Welded joint

Abstract

A welded joint in which the chemical composition of the base material has a specific chemical composition in which [alpha] is 4.0-16.0 represented by the following formula, the tensile strength is 590-930 MPa, the microstructure of a site that is 1 / 4 of the thickness from the surface contains lower bainite and martensite, the total area ratio of the lower bainite and martensite being 15.0% or more, and the thickness of the lower bainite and martensite being 1 / 4 of the thickness of the surface. The total area ratio of the upper bainite, the lower bainite, and the martensite is 90.0% or more, and the effective crystal grain size of the welding heat-affected portion is 100.0 [mu] m or less. Alpha = 0.50 * square root [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

TECHNICAL FIELD

[0001] The present disclosure relates to a welded joint. BACKGROUND

[0002] Steel materials can be used for welded structures such as buildings, bridges, ships, pipelines, marine structures, pressure vessels, tanks, and the like. Steel materials excellent in corresponding stress of strength and low-temperature toughness are effective in use at low temperatures.

[0003] Low-temperature pressure vessels such as storage tanks for liquefied gas use low-temperature steels. In low-temperature steels, there are Al-killed steels, nickel steels, high-Mn steels, and austenitic stainless steels, and the like, depending on the use temperature. For example, as a material for a tank for loading liquefied ethane or liquefied ethylene having a use temperature of -100°C or thereabout, a nickel steel such as 3.5%Ni steel is used.

[0004] In steels that need to ensure low-temperature toughness typified by low-temperature pressure vessels, such as this 3.5%Ni steel, Ni is contained in many cases.

[0005] For example, in Patent Literature 1, a low-temperature nickel-containing steel material excellent in toughness is proposed, which has a specific chemical composition containing 2.7% to 5.0% of Ni, a prior austenite grain size at the time of quenching heating of 20 μm or less, an effective crystal grain size after heat treatment of 12 μm or less, and a tensile strength of 450 MPa to 690 MPa.

[0006] In addition, various steel materials in which the chemical composition and the microstructure (metal structure) are specified for the purpose of low-temperature toughness and high strength are proposed (for example, refer to Patent Literatures 2 to 11).

[0007] Patent Literature 1: Japanese Patent Application Publication No. 2019-81930 Patent Literature 2: International Publication No. 2014 / 103629 Patent Literature 3: Japanese Patent Application Publication No. S52-156121 Patent Literature 4: Japanese Patent Application Publication No. S55-104427 Patent Literature 5: Japanese Patent Application Publication No. S58-73717 Patent Literature 6: Japanese Patent Application Publication No. H7-331328 Patent Literature 7: Japanese Patent Application Publication No. 2001-123222 Patent Literature 8: Japanese Patent Application Publication No. 2001-123245 Patent Literature 9: Japanese Patent Application Publication No. 2007-46096 Patent Literature 10: Japanese Patent Application Publication No. H2-254120 Patent Literature 11: Japanese Patent Application Publication No. 2002-224835 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION For a low-temperature steel used in a low-temperature pressure vessel, it is desired to achieve a balance between high strength and securing of low-temperature toughness. In addition, the low-temperature pressure vessel is manufactured by welding a steel material, and in order to remove residual stress generated due to welding, post-weld heat treatment (sometimes referred to as PWHT) is sometimes performed. Recently, the requirement for low-temperature toughness after PWHT of a welded joint is further increased.

[0009] The object of the present disclosure is to provide a welded joint suitable for a low-temperature use in which a steel material having high tensile strength is used as a base material and good low-temperature toughness can be obtained regardless of before and after post-weld heat treatment.

[0010] MEANS FOR SOLVING THE PROBLEMS The gist of the present disclosure is as described below.

[0011] <1> A welded joint having a base material made of a steel material and a welded portion, the chemical composition of the base material having the following chemical composition: in mass %: C: 0.03% to 0.20%, Si: 0.01% to 0.50%, Mn: 0.10% to 1.65%, P: 0.025% or less, S: 0.0250% or less, Ni: 2.65% to 4.45%, Al: 0.001% to 0.100%, 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%, V: 0 to 0.10%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, REM: 0 to 0.0200%, remainder: Fe and impurities, and α represented by the following (1) formula is 4.0 to 16.0, a tensile strength of 590 MPa to 930 MPa, a microstructure at a position at a distance of 1 / 4 of the thickness in the thickness direction from the surface of the steel material contains lower bainite and martensite, a total of area ratios of the lower bainite and the martensite is 15.0% or more, and a total of area ratios of the upper bainite, the lower bainite, and the martensite is 90.0% or more, an effective crystal grain diameter in a region between a fusion line of the welded portion and a position of the welded heat affected zone at a distance of 1 mm from the fusion line is 100.0 μm or less.

[0012] α = 0.50 x V[C] x (1 + 0.64[Si]) x (1 + 4.10[Mn]) x (1 + 0.27[Cu]) x (1 + 0.52[Ni]) x (1 + 2.33[Cr]) x (1 + 3.14[Mo]) (1) wherein [element symbol] in formula (1) represents a content (mass %) of each corresponding element contained in the steel material. Zero is substituted in the case where the corresponding element is not contained.

[0013] <2> The welded joint according to <1>, wherein the chemical composition contains the following Group A.

[0014] [Group A] one or two or more kinds selected from the following elements: Cu: 0.01% to 1.50%, Cr: 0.01% to 3.00%, Mo: 0.01% to 2.00%, and B: 0.0003% to 0.0050%.

[0015] <3> The welded joint according to <1> or <2>, wherein the chemical composition contains the following Group B.

[0016] [Group B] one or two or more kinds selected from the following elements: Nb: 0.001% to 0.050%, Ti: 0.001% to 0.050%, and V: 0.01% to 0.10%.

[0017] <4> The welded joint according to any one of <1> to <3>, wherein the chemical composition contains the following Group C.

[0018] [Group C] one or two or more kinds selected from the following elements: Mg: 0.0003% to 0.0200%, Ca: 0.0003%~0.0200%, and REM: 0.0003%~0.0200%.

[0019] <5> according to <1> ~ <4> The steel described in any one of the following statements, wherein the Charpy impact absorption energy of the weld heat-affected zone at -100°C is 70 J or more.

[0020] <6> according to <1> ~ <5> The steel described in any one of the following statements, wherein, after the welded joint has undergone heat treatment in a temperature range of 425°C or higher with a heating rate and a cooling rate of 55°C / h and a holding time of 600°C for 2 hours, the Charpy impact energy absorbed at -100°C in the heat-affected zone of the heat-treated portion is 70J or higher.

[0021] Invention Effects According to this disclosure, it is possible to provide welded joints suitable for low-temperature applications that use high-tensile-strength steel as the base material and achieve good low-temperature toughness both before and after post-weld heat treatment. Attached Figure Description

[0022] Figure 1 This is an example diagram showing the discrimination results of microstructures.

[0023] Figure 2A This is a schematic diagram of a portion of a test piece used to collect Charpy impact energy from the heat-affected zone of a semi-V weld joint.

[0024] Figure 2B This is a schematic diagram of another example of a test piece used to collect Charpy impact energy from the heat-affected zone of a semi-V weld joint.

[0025] Figure 3A This is a schematic diagram of a portion of a test piece used to collect Charpy impact energy from the heat-affected zone of a K-type weld joint.

[0026] Figure 3B This is a schematic diagram of another example of a test piece used to collect Charpy impact energy from the heat-affected zone of a K-type weld joint.

[0027] Figure 4 It means Figure 2A A rough three-dimensional diagram of the notch shape of the test piece. Detailed Implementation

[0028] The following is a detailed description of this disclosure.

[0029] Unless otherwise specified, "post-weld heat treatment" in this disclosure refers to post-weld heat treatment in accordance with the provisions of JIS Z 3700:2009 "Methods for Post-weld Heat Treatment".

[0030] In this disclosure, "steel" or "base material" refers to the portion of steel that does not include surface treatment layers such as plating or coating. However, surface treatment layers such as plating or coating may also be formed on the surface of the steel and welded joints in this disclosure. It should be noted that "base material" refers to the portion of steel that is not affected by welding in comparison with the welded portion (weld metal and weld heat-affected zone) in the welded joint. "Weld heat-affected zone" refers to the portion of steel that has been affected by heat due to welding.

[0031] In this disclosure, the numerical range indicated by “~” refers to the range that includes the values ​​recorded before and after “~” as lower and upper limits. However, when the values ​​recorded before and after “~” are marked as “exceeding” or “below”, the numerical range refers to the range that does not include these values ​​as lower or upper limits.

[0032] Regarding the content of elements in a chemical composition, "%" refers to "mass %".

[0033] The term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes, as long as they achieve the intended purpose of the process.

[0034] The following describes a welded joint according to one embodiment of the present disclosure. First, the research results and new insights obtained by the inventors of the present disclosure up to the completion of the welded joint of the present disclosure will be described in detail.

[0035] The inventors of this disclosure conducted research to improve the strength of steel used in the manufacture of welded joints. The tensile strength of the steel is ensured by the composition of its microstructure. The inventors of this disclosure collected samples from a portion of hot-rolled and accelerated-cooled steel at a distance of 1 / 4 of its thickness in the thickness direction (1 / 4t portion, where t is the thickness of the steel) from the surface, conducted tensile tests, and observed the microstructure. The results showed that for steel with a tensile strength of 590 MPa to 930 MPa, the ferrite area fraction in the 1 / 4t portion of the microstructure was less than 10.0%, and the combined area fraction of upper bainite, lower bainite, and martensite was more than 90.0%. It should be noted that the combined area fraction of upper bainite, lower bainite, and martensite was determined using electron backscatter diffraction (EBSD).

[0036] Furthermore, the inventors of this disclosure conducted research to ensure the toughness of the welded joint using the aforementioned steel. The toughness of the welded joint is ensured by reducing the region enclosed by large-angle grain boundaries with a crystal orientation difference of 15° or more. The inventors of this disclosure collected samples from the heat-affected zone of the welded joint and subjected the samples to mechanical and electrolytic grinding. Using an EBSD device attached to a FE-SEM (Field Emission Scanning Electron Microscope), the equivalent circle diameter of the region enclosed by the large-angle grain boundaries was measured. Hereinafter, the equivalent circle diameter of the region enclosed by the large-angle grain boundaries will be referred to as the grain size. The area for measuring the grain size is the region 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 is measured along the fusion line at a distance of 4 mm. 2 It will be carried out in the area of ​​4mm. 2 The effective grain size (sometimes referred to in this disclosure as "effective grain size of the weld heat-affected zone") is calculated by averaging the top 10 largest grain sizes among those measured in the region. It has been observed that if the effective grain size of the weld heat-affected zone of the weld joint is 100.0 μm or less, the toughness of the weld joint tends to be further improved both before and after post-weld heat treatment.

[0037] Furthermore, the inventors of this disclosure conducted research to reduce the effective grain size in the heat-affected zone (HAZ) of the weld joint. The effective grain size in the HAZ of the weld joint is ensured by the composition of the microstructure of the base material made of steel. The inventors of this disclosure collected samples from 1 / 4 ton of the steel, observed the microstructure, and conducted Charpy impact tests on the HAZ of the weld joint manufactured by welding the steel. The results showed that the combined area ratio of lower bainite and martensite in the microstructure of the steel with a Charpy impact absorption energy of 70 J or more at -100°C was 15.0% or more. The combined area ratio of lower bainite and martensite was measured using EBSD.

[0038] <Chemical Composition> Next, the alloying elements constituting the chemical composition of the steel in the welded joint of this disclosure will be described. It should be noted that in the following description of alloying elements, the percentage (%) refers to "mass %". Additionally, the steel used as the base material for the welded joint of this disclosure is sometimes referred to as "the steel in this disclosure".

[0039] (C: 0.03%~0.20%) Carbon (C) is an element that increases the strength of steel. From the viewpoint of ensuring the strength of the steel used in structures, the C content in this disclosure 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. From the viewpoint of ensuring the toughness of the weld heat-affected zone (HAZ), the C content in this disclosure is 0.20% or less. The C content is preferably 0.16% or less, 0.14% or less, or 0.12% or less.

[0040] (Si: 0.01%~0.50%) Si is used as a deoxidizer and is an element that increases strength by being dissolved in steel. From the viewpoint of controlling the O concentration in the molten steel, the Si content in this disclosure 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 form in the HAZ, reducing toughness. Therefore, from the viewpoint of ensuring the toughness of the HAZ, the Si content in this disclosure is 0.50% or less. The Si content is preferably 0.30% or less or 0.20% or less.

[0041] (Mn: 0.10%~1.65%) Mn is used as a deoxidizer and is an element that improves the hardenability of steel, thus contributing to high strength. From the viewpoint of controlling the O concentration in the molten steel, the Mn content in this disclosure is 0.10% or more. Furthermore, with 0.10% or more Mn, MnS is formed, thereby reducing the solid solution S and preventing hot cracking. From the viewpoint of ensuring the strength and HAZ toughness of the steel, 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 segregation at grain boundaries during PWHT may lead to a decrease in toughness after PWHT. Therefore, from the viewpoint of ensuring HAZ toughness after PWHT, the Mn content in this disclosure is 1.65% or less. The Mn content is preferably 1.50% or less, 1.25% or less, or 1.10% or less.

[0042] (P: below 0.025%) P is an impurity element. There is no lower limit to the P content, but from a manufacturing cost perspective, the P content in this disclosure can be 0.001% or more. On the other hand, if the P content is excessive, P segregation at grain boundaries during PWHT may lead to a decrease in toughness after PWHT. Therefore, in this 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.

[0043] (S: below 0.0250%) S is an impurity element. There is no lower limit to the S content, but from a manufacturing cost perspective, the S content in this disclosure can be 0.0001% or more. On the other hand, if the S content is excessive, tensile MnS may be generated in the central segregation region, deteriorating the toughness and ductility of the steel and the HAZ. From the viewpoint of ensuring the toughness and ductility of the steel and the HAZ, the S content is 0.0250% or less. The S content is preferably 0.0100% or less or 0.0050% or less.

[0044] (Ni: 2.65%~4.45%) Ni is an effective element for improving the hardenability and toughness of steel; therefore, in this disclosure, the Ni content is 2.65% or more. The Ni content is preferably set to 3.00% or more or 3.20% or more. However, Ni is an expensive element, and from a cost-reduction perspective, in this disclosure, the Ni content is 4.45% or less. The Ni content is preferably 4.10% or less or 3.80% or less.

[0045] (Al: 0.001%~0.100%) Al is a useful element for deoxidation and also contributes to grain refinement during quenching by forming nitrides. Therefore, in this disclosure, the Al content is 0.001% or more. However, if Al is present in excess, it may form coarse nitrides, reducing the toughness of the steel and the HAZ (hardening zone). Therefore, the Al content is 0.100% or less. The Al content is preferably 0.080% or 0.050% or less.

[0046] (O: below 0.0100%) O is an impurity element. There is no lower limit to the O content, but from a manufacturing cost perspective, the O content in this disclosure can be 0.0001% or more. On the other hand, if the O content is excessive, coarse oxides may form, deteriorating the toughness and ductility of the steel and the HAZ. From the viewpoint of ensuring the toughness and ductility of the steel and the HAZ, the O content is 0.0100% or less. The O content is preferably 0.0060% or less or 0.0040% or less.

[0047] (N: below 0.0100%) Nitrogen (N) is an impurity element. There is no lower limit to the N content, but from a manufacturing cost perspective, the N content in this disclosure can be 0.0001% or more. From the viewpoint of ensuring the properties of the steel and the toughness of the HAZ (Hyperal Zone), the N content in this disclosure is 0.0100% or less. The N content is preferably 0.0050% or less or 0.0040% or less.

[0048] The steel disclosed herein may also contain other elements (optional elements) to replace a portion of the Fe. For example, optional elements from groups A to C below can be listed, but the content of these elements may also be 0%.

[0049] [Group A] For the steel disclosed herein, in order to improve strength and toughness, it may also contain one or more of the following optional elements, Cu, Cr, Mo, and B, which have the effect of improving hardenability.

[0050] (Cu: below 1.50%) Cu is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Cu content; it can be 0%. Furthermore, Cu has minimal adverse effects on weldability and HAZ toughness, and it improves the hardenability of steel, thus also contributing to increased steel strength. Therefore, in this disclosure, the Cu content can be 0.01% or more. The Cu content is preferably 0.10% or more. However, from the viewpoint of suppressing the formation of Cu cracks during hot rolling of steel, in this disclosure, the Cu content is 1.50% or less. The Cu content is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[0051] (Cr: less than 3.00%) Cr is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Cr content; it can be 0%. Furthermore, Cr improves the hardenability of steel, and therefore is also an element that increases the strength of steel. Therefore, in this disclosure, the Cr content can be 0.01% or more. The Cr content is preferably 0.10% or more. However, from the viewpoint of suppressing the deterioration of the toughness and weldability of the HAZ (hardness zone), in this disclosure, the Cr content is 3.00% or less. The Cr content is preferably 2.20% or less, 1.40% or less, or 0.80% or less.

[0052] (Mo: 2.00% or less) Mo is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Mo content; it can be 0%. Furthermore, Mo improves the hardenability of steel, and therefore also increases its strength. Therefore, in this disclosure, the Mo content can be 0.01% or more. The preferred Mo content is 0.05% or more, 0.10% or more, 0.20% or more, or 0.30% or more. However, from the viewpoint of suppressing the deterioration of the toughness and weldability of the HAZ (hardness zone), and suppressing the increase in alloy cost, the Mo content in this disclosure is 2.00% or less. The preferred Mo content is 1.20% or less or 0.80% or less.

[0053] (B: below 0.0050%) Botanicals (B) are elements that can potentially be introduced into steel during the manufacturing process. However, there is no lower limit to the B content; it can be 0%. Furthermore, B is an element that significantly improves the hardenability and strength of steel. Therefore, in this disclosure, the B content can be 0.0003% or more. However, from the viewpoint of suppressing the deterioration of the surface quality of steel billets manufactured by continuous casting, in this disclosure, the B content is 0.0050% or less. The B content is preferably 0.0030% or less or 0.0020% or less.

[0054] Group B For the steel disclosed herein, in order to improve strength, it may also contain one or more of the following optional elements Nb, Ti, and V, which have the effect of improving the strength of the steel through precipitates such as carbides and nitrides, as needed.

[0055] (Nb: below 0.050%) Nitrogen (Nb) is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Nb content; it can be 0%. Furthermore, Nb forms carbides and nitrides, which refine the metal microstructure and contribute to increased steel strength. Therefore, in this disclosure, the Nb content can be 0.001% or more. However, from the viewpoint of suppressing the deterioration of the toughness and weldability of the HAZ (hardening zone), 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 the toughness of the HAZ after PWHT (Polymerized Welded Steel Tolerancing), the Nb content can also be 0.004% or less.

[0056] (Ti: below 0.050%) Ti is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Ti content; it can be 0%. Furthermore, Ti forms carbides and nitrides, which refine the metal microstructure and contribute to increased steel strength. Therefore, in this disclosure, the Ti content can be 0.001% or more. However, from the viewpoint of suppressing the deterioration of the toughness and weldability of the HAZ, the Ti content is 0.050% or less. The preferred Ti content is 0.040%, 0.030%, or 0.020% or less. In particular, from the viewpoint of ensuring the toughness of the HAZ after PWHT, the Ti content can also be 0.004% or less or 0.002% or less.

[0057] (V: below 0.10%) V is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the V content; it can be 0%. Furthermore, V is also an element that forms carbides and nitrides, increasing the strength of steel. Therefore, in this disclosure, the V content can be 0.01% or more. However, from the viewpoint of suppressing the deterioration of the toughness and weldability of the HAZ (hard alloy zone) and suppressing the 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.

[0058] [Group C] For the steels disclosed herein, in order to improve the toughness of the HAZ, they may also contain one or more of the optional elements Mg, Ca, REM shown below, as needed.

[0059] (Mg: below 0.0200%) Mg is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Mg content; it can be 0%. Furthermore, Mg is also an element that forms oxides and improves the toughness of the weld heat-affected zone. Therefore, in this disclosure, the Mg content can be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, if the Mg content is excessive, coarse oxides may form, reducing the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, in this disclosure, the Mg content is 0.0200% or less. Preferably, the Mg content is 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0060] (Ca: below 0.0200%) Ca is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Ca content; it can be 0%. Furthermore, Ca is also an element that mitigates the effects of MnS, which reduces the toughness of steel and the weld heat-affected zone, by causing spheroidization of sulfides in the steel. Therefore, in this disclosure, the Ca content can be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, if the Ca content is excessive, coarse oxides may form, reducing the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, in this 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.

[0061] (REM: below 0.0200%) Rare earth metals (REM) refer to a total of 17 elements, including Sc and Y, and the lanthanide elements such as La, Ce, and Nd. REM content refers to the total content of these 17 elements. REM is an element that can potentially be mixed into steel during the manufacturing process. However, there is no lower limit to the REM content; it can be 0%. Furthermore, REM is also an element that forms oxides and improves the toughness of the weld heat-affected zone. Therefore, in this disclosure, the REM content can be 0.0003% or more, 0.0006% or more, or 0.0010% or more. On the other hand, if the REM content is excessive, coarse oxides may form, reducing the toughness of the steel. Therefore, from the viewpoint of ensuring toughness, in this disclosure, the REM content is 0.0200% or less. The REM content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0062] (Remaining portion: Fe and impurities) The remaining chemical composition of the steel disclosed herein consists of iron (Fe) and impurities. Impurities refer to components introduced during the industrial manufacturing of steel due to raw materials such as ores and waste, as well as other factors.

[0063] In addition to limiting the content of each element, this disclosure also limits the range of α values ​​as follows.

[0064] (α value: 4.0~16.0) The value of α is calculated using the following equation (1).

[0065] 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] represent the content (mass%) of C, Si, Mn, Cu, Ni, Cr, and Mo in the steel. Substitute zero if the corresponding element is not present. It should be noted that √[C] and [C] are different. 1 / 2 They have the same meaning.

[0066] In the steel disclosed herein, the α value is set in the range of 4.0 to 16.0. This is an indicator of the hardenability of the steel; the higher the α value, the more easily a lower bainite and martensite structure with an excellent balance of strength and toughness can be formed. When α is within an appropriate range, the ratio of lower bainite and martensite structure with an excellent balance of strength and toughness in the HAZ structure also increases, 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 that is beneficial to the balance of strength and toughness increases, and toughness deterioration is suppressed. In addition, the structure of the HAZ also tends to increase the ratio of lower bainite and martensite, and HAZ toughness is also improved. On the other hand, if the α value is set to 16.0 or lower, the strength of the steel will not become excessively high, and toughness can be ensured. In addition, if the α value is set to 16.0 or lower, toughness after PWHT can also be ensured. Furthermore, the HAZ will not become excessively hardened, and HAZ toughness can also be ensured.

[0067] By satisfying the above-mentioned numerical range regarding the α value, nickel-containing steel for low-temperature applications with excellent strength and toughness can be provided. The α value is preferably 4.5 or higher, or 5.0 or higher. Furthermore, the α value is preferably 15.5 or lower, or 15.0 or lower.

[0068] <Microstructure> Next, the microstructure of the steel in this disclosure will be described. The microstructure of the steel in this disclosure at a distance of 1 / 4 of its thickness from the surface in the thickness direction includes lower bainite and martensite. In addition, as bainite, upper bainite may also be included in addition to lower bainite.

[0069] "Bainite" is a microstructure consisting of bainitic ferrite (α°B) within the grains, and is a general term for both upper and lower bainite. "Upper bainite" refers to upper bainite containing retained austenite or MA phase (martensite / austenite mixture) between laths, or upper bainite containing carbides between laths, or both. "Lower bainite" is lath-shaped lower bainite containing carbides within the laths.

[0070] Martensite exists in four forms: lath, butterfly, lenticular, and thin plate. However, lath martensite is the main component in this disclosure. Lath martensite is composed of lath bundles and lath blocks formed by groups of laths arranged in a specific manner. It is an austenite grain divided into several lath bundles.

[0071] (The total area ratio of lower bainite and martensite in 1 / 4 t of the steel: 15.0% or more) Lower bainite and martensite are hard phases that improve the toughness of the steel and the HAZ (Hard Zone). From the viewpoint of ensuring HAZ toughness, the area ratio of lower bainite and martensite in the 1 / 4 t portion of the steel is 15.0% or more. The area ratio of lower bainite and martensite in the 1 / 4 t portion is preferably 20.0% or more or 30.0% or more. The total area ratio of lower bainite and martensite in the 1 / 4 t portion can also be 100%.

[0072] (The total area ratio of upper bainite, lower bainite, and martensite in 1 / 4 t of steel: ≥90.0%) From the perspective of ensuring steel strength, the total area ratio of upper bainite, lower bainite, and martensite in 1 / 4 t of the steel should be 90.0% or more. Alternatively, the total area ratio of upper bainite, lower bainite, and martensite in 1 / 4 t can also be 100%. Furthermore, the upper bainite content in 1 / 4 t can also be 1.0% or more.

[0073] The microstructure of the steel was observed using a sample with 1 / 4 of its surface as the observation plane. Two types of samples were prepared, one subjected to electrolytic grinding and the other to nitric acid-ethanol etching. For each sample (a) and (b), measurements were taken at three locations using the method described below, and the average of the three locations was taken as the area ratio of the steel's microstructure. It should be noted that for each sample (a) and (b), three samples can be prepared separately and the average of each sample can be taken, or measurements can be taken at three locations within a single sample and the average can be taken.

[0074] After mirror finishing via mechanical grinding, electrolytic grinding was performed to remove the strain layer generated by mechanical grinding. Using this electrolytically ground sample, the total area ratio of upper bainite, lower bainite, and martensite was measured using EBSD. The measurement magnification was 200x, and measurements were taken over a 400μm × 400μm area at 0.4μm intervals. Measurements were performed with an electron beam diameter of less than 0.4μm. The confidence index (CI value) was set to 0.1 or higher. The determination of ferrite from upper bainite, lower bainite, and martensite was based on a threshold of 0.5 for Grain Average Misorientation (GAM). It should be noted that the GAM value is an index defined in OIM-Analysis (EBSD crystal orientation analysis software manufactured by TSL Corporation, USA). Regions with a GAM value below 0.5 are ferrite, while regions with a GAM value above 0.5 are upper bainite, lower bainite, or martensite. In this disclosure, upper bainite, lower bainite, and martensite are determined using EBSD's GAM as a threshold; therefore, they include not only upper bainite, lower bainite, and martensite, but also tempered upper bainite, tempered lower bainite, and tempered martensite.

[0075] The samples were etched using nitric acid and ethanol, and the area ratio of upper bainite was determined by SEM observation. The measurement was performed at 500x magnification over a range of 360 μm × 480 μm. Upper bainite was defined as the region with a clear lath structure and carbides and metamerism (MA) along the lath boundaries. Regions with a coarser internal structure and a sparser, mixed coarse-dense carbide density were also defined as upper bainite. Figure 1 Examples of microstructure identification results are shown below. (A) and (B) are SEM images of the same area of ​​steel manufactured by DQT with an α value of 9.9. In (B), the area enclosed by the white line is upper bainite (Bu), and the other areas are lower bainite + martensite (BL+M). Regarding the portion identified as upper bainite (Bu), the whitish carbides appear sparse and mixed, with some denser areas. On the other hand, regarding the portion identified as lower bainite + martensite (BL+M), the carbides are dense and uniform. The total area ratio of lower bainite and martensite is obtained by subtracting the area ratio of upper bainite from the total area ratio of upper bainite, lower bainite, and martensite measured above.

[0076] It should be noted that when the combined area ratio of lower bainite and martensite exceeds 0% using the above-mentioned tissue differentiation method, it usually includes both lower bainite and martensite. Lower bainite and martensite can be distinguished using SEM or TEM (transmission electron microscopy), confirming the presence of each tissue.

[0077] (Effective grain size of the weld heat-affected zone: below 100.0 μm) From the viewpoint of ensuring HAZ toughness, the effective grain size of the weld heat-affected zone is 100.0 μm or less. Preferably, the effective grain size of the weld heat-affected zone is 95.0 μm or less or 90.0 μm or less. It should be noted that the lower limit of the effective grain size of the weld heat-affected zone is not particularly limited, but the effective grain size of the weld heat-affected zone can also be, for example, 50.0 μm or more or 60.0 μm or more.

[0078] <Mechanical Properties> The welded joint having a base material formed from the steel disclosed herein and a welded portion possesses mechanical properties that combine the strength of the steel with the low-temperature toughness of the weld heat-affected zone. In particular, it exhibits excellent toughness not only at -100°C in the weld heat-affected zone but also maintains excellent low-temperature toughness after PWHT (Polymerized Welding Hydrolysis).

[0079] (Tensile strength: 590MPa~930MPa) In this disclosure, the tensile strength of the steel is set to 590–930 MPa. To reduce the weight of large welded structures such as transport tanks, steel that can ensure structural strength even with thinness is required. Typically, steel with the aforementioned tensile strength is selected for use in such applications; therefore, the steel in this disclosure is also manufactured to have the aforementioned tensile strength.

[0080] (Yield ratio) The yield ratio (YR = yield strength / tensile strength × 100) of the steel disclosed herein is not particularly limited, but is preferably 90% or less. By producing a multiphase structure, it is possible to suppress the increase of YR.

[0081] (Charpy impact energy absorbed at -100°C in the heat-affected zone of the weld) To ensure high toughness at low temperatures, the welded joint of this disclosure preferably has a Charpy impact absorption energy of 70 J or more at -100°C in the heat-affected zone. By possessing low-temperature toughness with a Charpy impact absorption energy of 70 J or more at -100°C in the heat-affected zone, the welded joint of this disclosure enables transport tanks formed from it to be suitable for, for example, the transport of liquid carbon dioxide. It should be noted that the Charpy impact absorption energy at -100°C in the heat-affected zone is as follows... Figure 2A , Figure 2B , Figure 3A and Figure 3BAs shown, the following values ​​are set: Samples 14 are collected from the region containing the fusion line (FL) of the weld 12 and the region containing a position 1 mm away from the fusion line (FL+1 mm) at 1 / 4 t of the base material 10 of the weld joint 20, and measurements are performed using these samples 14. It should be noted that... Figure 2A , Figure 2B , Figure 3A and Figure 3B In the diagram, 15 represents the weld metal, and 16 represents the weld heat-affected zone (HAZ). For example... Figure 4 As shown, with the width direction X, thickness direction Y, and length direction Z of the base material 10 set, the notch 18 is formed in such a way that the length direction of the notch 18 in the test piece 14 is parallel to the thickness direction Y of the base material 10, and the depth direction of the notch 18 is parallel to the length direction Z of the base material 10. It should be noted that the length direction Z of the base material 10 is the rolling direction, and the width direction X is the direction perpendicular to both the rolling direction Z and the thickness direction Y.

[0082] (Charpy impact energy absorbed at -100°C in the weld heat-affected zone after PWHT) In the case of cryogenic tanks, to prevent damage, welded sections are sometimes subjected to PWHT (Potential Welding Temperature Observation) after assembly into transport tanks. If the weld joint is heated to a temperature range above 425°C for an extended period, there is a tendency for reduced HAZ (Heat Affected Zone) toughness. In the case of the weld joint disclosed herein, after PWHT treatment of the aforementioned steel at a holding temperature of 600°C for 2 hours and a heating / cooling rate of 55°C / h in the temperature range above 425°C, the preferred toughness of the PWHT-treated weld heat-affected zone is a Charpy impact absorption energy of 70 J or more at -100°C. The Charpy impact absorption energy at -100°C after PWHT can also be 100 J or more. The Charpy impact absorption energy at -100°C after PWHT is also set as a value measured using a sample taken from a location one-quarter of the thickness.

[0083] It should be noted that PWHT may reduce the toughness of the HAZ (Heat Affected Zone). The reason is unclear, but it is speculated that the diffusion of phosphorus (P) and manganese (Mn) to the grain boundaries, and the resulting growth or aggregation of inclusions in the microstructure, leads to decreased brittleness and thus reduced toughness. The reduction in toughness caused by PWHT can be suppressed by limiting the content of P and Mn, thereby reducing the effective grain size of the weld heat-affected zone.

[0084] Tensile strength (TS) and yield strength (YS) in the examples were determined by tensile testing according to JIS Z2241:2011. In the tensile test, JIS 14A test specimens were used, taken from a position at 1 / 4 thickness, with the length direction parallel to the width direction of the steel (C direction). TS and YS were determined using three test specimens, and calculated by averaging them.

[0085] The toughness of the steel was evaluated according to JIS Z2242:2018, using an impact blade with a radius of 2 mm, through the Charpy impact test, and the toughness was assessed by the ductile-brittle transition temperature (vTrs). The Charpy impact test was conducted at three of each of five temperatures, and the brittle fracture rate was measured to calculate vTrs. In the Charpy impact test, a V-notch test piece was used, taken from the 1 / 4 t portion of the steel, with its length direction parallel to the width direction of the steel (C direction).

[0086] The Charpy impact absorption energy of the weld heat-affected zone is determined according to JIS Z2242:2018, using an impact blade with a radius of 2 mm at -100°C via a Charpy impact test. The Charpy impact absorption energy is calculated by averaging three test pieces. In the Charpy impact test, if... Figure 2A , Figure 2B , Figure 3A , Figure 3B and Figure 4 As shown, a V-notch test piece 14 is made from a welded joint 20 at a position equivalent to 1 / 4 t of steel.

[0087] The shape of the steel materials disclosed herein is not particularly limited, and includes steel plates, steel strips, structural steel, steel pipes, etc. However, steel pipes and structural steel include steel materials formed by joining steel plates together, such as structural steel joined by rivets in addition to welded steel pipes and welded structural steel. The thickness of steel materials such as steel plates, steel strips, structural steel, and steel pipes (the thickness of the flange for structural steel) is not particularly limited, and is generally from 3mm to 150mm. The thickness of the steel materials can be 6mm or more, 10mm or more, 15mm or more, or 30mm or more. In addition, the thickness of the steel materials can be less than 100mm, less than 80mm, or less than 60mm.

[0088] Furthermore, the application of the welded joint disclosed herein is not particularly limited, but due to its mechanical properties that balance strength and low-temperature toughness, especially its excellent low-temperature toughness after PWHT, it can be suitable for use as a tank for storing and transporting liquefied gas, particularly liquid carbon dioxide.

[0089] (Methods for manufacturing steel) The manufacturing method of the steel disclosed herein is not particularly limited, but the steel disclosed herein is, for example, produced by continuous casting after smelting steel that meets the above-described chemical composition. The steel billet is heated, and after hot rolling, it is directly water-cooled and quenched (DQ), or it is cooled, reheated, and water-cooled and quenched (RQ) to produce steel. Furthermore, intermediate heat treatment (L) and tempering (T) may also be performed. It should be noted that in the case of RQ, cooling before reheating is not necessary; water cooling may also be performed. The manufacturing process after hot rolling is selected from the above-described combinations of DQ, RQ, L, and T, for example, DQT, RQT, DQLT, and RQLT.

[0090] (1) DQT: Direct Quenching (DQ), Tempering (T) (2) RQT: Cooling, reheating and quenching (RQ), tempering (T) (3) DQLT: Direct Quenching (DQ), Intermediate Heat Treatment (L), Tempering (T) (4) RQLT: Cooling, reheating and quenching (RQ), intermediate heat treatment (L), tempering (T) From the viewpoint of manufacturing cost, DQT is preferred in the manufacture of the steel disclosed herein, and examples of preferred manufacturing processes are shown below.

[0091] From the viewpoint of hot rolling in a temperature range where the microstructure of the rolled material is austenitic, the heating temperature of the hot-rolled steel billet is Ac3 or higher. From the viewpoint of reducing deformation resistance, the heating temperature of the steel billet is preferably 1000°C or higher. On the other hand, from the viewpoint of suppressing the coarsening of heated γ-grains, the hot-rolling heating temperature is 1250°C or lower. The hot-rolling heating temperature is preferably 1200°C or lower. It should be noted that Ac3 is a value calculated using the following formula.

[0092] Ac3=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 refer to the content (mass%) of each element contained in the steel billet.

[0093] Hot rolling is sometimes composed of rolling in the temperature zone where recrystallization occurs (recrystallization temperature zone rolling) and rolling in the temperature zone where recrystallization is inhibited (non-recrystallization temperature zone rolling).

[0094] Recrystallization temperature zone rolling is hot rolling performed at a temperature of 900°C or higher. From the viewpoint of refining the austenite grain size of the steel, the cumulative reduction rate in recrystallization temperature zone rolling is preferably 20% or more, and more preferably 30% or more. The cumulative reduction rate in recrystallization temperature zone rolling is calculated from the difference between the thickness of the billet before hot rolling and the thickness of the rolled material at 900°C.

[0095] Cumulative reduction rate (%) during rolling in the recrystallization temperature range = 100 × ([bill thickness] - [thickness of rolled material at 900℃]) / [bill thickness] Non-recrystallization temperature zone rolling is hot rolling performed at a temperature below 900°C. From the viewpoint of refining the grain size of the steel, the cumulative reduction rate in non-recrystallization temperature zone rolling is preferably 20% or more, more preferably 30% or more. The cumulative reduction rate in non-recrystallization temperature zone rolling is calculated from the difference between the thickness of the rolled material at 900°C and the thickness of the steel after rolling.

[0096] Cumulative reduction rate (%) during rolling in the non-recrystallization temperature zone = 100 × ([thickness of the rolled material at 900℃] - [thickness of the steel after rolling]) / [thickness of the rolled material at 900℃] From the viewpoint of suppressing the formation of ferrite, which reduces strength, the finishing temperature of hot rolling is Ar3 or higher. After hot rolling, the steel is subjected to accelerated cooling such as water cooling. From the viewpoint of suppressing the formation of ferrite, which reduces strength, the starting temperature of accelerated cooling is Ar3 or higher. It should be noted that Ar3 is a value calculated using the following formula.

[0097] Ar3=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo+0.35 (t-8) The element symbols in the formula refer to the content (mass%) of each element contained in the steel, and t refers to the thickness of the steel (mm).

[0098] From the viewpoint of promoting bainitic and martensitic phase transformations, the cooling rate is 1.0 °C / s or higher. For accelerated cooling, a cooling rate of 5.0 °C / s or higher or 10.0 °C / s or higher is preferred. A faster accelerated cooling rate is preferable, but from the viewpoints of homogenization of cooling rate and cost, a rate of 50.0 °C / s or lower or 30.0 °C / s or lower is preferred. The cooling rate is obtained by calculating the cooling rate at 1 / 4 of the thickness using simulations based on heat transfer calculations.

[0099] From the viewpoint of improving steel strength by ensuring the presence of upper bainite, lower bainite, and martensite, the stop temperature for accelerated cooling is 400°C or below. The stop temperature for accelerated cooling is preferably 350°C or below. Accelerated cooling to room temperature is also possible. From the viewpoint of steel dehydrogenation, the stop temperature for accelerated cooling is preferably 100°C or above.

[0100] After accelerated cooling, the steel can also be tempered. From the viewpoint of suppressing strength reduction, the tempering temperature is preferably 650°C or below, 620°C or below, or 590°C or below. On the other hand, from the viewpoint of improving toughness, the tempering temperature is preferably 350°C or above or 400°C or above.

[0101] (Manufacturing method of welded joint) The method for manufacturing the welded joint (welding method) disclosed herein will be described.

[0102] First, a bevel is formed at the end of the steel (base material) described above. The bevel shape can be any one of a single bevel groove, a single J groove, a single U groove, a double bevel groove, a double J groove, or a double U groove. The bevels are butt-jointed, and welding is performed using welding materials. The welding materials are not particularly limited and can be appropriately determined based on the desired characteristics of the weld joint. Welding methods include flux-cored arc welding, gas-shielded arc welding, submerged arc welding, TIG welding, etc. The welding heat input and the number of weld passes are appropriately determined based on the welding method and plate thickness. The welding heat input is, for example, approximately 1.0–6.0 kJ / mm.

[0103] The following shows the specific welding conditions for gas shielded arc welding and submerged arc welding.

[0104] In gas-shielded arc welding (GSAW), a mixture of Ar and CO2 is used, with CO2 comprising 20%. For a K-groove, the groove angle is 10° at the front and 30° for both the upper and lower layers. Welding is performed using YM-69F (solid welding wire manufactured by Nippon Steel Welding Industries Co., Ltd.) with a residual heat temperature of 100–150°C, an inter-bead temperature of 100–150°C, a gas flow rate of 15–20 L / min for the lower layer and 18–22 L / min for the upper layer. The current is 250–270 A, the voltage is 27–33 V, and the welding speed is 30–40 cm / min with a heat input of 1.5–2.4 kJ / mm. The number of weld passes depends on the plate thickness.

[0105] In submerged arc welding, with a semi-V bevel, the bevel gap is 10mm, the angle is 30°, and welding is performed using Y-80M flux (submerged arc welding material manufactured by Nippon Steel Welding Industries Co., Ltd.) with a residual heat temperature of 100-150℃ and an inter-bead temperature of 100-150℃. The current is 630-670A, the voltage is 27-33V, and the welding speed is 3.5-4.4kJ / mm and 25-34cm / min. The number of weld passes depends on the plate thickness.

[0106] (Evaluation methods for welded joints) After fabricating the welded joint, such as Figure 2A , Figure 2B , Figure 3A and Figure 3B ,as well as Figure 4 As shown, a Charpy impact test is performed by introducing a notch at the fusion line (FL) or at a position 1 mm away from the fusion line (FL+1 mm). When the bevel shape is any of the following: semi-V-shaped, J-shaped, U-shaped, K-shaped, double-sided J-shaped, or H-shaped, the V-notch at FL is set as follows: it is placed at the tangent on the weld heat-affected zone side of the fusion line, covering more than 80% of the weld heat-affected zone. The V-notch at FL+1 mm is placed at a position 1 mm away from the FL notch. The grain size of the metal structure near FL is approximately determined by the weld heat line energy. Therefore, the absorbed energy of the weld heat-affected zone is independent of the bevel shape but depends more on the heat line energy.

[0107] Example The following examples illustrate the welded joints of this disclosure. However, the conditions in the following examples are examples used to verify the feasibility and effectiveness of this disclosure, and the welded joints of this disclosure are not limited to the following examples.

[0108] [Steel Manufacturing] First, slabs with the chemical compositions shown in Table 1 are cast using a continuous casting method. The remainder besides the components shown in Table 1 consists of Fe and impurities. Additionally, blank columns indicate that no alloying elements were intentionally added during the steelmaking process.

[0109] The underlined part indicates that it is outside the scope of this disclosure.

[0110] [Table 1] Next, these slabs are used to manufacture steel under the manufacturing conditions shown in Table 2. "Tempering heat treatment" refers to the heating temperature during the tempering process after quenching.

[0111] [Table 2] [Manufacturing of welded joints] For steels other than No. 12 and No. 18, use the aforementioned steels to create a K-groove and perform multi-layer gas shielded arc welding (GMAW) to manufacture the welded joint.

[0112] In GMAW welding, a mixture of Ar and CO2 gas is used, with CO2 accounting for 20%. At the K-groove, the groove angle is 10R at the front and 30° for both the upper and lower layers. YM-69F (solid welding wire manufactured by Nippon Steel Welding Industry Co., Ltd.) is used for welding the weld metal at a residual heat temperature of 100–150°C, an inter-bead temperature of 100–150°C, a gas flow rate of 17 L / min for the lower layer, and 20 L / min for the upper layer. The current is 260A, the voltage is 30V, and the welding speed is 35 cm / min with a heat input of 2.0 kJ / mm. The number of weld passes varies depending on the plate thickness. Furthermore, for No. 12 and 18, a semi-V-groove is created using the aforementioned steel, and multi-layer submerged arc welding (SAW) is performed to produce the welded joint.

[0113] In SAW welding, the bevel gap for the semi-V groove is 10mm, the angle is 30°, and the residual heat temperature is 100-150℃, with an inter-bead temperature of 100-150℃. Y-80M flux and NB-250H flux (submerged arc welding material manufactured by Nippon Steel Welding Industries Co., Ltd.) are used for the weld metal. The current is 650A, the voltage is 30V, and the welding speed is 4.40J / mm² and 29cm / min. The number of weld passes varies depending on the plate thickness.

[0114] [Measurement and Evaluation] The microstructure and mechanical properties of the obtained steel and welded joints (heat-affected zone) were determined using the methods described above. The results are shown in Table 3. The meanings of the symbols for the microstructure are as follows. It should be noted that for the region identified as lower bainite + martensite (BL+M), the presence of lower bainite and martensite was confirmed by SEM observation. In addition, the remaining parts of the microstructure consist of pearlite, MA phase, ferrite, and retained γ (austenite).

[0115] Bu: Upper Bainite BL: Lower Bainite M: Martensite Regarding the toughness of the steel, vTrs was measured.

[0116] Regarding the toughness of the heat-affected zone (HAZ), a notch was set at FL or FL+1mm on the I side of the K-groove or semi-V-groove, and a Charpy impact test was conducted at -100℃. Regarding HAZ toughness, the average Charpy impact absorbed energy (KV2) at -100℃ was measured for the manufactured weld joint in its as-weld toughness state, and the average Charpy impact absorbed energy at -100℃ after a PWHT with a holding temperature of 600℃, a holding time of 2 hours, and a heating and cooling rate of 55℃ / h in a temperature range above 425℃.

[0117] Regarding toughness, measurements were taken for (1) the case where the steel (base material) was not heat-treated, (2) the case where heat treatment was performed after welding, and (3) the case where heat treatment was not performed after welding, using samples taken from 1 / 4 of the thickness.

[0118] (1) Base material toughness vTrs obtained from Charpy impact tests at various temperatures (2) Toughness of welded joint To fabricate a K-groove or semi-V-groove, use gas-shielded arc welding (GMAW) with a heat input of 2.0 kJ / mm for multi-layer surfacing, or submerged arc welding (SAW) with a heat input of 4.0 kJ / mm for multi-layer surfacing, and determine the average Charpy impact absorption energy at -100°C after creating the weld joint. (3) Post-PWHT toughness of welded joint To fabricate a K-groove or semi-V-groove, perform multi-layer gas shielded arc welding (GMAW) with a heat input of 2.0 kJ / mm, or submerged arc welding (SAW) with a heat input of 4.0 kJ / mm. After fabricating the weld joint, set the holding temperature to 600℃, holding time to 2 hours, and heating and cooling rates above 425℃ to 55℃ / h. Then, calculate the average Charpy impact absorption energy at -100℃ after PWHT at a temperature range of 600℃, 55℃ / h, and a heating and cooling rate of 55℃ / h. [Table 3] Examples No. 1 to 19 are examples of the present invention, and examples No. 20, 21, and 22 are comparative examples.

[0119] No. 20 cannot achieve sufficient hardenability, sufficient strength, or sufficient low-temperature toughness due to its small α value.

[0120] No. 21 has too much Mn content, so it cannot obtain sufficient low-temperature toughness before and after PWHT.

[0121] No. 22 has an excessively high α value, resulting in excessive strength and insufficient low-temperature toughness before and after PWHT.

[0122] In contrast to the comparative examples, all of the present invention examples (No. 1 to 19) not only had the chemical composition and microstructure of the steel properly controlled, with tensile strength of 590 MPa to 930 MPa within an appropriate range, but also had high Charpy impact absorption energy at -100°C in the weld heat-affected zone before and after PWHT, and achieved low-temperature HAZ toughness of 70 J or more.

[0123] Industrial availability The welded joint disclosed herein is suitable for use as a transport tank for liquefied carbon dioxide. Additionally, the welded joint disclosed herein can also be used in other welded structures such as buildings, bridges, ships, conduits, marine structures, pressure vessels, and tanks.

[0124] The entire disclosure of Japanese Patent Application No. 2023-119448, filed on July 21, 2023, is incorporated herein by reference. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as those specifically described therein.

[0125] Symbol Explanation 10. Base Material 12 Welding Section 14 test pieces 15 Welding metal 16 Welding Heat Affected Zone 18 gaps 20 Welded joints

Claims

1. A welded joint having a base material made of steel and a welded portion, said base material having the following chemical composition (in mass%): C：0.03%～0.20%、 Si: 0.01%~0.50% Mn: 0.10%~1.65%, P: below 0.025% S: Below 0.0250% Ni: 2.65%~4.45% Al：0.001%～0.100%、 O: Below 0.0100% N: below 0.0100% Cu: 0~1.50%, Cr：0～3.00%、 Mo: 0–2.00%, B：0～0.0050%、 Nb: 0~0.050%, Ti: 0~0.050%, V：0～0.10%、 Mg: 0~0.0200%, Ca: 0~0.0200%, REM: 0~0.0200%, Remaining components: Fe and impurities. Furthermore, α, as expressed by equation (1) below, ranges from 4.0 to 16.

0. The tensile strength is 590MPa~930MPa. The microstructure of the portion of the steel material located at 1 / 4 of its thickness along the thickness direction from the surface comprises lower bainite and martensite, wherein the combined area ratio of the lower bainite and the martensite is 15.0% or more, and the combined area ratio of the upper bainite, the lower bainite, and the martensite is 90.0% or more. The effective crystal grain size in the region between the fusion line of the welded part and the location of the heat-affected zone 1 mm away from the fusion line is less than 100.0 μm. 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), in, In formula (1), [element symbol] represents the content of each corresponding element contained in the steel by mass % and is substituted into zero if the corresponding element is not contained.

2. The welded joint according to claim 1, wherein, The chemical composition includes the following group A. [Group A] Selected from one or more of the following elements: Cu: 0.01%~1.50%, Cr：0.01%～3.00%、 Mo: 0.01%~2.00%, and B：0.0003%～0.0050%。 3. The welded joint according to claim 1 or claim 2, wherein, The chemical composition includes the following group B. Group B Selected from one or more of the following elements: Nb: 0.001%~0.050% Ti: 0.001%~0.050%, and V：0.01%～0.10%。 4. The welded joint according to any one of claims 1 to 3, wherein, The chemical composition comprises the following group C. [Group C] Selected from one or more of the following elements: Mg: 0.0003%~0.0200% Ca: 0.0003%~0.0200%, and REM: 0.0003%~0.0200%.

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

6. The welded joint according to any one of claims 1 to 5, wherein, When the welded joint is subjected to heat treatment in a temperature range of 425°C or higher, with a heating and cooling rate of 55°C / h and a holding time of 600°C for 2 hours, the Charpy impact absorption energy at -100°C of the heat-affected zone at the heat-treated area is 70J or higher.

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