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, 0.001-1.000% Au, 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 high strength, such as a yield strength of 448 MPa (65 ksi) or more.

[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 O ２ 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" used here 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 Au 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%, Au: 0.001 to 1.000%, 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 chemical 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%, Au: 0.001 to 1.000%, 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] above, 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 0.100% or less of Pb, 0.2 ...

[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%, Au: 0.001 to 1.000%, 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 0.012% or more, even more preferably, the C content is 0.015% or more, and most preferably, the C content is 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 0.035% or less, more preferably, 0.030% or less, even more preferably, the C content is 0.025% or less, and most preferably, the C content is 0.024% 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. Preferably, the Si content is 0.10% or more. The Si content is more preferably 0.12% or more, even more preferably 0.13% or more, and most preferably 0.14% or more. On the other hand, if the Si content exceeds 1.00%, the hot workability of the steel decreases. Therefore, the Si content is set to 1.00% or less. Preferably, the Si content is 0.80% or less, more preferably 0.70% or less, even more preferably 0.60% or less, and most preferably 0.55% 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. Preferably, the Mn content is 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 0.030% or less, more preferably 0.025% or less, even more preferably 0.020% 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, more preferably 0.007% or more. The P content is 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 it exceeds 0.0200%, it will hinder the production of stainless steel. Therefore, the S content is set to 0.0200% or less. Preferably, the S content is set to 0.0100% or less. The S content is more preferably 0.0040% or less, even more preferably 0.0020% 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 set to 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 forms an oxide film on the surface and is effective in 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 achieve even higher strength, the Cr content is preferably 22.0% or more, more preferably 22.5% or more, even more preferably 23.0% or more, and most preferably 24.0% 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. The Cr content is 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 5.6% or more, and most preferably 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. In addition, 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 7.4% or less, even more preferably 6.9% or less, and most preferably 6.8% 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 2.5% or more, more preferably 2.7% or more, even 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. The Mo content is preferably 4.5% or less, more preferably 4.2% or less, even more preferably 4.0% or less, and most preferably 3.9% 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. The Al content is preferably 0.005% or more, more preferably 0.007% or more, even more preferably 0.010% 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. The Al content is more preferably 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 to contribute 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 desired properties of 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, and most preferably, 0.12% 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. Preferably, the N content is set to 0.30% or less. More preferably, the N content is set to 0.25% or less. Even more preferably, the N content is set to 0.24% or less, and most preferably, the N content is set to 0.23% or less.

[0031] Au: 0.001 to 1.000% When the passive film is destroyed and crevice corrosion initiation occurs, Au concentrates on the steel surface and coats it. As a result, it improves the crevice corrosion resistance of stainless steel. If the Au content is less than 0.001%, this effect cannot be obtained. Therefore, the Au content is set to 0.001% or more. Preferably, the Au content is 0.005% or more. More preferably, the Au content is 0.010% or more. Most preferably, the Au content is 0.030% or more. Most preferably, the Au content is 0.100% or more. On the other hand, if the Au content exceeds 1.000%, the hot workability of the stainless steel deteriorates. If the Au content exceeds 1.000%, Au precipitates as coarse precipitates, further reducing the crevice corrosion resistance of the stainless steel. Therefore, the Au content is set to 1.000% or less. Preferably, the Au content is 0.800% or less. It is more preferably 0.700% or less, even more preferably 0.500% or less, and most preferably 0.450% or less.

[0032] In order to obtain the above-mentioned effect, the precipitates containing Au in the structure are set to have a major diameter of 0.5 μm or more and a density of 3 precipitates / mm ２ It is preferable that the number of particles is 8 or more, and more preferably 8 particles / mm ２ More preferably, 9 pieces / mm ２ More preferably, 10 pieces / mm ２ In order to suppress the formation of coarse Au precipitates, the number of precipitates having a major axis of 0.5 μm or more is set to 75 / mm ２ It is preferable that the number of particles is 45 or less per mm. ２ More preferably, it is 30 pieces / mm or less. ２ or less, and most preferably 25 / mm ２ The following is the result.

[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 0.0050% or less, more preferably 0.0045% or less, even more preferably 0.0040% or less, and most preferably 0.0030% or less. Since excessive reduction increases manufacturing costs, the O content is preferably 0.0005% or more, more preferably 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 being Fe and inevitable impurities. Here, a steel according to one embodiment of the present invention may contain only the above basic components and the balance being Fe and inevitable impurities. Inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, manufacturing equipment, etc., and are allowed to be included to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap.

[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 contained as needed. To achieve this effect, when W is contained, the W content is preferably 0.02% or more. More preferably, the W content is 0.10% or more. Even more preferably, it is 0.20% or more, and most preferably, it is 0.40% or more. On the other hand, if W is contained in a large amount exceeding 1.50%, it may reduce low-temperature toughness. Therefore, when W is contained, the W content is set to 1.50% or less. Preferably, the W content is 1.30% or less. More preferably, it is 1.10% or less. Even more preferably, it is 0.90% or less, and most preferably, it is 0.70% 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, when Cu is added, the content of each of these elements is set to 0.1% or more. It is preferably set to 0.2% or more. More preferably, it is set to 0.3% or more. Even more preferably, it is set to 0.4% or more, and most preferably, it is set to 0.6% or more. However, since an excessive Cu content deteriorates hot workability, when Cu is added, the content is set to 2.0% or less. It is preferably set to 1.5% or less. More preferably, it is set to 1.0% or less. Even more preferably, it is set to 0.8% or less, and most preferably, it is set to 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 contained as needed. To achieve this effect, the V content is preferably 0.02% or more. The V content is 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. Preferably, it is 0.15% or less. More preferably, it is 0.10% or less. The V content is further 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. The Nb content is 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, it is 0.15% or less. More preferably, it is 0.13% or less. Even more preferably, it is 0.10% or less. Most preferably, the Nb content is 0.08% 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. The Ti content is 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. Preferably, it is 0.15% or less. More preferably, it is 0.10% or less. Even more preferably, the Ti content is 0.08% or less, and most preferably, it is 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 not only contributes to the above-mentioned increase in strength, but also contributes to improving sulfide stress corrosion cracking resistance. To achieve these effects, the Zr content is preferably 0.02% or more. More preferably, the Zr content is 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. Preferably, it is 0.40% or less. More preferably, it is 0.30% or less. Even more preferably, the Zr content is 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. More preferably, the B content is 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. Preferably, the B content is 0.0080% or less. More preferably, the B content is 0.0060% or less. Even more preferably, the B content is 0.0050% or less, and most preferably, the B content is 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. More preferably, the REM content is 0.02% 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. The Ca content is more preferably 0.002% or more, and even more preferably 0.003% 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. More preferably, the Sn content is 0.002% or more. On the other hand, if the Sn content exceeds 0.200%, hot workability decreases. Therefore, when Sn is contained, the Sn content is set to 0.200% or less. The Sn content is preferably 0.195% or less, more preferably 0.150% or less, even more preferably 0.100% or less, and most preferably 0.050% 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, it is preferable to contain 0.0002% or more of Mg. More preferably, the Mg content is 0.0005% or more. The Mg content is even more preferably 0.0008% or more, and most preferably 0.0010% or more. On the other hand, if Mg exceeds 0.0100%, hot workability deteriorates. Therefore, when Mg is contained, the Mg content is set to 0.0100% or less. The Mg content is preferably 0.0080% or less, more preferably 0.0050% or less, even more preferably 0.0040% or less, and most preferably 0.0030% 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, and most preferably 0.04% or more. On the other hand, if Ta exceeds 0.15%, hot workability deteriorates. Therefore, if Ta is contained, the Ta content is set to 0.15% or less. The Ta content is preferably 0.13% or less, more preferably 0.10% or less. More preferably, the Ta content is 0.08% or less, and most preferably 0.06% 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, when 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, even more preferably 0.10% or more, and most preferably 0.20% or more. On the other hand, if Co exceeds 3.00%, hot workability deteriorates. Therefore, when Co is contained, the Co content is set to 3.00% or less. Preferably, the Co content is 2.00% or less. More preferably, the Co content is 1.50% or less, even more preferably 1.00% or less, and most preferably 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. More preferably, the Sb content is 0.002% 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. The Sb content is preferably 0.195% or less, more preferably 0.150% or less, even more preferably 0.100% or less, and most preferably 0.050% 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. More preferably, the As content is 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. The As content is preferably 0.080% or less, more preferably 0.050% or less, even more preferably 0.040% or less, and most preferably 0.030% 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, even more preferably 0.010% or more, and most preferably 0.015% 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 0.130% or less, even more preferably 0.100% or less, and most preferably 0.050% 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. More preferably, the Zn content is 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. The Zn content is preferably 0.080% or less, more preferably 0.050% or less. More preferably, it is 0.040% or less, and even more preferably 0.030% 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. More preferably, the Pb content is 0.005% or more. Even more preferably, it is 0.020% or more, and most preferably, it is 0.040% or more. On the other hand, if the Pb content exceeds 0.200%, the hot workability decreases. 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, it is 0.100% or less, and even more preferably, it is 0.080% 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 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 austenite phase has a volume fraction (%) of 20-70%. The ferrite phase has a volume fraction (%) of 30-80%. If the austenite phase has a volume fraction (%) of less than 20%, crevice corrosion resistance may be insufficient. Therefore, the austenite phase has a volume fraction (%) of 20% or more. The austenite phase has a volume fraction (%) of 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 has a volume fraction (%) of more than 70%, crevice corrosion resistance may be insufficient. Therefore, the austenite phase has a volume fraction (%) of 70% or less. The volume fraction (%) of 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% by volume, crevice corrosion resistance may be insufficient, so the ferrite phase is set to be 30% or more by volume. 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% by volume, crevice corrosion resistance may be insufficient, so the ferrite phase is set to be 80% or less by volume. The ferrite phase is preferably 70% or less by volume, more preferably 60% or less, even more preferably 55% or less, and most preferably 54% or less by volume.

[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 its volume fraction (%) is 0.5% 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 the respective volume fractions (volume %). In the photographed image, the phase that appears white because it is resistant to corrosion was designated as the austenite phase, and the phase that appears gray because it is susceptible to corrosion was designated as the ferrite phase. Furthermore, if there is a black area in the photographed image, it can be determined that the σ phase has precipitated. The above-mentioned observation method for each microstructure is 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 to support formation pressure and their own weight. The yield strength is preferably 517 MPa or more, and more preferably 551 MPa or more. Furthermore, when used in deeper wells, even higher strength is required, and the yield strength is more preferably 758 MPa (110 ksi) 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, more preferably 950 MPa or less, and even more preferably 940 MPa or less. For example, the above-mentioned yield strength (YS) of 758 MPa or more can be obtained by cold working.

[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. －４０ The force is more preferably 120 J, even more preferably 130 J or more, and most preferably 140 J or more. There is no particular upper limit, but it 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 billets or other steel pipe materials using conventional methods, 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, more preferably 1200°C or higher, and even more preferably 1250°C or higher. The heating temperature of the steel pipe material before hot working is preferably 1350°C or lower, more preferably 1290°C or lower, and even more 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 the Mannesmann pipe-making process. After hot working, a cooling treatment may be performed. This cooling treatment (cooling step) does not need to be particularly limited. As long as the composition range of the present invention is as described above, it is preferable to cool the steel to room temperature after hot working at a cooling rate similar to that of air cooling. Furthermore, the temperature described below is the temperature at the center of the wall thickness unless otherwise specified. The room temperature refers to 10 to 50°C.

[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 thus formed 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-forming or cooling after pipe-forming to be solid-dissolved, resulting in a seamless steel pipe having a structure containing the desired amounts of austenite and ferrite phases. If the heating temperature for the solution heat treatment is lower than 1000°C, the desired high toughness cannot be ensured. Therefore, the heating temperature for the solution heat treatment is preferably 1000°C or higher. More preferably, the heating temperature for the solution heat treatment is 1020°C or higher, even more preferably 1030°C or higher, and most preferably 1040°C or higher. Furthermore, the heating temperature for the solution heat treatment is preferably 1150°C or lower from the viewpoint of preventing coarsening of the structure. The heating temperature of the solution heat treatment is more preferably 1130°C or less, even more preferably 1110°C or less, and most preferably 1100°C or less. In the present invention, the holding time at the heating temperature of the solution heat treatment is preferably 5 minutes or more, from the viewpoint of uniformly distributing the temperature within the material. 10 minutes or more is more preferred, 12 minutes or more is even more preferred, and 15 minutes or more is most preferred. Furthermore, the holding time at the heating temperature of the solution heat treatment is preferably 210 minutes or less, more preferably 120 minutes or less, even more preferably 60 minutes or less, and most preferably 40 minutes or less.

[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 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. The upper limit of the average cooling rate is not particularly limited, but is preferably 500°C / s or lower. It is more preferably 400°C / s or lower, even more preferably 200°C / s or lower, and most preferably 100°C / s or lower. The average cooling rate can be calculated by dividing the temperature difference from the heating temperature to 300°C by the time required for cooling. Water cooling is preferred as the cooling method. The cooling is preferably performed to a temperature of 300°C or lower, more preferably 200°C or lower, and even more preferably 100°C or lower. The lower limit of the temperature is not particularly limited, but is preferably 0°C or higher, and more preferably 10°C or higher.

[0066] Pickling Pickling may be performed to remove oxide scale from the steel surface. Pickling increases the Cr concentration of the surface oxide film. The acid used in pickling is not particularly limited. For example, hydrofluoric nitric acid or sulfuric acid may be used. The temperature and immersion time of pickling are also not limited. From the viewpoint of manufacturing efficiency, the temperature is preferably 10°C or higher, more preferably 20°C or higher, and even more preferably 25°C or higher. The temperature of pickling is preferably 80°C or lower, more preferably 70°C or lower, and even more preferably 50°C or lower. If the immersion time is too short, the acid cannot be sufficiently removed, so it is preferably 10 minutes or longer. It is more preferably 30 minutes or longer, and even more preferably 60 minutes or longer. On the other hand, if the immersion time is too long, manufacturing efficiency will be adversely affected, so it is preferably 210 minutes or shorter. It is more preferably 150 minutes or shorter, and even more preferably 140 minutes or shorter. 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 β is preferably 0.5° or more. The tilt angle β is preferably 40.0° or less. The cross angle γ is preferably 0° or more. The cross angle γ 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 ratio 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 low-temperature toughness and 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 made into 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, the alloy was subjected to solution heat treatment at the temperature, soaking time, cooling method, and average cooling rate shown in Table 2.

[0071] In addition, the stainless steels marked with a circle in the "cold working + pickling" column in Table 2 were cold worked using a cold rolling mill and then pickled. The reduction ratio was 3 to 10%, and the pickling was carried out using fluoronitric acid at 25°C for 135 minutes. The cold working was carried out at room temperature (10 to 50°C).

[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 (volume %) of each phase in the entire steel pipe structure. Test specimens for microstructure observation were taken from the seamless steel pipes subjected to the heat treatment described above, 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 described above 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 (volume %). In the photographed images, the phase that appears white because it is resistant to corrosion was designated as the austenite phase, and the phase that appears gray because it is easily corroded was designated as the ferrite phase. In addition, the areas observed as black in the photographed images were determined to be σ-phase precipitates, and the area fraction of the σ-phase was calculated and used as the volume fraction (volume %).

[0074] (2) Measurement of the number of Au-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 Au-containing precipitates with a major axis of 0.5 μm or more was measured using a scanning electron microscope (SEM). Specifically, the number of Au-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 Au-containing precipitates in the structure. SEM observation was performed 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 the seamless steel pipes after solution heat treatment for seamless steel pipes that had not been cold worked, and from the seamless steel pipes that had been cold worked. Furthermore, the test specimens taken from the seamless steel pipes that had been cold worked 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 For seamless steel pipes that were not cold worked, V-notch test pieces (thickness 10 mm) were taken from the seamless steel pipes after solution heat treatment, and for seamless steel pipes that were cold worked, V-notch test pieces (thickness 10 mm) were taken from the center of the wall thickness of the seamless steel pipes after cold working and pickling, in accordance with the provisions of JIS Z 2242 (2018), so that the circumferential direction was the length of the test piece. 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 values ​​obtained are shown in Table 2. Absorbed energy vE in the -40°C Charpy impact test －４０ A value of 100J or more was considered a pass.

[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%, Au: 0.001 to 1.000%, 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 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.

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%, Au: 0.001 to 1.000%, 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 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.

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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