Fe-Cr-Ni ALLOY MATERIAL

Abstract

The present invention provides an Fe-Cr-Ni alloy material which has high strength and decreased strength anisotropy. An Fe-Cr-Ni alloy material according to the present disclosure contains, in mass%, 0.030% or less of C, 0.01-1.00% of Si, 0.01-2.00% of Mn, 0.040% or less of P, 0.0050% or less of S, 0.01-0.50% of Al, 28.0-36.5% of Ni, 19.0-27.5% of Cr, 2.00-6.00% of Mo, 0.01-3.00% of Cu, not less than 0.010% but less than 0.220% of N, 0.01-2.00% of Co, and 0.010% or less of O, with the balance being made up of Fe and impurities. In the microstructure, the standard deviation of the crystal grain size number of the austenite grains is 0.60 or less, and the tensile yield strength is 758 MPa or more.

Description

Fe-Cr-Ni alloy material

[0001] The present disclosure relates to alloy materials, and more particularly to Fe—Cr—Ni alloy materials.

[0002] Oil well alloys, such as oil country tubular goods, are used in oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells"). Many oil wells are in sour environments containing corrosive hydrogen sulfide. In this specification, a sour environment refers to an acidic environment containing hydrogen sulfide. A sour environment may contain not only hydrogen sulfide but also carbon dioxide. Materials used in such sour environments are required to have excellent corrosion resistance.

[0003] Materials that require excellent corrosion resistance include, for example, 18-8 stainless steel materials such as SUS304H, SUS316H, SUS321H, and SUS347H, and Fe—Cr—Ni alloy materials, such as Alloy 800H, which is specified as NCF800H in the JIS standard. Fe—Cr—Ni alloy materials have superior corrosion resistance compared to 18-8 stainless steel materials. Fe—Cr—Ni alloy materials are also more economical than Ni-based alloy materials, such as Alloy 617. For this reason, Fe—Cr—Ni alloy materials are sometimes used as alloy materials for oil wells used in sour environments.

[0004] Japanese Patent Laid-Open Publication No. 2-217445 (Patent Document 1) and International Publication No. 2015 / 072458 (Patent Document 2) propose an oil well alloy material having excellent corrosion resistance.

[0005] The alloy material described in Patent Document 1 is an Fe—Cr—Ni alloy containing 27-32% Ni, 24-28% Cr, 1.25-3.0% Cu, 1.0-3.0% Mo, 1.5-2.75% Si, and 1.0-2.0% Mn, with N being restricted to 0.015% or less, B being 0.10% or less, C being 0.10% or less, Al being 0.30% or less, P being 0.03% or less, and S being 0.02% or less, with the balance being essentially Fe and impurities. Patent Document 1 describes this alloy material as having high strength, galling resistance, and corrosion resistance under stress.

[0006] The alloy material described in Patent Document 2 is a Ni-Cr alloy material, and contains, in mass %, Si: 0.01 to 0.5%, Mn: 0.01 to less than 1.0%, Cu: 0.01 to less than 1.0%, Ni: 48 to less than 55%, Cr: 22 to 28%, Mo: 5.6 to less than 7.0%, N: 0.04 to 0.16%, sol. The alloy has a chemical composition consisting of Al: 0.03 to 0.20%, REM: 0.01 to 0.074%, W: 0 to less than 8.0%, Co: 0 to 2.0%, one or more of Ca and Mg: 0.0003 to 0.01% in total, one or more of Ti, Nb, Zr, and V: 0 to 0.5% in total, and the balance being Fe and impurities, in which the impurities are C: 0.03% or less, P: 0.03% or less, S: 0.001% or less, and O: 0.01% or less, and a dislocation density ρ satisfying the formula (7.0 × 10 15 ≦ρ≦2.7×10 16 -2.67 x 10 17 × [REM (%)]). Patent Document 2 describes that this alloy material is excellent in hot workability and toughness, as well as in corrosion resistance (stress corrosion cracking resistance at high temperatures exceeding 200°C and in environments containing hydrogen sulfide), and has a yield strength (0.2% proof stress) of 965 MPa or more.

[0007] Japanese Patent Application Laid-Open No. 2-217445 International Publication No. 2015 / 072458

[0008] In recent years, as oil wells have become deeper, there has been a demand for higher strength alloy materials for oil wells, i.e., Fe-Cr-Ni alloy materials intended for use as oil well alloy materials are required to have high strength.

[0009] In recent oil wells, in addition to vertical wells drilled straight down the vertical, inclined wells are also on the rise. Inclined wells are formed by drilling the well by bending its extension direction from vertical down to horizontal. Including a horizontally extending portion (horizontal well), inclined wells can cover a wide area of ​​the geological formations where production fluids such as crude oil and gas are buried, thereby increasing the production efficiency of the production fluids.

[0010] On the other hand, when used in such a deviated well, the alloy material may be subjected to a compressive force. In this case, it is preferable that the alloy material have not only a high tensile yield strength but also a high compressive yield strength. In other words, it is preferable that an Fe—Cr—Ni alloy material intended for use in a deviated well not only has high strength but also has reduced strength anisotropy. However, in the above Patent Documents 1 and 2, only the tensile yield strength is considered as the strength of the Fe—Cr—Ni alloy material. In other words, the above Patent Documents 1 and 2 do not consider the strength anisotropy of the alloy material.

[0011] An object of the present disclosure is to provide an Fe—Cr—Ni alloy material having high strength and reduced strength anisotropy.

[0012] The Fe—Cr—Ni alloy material according to the present disclosure has, in mass %, C: 0.030% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Al: 0.01 to 0.50%, Ni: 28.0 to 36.5%, Cr: 19.0 to 27.5%, Mo: 2.00 to 6.00%, Cu: 0.01 to 3.00%, N: 0.010 to less than 0.220%, Co: 0.01 to 2.00%, O: 0.010% or less, V: 0 to 0.50%, Nb: 0 to 0.10%, Ti: 0 to 0.40%, W: 0 to 3.0%, The alloy consists of Sn: 0 to 0.010%, Ca: 0 to 0.0100%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.100%, and the balance being Fe and impurities, and in the microstructure, the standard deviation of the grain size number of the austenite grains is 0.60 or less, and the tensile yield strength is 758 MPa or more.

[0013] The Fe—Cr—Ni alloy material according to the present disclosure has high strength and reduced strength anisotropy.

[0014] FIG. 1 is a diagram showing the relationship between the standard deviation σ of the grain size number and the anisotropy index AI (=compression YS / tensile YS) in this example.

[0015] The present inventors first focused on Fe—Cr—Ni alloy materials having a tensile yield strength of 110 ksi (758 MPa) or more as Fe—Cr—Ni alloy materials having high strength. Next, the present inventors investigated the strength anisotropy of Fe—Cr—Ni alloy materials having a tensile yield strength of 758 MPa or more from the viewpoint of chemical composition.

[0016] As a result, the inventors found that the composition of the steel sheet was, in mass%, C: 0.030% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Al: 0.01 to 0.50%, Ni: 28.0 to 36.5%, Cr: 19.0 to 27.5%, Mo: 2.00 to 6.00%, Cu: 0.01 to 3.00%, N: 0.010 to less than 0.220%, Co: 0.01 to 2.00%, O: 0.010 %, V: 0 to 0.50%, Nb: 0 to 0.10%, Ti: 0 to 0.40%, W: 0 to 3.0%, Sn: 0 to 0.010%, Ca: 0 to 0.0100%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.100%, and the balance being Fe and impurities, it was thought that an Fe-Cr-Ni alloy material would have a tensile yield strength of 758 MPa or more and further have the potential to reduce strength anisotropy.

[0017] On the other hand, even in the case of an Fe—Cr—Ni alloy material having the above-mentioned chemical composition, if the alloy has a tensile yield strength of 758 MPa or more, strength anisotropy may become large. Therefore, the inventors of the present invention have conducted detailed studies on reducing the strength anisotropy of an alloy material having the above-mentioned chemical composition and a tensile yield strength of 758 MPa or more.

[0018] Here, an Fe—Cr—Ni alloy material having the above-mentioned chemical composition has a microstructure consisting of austenite. In this specification, "a microstructure consisting of austenite" means that phases other than austenite are present in a negligible amount. Therefore, the inventors have focused on the austenite grains of an Fe—Cr—Ni alloy material having the above-mentioned chemical composition and a tensile yield strength of 758 MPa or more, and have conducted detailed studies on a method for reducing the strength anisotropy of the alloy material.

[0019] As a result of detailed studies by the present inventors, it has become clear that in an Fe—Cr—Ni alloy material having the above-mentioned chemical composition and a tensile yield strength of 758 MPa or more, the standard deviation σ of the grain size number in the microstructure affects the strength anisotropy of the alloy material. This point will be specifically explained using the drawings. FIG. 1 is a diagram showing the relationship between the standard deviation σ of the grain size number and the anisotropy index AI in this example. FIG. 1 was created using the standard deviation σ of the grain size number and the anisotropy index AI for an example described below in which the configuration other than the standard deviation σ of the grain size number satisfies the conditions of this embodiment.

[0020] 1 , in an Fe—Cr—Ni alloy material having the above-described chemical composition and a tensile yield strength of 758 MPa or more, if the standard deviation σ of the grain size number is 0.60 or less, the anisotropy index AI can be increased to 0.700 or more. On the other hand, if the standard deviation σ of the grain size number exceeds 0.60, the anisotropy index AI decreases to less than 0.700. Therefore, the Fe—Cr—Ni alloy material according to this embodiment satisfies the above-described chemical composition, has a tensile yield strength of 758 MPa or more, and further, the standard deviation σ of the grain size number is set to 0.60 or less. As a result, the Fe—Cr—Ni alloy material according to this embodiment can reduce strength anisotropy.

[0021] The details of why the strength anisotropy of an alloy material can be reduced by setting the standard deviation σ of the grain size number to 0.60 or less are not clear. However, it has been proven by the examples described below that the strength anisotropy can be reduced by satisfying the above-mentioned chemical composition, having a tensile yield strength of 758 MPa or more, and further setting the standard deviation σ of the grain size number to 0.60 or less.

[0022] The Fe-Cr-Ni alloy material according to this embodiment, which was completed based on the above findings, has the following features.

[0023] [1] In mass%, C: 0.030% or less, Si: 0.01 to 1.00%, Mn: 0.01 to 2.00%, P: 0.040% or less, S: 0.0050% or less, Al: 0.01 to 0.50%, Ni: 28.0 to 36.5%, Cr: 19.0 to 27.5%, Mo: 2.00 to 6.00%, Cu: 0.01 to 3.00%, N: 0.010 to less than 0.220%, Co: 0.01 to 2.00%, O: 0.010% or less, V: 0 to 0.50%, Nb: 0 to 0.10%, Ti: 0 to 0.40%, W: 0 to 3.0%, Sn: 0 to 0.010%, An Fe-Cr-Ni alloy material comprising: Ca: 0 to 0.0100%, B: 0 to 0.0100%, Mg: 0 to 0.0100%, rare earth elements: 0 to 0.100%, and the balance being Fe and impurities, wherein the standard deviation of the grain size number of austenite grains in the microstructure is 0.60 or less, and the tensile yield strength is 758 MPa or more.

[0024] [2] The Fe-Cr-Ni alloy material according to [1], containing one or more elements selected from the group consisting of V: ​​0.01 to 0.50%, Nb: 0.01 to 0.10%, Ti: 0.01 to 0.40%, W: 0.1 to 3.0%, Sn: 0.001 to 0.010%, Ca: 0.0001 to 0.0100%, B: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, and rare earth elements: 0.001 to 0.100%.

[0025] [3] The Fe-Cr-Ni alloy material according to [1] or [2], which is a seamless alloy pipe for oil wells.

[0026] The shape of the Fe—Cr—Ni alloy material according to this embodiment is not particularly limited. The shape of the Fe—Cr—Ni alloy material according to this embodiment may be a plate, a rod with a circular cross section, or a tube. That is, the Fe—Cr—Ni alloy material according to this embodiment may be an alloy plate, a rod with a circular cross section, or an alloy pipe. The alloy pipe may be a seamless alloy pipe or a welded alloy pipe. When the alloy material is an oil well alloy pipe, it is preferably a seamless alloy pipe.

[0027] The Fe—Cr—Ni alloy material according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[0028] [Chemical Composition] The chemical composition of the Fe-Cr-Ni alloy material according to this embodiment contains the following elements.

[0029] C: 0.030% or less Carbon (C) is an unavoidable impurity. That is, the lower limit of the C content is greater than 0%. If the C content is too high, Cr carbides will form at the grain boundaries even if the contents of other elements are within the ranges of this embodiment. Cr carbides increase the cracking susceptibility at the grain boundaries. As a result, the corrosion resistance of the alloy material decreases. Therefore, the C content is 0.030% or less. A preferred upper limit of the C content is 0.028%, more preferably 0.025%, even more preferably 0.020%, and even more preferably 0.015%. The C content should be as low as possible. However, an extreme reduction in the C content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the C content is 0.001%, even more preferably 0.003%.

[0030] Si: 0.01 to 1.00% Silicon (Si) deoxidizes the alloy. If the Si 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 Si content is too high, the hot workability of the alloy material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.01 to 1.00%. A preferred lower limit of the Si content is 0.05%, more preferably 0.10%, and even more preferably 0.20%. A preferred upper limit of the Si content is 0.90%, more preferably 0.80%, and even more preferably 0.70%.

[0031] Mn: 0.01 to 2.00% Manganese (Mn) deoxidizes and desulfurizes the alloy. If the Mn 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 Mn content is too high, the hot workability of the alloy material will decrease, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.01 to 2.00%. A preferred lower limit of the Mn content is 0.10%, more preferably 0.20%, and even more preferably 0.30%. A preferred upper limit of the Mn content is 1.80%, more preferably 1.60%, even more preferably 1.50%, even more preferably 1.30%, and even more preferably 1.00%.

[0032] P: 0.040% or less Phosphorus (P) is an unavoidable impurity. 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 hot workability and corrosion resistance of the alloy material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.040% or less. A preferred upper limit of the P content is 0.035%, more preferably 0.030%, and even more preferably 0.025%. The P content is preferably 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 0.002%, and even more preferably 0.003%.

[0033] S: 0.0050% or less Sulfur (S) is an unavoidable impurity. 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 hot workability of the alloy material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.0050% or less. A preferred upper limit of the S content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%. 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 0.0003%, and even more preferably 0.0005%.

[0034] Al: 0.01 to 0.50% Aluminum (Al) deoxidizes the alloy. Furthermore, Al forms oxides to fix oxygen, improving the hot workability of the alloy material. Al also improves the impact resistance and corrosion resistance of the alloy material. If the Al 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 Al content is too high, even if the contents of other elements are within the ranges of this embodiment, excessive Al oxides are formed, thereby reducing the hot workability of the alloy material. Therefore, the Al content is 0.01 to 0.50%. The preferred lower limit of the Al content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Al content is 0.45%, more preferably 0.40%, and even more preferably 0.30%. Note that the Al content referred to in this specification refers to the content of "acid-soluble Al," i.e., sol. Al.

[0035] Ni: 28.0 to 36.5% Nickel (Ni) is an austenite-forming element and stabilizes austenite in the alloy material. If the Ni 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, Ni may increase strength anisotropy. Therefore, if the Ni content is too high, the strength anisotropy of the alloy material may increase even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 28.0 to 36.5%. The preferred lower limit of the Ni content is 28.5%, more preferably 29.0%, and even more preferably 30.0%. The preferred upper limit of the Ni content is 36.0%, more preferably 35.5%, and even more preferably 35.0%.

[0036] Cr: 19.0 to 27.5% Chromium (Cr) improves the corrosion resistance of the alloy material. Cr may also reduce the strength anisotropy of the alloy 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, the hot workability of the alloy material will be reduced even if the contents of other elements are within the ranges of this embodiment. In this case, intermetallic compounds, such as the σ phase, will be more likely to form, reducing the corrosion resistance of the alloy material. Therefore, the Cr content is 19.0 to 27.5%. The preferred lower limit of the Cr content is 19.5%, more preferably 20.0%, even more preferably 21.0%, and even more preferably 22.0%. The preferred upper limit of the Cr content is 27.0%, even more preferably 26.5%.

[0037] Mo: 2.00 to 6.00% Molybdenum (Mo) contributes to the stabilization of corrosion protective films and improves the corrosion resistance of alloy materials. Mo also increases the strength of alloy materials through solid solution strengthening. If the Mo 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 Mo content is too high, the hot workability of the alloy material will deteriorate even if the contents of other elements are within the ranges of this embodiment. In this case, the manufacturing cost will further increase significantly. Therefore, the Mo content is 2.00 to 6.00%. The preferred lower limit of the Mo content is 2.20%, more preferably 2.40%, and even more preferably 2.50%. The preferred upper limit of the Mo content is 5.50%, more preferably 5.00%, even more preferably 4.50%, and even more preferably 4.00%.

[0038] Cu: 0.01 to 3.00% Copper (Cu) contributes to stabilizing the corrosion protective film and improving the corrosion resistance of the alloy material. If the Cu 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 Cu content is too high, the hot workability of the alloy material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.01 to 3.00%. The preferred lower limit of the Cu content is 0.02%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.20%. The preferred upper limit of the Cu content is 2.80%, more preferably 2.50%, and even more preferably 2.00%.

[0039] N: 0.010 to less than 0.220% Nitrogen (N) increases the strength of the alloy material through solid solution strengthening. If the N content is too low, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content is too high, the corrosion resistance of the alloy material may decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.010 to less than 0.220%. A preferred lower limit of the N content is 0.015%, more preferably 0.020%, even more preferably 0.030%, even more preferably 0.050%, and even more preferably 0.090%. A preferred upper limit of the N content is 0.219%, even more preferably 0.215%, even more preferably 0.210%, and even more preferably 0.200%.

[0040] Co: 0.01 to 2.00% Cobalt (Co) stabilizes austenite in the alloy material. If the Co 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 Co content is too high, the manufacturing cost will increase significantly even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0.01 to 2.00%. A preferred lower limit of the Co content is 0.02%, more preferably 0.03%, even more preferably 0.05%, and even more preferably 0.10%. A preferred upper limit of the Co content is 1.50%, more preferably 1.20%, even more preferably 1.00%, and even more preferably 0.90%.

[0041] O: 0.010% or less Oxygen (O) is an unavoidable impurity. That is, the lower limit of the O content is greater than 0%. O forms oxides. Therefore, if the O content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse oxides are formed in the alloy material, degrading the hot workability of the alloy material. In this case, the corrosion resistance of the alloy material is further degraded. Therefore, the O content is 0.010% or less. A preferred upper limit of the O content is 0.008%, more preferably 0.005%. The O content is preferably as low as possible. However, an extreme reduction in the O content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the O content is 0.0001%, more preferably 0.001%, and even more preferably 0.002%.

[0042] The balance of the chemical composition of the Fe—Cr—Ni alloy material according to this embodiment is composed of Fe and impurities. Here, the term "impurities" refers to substances that are mixed in from raw materials such as ore or scrap or the manufacturing environment during industrial production of the Fe—Cr—Ni alloy material, and are acceptable within a range that does not significantly adversely affect the effects of the Fe—Cr—Ni alloy material according to this embodiment.

[0043] [Optional Elements] The chemical composition of the Fe—Cr—Ni alloy material according to this embodiment may further contain one or more elements selected from the group consisting of V, Nb, and Ti. All of these elements increase the strength of the alloy material.

[0044] V: 0 to 0.50% Vanadium (V) is an optional element and does not necessarily need to be contained. In other words, the V content may be 0%. When contained, V forms carbonitrides and the like with C and N, increasing the strength of the alloy 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, excessive carbonitrides and the like will be formed, reducing the ductility of the alloy material. Therefore, the V content is 0 to 0.50%. The preferred lower limit of the V content is more than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the V content is 0.40%, even more preferably 0.30%, and even more preferably 0.20%.

[0045] Nb: 0 to 0.10% Niobium (Nb) is an optional element and does not necessarily need to be contained. In other words, the Nb content may be 0%. When contained, Nb forms carbonitrides and the like with C and N, increasing the strength of the alloy 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, excessive carbonitrides and the like will be formed, reducing the ductility of the alloy material. Therefore, the Nb content is 0 to 0.10%. The preferred lower limit of the Nb content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferred upper limit of the Nb content is 0.09%, more preferably 0.08%, even more preferably 0.07%, even more preferably 0.06%, even more preferably 0.05%, even more preferably 0.04%, and even more preferably 0.03%.

[0046] Ti: 0 to 0.40% Titanium (Ti) is an optional element and does not necessarily need to be contained. In other words, the Ti content may be 0%. When contained, Ti forms carbonitrides and the like with C and N, increasing the strength of the alloy material. Even if even a small amount of Ti is contained, the above effect can be achieved 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, excessive carbonitrides and the like will be formed, reducing the ductility of the alloy material. Therefore, the Ti content is 0 to 0.40%. The preferred lower limit of the Ti content is greater than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Ti content is 0.35%, even more preferably 0.30%, even more preferably 0.20%, even more preferably 0.15%, and even more preferably 0.10%.

[0047] The chemical composition of the Fe—Cr—Ni alloy material according to this embodiment may further contain one or more elements selected from the group consisting of W and Sn, both of which enhance the corrosion resistance of the alloy material.

[0048] W: 0 to 3.0% Tungsten (W) is an optional element and does not necessarily need to be contained. In other words, the W content may be 0%. When contained, W contributes to stabilizing the corrosion protective film and improving the corrosion resistance of the alloy material. W also increases the strength of the alloy material through solid solution strengthening. Even if even a small amount of W is contained, the above effects can be achieved to some extent. However, if the W content is too high, the hot workability of the alloy material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0 to 3.0%. The preferred lower limit of the W content is greater than 0%, more preferably 0.1%, even more preferably 0.3%, and even more preferably 0.5%. The preferred upper limit of the W content is 2.8%, even more preferably 2.5%, even more preferably 2.2%, and even more preferably 2.0%.

[0049] Sn: 0 to 0.010% 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 alloy 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, the hot workability of the alloy material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.010%. The preferred lower limit of the Sn content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Sn content is 0.009%, even more preferably 0.008%, and even more preferably 0.007%.

[0050] The chemical composition of the Fe—Cr—Ni alloy material according to the present embodiment may further include one or more elements selected from the group consisting of Ca, B, Mg, and rare earth elements (REM), all of which enhance the hot workability of the alloy material.

[0051] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When contained, Ca neutralizes S in the alloy by fixing it as sulfide, improving the hot workability of the alloy material. Even if even a small amount of Ca is contained, the above effect 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, coarse oxides will form in the alloy material, and the hot workability of the alloy material will actually decrease. 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 0.0005%, even more preferably 0.0009%, even more preferably 0.0011%, even more preferably 0.0013%, and even more preferably 0.0015%. The upper limit of the Ca content is preferably 0.0080%, more preferably 0.0060%, and still more preferably 0.0050%.

[0052] B: 0 to 0.0100% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, B neutralizes S in the alloy by fixing it as sulfide, improving the hot workability of the alloy material. Even if even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content is too high, even if the contents of other elements are within the ranges of this embodiment, B will segregate to grain boundaries, thereby reducing the hot workability of the alloy material. Therefore, the B content is 0 to 0.0100%. The preferred lower limit of the B content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the B content is preferably 0.0080%, more preferably 0.0060%, even more preferably 0.0040%, even more preferably 0.0030%, even more preferably 0.0020%, even more preferably 0.0015%, and even more preferably 0.0010%.

[0053] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When contained, Mg neutralizes S in the alloy by fixing it as sulfide, improving the hot workability of the alloy material. Even if even a small amount of Mg is contained, the above effect can be achieved 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, coarse oxides will form in the alloy material, thereby reducing the hot workability of the alloy material. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is greater than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the Mg content is 0.0080%, even more preferably 0.0060%, and even more preferably 0.0040%.

[0054] Rare Earth Elements: 0 to 0.100% Rare earth elements (REM) are optional elements and do not necessarily need to be present. In other words, the REM content may be 0%. When present, REM neutralizes S in the alloy by fixing it as sulfides, thereby improving the hot workability of the alloy material. 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, coarse oxides will form in the alloy material, thereby reducing the hot workability of the alloy material. Therefore, the REM content is 0 to 0.100%. The preferred lower limit of the REM content is greater than 0%, more preferably 0.001%, even more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the REM content is 0.080%, even more preferably 0.060%, and even more preferably 0.050%.

[0055] 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.

[0056] [Standard deviation σ of grain size number] The Fe—Cr—Ni alloy material according to this embodiment has the above-described chemical composition, and further, the standard deviation σ of the grain size number of the austenite grains is set to 0.60 or less. As a result, the Fe—Cr—Ni alloy material according to this embodiment can reduce strength anisotropy even if it has a tensile yield strength of 758 MPa or more.

[0057] Here, when the standard deviation σ of the austenite grain size number is large, it is presumed that there are regions in the alloy material where coarse austenite grains (coarse grains) are unevenly distributed and regions where fine austenite grains (fine grains) are unevenly distributed. Furthermore, when the tensile yield strength of an Fe—Cr—Ni alloy material having the above-mentioned chemical composition is to be 758 MPa or more, cold working or the like is performed after heat treatment, such as solution treatment, in the manufacturing process described below, and strain may be introduced into the alloy material. Therefore, anisotropy in strength may occur depending on the direction in which strain is introduced. Specifically, when cold drawing or cold rolling is performed as cold working or the like, the tensile yield strength is greater than the compressive yield strength.

[0058] More specifically, when a tensile test is conducted on an alloy material in which strength anisotropy has been created by applying stress in the tensile direction, in which further stress is applied in the tensile direction, the suppression of dislocation motion becomes dominant. In other words, the ease of dislocation movement during the tensile test does not change significantly between coarse grains and fine grains. On the other hand, when a compression test is conducted under the same conditions, in which stress is applied in the compressive direction, the suppression of dislocation movement during the compression test becomes less effective and the material becomes more susceptible to the influence of grain boundaries. Therefore, during the compression test, dislocations move more easily in coarse grains, while dislocations move less easily in fine grains.

[0059] In other words, when strength anisotropy occurs due to the application of stress in the tensile direction, it is presumed that the ease of dislocation movement in the tensile direction differs from the ease of dislocation movement in the compressive direction. Therefore, when there are areas in an alloy material where coarse grains are unevenly distributed and areas where fine grains are unevenly distributed, the ease of dislocation movement in the tensile direction and the difficulty of dislocation movement in the compressive direction may be easily manifested. Thus, when the standard deviation σ of the austenite grain size number is large, it is presumed that the tensile yield strength is likely to be larger than the compressive yield strength, and strength anisotropy increases.

[0060] Based on the above mechanism, the inventors speculate that, for an Fe—Cr—Ni alloy material having the above-described chemical composition, if the standard deviation σ of the austenite grain grain size number is 0.60 or less, strength anisotropy can be reduced even if the material has a tensile yield strength of 758 MPa or more. It should be noted that, based on a mechanism other than the above, it is possible that, for an Fe—Cr—Ni alloy material having the above-described chemical composition, if the standard deviation σ of the austenite grain grain size number is 0.60 or less, strength anisotropy can be reduced even if the material has a tensile yield strength of 758 MPa or more. However, as described above, it has been proven by the examples described below that, for an Fe—Cr—Ni alloy material having the above-described chemical composition, if the standard deviation σ of the austenite grain grain size number is 0.60 or less, strength anisotropy can be reduced even if the material has a tensile yield strength of 758 MPa or more.

[0061] In this embodiment, the upper limit of the standard deviation σ of the austenite grain size number is preferably 0.58, more preferably 0.55, and even more preferably 0.53. In the Fe—Cr—Ni alloy material according to this embodiment, the smaller the standard deviation σ of the austenite grain size number, the more preferable. That is, the lower limit of the standard deviation σ of the austenite grain size number may be 0.00, 0.05, 0.10, or 0.15.

[0062] In the Fe—Cr—Ni alloy material according to this embodiment, the standard deviation σ of the grain size number of the austenite grains can be determined by the following method. Specifically, a test piece for microstructure observation is prepared from the Fe—Cr—Ni alloy material according to this embodiment. When the alloy material has a plate shape, the test piece is prepared from the center of the plate thickness. When the alloy material has a tubular shape, the test piece is prepared from the center of the wall thickness. When the alloy material has a rod shape with a circular cross section, the test piece is prepared from the R / 2 position. In this specification, the R / 2 position refers to the center position of the radius R in a cross section perpendicular to the axial direction. The size of the test piece is not particularly limited as long as it can provide an observation surface, which will be described later.

[0063] The observation surface of the prepared test specimen is polished to a mirror finish, and then etched using aqua regia (a solution of hydrochloric acid and nitric acid mixed at a ratio of 3:1) to reveal the austenite grain boundaries. Ten fields of view are selected from the observation surface and observed using an optical microscope to generate photographic images. The magnification for microscopic observation can be appropriately set depending on the grain size. Specifically, the magnification is set so that 50 or more grains are included in the field of view.

[0064] Image analysis is performed on the photographic image obtained in each field of view, and the grain size number is measured in accordance with ASTM E112 (2021). That is, one grain size number is obtained for each observation field of view. At this time, the grain size number of the austenite grains is obtained by rounding the obtained value to the nearest ten decimal places. The standard deviation of the obtained 10 grain size numbers is calculated and defined as the standard deviation σ of the austenite grain grains. The standard deviation σ of the austenite grain grains is calculated by rounding the obtained value to the nearest ten decimal places.

[0065] In the Fe—Cr—Ni alloy material according to this embodiment, the grain size number of the austenite grains is not particularly limited as long as the standard deviation σ is 0.60 or less. In this embodiment, the lower limit of the grain size number of the austenite grains may be, for example, 4.0, 4.5, or 5.0. In this embodiment, the upper limit of the grain size number of the austenite grains may be, for example, 12.0, 11.5, or 11.0. In this case, the grain size number of the austenite grains means the arithmetic average value of the 10 grain size numbers obtained by the above-mentioned method.

[0066] [Tensile yield strength] The Fe—Cr—Ni alloy material according to this embodiment has the above-described chemical composition, and further has a standard deviation σ of the austenite grain size number of 0.60 or less. As a result, even if the Fe—Cr—Ni alloy material according to this embodiment has a tensile yield strength of 758 MPa or more, the strength anisotropy is reduced.

[0067] As described above, when attempting to obtain a tensile yield strength of 758 MPa or more in an Fe—Cr—Ni alloy material having the above-described chemical composition, strength anisotropy may become high. However, the alloy material according to this embodiment has a standard deviation σ of the austenite grain size number of 0.60 or less, which makes it possible to suppress the manifestation of strength anisotropy due to variations in grain size. Therefore, the alloy material according to this embodiment can reduce strength anisotropy even when it has a high tensile yield strength of 758 MPa or more.

[0068] In this embodiment, the lower limit of the tensile yield strength is preferably 800 MPa, more preferably 830 MPa, and even more preferably 860 MPa. In this embodiment, the upper limit of the tensile yield strength is not particularly limited and may be, for example, 1240 MPa, 1200 MPa, or 1150 MPa.

[0069] Furthermore, in the Fe—Cr—Ni alloy material of this embodiment, the compressive yield strength is not particularly limited. In this embodiment, the lower limit of the compressive yield strength may be, for example, 600 MPa, 610 MPa, or 630 MPa. In this embodiment, the upper limit of the compressive yield strength may be, for example, less than 1240 MPa, less than 1200 MPa, or less than 1150 MPa. In addition, the method for measuring the tensile yield strength and compressive yield strength in this embodiment will be described later.

[0070] [Strength Anisotropy] The Fe—Cr—Ni alloy material according to this embodiment has the above-described chemical composition, and further, the standard deviation σ of the austenite grain size number is 0.60 or less. As a result, the Fe—Cr—Ni alloy material according to this embodiment has reduced strength anisotropy even though it has a tensile yield strength of 758 MPa or more. In this specification, reduced strength anisotropy means that the anisotropy index AI is 0.700 or more. In addition, in this specification, the anisotropy index AI means the ratio of the compressive yield strength (compressive YS) to the tensile yield strength (tensile YS) (compressive YS / tensile YS).

[0071] The lower limit of the anisotropy index AI is preferably 0.710, more preferably 0.730, and even more preferably 0.750. The upper limit of the anisotropy index AI is substantially less than 1.000, more preferably 0.999, even more preferably 0.990, and even more preferably 0.980.

[0072] The anisotropy index AI, tensile yield strength, and compressive yield strength of the Fe—Cr—Ni alloy material according to this embodiment can be determined by the following method: First, the tensile yield strength and compressive yield strength of the Fe—Cr—Ni alloy material according to this embodiment are determined.

[0073] Specifically, the tensile yield strength of the Fe—Cr—Ni alloy material according to this embodiment can be determined by the following method. A tensile test is performed according to ASTM E8 / E8M (2021). A round bar test specimen is prepared from the alloy material according to this embodiment. If the alloy material has a plate shape, a round bar test specimen is prepared from the center of the plate thickness. If the alloy material has a tubular shape, a round bar test specimen is prepared from the center of the wall thickness. If the alloy material has a rod shape with a circular cross section, a round bar test specimen is prepared from the R / 2 position. The size of the round bar test specimen is, for example, 4 mm in parallel diameter and 20 mm in gauge length. The axial direction of the round bar test specimen is parallel to the rolling direction of the alloy material. A tensile test is performed using the round bar test specimen at room temperature (25°C) in the air, and the obtained 0.2% offset proof stress is defined as the tensile yield strength (MPa). The tensile yield strength (MPa) is determined by rounding off the obtained value to one decimal place.

[0074] Similarly, the compressive yield strength of the Fe—Cr—Ni alloy material according to this embodiment can be determined by the following method. A compression test is performed according to ASTM E9 (2019). A cylindrical test specimen is prepared from the alloy material according to this embodiment. If the alloy material has a plate shape, a cylindrical test specimen is prepared from the center of the plate thickness. If the alloy material has a tubular shape, a cylindrical test specimen is prepared from the center of the wall thickness. If the alloy material has a rod shape with a circular cross section, a cylindrical test specimen is prepared from the R / 2 position. The size of the cylindrical test specimen is, for example, 4 mm in parallel diameter and 8 mm in length. The axial direction of the cylindrical test specimen is parallel to the rolling direction of the alloy material. A compression test is performed using the cylindrical test specimen at room temperature (25°C) in the air, and the resulting 0.2% offset proof stress is defined as the compressive yield strength (MPa). The compressive yield strength (MPa) is determined by rounding the obtained value to one decimal place.

[0075] The anisotropy index AI (=compression YS / tensile YS) can be calculated using the tensile yield strength (tensile YS) and compressive yield strength (compression YS) obtained by rounding the obtained value to the nearest four decimal places.

[0076] [Manufacturing Method] An example of a manufacturing method for an Fe—Cr—Ni alloy material according to this embodiment will be described below. Hereinafter, a manufacturing method for a seamless alloy pipe will be described as an example of the Fe—Cr—Ni alloy material according to this embodiment. The manufacturing method for a seamless alloy pipe includes a step of preparing a raw material (raw material preparation step), a step of manufacturing a mother pipe from the raw material (hot working step), a step of cold working the manufactured mother pipe (first cold working step), a step of performing a solution treatment (solution treatment step), and a step of cold working the solution-treated mother pipe (second cold working step). It should be noted that the manufacturing method for the Fe—Cr—Ni alloy material according to this embodiment is not limited to the manufacturing method described below.

[0077] [Material Preparation Step] In the material preparation step, an Fe—Cr—Ni alloy having the above-described chemical composition is melted. The Fe—Cr—Ni alloy may be melted in an electric furnace or in an Ar—O mixed gas bottom blown decarburization furnace (AOD furnace). Alternatively, it may be melted in a vacuum decarburization furnace (VOD furnace). The melted Fe—Cr—Ni alloy may be formed into an ingot by an ingot-making method, or into a slab, bloom, or billet by a continuous casting method. If necessary, the slab, bloom, or ingot may be subjected to blooming to produce a billet. A material (slab, bloom, or billet) is produced by the above steps.

[0078] [Hot Working Step] In the hot working step, the prepared material is hot worked to produce an intermediate alloy material (blank pipe). The hot working method is not particularly limited and may be a well-known method. That is, in this embodiment, the hot working may be hot rolling, hot extrusion, or hot forging. In the hot working, the heating temperature of the material is, for example, 1100 to 1300°C.

[0079] For example, when a mother pipe is manufactured by carrying out the Mannesmann process as hot working, a round billet is pierced and rolled using a piercing mill. In this case, the piercing ratio is not particularly limited and is, for example, 1.0 to 4.0. The mother pipe that has been pierced and rolled may be hot rolled using a mandrel mill, a reducer, a sizing mill, or the like to produce a mother pipe.

[0080] In this specification, the intermediate alloy material refers to a plate-shaped alloy material when the final product is an alloy plate, a blank pipe when the final product is an alloy pipe, and an alloy material having a circular cross section perpendicular to the axial direction when the final product is a solid material with a circular cross section. When the alloy material is a solid material 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 hot worked to produce an intermediate alloy material having 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 an alternating arrangement of horizontal stands having a pair of grooved rolls arranged vertically and vertical stands having a pair of grooved rolls arranged horizontally. When the alloy material is an alloy 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 alloy material in the form of an alloy plate.

[0081] [First Cold Working Step] In the first cold working step, cold working is performed on the produced intermediate alloy material (blank pipe). In this embodiment, the cold working may be cold drawing or cold rolling. When cold rolling is performed, for example, a continuous rolling mill equipped with multiple cold rolling stands may be used. That is, in the first cold working step according to this embodiment, known cold working may be performed under known conditions. Specifically, the temperature of the intermediate alloy material (blank pipe) during cold working may be, for example, room temperature to 300°C.

[0082] In the first cold working step according to this embodiment, the cold working ratio R1 (%) is preferably 5% or more. Here, the cold working ratio R1 refers to the rate of reduction in the cross-sectional area of ​​the intermediate alloy material (blank pipe) from before the start of the first cold working step to after the end of the first cold working step. Specifically, when the cross-sectional area of ​​the blank pipe before the first cold working step is defined as S0(1) and the cross-sectional area of ​​the blank pipe after the first cold working step is defined as S1(1), the cold working ratio R1 (%) in the first cold working step is defined by the following formula (A): R1(%)=100(1-S1(1) / S0(1)) (A)

[0083] If the cold working ratio R1 is 5% or more, recrystallization during heat treatment is promoted in the solution treatment step described below. As a result, the standard deviation σ of the grain size number of the manufactured Fe—Cr—Ni alloy material can be stably reduced. Therefore, in this embodiment, the cold working ratio R1 in the first cold working step is preferably 5% or more. In this embodiment, the upper limit of the cold working ratio R1 in the first cold working step is not particularly limited, but is, for example, 30%.

[0084] [Solution Treatment Step] In the solution treatment step, a solution treatment is performed on the cold-worked intermediate alloy material (blank pipe). The solution treatment method is not particularly limited and may be a well-known method. For example, the blank pipe is loaded into a heat treatment furnace, held at a desired temperature, and then rapidly cooled. Note that when the solution treatment is performed by loading the blank pipe into a heat treatment furnace, holding it at a desired temperature, and then rapidly cooling it, the temperature at which the solution treatment is performed (solution temperature) refers to the temperature (°C) of the heat treatment furnace used to perform the solution treatment. In this case, the time for which the solution treatment is performed (holding time) refers to the time for which the blank pipe is held at the solution temperature.

[0085] Preferably, in the solution treatment process according to this embodiment, when the intermediate alloy material (blank tube) is heated to the solution temperature, the residence time at 900 to 1050°C is 9 minutes or more. In an intermediate alloy material having the above-described chemical composition, recrystallization and grain growth are likely to proceed at temperatures above 900°C. Therefore, if the residence time at 900 to 1050°C is too short, temperature variations within the intermediate alloy material are likely to occur, making recrystallization and grain growth non-uniform. On the other hand, if the residence time at 900 to 1050°C is 9 minutes or more, recrystallization and grain growth are likely to become uniform. In this case, recrystallization is further likely to be promoted in heat treatment at 1060°C or higher. As a result, the standard deviation σ of the grain size number of the manufactured Fe—Cr—Ni alloy material can be stably reduced.

[0086] Therefore, in this embodiment, the residence time at 900 to 1050°C during heating in the solution treatment step is preferably 9 minutes or more. In this embodiment, a more preferable lower limit of the residence time at 900 to 1050°C during heating in the solution treatment step is 10 minutes. Note that if the residence time at 900 to 1050°C is too long, the above-mentioned effect will saturate. Therefore, in this embodiment, the upper limit of the residence time at 900 to 1050°C during heating in the solution treatment step is, for example, 60 minutes. The upper limit of the residence time at 900 to 1050°C during heating in the solution treatment step may be 45 minutes or may be 30 minutes.

[0087] Preferably, the solution treatment temperature in the solution treatment step according to this embodiment is set to 1060 to 1300°C. If the solution treatment temperature is too low, precipitates (e.g., σ phase, which is an intermetallic compound) may remain in the mother pipe after the solution treatment. In this case, the corrosion resistance of the manufactured Fe—Cr—Ni alloy material may decrease. On the other hand, if the solution treatment temperature is too high, the effect of the solution treatment will saturate. Therefore, in this embodiment, the solution treatment temperature in the solution treatment step is preferably set to 1060 to 1300°C.

[0088] When the solution treatment is carried out by charging the mother pipe into a heat treatment furnace, holding it at a desired temperature, and then quenching it, the holding time is not particularly limited, and the solution treatment may be carried out under well-known conditions. The holding time is, for example, 5 to 180 minutes. The quenching method is, for example, water cooling.

[0089] [Second Cold Working Step] In the cold working step, the solution-treated intermediate alloy material (blank pipe) is cold worked to produce an Fe—Cr—Ni alloy material. As described above, in this embodiment, the cold working may be cold drawing or cold rolling. That is, in the second cold working step according to this embodiment, as in the first cold working step, well-known cold working may be performed under well-known conditions. Specifically, the temperature of the intermediate alloy material (blank pipe) during cold working may be, for example, room temperature to 300°C.

[0090] In the second cold working step according to this embodiment, the cold working ratio R2 (%) is preferably 5 to 50%. Here, the cold working ratio R2 refers to the reduction rate of the cross-sectional area of ​​the intermediate alloy material (blank pipe) from before the start of the second cold working step to after the end of the second cold working step. Specifically, when the cross-sectional area of ​​the blank pipe before the second cold working step is defined as S0(2) and the cross-sectional area of ​​the Fe—Cr—Ni alloy material after the second cold working step is defined as S1(2), the cold working ratio R2 (%) is defined by the following formula (B): R2(%)=100(1−S1(2) / S0(2)) (B)

[0091] If the cold working ratio R2 is 5 to 50%, the tensile yield strength of the Fe—Cr—Ni alloy material after the second cold working step can be stably set to 758 MPa or more. Therefore, the cold working ratio R2 is preferably set to 5 to 50%.

[0092] In this embodiment, it is preferable that the cold working rate R1 (%) of the first cold working step and the cold working rate R2 (%) of the second cold working step satisfy the above-mentioned ranges, and the total cold working rate in the manufacturing process is not particularly limited.

[0093] The Fe—Cr—Ni alloy material according to this embodiment can be manufactured by the above-described manufacturing method. In the above-described manufacturing method, a method for manufacturing a seamless alloy pipe has been described as an example. However, the Fe—Cr—Ni alloy material according to this embodiment may have other shapes, such as a plate shape. Similar to the above-described manufacturing method, manufacturing methods for other shapes, such as a plate shape, also include, for example, a material preparation step, a hot working step, a solution treatment step, and a cold working step. Furthermore, the above-described manufacturing method is merely an example, and other manufacturing methods may also be used. The present invention will be described in more detail below with reference to examples.

[0094] Alloys having the chemical compositions shown in Tables 1A and 1B were produced by high-frequency vacuum melting. Note that "-" in Table 1B indicates that the content of each element was at the impurity level. Specifically, the W content indicated by symbol A was rounded to one decimal place to mean 0%. Similarly, the V content, Nb content, and Ti content indicated by symbol A were rounded to two decimal places to mean 0%. Similarly, the Sn content and REM content indicated by symbol A were rounded to four decimal places to mean 0%. Similarly, the Ca content, B content, and Mg content indicated by symbol A were rounded to five decimal places to mean 0%.

[0095]

[0096]

[0097] Using the alloys indicated by the respective symbols, 50 kg of ingots of each test number were produced by ingot casting. The obtained ingots of each test number were heated at 1200°C for 3 hours and then hot forged to produce square bars with a cross section of 50 mm x 50 mm. The obtained square bars were heated at 1200°C for 1 hour and then hot rolled to produce plates (alloy plates) with a plate thickness of 30 mm. The obtained alloy plates of each test number were subjected to first cold working. The cold working ratio R1 (%) of the first cold working performed on the alloy plates of each test number is shown in Table 2.

[0098]

[0099] The alloy sheets of each test number that had been subjected to the first cold working were subjected to a solution treatment. In the solution treatment, the alloy sheets that had been subjected to the first cold working were heated and held at the solution temperature (°C) shown in Table 2 for the holding time (minutes) shown in Table 2, and then water-cooled. The holding time at 900 to 1050°C when heating to the solution temperature is shown in the "Holding time (minutes)" column in Table 2.

[0100] The alloy sheets of each test number that had been subjected to the solution treatment were subjected to a second cold working. The cold working ratio R2 (%) of the second cold working performed on the alloy sheets of each test number is shown in Table 2. In test numbers 1, 7, and 15, cold drawing was performed as the cold working. In test numbers other than test numbers 1, 7, and 15, cold rolling was performed as the cold working.

[0101] The total cold working ratio R (%) of the cold working performed on the alloy plate of each test number is shown in Table 2. In this example, the total cold working ratio R (%) is defined by the following formula (C): R (%) = R1 (%) + R2 (%) (C) In formula (C), the cold working ratio (%) of the first cold working is substituted for R1, and the cold working ratio (%) of the second cold working is substituted for R2.

[0102] [Evaluation Tests] The alloy sheets having each test number manufactured by the above method were subjected to a grain size number measurement test and a strength anisotropy measurement test, which will be described below.

[0103] [Grain size number measurement test] A grain size number measurement test was performed on the alloy plate of each test number to determine the standard deviation σ of the grain size number. Specifically, the test pieces prepared by the above-described method were subjected to microscopic observation by the above-described method. Image analysis was performed on the photographic images obtained by microscopic observation, and the grain size number was measured in accordance with ASTM E112 (2021). For each test number, the grain size numbers obtained in 10 fields of view are shown in Table 3. The average value and standard deviation σ of the grain size numbers obtained from the 10 obtained grain size numbers are shown in Table 3.

[0104]

[0105] [Strength Anisotropy Measurement Test] A strength anisotropy measurement test was performed on the alloy plate of each test number to determine the anisotropy index AI. Specifically, the tensile yield strength (MPa) and compressive yield strength (MPa) were first determined using the method described above. Specifically, round bar test pieces for tensile tests and cylindrical test pieces for compression tests were prepared from the center of the plate thickness of the alloy plate of each test number. The round bar test pieces had a parallel part diameter of 4 mm and a gauge length of 20 mm. The cylindrical test pieces had a parallel part diameter of 4 mm and a length of 8 mm. The axial directions of the round bar test pieces and cylindrical test pieces were parallel to the rolling direction of the alloy plate.

[0106] A tensile test was performed on round bar test pieces at room temperature (25°C) in air in accordance with ASTM E8 / E8M (2021). The 0.2% offset yield strength obtained from the tensile test was defined as the tensile yield strength (MPa). Furthermore, a compression test was performed on cylindrical test pieces at room temperature (25°C) in air in accordance with ASTM E9 (2019). The 0.2% offset yield strength obtained from the compression test was defined as the compressive yield strength (MPa). The ratio of the compressive yield strength (compressive YS) to the obtained tensile yield strength (tensile YS) (compressive YS / tensile YS) was calculated and defined as the anisotropy index AI. For the alloy plates of each test number, the obtained tensile yield strength is shown in the "Tensile YS (MPa)" column of Table 3, the compressive yield strength is shown in the "Compressive YS (MPa)" column of Table 3, and the anisotropy index AI is shown in the "Anisotropy Index AI" column of Table 3.

[0107] [Evaluation Results] Referring to Tables 1A to 3, the alloy plates of Test Nos. 1 to 26 all had appropriate chemical compositions. Furthermore, the standard deviation σ of the grain size number of these alloy plates was 0.60 or less. As a result, these alloy plates had a tensile yield strength of 758 MPa or more. Furthermore, the anisotropy index AI was 0.700 or more, indicating reduced strength anisotropy.

[0108] On the other hand, the Ni content was too high in the alloy plates of test numbers 27 and 28. As a result, although these alloy plates had a tensile yield strength of 758 MPa or more, the anisotropy index AI was less than 0.700, and the strength anisotropy was not reduced.

[0109] The alloy plates of test numbers 29 and 30 had too low a Cr content. As a result, although these alloy plates had a tensile yield strength of 758 MPa or more, the anisotropy index AI was less than 0.700, and the strength anisotropy was not reduced.

[0110] The alloy sheets of test numbers 31 to 33 had a cold working ratio R1 in the first cold working step that was too low. As a result, the standard deviation σ of the grain size number of these alloy sheets exceeded 0.60. As a result, although these alloy sheets had a tensile yield strength of 758 MPa or more, the anisotropy index AI was less than 0.700, and the strength anisotropy was not reduced.

[0111] The alloy sheets of test numbers 34 to 36 had too short a residence time at 900 to 1050°C during heating in the solution treatment step. As a result, the standard deviation σ of the grain size number of these alloy sheets exceeded 0.60. As a result, although these alloy sheets had a tensile yield strength of 758 MPa or more, the anisotropy index AI was less than 0.700, and the strength anisotropy was not reduced.

[0112] 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. By mass%, C: 0.030% or less, Si: 0.01-1.00%, Mn: 0.01-2.00%, P: 0.040% or less, S: 0.0050% or less, Al: 0.01-0.50%, Ni: 28.0-36.5%, Cr: 19.0-27.5%, Mo: 2.00-6.00%, Cu: 0.01-3.00%, N: 0.010-less than 0.220%, Co: 0.01-2.00%, O: 0.010% or less, V: 0-0.50%, Nb: 0-0.10%, Ti: 0-0.40%, W: 0-3.0%, Sn: 0-0.010%, An Fe-Cr-Ni alloy material comprising Ca: 0-0.0100%, B: 0-0.0100%, Mg: 0-0.0100%, rare earth elements: 0-0.100%, and the balance being Fe and impurities, the alloy material having a microstructure in which the standard deviation of the grain size number of austenite grains is 0.60 or less and a tensile yield strength of 758 MPa or more.

2. The Fe-Cr-Ni alloy material according to claim 1, containing one or more elements selected from the group consisting of V: ​​0.01-0.50%, Nb: 0.01-0.10%, Ti: 0.01-0.40%, W: 0.1-3.0%, Sn: 0.001-0.010%, Ca: 0.0001-0.0100%, B: 0.0001-0.0100%, Mg: 0.0001-0.0100%, and rare earth elements: 0.001-0.100%.

3. The Fe-Cr-Ni alloy material according to claim 1 or 2, which is a seamless alloy pipe for oil wells.

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