Martensitic stainless steel material

Abstract

A martensitic stainless steel material that has high strength, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment is provided. The martensitic stainless steel material according to the present disclosure has the chemical composition described in the description, has a yield strength of 550 to less than 758 MPa, and has a microstructure composed of, in percent by volume, retained austenite in an amount of 10.0 to 40.0% and ferrite in an amount of 0 to 5.0%, with the balance being martensite. In this martensitic stainless steel material, in addition, in the steel material, when a square having sides with a length of 30 µm is defined as a measurement region, and the measurement region is divided into nine specified regions which are each a square having sides with a length of 10 µm, in each of eight or more of the specified regions among nine of the specified regions included in the measurement region, a standard deviation of a Ni concentration satisfies 0.50% by mass or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a steel material, and more particularly relates to a martensitic stainless steel material.BACKGROUND ART

[0002] An increase in the concentration of carbon dioxide (CO 2 ) above ground is currently a global problem. Therefore, efforts to suppress CO 2 emissions are proceeding. One such effort to suppress CO 2 emissions that is attracting particular attention is CCUS.

[0003] CCUS is an abbreviation for "carbon dioxide capture, utilization and storage". That is, CCUS includes the three technologies of capturing, utilizing, and storing CO 2 . Among these, as technology for storing CO 2 , technology which captures CO 2 emitted from an industrial facility such as an electric power plant or a factory, and injects the CO 2 into a depleted oil well to store the CO 2 therein is attracting attention.

[0004] A steel material to be used for such kind of CO 2 storage technology is required to have high strength in order to inject CO 2 into a depleted oil well. Further, CO 2 is a corrosive substance that corrodes the steel material. Therefore, a steel material to be used for CO 2 storage technology is required to have excellent corrosion resistance in a corrosive environment that contains a large amount of CO 2 .

[0005] Martensitic stainless steel materials containing about 13% by mass of Cr that are typified by API L80 13Cr steel material and Super 13Cr steel material in which the content of C is reduced are already known as steel materials which have high strength and which are excellent in corrosion resistance in corrosive environments. Specifically, Japanese Patent Application Publication No. 2000-192196 (Patent Literature 1), and Japanese Patent Application Publication No. 2012-136742 (Patent Literature 2) each propose a martensitic stainless steel material which has high strength and which is excellent in corrosion resistance in a corrosive environment.

[0006] The martensitic stainless steel material proposed in Patent Literature 1 is a martensitic stainless steel for oil wells consisting of, in weight%, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 0.05 to 2%, P: 0.025% or less, S: 0.01% or less, Cr: 9 to 14%, Mo: 3.1 to 7%, Ni: 1 to 8%, Co: 0.5 to 7%, sol. Al: 0.001 to 0.1%, N: 0.05% or less, O (oxygen): 0.01% or less, Cu: 0 to 5%, and W: 0 to 5%, with the balance being Fe and impurities. When a steel material contains Mo, the Ms point decreases. Therefore, because this steel material contains Co as well as Mo, a decrease in the Ms point is suppressed, and the microstructure is made a martensitic single-phase structure. It is disclosed in Patent Literature 1 that, as a result, in this steel material, the corrosion resistance in a corrosive environment can be increased while maintaining the strength at a yield strength of 552 MPa or more.

[0007] The martensitic stainless steel material proposed in Patent Literature 2 is a martensitic stainless steel seamless pipe consisting of, in mass%, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0 to 15.5%, Ni: 5.5 to 7.0%, Mo: 2.0 to 3.5%, Cu: 0.3 to 3.5%, V: 0.20% or less, Al: 0.05% or less, and N: 0.06% or less, with the balance being Fe and impurities. This steel material has a yield strength of 655 to 862 MPa, and a yield ratio of 0.90 or more. It is disclosed in Patent Literature 2 that with respect to this steel material, by making the content of C 0.01% or less, adjusting Cr, Ni and Mo to within an appropriate range, and also containing appropriate amounts of Cu and V or an appropriate amount of W, excellent corrosion resistance is obtained in a corrosive environment.CITATION LISTPATENT LITERATURE

[0008] Patent Literature 1: Japanese Patent Application Publication No. 2000-192196 Patent Literature 2: Japanese Patent Application Publication No. 2012-136742 SUMMARY OF INVENTIONTECHNICAL PROBLEM

[0009] As mentioned above, according to the technology for storing carbon dioxide (CO 2 ), captured CO 2 is injected into a depleted oil well or the like and stored therein. On the other hand, in some cases oxides such as SOx are mixed in CO 2 that is captured from an industrial facility such as an electric power plant or a factory. The term "SOx" is a generic term for sulfur oxides typified by SO 2 . In addition, SOx dissolves in water and forms an acidic compound (sulfuric acid, sulfurous acid or the like). That is, when an oxide typified by SOx is mixed in CO 2 , the corrosive environment tends to become acidic, and the steel material in the corrosive environment is liable to be further corroded. Therefore, a steel material to be used for such kind of carbon dioxide storage technology is required to have not only high strength, but also to have excellent corrosion resistance in a corrosive environment in which SOx is mixed.

[0010] In recent years, in addition, in some cases a steel material is required to have toughness in an extremely low temperature environment when storing carbon dioxide. Specifically, if a change in the pressure of carbon dioxide gas that is being stored occurs, the temperature of the gas that is being stored may decrease due to the Joule-Thomson effect. In such a case, the steel material may be required to have toughness in an extremely low temperature environment of -70°C, which is far below the usual temperature. Therefore, a martensitic stainless steel material for which application to technology for storing carbon dioxide in such kind of extremely low temperature environments is assumed is also required to have low-temperature toughness in an extremely low temperature environment of -70°C or less, in addition to having high strength and having excellent corrosion resistance in a corrosive environment in which SOx is mixed.

[0011] Here, the aforementioned Patent Literatures 1 and 2 propose martensitic stainless steel materials that have high strength and excellent corrosion resistance in a corrosive environment. However, in the aforementioned Patent Literatures 1 and 2, the corrosion resistance of the steel material in a corrosive environment in which SOx is mixed has not been examined at all. In addition, in the aforementioned Patent Literatures 1 and 2, the low-temperature toughness in an extremely low temperature environment of -70°C or less has also not been examined at all.

[0012] An objective of the present disclosure is to provide a martensitic stainless steel material having high strength, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment.SOLUTION TO PROBLEM

[0013] A martensitic stainless steel material according to the present disclosure consists of, in mass%, C: 0.030% or less, Si: 0.50% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.50 to 3.50%, Cr: 10.00 to 14.00%, Ni: 4.50 to 6.50%, Mo: 1.00 to 3.50%, W: 0.10 to 0.50%, Ti: 0.050 to 0.300%, V: 0.01 to 0.50%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0002 to 0.0050%, N: 0.0500% or less, O: 0.050% or less, Nb: 0 to 0.500%, Mg: 0 to 0.0050%, As: 0 to 0.100%, Sn: 0 to 0.100%, Sb: 0 to 0.100%, B: 0 to 0.0050%, rare earth metal: 0 to 0.0050%, and, the balance: Fe and impurities, wherein: a yield strength is 550 to less than 758 MPa, a microstructure is composed of, in percent by volume, retained austenite in an amount of 10.0 to 40.0%, and ferrite in an amount of 0 to 5.0%, with the balance being martensite, and in the martensitic stainless steel material, a square having sides with a length of 30 µm is defined as a measurement region, and the measurement region is divided into nine specified regions which are each a square having sides with a length of 10 µm, in each of eight or more of the specified regions among nine of the specified regions included in the measurement region, a standard deviation of a Ni concentration is 0.50% by mass or more. ADVANTAGEOUS EFFECTS OF INVENTION

[0014] The martensitic stainless steel material according to the present disclosure has high strength, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment.BRIEF DESCRIPTION OF DRAWINGS

[0015] [FIG. 1] FIG. 1 is a schematic diagram showing the distribution of a Ni concentration in a measurement region having sides with a length of 30 µm, with respect to one example of a martensitic stainless steel material that satisfies the chemical composition described above. [FIG. 2] FIG. 2 is a schematic diagram showing the distribution of a Ni concentration in a measurement region 50 having sides with a length of 30 µm, with respect to one example of a martensitic stainless steel material according to the present embodiment. [FIG. 3] FIG. 3 is a schematic diagram showing the distribution of a Ni concentration in a measurement region 50 having sides with a length of 30 µm, with respect to a different example to FIG. 2 of a martensitic stainless steel material according to the present embodiment. DESCRIPTION OF EMBODIMENTS

[0016] The present inventors firstly conducted studies regarding obtaining a martensitic stainless steel material having a yield strength of 550 to less than 758 MPa for which application to carbon dioxide storage technology is assumed. That is, the present inventors carried out investigations and studies regarding techniques for obtaining a yield strength of 550 to less than 758 MPa, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment in a martensitic stainless steel material for which application to carbon dioxide storage technology is assumed. As a result, the present inventors obtained the following findings.

[0017] Initially, the present inventors focused on the chemical composition, and conducted studies regarding obtaining a martensitic stainless steel material having a yield strength of 550 to less than 758 MPa, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment. As a result, the present inventors considered that if a martensitic stainless steel material consists of, in mass%, C: 0.030% or less, Si: 0.50% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.50 to 3.50%, Cr: 10.00 to 14.00%, Ni: 4.50 to 6.50%, Mo: 1.00 to 3.50%, W: 0.10 to 0.50%, Ti: 0.050 to 0.300%, V: 0.01 to 0.50%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0002 to 0.0050%, N: 0.0500% or less, O: 0.050% or less, Nb: 0 to 0.500%, Mg: 0 to 0.0050%, As: 0 to 0.100%, Sn: 0 to 0.100%, Sb: 0 to 0.100%, B: 0 to 0.0050%, rare earth metal: 0 to 0.0050%, and the balance: Fe and impurities, there is a possibility of obtaining a yield strength of 550 to less than 758 MPa, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment.

[0018] Next, the present inventors focused on the microstructure, and conducted studies regarding obtaining a martensitic stainless steel material having the chemical composition described above, a yield strength of 550 to less than 758 MPa, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment. As a result, the present inventors considered that if a martensitic stainless steel material having the chemical composition described above has a microstructure composed of, in percent by volume, retained austenite in an amount of 10.0 to 40.0% and ferrite in an amount of 0 to 5.0%, with the balance being martensite, there is a possibility of obtaining a yield strength of 550 to less than 758 MPa, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment.

[0019] On the other hand, even when martensitic stainless steel materials had the chemical composition and microstructure described above, in some cases a yield strength of 550 to less than 758 MPa, excellent corrosion resistance in a corrosive environment in which SOx was mixed, and excellent low-temperature toughness in an extremely low temperature environment were not obtained. Therefore, the present inventors produced various kinds of martensitic stainless steel materials having the chemical composition and microstructure described above and a yield strength of 550 to less than 758 MPa, and conducted detailed investigations and studies regarding a technique for increasing the corrosion resistance in a corrosive environment in which SOx is mixed, and low-temperature toughness in an extremely low temperature environment. As a result, it was revealed that in a martensitic stainless steel material having the chemical composition and microstructure described above and a yield strength of 550 to less than 758 MPa, if the distribution state of the Ni concentration in the steel material can be controlled, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment can be obtained. This point will now be described specifically using the drawings.

[0020] FIG. 1 and FIG. 2 are each a schematic diagram showing the distribution of a Ni concentration in a measurement region having sides with a length of 30 µm with respect to an example of a martensitic stainless steel material with a chemical composition satisfying the chemical composition described above. In FIG. 1 and FIG. 2, the distribution of the Ni concentration is represented by black and white gradations. Specifically, in FIG. 1 and FIG. 2, a whitish region (denoted by reference numeral 10 in FIG. 2) is a region in which the Ni concentration is relatively high, and a blackish region (denoted by reference numeral 20 in FIG. 2) is a region in which the Ni concentration is relatively low. FIG. 1 and FIG. 2 were obtained by performing elemental analysis using a field emission-electron probe micro-analyzer (FE-EPMA) that is described later on martensitic stainless steel materials having the chemical composition described above. Further, the martensitic stainless steel materials shown in FIGS. 1 and 2 each had a yield strength of 550 to less than 758 MPa.

[0021] Referring to FIG. 1 and FIG. 2, it is found that in a measurement region 60 in FIG. 1, the Ni concentration is uniform in comparison to a measurement region 50 in FIG. 2. On the other hand, in the measurement region 50 in FIG. 2, high Ni concentration regions 10 and low Ni concentration regions 20 are alternately disposed, and the changes in the Ni concentration are also large compared to the measurement region 60 in FIG. 1. Referring further to FIG. 2, the distribution of the Ni concentration described above is not locally formed. The result of studies conducted by the present inventors showed that excellent corrosion resistance in a corrosive environment in which SOx was mixed, and excellent low-temperature toughness in an extremely low temperature environment were obtained in the martensitic stainless steel material shown in FIG. 2.

[0022] As a result of detailed studies conducted by the present inventors based on the above findings, it was clarified that in a martensitic stainless steel material having the chemical composition and microstructure described above and having a yield strength of 550 to less than 758 MPa, when a square with sides of 30 µm in length is defined as a measurement region 50, and the measurement region 50 is divided into nine specified regions which are each a square with sides of 10 µm in length, if the standard deviation of the Ni concentration is 0.50% by mass or more in each of eight or more specified regions among the nine specified regions included in the measurement region 50, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment are obtained.

[0023] Specifically, this will be described in further detail referring to the drawings. FIG. 3 is a schematic diagram showing the distribution of the Ni concentration in a measurement region 50 having sides with a length of 30 µm with respect to a different example to FIG. 2 of the martensitic stainless steel material according to the present embodiment. Referring to FIG. 3, line segments that extend in the horizontal direction in the drawing to vertically trisect the measurement region 50 are denoted by reference characters H1 and H2. Line segments that extend in the vertical direction in the drawing to transversely trisect the measurement region 50 are denoted by reference characters V1 and V2. As illustrated in FIG. 3, the measurement region 50 is divided into nine regions by the line segments H1, H2, V1 and V2. In the present description, each of the nine regions into which the measurement region 50 is divided in this way is defined as a "specified region". Each specified region is a square with sides of 10 µm in length.

[0024] Referring to FIG. 2 and FIG. 3, in the martensitic stainless steel material according to the present embodiment, high Ni concentration regions 10 and low Ni concentration regions 20 are alternately disposed, and this distribution of the Ni concentration is not a distribution that is locally formed. On the other hand, the specified regions defined as described above are squares with sides of 10 µm in length, and are thus extremely small. Therefore, in a case where the standard deviation of the Ni concentration is determined for each of the nine specified regions, it is possible that a case may arise in which an outlier is included in the obtained nine standard deviations of the Ni concentration. However, if the standard deviation of the Ni concentration is 0.50% by mass or more in each of eight or more specified regions among the nine specified regions included in the measurement region 50, it can be determined that the high Ni concentration regions 10 and the low Ni concentration regions 20 are alternately disposed in almost all of the measurement region 50. Therefore, in the martensitic stainless steel material according to the present embodiment, the standard deviation of the Ni concentration in each of eight or more specified regions among the nine specified regions included in the measurement region 50 is defined.

[0025] Note that, the reason why excellent corrosion resistance in a corrosive environment in which SOx is mixed and excellent low-temperature toughness in an extremely low temperature environment are obtained if the standard deviation of the Ni concentration is 0.50% by mass or more in each of eight or more specified regions among the aforementioned nine specified regions included in the measurement region 50 has not been clarified in detail. However, the present inventors surmise that the reason is as follows. Among the phases of the microstructure of the steel material according to the present embodiment, the toughness is highest in the retained austenite phase. In addition, there is a possibility that Ni concentrates in the retained austenite and further increases the toughness of the retained austenite. In other words, if retained austenite and martensite are alternately disposed in the steel material and, in addition, Ni concentrates in the retained austenite, there is a possibility that propagation of cracks can be suppressed by the retained austenite in which Ni is concentrated, even in a corrosive environment in which SOx is mixed or in an extremely low temperature environment of -70°C or less.

[0026] The present inventors surmise that because of the mechanism described above, high strength, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment are obtained in the martensitic stainless steel material according to the present embodiment. Note that, there is also a possibility that high strength, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment are obtained due to a mechanism other than the mechanism described above if the standard deviation of the Ni concentration is 0.50% by mass or more in each of eight or more specified regions among the aforementioned nine specified regions included in the measurement region 50. However, the fact that high strength, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment are obtained when the standard deviation of the Ni concentration is 0.50% by mass or more in each of eight or more specified regions among the aforementioned nine specified regions included in the measurement region 50 has been demonstrated by Examples that are described later.

[0027] The gist of the martensitic stainless steel material according to the present embodiment, which has been completed based on the above findings, is as follows. [1] A martensitic stainless steel material, consisting of, in mass%, C: 0.030% or less, Si: 0.50% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.50 to 3.50%, Cr: 10.00 to 14.00%, Ni: 4.50 to 6.50%, Mo: 1.00 to 3.50%, W: 0.10 to 0.50%, Ti: 0.050 to 0.300%, V: 0.01 to 0.50%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0002 to 0.0050%, N: 0.0500% or less, O: 0.050% or less, Nb: 0 to 0.500%, Mg: 0 to 0.0050%, As: 0 to 0.100%, Sn: 0 to 0.100%, Sb: 0 to 0.100%, B: 0 to 0.0050%, rare earth metal: 0 to 0.0050%, and the balance: Fe and impurities, wherein: a yield strength is 550 to less than 758 MPa, a microstructure is composed of, in percent by volume, retained austenite in an amount of 10.0 to 40.0%, and ferrite in an amount of 0 to 5.0%, with the balance being martensite, and in the martensitic stainless steel material, a square having sides with a length of 30 µm is defined as a measurement region, and the measurement region is divided into nine specified regions which are each a square having sides with a length of 10 µm, in each of eight or more of the specified regions among nine of the specified regions included in the measurement region, a standard deviation of a Ni concentration is 0.50% by mass or more. [2] The martensitic stainless steel material according to [1], containing one or more elements selected from a group consisting of: Nb: 0.001 to 0.500%, Mg: 0.0001 to 0.0050%, As: 0.001 to 0.100%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.100%, B: 0.0001 to 0.0050%, and rare earth metal: 0.0001 to 0.0050%. [3] The martensitic stainless steel material according to [1] or [2], wherein: the martensitic stainless steel material is a martensitic stainless steel seamless pipe.

[0028] The shape of the martensitic stainless steel material according to the present embodiment is not particularly limited. The martensitic stainless steel material according to the present embodiment may be a steel pipe, may be a round steel bar (solid material), or may be a steel plate. Note that, the term "round steel bar" refers to a steel bar in which a cross section in a direction perpendicular to the axial direction is a circular shape. Further, the steel pipe may be a seamless steel pipe or may be a welded steel pipe.

[0029] Hereunder, the martensitic stainless steel material according to the present embodiment is described in detail. The symbol "%" in relation to an element means mass percent unless otherwise stated. Further, in the following description, the martensitic stainless steel material is also referred to as simply "steel material".[Chemical composition]

[0030] The martensitic stainless steel material according to the present embodiment contains the following elements.C: 0.030% or less

[0031] Carbon (C) is unavoidably contained. That is, the lower limit of the content of C is more than 0%. C increases hardenability of the steel material and increases strength of the steel material. On the other hand, if the content of C is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high. As a result, the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of C is to be 0.030% or less. A preferable upper limit of the content of C is 0.028%, more preferably is 0.025%, further preferably is 0.020%, and further preferably is 0.018%. The content of C is preferably as low as possible. However, extremely reducing the content of C will increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of C is 0.001 %, more preferably is 0.003%, and further preferably is 0.005%.Si: 0.50% or less

[0032] Silicon (Si) is unavoidably contained. That is, the lower limit of the content of Si is more than 0%. Si deoxidizes the steel. On the other hand, if the content of Si is too high, even if the contents of other elements are within the range of the present embodiment, hot workability of the steel material will decrease. Therefore, the content of Si is to be 0.50% or less. A preferable lower limit of the content of Si for effectively obtaining the aforementioned advantageous effect is 0.01%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.15%. A preferable upper limit of the content of Si is 0.45%, more preferably is 0.40%, and further preferably is 0.35%.Mn: 1.00% or less

[0033] Manganese (Mn) is unavoidably contained. That is, the lower limit of the content of Mn is more than 0%. Mn increases hardenability of the steel material and increases strength of the steel material. On the other hand, if the content of Mn is too high, even if the contents of other elements are within the range of the present embodiment, the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of Mn is to be 1.00% or less. A preferable lower limit of the content of Mn for effectively obtaining the aforementioned advantageous effect is 0.01%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.15%. A preferable upper limit of the content of Mn is 0.80%, more preferably is 0.70%, further preferably is 0.60%, and further preferably is 0.50%.P: 0.030% or less

[0034] Phosphorus (P) is an impurity that is unavoidably contained. That is, the lower limit of the content of P is more than 0%. If the content of P is too high, even if the contents of other elements are within the range of the present embodiment, P will segregate at grain boundaries and the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of P is to be 0.030% or less. A preferable upper limit of the content of P is 0.025%, more preferably is 0.020%, and further preferably is 0.018%. The content of P is preferably as low as possible. However, extremely reducing the content of P will raise the production cost. Accordingly, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.S: 0.0050% or less

[0035] Sulfur (S) is an impurity that is unavoidably contained. That is, the lower limit of the content of S is more than 0%. If the content of S is too high, even if the contents of other elements are within the range of the present embodiment, S will segregate at grain boundaries and the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of S is to be 0.0050% or less. A preferable upper limit of the content of S is 0.0040%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%. The content of S is preferably as low as possible. However, extremely reducing the content of S will raise the production cost. Accordingly, when taking industrial production into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.Cu: 0.50 to 3.50%

[0036] Copper (Cu) increases the corrosion resistance, and in particular Cu markedly increases the corrosion resistance of the steel material in a corrosive environment in which SOx is mixed. If the content of Cu is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Cu is too high, hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Cu is to be 0.50 to 3.50%. A preferable lower limit of the content of Cu is 0.53%, more preferably is 0.55%, further preferably is 0.60%, further preferably is 0.65%, further preferably is 0.70%, and further preferably is 0.80%. A preferable upper limit of the content of Cu is 3.30%, more preferably is 3.10%, and further preferably is 2.90%.Cr: 10.00 to 14.00%

[0037] Chromium (Cr) forms a passive film on the surface of the steel material and thereby increases the corrosion resistance of the steel material. If the content of Cr is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Cr is too high, even if the contents of other elements are within the range of the present embodiment, the volume ratio of ferrite in the microstructure will be too high, and the low-temperature toughness of the steel material will decrease. Therefore, the content of Cr is to be within the range of 10.00 to 14.00%. A preferable lower limit of the content of Cr is 10.20%, more preferably is 10.50%, further preferably is 11.00%, further preferably is 11.20%, and further preferably is 11.50%. A preferable upper limit of the content of Cr is 13.80%, more preferably is 13.60%, and further preferably is 13.50%.Ni: 4.50 to 6.50%

[0038] Nickel (Ni) concentrates in retained austenite in the microstructure, and markedly increases the low-temperature toughness of the steel material in an extremely low temperature environment of -70°C or less. In this case, in addition, Ni increases the corrosion resistance of the steel material in a corrosive environment in which SOx is mixed. Ni increases hardenability of the steel material and increases the yield strength of the steel material. Ni also increases the corrosion resistance of the steel material. If the content of Ni is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Ni is too high, even if the contents of other elements are within the range of the present embodiment, in some cases the corrosion resistance of the steel material may decrease. Therefore, the content of Ni is to be 4.50 to 6.50%. A preferable lower limit of the content of Ni is 4.60%, more preferably is 4.75%, further preferably is 5.00%, further preferably is 5.10%, further preferably is 5.50%, and further preferably is 6.00%. A preferable upper limit of the content of Ni is 6.40%, more preferably is 6.20%, further preferably is 6.00%, and further preferably is 5.90%.Mo: 1.00 to 3.50%

[0039] Molybdenum (Mo) stabilizes the passive film and thereby increases the corrosion resistance of the steel material. If the content of Mo is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Mo is too high, even if the contents of other elements are within the range of the present embodiment, the volume ratio of ferrite in the microstructure will be too high, and the low-temperature toughness of the steel material will decrease. Therefore, the content of Mo is to be 1.00 to 3.50%. A preferable lower limit of the content of Mo is 1.20%, more preferably is 1.50%, and further preferably is 1.80%. A preferable upper limit of the content of Mo is 3.30%, more preferably is 3.00%, and further preferably is 2.80%.W: 0.10 to 0.50%

[0040] Tungsten (W) stabilizes the passive film and thereby increases the corrosion resistance of the steel material. If the content of W is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of W is too high, even if the contents of other elements are within the range of the present embodiment, coarse carbides will be formed and in some cases the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of W is to be 0.10 to 0.50%. A preferable lower limit of the content of W is 0.15%, more preferably is 0.18%, further preferably is 0.20%, and further preferably is 0.25%. A preferable upper limit of the content of W is 0.45%, more preferably is 0.40%, and further preferably is 0.35%.Ti: 0.050 to 0.300%

[0041] Titanium (Ti) combines with C and / or N to form carbides or nitrides. As a result, the yield strength of the steel material is increased. If the content of Ti is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Ti is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high, and the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of Ti is to be 0.050 to 0.300%. A preferable lower limit of the content of Ti is 0.060%, and more preferably is 0.080%. A preferable upper limit of the content of Ti is 0.250%, more preferably is 0.200%, and further preferably is 0.180%.V: 0.01 to 0.50%

[0042] Vanadium (V) increases hardenability of the steel material and raises the yield strength of the steel material. If the content of V is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of V is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of V is to be 0.01 to 0.50%. A preferable lower limit of the content of V is 0.02%, and more preferably is 0.04%. A preferable upper limit of the content of V is 0.45%, and more preferably is 0.40%.Al: 0.001 to 0.100%

[0043] Aluminum (Al) deoxidizes the steel. If the content of Al is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Al is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will form and the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of Al is to be 0.001 to 0.100%. A preferable lower limit of the content of Al is 0.005%, more preferably is 0.010%, and further preferably is 0.015%. A preferable upper limit of the content of Al is 0.080%, more preferably is 0.060%, further preferably is 0.055%, and further preferably is 0.050%. As used in the present description, the term "content of Al" means the content of sol. Al (acid-soluble Al).Co: 0.010 to 0.500%

[0044] Cobalt (Co) stabilizes the passive film and increases the corrosion resistance of the steel material. Co also increases hardenability of the steel material and raises the yield strength of the steel material. If the content of Co is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Co is too high, even if the contents of other elements are within the range of the present embodiment, toughness of the steel material will decrease. Therefore, the content of Co is to be 0.010 to 0.500%. A preferable lower limit of the content of Co is 0.015%, more preferably is 0.020%, further preferably is 0.030%, further preferably is 0.040%, further preferably is 0.042%, further preferably is 0.050%, and further preferably is 0.060%. A preferable upper limit of the content of Co is 0.450%, more preferably is 0.400%, and further preferably is 0.350%.Ca: 0.0002 to 0.0050%

[0045] Calcium (Ca) immobilizes S in the steel material as a sulfide to make it harmless, and thereby improves hot workability of the steel material. If the content of Ca is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Ca is too high, even if the contents of other elements are within the range of the present embodiment, coarse inclusions will be formed in the steel material, and the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of Ca is to be 0.0002 to 0.0050%. A preferable lower limit of the content of Ca is 0.0003%, more preferably is 0.0005%, and further preferably is 0.0010%. A preferable upper limit of the content of Ca is 0.0045%, more preferably is 0.0040%, and further preferably is 0.0035%.N: 0.0500% or less

[0046] Nitrogen (N) is unavoidably contained. That is, the lower limit of the content of N is more than 0%. N combines with Ti to form fine Ti nitrides. As a result, the yield strength of the steel material is increased. On the other hand, if the content of N is too high, even if the contents of other elements are within the range of the present embodiment, coarse nitrides will be formed and the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of N is to be 0.0500% or less. A preferable upper limit of the content of N is 0.0450%, more preferably is 0.0400%, further preferably is 0.0350%, and further preferably is 0.0300%. A preferable lower limit of the content of N for more effectively obtaining the aforementioned advantageous effect is 0.0015%, more preferably is 0.0020%, further preferably is 0.0030%, and further preferably is 0.0040%.O: 0.050% or less

[0047] Oxygen (O) is an impurity that is unavoidably contained. That is, the lower limit of the content of O is more than 0%. O forms oxides and reduces the corrosion resistance and the low-temperature toughness of the steel material. Accordingly, if the content of O is too high, even if the contents of other elements are within the range of the present embodiment, the corrosion resistance and the low-temperature toughness of the steel material will markedly decrease. Therefore, the content of O is to be 0.050% or less. A preferable upper limit of the content of O is 0.040%, more preferably is 0.030%, and further preferably is 0.020%. The content of O is preferably as low as possible. However, extremely reducing the content of O will raise the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of O is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.

[0048] The balance in the martensitic stainless steel material according to the present embodiment is Fe and impurities. Here, the term "impurities" refers to elements which, when industrially producing the steel material, are mixed in from ore or scrap that is used as the raw material or from the production environment or the like, and which are not intentionally contained but are allowed within a range that does not adversely affect the martensitic stainless steel material according to the present embodiment.[Optional elements]

[0049] The martensitic stainless steel material according to the present embodiment may further contain Nb in lieu of a part of Fe.Nb: 0 to 0.500%

[0050] Niobium (Nb) is an optional element, and need not be contained. That is, the content of Nb may be 0%. When contained, Nb combines with C and / or N to form carbides and / or carbo-nitrides. As a result, the yield strength of the steel material is increased. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent. On the other hand, if the content of Nb is too high, even if the contents of other elements are within the range of the present embodiment, carbides and / or carbo-nitrides will be excessively formed and the corrosion resistance of the steel material will decrease. Therefore, the content of Nb is to be 0 to 0.500%. A preferable lower limit of the content of Nb is more than 0%, more preferably is 0.001%, further preferably is 0.002%, and further preferably is 0.003%. A preferable upper limit of the content of Nb is 0.400%, more preferably is 0.300%, further preferably is 0.200%, and further preferably is 0.100%.

[0051] The martensitic stainless steel material according to the present embodiment may further contain Mg in lieu of a part of Fe.Mg: 0 to 0.0050%

[0052] Magnesium (Mg) is an optional element, and need not be contained. That is, the content of Mg may be 0%. When contained, Mg immobilizes S in the steel material as a sulfide to make it harmless, and thereby improves hot workability of the steel material. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent. On the other hand, if the content of Mg is too high, even if the contents of other elements are within the range of the present embodiment, coarse inclusions will be formed in the steel material and the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of Mg is to be 0 to 0.0050%. A preferable lower limit of the content of Mg is more than 0%, more preferably is 0.0001 %, further preferably is 0.0003%, further preferably is 0.0005%, and further preferably is 0.0007%. A preferable upper limit of the content of Mg is 0.0040%, more preferably is 0.0030%, further preferably is 0.0025%, further preferably is 0.0020%, and further preferably is 0.0015%.

[0053] The martensitic stainless steel material according to the present embodiment may further contain one or more elements selected from the group consisting of As, Sn, and Sb in lieu of a part of Fe. Each of these elements is an optional element, and increases the corrosion resistance of the steel material.As: 0 to 0.100%

[0054] Arsenic (As) is an optional element, and need not be contained. That is, the content of As may be 0%. When contained, As increases the corrosion resistance of the steel material. If even a small amount of As is contained, the aforementioned advantageous effect will be obtained to a certain extent. On the other hand, if the content of As is too high, even if the contents of other elements are within the range of the present embodiment, As will segregate at grain boundaries and, on the contrary, the corrosion resistance of the steel material will decrease. Therefore, the content of As is to be 0 to 0.100%. A preferable lower limit of the content of As is more than 0%, more preferably is 0.001%, and further preferably is 0.002%. A preferable upper limit of the content of As is 0.080%, more preferably is 0.060%, further preferably is 0.040%, and further preferably is 0.020%.Sn: 0 to 0.100%

[0055] Tin (Sn) is an optional element, and need not be contained. That is, the content of Sn may be 0%. When contained, Sn increases the corrosion resistance of the steel material. If even a small amount of Sn is contained, the aforementioned advantageous effect will be obtained to a certain extent. On the other hand, if the content of Sn is too high, even if the contents of other elements are within the range of the present embodiment, Sn will segregate at grain boundaries and, on the contrary, the corrosion resistance of the steel material will decrease. Therefore, the content of Sn is to be 0 to 0.100%. A preferable lower limit of the content of Sn is more than 0%, more preferably is 0.001%, and further preferably is 0.002%. A preferable upper limit of the content of Sn is 0.080%, more preferably is 0.060%, further preferably is 0.040%, and further preferably is 0.020%.Sb: 0 to 0.100%

[0056] Antimony (Sb) is an optional element, and need not be contained. That is, the content of Sb may be 0%. When contained, Sb increases the corrosion resistance of the steel material. If even a small amount of Sb is contained, the aforementioned advantageous effect will be obtained to a certain extent. On the other hand, if the content of Sb is too high, even if the contents of other elements are within the range of the present embodiment, Sb will segregate at grain boundaries and, on the contrary, the corrosion resistance of the steel material will decrease. Therefore, the content of Sb is to be 0 to 0.100%. A preferable lower limit of the content of Sb is more than 0%, more preferably is 0.001%, and further preferably is 0.002%. A preferable upper limit of the content of Sb is 0.080%, more preferably is 0.060%, further preferably is 0.040%, and further preferably is 0.020%.

[0057] The martensitic stainless steel material according to the present embodiment may further contain one or more elements selected from the group consisting of B and rare earth metal (REM) in lieu of a part of Fe. These elements are optional elements, and each of these elements increases hot workability of the steel material.B: 0 to 0.0050%

[0058] Boron (B) is an optional element, and need not be contained. That is, the content of B may be 0%. When contained, B strengthens the grain boundaries and increases hot workability of the steel material. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent. On the other hand, if the content of B is too high, even if the contents of other elements are within the range of the present embodiment, Cr carbo-borides will be formed and the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of B is to be 0 to 0.0050%. A preferable lower limit of the content of B is more than 0%, more preferably is 0.0001%, and further preferably is 0.0002%. A preferable upper limit of the content of B is 0.0045%, more preferably is 0.0040%, and further preferably is 0.0035%.Rare earth metal (REM): 0 to 0.0050%

[0059] Rare earth metal (REM) is an optional element, and need not be contained. That is, the content of REM may be 0%. When contained, REM controls the morphology of inclusions and increases hot workability of the steel material. If even a small amount of REM is contained, the aforementioned advantageous effect will be obtained to a certain extent. On the other hand, if the content of REM is too high, even if the contents of other elements are within the range of the present embodiment, coarse inclusions will be formed and the corrosion resistance and the low-temperature toughness of the steel material will decrease. Therefore, the content of REM is to be 0 to 0.0050%. A preferable lower limit of the content of REM is more than 0%, more preferably is 0.0001%, further preferably is 0.0005%, and further preferably is 0.0010%. A preferable upper limit of the content of REM is 0.0045%, more preferably is 0.0040%, and further preferably is 0.0035%.

[0060] Note that, in the present description the term "REM" means one or more types of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. In the present description the term "content of REM" refers to the total content of these elements.[Yield strength]

[0061] The yield strength of the martensitic stainless steel material according to the present embodiment is 550 to less than 758 MPa (80 to less than 110 ksi). In the present embodiment, a preferable lower limit of the yield strength is 560 MPa, more preferably is 563 MPa, further preferably is 565 MPa, further preferably is 570 MPa, and further preferably is 580 MPa. In addition, a preferable upper limit of the yield strength is 750 MPa, and more preferably is 740 MPa. As used herein, the term "yield strength" means 0.2% offset proof stress (MPa) obtained by a tensile test at normal temperature (24 ± 3°C) in conformity with ASTM E8 / E8M (2021) that is described hereunder.

[0062] Specifically, in the present embodiment, the yield strength is determined by the following method. First, a tensile test specimen is prepared from the martensitic stainless steel material according to the present embodiment. The size of the tensile test specimen is not particularly limited. For example, a round bar tensile test specimen in which the diameter of the parallel portion is 8.9 mm and the gage length is 35.6 mm is used as the tensile test specimen. If the steel material is a steel pipe, the tensile test specimen is prepared from a central position of the wall thickness. In this case, the longitudinal direction of the tensile test specimen is to be made parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, the tensile test specimen is prepared from an R / 2 position. Note that, in the present description, the term "R / 2 position" of a round steel bar means the center position of a radius R in a cross section perpendicular to the axial direction of the round steel bar. Further, in this case, the longitudinal direction of the tensile test specimen is to be made parallel to the axial direction of the round steel bar. If the steel material is a steel plate, the tensile test specimen is prepared from a central position of the thickness. In this case, the longitudinal direction of the tensile test specimen is to be made parallel to the rolling elongation direction of the steel plate. A tensile test is conducted at normal temperature (24 ± 3°C) in accordance with ASTM E8 / E8M (2021) using the prepared tensile test specimen, and the 0.2% offset proof stress (MPa) is determined. The determined 0.2% offset proof stress is defined as the yield strength (MPa). Note that, a value obtained by rounding off the first decimal place of the obtained numerical value is adopted as the yield strength.[Microstructure]

[0063] The microstructure of the martensitic stainless steel material according to the present embodiment is composed of, in percent by volume, 10.0 to 40.0% of retained austenite and 0 to 5.0% of ferrite, and the balance is martensite. In the present description, the phrase "composed of retained austenite, ferrite, and martensite" means that the amount of any phase other than retained austenite, ferrite, and martensite is negligible. For example, in the chemical composition of the martensitic stainless steel material according to the present embodiment, the volume ratios of precipitates and inclusions are negligible as compared with the volume ratios of retained austenite, ferrite, and martensite. That is, the microstructure of the martensitic stainless steel pipe according to the present embodiment may contain minute amounts of precipitates, inclusions and the like, in addition to retained austenite, ferrite, and martensite.

[0064] If the volume ratio of retained austenite in the microstructure is too low, the standard deviation of the Ni concentration in the specified regions will decrease, and the low-temperature toughness of the steel material will decrease. In such a case, in addition, in some cases the yield strength may become 758 MPa or more and / or the corrosion resistance of the steel material may decrease. On the other hand, if the volume ratio of retained austenite in the microstructure is too high, in some cases the yield strength may become less than 550 MPa. Therefore, in the present embodiment, the volume ratio of retained austenite in the microstructure is to be within the range of 10.0 to 40.0%. A preferable lower limit of the volume ratio of retained austenite is 11.0%, and more preferably is 12.0%. A preferable upper limit of the volume ratio of retained austenite is 39.0%, and more preferably is 38.0%.

[0065] In the present embodiment, the microstructure does not have to contain ferrite. That is, the volume ratio of ferrite may be 0%. On the other hand, if the volume ratio of ferrite is too high, the low-temperature toughness of the steel material will decrease. Therefore, in the present embodiment, the volume ratio of ferrite in the microstructure is to be within the range of 0 to 5.0%. A preferable upper limit of the volume ratio of ferrite is 4.0%, and more preferably is 3.0%.

[0066] In the present embodiment, the balance of the microstructure is martensite. That is, in the present embodiment, the volume ratio of martensite in the microstructure is 55.0 to 90.0%. If the volume ratio of martensite in the microstructure is too low, the yield strength may become less than 550 MPa in some cases. On the other hand, if the volume ratio of martensite in the microstructure is too high, the volume ratio of retained austenite will decrease, and the standard deviation of the Ni concentration in the specified regions will decrease. As a result, the corrosion resistance and the low-temperature toughness of the steel material will decrease. In such a case, furthermore, the yield strength may become 758 MPa or more. A preferable lower limit of the volume ratio of martensite is 60.0%, and more preferably is 62.0%. A preferable upper limit of the volume ratio of retained austenite is 89.0%, and more preferably is 85.0%.

[0067] In the present embodiment, the volume ratio of each phase in the microstructure can be determined by the following method. Specifically, the volume ratio (%) of retained austenite and the volume ratio (%) of ferrite in the microstructure of the steel material are determined by the following method. The determined volume ratio of retained austenite and volume ratio of ferrite are subtracted from 100% to determine the volume ratio (%) of martensite.[Method for measuring volume ratio of retained austenite]

[0068] The volume ratio of retained austenite in the microstructure of the steel material is determined by an X-ray diffraction method. Specifically, a test specimen for measuring the volume ratio of retained austenite is prepared from the steel material according to the present embodiment. If the steel material is a steel pipe, the test specimen is taken from a central position of the wall thickness. If the steel material is a round steel bar, the test specimen is taken from an R / 2 position. If the steel material is a steel plate, the test specimen is taken from a central position of the thickness. The size of the test specimen is not particularly limited. The test specimen is for example, 15 mm × 15 mm × a thickness of 2 mm. 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 round steel bar, the thickness direction of the test specimen is the radial direction. If the steel material is a steel plate, the thickness direction of the test specimen is the plate thickness direction. Using the prepared test specimen, the X-ray diffraction intensity of each of the (110) plane of α phase (martensite), the (200) plane of α phase, the (211) plane of α phase, the (111) plane of γ phase (retained austenite), the (200) plane of γ phase, and the (220) plane of γ phase are measured, and an integrated intensity of the respective planes is calculated.

[0069] In the measurement of the X-ray diffraction intensity, the target of the X-ray diffraction apparatus is Co (Co Kα radiation), and the output is set to 30 kV - 100 mA. The measurement angle (2θ) is set to 45 to 105°. After calculation, the volume ratio Vy (%) of retained austenite is calculated using Formula (I) for combinations (3 × 3 = 9 pairs) of each plane of the α phase and each plane of the γ phase. Then, an average value of the volume ratios Vγ of retained austenite of the nine pairs is defined as the volume ratio (%) of retained austenite. Vγ = 100 / 1 + Iα × Rγ / Iγ × Rα

[0070] Where, Iα is the integrated intensity of α phase. Rα is a crystallographic theoretical calculation value of α phase. Iγ is the integrated intensity of γ phase. Ry is a crystallographic theoretical calculation value of γ phase. Values incorporated into a retained γ quantitative analysis system belonging to RINT-TTR (product name) manufactured by Rigaku Corporation can be used for the values of Rα and Rγ for each plane. Note that, a value obtained by rounding off the second decimal place of the obtained numerical value is adopted as the volume ratio of retained austenite.[Method for measuring volume ratio of ferrite]

[0071] The volume ratio of ferrite in the microstructure of the steel material is determined by a point counting method. Specifically, a test specimen for measuring the volume ratio of ferrite is prepared from the steel material according to the present embodiment. If the steel material is a steel pipe, the test specimen is taken from a central position of the wall thickness. If the steel material is a round steel bar, the test specimen is taken from an R / 2 position. If the steel material is a steel plate, the test specimen is taken from a central position of the thickness. Note that, the size of the test specimen is not particularly limited. Further, if the steel material is a steel pipe, a face that is parallel to the axial direction of the steel pipe is adopted as the observation surface of the test specimen. If the steel material is a round steel bar, a face that is parallel to the axial direction of the round steel bar is adopted as the observation surface of the test specimen. If the steel material is a steel plate, a face that is parallel to the rolling elongation direction of the steel plate is adopted as the observation surface of the test specimen. After the observation surface is mechanically polished, the observation surface is subjected to electrolytic etching to reveal the microstructure. The electrolytic etching is performed using a 30% sodium hydroxide aqueous solution as the electrolyte, at a current density of 1 A / cm 2< , for an electrolysis time of 1 minute.

[0072] On the observation surface subjected to the electrolytic etching, 30 visual fields are observed using an optical microscope. Each observation visual field is set as a rectangle of 250 µm × 250 µm. Note that, the observation magnification is ×400. Those skilled in the art can distinguish ferrite from other phases (retained austenite or martensite) based on contrast in each observation visual field. Therefore, ferrite in each observation visual field is identified based on contrast. The area fraction of the identified ferrite is determined by a point counting method conforming to ASTM E562 (2019).

[0073] Specifically, for each observation visual field, 20 vertical lines are drawn at regular intervals from the top end to the bottom end of the observation visual field. That is, the observation visual field is divided into 21 regions in the left-right direction by the 20 vertical lines. Further, for each observation visual field, 20 horizontal lines are also drawn at regular intervals from the left end to the right end of the observation visual field. That is, the observation visual field is divided into 21 regions in the vertical direction by the 20 horizontal lines. At this time, intersections between the vertical lines and the horizontal lines are called lattice points. That is, in each observation visual field, 400 lattice points are arranged at regular intervals. In accordance with ASTM E562 (2019), the lattice points that overlap with ferrite in the observation visual field are counted. The number of lattice points overlapping with ferrite obtained in the 30 visual fields is divided by the total number of lattice points (400×30 = 12000), and the resultant value is defined as the ferrite area fraction. In the present embodiment, the area fraction of ferrite determined by the above method is defined as the volume ratio (%) of ferrite. Note that, a value obtained by rounding off the second decimal place of the obtained numerical value is adopted as the volume ratio of ferrite.

[0074] Using the volume ratio (%) of retained austenite obtained by the aforementioned X-ray diffraction method, and the volume ratio (%) of ferrite obtained by the aforementioned point counting method, the volume ratio (%) of martensite in the microstructure of the steel material is determined by the following formula. [Standard deviation of Ni concentration]

[0075] On the premise that the martensitic stainless steel material according to the present embodiment has the chemical composition described above, a yield strength of 550 to less than 758 MPa, and a microstructure composed of retained austenite in an amount of 10.0 to 40.0% by volume and ferrite in an amount of 0 to 5.0% by volume, with the balance being martensite, when, in the steel material, a square with sides of 30 µm in length is defined as a measurement region 50, and the measurement region 50 is divided into nine specified regions that are each a square with sides of 10 µm in length, the standard deviation of the Ni concentration is 0.50% by mass or more in each of eight or more specified regions among the nine specified regions included in the measurement region 50. As a result, high strength, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment are obtained in the martensitic stainless steel material according to the present embodiment.

[0076] As described above, referring to FIG. 2 and FIG. 3, in the martensitic stainless steel material according to the present embodiment, high Ni concentration regions 10 and low Ni concentration regions 20 are alternately disposed in the measurement region 50. Furthermore, in the martensitic stainless steel material according to the present embodiment, this kind of distribution of the Ni concentration is confirmed in almost the entire area of the measurement region 50. Therefore, in the present embodiment, the measurement region 50 is further divided into nine specified regions, and the standard deviation of the Ni concentration in each of the nine specified regions is determined. As described above, the measurement region 50 is divided into nine regions by line segments H1, H2, V1 and V2 as illustrated in FIG. 3. In the present description, the nine regions into which the measurement region 50 is divided in this way are defined as specified regions. Each of the specified regions is a square with sides of 10 µm in length.

[0077] Referring to FIG. 2 and FIG. 3, it can be confirmed that the regions 10 in which the Ni concentration is relatively high are distributed in an elongated shape. As described above, in the martensitic stainless steel material according to the present embodiment, Ni tends to concentrate in retained austenite. Therefore, the regions 10 in which the Ni concentration is relatively high correspond to regions occupied by retained austenite in the microstructure. Here, there is a tendency for retained austenite to be easily distributed side by side in the same direction within a single packet or block in prior-austenite grains in the microstructure. On the other hand, the state of the distribution of the Ni concentration significantly changes at the grain boundaries of prior-austenite grains or at the boundaries of packets or blocks thereof.

[0078] Here, the specified regions defined as described above are squares with sides of 10 µm in length, and hence are extremely small. Therefore, if grain boundaries of prior-austenite grains or the boundaries of packets or blocks thereof are unevenly distributed in a certain specified region, it is possible for a case to arise where the manner in which retained austenite is distributed in the certain specified region is significantly different from other specified regions of the measurement region 50. In such a case, there is a possibility that in the specified region in which the manner in which retained austenite is distributed is different from the other specified regions, the standard deviation of the Ni concentration will be an extremely large value or an extremely small value (that is, an outlier).

[0079] Therefore, in the martensitic stainless steel material according to the present embodiment, in addition to satisfying the requirements regarding the chemical composition, microstructure, and yield strength described above, the standard deviation of the Ni concentration is to be 0.50% by mass or more in each of eight or more specified regions among the nine specified regions included in the measurement region 50. This is because, if the standard deviation of the Ni concentration is 0.50% by mass or more in each of eight or more specified regions among the nine specified regions of the measurement region 50, it can be determined that the high Ni concentration regions 10 and the low Ni concentration regions 20 are alternately disposed in almost all of the measurement region 50.

[0080] As a result, in the martensitic stainless steel material according to the present embodiment, the propagation of cracks can be suppressed by the retained austenite in which Ni is concentrated, even in a corrosive environment in which SOx is mixed or an extremely low temperature environment of -70°C or less, and thus high strength, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment are obtained. Note that, in the present embodiment, a case where the volume ratio of retained austenite is low, a case where Ni is not sufficiently concentrated in the retained austenite, and a case where variations in the Ni concentration are locally distributed are assumed as cases where the condition that the standard deviation of the Ni concentration is 0.50% by mass or more in each of eight or more specified regions among the nine specified regions included in the measurement region 50 is not satisfied.

[0081] In the present embodiment, a preferable lower limit of the standard deviation of the Ni concentration in each of eight or more specified regions among the nine specified regions included in the measurement region 50 is 0.51% by mass, and more preferably is 0.55% by mass. On the other hand, in the present embodiment, although an upper limit of the standard deviation of the Ni concentration in the specified regions is not particularly limited, for example, the upper limit may be 0.90% by mass or may be 0.85% by mass.

[0082] In the present embodiment, the standard deviation of the Ni concentration in the specified regions is determined by the following method. A test specimen for Ni concentration analysis is prepared from the martensitic stainless steel material according to the present embodiment. A method for preparing the test specimen is not particularly limited, and any method may be adopted as long as an analysis surface, described later, can be obtained. The analysis surface has dimensions of, for example, 5 mm in length × 5 mm in width.

[0083] An arbitrary 10 visual fields of 30 µm × 30 µm are identified on the obtained analysis surface, and defined as measurement regions 50. Each of the identified measurement regions 50 is divided into regions of 500 × 500 pixels at a pitch of 0.06 µm. Each pixel in the divided regions is subjected to elemental analysis by FE-EPMA. In the elemental analysis by FE-EPMA, the accelerating voltage is set to 15 kV. Note that, Si, Mn, Cu, Cr, Ni, Mo, Fe, and W are set as the elements to be analyzed. The Ni concentration in each pixel is determined based on the obtained elemental analysis results. Specifically, for each element that is analyzed, a ratio (relative intensity, a so-called "K-ratio") of the X-ray intensity of the aforementioned test specimen to the X-ray intensity of a standard sample when the respective X-ray intensities were measured under the same conditions is determined. In addition, the K-ratios of all the elements that are analyzed are subjected to ZAF correction, and the Ni concentration in percent by mass is determined.

[0084] In the method described above, each measurement region 50 of 30 µm × 30 µm is divided into specified regions of 10 µm × 10 µm. For each of the nine specified regions in each measurement region 50, the standard deviation of the Ni concentration is determined using the Ni concentrations of the pixels belonging to each specified region. Specifically, a case of determining a standard deviation σ(n) of the Ni concentration in an n th< specified region (n is a natural number of 9 or less) will be described. The total number of pixels belonging to the n th< specified region is defined as "k", and the Ni concentration of an i th< pixel belonging to the relevant specified region is defined as "C i " (i is a natural number of k or less). In addition, the arithmetic average value of the Ni concentration in the relevant specified region is defined as "C ave ". At this time, the standard deviation σ(n) of the Ni concentration in the relevant specified region is defined as follows. σ n = 1 k ∑ i = 1 k C i − C ave 2

[0085] The standard deviation σ(n) of the Ni concentration is determined for all of the nine specified regions by the above method. Note that, a value obtained by rounding off the third decimal place of the obtained numerical value is adopted as the standard deviation σ(n) of the Ni concentration.[Corrosion resistance]

[0086] The martensitic stainless steel material according to the present embodiment has the chemical composition described above, a yield strength of 550 to less than 758 MPa, and a microstructure composed of retained austenite in an amount of 10.0 to 40.0% by volume and ferrite in an amount of 0 to 5.0% by volume, with the balance being martensite, and in addition, the standard deviation of the Ni concentration being 0.50% by mass or more in each of eight or more specified regions among the nine specified regions included in the measurement region 50. As a result, the martensitic stainless steel material according to the present embodiment has high strength, has excellent corrosion resistance in a corrosive environment in which SOx is mixed, and has excellent low-temperature toughness in an extremely low temperature environment. In the present embodiment, the phrase "has excellent corrosion resistance in a corrosive environment in which SOx is mixed" means that pitting is not confirmed in a four-point bending test described hereunder.

[0087] Specifically, a test specimen for a four-point bending test is prepared from the martensitic stainless steel material according to the present embodiment. If the steel material is a steel pipe, the test specimen is prepared from a central position of the wall thickness. In this case, the longitudinal direction of the test specimen is made parallel to the axial direction of the steel pipe. If the steel material is a round steel bar, the test specimen is prepared from the R / 2 position. In this case, the longitudinal direction of the test specimen is made parallel to the axial direction of the round steel bar. If the steel material is a steel plate, the test specimen is prepared from a central position of the thickness. In this case, the longitudinal direction of the test specimen is made parallel to the rolling elongation direction of the steel plate. The test specimen is, for example, a test specimen having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm.

[0088] In conformity with ASTM G39-99 (2011), stress corresponding to 100% of the actual yield stress is applied to the test specimen by four-point bending. The test specimen to which stress has been applied is enclosed in an autoclave together with the test jig. A 5.0% by mass sodium chloride aqueous solution is poured into the autoclave so as to immerse the test specimen in the aqueous solution. A gaseous mixture of SO 2 , O 2 , and CO 2 is sealed under pressure in the autoclave and caused to saturate in the test solution, and this solution is adopted as the test bath. At such time, the total pressure of the gaseous mixture is made 130 bar, the SO 2 concentration in the gaseous mixture is made 10 ppm, and the O 2 concentration in the gaseous mixture is made 10 ppm. After sealing the autoclave, the test bath is stirred at 100°C for 720 hours.

[0089] In the present embodiment, if pitting is not confirmed in the test specimen after 720 hours elapses under the conditions described above, it is determined that the martensitic stainless steel material "has excellent corrosion resistance in a corrosive environment in which SOx is mixed". Note that, in the present description, the phrase "pitting is not confirmed" means that pitting is not confirmed in a case where the test specimen after the test is observed using a magnifying glass with a magnification of 10× and an optical microscope with a magnification of 100×.[Low-temperature toughness]

[0090] The martensitic stainless steel material according to the present embodiment has the chemical composition described above, a yield strength of 550 to less than 758 MPa, and a microstructure composed of retained austenite in an amount of 10.0 to 40.0% by volume and ferrite in an amount of 0 to 5.0% by volume, with the balance being martensite, and in addition, the standard deviation of the Ni concentration being 0.50% by mass or more in each of eight or more specified regions among the nine specified regions included in the measurement region 50. As a result, the martensitic stainless steel material according to the present embodiment has high strength, has excellent corrosion resistance in a corrosive environment in which SOx is mixed, and has excellent low-temperature toughness in an extremely low temperature environment. In the present embodiment, the phrase "has excellent low-temperature toughness in an extremely low temperature environment" means that the absorbed energy per unit area at -70°C obtained by a Charpy impact test described hereunder is 60 J / cm 2< or more.

[0091] Specifically, a full-size or sub-size V-notch test specimen is prepared in conformity with API 5CT (2019) from the martensitic stainless steel material according to the present embodiment. Here, if the steel material is a steel plate, the rolling elongation direction of the steel plate is defined as an "L direction" (Longitudinal), and the plate width direction of the steel plate is defined as a "T direction" (Transverse). If the steel material is a steel pipe, the radial direction of the steel pipe is defined as a "C direction", the axial direction of the steel pipe is defined as an "L direction", and a direction perpendicular to the C direction and the L direction is defined as a "T direction". If the steel material is a round steel bar, the cross-sectional radial direction of the round steel bar is defined as a "C direction", the axial direction of the round steel bar is defined as an "L direction", and a direction perpendicular to the C direction and the L direction is defined as a "T direction".

[0092] A Charpy impact test in conformity with JIS Z 2242 (2018) is performed on the prepared V-notch test specimen to determine the absorbed energy (J) at -70°C. Note that, in a case where a sub-size V-notch test specimen is used, the absorbed energy that is obtained is divided by a reduction factor described in API 5CT (2019) to convert the obtained absorbed energy to the absorbed energy for a full-size V-notch test specimen. The determined absorbed energy (J) is divided by the cross-sectional area (cm 2< ) of the V-notch test specimen to determine the absorbed energy (J / cm 2< ) per unit area at -70°C. In the present embodiment, a value obtained by rounding off the first decimal place of the obtained numerical value is adopted as the absorbed energy (J / cm 2< ) per unit area at -70°C.

[0093] In the present embodiment, if the absorbed energy per unit area at -70°C determined under the conditions described above is 60 J / cm 2< or more, it is determined that the martensitic stainless steel material "has excellent low-temperature toughness in an extremely low temperature environment". Note that, in the present description, the absorbed energy per unit area at -70°C is also referred to simply as "absorbed energy".[Shape and uses of steel material]

[0094] As mentioned above, the shape of the martensitic stainless steel material according to the present embodiment is not particularly limited. Specifically, the martensitic stainless steel material according to the present embodiment may be a steel pipe, may be a round steel bar (solid material), or may be a steel plate. Further, the steel pipe may be a seamless steel pipe or may be a welded steel pipe. The martensitic stainless steel material according to the present embodiment, for example, is suitable for use in CCUS.[Production method]

[0095] An example of a method for producing the martensitic stainless steel material according to the present embodiment will now be described. Note that, the production method described hereunder is an example, and a method for producing the martensitic stainless steel material according to the present embodiment is not limited to the production method described hereunder. That is, as long as the martensitic stainless steel material according to the present embodiment that is composed as described above can be produced, a method for producing the martensitic stainless steel material is not limited to the production method described hereunder. However, the production method described hereunder is a favorable method for producing the martensitic stainless steel material according to the present embodiment.

[0096] One example of a method for producing the martensitic stainless steel material according to the present embodiment includes a process of preparing an intermediate steel material (preparation process), and a process of subjecting the intermediate steel material to quenching and tempering (heat treatment process). Each of these processes is described in detail hereunder.[Preparation process]

[0097] In the preparation process, an intermediate steel material having the chemical composition described above is prepared. A method for producing the intermediate steel material is not particularly limited as long as the intermediate steel material has the chemical composition described above. As used here, the term "intermediate steel material" refers to a plate-shaped steel material in a case where the end product is a steel plate or a welded steel pipe, refers to a hollow shell in a case where the end product is a seamless steel pipe, and refers to a steel material in which a cross section perpendicular to the axial direction is a circular shape in a case where the end product is a round steel bar.

[0098] The preparation process may include a process of preparing a starting material (starting material preparation process), and a process of subjecting the starting material to hot working to produce an intermediate steel material (hot working process). Hereunder, a case where the preparation process includes a starting material preparation process and a hot working process is described in detail.[Starting material preparation process]

[0099] In the starting material preparation process, a starting material is produced using a molten steel having the chemical composition described above. The method for producing the starting material is not particularly limited, and it suffices to use a well-known method. Specifically, a cast piece (a slab, a bloom, or a billet) may be produced by a continuous casting process using the molten steel. An ingot may also be produced by an ingot-making process using the molten steel. As necessary, the slab, bloom, or ingot may be subjected to blooming to produce a billet. A starting material (a slab, a bloom, or a billet) is produced by the above-described process.[Hot working process]

[0100] In the hot working process, the prepared starting material is subjected to hot working to produce an intermediate steel material. As described above, if the steel material is a seamless steel pipe, the intermediate steel material corresponds to a hollow shell. First, a billet is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. After the billet is extracted from the heating furnace, the billet is subjected to hot working to produce a hollow shell (seamless steel pipe). The method of hot working is not particularly limited, and it suffices to use a well-known method.

[0101] For example, the Mannesmann process may be performed as hot working to produce a hollow shell. In this case, a round billet is subjected to piercing-rolling using a piercing machine. When performing piercing-rolling, although not particularly limited, for example, the piercing ratio is 1.0 to 4.0. The round billet subjected to piercing-rolling is further subjected to hot rolling with a mandrel mill, a reducer, a sizing mill or the like to produce a hollow shell. The cumulative reduction of area in the hot working process is, for example, 20 to 70%. A hollow shell may be produced from the billet by performing another hot working method. For example, in a case where the steel material is a heavy-wall steel pipe of a short length such as a coupling, a hollow shell may be produced by forging by the Ehrhardt process or the like. A hollow shell is produced by the above process. Although not particularly limited, the wall thickness of the hollow shell is, for example, 9 to 60 mm.

[0102] If the steel material is a round steel bar, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. The starting material extracted from the heating furnace is subjected to hot working to produce an intermediate steel material in which a cross section perpendicular to the axial direction is a circular shape. The hot working is, for example, blooming performed using a blooming mill or hot rolling performed using a continuous mill. In a continuous mill, a horizontal stand having a pair of grooved rolls arranged one on the other in the vertical direction, and a vertical stand having a pair of grooved rolls arranged side by side in the horizontal direction are alternately arranged. If the steel material is a steel plate, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. The starting material extracted from the heating furnace is subjected to hot rolling using a blooming mill and a continuous mill to produce an intermediate steel material having a steel plate shape.

[0103] The intermediate steel material produced by hot working may be air-cooled (as-rolled). The intermediate steel material produced by hot working may be subjected to direct quenching after hot working without being cooled to normal temperature, or may be subjected to quenching after undergoing supplementary heating (reheating) after hot working. In a case of performing direct quenching after hot working, or performing quenching after supplementary heating, cooling may be stopped midway through the quenching process or slow cooling may be performed. In this case, the occurrence of quench cracking in the hollow shell can be suppressed. In addition, in the case of performing direct quenching after hot working, or performing quenching after supplementary heating, stress relief annealing (SR) may be performed at a time that is after quenching and before the heat treatment of the next process. In this case, residual stress of the hollow shell is eliminated.

[0104] As described above, an intermediate steel material is prepared in the preparation process. The intermediate steel material may be produced by the aforementioned preferable process, or may be an intermediate steel material that was produced by a third party, or an intermediate steel material that was produced in another factory other than the factory in which a quenching process and a tempering process to be described later are performed or that was produced at different works may be prepared. Hereunder, the heat treatment process is described in detail.[Heat treatment process]

[0105] The heat treatment process includes a quenching process and a tempering process.[Quenching process]

[0106] In the heat treatment process, first, the intermediate steel material produced in the hot working process is subjected to quenching (quenching process). The quenching is performed by a well-known method. Specifically, the intermediate steel material after the hot working process is charged into a heat treatment furnace and held at a quenching temperature. The quenching temperature is equal to or higher than the A 3 point, and for example is 900 to 1000°C. After being held at the quenching temperature, the intermediate steel material is rapidly cooled (quenched). Although not particularly limited, the holding time at the quenching temperature is, for example, 10 to 60 minutes. The quenching method is, for example, water cooling. The quenching method is not particularly limited. In a case where the intermediate steel material is a hollow shell, for example, the hollow shell may be rapidly cooled by immersing the hollow shell in a water bath or an oil bath, or the hollow shell may be rapidly cooled by pouring or jetting cooling water onto the outer surface and / or inner surface of the hollow shell by shower cooling or mist cooling.

[0107] Note that, as mentioned above, after the hot working process, quenching (direct quenching) may be performed immediately after the hot working, without cooling the intermediate steel material to normal temperature, and quenching may be performed after the hollow shell has been held at the quenching temperature after being charged into a holding furnace before the temperature of the hollow shell decreased after the hot working.[Tempering process]

[0108] The intermediate steel material after quenching is also subjected to a tempering process. In the present description, the term "tempering" means reheating the intermediate steel material after quenching to a temperature that is less than the A c1 point and holding the intermediate steel material at that temperature. Here, the tempering temperature corresponds to the temperature of the furnace when the intermediate steel material after quenching is heated and held at the relevant temperature. The tempering time means the period of time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until the intermediate steel material is extracted from the heat treatment furnace.

[0109] As described above, in the martensitic stainless steel material according to the present embodiment, as a result of the volume ratio of retained austenite in the microstructure being made 10.0 to 40.0% by volume and Ni concentrating in the retained austenite, the standard deviation of the Ni concentration is 0.50% by mass or more in each of eight or more specified regions among the nine specified regions included in the measurement region 50. Therefore, in the tempering process according to the present embodiment, it is preferable to perform tempering at a high temperature that increases the volume ratio of retained austenite (a first tempering process), and thereafter perform tempering that causes Ni to concentrate in the retained austenite (a second tempering process). Hereunder, the first tempering process and the second tempering process are described.[First tempering process]

[0110] In the first tempering process, the intermediate steel material (hollow shell) that was subjected to quenched is heated, and is held for a tempering time t1 (mins) at a tempering temperature T1 (°C). As described above, in the first tempering process, the volume ratio of retained austenite is increased. If the tempering temperature T1 is too low, the volume ratio of retained austenite will decrease, and the standard deviation of the Ni concentration in the specified regions may decrease in some cases. In such a case, in addition, the yield strength may become 758 MPa or more. On the other hand, if the tempering temperature T1 is too high, the volume ratio of retained austenite will be too high, and the yield strength may be less than 550 MPa in some cases. Therefore, in the present embodiment, preferably the tempering temperature T1 is set within the range of 610 to 730°C. On the other hand, in the first tempering process according to the present embodiment, the tempering time t1 is not particularly limited as long as the first tempering process satisfies LMP1 that is described later. The tempering time t1 in the first tempering process is, for example, 5 to 50 minutes.

[0111] In the first tempering process according to the present embodiment, in addition, it is preferable to adjust a Larson-Miller parameter LMP1 (= (T1 (°C)+273.15)×(log(t1 (mins) / 60)+20)) for the tempering. If LMP1 is too small, the volume ratio of retained austenite will decrease, and the standard deviation of the Ni concentration in the specified regions may decrease in some cases. In such a case, in addition, the yield strength may become 758 MPa or more. On the other hand, if LMP1 is too large, the volume ratio of retained austenite will, on the contrary, decrease. As a result, the standard deviation of the Ni concentration in specified regions may decrease in some cases. In such a case, in addition, the yield strength may become 758 MPa or more. Therefore, in the first tempering process according to the present embodiment, it is preferable to adjust LMP1 to within a range of 17500 to 19000.[Second tempering process]

[0112] In the second tempering process, the intermediate steel material (hollow shell) that was tempered by the first tempering process is heated, and is held at a tempering temperature T2 (°C) for a tempering time t2 (mins). As described above, in the second tempering process, Ni is caused to concentrate in the retained austenite. If the tempering temperature T2 is too low, in some cases Ni will not concentrate sufficiently in the retained austenite, and the standard deviation of the Ni concentration in the specified regions will decrease. On the other hand, if the tempering temperature T2 is too high, the yield strength may become less than 550 MPa in some cases. Therefore, in the present embodiment, preferably the tempering temperature T2 is set within the range of 500 to 600°C. On the other hand, in the second tempering process according to the present embodiment, the tempering time t2 is not particularly limited as long as the second tempering process satisfies LMP2 that is described later. The tempering time t2 of the second tempering process is, for example, 10 to 50 minutes.

[0113] In the second tempering process according to the present embodiment, in addition, it is preferable to adjust a Larson-Miller parameter LMP2 (= (T2 (°C)+273.15)×(log (t2 (mins) / 60)+20)) for the tempering. If LMP2 is too small, Ni will not concentrate sufficiently in the retained austenite, and the standard deviation of the Ni concentration in the specified regions may decrease in some cases. On the other hand, if LMP2 is too large, the yield strength may become less than 550 MPa in some cases. Therefore, in the second tempering process according to the present embodiment, it is preferable to adjust LMP2 to within a range of 15000 to 17000.

[0114] Note that, the aforementioned first tempering process and second tempering process can be performed as consecutive heat treatments. That is, after performing the first tempering process, the second tempering process may then be performed in a successive manner. At such time, the first tempering process and the second tempering process may be performed within the same heat treatment furnace. On the other hand, the aforementioned first tempering process and second tempering process can also be performed as non-consecutive heat treatments. That is, after performing the first tempering process, the intermediate steel material may be temporarily cooled to a lower temperature than the tempering temperature of the second tempering process that is described above, and thereafter heated again to perform the second tempering process. Even in this case, the effects obtained by the first tempering process and second tempering process are not impaired, and the steel material according to the present embodiment can be produced. However, the cooling after the first tempering process is preferably performed by cooling at a cooling rate that is slower than air cooling. If the intermediate steel material is cooled at a cooling rate that is the same as air cooling or is faster than air cooling, in some cases the volume ratio of retained austenite may decrease and the standard deviation of the Ni concentration in the specified regions will decrease.

[0115] The martensitic stainless steel material according to the present embodiment can be produced by the production method described above. Note that, the production method described above is a description of one example of a method for producing the martensitic stainless steel material according to the present embodiment. In other words, the martensitic stainless steel material according to the present embodiment may be produced by a production method other than the production method that is described above. Even in such a case, a martensitic stainless steel material having the chemical composition described above, the microstructure described above, the yield strength described above, and the standard deviation of the Ni concentration described above has high strength, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment. That is, a method for producing the martensitic stainless steel material according to the present embodiment is not limited to the production method described above, and the martensitic stainless steel material may be produced by a different production method. Hereunder, the martensitic stainless steel material according to the present embodiment is described more specifically by way of examples.EXAMPLES

[0116] Molten steels having the chemical compositions shown in Tables 1-1 to 1-3 were produced. Note that, the symbol "-" in Table 1-3 means that the content of the corresponding element was at an impurity level. For example, it means that the content of Nb, content of As, content of Sn, and content of Sb of Test No. 1 were each 0% when rounded off to three decimal places. Further, it means that the content of Mg, content of B, and content of rare earth metal (REM) of Test No. 1 were each 0% when rounded off to four decimal places.[Table 1-1]

[0117] TABLE 1-1SteelChemical Composition (unit is mass%; balance is Fe and impurities)CSiMnPSCuCrNiMoA0.0210.350.520.0120.00062.5813.566.322.82B0.0250.240.750.0200.00083.1211.584.673.16C0.0100.380.410.0150.00210.8913.756.481.58D0.0100.230.400.0150.00072.3012.746.022.61E0.0080.250.390.0170.00051.9812.806.252.48F0.0120.310.410.0170.00061.5613.125.882.78G0.0080.240.400.0160.00122.0213.086.121.86H0.0070.220.480.0170.00060.1512.676.062.68I0.0110.250.420.0160.00080.1211.866.922.96J0.0120.260.450.0150.00132.1312.326.152.61K0.0150.300.390.0180.00092.2312.156.212.58 [Table 1-2]

[0118] TABLE 1-2SteelChemical Composition (unit is mass%; balance is Fe and impurities)WTiVAlCoCaNOA0.150.1150.060.0350.2560.00190.00680.003B0.120.1890.120.0560.0760.00380.01310.012C0.450.0760.040.0260.3450.00120.00580.003D0.380.0950.050.0280.2060.00060.00710.003E0.210.1050.050.0310.1860.00130.00720.002F0.180.0970.050.0240.1840.00120.00650.004G0.280.1000.050.0270.2220.00180.00760.004H0.120.0960.050.0310.1830.00100.00700.003I0.050.0970.050.0320.2860.00210.00980.003J0.020.1030.050.0260.2350.00160.00680.003K0.260.1130.040.0310.0050.00080.00810.003 [Table 1-3]

[0119] TABLE 1-3SteelChemical Composition (unit is mass%; balance is Fe and impurities)NbMgAsSnSbBREMA-------B-------C-------D0.001----0.0010-E-0.0010-0.002---F--0.002-0.001--G--0.002---0.0010H0.003------I--0.002---0.0020J-------K-------

[0120] The respective molten steels described above were melted in a 180-kg vacuum furnace, and ingots were produced by an ingot-making process. The ingot was heated for three hours at 1250°C. The ingot after heating was subjected to hot forging to produce a block. A block after hot forging was heated for three hours at 1230°C, and thereafter the block was subjected to hot rolling. In this way, a steel material (a steel plate) having a thickness of 13 mm was produced.

[0121] The steel material of each test number was subjected to quenching. Specifically, the steel plate of each test number was heated so as to reach a temperature shown in "Temperature (°C)" in the column "Quenching" in Table 2. The steel plate of each test number was held at the quenching temperature for 15 mins, and thereafter subjected to water cooling. The steel material of each test number after quenching was subjected to a first tempering process and a second tempering process under the conditions described in Table 2. Specifically, the steel material of each test number was held at the temperature T1 (°C) for the time t1 (mins) of the first tempering, and thereafter, in a successive manner, was held at the temperature T2 (°C) for the time t2 (mins) of the second tempering.[Table 2]

[0122] TABLE 2Test NumberSteelQuenchingFirst TemperingSecond TemperingTemperature (°C)Temperature T1 (°C)Time t1 (mins)LMP1Temperature T2 (°C)Time t2 (mins)LMP21A910630201763252030156242A910640151771353030158213A910680101832155030162154B910640201782755030162155C910640201782752030156246D900640101755253030158217D900660101793752030156248D900680101832155030162159E9106302017632510301542710E9106401017552540301601811E9106801018321575301670812F9006401017552550301621513G9006601017937560301641214H9006401017552550301621515I9006601017937550301621516J9006401017552550301621517K9006401017552550301621518A9106003017200550301621519D9006004517354---20E9105803016806---21E9106003017200---22F9106003017200---23A9107403019958550301621524D9007204519739---25E9107203019564---26A9006804518944---27F9006401017552---28A9106801018321480301483629F9106401017552475301473830A9106801018321630301779131F91066010179376103017397

[0123] The steel plate of each test number was produced by the above production process.[Evaluation tests]

[0124] The produced steel plate of each test number was subjected to a tensile test, a microstructure observation test, a Ni concentration analysis test, a corrosion resistance test, and a Charpy impact test.[Tensile test]

[0125] The steel plate of each test number was subjected to a tensile test in accordance with ASTM E8 / E8M (2021). Specifically, from a central position of the thickness of the steel plate of each test number, a round bar tensile test specimen in which the diameter of the parallel portion was 8.9 mm, and the gage length was 35.6 mm was prepared. The longitudinal direction of the round bar tensile test specimen was parallel to the rolling elongation direction of the steel plate. A tensile test was performed at normal temperature (24 ± 3°C) in the atmosphere using the round bar tensile test specimen of each test number, and the 0.2% offset proof stress (MPa) was determined. The determined 0.2% offset proof stress was defined as the yield strength (MPa). The obtained yield strength of each test number is shown in the column "YS (MPa)" in Table 3.[Table 3]

[0126] TABLE 3Test NumberYS (MPa)MicrostructureNi ConcentrationEvaluation ResultsMartensite (vol%)Retained γ (vol%)Ferrite (vol%)δ-MAX (mass%)δ-MIN (mass%)Corrosion ResistanceLow-Temperature Toughness171288.810.60.60.640.52AEX270186.712.80.50.660.54AEX368270.928.60.50.750.64AEX472181.018.80.20.690.54AEX563566.732.80.50.660.54AEX667078.920.90.20.660.52AEX758682.617.20.20.680.53AEX868886.313.50.20.630.57AEX971673.226.40.40.630.52AEX1061878.820.80.40.650.53AEX1168887.711.90.40.720.62AEX1262881.317.51.20.670.55AEX1363585.214.30.50.690.59AEX1464584.515.40.10.650.54CEX1561284.215.60.20.710.53CEX1662883.116.80.10.650.56CEX1763488.111.60.30.640.52CEX1896897.32.50.20.560.39ANA19100298.31.50.20.340.31ANA2099296.82.80.40.380.35ANA2192490.39.30.40.490.43ANA2298597.52.10.40.360.31ANA2353558.541.20.30.770.65-EX2480591.48.40.20.520.48CNA2575892.96.70.40.540.48BNA2662364.135.80.10.560.44ANA2764883.815.60.60.480.41ANA2865874.725.10.20.540.42CNA2963287.312.30.40.490.39CNA3051356.543.500.570.44-NA3153862.936.70.40.550.47-NA [Microstructure observation test]

[0127] The steel plate of each test number was subjected to a microstructure observation test by the method described above. Specifically, the volume ratio (%) of retained austenite was determined by X-ray diffraction analysis performed by the method described above. In addition, the volume ratio (%) of ferrite was determined by a point counting method conforming to ASTM E562 (2019) that was performed according to the above method. The volume ratio (%) of martensite was determined based on the obtained volume ratios of retained austenite and ferrite. The obtained volume ratio of martensite is shown in the column "Martensite (vol%)" in Table 3. The obtained volume ratio of retained austenite is shown in the column "Retained γ (vol%)" in Table 3. The obtained volume ratio of ferrite is shown in the column "Ferrite (vol%)" in Table 3.[Ni concentration analysis test]

[0128] The steel plate of each test number was subjected to a Ni concentration analysis test by the method described above. Specifically, 10 measurement regions identified by the method described above were subjected to elemental analysis using a FE-EPMA (manufactured by JEOL Ltd; model name: JXA-8530F) according to the above method, and the Ni concentration was determined for each pixel. The software supplied with the microanalyzer was used to perform quantitative calculations that included ZAF correction. Each measurement region was divided into nine specified regions of 10 µm × 10 µm by the above method. The standard deviation (mass%) of the Ni concentration was determined by the above method for each specified region into which the measurement region was divided.

[0129] In the present example, in order to exclude outliers, seven standard deviations (mass%) of the Ni concentration that excluded a largest value and a smallest value among the nine standard deviations (mass%) of the Ni concentration determined for each measurement region were used for evaluation. A maximum value and a minimum value were determined from the 70 standard deviations (mass%) of the Ni concentration obtained for the 10 measurement regions. The obtained maximum value of the standard deviation of the Ni concentration of each test number is shown in the column "σ-MAX (mass%)" in Table 3. The obtained minimum value of the standard deviation of the Ni concentration of each test number is shown in the column "σ-MIN (mass%)" in Table 3.[Corrosion resistance test]

[0130] Among the steel plates of the respective test numbers, a four-point bending test conforming to ASTM G39-99 (2011) was performed on those steel plates which had a yield strength of 550 MPa or more. Specifically, a test specimen having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm was prepared from the central position of the thickness of the steel plate of each test number. By the method described above, stress corresponding to 100% of the actual yield stress was applied to the test specimen, and the test specimen was enclosed in an autoclave together with the test jig. A 5.0% by mass sodium chloride aqueous solution was poured into the autoclave so as to immerse the test specimen in the aqueous solution. A gaseous mixture of SO 2 , O 2 , and CO 2 was sealed under pressure in the autoclave and caused to saturate in the test solution, and this solution was adopted as the test bath. At such time, the total pressure of the gaseous mixture was made 130 bar, the SO 2 concentration in the gaseous mixture was made 10 ppm, and the O 2 concentration in the gaseous mixture was made 10 ppm. After sealing the autoclave, the test bath was stirred at 100°C for 720 hours.

[0131] The test specimen after 720 hours had elapsed was observed using a magnifying glass with a magnification of 10×. If a location where pitting was suspected to have occurred on the test specimen was found by observation with the magnifying glass, the relevant location was observed using an optical microscope with a magnification of 100× to confirm the presence or absence of pitting. The presence or absence of pitting was checked under the same conditions for three test specimens. If pitting was not confirmed on the surface of all of the three test specimens, it was determined that the steel material had excellent corrosion resistance (indicated by "A" in Table 3). If pitting was confirmed on the surface of one or two test specimens among the three test specimens, it was determined that the steel material did not have excellent corrosion resistance (indicated by "B" in Table 3). If pitting was confirmed on the surface of all of the three test specimens, it was determined that the steel material did not have excellent corrosion resistance (indicated by "C" in Table 3). The evaluation result obtained for each test number is shown in the column "Corrosion Resistance" in Table 3. Note that, for those test numbers in which the yield strength of the steel plate was less than 550 MPa, the corrosion resistance was not evaluated (indicated by "-" in Table 3).[Charpy impact test]

[0132] The steel plate of each test number was subjected to a Charpy impact test conforming to JIS Z 2242 (2018). Specifically, a full-size V-notch test specimen was prepared in conformity with API 5CT (2019) from a central position of the thickness of the steel plate of each test number. A Charpy impact test in conformity with JIS Z 2242 (2018) was performed on each prepared V-notch test specimen to determine the absorbed energy (J) at -70°C. The determined absorbed energy (J) was divided by the cross-sectional area (cm 2< ) of the V-notch test specimen to determine the absorbed energy (J / cm 2< ) per unit area at -70°C. If the obtained absorbed energy per unit area at -70°C was 60 J / cm 2< or more, it was determined that the relevant steel plate had excellent low-temperature toughness (indicated by "EX (Excellent)" in Table 3). If the obtained absorbed energy per unit area at -70°C was less than 60 J / cm 2< , it was determined that the relevant steel plate did not have excellent low-temperature toughness (indicated by "NA (Not Acceptable)" in Table 3). The evaluation result obtained for each test number is shown in the column "Low-temperature Toughness" in Table 3.[Evaluation Results]

[0133] Referring to Table 1-1, Table 1-2, Table 1-3, Table 2, and Table 3, the steel plates of Test Nos. 1 to 13 had the chemical composition described above, had a yield strength of 550 to less than 758 MPa, and had a microstructure composed of retained austenite in an amount of 10.0 to 40.0% by volume and ferrite in an amount of 0 to 5.0% by volume, with the balance being martensite. In these steel plates, in addition, the minimum value of the standard deviation of the Ni concentration was 0.50% by mass or more, and in all of the measurement regions that were analyzed, the standard deviation of the Ni concentration was 0.50% by mass or more in each of eight or more specified regions among the nine specified regions into which the measurement region was divided. As a result, it was determined that these steel plates had excellent corrosion resistance in the corrosion resistance test, and it was determined that these steel plates had excellent low-temperature toughness in the Charpy impact test. That is, these steel plates had high strength, excellent corrosion resistance in a corrosive environment in which SOx is mixed, and excellent low-temperature toughness in an extremely low temperature environment.

[0134] On the other hand, in the steel plate of Test No. 14, the content of Cu was too low. As a result, it was determined that this steel plate did not have excellent corrosion resistance in the corrosion resistance test. That is, this steel plate did not have excellent corrosion resistance in a corrosive environment in which SOx is mixed.

[0135] In the steel plate of Test No. 15, the content of Cu was too low, the content of Ni was too high, and the content of W was too low. As a result, it was determined that this steel plate did not have excellent corrosion resistance in the corrosion resistance test. That is, this steel plate did not have excellent corrosion resistance in a corrosive environment in which SOx is mixed.

[0136] In the steel plate of Test No. 16, the content of W was too low. As a result, it was determined that this steel plate did not have excellent corrosion resistance in the corrosion resistance test. That is, this steel plate did not have excellent corrosion resistance in a corrosive environment in which SOx is mixed.

[0137] In the steel plate of Test No. 17, the content of Co was too low. As a result, it was determined that this steel plate did not have excellent corrosion resistance in the corrosion resistance test. That is, this steel plate did not have excellent corrosion resistance in a corrosive environment in which SOx is mixed.

[0138] For the steel plate of Test No. 18, the tempering temperature T1 of the first tempering process in the production process was too low, and LMP1 was too small. As a result, in this steel plate, the volume ratio of retained austenite in the microstructure was too low, the yield strength was 758 MPa or more, the minimum value of the standard deviation of the Ni concentration was less than 0.50% by mass, and the standard deviations of the Ni concentration were less than 0.50% by mass in at least two specified regions among the nine specified regions into which the measurement region was divided. As a result, it was determined that this steel plate did not have excellent low-temperature toughness in the Charpy impact test. That is, this steel plate did not have excellent low-temperature toughness in an extremely low temperature environment.

[0139] For the steel plates of Test Nos. 19 to 22, the tempering temperature T1 of the first tempering process in the production process was too low, LMP1 was too small, and a second tempering process was not performed. As a result, in these steel plates, the volume ratio of retained austenite in the microstructure was too low, the yield strength was 758 MPa or more, the minimum value of the standard deviation of the Ni concentration was less than 0.50% by mass, and the standard deviations of the Ni concentration were less than 0.50% by mass in at least two specified regions among the nine specified regions into which the measurement region was divided. As a result, it was determined that these steel plates did not have excellent low-temperature toughness in the Charpy impact test. That is, these steel plates did not have excellent low-temperature toughness in an extremely low temperature environment.

[0140] For the steel plate of Test No. 23, the tempering temperature T1 of the first tempering process in the production process was too high, and LMP1 was too large. As a result, in this steel plate, the volume ratio of retained austenite in the microstructure was too high, and the yield strength was less than 550 MPa. That is, this steel plate did not have the desired high strength.

[0141] For the steel plates of Test Nos. 24 and 25, LMP1 in the production process was too large, and a second tempering process was not performed. As a result, in these steel plates, the volume ratio of retained austenite in the microstructure was too low, the yield strength was 758 MPa or more, the minimum value of the standard deviation of the Ni concentration was less than 0.50% by mass, and the standard deviations of the Ni concentration were less than 0.50% by mass in at least two specified regions among the nine specified regions into which the measurement region was divided. As a result, it was determined that these steel plates did not have excellent corrosion resistance in the corrosion resistance test, and it was determined that these steel plates did not have excellent low-temperature toughness in the Charpy impact test. That is, these steel plates did not have excellent corrosion resistance in a corrosive environment in which SOx is mixed, and did not have excellent low-temperature toughness in an extremely low temperature environment.

[0142] For the steel plates of Test Nos. 26 and 27, a second tempering process was not performed in the production process. As a result, in these steel plates, the minimum value of the standard deviation of the Ni concentration was less than 0.50% by mass, and the standard deviations of the Ni concentration were less than 0.50% by mass in at least two specified regions among the nine specified regions into which the measurement region was divided. As a result, it was determined that these steel plates did not have excellent low-temperature toughness in the Charpy impact test. That is, these steel plates did not have excellent low-temperature toughness in an extremely low temperature environment.

[0143] For the steel plates of Test Nos. 28 and 29, the tempering temperature T2 of the second tempering in the production process was too low, and LMP2 was too small. As a result, in these steel plates, the minimum value of the standard deviation of the Ni concentration was less than 0.50% by mass, and the standard deviations of the Ni concentration were less than 0.50% by mass in at least two specified regions among the nine specified regions into which the measurement region was divided. As a result, it was determined that these steel plates did not have excellent corrosion resistance in the corrosion resistance test, and it was determined that these steel plates did not have excellent low-temperature toughness in the Charpy impact test. That is, these steel plates did not have excellent corrosion resistance in a corrosive environment in which SOx is mixed, and did not have excellent low-temperature toughness in an extremely low temperature environment.

[0144] For the steel plate of Test No. 30, the tempering temperature T2 of the second tempering in the production process was too high, and LMP2 was too large. As a result, in this steel plate, the volume ratio of retained austenite in the microstructure was too high, the yield strength was less than 550 MPa, the minimum value of the standard deviation of the Ni concentration was less than 0.50% by mass, and the standard deviations of the Ni concentration were less than 0.50% by mass in at least two specified regions among the nine specified regions into which the measurement region was divided. As a result, it was determined that this steel plate did not have excellent low-temperature toughness in the Charpy impact test. That is, this steel plate did not have high strength, and did not have excellent low-temperature toughness in an extremely low temperature environment.

[0145] For the steel plate of Test No. 31, the tempering temperature T2 of the second tempering in the production process was too high, and LMP2 was too large. As a result, in this steel plate, the yield strength was less than 550 MPa, the minimum value of the standard deviation of the Ni concentration was less than 0.50% by mass, and the standard deviations of the Ni concentration were less than 0.50% by mass in at least two specified regions among the nine specified regions into which the measurement region was divided. As a result, it was determined that this steel plate did not have excellent low-temperature toughness in the Charpy impact test. That is, this steel plate did not have high strength and did not have excellent low-temperature toughness in an extremely low temperature environment.

[0146] An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range not departing from the spirit thereof.

Claims

1. A martensitic stainless steel material consisting of, in mass%, C: 0.030% or less, Si: 0.50% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.50 to 3.50%, Cr: 10.00 to 14.00%, Ni: 4.50 to 6.50%, Mo: 1.00 to 3.50%, W: 0.10 to 0.50%, Ti: 0.050 to 0.300%, V: 0.01 to 0.50%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0002 to 0.0050%, N: 0.0500% or less, O: 0.050% or less, Nb: 0 to 0.500%, Mg: 0 to 0.0050%, As: 0 to 0.100%, Sn: 0 to 0.100%, Sb: 0 to 0.100%, B: 0 to 0.0050%, rare earth metal: 0 to 0.0050%, and the balance: Fe and impurities, wherein: a yield strength is 550 to less than 758 MPa, a microstructure is composed of, in percent by volume, retained austenite in an amount of 10.0 to 40.0%, and ferrite in an amount of 0 to 5.0%, with the balance being martensite, and in the martensitic stainless steel material, a square having sides with a length of 30 µm is defined as a measurement region, and the measurement region is divided into nine specified regions which are each a square having sides with a length of 10 µm, in each of eight or more of the specified regions among nine of the specified regions included in the measurement region, a standard deviation of a Ni concentration is 0.50% by mass or more.

2. The martensitic stainless steel material according to claim 1, containing one or more elements selected from a group consisting of: Nb: 0.001 to 0.500%, Mg: 0.0001 to 0.0050%, As: 0.001 to 0.100%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.100%, B: 0.0001 to 0.0050%, and rare earth metal : 0.0001 to 0.0050%.

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

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