Duplex stainless steel material

Abstract

Provided is a duplex stainless steel material having high strength, excellent pitting corrosion resistance, and excellent low-temperature toughness. A duplex stainless steel material according to the present disclosure has the chemical composition described in the description, and has Fn1 defined by formula (1) of 31.0 or more, Fn2 defined by formula (2) of 95.0 or more, a yield strength of 655 MPa or more, a microstructure comprising 60-90% ferrite by volume, the remainder comprising austenite, and a precipitated N amount of 0.0120 mass% or less.　(1): Fn1 = Cr + 3.3(Mo + 0.5W) + 16N, and 2: Fn2 = 81.543 + 5.67Si + 0.7Mn + 8.85Fn3, where Fn3 in formula (2) is defined by formula (3).　(3): Fn3 = 0.216 + 0.0688Si - 0.0048Mn - 0.0268Cu + 0.0202Cr - 0.0685Ni + 0.0767Mo - 0.0662N

Description

Duplex stainless steel material

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

[0002] A known method for improving the corrosion resistance of steel materials is to increase the chromium (Cr) content in the chemical composition and form a passive film mainly composed of Cr oxides on the surface of the steel material. Therefore, duplex stainless steel materials with an increased Cr content are sometimes used in environments requiring excellent corrosion resistance. Here, duplex stainless steel materials having a two-phase structure of ferrite and austenite phases have excellent corrosion resistance against pitting corrosion and / or crevice corrosion (hereinafter referred to as "pitting corrosion resistance"), which are problematic in aqueous solutions containing chlorides. Therefore, duplex stainless steel materials are used, for example, as materials for steel pipes for seawater heat exchangers and umbilical cables for offshore development.

[0003] By the way, currently, carbon dioxide (CO ２ The rise in CO ２ Efforts to reduce CO emissions have been underway, and CCUS in particular has been attracting attention. CCUS is an abbreviation for Carbon Dioxide Capture, Utilization and Storage. ２ It includes three technologies: CO capture, utilization, and storage. ２ CO2 emitted from industrial facilities such as power plants and factories is stored as a technology. ２ and CO2 ２ Technology to inject and store gas has been attracting attention.

[0004] In other words, such CO ２ Steel materials that are expected to be applied to storage technology include those for CO storage in depleted oil wells. ２ Therefore, high strength is required. In other words, there is a demand for duplex stainless steel materials that combine high strength with excellent pitting corrosion resistance.

[0005] Japanese Patent Application Laid-Open No. 2002-339042 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2016-3377 (Patent Document 2) propose a duplex stainless steel material having high strength and excellent pitting corrosion resistance.

[0006] The duplex stainless steel material disclosed in Patent Document 1 is a duplex stainless steel for shafts, and consists, in mass%, of C: 0.080% or less, Si: 0.10 to less than 1.50%, Mn: 2.0% or less, P: 0.03% or less, S: 0.01% or less, Ni: 4.0 to 10.0%, Cr: 22.0 to 30.0%, Mo: 1.0 to 3.0%, W: 0 to 1.5%, Mo+0.5×W: 1.0 to 3.0%, Cu: more than 1.0 to 3.5%, N: 0.30% or less, B: 0.0005 to 0.01%, and the balance being substantially Fe. Furthermore, the C value (= ((%Cr) + 2 x (%Si) + 1.5 x (%Mo) + 0.75 x (%W)) / ((%Ni) + 0.5 x (%Mn) + 0.3 x (%Cu) + 30 x (%C) + 25 x (%N))) is 2.00 to 2.60, the D value (= (%Cr) + 3.3 x (%Mo) + 1.65 x (%W) + (%Cu) + 30 x (%N)) is 36.0 or more, and the 0.2% yield strength at room temperature is 650 MPa or more. Patent Document 1 discloses that this duplex stainless steel material simultaneously satisfies the three properties of high strength, high toughness, and high pitting corrosion resistance.

[0007] The duplex stainless steel material disclosed in Patent Document 2 contains, in mass %, C: 0.03% or less, Si: 0.2 to 1%, Mn: 0.5 to 2.0%, P: 0.040% or less, S: 0.010% or less, Sol. A steel sheet having a chemical composition of Al: 0.040% or less, Ni: 4 to less than 6%, Cr: 20 to less than 25%, Mo: 2.0 to 4.0%, N: 0.1 to 0.35%, O: 0.003% or less, V: 0.05 to 1.5%, Ca: 0.0005 to 0.02%, B: 0.0005 to 0.02%, and the balance being Fe and impurities, the metal structure is composed of a two-phase structure of a ferrite phase and an austenite phase, there is no precipitation of a sigma phase, and the proportion of the ferrite phase in the metal structure in terms of area ratio is 50% or less, and ２ The number of oxide particles with a particle size of 30 μm or more present in the field of view is 15 or less. Patent Document 2 discloses that this duplex stainless steel material is excellent in strength, pitting corrosion resistance, and low-temperature toughness.

[0008] JP 2002-339042 A JP 2016-3377 A

[0009] In recent years, carbon dioxide (CO ２ When storing CO2, low-temperature toughness may be required for the steel material. ２ When a change in gas pressure occurs, the temperature of the stored gas may decrease due to the Joule-Thomson effect. ２ Steel materials expected to be used in storage technology must have high strength, excellent pitting corrosion resistance, and also excellent low-temperature toughness.

[0010] Here, Patent Document 1 proposes a duplex stainless steel material having high strength, high toughness, and high pitting corrosion resistance. Patent Document 2 proposes a duplex stainless steel material having excellent strength, pitting corrosion resistance, and low-temperature toughness. However, a duplex stainless steel material having high strength, excellent pitting corrosion resistance, and excellent low-temperature toughness may be obtained by techniques other than those disclosed in Patent Documents 1 and 2.

[0011] An object of the present disclosure is to provide a duplex stainless steel material having high strength, excellent pitting corrosion resistance, and excellent low temperature toughness.

[0012] The duplex stainless steel material according to the present disclosure has, in mass %, C: 0.001 to 0.030%, Si: 1.00 to 5.00%, Mn: 0.05 to 5.50%, P: 0.035% or less, S: 0.0010% or less, Cr: 21.00 to 27.00%, Ni: 4.00 to 9.00%, Mo: 1.00 to 5.50%, Cu: 0.30 to 3.50%, N: 0.250% or less, Al: 0.001 to 0.050%, B: 0.0001 to 0.0050%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.200%, W: 0 to 4.00%, The duplex stainless steel material comprises Ca: 0 to 0.0100%, Mg: 0 to 0.0050%, rare earth elements: 0 to 0.050%, Co: 0 to 2.00%, Sn: 0 to 0.020%, and the balance being Fe and impurities, Fn1 defined by formula (1) is 31.0 or more, Fn2 defined by formula (2) is 95.0 or more, the yield strength is 655 MPa or more, the microstructure is 60 to 90% by volume of ferrite and the balance is austenite, and in the duplex stainless steel material, the amount of precipitated N is 0.0120 mass% or less. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N (1) Fn2 = 81.543 + 5.67Si + 0.7Mn + 8.85Fn3 (2) Here, the element symbols in formulas (1) and (2) are substituted with the content of the corresponding element in mass%. If the corresponding element is not contained, "0" is substituted for that element symbol. Furthermore, Fn3 in formula (2) is defined by the following formula (3): Fn3 = 0.216 + 0.0688Si - 0.0048Mn - 0.0268Cu + 0.0202Cr - 0.0685Ni + 0.0767Mo - 0.0662N (3) Here, the element symbols in formula (3) are substituted with the content of the corresponding element in mass%.

[0013] The duplex stainless steel material according to the present disclosure has high strength, excellent pitting corrosion resistance, and excellent low temperature toughness.

[0014] Fig. 1 shows the relationship between Fn1 (= Cr + 3.3(Mo + 0.5W) + 16N) and the number of pits (number of pits), which is an index of pitting resistance, in this example. Fig. 2 shows the relationship between Fn2 (= 81.543 + 5.67Si + 0.7Mn + 8.85Fn3) and yield strength (MPa) in this example.

[0015] Specifically, the present inventors sought to obtain a duplex stainless steel material having a high strength of 95 ksi or more (655 MPa or more). Therefore, the present inventors first investigated, from the standpoint of chemical composition, duplex stainless steel materials that could achieve a yield strength of 95 ksi or more, excellent pitting corrosion resistance, and excellent low-temperature toughness.

[0016] Specifically, silicon (Si) is an element that increases the volume fraction of ferrite and further increases the strength of ferrite. Therefore, increasing the Si content may increase the yield strength of duplex stainless steel materials. On the other hand, it has been known that nitrogen (N) dissolves in austenite in duplex stainless steel materials, increasing the strength of the steel. Here, N dissolves easily in austenite but not easily in ferrite. In other words, increasing the Si content to increase the volume fraction of ferrite increases the activity of N, increasing the amount of N that remains undissolved and is more likely to precipitate as Cr nitrides.

[0017] Here, if a large number of coarse Cr nitrides are formed in a steel material, the low-temperature toughness of the steel material is likely to decrease. Therefore, the inventors considered that if the Si content is increased and the N content is reduced, it may be possible to achieve both high strength and excellent low-temperature toughness. Specifically, the inventors considered that by setting the Si content to 1.00% or more, the volume fraction of ferrite in the microstructure of the steel material is increased and the ferrite is strengthened, and further, by setting the N content to 0.250% or less, it may be possible to suppress the formation of nitrides.

[0018] As a result of detailed investigations by the present inventors based on the above findings, it was found that the composition is, in mass%, C: 0.001 to 0.030%, Si: 1.00 to 5.00%, Mn: 0.05 to 5.50%, P: 0.035% or less, S: 0.0010% or less, Cr: 21.00 to 27.00%, Ni: 4.00 to 9.00%, Mo: 1.00 to 5.50%, Cu: 0.30 to 3.50%, N: 0.250% or less, Al: 0.001 to 0.050%, B: 0.0001 to 0.0050%, It has been revealed that a duplex stainless steel material having a chemical composition consisting of Ti: 0-0.100%, Nb: 0-0.100%, V: 0-0.200%, W: 0-4.00%, Ca: 0-0.0100%, Mg: 0-0.0050%, rare earth elements: 0-0.050%, Co: 0-2.00%, Sn: 0-0.020%, and the balance being Fe and impurities may be able to achieve a yield strength of 95 ksi or more, excellent pitting corrosion resistance, and excellent low-temperature toughness.

[0019] Here, a duplex stainless steel material having the above-mentioned chemical composition has a microstructure consisting of 60 to 90% by volume of ferrite and the remainder being austenite. As described above, by increasing the Si content to 1.00 to 5.00%, the volume fraction of ferrite in the microstructure can be stably increased to 60% or more. In this specification, "consisting of ferrite and austenite" means that the amount of phases other than ferrite and austenite in the microstructure is negligibly small.

[0020] As a result of further investigations by the present inventors, it has become clear that in a duplex stainless steel material having the above-mentioned chemical composition and microstructure, if Fn1, defined by the following formula (1), is 31.0 or more, the pitting corrosion resistance of the steel material can be improved while maintaining strength and low-temperature toughness. Fn1=Cr+3.3(Mo+0.5W)+16N (1) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in mass%. If the corresponding element is not contained, "0" is substituted for that element symbol.

[0021] Fn1 is an index of pitting corrosion resistance in a duplex stainless steel material having the above-described chemical composition. The relationship between Fn1 and pitting corrosion resistance will be specifically explained using the drawings. FIG. 1 is a diagram showing the relationship between Fn1 and the number of pitting corrosion occurrences (number of occurrences), which is an index of pitting corrosion resistance, in this example. FIG. 1 was created using Fn1 and the number of pitting corrosion occurrences (number of occurrences) for examples described below in which the configurations other than Fn1 satisfy the conditions of this embodiment.

[0022] 1 , it can be seen that in a duplex stainless steel material having the above-described chemical composition and microstructure and a yield strength of 95 ksi or more, if Fn1 is 31.0 or more, the number of pitting corrosion occurrences is zero, and the material has excellent pitting corrosion resistance. Therefore, the duplex stainless steel material according to this embodiment has the above-described chemical composition and microstructure, and furthermore, Fn1 is set to 31.0 or more. As a result, the duplex stainless steel material according to this embodiment can achieve not only a yield strength of 95 ksi or more, but also excellent pitting corrosion resistance, provided that other configurations are satisfied.

[0023] As a result of further investigations by the present inventors, it has become clear that in a duplex stainless steel material having the above-described chemical composition and microstructure and satisfying an Fn1 of 31.0 or more, if Fn2 defined by the following formula (2) is 95.0 or more, the yield strength of the steel material can be increased while maintaining pitting corrosion resistance and low-temperature toughness. Fn2 = 81.543 + 5.67Si + 0.7Mn + 8.85Fn3 (2) Here, the element symbols in formula (2) are substituted with the contents of the corresponding elements in mass%. Furthermore, Fn3 in formula (2) is defined by the following formula (3). Fn3=0.216+0.0688Si-0.0048Mn-0.0268Cu+0.0202Cr-0.0685Ni+0.0767Mo-0.0662N (3) Here, the element symbols in formula (3) are substituted with the contents of the corresponding elements in mass %.

[0024] Fn2 is an index of strength in a duplex stainless steel material having the above-mentioned chemical composition. The relationship between Fn2 and strength will be specifically explained using the drawings. FIG. 2 is a diagram showing the relationship between Fn2 and yield strength (MPa) in this example. FIG. 2 was created using Fn2 and yield strength (MPa) for examples described below in which the configuration other than Fn2 satisfies the conditions of this embodiment.

[0025] 2 , it can be seen that a duplex stainless steel material having the above-described chemical composition and microstructure and satisfying an Fn1 of 31.0 or more will have a yield strength of 655 MPa or more (95 ksi or more) if Fn2 is 95.0 or more. Therefore, the duplex stainless steel material according to this embodiment has the above-described chemical composition and microstructure, and in addition to satisfying an Fn1 of 31.0 or more, Fn2 is also set to 95.0 or more. As a result, a yield strength of 95 ksi or more can be obtained, provided that the other configurations of this embodiment are satisfied.

[0026] As a result of further investigations by the present inventors, it has become clear that in a duplex stainless steel material having the above-mentioned chemical composition and microstructure, in which Fn1 is 31.0 or more and Fn2 is 95.0 or more, if the amount of precipitated N is 0.0120 mass% or less, it is possible to improve low-temperature toughness while maintaining a yield strength of 95 ksi or more.

[0027] The details of why setting the amount of precipitated N to 0.0120 mass% or less can improve low-temperature toughness while maintaining a yield strength of 95 ksi or more are not clear. However, the inventors speculate as follows. As described above, in a duplex stainless steel material having the above-mentioned chemical composition, the volume fraction of ferrite is 60 to 90%. Furthermore, Fn3 defined by the above-mentioned formula (3) is an index indicating the degree of strengthening of ferrite. The higher Fn3, the more ferrite is strengthened, and the easier it is for the strength of the steel material to increase.

[0028] On the other hand, in a duplex stainless steel material in which ferrite has been strengthened in this manner, the solid solubility limit of N in ferrite tends to be small. In other words, by adjusting the chemical composition, not only is the volume fraction of austenite, in which N is easily dissolved, reduced, but the solid solubility limit of N in ferrite also decreases. Therefore, a duplex stainless steel material having the above-mentioned chemical composition and microstructure, and in which Fn1 is 31.0 or more and Fn2 is 95.0 or more, may be prone to nitride formation depending on the balance of alloy components.

[0029] Furthermore, in a duplex stainless steel material having the above-described chemical composition, nitrides, particularly Cr nitrides, are easily formed. Coarse Cr nitrides tend to reduce the low-temperature toughness of the steel material. Therefore, if the precipitation of coarse Cr nitrides can be suppressed while satisfying formulas (1) and (2), the low-temperature toughness of the steel material may be improved. Therefore, in the duplex stainless steel material according to this embodiment, the amount of precipitated N is reduced. Specifically, the duplex stainless steel according to this embodiment has the above-described chemical composition and microstructure. By setting Fn1 to 31.0 or more and Fn2 to 95.0 or more, the pitting corrosion resistance of the steel material is improved, the yield strength of the steel material is increased to 95 ksi or more, and the amount of precipitated N is further reduced to 0.0120 mass% or less. As a result, the inventors speculate that the duplex stainless steel material according to this embodiment has a yield strength of 95 ksi or more, excellent pitting corrosion resistance, and excellent low-temperature toughness.

[0030] It is possible that the duplex stainless steel material according to this embodiment may have a yield strength of 95 ksi or more, excellent pitting corrosion resistance, and excellent low-temperature toughness for reasons other than those conjectured by the inventors. However, the examples described below demonstrate that a duplex stainless steel material having the above-described chemical composition and microstructure, in which Fn1 is 31.0 or more, Fn2 is 95.0 or more, and the amount of precipitated N is 0.0120 mass% or less, has a yield strength of 95 ksi or more, excellent pitting corrosion resistance, and excellent low-temperature toughness.

[0031] The duplex stainless steel material according to this embodiment, which was completed based on the above findings, has the following features.

[0032] [1] A duplex stainless steel material comprising, in mass%, C: 0.001 to 0.030%, Si: 1.00 to 5.00%, Mn: 0.05 to 5.50%, P: 0.035% or less, S: 0.0010% or less, Cr: 21.00 to 27.00%, Ni: 4.00 to 9.00%, Mo: 1.00 to 5.50%, Cu: 0.30 to 3.50%, N: 0.250% or less, Al: 0.001 to 0.050%, B: 0.0001 to 0.0050%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.200%, W: 0 to 4.00%, 1. A duplex stainless steel material comprising: Ca: 0-0.0100%, Mg: 0-0.0050%, rare earth elements: 0-0.050%, Co: 0-2.00%, Sn: 0-0.020%, and the balance being Fe and impurities; Fn1 defined by formula (1) being 31.0 or more; Fn2 defined by formula (2) being 95.0 or more; a yield strength of 655 MPa or more; and a microstructure consisting of 60-90% by volume of ferrite and the balance being austenite; and wherein the amount of precipitated N in the duplex stainless steel material is 0.0120% by mass or less. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N (1) Fn2 = 81.543 + 5.67Si + 0.7Mn + 8.85Fn3 (2) Here, the element symbols in formulas (1) and (2) are substituted with the content of the corresponding element in mass%. If the corresponding element is not contained, "0" is substituted for that element symbol. Furthermore, Fn3 in formula (2) is defined by the following formula (3): Fn3 = 0.216 + 0.0688Si - 0.0048Mn - 0.0268Cu + 0.0202Cr - 0.0685Ni + 0.0767Mo - 0.0662N (3) Here, the element symbols in formula (3) are substituted with the content of the corresponding element in mass%.

[0033] [2] The duplex stainless steel material according to [1], containing one or more elements selected from the group consisting of Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.001 to 0.200%, W: 0.01 to 4.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0050%, rare earth elements: 0.001 to 0.050%, Co: 0.01 to 2.00%, and Sn: 0.001 to 0.020%.

[0034] The shape of the duplex stainless steel material according to this embodiment is not particularly limited. The duplex stainless steel material according to this embodiment may be a steel pipe, a round bar (solid material), or a steel plate. The round bar refers to a steel bar having a circular cross section perpendicular to the axial direction. The steel pipe may be a seamless steel pipe or a welded steel pipe.

[0035] The duplex stainless steel material according to this embodiment will be described in detail below.

[0036] [Chemical Composition] The chemical composition of the duplex stainless steel material according to this embodiment contains the following elements: "%" relating to elements means mass % unless otherwise specified.

[0037] C: 0.001 to 0.030% Carbon (C) stabilizes austenite. If the C content is too low, the amount of austenite decreases, even if the contents of other elements are within the ranges of this embodiment, and the low-temperature toughness of the steel material decreases. On the other hand, C forms Cr carbides at grain boundaries, increasing the corrosion susceptibility at the grain boundaries. Therefore, if the C content is too high, the corrosion resistance of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.001 to 0.030%. The preferred lower limit of the C content is 0.002%, more preferably 0.005%, even more preferably 0.008%, and even more preferably 0.010%. The preferred upper limit of the C content is 0.028%, more preferably 0.025%, even more preferably 0.022%, and even more preferably 0.020%.

[0038] Si: 1.00 to 5.00% Silicon (Si) is a ferrite-forming element and increases the volume fraction of ferrite. Si also strengthens ferrite, increasing the strength of the steel. If the Si content is too low, the above effects cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content is too high, the volume fraction of ferrite may become too high even if the contents of other elements are within the ranges of this embodiment. If the Si content is too high, Cr nitrides may be stabilized, which may reduce the low-temperature toughness of the steel. Therefore, the Si content is 1.00 to 5.00%. A preferred lower limit of the Si content is 1.02%, more preferably 1.10%, and even more preferably 1.20%. A preferred upper limit of the Si content is 4.50%, more preferably 4.00%, and even more preferably 3.80%.

[0039] Mn: 0.05 to 5.50% Manganese (Mn) is an austenite-forming element and increases the austenite volume fraction. Mn also increases the solid solubility limit of N in ferrite. If the Mn content is too low, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, Mn segregates to grain boundaries along with impurities such as P and S. Therefore, if the Mn content is too high, the pitting corrosion resistance of the steel material will decrease, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.05 to 5.50%. The preferred lower limit of the Mn content is 0.30%, more preferably 0.60%, even more preferably 1.00%, even more preferably 1.30%, even more preferably 1.50%, even more preferably 1.60%, and even more preferably 1.80%. The preferred upper limit of the Mn content is 5.35%, even more preferably 5.10%.

[0040] P: 0.035% or less Phosphorus (P) is unavoidably contained. That is, the lower limit of the P content is greater than 0%. P segregates at grain boundaries. Therefore, if the P content is too high, the corrosion resistance of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.035% or less. A preferred upper limit of the P content is 0.032%, more preferably 0.030%. The P content should be as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.003%.

[0041] S: 0.0010% or less Sulfur (S) is unavoidably contained. That is, the lower limit of the S content is greater than 0%. S segregates at grain boundaries. Therefore, if the S content is too high, the toughness and hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.0010% or less. A preferred upper limit of the S content is 0.0008%, more preferably 0.0006%. The S content should be as low as possible. However, an extreme reduction in the S content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the S content is 0.0001%, more preferably 0.0002%.

[0042] Cr: 21.00 to 27.00% Chromium (Cr) forms a passive film as an oxide on the surface of a steel material, thereby improving the corrosion resistance of the steel material. Cr also increases the volume fraction of the ferrite structure of the steel material, thereby increasing the strength of the steel material. If the Cr content is too low, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, many Cr nitrides are formed in the steel material, even if the contents of other elements are within the ranges of this embodiment, and the low-temperature toughness of the steel material decreases. Therefore, the Cr content is 21.00 to 27.00%. The preferred lower limit of the Cr content is 21.20%, more preferably 21.40%, and even more preferably 21.50%. The preferred upper limit of the Cr content is 26.50%, more preferably 26.00%, and even more preferably 25.50%.

[0043] Ni: 4.00 to 9.00% Nickel (Ni) stabilizes austenite in steel. Ni also enhances the corrosion resistance of steel. If the Ni content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content is too high, the volume fraction of austenite becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the yield strength of the steel decreases. Therefore, the Ni content is 4.00 to 9.00%. A preferred lower limit of the Ni content is 4.10%, more preferably 4.30%, and even more preferably 4.50%. A preferred upper limit of the Ni content is 8.50%, more preferably 8.00%, and even more preferably 7.50%.

[0044] Mo: 1.00 to 5.50% Molybdenum (Mo) dissolves in ferrite to increase the yield strength of the steel material. Mo also increases the corrosion resistance of the steel material. If the Mo content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content is too high, the low-temperature toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 1.00 to 5.50%. A preferred lower limit of the Mo content is 1.50%, more preferably 1.80%, even more preferably 2.00%, and even more preferably 2.20%. A preferred upper limit of the Mo content is 5.00%, even more preferably 4.80%, even more preferably 4.50%, and even more preferably 4.20%.

[0045] Cu: 0.30 to 3.50% Copper (Cu) precipitates in the steel material and increases the yield strength of the steel material. Cu also improves acid resistance in sulfuric acid and hydrogen sulfide environments. If the Cu content is too low, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cu content is too high, the solid solubility limit of N in ferrite decreases, making Cr nitrides more likely to form, even if the contents of other elements are within the ranges of this embodiment. As a result, the low-temperature toughness of the steel material decreases. Therefore, the Cu content is 0.30 to 3.50%. The preferred lower limit of the Cu content is 0.40%, more preferably 0.50%, and even more preferably 0.52%. The preferred upper limit of the Cu content is 3.40%, more preferably 3.30%, and even more preferably 3.20%.

[0046] N: 0.250% or less Nitrogen (N) is unavoidably contained. That is, the lower limit of the N content is greater than 0%. N stabilizes the austenite of the steel material. N also dissolves in austenite to increase the yield strength of the steel material. On the other hand, N combines with Cr to form Cr nitrides. Therefore, if the N content is too high, the low-temperature toughness of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.250% or less. A preferred upper limit of the N content is 0.240%, and more preferably 0.230%. To more effectively obtain the above effects, a preferred lower limit of the N content is 0.010%, more preferably 0.030%, and even more preferably 0.050%.

[0047] Al: 0.001 to 0.050% Aluminum (Al) deoxidizes steel. If the Al content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content is too high, coarse inclusions are formed, reducing the toughness and corrosion resistance of the steel material even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.001 to 0.050%. A preferred lower limit of the Al content is 0.003%, more preferably 0.005%, and even more preferably 0.010%. A preferred upper limit of the Al content is 0.048%, and even more preferably 0.045%. Note that the Al content referred to in this specification refers to the content of "acid-soluble Al," that is, sol. Al.

[0048] B: 0.0001 to 0.0050% Boron (B) suppresses the segregation of S to grain boundaries in steel and improves the hot workability of the steel. If the B content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content is too high, boron nitride (BN) is formed, reducing the toughness of the steel, even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0.0001 to 0.0050%. A preferred lower limit of the B content is 0.0003%, more preferably 0.0005%, and even more preferably 0.0010%. A preferred upper limit of the B content is 0.0045%, and even more preferably 0.0040%.

[0049] The balance of the chemical composition of the duplex stainless steel material according to this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment during industrial production of the duplex stainless steel material, and are acceptable within a range that does not adversely affect the duplex stainless steel material according to this embodiment.

[0050] [Optional Elements] The chemical composition of the above-described duplex stainless steel material may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti, Nb, V, and W. All of these elements are optional elements and increase the strength of the steel material.

[0051] Ti: 0 to 0.100% Titanium (Ti) is an optional element and does not necessarily need to be contained. That is, the Ti content may be 0%. When contained, Ti forms carbonitrides and increases the strength of the steel material. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the Ti content is 0 to 0.100%. The preferred lower limit of the Ti content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the Ti content is 0.060%, more preferably 0.040%, even more preferably 0.020%, and even more preferably 0.010%.

[0052] Nb: 0 to 0.100% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When contained, Nb forms carbonitrides and increases the strength of the steel material. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the Nb content is 0 to 0.100%. The preferred lower limit of the Nb content is more than 0%, more preferably 0.001%, even more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.030%. The preferred upper limit of the Nb content is 0.080%, even more preferably 0.070%, and even more preferably 0.060%.

[0053] V: 0 to 0.200% Vanadium (V) is an optional element and does not necessarily need to be contained. That is, the V content may be 0%. When contained, V forms carbonitrides and increases the strength of the steel material. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the V content is 0 to 0.200%. A preferred lower limit of the V content is more than 0%, more preferably 0.001%, and even more preferably 0.003%. A preferred upper limit of the V content is 0.150%, more preferably 0.100%, even more preferably 0.080%, even more preferably 0.050%, even more preferably 0.030%, and even more preferably 0.010%.

[0054] W: 0 to 4.00% Tungsten (W) is an optional element and does not necessarily need to be contained. That is, the W content may be 0%. When contained, W forms carbonitrides and increases the strength of the steel material. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content is too high, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material will be too high and the toughness of the steel material will decrease. Therefore, the W content is 0 to 4.00%. The preferred lower limit of the W content is more than 0%, more preferably 0.01%, even more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.20%. The preferred upper limit of the W content is 3.50%, even more preferably 3.00%.

[0055] The chemical composition of the duplex stainless steel material may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca, Mg, and rare earth elements. All of these elements are optional elements and improve the hot workability of the steel material.

[0056] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and does not necessarily need to be contained. That is, the Ca content may be 0%. When contained, Ca neutralizes S in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even if even a small amount of Ca is contained, the above effects can be achieved to some extent. However, if the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel will coarsen, reducing the toughness of the steel. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0080%, even more preferably 0.0060%, even more preferably 0.0040%, and even more preferably 0.0030%.

[0057] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and does not necessarily need to be contained. That is, the Mg content may be 0%. When contained, Mg neutralizes S in the steel material by fixing it as sulfides, thereby improving the hot workability of the steel material. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel material will coarsen, reducing the toughness of the steel material. Therefore, the Mg content is 0 to 0.0050%. The preferred lower limit of the Mg content is more than 0%, more preferably 0.0001%, even more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit of the Mg content is 0.0030%, even more preferably 0.0020%, and even more preferably 0.0010%.

[0058] Rare Earth Elements: 0 to 0.050% Rare earth elements (REM) are optional elements and do not necessarily need to be present. That is, the REM content may be 0%. When present, REM neutralizes S in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even if even a small amount of REM is present, the above effects can be achieved to some extent. However, if the REM content is too high, even if the contents of other elements are within the ranges of this embodiment, oxides in the steel will coarsen, reducing the toughness of the steel. Therefore, the REM content is 0 to 0.050%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.001%, even more preferably 0.003%, even more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the REM content is 0.040%, even more preferably 0.020%.

[0059] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0060] The chemical composition of the duplex stainless steel material may further contain one or more elements selected from the group consisting of Co and Sn in place of a portion of Fe. All of these elements are optional elements and enhance the corrosion resistance of the steel material.

[0061] Co: 0 to 2.00% Cobalt (Co) is an optional element and does not necessarily need to be contained. That is, the Co content may be 0%. When contained, Co forms a coating on the surface of the steel material, improving the corrosion resistance of the steel material. Even if even a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content is too high, even if the contents of other elements are within the ranges of this embodiment, the manufacturing cost will increase dramatically. Therefore, the Co content is 0 to 2.00%. The preferred lower limit of the Co content is more than 0%, more preferably 0.01%, even more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Co content is 1.50%, even more preferably 1.00%, and even more preferably 0.50%.

[0062] Sn: 0 to 0.020% Tin (Sn) is an optional element and does not necessarily need to be contained. That is, the Sn content may be 0%. When contained, Sn enhances the corrosion resistance of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content is too high, liquation embrittlement cracking occurs at grain boundaries, even if the contents of other elements are within the ranges of this embodiment, and the hot workability of the steel material is reduced. Therefore, the Sn content is 0 to 0.020%. The preferred lower limit of the Sn content is more than 0%, more preferably 0.001%, and even more preferably 0.003%. The preferred upper limit of the Sn content is 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0063] [Fn1] Assuming that the duplex stainless steel material according to this embodiment has the above-described chemical composition, Fn1 defined by the following formula (1) is 31.0 or more. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N (1) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in mass%. If the corresponding element is not contained, "0" is substituted for that element symbol.

[0064] Fn1 is an index of pitting corrosion resistance in a duplex stainless steel material having the above-mentioned chemical composition. If Fn1 is 31.0 or more, the pitting corrosion resistance of the duplex stainless steel material can be improved, provided that the other configurations of this embodiment are included. Therefore, in this embodiment, Fn1 is set to 31.0 or more.

[0065] The lower limit of Fn1 is preferably 31.1, more preferably 31.5, and even more preferably 32.0. A higher Fn1 is preferable. However, in the duplex stainless steel material according to this embodiment having the above-described chemical composition, the upper limit of Fn1 is substantially 55.8. The upper limit of Fn1 may further be 55.0, 53.0, 50.0, or 46.0. In this embodiment, Fn1 is calculated by rounding the obtained numerical value to one decimal place.

[0066] [Fn2] The duplex stainless steel material according to this embodiment has the above-described chemical composition, and, assuming that Fn1 is 31.0 or more, Fn2 defined by the following formula (2) is 95.0 or more. Fn2 = 81.543 + 5.67Si + 0.7Mn + 8.85Fn3 (2) Here, the element symbols in formula (2) are substituted with the contents of the corresponding elements in mass%. Furthermore, Fn3 in formula (2) is defined by the following formula (3): Fn3 = 0.216 + 0.0688Si - 0.0048Mn - 0.0268Cu + 0.0202Cr - 0.0685Ni + 0.0767Mo - 0.0662N (3) Here, the element symbols in formula (3) are substituted with the contents of the corresponding elements in mass%.

[0067] Fn2 is an index of strength in a duplex stainless steel material having the above-described chemical composition. If Fn2 is 95.0 or more, a yield strength of 95 ksi or more can be obtained, provided that the other components of this embodiment are included. Therefore, in this embodiment, Fn2 is 95.0 or more.

[0068] The lower limit of Fn2 is preferably 95.3, more preferably 95.5, and even more preferably 96.0. A higher Fn2 is preferable. However, in the duplex stainless steel material according to this embodiment having the above-mentioned chemical composition, the upper limit of Fn2 is substantially 124.7. In this embodiment, Fn2 is calculated by rounding the obtained value to one decimal place.

[0069] Fn3 is an index that indicates the degree of strengthening of ferrite in a duplex stainless steel material having the above-mentioned chemical composition. As Fn3 increases, ferrite is strengthened, and the strength of the steel material is likely to increase. However, in this embodiment, the value of Fn3 is not particularly limited as long as Fn2 satisfies 95.0 or more. Fn3 may be, for example, 0.50 to 1.24, or 0.51 to 1.00. In this embodiment, Fn3 is calculated by rounding the obtained value to two decimal places.

[0070] [Yield Strength] The yield strength of the duplex stainless steel material according to this embodiment is 655 MPa or more. The upper limit of the yield strength is not particularly limited, but is, for example, 862 MPa. In other words, the yield strength of the duplex stainless steel material according to this embodiment may be 655 to 862 MPa, 655 to 827 MPa, or 655 to 793 MPa.

[0071] The yield strength of the duplex stainless steel material according to this embodiment is defined as follows. Specifically, a tensile test is performed according to ASTM E8 / E8M (2022). A test specimen is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a tensile test specimen is prepared from the center of the plate thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, an arc-shaped test specimen is prepared, having 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. In this case, the longitudinal direction of the arc-shaped test specimen is parallel to the axial direction of the steel pipe. When the steel material is a round bar, a tensile test specimen is prepared from the R / 2 position. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the round bar. In this specification, the R / 2 position of the round bar means the center position of the radius R in a cross section perpendicular to the axial direction of the round bar.

[0072] When preparing a tensile test piece, the size of the tensile test piece is, for example, 6 mm in parallel diameter and 30 mm in gauge length. Using the test piece, a tensile test is performed at room temperature (25°C) in air using a method in accordance with ASTM E8 / E8M (2022). In this embodiment, the 0.2% offset yield strength obtained from the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is determined by rounding the obtained value to one decimal place.

[0073] [Microstructure] The duplex stainless steel material according to this embodiment has a microstructure consisting of 60 to 90% by volume of ferrite and the remainder being austenite. In this specification, the microstructure "consisting of ferrite and austenite" means that the amount of phases other than ferrite and austenite in the microstructure is negligibly small. For example, in the chemical composition of the duplex stainless steel material according to this embodiment, the volume fraction of precipitates and inclusions is negligibly small compared to the volume fractions of ferrite and austenite. In other words, the microstructure of the duplex stainless steel material according to this embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to ferrite and austenite.

[0074] As described above, the microstructure of the duplex stainless steel material according to this embodiment has a ferrite volume fraction of 60 to 90%. If the ferrite volume fraction is too low, the corrosion resistance of the steel material may decrease. On the other hand, if the ferrite volume fraction is too high, the toughness and hot workability of the steel material may decrease. Therefore, in the microstructure of the duplex stainless steel material according to this embodiment, the ferrite volume fraction is 60 to 90%. A preferred lower limit of the ferrite volume fraction is 63%, more preferably 65%, even more preferably 70%, and even more preferably 73%. A preferred upper limit of the ferrite volume fraction is 89%, even more preferably 88%.

[0075] In this embodiment, the volume fraction of ferrite in a duplex stainless steel material is defined by a method conforming to ASTM E562 (2019). A test piece for microstructure observation is prepared from the duplex stainless steel material according to this embodiment. When the steel material is a steel plate, a test piece having an observation surface measuring 5 mm in the rolling direction and 5 mm in the plate width direction is prepared from the center of the plate thickness. When the steel material is a steel pipe, a test piece having an observation surface measuring 5 mm in the pipe axial direction and 5 mm in the pipe circumferential direction is prepared from the center of the wall thickness. In this specification, the circumferential direction of a steel pipe refers to a direction perpendicular to the pipe axial direction and the pipe radial direction. When the steel material is a round steel bar, a test piece having an observation surface measuring 5 mm in the axial direction and 5 mm in the circumferential direction is prepared from the R / 2 position. In addition, in this specification, the circumferential direction of a round steel bar refers to a direction perpendicular to the axial direction and the radial direction. Note that the size of the test piece is not particularly limited as long as the above observation surface can be obtained. In this embodiment, the test piece is prepared so that the observation position is the center of the plate thickness, the center of the wall thickness, or the R / 2 position.

[0076] The observation surface of the prepared test piece is mirror-polished. The mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etching solution to reveal the structure. The observation surface with the revealed structure is observed using an optical microscope in 10 fields of view. In this embodiment, the total area of ​​the 10 fields of view is 10 mm ２ Microscopic observation is carried out so that the area of ​​each field of view is, for example, 1.00 mm ２ (Magnification: 100x). Observation images in each field of view are acquired, and the ferrite area ratio is determined by image analysis. Specifically, binarization processing is performed on the acquired observation images to distinguish ferrite from austenite. It is obvious that a person skilled in the art can distinguish ferrite from austenite from the contrast using binarization processing. Furthermore, the ferrite area is derived by performing image analysis. In this embodiment, the arithmetic mean value of the obtained ferrite area ratios in 10 fields of view is defined as the ferrite volume ratio (%). In this embodiment, the ferrite volume ratio (%) is determined by rounding the obtained value to one decimal place.

[0077] [Amount of Precipitated N] The duplex stainless steel material according to this embodiment has the above-described chemical composition and microstructure, with Fn1 being 31.0 or more, Fn2 being 95.0 or more, and the amount of precipitated N being 0.0120 mass% or less. Here, in a steel material having the above-described chemical composition, nitrides may include nitrides other than Cr nitrides. However, in a duplex stainless steel material having the above-described chemical composition, most of the nitrides are Cr nitrides. Furthermore, other nitrides also reduce the low-temperature toughness of the duplex stainless steel material. Therefore, by reducing the amount of precipitated N, it is possible to reduce the amount of nitrides that reduce the low-temperature toughness of the duplex stainless steel material.

[0078] In this embodiment, the upper limit of the amount of precipitated N is preferably 0.0115 mass%, more preferably 0.0110 mass%, even more preferably 0.0105 mass%, and even more preferably 0.0100 mass%. A smaller amount of precipitated N is preferable. In other words, the amount of precipitated N may be 0 mass%. Note that in a duplex stainless steel material having the above-described chemical composition, the lower limit of the amount of precipitated N may be 0.0001 mass%.

[0079] In this embodiment, the amount of precipitated N is defined by the following method. Specifically, a test piece for measuring the amount of precipitated N is prepared from the duplex stainless steel material according to this embodiment. The test piece is, for example, a cutting chip. When the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. When the steel material is a round bar, the test piece is prepared from the R / 2 position.

[0080] Mass of the prepared test piece M ０ (g) is measured. From the viewpoint of ensuring statistical accuracy and representativeness of the amount of precipitated N, the mass M ０ The weight of the test piece is preferably about 2 g. Next, the test piece is immersed in a 10% bromine methyl acetate solution to dissolve the test piece. The solution is suction filtered through a polyester filter with a pore size of 0.2 μm to capture the residue. The captured residue is acid-decomposed and subjected to ICP (inductively coupled plasma) emission spectrometry to quantify the amount of N in the residue. The amount of N in the resulting residue is determined by multiplying the mass M of the test piece by the weight of the test piece. ０The N content (mass%) is calculated by dividing the result by 1 / 2, and the N content (mass%) is defined as the amount of precipitated N (mass%). In this embodiment, the amount of precipitated N (mass%) is calculated by rounding the obtained value to the nearest five decimal places.

[0081] [Pitting corrosion resistance] The duplex stainless steel material according to this embodiment has the above-described chemical composition, Fn1 of 31.0 or more, Fn2 of 95.0 or more, the above-described microstructure, and the amount of precipitated N of 0.0120 mass% or less. As a result, the duplex stainless steel material according to this embodiment has a yield strength of 655 MPa or more, excellent pitting corrosion resistance, and excellent low-temperature toughness. In this embodiment, excellent pitting corrosion resistance is defined as follows.

[0082] The pitting corrosion resistance of the duplex stainless steel material according to this embodiment can be evaluated by a corrosion test in accordance with ASTM G48 (2011) Method E. A test piece for the corrosion test is prepared from the steel material according to this embodiment. The size of the test piece is, for example, 3 mm thick, 15 mm wide, and 25 mm long. Furthermore, when the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. In this case, the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. In this case, the longitudinal direction of the test piece is parallel to the axial direction of the steel pipe. When the steel material is a round bar, the test piece is prepared from the R / 2 position. In this case, the longitudinal direction of the test piece is parallel to the axial direction of the round bar.

[0083] The test solution was 6% FeCl ３ + 1% HCl. The test piece is placed in a liquid volume of 5 mL / cm ２ The test pieces were immersed in the above test solutions. The test temperature was 30°C, and the test time was 72 hours. After 72 hours, the test pieces were observed under an optical microscope at 100x magnification to check for the presence or absence of pitting corrosion. If no pitting corrosion was observed in the test pieces under the above conditions, the duplex stainless steel material was judged to have excellent pitting corrosion resistance.

[0084] [Low temperature toughness] The duplex stainless steel material according to this embodiment has the above-described chemical composition, Fn1 of 31.0 or more, Fn2 of 95.0 or more, the above-described microstructure, and the amount of precipitated N of 0.0120 mass% or less. As a result, the duplex stainless steel material according to this embodiment has a yield strength of 655 MPa or more, excellent pitting corrosion resistance, and excellent low temperature toughness. In this embodiment, excellent low temperature toughness is defined as follows.

[0085] Specifically, full-size or sub-size V-notch test pieces are prepared from the stainless steel material according to this embodiment in accordance with API 5CT (2019). Here, when the steel material is a steel plate, the rolling direction of the steel plate is defined as the "L direction" (Longitudinal), and the plate width direction of the steel plate is defined as the "T direction" (Transverse). When the steel material is a steel pipe, the pipe diameter direction of the steel pipe is defined as the "C direction", the pipe axial direction of the steel pipe is defined as the "L direction", and the direction perpendicular to the C direction and L direction is defined as the "T direction". When the steel material is a round bar, the cross-sectional diameter direction of the round bar is defined as the "C direction", the axial direction of the round bar is defined as the "L direction", and the direction perpendicular to the C direction and L direction is defined as the "T direction".

[0086] The prepared T-direction V-notch test specimen is subjected to a Charpy impact test in accordance with ASTM E23 (2018) to determine the absorbed energy (J) at -10°C. When a sub-size V-notch test specimen is used, the obtained absorbed energy is divided by the reduction factor (Reduction factor) described in API 5CT (2019) to convert it to the absorbed energy of a full-size V-notch test specimen. In this embodiment, the absorbed energy (J) at -10°C is determined by rounding the obtained value to one decimal place.

[0087] [Shape of duplex stainless steel material] As described above, the shape of the duplex stainless steel material according to this embodiment is not particularly limited. Preferably, the duplex stainless steel material according to this embodiment is a seamless steel pipe. When the duplex stainless steel material according to this embodiment is a seamless steel pipe, it has high strength, excellent pitting corrosion resistance, and excellent low-temperature toughness, even if the wall thickness is 5 mm or more.

[0088] [Manufacturing Method] An example of a method for manufacturing a duplex stainless steel material according to this embodiment having the above-described configuration will be described. Note that the method for manufacturing a duplex stainless steel material according to this embodiment is not limited to the manufacturing method described below. The example of a method for manufacturing a duplex stainless steel material according to this embodiment includes a material preparation step, a hot working step, and a solution treatment step. Each manufacturing step will be described in detail below.

[0089] [Material Preparation Step] In the material preparation step according to this embodiment, a material having the above-described chemical composition is prepared. The material may be prepared by manufacturing or by purchasing from a third party. In other words, the method for preparing the material is not particularly limited.

[0090] When manufacturing a raw material, for example, it is manufactured by the following method. Molten steel having the above-mentioned chemical composition is manufactured. A cast piece (slab, bloom, or billet) is manufactured using the molten steel by a continuous casting method. A steel ingot (ingot) may be manufactured using the molten steel by an ingot-making method. If necessary, the slab, bloom, or ingot may be subjected to blooming to manufacture a billet. The raw material is manufactured by the above-mentioned steps.

[0091] [Hot Working Step] In the hot working step according to this embodiment, the raw material prepared in the raw material preparation step is hot worked to produce an intermediate steel material. In this specification, the intermediate steel material refers to a plate-shaped steel material if the final product is a steel plate, a blank pipe if the final product is a steel pipe, or a bar-shaped steel material having a circular cross section perpendicular to the axial direction if the final product is a round steel bar. The hot working may be hot forging, hot extrusion, or hot rolling. The hot working method is not particularly limited and may be a well-known method.

[0092] When the intermediate steel material is a mother pipe (seamless steel pipe), the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot worked to produce a mother pipe. At this time, the hot working method may be, for example, the Eugène-Séjournet method or the Erhardt push bench method (i.e., hot extrusion), or piercing-rolling by the Mannesmann method (i.e., hot rolling). Note that the hot working may be performed only once or multiple times. For example, the material may be subjected to the above-mentioned piercing-rolling and then the above-mentioned hot extrusion. For example, the material may be subjected to the above-mentioned piercing-rolling and then elongation-rolling.

[0093] When the intermediate steel material is a rod-shaped steel material (round steel) with a circular cross section, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is subjected to hot working to produce an intermediate steel material with a circular cross section perpendicular to the axial direction. The hot working is, for example, blooming using a blooming mill or hot rolling using a continuous rolling mill. The continuous rolling mill has horizontal stands each having a pair of grooved rolls arranged side by side in the vertical direction, and vertical stands each having a pair of grooved rolls arranged side by side in the horizontal direction, arranged alternately.

[0094] Furthermore, when the intermediate steel material is a plate-shaped steel material (steel plate), the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300°C. The material extracted from the heating furnace is hot-rolled using a blooming mill and a continuous rolling mill to produce an intermediate steel material in the shape of a steel plate. That is, in the hot working step, hot working is carried out by a well-known method to produce an intermediate steel material in the desired shape.

[0095] Preferably, in the hot working step according to this embodiment, rapid cooling is performed after the hot working is completed. At this time, the temperature at which the hot working is completed is preferably 1150°C or higher. In this specification, the temperature at which the hot working is completed refers to the temperature of the material when the material separates from the rolling rolls or die. That is, when hot rolling is performed as the hot working, the completion of hot working is defined as the time when rolling by the final rolling roll is completed. That is, further, when hot extrusion is performed as the hot working, the completion of hot working is defined as the time when the material is extruded from the die. That is, further, when hot forging is performed as the hot working, the completion of hot working is defined as the time when the material separates from the die.

[0096] If the temperature at which the hot working is completed is too low, nitrides may precipitate in the material during the hot working or before the subsequent rapid cooling. In this case, there is a concern that the nitrides may become coarse and may not be able to form a solid solution sufficiently in the solution treatment process described below. Therefore, in the hot working process according to this embodiment, the temperature at which the hot working is completed is preferably 1150°C or higher. Note that the upper limit of the temperature at which the hot working is completed is not particularly limited, but is, like the heating temperature, for example, 1300°C.

[0097] Preferably, the time from the completion of hot working to the start of rapid cooling is set to 60 seconds or less. If the time from the completion of hot working to the start of rapid cooling is too long, nitrides may precipitate in the material before rapid cooling begins. In this case, there is a concern that the nitrides may become coarse and may not be able to form a solid solution sufficiently in the solution treatment step described below. Therefore, in the hot working step of this embodiment, it is preferable that the time from the completion of hot working to the start of rapid cooling be set to 60 seconds or less.

[0098] In this embodiment, the method of quenching is not particularly limited, and any known method may be used. For example, the material can be cooled by shower water cooling, mist water cooling, oil cooling, etc.

[0099] [Solution Treatment Process] In the solution treatment process according to this embodiment, the intermediate steel produced in the hot working process is subjected to solution treatment to produce a duplex stainless steel material. Solution treatment refers to heat treatment that dissolves intermetallic compounds and precipitates in the intermediate steel material. That is, the solution treatment process includes a process of heat treating the intermediate steel material at a desired temperature (heat treatment process) and a process of quenching the heat-treated intermediate steel material (quenching process). Each process will be described in detail below.

[0100] [Heat Treatment Step] In the heat treatment step according to this embodiment, heat treatment is performed on the intermediate steel material produced in the hot working step. Preferably, the heat treatment temperature T (°C) in the heat treatment step satisfies the following formulas (A) and (B). T≧441+42Si-3.2Mn+10Cu+10Cr-2Ni+44Mo+993N+6W (A) T≧1660-8FnC (B) Here, "T" in formulas (A) and (B) represents the temperature of the heat treatment furnace used in the heat treatment in °C, and the element symbols represent the content of the corresponding element in mass%. If the corresponding element is not contained, "0" is substituted for that element symbol. Furthermore, FnC in formula (B) is defined by the following formula (C). FnC = -42.8 - 2189.5C + 10.77Si + 0.182Mn + 7.4Cr + 1.76Ni - 9.4Mo + 5.126Cu - 130.5N + 110.254Al + 84.62Nb - 2.157W (C) Here, the element symbols in formula (C) are substituted with the content of the corresponding element in mass%. If the corresponding element is not contained, "0" is substituted for that element symbol.

[0101] The formula is defined as FnA = 441 + 42Si - 3.2Mn + 10Cu + 10Cr - 2Ni + 44Mo + 993N + 6W. FnA is an index indicating the ease of dissolution of Cr nitrides. As described above, in a duplex stainless steel material having the chemical composition of this embodiment, Cr nitrides are easily formed as nitrides. Furthermore, Si, Cu, Cr, Mo, N, and W stabilize Cr nitrides, making them less likely to dissolve by heat treatment. On the other hand, Mn and Ni promote the dissolution of Cr nitrides. Specifically, in the heat treatment process, heating to a heat treatment temperature T (°C) equal to or higher than FnA dissolves Cr nitrides, and the amount of precipitated N in the manufactured duplex stainless steel material can be stably reduced. Therefore, the heat treatment temperature T (°C) is preferably equal to or higher than FnA.

[0102] The definition is FnB = 1660 - 8FnC. FnB is an index of the ferrite volume fraction in the microstructure. Specifically, in the heat treatment step, by heating to a heat treatment temperature T (°C) equal to or higher than FnB, the ferrite volume fraction in the microstructure can be stably increased to 60% or higher. Therefore, the heat treatment temperature T (°C) is preferably equal to or higher than FnB.

[0103] The upper limit of the heat treatment temperature T is, for example, 1200°C. If the heat treatment time is too short, precipitates that reduce pitting corrosion resistance may remain in the duplex stainless steel material after the solution treatment process. On the other hand, if the heat treatment time is too long, the effect of solutionizing the precipitates becomes saturated. Therefore, the heat treatment time is, for example, 5 to 180 minutes. In this specification, the heat treatment time means the time during which the heat treatment temperature T is maintained.

[0104] [Quenching process] In the quenching process according to this embodiment, the intermediate steel material heated in the heat treatment process is quenched. The quenching method is not particularly limited, and a well-known method may be used. For example, the intermediate steel material can be cooled by shower water cooling, mist water cooling, oil cooling, etc.

[0105] [Other Steps] The manufacturing method according to this embodiment may include manufacturing steps other than those described above. For example, the duplex stainless steel material according to this embodiment may be subjected to an aging heat treatment. Aging heat treatment means maintaining the manufactured duplex stainless steel material at a desired temperature. In this case, the aging heat treatment may be performed by a well-known method, and is not particularly limited. For example, the duplex stainless steel material according to this embodiment may also be subjected to a pickling treatment. In this case, the pickling treatment may be performed by a well-known method, and is not particularly limited. For example, the duplex stainless steel material according to this embodiment may also be subjected to cold working. Even when cold working is performed, if the duplex stainless steel material has the above-described chemical composition, Fn1 of 31.0 or more, Fn2 of 95.0 or more, the above-described microstructure, and the amount of precipitated N of 0.0120 mass% or less, it will have a yield strength of 655 MPa or more, excellent pitting corrosion resistance, and excellent low-temperature toughness. Furthermore, other well-known post-treatments may be performed.

[0106] The duplex stainless steel material according to this embodiment can be manufactured by the above steps. Note that the above-described method for manufacturing the duplex stainless steel material is one example, and the duplex stainless steel material may be manufactured by other methods. The present invention will be described in more detail below with reference to examples.

[0107] Molten steel having the chemical compositions shown in Tables 1A and 1B was melted using a 50 kg vacuum melting furnace, and steel ingots (ingots) were produced by an ingot casting method. The ingots had an outer diameter of 120 mm and a weight of 30 kg. Note that "-" in Table 1B indicates that the content of the corresponding element was at the impurity level. For example, the W and Co contents of Test No. A1 were rounded to two decimal places to mean 0%. Similarly, the Ti, Nb, V, rare earth elements (REM), and Sn contents of Test No. A1 were rounded to four decimal places to mean 0%. Similarly, the Ca and Mg contents of Test No. A1 were rounded to five decimal places to mean 0%.

[0108]

[0109]

[0110] Furthermore, the chemical compositions listed in Tables 1A and 1B and Fn1 (= Cr + 3.3 (Mo + 0.5W) + 16N), Fn2 (= 81.543 + 5.67Si + 0.7Mn + 8.85Fn3), and Fn3 (= 0.216 + 0.0688Si - 0.0048Mn - 0.0268Cu + 0.0202Cr - 0.0685Ni + 0.0767Mo - 0.0662N) calculated from the above definitions are shown in Table 2.

[0111]

[0112] Hot forging and hot rolling were performed on the ingots of each test number to produce intermediate steel materials (steel plates) with a thickness of 15 mm. At this time, the temperature at which hot rolling was completed and the time from the completion of hot rolling to the start of quenching for each test number are shown in Table 2. In Table 2, "H (High)" in the "Processing Completion Temperature" column means that the temperature of the material when the final hot rolling was completed was 1150 ° C or higher. In Table 2, "L (Low)" in the "Processing Completion Temperature" column means that the temperature of the material when the final hot rolling was completed was less than 1150 ° C. In Table 2, "S (Short)" in the "Quenching Start Time" column means that the time from the completion of the final hot rolling to the start of quenching was 60 seconds or less. In Table 2, "L (Long)" in the "Quenching Start Time" column means that the time from the completion of the final hot rolling to the start of quenching was 180 seconds. In this embodiment, the rapid cooling in the hot working step was carried out by water cooling the material.

[0113] The quenched intermediate steel material of each test number was heated for 30 minutes at the heat treatment temperature T (°C) shown in Table 2, followed by water cooling, to perform solution treatment. Here, the chemical compositions shown in Tables 1A and 1B and the FnA (= 441 + 42Si - 3.2Mn + 10Cu + 10Cr - 2Ni + 44Mo + 993N + 6W) and FnB (1660 - 8FnC, where FnC = -42.8 - 2189.5C + 10.77Si + 0.182Mn + 7.4Cr + 1.76Ni - 9.4Mo + 5.126Cu - 130.5N + 110.254Al + 84.62Nb - 2.157W) calculated from the above definitions are shown in Table 2. Steel plates of each test number were obtained by the above process.

[0114] [Evaluation Tests] The steel sheets obtained with each test number were subjected to a tensile test, a microstructure observation test, a precipitated N amount measurement test, a pitting corrosion test, and a Charpy impact test.

[0115] [Tensile Test] A tensile test was performed on the steel plate of each test number in accordance with ASTM E8 / E8M (2022). Specifically, a round bar tensile test specimen with a parallel section diameter of 6 mm and a gauge length of 30 mm was prepared from the center of the plate thickness of the steel plate of each test number. The longitudinal direction of the round bar tensile test specimen was parallel to the rolling direction of the steel plate. Using the round bar tensile test specimen of each test number, a tensile test was performed at room temperature (24±3°C) in air to determine the 0.2% offset yield strength (MPa). The determined 0.2% offset yield strength was defined as the yield strength (MPa). The obtained yield strength for each test number is shown in the "YS (MPa)" column of Table 3.

[0116]

[0117] [Microstructure Observation Test] Microstructure observation was performed on the steel plate of each test number to determine the volume fractions of ferrite and austenite. Specifically, a test piece for microstructure observation was prepared from the center of the thickness of the steel plate of each test number, having an observation surface of 5 mm in the rolling direction × 5 mm in the plate width direction. The observation surface of the test piece of each test number was polished to a mirror finish and subjected to electrolytic corrosion in a 7% potassium hydroxide etching solution. The observation surface, on which the structure was revealed by electrolytic corrosion, was observed in 10 fields of view using an optical microscope. The area of ​​each field of view was 1.00 mm ２ (magnification 100x).

[0118] In each visual field of each test number, the microstructure contained negligibly little phases other than ferrite and austenite. That is, the steel sheets of each test number had a microstructure consisting of ferrite and austenite. The observed images obtained in each visual field of each test number were binarized to determine the ferrite area fraction. The arithmetic mean value of the ferrite area fractions in the 10 visual fields was taken as the ferrite volume fraction (%). The difference between 100 and the ferrite volume fraction (%) was taken as the austenite volume fraction (%). The ferrite volume fractions obtained for each test number are shown in the "α" column of the "Microstructure" column in Table 3. Similarly, the austenite volume fractions obtained for each test number are shown in the "γ" column of the "Microstructure" column in Table 3.

[0119] [Precipitated N amount measurement test] The precipitated N amount measurement test was carried out for the steel plate of each test number by the method described above. Specifically, chips were collected from the center of the plate thickness of the steel plate of each test number. The mass M of the chips of each test number was ０ The amount of N in the residue was determined by acid decomposition and then quantification by ICP emission spectrometry. The amount of N in the residue was determined by multiplying the mass M of the chips by the mass M of the chips. ０ The amount of precipitated N (mass %) for each test number is shown in Table 3.

[0120] [Pitting corrosion test] A pitting corrosion test was performed on the steel plate with each test number by the above-mentioned method in accordance with ASTM G48 (2011) Method E to evaluate pitting corrosion resistance. Specifically, three test specimens with a thickness of 3 mm, a width of 15 mm, and a length of 25 mm were prepared by the above-mentioned method. A specific liquid volume of 5 mL / cm was applied to the prepared test specimens. ２ The above test solution (6% FeCl ３ The test pieces were immersed in a 100x optical microscope for 72 hours to check for the presence or absence of pitting corrosion. The number of test pieces in which pitting corrosion was confirmed was counted. The number of test pieces in which pitting corrosion occurred for each test number is shown in Table 3.

[0121] [Charpy Impact Test] A Charpy impact test in accordance with ASTM E23 (2018) was performed on the steel plate of each test number to evaluate low-temperature toughness. Specifically, a full-size T-direction V-notch test specimen was prepared from the center of the plate thickness of the steel plate of each test number in accordance with API 5CT (2019). A Charpy impact test in accordance with ASTM E23 (2018) was performed on the prepared T-direction V-notch test specimen to determine the absorbed energy (J) at -10 ° C. The absorbed energy (J) at -10 ° C. for each test number obtained is shown in Table 3.

[0122] [Test Results] Referring to Tables 1A, 1B, 2, and 3, the steel plates with test numbers A1 to A17 had appropriate chemical compositions, Fn1 of 31.0 or more, and Fn2 of 95.0 or more. These steel plates were also manufactured by the preferred manufacturing method described in the specification. As a result, these steel plates had a yield strength of 655 MPa or more. These steel plates also had a microstructure consisting of 60 to 90% by volume of ferrite and the remainder being austenite. These steel plates also had a precipitated N amount of 0.0120 mass% or less. As a result, these steel plates had excellent pitting resistance, with zero pitting in a pitting corrosion test. These steel plates also had excellent low-temperature toughness, with absorbed energy of 32 J or more at -10°C in a Charpy impact test.

[0123] On the other hand, the steel plates of test numbers B1 and B2 had Fn1 values ​​of less than 31.0. As a result, these steel plates had one or more pits in the pitting corrosion test, and did not have excellent pitting corrosion resistance.

[0124] The steel plate of test number B3 had an Fn2 of less than 95.0, and as a result, the yield strength of this steel plate was less than 655 MPa. In other words, this steel plate did not have the desired yield strength.

[0125] The steel plate of test number B4 had an excessively low Si content and an Fn2 of less than 95.0. As a result, the yield strength of this steel plate was less than 655 MPa. In other words, the steel plate did not have the desired yield strength.

[0126] The steel plate of test number B5 had an excessively low Cu content and an Fn2 of less than 95.0. As a result, the yield strength of this steel plate was less than 655 MPa. In other words, the steel plate did not have the desired yield strength.

[0127] The steel plate of test number B6 had a hot working completion temperature that was too low. As a result, the amount of precipitated N in this steel plate exceeded 0.0120 mass%. As a result, in a Charpy impact test, the absorbed energy of this steel plate at -10°C was less than 32 J, and it did not have excellent low-temperature toughness.

[0128] For the steel plate of test number B7, the time from the completion of the hot working step to the start of rapid cooling was too long. As a result, the amount of precipitated N in this steel plate exceeded 0.0120 mass%. As a result, in the Charpy impact test, this steel plate had an absorbed energy of less than 32 J at -10°C, and did not have excellent low-temperature toughness.

[0129] The steel plates of test numbers B8 to B10 had a heat treatment temperature T (°C) of the solution treatment lower than that of FnA. As a result, the amount of precipitated N in these steel plates exceeded 0.0120 mass%. As a result, in a Charpy impact test, these steel plates had an absorbed energy of less than 32 J at -10°C, and did not have excellent low-temperature toughness.

[0130] The steel plates of test numbers B11 and B12 had a heat treatment temperature T (°C) of the solution treatment lower than that of FnB. As a result, the volume fraction of ferrite in the microstructure of these steel plates was less than 60%. As a result, the yield strength of these steel plates was less than 655 MPa. In other words, they did not have the desired yield strength.

[0131] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A duplex stainless steel material, in mass %, C: 0.001 to 0.030%, Si: 1.00 to 5.00%, Mn: 0.05 to 5.50%, P: 0.035% or less, S: 0.0010% or less, Cr: 21.00 to 27.00%, Ni: 4.00 to 9.00%, Mo: 1.00 to 5.50%, Cu: 0.30 to 3.50%, N: 0.250% or less, Al: 0.001 to 0.050%, B: 0.0001 to 0.0050%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.200%, W: 0 to 4.00%, Ca: 0 to 0.0100%, Mg: 0 to 0.0050%, rare earth elements: 0 to 0.050%, Co: 0 to 2.00%, Sn: 0 to 0.020%, and the balance consists of Fe and impurities, Fn1 defined by formula (1) is 31.0 or more, Fn2 defined by formula (2) is 95.0 or more, the yield strength is 655 MPa or more, the microstructure consists of ferrite with a volume fraction of 60 to 90% and the balance austenite, and in the said duplex stainless steel material, the precipitated N amount is 0.0120 mass % or less. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N (1) Fn2 = 81.543 + 5.67Si + 0.7Mn + 8.85Fn3 (2) Here, in the element symbols in formulas (1) and (2), the content of the corresponding element is substituted in mass %. When the corresponding element is not contained, "0" is substituted for the element symbol. Further, Fn3 in formula (2) is defined by the following formula (3). Fn3 = 0.216 + 0.0688Si - 0.0048Mn - 0.0268Cu + 0.0202Cr - 0.0685Ni + 0.0767Mo - 0.0662N (3) Here, in the element symbols in formula (3), the content of the corresponding element is substituted in mass %.

2. The duplex stainless steel material according to claim 1, containing at least one element selected from the group consisting of: Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.001 to 0.200%, W: 0.01 to 4.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0050%, rare earth elements: 0.001 to 0.050%, Co: 0.01 to 2.00%, and Sn: 0.001 to 0.020%.

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