S inclusions]
If the number density ND2 of the coarse low-S inclusions exceeds 2.0 pieces/ ^mm2 , excessive coarse low-S inclusions are formed. In this case, the coarse low-S inclusions are likely to dissolve in a sour environment, forming coarse pits. These coarse pits are likely to promote the occurrence of SSC.
Therefore, the number density ND2 of coarse low-S inclusions is set to 2.0 pieces/ ^mm2 or less.
The upper limit of the number density ND2 is preferably 1.9 pieces/mm ² , more preferably 1.8 pieces/mm ² , and even more preferably 1.7 pieces/mm ² .
It is preferable that the number density ND2 be as low as possible. In other words, the number density ND2 is preferably 0 pieces/ ^mm2 . However, if the number density ND2 is reduced too much, the manufacturing cost may increase. Therefore, the preferred lower limit of the number density ND2 is 0.1 pieces/ ^mm2 , more preferably 0.2 pieces/ ^mm2 , and even more preferably 0.3 pieces/ ^mm2 .

[Regarding the number density ND3 of high-S inclusions]
As described above, high-S inclusions, even if small in size, are likely to dissolve and form pits in a sour environment. Pits formed by high-S inclusions, even if small in size, are likely to promote the occurrence of SSC. Therefore, if the number density ND3 of high-S inclusions exceeds 2.0 pieces/ ^mm2 , sufficient SSC resistance cannot be obtained in a sour environment.
Therefore, the number density ND3 of high-S inclusions is set to 2.0 pieces/ ^mm2 or less.
The upper limit of the number density ND3 is preferably 1.9 pieces/mm ² , more preferably 1.8 pieces/mm ² , and even more preferably 1.7 pieces/mm ² .
It is preferable that the number density ND3 be as low as possible. In other words, the number density ND3 is preferably 0 pieces/ ^mm2 . However, if the number density ND3 is reduced too much, the manufacturing cost may increase. Therefore, the preferred lower limit of the number density ND3 is 0.1 pieces/ ^mm2 , more preferably 0.2 pieces/ ^mm2 , and even more preferably 0.3 pieces/ ^mm2 .

[Method for measuring the number density ND1 of fine low-S inclusions, the number density ND2 of coarse low-S inclusions, and the number density ND3 of high-S inclusions]
In this embodiment, the number density ND1 (pieces/ ^mm2 ) of fine low-S inclusions, the number density ND2 (pieces/ ^mm2 ) of coarse low-S inclusions, and the number density ND3 (pieces/ ^mm2 ) of high-S inclusions in the martensitic stainless steel material can be measured by the following method.

A test piece is taken from the martensitic stainless steel material.
When the steel material is a steel pipe, a test piece having an observation surface including the pipe axial direction and the wall thickness direction (pipe radial direction) is taken from the center of the wall thickness.
When the steel material is a steel plate, a test piece having an observation surface including the rolling direction and the plate thickness direction is taken from the center of the plate thickness.
When the steel material is a round bar, a test piece having an observation surface including the axial and radial directions is prepared from the R/2 portion of the cross section perpendicular to the axial direction of the round bar. In this specification, the term "round bar" refers to a steel bar having a circular cross section perpendicular to the axial direction. The R/2 portion refers to the center position of the radius R in the cross section perpendicular to the axial direction of the round bar.

The observation surface of the collected test piece is mirror-polished. An observation field is selected from the mirror-polished observation surface. At this time, three or more observation fields are selected, with a total area of 500 ^mm2 . A backscattered electron image of each observation field is obtained at 800x magnification using a scanning electron microscope equipped with a composition analysis function (SEM-EDS device). The obtained backscattered electron image is observed, and particles are identified from the contrast. The position coordinates of all identified particles in the observation field are recorded.

Based on the position coordinates of the identified particles, elemental concentration analysis (EDS analysis) is performed on the particles. Specifically, the identified particles are scanned with an electron beam based on the position coordinates to analyze the elemental concentration of the particles. In this case, rather than scanning only a portion of the particle with the electron beam, the entire particle is scanned with the electron beam. This allows the elemental concentration of the entire particle to be analyzed. In EDS analysis, the acceleration voltage is set to 20 kV, and the target elements are quantified as N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb.

Of the identified particles, when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, particles with a Cl content of more than 10.0%, particles with a K content of more than 10.0%, particles with a Na content of more than 20.0%, particles with a Ca content of more than 75.0%, and particles with an O content of more than 70.0% are dust, abrasives, etc. that adhered during mirror polishing. Therefore, these particles are determined not to be inclusions and are excluded from the target.
Furthermore, among the identified particles, when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, particles with an S content of less than 1.0% are not inclusions containing S but oxides or nitrides, so particles with an S content of less than 1.0% are also excluded from the scope of the present invention.
In other words, among the identified particles, when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, particles other than particles with a Cl content of more than 10.0%, particles with a K content of more than 10.0%, particles with a Na content of more than 20.0%, particles with a Ca content of more than 75.0%, particles with an O content of more than 70.0%, and particles with an S content of less than 1.0% are recognized as inclusions containing S.

For particles identified as inclusions containing S, the equivalent circle diameter (μm) is determined. The equivalent circle diameter refers to the diameter (μm) of a circle with the same area as the particle. The equivalent circle diameter is the value obtained by rounding off the first decimal place to the nearest tenth.

Based on the EDS analysis results and the equivalent circle diameter of each particle, fine low-S inclusions, coarse low-S inclusions, and high-S inclusions are identified as follows:
(A) Fine low-S inclusions Particles having an Fn1 defined by formula (1) of 5.0 or more and an equivalent circle diameter of 2.0 to 10.0 μm based on the Mg content, Al content, Ca content, Ti content, V content, Mn content and S content in mass%, where the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr and Nb is taken as 100.0 mass%, are specified as "fine low-S inclusions."
Fn1=(Mg+Al+Ca+Ti+V+Mn)/S (1)
Here, the element symbol in formula (1) is substituted with the content of the element in the corresponding particle (inclusion) in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol.
(B) Coarse Low-S Inclusions Particles having an Fn1 defined by formula (1) of 5.0 or more and an equivalent circle diameter of more than 10.0 μm based on the Mg content, Al content, Ca content, Ti content, V content, Mn content, and S content in mass%, where the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, are specified as "coarse low-S inclusions."
(C) High-Sulfur Inclusions Particles having an Fn1 defined by formula (1) of less than 5.0 and an equivalent circle diameter of 2.0 μm or more based on the Mg content, Al content, Ca content, Ti content, V content, Mn content, and S content in mass% when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass% are specified as "high-Sulfur inclusions."

The fine low-S inclusions, coarse low-S inclusions, and high-S inclusions identified using the method described above are counted in each field of view.

The number density ND1 (pieces/mm ^{2 )} of the fine low S inclusions of the specific inclusion is calculated based on the total number of the counted fine low S inclusions in all the observation fields and the total area of all the observation fields.
The number density ND2 (pieces/mm ^{2 )} of the coarse low-S inclusions of the specific inclusions is calculated based on the total number of the coarse low-S inclusions counted in all the observation fields and the total area of all the observation fields.
The number density ND3 (numbers/mm ² ) of the high S inclusions of the specific inclusion is calculated based on the total number of the high S inclusions counted in all the observation fields and the total area of all the observation fields.

The number density ND1 (pieces/ ^mm2 ) of fine low S inclusions, the number density ND2 (pieces/ ^mm2 ) of coarse low S inclusions, and the number density ND3 (pieces/ ^mm2 ) of high S inclusions are calculated by rounding the obtained values to one decimal place.
Furthermore, as the SEM-EDS device, for example, an automatic analyzer manufactured by FEI (ASPEX) under the trade name Metals Quality Analyzer can be used.

[Effects of the martensitic stainless steel material according to this embodiment]
The martensitic stainless steel material of this embodiment satisfies Features 1 and 2. Therefore, excellent SSC resistance can be obtained in a sour environment.

[Method for evaluating SSC resistance]
The SSC resistance evaluation test of the martensitic stainless steel material of this embodiment is carried out as follows in accordance with NACE TM0177-2016 Method A.

A round bar test piece is taken from the steel material according to this embodiment. When the steel material is a steel pipe, the round bar test piece is taken from the center of the wall thickness. The axial direction of the round bar test piece is parallel to the axial direction of the steel pipe.
When the steel material is a steel plate, a round bar test piece is taken from the center of the plate thickness, with the axial direction of the round bar test piece parallel to the rolling direction of the steel plate.
When the steel material is a round bar, a round bar test piece is taken from the R/2 portion, with the axial direction of the round bar test piece parallel to the axial direction of the round bar.
The round bar test piece has, for example, a parallel part having a diameter of 6.35 mm and a length of 25.4 mm.

The test solution is a 0.17% by mass sodium chloride aqueous solution with a pH of 3.0. The test solution is an aqueous solution containing 0.17% by mass sodium chloride and 0.41 g/L sodium acetate, with acetic acid added to adjust the pH to 3.0. A stress equivalent to 90% of the actual yield stress is applied to the round bar test specimen. The test solution at 24°C is poured into a test vessel so that the stressed round bar test specimen is immersed, forming a test bath. After degassing the test bath, 0.04 bar of _H2S gas and 0.96 bar of _CO2 gas are blown into the test bath to saturate the test bath with _H2S gas. The _H2S -saturated test bath is maintained at 24°C for 720 hours. After 720 hours, the surface of the parallel portion of the test specimen is observed with a 10x magnification loupe to check for cracks. If a location is suspected of having a crack when observed with a magnifying glass, the cross section of the location is observed with an optical microscope at 100x magnification to confirm the presence or absence of a crack.

In this embodiment, having excellent SSC resistance means that no cracks are observed after 720 hours in the above-mentioned SSC resistance evaluation test. In this specification, "no cracks are observed" means that no cracks are observed when the test specimen after the test is observed with a 10x magnification loupe and a 100x optical microscope.

[Regarding Microstructure]
The microstructure of the martensitic stainless steel material of this embodiment is mainly composed of martensite. In this specification, martensite includes not only fresh martensite but also tempered martensite. Also, in this specification, "mainly composed of martensite" means that the volume fraction of martensite in the microstructure is 80% or more. The remainder of the microstructure is retained austenite. In other words, in the steel material of this embodiment, the volume fraction of retained austenite is 0 to 20%. It is preferable that the volume fraction of retained austenite is as low as possible. A preferred lower limit of the volume fraction of martensite in the microstructure of the steel material of this embodiment is 85%, and more preferably 90%. More preferably, the microstructure of the steel material is a single martensite phase.

In the microstructure, a small amount of retained austenite does not significantly reduce strength, and significantly increases the toughness of the steel. However, if the volume fraction of retained austenite is too high, the strength of the steel will significantly decrease. Therefore, as described above, in the microstructure of the martensitic stainless steel material of this embodiment, the volume fraction of retained austenite is 0 to 20%. From the perspective of ensuring strength, the preferred upper limit of the volume fraction of retained austenite is 15%, and more preferably 10%. As described above, the microstructure of the steel material of this embodiment may be a single martensite phase. Therefore, the volume fraction of retained austenite may be 0%. On the other hand, if even a small amount of retained austenite is present, the volume fraction of retained austenite is greater than 0 to 20%, more preferably greater than 0 to 15%, and even more preferably greater than 0 to 10%.

[Method for measuring martensite volume fraction]
The volume fraction (vol. %) of martensite in the microstructure of the martensitic stainless steel material of this embodiment is determined by subtracting the volume fraction (vol. %) of retained austenite, determined by the method described below, from 100%.

The volume fraction of retained austenite is determined by X-ray diffraction. Specifically, a test piece is taken from the steel material.
When the steel material is a steel pipe, the test piece is taken from the center of the wall thickness. When the steel material is a steel plate, the test piece is taken from the center of the plate thickness. When the steel material is a round bar, the test piece is taken from the R/2 part. The size of the test piece is not particularly limited. The test piece is, for example, 15 mm x 15 mm x 2 mm thick. In this case, when the steel material is a steel pipe, the thickness direction of the test piece is the pipe diameter direction. When the steel material is a steel plate, the thickness direction of the test piece is the plate thickness direction. When the steel material is a round bar, the thickness direction of the test piece is the diameter direction.
Using the obtained test specimen, the X-ray diffraction intensity of each of the (200) plane of the α phase, the (211) plane of the α phase, the (200) plane of the γ phase, the (220) plane of the γ phase, and the (311) plane of the γ phase is measured, and the integrated intensity of each plane is calculated. In measuring the X-ray diffraction intensity, the target of the X-ray diffractometer is Mo (MoKα radiation), and the output is 50 kV-40 mA. After calculation, the volume fraction Vγ (%) of retained austenite is calculated using formula (I) for each combination (2 × 3 = 6 pairs) of each plane of the α phase and each plane of the γ phase. Then, the average value of the volume fraction Vγ of the six pairs of retained austenite is defined as the volume fraction (%) of retained austenite.
Vγ=100/{1+(Iα×Rγ)/(Iγ×Rα)} (I)
Here, Iα is the integrated intensity of the α phase. Rα is the crystallographically calculated value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographically calculated value of the γ phase. In this specification, Rα on the (200) plane of the α phase is 15.9, Rα on the (211) plane of the α phase is 29.2, Rγ on the (200) plane of the γ phase is 35.5, Rγ on the (220) plane of the γ phase is 20.8, and Rγ on the (311) plane of the γ phase is 21.8. The volume fraction of retained austenite is an integer value obtained by rounding the obtained numerical value to one decimal place.

Using the volume fraction (%) of retained austenite obtained by the above-mentioned X-ray diffraction method, the volume fraction (vol. %) of martensite in the microstructure of the steel material is calculated according to the following formula.
Volume fraction of martensite = 100 - volume fraction of retained austenite (%)

[Yield strength]
The yield strength of the martensitic stainless steel material according to this embodiment is not particularly limited. The preferred yield strength of the martensitic stainless steel material according to this embodiment is 758 MPa or more (110 ksi or more). The upper limit of the yield strength is not particularly limited, but the upper limit of the yield strength of the martensitic stainless steel material according to this embodiment is, for example, 862 MPa.

[Method for measuring yield strength]
The yield strength of the martensitic stainless steel material of this embodiment can be determined by the following method.
A tensile test is carried out in accordance with ASTM E8/E8M (2022). First, a tensile test piece is taken from the martensitic stainless steel material.
When the martensitic stainless steel material is a steel pipe, a round bar-shaped tensile test specimen or an arc-shaped tensile test specimen is taken from the center of the wall thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the steel pipe. When the martensitic stainless steel material is a steel pipe and a round bar test specimen cannot be taken from the steel pipe, an arc-shaped tensile test specimen is taken from the steel pipe.
When the martensitic stainless steel material is a steel plate, a round bar-shaped tensile test specimen is taken from the center of the plate thickness, with the longitudinal direction of the tensile test specimen parallel to the rolling direction of the steel plate.
When the martensitic stainless steel material is a round bar, a round bar-shaped tensile test specimen is taken from the R/2 portion, with the longitudinal direction of the tensile test specimen parallel to the axial direction of the round bar.

The size of the round bar-shaped tensile test specimen is, for example, 6 mm in diameter at the parallel part and 30 mm in gauge length, while the size of the arc-shaped tensile test specimen is, for example, the full thickness, 25.4 mm in width, and 50.8 mm in gauge length.
A tensile test is performed using a tensile test piece at room temperature (24±3°C) in the atmosphere. In this embodiment, the 0.2% offset yield strength obtained from the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is an integer value obtained by rounding the obtained value to one decimal place.

[Shapes and uses of steel materials]
The shape of the martensitic stainless steel material of this embodiment is not particularly limited. The steel material is, for example, a steel pipe, a steel plate, or a round bar (solid material). The steel pipe may be a seamless steel pipe or a welded steel pipe. The steel pipe is, for example, a steel pipe for oil country tubular goods. Steel pipe for oil country tubular goods refers to a steel pipe for oil country tubular goods applications. Oil country tubular goods are, for example, casings, tubing, drill pipes, etc. used for drilling oil wells or gas wells, extracting crude oil or natural gas, etc. Preferably, the martensitic stainless steel material of this embodiment is a seamless steel pipe for oil country tubular goods.

[Manufacturing method]
An example of a method for manufacturing the martensitic stainless steel material of this embodiment having the above-mentioned configuration will be described below. Note that the method for manufacturing the martensitic stainless steel material of this embodiment is not limited to the manufacturing method described below.
An example of a method for manufacturing a martensitic stainless steel material according to this embodiment includes the following steps.
(Step 1) Material preparation step (Step 2) Hot working step (Step 3) Heat treatment step Each manufacturing step will be described in detail below.

[Material preparation process]
In the material preparation process, molten pig iron produced by a known method is subjected to refining (primary refining) in a converter. Secondary refining is performed on the molten steel that has been primarily refined. In the secondary refining, alloy elements are added to adjust the composition to produce molten steel that satisfies the chemical composition of Feature 1. In the secondary refining, for example, RH (Ruhrstahl-Hausen) vacuum degassing treatment is performed, and then final adjustment of the alloy composition is performed. In the secondary refining, combined refining may be performed. In this case, prior to the RH vacuum degassing treatment, for example, a refining treatment using an LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) is performed.

Materials are manufactured using molten steel that has undergone secondary refinement. Specifically, cast pieces (slabs, blooms, or billets) are manufactured using continuous casting using molten steel that has undergone secondary refinement.

In continuous casting, molten steel is first poured from a ladle into a tundish. The molten steel, which is temporarily stored in the tundish, is then guided into the mold through an immersion nozzle. The molten steel gradually solidifies from the outer surface, which is cooled in the mold. The solidified portion is gradually pulled downward from the mold, and cooling water is sprayed onto the outer surface of the slab as it is further cooled, solidifying it. Through these manufacturing processes, slabs that will serve as the raw material for martensitic stainless steel are produced.

In the material preparation step, the bloom may be further subjected to blooming to form a billet. In this case, the bloom is heated to, for example, 1150 to 1300° C. The heated bloom is then subjected to blooming to form a billet.
Through the above steps, a martensitic stainless steel material (slab, bloom, or billet) is produced.

In the continuous casting process during the material preparation process, the following conditions must be met:
(Condition 1)
The temperature of the molten steel in the tundish is maintained at 1600 to 1500°C for 5 to 100 minutes.
(Condition 2)
The vertical distance D from the surface of the molten steel in the mold to the center of the discharge port on the outer surface of the submerged nozzle is set to 150 to 400 mm.
Conditions 1 and 2 will be explained below.

[Regarding Condition 1]
As described above, fine low-S inclusions are formed by the aggregation of sulfides and nitrides around oxides as nuclei. To form such fine low-S inclusions, it is effective to float coarse oxides in the molten steel in the tundish and prevent the coarse oxides from flowing into the mold.

The tundish is equipped with a heating device that maintains the temperature of the molten steel. The heating device is, for example, an induction heating device or a plasma heating device. In the tundish, the molten steel is held at 1600-1500°C for a holding time t. If the holding time t is less than 5 minutes, the coarse oxides in the molten steel will not rise sufficiently, and will remain in the molten steel as it flows into the mold. In this case, the number density ND2 of coarse low-S inclusions will be excessive in the martensitic stainless steel material after production.

On the other hand, if the holding time t exceeds 100 minutes, although the coarse oxides in the molten steel rise sufficiently, the fine oxides also become coarse and rise. As a result, the amount of fine oxides that serve as nuclei for the fine low-S inclusions in the molten steel flowing from the immersion nozzle into the mold is insufficient. As a result, in the martensitic stainless steel material after production, the number density ND1 of the fine low-S inclusions becomes excessively low, and the number density ND3 of the high-S inclusions becomes excessive.
Therefore, the retention time t is set to 5 to 100 minutes.

[Regarding Condition 2]
1 is a schematic diagram illustrating the positional relationship between molten steel in a mold and an immersion nozzle during continuous casting. Referring to Fig. 1, molten steel 10 passes through an immersion nozzle 20 and flows from a discharge port 22 outside the immersion nozzle 20 into a mold 30.

The submerged nozzle 20 includes a cylindrical main body 21 and two discharge ports 22. The two discharge ports 22 are arranged opposite each other on the sidewall near the bottom of the cylindrical main body. More specifically, one of the discharge ports 22 is positioned 180° apart from the other discharge port 22 around the central axis of the submerged nozzle 20. The discharge ports 22 are inclined upward at an inclination angle θ of 5 to 35° relative to the horizontal.

The vertical distance from the liquid level 11 of the molten steel 10 in the mold 30 to the center of the discharge port 22 on the outer surface of the submerged nozzle 20 is defined as distance D (mm). Here, "the center of the discharge port 22 on the outer surface of the submerged nozzle 20" refers to the position P22 of the central axis C22 of the discharge port 22 on the outer surface of the submerged nozzle 20. Note that during continuous casting, the liquid level 11 fluctuates within ±10 mm. Therefore, the position of the liquid level 11 (liquid level height) when calculating distance D is the liquid level height (target level) set during actual continuous casting operation.

Distance D forms a stirring area DA between the liquid surface 11 and the discharge port 22 in the molten steel 10. In the stirring area DA, the molten steel flowing out from the discharge port 22 of the submerged nozzle 20 into the molten steel 10 rises, stirring the molten steel in the vertical direction. At this time, the molten steel temperature gradually decreases, resulting in the formation of sulfides and nitrides. If the molten steel flowing out from the discharge port 22 contains a sufficient amount of fine oxides, sulfides and nitrides will form in the stirring area DA using the fine oxides as nuclei, or the sulfides and nitrides formed in the molten steel will collide with the fine oxides and agglomerate, forming low-S inclusions with an Fn1 of 5.0 or more. The formation of low-S inclusions suppresses the formation of high-S inclusions. Furthermore, among the low-S inclusions, coarse low-S inclusions float to the liquid surface 11. A molten slag layer is formed on the liquid surface 11 by the mold powder. Coarse low-S inclusions that float to the liquid surface 11 are absorbed into the molten slag layer. As a result, although fine low-S inclusions remain in the molten steel, the amount of coarse low-S inclusions is reduced.

As described above, the stirring area DA affects the number density of fine low-S inclusions, coarse low-S inclusions, and high-S inclusions in the martensitic stainless steel material after production. Furthermore, the distance D is a factor that determines the size of the stirring area DA.

If the distance D is less than 150 mm, the stirring area DA is too narrow. In this case, low-S inclusions may not be generated to a degree that sufficiently reduces the number density ND3 of high-S inclusions. As a result, the number density ND3 of high-S inclusions may become excessive, or the number density ND1 of fine low-S inclusions may become excessively low.

On the other hand, if the distance D exceeds 400 mm, the stirring area DA becomes too wide. In this case, low-S inclusions are sufficiently generated, and the generation of high-S inclusions can be sufficiently suppressed. However, the coarse low-S inclusions are less likely to rise to the liquid surface 11. As a result, the number density ND2 of the coarse low-S inclusions in the produced martensitic stainless steel material becomes excessive.
Therefore, the distance D is set to 150 to 400 mm.

In addition, electromagnetic stirring may be performed on the molten steel 10 in the mold 30. Electromagnetic stirring stirs the molten steel horizontally. Therefore, electromagnetic stirring does not easily create the stirring area DA in which the molten steel is stirred vertically as shown in Figure 1. By setting the vertical distance D (mm) from the liquid surface 11 of the molten steel 10 in the mold 30 to the center of the outlet 22 on the outer surface of the submerged nozzle 20 to 150 to 400 mm, it is possible to create a stirring area DA of an appropriate range.

[Hot working process]
In the hot working process, the raw material is hot worked to produce an intermediate steel material. When the steel material is a steel pipe, the intermediate steel material corresponds to a mother pipe. First, the raw material is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The raw material extracted from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe), which is the intermediate steel material. The hot working method is not particularly limited, and a well-known method may be used. For example, the Mannesmann process is performed as the hot working to produce a mother pipe. In this case, a round billet is pierced and rolled using a piercing mill. When piercing and rolling is performed, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The pierced and rolled round billet is further hot rolled using a mandrel mill, a reducer, a sizing mill, or the like to produce a mother pipe. The cumulative area reduction rate in the hot working process is, for example, 20 to 70%.

A blank pipe can also be manufactured from the billet using other hot working methods. For example, in the case of short, thick-walled steel products such as couplings, the blank pipe can be manufactured using forging methods such as the Erhardt process. A blank pipe is manufactured through the above process.

When the steel material is a steel plate, the raw material is first heated in a heating furnace. There are no particular restrictions on the heating temperature, but it is, for example, 1100 to 1300°C. The raw material extracted from the heating furnace is hot rolled using a roughing mill and a tandem finishing mill to produce a steel plate, which is an intermediate steel material.

When the steel material is round bar, the material is first heated in a heating furnace. There are no particular restrictions on the heating temperature, but it is, for example, 1100 to 1300°C. The material extracted from the heating furnace is then hot processed to produce round bar, an intermediate steel material. Hot processing can be, for example, blooming using a blooming mill, or hot rolling using a continuous rolling mill. A continuous rolling mill has an alternating arrangement of horizontal stands each having a pair of grooved rolls arranged side by side in the vertical direction, and vertical stands each having a pair of grooved rolls arranged side by side in the horizontal direction.

Intermediate steel produced by hot working may be air-cooled. Intermediate steel produced by hot working may also be quenched directly after hot working without being cooled to room temperature, or may be quenched after being reheated after hot working.

If quenching is performed directly after hot working, or after reheating and quenching after hot working, stress relief annealing (SR treatment) may be performed before the next heat treatment process (quenching and tempering) in order to remove residual stress.

[Heat treatment process]
The heat treatment process includes a quenching process and a tempering process.

[Quenching process]
In the heat treatment process, first, the intermediate steel produced in the hot working process is quenched (quenching process). Quenching is performed by a well-known method. Specifically, the intermediate steel after the hot working process is charged into a heat treatment furnace and held at the quenching temperature. The quenching temperature is equal to or higher than the _AC3 transformation point, for example, 900 to 920°C. After holding the intermediate steel at the quenching temperature, it is rapidly cooled (quenched). The holding time at the quenching temperature is not particularly limited, but is, for example, 10 to 60 minutes. The quenching method is, for example, water cooling. There are no particular restrictions on the quenching method. When the intermediate steel is a mother pipe, the mother pipe may be quenched, for example, by immersing it in a water bath or an oil bath, or by shower cooling or mist cooling, in which cooling water is poured or sprayed onto the outer and/or inner surface of the mother pipe.

As mentioned above, after the hot working process, the intermediate steel may be quenched immediately after hot working (direct quenching) without being cooled to room temperature, or the raw pipe after hot working may be loaded into a reheating furnace before its temperature drops and held at the quenching temperature, after which quenching may be performed.

[Tempering process]
The intermediate steel material after quenching is further subjected to a tempering process. In the tempering process, the yield strength of the steel material is adjusted. In this embodiment, the tempering temperature is set to 500 to 650°C. The holding time at the tempering temperature is not particularly limited, but is, for example, 10 to 60 minutes. It is well known to those skilled in the art that the yield strength of a steel material can be adjusted by appropriately adjusting the tempering temperature depending on the chemical composition. Preferably, the tempering conditions are adjusted so that the yield strength of the steel material is 758 MPa or more.

[Optional manufacturing process]
The method for manufacturing a martensitic stainless steel material according to this embodiment may further include a pickling step. In other words, the pickling step is an optional step. When the pickling step is performed, the intermediate steel material after the tempering step is subjected to pickling treatment. The pickling treatment may be performed under well-known conditions.

The martensitic stainless steel material of this embodiment can be manufactured through the above steps. Note that the above-described method for manufacturing martensitic stainless steel material is one example, and martensitic stainless steel material satisfying Features 1 and 2 may be manufactured by other methods. The present invention will be described in further detail below using examples.

Martensitic stainless steel materials were manufactured having the chemical compositions shown in Tables 1A, 1B, and 1C.

Specifically, molten steel for each test number was produced, and blooms were produced by continuous casting. During continuous casting, the holding time t (minutes) at which the molten steel temperature in the tundish was 1600-1500°C, and the vertical distance D (mm) from the liquid surface of the molten steel in the mold to the center of the discharge port on the outer surface of the submerged entry nozzle were as shown in the "Holding time t (minutes)" and "Distance D (mm)" columns in Table 2. The upward tilt angle θ of the discharge port of the submerged entry nozzle (see Figure 1) was 15°.

The blooms were bloomed to produce billets with an outer diameter of 310 mm. The bloom heating temperature during blooming was 1150-1300°C. The produced billets were heated to 1250°C and then hot rolled using the Mannesmann process to produce blank pipes (seamless steel pipes) with an outer diameter of 244.48 mm and a wall thickness of 13.84 mm.

The mother pipe of each test number was quenched and tempered. The mother pipe of each test number was held for 20 minutes at the quenching temperature T1 (°C) shown in the "Quenching temperature T1 (°C)" column in Table 2, and then rapidly cooled (water-cooled). The mother pipe after quenching was then tempered. Tempering was performed at the tempering temperature T2 (°C) shown in the "Tempering temperature T2 (°C)" column in Table 2 for the holding time t2 (minutes) shown in the "Holding time t2 (minutes)" column in Table 2, so that the yield strength of the tempered steel (seamless steel pipe) would be 758 MPa or more.

Martensitic stainless steel materials (seamless steel pipes) with each test number were manufactured using the above manufacturing process.

[Evaluation test]
The following evaluation tests were carried out on the martensitic stainless steel materials having each test number thus produced.
(Test 1) Measurement test of martensite volume fraction (%) (Test 2) Yield strength measurement test (Test 3) Measurement test of number density ND1 (pieces/ ^mm2 ) of fine low-S inclusions, number density ND2 (pieces/ ^mm2 ) of coarse low-S inclusions, and number density ND3 (pieces/ ^mm2 ) of high-S inclusions (Test 4) SSC resistance evaluation test Each test will be explained below.

[(Test 1) Measurement test of martensite volume fraction (%)]
The martensite volume fraction (%) of the martensitic stainless steel material of each test number was determined based on the method described in the above [Method for measuring martensite volume fraction]. Test specimens were taken from the center of the wall thickness of the steel material (seamless steel pipe) of each test number. The size of the test specimen was 15 mm × 15 mm × 2 mm thick, and the thickness direction of the test specimen was the wall thickness direction of the seamless steel pipe. As a result, the volume fraction of martensite was 80% or more for the steel material of each test number.

[(Test 2) Yield Strength Measurement Test]
The yield strength (MPa) of the martensitic stainless steel material of each test number was determined based on the method described in the above-mentioned [Method for measuring yield strength]. A round bar-shaped test piece with a parallel part diameter of 6 mm and a gauge length of 30 mm was taken from the martensitic stainless steel material of each test number as a tensile test piece. The obtained yield strength (MPa) is shown in the "Yield strength (MPa)" column in Table 3.

[(Test 3) Measurement test of the number density ND1 (pieces/mm ² ) of fine low-S inclusions, the number density ND2 (pieces/mm ² ) of coarse low-S inclusions, and the number density ND3 (pieces/mm ² ) of high-S inclusions]
The number density ND1 (pieces/mm 2 ) of fine low S inclusions, the number density ND2 (pieces/mm ² ) of coarse low S inclusions, and the number density ND3 (pieces/mm 2 ) of high S inclusions were determined based on the method described above in [Method for measuring the number density ND1 of fine low S inclusions, the number density ND2 of coarse low S inclusions, and the number density ND3 (pieces/mm ^{2 )} ]. Five 10 mm × 10 mm visual fields were observed, with a total area of 500 mm ² . The measured number densities ND1 to ND3 (pieces/mm ² ) are shown in Table 3 as "Number density ND1 (pieces/mm ² ),""Number density ND2 (pieces/mm ² )," and "Number density ND3 (pieces/mm ² )," respectively.

[(Test 4) SSC Resistance Evaluation Test]
The SSC resistance of the martensitic stainless steel material of each test number was evaluated based on the method described in the above-mentioned [Method for evaluating SSC resistance]. The collected round bar test specimens had a parallel portion diameter of 6.35 mm and a parallel portion length of 25.4 mm. If no cracks were observed, the "SSC resistance test" column in Table 3 is marked with "pass." On the other hand, if cracks were observed, the "SSC resistance test" column is marked with "fail." If no cracks were observed, it was evaluated that excellent SSC resistance was obtained.

[Evaluation results]
With reference to Tables 1A, 1B, 1C, 2, and 3, the martensitic stainless steel materials of test numbers 1 to 19 satisfied features 1 and 2. Therefore, excellent SSC resistance was obtained. The yield strength of the martensitic stainless steel materials of these test numbers was all 758 MPa or more.

On the other hand, in Test Nos. 20 and 21, the holding time t at a molten steel temperature of 1600 to 1500°C in the continuous casting tundish in the material preparation process was too short. As a result, the number density ND2 of coarse low-S inclusions exceeded 2.0 pieces/ ^mm2 . As a result, excellent SSC resistance was not obtained.

In Test Nos. 22 and 23, the holding time t at a molten steel temperature of 1600 to 1500°C in the continuous casting tundish in the material preparation step was too long. As a result, the number density ND1 of fine low-S inclusions was less than 3.0 pieces/ ^mm2 , and the number density ND3 of high-S inclusions exceeded 2.0 pieces/ ^mm2 . As a result, excellent SSC resistance was not obtained.

In Test Nos. 24 and 25, the vertical distance D from the liquid surface of the molten steel in the mold to the center of the discharge port on the outer surface of the submerged entry nozzle during continuous casting in the material preparation step was too short. As a result, the number density ND1 of fine low-S inclusions was less than 3.0 pieces/ ^mm2 , and the number density ND3 of high-S inclusions exceeded 2.0 pieces/ ^mm2 . As a result, excellent SSC resistance was not obtained.

In test numbers 26 and 27, the distance D was too short in the continuous casting of the material preparation step. As a result, the number density ND3 of high-S inclusions exceeded 2.0 pieces/ ^mm2 . As a result, excellent SSC resistance was not obtained.

In test numbers 28 and 29, the distance D was too long in the continuous casting in the material preparation step. As a result, the number density ND2 of coarse low-S inclusions exceeded 2.0 pieces/ ^mm2 . As a result, excellent SSC resistance was not obtained.

The above describes embodiments of the present disclosure. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

Claims (3)

A martensitic stainless steel material,
The chemical composition, in mass%, is
C: 0.030% or less,
Si: 0.10-1.00%,
Mn: 0.10-2.00%,
P: 0.040% or less,
S: 0.0050% or less,
Cr: 10.0-14.0%,
Ni: 5.00 to 7.50%,
Mo: 1.50-3.50%,
Cu: 0.01 to 2.50%,
Ti: 0.02-0.40%,
V: 0.01-0.50%,
Co: 0.01 to 0.50%,
Al: 0.010-0.100%,
Ca: 0.0001-0.0040%,
N: 0.001-0.020%,
O: 0.010% or less,
W: 0-1.50%,
Sn: 0 to 0.010%,
Nb: 0 to 0.50%,
B: 0 to 0.0100%,
Mg: 0 to 0.0100%,
Rare earth elements: 0 to 0.100%, and
the balance being Fe and impurities;
In the martensitic stainless steel material,
The steel contains S, has an Fn1 defined by formula (1) of 5.0 or more, and has a number density ND1 of 3.0 pieces/ ^mm2 or more of fine low-S inclusions having an equivalent circle diameter of 2.0 to 10.0 μm,
The steel contains S, the Fn1 is 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions having an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces/ ^mm2 or less,
The steel contains S, the Fn1 is less than 5.0, and the number density ND3 of high S inclusions, which are inclusions having an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces/ ^mm2 or less.
Martensitic stainless steel material.
Fn1=(Mg+Al+Ca+Ti+V+Mn)/S (1)
Here, the element symbols in formula (1) are substituted with the content of the corresponding element in the inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol. The martensitic stainless steel material according to claim 1,
The chemical composition is
W: 0.01-1.50%,
Sn: 0.001 to 0.010%,
Nb: 0.01 to 0.50%,
B: 0.0001 to 0.0100%,
Mg: 0.0001 to 0.0100%, and
Rare earth elements: 0.001 to 0.100%, containing one or more selected from the group consisting of
Martensitic stainless steel material. The martensitic stainless steel material according to claim 1 or 2,
The martensitic stainless steel material is a steel pipe.
Martensitic stainless steel material.