Duplex stainless steel material

Abstract

To provide a duplex stainless steel material having excellent SSC resistance.SOLUTION: The duplex stainless steel material of the present disclosure has the chemical composition described in the description, and has a microstructure comprising, by volume fraction, 35 to 65% of ferrite and the balance being austenite. In the steel, the number densities Fn1 and Fn1 of fine low-S inclusions, which are inclusions having ND1 defined by Formula (1) of 5.0 or more and equivalent circle diameters of 2.0 to 10.0 μ m, are 3.0 pieces / mm2 or more and 5.0 or more, respectively, the number densities ND2 and Fn1 of coarse low-S inclusions, which are inclusions having equivalent circle diameters of more than 10.0 μ m, are 2.0 pieces / mm2 or less and less than 5.0, respectively, and the number densities ND3 of high-S inclusions, which are inclusions having equivalent circle diameters of 2.0 μm or more, are 2.0 pieces / mm2 or less. Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, for each symbol of an element in Formula (1), the content of the element in the corresponding inclusion is substituted in mass%.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Some oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as "oil wells") have environments containing large amounts of corrosive substances. Examples of corrosive substances are corrosive gases such as hydrogen sulfide and carbon dioxide. In this specification, an environment containing hydrogen sulfide and carbon dioxide is referred to as a "sour environment." The temperature of a sour environment can reach approximately 100°C, depending on the depth of the well. When steel materials are used in such sour environments, an electrochemical reaction occurs when the steel surface comes into contact with the corrosive substances, generating hydrogen on the steel surface. This hydrogen makes the steel susceptible to sulfide stress corrosion cracking (SSC). Therefore, steel materials used in sour environments are required to have excellent SSC resistance.

[0003] Chromium (Cr) is known to be effective in improving the carbon dioxide corrosion resistance of steel. Therefore, in oil wells in environments with high carbon dioxide concentrations, martensitic stainless steels containing approximately 13% Cr by mass, such as API L80 13Cr steel (normal 13Cr steel) or Super 13Cr steel with reduced carbon content, are used depending on the carbon dioxide partial pressure and temperature. 13Cr steel and Super 13Cr steel are primarily used in oil wells in mild sour environments where the H2S partial pressure is 0.03 bar or less.

[0004] In a highly corrosive sour environment where the H2S partial pressure is about 0.1 bar, the H2S partial pressure is higher than in a mild sour environment. Therefore, in such a highly corrosive sour environment, duplex stainless steels with a higher Cr content are used than 13Cr steels and Super 13Cr steels. Therefore, duplex stainless steels with excellent SSC resistance are required even in such a highly corrosive sour environment.

[0005] A technology for improving SSC resistance in duplex stainless steel materials is proposed in Japanese Patent Laid-Open Publication No. 2018-193591 (Patent Document 1).

[0006] The duplex stainless steel material disclosed in Patent Document 1 contains, in mass %, C: 0.005 to 0.04%, Si: 0.2 to 1.0%, Mn: 0.1 to 2.0%, P: 0.040% or less, S: 0.010% or less, Ni: 3 to 7%, Cr: 23 to 28%, Mo: 0.5 to 1.5%, Cu: 2 to 4%, N: 0.10 to 0.35%, and Al: 0.001 to 0.04%. %, W: 0-1.0%, Co: 0-1.0%, V: 0-1.0%, Nb: 0-0.2%, Ti: 0-0.2%, Ca: 0-0.02%, Mg: 0-0.02%, B: 0-0.02%, and rare earth elements (REM): 0-0.2%, with the balance being Fe and impurities, and satisfying YS / 150≦Ni+Mo+0.5W+Cu-Mn≦YS / 75 (1), Cr+3.3×(Mo+0.5W)+16N≧30.0 (2), and Mo+0.5W+Ni≦7.50 (3). Here, the element symbols in formulas (1) to (3) are substituted with the content (mass%) of the corresponding element, and the yield strength (MPa) of the steel is substituted for YS in formula (1).

[0007] In the duplex stainless steel material disclosed in Patent Document 1, the contents of specific elements in the chemical composition of the duplex stainless steel material and the yield strength are adjusted to satisfy formulas (1) to (3), thereby achieving excellent SSC resistance in an atmosphere of 0.07 bar H2S and 10 bar CO2. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2018-193591 Summary of the Invention [Problem to be solved by the invention]

[0009] Excellent SSC resistance can be obtained even with the duplex stainless steel material disclosed in Patent Document 1. However, it is desirable to obtain excellent SSC resistance even in a highly corrosive sour environment containing H2S at about 0.1 bar.

[0010] An object of the present disclosure is to provide a duplex stainless steel material having excellent SSC resistance. [Means for solving the problem]

[0011] The duplex stainless steel material of the present disclosure has a chemical composition, in mass%, of C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.80 to 4.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 20.0 to 25.0%, Ni: 1.50 to less than 4.00%, Mo: 0.01 to 3.00%, Cu: 0.01 to 3.00%, Nb: 0.001 to 0.150%, V: 0.01 to 0.50%, Co: 0.01 to 0.50%, Sn: 0.001 to 0.050%, and Al: 0. The duplex stainless steel material has a microstructure consisting of 35-65% by volume ferrite and the remainder being austenite. In the duplex stainless steel material, the number density ND1 of fine low-S inclusions containing S, which are inclusions with an equivalent circle diameter of 2.0 to 10.0 μm, is 3.0 pieces / mm 2 Furthermore, the number density ND2 of coarse low-S inclusions, which contain S, have Fn1 of 5.0 or more, and have a circle equivalent diameter of more than 10.0 μm, is 2.0 pieces / mm 2 Furthermore, the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm 2The following is the result. Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the element symbol in formula (1) is substituted with the content of the corresponding element in the inclusion in mass %. If the element is not contained, "0" is substituted for the corresponding element symbol. [Effects of the Invention]

[0012] The duplex stainless steel material of the present disclosure has excellent SSC resistance. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram illustrating the positional relationship between molten steel in a mold and a submerged entry nozzle during continuous casting. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present inventors have conducted research into duplex stainless steel materials that have excellent SSC resistance even in highly corrosive sour environments where the H2S partial pressure is about 0.1 bar.

[0015] The present inventors first investigated duplex stainless steel materials having excellent SSC resistance in highly corrosive sour environments from the viewpoint of chemical composition. The results were as follows: C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.80 to 4.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 20.0 to 25.0%, Ni: 1.50 to less than 4.00%, Mo: 0.01 to 3.00%, Cu: 0.01 to 3.00%, Nb: 0.001 to 0.150%, V: 0.01 to 0.50%, Co: 0.01 to 0.50%, Sn: 0.001 to 0.050%, Al: 0.010 to 0.100%, Ca: 0.0001 to 0.0100%, and N: 0.001 to 0. The present inventors considered that a duplex stainless steel material having a chemical composition consisting of 0.250%, O: 0.010% or less, Ta: 0-0.100%, Ti: 0-0.100%, Zr: 0-0.100%, W: 0-0.200%, Zn: 0-0.010%, Sb: 0-0.1000%, As: 0-0.050%, Pb: 0-0.010%, B: 0-0.0200%, Mg: 0-0.020%, rare earth elements: 0-0.100%, and the balance being Fe and impurities, could potentially provide excellent SSC resistance even in highly corrosive sour environments.

[0016] The present inventors further investigated means for obtaining excellent SSC resistance even in highly corrosive sour environments from the viewpoint of microstructure.

[0017] The present inventors first investigated the mechanism of SSC generation in duplex stainless steel materials in highly corrosive sour environments, and as a result, the following points were discovered.

[0018] In highly corrosive sour environments, some of the inclusions present on the surface of duplex stainless steel may dissolve, forming pits on the surface of the steel. These pits are thought to promote the occurrence of SSC in highly corrosive sour environments.

[0019] In a highly corrosive sour environment, among the inclusions in the surface layer of duplex stainless steel materials, inclusions with a high S content, such as Mn sulfides or Ca sulfides, tend to dissolve preferentially. In the following explanation, inclusions with a high S content will be referred to as "high S inclusions." A specific definition of high S inclusions will be provided later.

[0020] High-S inclusions in the surface layer of duplex stainless steel dissolve in highly corrosive sour environments, forming pits. The pits formed by high-S inclusions promote the occurrence of SSC in highly corrosive sour environments. Therefore, if the number density of high-S inclusions in duplex stainless steel can be reduced, the occurrence of SSC in highly corrosive sour environments can be suppressed.

[0021] In order to reduce the number density of high-S inclusions, it is effective to reduce the S content in steel as much as possible. Therefore, the inventors attempted to improve SSC resistance by reducing the S content in steel as much as possible. However, not only is there a limit to how much S content can be reduced in steel, but excessive reduction of the S content significantly increases production costs.

[0022] Therefore, the present inventors have considered reducing the number density of high-S inclusions by other means while allowing a certain amount of S content in the steel material, rather than reducing the number density of high-S inclusions by reducing the S content as much as possible.

[0023] Here, the inventors have focused on composite inclusions, a different perspective from conventional ones. Composite inclusions are inclusions formed by the aggregation of multiple types of inclusions. Examples of multiple types of inclusions include oxides such as Al2O3 and MgO, sulfides such as MnS and CaS, and nitrides such as TiN. Note that TiN may also form a solid solution with V. Inclusions other than oxides, sulfides, and nitrides may combine with oxides, sulfides, nitrides, etc. to form composite inclusions.

[0024] The sulfur content in composite inclusions is lower than that of high-sulfur inclusions. Therefore, in highly corrosive sour environments, composite inclusions are less likely to dissolve than high-sulfur inclusions. Even if composite inclusions dissolve, the sulfur-containing portions of the composite inclusions dissolve locally. Therefore, the pits that form are very small. These very small pits easily repassivate and do not become the starting point for SSC. Therefore, if sulfur is incorporated into the composite inclusions, the pits can be kept small even after dissolution, improving SSC resistance. Furthermore, increasing the number density of composite inclusions allows sulfur to be incorporated into the composite inclusions. This reduces the amount of sulfur available for the formation of high-sulfur inclusions. As a result, the number density of high-sulfur inclusions decreases.

[0025] When Fn1, defined by formula (1), is 5.0 or more, the S content in the inclusions is sufficiently low. Such inclusions are considered to be composite inclusions. In the following explanation, inclusions that contain S and have Fn1 of 5.0 or more will be referred to as "low-S inclusions." Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the element symbol in formula (1) is substituted with the content of the corresponding element in the inclusion in mass %. If the element is not contained, "0" is substituted for the corresponding element symbol.

[0026] As described above, it is believed that increasing the number density of low-S inclusions in duplex stainless steel materials can minimize the depressions caused by dissolution, thereby improving SSC resistance. Increasing the number density of low-S inclusions also causes more S to be incorporated into the low-S inclusions. This reduces the amount of S available for the formation of high-S inclusions. As a result, it is believed that the number density of high-S inclusions can be sufficiently reduced, further improving SSC resistance.

[0027] Based on the above considerations, the present inventors attempted to increase the number density of low-S inclusions in duplex stainless steel materials, and as a result, found that the number density of high-S inclusions can be sufficiently reduced by increasing the number density of low-S inclusions in duplex stainless steel materials.

[0028] However, even when the number density of low-S inclusions is increased and the number density of high-S inclusions is reduced, sufficient SSC resistance in highly corrosive sour environments still cannot be obtained in some cases. Therefore, the present inventors conducted further studies and found the following.

[0029] In highly corrosive sour environments, even low-S inclusions, coarse low-S inclusions with an equivalent circle diameter of more than 10.0 μm, are easily dissolved. Therefore, by increasing the number density of fine low-S inclusions with an equivalent circle diameter of 10.0 μm or less and reducing the number density of high-S inclusions, while also reducing the number density of coarse low-S inclusions with an equivalent circle diameter of more than 10.0 μm, the SSC resistance of duplex stainless steel materials in highly corrosive sour environments can be improved.

[0030] Based on the above findings, the present inventors have further investigated and found that in a duplex stainless steel material having the above-mentioned chemical composition, the number density ND1 of fine low-S inclusions, which contain S and have an Fn1 defined by formula (1) of 5.0 or more and have an equivalent circle diameter of 2.0 to 10.0 μm, is 3.0 pieces / mm 2 or more, containing S, Fn1 being 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions with an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces / mm 2 The number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm 2 The present inventors have found that if the temperature is less than or equal to the above range, excellent SSC resistance can be obtained even in a highly corrosive sour environment.

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

[0032] The duplex stainless steel material of the first configuration has a chemical composition, in mass%, of C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.80 to 4.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 20.0 to 25.0%, Ni: 1.50 to less than 4.00%, Mo: 0.01 to 3.00%, Cu: 0.01 to 3.00%, Nb: 0.001 to 0.150%, V: 0.01 to 0.50%, Co: 0.01 to 0.50%, Sn: 0.001 to 0.050%, and Al: The duplex stainless steel material has a microstructure consisting of 35 to 65% by volume ferrite and the remainder being austenite. In the duplex stainless steel material, the number density ND1 of fine low-S inclusions containing S, which are inclusions with an equivalent circle diameter of 2.0 to 10.0 μm, is 3.0 pieces / mm 2 Furthermore, the number density ND2 of coarse low-S inclusions, which contain S, have Fn1 of 5.0 or more, and have a circle equivalent diameter of more than 10.0 μm, is 2.0 pieces / mm 2 Furthermore, the number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm 2 The following is the result. Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the element symbol in formula (1) is substituted with the content of the corresponding element in the inclusion in mass %. If the element is not contained, "0" is substituted for the corresponding element symbol.

[0033] The duplex stainless steel material of the second configuration is the duplex stainless steel material of the first configuration, and its chemical composition contains one or more elements selected from the group consisting of Ta: 0.001-0.100%, Ti: 0.001-0.100%, Zr: 0.001-0.100%, W: 0.001-0.200%, Zn: 0.001-0.010%, Sb: 0.0001-0.1000%, As: 0.001-0.050%, Pb: 0.001-0.010%, B: 0.0001-0.0200%, Mg: 0.001-0.020%, and rare earth elements: 0.001-0.100%.

[0034] The duplex stainless steel material of the third configuration is a duplex stainless steel material of the first or second configuration, and is a steel pipe.

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

[0036] [Features of the duplex stainless steel material of this embodiment] The duplex stainless steel material of this embodiment satisfies the following characteristics 1 to 3. (Feature 1) The chemical composition, in mass%, is C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.80 to 4.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 20.0 to 25.0%, Ni: 1.50 to less than 4.00%, Mo: 0.01 to 3.00%, Cu: 0.01 to 3.00%, Nb: 0.001 to 0.150%, V: 0.01 to 0.50%, Co: 0.01 to 0.50%, Sn: 0.001 to 0.050%, Al: 0.010 to 0. 100%, Ca: 0.0001-0.0100%, N: 0.001-0.250%, O: 0.010% or less, Ta: 0-0.100%, Ti: 0-0.100%, Zr: 0-0.100%, W: 0-0.200%, Zn: 0-0.010%, Sb: 0-0.1000%, As: 0-0.050%, Pb: 0-0.010%, B: 0-0.0200%, Mg: 0-0.020%, rare earth elements: 0-0.100%, and the remainder being Fe and impurities.

[0037] (Feature 2) The microstructure is composed of 35 to 65% by volume of ferrite and the remainder of austenite.

[0038] (Feature 3) In a duplex stainless steel material, S is contained, and Fn1 defined by formula (1) is 5.0 or more, and the number density ND1 of fine low-S inclusions, which are inclusions with an equivalent circle diameter of 2.0 to 10.0 μm, is 3.0 pieces / mm 2 or more, containing S, Fn1 being 5.0 or more, and the number density ND2 of coarse low-S inclusions, which are inclusions with an equivalent circle diameter of more than 10.0 μm, is 2.0 pieces / mm 2 The number density ND3 of high-S inclusions, which contain S, have an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more, is 2.0 pieces / mm 2 The following is the result. Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the element symbol in formula (1) is substituted with the content of the corresponding element in the inclusion in mass %. If the element is not contained, "0" is substituted for the corresponding element symbol. The following describes Features 1 to 3. Note that, hereinafter, the duplex stainless steel material may also be simply referred to as "steel material."

[0039] [(Feature 1) Chemical composition] The chemical composition of the steel material of this embodiment contains the following elements: "%" relating to elements means mass % unless otherwise specified.

[0040] C: 0.030% or less Carbon (C) is unavoidably contained. That is, the C content is more than 0%. C forms Cr carbides at grain boundaries, increasing the corrosion susceptibility at the grain boundaries. Therefore, if the C content exceeds 0.030%, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.030% or less. The C content is preferably as low as possible. However, excessive reduction in the C content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the C content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.005%. The upper limit of the C content is preferably 0.028%, more preferably 0.026%, and even more preferably 0.024%.

[0041] Si: 0.10 to 1.00% Silicon (Si) deoxidizes steel. If the Si content is less than 0.10%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 1.00%, the toughness of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.10 to 1.00%. The lower limit of the Si content is preferably 0.15%, more preferably 0.20%, and even more preferably 0.30%. The upper limit of the Si content is preferably 0.98%, more preferably 0.95%, and even more preferably 0.90%.

[0042] Mn: 0.80 to 4.00% Manganese (Mn) deoxidizes and desulfurizes steel. Mn also improves the hot workability of steel. If the Mn content is 0.80% or more, the above effects can be obtained. On the other hand, if the Mn content is too high, a large number of coarse Mn sulfides are formed. In this case, the Mn sulfides dissolve in a highly corrosive sour environment, forming pits. These pits may become the starting points for SSC, resulting in the occurrence of SSC. If the Mn content exceeds 4.00%, the above-mentioned pits are formed, and SSC resistance is reduced, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.80 to 4.00%. The lower limit of the Mn content is preferably 0.85%, more preferably 0.90%, even more preferably 1.00%, and still more preferably 1.30%. The upper limit of the Mn content is preferably 3.90%, more preferably 3.70%, and even more preferably 3.50%.

[0043] P:0.040% or less Phosphorus (P) is unavoidably contained. That is, the P content exceeds 0%. P segregates at grain boundaries. Therefore, if the P content exceeds 0.040%, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.040% or less. The lower the P content, the better. However, excessive reduction in the P content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the P content is preferably 0.035%, more preferably 0.030%, and even more preferably 0.025%.

[0044] S: 0.0050% or less Sulfur (S) is unavoidably contained. In other words, the S content is greater than 0%. S generates high-S inclusions, typified by sulfides. As described above, high-S inclusions dissolve in a highly corrosive sour environment and form pits. The pits formed by high-S inclusions cause SSC. If the S content exceeds 0.0050%, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.0050% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the S content is preferably 0.0048%, more preferably 0.0045%, and even more preferably 0.0040%.

[0045] Cr: 20.0 to 25.0% Chromium (Cr) forms a passive film as an oxide on the surface of a steel material, thereby improving the SSC resistance of the steel material. If the Cr content is less than 20.0%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 25.0%, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 20.0 to 25.0%. The lower limit of the Cr content is preferably 20.2%, more preferably 20.5%, and even more preferably 21.0%. The upper limit of the Cr content is preferably 24.5%, more preferably 24.0%, and even more preferably 23.5%.

[0046] Ni: 1.50 to less than 4.00% Nickel (Ni) stabilizes the austenite in steel. Ni also increases the SSC resistance of steel. If the Ni content is 1.50% or more, the above effects can be obtained. On the other hand, if the Ni content is less than 4.00%, the volume fraction of austenite can be kept within an appropriate range. Therefore, the Ni content is 1.50 to less than 4.00%. The lower limit of the Ni content is preferably 1.65%, more preferably 2.00%, and even more preferably 2.20%. The upper limit of the Ni content is preferably 3.99%, more preferably 3.80%, and even more preferably 3.70%.

[0047] Mo: 0.01 to 3.00% Molybdenum (Mo) improves the SSC resistance of steel. If the Mo content is 0.01% or more, this effect can be obtained. On the other hand, if the Mo content is 3.00% or less, sufficient hot workability can be obtained in the steel material. Therefore, the Mo content is 0.01 to 3.00%. The lower limit of the Mo content is preferably 0.04%, more preferably 0.30%, and even more preferably 0.50%. The upper limit of the Mo content is preferably 2.90%, more preferably 2.70%, and even more preferably 2.40%.

[0048] Cu: 0.01 to 3.00% Copper (Cu) improves the SSC resistance of steel materials. If the Cu content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cu content exceeds 3.00%, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.01 to 3.00%. The lower limit of the Cu content is preferably 0.05%, more preferably 0.20%, and even more preferably 0.50%. The upper limit of the Cu content is preferably 2.50%, more preferably 2.30%, and even more preferably 2.20%.

[0049] Nb: 0.001 to 0.150% Niobium (Nb) forms precipitates to increase the strength of the steel material. If the Nb content is less than 0.001%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Nb content exceeds 0.150%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high, and in this case, the toughness of the steel material decreases. Therefore, the Nb content is 0.001 to 0.150%. The lower limit of the Nb content is preferably 0.005%, more preferably 0.010%, and even more preferably 0.015%. The upper limit of the Nb content is preferably 0.140%, more preferably 0.130%, and even more preferably 0.120%.

[0050] V: 0.01 to 0.50% Vanadium (V) increases the strength of steel. If the V content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the V content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high, and in this case, the toughness of the steel material decreases. Therefore, the V content is 0.01 to 0.50%. The lower limit of the V content is preferably 0.02%, more preferably 0.03%, even more preferably 0.05%, and still more preferably 0.10%. The upper limit of the V content is preferably 0.48%, more preferably 0.45%, and even more preferably 0.40%.

[0051] Co: 0.01 to 0.50% Cobalt (Co) forms a coating on the surface of a steel material to enhance the SSC resistance of the steel material. If the Co content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Co content exceeds 0.50%, the production cost increases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0.01 to 0.50%. The lower limit of the Co content is preferably 0.05%, more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Co content is preferably 0.47%, more preferably 0.40%, and even more preferably 0.35%.

[0052] Sn: 0.001 to 0.050% Tin (Sn) improves the SSC resistance of steel materials. If the Sn content is less than 0.001%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Sn content exceeds 0.050%, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0.001 to 0.050%. The lower limit of the Sn content is preferably 0.005%, more preferably 0.010%, and even more preferably 0.015%. The upper limit of the Sn content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0053] Al: 0.010 to 0.100% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.010%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.100%, coarse oxides are formed, and in this case, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.010 to 0.100%. The lower limit of the Al content is preferably 0.012%, more preferably 0.017%, and even more preferably 0.025%. The upper limit of the Al content is preferably 0.095%, more preferably 0.090%, and even more preferably 0.085%. In this specification, the Al content refers to the content of "acid-soluble Al", that is, sol. Al.

[0054] Ca: 0.0001 to 0.0100% Calcium (Ca) combines with S in the steel material to form Ca sulfides, which suppress the formation of coarse Mn sulfides. As a result, the SSC resistance of the steel material is improved. If the Ca content is less than 0.0001%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ca content exceeds 0.0100%, coarse Ca sulfides are formed in excess, and in this case, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0.0001 to 0.0100%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.0015%, and even more preferably 0.0020%. The upper limit of the Ca content is preferably 0.0090%, more preferably 0.0085%, and even more preferably 0.0075%.

[0055] N: 0.001 to 0.250% Nitrogen (N) stabilizes austenite in steel. If the N content is less than 0.001%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content is 0.250% or less, the hot workability of the steel material is sufficient. Therefore, the N content is 0.001 to 0.250%. The lower limit of the N content is preferably 0.003%, more preferably 0.050%, and even more preferably 0.080%. The upper limit of the N content is preferably 0.230%, more preferably 0.220%, and even more preferably 0.180%.

[0056] O: 0.010% or less Oxygen (O) is an unavoidable impurity. In other words, the O content is greater than 0%. O forms oxides and reduces the toughness of the steel material. If the O content exceeds 0.010%, the toughness of the steel material will be significantly reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the O content is 0.010% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the O content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the O content is preferably 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0057] The balance of the chemical composition of the duplex stainless steel material of this embodiment is Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the duplex stainless steel material, and are acceptable within a range that does not adversely affect the duplex stainless steel material of this embodiment.

[0058] [Optional elements] The chemical composition of the duplex stainless steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ta: 0-0.100%, Ti: 0-0.100%, Zr: 0-0.100%, W: 0-0.200%, Zn: 0-0.010%, Sb: 0-0.1000%, As: 0-0.050%, Pb: 0-0.010%, B: 0-0.0200%, Mg: 0-0.020%, and rare earth elements: 0-0.100%. All of these elements are optional elements. Each optional element will be described below.

[0059] [Ta, Ti and Zr] The chemical composition of the duplex stainless steel material of this embodiment may contain one or more elements selected from the group consisting of Ta, Ti, and Zr in place of a portion of Fe. All of these elements form precipitates to increase the strength of the steel material. Ta, Ti, and Zr will be described below.

[0060] Ta: 0 to 0.100% Tantalum (Ta) is an optional element and may not be contained, that is, the Ta content may be 0%. When contained, that is, when the Ta content exceeds 0%, Ta forms precipitates and increases the strength of the steel material. Even if even a small amount of Ta is contained, the above effects can be obtained to some extent. However, if the Ta content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high, and in this case, the toughness of the steel material decreases. Therefore, the Ta content is 0 to 0.100%. The lower limit of the Ta content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.005%. The upper limit of the Ta content is preferably 0.095%, more preferably 0.090%, and even more preferably 0.080%.

[0061] Ti: 0 to 0.100% Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When contained, that is, when the Ti content exceeds 0%, Ti forms precipitates and increases the strength of the steel material. Even if even a small amount of Ti is contained, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high, and in this case, the toughness of the steel material decreases. Therefore, the Ti content is 0 to 0.100%. The lower limit of the Ti content is preferably 0.001%, more preferably 0.002%, even more preferably 0.003%, and still more preferably 0.005%. The upper limit of the Ti content is preferably 0.095%, more preferably 0.090%, and even more preferably 0.080%.

[0062] Zr: 0 to 0.100% Zirconium (Zr) is an optional element and may not be contained, that is, the Zr content may be 0%. When contained, that is, when the Zr content is more than 0%, Zr forms precipitates and increases the strength of the steel material. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high, and in this case, the toughness of the steel material decreases. Therefore, the Zr content is 0 to 0.100%. The lower limit of the Zr content is preferably 0.001%, more preferably 0.002%, even more preferably 0.003%, and still more preferably 0.005%. The upper limit of the Zr content is preferably 0.095%, more preferably 0.090%, and even more preferably 0.080%.

[0063] [About W, Zn, Sb, As and Pb] The chemical composition of the duplex stainless steel material of this embodiment may contain one or more elements selected from the group consisting of W, Zn, Sb, As, and Pb in place of a portion of Fe. All of these elements improve the SSC resistance of the steel material. W, Zn, Sb, As, and Pb will be described below.

[0064] W: 0 to 0.200% Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W enhances the SSC resistance of the steel material. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content exceeds 0.200%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high, and in this case, the toughness of the steel material decreases. Therefore, the W content is 0 to 0.200%. The lower limit of the W content is preferably 0.001%, more preferably 0.005%, even more preferably 0.010%, and still more preferably 0.020%. The upper limit of the W content is preferably 0.190%, more preferably 0.180%, and even more preferably 0.170%.

[0065] Zn: 0 to 0.010% Zinc (Zn) is an optional element and may not be contained, that is, the Zn content may be 0%. When Zn is contained, that is, when the Zn content exceeds 0%, Zn increases the SSC resistance of the steel material. Even if even a small amount of Zn is contained, the above effect can be obtained to some extent. However, if the Zn content exceeds 0.010%, the SSC resistance of the steel material may be reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Zn content is 0 to 0.010%. The lower limit of the Zn content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the Zn content is preferably 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0066] Sb: 0 to 0.1000% Antimony (Sb) is an optional element and may not be contained, that is, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 0%, Sb enhances the SSC resistance of the steel material. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.1000%, the manufacturing cost increases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.1000%. The lower limit of the Sb content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0050%. The upper limit of the Sb content is preferably 0.0950%, more preferably 0.0800%, and even more preferably 0.0600%.

[0067] As: 0 to 0.050% Arsenic (As) is an optional element and may not be contained, that is, the As content may be 0%. When contained, that is, when the As content exceeds 0%, As enhances the SSC resistance of the steel material. Even if only a small amount of As is contained, the above effect can be obtained to some extent. However, if the As content exceeds 0.050%, the SSC resistance of the steel material may be reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the As content is 0 to 0.050%. The lower limit of the As content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.005%. The upper limit of the As content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0068] Pb: 0 to 0.010% Lead (Pb) is an optional element and may not be contained, that is, the Pb content may be 0%. When Pb is contained, that is, when the Pb content is more than 0%, Pb enhances the SSC resistance of the steel material. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.010%, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.010%. The lower limit of the Pb content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the Pb content is preferably 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0069] [About B, Mg and rare earth elements (REM)] The chemical composition of the duplex stainless steel material of this embodiment may contain one or more elements selected from the group consisting of B, Mg, and rare earth elements (REM) in place of a portion of Fe. All of these elements improve the hot workability of the steel material. B, Mg, and REM will be described below.

[0070] B: 0 to 0.0200% Boron (B) is an optional element and may not be contained, that is, the B content may be 0%. When B is contained, that is, when the B content exceeds 0%, B suppresses the segregation of S to grain boundaries in the steel material and improves the hot workability of the steel material. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0200%, coarse B nitrides are formed, and in this case, the toughness of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.0200%. The lower limit of the B content is preferably 0.0001%, more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the B content is preferably 0.0190%, more preferably 0.0180%, and even more preferably 0.0170%.

[0071] Mg: 0 to 0.020% Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When contained, that is, when the Mg content is more than 0%, Mg fixes the S in the steel as sulfides, rendering it harmless and improving the hot workability of the steel. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content exceeds 0.020%, coarse oxides are formed, and in this case, the SSC resistance of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mg content is 0 to 0.020%. The lower limit of the Mg content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the Mg content is preferably 0.018%, more preferably 0.016%, and even more preferably 0.014%.

[0072] Rare earth elements (REM): 0~0.100% Rare earth elements (REM) are optional elements and may not be contained, that is, the REM content may be 0%. When REM is contained, that is, when the REM content is over 0%, REM neutralizes the S in the steel by fixing it as sulfides, improving the hot workability of the steel. Even if the REM is contained even a small amount, the above effects can be achieved to some extent. However, if the REM content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the oxides in the steel material will become coarse, and in this case, the toughness of the steel material will decrease. Therefore, the REM content is 0 to 0.100%. The lower limit of the REM content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the REM content is preferably 0.098%, more preferably 0.090%, and even more preferably 0.085%.

[0073] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. In addition, the REM content in this specification refers to the total content of these elements.

[0074] [(Feature 2) Microstructure] The microstructure of the duplex stainless steel material according to this embodiment is composed of ferrite and austenite. In this specification, "composed of ferrite and austenite" means that the amount of structures other than ferrite and austenite is negligibly small. For example, the volume fraction of precipitates and inclusions in the microstructure of the duplex stainless steel material according to this embodiment is negligibly low compared to the volume fraction of ferrite and austenite. In other words, the microstructure of the duplex stainless steel material according to this embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to ferrite and austenite.

[0075] In the microstructure of the duplex stainless steel material according to this embodiment, the volume fraction of ferrite is 35 to 65%, and the remainder is austenite. In this case, the volume fraction of austenite is 35 to 65%.

[0076] [Method for measuring the volume fraction (%) of ferrite in the microstructure] In this embodiment, the volume fraction of ferrite in the duplex stainless steel material is determined by a method in accordance with ASTM E562 (2019).

[0077] Specifically, a test piece for microstructure observation is taken from the duplex stainless steel material. When the steel material is a steel pipe, a test piece having an observation surface of 5 mm in the pipe axial direction and 5 mm in the pipe circumferential direction is taken from the center of the wall thickness. In this specification, the pipe circumferential direction of the steel pipe means the direction perpendicular to the pipe axial direction and the pipe radial direction. When the steel material is a steel plate, a test piece having an observation surface of 5 mm in the rolling direction and 5 mm in the plate width direction is taken from the center of the plate thickness. When the steel material is round steel, a test piece having an observation surface of 5 mm in the axial direction and 5 mm in the circumferential direction is prepared from the R / 2 portion. In this specification, round steel refers to a steel bar having a circular cross section perpendicular to the axial direction. The R / 2 portion refers to the center of the radius R in the cross section perpendicular to the axial direction of the round steel. The circumferential direction of the round steel refers to the direction perpendicular to the axial and radial directions. The size of the test piece is not particularly limited as long as the above observation surface can be obtained.

[0078] The observation surface of the prepared test piece is mirror-polished. The mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etching solution to reveal the microstructure. The observation surface with the revealed microstructure is observed using an optical microscope in 10 fields of view. The area of ​​each field of view is, for example, 1.00 mm. 2 (Magnification 100x).

[0079] In each field of view, ferrite and austenite can be distinguished by contrast. Therefore, ferrite is identified from the contrast in each field of view. The area fraction of the identified ferrite is measured using a point counting method in accordance with ASTM E562 (2019). In this embodiment, the arithmetic mean value of the obtained ferrite area fractions in 10 fields of view is defined as the ferrite volume fraction (%). The ferrite volume fraction (%) is an integer value obtained by rounding the obtained value to one decimal place.

[0080] [(Feature 3) Number density of fine low-S inclusions, coarse low-S inclusions, and high-S inclusions] For inclusions containing S in the duplex stainless steel material of this embodiment, Fn1 is defined by formula (1). Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the element symbol in formula (1) is substituted with the content of the corresponding element in the inclusion in mass percent. If the element is not contained, "0" is substituted for the corresponding element symbol. Fn1 is the value obtained by rounding the obtained value to one decimal place.

[0081] Furthermore, inclusions in duplex stainless steel materials are defined as follows. (A) Inclusions containing S, having an Fn1 of 5.0 or more, and an equivalent circle diameter of 2.0 to 10.0 μm are defined as "fine low-S inclusions." (B) Inclusions containing S, having an Fn1 of 5.0 or more, and an equivalent circle diameter of more than 10.0 μm are defined as "coarse low-S inclusions." (C) Inclusions containing S, having an Fn1 of less than 5.0, and an equivalent circle diameter of 2.0 μm or more are defined as "high S inclusions."

[0082] At this time, the number density ND1 of fine low-S inclusions in the duplex stainless steel material is 3.0 pieces / mm 2 or more, and the number density ND2 of coarse low-S inclusions is 2.0 pieces / mm 2 The number density ND3 of high S inclusions is 2.0 pieces / mm 2 The following is the result.

[0083] Furthermore, even if inclusions containing S and having an equivalent circle diameter of less than 2.0 μm dissolve and form dents, the dents quickly repassivate. Therefore, in duplex stainless steel materials having the chemical composition of Feature 1, inclusions containing S and having an equivalent circle diameter of less than 2.0 μm do not affect SSC resistance.

[0084] As described above, in the duplex stainless steel material of this embodiment, the S content in the chemical composition is not drastically reduced, but rather a certain amount of S is allowed while increasing the number density ND1 of fine low-S inclusions, which are composite inclusions, thereby reducing the S content available for forming high-S inclusions. This makes it possible to reduce the number density ND3 of high-S inclusions that are easily dissolved in highly corrosive sour environments.

[0085] Furthermore, among low-S inclusions with an Fn1 of 5.0 or more, coarse low-S inclusions are coarse even when the S content is low. Therefore, coarse low-S inclusions are likely to dissolve in a highly corrosive sour environment and form coarse pits. These coarse pits are difficult to repassivate and promote the occurrence of SSC. Therefore, the number density ND1 of fine low-S inclusions is increased, the number density ND3 of high-S inclusions is decreased, and the number density ND2 of coarse low-S inclusions is also decreased.

[0086] [Number density ND1 of fine low-S inclusions] The number density ND1 of fine low-S inclusions is 3.0 pieces / mm 2 If the ND3 is less than 2.0 / mm3, the amount of fine low-S inclusions in the steel is insufficient. In this case, the amount of S available for forming high-S inclusions in the steel increases, and the number density ND3 of the high-S inclusions becomes 2.0 / mm3. 2As a result, sufficient SSC resistance in highly corrosive sour environments cannot be obtained. Therefore, the number density ND1 of fine low-S inclusions is 3.0 pieces / mm 2 That's all. The preferable lower limit of the number density ND1 is 3.5 pieces / mm 2 and more preferably 4.0 pieces / mm 2 and more preferably 4.5 pieces / mm 2 is. The upper limit of the number density ND1 is not particularly limited. However, in a duplex stainless steel material that satisfies Feature 1, the upper limit of the number density ND1 is, for example, 25.0 pieces / mm 2 For example, 20.0 pieces / mm 2 is.

[0087] [Number density ND2 of coarse low-S inclusions] The number density ND2 of coarse low-S inclusions is 2.0 pieces / mm 2 If the temperature exceeds this value, excessive coarse low-S inclusions are formed. In this case, the coarse low-S inclusions are likely to dissolve in a highly corrosive sour environment, forming coarse pits. These coarse pits are likely to promote the occurrence of SSC. Therefore, the number density ND2 of coarse low-S inclusions is 2.0 pieces / mm 2 The following applies. The preferred upper limit of the number density ND2 is 1.9 pieces / mm 2 and more preferably 1.8 pieces / mm 2 and more preferably 1.7 pieces / mm 2 is. It is preferable that the number density ND2 is as small as possible. In other words, the number density ND2 is 0 pieces / mm 2 However, excessive reduction in the number density ND2 may increase the manufacturing cost. Therefore, the preferred lower limit of the number density ND2 is 0.1 pieces / mm 2 and more preferably 0.2 pieces / mm 2 is.

[0088] [Number density ND3 of high-S inclusions] As mentioned above, high-S inclusions, even if small in size, are easily dissolved in a highly corrosive sour environment and tend to form pits. Pits formed from high-S inclusions tend to promote the occurrence of SSC, even if they are small in size. Therefore, when the number density ND3 of high-S inclusions is 2.0 pieces / mm 2 If the temperature exceeds 100°C, sufficient SSC resistance cannot be obtained in a highly corrosive sour environment. Therefore, the number density ND3 of high-S inclusions is 2.0 pieces / mm 2 The following applies. The preferred upper limit of the number density ND3 is 1.9 pieces / mm 2 and more preferably 1.7 pieces / mm 2 and more preferably 1.5 pieces / mm 2 is. It is preferable that the number density ND3 is as small as possible. In other words, the number density ND3 is 0 pieces / mm 2 However, if the number density ND3 is reduced too much, the manufacturing cost may increase. Therefore, the preferable lower limit of the number density ND3 is 0.1 pieces / mm 2 and more preferably 0.2 pieces / mm 2 is.

[0089] [Number density of fine low-S inclusions ND1 (pieces / mm 2 ), the number density of coarse low-S inclusions ND2 (pieces / mm 2 ) and the number density of high-S inclusions ND3 (pieces / mm 2 ) Measurement method] In this embodiment, the number density ND1 (number / mm 2 ), the number density of coarse low-S inclusions ND2 (pieces / mm 2 ) and the number density of high-S inclusions ND3 (pieces / mm 2 ) can be found in the following way:

[0090] Test pieces are taken from the duplex stainless steel material. When the duplex stainless steel material is a steel pipe, a test piece having an observation surface including the pipe axial direction and the wall thickness direction (pipe radial direction) is taken 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 taken from the center of the plate thickness. When the steel material is a round bar, a test piece having an observation surface including the axial and radial directions is taken from the R / 2 portion of the round bar.

[0091] The observation surface of the sampled test piece is mirror-polished. An observation field is selected from the mirror-polished observation surface. At this time, three or more observation fields are selected, and the total area of ​​the observation fields is 500 mm. 2 Using a scanning electron microscope equipped with composition analysis capabilities (SEM-EDS device), a backscattered electron image of each observation field is obtained at 800x magnification. The obtained backscattered electron images are observed, and particles are identified from the contrast. The position coordinates of all identified particles in the observation field are recorded.

[0092] Based on the position coordinates of the identified particles, element concentration analysis (EDS analysis) is performed on the particles. Specifically, the identified particles are scanned with an electron beam based on the position coordinates to analyze the element concentration of the particles. At this time, the electron beam is scanned over the entire particle, rather than just a portion of the particle. This allows the element concentration of the entire particle to be analyzed. In EDS analysis, the acceleration voltage is set to 20 kV, and the target elements are quantified as N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb.

[0093] Of the identified particles, when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, particles with a Cl content of more than 10.0%, particles with a K content of more than 10.0%, particles with a Na content of more than 20.0%, particles with a Ca content of more than 75.0%, and particles with an O content of more than 70.0% are dust or abrasives that adhered during mirror polishing. Therefore, these particles are determined not to be inclusions and are excluded from the scope of the test. Furthermore, among the identified particles, when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, particles with an S content of less than 1.0% are oxides or nitrides, not inclusions containing S. Therefore, particles with an S content of less than 1.0% are also excluded from the scope of the present invention. In other words, among the identified particles, when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, particles other than those with a Cl content of more than 10.0%, those with a K content of more than 10.0%, those with a Na content of more than 20.0%, those with a Ca content of more than 75.0%, those with an O content of more than 70.0%, and those with an S content of less than 1.0% are recognized as inclusions containing S.

[0094] For particles identified as inclusions containing S, the equivalent circle diameter (μm) is determined. The equivalent circle diameter means the diameter (μm) of a circle with the same area as the particle. The equivalent circle diameter is the value obtained by rounding off the first decimal place to the nearest tenth.

[0095] Based on the EDS analysis results and the circle equivalent diameter of each particle, fine low-S inclusions, coarse low-S inclusions, and high-S inclusions are identified as follows: (A) Fine low S inclusions Particles having an Fn1 defined by formula (1) of 5.0 or more and an equivalent circle diameter of 2.0 to 10.0 μm based on the Mg content, Al content, Ca content, Ti content, V content, Mn content and S content in mass % when the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr and Nb is taken as 100.0 mass % are specified as "fine low-S inclusions." Fn1 = (Mg + Al + Ca + Ti + V + Mn) / S (1) Here, the element symbol in formula (1) is substituted with the content of the element in the corresponding particle (inclusion) in mass %. If the element is not contained, "0" is substituted for the corresponding element symbol. (B) Coarse low S inclusions Particles having an Fn1 defined by formula (1) of 5.0 or more and an equivalent circle diameter of more than 10.0 μm based on the Mg content, Al content, Ca content, Ti content, V content, Mn content and S content in mass%, where the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr and Nb is taken as 100.0 mass%, are specified as "coarse low-S inclusions." (C) High S inclusions Particles having an Fn1 defined by formula (1) of less than 5.0 based on the Mg content, Al content, Ca content, Ti content, V content, Mn content, and S content in mass%, where the total content of N, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Mn, Cu, Zr, and Nb is taken as 100.0 mass%, and an equivalent circle diameter of 2.0 μm or more are specified as "high-S inclusions."

[0096] The fine low-S inclusions, coarse low-S inclusions, and high-S inclusions identified by the above method are counted in each field of view.

[0097] Based on the total number of counted fine low-S inclusions in all observation fields and the total area of ​​all observation fields, the number density ND1 (number / mm 2 ) is found. Based on the total number of coarse low-S inclusions counted in all observation fields and the total area of ​​all observation fields, the number density ND2 (numbers / mm 2 ) is found. Based on the total number of high-S inclusions counted in all observation fields and the total area of ​​all observation fields, the number density ND3 (numbers / mm 2 ) is found.

[0098] The number density of fine low-S inclusions (ND1) (pieces / mm 2 ), the number density of coarse low-S inclusions ND2 (pieces / mm 2 ), and the number density of high S inclusions ND3 (pieces / mm 2 ) is the value obtained by rounding off the obtained number to the first decimal place. Furthermore, as the SEM-EDS device, for example, an automatic analyzer manufactured by FEI (ASPEX) under the trade name of Metals Quality Analyzer can be used.

[0099] [Effects of the duplex stainless steel material of this embodiment] The duplex stainless steel material of this embodiment satisfies Features 1 to 3. Therefore, excellent SSC resistance can be obtained in highly corrosive sour environments.

[0100] [SSC resistance evaluation method] The SSC resistance of the duplex stainless steel material of this embodiment is evaluated by the following method.

[0101] A four-point bending test piece having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm is taken from the duplex stainless steel material. When the duplex stainless steel material is a steel pipe, a four-point bending test piece is taken from the center of the wall thickness. In this case, the longitudinal direction of the four-point bending test piece is parallel to the axial direction of the steel pipe. The thickness direction of the four-point bending test piece is the wall thickness direction of the steel pipe. When the duplex stainless steel material is a steel plate, a four-point bending test piece is taken from the center of the plate thickness. In this case, the longitudinal direction of the four-point bending test piece is parallel to the rolling direction of the steel plate. The thickness direction of the four-point bending test piece is the plate thickness direction of the steel plate. When the duplex stainless steel material is a round bar, a round bar test piece is taken from the R / 2 section. In this case, the longitudinal direction of the four-point bending test piece is parallel to the rolling direction of the round bar. The thickness direction of the four-point bending test piece is the radial direction of the round bar.

[0102] A test solution containing 0.17% by mass of NaCl (sodium chloride), 0.41 g / L of CH3COONa (sodium acetate), and CH3COOH (acetic acid) and adjusted to a pH of 3.5 is prepared. A stress equivalent to 90% of the actual yield stress is applied to a four-point bending test specimen. The test solution at 90°C is poured into a test vessel (autoclave) so that the stressed four-point bending test specimen is immersed, creating a test bath. The test bath is degassed with N2 gas. Simulating a highly corrosive sour environment, 0.1 bar of H2S and 30 bar of CO2 are blown into the degassed test bath to saturate it. The test bath is then held for 720 hours.

[0103] After 720 hours of holding, the surface of the test piece is observed with a magnifying glass at 10x magnification to check for the presence or absence of cracks. If the magnifying glass observation reveals any areas where cracks are suspected, the cross section of the suspected area is observed with an optical microscope at 100x magnification to check for the presence or absence of cracks.

[0104] In this embodiment, having excellent SSC resistance means that no cracks are observed after 720 hours in the above-mentioned SSC resistance evaluation test. In this specification, "no cracks are observed" means that no cracks are observed when the test piece after the test is observed with a 10x magnification loupe and a 100x optical microscope.

[0105] [Yield strength] The yield strength of the duplex stainless steel material according to this embodiment is not particularly limited. The yield strength of the duplex stainless steel material according to this embodiment is, for example, 414 to 758 MPa. The yield strength of the duplex stainless steel material according to this embodiment may be 414 to 655 MPa.

[0106] [Yield strength measurement method] The yield strength of the duplex stainless steel material of this embodiment can be determined by the following method. Tensile tests are performed in accordance with ASTM E8 / E8M (2022). First, tensile test specimens are taken from the duplex stainless steel material. When the duplex stainless steel material is a steel pipe, a round bar-shaped tensile test specimen or an arc-shaped tensile test specimen is taken from the center of the wall thickness. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the steel pipe. When the duplex stainless steel material is a steel pipe and a round bar test specimen cannot be taken from the steel pipe, an arc-shaped tensile test specimen is taken from the steel pipe. When the duplex stainless steel material is a steel plate, a round bar-shaped tensile test specimen is taken from the center of the plate thickness, with the longitudinal direction of the tensile test specimen parallel to the rolling direction of the steel plate. When the duplex stainless steel material is a round bar, a round bar-shaped tensile test specimen is taken from the R / 2 part. In this case, the longitudinal direction of the tensile test specimen is parallel to the axial direction of the round bar.

[0107] The size of the round bar-shaped tensile test specimen is, for example, 6 mm in diameter at the parallel part and 30 mm in gauge length, while the size of the arc-shaped tensile test specimen is, for example, the full thickness, 25.4 mm in width, and 50.8 mm in gauge length. A tensile test is performed using a tensile test piece at room temperature (24±3°C) in the atmosphere. In this embodiment, the 0.2% offset yield strength obtained from the tensile test is defined as the yield strength (MPa). In this embodiment, the yield strength (MPa) is an integer value obtained by rounding the obtained value to one decimal place.

[0108] [Shape of duplex stainless steel material] As described above, the shape of the duplex stainless steel material according to this embodiment is not particularly limited. The duplex stainless steel material may be a steel pipe, a steel plate, or a round bar. Preferably, the duplex stainless steel material according to this embodiment is a steel pipe, and more preferably a seamless steel pipe.

[0109] [Manufacturing method] An example of a method for manufacturing the duplex stainless steel material according to this embodiment having the above-described configuration will be described below. Note that the method for manufacturing the duplex stainless steel material according to this embodiment is not limited to the manufacturing method described below. An example of a method for manufacturing the duplex stainless steel material of this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Hot processing process (Step 3) Solution treatment step Each manufacturing step will be described in detail below.

[0110] [Material preparation process] In the material preparation process, molten pig iron produced by a known method is subjected to refining (primary refining) in a converter. The molten steel produced by the primary refining is subjected to secondary refining. In the secondary refining, alloy elements are added to adjust the composition to produce molten steel that satisfies the chemical composition of Feature 1. In the secondary refining, for example, RH (Ruhrstahl-Hausen) vacuum degassing treatment is performed, followed by final adjustment of the alloy composition. In the secondary refining, combined refining may be performed. In this case, prior to the RH vacuum degassing treatment, for example, a refining treatment using an LF (Ladle Furnace) or VAD (Vacuum Arc Degassing) is performed.

[0111] The molten steel that has undergone secondary refining is used to produce a material. Specifically, the molten steel that has undergone secondary refining is used to produce a cast piece (slab, bloom, or billet) by continuous casting.

[0112] In continuous casting, molten steel is first poured from a ladle into a tundish. The molten steel, temporarily stored in the tundish, is then guided into the mold through an immersion nozzle. The molten steel gradually solidifies from the outer surface, which is cooled in the mold. The solidified portion is gradually pulled downward from the mold, and cooling water is sprayed onto the outer surface of the slab to further cool it and solidify it. Through these manufacturing processes, slabs that will serve as the raw material for duplex stainless steel are produced.

[0113] In the material preparation step, the bloom may be further subjected to blooming to form a billet. In this case, for example, the bloom is heated to 1150 to 1300° C. The heated bloom is then subjected to blooming to form a billet. Through the above steps, a duplex stainless steel material (slab, bloom, or billet) is produced.

[0114] In the continuous casting process during the material preparation process, the following conditions must be met: (Condition 1) The temperature of the molten steel in the tundish is maintained at 1600 to 1500°C for 5 to 100 minutes. (Condition 2) The vertical distance D from the surface of the molten steel in the mold to the center of the discharge port on the outer surface of the submerged entry nozzle is set to 150 to 400 mm. Conditions 1 and 2 will be explained below.

[0115] [Regarding Condition 1] Fine low-S inclusions are formed by the aggregation of sulfides and nitrides around oxides, which act as nuclei. To form such fine low-S inclusions, it is effective to float coarse oxides in the molten steel in the tundish and prevent them from flowing into the mold.

[0116] The tundish is equipped with a heating device that maintains the temperature of the molten steel. The heating device is, for example, an induction heating device or a plasma heating device. In the tundish, the molten steel is held at 1600 to 1500°C for a holding time t. If the holding time t is less than 5 minutes, coarse oxides in the molten steel do not rise sufficiently, and the coarse oxides remain in the molten steel and flow into the mold. In this case, the number density ND2 of coarse low-S inclusions in the manufactured duplex stainless steel becomes excessive.

[0117] On the other hand, if the holding time t exceeds 100 minutes, although the coarse oxides in the molten steel rise to the surface, the fine oxides also become coarse and rise to the surface. As a result, the amount of fine oxides that serve as nuclei for fine low-S inclusions in the molten steel flowing from the immersion nozzle into the mold is insufficient. As a result, in the duplex stainless steel material produced, the number density ND1 of the fine low-S inclusions becomes excessively low, and the number density ND3 of the high-S inclusions becomes excessive. Therefore, the retention time t is set to 5 to 100 minutes.

[0118] [Regarding Condition 2] 1 is a schematic diagram illustrating the positional relationship between molten steel in a mold and the submerged entry nozzle during continuous casting. Referring to Fig. 1, molten steel 10 passes through a submerged entry nozzle 20 and flows from a discharge port 22 outside the submerged entry nozzle 20 into a mold 30.

[0119] The submerged nozzle 20 includes a cylindrical main body 21 and two discharge ports 22. The two discharge ports 22 are arranged opposite each other on the side wall near the bottom of the cylindrical main body. More specifically, one of the discharge ports 22 is arranged 180° offset from the other discharge port 22 around the central axis of the submerged nozzle 20. The discharge ports 22 are inclined upward at an inclination angle θ of 5 to 35° with respect to the horizontal direction.

[0120] The vertical distance from the liquid level 11 of the molten steel 10 in the mold 30 to the center of the discharge port 22 on the outer surface of the submerged nozzle 20 is defined as distance D (mm). Here, the "center of the discharge port 22 on the outer surface of the submerged nozzle 20" refers to the position P22 of the central axis C22 of the discharge port 22 on the outer surface of the submerged nozzle 20. During continuous casting, the liquid level 11 fluctuates within ±10 mm. Therefore, the position of the liquid level 11 (liquid level height) when calculating distance D is the liquid level height (target level) set during actual continuous casting operation.

[0121] The distance D forms a stirring zone DA between the liquid surface 11 and the discharge port 22 in the molten steel 10. In the stirring zone DA, the molten steel flowing out of the discharge port 22 of the submerged entry nozzle 20 into the molten steel 10 rises and is stirred vertically. At this time, the temperature of the molten steel gradually decreases, resulting in the formation of sulfides and nitrides. If the molten steel flowing out of the discharge port 22 contains a sufficient amount of fine oxides, sulfides and nitrides are formed in the stirring zone DA using the fine oxides as nuclei, or the sulfides and nitrides formed in the molten steel collide with the fine oxides and agglomerate, forming low-S inclusions with an Fn1 of 5.0 or more. The formation of low-S inclusions suppresses the formation of high-S inclusions. Furthermore, among the low-S inclusions, coarse low-S inclusions float to the liquid surface 11. A molten slag layer is formed on the liquid surface 11 by the mold powder. The coarse low-S inclusions that float to the liquid surface 11 are absorbed into the molten slag layer. Therefore, although fine low-S inclusions remain in the molten steel, the amount of coarse low-S inclusions is reduced.

[0122] As described above, the stirring region DA affects the number density of fine low-S inclusions, coarse low-S inclusions, and high-S inclusions in the duplex stainless steel material after production. The distance D is also a factor that determines the size of the stirring region DA.

[0123] If the distance D is less than 150 mm, the stirring region DA is too narrow. In this case, low-S inclusions may not be generated to a degree that sufficiently reduces the number density ND3 of high-S inclusions. As a result, the number density ND3 of high-S inclusions becomes excessive, or the number density ND1 of fine low-S inclusions becomes excessively low.

[0124] On the other hand, if the distance D exceeds 400 mm, the stirring area DA becomes too wide. In this case, low-S inclusions are sufficiently formed, and the formation of high-S inclusions can be sufficiently suppressed. However, the coarse low-S inclusions are less likely to rise to the liquid surface 11. As a result, the number density ND2 of the coarse low-S inclusions in the manufactured duplex stainless steel material becomes excessive. Therefore, the distance D is set to 150 to 400 mm.

[0125] Electromagnetic stirring may be performed on the molten steel 10 in the mold 30. Electromagnetic stirring stirs the molten steel in the horizontal direction. Therefore, electromagnetic stirring does not easily form a stirring region DA that stirs the molten steel in the vertical direction in FIG. 1. By setting the distance D (mm) from the liquid surface 11 of the molten steel 10 in the mold 30 to the center of the discharge port 22 on the outer surface of the submerged nozzle 20 to 150 to 400 mm, a stirring region DA of an appropriate range can be formed.

[0126] [Hot processing process] In the hot working step, the material (slab, bloom, or billet) produced in the material preparation step is hot worked to produce an intermediate steel material. In this specification, the intermediate steel material refers to a blank pipe if the final product is a steel pipe, a plate-shaped steel material if the final product is a steel plate, or a bar-shaped steel material with a circular cross section perpendicular to the axial direction if the final product is a round bar. The hot working may be hot forging, hot extrusion, or hot rolling. The hot working method is not particularly limited and may be a well-known method.

[0127] When the intermediate steel material is a mother pipe (seamless steel pipe), for example, hot extrusion such as the Eugène-Séjournet method or the Erhardt push bench method may be performed. Also, hot rolling such as piercing and rolling by the Mannesmann method may be performed. Hot working may be performed only once or multiple times. For example, the material is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1000 to 1280°C. The material extracted from the heating furnace is subjected to the above-mentioned piercing and rolling, and then subjected to elongation rolling to produce a mother pipe, which is an intermediate steel material.

[0128] When the intermediate steel material is a steel plate, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1000 to 1280°C. The material removed from the heating furnace is hot-rolled using a roughing mill and a tandem finishing mill to produce a steel plate, which is the intermediate steel material.

[0129] When the intermediate steel material is round steel, the material is first heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1000 to 1280°C. The material extracted from the heating furnace is subjected to hot working to produce the round steel material, which is the intermediate steel material. The hot working is, for example, blooming using a blooming mill or hot rolling using a continuous rolling mill. The continuous rolling mill has horizontal stands each having a pair of grooved rolls arranged side by side in the vertical direction, and vertical stands each having a pair of grooved rolls arranged side by side in the horizontal direction, arranged alternately.

[0130] [Solution treatment process] In the solution treatment step according to this embodiment, the intermediate steel material is subjected to solution treatment. The method of solution treatment is not particularly limited and may be a well-known method. For example, the intermediate steel material is charged into a heat treatment furnace, held at a desired temperature, and then rapidly cooled. In this case, the temperature at which the solution treatment is performed (heat treatment temperature) refers to the temperature (°C) of the heat treatment furnace for carrying out the solution treatment. The time for which the intermediate steel material is held at the solution treatment temperature (holding time) refers to the time (minutes) for which the intermediate steel material is held at the heat treatment temperature.

[0131] The solution temperature in the solution treatment step is set to 950 to 1070°C. If the solution temperature is too low, the ferrite volume fraction of the duplex stainless steel material after solution treatment will be less than 35%. On the other hand, if the solution temperature is too high, the ferrite volume fraction of the duplex stainless steel material after solution treatment will be more than 65%.

[0132] Therefore, the solution temperature is set to 950 to 1070° C. The holding time at the solution temperature is not particularly limited, but is, for example, 5 to 180 minutes. The rapid cooling method is, for example, water cooling.

[0133] [Optional manufacturing process] The method for manufacturing a duplex stainless steel material according to this embodiment may further include a pickling step. In other words, the pickling step is an optional step. When the pickling step is performed, the intermediate steel material after the solution treatment step is subjected to pickling treatment. The pickling treatment may be performed under well-known conditions.

[0134] The duplex stainless steel material of this embodiment can be manufactured by the above steps. Note that the above-described method for manufacturing a duplex stainless steel material is one example, and a duplex stainless steel material satisfying Features 1 to 3 may be manufactured by other methods. The present invention will be described in more detail below with reference to examples. [Example]

[0135] Duplex stainless steel materials having the chemical compositions shown in Tables 1A, 1B, and 1C were produced.

[0136] [Table 1A]

[0137] [Table 1B]

[0138] [Table 1C]

[0139] Specifically, molten steel of each test number was produced, and blooms were produced by continuous casting. During continuous casting, the holding time t (min) at a molten steel temperature of 1600-1500°C in the tundish and the vertical distance D (mm) from the liquid surface of the molten steel in the mold to the center of the discharge port on the outer surface of the submerged entry nozzle were as shown in the "Holding time t (min)" and "Distance D (mm)" columns in Table 2. The upward tilt angle θ of the discharge port of the submerged entry nozzle (see Figure 1) was 15°.

[0140] [Table 2]

[0141] The produced blooms were subjected to blooming to produce billets. The bloom heating temperature during blooming was 1250°C. The produced billets were heated to 1250°C and then hot rolled by the Mannesmann process to produce mother pipes (seamless steel pipes) with an outer diameter of 244.48 mm and a wall thickness of 13.84 mm. The mother pipes were then subjected to solution treatment. The solution treatment temperatures (°C) are shown in the "Solution Treatment Temperature (°C)" column in Table 2. The holding time at the solution treatment temperature was 20 minutes for all test numbers. After the holding time had elapsed, the mother pipes were water-cooled. Through the above process, duplex stainless steel materials (seamless steel pipes) for each test number were produced.

[0142] [Evaluation test] The following evaluation tests were carried out on the manufactured duplex stainless steel materials with each test number. (Test 1) Microstructure observation and ferrite volume fraction (%) measurement test (Test 2) Number density of fine low-S inclusions ND1 (pieces / mm 2 ), the number density of coarse low-S inclusions ND2 (pieces / mm 2 ) and the number density of high-S inclusions ND3 (pieces / mm 2 ) measurement test (Test 3) Yield strength measurement test (Test 4) SSC resistance evaluation test Each test will be explained below.

[0143] [(Test 1) Microstructure observation and ferrite volume fraction (%) measurement test] Based on the method described above in [Method for measuring the volume fraction (%) of ferrite in the microstructure], the microstructure of each test number was observed and the volume fraction (%) of ferrite in the duplex stainless steel material was determined. The area of ​​each field of view on the observation surface was 1.00 mm 2 It was decided.

[0144] As a result of the measurement, in all test numbers, the microstructure contained 35 to 65% by volume of ferrite, and the remainder was a structure consisting of austenite.

[0145] (Test 2) Number density of fine low-S inclusions ND1 (number / mm2 ), the number density of coarse low-S inclusions ND2 (pieces / mm 2 ) and the number density of high-S inclusions ND3 (pieces / mm 2 ) Measurement test] The number density of fine low-S inclusions (ND1 (pieces / mm 2 ), the number density of coarse low-S inclusions ND2 (pieces / mm 2 ) and the number density of high-S inclusions ND3 (pieces / mm 2 Based on the method described in [Measuring Method of Fine Low-Sulfur Inclusions], the number density ND1 (number / mm 2 ), the number density of coarse low-S inclusions ND2 (pieces / mm 2 ), and the number density of high S inclusions ND3 (pieces / mm 2 The observation field was set to five fields of 10 mm × 10 mm, and the total area of ​​the observation fields was 500 mm 2 The measured number densities ND1 to ND3 (pieces / mm 2 ) in Table 3, "Number density ND1 (pieces / mm 2 ) and "Number density ND2 (pieces / mm 2 ) and "Number density ND3 (pieces / mm 2 )" respectively.

[0146] [Table 3]

[0147] [(Test 3) Yield strength measurement test] The yield strength (MPa) of the duplex stainless steel material of each test number was determined based on the method described in the "Method for measuring yield strength" above. Round bar-shaped test specimens with a parallel part diameter of 6 mm and a gauge length of 30 mm were taken from the duplex stainless steel material of each test number as tensile test specimens. The results are shown in the "Yield strength (MPa)" column of Table 3.

[0148] [(Test 4) SSC resistance evaluation test] The SSC resistance of the duplex stainless steel material of each test number was evaluated based on the method described in the above [Method for evaluating SSC resistance]. If no cracks were observed, the "SSC resistance test" column in Table 3 is marked with "pass." On the other hand, if cracks were observed, the "SSC resistance test" column is marked with "fail." If no cracks were observed, the material was evaluated as having excellent SSC resistance.

[0149] [Evaluation results] Referring to Tables 1A, 1B, 1C, 2, and 3, the duplex stainless steel materials of test numbers 1 to 21 satisfied features 1 to 3. Therefore, excellent SSC resistance was obtained. The yield strength of the duplex stainless steel materials of these test numbers was 414 MPa or more.

[0150] On the other hand, in test numbers 22 and 23, the holding time t at a molten steel temperature of 1600 to 1500°C in the continuous casting tundish in the material preparation process was too short. As a result, the number density ND2 of coarse low-S inclusions was 2.0 pieces / mm 2 As a result, excellent SSC resistance was not obtained.

[0151] In test numbers 24 and 25, the holding time t at the molten steel temperature of 1600 to 1500°C in the continuous casting tundish in the material preparation process was too long. As a result, the number density ND1 of fine low-S inclusions was 3.0 pieces / mm 2 Furthermore, the number density ND3 of high S inclusions is less than 2.0 pieces / mm 2 As a result, excellent SSC resistance was not obtained.

[0152] In test numbers 26 and 27, the distance D from the surface of the molten steel in the mold to the center of the outlet on the outer surface of the submerged entry nozzle during continuous casting in the material preparation process was too short. As a result, the number density ND1 of fine low-S inclusions was 3.0 pieces / mm 2 Furthermore, the number density ND3 of high S inclusions is less than 2.0 pieces / mm 2 As a result, excellent SSC resistance was not obtained.

[0153] In test numbers 28 and 29, the distance D was too short in the continuous casting process of the material preparation process. As a result, the number density ND3 of high-S inclusions was 2.0 pieces / mm 2 As a result, excellent SSC resistance was not obtained.

[0154] In test numbers 30 and 31, the distance D was too long in the continuous casting process for preparing the material. As a result, the number density ND2 of the coarse low-S inclusions was 2.0 pieces / mm 2 As a result, excellent SSC resistance was not obtained.

[0155] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A duplex stainless steel material, The chemical composition, in mass%, is C: 0.030% or less, Si: 0.10-1.00%, Mn: 0.80-4.00%, P: 0.040% or less, S: 0.0050% or less, Cr: 20.0-25.0%, Ni: 1.50 to less than 4.00% Mo: 0.01-3.00%, Cu: 0.01-3.00%, Nb: 0.001 to 0.150%, V: 0.01-0.50%, Co: 0.01 to 0.50%, Sn: 0.001 to 0.050%, Al: 0.010-0.100%, Ca: 0.0001-0.0100%, N: 0.001-0.250%, O: 0.010% or less, Ta: 0-0.100%, Ti: 0 to 0.100%, Zr: 0 to 0.100%, W: 0-0.200%, Zn: 0 to 0.010%, Sb: 0 to 0.1000%, As: 0 to 0.050%, Pb: 0 to 0.010%, B: 0 to 0.0200%, Mg: 0 to 0.020%, Rare earth elements: 0 to 0.100%, and the balance being Fe and impurities; The microstructure is composed of 35 to 65% by volume of ferrite and the remainder of austenite, In the duplex stainless steel material, The steel contains S, has an Fn1 defined by the formula (1) of 5.0 or more, and has a number density ND1 of 3.0 pieces / mm ２ That's all, The number density ND2 of coarse low-S inclusions containing S, which have Fn1 of 5.0 or more and have a circle equivalent diameter of more than 10.0 μm, is 2.0 pieces / mm ２ is as follows: The number density ND3 of high-S inclusions, which contain S, have Fn1 of less than 5.0 and have a circle-equivalent diameter of 2.0 μm or more, is 2.0 pieces / mm ２ Below is the Duplex stainless steel material. Fn1=(Mg+Al+Ca+Ti+V+Mn) / S (1) Here, the element symbols in formula (1) are substituted with the contents of the corresponding elements in the inclusions in mass %. When an element is not contained, the corresponding element symbol is substituted with "0."

2. 2. The duplex stainless steel material according to claim 1, The chemical composition is Ta: 0.001 to 0.100%, Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100%, W: 0.001-0.200%, Zn: 0.001-0.010%, Sb: 0.0001 to 0.1000%, As: 0.001 to 0.050%, Pb: 0.001 to 0.010%, B: 0.0001 to 0.0200%, Mg: 0.001 to 0.020%, and Rare earth elements: 0.001 to 0.100%, containing one or more selected from the group consisting of Duplex stainless steel material.

3. The duplex stainless steel material according to claim 1 or claim 2, The duplex stainless steel material is a steel pipe. Duplex stainless steel material.

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