Duplex stainless steel material

Abstract

Provided is a duplex stainless steel material having high strength, excellent pitting corrosion resistance, and excellent low-temperature toughness. The duplex stainless steel material according to the present disclosure has the chemical composition described in the specification, where 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 being austenite, and 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, 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)

Description

[Technical field]

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

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

[0003] By the way, the current carbon dioxide (CO 2 ) is a global problem. 2 Efforts to reduce carbon dioxide emissions have been underway, and CCUS in particular has been attracting attention. CCUS stands for Carbon dioxide Capture, Utilization and Storage. 2 The three technologies include CO capture, utilization, and storage. 2 As a technology to store CO emitted from industrial facilities such as power plants and factories, 2 CO2 is captured and pumped into depleted oil wells 2 Technology for injecting and storing gas has been attracting attention.

[0004] In other words, such CO 2 Steel materials that are expected to be applied to CO storage technology include those for depleted oil wells. 2High strength is required for press-fitting. That is, a duplex stainless steel material that combines high strength and excellent pitting corrosion resistance has been demanded.

[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 in mass%, 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 is substantially composed of Fe. Further, the C value (= ((%Cr) + 2×(%Si) + 1.5×(%Mo) + 0.75×(%W)) / ((%Ni) + 0.5×(%Mn) + 0.3×(%Cu) + 30×(%C) + 25×(%N))) is 2.00 to 2.60, the D value (= (%Cr) + 3.3×(%Mo) + 1.65×(%W) + (%Cu) + 30×(%N)) is 36.0 or more, and the 0.2% proof stress at room temperature is 650 MPa or more. This duplex stainless steel material is disclosed in Patent Document 1 as satisfying three characteristics of high strength, high toughness, and high pitting corrosion resistance at the same time.

[0007] The duplex stainless steel material disclosed in Patent Document 2 has a chemical composition consisting of, by 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.Al: 0.040% or less, Ni: less than 4 to 6%, Cr: less than 20 to 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. Its metal structure is composed of a duplex structure of a ferrite phase and an austenite phase, without the precipitation of a sigma phase, and the proportion of the ferrite phase in the metal structure is 50% or less in terms of area ratio, and 300 mm 2 There are 15 or fewer oxides with a particle size of 30 μm or more present in the field of view. This duplex stainless steel material is disclosed in Patent Document 2 as being excellent in strength, pitting corrosion resistance, and low-temperature toughness.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] In recent years, furthermore, when storing carbon dioxide (CO 2 ), low-temperature toughness may be required for steel materials. Specifically, when the pressure of the stored CO 2 gas changes, the temperature of the stored gas may decrease due to the Joule-Thomson effect. Therefore, for steel materials assumed to be applied to such CO 2 storage technologies, in addition to high strength and excellent pitting corrosion resistance, excellent low-temperature toughness is also required.

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

Means for Solving the Problems

[0012] The duplex stainless steel material according to the present disclosure is by 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 ratio of 60 to 90% and the balance being austenite, in the 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 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 formula (3), the content of the corresponding element is substituted in mass%.

Advantages of the Invention

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

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0015] Specifically, the inventors aimed to obtain a duplex stainless steel material having a yield strength of 95 ksi or higher (655 MPa or higher) as high strength. Therefore, the inventors first examined a duplex stainless steel material capable of obtaining a yield strength of 95 ksi or higher, excellent pitting corrosion resistance, and excellent low-temperature toughness from the viewpoint of chemical composition.

[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 the duplex stainless steel material. On the other hand, it has been known that in duplex stainless steel materials, nitrogen (N) dissolves in austenite to increase the strength of the steel material. Here, N easily dissolves in austenite and hardly dissolves in ferrite. That is, when the Si content is increased to increase the volume fraction of ferrite, the activity of N increases, and the amount of N that cannot dissolve completely increases, making it easier to precipitate as Cr nitride.

[0017] Here, when a large number of coarse Cr nitrides are formed in the steel material, the low-temperature toughness of the steel material is likely to decrease. Therefore, the inventors considered that it might be possible to achieve both high strength and excellent low-temperature toughness by reducing the N content after increasing the Si content. 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 can be increased and the ferrite can be strengthened, and further, by setting the N content to 0.250% or less, the formation of nitrides might be suppressed.

[0018] As a result of the inventors' detailed studies based on the above findings, a duplex stainless steel having a chemical composition of, by 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 being Fe and impurities, may have a yield strength of 95 ksi or more, excellent pitting corrosion resistance, and excellent low-temperature toughness.

[0019] Here, in the duplex stainless steel having the above chemical composition, the microstructure consists of ferrite with a volume fraction of 60 to 90% and the balance 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 phases other than ferrite and austenite in the microstructure are so few as to be negligible.

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

[0021] Fn1 is an indicator 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 described with reference to the drawings. FIG. 1 is a diagram showing the relationship between Fn1 and the number of pitting corrosion occurrences (number), which is an indicator of pitting corrosion resistance, in this embodiment. FIG. 1 was created using Fn1 and the number of pitting corrosion occurrences (number) for examples in which configurations other than Fn1 satisfy the conditions of this embodiment among the examples described later.

[0022] Referring to FIG. 1, in a duplex stainless steel material having the above-described chemical composition and microstructure and having a yield strength of 95 ksi or more, if Fn1 is 31.0 or more, the number of pitting corrosion occurrences becomes 0, and it can be confirmed that it has excellent pitting corrosion resistance. Therefore, the duplex stainless steel material according to this embodiment has the above-described chemical composition and microstructure, and further, Fn1 is 31.0 or more. As a result, the duplex stainless steel material according to this embodiment can obtain not only a yield strength of 95 ksi or more but also excellent pitting corrosion resistance on the condition that other configurations are satisfied.

[0023] Furthermore, as a result of investigations by the present inventors, in a duplex stainless steel material having the above-described chemical composition and microstructure and satisfying Fn1 of 31.0 or more, if Fn2 defined by the following formula (2) is 95.0 or more, it has been clarified that 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, in the element symbols in formula (2), the content of the corresponding element is substituted in mass%. 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%.

[0024] Fn2 is an index of the strength in the duplex stainless steel material having the above-described chemical composition. The relationship between Fn2 and the strength will be specifically described with reference to the drawings. FIG. 2 is a diagram showing the relationship between Fn2 and the yield strength (MPa) in the present embodiment. FIG. 2 was created using Fn2 and the yield strength (MPa) for examples in which the configurations other than Fn2 among the examples described later satisfy the conditions of the present embodiment.

[0025] Referring to FIG. 2, in the duplex stainless steel material having the above-described chemical composition and microstructure and satisfying Fn1 of 31.0 or more, it can be confirmed that when Fn2 is 95.0 or more, the yield strength becomes 655 MPa or more (95 ksi or more). Therefore, the duplex stainless steel material according to the present embodiment has the above-described chemical composition and microstructure, and after setting Fn1 to 31.0 or more, further sets Fn2 to 95.0 or more. As a result, a yield strength of 95 ksi or more can be obtained on the condition that other configurations of the present embodiment are satisfied.

[0026] As a further result of the study by the present inventors, in the duplex stainless steel material having the above-described chemical composition and microstructure, with Fn1 of 31.0 or more and Fn2 of 95.0 or more, it has been clarified that if the amount of precipitated N is 0.0120 mass% or less, the low-temperature toughness can be enhanced while maintaining a yield strength of 95 ksi or more.

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

[0028] On the one hand, in the duplex stainless steel in which ferrite is strengthened in this way, the solid solubility limit of N in ferrite tends to be small. That is, by adjusting the chemical composition, the volume fraction of austenite in which N easily dissolves decreases, and the solid solubility limit of N in ferrite also decreases. Therefore, a duplex stainless steel having the above-described chemical composition and microstructure, with Fn1 being 31.0 or more and Fn2 being 95.0 or more, may easily form nitrides depending on the alloy component balance.

[0029] Further, in the duplex stainless steel having the above-described chemical composition, among the nitrides, Cr nitrides in particular tend to be formed. Coarse Cr nitrides tend to reduce the low-temperature toughness of the steel. Therefore, if precipitation of coarse Cr nitrides can be suppressed while satisfying formulas (1) and (2), there is a possibility of enhancing the low-temperature toughness of the steel. Thus, in the duplex stainless steel according to the present embodiment, the amount of precipitated N is reduced. Specifically, the duplex stainless steel according to the present embodiment has the above-described chemical composition and microstructure, sets Fn1 to 31.0 or more and Fn2 to 95.0 or more to enhance the pitting corrosion resistance of the steel while increasing the yield strength of the steel to 95 ksi or more, and further reduces the amount of precipitated N to 0.0120 mass% or less. As a result, the inventors speculate that the duplex stainless steel according to the present embodiment has a yield strength of 95 ksi or more, excellent pitting corrosion resistance, and excellent low-temperature toughness.

[0030] Note that, for reasons different from the speculation of the inventors, the duplex stainless steel according to the present embodiment may also have a yield strength of 95 ksi or more, excellent pitting corrosion resistance, and excellent low-temperature toughness. However, it has been proven by the examples described later that a duplex stainless steel having 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, has a yield strength of 95 ksi or more, excellent pitting corrosion resistance, and excellent low-temperature toughness.

[0031] Based on the above findings, the gist of the duplex stainless steel according to the present embodiment completed is as follows.

[0032] [1] A duplex stainless steel material, by mass percentage, 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, In the said duplex stainless steel material, the precipitated N amount is 0.0120 mass% or less, A duplex stainless steel material. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N (1) Fn2 = 81.543 + 5.67Si + 0.7Mn + 8.85Fn3 (2) Here, in formulas (1) and (2), the element symbols are substituted with the content of the corresponding element 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 formula (3), the element symbols are substituted with the content of the corresponding element in mass %.

[0033] [2] The duplex stainless steel material described in [1], 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%, containing at least one element selected from the group consisting of Duplex stainless steel material.

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

[0035] Hereinafter, the duplex stainless steel material according to the present embodiment will be described in detail.

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

[0037] C: 0.001 to 0.030% Carbon (C) stabilizes austenite. If the C content is too low, even if the contents of other elements are within the range of this embodiment, the amount of austenite decreases and the low-temperature toughness of the steel material deteriorates. On the other hand, C forms Cr carbides at the grain boundaries and increases the corrosion susceptibility at the grain boundaries. Therefore, if 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 material deteriorates. Accordingly, the C content is 0.001 to 0.030%. The preferable lower limit of the C content is 0.002%, more preferably 0.005%, still more preferably 0.008%, and even more preferably 0.010%. The preferable upper limit of the C content is 0.028%, more preferably 0.025%, still 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 further strengthens ferrite and increases the strength of the steel material. If 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 sufficiently obtained. On the other hand, if the Si content is too high, even if the contents of other elements are within the range of this embodiment, the volume fraction of ferrite may become too high. If the Si content is too high, it may further stabilize Cr nitrides and reduce the low-temperature toughness of the steel material. Accordingly, the Si content is 1.00 to 5.00%. The preferable lower limit of the Si content is 1.02%, more preferably 1.10%, and still more preferably 1.20%. The preferable upper limit of the Si content is 4.50%, more preferably 4.00%, and still more preferably 3.80%.

[0039] Mn: 0.05 to 5.50% Manganese (Mn) is an austenite-forming element and increases the volume fraction of austenite. Mn further increases the solid solubility limit of N in ferrite. If 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 sufficiently obtained. On the other hand, Mn segregates at grain boundaries together with impurities such as P and S. Therefore, if the Mn content is too high, even if the contents of other elements are within the range of this embodiment, the pitting corrosion resistance of the steel material decreases. Accordingly, the Mn content is 0.05 to 5.50%. The preferable lower limit of the Mn content is 0.30%, more preferably 0.60%, still more preferably 1.00%, still more preferably 1.30%, still more preferably 1.50%, still more preferably 1.60%, and still more preferably 1.80%. The preferable upper limit of the Mn content is 5.35%, more preferably 5.10%.

[0040] P: 0.035% or less Phosphorus (P) is inevitably contained. That is, the lower limit of the P content is more than 0%. P segregates at grain boundaries. Therefore, if 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 material decreases. Accordingly, the P content is 0.035% or less. The preferable upper limit of the P content is 0.032%, more preferably 0.030%. It is preferable that the P content is as low as possible. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.003%.

[0041] S: 0.0010% or less Sulfur (S) is inevitably contained. That is, the lower limit of the S content is more than 0%. S segregates at grain boundaries. Therefore, if 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 material will decrease. Accordingly, the S content is 0.0010% or less. The preferable upper limit of the S content is 0.0008%, and more preferably 0.0006%. It is preferable that the S content is as low as possible. However, an extreme reduction in the S content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the S content is 0.0001%, and more preferably 0.0002%.

[0042] Cr: 21.00 - 27.00% Chromium (Cr) forms a passive film on the surface of the steel material as an oxide, enhancing the corrosion resistance of the steel material. Cr further increases the volume fraction of the ferrite structure in the steel material, enhancing the strength of the steel material. If 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 sufficiently obtained. On the other hand, if the Cr content is too high, even if the contents of other elements are within the range of this embodiment, a large number of Cr nitrides are formed in the steel material, and the low-temperature toughness of the steel material decreases. Therefore, the Cr content is 21.00 - 27.00%. The preferable lower limit of the Cr content is 21.20%, more preferably 21.40%, and even more preferably 21.50%. The preferable upper limit of the Cr content is 26.50%, more preferably 26.00%, and even more preferably 25.50%.

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

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

[0045] Cu: 0.30 - 3.50% Copper (Cu) precipitates in the steel material to increase the yield strength of the steel material. Cu further enhances the acid resistance in a sulfuric acid or hydrogen sulfide environment. If the Cu content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Cu content is too high, even if the contents of other elements are within the scope of this embodiment, the solid solubility limit of N in ferrite decreases, and Cr nitride is likely to be formed. As a result, the low-temperature toughness of the steel material decreases. Therefore, the Cu content is 0.30 to 3.50%. The preferable lower limit of the Cu content is 0.40%, more preferably 0.50%, and even more preferably 0.52%. The preferable 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 inevitably contained. That is, the lower limit of the N content is more than 0%. N stabilizes the austenite of the steel material. N further dissolves in austenite to increase the yield strength of the steel material. On the other hand, N combines with Cr to form Cr nitride. Therefore, if the N content is too high, even if the contents of other elements are within the scope of this embodiment, the low-temperature toughness of the steel material decreases. Therefore, the N content is 0.250% or less. The preferable upper limit of the N content is 0.240%, more preferably 0.230%. The preferable lower limit of the N content for more effectively obtaining the above effects 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, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Al content is too high, even if the contents of other elements are within the scope of this embodiment, coarse inclusions are generated, and the toughness and corrosion resistance of the steel material decrease. Therefore, the Al content is 0.001 to 0.050%. The preferable lower limit of the Al content is 0.003%, more preferably 0.005%, and even more preferably 0.010%. The preferable upper limit of the Al content is 0.048%, more preferably 0.045%. Note that the Al content referred to in this specification means 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 the grain boundaries in the steel material and improves the hot workability of the steel material. If the B content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the B content is too high, even if the contents of other elements are within the scope of this embodiment, boron nitride (BN) is generated, and the toughness of the steel material decreases. Therefore, the B content is 0.0001 to 0.0050%. The preferable lower limit of the B content is 0.0003%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the B content is 0.0045%, more preferably 0.0040%.

[0049] The remainder of the chemical composition of the duplex stainless steel material according to this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition mean those mixed from ores, scraps, or manufacturing environments as raw materials when industrially manufacturing the duplex stainless steel material, and are allowed within a range that does not adversely affect the duplex stainless steel material according to this embodiment.

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

[0051] Ti: 0 to 0.100% Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, Ti forms carbonitrides and increases the strength of the steel. Even if a little 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 range of this embodiment, the strength of the steel becomes too high and the toughness of the steel decreases. Therefore, the Ti content is 0 to 0.100%. The preferable lower limit of the Ti content is more than 0%, more preferably 0.001%, still more preferably 0.002%. The preferable upper limit of the Ti content is 0.060%, more preferably 0.040%, still more preferably 0.020%, and even more preferably 0.010%.

[0052] Nb: 0 to 0.100% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb forms carbonitrides and increases the strength of the steel. Even if a little 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 range of this embodiment, the strength of the steel becomes too high and the toughness of the steel decreases. Therefore, the Nb content is 0 to 0.100%. The preferable lower limit of the Nb content is more than 0%, more preferably 0.001%, still more preferably 0.005%, even more preferably 0.010%, and still more preferably 0.030%. The preferable upper limit of the Nb content is 0.080%, more preferably 0.070%, still more preferably 0.060%.

[0053] V: 0 to 0.200% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V forms a carbonitride and increases the strength of the steel material. Even if a small amount of V is contained, the above effect can be obtained to a certain extent. However, if the V content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the V content is 0 to 0.200%. The preferable lower limit of the V content is more than 0%, more preferably 0.001%, and even more preferably 0.003%. The preferable 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 may not be contained. That is, the W content may be 0%. When contained, W forms a carbonitride and increases the strength of the steel material. Even if a small amount of W is contained, the above effect can be obtained to a certain extent. However, if 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 material becomes too high and the toughness of the steel material decreases. Therefore, the W content is 0 to 4.00%. The preferable lower limit of the W content is more than 0%, more preferably 0.01%, and even more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.20%. The preferable upper limit of the W content is 3.50%, more preferably 3.00%.

[0055] The chemical composition of the above-described duplex stainless steel material may further contain one or more elements selected from the group consisting of Ca, Mg, and rare earth elements in place of a part of Fe. 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 may not be contained. That is, the Ca content may be 0%. When contained, Ca immobilizes S in the steel as sulfide to render it harmless and improves the hot workability of the steel. Even if a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content is too high, even if the contents of other elements are within the scope of this embodiment, the oxides in the steel become coarsened and the toughness of the steel decreases. Therefore, the Ca content is 0 to 0.0100%. The preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, still more preferably 0.0005%, and still more preferably 0.0010%. The preferable upper limit of the Ca content is 0.0080%, more preferably 0.0060%, still more preferably 0.0040%, and still more preferably 0.0030%.

[0057] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg immobilizes S in the steel as sulfide to render it harmless and improves the hot workability of the steel. Even if 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 scope of this embodiment, the oxides in the steel become coarsened and the toughness of the steel decreases. Therefore, the Mg content is 0 to 0.0050%. The preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%, and still more preferably 0.0003%. The preferable upper limit of the Mg content is 0.0030%, more preferably 0.0020%, and still more preferably 0.0010%.

[0058] Rare earth elements: 0 to 0.050% The rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When contained, REM immobilizes S in the steel material as sulfide to detoxify it and improves the hot workability of the steel material. Even if a small amount of REM is contained, the above effect can be obtained to some extent. However, if the REM content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel material become coarse and the toughness of the steel material decreases. Therefore, the REM content is 0 to 0.050%. The preferable lower limit of the REM content is more than 0%, more preferably 0.001%, still more preferably 0.003%, still more preferably 0.005%, and still more preferably 0.010%. The preferable upper limit of the REM content is 0.040%, more preferably 0.020%.

[0059] In addition, REM in this specification means 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 which are lanthanoids. Further, the REM content in this specification means the total content of these elements.

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

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

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

[0063] [Fn1] The duplex stainless steel material according to this embodiment is based on having the above chemical composition, and Fn1 defined by the following formula (1) is 31.0 or more. Fn1 = Cr + 3.3(Mo + 0.5W) + 16N (1) Here, in the element symbols in formula (1), the contents of the corresponding elements are substituted in mass%. When the corresponding element is not contained, "0" is substituted for that element symbol.

[0064] Fn1 is an indicator of pitting corrosion resistance in the duplex stainless steel material having the above-described chemical composition. If Fn1 is 31.0 or more, the pitting corrosion resistance of the duplex stainless steel material can be enhanced on condition that it has other configurations of the present embodiment. Therefore, in the present embodiment, Fn1 is set to 31.0 or more.

[0065] A preferable lower limit of Fn1 is 31.1, more preferably 31.5, and still more preferably 32.0. Higher Fn1 is preferable. However, in the duplex stainless steel material according to the present 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 the present embodiment, Fn1 is obtained by rounding off the second decimal place of the obtained numerical value.

[0066] [Fn2] The duplex stainless steel material according to the present embodiment has the above-described chemical composition, and on the premise 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 content of the corresponding element is substituted for the element symbol in formula (2) in mass%. 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, the content of the corresponding element is substituted for the element symbol in formula (3) in mass%.

[0067] Fn2 is an indicator of strength in the 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 on condition that it has other configurations of the present embodiment. Therefore, in the present embodiment, Fn2 is 95.0 or more.

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

[0069] Fn3 is an index indicating the degree of ferrite strengthening in the duplex stainless steel material having the above chemical composition. The higher Fn3 is, the more the ferrite is strengthened and the easier it is for the strength of the steel material to increase. However, in this embodiment, the value of Fn3 only needs to satisfy that Fn2 is 95.0 or more and is not particularly limited. Fn3 may be, for example, 0.50 to 1.24, or may be 0.51 to 1.00. In this embodiment, Fn3 is obtained by rounding off the third decimal place of the obtained numerical value.

[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. That is, the yield strength of the duplex stainless steel material according to this embodiment may be 655 to 862 MPa, may be 655 to 827 MPa, or may be 655 to 793 MPa.

[0071] The yield strength of the duplex stainless steel material according to this embodiment is defined by the following method. Specifically, a tensile test is performed in accordance with the method specified in ASTM E8 / E8M (2022). A test piece is prepared from the steel material of this embodiment. When the steel material is a steel plate, a tensile test piece is prepared from the central part of the plate thickness. In this case, the longitudinal 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 prepared. In this case, the longitudinal 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 prepared from the R / 2 position. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the round bar. Note that in this specification, the R / 2 position of the round bar means the central 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, a parallel part diameter of 6 mm and a gauge length of 30 mm. Using the test piece, a tensile test is carried out at room temperature (25 °C) in the atmosphere in accordance with the method specified in 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 obtained by rounding off the first decimal place of the obtained numerical value.

[0073] [Microstructure] The duplex stainless steel material according to this embodiment has a microstructure consisting of 60 - 90% ferrite by volume fraction and the balance being austenite. In this specification, the microstructure being "consisting of ferrite and austenite" means that the phases other than ferrite and austenite in the microstructure are so few that they can be ignored. For example, in the chemical composition of the duplex stainless steel material according to this embodiment, the volume fractions of precipitates and inclusions are negligibly small compared to the volume fractions of ferrite and austenite. That is, the microstructure of the duplex stainless steel material according to this embodiment may contain a small amount 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%. The preferred lower limit of the ferrite volume fraction is 63%, more preferably 65%, still more preferably 70%, and even more preferably 73%. The preferred upper limit of the ferrite volume fraction is 89%, more preferably 88%.

[0075] In this embodiment, the ferrite volume fraction of the duplex stainless steel material is defined by a method compliant with 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 with a size of 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 with a size of 5 mm in the pipe axis direction and 5 mm in the pipe circumferential direction is prepared from the center of the wall thickness. In this specification, the pipe circumferential direction of the steel pipe means the direction perpendicular to the pipe axis direction and the pipe diameter direction. When the steel material is a round steel, a test piece having an observation surface with a size of 5 mm in the axial direction and 5 mm in the circumferential direction is prepared from the R / 2 position. Also, in this specification, the circumferential direction of the round steel means the direction perpendicular to the axial direction and the radial direction. Note that if the above observation surface is obtained, the size of the test piece is not particularly limited. Also, in this embodiment, the test piece is prepared so that the observation position is at 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 corroded in a 7% potassium hydroxide corrosion solution to reveal the structure. The observation surface on which the structure is revealed is observed in 10 fields using an optical microscope. In this embodiment, the microscopic observation is carried out so that the total area of the 10 fields is 10 mm 2 or more. That is, the area of each field is, for example, 1.00 mm 2(Magnification: 100 times). Observation images in each field of view are acquired, and the area ratio of ferrite is determined by image analysis. Specifically, binarization processing is performed on the acquired observation images to distinguish ferrite from austenite. For those skilled in the art, it is naturally possible to distinguish ferrite from austenite based on the contrast by binarization processing. Furthermore, by performing image analysis, the area of ferrite is derived. In this embodiment, the arithmetic mean value of the area ratios of ferrite in 10 fields of view obtained is defined as the volume ratio (%) of ferrite. In this embodiment, the volume ratio (%) of ferrite is obtained by rounding off the first decimal place of the obtained numerical value.

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

[0078] In this embodiment, the preferable upper limit of the amount of precipitated N is 0.0115 mass%, more preferably 0.0110 mass%, still more preferably 0.0105 mass%, and still more preferably 0.0100 mass%. It is preferable that the amount of precipitated N is less. That is, the amount of precipitated N may be 0 mass%. In addition, in the 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 precipitated N amount is defined by the following method. Specifically, a test piece for measuring the precipitated N amount is prepared from the duplex stainless steel material according to this embodiment. The test piece is, for example, cutting chips. 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 steel, the test piece is prepared from the R / 2 position.

[0080] Quantify the mass M 0 (g) of the prepared test piece. From the viewpoint of ensuring the statistical accuracy and representativeness of the precipitated N amount, the mass M 0 of the test piece is preferably about 2 g. Subsequently, the test piece is immersed in a 10% bromine acetic acid methyl 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 analysis to quantify the N amount in the residue. Divide the N amount in the obtained residue by the mass M 0 of the test piece to obtain the N content (mass%) and define it as the precipitated N amount (mass%). Also, in this embodiment, the precipitated N amount (mass%) is obtained by rounding off the fifth decimal place of the obtained numerical value.

[0081] [Pitting corrosion resistance] The duplex stainless steel material according to this embodiment has the above chemical composition, Fn1 is 31.0 or more, Fn2 is 95.0 or more, has the above microstructure, and the precipitated N amount is 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, the 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 in thickness, 15 mm in width, and 25 mm in length. Also, when the steel material is a steel plate, the test piece is prepared from the central part 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 central part of the wall thickness. In this case, the longitudinal direction of the test piece is parallel to the pipe axis direction of the steel pipe. When the steel material is a round steel, 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 steel.

[0083] The test solution is 6% FeCl 3 + 1% HCl. The test piece is immersed in the test solution with a liquid ratio of 5 mL / cm 2 or more. The test temperature is 30°C and the test time is 72 hours. The test piece after 72 hours has elapsed is observed with a 100-fold optical microscope to confirm the presence or absence of pitting corrosion. Under the above conditions, when no pitting corrosion is confirmed on the test piece, it is determined that the duplex stainless steel material has excellent pitting corrosion resistance.

[0084] [Low-temperature toughness] The duplex stainless steel material according to this embodiment has the above-described chemical composition, Fn1 is 31.0 or more, Fn2 is 95.0 or more, has the above-described microstructure, and the precipitated N amount is 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, from the stainless steel material according to this embodiment, full-size or sub-size V-notch test pieces are prepared 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 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 axis direction of the steel pipe is defined as the "L direction", and the direction perpendicular to the C direction and the L direction is defined as the "T direction". When the steel material is a round steel, the cross-sectional diameter direction of the round steel is defined as the "C direction", the axial direction of the round steel is defined as the "L direction", and the direction perpendicular to the C direction and the L direction is defined as the "T direction".

[0086] For the produced V-notch test piece in the T direction, a Charpy impact test is carried out in accordance with ASTM E23 (2018) to obtain the absorbed energy (J) at -10°C. When a sub-size V-notch test piece is used, the obtained absorbed energy is divided by the reduction factor described in API 5CT (2019) and converted to the absorbed energy of a full-size V-notch test piece. In this embodiment, the absorbed energy (J) at -10°C is obtained by rounding off the first decimal place of the obtained numerical value.

[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, even if the wall thickness is 5 mm or more, it has high strength, excellent pitting corrosion resistance, and excellent low-temperature toughness.

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

[0089] [Material preparation process] In the material preparation process according to this embodiment, a material having the above-described chemical composition is prepared. The material may be prepared by manufacturing it or by purchasing it from a third party. That is, the method of preparing the material is not particularly limited.

[0090] When manufacturing the material, for example, it is manufactured by the following method. Molten steel having the above-described chemical composition is manufactured. Using the molten steel, a slab (slab, bloom, or billet) is manufactured by the continuous casting method. A steel ingot may be manufactured from the molten steel by the ingot-making method. If necessary, the slab, bloom, or ingot may be block-rolled to manufacture a billet. The material is manufactured by the above steps.

[0091] [Hot working process] In the hot working process according to this embodiment, the material prepared in the above material preparation process is hot-worked to manufacture an intermediate steel material. In this specification, the intermediate steel material is a plate-shaped steel material when the final product is a steel plate, a plain pipe when the final product is a steel pipe, and a bar-shaped steel material with a circular cross-section perpendicular to the axial direction when the final product is a round steel. The hot working may be hot forging, hot extrusion, or hot rolling. The method of hot working is not particularly limited and may be a well-known method.

[0092] When the intermediate steel material is a plain pipe (seamless steel pipe), first, the material is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300 °C. Hot working is performed on the material extracted from the heating furnace to manufacture a plain pipe. At this time, as the method of hot working, for example, the Eugene Cédulne method or the Erhardt push bench method (that is, hot extrusion) may be performed, or piercing rolling by the Mannesmann method (that is, hot rolling) may be performed. Note that the hot working may be performed only once or multiple times. For example, after performing the above-described piercing rolling on the material, the above-described hot extrusion may be performed. For example, further, after performing the above-described piercing rolling on the material, stretch rolling may be performed.

[0093] In the case where the intermediate steel material is a bar-shaped steel material (round steel) with a circular cross-section, first, the raw material is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300 °C. Hot working is performed on the raw material extracted from the heating furnace to produce an intermediate steel material with a circular cross-section perpendicular to the axial direction. The hot working is, for example, block rolling by a block rolling mill or hot rolling by a continuous rolling mill. The continuous rolling mill has a horizontal stand having a pair of grooved rolls arranged side by side in the vertical direction and a vertical stand having a pair of grooved rolls arranged side by side in the horizontal direction, which are alternately arranged.

[0094] In the case where the intermediate steel material is a plate-shaped steel material (steel plate), first, the raw material is heated in a heating furnace. The heating temperature is not particularly limited, but for example, it is 1100 to 1300 °C. Hot rolling is performed on the raw material extracted from the heating furnace using a block rolling 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 process, hot working is performed by a well-known method to produce an intermediate steel material with a desired shape.

[0095] Preferably, in the hot working process according to this embodiment, quenching 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 means the temperature of the raw material when the raw material is separated from the rolling roll or die. That is, when hot rolling is performed as the hot working, the time when the rolling by the final rolling roll is completed is defined as the time when the hot working is completed. That is, further, when hot extrusion is performed as the hot working, the time when the raw material is extruded from the die is defined as the time when the hot working is completed. That is, further, when hot forging is performed as the hot working, the time when the raw material is separated from the die is defined as the time when the hot working is completed.

[0096] If the temperature at which hot working is completed is too low, nitrides may precipitate in the material during hot working or until subsequent rapid cooling. In this case, the nitrides coarsen, and there is a concern that the nitrides cannot be sufficiently dissolved in the solution treatment step described later. Therefore, in the hot working step according to this embodiment, the temperature at which hot working is completed is preferably 1150 °C or higher. The upper limit of the temperature at which hot working is completed is not particularly limited, but is, for example, 1300 °C, similar to the heating temperature.

[0097] Preferably, the time from when hot working is completed to when rapid cooling is started is 60 seconds or less. If the time from when hot working is completed to when rapid cooling is started is too long, nitrides may precipitate in the material before rapid cooling starts. In this case, the nitrides coarsen, and there is a concern that the nitrides cannot be sufficiently dissolved in the solution treatment step described later. Therefore, in the hot working step of this embodiment, it is preferable that the time from when hot working is completed to when rapid cooling is started is 60 seconds or less.

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

[0099] [Solution Treatment Step] In the solution treatment step according to this embodiment, a solution treatment is performed on the intermediate steel material manufactured in the above hot working step to manufacture a duplex stainless steel material. Solution treatment means a heat treatment for dissolving intermetallic compounds and precipitates in the intermediate steel material. That is, the solution treatment step includes a heat treatment step of heat-treating the intermediate steel material at a desired temperature (heat treatment step) and a rapid cooling step of rapidly cooling the heat-treated intermediate steel material (rapid cooling step). Each step will be described in detail below.

[0100] [Heat Treatment Step] In the heat treatment step according to this embodiment, a heat treatment is performed on the intermediate steel material manufactured in the above 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, in formulas (A) and (B), for "T", the temperature of the heat treatment furnace used for heat treatment is substituted in °C, and for the element symbols, the content of the corresponding element is substituted in mass%. When the corresponding element is not contained, "0" is substituted for that element symbol. Also, 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, for the element symbols in formula (C), the content of the corresponding element is substituted in mass%. When the corresponding element is not contained, "0" is substituted for that element symbol.

[0101] Define FnA = 441 + 42Si - 3.2Mn + 10Cu + 10Cr - 2Ni + 44Mo + 993N + 6W. FnA is an index indicating the ease of dissolution of Cr nitride. As described above, in the duplex stainless steel material having the chemical composition of this embodiment, Cr nitride is likely to be formed as the nitride. Also, Si, Cu, Cr, Mo, N, and W stabilize Cr nitride and make it difficult to dissolve by heat treatment. On the other hand, Mn and Ni promote the dissolution of Cr nitride. Specifically, in the heat treatment process, by heating to a heat treatment temperature T (°C) of FnA or higher, Cr nitride can be dissolved, 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 FnA or higher.

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

[0103] Incidentally, the upper limit of the heat treatment temperature T is, for example, 1200°C. Also, if the heat treatment time is too short, precipitates that reduce the pitting corrosion resistance may remain in the duplex stainless steel material after the solution treatment step. On the other hand, if the heat treatment time is too long, the effect of dissolving 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 held at the heat treatment temperature T.

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

[0105] [Other steps] In the manufacturing method according to this embodiment, manufacturing steps other than the above may be included. For example, age heat treatment may be performed on the duplex stainless steel material according to this embodiment. Age heat treatment means holding the manufactured duplex stainless steel material at a desired temperature. In this case, the age heat treatment may be performed by a well-known method and is not particularly limited. For example, further, pickling treatment may be performed on the duplex stainless steel material according to this embodiment. In this case, the pickling treatment may be performed by a well-known method and is not particularly limited. For example, further, cold working may be performed on the duplex stainless steel material according to this embodiment. Even when cold working is performed, if it has the above-described chemical composition, Fn1 is 31.0 or more, Fn2 is 95.0 or more, has the above-described microstructure, and the precipitated N amount is 0.0120 mass% or less, it has 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] Through the above steps, the duplex stainless steel material according to this embodiment can be manufactured. Note that the above-described manufacturing method of the duplex stainless steel material is an example, and the duplex stainless steel material may be manufactured by other methods. Hereinafter, the present invention will be described in more detail with reference to examples.

Example

[0107] Molten steel having the chemical compositions shown in Table 1A and Table 1B was melted using a 50 kg vacuum melting furnace, and steel ingots were produced by the ingot casting method. The ingots had an outer diameter of 120 mm and a weight of 30 kg. Note that "-" in Table 1B means that the content of the corresponding element was at the impurity level. For example, the W and Co contents of test number A1, rounded to the third decimal place, meant 0%. Similarly, the Ti, Nb, V, rare earth element (REM), and Sn contents of test number A1, rounded to the fourth decimal place, meant 0%. Similarly, the Ca and Mg contents of test number A1, rounded to the fifth decimal place, meant 0%.

[0108]

Table 1A

[0109]

Table 1B

[0110] Furthermore, the chemical compositions described in Table 1A and Table 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) obtained from the above definitions are shown in Table 2.

[0111]

Table 2

[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, Table 2 shows the temperature at which hot rolling was completed and the time from the completion of hot rolling to the start of rapid cooling for each test number. 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. "L (Low)" in the "Processing Completion Temperature" column in Table 2 means that the temperature of the material when the final hot rolling was completed was less than 1150 °C. Also, "S (Short)" in the "Rapid Cooling Start Time" column in Table 2 means that the time from the completion of the final hot rolling to the start of rapid cooling was 60 seconds or less. "L (Long)" in the "Rapid Cooling Start Time" column in Table 2 means that the time from the completion of the final hot rolling to the start of rapid cooling was 180 seconds. In this embodiment, rapid cooling in the hot working process was performed by water-cooling the material.

[0113] The solution treatment was carried out by heating the quenched intermediate steel materials of each test number at the heat treatment temperature T (°C) shown in Table 2 for 30 minutes and then water-cooling. Here, the chemical compositions described in Table 1A and Table 1B, 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) are shown in Table 2. By the above process, steel plates of each test number were obtained.

[0114] [Evaluation Test] Tensile tests, microstructural observation tests, precipitation N amount measurement tests, pitting corrosion tests, and Charpy impact tests were carried out on the obtained steel plates of each test number.

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

[0116]

Table 3

[0117] [Microstructure Observation Test] For the steel plates of each test number, microstructure observation was carried out to determine the volume fractions of ferrite and austenite. Specifically, from the central part of the plate thickness of the steel plates of each test number, test specimens for microstructure observation with an observation surface of 5 mm in the rolling direction × 5 mm in the plate width direction were prepared. The observation surfaces of the test specimens of each test number were polished to a mirror surface and electrolytically corroded in a 7% potassium hydroxide etching solution. The observation surface where the microstructure was revealed by electrolytic corrosion was observed in 10 fields using an optical microscope. The area of each field was 1.00 mm 2 (magnification 100 times).

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

[0119] [N Precipitation Measurement Test] For each steel plate with a test number, the N precipitation measurement test was carried out by the above method. Specifically, chips were collected from the central part of the plate thickness of each steel plate with a test number. The mass M 0 of the chips for each test number was about 2 g each. The solution obtained by dissolving the chips for each test number in a 10% bromine acetic acid methyl solution was suction filtered through a polyester filter with a pore size of 0.2 μm to capture the residue. The residue was acid decomposed, and the amount of N in the residue was quantified by ICP emission analysis. The amount of N in the obtained residue was divided by the mass M 0 of the chips to obtain the N precipitation amount. The N precipitation amounts (mass %) for each test number obtained are shown in Table 3.

[0120] [Pitting Corrosion Test] For each steel plate with a test number, the pitting corrosion test was carried out by the above method in accordance with ASTM G48(2011) Method E to evaluate the pitting corrosion resistance. Specifically, three test pieces with a thickness of 3 mm, a width of 15 mm, and a length of 25 mm were produced by the above method. For the produced test pieces, the test solution (6% FeCl 2 +1% HCl) with a specific liquid volume of 5 mL / cm 3 or more and a temperature of 30 °C was immersed for 72 hours. The test pieces after 72 hours were observed with a 100-fold optical microscope to confirm the presence or absence of pitting corrosion. The number of test pieces with pitting corrosion confirmed was counted. The number of pitting corrosion occurrences (pieces) for each test number obtained are shown in Table 3.

[0121] [Charpy Impact Test] For each steel plate with a test number, the Charpy impact test was carried out in accordance with ASTM E23(2018) to evaluate the low-temperature toughness. Specifically, from the central part of the plate thickness of each steel plate with a test number, a full-size T-direction V-notch test piece was produced in accordance with API 5CT(2019). For the produced T-direction V-notch test piece, the Charpy impact test was carried out in accordance with ASTM E23(2018) to obtain the absorbed energy (J) at -10 °C. The absorbed energy (J) at -10 °C for each test number obtained are shown in Table 3.

[0122] [Test Results] Referring to Table 1A, Table 1B, Table 2 and Table 3, the steel plates of Test Nos. A1 to A17 had appropriate chemical compositions, Fn1 was 31.0 or more, and Fn2 was 95.0 or more. These steel plates were further 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 further had a microstructure consisting of 60 to 90% ferrite by volume and the balance being austenite. These steel plates further had a precipitated N amount of 0.0120 mass% or less. As a result, these steel plates had 0 pitting corrosion occurrences in the pitting corrosion test and had excellent pitting corrosion resistance. These steel plates further had an absorbed energy of 32 J or more at -10°C in the Charpy impact test and had excellent low-temperature toughness.

[0123] On the other hand, the steel plates of Test Nos. B1 and B2 had Fn1 less than 31.0. As a result, these steel plates had 1 or more pitting corrosion occurrences in the pitting corrosion test and did not have excellent pitting corrosion resistance.

[0124] The steel plate of Test No. B3 had Fn2 less than 95.0. As a result, this steel plate had a yield strength of less than 655 MPa. That is, it did not have the desired yield strength.

[0125] The steel plate of Test No. B4 had an Si content that was too low and further had Fn2 less than 95.0. As a result, this steel plate had a yield strength of less than 655 MPa. That is, it did not have the desired yield strength.

[0126] The steel plate of Test No. B5 had a Cu content that was too low and further had Fn2 less than 95.0. As a result, this steel plate had a yield strength of less than 655 MPa. That is, it did not have the desired yield strength.

[0127] For the steel plate with test number B6, the finishing temperature in the hot working process was too low. As a result, the amount of precipitated N in this steel plate exceeded 0.0120 mass%. As a result, in the 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 with test number B7, the time from the completion of hot working to the start of rapid cooling in the hot working process 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, the absorbed energy of this steel plate at -10°C was less than 32 J, and it did not have excellent low-temperature toughness.

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

[0130] For the steel plates with test numbers B11 and B12, the heat treatment temperature T (°C) of the solution treatment was lower than FnB. As a result, in the microstructure of these steel plates, the volume fraction of ferrite was less than 60%. As a result, the yield strength of these steel plates was less than 655 MPa. That is, 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 the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. A duplex stainless steel material, In mass percent, C: 0.001-0.030%, Si: 1.00-5.00%, Mn: 0.05-5.50% P: 0.035% or less, S: 0.0010% or less, Cr: 21.00-27.00%, Ni: 4.00-9.00%, Mo: 1.00-5.50%, Cu: 0.30-3.50%, N: 0.250% or less, Al: 0.001-0.050%, B: 0.0001 to 0.0050%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0-0.200%, W: 0-4.00%, Ca: 0-0.0100%, Mg: 0 to 0.0050%, Rare earth elements: 0 to 0.050%, Co: 0-2.00%, Sn: 0 to 0.020%, and The balance is 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 composed of 60 to 90% by volume of ferrite and the remainder of austenite, In the duplex stainless steel material, The amount of precipitated N is 0.0120 mass% or less. Duplex stainless steel material. 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%. When the corresponding element is not contained, the element symbol is substituted with "0". 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 %.

2. 2. The duplex stainless steel material according to claim 1, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.001-0.200%, W: 0.01-4.00%, Ca: 0.0001-0.0100%, 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%; containing one or more elements selected from the group consisting of: Duplex stainless steel material.