Stainless steel and seamless stainless steel pipe

Abstract

The purpose of the present invention is to provide stainless steel and a seamless stainless steel pipe which exhibit a high yield strength of 448 MPa (65 ksi) or higher, excellent crevice corrosion resistance, and excellent low-temperature ductility.　The present invention provides stainless steel having a component composition containing, in terms of mass %, 0.002-0.040% C, 0.05-1.00% Si, 0.1-1.7% Mn, not more than 0.040% P, not more than 0.0200% S, 20.0-28.0% Cr, 4.0-10.0% Ni, 2.0-5.0% Mo, 0.001-0.050% Al, 0.06-0.35% N, 0.001-0.800% Pt, and not more than 0.0100% O, with the balance being Fe and unavoidable impurities, said stainless steel having a structure containing an austenitic phase at a volume ratio of 20-70% and a ferritic phase at a volume ratio of 30-80%, and exhibiting a yield strength of 448 MPa or greater.

Description

Stainless steel and stainless steel seamless pipes

[0001] The present invention relates to a stainless steel and a stainless steel seamless pipe, and more particularly to a stainless steel and a stainless steel seamless pipe having excellent crevice corrosion resistance.

[0002] In recent years, in light of the expected depletion of energy resources in the near future, there has been active development of oil wells in severe corrosive environments that have not previously been considered, such as deep oil fields, environments containing carbon dioxide, and sour environments containing hydrogen sulfide. Oil well steel pipes used in such environments are required to have high strength and high corrosion resistance.

[0003] Conventionally, CO ２ and Cl － In oil and gas fields that are exposed to environments including the above, 13Cr martensitic stainless steel pipes have generally been used as oil well pipes for mining. However, in recent years, the development of oil wells that can be used at even higher temperatures (up to 230°C) has progressed, and the corrosion resistance of 13Cr martensitic stainless steel pipes has sometimes been insufficient. There is a demand for oil well pipes that have high corrosion resistance and can be used in such environments.

[0004] To meet this demand, duplex stainless steel is used for oil well pipes. Duplex stainless steel has excellent strength characteristics. However, in order to apply duplex stainless steel in severe corrosive environments containing large amounts of hydrogen sulfide, carbon dioxide, and chloride ions, such as deep oil wells, which have been developed in recent years, improved corrosion resistance is required.

[0005] In this regard, for example, Patent Document 1 discloses a duplex stainless steel having excellent corrosion resistance and a PREW value of 40 or more, which is achieved by controlling the contents of Cr, Mo, N, and W.

[0006] Furthermore, Patent Document 2 discloses a duplex stainless steel that is excellent in corrosion resistance and hot workability by controlling the contents of Cr, Mo, W, and N as well as the contents of B, Ta, etc.

[0007] In addition, in Patent Document 3, in order to reduce sulfide-based inclusions in steel that adversely affect hot workability and corrosion resistance, a CaO crucible and a CaO-CaF ２ -Al ２ O ３ This paper discloses a duplex stainless steel in which the S content is reduced to 3 ppm by weight or less by using slag of this type.

[0008] Furthermore, Patent Document 4 discloses a duplex stainless steel in which the total content of Ca and Mg and the content of S in the oxide-based inclusions are controlled, and the shape and density of the inclusions are further adjusted, as a technique for controlling oxide-based inclusions that can be the initiation points of pitting corrosion. Patent Document 4 also discloses a duplex stainless steel in which the size and number of the inclusions are controlled and the occurrence of localized corrosion is suppressed by optimally combining the slag basicity during reduction treatment, the killing temperature and time in the ladle, and the total processing ratio after casting, because even insoluble Al oxides that contain certain amounts of Ca, Mg, and S can be the initiation points of localized corrosion.

[0009] JP-A-5-132741 JP-A-8-170153 JP-A-3-291358 International Publication No. 2005 / 014872

[0010] In order to support the pressure of the formation and its own weight, oil well steel pipes are sometimes required to have a high strength of 448 MPa (65 ksi) or more in yield strength. Furthermore, they are sometimes required to have a yield strength of 758 MPa or more by cold working.

[0011] As mentioned above, with the development of oil wells at higher temperatures, there is a demand for oil well steel pipes with high corrosion resistance. In addition, in recent years, in order to realize a carbon-neutral society, CO generated from oil fields, gas fields, factories, etc. ２ Carbon Capture and Storage (CCS) technology, which injects and stores CO underground, is attracting attention. ２ Steel pipes are used to inject CO ２ SOx, NOx, and O remaining in ２ The problem is that impurities such as high concentrations of Cl increase corrosiveness. － Environment and O２ Crevice corrosion can be a particular problem when using duplex stainless steel in environments containing these elements.

[0012] Furthermore, when used in cold regions, steel pipes used in oil wells are also required to have excellent low-temperature toughness. One of the methods for evaluating excellent low-temperature toughness is the "absorbed energy vE" test in a Charpy impact test at -40°C. －４０ In some cases, it may be required that the value be 100J or more.

[0013] Patent Documents 1 to 4 disclose stainless steels with improved corrosion resistance. However, Patent Documents 1 to 4 sometimes have insufficient crevice corrosion resistance.

[0014] The present invention solves the problems of the prior art, and aims to provide a stainless steel and a stainless steel seamless pipe that have high strength, with a yield strength of 448 MPa (65 ksi) or more, excellent crevice corrosion resistance, and excellent low-temperature toughness.

[0015] The term "excellent crevice corrosion resistance" refers to the resistance to crevice corrosion when an insulator is pressed into a test liquid: a 3.5 mass % NaCl aqueous solution (liquid temperature: 50°C) at a pressure of 20 N / mm ２ The crevice corrosion test specimen, which has a crevice structure formed by pressing the specimen against the pipe, is immersed for 720 hours, and no crevice corrosion occurs, or the maximum depth of crevice corrosion is less than 25 μm. Furthermore, the term "excellent low-temperature toughness" as used herein refers to the case where a V-notch test specimen (10 mm thick) is taken in a Charpy impact test in accordance with the provisions of JIS Z 2242 (2018) so that the longitudinal direction of the test specimen is the pipe axial direction, and the absorbed energy vE －４０ is 100 J or more.

[0016] In order to achieve the above object, the present inventors have conducted extensive research into the various factors that affect the corrosion resistance of stainless steel, particularly crevice corrosion. As a result, it has been found that excellent crevice corrosion resistance can be achieved by adding a predetermined amount or more of Pt in addition to Cr and Mo.

[0017] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. [1] A stainless steel having a chemical composition containing, by mass%, C: 0.002 to 0.040%, Si: 0.05 to 1.00%, Mn: 0.1 to 1.7%, P: 0.040% or less, S: 0.0200% or less, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.050%, N: 0.06 to 0.35%, Pt: 0.001 to 0.800%, O: 0.0100% or less, with the balance being Fe and unavoidable impurities; having a structure containing, by volume, 20 to 70% austenite phase and 30 to 80% ferrite phase; and having a yield strength of 448 MPa or more. [2] In addition to the above-mentioned composition, the following contents are contained, in mass%, by weight: W: 0.02 to 1.50%, Cu: 0.1 to 2.0%, V: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less, Zr: 0.50% or less, B: 0.0100% or less, REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.200% or less, Mg: 0.0002 to 0.0100%, Ta: 0.01 to 0.15%, Co: 0.01 to 3.00%, Sb: 0.200% or less, As: 0.100% or less, Bi: 0.200% or less, Zn: 0.100% or less, [3] The stainless steel according to [1], which contains one or more selected from the group consisting of: Pb: 0.200% or less; [4] The stainless steel according to [1] or [2], which has the chemical composition and structure according to [1] or [2], and has a yield strength of 758 MPa or more. [4] A stainless steel seamless pipe having a composition containing, by mass%, C: 0.002 to 0.040%, Si: 0.05 to 1.00%, Mn: 0.1 to 1.7%, P: 0.040% or less, S: 0.0200% or less, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.050%, N: 0.06 to 0.35%, Pt: 0.001 to 0.800%, O: 0.0100% or less, with the balance being Fe and unavoidable impurities, and having a structure containing, by volume, 20 to 70% austenite phase and 30 to 80% ferrite phase, and having a yield strength of 448 MPa or more.[5] In addition to the above-mentioned composition, the following contents are contained, in mass%, by weight: W: 0.02 to 1.50%, Cu: 0.1 to 2.0%, V: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less, Zr: 0.50% or less, B: 0.0100% or less, REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.200% or less, Mg: 0.0002 to 0.0100%, Ta: 0.01 to 0.15%, Co: 0.01 to 3.00%, Sb: 0.200% or less, As: 0.100% or less, Bi: 0.200% or less, Zn: 0.100% or less, [6] A stainless steel seamless pipe having the chemical composition and structure described in [4] or [5] and having a yield strength of 758 MPa or more, wherein the stainless steel seamless pipe contains one or more selected from the group consisting of 0.200% or less of Pb, ...

[0018] According to the present invention, it is possible to obtain a stainless steel and a stainless steel seamless pipe having high strength with a yield strength of 448 MPa (65 ksi) or more, excellent crevice corrosion resistance, and excellent low-temperature toughness.

[0019] The present invention will be described in detail below. The stainless steel of the present invention and the stainless steel seamless pipe made from the stainless steel have a chemical composition, by mass%, of C: 0.002 to 0.040%, Si: 0.05 to 1.00%, Mn: 0.1 to 1.7%, P: 0.040% or less, S: 0.0200% or less, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.050%, N: 0.06 to 0.35%, Pt: 0.001 to 0.800%, O: 0.0100% or less, with the balance being Fe and unavoidable impurities, and have a structure containing, by volume, 20 to 70% austenite phase and 30 to 80% ferrite phase, and have a yield strength of 448 MPa or more.

[0020] The reasons for limiting the ranges of the chemical compositions of the stainless steel and stainless steel seamless pipe of the present invention will be explained below. Note that % regarding the content of components is "% by mass".

[0021] C: 0.002 to 0.040% C is an element that stabilizes the austenite phase and improves strength. To achieve a high strength of 65 ksi (448 MPa) yield strength, the C content is set to 0.002% or more. Preferably, the C content is 0.010% or more. More preferably, the C content is 0.015% or more, even more preferably 0.016% or more, and most preferably 0.018% or more. On the other hand, if the C content exceeds 0.040%, excessive carbide precipitation may occur due to heat treatment, adversely affecting low-temperature toughness. Therefore, the C content is set to 0.040% or less. The C content is preferably 0.030% or less. More preferably, the C content is 0.025% or less, even more preferably, the C content is 0.024% or less, and most preferably, the C content is 0.023% or less.

[0022] Si: 0.05 to 1.00% Si is an element that functions as a deoxidizer, and to obtain this effect, the Si content is set to 0.05% or more. The Si content is preferably 0.10% or more, more preferably 0.12% or more, even more preferably 0.13% or more, and most preferably 0.014% or more. On the other hand, if the Si content exceeds 1.00%, the hot workability of the steel decreases. For this reason, the Si content is set to 1.00% or less. The Si content is preferably 0.70% or less, more preferably 0.60% or less, even more preferably 0.50% or less, and most preferably 0.45% or less.

[0023] Mn: 0.1 to 1.7% Like Si, Mn is an effective deoxidizer and also improves hot workability by immobilizing S, which is inevitably contained in steel, as sulfides. These effects are achieved when the Mn content is 0.1% or more. Therefore, the Mn content is set to 0.1% or more. The Mn content is preferably 0.2% or more, more preferably 0.3% or more, even more preferably 0.4% or more, and most preferably 0.5% or more. On the other hand, a Mn content exceeding 1.7% not only reduces hot workability but also adversely affects carbon dioxide corrosion resistance. For this reason, the Mn content is set to 1.7% or less. Preferably, the Mn content is 1.6% or less. The Mn content is more preferably 1.3% or less, even more preferably 1.0% or less, and most preferably 0.9% or less.

[0024] P: 0.040% or less P is an element that reduces the low-temperature toughness of stainless steel. If the P content exceeds 0.040%, low-temperature toughness is significantly reduced. Therefore, the P content is set to 0.040% or less. The P content is preferably 0.025% or less, more preferably 0.020% or less, even more preferably 0.019% or less, and most preferably 0.018% or less. However, reducing the P content to less than 0.005% requires a long time for dephosphorization during the molten steel production process, which increases the manufacturing cost of stainless steel. Therefore, the P content is preferably 0.005% or more. The P content is more preferably 0.007% or more, and even more preferably 0.010% or more.

[0025] S: 0.0200% or less S is an element that reduces hot workability during the production process of stainless steel, and if the S content exceeds 0.0200%, it will hinder the production of stainless steel. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less, more preferably 0.0020% or less, even more preferably 0.0019% or less, and most preferably 0.0010% or less. However, reducing the S content to less than 0.0003% requires a long time for desulfurization treatment during the process of melting molten steel, which increases the production cost of stainless steel. Therefore, the S content is preferably 0.0003% or more. The S content is more preferably 0.0005% or more, and even more preferably 0.0006% or more.

[0026] Cr: 20.0 to 28.0% Cr is a basic element that is effective in forming an oxide film on the surface and improving crevice corrosion resistance. It also improves the strength of steel through solid solution strengthening. To achieve these effects, the Cr content is set to 20.0% or more. To obtain even higher strength, the Cr content is preferably 22.0% or more, more preferably 23.0% or more, even more preferably 24.0% or more, and most preferably 24.1% or more. On the other hand, if the Cr content exceeds 28.0%, the σ phase is likely to precipitate, deteriorating low-temperature toughness. Therefore, the Cr content is set to 28.0% or less. From the viewpoint of toughness, the Cr content is preferably 27.0% or less, more preferably 26.0% or less, even more preferably 25.8% or less, and most preferably 25.7% or less.

[0027] Ni: 4.0 to 10.0% Ni is an element contained to stabilize the austenite phase and obtain a dual-phase structure. If the Ni content is less than 4.0%, the austenite phase becomes unstable and the volume fraction of the ferrite phase becomes excessive. Therefore, the Ni content is set to 4.0% or more. Preferably, the Ni content is 5.3% or more. The Ni content is more preferably 5.5% or more. Even more preferably, it is 5.6% or more, and most preferably, it is 5.7% or more. On the other hand, if the Ni content exceeds 10.0%, the austenite phase becomes dominant and the volume fraction of the austenite phase becomes excessive. Furthermore, since Ni is an expensive element, economic efficiency is also impaired. Therefore, the Ni content is set to 10.0% or less. Preferably, the Ni content is 8.0% or less. More preferably, it is 6.9% or less. Even more preferably, it is 6.8% or less, and most preferably, it is 6.7% or less.

[0028] Mo: 2.0 to 5.0% Mo is an element that has the effect of improving the stability of the oxide film on stainless steel. － This contributes to the prevention of pitting corrosion and crevice corrosion caused by the above. If the Mo content is less than 2.0%, this effect cannot be obtained. Therefore, the Mo content is set to 2.0% or more. The Mo content is preferably set to 2.5% or more, more preferably 3.3% or more, and most preferably 3.4% or more. On the other hand, if the Mo content exceeds 5.0%, the σ phase precipitates and low-temperature toughness decreases. Therefore, the Mo content is set to 5.0% or less. Preferably, the Mo content is 4.5% or less, more preferably 4.0% or less, even more preferably 3.9% or less, and most preferably 3.8% or less.

[0029] Al: 0.001 to 0.050%. Al functions as a deoxidizer during the process of refining molten steel, the raw material for stainless steel. If the Al content is less than 0.001%, this effect cannot be obtained. Therefore, the Al content is set to 0.001% or more. Preferably, the Al content is 0.005% or more, more preferably 0.010% or more, even more preferably 0.011% or more, and most preferably 0.012% or more. On the other hand, if the Al content exceeds 0.050%, alumina-based inclusions are likely to precipitate, deteriorating hot workability during the stainless steel manufacturing process and low-temperature toughness. Therefore, the Al content is set to 0.050% or less. Preferably, the Al content is 0.039% or less. More preferably, the Al content is 0.035% or less, even more preferably 0.030% or less, and most preferably 0.028% or less.

[0030] N: 0.06 to 0.35% N is known as an element that contributes to solid-solution strengthening in ordinary stainless steels, and is actively added, with the N content set to 0.06% or more. To obtain the properties targeted by the present invention, the N content is preferably set to 0.08% or more. More preferably, the N content is set to 0.10% or more. Even more preferably, the N content is set to 0.11% or more. However, excessive N content forms nitrides, which reduces low-temperature toughness and deteriorates hot workability. Therefore, the N content is set to 0.35% or less. The N content is preferably 0.30% or less, more preferably 0.25% or less, even more preferably 0.24% or less, and most preferably 0.23% or less.

[0031] Pt: 0.001 to 0.800% When the passive film is destroyed and crevice corrosion initiation occurs, Pt concentrates on the steel surface and coats it. As a result, it improves the crevice corrosion resistance of stainless steel. If the Pt content is less than 0.001%, this effect cannot be achieved. Therefore, the Pt content is set to 0.001% or more. The Pt content is preferably 0.010% or more, more preferably 0.100% or more, even more preferably 0.150% or more, and most preferably 0.160% or more. On the other hand, if the Pt content exceeds 0.800%, Pt precipitates as coarse particles, reducing the crevice corrosion resistance of the stainless steel. Therefore, the Pt content is set to 0.800% or less. Preferably, the Pt content is 0.600% or less, and more preferably 0.400% or less. The content is more preferably 0.300% or less, and most preferably 0.290% or less.

[0032] In order to further improve the above-mentioned effect, the precipitates containing Pt in the structure are 3 precipitates per mm with a major axis of 0.5 μm or more. ２ More preferably, the number is 8 pieces / mm or more. ２ or more, and ２ More preferably, it is 10 / mm or more. ２ From the viewpoint of suppressing coarse precipitates, the number of precipitates having a major axis of 0.5 μm or more is most preferably 75 / mm ２ It is preferable that the number of particles is 50 / mm or less. ２ It is more preferable that the number of pieces is 25 / mm or less. ２ More preferably, it is 15 pieces / mm or less, and most preferably, it is 15 pieces / mm ２ There are less than 100 pieces.

[0033] O (oxygen): 0.0100% or less O (oxygen) exists as an oxide in steel and has a negative effect on various properties. For this reason, it is desirable to reduce O as much as possible. In particular, if the O content exceeds 0.0100%, low-temperature toughness significantly decreases. For this reason, the O content is set to 0.0100% or less. The O content is preferably 0.0040% or less, more preferably 0.0035% or less, even more preferably 0.0030% or less, and most preferably 0.0025% or more. Since excessive reduction increases manufacturing costs, the O content is preferably 0.0005% or more. More preferably, it is 0.0010% or more, even more preferably 0.0012% or more, and most preferably 0.0015% or more.

[0034] The above components are the basic components. The steel contains the above basic components, with the balance consisting of Fe and unavoidable impurities. Here, a steel according to one embodiment of the present invention may contain only the above basic components and the balance consisting of Fe and unavoidable impurities.

[0035] In addition to the above basic components, the present invention may contain one or more elements selected from the following, if necessary: ​​W: 0.02 to 1.50%, Cu: 0.1 to 2.0%, V: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less, Zr: 0.50% or less, B: 0.0100% or less, REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.200% or less, Mg: 0.0002 to 0.0100%, Ta: 0.01 to 0.15%, Co: 0.01 to 3.00%, Sb: 0.200% or less, As: 0.100% or less, Bi: 0.200% or less, Zn: 0.100% or less, and Pb: 0.200% or less.

[0036] W: 0.02 to 1.50% W is an element that improves sulfide stress corrosion cracking resistance and may be selectively added as needed. To achieve this effect, when W is contained, the W content is 0.02% or more. The W content is preferably 0.10% or more, more preferably 0.20% or more, even more preferably 0.21% or more, and most preferably 0.22% or more. On the other hand, if W is contained in a large amount exceeding 1.50%, low-temperature toughness may be reduced. Therefore, when W is contained, the W content is set to 1.50% or less. The W content is preferably 1.00% or less, more preferably 0.70% or less, even more preferably 0.50% or less, and most preferably 0.30% or less.

[0037] Cu: 0.1 to 2.0% Cu is an element that improves carbon dioxide corrosion resistance and stabilizes the austenite phase, and may be selectively added as needed. To achieve these effects, if Cu is included, the Cu content is set to 0.1% or more. 0.2% or more is preferable. 0.3% or more is more preferable, 0.4% or more is even more preferable, and 0.6% or more is most preferable. However, since excessive Cu content deteriorates hot workability, if Cu is included, the Cu content is set to 2.0% or less. It is preferably 1.5% or less, more preferably 1.0% or less, even more preferably 0.8% or less, and most preferably 0.7% or less.

[0038] V: 0.20% or less V is an element that improves the strength of steel by precipitation strengthening and may be selectively added as needed. To achieve this effect, the V content is preferably 0.02% or more, more preferably 0.03% or more, even more preferably 0.04% or more, and most preferably 0.05% or more. On the other hand, if V is contained in an amount exceeding 0.20%, low-temperature toughness may be reduced. Therefore, when V is contained, the V content is set to 0.20% or less. The V content is preferably 0.15% or less, more preferably 0.10% or less, even more preferably 0.08% or less, and most preferably 0.07% or less.

[0039] Nb: 0.20% or less Nb is an element that improves the strength of steel by precipitation strengthening and may be selectively added as needed. Nb also has the effect of refining crystal grains and improving low-temperature toughness and sulfide stress corrosion cracking resistance. To achieve this effect, the Nb content is preferably 0.02% or more, more preferably 0.03% or more, even more preferably 0.04% or more, and most preferably 0.05% or more. On the other hand, if Nb is contained in an amount exceeding 0.20%, coarse precipitates and intermetallic compounds may precipitate, reducing low-temperature toughness. Furthermore, if Nb is contained in a large amount, sulfide stress cracking resistance may be reduced. Therefore, when Nb is contained, the Nb content is set to 0.20% or less, preferably 0.15% or less, and more preferably 0.10% or less. Even more preferably, the Nb content is 0.08% or less, and most preferably 0.07% or less.

[0040] Ti: 0.20% or less Ti is an element that refines crystal grains and improves low-temperature toughness and sulfide stress corrosion cracking resistance, and may be selectively added as needed. Ti also has the effect of improving the strength of steel through precipitation strengthening. To achieve this effect, the Ti content is preferably 0.02% or more, more preferably 0.03% or more, even more preferably 0.04% or more, and most preferably 0.05% or more. On the other hand, if Ti is contained in an amount exceeding 0.20%, low-temperature toughness may be reduced. Furthermore, if Ti is contained in a large amount, sulfide stress corrosion cracking resistance may be reduced. Therefore, when Ti is contained, the Ti content is set to 0.20% or less. The Ti content is preferably 0.15% or less, more preferably 0.10% or less, even more preferably 0.08% or less, and most preferably 0.07% or less.

[0041] Zr: 0.50% or less Zr and B are both useful elements that contribute to increasing strength, and may be selectively contained as needed. Zr contributes to the above-mentioned increase in strength and also contributes to improving sulfide stress corrosion cracking resistance. To achieve these effects, the Zr content is preferably 0.02% or more. The Zr content is more preferably 0.05% or more, even more preferably 0.06% or more, and most preferably 0.07% or more. On the other hand, if Zr is contained in an amount exceeding 0.50%, low-temperature toughness may be reduced. Therefore, when Zr is contained, the Zr content is set to 0.50% or less. The Zr content is preferably 0.40% or less, more preferably 0.30% or less, even more preferably 0.20% or less, and most preferably 0.18% or less.

[0042] B: 0.0100% or less B is useful as an element that contributes to the above-mentioned increase in strength and also contributes to improving hot workability. To achieve this effect, the B content is preferably 0.0005% or more. The B content is more preferably 0.0010% or more, even more preferably 0.0012% or more, and most preferably 0.0014% or more. On the other hand, if B is contained in an amount exceeding 0.0100%, low-temperature toughness and hot workability may be reduced. Therefore, when B is contained, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, more preferably 0.0060% or less, even more preferably 0.0050% or less, and most preferably 0.0040% or less.

[0043] REM: 0.08% or less REM is useful as an element that contributes to improving sulfide stress corrosion cracking resistance and may be contained as needed. To ensure this effect, it is preferable to contain 0.01% or more of REM. The REM content is more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, if REM is contained in an amount exceeding 0.08%, it may reduce low-temperature toughness and hot workability. Therefore, if REM is contained, the REM content is set to 0.08% or less. The REM content is preferably 0.07% or less, more preferably 0.06% or less, even more preferably 0.05% or less, and most preferably 0.04% or less. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in the present invention is the total content of one or more elements selected from the above-mentioned REMs.

[0044] Ca: 0.010% or less Ca is useful as an element that contributes to improving sulfide stress corrosion cracking resistance, and may be contained as needed. To ensure this effect, it is preferable to contain 0.001% or more of Ca. More preferably, the Ca content is 0.002% or more. On the other hand, if Ca is contained in an amount exceeding 0.010%, hot workability deteriorates. Therefore, when Ca is contained, the Ca content is set to 0.010% or less. The Ca content is preferably 0.007% or less, more preferably 0.006% or less, even more preferably 0.005% or less, and most preferably 0.004% or less.

[0045] Sn: 0.200% or less Sn is useful as an element that contributes to improving hydrochloric acid corrosion resistance and may be contained as needed. To ensure this effect, it is preferable to contain 0.001% or more of Sn. The Sn content is more preferably 0.002% or more, even more preferably 0.010% or more, and most preferably 0.015% or more. On the other hand, if the Sn content exceeds 0.200%, hot workability deteriorates. Therefore, when Sn is contained, the Sn content is set to 0.200% or less. Preferably, the Sn content is 0.150% or less. More preferably, the Sn content is 0.100% or less, even more preferably 0.050% or less. Most preferably, the Sn content is 0.040% or less.

[0046] Mg: 0.0002 to 0.0100% Mg is useful as an element that contributes to improving sulfide stress corrosion cracking resistance and may be contained as needed. To ensure this effect, if Mg is contained, the Mg content is 0.0002% or more. Preferably, the Mg content is 0.0005% or more. More preferably, the Mg content is 0.0008% or more, even more preferably 0.0010% or more, and most preferably 0.0015% or more. On the other hand, if Mg exceeds 0.0100%, hot workability deteriorates. Therefore, if Mg is contained, the Mg content is set to 0.0100% or less. Preferably, the Mg content is 0.0050% or less. More preferably, the Mg content is 0.0040% or less, even more preferably 0.0030% or less, and most preferably 0.0020% or less.

[0047] Ta: 0.01 to 0.15% Ta is useful as an element that contributes to improving carbon dioxide corrosion resistance, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance, and may be contained as needed. To ensure this effect, if Ta is contained, the Ta content is 0.01% or more. Preferably, the Ta content is 0.02% or more. More preferably, the Ta content is 0.03% or more. On the other hand, if Ta exceeds 0.15%, hot workability deteriorates. Therefore, if Ta is contained, the Ta content is 0.15% or less. Preferably, the Ta content is 0.10% or less. More preferably, the Ta content is 0.08% or less, even more preferably, the Ta content is 0.05% or less, and most preferably, the Ta content is 0.04% or less.

[0048] Co: 0.01 to 3.00% Co is useful as an element that contributes to improving carbon dioxide corrosion resistance, sulfide stress cracking resistance, and sulfide stress corrosion cracking resistance, and may be contained as needed. To ensure this effect, if Co is contained, the Co content is 0.01% or more. Preferably, the Co content is 0.02% or more. More preferably, the Co content is 0.05% or more. On the other hand, if Co exceeds 3.00%, hot workability deteriorates. Therefore, if Co is contained, the Co content is 3.00% or less. Preferably, the Co content is 2.00% or less. More preferably, the Co content is 1.50% or less, and even more preferably, the Co content is 1.00% or less. Most preferably, the Co content is 0.80% or less, and most preferably, the Co content is 0.70% or less.

[0049] Sb: 0.200% or less Sb is useful as an element that contributes to improving hydrochloric acid corrosion resistance and may be contained as needed. To ensure this effect, it is preferable to contain 0.001% or more of Sb. The Sb content is more preferably 0.002% or more, even more preferably 0.010% or more, and most preferably 0.015% or more. On the other hand, if the Sb content exceeds 0.200%, hot workability deteriorates. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. Preferably, the Sb content is 0.150% or less. More preferably, the Sb content is 0.100% or less, even more preferably 0.050% or less. Most preferably, the Sb content is 0.040% or less.

[0050] As: 0.100% or less As is useful as an element that contributes to improving sulfide stress corrosion cracking resistance and may be contained as needed. To ensure this effect, it is preferable to contain 0.001% or more of As. The As content is more preferably 0.005% or more, even more preferably 0.010% or more, and most preferably 0.015% or more. On the other hand, if As is contained in an amount exceeding 0.100%, hot workability deteriorates. Therefore, when As is contained, the As content is set to 0.100% or less. Preferably, the As content is 0.050% or less. More preferably, it is 0.040% or less, even more preferably 0.030% or less. Most preferably, it is 0.025% or less.

[0051] Bi: 0.200% or less Bi is useful as an element that contributes to reducing segregation and may be contained as needed. To ensure this effect, it is preferable to contain 0.001% or more of Bi. The Bi content is more preferably 0.005% or more. On the other hand, if the Bi content exceeds 0.200%, the hot workability decreases. Therefore, if Bi is contained, the Bi content is set to 0.200% or less. Preferably, the Bi content is 0.150% or less. More preferably, it is 0.100% or less, and even more preferably, it is 0.050% or less. Most preferably, it is 0.040% or less.

[0052] Zn: 0.100% or less Zn is useful as an element that contributes to improving carbon dioxide corrosion resistance and may be contained as needed. To ensure this effect, it is preferable to contain Zn at 0.001% or more. The Zn content is more preferably 0.005% or more, even more preferably 0.010% or more, and most preferably 0.015% or more. On the other hand, if the Zn content exceeds 0.100%, hot workability deteriorates. Therefore, if Zn is contained, the Zn content is set to 0.100% or less. Preferably, the Zn content is 0.050% or less. More preferably, the Zn content is 0.040% or less, even more preferably 0.030% or less. Most preferably, the Zn content is 0.025% or less.

[0053] Pb: 0.200% or less Pb is useful as an element that contributes to reducing segregation and may be contained as needed. To ensure this effect, it is preferable to contain 0.001% or more of Pb. The Pb content is more preferably 0.005% or more, even more preferably 0.010% or more, and most preferably 0.015% or more. On the other hand, if the Pb content exceeds 0.200%, hot workability deteriorates. Therefore, when Pb is contained, the Pb content is set to 0.200% or less. Preferably, the Pb content is 0.150% or less. More preferably, the Pb content is 0.100% or less, even more preferably 0.050% or less. Most preferably, the Pb content is 0.040% or less.

[0054] Even if the above-mentioned components are within the ranges, depending on the balance of the components, the structure described below may not be within the range of the present invention. For example, when the value of the following formula is large, the proportion of ferrite phase increases, while when the value is small, the proportion of austenite phase tends to increase. In particular, since Ni stabilizes the austenite phase and Cr stabilizes the ferrite phase, the proportion of ferrite phase and austenite phase changes depending on the balance of the above elements. Therefore, although not particularly limited, the value obtained from -20-141C+5Cr-8Ni-Mn+4Si+5Mo-6Cu+3W-87N is preferably 30 or more and 80 or less. However, each element in the above formula represents the content (mass %) of the stainless steel and stainless steel seamless pipe.

[0055] Structure of Stainless Steel: A structure containing, by volume, 20-70% austenite phase and 30-80% ferrite phase. The stainless steel of the present invention has a structure containing at least an austenite phase and a ferrite phase, and may have a structure consisting of an austenite phase and a ferrite phase. The volume fraction (%) of the austenite phase is 20-70%. The volume fraction (%) of the ferrite phase is 30-80%. If the austenite phase is less than 20%, crevice corrosion resistance may be insufficient. Therefore, the austenite phase is set to 20% or more. The austenite phase is preferably 30% or more, more preferably 40% or more, even more preferably 45% or more, and most preferably 46% or more. Furthermore, if the austenite phase is more than 70%, crevice corrosion resistance may be insufficient. Therefore, the austenite phase is set to 70% or less. The austenite phase is preferably 67% or less, more preferably 65% ​​or less, even more preferably 60% or less, and most preferably 58% or less.

[0056] If the ferrite phase is less than 30%, crevice corrosion resistance may be insufficient, so the ferrite phase is set to 30% or more. The ferrite phase is preferably 33% or more, more preferably 35% or more, even more preferably 40% or more, and most preferably 42% or more. On the other hand, if the ferrite phase is more than 80%, crevice corrosion resistance may be insufficient, so the ferrite phase is set to 80% or less. The ferrite phase is preferably 70% or less, more preferably 60% or less, even more preferably 55% or less, and most preferably 54% or less.

[0057] The volume fraction of each phase can be controlled by adjusting the steel composition and the solution treatment temperature. Specifically, the volume fraction of the austenite phase increases as the amount of austenite-forming elements (C, Mn, Ni, N, Cu) increases or the solution treatment temperature decreases. The volume fraction of the ferrite phase increases as the amount of ferrite-forming elements (Si, Cr, Mo, W) increases or the solution treatment temperature increases. The total volume fraction of the ferrite phase and the austenite phase is preferably 99.5% or more.

[0058] The remaining structure other than the above includes a σ phase, and the σ phase is acceptable if it is 0.5% or less. The σ phase is preferably 0.4% or less, and more preferably 0.3% or less. The σ phase may be 0%.

[0059] To measure the volume fraction of each phase, a test specimen for microstructural observation was first prepared using the cross section of the stainless steel as the observation surface (or the axial cross section of the stainless steel in the case of a seamless steel pipe). The volume fractions of the ferrite and austenite phases were then determined by observing the observation surface with an optical microscope. Specifically, the test specimen for microstructural observation was electrolytically corroded in a KOH aqueous solution, and the microstructure was photographed with an optical microscope (magnification: 400x). From the obtained microstructural photograph, an image analyzer was used to calculate the average area fractions of the ferrite and austenite phases, which were used as their respective volume fractions (volume %). In the photographed image, the phase that is resistant to corrosion and therefore appears white when binarized was designated as the austenite phase, while the phase that is susceptible to corrosion and therefore appears black when binarized was designated as the ferrite phase. The above-described observation methods for each microstructure are also described in detail in the Examples below.

[0060] Yield strength (YS) of 448 MPa or more The stainless steel and stainless steel seamless pipe of the present invention must have a yield strength (YS) of 448 MPa or more in order to support formation pressure and their own weight. The yield strength is preferably 517 MPa or more, and more preferably 551 MPa or more. For use in even deeper wells, the yield strength is even more preferably 758 MPa or more. The yield strength is most preferably 862 MPa or more. There is no particular upper limit, but it is preferably 965 MPa or less. It is more preferably 930 MPa or less, even more preferably 925 MPa or less, and most preferably 920 MPa or less.

[0061] Low temperature toughness: For use in cold regions, the absorbed energy vE of the Charpy impact test at -40°C －４０ must be 100 J or more. Absorbed energy vE －４０ The absorbed energy vE is preferably 110 J or more. －４０It is more preferable that the absorbed energy vE is 120 J or more. －４０ The upper limit is not particularly limited, but is preferably 300 J or less, more preferably 270 J or less, and even more preferably 250 J or less.

[0062] A preferred method for producing the stainless steel and stainless steel seamless pipe of the present invention is described below. Molten steel having the above-described chemical composition is preferably produced using a conventional melting method such as a converter, and then processed into steel pipe material such as billets using a conventional method such as continuous casting or ingot-making and blooming. The heating temperature of the steel pipe material before hot working is preferably 1100°C or higher. It is more preferably 1150°C or higher, even more preferably 1200°C or higher, and most preferably 1250°C or higher. The heating temperature is preferably 1350°C or lower, more preferably 1300°C or lower, even more preferably 1290°C or lower, and most preferably 1280°C or lower. This allows for both good hot workability during pipe-making and low-temperature toughness of the final product. The resulting steel pipe material is then processed into a seamless steel pipe having the above-described composition and the desired dimensions using a commonly known pipe-making method, such as an extrusion pipe-making method such as the Ugine-Séjournet process or a hot working method such as the Mannesmann pipe-making process. After the hot working, a cooling treatment may be carried out. This cooling treatment (cooling step) does not need to be particularly limited. If the composition range of the present invention is as described above, it is preferable to cool the steel sheet to room temperature (10 to 50°C) at a cooling rate similar to that of air cooling after the hot working. Furthermore, the temperature described below is the temperature at the center of the wall thickness unless otherwise specified.

[0063] The present invention also includes a method for producing stainless steel by melting molten steel having the above-mentioned chemical composition using a conventional melting method such as a converter, forming it into a steel pipe material such as a billet or slab using a conventional method such as continuous casting or ingot making-blooming rolling, and then heating it and performing conventional hot rolling without performing the above-mentioned pipe-making process. Note that with regard to stainless steel as well, the subsequent solution heat treatment and cold working are preferably performed under the conditions described below.

[0064] Solution Heat Treatment Next, the steel pipe after pipe formation is preferably subjected to solution heat treatment. Specifically, the steel pipe is heated to a heating temperature of 1000°C or higher, and then cooled to a temperature of 300°C or lower at an average cooling rate faster than air cooling, more specifically, at an average cooling rate of 1°C / s or higher. This allows intermetallic compounds, carbides, nitrides, sulfides, etc. precipitated during pipe formation or during cooling after pipe formation to be solid-dissolved, resulting in a seamless steel pipe with a structure containing the desired amounts of austenite and ferrite phases. The cooling stop temperature is preferably 300°C or lower, more preferably 200°C or lower, and even more preferably 100°C or lower. The lower limit of the cooling stop temperature is not particularly limited, but is preferably 0°C or higher, and more preferably 10°C or higher. If the heating temperature for solution heat treatment is lower than 1000°C, the desired high toughness cannot be ensured. Therefore, the heating temperature for solution heat treatment is preferably 1000°C or higher. The heating temperature of the solution heat treatment is more preferably 1020°C or higher. Even more preferably 1025°C or higher, and most preferably 1030°C or higher. Furthermore, from the viewpoint of preventing coarsening of the structure, the heating temperature of the solution heat treatment is preferably 1150°C or lower. More preferably, the heating temperature of the solution heat treatment is 1130°C or lower. Even more preferably, the heating temperature of the solution heat treatment is 1100°C or lower, and most preferably 1080°C or lower. In the present invention, from the viewpoint of uniform temperature distribution within the material, the holding time at the heating temperature of the solution heat treatment is preferably 5 minutes or longer, more preferably 10 minutes or longer, even more preferably 15 minutes or longer, and most preferably 20 minutes or longer. Furthermore, the holding time at the heating temperature of the solution heat treatment is preferably 210 minutes or shorter, more preferably 120 minutes or shorter, even more preferably 60 minutes or shorter, and most preferably 40 minutes or shorter.

[0065] If the average cooling rate during solution treatment is less than 1°C / s, intermetallic compounds such as σ-phase and χ-phase will precipitate during cooling, significantly reducing low-temperature toughness. Therefore, the average cooling rate during solution treatment is preferably 1°C / s or higher. The average cooling rate during solution treatment is more preferably 2°C / s or higher, even more preferably 10°C / s or higher, and most preferably 30°C / s or higher. While the upper limit of the average cooling rate is not particularly limited, it is preferably 500°C / s or lower. The average cooling rate is more preferably 450°C / s or lower, even more preferably 400°C / s or lower, and most preferably 100°C / s or lower. The average cooling rate is the average cooling rate from the heating temperature of solution treatment to 300°C. Water cooling is preferred as the cooling method. The average cooling rate can be calculated by dividing the temperature difference from the heating temperature of solution treatment to 300°C by the time required for cooling.

[0066] Pickling Pickling may be performed to remove oxide scale from the steel surface. Pickling increases the Cr concentration in the surface oxide film. The acid used in pickling is not particularly limited. For example, fluoronitric acid or sulfuric acid may be used. The temperature and immersion time for pickling are also not limited. From the viewpoint of production efficiency, the temperature is preferably 10°C or higher, more preferably 20°C or higher, and even more preferably 22°C or higher. It is preferably 80°C or lower, and more preferably 70°C or lower. Since an immersion time that is too short does not sufficiently remove the acid, it is preferably 10 minutes or higher. It is more preferably 30 minutes or higher. It is even more preferably 40 minutes or higher, and most preferably 60 minutes or higher. On the other hand, since an immersion time that is too long reduces production efficiency, it is preferably 210 minutes or lower. It is more preferably 150 minutes or lower. It is even more preferably 145 minutes or lower, and most preferably 140 minutes or lower. This pickling process may be performed after cold working. When pickling is performed after cold working, the strain on the very surface caused by cold working is removed, improving stress corrosion cracking resistance and crevice corrosion resistance.

[0067] Cold Working To improve the yield strength of the material, strain may be introduced by cold drawing, cold rolling, or cold working by inclined rolling with opposing rolls to increase strength. Preferably, diameter reduction rolling or cold drawing is performed. The inclined rolling mill used for diameter reduction may be a two-roll inclined rolling mill or a three-roll inclined rolling mill having barrel-shaped or cone-shaped rolls. The diameter reduction rolling can be performed by adjusting the tilt angle (the angle of inclination with respect to the tube axis direction when viewed in a direction perpendicular to the tube axis direction and in the direction in which the rolling load is applied to the mother tube), the cross angle (the angle of inclination with respect to the tube axis direction when viewed in a direction perpendicular to the tube axis direction and in the direction in which the rolling load is applied to the mother tube), and the roll gap. While not particularly limited, the tilt angle is preferably 0.5° or more. The tilt angle is preferably 40.0° or less. The cross angle is preferably 0° or more. The cross angle is preferably 45.0° or less. Cold drawing may be performed by the cold pilger method. The temperature during working may be high to reduce deformation resistance. Specifically, the working temperature is preferably in the range of 0 to 600°C, and is preferably performed while avoiding the 460 to 490°C temperature range where stainless steel becomes embrittled. That is, the working temperature is preferably 0°C or higher, more preferably 15°C or higher, even more preferably 20°C or higher, and most preferably 25°C or higher, and is preferably outside the 460 to 490°C temperature range. Furthermore, the working temperature is preferably 600°C or lower and outside the 460 to 490°C temperature range, more preferably 400°C or lower, even more preferably 300°C or lower, and most preferably 200°C or lower. Furthermore, the reduction rate of cold working is preferably 1% or higher, more preferably 2% or higher, even more preferably 3% or higher, and most preferably 4% or higher. The rolling reduction in cold working is preferably 90% or less, more preferably 70% or less, even more preferably 50% or less, and most preferably 15% or less.

[0068] As described above, the stainless steel and stainless steel seamless pipe obtained by the present invention are high-strength steel pipes with a yield strength of 448 MPa or more and have excellent crevice corrosion resistance. In particular, the stainless steel and stainless steel seamless pipes that have been subjected to the above-mentioned cold working can be obtained as steel pipes with a yield strength of 758 MPa or more, which is considered to be of even higher strength. The stainless steel seamless pipe of the present invention can be used as a stainless steel seamless pipe for oil wells (high-strength stainless steel seamless pipe for oil wells).

[0069] The present invention will be described in further detail below with reference to examples. It should be noted that the present invention is not limited to the following examples. A steel pipe material was cast using molten steel having the chemical composition shown in Table 1. The steel pipe material was then heated and hot-formed using a model seamless rolling mill to produce a seamless steel pipe having an outer diameter of 72 mm and a wall thickness of 11 mm, which was then air-cooled. The heating temperature of the steel pipe material before hot-forming was 1250°C. After casting and heating the steel pipe material as described above, it was hot-rolled using a hot rolling mill to produce stainless steel, which was then air-cooled. The heating temperature of the steel pipe material before hot-rolling was 1250°C, and the final temperature of hot-rolling was 900°C or higher and 1000°C or lower.

[0070] After air cooling, solution heat treatment was performed at the temperature, soaking time, and cooling method shown in Table 2. The average cooling rate in the solution heat treatment was 7 to 80°C / s, and the cooling stop temperature was 25°C.

[0071] The stainless steels marked with a circle in the cold working + pickling column in Table 2 were cold worked at a reduction of 3 to 10% in a cold rolling mill, and then pickled after cold working. Pickling was carried out using fluoronitric acid at 25°C for 135 minutes.

[0072] Test specimens for microstructural observation were taken from the finally obtained seamless steel pipes, and quantitative evaluation of the microstructure, tensile tests, and crevice corrosion tests were carried out. The test methods were as follows. The results obtained from these tests are shown in Table 2.

[0073] (1) Measurement of the volume fraction (vol %) of each phase in the entire structure of the steel pipe. Test specimens for microstructure observation were taken from the seamless steel pipes subjected to the above-mentioned heat treatment so that the axial cross section of the pipe served as the observation surface. The volume fractions of the ferrite phase and the austenite phase were determined by observing the observation surface with an optical microscope. Specifically, the test specimens for microstructure observation were electrolytically corroded in a KOH aqueous solution, and the microstructure was photographed with an optical microscope (magnification 400x). From the obtained microstructure photographs, an image analyzer was used to calculate the average area fractions of the ferrite phase and the austenite phase, and these were used as the respective volume fractions (vol %). In the photographed images, the phase that is resistant to corrosion and therefore appears white when binarized was defined as the austenite phase, and the phase that is easily corroded and therefore appears black when binarized was defined as the ferrite phase.

[0074] (2) Measurement of the number of Pt-containing precipitates in the structure. A test specimen for microstructure observation was taken so that the tube axial cross section served as the observation surface, and the number of Pt-containing precipitates with a major axis of 0.5 μm or more was counted using a scanning electron microscope (SEM). Specifically, the number of Pt-containing precipitates with a major axis of 0.5 μm or more was counted in 25 fields of view of 200 μm × 200 μm at any position, and the average value was taken as the number of Pt-containing precipitates in the structure. The observation was carried out at an accelerating voltage of 15 kV and a magnification of 400x.

[0075] (3) Tensile Test [Evaluation of Tensile Properties] For seamless steel pipes that had not been subjected to cold working, tensile test pieces (Φ6.0 mm) according to JIS (Japan Industrial Standards) No. 14A were taken from the seamless steel pipes after solution heat treatment, and for seamless steel pipes that had been subjected to cold working, they were taken from the seamless steel pipes after cold working and pickling. Tensile tests were carried out in accordance with the provisions of JIS Z2241:2011 to determine the yield strength (YS).

[0076] (4) Crevice corrosion test Corrosion test specimens of 3 mm thick x 30 mm wide x 40 mm long were prepared by machining from seamless steel pipes that had not been subjected to cold working after solution heat treatment, and from seamless steel pipes that had been subjected to cold working, corrosion test specimens of 3 mm thick x 30 mm wide x 40 mm long were prepared by machining. The test specimens of the cold worked seamless steel pipes were pickled. A corrosion test was carried out on these test specimens to evaluate their crevice corrosion resistance. The corrosion test was carried out by pressing an insulator into a test liquid: a 3.5 mass % NaCl aqueous solution (liquid temperature: 50°C) at a pressure of 20 N / mm ２ The crevice corrosion test specimens were immersed for 720 hours. After the corrosion test, the specimens were inspected for crevice corrosion using a 10x magnifying glass. Crevice corrosion refers to the presence of a corroded area with a circle equivalent diameter of 0.2 mm or more. The depth of crevice corrosion was measured at locations where crevice corrosion was observed. The depth was measured optically using a 3D digital microscope. In the present invention, a specimen was deemed to have passed if no crevice corrosion occurred or if the maximum depth of crevice corrosion was less than 25 μm.

[0077] (5) Charpy Impact Test V-notch test pieces (thickness 10 mm) were taken from the center of the wall thickness of seamless steel pipes that were not subjected to cold working after solution heat treatment, and from seamless steel pipes that were subjected to cold working after cold working and pickling, in accordance with the provisions of JIS Z 2242 (2018), so that the length of the test piece was in the circumferential direction. A Charpy impact test was performed on the taken test pieces at a test temperature of -40°C, and the absorbed energy vE －４０ Three test pieces were taken from each steel pipe, and the Charpy impact test was carried out on these test pieces. The arithmetic mean values ​​of the absorbed energy vE at test temperature of -40°C were shown in Table 2. －４０ Those having a value of 100 J or more were judged to be acceptable.

[0078]

[0079]

[0080] All of the examples of the present invention were stainless steel seamless pipes having high strength with a yield strength of 448 MPa or more, excellent crevice corrosion resistance, and excellent low-temperature toughness. On the other hand, the comparative examples outside the scope of the present invention either did not achieve the high strength aimed at by the present invention, did not have excellent crevice corrosion resistance, or did not have excellent low-temperature toughness. The stainless steels obtained above also gave the same results as the above stainless steel seamless pipes.

Claims

1. A stainless steel having a chemical composition containing, by mass%, the following: C: 0.002 to 0.040%, Si: 0.05 to 1.00%, Mn: 0.1 to 1.7%, P: 0.040% or less, S: 0.0200% or less, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.050%, N: 0.06 to 0.35%, Pt: 0.001 to 0.800%, O: 0.0100% or less, with the balance being Fe and unavoidable impurities; having a structure containing, by volume, 20 to 70% austenite phase and 30 to 80% ferrite phase; and having a yield strength of 448 MPa or more.

2. In addition to the above chemical composition, the following components are contained, in mass %, in the following order: W: 0.02 to 1.50%, Cu: 0.1 to 2.0%, V: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less, Zr: 0.50% or less, B: 0.0100% or less, REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.200% or less, Mg: 0.0002 to 0.0100%, Ta: 0.01 to 0.15%, Co: 0.01 to 3.00%, Sb: 0.200% or less, As: 0.100% or less, Bi: 0.200% or less, Zn: 0.100% or less.

2. The stainless steel according to claim 1, further comprising one or more selected from the group consisting of Pb: 0.200% or less.

3. A stainless steel having the chemical composition and structure described in claim 1 or 2 and having a yield strength of 758 MPa or more.

4. A stainless steel seamless pipe having a chemical composition containing, by mass%, the following: C: 0.002 to 0.040%, Si: 0.05 to 1.00%, Mn: 0.1 to 1.7%, P: 0.040% or less, S: 0.0200% or less, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.050%, N: 0.06 to 0.35%, Pt: 0.001 to 0.800%, O: 0.0100% or less, with the balance being Fe and unavoidable impurities; having a structure containing, by volume, 20 to 70% austenite phase and 30 to 80% ferrite phase; and having a yield strength of 448 MPa or more.

5. In addition to the above chemical composition, the following components are contained, in mass %, in the following order: W: 0.02 to 1.50%, Cu: 0.1 to 2.0%, V: 0.20% or less, Nb: 0.20% or less, Ti: 0.20% or less, Zr: 0.50% or less, B: 0.0100% or less, REM: 0.08% or less, Ca: 0.010% or less, Sn: 0.200% or less, Mg: 0.0002 to 0.0100%, Ta: 0.01 to 0.15%, Co: 0.01 to 3.00%, Sb: 0.200% or less, As: 0.100% or less, Bi: 0.200% or less, Zn: 0.100% or less.

5. The stainless steel seamless pipe according to claim 4, further comprising one or more selected from the group consisting of Pb: 0.200% or less.

6. A stainless steel seamless pipe having the chemical composition and structure described in claim 4 or 5 and having a yield strength of 758 MPa or more.

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