Welded joint

Abstract

Provided is a welded joint in which the chemical composition of a base material is a specific chemical composition where α represented by the equation is 4.0-16.0, the tensile strength thereof is 590-930 MPa, the microstructure at a location of 1 / 4 the thickness from the surface includes lower bainite and martensite, the total of the surface area ratios of the lower bainite and the martensite is 15.0% or greater, the total of the surface area ratios of upper bainite, the lower bainite, and the martensite is 90.0% or greater, and an effective crystal grain size of a welding-heat-affected portion is 100.0 μm or less. α=0.50×√[C] × (1+0.64[Si]) × (1+4.10[Mn]) × (1+0.27[Cu]) × (1+0.52[Ni]) × (1+2.33[Cr]) × (1+3.14[Mo])

Description

Welded joints

[0001] The present disclosure relates to weld joints.

[0002] Steel can be used for welded structures such as buildings, bridges, ships, line pipes, marine structures, pressure vessels, tanks, etc. Steel, which has excellent strength and low-temperature toughness, is effective for low-temperature applications.

[0003] Low-temperature steels are used for low-temperature pressure vessels such as liquefied gas storage tanks. Low-temperature steels include Al-killed steel, nickel steel, high-manganese steel, and austenitic stainless steel, depending on the operating temperature. For example, nickel steels such as 3.5% Ni steel are used as materials for tanks carrying liquefied ethane and liquefied ethylene, which have an operating temperature of around -100°C.

[0004] Like this 3.5% Ni steel, Ni is often contained in steel materials that require low-temperature toughness, such as those used in low-temperature pressure vessels.

[0005] For example, Patent Document 1 proposes a nickel-containing steel material for low-temperature use that has a specific chemical composition containing 2.7% to 5.0% Ni, a prior austenite grain size of 20 μm or less when heated for quenching, an effective crystal grain size of 12 μm or less after heat treatment, and a tensile strength of 450 MPa to 690 MPa, and has excellent toughness. Furthermore, various steel materials with defined chemical compositions and microstructures (metal structures) have been proposed with the aim of achieving low-temperature toughness and high strength (see, for example, Patent Documents 2 to 11).

[0006] Patent Document 1: JP 2019-81930 A Patent Document 2: WO 2014 / 103629 Patent Document 3: JP 52-156121 A Patent Document 4: JP 55-104427 A Patent Document 5: JP 58-73717 A Patent Document 6: JP 7-331328 A Patent Document 7: JP 2001-123222 A Patent Document 8: JP 2001-123245 A Patent Document 9: JP 2007-46096 A Patent Document 10: JP 2-254120 A Patent Document 11: JP 2002-224835 A

[0007]

[0003] Low-temperature steels used in low-temperature pressure vessels are desired to have both high strength and low-temperature toughness. Cryogenic pressure vessels are manufactured by welding steel materials, and post-weld heat treatment (PWHT) is sometimes performed to remove residual stresses caused by welding. Recently, there has been an increasing demand for low-temperature toughness after PWHT of welded joints.

[0008] An object of the present disclosure is to provide a welded joint suitable for low-temperature applications, which uses a steel material with high tensile strength as a base material and has good low-temperature toughness regardless of whether it is before or after post-weld heat treatment.

[0009] The gist of the present disclosure is as follows. <1> A welded joint having a base metal made of steel and a weld, wherein the base metal has a chemical composition, in mass%, of C: 0.03% or more and 0.20% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.10% or more and 1.65% or less, P: 0.025% or less, S: 0.0250% or less, Ni: 2.65% or more and 4.45% or less, Al: 0.001% or more and 0.100% or less, O: 0.0100% or less, N: 0.0100% or less, Cu: 0 to 1.50%, Cr: 0 to 3.00%, Mo: 0 to 2.00%, B: 0 to 0.0050%, Nb: 0 to 0.050%, Ti: 0 to 0.050%, a welded joint having a chemical composition of V: ​​0 to 0.10%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, REM: 0 to 0.0200%, balance: Fe and impurities, and wherein α, represented by the following formula (1), is 4.0 or more and 16.0 or less, a tensile strength is 590 MPa or more and 930 MPa or less, a microstructure at a position at ¼ of the thickness from the surface of the steel material in the thickness direction contains lower bainite and martensite, a total area ratio of the lower bainite and the martensite is 15.0% or more and a total area ratio of upper bainite, the lower bainite and the martensite is 90.0% or more, and an effective grain size in a region between a fusion line of the weld and a position 1 mm away from the fusion line in the weld heat affected zone is 100.0 μm or less. α=0.50×√[C]×(1+0.64[Si])×(1+4.10[Mn])×(1+0.27[Cu])×(1+0.52[Ni])×(1+2.33[Cr])×(1+3.14[Mo]) (1) where the [element symbols] in formula (1) represent the content (mass%) of the corresponding element contained in the steel material. If the corresponding element is not contained, zero is substituted. <2> The welded joint according to <1>, wherein the chemical composition includes the following Group A:[Group A] One or more elements selected from the group consisting of Cu: 0.01% or more and 1.50% or less, Cr: 0.01% or more and 3.00% or less, Mo: 0.01% or more and 2.00% or less, and B: 0.0003% or more and 0.0050% or less <3> The welded joint according to <1> or <2>, wherein the chemical composition includes the following Group B. [Group B] One or more elements selected from the group consisting of Nb: 0.001% or more and 0.050% or less, Ti: 0.001% or more and 0.050% or less, and V: 0.01% or more and 0.10% or less <4> The welded joint according to any one of <1> to <3>, wherein the chemical composition includes the following Group C. [Group C] One or more selected from the group consisting of Mg: 0.0003% or more and 0.0200% or less, Ca: 0.0003% or more and 0.0200% or less, and REM: 0.0003% or more and 0.0200% or less. <5> The steel material according to any one of <1> to <4>, wherein the Charpy impact absorption energy at -100°C of the welded heat affected zone is 70 J or more. <6> The steel material according to any one of <1> to <5>, wherein, when the welded joint is subjected to a heat treatment in which the heating rate and cooling rate are 55°C / h in a temperature range of 425°C or more and the welded joint is held at 600°C for 2 hours, the Charpy impact absorption energy at -100°C of the welded heat affected zone at the heat treated location is 70 J or more.

[0010] According to the present disclosure, it is possible to provide a welded joint that is suitable for low-temperature applications, in which a steel material with high tensile strength is used as the base material and good low-temperature toughness is obtained regardless of whether it is before or after post-weld heat treatment.

[0011] Fig. 2A is a diagram showing an example of a discrimination result of a microstructure. Fig. 2B is a schematic diagram showing an example of a portion from which a test specimen used in the Charpy impact absorption energy of a weld heat affected zone is taken from a square weld joint. Fig. 2C is a schematic diagram showing another example of a portion from which a test specimen used in the Charpy impact absorption energy of a weld heat affected zone is taken from a square weld joint. Fig. 2D is a schematic diagram showing an example of a portion from which a test specimen used in the Charpy impact absorption energy of a weld heat affected zone is taken from a K-type weld joint. Fig. 2E is a schematic perspective view showing a notch shape of the test specimen in Fig. 2A.

[0012] The present disclosure will be described in detail below. In this disclosure, unless otherwise specified, "post-weld heat treatment" refers to post-weld heat treatment conforming to the content specified in JIS Z 3700:2009 "Post-weld heat treatment method." In this disclosure, "steel" or "base material" refers to a steel portion that does not include a surface treatment layer such as a plating layer or a paint film. However, a surface treatment layer such as a plating layer or a paint film may be formed on the surface of the steel and welded joint in this disclosure. Note that "base material" refers to a steel portion that is not affected by welding, in contrast to the welded portion (weld metal and weld heat-affected zone) in a welded joint. "Weld heat-affected zone" refers to a steel portion that is thermally affected by welding.

[0013] In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. However, when the numerical values ​​before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limits. Regarding the content of elements in a chemical composition, "%" means "mass %." The term "process" does not only refer to an independent process, but also includes processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0014] Hereinafter, a welded joint according to an embodiment of the present disclosure will be described. First, the results of the studies conducted by the inventors of the present disclosure, which led to the completion of the invention of the welded joint according to the present disclosure, and the new findings obtained will be described in detail.

[0015] The inventors of the present disclosure conducted studies to improve the strength of steel materials used in manufacturing welded joints. The tensile strength of steel materials is ensured by the composition of the microstructure. The inventors of the present disclosure collected samples from the 1 / 4t portion (t: thickness of the steel material), which is a portion of the steel material that is 1 / 4 of the thickness from the surface in the thickness direction after hot rolling and accelerated cooling, performed tensile tests, and observed the microstructure. As a result, it was found that the microstructure of the 1 / 4t portion of steel materials having a tensile strength of 590 MPa or more and 930 MPa or less had an area fraction of ferrite of less than 10.0%, and a total area fraction of upper bainite, lower bainite, and martensite of 90.0% or more. The total area fraction of upper bainite, lower bainite, and martensite was measured using electron backscatter diffraction (hereinafter referred to as "EBSD").

[0016] Furthermore, the inventors of the present disclosure conducted studies to ensure the toughness of welded joints using the above-mentioned steel material. The toughness of a welded joint is ensured by reducing the area surrounded by high-angle grain boundaries, where the difference in crystal orientation is 15° or more. The inventors of the present disclosure collected samples from the weld heat-affected zone of a welded joint, and the samples were subjected to mechanical polishing and electrolytic polishing. The circular equivalent diameter of the area surrounded by high-angle grain boundaries was measured using an EBSD device attached to an FE-SEM (field emission scanning electron microscope). Hereinafter, the circular equivalent diameter of the area surrounded by high-angle grain boundaries is referred to as the grain size. The location where the grain size was measured was the area between the fusion line (hereinafter sometimes referred to as "FL") and a position 1 mm away from the fusion line (hereinafter "FL + 1 mm"). The grain size was measured 4 mm along the fusion line. ２ The area was 4 mm ２ The effective grain size (sometimes referred to as the "effective grain size in the weld heat affected zone" in this disclosure) was calculated as the average of the top 10 largest grain sizes among the grain sizes measured in the region of 1. It was found that if the effective grain size in the weld heat affected zone of a welded joint is 100.0 μm or less, the toughness of the welded joint tends to be further improved regardless of whether it is before or after post-weld heat treatment.

[0017] Furthermore, the inventors of the present disclosure conducted studies to reduce the effective grain size in the weld heat-affected zone of a welded joint. The effective grain size in the weld heat-affected zone of a welded joint is ensured by the microstructural configuration of the base metal made of steel. The inventors of the present disclosure collected samples from the 1 / 4t section of the steel, observed the microstructure, and also conducted Charpy impact tests on the weld heat-affected zone of a welded joint produced by welding the steel. As a result, it was found that the microstructure of a steel having a Charpy impact absorption energy of 70 J or more at -100°C in the weld heat-affected zone has a total area fraction of lower bainite and martensite of 15.0% or more. The total area fraction of lower bainite and martensite was measured using EBSD.

[0018] <Chemical composition> Next, the alloying elements that make up the chemical composition of the steel material in the welded joint according to the present disclosure will be described. Note that in the following description of the alloying elements, "%" in the content means "mass %." Furthermore, the steel material that is the base metal of the welded joint according to the present disclosure may be referred to as "the steel material in the present disclosure."

[0019] (C: 0.03% or more, 0.20% or less) C is an element that increases the strength of steel. From the viewpoint of ensuring the strength of steel used in structures, in the present disclosure, the C content is 0.03% or more. The C content is preferably 0.05% or more, or 0.07% or more. On the other hand, C is an element that reduces toughness, and from the viewpoint of ensuring the toughness of the weld heat affected zone (hereinafter sometimes referred to as "HAZ"), in the present disclosure, the C content is 0.20% or less. The C content is preferably 0.16% or less, 0.14% or less, or 0.12% or less.

[0020] (Si: 0.01% or more, 0.50% or less) Si is used as a deoxidizer and is also an element that dissolves in steel to increase strength. From the viewpoint of controlling the O concentration contained in molten steel, in the present disclosure, the Si content is 0.01% or more. The Si content is preferably 0.03% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Si content is excessive, a hard phase may be formed in the HAZ, resulting in a decrease in toughness. Therefore, from the viewpoint of ensuring the toughness of the HAZ, in the present disclosure, the Si content is 0.50% or less. The Si content is preferably 0.30% or less, or 0.20% or less.

[0021] (Mn: 0.10% or more, 1.65% or less) Mn is used as a deoxidizer and is an element that improves the hardenability of steel and contributes to high strength. From the viewpoint of controlling the O concentration contained in molten steel, the Mn content is 0.10% or more in the present disclosure. Furthermore, 0.10% or more of Mn forms MnS, thereby reducing solute S and preventing hot tearing. From the viewpoint of ensuring the strength of the steel and the toughness of the HAZ, the Mn content is preferably 0.30% or more, or 0.50% or more. On the other hand, if the Mn content is excessive, Mn may segregate to grain boundaries during PWHT, resulting in a decrease in toughness after PWHT. Therefore, from the viewpoint of ensuring the toughness of the HAZ after PWHT, the Mn content is 1.65% or less in the present disclosure. The Mn content is preferably 1.50% or less, 1.25% or less, or 1.10% or less.

[0022] (P: 0.025% or less) P is an impurity element. Although there is no lower limit for the P content, from the viewpoint of production costs, in the present disclosure, the P content may be 0.001% or more. On the other hand, if the P content is excessive, P may segregate to grain boundaries during PWHT, which may reduce toughness after PWHT. Therefore, in the present disclosure, the P content is 0.025% or less. The P content is preferably 0.016% or less, 0.012% or less, or 0.008% or less.

[0023] (S: 0.0250% or less) S is an impurity element. There is no lower limit for the S content, but from the viewpoint of production costs, in the present disclosure, the S content may be 0.0001% or more. On the other hand, if the S content is excessive, elongated MnS is generated in the central segregation portion, which may deteriorate the toughness and ductility of the steel material and HAZ. From the viewpoint of ensuring the toughness and ductility of the steel material and HAZ, the S content is 0.0250% or less. The S content is preferably 0.0100% or less, or 0.0050% or less.

[0024] (Ni: 2.65% or more, 4.45% or less) Ni is an element effective for improving the hardenability and toughness of steel, so in the present disclosure, the Ni content is 2.65% or more. The Ni content is preferably 3.00% or more, or 3.20% or more. However, since Ni is an expensive element, in the present disclosure, from the viewpoint of cost reduction, the Ni content is 4.45% or less. The Ni content is preferably 4.10% or less, or 3.80% or less.

[0025] (Al: 0.001% or more, 0.100% or less) Al is an element useful for deoxidation and also an element that refines the grain size during quenching by forming nitrides, so in the present disclosure, the Al content is 0.001% or more. However, if Al is contained in excess, Al may form coarse nitrides, which may reduce the toughness of the steel material and the HAZ. Therefore, the Al content is 0.100% or less. The Al content is preferably 0.080% or 0.050% or less.

[0026] (O: 0.0100% or less) O ​​is an impurity element. There is no lower limit for the O content, but from the viewpoint of production costs, in the present disclosure, the O content may be 0.0001% or more. On the other hand, if the O content is excessive, coarse oxides are generated, and the toughness and ductility of the steel material and HAZ may deteriorate. From the viewpoint of ensuring the toughness and ductility of the steel material and HAZ, the O content is 0.0100% or less. The O content is preferably 0.0060% or less, or 0.0040% or less.

[0027] (N: 0.0100% or less) N is an impurity element. There is no lower limit for the N content, but from the viewpoint of production costs, in the present disclosure, the N content may be 0.0001% or more. From the viewpoint of ensuring the properties of the steel material and the toughness of the HAZ, in the present disclosure, the N content is 0.0100% or less. The N content is preferably 0.0050% or less, or 0.0040% or less.

[0028] The steel material according to the present disclosure may contain other elements (selective elements) in place of a portion of Fe. For example, the following selective elements from groups A to C may be included, but the content of these elements may be 0%.

[0029] [Group A] In order to improve strength and toughness, the steel material according to the present disclosure may contain, as necessary, one or more of the following selective elements Cu, Cr, Mo, and B, which have the effect of improving hardenability.

[0030] (Cu: 1.50% or less) Cu is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Cu content is not limited and may be 0%. Cu also has a small adverse effect on weldability and HAZ toughness, and has the effect of increasing the hardenability of steel, thereby improving the strength of the steel. Therefore, in the present disclosure, the Cu content may be 0.01% or more. The Cu content is preferably 0.10% or more. However, from the viewpoint of suppressing the occurrence of Cu cracks during hot rolling of the steel, the Cu content is 1.50% or less in the present disclosure. The Cu content is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[0031] (Cr: 3.00% or less) Cr is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Cr content is not limited and may be 0%. Cr is also an element that has the effect of increasing the hardenability of steel and therefore improves the strength of the steel. Therefore, in the present disclosure, the Cr content may be 0.01% or more. The Cr content is preferably 0.10% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the HAZ, the Cr content is 3.00% or less in the present disclosure. The Cr content is preferably 2.20% or less, 1.40% or less, or 0.80% or less.

[0032] (Mo: 2.00% or less) Mo is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Mo content is not limited and may be 0%. Mo is also an element that has the effect of increasing the hardenability of steel and therefore improves the strength of steel. Therefore, in the present disclosure, the Mo content may be 0.01% or more. The Mo content is preferably 0.05% or more, 0.10% or more, 0.20% or more, or 0.30% or more. However, from the viewpoints of suppressing deterioration of HAZ toughness and weldability and suppressing increases in alloy costs, the Mo content is 2.00% or less in the present disclosure. The Mo content is preferably 1.20% or less, or 0.80% or less.

[0033] (B: 0.0050% or less) B is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the B content is not limited and may be 0%. B is also an element that exhibits a significant effect of increasing the hardenability of steel and improves the strength of the steel. Therefore, in the present disclosure, the B content may be 0.0003% or more. However, from the viewpoint of suppressing deterioration in the surface quality of steel slabs manufactured by continuous casting, in the present disclosure, the B content is 0.0050% or less. The B content is preferably 0.0030% or less, or 0.0020% or less.

[0034] [Group B] In order to improve strength, the steel material according to the present disclosure may contain, as necessary, one or more of the following optional elements Nb, Ti, and V, which have the effect of increasing the strength of the steel material by forming precipitates such as carbides and nitrides.

[0035] (Nb: 0.050% or less) Nb is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Nb content is not limited and may be 0%. Nb also forms carbides and nitrides, has the effect of refining the metal structure, and is an element that improves the strength of the steel. Therefore, in the present disclosure, the Nb content may be 0.001% or more. However, from the viewpoint of suppressing deterioration of the HAZ toughness and weldability, the Nb content is 0.050% or less. The Nb content is preferably 0.040% or less, or 0.030% or less. In particular, from the viewpoint of ensuring HAZ toughness after PWHT, the Nb content may be 0.004% or less.

[0036] (Ti: 0.050% or less) Ti is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Ti content is not limited and may be 0%. Ti also forms carbides and nitrides, has the effect of refining the metal structure, and is an element that improves the strength of the steel. Therefore, in the present disclosure, the Ti content may be 0.001% or more. However, from the viewpoint of suppressing deterioration of HAZ toughness and weldability, the Ti content is 0.050% or less. The Ti content is preferably 0.040% or less, 0.030% or less, or 0.020% or less. In particular, from the viewpoint of ensuring HAZ toughness after PWHT, the Ti content may be 0.004% or less, or 0.002% or less.

[0037] (V: 0.10% or less) V is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the V content is not limited and may be 0%. V is also an element that forms carbides and nitrides and improves the strength of steel. Therefore, in the present disclosure, the V content may be 0.01% or more. However, from the viewpoint of suppressing deterioration of the toughness and weldability of the HAZ and suppressing an increase in alloy costs, the V content is 0.10% or less. The V content is preferably 0.08% or less, or 0.05% or less.

[0038] [Group C] In order to improve the toughness of the HAZ, the steel material according to the present disclosure may contain one or more of the following optional elements Mg, Ca and REM, as needed.

[0039] (Mg: 0.0200% or less) Mg is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Mg content is not limited and may be 0%. Mg is also an element that forms oxides to improve the toughness of the weld heat affected zone. Therefore, in the present disclosure, the Mg content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, an excessive Mg content may form coarse oxides, which may reduce the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, in the present disclosure, the Mg content is 0.0200% or less. The Mg content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0040] (Ca: 0.0200% or less) Ca is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the Ca content is not limited and may be 0%. Ca is also an element that spheroidizes sulfides in the steel, thereby reducing the effect of MnS, which reduces the toughness of the steel and the weld heat-affected zone. Therefore, in the present disclosure, the Ca content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, an excessive Ca content may form coarse oxides, which may reduce the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, in the present disclosure, the Ca content is 0.0200% or less. The Ca content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0041] (REM: 0.0200% or less) Rare earth metals (REM) refer to a collective term for 17 elements, including two elements, Sc and Y, and 15 lanthanoid elements, such as La, Ce, and Nd. The REM content refers to the total content of these 17 elements. REM is an element that may be mixed into steel during the manufacturing process. However, the lower limit of the REM content is not limited and may be 0%. REM is also an element that forms oxides to improve the toughness of the weld heat affected zone. Therefore, in the present disclosure, the REM content may be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, an excessive REM content may form coarse oxides, which may reduce the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, the REM content is 0.0200% or less in the present disclosure. The REM content is preferably 0.0100% or less, 0.006% or less, or 0.0040% or less.

[0042] (Balance: Fe and Impurities) The balance of the chemical composition of the steel material in the present disclosure is iron (Fe) and impurities. The impurities refer to components that are mixed in due to raw materials such as ore and scrap or other factors when industrially producing the steel material.

[0043] In addition to limiting the content of each element, the present disclosure limits the range of the α value as follows:

[0044] (α value: 4.0 or more and 16.0 or less) The α value is calculated by the following formula (1): α = 0.50 × √[C] × (1 + 0.64 [Si]) × (1 + 4.10 [Mn]) × (1 + 0.27 [Cu]) × (1 + 0.52 [Ni]) × (1 + 2.33 [Cr]) × (1 + 3.14 [Mo]) ... (1) where [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] are the contents (mass%) of C, Si, Mn, Cu, Ni, Cr, and Mo in the steel. If the corresponding element is not contained, zero is substituted. Note that √[C] is [C] １／２ is synonymous with.

[0045] In the steel material disclosed herein, the α value is set to a range of 4.0 to 16.0. This is an index indicating the hardenability of the steel material, and the higher the α value, the more likely it is that a lower bainite and martensite structure with an advantageous balance of strength and toughness can be formed. When α is within an appropriate range, the ratio of lower bainite and martensite structures with an advantageous balance of strength and toughness also increases in the HAZ structure, ensuring HAZ toughness. When α is 4.0 or higher, the hardenability of the base material is ensured, the ratio of lower bainite and martensite with an advantageous balance of strength and toughness increases, and toughness deterioration is suppressed. Furthermore, the ratio of lower bainite and martensite in the HAZ structure also tends to increase, improving HAZ toughness. On the other hand, when the α value is set to 16.0 or less, the strength of the steel material does not become too high, and toughness can be ensured. Furthermore, when the α value is set to 16.0 or less, toughness after PWHT can be ensured. Furthermore, the HAZ does not become too hard, and HAZ toughness can be ensured.

[0046] By satisfying the above-mentioned numerical range for the α value, it is possible to provide a nickel-containing steel material for low temperature use that is excellent in strength and toughness. The α value is preferably 4.5 or more, or 5.0 or more. The α value is preferably 15.5 or less, or 15.0 or less.

[0047] <Microstructure> Next, the microstructure of the steel material according to the present disclosure will be described. The microstructure at a position from the surface of the steel material to ¼ of the thickness in the thickness direction contains lower bainite and martensite. Furthermore, the bainite may contain upper bainite in addition to lower bainite.

[0048] "Bainite" is a structure containing bainitic ferrite (α°B) with a substructure within the grains, and is a general term for upper bainite and lower bainite. "Upper bainite" refers to either or both upper bainite containing retained austenite or MA phase (martensite-austenite mixed phase) between the laths, and upper bainite containing carbides between the laths. "Lower bainite" refers to lath-shaped lower bainite containing carbides within the laths.

[0049] "Martensite" exists in four forms: lath, butterfly, lens, and thin plate, but the components disclosed herein mainly produce lath martensite. Lath martensite is composed of packets and blocks consisting of groups of laths in a specific arrangement, and is a structure in which one austenite grain is divided into several packets.

[0050] (Total area ratio of lower bainite and martensite in 1 / 4t portion of steel material: 15.0% or more) Lower bainite and martensite are hard phases, and increase the toughness of the steel material and HAZ. From the viewpoint of ensuring HAZ toughness, the area ratio of lower bainite and martensite in 1 / 4t portion of steel material is 15.0% or more. The area ratio of lower bainite and martensite in 1 / 4t portion is preferably 20.0% or more, or 30.0% or more. The total area ratio of lower bainite and martensite in 1 / 4t portion may be 100%.

[0051] (Total area ratio of upper bainite, lower bainite, and martensite in the 1 / 4 t portion of the steel material: 90.0% or more) From the viewpoint of ensuring the strength of the steel material, the total area ratio of upper bainite, lower bainite, and martensite in the 1 / 4 t portion of the steel material is 90.0% or more. The total area ratio of upper bainite, lower bainite, and martensite in the 1 / 4 t portion may be 100%. Furthermore, the upper bainite in the 1 / 4 t portion may be 1.0% or more.

[0052] Observation of the microstructure of steel is carried out using a sample with the 1 / 4 t portion of the steel as the observation surface. Two types of samples are prepared: (a) electrolytic polishing and (b) nital etching. Measurements are taken at three locations for each of (a) and (b) samples using the method described below, and the average value of the three locations is taken as the area ratio of the microstructure of the steel. Three samples of each of (a) and (b) may be prepared and the average taken for each sample, or measurements may be taken at three visual fields within each sample and the average taken.

[0053] The total area ratio of upper bainite, lower bainite, and martensite is measured by EBSD using electropolished samples that have been mechanically polished to a mirror finish and then electropolished to remove the strain layer caused by mechanical polishing. The measurement magnification is 200x, and measurements are made over a 400 μm x 400 μm area at a 0.4 μm pitch. The measurement is performed with an electron beam diameter of 0.4 μm or less. The confidence index (hereinafter referred to as the "CI value") is set to 0.1 or greater. The determination of ferrite, upper bainite, lower bainite, and martensite is performed by setting the threshold value of grain average misorientation (hereinafter referred to as "GAM") to 0.5. The GAM value is an index defined in OIM-Analysis (EBSD crystal orientation analysis software manufactured by TSL, USA). The region where GAM is 0.5 or less is ferrite, and the region where GAM is more than 0.5 is upper bainite, lower bainite, or martensite. In the present disclosure, upper bainite, lower bainite, and martensite are determined using the GAM of EBSD as a threshold, and therefore include not only upper bainite, lower bainite, and martensite, but also tempered upper bainite, tempered lower bainite, and tempered martensite.

[0054] The area ratio of upper bainite was measured by SEM observation using a nital-etched sample. The measurement was performed at 500x magnification over a 360 μm × 480 μm area. Upper bainite was defined as a region with a clear lath structure and where carbides and MA were formed along the lath boundaries. Upper bainite was defined as a region with a relatively coarse internal structure, sparse carbide density, and a mixture of dense and coarse carbides. Figure 1 shows an example of the results of structure determination. (A) and (B) are SEM images of the same region of a steel material manufactured by DQT and with an α value of 9.9. In (B), the region surrounded by a white line is upper bainite (Bu), and the other regions are lower bainite + martensite (BL + M). The region determined to be upper bainite (Bu) has sparse white carbides and a mixture of dense and coarse regions. On the other hand, in the portion determined to be lower bainite + martensite (BL + M), carbides are densely and uniformly present. The total area fraction of lower bainite and martensite can be determined by subtracting the area fraction of upper bainite from the total area fraction of upper bainite, lower bainite, and martensite measured above. Note that when the sum of the area fractions of lower bainite and martensite is greater than 0% according to the above structure determination, lower bainite and martensite are usually present. Lower bainite and martensite can be distinguished using SEM or TEM (transmission electron microscope), and the presence of each structure can be confirmed.

[0055] (Effective grain size of weld heat affected zone: 100.0 μm or less) From the viewpoint of ensuring HAZ toughness, the effective grain size of the weld heat affected zone is 100.0 μm or less. The effective grain size of the weld heat affected zone is preferably 95.0 μm or less, or 90.0 μm or less. There is no particular restriction on the lower limit of the effective grain size of the weld heat affected zone, but the effective grain size of the weld heat affected zone may be, for example, 50.0 μm or more or 60.0 μm or more.

[0056] <Mechanical Properties> A welded joint having a base material and a weld made of a steel material according to the present disclosure has mechanical properties that combine the strength of the steel material with the low-temperature toughness of the weld heat-affected zone. In particular, the weld heat-affected zone has excellent toughness at −100° C., and can also exhibit excellent low-temperature toughness even after PWHT.

[0057] (Tensile strength: 590 MPa or more and 930 MPa or less) In the present disclosure, the tensile strength of the steel material is set to 590 to 930 MPa. In order to reduce the weight of large welded structures such as transport tanks, steel material that can ensure the strength of the structure even if it is thin is required. Typically, steel material selected for such applications has the above-mentioned tensile strength, and therefore the steel material in the present disclosure is also manufactured to have the above-mentioned tensile strength.

[0058] (Yield Ratio) The yield ratio (YR = yield strength / tensile strength × 100) of the steel material in the present disclosure is not particularly limited, but is preferably 90% or less. By using a multi-phase structure, it is possible to prevent the YR from becoming high.

[0059] (Charpy Impact Absorption Energy of Weld Heat Affected Zone at −100°C) In order to ensure high toughness at low temperatures, the welded joint of the present disclosure preferably has a Charpy impact absorption energy of 70 J or more at −100°C. Because the welded joint of the present disclosure has low-temperature toughness such that the Charpy impact absorption energy of the welded heat affected zone is 70 J or more at −100°C, a transport tank formed from the welded joint of the present disclosure can be suitably used, for example, for transporting liquid carbon dioxide. The Charpy impact absorption energy of the welded heat affected zone at −100°C is a value measured using samples 14 taken from a region including the fusion line position (FL) of the weld 12 and a region including a position 1 mm away from the fusion line (FL + 1 mm) at the ¼ t portion of the base metal 10 of the welded joint 20, as shown in FIGS. 2A , 2B , 3A , and 3B . 2A, 2B, 3A, and 3B, reference numeral 15 denotes a weld metal, and 16 denotes a weld heat-affected zone (HAZ). As shown in Fig. 4, when the base metal 10 has a width direction X, a thickness direction Y, and a length direction Z, the notch 18 of the test piece 14 is formed so that the length direction of the notch 18 is parallel to the thickness direction Y of the base metal 10 and the depth direction of the notch 18 is parallel to the length direction Z of the base metal 10. The length direction Z of the base metal 10 is the rolling direction, and the width direction X is perpendicular to the rolling direction Z and the thickness direction Y.

[0060] (Charpy Impact Absorption Energy at −100°C of Weld Heat-Affected Zone After PWHT) In low-temperature tanks, PWHT may be performed on welds after assembly into transport tanks to prevent fracture. The longer the time during which a weld joint is heated to a temperature range of 425°C or higher, the lower the HAZ toughness tends to decrease. For the welded joint of the present disclosure, when PWHT is performed on the steel material at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and cooling rate of 55°C / h in a temperature range of 425°C or higher, the toughness of the weld heat-affected zone after PWHT is preferably 70 J or more in Charpy impact absorption energy at −100°C. The Charpy impact absorption energy at −100°C after PWHT may be 100 J or more. The Charpy impact absorption energy at −100°C after PWHT is also a value measured using a sample taken from the ¼ position of the thickness.

[0061] PWHT may reduce HAZ toughness. Although the cause is unclear, it is presumed that the diffusion of P (phosphorus) and Mn to grain boundaries and the growth or aggregation of inclusions in the structure reduce brittleness and therefore toughness. The reduction in toughness due to PWHT can be suppressed by limiting the P and Mn contents and reducing the effective grain size of the weld heat-affected zone.

[0062] The tensile strength (TS) and yield strength (YS) in the examples are measured by a tensile test in accordance with JIS Z2241:2011. For the tensile test, a JIS 14A test piece taken from the 1 / 4 thickness position and with the longitudinal direction parallel to the width direction of the steel (C direction) is used. TS and YS are measured using three test pieces and calculated by averaging the measurements. The toughness of the steel is evaluated by the brittle-ductile transition temperature (vTrs) using a Charpy impact test using an impact blade with a radius of 2 mm in accordance with the provisions of JIS Z2242:2018. The Charpy impact test is performed on three pieces at five temperatures, the brittle fracture surface ratio is measured, and vTrs is calculated. For the Charpy impact test, a V-notch test piece taken from the 1 / 4 thickness position of the steel and with the longitudinal direction parallel to the width direction of the steel (C direction) is used. The Charpy impact absorption energy of the weld heat affected zone is measured by a Charpy impact test at -100°C using an impact blade with a radius of 2 mm in accordance with JIS Z2242:2018. The Charpy impact absorption energy is measured using three test specimens and calculated by averaging the measured values. For the Charpy impact test, a V-notch test specimen 14 is used, cut from a weld joint 20 at a position corresponding to the 1 / 4 t portion of the steel material, as shown in Figures 2A, 2B, 3A, 3B, and 4.

[0063] The shape of the steel material in the present disclosure is not particularly limited and may be a steel plate, a steel strip, a structural steel, a steel pipe, or the like. However, steel pipes and structural steel include steel materials formed by joining steel plates, such as welded steel pipes and welded structural steel, as well as structural steel joined with rivets. The thickness of steel materials such as steel plates, steel strips, structural steel, and steel pipes (the thickness of the flange in the case of structural steel) is not particularly limited and is typically 3 mm or more and 150 mm or less. The thickness of the steel material may be 6 mm or more, 10 mm or more, 15 mm or more, or 30 mm or more. The thickness of the steel material may also be 100 mm or less, 80 mm or less, or 60 mm or less.

[0064] Furthermore, the uses of the welded joint according to the present disclosure are not particularly limited, but since it has mechanical properties that combine strength and low-temperature toughness, and can exhibit excellent low-temperature toughness even after PWHT in particular, it can be suitably used as a tank for storing and transporting liquefied gas, in particular liquid carbon dioxide.

[0065] (Method for manufacturing steel material) The method for manufacturing steel material in the present disclosure is not particularly limited. For example, the steel material in the present disclosure is produced by melting steel satisfying the above-mentioned chemical composition and then continuously casting it into a steel billet. The steel billet is heated, hot-rolled, and then directly water-cooled (direct quenching (DQ)), or is naturally cooled, reheated, and water-cooled (reheat quenching (RQ)), to produce a steel material. Furthermore, intermediate heat treatment (L) and tempering (T) may be performed. In the case of RQ, naturally cooling before reheating is not necessarily required, and water cooling may also be performed. The manufacturing process after hot rolling is selected from the above-mentioned combinations of DQ, RQ, L, and T, and is, for example, DQT, RQT, DQLT, or RQLT.

[0066] (1) DQT: Direct quenching (DQ), tempering (T) (2) RQT: Natural cooling, reheat quenching (RQ), tempering (T) (3) DQLT: Direct quenching (DQ), intermediate heat treatment (L), tempering (T) (4) RQLT: Natural cooling, reheat quenching (RQ), intermediate heat treatment (L), tempering (T)

[0067] From the viewpoint of manufacturing costs, DQT is preferred for manufacturing steel materials in the present disclosure, and an example of a preferred manufacturing process is shown below.

[0068] The heating temperature of the steel billet to be hot rolled is set to Ac from the viewpoint that hot rolling is performed in a temperature range in which the metal structure of the rolled material is austenite. ３ The heating temperature of the slab is preferably 1000°C or higher from the viewpoint of reducing the deformation resistance. On the other hand, the heating temperature of the hot rolling is 1250°C or lower from the viewpoint of suppressing the coarsening of heated γ grains. The heating temperature of the hot rolling is preferably 1200°C or lower. ３ is a value calculated by the following formula: Ac ３ = 937.2 - 436.5C + 56Si - 19.7Mn - 16.3Cu - 26.6Ni - 4.9Cr + 38.1Mo + 124.8V + 136.3Ti - 19.1Nb + 198.4Al + 3315B The element symbols in the formula represent the content (mass%) of each element contained in the steel slab.

[0069] Hot rolling may consist of rolling in a temperature range where recrystallization occurs (recrystallization temperature range rolling) and rolling in a temperature range where recrystallization is suppressed (non-recrystallization temperature range rolling). Recrystallization temperature range rolling is hot rolling performed at a temperature of 900°C or higher during rolling. The cumulative reduction in recrystallization temperature range rolling is preferably 20% or higher, more preferably 30% or higher, from the viewpoint of refining the austenite grain size of the steel. The cumulative reduction in recrystallization temperature range rolling is determined from the difference between the thickness of the slab before hot rolling and the thickness of the rolled material at 900°C. Cumulative reduction in recrystallization temperature range rolling (%) = 100 x ([thickness of slab] - [thickness of rolled material at 900°C]) / [thickness of slab]

[0070] Non-recrystallization temperature range rolling is hot rolling performed at a temperature of the rolled material during rolling of less than 900°C. The cumulative reduction in non-recrystallization temperature range rolling is preferably 20% or more, more preferably 30% or more, from the viewpoint of refining the crystal grain size of the steel material. The cumulative reduction in non-recrystallization temperature range rolling is determined from the difference between the thickness of the rolled material at 900°C and the thickness of the steel material after rolling is completed. Cumulative reduction in non-recrystallization temperature range rolling (%) = 100 x ([Thickness of rolled material at 900°C] - [Thickness of steel material after rolling]) / [Thickness of rolled material at 900°C]

[0071] The finishing temperature of the hot rolling is set to Ar from the viewpoint of suppressing the formation of ferrite, which reduces the strength. ３ After the hot rolling is completed, the steel material is subjected to accelerated cooling such as water cooling. The starting temperature of the accelerated cooling is set to 1000°C / 2000°F from the viewpoint of suppressing the formation of ferrite, which reduces the strength. ３ That's all. ３ is a value calculated by the following formula: ３=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo+0.35(t-8) The element symbols in the formula represent the content (mass%) of each element contained in the steel, and t represents the thickness (mm) of the steel. From the viewpoint of promoting bainite transformation and martensitic transformation, the cooling rate is 1.0°C / s or more. The cooling rate of accelerated cooling is preferably 5.0°C / s or more, or 10.0°C / s or more. The faster the cooling rate of accelerated cooling, the more preferable it is, but from the viewpoints of homogenizing the cooling rate, cost, etc., it is preferably 50.0°C / s or less, or 30.0°C / s or less. The cooling rate is a value calculated by simulating the cooling rate at a 1 / 4 position of the thickness using heat transfer calculation.

[0072] The stop temperature of the accelerated cooling is 400°C or lower from the viewpoint of improving the strength of the steel material by ensuring upper bainite, lower bainite, and martensite. The stop temperature of the accelerated cooling is preferably 350°C or lower. Accelerated cooling may be performed down to room temperature. The stop temperature of the accelerated cooling is preferably 100°C or higher from the viewpoint of dehydrogenating the steel material.

[0073] After the accelerated cooling, the steel may be subjected to a tempering treatment. The heating temperature in the tempering treatment is preferably 650°C or less, 620°C or less, or 590°C or less from the viewpoint of suppressing a decrease in strength. On the other hand, the heating temperature in the tempering treatment is preferably 350°C or more, or 400°C or more from the viewpoint of improving toughness.

[0074] (Method for Manufacturing Welded Joint) A method for manufacturing a welded joint (welding method) according to the present disclosure will be described. First, a groove is formed at the end of the steel material (base metal) according to the present disclosure. The groove shape is one of a single bevel groove, a J groove, a single U groove, a double bevel groove, a double J groove, and a double U groove. The grooves are butted together, and welding is performed using a welding material. The welding material is not particularly limited and is determined appropriately depending on the desired characteristics of the welded joint. Welding methods include shielded arc welding, gas-shielded arc welding, submerged arc welding, and TIG welding. The welding heat input and the number of welding passes are determined appropriately depending on the welding method and plate thickness. The welding heat input is, for example, approximately 1.0 to 6.0 kJ / mm. Specific welding conditions for gas-shielded arc welding and submerged arc welding are shown below. In gas-shielded arc welding, Ar and CO are used. ２ Using a mixed gas of CO ２ The gas ratio is 20%. For a K-shaped groove, the groove angle is 10R at the tip, 30° for both the upper and lower layers, the preheat temperature is 100-150°C, the interpass temperature is 100-150°C, the gas flow rate is 15-20 L / min for the lower layer and 18-22 L / min for the upper layer, and welding is performed using YM-69F (solid wire manufactured by Nippon Steel Welding Co., Ltd.) as the material to form the weld metal. The current is 250-270A, the voltage is 27-33V, the heat input is 1.5-2.4 kJ / mm, and the welding speed is 30-40 cm / min. The number of passes is determined depending on the plate thickness. In submerged arc welding, for a square groove, the groove gap is 10 mm, the angle is 30°, the preheat temperature is 100-150°C, and the interpass temperature is 100-150°C, and welding is performed using Y-80M as the material to form the weld metal and NB-250H (submerged arc welding material manufactured by Nippon Steel Welding Co., Ltd.) as the flux. The current is 630-670A, the voltage is 27-33V, the heat input is 3.5-4.4kJ / mm, and the welding speed is 25-34cm / min. The number of passes is determined depending on the plate thickness.

[0075] (Method for Evaluating Welded Joints) After preparing a welded joint, a notch is introduced at the fusion line (FL) or at a position 1 mm away from the fusion line (FL + 1 mm), as shown in Figures 2A, 2B, 3A, 3B, and 4, and a Charpy impact test is performed. When the groove shape is a square, J-shaped, U-shaped, K-shaped, double-sided J-shaped, or H-shaped, the V-notch at FL is introduced at the tangent line of the fusion line on the weld heat-affected zone side and includes 80% or more of the weld heat-affected zone. The V-notch at FL + 1 mm is introduced at a position 1 mm away from the FL notch. The grain size of the metal structure near FL is roughly determined depending on the heat input of welding. Therefore, the absorbed energy of the weld heat-affected zone is not dependent on the groove shape but is largely dependent on the heat input.

[0076] The welded joint according to the present disclosure will be specifically described below using examples. However, the conditions in the following examples are merely examples of conditions adopted to confirm the feasibility and effects of the present disclosure, and the welded joint according to the present disclosure is not limited to the following examples.

[0077] [Manufacturing of Steel Materials] First, a slab having the chemical composition shown in Table 1 was cast by a continuous casting method. The balance other than the components shown in Table 1 is Fe and impurities. Blank cells indicate that no alloying elements were intentionally added in the steelmaking process. Underlined cells indicate that the contents are outside the scope of the present disclosure.

[0078]

[0079] Next, steel materials were produced from these slabs under the production conditions shown in Table 2. "Temper heat treatment" refers to the heating temperature in the tempering treatment after quenching.

[0080]

[0081] [Manufacturing of welded joints] For the steel materials other than Nos. 12 and 18, K-grooves were prepared using the steel materials, and multi-layer gas-shielded arc welding (GMAW) was performed to manufacture welded joints. In GMAW welding, Ar and CO ２ Using a mixed gas of CO ２The gas ratio was 20%. A K-shaped groove was used, with a groove angle of 10R at the tip, 30° for both the upper and lower layers, a preheat temperature of 100-150°C, an interpass temperature of 100-150°C, and a gas flow rate of 17 L / min for the lower layer and 20 L / min for the upper layer. Welding was performed using YM-69F (solid wire manufactured by Nippon Steel Welding Co., Ltd.) on the weld metal. The current was 260 A, the voltage was 30 V, the heat input was 2.0 kJ / mm, and the welding speed was 35 cm / min. The number of passes varied depending on the plate thickness. For Nos. 12 and 18, a K-shaped groove was prepared using the above steel material, and multi-layer submerged arc (SAW) welding was performed to produce welded joints. For SAW welding, a square groove was used, the groove gap was 10 mm, the angle was 30°, the preheat temperature was 100-150°C, and the interpass temperature was 100-150°C. Y-80M was used for the weld metal and NB-250H (submerged arc welding material manufactured by Nippon Steel Welding Co., Ltd.) was used for the flux. The current was 650 A, the voltage was 30 V, the heat input was 4.40 J / mm, and the welding speed was 29 cm / min. The number of passes varied depending on the plate thickness.

[0082] [Measurement and Evaluation] The microstructure and mechanical properties of the obtained steel material and welded joint (weld heat affected zone) were measured by the methods described above. The results are shown in Table 3. The meanings of the symbols for the microstructure are as follows. In the region determined to be lower bainite + martensite (BL + M), the presence of lower bainite and martensite was confirmed by SEM observation. The remainder of the microstructure is pearlite, MA phase, ferrite, and retained γ (austenite). Bu: upper bainite BL: lower bainite M: martensite

[0083] The toughness of the steel was measured as vTrs. The toughness of the weld heat-affected zone was measured by inserting a notch at FL or FL + 1 mm into the I side of a K-groove or a R-groove and performing a Charpy impact test at -100°C. For HAZ toughness, the average value of the Charpy impact absorption energy (KV2) at -100°C of the as-manufactured welded joint (As-weld toughness) and the average value of the Charpy impact absorption energy at -100°C after PWHT with a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and cooling rate of 55°C / h in a temperature range of 425°C or higher were measured.

[0084] The toughness was measured as follows: (1) when no heat treatment was performed on the steel (base metal), (2) when heat treatment was performed after welding, and (3) when no heat treatment was performed after welding. In each case, the measurements were performed using samples taken from a location 1 / 4 of the thickness. (1) Base metal toughness: vTrs obtained in a Charpy impact test at each temperature. (2) Welded joint toughness: A K-groove or a R-groove was prepared, and multi-layer gas shielded arc welding (GMAW) was performed with a heat input of 2.0 kJ / mm, or multi-layer submerged arc (SAW) welding was performed with a heat input of 4.0 kJ / mm, and the average value of the Charpy impact absorbed energy at -100°C after the welded joint was produced. (3) Toughness of welded joint after PWHT A K-groove or a V-groove was prepared, and a heat input of 2.0 kJ / mm was used for multi-layer gas shielded arc welding (GMAW), or a heat input of 4.0 kJ / mm was used for multi-layer submerged arc welding (SAW), to produce a welded joint. After that, the average value of the Charpy impact absorbed energy at -100°C after PWHT was measured at a holding temperature of 600°C, a holding time of 2 hours, and a heating rate and cooling rate of 55°C / h in the temperature range of 425°C or higher.

[0085]

[0086] Nos. 1 to 19 are examples of the present invention, and Nos. 20, 21, and 22 are comparative examples. No. 20 had too small an α value, so it was unable to obtain sufficient hardenability, strength, or low-temperature toughness. No. 21 had too much Mn content, so it was unable to obtain sufficient low-temperature toughness either before or after PWHT. No. 22 had too high an α value, so it had excessive strength and was unable to obtain sufficient low-temperature toughness either before or after PWHT.

[0087] In contrast to the comparative examples, in all of the inventive examples (Nos. 1 to 19), the chemical composition and microstructure of the steel were appropriately controlled, and the tensile strength was within an appropriate range of 590 MPa or more and 930 MPa or less. In addition, the Charpy impact absorption energy of the weld heat affected zone at -100°C was high, and a low-temperature HAZ toughness of 70 J or more was obtained, regardless of whether it was before or after PWHT.

[0088] The welded joint according to the present disclosure is suitable for use in a transport tank for liquefied carbon dioxide. The welded joint according to the present disclosure can also be used in other welded structures such as buildings, bridges, ships, line pipes, offshore structures, pressure vessels and tanks, etc.

[0089] The disclosure of Japanese Patent Application No. 2023-119448, filed on July 21, 2023, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated herein by reference.

[0090] REFERENCE SIGNS LIST 10 Base metal 12 Welded portion 14 Test piece 15 Weld metal 16 Weld heat affected zone 18 Notch 20 Welded joint

Claims

1. A welded joint having a base material made of steel and a welded portion, wherein the chemical composition of the base material is, by mass%, C: 0.03% or more, 0.20% or less, Si: 0.01% or more, 0.50% or less, Mn: 0.10% or more, 1.65% or less, P: 0.025% or less, S: 0.0250% or less, Ni: 2.65% or more, 4.45% or less, Al: 0.001% or more, 0.100% or less, O: 0.0100% or less, N: 0.0100% or less, Cu: 0 to 1.50%, Cr: 0-3.00%, Mo: 0-2.00%, B: 0 to 0.0050%, Nb: 0 to 0.050%, Ti: 0 to 0.050%, V: 0-0.10%, Mg: 0 to 0.0200%, Ca: 0-0.0200%, REM: 0-0.0200%, Remainder: Fe and impurities Furthermore, it has a chemical composition in which α, represented by the following formula (1), is 4.0 or more and 16.0 or less. The tensile strength is between 590 MPa and 930 MPa. The microstructure of the steel material in the thickness direction from the surface, up to 1 / 4 of the thickness, includes lower bainite and martensite, the total area ratio of the lower bainite and martensite is 15.0% or more, and the total area ratio of the upper bainite, lower bainite and martensite is 90.0% or more. The effective grain size in the region between the melting line of the weld and a point 1 mm away from the melting line in the heat-affected zone of the weld is 100.0 μm or less. Welded joint. α=0.50×√[C]×(1+0.64[Si])×(1+4.10[Mn])×(1+0.27[Cu])×(1+0.52[Ni])×(1+2.33[Cr])×(1+3.14[Mo]) ...(1) However, the [element symbol] in formula (1) represents the content (mass%) of the corresponding element contained in the steel material. If the element is not present, substitute zero.

2. The welded joint according to claim 1, wherein the chemical composition includes the following group A. [Group A] Cu: 0.01% or more, 1.50% or less, Cr: 0.01% or more, 3.00% or less, Mo: 0.01% or more, 2.00% or less, B: 0.0003% or more, 0.0050% or less One or more selected from the group consisting of

3. The welded joint according to claim 1, wherein the chemical composition includes the following group B. [Group B] Nb: 0.001% or more, 0.050% or less, Ti: 0.001% or more, 0.050% or less, V: 0.01% or more, 0.10% or less One or more selected from the group consisting of

4. The welded joint according to claim 1, wherein the chemical composition includes the following group C. [Group C] Mg: 0.0003% or more, 0.0200% or less Ca: 0.0003% or more, 0.0200% or less, REM: 0.0003% or more, 0.0200% or less One or more selected from the group consisting of

5. The welded joint according to any one of claims 1 to 4, wherein the Charpy impact absorption energy at -100°C of the heat-affected zone during welding is 70 J or more.

6. A welded joint according to any one of claims 1 to 4, wherein, in a temperature range of 425°C or higher, the heating rate and cooling rate are 55°C / h, and when a heat treatment is performed on the welded joint by holding it at 600°C for 2 hours, the Charpy impact absorption energy at -100°C in the heat-affected zone of the weld at the heat-treated location is 70 J or more.

7. The welded joint according to claim 5, wherein when a heat treatment is performed on the welded joint, in which the heating rate and cooling rate are 55°C / h in a temperature range of 425°C or higher, and the joint is held at 600°C for 2 hours, the Charpy impact absorption energy at -100°C in the heat-affected zone of the weld at the heat-treated location is 70 J or more.