Welded joint and pressure vessel

Abstract

A welded joint which has a specific chemical composition having a Ceq of 0.350-0.490 represented by formula (1), has a tensile strength of 490-720 MPa, has a circle-equivalent diameter of 0.01-0.50 [mu] m and a number density of Ti and N-containing inclusions of 1.0 * 105 / mm2 or more at positions 1 / 4 and 1 / 2 of the thickness from the surface in the thickness direction, and has a diameter of 1 / 4 and 1 / 2 of the number density of 1.0 * 105 inclusions / mm2 or more at positions 1 / 4 and 1 / 2 of the thickness in the thickness direction. The average particle diameter of inclusions containing Ti and N is 150 nm or less, and the effective crystal particle diameter in the region between the fusion line of the welded section and the position of the welding heat-affected section at a distance of 1 mm from the fusion line is 100.0 [mu] m or less. The pressure container comprises the welding joint. Ceq = [C] + [Mn] / 6 + [Ni] / 15 + [Cu] / 15 + [Cr] / 5 + [Mo] / 5 + [V] / 5 (1)

Description

Technical Field

[0001] This disclosure relates to welded joints and pressure vessels. Background Technology

[0002] Steel can be used in welded structures such as buildings, bridges, ships, conduits, marine structures, and pressure vessels (tanks). Steels with excellent strength and low-temperature toughness under stress are effective in applications at low temperatures.

[0003] Cryogenic pressure vessels, such as liquefied gas storage tanks, use cryogenic steel. Cryogenic steels include Al-killed steel, nickel steel, high-Mn steel, and austenitic stainless steel, depending on the operating temperature. In structural construction, welding is often used to join steel components. Near the weld metal obtained from welding the steel, a heat-affected zone (HAZ) is formed. In particular, the toughness of the HAZ (HAZ toughness) deteriorates compared to the toughness of the base metal due to grain coarsening caused by the welding heat input.

[0004] For example, Patent Document 1 proposes a steel plate that exhibits excellent toughness in both the base material and the welded parts when multiple layers of welding are performed.

[0005] In addition, Patent Document 2 proposes a welding steel that exhibits excellent HAZ toughness (especially low-temperature toughness) even when performing welding with a wide range of heat inputs from low to high.

[0006] With the aim of achieving low-temperature toughness and high strength, various steels and their manufacturing methods that specify chemical composition and inclusions have been proposed (see, for example, Patent Documents 3-4).

[0007] Patent Document 1: Japanese Patent Application Publication No. 2019-183205 Patent Document 2: Japanese Patent Application Publication No. 2004-218010 Patent Document 3: Japanese Patent Application Publication No. 2012-92422 Patent Document 4: Japanese Patent Application Publication No. 2016-79461 Summary of the Invention

[0008] The problem that the invention aims to solve For cryogenic steel used in cryogenic pressure vessels, a balance between high strength and ensured cryogenic toughness is desired. Furthermore, cryogenic pressure vessels are manufactured by welding steel, and post-weld heat treatment (sometimes called PWHT) is sometimes performed to remove residual stresses generated during welding. Recently, the requirements for the cryogenic toughness of welded joints after PWHT have been further increased.

[0009] The subject of this disclosure is to provide welded joints and pressure vessels suitable for low-temperature applications, which use high-tensile-strength steel as the base material and can obtain good low-temperature toughness both before and after post-weld heat treatment.

[0010] Methods for solving problems The main points of this disclosure are as follows.

[0011] <1> A welded joint having a base material made of steel and a welded portion, The steel has the following chemical composition, expressed as a percentage by mass: C: 0.03%~0.17% Si: 0.10%~0.50% Mn: 0.10%~1.70%, P: below 0.025% S: Below 0.0250% Al: 0.015%~0.100% Ti: 0.005%~0.050% O: Below 0.0100% N: 0.0010%~0.0064% Ni: 0~1.20%, Cu: 0–0.95%, Cr: 0–0.95% Mo: 0~1.00%, B: 0~0.0050% Nb: 0~0.034%, V: 0~0.10%, Mg: 0–0.020% Ca: 0~0.020%, REM: 0~0.020%, Remaining components: Fe and impurities. And the Ceq expressed by the following equation (1) is 0.350 to 0.490. The tensile strength of the steel is 490MPa to 720MPa. At locations 1 / 4 and 1 / 2 of the thickness of the steel at a distance of 0.01 μm to 0.50 μm from the surface, the number density of inclusions containing Ti and N in the steel is 1.0 × 10⁻⁶. 5 pcs / mm 2 above, In the 1 / 4 and 1 / 2 portions, the average particle size of the Ti and N inclusions is less than 150 nm. The effective crystal grain size in the region between the fusion line of the welded part and the position of the heat-affected part 1 mm away from the fusion line is less than 100.0 μm.

[0012] Ceq=[C]+[Mn] / 6+[Ni] / 15+[Cu] / 15+[Cr] / 5+[Mo] / 5+[V] / 5 (1) In equation (1), [element symbol] represents the content (mass%) of the corresponding element contained in the steel. Zero is used if the corresponding element is not present.

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

[0014] [Group A] Selected from one or more of the following elements: Ni: 0.01%~1.20% Cu: 0.01%~0.95%, Cr: 0.1%~0.95% Mo: 0.01%~1.00%, and B: 0.0003%~0.0050%.

[0015] <3> according to <1> or <2> The welded joint, wherein the chemical composition comprises group B below.

[0016] Group B Selected from one or more of the following elements: Nb: 0.001%~0.034%, and V: 0.01%~0.10%.

[0017] <4> according to <1> ~ <3> The welded joint according to any one of the following methods, wherein the chemical composition comprises group C below.

[0018] [Group C] Selected from one or more of the following elements: Mg: 0.0003%~0.020% Ca: 0.0003%~0.020%, and REM: 0.0003%~0.020%.

[0019] <5> according to <1> ~ <4> In any one of the welded joints, the Charpy impact absorption energy at -70°C of the weld heat-affected zone is 150 J or more.

[0020] <6> according to <1> ~ <5> In any one of the welded joints, 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 absorption energy at -70°C of the heat-affected zone at the heat-treated portion is 150J or higher.

[0021] <7> A pressure vessel comprising <1> ~ <6> The welded joint as described in any one of the following.

[0022] Invention Effects According to this disclosure, it is possible to provide welded joints and pressure vessels suitable for low-temperature applications that use high-tensile-strength steel as the base material and exhibit good low-temperature toughness both before and after post-weld heat treatment. Here, low temperature refers to a temperature range, for example, -10 to -70°C. Attached Figure Description

[0023] Figure 1A This is a schematic diagram of a portion of a test piece used to collect Charpy impact energy from the heat-affected zone of an X-type weld joint.

[0024] Figure 1B This is a schematic diagram of another example of a test piece used to collect Charpy impact energy from the heat-affected zone of an X-type weld joint.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] Figure 4 It means Figure 2AA rough three-dimensional diagram of the notch shape of the test piece. Detailed Implementation

[0030] The following is a detailed description of this disclosure.

[0031] 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".

[0032] In this disclosure, "steel" and "base material" refer to the steel portion excluding surface treatment layers such as plating or coatings. However, surface treatment layers such as plating or coatings may also be formed on the surface of the steel and welded joints in this disclosure. It should be noted that "base material" in a welded joint refers to the steel portion that is not affected by welding in comparison to the welded portion (weld metal and weld heat-affected zone) of the welded joint. "Weld heat-affected zone" refers to the steel portion that has been affected by heat due to welding.

[0033] 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.

[0034] Regarding the content of elements in a chemical composition, "%" refers to "mass %".

[0035] 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.

[0036] The following describes a welded joint according to one embodiment of the present disclosure. First, the research results and new insights gained by the inventors of the present disclosure up to the completion of the welded joint of the present disclosure will be described in detail.

[0037] The inventors of this disclosure conducted research to ensure the toughness of welded joints. To refine the microstructure of the weld heat-affected zone (HAZ), they repeatedly studied steel particles used to suppress the coarsening of austenite grains caused by welding heat. The inventors of this disclosure discovered that controlling the Ti and N content, using Al-deoxidized steel as a basis, effectively inhibits the growth of austenite grains in the weld HAZ. It was recognized that the particles with the pinning effect that inhibits austenite grain growth are fine Ti-based nitrides with an equivalent circle diameter of 0.01 μm to 0.50 μm and a number density of 1.0 × 10⁻⁶. 5 pcs / mm 2Under the above conditions, austenite grain growth inhibition can be achieved. Furthermore, it is understood that the finer the particles, the greater the pinning effect; therefore, to meet low-temperature toughness requirements, the average particle size of this Ti-based nitride is below 150 nm. It should be noted that in this specification, Ti-based nitride refers to inclusions containing Ti and N, sometimes abbreviated as "inclusions." Ti-based nitrides may also contain elements other than Ti and N, such as Nb and C.

[0038] The toughness of the weld 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 weld joint and subjected the samples to mechanical and electrolytic grinding. The equivalent circle diameter of the region enclosed by the large-angle grain boundaries was measured using an EBSD device attached to a FE-SEM (Field Emission Scanning Electron Microscope). 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 at 80x magnification 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 ten largest grain sizes measured in the region. It has been found 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 can be ensured both before and after post-weld heat treatment.

[0039] <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".

[0040] (C: 0.03%~0.17%) 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 0.17% or less. The C content is preferably 0.15% or less, 0.13% or less, or 0.11% or less.

[0041] (Si: 0.10%~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 0.10% or more. The Si content is preferably 0.11% or more, 0.13% or more, or 0.15% 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 0.40% or less, 0.30% or less, or 0.20% or less.

[0042] (Mn: 0.10%~1.70%) 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, the presence of 0.10% or more Mn allows for the formation of MnS, thereby reducing the amount of dissolved sulfur (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 1.70% or less. The Mn content is preferably 1.60% or less, 1.50% or less, 1.40% or less, 1.25% or less, or 1.10% or less.

[0043] (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.

[0044] (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.

[0045] (Al: 0.015%~0.100%) Al is an essential element for suppressing the formation of coarse oxides from Ti and for promoting the formation of fine Ti-based nitrides from Ti. From the viewpoint of Al-deoxidized steel, the presence of Al is also necessary; in this disclosure, the Al content is 0.015% or more. The Al content is preferably 0.020%, 0.025% or more, or 0.030% or more. However, if Al is present in excess, a large number of Al-containing inclusions may be formed, reducing the toughness of the steel and the HAZ (Hydrogen Acid Zone). Therefore, the Al content is 0.100% or less. The Al content is preferably 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[0046] (Ti: 0.005%~0.050%) Ti is an essential element for the formation of Ti-based nitrides, and in this disclosure, the Ti content is 0.005% or more. The Ti content is preferably 0.008%, 0.010% or more, or 0.015% or more. However, if Ti is present in excess, the Ti-based nitrides may become coarse, resulting in a decrease in the number density due to agglomeration and coarsening. Therefore, the Ti content is 0.050% or less. The Ti content is preferably 0.040% or less, 0.030% or less, or 0.025% or less.

[0047] (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, 0.0050% or less, or 0.0040% or less.

[0048] (N: 0.0010%~0.0064%) Nitrogen (N) is an essential element for the formation of Ti-based nitrides, and in this disclosure, the N content is 0.0010% or more. The N content is preferably 0.0015%, 0.0020% or more, or 0.0025% or more. However, if excessive N is present, the Ti-based nitrides may become coarse, resulting in a decrease in the number density due to agglomeration and coarsening. Therefore, the N content is 0.0064% or less. The Ti content is preferably 0.0060%, 0.0055%, 0.0050%, or 0.0045% or less.

[0049] 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%.

[0050] [Group A] To improve strength and toughness, it may also contain one or more of the following optional elements, Ni, Cu, Cr, Mo, and B, which have the effect of improving hardenability, as needed.

[0051] (Ni: below 1.20%) Ni is an element that can potentially be incorporated into steel during the manufacturing process. However, there is no lower limit to the Ni content; it can be 0%. Furthermore, Ni 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 Ni content can be 0.01% or more. The Ni content is preferably 0.10% or more. However, Ni is an expensive element, and from a cost-reduction perspective, in this disclosure, the Ni content is 1.20% or less. The Ni content is preferably 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[0052] (Cu: below 0.95%) 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 0.95% or less. The Cu content is preferably 0.90% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[0053] (Cr: less than 0.95%) 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), the Cr content in this disclosure is 0.95% or less. The Cr content is preferably 0.90% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[0054] (Mo: 1.00% 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, and it can be 0%. Furthermore, Mo improves the hardenability of steel, and therefore is also an element that increases the strength of steel. 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, in this disclosure, the Mo content is 1.00% or less. The preferred Mo content is 0.90% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[0055] (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.

[0056] Group B For the steel disclosed herein, in order to improve strength, it may also contain one or two of the following optional elements Nb and V, which have the effect of improving the strength of the steel through precipitates such as carbides and nitrides, as needed.

[0057] (Nb: below 0.034%) 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 0.034% or less. The preferred Nb content is 0.030% or less, 0.020% or less, 0.015% or less, or 0.009% 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.

[0058] (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.

[0059] [Group C] For the steel disclosed herein, in order to improve the toughness of HAZ, it may also contain one or more of the optional elements Mg, Ca, REM shown below, as needed.

[0060] (Mg: below 0.020%) 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 0.020% or less. The Mg content is preferably 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0061] (Ca: below 0.020%) 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.

[0062] (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.

[0063] (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.

[0064] In addition to limiting the content of each element, this disclosure also limits the range of Ceq values ​​as follows.

[0065] (Ceq: 0.350~0.490) The Ceq value is calculated using the following equation (1).

[0066] Ceq=[C]+[Mn] / 6+[Ni] / 15+[Cu] / 15+[Cr] / 5+[Mo] / 5+[V] / 5 (1) Where [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] represent the contents (mass%) of C, Mn, Ni, Cu, Cr, Mo, and V in the steel, respectively. Substitute zero if the corresponding element is not present.

[0067] In the steel disclosed herein, the Ceq value is set in the range of 0.350 to 0.490. This is an indicator of the hardenability of the steel; the higher the Ceq value, the greater the increase in strength.

[0068] When the Ceq value is within an appropriate range, an excellent balance between strength and toughness is achieved, while also ensuring HAZ toughness. A Ceq value above 0.350 ensures the hardenability of the base material and maintains strength. Conversely, if the Ceq value is below 0.490, the steel strength does not become excessively high, ensuring toughness. Furthermore, the HAZ does not become excessively hardened, ensuring HAZ toughness before and after PWHT.

[0069] By satisfying the above-mentioned numerical range regarding the Ceq value, welded joints with excellent strength and toughness suitable for low-temperature applications can be provided. The Ceq value is preferably 0.355 or higher, 0.360 or higher, or 0.370 or higher. Furthermore, the Ceq value is preferably 0.480 or lower, 0.470 or lower, 0.460 or lower, or 0.450 or lower.

[0070] <Miscellaneous materials> Next, the equivalent circle diameter and the number density of inclusions contained in the steel in this disclosure will be explained.

[0071] (Equivalent circle diameter of inclusions: 0.01μm~0.50μm) In this disclosure, inclusions that effectively inhibit the growth of austenite grains in the weld heat-affected zone are important. The inventors of this disclosure have found that inclusions with a pinning effect that inhibits the growth of austenite grains in the weld heat-affected zone are fine Ti-based nitrides with an equivalent circle diameter of 0.01 μm to 0.50 μm. The equivalent circle diameter of the inclusion is determined by the area of ​​the inclusion measured by TEM observation. The pinning effect of inclusions with an equivalent circle diameter less than 0.01 μm or greater than 0.50 μm is not significant. Sometimes, steel may contain inclusions with an equivalent circle diameter less than 0.01 μm or greater than 0.50 μm, but these inclusions are excluded from the number density measurement.

[0072] (Number density of inclusions: 1.0 × 10⁻⁶) 5 pcs / mm 2 above) In this disclosure, from the viewpoint of promoting the pinning effect, the number density of Ti-based nitrides with an equivalent circle diameter of 0.01 μm to 0.50 μm is 1.0 × 10⁻⁶. 5 pcs / mm 2 The above. The preferred number density of such inclusions is 2.0 × 10⁻⁶. 5 pcs / mm 2 or above 5.0×10 5 pcs / mm 2 In general, the higher the density of such inclusions, the better.

[0073] (Average particle size of inclusions: below 150nm) It is understood that in this disclosure, the finer the particles, the greater the pinning effect. In order to meet the requirements of low-temperature toughness, the average particle size of the Ti-based nitride is below 150 nm.

[0074] Regarding the particle size, composition, and number density of inclusions in the steel disclosed herein, extraction replicas were prepared using test pieces collected from portions of the steel at 1 / 4 t (the distance from the surface in the thickness direction) and 1 / 2 t (the distance from the surface in the thickness direction), and measurements were performed using a transmission electron microscope (TEM) equipped with a characteristic X-ray detector (EDX). For inclusions at least 1000 μm... 2 The number of particles with a size of 0.01 μm to 0.50 μm, measured by the equivalent circle diameter, is determined by the area above. The number of particles per unit area is then used as the number density for each particle.

[0075] Furthermore, regarding the identification of composition, the number of particles can exceed 1000. Identifying each particle individually is a massive undertaking; therefore, at least 50 particles are identified as Ti-based nitrides to determine their proportion. This proportion is then multiplied by the previously determined particle count to calculate the total number of Ti-based nitrides. The average particle size (average equivalent circle diameter) of the Ti-based nitrides is calculated by averaging the values ​​of the 50 identified Ti-based nitrides.

[0076] (Effective grain size of weld heat-affected zone: below 100.0 μm) From the perspective 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 90.0 μm or less, 80.0 μm or less, or 70.0 μm or less.

[0077] It should be noted that the grain size of the weld heat-affected zone was measured using an EBSD device, within a 4mm range between the fusion line (FL) and a position 1mm away from the fusion line (FL+1mm). 2The effective crystal size is calculated by averaging the top 10 of the largest crystal grain sizes measured within the specified region.

[0078] <Mechanical Properties> The welded joint, comprising a base material made of 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 -70°C in the weld heat-affected zone but also maintains excellent low-temperature toughness after PWHT treatment.

[0079] (Tensile strength: 490MPa~720MPa) In this disclosure, the tensile strength of the steel is set to 490–720 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] (Charpy impact energy absorbed at -70°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 150 J or more at -70°C for the weld heat-affected zone. By possessing low-temperature toughness with a Charpy impact absorption energy of 150 J or more at -70°C for the weld heat-affected zone, the welded joint of this disclosure enables tanks manufactured using the welded joint of this disclosure to be suitable for, for example, the transport of liquid carbon dioxide. It should be noted that the Charpy impact absorption energy at -70°C for the weld heat-affected zone is set to the following values: Figure 1A , Figure 1B , Figure 2A , Figure 2B , Figure 3A and Figure 3B As shown, in the base material 10 of the weld joint 20, at a 1 / 4 t portion, specimens 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), respectively, and measurements are performed using these specimens 14. It should be noted that in each figure, 15 represents the weld metal, and 16 represents the weld heat-affected zone (HAZ). 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.

[0081] When taking samples from welded joints with an X-groove, such as Figure 1A As shown, a notch 18 of FL is formed at a position where the weld metal 15 constitutes 50%, the weld heat-affected zone 16 and the base material 10 together constitute 50%. Additionally, as... Figure 1B As shown, the notch 18 of FL+1mm is formed at a position 1mm away from the aforementioned FL on the base material side.

[0082] (Charpy impact energy absorbed at -70°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 (Potentially Welded Heat Attempt) 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 PWHT performed on the aforementioned steel at a holding temperature of 600°C for 2 hours, with a heating and cooling rate of 55°C / h in the temperature range above 425°C, the weld heat-affected zone exhibits preferably a Charpy impact absorption energy of 150 J or more at -70°C.

[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 and reducing the average 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, with the ductile-brittle transition temperature (vTrs) as the crisscross temperature. The Charpy impact test was conducted at three of each of five temperatures, measuring the brittle fracture rate and calculating vTrs. In the Charpy impact test, a V-notch test piece was used, collected from the 1 / 4 t portion of the steel, with the length direction (L direction) parallel to the length of the steel.

[0086] Furthermore, 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 -70°C via a Charpy impact test. The Charpy impact absorption energy is measured using three test pieces, and the result is calculated by averaging them. In the Charpy impact test, if... Figure 1A , Figure 1B , Figure 2A , Figure 2B , Figure 3A , Figure 3B and Figure 4 As shown, V-notch test pieces 14 were taken from welded joints 20 at positions corresponding to 1 / 4 t and 1 / 2 t (not shown) of the 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 as a constituent material for pressure vessels, specifically for tanks for storing and transporting liquefied gas, particularly liquid carbon dioxide.

[0089] (Methods for manufacturing steel) An example of a method for manufacturing the steel disclosed herein will be described. In manufacturing the steel disclosed herein, controlling the conditions of the steelmaking process is crucial in order to control the equivalent circle diameter and number density of Ti-based nitrides. First, a predetermined amount of Al is added for deoxidation while the molten steel temperature is controlled at 1650°C or below. Next, a predetermined amount of Ti is added while the O concentration in the molten steel is controlled at 0.0050% or below, and casting is performed. The time from the addition of Ti to casting is 120 minutes or less.

[0090] Since Ti is also a deoxidizing element, the O concentration in the molten steel before Ti addition needs to be reduced in order to achieve a large dispersion of Ti-based nitrides. In this disclosure, the O concentration in the molten steel before Ti addition is 0.0050% or less. Preferably, the O concentration in the molten steel before Ti addition is 0.0040% or less, 0.0035% or less, or 0.0030% or less.

[0091] The time from Ti addition to casting affects the equivalent circle diameter and number density of Ti-based nitrides. After Ti is added, the particles formed in the molten steel coalesce and become larger. From the viewpoint of suppressing inclusion coarsening and ensuring the number density, the time from Ti addition to casting is, in this disclosure, 120 minutes or less. Preferably, the time from Ti addition to casting is 90 minutes or less, 75 minutes or less, or 60 minutes or less.

[0092] Subsequently, a light reduction is applied at the end of the solidification process of the cast slab to produce a steel billet. Light reduction is a method of applying pressure at the end of the solidification process of the cast slab, which has the effect of dispersing Ti-based nitrides more uniformly along the thickness direction of the plate. From the viewpoint of uniform dispersion of Ti-based nitrides, the reduction rate of light reduction is 2.0% or more. The reduction rate of light reduction is calculated from the difference between the original slab thickness before light reduction and the slab thickness after light reduction.

[0093] Light reduction rate (%) = 100 × ([original slab thickness] - [slab thickness after light reduction]) / [original slab thickness] 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.

[0094] (1) DQT: Direct Quenching (DQ) and 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.

[0095] 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.

[0096] 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.

[0097] 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).

[0098] Recrystallization temperature zone rolling is hot rolling in which the temperature of the workpiece is 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 30% or more, 40% or more, or 50% 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 workpiece rolled at 900°C.

[0099] 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 average grain size of the steel, the cumulative reduction rate in non-recrystallization temperature zone rolling is preferably 30% or more, 40% or more, or 50% 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.

[0100] 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.

[0101] Ar3=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo+0.35 (t-8) In the formula, the element symbols refer to the content (mass%) of each element contained in the steel, and t refers to the thickness of the steel (mm).

[0102] From the viewpoint of promoting the formation of low-temperature phase transformation structures (mainly bainite and martensite) that increase strength, 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 cooling rate is preferred, 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.

[0103] 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.

[0104] (Manufacturing method of welded joint) Next, the manufacturing method (welding method) of the welded joint disclosed herein will be described.

[0105] First, a bevel is formed at the end of the steel (base material) described above. The bevel shape can be, for example, any of the following: single V groove, single bevel groove, single J groove, single U groove, double V groove, double bevel groove, double J groove, or 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 can be appropriately determined based on the welding method and plate thickness. The welding heat input is, for example, approximately 1.0 kJ / mm to 7.0 kJ / mm.

[0106] The following shows the specific welding conditions for flux-coated arc welding, gas-shielded arc welding, and submerged arc welding.

[0107] In flux-coated arc welding, after welding the outer surface, the inner surface is welded after root cutting. With an X-groove, the welding angle for the outer surface is 55°, and for the inner surface it is 70°. The preheating temperature is 125–175°C, and the inter-bead temperature is 125–175°C. N-16 flux-coated welding electrodes with a core diameter of 5.0 mm (manufactured by Nippon Steel Welding Industries Co., Ltd.) are used to weld the material forming the weld metal. Welding is performed with a current of 140–270 A, a voltage of 22–32 V, a linear energy of 1.2–4.5 kJ / mm, and a weld speed of 9–18 cm / min. The number of weld passes depends on the plate thickness.

[0108] In gas-shielded arc welding, CO2 gas or a mixture of Ar and CO2 is used. With a K-groove, the following parameters are used: a groove angle of 10R at the front end, with both the upper and lower layers at 30°; a preheating 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 a gas flow rate of 18–22 L / min for the upper layer. YM-70CS (solid welding wire manufactured by Nippon Steel Welding Industry Co., Ltd.) is used for welding. Welding is performed at a current of 250–270 A, a voltage of 27–33 V, a heat input of 1.5–2.4 kJ / mm, and a welding speed of 30–40 cm / min. The number of weld passes depends on the plate thickness.

[0109] In submerged arc welding, with a semi-V-groove, welding is performed using Y-204B material and NB-250H flux (submerged arc welding material manufactured by Nippon Steel Welding Industries Co., Ltd.) with a groove gap of 10mm, an angle of 30°, a preheating temperature of 100–150°C, and an inter-bead temperature of 100–150°C. Welding is carried out at a current of 630–670A, a voltage of 27–33V, a heat input of 3.5–4.4kJ / mm, and a welding speed of 25–34cm / min. The number of weld passes depends on the plate thickness.

[0110] (Evaluation methods for welded joints) After fabricating the welded joint, such as Figure 1A , Figure 1B , 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, J, U, K, double-sided J, or H, the V-notch at FL is set at the tangent on the weld heat-affected zone side of the fusion line, covering more than 80% of the weld heat-affected zone. Alternatively, when the bevel shape is any of the following: V or X, the V-notch at FL is set such that the weld metal accounts for 50%, and the combined portion of the weld heat-affected zone and base metal accounts for 50%. The V-notch at FL+1 mm is set 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 energy. Therefore, the energy absorbed by the weld heat-affected zone is independent of the bevel shape but depends more on the heat energy.

[0111] Example The following examples illustrate the steel used in this disclosure. However, the conditions in the following examples are merely examples used to confirm the feasibility and effectiveness of this disclosure, and the steel used in this disclosure is not limited to the following examples.

[0112] [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. Underlined items indicate items outside the scope of this disclosure.

[0113] [Table 1] [Table 2] [Manufacturing of welded joints] For steels other than No. 1, 16, and 25-27, a K-groove is fabricated using the aforementioned steels. Gas shielded arc welding (GMAW) with multi-layer overlay is performed, setting the heat input to 2.0 kJ / mm. In GMAW welding, a mixture of Ar and CO2 gas is used, with CO2 at a ratio of 20%. For the K-groove, the bevel angle is 10R at the front, with both the upper and lower layers at 30°. The preheating temperature is 100-150°C, the inter-bead temperature is 100-150°C, the gas flow rate is 17 L / min for the lower layer, and 20 L / min for the upper layer. YM-69F (solid welding wire manufactured by Nippon Steel Welding Industries Co., Ltd.) is used for welding the weld metal. Welding is performed at a current of 260 A, a voltage of 30 V, a heat input of 2.0 kJ / mm, and a welding speed of 35 cm / min. The number of weld passes varies depending on the plate thickness.

[0114] Additionally, for No. 1 and 16, the aforementioned steel was used to create a semi-V-groove, and submerged arc welding (SAW) was performed using multi-layer surfacing to manufacture welded joints. In SAW welding, the groove gap of the semi-V-groove was 10mm, the angle was 30°, the preheating temperature was 100–150°C, the inter-bead temperature was 100–150°C, Y-204B was used for the weld metal, and NB-250H (submerged arc welding material manufactured by Nippon Steel Welding Industries Co., Ltd.) was used for welding. Welding was performed at a current of 650A, a voltage of 30V, a heat input of 4.0J / mm, and a welding speed of 29cm / min. The number of weld passes varied depending on the plate thickness.

[0115] For No. 25 to 27, the aforementioned steel is used to create an X-groove, followed by flux-coated arc welding to produce welded joints. In flux-coated arc welding, after welding the outer surface, the root is removed, and then the inner surface is welded. The welding angle for the outer surface is 55°, and for the inner surface, it is 70°. The preheating temperature is 125–175°C, and the inter-bead temperature is 125–175°C. N-16 flux-coated welding electrodes with a core diameter of 5.0 mm are used to weld the material forming the weld metal. Welding is performed at currents of 190, 220, and 250 A, voltages of 23, 27, and 29 V, and linear energy of 1.6, 3.0, and 4.4 kJ / mm, 16, 12, and 10 cm / min, respectively. The number of weld passes varies depending on the plate thickness.

[0116] [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. It should be noted that in Table 3, "inclusions" refers to "inclusions with an equivalent circle diameter of 0.01 μm to 0.50 μm containing Ti and N".

[0117] Regarding the toughness of the steel, vTrs was measured.

[0118] Regarding the toughness of the weld heat-affected zone (HAZ), a notch was set at FL or FL+1mm on the I side of the K-groove or half-V-groove, and a Charpy impact test was performed at -70℃. Regarding HAZ toughness, the average Charpy impact absorbed energy (KV2) at -70℃ was measured for the manufactured weld joint in its as-weld toughness state, and the average Charpy impact absorbed energy at -70℃ after a PWHT test 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℃.

[0119] 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.

[0120] (1) Base material toughness vTrs obtained from Charpy impact tests at various temperatures (2) Toughness of welded joint Gas-shielded arc welding (GMAW) with a line energy set to 2.0 kJ / mm for multi-layer surfacing, creating K-groove, semi-V-groove, or X-groove joints; submerged arc welding (SAW) with a line energy set to 4.0 kJ / mm for multi-layer surfacing; or flux-coated arc welding with a line energy set to 1.6, 3.0, or 4.4 kJ / mm for multi-layer surfacing, to generate the average Charpy impact absorption energy at -70°C after welding. (3) Post-PWHT toughness of welded joint For fabricating K-grooves, semi-V-grooves, or X-grooves, gas-shielded arc welding (GMAW) with a heat input of 2.0 kJ / mm is used for multi-layer surfacing; submerged arc welding (SAW) with a heat input of 4.0 kJ / mm is used for multi-layer surfacing; or flux-coated arc welding with a heat input of 1.6, 3.0, or 4.4 kJ / mm is used for multi-layer surfacing. After fabricating the weld joint, the average Charpy impact absorbed energy at -70°C is obtained after PWHT with a holding temperature of 600°C, a holding time of 2 hours, and a heating and cooling rate of 55°C / h in the temperature range above 425°C. [Table 3] Examples No. 1 to 18 and 25 to 27 are examples of the present invention, and examples No. 19 to 24 are comparative examples.

[0121] No. 19 has insufficient hardenability and therefore cannot achieve sufficient strength due to its small Ceq ​​value.

[0122] No. 20 Due to the excessively high O concentration in the molten steel before the addition of Ti, the Ti-based nitrides (TiN) become coarse and have a low number density, thus failing to achieve sufficient joint toughness.

[0123] No. 21 Due to the excessive time between the addition of Ti and casting, TiN becomes coarse with low density, resulting in insufficient joint toughness.

[0124] No. 22 Because light pressure was not applied at the end of the solidification period of the cast billet, TiN was not evenly dispersed, and sufficient joint toughness could not be obtained.

[0125] No. 23 Due to the small reduction rate of the light pressure at the end of the solidification of the billet, TiN is not evenly dispersed and sufficient joint toughness cannot be obtained.

[0126] No. 24 It is believed that due to the high amount of Mn, Mn segregates at the grain boundaries during the manufacturing of welded joints, especially during PWHT, resulting in reduced toughness after PWHT.

[0127] In contrast to the comparative examples, all the present invention examples (Nos. 1 to 18 and 25 to 27) not only had the chemical composition and inclusions of the steel properly controlled, with tensile strength of 490 MPa to 720 MPa within an appropriate range, but also had high Charpy impact absorption energy at -70°C in the weld heat-affected zone before and after PWHT, and achieved low-temperature HAZ toughness of 150 J or more.

[0128] Industrial availability The welded joints disclosed herein are suitable, for example, as a constituent material for transport tanks for liquefied carbon dioxide. Additionally, the welded joints disclosed herein can also be used in the manufacture of other welded structures such as buildings, bridges, ships, conduits, marine structures, and pressure vessels (tanks).

[0129] Symbol Explanation 10. Base Material 12 Welding Section 14 test pieces 15 Welding metal 16 Welding Heat Affected Zone The entire disclosure of Japanese Patent Application No. 2023-176328, filed on October 11, 2023, is incorporated herein by reference. All documents, patent applications, and technical standards described in this specification are incorporated herein by reference to the same extent as those specifically described therein.

Claims

1. A welded joint having a base material made of steel and a welded portion, The steel has the following chemical composition, expressed as a percentage by mass: C：0.03%～0.17%、 Si: 0.10%~0.50% Mn: 0.10%~1.70%, P: below 0.025% S: Below 0.0250% Al：0.015%～0.100%、 Ti: 0.005%~0.050% O: Below 0.0100% N：0.0010%～0.0064%、 Ni: 0~1.20%, Cu: 0–0.95%, Cr：0～0.95%、 Mo: 0~1.00%, B：0～0.0050% Nb: 0~0.034%, V：0～0.10%、 Mg: 0–0.020% Ca: 0~0.020%, REM: 0~0.020%, Remaining components: Fe and impurities. And the Ceq expressed by the following equation (1) is 0.350 to 0.

490. The tensile strength of the steel is 490MPa to 720MPa. The number density of inclusions having an equivalent circle diameter of 0.01 μm to 0.50 μm and containing Ti and N in the steel material is 1.0 x 10 5 0 / mm 2 2 at a position at a depth of 1 / 4 of the thickness and a position at a depth of 1 / 2 of the thickness from the surface of the steel material in the thickness direction. The above, In the 1 / 4 and 1 / 2 portions, the average particle size of the Ti and N inclusions is less than 150 nm. 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. Ceq=[C]+[Mn] / 6+[Ni] / 15+[Cu] / 15+[Cr] / 5+[Mo] / 5+[V] / 5 (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: Ni: 0.01%~1.20% Cu: 0.01%~0.95%, Cr：0.01%～0.95%、 Mo: 0.01%~1.00%, 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%~0.034%, 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%~0.020% Ca: 0.0003%~0.020%, and REM: 0.0003%~0.020%.

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 -70°C is above 150J.

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 -70°C of the heat-affected zone at the heat-treated area is 150J or higher.

7. A pressure vessel comprising a welded joint according to any one of claims 1 to 6.

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