Two-phase stainless steel welded joint

Abstract

Provided is a two-phase stainless steel welded joint that can suppress undercut and has superior fatigue characteristics. This two-phase stainless steel welded joint comprises: a parent material; and a weld metal that comprises, by mass%, C: 0.001%-0.030%, Si: 0.05%-0.70%, Mn: 0.05%-0.85%, P: 0.030% or less, S: 0.0030% or less, Cr: 21.00%-28.00%, Ni: 5.00%-11.00%, Mo: 2.00%-4.50%, Cu: 0.01-4.00%, Sol. Al: 0.0010-0.0500%, N: 0.080%-0.400%, and B: 0.0001-0.0100%, the remainder consisting of Fe and unavoidable impurities, wherein in addition to the respective contents of each of the elements in the weld metal and the parent material being satisfied, mathematical expression (1) and mathematical expression (2) are also satisfied.　(1): Si+3Mn≤3.00　(2): 0<BH / BW<0.15

Description

Technical Field

[0001] The present disclosure relates to a two-phase stainless steel welded joint.

Background Art

[0002] Two-phase stainless steel has high strength and excellent corrosion resistance in a chloride environment. Therefore, two-phase stainless steel is used in a wide range of technical fields. For example, two-phase stainless steel is used as a material for steel pipes for seawater heat exchangers and as a material for steel pipes for umbilical cables for offshore development.

[0003] Two-phase stainless steel is standardized in JIS Standards (Japanese Industrial Standards) and ASTM Standards according to the required applications. For example, SUS329J3L and SUS329J4L are defined in the JIS standard. Recently, SUS327L1 has been newly added to the JIS standard as a super two-phase stainless steel with a pitting resistance equivalent value (PREW) exceeding 40. In addition, ASTM A789 S39274, which is a super two-phase stainless steel, has also been developed and put into practical use. In the two-phase stainless steel defined in ASTM A789 S39274, the PREW is increased to enhance the strength and corrosion resistance, and the precipitation of sigma phase associated with the increase in PREW is suppressed by adding a large amount of W.

[0004] By the way, when two-phase stainless steel is welded and used, due to rapid cooling after welding, the amount of ferrite in the weld metal becomes significantly larger compared to the amount of ferrite in the base metal of the two-phase stainless steel. As a result, the strength and corrosion resistance of the weld metal decrease. Therefore, a welded joint that is excellent in strength and corrosion resistance even as-welded is required.

[0005] A duplex stainless steel welded joint with improved pitting corrosion resistance has been proposed, for example, in Japanese Patent Application Laid-Open No. 2015-196894 (Patent Document 1). In the duplex stainless steel welded joint of Patent Document 1, the chemical compositions of the base metal and the weld metal are, 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%, balance: Fe and impurities. The duplex stainless steel welded joint of Patent Document 1 further has an austenite index a determined from the following formula (1) of 0.1 to 0.4, satisfies Mn / N ≧ 2, and the PF index determined from the following formula (2) is 1.0 or less, and the oxidation scale thickness during welding formed on the surfaces of the base metal and the weld metal is 500 nm or less. a = {Ni + 30(C + N) - 0.6(Cr + 1.5Si + Mo) + 5.6} / {Cr + 1.5Si + Mo - 6} (1) PF = Mn × (100Pb + 50Sb + 30Zn + 40As) (2) However, each element symbol in the above formulas means the content (mass%) of each element. In Patent Document 1, by satisfying formulas (1) and (2), the pitting corrosion resistance of the duplex stainless steel welded joint is improved.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] By the way, when duplex stainless steel is used as an umbilical tube, duplex stainless steel pipes are circumferentially welded together to produce a duplex stainless steel welded joint. The duplex stainless steel welded joint is used as part of an umbilical cable with a length of several km to several tens of km.

[0008] In order to improve the weldability and increase the welding efficiency, it is preferable that the duplex stainless steel pipe is thin. On the other hand, when used as a steel pipe for umbilical cables, the duplex stainless steel pipe is used for a long period while being swayed by waves. Therefore, the duplex stainless steel pipe is required to have high fatigue strength. If there are defects at the welded end of the duplex stainless steel welded joint, stress is likely to concentrate on these defects. As a result, fatigue fracture is likely to occur starting from these defects. The thinner the duplex stainless steel pipe, the larger the ratio of the shape defects of the weld metal to the thickness of the duplex stainless steel pipe.

[0009] One of the defects that are likely to occur at the welded end is undercut. FIG. 1 is a cross-sectional view perpendicular to the extending direction of the weld metal 20, including the weld metal 20 of the duplex stainless steel welded joint 1 in which the undercut 3 has occurred. Referring to FIG. 1, two base materials 10 are connected via the weld metal 20, and a groove is formed at the end of the weld metal 20. This groove is called an undercut 3. When the duplex stainless steel welded joint 1 has an undercut 3 at the end of the weld metal 20, stress is likely to concentrate on the undercut 3 when the umbilical cable vibrates. Therefore, fatigue fracture starting from the undercut 3 is likely to occur. In this case, the life of the umbilical cable may be significantly lower than the design standard. Therefore, a duplex stainless steel welded joint 1 capable of suppressing the undercut 3 is desired.

[0010] An object of the present disclosure is to provide a duplex stainless steel welded joint that can suppress undercut and has excellent fatigue characteristics.

Means for Solving the Problems

[0011] The duplex stainless steel welded joint of the present disclosure is by mass percentage, 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%, and, a base material composed of the balance being Fe and impurities, and by mass percentage, 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%, and, comprises a weld metal consisting of the balance being Fe and impurities, satisfies the content of each element in the base material and the weld metal, and satisfies Formula (1) and Formula (2). Si + 3Mn ≤ 3.00 (1) 0 < BH / BW < 0.15 (2) Here, for each element symbol in Formula (1), the content of the corresponding element in the weld metal is substituted in mass%. Here, in Formula (2), BH is the Cap height (mm) of the weld metal, and BW is the Cap width (mm) of the weld metal, which are substituted.

Advantages of the Invention

[0012] The duplex stainless steel welded joint of the present disclosure can suppress undercut and has excellent fatigue characteristics.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0014] Hereinafter, with reference to the drawings, the present embodiment will be described in detail. The same or corresponding parts in the drawings are denoted by the same reference numerals and their description will not be repeated.

[0015] First, the inventors examined the chemical composition for obtaining the strength and corrosion resistance required for the duplex stainless steel welded joint 1. As a result, 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, 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%, and the balance being composed of Fe and impurities for the base material 10, and 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, 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%, and the balance being composed of Fe and impurities for the weld metal 20. It was found that excellent strength and excellent corrosion resistance can be obtained with the duplex stainless steel welded joint 1 having such a composition.

[0016] Subsequently, the inventors variously examined means for suppressing the undercut 3 as shown in FIG. 1 with respect to the duplex stainless steel welded joint 1 having the above-described chemical composition. As a result, the inventors obtained the following findings different from the conventional knowledge. In this specification, the molten metal refers to the metal in a molten state (i.e., the molten pool) when welding is performed using a welding material. The molten metal solidifies to form the weld metal 20.

[0017] The duplex stainless steel has a high viscosity of the molten metal during welding due to its high Cr content. Therefore, the wettability of the molten metal during welding with respect to the base material 10 is low. If the wettability of the molten metal during welding is low, the undercut 3 is likely to occur. Therefore, the inventors considered that if the wettability of the molten metal during welding with respect to the base material 10 is increased, the undercut 3 can be suppressed.

[0018] First, the inventors considered that if the viscosity of the molten metal during welding is reduced, the wettability of the molten metal during welding with respect to the base material 10 will increase and the undercut 3 can be suppressed. As described above, Cr increases the viscosity. Therefore, it is also conceivable to reduce the Cr content. However, Cr needs to be contained in a predetermined amount to obtain the strength and corrosion resistance of the duplex stainless steel welded joint 1. Therefore, it is difficult to reduce the Cr content.

[0019] Therefore, the inventors examined elements other than Cr that affect the viscosity of the molten metal during welding. As a result, the inventors found that in the duplex stainless steel having the above chemical composition, Si affects the viscosity of the molten metal during welding.

[0020] Si increases the viscosity of the molten metal during welding. Therefore, the inventors considered that by reducing the Si content, the viscosity of the molten metal during welding would decrease. As a result, the outward Marangoni convection velocity during welding increases, and the molten metal during welding is likely to spread in the width direction of the weld metal 20. As a result, it is considered that the wettability of the molten metal during welding with respect to the base material 10 is increased.

[0021] However, even when only the Si content was adjusted, the undercut 3 still occurred. Therefore, the inventors examined a method for suppressing the undercut 3 from a perspective different from the viscosity of the molten metal during welding.

[0022] Here, the inventors focused on the surface tension of the molten metal during welding. As a result, the following findings were obtained.

[0023] Mn increases the temperature coefficient of the surface tension of the molten metal during welding. By reducing the amount of Mn, the temperature coefficient of the surface tension of the molten metal during welding decreases. If the amount of Mn is reduced, the difference in surface tension between the central part of the welding metal 20 with a high temperature and the peripheral part of the welding metal 20 with a low temperature becomes smaller. As a result, the wettability of the molten metal during welding with respect to the base material 10 increases.

[0024] The inventors further examined in more detail the Si content and the Mn content in the welding metal 20. And the inventors thought that if the Si content and the Mn content in the welding metal 20 were adjusted to have an appropriate relationship, the wettability of the molten metal during welding would increase and the undercut 3 of the duplex stainless steel welded joint 1 could be suppressed. Therefore, the inventors examined the relationship between the Si content and the Mn content in the welding metal 20. As a result, it was found that in the duplex stainless steel having the above chemical composition, it is necessary to satisfy the formula (1). Si + 3Mn ≦ 3.00 (1) Here, for each element symbol in the formula (1), the content of the corresponding element in the welding metal 20 is substituted in mass%.

[0025] When the chemical composition of the welding metal 20 is within the above range and the formula (1) is satisfied, the viscosity of the molten metal during welding can be reduced, and further, the temperature coefficient of the surface tension of the molten metal during welding can be reduced. Thereby, the wettability of the molten metal during welding increases.

[0026] The inventors further examined in more detail and found that in the duplex stainless steel welded joint 1 having the above chemical composition, the undercut 3 can be suppressed only by adjusting the shape of the welding metal 20 after increasing the wettability of the molten metal during welding by satisfying the formula (1). Specifically, when the welding metal 20 satisfies the formula (1) and further satisfies the formula (2) in the shape of the welding metal 20, the undercut 3 can be suppressed. 0 < BH / BW < 0.15 (2) Here, BH in Formula (2) is substituted with the Cap height (mm) of the weld metal 20, and BW is substituted with the Cap width (mm) of the weld metal 20.

[0027] If the surface tension of the molten metal during welding increases, the formed weld metal 20 is likely to become convex, and the undercut 3 is likely to occur. Therefore, after enhancing the wettability of the molten metal during welding by satisfying the chemical composition and Formula (1), and further suppressing the convex shape of the weld metal 20 to satisfy Formula (2), the undercut 3 can be suppressed and the fatigue characteristics can be improved.

[0028] Based on the above findings, the gist of the duplex stainless steel welded joint 1 according to the present embodiment is as follows.

[0029] [1] A duplex stainless steel welded joint, by 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, 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%, and, a base material composed of the balance being Fe and impurities, In mass percentage, 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%, and a weld metal composed of the balance being Fe and impurities, satisfying the content of each element in the base material and the weld metal, and satisfying Formula (1) and Formula (2), a duplex stainless steel welded joint. Si + 3Mn ≤ 3.00 (1) 0 < BH / BW < 0.15 (2) Here, for each element symbol in Formula (1), the content of the corresponding element in the weld metal is substituted in mass percentage. Here, in Formula (2), BH is the Cap height (mm) of the weld metal, and BW is the Cap width (mm) of the weld metal and is substituted.

[0030] [2] The duplex stainless steel welded joint according to [1], when the thickness of the base material is 2.5 mm or less, further satisfying Formula (3) and Formula (4), Duplex stainless steel welded joint. Si + 4Mn ≤ 3.75 (3) 0 < BH / BW ≤ 0.5 / (6.0 - 0.85WT) (4) Here, for each element symbol in formula (3), the content of the corresponding element in the weld metal is substituted in mass%. Here, for BH in formula (4), the Cap height (mm) of the weld metal, for BW, the Cap width (mm) of the weld metal, and for WT, the thickness (mm) of the base material are substituted.

[0031] [3] The duplex stainless steel welded joint according to [1] or [2], wherein the base material in mass%, W: 0.01 - 4.00%, Nb: 0.01 - 0.10%, V: 0.01 - 0.20%, Ta: 0.01 - 0.30%, Co: 0.01 - 1.00%, Sn: 0.001 - 0.020%, Mg: 0.0001 - 0.0200%, and Ca: 0.0001 - 0.0100%, contains one or more elements selected from the group consisting of duplex stainless steel welded joint.

[0032] [4] The duplex stainless steel welded joint according to any one of [1] to [3], wherein the weld metal in mass%, W: 0.01 - 4.00%, Nb: 0.01 - 0.10%, V: 0.01 - 0.20%, Ta: 0.01 - 0.30%, Co: 0.01 - 1.00%, Sn: 0.001 - 0.020%, Mg: 0.0001 - 0.0200%, and Ca: 0.0001 to 0.0100%, containing one or more elements selected from the group consisting of a duplex stainless steel welded joint.

[0033] [Regarding the configuration of the welded joint of the present embodiment] FIG. 2 is a cross-sectional view of the duplex stainless steel welded joint 1 of the present embodiment cut in a direction perpendicular to the extending direction of the weld metal 20. Referring to FIG. 2, the duplex stainless steel welded joint 1 according to the present embodiment includes a pair of base materials 10 and a weld metal 20. The weld metal 20 is disposed between the pair of base materials 10. The weld metal 20 is disposed between the pair of base materials 10 and is connected to the pair of base materials 10.

[0034] The weld metal 20 is formed by performing welding after joining the ends of the pair of base materials 10. Any well-known welding may be applied for the welding. For example, the welding may be 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).

[0035] FIG. 3 is a cross-sectional view of the duplex stainless steel welded joint 1 according to another embodiment different from FIG. 2. The end of the base material 10 may be beveled as shown in FIG. 3 or may not be beveled as shown in FIG. 2. When the thickness of the base material 10 is, for example, more than 2.5 mm, it is preferable to perform beveling on the end of the base material 10 and perform welding by so-called multi-layer welding in which welding is performed a plurality of times. When the thickness of the base material 10 is, for example, 2.5 mm or less, so-called single-layer single-pass welding in which beveling is not performed on the end of the base material 10 and welding is performed once may be performed. When beveling is performed on the end of the base material 10, the shape of the beveling is not particularly limited. The shape of the bevel at the end of the base material 10 is selected from the group consisting of, for example, a V-bevel, a U-bevel, and an X-bevel.

[0036] [Regarding the base material] The base material 10 according to this embodiment will be described.

[0037] [Chemical composition of the base material] The chemical composition of the base material 10 according to this embodiment contains the following elements.

[0038] C: 0.001 - 0.030% Carbon (C) is an element effective for stabilizing the austenite phase. If the C content is too low, even if the contents of other elements are within the range of this embodiment, the amount of austenite in the as-welded weld metal 20 will decrease, and the corrosion resistance and strength of the weld metal 20 will deteriorate. On the other hand, if the C content is too high, even if the contents of other elements are within the range of this embodiment, carbides are likely to precipitate, and the corrosion resistance of the base material 10 will deteriorate. Therefore, the C content is 0.001 - 0.030%. The preferable lower limit of the C content is 0.002%, more preferably 0.003%, still more preferably 0.005%, and even more preferably 0.010%. The preferable upper limit of the C content is 0.028%, more preferably 0.025%, still more preferably 0.023%, and even more preferably 0.020%.

[0039] Si: 0.05 - 0.80% Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be fully obtained even if the contents of other elements are within the range of this embodiment. On the other hand, Si stabilizes the ferrite phase. If the Si content is too high, even if the contents of other elements are within the range of this embodiment, the amount of ferrite in the as-welded weld metal 20 increases, and the corrosion resistance and strength of the weld metal 20 decrease. Si further increases the viscosity of the molten metal during 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%, still more preferably 0.15%, still more preferably 0.20%, still more preferably 0.25%, still more preferably 0.30%, still more preferably 0.40%, still more preferably 0.50%. The preferable upper limit of the Si content is 0.75%, more preferably 0.70%, still more preferably 0.65%, still more preferably 0.60%.

[0040] Mn: 0.05 to 1.20% Manganese (Mn) stabilizes the austenite phase. If the Mn content is too low, even if the contents of other elements are within the range of this embodiment, the amount of austenite in the as-welded weld metal 20 decreases, 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 during welding. In other words, Mn increases the temperature dependence of the surface tension of the molten metal during welding. If the Mn content is too high, even if the contents of other elements are within the range of this embodiment, the wettability of the molten metal during welding decreases. 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%, still more preferably 0.20%, still more preferably 0.30%, still more preferably 0.40%, still more preferably 0.50%, still more preferably 0.60%, still more preferably 0.70%. The preferable upper limit of the Mn content is 1.10%, more preferably 1.00%, still more preferably 0.90%, still more preferably 0.80%, still more preferably 0.70%.

[0041] P: Below 0.030% Phosphorus (P) is an impurity inevitably contained. That is, the lower limit of the P content is over 0%. P significantly increases the cracking susceptibility 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%, still more preferably 0.023%, and even more preferably 0.020%. It is preferable that the P content is as low as possible. However, an extreme reduction in the P content leads to an increase in the manufacturing cost. Therefore, considering industrial productivity, the preferable lower limit of the P content is 0.001%, more preferably 0.002%.

[0042] S: 0.0030% or less Sulfur (S) is an impurity inevitably contained. That is, the lower limit of the S content is over 0%. S significantly increases the cracking susceptibility 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%, still more preferably 0.0015%, and even more preferably 0.0010%. It is preferable that the S content is as low as possible. However, an extreme reduction in the S content leads to an increase in the manufacturing cost. Therefore, considering industrial productivity, the preferable lower limit of the S content is 0.0001%, more preferably 0.0002%.

[0043] Cr: 21.00 - 28.00% Chromium (Cr) enhances the corrosion resistance of the base material 10. If the Cr content is too low, even if the contents of other elements are within the scope of this embodiment, the pitting corrosion resistance of the base material 10 will decrease. On the other hand, if the Cr content is too high, even if the contents of other elements are within the scope of this embodiment, intermetallic compounds such as sigma phase are likely to precipitate, and the hot workability, toughness, and corrosion resistance of the base material 10 will decrease. Therefore, the Cr content is 21.00 - 28.00%. The preferable lower limit of the Cr content is 21.50%, more preferably 22.00%, still more preferably 22.50%, still more preferably 23.00%, still more preferably 23.50%, and still more preferably 24.00%. The preferable upper limit of the Cr content is 27.50%, more preferably 27.00%, still more preferably 26.50%, and still more preferably 26.00%.

[0044] Ni: 4.00 - 8.00% Nickel (Ni) stabilizes the austenite phase. If the Ni content is too low, even if the contents of other elements are within the scope of this embodiment, the amount of austenite in the base material 10 will decrease, and the corrosion resistance and strength of the base material 10 will decrease. On the other hand, if the Ni content is too high, even if the contents of other elements are within the scope of this embodiment, the amount of ferrite in the base material 10 will decrease, and the corrosion resistance and strength of the base material 10 will decrease. In this case, furthermore, sigma phase precipitates in the base material 10. Therefore, the Ni content is 4.00 - 8.00%. The preferable lower limit of the Ni content is 4.50%, more preferably 5.00%, still more preferably 5.50%, and still more preferably 6.00%. The preferable upper limit of the Ni content is 7.50%, more preferably 7.00%.

[0045] Mo: 2.00 - 4.50% Molybdenum (Mo) enhances the corrosion resistance of the base material 10 in the same way as Cr. If the Mo content is too low, even if the contents of other elements are within the range of this embodiment, the pitting corrosion resistance and crevice corrosion resistance of the base material 10 will decrease. On the other hand, if the Mo content is too high, even if the contents of other elements are within the range of this embodiment, sigma phase is likely to precipitate, resulting in a decrease in the manufacturability of the base material 10 and a decrease in the toughness and corrosion resistance of the base material 10. Therefore, the Mo content is 2.00 - 4.50%. The preferable lower limit of the Mo content is 2.20%, more preferably 2.40%, and even more preferably 2.50%. The preferable upper limit of the Mo content is 4.30%, more preferably 4.00%.

[0046] Cu: 0.01 - 4.00% Copper (Cu) enhances the acid resistance of the base material 10 in a sulfuric acid or hydrogen sulfide environment. If the Cu content is too low, even if the contents of other elements are within the range of this embodiment, the acid resistance of the base material 10 will decrease. However, if the Cu content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability of the base material 10 will decrease. Therefore, the Cu content is 0.01 - 4.00%. The preferable lower limit of the Cu content is 0.05%, even more preferably 0.10%, even more preferably 0.15%, even more preferably 0.20%, even more preferably 0.25%, even more preferably 0.30%, even more preferably 0.35%, even more preferably 0.40%, even more preferably 0.45%. The preferable upper limit of the Cu content is 3.50%, more preferably 3.00%, even more preferably 2.50%, even more preferably 2.00%, even more preferably 1.50%, even more preferably 1.00%.

[0047] Sol.Al: 0.0010 - 0.0500% Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained even if the contents of other elements are within the scope of this embodiment. However, if the Al content is too high, even if the contents of other elements are within the scope of this embodiment, AlN will precipitate and the toughness and corrosion resistance of the base material 10 will decrease. Therefore, the Sol.Al content is 0.0010 - 0.0500%. The preferred lower limit of the Al content is 0.0030%, more preferably 0.0050%, still more preferably 0.0080%, still more preferably 0.0100%, and still more preferably 0.0120%. The preferred upper limit of the Al content is 0.0400%, more preferably 0.0300%, and still more preferably 0.0200%. Note that the Al content referred to in this specification means the content of "acid-soluble Al", that is, Sol.Al.

[0048] N: 0.080 - 0.400% Nitrogen (N) stabilizes the austenite phase and increases PREW, thereby enhancing the pitting corrosion resistance and crevice corrosion resistance of the base material 10. If the N content is too low, even if the contents of other elements are within the scope of this embodiment, the balance between the ferrite phase and the austenite phase of the base material 10 will be disrupted, and the corrosion resistance and strength of the base material 10 will decrease. However, if the N content is too high, even if the contents of other elements are within the scope of this embodiment, defects such as blowholes will occur during welding. Therefore, the N content is 0.080 - 0.400%. The preferred lower limit of the N content is 0.100%, more preferably 0.150%, and still more preferably 0.200%. The preferred upper limit of the N content is 0.370%, more preferably 0.350%, and still more preferably 0.320%.

[0049] B: 0.0001 - 0.0100% Boron (B) segregates at grain boundaries at high temperatures, enhancing the hot workability of the base material 10. B is also effective as a deoxidizer. If the B content is too low, this effect cannot be obtained even if the contents of other elements are within the range of this embodiment. However, if the B content is too high, even if the contents of other elements are within the range of this embodiment, solidification segregation occurs during the solidification process of the welded joint, increasing the solidification cracking susceptibility of the weld metal 20. Therefore, the B content is 0.0001 to 0.0100%. The preferred lower limit of the B content is 0.0005%, more preferably 0.0010%, and even more preferably 0.0015%. The preferred upper limit of the B content is 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, even more preferably 0.0050%, even more preferably 0.0040%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0050] The remainder of the base material 10 according to this embodiment consists of Fe and impurities. Here, the impurities are those mixed in from ores, scraps, or the manufacturing environment as raw materials when the base material 10 is industrially manufactured, and are those allowed within a range that does not adversely affect the duplex stainless steel welded joint 1 according to this embodiment. The impurities in the base material 10 are, for example, O (oxygen) and REM.

[0051] [Optional element] The above-mentioned base material 10 may further contain one or more elements selected from the group consisting of the following Group 1 to Group 5, instead of a part of Fe.

[0052] [Group 1] The chemical composition of the above-mentioned base material 10 may further contain W instead of a part of Fe. W is an optional element that forms oxides and enhances the corrosion resistance of the base material 10.

[0053] W: 0 to 4.00% Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, W forms a stable oxide to enhance the corrosion resistance of the base material 10 in an environment with a low pH. Even if a small amount of W is contained, the above effect can be obtained to a certain extent. However, if the W content is too high, even if the contents of other elements are within the range of this embodiment, the precipitation of intermetallic compounds is promoted and the toughness of the base material 10 decreases. Therefore, the W content is 0 to 4.00%. The preferable lower limit of the W content is more than 0%, more preferably 0.01%, still more preferably 0.50%, and even more preferably 1.00%. The preferable upper limit of the W content is 3.50%, more preferably 3.00%, still more preferably 2.50%.

[0054] [Group 2] The chemical composition of the above base material 10 may further contain one or more elements selected from the group consisting of Nb, V, and Ta in place of a part of Fe. All of these elements are optional elements, which generate carbides to enhance the corrosion resistance of the base material 10.

[0055] Nb: 0 to 0.10% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When 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 base material 10 is enhanced. Even if a small amount of Nb is contained, the above effect can be obtained to a certain extent. However, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, excessive carbides are precipitated, and conversely, the corrosion resistance of the base material 10 decreases. Therefore, the Nb content is 0 to 0.10%. The preferable lower limit of the Nb content is more than 0%, more preferably 0.01%, still more preferably 0.02%. The preferable upper limit of the Nb content is 0.08%, more preferably 0.07%, still more preferably 0.05%, and even more preferably 0.03%.

[0056] V: 0 to 0.20% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V combines with C to form carbides. Thereby, the formation of Cr carbides at the grain boundaries is suppressed, and the corrosion resistance of the base material 10 is enhanced. Even if a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content is too high, even if the contents of other elements are within the scope of this embodiment, excessive carbides will precipitate, and conversely, the corrosion resistance of the base material 10 will 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%, still more preferably 0.02%, still more preferably 0.05%. The preferable upper limit of the V content is 0.18%, more preferably 0.15%, still more preferably 0.10%, still more preferably 0.08%, still more preferably 0.07%, still more preferably 0.05%.

[0057] Ta: 0 to 0.30% Tantalum (Ta) is an optional element and may not be contained. That is, the Ta content may be 0%. When contained, Ta combines with C to form carbides. Thereby, the formation of Cr carbides at the grain boundaries is suppressed, and the corrosion resistance of the base material 10 is enhanced. Even if a small amount of Ta is contained, the above effects can be obtained to some extent. However, if the Ta content is too high, even if the contents of other elements are within the scope of this embodiment, excessive carbides will precipitate, and conversely, the corrosion resistance of the base material 10 will 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%, still more preferably 0.02%, still more preferably 0.05%, still more preferably 0.10%, still more preferably 0.15%. The preferable upper limit of the Ta content is 0.27%, more preferably 0.25%, still more preferably 0.20%, still more preferably 0.15%, still more preferably 0.10%, still more preferably 0.08%, still more preferably 0.07%, still more preferably 0.05%.

[0058] [Group 3] The chemical composition of the base material 10 described above may further contain Co in place of a part of Fe. Co is an optional element and enhances the acid resistance of the base material 10.

[0059] Co: 0 to 1.00% Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When contained, Co enhances the acid resistance of the base material 10 and stabilizes the austenite phase. Even if a small amount of Co is contained, the above effects can be obtained to some extent. However, if the Co content is too high, the cost increases even if the contents of other elements are within the range of this 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%, still more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20%, still more preferably 0.25%, and still more preferably 0.30%. The preferable upper limit of the Co content is 0.90%, more preferably 0.80%, still more preferably 0.70%, still more preferably 0.65%, and still more preferably 0.60%.

[0060] [Group 4] The chemical composition of the base material 10 described above may further contain Sn in place of a part of Fe. Sn is an optional element and enhances the pitting corrosion resistance of the base material 10.

[0061] Sn: 0 to 0.020% Tin (Sn) is an optional element and may not be contained. That is, the Sn content may be 0%. When contained, Sn enhances the pitting corrosion resistance of the base material 10. Even if a small amount of Sn is contained, the above effect can be obtained to a certain extent. However, if the Sn content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability of the base material 10 will decrease. If the Sn content is too high, furthermore, even if the contents of other elements are within the range of this embodiment, the penetration depth increases and the wettability of the molten metal during 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%, and even more preferably 0.003%. The preferable upper limit of the Sn content is 0.018%, more preferably 0.015%, even more preferably 0.010%, even more preferably 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0062] [Group 5] The chemical composition of the above base material 10 may further contain one or more elements selected from the group consisting of Mg and Ca in place of a part of Fe. Mg and Ca are optional elements and enhance the hot workability of the base material 10.

[0063] Mg: 0 to 0.0200% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg improves the hot workability of the base material 10. Even if a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the range of this embodiment, Mg combines with oxygen, significantly reducing the cleanliness and, conversely, reducing the hot workability of the base material 10. Therefore, the Mg content is 0 to 0.0200%. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0002%, still more preferably 0.0003%, still more preferably 0.0005%, still more preferably 0.0010%. The preferable upper limit of the Mg content is 0.0150%, more preferably 0.0130%, still more preferably 0.0100%, still more preferably 0.0090%, still more preferably 0.0080%, still more preferably 0.0070%, still more preferably 0.0060%, still more preferably 0.0050%, still more preferably 0.0040%, still more preferably 0.0030%, still more preferably 0.0020%.

[0064] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca is effective as a deoxidizer. Ca further fixes S and improves the hot workability of the base material 10. Even if a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content is too high, even if the contents of other elements are within the range of this embodiment, Ca combines with oxygen, significantly reducing the cleanliness of the base material 10 and lowering the hot workability of the base material 10. Therefore, the Ca content is 0 to 0.0100%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0002%, still more preferably 0.0005%, still more preferably 0.0010%, still more preferably 0.0020%. The preferable upper limit of the Ca content is 0.0090%, more preferably 0.0080%, still more preferably 0.0070%, still more preferably 0.0060%, still more preferably 0.0050%, still more preferably 0.0040%, still more preferably 0.0030%.

[0065] [Regarding the microstructure of the base material] The microstructure of the base material 10 according to this embodiment consists of ferrite and austenite. In this specification, "consisting of ferrite and austenite" means that the phases other than ferrite and austenite are so few as to be negligible. For example, in the microstructure of the base material 10 according to this embodiment, the volume fraction of precipitates and inclusions is so low as to be negligible compared to the volume fractions of ferrite and austenite. That is, the microstructure of the base material 10 according to this embodiment may contain a minute amount of precipitates, inclusions, etc. in addition to ferrite and austenite.

[0066] [Shape of the base material] The shape of the base material 10 is not particularly limited. The base material 10 may be, for example, one selected from the group consisting of a sheet material, a steel pipe, a bar, a wire rod, a forged product, and a shaped steel.

[0067] [Thickness of the base material] 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.

[0068] [Regarding the weld metal] The weld metal 20 will be described.

[0069] [Chemical composition of the weld metal] The chemical composition of the weld metal 20 according to this embodiment contains the following elements.

[0070] C: 0.001 to 0.030% Carbon (C) is an element effective for stabilizing the austenite phase. If the C content is too low, even if the contents of other elements are within the range of this embodiment, the amount of austenite in the weld metal 20 as-welded will decrease, and the corrosion resistance and strength of the weld metal 20 will deteriorate. On the other hand, if the C content is too high, even if the contents of other elements are within the range of this embodiment, carbides are likely to precipitate, and the corrosion resistance of the weld metal 20 will deteriorate. 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%, still more preferably 0.005%, still more preferably 0.010%, still more preferably 0.015%. The preferable upper limit of the C content is 0.025%, more preferably 0.020%.

[0071] Si: 0.05 to 0.70% Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be fully obtained even if the contents of other elements are within the scope of this embodiment. On the other hand, Si stabilizes the ferrite phase. If the Si content is too high, even if the contents of other elements are within the scope of this embodiment, the amount of ferrite in the as-welded weld metal 20 increases, and the corrosion resistance and strength of the weld metal 20 decrease. Si further increases the viscosity of the molten metal during welding. In order to suppress the undercut 3, it is necessary to further limit the Si content in the chemical composition of the weld metal 20 compared with 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%, still more preferably 0.20%, still more preferably 0.30%, still more preferably 0.40%, still more preferably 0.50%, still more preferably 0.60%. The preferable upper limit of the Si content is 0.65%, more preferably 0.60%, still more preferably 0.55%, still more preferably 0.50%.

[0072] Mn: 0.05 to 0.85% Manganese (Mn) stabilizes the austenite phase. If the Mn content is too low, even if the contents of other elements are within the scope of this embodiment, the amount of austenite in the as-welded weld metal decreases, 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 during welding. In other words, Mn increases the temperature dependence of the surface tension of the molten metal during welding. If the Mn content is too high, even if the contents of other elements are within the scope of this embodiment, the wettability of the molten metal during welding decreases. In order to suppress the undercut 3, it is necessary to further limit the Mn content in the chemical composition of the weld metal 20 compared with the Mn content of the base material 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%, still more preferably 0.15%, still more preferably 0.20%. The preferable upper limit of the Mn content is 0.80%, more preferably 0.75%, still more preferably 0.70%.

[0073] P: Below 0.030% Phosphorus (P) is an impurity inevitably contained. That is, the lower limit of the P content is over 0%. P significantly increases the weld cracking susceptibility 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%, still more preferably 0.023%, and even more preferably 0.020%. It is preferable that the P content is as low as possible. However, an extreme reduction in the P content leads to an increase in the manufacturing cost. Therefore, considering industrial productivity, the preferable lower limit of the P content is 0.001%, more preferably 0.002%.

[0074] S: Below 0.0030% Sulfur (S) is an impurity inevitably contained. That is, the lower limit of the S content is over 0%. S significantly increases the weld cracking susceptibility 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%, more preferably 0.0020%, still more preferably 0.0015%, and even more preferably 0.0010%. It is preferable that the S content is as low as possible. However, an extreme reduction in the S content leads to an increase in the manufacturing cost. Therefore, considering industrial productivity, the preferable lower limit of the S content is 0.0001%, more preferably 0.0002%.

[0075] Cr: 21.00 - 28.00% Chromium (Cr) enhances the corrosion resistance of the weld metal 20. If the Cr content is too low, even if the contents of other elements are within the scope of this 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 other elements are within the scope of this 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 - 28.00%. The preferable lower limit of the Cr content is 21.50%, more preferably 22.00%, still more preferably 22.50%, still more preferably 23.00%, still more preferably 23.50%, still more preferably 24.00%. The preferable upper limit of the Cr content is 27.50%, more preferably 27.00%, still more preferably 26.50%, still more preferably 26.00%.

[0076] Ni: 5.00 - 11.00% Nickel (Ni) stabilizes the austenite phase. If the Ni content is too low, even if the contents of other elements are within the scope of this 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 other elements are within the scope of this 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, furthermore, sigma phase precipitates in the weld metal. In the weld metal 20, in order to promote the transformation from ferrite to austenite during the rapid cooling process during welding and control the amount of ferrite to have the same performance as the base metal 10, it is preferable that the Ni content is higher than that of the base metal 10. Therefore, the Ni content is 5.00 - 11.00%. The preferable lower limit of the Ni content is 5.50%, more preferably 6.00%, still more preferably 6.50%, still more preferably 7.00%, still more preferably 7.50%. The preferable upper limit of the Ni content is 10.50%, more preferably 10.00%, still more preferably 9.50%, still more preferably 9.00%, still more preferably 8.50%, still more preferably 8.00%.

[0077] Mo: 2.00 to 4.50% Similar to Cr, molybdenum (Mo) enhances the corrosion resistance of the weld metal 20. If the Mo content is too low, even if the contents of other elements are within the range of this embodiment, the pitting corrosion resistance and crevice corrosion resistance of the weld metal 20 will decrease. On the other hand, if the Mo content is too high, even if the contents of other elements are within the range of this embodiment, sigma phase is likely to precipitate, resulting in a decrease in the manufacturability of the weld metal 20, and a decrease in the toughness and corrosion resistance of the weld metal 20. Therefore, the Mo content is 2.00 to 4.50%. The preferred lower limit of the Mo content is 2.50%, more preferably 3.00%. The preferred upper limit of the Mo content is 4.30%, more preferably 4.00%.

[0078] Cu: 0.01 to 4.00% Copper (Cu) enhances the acid resistance of the weld metal 20 in sulfuric acid and hydrogen sulfide environments. If the Cu content is too low, even if the contents of other elements are within the range of this embodiment, the acid resistance of the weld metal 20 will decrease. However, if the Cu content is too high, even if the contents of other elements are within the range of this embodiment, the solid solution amount of N will decrease and denitrification during welding will be accelerated, and welding defects such as blowholes are likely to occur. Therefore, the Cu content is 0.01 to 4.00%. The preferred lower limit of the Cu content is 0.20%, more preferably 0.30%, still more preferably 0.40%, still more preferably 0.50%. The preferred upper limit of the Cu content is 3.50%, more preferably 3.00%, still more preferably 2.50%, still more preferably 2.00%, still more preferably 1.50%, still more preferably 1.00%.

[0079] Sol.Al: 0.0010 to 0.0500% Aluminum (Al) deoxidizes steel. Al further reduces the amount of oxygen in the molten metal during welding and suppresses the convex shape of the weld metal 20. If the Al content is too low, this effect cannot be obtained even if the contents of other elements are within the scope of this embodiment. However, if the Al content is too high, even if the contents of other elements are within the scope of this embodiment, AlN precipitates and the toughness and corrosion resistance of the weld metal 20 decrease. 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%, still more preferably 0.0100%, and still more preferably 0.0150%. The preferable upper limit of the Al content is 0.0400%, more preferably 0.0350%, still more preferably 0.0300%, still more preferably 0.0250%, and still more preferably 0.0200%. Note that the Al content referred to in this specification means the content of "acid-soluble Al", that is, Sol.Al.

[0080] N: 0.080 to 0.400% Nitrogen (N) stabilizes the austenite phase and increases PREW, thereby enhancing 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 other elements are within the scope of this embodiment, the balance between the ferrite phase and the austenite phase of the weld metal 20 is disrupted, 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 other elements are within the scope of this embodiment, defects such as blowholes occur during 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%, still more preferably 0.200%, and still more preferably 0.250%. The preferable upper limit of the N content is 0.370%, more preferably 0.350%, and still more preferably 0.320%.

[0081] B: 0.0001 to 0.0100% Boron (B) segregates at grain boundaries at high temperatures, enhancing the hot workability of the weld metal 20. B is also effective as a deoxidizer. If the B content is too low, this effect cannot be obtained even if the contents of other elements are within the range of this embodiment. However, if the B content is too high, even if the contents of other elements are within the range of this embodiment, solidification segregation occurs during the solidification process of the welded portion, increasing the solidification crack susceptibility of the weld metal 20. 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%, still more preferably 0.0060%, still more preferably 0.0050%, still more preferably 0.0040%, still more preferably 0.0030%, still more preferably 0.0020%.

[0082] The remainder of the weld metal 20 according to this embodiment consists of Fe and impurities. Here, the impurities are those mixed in from the welding material, the base material 10, etc. when the weld metal 20 is formed by welding, and are meant to be those allowed within a range that does not adversely affect the duplex stainless steel welded joint 1 of this embodiment. The impurities in the weld metal 20 are, for example, O (oxygen) and REM.

[0083] [Optional element] The above-mentioned weld metal 20 may further 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.

[0084] [Group 1] The chemical composition of the above-mentioned weld metal 20 may further contain W in place of a part of Fe. W is an optional element and forms oxides, enhancing the corrosion resistance of the weld metal 20.

[0085] W: 0 to 4.00% Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When contained, W forms a stable oxide to enhance the corrosion resistance of the weld metal 20 in an environment with a low pH. Even if a small amount of W is contained, the above effect can be obtained to a certain extent. However, if the W content is too high, even if the contents of other elements are within the scope of this embodiment, the precipitation of intermetallic compounds is promoted and the toughness of the weld metal 20 decreases. Therefore, the W content is 0 to 4.00%. The preferable lower limit of the W content is more than 0%, more preferably 0.01%, still more preferably 0.05%, still more preferably 0.50%, still more preferably 1.00%, still more preferably 1.50%, and still more preferably 2.00%. The preferable upper limit of the W content is 3.50%, more preferably 3.00%, and still more preferably 2.50%.

[0086] [Group 2] The chemical composition of the above-mentioned weld metal 20 may further contain one or more elements selected from the group consisting of Nb, V, and Ta in place of a part of Fe. All of these elements are optional elements, which generate carbides to enhance the corrosion resistance of the weld metal 20.

[0087] Nb: 0 to 0.10% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb combines with C to generate carbides. Thereby, the generation of Cr carbides at the grain boundaries is suppressed and the corrosion resistance of the weld metal 20 is enhanced. Even if a small amount of Nb is contained, the above effect can be obtained to a certain extent. However, if the Nb content is too high, even if the contents of other elements are within the scope of this embodiment, excessive carbides are precipitated, and conversely, the corrosion resistance of the weld metal 20 decreases. Therefore, the Nb content is 0 to 0.10%. The preferable lower limit of the Nb content is more than 0%, more preferably 0.01%, and still more preferably 0.02%. The preferable upper limit of the Nb content is 0.08%, more preferably 0.07%, and still more preferably 0.05%.

[0088] V: 0 to 0.20% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V combines with C to form carbides. Thereby, the formation of Cr carbides at the grain boundaries is suppressed, and the corrosion resistance of the weld metal 20 is enhanced. Even if a little V is contained, the above effects can be obtained to some extent. However, if the V content is too high, even if the contents of other elements are within the scope of this embodiment, excessive carbides will be precipitated, and conversely, the corrosion resistance of the weld metal 20 will 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%, still more preferably 0.02%. The preferable upper limit of the V content is 0.18%, more preferably 0.15%, still more preferably 0.10%, still more preferably 0.08%, more preferably 0.07%, still more preferably 0.05%.

[0089] Ta: 0 to 0.30% Tantalum (Ta) is an optional element and may not be contained. That is, the Ta content may be 0%. When contained, Ta combines with C to form carbides. Thereby, the formation of Cr carbides at the grain boundaries is suppressed, and the corrosion resistance of the weld metal 20 is enhanced. Even if a little Ta is contained, the above effects can be obtained to some extent. However, if the Ta content is too high, even if the contents of other elements are within the scope of this embodiment, excessive carbides will be precipitated, and conversely, the corrosion resistance of the weld metal 20 will 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%, still more preferably 0.02%. The preferable upper limit of the Ta content is 0.27%, more preferably 0.25%, still more preferably 0.20%, still more preferably 0.15%, still more preferably 0.10%, still more preferably 0.08%, more preferably 0.07%, still more preferably 0.05%.

[0090] [Group 3] The chemical composition of the above-mentioned weld metal 20 may further contain Co in place of a part of Fe. Co is an optional element and enhances the acid resistance of the weld metal 20.

[0091] Co: 0 to 1.00% Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When contained, Co enhances the acid resistance of the weld metal 20 and stabilizes the austenite phase. Even if a little Co is contained, the above effects can be obtained to some extent. However, if the Co content is too high, the cost increases even if the contents of other elements are within the range of this 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%, still more preferably 0.10%, and even more preferably 0.20%. The preferable upper limit of the Co content is 0.90%, more preferably 0.80%, still more preferably 0.70%, even more preferably 0.60%, and even more preferably 0.50%.

[0092] [Group 4] The chemical composition of the above-mentioned weld metal 20 may further contain Sn in place of a part of Fe. Sn is an optional element and enhances the pitting corrosion resistance of the weld metal 20.

[0093] Sn: 0 to 0.020% Tin (Sn) is an optional element and may not be contained. That is, the Sn content may be 0%. When contained, Sn enhances the pitting corrosion resistance of the welding metal 20. Even if a small amount of Sn is contained, the above effect can be obtained to a certain extent. However, if the Sn content is too high, even if the contents of other elements are within the scope of this embodiment, the hot workability of the welding metal 20 will deteriorate. Further, if the Sn content is too high, even if the contents of other elements are within the scope of this embodiment, the penetration depth increases and the wettability of the molten metal during welding deteriorates. Therefore, the Sn content is 0 to 0.020%. The preferable lower limit of the Sn content is 0.001%, more preferably 0.002%, still more preferably 0.003%. The preferable upper limit of the Sn content is 0.018%, more preferably 0.015%, still more preferably 0.010%, still more preferably 0.009%, still more preferably 0.008%, still more preferably 0.007%, still more preferably 0.005%.

[0094] [Group 5] The chemical composition of the above-mentioned welding metal 20 may further contain one or more elements selected from the group consisting of Mg and Ca in place of a part of Fe. Mg and Ca are optional elements and enhance the hot workability of the welding metal 20.

[0095] Mg: 0 to 0.0200% Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg improves the wettability of the molten metal during welding due to its deoxidizing effect. Even if a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content is too high, even if the contents of other elements are within the range of this embodiment, Mg combines with oxygen, significantly reducing the cleanliness and lowering the toughness of the weld metal 20. Therefore, the Mg content is 0 to 0.0200%. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0002%, still more preferably 0.0003%. The preferable upper limit of the Mg content is 0.0150%, more preferably 0.0130%, still more preferably 0.0100%, still more preferably 0.0080%, still more preferably 0.0060%, still more preferably 0.0040%, still more preferably 0.0020%, still more preferably 0.0010%.

[0096] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca is effective as a deoxidizer. Ca further fixes S to enhance the solidification cracking resistance of the weld metal 20. Even if a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content is too high, even if the contents of other elements are within the range of this embodiment, Ca combines with oxygen, significantly reducing the cleanliness of the weld metal 20 and lowering the toughness of the weld metal 20. Therefore, the Ca content is 0 to 0.0100%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0002%, still more preferably 0.0005%, still more preferably 0.0010%, still more preferably 0.0015%. The preferable upper limit of the Ca content is 0.0070%, more preferably 0.0050%, still more preferably 0.0030%, still more preferably 0.0020%.

[0097] [Method for Measuring Chemical Composition of Weld Metal] The chemical composition of the weld metal 20 is measured by the following method. At the central position of the Cap width of the weld metal 20, the surface of the weld metal 20 is shaved to collect swarf. The collected swarf is dissolved in an acid to obtain a solution. For the solution, ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) is performed to conduct elemental analysis of the chemical composition. The C content and the S content are determined by a well-known high-frequency combustion method. Specifically, the above solution is burned by high-frequency heating in an oxygen stream to detect the generated carbon dioxide and sulfur dioxide, and the C content and the S content are determined. By the above analysis methods, the chemical composition of the weld metal 20 can be determined.

[0098] [Regarding the Microstructure of Duplex Stainless Steel Weld Metal] The microstructure of the weld metal 20 according to the present embodiment consists of ferrite and austenite. In this specification, "consisting of ferrite and austenite" means that the phases other than ferrite and austenite are so few that they can be ignored. For example, in the microstructure of the weld metal 20 according to the present embodiment, the volume fraction of precipitates and inclusions 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 according to the present embodiment may contain a minute amount of precipitates, inclusions, etc. in addition to ferrite and austenite.

[0099] [Regarding Formula (1) and Formula (2)] In the duplex stainless steel weld joint 1 of the present embodiment, the content of each element in the base metal 10 and the weld metal 20 satisfies the above range and satisfies Formula (1) and Formula (2). Si + 3Mn ≦ 3.00 (1) 0 < BH / BW < 0.15 (2) Here, the content of the corresponding element in the weld metal 20 in mass% is substituted for each element symbol in Formula (1). Here, the Cap height (mm) of the weld metal 20 is substituted for BH and the Cap width (mm) of the weld metal 20 is substituted for BW in Formula (2).

[0100] [Regarding Formula (1)] Define F1 = Si + 3Mn. If F1 exceeds 3.00, the viscosity of the molten metal during welding increases, and furthermore, the temperature coefficient of the surface tension of the molten metal during welding increases. In this case, the wettability of the molten metal during welding decreases, and 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, still more preferably 2.85, still more preferably 2.80, still more preferably 2.75, still more preferably 2.70, still more preferably 2.65, still more preferably 2.60, still more preferably 2.55, still more preferably 2.50, still more preferably 2.45, still more preferably 2.40. The lower limit of F1 is 0.20. The lower limit of F1 is preferably 0.40, more preferably 0.60, still more preferably 0.80, still more preferably 1.00, still more preferably 1.20, still more preferably 1.40, still more preferably 1.60, still more preferably 1.80, still more preferably 2.00, still more preferably 2.20, still more preferably 2.40.

[0101] [Regarding Formula (2)] Referring to FIGS. 2 and 3, the Cap height (BH) of the weld metal 20 refers to the maximum height of the weld metal 20 when the surface of the base material 10 is used as a reference (0 mm). Also, the Cap width (BW) of the weld metal 20 refers to the width of the weld metal 20 on the surface of the base material 10 in a direction perpendicular to the extending direction of the weld metal 20. When the base material 10 is a steel pipe, the Cap width of the weld metal 20 refers to the straight-line distance between the boundaries of the base material 10 and the weld metal 20 in a direction perpendicular to the extending 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 using a well-known welding gauge or the like. When the base material 10 is a steel pipe, the gauge is brought into contact with the outer surface of the base material 10 so that the longitudinal direction of the well-known gauge is parallel to the axial direction of the base material 10, and the Cap height is measured.

[0102] Define F2 as F2 = BH / BW. If F2 is 0.15 or more, the Cap height relative to the Cap width is too high. In this case, as the surface tension of the molten metal during welding increases, the formed weld metal 20 tends to have a convex shape, and undercut 3 is likely to occur. However, when F2 becomes negative, the thickness of the weld metal 20 is thinner compared to the thickness of the base material 10. In this case, it becomes a welding defect called underfill. In this case, the strength of the duplex stainless steel weld 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, still more preferably 0.12, still more preferably 0.11, still more preferably 0.10, still more preferably 0.09, still more preferably 0.08. The lower limit of F2 is preferably 0.01, more preferably 0.02, still more preferably 0.04, still more preferably 0.05, still more preferably 0.06, still more preferably 0.07, still more preferably 0.08, still more preferably 0.09, still more preferably 0.10.

[0103] In the duplex stainless steel weld joint 1 of the present embodiment, after satisfying formula (1) to enhance the wettability of the molten metal during welding, the shape of the weld metal 20 is further adjusted to satisfy formula (2). Only when the base material 10 and the weld metal 20 have the above-described chemical compositions and the weld metal 20 satisfies formula (1) and formula (2), can the undercut 3 be suppressed and the fatigue characteristics be enhanced.

[0104] [Regarding formula (3) and formula (4)] In the duplex stainless steel weld joint 1 of the present embodiment, the content of each element in the base material 10 and the weld metal 20 satisfies the above range, satisfies formula (1) and formula (2), and when the thickness of the base material 10 is 2.5 mm or less, it is further preferable to satisfy formula (3) and formula (4). Si + 4Mn ≤ 3.75 (3) 0 < BH / BW ≤ 0.5 / (6.0 - 0.85WT) (4) Here, the content of the corresponding element in the welding metal 20 is substituted for each element symbol in formula (3) in mass %. Here, the Cap height (mm) of the welding metal 20 is substituted for BH in formula (4), the Cap width (mm) of the welding metal 20 is substituted for BW, and the thickness (mm) of the base material 10 is substituted for WT.

[0105] [Regarding formula (3)] Define F3 = Si + 4Mn. If F3 is 3.75 or less, the wettability of the molten metal during welding is further enhanced, and undercut 3 can be further suppressed. Therefore, F3 is preferably 3.75 or less. The upper limit of F3 is more preferably 3.70, still more preferably 3.65, still more preferably 3.60, still more preferably 3.55, still more preferably 3.50, still more preferably 3.45, still more preferably 3.40, still more preferably 3.35, still more preferably 3.30, still more preferably 3.25, still more preferably 3.20, still more preferably 3.15, still more preferably 3.10, still more preferably 3.05, and still more preferably 3.00. The lower limit of F3 is preferably 0.25. The lower limit of F3 is more preferably 0.50, still more preferably 0.70, still more preferably 1.00, still more preferably 1.50, still more preferably 2.00, still more preferably 2.30, still more preferably 2.50, and still more preferably 3.00.

[0106] [Regarding formula (4)] If the base material 10 is thin, the base material 10 is thin with respect to the undercut 3, and the fatigue life is likely to decrease. Therefore, on the premise that F2 (BH / BW) is more than 0 and less than 0.15, it is preferable to further limit F2 according to the thickness of the base material 10. Specifically, when 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. A more preferable upper limit of F2 is 0.4 / (6.0 - 0.85WT), more preferably 0.3 / (6.0 - 0.85WT), and even more preferably 0.2 / (6.0 - 0.85WT).

[0107] [Manufacturing Method of Duplex Stainless Steel Welded Joint] Hereinafter, the 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 hereinafter 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 may be manufactured by a manufacturing method other than the manufacturing method described hereinafter. However, the manufacturing method described hereinafter is a preferable example of the manufacturing method of the duplex stainless steel welded joint 1 of the present embodiment.

[0108] The manufacturing method of the duplex stainless steel welded joint 1 of the present embodiment includes a step of preparing a pair of base materials 10 (base material preparation step) and a step of butting the ends of the pair of base materials 10 and performing welding to form a weld metal 20 (weld metal formation step). Hereinafter, each step will be described in detail.

[0109] [Base Material Preparation Step] First, a pair of base materials 10 are prepared. The base material 10 may be provided by a third party, or the manufacturer of the duplex stainless steel welded joint 1 may manufacture and prepare the base material 10. Hereinafter, an example of the manufacturing method of the base material 10 when manufacturing the base material 10 will be described.

[0110] The manufacturing method of the base material 10 includes a step of preparing the material of the base material 10 (preparation step), a step of performing hot working on the material to manufacture an intermediate material as needed (hot working step), a step of performing pickling treatment on the intermediate material after the hot working step and then performing cold working as needed (cold working step), a step of performing solution heat treatment on the material prepared in the material preparation step, the intermediate material after the hot working step, or the intermediate material after the cold working step as needed (solution heat treatment step), and a step of removing the scale on the surface of the material or the intermediate material as needed (scale removal step). Hereinafter, each step will be described.

[0111] [Preparation Step] In the preparation step, a material having the above-described chemical composition is prepared. The material may be supplied by a third party or manufactured. The material may be an ingot, or may be a slab, bloom, or billet. When manufacturing the material, the material 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 the ingot casting method. Using the manufactured molten alloy, a slab, bloom, billet, or seamless tube may be manufactured by the casting method. Hot working may be performed on the manufactured ingot, slab, or bloom to manufacture a billet or seamless tube. For example, hot forging may be performed on an ingot to manufacture a columnar billet, and this billet may be used as the material (columnar material). In this case, the temperature of the material immediately before the start of hot forging is not particularly limited, but is, for example, 900 to 1300°C. Also, a material (steel pipe) may be manufactured by a well-known centrifugal casting method.

[0112] [Hot Working Step] The hot working process is carried out as required. That is, the hot working process may not be carried out. When carried out, hot working is performed on the material to manufacture an intermediate material with a predetermined shape. When the base material 10 is a plate material, a plate-shaped intermediate material is manufactured by hot rolling. When the base material 10 is an alloy tube, a through hole is formed along the central axis of the cylindrical material by machining. Hot extrusion is carried out on the cylindrical material with the through hole formed to manufacture an intermediate material (tube material). The heating temperature of the material in the hot working process is not particularly limited, for example, it is 900 to 1300 °C.

[0113] In the hot working process, instead of hot extrusion, piercing rolling by the Mannesmann method may be carried out on the cylindrical material to manufacture an intermediate material (tube material). The temperature of the material before piercing rolling is, for example, 900 to 1300 °C.

[0114] [Cold working process] The cold working process is carried out as required. That is, the cold working process may not be carried out. When carried out, pickling treatment is carried out on the intermediate material, and then cold working is carried out. When the base material 10 is a plate material, cold rolling is carried out. When the base material 10 is a steel pipe, cold drawing is carried out. The reduction ratio in the cold working process is not particularly limited, for example, it is 10 to 90%.

[0115] [Solution heat treatment process] The solution heat treatment process is carried out as required. That is, the solution heat treatment process may not be carried out. When carried out, solution heat treatment is carried out on the material prepared in the preparation process, the intermediate material after the hot working process, or the intermediate material after the cold working process. By solution heat treatment, the precipitates of the material or the intermediate material are dissolved.

[0116] The solution heat treatment is carried out in the following manner. A material or an intermediate material is charged into a heat treatment furnace with an atmospheric atmosphere in the furnace. The atmospheric atmosphere here means an atmosphere containing 78% or more by volume of nitrogen, which is a gas constituting the atmosphere, and 20% or more by volume of oxygen. In the furnace with an atmospheric atmosphere, the material or the intermediate material is heated to 1000 - 1300 °C and held at 1000 - 1300 °C. The holding time is 1 - 60 minutes. The material or the intermediate material after the heat treatment is rapidly cooled. The rapid cooling method is, for example, well-known water cooling or well-known oil cooling.

[0117] [Scale removal process] The scale removal process is carried out as required. That is, the scale removal process may not be carried out. When carried out, the scale on the surface of the material prepared in the preparation process, the intermediate material after the hot working process, the intermediate material after the cold working process, or the material or the intermediate material after the solution heat treatment is removed. The method for removing the scale may be a method for removing the scale by blasting, grinding, etc., or a method for removing the scale by pickling treatment.

[0118] When carrying out the pickling treatment, the pickling conditions are not particularly limited. Preferably, as the pickling solution, a mixed solution of nitric acid and hydrofluoric acid is used. The mixed solution is, for example, an aqueous solution containing 5.0 - 8.0% by volume of nitric acid and 5.0 - 8.0% by volume of hydrofluoric acid. The temperature of the pickling solution in the pickling solution tank is adjusted to 30 - 50 °C, and the material or the intermediate material is immersed in the pickling solution tank. The immersion time is, for example, 0.5 - 5.0 hours. By the above pickling treatment, the scale is sufficiently removed from the surface of the material or the surface of the intermediate material.

[0119] Blasting means a process of imparting kinetic energy to abrasive materials to collide with the surface of a material or an intermediate material and cutting or striking the metal surface. When removing the scale by blasting, the blasting may be, for example, sandblasting using sand as the abrasive material, shot blasting using steel grains as the abrasive material, grid blasting using cast iron grids, cast steel grids, alumina grids, silicon carbide grids, etc. as the abrasive material, shot blasting using cast iron shots, cast steel shots, cut wires, etc. as the abrasive material.

[0120] By the above process, the base material 10 is manufactured.

[0121] A groove may be formed on the prepared base material 10. Specifically, a groove may be formed at the end of the base material 10 by a well-known processing method. The groove shape may be the V shape shown in FIG. 3, or may be other shapes other than those shown in FIG. 3.

[0122] [Weld metal forming process] In the weld metal forming process, welding is performed on the prepared base material 10 to form a weld metal 20, and a duplex stainless steel welded joint 1 is manufactured. Specifically, two base materials 10 are prepared. The ends of the prepared base materials 10 are abutted against each other. Then, welding is performed on the pair of abutted ends using a welding material to form a weld metal 20 having the above-described chemical composition.

[0123] [Regarding the welding material] The welding material for forming the above-described weld metal 20 contains, by 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 consists of Fe and impurities.

[0124] The above welding material may be supplied by a third party or may be the manufactured one. When manufacturing the welding material, casting is carried out using the molten metal of the welding material having the above chemical composition to form an ingot. The ingot is hot-worked to manufacture the welding material. Further cold working may be carried out on the welding material after hot working. Also, a well-known heat treatment may be carried out on the welding material. The heat treatment is, for example, a solution heat treatment similar to that of the base material 10. The heat treatment may not be carried out. The welding material may be in a rod shape (including wire) or in a small block shape.

[0125] Using the welding material having the above chemical composition, the weld metal 20 is formed. The welding method is, for example, TIG welding (GTAW), shielded metal arc welding (SMAW), flux cored arc welding (FCAW), gas metal arc welding (GMAW), and submerged arc welding (SAW).

[0126] At this time, the interval between the ends of the pair of butted base materials 10 and the feeding amount of the welding material during welding are adjusted to adjust the dilution amount by the base material 10, so that the content of each element of the chemical composition of the weld metal 20 is within the range of this embodiment, and F1≤3.00 and 0<F2<0.15. When a groove is formed in the base material 10, the thickness of the root face of the groove may be further adjusted. By adjusting these conditions, the content of each element of the chemical composition of the weld metal 20 can be adjusted to be within the range of this embodiment, and F1≤3.00 and 0<F2<0.15.

[0127] By the above manufacturing process, the welded joint 1 according to this embodiment can be manufactured. Note that the manufacturing method of the welded joint 1 according to this embodiment is not limited to the above 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 range, the content of each element of the chemical composition of the weld metal 20 is within the above range, and F1≤3.00 and 0<F2<0.15, the welded joint 1 of this embodiment is not particularly limited to the above manufacturing method.

Example

[0128] [Manufacture of Base Metal] The molten steel for the base metal shown in Table 1 was manufactured. The blanks in Table 1 indicate that the contents of the corresponding elements were at impurity levels.

[0129]

Table 1

[0130] Using the molten steel, a cylindrical ingot with an outer diameter of 120 mm and a weight of 30 kg was manufactured. Hot forging was performed on the ingot to obtain a bar with a diameter of 180 mm. Further, hot rolling and cold rolling were carried out to manufacture a seamless pipe with an outer diameter of 15.12 mm × wall thickness of 1.21 mm, or a seamless pipe with an outer diameter of 30.72 mm × wall thickness of 2.66 mm. For each seamless pipe, solution heat treatment was carried out at 1100 °C for 10 minutes and then water-cooled. Each seamless pipe was cut to a length of 300 mm. For the seamless pipe with a wall thickness of 2.66 mm, a groove was formed by simulating the weld root. Specifically, a groove with a root thickness of 1.5 mm, a one-sided protruding length of 2 mm, and a one-sided inclination of 20° was machined at the center in the longitudinal direction.

[0131] [Manufacture of Welding Material] The molten steel for the welding material having the chemical composition shown in Table 2 was manufactured. The blanks in Table 2 indicate that the contents of the corresponding elements were at impurity levels.

[0132]

Table 2

[0133] Using a vacuum melting furnace, a 30 kg ingot was manufactured. Hot forging and hot rolling were performed on the ingot to obtain a bar with a diameter of 30 mm. Thereafter, cold rolling and intermediate annealing were repeated to manufacture a spool coil with a diameter of 0.8 mm.

[0134] [Formation of Weld Metal] Both ends of the seamless steel pipe produced were fixed, and welding was carried out on the longitudinal center part of the seamless steel pipe (in the case of a seamless steel pipe with a wall thickness of 2.66 mm, the center part of the groove processing). Specifically, using the welding materials in Table 2, automatic gas tungsten arc welding was carried out in the downward direction (1G) while rotating the seamless steel pipe in the circumferential direction. Here, the welding conditions were changed for each test number, and the dilution rate by the base material and BH / BW were adjusted. Specifically, for the seamless steel pipe with a wall thickness of 1.21 mm, while adjusting the heat input in the range of 150 - 230 J / mm, the supply rate of the welding material was changed in the range of 150 - 900 mm / min. For the seamless steel pipe with a wall thickness of 2.66 mm, two-pass welding was applied. For the first pass of welding, the heat input was set to approximately 500 J / mm, and the supply rate of the welding material was set to 400 - 500 mm / min. For the second pass of welding, in order to change the dilution rate by the base material, while adjusting the heat input in the range of 470 - 550 J / mm, the supply rate of the welding material was changed in the range of 1500 - 2000 mm / min. Also, in any welding, Ar + 2%N 2 was used as the shielding gas, and the flow rate was set to 10 L / min. Thereby, the weld metal was formed. Five samples of the weld metal were prepared for each test number.

[0135] Regarding the formed weld metal, at a position 180° away from the start and end of welding in the circumferential direction of the base material, the Cap height (mm) and Cap width (mm) of the weld metal were measured using a welding gauge. The results are shown in Table 3.

[0136]

Table 3

[0137] At a position 180° away from the start and end points of 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 shaved to collect chips. In the case of seamless steel pipes with a wall thickness of 2.66 mm, chips were collected from the weld metal part of the second pass. Check analysis was performed on the obtained chips to examine the chemical composition of the weld metal. The results are shown in Table 4. The blank spaces in Table 4 indicate that the content of the corresponding element was at the impurity level. Among the chemical composition of the weld metal, the Si and Mn contents are also shown in Table 3.

[0138]

Table 4

[0139] [Undercut Judgment Test] At a position 180° away from the start and end points of welding in 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-sectional observation piece including the weld metal was collected. Five samples of each test number were observed, and at the weld termination part in the cross-sectional observation piece, the presence or absence of undercut and the depth of undercut were evaluated. When the depth of undercut was less than 0.05 mm in all of the five samples, it was evaluated as E (Excellent). Even if undercut occurred, when the depth of undercut was 0.05 mm or more and less than 0.10 mm, it was evaluated as G (Good). When undercut with a depth of 0.10 mm or more occurred in even one of the five samples, it was evaluated as B (Bad). The results are shown in the "Undercut" column of Table 3.

[0140] [Evaluation Results] Referring to Table 3, Test Numbers 3, 6 - 8, 10, 12, and 14 - 30 had appropriate contents of each element in the base material and the weld metal, and satisfied Equation (1) (Si + 3Mn ≤ 3.00) and Equation (2) (0 < BH / BW < 0.15). Therefore, in Test Numbers 3, 6 - 8, 10, 12, and 14 - 30, the depth of undercut was less than 0.10 mm. Therefore, in Test Numbers 3, 6 - 8, 10, 12, and 14 - 30, undercut was suppressed.

[0141] Furthermore, by comparing Test Nos. 3, 6 to 8, 15 to 17, 19 to 20, and 27 to 29 where the base material thickness was 2.5 mm or less, the following results were obtained. For Test Nos. 3, 6 to 8, 16, 17, 19 to 20, and 27 to 29 that satisfied Formula (3) and Formula (4) in addition to Formula (1) and Formula (2), the undercut depth was less than 0.05 mm. Therefore, for Test Nos. 3, 6 to 8, 16, 17, 19 to 20, and 27 to 29, the undercut could be further suppressed compared to Test No. 15.

[0142] On the other hand, in Test Nos. 1 and 2, although the content of each element in the base material and the weld metal was appropriate, Formula (1) was not satisfied. As a result, an undercut with a depth of 0.10 mm or more occurred. Therefore, in Test Nos. 1 and 2, the undercut could not be suppressed.

[0143] In Test Nos. 4 and 5, although the content of each element in the base material and the weld metal was appropriate, Formula (1) and Formula (2) were not satisfied. As a result, an undercut with a depth of 0.10 mm or more occurred. Therefore, in Test Nos. 4 and 5, the undercut could not be suppressed.

[0144] In Test Nos. 9, 11, and 13, although the content of each element in the base material and the weld metal was appropriate, Formula (2) was not satisfied. As a result, an undercut with a depth of 0.10 mm or more occurred. Therefore, in Test Nos. 9, 11, and 13, the undercut could not be suppressed.

[0145] In Test No. 31, although the content of each element in the base material was appropriate, Formula (1) was not satisfied. As a result, an undercut with a depth of 0.10 mm or more occurred. Therefore, in Test No. 31, the undercut could not be suppressed.

[0146] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the gist thereof.

Explanation of Reference Numerals

[0147] 1 Welded joint 10 Base material 20 Weld metal

Claims

1. A duplex stainless steel welded joint comprising: by mass percentage, 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%, and a base material composed of the balance being Fe and impurities, by mass percentage, 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%, and a weld metal composed of the balance being Fe and impurities, and satisfying the content of each element in the base material and the weld metal, and satisfying Formula (1) and Formula (2), a duplex stainless steel welded joint. Si + 3Mn ≤ 3.00 (1) 0 < BH / BW < 0.15 (2) Here, in each element symbol in Formula (1), the content of the corresponding element in the weld metal is substituted in mass percentage. Here, in BH in Formula (2), the Cap height (mm), which is the maximum height of the weld metal with respect to the surface of the base material, is substituted, and in BW, the Cap width (mm), which is the width of the weld metal on the surface of the base material in a direction perpendicular to the extending direction of the weld metal, is substituted.

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

3. The duplex stainless steel welded joint according to claim 1 or claim 2, wherein the base material in mass%, W: 0.01 to 4.00%, Nb: 0.01 to 0.10%, V: 0.01 to 0.20%, 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%, contains one or more elements selected from the group consisting of duplex stainless steel welded joint.

4. The duplex stainless steel welded joint according to any one of claims 1 to 3, wherein the weld metal in mass%, W: 0.01 to 4.00%, Nb: 0.01 to 0.10%, V: 0.01 to 0.20%, 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%, contains one or more elements selected from the group consisting of duplex stainless steel welded joint.

Images