Martensitic stainless steel material

Abstract

Provided is a martensitic stainless steel material having high strength, excellent SSC resistance, and excellent low-temperature toughness. This martensitic stainless steel material contains, in mass%, 0.030% or less of C, 1.00% or less of Si, 1.00% or less of Mn, 0.030% or less of P, 0.0050% or less of S, 10.00-16.00% of Cr, 4.00-7.50% of Ni, 1.10-3.50% of Mo, 0.005-0.050% of Al, 0.01-0.30% of V, 0.0030-0.0500% of N, 0.020-0.150% of Ti, 0.01-3.50% of Cu, and 0.01-0.50% of Co, and has a yield strength of 758 MPa or more, and the area ratio Sd (%) of delta ferrite and the area ratio Sc (%) of intermetallic compounds in a cross section parallel to the rolling direction satisfy formula (1) and formula (2). 0 < Sd < = 10.00 (1) Sc / Sd < = 5.00 (2).

Description

Technical Field

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

[0002] Oil wells and gas wells (hereinafter referred to as "oil wells") are sometimes in a corrosive environment containing corrosive gases. Here, corrosive gases refer to carbon dioxide gas and / or hydrogen sulfide gas. Steel used in oil wells needs to have excellent stress corrosion cracking resistance (SSC resistance) in corrosive environments.

[0003] Chromium (Cr) is known to be effective in improving the SSC resistance of steel in a corrosive environment. For this reason, in a corrosive environment, a martensitic stainless steel containing about 13 mass % Cr, represented by API L80 13Cr steel (normal 13Cr steel) and super 13Cr steel with reduced C content, is used.

[0004] In recent years, as oil wells have deepened, steel materials have been required to have not only SSC resistance but also high strength. For example, steel materials with a strength of 110 ksi (758 MPa) or more are required.

[0005] Patent Document 1 (International Publication No. 2019 / 065115) and Patent Document 2 (International Publication No. 2020 / 095559) propose martensitic stainless steel materials having a high strength of 110 ksi or more and excellent SSC resistance.

[0006] Patent Documents 1 and 2 attempt to achieve both high strength and SSC resistance from the perspective of chemical composition for martensitic stainless steel containing 10.0 to 14.0% Cr. Specifically, the chemical composition is adjusted so that the contents of C, Mn, Cr, Ni, Mo, W, Nb, N, and Ti in the chemical composition satisfy a specific parameter formula.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: International Publication No. 2019 / 065115

[0010] Patent Document 2: International Publication No. 2020 / 095559 Summary of the invention

[0011] Problem that the invention aims to solve

[0012] Recently, oil well development has also been carried out in cold regions. For oil well steel pipes used in deep wells in such cold regions, not only high strength and excellent SSC resistance but also excellent low temperature toughness are required. Patent Documents 1 and 2 do not study low temperature toughness.

[0013] An object of the present disclosure is to provide a martensitic stainless steel material having high strength and excellent SSC resistance, and further having excellent low-temperature toughness.

[0014] Solutions for solving problems

[0015] The chemical composition of the martensitic stainless steel material disclosed in the present invention is expressed in mass %.

[0016] C: 0.030% or less,

[0017] Si: 1.00% or less,

[0018] Mn: 1.00% or less,

[0019] P: 0.030% or less,

[0020] S: 0.0050% or less,

[0021] Cr: 10.00~16.00%,

[0022] Ni: 4.00~7.50%,

[0023] Mo: 1.10~3.50%,

[0024] Al: 0.005-0.050%,

[0025] V: 0.01~0.30%,

[0026] N: 0.0030~0.0500%、

[0027] Ti: 0.020~0.150%,

[0028] Cu: 0.01~3.50%,

[0029] Co: 0.01-0.50%,

[0030] Nb: 0~0.150%,

[0031] W: 0~1.50%,

[0032] Zr: 0~0.0100%,

[0033] Sn: 0~0.0100%,

[0034] As: 0~0.0100%,

[0035] Sb: 0~0.0100%,

[0036] B: 0~0.0050%,

[0037] Ca: 0~0.0050%,

[0038] Mg: 0~0.0050%,

[0039] Rare earth elements (REM): 0 to 0.0100%, and

[0040] The balance is Fe and impurities.

[0041] The yield strength of the martensitic stainless steel is 758 MPa or more.

[0042] The area ratio Sd (%) of delta ferrite and the area ratio Sc (%) of intermetallic compounds having a circle equivalent diameter of 1.0 μm or more in a cross section parallel to the rolling direction of a martensitic stainless steel material satisfy equations (1) and (2).

[0043] 0<Sd≤10.00 (1)

[0044] Sc / Sd≤5.00 (2).

[0045] Effects of the Invention

[0046] The martensitic stainless steel material disclosed in the present invention has high strength with a yield strength of 758 MPa or more (110 ksi or more) and excellent SSC resistance, and further has excellent low-temperature toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is a graph showing the relationship between the delta ferrite area ratio Sd, the coarse intermetallic compound area ratio Sc, and the low temperature toughness.

[0048] Figure 2A This is a schematic diagram for explaining a method of identifying intermetallic compounds in each field of view divided into 512×512 pixels in the measurement of the coarse intermetallic compound area ratio Sc.

[0049] Figure 2B yes Figure 2A Schematic diagram of the subsequent process.

[0050] Figure 2C yes Figure 2B Schematic diagram of the subsequent process.

[0051] Figure 2D yes Figure 2C Schematic diagram of the subsequent process.

[0052] Figure 2E yes Figure 2D Schematic diagram of the subsequent process.

[0053] Figure 3 The figure shows the change in the intermediate temperature of the steel material satisfying the chemical composition of the present embodiment from A to c3 This is a graph showing the relationship between the time t1 from the point at which the temperature reaches 900° C., the holding time t2 at the quenching temperature T1, the Cu content, and the coarse intermetallic compound area ratio Sc. DETAILED DESCRIPTION

[0054] The inventors of the present invention have studied steel materials having a yield strength of 758 MPa or more (110 ksi or more) and excellent SSC resistance from the perspective of chemical composition. As a result, it is believed that a yield strength of 758 MPa or more and excellent SSC resistance can be obtained as long as the martensitic stainless steel material has the following chemical composition: C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cr: 10.00-16.00%, Ni: 4.00-7.50%, Mo: 1.10-3.50%, Al: 0.005-0.050%, V: 0.01-0.30%, N: 0. 0030~0.0500%, Ti: 0.020~0.150%, Cu: 0.01~3.50%, Co: 0.01~0.50%, Nb: 0~0.150%, W: 0~1.50%, Zr: 0~0.0100%, Sn: 0~0.0100%, As: 0~0.0100%, Sb: 0~0.0100%, B: 0~0.0050%, Ca: 0~0.0050%, Mg: 0~0.0050%, rare earth elements (REM): 0~0.0100%, and the balance is Fe and impurities.

[0055] Therefore, the present inventors have further studied methods for improving the low temperature toughness of martensitic stainless steel materials having the content of each element in the chemical composition within the above range. As a result, it was found that, when only the content of the elements in the chemical composition is adjusted, excellent SSC resistance can be obtained when the yield strength is adjusted to 758 MPa or more, but excellent low temperature toughness may not be obtained.

[0056] For this reason, the inventors of the present invention have tried to improve low temperature toughness from the perspective of microstructure rather than chemical composition. Here, the inventors of the present invention have focused on δ ferrite in martensitic stainless steel. δ ferrite exists in martensitic stainless steel containing 10.00 to 16.00% Cr. δ ferrite present in steel makes the steel embrittled. Therefore, it can be considered that if the amount of δ ferrite generated in the steel is large, it will also affect the low temperature toughness of the steel.

[0057] Therefore, the present inventors investigated the relationship between the amount of δ ferrite generated in a martensitic stainless steel material having the above chemical composition and a yield strength of 758 MPa or more, the hot workability of the steel material, and the absorbed energy at -10°C, which is an index of low-temperature toughness. As a result, when the area ratio Sd of δ ferrite, which is an index of the amount of δ ferrite generated, exceeds 10.00%, although the hot workability of the steel material decreases, no clear correlation is found between the area ratio of δ ferrite and the absorbed energy at -10°C.

[0058] Therefore, the present inventors further investigated the microstructure of the above-mentioned martensitic stainless steel material, and as a result, obtained the following new findings.

[0059] In the area where δ ferrite is generated in the steel, coarse intermetallic compounds are easily generated in the δ ferrite or at the interface between the δ ferrite and the parent phase. Here, the intermetallic compounds generated in the martensitic stainless steel material of the above chemical composition are Laves phase and / or Chi phase (χ phase). Laves phase is Fe2Mo, and χ phase is Fe 36 Cr 12 Mo 10 The coarse intermetallic compound refers to an intermetallic compound having a circle equivalent diameter of 1.0 μm or more.

[0060] The present inventors paid attention to the above-mentioned coarse intermetallic compounds generated along with the formation of delta ferrite. In addition, the present inventors considered whether the ratio of the amount of coarse intermetallic compounds to the amount of delta ferrite is related to low-temperature toughness. To this end, the present inventors investigated the relationship between the delta ferrite area ratio Sd, the coarse intermetallic compound area ratio Sc and low-temperature toughness.

[0061] Figure 1 : is a graph showing the relationship between the delta ferrite area ratio Sd, the coarse intermetallic compound area ratio Sc and the low temperature toughness. Figure 1 , the horizontal axis is Sc / Sd. The vertical axis is the absorbed energy (J) at -10°C. Figure 1The lower the Sc / Sd, that is, the smaller the area ratio of the coarse intermetallic compound Sc is relative to the area ratio of the delta ferrite Sd, the higher the absorbed energy. Moreover, when Sc / Sd is 5.00 or less, the slope of the graph becomes gentler than when Sc / Sd is greater than 5.00. That is, Sc / Sd has an inflection point near 5.00.

[0062] Based on the above research results, the present inventors have found that in the martensitic stainless steel material having the above chemical composition, by setting the delta ferrite area ratio Sd to 10.00% or less to ensure sufficient hot workability, and setting Sc / Sd to 5.00 or less, not only excellent SSC resistance but also excellent low temperature toughness can be obtained.

[0063] The reason why excellent low-temperature toughness can be obtained by setting Sc / Sd to less than 5.00 is not clear, but it can be considered to be the following reasons. As described above, coarse intermetallic compounds are generated in δ ferrite or at the interface between δ ferrite and the parent phase. Coarse intermetallic compounds promote the propagation of cracks. When Sc / Sd is higher than 5.00, the proportion of coarse intermetallic compounds present in δ ferrite or at the interface between δ ferrite and the parent phase becomes higher. Therefore, coarse intermetallic compounds promote the propagation of cracks, and cracks are also easy to extend in δ ferrite or at the interface between δ ferrite and the parent phase. As a result, it can be considered that the low-temperature toughness decreases.

[0064] The above mechanism is a speculation, and it is possible that excellent low temperature toughness can be achieved through different mechanisms. Figure 1 It was demonstrated in the examples described later that excellent low temperature toughness can be obtained by setting Sc / Sd to 5.00 or less.

[0065] The martensitic stainless steel material of the present embodiment is completed based on the above technical ideas and has the following configuration.

[0066] [1] A martensitic stainless steel material having a chemical composition in mass % as follows:

[0067] C: 0.030% or less,

[0068] Si: 1.00% or less,

[0069] Mn: 1.00% or less,

[0070] P: 0.030% or less,

[0071] S: 0.0050% or less,

[0072] Cr: 10.00~16.00%,

[0073] Ni: 4.00~7.50%,

[0074] Mo: 1.10~3.50%,

[0075] Al: 0.005-0.050%,

[0076] V: 0.01~0.30%,

[0077] N: 0.0030~0.0500%、

[0078] Ti: 0.020~0.150%,

[0079] Cu: 0.01~3.50%,

[0080] Co: 0.01-0.50%,

[0081] Nb: 0~0.150%,

[0082] W: 0~1.50%,

[0083] Zr: 0~0.0100%,

[0084] Sn: 0~0.0100%,

[0085] As: 0~0.0100%,

[0086] Sb: 0~0.0100%,

[0087] B: 0~0.0050%,

[0088] Ca: 0~0.0050%,

[0089] Mg: 0~0.0050%,

[0090] Rare earth elements (REM): 0 to 0.0100%, and

[0091] The balance is Fe and impurities.

[0092] The yield strength of the martensitic stainless steel is 758 MPa or more.

[0093] The area ratio Sd (%) of delta ferrite and the area ratio Sc (%) of intermetallic compounds having a circle equivalent diameter of 1.0 μm or more in the cross section parallel to the rolling direction of the martensitic stainless steel material satisfy equations (1) and (2),

[0094] 0<Sd≤10.00 (1)

[0095] Sc / Sd≤5.00 (2).

[0096] [2] The martensitic stainless steel material according to [1], wherein the chemical composition contains, in mass %, selected from

[0097] Nb: 0.001~0.150%,

[0098] W: 0.01~1.50%,

[0099] Zr: 0.0001~0.0100%,

[0100] Sn: 0.0001~0.0100%,

[0101] As: 0.0001~0.0100%,

[0102] Sb: 0.0001~0.0100%,

[0103] B: 0.0001~0.0050%,

[0104] Ca: 0.0001~0.0050%,

[0105] Mg: 0.0001 to 0.0050%, and

[0106] Rare earth elements (REM): one or more selected from the group consisting of 0.0001 to 0.0100%.

[0107] [3] The martensitic stainless steel material according to [1] or [2], wherein the chemical composition contains, by mass%, Cu: 1.00 to 3.50%.

[0108] Hereinafter, the martensitic stainless steel material of the present embodiment will be described in detail. Unless otherwise specified, "%" about an element means mass %.

[0109] The martensitic stainless steel material of the present embodiment satisfies characteristics 1 to 3.

[0110] (Feature 1)

[0111] The chemical composition is C: less than 0.030%, Si: less than 1.00%, Mn: less than 1.00%, P: less than 0.030%, S: less than 0.0050%, Cr: 10.00-16.00%, Ni: 4.00-7.50%, Mo: 1.10-3.50%, Al: 0.005-0.050%, V: 0.01-0.30%, N: 0.0030-0.0500%, Ti: 0.020-0.150%, Cu :0.01~3.50%, Co: 0.01~0.50%, Nb: 0~0.150%, W: 0~1.50%, Zr: 0~0.0100%, Sn: 0~0.0100%, As: 0~0.0100%, Sb: 0~0.0100%, B: 0~0.0050%, Ca: 0~0.0050%, Mg: 0~0.0050%, rare earth elements (REM): 0~0.0100%, and the balance is Fe and impurities.

[0112] (Feature 2)

[0113] The yield strength is above 758 MPa.

[0114] (Feature 3)

[0115] The area ratio Sd(%) of delta ferrite and the area ratio Sc(%) of intermetallic compounds having an equivalent circle diameter of 1.0 μm or more in a cross section parallel to the rolling direction of the martensitic stainless steel material satisfy equations (1) and (2).

[0116] 0<Sd≤10.00 (1)

[0117] Sc / Sd≤5.00 (2)

[0118] Hereinafter, features 1 to 3 will be described.

[0119] [About (Feature 1) Chemical composition]

[0120] The chemical composition of the martensitic stainless steel material of the present embodiment contains the following elements.

[0121] C: 0.030% or less

[0122] It inevitably contains carbon (C). That is, the C content is greater than 0%.

[0123] C improves the hardenability of steel, thereby improving the strength of steel. However, if the C content exceeds 0.030%, C combines with Cr to form Cr carbides. As a result, even if the contents of other elements are within the range of this embodiment, the SSC resistance of the steel will decrease.

[0124] Therefore, the C content is set to 0.030% or less.

[0125] The lower limit of the C content is preferably 0.001%, more preferably 0.003%, further preferably 0.005%, further preferably 0.006%.

[0126] The upper limit of the C content is preferably 0.029%, more preferably 0.028%, more preferably 0.025%, more preferably 0.020%, and still more preferably 0.015%.

[0127] Si: 1.00% or less

[0128] Silicon (Si) is inevitably contained, that is, the Si content is greater than 0%.

[0129] Si deoxidizes the steel. However, if the Si content exceeds 1.00%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of the present embodiment.

[0130] Therefore, the Si content is 1.00% or less.

[0131] The lower limit of the Si content is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.15%, and still more preferably 0.20%.

[0132] The upper limit of the Si content is preferably 0.70%, more preferably 0.50%, further preferably 0.45%, further preferably 0.40%.

[0133] Mn: 1.00% or less

[0134] Manganese (Mn) is inevitably contained. That is, the Mn content is greater than 0%.

[0135] Mn improves the hardenability of steel, thereby improving the strength of steel. However, if the Mn content is greater than 1.00%, Mn will form coarse inclusions even if the contents of other elements are within the range of this embodiment. Coarse inclusions reduce the low-temperature toughness of steel.

[0136] Therefore, the Mn content is 1.00% or less.

[0137] The lower limit of the Mn content is preferably 0.01%, more preferably 0.10%, further preferably 0.20%, further preferably 0.25%.

[0138] The upper limit of the Mn content is preferably 0.90%, more preferably 0.80%, more preferably 0.70%, more preferably 0.60%, and still more preferably 0.50%.

[0139] P: 0.030% or less

[0140] Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is greater than 0%.

[0141] If the P content exceeds 0.030%, P will segregate at the grain boundaries even if the contents of other elements are within the ranges of the present embodiment, and the low-temperature toughness of the steel material will significantly decrease.

[0142] Therefore, the P content is set to 0.030% or less.

[0143] The P content is preferably as low as possible. However, if the P content is excessively reduced, the production cost will increase significantly. Therefore, in consideration of industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, more preferably 0.005%, and more preferably 0.007%.

[0144] The upper limit of the P content is preferably 0.028%, more preferably 0.025%, further preferably 0.023%, further preferably 0.020%.

[0145] S: 0.0050% or less

[0146] Sulfur (S) is an impurity that is inevitably contained. That is, the S content is greater than 0%.

[0147] If the S content exceeds 0.0050%, S segregates excessively at grain boundaries or S combines with Mn to form excessive inclusions MnS. In this case, even if the contents of other elements are within the range of this embodiment, the low temperature toughness and hot workability of the steel material are significantly reduced.

[0148] Therefore, the S content is set to 0.0050% or less.

[0149] The S content is preferably as low as possible. However, if the S content is excessively reduced, the production cost will increase significantly. Therefore, in consideration of industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, more preferably 0.0003%, and more preferably 0.0004%.

[0150] The upper limit of the S content is preferably 0.0040%, more preferably 0.0030%, further preferably 0.0020%, further preferably 0.0015%.

[0151] Cr: 10.00~16.00%

[0152] Chromium (Cr) forms a passive film on the surface of steel in a corrosive environment, thereby improving the SSC resistance of the steel. If the Cr content is less than 10.00%, the above effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0153] On the other hand, if the Cr content exceeds 16.00%, δ-ferrite is excessively generated. In this case, even if the contents of other elements are within the ranges of the present embodiment, the hot workability of the steel material is reduced.

[0154] Therefore, the Cr content is 10.00 to 16.00%.

[0155] The lower limit of the Cr content is preferably 10.05%, more preferably 10.10%, further preferably 10.50%, further preferably 11.00%.

[0156] The upper limit of the Cr content is preferably 15.90%, more preferably 15.80%, more preferably 15.50%, more preferably 15.00%, more preferably 14.50%, and more preferably 14.00%.

[0157] Ni: 4.00~7.50%

[0158] Nickel (Ni) generates sulfides on the passive film in a corrosive environment. Ni sulfides inhibit the chloride ion (Cl - ), hydrogen sulfide ion (HS - ) contacts the passivation film, inhibiting the passivation film from being destroyed by chloride ions and hydrogen sulfide ions. Therefore, Ni improves the SSC resistance of steel in a corrosive environment. Ni is also an austenite-forming element, making the microstructure of the steel after quenching martensite. If the Ni content is less than 4.00%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0159] On the other hand, if the Ni content exceeds 7.50%, austenite is excessively stabilized and sufficient martensite cannot be obtained after quenching. Therefore, even if the content of other elements is within the range of this embodiment, the yield strength decreases.

[0160] Therefore, the Ni content is 4.00 to 7.50%.

[0161] The lower limit of the Ni content is preferably 4.05%, more preferably 4.10%, more preferably 4.50%, more preferably 5.00%, and still more preferably 5.50%.

[0162] The upper limit of the Ni content is preferably 7.35%, more preferably 7.10%, and further preferably 6.90%.

[0163] Mo: 1.10~3.50%

[0164] Molybdenum (Mo) generates sulfides on the passive film in a corrosive environment. Mo sulfides inhibit the chloride ion (Cl- ), hydrogen sulfide ion (HS - ) contacts the passivation film, inhibiting the passivation film from being destroyed by chloride ions and hydrogen sulfide ions. Therefore, Mo improves the SSC resistance of steel in a corrosive environment. If the Mo content is less than 1.10%, this effect cannot be fully obtained even if the content of other elements is within the range of this embodiment.

[0165] On the other hand, if the Mo content exceeds 3.50%, δ-ferrite is excessively generated. Therefore, even if the contents of other elements are within the ranges of the present embodiment, the hot workability of the steel material is reduced.

[0166] Therefore, the Mo content is 1.10 to 3.50%.

[0167] The lower limit of the Mo content is preferably 1.15%, more preferably 1.30%, more preferably 1.50%, more preferably 1.70%, and still more preferably 2.00%.

[0168] The upper limit of the Mo content is preferably 3.40%, more preferably 3.30%, more preferably 3.20%, more preferably 3.00%, and still more preferably 2.80%.

[0169] Al: 0.005~0.050%

[0170] Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the ranges of the present embodiment.

[0171] On the other hand, if the Al content exceeds 0.050%, even if the contents of other elements are within the ranges of the present embodiment, coarse Al oxides are generated, and the low-temperature toughness of the steel material decreases.

[0172] Therefore, the Al content is 0.005 to 0.050%.

[0173] The lower limit of the Al content is preferably 0.007%, more preferably 0.010%, and further preferably 0.015%.

[0174] The upper limit of the Al content is preferably 0.047%, more preferably 0.043%, and further preferably 0.040%.

[0175] It should be noted that the Al content in this specification refers to the content of sol.Al (acid-soluble Al).

[0176] V: 0.01~0.30%

[0177] Vanadium (V) forms V precipitates such as carbides, nitrides, and carbonitrides in steel, thereby increasing the strength of the steel. If the V content is less than 0.01%, the above effects cannot be fully obtained even if the contents of other elements are within the ranges of this embodiment.

[0178] On the other hand, if the V content exceeds 0.30%, too much V precipitates are generated, and the strength of the steel material becomes too high. In this case, even if the contents of other elements are within the range of this embodiment, the low-temperature toughness of the steel material decreases.

[0179] Therefore, the V content is 0.01 to 0.30%.

[0180] The lower limit of the V content is preferably 0.02%, more preferably 0.03%.

[0181] The upper limit of the V content is preferably 0.25%, more preferably 0.20%, more preferably 0.15%, more preferably 0.10%, and still more preferably 0.08%.

[0182] N: 0.0030~0.0500%

[0183] Nitrogen (N) improves the pitting corrosion resistance of the steel material, and as a result, improves the SSC resistance of the steel material. If the N content is less than 0.0030%, the above effects cannot be fully obtained even if the contents of other elements are within the ranges of the present embodiment.

[0184] On the other hand, if the N content exceeds 0.0500%, coarse TiN is generated. In this case, even if the contents of other elements are within the range of the present embodiment, the low temperature toughness of the steel material decreases.

[0185] Therefore, the N content is 0.0030 to 0.0500%.

[0186] The lower limit of the N content is preferably 0.0033%, more preferably 0.0035%, and further preferably 0.0038%.

[0187] The upper limit of the N content is preferably 0.0450%, more preferably 0.0420%, more preferably 0.0400%, more preferably 0.0350%, more preferably 0.0300%, more preferably 0.0250%, and more preferably 0.0200%.

[0188] Ti: 0.020~0.150%

[0189] Titanium (Ti) combines with C or N to form carbides or nitrides. In this case, due to the pinning effect, the coarsening of the grains is suppressed and the strength of the steel is improved. Ti also forms carbides or nitrides. Thus, excessive strength increases caused by excessive generation of V precipitates (carbides, nitrides, carbonitrides) are suppressed. As a result, the low-temperature toughness of the steel is improved. If the Ti content is less than 0.020%, even if the content of other elements is within the range of this embodiment, the above-mentioned effects cannot be fully obtained.

[0190] On the other hand, if the Ti content exceeds 0.150%, excessive amounts of Ti carbides or Ti nitrides are generated. In this case, even if the contents of other elements are within the ranges of this embodiment, the low-temperature toughness of the steel material decreases.

[0191] Therefore, the Ti content is 0.020 to 0.150%.

[0192] The lower limit of the Ti content is preferably 0.030%, more preferably 0.040%, more preferably 0.050%, more preferably 0.060%, more preferably 0.070%, and still more preferably 0.080%.

[0193] The upper limit of the Ti content is preferably 0.140%, more preferably 0.130%.

[0194] Cu: 0.01~3.50%

[0195] Copper (Cu) generates sulfides on the passive film in a corrosive environment. Cu sulfides inhibit chloride ions and hydrogen sulfide ions from contacting the passive film, inhibiting the passive film from being destroyed by chloride ions and hydrogen sulfide ions. Therefore, Cu improves the SSC resistance of steel in a corrosive environment. If the Cu content is less than 0.01%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0196] On the other hand, if the Cu content is greater than 3.50%, the hot workability of the steel material will decrease even if the contents of other elements are within the range of this embodiment. If the Cu content is greater than 3.50%, the strength of the steel material becomes too high. In this case, the low temperature toughness of the steel material decreases.

[0197] Therefore, the Cu content is 0.01 to 3.50%.

[0198] The lower limit of the Cu content is preferably 0.10%, more preferably 0.50%, more preferably 0.80%, more preferably 1.00%, more preferably 1.30%, and more preferably 1.50%.

[0199] The upper limit of the Cu content is preferably 3.30%, more preferably 3.10%, and further preferably 3.00%.

[0200] Co: 0.01~0.50%

[0201] Cobalt (Co) generates sulfides on the passive film in a corrosive environment. Co sulfides inhibit the chloride ion (Cl - ), hydrogen sulfide ion (HS - ) contacts the passivation film, inhibiting the passivation film from being destroyed by chloride ions and hydrogen sulfide ions. Therefore, Co improves the SSC resistance of steel in a corrosive environment. Co also inhibits the formation of retained austenite and suppresses uneven strength of the steel. If the Co content is less than 0.01%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0202] On the other hand, if the Co content exceeds 0.50%, δ-ferrite is excessively generated. Therefore, even if the contents of other elements are within the ranges of the present embodiment, the hot workability of the steel material is reduced.

[0203] Therefore, the Co content is 0.01 to 0.50%.

[0204] The lower limit of the Co content is preferably 0.02%, more preferably 0.05%, further preferably 0.10%, further preferably 0.15%.

[0205] The upper limit of the Co content is preferably 0.49%, more preferably 0.45%, further preferably 0.40%, further preferably 0.35%.

[0206] The balance of the chemical composition of the martensitic stainless steel material of the present embodiment is Fe and impurities. Here, impurities refer to substances mixed from ore and waste materials as raw materials or from the manufacturing environment when the martensitic stainless steel material is manufactured industrially, and are not intentionally contained substances, but are allowed within the range that does not adversely affect the effect of the martensitic stainless steel material of the present embodiment.

[0207] [About Optional Elements]

[0208] The chemical composition of the martensitic stainless steel material of the present embodiment may further contain one or more selected from the group consisting of the first group and the second group in place of a portion of Fe.

[0209] [Group 1]

[0210] Nb: 0~0.150%,

[0211] W: 0~1.50%,

[0212] Zr: 0~0.0100%,

[0213] Sn: 0~0.0100%,

[0214] As: 0 to 0.0100%, and

[0215] Sb: 0~0.0100%.

[0216] [Group 2]

[0217] B: 0~0.0050%,

[0218] Ca: 0~0.0050%,

[0219] Mg: 0 to 0.0050%, and

[0220] Rare earth elements (REM): 0 to 0.0100%.

[0221] These optional elements are described below.

[0222] [Group 1: Nb, W, Zr, Sn, As and Sb]

[0223] The chemical composition of the martensitic stainless steel material of the present embodiment may further contain one or more selected from the first group consisting of Nb, W, Zr, Sn, As and Sb to replace a portion of Fe. These elements are arbitrary elements and may not be contained. When contained, these elements improve the SSC resistance of the steel material. The following describes each element of the first group.

[0224] Nb: 0~0.150%

[0225] Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%.

[0226] When Nb is contained, that is, when the Nb content is greater than 0%, Nb forms fine precipitates (carbides, nitrides, carbonitrides, hereinafter referred to as Nb precipitates). Nb precipitates refine the substructure of the steel material through the pinning effect. As a result, the SSC resistance of the steel material is improved. The above effect can be achieved to a certain extent as long as Nb is contained in a small amount.

[0227] However, if the Nb content exceeds 0.150%, too much Nb precipitates are generated. In this case, even if the contents of other elements are within the range of the present embodiment, the low-temperature toughness of the steel material decreases.

[0228] Therefore, the Nb content is 0 to 0.150%, and when contained, it is 0.150% or less.

[0229] The lower limit of the Nb content is preferably 0.001%, more preferably 0.002%.

[0230] The upper limit of the Nb content is preferably 0.100%, more preferably 0.050%, and further preferably 0.030%.

[0231] W: 0~1.50%

[0232] Tungsten (W) is an arbitrary element and may not be contained. That is, the W content may be 0%.

[0233] When W is contained, that is, when the W content is greater than 0%, W stabilizes the passive film in a corrosive environment and inhibits the passive film from being destroyed by chloride ions and hydrogen sulfide ions. Therefore, the SSC resistance of the steel material is improved. The above effect can be achieved to a certain extent as long as W is contained in a small amount.

[0234] However, if the W content exceeds 1.50%, W combines with C to form coarse carbides. In this case, even if the contents of other elements are within the range of this embodiment, the low-temperature toughness of the steel material decreases.

[0235] Therefore, the W content is 0 to 1.50%, and when contained, it is 1.50% or less.

[0236] The lower limit of the W content is preferably 0.01%, more preferably 0.03%, and further preferably 0.05%.

[0237] The upper limit of the W content is preferably 1.20%, more preferably 1.00%.

[0238] Zr: 0~0.0100%

[0239] Zirconium (Zr) is an arbitrary element and may not be contained. That is, the Zr content may be 0%.

[0240] When Zr is contained, that is, when the Zr content is greater than 0%, Zr forms fine precipitates (carbides, nitrides, carbonitrides, hereinafter referred to as Zr precipitates). Zr precipitates refine the substructure of the steel material through the pinning effect. As a result, the SSC resistance of the steel material is improved. The above effect can be achieved to a certain extent as long as Zr is contained in a small amount.

[0241] However, if the Zr content exceeds 0.0100%, Zr precipitates are excessively generated. In this case, even if the contents of other elements are within the range of the present embodiment, the low-temperature toughness of the steel material decreases.

[0242] Therefore, the Zr content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0243] The lower limit of the Zr content is preferably 0.0001%, more preferably 0.0003%, and further preferably 0.0005%.

[0244] The upper limit of the Zr content is preferably 0.0070%, more preferably 0.0050%, and further preferably 0.0030%.

[0245] Sn: 0~0.0100%

[0246] Tin (Sn) is an arbitrary element and may not be contained. That is, the Sn content may be 0%.

[0247] When Sn is contained, that is, when the Sn content is greater than 0%, Sn improves the SSC resistance of the steel material. The above effect can be obtained to a certain extent as long as Sn is contained in a small amount.

[0248] On the other hand, if the Sn content exceeds 0.0100%, Sn segregates at the grain boundaries. In this case, even if the contents of other elements are within the range of the present embodiment, the toughness of the steel material decreases.

[0249] Therefore, the Sn content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0250] The lower limit of the Sn content is preferably 0.0001%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0007%, and still more preferably 0.0010%.

[0251] The upper limit of the Sn content is preferably 0.0090%, more preferably 0.0080%, more preferably 0.0070%, more preferably 0.0060%, and still more preferably 0.0050%.

[0252] As: 0~0.0100%

[0253] Arsenic (As) is an arbitrary element and may not be contained. That is, the As content may be 0%.

[0254] When contained, As improves the SSC resistance of steel materials. The above effect can be obtained to a certain extent as long as As is contained in a small amount.

[0255] On the other hand, if the As content exceeds 0.0100%, As segregates at the grain boundaries. In this case, even if the contents of other elements are within the ranges of the present embodiment, the SSC resistance of the steel material decreases.

[0256] Therefore, the As content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0257] The lower limit of the As content is preferably 0.0001%, more preferably 0.0003%, further preferably 0.0005%, further preferably 0.0010%.

[0258] The upper limit of the As content is preferably 0.0090%, more preferably 0.0080%, more preferably 0.0060%, more preferably 0.0040%, and still more preferably 0.0030%.

[0259] Sb: 0~0.0100%

[0260] Antimony (Sb) is an arbitrary element and may not be contained. That is, the Sb content may be 0%.

[0261] When Sb is contained, that is, when the Sb content is greater than 0%, Sb improves the SSC resistance of the steel material. The above effect can be obtained to a certain extent as long as Sb is contained in a small amount.

[0262] On the other hand, if the Sb content exceeds 0.0100%, Sb segregates at the grain boundaries. In this case, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material decreases.

[0263] Therefore, the Sb content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0264] The lower limit of the Sb content is preferably 0.0001%, more preferably 0.0003%, further preferably 0.0005%, further preferably 0.0010%.

[0265] The upper limit of the Sb content is preferably 0.0090%, more preferably 0.0080%, and further preferably 0.0060%.

[0266] [Group 2: B, Ca, Mg and rare earth elements (REM)]

[0267] The chemical composition of the martensitic stainless steel material of the present embodiment may further contain one or more selected from the second group consisting of B, Ca, Mg and rare earth elements (REM) to replace a portion of Fe. These elements are all arbitrary elements and may not be contained. When contained, these elements improve the hot workability of the steel material. The following describes each element of the second group.

[0268] B: 0~0.0050%

[0269] Boron (B) is an arbitrary element and may not be contained. That is, the B content may be 0%.

[0270] When B is contained, that is, when the B content is greater than 0%, B segregates at the austenite grain boundaries, thereby strengthening the grain boundaries. As a result, the hot workability of the steel material is improved. The above effect can be achieved to a certain extent even if B is contained in a small amount.

[0271] However, if the B content exceeds 0.0050%, Cr carboride is generated. In this case, even if the contents of other elements are within the range of the present embodiment, the low temperature toughness of the steel material decreases.

[0272] Therefore, the B content is 0 to 0.0050%, and when contained, it is 0.0050% or less.

[0273] The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%.

[0274] The upper limit of the B content is preferably 0.0045%, more preferably 0.0040%.

[0275] Ca: 0~0.0050%

[0276] Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%.

[0277] When Ca is contained, that is, when the Ca content is greater than 0%, Ca controls the morphology of inclusions and improves the hot workability of steel. Controlling the morphology of inclusions means, for example, making inclusions spherical or making inclusions fine. The above effects can be achieved to a certain extent as long as Ca is contained in a small amount.

[0278] However, if the Ca content exceeds 0.0050%, coarse oxides are generated. In this case, even if the contents of other elements are within the range of the present embodiment, the low temperature toughness of the steel material decreases.

[0279] Therefore, the Ca content is 0 to 0.0050%, and when contained, it is 0.0050% or less.

[0280] The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%, further preferably 0.0010%, further preferably 0.0015%.

[0281] The upper limit of the Ca content is preferably 0.0045%, more preferably 0.0040%.

[0282] Mg: 0~0.0050%

[0283] Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%.

[0284] When Mg is contained, that is, when the Mg content is greater than 0%, Mg controls the morphology of inclusions and improves the hot workability of steel materials in the same way as Ca. The above effect can be obtained to a certain extent even when Mg is contained in a small amount.

[0285] However, if the Mg content exceeds 0.0050%, coarse oxides are generated. In this case, even if the contents of other elements are within the range of the present embodiment, the low temperature toughness of the steel material decreases.

[0286] Therefore, the Mg content is 0 to 0.0050%, and when contained, it is 0.0050% or less.

[0287] The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0005%, and further preferably 0.0010%.

[0288] The upper limit of the Mg content is preferably 0.0045%, more preferably 0.0035%, and further preferably 0.0030%.

[0289] Rare earth elements (REM): 0~0.0100%

[0290] Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%.

[0291] When contained, that is, when the REM content exceeds 0%, REM controls the morphology of inclusions and improves the hot workability of steel materials in the same manner as Ca. The above effect can be obtained to a certain extent as long as REM is contained in a small amount.

[0292] However, if the REM content exceeds 0.0100%, coarse oxides are generated. In this case, even if the contents of other elements are within the range of the present embodiment, the low-temperature toughness of the steel material decreases.

[0293] Therefore, the REM content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0294] The lower limit of the REM content is preferably 0.0001%, more preferably 0.0005%, and further preferably 0.0010%.

[0295] The upper limit of the REM content is preferably 0.0080%, more preferably 0.0070%.

[0296] It should be noted that REM in this specification refers to one or more elements selected from the group consisting of scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71 belonging to the lanthanide series. In addition, the REM content in this specification refers to the total content of these elements.

[0297] [About (Feature 2) Yield Strength]

[0298] The yield strength of the martensitic stainless steel material of the present embodiment is 110 ksi level or higher, that is, 758 MPa or higher.

[0299] In this specification, the yield strength refers to the 0.2% residual deformation stress (MPa) obtained by performing a tensile test at room temperature (24±3° C.) according to ASTM E8 / E8M (2021).

[0300] [Determination method of yield strength]

[0301] The yield strength is calculated by the following method.

[0302] When the martensitic stainless steel material is a steel pipe, a round bar test piece is collected from the center of the wall thickness of the steel pipe. When the martensitic stainless steel material is a round bar, a round bar test piece is collected from the R / 2 portion, that is, the center of the radius R in the cross section perpendicular to the axial direction of the round bar. When the steel material is a steel plate, a round bar test piece is collected from the center of the plate thickness.

[0303] The size of the round bar test piece is, for example, that the diameter of the parallel portion is 4.0 mm and the gauge length is 16.0 mm. The length direction of the round bar test piece is parallel to the rolling direction of the martensitic stainless steel. Specifically, when the martensitic stainless steel is a steel pipe, the length direction of the round bar test piece is parallel to the tube axis direction of the steel pipe. When the martensitic stainless steel is a round steel, the length direction of the round bar test piece is parallel to the axial direction of the round steel. When the martensitic stainless steel is a steel plate, the length direction of the round bar test piece is parallel to the rolling direction of the steel plate.

[0304] A tensile test was performed using a round bar test piece at room temperature (24±3°C) according to ASTM E8 / E8M (2021) to determine the 0.2% residual strain stress (MPa). The determined 0.2% residual strain stress was defined as the yield strength (MPa).

[0305] The preferred lower limit of the yield strength of the martensitic stainless steel material of the present embodiment is 760 MPa, more preferably 770 MPa, and further preferably 780 MPa. The upper limit of the yield strength of the martensitic stainless steel material of the present embodiment is not particularly limited, as long as it is within the above-mentioned chemical composition range, and the upper limit of the yield strength is, for example, less than 1069 MPa (155 ksi), preferably less than 1000 MPa (145 ksi).

[0306] [Microstructure]

[0307] In the microstructure of the martensitic stainless steel material of the present embodiment, martensite is the main body. In the present specification, martensite includes not only fresh martensite but also tempered martensite. In addition, in the present specification, martensite as the main body means that the volume fraction of martensite in the microstructure is 80% or more. The remainder of the microstructure is composed of retained austenite and δ ferrite. That is, in the martensitic stainless steel material of the present embodiment, the total volume fraction of retained austenite and δ ferrite is greater than 0 and less than 20%. The volume fraction of retained austenite is preferably as low as possible. The preferred lower limit of the volume fraction of martensite in the microstructure of the martensitic stainless steel material of the present embodiment is 85%, and more preferably 90%.

[0308] It should be noted that when the martensitic stainless steel material having a chemical composition satisfying characteristic 1 satisfies characteristic 2, that is, when the yield strength is 758 MPa or more, the volume fraction of martensite in the microstructure is 80% or more.

[0309] In the microstructure, a small amount of retained austenite does not cause a significant decrease in strength, and significantly improves the low-temperature toughness of the steel. However, when the volume fraction of retained austenite is too high, the strength of the steel is significantly reduced. Therefore, in the microstructure of the martensitic stainless steel material of this embodiment, from the perspective of ensuring strength, the preferred upper limit of the volume fraction of retained austenite is 15%, and more preferably 10%. The preferred lower limit of the volume fraction of retained austenite is 0%.

[0310] [Measurement method of volume fraction of martensite]

[0311] The volume fraction (%) of martensite in the microstructure of the martensitic stainless steel material of the present embodiment is determined by the following method. The volume fraction (%) of retained austenite is determined by the method shown below. In addition, the area fraction Sd of δ ferrite determined by the method described in [Method for determining the area fraction Sd of δ ferrite] described later is regarded as the volume fraction (%) of δ ferrite. The volume fraction (%) of martensite is determined by subtracting the sum of the obtained volume fraction of retained austenite and the volume fraction of δ ferrite from 100%.

[0312] The volume ratio of retained austenite is determined by X-ray diffraction. Specifically, a test piece is collected from a martensitic stainless steel material.

[0313] When the martensitic stainless steel material is a steel pipe, the test piece is collected from the center of the wall thickness of the steel pipe. When the martensitic stainless steel material is a round steel, the test piece is collected from the R / 2 portion. When the steel material is a steel plate, the test piece is collected from the center of the plate thickness. The size of the test piece is not particularly limited, for example, 15mm×15mm×thickness 2mm. In this case, the thickness direction of the test piece is the wall thickness direction in the case of a steel pipe, the radial direction in the case of a round steel, and the plate thickness direction in the case of a steel plate.

[0314] Using the obtained test piece, the X-ray diffraction intensity of each of the (200) plane of the α phase, the (211) plane of the α phase, the (200) plane of the γ phase, the (220) plane of the γ phase, and the (311) plane of the γ phase was measured, and the integrated intensity of each plane was calculated. In the measurement of the X-ray diffraction intensity, the target of the X-ray diffraction device was set to Mo (Mo Kα ray), and the output was set to 50kV-40mA. After calculation, for each combination of each plane of the α phase and each plane of the γ phase (2×3=6 groups), the volume fraction Vγ (%) of retained austenite was calculated using formula (I). Then, the arithmetic mean of the volume fractions Vγ of the 6 groups of retained austenite was defined as the volume fraction (%) of retained austenite.

[0315] Vγ=100 / {1+(Iα×Rγ) / (Iγ×Rα)}(I)

[0316] Here, Iα is the integrated intensity of the α phase. Rα is the crystallographically calculated value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographically calculated value of the γ phase. It should be noted that in this specification, Rα of the (200) plane of the α phase is set to 15.9, Rα of the (211) plane of the α phase is set to 29.2, Rγ of the (200) plane of the γ phase is set to 35.5, Rγ of the (220) plane of the γ phase is set to 20.8, and Rγ of the (311) plane of the γ phase is set to 21.8. It should be noted that the volume fraction of retained austenite is obtained by rounding off the first decimal place of the obtained value.

[0317] In addition, the area ratio Sd (%) of δ ferrite is determined by the method described in [Method for determining the area ratio Sd of δ ferrite] described later. The area ratio Sd (%) of δ ferrite determined is regarded as the volume ratio (%) of δ ferrite. It should be noted that the volume ratio of δ ferrite is obtained by rounding off the first decimal place of the obtained value.

[0318] The volume fraction (%) of martensite in the microstructure of the martensitic stainless steel material is determined by the following formula using the obtained volume fraction (%) of retained austenite and the obtained volume fraction (%) of delta ferrite.

[0319] Volume fraction of martensite (%) = 100 - (volume fraction of retained austenite (%) + volume fraction of delta ferrite (%))

[0320] [Regarding (Feature 3) Formula (1) and Formula (2)]

[0321] In the martensitic stainless steel material of the present embodiment, the area ratio Sd(%) of δ-ferrite and the area ratio Sc(%) of intermetallic compounds having an equivalent circle diameter of 1.0 μm or more in a cross section parallel to the rolling direction satisfy equations (1) and (2).

[0322] 0<Sd≤10.00 (1)

[0323] Sc / Sd≤5.00 (2)

[0324] [About formula (1)]

[0325] In the martensitic stainless steel material of the present embodiment, the formation of δ ferrite is allowed to some extent. Therefore, the area ratio Sd of δ ferrite is greater than 0%. However, if the area ratio Sd of δ ferrite is greater than 10.00%, the hot workability of the steel material decreases in the manufacturing process of the martensitic stainless steel material. If the area ratio Sd of δ ferrite is less than 10.00%, that is, the area ratio Sd of δ ferrite satisfies the formula (1), sufficient hot workability can be obtained in the manufacturing process of the martensitic stainless steel material.

[0326] The lower limit of the delta ferrite area ratio Sd is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.15%, more preferably 0.20%, and still more preferably 0.25%.

[0327] The upper limit of the delta ferrite area ratio Sd is preferably 9.00%, more preferably 8.80%, more preferably 8.00%, more preferably 7.50%, and still more preferably 7.00%.

[0328] [About formula (2)]

[0329] In the martensitic stainless steel material of this embodiment, the intermetallic compound is a Laves phase and / or a χ phase. The Laves phase is Fe2Mo, and the χ phase is Fe 36 Cr 12 Mo 10 An intermetallic compound having an equivalent circle diameter of 1.0 μm or more is referred to as a coarse intermetallic compound. Here, the equivalent circle diameter refers to the diameter of a circle having an area equal to that of the intermetallic compound.

[0330] When δ ferrite exists in the steel material, along with the formation of δ ferrite, coarse intermetallic compounds are easily generated in the δ ferrite or at the interface between the δ ferrite and the parent phase. The area ratio of the coarse intermetallic compounds is defined as Sc. The ratio Fn of the area ratio of the coarse intermetallic compounds Sc to the area ratio of the δ ferrite Sd is defined by the following formula.

[0331] Fn=Sc / Sd

[0332] See also Figure 1 As mentioned above, the lower Fn (=Sc / Sd), that is, the smaller the area ratio of coarse intermetallic compounds Sc relative to the area ratio of delta ferrite Sd, the higher the absorption energy at -10°C. Moreover, when Fn is 5.00 or less, the slope of the graph becomes gentle. That is, Fn has an inflection point near 5.00.

[0333] Therefore, when Fn is 5.00 or less, excellent low-temperature toughness can be obtained. Specifically, the absorbed energy at -10°C obtained by the method described in the "Low-temperature Toughness Evaluation Method" described later is 120 J or more.

[0334] The preferred upper limit of Fn is 4.90, more preferably 4.80, more preferably 4.50, more preferably 4.00, more preferably 3.80, more preferably 3.40, more preferably 3.00, more preferably 2.50, and more preferably 2.00.

[0335] See also Figure 1 , if Fn is below 2.00, the slope of the graph becomes steep again. Figure 1 In the embodiment, Fn also has an inflection point near 2.00. When Fn is 2.00 or less, the absorbed energy at -10°C is 160 J or more, and more excellent low-temperature toughness can be obtained.

[0336] The lower limit of Fn is not particularly limited. As described above, the delta ferrite area ratio Sd is greater than 0%, and the coarse intermetallic compound area ratio Sc is also greater than 0%. Therefore, the lower limit of Fn is, for example, 0.01, more preferably 0.02, and even more preferably 0.05.

[0337] [Measurement method of area ratio Sd of delta ferrite]

[0338] The area ratio Sd of delta ferrite can be measured by the following method.

[0339] The test piece is collected from the martensitic stainless steel material of the present embodiment. When the martensitic stainless steel material is a steel pipe, the test piece is collected from the center of the wall thickness. When the martensitic stainless steel material is a round steel, the test piece is collected from the R / 2 portion. When the martensitic stainless steel material is a steel plate, the test piece is collected from the center of the plate thickness.

[0340] The test piece has an observation surface parallel to the rolling direction of the martensitic stainless steel material. When the martensitic stainless steel material is a steel pipe, the observation surface of the test piece includes the pipe axis direction and the wall thickness direction. When the martensitic stainless steel material is a round steel, the observation surface of the test piece includes the axial direction and the radial direction. When the martensitic stainless steel material is a steel plate, the observation surface of the test piece includes the rolling direction and the plate thickness direction.

[0341] The observation surface was mirror-polished. Electrolytic etching was performed on the mirror-polished observation surface using a 30% by mass NaOH aqueous solution to reveal the microstructure on the observation surface. In the observation surface where the microstructure is revealed, the area ratio of δ ferrite was determined by the point counting method according to ASTM E562 (2019). At this time, the measurement magnification was set to 400 times, the number of grid points was set to 400, and the number of measurement fields was set to 30. Each measurement field was set to a rectangle of 250 μm × 250 μm. The rectangle was divided into 400 grid points at equal intervals. When δ ferrite overlaps with a grid point, it is counted as "1". When the interface between the parent phase and δ ferrite overlaps with a grid point, it is counted as "0.5". The value (%) obtained by dividing the counts on the grid points (400) of all measurement fields (30) by the total number of grid points is defined as the area ratio Sd (%) of δ ferrite.

[0342] It should be noted that whether it is δ ferrite is determined by element concentration analysis (EDS). Specifically, the particles in each measurement field are determined based on contrast. Element concentration analysis (EDS analysis) is performed on each determined particle. In the EDS analysis, the acceleration voltage is set to 20 kV, and the target elements are set to N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, and Nb for quantification. According to the EDS analysis results of each particle, the Cr content in the particle is more than 14.00% by mass% and the Ni content in the particle is less than 0.8 times the Ni content of the parent phase (i.e., the Ni content of the martensitic stainless steel material) The particles are judged as δ ferrite.

[0343] [Method for measuring the area ratio Sc of coarse intermetallic compounds]

[0344] The coarse intermetallic compound area ratio Sc can be measured by the following method.

[0345] The test piece is collected from the martensitic stainless steel material of the present embodiment. When the martensitic stainless steel material is a steel pipe, the test piece is collected from the center of the wall thickness. When the martensitic stainless steel material is a round steel, the test piece is collected from the R / 2 portion. When the martensitic stainless steel material is a steel plate, the test piece is collected from the center of the plate thickness.

[0346] The test piece has an observation surface parallel to the rolling direction of the martensitic stainless steel material. When the martensitic stainless steel material is a steel pipe, the observation surface of the test piece includes the pipe axis direction and the wall thickness direction. When the martensitic stainless steel material is a round steel, the observation surface of the test piece includes the axial direction and the radial direction. When the martensitic stainless steel material is a steel plate, the observation surface of the test piece includes the rolling direction and the plate thickness direction.

[0347] The observation surface of the test piece is mirror-polished. Randomly select 20 fields of view containing δ ferrite from the observation surface after mirror polishing. Use an electron probe microanalyzer (EPMA: Electron Probe Micro Analyzer) to perform element mapping analysis (surface analysis) on these fields of view. The measurement magnification is set to 500 times, and the field of view area is set to 100μm×100μm. In the element mapping analysis using EPMA, the acceleration voltage is set to 15kV. In addition, the elements to be measured are set to Fe, Cr, Mo, Ni, and C. Through the above measurements, the element mapping under each field of view is obtained.

[0348] As mentioned above, the intermetallic compound is a Laves phase (Fe2Mo) and / or a χ phase (Fe 36 Cr 12 Mo 10 ). The Mo concentration of these intermetallic compounds is higher than that of the parent phase. Therefore, the intermetallic compounds are determined based on the Mo concentration of each pixel.

[0349] Specifically, the element mapping area of ​​each field of view is divided into 512×512 pixels. The element amount contained in each pixel is expressed by the count of characteristic X-rays. For 512×512 pixels, the arithmetic mean CMo of Mo counts is calculated. ave Count Mo in 512×512 pixels as CMo ave Pixels with a value greater than 10 times the value are excluded as outlier pixels. The maximum value CMo is determined from the Mo counts of the remaining pixels after excluding the outlier pixels from the 512×512 pixels. max . The maximum value CMo will be displayed among multiple pixels after excluding outlier pixels max Pixels with Mo counts greater than 0.600 times of the value of FIG. 1 are identified as intermetallic compound region pixels.

[0350] Based on the obtained intermetallic compound region pixels, the intermetallic compound is identified by the following method. Figure 2A to Figure 2E Schematic diagram for explaining the method of determining the intermetallic compound in each field of view divided into 512×512 pixels. Figure 2A, assuming that there are a plurality of intermetallic compound region pixels PX1 to PX4 in the field of view VA. In this case, focus on any intermetallic compound region pixel PX1 among the plurality of intermetallic compound region pixels PX1 to PX4. Then, as Figure 2B As shown, a 5-pixel × 5-pixel determination area A1 is drawn with the intermetallic compound area pixel PX1 as the center. When there are other intermetallic compound area pixels in the determination area A1 of the intermetallic compound area pixel PX1, the intermetallic compound area pixel and the intermetallic compound area pixel PX1 are considered to be in the same intermetallic compound area. Figure 2B In FIG. 1 , the intermetallic compound region pixel PX2 is included in the determination region A1 . Therefore, the intermetallic compound region pixel PX2 and the intermetallic compound region pixel PX1 are identified as being in the same intermetallic compound region.

[0351] Then, if Figure 2C As shown in FIG. 1 , a determination area A2 of 5 pixels×5 pixels is drawn with the intermetallic compound area pixel PX2 as the center. Figure 2C In the example, the intermetallic compound region pixel PX3 is included in the determination region A2. Therefore, the intermetallic compound region pixel PX3 and the intermetallic compound region pixel PX2 are identified as being in the same intermetallic compound region. Therefore, in this case, the intermetallic compound region pixels PX1 to PX3 are identified as being in the same intermetallic compound region.

[0352] Likewise, if Figure 2D As shown in FIG. 1 , a determination area A3 of 5 pixels×5 pixels is drawn with the intermetallic compound area pixel PX3 as the center. Figure 2D In the above example, except for the intermetallic compound region pixel PX3 and the intermetallic compound region pixel PX2 that has been identified as being in the same intermetallic compound, there are no new intermetallic compound region pixels included in the determination area A3. If there are no other new intermetallic compound region pixels in the determination area, the intermetallic compound region pixels included in the same intermetallic compound are determined. Specifically, in Figure 2D In the case of , it is determined that the intermetallic compound region pixels PX1 to PX3 are included in the same intermetallic compound, and the intermetallic compound region pixel PX4 is not included in the intermetallic compound including the intermetallic compound region pixels PX1 to PX3.

[0353] In the case of determining the intermetallic compound region pixels contained in the same intermetallic compound, such as Figure 2EAs shown, the circumscribed rectangle IM including the intermetallic compound region pixels PX1 to PX3 contained in the same intermetallic compound is determined as one intermetallic compound. It should be noted that in the field of view divided into 512×512 pixels, each side of the circumscribed rectangle IM is parallel to the row direction X or column direction Y of the pixels.

[0354] The area of ​​the intermetallic compound determined by the above method is regarded as the area of ​​the circumscribed rectangle IM. The circle equivalent diameter of the intermetallic compound is calculated based on the area of ​​the obtained intermetallic compound. It should be noted that the circle equivalent diameter refers to the diameter of a circle whose area is equal to the area of ​​the intermetallic compound. It should be noted that each field of view is 100μm×100μm, and the field of view is divided into 512×512 pixels. Therefore, the size of each pixel is 0.195μm×0.195μm.

[0355] Among the identified intermetallic compounds, intermetallic compounds having a circle equivalent diameter of 1.0 μm or more (coarse intermetallic compounds) were identified. In each field of view, the area ratio of the identified coarse intermetallic compounds was calculated by the following formula.

[0356] Area ratio of coarse intermetallic compounds = (total area of ​​identified coarse intermetallic compounds / area of ​​field of view) × 100

[0357] The arithmetic mean of the area ratios of the coarse intermetallic compounds obtained in each of the viewing fields at 20 locations was defined as the coarse intermetallic compound area ratio Sc (%).

[0358] [Effects of the Martensitic Stainless Steel Material of the Present Embodiment]

[0359] The martensitic stainless steel material of the present embodiment satisfies Characteristics 1 to 3. Therefore, the martensitic stainless steel material of the present embodiment can obtain excellent SSC resistance and excellent low-temperature toughness even if it has a high yield strength of 758 MPa or more.

[0360] [About SSC resistance of steel materials]

[0361] The martensitic stainless steel material of the present embodiment can obtain excellent SSC resistance even if it has a high yield strength of 758 MPa or more. In the present embodiment, having excellent SSC resistance means that no cracks are confirmed by the following [SSC resistance evaluation method 1].

[0362] [SSC resistance evaluation method 1]

[0363] The SSC resistance of the martensitic stainless steel material of the present embodiment can be evaluated by an SSC resistance evaluation test at room temperature. The SSC resistance evaluation test is performed by a method based on NACE TM0177-2016 Method A.

[0364] Specifically, a round bar test piece is collected from the martensitic stainless steel material of the present embodiment. In the case where the martensitic stainless steel material is a steel pipe, the round bar test piece is collected from the central part of the wall thickness. In the case where the martensitic stainless steel material is a round steel, the round bar test piece is collected from the R / 2 part. In the case where the martensitic stainless steel material is a steel plate, the round bar test piece is collected from the central part of the plate thickness. The size of the round bar test piece is not particularly limited. Regarding the size of the round bar test piece, for example, the diameter of the parallel portion is 6.35 mm, and the length of the parallel portion is 25.4 mm. It should be noted that the length direction of the round bar test piece is parallel to the rolling direction of the steel material. Specifically, in the case where the martensitic stainless steel material is a steel pipe, the length direction of the round bar test piece is parallel to the tube axis direction of the steel pipe. In the case where the martensitic stainless steel material is a round steel, the length direction of the round bar test piece is parallel to the axial direction of the round steel. In the case where the martensitic stainless steel material is a steel plate, the length direction of the round bar test piece is parallel to the rolling direction of the steel plate.

[0365] The test solution is a 20 mass % sodium chloride aqueous solution with a pH of 4.0. The test solution is adjusted to a pH of 4.0 by adding acetic acid to an aqueous solution containing 20 mass % sodium chloride and 0.41 g / L sodium acetate. The prepared round rod test piece is loaded with a stress equivalent to 90% of the actual yield stress. The test solution at 24°C is injected into the test container so that the round rod test piece loaded with stress is immersed as a test bath. After degassing the test bath, 0.07 bar of H2S gas and 0.93 bar of CO2 gas are blown into the test bath to saturate the H2S gas in the test bath. The test bath saturated with H2S gas is kept at 24°C for 720 hours.

[0366] After the test piece was kept for 720 hours, the surface of the parallel portion of the test piece was observed using a magnifying glass with a magnification of 10 times to confirm the presence or absence of cracks. In the martensitic stainless steel material of this embodiment, no cracks were confirmed even after 720 hours in the SSC resistance evaluation test performed by the above method.

[0367] Preferably, the martensitic stainless steel material of the present embodiment satisfies characteristics 1 to 3 and has a Cu content of 1.00 to 3.50%. In this case, more excellent SSC resistance can be obtained. Specifically, no cracks were confirmed by the following [SSC resistance evaluation method 2].

[0368] [SSC resistance evaluation method 2]

[0369] Compared with the SSC resistance evaluation method 1, the SSC resistance evaluation method 2 is different only in the test bath. Specifically, the test solution is the same as that of the SSC resistance evaluation method 1, and a 20 mass% sodium chloride aqueous solution with a pH of 4.0 is prepared. The prepared round rod test piece is loaded with a stress equivalent to 90% of the actual yield stress. The test solution at 24°C is injected into the test container so that the round rod test piece loaded with stress is immersed as a test bath. After the test bath is degassed, 0.10 bar of H2S gas and 0.90 bar of CO2 gas are blown into the test bath to saturate the H2S gas in the test bath. The test bath saturated with H2S gas is kept at 24°C for 720 hours. That is, in the SSC resistance evaluation method 2, the SSC resistance is evaluated under more severe conditions than the SSC resistance evaluation method 1.

[0370] After the test piece was kept for 720 hours, the surface of the parallel portion of the test piece was observed using a magnifying glass with a magnification of 10 times to confirm the presence or absence of cracks. In the martensitic stainless steel material of this embodiment, no cracks were confirmed even after 720 hours in the SSC resistance evaluation test performed by the above method.

[0371] [About low temperature toughness]

[0372] As described above, the martensitic stainless steel material of the present embodiment has excellent low-temperature toughness. In the present embodiment, excellent low-temperature toughness can be evaluated by the following method.

[0373] [Evaluation method for low temperature toughness]

[0374] The low-temperature toughness of the martensitic stainless steel material according to the present embodiment was evaluated by a Charpy impact test in accordance with ASTM E23 (2018).

[0375] Specifically, according to API 5CRA (2010), a full-size or small-size V-notch test piece is prepared from a martensitic stainless steel material. The prepared V-notch test piece is subjected to a Charpy impact test in accordance with ASTM E23 (2018) to obtain the absorbed energy (J) at -10°C. It should be noted that when a small-size V-notch test piece is used, the absorbed energy obtained is divided by the reduction factor described in API 5CRA (2010) and converted into the absorbed energy of the full-size V-notch test piece. In addition, the absorbed energy (J) at -10°C is rounded to the first decimal place of the obtained value. In the present embodiment, when the absorbed energy at -10°C is 120J or more, it is evaluated as having excellent low-temperature toughness.

[0376] [Shape and uses of martensitic stainless steel]

[0377] The martensitic stainless steel material of the present embodiment is a steel pipe, round steel (solid material), or steel plate. When the martensitic stainless steel material is a steel pipe, the martensitic stainless steel material is preferably a steel pipe for oil wells. For example, a steel pipe for oil wells is a casing, a tubing, a drill pipe, etc. used in the excavation of oil wells or gas wells, the collection of crude oil or natural gas, etc. When the martensitic stainless steel material is a steel pipe, it is preferably a seamless steel pipe.

[0378] When the martensitic stainless steel material is a round steel bar, the martensitic stainless steel material is, for example, a steel material for downhole components.

[0379] As described above, the martensitic stainless steel material of the present embodiment satisfies Characteristics 1 to 3. Therefore, it has high strength with a yield strength of 110 ksi class or higher (758 MPa or higher), excellent SSC resistance, and further has excellent low-temperature toughness.

[0380] [Manufacturing method]

[0381] An example of a method for manufacturing the martensitic stainless steel material of the present embodiment is described. It should be noted that as long as the martensitic stainless steel material satisfying the above-mentioned characteristics 1 to 3 can be manufactured, the martensitic stainless steel material of the present embodiment is not limited to the manufacturing method described below, and the martensitic stainless steel material of the present embodiment can also be manufactured by other manufacturing methods. Among them, the manufacturing method described below is a preferred example for manufacturing the martensitic stainless steel material of the present embodiment.

[0382] An example of a method for producing a martensitic stainless steel material according to the present embodiment includes the following steps.

[0383] (Process 1) Blank preparation process

[0384] (Process 2) Hot working process

[0385] (Process 3) Quenching process

[0386] (Process 4) Tempering process

[0387] Hereinafter, each step will be described.

[0388] [(Process 1) Blank preparation process]

[0389] In the billet preparation step, a casting material having the chemical composition of feature 1 is prepared. Specifically, molten steel having the above chemical composition is manufactured by a known method. The manufactured molten steel is used to manufacture a cast billet by a continuous casting method. Here, the cast billet refers to a slab or a bloom. Instead of the cast billet, an ingot can be manufactured by an ingot casting method using the above molten steel. A billet can also be manufactured by performing initial rolling on a bloom or an ingot. The billet (slab, bloom, billet or ingot) is manufactured by the above manufacturing process.

[0390] [(Process 2) Hot working process]

[0391] In the hot working step, the prepared billet is hot worked to produce an intermediate steel material. The method of hot working to produce the intermediate steel material is not particularly limited. That is, in this embodiment, the hot working may be hot forging, hot extrusion, or hot rolling.

[0392] When the martensitic stainless steel material is a steel pipe (seamless steel pipe), the billet is hot-worked to produce a tube billet (seamless steel pipe). In this case, as a hot working, for example, a glass lubricant high-speed extrusion method or an Einstein pipe punching method (i.e., hot extrusion) can be implemented. When the intermediate steel material is a seamless steel pipe, further hot working, for example, piercing rolling using the Mannesmann method (i.e., hot rolling) can be implemented.

[0393] For example, when piercing and rolling using the Mannesmann method is performed during hot working, first, the billet is heated in a heating furnace. The heating temperature is not particularly limited, and is, for example, 1100 to 1250°C. The billet extracted from the heating furnace is subjected to piercing and rolling to produce an intermediate steel material (billet). The piercing ratio in the piercing and rolling is not particularly limited, and is, for example, 1.0 to 4.0. The billet after piercing and rolling is subjected to stretching and rolling using a mandrel mill. Furthermore, as needed, the billet after stretching and rolling is subjected to sizing and rolling using a reducing mill or a sizing mill. The billet is manufactured through the above process. The cumulative cross-sectional shrinkage in the hot working process is not particularly limited, and is, for example, 20 to 70%.

[0394] When the martensitic stainless steel material is round steel, the billet is hot-worked to produce an intermediate steel material (round steel). In this case, hot rolling is performed as the hot working. The heating temperature before hot rolling is not particularly limited, and is, for example, 1100 to 1250° C. When hot rolling is performed, hot rolling using a continuous rolling mill is preferably performed. The continuous rolling mill alternately arranges a horizontal frame having a pair of hole-shaped rollers arranged in the vertical direction and a vertical frame having a pair of hole-shaped rollers arranged in the horizontal direction.

[0395] When the steel material is a steel plate, the billet is hot-worked to produce an intermediate steel material (a plate-shaped steel material). In this case, hot rolling is performed as the hot working. The heating temperature before hot rolling is not particularly limited, and is, for example, 1100 to 1250° C. The billet extracted from the heating furnace is hot-rolled using a continuous rolling mill to produce an intermediate steel material (a plate-shaped steel material).

[0396] As described above, an intermediate steel material having a desired shape is manufactured through the hot working process. It should be noted that the hot working may be performed only once or multiple times. For example, the billet may be subjected to the piercing rolling and then subjected to the hot extrusion. For example, the billet may be subjected to the initial rolling and then subjected to the hot rolling using the continuous rolling mill.

[0397] The intermediate steel material produced by hot working can be air-cooled. The intermediate steel material produced by hot working can be directly quenched after hot working without cooling to room temperature, or can be quenched after supplementary heating (reheating) after hot working. When quenching is performed directly after hot working or quenching is performed after supplementary heating after hot working, stress relief annealing (SR treatment) can be performed before the heat treatment step (quenching and tempering) of the next step for the purpose of removing residual stress.

[0398] [(Step 3) Quenching step]

[0399] In the quenching step, the intermediate steel material produced in the hot working step is quenched. In the quenching step, quenching is performed so as to satisfy the following conditions.

[0400] Condition 1: The quenching temperature T1 is set to 900 to 1090°C.

[0401] Condition 2: FA defined by the formula (A) is set to 1.71 or less.

[0402] FA={(t1 / t2)-1} / Cu(A)

[0403] Here, the temperature of the intermediate steel material during the heating of the quenching process is substituted into "t1" in the formula (A) from A to c3 The time t1 (minutes) from the point where the temperature reaches 900°C is substituted. The holding time t2 (minutes) at the quenching temperature T1 is substituted for "t2". The Cu content (mass %) of the martensitic stainless steel material is substituted for "Cu".

[0404] FA is an index related to the amount of intermetallic compounds generated in the intermediate steel after quenching. c3 The temperature range from 900°C to 900°C is a temperature range that promotes the formation of intermetallic compounds. Therefore, when heating in the quenching process, a shorter time t1 can suppress the formation of coarse intermetallic compounds. On the other hand, in the quenching temperature range above 900°C, coarse intermetallic compounds will melt. Therefore, even if A c3 As time t1 in the temperature range of 0.5°C to 900°C becomes longer, as long as holding time t2 at quenching temperature T1 is long enough to satisfy condition 2 relative to time t1, the coarse intermetallic compounds generated during heating will melt.

[0405] In addition, as described above, in the martensitic stainless steel material satisfying the chemical composition of feature 1, when δ ferrite is generated, coarse intermetallic compounds are easily generated in the δ ferrite or at the interface between the δ ferrite and the parent phase. That is, the more δ ferrite is generated, the more coarse intermetallic compounds are generated. In the chemical composition satisfying feature 1, Cu is an austenite forming element. That is, the less Cu content, the easier it is to generate δ ferrite. Therefore, the less Cu content, the easier it is to generate coarse intermetallic compounds. Therefore, the Cu content of the intermediate steel also affects the amount of coarse intermetallic compounds generated.

[0406] Figure 3 This is a graph showing the relationship between time t1 and time t2 of a steel material having a chemical composition satisfying feature 1, the Cu content, and the area ratio Sc of coarse intermetallic compounds. Figure 3 The vertical axis represents t1 / t2, and the horizontal axis represents the Cu content (%). Figure 3 The straight line FA in represents the above formula (A). Figure 3 The "○" in the figure indicates the test result that Fn (=Sc / Sd) is 5.00 or less. The "●" indicates the test result that Fn is greater than 5.00. Figure 3 It was produced based on the test results obtained in the examples described later.

[0407] See also Figure 3 In the region above the straight line FA, FA is greater than 1.71. In this case, the amount of coarse intermetallic compound generation relative to the amount of delta ferrite generation becomes large, and Fn is greater than 5.00. On the other hand, in the region below the straight line FA, FA is less than 1.71. In this case, the amount of coarse intermetallic compound generation relative to the amount of delta ferrite generation is suppressed within a suitable range, and Fn is less than 5.00.

[0408] As described above, in the quenching step, the quenching temperature T1 is set to 900 to 1090° C., and FA is set to 1.71 or less. This can sufficiently suppress the amount of coarse intermetallic compounds generated relative to the amount of delta ferrite generated.

[0409] After being held at the quenching temperature T1 for a holding time t2, the intermediate steel material is quenched. The quenching method is, for example, water cooling or oil cooling. The quenching method is not particularly limited. For example, the intermediate steel material may be rapidly cooled by immersing it in a water tank or an oil tank. In the case where the intermediate steel material is a steel pipe, the intermediate steel material may be rapidly cooled by pouring or spraying cooling water onto the outer surface and / or inner surface of the steel pipe using spray cooling or mist cooling.

[0410] It should be noted that, when the intermediate steel material is a tube (seamless steel pipe), after the hot working step, quenching (direct quenching) may be performed immediately after the hot working without cooling the tube to room temperature. Alternatively, the tube may be charged into a reheating furnace before the temperature of the hot-worked tube drops, and then quenched after being kept at the quenching temperature.

[0411] [(Step 4) Tempering step]

[0412] In the tempering process, the intermediate steel material after quenching is tempered. In the tempering process, the yield strength of the martensitic stainless steel material can be adjusted by appropriately adjusting the tempering temperature according to the chemical composition. Specifically, the tempering conditions are adjusted so that the yield strength of the martensitic stainless steel material is 110 ksi level or more (758 MPa or more).

[0413] The tempering temperature and the holding time at the tempering temperature in the tempering step can be appropriately adjusted according to the set yield strength and chemical composition. The tempering temperature is not particularly limited. The tempering temperature is, for example, 500 to 650°C.

[0414] Through the above steps, the martensitic stainless steel material of the present embodiment can be manufactured. It should be noted that, in the above manufacturing method, an example of the manufacturing method of the martensitic stainless steel material of the present embodiment is described. The martensitic stainless steel material of the present embodiment may also be manufactured by a manufacturing method other than the above manufacturing method.

[0415] Example

[0416] The effect of the martensitic stainless steel material of the present embodiment will be further specifically described by way of examples. The conditions in the following examples are one conditional example adopted to confirm the feasibility and effect of the martensitic stainless steel material of the present embodiment. Therefore, the martensitic stainless steel material of the present embodiment is not limited to this one conditional example.

[0417] Martensitic stainless steel materials (seamless steel pipes) having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured.

[0418] [Table 1-1]Table 1-1

[0419]

[0420] [Table 1-2]

[0421] Table 1-2

[0422]

[0423] The "-" part in Table 1-1 and Table 1-2 means that the content of the element is 0% after the decimal point of the numerical value is rounded off. For example, for the Nb content of Test No. 1, the fourth decimal place is rounded off to 0%. For the W content of Test No. 1, the third decimal place is rounded off to 0%. For the Zr content of Test No. 1, the fifth decimal place is rounded off to 0%.

[0424] As a billet preparation process, large square billets are manufactured by continuous casting using molten steel. The prepared large square billets are subjected to initial rolling to manufacture billets (round steel billets). The billets are subjected to a hot working process. Specifically, the billets are loaded into a heating furnace and heated at 1100 to 1250°C. The billets extracted from the heating furnace are subjected to hot rolling (hot working) using the Mannesmann-mandrel type to manufacture intermediate steel materials (tube billets) of each test number. At this time, the perforation ratio is in the range of 1.0 to 4.0, and the cumulative cross-sectional reduction rate in the hot working process is in the range of 20 to 70%.

[0425] The hot-worked blank was subjected to a quenching step. c3 Points are shown in Table 2 as “A c3 The quenching temperature T1 (°C), t1 / t2 and FA in the quenching process are shown in the "Quenching temperature T1 (°C)" column, the "t1 / t2" column and the "FA" column in Table 2.

[0426] [Table 2]

[0427] Table 2

[0428]

[0429] The tube blank after the quenching step is tempered to adjust the yield strength. The tempering temperature is 500 to 650° C. Through the above manufacturing process, a martensitic stainless steel material (seamless steel pipe) is manufactured.

[0430] [Evaluation test]

[0431] The following evaluation tests were performed on the martensitic stainless steel materials (seamless steel pipes) of the respective test numbers.

[0432] (Test 1) Yield strength evaluation test

[0433] (Test 2) Martensite volume fraction evaluation test

[0434] (Test 3) Delta Ferrite Area Ratio Sd Evaluation Test

[0435] (Test 4) Evaluation test of the area ratio Sc of coarse intermetallic compounds

[0436] (Test 5) Hot workability evaluation test

[0437] (Test 6) SSC resistance evaluation test

[0438] (Test 7) ​​Low temperature toughness evaluation test

[0439] Hereinafter, each test will be described.

[0440] [(Test 1) Yield strength evaluation test]

[0441] Based on the above [yield strength determination method], the yield strength (MPa) of the martensitic stainless steel of each test number is obtained. It should be noted that a round bar test piece is collected from the central part of the wall thickness of the martensitic stainless steel (seamless steel pipe) of each test number. Regarding the size of the round bar test piece, the diameter of the parallel portion is 4.0 mm and the gauge length is 16.0 mm. The length direction of the round bar test piece is set to be parallel to the tube axis direction of the martensitic stainless steel (seamless steel pipe). The obtained yield strength (MPa) is shown in the "YS (MPa)" column in Table 3.

[0442] [Table 3]

[0443] Table 3

[0444]

[0445] [(Test 2) Martensite volume ratio evaluation test]

[0446] Based on the above [method for determining the volume fraction of martensite], the martensite volume fraction (%) of the martensite stainless steel material of each test number was obtained. It should be noted that the test piece was collected from the central part of the wall thickness of the martensite stainless steel material (seamless steel pipe) of each test number. The size of the test piece was 15mm×15mm×thickness 2mm, and the thickness direction of the test piece was the wall thickness direction of the seamless steel pipe. The obtained martensite volume fraction (%) is shown in the "Martensite volume fraction (%)" column in Table 3.

[0447] [(Test 3) Delta Ferrite Area Ratio Sd Evaluation Test]

[0448] The area ratio Sd of δ ferrite in the cross section parallel to the rolling direction of the martensitic stainless steel material of each test number is obtained by the above-mentioned [method for determining the area ratio Sd of δ ferrite]. It should be noted that the test piece was collected from the central part of the wall thickness of the martensitic stainless steel material (seamless steel pipe) of each test number. The observation surface of the test piece is a surface including the rolling direction and the wall thickness direction of the martensitic stainless steel material. The obtained area ratio Sd (%) of δ ferrite is shown in the "Area ratio Sd (%) of δ ferrite" column of Table 3.

[0449] [(Test 4) Evaluation test of the area ratio Sc of coarse intermetallic compounds]

[0450] The area ratio Sc of the intermetallic compounds in the cross section parallel to the rolling direction of the martensitic stainless steel material of each test number is determined by the above-mentioned [method for determining the area ratio Sc of coarse intermetallic compounds]. It should be noted that the test piece is collected from the central part of the wall thickness of the martensitic stainless steel material (seamless steel pipe) of each test number. The observation surface of the test piece is a surface including the pipe axis direction and the wall thickness direction of the martensitic stainless steel material. The obtained area ratio Sc (%) of the intermetallic compound is shown in the "Area ratio Sc (%) of coarse intermetallic compounds" column of Table 3. In addition, Fn (=Sc / Sd) is shown in the "Fn" column of Table 3.

[0451] [(Test 5) Hot workability evaluation test]

[0452] The surface of the tube blank of each test number after the hot working step and before the quenching step was visually observed to confirm the presence or absence of cracks. The confirmation results are shown in the "Hot Workability" column in Table 3. If the "Hot Workability" column is "NA (Not Accepted)", it means that cracks with a crack length of 1.0 mm or more in the tube axis direction are confirmed, and sufficient hot workability cannot be obtained. If the "Hot Workability" column is "EX (Excellent)", it means that cracks with a crack length of 1.0 mm or more in the tube axis direction are not confirmed, and sufficient hot workability is obtained.

[0453] [(Test 6) SSC resistance evaluation test]

[0454] The SSC resistance of the martensitic stainless steel material of each test number was evaluated based on the above-mentioned [SSC resistance evaluation method 1] and [SSC resistance evaluation method 2].

[0455] Specifically, a round bar test piece was collected from the central part of the wall thickness of the martensitic stainless steel material (seamless steel pipe) of each test number. Regarding the size of the round bar test piece, the diameter of the parallel part was set to 6.35 mm, and the length of the parallel part was set to 25.4 mm. The length direction of the round bar test piece was set to be parallel to the rolling direction of the steel material (the pipe axis direction of the seamless steel pipe).

[0456] The test solution is set to a 20 mass % sodium chloride aqueous solution with a pH of 4.0. The test solution is adjusted to a pH of 4.0 by adding acetic acid to an aqueous solution containing 20 mass % sodium chloride and 0.41 g / L sodium acetate. A stress equivalent to 90% of the actual yield stress is applied to the prepared round bar test piece. The test solution at 24°C is injected into the test container so that the round bar test piece loaded with stress is immersed as a test bath. In each test number, the following evaluation method 1 and evaluation method 2 tests are respectively implemented.

[0457] Evaluation method 1: After degassing the test bath, 0.07 bar of H2S gas and 0.93 bar of CO2 gas were blown into the test bath to saturate the test bath with H2S gas. The H2S gas-saturated test bath was maintained at 24°C for 720 hours.

[0458] Evaluation method 2: After degassing the test bath, 0.10 bar of H2S gas and 0.90 bar of CO2 gas were blown into the test bath to saturate the test bath with H2S gas. The H2S gas-saturated test bath was maintained at 24°C for 720 hours.

[0459] The surfaces of the parallel portions of the test pieces after holding for 720 hours in the evaluation methods 1 and 2 were observed using a magnifying glass with a magnification of 10 times to confirm the presence or absence of cracks.

[0460] In the test piece tested by evaluation method 1 and the test piece tested by evaluation method 2, the surface of the parallel part of the test piece was observed using a magnifying glass with a magnification of 10 times. The observation results are shown in the "Evaluation Method 1" or "Evaluation Method 2" column of the "SSC Resistance" column in Table 3. If the "Evaluation Method 1" or "Evaluation Method 2" column of the "SSC Resistance" column is "EX (Excellent)", it means that no cracks were confirmed on the test piece and excellent SSC resistance was obtained. On the other hand, if the "Evaluation Method 1" or "Evaluation Method 2" column of the "SSC Resistance" column is "NA (Not Accepted)", it means that cracks were confirmed on the test piece and excellent SSC resistance could not be obtained.

[0461] [(Test 7) ​​Low temperature toughness evaluation test]

[0462] The low temperature toughness of the martensitic stainless steel material of each test number was evaluated based on the above-mentioned [low temperature toughness evaluation method]. It should be noted that the evaluation was performed using a full-size V-notch test piece. The obtained absorbed energy vE (-10°C) at -10°C is shown in the "Absorbed Energy vE (-10°C) (J)" column in Table 3.

[0463] [Evaluation results]

[0464] Referring to Table 1-1, Table 1-2, Table 2, and Table 3, in Test Nos. 1 to 24, the martensitic stainless steel material satisfies Characteristics 1 to 3. Therefore, sufficient hot workability is obtained. In addition, even if the yield strength is 758 MPa or more, excellent SSC resistance can be obtained in Evaluation Method 1. In addition, the absorbed energy at -10°C is 120 J or more, and excellent low-temperature toughness is obtained.

[0465] In Test Nos. 1 to 3, 6 to 8, and 10 to 24 among Test Nos. 1 to 24, the Cu content was 1.00% or more. Therefore, excellent SSC resistance was obtained not only in Evaluation Method 1 but also in Evaluation Method 2 which is stricter than Evaluation Method 1.

[0466] In Test Nos. 1 to 24, Fn was 2.00 or less in Test Nos. 1, 2, 4, 6, 7, 9, 11, 15, 16, 19 to 21, 23, and 24. Therefore, the absorbed energy at -10°C was 160 J or more, and more excellent low-temperature toughness was obtained.

[0467] On the other hand, in test numbers 25 to 27, the quenching temperature T1 in the quenching step was too low. Therefore, although characteristics 1 and 2 were satisfied, Fn was greater than 5.00 and characteristic 3 was not satisfied. As a result, the absorbed energy at -10°C was less than 120 J, and excellent low-temperature toughness could not be obtained.

[0468] In test numbers 28 to 32, although the contents of each element in the chemical composition are appropriate, FA in the quenching process is greater than 1.71. Therefore, although characteristics 1 and 2 are satisfied, Fn is greater than 5.00 and characteristic 3 is not satisfied. As a result, the absorbed energy at -10°C is less than 120 J, and excellent low-temperature toughness cannot be obtained.

[0469] In Test No. 33, the C content was too high, so excellent SSC resistance could not be obtained.

[0470] In Test No. 34, the Cr content was too high. Therefore, the area ratio Sd of delta ferrite exceeded 10.00%. As a result, sufficient hot workability could not be obtained.

[0471] In Test No. 35, the Cr content was too low, so excellent SSC resistance could not be obtained.

[0472] In Test No. 36, the Ni content was too high. Therefore, sufficient yield strength could not be obtained, and the martensite volume fraction was less than 80%.

[0473] In Test No. 37, the Ni content was too low, so excellent SSC resistance could not be obtained.

[0474] In Test No. 38, the Mo content was too high. Therefore, the area ratio Sd of delta ferrite exceeded 10.00%, and therefore, sufficient hot workability could not be obtained.

[0475] In Test No. 39, the Mo content was too low, so excellent SSC resistance could not be obtained.

[0476] In Test No. 40, the Cu content was too high, so sufficient hot workability could not be obtained.

[0477] In Test No. 41, the Cu content was too low, so excellent SSC resistance could not be obtained.

[0478] In Test No. 42, the Co content was too high. Therefore, the area ratio Sd of delta ferrite exceeded 10.00%, and therefore, sufficient hot workability could not be obtained.

[0479] In Test No. 43, the Co content was too low, so excellent SSC resistance could not be obtained.

[0480] The embodiments of the present invention have been described above. However, the above embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and the above embodiments may be appropriately modified and implemented within the scope of the present invention.

Claims

1. A martensitic stainless steel material, Its chemical composition is C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cr：10.00～16.00％、 Ni: 4.00~7.50%, Mo: 1.10~3.50%, Al：0.005～0.050％、 V：0.01～0.30％、 N：0.0030～0.0500％、 Ti: 0.020~0.150%, Cu: 0.01~3.50%, Co: 0.01-0.50%, Nb: 0~0.150%, W：0～1.50％、 Zr：0～0.0100％、 Sn: 0~0.0100%, As: 0~0.0100%, Sb: 0~0.0100%, B：0～0.0050％、 Ca: 0~0.0050%, Mg: 0~0.0050%, Rare earth elements (REM): 0 to 0.0100%, and the balance is Fe and impurities, The yield strength of the martensitic stainless steel is 758 MPa or more. The area ratio Sd (%) of delta ferrite and the area ratio Sc (%) of intermetallic compounds having a circle equivalent diameter of 1.0 μm or more in the cross section parallel to the rolling direction of the martensitic stainless steel material satisfy equations (1) and (2), 0<Sd≤10.00 (1) Sc / Sd≤5.00 (2).

2. The martensitic stainless steel material according to claim 1, wherein: The chemical composition contains, in mass%, selected from Nb: 0.001~0.150%, W：0.01～1.50％、 Zr：0.0001～0.0100％、 Sn: 0.0001~0.0100%, As: 0.0001~0.0100%, Sb: 0.0001~0.0100%, B：0.0001～0.0050％、 Ca: 0.0001~0.0050%, Mg: 0.0001 to 0.0050%, and Rare earth elements (REM): one or more selected from the group consisting of 0.0001 to 0.0100%.

3. The martensitic stainless steel material according to claim 1 or claim 2, wherein: The chemical composition contains Cu: 1.00 to 3.50% by mass.

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