Duplex stainless steel material

Abstract

Provided is a duplex stainless steel material which has excellent comprehensive corrosion resistance in a high-temperature, high-pressure and strongly acidic corrosion environment and excellent pitting corrosion resistance in a high-temperature, high-pressure chloride corrosion environment. This duplex stainless steel material has a chemical composition containing, in mass%, 0.050% or less of C, 0.2 to 1.2% of Si, 0.5 to 7.0% of Mn, 0.040% or less of P, 0.010% or less of S, 20.0 to 27.0% of Cr, 4.0 to 9.0% of Ni, 0.5 to 5.0% of Mo, 0.0005 to 0.0100% of As, 0.0005 to 0.0100% of a total of one or more of Ca and Mg, 0.001 to 0.050% of sol.Al, 0.40% or less of N, and 0.100% or less of O, with the remainder being Fe and impurities, and satisfies formulae (1) and (2). 0.70 lt, 0.70 lt; 10000 * As / (Ni + Cu) lt; 16.00 (1) (Ca + Mg) / Olt; 1.50 (2).

Description

Technical Field

[0001] The present disclosure relates to duplex stainless steels. Background Art

[0002] As one of the low-carbon energy sources, geothermal power generation has attracted attention. In geothermal power generation, geothermal fluid collected from geothermal wells that store high-temperature and high-pressure hot water is used to generate steam. Geothermal fluid refers to high-temperature and high-pressure hot water and steam. The steam is supplied to a steam turbine to generate electricity.

[0003] Recently, the development of deep geothermal wells deeper than before is being carried out. The steam obtained from such deep geothermal wells contains hydrogen sulfide (H2S) and carbon dioxide (CO2), and also contains reducing acids such as sulfuric acid (H2SO4) and / or hydrochloric acid (HCl). The piping for collecting such steam or the piping for transporting such steam sometimes becomes an environment of high temperature of 180°C and high pressure of 5 bar. Therefore, a strongly acidic aqueous solution containing sulfuric acid and / or chloride ions is generated under the high temperature and high pressure environment in the piping. As mentioned above, the steam in the piping contains highly corrosive hydrogen sulfide (H2S). Therefore, the piping used for geothermal power generation becomes an extremely harsh corrosive environment.

[0004] In an environment containing hydrogen sulfide and reducing acid, general corrosion becomes the main corrosion factor. In addition, in an environment containing hydrogen sulfide and chloride ions, pitting corrosion becomes the main corrosion factor. Therefore, in order to have excellent corrosion resistance in the above-mentioned high-temperature and high-pressure corrosive environment, it is required to have excellent general corrosion resistance in a high-temperature and high-pressure strong acid corrosive environment containing hydrogen sulfide and sulfuric acid at a high temperature of 180°C and a high pressure of 5 bar, and excellent pitting resistance in a high-temperature and high-pressure chloride corrosive environment containing hydrogen sulfide and chloride ions at a high temperature of 180°C and a high pressure of 5 bar.

[0005] International Publication No. 2009 / 119630 (Patent Document 1) proposes an alloy material having excellent corrosion resistance in a strong acidic environment containing a reducing acid. The alloy material disclosed in the document is a Ni alloy material, which contains, by mass%, C: 0.03% or less, Si: 0.01-0.5%, Mn: 0.01-1.0%, P: 0.03% or less, S: 0.01% or less, Cr: 20% or more and less than 30%, Ni: more than 40% and less than 60%, Cu: more than 2.0% and less than 5.0%, Mo: 4.0-10%, Al: 0.005-0.5%, and N: more than 0.02% and less than 0.3%, and satisfies 0.5Cu+Mo≥6.5.

[0006] In addition, International Publication No. 2013 / 035588 (Patent Document 2) proposes a duplex stainless steel material that has excellent corrosion resistance in a corrosive environment of about 150°C containing hydrogen sulfide and chloride ions. The duplex stainless steel material disclosed in the document contains, by mass%, C: 0.03% or less, Si: 0.2-1%, Mn: higher than 5.0% and less than 10%, P: 0.040% or less, S: 0.010% or less, Ni: 4.5-8%, sol.Al: 0.040% or less, N: higher than 0.2% and less than 0.4%, Cr: 24-29%, Mo: 0.5% or more and less than 1.5%, Cu: 1.5-3.5%, and W: 0.05-0.2%, the balance being Fe and impurities, and satisfies Cr+8Ni+Cu+Mo+W / 2≥65.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: International Publication No. 2009 / 119630

[0010] Patent Document 2: International Publication No. 2013 / 035588 Summary of the invention

[0011] Problem that the invention aims to solve

[0012] However, Patent Documents 1 and 2 do not study the general corrosion resistance in the high-temperature and high-pressure strong acid corrosion environment and the pitting corrosion resistance in the high-temperature and high-pressure chloride corrosion environment.

[0013] The object of the present invention is to provide a duplex stainless steel material, which has excellent overall corrosion resistance in a high temperature and high pressure strong acid corrosion environment, and has excellent pitting corrosion resistance in a high temperature and high pressure chloride corrosion environment.

[0014] Solutions for solving problems

[0015] The chemical composition of the duplex stainless steel material disclosed in the present invention is as follows in terms of mass %:

[0016] C: 0.050% or less,

[0017] Si: 0.2-1.2%,

[0018] Mn: 0.5-7.0%,

[0019] P: 0.040% or less,

[0020] S: 0.010% or less,

[0021] Cr: 20.0~27.0%,

[0022] Ni: 4.0-9.0%,

[0023] Mo: 0.5-5.0%,

[0024] As: 0.0005~0.0100%,

[0025] One or more of Ca and Mg: 0.0005 to 0.0100% in total,

[0026] Sol.Al: 0.001~0.050%,

[0027] N: 0.40% or less,

[0028] O: 0.100% or less,

[0029] Cu: 0-4.0%,

[0030] V: 0~1.50%,

[0031] Co: 0-2.00%,

[0032] Ta: 0~2.00%,

[0033] W: 0~4.00%,

[0034] Nb: 0-2.00%,

[0035] Ti: 0-2.00%,

[0036] Zn: 0~0.0100%,

[0037] Pb: 0~0.0100%,

[0038] Sb: 0~0.0100%,

[0039] Sn: 0~0.0100%,

[0040] Bi: 0~0.0100%,

[0041] B: 0~0.0100%,

[0042] Rare earth elements: 0-0.050%, Zr: 0-2.00%,

[0043] Hf: 0 to 2.00%, and

[0044] Balance: Fe and impurities,

[0045] And satisfying equations (1) and (2),

[0046] 0.70<10000×As / (Ni+Cu)<16.00 (1)

[0047] (Ca+Mg) / O<1.50 (2)

[0048] Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula. When the element is not contained, "0" is substituted for the corresponding element symbol.

[0049] Effects of the Invention

[0050] The duplex stainless steel material of the present invention has excellent overall corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosion environment, and has excellent pitting corrosion resistance in a high-temperature, high-pressure chloride corrosion environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is the relationship between Fn1 in duplex stainless steel and the corrosion rate in high temperature, high pressure and strong acid corrosion environment (g·cm -2 ·h -1 )’s relationship diagram.

[0052] Figure 2 The corrosion rate of Fn2 in duplex stainless steel in high temperature and high pressure chloride corrosion environment (g·cm -2 ·h -1 )’s relationship diagram. DETAILED DESCRIPTION

[0053] In this specification, the high temperature and high pressure strong acid corrosion environment and the high temperature and high pressure chloride corrosion environment are defined as follows.

[0054] High temperature, high pressure, strong acid corrosion environment: 180℃ high temperature and 5 bar high pressure environment containing hydrogen sulfide and sulfuric acid

[0055] High temperature and high pressure chloride corrosion environment: 180℃ high temperature and 5 bar high pressure environment containing hydrogen sulfide and chloride ions

[0056] The present inventors have studied duplex stainless steel materials that have excellent general corrosion resistance in high temperature and high pressure strong acid corrosion environments and excellent pitting corrosion resistance in high temperature and high pressure chloride corrosion environments from the perspective of chemical composition. As a result, the present inventors believe that it is possible to obtain excellent general corrosion resistance in high temperature and high pressure strong acid corrosion environments and excellent pitting corrosion resistance in high temperature and high pressure chloride corrosion environments if the duplex stainless steel material has the following chemical composition, which is calculated by mass % as follows: C: 0.050% or less, Si: 0.2-1.2%, Mn: 0.5-7.0%, P: 0.040% or less, S: 0.010% or less, Cr: 20.0-27.0%, Ni: 4.0-9.0%, Mo: 0.5-5.0%, one or more of Ca and Mg: 0.0005-0.0100% in total, sol.Al: 0.0 01~0.050%, N: less than 0.40%, O: less than 0.100%, Cu: 0~4.0%, V: 0~1.50%, Co: 0~2.00%, Ta: 0~2.00%, W: 0~4.00%, Nb: 0~2.00%, Ti: 0~2.00%, Zn: 0~0.0100%, Pb: 0~0.0100%, Sb: 0~0.0100%, Sn: 0~0.0100%, Bi: 0~0.0100%, B: 0~0.0100%, rare earth elements: 0~0.050%, Zr: 0~2.00%, Hf: 0~2.00%, and the balance: Fe and impurities.

[0057] However, when the duplex stainless steel material having the above chemical composition is applied to a high temperature, high pressure, and strongly acidic corrosion environment, excellent overall corrosion resistance is sometimes not obtained. Therefore, the inventors of the present invention have conducted further research. As a result, the inventors of the present invention have found that arsenic (As) improves overall corrosion resistance in a high temperature, high pressure, and strongly acidic corrosion environment. Therefore, further research was conducted, and it was believed that if the duplex stainless steel material satisfies the characteristic 1 that contains 0.0005 to 0.0100% of As to replace part of the Fe in the above chemical composition, it is possible to obtain excellent overall corrosion resistance in a high temperature, high pressure, and strongly acidic corrosion environment.

[0058] (Feature 1)

[0059] The chemical composition is as follows by mass %: C: 0.050% or less, Si: 0.2-1.2%, Mn: 0.5-7.0%, P: 0.040% or less, S: 0.010% or less, Cr: 20.0-27.0%, Ni: 4.0-9.0%, Mo: 0.5-5.0%, As: 0.0005-0.0100%, one or more of Ca and Mg: 0.0005-0.0100% in total, sol.Al: 0.001-0.050%, N: 0.40% or less, O: 0.100% or less, Cu: 0~4.0%, V: 0~1.50%, Co: 0~2.00%, Ta: 0~2.00%, W: 0~4.00%, Nb: 0~2.00%, Ti: 0~2.00%, Zn: 0~0.0100%, Pb: 0~0.0100%, Sb: 0~0.0100%, Sn: 0~0.0100%, Bi: 0~0.0100%, B: 0~0.0100%, rare earth elements: 0~0.050%, Zr: 0~2.00%, and Hf: 0~2.00%, and the balance is Fe and impurities.

[0060] However, even for duplex stainless steel materials that meet characteristic 1, there are still cases where excellent overall corrosion resistance cannot be obtained in high temperature, high pressure, strong acid corrosion environments. In addition, even for duplex stainless steel materials that meet characteristic 1, excellent pitting corrosion resistance may not be obtained in high temperature, high pressure, chloride corrosion environments.

[0061] Therefore, the present inventors have further studied means for improving the general corrosion resistance in a high temperature and high pressure strong acid corrosion environment and the pitting corrosion resistance in a high temperature and high pressure chloride corrosion environment in a duplex stainless steel material satisfying feature 1. As a result, it was found that if the duplex stainless steel material satisfying feature 1 further satisfies features 2 and 3, excellent general corrosion resistance is obtained in a high temperature and high pressure strong acid corrosion environment, and excellent pitting corrosion resistance is obtained in a high temperature and high pressure chloride corrosion environment.

[0062] (Feature 2)

[0063] The chemical composition satisfies the formula (1).

[0064] 0.70<10000×As / (Ni+Cu)<16.00 (1)

[0065] Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula. When the element is not contained, "0" is substituted for the corresponding element symbol.

[0066] (Feature 3)

[0067] The chemical composition satisfies formula (2).

[0068] (Ca+Mg) / O<1.50 (2)

[0069] Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula. When the element is not contained, "0" is substituted for the corresponding element symbol.

[0070] [About Feature 2]

[0071] For feature 2, Fn1 is defined as follows.

[0072] Fn1=10000×As / (Ni+Cu)

[0073] Here, in the case of the essential elements in the chemical composition of the duplex stainless steel material of the present embodiment, "0" is substituted for Cu in Fn1, so Fn1 = 10000 × As / Ni.

[0074] Fn1 is an indicator of overall corrosion resistance in a high temperature, high pressure, strong acidic corrosion environment. As, Ni and Cu all improve overall corrosion resistance in a high temperature, high pressure, strong acidic corrosion environment. Furthermore, by adjusting the ratio of the As content to the total content of Ni and Cu, the overall corrosion resistance in a high temperature, high pressure, strong acidic corrosion environment is significantly improved.

[0075] Figure 1 This is a graph showing the relationship between Fn1 and the general corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosive environment in a duplex stainless steel material satisfying Characteristics 1 and 3. Figure 1 The steel sheet was produced based on the results of a general corrosion resistance evaluation test in a high-temperature, high-pressure, strongly acidic corrosive environment in the examples described later.

[0076] Reference Figure 1 When Fn1 is below 0.70, the corrosion rate in a high temperature, high pressure, strong acidic corrosion environment becomes significantly faster, and excellent overall corrosion resistance cannot be obtained. On the other hand, when Fn1 is higher than 0.70, the corrosion rate in a high temperature, high pressure, strong acidic corrosion environment becomes significantly slower, and overall corrosion resistance is significantly improved. Therefore, if Fn1 satisfies formula (1), excellent overall corrosion resistance is obtained in a high temperature, high pressure, strong acidic corrosion environment, provided that the duplex stainless steel satisfies characteristics 1 and 3. It should be noted that when Fn1 is too high, the hot workability of the duplex stainless steel is reduced. Therefore, the upper limit of Fn1 is set to be less than 16.00.

[0077] [About Feature 3]

[0078] For Feature 3, Fn2 is defined as follows.

[0079] Fn2=(Ca+Mg) / O

[0080] Fn2 is an indicator of pitting corrosion resistance in a high-temperature, high-pressure chloride corrosion environment. Ca and Mg combine with S to form sulfides, thereby inhibiting the formation of coarse Mn sulfides. When coarse Mn sulfides exist on the surface of the steel material, the coarse Mn sulfides on the surface dissolve in a high-temperature, high-pressure chloride corrosion environment, and pitting corrosion is likely to occur. Therefore, Ca and Mg improve pitting corrosion resistance in a high-temperature, high-pressure chloride corrosion environment.

[0081] However, when the S content in the steel is within the range described in Feature 1 (0.010% or less), if the total content of Ca and Mg is too high relative to the O content, Ca and Mg are combined not only with S but also with O to form excessively coarse Ca oxysulfides and Mg oxides. Coarse Ca oxysulfides and coarse Mg oxides, like coarse Mn sulfides, are easily dissolved in a high-temperature and high-pressure chloride corrosion environment and are likely to become the starting point of pitting corrosion.

[0082] Figure 2 This is a graph showing the relationship between Fn2 in a duplex stainless steel material satisfying Characteristics 1 and 2 and pitting corrosion resistance in a high temperature and high pressure chloride corrosion environment. Figure 2 The steel sheet was produced based on the results of a pitting corrosion resistance evaluation test in a high-temperature and high-pressure chloride corrosion environment in the examples described later.

[0083] Reference Figure 2 If Fn2 is 1.50 or more, even if the duplex stainless steel material satisfies characteristics 1 and 2, the corrosion rate is high and the pitting corrosion resistance in a high-temperature and high-pressure chloride corrosion environment is low. On the other hand, if Fn2 is less than 1.50, the corrosion rate is significantly slowed down in a high-temperature and high-pressure chloride corrosion environment, and excellent pitting corrosion resistance is obtained, provided that the duplex stainless steel material satisfies characteristics 1 and 2.

[0084] The duplex stainless steel material of the present embodiment is completed based on the above technical concept and has the following configuration.

[0085] The chemical composition of the duplex stainless steel material of the first constitution is as follows, in mass %, C: 0.050% or less, Si: 0.2-1.2%, Mn: 0.5-7.0%, P: 0.040% or less, S: 0.010% or less, Cr: 20.0-27.0%, Ni: 4.0-9.0%, Mo: 0.5-5.0%, As: 0.0005-0.0100%, one or more of Ca and Mg: 0.0005-0.0100% in total, sol. Al: 0.001-0.050%, N: 0.40% or less, O: 0.100% or less, C u: 0~4.0%, V: 0~1.50%, Co: 0~2.00%, Ta: 0~2.00%, W: 0~4.00%, Nb: 0~2.00%, Ti: 0~2.00%, Zn: 0~0.0100%, Pb: 0~0.0100%, Sb: 0~0.0100%, Sn: 0~0.0100%, Bi: 0~0.0100%, B: 0~0.0100%, rare earth elements: 0~0.050%, Zr: 0~2.00%, Hf: 0~2.00%, and the remainder: Fe and impurities, and satisfies equations (1) and (2).

[0086] 0.70<10000×As / (Ni+Cu)<16.00 (1)

[0087] (Ca+Mg) / O<1.50 (2)

[0088] Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula. When the element is not contained, "0" is substituted for the corresponding element symbol.

[0089] The duplex stainless steel material of the second constitution is the duplex stainless steel material of the first constitution, wherein particles having an equivalent circle diameter of 1.0 to 2.0 μm, a total amount of Ca content and S content of more than 5.0% by mass%, an O content of 1.0% or more, and a Ca content higher than the S content are defined as fine Ca oxysulfides, and particles having an equivalent circle diameter of 1.0 to 2.0 μm, a Mg content of more than 5.0% by mass%, an O content of more than 1.0%, and a S content of 15.0% or less are defined as fine Mg oxysulfides. When particles with an equivalent circle diameter of 1.0 to 2.0 μm, an Al content of 20.0% or more in mass %, and a N content of 20.0% or more are defined as fine Al nitrides, and particles with an equivalent circle diameter of 1.0 to 2.0 μm, a Ti content of 30.0% or more in mass %, and a N content of 20.0% or more are defined as fine Ti nitrides, the total number density of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides is 2.00 pieces / mm 2 above.

[0090] The duplex stainless steel material of the third configuration is the duplex stainless steel material of the first or second configuration, wherein the chemical composition contains Cu: 0.1-4.0%, V: 0.01-1.50%, Co: 0.01-2.00%, Ta: 0.01-2.00%, W: 0.01-4.00%, Nb: 0.01-2.00%, Ti: 0.01-2.00%, Zn: 0.0001-0.010% 0%, Pb: 0.0001-0.0100%, Sb: 0.0001-0.0100%, Sn: 0.0001-0.0100%, Bi: 0.0001-0.0100%, B: 0.0001-0.0100%, rare earth elements: 0.001-0.050%, Zr: 0.01-2.00%, and Hf: 0.01-2.00%.

[0091] The duplex stainless steel material of the present embodiment will be described below. It should be noted that "%" of an element means mass % unless otherwise specified. In the following description, the duplex stainless steel material is also simply referred to as "steel material".

[0092] [Characteristics of the duplex stainless steel material of this embodiment]

[0093] The duplex stainless steel material of the present embodiment satisfies the following features 1 to 3.

[0094] (Feature 1)

[0095] The chemical composition is as follows by mass %: C: 0.050% or less, Si: 0.2-1.2%, Mn: 0.5-7.0%, P: 0.040% or less, S: 0.010% or less, Cr: 20.0-27.0%, Ni: 4.0-9.0%, Mo: 0.5-5.0%, As: 0.0005-0.0100%, one or more of Ca and Mg: 0.0005-0.0100% in total, sol.Al: 0.001-0.050%, N: 0.40% or less, O: 0.100% or less, Cu :0~4.0%、V:0~1.50%、Co:0~2.00%、Ta:0~2.00%、W:0~4.00%、Nb:0~2.00%、Ti:0~2.00%、Zn:0~0.0100%、Pb:0~0.0100%、Sb:0~0.0100%、Sn:0~0.0100%、Bi:0~0.0100%、B:0~0.0100%、Rare earth elements:0~0.050%、Zr:0~2.00%、Hf:0~2.00%、The balance is Fe and impurities.

[0096] (Feature 2)

[0097] The chemical composition satisfies the formula (1).

[0098] 0.70<10000×As / (Ni+Cu)<16.00 (1)

[0099] Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula. When the element is not contained, "0" is substituted for the corresponding element symbol.

[0100] (Feature 3)

[0101] The chemical composition satisfies formula (2).

[0102] (Ca+Mg) / O<1.50 (2)

[0103] Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula. When the element is not contained, "0" is substituted for the corresponding element symbol.

[0104] Hereinafter, features 1 to 3 will be described.

[0105] [(Feature 1) Chemical composition]

[0106] The chemical composition of the duplex stainless steel material according to the present embodiment contains the following elements.

[0107] C: 0.050% or less

[0108] It inevitably contains carbon (C). That is, the C content exceeds 0%.

[0109] C forms Cr carbides at grain boundaries, increasing the corrosion sensitivity of grain boundaries. Therefore, if the C content exceeds 0.050%, even if the contents of other elements are within the range of this embodiment, excellent general corrosion resistance cannot be obtained in a high temperature, high pressure, strong acidic corrosive environment.

[0110] Therefore, the C content is set to 0.050% or less.

[0111] The C content is preferably as low as possible. However, if the C content is excessively reduced, the manufacturing cost will increase significantly. Therefore, in consideration of industrial production, the preferred lower limit of the C content is 0.001%, more preferably 0.002%, and further preferably 0.003%.

[0112] The upper limit of the C content is preferably 0.048%, more preferably 0.046%, further preferably 0.044%, further preferably 0.042%.

[0113] Si: 0.2~1.2%

[0114] Silicon (Si) deoxidizes steel in the process of manufacturing steel materials. If the Si content is less than 0.2%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the ranges of the present embodiment.

[0115] On the other hand, when the Si content exceeds 1.2%, the toughness and hot workability of the steel material decrease even if the contents of other elements are within the ranges of the present embodiment.

[0116] Therefore, the Si content is 0.2 to 1.2%.

[0117] The lower limit of the Si content is preferably 0.3%, more preferably 0.4%, and further preferably 0.5%.

[0118] The upper limit of the Si content is preferably 1.1%, more preferably 1.0%, and further preferably 0.9%.

[0119] Mn: 0.5~7.0%

[0120] Manganese (Mn) improves the hardenability of steel and increases the strength of steel. If the Mn content is less than 0.5%, the above effects cannot be fully obtained even if the contents of other elements are within the ranges of the present embodiment.

[0121] On the other hand, if the Mn content exceeds 7.0%, Mn forms a large amount of coarse Mn sulfides. In a high-temperature, high-pressure chloride corrosion environment, the coarse Mn sulfides present near the surface of the steel material dissolve. Depressions are formed in the portion where the coarse Mn sulfides dissolve. The depressions become the starting point of pitting corrosion. Therefore, even if the contents of other elements are within the range of the present embodiment, excellent pitting corrosion resistance cannot be obtained in a high-temperature, high-pressure chloride corrosion environment.

[0122] Therefore, the Mn content is 0.5 to 7.0%.

[0123] The lower limit of the Mn content is preferably 0.6%, more preferably 0.7%, and further preferably 0.8%.

[0124] The upper limit of the Mn content is preferably 6.8%, more preferably 6.0%, more preferably 5.5%, more preferably 4.5%, more preferably 3.5%, more preferably 2.5%, and more preferably 2.0%.

[0125] P: 0.040% or less

[0126] Phosphorus (P) is an impurity inevitably contained. That is, the P content exceeds 0%.

[0127] When the P content exceeds 0.040%, P is excessively segregated at the grain boundaries. Therefore, even if the contents of other elements are within the ranges of the present embodiment, the toughness of the steel material decreases.

[0128] Therefore, the P content is set to 0.040% or less.

[0129] The P content is preferably as low as possible. However, if the P content is excessively reduced, the manufacturing cost will increase significantly. Therefore, in consideration of industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, more preferably 0.003%, and more preferably 0.005%.

[0130] The upper limit of the P content is preferably 0.035%, more preferably 0.030%, further preferably 0.026%, further preferably 0.022%.

[0131] S: 0.010% or less

[0132] Sulfur (S) is an impurity inevitably contained. That is, the S content exceeds 0%.

[0133] If the S content exceeds 0.010%, S will excessively segregate at the grain boundaries. Therefore, even if the contents of other elements are within the ranges of the present embodiment, the toughness and hot workability of the steel material will decrease.

[0134] Therefore, the S content is set to 0.010% or less.

[0135] The S content is preferably as low as possible. However, if the S content is excessively reduced, the manufacturing cost will increase significantly. Therefore, in consideration of industrial production, the preferred lower limit of the S content is 0.001%, and more preferably 0.002%.

[0136] The upper limit of the S content is preferably 0.009%, more preferably 0.008%, and further preferably 0.007%.

[0137] Cr: 20.0~27.0%

[0138] Chromium (Cr) forms a passive film as an oxide on the surface of the steel material. As a result, the overall corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosive environment is improved. Furthermore, the pitting corrosion resistance in a high-temperature, high-pressure, chloride corrosive environment is improved. If the Cr content is less than 20.0%, even if the contents of other elements are within the range of this embodiment, the above effects cannot be fully obtained.

[0139] On the other hand, when the Cr content exceeds 27.0%, intermetallic compounds represented by σ phase (sigma phase) are easily generated. Therefore, even if the contents of other elements are within the range of the present embodiment, the toughness of the steel material decreases.

[0140] Therefore, the Cr content is 20.0 to 27.0%.

[0141] The lower limit of the Cr content is preferably 20.2%, more preferably 20.5%, more preferably 21.0%, and further preferably 21.5%.

[0142] The upper limit of the Cr content is preferably 26.8%, more preferably 26.6%, further preferably 26.4%, further preferably 26.2%.

[0143] Ni: 4.0-9.0%

[0144] Nickel (Ni) improves the general corrosion resistance of steel in a high temperature, high pressure, strong acidic corrosive environment. If the Ni content is less than 4.0%, the above effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0145] On the other hand, when the Ni content exceeds 9.0%, the volume fraction of austenite becomes too high. In this case, even if the contents of other elements are within the range of the present embodiment, the strength of the steel material decreases.

[0146] Therefore, the Ni content is 4.0 to 9.0%.

[0147] The lower limit of the Ni content is preferably 4.2%, more preferably 4.4%, further preferably 4.6%, further preferably 4.8%.

[0148] The upper limit of the Ni content is preferably 8.8%, more preferably 8.6%, more preferably 8.2%, more preferably 7.9%, more preferably 7.8%, more preferably 7.7%, and more preferably 7.6%.

[0149] Mo: 0.5~5.0%

[0150] Molybdenum (Mo) improves the pitting corrosion resistance of steel in a high-temperature and high-pressure chloride corrosion environment. If the Mo content is less than 0.5%, the above effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0151] On the other hand, when the Mo content exceeds 5.0%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of the present embodiment.

[0152] Therefore, the Mo content is 0.5 to 5.0%.

[0153] The lower limit of the Mo content is preferably 0.7%, more preferably 1.0%, more preferably 1.5%, more preferably 2.0%, more preferably 2.4%, more preferably 2.6%, and more preferably 2.8%.

[0154] The upper limit of the Mo content is preferably 4.8%, more preferably 4.6%, further preferably 4.4%, further preferably 4.2%.

[0155] As: 0.0005~0.0100%

[0156] Arsenic (As) improves the general corrosion resistance of steel in a high-temperature, high-pressure, strongly acidic corrosive environment. If the As content is less than 0.0005%, the above effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0157] On the other hand, when the As content exceeds 0.0100%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of the present embodiment.

[0158] Therefore, the As content is 0.0005 to 0.0100%.

[0159] The lower limit of the As content is preferably 0.0010%, more preferably 0.0015%, further preferably 0.0020%, further preferably 0.0025%.

[0160] The upper limit of the As content is preferably 0.0090%, more preferably 0.0080%, further preferably 0.0070%, further preferably 0.0060%.

[0161] One or more of Ca and Mg: 0.0005 to 0.0100% in total

[0162] Calcium (Ca) and magnesium (Mg) form fine Ca oxysulfide or fine Mg oxide. Fine Ca oxysulfide and fine Mg oxide function as segregation sites of As at the interface with the parent phase. Fine Ca oxysulfide and fine Mg oxide are dispersed in the steel, and As segregated on the surface of these fine particles is also dispersed in the steel. As a result, the overall corrosion resistance of the steel in a high temperature, high pressure, and strongly acidic corrosive environment is improved. If the total content of Ca and Mg is less than 0.0005%, the above effect cannot be fully obtained.

[0163] On the other hand, if the total content of Ca and Mg exceeds 0.0100%, coarse Ca oxysulfide or coarse Mg oxide is generated. In a high-temperature and high-pressure chloride corrosion environment, coarse Ca oxysulfide and coarse Mg oxide generated in the surface layer of the steel material are easily dissolved. Therefore, even if the content of other elements is within the range of this embodiment, the pitting corrosion resistance of the steel material in a high-temperature and high-pressure chloride corrosion environment is reduced.

[0164] Therefore, the total content of Ca and Mg is 0.0005 to 0.0100%.

[0165] The lower limit of the total content of Ca and Mg is preferably 0.0010%, more preferably 0.0015%, more preferably 0.0020%, more preferably 0.0025%, and still more preferably 0.0030%.

[0166] The upper limit of the total content of Ca and Mg is preferably 0.0095%, more preferably 0.0090%, more preferably 0.0085%, more preferably 0.0080%, and still more preferably 0.0075%.

[0167] Sol.Al: 0.001~0.050%

[0168] Aluminum (Al) deoxidizes steel during the manufacturing process of steel. Al further combines with N to form fine Al nitrides. Fine Al nitrides function as segregation sites for As. Therefore, if a large amount of fine Al nitrides are generated and dispersed in the steel, As is more easily dispersed in the steel. As a result, the overall corrosion resistance of the steel in a high-temperature, high-pressure, and strongly acidic corrosive environment is improved. If the sol.Al content is less than 0.001%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0169] On the other hand, when the sol.Al content exceeds 0.050%, coarse oxides are excessively generated. Therefore, even if the contents of other elements are within the range of the present embodiment, the toughness of the steel material is reduced.

[0170] Therefore, the sol.Al content is 0.001 to 0.050%.

[0171] The lower limit of the sol.Al content is preferably 0.002%, more preferably 0.005%, and further preferably 0.010%.

[0172] The upper limit of the sol.Al content is preferably 0.045%, more preferably 0.040%, more preferably 0.035%, and more preferably 0.030%. It should be noted that the sol.Al content in this specification refers to the content of acid-soluble Al.

[0173] N: 0.40% or less

[0174] Nitrogen (N) is inevitably contained. That is, the N content exceeds 0%.

[0175] N stabilizes austenite in the steel. N also improves the overall corrosion resistance in high temperature, high pressure, strong acidic corrosion environments and the pitting corrosion resistance of steel in high temperature, high pressure chloride corrosion environments. N further combines with Al and Ti to generate fine Al nitrides and fine Ti nitrides. Fine Al nitrides and fine Ti nitrides function as segregation sites for As. Therefore, if fine Al nitrides and fine Ti nitrides are generated in large quantities and dispersed in the steel, As is more easily dispersed in the steel. As a result, the overall corrosion resistance of the steel in high temperature, high pressure, strong acidic corrosion environments is improved. As long as a small amount of N is contained, the above-mentioned effect can be obtained to some extent.

[0176] However, when the N content exceeds 0.40%, the toughness and hot workability of the steel material decrease even if the contents of other elements are within the ranges of the present embodiment.

[0177] Therefore, the N content is set to 0.40% or less.

[0178] The lower limit of the N content is preferably 0.01%, more preferably 0.02%, more preferably 0.05%, more preferably 0.10%, and still more preferably 0.15%.

[0179] The upper limit of the N content is preferably 0.38%, more preferably 0.36%, more preferably 0.34%, more preferably 0.32%, and still more preferably 0.30%.

[0180] O: 0.100% or less

[0181] Oxygen (O) is an impurity inevitably contained. That is, the O content exceeds 0%.

[0182] If the O content exceeds 0.100%, coarse Ca oxysulfides and coarse Mg oxides are excessively generated. Therefore, even if the contents of other elements are within the range of the present embodiment, excellent pitting corrosion resistance cannot be obtained in a high-temperature and high-pressure chloride corrosion environment.

[0183] Therefore, the O content is set to 0.100% or less.

[0184] The O content is preferably as low as possible. However, if the O content is excessively reduced, the manufacturing cost becomes high. Therefore, in consideration of industrial production, the preferred lower limit of the O content is 0.001%, more preferably 0.005%, and further preferably 0.010%.

[0185] The upper limit of the O content is preferably 0.090%, more preferably 0.085%, further preferably 0.080%, further preferably 0.075%.

[0186] The balance of the chemical composition of the duplex stainless steel material of the present embodiment is Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed from ore, scrap or the manufacturing environment as raw materials when the duplex stainless steel material is industrially manufactured, are not intentionally contained, and are allowed within a range that does not adversely affect the effect of the duplex stainless steel material of the present embodiment.

[0187] [About Optional Elements]

[0188] The chemical composition of the duplex stainless steel material of the present embodiment may further contain one or more selected from the group consisting of Cu: 0 to 4.0%, V: 0 to 1.50%, Co: 0 to 2.00%, Ta: 0 to 2.00%, W: 0 to 4.00%, Nb: 0 to 2.00%, Ti: 0 to 2.00%, Zn: 0 to 0.0100%, Pb: 0 to 0.0100%, Sb: 0 to 0.0100%, Sn: 0 to 0.0100%, Bi: 0 to 0.0100%, B: 0 to 0.0100%, rare earth elements: 0 to 0.050%, Zr: 0 to 2.00%, and Hf: 0 to 2.00%, in place of a portion of Fe.

[0189] Each of the optional elements will be described below.

[0190] [Group 1: Cu, V, Co, Ta, W, Nb, Ti, Zn, Pb, Sb, Sn and Bi]

[0191] The chemical composition of the duplex stainless steel material of the present embodiment may contain one or more selected from the group consisting of Cu, V, Co, Ta, W, Nb, Ti, Zn, Pb, Sb, Sn and Bi to replace a portion of Fe. These elements are arbitrary elements that improve the overall corrosion resistance of the steel material in a high temperature, high pressure and strong acidic corrosive environment. Each element is described below.

[0192] Cu: 0-4.0%

[0193] Copper (Cu) is an arbitrary element and may not be contained. That is, the Cu content may be 0%.

[0194] When Cu is contained, that is, when the Cu content exceeds 0%, Cu generates sulfides on the passive film in a high temperature, high pressure, and strong acidic corrosive environment. The active dissolution of the steel is suppressed by the sulfides. Therefore, the overall corrosion resistance in a high temperature, high pressure, and strong acidic corrosive environment is improved. As long as a small amount of Cu is contained, the above effect can be obtained to some extent.

[0195] However, when the Cu content exceeds 4.0%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of the present embodiment.

[0196] Therefore, the Cu content is 0 to 4.0%, and when contained, it is 4.0% or less.

[0197] The lower limit of the Cu content is preferably 0.1%, more preferably 0.2%, further preferably 0.5%, further preferably 1.0%.

[0198] The upper limit of the Cu content is preferably 3.8%, more preferably 3.5%, more preferably 2.5%, and further preferably 2.0%.

[0199] V: 0~1.50%

[0200] Vanadium (V) is an arbitrary element and may not be contained. That is, the V content may be 0%.

[0201] When V is contained, that is, when the V content exceeds 0%, V inhibits active dissolution of steel in a high-temperature, high-pressure, strongly acidic corrosive environment and improves general corrosion resistance. The above effect can be obtained to some extent as long as V is contained in a small amount.

[0202] However, if the V content exceeds 1.50%, the strength of the steel material becomes too high. In this case, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel material decreases.

[0203] Therefore, the V content is 0 to 1.50%, and when contained, it is 1.50% or less.

[0204] The lower limit of the V content is preferably 0.01%, more preferably 0.05%, further preferably 0.10%, further preferably 0.20%.

[0205] The upper limit of the V content is preferably 1.40%, more preferably 1.30%, further preferably 1.20%, further preferably 1.00%.

[0206] Co: 0~2.00%

[0207] Cobalt (Co) is an arbitrary element and may not be contained. That is, the Co content may be 0%.

[0208] When Co is contained, that is, when the Co content exceeds 0%, Co improves the general corrosion resistance of the steel in a high temperature, high pressure, strong acidic corrosive environment. The above effect can be obtained to some extent as long as Co is contained in a small amount.

[0209] However, when the Co content exceeds 2.00%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of the present embodiment.

[0210] Therefore, the Co content is 0 to 2.00%, and when contained, it is 2.00% or less.

[0211] The lower limit of the Co content is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.20%, and still more preferably 0.30%.

[0212] The upper limit of the Co content is preferably 1.90%, more preferably 1.80%, more preferably 1.70%, more preferably 1.60%, more preferably 1.50%, and more preferably 1.00%.

[0213] Ta: 0~2.00%

[0214] Tantalum (Ta) is an arbitrary element and may not be contained. That is, the Ta content may be 0%.

[0215] When Ta is contained, that is, when the Ta content exceeds 0%, Ta improves the general corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosive environment. The above effect can be obtained to some extent as long as Ta is contained in a small amount.

[0216] However, if the Ta content exceeds 2.00%, the strength of the steel material becomes too high. In this case, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel material decreases.

[0217] Therefore, the Ta content is 0 to 2.00%, and when contained, it is 2.00% or less.

[0218] The lower limit of the Ta content is preferably 0.01%, more preferably 0.05%, and further preferably 0.08%.

[0219] The upper limit of the Ta content is preferably 1.50%, more preferably 1.00%, further preferably 0.70%, further preferably 0.50%.

[0220] W: 0~4.00%

[0221] Tungsten (W) is an arbitrary element and may not be contained. That is, the W content may be 0%.

[0222] When W is contained, that is, when the W content exceeds 0%, W inhibits active dissolution of steel in a high-temperature, high-pressure, strongly acidic corrosive environment and improves general corrosion resistance. The above effect can be obtained to some extent as long as W is contained in a small amount.

[0223] However, if the W content exceeds 4.00%, the strength of the steel material becomes too high. In this case, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel material decreases.

[0224] Therefore, the W content is 0 to 4.00%, and when contained, it is 4.00% or less.

[0225] The lower limit of the W content is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.20%, more preferably 0.30%, and more preferably 0.50%.

[0226] The upper limit of the W content is preferably 3.90%, more preferably 3.80%, more preferably 3.70%, more preferably 3.50%, more preferably 3.00%, more preferably 2.50%, more preferably 2.00%, and more preferably 1.80%.

[0227] Nb: 0~2.00%

[0228] Niobium (Nb) is an arbitrary element and may not be contained. That is, the Nb content may be 0%.

[0229] When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms carbides or nitrides and inhibits the formation of Cr carbides. Therefore, the formation of Cr-deficient regions at the grain boundaries is inhibited. As a result, the overall corrosion resistance in high-temperature, high-pressure, and strongly acidic corrosive environments is improved. As long as a small amount of Nb is contained, the above effect can be obtained to some extent.

[0230] However, when the Nb content exceeds 2.00%, the strength of the steel material becomes too high. In this case, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel material decreases.

[0231] Therefore, the Nb content is 0 to 2.00%, and when contained, it is 2.00% or less.

[0232] The lower limit of the Nb content is preferably 0.01%, more preferably 0.05%, more preferably 0.10%, more preferably 0.20%, more preferably 0.30%, and still more preferably 0.40%.

[0233] The upper limit of the Nb content is preferably 1.50%, more preferably 1.00%, further preferably 0.70%, further preferably 0.50%.

[0234] Ti: 0~2.00%

[0235] Titanium (Ti) is an arbitrary element and may not be contained. That is, the Ti content may be 0%.

[0236] When Ti is contained, that is, when the Ti content exceeds 0%, Ti forms carbides or nitrides and suppresses the formation of Cr carbides. Therefore, the formation of Cr-deficient regions at grain boundaries is suppressed. As a result, the overall corrosion resistance in high-temperature, high-pressure, and strongly acidic corrosion environments is improved. When Ti forms a large amount of fine Ti nitrides, the fine Ti nitrides dispersed in the steel further function as segregation sites for As. Therefore, As is more easily dispersed in the steel. As a result, the overall corrosion resistance in high-temperature, high-pressure, and strongly acidic corrosion environments is further improved. As long as a small amount of Ti is contained, the above-mentioned effect can be obtained to some extent.

[0237] However, when the Ti content exceeds 2.00%, the strength of the steel material becomes too high. In this case, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel material decreases.

[0238] Therefore, the Ti content is 0 to 2.00%, and when contained, it is 2.00% or less.

[0239] The lower limit of the Ti content is preferably 0.01%, more preferably 0.05%.

[0240] The upper limit of the Ti content is preferably 1.50%, more preferably 1.00%, further preferably 0.70%, further preferably 0.50%.

[0241] Zn: 0~0.0100%

[0242] Zinc (Zn) is an arbitrary element and may not be contained. That is, the Zn content may be 0%.

[0243] When Zn is contained, that is, when the Zn content exceeds 0%, Zn forms stable sulfides, which improves the overall corrosion resistance in high temperature, high pressure, strong acidic corrosive environments. The above effect can be obtained to some extent as long as Zn is contained in a small amount.

[0244] However, when the Zn content exceeds 0.0100%, the mechanical properties of the steel material decrease even if the contents of other elements are within the ranges of the present embodiment.

[0245] Therefore, the Zn content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0246] The lower limit of the Zn content is preferably 0.0001%, more preferably 0.0005%, and further preferably 0.0010%.

[0247] The upper limit of the Zn content is preferably 0.0050%, more preferably 0.0030%, and further preferably 0.0025%.

[0248] Pb: 0~0.0100%

[0249] Lead (Pb) is an arbitrary element and may not be contained. That is, the Pb content may be 0%.

[0250] When Pb is contained, that is, when the Pb content exceeds 0%, Pb forms a stable sulfide, which improves the overall corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosive environment. The above effect can be obtained to some extent as long as a small amount of Pb is contained.

[0251] However, when the Pb content exceeds 0.0100%, the mechanical properties of the steel material decrease even if the contents of other elements are within the ranges of the present embodiment.

[0252] Therefore, the Pb content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0253] The lower limit of the Pb content is preferably 0.0001%, more preferably 0.0003%, more preferably 0.0005%, more preferably 0.0008%, and still more preferably 0.0010%.

[0254] The upper limit of the Pb content is preferably 0.0070%, more preferably 0.0050%, further preferably 0.0030%, further preferably 0.0020%.

[0255] Sb: 0~0.0100%

[0256] Antimony (Sb) is an arbitrary element and may not be contained. That is, the Sb content may be 0%.

[0257] When Sb is contained, that is, when the Sb content exceeds 0%, Sb improves the overall corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosive environment. Even if Sb is contained in a small amount, the above effect can be obtained to some extent.

[0258] However, when the Sb content exceeds 0.0100%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of the present embodiment.

[0259] Therefore, the Sb content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0260] The lower limit of the Sb content is preferably 0.0001%, more preferably 0.0002%, and further preferably 0.0005%.

[0261] The upper limit of the Sb content is preferably 0.0070%, more preferably 0.0050%, more preferably 0.0030%, more preferably 0.0020%, and still more preferably 0.0015%.

[0262] Sn: 0~0.0100%

[0263] Tin (Sn) is an arbitrary element and may not be contained. That is, the Sn content may be 0%.

[0264] When Sn is contained, that is, when the Sn content exceeds 0%, Sn improves the overall corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosive environment. The above effect can be obtained to some extent as long as Sn is contained in a small amount.

[0265] However, when the Sn content exceeds 0.0100%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of the present embodiment.

[0266] Therefore, the Sn content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0267] The lower limit of the Sn content is preferably 0.0001%, more preferably 0.0002%, and further preferably 0.0005%.

[0268] The upper limit of the Sn content is preferably 0.0070%, more preferably 0.0050%, more preferably 0.0030%, more preferably 0.0020%, and still more preferably 0.0015%.

[0269] Bi: 0~0.0100%

[0270] Bismuth (Bi) is an arbitrary element and may not be contained. That is, the Bi content may be 0%.

[0271] When Bi is contained, that is, when the Bi content exceeds 0%, Bi improves the overall corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosive environment. The above effect can be obtained to some extent as long as Bi is contained in a small amount.

[0272] However, when the Bi content exceeds 0.0100%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of the present embodiment.

[0273] Therefore, the Bi content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0274] The lower limit of the Bi content is preferably 0.0001%, more preferably 0.0002%, and further preferably 0.0005%.

[0275] The upper limit of the Bi content is preferably 0.0070%, more preferably 0.0050%, more preferably 0.0030%, more preferably 0.0020%, and still more preferably 0.0015%.

[0276] [Second Group: B, rare earth elements, Zr and Hf]

[0277] The chemical composition of the duplex stainless steel material of the present embodiment may contain one or more selected from the group consisting of B, rare earth elements (REM), Zr and Hf to replace a portion of Fe. These elements are arbitrary elements that improve the hot workability of the steel material. Each element is described below.

[0278] B: 0~0.0100%

[0279] Boron (B) is an arbitrary element and may not be contained. That is, the B content may be 0%.

[0280] When B is contained, that is, when the B content exceeds 0%, B suppresses the segregation of P and S in the steel material to the grain boundaries and improves the hot workability of the steel material. The above effect can be obtained to some extent as long as B is contained in a small amount.

[0281] However, when the B content exceeds 0.0100%, B nitrides are excessively generated. Therefore, even if the contents of other elements are within the ranges of the present embodiment, the toughness of the steel material decreases.

[0282] Therefore, the B content is 0 to 0.0100%, and when contained, it is 0.0100% or less.

[0283] The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%, further preferably 0.0010%, further preferably 0.0020%.

[0284] The upper limit of the B content is preferably 0.0090%, more preferably 0.0080%, further preferably 0.0070%, further preferably 0.0050%.

[0285] Rare earth elements: 0~0.050%

[0286] The rare earth element (REM) is an arbitrary element and may not be contained. That is, the REM content may be 0%.

[0287] When rare earth elements are contained, that is, when the REM content exceeds 0%, REM controls the morphology of inclusions and improves the hot workability of steel. The above effect can be obtained to some extent as long as REM is contained in a small amount.

[0288] However, when the REM content exceeds 0.050%, the oxides in the steel material coarsen. Therefore, even if the contents of other elements are within the range of the present embodiment, the toughness of the steel material decreases.

[0289] Therefore, the REM content is 0 to 0.050%, and when contained, it is 0.050% or less.

[0290] The lower limit of the REM content is preferably 0.001%, more preferably 0.003%, more preferably 0.005%, more preferably 0.008%, and still more preferably 0.010%.

[0291] The upper limit of the REM content is preferably 0.045%, more preferably 0.040%, further preferably 0.035%, further preferably 0.030%.

[0292] REM in this specification 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 lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are lanthanoid elements. The REM content in this specification refers to the total content of these elements.

[0293] Zr: 0~2.00%

[0294] Zirconium (Zr) is an arbitrary element and may not be contained. That is, the Zr content may be 0%.

[0295] When Zr is contained, that is, when the Zr content exceeds 0%, Zr forms carbonitrides and improves the strength and hot workability of the steel material. The above effects can be obtained to some extent even if Zr is contained in a small amount.

[0296] However, if the Zr content exceeds 2.00%, the strength of the steel material becomes too high. Therefore, even if the contents of other elements are within the range of the present embodiment, the toughness of the steel material decreases.

[0297] Therefore, the Zr content is 0 to 2.00%, and when contained, it is 2.00% or less.

[0298] The lower limit of the Zr content is preferably 0.01%, more preferably 0.02%, and further preferably 0.05%.

[0299] The upper limit of the Zr content is preferably 1.50%, more preferably 1.00%, further preferably 0.50%, further preferably 0.30%.

[0300] Hf: 0~2.00%

[0301] Hafnium (Hf) is an arbitrary element and may not be contained. That is, the Hf content may be 0%.

[0302] When Hf is contained, that is, when the Hf content exceeds 0%, Hf forms carbonitrides to improve the strength and hot workability of the steel material. The above effects can be obtained to some extent even if Hf is contained in a small amount.

[0303] However, if the Hf content exceeds 2.00%, the strength of the steel material becomes too high. Therefore, even if the contents of other elements are within the range of this embodiment, the toughness of the steel material decreases.

[0304] Therefore, the Hf content is 0 to 2.00%, and when contained, it is 2.00% or less.

[0305] The lower limit of the Hf content is preferably 0.01%, more preferably 0.10%, further preferably 0.15%, further preferably 0.20%.

[0306] The upper limit of the Hf content is preferably 1.50%, more preferably 1.00%, further preferably 0.80%, further preferably 0.75%.

[0307] [(Feature 2) Regarding formula (1)]

[0308] The chemical composition of the duplex stainless steel material of the present embodiment also satisfies the formula (1).

[0309] 0.70<10000×As / (Ni+Cu)<16.00 (1)

[0310] Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula. When the element is not contained, "0" is substituted for the corresponding element symbol.

[0311] Fn1 (=10000×As / (Ni+Cu)) is an indicator of overall corrosion resistance in a high temperature, high pressure, strong acidic corrosion environment. By adjusting the ratio of As content to the total content of Ni and Cu, the overall corrosion resistance in a high temperature, high pressure, strong acidic corrosion environment is significantly improved. Specifically, Figure 1 As shown in the figure, if Fn1 is higher than 0.70, the corrosion rate in a high temperature, high pressure, strong acidic corrosive environment will be significantly slowed down, provided that the duplex stainless steel material satisfies characteristics 1 and 3. Therefore, excellent overall corrosion resistance can be obtained in a high temperature, high pressure, strong acidic corrosive environment.

[0312] On the other hand, when Fn1 is too high, although the overall corrosion resistance in a high temperature, high pressure, strong acidic corrosive environment is improved, the hot workability of the steel material is reduced. If Fn1 is less than 16.00, the steel material can obtain sufficient hot workability. Therefore, Fn1 is set to be higher than 0.70 and less than 16.00.

[0313] The preferred lower limit of Fn1 is 0.71, more preferably 1.00, more preferably 2.00, more preferably 3.00, and more preferably 4.00. Figure 1 When Fn1 is 5.50 or more, the corrosion rate in a high temperature, high pressure, strong acidic corrosive environment is significantly reduced compared to when Fn1 is higher than 0.70 and lower than 5.50. Therefore, the further preferred lower limit of Fn1 is 5.50, and further preferably 6.00.

[0314] The upper limit of Fn1 is preferably 15.50, more preferably 15.00, and further preferably 14.50.

[0315] It should be noted that, in the present embodiment, Fn1 is a value at the second decimal place obtained by rounding off the third decimal place of the obtained value.

[0316] [(Feature 3) Regarding formula (2)]

[0317] The chemical composition of the duplex stainless steel material of the present embodiment also satisfies the formula (2).

[0318] (Ca+Mg) / O<1.50 (2)

[0319] Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula. When the element is not contained, "0" is substituted for the corresponding element symbol.

[0320] Fn2 (=(Ca+Mg) / O) is an indicator of pitting corrosion resistance in a high-temperature, high-pressure chloride corrosion environment. As described above, Ca and Mg combine with S to form sulfides. As a result, the formation of coarse Mn sulfides is suppressed. As a result, pitting corrosion resistance in a high-temperature, high-pressure chloride corrosion environment is improved.

[0321] However, when the S content in the steel is within the above range (less than 0.010%), if the total content of Ca and Mg is too high relative to the O content, Ca and Mg are combined not only with S but also with O to form coarse Ca oxysulfides and Mg oxides. Coarse Ca oxysulfides and coarse Mg oxides, like coarse Mn sulfides, are easily dissolved in a high-temperature and high-pressure chloride corrosion environment and are likely to become the starting point of pitting corrosion. Therefore, if the total content of Ca and Mg is too high relative to the O content, the pitting corrosion resistance in a high-temperature and high-pressure chloride corrosion environment is reduced.

[0322] like Figure 2 As shown in Figure 1, if Fn2 is less than 1.50, the corrosion rate in a high-temperature, high-pressure chloride corrosion environment is significantly slowed down, provided that the duplex stainless steel material satisfies characteristics 1 and 2. As a result, excellent pitting resistance can be obtained in a high-temperature, high-pressure chloride corrosion environment.

[0323] The upper limit of Fn2 is preferably 1.45, more preferably 1.43, more preferably 1.40, more preferably 1.35, and still more preferably 1.30.

[0324] The lower limit of Fn2 is not particularly limited, but the preferred lower limit of Fn2 is 0.01, and more preferably 0.02.

[0325] It should be noted that, in the present embodiment, Fn2 is a value of the second decimal place obtained by rounding off the third decimal place of the obtained value.

[0326] [Effects of the Duplex Stainless Steel Material of the Present Embodiment]

[0327] The duplex stainless steel material of the present embodiment satisfies Features 1 to 3. Therefore, the duplex stainless steel material of the present embodiment obtains excellent general corrosion resistance in a high temperature and high pressure strong acid corrosion environment, and further obtains excellent pitting corrosion resistance in a high temperature and high pressure chloride corrosion environment.

[0328] Here, excellent general corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosion environment and excellent pitting corrosion resistance in a high-temperature, high-pressure chloride corrosion environment are defined as follows by the general corrosion resistance evaluation test in a high-temperature, high-pressure, strongly acidic corrosion environment and the pitting corrosion resistance evaluation test in a high-temperature, high-pressure chloride corrosion environment shown below.

[0329] [Overall corrosion resistance evaluation test in high temperature, high pressure and strong acid corrosion environment]

[0330] The overall corrosion resistance evaluation test in a high temperature, high pressure, strong acidic corrosive environment was carried out by the following method.

[0331] Collect test pieces from duplex stainless steel. When the duplex stainless steel is a steel pipe, collect the test piece from the center of the wall thickness. In this case, the length direction of the test piece is parallel to the axial direction of the steel pipe. When the duplex stainless steel is a round steel, collect the test piece from the R / 2 position. Here, the R / 2 position refers to the center position of the radius R in the cross section perpendicular to the axial direction of the round steel. In this case, the length direction of the test piece is parallel to the axial direction of the round steel. When the duplex stainless steel is a steel plate, collect the test piece from the center of the plate thickness. In this case, the length direction of the test piece is parallel to the rolling direction of the steel plate. The dimensions of the test piece are, for example, 40 mm in length, 10 mm in width, and 3 mm in thickness. Measure the mass of the test piece before the start of the test.

[0332] Prepare a 0.01 mol / L sulfuric acid (H2SO4) aqueous solution as the test solution. Place the test solution in an autoclave. Immerse the test piece in the test solution, pressurize and seal a mixed gas of 0.05 bar H2S gas and 5.00 bar CO2 gas in the autoclave, and start the corrosion test. The test time is set to 336 hours. Keep the temperature in the autoclave at 180°C during the test.

[0333] After the test time has passed, the corrosion products are removed from the test piece. The removal of the corrosion products from the test piece is performed, for example, based on the method specified in ASTM G31-21. The mass of the test piece from which the corrosion products have been removed is measured. Corrosion rate (g·cm -2 ·h -1 ) is calculated by dividing the difference between the mass of the test piece before the test and the mass of the test piece after the test time has passed and the corrosion products have been removed by the surface area of ​​the test piece and the test time. The corrosion rate is 0.100 g·cm -2 ·h -1 When the value is below 0.05, it is judged that the overall corrosion resistance is excellent in a high temperature, high pressure, strong acidic corrosive environment.

[0334] [Evaluation test of pitting corrosion resistance in high temperature and high pressure chloride corrosion environment]

[0335] The pitting corrosion resistance evaluation test in a high-temperature and high-pressure chloride corrosion environment was carried out by the following method.

[0336] Collect test pieces from duplex stainless steel. When the duplex stainless steel is a steel pipe, collect the test piece from the center of the wall thickness. In this case, the length direction of the test piece is parallel to the pipe axis direction of the steel pipe. When the duplex stainless steel is a round steel, collect the test piece from the R / 2 position. In this case, the length direction of the test piece is parallel to the axial direction of the round steel. When the duplex stainless steel is a steel plate, collect the test piece from the center of the plate thickness. In this case, the length direction of the test piece is parallel to the rolling direction of the steel plate. The dimensions of the test piece are, for example, 40 mm in length, 10 mm in width, and 3 mm in thickness. Measure the mass of the test piece before the start of the test.

[0337] Prepare a 25 mass % sodium chloride (NaCl) aqueous solution as the test solution. Place the test solution in an autoclave. Immerse the test piece in the test solution, pressurize and seal a mixed gas of 0.05 bar H2S gas and 5.00 bar CO2 gas in the autoclave, and start the corrosion test. The test time is set to 336 hours. Keep the temperature in the autoclave at 180°C during the test.

[0338] After the test time has passed, the corrosion products are removed from the test piece. The removal of the corrosion products from the test piece is performed, for example, based on the method specified in ASTM G31-21. The mass of the test piece from which the corrosion products have been removed is measured. Corrosion rate (g·cm -2 ·h -1 ) is calculated by dividing the difference between the mass of the test piece before the start of the test and the mass of the test piece after the test time has passed and the corrosion products have been removed by the surface area of ​​the test piece and the test time.

[0339] Furthermore, the surface of the test piece after the test was observed with a magnifying glass at a magnification of 10 times to confirm the presence of pitting. In the case where there was a suspected pitting site under the magnifying glass observation, the cross section of the suspected pitting site was observed with an optical microscope at a magnification of 100 times to confirm the presence of pitting.

[0340] The corrosion rate is 0.005 g·cm -2 ·h -1 When pitting corrosion is not confirmed on the entire surface of the test piece, the test piece is judged to be excellent in pitting corrosion resistance in a high-temperature and high-pressure chloride corrosion environment.

[0341] As described above, the duplex stainless steel material of the present embodiment satisfies Characteristics 1 to 3. Therefore, excellent general corrosion resistance in a high temperature and high pressure strong acid corrosion environment and excellent pitting corrosion resistance in a high temperature and high pressure chloride corrosion environment can be obtained.

[0342] [Microstructure]

[0343] It should be noted that the microstructure of the duplex stainless steel material of the present embodiment is composed of ferrite and austenite. Here, "composed of ferrite and austenite" means that the microstructure contains, for example, 30 to 80% ferrite by volume, and the remainder is austenite. It should be noted that the amount of tissue other than ferrite and austenite is negligible. For example, in the duplex stainless steel material of the present embodiment, the volume fraction of precipitates and inclusions is negligible compared to the volume fraction of ferrite and austenite. That is, the microstructure of the duplex stainless steel material of the present embodiment may include precipitates and / or inclusions in addition to ferrite and austenite.

[0344] [Microstructure observation method]

[0345] The volume fraction of ferrite in duplex stainless steel can be obtained by the method according to JIS G 0555 (2020). Specifically, a test piece for microstructure observation is made from a duplex stainless steel. When the steel is a steel pipe, a test piece having an observation surface of, for example, 5 mm in the tube axis direction and 5 mm in the tube diameter direction is made from the center of the wall thickness. When the steel is a round steel, a test piece having an observation surface of, for example, 5 mm in the axial direction and 5 mm in the radial direction is made from the R / 2 position. When the steel is a steel plate, a test piece having an observation surface of, for example, 5 mm in the rolling direction and 5 mm in the plate thickness direction is made from the center of the plate thickness. It should be noted that as long as the above-mentioned observation surface can be obtained, the size of the test piece is not particularly limited.

[0346] The observation surface of the prepared test piece was mirror-polished. The mirror-polished observation surface was electrolytically etched in a 7% potassium hydroxide etching solution to visualize the structure. The observation surface with the structure visualized was observed in 10 fields of view using an optical microscope. The field of view area is not particularly limited, for example, 1.00 mm 2 (Magnification 100 times). In each field of view, ferrite and austenite are determined by contrast. The area ratio of ferrite determined by the point algorithm according to JIS G 0555 (2020) is measured. The arithmetic mean of the area ratio of ferrite obtained in 10 fields of view is defined as the volume ratio (%) of ferrite. It should be noted that the volume ratio (%) of ferrite is the value obtained by rounding off the first decimal place of the obtained value. The value obtained by subtracting the volume ratio of ferrite obtained from 100% is defined as the volume ratio (%) of austenite.

[0347] [Shapes and uses of duplex stainless steel]

[0348] The shape of the duplex stainless steel material of the present embodiment is not particularly limited. The duplex stainless steel material of the present embodiment may be a steel pipe, a round steel (solid material), or a steel plate. In addition, the steel pipe may be a seamless steel pipe or a welded steel pipe.

[0349] The duplex stainless steel material of this embodiment can be widely used in high temperature and high pressure strong acid corrosion environment applications or high temperature and high pressure chloride corrosion environment applications. The duplex stainless steel material of this embodiment can be used in geothermal well applications or oil well applications, for example.

[0350] [Preferred Embodiments of the Duplex Stainless Steel Material of the Present Embodiment]

[0351] The duplex stainless steel material of the present embodiment preferably satisfies the above-mentioned characteristics 1 to 3, and further satisfies the following characteristic 4.

[0352] (Feature 4)

[0353] Fine Ca oxysulfides are defined as particles with an equivalent circle diameter of 1.0 to 2.0 μm, a total amount of Ca and S content greater than 5.0% by mass, an O content of 1.0% or more, and a Ca content greater than S content. Fine Mg oxides are defined as particles with an equivalent circle diameter of 1.0 to 2.0 μm, a Mg content of 5.0% or more by mass, an O content of 1.0% or more, and a S content of 15.0% or less by mass. Fine Mg oxides are defined as particles with an equivalent circle diameter of 1.0 to 2 When particles with an equivalent circle diameter of 1.0 to 2.0 μm, an Al content of 20.0% or more, and a N content of 20.0% or more are defined as fine Al nitrides, and particles with an equivalent circle diameter of 1.0 to 2.0 μm, a Ti content of 30.0% or more, and a N content of 20.0% or more are defined as fine Ti nitrides, the total number density ND of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides is 2.00 pieces / mm 2 above.

[0354] When the duplex stainless steel material of the present embodiment satisfies characteristics 1 to 3 and further satisfies characteristic 4, more excellent general corrosion resistance can be obtained in a high-temperature, high-pressure, strongly acidic corrosive environment. Characteristic 4 will be described below.

[0355] [(Feature 4) Regarding total number density ND]

[0356] As described above, As improves the overall corrosion resistance in a high-temperature, high-pressure, and strongly acidic corrosion environment. If As is dispersed in the steel, the overall corrosion resistance in a high-temperature, high-pressure, and strongly acidic corrosion environment is further improved. Here, among the particles of inclusions and precipitates in the steel, fine particles with an equivalent circle diameter of 1.0 to 2.0 μm are prone to segregation of As at the interface with the parent phase. That is, the surface of such fine particles functions as a segregation site for As. If the segregation sites are dispersed in the steel, As is also easily dispersed in the steel. Therefore, even when the As content is low, As can be dispersed in the steel. As a result, the overall corrosion resistance in a high-temperature, high-pressure, and strongly acidic corrosion environment is significantly improved.

[0357] In duplex stainless steel materials satisfying characteristics 1 to 3, the number ratio of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides in all fine particles in the duplex stainless steel materials is high. Therefore, if the total number density ND of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides can be increased, the segregation sites of As can be fully dispersed in the steel material. As a result, the general corrosion resistance in a high-temperature, high-pressure, and strongly acidic corrosive environment can be further improved.

[0358] Total number density ND (pieces / mm 2) is the total number density of the main fine particles (fine Ca oxysulfides, fine Mg oxides, fine Al nitrides and fine Ti nitrides) that become the segregation sites of As. If the total number density ND is 2.00 pieces / mm 2 As above, a sufficient amount of As segregation sites are dispersed in the steel material. Therefore, As is easily dispersed in the steel material. As a result, the overall corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosive environment is further improved.

[0359] In particular, when the duplex stainless steel material satisfies not only characteristics 1 to 3 but also characteristics 4, even when Fn1 is higher than 0.70 and lower than 5.50, the corrosion rate obtained in the above-mentioned [General Corrosion Resistance Evaluation Test in High Temperature, High Pressure and Strong Acid Corrosion Environment] is 0.080 g·cm -2 ·h -1 Below, more excellent general corrosion resistance can be obtained.

[0360] The preferred lower limit of the total number density ND is 2.01 pieces / mm 2 , more preferably 2.05 pieces / mm 2 , more preferably 2.07 pieces / mm 2 , more preferably 2.10 pieces / mm 2 .

[0361] It should be noted that the greater the total number density ND, the more As segregation sites increase, and thus the overall corrosion resistance is more easily improved. Therefore, the upper limit of the total number density ND is not particularly limited. If it is a duplex stainless steel material that satisfies feature 1, the upper limit of the total number density ND is, for example, 30.00 pieces / mm 2 , preferably 28.50 pcs / mm 2 , more preferably 25.00 pieces / mm 2 , more preferably 20.00 pieces / mm 2 , more preferably 17.00 pieces / mm 2 , more preferably 15.00 pieces / mm 2 , more preferably 10.00 pieces / mm 2 , more preferably 5.00 pieces / mm 2 , more preferably 3.00 pieces / mm 2 .

[0362] [Method for measuring total number density ND]

[0363] Total number density ND (number / mm) of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides and fine Ti nitrides 2 ) can be obtained by the following method.

[0364] Specifically, a test piece is made from a duplex stainless steel material. When the steel material is a steel pipe, a test piece having an observation surface including the pipe axis direction and the pipe diameter direction (wall thickness direction) is made from the center of the wall thickness. When the steel material is a steel plate, a test piece having an observation surface including the rolling direction and the plate thickness direction is made from the center of the plate thickness. When the steel material is a round steel, a test piece having an observation surface including the axial direction and the radial direction is made from the R / 2 position in a cross section perpendicular to the axial direction of the round steel.

[0365] Use diamond paste to mirror-polish the observation surface of the manufactured test piece. Use a scanning electron microscope (SEM) to observe the observation field of the center position of the thickness of the mirror-polished observation surface at 500 times. When the steel material is a steel pipe, the center position of the thickness of the observation surface refers to the center position of the wall thickness direction of the steel pipe in the observation surface. When the steel material is a steel plate, the center position of the thickness of the observation surface refers to the center position of the plate thickness direction of the steel plate in the observation surface. When the steel material is round steel, the center position of the thickness of the observation surface refers to the center position of the radial direction of the round steel in the observation surface. As long as the total area of ​​the observation field is 1125mm 2 , the number of observation fields is not particularly limited. 2 When a plurality of rectangular observation fields are selected in such a manner, the plurality of observation fields are selected in such a manner that the plurality of observation fields are arranged in a row on the observation plane and one side of adjacent observation fields are in contact with each other.

[0366] For example, when the size of each observation field is a rectangle of 15 mm×15 mm, the number of observation fields is set to 5 (15 mm×15 mm×5=1125 mm). 2 ). In addition, 5 observation fields were selected in such a manner that the 5 observation fields were arranged in a row on the observation surface and one side (15 mm) of adjacent observation fields was in contact with each other.

[0367] The particles in the observation field are determined according to the contrast. The equivalent circle diameter (μm) of each determined particle is obtained. Here, the equivalent circle diameter refers to the diameter (μm) of a circle having the same area as the area of ​​the particle. Furthermore, element concentration analysis (EDS analysis) is performed on each determined particle. The element concentration analysis can be performed using a device (SEM-EDS device) having an element concentration analysis function in a scanning electron microscope. In the element concentration analysis, the acceleration voltage is set to 20 kV, and the target elements are quantified as N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr and Nb. Based on the EDS analysis results of each particle, when the total content of N, O, Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Cu, Zr and Nb is set to 100% in mass%, the fine Ca oxysulfide, fine Mg oxide, fine Al nitride and fine Ti nitride are determined as follows.

[0368] Particles having an equivalent circle diameter of 1.0 to 2.0 μm, a total Ca content and S content of more than 5.0% by mass, an O content of 1.0% or more, and a Ca content higher than the S content are identified as "fine Ca oxysulfides".

[0369] Particles having an equivalent circle diameter of 1.0 to 2.0 μm, a Mg content of 5.0% or more, an O content of 1.0% or more, and a S content of 15.0% or less in terms of mass % are identified as “fine Mg oxides”.

[0370] Particles having an equivalent circle diameter of 1.0 to 2.0 μm, an Al content of 20.0% or more, and a N content of 20.0% or more in terms of mass % are determined as “fine Al nitrides”.

[0371] Particles having an equivalent circle diameter of 1.0 to 2.0 μm, a Ti content of 30.0% or more, and a N content of 20.0% or more in terms of mass % are determined as “fine Ti nitrides”.

[0372] In the observation field, the number of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides identified by the above method was counted.

[0373] Based on the total number of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides in all observation fields and the total area of ​​the observation fields, the total number density ND (number / mm2) of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides, and fine Ti nitrides was calculated. 2 ). The total number density ND is the value of the second decimal place obtained by rounding off the third decimal place of the obtained value.

[0374] [Manufacturing method]

[0375] An example of a method for producing a duplex stainless steel material according to the present embodiment will be described. An example of a method for producing a duplex stainless steel material according to the present embodiment includes a billet production step, a hot working step, and a solution treatment step. Each step will be described in detail.

[0376] [Blank material manufacturing process]

[0377] In the billet manufacturing process, a billet satisfying characteristics 1 to 3 is prepared. Specifically, molten steel satisfying characteristics 1 to 3 is manufactured. The method for manufacturing the molten steel is not particularly limited. The molten steel may be manufactured using a converter, an electric furnace, or other methods.

[0378] The produced molten steel is used to produce a billet. The billet is, for example, a cast billet or a steel ingot. Specifically, the cast billet is produced by a continuous casting method using the molten steel. The cast billet may be a slab, a bloom, or a billet. Alternatively, the molten steel may be used to produce a steel ingot by an ingot casting method. The cast billet or the steel ingot may also be further subjected to hot forging or blanking to produce a billet. The billet is produced by the above steps.

[0379] [Hot working process]

[0380] In the hot working process, the manufactured billet is subjected to known hot working to manufacture an intermediate steel material. When the final product is a steel pipe, the intermediate steel material is a pipe billet. When the final product is a round steel, the intermediate steel material is a rod-shaped steel material. When the final product is a steel plate, the intermediate steel material is a plate-shaped steel material. Hot working may be hot forging, hot extrusion, or hot rolling. The method of hot working is not particularly limited and may be a known method.

[0381] An example of a hot working process when the final product is a seamless steel pipe is described below. First, a small square billet as a blank is heated in a heating furnace. The heating temperature is not particularly limited, and is, for example, 1000 to 1300°C. The small square billet extracted from the heating furnace is subjected to hot working to produce a tube billet (seamless steel pipe) as an intermediate steel material. The method of hot working is not particularly limited, and can be a well-known method. For example, Mannesmann-type piercing and rolling can be implemented as hot working to produce a tube billet. In this case, a round billet is pierced and rolled using a piercing machine. During piercing and rolling, the piercing ratio is not particularly limited, and is, for example, 1.0 to 4.0. The round billet that has been pierced and rolled is further hot rolled using a mandrel seamless tube rolling mill, a reducing mill, a sizing mill, etc. to form a tube billet. The cumulative cross-sectional reduction rate in the hot working process is, for example, 20 to 70%. Other hot working methods can also be implemented to produce a tube billet from a small square billet. For example, when the steel material is a short thick-walled steel pipe such as a pipe joint, the blank pipe may be produced by forging such as the Ernst & Young's method. The blank pipe is produced through the above steps.

[0382] An example of a hot working process when the final product is round steel is described below. First, the billet is heated in a heating furnace. The heating temperature is not particularly limited, and is, for example, 1000-1300°C. The billet extracted from the heating furnace is subjected to hot working to produce an intermediate steel material having a circular cross-section perpendicular to the axial direction. Hot working is, for example, billeting using a billeting machine or hot rolling using a continuous rolling mill. In the continuous rolling mill, a horizontal rolling mill having a pair of hole-shaped rollers arranged in the up-and-down direction and a vertical rolling mill having a pair of hole-shaped rollers arranged in the horizontal direction are alternately arranged.

[0383] An example of a hot working process when the final product is a steel plate is described below. First, a billet is heated in a heating furnace. The heating temperature is not particularly limited, and is, for example, 1000 to 1300°C. The billet extracted from the heating furnace is hot rolled using a reverse rolling mill and a tandem rolling mill to produce a plate-shaped intermediate steel material. It should be noted that hot forging can also be performed, and then the hot forged billet is reheated to 1000 to 1300°C, and the reheated billet is further hot rolled to produce a plate-shaped intermediate steel material.

[0384] [Solution treatment process]

[0385] In the solution treatment process, the intermediate steel material manufactured in the hot working process is subjected to a known solution treatment. For example, the intermediate steel material can be charged into a heat treatment furnace, maintained at a desired temperature, and then quenched. It should be noted that, when the intermediate steel material is charged into a heat treatment furnace, maintained at a desired temperature, and then quenched to perform a solution treatment, the solution temperature refers to the temperature (°C) of the heat treatment furnace used to perform the solution treatment. The solution time refers to the time that the intermediate steel material is maintained at the solution temperature. The solution temperature is, for example, 900 to 1100°C. The solution time is, for example, 5 to 180 minutes. The quenching method in the solution treatment is, for example, water cooling.

[0386] The duplex stainless steel material of the present embodiment is manufactured by the above-described manufacturing method.

[0387] [Preferred manufacturing conditions]

[0388] The method for producing the duplex stainless steel material according to the present embodiment preferably satisfies the following conditions 1 and 2.

[0389] (Condition 1)

[0390] In the billet production process, the average cooling rate CR1 of the billet surface temperature from 1350° C. to 1100° C. during casting of the molten steel is set to 8 to 25° C. / min.

[0391] (Condition 2)

[0392] In the solution treatment process, after maintaining the solution temperature for the solution time, the average cooling rate CR2 of the surface temperature of the intermediate blank from the solution temperature to 850°C is set to less than 200°C / minute, and the average cooling rate CR3 of the surface temperature of the intermediate blank from 850°C to 300°C is set to more than 1000°C / minute.

[0393] If conditions 1 and 2 are satisfied, the produced duplex stainless steel material satisfies characteristics 1 to 3, and further satisfies characteristic 4. Conditions 1 and 2 will be described below.

[0394] [(Regarding condition 1)]

[0395] In the duplex stainless steel material satisfying characteristics 1 to 3, Ca oxysulfides and Mg oxides are generated in the temperature range of 1350°C to 1100°C when the molten steel is cast. If the average cooling rate CR1 is too slow, the Ca oxysulfides and Mg oxides are coarsened. At this time, the number density of fine Ca oxysulfides and fine Mg oxides decreases. As a result, the total number density ND decreases. On the other hand, if the average cooling rate CR1 is too fast, the amount of fine Ca oxysulfides and fine Mg oxides generated is insufficient.

[0396] If the average cooling rate CR1 is 8 to 25°C / min, the total number density ND is 2.00 pieces / mm, provided that condition 2 is satisfied. 2 above.

[0397] It should be noted that the method for controlling the average cooling rate CR1 is not particularly limited and may be a known method. For example, when the billet is manufactured by continuous casting, the amount of cooling water (specific water amount) for cooling the billet may be adjusted to control the cooling rate. For example, when the billet is manufactured by an ingot casting method, the cooling rate may be controlled by the material of the mold and the water cooling of the mold.

[0398] The surface temperature of the blank can be measured using a non-contact infrared radiation thermometer. The average cooling rate CR1 (°C / min) can be determined by measuring the time it takes for the surface temperature of the blank to decrease from 1350°C to 1100°C.

[0399] [(Regarding condition 2)]

[0400] In the solution treatment process, the temperature zone T2 from the time the intermediate steel is extracted from the heat treatment furnace until the surface temperature of the intermediate steel reaches 850°C is the temperature zone where fine Al nitrides and fine Ti nitrides are generated. If the average cooling rate CR2 in the temperature zone T2 exceeds 200°C / min, the amount of fine Al nitrides and fine Ti nitrides generated in the temperature zone T2 is insufficient. If the average cooling rate CR2 is below 200°C / min, a sufficient amount of fine Al nitrides and fine Ti nitrides are generated. As a result, the total number density ND is 2.00 pieces / mm 2 above.

[0401] It should be noted that the average cooling rate CR3 of the intermediate steel material from the surface temperature of 850° C. to 300° C. is 1000° C. / min or more. If the intermediate steel material is water-cooled, the average cooling rate CR3 is 1000° C. / min or more.

[0402] The surface temperature of the intermediate steel can be measured using a non-contact infrared radiation thermometer. By measuring the time from the solution treatment temperature to 850°C for the surface temperature of the intermediate steel, the average cooling rate CR2 (°C / minute) can be calculated. Similarly, by measuring the time from 850°C to 300°C for the surface temperature of the intermediate steel, the average cooling rate CR3 (°C / minute) can be calculated. It should be noted that, as described above, if water cooling is applied to the intermediate steel, the average cooling rate CR3 is above 1000°C / minute.

[0403] [About other processes]

[0404] The method for manufacturing the duplex stainless steel material of the present embodiment may implement other steps in addition to the above steps. For example, a cold working step may be implemented on the intermediate steel material after the solution treatment step. That is, the cold working step is an arbitrary step.

[0405] In the cold working process, the intermediate steel material is subjected to known cold working. The cold working may be, for example, cold drawing or cold rolling. By cold working the intermediate steel material after the solution treatment, the strength of the duplex stainless steel material can be improved.

[0406] It should be noted that the above-mentioned production method is an example. Therefore, the production method of the duplex stainless steel material of the present embodiment is not limited to the above-mentioned example.

[0407] Example

[0408] The effects of the duplex stainless steel material of the present embodiment will be further specifically described by way of examples. The conditions in the following examples are an example of conditions used to confirm the feasibility and effects of the duplex stainless steel material of the present embodiment. Therefore, the duplex stainless steel material of the present embodiment is not limited to this example of conditions.

[0409] Duplex stainless steel materials having the chemical compositions shown in Table 1-1 and Table 1-2 were manufactured.

[0410] [Table 1-1]

[0411] Table 1-1

[0412]

[0413] [Table 1-2]

[0414] Table 1-2

[0415]

[0416] The "-" in Table 1-1 and Table 1-2 means that the content of the corresponding element is an impurity level. For example, the V content of Test No. 1 is rounded off to the third decimal place and is 0%. The Ca content of Test No. 2 is rounded off to the fifth decimal place and is 0%.

[0417] A high-frequency vacuum melting furnace was used to melt 30 kg of molten steel of each test number. The molten steel was used to produce a steel ingot by an ingot casting method. During casting, the average cooling rate CR1 (°C / min) of the surface temperature of the steel ingot from 1350°C to 1100°C is shown in the "CR1 (°C / min)" column in Table 2.

[0418] [Table 2]

[0419] Table 2

[0420]

[0421] The steel ingots of each test number were heated at 1200°C for 3 hours. The heated steel ingots were hot forged to produce intermediate steels with a cross section perpendicular to the length direction of 70 mm × 100 mm. The intermediate steels were heated at 1250°C for 1 hour. The heated intermediate steels were hot rolled to produce intermediate steels in the form of steel plates with a thickness of 17 mm.

[0422] The intermediate steel after hot rolling is subjected to solution treatment. The solution temperature is set to 950°C, and the holding time at the solution temperature is set to 15 minutes. The intermediate steel is cooled after the holding time. Specifically, the average cooling rate CR2 of the surface temperature of the intermediate billet from the solution temperature (950°C) to 850°C is as shown in the "CR2 (°C / min)" column in Table 2. In addition, for any test number, water cooling is implemented for subsequent cooling. Therefore, the average cooling rate CR3 of the surface temperature of the intermediate billet from 850°C to 300°C is above 1000°C / min.

[0423] Through the above-described manufacturing process, a duplex stainless steel material (steel plate) of each test number was manufactured.

[0424] It should be noted that the microstructure of the duplex stainless steel material of each test number was observed by the method described in the above-mentioned "Microstructure Observation Method". It should be noted that in the microstructure observation, a test piece having an observation surface of 5 mm in the rolling direction and 5 mm in the plate thickness direction was prepared from the center position of the plate thickness of the steel plate. As a result, in any test number, the microstructure of the duplex stainless steel material is composed of ferrite and austenite, and the volume fraction of ferrite is 30 to 80%.

[0425] [Evaluation test]

[0426] The following evaluation tests were performed on the duplex stainless steel materials of each test number.

[0427] (Test 1) Test for determination of total number density ND

[0428] (Test 2) Overall corrosion resistance evaluation test in high temperature, high pressure and strong acid corrosion environment

[0429] (Test 3) Pitting corrosion resistance evaluation test in high temperature and high pressure chloride corrosion environment

[0430] Hereinafter, Tests 1 to 3 will be described.

[0431] [(Test 1) Test for determination of total number density ND]

[0432] The total number density ND (number / mm) of fine Ca oxysulfides, fine Mg oxides, fine Al nitrides and fine Ti nitrides in the duplex stainless steel material of each test number was determined by the method described in the above [Method for measuring the total number density ND]. 2 ). It should be noted that the size of the observation field on the observation surface of the test piece is a rectangle of 15 mm × 15 mm, and the number of observation fields is 5. The observation was performed using SEM at 500 times. The total number density ND obtained is shown in "ND (pieces / mm 2 )"column.

[0433] [(Test 2) Overall corrosion resistance evaluation test in high temperature, high pressure and strong acid corrosion environment]

[0434] The corrosion rate (g·cm) of the duplex stainless steel material of each test number in a high temperature, high pressure and strong acidic corrosive environment was calculated by the method described in the above-mentioned [General Corrosion Resistance Evaluation Test in High Temperature, High Pressure and Strong Acidic Corrosive Environment]. -2 ·h -1). The removal of corrosion products from the test piece was carried out based on the method specified in ASTM G31-21. The dimensions of the test piece were set to 40 mm in length, 10 mm in width, and 3 mm in thickness. The obtained corrosion rate is shown in the "Corrosion rate (g·cm -2 ·h -1 )"column.

[0435] [(Test 3) Pitting corrosion resistance evaluation test in high temperature and high pressure chloride corrosion environment]

[0436] The corrosion rate (g·cm) of the duplex stainless steel material of each test number in a high temperature and high pressure chloride corrosion environment was calculated by the method described in the above-mentioned [Evaluation test of pitting resistance in a high temperature and high pressure chloride corrosion environment]. -2 ·h -1 ) and confirm the presence or absence of pitting. The removal of corrosion products from the test piece was carried out based on the method specified in ASTM G31-21. The dimensions of the test piece were set to 40 mm in length, 10 mm in width, and 3 mm in thickness. The obtained corrosion rate is shown in the "Corrosion rate (g·cm -2 ·h -1 The presence or absence of pitting corrosion is shown in the "Pitting corrosion" column.

[0437] [Evaluation results]

[0438] The evaluation results are shown in Table 2. In Table 2, the column "Fn1" shows Fn1 of each test number, and the column "Fn2" shows Fn2 of each test number.

[0439] Referring to Table 1-1, Table 1-2 and Table 2, the duplex stainless steel materials of test numbers 1 to 39 satisfy characteristics 1 to 3. Therefore, the corrosion rate in a high temperature, high pressure and strong acidic corrosive environment is 0.100 g·cm -2 ·h -1 Furthermore, the corrosion rate in a high-temperature and high-pressure chloride corrosion environment is 0.005 g·cm -2 ·h -1 Therefore, the duplex stainless steel materials of these test numbers have excellent general corrosion resistance in a high-temperature, high-pressure, strongly acidic corrosive environment, and excellent pitting corrosion resistance in a high-temperature, high-pressure, chloride corrosive environment.

[0440] Furthermore, in Test Nos. 1 to 10, 12 to 22, 24 to 36, 38, and 39 among Test Nos. 1 to 39, the above-mentioned Condition 1 and Condition 2 are satisfied in the manufacturing process. Therefore, the duplex stainless steel materials of these Test Nos. satisfy Characteristics 1 to 3, and further satisfy Characteristics 4. As a result, the duplex stainless steel materials of these Test Nos. are more excellent in corrosion rate in a high-temperature, high-pressure, strongly acidic corrosive environment.

[0441] Specifically, in Test Nos. 1 to 39, when the Fn1 value is the same, the duplex stainless steel material satisfying Characteristics 1 to 4 obtains superior general corrosion resistance compared to the duplex stainless steel material satisfying Characteristics 1 to 3 but not Characteristics 4.

[0442] For example, focusing on test number 11 and test number 14, the Fn1 of test number 11 is 0.89, which is a value close to the Fn1 (= 0.87) of test number 14. However, although the duplex stainless steel material of test number 11 satisfies characteristics 1 to 3, it does not satisfy characteristic 4, while the duplex stainless steel material of test number 14 satisfies characteristics 1 to 4. As a result, the corrosion rate of test number 14 in a high temperature, high pressure, strong acidic corrosive environment is slower than that of test number 11, and more excellent general corrosion resistance is obtained in a high temperature, high pressure, strong acidic corrosive environment.

[0443] Similarly, focusing on test number 18 and test number 23, the Fn1 of test number 18 is 8.78, which is a value close to the Fn1 (=8.33) of test number 23. However, the duplex stainless steel material of test number 18 satisfies characteristics 1 to 4, and the duplex stainless steel material of test number 23 satisfies characteristics 1 to 3, but does not satisfy characteristic 4. As a result, the corrosion rate of test number 18 in a high temperature, high pressure, strong acid corrosion environment is slower than that of test number 23, and more excellent general corrosion resistance is obtained in a high temperature, high pressure, strong acid corrosion environment.

[0444] Focusing on test number 9 and test number 37, Fn1 of test number 9 is 1.41, which is a value close to Fn1 (=1.45) of test number 37. However, the duplex stainless steel material of test number 9 satisfies characteristics 1 to 4, and the duplex stainless steel material of test number 37 satisfies characteristics 1 to 3, but does not satisfy characteristic 4. As a result, the corrosion rate of test number 9 in a high temperature, high pressure, strong acidic corrosive environment is slower than that of test number 37, and more excellent general corrosion resistance is obtained in a high temperature, high pressure, strong acidic corrosive environment.

[0445] In particular, when Fn1 exceeds 0.70 and is less than 5.50, the corrosion rate of duplex stainless steels meeting characteristics 1 to 4 (test numbers 1, 4 to 6, 9, 12 to 17, 22, 24 to 26, 31, 32, 35, 36, 38 and 39) in high temperature, high pressure and strong acidic corrosion environments is 0.080 g cm -2 ·h -1 On the other hand, when Fn1 exceeds 0.70 and is less than 5.50, the corrosion rate of the duplex stainless steel (test numbers 11 and 37) that meets characteristics 1 to 3 but does not meet characteristic 4 in a high temperature, high pressure, strong acidic corrosive environment is 0.100 g·cm -2 ·h -1 Below, but more than 0.080 g·cm -2 ·h -1 .

[0446] It should be noted that in test numbers 2, 3, 7, 8, 10, 18 to 21, 23, 27 to 30, 33 and 34, Fn1 is 5.50 or more. Therefore, in the duplex stainless steel materials of these test numbers, regardless of whether characteristic 4 is satisfied, the corrosion rate in a high temperature, high pressure and strong acidic corrosive environment is 0.040 g·cm -2 ·h -1 Next, better overall corrosion resistance is obtained in high temperature, high pressure and strong acidic corrosive environments.

[0447] On the other hand, in test number 40, the Cr content was too low. Therefore, the corrosion rate in a high temperature, high pressure, strong acidic corrosive environment exceeded 0.100 g·cm -2 ·h -1 , it is impossible to obtain excellent overall corrosion resistance in a high temperature, high pressure, strong acidic corrosive environment. Furthermore, the corrosion rate in a high temperature, high pressure, chloride corrosive environment exceeds 0.005 g·cm -2 ·h -1 , pitting corrosion was also confirmed, and excellent pitting corrosion resistance in a high temperature and high pressure chloride corrosion environment could not be obtained.

[0448] In test numbers 41 and 42, the As content was too low. Therefore, the corrosion rate in the high temperature, high pressure, strong acidic corrosive environment exceeded 0.100 g·cm -2 ·h -1 , it is impossible to obtain excellent overall corrosion resistance in high temperature, high pressure and strong acidic corrosive environment.

[0449] In test No. 43, the total content of Ca and Mg was too low. Therefore, the corrosion rate in the high temperature, high pressure, strong acidic corrosive environment exceeded 0.100 g·cm -2 ·h -1 , it is impossible to obtain excellent overall corrosion resistance in high temperature, high pressure and strong acidic corrosive environment.

[0450] In test numbers 44 to 46, although feature 1 was satisfied, Fn1 was too high. Therefore, cracks occurred in the process of hot forging the steel ingot in the manufacturing process. Therefore, the manufacturing process and tests after hot forging were not carried out for these test numbers.

[0451] In test numbers 47 to 49, although characteristic 1 was satisfied, Fn1 was too low. Therefore, the corrosion rate in a high-temperature, high-pressure, strongly acidic corrosive environment exceeded 0.100 g·cm -2 ·h -1 , it is impossible to obtain excellent overall corrosion resistance in high temperature, high pressure and strong acidic corrosive environment.

[0452] In test numbers 50 to 52, although characteristic 1 is satisfied, Fn2 is too high. The corrosion rate in a high-temperature and high-pressure chloride corrosion environment exceeds 0.005 g·cm -2 ·h -1 , pitting corrosion was also confirmed, and excellent pitting corrosion resistance in a high temperature and high pressure chloride corrosion environment could not be obtained.

[0453] The embodiments of the present disclosure are described above. However, the above embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above embodiments, and the above embodiments can be appropriately modified within the scope of the present disclosure.

Claims

1. A duplex stainless steel material, the chemical composition of which is C: 0.050% or less, Si: 0.2-1.2%, Mn: 0.5-7.0%, P: 0.040% or less, S: 0.010% or less, Cr：20.0～27.0％、 Ni: 4.0-9.0%, Mo: 0.5-5.0%, As: 0.0005~0.0100%, One or more of Ca and Mg: 0.0005 to 0.0100% in total, Sol.Al: 0.001~0.050%, N: 0.40% or less, O: 0.100% or less, Cu: 0-4.0%, V：0～1.50％、 Co: 0-2.00%, Ta: 0~2.00%, W：0～4.00％、 Nb: 0-2.00%, Ti: 0-2.00%, Zn: 0~0.0100%, Pb: 0~0.0100%, Sb: 0~0.0100%, Sn: 0~0.0100%, Bi: 0~0.0100%, B：0～0.0100％、 Rare earth elements: 0~0.050%, Zr：0～2.00％、 Hf: 0 to 2.00%, and Balance: Fe and impurities, And satisfying equations (1) and (2), 0.70<10000×As / (Ni+Cu)<16.00 (1) (Ca+Mg) / O<1.50 (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula, and when the element is not contained, "0" is substituted for the corresponding element symbol.

2. The duplex stainless steel material according to claim 1, wherein: Fine Ca oxysulfides are defined as particles having an equivalent circle diameter of 1.0 to 2.0 μm, a total Ca content and a S content of more than 5.0% by mass, an O content of 1.0% or more, and a Ca content higher than the S content. Fine Mg oxides are defined as particles having an equivalent circle diameter of 1.0 to 2.0 μm, a Mg content of 5.0% or more, an O content of 1.0% or more, and a S content of 15.0% or less in terms of mass %. Fine Al nitrides are defined as particles having an equivalent circle diameter of 1.0 to 2.0 μm, an Al content of 20.0% or more, and a N content of 20.0% or more in terms of mass %. When particles having an equivalent circle diameter of 1.0 to 2.0 μm, a Ti content of 30.0% or more, and a N content of 20.0% or more in terms of mass % are defined as fine Ti nitrides, The total number density of the fine Ca oxysulfide, the fine Mg oxide, the fine Al nitride, and the fine Ti nitride is 2.00 pieces / mm 2 above.

3. The duplex stainless steel material according to claim 1 or claim 2, wherein: The chemical composition contains a Cu: 0.1-4.0%, V：0.01～1.50％、 Co: 0.01~2.00% Ta: 0.01~2.00%, W：0.01～4.00％、 Nb: 0.01-2.00%, Ti: 0.01-2.00%, Zn: 0.0001~0.0100%, Pb: 0.0001~0.0100%, Sb: 0.0001~0.0100%, Sn: 0.0001~0.0100%, Bi: 0.0001~0.0100%, B：0.0001～0.0100％、 One or more selected from the group consisting of rare earth elements: 0.001 to 0.050%, Zr: 0.01 to 2.00%, and Hf: 0.01 to 2.00%.

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