Duplex stainless steel pipe

Abstract

Provided is a duplex stainless steel pipe with which it is possible to obtain high strength and excellent corrosion resistance. The duplex stainless steel pipe according to the present invention contains, in mass%, 0.030% or less of C, 0.20-1.00% of Si, 0.5-7.0% of Mn, 0.040% or less of P, 0.020% or less of S, 0.100% or less of Al, 4.0-9.0% of Ni, 20.0-30.0% of Cr, 0.5-2.0% of Mo, 1.5-3.0% of Cu, 0.15-0.30% of N, 0.01-0.50% of V, 0.030-0.300% of Nb, 0.10-0.50% of Co, 0.001-0.050% of Sn, ferrite having a microstructure of 35.0-65.0 vol%, 0-1.0% of a sigma phase, and a remainder of austenite, and has a non-solid-solution Nb content of 0.008% or more in mass%, the ratio of the non-solid-solution Nb content to the non-solid-solution Al content is 1.0 or more, and the yield strength is 655 MPa or more.

Description

Technical Field

[0001] This invention relates to steel pipes, and more specifically, to duplex stainless steel pipes. Background Technology

[0002] Oil wells and gas wells (hereinafter collectively referred to as "oil wells") sometimes create corrosive environments containing corrosive gases. Here, corrosive gases refer to carbon dioxide and / or hydrogen sulfide gases. Therefore, the steel used in oil wells is required to have excellent corrosion resistance in corrosive environments.

[0003] To date, known methods for improving the corrosion resistance of steel include increasing the chromium (Cr) content and forming a passivation coating primarily composed of Cr oxides on the steel surface. Therefore, duplex stainless steels with increased Cr content are sometimes used in environments requiring excellent corrosion resistance.

[0004] In recent years, the development of deep wells beneath the sea surface has become more active. The steel used in these deep wells requires high strength. Therefore, duplex stainless steel, which combines high strength with excellent corrosion resistance, is required for use in oil wells.

[0005] Japanese Patent Application Publication No. 2018-193591 (Patent Document 1) and International Publication No. 2012 / 121232 (Patent Document 2) disclose duplex stainless steel with high strength and excellent corrosion resistance.

[0006] Patent Document 1 discloses a duplex stainless steel with the following chemical composition, containing, by mass %: C: 0.005–0.04%, Si: 0.2–1.0%, Mn: 0.1–2.0%, P: less than 0.040%, S: less than 0.010%, Ni: 3–7%, Cr: 23–28%, Mo: 0.5–1.5%, Cu: 2–4%, N: 0.10–0.35%, Al: 0.001–0.04%. The duplex stainless steel contains 0.04% W, 0-1.0% Co, 0-1.0% V, 0-1.0% Nb, 0-0.2% Ti, 0-0.2% Ca, 0-0.02% Mg, 0-0.02% B, and rare earth elements (REM): 0-0.2%, with the balance being Fe and impurities, and satisfies formulas (1) to (3). The duplex stainless steel has a yield strength YS of 655 MPa or higher. Here, formulas (1) to (3) are as follows.

[0007] YS / 150≤Ni+Mo+0.5W+Cu-Mn≤YS / 75 (1)

[0008] Cr+3.3×(Mo+0.5W)+16N≥30.0 ​​(2)

[0009] Mo + 0.5W + Ni ≤ 7.50 (3)

[0010] Patent document 1 describes that by adjusting the element content and yield strength in the chemical composition of the duplex stainless steel to satisfy formulas (1) to (3), high strength and excellent corrosion resistance can be obtained.

[0011] The duplex stainless steel disclosed in Patent Document 2 has the following chemical composition, containing, by mass %: C: less than 0.03%, Si: less than 0.3%, Mn: less than 3.0%, P: less than 0.040%, S: less than 0.008%, Cu: 0.2 to 2.0%, Ni: 5.0 to 6.5%, Cr: 23.0 to 27.0%, Mo: 2.5 to 3.5%, W: 1.5 to 4.0%, and N: 0.24 to 0.40%, with the balance being Fe and impurities. The σ-phase sensitivity index X (=2.2Si+0.5Cu+2.0Ni+Cr+4.2Mo+0.2W) is less than 52.0, the strength index Y (=Cr+1.5Mo+10N+3.5W) is more than 40.5, and the pitting corrosion resistance index PREW (=Cr+3.3(Mo+0.5W)+16N) is more than 40. Regarding the microstructure of the steel, when a straight line parallel to the thickness direction is drawn from the surface to a depth of 1 mm in a section parallel to the rolling direction, the number of ferrite and austenite phase boundaries intersecting this line is 160 or more. Patent Document 2 describes that this duplex stainless steel can achieve high strength without compromising corrosion resistance.

[0012] Existing technical documents

[0013] Patent documents

[0014] Patent Document 1: Japanese Patent Application Publication No. 2018-193591

[0015] Patent Document 2: International Publication No. 2012 / 121232 Summary of the Invention

[0016] The problem that the invention aims to solve

[0017] The duplex stainless steels disclosed in Patent Documents 1 and 2 exhibit high strength and excellent corrosion resistance. However, high strength and excellent corrosion resistance can also be achieved through means other than those disclosed in Patent Documents 1 and 2.

[0018] The purpose of this invention is to provide a duplex stainless steel pipe that can achieve high strength and excellent corrosion resistance.

[0019] Solution for solving the problem

[0020] The duplex stainless steel pipe of the present invention comprises, by mass%, C: less than 0.030%, Si: 0.20-1.00%, Mn: 0.5-7.0%, P: less than 0.040%, S: less than 0.020%, Al: less than 0.100%, Ni: 4.0-9.0%, Cr: 20.0-30.0%, Mo: 0.5-2.0%, Cu: 1.5-3.0%, N: 0.15-0.30%, V: 0.01-0.50%, Nb: 0.030-0.300%, Co: 0.10-0.50%, Sn: 0.001-0.050%, Ta: 0-0.100%, Ti: 0–0.100%, Zr: 0–0.100%, Hf: 0–0.100%, W: 0–0.200%, Sb: 0–0.100%, Ca: 0–0.020%, Mg: 0–0.020%, B: 0–0.020%, rare earth elements: 0–0.200%, with the balance being Fe and impurities; microstructure by volume fraction of 35.0–65.0% ferrite, 0 or more and less than 1.0% σ phase, with the balance being austenite; non-solid dissolved Nb content by mass percentage of 0.008% or more; ratio of non-solid dissolved Nb content to non-solid dissolved Al content of 1.0 or more; yield strength of 655 MPa or more.

[0021] Invention Effects

[0022] The duplex stainless steel pipe of the present invention can achieve high strength and excellent corrosion resistance. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of an example of a correction machine.

[0024] Figure 2 yes Figure 1 The front view of the correction machine is shown. Detailed Implementation

[0025] From a chemical composition perspective, the inventors have studied duplex stainless steel pipes that exhibit high strength and excellent corrosion resistance. As a result, the inventors believe that, in terms of mass percent, the following composition is achieved: C: 0.030% or less, Si: 0.20–1.00%, Mn: 0.5–7.0%, P: 0.040% or less, S: 0.020% or less, Al: 0.100% or less, Ni: 4.0–9.0%, Cr: 20.0–30.0%, Mo: 0.5–2.0%, Cu: 1.5–3.0%, N: 0.15–0.30%, V: 0.01–0.50%, Nb: 0.030–0.300%, C… Duplex stainless steel pipes containing o: 0.10~0.50%, Sn: 0.001~0.050%, Ta: 0~0.100%, Ti: 0~0.100%, Zr: 0~0.100%, Hf: 0~0.100%, W: 0~0.200%, Sb: 0~0.100%, Ca: 0~0.020%, Mg: 0~0.020%, B: 0~0.020%, rare earth elements: 0~0.200%, with the balance being Fe and impurities, can achieve high strength and excellent corrosion resistance.

[0026] Therefore, the inventors, from viewpoints other than chemical composition, further investigated methods to obtain higher strength in duplex stainless steel pipes that satisfy the above chemical composition. The inventors first studied the microstructure of the duplex stainless steel pipe. The microstructure of the duplex stainless steel pipe having the above chemical composition mainly consists of ferrite and austenite. The inventors discovered that if the microstructure of the duplex stainless steel pipe having the above chemical composition is a microstructure with a ferrite volume fraction of 35.0% to 65.0% and the balance being substantially austenite, the strength and corrosion resistance can be stably improved.

[0027] The inventors also believe that the strength of duplex stainless steel pipes can be further improved by precipitating Nb carbonitrides within them. Nb carbonitrides are fine precipitates. Therefore, the strength of duplex stainless steel pipes is improved through precipitation strengthening based on Nb carbonitrides. However, σ phase is also easily generated within the Nb carbonitride formation temperature range during the manufacturing process of duplex stainless steel pipes. σ phase reduces the corrosion resistance of duplex stainless steel pipes. Therefore, the inventors studied the relationship between the amount of Nb carbonitrides generated, the amount of σ phase generated, and the strength and corrosion resistance. The amount of Nb carbonitrides generated is related to the content of non-solid-dissolved Nb. In duplex stainless steel pipes satisfying the above chemical composition, if the content of non-solid-dissolved Nb is 0.008% or more by mass%, Nb carbonitrides are sufficiently generated, which can improve strength. Furthermore, by making the non-solid-dissolved Nb content 0.008% or more by mass, and the microstructure 35.0% to 65.0% by volume of ferrite, 0% or more and less than 1.0% of σ phase, with the balance being austenite, excellent corrosion resistance can be maintained.

[0028] However, it has been clearly established that even duplex stainless steel pipes that meet the above characteristics may still fail to achieve sufficient strength. Therefore, the inventors conducted further research. As a result, it was newly clarified that in duplex stainless steel pipes with the above chemical composition, Al nitrides are the main cause of the strength reduction.

[0029] In the above chemical composition, the N content is higher than the C content in order to obtain high strength through solid solution N. For this chemical composition, it is assumed that the N content in the Nb carbonitride will also be higher than the C content. That is, in the above chemical composition, N not only contributes to solid solution strengthening in the form of solid solution N, but also contributes to precipitation strengthening based on Nb carbonitride.

[0030] On the other hand, nitrogen (N) can also combine with al to form al nitrides. Al nitrides are coarser precipitates than nb carbonitrides. Therefore, al nitrides contribute almost nothing to precipitation strengthening. Thus, in duplex stainless steel pipes with the above chemical composition, if the amount of al nitrides formed is relatively greater than the amount of nb carbonitrides formed, insufficient dissolved nitrogen (N) and nb nitrides will occur. In this case, the strength of the duplex stainless steel pipe cannot be sufficiently improved.

[0031] Al nitride content is related to the content of non-solid dissolved Al. Therefore, the inventors further investigated the relationship between the content of non-solid dissolved Nb and non-solid dissolved Al and strength and corrosion resistance. As a result, the inventors found that if the ratio of non-solid dissolved Nb content to non-solid dissolved Al content, i.e., the non-solid dissolved Nb / Al ratio, is 1.0 or higher, sufficient Nb nitride and dissolved N content can be ensured, resulting in a balance between high strength and excellent corrosion resistance.

[0032] It should be noted that by utilizing mechanisms other than those described above, in duplex stainless steel pipes satisfying the above chemical composition and microstructure, it is possible to achieve both a yield strength of 655 MPa or higher and excellent corrosion resistance by ensuring that the non-solid-dissolved Nb content is 0.008% or higher by mass and the non-solid-dissolved Nb / Al ratio is 1.0 or higher. The examples described later demonstrate that by ensuring that the non-solid-dissolved Nb content is 0.008% or higher by mass and the non-solid-dissolved Nb / Al ratio is 1.0 or higher in duplex stainless steel pipes with the above chemical composition and microstructure, it is possible to achieve both a high yield strength of 655 MPa or higher and excellent corrosion resistance.

[0033] Based on the above insights, the duplex stainless steel pipe of this embodiment has the following structure.

[0034] The chemical composition (by mass%) of the first-component duplex stainless steel pipe is as follows: C: less than 0.030%, Si: 0.20–1.00%, Mn: 0.5–7.0%, P: less than 0.040%, S: less than 0.020%, Al: less than 0.100%, Ni: 4.0–9.0%, Cr: 20.0–30.0%, Mo: 0.5–2.0%, Cu: 1.5–3.0%, N: 0.15–0.30%, V: 0.01–0.50%, Nb: The microstructure of the above duplex stainless steel pipe, by volume, is 35.0-65.0% ferrite, 0.10-0.50% Co, 0.001-0.050% Sn, 0-0.100% Ta, 0-0.100% Ti, 0-0.100% Zr, 0-0.100% Hf, 0-0.100% W, 0-0.200% Sb, 0-0.100% Ca, 0-0.020% Mg, 0-0.020% B, 0-0.020% rare earth elements, and the balance is Fe and impurities. Furthermore, in the aforementioned duplex stainless steel pipes, the content of non-solid-dissolved Nb is 0.008% or more by mass, the ratio of non-solid-dissolved Nb content to non-solid-dissolved Al content is 1.0 or more, and the yield strength is 655 MPa or more.

[0035] The second type of duplex stainless steel pipe is the same as the first type of duplex stainless steel pipe. Its chemical composition contains one or more elements selected from the group consisting of Ta: 0.001-0.100%, Ti: 0.001-0.100%, Zr: 0.001-0.100%, Hf: 0.001-0.100%, W: 0.001-0.200%, Sb: 0.001-0.100%, Ca: 0.001-0.020%, Mg: 0.001-0.020%, B: 0.001-0.020%, and rare earth elements: 0.001-0.200%.

[0036] The duplex stainless steel pipe of this embodiment will now be described in detail. It should be noted that in the following description, the duplex stainless steel pipe will also be referred to simply as "pipe". Furthermore, unless otherwise specified, "%" related to elements refers to mass%.

[0037] [Features of the duplex stainless steel pipe in this embodiment]

[0038] The duplex stainless steel pipe of this embodiment satisfies the following features 1 to 4.

[0039] (Feature 1)

[0040] The chemical composition, by mass%, is as follows: C: less than 0.030%, Si: 0.20–1.00%, Mn: 0.5–7.0%, P: less than 0.040%, S: less than 0.020%, Al: less than 0.100%, Ni: 4.0–9.0%, Cr: 20.0–30.0%, Mo: 0.5–2.0%, Cu: 1.5–3.0%, N: 0.15–0.30%, V: 0.01–0.50%, Nb: 0.030– 0.300%, Co: 0.10~0.50%, Sn: 0.001~0.050%, Ta: 0~0.100%, Ti: 0~0.100%, Zr: 0~0.100%, Hf: 0~0.100%, W: 0~0.200%, Sb: 0~0.100%, Ca: 0~0.020%, Mg: 0~0.020%, B: 0~0.020%, rare earth elements: 0~0.200%, with the balance being Fe and impurities.

[0041] (Feature 2)

[0042] The microstructure consists of 35.0–65.0% ferrite by volume, 0% or more and less than 1.0% σ phase, with the balance being austenite.

[0043] (Feature 3)

[0044] The content of non-solid-soluble Nb is 0.008% or more by mass, and the ratio of the content of non-solid-soluble Nb to the content of non-solid-soluble Al is 1.0 or more.

[0045] (Feature 4)

[0046] The yield strength is above 655 MPa.

[0047] The following describes features 1 through 4.

[0048] [(Feature 1) Regarding chemical composition]

[0049] The chemical composition of the duplex stainless steel pipe in this embodiment contains the following elements.

[0050] C: Below 0.030%

[0051] The steel pipe inevitably contains carbon (C). Specifically, the lower limit of the C content exceeds 0%. C forms Cr carbides at grain boundaries, increasing the corrosion susceptibility of the grain boundaries. Therefore, if the C content exceeds 0.030%, the corrosion resistance of the steel pipe will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the C content is 0.030% or less.

[0052] The carbon content is preferably as low as possible. However, excessively reducing the carbon content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the carbon content is 0.001%, more preferably 0.002%, and even more preferably 0.005%.

[0053] The preferred upper limit for the C content is 0.029%, more preferably 0.028%, and even more preferably 0.027%.

[0054] Si: 0.20–1.00%

[0055] Silicon (Si) deoxidizes steel. If the Si content is less than 0.20%, the above-mentioned effect cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0056] On the other hand, if the Si content exceeds 1.00%, the toughness and hot workability of the steel pipe will decrease even if the contents of other elements are within the range of this embodiment.

[0057] Therefore, the Si content is 0.20–1.00%.

[0058] The preferred lower limit for the Si content is 0.21%, more preferably 0.22%, more preferably 0.25%, and more preferably 0.30%.

[0059] The preferred upper limit for the Si content is 0.95%, more preferably 0.92%, more preferably 0.91%, and more preferably 0.90%.

[0060] Mn: 0.5–7.0%

[0061] Manganese (Mn) deoxidizes and desulfurizes steel. Furthermore, Mn improves the hot workability of steel pipes. If the Mn content is less than 0.5%, even if the contents of other elements are within the range specified in this embodiment, the aforementioned effects cannot be fully achieved.

[0062] On the other hand, Mn segregates at grain boundaries along with impurities such as P and S. Therefore, if the Mn content exceeds 7.0%, the corrosion resistance of the steel pipe at high temperatures will decrease even if the contents of other elements are within the range of this embodiment.

[0063] Therefore, the Mn content is 0.5%–7.0%.

[0064] The preferred lower limit for Mn content is 0.6%, more preferably 0.8%, more preferably 1.0%, and more preferably 1.2%.

[0065] The preferred upper limit for Mn content is 6.8%, more preferably 6.5%, more preferably 6.3%, more preferably 6.2%, and more preferably 6.0%.

[0066] P: below 0.040%

[0067] Phosphorus (P) is unavoidably present. That is, the lower limit of P content exceeds 0%. P segregates at grain boundaries. Therefore, if the P content exceeds 0.040%, the corrosion resistance of the steel pipe will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the P content is 0.040% or less.

[0068] The phosphorus (P) content is preferably as low as possible. However, excessively reducing the P content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0069] The preferred upper limit for the P content is 0.038%, more preferably 0.036%, more preferably 0.035%, and more preferably 0.030%.

[0070] S: below 0.020%

[0071] The steel pipe inevitably contains sulfur (S). That is, the lower limit of the S content exceeds 0%. S segregates at grain boundaries. Therefore, if the S content exceeds 0.020%, the toughness and hot workability of the steel pipe will decrease, even if the contents of other elements are within the range of this embodiment. Therefore, the S content is 0.020% or less.

[0072] The sulfur (S) content is preferably as low as possible. However, excessively reducing the S content would significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the S content is 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.004%, and more preferably 0.005%.

[0073] The preferred upper limit for the sulfur content is 0.018%, more preferably 0.016%, and even more preferably 0.014%.

[0074] Al: below 0.100%

[0075] Aluminum (Al) is unavoidable. That is, the lower limit of Al content exceeds 0%. Al deoxidizes the steel. On the other hand, if the Al content exceeds 0.100%, coarse oxide inclusions are formed. Therefore, even if the contents of other elements are within the range of this embodiment, the toughness of the steel pipe will decrease. Therefore, the Al content is 0.100% or less.

[0076] The preferred lower limit for Al content is 0.001%, more preferably 0.005%, more preferably 0.007%, and more preferably 0.010%.

[0077] The preferred upper limit for Al content is 0.095%, more preferably 0.092%, more preferably 0.090%, and more preferably 0.085%.

[0078] It should be noted that the Al content in the chemical composition of the duplex stainless steel pipe in this embodiment refers to "acid-soluble Al", that is, the content of sol.Al.

[0079] Ni: 4.0–9.0%

[0080] Nickel (Ni) stabilizes the austenitic structure of the steel pipe. That is, Ni stabilizes the dual-phase structure of ferrite and austenite. Furthermore, Ni improves the corrosion resistance of the steel pipe. If the Ni content is less than 4.0%, even if the contents of other elements are within the range specified in this embodiment, the above-mentioned effects cannot be fully obtained.

[0081] On the other hand, if the Ni content exceeds 9.0%, the volume fraction of austenite becomes too high. In this case, even if the contents of other elements are within the range of this embodiment, the strength of the steel pipe will decrease.

[0082] Therefore, the Ni content is 4.0%–9.0%.

[0083] The preferred lower limit for Ni content is 4.1%, more preferably 4.3%, and even more preferably 4.5%.

[0084] The preferred upper limit for Ni content is 8.8%, more preferably 8.6%, more preferably 8.4%, more preferably 8.2%, and more preferably 8.0%.

[0085] Cr: 20.0–30.0%

[0086] Chromium (Cr) forms a passivation coating on the surface of the steel pipe as an oxide, improving the corrosion resistance of the steel pipe. Furthermore, Cr increases the volume fraction of the ferrite structure in the steel pipe. By obtaining a sufficient ferrite structure, the corrosion resistance of the steel pipe is stable. If the Cr content is less than 20.0%, even if the contents of other elements are within the range of this embodiment, the above-mentioned effects cannot be fully obtained.

[0087] On the other hand, if the Cr content exceeds 30.0%, the hot workability of the steel pipe will decrease even if the contents of other elements are within the range of this embodiment.

[0088] Therefore, the Cr content is 20.0%–30.0%.

[0089] The preferred lower limit for Cr content is 20.2%, more preferably 20.5%, more preferably 21.0%, and more preferably 21.5%.

[0090] The preferred upper limit for Cr content is 29.8%, more preferably 29.6%, more preferably 29.5%, more preferably 29.0%, and more preferably 28.5%.

[0091] Mo: 0.5–2.0%

[0092] Molybdenum (Mo) improves the corrosion resistance of steel pipes. Furthermore, Mo dissolves in the steel, increasing its strength. If the Mo content is less than 0.5%, even with the contents of other elements within the range specified in this embodiment, the aforementioned effects cannot be fully achieved.

[0093] On the other hand, if the Mo content exceeds 2.0%, the hot workability of the steel pipe will decrease even if the contents of other elements are within the range of this embodiment.

[0094] Therefore, the Mo content is 0.5%–2.0%.

[0095] The preferred lower limit for the Mo content is 0.6%, more preferably 0.7%, and even more preferably 0.8%.

[0096] The preferred upper limit for the Mo content is 1.9%, more preferably 1.8%, more preferably 1.7%, more preferably 1.6%, and more preferably 1.5%.

[0097] Cu: 1.5–3.0%

[0098] Copper (Cu) precipitates in the steel pipe, increasing its strength. If the Cu content is less than 1.5%, the above-mentioned effect cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0099] On the other hand, if the Cu content exceeds 3.0%, the hot workability of the steel pipe will decrease even if the contents of other elements are within the range of this embodiment.

[0100] Therefore, the Cu content is 1.5%–3.0%.

[0101] The preferred lower limit for Cu content is 1.6%, more preferably 1.8%, and even more preferably 2.0%.

[0102] The preferred upper limit for Cu content is 2.9%, more preferably 2.8%, and even more preferably 2.7%.

[0103] N: 0.15–0.30%

[0104] Nitrogen (N) dissolves in the steel pipe, thereby increasing its strength. Furthermore, N combines with Nb to form Nb carbonitrides, which further enhance the steel pipe's strength through precipitation strengthening. N also stabilizes the austenitic structure of the steel pipe. If the N content is less than 0.15%, even with the contents of other elements within the range specified in this embodiment, the aforementioned effects cannot be fully achieved.

[0105] On the other hand, if the nitrogen content exceeds 0.30%, the toughness and hot workability of the steel pipe will decrease even if the contents of other elements are within the range of this embodiment.

[0106] Therefore, the nitrogen content is 0.15–0.30%.

[0107] The preferred lower limit for the nitrogen content is 0.16%, more preferably 0.18%, and even more preferably 0.20%.

[0108] The preferred upper limit for the nitrogen content is 0.29%, more preferably 0.28%, more preferably 0.27%, more preferably 0.26%, and more preferably 0.25%.

[0109] V: 0.01~0.50%

[0110] Vanadium (V) increases the strength of steel pipes. If the V content is less than 0.01%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0111] On the other hand, if the V content exceeds 0.50%, even if the contents of other elements are within the range of this embodiment, the strength of the steel pipe will become too high, and the toughness and hot workability of the steel pipe will decrease.

[0112] Therefore, the V content is 0.01%–0.50%.

[0113] The preferred lower limit for the V content is 0.02%, more preferably 0.03%, more preferably 0.05%, more preferably 0.07%, and more preferably 0.10%.

[0114] The preferred upper limit for the V content is 0.48%, more preferably 0.47%, more preferably 0.45%, more preferably 0.42%, and more preferably 0.40%.

[0115] Nb: 0.030~0.300%

[0116] Niobium (Nb) forms carbonitrides, increasing the strength of the steel pipe. If the Nb content is less than 0.030%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0117] On the other hand, if the Nb content exceeds 0.300%, the strength of the steel pipe will become too high and the toughness of the steel pipe will decrease, even if the contents of other elements are within the range of this embodiment.

[0118] Therefore, the Nb content is 0.030–0.300%.

[0119] The preferred lower limit for Nb content is 0.031%, more preferably 0.033%, more preferably 0.035%, 0.037%, and more preferably 0.040%.

[0120] The preferred upper limit for Nb content is 0.294%, more preferably 0.290%, more preferably 0.280%, and more preferably 0.250%.

[0121] Co: 0.10–0.50%

[0122] Cobalt (Co) forms a coating on the surface of the steel pipe, improving its corrosion resistance. Furthermore, Co improves the hardenability of the steel pipe, stabilizing its strength. If the Co content is less than 0.10%, even if the contents of other elements are within the range specified in this embodiment, the above-mentioned effects cannot be fully achieved.

[0123] On the other hand, if the Co content exceeds 0.50%, the manufacturing cost will increase significantly even if the contents of other elements are within the range of this embodiment.

[0124] Therefore, the Co content is 0.10–0.50%.

[0125] The preferred lower limit for the Co content is 0.11%, more preferably 0.13%, and even more preferably 0.15%.

[0126] The preferred upper limit for the Co content is 0.48%, more preferably 0.45%, more preferably 0.40%, and more preferably 0.35%.

[0127] Sn: 0.001~0.050%

[0128] Tin (Sn) improves the corrosion resistance of steel pipes. If the Sn content is less than 0.001%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0129] On the other hand, if the Sn content exceeds 0.050%, even if the contents of other elements are within the range of this embodiment, liquefaction embrittlement cracks will occur at the grain boundaries, reducing the hot workability of the steel pipe.

[0130] Therefore, the Sn content is 0.001–0.050%.

[0131] The preferred lower limit for Sn content is 0.002%, more preferably 0.003%, more preferably 0.005%, more preferably 0.006%, more preferably 0.008%, and more preferably 0.010%.

[0132] The preferred upper limit for the Sn content is 0.048%, more preferably 0.045%, more preferably 0.043%, and more preferably 0.040%.

[0133] The chemical composition of the duplex stainless steel pipe of this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are introduced into the duplex stainless steel pipe during industrial manufacturing from raw materials such as ore, waste, or the manufacturing environment, and are considered substances that are permissible within a range that do not adversely affect the duplex stainless steel pipe of this embodiment.

[0134] [Regarding arbitrary elements]

[0135] The chemical composition of the above-mentioned duplex stainless steel pipe may also contain one or more elements selected from the group consisting of Ta: 0-0.100%, Ti: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100%, W: 0-0.200%, Sb: 0-0.100%, Ca: 0-0.020%, Mg: 0-0.020%, B: 0-0.020%, and rare earth elements: 0-0.200% to replace a portion of Fe.

[0136] The following is an explanation of these arbitrary elements.

[0137] [Group 1 (Ta, Ti, Zr, Hf, and W)]

[0138] The duplex stainless steel pipe of this embodiment may contain one or more elements selected from the group consisting of Ta, Ti, Zr, Hf, and W to replace a portion of the Fe. These elements are arbitrary and improve the strength of the steel pipe.

[0139] Ta: 0~0.100%

[0140] Tantalum (Ta) can be any element, or it can be absent. That is, the Ta content can be 0%.

[0141] When present in the presence of Ta (acid), specifically when the Ta content exceeds 0%, Ta forms carbonitrides, increasing the strength of the steel pipe. Even a small amount of Ta can achieve these effects to some extent.

[0142] However, if the Ta content exceeds 0.100%, the strength of the steel pipe will become too high and the toughness of the steel pipe will decrease, even if the contents of other elements are within the range of this embodiment.

[0143] Therefore, the Ta content is 0–0.100%.

[0144] The preferred lower limit for Ta content is 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%.

[0145] The preferred upper limit for Ta content is 0.080%, more preferably 0.070%, more preferably 0.060%, more preferably 0.050%, and more preferably 0.040%.

[0146] Ti: 0~0.100%

[0147] Titanium (Ti) can be any element, or it can be absent. That is, the Ti content can be 0%.

[0148] When Ti is present, i.e., the Ti content exceeds 0%, Ti forms carbonitrides, increasing the strength of the steel pipe. Even a small amount of Ti can achieve this effect to some extent.

[0149] However, if the Ti content exceeds 0.100%, the strength of the steel pipe will become too high and the toughness of the steel pipe will decrease, even if the contents of other elements are within the range of this embodiment.

[0150] Therefore, the Ti content is 0–0.100%.

[0151] The preferred lower limit for Ti content is 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%.

[0152] The preferred upper limit for Ti content is 0.098%, more preferably 0.095%, more preferably 0.090%, more preferably 0.085%, more preferably 0.080%, more preferably 0.075%, and more preferably 0.070%.

[0153] Zr: 0~0.100%

[0154] Zirconium (Zr) can be any element, or it can be absent. That is, the Zr content can be 0%.

[0155] When Zr is present, i.e., the Zr content exceeds 0%, Zr forms carbonitrides, increasing the strength of the steel pipe. Even a small amount of Zr can achieve this effect to some extent.

[0156] However, if the Zr content exceeds 0.100%, the strength of the steel pipe will become too high and the toughness of the steel pipe will decrease, even if the contents of other elements are within the range of this embodiment.

[0157] Therefore, the Zr content is 0–0.100%.

[0158] The preferred lower limit for Zr content is 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%.

[0159] The preferred upper limit for Zr content is 0.090%, more preferably 0.080%, more preferably 0.070%, more preferably 0.060%, and more preferably 0.050%.

[0160] Hf: 0~0.100%

[0161] Hafnium (Hf) can be any element, or it can be absent. That is, the Hf content can be 0%.

[0162] When Hf is present, i.e., the Hf content exceeds 0%, Hf forms carbonitrides, increasing the strength of the steel pipe. Even a small amount of Hf can achieve this effect to some extent.

[0163] However, if the Hf content exceeds 0.100%, the strength of the steel pipe will become too high and the toughness of the steel pipe will decrease, even if the contents of other elements are within the range of this embodiment.

[0164] Therefore, the Hf content is 0–0.100%.

[0165] The preferred lower limit for Hf content is 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%.

[0166] The preferred upper limit for Hf content is 0.095%, more preferably 0.090%, more preferably 0.085%, more preferably 0.080%, more preferably 0.070%, more preferably 0.060%, and more preferably 0.050%.

[0167] W: 0~0.200%

[0168] Tungsten (W) can be any element, or it can be absent. That is, the W content can be 0%.

[0169] When W is present, i.e., when the W content exceeds 0%, W forms carbonitrides, increasing the strength of the steel pipe. Even a small amount of W can achieve this effect to some extent. However, if the W content exceeds 0.200%, the strength of the steel pipe becomes excessively high, and its toughness decreases, even if the contents of other elements are within the range of this embodiment.

[0170] Therefore, the W content is 0–0.200%.

[0171] The preferred lower limit for W content is 0.001%, more preferably 0.003%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%.

[0172] The preferred upper limit for W content is 0.180%, more preferably 0.150%, more preferably 0.130%, more preferably 0.100%, more preferably 0.080%, and more preferably 0.050%.

[0173] [Group 2: Sb]

[0174] The chemical composition of the duplex stainless steel pipe in this embodiment may also contain Sb to replace a portion of Fe.

[0175] Sb: 0~0.100%

[0176] Antimony (Sb) can be any element, or it can be absent. That is, the Sb content can be 0%.

[0177] When present, that is, when the Sb content exceeds 0%, Sb improves the corrosion resistance of steel pipes. Even a small amount of Sb can achieve this effect to some extent.

[0178] However, if the Sb content exceeds 0.100%, even if the contents of other elements are within the range of this embodiment, the ductility of the steel pipe at high temperature will decrease, and the hot workability of the steel pipe will decrease.

[0179] Therefore, the Sb content is 0–0.100%.

[0180] The preferred lower limit for Sb content is 0.001%, more preferably 0.003%, more preferably 0.005%, more preferably 0.010%, and more preferably 0.015%.

[0181] The preferred upper limit for Sb content is 0.090%, more preferably 0.085%, more preferably 0.080%, more preferably 0.070%, more preferably 0.060%, and more preferably 0.050%.

[0182] [Group 3 (Ca, Mg, B, and rare earth elements)]

[0183] The chemical composition of the duplex stainless steel pipe of this embodiment may also contain one or more elements selected from the group consisting of Ca, Mg, B and rare earth elements to replace a portion of Fe. These elements are arbitrary elements, which improve the hot workability of the steel pipe.

[0184] Ca: 0–0.020%

[0185] Calcium (Ca) can be any element, or it can be absent. That is, the Ca content can be 0%.

[0186] When Ca is present (i.e., the Ca content exceeds 0%), Ca fixes the sulfur in the steel pipe in the form of sulfides, improving the hot workability of the steel pipe. Even a small amount of Ca can achieve the above effect to some extent.

[0187] However, if the Ca content exceeds 0.020%, the oxides in the steel pipe become coarser. Therefore, even if the contents of other elements are within the range of this embodiment, the toughness of the steel pipe will decrease.

[0188] Therefore, the Ca content is 0–0.020%.

[0189] The preferred lower limit for Ca content is 0.001%, more preferably 0.002%, more preferably 0.003%, and more preferably 0.005%.

[0190] The preferred upper limit for Ca content is 0.018%, more preferably 0.016%, more preferably 0.014%, more preferably 0.012%, and more preferably 0.010%.

[0191] Mg: 0–0.020%

[0192] Magnesium (Mg) can be any element, or it can be absent. That is, the Mg content can be 0%.

[0193] When Mg is present (i.e., the Mg content exceeds 0%), Mg fixes the sulfur in the steel pipe in the form of sulfides, improving the hot workability of the steel pipe. Even a small amount of Mg can achieve the above effects to some extent.

[0194] However, if the Mg content exceeds 0.020%, the oxides in the steel become coarser. Therefore, even if the contents of other elements are within the range of this embodiment, the toughness of the steel pipe will decrease.

[0195] Therefore, the Mg content is 0–0.020%.

[0196] The preferred lower limit for Mg content is 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%, and more preferably 0.006%.

[0197] The preferred upper limit for Mg content is 0.018%, more preferably 0.016%, and even more preferably 0.015%.

[0198] B: 0~0.020%

[0199] Boron (B) can be any element, or it can be absent. That is, the B content can be 0%.

[0200] When present in the presence of B (i.e., when the B content exceeds 0%), B inhibits the segregation of S towards grain boundaries in the steel pipe, thereby improving the hot workability of the steel pipe. Even a small amount of B can achieve the above effects to a certain extent.

[0201] However, if the boron content exceeds 0.020%, boron nitride (BN) is formed. Therefore, even if the contents of other elements are within the range of this embodiment, the toughness of the steel pipe will decrease.

[0202] Therefore, the B content is 0–0.020%.

[0203] The preferred lower limit for the content of B is 0.001%, more preferably 0.002%, more preferably 0.003%, and more preferably 0.005%.

[0204] The preferred upper limit for the content of B is 0.018%, more preferably 0.016%, more preferably 0.014%, more preferably 0.012%, and more preferably 0.010%.

[0205] Rare earth elements: 0–0.200%

[0206] Rare earth elements (REM) can be any element, or they can be absent. That is, the REM content can be 0%.

[0207] When REM is present, i.e., when the REM content exceeds 0%, REM fixes the sulfur in the steel pipe in the form of sulfides, improving the hot workability of the steel pipe. Even a small amount of REM can achieve the above effects to some extent.

[0208] However, if the REM content exceeds 0.200%, the oxides in the steel pipe become coarser. Therefore, even if the contents of other elements are within the range of this embodiment, the toughness of the steel pipe will decrease.

[0209] Therefore, the REM content is 0–0.200%.

[0210] The preferred lower limit for REM content is 0.001%, more preferably 0.005%, more preferably 0.008%, more preferably 0.010%, and more preferably 0.020%.

[0211] The preferred upper limit for REM content is 0.180%, more preferably 0.160%, more preferably 0.140%, more preferably 0.120%, more preferably 0.100%, more preferably 0.080%, more preferably 0.060%, and more preferably 0.050%.

[0212] It should be noted that REM in this specification refers to one or more elements selected from the group consisting of scandium (Sc) atomic number 21, yttrium (Y) atomic number 39, and lanthanum (La) atomic number 57 to lutetium (Lu) atomic number 71, which are lanthanide elements. Furthermore, REM content in this specification indicates the total content of these elements.

[0213] [(Feature 2) Regarding microstructure]

[0214] The microstructure of the duplex stainless steel pipe of this embodiment consists of 35.0% to 65.0% ferrite by volume, 0% to less than 1.0% σ phase, and the balance being austenite. Other microstructures besides ferrite, austenite, and σ phase are negligible in the microstructure. Specifically, the microstructure of the duplex stainless steel pipe of this embodiment may contain trace amounts of precipitates, inclusions, etc., in addition to ferrite and austenite. However, in the chemical composition of the duplex stainless steel pipe of this embodiment, the volume fraction of precipitates, inclusions, etc., is negligible compared to the volume fractions of ferrite, austenite, and σ phase.

[0215] In the microstructure of the duplex stainless steel pipe of this embodiment, the ferrite volume fraction is 35.0% to 65.0%. If the ferrite volume fraction is too low, the yield strength and / or corrosion resistance of the steel pipe may decrease. On the other hand, if the ferrite volume fraction is too high, the toughness and / or hot workability of the steel pipe may decrease.

[0216] Therefore, in the microstructure of the duplex stainless steel pipe of this embodiment, the volume fraction of ferrite is 35.0 to 65.0%.

[0217] The preferred lower limit for the volume fraction of ferrite is 36.0%, and more preferably 37.0%.

[0218] The preferred upper limit for the volume fraction of ferrite is 64.0%, and more preferably 63.0%.

[0219] In the microstructure of the duplex stainless steel pipe of this embodiment, the σ phase reduces corrosion resistance. Therefore, the volume fraction of the σ phase is preferably as small as possible. If the volume fraction of the σ phase is 1.0% or more, the corrosion resistance of the duplex stainless steel pipe decreases. Therefore, the volume fraction of the σ phase is 0 or more and less than 1.0%.

[0220] The volume fraction of the σ phase is preferably as low as possible, and most preferably 0%. However, if the volume fraction of the σ phase is reduced excessively, the manufacturing cost will increase significantly. Therefore, the preferred lower limit for the volume fraction of the σ phase is more than 0%, and more preferably 0.1%.

[0221] The balance of the microstructure is austenite. When the microstructure consists of 35.0% to 65.0% ferrite by volume, 0% to less than 1.0% σ phase, and the balance is austenite, high strength and excellent corrosion resistance can be obtained, provided that other characteristics 1, 3 and 4 are satisfied.

[0222] [Methods for determining ferrite volume fraction and σ phase volume fraction]

[0223] The volume fraction of ferrite in duplex stainless steel tubes can be determined according to the method of ASTM E562 (2019).

[0224] Specifically, a microstructure observation specimen with an observation surface of 5 mm in the axial direction and 5 mm in the circumferential direction is collected from the central portion of the wall thickness of the duplex stainless steel pipe. In this specification, the circumferential direction of the steel pipe refers to the direction perpendicular to both the axial direction and the diameter direction. It should be noted that the size of the specimen is not particularly limited as long as the above observation surfaces can be obtained.

[0225] The observation surface of the test specimen was mirror-polished. Electrolytic etching was then performed on the mirror-polished observation surface in a 7% potassium hydroxide etching solution to expose the tissue. The exposed tissue was observed using an optical microscope with 10 fields of view. The area of ​​each field of view was set to 1.00 mm². 2 (Magnification 100x). Ferrite and austenite are determined based on contrast in each field of view. In the case of electrolytic corrosion in a 7% potassium hydroxide etchant, areas of low brightness correspond to ferrite, and areas of high brightness correspond to austenite. Therefore, those skilled in the art can easily determine ferrite and austenite based on contrast.

[0226] The determined ferrite area fraction was determined using the point method according to ASTM E562 (2019). The arithmetic mean of the ferrite area fractions obtained from each field of view (a total of 10) was defined as the ferrite volume fraction (%). The ferrite volume fraction (%) is the first decimal place obtained by rounding the obtained value to two decimal places.

[0227] The volume fraction of the σ phase in duplex stainless steel pipes is determined by the following method.

[0228] The tissue surfaces were observed using an optical microscope with five fields of view. The area of ​​each field of view was set to 0.0625 mm². 2 (Magnification 400x, 250μm × 250μm). The σ phase was determined based on contrast in each field of view. In the observation surface after electrolytic etching in a 7% potassium hydroxide solution, the σ phase could be identified as a black area with lower brightness than other tissues. It should be noted that elemental concentration analysis (EDS analysis) can also be performed in each field of view to determine the σ phase. When performing elemental concentration analysis, the σ phase was determined using the following method: Particles were identified based on contrast in each field of view. EDS analysis was performed on the identified particles. In the EDS analysis, the accelerating voltage was set to 20kV, and N, Mo, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, and Nb were used as target elements for quantification. Based on the EDS analysis results of each particle, if the Cr content in the particle was ≥35.0% and the Mo content was ≥3.0% by mass, the particle was identified as the σ phase.

[0229] Calculate the area of ​​the determined σ phase. Based on the total area of ​​the σ phase in the five fields of view and the total area of ​​the five fields of view, calculate the area ratio (%) of the σ phase. The calculated area ratio (%) of the σ phase is regarded as the volume ratio (%) of the σ phase. In this embodiment, the volume ratio (%) of the σ phase is set as the value of the first decimal place obtained by rounding the obtained value to the second decimal place.

[0230] [(Feature 3) Regarding the content of non-solid-dissolved Nb and the Nb / Al ratio]

[0231] In the duplex stainless steel pipe of this embodiment, the non-solid-dissolved Nb content is 0.008% or more by mass, and the ratio of the non-solid-dissolved Nb content to the non-solid-dissolved Al content, i.e., the non-solid-dissolved Nb / Al ratio, is 1.0 or more. These matters will be explained below.

[0232] [Regarding the content of non-solid-dissolved Nb]

[0233] Non-solid-dissolved Nb is not dissolved in the base material and is contained in the precipitates. For the duplex stainless steel pipe of this embodiment, a high yield strength of 655 MPa or higher can be obtained by sufficiently generating Nb carbonitrides in the pipe. If the non-solid-dissolved Nb content is less than 0.008% by mass, Nb carbonitrides are not sufficiently generated in the pipe. Therefore, sufficient yield strength cannot be obtained. Therefore, the non-solid-dissolved Nb content is 0.008% or more by mass.

[0234] The preferred lower limit of the non-solid-solid Nb content, in mass %, is 0.009%, more preferably 0.010%, more preferably 0.012%, more preferably 0.015%, more preferably 0.020%, and more preferably 0.025%.

[0235] There is no particular upper limit to the content of non-solid-soluble Nb. However, if the chemical composition meets the requirements of Characteristic 1, the upper limit for the content of non-solid-soluble Nb is, for example, 0.300%, or, for example, 0.250%.

[0236] [Regarding the non-solid-solution Nb / Al ratio]

[0237] The ratio of non-solid-dissolved Nb content to non-solid-dissolved Al content, i.e., the non-solid-dissolved Nb / Al ratio, can be defined by the following formula.

[0238] Non-solid-soluble Nb / Al ratio = Non-solid-soluble Nb content (mass%) / Non-solid-soluble Al content (mass%)

[0239] The non-solid-solution Nb / Al ratio is an indicator of the ratio of the amount of Nb carbonitrides formed in a steel pipe to the amount of Al nitrides formed. As mentioned above, Nb carbonitrides in steel pipes improve the strength of the pipe through precipitation strengthening. On the other hand, Al nitrides are coarser than Nb carbonitrides and do not contribute to precipitation strengthening. Al nitrides further reduce the amount of dissolved N in the steel pipe and also reduce the amount of dissolved N used to form Nb carbonitrides. Therefore, even if a certain amount of Nb carbonitrides is formed, if the amount of Al nitrides formed is excessive relative to the amount of Nb carbonitrides formed, the amount of dissolved N and Nb carbonitrides formed in the steel pipe cannot be sufficiently obtained. As a result, duplex stainless steel pipes sometimes cannot achieve sufficient strength.

[0240] If, in duplex stainless steel pipes that satisfy characteristics 1 and 2, the non-solid-dissolved Nb content is 0.008% or more, and the non-solid-dissolved Nb / Al ratio is 1.0 or more, then sufficient Nb carbonitride formation and sufficient dissolved N content can be ensured in the pipe. As a result, the yield strength of the duplex stainless steel pipe can be increased to 655 MPa or more.

[0241] The preferred lower limit for the non-solid-solid Nb / Al ratio is 1.1, further preferably 1.2, further preferably 1.5, further preferably 2.0, further preferably 2.5, and further preferably 3.0.

[0242] There is no particular upper limit to the non-solid-soluble Nb / Al ratio. Under the condition of chemical composition satisfying characteristic 1, the upper limit of the non-solid-soluble Nb / Al ratio is, for example, 70.0, or 65.0.

[0243] [Methods for determining non-solid-soluble Nb content and non-solid-soluble Nb / Al ratio]

[0244] The content of non-solid-dissolved Nb and the non-solid-dissolved Nb / Al ratio were determined by the following method.

[0245] Cylindrical test specimens with a diameter of 8 mm and a length of 50 mm were collected from duplex stainless steel pipes. Specifically, cylindrical test specimens were fabricated with the central portion of the pipe wall thickness as the central axis. The axial direction of the cylindrical test specimen was set as the pipe axis direction.

[0246] The cylindrical test piece was subjected to constant current electrolysis using a 10% AA-based solution (containing 10% acetylacetone, 1% tetramethylammonium chloride, and 89% methanol by volume fraction).

[0247] First, pre-electrolysis was performed to remove the deposits (oxide scale and impurities) from the surface of the cylindrical test piece. Pre-electrolysis was conducted at room temperature (25°C) with a current of 1000 mA on a region extending from the surface of the oxide scale to a depth of approximately 100 μm. After pre-electrolysis, the cylindrical test piece was immersed in an alcohol solution. The immersed cylindrical test piece was then ultrasonically cleaned to remove the deposits from its surface. The mass of the cylindrical test piece after the deposits were removed, i.e., the mass of the cylindrical test piece before constant current electrolysis, was measured.

[0248] Next, constant current electrolysis was performed on the cylindrical test piece. Specifically, a new 10% AA-based solution was prepared. Then, using the new 10% AA-based solution, the current density was maintained at 20 mA / cm² at room temperature. 2 Electrolysis was then performed. After constant current electrolysis, the cylindrical test piece was immersed in an alcohol solution, and then ultrasonically cleaned to remove the deposits on the surface of the cylindrical test piece. The mass of the cylindrical test piece after the deposits were removed was measured and used as the mass of the cylindrical test piece after constant current electrolysis.

[0249] The residue was extracted by suction filtration using a filter with a mesh size of 0.2 μm to extract the 10% AA solution used in constant current electrolysis and the alcohol solution used in subsequent ultrasonic cleaning.

[0250] Chemical elemental analysis was performed on the extracted residue. Specifically, the residue was dissolved in acid to obtain a solution. ICP-AES was used to perform chemical elemental analysis on the solution to quantitatively analyze Nb and Al. Based on the mass of Nb and Al obtained through quantitative analysis, and the mass difference of the cylindrical test piece before and after constant current electrolysis, the Nb content (mass%) and Al content (mass%) in the residue were determined. The Nb content in the obtained residue was taken as the non-solid dissolved Nb content (mass%). The Al content in the obtained residue was taken as the non-solid dissolved Al content (mass%). Based on the non-solid dissolved Nb content and non-solid dissolved Al content, the non-solid dissolved Nb / Al ratio was determined.

[0251] [(Feature 4) Regarding yield strength]

[0252] The duplex stainless steel pipe of this embodiment has a yield strength of 655 MPa or higher (95 ksi or higher). The duplex stainless steel pipe of this embodiment satisfies characteristics 1 to 3. As a result, excellent corrosion resistance and a high yield strength of 655 MPa or higher can be obtained.

[0253] The preferred lower limit of the yield strength of the duplex stainless steel pipe in this embodiment is 660 MPa or above, more preferably 665 MPa, more preferably 670 MPa, and more preferably 675 MPa.

[0254] The upper limit of the yield strength of the duplex stainless steel pipe in this embodiment is not particularly limited, for example, it is 800 MPa.

[0255] [Methods for determining yield strength]

[0256] The yield strength of the duplex stainless steel pipe in this embodiment was determined by performing a tensile test according to the method of ASTM E8 / E8M (2022).

[0257] Specifically, an arc-shaped test piece is collected from the duplex stainless steel pipe of this embodiment. The arc-shaped test piece, for example, has a thickness equal to the wall thickness of the pipe, a width of 25.4 mm, and a gauge length of 50.8 mm. The long side of the arc-shaped test piece is parallel to the pipe axis direction.

[0258] A tensile test was conducted at room temperature (25°C) in the atmosphere using an arc-shaped test piece. In this embodiment, the 0.2% residual deformation stress obtained through the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is an integer value obtained by rounding the first decimal place of the obtained value.

[0259] [The effect of duplex stainless steel pipe in this embodiment]

[0260] The duplex stainless steel pipe of this embodiment satisfies features 1 to 4. Therefore, the duplex stainless steel pipe of this embodiment can achieve a high yield strength of 655 MPa or more (95 ksi or more) and excellent corrosion resistance.

[0261] [Regarding corrosion resistance]

[0262] In this embodiment, the corrosion resistance of duplex stainless steel pipes is evaluated by the following method.

[0263] [Corrosion Resistance Evaluation Methods]

[0264] A test piece for a four-point bending test is taken from the duplex stainless steel pipe of this embodiment. The test piece is, for example, 2 mm thick, 10 mm wide, and 75 mm long. The test piece is made from the center of the pipe wall. In this case, the length direction of the test piece is parallel to the pipe axis direction.

[0265] As the test solution, a 20% by mass sodium chloride aqueous solution adjusted to pH 4.0 was used. According to ASTM G39-99 (2021), a stress equivalent to 90% of the actual yield stress was applied to the test specimen through a four-point bending test. The stressed test specimen, along with the test fixture, was sealed in an autoclave. The test solution was injected into the autoclave, retaining the gas phase, to prepare a test bath. After degassing the test bath, a mixture of 0.2 bar H₂S gas and 30 bar CO₂ gas was pressurized and introduced into the autoclave, and the test bath was stirred to saturate the gas mixture. The autoclave was sealed, and the test bath was then stirred at 90°C for 720 hours.

[0266] In this embodiment, after 720 hours in the above-described test environment, observation was performed using a 10x magnifying glass to confirm the presence of cracks. If cracks were suspected during observation with a magnifying glass, further observation was performed using a 100x optical microscope to confirm the presence of cracks. When no cracks were confirmed, the evaluation was "excellent corrosion resistance is achieved".

[0267] [Shapes of duplex stainless steel pipes]

[0268] The duplex stainless steel pipe in this embodiment can be either a welded pipe or a seamless pipe. Preferably, the duplex stainless steel pipe in this embodiment is a seamless pipe.

[0269] [Manufacturing Method]

[0270] An example of a method for manufacturing a duplex stainless steel pipe having the above-described configuration according to this embodiment will be described. It should be noted that the method for manufacturing a duplex stainless steel pipe according to this embodiment is not limited to the method described below.

[0271] The manufacturing method of the duplex stainless steel pipe of this embodiment includes the following steps.

[0272] (Process 1) Blank preparation process

[0273] (Process 2) Hot working process

[0274] (Step 3) Solution treatment process

[0275] (Process 4) Correction Process

[0276] (Step 5) Aging heat treatment process

[0277] The following is a description of each process.

[0278] [(Process 1) Blank Preparation Process]

[0279] In the billet preparation process, a billet with a chemical composition satisfying characteristic 1 is prepared. The billet can be prepared either through manufacturing or by purchasing from a third party. That is, there is no particular limitation on the method of preparing the billet.

[0280] In the case of manufacturing billets, the manufacturing process may be carried out, for example, by the following methods: Molten steel with the aforementioned chemical composition is produced. A billet (slab, large billet, or small billet) is manufactured using the molten steel through continuous casting. Alternatively, a steel ingot (cast ingot) may be manufactured using the molten steel through ingot casting. Small billets may also be manufactured by initial rolling of the slab, large billet, or ingot as needed. Billets are manufactured through the above processes.

[0281] [(Process 2) Hot working process]

[0282] In the hot working process, the billet prepared in the above-mentioned billet preparation process is hot-worked to manufacture a tube blank. Hot working can be hot forging, hot extrusion, or hot rolling. There are no particular limitations on the method of hot working; any known method can be used.

[0283] As a hot working method, for example, hot extrusion such as the Ugine-Sejournet method or the Ehrhardt Push Bench method can be performed, or piercing rolling using the Mannesman method can be performed as a form of hot rolling. It should be noted that hot working can be performed once or multiple times. For example, the billet can be subjected to the aforementioned piercing rolling, followed by the aforementioned hot extrusion. Alternatively, the billet can be subjected to the aforementioned piercing rolling, followed by stretch rolling as a form of hot rolling. That is, in the hot working process, hot working is performed using known methods to manufacture the tube blank. It should be noted that the heating temperature during hot working is, for example, 1000°C to 1280°C.

[0284] [(Process 3) Solution treatment process]

[0285] In the solution treatment process, the tube blank after the heat treatment process is subjected to solution treatment. Specifically, the tube blank is placed in a heat treatment furnace and heated. Then, the tube blank is held at the desired temperature (solution temperature) and then rapidly cooled. The solution treatment meets the following conditions.

[0286] (Condition 1)

[0287] The average heating rate HR1 during the heating of the tube blank at 700℃~900℃ is set to 0.25℃ / second or higher.

[0288] (Condition 2)

[0289] Set the solution temperature T1 to 980℃~1100℃.

[0290] The following is an explanation of each condition.

[0291] (Regarding condition 1)

[0292] During solution treatment heating, Al nitrides readily form in the temperature range of 700°C to 900°C. By minimizing the residence time in this temperature range, Al nitride formation in the tube blank is suppressed, resulting in an increased non-solidified Nb / Al ratio in the manufactured tube blank. A sufficiently high average heating rate HR1 of 0.25°C / second or higher in the 700°C to 900°C range can significantly improve the non-solidified Nb / Al ratio in the manufactured tube blank. An upper limit for the average heating rate HR1 is, for example, 0.60°C / second.

[0293] (Regarding condition 2)

[0294] The solution treatment temperature T1 affects the ferrite volume fraction in the microstructure of the steel pipe. If the solution treatment temperature T1 is too low, the ferrite volume fraction of the duplex stainless steel pipe will be less than 35.0%, sometimes resulting in reduced strength and / or corrosion resistance. Conversely, if the solution treatment temperature T1 is too high, the ferrite volume fraction of the duplex stainless steel pipe after solution treatment will reach over 65.0%, which may lead to a decrease in corrosion resistance. A solution treatment temperature T1 of 980℃ to 1100℃ achieves a suitable ferrite volume fraction for the duplex stainless steel pipe.

[0295] It should be noted that the holding time t1 at the solution temperature T1 is, for example, 10 to 180 minutes. Here, the solution temperature T1 refers to the temperature (°C) of the heat treatment furnace used to perform the solution treatment. The holding time t1 at the solution temperature T1 refers to the time (minutes) the tube blank is held at the solution temperature.

[0296] [(Process 4) Correction Process]

[0297] In the straightening process, the tube blank that has undergone the above-mentioned solution treatment is straightened at room temperature. The straightening process imparts strain to the tube blank, thereby generating sufficient Nb carbonitrides in the next aging heat treatment process.

[0298] Figure 1 This is a schematic diagram of a rotary straightening machine, used as an example of a straightening machine. (Refer to...) Figure 1 The rotary straightening machine has multiple rolling mills ST1 to STn (n is a natural number of 3 or more). Figure 1In this system, the rotary straightening mill has four rolling mills. However, there is no particular limitation on the number of rolling mills, as long as there are three or more. For example, straightening can be performed using three rolling mills, or it can be performed using five or more rolling mills. Each rolling mill has a pair of tilting rolls. The rolling mills are arranged in a row along the rolling line PL through which the billet passes. Among the multiple rolling mills, the tilting rolls of the rolling mills other than rolling mill ST2 are arranged on the rolling line PL, while the tilting rolls of rolling mill ST2 are offset from the rolling line PL.

[0299] Figure 2 yes Figure 1 The front view of the correction machine. Figure 2 The diagram shown on the left is a cross-sectional view perpendicular to the axial direction of the tube blank before the correction was performed. Figure 2 The diagram shown on the right is a front view of the rolling mill with the smallest roll gap (DB). In this embodiment, roll straightening is performed. Roll straightening refers to the straightening process of applying pressure to the tube blank to deform it into an elliptical shape. (Refer to...) Figure 2 The amount of reduction applied to the tube blank by the mill with the smallest roll gap DB is defined as the rolling amount δc (mm). The rolling amount δc can be obtained by subtracting the roll gap DB of the mill with the smallest roll gap DB from the outer diameter DA of the tube blank before correction.

[0300] The straightening process must meet the following conditions.

[0301] (Condition 3)

[0302] Set the rolling amount δc to 3mm or more.

[0303] (Regarding condition 3)

[0304] If the rolling amount δc during straightening is too small, sufficient strain cannot be introduced into the tube blank before aging heat treatment. Therefore, insufficient Nb carbonitrides cannot be generated in the subsequent aging heat treatment process. If the rolling amount δc is 3 mm or more, sufficient strain is introduced into the tube blank. As a result, sufficient Nb carbonitrides are generated in the duplex stainless steel tube after the aging heat treatment process, resulting in a sufficient non-solid-dissolved Nb content. It should be noted that the upper limit of the rolling amount is, for example, 8 mm.

[0305] [(Process 5) Aging heat treatment process]

[0306] In the aging heat treatment process, the tube blank is subjected to aging heat treatment. In the aging heat treatment process of this embodiment, the formation of the σ phase is suppressed and a sufficient amount of Nb carbonitrides is generated during the aging heat treatment. The aging heat treatment satisfies the following conditions.

[0307] (Condition 4)

[0308] The aging heat treatment temperature T2 satisfies the following formula (A).

[0309] T2<700-(0.5Cr+0.3Mn+3Mo+1.5Si+8Nb-Ni-0.6Cu-4Co-10Sn) 2 (A)

[0310] Here, the element symbols in Equation (A) are replaced with the content of the corresponding element in duplex stainless steel pipe in terms of mass%.

[0311] (Condition 5)

[0312] The holding time t2 at the aging heat treatment temperature T2 satisfies equation (B).

[0313] t2>(1 / 60)×10 (3200 / (T2+273.15)-3Nb) (B)

[0314] Here, the aging heat treatment temperature T2 (°C) is substituted into T2 in Equation (B), and the Nb content of the duplex stainless steel pipe in mass % is substituted into Nb in Equation (B).

[0315] The following is an explanation of each condition.

[0316] (Regarding condition 4)

[0317] FnA is defined as follows.

[0318] FnA=700-(0.5Cr+0.3Mn+3Mo+1.5Si+8Nb-Ni-0.6Cu-4Co-10Sn) 2

[0319] FnA represents the lower limit of the temperature (°C) that promotes the formation of the σ phase. Cr, Mn, Mo, Si, and Nb in FnA are elements that promote σ phase formation. On the other hand, Ni, Cu, Co, and Sn are elements that inhibit σ phase formation. If the aging heat treatment temperature T2 is above FnA, the formation of the σ phase is promoted in the tube blank during aging heat treatment. As a result, the volume fraction of the σ phase in the manufactured duplex stainless steel tube becomes excessively high. If the aging heat treatment temperature T2 is below FnA, the formation of the σ phase is sufficiently inhibited in the tube blank during aging heat treatment. As a result, the volume fraction of the σ phase in the manufactured duplex stainless steel tube is sufficiently reduced.

[0320] (Regarding condition 5)

[0321] FnB is defined as follows.

[0322] FnB=(1 / 60)×10 (3200 / (T2+273.15)-3Nb)

[0323] FnB represents the lower limit of the holding time (in minutes) required for sufficient Nb carbonitride formation. Since the Nb content in the steel pipe has a significant impact on Nb carbonitride formation, FnB includes Nb. If the holding time t2 at the aging heat treatment temperature T2 is less than FnB, sufficient Nb carbonitride will not be formed in the manufactured duplex stainless steel pipe. Therefore, a sufficient amount of non-solid-dissolved Nb cannot be obtained. If the holding time t2 is longer than FnB, sufficient Nb carbonitride will be formed in the manufactured duplex stainless steel pipe. Therefore, a sufficient amount of non-solid-dissolved Nb can be obtained.

[0324] [Other processes]

[0325] The manufacturing method of this embodiment may also include manufacturing steps other than those described above. For example, a pickling process may be performed on the duplex stainless steel pipe after the aging heat treatment process. In this case, the pickling process can be performed by a known method and is not particularly limited. It should be noted that in the manufacturing method of this embodiment, the cold working process may not be performed after the hot working process and before the solution treatment process. Even if the cold working process is omitted, a duplex stainless steel pipe with sufficient strength can be obtained.

[0326] Through the above processes, the duplex stainless steel pipe of this embodiment can be manufactured. It should be noted that the above method for manufacturing the duplex stainless steel pipe is one example; other methods can also be used to manufacture the duplex stainless steel pipe of this embodiment. Hereinafter, the duplex stainless steel pipe of this embodiment will be further described in detail through examples.

[0327] Example

[0328] Seamless steel pipes with the chemical compositions shown in Tables 1A and 1B are manufactured, namely duplex stainless steel pipes.

[0329] [Table 1A]

[0330] Table 1A

[0331]

[0332] [Table 1B]

[0333] Table 1B

[0334]

[0335] Specifically, a 50kg vacuum melting furnace is used to melt the steel. The molten steel is then used to manufacture steel ingots (cast ingots) via casting. It should be noted that the "-" in Table 1B indicates that the content of the corresponding element is the impurity level. For example, the Ta, Ti, Zr, Hf, W, Sb, Ca, Mg, B, and REM contents of steel symbol A are rounded to 0% after rounding to the fourth decimal place.

[0336] Hot working (hot extrusion) was performed on the ingots to manufacture tube blanks. The heating temperature during hot working was 1000℃~1280℃. Solution treatment was performed on the tube blanks of each test number that underwent hot working. The average heating rate HR1 (℃ / s), solution temperature T1 (℃), and holding time t1 (minutes) at solution temperature T1 during solution treatment (700℃~900℃) are shown in Table 2.

[0337] [Table 2]

[0338] Table 2

[0339]

[0340] For the solution-treated tube blank, a rotary straightener with three rolling mills was used for straightening. The rolling amount δc (mm) during straightening is shown in Table 2. The straightened tube blank was then subjected to aging heat treatment. The aging heat treatment temperature T2 (°C) and holding time t2 (minutes) are shown in Table 2. It should be noted that Table 2 also shows FnA (°C) and FnB (minutes) for each test number. Through the above manufacturing processes, duplex stainless steel tubes for each test number were manufactured.

[0341] [Evaluation Test]

[0342] The following evaluation tests were conducted on duplex stainless steel pipes for each test number.

[0343] (Experiment 1) Determination of Ferrite Volume Fraction and σ Phase Volume Fraction

[0344] (Experiment 2) Determination of non-solid-soluble Nb content and non-solid-soluble Nb / Al ratio

[0345] (Experiment 3) Yield Strength Test

[0346] (Test 4) Corrosion Resistance Test

[0347] The following is a description of each experiment.

[0348] [(Experiment 1) Determination of Ferrite Volume Fraction and σ Phase Volume Fraction]

[0349] Following the method described above in [Determination of Ferrite Volume Ratio and σ Phase Volume Ratio], the ferrite volume ratio (%) and σ phase volume ratio (%) in duplex stainless steel pipes for each test number were calculated. The obtained ferrite volume ratio (%) and σ phase volume ratio (%) are shown in Table 3.

[0350] [Table 3]

[0351] Table 3

[0352]

[0353] [(Experiment 2) Determination of non-solid-soluble Nb content and non-solid-soluble Nb / Al ratio]

[0354] Following the method described above in [Determination Method of Non-solid Solution Nb Content and Non-solid Solution Nb / Al Ratio], the non-solid solution Nb content (mass%) and non-solid solution Nb / Al ratio in duplex stainless steel pipes of each test number were calculated. The obtained non-solid solution Nb content (mass%), non-solid solution Al content (mass%), and non-solid solution Nb / Al ratio are shown in Table 3.

[0355] [(Experiment 3) Yield Strength Test]

[0356] Following the method described in the above-mentioned [Method for Determining Yield Strength], the yield strength (MPa) of the duplex stainless steel pipe for each test number was determined. It should be noted that the thickness of the arc-shaped test piece was set to be the same as the wall thickness of the steel pipe, the width to be 25.4 mm, and the gauge length to be 50.8 mm. The obtained yield strength (MPa) is shown in "YS (MPa)" in Table 3.

[0357] [(Test 4) Corrosion Resistance Test]

[0358] The corrosion resistance of duplex stainless steel pipes for each test number was evaluated according to the method described in the [Corrosion Resistance Evaluation Method] above. It should be noted that the test pieces were set to a thickness of 2 mm, a width of 10 mm, and a length of 75 mm. Test pieces for which no cracks were detected after 720 hours were considered to have achieved excellent corrosion resistance. In cases of excellent corrosion resistance, "NO SSC" was marked in the "Corrosion Resistance" column of Table 3. Conversely, test pieces for which cracks were detected after 720 hours were considered to have not achieved excellent corrosion resistance. In cases of not achieving excellent corrosion resistance, "SSC" was marked in the "Corrosion Resistance" column of Table 3.

[0359] [Evaluation Results]

[0360] Referring to Tables 1A, 1B, 2, and 3, the duplex stainless steel pipes tested 1–18 satisfy characteristics 1–4. Therefore, these seamless pipes achieve a high yield strength of over 655 MPa. Consequently, they exhibit excellent corrosion resistance. In other words, the duplex stainless steel pipes tested 1–18 possess both a high yield strength of over 655 MPa and excellent corrosion resistance.

[0361] On the other hand, in test number 19, the Nb content was too high. Therefore, excellent corrosion resistance was not obtained.

[0362] In test number 20, the Nb content was too low. Therefore, the non-solid-dissolved Nb content was too low. As a result, the yield strength was less than 655 MPa.

[0363] In tests 21 and 22, the aging heat treatment temperature T2 reached above FnA, failing to satisfy equation (A). Therefore, the σ phase volume fraction was as high as 1.0% or more. Consequently, excellent corrosion resistance was not achieved.

[0364] In tests 23 and 24, the holding time t2 at the aging heat treatment temperature T2 was shorter than that of FnB, failing to satisfy equation (B). Therefore, the non-solution Nb content was too low. Consequently, the yield strength was less than 655 MPa.

[0365] In tests 25 and 26, the average heating rate HR1 during solution treatment at 700℃–900℃ was too slow. Therefore, the non-solution Nb / Al ratio was low. Consequently, the yield strength was less than 655 MPa.

[0366] In tests 27 and 28, the rolling amount δc was too low during the straightening process. Therefore, the non-solid-solid Nb / Al ratio was low. As a result, the yield strength was as low as less than 655 MPa.

[0367] The embodiments of the present invention have been described above. However, the above embodiments are merely illustrative examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and can be implemented by appropriate modifications without departing from its spirit.

Claims

1. A duplex stainless steel pipe, wherein, Chemical composition (by mass%): C: less than 0.030% Si: 0.20~1.00% Mn: 0.5-7.0% P: below 0.040% S: Below 0.020% Al: below 0.100% Ni: 4.0–9.0% Cr：20.0～30.0％、 Mo: 0.5–2.0% Cu: 1.5–3.0% N：0.15～0.30％、 V：0.01～0.50％、 Nb: 0.030~0.300% Co: 0.10-0.50% Sn: 0.001~0.050% Ta: 0~0.100% Ti: 0~0.100% Zr：0～0.100％、 Hf: 0~0.100% W：0～0.200％、 Sb: 0~0.100% Ca: 0–0.020% Mg: 0–0.020% B：0～0.020％、 Rare earth elements: 0–0.200%, with the balance being Fe and impurities. The microstructure consists of 35.0–65.0% ferrite by volume, 0% or more but less than 1.0% σ phase, with the balance being austenite. The content of non-solid-soluble Nb is 0.008% or more by mass, and the ratio of the non-solid-soluble Nb content to the non-solid-soluble Al content is 1.0 or more. The yield strength is above 655 MPa.

2. The duplex stainless steel pipe according to claim 1, wherein, The chemical composition contains selected... Ta: 0.001~0.100% Ti: 0.001~0.100% Zr：0.001～0.100％、 Hf: 0.001~0.100% W：0.001～0.200％、 Sb: 0.001~0.100% Ca: 0.001~0.020% Mg: 0.001~0.020% B: 0.001~0.020%, and Rare earth elements: one or more elements in a group consisting of 0.001% to 0.200%.

Images