Duplex stainless steel material

Abstract

Provided is a duplex stainless steel material having both high strength and excellent corrosion resistance. This duplex stainless steel material comprises, 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, 0.0200% 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, 0.05-1.00% of Co, and 0.001-0.050% of Sn, with the remainder being Fe and impurities, has a yield strength of 758 MPa or more, has a microstructure of 35-65% by volume of ferrite, with the remainder being austenite, and has a thickness of 0.5-1.0 mm. The dislocation density [rho] ([alpha]) in the ferrite and the dislocation density [rho] ([gamma]) in the austenite satisfy formula (1). 0.3 < rho (gamma) / rho (alpha) < 4.0 (1).

Description

Technical Field

[0001] The present disclosure relates to steel materials, and more particularly to duplex stainless steel materials. Background Art

[0002] Oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") are sometimes in a corrosive environment containing corrosive gases. Here, the corrosive gas means carbon dioxide gas and / or hydrogen sulfide gas. That is, the steel materials used in oil wells need to have excellent corrosion resistance in a corrosive environment.

[0003] So far, as a method for improving the corrosion resistance of steel materials, a method of increasing the chromium (Cr) content and forming a passivation film mainly composed of Cr oxide on the surface of the steel material is known. Therefore, in an environment where excellent corrosion resistance is required, duplex stainless steel materials with an increased Cr content are sometimes used.

[0004] In recent years, the development of deep wells underwater has become increasingly active. Therefore, it is necessary to increase the strength of duplex stainless steel materials. That is, duplex stainless steel materials that can balance high strength and excellent corrosion resistance are required.

[0005] Japanese Patent Application Laid-Open No. 2014-043616 (Patent Document 1) and International Publication No. 2021 / 246118 (Patent Document 2) propose duplex stainless steel materials having high strength and excellent corrosion resistance.

[0006] The duplex stainless steel material disclosed in Patent Document 1 has the following chemical composition: C: 0.03% or less, Si: 0.3% or less, Mn: 3.0% or less, P: 0.040% or less, S: 0.008% or less, 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%, N: 0.24 to 0.40%, and Al: 0.03% or less, with the balance being Fe and impurities. The σ-phase susceptibility index X (= 2.2Si + 0.5Cu + 2.0Ni + Cr + 4.2Mo + 0.2W) is 52.0 or less, the strength index Y (= Cr + 1.5Mo + 10N + 3.5W) is 40.5 or more, and the pitting corrosion resistance index PREW (= Cr + 3.3(Mo + 0.5W) + 16N) is 40 or more. Regarding the structure of the steel, when a straight line parallel to the thickness direction is drawn from the surface layer to a depth of 1 mm in the thickness direction section parallel to the rolling direction, the number of boundaries between the ferrite phase and the austenite phase intersecting with the straight line is 160 or more. Patent Document 1 describes that this duplex stainless steel can be strengthened to high strength without impairing corrosion resistance, and exhibits excellent hydrogen embrittlement resistance characteristics by combining cold working with a high degree of work.

[0007] The duplex stainless steel disclosed in Patent Document 2 contains, by mass%, C: 0.002 to 0.03%, Si: 0.05 to 1.0%, Mn: 0.10 to 1.5%, P: 0.040% or less, S: 0.0005 to 0.02%, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.05%, and N: 0.06 to 0.35%, with the balance being Fe and impurities. This duplex stainless steel also has a structure containing, by volume fraction, an austenite phase: 20 to 70% and a ferrite phase: 30 to 80%, a yield strength of 448 MPa or more, and a number density of oxide-based inclusions with an average particle size of 1 μm or more of 15 pieces / mm 2 Hereinafter, the proportion of oxide-based inclusions containing Al in the oxide-based inclusions is 50% by mass or less. Patent Document 2 describes that this duplex stainless steel has high strength, high toughness, and excellent corrosion resistance.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2014-043616

[0011] Patent Document 2: International Publication No. 2021 / 246118 Summary of the Invention

[0012] Problems to be Solved by the Invention

[0013] According to the above Patent Documents 1 and 2, a duplex stainless steel material having high strength and excellent corrosion resistance can be obtained. However, a duplex stainless steel material that also achieves both high strength and excellent corrosion resistance can be obtained by technologies other than those disclosed in the above Patent Documents 1 and 2.

[0014] An object of the present disclosure is to provide a duplex stainless steel material that achieves both high strength and excellent corrosion resistance.

[0015] Solutions to the Problems

[0016] The duplex stainless steel material of the present disclosure, by mass, is

[0017] C: 0.030% or less,

[0018] Si: 0.20 to 1.00%,

[0019] Mn: 0.5 to 7.0%,

[0020] P: 0.040% or less,

[0021] S: 0.0200% or less,

[0022] Al: 0.100% or less,

[0023] Ni: 4.0 - 9.0%,

[0024] Cr: 20.0 - 30.0%,

[0025] Mo: 0.5 - 2.0%,

[0026] Cu: 1.5 - 3.0%,

[0027] N: 0.15 - 0.30%,

[0028] V: 0.01 - 0.50%,

[0029] Co: 0.05 - 1.00%,

[0030] Sn: 0.001 - 0.050%,

[0031] Nb: 0 - 0.300%,

[0032] Ta: 0 - 0.100%,

[0033] Ti: 0 - 0.100%,

[0034] Zr: 0 - 0.100%,

[0035] Hf: 0 - 0.100%,

[0036] W: 0 - 0.200%,

[0037] Sb: 0 - 0.100%,

[0038] Ca: 0 - 0.020%,

[0039] Mg: 0 - 0.020%,

[0040] B: 0 - 0.020%,

[0041] Rare earth elements: 0 - 0.200%, and

[0042] the balance is Fe and impurities,

[0043] the yield strength of the duplex stainless steel is 758 MPa or more,

[0044] the microstructure of the duplex stainless steel consists of 35 - 65% ferrite by volume fraction and the balance is composed of austenite,

[0045] the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the following formula (1),

[0046] 0.3 < ρ(γ) / ρ(α) < 4.0 (1)

[0047] Here, ρ(γ) in formula (1) is substituted with the dislocation density in the austenite in terms of m -2 count, and ρ(α) is substituted with the dislocation density in the ferrite in terms of m -2 count.

[0048] Effects of the Invention

[0049] The duplex stainless steel of the present disclosure has both high strength and excellent corrosion resistance. Detailed Description of the Invention

[0050] Specifically, the inventors aimed to obtain a high-strength duplex stainless steel having a yield strength of 758 MPa or more. For this purpose, the inventors first studied duplex stainless steels that have both a high yield strength of 758 MPa or more and excellent corrosion resistance from the perspective of chemical composition. As a result, the inventors considered that a duplex stainless steel having the following chemical composition might be able to have both a high yield strength of 758 MPa or more and excellent corrosion resistance: C: 0.030% or less, Si: 0.20 - 1.00%, Mn: 0.5 - 7.0%, P: 0.040% or less, S: 0.0200% 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%, Co: 0.05 - 1.00%, Sn: 0.001 - 0.050%, Nb: 0 - 0.300%, 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% by mass, and the balance being Fe and impurities.

[0051] Here, the microstructure of the duplex stainless steel having the above chemical composition is composed of ferrite and austenite. The inventors found that in the duplex stainless steel having the above chemical composition, if the microstructure is ferrite with a volume fraction of 35 - 65% and the balance is composed of austenite, the strength and corrosion resistance can be stably improved. That is, in the duplex stainless steel of the present embodiment, the microstructure is ferrite with a volume fraction of 35 - 65% and the balance is composed of austenite. It should be noted that in this specification, "composed of ferrite and austenite" means that the phases other than ferrite and austenite are so few that they can be ignored.

[0052] The inventors further conducted a detailed study on a method for improving the corrosion resistance while maintaining the yield strength of a duplex stainless steel material having the above chemical composition and microstructure and a yield strength of 758 MPa or more. Specifically, the inventors focused on the dislocations in the duplex stainless steel material. If the dislocation density in the duplex stainless steel material is increased, the yield strength of the steel material increases. That is, in the duplex stainless steel material of this embodiment in which the yield strength is increased to 758 MPa or more, the dislocation density may have increased to a certain extent or more.

[0053] On the other hand, it is considered that regions with a high dislocation density in the steel material are likely to become the starting points of corrosion. That is, if there are regions in the duplex stainless steel material where the dislocation density locally increases, the corrosion resistance of the duplex stainless steel material may decrease. That is, the inventors believe that the distribution of the dislocation density in the microstructure may affect the corrosion resistance of the steel material.

[0054] Based on the above insights, the inventors conducted further detailed research and found that in a duplex stainless steel material having the above chemical composition, a ferrite with a volume fraction of 35 to 65% and the balance being composed of austenite in the microstructure, and a yield strength of 758 MPa or more, if the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the following formula (1), it is possible to achieve both a yield strength of 758 MPa or more and excellent corrosion resistance.

[0055] 0.3 < ρ(γ) / ρ(α) < 4.0 (1)

[0056] Here, the dislocation density in the austenite is substituted for ρ(γ) in formula (1) -2 and the dislocation density in the ferrite is substituted for ρ(α). -2 In a duplex stainless steel material having the above chemical composition, a ferrite with a volume fraction of 35 to 65% and the balance being composed of austenite in the microstructure, and a yield strength of 758 MPa or more, if the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the above formula (1), it is possible to achieve both a yield strength of 758 MPa or more and excellent corrosion resistance, and the detailed reason is not yet clear. However, the inventors make the following speculation.

[0057]

[0058] ​As described above, it can be considered that in the duplex stainless steel material having the above chemical composition and microstructure, by increasing the yield strength to more than 758 MPa, the dislocation density will increase to a certain level or more. In addition, when the dislocation density of the duplex stainless steel material is increased by work hardening or the like, dislocations may sometimes be locally introduced, easily leading to a local increase in the dislocation density. On the other hand, if the ratio of the dislocation density ρ(α) in ferrite to the dislocation density ρ(γ) in austenite is controlled within a certain range, the localization of the dislocation density in the duplex stainless steel material may be slowed down. As a result, the inventors et al. speculate that while maintaining the yield strength, the increase in the local dislocation density is slowed down, thereby improving the corrosion resistance of the duplex stainless steel material.

[0059] It should be noted that through a mechanism other than the above mechanism, it is also possible to achieve both a yield strength of more than 758 MPa and excellent corrosion resistance in the duplex stainless steel material having the above chemical composition and microstructure by making the dislocation density ρ(α) in ferrite and the dislocation density ρ(γ) in austenite satisfy the above formula (1). Among them, the following examples prove that in the duplex stainless steel material having the above chemical composition and microstructure, by making the dislocation density ρ(α) in ferrite and the dislocation density ρ(γ) in austenite satisfy the above formula (1), it is possible to achieve both a yield strength of more than 758 MPa and excellent corrosion resistance.

[0060] The gist of the duplex stainless steel material of the present embodiment completed based on the above insights is as follows.

[0061] [1] A duplex stainless steel material, by mass

[0062] C: 0.030% or less,

[0063] Si: 0.20 - 1.00%,

[0064] Mn: 0.5 - 7.0%,

[0065] P: 0.040% or less,

[0066] S: 0.0200% or less,

[0067] Al: 0.100% or less,

[0068] Ni: 4.0 - 9.0%,

[0069] Cr: 20.0 - 30.0%,

[0070] Mo: 0.5 - 2.0%,

[0071] Cu: 1.5 - 3.0%,

[0072] N: 0.15 - 0.30%,

[0073] V: 0.01 to 0.50%,

[0074] Co: 0.05 to 1.00%,

[0075] Sn: 0.001 to 0.050%,

[0076] Nb: 0 to 0.300%,

[0077] Ta: 0 to 0.100%,

[0078] Ti: 0 to 0.100%,

[0079] Zr: 0 to 0.100%,

[0080] Hf: 0 to 0.100%,

[0081] W: 0 to 0.200%,

[0082] Sb: 0 to 0.100%,

[0083] Ca: 0 to 0.020%,

[0084] Mg: 0 to 0.020%,

[0085] B: 0 to 0.020%,

[0086] Rare earth elements: 0 to 0.200%, and

[0087] the balance is Fe and impurities,

[0088] the yield strength of the duplex stainless steel is 758 MPa or more,

[0089] the microstructure of the duplex stainless steel consists of 35 to 65% ferrite by volume fraction and the balance is composed of austenite,

[0090] the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the following formula (1),

[0091] 0.3 < ρ(γ) / ρ(α) < 4.0 (1)

[0092] Here, the dislocation density in the austenite is substituted for ρ(γ) in formula (1) in m -2 terms, and the dislocation density in the ferrite is substituted for ρ(α) in m -2 terms.

[0093] [2] The duplex stainless steel according to [1], which contains a selected group consisting of

[0094] Nb: 0.001 to 0.300%,

[0095] Ta: 0.001 to 0.100%,

[0096] Ti: 0.001 to 0.100%,

[0097] Zr: 0.001 to 0.100%,

[0098] Hf: 0.001 to 0.100%,

[0099] W: 0.001 to 0.200%,

[0100] Sb: 0.001 to 0.100%,

[0101] Ca: 0.001 to 0.020%,

[0102] Mg: 0.001 to 0.020%,

[0103] B: 0.001 to 0.020%, and

[0104] One or more elements selected from the group consisting of rare earth elements: 0.001 to 0.200%.

[0105] It should be noted that the shape of the duplex stainless steel material of this embodiment is not particularly limited. The duplex stainless steel material of this embodiment can be a steel pipe, a round steel (solid material), or a steel plate. It should be noted that round steel refers to a steel bar with a circular cross-section perpendicular to the axial direction. In addition, the steel pipe can be a seamless steel pipe or a welded steel pipe.

[0106] Hereinafter, the duplex stainless steel material of this embodiment will be described in detail. It should be noted that in the following description, the duplex stainless steel material will also be simply referred to as "steel material".

[0107] [Chemical composition]

[0108] The chemical composition of the duplex stainless steel material of this embodiment contains the following elements. Unless otherwise specified, "%" for elements represents mass %.

[0109] C: 0.030% or less

[0110] Inevitably contains carbon (C). That is, the lower limit of the C content is greater than 0%. C forms Cr carbides at grain boundaries, increasing the corrosion sensitivity at grain boundaries. Therefore, when the C content is too high, even if the contents of other elements are within the range of this embodiment, the corrosion resistance of the steel will decrease. Therefore, the C content is 0.030% or less. The preferred upper limit of the C content is 0.028%, and more preferably 0.025%. The C content is preferably as low as possible. However, an extreme reduction in the C content will significantly increase the production cost. Therefore, considering industrial production, the preferred lower limit of the C content is 0.001%, and more preferably 0.005%.

[0111] Si: 0.20 - 1.00%

[0112] Silicon (Si) deoxidizes the steel. When the Si content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. On the other hand, when the Si content is too high, even if the contents of other elements are within the range of this embodiment, the toughness and hot workability of the steel will decrease. Therefore, the Si content is 0.20 - 1.00%. The preferred lower limit of the Si content is 0.25%, and more preferably 0.30%. The preferred upper limit of the Si content is 0.95%, and more preferably 0.90%.

[0113] Mn: 0.5 - 7.0%

[0114] Manganese (Mn) deoxidizes the steel and desulfurizes the steel. Mn also improves the hot workability of the steel. When the Mn content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. On the other hand, Mn segregates at grain boundaries together with impurities such as P and S. Therefore, when the Mn content is too high, even if the contents of other elements are within the range of this embodiment, the corrosion resistance of the steel in a high-temperature environment will decrease. Therefore, the Mn content is 0.5 - 7.0%. The preferred lower limit of the Mn content is 0.6%, more preferably 0.8%, and more preferably 1.0%. The preferred upper limit of the Mn content is 6.5%, and more preferably 6.2%.

[0115] P: 0.040% or less

[0116] Inevitably contains phosphorus (P). That is, the lower limit of the P content is greater than 0%. P segregates at grain boundaries. Therefore, when the P content is too high, even if the contents of other elements are within the range of this embodiment, the corrosion resistance of the steel will decrease. Therefore, the P content is 0.040% or less. The preferred upper limit of the P content is 0.035%, and more preferably 0.030%. The P content is preferably as low as possible. Among them, an extreme reduction in the P content will significantly increase the production cost. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, and more preferably 0.003%.

[0117] S: 0.0200% or less

[0118] Inevitably contains sulfur (S). That is, the lower limit of the S content is greater than 0%. S segregates at grain boundaries. Therefore, when the S content is too high, even if the contents of other elements are within the range of this embodiment, the toughness and hot workability of the steel will decrease. Therefore, the S content is 0.0200% or less. The preferred upper limit of the S content is 0.0180%, and more preferably 0.0160%. The S content is preferably as low as possible. However, an extremely low S content will significantly increase the production cost. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0005%, more preferably 0.0010%, and more preferably 0.0015%.

[0119] Al: 0.100% or less

[0120] Inevitably contains aluminum (Al). That is, the lower limit of the Al content is greater than 0%. Al deoxidizes the steel. On the other hand, when the Al content is too high, even if the contents of other elements are within the range of this embodiment, coarse oxide-based inclusions will be generated and the toughness of the steel will decrease. Therefore, the Al content is 0.100% or less. The preferred lower limit of the Al content is 0.001%, more preferably 0.005%, and more preferably 0.010%. The preferred upper limit of the Al content is 0.090%, and more preferably 0.085%. It should be noted that the Al content referred to in this specification is "acid-soluble Al", that is, the content of sol.Al.

[0121] Ni: 4.0 - 9.0%

[0122] Nickel (Ni) stabilizes the austenite structure of the steel. That is, Ni is an element required to obtain a stable ferrite / austenite duplex structure. Ni also improves the corrosion resistance of the steel. When the Ni content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. On the other hand, when the Ni content is too high, even if the contents of other elements are within the range of this embodiment, the volume fraction of austenite will become too high and the yield strength of the steel will decrease. Therefore, the Ni content is 4.0 - 9.0%. The preferred lower limit of the Ni content is 4.1%, more preferably 4.3%, and more preferably 4.5%. The preferred upper limit of the Ni content is 8.8%, more preferably 8.5%, and more preferably 8.0%.

[0123] Cr: 20.0 - 30.0%

[0124] Chromium (Cr) forms a passivation film on the surface of the steel in the form of an oxide, improving the corrosion resistance of the steel. Cr also increases the volume fraction of the ferrite structure in the steel. By obtaining sufficient ferrite structure, the corrosion resistance of the steel is stabilized. When the Cr content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. On the other hand, when the Cr content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel will decrease. Therefore, the Cr content is 20.0 to 30.0%. The preferred lower limit of the Cr content is 20.5%, more preferably 21.0%, and even more preferably 21.5%. The preferred upper limit of the Cr content is 29.5%, more preferably 29.0%, and even more preferably 28.5%.

[0125] Mo: 0.5 to 2.0%

[0126] Molybdenum (Mo) improves the corrosion resistance of the steel. Mo also dissolves in the steel, increasing the yield strength of the steel. Mo also forms fine carbides in the steel, increasing the yield strength of the steel. When the Mo content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. On the other hand, when the Mo content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel will decrease. Therefore, the Mo content is 0.5 to 2.0%. The preferred lower limit of the Mo content is 0.6%, more preferably 0.7%, and even more preferably 0.8%. The preferred upper limit of the Mo content is 1.9%, more preferably 1.7%, and even more preferably 1.5%.

[0127] Cu: 1.5 to 3.0%

[0128] Copper (Cu) precipitates in the steel, increasing the yield strength of the steel. When the Cu content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained. On the other hand, when the Cu content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel will decrease. Therefore, the Cu content is 1.5 to 3.0%. The preferred lower limit of the Cu content is 1.6%, more preferably 1.8%, and even more preferably 2.0%. The preferred upper limit of the Cu content is 2.9%, more preferably 2.8%, and even more preferably 2.7%.

[0129] N: 0.15 to 0.30%

[0130] Nitrogen (N) stabilizes the austenite structure of the steel. That is, N is an element required to obtain a stable ferrite / austenite duplex structure. N also improves the corrosion resistance of the steel. When the N content is too low, even if the contents of other elements are within the range of the present embodiment, the above effects cannot be fully obtained. On the other hand, when the N content is too high, even if the contents of other elements are within the range of the present embodiment, the toughness and hot workability of the steel will decrease. Therefore, the N content is 0.15 to 0.30%. The preferred lower limit of the N content is 0.16%, more preferably 0.18%, and still more preferably 0.20%. The preferred upper limit of the N content is 0.29%, more preferably 0.27%.

[0131] V: 0.01 to 0.50%

[0132] Vanadium (V) increases the yield strength of the steel. When the V content is too low, even if the contents of other elements are within the range of the present embodiment, the above effects cannot be fully obtained. On the other hand, when the V content is too high, even if the contents of other elements are within the range of the present embodiment, the strength of the steel will become too high, and the toughness and hot workability of the steel will decrease. Therefore, the V content is 0.01 to 0.50%. The preferred lower limit of the V content is 0.02%, more preferably 0.03%, and still more preferably 0.05%. The preferred upper limit of the V content is 0.45%, more preferably 0.40%.

[0133] Co: 0.05 to 1.00%

[0134] Cobalt (Co) forms a coating on the surface of the steel, improving the corrosion resistance of the steel. Co also improves the hardenability of the steel, stabilizing the strength of the steel. When the Co content is too low, even if the contents of other elements are within the range of the present embodiment, the above effects cannot be fully obtained. On the other hand, when the Co content is too high, even if the contents of other elements are within the range of the present embodiment, the production cost will increase extremely. Therefore, the Co content is 0.05 to 1.00%. The preferred lower limit of the Co content is 0.06%, more preferably 0.08%, and still more preferably 0.10%. The preferred upper limit of the Co content is 0.95%, more preferably 0.90%, and still more preferably 0.85%.

[0135] Sn: 0.001 to 0.050%

[0136] Tin (Sn) improves the corrosion resistance of steel. When the Sn content is too low, even if the contents of other elements are within the range of the present embodiment, the above effects cannot be fully obtained. On the other hand, when the Sn content is too high, even if the contents of other elements are within the range of the present embodiment, liquation embrittlement cracks will occur at the grain boundaries, and the hot workability of the steel will decrease. Therefore, the Sn content is 0.001 to 0.050%. The preferred lower limit of the Sn content is 0.002%, more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit of the Sn content is 0.045%, more preferably 0.040%.

[0137] The balance of the chemical composition of the duplex stainless steel of the present embodiment is Fe and impurities. Here, the impurities in the chemical composition refer to substances mixed in from ores, scraps as raw materials or from the manufacturing environment, etc. during the industrial production of duplex stainless steel, and are substances allowed within the range that does not have an adverse effect on the duplex stainless steel of the present embodiment.

[0138] [Optional element]

[0139] The chemical composition of the above duplex stainless steel may also contain one or more elements selected from the group consisting of Nb, Ta, Ti, Zr, Hf, and W to replace a part of Fe. These elements are all optional elements and will increase the strength of the steel.

[0140] Nb: 0 to 0.300%

[0141] Niobium (Nb) is an optional element and may not be contained. That is, the Nb content can be 0%. When contained, Nb forms carbonitrides and increases the strength of the steel. The above effects can be obtained to a certain extent as long as a small amount of Nb is contained. However, when the Nb content is too high, even if the contents of other elements are within the range of the present embodiment, the strength of the steel will become too high and the toughness of the steel will decrease. Therefore, the Nb content is 0 to 0.300%. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.001%, still more preferably 0.002%, still more preferably 0.003%, and still more preferably 0.005%. The preferred upper limit of the Nb content is 0.280%, more preferably 0.250%.

[0142] Ta: 0 to 0.100%

[0143] Tantalum (Ta) is an optional element and may not be contained. That is, the Ta content can be 0%. When contained, Ta forms carbonitrides to increase the strength of the steel. Even a small amount of Ta can achieve the above effects to a certain extent. However, when the Ta content is too high, even if the contents of other elements are within the scope of this embodiment, the strength of the steel will become too high and the toughness of the steel will decrease. Therefore, the Ta content is 0 to 0.100%. The preferred lower limit of the Ta content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%. The preferred upper limit of the Ta content is 0.080%, more preferably 0.070%.

[0144] Ti: 0 to 0.100%

[0145] Titanium (Ti) is an optional element and may not be contained. That is, the Ti content can be 0%. When contained, Ti forms carbonitrides to increase the strength of the steel. Even a small amount of Ti can achieve the above effects to a certain extent. However, when the Ti content is too high, even if the contents of other elements are within the scope of this embodiment, the strength of the steel will become too high and the toughness of the steel will decrease. Therefore, the Ti content is 0 to 0.100%. The preferred lower limit of the Ti content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%. The preferred upper limit of the Ti content is 0.080%, more preferably 0.070%.

[0146] Zr: 0 to 0.100%

[0147] Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content can be 0%. When contained, Zr forms carbonitrides to increase the strength of the steel. Even a small amount of Zr can achieve the above effects to a certain extent. However, when the Zr content is too high, even if the contents of other elements are within the scope of this embodiment, the strength of the steel will become too high and the toughness of the steel will decrease. Therefore, the Zr content is 0 to 0.100%. The preferred lower limit of the Zr content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%. The preferred upper limit of the Zr content is 0.080%, more preferably 0.070%, more preferably 0.060%, more preferably 0.050%, more preferably 0.045%.

[0148] Hf: 0 to 0.100%

[0149] Hafnium (Hf) is an optional element and may not be contained. That is, the Hf content can be 0%. When contained, Hf forms carbonitrides and improves the strength of the steel. Even a small amount of Hf can achieve the above effects to a certain extent. However, when the Hf content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel will become too high and the toughness of the steel will decrease. Therefore, the Hf content is 0 to 0.100%. The preferred lower limit of the Hf content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%. The preferred upper limit of the Hf content is 0.080%, more preferably 0.070%.

[0150] W: 0 to 0.200%

[0151] Tungsten (W) is an optional element and may not be contained. That is, the W content can be 0%. When contained, W forms carbonitrides and improves the strength of the steel. Even a small amount of W can achieve the above effects to a certain extent. However, when the W content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel will become too high and the toughness of the steel will decrease. Therefore, the W content is 0 to 0.200%. The preferred lower limit of the W content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%. The preferred upper limit of the W content is 0.180%, more preferably 0.150%.

[0152] The chemical composition of the above duplex stainless steel may also contain Sb to replace a part of Fe.

[0153] Sb: 0 to 0.100%

[0154] Antimony (Sb) is an optional element and may not be contained. That is, the Sb content can be 0%. When contained, Sb improves the corrosion resistance of the steel. Even a small amount of Sb can achieve the above effects to a certain extent. However, when the Sb content is too high, even if the contents of other elements are within the range of this embodiment, the ductility of the steel at high temperatures will decrease and the hot workability of the steel will decrease. Therefore, the Sb content is 0 to 0.100%. The preferred lower limit of the Sb content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%. The preferred upper limit of the Sb content is 0.080%, more preferably 0.070%.

[0155] The chemical composition of the above duplex stainless steel may also contain one or more elements selected from the group consisting of Ca, Mg, B, and rare earth elements to replace a part of Fe. These elements are all optional elements and will improve the hot workability of the steel.

[0156] Ca: 0 to 0.020%

[0157] Calcium (Ca) is an optional element and may not be contained. That is, the Ca content can be 0%. When contained, Ca fixes S in the steel in the form of sulfide to render it harmless and improves the hot workability of the steel. Even a small amount of Ca can achieve the above effects to a certain extent. However, when the Ca content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel will coarsen and the toughness of the steel will decrease. Therefore, the Ca content is 0 to 0.020%. The preferred lower limit of the Ca content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%. The preferred upper limit of the Ca content is 0.018%, more preferably 0.015%.

[0158] Mg: 0 to 0.020%

[0159] Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content can be 0%. When contained, Mg fixes S in the steel in the form of sulfide to render it harmless and improves the hot workability of the steel. Even a small amount of Mg can achieve the above effects to a certain extent. However, when the Mg content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel will coarsen and the toughness of the steel will decrease. Therefore, the Mg content is 0 to 0.020%. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%. The preferred upper limit of the Mg content is 0.018%, more preferably 0.015%.

[0160] B: 0 to 0.020%

[0161] Boron (B) is an optional element and may not be contained. That is, the B content can be 0%. When contained, B inhibits the segregation of S in the steel to the grain boundaries and improves the hot workability of the steel. Even a small amount of B can achieve the above effects to a certain extent. However, when the B content is too high, even if the contents of other elements are within the range of this embodiment, boron nitride (BN) will be formed and the toughness of the steel will decrease. Therefore, the B content is 0 to 0.020%. The preferred lower limit of the B content is greater than 0%, more preferably 0.001%, more preferably 0.002%, more preferably 0.003%, more preferably 0.005%. The preferred upper limit of the B content is 0.018%, more preferably 0.015%.

[0162] Rare earth elements: 0 to 0.200%

[0163] The rare earth element (REM) can be any element and may not be contained. That is, the REM content can be 0%. When contained, REM fixes S in the steel in the form of sulfide to make it harmless and improves the hot workability of the steel. Even a small amount of REM can achieve the above effects to a certain extent. However, when the REM content is too high, even if the contents of other elements are within the range of the present embodiment, the oxides in the steel will coarsen and the toughness of the steel will decrease. Therefore, the REM content is 0 to 0.200%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.001%, still more preferably 0.005%, still more preferably 0.010%, still more preferably 0.020%. The preferred upper limit of the REM content is 0.180%, more preferably 0.160%.

[0164] It should be noted that REM in this specification refers to one or more elements selected from the group consisting of scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71 belonging to the lanthanide series. In addition, the REM content in this specification refers to the total content of these elements.

[0165] [Yield strength]

[0166] The yield strength of the duplex stainless steel of the present embodiment is 758 MPa or more. The duplex stainless steel of the present embodiment has the above chemical composition, has a microstructure in which ferrite is 35 to 65% by volume fraction and the balance is composed of austenite, and the dislocation density ratio ρ(γ) / ρ(α) described later is greater than 0.3 and less than 4.0. As a result, the duplex stainless steel of the present embodiment has excellent corrosion resistance even though the yield strength is 758 MPa or more.

[0167] The preferred lower limit of the yield strength of the duplex stainless steel of the present embodiment is 760 MPa, more preferably 765 MPa. The upper limit of the yield strength of the duplex stainless steel of the present embodiment is not particularly limited, for example, it is 1000 MPa.

[0168] The yield strength of the duplex stainless steel material of this embodiment can be obtained by the following method. Specifically, a tensile test is carried out in accordance with the method of ASTM E8 / E8M (2022). A test piece is made from the steel material of this embodiment. When the steel material is a steel plate, a tensile test piece is made from the central part of the plate thickness. In this case, the length direction of the tensile test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, an arc-shaped test piece with a thickness equal to the wall thickness of the steel pipe, a width of 25.4 mm, and a gauge length of 50.8 mm is made. In this case, the length direction of the arc-shaped test piece is parallel to the pipe axis direction of the steel pipe. When the steel material is a round bar, a tensile test piece is made from the R / 2 position. In this case, the length direction of the tensile test piece is parallel to the axial direction of the round bar. In this specification, the R / 2 position of the round bar refers to the central position of the radius R in the cross section perpendicular to the axial direction of the round bar. When making the tensile test piece, the size of the tensile test piece is, for example, a parallel part diameter of 6 mm and a gauge length of 24 mm. Using the test piece, a tensile test is carried out in the atmosphere at room temperature (25°C). In this embodiment, the 0.2% residual deformation stress obtained by the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is obtained by rounding off the first digit after the decimal point of the obtained value.

[0169] [Microstructure]

[0170] The duplex stainless steel material of this embodiment has the above chemical composition, has a microstructure in which ferrite is 35 to 65% by volume fraction and the balance is composed of austenite, and the dislocation density ratio ρ(γ) / ρ(α) described later is greater than 0.3 and less than 4.0. As a result, the duplex stainless steel material of this embodiment has excellent corrosion resistance even when the yield strength is 758 MPa or more. In this specification, the microstructure "composed of ferrite and austenite" means that the phases other than ferrite and austenite in the microstructure are so few as to be negligible. For example, in the chemical composition of the duplex stainless steel material of this embodiment, the volume fractions of precipitates and inclusions are so small as to be negligible compared with the volume fractions of ferrite and austenite. That is, the microstructure of the duplex stainless steel material of this embodiment may contain trace amounts of precipitates, inclusions, etc. in addition to ferrite and austenite.

[0171] In the microstructure of the duplex stainless steel material of the present embodiment, the volume fraction of ferrite is 35 to 65%. When the volume fraction of ferrite is too low, the yield strength and / or corrosion resistance of the steel material may sometimes decrease. On the other hand, when the volume fraction of ferrite is too high, the toughness and hot workability of the steel material may sometimes decrease. Therefore, in the microstructure of the duplex stainless steel material of the present embodiment, the volume fraction of ferrite is 35 to 65%. The preferred lower limit of the volume fraction of ferrite is 36%, and more preferably 37%. The preferred upper limit of the volume fraction of ferrite is 64%, and more preferably 63%.

[0172] In the present embodiment, the volume fraction of ferrite in the duplex stainless steel material can be obtained by the method according to ASTM E562 (2019). A test piece for microstructure observation is made from the duplex stainless steel material of the present embodiment. When the steel material is a steel plate, a test piece with an observation surface having a rolling direction of 5 mm and a plate width direction of 5 mm is made from the center of the plate thickness. When the steel material is a steel pipe, a test piece with an observation surface having a pipe axis direction of 5 mm and a pipe circumference direction of 5 mm is made from the center of the wall thickness. In this specification, the pipe circumference direction of the steel pipe refers to the direction perpendicular to the pipe axis direction and the pipe diameter direction. When the steel material is a round steel, a test piece with an observation surface having an axial direction of 5 mm and a circumferential direction of 5 mm is made from the R / 2 position. In this specification, the circumferential direction of the round steel refers to the direction perpendicular to the axial direction and the radial direction. It should be noted that the size of the test piece is not particularly limited as long as the above observation surface can be obtained.

[0173] The observation surface of the produced test piece is mirror-polished. The mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etching solution to reveal the microstructure. The observation surface with the revealed microstructure is observed under an optical microscope in 10 fields of view. The area of each field of view is, for example, 1.00 mm 2 (magnification 100 times). Ferrite is determined in each field of view based on the contrast. The area fraction of the determined ferrite is measured by the point counting method according to ASTM E562 (2019). In the present embodiment, the arithmetic mean of the area fractions of ferrite obtained in 10 fields of view is defined as the volume fraction (%) of ferrite. In the present embodiment, the volume fraction (%) of ferrite is obtained by rounding off the first digit after the decimal point of the obtained value.

[0174] [Dislocation density ratio]

[0175] The duplex stainless steel material of the present embodiment has the above chemical composition and microstructure, has a yield strength of 758 MPa or more, and the dislocation density ρ(α) in ferrite and the dislocation density ρ(γ) in austenite satisfy the following formula (1).

[0176] 0.3 < ρ(γ) / ρ(α) < 4.0 (1)

[0177] Here, the dislocation density in austenite is substituted at ρ(γ) in formula (1) in terms of m -2 count, and the dislocation density in ferrite is substituted at ρ(α) in terms of m -2 count.

[0178] Define Fn1 = ρ(γ) / ρ(α). Fn1 represents the distribution ratio of the dislocation density in austenite to the dislocation density in ferrite in the duplex stainless steel material having the above chemical composition and microstructure. The larger Fn1 is, the more the dislocations are localized in austenite. The smaller Fn1 is, the more the dislocations are localized in ferrite. That is, when Fn1 is too high, the dislocation density in austenite locally becomes high, and the corrosion resistance of the steel material significantly decreases. On the other hand, when Fn1 is too low, the dislocation density in ferrite locally becomes high, and the corrosion resistance of the steel material decreases. Therefore, in the duplex stainless steel material of the present embodiment, Fn1 is greater than 0.3 and less than 4.0. The preferred lower limit of Fn1 is 0.4, and more preferably 0.5. The preferred upper limit of Fn1 is 3.9, and more preferably 3.8.

[0179] In the present embodiment, the dislocation density ratio Fn1 can be obtained by the following method. A thin film specimen for measuring the dislocation density is produced from the duplex stainless steel material of the present embodiment. Specifically, a test piece is cut out from the duplex stainless steel material. In addition, a thin film specimen is produced from the cut-out test piece by electrolytic polishing using the Twin jet method. It should be noted that when the steel material is a steel plate, a thin film specimen having an observation surface perpendicular to the rolling direction is produced from a test piece cut from the center of the plate thickness. When the steel material is a steel pipe, a thin film specimen having an observation surface perpendicular to the pipe axis direction is produced from a test piece cut from the center of the wall thickness. When the steel material is a round bar, a thin film specimen having an observation surface perpendicular to the axial direction is produced from a test piece cut from the R / 2 position. In addition, as long as the observation field of view described later can be obtained, the sizes of the test piece and the thin film specimen are not particularly limited.

[0180] Ferrite and austenite are determined in the observation surface of the obtained thin film specimen. Ferrite and austenite in the observation surface can be determined by identifying the crystal structure using electron beam diffraction. Microstructure observation is performed on the determined field of view using a transmission electron microscope (hereinafter, also referred to as "TEM"). The area of the observation field of view is not particularly limited, and may be an area obtained at a magnification at which dislocations can be easily observed. The area of the observation field of view is, for example, 100 nm × 100 nm to 800 nm × 800 nm. In addition, the volume (m 3 ) of each observation field of view is obtained from the area of the observation field of view and the thickness of the observation field of view. It should be noted that the thickness of the observation region is obtained from the total integrated intensity of the electron energy loss intensity spectrum (EELS) and the integrated intensity of the zero loss spectrum for the thin film specimen.

[0181] Observation of the tissue in the observation field of view was carried out under the condition that the acceleration voltage was set to 300 kV and the diffraction conditions were suitable for dislocation observation. The diffraction conditions suitable for dislocation observation refer to the conditions that can achieve the two-wave approximation of exciting the transmitted wave and one diffracted wave. Specifically, for austenite, it is the condition of exciting the reciprocal lattice vector g = 40 - 2, and for ferrite, it is the condition of exciting the reciprocal lattice vector g = 200 or 30 - 1. In this embodiment, the thin film specimen was tilted to achieve the diffraction conditions suitable for dislocation observation, and bright field observation was performed on the observation area of the thin film specimen. It should be noted that the high-angle annular dark field scanning transmission electron microscopy method (HAADF-STEM: High-angle Annular Dark Field Scanning Transmission Electron Microscopy) can be used instead of bright field observation to observe dislocations. Observation based on HAADF-STEM can observe dislocations more simply compared to bright field observation.

[0182] In addition, by performing exposure for an appropriate time, a photograph of the observation field of view was taken. For the generated photographic image, dislocations were determined based on the contrast, and the dislocation length was measured. It should be noted that the dislocation length can be measured by a known method. For example, the dislocation length determined based on the contrast can be obtained by image analysis. Based on the sum (m) of the dislocation lengths in 5 fields of view in ferrite and the total volume (m 3 ) of ferrite in 5 fields of view, the dislocation density ρ(α) (m -2 ) in ferrite was obtained. Similarly, based on the sum (m) of the dislocation lengths in 5 fields of view in austenite and the total volume (m 3 ) of austenite in 5 fields of view, the dislocation density ρ(γ) (m -2 ) in austenite was obtained.

[0183] The ratio Fn1 (= ρ(γ) / ρ(α)) of the dislocation density ρ(γ) (m -2 ) in austenite obtained by the above method to the dislocation density ρ(α) (m -2 ) in ferrite was obtained. In this embodiment, the dislocation density ratio Fn1 was obtained by rounding the second decimal place of the obtained value.

[0184] It should be noted that in this embodiment, as long as the yield strength is 758 MPa or more and Fn1 satisfies being greater than 0.3 and less than 4.0, the dislocation density ρ(α) (m -2 ) in ferrite and the dislocation density ρ(γ) (m -2 ) in austenite are not particularly limited. In the duplex stainless steel material of this embodiment, the dislocation density ρ(α) (m -2)For example, it is 1.0×10 14 ~8.0×10 15 (m -2 ). In the duplex stainless steel of the present embodiment, the dislocation density ρ(γ) in austenite (m -2 ) is, for example, 1.0×10 14 ~8.0×10 15 (m -2 ). If the dislocation density ρ(α) in ferrite (m -2 ) is 1.0×10 14 ~8.0×10 15 (m -2 ), and the dislocation density ρ(γ) in austenite (m -2 ) is 1.0×10 14 ~8.0×10 15 (m -2 ), then under the condition of satisfying other configurations of the present embodiment, a duplex stainless steel having a yield strength of 758 MPa or more stably and having excellent corrosion resistance can be obtained.

[0185] [Corrosion resistance]

[0186] The yield strength of the duplex stainless steel of the present embodiment is 758 MPa or more. The duplex stainless steel of the present embodiment has the above chemical composition, has a microstructure in which ferrite is 35 to 65% by volume fraction and the balance is composed of austenite, and the dislocation density ratio Fn1 (=ρ(γ) / ρ(α)) is greater than 0.3 and less than 4.0. As a result, the duplex stainless steel of the present embodiment has excellent corrosion resistance even though the yield strength is 758 MPa or more. In the present embodiment, the excellent corrosion resistance of the duplex stainless steel is evaluated as follows.

[0187] A test piece for a four-point bending test is made from the duplex stainless steel of the present embodiment. The size of the test piece is, for example, a thickness of 2 mm, a width of 10 mm, and a length of 75 mm. When the steel is a steel plate, the test piece is made from the central part of the plate thickness. In this case, the length direction of the test piece is parallel to the rolling direction of the steel plate. When the steel is a steel pipe, the test piece is made from the central part of the wall thickness. In this case, the length direction of the test piece is parallel to the pipe axis direction of the steel pipe. When the steel is a round bar, the test piece is made from the R / 2 position. In this case, the length direction of the test piece is parallel to the axial direction of the round bar.

[0188] The test solution used was an aqueous sodium chloride solution adjusted to pH = 4.0 with a concentration of 20% by mass. According to ASTM G39-99(2021), a stress equivalent to 90% of the actual yield stress was applied to the test piece by four-point bending. The test piece loaded with stress was sealed in an autoclave together with the test fixture. The test solution was injected into the autoclave, and the gas phase part was reserved to form a test bath. After degassing the test bath, a mixed gas of 0.1 bar of H 2 S gas and 10 bar of CO 2 gas was pressure-sealed into the autoclave, and the test bath was stirred to saturate the mixed gas. After sealing the autoclave, the test bath was stirred at 90 °C for 720 hours.

[0189] In this embodiment, if no cracks are found after 720 hours in the above test environment, it is evaluated as "having excellent corrosion resistance". It should be noted that in this specification, "no cracks are found" means that no cracks are found when observing the test piece after the test with the naked eye.

[0190] [Shape of duplex stainless steel]

[0191] As described above, the shape of the duplex stainless steel in this embodiment is not particularly limited. Preferably, the duplex stainless steel in this embodiment is a seamless steel pipe. When the duplex stainless steel in this embodiment is a seamless steel pipe, even if the wall thickness is 5 mm or more, it has a yield strength of 758 MPa or more and excellent corrosion resistance.

[0192] [Manufacturing method]

[0193] An example of the manufacturing method of the duplex stainless steel in this embodiment having the above constitution will be described. It should be noted that the manufacturing method of the duplex stainless steel in this embodiment is not limited to the manufacturing method described below. An example of the manufacturing method of the duplex stainless steel in this embodiment includes a blank preparation process, a hot working process, a first cold working process, a solution treatment process, and a second cold working process. Hereinafter, each manufacturing process will be described in detail.

[0194] [Blank preparation process]

[0195] In the blank preparation process of this embodiment, a blank having the above chemical composition is prepared. The blank can be prepared by manufacturing or by purchasing from a third party. That is, the method of preparing the blank is not particularly limited.

[0196] When manufacturing the blank, for example, it is manufactured by the following method. Molten steel having the above chemical composition is manufactured. The molten steel is used to manufacture a continuous casting billet (slab, bloom, or billet) by the continuous casting method. It is also possible to use the molten steel to manufacture a steel ingot (ingot) by the ingot casting method. It is also possible to perform primary rolling on the slab, bloom, or ingot as needed to manufacture a billet. The blank is manufactured through the above processes.

[0197] [Hot working process]

[0198] In the hot working process of the present embodiment, the billet prepared in the above billet preparation process is hot-worked to manufacture intermediate steel. In this specification, regarding the intermediate steel, it is plate-shaped steel when the final product is a steel plate, a tube blank when the final product is a steel pipe, bar-shaped steel with a circular cross-section perpendicular to the axial direction when the final product is a round steel, and wire-shaped steel when the final product is a wire rod. The hot working can be hot forging, hot extrusion, or hot rolling. The method of hot working is not particularly limited and can be a known method.

[0199] When the intermediate steel is a tube blank (seamless steel pipe), in the hot working process, for example, the high-speed extrusion method of glass lubricant or the Erhardt punching method (i.e., hot extrusion) can be implemented, or piercing rolling using the Mannesmann method (i.e., hot rolling) can be implemented. It should be noted that the hot working can be carried out only once or multiple times. For example, after performing the above piercing rolling on the billet, the above hot extrusion can be performed. For example, further, after performing the above piercing rolling on the billet, stretch rolling can be performed. That is, in the hot working process, hot working is carried out by a known method to manufacture intermediate steel of a desired shape.

[0200] [First cold working process]

[0201] In the first cold working process of the present embodiment, cold working is performed on the intermediate steel after the above hot working process. The cold working can be cold rolling or cold drawing. That is, in the first cold working process, known cold working is performed under known conditions. For example, the temperature of the intermediate steel during cold working can be room temperature to less than 150 °C.

[0202] Here, the cross-sectional reduction rate Rd1 (%) of the intermediate steel in the first cold working process is defined as follows. It should be noted that the cross-sectional reduction rate Rd1 (%) in the first cold working process is not particularly limited and is, for example, 2 to 30%.

[0203] Rd1 (%) = {1 - (cross-sectional area of the intermediate steel perpendicular to the processing direction after the first cold working process / cross-sectional area of the intermediate steel perpendicular to the processing direction before the first cold working process)} × 100

[0204] [Solution treatment process]

[0205] In the solution heat treatment process of this embodiment, solution heat treatment is performed on the intermediate steel material after the above-described first cold working process. The method of solution heat treatment is not particularly limited and may be a known method. For example, the intermediate steel material is loaded into a heat treatment furnace, held at a desired temperature, and then rapidly cooled. In this case, the temperature (heat treatment temperature) for performing solution heat treatment refers to the temperature (°C) of the heat treatment furnace used for performing solution heat treatment. The time held at the solution heat treatment temperature (holding time) refers to the time (minutes) that the intermediate steel material is held at the heat treatment temperature.

[0206] Preferably, the heat treatment temperature in the solution heat treatment process of this embodiment is set to 950 to 1150°C. When the heat treatment temperature is too low, the ferrite volume fraction of the duplex stainless steel material after solution heat treatment will be less than 35%, and the strength and / or corrosion resistance of the manufactured duplex stainless steel material may sometimes decrease. On the other hand, when the heat treatment temperature is too high, the ferrite volume fraction of the duplex stainless steel material after solution heat treatment will be greater than 65%, and the corrosion resistance of the steel material may sometimes decrease instead.

[0207] Therefore, when performing solution heat treatment by loading the intermediate steel material into a heat treatment furnace, holding at a desired temperature, and then rapidly cooling, the solution heat treatment temperature is preferably set to 950 to 1150°C. A further preferred lower limit of the solution heat treatment temperature is 960°C, and more preferably 970°C. A further preferred upper limit of the solution heat treatment temperature is 1140°C, and more preferably 1120°C.

[0208] When performing solution heat treatment by loading the intermediate steel material into a heat treatment furnace, holding at a desired temperature, and then rapidly cooling, the solution time is not particularly limited and can be performed under known conditions. The solution time is, for example, 5 to 180 minutes. The rapid cooling method is, for example, water cooling.

[0209] [Second Cold Working Process]

[0210] In the second cold working process of this embodiment, cold working is performed on the intermediate steel material after the above-described solution heat treatment process. The cold working can be cold rolling or cold drawing. That is, in the second cold working process, known cold working can be performed under known conditions in the same manner as in the first cold working process. For example, the temperature of the intermediate steel material during cold working can be room temperature to less than 150°C.

[0211] Here, the cross-sectional reduction rate Rd2 (%) of the intermediate steel material in the second cold working process is defined as follows.

[0212] Rd2 (%) = {1 - (cross-sectional area perpendicular to the processing direction of the intermediate steel material after the second cold working process / cross-sectional area perpendicular to the processing direction of the intermediate steel material before the second cold working process)} × 100

[0213] The cross-sectional reduction rate Rd2 (%) in the second cold working process has a significant impact on the strength of the manufactured duplex stainless steel. Therefore, if the cross-sectional reduction rate Rd2 is too small, the yield strength of the manufactured duplex stainless steel sometimes fails to stably reach 758 MPa or more. On the other hand, if the cross-sectional reduction rate Rd2 is too large, the dislocation density of austenite increases, and the dislocation density ratio Fn1 sometimes becomes 4.0 or more. Therefore, in the present embodiment, the cross-sectional reduction rate Rd2 is set to 4 to 20%.

[0214] Thus, in the preferred manufacturing method of the duplex stainless steel in the present embodiment, a blank preparation process, a hot working process, a first cold working process, a solution treatment process, and a second cold working process are implemented. Here, the ratio Fn1 (=ρ(γ) / ρ(α)) of the dislocation density ρ(γ) in austenite to the dislocation density ρ(α) in ferrite is greatly affected by cold working, and its value changes. That is, in the above preferred manufacturing method, the value of the dislocation density ratio Fn1 changes according to the balance between the first cold working process and the second cold working process.

[0215] For this reason, in the preferred manufacturing method of the present embodiment, the cross-sectional reduction rate Rd1 (%) in the first cold working process and the cross-sectional reduction rate Rd2 (%) in the second cold working process satisfy the following formula (A). As a result, it is possible to stably manufacture a duplex stainless steel having the above chemical composition and microstructure, having a yield strength of 758 MPa or more, and a dislocation density ratio Fn1 satisfying greater than 0.3 and less than 4.0.

[0216] Rd1 / Rd2>(Ni + 20N + 10Sn + 4Co + 0.5Mn + 0.5Cu) / (Cr + 3Mo + 2Si) (A)

[0217] Here, the cross-sectional reduction rate in the first cold working process is substituted for Rd1 in formula (A) in %, the cross-sectional reduction rate in the second cold working process is substituted for Rd2 in %, and the content of the corresponding element is substituted for the element symbol in mass %.

[0218] Here, by performing cold working before solution treatment, recrystallization is promoted during solution treatment, and the grain size non-uniformity is likely to be reduced. That is, the cross-sectional reduction rate Rd1 (%) in the first cold working process affects the grain non-uniformity after solution treatment. If the grain size non-uniformity of the grains after solution treatment is small, dislocations are easily evenly distributed in ferrite and austenite by the cold working in the second cold working process. In this case, the dislocation density ratio Fn1 is likely to become small.

[0219] On the other hand, as described above, if the cross-sectional reduction rate Rd2 (%) in the second cold working process is too large, the dislocation density of austenite tends to increase, and Fn1 tends to increase. For this reason, in the preferred manufacturing method of the present embodiment, Rd1 is defined relative to Rd2. That is, by increasing Rd1 to a certain extent or more according to Rd2, the grains of the intermediate steel in the second cold working process can be refined in advance. That is, it is possible to suppress the local increase in the dislocation density ρ(γ) in austenite during the second cold working process. As a result, the dislocation density ratio Fn1 can be reduced.

[0220] In addition, define FnA = (Ni + 20N + 10Sn + 4Co + 0.5Mn + 0.5Cu) / (Cr + 3Mo + 2Si). FnA is an index indicating the degree of grain refinement in the microstructure of the duplex stainless steel having the above chemical composition. The larger FnA is, the more likely the non-uniformity of the grains becomes. Therefore, even if FnA is large, if Rd1 is increased according to Rd2, the effect of grain refinement will be improved.

[0221] Therefore, in the preferred manufacturing method of the present embodiment, the ratio of Rd1 to Rd2 is made greater than FnA. In this case, it is possible to suppress the local increase in the dislocation density ρ(γ) in austenite during the second cold working process. As a result, the dislocation density ratio Fn1 can be reduced. In this way, according to the preferred manufacturing method of the present embodiment, it is possible to stably manufacture a duplex stainless steel having a yield strength of 758 MPa or more and a dislocation density ratio Fn1 satisfying greater than 0.3 and less than 4.0.

[0222] [Other Processes]

[0223] In the manufacturing method of the present embodiment, manufacturing processes other than the above may also be included. For example, age hardening heat treatment may be performed on the duplex stainless steel of the present embodiment. Age hardening heat treatment means holding the manufactured duplex stainless steel at a desired temperature. In this case, the age hardening heat treatment may be performed by a known method and is not particularly limited. For example, pickling treatment may also be performed on the duplex stainless steel of the present embodiment. In this case, the pickling treatment may be performed by a known method and is not particularly limited. In addition, other known post-treatments may be performed on the duplex stainless steel after the second cold working process.

[0224] Through the above processes, the duplex stainless steel of the present embodiment can be manufactured. It should be noted that the above manufacturing method of the duplex stainless steel is an example, and the duplex stainless steel can also be manufactured by other methods. Hereinafter, the present invention will be further described in detail by way of examples.

[0225] Examples

[0226] Melt molten steel with the chemical compositions shown in Table 1-1 and Table 1-2 using a 50 kg vacuum melting furnace, and manufacture steel blocks (ingots) by the ingot casting method. It should be noted that "-" in Table 1-2 indicates that the content of the corresponding element is at the impurity level. For example, for the Nb content, Ta content, Ti content, Zr content, Hf content, W content, Sb content, Ca content, Mg content, B content, and REM content of Steel A, rounding to the fourth decimal place gives 0%. In addition, the chemical compositions described in Table 1-1 and FnA (= (Ni + 20N + 10Sn + 4Co + 0.5Mn + 0.5Cu) / (Cr + 3Mo + 2Si)) calculated according to the above definition are shown in Table 2.

[0227] [Table 1-1]

[0228] Table 1-1

[0229]

[0230] [Table 1-2]

[0231] Table 1-2

[0232]

[0233] [Table 2]

[0234] Table 2

[0235]

[0236] Perform hot working on the ingots of each steel type to manufacture tube blanks (seamless steel tubes). For the tube blanks of each test number after hot working, perform the first cold working at the cross-sectional reduction rate Rd1 (%) described in Table 2. Further, for the tube blanks of each test number, perform solution treatment at the heat treatment temperature (°C) and holding time (minutes) described in Table 2. Further, for the tube blanks of each test number after solution treatment, perform the second cold working at the cross-sectional reduction rate Rd2 (%) described in Table 2. The ratio of the cross-sectional reduction rate Rd1 (%) of the first cold working to the cross-sectional reduction rate Rd2 (%) of the second cold working in each test number is shown in the "Rd1 / Rd2" column of Table 2. It should be noted that both the first cold working and the second cold working are performed by cold drawing.

[0237] [Evaluation Test]

[0238] Obtain seamless steel tubes of each test number through the above processes. Perform tensile tests, microstructure observation tests, dislocation density ratio measurement tests, and corrosion resistance tests on the obtained seamless steel tubes of each test number.

[0239] [Tensile Test]

[0240] For seamless steel pipes of each test number, a tensile test was carried out in accordance with ASTM E8 / E8M (2022) to obtain the yield strength. Specifically, arc-shaped test pieces for tensile tests were manufactured from seamless steel pipes of each test number. The arc-shaped test pieces were set to have the same thickness as the wall thickness of the steel pipe, a width of 25.4 mm, and a gauge length of 50.8 mm. Using the arc-shaped test pieces of each test number, a tensile test was carried out in the atmosphere at room temperature (25 °C) to obtain the 0.2% residual deformation stress (MPa). The obtained 0.2% residual deformation stress was defined as the yield strength (MPa). The yield strength (Yield Strength) of each test number obtained was shown in the "YS (MPa)" column of Table 3.

[0241] [Table 3]

[0242] Table 3

[0243]

[0244] [Microstructure Observation Test]

[0245] For seamless steel pipes of each test number, microstructure observation was carried out to obtain the volume fraction of ferrite. Specifically, test pieces for microstructure observation with an observation surface of 5 mm in the pipe axis direction × 5 mm in the pipe circumferential direction were made from the central part of the wall thickness of seamless steel pipes of each test number. The observation surfaces of the test pieces of each test number were polished to a mirror surface and electrolytically etched in a 7% potassium hydroxide etching solution. The observation surface showing the microstructure after electrolytic etching was observed using an optical microscope in 10 fields of view. The area of each field of view was 1.00 mm 2 (Magnification 100 times).

[0246] In each field of view of each test number, the phases other than ferrite and austenite in the microstructure were negligible. That is, the seamless steel pipes of each test number had a microstructure composed of ferrite and austenite. In each field of view of each test number, ferrite and austenite were determined based on the contrast. The area fraction (%) of the determined ferrite was obtained by image analysis in accordance with ASTM E562 (2019). The arithmetic mean of the area fractions of ferrite in 10 fields of view was taken as the ferrite volume fraction (%). The ferrite volume fraction (%) of each test number obtained was shown in Table 3.

[0247] [Dislocation Density Ratio Measurement Test]

[0248] For seamless steel pipes of each test number, a dislocation density ratio measurement test was carried out to obtain the dislocation density ratio Fn1 (= ρ(γ) / ρ(α)). Specifically, thin film specimens were made from seamless steel pipes of each test number by the above method. Further, using the thin film specimens of each test number, the dislocation density ρ(α) in ferrite (m -2) and the dislocation density ρ(γ) in austenite (m -2 ). It should be noted that in this embodiment, dislocations are observed by bright-field observation. In each test number, the dislocation density ρ(α) in ferrite is 1.0×10 14 ~8.0×10 15 (m -2 ), and the dislocation density ρ(γ) in austenite is 1.0×10 14 ~8.0×10 15 (m -2 ). Based on the obtained ρ(α) (m -2 ) and ρ(γ) (m -2 ), the dislocation density ratio Fn1 (= ρ(γ) / ρ(α)) is calculated. The calculated dislocation density ratio Fn1 is shown in the column of "Dislocation density ratio ρ(γ) / ρ(α)" in Table 3.

[0249] [Corrosion resistance test]

[0250] The seamless steel pipes of each test number are subjected to a corrosion resistance test to evaluate the corrosion resistance. Specifically, test pieces are made from the seamless steel pipes of each test number by the above method. The test solution used is a 20 mass% aqueous sodium chloride solution adjusted to pH = 4.0. According to ASTM G39-99(2021), a stress equivalent to 90% of the actual yield stress is applied to the test pieces by four-point bending. The test pieces loaded with stress are sealed in an autoclave together with the test fixtures. The test solution is injected into the autoclave and the gas phase part is reserved to form a test bath. After degassing the test bath, a mixed gas of 0.1 bar of H 2 S gas and 10 bar of CO 2 gas is pressure-sealed into the autoclave, and the test bath is stirred to saturate the mixed gas. After sealing the autoclave, the test bath is stirred at 90 °C for 720 hours.

[0251] The test pieces that do not show cracks after 720 hours are judged as "having excellent corrosion resistance" ("EX" (EXcellent) in Table 3). On the other hand, the test pieces that show cracks after 720 hours are judged as "not having excellent corrosion resistance" ("NA" (Not Acceptable) in Table 3). The evaluation results of the seamless steel pipes of each test number are shown in Table 3.

[0252] Referring to Table 1-1, Table 1-2, Table 2 and Table 3, the chemical compositions of the seamless steel pipes with test numbers 1 to 19 are appropriate. In addition, the manufacturing methods implemented for these seamless steel pipes are the preferred manufacturing methods described in the specification. As a result, the yield strengths of these seamless steel pipes are 758 MPa or more, the volume fraction of ferrite is 35 to 65%, and the dislocation density ratio Fn1 satisfies being greater than 0.3 and less than 4.0. As a result, these seamless steel pipes are judged to have excellent corrosion resistance in the corrosion resistance test. That is, the seamless steel pipes with test numbers 1 to 19 achieve both a high yield strength of 758 MPa or more and excellent corrosion resistance.

[0253] On the other hand, the cross-sectional reduction rate Rd2 in the second cold working process of the seamless steel pipes with test numbers 20 and 21 is too small. As a result, the yield strengths of these seamless steel pipes are less than 758 MPa.

[0254] The cross-sectional reduction rate Rd2 in the second cold working process of the seamless steel pipes with test numbers 22 and 23 is too large. As a result, the dislocation density ratio Fn1 of these seamless steel pipes is 4.0 or more. As a result, these seamless steel pipes are judged not to have excellent corrosion resistance in the corrosion resistance test.

[0255] The cross-sectional reduction rate Rd1 in the first cold working process, the cross-sectional reduction rate Rd2 in the second cold working process, and FnA of the seamless steel pipes with test numbers 24 to 26 do not satisfy formula (A). As a result, the dislocation density ratio Fn1 of these seamless steel pipes is 4.0 or more. As a result, these seamless steel pipes are judged not to have excellent corrosion resistance in the corrosion resistance test.

[0256] The above describes the embodiments of the present disclosure. However, the above embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above embodiments, and the above embodiments can be appropriately modified and implemented without departing from its gist.

Claims

1. A duplex stainless steel, by mass, C: 0.030% or less, Si: 0.20 - 1.00%, Mn: 0.5 - 7.0%, P: 0.040% or less, S: 0.0200% 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％、 Co: 0.05 - 1.00%, Sn: 0.001 - 0.050%, Nb: 0 - 0.300%, 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%, and the balance is Fe and impurities, the yield strength of the duplex stainless steel is 758 MPa or more, the microstructure of the duplex stainless steel is 35 - 65% ferrite by volume fraction, and the balance is composed of austenite, the dislocation density ρ(α) in the ferrite and the dislocation density ρ(γ) in the austenite satisfy the following formula (1), 0.3 < ρ(γ) / ρ(α) < 4.0 (1) Here, the dislocation density in the austenite is substituted at ρ(γ) in formula (1) in terms of m -2 count, and the dislocation density in the ferrite is substituted at ρ(α) in terms of m -2 count.

2. The duplex stainless steel according to claim 1, which contains at least one element selected from the group consisting of Nb: 0.001 - 0.300%, 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%.

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