Martensitic stainless steel seamless pipe

Abstract

To provide a martensitic stainless seamless steel pipe that has a yield strength of 655 MPa or more and excellent corrosion resistance, and is resistant to the formation of inner surface flaws.SOLUTION: A martensitic stainless seamless steel pipe according to the present disclosure has a chemical composition containing, in mass%, C: 0.001 to 0.050%, Si: 0.05 to 1.00%, Mn: 0.05 to 2.00%, P: 0.030% or less, S: 0.0100% or less, Al :0.005 to 0.100%, N: 0.020% or less, Ni: 1.00 to 9.00%, Cr: 8.00 to 16.00%, Cu: 3.50% or less, Mo: 1.00 to 5.00%, W: 0.01 to 0.30%, V: 0.010 to 1.500% and Co: 0.001 to 0.500% and furthermore containing one or more selected from the group consisting of Ca, Mg, B, and rare earth elements, and the balance consisting of Fe and impurities, and has a yield strength of 655 MPa or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a seamless steel pipe, and more particularly to a martensitic stainless steel seamless pipe having a microstructure mainly composed of martensite. [Background technology]

[0002] Oil wells and gas wells (hereinafter, oil wells and gas wells will be collectively referred to simply as "oil wells") may be in a corrosive environment containing corrosive gases. Here, corrosive gas means carbon dioxide gas and / or hydrogen sulfide gas. In other words, steel materials used in oil wells are required to have excellent corrosion resistance in corrosive environments.

[0003] Chromium (Cr) is known to be effective in improving the corrosion resistance of steel in corrosive environments. Therefore, martensitic stainless steels containing approximately 13% Cr by mass, such as API L80 13Cr steel (normal 13Cr steel) and Super 13Cr steel with a reduced C content, are used in corrosive environments.

[0004] Japanese Patent Laid-Open Publication No. 10-1755 (Patent Document 1), Japanese Patent Laid-Open Publication No. 10-503809 (Patent Document 2), Japanese Patent Laid-Open Publication No. 2000-192196 (Patent Document 3), Japanese Patent Laid-Open Publication No. 8-246107 (Patent Document 4), and Japanese Patent Laid-Open Publication No. 2012-136742 (Patent Document 5) propose martensitic stainless steel materials that have excellent corrosion resistance in corrosive environments.

[0005] The steel material disclosed in Patent Document 1 is a martensitic stainless steel having a chemical composition, in mass%, of C: 0.005-0.05%, Si: 0.05-0.5%, Mn: 0.1-1.0%, P: 0.025% or less, S: 0.015% or less, Cr: 10-15%, Ni: 4.0-9.0%, Cu: 0.5-3%, Mo: 1.0-3%, Al: 0.005-0.2%, N: 0.005%-0.1%, with the remainder being Fe and impurities, satisfying 40C+34N+Ni+0.3Cu-1.1Cr-1.8Mo≧-10. The microstructure of this steel material is composed of tempered martensite, martensite, and retained austenite, with the total fraction of tempered martensite and martensite being 60 to 80%, and the remainder being retained austenite. Patent Document 1 discloses that this steel material has excellent corrosion resistance and resistance to sulfide stress corrosion cracking.

[0006] The steel disclosed in Patent Document 2 is a martensitic stainless steel having a chemical composition, in weight percent, of C: 0.005-0.05%, Si≦0.50%, Mn: 0.1-1.0%, P≦0.03%, S≦0.005%, Mo: 1.0-3.0%, Cu: 1.0-4.0%, Ni: 5-8%, Al≦0.06%, with the balance being Fe and impurities, satisfying Cr+1.6Mo≧13 and 40C+34N+Ni+0.3Cu-1.1Cr-1.8Mo≧−10.5. The microstructure of this steel is a tempered martensitic structure. Patent Document 2 also discloses that this steel has excellent hot workability and resistance to sulfide stress corrosion cracking.

[0007] The steel material disclosed in Patent Document 3 is a martensitic stainless steel having a chemical composition, by weight, of 0.001-0.05% C, 0.05-1% Si, 0.05-2% Mn, 0.025% or less P, 0.01% or less S, 9-14% Cr, 3.1-7% Mo, 1-8% Ni, 0.5-7% Co, 0.001-0.1% sol. Al, 0.05% or less N, 0.01% or less O (oxygen), 0-5% Cu, 0-5% W, with the balance being Fe and impurities. Patent Document 3 discloses that this steel material has excellent resistance to carbon dioxide corrosion and sulfide stress corrosion cracking.

[0008] The steel material disclosed in Patent Document 4 is a martensitic stainless steel having a chemical composition, by weight percent, of 0.005% to 0.05% C, 0.05% to 0.5% Si, 0.1% to 1.0% Mn, 0.025% or less P, 0.015% or less S, 12 to 15% Cr, 4.5% to 9.0% Ni, 1% to 3% Cu, 2% to 3% Mo, 0.1% to 3% W, 0.005 to 0.2% Al, and 0.005% to 0.1% N, with the balance being Fe and impurities, satisfying the formula 40C + 34N + Ni + 0.3Cu + Co - 1.1Cr - 1.8Mo - 0.9W ≧ -10. Patent Document 4 discloses that this steel material has excellent resistance to carbon dioxide corrosion and sulfide stress corrosion cracking.

[0009] The steel material disclosed in Patent Document 5 is a martensitic stainless steel seamless pipe having a chemical composition, in mass%, of up to 0.01% C, up to 0.5% Si, 0.1 to 2.0% Mn, up to 0.03% P, up to 0.005% S, 14.0 to 15.5% Cr, 5.5 to 7.0% Ni, 2.0 to 3.5% Mo, 0.3 to 3.5% Cu, up to 0.20% V, up to 0.05% Al, and up to 0.06% N, with the balance being Fe and impurities, and having a yield strength of 655 to 862 MPa and a yield ratio of 0.90 or greater. Patent Document 5 also discloses that this steel material has excellent resistance to carbon dioxide corrosion and sulfide stress corrosion cracking. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 10-1755 [Patent Document 2] Special Publication No. 10-503809 [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-192196 [Patent Document 4] Japanese Patent Application Publication No. 8-246107 [Patent Document 5] Japanese Patent Application Laid-Open No. 2012-136742 Summary of the Invention [Problem to be solved by the invention]

[0011] Martensitic stainless steel seamless pipes having excellent corrosion resistance in corrosive environments may also be required to have a yield strength of 655 MPa or more (95 ksi or more). Therefore, martensitic stainless steel seamless pipes having a yield strength of 655 MPa or more and excellent corrosion resistance may be obtained by techniques other than those disclosed in Patent Documents 1 to 5.

[0012] Martensitic stainless steel seamless pipes may also be subjected to hot rolling, typically piercing and rolling, during production. In piercing and rolling, a hollow mother pipe is produced from a solid material. Defects are likely to form on the inner surface of the mother pipe produced by piercing and rolling. In this specification, defects formed on the inner surface of the mother pipe are also referred to as "internal defects." When internal defects are formed in the mother pipe by piercing and rolling, the internal defects also remain on the inner surface of the manufactured martensitic stainless steel seamless pipe. If deep internal defects are formed in the martensitic stainless steel seamless pipe, the seamless pipe may not achieve the desired mechanical properties. Therefore, deep internal defects formed on the inner surface of the seamless pipe are removed by mechanical processing such as polishing. On the other hand, when internal defects are removed by polishing or the like, the wall thickness of the seamless pipe may be thinner than desired depending on the depth of the internal defects. Thus, in martensitic stainless steel seamless pipes, it is preferable to be able to suppress inner surface defects.

[0013] Thus, it is preferable that a martensitic stainless steel seamless pipe has a yield strength of 655 MPa or more and excellent corrosion resistance, and furthermore, that the formation of inner surface defects can be suppressed. However, Patent Documents 1 to 5 do not consider inner surface defects formed by piercing-rolling.

[0014] An object of the present disclosure is to provide a martensitic stainless steel seamless pipe having a yield strength of 655 MPa or more, excellent corrosion resistance, and suppressed formation of inner surface defects. [Means for solving the problem]

[0015] The martensitic stainless steel seamless pipe according to the present disclosure has: In mass%, C: 0.001 to 0.050%, Si: 0.05 to 1.00%, Mn: 0.05 to 2.00%, P:0.030% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, N: 0.020% or less, Ni: 1.00~9.00%, Cr: 8.00~16.00%, Cu:3.50% or less, Mo: 1.00~5.00%, W: 0.01 to 0.30%, V: 0.010~1.500%, Co: 0.001 to 0.500%, Ca: 0 to 0.0250%, Mg: 0 to 0.0250%, B: 0~0.0200%, Rare earth elements: 0~0.200%, Nb: 0 to 0.100%, Ta: 0 to 0.100%, Ti: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, Sn: 0 to 0.100%, and The balance is composed of Fe and impurities. Within the range of the content of elements in the martensitic stainless steel seamless pipe, the content of the elements satisfies formula (1), The yield strength is 655 MPa or more. 10Ca+10Mg+2B+REM≧0.0010 (1) Here, the contents of the corresponding elements in mass% are substituted for Ca, Mg, and B in formula (1), and the total content of rare earth elements in mass% is substituted for REM in formula (1). [Effects of the Invention]

[0016] The martensitic stainless steel seamless pipe according to the present disclosure has a yield strength of 655 MPa or more, excellent corrosion resistance, and further suppresses the formation of inner surface defects. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram showing the relationship between the W content (mass %) and the maximum depth (mm) of the inner surface flaws in this example. [Figure 2] FIG. 2 is a diagram showing the relationship between the W content (mass %) and the hot tensile strength (MPa) which is an index of the load applied to the piercing-rolling mill in this example. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present inventors have investigated and studied a martensitic stainless steel seamless pipe having a yield strength of 655 MPa or more, excellent corrosion resistance, and suppressed formation of inner surface defects, and have obtained the following findings.

[0019] First, the present inventors conducted a detailed study on elements that improve the corrosion resistance of steel materials. As a result, they discovered that the corrosion resistance of steel materials can be improved by appropriately containing Cr, Mo, Cu, Ni, and Co in the steel material. That is, by mass%, C: 0.001 to 0.050%, Si: 0.05 to 1.00%, Mn: 0.05 to 2.00%, P: 0.030% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, N: 0.020% or less, Ni: 1.00 to 9.00%, Cr: 8.00 to 16.00%, Cu: 3.50% or less, Mo: 1.00 to 5.00%, V: 0.010 to 1.500%, The present inventors considered that a martensitic stainless steel seamless pipe having a chemical composition containing Co: 0.001-0.500%, Nb: 0-0.100%, Ta: 0-0.100%, Ti: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, and Sn: 0-0.100% may be able to achieve both a yield strength of 655 MPa or more and excellent corrosion resistance.

[0020] On the other hand, in martensitic stainless steel seamless pipes having the above-mentioned chemical composition, inner surface defects may be formed during piercing and rolling during the manufacturing process. If inner surface defects occur in a mother pipe during piercing and rolling, the inner surface defects must be removed by grinding or the like. In this case, the productivity of seamless steel pipes decreases. Furthermore, if inner surface defects are formed too deeply during piercing and rolling, the inner surface of the mother pipe must be polished deeply to remove the inner surface defects. As a result, the wall thickness of the manufactured seamless steel pipe may become thin.

[0021] Therefore, the present inventors investigated methods for suppressing the occurrence of internal defects in martensitic stainless steel seamless pipes having the above-mentioned chemical composition. As a result, in addition to the above-mentioned chemical composition, they focused on calcium (Ca), magnesium (Mg), boron (B), and rare earth elements (REM) as elements that improve hot workability. Ca, Mg, and REM neutralize sulfur (S) in steel by fixing it as sulfide, thereby improving the hot workability of the steel. B suppresses segregation of sulfur in steel to grain boundaries, thereby improving the hot workability of the steel. In other words, the present inventors thought that the occurrence of internal defects could be suppressed by adding Ca, Mg, B, and / or REM.

[0022] Here, F1 is defined as 10Ca + 10Mg + 2B + REM. Increasing F1 can suppress the deterioration of hot workability of the steel material due to S, and can suppress the formation of inner surface defects in the steel material. Therefore, the martensitic stainless steel seamless pipe according to this embodiment contains, in addition to the above-mentioned element contents, 0 to 0.0250% of Ca, 0 to 0.0250% of Mg, 0 to 0.0200% of B, and 0 to 0.200% of REM, and further, the element contents satisfy formula (1). 10Ca+10Mg+2B+REM≧0.0010 (1) Here, the contents of the corresponding elements in mass% are substituted for Ca, Mg, and B in formula (1), and the total content of rare earth elements in mass% is substituted for REM in formula (1).

[0023] On the other hand, in mass%, C: 0.001 to 0.050%, Si: 0.05 to 1.00%, Mn: 0.05 to 2.00%, P: 0.030% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, N: 0.020% or less, Ni: 1.00 to 9.00%, Cr: 8.00 to 16.00%, Cu: 3.50% or less, Mo: 1.00 to 5.00%, V: 0.010 to 1.500%, Co: 0.001 to 0.500 %, Ca: 0-0.0250%, Mg: 0-0.0250%, B: 0-0.0200%, REM: 0-0.200%, Nb: 0-0.100%, Ta: 0-0.100%, Ti: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, and Sn: 0-0.100%, and further satisfying formula (1), deep internal defects may be formed. Therefore, the present inventors investigated a method for further suppressing the formation of internal defects in martensitic stainless steel seamless pipes having the above-mentioned element contents. As a result, they found that the formation of internal defects in seamless steel pipes can be suppressed by further containing tungsten (W) in addition to the above-mentioned element contents. This point will be specifically explained using the drawings.

[0024] Fig. 1 is a diagram showing the relationship between the W content (mass%) and the maximum depth (mm) of internal flaws in this example. Fig. 1 was created using the W content (mass%) and the maximum depth (mm) of internal flaws caused by piercing and rolling for steel materials in the examples described below that have the above-mentioned element contents, satisfy formula (1), and exhibit excellent corrosion resistance. The maximum depth (mm) of internal flaws was obtained using a method described below. All of the steel materials used in Fig. 1 had a yield strength of 655 MPa or more.

[0025] 1, in a steel material having the above-mentioned element contents, satisfying formula (1), and exhibiting excellent corrosion resistance, if the W content is 0.01%, the maximum depth of internal flaws will be less than 0.3 mm. In other words, FIG. 1 proves that if the W content is 0.01% or more, the formation of internal flaws can be suppressed.

[0026] The details of why the inclusion of 0.01% or more of W can suppress the formation of internal surface defects are not clear. However, the inventors of the present invention speculate as follows: When piercing-rolling is performed on a steel material having the above-mentioned element contents and satisfying formula (1), oxides are formed on the surface of the steel material during heating before piercing-rolling and during piercing-rolling. W may dissolve in the oxides and lower the melting point of the oxides. In this case, the oxides may melt and become liquid during piercing-rolling. As a result, it is speculated that the oxides containing dissolved W function as a lubricant, and the formation of internal surface defects can be suppressed even when piercing-rolling is performed.

[0027] The effect of suppressing the formation of inner surface flaws in steel materials by a W content of 0.01% or more is demonstrated by the examples described below. That is, even if W suppresses the formation of inner surface flaws in steel materials by a mechanism different from the above-mentioned mechanism believed by the inventors, the examples demonstrate that W can suppress the formation of inner surface flaws in martensitic stainless steel seamless pipes having the above-mentioned chemical composition.

[0028] Therefore, the martensitic stainless steel seamless pipe according to this embodiment has the above-mentioned element contents, satisfies formula (1), and further contains 0.01 to 0.30% of W. As a result, the martensitic stainless steel seamless pipe according to this embodiment not only has a yield strength of 655 MPa or more and excellent corrosion resistance, but also suppresses the formation of inner surface defects.

[0029] The martensitic stainless steel seamless pipe according to this embodiment, which was completed based on the above findings, has the following features.

[0030] [1] A martensitic stainless steel seamless pipe, In mass%, C: 0.001 to 0.050%, Si: 0.05 to 1.00%, Mn: 0.05 to 2.00%, P:0.030% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, N: 0.020% or less, Ni: 1.00~9.00%, Cr: 8.00~16.00%, Cu:3.50% or less, Mo: 1.00~5.00%, W: 0.01 to 0.30%, V: 0.010~1.500%, Co: 0.001 to 0.500%, Ca: 0 to 0.0250%, Mg: 0 to 0.0250%, B: 0~0.0200%, Rare earth elements: 0~0.200%, Nb: 0 to 0.100%, Ta: 0 to 0.100%, Ti: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, Sn: 0 to 0.100%, and The balance is composed of Fe and impurities. Within the range of the content of elements in the martensitic stainless steel seamless pipe, the content of the elements satisfies formula (1), The yield strength is 655 MPa or more. Martensitic stainless steel seamless pipe. 10Ca+10Mg+2B+REM≧0.0010 (1) Here, the contents of the corresponding elements in mass% are substituted for Ca, Mg, and B in formula (1), and the total content of rare earth elements in mass% is substituted for REM in formula (1).

[0031] [2] [1] A martensitic stainless steel seamless pipe according to the present invention, Nb: 0.001 to 0.100%, Ta: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100%, Hf: 0.001 to 0.100%, and Sn: Contains one or more elements selected from the group consisting of 0.001 to 0.100% Martensitic stainless steel seamless pipe.

[0032] [3] A martensitic stainless steel seamless pipe according to [1] or [2], W: Contains 0.01 to 0.25% Martensitic stainless steel seamless pipe.

[0033] [4] A martensitic stainless steel seamless pipe according to any one of [1] to [3], Within the range of the content of the elements in the martensitic stainless steel seamless pipe, the content of the elements satisfies formula (2). Martensitic stainless steel seamless pipe. 0.05Mo+W≧α (2) Here, α in formula (2) is 0.240 when the Cu content of the martensitic stainless steel seamless pipe is less than 0.50%, and is 0.200 when the Cu content is 0.50 to 3.50%. The contents of the corresponding elements in mass% are substituted for W and Mo in formula (2).

[0034] [5] A martensitic stainless steel seamless pipe according to any one of [1] to [4], The martensitic stainless steel seamless pipe is a seamless steel pipe for oil wells. Martensitic stainless steel seamless pipe.

[0035] In this specification, "seamless steel pipe for oil wells" is a general term for casings, tubing, and drill pipes used in drilling oil or gas wells and extracting crude oil or natural gas.

[0036] The martensitic stainless steel seamless pipe according to this embodiment will be described in detail below. "%" relating to elements means mass % unless otherwise specified.

[0037] [Chemical composition] The chemical composition of the martensitic stainless steel seamless pipe according to this embodiment contains the following elements.

[0038] C: 0.001 to 0.050% Carbon (C) improves the hardenability of steel materials and increases their strength. If the C content is too low, this effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content is too high, the corrosion resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.001 to 0.050%. The preferred lower limit of the C content is 0.002%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the C content is 0.045%, and even more preferably 0.040%.

[0039] Si: 0.05 to 1.00% Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content is too high, this effect saturates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.05 to 1.00%. A preferred lower limit of the Si content is 0.07%, more preferably 0.10%, and even more preferably 0.15%. A preferred upper limit of the Si content is 0.70%, more preferably 0.65%, and even more preferably 0.60%.

[0040] Mn: 0.05 to 2.00% Manganese (Mn) improves the hardenability of steel and increases the strength of the steel material. If the Mn content is too low, this effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, Mn may segregate at grain boundaries together with impurity elements such as P and S. Therefore, if the Mn content is too high, the corrosion resistance of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.05 to 2.00%. The preferred lower limit of the Mn content is 0.15%, more preferably 0.18%, even more preferably 0.20%, even more preferably 0.30%, and even more preferably 0.50%. The preferred upper limit of the Mn content is 1.90%, more preferably 1.85%, and even more preferably 1.80%.

[0041] P:0.030% or less Phosphorus (P) is an unavoidable impurity. That is, the lower limit of the P content is greater than 0%. P segregates at grain boundaries and reduces the corrosion resistance of steel. Therefore, the P content is 0.030% or less. A preferred upper limit of the P content is 0.028%, and more preferably 0.025%. The lower the P content, the better. However, an extreme reduction in the P content significantly increases production costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.005%.

[0042] S: 0.0100% or less Sulfur (S) is an unavoidable impurity. That is, the lower limit of the S content is greater than 0%. S segregates at grain boundaries and reduces the toughness and hot workability of steel. S also combines with Mn to form MnS inclusions, which reduce the toughness and hot workability of steel. Therefore, the S content is 0.0100% or less. A preferred upper limit of the S content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0080%. The S content should be as low as possible. However, an extreme reduction in the S content significantly increases production costs. Therefore, considering industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%.

[0043] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect is not sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content is too high, this effect saturates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.005 to 0.100%. A preferred lower limit of the Al content is 0.008%, more preferably 0.010%, even more preferably 0.015%, even more preferably 0.020%, and even more preferably 0.025%. A preferred upper limit of the Al content is 0.090%, more preferably 0.080%, and even more preferably 0.070%. Note that the Al content in this specification refers to the content of sol.Al (acid-soluble Al).

[0044] N: 0.020% or less Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is greater than 0%. N combines with Ti to form Ti nitrides. Fine Ti nitrides suppress grain coarsening through a pinning effect. On the other hand, if the N content is too high, coarse nitrides are formed, even if the contents of other elements are within the ranges of this embodiment, and the toughness of the steel material decreases. Therefore, the N content is 0.020% or less. A preferred upper limit of the N content is 0.018%, more preferably 0.015%, and even more preferably 0.012%. A preferred lower limit of the N content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. To more effectively obtain the above effects, a preferred lower limit of the N content is 0.004%, and even more preferably 0.005%.

[0045] Ni: 1.00~9.00% Nickel (Ni) is an austenite-forming element, which causes the microstructure after quenching to become martensite. Ni also enhances the corrosion resistance of the steel material. If the Ni content is too low, even if the contents of other elements are within the ranges of this embodiment, a large amount of ferrite may be contained in the microstructure after tempering. In this case, the steel material does not obtain the desired mechanical properties. If the Ni content is too low, the steel material will not obtain sufficient corrosion resistance, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content is too high, even if the contents of other elements are within the ranges of this embodiment, the A c1 The transformation point becomes too low, making it difficult to temper the steel material. As a result, the steel material may not have the desired mechanical properties. Therefore, the Ni content is 1.00 to 9.00%. The lower limit of the Ni content is preferably 1.50%, more preferably 2.00%, even more preferably 2.50%, even more preferably 3.00%, and even more preferably 3.50%. The upper limit of the Ni content is preferably 8.50%, more preferably 8.00%, and even more preferably 7.50%.

[0046] Cr: 8.00~16.00% Chromium (Cr) forms a coating on the surface of a steel material, improving the corrosion resistance of the steel material. If the Cr content is too low, this effect cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive intermetallic compounds and Cr oxides may be formed, or coarse intermetallic compounds and / or coarse Cr oxides may be formed, thereby reducing the SSC resistance of the steel material. Therefore, the Cr content is 8.00 to 16.00%. The preferred lower limit of the Cr content is 8.50%, more preferably 9.00%, even more preferably 10.00%, even more preferably 10.50%, even more preferably 10.65%, even more preferably 10.70%, even more preferably 10.80%, and even more preferably 11.00%. The upper limit of the Cr content is preferably 15.50%, more preferably 15.00%, even more preferably 14.50%, and still more preferably 14.20%.

[0047] Cu:3.50% or less Copper (Cu) is unavoidably contained. That is, the lower limit of the Cu content is greater than 0%. Cu dissolves in the steel material and improves the corrosion resistance of the steel material. On the other hand, if the Cu content is too high, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 3.50% or less. A preferred lower limit of the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. Here, if the Cu content is 0.50% or more, the corrosion resistance of the steel material will be further improved. If the Cu content is 0.50% or more, the effect of formula (2) described below will be further enhanced. Specifically, if the Cu content is 0.50% or more, internal defects can be further suppressed even if 0.05Mo+W, defined as F2, is slightly low. To effectively obtain these effects, the lower limit of the Cu content is preferably 0.50%, more preferably 0.60%, even more preferably 0.80%, and even more preferably 1.00%. The upper limit of the Cu content is preferably 3.30%, even more preferably 3.10%, and even more preferably 2.90%. On the other hand, if the Cu content is less than 0.50%, the manufacturing cost can be reduced. Therefore, when the Cu content is less than 0.50%, the upper limit of the Cu content is preferably 0.48%, even more preferably 0.45%, and even more preferably 0.43%.

[0048] Mo: 1.00-5.00% Molybdenum (Mo) increases the strength of steel. Mo also improves the corrosion resistance of steel. Mo also assists W in suppressing the formation of internal defects in steel. If the Mo content is too low, these effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, Mo is a ferrite-forming element. Therefore, if the Mo content is too high, austenite is difficult to stabilize, and a microstructure mainly composed of martensite is difficult to stably obtain, even if the contents of other elements are within the ranges of this embodiment. As a result, the steel may not exhibit the desired mechanical properties. Therefore, the Mo content is 1.00 to 5.00%. The preferred lower limit of the Mo content is 1.10%, more preferably 1.20%, even more preferably 1.50%, and even more preferably 1.80%. The preferred upper limit of the Mo content is 4.70%, more preferably 4.50%, even more preferably 4.00%, and even more preferably 3.80%.

[0049] W: 0.01 to 0.30% Tungsten (W) suppresses the formation of inner surface defects. If the W content is too low, this effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0.01 to 0.30%. On the other hand, if the W content is too high, the strength of the steel material may become too high even if the contents of other elements are within the ranges of this embodiment. In this case, the stress required for piercing and rolling becomes too high. This point will be specifically explained using the drawings.

[0050] FIG. 2 is a diagram showing the relationship between the W content (mass%) and hot tensile strength (MPa) in this example. FIG. 2 was created using the W content (mass%) and hot tensile strength (MPa) for steel materials in the examples described later, in which the contents of elements other than W satisfy the ranges described in this embodiment. Note that the piercing-rolling was performed using a preferred manufacturing method described later. In addition, in a hot workability test (Greeble test) performed under conditions described later, the maximum stress until the steel material broke was defined as the "hot tensile strength." Note that "◯" in FIG. 2 indicates a steel material in which the maximum depth of the internal flaws formed by piercing-rolling is less than 0.3 mm. On the other hand, "●" in FIG. 2 indicates a steel material in which the maximum depth of the internal flaws formed by piercing-rolling is 0.3 mm or more.

[0051] Referring to FIG. 2, in a steel material satisfying the chemical composition according to this embodiment, if the W content exceeds 0.25%, the hot tensile strength exceeds 130 MPa. In this case, the load on the piercing-rolling mill increases. Therefore, in the chemical composition of the martensitic stainless steel seamless pipe according to this embodiment, the W content is preferably 0.25% or less. Furthermore, as described above, if the W content is less than 0.01%, the maximum depth of inner surface defects will be 0.3 mm or more. Therefore, the W content according to this embodiment is preferably 0.01 to 0.25%. In this case, the formation of inner surface defects in the seamless steel pipe can be suppressed, and the load on the piercing-rolling mill can be reduced.

[0052] The lower limit of the W content is preferably 0.02%, more preferably 0.04%, even more preferably 0.05%, even more preferably 0.06%, and even more preferably 0.07%. The upper limit of the W content is more preferably 0.24%, even more preferably less than 0.24%, even more preferably 0.23%, and even more preferably 0.22%.

[0053] V: 0.010 to 1.500% Vanadium (V) improves the hardenability of steel materials and increases their strength. If the V content is too low, this effect cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content is too high, the toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0.010 to 1.500%. The preferred lower limit of the V content is 0.020%, more preferably 0.030%, and even more preferably 0.040%. The preferred upper limit of the V content is 1.000%, more preferably 0.700%, even more preferably 0.500%, and even more preferably 0.300%.

[0054] Co: 0.001 to 0.500% Cobalt (Co) improves the corrosion resistance of steel. Co also improves the hardenability of steel and stabilizes its strength. If the Co content is too low, these effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Co content is too high, the toughness of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0.001 to 0.500%. The preferred lower limit of the Co content is 0.005%, more preferably 0.010%, even more preferably 0.030%, even more preferably 0.050%, even more preferably 0.100%, even more preferably 0.120%, and even more preferably 0.150%. The preferred upper limit of the Co content is 0.450%, even more preferably 0.400%, and even more preferably 0.350%.

[0055] The balance of the martensitic stainless steel seamless pipe according to this embodiment is composed of Fe and impurities. Here, the term "impurities" refers to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment during industrial production of steel, but are not intentionally added, and are permissible within a range that does not adversely affect the martensitic stainless steel seamless pipe according to this embodiment.

[0056] [Optional element] [Group 1 arbitrary elements] The chemical composition of the martensitic stainless steel seamless pipe according to this embodiment further contains one or more elements selected from the group consisting of Ca, Mg, B, and rare earth elements (REM), all of which improve the hot workability of the steel material and suppress the formation of inner surface defects in the steel material.

[0057] Ca: 0 to 0.0250% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca neutralizes S in the steel material by fixing it as sulfide. As a result, the hot workability of the steel material is improved. Even if even a small amount of Ca is contained, this effect can be obtained to some extent. On the other hand, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, inclusions in the steel material will coarsen, reducing the toughness of the steel material. Therefore, the Ca content is 0 to 0.0250%. To effectively obtain the above effect, the lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the Ca content is preferably 0.0200%, more preferably 0.0150%, and even more preferably 0.0100%.

[0058] Mg: 0 to 0.0250% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg neutralizes S in the steel material by fixing it as sulfide. As a result, the hot workability of the steel material is improved. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. On the other hand, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, inclusions in the steel material will coarsen, reducing the toughness of the steel material. Therefore, the Mg content is 0 to 0.0250%. To effectively obtain the above effects, the lower limit of the Mg content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the Mg content is preferably 0.0240%, more preferably 0.0220%, and even more preferably 0.0200%.

[0059] B: 0 to 0.0200% Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B suppresses the segregation of S to grain boundaries in the steel material. As a result, the hot workability of the steel material is improved. Even if even a small amount of B is contained, the above effect can be obtained to some extent. On the other hand, if the B content is too high, boron nitride (BN) is formed, even if the contents of other elements are within the ranges of this embodiment, and the toughness of the steel material decreases. Therefore, the B content is 0 to 0.0200%. To effectively obtain the above effect, the lower limit of the B content is preferably 0.0005%, more preferably 0.0010%, even more preferably 0.0012%, and even more preferably 0.0014%. The upper limit of the B content is preferably 0.0180%, more preferably 0.0170%, and even more preferably 0.0150%.

[0060] Rare earth elements: 0~0.200% Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, REM neutralizes S in the steel material by fixing it as sulfides. As a result, the hot workability of the steel material is improved. Even if even a small amount of REM is contained, the above effects can be obtained to some extent. On the other hand, if the REM content is too high, inclusions in the steel material will coarsen and the toughness of the steel material will decrease, even if the contents of other elements are within the ranges of this embodiment. Therefore, the REM content is 0 to 0.200%. To effectively obtain the above effects, the lower limit of the REM content is preferably 0.001%, more preferably 0.010%, even more preferably 0.020%, and even more preferably 0.025%. The upper limit of the REM content is preferably 0.190%, even more preferably 0.180%, and even more preferably 0.170%.

[0061] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0062] [Group 2 arbitrary elements] The chemical composition of the martensitic stainless steel seamless pipe according to this embodiment may further contain one or more elements selected from the group consisting of Nb, Ta, Ti, Zr, and Hf in place of a portion of Fe. All of these elements are optional elements and increase the strength of the steel material.

[0063] Nb: 0 to 0.100% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When contained, Nb forms carbonitrides and increases the strength of the steel material. Even if even a small amount of Nb is contained, this effect can be obtained to some extent. On the other hand, if the Nb content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Nb content is 0 to 0.100%. The lower limit of the Nb content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the Nb content is preferably 0.090%, and even more preferably 0.080%.

[0064] Ta: 0 to 0.100% Tantalum (Ta) is an optional element and may not be contained. That is, the Ta content may be 0%. When contained, Ta forms carbonitrides and increases the strength of the steel material. Even if even a small amount of Ta is contained, this effect can be obtained to some extent. On the other hand, if the Ta content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Ta content is 0 to 0.100%. The preferred lower limit of the Ta content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Ta content is 0.090%, and even more preferably 0.080%.

[0065] Ti: 0 to 0.100% Titanium (Ti) is an optional element and does not necessarily need to be contained. That is, the Ti content may be 0%. When contained, Ti forms carbonitrides and increases the strength of the steel material. Even if even a small amount of Ti is contained, this effect can be obtained to some extent. On the other hand, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the Ti content is 0 to 0.100%. The preferred lower limit of the Ti content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Ti content is 0.090%, and even more preferably 0.080%.

[0066] Zr: 0 to 0.100% Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When contained, Zr forms carbonitrides and increases the strength of the steel material. Even if even a small amount of Zr is contained, this effect can be obtained to some extent. On the other hand, if the Zr content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the Zr content is 0 to 0.100%. The lower limit of the Zr content is preferably more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The upper limit of the Zr content is preferably 0.090%, and even more preferably 0.080%.

[0067] Hf: 0 to 0.100% Hafnium (Hf) is an optional element and may not be contained. That is, the Hf content may be 0%. When contained, Hf forms carbonitrides and increases the strength of the steel material. Even if even a small amount of Hf is contained, this effect can be obtained to some extent. On the other hand, if the Hf content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Hf content is 0 to 0.100%. The lower limit of the Hf content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the Hf content is preferably 0.090%, and even more preferably 0.080%.

[0068] [Group 3 arbitrary elements] The chemical composition of the martensitic stainless steel seamless pipe according to this embodiment may further contain Sn in place of a portion of Fe.

[0069] Sn: 0 to 0.100% Tin (Sn) is an optional element and does not necessarily need to be contained. That is, the Sn content may be 0%. When contained, Sn enhances the corrosion resistance of the steel material. Even if even a small amount of Sn is contained, this effect can be obtained to some extent. On the other hand, if the Sn content is too high, liquation embrittlement cracking may occur at grain boundaries during hot working, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.100%. The preferred lower limit of the Sn content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Sn content is 0.090%, and even more preferably 0.080%.

[0070] [Regarding formula (1)] In the martensitic stainless steel seamless pipe according to this embodiment, the element contents satisfy formula (1) within the above-mentioned element content ranges. 10Ca+10Mg+2B+REM≧0.0010 (1) Here, Ca, Mg, and B in formula (1) are substituted with the content of the corresponding element in mass %. REM in formula (1) is substituted with the total content of rare earth elements in mass %. If Ca, Mg, or B is not contained, "0" is substituted for the corresponding element symbol. If no rare earth element is contained, "0" is substituted for REM.

[0071] F1 (= 10Ca + 10Mg + 2B + REM) is an index showing the degree to which the deterioration of the hot workability of the steel material due to S is suppressed. Within the content ranges of the above-mentioned elements, if F1 is 0.0010 or more, the deterioration of the hot workability of the steel material due to S can be sufficiently suppressed. As a result, provided that the content ranges of the above-mentioned elements are met, the formation of inner surface defects in the steel material can be suppressed. Therefore, in the martensitic stainless steel seamless pipe according to this embodiment, F1 is set to 0.0010 or more within the content ranges of the above-mentioned elements.

[0072] The lower limit of F1 is preferably 0.0030, more preferably 0.0050, even more preferably 0.0100, and even more preferably 0.0120. The upper limit of F1 is not particularly limited. However, since this is within the range of the element contents of the martensitic stainless steel seamless steel pipe according to this embodiment, the upper limit of F1 is substantially 0.7400. The upper limit of F1 is preferably 0.7000, more preferably 0.6000, and even more preferably 0.5000.

[0073] In short, the martensitic stainless steel seamless pipe according to this embodiment has the following properties within the ranges of the contents of the elements described above: Ca: 0.0001 to 0.0250%, Mg: 0.0001 to 0.0250%, B: 0.0005 to 0.0200%, and Rare earth elements: Contains one or more elements selected from the group consisting of 0.001 to 0.200%. In this case, F1 is 0.0010 or more, and the deterioration of the hot workability of the steel material due to S can be sufficiently suppressed.

[0074] [Regarding formula (2)] Preferably, in the martensitic stainless steel seamless pipe according to this embodiment, the element contents satisfy formula (2) within the above-mentioned element content ranges. 0.05Mo+W≧α (2) Here, α in formula (2) is 0.240 when the Cu content of the elements in the martensitic stainless steel seamless pipe is less than 0.50%, and is 0.200 when the Cu content is 0.50 to 3.50%. The contents of the corresponding elements in mass% are substituted for W and Mo in formula (2).

[0075] F2 is defined as 0.05Mo+W. F2 is an index relating to the melting point of oxides formed during hot working. Within the content ranges of the above elements, if F2 is 0.240 or more, the melting point of oxides during hot working is further reduced. In this case, the maximum depth of internal defects in the steel material becomes even shallower. In other words, internal defects in martensitic stainless steel seamless pipes can be further suppressed. Therefore, in the martensitic stainless steel seamless pipe according to this embodiment, it is preferable that F2 is 0.240 or more within the content ranges of the above elements.

[0076] A more preferable lower limit of F2 is 0.250, even more preferably 0.255, and even more preferably 0.260. The upper limit of F2 is not particularly limited. However, with the above-mentioned chemical composition, the upper limit of F2 is substantially 0.550. In the martensitic stainless steel seamless pipe according to this embodiment, as long as the above-mentioned chemical composition is satisfied, the formation of inner surface defects can be suppressed even if F2 is less than 0.240, but the formation of inner surface defects is further suppressed when F2 is 0.240 or more.

[0077] Furthermore, when the Cu content is 0.50% or more, if F2 is 0.200 or more, the formation of internal defects is further suppressed. Note that the reason why increasing the Cu content to 0.50% or more can suppress internal defects even when F2 is low is not clear. However, the fact that internal defects can be suppressed even when F2 is low when the Cu content is 0.50% or more is proven by the examples described below.

[0078] Therefore, in the martensitic stainless steel seamless pipe according to this embodiment, when the content of the elements is within the ranges described above and the Cu content is 0.50% or more, F2 is preferably set to 0.200 or more. When the Cu content is 0.50% or more, the lower limit of F2 is more preferably 0.220, and even more preferably 0.240.

[0079] [Microstructure] The microstructure of the martensitic stainless steel seamless pipe according to this embodiment is mainly composed of martensite. In this specification, martensite includes not only fresh martensite but also tempered martensite. Furthermore, in this specification, mainly composed of martensite means that the volume fraction of martensite in the microstructure is 80.0% or more. The remainder of the microstructure is retained austenite. In other words, in the martensitic stainless steel seamless pipe according to this embodiment, the volume fraction of retained austenite is 0 to 20.0%. It is preferable that the volume fraction of retained austenite is as low as possible. A preferred lower limit of the volume fraction of martensite in the microstructure of the martensitic stainless steel seamless pipe according to this embodiment is 85.0%, and more preferably 90.0%. More preferably, the microstructure of the steel material is a single martensite phase.

[0080] [Method for measuring the volume fraction of martensite] The volume fraction (%) of martensite in the microstructure of the martensitic stainless steel seamless pipe of this embodiment can be determined by subtracting the volume fraction (%) of retained austenite, determined by the method described below, from 100.0%.

[0081] The volume fraction of retained austenite can be determined by X-ray diffraction. Specifically, a test specimen is taken from the center of the wall thickness of a martensitic stainless steel seamless pipe. The size of the test specimen is not particularly limited, but for example, it may be 15 mm x 15 mm x 2 mm thick. In this case, the thickness direction of the test specimen is parallel to the diameter direction of the martensitic stainless steel seamless pipe. Using the obtained test specimen, the X-ray diffraction intensities of the (200) plane of the α phase (ferrite and martensite), the (211) plane of the α phase, the (200) plane of the γ phase (retained austenite), the (220) plane of the γ phase, and the (311) plane of the γ phase are measured, and the integrated intensity of each plane is calculated. In measuring the X-ray diffraction intensity, the target of the X-ray diffractometer is Mo (MoKα radiation) and the output is 50 kV-40 mA. After the calculation, the volume fraction Vγ (%) of the retained austenite is calculated for each combination (2 × 3 = 6 pairs) of each α phase surface and each γ phase surface using formula (I). The average value of the volume fraction Vγ of the retained austenite for the six pairs is then defined as the volume fraction (%) of the retained austenite. Vγ=100 / {1+(Iα×Rγ) / (Iγ×Rα)} (I) Here, Iα is the integrated intensity of the α phase. Rα is the crystallographically calculated value of the α phase. Iγ is the integrated intensity of the γ phase. Rγ is the crystallographically calculated value of the γ phase. In this specification, Rα on the (200) plane of the α phase is 15.9, Rα on the (211) plane of the α phase is 29.2, Rγ on the (200) plane of the γ phase is 35.5, Rγ on the (220) plane of the γ phase is 20.8, and Rγ on the (311) plane of the γ phase is 21.8. The volume fraction of retained austenite is rounded to one decimal place.

[0082] Using the volume fraction (%) of retained austenite obtained by the above-mentioned X-ray diffraction method, the volume fraction (%) of martensite in the microstructure of a martensitic stainless steel seamless pipe is calculated according to the following formula. Martensite volume fraction = 100.0 - retained austenite volume fraction (%)

[0083] [Yield strength] The martensitic stainless steel seamless pipe according to this embodiment has a yield strength of 655 MPa or more (95 ksi or more). In this specification, the yield strength refers to the 0.2% offset yield strength (MPa) obtained by a tensile test at room temperature (24±3°C) in accordance with ASTM E8 / E8M(2013).

[0084] The martensitic stainless steel seamless pipe according to this embodiment has the above-mentioned element contents, satisfies formula (1), and has a yield strength of at least 655 MPa, so that it has excellent corrosion resistance and further suppresses the formation of inner surface defects, as proven by the examples described below. The upper limit of the yield strength of the martensitic stainless steel seamless pipe according to this embodiment is not particularly limited. The upper limit of the yield strength may be, for example, 1034 MPa, 1000 MPa, or 965 MPa.

[0085] Specifically, in this embodiment, the yield strength can be determined by the following method. A round bar test specimen is taken from the center of the wall thickness of a martensitic stainless steel seamless pipe. The round bar test specimen has, for example, a parallel portion diameter of 6.0 mm and a parallel portion length of 40.0 mm. The longitudinal direction of the parallel portion of the round bar test specimen is parallel to the pipe axis direction of the martensitic stainless steel seamless pipe. A tensile test is performed at room temperature (24±3°C) using the round bar test specimen in accordance with ASTM E8 / E8M (2013), and the 0.2% offset yield strength (MPa) is determined. The obtained 0.2% offset yield strength is defined as the yield strength (MPa).

[0086] [Corrosion resistance] The martensitic stainless steel seamless pipe according to this embodiment has excellent corrosion resistance. In this embodiment, excellent corrosion resistance is defined as follows.

[0087] In this embodiment, corrosion resistance is evaluated by a four-point bending test. Specifically, a test specimen is first taken from the center of the wall thickness of the steel material according to this embodiment. The size of the test specimen is, for example, 2 mm thick, 10 mm wide, and 75 mm long. The longitudinal direction of the test specimen is parallel to the axial direction of the martensitic stainless steel seamless pipe. The test solution is a 25 wt % sodium chloride aqueous solution adjusted to pH 4.5.

[0088] In accordance with ASTM G39-99 (2011), a stress equivalent to 100% of the actual yield stress is applied to the test specimen using four-point bending. The stressed test specimen is then sealed in an autoclave along with the test jig. The test solution is poured into the autoclave, leaving the gas phase intact, to form the test bath. After degassing the test bath, a mixture of 0.03 bar of H2S gas and 30 bar of CO2 gas is pressurized and sealed into the autoclave, and the test bath is stirred to saturate the mixed gas. After sealing the autoclave, the test bath is stirred at 180°C for 720 hours.

[0089] If no cracks are found in the test piece after 720 hours under the above conditions, the martensitic stainless steel seamless pipe according to this embodiment is judged to have "excellent corrosion resistance." In this specification, "no cracks are found" means that no cracks are found when the test piece is observed with the naked eye after the test.

[0090] [Inner surface defects of seamless steel pipes] In the martensitic stainless steel seamless pipe according to this embodiment, the formation of inner surface defects is suppressed. In this embodiment, "the formation of inner surface defects is suppressed" is defined as follows.

[0091] Specifically, piercing and rolling simulating the production of a martensitic stainless steel seamless pipe according to this embodiment is carried out under specific conditions, and the maximum depth of internal defects in the resulting steel material is measured. More specifically, a raw material (round billet) having the above-described chemical composition is heated to 1230°C, and then piercing and rolling is carried out with a cross-sectional area reduction rate of 65%. The heat treatment described below is then carried out to obtain a martensitic stainless steel seamless pipe. Internal defects formed on the inner surface of the resulting seamless steel pipe are visually inspected, and the depth of the formed defects is measured with a vernier caliper. The maximum value of the determined defect depth is defined as the maximum depth (mm) of internal defects. If the maximum depth of internal defects is less than 0.3 mm, the martensitic stainless steel seamless pipe is determined to have "suppressed internal defect formation."

[0092] [Load on piercing mill] The martensitic stainless steel seamless pipe according to this embodiment preferably has a W content of 0.01 to 0.25%. In this case, the martensitic stainless steel seamless pipe can further reduce the load on the piercing-rolling mill. In this embodiment, "reducing the load on the piercing-rolling mill" is defined as follows.

[0093] Specifically, a hot workability test (Greeble test) is conducted on the martensitic stainless steel seamless pipe according to this embodiment. A test specimen for the Gleeble test is prepared from the steel material according to this embodiment. The test specimen is prepared from the central portion of the wall thickness of the seamless steel pipe. The test specimen is, for example, a round bar test specimen with a parallel portion diameter of 10 mm and a parallel portion length of 130 mm. The longitudinal direction of the test specimen is parallel to the pipe axis direction of the martensitic stainless steel seamless pipe.

[0094] A test piece heated to 1250°C is cooled at 100°C / min, and tensile stress is applied at 1100°C until it breaks. The maximum stress (MPa) until the test piece breaks is determined and defined as the "hot tensile strength." If the obtained hot tensile strength (MPa) is 130 MPa or less, the martensitic stainless steel seamless pipe is judged to have "reduced load on the piercing mill."

[0095] [Uses of seamless steel pipes] The martensitic stainless steel seamless pipe according to this embodiment is not particularly limited in its application. The martensitic stainless steel seamless pipe according to this embodiment is suitable for use as a seamless steel pipe for oil wells. Examples of seamless steel pipe for oil wells include casings, tubing, drill pipes, etc. used in drilling oil or gas wells and extracting crude oil or natural gas.

[0096] [Manufacturing method] An example of a method for manufacturing a martensitic stainless steel seamless pipe according to this embodiment will be described. Note that the manufacturing method described below is just one example, and the manufacturing method for the martensitic stainless steel seamless pipe according to this embodiment is not limited to this. In other words, as long as the martensitic stainless steel seamless pipe according to this embodiment having the above-mentioned configuration can be manufactured, the manufacturing method is not limited to the manufacturing method described below, and other manufacturing methods may be used. Preferably, the manufacturing method for a martensitic stainless steel seamless pipe according to this embodiment includes a material preparation step, a hot working step, and a heat treatment step. Below, a detailed description will be given of a case where the manufacturing method includes a material preparation step, a hot working step, and a heat treatment step.

[0097] [Material preparation process] In the material preparation process, molten steel having the above-mentioned chemical composition is produced by a known refining method. The produced molten steel is used to produce a slab by continuous casting. Here, the slab refers to a slab, bloom, or billet. Instead of a slab, the molten steel may be used to produce an ingot by ingot casting. If necessary, the slab, bloom, or ingot may be hot-rolled to produce a billet. A material (slab, bloom, or billet) is produced by the above-mentioned production process.

[0098] [Hot processing process] In the hot working step, a prepared raw material is hot worked. First, the raw material is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The raw material removed from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). Specifically, in this embodiment, piercing-rolling is performed as the hot working to produce a mother pipe. The piercing ratio in piercing-rolling is not particularly limited, but is, for example, 1.0 to 4.0. The billet after piercing-rolling is subjected to elongation rolling using a mandrel mill. Furthermore, if necessary, the billet after elongation rolling is subjected to sizing rolling using a reducer or a sizing mill. A mother pipe is produced through the above steps. The cumulative area reduction rate in the hot working step is not particularly limited, but is, for example, 20 to 70%.

[0099] [Heat treatment process] The heat treatment process includes a quenching process and a tempering process. In the heat treatment process, first, the mother pipe manufactured in the hot working process is quenched (quenching process). After quenching, the mother pipe is tempered (tempering process). The quenching process and the tempering process will be described below.

[0100] [Quenching process] In the quenching step, quenching is performed by a well-known method. In this specification, "quenching" means rapidly cooling a mother pipe at or above the A3 point. Quenching may be performed immediately after the hot working without cooling the mother pipe to room temperature (direct quenching), or the mother pipe may be loaded into a heat treatment furnace or a reheating furnace before its temperature drops after hot working, and then quenched after being heated to the quenching temperature.

[0101] Quenching temperature is A C3 The quenching temperature is equal to or higher than the transformation point, for example, 900 to 1000°C. Here, the quenching temperature means the furnace temperature when a heat treatment furnace or a reheating furnace is used, and means the temperature of the outer surface of the mother pipe in the case of direct quenching. When a heat treatment furnace or a reheating furnace is used, the time for which the mother pipe is held at the quenching temperature is not particularly limited, but is, for example, 10 to 120 minutes.

[0102] The quenching method is not particularly limited, but may be, for example, water cooling. Specifically, the water-quenching method may involve immersing the mother tube in a water or oil bath to rapidly cool it. Alternatively, shower cooling or mist cooling may be used to rapidly cool the mother tube by pouring or spraying cooling water onto the outer and / or inner surface of the mother tube.

[0103] [Tempering process] In the tempering process, the quenched mother pipe is tempered to adjust the yield strength. c1 In the tempering process according to the present embodiment, the tempering temperature is set to 500°C to 500°C. c1 The transformation point is the transformation point. In the tempering step according to this embodiment, the tempering time is not particularly limited, but is, for example, 10 to 180 minutes. In this specification, the tempering temperature means the furnace temperature (°C) in a heat treatment furnace. In this specification, the tempering time means the time for which the mother pipe is held at the tempering temperature.

[0104] In the tempering process according to this embodiment, the tempering temperature and tempering time are adjusted depending on the element contents of the mother pipe and the desired yield strength. Specifically, for example, if the yield strength of a mother pipe having the above-mentioned element contents is to be 655 to less than 862 MPa, the tempering temperature is preferably 570 to 620°C and the tempering time is preferably 10 to 30 minutes. Furthermore, if the yield strength of a mother pipe having a Cu content of less than 0.50% is to be 862 MPa or more, the tempering temperature is preferably 520 to 570°C and the tempering time is preferably 30 to 60 minutes. Furthermore, if the yield strength of a mother pipe having a Cu content of 0.50% or more is to be 862 MPa or more, the tempering temperature is preferably 510 to 570°C and the tempering time is preferably 60 to 100 minutes.

[0105] In this way, it is naturally possible for a person skilled in the art to obtain a martensitic stainless steel seamless pipe having a yield strength of 655 MPa or more by appropriately adjusting the tempering temperature and tempering time depending on the element contents of the mother pipe.

[0106] The martensitic stainless steel seamless pipe according to this embodiment can be manufactured by the above steps. As mentioned above, the martensitic stainless steel seamless pipe may also be manufactured by a method other than the above manufacturing method. Furthermore, the manufactured martensitic stainless steel seamless pipe may be subjected to post-treatment as necessary. The post-treatment may be, for example, descaling to remove oxide scale formed on the surface of the steel material. The present invention will be explained in more detail below with reference to examples. [Example]

[0107] In Example 1, the maximum depth of inner surface defects, corrosion resistance, and piercing / rolling mill load were investigated for martensitic stainless steel seamless pipes with a Cu content of less than 0.50%. Specifically, molten steel having the chemical composition shown in Table 1 was melted using a 50 kg vacuum melting furnace, and a steel ingot was produced by an ingot casting method.

[0108] [Table 1]

[0109] In Table 1, "-" means that the content of the corresponding element was at the impurity level. For example, the Ca, Mg, and B contents of Steel D were 0% when rounded to the nearest five decimal places. For example, the REM, Nb, Ta, Ti, Zr, Hf, and Sn contents of Steel A were 0% when rounded to the nearest four decimal places. Table 1 also shows the chemical compositions listed in Table 1 and F1 calculated from the above definition. Table 1 also shows the chemical compositions listed in Table 1 and F2 calculated from the above definition.

[0110] The ingots of test numbers 1 to 44 were heated at 1250°C for 3 hours and hot forged to produce round billets with a diameter of 200 mm. The round billets of test numbers 1 to 44 after hot forging were held at 1230°C for 120 minutes and piercing-rolled using a test piercing machine. The area reduction rate during piercing-rolling was 65%. In this way, mother pipes with an outer diameter of 139.7 mm and a wall thickness of 12.09 mm were produced.

[0111] The mother pipes of test numbers 1 to 44 were quenched. The quenching was carried out by reheating the mother pipes in a heat treatment furnace and immersing them in a water tank. For the mother pipes of test numbers 1 to 44, the quenching temperature (furnace temperature of the heat treatment furnace) was 900°C, and the mother pipes were held at the quenching temperature for 60 minutes. The mother pipes of test numbers 1 to 44 after quenching were tempered. The tempering was carried out by reheating the mother pipes after quenching in a tempering furnace and holding them there. The tempering temperature and tempering time for test numbers 1 to 44 are shown in Table 2. Seamless steel pipes of test numbers 1 to 44 were produced by the above manufacturing process.

[0112] [Table 2]

[0113] [Evaluation test] The manufactured seamless steel pipes of test numbers 1 to 44 were subjected to a tensile test, a test for measuring the maximum depth of inner surface flaws, a hot tensile strength test, and a corrosion resistance test.

[0114] [Tensile test] Tensile tests were conducted on the seamless steel pipes of test numbers 1 to 44. Specifically, round bar test specimens for the tensile tests were prepared from the center of the wall thickness of the seamless steel pipes of test numbers 1 to 44. The round bar test specimens had a parallel section diameter of 6.0 mm and a parallel section length of 40.0 mm. The longitudinal direction of the round bar test specimens was parallel to the axial direction of the seamless steel pipes. Using the round bar test specimens, tensile tests were conducted at room temperature (24±3°C) in accordance with ASTM E8 / E8M (2013). The 0.2% offset proof stress obtained in the tensile test was taken as the yield strength (MPa). The yield strengths (MPa) obtained for test numbers 1 to 44 are shown in Table 2.

[0115] [Maximum depth measurement test for internal flaws] A test for measuring the maximum depth of internal defects was conducted on the seamless steel pipes of test numbers 1 to 44. Specifically, the inner surfaces of the seamless steel pipes of test numbers 1 to 44 were visually inspected to identify internal defects. The depth of the identified internal defects was measured with a vernier caliper. The maximum value of the determined internal defect depth was defined as the maximum internal defect depth (mm). The maximum internal defect depths (mm) obtained for test numbers 1 to 44 are shown in Table 2.

[0116] [Hot tensile strength measurement test] Hot tensile strength measurement tests were conducted on seamless steel pipes of test numbers 1 to 44. Specifically, test specimens for Gleeble tests were prepared from the center of the wall thickness of seamless steel pipes of test numbers 1 to 44. The test specimens were round bar test specimens with a parallel section diameter of 10 mm and a parallel section length of 130 mm. The longitudinal direction of the parallel section of the round bar test specimen was parallel to the axial direction of the seamless steel pipe. The round bar test specimens were heated to 1250°C, cooled at a rate of 100°C / min, and then subjected to a tensile test at 1100°C to fracture the round bar test specimens. The maximum stress (MPa) until the round bar test specimen fractured was determined and recorded as the "hot tensile strength." The hot tensile strengths (MPa) obtained for test numbers 1 to 44 are shown in Table 2.

[0117] [Corrosion resistance test] Corrosion resistance tests were conducted on seamless steel pipes of test numbers 1 to 44. Specifically, test specimens for four-point bending tests were prepared from the center of the wall thickness of seamless steel pipes of test numbers 1 to 44. The test specimens were 2 mm thick, 10 mm wide, and 75 mm long. The longitudinal direction of the test specimens was parallel to the axial direction of the seamless steel pipes. The test solution was a 25 wt % sodium chloride aqueous solution adjusted to pH 4.5. In accordance with ASTM G39-99 (2011), a stress equivalent to 100% of the actual yield stress was applied to the test specimens by four-point bending.

[0118] The stressed test specimens were sealed in an autoclave along with the test fixture. The test solution was poured into the autoclave, leaving the gas phase, to form the test bath. After degassing the test bath, a mixture of 0.03 bar H2S gas and 30 bar CO2 gas was pressurized and charged into the autoclave. The test bath was stirred to saturate the mixed gas. After sealing the autoclave, the test bath was stirred at 180°C for 720 hours. After 720 hours, the test specimens Nos. 1 to 44 were observed for the presence or absence of cracks. Specifically, the test specimens were visually observed after 720 hours. Test specimens that showed no cracks were rated "E" (Excellent). On the other hand, test specimens that showed cracks were rated "NA" (Not Acceptable). The evaluation results obtained for Tests Nos. 1 to 44 are shown in Table 2.

[0119] [Test Results] Referring to Tables 1 and 2, the seamless steel pipes of test numbers 1 to 17, 19, 23 to 39, and 41 had appropriate chemical compositions and F1 of 0.0010 or more. These seamless steel pipes also had yield strengths of 655 MPa or more. As a result, the maximum depth of internal defects was less than 0.3 mm, and the formation of internal defects was suppressed. Furthermore, the corrosion resistance test evaluation was "E," indicating excellent corrosion resistance.

[0120] Furthermore, the seamless steel pipes of test numbers 1 to 17 and 23 to 39 had a W content of 0.01 to 0.25%, which resulted in a hot tensile strength of 130 MPa or less, reducing the load on the piercing-rolling mill.

[0121] Furthermore, the seamless steel pipes of test numbers 2 to 4, 7 to 9, 12 to 14, 16, 17, 19, 24 to 26, 29 to 31, 34 to 36, 38, 39, and 41 had F2 of 0.240 or more. As a result, the maximum depth of the inner surface defects was 0.1 mm or less, and the formation of inner surface defects was further suppressed.

[0122] On the other hand, the seamless steel pipes of test numbers 18 and 40 had too low a W content, which resulted in the maximum depth of the inner surface defects being 0.3 mm or more, and the formation of inner surface defects was not suppressed.

[0123] The seamless steel pipes of test numbers 20 and 42 did not contain any of Ca, Mg, B, and REM, and had F1 of less than 0.0010. As a result, the maximum depth of the inner surface defects was 0.3 mm or more, and the formation of inner surface defects was not suppressed.

[0124] The seamless steel pipes of test numbers 21, 22, 43, and 44 did not contain Co. As a result, the evaluation of the corrosion resistance test was "NA," and they did not exhibit excellent corrosion resistance. [Example]

[0125] In Example 2, the maximum depth of inner surface defects, corrosion resistance, and piercing / rolling mill load were investigated for martensitic stainless steel seamless pipes with a Cu content of 0.50 to 3.50%. Specifically, molten steel having the chemical composition shown in Table 3 was melted using a 50 kg vacuum melting furnace, and steel ingots were produced by an ingot casting method.

[0126] [Table 3]

[0127] In Table 3, "-" means that the content of the corresponding element was at the impurity level. For example, the Ca, Mg, and B contents of Steel Z were 0% when rounded to the nearest five decimal places. For example, the REM, Nb, Ta, Ti, Zr, Hf, and Sn contents of Steel W were 0% when rounded to the nearest four decimal places. Table 3 also shows the chemical compositions listed in Table 3 and F1 calculated from the above definition. Table 3 also shows the chemical compositions listed in Table 3 and F2 calculated from the above definition.

[0128] The ingots of test numbers 45 to 88 were heated at 1250°C for 3 hours and hot forged to produce round billets with a diameter of 200 mm. The round billets of test numbers 45 to 88 after hot forging were held at 1230°C for 120 minutes and piercing-rolled using a test piercing machine. The area reduction rate during piercing-rolling was 65%. In this way, mother pipes with an outer diameter of 139.7 mm and a wall thickness of 12.09 mm were produced.

[0129] The mother pipes of test numbers 45 to 88 were quenched. The quenching was carried out by reheating the mother pipes in a heat treatment furnace and immersing them in a water tank. For the mother pipes of test numbers 45 to 88, the quenching temperature (furnace temperature of the heat treatment furnace) was 900°C, and the mother pipes were held at the quenching temperature for 60 minutes. The mother pipes of test numbers 45 to 88 after quenching were tempered. The tempering was carried out by reheating the mother pipes after quenching in a tempering furnace and holding them there. The tempering temperature and tempering time for test numbers 45 to 88 are shown in Table 4. Seamless steel pipes of test numbers 45 to 88 were produced using the above manufacturing process.

[0130] [Table 4]

[0131] [Evaluation test] The manufactured seamless steel pipes with test numbers 45 to 88 were subjected to a tensile test, a test for measuring the maximum depth of inner surface flaws, a hot tensile strength test, and a corrosion resistance test.

[0132] [Tensile test] Tensile tests were carried out on the seamless steel pipes of test numbers 45 to 88 in the same manner as in Example 1. The 0.2% offset proof stress obtained in the tensile test carried out by the above-mentioned method was taken as the yield strength (MPa). The yield strengths (MPa) obtained for test numbers 45 to 88 are shown in Table 4.

[0133] [Maximum depth measurement test for internal flaws] The seamless steel pipes of test numbers 45 to 88 were subjected to a test for measuring the maximum depth of internal defects in the same manner as in Example 1. The maximum value of the depth of the internal defects obtained by the above-mentioned method was defined as the maximum depth (mm) of the internal defects. The maximum depth (mm) of the internal defects obtained for test numbers 45 to 88 is shown in Table 4.

[0134] [Hot tensile strength measurement test] The seamless steel pipes of test numbers 45 to 88 were subjected to a hot tensile strength measurement test in the same manner as in Example 1. The maximum stress (MPa) until the round bar test piece broke, determined by the above-mentioned method, was defined as the "hot tensile strength." The hot tensile strengths (MPa) obtained for test numbers 45 to 88 are shown in Table 4.

[0135] [Corrosion resistance test] Corrosion resistance tests were carried out on seamless steel pipes of test numbers 45 to 88 in the same manner as in Example 1. A four-point bending test was carried out using the method described above, and the test specimens were visually observed after 720 hours of holding. As a result of the observation, test specimens in which no cracks were found were judged to be "E" (Excellent). On the other hand, test specimens in which cracks were found were judged to be "NA" (Not Acceptable). The evaluation results obtained for test numbers 45 to 88 are shown in Table 4.

[0136] [Test Results] Referring to Tables 3 and 4, the seamless steel pipes of test numbers 45 to 62, 64, 67 to 84, and 86 had appropriate chemical compositions and F1 of 0.0010 or more. These seamless steel pipes also had yield strengths of 655 MPa or more. As a result, the maximum depth of internal flaws was less than 0.3 mm, and the formation of internal flaws was suppressed. Furthermore, the corrosion resistance test evaluation was "E," indicating excellent corrosion resistance.

[0137] Furthermore, the seamless steel pipes of test numbers 45 to 62 and 67 to 84 had a W content of 0.01 to 0.25%, which resulted in a hot tensile strength of 130 MPa or less, reducing the load on the piercing-rolling mill.

[0138] Furthermore, the seamless steel pipes of test numbers 45, 47 to 50, 52 to 56, 58 to 60, 62, 64, 67, 69 to 72, 74 to 78, 80 to 82, 84, and 86 had F2 of 0.200 or more. As a result, the maximum depth of the inner surface defects was 0.1 mm or less, and the formation of inner surface defects was further suppressed.

[0139] On the other hand, the seamless steel pipes of test numbers 63 and 85 had too low a W content, which resulted in the maximum depth of the inner surface defects being 0.3 mm or more, and the formation of inner surface defects was not suppressed.

[0140] The seamless steel pipes of test numbers 65 and 87 did not contain any of Ca, Mg, B, and REM, and had F1 of less than 0.0010. As a result, the maximum depth of the inner surface defects was 0.3 mm or more, and the formation of inner surface defects was not suppressed.

[0141] The seamless steel pipes of test numbers 66 and 88 did not contain Co. As a result, the evaluation of the corrosion resistance test was "NA", and they did not exhibit excellent corrosion resistance.

[0142] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure. [Industrial Applicability]

[0143] The seamless steel pipe according to the present disclosure is widely applicable to steel materials used in harsh environments such as polar regions, and is preferably usable as steel materials used in oil well environments, and more preferably usable as steel materials for casing, tubing, line pipes, etc.

Claims

1. A martensitic stainless steel seamless pipe, In mass%, C: 0.001-0.050%, Si: 0.05-1.00%, Mn: 0.05-2.00%, P: 0.030% or less, S: 0.0100% or less, Al: 0.005-0.100%, N: 0.020% or less, Ni: 1.00-9.00%, Cr: 8.00-14.50%, Cu: 0.50 to 3.50%, Mo: 1.00-5.00%, W: 0.01-0.20%, V: 0.010-1.500%, Co: 0.001 to 0.500%, Ca: 0-0.0250%, Mg: 0 to 0.0250%, B: 0 to 0.0200%, Rare earth elements: 0 to 0.200%, Nb: 0 to 0.100%, Ta: 0-0.100%, Ti: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0-0.100%, Sn: 0 to 0.100%, and The balance is composed of Fe and impurities. Within the range of the content of elements in the martensitic stainless steel seamless pipe, the content of the elements satisfies formulas (1) and (2), The yield strength is 655 MPa or more. Martensitic stainless steel seamless pipe. 10Ca+10Mg+2B+REM≧0.0010 (1) 0.200≦0.05Mo+W≦0.342 (2) Here, the contents of the corresponding elements in mass% are substituted for Ca, Mg, and B in formula (1). The total content of rare earth elements in mass% is substituted for REM in formula (1). The contents of the corresponding elements in mass% are substituted for W and Mo in formula (2).

2. The martensitic stainless steel seamless pipe according to claim 1, Nb: 0.001 to 0.100%, Ta: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100%, Hf: 0.001 to 0.100%, and Sn: Contains one or more elements selected from the group consisting of 0.001 to 0.100%; Martensitic stainless steel seamless pipe.

3. The martensitic stainless steel seamless pipe according to claim 1 or 2, The martensitic stainless steel seamless pipe is a seamless steel pipe for oil wells. Martensitic stainless steel seamless pipe.

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