Duplex stainless steel welded joint

Abstract

Provided is a duplex stainless steel welded joint capable of suppressing underbead and excellent in fatigue characteristics. A duplex stainless steel welded joint having a base material and a weld metal, the weld metal containing, by mass%, C: 0.001 to 0.030%, Si: 0.05 to 0.70%, Mn: 0.05 to 0.85%, P: 0.030% or less, S: 0.0030% or less, Cr: 21.00 to 28.00%, Ni: 5.00 to 11.00%, Mo: 2.00 to 4.50%, Cu: 0.01 to 4.00%, Sol. Al: 0.0010 to 0.0500%, N: 0.080 to 0.400%, B: 0.0001 to 0.0100%, and the balance of Fe and impurities, the duplex stainless steel welded joint satisfying formula (1) and formula (2) while satisfying the content of each element in the base material and the weld metal. Si + 3Mn ≤ 3.00 (1) 0 < BH / BW < 0.15 (2)

Description

TECHNICAL FIELD

[0001] The present application relates to a welded joint of duplex stainless steel. BACKGROUND

[0002] Duplex stainless steel has high strength and excellent corrosion resistance in a chloride environment. Therefore, duplex stainless steel is being used in a wide range of technical fields. Duplex stainless steel is used, for example, as a material for a steel pipe for a seawater heat exchanger and a steel pipe for a umbilical cable for offshore development.

[0003] Duplex stainless steel is prescribed in JIS standards (Japanese Industrial Standards) and ASTM Standards according to the required use. For example, SUS329J3L and SUS329J4L are prescribed in JIS standards. Recently, as a super duplex stainless steel with a pitting resistance equivalent (PREW) exceeding 40, SUS327L1 has been newly added to JIS standards. In addition, development and practical use of ASTM A789 S39274, which is a super duplex stainless steel, is also being conducted. The duplex stainless steel prescribed in ASTM A789 S39274 achieves high strength and high corrosion resistance by increasing the PREW, and suppresses the precipitation of a sigma phase accompanying the high PREW by adding a large amount of W.

[0004] On the other hand, in the case of using duplex stainless steel after welding, the ferrite content in the weld metal significantly increases compared to the ferrite content of the base material of the duplex stainless steel due to rapid cooling after welding. Thereby, the strength and corrosion resistance of the weld metal are reduced. Therefore, a welded joint with excellent strength and corrosion resistance in the welded state is sought.

[0005] For example, Japanese Patent Application Publication No. 2015-196894 (Patent Literature 1) proposes a duplex stainless steel welded joint with improved pitting corrosion resistance. The chemical composition of the base material and the weld metal of the duplex stainless steel welded joint of Patent Literature 1 is, by mass%, C: 0.03% or less, Si: 0.5% or less, Mn: 2% or less, P: 0.04% or less, S: 0.003% or less, Cr: 21% or more and less than 29%, Ni: 4.0 to 10.5%, Mo: 0.8 to 4.0%, N: more than 0.1% and 0.4% or less, sol. Al: 0.040% or less, W: 0 to 4.0%, Cu: 0 to 4.0%, B: 0 to 0.005%, REM: 0 to 0.2%, and the balance: Fe and impurities. Further, the austenite index a of the duplex stainless steel welded joint of Patent Literature 1, which is calculated from the following (1) formula, is 0.1 to 0.4, satisfies Mn / N ≥ 2, and the PF index, which is calculated from the following (2) formula, is 1.0 or less, and the oxide scale thickness at the time of welding formed on the surface of the base material and the weld metal is 500 nm or less.

[0006] a = {Ni + 30(C + N) - 0.6(Cr + 1.5Si + Mo) + 5.6} / {Cr + 1.5Si + Mo - 6} (1)

[0007] PF = Mn x (100Pb + 50Sb + 30Zn + 40As) (2)

[0008] wherein each of the element symbols in the above formulae represents the content (mass%) of each element. Patent Literature 1 improves the pitting corrosion resistance of the duplex stainless steel welded joint by satisfying the formulae (1) and (2).

[0009] Prior Art Documents

[0010] Patent Literature

[0011] Patent Literature 1: Japanese Patent Application Publication No. 2015-196894 SUMMARY

[0012] PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] On the other hand, in the case where duplex stainless steel is used as a tube for an umbilical, the duplex stainless steel tubes are girth welded to each other to manufacture a duplex stainless steel welded joint. The duplex stainless steel welded joint is used as a part of an umbilical cable with a length of several km to several tens of km.

[0014] For improvement of weldability and efficiency of welding, it is preferable that the duplex stainless steel pipe be thin. On the other hand, in the case where the duplex stainless steel pipe is used as a steel pipe for a umbilical cable, the duplex stainless steel pipe is subjected to oscillation by sea waves and used for a long period of time. Therefore, it is required that the duplex stainless steel pipe have high fatigue strength. If a defect exists in the toe portion of the duplex stainless steel welded joint, stress becomes easily concentrated on the defect. As a result, fatigue failure easily occurs with the defect as a starting point. The thinner the duplex stainless steel pipe, the greater the proportion of the shape defect of the weld metal with respect to the thickness of the duplex stainless steel pipe.

[0015] The undercut is one of the defects that easily occur in the toe portion. Figure 1 is a cross-sectional view of the duplex stainless steel welded joint 1 including the weld metal 20, taken in a direction perpendicular to the extension direction of the weld metal 20, in which the undercut 3 has occurred. Referring to Figure 1 , the two base metals 10 are connected by the weld metal 20, and a groove is formed at the end portion of the weld metal 20. The groove is referred to as the undercut 3. In the case where the duplex stainless steel welded joint 1 has the undercut 3 at the end portion of the weld metal 20, stress is easily concentrated on the undercut 3 at the time of oscillation of the umbilical cable. Therefore, fatigue failure easily occurs with the undercut 3 as a starting point. In this case, the life of the umbilical cable can be greatly lower than the design standard. Therefore, it is desirable that the duplex stainless steel welded joint 1 in which the undercut 3 is suppressed.

[0016] An object of the present application is to provide a duplex stainless steel welded joint in which an undercut is suppressed and which has excellent fatigue properties.

[0017] Approach to solving the problem

[0018] The duplex stainless steel welded joint of the present application includes a base metal and a weld metal,

[0019] The base metal contains, in mass %,

[0020] C: 0.001 to 0.030%,

[0021] Si: 0.05 to 0.80%,

[0022] Mn: 0.05 to 1.20%,

[0023] P: 0.030% or less,

[0024] S: 0.0030% or less,

[0025] Cr: 21.00 to 28.00%,

[0026] Ni: 4.00 to 8.00%,

[0027] Mo: 2.00 to 4.50%,

[0028] Cu: 0.01 to 4.00%,

[0029] Sol. Al: 0.0010 to 0.0500%,

[0030] N: 0.080% to 0.400%,

[0031] B: 0.0001 to 0.0100%,

[0032] W: 0 to 4.00%,

[0033] Nb: 0 to 0.10%,

[0034] V: 0 to 0.20%,

[0035] Ta: 0 to 0.30%,

[0036] Co: 0 to 1.00%,

[0037] Sn: 0 to 0.020%,

[0038] Mg: 0 to 0.0200%,

[0039] Ca: 0 to 0.0100% and

[0040] the balance of Fe and impurities,

[0041] the weld metal is, by mass%,

[0042] C: 0.001 to 0.030%,

[0043] Si: 0.05 to 0.70%,

[0044] Mn: 0.05 to 0.85%,

[0045] P: 0.030% or less,

[0046] S: 0.0030% or less,

[0047] Cr: 21.00 to 28.00%,

[0048] Ni: 5.00 to 11.00%,

[0049] Mo: 2.00 to 4.50%,

[0050] Cu: 0.01 to 4.00%,

[0051] Sol. Al: 0.0010 to 0.0500%,

[0052] N: 0.080% to 0.400%,

[0053] B: 0.0001 to 0.0100%,

[0054] W: 0 to 4.00%,

[0055] Nb: 0 to 0.10%,

[0056] V: 0 to 0.20%,

[0057] Ta: 0 to 0.30%,

[0058] Co: 0 to 1.00%,

[0059] Sn: 0 to 0.020%,

[0060] Mg: 0 to 0.0200%,

[0061] Ca: 0 to 0.0100%, and

[0062] the balance of Fe and impurities,

[0063] The aforementioned duplex stainless steel welded joint satisfies formula (1) and formula (2) while satisfying the content of each element in the aforementioned base material and the aforementioned weld metal.

[0064] Si + 3Mn ≤ 3.00 (1)

[0065] 0 < BH / BW < 0.15 (2)

[0066] Here, each element symbol in formula (1) is substituted with the content of the corresponding element in the aforementioned weld metal in mass %.

[0067] Here, BH in formula (2) is substituted with the Cap height (mm) of the aforementioned weld metal, and BW is substituted with the Cap width (mm) of the aforementioned weld metal.

[0068] Effects of the Invention

[0069] The duplex stainless steel welded joint of the present invention can suppress undercut, and has excellent fatigue characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0070] Figure 1 is a cross-sectional view of a duplex stainless steel welded joint that has developed undercut, including the weld metal, perpendicular to the extension direction of the weld metal.

[0071] Figure 2 is a cross-sectional view of the duplex stainless steel welded joint of the present embodiment cut in a direction perpendicular to the extension direction of the weld metal.

[0072] Figure 3 is a cross-sectional view of a duplex stainless steel welded joint based on another embodiment different from Figure 2 the present embodiment. DETAILED DESCRIPTION

[0073] Hereinafter, the present embodiment will be described in detail with reference to the drawings. The same or equivalent portions in the drawings are designated by the same reference numerals, and the description thereof will not be repeated.

[0074] The present inventors et al. first conducted a study on a chemical composition for obtaining the strength and corrosion resistance required for the duplex stainless steel welded joint 1. As a result, it was found that, if the duplex stainless steel welded joint 1 having a base material 10 and a weld metal 20, the base material 10 being C: 0.001% to 0.030%, Si: 0.05% to 0.80%, Mn: 0.05% to 1.20%, P: 0.030% or less, S: 0.0030% or less, Cr: 21.00% to 28.00%, Ni: 4.00% to 8.00%, Mo: 2.00% to 4.50%, Cu: 0.01 to 4.00%, Sol. Al: 0.0010 to 0.0500%, N: 0.080% to 0.400%, B: 0.0001 to 0.0100%, W: 0 to 4.00%, Nb: 0 to 0.10%, V: 0 to 0.20%, Ta: 0 to 0.30%, Co: 0 to 1.00%, Sn: 0 to 0.020%, Mg: 0 to 0.0200%, Ca: 0 to 0.0100% by mass, and the balance of Fe and impurities, and the weld metal 20 being C: 0.001% to 0.030%, Si: 0.05% to 0.70%, Mn: 0.05% to 0.85%, P: 0.030% or less, S: 0.0030% or less, Cr: 21.00% to 28.00%, Ni: 5.00% to 11.00%, Mo: 2.00% to 4.50%, Cu: 0.01 to 4.00%, Sol. Al: 0.0010 to 0.0500%, N: 0.080% to 0.400%, B: 0.0001 to 0.0100%, W: 0 to 4.00%, Nb: 0 to 0.10%, V: 0 to 0.20%, Ta: 0 to 0.30%, Co: 0 to 1.00%, Sn: 0 to 0.020%, Mg: 0 to 0.0200%, Ca: 0 to 0.0100% by mass, and the balance of Fe and impurities, was used, excellent strength and excellent corrosion resistance could be obtained.

[0075] Next, the present inventors et al. conducted a study on the duplex stainless steel welded joint 1 having the above-described chemical composition, for being able to suppress the occurrence of the cracks and the like in the weld metal 20, and the like. Figure 1The means of undercut 3 shown was investigated. As a result, the present inventors and others obtained the following understanding, which is different from the existing understanding. Note that, in this specification, molten metal refers to metal in a molten state (i.e., a molten pool) at the time of welding using a welding material. The molten metal solidifies to form the welded metal 20.

[0076] The duplex stainless steel has a high Cr content, and therefore the molten metal at the time of welding has high viscosity. Therefore, the molten metal at the time of welding has low wettability with the base material 10. If the wettability of the molten metal at the time of welding is low, the undercut 3 is easily produced. Thus, the present inventors and others considered that if the wettability of the molten metal at the time of welding is increased, the undercut 3 can be suppressed.

[0077] The present inventors and others first considered that if the viscosity of the molten metal at the time of welding is reduced, the wettability of the molten metal at the time of welding with the base material 10 is increased, and the undercut 3 can be suppressed. As described above, Cr increases the viscosity. Therefore, it can also be considered that the Cr content is reduced. However, in order to obtain the strength and corrosion resistance of the duplex stainless steel welded joint 1, it is necessary to contain a prescribed amount of Cr. Therefore, it is difficult to reduce the Cr content.

[0078] Thus, the present inventors and others investigated elements other than Cr that would affect the viscosity of the molten metal at the time of welding. As a result, the present inventors and others found that, in the duplex stainless steel having the above-described chemical composition, Si affects the viscosity of the molten metal at the time of welding.

[0079] Si increases the viscosity of the molten metal at the time of welding. Thus, the present inventors and others considered that the viscosity of the molten metal at the time of welding is reduced by reducing the amount of Si. Thereby, the outward Marangoni convection velocity at the time of welding is increased, and the molten metal at the time of welding becomes easy to spread in the width direction of the welded metal 20. As a result, it is considered that the wettability of the molten metal at the time of welding with the base material 10 is increased.

[0080] However, even if only the Si content is adjusted, the undercut 3 is still produced. Thus, the present inventors and others investigated a method for suppressing the undercut 3 from another perspective different from the viscosity of the molten metal at the time of welding.

[0081] Here, the present inventors and others focused on the surface tension of the molten metal at the time of welding. As a result, the following understanding was obtained.

[0082] Mn increases the temperature coefficient of the surface tension of the molten metal at the time of welding. By reducing the amount of Mn, the temperature coefficient of the surface tension of the molten metal at the time of welding is reduced. If the amount of Mn is reduced, the difference in surface tension between the central portion of the welded metal 20, which is high in temperature, and the peripheral portion of the welded metal 20, which is low in temperature, is reduced. As a result, the wettability of the molten metal at the time of welding with the base material 10 is increased.

[0083] The inventors further conducted detailed studies on the Si content and the Mn content in the weld metal 20. Furthermore, the inventors considered that if the Si content and the Mn content in the weld metal 20 are adjusted to a range having an appropriate relationship, the wettability of the molten metal at the time of welding improves, and the undercut 3 of the duplex stainless steel welded joint 1 can be suppressed. Therefore, the inventors conducted studies on the relationship between the Si content and the Mn content in the weld metal 20. As a result, it was found that in the duplex stainless steel having the above-described chemical composition, the following formula (1) needs to be satisfied.

[0084] Si + 3Mn ≤ 3.00 (1)

[0085] Here, the content of the corresponding element in the weld metal 20 is substituted for each element symbol in formula (1) in mass %.

[0086] By satisfying formula (1) while the chemical composition of the weld metal 20 is within the above-described range, the viscosity of the molten metal at the time of welding can be reduced, and furthermore, the temperature coefficient of the surface tension of the molten metal at the time of welding can be reduced. Thus, the wettability of the molten metal at the time of welding improves.

[0087] The inventors further conducted detailed studies and recognized that in the duplex stainless steel welded joint 1 having the above-described chemical composition, by further adjusting the shape of the weld metal 20 on the basis of satisfying formula (1) to improve the wettability of the molten metal at the time of welding, the undercut 3 can be suppressed. Specifically, by further causing the shape of the weld metal 20 to satisfy formula (2) on the basis of causing the weld metal 20 to satisfy formula (1), the undercut 3 can be suppressed.

[0088] 0 < BH / BW < 0.15 (2)

[0089] Here, BH in formula (2) is substituted for the Cap height (mm) of the weld metal 20, and BW is substituted for the Cap width (mm) of the weld metal 20.

[0090] If the surface tension of the molten metal at the time of welding is increased, the weld metal 20 formed becomes easy to be convex, and the undercut 3 is easy to occur. Therefore, if the convex shape of the weld metal 20 is further suppressed to satisfy formula (2) on the basis of satisfying the chemical composition and formula (1) to improve the wettability of the molten metal at the time of welding, the undercut 3 can be suppressed, and the fatigue characteristics improve.

[0091] The gist of the duplex stainless steel welded joint 1 of the present embodiment completed on the basis of the above recognition is as described below. [1]

[0093] A duplex stainless steel welded joint having a base material and a weld metal,

[0094] The base material is 0.0010 to 0.0500% by mass of

[0095] C: 0.001 to 0.030%,

[0096] Si: 0.05 to 0.80%,

[0097] Mn: 0.05 to 1.20%,

[0098] P: 0.030% or less,

[0099] S: 0.0030% or less,

[0100] Cr: 21.00 to 28.00%,

[0101] Ni: 4.00 to 8.00%,

[0102] Mo: 2.00 to 4.50%,

[0103] Cu: 0.01 to 4.00%,

[0104] Sol. Al: 0.0010 to 0.0500%,

[0105] N: 0.080 to 0.400%,

[0106] B: 0.0001 to 0.0100%,

[0107] W: 0 to 4.00%,

[0108] Nb: 0 to 0.10%,

[0109] V: 0 to 0.20%,

[0110] Ta: 0 to 0.30%,

[0111] Co: 0 to 1.00%,

[0112] Sn: 0 to 0.020%,

[0113] Mg: 0 to 0.0200%,

[0114] Ca: 0 to 0.0100% and

[0115] the balance being Fe and impurities,

[0116] The weld metal is 0.0010 to 0.0500% by mass of

[0117] C: 0.001 to 0.030%,

[0118] Si: 0.05 to 0.70%,

[0119] Mn: 0.05 to 0.85%,

[0120] P: 0.030% or less,

[0121] S: 0.0030% or less,

[0122] Cr: 21.00 to 28.00%,

[0123] Ni: 5.00 to 11.00%,

[0124] Mo: 2.00 to 4.50%,

[0125] Cu: 0.01 to 4.00%,

[0126] Sol. Al: 0.0010 to 0.0500%,

[0127] N: 0.080 to 0.400%,

[0128] B: 0.0001 to 0.0100%,

[0129] W: 0 to 4.00%,

[0130] Nb: 0 to 0.10%,

[0131] V: 0 to 0.20%,

[0132] Ta: 0 to 0.30%,

[0133] Co: 0 to 1.00%,

[0134] Sn: 0 to 0.020%,

[0135] Mg: 0 to 0.0200%,

[0136] Ca: 0 to 0.0100% and

[0137] the balance of Fe and impurities,

[0138] The aforementioned duplex stainless steel welded joint satisfies Formula (1) and Formula (2) while satisfying the content of each element in the aforementioned base material and the aforementioned weld metal.

[0139] Si + 3Mn ≤ 3.00 (1)

[0140] 0 < BH / BW < 0.15 (2)

[0141] Here, the content of each element symbol in Formula (1) is substituted with the content of the corresponding element in the aforementioned weld metal in mass %.

[0142] Here, BH in formula (2) is substituted with the Cap height (mm) of the aforementioned weld metal, and BW is substituted with the Cap width (mm) of the aforementioned weld metal. [2]

[0144] The duplex stainless steel welded joint according to [1], wherein

[0145] In the case where the thickness of the aforementioned base material is 2.5 mm or less, the aforementioned duplex stainless steel welded joint also satisfies formula (3) and formula (4).

[0146] Si + 4Mn ≤ 3.75 (3)

[0147] 0 < BH / BW ≤ 0.5 / (6.0-0.85WT) (4)

[0148] Here, the content of the corresponding element in the aforementioned weld metal is substituted with the mass % at each element symbol in formula (3).

[0149] Here, BH in formula (4) is substituted with the Cap height (mm) of the aforementioned weld metal, BW is substituted with the Cap width (mm) of the aforementioned weld metal, and WT is substituted with the thickness (mm) of the aforementioned base material. [3]

[0151] The duplex stainless steel welded joint according to [1] or [2], wherein

[0152] The aforementioned base material contains, by mass %, one or more elements selected from the group consisting of

[0153] W: 0.01 to 4.00%,

[0154] Nb: 0.01 to 0.10%,

[0155] V: 0.01 to 0.20%,

[0156] Ta: 0.01 to 0.30%,

[0157] Co: 0.01 to 1.00%,

[0158] Sn: 0.001 to 0.020%,

[0159] Mg: 0.0001 to 0.0200%, and

[0160] Ca: 0.0001 to 0.0100%. [4]

[0162] The duplex stainless steel welded joint according to any one of [1] to [3], wherein

[0163] The aforementioned weld metal contains, by mass %, one or more elements selected from the group consisting of

[0164] W: 0.01 to 4.00%,

[0165] Nb: 0.01 to 0.10%,

[0166] V: 0.01 to 0.20%,

[0167] Ta: 0.01 to 0.30%,

[0168] Co: 0.01 to 1.00%,

[0169] Sn: 0.001 to 0.020%,

[0170] Mg: 0.0001 to 0.0200%, and

[0171] Ca: 0.0001 to 0.0100% of one or more elements in the group.

[0172] [Configuration of the Welded Joint of the Present Embodiment]

[0173] Figure 2 is a cross-sectional view of the welded joint 1 of the duplex stainless steel of the present embodiment, which is cut in a direction perpendicular to the extension direction of the weld metal 20. Referring to Figure 2 , the welded joint 1 of the duplex stainless steel of the present embodiment has a pair of base metals 10 and a weld metal 20. The weld metal 20 is disposed between the pair of base metals 10. The weld metal 20 is disposed between the pair of base metals 10 and connects the pair of base metals 10.

[0174] The weld metal 20 is formed by welding the end portions of the pair of base metals 10 after butting the end portions of the pair of base metals 10 against each other. The welding can be performed using a publicly known welding method. The welding is, for example, Gas Tungsten Arc Welding (GTAW), Shielded Metal Arc Welding (SMAW), Flux Cored Arc Welding (FCAW), Gas Metal Arc Welding (GMAW), or Submerged Arc Welding (SAW).

[0175] Figure 3 is a cross-sectional view of the welded joint 1 of the duplex stainless steel of another embodiment, which is different from Figure 2 . The end portions of the base metal 10 can be beveled as shown in Figure 3 , or can be beveled as shown in Figure 2The end portion of the base material 10 is not bevel processed as shown. In the case where the thickness of the base material 10 is, for example, more than 2.5 mm, it is preferable to bevel process the end portion of the base material 10 and perform welding by so-called multi-layer surfacing which performs welding multiple times. In the case where the thickness of the base material 10 is, for example, 2.5 mm or less, it is possible to perform so-called single-layer welding which performs welding once without bevel processing the end portion of the base material 10. In the case where the end portion of the base material 10 is bevel processed, the shape of the bevel processing is not particularly limited. The shape of the bevel of the end portion of the base material 10 is, for example, selected from the group consisting of a V-shaped bevel, a U-shaped bevel, and an X-shaped bevel.

[0176] [About the Base Material]

[0177] The base material 10 of the present embodiment will be described.

[0178] [Chemical Composition of the Base Material]

[0179] The chemical composition of the base material 10 of the present embodiment contains the following elements.

[0180] C: 0.001 to 0.030%

[0181] Carbon (C) is an element effective for stabilizing the austenite phase. If the C content is too low, even if the contents of the other elements are within the range of the present embodiment, the amount of austenite in the weld metal 20 in the as-welded state decreases, and the corrosion resistance and strength of the weld metal 20 decrease. On the other hand, if the C content is too high, even if the contents of the other elements are within the range of the present embodiment, carbides are easily precipitated, and the corrosion resistance of the base material 10 decreases. Therefore, the C content is 0.001 to 0.030%. The preferable lower limit of the C content is 0.002%, more preferably 0.003%, further preferably 0.005%, and further preferably 0.010%. The preferable upper limit of the C content is 0.028%, more preferably 0.025%, further preferably 0.023%, and further preferably 0.020%.

[0182] Si: 0.05 to 0.80%

[0183] Silicon (Si) deoxidizes steel. If the Si content is too low, the effect cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, Si stabilizes the ferrite phase. If the Si content is too high, the amount of ferrite in the weld metal 20 in the as-welded state increases even if the contents of other elements are within the range of the present embodiment, and the corrosion resistance and strength of the weld metal 20 decrease. Si also increases the viscosity of molten metal at the time of welding. Therefore, the Si content is 0.05 to 0.80%. The preferable lower limit of the Si content is 0.08%, more preferably 0.10%, further preferably 0.15%, further preferably 0.20%, further preferably 0.25%, further preferably 0.30%, further preferably 0.40%, further preferably 0.50%. The preferable upper limit of the Si content is 0.75%, more preferably 0.70%, further preferably 0.65%, further preferably 0.60%.

[0184] Mn: 0.05 to 1.20%

[0185] Manganese (Mn) stabilizes the austenite phase. If the Mn content is too low, the amount of austenite in the weld metal 20 in the as-welded state decreases even if the contents of other elements are within the range of the present embodiment, and the corrosion resistance and strength of the weld metal 20 also decrease. On the other hand, Mn increases the temperature coefficient of the surface tension of molten metal at the time of welding. In other words, Mn increases the temperature dependence of the surface tension of molten metal at the time of welding. If the Mn content is too high, the wettability of molten metal at the time of welding decreases even if the contents of other elements are within the range of the present embodiment. Therefore, the Mn content is 0.05 to 1.20%. The preferable lower limit of the Mn content is 0.08%, more preferably 0.10%, further preferably 0.20%, further preferably 0.30%, further preferably 0.40%, further preferably 0.50%, further preferably 0.60%, further preferably 0.70%. The preferable upper limit of the Mn content is 1.10%, more preferably 1.00%, further preferably 0.90%, further preferably 0.80%, further preferably 0.70%.

[0186] P: 0.030% or less

[0187] Phosphorus (P) is an impurity that is inevitably contained. That is, the lower limit of the P content is greater than 0%. P significantly increases the crack sensitivity during hot working. Therefore, the P content is 0.030% or less. The preferable upper limit of the P content is 0.028%, more preferably 0.025%, further preferably 0.023%, further preferably 0.020%. The P content is preferably as low as possible. However, an extreme reduction in the P content leads to an increase in the manufacturing cost. Therefore, the preferable lower limit of the P content is 0.001%, more preferably 0.002% in consideration of the industrial productivity.

[0188] S: 0.0030% or less

[0189] Sulfur (S) is an impurity that is inevitably contained. That is, the lower limit of the S content is greater than 0%. S significantly increases the crack sensitivity during hot working. Therefore, the S content is 0.0030% or less. The preferable upper limit of the S content is 0.0025%, more preferably 0.0020%, further preferably 0.0015%, further preferably 0.0010%. The S content is preferably as low as possible. However, an extreme reduction in the S content leads to an increase in the manufacturing cost. Therefore, the preferable lower limit of the S content is 0.0001%, more preferably 0.0002% in consideration of the industrial productivity.

[0190] Cr: 21.00 to 28.00%

[0191] Chromium (Cr) improves the corrosion resistance of the base material 10. If the Cr content is too low, the pitting corrosion resistance of the base material 10 decreases even if the contents of the other elements are within the range of the present embodiment. On the other hand, if the Cr content is too high, intermetallic compounds such as sigma phase are easily precipitated even if the contents of the other elements are within the range of the present embodiment, and the hot workability, toughness, and corrosion resistance of the base material 10 decrease. Therefore, the Cr content is 21.00 to 28.00%. The preferable lower limit of the Cr content is 21.50%, more preferably 22.00%, further preferably 22.50%, further preferably 23.00%, further preferably 23.50%, further preferably 24.00%. The preferable upper limit of the Cr content is 27.50%, more preferably 27.00%, further preferably 26.50%, further preferably 26.00%.

[0192] Ni: 4.00 to 8.00%

[0193] Nickel (Ni) stabilizes the austenite phase. If the Ni content is too low, the amount of austenite in the base material 10 decreases even if the contents of the other elements are within the ranges of the present embodiment, and the corrosion resistance and strength of the base material 10 decrease. On the other hand, if the Ni content is too high, the amount of ferrite in the base material 10 decreases even if the contents of the other elements are within the ranges of the present embodiment, and the corrosion resistance and strength of the base material 10 decrease. In this case, a sigma phase is also precipitated in the base material 10. Therefore, the Ni content is 4.00 to 8.00%. The preferable lower limit of the Ni content is 4.50%, more preferably 5.00%, further preferably 5.50%, further preferably 6.00%. The preferable upper limit of the Ni content is 7.50%, more preferably 7.00%.

[0194] Mo: 2.00 to 4.50%

[0195] Molybdenum (Mo) improves the corrosion resistance of the base material 10 as does Cr. If the Mo content is too low, the pitting corrosion resistance and crevice corrosion resistance of the base material 10 decrease even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the Mo content is too high, a sigma phase is easily precipitated even if the contents of the other elements are within the ranges of the present embodiment, the workability of the base material 10 decreases, and the toughness and corrosion resistance of the base material 10 decrease. Therefore, the Mo content is 2.00 to 4.50%. The preferable lower limit of the Mo content is 2.20%, more preferably 2.40%, further preferably 2.50%. The preferable upper limit of the Mo content is 4.30%, more preferably 4.00%.

[0196] Cu: 0.01 to 4.00%

[0197] Copper (Cu) improves the acid resistance of the base material 10 in a sulfuric acid, hydrogen sulfide environment. If the Cu content is too low, the acid resistance of the base material 10 decreases even if the contents of the other elements are within the ranges of the present embodiment. However, if the Cu content is too high, the hot workability of the base material 10 decreases even if the contents of the other elements are within the ranges of the present embodiment. Therefore, the Cu content is 0.01 to 4.00%. The preferable lower limit of the Cu content is 0.05%, further preferably 0.10%, further preferably 0.15%, further preferably 0.20%, further preferably 0.25%, further preferably 0.30%, further preferably 0.35%, further preferably 0.40%, further preferably 0.45%. The preferable upper limit of the Cu content is 3.50%, more preferably 3.00%, further preferably 2.50%, further preferably 2.00%, further preferably 1.50%, further preferably 1.00%.

[0198] Sol. Al: 0.0010 to 0.0500%

[0199] Al deoxidizes steel. If the Al content is too low, the effect cannot be obtained even if the contents of other elements are within the range of the present embodiment. However, if the Al content is too high, AlN is precipitated and the toughness and corrosion resistance of the base material 10 are degraded even if the contents of other elements are within the range of the present embodiment. Therefore, the Sol. Al content is 0.0010 to 0.0500%. The preferable lower limit of the Al content is 0.0030%, more preferably 0.0050%, further preferably 0.0080%, further preferably 0.0100%, further preferably 0.0120%. The preferable upper limit of the Al content is 0.0400%, more preferably 0.0300%, further preferably 0.0200%. Note that the Al content referred to in the present specification means the content of "acid-soluble Al", i.e., Sol. Al.

[0200] N: 0.080 to 0.400%

[0201] N stabilizes the austenite phase and increases PREW, improving the pitting corrosion resistance and crevice corrosion resistance of the base material 10. If the N content is too low, the ferrite phase and the austenite phase of the base material 10 are unbalanced even if the contents of other elements are within the range of the present embodiment, and the corrosion resistance and strength of the base material 10 are degraded. However, if the N content is too high, defects such as pores are generated at the time of welding even if the contents of other elements are within the range of the present embodiment. Therefore, the N content is 0.080 to 0.400%. The preferable lower limit of the N content is 0.100%, more preferably 0.150%, further preferably 0.200%. The preferable upper limit of the N content is 0.370%, more preferably 0.350%, further preferably 0.320%.

[0202] B: 0.0001 to 0.0100%

[0203] B segregates at the grain boundaries at high temperatures, improving the hot workability of the base material 10. Furthermore, B is also effective as a deoxidizer. If the B content is too low, the effect cannot be obtained even if the contents of other elements are within the range of the present embodiment. However, if the B content is too high, solidification segregation is generated during solidification at the welded portion even if the contents of other elements are within the range of the present embodiment, and the solidification cracking sensitivity of the weld metal 20 increases. Therefore, the B content is 0.0001 to 0.0100%. The preferable lower limit of the B content is 0.0005%, more preferably 0.0010%, further preferably 0.0015%. The preferable upper limit of the B content is 0.0080%, further preferably 0.0070%, further preferably 0.0060%, further preferably 0.0050%, further preferably 0.0040%, further preferably 0.0030%, further preferably 0.0020%.

[0204] The balance of the base material 10 of the present embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from ores, scrap, or manufacturing environments, etc. when the base material 10 is industrially manufactured, and are acceptable within a range that does not adversely affect the duplex stainless steel welded joint 1 of the present embodiment. The impurities in the base material 10 are, for example, O (oxygen) and REM.

[0205] [Optional Elements]

[0206] The above-described base material 10 can also contain one or more elements selected from the group consisting of the following Group 1 to Group 5 in place of a portion of Fe.

[0207] [Group 1]

[0208] The above-described base material 10 can also contain W in place of a portion of Fe. W is an optional element that forms an oxide and improves the corrosion resistance of the base material 10.

[0209] W: 0 to 4.00%

[0210] Tungsten (W) is an optional element and can not be contained. That is, the W content can be 0%. In the case of being contained, W forms a stable oxide and improves the corrosion resistance of the base material 10 in a low-pH environment. As long as a small amount of W is contained, the above-described effect can be obtained to some extent. However, if the W content is too high, even if the contents of the other elements are within the range of the present embodiment, the toughness of the base material 10 can decrease due to the promotion of intermetallic compound precipitation. Therefore, the W content is 0 to 4.00%. The preferable lower limit of the W content is greater than 0%, more preferably 0.01%, further preferably 0.50%, and further preferably 1.00%. The preferable upper limit of the W content is 3.50%, more preferably 3.00%, and further preferably 2.50%.

[0211] [Group 2]

[0212] The above-described base material 10 can also contain one or more elements selected from the group consisting of Nb, V, and Ta in place of a portion of Fe. These elements are all optional elements that form carbides and improve the corrosion resistance of the base material 10.

[0213] Nb: 0 to 0.10%

[0214] Niobium (Nb) is an optional element and can not be contained. That is, the content of Nb can be 0%. In the case of being contained, Nb combines with C to form a carbide. Thereby, the generation of Cr carbide at the grain boundaries is suppressed, and the corrosion resistance of the base material 10 is improved. As long as a small amount of Nb is contained, the above effect is obtained to some extent. However, if the content of Nb is too high, even if the contents of other elements are within the range of the present embodiment, an excessive amount of carbide is precipitated, and thus the corrosion resistance of the base material 10 can decrease. Therefore, the content of Nb is 0 to 0.10%. The preferable lower limit of the content of Nb is more than 0%, more preferably 0.01%, and further preferably 0.02%. The preferable upper limit of the content of Nb is 0.08%, more preferably 0.07%, further preferably 0.05%, and further preferably 0.03%.

[0215] V: 0 to 0.20%

[0216] Vanadium (V) is an optional element and can not be contained. That is, the content of V can be 0%. In the case of being contained, V combines with C to form a carbide. Thereby, the generation of Cr carbide at the grain boundaries is suppressed, and the corrosion resistance of the base material 10 is improved. As long as a small amount of V is contained, the above effect is obtained to some extent. However, if the content of V is too high, even if the contents of other elements are within the range of the present embodiment, an excessive amount of carbide is precipitated, and thus the corrosion resistance of the base material 10 can decrease. Therefore, the content of V is 0 to 0.20%. The preferable lower limit of the content of V is more than 0%, more preferably 0.01%, further preferably 0.02%, and further preferably 0.05%. The preferable upper limit of the content of V is 0.18%, more preferably 0.15%, further preferably 0.10%, further preferably 0.08%, further preferably 0.07%, and further preferably 0.05%.

[0217] Ta: 0 to 0.30%

[0218] Tantalum (Ta) is an optional element and can not be contained. That is, the content of Ta can be 0%. In the case of being contained, Ta forms a carbide in combination with C. Thereby, the generation of Cr carbide at the grain boundaries is suppressed, and the corrosion resistance of the base material 10 is improved. As long as a small amount of Ta is contained, the above-mentioned effects can be obtained to some extent. However, if the content of Ta is too high, even if the contents of the other elements are within the range of the present embodiment, an excessive amount of carbide is precipitated, and thus the corrosion resistance of the base material 10 can be decreased. Therefore, the content of Ta is 0 to 0.30%. The preferable lower limit of the content of Ta is more than 0%, more preferably 0.01%, further preferably 0.02%, further preferably 0.05%, further preferably 0.10%, further preferably 0.15%. The preferable upper limit of the content of Ta is 0.27%, more preferably 0.25%, further preferably 0.20%, further preferably 0.15%, further preferably 0.10%, further preferably 0.08%, further preferably 0.07%, further preferably 0.05%.

[0219] [Group 3]

[0220] The chemical composition of the above-mentioned base material 10 can also contain Co instead of a part of Fe. Co is an optional element, which improves the acid resistance of the base material 10.

[0221] Co: 0 to 1.00%

[0222] Cobalt (Co) is an optional element and can not be contained. That is, the content of Co can be 0%. In the case of being contained, Co improves the acid resistance of the base material 10 and stabilizes the austenite phase. As long as a small amount of Co is contained, the above-mentioned effects can be obtained to some extent. However, if the content of Co is too high, even if the contents of the other elements are within the range of the present embodiment, the cost is increased. Therefore, the content of Co is 0 to 1.00%. The preferable lower limit of the content of Co is 0.01%, more preferably 0.05%, further preferably 0.10%, further preferably 0.15%, further preferably 0.20%, further preferably 0.25%, further preferably 0.30%. The preferable upper limit of the content of Co is 0.90%, more preferably 0.80%, further preferably 0.70%, further preferably 0.65%, further preferably 0.60%.

[0223] [Group 4]

[0224] The chemical composition of the above-mentioned base material 10 can also contain Sn instead of a part of Fe. Sn is an optional element, which improves the pitting corrosion resistance of the base material 10.

[0225] Sn: 0 to 0.020%

[0226] Tin (Sn) is an optional element and can not be contained. That is, the Sn content can be 0%. In the case of being contained, Sn improves the pitting corrosion resistance of the base material 10. As long as a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content is too high, the hot workability of the base material 10 can be decreased even if the contents of other elements are within the range of the present embodiment. Furthermore, if the Sn content is too high, the penetration depth can be increased even if the contents of other elements are within the range of the present embodiment, and the wettability of the molten metal at the time of welding decreases. Therefore, the Sn content is 0 to 0.020%. The preferable lower limit of the Sn content is 0.001%, more preferably 0.002%, further preferably 0.003%. The preferable upper limit of the Sn content is 0.018%, more preferably 0.015%, further preferably 0.010%, further preferably 0.009%, more preferably 0.008%, further preferably 0.007%.

[0227] [Group 5]

[0228] The chemical composition of the above-described base material 10 can also contain one or more elements selected from the group consisting of Mg and Ca instead of a part of Fe. Mg and Ca are optional elements and improve the hot workability of the base material 10.

[0229] Mg: 0 to 0.0200%

[0230] Magnesium (Mg) is an optional element and can not be contained. That is, the Mg content can be 0%. In the case of being contained, Mg improves the hot workability of the base material 10. As long as a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content is too high, Mg can combine with oxygen even if the contents of other elements are within the range of the present embodiment, and the cleanliness can significantly decrease, and the hot workability of the base material 10 can decrease instead. Therefore, the Mg content is 0 to 0.0200%. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0002%, further preferably 0.0003%, further preferably 0.0005%, further preferably 0.0010%. The preferable upper limit of the Mg content is 0.0150%, more preferably 0.0130%, further preferably 0.0100%, further preferably 0.0090%, further preferably 0.0080%, further preferably 0.0070%, further preferably 0.0060%, further preferably 0.0050%, further preferably 0.0040%, further preferably 0.0030%, further preferably 0.0020%.

[0231] Ca: 0 to 0.0100%

[0232] Calcium (Ca) is an optional element, and can not be contained. That is, the Ca content can be 0%. In the case of being contained, Ca is effective as a deoxidizer. Ca also fixes S to improve the hot workability of the base material 10. As long as a small amount of Ca is contained, the above effects are obtained to some extent. However, if the Ca content is too high, even if the contents of the other elements are within the ranges of the present embodiment, Ca combines with oxygen, and the cleanliness of the base material 10 significantly decreases, and the hot workability of the base material 10 decreases. Therefore, the Ca content is 0 to 0.0100%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0002%, further preferably 0.0005%, further preferably 0.0010%, further preferably 0.0020%. The preferable upper limit of the Ca content is 0.0090%, more preferably 0.0080%, further preferably 0.0070%, further preferably 0.0060%, further preferably 0.0050%, further preferably 0.0040%, further preferably 0.0030%.

[0233] [Microstructure of Base Material]

[0234] The microstructure of the base material 10 of the present embodiment is composed of ferrite and austenite. In the present specification, "composed of ferrite and austenite" means that the phases other than ferrite and austenite are so small as to be negligible. For example, in the microstructure of the base material 10 of the present embodiment, the volume fraction of precipitates, inclusions, and the like is so small as to be negligible compared to the volume fraction of ferrite and austenite. That is, the microstructure of the base material 10 of the present embodiment can contain a small amount of precipitates, inclusions, and the like in addition to ferrite and austenite.

[0235] [Shape of Base Material]

[0236] The shape of the base material 10 is not particularly limited. The base material 10 can be, for example, one selected from the group consisting of a plate, a steel pipe, a bar steel, a wire, a forged product, and a shaped steel.

[0237] [Thickness of Base Material]

[0238] The thickness of the base material 10 is not particularly limited. The thickness of the base material 10 is, for example, 1.0 to 50.0 mm.

[0239] [About Weld Metal]

[0240] The weld metal 20 will be described.

[0241] [Chemical Composition of Weld Metal]

[0242] The chemical composition of the weld metal 20 of the present embodiment contains the following elements.

[0243] C: 0.001 to 0.030%

[0244] Carbon (C) is an element effective in stabilizing the austenite phase. If the C content is too low, even if the contents of the other elements are within the range of the present embodiment, the amount of austenite in the weld metal 20 in the as-welded state decreases, and the corrosion resistance and strength of the weld metal 20 decrease. On the other hand, if the C content is too high, even if the contents of the other elements are within the range of the present embodiment, it becomes easy to precipitate carbides, and the corrosion resistance of the weld metal 20 decreases. Therefore, the C content is 0.001 to 0.030%. The preferable lower limit of the C content is 0.002%, more preferably 0.003%, further preferably 0.005%, further preferably 0.010%, further preferably 0.015%. The preferable upper limit of the C content is 0.025%, more preferably 0.020%.

[0245] Si: 0.05 to 0.70%

[0246] Silicon (Si) deoxidizes the steel. If the Si content is too low, even if the contents of the other elements are within the range of the present embodiment, the effect cannot be sufficiently obtained. On the other hand, Si stabilizes the ferrite phase. If the Si content is too high, even if the contents of the other elements are within the range of the present embodiment, the amount of ferrite in the weld metal 20 in the as-welded state increases, and the corrosion resistance and strength of the weld metal 20 decrease. Si also increases the viscosity of the molten metal at the time of welding. In order to suppress the undercut 3, the Si content needs to be further limited in the chemical composition of the weld metal 20 compared to the Si content of the base material 10. Therefore, the Si content is 0.05 to 0.70%. The preferable lower limit of the Si content is 0.08%, more preferably 0.10%, further preferably 0.20%, further preferably 0.30%, further preferably 0.40%, further preferably 0.50%, further preferably 0.60%. The preferable upper limit of the Si content is 0.65%, more preferably 0.60%, further preferably 0.55%, further preferably 0.50%.

[0247] Mn: 0.05 to 0.85%

[0248] Manganese (Mn) stabilizes the austenite phase. If the Mn content is too low, the amount of austenite in the weld metal in the as-welded state decreases even if the contents of the other elements are within the ranges of the present embodiment, and the corrosion resistance and strength of the weld metal 20 decrease. On the other hand, Mn increases the temperature coefficient of the surface tension of the molten metal at the time of welding. In other words, Mn increases the temperature dependence of the surface tension of the molten metal at the time of welding. If the Mn content is too high, the wettability of the molten metal at the time of welding decreases even if the contents of the other elements are within the ranges of the present embodiment. In order to suppress the undercut 3, the Mn content needs to be further limited in the chemical composition of the weld metal 20 compared to the Mn content of the base metal 10. Therefore, the Mn content is 0.05 to 0.85%. The preferable lower limit of the Mn content is 0.08%, more preferably 0.10%, further preferably 0.15%, further preferably 0.20%. The preferable upper limit of the Mn content is 0.80%, more preferably 0.75%, further preferably 0.70%.

[0249] P: 0.030% or less

[0250] Phosphorus (P) is an impurity that is inevitably contained. That is, the lower limit of the P content is greater than 0%. P significantly increases the weld crack sensitivity of the weld metal 20. Therefore, the P content is 0.030% or less. The preferable upper limit of the P content is 0.028%, more preferably 0.025%, further preferably 0.023%, further preferably 0.020%. The P content is preferably as low as possible. However, extreme reduction of the P content leads to an increase in manufacturing cost. Therefore, the preferable lower limit of the P content is 0.001%, more preferably 0.002% in consideration of industrial productivity.

[0251] S: 0.0030% or less

[0252] Sulfur (S) is an impurity that is inevitably contained. That is, the lower limit of the S content is greater than 0%. S significantly increases the weld crack sensitivity of the weld metal 20. Therefore, the S content is 0.0030% or less. The preferable upper limit of the S content is 0.0025%, further preferably 0.0020%, further preferably 0.0015%, further preferably 0.0010%. The S content is preferably as low as possible. However, extreme reduction of the S content leads to an increase in manufacturing cost. Therefore, the preferable lower limit of the S content is 0.0001%, more preferably 0.0002% in consideration of industrial productivity.

[0253] Cr: 21.00 to 28.00%

[0254] Chromium (Cr) improves the corrosion resistance of the weld metal 20. If the Cr content is too low, even if the contents of the other elements are within the range of the present embodiment, the pitting corrosion resistance of the weld metal 20 will decrease. On the other hand, if the Cr content is too high, even if the contents of the other elements are within the range of the present embodiment, intermetallic compounds such as sigma phase are likely to precipitate, and the hot workability, toughness, and corrosion resistance of the weld metal 20 will decrease. Therefore, the Cr content is 21.00 to 28.00%. The preferable lower limit of the Cr content is 21.50%, more preferably 22.00%, further preferably 22.50%, further preferably 23.00%, further preferably 23.50%, further preferably 24.00%. The preferable upper limit of the Cr content is 27.50%, more preferably 27.00%, further preferably 26.50%, further preferably 26.00%.

[0255] Ni: 5.00 to 11.00%

[0256] Nickel (Ni) stabilizes the austenite phase. If the Ni content is too low, even if the contents of the other elements are within the range of the present embodiment, the amount of austenite in the weld metal will decrease, and the corrosion resistance and strength of the weld metal 20 will decrease. On the other hand, if the Ni content is too high, even if the contents of the other elements are within the range of the present embodiment, the amount of ferrite in the weld metal 20 will decrease, and the corrosion resistance and strength of the weld metal 20 will decrease. In this case, sigma phase will also precipitate in the weld metal. In the weld metal 20, in order to promote the phase transition from ferrite to austenite during the rapid cooling at the time of welding while controlling the amount of ferrite to have the same properties as the base material 10, it is preferable that the Ni content be higher than that of the base material 10. Therefore, the Ni content is 5.00 to 11.00%. The preferable lower limit of the Ni content is 5.50%, more preferably 6.00%, further preferably 6.50%, further preferably 7.00%, further preferably 7.50%. The preferable upper limit of the Ni content is 10.50%, more preferably 10.00%, further preferably 9.50%, further preferably 9.00%, further preferably 8.50%, further preferably 8.00%.

[0257] Mo: 2.00 to 4.50%

[0258] Molybdenum (Mo) also improves the corrosion resistance of the weld metal 20 as with Cr. If the Mo content is too low, the pitting resistance and crevice corrosion resistance of the weld metal 20 will decrease even if the contents of the other elements are within the ranges of the present embodiment. On the other hand, if the Mo content is too high, the σ phase will precipitate even if the contents of the other elements are within the ranges of the present embodiment, the workability of the weld metal 20 will decrease and the toughness and corrosion resistance of the weld metal 20 will decrease. Therefore, the Mo content is 2.00 to 4.50%. The preferable lower limit of the Mo content is 2.50%, more preferably 3.00%. The preferable upper limit of the Mo content is 4.30%, more preferably 4.00%.

[0259] Cu: 0.01 to 4.00%

[0260] Copper (Cu) improves the acid resistance of the weld metal 20 in a sulfuric acid, hydrogen sulfide environment. If the Cu content is too low, the acid resistance of the weld metal 20 will decrease even if the contents of the other elements are within the ranges of the present embodiment. However, if the Cu content is too high, the solid solution amount of N will decrease, denitrogenation during welding will accelerate, and it will become easy to produce welding defects such as gas pores even if the contents of the other elements are within the ranges of the present embodiment. Therefore, the Cu content is 0.01 to 4.00%. The preferable lower limit of the Cu content is 0.20%, more preferably 0.30%, further preferably 0.40%, further preferably 0.50%. The preferable upper limit of the Cu content is 3.50%, more preferably 3.00%, further preferably 2.50%, further preferably 2.00%, further preferably 1.50%, further preferably 1.00%.

[0261] Sol. Al: 0.0010 to 0.0500%

[0262] Aluminum (Al) deoxidizes the steel. Al also reduces the oxygen amount in the molten metal at the time of welding, and suppresses the convex shape of the weld metal 20. If the Al content is too low, the effect will not be obtained even if the contents of the other elements are within the ranges of the present embodiment. However, if the Al content is too high, AlN will precipitate and the toughness and corrosion resistance of the weld metal 20 will decrease even if the contents of the other elements are within the ranges of the present embodiment. Therefore, the Sol. Al content is 0.0010 to 0.0500%. The preferable lower limit of the Al content is 0.0030%, more preferably 0.0050%, further preferably 0.0100%, further preferably 0.0150%. The preferable upper limit of the Al content is 0.0400%, more preferably 0.0350%, further preferably 0.0300%, further preferably 0.0250%, further preferably 0.0200%. Note that the Al content referred to in the present specification means the content of "acid-soluble Al", i.e., Sol. Al.

[0263] N: 0.080 to 0.400%

[0264] Nitrogen (N) stabilizes the austenite phase and increases the PREW, improving the pitting corrosion resistance and crevice corrosion resistance of the weld metal 20. If the N content is too low, even if the contents of the other elements are within the ranges of the present embodiment, the ferrite phase and the austenite phase of the weld metal 20 become imbalanced, and the corrosion resistance and strength of the weld metal 20 decrease. However, if the N content is too high, even if the contents of the other elements are within the ranges of the present embodiment, defects such as pores are generated at the time of welding. Therefore, the N content is 0.080 to 0.400%. The preferable lower limit of the N content is 0.100%, more preferably 0.150%, further preferably 0.200%, further preferably 0.250%. The preferable upper limit of the N content is 0.370%, more preferably 0.350%, further preferably 0.320%.

[0265] B: 0.0001 to 0.0100%

[0266] Boron (B) segregates at the grain boundaries at high temperatures, improving the hot workability of the weld metal 20. Furthermore, B is also effective as a deoxidizer. If the B content is too low, even if the contents of the other elements are within the ranges of the present embodiment, the effect cannot be obtained. However, if the B content is too high, even if the contents of the other elements are within the ranges of the present embodiment, solidification segregation occurs during the solidification of the welded portion, and the solidification cracking sensitivity of the weld metal 20 increases. Therefore, the B content is 0.0001 to 0.0100%. The preferable lower limit of the B content is 0.0005%, more preferably 0.0010%. The preferable upper limit of the B content is 0.0080%, more preferably 0.0070%, further preferably 0.0060%, further preferably 0.0050%, further preferably 0.0040%, further preferably 0.0030%, further preferably 0.0020%.

[0267] The balance of the weld metal 20 of the present embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed from the welding material, the base material 10, and the like at the time of forming the weld metal 20 by welding, and are acceptable within a range that does not adversely affect the duplex stainless steel welded joint 1 of the present embodiment. The impurities in the weld metal 20 are, for example, O (oxygen) and REM.

[0268] [Optional Elements]

[0269] The weld metal 20 described above can also contain one or more elements selected from the group consisting of the following Group 1 to Group 5 in place of a part of Fe.

[0270] [Group 1]

[0271] The chemical composition of the weld metal 20 described above can also contain W instead of a part of Fe. W is an optional element that forms an oxide and improves the corrosion resistance of the weld metal 20.

[0272] W: 0 to 4.00%

[0273] Tungsten (W) is an optional element and can not be contained. That is, the content of W can be 0%. In the case of being contained, W forms a stable oxide and improves the corrosion resistance of the weld metal 20 in a low-pH environment. As long as a small amount of W is contained, the above effect can be obtained to some extent. However, if the content of W is too high, even if the contents of the other elements are within the range of the present embodiment, the precipitation of intermetallic compounds is promoted and the toughness of the weld metal 20 is decreased. Therefore, the content of W is 0 to 4.00%. The preferable lower limit of the content of W is more than 0%, more preferably 0.01%, further preferably 0.05%, further preferably 0.50%, further preferably 1.00%, further preferably 1.50%, further preferably 2.00%. The preferable upper limit of the content of W is 3.50%, more preferably 3.00%, further preferably 2.50%.

[0274] [Group 2]

[0275] The chemical composition of the weld metal 20 described above can also contain one or more elements selected from the group consisting of Nb, V, and Ta instead of a part of Fe. These elements are all optional elements that form carbides and improve the corrosion resistance of the weld metal 20.

[0276] Nb: 0 to 0.10%

[0277] Niobium (Nb) is an optional element and can not be contained. That is, the content of Nb can be 0%. In the case of being contained, Nb combines with C to form carbides. Thereby, the generation of Cr carbides at the grain boundaries is suppressed and the corrosion resistance of the weld metal 20 is improved. As long as a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the content of Nb is too high, even if the contents of the other elements are within the range of the present embodiment, an excessive amount of carbides is precipitated and thus the corrosion resistance of the weld metal 20 is rather decreased. Therefore, the content of Nb is 0 to 0.10%. The preferable lower limit of the content of Nb is more than 0%, more preferably 0.01%, further preferably 0.02%. The preferable upper limit of the content of Nb is 0.08%, more preferably 0.07%, further preferably 0.05%.

[0278] V: 0 to 0.20%

[0279] Vanadium (V) is an optional element and can not be contained. That is, the V content can be 0%. In the case of being contained, V combines with C to form a carbide. Thereby, the generation of Cr carbide at the grain boundaries is suppressed, and the corrosion resistance of the weld metal 20 is improved. As long as a small amount of V is contained, the above effect is obtained to some extent. However, if the V content is too high, even if the contents of other elements are within the range of the present embodiment, an excessive amount of carbide is precipitated, and thus the corrosion resistance of the weld metal 20 can rather decrease. Therefore, the V content is 0 to 0.20%. The preferable lower limit of the V content is more than 0%, more preferably 0.01%, and further preferably 0.02%. The preferable upper limit of the V content is 0.18%, more preferably 0.15%, further preferably 0.10%, further preferably 0.08%, more preferably 0.07%, and further preferably 0.05%.

[0280] Ta: 0 to 0.30%

[0281] Tantalum (Ta) is an optional element and can not be contained. That is, the Ta content can be 0%. In the case of being contained, Ta combines with C to form a carbide. Thereby, the generation of Cr carbide at the grain boundaries is suppressed, and the corrosion resistance of the weld metal 20 is improved. As long as a small amount of Ta is contained, the above effect is obtained to some extent. However, if the Ta content is too high, even if the contents of other elements are within the range of the present embodiment, an excessive amount of carbide is precipitated, and thus the corrosion resistance of the weld metal 20 can rather decrease. Therefore, the Ta content is 0 to 0.30%. The preferable lower limit of the Ta content is more than 0%, more preferably 0.01%, and further preferably 0.02%. The preferable upper limit of the Ta content is 0.27%, more preferably 0.25%, further preferably 0.20%, further preferably 0.15%, further preferably 0.10%, further preferably 0.08%, more preferably 0.07%, and further preferably 0.05%.

[0282] [Group 3]

[0283] The chemical composition of the above-described weld metal 20 can also contain Co instead of a part of Fe. Co is an optional element, which improves the acid resistance of the weld metal 20.

[0284] Co: 0 to 1.00%

[0285] Cobalt (Co) is an optional element, and can not be contained. That is, the Co content can be 0%. In the case of being contained, Co improves the acid resistance of the weld metal 20, and stabilizes the austenite phase. As long as a small amount of Co is contained, the above-mentioned effects can be obtained to some extent. However, if the Co content is too high, the cost increases even if the contents of the other elements are within the range of the present embodiment. Therefore, the Co content is 0 to 1.00%. The preferable lower limit of the Co content is 0.01%, more preferably 0.05%, further preferably 0.10%, further preferably 0.20%. The preferable upper limit of the Co content is 0.90%, more preferably 0.80%, further preferably 0.70%, further preferably 0.60%, further preferably 0.50%.

[0286] [Group 4]

[0287] The chemical composition of the weld metal 20 described above can also contain Sn instead of a part of Fe. Sn is an optional element, and improves the pitting corrosion resistance of the weld metal 20.

[0288] Sn: 0 to 0.020%

[0289] Tin (Sn) is an optional element, and can not be contained. That is, the Sn content can be 0%. In the case of being contained, Sn improves the pitting corrosion resistance of the weld metal 20. As long as a small amount of Sn is contained, the above-mentioned effects can be obtained to some extent. However, if the Sn content is too high, the hot workability of the weld metal 20 decreases even if the contents of the other elements are within the range of the present embodiment. If the Sn content is too high, the penetration depth further increases even if the contents of the other elements are within the range of the present embodiment, and the wettability of the molten metal at the time of welding decreases. Therefore, the Sn content is 0 to 0.020%. The preferable lower limit of the Sn content is 0.001%, more preferably 0.002%, further preferably 0.003%. The preferable upper limit of the Sn content is 0.018%, more preferably 0.015%, further preferably 0.010%, further preferably 0.009%, further preferably 0.008%, further preferably 0.007%, further preferably 0.005%.

[0290] [Group 5]

[0291] The chemical composition of the weld metal 20 described above can also contain one or more elements selected from the group consisting of Mg and Ca instead of a part of Fe. Mg and Ca are optional elements, and improve the hot workability of the weld metal 20.

[0292] Mg: 0 to 0.0200%

[0293] Magnesium (Mg) is an optional element and can not be contained. That is, the content of Mg can be 0%. In the case of being contained, Mg improves the wettability of molten metal at the time of welding by the effect of deoxidation. As long as a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the content of Mg is too high, even if the contents of other elements are within the range of the present embodiment, Mg combines with oxygen, and the cleanliness of the weld metal 20 significantly decreases, and the toughness of the weld metal 20 decreases. Therefore, the content of Mg is 0 to 0.0200%. The preferable lower limit of the content of Mg is 0.0001%, more preferably 0.0002%, further preferably 0.0003%. The preferable upper limit of the content of Mg is 0.0150%, more preferably 0.0130%, further preferably 0.0100%, further preferably 0.0080%, further preferably 0.0060%, further preferably 0.0040%, further preferably 0.0020%, further preferably 0.0010%.

[0294] Ca: 0 to 0.0100%

[0295] Calcium (Ca) is an optional element and can not be contained. That is, the content of Ca can be 0%. In the case of being contained, Ca is effective as a deoxidizer. Ca also fixes S to improve the solidification cracking resistance of the weld metal 20. As long as a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the content of Ca is too high, even if the contents of other elements are within the range of the present embodiment, Ca combines with oxygen, and the cleanliness of the weld metal 20 significantly decreases, and the toughness of the weld metal 20 decreases. Therefore, the content of Ca is 0 to 0.0100%. The preferable lower limit of the content of Ca is 0.0001%, more preferably 0.0002%, further preferably 0.0005%, further preferably 0.0010%, further preferably 0.0015%. The preferable upper limit of the content of Ca is 0.0070%, more preferably 0.0050%, further preferably 0.0030%, further preferably 0.0020%.

[0296] [Method for measuring chemical composition of weld metal]

[0297] The chemical composition of the weld metal 20 is measured as follows. At the center of the Cap width of the weld metal 20, the surface of the weld metal 20 is cut, and the cut powder is collected. The collected cut powder is dissolved in acid to obtain a solution. For the solution, ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) is performed, and elemental analysis of the chemical composition is performed. As for the C content and the S content, they are obtained by a known high-frequency combustion method. Specifically, the above solution is combusted by high-frequency heating in an oxygen stream, and the generated carbon dioxide and sulfur dioxide are detected, and the C content and the S content are obtained. By the above analysis method, the chemical composition of the weld metal 20 can be obtained.

[0298] [About the microstructure of the duplex stainless steel weld metal]

[0299] The microstructure of the weld metal 20 of the present embodiment is composed of ferrite and austenite. In the present specification, "composed of ferrite and austenite" means that the phases other than ferrite and austenite are negligible. For example, in the microstructure of the weld metal 20 of the present embodiment, the volume fraction of precipitates, inclusions, and the like is so low as to be negligible compared to the volume fraction of ferrite and austenite. That is, the microstructure of the weld metal 20 of the present embodiment can contain a small amount of precipitates, inclusions, and the like in addition to ferrite and austenite.

[0300] [About formula (1) and formula (2)]

[0301] The duplex stainless steel welded joint 1 of the present embodiment satisfies formula (1) and formula (2) while the content of each element in the base material 10 and the weld metal 20 satisfies the above range.

[0302] Si + 3Mn ≤ 3.00 (1)

[0303] 0 < BH / BW < 0.15 (2)

[0304] Here, in formula (1), the content of each element in the weld metal 20 is substituted for the corresponding element symbol in mass %.

[0305] Here, in formula (2), BH is substituted for the Cap height (mm) of the weld metal 20, and BW is substituted for the Cap width (mm) of the weld metal 20.

[0306] [About formula (1)]

[0307] F1 = Si + 3Mn is defined. If F1 is greater than 3.00, the viscosity of the molten metal at the time of welding increases, and further, the temperature coefficient of the surface tension of the molten metal at the time of welding increases. In this case, the wettability of the molten metal at the time of welding decreases, and the undercut 3 cannot be suppressed. Therefore, F1 is 3.00 or less. The upper limit of F1 is preferably 2.95, more preferably 2.90, further preferably 2.85, further preferably 2.80, further preferably 2.75, further preferably 2.70, further preferably 2.65, further preferably 2.60, further preferably 2.55, further preferably 2.50, further preferably 2.45, further preferably 2.40. The lower limit of F1 is 0.20. The lower limit of F1 is preferably 0.40, more preferably 0.60, further preferably 0.80, further preferably 1.00, further preferably 1.20, further preferably 1.40, further preferably 1.60, further preferably 1.80, further preferably 2.00, further preferably 2.20, further preferably 2.40.

[0308] [Regarding Formula (2)]

[0309] With reference to Figure 2 and Figure 3 , the Cap height (BH) of the weld metal 20 refers to the maximum height of the weld metal 20 from the surface of the base material 10 as a reference (0 mm). Further, the Cap width (BW) of the weld metal 20 refers to the width of the weld metal 20 at the surface of the base material 10 in a direction perpendicular to the extension direction of the weld metal 20. In the case where the base material 10 is a steel pipe, the Cap width of the weld metal 20 refers to the linear distance between the base material 10 and the weld metal 20 in a direction perpendicular to the extension direction of the weld metal 20 on the outer surface of the steel pipe. The Cap height and the Cap width of the weld metal 20 are measured with a known welding gauge or the like. Note that, in the case where the base material 10 is a steel pipe, the length direction of the known gauge is made parallel to the axial direction of the base material 10, and the gauge is brought into contact with the outer surface of the base material 10, whereby the Cap height is measured.

[0310] F2 = BH / BW is defined. If F2 is 0.15 or more, the Cap height is too high relative to the Cap width. In this case, the surface tension of the molten metal at the time of welding increases, thereby making it easy for the formed weld metal 20 to become convex, making it easy to generate the undercut 3. However, if F2 is negative, the thickness of the weld metal 20 is thinner than the thickness of the base material 10. In this case, a welding defect called underfilling is formed. In this case, the strength of the duplex stainless steel welded joint 1 decreases. Therefore, F2 is greater than 0 and less than 0.15. The upper limit of F2 is preferably 0.14, more preferably 0.13, further preferably 0.12, further preferably 0.11, further preferably 0.10, further preferably 0.09, further preferably 0.08. The lower limit of F2 is preferably 0.01, more preferably 0.02, further preferably 0.04, further preferably 0.05, further preferably 0.06, further preferably 0.07, further preferably 0.08, further preferably 0.09, further preferably 0.10.

[0311] In the duplex stainless steel welded joint 1 of the present embodiment, on the basis of satisfying the formula (1) to improve the wettability of the molten metal at the time of welding, the shape of the weld metal 20 is further adjusted to satisfy the formula (2). Only when the base material 10 and the weld metal 20 have the above-described chemical composition and the weld metal 20 satisfies the formula (1) and the formula (2), the undercut 3 can be suppressed, and the fatigue characteristics can be improved.

[0312] [About the formula (3) and the formula (4)]

[0313] The duplex stainless steel welded joint 1 of the present embodiment satisfies the formula (1) and the formula (2) while the content of each element in the base material 10 and the weld metal 20 satisfies the above-described range, and preferably satisfies the formula (3) and the formula (4) in the case where the thickness of the base material 10 is 2.5 mm or less.

[0314] Si + 4Mn ≤ 3.75 (3)

[0315] 0 < BH / BW ≤ 0.5 / (6.0 - 0.85WT) (4)

[0316] Here, the content of the corresponding element in the weld metal 20 is substituted for each element symbol in the formula (3) in mass %.

[0317] Here, BH in the formula (4) is substituted for the Cap height (mm) of the weld metal 20, BW is substituted for the Cap width (mm) of the weld metal 20, and WT is substituted for the thickness (mm) of the base material 10.

[0318] [About the formula (3)]

[0319] F3 is preferably 3.75 or less. The upper limit of F3 is further preferably 3.70, further preferably 3.65, further preferably 3.60, further preferably 3.55, further preferably 3.50, further preferably 3.45, further preferably 3.40, further preferably 3.35, further preferably 3.30, further preferably 3.25, further preferably 3.20, further preferably 3.15, further preferably 3.10, further preferably 3.05, further preferably 3.00. The lower limit of F3 is preferably 0.25. The lower limit of F3 is further preferably 0.50, further preferably 0.70, further preferably 1.00, further preferably 1.50, further preferably 2.00, further preferably 2.30, further preferably 2.50, further preferably 3.00.

[0320] [Regarding Formula (4)]

[0321] If the base material 10 is thin, the base material 10 is relatively thin with respect to the undercut 3, and the fatigue life is likely to decrease. Therefore, it is preferable to further limit F2 in accordance with the thickness of the base material 10 on the premise that F2 (BH / BW) is greater than 0 and less than 0.15. Specifically, in the case where the thickness of the base material 10 is 2.5 mm or less, if F2 is 0.5 / (6.0-0.85WT) or less, the undercut 3 can be further suppressed. The further preferable upper limit of F2 is 0.4 / (6.0-0.85WT), further preferably 0.3 / (6.0-0.85WT), further preferably 0.2 / (6.0-0.85WT).

[0322] [Manufacturing method of duplex stainless steel welded joint]

[0323] Hereinafter, a manufacturing method of the duplex stainless steel welded joint 1 of the present embodiment will be described. The manufacturing method of the duplex stainless steel welded joint 1 described below is an example of the manufacturing method of the duplex stainless steel welded joint 1 of the present embodiment. Therefore, the duplex stainless steel welded joint 1 having the above-described configuration can also be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferable example of the manufacturing method of the duplex stainless steel welded joint 1 of the present embodiment.

[0324] The manufacturing method of the duplex stainless steel welded joint 1 of the present embodiment includes a process of preparing a pair of base materials 10 (base material preparation process) and a process of butting the end portions of the pair of base materials 10 and performing welding to form a weld metal 20 (weld metal formation process). Hereinafter, each process will be described in detail.

[0325] [Base material preparation step]

[0326] First, a pair of base materials 10 is prepared. The base materials 10 can be provided by a third party, or the base materials 10 can be prepared by the manufacturer of the duplex stainless steel welded joint 1. Hereinafter, an example of a manufacturing method of the base materials 10 at the time of manufacturing the base materials 10 will be described.

[0327] The manufacturing method of the base materials 10 includes a step of preparing a blank of the base material 10 (preparation step), a step of manufacturing an intermediate material by subjecting the blank to hot working as necessary (hot working step), a step of subjecting the intermediate material after the hot working step to pickling treatment and then subjecting the intermediate material to cold working as necessary (cold working step), a step of subjecting the blank prepared in the blank preparation step, the intermediate material after the hot working step, or the intermediate material after the cold working to solution heat treatment as necessary (solution heat treatment step), and a step of removing scale from the surface of the blank or the intermediate material as necessary (scale removal step). Hereinafter, each step will be described.

[0328] [Preparation step]

[0329] In the preparation step, a blank having the above-described chemical composition is prepared. The blank can be provided by a third party, or can be manufactured. The blank can be an ingot, or can be a slab, a bloom, a billet, or a pipe. In the case of manufacturing the blank, the blank is manufactured by the following method. A molten alloy having the above-described chemical composition is manufactured. Using the manufactured molten alloy, an ingot is manufactured by an ingot casting method. Using the manufactured molten alloy, a slab, a bloom, a billet, or a pipe can also be manufactured by a casting method. The manufactured ingot, slab, or bloom can be subjected to hot working to manufacture a billet or a pipe. For example, an ingot can be subjected to hot forging to manufacture a cylindrical billet, and the billet can be used as a blank (cylindrical blank). In this case, the temperature of the blank immediately before the start of hot forging is not particularly limited, and is, for example, 900 to 1300°C. Further, a blank (steel pipe) can be manufactured by a publicly known centrifugal casting method.

[0330] [Hot working step]

[0331] The hot working step is performed as necessary. That is, the hot working step can not be performed. In the case where the hot working step is performed, the blank is subjected to hot working to manufacture an intermediate material having a prescribed shape. In the case where the base material 10 is a plate material, a plate-shaped intermediate material is manufactured by hot rolling. In the case where the base material 10 is an alloy pipe, a through hole is formed along the central axis of the cylindrical blank by machining. The cylindrical blank in which the through hole is formed is subjected to hot extrusion to manufacture an intermediate material (pipe material). The heating temperature of the blank in the hot working step is not particularly limited, and is, for example, 900 to 1300°C.

[0332] The hot working process can be substituted for hot extrusion to manufacture an intermediate material (pipe material) by performing a Mannesmann-based piercing rolling on a cylindrical blank. The temperature of the blank before the piercing rolling is, for example, 900 to 1300°C.

[0333] [cool working process]

[0334] The cool working process is performed as necessary. That is, the cool working process can not be performed. In the case of performing, the cool working is performed after performing a pickling treatment on the intermediate material. In the case where the base material 10 is a plate material, cold rolling is performed. In the case where the base material 10 is a steel pipe, cold drawing is performed. The cross-sectional reduction ratio in the cool working process is not particularly limited and is, for example, 10 to 90%.

[0335] [solution heat treatment process]

[0336] The solution heat treatment process is performed as necessary. That is, the solution heat treatment process can not be performed. In the case of performing, the solution heat treatment is performed on the blank prepared in the preparation process, the intermediate material after the hot working process, or the intermediate material after the cool working process. The precipitates of the blank or the intermediate material are dissolved by the solution heat treatment.

[0337] The solution heat treatment is performed as follows. The blank or the intermediate material is charged into a heat treatment furnace in which the atmosphere is an atmospheric atmosphere. The atmospheric atmosphere herein means an atmosphere in which nitrogen gas as a gas constituting the atmosphere contains 78% or more by volume and oxygen gas contains 20% or more by volume. In the furnace in the atmospheric atmosphere, the blank or the intermediate material is heated to 1000 to 1300°C and is held at 1000 to 1300°C. The holding time is 1 to 60 minutes. The blank or the intermediate material after the heat treatment is quenched. The quenching method is, for example, a publicly known water quenching or a publicly known oil quenching.

[0338] [oxide scale removal process]

[0339] The oxide scale removal process is performed as necessary. That is, the oxide scale removal process can not be performed. In the case of performing, the oxide scale on the surface of the blank prepared in the preparation process, the intermediate material after the hot working process, the intermediate material after the cool working process, or the blank or the intermediate material after the solution heat treatment is removed. The method of removing the oxide scale can be a method of removing the oxide scale by high-pressure jet processing, grinding, or the like, or a method of removing the oxide scale by a pickling treatment.

[0340] In the case where the pickling treatment is performed, the pickling conditions are not particularly limited. It is preferable to use a mixed solution of nitric acid and hydrofluoric acid as the pickling solution. The mixed solution is, for example, an aqueous solution containing 5.0 to 8.0% by volume of nitric acid and 5.0 to 8.0% by volume of hydrofluoric acid. The temperature of the pickling solution in the pickling solution tank is adjusted to 30 to 50°C, and the blank or the intermediate material is immersed in the pickling solution tank. The immersion time is, for example, 0.5 to 5.0 hours. By the above pickling treatment, the scale is sufficiently removed from the surface of the blank or the surface of the intermediate material.

[0341] The high-pressure jet processing refers to processing in which a grinding material is given kinetic energy to collide with the surface of the blank or the intermediate material, and the metal surface is cut or impacted. In the case where the scale is removed by the high-pressure jet processing, the high-pressure jet processing is, for example, blasting in which the grinding material uses sand, shot blasting in which the grinding material uses steel shot, grit blasting in which the grinding material uses cast iron grit, cast steel grit, alumina grit, silicon carbide grit, or the like, shot blasting in which the grinding material uses cast iron shot, cast steel shot, a cutting wire, or the like, and the like.

[0342] The base material 10 is manufactured by the above process.

[0343] A bevel can be formed for the prepared base material 10. Specifically, a bevel can be formed at the end of the base material 10 by a known processing method. The bevel shape can be a V shape as shown in the drawing, or a shape other than the V shape. Figure 3 Figure 3

[0344] [Formation of Weld Metal]

[0345] In the formation of the weld metal process, the prepared base material 10 is subjected to welding, and the weld metal 20 is formed, thereby manufacturing the duplex stainless steel welded joint 1. Specifically, two base materials 10 are prepared. The ends of the prepared base materials 10 are butted against each other. Then, the butted pair of ends is subjected to welding using a welding material, and the weld metal 20 having the above chemical composition is formed.

[0346] [About the Welding Material]

[0347] ​​The welding material for forming the above-described weld metal 20 is, in mass%, C: 0.001 to 0.030%, Si: 0.05 to 0.60%, Mn: 0.05 to 0.60%, P: 0.025% or less, S: 0.0030% or less, Cr: 21.00 to 28.00%, Ni: 6.00 to 11.00%, Mo: 2.00 to 4.50%, Cu: 0 to 4.00%, sol. Al: 0.0010 to 0.0500%, N: 0.0800 to 0.4000%, B: 0.0001 to 0.0030%, W: 0 to 4.00%, Nb: 0 to 0.100%, V: 0 to 0.10%, Ta: 0 to 0.10%, Co: 0 to 1.00%, Sn: 0 to 0.010%, Mg: 0 to 0.02000%, Ca: 0 to 0.0100%, REM: 0 to 0.100%, O: 0.0150% or less, and the balance of Fe and impurities.

[0348] The above-described welding material can be a welding material provided by a third party, or a manufactured welding material can be used. In the case of manufacturing the welding material, casting is performed using a molten material of a welding material having the above-described chemical composition to produce an ingot. The ingot is subjected to hot working to manufacture the welding material. The welding material after hot working can be further subjected to cold working. In addition, the welding material can be subjected to a known heat treatment. The heat treatment is, for example, the same solid solution heat treatment as the base material 10. The heat treatment can not be performed. The welding material can be in a bar shape (including wire) or a small block shape.

[0349] The weld metal 20 is formed using a welding material having the above-described chemical composition. The welding method is, for example, gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW), flux cored arc welding (FCAW), gas metal arc welding (GMAW), and submerged arc welding (SAW).

[0350] At this time, the interval of the end portions of the butted pair of base materials 10 and the amount of feeding of the welding material at the time of welding are adjusted, the dilution amount based on the base material 10 is adjusted, the content of each element of the chemical composition of the weld metal 20 is within the range of the present embodiment, and F1≤3.00 and 0

[0351] By the above manufacturing procedure, the welded joint 1 of the present embodiment can be manufactured. Note that the manufacturing method of the welded joint 1 of the present embodiment is not limited to the above-described manufacturing method. In the welded joint 1, as long as the content of each element of the chemical composition of the base material 10 is within the above-described range, the content of each element of the chemical composition of the weld metal 20 is within the above-described range, and F1≤3.00 and 0

[0352] Example

[0353] [Manufacture of base material]

[0354] A molten steel for a base material shown in Table 1 was manufactured. The blank cells in Table 1 indicate that the content of the corresponding element was at an impurity level.

[0355] [Table 1]

[0356]

[0357] A cylindrical ingot having an outer diameter of 120 mm and a weight of 30 kg was manufactured using a molten steel. The ingot was subjected to hot forging to produce a bar steel having a diameter of 180 mm. Further, hot rolling and cold rolling were performed to produce a seamless steel pipe having an outer diameter of 15.12 mm and a wall thickness of 1.21 mm, or a seamless steel pipe having an outer diameter of 30.72 mm and a wall thickness of 2.66 mm. Each of the seamless steel pipes was subjected to a solution heat treatment at 1100°C for 10 minutes and then water-cooled. Each of the seamless steel pipes was cut into a length of 300 mm. For the seamless steel pipe having a wall thickness of 2.66 mm, a groove was formed to simulate a bevel. Specifically, a groove having a root thickness of 1.5 mm, a single-side protruding length of 2 mm, and a single-side slope of 20° was formed in the center of the length direction.

[0358] [Manufacture of welding material]

[0359] A molten steel for a welding material having a chemical composition shown in Table 2 was manufactured. The blank cells in Table 2 indicate that the content of the corresponding element was at an impurity level.

[0360] [Table 2]

[0361] Table Z

[0362]

[0363] A 30-kg ingot was manufactured using a vacuum melting furnace. The ingot was subjected to hot forging and hot rolling to produce a bar steel having a diameter of 30 mm. Then, cold rolling and intermediate annealing were repeatedly performed to produce a wire coil having a diameter of 0.8 mm.

[0364] [Formation of weld metal]

[0365] The both ends of the manufactured seamless steel pipe were fixed, and welding was performed on the center portion in the length direction of the seamless steel pipe (the center portion of the groove processing for the seamless steel pipe with a wall thickness of 2.66 mm). Specifically, using the welding material of Table 2, welding was performed by automatic tungsten inert gas arc welding while rotating the seamless steel pipe in the circumferential direction downward (1G). Here, the welding conditions were changed for each test number, and the dilution rate based on the base material and BH / BW were adjusted. Specifically, for the seamless steel pipe with a wall thickness of 1.21 mm, the input heat was adjusted in the range of 150 to 230 J / mm while varying the feeding speed of the welding material in the range of 150 to 900 mm / minute. Two passes of welding were applied to the seamless steel pipe with a wall thickness of 2.66 mm. The welding of the first pass was performed with the input heat set to about 500 J / mm and the feeding speed of the welding material of 400 to 500 mm / minute. The welding of the second pass was performed in order to vary the dilution rate based on the base material, adjusting the input heat in the range of 470 to 550 J / mm while varying the feeding speed of the welding material in the range of 1500 to 2000 mm / minute. In addition, in any of the weldings, the protective gas was Ar + 2% N2, and the flow rate was set to 10 L / minute. Thus, the weld metal was formed. The weld metal was produced in five samples for each test number.

[0366] For the formed weld metal, at a position 180° apart from the start and end of the welding in the circumferential direction of the base material, the Cap height (mm) and the Cap width (mm) of the weld metal were measured using a weld gauge. The results are shown in Table 3.

[0367] [Table 3]

[0368] Table 3

[0369]

[0370] At a position 180° apart from the start and end of the welding in the circumferential direction of the base material, at the center position of the Cap width of the weld metal, the surface of the weld metal was cut, and the cutting powder was collected. For the seamless steel pipe with a wall thickness of 2.66 mm, the cutting powder was collected from the weld metal portion of the second pass. The resulting cutting powder was subjected to inspection analysis, and the chemical composition of the weld metal was investigated. The results are shown in Table 4. The blank in Table 4 indicates that the content of the corresponding element is at the impurity level. The Si and Mn contents in the chemical composition of the weld metal are also shown in Table 3.

[0371] [Table 4]

[0372]

[0373] [Undercutting determination test]

[0374] At a position 180° from the start end of the welding along the circumferential direction of the base material, the base material steel pipe was cut in a direction perpendicular to the extending direction of the weld metal, and a cross-section observation piece including the weld metal was taken. The 5 samples of each test number were observed, and the presence or absence of undercut in the weld toe portion in the cross-section observation piece and the depth of the undercut were evaluated. The case where the depth of the undercut was less than 0.05 mm in all 5 samples was evaluated as E (Excellent). The case where the depth of the undercut was 0.05 mm or more and less than 0.10 mm even if the undercut was generated was evaluated as G (Good). The case where the depth of the undercut was 0.10 mm or more in at least one of the 5 samples was evaluated as B (Bad). The results are shown in the column of "Undercut" in Table 3.

[0375] [Results of Evaluation]

[0376] Referring to Table 3, with respect to Test Nos. 3, 6 to 8, 10, 12, and 14 to 30, the contents of the respective elements in the base material and the weld metal were appropriate, and the formula (1) (Si + 3Mn ≤ 3.00) and the formula (2) (0 < BH / BW < 0.15) were satisfied. Therefore, the depth of the undercut was less than 0.10 mm in Test Nos. 3, 6 to 8, 10, 12, and 14 to 30. Therefore, Test Nos. 3, 6 to 8, 10, 12, and 14 to 30 suppressed the undercut.

[0377] Further, Test Nos. 3, 6 to 8, 15 to 17, 19 to 20, and 27 to 29, in which the thickness of the base material was 2.5 mm or less, were compared, and the following results were obtained. In Test Nos. 3, 6 to 8, 16, 17, 19 to 20, and 27 to 29, which satisfied the formula (3) and the formula (4) in addition to the formula (1) and the formula (2), the depth of the undercut was less than 0.05 mm. Therefore, Test Nos. 3, 6 to 8, 16, 17, 19 to 20, and 27 to 29 further successfully suppressed the undercut compared to Test No. 15.

[0378] On the other hand, in Test Nos. 1 and 2, although the contents of the respective elements in the base material and the weld metal were appropriate, the formula (1) was not satisfied. As a result, the undercut having a depth of 0.10 mm or more was generated. Therefore, Test Nos. 1 and 2 failed to suppress the undercut.

[0379] In Test Nos. 4 and 5, although the contents of the respective elements in the base material and the weld metal were appropriate, the formula (1) and the formula (2) were not satisfied. As a result, the undercut having a depth of 0.10 mm or more was generated. Therefore, Test Nos. 4 and 5 failed to suppress the undercut.

[0380] In Test Nos. 9, 11, and 13, although the contents of the respective elements in the base material and the weld metal were appropriate, the formula (2) was not satisfied. As a result, the undercut having a depth of 0.10 mm or more was generated. Therefore, Test Nos. 9, 11, and 13 failed to suppress the undercut.

[0381] In Test No. 31, although the contents of the respective elements in the base material were appropriate, the formula (1) was not satisfied. As a result, undercuts having a depth of 0.10 mm or more were generated. Therefore, Test No. 31 failed to successfully suppress undercuts.

[0382] The above describes the embodiments of the present application. However, the above-described embodiments are merely examples for implementing the present application. Therefore, the present application is not limited to the above-described embodiments, and the above-described embodiments can be appropriately changed to be implemented within a range not departing from the gist thereof.

[0383] Explanation of Reference Numerals

[0384] 1 Welded joint

[0385] 10 Base material

[0386] 20 Welded metal

Claims

1. A duplex stainless steel welded joint having a base material and a weld metal, the base material comprising, in mass %, C: 0.001 to 0.030%, Si: 0.05 to 0.80%, Mn: 0.05 to 1.20%, P: 0.030% or less, S: 0.0030% or less, Ni: 4.00 to 8.00%, Mo: 2.00 to 4.50%, Cu: 0.01 to 4.00%, Sol. Al: 0.0010 to 0.0500%, Nb: 0 to 0.10%, Ta: 0 to 0.30%, Co: 0 to 1.00%, Sn: 0 to 0.020%, Mg: 0 to 0.0200%, Ca: 0 to 0.0100%, and the balance of Fe and impurities, the weld metal comprising, in mass %, C: 0.001 to 0.030%, Si: 0.05 to 0.70%, Mn: 0.05 to 0.85%, P: 0.030% or less, S: 0.0030% or less, Ni: 5.00 to 11.00%, Mo: 2.00 to 4.50%, Cu: 0.01 to 4.00%, Sol. Al: 0.0010 to 0.0500%, Nb: 0 to 0.10%, Ta: 0 to 0.30%, Co: 0 to 1.00%, Sn: 0 to 0.020%, Mg: 0 to 0.0200%, Ca: 0 to 0.0100%, and the balance of Fe and impurities, the duplex stainless steel welded joint satisfying formula (1) and formula (2) while satisfying the content of each element in the base material and the weld metal: Si + 3Mn ≤ 3.00 (1) 0 < BH / BW < 0.15 (2) where the content of the corresponding element in the weld metal is substituted for each element symbol in formula (1) in mass %, and where BH in formula (2) is substituted for the Cap height of the weld metal, and BW is substituted for the Cap width of the weld metal, the Cap height and the Cap width being in mm.

2. The duplex stainless steel welded joint according to claim 1, wherein, in the case where the thickness of the base material is 2.5 mm or less, the duplex stainless steel welded joint further satisfies formula (3) and formula (4): Si + 4Mn ≤ 3.75 (3) 0 < BH / BW ≤ 0.5 / (6.0 - 0.85WT) (4) where the content of the corresponding element in the weld metal is substituted for each element symbol in formula (3) in mass %, and where BH in formula (4) is substituted for the Cap height of the weld metal, BW is substituted for the Cap width of the weld metal, and WT is substituted for the thickness of the base material, the Cap height, the Cap width, and the thickness of the base material being in mm.

3. The duplex stainless steel welded joint according to claim 1, wherein, the base material contains, in mass %, one or more selected from the group consisting of Nb: 0.01 to 0.10%, Ta: 0.01 to 0.30%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.020%, Mg: 0.001 to 0.0200%, and Ca: 0.001 to 0.0100%. ​ ​ Cr：21.00％～28.00％、 ​ ​ ​ ​ N：0.080％～0.400％、 B：0.0001％～0.0100％、 W：0～4.00％、 ​ V：0～0.20％、 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cr：21.00％～28.00％、 ​ ​ ​ ​ N：0.080％～0.400％、 B：0.0001％～0.0100％、 W：0～4.00％、 ​ V：0～0.20％、 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ W：0.01～4.00％、 ​ V：0.01～0.20％、 ​ ​ ​ Mg: 0.0001 to 0.0200%, and one or more elements selected from the group consisting of 4. The duplex stainless steel welded joint according to claim 2, wherein the base material contains, in mass %, W：0.01～4.00％、 one or more elements selected from the group consisting of V：0.01～0.20％、 Nb: 0.01 to 0.10%, Ta: 0.01 to 0.30%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.020%, Mg: 0.0001 to 0.0200%, and 5. Duplex stainless steel welded joint according to any one of claims 1 - 4, wherein Ca: 0.0001 to 0.0100%. W：0.01～4.00％、 the weld metal contains, in mass %, V：0.01～0.20％、 one or more elements selected from the group consisting of Nb: 0.01 to 0.10%, Ta: 0.01 to 0.30%, Co: 0.01 to 1.00%, Sn: 0.001 to 0.020%, Mg: 0.0001 to 0.0200%, and Ca: 0.0001 to 0.0100%. W：0.50～4.00％、 6. The duplex stainless steel welded joint according to claim 1 or 2, wherein V：0.01～0.20％、 the base material contains, in mass %, one or more elements selected from the group consisting of Nb: 0.01 to 0.10%, Ta: 0.01 to 0.30%, Co: 0.05 to 1.00%, 7. Duplex stainless steel welded joint according to any one of claims 1 - 4, wherein Sn: 0.001 to 0.020%, W：0.50～4.00％、 Mg: 0.0001 to 0.0200%, and V：0.01～0.20％、 Ca: 0.0002 to 0.0100%. the weld metal contains, in mass %, one or more elements selected from the group consisting of Nb: 0.01 to 0.10%, Ta: 0.01 to 0.30%, Co: 0.05 to 1.00%, Sn: 0.001 to 0.020%, Mg: 0.0001 to 0.0200%, and Ca: 0.0002 to 0.0100%.

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