Martensitic stainless steel material

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

[0001] The present disclosure relates to steel materials, and more particularly to martensitic stainless steel materials.

[0002] 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." In steel materials used in sour environments, 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 materials used in sour environments are required to have excellent SSC resistance.

[0003] 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 steel materials containing about 13 mass% Cr, such as API L80 13Cr steel (normal 13Cr steel) and Super 13Cr steel with 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 for H ２ It is used in oil wells in sour environments where the S partial pressure is 0.03 bar or less.

[0004] Japanese Patent Laid-Open No. 2000-192196 (Patent Document 1) and Japanese Patent Laid-Open No. 2012-136742 (Patent Document 2) propose steel materials with excellent SSC resistance.

[0005] The martensitic stainless steel material of Patent Document 1 contains, by weight, 0.001-0.05% C, 0.05-1% Si, 0.05-2% Mn, 0.025% or less P, 0.01% or less S, 9-14% Cr, 3.1-7% Mo, 1-8% Ni, 0.5-7% Co, 0.001-0.1% sol. Al, 0.05% or less N, 0.01% or less O (oxygen), 0-5% Cu, and 0-5% W, 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 the Ms point is suppressed, resulting in a martensitic single-phase microstructure. Patent Document 1 states that this improves SSC resistance.

[0006] The martensitic stainless steel material of Patent Document 2 contains, by mass%, 0.01% or less of C, 0.5% or less of Si, 0.1 to 2.0% of Mn, 0.03% or less of P, 0.005% or less of S, 14.0 to 15.5% of Cr, 5.5 to 7.0% of Ni, 2.0 to 3.5% of Mo, 0.3 to 3.5% of Cu, 0.20% or less of V, 0.05% or less of Al, and 0.06% or less of N, with the balance being Fe and unavoidable impurities. The martensitic stainless steel material of this document contains 0.01% or less of C, 0.01% or less of Cr, Ni, and Mo, and contains appropriate amounts of Cu and V or W. Patent Document 2 states that this results in excellent SSC resistance.

[0007] JP 2000-192196 A JP 2012-136742 A

[0008] The above-mentioned Patent Documents 1 and 2 propose a means for improving the SSC resistance in a sour environment by adjusting the element contents in the chemical composition. However, the SSC resistance of steel materials in a sour environment may be improved by means other than the means proposed in the above-mentioned Patent Documents.

[0009] An object of the present disclosure is to provide a martensitic stainless steel material having excellent SSC resistance.

[0010] 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. The martensitic stainless steel material contains S, has an Fn1 defined by formula (1) of 5.0 or more, and has a number density ND1 of 3.0 pieces / mm ２ or more, containing S, Fn1 is 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions with an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces / mm ２ or less, and the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm ２ The formula is as follows: Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) where the element symbol in formula (1) is 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.

[0011] The martensitic stainless steel material of the present disclosure has excellent SSC resistance.

[0012] 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.

[0013] The present inventors have conducted research into martensitic stainless steel materials having excellent SSC resistance.

[0014] The present inventors first investigated martensitic stainless steel materials with excellent SSC resistance from the viewpoint of chemical composition, and found that the martensitic stainless steel materials contained, in mass%, 0.030% or less of C, 0.10 to 1.00% of Si, 0.10 to 2.00% of Mn, 0.040% or less of P, 0.0050% or less of S, 10.0 to 14.0% of Cr, 5.00 to 7.50% of Ni, 1.50 to 3.50% of Mo, 0.01 to 2.50% of Cu, 0.02 to 0.40% of Ti, 0.01 to 0.50% of V, 0.01 to 0.50% of Co, 0.01 to 0.50% of Al, 0.010 to 0.100% of Ca, The present inventors considered that excellent SSC resistance could be obtained if the martensitic stainless steel material had a chemical composition consisting of: N: 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.

[0015] The present inventors further investigated means for obtaining excellent SSC resistance from the viewpoint of the microstructure.

[0016] The present inventors first investigated the mechanism of SSC generation in martensitic stainless steel materials in sour environments, and as a result, the following facts were discovered.

[0017] In a sour environment, some of the inclusions present in the surface layer of a martensitic stainless steel material may dissolve, forming pits on the steel surface. These pits are thought to promote the occurrence of SSC in a sour environment.

[0018] In a sour environment, among inclusions in the surface layer of a martensitic stainless steel material, inclusions with a high S content, such as Mn sulfides or Ca sulfides, tend to dissolve preferentially. In the following description, 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.

[0019] High-S inclusions in the surface layer of martensitic stainless steel material dissolve in a sour environment and form pits. The pits formed by the high-S inclusions promote the occurrence of SSC in a sour environment. Therefore, if the number density of high-S inclusions in the martensitic stainless steel material can be reduced, the occurrence of SSC in a sour environment can be suppressed.

[0020] In order to reduce the number density of high-S inclusions, it is effective to reduce the S content in the steel material as much as possible. Therefore, the inventors attempted to improve SSC resistance by reducing the S content in the steel material as much as possible. However, not only is there a limit to how much the S content in the steel material can be reduced, but excessive reduction of the S content significantly increases the manufacturing cost.

[0021] Therefore, the present inventors have considered reducing the number density of high-S inclusions by other means while allowing a certain amount of S content in the steel material, rather than reducing the number density of high-S inclusions by reducing the S content as much as possible.

[0022] Here, the inventors have focused on composite inclusions, which are inclusions formed by the aggregation of multiple types of inclusions. ２ O ３ and oxides typified by MgO, sulfides typified by MnS and CaS, nitrides typified by TiN, etc. Note that TiN may form a solid solution with V. Inclusions other than oxides, sulfides, and nitrides may combine with oxides, sulfides, nitrides, etc. to form composite inclusions.

[0023] The S content in composite inclusions is lower than that of high-S inclusions. Therefore, in a sour environment, composite inclusions are less likely to dissolve than high-S inclusions. Even if composite inclusions dissolve, the S-containing portions of the composite inclusions dissolve locally. Therefore, the pits formed are very small. These very small pits are easily repassivated and do not serve as the starting point for SSC. Therefore, if S is incorporated into the composite inclusions, the pits can be kept small even after dissolution, improving SSC resistance. Furthermore, increasing the number density of composite inclusions allows S to be incorporated into the composite inclusions. Therefore, the amount of S available for the formation of high-S inclusions is reduced. As a result, the number density of high-S inclusions is reduced.

[0024] In inclusions, if Fn1 defined by formula (1) is 5.0 or more, the S content in the inclusions is sufficiently low. Such inclusions are considered to be composite inclusions. In the following explanation, inclusions containing S and having 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 %. If an element is not contained, "0" is substituted for the corresponding element symbol.

[0025] As described above, it is believed that increasing the number density of low-S inclusions in martensitic stainless steel materials can minimize the depressions caused by dissolution, thereby improving SSC resistance. Increasing the number density of low-S inclusions also allows more S to be incorporated into the low-S inclusions. This reduces the amount of S available for the formation of high-S inclusions. As a result, it is believed that the number density of high-S inclusions can be sufficiently reduced, further improving SSC resistance.

[0026] Based on the above considerations, the present inventors attempted to increase the number density of low-S inclusions in martensitic stainless steel materials, and as a result, found that the number density of high-S inclusions can be sufficiently reduced by increasing the number density of low-S inclusions in martensitic stainless steel materials.

[0027] However, even when the number density of low-S inclusions is increased and the number density of high-S inclusions is reduced, there are still cases where sufficient SSC resistance cannot be obtained in sour environments. Therefore, the present inventors conducted further studies and found the following.

[0028] In a sour environment, even if the inclusions are low in S, coarse low inclusions with an equivalent circle diameter of more than 10.0 μm are easily dissolved. Therefore, by increasing the number density of fine low inclusions with an equivalent circle diameter of 10.0 μm or less and reducing the number density of high S inclusions, and further 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 a sour environment can be improved.

[0029] Based on the above findings, the present inventors have further investigated and found that in a martensitic stainless steel material having the above-mentioned chemical composition, 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 pieces / mm ２ or more, containing S, Fn1 is 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions with an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces / mm ２ or less, and the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm ２ The present inventors have found that excellent SSC resistance can be obtained if the temperature is below 100°C.

[0030] The martensitic stainless steel material of this embodiment, which was completed based on the above findings, has the following configuration.

[0031] 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 of fine low-S inclusions having an equivalent circle diameter of 2.0 to 10.0 μm. ２ Furthermore, the number density ND2 of coarse low-S inclusions, which contain S, have Fn1 of 5.0 or more, and have a circle equivalent diameter of more than 10.0 μm, is 2.0 pieces / mm ２ Furthermore, the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm ２ The formula is as follows: Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) where the element symbol in formula (1) is 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.

[0032] 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 to 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 REM: 0.001 to 0.100%.

[0033] 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.

[0034] The martensitic stainless steel material of this embodiment will be described in detail below.

[0035] [Features of the martensitic stainless steel material of this embodiment] The martensitic stainless steel material of this embodiment satisfies the following feature 1 and feature 2. (Feature 1) The chemical composition, in mass%, is: 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.

[0036] (Feature 2) In a martensitic stainless steel material, S is contained, and Fn1 defined by formula (1) is 5.0 or more, and the number density ND1 of fine low-S inclusions, which are inclusions with an equivalent circle diameter of 2.0 to 10.0 μm, is 3.0 pieces / mm ２ or more, containing S, Fn1 is 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions with an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces / mm ２ or less, and the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm ２The formula is as follows: 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 inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol. Feature 1 and Feature 2 will be explained below. Note that, hereinafter, martensitic stainless steel material will also be simply referred to as "steel material".

[0037] [(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.

[0038] C: 0.030% or less Carbon (C) is unavoidably contained. In other words, the C content is greater than 0%. C improves the hardenability of the steel material and increases its strength. However, if the C content exceeds 0.030%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high, resulting in a decrease in the SSC resistance of the steel material. Therefore, the C content is 0.030% or less. A preferred lower limit of the C content is 0.001%, more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.007%. A preferred upper limit of the C content is 0.027%, more preferably 0.025%, and even more preferably 0.020%.

[0039] Si: 0.10 to 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 decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.10 to 1.00%. A preferred lower limit of the Si content is 0.11%, more preferably 0.12%, even more preferably 0.15%, and even more preferably 0.20%. A preferred upper limit of the Si content is 0.95%, even more preferably 0.90%, and even more preferably 0.85%.

[0040] Mn: 0.10 to 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 fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is excessive, numerous coarse Mn sulfides are formed. In this case, the Mn sulfides dissolve in a sour environment, forming pits. These pits may act as starting points for SSC, resulting in SSC. If the Mn content exceeds 2.00%, the above-mentioned pits will form, reducing SSC resistance, 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 preferred lower limit of the Mn content is 0.15%, more preferably 0.20%, and even more preferably 0.30%. The preferred upper limit of the Mn content is 1.90%, more preferably 1.80%, and even more preferably 1.70%.

[0041] P: 0.040% or less Phosphorus (P) is unavoidably contained. In other words, the lower limit of the P content is greater 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 of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the P content is 0.035%, more preferably 0.030%, and even more preferably 0.025%.

[0042] S: 0.0050% or less Sulfur (S) is unavoidably contained. In other words, the lower limit of the S content is greater 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 formed by 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. A preferred lower limit of the S content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. A preferred upper limit of the S content is 0.0048%, more preferably 0.0045%, even more preferably 0.0040%, and even more preferably 0.0035%.

[0043] Cr: 10.0 to 14.0% Chromium (Cr) forms a passive film on the surface of a steel material, improving the SSC resistance of the steel material. If the Cr content is less than 10.0%, the above effect cannot be fully achieved 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. 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 preferred lower limit of the Cr content is 10.2%, more preferably 10.4%, and even more preferably 10.6%. The preferred upper limit of the Cr content is 13.8%, more preferably 13.5%, even more preferably 13.2%, and even more preferably 13.0%.

[0044] Ni: 5.00 to 7.50% Nickel (Ni) is an austenite-forming element and transforms the structure after quenching into martensite. This increases the strength of the steel. Ni also forms sulfides on the passive film in sour environments. Ni sulfides react with 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 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 Ni content exceeds 7.50%, the hydrogen diffusion coefficient in the steel material decreases. 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 Ni content is 5.00 to 7.50%. A preferred lower limit of the Ni content is 5.10%, more preferably 5.20%, and even more preferably 5.30%. A preferred upper limit of the Ni content is 7.30%, more preferably 7.20%, even more preferably 7.10%, and even more preferably 6.90%.

[0045] Mo: 1.50 to 3.50% Molybdenum (Mo) forms sulfides on the passive film in sour environments. Mo sulfides react with 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. This improves the SSC resistance of the steel material. Mo also dissolves in the steel material, thereby increasing its strength. If the Mo content is less than 1.50%, the above-mentioned effect 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%, austenite becomes difficult to stabilize. 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 preferred lower limit of the Mo content is 1.55%, more preferably 1.60%, and even more preferably 1.80%. The preferred upper limit of the Mo content is 3.45%, more preferably 3.40%, even more preferably 3.30%, and even more preferably 3.20%.

[0046] Cu: 0.01 to 2.50% Copper (Cu) dissolves in steel to enhance the SSC resistance of the steel. Cu also forms sulfides on the passive film in sour environments. Cu sulfides react with 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. This improves the SSC resistance of the steel material. If the Cu content is less than 0.01%, the above effect cannot be sufficiently 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 deteriorates 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 preferred lower limit of the Cu content is 0.02%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.50%. The preferred upper limit of the Cu content is 2.00%, more preferably 1.50%, and even more preferably 1.35%.

[0047] Ti: 0.02 to 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 effect cannot be fully achieved 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. In this case, the toughness of the steel 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 preferred lower limit of the Ti content is 0.04%, more preferably 0.06%, and even more preferably 0.08%. The preferred upper limit of the Ti content is 0.35%, more preferably 0.30%, and even more preferably 0.28%.

[0048] V: 0.01 to 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%, the hardenability of the steel becomes excessively high, and the SSC resistance of the steel decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0.01 to 0.50%. The preferred lower limit of the V content is 0.02%, more preferably 0.04%, and even more preferably 0.06%. The preferred upper limit of the V content is 0.48%, more preferably 0.45%, and even more preferably 0.40%.

[0049] Cobalt (Co) forms sulfides on the passive film in a sour environment. Co sulfides react with chloride ions (Cl － ) and hydrogen sulfide ions (HS － ) from contacting the passive film, thereby preventing the passive film from being destroyed by chloride ions and hydrogen sulfide ions. This enhances 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 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 preferred lower limit of the Co content is 0.02%, more preferably 0.04%, and even more preferably 0.06%. The preferred upper limit of the Co content is 0.45%, more preferably 0.40%, and even more preferably 0.36%.

[0050] Al: 0.010 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.010%, the above effect cannot be fully achieved, 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. 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%. A preferred lower limit of the Al content is 0.012%, more preferably 0.015%, and even more preferably 0.020%. A preferred upper limit of the Al content is 0.095%, more preferably 0.090%, and even more preferably 0.085%. Note that the Al content in this specification refers to the content of sol. Al (acid-soluble Al).

[0051] Ca: 0.0001 to 0.0040% Calcium (Ca) combines with S in the steel material to form Ca sulfides, suppressing 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 effect cannot be fully achieved 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%, excessive Ca sulfides are formed. 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 Ca content is 0.0001 to 0.0040%. A preferred lower limit of the Ca content is 0.0002%, more preferably 0.0005%, and even more preferably 0.0010%. A preferred upper limit of the Ca content is 0.0035%, more preferably 0.0030%, and even more preferably 0.0025%.

[0052] N: 0.001 to 0.020% Nitrogen (N) combines with Ti to form fine Ti nitrides. The fine TiN has a pinning effect, suppressing grain coarsening. As a result, the strength of the steel is increased. If the N content is less than 0.001%, the above effect cannot be fully achieved, 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. In this case, the toughness of the steel is reduced, 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 preferred lower limit of the N content is 0.002%, more preferably 0.004%, and even more preferably 0.006%. The preferred upper limit of the N content is 0.019%, more preferably 0.018%, and even more preferably 0.017%.

[0053] O: 0.010% or less Oxygen (O) is unavoidably contained. In other words, the lower limit of the O content is greater 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 manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the O content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0054] The balance of the chemical composition of the martensitic stainless steel material according to this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment during industrial production of the martensitic stainless steel material, and are acceptable within a range that does not adversely affect the martensitic stainless steel material according to this embodiment.

[0055] [Regarding 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.

[0056] [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.

[0057] W: 0 to 1.50% Tungsten (W) is an optional element and does not necessarily need to 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 material. Even if even a small amount of W is contained, the above effect can be achieved to some extent. However, if the W content exceeds 1.50%, W combines with C to form coarse carbides. 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 W content is 0 to 1.50%. The preferred lower limit of the W content is 0.01%, more preferably 0.02%, and even more preferably 0.10%. The preferred upper limit of the W content is 1.40%, more preferably 1.30%, and even more preferably 1.20%.

[0058] Sn: 0 to 0.010% Tin (Sn) is an optional element and does not necessarily need to be contained. In other words, the Sn content may be 0%. When contained, that is, when the Sn content exceeds 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. 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 Sn content is 0 to 0.010%. The preferred lower limit of the Sn content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Sn content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0059] [Regarding Nb] The chemical composition of the martensitic stainless steel material according to this embodiment may further contain Nb instead of a portion of Fe. Nb: 0 to 0.50% Niobium (Nb) is an optional element and need not be contained. That is, the Nb content may be 0%. When 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 grain coarsening and increases the strength of the steel material. Even if even a small amount of Nb is contained, the above effect can be achieved to some extent. However, if the Nb content exceeds 0.50%, excessive precipitates are formed. 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 preferred lower limit of the Nb content is 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%.

[0060] [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.

[0061] B: 0 to 0.0100% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When 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 effect can be obtained to some extent. However, if the B content exceeds 0.0100%, coarse B nitrides are formed. 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 preferred lower limit of the B content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the B content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0080%.

[0062] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When contained, that is, when the Mg content exceeds 0%, Mg neutralizes S in the steel material by fixing it as sulfides, thereby improving the hot workability of the steel material. Even if even a small amount of Mg is contained, the above effect can be achieved to some extent. However, if the Mg content exceeds 0.0100%, coarse oxides are formed. 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 Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the Mg content is 0.0095%, more preferably 0.0091%, and even more preferably 0.0085%.

[0063] Rare Earth Elements: 0 to 0.100% Rare earth elements (REM) are optional elements and do not necessarily need to be contained. In other words, the REM content may be 0%. When contained, that is, when the REM content exceeds 0%, the REM fixes the 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 REM is contained, the above effect can be achieved to some extent. However, if the REM content exceeds 0.100%, coarse oxides are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the SSC resistance of the steel material will decrease. Therefore, the REM content is 0 to 0.100%. The preferred lower limit of the REM content is 0.001%, and more preferably 0.003%. The preferred upper limit of the REM content is 0.095%, and more preferably 0.090%.

[0064] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, the REM content in this specification refers to the total content of these elements.

[0065] [(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 symbol in formula (1) is substituted with the content of the element in the corresponding 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.

[0066] Furthermore, inclusions in martensitic stainless steel materials are defined as follows: (A) Inclusions that contain S, have an Fn1 of 5.0 or more, and have an equivalent circle diameter of 2.0 to 10.0 μm are defined as "fine low-S inclusions." (B) Inclusions that contain S, have an Fn1 of 5.0 or more, and have an equivalent circle diameter of more than 10.0 μm are defined as "coarse low-S inclusions." (C) Inclusions that contain S, have an Fn1 of less than 5.0, and have an equivalent circle diameter of 2.0 μm or more are defined as "high-S inclusions."

[0067] At this time, the number density ND1 of fine low-S inclusions in the martensitic stainless steel material is 3.0 pieces / mm ２ or more, and the number density ND2 of coarse low-S inclusions is 2.0 pieces / mm ２ The number density ND3 of high S inclusions is 2.0 pieces / mm ２ The following is the result.

[0068] In addition, even if an inclusion containing S and having an equivalent circle diameter of less than 2.0 μm dissolves and forms a dent, the dent quickly repassivates. Therefore, in a martensitic stainless steel material having the chemical composition of Feature 1, inclusions containing S and having an equivalent circle diameter of less than 2.0 μm do not affect SSC resistance.

[0069] As described above, in the martensitic stainless steel material of this embodiment, the S content in the chemical composition is not drastically reduced, but rather a certain amount of S is allowed 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 that are easily dissolved in a sour environment.

[0070] 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 and promote the occurrence of SSC. Therefore, the number density ND1 of fine low-S inclusions is increased, while the number density ND3 of high-S inclusions is reduced, and the number density ND2 of coarse low-S inclusions is also reduced.

[0071] [Number density ND1 of fine low-S inclusions] The number density ND1 of fine low-S inclusions is 3.0 pieces / mm ２ If the S content is less than 2.0 / mm3, the amount of fine low-S inclusions in the steel is insufficient. In this case, the S content available for forming high-S inclusions is large in the steel, so the number density ND3 of the high-S inclusions is 2.0 / mm3. ２ As a result, sufficient SSC resistance in a sour environment cannot be obtained. Therefore, the number density ND1 of fine low-S inclusions is 3.0 pieces / mm ２ The preferred lower limit of the number density ND1 is 3.5 pieces / mm ２ and more preferably 4.0 pieces / mm ２ and more preferably 4.5 pieces / mm ２ The upper limit of the number density ND1 is not particularly limited. 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 is.

[0072] [Number density ND2 of coarse low-S inclusions] The number density ND2 of coarse low-S inclusions is 2.0 pieces / mm ２ If the density ND2 of the coarse low-S inclusions exceeds 2.0 / mm, 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 the coarse low-S inclusions is 2.0 / mm. ２ The preferred upper limit of the number density ND2 is 1.9 pieces / mm ２ and more preferably 1.8 pieces / mm ２ and more preferably 1.7 pieces / mm ２ It is preferable that the number density ND2 is as small as possible. In other words, the number density ND2 is 0 pieces / mm ２ However, if the number density ND2 is reduced too much, the manufacturing cost may increase. Therefore, the preferable lower limit of the number density ND2 is 0.1 pieces / mm ２ and more preferably 0.2 pieces / mm ２ and more preferably 0.3 pieces / mm２ is.

[0073] [Regarding the number density ND3 of high-S inclusions] As mentioned above, high-S inclusions, even if small in size, are easily dissolved in a sour environment and tend to form pits. Pits formed from high-S inclusions tend to promote the occurrence of SSC even if they are small in size. Therefore, when the number density ND3 of high-S inclusions is 2.0 pieces / mm ２ If the number density ND3 of high-S inclusions exceeds 2.0 pieces / mm ２ The preferred upper limit of the number density ND3 is 1.9 pieces / mm ２ and more preferably 1.8 pieces / mm ２ and more preferably 1.7 pieces / mm ２ It is preferable that the number density ND3 is as small as possible. In other words, the number density ND3 is 0 pieces / mm ２ However, if the number density ND3 is reduced too much, the manufacturing cost may increase. Therefore, the preferable lower limit of the number density ND3 is 0.1 pieces / mm ２ and more preferably 0.2 pieces / mm ２ and more preferably 0.3 pieces / mm ２ is.

[0074] [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 (numbers / mm ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ) and the number density ND3 of high-S inclusions (number / mm ２ ) can be measured by the following method.

[0075] Test pieces are taken from 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 a cross section perpendicular to the axial direction of the round bar. In this specification, round bar means a steel bar having a circular cross section perpendicular to the axial direction. The R / 2 portion means the center position of the radius R in a cross section perpendicular to the axial direction of the round bar.

[0076] 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, and the total area of ​​the observation fields is 500 mm. ２ Using a scanning electron microscope equipped with a composition analysis function (SEM-EDS device), a backscattered electron image of each observation field is obtained at 800x magnification. The obtained backscattered electron images are observed, and particles are identified from the contrast. The position coordinates of all identified particles in the observation field are recorded.

[0077] Based on the position coordinates of the identified particles, element concentration analysis (EDS analysis) is performed on the particles. Specifically, based on the position coordinates, the identified particles are scanned with an electron beam to analyze the element concentration of the particles. At this time, the electron beam is not scanned only on a portion of the particle, but the entire particle. This allows the element concentration of the entire particle to be analyzed. In the 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.

[0078] 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 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 or abrasives that adhered during mirror polishing. Therefore, these particles are determined not to be inclusions and are excluded from the scope of the present invention. Furthermore, 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%, among the identified particles, particles with an S content of less than 1.0% are not S-containing inclusions but oxides or nitrides. Therefore, particles with an S content of less than 1.0% are also excluded from the scope of the present invention. In other words, 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%, among the identified particles, 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.

[0079] The equivalent circle diameter (μm) of particles identified as inclusions containing S is determined. The equivalent circle diameter means 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 obtained value to one decimal place.

[0080] Based on the EDS analysis results and 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 value of 5.0 or more and an equivalent circle diameter of 2.0 to 10.0 μm, as defined by formula (1), based on the Mg, Al, Ca, Ti, V, Mn, and S contents 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 identified as "fine low-S inclusions." Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in the particle (inclusion) in mass%. When an element is not contained, the corresponding element symbol is substituted with "0." (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."

[0081] The fine low-S inclusions, coarse low-S inclusions, and high-S inclusions identified by the above method are counted in each field of view.

[0082] Based on the total number of counted fine low-S inclusions in all observation fields and the total area of ​​all observation fields, the number density ND1 (pieces / mm ２Based on the total number of coarse low-S inclusions counted in all observation fields and the total area of ​​all observation fields, the number density ND2 (numbers / mm ２ Based on the total number of high-S inclusions counted in all observation fields and the total area of ​​all observation fields, the number density ND3 (numbers / mm ２ ) is required.

[0083] The number density ND1 (number / mm ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ), and the number density ND3 of high S inclusions (pieces / mm ２ ) is the value obtained by rounding off the obtained value to one decimal place. In addition, as the SEM-EDS device, for example, an automatic analyzer manufactured by FEI (ASPEX) under the trade name of Metals Quality Analyzer can be used.

[0084] [Effects of the Martensitic Stainless Steel Material of the Present Embodiment] The martensitic stainless steel material of the present embodiment satisfies Features 1 and 2. Therefore, excellent SSC resistance can be obtained in sour environments.

[0085] [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.

[0086] Round bar test pieces are 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, the round bar test piece is taken from the center of the plate thickness. The axial direction of the round bar test piece is parallel to the rolling direction of the steel plate. When the steel material is a round bar, the round bar test piece is taken from the R / 2 portion. The axial direction of the round bar test piece is parallel to the axial direction of the round bar. The round bar test piece has, for example, a diameter of 6.35 mm and a length of 25.4 mm at the parallel part.

[0087] The test solution is a 0.17% by mass sodium chloride aqueous solution with a pH of 3.0. The test solution is prepared by adding acetic acid to an aqueous solution containing 0.17% by mass sodium chloride and 0.41 g / L sodium acetate 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, it is heated to 0.04 bar H ２ S gas and 0.96 bar of CO ２ Gas was blown into the test bath, and H ２ Saturate the gas. ２ The test bath saturated with S gas is maintained at 24°C for 720 hours. After 720 hours of maintenance, the surface of the parallel part of the test piece is observed with a magnifying glass at 10x magnification to check for the presence or absence of cracks. If there is a location where cracks are suspected during the magnifying glass observation, the cross section of the suspected location is observed with an optical microscope at 100x magnification to check for the presence or absence of cracks.

[0088] 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 piece after the test is observed with a 10x magnification loupe and a 100x optical microscope.

[0089] [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.

[0090] In the microstructure, a small amount of retained austenite does not significantly decrease the strength and significantly increases the toughness of the steel material. However, if the volume fraction of retained austenite is too high, the strength of the steel material significantly decreases. 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 viewpoint of ensuring strength, the upper limit of the volume fraction of retained austenite is preferably 15%, and more preferably 10%. As described above, the microstructure of the steel material of this embodiment may be a martensite single 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 more than 0 to 20%, more preferably more than 0 to 15%, and even more preferably more than 0 to 10%.

[0091] [Method for Measuring Volume Fraction of Martensite] 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%.

[0092] The volume fraction of retained austenite is determined by X-ray diffraction. Specifically, a test specimen is taken from the steel material. If the steel material is a steel pipe, the test specimen is taken from the center of the wall thickness. If the steel material is a steel plate, the test specimen is taken from the center of the plate thickness. If the steel material is a round bar, the test specimen is taken from the R / 2 part. The size of the test specimen is not particularly limited. The test specimen is, for example, 15 mm x 15 mm x 2 mm thick. In this case, if the steel material is a steel pipe, the thickness direction of the test specimen is the pipe diameter direction. If the steel material is a steel plate, the thickness direction of the test specimen is the plate thickness direction. If the steel material is a round bar, the thickness direction of the test specimen is the radial 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 α phase surface and each γ phase surface. The average value of the volume fraction Vγ of the six pairs of retained austenite is then 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 off the obtained numerical value to one decimal place.

[0093] 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 by the following formula: Volume fraction of martensite = 100 - Volume fraction (%) of retained austenite

[0094] [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.

[0095] [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 performed according to ASTM E8 / E8M (2022). First, a tensile test specimen is taken from the martensitic stainless steel material. If 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. If the martensitic stainless steel material is a steel pipe and a round bar-shaped tensile test specimen cannot be taken from the steel pipe, an arc-shaped tensile test specimen is taken from the steel pipe. If 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. In this case, the longitudinal direction of the tensile test specimen is parallel to the rolling direction of the steel plate. If the martensitic stainless steel material is a round bar, a round bar-shaped tensile test specimen is taken from the R / 2 portion. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the round bar.

[0096] The size of the round bar-shaped tensile test specimen is, for example, 6 mm in diameter at the parallel portion and 30 mm in gauge length. The size of the arc-shaped tensile test specimen is, for example, the total thickness, 25.4 mm in width, and 50.8 mm in gauge length. A tensile test is performed using the tensile test specimen at room temperature (24±3°C) in the atmosphere. In this embodiment, the 0.2% offset proof stress 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.

[0097] [Shape and Use of Steel Material] 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.

[0098] [Manufacturing Method] An example of a manufacturing method for the martensitic stainless steel material of this embodiment having the above-described configuration will be described. Note that the manufacturing method for the martensitic stainless steel material of this embodiment is not limited to the manufacturing method described below. The example of a manufacturing method for the martensitic stainless steel material of 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.

[0099] [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. The molten steel produced by the primary refining process is subjected to secondary refining. In the secondary refining process, alloy elements are added for composition adjustment to produce molten steel that satisfies the chemical composition of Feature 1. In the secondary refining process, for example, RH (Ruhrstahl-Hausen) vacuum degassing treatment is performed, and then final adjustment of the alloy composition is performed. In the secondary refining process, combined refining may be performed. In this case, prior to the RH vacuum degassing treatment, for example, a refining process using an LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) is performed.

[0100] The molten steel that has been subjected to secondary refining is used to produce a material. Specifically, the molten steel that has been subjected to secondary refining is used to produce a cast piece (slab, bloom, or billet) by continuous casting.

[0101] In continuous casting, molten steel is first poured from a ladle into a tundish. The molten steel, 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 the outer surface of the slab is further cooled while cooling water is poured onto it, causing the slab to solidify. Through these manufacturing processes, slabs that will serve as the raw material for martensitic stainless steel are produced.

[0102] In the material preparation process, the bloom may further be bloomed into a billet. In this case, for example, the bloom is heated to 1150 to 1300°C. The heated bloom is then bloomed into a billet. Through the above processes, a martensitic stainless steel material (slab, bloom, or billet) is produced.

[0103] The following conditions are met in the continuous casting during the material preparation process: (Condition 1) The molten steel temperature 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 entry nozzle is 150 to 400 mm. Conditions 1 and 2 are explained below.

[0104] [Regarding Condition 1] As described above, fine low-S inclusions are formed by agglomeration 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.

[0105] 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 to 1500°C for a holding time t. If the holding time t is less than 5 minutes, coarse oxides in the molten steel do not rise sufficiently, and the coarse oxides remain in the molten steel and flow into the mold. In this case, the number density ND2 of coarse low-S inclusions in the manufactured martensitic stainless steel material becomes excessive.

[0106] On the other hand, if the holding time t exceeds 100 minutes, although the coarse oxides in the molten steel will rise sufficiently, the fine oxides will also become coarse and rise. As a result, the amount of fine oxides that serve as nuclei for the fine low-S inclusions will be insufficient in the molten steel flowing from the submerged entry nozzle into the mold. As a result, in the martensitic stainless steel material produced, the number density ND1 of the fine low-S inclusions will be excessively low, and the number density ND3 of the high-S inclusions will be excessive. Therefore, the holding time t is set to 5 to 100 minutes.

[0107] [Regarding Condition 2] Fig. 1 is a schematic diagram illustrating the positional relationship between molten steel in a mold and the submerged entry nozzle during continuous casting. Referring to Fig. 1, molten steel 10 passes through a submerged entry nozzle 20 and flows from a discharge port 22 outside the submerged entry nozzle 20 into a mold 30.

[0108] 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 side wall portion near the bottom of the cylindrical main body. More specifically, one of the discharge ports 22 is arranged at an angle of 180° around the central axis of the submerged nozzle 20 relative to the other discharge port 22. The discharge ports 22 are inclined upward at an angle θ of 5 to 35° with respect to the horizontal direction.

[0109] 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 determining distance D is the liquid level height (target level) set during actual continuous casting operation.

[0110] The distance D forms a stirring region DA between the liquid surface 11 and the discharge port 22 in the molten steel 10. In the stirring region DA, the molten steel flowing out of the discharge port 22 of the submerged nozzle 20 into the molten steel 10 rises, stirring the molten steel vertically. At this time, the temperature of the molten steel gradually decreases, resulting in the formation of sulfides and nitrides. If the molten steel flowing out of the discharge port 22 contains a sufficient amount of fine oxides, sulfides and nitrides are formed in the stirring region DA using the fine oxides as nuclei, or the sulfides and nitrides formed in the molten steel 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. The coarse low-S inclusions that float to the liquid surface 11 are absorbed into the molten slag layer. Therefore, although fine low-S inclusions remain in the molten steel, the amount of coarse low-S inclusions is reduced.

[0111] 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. The distance D is a factor that determines the size of the stirring area DA.

[0112] 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 becomes excessive, or the number density ND1 of fine low-S inclusions becomes excessively low.

[0113] 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.

[0114] Electromagnetic stirring may be performed on the molten steel 10 in the mold 30. Electromagnetic stirring stirs the molten steel in the horizontal direction. Therefore, electromagnetic stirring does not easily form a stirring region DA that stirs the molten steel in the vertical direction as shown in FIG. 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 discharge port 22 on the outer surface of the submerged entry nozzle 20 to 150 to 400 mm, a stirring region DA of an appropriate range can be formed.

[0115] [Hot Working Process] In the hot working process, a 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 an 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%.

[0116] A mother pipe may be manufactured from the billet by other hot working methods. For example, in the case of a short, thick-walled steel material such as a coupling, a mother pipe may be manufactured by forging using the Erhardt method or the like. A mother pipe is manufactured by the above-mentioned steps.

[0117] When the steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The 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.

[0118] When the steel material is round steel, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot-worked to produce round steel, which is an intermediate steel material. The hot-working is, for example, blooming using a blooming mill or hot rolling using a continuous rolling mill. The 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.

[0119] The intermediate steel produced by hot working may be air-cooled. Alternatively, the intermediate steel produced by hot working may be directly quenched after the hot working without being cooled to room temperature, or may be reheated after the hot working and then quenched.

[0120] When direct quenching is performed after hot working, or when quenching is performed after reheating after hot working, stress relief annealing (SR treatment) may be performed before the next heat treatment step (quenching and tempering) in order to remove residual stress.

[0121] [Heat Treatment Step] The heat treatment step includes a quenching step and a tempering step.

[0122] [Quenching process] In the heat treatment process, first, the intermediate steel material produced in the hot working process is quenched (quenching process). Quenching is performed by a well-known method. Specifically, the intermediate steel material after the hot working process is loaded into a heat treatment furnace and held at a quenching temperature. The quenching temperature is A Ｃ３ The temperature is equal to or higher than the transformation point, for example, 900 to 920°C. The intermediate steel material is held at the quenching temperature and then 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. The quenching method is not particularly limited. When the intermediate steel material is a mother pipe, the mother pipe may be quenched, for example, by immersing it in a water bath or an oil bath, or the mother pipe may be quenched by pouring or spraying cooling water onto the outer surface and / or inner surface of the mother pipe using shower cooling or mist cooling.

[0123] As described above, after the hot working step, the intermediate steel material may be quenched (direct quenched) immediately after the hot working without being cooled to room temperature, or the mother pipe after the hot working may be loaded into a reheating furnace before the temperature of the mother pipe drops, and then quenched after being held at the quenching temperature.

[0124] [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.

[0125] [Optional Manufacturing Step] The manufacturing method of the martensitic stainless steel material of 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.

[0126] The martensitic stainless steel material of this embodiment can be manufactured by the above-mentioned steps. Note that the above-mentioned manufacturing method of the martensitic stainless steel material is one example, and a 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 with reference to examples.

[0127] Martensitic stainless steel materials having the chemical compositions shown in Tables 1A, 1B and 1C were produced.

[0128]

[0129]

[0130]

[0131] Specifically, molten steel of each test number was produced, and a bloom was produced by continuous casting. In continuous casting, the holding time t (minutes) at which the molten steel temperature was 1600 to 1500°C in the tundish 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 inclination angle θ of the discharge port of the submerged entry nozzle (see Figure 1) was 15°.

[0132]

[0133] The bloom was subjected to blooming to produce a billet having an outer diameter of 310 mm. The bloom was heated to a temperature of 1150 to 1300°C during blooming. The produced billet was heated to 1250°C and then hot rolled by the Mannesmann process to produce a mother pipe (seamless steel pipe) having an outer diameter of 244.48 mm and a wall thickness of 13.84 mm.

[0134] 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 tempered. The tempering temperature was 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 material (seamless steel pipe) would be 758 MPa or more.

[0135] By the above manufacturing process, martensitic stainless steel materials (seamless steel pipes) of each test number were manufactured.

[0136] [Evaluation Tests] The following evaluation tests were carried out on the martensitic stainless steel materials of each test number produced. (Test 1) Measurement test of martensite volume fraction (%) (Test 2) Yield strength measurement test (Test 3) Number density ND1 (number / mm) of fine low-S inclusions ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ) and the number density ND3 of high-S inclusions (number / mm ２(Test 4) SSC Resistance Evaluation Test Each test will be explained below.

[0137] [(Test 1) Measurement of Martensite Volume Fraction (%)] Based on the method described in the above [Method for Measuring Martensite Volume Fraction], the martensite volume fraction (%) of the martensitic stainless steel material of each test number was determined. 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.

[0138] [(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 [Yield Strength Measurement Method]. Round bar-shaped test specimens with a parallel portion diameter of 6 mm and a gauge length of 30 mm were taken from the martensitic stainless steel material of each test number as tensile test specimens. The obtained yield strengths (MPa) are shown in the "Yield Strength (MPa)" column in Table 3.

[0139]

[0140] [(Test 3) Number density ND1 of fine low-S inclusions (pieces / mm ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ) and the number density ND3 of high-S inclusions (number / mm ２ Based on the method described in the above [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], the number density ND1 (numbers / mm ２ ), the number density of coarse low-S inclusions ND2 (pieces / mm ２ ) and the number density ND3 of high-S inclusions (number / mm ２ At this time, the observation field was set to five fields of 10 mm × 10 mm, and the total area of ​​the observation fields was 500 mm ２ The measured number densities ND1 to ND3 (pieces / mm ２ ) in Table 3, "Number density ND1 (pieces / mm ２) and "Number density ND2 (pieces / mm ２ ) and "Number density ND3 (pieces / mm ２ ) are shown below.

[0141] [(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.

[0142] [Evaluation Results] With reference to Tables 1A, 1B, 1C, 2, and 3, the martensitic stainless steel materials of test numbers 1 to 19 satisfied characteristics 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.

[0143] On the other hand, in test numbers 20 and 21, the holding time t at the 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 was 2.0 pieces / mm ２ As a result, excellent SSC resistance was not obtained.

[0144] In test numbers 22 and 23, the holding time t at the molten steel temperature of 1600 to 1500°C in the continuous casting tundish in the material preparation process was too long. As a result, the number density ND1 of fine low-S inclusions was 3.0 pieces / mm ２ Furthermore, the number density ND3 of high S inclusions is less than 2.0 pieces / mm ２ As a result, excellent SSC resistance was not obtained.

[0145] In test numbers 24 and 25, the vertical distance D from the liquid surface of the molten steel in the mold to the center of the outlet on the outer surface of the submerged entry nozzle during continuous casting in the material preparation process was too short. As a result, the number density ND1 of fine low-S inclusions was 3.0 pieces / mm２ Furthermore, the number density ND3 of high S inclusions is less than 2.0 pieces / mm ２ As a result, excellent SSC resistance was not obtained.

[0146] In test numbers 26 and 27, the distance D was too short in the continuous casting of the material preparation process. As a result, the number density ND3 of high-S inclusions was 2.0 pieces / mm ２ As a result, excellent SSC resistance was not obtained.

[0147] In test numbers 28 and 29, the distance D was too long in the continuous casting of the material preparation process. As a result, the number density ND2 of the coarse low-S inclusions was 2.0 pieces / mm ２ As a result, excellent SSC resistance was not obtained.

[0148] The embodiments of the present disclosure have been described above. 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 appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A martensitic stainless steel material having 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 0.50%, 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, wherein the martensitic stainless steel material contains S, has an Fn1 defined by formula (1) of 5.0 or more, and has a number density ND1 of 3.0 pieces / mm ２ or more, containing S, the Fn1 being 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 / mm ２ or less, and the number density ND3 of high-S inclusions, which contain S, have Fn1 of less than 5.0 and have an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm ２ Martensitic stainless steel material: Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) where the element symbol in formula (1) is substituted with the content of the element in the corresponding inclusion in mass %. When an element is not contained, "0" is substituted for the corresponding element symbol.

2. A martensitic stainless steel material according to claim 1, wherein the chemical composition contains one or more elements selected from the group consisting of W: 0.01 to 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%.

3. A martensitic stainless steel material according to claim 1 or 2, wherein the martensitic stainless steel material is a steel pipe.

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