Duplex stainless steel material and duplex stainless steel welded joint

Abstract

Provided is a duplex stainless steel material in which the strength of a welded metal obtained by filler-free welding using a nitrogen-free shielding gas is greater than or equal to the strength of a base material part. This duplex stainless steel material contains, in mass%, 0.001 to 0.030% of C, 1.00% or less of Si, 0.05 to 5.00% of Mn, 0.035% or less of P, 0.0008% or less of S, 21.00 to 28.00% of Cr, 4.00 to 9.50% of Ni, 0.80 to 5.50% of Mo, 0.01 to 3.50% of Cu, 0.001 to 0.050% of Al, 0.400% or less of N, 0.0001 to 0.0050% of B, 0.0050% or less of Mg, 0.0005 to 0.0100% of Ca, 0.002 to 0.100% of Ti, and 0.05 to 2.00% of Co, and satisfies formula (1) and formula (2). Ca / S > = 2.00 (1) (1000 * Ti + 2 * N) * Co 0.2 * (0.5 * Ca / S) 1.6 > = 50.00 (2)

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a duplex stainless steel material and a duplex stainless steel welded joint, and more particularly to a duplex stainless steel material and a duplex stainless steel welded joint suitable for fillerless welding. BACKGROUND

[0002] A duplex stainless steel material has high strength and excellent corrosion resistance in a chloride environment. Therefore, the duplex stainless steel material is used as, for example, a billet of a steel pipe for a seawater heat exchanger, a billet of a steel pipe for a umbilical cable for offshore development.

[0003] The chemical composition of a duplex stainless steel material is prescribed in Japanese Industrial Standards (JIS) and ASTM Standards according to a required use. For example, in the JIS standards, SUS329J3L and SUS329J4L are prescribed as the chemical composition of a duplex stainless steel material. Recently, SUS327L1 is newly prescribed in the JIS standards as a super duplex stainless steel material having a high nitrogen content and a pitting resistance equivalent (PREW) exceeding 40. In addition, in the ASTM standards, ASTM A789 S32750, S39274 are prescribed as super duplex stainless steel materials. The duplex stainless steel material prescribed in ASTM A789 S39274 has high strength and excellent corrosion resistance by increasing the PREW. Also, by increasing the W content, precipitation of a sigma phase accompanying the increase in the PREW is suppressed.

[0004] On the other hand, in the case of welding a duplex stainless steel material, the ferrite content increases in a weld metal obtained by melting a base material. Therefore, in a duplex stainless steel welded joint, the strength of the weld metal decreases. Therefore, in the process of manufacturing a welded joint of a duplex stainless steel material, welding is generally performed using a welding material having a high Ni content with the aim of securing the austenite content of the weld metal.

[0005] However, in a seawater heat exchanger provided in a chemical plant, several hundreds to several thousands of duplex stainless steel pipes are inserted into through holes formed in a tube sheet, and then the duplex stainless steel pipes are seal welded to the tube sheet. In this seal welding, in order to avoid the large-scale of the equipment, the interval of the through holes is suppressed to the minimum. Therefore, the gap between the duplex stainless steel pipes seal welded and the through holes formed in the tube sheet becomes small. Therefore, in welding in such a seawater heat exchanger, fillerless welding in which seal welding is performed without using a welding material is performed.

[0006] In fillerless welding, a base material portion is melted without using a welding material to form a weld metal, and a welded joint is manufactured. At this time, there is a case where the strength of the weld metal decreases. The decrease in the strength of the weld metal can cause a decrease in pressure resistance. In order to suppress the decrease in the strength of the weld metal, nitrogen is added to Ar shielding gas used in fillerless welding. In this case, nitrogen invades the molten metal due to the addition of nitrogen. As a result, the strength of the weld metal increases.

[0007] However, the addition of nitrogen to the Ar shielding gas can increase the cost of gas purification or lengthen the construction period due to a decrease in electrode life. Therefore, a duplex stainless steel material is required in which the strength of the weld metal is greater than or equal to the strength of the base material portion without adding nitrogen to the shielding gas in fillerless welding of the duplex stainless steel material.

[0008] Techniques relating to duplex stainless steel materials are proposed in Japanese Patent Application Publication No. 2017-95794 (Patent Literature 1) and Japanese Patent Application Publication No. 2021-31757 (Patent Literature 2).

[0009] The duplex stainless steel material disclosed in Patent Literature 1 contains at least one X group element selected from 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%, has a complex inclusion or has a complex inclusion and an inclusion in the steel material, the inclusion includes at least one of an oxide, a sulfide, and an oxysulfide. The complex inclusion has a shell containing Cr and at least one X group element around a core of the inclusion, and the proportion of the number of the complex inclusions is 30% or more of the total number of the inclusions. For the duplex stainless steel material disclosed in Patent Literature 1, the corrosion resistance is improved and the stress corrosion cracking resistance of the welded portion is improved by modifying the inclusion that becomes the starting point of localized corrosion.

[0010] The 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%, REM: 0 to 0.1%, and the balance: Fe and impurities, and the Md30 The value is -230 to 90°C, and the volume ratio of the ferrite phase in the metallographic structure is 35.0 to 65.0%.

[0011] Md 30 (℃)=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo

[0012] -68Nb(i)

[0013] In the duplex stainless steel material of Patent Document 2, strength and corrosion resistance are improved.

[0014] Prior art literature

[0015] Patent Literature

[0016] Patent Document 1: Japanese Patent Application Laid-Open No. 2017-95794

[0017] Patent Document 2: Japanese Patent Application Laid-Open No. 2021-31757 Summary of the Invention

[0018] Problems to be solved by the invention

[0019] However, in the duplex stainless steel material disclosed in Patent Document 1, no research has been conducted on making the strength of the weld metal during fillerless welding equal to or greater than the strength of the base material. Furthermore, in the duplex stainless steel material disclosed in Patent Document 2, no research has been conducted on the strength of the weld metal obtained by fillerless welding and the strength of the base material.

[0020] An object of the present disclosure is to provide a duplex stainless steel material and a duplex stainless steel weld joint in which the strength of the produced weld metal is equal to or greater than that of the base material even when fillerless welding is performed using a shielding gas containing no nitrogen.

[0021] Means for solving problems

[0022] The duplex stainless steel material 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%, rare earth elements (REM): 0 to 0.050%, and the balance of Fe and impurities, and satisfies formula (1) and formula (2).

[0023] Ca / S ≥ 2.00 (1) (1000 × Ti + 2 × N) × Co 0.2 × (0.5 × Ca / S) 1.6 ≥ 50.00 (2)

[0024] Here, the content of each element in mass% is substituted at each element symbol in formula (1) and formula (2).

[0025] The duplex stainless steel welded joint of the present disclosure has a base material portion and a weld metal. 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%, rare earth elements (REM): 0 to 0.050%, and the balance of Fe and impurities, and satisfies formula (1) and formula (2). The average grain size of ferrite in the weld metal is 150 μm or less.

[0026] Ca / S ≥ 2.00 (1) (1000 × Ti + 2 × N) × Co 0.2 × (0.5 × Ca / S)1.6 ≥ 50.00 (2)

[0027] Here, the content of each element in mass % is substituted at each element symbol in formula (1) and formula (2).

[0028] Inventive Effects

[0029] In the duplex stainless steel material of the present disclosure, even in the case where fillerless welding using a nitrogen-free shielding gas is performed, the strength of the welded metal produced is greater than or equal to the strength of the base material portion. In the duplex stainless steel welded joint of the present disclosure, the strength of the welded metal is greater than or equal to the strength of the base material portion. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a graph showing the relationship between Fn2 and the average grain size D of ferrite in the welded metal of a duplex stainless steel welded joint when a duplex stainless steel material in which the content of each element in the chemical composition is within the range of the present embodiment and which satisfies formula (1) is used as a blank and fillerless welding using a nitrogen-free shielding gas is performed to manufacture a duplex stainless steel welded joint.

[0031] Figure 2 is a schematic view of the test piece on which fillerless welding was performed in the example.

[0032] Figure 3 is a plan view of a welded joint tensile test piece collected from the test piece of Figure 2 DETAILED DESCRIPTION

[0033] The present inventors conducted research on a duplex stainless steel material in which the strength of the welded metal produced by fillerless welding using a nitrogen-free shielding gas is greater than or equal to the strength of the base material portion. As a result, the present inventors obtained the following insights.

[0034] ​The inventors first studied, from the viewpoint of chemical composition, duplex stainless steel steels in which the strength of a weld metal produced by fillerless welding using a nitrogen-free shielding gas is greater than or equal to the strength of a base material portion. As a result, it was found that, if the chemical composition is, 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%, rare earth elements (REM): 0 to 0.050%, and the balance being Fe and impurities, in duplex stainless steel steels, the strength of a weld metal produced by fillerless welding using a nitrogen-free shielding gas is greater than or equal to the strength of a base material portion.

[0035] However, even in duplex stainless steel steels having the above chemical composition, there are cases in which the strength of a weld metal produced by fillerless welding using a nitrogen-free shielding gas is not greater than or equal to the strength of a base material portion. Therefore, the inventors further studied. As a result, the inventors obtained the following insight.

[0036] In duplex stainless steel steels having the above chemical composition, in order to make the strength of a weld metal produced by fillerless welding using a nitrogen-free shielding gas greater than or equal to the strength of a base material portion, it is effective to use (I) a strengthening mechanism based on grain boundary strengthening of the weld metal, (II) a strengthening mechanism based on fine-graining of the weld metal, and (III) a strengthening mechanism based on work hardening of the weld metal.

[0037] [(I) About the grain boundary strengthening mechanism of the weld metal]

[0038] During rapid cooling at the time of welding, solidification segregation occurs. Specifically, S segregates at the interface of austenite and ferrite, the grain boundary of ferrite during solidification. The segregation of S decreases the grain boundary strength. Therefore, in order to improve the strength of the welded 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 combined with S to form CaS, thereby fixing S. Due to this, S is difficult to segregate at the interface or the grain boundary. As a result, the strength of the welded metal is improved. In order to obtain the above effect, it is necessary to increase the Ca content to some extent with respect to the S content in the steel material. Therefore, the chemical composition of the above-described duplex stainless steel material further contains Ca: 0.0005 to 0.0100%, and is adjusted so that the Ca content and the S content satisfy formula (1).

[0039] Ca / S > 2.00 (1)

[0040] [Regarding (II) Strengthening Mechanism Based on Refinement of the Welded Metal]

[0041] If the grain of the welded metal is fine, the strength of the welded metal is improved. Therefore, in the present embodiment, Ti nitride is effectively utilized in order to refine the welded metal. Specifically, during rapid cooling at the time of fillerless welding, fine Ti nitride is precipitated in the welded metal. The Ti nitride refines the primary ferrite during solidification. Due to this, the strength of the welded metal is improved. If the chemical composition of the above-described duplex stainless steel material further contains Ti: 0.002 to 0.100%, during rapid cooling at the time of fillerless welding, Ti nitride, which is effective for refinement, can be sufficiently precipitated in the welded metal.

[0042] [Regarding (III) Strengthening Mechanism Based on Work Hardening of the Welded Metal]

[0043] In the case where the welded metal is formed by fillerless welding, if work hardening occurs in the welded metal during rapid cooling at the time of welding, the strength of the welded metal is improved. Co decreases the stacking fault energy, and improves the work hardening characteristics of the austenite in the microstructure of the duplex stainless steel material. Co also increases the amount of austenite generated in the welded metal formed by fillerless welding. If the amount of austenite generated in the welded metal increases, the austenite is subjected to plastic constraint based on the ferrite. Since the plastic constraint is received in a state where the stacking fault energy is decreased due to Co, the austenite in the welded metal exhibits significant work hardening. As a result, it is possible to improve the strength of the welded metal. Therefore, the chemical composition of the above-described duplex stainless steel material further contains Co: 0.05 to 2.00%.

[0044] The strengthening mechanisms of (I) to (III) described above exert a synergistic effect in association with each other at the time of fillerless welding. Specifically, if the Ti content, N content, Co content, Ca content, and S content in the chemical composition of the duplex stainless steel material satisfy formula (2), then by the synergistic effect of the strengthening mechanisms of (I) to (III), it is possible to increase the strength of the weld metal formed by fillerless welding using a protective gas that does not contain nitrogen to be greater than or equal to the strength of the base material portion.

[0045] (1000 x Ti + 2 x N) x Co 0.2 x (0.5 x Ca / S) 1.6 ≥ 50.00 (2)

[0046] The duplex stainless steel material of the present embodiment based on the technical idea described above has the following composition.

[0047] The duplex stainless steel material of the first 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%, rare earth elements (REM): 0 to 0.050%, and the balance being Fe and impurities, and satisfies formula (1) and formula (2).

[0048] Ca / S ≥ 2.00 (1) (1000 x Ti + 2 x N) x Co 0.2 x (0.5 x Ca / S) 1.6 ≥ 50.00 (2)

[0049] Here, the content of each element in mass% is substituted at each element symbol in formula (1) and formula (2).

[0050] The duplex stainless steel material of the 2nd configuration contains one or more 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 rare earth elements (REM): 0.001 to 0.050%, on the basis of the duplex stainless steel material of the 1st configuration.

[0051] The duplex stainless steel welded joint of the 1st configuration has a base material portion and a weld metal. 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%, rare earth elements (REM): 0 to 0.050%, and the balance being Fe and impurities, and satisfy formula (1) and formula (2). The average grain size of ferrite in the weld metal is 150 μm or less.

[0052] Ca / S ≥ 2.00 (1) (1000 × Ti + 2 × N) × Co 0.2 × (0.5 × Ca / S) 1.6 ≥ 50.00 (2)

[0053] Here, the content of each element in mass% is substituted at each element symbol in formula (1) and formula (2).

[0054] Hereinafter, the duplex stainless steel material and the duplex stainless steel welded joint of the present embodiment will be described.

[0055] [Characteristics of the duplex stainless steel material of the present embodiment]

[0056] The duplex stainless steel material of the present embodiment satisfies the following characteristics 1 to 3.

[0057] (Characteristics 1)

[0058] The chemical composition is 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%, rare earth elements (REM): 0 to 0.050%, and the balance being Fe and impurities.

[0059] (Feature 2)

[0060] The above chemical composition satisfies formula (1).

[0061] Ca / S ≥ 2.00 (1)

[0062] Here, the content in mass% of the corresponding element in the chemical composition is substituted at each element symbol in formula (1).

[0063] (Feature 3)

[0064] The above chemical composition satisfies formula (2).

[0065] (1000 x Ti + 2 x N) x Co 0.2 x (0.5 x Ca / S) 1.6 ≥ 50.00 (2)

[0066] Here, the content in mass% of the corresponding element in the chemical composition is substituted at each element symbol in formula (2).

[0067] Hereinafter, Features 1 to 3 will be described.

[0068] [(Feature 1) Regarding the Chemical Composition]

[0069] The chemical composition of the duplex stainless steel material of the present embodiment contains the following elements. In the following description, the duplex stainless steel material will also be simply referred to as "steel material".

[0070] C: 0.001 to 0.030%

[0071] Carbon (C) stabilizes austenite and increases the amount of austenite in the weld metal formed by fillerless welding. As a result, the strength of the weld metal is improved. If the C content is less than 0.001%, the above-mentioned effect cannot be sufficiently obtained.

[0072] On the other hand, if the C content exceeds 0.030%, carbides are easily generated at the time of fillerless welding. In this case, the corrosion resistance of the weld metal is reduced.

[0073] Therefore, the C content is 0.001 to 0.030%.

[0074] The preferable lower limit of the C content is 0.002%, and further preferably 0.003%.

[0075] The preferable upper limit of the C content is 0.025%, and further preferably 0.020%.

[0076] Si: 1.00% or less

[0077] Silicon (Si) deoxidizes the steel. As long as Si is contained in a small amount, the above-mentioned effect is obtained to some extent.

[0078] On the other hand, Si stabilizes ferrite. Therefore, if the Si content exceeds 1.00%, the amount of ferrite in the weld metal formed by fillerless welding increases. As a result, the strength and the corrosion resistance of the weld metal are reduced.

[0079] Therefore, the Si content is 1.00% or less.

[0080] The preferable lower limit of the Si content exceeds 0%, and further preferably 0.01%, and further preferably 0.05%, and further preferably 0.10%.

[0081] The preferable upper limit of the Si content is 0.90%, and further preferably 0.80%.

[0082] Mn: 0.05 to 5.00%

[0083] Manganese (Mn) stabilizes austenite and increases the amount of austenite in the weld metal formed by fillerless welding. As a result, the strength of the weld metal is improved. If the Mn content is less than 0.05%, the above-mentioned effect cannot be sufficiently obtained.

[0084] On the other hand, if the Mn content exceeds 5.00%, the stacking fault energy is excessively reduced. Therefore, the toughness of the weld metal formed by fillerless welding is reduced.

[0085] Therefore, the Mn content is 0.05 to 5.00%.

[0086] The preferable lower limit of the Mn content is 0.08%, and further preferably 0.10%.

[0087] The upper limit of the Mn content is preferably 4.00%, further preferably 3.00%, further preferably 2.00%.

[0088] P: 0.035% or less

[0089] Phosphorus (P) is an impurity. P segregates at grain boundaries, increasing the crack sensitivity during hot working. P also segregates during solidification at the time of welding, increasing the high-temperature crack sensitivity.

[0090] Therefore, the P content is 0.035% or less.

[0091] The P content is preferably as low as possible. However, excessive reduction of the P content greatly increases the manufacturing cost. Therefore, the lower limit of the P content is more than 0%, further preferably 0.001%, further preferably 0.002%, taking into account the industrial productivity.

[0092] The upper limit of the P content is preferably 0.028%, further preferably 0.025%, further preferably 0.023%, further preferably 0.020%.

[0093] S: 0.0008% or less

[0094] Sulfur (S) is an impurity. S segregates at grain boundaries, increasing the crack sensitivity during hot working. S also segregates during solidification at the time of welding, enriching at the ferrite grain boundaries and the interface between ferrite and austenite. As a result, the tensile strength of the weld metal decreases.

[0095] Therefore, the S content is 0.0008% or less.

[0096] The S content is preferably as low as possible. However, excessive reduction of the S content greatly increases the manufacturing cost. Therefore, the lower limit of the S content is more than 0%, further preferably 0.0001%, further preferably 0.0002%, taking into account the industrial productivity.

[0097] The upper limit of the S content is preferably 0.0007%, further preferably 0.0005%.

[0098] Cr: 21.00 to 28.00%

[0099] Chromium (Cr) improves the corrosion resistance of the steel material and the weld metal formed by fillerless welding. If the Cr content is less than 21.00%, the above effect cannot be sufficiently obtained.

[0100] On the other hand, if the Cr content exceeds 28.00%, intermetallic compounds represented by σ phase are easily generated in the weld metal formed by fillerless welding, and further the ferrite amount in the weld metal excessively increases. As a result, the strength of the weld metal decreases.

[0101] Therefore, the Cr content is 21.00 to 28.00%.

[0102] The preferable lower limit of the Cr content is 21.50%, further preferably 22.00%, further preferably 22.70%.

[0103] The preferable upper limit of the Cr content is 27.50%, further preferably 27.20%, further preferably 27.00%.

[0104] Ni: 4.00 to 9.50%

[0105] Nickel (Ni) stabilizes austenite. Therefore, the amount of austenite in the weld metal formed by the fillerless welding increases. As a result, the strength of the weld metal improves. If the Ni content is less than 4.00%, the above effect cannot be sufficiently obtained.

[0106] On the other hand, if the Ni content exceeds 9.50%, intermetallic compounds represented by σ phase are easily generated in the weld metal formed by the fillerless welding. As a result, the strength of the weld metal decreases.

[0107] Therefore, the Ni content is 4.00 to 9.50%.

[0108] The preferable lower limit of the Ni content is 4.50%, further preferably 5.00%, further preferably 5.50%.

[0109] The preferable upper limit of the Ni content is 9.25%, further preferably 9.00%, further preferably 8.50%, further preferably 8.00%.

[0110] Mo: 0.80 to 5.50%

[0111] Molybdenum (Mo) improves the corrosion resistance of the steel material and the weld metal formed by the fillerless welding. Mo also improves the strength of the steel material and the weld metal by solid solution strengthening. If the Mo content is less than 0.80%, the above effect cannot be sufficiently obtained.

[0112] On the other hand, if the Mo content exceeds 5.50%, intermetallic compounds represented by σ phase are easily generated in the weld metal formed by the fillerless welding. As a result, the strength of the weld metal decreases.

[0113] Therefore, the Mo content is 0.80 to 5.50%.

[0114] The preferable lower limit of the Mo content is 0.85%, further preferably 0.90%, further preferably 1.00%.

[0115] The preferable upper limit of the Mo content is 5.20%, further preferably 5.00%, further preferably 4.50%.

[0116] Cu: 0.01 to 3.50%

[0117] Copper (Cu) improves the corrosion resistance of the steel and the weld metal formed by fillerless welding. Cu also improves the strength of ferrite. If the Cu content is less than 0.01%, the above effects cannot be sufficiently obtained.

[0118] On the other hand, if the Cu content exceeds 3.50%, the hot workability of the steel deteriorates. Also, the solid solution amount of N decreases, and the generation of Cr nitrides is promoted. As a result, the corrosion resistance and toughness of the steel deteriorate.

[0119] Therefore, the Cu content is 0.01 to 3.50%.

[0120] The preferable lower limit of the Cu content is 0.05%, further preferably 0.10%, further preferably 0.20%.

[0121] The preferable upper limit of the Cu content is 3.30%, further preferably 3.00%, further preferably 2.50%.

[0122] Al: 0.001 to 0.050%

[0123] Aluminum (Al) deoxidizes the steel. Al also combines with Mg to generate Al-Mg oxides at the time of fillerless welding, thereby promoting the precipitation of Ti nitrides in the weld metal. Therefore, the weld metal phase structure is refined, and the strength of the weld metal is improved. If the Al content is less than 0.001%, the above effects cannot be sufficiently obtained.

[0124] On the other hand, if the Al content exceeds 0.050%, AlN is excessively generated. In this case, the toughness and corrosion resistance of the steel and the weld metal formed by fillerless welding deteriorate.

[0125] Therefore, the Al content is 0.001 to 0.050%.

[0126] The preferable lower limit of the Al content is 0.003%, further preferably 0.004%, further preferably 0.005%.

[0127] The preferable upper limit of the Al content is 0.045%, further preferably 0.040%, further preferably 0.030%.

[0128] N: 0.400% or less

[0129] Nitrogen (N) is inevitably contained. N stabilizes austenite and increases the strength of austenite in the steel. N also increases PREW, and improves the pitting corrosion resistance and crevice corrosion resistance of the steel and the weld metal formed by fillerless welding.

[0130] However, if the N content exceeds 0.400%, defects such as porosity occur in the weld metal at the time of fillerless welding.

[0131] Therefore, the N content is 0.400% or less.

[0132] The preferable lower limit of the N content is more than 0%, further preferably 0.001%, further preferably 0.005%, further preferably 0.010%, further preferably 0.050%, further preferably 0.080%, further preferably 0.100%.

[0133] The preferable upper limit of the N content is 0.380%, further preferably 0.370%, further preferably 0.350%.

[0134] B: 0.0001 to 0.0050%

[0135] Boron (B) segregates at the grain boundaries at high temperatures, and improves the hot workability of the steel. B also deoxidizes the steel. If the B content is less than 0.0001%, the above effects cannot be sufficiently obtained.

[0136] On the other hand, if the B content exceeds 0.0050%, solidification segregation occurs in the solidification process of the weld metal at the time of fillerless welding. As a result, the solidification cracking sensitivity of the weld metal increases.

[0137] Therefore, the B content is 0.0001 to 0.0050%.

[0138] The preferable lower limit of the B content is 0.0003%, further preferably 0.0005%, further preferably 0.0010%.

[0139] The preferable upper limit of the B content is 0.0048%, further preferably 0.0045%, further preferably 0.0043%, further preferably 0.0040%.

[0140] Mg: 0.0050% or less

[0141] Magnesium (Mg) forms Al-Mg oxide in combination with Mg at the time of fillerless welding, thereby promoting the crystallization of Ti nitride. Therefore, the weld metal phase structure is refined, and the strength of the weld metal is improved. The above effects are obtained to some extent as long as Mg is contained in a small amount.

[0142] However, if the Mg content exceeds 0.0050%, the hot workability of the duplex stainless steel material decreases.

[0143] Therefore, the Mg content is 0.0050% or less.

[0144] The preferable lower limit of the Mg content is more than 0%, further preferably 0.0001%, further preferably 0.0002%, further preferably 0.0003%.

[0145] The preferable upper limit of the Mg content is 0.0045%, further preferably 0.0040%, further preferably 0.0035%, further preferably 0.0030%.

[0146] Ca: 0.0005 to 0.0100%

[0147] Calcium (Ca) combines with S to form CaS at the time of fillerless welding, thereby immobilizing S. Therefore, the solidification segregation of S at the time of welding is suppressed. As a result, the strength of the weld metal is improved. If the Ca content is less than 0.0005%, the above effect cannot be sufficiently obtained.

[0148] On the other hand, if the Ca content exceeds 0.0100%, CaO is excessively generated at the time of fillerless welding. Therefore, the cleanliness of the weld metal is significantly decreased, and the quality of the appearance of the weld metal is also decreased.

[0149] Therefore, the Ca content is 0.0005 to 0.0100%.

[0150] The preferable lower limit of Ca is 0.0010%, further preferably 0.0015%, further preferably 0.0020%.

[0151] The preferable upper limit of the Ca content is 0.0080%, further preferably 0.0070%, further preferably 0.0050%.

[0152] Ti: 0.002 to 0.100%

[0153] Titanium (Ti) combines with N to precipitate Ti nitride at the time of fillerless welding. By the pinning effect of the Ti nitride, the proeutectic ferrite in the weld metal is fine-grained. As a result, the strength of the weld metal is improved. If the Ti content is less than 0.002%, the above effect cannot be sufficiently obtained.

[0154] On the other hand, if the Ti content exceeds 0.100%, the Ti nitride is coarsened. Therefore, the toughness of the steel material and the weld metal is decreased.

[0155] Therefore, the Ti content is 0.002 to 0.100%.

[0156] The preferable lower limit of the Ti content is 0.005%, further preferably 0.010%, further preferably 0.011%, further preferably 0.012%, further preferably 0.015%.

[0157] The preferable upper limit of the Ti content is 0.090%, further preferably 0.080%, further preferably 0.070%, further preferably 0.050%.

[0158] Co: 0.05 to 2.00%

[0159] Cobalt (Co) lowers the stacking fault energy and improves the work hardening property. Co also stabilizes austenite. Therefore, the amount of austenite in the weld metal increases when welding without filler metal. In this case, the generated austenite is surrounded by ferrite. The austenite surrounded by ferrite is plastically constrained and work-hardened, for example, in the case where a tensile stress or the like is applied. As a result, the strength of the weld metal is improved. If Co is less than 0.05%, the above effects cannot be sufficiently obtained.

[0160] On the other hand, if the Co content exceeds 2.00%, the manufacturing cost becomes excessively high.

[0161] Therefore, the Co content is 0.05 to 2.00%.

[0162] The preferable lower limit of the Co content is 0.08%, further preferably 0.10%, further preferably 0.15%, further preferably 0.20%, further preferably 0.30%.

[0163] The preferable upper limit of the Co content is 1.90%, further preferably 1.80%, further preferably 1.70%, further preferably 1.50%.

[0164] The balance of the chemical composition of the duplex stainless steel material of the present embodiment is Fe and impurities. Here, the impurities refer to substances that are mixed from ores, waste materials, or manufacturing environments, etc. as raw materials when the duplex stainless steel material is industrially manufactured, and are allowed within a range that does not adversely affect the duplex stainless steel material of the present embodiment. Impurities other than the above are, for example, O: 0.0300% or less.

[0165] [Regarding Arbitrary Elements]

[0166] The chemical composition of the duplex stainless steel material of the present embodiment can further contain one or more 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 rare earth elements (REM): 0 to 0.050% instead of a part of Fe. These elements are all optional elements, and can not be contained. Hereinafter, these optional elements are described.

[0167] [About W]

[0168] The chemical composition of the duplex stainless steel material of the present embodiment can further contain W instead of a part of Fe.

[0169] W: 0 to 5.00%

[0170] Tungsten (W) is an optional element, and can not be contained. That is, the W content can be 0%.

[0171] In the case where it is contained, that is, in the case where the W content exceeds 0%, W forms an oxide, and improves the corrosion resistance of the steel material in an environment with low pH and the weld metal formed by fillerless welding. W also improves the strength of the steel material and the weld metal by solid solution strengthening. As long as W is contained in a small amount, the above effects are obtained to some extent.

[0172] However, if the W content exceeds 5.00%, intermetallic compounds represented by a sigma phase are easily generated in the weld metal formed by fillerless welding. As a result, the toughness of the weld metal is reduced.

[0173] Therefore, the W content is 0 to 5.00%.

[0174] The preferable lower limit of the W content is 0.01%, further preferably 0.10%, further preferably 0.30%, further preferably 0.50%, further preferably 1.00%.

[0175] The preferable upper limit of the W content is 4.50%, further preferably 4.00%, further preferably 3.50%, further preferably 3.00%.

[0176] [About Nb, V, and Ta]

[0177] The chemical composition of the duplex stainless steel material of the present embodiment can further contain one or more selected from the group consisting of Nb, V, and Ta instead of a part of Fe. These elements all form carbides, and improve the strength of the steel material and the weld metal formed by fillerless welding by precipitation strengthening. Furthermore, the generation of Cr-deficient regions is suppressed by the generation of carbides. As a result, the corrosion resistance of the steel material and the weld metal is improved.

[0178] Nb: 0 to 0.100%

[0179] Niobium (Nb) is an optional element and can not be contained. That is, the content of Nb can be 0%.

[0180] In the case of being contained, that is, in the case of the content of Nb exceeding 0%, Nb combines with C to form a carbide. Therefore, the strength of the weld metal formed by the fillerless welding is improved. Also, by the generation of the Nb carbide, the generation of the Cr carbide at the grain boundaries is suppressed. As a result, the corrosion resistance of the weld metal is improved. As long as Nb is contained in a small amount, the above-mentioned effects are obtained to some extent.

[0181] However, if the content of Nb exceeds 0.100%, the Nb carbide is excessively generated. In this case, the corrosion resistance and the toughness of the steel material and the weld metal are reduced.

[0182] Therefore, the content of Nb is 0 to 0.100%.

[0183] The preferable lower limit of the content of Nb is 0.001%, further preferably 0.002%, further preferably 0.005%.

[0184] The preferable upper limit of the content of Nb is 0.080%, further preferably 0.050%, further preferably 0.030%, further preferably 0.020%, further preferably 0.015%.

[0185] V: 0 to 0.200%

[0186] Vanadium (V) is an optional element and can not be contained. That is, the content of V can be 0%.

[0187] In the case of being contained, that is, in the case of the content of V exceeding 0%, V combines with C to form a carbide. Therefore, the strength of the weld metal formed by the fillerless welding is improved. Also, by the generation of the V carbide, the generation of the Cr carbide at the grain boundaries is suppressed. As a result, the corrosion resistance of the weld metal is improved. As long as V is contained in a small amount, the above-mentioned effects are obtained to some extent.

[0188] However, if the content of V exceeds 0.200%, the V carbide is excessively generated. In this case, the corrosion resistance and the toughness of the steel material and the weld metal are reduced.

[0189] Therefore, the content of V is 0 to 0.200%.

[0190] The preferable lower limit of the content of V is 0.001%, further preferably 0.002%, further preferably 0.010%, further preferably 0.020%.

[0191] The preferable upper limit of the content of V is 0.180%, further preferably 0.150%, further preferably 0.120%, further preferably 0.100%, further preferably 0.080%, further preferably 0.050%.

[0192] Ta: 0 to 0.100%

[0193] Tantalum (Ta) is an optional element, and can not be contained. That is, the content of Ta can be 0%.

[0194] In the case of being contained, that is, in the case where the content of Ta exceeds 0%, Ta combines with C to form a carbide. Therefore, the strength of the weld metal formed by fillerless welding is improved. Also, by the generation of Ta carbide, the generation of Cr carbide at the grain boundaries is suppressed. As a result, the corrosion resistance of the weld metal is improved. As long as a small amount of Ta is contained, the above-mentioned effects are obtained to some extent.

[0195] However, if the content of Ta exceeds 0.100%, Ta carbide is generated in excess. In this case, the corrosion resistance and toughness of the steel material and the weld metal are reduced.

[0196] Therefore, the content of Ta is 0 to 0.100%.

[0197] The preferable lower limit of the content of Ta is 0.001%, further preferably 0.002%, further preferably 0.003%.

[0198] The preferable upper limit of the content of Ta is 0.080%, further preferably 0.050%, further preferably 0.030%, further preferably 0.020%.

[0199] [About Sn]

[0200] The chemical composition of the duplex stainless steel material of the present embodiment can also contain Sn instead of a part of Fe.

[0201] Sn: 0 to 0.020%

[0202] Tin (Sn) is an optional element, and can not be contained. That is, the content of Sn can be 0%.

[0203] In the case of being contained, that is, in the case where the content of Sn exceeds 0%, Sn improves the pitting corrosion resistance of the steel material and the weld metal formed by fillerless welding. As long as a small amount of Sn is contained, the above-mentioned effects are obtained to some extent.

[0204] However, if the content of Sn exceeds 0.020%, the hot workability of the steel material is reduced. Also, the penetration during welding increases, and the wettability of the molten metal during welding is reduced.

[0205] Therefore, the Sn content is 0 to 0.020%.

[0206] The preferable lower limit of the Sn content is 0.001%, further preferably 0.002%, further preferably 0.003%.

[0207] The preferable upper limit of the Sn content is 0.018%, further preferably 0.015%, further preferably 0.010%, further preferably 0.009%, further preferably 0.008%, further preferably 0.007%.

[0208] [About Rare Earth Element (REM)]

[0209] The chemical composition of the duplex stainless steel material of the present embodiment can further contain a rare earth element (REM) instead of a part of Fe.

[0210] Rare Earth Element (REM): 0 to 0.050%

[0211] The rare earth element (REM) is any element, and can not be contained. That is, the REM content can be 0%.

[0212] In the case of being contained, that is, in the case where the REM content exceeds 0%, the REM improves the hot workability of the base material portion. The above effect is obtained to some extent as long as the REM is contained in a small amount.

[0213] However, if the REM content exceeds 0.050%, the hot workability of the base material portion is rather reduced.

[0214] Therefore, the REM content is 0 to 0.050%.

[0215] The preferable lower limit of the REM content is 0.001%, further preferably 0.002%, further preferably 0.005%.

[0216] The preferable upper limit of the REM content is 0.040%, further preferably 0.030%, further preferably 0.020%.

[0217] Further, the REM in the present specification is one or more elements selected from the group consisting of scandium (Sc) of atomic number 21, yttrium (Y) of atomic number 39, and lanthanum (La) of atomic number 57 to lutetium (Lu) of atomic number 71 as lanthanoid elements. In addition, the REM content (%) in the present specification means the total content (%) of these elements.

[0218] [(Feature 2) About Formula (1)]

[0219] The duplex stainless steel material of the present embodiment also satisfies Formula (1).

[0220] Ca / S ≥ 2.00 (1)

[0221] Here, at each element symbol in formula (1), the content in mass% of the corresponding element in the chemical composition is substituted.

[0222] Fn1 is defined as follows.

[0223] Fn1 = Ca / S

[0224] Fn1 corresponds to the left side of formula (1). Fn1 is an index related to the fixed amount of S in the weld metal when welding without filler metal in the duplex stainless steel material satisfying Characteristic 1. If Fn1 is less than 2.00, the content of Ca relative to the content of S in the steel material is too small. In this case, S in the weld metal cannot be sufficiently fixed with Ca. Therefore, S not combined with Ca is solidified segregation. As a result, the grain boundary strength of the weld metal decreases, and the strength of the weld metal decreases.

[0225] If Fn1 satisfies 2.00 or more, S is sufficiently fixed with Ca in the duplex stainless steel material satisfying Characteristic 1. Therefore, in the weld metal, solidification segregation of S is suppressed. As a result, in the weld metal, excellent strength can be obtained.

[0226] The lower limit of Fn1 is preferably 2.10, further preferably 2.30, further preferably 2.50, further preferably 3.00, further preferably 4.00, further preferably 5.00, further preferably 6.00, further preferably 7.00, further preferably 10.00.

[0227] The upper limit of Fn1 is not particularly specified. However, in the case where the chemical composition of the duplex stainless steel material satisfies Characteristic 1, the upper limit of Fn1 is 100.00.

[0228] [(Characteristic 3) Regarding Formula (2)]

[0229] The duplex stainless steel material of the present embodiment also satisfies formula (2).

[0230] (1000 x Ti + 2 x N) x Co 0.2 x (0.5 x Ca / S) 1.6 ≥ 50.00 (2)

[0231] Here, at each element symbol in formula (2), the content in mass% of the corresponding element in the chemical composition is substituted.

[0232] Fn2 is defined as follows.

[0233] Fn2 = (1000 x Ti + 2 x N) x Co 0.2 x (0.5 x Ca / S)1.6

[0234] Fn2 corresponds to the left side of formula (2). Fn2 is an index related to the strength of the weld metal formed by the fillerless welding using the nitrogen-free shielding gas in the duplex stainless steel material satisfying the characteristic 1 and the characteristic 2. If Fn2 is less than 50.00, the synergistic effect of the strengthening mechanisms of (I) to (III) described above cannot be sufficiently obtained. Therefore, even if the characteristic 1 and the characteristic 2 are satisfied, the strength of the weld metal formed by the fillerless welding using the nitrogen-free shielding gas is not sufficiently increased.

[0235] If Fn2 is 50.00 or more, the synergistic effect of the strengthening mechanisms of (I) to (III) described above can be sufficiently obtained. Therefore, the strength of the weld metal formed by the fillerless welding using the nitrogen-free shielding gas is sufficiently increased.

[0236] Fn2 particularly contributes to the strengthening mechanism of (II). Figure 1 is a graph showing the relationship between Fn2 and the average grain size D of the ferrite of the weld metal of the duplex stainless steel welded joint when duplex stainless steel welded joints are manufactured by the fillerless welding using the nitrogen-free shielding gas with the duplex stainless steel material in which the content of each element in the chemical composition is within the range of the present embodiment and satisfies formula (1) as a blank. Referring to Figure 1 , as Fn2 increases, the average grain size D of the ferrite of the weld metal significantly becomes smaller. Moreover, if Fn2 becomes 50.00 or more, the average grain size D becomes 150 μm or less. As described above, if Fn2 is 50.00 or more, the weld metal is significantly fine-grained. As a result, the strength of the weld metal is increased by the strengthening mechanism of (II).

[0237] The lower limit of Fn2 is preferably 70.00, further preferably 90.00, further preferably 100.00, further preferably 120.00, further preferably 150.00, further preferably 170.00, further preferably 190.0. If Fn2 is 100.00 or more, the average grain size D of the weld metal is 100 μm or less.

[0238] The upper limit of Fn2 is not particularly limited. However, in the case where the chemical composition of the duplex stainless steel material satisfies the characteristic 1, the upper limit of Fn2 is 60536.67. The preferable upper limit of Fn2 is 60000.00, further preferably 59000.00.

[0239] [Effects of the duplex stainless steel material of the present embodiment]

[0240] The duplex stainless steel material of this embodiment satisfies characteristics 1 to 3. Therefore, in the duplex stainless steel material of this embodiment, when fillerless welding is performed using a shielding gas that does not contain nitrogen, the strength of the resulting weld metal can be sufficiently improved, and the strength of the weld metal can be greater than or equal to the strength of the base material (steel material).

[0241] [About microstructure]

[0242] The microstructure of the duplex stainless steel material of this embodiment contains 30-70% ferrite by volume, with the balance being austenite. The microstructure of structures other than ferrite and austenite is negligibly small. Specifically, the microstructure of the duplex stainless steel material of this embodiment may contain trace amounts of precipitates, inclusions, and the like in addition to ferrite and austenite. However, in the duplex stainless steel material of this embodiment, the volume fraction of precipitates, inclusions, and the like is negligibly small compared to the volume fraction of ferrite and austenite.

[0243] [Method for measuring ferrite volume fraction]

[0244] The volume fraction of ferrite in the duplex stainless steel pipe can be determined by a method in accordance with ASTM E562 (2019).

[0245] Specifically, a test piece for microstructure was collected from a duplex stainless steel material.

[0246] When the duplex stainless steel material is a steel pipe, a test piece having an observation surface of 2 mm in the pipe axial direction and 2 mm in the pipe diameter direction is collected from the center of the wall thickness.

[0247] When 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 collected from the center of the plate thickness.

[0248] When 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 collected from the R / 2 portion.

[0249] The observation surface of the test piece was mirror polished. The mirror polished observation surface was electrolytically etched in a 7% potassium hydroxide etching solution to reveal the structure. The observation surface with the revealed structure was observed under one field of view using an optical microscope. The area of ​​each field of view was set to 1.00 mm. 2 (Magnification 100x). Ferrite and austenite are identified in each field of view based on contrast. When electrolytic etching is performed in a 7% potassium hydroxide solution, areas with lower brightness correspond to ferrite, while areas with higher brightness correspond to austenite. Therefore, those skilled in the art can easily identify ferrite and austenite based on contrast.

[0250] The area rate of ferrite that has been determined is measured by the point counting method according to ASTM E562 (2019). The area rate of ferrite obtained in each field is defined as the volume rate (%) of ferrite. The volume rate (%) of ferrite is an integer value obtained by rounding off the first digit after the decimal point of the obtained value.

[0251] Further, the volume rate (%) of austenite is a value obtained by subtracting the volume rate of ferrite from 100.

[0252] [Shape of the Duplex Stainless Steel Material of the Present Embodiment]

[0253] The shape of the duplex stainless steel material of the present embodiment is not particularly limited. The duplex stainless steel material of the present embodiment can be a steel pipe, a steel sheet, or a round steel (bar steel).

[0254] [Use of the Duplex Stainless Steel Material of the Present Embodiment]

[0255] The duplex stainless steel material of the present embodiment can be widely used for uses that require high strength and excellent corrosion resistance in a chloride environment. The duplex stainless steel material of the present embodiment is suitable for, for example, uses in a humid environment containing a chloride such as seawater. Such uses are, for example, flow line pipes, umbilical pipes, heat exchangers, and the like.

[0256] [Manufacturing Method of the Duplex Stainless Steel Material]

[0257] A manufacturing method of the duplex stainless steel material of the present embodiment will be described. The manufacturing method of the duplex stainless steel material described below is an example of the manufacturing method of the duplex stainless steel material of the present embodiment. Therefore, the duplex stainless steel material having the above-described configuration can also be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the manufacturing method of the duplex stainless steel material of the present embodiment.

[0258] The manufacturing method of the duplex stainless steel material of the present embodiment includes the following processes.

[0259] (Process 1) Preparation Process

[0260] (Process 2) Hot Working Process

[0261] (Process 3) Cold Working Process

[0262] (Process 4) Heat Treatment Process

[0263] Further, the cold working process is an optional process and can not be performed. Hereinafter, each process will be described.

[0264] [(Process 1) Preparation Process]

[0265] In the preparation process, a blank having a chemical composition satisfying the above-described characteristics 1 to 3 is prepared. The blank can be supplied from a third party or can be manufactured. The blank can be an ingot or can be a slab, a bloom, or a billet.

[0266] In the case of manufacturing the blank, the blank is manufactured, for example, by the following method. Molten steel having the above-described chemical composition is manufactured. Using the manufactured molten steel, an ingot is manufactured by an ingot casting method. Using the manufactured molten steel, a slab, a bloom, or a billet can also be manufactured by a continuous casting method. A billet can also be manufactured by subjecting the manufactured ingot, slab, or bloom to hot working. For example, an ingot can also be subjected to hot forging to manufacture a cylindrical billet, and the billet can be used as the blank. In this case, the temperature of the blank immediately before the start of hot forging is not particularly limited, and is, for example, 1000 to 1300°C. The cooling method of the blank after hot forging is not particularly limited.

[0267] [(Process 2) Hot Working Process]

[0268] In the hot working process, the blank prepared in the preparation process is subjected to hot working to manufacture an intermediate steel product. The intermediate steel product can be, for example, a steel pipe, a steel sheet, or a steel bar.

[0269] In the case where the intermediate steel product is a steel pipe, in the hot working process, the following working is performed. First, a round billet (a billet having a circular shape in a cross section perpendicular to an axial direction) is prepared. A through hole along a central axis of the round billet is formed by machining. The round billet in which the through hole is formed is heated. The heated round billet is subjected to hot extrusion represented by a glass lubrication high-speed extrusion method to manufacture the intermediate steel product (steel pipe). Instead of the hot extrusion method, a hot press pipe forming method can also be performed.

[0270] Instead of the hot extrusion, a piercing rolling based on a Mannesmann method can also be performed to manufacture the steel pipe. In this case, the round billet is heated. The heated round billet is subjected to the piercing rolling by a piercer.

[0271] In the case where the intermediate steel product is a steel sheet, the hot working process uses, for example, one or a plurality of rolling mills each having a pair of work rolls. Specifically, a slab serving as a blank is heated. The heated slab is subjected to hot rolling using one or a plurality of rolling mills of a reversing type or / and a tandem type to manufacture the steel sheet.

[0272] In the case where the intermediate steel material is a round bar (rod), the hot working process uses, for example, one or more rolling mills having a pair of work rolls. Specifically, a round billet as a billet is heated. The heated round billet is subjected to hot rolling using one or more rolling mills of the reversing type or / and the tandem type to produce a rod.

[0273] [(Process 3) Cold working process]

[0274] The cold working process is performed as necessary. That is, the cold working process can not be performed. In the case where the cold working process is performed, the cold working is performed after the intermediate steel material is subjected to pickling treatment.

[0275] In the case where the intermediate steel material is a steel pipe or a round bar (rod), the cold working is, for example, cold rolling typified by cold drawing or pilger rolling. In the case where the intermediate steel material is a steel sheet, the cold working is, for example, cold rolling. The cross-sectional reduction ratio in the cold working process is not particularly limited, and is, for example, 10 to 90%.

[0276] [(Process 4) Heat treatment process]

[0277] In the heat treatment process, the intermediate steel material after the hot working process or the cold working process is subjected to heat treatment to adjust the ratio of austenite to ferrite in the steel material. The preferred heat treatment temperature in the heat treatment is 1050 to 1250°C. After the heat treatment, the intermediate steel material is quenched.

[0278] By the above processes, the duplex stainless steel material of the present embodiment can be produced.

[0279] [Duplex stainless steel welded joint]

[0280] The duplex stainless steel welded joint of the present embodiment is produced by fillerless welding using the duplex stainless steel material of the present embodiment.

[0281] The duplex stainless steel welded joint of the present embodiment has a base material portion and a weld metal. The base material portion and the weld metal satisfy the above-described characteristics 1 to 3. That is, the base material portion and the weld metal are 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%, rare earth elements (REM): 0 to 0.050% in mass%, and the balance being Fe and impurities, and satisfy formula (1) and formula (2).

[0282] Ca / S ≥ 2.00 (1) (1000 x Ti + 2 x N) x Co 0.2 x (0.5 x Ca / S) 1.6 ≥ 50.00 (2)

[0283] Here, the content of each element in mass% is substituted at each element symbol in formula (1) and formula (2).

[0284] [Method for measuring chemical composition of weld metal]

[0285] The chemical composition of the weld metal of the duplex stainless steel welded joint is measured by the following known method.

[0286] Chips of the weld metal 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), and elemental analysis of the chemical composition is performed. As for the C content and the S content, they are found by a known high-frequency combustion method (combustion-infrared absorption method). As for the N content, a known non-active gas melting-thermal conductivity method is used. For example, by using a component analysis device (trade name: ICPS-8000) manufactured by Shimadzu Corporation, the chemical composition of the weld metal is analyzed. Furthermore, the chemical composition of the base material portion can also be measured in the same manner.

[0287] In the duplex stainless steel welded joint of the present embodiment, the following characteristic 4 is also satisfied.

[0288] (Feature 4)

[0289] The average grain size D of ferrite in the weld metal is 150 μm or less.

[0290] In the duplex stainless steel welded joint of the present embodiment, both the base material portion and the weld metal satisfy Feature 1 to Feature 3. Therefore, as shown in Table 1, even in the case where fillerless welding using a nitrogen-free shielding gas is performed, the average grain size D of ferrite in the weld metal is 150 μm or less, and the grain size in the weld metal is sufficiently small. Therefore, it is possible to increase the strength of the weld metal to be greater than or equal to the strength of the base material portion. Figure 1

[0291] The preferable upper limit of the average grain size D is 140 μm, further preferably 130 μm, further preferably 120 μm, further preferably 110 μm, further preferably 100 μm.

[0292] The lower limit of the average grain size D is not particularly limited. The preferable lower limit of the average grain size D is 15 μm, further preferably 20 μm, further preferably 25 μm.

[0293] Further, the austenite in the microstructure of the weld metal is generated as acicular at the time of welding. Therefore, it is difficult to measure the grain size of the austenite. Therefore, in the present embodiment, the average grain size D of ferrite of the weld metal is found.

[0294] [Method for measuring the average grain size D of ferrite in the weld metal]

[0295] The average grain size D of ferrite in the weld metal of the duplex stainless steel welded joint is found by the following method.

[0296] Three test pieces including a section perpendicular to the extension direction of the weld metal of the duplex stainless steel welded joint are collected. The section in the test piece is taken as an observation surface. The observation surface is mirror-polished. The observation surface after mirror-polishing is subjected to etching using 10% oxalic acid to make the microstructure appear. In the observation surface after etching, a rectangular region of 1 mm in the thickness direction of the weld metal and 1 mm in the width direction of the weld metal is determined at the central position of the width direction of the weld metal at the central position including the thickness direction of the weld metal.

[0297] An arbitrary one observation field is selected in the determined rectangular region. The observation field is 840 μm x 800 μm. The observation field is observed with an optical microscope at 100 times. The observation field is divided into 25 grids. The intersections of each grid line and the grain boundaries of ferrite grains are counted. The value obtained by dividing the total length of the grid lines by the total number of the intersections is taken as the ferrite grain size. Further, the ferrite grain size is an integer found by rounding off the first digit after the decimal point of the obtained value.​

[0298] The arithmetic mean of the ferrite grain diameters obtained in the three observation fields was taken as the average grain diameter D (μm) of the ferrite in the weld metal. The average grain diameter D was an integer obtained by rounding off the first digit after the decimal point of the obtained value.

[0299] [Microstructure of the base material portion and the weld metal]

[0300] Further, the microstructure of the base material portion and the weld metal each contained 30 to 70% by volume of ferrite, with the balance being austenite.

[0301] Further, the ferrite volume fraction of the weld metal was determined by the following method.

[0302] A test piece was taken in which a section perpendicular to the extension direction of the weld metal of the duplex stainless steel welded joint was taken as an observation surface. The observation surface was a rectangular region of 2 mm in the thickness direction and 2 mm in the width direction of the weld metal at the center position of the weld metal in the thickness direction including the center in the width direction at the central position. The observation surface was the region of the weld metal. The method of determining the ferrite volume fraction and the austenite volume fraction using the observation surface was performed based on the method described in the above [determination method of the ferrite volume fraction].

[0303] [Method for producing a duplex stainless steel welded joint]

[0304] An example of the method for producing a duplex stainless steel welded joint is as follows.

[0305] The duplex stainless steel material of the present embodiment was used in order. The duplex stainless steel materials were brought into contact with each other, and fillerless welding was performed on the contact portion. At the time of the fillerless welding, a nitrogen-free shielding gas can be used, or a nitrogen-containing shielding gas can be used. Further, the input heat quantity in the fillerless welding is not particularly limited, and is, for example, 0.1 to 10.0 kJ / mm.

[0306] Examples

[0307] The effect of the duplex stainless steel material of the present embodiment was further specifically explained by examples. The conditions in the following examples were one example of the conditions employed in order to confirm the implementability and the effect 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 one example of the conditions.

[0308] Duplex stainless steel materials (duplex stainless steel sheets) having the chemical compositions shown in Tables 1A to 1C were produced by the following method.

[0309] [Table 1A]

[0310]

[0311] [Table 1B]

[0312]

[0313] [Table 1C]

[0314]

[0315] "-" in Tables 1A to 1C indicates that the content of the corresponding element is below the impurity level. In addition, the content of O is 0.0300% or less in any of the test numbers.

[0316] First, a cylindrical ingot having a diameter of 120 mm and a mass of 30 kg was manufactured using molten steel. The ingot was subjected to hot forging and hot rolling to manufacture an intermediate steel sheet having a plate thickness of 10 mm. The manufactured intermediate steel sheet was cooled to normal temperature. Then, the intermediate steel sheet was subjected to heat treatment. In the heat treatment, the heat treatment temperature was set to 1100°C, and the holding time at the heat treatment temperature was set to 30 minutes. The intermediate steel sheet after the holding time was water-cooled to normal temperature. Through the above manufacturing process, a duplex stainless steel material (steel sheet) having a plate thickness of 10 mm of each of the test numbers and having the chemical composition shown in Tables 1A to 1C was manufactured.

[0317] In addition, for the duplex stainless steel material of each of the test numbers, the volume fraction of ferrite was measured based on the method described in [Method for Measuring Volume Fraction of Ferrite]. As a result, in any of the test numbers, the volume fraction of ferrite was 30 to 70%, and the balance was austenite.

[0318] [Evaluation Test]

[0319] The following evaluation tests were performed on the manufactured duplex stainless steel material.

[0320] (Test 1) Toughness Evaluation Test of Steel Material

[0321] (Test 2) Measurement Test of Average Grain Size D of Ferrite in Weld Metal of Welded Joint

[0322] (Test 3) Strength Evaluation Test of Weld Metal of Welded Joint

[0323] Hereinafter, Test 1 to Test 3 will be described.

[0324] [(Test 1) Toughness Evaluation Test of Steel Material]

[0325] A test piece having a width of 100 mm, a length of 100 mm, and a thickness of 8 mm was collected from the steel material of each test number. The test piece was collected in such a manner that the center position of the plate width of the steel material becomes the center position of the width of the test piece, the width direction is parallel to the width direction of the steel material, and the length direction is parallel to the rolling direction of the steel material. 2 ). The arithmetic average of the three values obtained was taken as the Charpy impact value of the steel material of the test number (J / cm

[0326] [Table 2]

[0327]

[0328] [(Test 2) Measurement Test of Average Grain Size D of Ferrite in Weld Metal of Welded Joint]

[0329] A test piece having a width of 100 mm, a length of 100 mm, and a thickness of 8 mm was collected from the steel material of each test number. The test piece was collected in such a manner that the center position of the plate width of the steel material becomes the center position of the width of the test piece, the width direction is parallel to the width direction of the steel material, and the length direction is parallel to the rolling direction of the steel material.

[0330] A fillerless welding based on TIG welding was performed on the test piece to form a weld metal. Specifically, as shown in FIG. 1, a fillerless welding based on TIG welding was performed at the center position of the length L of the test piece 1 along the width W direction. Pure Ar was used as the shielding gas, and the input heat in the welding was 3.2 kJ / mm. By the above welding method, a simulated duplex stainless steel welded joint 1 having a base material portion 5 and a weld metal 10 was formed. Figure 2

[0331] The chemical composition of the weld metal 10 of the manufactured simulated duplex stainless steel welded joint 1 was found by the method described in the above [Measurement Method of Chemical Composition of Weld Metal]. As a result, the chemical composition of the weld metal 10 of each test number was the same as the chemical composition described in Tables 1A to 1C of the base material portion 5 of the corresponding test number.

[0332] Furthermore, based on the method described in the above [Measurement Method of Average Grain Size D of Ferrite in Weld Metal], the average grain size D (pm) of the ferrite in the weld metal 10 was found. The average grain size D (pm) obtained is shown in Table 2. ​

[0333] [(Test 3) Strength evaluation test of weld metal of welded joint]

[0334] From Figure 2 A welded joint tensile test piece 20 was taken out in the shape shown in Figure 3 Referring to Figure 2 , the welded joint tensile test piece 20 was taken out from the width central position of the test piece 1 and the thickness central position of the test piece 1. The length direction of the welded joint tensile test piece 20 was parallel to the length L direction of the test piece 1. In addition, the welded joint tensile test piece 20 was taken out in a manner that the weld metal portion was disposed at the central position of the parallel portion of the welded joint tensile test piece 20. Further, the dimensions of the welded joint tensile test piece 20 were as shown in Figure 3 . Figure 3 The values assigned (mm) in the values in

[0335] Using the welded joint tensile test piece 20, a tensile test was performed in accordance with JIS Z 2241:2011 in normal temperature, atmosphere, and the welded joint tensile test piece 20 was broken. The breaking position of the welded joint tensile test piece 20 after breaking was confirmed. In the case where the breaking position was the base material portion (steel material), it was judged that the strength of the weld metal was higher than that of the base material portion (steel material), and it was evaluated that excellent strength was obtained in the weld metal (indicated by "E" (Excellent) in the "Weld metal strength" column in Table 2). On the other hand, in the case where the breaking position was the weld metal, it was evaluated that sufficient strength was not obtained in the weld metal (indicated by "B" (Bad) in the "Weld metal strength" column in Table 2).

[0336] [Results of evaluation]

[0337] The results of evaluation are shown in Table 2. Referring to Table 2, the duplex stainless steel materials of Test Nos. 1 to 13 satisfy the characteristics 1 to 3. Therefore, the Charpy impact value at 0°C of the steel material (base material portion) was 100 J / cm 2 Thus, excellent toughness was obtained. Also, in the tensile test using the welded joint tensile test piece, excellent strength in the weld metal was obtained.

[0338] On the other hand, in Test No. 14, the Ca content and the Ti content were too low. Therefore, sufficient strength was not obtained in the weld metal.

[0339] In Test No. 15, the Ti content and the Co content were too low. Therefore, sufficient strength was not obtained in the weld metal.

[0340] In Test Nos. 16, 17, and 22, the Co content was too low. Therefore, sufficient strength was not obtained in the weld metal.

[0341] In Test No. 18, the S content is too high and the Ca content is too low. Therefore, sufficient strength is not obtained in the weld metal.

[0342] In Test No. 19, the Ti content is too high. Therefore, the toughness of the duplex stainless steel material is low.

[0343] In Test No. 20, the Ti content is too low. Therefore, sufficient strength is not obtained in the weld metal.

[0344] In Test No. 21, the Ca content is too low. Therefore, sufficient strength is not obtained in the weld metal.

[0345] In Test Nos. 23 and 24, Fn1 and Fn2 are too low. Therefore, sufficient strength is not obtained in the weld metal.

[0346] In Test No. 25, Fn1 is too low. Therefore, sufficient strength is not obtained in the weld metal.

[0347] In Test Nos. 26 to 29, Fn2 is too low. Therefore, sufficient strength is not obtained in the weld metal.

[0348] The above describes the embodiments of the present disclosure. However, the above-described embodiments are nothing more than examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented with appropriate modifications to the above-described embodiments within a scope that does not depart from the gist thereof.

Claims

1. A duplex stainless steel material, which 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：21.00～28.00％、 Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al：0.001～0.050％、 N: 0.400% or less, B：0.0001～0.0050％、 Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, Co: 0.05 to 2.00%, W：0～5.00％、 Nb: 0 to 0.100%, V：0～0.200％、 Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and the balance being Fe and impurities, and satisfies formula (1) and formula (2), Ca / S ≥ 2.00 (1) (1000 x Ti + 2 x N) x Co 0.2 x (0.5 x Ca / S) 1.6 ≥ 50.00 (2) Here, the content in mass % of the corresponding element is substituted at each element symbol in Formula (1) and Formula (2).

2. The duplex stainless steel material according to claim 1, which contains, in mass%, one or more selected from the group consisting of W：0.01～5.00％、 Nb: 0.001 to 0.100%, V：0.001～0.200％、 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 duplex stainless steel welded joint, which comprises a base material portion and a weld metal, the base material portion and the weld metal 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：21.00～28.00％、 Ni: 4.00 to 9.50%, Mo: 0.80 to 5.50%, Cu: 0.01 to 3.50%, Al：0.001～0.050％、 N: 0.400% or less, B：0.0001～0.0050％、 Mg: 0.0050% or less, Ca: 0.0005 to 0.0100%, Ti: 0.002 to 0.100%, Co: 0.05 to 2.00%, W：0～5.00％、 Nb: 0 to 0.100%, V：0～0.200％、 Ta: 0 to 0.100%, Sn: 0 to 0.020%, a rare earth element (REM): 0 to 0.050%, and the balance being Fe and impurities, and satisfies formula (1) and formula (2), an average grain size of ferrite in the weld metal is 150 μm or less, Ca / S ≥ 2.00 (1) (1000 x Ti + 2 x N) x Co 0.2 x (0.5 x Ca / S) 1.6 ≥ 50.00 (2) Here, the content in mass% of the corresponding element is substituted at each element symbol in formula (1) and formula (2).

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