Title of invention: Cr-Ni alloy and seamless steel pipe made of Cr-Ni alloy
Technical field
[0001]
　The present invention relates to a Cr-Ni alloy and a seamless steel pipe made of a Cr-Ni alloy.
Background technology
[0002]
　Geothermal power generation is attracting attention against the backdrop of growing awareness of low carbon energy. In geothermal power generation, steam generated from a geothermal well deep in the ground is used to generate power. Particularly in recent years, the development of deeper geothermal wells (deep geothermal wells) than in the past has been promoted. Therefore, a high yield strength (0.2% proof stress) capable of withstanding underground pressure is required for alloy pipes and the like used in deep geothermal wells.
[0003]
　The deep geothermal well also has a high temperature of 250° C. or higher and contains corrosive substances. The corrosive substance is, for example, sulfuric acid. Deep geothermal wells contain large amounts of reducing acids such as sulfuric acid. Deep geothermal wells also contain corrosive materials such as hydrogen sulfide, carbon dioxide, and chloride ions. That is, the deep geothermal property is a severe corrosive environment (hereinafter, such an environment of the deep geothermal well is referred to as “harsh geothermal environment”).
[0004]
　Therefore, in addition to high yield strength, alloy pipes used in deep geothermal wells are required to have excellent corrosion resistance to these corrosive substances, especially sulfuric acid. For example, in an environment containing a large amount of sulfuric acid, general corrosion due to sulfuric acid becomes a dominant factor for corrosion. Therefore, alloy pipes and the like used in a severe geothermal environment are required to have high yield strength and high general sulfuric acid corrosion resistance at a high temperature of 250°C.
[0005]
　Conventionally, Cr-Ni alloys have been developed mainly for application to oil wells. Examples of conventional Cr-Ni alloys include, for example, JP-A-58-210158 (Patent Document 1), JP-A-58-9924 (Patent Document 2), JP-A-11-302801 (Patent Document 3), JP-A-62-158844 (Patent Document 4), JP-A-62-158845 (Patent Document 5) and JP-A-62-158846 (Patent Document 6) have been proposed. However, the conventional Cr-Ni alloy is intended for an oil well containing a large amount of hydrogen sulfide (H 2 S), not a deep geothermal well containing a large amount of sulfuric acid (H 2 SO 4 ) . Therefore, the conventional Cr-Ni alloy may not be able to obtain excellent sulfuric acid general corrosion resistance in a deep geothermal well containing a large amount of sulfuric acid.
[0006]
　On the other hand, Ni-based alloys having excellent corrosion resistance even in a corrosive environment such as a chemical plant containing a reducing acid such as sulfuric acid are disclosed in, for example, WO 2009/119630 (Patent Document 7) and JP 2011-63863 A. It is proposed in Japanese Patent Publication (Patent Document 8).
[0007]
　Patent Document 7 proposes a Ni-based alloy having excellent corrosion resistance in a harsh corrosive environment containing a reducing acid such as hydrochloric acid (HCl) or sulfuric acid (H 2 SO 4 ). The mass percentage of the Ni-based alloy of Patent Document 7 is C: 0.03% or less, Si: 0.01 to 0.5%, Mn: 0.01 to 1.0%, P: 0.03% or less. , S: 0.01% or less, Cr: 20% or more and less than 30%, Ni: more than 40% and 60% or less, Cu: more than 2.0% and 5.0% or less, Mo: 4.0 to 10%, Al: 0.005 to 0.5% and N: more than 0.02% and 0.3% or less, and 0.5Cu+Mo≧6.5 (1) Satisfactory, and the balance is Fe and impurities. As a result, in a severe corrosive environment containing a reducing acid such as hydrochloric acid and sulfuric acid, it has corrosion resistance equivalent to that of a Ni-based alloy having a high Mo content such as Hastelloy C22 and Hastelloy C276, and has good workability, and Patent Document 7 describes that a low-cost Ni-based alloy can be obtained.
[0008]
　Patent Document 8 proposes a Ni-based alloy material having excellent corrosion resistance in an environment where hydrochloric acid corrosion and/or sulfuric acid corrosion occurs. The Ni-based alloy material of Patent Document 8 is C: 0.03% or less, Si: 0.01 to 0.5%, Mn: 0.01 to 1.0%, and P: 0.03% in mass%. Hereinafter, S: 0.01% or less, Cr: 20% or more and less than 30%, Ni: more than 40% and 50% or less, Cu: more than 2.0% and 5.0% or less, Mo: 4.0. .About.10%, Al: 0.005 to 0.5%, W: 0.1 to 10%, and N: 0.10% to 0.35% and more, and 0.5Cu+Mo≧6. 5... (1) is satisfied, the balance is Fe and impurities, and the Vickers hardness of the surface at 500° C. is 350 or more. As a result, at a temperature of 100 to 500° C., in a harsh environment where erosion and hydrochloric acid corrosion and/or sulfuric acid corrosion occur, corrosion resistance equivalent to that of a Ni-based alloy having a high Mo content such as Hastelloy C22 and Hastelloy C276 is obtained. It is described in Patent Document 8 that a Ni-based alloy material that can be secured and that can prevent erosion due to its high surface hardness can be obtained.
Prior art documents
Patent literature
[0009]
Patent Document 1: JP-A-58-210158
Patent Document 2:
JP-A-58-9924 Patent Document 3: JP-A-11-302801
Patent Document 4: JP-A-62-158844
Patent Document 5 : Japanese Patent Laid-Open No. 62-158845
Patent Document 6: Japanese Patent Laid-Open No. 62-158846
Patent Document 7: International Publication No. 2009/119630
Patent Document 8: Japanese Patent Laid-Open No. 2011-63863
Non-patent literature
[0010]
Non-Patent Document 1: Koichi Nakajima et al., "Evaluation Method of Dislocation Density Using X-Ray Diffraction", Materials and Processes (CAMP-ISIJ), Iron and Steel Institute of Japan, 2004, Volume 17, No. 3, p. 396
Non-Patent Document 2: G.I. K. Williamson and W.W. H. Hall, "X-RAY LINE BROADINGING FROM FILED ALUMINIUM AND WOLFRAM", Acta Metallurgica, 1953, Vol. 1, No. 1, p. 22-31
Non-Patent Document 3: H.M. M. Rietveld, “A Profile Refinement Method for Nuclear and Magnetic Structures”, Journal of Applied Crystallography, 1969, Volume 2, p. 65-71
Summary of the invention
Problems to be Solved by the Invention
[0011]
　However, even if the techniques disclosed in Patent Document 7 and Patent Document 8 described above are used, high yield strength and high general sulfuric acid corrosion resistance at a high temperature of 250° C. may not be obtained.
[0012]
　An object of the present invention is to provide a Cr—Ni alloy having high yield strength and high sulfuric acid general corrosion resistance at a high temperature of 250° C., and a seamless steel pipe made of the Cr—Ni alloy.
Means for solving the problem
[0013]
　The Cr-Ni alloy of the present embodiment is, in mass%, Si: 0.01 to 0.50%, Mn: 0.01 to 1.00%, Cr: 21.0 to 27.0%, Ni: 40. 0.0 to less than 50.0%, Mo: 4.5 to less than 9.0%, W: 2.0 to 6.0%, Cu: more than 2.0% and less than 6.0%, Co: 0 0.01 to 2.00%, one or two selected from the group consisting of Ca and Mg: 0.001 to 0.010% in total, sol. Al: 0.005 to 0.200%, N: 0.01 to 0.20%, one or more selected from the group consisting of Ti, Nb, Zr and V: 0 to 0.50 in total %, REM: 0 to 0.050%, C: 0.030% or less, P: 0.030% or less, S: 0.0010% or less, O: 0.010% or less, and the balance Fe and impurities With a chemical composition consisting of The dislocation density in the Cr-Ni alloy satisfies the following formula (1).
　8.00×10 14 ≦ρ≦2.50×10 15 +1.40×10 14 ×[Cu+Co] (1)
　where ρ is the dislocation density (m −2 ) in the above formula (1 ). , Cu and Co represent the Cu content (mass %) and Co content (mass %) in the Cr—Ni alloy, respectively.
[0014]
　The seamless steel pipe of this embodiment is made of the above Cr-Ni alloy.
Effect of the invention
[0015]
　The Cr—Ni alloy of this embodiment has high yield strength and high sulfuric acid general corrosion resistance at a high temperature of 250° C.
MODE FOR CARRYING OUT THE INVENTION
[0016]
　The present inventors have examined the yield strength of Cr-Ni alloys and the sulfuric acid general corrosion resistance at 250°C. As a result, the following findings were obtained.
[0017]
　In order to obtain the yield strength required in deep geothermal wells, it is effective to introduce dislocations into the Cr-Ni alloy by using a means such as cold working. Specifically, if the contents of Cr and Ni are appropriately adjusted and the dislocation density is 8.00×10 14 m −2 or more, sufficient yield strength can be obtained.
[0018]
　However, it has been found that when the dislocation density is increased, the corrosion resistance of the Cr-Ni alloy decreases in a high temperature environment of 250° C. such as a severe geothermal environment and in a strong acid environment containing a large amount of sulfuric acid. The present inventors investigated the cause in detail. As a result, it was found that when the dislocation density is too high, the resistance to general corrosion by sulfuric acid (sulfuric acid general corrosion resistance) decreases in a severe geothermal environment of 250° C., so that the corrosion resistance of the Cr—Ni alloy decreases. .. It is speculated that this is because the dislocation introduced into the Cr—Ni alloy accelerates the dissolution reaction of the Cr—Ni alloy in the strongly acidic environment, and further accelerates the dissolution reaction in the high temperature environment of 250° C.
[0019]
　Therefore, the present inventors added various alloy elements to the Cr—Ni alloy and investigated the relationship between the dislocation density after cold working and the sulfuric acid general corrosion resistance at a high temperature of 250° C.
[0020]
　As a result, the present inventors have found that Cu has a "softening effect" that makes dislocations less likely to enter the Cr-Ni alloy. That is, if the Cu content is high, excessive dislocations are suppressed from entering the Cr—Ni alloy. Cu further enhances the corrosion resistance of Cr-Ni alloys to sulfuric acid. This enhances the sulfuric acid general corrosion resistance of the Cr-Ni alloy. The inventors have further found that Co enhances resistance to general corrosion by sulfuric acid.
[0021]
　From the above, the present inventors have for the first time found that when a proper amount of Cu and Co are contained in a Cr-Ni alloy, high sulfuric acid general corrosion resistance can be obtained even when the dislocation density is increased. .. Specifically, if the dislocation density in the Cr—Ni alloy is 2.50×10 15 +1.40×10 14 ×[Cu+Co]m −2 or less, high sulfuric acid general corrosion resistance at 250° C. is obtained. To be Here, Cu and Co represented by [Cu+Co] represent the Cu content (mass %) and Co content (mass %) in the Cr—Ni alloy, respectively.
[0022]
　Conventionally, dislocation density has been studied in relation to strength. Therefore, the relationship between the dislocation density and sulfuric acid general corrosion resistance, and the relationship between the dislocation density and the Cu content and the Co content have not been clarified.
[0023]
　On the other hand, in this embodiment, the upper limit of the dislocation density in the Cr—Ni alloy is limited by the relationship between the Cu content and the Co content. For the first time, it becomes possible to satisfy both the high yield strength required for deep geothermal wells and sulfuric acid general corrosion resistance at 250°C. Therefore, the Cu-Ni alloy of this embodiment is completed based on a technical idea different from the conventional one.
[0024]
　The Cr—Ni alloy of the present embodiment completed based on the above findings is, in mass %, Si: 0.01 to 0.50%, Mn: 0.01 to 1.00%, Cr: 21.0 to 27.0%, Ni: 40.0 to less than 50.0%, Mo: 4.5 to less than 9.0%, W: 2.0 to 6.0%, Cu: more than 2.0% and 6 0.0% or less, Co: 0.01 to 2.00%, one or two selected from the group consisting of Ca and Mg: 0.001 to 0.010% in total, sol. Al: 0.005 to 0.200%, N: 0.01 to 0.20%, one or more selected from the group consisting of Ti, Nb, Zr and V: 0 to 0.50 in total %, REM: 0 to 0.050%, C: 0.030% or less, P: 0.030% or less, S: 0.0010% or less, O: 0.010% or less, and the balance Fe and impurities With a chemical composition consisting of The dislocation density in the Cr-Ni alloy satisfies the following formula (1).
　8.00×10 14 ≦ρ≦2.50×10 15 +1.40×10 14 ×[Cu+Co] (1)
　where ρ is the dislocation density (m −2 ) in the above formula (1 ). , Cu and Co represent the Cu content (mass %) and Co content (mass %) in the Cr—Ni alloy, respectively.
[0025]
　The Cr—Ni alloy of this embodiment has an appropriate chemical composition and the dislocation density satisfies the formula (1). Therefore, the Cr—Ni alloy has high yield strength and high sulfuric acid general corrosion resistance at a high temperature of 250° C.
[0026]
　Preferably, the total content of one or more selected from the group consisting of Ti, Nb, Zr and V having the above chemical composition is 0.01 to 0.50%.
[0027]
　In this case, the strength and ductility of the Cr-Ni alloy are further enhanced.
[0028]
　Preferably, the REM content of the above chemical composition is 0.005 to 0.050%.
[0029]
　In this case, the hot workability of the Cr-Ni alloy is enhanced.
[0030]
　Preferably, the yield strength (0.2% proof stress) of the Cr-Ni alloy is 758 MPa or more.
[0031]
　Preferably, the yield strength (0.2% proof stress) of the Cr-Ni alloy is 861 MPa or more.
[0032]
　Preferably, the yield strength (0.2% proof stress) of the Cr-Ni alloy is 965 MPa or more.
[0033]
　The seamless steel pipe of this embodiment is made of the above Cr-Ni alloy.
[0034]
　Hereinafter, the Cr-Ni alloy of this embodiment will be described in detail.
[0035]
　[Chemical composition]
　The chemical composition of the Cr—Ni alloy of the present embodiment contains the following elements. Unless otherwise specified,% relating to elements means mass%.
[0036]
　Si: 0.01 to 0.50%
　Silicon (Si) is an element necessary for deoxidizing a Cr—Ni alloy. If the Si content is less than 0.01%, this effect cannot be obtained. On the other hand, if the Si content exceeds 0.50%, the hot workability may decrease. Therefore, the Si content is set to 0.01 to 0.50%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%. The upper limit of the Si content is preferably 0.45%, more preferably 0.40%.
[0037]
　Mn: 0.01 to 1.00%
　Manganese (Mn) is an element necessary as a deoxidizing and/or desulfurizing agent for Cr-Ni alloys. If the Mn content is less than 0.01%, this effect cannot be obtained. On the other hand, when the Mn content exceeds 1.00%, the hot workability of the Cr-Ni alloy deteriorates. Therefore, the Mn content is 0.01 to 1.00%. The lower limit of the Mn content is preferably 0.05%, more preferably 0.10%. The upper limit of the Mn content is preferably 0.80%, more preferably 0.70%.
[0038]
　Cr: 21.0 to 27.0%
　Chromium (Cr) is an element that improves sulfuric acid general corrosion resistance and stress corrosion cracking resistance. If the Cr content is less than 21.0%, this effect cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 27.0%, the hot workability of the Cr-Ni alloy deteriorates. If the Cr content exceeds 27.0%, a TCP phase typified by a sigma phase is more likely to occur, and the sulfuric acid general corrosion resistance and stress corrosion cracking resistance are rather deteriorated. Therefore, the Cr content is 21.0 to 27.0%. The lower limit of the Cr content is preferably 21.2%, more preferably 21.4%, further preferably 21.6%. The upper limit of the Cr content is preferably 26.8%, more preferably 26.5%, further preferably 26.0%.
[0039]
　Ni: 40.0 to less than 50.0%
　Nickel (Ni) is an austenite stabilizing element. If the Ni content is less than 40.0%, the corrosion resistance of the Cr-Ni alloy decreases. On the other hand, if the Ni content is 50.0% or more, the cost is increased. Therefore, the Ni content is 40.0 to less than 50.0%. The lower limit of the Ni content is preferably 41.5%, more preferably 43.5%, further preferably 44.0%. The upper limit of the Ni content is preferably 49.7%, more preferably 49.3%, even more preferably 49.0%.
[0040]
　Mo: 4.5 to less than 9.0%
　Molybdenum (Mo) enhances the pitting corrosion resistance of the Cr-Ni alloy in the environment where hydrogen sulfide and chloride ions are present. If the Mo content is less than 4.5%, this effect cannot be obtained. On the other hand, when the Mo content is 9.0% or more, the hot workability of the Cr-Ni alloy is significantly reduced. If the Mo content is 9.0% or more, the cost is further increased. Therefore, the Mo content is 4.5 to less than 9.0%. The lower limit of the Mo content is preferably 4.7%, more preferably 4.8%, further preferably 5.0%. The upper limit of the Mo content is preferably 8.5%, more preferably less than 8.3%, even more preferably 8.2%, and most preferably 8.0%.
[0041]
　W: 2.0 to 6.0%
　Tungsten (W) enhances the pitting corrosion resistance of the Cr—Ni alloy in the environment where hydrogen sulfide and chloride ions are present, similar to Mo. If the W content is less than 2.0%, this effect cannot be sufficiently obtained. On the other hand, if the W content exceeds 6.0%, the hot workability of the Cr-Ni alloy is significantly reduced. If the W content exceeds 6.0%, the cost is further increased. Therefore, the W content is 2.0 to 6.0%. The lower limit of the W content is preferably 2.5%, more preferably 2.7%, and further preferably 3.0%. The upper limit of the W content is preferably 5.5%, more preferably 5.0%, and further preferably 4.5%.
[0042]
　Cu: more than 2.0% and 6.0% or less
　Copper (Cu) is an important element for ensuring sulfuric acid general corrosion resistance in a severe geothermal environment. Cu significantly improves the sulfuric acid general corrosion resistance and the stress corrosion cracking resistance of the Cr-Ni alloy. When the Cu content is 2.0% or less, it is not sufficient to secure sulfuric acid general corrosion resistance. On the other hand, if the Cu content exceeds 6.0%, the hot workability of the Cr-Ni alloy deteriorates. Therefore, the Cu content is more than 2.0% and 6.0% or less. The lower limit of the Cu content is preferably 2.1%, more preferably 2.3%, and further preferably 2.5%. The upper limit of the Cu content is preferably 4.6%, more preferably 4.3%, and further preferably 4.0%.
[0043]
　Co: 0.01 to 2.00%
　Cobalt (Co), like Cu, is an element that ensures sulfuric acid general corrosion resistance in a severe geothermal environment. If the Co content is less than 0.01%, this effect cannot be sufficiently obtained. On the other hand, if the Co content exceeds 2.00%, the economic efficiency deteriorates. Therefore, the Co content is 0.01 to 2.00%. The lower limit of the Co content is preferably 0.02%, more preferably 0.04%, further preferably 0.05%. The upper limit of the Co content is preferably 1.80%, more preferably 1.60%.
[0044]
　One or two selected from the group consisting of Ca and Mg: 0.001 to 0.010% in total
　calcium (Ca) and magnesium (Mg) improve the hot workability of the Cr-Ni alloy; Improves manufacturability of Cr-Ni alloy. If the total content of one or two selected from the group consisting of Ca and Mg is less than 0.001%, this effect cannot be obtained. On the other hand, if the total content of one or two selected from the group consisting of Ca and Mg exceeds 0.010%, the hot workability of the Cr—Ni alloy is rather deteriorated. If the total content of one or two selected from the group consisting of Ca and Mg exceeds 0.010%, coarser inclusions are further generated, and the sulfuric acid general corrosion resistance of the Cr—Ni alloy is reduced. .. Therefore, the total content of one or two selected from the group consisting of Ca and Mg is 0.001 to 0.010%. The upper limit of the total content of one or two selected from the group consisting of Ca and Mg is preferably 0.007%.
[0045]
　Both Ca and Mg do not necessarily need to be contained. The chemical composition of the Cr—Ni alloy may contain Ca alone and may not contain Mg. When Ca is contained alone, the Ca content is 0.001 to 0.010%. When Ca is contained alone, the upper limit of the Ca content is preferably 0.007%. The chemical composition of the Cr—Ni alloy may contain Mg alone and may not contain Ca. When Mg is contained alone, the Mg content is 0.001 to 0.010%. When Mg is contained alone, the upper limit of the Mg content is preferably 0.007%.
[0046]
　sol. Al: 0.005 to 0.200%
　Aluminum (Al) fixes O (oxygen) in the alloy and improves the hot workability of the Cr-Ni alloy. On the other hand, sol. If the Al content exceeds 0.200%, the hot workability of the Cr-Ni alloy deteriorates. Therefore, the Al content is sol. Al is 0.005 to 0.200%. sol. The lower limit of the Al content in Al is preferably 0.008%. sol. The upper limit of the Al content in Al is preferably 0.160%, more preferably 0.150%. In addition, "sol.Al" means so-called "acid-soluble Al".
[0047]
　N: 0.01 to 0.20%
　Nitrogen (N) enhances the yield strength (0.2% proof stress) of the Cr-Ni alloy. If the N content is less than 0.01%, this effect cannot be obtained. On the other hand, if the N content exceeds 0.20%, the hot workability of the Cr—Ni alloy deteriorates due to the increase in nitrides. Therefore, the N content is 0.01 to 0.20%. The lower limit of the N content is preferably 0.02%, more preferably 0.04%. The upper limit of the N content is preferably 0.18%, more preferably 0.15%.
[0048]
　The balance of the chemical composition of the Cr—Ni alloy according to the present embodiment is Fe and impurities. That is, the chemical composition of the Cr—Ni alloy according to the present embodiment essentially contains Fe. Here, the impurities in the chemical composition are those that are mixed in from the ore as a raw material, scrap, or the manufacturing environment when industrially manufacturing a stainless steel material, and are added to the Cr-Ni alloy of the present embodiment. It means that it is permissible as long as it does not adversely affect.
[0049]
　[Regarding Arbitrary Elements]
　The chemical composition of the Cr—Ni alloy of the present embodiment may further contain the following arbitrary elements.
[0050]
　One or more selected from the group consisting of Ti, Nb, Zr and V: 0 to 0.50% in total of
　titanium (Ti), niobium (Nb), zirconium (Zr) and vanadium (V). Is also an optional element and may not be contained. That is, the total content of one or more selected from the group consisting of Ti, Nb, Zr and V may be 0%. Each of Ti, Nb, Zr and V refines the crystal grains to enhance the strength and ductility of the Cr-Ni alloy. Therefore, one or more selected from the group consisting of Ti, Nb, Zr, and V may be contained, if necessary. However, if the total content of one or more selected from the group consisting of Ti, Nb, Zr, and V exceeds 0.50%, a large amount of inclusions are generated, rather the ductility decreases, and further, Hot workability is reduced. Therefore, the total content of one or more selected from the group consisting of Ti, Nb, Zr and V is 0 to 0.50%. The lower limit of the total content of one or more selected from the group consisting of Ti, Nb, Zr and V is preferably 0.01%, more preferably 0.02%, and further preferably Is 0.04%. The upper limit of the total content of one or more selected from the group consisting of Ti, Nb, Zr and V is preferably 0.30%.
[0051]
　When one or more selected from the group consisting of Ti, Nb, Zr and V is contained, Ti, Nb, Zr or V may be contained alone. When Ti is contained alone, the Ti content is 0 to 0.50%. When Nb is contained alone, the Nb content is 0 to 0.50%. When Zr is contained alone, the Zr content is 0 to 0.50%. When V is contained alone, the V content is 0 to 0.50%.
[0052]
　REM: 0 to 0.050%
　Rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. REM enhances the hot workability of Cr-Ni alloys. Therefore, it may be contained if necessary. On the other hand, if the REM content exceeds 0.050%, the sulfuric acid general corrosion resistance of the Cr-Ni alloy decreases. Therefore, the REM content is 0 to 0.050%. The lower limit of the REM content is preferably 0.005%, more preferably 0.008%, and further preferably 0.010%. The upper limit of the REM content is preferably 0.030%. Here, REM means 17 elements obtained by adding yttrium (Y) and scandium (Sc) to elements from lanthanum (La) of element number 57 to lutetium (Lu) of element number 71 in the periodic table. The REM content means the total content of these elements. REM may be industrially added as a misch metal.
[0053]
　[Regarding Impurity Elements]
　The chemical composition of the Cr—Ni alloy of the present embodiment contains the following elements as impurities. The content of these elements is limited for the following reasons.
[0054]
　C: 0.030% or less
　Carbon (C) is an impurity unavoidably contained in the Cr-Ni alloy. Therefore, the lower limit of the C content is more than 0%. When the C content exceeds 0.030%, stress corrosion cracking accompanied by intergranular fracture due to precipitation of M 23 C 6 type carbide (“M” indicates alloying elements such as Cr, Mo and/or Fe.). Is likely to occur. Therefore, the C content is 0.030% or less. The upper limit of the C content is preferably 0.025%, more preferably 0.020%. On the other hand, if the C content is reduced to the limit, the production cost due to decarburization becomes high. Therefore, the lower limit of the C content is preferably 0.0001%.
[0055]
　P: 0.030% or less
　Phosphorus (P) is an impurity inevitably contained in the Cr-Ni alloy. Therefore, the lower limit of the P content is more than 0%. P significantly reduces the hot workability and the stress corrosion cracking resistance of the Cr-Ni alloy. Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 0.025%, more preferably 0.020%. On the other hand, if the P content is reduced to the limit, the production cost due to dephosphorization increases. Therefore, the lower limit of the P content is preferably 0.0001%.
[0056]
　S: 0.0010% or less
　Sulfur (S) is an impurity inevitably contained in the Cr-Ni alloy. Therefore, the lower limit of the S content is more than 0%. S, like P, remarkably reduces the hot workability of the Cr-Ni alloy. From the viewpoint of suppressing the deterioration of hot workability, the S content is preferably as low as possible. Therefore, the S content is 0.0010% or less. The upper limit of the S content is preferably 0.0008%. On the other hand, if the S content is reduced to the limit, the manufacturing cost due to desulfurization becomes high. Therefore, the lower limit of the S content is preferably 0.0001%.
[0057]
　O: 0.010% or less
　Oxygen (O) is an impurity inevitably contained in the Cr-Ni alloy. Therefore, the lower limit of the O content is more than 0%. O significantly reduces the hot workability of the Cr-Ni alloy. Therefore, the O content is 0.010% or less. It is desirable that the O content is as low as possible. On the other hand, if the O content is reduced to the utmost, the production cost due to deoxidation increases. Therefore, the lower limit of the O content is preferably 0.0001%.
[0058]
　[Microstructure]
　The microstructure of the Cr—Ni alloy of this embodiment is an austenite single phase.
[0059]
　[Dislocation Density] The dislocation density in the
　Cr—Ni alloy satisfies the formula (1).
　8.00×10 14 ≦ρ≦2.50×10 15 +1.40×10 14 ×[Cu+Co] (1)
　where ρ is the dislocation density (m −2 ) in the above formula (1 ). , Cu and Co represent the Cu content (mass %) and Co content (mass %) in the Cr—Ni alloy, respectively.
[0060]
　In the Cr-Ni alloy having the above chemical composition, when the dislocation density ρ is less than 8.00×10 14 m -2 , sufficient yield strength (0.2% proof stress) suitable for use in deep geothermal wells is obtained. ) Cannot be obtained. On the other hand, the dislocation accelerates the dissolution reaction of the Cr—Ni alloy in the strongly acidic environment of 250° C. Therefore, if the dislocation density ρ is too high, the sulfuric acid general corrosion resistance of the Cr—Ni alloy decreases. Specifically, if the dislocation density ρ exceeds {2.50×10 15 +1.40×10 14 ×[Cu+Co]}m −2 , the temperature is 250° C. or higher, as in a severe geothermal environment, and sulfuric acid is used. Sulfuric acid general corrosion resistance in a strongly acidic environment containing a large amount of is reduced. Therefore, if the dislocation density in the Cr—Ni alloy satisfies the above expression (1), sufficiently high yield strength and high sulfuric acid general corrosion resistance at a high temperature of 250° C. can both be achieved.
[0061]
　The lower limit of the dislocation density ρ is preferably 1.60×10 15 , more preferably 1.70×10 15 , and even more preferably 1.80×10 15 . The upper limit of the dislocation density ρ is preferably 2.40×10 15 +1.40×10 14 ×[Cu+Co], more preferably 2.25×10 15 +1.40×10 14 ×[Cu+Co], and further preferably 2 .15×10 15 +1.40×10 14 ×[Cu+Co].
[0062]
　[Measurement Method of Dislocation Density] The dislocation density in the
　Cr—Ni alloy is measured by the following method. First, from the central portion in the thickness direction of the Cr—Ni alloy (the central portion of the plate thickness in the case of a plate material, the central portion of the wall thickness of the tube in the case of a pipe material), a test of 20 mm in length and width and 2 mm in thickness Cut out a piece. Then, the surface of the test piece is electropolished at 10° C. using a 10% perchloric acid-acetic acid solution as an electrolytic solution. The dislocation density is measured using the test piece after electrolytic polishing. The dislocation density is measured using an evaluation method based on the Williamson-Hall method described in Non-Patent Document 2 proposed by Nakajima et al. in Non-Patent Document 1. Specifically, measuring device: Rint-2500 manufactured by Rigaku Corporation, cathode tube: Co tube, profile: θ-2θ diffraction method, scan range: 2θ, X-ray diffraction profile is measured at 40° to 130°. Then, the diffraction of each of the {111} plane, {220} plane, and {311} plane of the FCC crystal structure is fitted using the Rietveld method described in Non-Patent Document 3. The strain ε is obtained using the obtained half width. Further, the following equation (2) represented by the strain ε and the Burgers vector b is calculated to obtain the dislocation density ρ(m −2 ).
　ρ=14.4ε 2 /b 2 (2)
[0063]
　Note that a 1100° C. solution water-cooled material, which is considered to have a very low dislocation density, is used for profile measurement derived from the measuring device. In addition, 0.2545×10 -9 m is used as the value of the Burgers vector b .
[0064]
　[Yield Strength] The yield strength
　required for a deep geothermal well is, for example, 758 MPa or more. Therefore, the yield strength of the Cr—Ni alloy of this embodiment is preferably 758 MPa or more. In this case, it is possible to more stably withstand the high underground pressure in the deep geothermal well. The yield strength of the Cr-Ni alloy is preferably 861 MPa or more, more preferably 965 MPa or more. The upper limit of the yield strength is preferably 1175 MPa, more preferably 1103 MPa, further preferably 1000 MPa.
[0065]
　[Yield Strength Measuring Method]
　Yield strength shall be 0.2% proof stress determined by the method according to JIS Z2241 (2011). Two round bar tensile test pieces each having a diameter of the parallel part of 6 mm and a gauge length of 40 mm are sampled. The sampling direction of the round bar tensile test piece is the rolling direction. A tensile test is performed on the collected round bar tensile test piece at room temperature by a method according to JIS Z2241 (2011) to determine the yield strength (0.2% yield strength).
[0066]
　[Shape of Cr-Ni Alloy]
　The shape of the Cr-Ni alloy of the present embodiment is not particularly limited. The Cr—Ni alloy may be, for example, an alloy tube, an alloy plate, a bar alloy, or a wire rod. The Cr-Ni alloy of this embodiment can be suitably used as a seamless steel pipe.
[0067]
　The Cr—Ni alloy of this embodiment has excellent sulfuric acid general corrosion resistance. Therefore, it can be suitably used for a deep geothermal well. The Cr-Ni alloy of this embodiment is also excellent in stress corrosion cracking resistance (SCC resistance). Therefore, you may use it for an oil well.
[0068]
　[Manufacturing Method]
　The Cr—Ni alloy of the present embodiment can be manufactured, for example, by the following method. Hereinafter, a manufacturing method for manufacturing a seamless steel pipe will be described as an example. However, the manufacturing method of the present embodiment is not limited to the case of manufacturing a seamless steel pipe.
[0069]
　First, a material having the above chemical composition is prepared. Specifically, the molten metal is manufactured using an electric furnace, an AOD furnace, a VOD furnace, or the like, and the chemical composition is adjusted. When performing desulfurization treatment by a composite of REM and Ca and/or Mg, it is desirable to add REM and Ca and/or Mg after sufficiently deoxidizing with Al or the like in advance.
[0070]
　Next, a raw material is manufactured from the molten metal whose chemical composition is adjusted. The material may be a slab, bloom or billet manufactured by a continuous casting method (including round CC). Further, it may be a billet manufactured by hot working an ingot manufactured by the ingot making method. The material may be a billet manufactured from a slab or bloom by hot working.
[0071]
　Then, the material is charged into a heating furnace or a soaking furnace and heated. The heating temperature is, for example, 850 to 1300°C. Hot-work the heated material. For example, the Mannesmann method is implemented as hot working. Specifically, the material is pierced and rolled by a piercing machine to form a raw pipe. Then, the raw pipe is stretch-rolled by a mandrel mill and a sizing mill and is subjected to constant-diameter rolling to manufacture a seamless steel pipe. Hot extrusion may be carried out as hot working, or hot forging may be carried out. The hot working temperature is, for example, 800 to 1300°C. In the case of processing into a plate material, it may be processed into a plate or coil shape by hot rolling.
[0072]
　Subsequently, solution heat treatment is performed on the raw tube after hot working. The temperature of the solution heat treatment is, for example, 1000 to 1250°C. The solution treatment time is, for example, 45 minutes to 2 hours.
[0073]
　Further, the blank is cold-worked so that the dislocation density ρ satisfies the above expression (1). Cold working is, for example, cold drawing or Pilger rolling. The cold working may be carried out once or plural times.
[0074]
　When the cold working is performed a plurality of times, the intermediate heat treatment may be performed between the cold workings. For example, the intermediate heat treatment may be performed after the cold working, and then the cold working may be further performed once or plural times. In the Cr-Ni alloy of this embodiment, the upper limit of the dislocation density ρ is limited depending on the Cu content and the Co content. When the Cu content and the Co content are high, excellent sulfuric acid general corrosion resistance can be obtained even if the dislocation density ρ is increased. On the other hand, when the Cu content and the Co content are low, the sulfuric acid general corrosion resistance decreases unless the dislocation density ρ is limited to a certain value. In order to keep the dislocation density ρ at or below the upper limit defined by the equation (1), the cross-sectional reduction rate (%) during cold working is set to, for example, the value (%) or less obtained from the equation (3). Formula (3) is 8.8×[Cu+Co]+12.5... (3). Thereby, the upper limit of the dislocation density ρ can be controlled within the range of the expression (1) under the condition having the chemical composition of the present application. The lower limit of the area reduction rate during cold working is, for example, 30%. When the intermediate heat treatment is performed after the cold working and then the cold working is further performed once or a plurality of times, the cross-sectional reduction rate after the intermediate heat treatment is, for example, 30% or more, and {8.8×[Cu+Co] It is +12.5}% or less. The preferable lower limit of the area reduction rate is 35%. In this case, the dislocation density is further increased.
[0075]
　In order to control the structure of the material, heat treatment may be performed at 600° C. or lower after performing cold working. As a result, C and N in the material are diffused and dislocations are hard to move. As a result, the anisotropy of yield strength can be reduced. If the temperature of the heat treatment exceeds 600°C, the yield strength will decrease. It is presumed that the dislocations disappear due to the dislocations coalescing with each other because the temperature is too high. In the method for producing the Cr—Ni alloy of the present disclosure, the solution heat treatment is not performed after the final cold working. After the heat treatment, descaling (oxidized scale formed on the surface may be removed by shot blasting, pickling, etc.). Finally, cleaning may be performed to remove foreign matter on the surface. A seamless steel pipe can be manufactured by the above process.
Example
[0076]
　Alloys having the chemical compositions shown in Table 1 were melted in a vacuum high frequency melting furnace to cast a 30 kg ingot.
[0077]
[table 1]

[0078]
　Alloys AR in Table 1 are alloys with suitable chemical compositions. On the other hand, the alloys S and T are alloys whose chemical compositions deviate from the conditions specified in the present invention.
[0079]
　Each ingot was soaked at 1200° C. for 3 hours, and then hot forged to form a square bar having a cross section of 50 mm×100 mm. Each square bar was further heated at 1200° C. for 1 hour and then hot rolled to obtain a plate material having a thickness of 17 mm. After that, solution heat treatment was performed at 1100° C. for 1 hour, followed by water cooling treatment to obtain an austenite single-phase structure.
[0080]
　Subsequently, cold rolling was performed using a part of the water-cooled plate material to obtain a plate material of each test number. For alloy A, three types of plate materials having a thickness of 16.2 mm (area reduction rate of 4.7%), 11.9 mm (area reduction rate of 30%) and 8.5 mm (area reduction rate of 50%) were produced. For alloy H, two types of plate materials having a thickness of 11.9 mm (cross-sectional reduction rate of 30%) and 10.2 mm (cross-sectional reduction rate of 40%) were produced. Further, for the alloys B, D, E, J, and O, two types of plate materials having a thickness of 11.9 mm (section reduction rate 30%) and 11.1 mm (section reduction rate 35%) were produced. For other alloys, plate materials having a thickness of 11.9 mm (reduction rate of cross section: 30%) were produced.
[0081]
　[Measurement of Dislocation Density] The dislocation density of
　the plate material of each test number was measured by the method described above. The results are shown in Table 2.
[0082]
　[Tensile Test] A
　tensile test was performed on the plate material of each test number. Two round bar tensile test pieces each having a parallel part diameter of 6 mm and a gauge length of 40 mm were sampled. The sampling direction of the round bar tensile test piece was the rolling direction. A tensile test was performed on the collected round bar tensile test piece at room temperature to determine the yield strength (0.2% proof stress). The yield strength shown in Table 2 is the arithmetic mean value of the yield strengths of two test pieces. In the examination of tensile properties, it was judged that the yield strength was sufficiently high when the average value of the yield strengths of the two test pieces was 758 MPa or more.
[0083]
　[Sulfuric acid general corrosion resistance evaluation test]
　Two pieces of corrosion test pieces each having a length of 40 mm, a width of 10 mm and a thickness of 3 mm were sampled from each plate number. Each corrosion test piece had a hole with a diameter of 3 mm for hanging on a jig. The weight of the corrosion test piece before the test was measured. The corrosion test piece was immersed in the test solution in the autoclave adjusted to the following test conditions.
[0084]
　Test conditions
　Test solution: 0.01 mol/LH 2 SO 4 +1.7 mass% NaCl
　Test gas: 0.1 bar H 2 S+5 bar CO 2
　Test temperature: 250° C.
　Test time: 360 hours
[0085]
　The weight of the corrosion test piece after the lapse of the test time was measured. The corrosion weight loss of each test piece was determined based on the amount of change in the weight of the corrosion test piece before and after the test. From the obtained corrosion weight loss, the corrosion rate (mm/y) of the plate material of each test number was calculated. This corrosion rate of 0.10 mm/y was set as the target value of the general sulfuric acid corrosion resistance. When the corrosion rate was 0.10 mm/y or less, it was judged that the sulfuric acid general corrosion resistance was good. The results are shown in Table 2.
[0086]
　[
　Stress corrosion cracking test] From the plate material of each test number, according to the low strain rate tensile test method specified in NACE TM0198, the low strain rate of the parallel part having a diameter of 3.81 mm and a length of 25.4 mm. Four tensile test pieces were collected. Specifically, test pieces were taken from the plate material of each test number so that the parallel portion was parallel to the rolling direction. Then, a low strain rate tensile test based on NACE TM0198 was performed to evaluate the stress corrosion cracking resistance. The test environment was set to two conditions: the atmosphere and an environment simulating an oil well environment (test conditions described later).
[0087]
　For one of the four test pieces, the values ​​of fracture ductility and fracture drawing were determined by a tensile test in the atmosphere (hereinafter, "reference value of fracture ductility" and "reference value of fracture drawing"). That). For the other three test pieces, the values ​​of fracture ductility and fracture reduction were obtained by a tensile test in an environment simulating an oil well environment (hereinafter referred to as "comparison value of fracture ductility" and "comparison value of fracture reduction"). ). That is, in this example, for each cold rolled material, one reference value of fracture ductility, three comparison values ​​of fracture ductility, one reference value of fracture drawing, and three comparison values ​​of fracture drawing were obtained. It was
[0088]
　Then, for each cold rolled material, the difference between the reference value of fracture ductility and the three comparative values ​​of fracture ductility was determined (hereinafter, each difference is referred to as "difference in fracture ductility"). Similarly, the difference between the reference value of the breakage reduction and the three comparison values ​​of the breakage reduction was obtained (hereinafter, each difference is referred to as the “difference of the breakage reduction”). In this evaluation, all of the "differences in fracture ductility" were set to 20% or less of the "reference value of fracture ductility", and all of the "differences of fracture drawing" were set to 20% or less of the "reference value of fracture drawing". This was the goal of stress corrosion cracking resistance. Then, when the above target was cleared, it was judged that the stress corrosion cracking resistance was good, and in Table 2, "○" was written in the column of the SCC resistance test. On the other hand, when the above target could not be cleared, it was described as “x”.
[0089]
　Test conditions
　Test solution: 25% NaCl+0.5% CH 3 COOH
　Test gas: 6.89 MPaH 2 S
　Test temperature: 204° C. Strain
　rate: 4.0×10 −6 /s
[0090]
[Table 2]

[0091]
　[Evaluation Results]
　Referring to Tables 1 and 2, the chemical compositions of the alloys of Test Nos. 1 to 24 were appropriate, and the dislocation density satisfied the formula (1). Therefore, the yield strength of the plate materials of Test Nos. 1 to 24 was 758 MPa or more, and the yield strength was sufficient for application to deep geothermal wells. Further, the plate materials of Test Nos. 1 to 24 had a corrosion rate of 0.100 mm/y or less, and showed excellent general sulfuric acid corrosion resistance at 250°C. Furthermore, the plate materials of test numbers 1 to 24 did not crack even in the stress corrosion cracking test, and were suitable for use not only in geothermal wells but also in oil wells.
[0092]
　On the other hand, the plate material of test number 25 had a dislocation density of 0.38×10 15 m −2 and did not satisfy the formula (1). Therefore, the yield strength of the plate material of test number 25 was 401 MPa, and sufficient yield strength could not be obtained.
[0093]
　The plate material of test number 26 had a dislocation density of 3.65×10 15 m −2 and did not satisfy the formula (1). Therefore, the corrosion rate of the plate No. 26 was 0.160 mm/y, and the sulfuric acid general corrosion resistance was poor.
[0094]
　In the plate material of test number 27, the Cu content of the alloy used was too low. Therefore, the corrosion rate of the plate of test number 27 was 0.310 mm/y, and the sulfuric acid general corrosion resistance was poor.
[0095]
　In the plate material of test number 28, the Co content of the alloy used was too low. Therefore, the corrosion rate of the plate material of test number 28 was 0.190 mm/y, and the sulfuric acid general corrosion resistance was poor.
[0096]
　The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit of the invention.
Industrial availability
[0097]
　The Cr-Ni alloy of this embodiment can be suitably used for geothermal wells. The Cr-Ni alloy of this embodiment may be used in an oil well.　
The scope of the claims
[Claim 1]
　Cr-Ni alloy, in
　mass %,
　Si: 0.01 to 0.50%,
　Mn: 0.01 to 1.00%,
　Cr: 21.0 to 27.0%,
　Ni: 40.0 Up to less than 50.0%,
　Mo: 4.5 to less than 9.0%,
　W: 2.0 to 6.0%,
　Cu: more than 2.0% and up to 6.0%,
　Co: 0.01 ~2.00%,
　one or two kinds selected from the group consisting of Ca and Mg: 0.001 to 0.010% in total,
　sol. Al: 0.005 to 0.200%,
　N: 0.01 to 0.20%,
　one or more selected from the group consisting of Ti, Nb, Zr and V: 0 to 0.50 in total %,
　REM: 0 to 0.050%,
　C: 0.030% or less,
　P: 0.030% or less,
　S: 0.0010% or less,
　O: 0.010% or less, and the
　balance Fe and impurities A
　Cr—Ni alloy having a chemical composition of: and a dislocation density in the Cr—Ni alloy satisfying the following formula (1).
　8.00 x 10 14≦ρ≦2.50×10 15 +1.40×10 14 ×[Cu+Co] (1)
　Here, in the above formula (1), ρ is the dislocation density (m −2 ), and Cu and Co are respectively. The Cu content (mass %) and Co content (mass %) in the Cr-Ni alloy are shown.
[Claim 2]
　The Cr-Ni alloy according to claim 1,
　wherein the total content of one or more selected from the group consisting of Ti, Nb, Zr and V in the chemical composition is 0.01 to 0. Cr-Ni alloy, which is 0.50%.
[Claim 3]
　The Cr-Ni alloy according to claim 1 or 2,
　wherein the REM content of the chemical composition is 0.005 to 0.050%.
[Claim 4]
　The Cr-Ni alloy according to any one of claims 1 to 3, wherein the
　yield strength (0.2% proof stress) is 758 MPa or more.
[Claim 5]
　The Cr-Ni alloy according to any one of claims 1 to 3, wherein the
　yield strength (0.2% proof stress) is 861 MPa or more.
[Claim 6]
　The Cr-Ni alloy according to any one of claims 1 to 3, wherein the
　yield strength (0.2% proof stress) is 965 MPa or more.
[Claim 7]
　A seamless steel pipe made of the Cr-Ni alloy according to any one of claims 1 to 6.