Welded joint and method for manufacturing the same

Abstract

To provide a welded joint which has excellent low-temperature toughness and is suppressed in stress corrosion cracking caused by liquefied ammonia, and has 570 MPa or more of tensile strength, and a method for manufacturing the joint.SOLUTION: A welded joint is provided, having has a base metal, a weld metal, and a welding heat affected zone, each of the base metal and the weld metal has a chemical composition of a specific range, the area fraction of a bainite structure from 1 / 4 thickness to 3 / 4 thickness in the sheet thickness direction from the surface of the base metal is 90% or more, Vickers hardness at the 1 mm position in the sheet thickness direction from the surface of the base metal is 230 or less, Vickers hardness in the welding heat affected zone is 230 or less, Vickers hardness of the weld metal is HVW or less (HVW=260-100×Ni), the tensile strength of the base metal and the welded joint is 570 MPa or more, and the absorption energy of the base metal and the weld metal in Charpy impact test is 47 J or more at -40°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a welded joint using high-strength thick steel plate used in welded structures that require weldability and stress corrosion cracking resistance, such as tanks and pressure vessels for storing ammonia or hydrogen sulfide, and a method for manufacturing the same. [Background technology]

[0002] In the future, the use of ammonia fuel will expand in response to carbon neutrality, and this expansion will require the installation of large tanks. To manufacture large tanks, high-strength thick steel plates and welded joints are required, and the use of 570 MPa class (SM570, etc.) and 610 MPa class (SPV490, etc.) thick steel plates is being considered. However, it is known that stress corrosion cracking occurs when low-alloy steel is used in tanks that store liquefied ammonia, which is an obstacle to expanded use.

[0003] Therefore, studies are being conducted on steel plates that are resistant to stress corrosion cracking caused by liquefied ammonia. For example, Patent Document 1 discloses a method for producing steel plates that have a strength of 570 MPa or more, low-temperature toughness at -40°C, and excellent toughness in the weld heat-affected zone by reducing the amount of C in the steel and optimally controlling the amounts of Ti and N. In Patent Document 2, the amount of C in the steel is reduced to achieve a low yield ratio while maintaining a strength of 400N / mm 2 A method for producing steel plates having tensile strengths of at least grade is disclosed. In addition, Patent Document 3 discloses that the amount of C in the steel is reduced and accelerated cooling is applied to reduce the increase in hardness of the steel sheet surface while achieving a hardness of 530 to 610 N / mm 2 A method for producing a steel plate having a tensile strength of In addition, in Patent Document 4, a decarburized layer is formed on the surface of the steel sheet, and a pressure of 60 kgf / mm 2 This publication discloses a method for manufacturing a steel sheet that has high tensile strength and excellent stress corrosion cracking resistance. Furthermore, Patent Document 5 discloses a steel sheet that has excellent stress corrosion cracking resistance achieved by softening the surface of the steel sheet to a surface hardness of 190 or less in Vickers hardness. Patent Documents 6 and 7 disclose methods for manufacturing steel sheets that ensure stress corrosion cracking resistance by optimally controlling the ferrite structure fraction and crystal grain size of the steel sheets and controlling the yield strength to 440 MPa or less. Patent Document 8 discloses a method for manufacturing a steel plate that ensures stress corrosion cracking resistance by optimizing the ferrite structure fraction in the center of the steel plate thickness while suppressing the hardness of the surface layer of the steel plate to 210 or less. Patent Document 9 discloses a method for manufacturing a steel plate that ensures stress corrosion cracking resistance by optimizing the morphology of the surface layer of the steel plate and optimizing the ferrite structure fraction in the center of the steel plate thickness. Patent Document 10 discloses a method for manufacturing a steel plate suitable for a tank for transporting liquefied ammonia, in which the yield ratio of the steel plate is reduced by optimizing the structure of ferrite and the like at the quarter thickness position of the steel plate. Furthermore, Patent Document 11 discloses a method for producing a steel plate that ensures excellent stress corrosion cracking resistance by performing two-stage water cooling during quenching or direct quenching of the steel plate to generate a soft ferrite structure. Furthermore, Patent Document 12 discloses a method for manufacturing a clad steel plate that ensures excellent stress corrosion cracking resistance by joining a clad material having a low carbon and manganese content to the surface of the steel plate by cast cladding or overlay welding. In Patent Document 13, the yield strength of the weld heat affected zone is 45 kgf / mm 2 A method for manufacturing a steel plate that ensures excellent stress corrosion cracking resistance is disclosed, which is obtained by joining the following laminated materials. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-115233 [Patent Document 2] Japanese Patent Application Publication No. 11-131178 [Patent Document 3] Japanese Patent Application Publication No. 10-195533 [Patent Document 4] Japanese Patent Publication No. 61-279631 [Patent Document 5] Japanese Patent Application Publication No. 50-085516 [Patent Document 6] Patent No. 7323090 [Patent Document 7] Patent No. 7323091 [Patent Document 8] Patent No. 7323088 [Patent Document 9] International Publication No. 2021 / 106368 [Patent Document 10] Patent Publication No. 2021-88753 [Patent Document 11] Japanese Patent Publication No. 156228 / 1983 [Patent Document 12] Japanese Patent Publication No. 57-149425 [Patent Document 13] Japanese Patent Application Publication No. 50-085546 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the methods described in Patent Documents 1 to 4 are intended for steel plates having a strength of up to 590 MPa, and although they can ensure excellent stress corrosion cracking resistance in thick steel plates, they do not disclose a method for ensuring the stress corrosion cracking resistance of the weld heat-affected zone and the weld metal when weld joints are prepared in the actual manufacture of tanks. Furthermore, Patent Document 5 does not take into consideration the low temperature toughness of the base metal or the weld metal at all. The methods described in Patent Documents 6 to 10 are based on the premise that the steel sheet contains a soft ferrite structure, and may not be able to produce the 570 MPa-class high-strength steel sheet that is the subject of the present invention. Furthermore, no method is disclosed for ensuring the stress corrosion cracking resistance of the weld heat-affected zone and the weld metal. Patent Document 11 applies two-stage cooling, but requires special water cooling equipment, and does not disclose a method for ensuring the stress corrosion cracking resistance of the weld heat affected zone and the weld metal. Patent Document 12 requires cast cladding or overlay welding, which requires special equipment and has productivity problems. Furthermore, it does not disclose a method for ensuring the stress corrosion cracking resistance of the weld heat affected zone and the weld metal. In Patent Document 13, stress corrosion cracking in the heat-affected zone can be suppressed, but when rolling and heat treatment conditions are selected to increase the strength of the clad base material in order to manufacture a large tank, the hardness of the clad material before welding increases, and stress corrosion cracking performance cannot be suppressed outside the heat-affected zone.

[0006] Furthermore, as a countermeasure against stress corrosion cracking, stainless steel plates or clad steel plates made of stainless steel plates can be used, but this has the problem of being significantly more expensive than using low-alloy steel plates.

[0007] As described above, the methods proposed so far have not disclosed a technology for ensuring the stress corrosion cracking resistance of the base material, weld heat-affected zone, and weld metal of a welded joint with a strength class of 570 MPa or higher (a technology for suppressing stress corrosion cracking caused by liquefied ammonia) without necessarily requiring special equipment.

[0008] An object of the present invention is to provide a welded joint that is excellent in low-temperature toughness, suppresses stress corrosion cracking due to liquefied ammonia, and has a tensile strength of 570 MPa or more, and a method for manufacturing the same.

[0009] Here, excellent low-temperature toughness means that the absorbed energy in a Charpy impact test of the base material (thick steel plate) and weld metal is 47 J or more at -40°C in accordance with JIS Z 2242:2018. In addition, suppressing stress corrosion cracking caused by liquefied ammonia is achieved by removing excess weld metal and processing it into a 5mm thick four-point bending test piece at a position 5mm from the surface of the weld joint in the plate thickness direction under three conditions: the back center of the four-point bending becomes the weld metal, the weld heat-affected zone, and the base material.Then, in a 20L stainless steel autoclave, it is heated in an NH3+CO2+O2 system with an anode current of 10A / m. 2 The test was conducted over a one-month period, and no cracks occurred under all three conditions, indicating excellent stress corrosion cracking resistance. In addition, having a tensile strength of 570 MPa or more means that the tensile strength of the base material (thick steel plate) is 570 MPa or more in accordance with JIS Z 2241:2022, and that the tensile strength of the welded joint is 570 MPa or more in accordance with JIS Z 3121:2013. [Means for solving the problem]

[0010] The present inventors have conducted extensive research to solve the above problems, and have come to the following findings. First, they investigated conditions for suppressing the initiation and subsequent propagation of liquefied ammonia stress corrosion cracking in low-alloy steel, and found that the Vickers hardness of the steel plate on the surface that comes into contact with liquefied ammonia must be 230 or less. Furthermore, they found that the Vickers hardness of the weld heat-affected zone after welding must also be 230 or less. On the other hand, it has become clear that the stress corrosion cracking resistance of weld metal changes depending on the amount of Ni it contains. Specifically, it has been found that it is necessary to keep the Vickers hardness of the weld metal below HVW, which is expressed as 260 - 100 x Ni. In addition, keeping the yield strength of steel below 440 MPa is also considered effective in preventing liquefied ammonia stress corrosion cracking, but it has been found that cracks will occur if the Vickers hardness of the steel plate on the surface that comes into contact with liquefied ammonia exceeds 230.

[0011] The present invention was derived by further studying the above findings, namely, [1] A welded joint having a base material, a weld metal, and a weld heat affected zone, The base material is, in mass%, C: 0.05-0.09%, Si: 0.01 to 0.55%, Mn: 1.00-1.95% P: 0.030% or less, S: 0.010% or less, Al: 0.001 to 0.060%, Nb: 0.006 to 0.060%, N: 0.001 to 0.006% and has a chemical composition in which Pcm represented by the following formula (1) satisfies 0.15 to 0.20, with the remainder consisting of Fe and unavoidable impurities, The weld metal comprises, in mass %, C: 0.04~0.12%, Si: 0.01 to 0.55%, Mn: 1.00-1.95% P: 0.030% or less, S: 0.010% or less, Al: 0.001 to 0.060%, Ti: 0.006 to 0.030%, B: 0.0006~0.0060%, N: 0.001 to 0.006%, O: 0.0100 to 0.0500% Contains moreover, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.80% or less Nb: 0.08% or less, V: 0.08% or less, Ca: 0.006% or less, Mg: 0.006% or less and has a chemical composition in which Pcm represented by the following formula (1) satisfies 0.17 to 0.25, with the balance being Fe and unavoidable impurities, The bainite structure from the surface of the base material to 1 / 4 to 3 / 4 of the thickness in the plate thickness direction area The fraction is 90% or more, The Vickers hardness at a position 1 mm from the surface of the base material in the plate thickness direction is 230 or less, The Vickers hardness in the weld heat affected zone is 230 or less, The Vickers hardness of the weld metal is equal to or less than HVW represented by the following formula (2), The tensile strength of the base material and the welded joint is 570 MPa or more, A welded joint, wherein the absorbed energy of the base metal and the weld metal in a Charpy impact test is 47 J or more at -40°C. Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5×B...Formula (1) HVW=260-100×Ni...Equation (2) In formula (1), the element symbols indicate the content (mass%) of each element contained in the base metal, and the element symbols indicate the content (mass%) of each element contained in the weld metal, with elements that are not contained in either the base metal or the weld metal being represented as 0. In formula (2), Ni represents the Ni content (mass%) contained in the weld metal, and is set to 0 if no Ni is contained. [2] The base material further comprises, in mass%, Cu: 0.50% or less, Ni: 0.30% or less, Cr: 0.50% or less, Mo: 0.50% or less V: 0.08% or less, Ti: 0.05% or less, B: 0.0030% or less, Ca: 0.006% or less, Mg: 0.006% or less The welded joint according to [1] above, containing one or more selected from the following. [3] When hot rolling a slab having the chemical composition of the base material described in [1] or [2], the slab is heated to 1000 to 1250 ° C., After rolling at a cumulative reduction rate of 50% or more at 930°C or less, Hot rolling is completed at a temperature of Ar3 point + 10°C or higher. A method for manufacturing a welded joint, in which accelerated cooling is started at an accelerated cooling start temperature of Ar3 or higher, with the average cooling rate at the surface of the steel plate from 680°C to 600°C being 20 to 80°C / s, and the accelerated cooling is stopped at an accelerated cooling stop temperature of 100°C or higher and 600°C or lower, to obtain a steel plate, and the steel plate is then welded as a base material to obtain a welded joint. [4] The method for manufacturing a welded joint according to [3], wherein the steel plate is cooled to room temperature immediately after the accelerated cooling is stopped or after the accelerated cooling is stopped, and then tempered at a temperature of 500 to 680°C to obtain the steel plate. [5] For the thick steel plate, By mass% C: 0.04~0.12%, Si: 0.01 to 0.55%, Mn: 1.00~2.20%, P: 0.030% or less, S: 0.010% or less, Al: 0.001 to 0.060%, Ti: 0.006 to 0.060%, B: 0.0006~0.0060%, Contains Or even more so, Cu: 0.50% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 1.00% or less Nb: 0.05% or less, V: 0.05% or less, Ca: 0.006% or less, Mg: 0.006% or less The method for producing a welded joint according to [3] or [4] above, wherein the welded joint is obtained by welding using a welding wire having a chemical composition containing one or more selected from the group consisting of: [Effects of the Invention]

[0012] According to the present invention, there are provided a welded joint which has excellent low-temperature toughness, is inhibited from being susceptible to stress corrosion cracking due to liquefied ammonia, and has a tensile strength of 570 MPa or more, and a method for producing the same. DETAILED DESCRIPTION OF THE INVENTION

[0013] The welded joint of the present invention is a welded joint having a base metal, a weld metal, and a weld heat affected zone, wherein the base metal contains, by mass%, C: 0.05 to 0.09%, Si: 0.01 to 0.55%, Mn: 1.00 to 1.95%, P: 0.030% or less, S: 0.010% or less, Al: 0.001 to 0.060%, Nb: 0.006 to 0.060%, and N: 0.001 to 0.006%, and has a chemical composition in which Pcm represented by the following formula (1) satisfies 0.15 to 0.20, with the balance being Fe and unavoidable impurities: The weld metal contains, by mass%, C: 0.04 to 0.12%, Si: 0.01 to 0.55%, Mn: 1.00 to 1.95%, P: 0.030% or less, S: 0.010% or less, Al: 0.001 to 0.060%, Ti: 0.006 to 0.030%, B: 0.0006 to 0.0060%, N: 0.001 to 0.006%, and O: 0.0100 to 0.0500%, and further contains one or more selected from Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.80% or less, Nb: 0.08% or less, V: 0.08% or less, Ca: 0.006% or less, and Mg: 0.006% or less, and the following formula (1) is satisfied: The base metal has a chemical composition in which Pcm, expressed as: satisfies 0.17 to 0.25, with the balance consisting of Fe and unavoidable impurities; the area fraction of bainite structure from 1 / 4 to 3 / 4 of the thickness from the surface of the base metal in the plate thickness direction is 90% or more; the Vickers hardness at a position 1 mm from the surface of the base metal in the plate thickness direction is 230 or less; the Vickers hardness in the weld heat affected zone is 230 or less; the Vickers hardness in the weld metal is HVW or less, expressed by the following formula (2); the tensile strength of the base metal and the weld joint is 570 MPa or more; and the absorbed energy in a Charpy impact test of the base metal and the weld metal is 47 J or more at -40°C. Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5×B...Formula (1) HVW=260-100×Ni...Equation (2) In formula (1), the element symbols indicate the content (mass%) of each element contained in the base metal, and the element symbols indicate the content (mass%) of each element contained in the weld metal, with elements that are not contained in either the base metal or the weld metal being represented as 0. In formula (2), Ni represents the Ni content (mass%) contained in the weld metal, and is set to 0 if no Ni is contained. The thickness of the thick steel plate (base material) constituting the welded joint of the present invention is not particularly limited, but from the viewpoint of use as a liquefied ammonia tank, it is preferably 6 mm or more, more preferably 8 mm or more, and the thickness of the thick steel plate (base material) is preferably 60 mm or less, more preferably 40 mm or less.

[0014] (1) Chemical composition of the base material (steel plate) First, the reasons for limiting the chemical composition of the base material will be explained. Note that "%" in the content of each element means "% by mass."

[0015] C: 0.05 to 0.09% C is an element effective in increasing strength, and for this purpose, the C content must be 0.05% or more. On the other hand, from the viewpoint of improving stress corrosion cracking resistance and suppressing weld cold cracking, the upper limit of the C content is set to 0.09%. The C content is preferably 0.06% or more. Also, the C content is preferably 0.08% or less.

[0016] Si: 0.01 to 0.55% Since Si is an effective element for deoxidation, it is added to improve the yield of Al, which is also a powerful deoxidizer. Si is also effective in increasing strength. To obtain these effects, the Si content must be 0.01% or more. The Si content is preferably 0.03% or more. However, if the Si content exceeds 0.55%, the toughness of the weld heat-affected zone decreases and cold cracking during welding is promoted, so the upper limit of the Si content is set to 0.55%. The Si content is preferably 0.40% or less.

[0017] Mn: 1.00 to 1.95% Mn is an element effective in increasing strength, and to obtain this effect, the Mn content must be 1.00% or more. However, if the Mn content exceeds 1.95%, toughness and weldability deteriorate, so the upper limit of the Mn content is set to 1.95%. To ensure higher performance in all of strength, toughness, and weldability, the upper limit of the Mn content is preferably set to 1.80%, and more preferably to 1.60%.

[0018] P:0.030% or less P is an impurity element that reduces the toughness of the base material and the weld heat-affected zone. If the P content exceeds 0.030%, the desired toughness cannot be obtained, so the upper limit of the P content is set to 0.030%. The P content is preferably 0.025% or less. Although there is no particular lower limit, excessive reduction of P significantly increases the refining cost, so the preferred lower limit of the P content is set to 0.001%.

[0019] S: 0.010% or less S is an impurity element that combines with Mn to form MnS, which reduces the toughness and through-thickness tensile performance of the base material and weld heat-affected zone. If the S content exceeds 0.010%, the desired toughness and through-thickness tensile performance cannot be obtained, so the upper limit of the S content is set to 0.010%. The S content is preferably 0.006% or less, and more preferably 0.003% or less. Although there is no particular lower limit, excessive reduction of S significantly increases the cost of refining, so the preferred lower limit of the S content is 0.001%.

[0020] Al: 0.001 to 0.060% Since Al has a strong deoxidizing effect and is also effective in refining crystal grains, the Al content must be 0.001% or more. The Al content is preferably 0.005% or more. However, since excessive Al reduces toughness, the Al content is set to 0.060% or less. The Al content is preferably 0.040% or less.

[0021] Nb: 0.006 to 0.060% In its solid solution state, Nb improves the hardenability of steel and increases its strength. It also preserves and accumulates dislocations introduced by rolling, and these accumulated dislocations significantly refine the lath length, which is the grain size unit of bainite. Therefore, Nb is an essential element, and is also essential for ensuring the strength of welded joints. For this reason, the Nb content is set to 0.006% or more, preferably 0.010% or more. However, if the Nb content exceeds 0.060%, not only does it reduce the toughness of the weld heat-affected zone, but it may also cause cracks on the surface of the slab during continuous casting. Therefore, the upper limit of the Nb content is set to 0.060%. The preferred upper limit of the Nb content is 0.050%.

[0022] N: 0.001 to 0.006% Since the presence of N reduces the toughness of the base metal and the weld heat-affected zone, a low content is preferable. Since a N content exceeding 0.006% significantly reduces toughness, the N content is set to 0.006% or less. On the other hand, reducing the N content to less than 0.001% requires significantly high refining costs, so the lower limit of the N content is set to 0.001%. The N content is preferably 0.001 to 0.005%.

[0023] Pcm (weld crack susceptibility composition): 0.15-0.20 Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5×B...Formula (1) In formula (1), the element symbols indicate the content (mass%) of each element contained in the base material, and elements that are not contained are represented as 0. Even if the chemical composition is as described above, if the parameter Pcm value expressed by formula (1) exceeds 0.20, cold cracking occurs when welding is performed in air at room temperature, so the Pcm value must be 0.20 or less. On the other hand, if the Pcm value is less than 0.15, the strength is insufficient, so the Pcm value must be 0.15 or more. The Pcm value is preferably 0.16 to 0.20.

[0024] The base metal constituting the weld joint of the present invention may further contain one or more of the following chemical compositions. Cu: 0.50% or less, Ni: 0.30% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.08% or less, Ti: 0.05% or less, B: 0.0030% or less, Ca: 0.006% or less, Mg: 0.006% or less

[0025] Cu: 0.50% or less Cu is effective in increasing strength, so it may be added when particularly high strength is desired. From the viewpoint of increasing strength, when Cu is contained, the lower limit of the Cu content is preferably 0.05%. More preferably, the Cu content is 0.10% or more. However, when the Cu content exceeds 0.50%, the toughness decreases and scratches may occur on the surface of the base material (steel plate). Therefore, when Cu is contained, the Cu content is set to 0.50% or less. The Cu content is preferably 0.30% or less.

[0026] Ni: 0.30% or less Ni is effective in improving toughness and preventing cracking in steel materials containing Cu, so it is preferable to add it, especially when Cu is included. When Ni is included, the lower limit of the Ni content is preferably set to 0.05%. On the other hand, if Ni is included in an amount exceeding 0.30%, the stress corrosion cracking resistance of the base metal deteriorates, and Ni dissolves in the weld metal through dilution, reducing the stress corrosion cracking resistance of the weld metal, so the upper limit of the Ni content is set to 0.30%. Preferably, the Ni content is 0.15% or less.

[0027] Cr:0.50% or less Cr improves hardenability and is effective in increasing strength, so it is advisable to add it when the plate thickness is large or when higher strength is required. When Cr is contained, the lower limit of the Cr content is preferably 0.05%. More preferably, the lower limit of the Cr content is 0.10%. On the other hand, if the Cr content exceeds 0.50%, the toughness of the weld heat affected zone deteriorates, so when Cr is contained, the upper limit of the Cr content must be 0.50%. Preferably, the upper limit of the Cr content is 0.30%.

[0028] Mo: 0.50% or less Mo improves hardenability and is effective in increasing strength, so it is advisable to add it when the plate thickness is large or when higher strength is required. When Mo is contained, the lower limit of the Mo content is preferably 0.05%. More preferably, the lower limit of the Mo content is 0.10%. On the other hand, if the Mo content exceeds 0.50%, the toughness of the weld heat affected zone is deteriorated, so when Mo is contained, the upper limit of the Mo content must be 0.50%. Preferably, the upper limit of the Mo content is 0.30%.

[0029] V:0.08% or less V improves the strength of the base material through precipitation strengthening. From the viewpoint of base material strength, when V is contained, the lower limit of the V content is preferably set to 0.01%. On the other hand, if the V content exceeds 0.08%, the toughness of the base material and the weld heat affected zone deteriorates, so when V is contained, the upper limit of the V content is set to 0.08%. The upper limit of the V content is preferably 0.05%.

[0030] Ti: 0.05% or less Ti not only increases the strength of steel through solid solution strengthening and precipitation strengthening, but also suppresses the growth of austenite grains during slab heating. This effectively reduces the lath length of bainite grains in thick steel plates, which are difficult to refine by rolling alone, thereby improving toughness. It also prevents grain coarsening in the weld heat-affected zone (HAZ) and reduces hardness, thereby improving toughness and preventing cracking. In addition, trace amounts of Ti are effective in suppressing cracking on the surface of continuously cast slabs, which is promoted by Nb, in Nb-containing steels. Therefore, when Ti is contained, the Ti content is preferably 0.01% or more. However, if the Ti content exceeds 0.05%, toughness deteriorates due to precipitation embrittlement. Therefore, when Ti is contained, the upper limit of the Ti content is set to 0.05%. Preferably, the upper limit of the Ti content is 0.03%.

[0031] B: 0.0030% or less Since B is effective in improving hardenability and the resulting increase in strength, it is added when an increase in plate thickness or strength is required. When B is contained, the lower limit of the B content is preferably set to 0.0006%. However, if the B content exceeds 0.0030%, weldability and toughness deteriorate, so when B is contained, the upper limit of the B content must be set to 0.0030%. Preferably, the upper limit of the B content is 0.0020%.

[0032] Ca: 0.006% or less Ca spheroidizes MnS, thereby improving the toughness of the base material, the toughness of the weld heat-affected zone, and the tensile properties in the thickness direction. When Ca is contained, the lower limit of the Ca content is preferably 0.001%. On the other hand, if the Ca content exceeds 0.006%, the Ca clusters, which not only deteriorates the toughness but also the internal quality. Therefore, when Ca is contained, the upper limit of the Ca content is set to 0.006%. Preferably, the upper limit of the Ca content is 0.004%.

[0033] Mg: 0.006% or less Mg spheroidizes MnS, thereby improving the toughness of the base material, the toughness of the weld heat-affected zone, and the tensile properties in the thickness direction. When Mg is contained, the lower limit of the Mg content is preferably set to 0.001%. On the other hand, if the Mg content exceeds 0.006%, the Mg clusters, which not only deteriorates the toughness but also the internal quality. Therefore, when Mg is contained, the upper limit of the Mg content is set to 0.006%. Preferably, the upper limit of the Mg content is 0.004%.

[0034] The remainder of the base material other than the above-mentioned chemical composition is composed of Fe and unavoidable impurities.

[0035] (2) Chemical composition of the weld metal Next, the reasons for limiting the chemical composition of the weld metal of the welded joint will be explained. Note that "%" for the content of each element means "mass %."

[0036] C: 0.04 to 0.12% C is an element effective in increasing strength, and in order to obtain this effect, the C content must be 0.04% or more, and preferably 0.05% or more. On the other hand, from the viewpoint of improving stress corrosion cracking resistance and suppressing weld cold cracking, the upper limit of the C content is set to 0.12%, and the C content is preferably 0.10% or less.

[0037] Si: 0.01 to 0.55% Since Si is an effective element for deoxidation, it is contained to improve the yield of Al, which is also a strong deoxidizer. Si is also effective in increasing strength. To obtain these effects, the Si content must be 0.01% or more. The Si content is preferably 0.03% or more. However, if the Si content exceeds 0.55%, the toughness of the weld metal decreases, so the upper limit of the Si content is set to 0.55%. The Si content is preferably 0.40% or less.

[0038] Mn: 1.00 to 1.95% Mn is an element effective in increasing strength and improving toughness, and to obtain this effect, the Mn content must be 1.00% or more. However, if the Mn content exceeds 1.95%, toughness and weldability deteriorate, so the upper limit of the Mn content is set to 1.95%. In order to ensure higher performance in all of strength, toughness, and weldability, the upper limit of the Mn content is preferably set to 1.80%.

[0039] P:0.030% or less P is an impurity element that reduces the toughness of the weld metal. If the P content exceeds 0.030%, the desired toughness cannot be obtained, so the upper limit of the P content is set to 0.030%. The P content is preferably 0.025% or less. Although the lower limit is not particularly specified, an excessive reduction in P content significantly increases the refining cost, so the preferable lower limit of the P content is set to 0.001%.

[0040] S: 0.010% or less S is an impurity element that combines with Mn to form MnS, thereby reducing the toughness of the weld metal. If the S content exceeds 0.010%, the desired toughness cannot be obtained, so the upper limit of the S content is set to 0.010%. The S content is preferably 0.006% or less, and more preferably 0.003% or less. Although there is no particular lower limit, excessive reduction of S significantly increases the cost of refining, so the preferred lower limit of the S content is 0.001%.

[0041] Al: 0.001 to 0.060% Since Al has a strong deoxidizing effect and is also effective in refining the crystal grains of the weld metal, the Al content must be 0.001% or more. The Al content is preferably 0.005% or more. However, since excessive Al reduces toughness, the Al content is set to 0.060% or less. The Al content is preferably 0.040% or less.

[0042] Ti: 0.006 to 0.030% Ti not only increases the strength of the weld metal through solid solution strengthening and precipitation strengthening, but also neutralizes harmful N present in the weld metal by bonding with it, improving toughness. This effect is manifested when the Ti content is 0.006% or more, so the lower limit of the Ti content is set to 0.006%. The lower limit of the Ti content is preferably 0.010%. On the other hand, if the Ti content exceeds 0.030%, toughness deteriorates due to precipitation embrittlement, so the upper limit of the Ti content is set to 0.030%.

[0043] B: 0.0006 to 0.0060% B improves hardenability, thereby suppressing the formation of coarse grain boundary ferrite in the weld metal and improving the strength and toughness of the weld metal. This effect is not achieved unless the B content is 0.0006% or more, so the lower limit of the B content is set to 0.0006%. However, if the B content exceeds 0.0060%, the weldability and toughness deteriorate, so the upper limit of the B content must be 0.0060%. The B content is preferably 0.0006 to 0.0050%.

[0044] N: 0.001 to 0.006% Since the presence of N reduces the toughness of the weld metal, a low N content is preferable. If the N content exceeds 0.006%, toughness deteriorates significantly, so the N content is set to 0.006% or less. On the other hand, reducing the N content to less than 0.001% requires significantly high costs for manufacturing welding materials and performing welding, so the lower limit of the N content is set to 0.001%. The N content is preferably 0.001 to 0.005%.

[0045] O (oxygen): 0.0100-0.0500% Since the presence of O reduces the toughness of the weld metal, the lower the O content, the better. If the O content exceeds 0.0500%, the toughness deteriorates significantly, so the O content is set to 0.0500% or less. On the other hand, reducing O to less than 0.0100% requires significantly high costs for manufacturing welding materials and performing welding, so the lower limit of the O content is set to 0.0100%. The O content is preferably 0.0100 to 0.0400%.

[0046] Pcm: 0.17~0.25 Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5×B...Formula (1) In formula (1), the element symbols indicate the content (mass%) of each element contained in the weld metal, and elements that are not contained are represented as 0. Even if the chemical composition is as described above, if the parameter Pcm value expressed by formula (1) exceeds 0.25, cold cracking occurs when welding is performed in air at room temperature, so the Pcm value must be 0.25 or less. The Pcm value is preferably 0.23% or less. On the other hand, if the Pcm value is less than 0.17, the strength is insufficient, so the Pcm value must be 0.17 or more. The Pcm value is preferably 0.18% or more.

[0047] The weld metal constituting the weld joint of the present invention further contains one or more of the following chemical compositions. Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.80% or less, Nb: 0.08% or less, V: 0.08% or less, Ca: 0.006% or less, Mg: 0.006% or less

[0048] Cu: 0.50% or less Cu improves hardenability and is effective in increasing the strength of the weld metal, so it is recommended to add it when the plate thickness is large or when higher strength is required. To obtain this effect, the lower limit of the Cu content is preferably 0.05%. More preferably, the lower limit of the Cu content is 0.10%. On the other hand, if the Cu content exceeds 0.50%, the toughness of the weld metal is deteriorated and the occurrence of cracks in the weld metal is promoted, so when Cu is contained, the upper limit of the Cu content must be 0.50%. The Cu content is preferably 0.30% or less.

[0049] Ni: 0.50% or less Ni improves the toughness of the weld metal, and in order to obtain this effect, the lower limit of the Ni content is preferably set to 0.05%. On the other hand, if Ni is contained in an amount exceeding 0.50%, stress corrosion cracking resistance is significantly deteriorated, so when Ni is contained, the upper limit of the Ni content is set to 0.50%, and the Ni content is preferably 0.30% or less.

[0050] Cr:0.50% or less Cr improves hardenability and is effective in increasing the strength of the weld metal, so it is recommended to add Cr when the plate thickness is large or when higher strength is required. To obtain this effect, the lower limit of the Cr content is preferably 0.05%. More preferably, the lower limit of the Cr content is 0.10%. On the other hand, if the Cr content exceeds 0.50%, the toughness of the weld metal deteriorates, so when Cr is contained, the upper limit of the Cr content must be 0.50%, and the upper limit of the Cr content is preferably 0.30%.

[0051] Mo: 0.80% or less Mo improves hardenability and is effective in increasing the strength of the weld metal, so it is recommended to add Mo when the plate thickness is large or when higher strength is required. To obtain this effect, the lower limit of the Mo content is preferably set to 0.05%. More preferably, the lower limit of the Mo content is set to 0.10%. On the other hand, if the Mo content exceeds 0.80%, the toughness of the weld metal deteriorates, so the upper limit must be set to 0.80%. The upper limit of the Mo content is preferably 0.60%, and more preferably 0.50%.

[0052] Nb: 0.08% or less Nb improves the strength of the weld metal through precipitation strengthening, and to obtain this effect, the lower limit of the Nb content is preferably set to 0.01%. On the other hand, if the Nb content exceeds 0.08%, the toughness of the weld metal deteriorates, so the upper limit is set to 0.08%. The Nb content is preferably 0.01 to 0.05%.

[0053] V:0.08% or less V improves the strength of the weld metal through precipitation strengthening, and to obtain this effect, the lower limit of the V content is preferably set to 0.01%. On the other hand, if the V content exceeds 0.08%, the toughness of the weld metal deteriorates, so the upper limit is set to 0.08%. The V content is preferably 0.01 to 0.05%.

[0054] Ca: 0.006% or less Ca spheroidizes MnS, thereby improving the toughness of the weld metal. To obtain this effect, the lower limit of the Ca content is preferably set to 0.001%. On the other hand, if the Ca content exceeds 0.006%, not only will the Ca cluster and deteriorate the toughness but also the internal quality of the weld, so when Ca is contained, the upper limit of the Ca content is set to 0.006%. The Ca content is preferably 0.001 to 0.004%.

[0055] Mg: 0.006% or less Mg improves the toughness of the weld metal by spheroidizing MnS. To obtain this effect, the lower limit of the Mg content is preferably set to 0.001%. On the other hand, if the Mg content exceeds 0.006%, the Mg clusters and not only deteriorates the toughness but also the internal quality of the weld, so when Mg is contained, the upper limit of the Mg content is set to 0.006%. The Mg content is preferably 0.001 to 0.004%.

[0056] The balance of the weld metal other than the above-mentioned chemical composition is Fe and unavoidable impurities.

[0057] (3) Microstructure and mechanical properties of the base material (steel plate) Area fraction of bainite structure from the surface of the base material (thick steel plate) to 1 / 4 to 3 / 4 of the thickness in the plate thickness direction: 90% or more The present invention aims to produce a welded joint having a tensile strength of 570 MPa or more. To achieve this, the desired tensile strength cannot be obtained unless the base material contains 90% or more of a bainite structure from the surface of the base material to 1 / 4 to 3 / 4 of the thickness in the plate thickness direction. Therefore, the area fraction of the bainite structure is set to 90% or more. The area fraction of the bainite structure is preferably 95% or more. The area fraction of the bainite structure may be 100%. It is preferable that the microstructure fraction be the same in regions other than the 1 / 4 to 3 / 4 thickness from the surface of the steel plate in the thickness direction. However, in this region, structures other than bainite may form due to surface decarburization or air cooling between hot rolling and the start of accelerated cooling. Since this has little effect on strength, it is acceptable for the microstructure fraction to be non-equivalent. Note that bainite also includes cementite and island martensite present between the bainite laths, but does not include cementite, ferrite, or martensite outside the bainite laths. These can be distinguished by taking a sample from the base material, mirror-polishing it, and etching it with 4 to 5 vol% nital, and then observing it with an optical microscope at a magnification of approximately 200 to 500 times.

[0058] Vickers hardness at a position 1 mm from the surface of the base material (thick steel plate) in the thickness direction: 230 or less The present invention aims to manufacture welded joints with excellent resistance to stress corrosion cracking caused by liquefied ammonia. High surface hardness of the base material makes stress corrosion cracking more likely to occur, and if the Vickers hardness exceeds 230, the initiation and propagation of stress corrosion cracking caused by liquefied ammonia cannot be suppressed, potentially resulting in destruction of the tank during service. Therefore, the upper limit of the Vickers hardness at a position 1 mm from the surface in the plate thickness direction is set to 230. The upper limit of the Vickers hardness is preferably 220, and more preferably 210 when considering the suppression of stress corrosion cracking itself. While the position for evaluating hardness is preferably closer to the surface, the position 1 mm from the surface in the plate thickness direction is set as a location where the Vickers test can be performed stably and accurate measurement values ​​can be evaluated. The Vickers test is performed in accordance with JIS Z 2244:2020.

[0059] Tensile strength of base material (thick steel plate): 570 MPa or more The present invention aims to manufacture welded joints suitable for large ammonia tanks of around 40,000 tons. To achieve this, a base metal strength of 570 MPa or more is required, so the lower limit of the tensile strength of the base metal is set to 570 MPa. The tensile strength of the base metal is preferably 590 MPa or more, and more preferably 610 MPa or more. The tensile strength is obtained by a tensile test carried out in accordance with JIS Z 2241:2022.

[0060] Absorbed energy in Charpy impact test of base material (thick steel plate): 47J or more at -40℃ Ammonia liquefies at -35°C under normal pressure. Therefore, the low-temperature toughness of steel must be lower than -35°C, so the Charpy impact test is conducted at -40°C. The test temperature is preferably -50°C, more preferably -55°C. The absorbed energy is 47J or greater, as specified in JIS G 3115:2016, the main standard used for high-strength tanks, and is the average value of three test pieces. Preferably, the average absorbed energy is 47J or greater, with each test piece being 27J or greater. Although no special impact test method is specified, the same method as specified in JIS G 3115:2016, the main standard used for high-strength tanks, is generally used, with samples taken at 1 / 4 the thickness of the base material and perpendicular to the rolling direction. The absorbed energy in the Charpy impact test is measured in accordance with JIS Z 2242:2018.

[0061] (4) Mechanical properties of the weld heat affected zone Vickers hardness in the weld heat affected zone: 230 or less The present invention aims to manufacture a welded joint with excellent resistance to stress corrosion cracking caused by liquefied ammonia. High hardness in the welded heat-affected zone (HAZ) makes it more susceptible to stress corrosion cracking. If the Vickers hardness in the HAZ exceeds 230, the initiation and propagation of stress corrosion cracking caused by liquefied ammonia cannot be suppressed, potentially resulting in destruction of the tank during service. Therefore, the upper limit of the Vickers hardness in the HAZ is set to 230. The upper limit of the Vickers hardness is preferably 220, and more preferably 210 when considering the suppression of stress corrosion cracking itself. While the location for evaluating hardness is preferably closer to the surface, the location for evaluating hardness is not particularly specified because there is no significant difference in the hardness of the HAZ through the thickness of the welded joint. The hardness evaluation location is preferably 1 mm from the surface. The Vickers test is performed in accordance with JIS Z 2244:2020.

[0062] (5) Mechanical properties of weld metal Vickers hardness of weld metal: HVW or less HVW=260-100×Ni...Equation (2) In formula (2), Ni represents the Ni content (mass %) contained in the weld metal, and is set to 0 if no Ni is contained. The present invention aims to manufacture a welded joint with excellent resistance to stress corrosion cracking caused by liquefied ammonia. High weld metal hardness increases the susceptibility to stress corrosion cracking, but the extent of stress corrosion cracking initiation and propagation depends on the Ni content in the weld metal. If the Vickers hardness of the weld metal exceeds HVW, as expressed by formula (2), the initiation and propagation of stress corrosion cracking caused by liquefied ammonia cannot be suppressed, potentially resulting in destruction of the tank during service. Therefore, the upper limit of the Vickers hardness of the weld metal is set to HVW. The upper limit of the Vickers hardness of the weld metal is preferably HVW-10, and, considering the need to suppress the occurrence of stress corrosion cracking itself, the upper limit of the Vickers hardness is more preferably HVW-20.

[0063] The closer to the surface layer side the Vickers hardness evaluation position is, the more preferable it is. However, since there is no significant difference in the hardness of the weld metal in the thickness direction of the welded joint, the evaluation position for Vickers hardness is not particularly specified. Since the weld metal may be ground and removed to improve fatigue performance, the portion that protrudes from the base metal is preferably evaluated at a position in the thickness direction that is the same as a position 1 mm from the surface of the base metal. The Vickers test is performed in accordance with JIS Z 2244:2020.

[0064] Charpy impact energy of weld metal: 47J or more at -40℃ Ammonia liquefies at -35°C under normal pressure. Therefore, the low-temperature toughness of steel must be lower than -35°C, so the Charpy impact test is conducted at -40°C. The test temperature is preferably -50°C, more preferably -55°C. The absorbed energy is 47J or greater, as specified in JIS G 3115:2016, the main standard used for high-strength tanks, and is the average value of three test pieces. Preferably, the average absorbed energy is 47J or greater, with each individual test value being 27J or greater. While the Charpy impact test method is not specifically specified, the same method as specified in JIS G 3115:2016, the main standard used for high-strength tanks, is generally used, with samples taken from 1 / 4 the thickness of the base material in a direction perpendicular to the weld line. The absorbed energy in the Charpy impact test is measured in accordance with JIS Z 2242:2018.

[0065] (6) Mechanical properties of welded joints Tensile strength of welded joint: 570 MPa or more The present invention aims to manufacture a welded joint suitable for large ammonia tanks with a tank capacity of approximately 40,000 tons. To achieve this, a strength of 570 MPa or more is required, so the lower limit of the tensile strength of the welded joint is set to 570 MPa. The tensile strength of the welded joint is preferably 590 MPa or more, and more preferably 610 MPa or more. The strength of the welded joint is evaluated using tensile test specimens taken perpendicular to the weld line. The tensile strength of the welded joint is obtained by a tensile test performed in accordance with JIS Z 3121:2013.

[0066] (7) Manufacturing method of welded joints Next, a method for manufacturing a welded joint according to the present invention will be described. In the method for producing a welded joint of the present invention, when hot rolling a slab having the chemical composition of the base metal described above, the slab is heated to 1000 to 1250°C, and is rolled at a cumulative reduction rate of 50% or more at 930°C or less, and then the hot rolling is completed at a temperature of Ar3 point + 10°C or more, Accelerated cooling is started at an accelerated cooling start temperature of Ar3 or higher, with the average cooling rate at the surface of the steel plate from 680°C to 600°C being 20 to 80°C / s. Accelerated cooling is stopped at an accelerated cooling stop temperature of 100°C to 600°C to produce a steel plate, and the steel plate is then welded as the base material to obtain a welded joint.

[0067] As details of the method for manufacturing a welded joint of the present invention, the manufacturing method of a thick steel plate (base material) for obtaining a welded joint, the welding method, and the welding wire (chemical composition of the welding wire) will be described below in that order. (7-1) Manufacturing method of thick steel plate (base material) First, a method for manufacturing a steel plate will be described. Note that, unless otherwise specified, the temperatures described below refer to the surface temperatures of the slab or steel plate.

[0068] Heating temperature: 1000~1250℃ If the heating temperature is less than 1000°C, the crystal grains before rolling become too fine and Nb does not dissolve, making it impossible for the base material to obtain the desired strength, so the lower limit of the heating temperature is set to 1000°C. If the heating temperature exceeds 1250°C, the crystal grains before rolling become too coarse, reducing the toughness of the base material, so the upper limit of the heating temperature is set to 1250°C. The heating temperature is preferably 1050 to 1200°C.

[0069] Cumulative reduction rate at 930°C or less in hot rolling: 50% or more The presence of high-density dislocations is desirable to nucleate ferrite from within austenite grains and suppress the growth of bainite laths. Therefore, rolling is performed with a cumulative reduction of 50% or more in the partial recrystallization and non-recrystallization temperature ranges of austenite. When the Nb-added steel of the present invention is produced at the above heating temperature, the upper limit of the partial recrystallization range is 930°C, so the upper limit is set to 930°C. While a larger reduction is better, a cumulative reduction of 50% or more is necessary to obtain the desired strength and toughness, so the lower limit of the cumulative reduction is set to 50%. The lower limit of the cumulative reduction is preferably 55%, more preferably 60%.

[0070] Rolling end temperature: Ar3 point + 10℃ or more The steel plate manufactured in accordance with the present invention has various base material components restricted to reduce the hardness of the weld heat-affected zone while maintaining a strength of 570 MPa. Therefore, it is necessary to select hot rolling conditions that facilitate maintaining the required strength. Since the desired strength cannot be achieved if the rolling end temperature is below the Ar3 point + 10°C, the lower limit of the rolling end temperature is set to the Ar3 point + 10°C. While there is no particular upper limit, the rolling end temperature is preferably 930°C or lower, and more preferably 910°C or lower. The Ar3 point (°C) is calculated using the formula based on the chemical composition of the steel: Ar3 = 910-310C-80Mn-20Cu-55Ni-15Cr-80Mo (°C) (wherein the formula, the element symbols indicate the content (mass%) of each element in the steel, and elements that are not contained are set to 0).

[0071] Accelerated cooling start temperature: Ar3 point or higher The steel plate manufactured in accordance with the present invention has various base material components restricted to reduce the hardness of the weld heat-affected zone while maintaining a tensile strength of 570 MPa. Therefore, it is necessary to select accelerated cooling conditions that facilitate maintaining strength. If the accelerated cooling start temperature is below the Ar3 point, a large amount of ferrite forms, preventing the desired strength from being achieved. Therefore, the lower limit of the accelerated cooling start temperature is set to the Ar3 point. While there is no particular upper limit, the accelerated cooling start temperature is preferably 900°C or lower, and more preferably 850°C or lower.

[0072] Accelerated cooling rate: Average cooling rate on the surface of the steel plate from 680°C to 600°C: 20 to 80°C / s The cooling rate of accelerated cooling affects surface hardness and strength. After examining the definition of the cooling rate that correlates highly with surface hardness, it was found that the average cooling rate defined from 680°C to 600°C on the steel plate surface correlates highly with surface hardness. If this average cooling rate is less than 20°C / s, the desired tensile strength cannot be obtained, so the lower limit of the average cooling rate is set to 20°C / s. On the other hand, if the average cooling rate exceeds 80°C / s, the desired surface hardness cannot be obtained, so the upper limit of the average cooling rate is set to 80°C / s. The average cooling rate is preferably 20 to 70°C / s, and more preferably 20 to 60°C / s. The average cooling rate is derived by heat conduction calculation from the relationship between the cooling start temperature, cooling stop temperature, and the time required between them. Specifically, the average cooling rate is obtained by dividing the difference (°C) between the cooling start temperature and cooling stop temperature by the cooling time (s).

[0073] Accelerated cooling stop temperature: 100~600℃ The accelerated cooling stop temperature affects the strength of the base material. If the accelerated cooling stop temperature exceeds 600°C, the desired strength cannot be obtained, so the accelerated cooling stop temperature is set to 600°C or less. If the accelerated cooling stop temperature is below 100°C, strain due to cooling increases and cracks occur after shear cutting due to hydrogen that has condensed during rolling, so the lower limit of the accelerated cooling stop temperature is set to 100°C. The accelerated cooling stop temperature is preferably 100 to 550°C. The accelerated cooling stop temperature is set to the temperature at which the temperature distribution in the plate thickness direction becomes approximately uniform due to reheating after accelerated cooling.

[0074] Tempering temperature: 500~680℃ The steel plate (base material) produced in the present invention can be tempered after the accelerated cooling is stopped. Tempering can be performed by reheating using a method such as induction heating immediately after the accelerated cooling is stopped, or by cooling (air-cooling) once to room temperature (-5 to 50°C) and then reheating in a heating furnace or the like. Specifically, in the present invention, the steel plate may be tempered to a temperature of 500 to 680°C immediately after the accelerated cooling is stopped, or after the accelerated cooling is stopped and the steel plate is cooled to room temperature. Here, there is no particular limitation on the term "immediately reheating", but it is preferable to reheat within 1000 seconds after the accelerated cooling is stopped. If the tempering temperature is less than 500°C, the toughness of the base material will actually deteriorate, so the lower limit of the tempering temperature is set to 500°C. The lower limit of the tempering temperature is preferably 520°C. Furthermore, if the tempering temperature exceeds 680°C, the desired strength will not be obtained, so the upper limit of the tempering temperature is set to 680°C. The upper limit of the tempering temperature is preferably 650°C.

[0075] (7-2) Welding method The welding method is not particularly specified, but a welding method commonly used for manufacturing large tanks is selected. Examples include shielded metal arc welding, CO2 welding, MAG welding, MIG welding, TIG welding, submerged arc welding (SAW), electrogas welding, and electroslag welding. Among the chemical compositions of the weld metal, N and O are affected by the welding method. While the N and O ranges of the present invention are easily achievable with conventional welding methods, it is necessary to select a welding method that can obtain the desired N and O contents. Furthermore, in the case of submerged arc welding, the welding flux also affects the chemical composition of the weld metal, such as B, so it is necessary to select a welding method that can obtain the desired N and O contents.

[0076] (7-3) Chemical composition of welding wire The chemical composition of the welding wire required to form the weld metal of the welded joint of the present invention is specified. The welded joint of the present invention can be obtained by welding the thick steel plate (base material) described above using the welding wire. Note that the filler metal in TIG welding and the iron core of the welding rod in covered metal arc welding are also included in this welding wire. Hereinafter, the unit % for the chemical composition of the welding wire means % by mass.

[0077] C: 0.04 to 0.12% C is an element effective in increasing strength, and for this purpose, the C content in the welding wire needs to be 0.04% or more, and the C content is preferably 0.05% or more. On the other hand, in view of stress corrosion cracking resistance and weld cold cracking resistance, the upper limit of the C content is set to 0.12%, and the preferred range of the C content is 0.10% or less.

[0078] Si: 0.01 to 0.55% Since Si is an effective element for deoxidation, it is contained to improve the yield of Al, which is also a powerful deoxidizer. Si is also effective in increasing strength. To obtain these effects, the Si content in the welding wire must be 0.01% or more. The Si content is preferably 0.03% or more. However, if the Si content exceeds 0.55%, the toughness of the weld metal decreases, so the upper limit of the Si content is set to 0.55%, and the Si content is preferably 0.40% or less.

[0079] Mn: 1.00 to 2.20% Mn is an element effective in increasing strength and improving toughness, and to obtain this effect, the Mn content in the welding wire must be 1.00% or more. However, if the Mn content exceeds 2.20%, toughness and weldability deteriorate, so the upper limit of the Mn content is set to 2.20%. In order to ensure higher performance in all of strength, toughness, and weldability, it is preferable that the upper limit of the Mn content be set to 2.00%.

[0080] P:0.030% or less P is an impurity element that reduces the toughness of the weld metal. If the P content in the welding wire exceeds 0.030%, the desired toughness cannot be obtained, so the upper limit of the P content is set to 0.030%. The P content is preferably 0.025% or less. Although the lower limit is not particularly specified, an excessive reduction in P content significantly increases the refining cost, so the preferred lower limit of the P content is 0.001%.

[0081] S: 0.010% or less S is an impurity element that combines with Mn to form MnS, thereby reducing the toughness of the weld metal. If the S content in the welding wire exceeds 0.010%, the desired toughness cannot be obtained, so the upper limit of the S content is set to 0.010%. The S content is preferably 0.006% or less, and more preferably 0.003% or less. Although the lower limit of the S content is not particularly specified, an excessive reduction in S content significantly increases the cost of refining, so the preferable lower limit of the S content is set to 0.001%.

[0082] Al: 0.001 to 0.060% Since Al has a strong deoxidizing effect and is also effective in refining the crystal grains of the weld metal, the Al content in the welding wire must be 0.001% or more, and the Al content is preferably 0.005% or more. However, since an excessive amount of Al leads to a decrease in toughness, etc., the Al content is set to 0.060% or less, and preferably 0.040% or less.

[0083] Ti: 0.006 to 0.060% Ti not only increases the strength of the weld metal through solid solution strengthening and precipitation strengthening, but also neutralizes harmful N present in the weld metal by bonding with it, improving toughness. This effect is achieved when the welding wire contains 0.006% or more Ti, so the lower limit of the Ti content is set to 0.006%. The Ti content is preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.060%, the toughness deteriorates due to precipitation embrittlement, so the upper limit of the Ti content is set to 0.060%, and the Ti content is preferably 0.050% or less.

[0084] B: 0.0006 to 0.0060% B improves hardenability, thereby suppressing the formation of coarse grain boundary ferrite in the weld metal and improving the strength and toughness of the weld metal. This effect is not achieved unless the welding wire contains 0.0006% or more B, so the lower limit of the B content is set to 0.0006%. However, if the B content exceeds 0.0060%, the weldability and toughness deteriorate, so the upper limit of the B content must be 0.0060%. The B content is preferably 0.0006 to 0.0050%.

[0085] The welding wire for producing the welded joint of the present invention may further contain one or more of the following chemical compositions. Cu: 0.50% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, Nb: 0.05% or less, V: 0.05% or less, Ca: 0.006% or less, Mg: 0.006% or less

[0086] Cu: 0.50% or less Cu improves hardenability and is effective in increasing the strength of the weld metal, so it is recommended to add it when the plate thickness is large or when higher strength is required. To obtain this effect, the lower limit of the Cu content is preferably 0.05%. The Cu content is more preferably 0.10% or more. On the other hand, if the Cu content exceeds 0.50%, the toughness of the weld metal deteriorates and the occurrence of cracks in the weld metal is promoted, so when Cu is contained, the upper limit of the Cu content is set to 0.50%. The Cu content is preferably 0.30% or less.

[0087] Ni: 1.00% or less Ni is effective in improving toughness, and in order to obtain this effect, the lower limit of the Ni content is preferably set to 0.05%. On the other hand, if the Ni content exceeds 1.00%, the stress corrosion cracking resistance of the weld metal deteriorates, so the upper limit of the Ni content is set to 1.00%, and the Ni content is preferably 0.50% or less.

[0088] Cr:1.00% or less Cr improves hardenability and is effective in increasing the strength of the weld metal, so it is recommended to add it when the plate thickness is large or when higher strength is required. To obtain this effect, the lower limit of the Cr content is preferably 0.05%. The Cr content is more preferably 0.10% or more. On the other hand, if the Cr content exceeds 1.00%, the toughness of the weld metal deteriorates, so when Cr is contained, the upper limit of the Cr content is set to 1.00%. The Cr content is preferably 0.80% or less, and more preferably 0.50% or less.

[0089] Mo: 1.00% or less Mo improves hardenability and is effective in increasing the strength of the weld metal, so it is recommended to add it when the plate thickness is large or when higher strength is required. To obtain this effect, the lower limit of the Mo content is preferably 0.05%. The Mo content is more preferably 0.10% or more. On the other hand, if the Mo content exceeds 1.00%, the toughness of the weld metal deteriorates, so when Mo is contained, the upper limit of the Mo content is set to 1.00%. The Mo content is preferably 0.80% or less, and more preferably 0.50% or less.

[0090] Nb: 0.05% or less Nb improves the strength of the weld metal through precipitation strengthening, and to obtain this effect, the lower limit of the Nb content is preferably set to 0.01%. On the other hand, if the Nb content exceeds 0.05%, the toughness of the weld metal deteriorates, so when Nb is contained, the upper limit of the Nb content is set to 0.05%, and the Nb content is preferably 0.04% or less.

[0091] V:0.05% or less V improves the strength of the weld metal through precipitation strengthening, and to obtain this effect, the lower limit of the V content is preferably set to 0.01%. On the other hand, if the V content exceeds 0.05%, the toughness of the weld metal deteriorates, so when V is contained, the upper limit of the V content is set to 0.05%. The V content is preferably 0.04% or less.

[0092] Ca: 0.006% or less Ca improves the toughness of the weld metal by spheroidizing MnS. To obtain this effect, the lower limit of the Ca content is preferably set to 0.001%. On the other hand, if the Ca content exceeds 0.006%, the Ca clusters and not only deteriorates the toughness but also the internal quality of the weld, so when Ca is contained, the upper limit of the Ca content is set to 0.006%, and the Ca content is preferably 0.004% or less.

[0093] Mg: 0.006% or less Mg improves the toughness of the weld metal by spheroidizing MnS. To obtain this effect, the lower limit of the Mg content is preferably set to 0.001%. On the other hand, if the Mg content exceeds 0.006%, the Mg clusters and not only deteriorates the toughness but also the internal quality of the weld, so when Mg is contained, the upper limit of the Mg content is set to 0.006%, and the Mg content is preferably 0.004% or less.

[0094] The chemical composition of the welding wire is based on the above-mentioned composition, with the remainder being Fe and unavoidable impurities. [Example]

[0095] Next, the present invention will be described in more detail based on examples. The following examples are intended to illustrate preferred examples of the present invention, and the present invention is not limited to these examples in any way. First, slabs having the chemical compositions shown in Table 1 were produced by continuous casting. Next, the obtained slabs were reheated under the conditions in Table 3, and immediately subjected to hot rolling and accelerated cooling. Some of the steel sheets after accelerated cooling (water cooling) were tempered.

[0096] Next, the prepared thick steel plate (base material) was welded using the welding wire shown in Table 2 under the conditions shown in Table 3. Here, the welding wire includes the filler metal when performing TIG welding and the iron core of the welding rod when performing covered metal arc welding. In all cases, thick steel plates (base materials) with a 30° X groove were butted together, and multiple passes were performed with the welding heat input shown in Table 3 until the groove was filled. The chemical composition of the prepared weld metal is shown in Table 4.

[0097] [Table 1]

[0098] [Table 2]

[0099] [Table 3]

[0100] [Table 4]

[0101] Material test pieces were taken from the manufactured welded joints and tested, and the results are summarized in Table 5.

[0102] [Table 5]

[0103] [Vickers hardness of base material] A sample was taken from the base metal portion of the obtained welded joint at a cross section perpendicular to the rolling direction so that the test surface was at a depth of 1 mm from the surface, and after mirror polishing the surface of the test piece, the Vickers hardness was measured in accordance with JIS Z 2244: 2020. A load of 10 kgf was used for the measurement, and the test was performed at five points, with the maximum value being the Vickers hardness.

[0104] [Tensile strength of base material] Test pieces were taken from the base metal of the obtained welded joints and processed into JIS Z 2241:2022 No. 5 tensile test pieces. The test pieces were taken in a direction perpendicular to the rolling direction, with the longitudinal direction of the test piece. A tensile test was conducted on the taken test pieces to measure the tensile strength.

[0105] [Absorbed energy in Charpy impact test of base material] Test specimens were taken from the base metal portion of the resulting welded joints and processed into 2mm V-notch Charpy impact test specimens in accordance with JIS Z 2242:2018. The specimens were taken perpendicular to the rolling direction and from a position at or close to 1 / 4 of the thickness in the plate thickness direction. 10mm x 10mm test specimens were used when the base metal thickness was 12mm or more, and 10mm x 7.5mm test specimens were used when the base metal thickness was less than 12mm. The Charpy impact test was performed at -40°C, and the impact absorption energy was measured.

[0106] [Base material microstructure] A sample was taken from the base metal of the resulting welded joint so that the cross section parallel to the rolling direction served as the observation surface. The observation surface of the sample was mirror-polished and then etched with 5 vol% nital. A 1000 μm × 1000 μm region of the etched surface was photographed with an optical microscope at 200x magnification, and the area fraction of the bainite structure was calculated from the surface to the thickness direction from 1 / 4 to 3 / 4 of the thickness by image processing. Note that bainite includes cementite and island martensite present between the bainite laths, but does not include cementite, ferrite, or martensite outside the bainite laths. Image processing was performed by tracing the areas of the microstructure other than the bainite structure in black on the photographed image and calculating the ratio of the area of ​​the black portion to the area of ​​the entire image.

[0107] [Vickers hardness of weld heat affected zone] A macro sample was taken from a cross section perpendicular to the weld line of the obtained welded joint, and the surface of the test piece was mirror-polished so that the test positions were 1 mm deep from the front and back surfaces of the welded joint excluding the excess reinforcement.The Vickers hardness was then measured in accordance with JIS Z 2244: 2020. A load of 10 kgf was used for the measurement, and measurements were taken at three points on each of four locations on both sides of the front and back surfaces of the welded heat-affected zone, and the maximum value of the 12 points was taken as the Vickers hardness of the welded heat-affected zone.

[0108] [Vickers hardness of weld metal] A macro sample was taken from a cross section perpendicular to the weld line of the obtained welded joint, and the surface of the test piece was mirror-polished so that the test positions were 1 mm deep from the front and back surfaces of the welded joint excluding the excess reinforcement, and then the Vickers hardness was measured in accordance with JIS Z 2244: 2020. A load of 10 kgf was used for the measurement, and measurements were made at five points on each of two locations on the front and back surfaces of the weld metal, and the maximum value of the 10 points was taken as the Vickers hardness of the weld metal.

[0109] [Tensile strength of welded joints] Test pieces were taken from the obtained welded joint in a direction perpendicular to the weld line direction, and JIS Z 3121:2013 No. 1 tensile test pieces were taken from the entire thickness of the welded joint and processed. A tensile test was conducted on the taken test pieces to measure the tensile strength.

[0110] [Absorbed energy in Charpy impact tests on weld metal] Test pieces were taken from the resulting welded joints and machined into 2mm V-notch Charpy impact test pieces in accordance with JIS Z 2242:2018. The test pieces were taken so that the notch was in the center of the weld metal, perpendicular to the weld line, and at or near 1 / 4 of the thickness in the plate thickness direction. A 10mm x 10mm test piece was used when the base metal thickness was 12mm or more, and a 10mm x 7.5mm test piece was used when the base metal thickness was less than 12mm. The Charpy impact test was performed at -40°C, and the impact absorption energy was measured.

[0111] [Liquefied ammonia stress corrosion cracking test] Stress corrosion tests using liquefied ammonia were carried out using the four-point bending test prescribed in "Nakai et al.: Iron and Steel, 67th year, No. 14, pp. 2226-2233 (1981)". The excess weld metal was removed, and 5mm thick test specimens were taken from the weld joint surface at a position 5mm in the plate thickness direction. The test specimens were processed under three conditions, with the back center of the four-point bending becoming the weld metal, the weld heat affected zone, and the base metal. The test was carried out in a 20L stainless steel autoclave using an NH3+CO2+O2 system with an anode current of 10A / m. 2The test was carried out over a one-month period, and specimens that showed visible cracks were deemed to have failed, while specimens that showed no cracks were deemed to have passed.

Claims

1. A welded joint having a base material, a weld metal, and a weld heat affected zone, The base material comprises, in mass %, C: 0.05-0.09%, Si: 0.01 to 0.55%, Mn: 1.00-1.95%, P: 0.030% or less, S: 0.010% or less, Al: 0.001-0.060%, Nb: 0.006-0.060%, N: 0.001-0.006% and has a chemical composition in which Pcm represented by the following formula (1) satisfies 0.15 to 0.20, with the balance being Fe and unavoidable impurities, The weld metal comprises, in mass %, C: 0.04-0.12%, Si: 0.01 to 0.55%, Mn: 1.00-1.95%, P: 0.030% or less, S: 0.010% or less, Al: 0.001-0.060%, Ti: 0.006 to 0.030%, B: 0.0006 to 0.0060%, N: 0.001-0.006%, O: 0.0100-0.0500% Contains moreover, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.80% or less, Nb: 0.08% or less, V: 0.08% or less, Ca: 0.006% or less, Mg: 0.006% or less and has a chemical composition in which Pcm represented by the following formula (1) satisfies 0.17 to 0.25, with the balance consisting of Fe and unavoidable impurities: the area fraction of bainite structure in the thickness direction from the surface of the base material to ¼ of the thickness is 90% or more, The Vickers hardness at a position 1 mm from the surface of the base material in the plate thickness direction is 230 or less, The Vickers hardness in the weld heat affected zone is 230 or less, The Vickers hardness of the weld metal is equal to or less than HVW represented by the following formula (2), The tensile strength of the base material and the welded joint is 570 MPa or more, A welded joint, wherein the absorbed energy of the base metal and the weld metal in a Charpy impact test is 47 J or more at −40° C. Pcm=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5×B...Formula (1) HVW=260-100×Ni...Formula (2) In formula (1), the element symbols indicate the content (mass%) of each element contained in the base metal, for the base metal, and indicate the content (mass%) of each element contained in the weld metal, with elements that are not contained in either the base metal or the weld metal being represented as 0. In formula (2), Ni represents the Ni content (mass %) contained in the weld metal, and is set to 0 if no Ni is contained.

2. The base material further comprises, in mass %, Cu: 0.50% or less, Ni: 0.30% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.08% or less, Ti: 0.05% or less, B: 0.0030% or less, Ca: 0.006% or less, Mg: 0.006% or less The welded joint according to claim 1, comprising one or more selected from the following:

3. When hot rolling a slab having the chemical composition of the base material according to claim 1 or 2, the slab is heated to 1000 to 1250°C, After rolling at a temperature of 930°C or less with a cumulative reduction rate of 50% or more, The hot rolling is completed at a temperature of Ar3 point + 10°C or higher. At an accelerated cooling start temperature of Ar3 point or higher, accelerated cooling is started so that the average cooling rate from 680 ° C. to 600 ° C. on the surface of the thick steel plate is 20 to 80 ° C. / s; A method for manufacturing a welded joint, comprising: stopping accelerated cooling at an accelerated cooling stop temperature of 100°C or higher and 600°C or lower to form a thick steel plate; and welding the thick steel plate as a base material to obtain a welded joint.

4. The method for manufacturing a welded joint according to claim 3, wherein the steel plate is obtained by cooling the steel plate to room temperature immediately after the accelerated cooling is stopped or after the accelerated cooling is stopped, and then tempering the steel plate to a temperature of 500 to 680 ° C.

5. For the thick steel plate, By mass% C: 0.04-0.12%, Si: 0.01 to 0.55%, Mn: 1.00-2.20%, P: 0.030% or less, S: 0.010% or less, Al: 0.001-0.060%, Ti: 0.006 to 0.060%, B: 0.0006 to 0.0060%, Contains Or even more so, Cu: 0.50% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, Nb: 0.05% or less, V: 0.05% or less, Ca: 0.006% or less, Mg: 0.006% or less 4. The method for producing a welded joint according to claim 3, wherein the welded joint is obtained by welding using a welding wire having a chemical composition containing one or more selected from the group consisting of:

6. For the thick steel plate, By mass% C: 0.04-0.12%, Si: 0.01 to 0.55%, Mn: 1.00-2.20%, P: 0.030% or less, S: 0.010% or less, Al: 0.001-0.060%, Ti: 0.006 to 0.060%, B: 0.0006 to 0.0060%, Contains Or even more so, Cu: 0.50% or less, Ni: 1.00% or less, Cr: 1.00% or less, Mo: 1.00% or less, Nb: 0.05% or less, V: 0.05% or less, Ca: 0.006% or less, Mg: 0.006% or less 5. The method for producing a welded joint according to claim 4, wherein the welded joint is obtained by welding using a welding wire having a chemical composition containing one or more selected from the group consisting of:

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