Two-phase stainless steel material and two-phase stainless steel welded joint

Abstract

Provided is a duplex stainless steel material in which the strength of the weld metal produced by non-filler welding using a shielding gas not containing nitrogen is greater than or equal to that of the base material portion. The duplex stainless steel material of this disclosure contains, in mass%: C: 0.001 to 0.030%, Si: 1.00% or less, Mn: 0.05 to 5.00%, P: 0.035% or less, S: 0.0008% or less, Cr: 21.00 to 28.00%, Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al: 0.001 to 0.050%, N: 0.400% or less, B: 0.0001 to 0.0050%, Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, and Co: 0.05 to 2.00%, and formula (1) and formula (2) below are satisfied. Ca / S≥2.00 1000×Ti+2×N×Co0.2×0.5×Ca / S1.6≥50.00

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a duplex stainless steel material and a welded joint of duplex stainless steel, and more particularly to a duplex stainless steel material and a welded joint of duplex stainless steel that are suited to non-filler welding.BACKGROUND ART

[0002] Duplex stainless steel materials have high strength as well as excellent corrosion resistance in a chloride environment. For this reason, duplex stainless steel materials are used as, for example, raw materials for steel pipes for seawater heat exchangers and raw materials for steel pipes for umbilical cables for offshore development.

[0003] Chemical compositions of duplex stainless steel materials for various required applications are specified in the Japanese Industrial Standards (JIS) and the ASTM Standards. For example, in JIS, SUS329J3L and SUS329J4L are specified as chemical compositions of duplex stainless steel materials. Recently, SUS327L1 has been newly specified in JIS as a super duplex stainless steel material that contains high nitrogen and has a pitting resistance equivalent (PREW) of over 40. Also, in the ASTM Standards, ASTM A789 S32750 and S39274 are specified as super duplex stainless steel materials. The duplex stainless steel materials specified in ASTM A789 S39274 have high strength and excellent corrosion resistance due to having an enhanced PREW. Furthermore, by increasing the W content, sigma phase precipitation that accompanies an increase in the PREW is suppressed.

[0004] When duplex stainless steel materials are welded, the amount of ferrite increases in the weld metal obtained by melting a base material portion. As a result, the strength of the weld metal in the welded joint of duplex stainless steel decreases. Therefore, in the production of welded joints of duplex stainless steel, welding is generally performed using a welding consumable that has a high Ni content in order to ensure the amount of austenite in the weld metal.

[0005] However, in seawater heat exchangers for installation in chemical plants, hundreds to thousands of duplex stainless steel pipes are inserted into through holes formed in a tube plate, and then the duplex stainless steel pipes and the tube plate are seal welded together. In this seal welding, the spacing between the through holes is minimized to avoid an increase in the size of the device. This results in smaller gaps between the seal-welded duplex stainless steel pipes and the through holes formed in the tube plate. Therefore, in welding for such seawater heat exchangers, non-filler welding is performed, in which seal welding is performed without using a welding consumable.

[0006] In non-filler welding, a welding consumable is not used, and a welded joint is produced by melting a base material portion to form weld metal. In this case, the strength of the weld metal may decrease. A decrease in the strength of the weld metal causes a decrease in pressure resistance. In order to prevent a decrease in the strength of the weld metal, nitrogen is added to the Ar shielding gas used in non-filler welding. In this case, due to the addition of nitrogen, nitrogen penetrates into the molten metal. As a result, the strength of the weld metal increases.

[0007] However, the addition of nitrogen to the Ar shielding gas can possibly increase the cost of gas purification and increase the length of the construction period due to a shortened electrode life. Therefore, there is demand for a duplex stainless steel material that has a weld metal strength greater than or equal to that of the base material portion without requiring the addition of nitrogen to the shielding gas when non-filler welding is performed on the duplex stainless steel material.

[0008] Technologies related to duplex stainless steel materials are proposed in JP 2017-95794A (Patent Literature 1) and JP 2021-31757A (Patent Literature 2).

[0009] A duplex stainless steel material disclosed in Patent Literature 1 contains at least one kind of X group element selected from the group consisting of, V: 0.01 to 0.50 mass%, Ti: 0.0001 to 0.0500 mass%, Nb: 0.0005 to 0.0500 mass%, and Ta: 0.01 to 0.50 mass%, the steel material having composite inclusions, or composite inclusions and inclusions, and the inclusions including at least one of an oxide, a sulfide, and an oxysulfide. The composite inclusion has an inclusion as a core and, around the core, an outer shell of a carbide or a nitride containing Cr and at least one kind of X group element, and the percentage of the number of composite inclusions is 30% or more of the total number of the inclusions. With the duplex stainless steel material disclosed in Patent Literature 1, by modifying the inclusions that may become starting points of localized corrosion, it is possible to improve corrosion resistance and improve the stress corrosion cracking resistance of welds.

[0010] A duplex stainless steel material disclosed in Patent Literature 2 contains, in mass%: C: 0.06% or less, Si: 1.0% or less, Mn: 0.01 to 5.5%, P: 0.03% or less, S: 0.01% or less, Ni: 1.5 to 8.0%, Cr: 20.0 to 28.0%, Mo: 0.05 to 4.5%, N: 0.06 to 0.35%, Cu: 0.05 to 1.5%, Ti: 0 to 1.0%, Nb: 0 to 1.0%, Al: 0 to 0.10%, B: 0 to 0.003%, V: 0 to 1.0%, Sn: 0 to 1.0%, Co: 0 to 0.5%, W: 0 to 0.5%, Ca: 0 to 0.05%, Mg: 0 to 0.1%, Zr: 0 to 0.5%, a rare earth element (REM): 0 to 0.1%, and the remainder being Fe and impurities, wherein a value of Md 30 defined by formula (i) below is -230 to 90°C, and a volume fraction of a ferrite phase in a metal structure is 35.0 to 65.0%. Md 30 ° C = 551 − 462 C + N − 9.2 Si − 8.1 Mn − 13.7 Cr − 29 Ni + Cu − 18.5 Mo − 68 Nb

[0011] The duplex stainless steel material disclosed in Patent Literature 2 has improved strength and corrosion resistance.CITATION LISTPATENT LITERATURE

[0012] Patent Literature 1: JP 2017-95794A Patent Literature 2: JP 2021-31757A SUMMARY OF INVENTIONTECHNICAL PROBLEM

[0013] However, with the duplex stainless steel material disclosed in Patent Literature 1, no consideration is given to making the strength of the weld metal greater than or equal to the strength of the base material portion when non-filler welding is performed. Furthermore, with the duplex stainless steel material disclosed in Patent Literature 2, no consideration is given to the strength of the weld metal produced by non-filler welding and the strength of the base material portion.

[0014] An object of the present disclosure is to provide a duplex stainless steel material and a welded joint of duplex stainless steel with which the strength of the produced weld metal is greater than or equal to that of the base material portion, even when non-filler welding is performed using a shielding gas not containing nitrogen.SOLUTION TO PROBLEM

[0015] A duplex stainless steel material according to an aspect of the present disclosure contains, in mass%, C: 0.001 to 0.030%, Si: 1.00% or less, Mn: 0.05 to 5.00%, P: 0.035% or less, S: 0.0008% or less, Cr: 21.00 to 28.00%, Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al: 0.001 to 0.050%, N: 0.400% or less, B: 0.0001 to 0.0050%, Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, Co: 0.05 to 2.00%, W: 0 to 5.00%, Nb: 0 to 0.100%, V: 0 to 0.200%, Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and a remainder including Fe and impurities, wherein formula (1) and formula (2) below are satisfied: Ca / S ≥ 2.00 100 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6 ≥ 50.00 where contents in mass% of corresponding elements are substituted for element symbols in formula (1) and formula (2).

[0016] A welded joint of duplex stainless steel according to an aspect of the present disclosure including: a base material portion; and a weld metal, wherein the base material portion and the weld metal contain, in mass%, C: 0.001 to 0.030%, Si: 1.00% or less, Mn: 0.05 to 5.00%, P: 0.035% or less, S: 0.0008% or less, Cr: 21.00 to 28.00%, Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al: 0.001 to 0.050%, N: 0.400% or less, B: 0.0001 to 0.0050%, Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, Co: 0.05 to 2.00%, W: 0 to 5.00%, Nb: 0 to 0.100%, V: 0 to 0.200%, Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and a remainder including Fe and impurities, formula (1) and formula (2) below are satisfied, and an averaged grain size of ferrite in the weld metal is 150 µm or less, Ca / S ≥ 2.00 1000 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6 ≥ 50.00 where contents in mass% of corresponding elements are substituted for element symbols in formula (1) and formula (2).ADVANTAGEOUS EFFECTS OF INVENTION

[0017] In the duplex stainless steel material according to the above aspect of the present disclosure, even when non-filler welding is performed using a shielding gas not containing nitrogen, the strength of the produced weld metal is greater than or equal to that of the base material portion. In the welded joint of duplex stainless steel according to the above aspect of the present disclosure, the strength of the weld metal is greater than or equal to that of the base material portion.BRIEF DESCRIPTION OF DRAWINGS

[0018] [FIG. 1] FIG. 1 is a diagram showing a relationship between Fn2 and an averaged grain size D of ferrite in weld metal of a welded joint of duplex stainless steel, when the welded joint of duplex stainless steel is produced by performing non-filler welding using a shielding gas not containing nitrogen on a duplex stainless steel material whose chemical composition has element contents within the ranges of an embodiment of the present invention and which satisfies formula (1). [FIG. 2] FIG. 2 is a schematic diagram of a test piece subjected to non-filler welding in Examples. [FIG. 3] FIG. 3 is a plan view of a tensile test specimen of welded joint taken from the test piece illustrated in FIG. 2. DESCRIPTION OF EMBODIMENTS

[0019] The inventors of the present invention conducted research on a duplex stainless steel material in which the strength of the weld metal produced by non-filler welding using a shielding gas not containing nitrogen is greater than or equal to that of the base material portion. As a result, the present inventors made the following findings.

[0020] The inventors first investigated, from the viewpoint of chemical composition, a duplex stainless steel material in which the strength of the weld metal produced by non-filler welding using a shielding gas not containing nitrogen is greater than or equal to that of the base material portion. As a result, it was thought that if a duplex stainless steel material has a chemical composition containing, in mass%, C: 0.001 to 0.030%, Si: 1.00% or less, Mn: 0.05 to 5.00%, P: 0.035% or less, S: 0.0008% or less, Cr: 21.00 to 28.00%, Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al: 0.001 to 0.050%, N: 0.400% or less, B: 0.0001 to 0.0050%, Mg: 0.0050% or less, W: 0 to 5.00%, Nb: 0 to 0.100%, V: 0 to 0.200%, Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and the remainder including Fe and impurities, then the strength of the weld metal produced by non-filler welding using a shielding gas not containing nitrogen is greater than or equal to that of the base material portion.

[0021] However, even with a duplex stainless steel material having the above-described chemical composition, there are still cases in which the strength of the weld metal produced by non-filler welding using a shielding gas not containing nitrogen is not greater than or equal to that of the base material portion. Therefore, the inventors of the present invention conducted further studies. As a result, the inventors of the present invention obtained the following findings.

[0022] In a duplex stainless steel material having the above-described chemical composition, in order to make the strength of the weld metal produced by non-filler welding using a shielding gas not containing nitrogen greater than or equal to that of the base material portion, it is effective to utilize (I) a strengthening mechanism utilizing weld metal grain boundary strengthening, (II) a strengthening mechanism utilizing weld metal grain refining, and (III) a strengthening mechanism utilizing weld metal work hardening.[(I) Weld Metal Grain Boundary Strengthening Mechanism]

[0023] Solidification segregation occurs in the process of rapid cooling during welding. Specifically, during solidification, S segregates at the interface between austenite and ferrite and at the grain boundaries of ferrite. The segregation of S reduces the strength of grain boundary. Therefore, in order to increase the strength of the weld metal, it is effective to suppress the segregation of S as much as possible. Therefore, in the duplex stainless steel material of the present embodiment, Ca is bonded with S to form CaS, thereby fixing the S. This makes it unlikely for S to segregate at the interface or grain boundaries. As a result, the strength of the weld metal increases. In order to obtain the above effect, it is necessary to increase the Ca content relative to the S content in the steel material to a certain degree. Therefore, the above-described chemical composition of the duplex stainless steel material further contains Ca in an amount of 0.0005 to 0.0100%, and the Ca content and the S content are adjusted so as to satisfy formula (1) below. Ca / S ≥ 2.00[(II) Strengthening Mechanism Utilizing Weld Metal Grain Refining]

[0024] If the crystal grains of the weld metal are fine, the strength of the weld metal is increased. In view of this, in the present embodiment, Ti nitrides are utilized to refine the grains of the weld metal. Specifically, fine Ti nitrides are crystallized in the weld metal during the process of rapid cooling during non-filler welding. Ti nitrides refine the primary ferrite grains during solidification. This increases the strength of the weld metal. If the chemical composition of the duplex stainless steel material further contains Ti in an amount of 0.002 to 0.100%, Ti nitrides, which are effective for grain refining, can be sufficiently crystallized in the weld metal during the process of rapid cooling during non-filler welding.[(III) Strengthening Mechanism Utilizing Weld Metal Work Hardening]

[0025] When weld metal is formed by non-filler welding, the strength of the weld metal increases if the weld metal undergoes hardening during the processing of rapid cooling during welding. Co reduces stacking fault energy and enhances the work hardening characteristics of austenite in the microstructure of the duplex stainless steel material. Co also increases the amount of austenite formed in the weld metal formed by non-filler welding. As the amount of austenite formed in the weld metal increases, the austenite is subjected to plastic constraint by ferrite. Due to being subjected to plastic constraint while the stacking fault energy is reduced by Co, the austenite in the weld metal exhibits significant work hardening. As a result, the strength of the weld metal can be increased. In view of this, the above-described chemical composition of the duplex stainless steel material further contains Co in an amount of 0.05 to 2.00%.

[0026] The strengthening mechanisms (I) to (III) described above interact with each other and exert a synergistic effect during non-filler welding. Specifically, if the Ti content, the N content, the Co content, the Ca content, and the S content in the chemical composition of the duplex stainless steel material satisfy formula (2) below, due to the synergistic effect of the strengthening mechanisms (I) to (III), the strength of the weld metal produced by non-filler welding using a shielding gas not containing nitrogen can be increased to an amount greater than or equal to the strength of the base material portion. 1000 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6 ≥ 50.00

[0027] The duplex stainless steel material according to the present embodiment, which has been completed based on the above technical concept, can have configurations as described below.

[0028] The duplex stainless steel material according to first configuration contains, in mass %, C: 0.001 to 0.030%, Si: 1.00% or less, Mn: 0.05 to 5.00%, P: 0.035% or less, S: 0.0008% or less, Cr: 21.00 to 28.00%, Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al: 0.001 to 0.050%, N: 0.400% or less, B: 0.0001 to 0.0050%, Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, Co: 0.05 to 2.00%, W: 0 to 5.00%, Nb: 0 to 0.100%, V: 0 to 0.200%, Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and the remainder including Fe and impurities, wherein formula (1) and formula (2) below are satisfied: Ca / S ≥ 2.00 1000 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6 ≥ 50.00 where the contents in mass% of corresponding elements are substituted for the element symbols in formula (1) and formula (2).

[0029] The duplex stainless steel material according to a second configuration is the duplex stainless steel material according to the first configuration, further containing one or more kinds of elements selected from the group consisting of, W: 0.01 to 5.00%, Nb: 0.001 to 0.100%, V: 0.001 to 0.200%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.020%, and a rare earth element (REM): 0.001 to 0.050%.

[0030] The welded joint of duplex stainless steel according to the first configuration includes a base material portion and a weld metal portion. The base material portion and the weld metal contain, in mass%, C: 0.001 to 0.030%, Si: 1.00% or less, Mn: 0.05 to 5.00%, P: 0.035% or less, S: 0.0008% or less, Cr: 21.00 to 28.00%, Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al: 0.001 to 0.050%, N: 0.400% or less, B: 0.0001 to 0.0050%, Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, Co: 0.05 to 2.00%, W: 0 to 5.00%, Nb: 0 to 0.100%, V: 0 to 0.200%, Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and the remainder including Fe and impurities, wherein formula (1) and formula (2) below are satisfied. The averaged grain size of ferrite in the weld metal is 150 µm or less. Ca / S ≥ 2.00 1000 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6 ≥ 50.00

[0031] Here, the contents in mass% of corresponding elements are substituted for the element symbols in formula (1) and formula (2).

[0032] The duplex stainless steel material and the welded joint of duplex stainless steel of the present embodiment will be described below.[Features of Duplex Stainless Steel Material of Present Embodiment]

[0033] The duplex stainless steel material of the present embodiment has the following features 1 to 3.(Feature 1)

[0034] The chemical composition contains, in mass%, C: 0.001 to 0.030%, Si: 1.00% or less, Mn: 0.05 to 5.00%, P: 0.035% or less, S: 0.0008% or less, Cr: 21.00 to 28.00%, Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al: 0.001 to 0.050%, N: 0.400% or less, B: 0.0001 to 0.0050%, Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, Co: 0.05 to 2.00%, W: 0 to 5.00%, Nb: 0 to 0.100%, V: 0 to 0.200%, Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and the remainder including Fe and impurities.(Feature 2)

[0035] The chemical composition satisfies formula (1) below. Ca / S ≥ 2.00

[0036] Here, the contents in mass% of corresponding elements in the chemical composition are substituted for the element symbols in formula (1).(Feature 3)

[0037] The chemical composition satisfies formula (2) below. 1000 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6 ≥ 50.00

[0038] Here, the contents in mass% of corresponding elements in the chemical composition are substituted for the element symbols in formula (2).

[0039] Features 1 to 3 will be described below.[Feature 1: Chemical Composition]

[0040] The chemical composition of the duplex stainless steel material of the present embodiment contains the elements described below. In the following description, the duplex stainless steel material will also be simply referred to as "the steel material".C: 0.001 to 0.030%

[0041] Carbon (C) stabilizes austenite and increases the amount of austenite in the weld metal formed by non-filler welding. This increases the strength of the weld metal. If the C content is less than 0.001%, the above effects cannot be sufficiently obtained.

[0042] On the other hand, if the C content is more than 0.030%, carbides are likely to form during non-filler welding. In this case, the corrosion resistance of the weld metal decreases.

[0043] Therefore, the C content is 0.001 to 0.030%.

[0044] The lower limit of the C content is preferably 0.002%, and more preferably 0.003%.

[0045] The upper limit of the C content is preferably 0.025%, and more preferably 0.020%.Si: 1.00% or less

[0046] Silicon (Si) deoxidizes steel. If even a small amount of Si is present, the above effect can be obtained to a certain degree.

[0047] On the other hand, Si stabilizes ferrite. Therefore, if the Si content is more than 1.00%, the amount of ferrite in the weld metal formed by non-filler welding increases. This reduces the strength and corrosion resistance of the weld metal.

[0048] Therefore, the Si content is 1.00% or less.

[0049] The lower limit of the Si content is preferably more than 0%, more preferably 0.01%, more preferably 0.05%, and more preferably 0.10%.

[0050] The upper limit of the Si content is preferably 0.90%, and more preferably 0.80%.Mn: 0.05 to 5.00%

[0051] Manganese (Mn) stabilizes austenite and increases the amount of austenite in the weld metal formed by non-filler welding. As a result, the strength of the weld metal increases. If the Mn content is less than 0.05%, the above effects cannot be sufficiently obtained.

[0052] On the other hand, if the Mn content is more than 5.00%, the stacking fault energy is excessively reduced. Therefore, the toughness of the weld metal formed by non-filler welding is reduced.

[0053] Therefore, the Mn content is 0.05 to 5.00%.

[0054] The lower limit of the Mn content is preferably 0.08%, and more preferably 0.10%.

[0055] The upper limit of the Mn content is preferably 4.00%, more preferably 3.00%, and more preferably 2.00%.P: 0.035% or less

[0056] Phosphorus (P) is an impurity. P segregates at grain boundaries and increases the crack susceptibility during hot working. Furthermore, P undergoes solidification segregation during welding, increasing the hot cracking susceptibility.

[0057] Therefore, the P content is 0.035% or less.

[0058] The P content is preferably as low as possible. However, excessive reduction in the P content significantly increases the production cost. Therefore, in consideration of industrial productivity, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, and more preferably 0.002%.

[0059] The upper limit of the P content is preferably 0.028%, more preferably 0.025%, more preferably 0.023%, and more preferably 0.020%.S: 0.0008% or less

[0060] Sulfur (S) is an impurity. S segregates at grain boundaries and increases the crack susceptibility during hot working. Furthermore, S undergoes solidification segregation during welding and concentrates at ferrite grain boundaries and interfaces between ferrite and austenite. As a result, the tensile strength of the weld metal decreases.

[0061] Therefore, the S content is 0.0008% or less.

[0062] The S content is preferably as low as possible. However, excessive reduction in the S content significantly increases the production cost. Therefore, in consideration of industrial productivity, the lower limit of the S content is preferably more than 0%, more preferably 0.0001%, and more preferably 0.0002%.

[0063] The upper limit of the S content is preferably 0.0007%, and more preferably 0.0005%.Cr: 21.00 to 28.00%

[0064] Chromium (Cr) increases the corrosion resistance of steel and weld metal formed by non-filler welding. If the Cr content is less than 21.00%, the above effect cannot be sufficiently obtained.

[0065] On the other hand, if the Cr content is more than 28.00%, intermetallic compounds typified by sigma phases are likely to form in the weld metal formed by non-filler welding, and further, the amount of ferrite in the weld metal increases excessively. As a result, the strength of the weld metal decreases.

[0066] Therefore, the Cr content is 21.00 to 28.00%.

[0067] The lower limit of the Cr content is preferably 21.50%, more preferably 22.00%, and more preferably 22.70%.

[0068] The upper limit of the Cr content is preferably 27.50%, more preferably 27.20%, and more preferably 27.00%.Ni: 4.00 to 9.50%

[0069] Nickel (Ni) stabilizes austenite. Therefore, the amount of austenite in the weld metal formed by non-filler welding increases. As a result, the strength of the weld metal increases. If the Ni content is less than 4.00%, the above effect cannot be sufficiently obtained.

[0070] On the other hand, if the Ni content is more than 9.50%, intermetallic compounds typified by sigma phases are likely to form in the weld metal formed by non-filler welding. As a result, the strength of the weld metal decreases.

[0071] Therefore, the Ni content is 4.00 to 9.50%.

[0072] The lower limit of the Ni content is preferably 4.50%, more preferably 5.00%, and more preferably 5.50%.

[0073] The upper limit of the Ni content is preferably 9.25%, more preferably 9.00%, more preferably 8.50%, and more preferably 8.00%.Mo: 0.80 to 5.50%

[0074] Molybdenum (Mo) increases the corrosion resistance of steel and weld metal formed by non-filler welding. Mo also increases the strength of steel and weld metal through solid solution strengthening. If the Mo content is less than 0.80%, the above effects cannot be sufficiently obtained.

[0075] On the other hand, if the Mo content is more than 5.50%, intermetallic compounds typified by sigma phases are likely to form in the weld metal formed by non-filler welding. As a result, the strength of the weld metal decreases.

[0076] Therefore, the Mo content is 0.80 to 5.50%.

[0077] The lower limit of the Mo content is preferably 0.85%, more preferably 0.90%, and more preferably 1.00%.

[0078] The upper limit of the Mo content is preferably 5.20%, more preferably 5.00%, and more preferably 4.50%.Cu: 0.01 to 3.50%

[0079] Copper (Cu) increases the corrosion resistance of steel and weld metal formed by non-filler welding. Cu also increases the strength of ferrite. If the Cu content is less than 0.01%, the above effects cannot be sufficiently obtained.

[0080] On the other hand, if the Cu content is more than 3.50%, the hot workability of the steel material decreases. Furthermore, the amount of N in solid solution decreases, promoting the formation of Cr nitrides. As a result, the corrosion resistance and the toughness of the steel material decrease.

[0081] Therefore, the Cu content is 0.01 to 3.50%.

[0082] The lower limit of the Cu content is preferably 0.05%, more preferably 0.10%, and more preferably 0.20%.

[0083] The upper limit of the Cu content is preferably 3.30%, more preferably 3.00%, and more preferably 2.50%.Al: 0.001 to 0.050%

[0084] Aluminum (Al) deoxidizes steel. Furthermore, during non-filler welding, Al combines with Mg to form Al-Mg oxides, which promotes the crystallization of Ti nitrides in the weld metal. For this reason, the weld metal structure is refined to increase the strength of the weld metal. If the Al content is less than 0.001%, the above effects cannot be sufficiently obtained.

[0085] On the other hand, if the Al content is more than 0.050%, AlN is formed in excess. In this case, the toughness and the corrosion resistance of the steel material and the weld metal formed by non-filler welding decrease.

[0086] Therefore, the Al content is 0.001 to 0.050%.

[0087] The lower limit of the Al content is preferably 0.003%, more preferably 0.004%, and more preferably 0.005%.

[0088] The upper limit of the Al content is preferably 0.045%, more preferably 0.040%, and more preferably 0.030%.N: 0.400% or less

[0089] Nitrogen (N) is inevitably present. N stabilizes austenite and increases the strength of the austenite in steel. N further increases the pitting resistance equivalent PREW, thereby improving the pitting corrosion resistance and crevice corrosion resistance of the steel material and the weld metal formed by non-filler welding.

[0090] However, if the N content is more than 0.400%, defects such as blowholes form in the weld metal during non-filler welding.

[0091] Therefore, the N content is 0.400% or less.

[0092] The lower limit of the N content is preferably more than 0%, more preferably 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.050%, more preferably 0.080%, and more preferably 0.100%.

[0093] The upper limit of the N content is preferably 0.380%, more preferably 0.370%, and more preferably 0.350%.B: 0.0001 to 0.0050%

[0094] Boron (B) segregates at grain boundaries at high temperatures and improves the hot workability of steel. B also deoxidizes steel. If the B content is less than 0.0001%, the above effects cannot be sufficiently obtained.

[0095] On the other hand, if the B content is more than 0.0050%, solidification segregation occurs during the process of solidification of the weld metal during non-filler welding. As a result, the susceptibility of the weld metal to solidification cracking increases.

[0096] Therefore, the B content is 0.0001 to 0.0050%.

[0097] The lower limit of the B content is preferably 0.0003%, more preferably 0.0005%, and more preferably 0.0010%.

[0098] The upper limit of the B content is preferably 0.0048%, more preferably 0.0045%, more preferably 0.0043%, and more preferably 0.0040%.Mg: 0.0050% or less

[0099] During non-filler welding, magnesium (Mg) combines with Mg to form Al-Mg oxides, which promotes the crystallization of Ti nitrides. For this reason, the weld metal structure is refined to increase the strength of the weld metal. If even a small amount of Mg is present, the above effects can be obtained to a certain degree.

[0100] However, if the Mg content is more than 0.0050%, the hot workability of the duplex stainless steel material decreases.

[0101] Therefore, the Mg content is 0.0050% or less.

[0102] The lower limit of the Mg content is preferably more than 0%, more preferably 0.0001%, more preferably 0.0002%, and more preferably 0.0003%.

[0103] The upper limit of the Mg content is preferably 0.0045%, more preferably 0.0040%, more preferably 0.0035%, and more preferably 0.0030%.Ca: 0.0005 to 0.0100%

[0104] During non-filler welding, calcium (Ca) combines with S to form CaS, which fixes the S. Therefore, solidification segregation of S during welding is suppressed. As a result, the strength of the weld metal increases. If the Ca content is less than 0.0005%, the above effects cannot be sufficiently obtained.

[0105] On the other hand, if the Ca content is more than 0.0100%, CaO is produced in excess during non-filler welding. As a result, the cleanliness of the weld metal significantly decreases, and the quality of the appearance of the weld metal also decreases.

[0106] Therefore, the Ca content is 0.0005 to 0.0100%.

[0107] The lower limit of Ca is preferably 0.0010%, more preferably 0.0015%, and more preferably 0.0020%.

[0108] The upper limit of the Ca content is preferably 0.0080%, more preferably 0.0070%, and more preferably 0.0050%.Ti: 0.002 to 0.100%

[0109] During non-filler welding, titanium (Ti) combines with N to crystallize Ti nitrides. Due to the pinning effect of the Ti nitrides, the grain size of primary ferrite in the weld metal is refined. As a result, the strength of the weld metal increases. If the Ti content is less than 0.002%, the above effects cannot be sufficiently obtained.

[0110] On the other hand, if the Ti content is more than 0.100%, the Ti nitrides become coarse. This reduces the toughness of the steel material and the weld metal.

[0111] Therefore, the Ti content is 0.002 to 0.100%.

[0112] The lower limit of the Ti content is preferably 0.005%, more preferably 0.010%, more preferably 0.011%, more preferably 0.012%, and more preferably 0.015%.

[0113] The upper limit of the Ti content is preferably 0.090%, more preferably 0.080%, more preferably 0.070%, and more preferably 0.050%.Co: 0.05 to 2.00%

[0114] Cobalt (Co) reduces stacking fault energy and improves work hardening characteristics. Co also stabilizes austenite. Therefore, the amount of austenite in the weld metal increases during non-filler welding. In this case, the formed austenite is surrounded by ferrite. When a force such as tensile stress is applied to the austenite surrounded by ferrite, the austenite is subjected to plastic constraint and undergoes work hardening. As a result, the strength of the weld metal increases. If the Co content is less than 0.05%, the above effects cannot be sufficiently obtained.

[0115] On the other hand, if the Co content is more than 2.00%, the production cost becomes too high.

[0116] Therefore, the Co content is 0.05 to 2.00%.

[0117] The lower limit of the Co content is preferably 0.08%, more preferably 0.10%, more preferably 0.15%, more preferably 0.20%, and more preferably 0.30%.

[0118] The upper limit of the Co content is preferably 1.90%, more preferably 1.80%, more preferably 1.70%, and more preferably 1.50%.

[0119] The remainder of the chemical composition of the duplex stainless steel material of the present embodiment includes Fe and impurities. Here, impurities refer to added substances that come from raw materials such as ore, scrap, or the manufacturing environment when industrially producing a duplex stainless steel material, and are acceptable within a range that does not adversely affect the duplex stainless steel material of the present embodiment. One example of an impurity other than the impurities mentioned above is O in an amount of 0.0300% or less.[Optional Elements]

[0120] The chemical composition of the duplex stainless steel material according to the present embodiment may further contain, in place of a portion of Fe, one or more kinds of elements selected from the group consisting of, W: 0 to 5.00%, Nb: 0 to 0.100%, V: 0 to 0.200%, Ta: 0 to 0.100%, Sn: 0 to 0.020%, and a rare earth element (REM): 0 to 0.050%. All of these elements are optional and are not required to be present. These optional elements will be described below.[W]

[0121] The chemical composition of the duplex stainless steel material of the present embodiment may further contain W instead of a portion of Fe.W: 0 to 5.00%

[0122] Tungsten (W) is an optional element and is not required to be present. In other words, the W content may be 0%.

[0123] When present, that is, when the W content is more than 0%, W forms oxides and enhances the corrosion resistance of steel and weld metal formed by non-filler welding in a low pH environment. W also increases the strength of steel and weld metal through solid solution strengthening. If even a small amount of W is present, the above effects can be obtained to a certain degree.

[0124] However, if the W content is more than 5.00%, intermetallic compounds typified by sigma phases are likely to form in the weld metal formed by non-filler welding. As a result, the toughness of the weld metal decreases.

[0125] Therefore, the W content is 0 to 5.00%.

[0126] The lower limit of the W content is preferably 0.01%, more preferably 0.10%, more preferably 0.30%, more preferably 0.50%, and more preferably 1.00%.

[0127] The upper limit of the W content is preferably 4.50%, more preferably 4.00%, more preferably 3.50%, and more preferably 3.00%.[Nb, V, and Ta]

[0128] The chemical composition of the duplex stainless steel material of the present embodiment may further contain one or more kinds of elements selected from the group consisting of Nb, V, and Ta instead of a portion of Fe. All of these elements form carbides and increase the strength of steel and weld metal formed by non-filler welding, through precipitation strengthening. Furthermore, the formation of carbides suppresses the formation of Cr-depleted zones. As a result, the corrosion resistance of the steel material and the weld metal is improved.Nb: 0 to 0.100%

[0129] Niobium (Nb) is an optional element and is not required to be present. In other words, the Nb content may be 0%.

[0130] When present, that is, when the Nb content is more than 0%, Nb combines with C to form carbides. Therefore, the strength of the weld metal formed by non-filler welding is increased. Furthermore, the formation of Nb carbides suppresses the formation of Cr carbides at grain boundaries. As a result, the corrosion resistance of the weld metal is improved. If even a small amount of Nb is present, the above effects can be obtained to a certain degree.

[0131] However, if the Nb content is more than 0.100%, Nb carbides are formed in excess. In this case, the corrosion resistance and the toughness of the steel material and weld metal decrease.

[0132] Therefore, the Nb content is 0 to 0.100%.

[0133] The lower limit of the Nb content is preferably 0.001%, more preferably 0.002%, and more preferably 0.005%.

[0134] The upper limit of the Nb content is preferably 0.080%, more preferably 0.050%, more preferably 0.030%, more preferably 0.020%, and more preferably 0.015%.V: 0 to 0.200%

[0135] Vanadium (V) is an optional element and is not required to be present. In other words, the V content may be 0%.

[0136] When present, that is, when the V content is more than 0%, V combines with C to form carbides. Therefore, the strength of the weld metal formed by non-filler welding is increased. Furthermore, the formation of V carbides suppresses the formation of Cr carbides at grain boundaries. As a result, the corrosion resistance of the weld metal is improved. If even a small amount of V is present, the above effects can be obtained to a certain degree.

[0137] However, if the V content is more than 0.200%, V carbides are formed in excess. In this case, the corrosion resistance and the toughness of the steel material and weld metal decrease.

[0138] Therefore, the V content is 0 to 0.200%.

[0139] The lower limit of the V content is preferably 0.001%, more preferably 0.002%, more preferably 0.010%, and more preferably 0.020%.

[0140] The upper limit of the V content is preferably 0.180%, more preferably 0.150%, more preferably 0.120%, more preferably 0.100%, more preferably 0.080%, and more preferably 0.050%.Ta: 0 to 0.100%

[0141] Tantalum (Ta) is an optional element and is not required to be present. In other words, the Ta content may be 0%.

[0142] When present, that is, when the Ta content is more than 0%, Ta combines with C to form carbides. Therefore, the strength of the weld metal formed by non-filler welding is increased. Furthermore, the formation of Ta carbides suppresses the formation of Cr carbides at grain boundaries. As a result, the corrosion resistance of the weld metal is improved. If even a small amount of Ta is present, the above effects can be obtained to a certain degree.

[0143] However, if the Ta content is more than 0.100%, Ta carbides are formed in excess. In this case, the corrosion resistance and the toughness of the steel material and weld metal decrease.

[0144] Therefore, the Ta content is 0 to 0.100%.

[0145] The lower limit of the Ta content is preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0146] The upper limit of the Ta content is preferably 0.080%, more preferably 0.050%, more preferably 0.030%, and more preferably 0.020%.[Sn]

[0147] The chemical composition of the duplex stainless steel material of the present embodiment may further contain Sn instead of a portion of Fe.Sn: 0 to 0.020%

[0148] Tin (Sn) is an optional element and is not required to be present. In other words, the Sn content may be 0%.

[0149] When present, that is, when the Sn content is more than 0%, Sn enhances the pitting corrosion resistance of the steel material and the weld metal formed by non-filler welding. If even a small amount of Sn is present, the above effects can be obtained to a certain degree.

[0150] However, if the Sn content is more than 0.020%, the hot workability of the steel material decreases. Furthermore, the penetration depth during welding increases, and the wettability of the molten metal during welding decreases.

[0151] Therefore, the Sn content is 0 to 0.020%.

[0152] The lower limit of the Sn content is preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0153] The upper limit of the Sn content is preferably 0.018%, more preferably 0.015%, more preferably 0.010%, more preferably 0.009%, more preferably 0.008%, and more preferably 0.007%.[A rare earth element (REM)]

[0154] The chemical composition of the duplex stainless steel material of the present embodiment may further contain a rare earth element (REM) instead of a portion of Fe.a rare earth element (REM): 0 to 0.050%

[0155] A rare earth element (REM) are optional elements and are not required to be present. In other words, the REM content may be 0%.

[0156] When present, that is, when the REM content is more than 0%, the REM enhances the hot workability of the base material portion. If even a small amount of a REM is present, the above effects can be obtained to a certain degree.

[0157] However, if the REM content is more than 0.050%, conversely, the hot workability of the base material portion decreases.

[0158] Therefore, the REM content is 0 to 0.050%.

[0159] The lower limit of the REM content is preferably 0.001%, more preferably 0.002%, and more preferably 0.005%.

[0160] The upper limit of the REM content is preferably 0.040%, more preferably 0.030%, and more preferably 0.020%.

[0161] In this specification, REM refers to one or more kinds of elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. Also, the REM content (%) in this specification is the total content (%) of these elements.[Feature 2: Formula (1)]

[0162] The duplex stainless steel material of the present embodiment further satisfies formula (1) below. Ca / S ≥ 2.00

[0163] Here, the contents in mass% of corresponding elements in the chemical composition are substituted for the element symbols in formula (1).

[0164] Fn1 is defined as follows: Fn 1 = Ca / S

[0165] Fn1 corresponds to the left side of formula (1). Fn1 is an index relating to the amount of fixed S in the weld metal during non-filler welding in a duplex stainless steel material that satisfies Feature 1. If Fn1 is less than 2.00, the Ca content relative to the S content in the steel material is too low. In this case, the S in the weld metal cannot be sufficiently fixed by Ca. As a result, S that does not bind to Ca undergoes solidification segregation. As a result, the strength of grain boundary of the weld metal decreases, and the strength of the weld metal decreases.

[0166] If Fn1 is 2.00 or more, in a duplex stainless steel material satisfying Feature 1, S is sufficiently fixed by Ca. Therefore, solidification segregation of S in the weld metal is suppressed. As a result, excellent strength is obtained in the weld metal.

[0167] The lower limit of Fn1 is preferably 2.10, more preferably 2.30, more preferably 2.50, more preferably 3.00, more preferably 4.00, more preferably 5.00, more preferably 6.00, more preferably 7.00, and more preferably 10.00.

[0168] There is no particular upper limit for Fn1. However, when the chemical composition of the duplex stainless steel material satisfies Feature 1, the upper limit of Fn1 is 100.00.[Feature 3: Formula (2)]

[0169] The duplex stainless steel material of the present embodiment further satisfies formula (2) below. 1000 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6 ≥ 50.00

[0170] Here, the contents in mass% of corresponding elements in the chemical composition are substituted for the element symbols in formula (2).

[0171] Fn2 is defined as follows: Fn 2 = 1000 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6

[0172] Fn2 corresponds to the left side of formula (2). Fn2 is an index relating to the strength of the weld metal formed by non-filler welding using a shielding gas not containing nitrogen in a duplex stainless steel material that satisfies Features 1 and 2. If Fn2 is less than 50.00, the synergistic effect of the strengthening mechanisms (I) to (III) described above cannot be sufficiently obtained. Therefore, even if Features 1 and 2 are satisfied, the strength of the weld metal formed by non-filler welding using a shielding gas not containing nitrogen is not sufficiently high.

[0173] If Fn2 is 50.00 or more, the synergistic effect of the strengthening mechanisms (I) to (III) described above can be sufficiently obtained. Therefore, the strength of the weld metal formed by non-filler welding using a shielding gas not containing nitrogen is sufficiently increased.

[0174] Fn2 in particular contributes to the strengthening mechanism (II). FIG. 1 is a diagram showing the relationship between Fn2 and an averaged grain size D of ferrite in weld metal of a welded joint of duplex stainless steel, when the welded joint of duplex stainless steel is produced by performing non-filler welding using a shielding gas not containing nitrogen on a duplex stainless steel material whose chemical composition has element contents within the ranges of the present embodiment and which satisfies formula (1). As shown in FIG. 1, as Fn2 increases, the averaged grain size D of ferrite in the weld metal decreases significantly. When Fn2 is 50.00 or more, the averaged grain size D is 150 µm or less. As described above, when Fn2 is 50.00 or more, the grain size of the weld metal is significantly refined. As a result, the strength of the weld metal is increased due to the strengthening mechanism (II).

[0175] The lower limit of Fn2 is preferably 70.00, more preferably 90.00, more preferably 100.00, more preferably 120.00, more preferably 150.00, more preferably 170.00, and more preferably 190.0. If Fn2 is 100.00 or more, the averaged grain size D of the weld metal is 100 µm or less.

[0176] The upper limit of Fn2 is not particularly limited. However, when the chemical composition of the duplex stainless steel material satisfies Feature 1, the upper limit of Fn2 is 60536.67. The upper limit of Fn2 is preferably 60,000.00, and more preferably 59,000.00.[Effects of Duplex Stainless Steel Material of Present Embodiment]

[0177] The duplex stainless steel material of the present embodiment satisfies Features 1 to 3. Therefore, in the duplex stainless steel material of the present embodiment, when non-filler welding is performed using a shielding gas not containing nitrogen, the strength of the weld metal formed can be sufficiently increased, and the strength of the weld metal becomes greater than or equal to the strength of the base material portion (steel material).[Microstructure]

[0178] The microstructure of the duplex stainless steel material of the present embodiment contains 30 to 70% by volume of ferrite, with the remainder being austenite. The amounts of structures other than ferrite and austenite in the microstructure are negligible. Specifically, the microstructure of the duplex stainless steel material according to the present embodiment may contain minute amounts of precipitates, inclusions, and the like in addition to ferrite and austenite. However, in the duplex stainless steel material according to the present embodiment, the volume fractions of precipitates, inclusions, and the like are negligibly small compared to the volume fractions of ferrite and austenite.[Method for Measuring Ferrite Volume Fraction]

[0179] The volume fraction of ferrite in a duplex stainless steel pipe can be determined by a method conforming to ASTM E562 (2019).

[0180] Specifically, a test piece for microstructure analysis is taken from a duplex stainless steel material.

[0181] In the case where the duplex stainless steel material is a steel pipe, a test piece having an observation surface of 2 mm in the axial direction and 2 mm in the radial direction is taken from the wall center in the wall thickness direction.

[0182] In the case where the duplex stainless steel material is a steel plate, a test piece having an observation surface of 2 mm in the rolling direction and 2 mm in the plate thickness direction is taken from the center in the plate thickness direction.

[0183] In the case where the duplex stainless steel material is a round steel bar (steel bar), a test piece having an observation surface of 2 mm in the axial direction and 2 mm in the radial direction is taken from the R / 2 portion.

[0184] The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etching solution to reveal the structure. The observation surface with the revealed structure is subjected to single-field observation using an optical microscope. The area of each field of view is 1.00 mm 2< (magnification of 100x). In each field, ferrite and austenite are identified by contrast. When electrolytically corroded in a 7% potassium hydroxide corrosive solution, the low-brightness areas correspond to ferrite and the high-brightness areas correspond to austenite. Therefore, one skilled in the art can easily identify ferrite and austenite from the contrast.

[0185] The area ratio of the identified ferrite is measured using the point counting method in accordance with ASTM E562 (2019). The area ratio of ferrite obtained in each field of view is defined as the volume fraction (%) of ferrite. The volume fraction (%) of ferrite is an integer value obtained by rounding off the obtained value to the first decimal place.

[0186] Note that the volume fraction (%) of austenite is obtained by subtracting the volume fraction of ferrite from 100.[Shape of Duplex Stainless Steel Material of Present Embodiment]

[0187] There are no particular limitations on the shape of the duplex stainless steel material of the present embodiment. The duplex stainless steel material of the present embodiment may be a steel pipe, a steel plate, or a round steel bar (steel bar).[Application of Duplex Stainless Steel Material of Present Embodiment]

[0188] The duplex stainless steel material of the present embodiment is widely applicable to applications requiring high strength and excellent corrosion resistance in chloride environments. The duplex stainless steel material of the present embodiment is suited to use in wet environments containing chlorides, such as seawater. Such applications include, for example, flow line pipes, umbilical tubes, heat exchangers, and the like.[Method for Manufacturing Duplex Stainless Steel Material]

[0189] The following describes a method for manufacturing a duplex stainless steel material of the present embodiment. The method for manufacturing a duplex stainless steel material described below is one example of a method for manufacturing a duplex stainless steel material of the present embodiment. Therefore, the duplex stainless steel material having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a duplex stainless steel material of the present embodiment.

[0190] A method for manufacturing a duplex stainless steel material of the present embodiment includes the following steps. Step 1: Preparation process Step 2: Hot working process Step 3: Cold working process Step 4: Heat treatment process

[0191] The cold working process is an optional step and is not required to be performed. These steps will be described below.[Step 1: Preparation Process]

[0192] In the preparation process, a material having a chemical composition that satisfies the above-described Features 1 to 3 is prepared. The material may be supplied by or manufactured by a third party. The material may be an ingot, or may be a slab, a bloom, or a billet.

[0193] In the case of manufacturing the material, the material is manufactured by, for example, the following method. Molten steel having the above-described chemical composition is produced. The produced molten steel is used to produce an ingot by ingot casting. The produced molten steel may be used to produce a slab, a bloom, or a billet by a continuous casting method. The produced ingot, slab, or bloom may be hot worked to produce a billet. For example, an ingot may be hot forged to produce a cylindrical billet, and the resulting billet may be used as the material. In this case, the temperature of the material immediately before the start of hot forging is not particularly limited, but is, for example, 1000 to 1300°C. The method of cooling the material after hot forging is not particularly limited.[Step 2: Hot Working Process]

[0194] In the hot working process, hot working is performed on the raw material prepared in the preparation process to produce an intermediate steel material. The intermediate steel material may be a steel pipe, may be a steel plate, or may be a steel bar, for example.

[0195] In the case where the intermediate steel material is a steel pipe, the following processing is carried out in the hot working process. First, a round billet (a billet having a circular cross section perpendicular to the axial direction) is prepared. A through hole is formed along the central axis of the round billet by machining. The round billet having the through hole is heated. The heated round billet is subjected to hot extrusion, typically the Eugene-Sejournet process, to produce an intermediate steel material (steel pipe). Instead of hot extrusion, a hot punch pipe making method may be carried out.

[0196] Instead of hot extrusion, piercing rolling according to the Mannesmann process may be carried out to produce a steel pipe. In this case, the round billet is heated. The heated round billet is subjected to piercing rolling using a piercing machine.

[0197] In the case where the intermediate steel material is a steel plate, the hot working process may, for example, involve the use of one or more rolling mills each having a pair of work rolls. Specifically, the slab (raw material) is heated. The heated slab is hot rolled using one or more reversing and / or tandem rolling mills to produce a steel plate.

[0198] In the case where the intermediate steel material is a round steel bar (steel bar), the hot working process may, for example, involve one or more rolling mills each having a pair of work rolls. Specifically, a round billet (the raw material) is heated. The heated round billet is hot rolled using one or more reversing and / or tandem rolling mills to produce a steel bar.[Step 3: Cold Working Process]

[0199] The cold working process is carried out as necessary. In other words, the cold working process does not need to be performed. In the case where this step is carried out, the intermediate steel material is subjected to pickling treatment before cold working.

[0200] In the case where the intermediate steel material is a steel pipe or a round steel bar (steel bar), the cold working is, for example, cold drawing or cold rolling such as Pilger rolling. In the case where the intermediate steel material is a steel plate, the cold working is, for example, cold rolling. The area reduction rate in the cold working process is not particularly limited, but is, for example, 10 to 90%.[Step 4: Heat Treatment Process]

[0201] In the heat treatment process, the intermediate steel material that has undergone the hot working process or the cold working process is subjected to heat treatment to adjust the ratio of austenite and ferrite in the steel material. The heat treatment temperature is preferably 1050 to 1250°C. After heat treatment, the intermediate steel material is quenched.

[0202] The duplex stainless steel material of the present embodiment can be manufactured by the above steps.[Welded Joint of Duplex Stainless Steel]

[0203] The welded joint of duplex stainless steel of the present embodiment is manufactured by non-filler welding using the duplex stainless steel material of the present embodiment.

[0204] The welded joint of duplex stainless steel of the present embodiment includes a base material portion and a weld metal. The base material portion and the weld metal satisfy the above-described Features 1 to 3. In other words, the base material portion and the weld metal contain, in mass%, C: 0.001 to 0.030%, Si: 1.00% or less, Mn: 0.05 to 5.00%, P: 0.035% or less, S: 0.0008% or less, Cr: 21.00 to 28.00%, Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al: 0.001 to 0.050%, N: 0.400% or less, B: 0.0001 to 0.0050%, Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, Co: 0.05 to 2.00%, W: 0 to 5.00%, Nb: 0 to 0.100%, V: 0 to 0.200%, Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and the remainder including Fe and impurities, wherein formula (1) and formula (2) below are satisfied: Ca / S ≥ 2.00 1000 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6 ≥ 50.00 where the contents in mass% of the corresponding elements are substituted for the element symbols in formula (1) and formula (2).[Method of Measuring Chemical Composition of Weld Metal]

[0205] The chemical composition of the weld metal of the welded joint of duplex stainless steel is measured by the following well-known method.

[0206] Weld metal chips are collected. The collected chips are dissolved in acid to obtain a solution. The solution is subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) to perform elemental analysis of the chemical composition. The C content and the S content are determined by the well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using the well-known inert gas fusion-thermal conductivity method. For example, the chemical composition of the weld metal is analyzed using a component analyzer (product name: ICPS-8000) manufactured by Shimadzu Corporation. Note that the chemical composition of the base material portion can also be measured in a similar manner.

[0207] The welded joint of duplex stainless steel of the present embodiment further satisfies the following Feature 4.(Feature 4)

[0208] The averaged grain size D of ferrite in the weld metal is 150 µm or less.

[0209] In the welded joint of duplex stainless steel of the present embodiment, both the base material portion and the weld metal satisfy Features 1 to 3. Therefore, as shown in FIG. 1, even when non-filler welding is performed using a shielding gas that does not contain nitrogen, the averaged grain size D of ferrite in the weld metal is 150 µm or less, and the crystal grains in the weld metal are sufficiently small. Therefore, the strength of the weld metal can be increased to an amount greater than or equal to the strength of the base material portion.

[0210] The upper limit of the averaged grain size D is preferably 140 µm, more preferably 130 µm, more preferably 120 µm, more preferably 110 µm, and more preferably 100 µm.

[0211] The lower limit of the averaged grain size D is not particularly limited. The lower limit of the averaged grain size D is preferably 15 µm, more preferably 20 µm, and more preferably 25 µm.

[0212] Note that the austenite in the microstructure of the weld metal is formed with a needle-like shape during welding. This makes it difficult to measure the grain size of austenite. Therefore, in the present embodiment, the averaged grain size D of ferrite in the weld metal is determined.[Method for Measuring Averaged Grain Size D of Ferrite in Weld Metal]

[0213] The averaged grain size D of ferrite in the weld metal of the welded joint of duplex stainless steel is determined by the following method.

[0214] Three test pieces are taken, each having a cross section perpendicular to the extending direction of the weld metal of the welded joint of duplex stainless steel. The cross sections of the test pieces are used as the observation surface. The observation surfaces are mirror polished. After mirror polishing, the observation surfaces are etched with 10% oxalic acid to reveal the microstructure. After etching, on each of the observation surfaces, a rectangular area that is centered on the center position in the width direction of the weld metal and the thickness direction of the weld metal and is 1 mm in the thickness direction and 1 mm in the width direction of the weld metal is specified.

[0215] One observation field is selected within the specified rectangular area. The observation field has a size of 840 µm × 800 µm. The observation field is observed under an optical microscope at 100x magnification. The observation field is divided into a grid of 25 areas. The number of intersections between the grid line and the grain boundaries of the ferrite grains is counted. The total length of the grid lines is divided by the total number of intersections, and that value is taken as the ferrite grain size. The ferrite grain size is calculated by rounding off the obtained value to the nearest integer.

[0216] The arithmetic mean value of the ferrite grain sizes obtained in the three observation fields is defined as the averaged grain size D (µm) of ferrite in the weld metal. The averaged grain size D is an integer obtained by rounding off the obtained value to the nearest integer.[Microstructure of Base Material Portion and Weld Metal]

[0217] The microstructures of both the base material portion and the weld metal contain ferrite in amount of 30 to 70% by volume, with the remainder being austenite.

[0218] The volume fraction of ferrite in the weld metal is measured by the following method.

[0219] A test piece is taken such that the observation surface is a cross section perpendicular to the extending direction of the weld metal of the welded joint of duplex stainless steel. The observation surface is a rectangular area of the above-described cross section that is centered on the center position in the thickness direction of the weld metal and in the width direction of the weld metal and is 2 mm in the thickness direction and 2 mm in the width direction of the weld metal. The entirety of the observation surface is considered to be the weld metal area. The method for measuring the ferrite volume fraction and the austenite volume fraction using the observation surface is based on the method described in the above section "[Method for Measuring Ferrite Volume Fraction]".[Method for Manufacturing Welded Joint of Duplex Stainless Steel]

[0220] The following describes an example of a method for manufacturing a welded joint of duplex stainless steel.

[0221] Duplex stainless steel materials of the present embodiment are prepared. The duplex stainless steel materials are brought into contact with each other, and non-filler welding is performed on the joint. During non-filler welding, a shielding gas that does not contain nitrogen may be used, or a shielding gas that contains nitrogen may be used. The amount of heat input in non-filler welding is not particularly limited, but is, for example, 0.1 to 10.0 kJ / mm.[Examples]

[0222] Effects of the duplex stainless steel material of the present embodiment will be described in more detail below with reference to examples. The conditions in the following examples are examples of conditions adopted to check enablement and effects of the duplex stainless steel material of the present embodiment. Therefore, the duplex stainless steel material of the present embodiment is not limited to this example of conditions.

[0223] Duplex stainless steel materials (plate of duplex stainless steel) having the chemical compositions shown in Tables 1A to 1C were manufactured by the following method.[Table 1A]

[0224] TABLE 1ATest No.Chemical composition (in mass%, remainder being Fe and impurities)CSiMnPSCrNiMoCu10.0120.680.890.0220.000223.166.243.210.1820.0150.340.610.0240.000224.896.773.930.3730.0160.290.780.0210.000323.977.033.560.4140.0190.210.800.0230.000225.036.973.980.1550.0190.230.800.0210.000325.206.953.980.3260.0190.210.780.0230.000224.926.883.970.1970.0190.524.970.0220.000325.584.981.022.4580.0160.544.940.0210.000225.624.991.022.4790.0170.514.970.0270.000225.384.971.052.47100.0140.340.790.0230.000524.147.933.110.44110.0150.290.490.0230.000427.557.740.890.17120.0150.290.510.0220.000327.557.750.890.12130.0140.290.510.0230.000227.577.930.890.08140.0170.220.810.0220.000625.036.963.970.11150.0190.230.800.0230.000525.056.963.990.22160.0190.210.810.0220.000525.166.923.980.24170.0140.220.830.0250.000725.367.013.132.47180.0160.240.790.0270.004124.776.953.082.32190.0160.504.960.0270.000425.264.971.042.46200.0140.320.520.0220.000527.497.770.890.06210.0150.380.790.0220.000224.896.674.130.30220.0160.310.830.0230.000325.166.614.150.29230.0140.440.740.0220.000724.977.013.991.86240.0170.360.760.0210.000625.216.584.051.99250.0150.310.750.0240.000825.276.963.980.51260.0160.240.810.0260.000324.476.734.010.41270.0150.524.920.0210.000225.644.961.022.47280.0160.524.990.0270.000225.354.951.052.46290.0170.220.810.0250.000425.086.924.280.41 [Table 1B]

[0225] TABLE 1BTest No.Chemical composition (in mass%, remainder being Fe and impurities)AlNBMgCaTiCo10.0180.2150.00240.00080.00130.0140.0720.0140.3110.00210.00030.00120.0150.1030.0230.2890.00250.00040.00140.0180.2940.0020.2860.00300.00020.00200.0310.3450.0040.2690.00300.00020.00320.0610.2860.0030.2900.00290.00010.00260.0320.4270.0380.2060.00340.00090.00230.0440.4480.0320.2550.00330.00120.00230.0430.3390.0340.2240.00270.00150.00260.0480.37100.0310.1950.00310.00180.00260.0380.26110.0180.3140.00010.00140.00300.0300.28120.0120.3240.00010.00100.00150.0520.34130.0160.3650.00010.00120.00240.0460.51140.0020.2900.00010.00020.00010.0010.24150.0040.2810.00280.00030.00110.0010.01160.0020.3100.00300.00020.00160.0030.01170.0060.3810.00020.00010.00170.0120.01180.0070.3920.00030.00010.00010.0110.12190.0420.3050.00290.00020.00260.1400.29200.0160.3270.00020.00020.00180.0010.33210.0170.3340.00030.00010.00030.0050.40220.0220.3430.00020.00010.00070.0040.01230.0260.3490.00030.00020.00060.0060.10240.0340.3380.00030.00020.00070.0040.07250.0290.3270.00260.00010.00150.0641.96260.0150.3650.00280.00020.00230.0070.13270.0290.2050.00330.00020.00070.0060.48280.0240.2260.00260.00020.00120.0030.42290.0230.3190.00090.00010.00180.0040.36 [Table 1C]

[0226] TABLE 1CTest No.Chemical composition (in mass%, remainder being Fe and impurities)Fn1Fn2WNbVTaSnREM1------6.5055.892------6.0057.173------4.6756.274------10.00334.165-0.008----10.67694.6662.00---0.008-13.00547.357---0.005--7.67323.538--0.110---11.50572.489-----0.02113.00793.5610------5.20135.26112.18-----7.50196.79122.18-----5.00183.82132.18-0.001---12.00718.0514------0.170.0215--0.001---2.200.72162.00-----3.203.0617------2.436.9318------0.020.0119--0.110---6.50723.62202.16-----3.603.3921------1.502.98220.04-0.080---2.332.3923------0.861.09240.200.002----1.171.1625------1.8866.71267.6744.1327--0.110---3.5013.5528------6.0016.8329------4.5013.84

[0227] In Tables 1A to 1C, a dash "-" indicates that the content of the corresponding element was at or below the impurity level. Also, the O content was 0.0300% or less for all of the test numbers.

[0228] First, a cylindrical ingot having a diameter of 120 mm and a mass of 30 kg was produced using molten steel. The ingot was subjected to hot forging and hot rolling to produce an intermediate steel plate having a plate thickness of 10 mm. The produced intermediate steel plate was cooled to room temperature. Thereafter, the intermediate steel plate was subjected to heat treatment. In the heat treatment, the heat treatment temperature was 1100°C, and the holding time at the heat treatment temperature was 30 minutes. After the holding time elapsed, the intermediate steel plate was water-cooled to room temperature. Through the above manufacturing steps, duplex stainless steel materials (steel plates) having the chemical compositions shown in Tables 1A to 1C and a plate thickness of 10 mm, and having corresponding test numbers, were manufactured.

[0229] For the duplex stainless steel material for each test number, the volume fraction of ferrite was measured based on the method described in the above section "[Method of Measuring Ferrite Volume Fraction]". As a result, for all of the test numbers, the ferrite volume fraction was 30 to 70%, and the remainder was austenite.[Evaluation Tests]

[0230] The following evaluation tests were carried out on the manufactured duplex stainless steel materials. Test 1: Steel material toughness evaluation test Test 2: Measurement test for averaged grain size D of ferrite in weld metal of welded joint Test 3: Strength evaluation test for weld metal of welded joint

[0231] Tests 1 to 3 will be described below.[Test 1: Steel toughness evaluation test]

[0232] A test piece having a width W of 100 mm, a length L of 100 mm, and a thickness of 8 mm was taken from the steel material for each test number. The test pieces were taken such that the widthwise center position of the steel material coincided with the widthwise center position of the test piece, the width W direction was parallel with the width direction of the steel material, and the length L direction was parallel with the rolling direction of the steel material. Three half-size (10 mm × 5 mm × 55 mm) T-direction Charpy test pieces were taken from the test pieces. The lengthwise direction of the T-direction Charpy test pieces was parallel with the direction perpendicular to the rolling direction of the steel material (i.e., parallel with the width direction). The notch in each test piece extended in the T direction. In accordance with JIS Z 2242:2018, a Charpy impact test was performed at 0°C to obtain the Charpy impact value. For each test number, the arithmetic mean value of the three obtained values was taken as the Charpy impact value (J / cm 2< ) of the steel material. The obtained Charpy impact values are shown in Table 2.[Table 2]

[0233] TABLE 2Test No.Charpy impact test value (J / cm 2< )Weld metal ferrite averaged grain size (µm)Weld metal strengthRemarks1343122EInventive Example2321120EInventive Example3294107EInventive Example426253EInventive Example527632EInventive Example625049EInventive Example712237EInventive Example812839EInventive Example914835EInventive Example1034441EInventive Example1133060EInventive Example1230131EInventive Example1332633EInventive Example14406263BComparative Example15336254BComparative Example16289225BComparative Example17367176BComparative Example18333181BComparative Example198728EComparative Example20347231BComparative Example21351206BComparative Example22288214BComparative Example23277217BComparative Example24265244BComparative Example2528129BComparative Example26354193BComparative Example27216241BComparative Example28136249BComparative Example29363223BComparative Example [Test 2: Measurement test for averaged grain size D of ferrite in weld metal of welded joint]

[0234] A test piece having a width of 100 mm, a length of 100 mm, and a thickness of 8 mm was taken from the steel material for each test number. The test pieces were taken such that the widthwise center position of the steel material coincided with the widthwise center position of the test piece, the width direction was parallel with the width direction of the steel material, and the length direction was parallel with the rolling direction of the steel material.

[0235] The test pieces were subjected to non-filler welding by TIG welding to form weld metal. Specifically, as shown in FIG. 2, at the center position in the length L direction of the test piece 1, melt-run welding was performed along the width W direction by TIG welding. The shielding gas that was used was pure Ar, and the heat input during welding was 3.2 kJ / mm. Through the above welding method, a pseudo welded joint of duplex stainless steel 1 including a base material portion 5 and weld metal 10 was formed.

[0236] The chemical composition of the weld metal 10 of the manufactured pseudo welded joint of duplex stainless steel 1 was determined by the method described above in the section "[Method for Measuring Chemical Composition of Weld Metal]". As a result, for each test number, the chemical composition of the weld metal 10 was the same as the chemical composition of the base material portion 5 of the corresponding test number in Tables 1A to 1C.

[0237] Furthermore, the averaged grain size D (µm) of ferrite in the weld metal 10 was determined based on the method described in the above section "[Method for Measuring Averaged Grain Size D of Ferrite in Weld Metal]". The obtained averaged grain sizes D (µm) are shown in Table 2.[Test 3: Strength evaluation test for weld metal of welded joint]

[0238] A weld joint tensile test piece 20 having the shape shown in FIG. 3 was taken from the pseudo welded joint of duplex stainless steel 1 shown in FIG. 2. As shown in FIG. 2, the tensile test specimen of welded joint 20 was taken at the widthwise center position and the thickness center position of the test piece 1. The length direction of the tensile test specimen of welded joint 20 was parallel with the length L direction of the test piece 1. Also, the tensile test specimen of welded joint 20 was taken such that the weld metal portion was located at the center position of a parallel portion of the tensile test specimen of welded joint 20. The dimensions of the tensile test specimen of welded joint 20 were as shown in FIG. 3. The numbers in FIG. 3 appended with (mm) indicate the dimensions of the corresponding parts.

[0239] A tensile test was carried out in air at room temperature on the tensile test specimen of welded joint 20 in accordance with JIS Z 2241:2011, and the tensile test specimen of welded joint 20 was broken. The location of the fracture in the tensile test specimen of welded joint 20 after breakage was checked. When the fracture was located in the base material portion (steel material), it was determined that the strength of the weld metal was higher than that of the base material portion (steel), and it was evaluated that the weld metal had excellent strength (shown as "E" (Excellent) in the "Weld Metal Strength" column in Table 2). On the other hand, when the fracture was located in the weld metal, it was evaluated that the weld metal did not have sufficient strength (indicated as "B" (Bad) in the "Weld Metal Strength" column in Table 2).[Evaluation Results]

[0240] The evaluation results are shown in Table 2. As shown in Table 2, the duplex stainless steel materials with the test numbers 1 to 13 satisfied Features 1 to 3. As a result, the Charpy impact value of the steel material (base material portion) at 0°C was 100 J / cm 2< or more, and excellent toughness was obtained. Furthermore, in the tensile test performed on the tensile test specimen of welded joint, excellent strength was obtained for the weld metal.

[0241] On the other hand, in test number 14, the Ca content and the Ti content were too low. Therefore, sufficient strength was not obtained in the weld metal.

[0242] In test number 15, the Ti content and the Co content were too low. Therefore, sufficient strength was not obtained in the weld metal.

[0243] In test numbers 16, 17 and 22, the Co content was too low. Therefore, sufficient strength was not obtained in the weld metal.

[0244] In test number 18, the S content was too high and the Ca content was too low. Therefore, sufficient strength was not obtained in the weld metal.

[0245] In test number 19, the Ti content was too high. Therefore, the toughness of the duplex stainless steel material was low.

[0246] In test number 20, the Ti content was too low. Therefore, sufficient strength was not obtained in the weld metal.

[0247] In test number 21, the Ca content was too low. Therefore, sufficient strength was not obtained in the weld metal.

[0248] In test numbers 23 and 24, Fn1 and Fn2 were too low. Therefore, sufficient strength was not obtained in the weld metal.

[0249] In test number 25, Fn1 was too low. Therefore, sufficient strength was not obtained in the weld metal.

[0250] In test numbers 26 to 29, Fn2 was too low. Therefore, sufficient strength was not obtained in the weld metal.

[0251] 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 modified as appropriate within the scope of the spirit of the disclosure.

Claims

1. A duplex stainless steel material containing, in mass%, C: 0.001 to 0.030%, Si: 1.00% or less, Mn: 0.05 to 5.00%, P: 0.035% or less, S: 0.0008% or less, Cr: 21.00 to 28.00%, Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al:0.001 to 0.050%, N: 0.400% or less, B: 0.0001 to 0.0050%, Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, Co: 0.05 to 2.00%, W: 0 to 5.00%, Nb: 0 to 0.100%, V: 0 to 0.200%, Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and a remainder including Fe and impurities, wherein formula (1) and formula (2) below are satisfied: Ca / S ≥ 2.00 1000 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6 ≥ 50.00 where contents in mass% of corresponding elements are substituted for element symbols in formula (1) and formula (2).

2. The duplex stainless steel material according to claim 1, further containing, in mass%, one or more kinds of elements selected from the group consisting of: W: 0.01 to 5.00%, Nb: 0.001 to 0.100%, V: 0.001 to 0.200%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.020%, and a rare earth element (REM): 0.001 to 0.050%.

3. A welded joint of duplex stainless steel comprising: a base material portion; and a weld metal, wherein the base material portion and the weld metal contain, in mass%, C: 0.001 to 0.030%, Si: 1.00% or less, Mn: 0.05 to 5.00%, P: 0.035% or less, S: 0.0008% or less, Cr: 21.00 to 28.00%, Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al: 0.001 to 0.050%, N: 0.400% or less, B: 0.0001 to 0.0050%, Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, Co: 0.05 to 2.00%, W: 0 to 5.00%, Nb: 0 to 0.100%, V: 0 to 0.200%, Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and a remainder including Fe and impurities, formula (1) and formula (2) below are satisfied, and an averaged grain size of ferrite in the weld metal is 150 µm or less, Ca / S ≥ 2.00 1000 × Ti + 2 × N × Co 0.2 × 0.5 × Ca / S 1.6 ≥ 50.00 where contents in mass% of corresponding elements are substituted for element symbols in formula (1) and formula (2).

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