Welded joint of steel sheet and method for manufacturing same

Abstract

Provided are: a welded joint of a steel plate, which has a plate thickness of 50 mm or more, has a sound welded metal that does not have welding defects such as high-temperature cracks and poor fusion, and has both high strength and excellent low-temperature toughness; in a welded joint of a steel plate having a plate thickness T of 50 mm or more, a weld metal is composed of a single layer and a single track, a first ratio [A / p] of a cross-sectional area A in a direction perpendicular to a weld line of the weld metal to the number of weld layers p is 120.0 mm2 / layer or less, and a second ratio [H / W] of a height H to a width W of each layer of the weld metal is 1.00 or less.

Description

Technical Field

[0001] The present invention relates to a welded joint of a steel plate and a method for manufacturing the same, and in particular to a welded joint of a steel plate having a thickness of 50 mm or more subjected to single-layer single-pass submerged arc welding and a method for manufacturing the same. Background Art

[0002] With the development of the energy industry, for example, in order to improve the efficiency of marine resource mining and power generation, the size of marine structures and offshore wind turbines built on the ocean is being promoted. As marine structures and offshore wind turbines become larger, the strength of the foundation of these equipment is required to be improved. Therefore, as an example of the material used for the foundation, the use of steel plates with a thickness of 50 mm or more (hereinafter also referred to as "thick plates") has been studied. In addition, in the cold ocean, it is also conceivable that waves, drifting ice, etc. will collide with the thick plates used for structures, etc. Therefore, for the above-mentioned thick plates, excellent toughness at -40°C is also required. In addition, when manufacturing marine structures, offshore wind turbine equipment, etc. using such thick plates, improving their construction efficiency is an important issue, and in particular, it is required to weld the above-mentioned thick plates with high efficiency.

[0003] High heat input welding has been proposed as one of the most efficient methods for welding thick plates. However, when high heat input welding is used to weld thick plates together, it is often difficult to maintain the toughness of the weld metal and the heat-affected zone.

[0004] Therefore, studies have been conducted in the past to improve the construction efficiency when welding thick plates together and to improve the toughness of the weld metal. For example, Patent Document 1 discloses a "double-electrode single-side single-pass high-heat input submerged arc welding method with excellent toughness of the weld metal." The welding method disclosed in Patent Document 1 is a method for welding steel plates having a thickness of 40 mm or more by double-electrode submerged arc welding using a flux and welding wires of a first electrode and a second electrode, wherein the steel plates contain, by mass%, C: 0.02-0.2%, Si: 0.01-1%, Mn: 0.1-2.5%, Al: 0.002-0.1%, N: 0.001-0.015%, P: 0.02% or less, S: 0.01% or less, O: 0.01% or less, and the balance being Fe and unavoidable impurities, and the flux contains, by mass%, SiO2: 10-25%, MgO The present invention relates to a method for welding a first electrode and a second electrode comprising a first electrode and a second electrode, wherein the first electrode and the second electrode comprise, by mass%, C: 0.02-0.2%, Si: 0.01-1%, Mn: 0.5-2.5%, Al: 0.002-0.1%, Ti: 0.005-0.3%, N: 0.001-0.015%, P: 0.02% or less, S: 0.01% or less, and O: 0.01% or less, with the balance being Fe and unavoidable impurities. Furthermore, in this method, welding is performed under conditions where the diameter of the second electrode wire is 6-8 mm and the ratio of the cross-sectional area of ​​the first electrode wire to the cross-sectional area of ​​the second electrode wire is 35-75%.

[0005] Patent Document 2 also discloses a "double-electrode high-heat input submerged arc welding method." The welding method described in Patent Document 2 uses a welding wire containing, by mass%, 0.02-0.18% C, 0.02-0.5% Si, 1.15-2.2% Mn, 0.1-1.0% Mo, 0.1-1.5% Ni, 0.005-0.05% Ti, 0.006% or less P, 0.003% or less S, with the balance consisting of Fe and unavoidable impurities, and a flux composed of 13-25% SiO2, 8-20% MgO, 5-13% CaO, 1-7% CaF2, 9-23% Al2O3, 3-11% TiO2, 11-25% Fe, 0.1-0.6% B2O3, 1-4.3% Mo, and 1-4.5% Ni. This welding method improves the mechanical properties of the weld metal and enhances welding workability, even in high-heat-input submerged arc welding (SHAW) with a heat input of 500 kJ / cm or more. Furthermore, it significantly enhances the safety of building structures and significantly improves welding efficiency.

[0006] For example, Patent Document 3 discloses a "high-efficiency welding method for thick steel plates." The method described in Patent Document 3 includes: a processing step for forming an X-groove on a pair of steel materials with a thickness greater than 50 mm and less than 100 mm; and a welding step for performing single-pass welding on the pair of steel materials using multi-electrode submerged arc welding with two or more and six or less electrodes, using flux. Furthermore, in this welding step, the welding current for the first electrode is set to an AC current with a waveform ratio of 60% to 90%, and the welding current for the other electrodes is set to an AC current with a waveform ratio of 70% or more or a negative DC current. Furthermore, the flux contains, by mass ratio relative to the total mass of the flux, Al2O3: 10% to 50%, SiO2: 16% to 30%, and a combined total of 10% to 60% of one or more of MgO, TiO2, CaF2, and MnO. The MgO content is limited to 40% or less, the TiO2 content to 20% or less, the CaF2 content to 30% or less, and the MnO content to 20% or less. This submerged arc welding method produces weld metal with excellent toughness even at low temperatures, enabling efficient fabrication of foundations for wind turbine generator systems installed in cold regions.

[0007] Meanwhile, narrow groove welding has been proposed as a highly efficient method for welding thick plates. By reducing the cross-sectional area of ​​the groove, the weld heat input can be reduced, ensuring the toughness of the weld metal and heat-affected zone. However, when welding thick plates using narrow groove welding alone, high-temperature cracking and poor fusion are common problems.

[0008] To address this issue, for example, Patent Document 4 discloses a "narrow groove submerged arc welding method." The welding method disclosed in Patent Document 4 is a narrow groove submerged arc welding method for extremely thick steel plates. The method involves placing the backside of a narrow groove with a groove angle of 1 to 5° and a root gap of 10 to 14 mm against a welding pad, and then spreading steel particles within the groove from the backside to a height of 10 to 15 mm. Then, using a molten flux containing, by mass % relative to the total mass of the flux, a total of 50 to 70% Al2O3, TiO2, and CaF2, and 10 to 20% SiO2, with the remainder consisting of CaO, MnO, BaO, K2O, Na2O, and unavoidable impurities, narrow groove submerged arc welding is performed by single-layer, single-pass, multi-layer overlay welding. Patent Document 4 provides excellent slag stripping properties and weld bead shape, resulting in excellent weldability. Furthermore, narrow groove submerged arc welding can be performed without root cleaning. In addition, a sound welded portion free from welding defects such as high-temperature cracks, slag inclusions, and poor fusion can be obtained efficiently.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent No. 4673710

[0012] Patent Document 2: Japanese Patent Application Laid-Open No. 2006-212676

[0013] Patent Document 3: International Publication No. 2013 / 073565

[0014] Patent Document 4: Japanese Patent Application Laid-Open No. 2021-126696 Summary of the Invention

[0015] Problems to be solved by the invention

[0016] However, the purpose of the invention of Patent Document 1, which aims to achieve high-efficiency construction by high-input energy welding, is to provide a high-toughness welding method that can obtain a 2mm V-notch Charpy absorbed energy of 70J or more at 0°C. That is, the thick plate welded by the method described in Patent Document 1 may not be suitable for use in a cold environment of -40°C. In Patent Document 2, the toughness evaluation of the weld metal part is carried out at 0°C, and the use of the thick plate welded by the method described in Patent Document 2 in a cold environment of -40°C is not envisaged. In addition, in the invention of Patent Document 3, the low-temperature toughness of the heat-affected zone is not studied, and it is unclear whether the thick plate welded by the method described in Patent Document 3 can be used in a cold environment of -40°C. In addition, the invention of Patent Document 4, which aims to achieve high-efficiency construction by narrow-groove welding, does not mention the mechanical properties of the weld metal and the heat-affected zone.

[0017] The object of the present invention is to solve the problems of the above-mentioned prior art and provide a welded joint of a steel plate and a method for manufacturing the same, wherein the welded joint of the steel plate has a plate thickness of more than 50 mm, has a sound weld metal without welding defects such as high-temperature cracks and poor fusion, and has both high strength and excellent low-temperature toughness.

[0018] It should be noted that the "high strength" mentioned here means that the yield strength (0.2% proof stress) of the weld metal produced in accordance with the regulations of JIS Z 3111:2005 at room temperature is 325 MPa or more, and its tensile strength is 520 MPa or more, and the tensile strength of the weld joint at room temperature is 520 MPa or more. In addition, "excellent low-temperature toughness" means that the absorbed energy (absorbed energy) of the Charpy impact test at a test temperature of -40°C for the weld metal and heat-affected zone of the weld joint produced in accordance with the regulations of JIS Z3128:2017 is v E -40 ) is 30J or more.

[0019] Methods used to solve problems

[0020] [Process of Completion of the Invention]

[0021] To achieve the above-mentioned object, the present inventors first conducted in-depth research on the weld metal shape of welded joints that can efficiently and soundly weld steel plates (hereinafter sometimes referred to as "base metal") having a plate thickness of 50 mm or more. The welding length of marine structures and offshore wind power foundations may exceed 1.0 km per structure in terms of total length. In order to improve efficiency, it is effective to reduce the number of weld metal layers. However, in conventional welding methods, since welding is distributed along the groove wall, the welding construction efficiency is not high. In contrast, the present invention has obtained the following insight: by constructing multiple layers of weld metal through welding of a single layer and a single pass of weld metal without distributing welding, welding construction can be performed efficiently. However, it should be noted that the low-temperature toughness of the weld metal and the heat-affected zone decreases when the welding line energy is large.

[0022] Next, we studied the above problem and came to the following conclusion: If the deposited area of ​​the weld metal is 120.0 mm per layer, 2 If the welding heat input is below 100mm, the heat effect on the base metal will not be too large, and the thermal effect on the base metal will be sufficiently small, so that a weld with excellent low temperature toughness can be obtained. In addition, the following findings were obtained: when forming a multi-layer weld metal by single-layer single-pass welding, even if the deposited area of ​​each layer is set to 120.0mm 2 If the second ratio [H / W] of the height H (mm) to the width W (mm) of the weld metal in each weld layer exceeds 1.00, hot cracking is more likely to occur. This is because during the solidification process of the weld metal, the solidification direction and the direction of tensile strain during cooling do not align, causing hot cracking to occur at the junction of dendrites. Therefore, it was found that controlling the second ratio [H / W] to 1.00 or less can suppress the occurrence of hot cracking.

[0023] Furthermore, an in-depth study was conducted on the groove shape of the weld that can meet the above requirements, that is, high efficiency and excellent low-temperature toughness. As a result, the following insights were obtained: if it is single-sided welding, when the gap between the parent materials is set to 6 mm or less and the groove angle is set to 20 ° or less, a weld that is high efficiency and excellent low-temperature toughness can be obtained. In addition, the following insights were obtained: if it is double-sided welding, when the gap between the parent materials is set to 10 mm or less and the groove angle is set to 20 ° or less, a weld that is high efficiency and excellent low-temperature toughness can be obtained. However, it was found that when the gap between the parent materials exceeds 6 mm, poor fusion may occur during the welding of the initial layer. This resulted in the following insights: regarding the groove shape that can efficiently and soundly weld steel plates of more than 50 mm, whether it is single-sided welding or double-sided welding, it is preferred that the gap between the parent materials is 6 mm or less and the groove angle is 20 ° or less. In addition, the following insights were obtained: by welding the steel plates to each other using double-sided welding, the groove area can be further reduced.

[0024] [Gist of the Invention]

[0025] The present invention has been completed as a result of further studies based on this knowledge, and the gist of the present invention is as follows.

[0026] [1] A welded joint of steel plates having a thickness T (mm) of 50 mm or more, wherein the weld metal is composed of multiple layers of a single layer and a single pass, and the cross-sectional area A (mm) of the weld metal in a direction perpendicular to the weld line is 2 ) and the first ratio of the number of weld layers p (layers) [A / p] is 120.0 mm 2 / layer or less, and a second ratio [H / W] of the height H (mm) to the width W (mm) of each layer of the weld metal is 1.00 or less.

[0027] [2] The welded joint of steel plates according to [1], wherein the weld metal is formed by multi-layer cladding using a narrow groove with a groove angle of less than 20° and a maximum groove width between the steel plates of less than 50% of the plate thickness T.

[0028] [3] A welded joint of a steel plate according to [1] or [2], wherein the chemical composition of the steel plate has a chemical composition containing, in mass %, C: 0.04-0.14%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.020% or less, Nb: 0.001-0.100%, Al: 0.001-0.100%, O: 0.0100% or less, and N: 0.0100% or less, with the balance being Fe and unavoidable impurities, and Ceq defined by the following formula (2) and the plate thickness (T) satisfying 0.0004T+0.25≤Ceq≤0.0004T+0.45 ... (1),

[0029] The dislocation density ρ at a position 1 mm below the surface of the steel plate is 4.0×10 14 m -2 The steel plate has an average grain size of 15.0 μm or less at a position 1 mm below the surface, and an average grain size of 20.0 μm or less at a center position of the plate thickness.

[0030] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15…(2)

[0031] The symbol of each element in the above formula (2) represents the content (mass %) of the element, and is set to 0 when the element is not contained.

[0032] [4] The welded joint of the steel plate according to [3], wherein the chemical composition of the steel plate further contains, in mass %, one or more selected from the group consisting of Cu: 2.000% or less, Ni: 2.500% or less, Cr: 1.500% or less, Mo: 1.000% or less, Ti: 0.100% or less, V: 0.300% or less, B: 0.0100% or less, W: 0.500% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, and REM: 0.0500% or less.

[0033] [5] The welded joint of the steel plate according to any one of [1] to [4], wherein the chemical composition of the weld metal contains, in mass%, C: 0.04-0.14%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.020% or less, Nb: 0.001-0.100%, Al: 0.001-0.100%, Cu: 0.001-2.000%, Ni: 0.001-2.500%, Cr: 0.001-1.500%, Mo: 0.001-1.000%, Ti: 0.001-0.100%, V: 0.001-0.300%, B: 0.001-0.020%, O: 0.050% or less, and N: 0.010% or less, with the balance being Fe and unavoidable impurities.

[0034] [6] The welded joint of steel plates according to [5], wherein the chemical composition of the weld metal further contains, in mass %, one or more selected from the group consisting of W: 0.500% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, and REM: 0.0500% or less.

[0035] [7] A method for manufacturing a welded joint of a steel plate, which is the method for manufacturing a welded joint of a steel plate according to any one of [1] to [6], wherein welding is performed by generating an arc using two or more welding electrodes using submerged arc welding.

[0036] Effects of the Invention

[0037] The present invention is a welded joint for steel plates having a thickness of 50 mm or more, which has a sound weld metal free from welding defects such as high-temperature cracking and poor fusion, and is a narrow-groove welded joint with both high strength and excellent low-temperature toughness, and has a significant effect in the industry.

[0038] It should be noted that the welded joints of the steel plates of the present invention are not limited to marine structures and offshore wind turbines, but can also be applied to shipbuilding, line pipes, construction, and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a schematic cross-sectional view showing the shape of the weld metal of the weld joint of the present invention.

[0040] Figure 2 Schematic cross-sectional views illustrating examples of the groove shape of a welded joint according to the present invention, for a plate thickness of 50 mm. (a) V-groove, gap = 6 mm; (b) V-groove, gap = 3 mm; (c) X-groove, gap = 6 mm; (d) X-groove, gap = 3 mm; (e) V-groove, gap = 0 mm; (f) X-groove, gap = 0 mm.

[0041] Figure 3 Schematic cross-sectional views illustrating examples of the groove shape of a welded joint according to the present invention, for a plate thickness of 100 mm. (a) V-groove, gap = 6 mm; (b) V-groove, gap = 3 mm; (c) X-groove, gap = 6 mm; (d) X-groove, gap = 3 mm; (e) V-groove, gap = 0 mm; (f) X-groove, gap = 0 mm.

[0042] Figure 4 Schematic cross-sectional views illustrating examples of the groove shape of the welded joint according to the present invention, for a plate thickness of 150 mm. (a) V-groove, gap = 8 mm; (b) V-groove, gap = 3 mm; (c) X-groove, gap = 8 mm; (d) X-groove, gap = 3 mm; (e) V-groove, gap = 0 mm; (f) X-groove, gap = 0 mm. DETAILED DESCRIPTION

[0043] The present invention will be described in detail below. However, it should be noted that the present invention is not limited to the following embodiments.

[0044] [Cross-sectional shape of welded joint]

[0045] The welded joint of the present invention is a butt welded joint of steel plates having a thickness T (mm) of 50 mm or more, wherein the weld metal is composed of multiple layers of a single layer and a single pass, and the cross-sectional area A (mm) of the weld metal in the direction perpendicular to the weld line is 2 ) and the first ratio of the number of weld layers p (layers) [A / p] is 120.0 mm 2 / layer or less, and a second ratio [H / W] of the height H (mm) to the width W (mm) of each layer of the weld metal is 1.00 or less.

[0046] In the present invention, unlike conventional welding methods in which welding is performed in a distributed manner along the groove wall, the weld metal is constructed by welding a single layer of weld metal in a single pass, thereby enabling highly efficient welding.

[0047] based on Figure 1 The cross-sectional shape of the above-mentioned welded joint will be described.

[0048] like Figure 1 As shown, the cross-sectional area A (mm) of the entire weld metal 2 in the direction perpendicular to the weld line 3 is 2 ) The welding layer is L1 (primary layer) ~ L p The cross-sectional area per layer in the case of construction of the number of layers p (outermost layer), that is, the first ratio [A / p] is set to 120.0 mm 2 This is because if the deposited area of ​​the weld metal 2 is set to 120.0 mm per layer, 2 Below, the welding heat input will not be too large, the thermal effect on the base material, that is, the steel plate 1 is sufficiently small, and the weld metal 2 with excellent low temperature toughness can be obtained. It is preferable to set the first ratio [A / p] to 60.0 mm 2 / layer or less. It is further preferred to set the first ratio [A / p] to 45.0 mm 2 / below the floor.

[0049] Furthermore, the second ratio [H / W] of the height H (mm) to the width W (mm) of each layer of the weld metal 2 is set to be 1.00 or less. For example, if the i-th weld layer (L i ) is set to H i , set the width to W i , then the second ratio [H i / W i ] i is 1.00 or less for any layer from the first layer (initial layer) to the pth layer (outermost layer). Here, the height H of the weld layer represents the distance between the upper and lower parts of each layer, and the width W represents the width of the weld metal 2 at the center line of the height H (the distance between the fusion lines). The height H of each layer is measured by cross-sectional macroscopic observation, etc. i and width W i , find the second ratio [H i / W i If the second ratio [H / W] exceeds 1.00, hot cracking may be more likely to occur. This is because during the solidification process of the weld metal 2, the solidification direction aligns with the direction of tensile strain during cooling, causing hot cracking to occur at the junction of dendrites. Therefore, by controlling the maximum value of the second ratio [H / W] of each layer to 1.00 or less, the occurrence of hot cracking can be suppressed. The second ratio [H / W] is preferably 0.90 or less.

[0050] [Bevel shape]

[0051] Next, the weld metal 2 of the weld joint of the present invention is weld metal 2 formed by multi-layer overlay welding using a single layer and a single pass with a narrow groove. In the present invention, a narrow groove means a groove angle of 20° or less, and a maximum groove width between the aforementioned base materials, i.e., between the butted steel plates 1, of 50% or less of the aforementioned plate thickness T. It should be noted that the maximum groove width between the aforementioned base materials corresponds to the maximum groove width between the steel plates in this embodiment.

[0052] In the case of single-sided welding, when the gap between the parent materials is set to 6 mm or less and the groove angle is set to 20° or less, the generation of high-temperature cracks can be suppressed. In addition, in the case of double-sided welding, when the gap between the parent materials is set to 10 mm or less and the groove angle is set to 20° or less, the generation of high-temperature cracks can be suppressed. However, when the gap between the parent materials exceeds 6 mm, poor fusion may occur during the welding of the initial layer. Therefore, with regard to the groove shape of the steel plate 1 that can be efficiently and soundly welded with a thickness of 50 mm or more, whether it is single-sided welding or double-sided welding, it is preferred that the gap between the parent materials be set to 6 mm or less and the groove angle be set to 20° or less. Furthermore, the groove angle is more preferably set to 4 to 20°. It should be noted that in the case of double-sided welding, the groove area can be constructed to be smaller, so single-sided welding is more preferred in this regard. In addition, as Figure 2 、 Figure 3 and Figure 4 As shown in (e) and (f), respectively, when the gap is 0 mm, that is, when the base metals are in contact, the width of the weld metal 2 becomes smaller than the height of the weld metal 2, and the weld bead shape approaches a pear shape, as will be made clear in the examples described later. Consequently, welding strain concentrates in the final solidification portion of the weld metal 2, where impurities segregate and lower the melting point. This results in high-temperature cracking and observed poor fusion during welding. Therefore, the gap is preferably larger than 0 mm. More preferably, the gap is 2 mm or greater.

[0053] [Chemical composition of steel plate]

[0054] Next, the steel plate 1, which serves as the base material of the welded joint of the present invention, will be described. Examples of the steel plate 1 include carbon steel and low-alloy steel. Next, the chemical composition will be described. Unless otherwise specified, "%" in the chemical composition below refers to "mass %."

[0055] [Basic composition of steel plates]

[0056] The chemical composition of the steel plate 1 of the welded joint of the present invention has the following basic composition.

[0057] The steel plate 1 preferably has a chemical composition containing C: 0.04-0.14%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.020% or less, Nb: 0.001-0.100%, Al: 0.001-0.100%, O: 0.0100% or less, and N: 0.0100% or less, with the balance being Fe and inevitable impurities.

[0058] The reasons why the chemical composition of the steel sheet 1 in the present invention is specified as above will be described below.

[0059] [C: 0.04~0.14%]

[0060] C is the element that increases the strength of the steel plate 1 most cheaply and is also an element that contributes to the strengthening of the austenite grain boundaries. When the C content is less than 0.04%, the grain boundary strength of the austenite decreases, causing hot cracks in the steel billet, thereby significantly reducing manufacturability. Furthermore, the target strength of the present invention may not be achieved. On the other hand, when the C content exceeds 0.14%, the weldability of the steel plate 1 may decrease, and the toughness may also decrease. Therefore, the C content is preferably set to 0.04 to 0.14%. It should be noted that the C content is more preferably set to 0.05 to 0.12%.

[0061] [Si: 0.03-0.70%]

[0062] Si is an element effective for deoxidation, but a Si content of less than 0.03% may not provide sufficient effects. On the other hand, a Si content exceeding 0.70% may reduce the weldability of the steel sheet 1. Therefore, the Si content is preferably set to 0.03% to 0.70%. It is further preferred that the Si content be set to 0.04% to 0.60%.

[0063] [Mn: 0.30-2.50%]

[0064] Mn is an element that can improve the hardenability and strength of steel at low cost. To achieve this effect, a Mn content of 0.30% or more is preferably used. On the other hand, a Mn content exceeding 2.50% may reduce the weldability of the steel plate 1. Therefore, the Mn content is preferably set to 0.30-2.50%. It should be noted that the Mn content is more preferably set to 0.50-2.20%.

[0065] [P: 0.030% or less]

[0066] P is an element that significantly embrittles grain boundaries. A high P content can sometimes reduce the toughness of the steel plate 1. Therefore, the P content is preferably set to 0.030% or less. Furthermore, the P content is more preferably set to 0.025% or less. On the other hand, a lower P content is preferred, so the lower limit of the P content is not particularly limited and can be 0%. However, P is an element that is inevitably contained in the steel plate 1 as an impurity. Excessively low P content increases refining time and refining costs. Therefore, the P content is preferably set to 0.001% or more.

[0067] [S: 0.020% or less]

[0068] Since S may reduce the toughness of the steel plate 1, the S content is preferably set to 0.020% or less. Furthermore, it is more preferably set to 0.010% or less. On the other hand, the lower the S content, the better, so the lower limit of the S content is not particularly limited and may be 0%. However, S is an element inevitably contained in the steel plate 1 as an impurity. Excessively low S content increases refining time and refining costs, so the S content is preferably set to 0.0001% or more.

[0069] [Nb: 0.001~0.100%]

[0070] Nb is an element that has the effect of suppressing recrystallization when strain is applied to the austenite structure through solid solution Nb and finely precipitated NbC, and increasing the non-recrystallization temperature range to the high temperature side. To achieve this effect, Nb is preferably contained at 0.001% or more. On the other hand, when the Nb content exceeds 0.100%, the weldability of the steel plate 1 may deteriorate. Therefore, the Nb content is preferably set to 0.001-0.100%. It should be noted that the Nb content is more preferably set to 0.005-0.075%. The Nb content is particularly preferably set to 0.005-0.050%.

[0071] [Al: 0.001~0.100%]

[0072] Al is an element that is effective as a deoxidizer and has the effect of forming nitrides, thereby reducing the austenite grain size. To achieve this effect, the Al content is preferably set to 0.001% or more. On the other hand, if the Al content exceeds 0.100%, the cleanliness of the steel material and steel plate 1 will decrease, resulting in the possibility of reduced ductility and toughness of the steel plate 1. Therefore, the Al content is preferably set to 0.001-0.100%. It should be noted that the Al content is more preferably set to 0.005-0.080%.

[0073] [O: 0.0100% or less]

[0074] O is an element that reduces the ductility and toughness of the steel sheet 1, so the O content is preferably set to 0.0100% or less. On the other hand, the lower the O content, the better, so the lower limit of the O content is not particularly limited and can be 0%. However, O is an element that is inevitably contained in the steel sheet 1 as an impurity. If the O content is too low, it will increase the refining time and refining costs. Therefore, the O content is preferably set to 0.0005% or more.

[0075] [N: 0.0100% or less]

[0076] Nitrogen is an element that reduces the ductility and toughness of the steel sheet 1, so the N content is preferably set to 0.0100% or less. On the other hand, the lower the N content, the better, so the lower limit of the N content is not particularly limited and can be 0%. However, since N is an element that is inevitably contained in the steel sheet 1 as an impurity, the industrial content can be greater than 0%. It should be noted that an excessively low N content increases refining time and refining costs, so the N content is preferably set to 0.0005% or more.

[0077] [Optional composition of steel plate]

[0078] The steel plate 1 of the welded joint of the present invention can achieve the target properties of the present invention by the above basic composition, but in order to further improve the strength, weldability, that is, the toughness of the welded portion, welding workability, etc., it is preferable to contain the following optional components as needed in addition to the above basic composition.

[0079] The optional composition may be, for example, one or more selected from the group consisting of Cu: 2.000% or less, Ni: 2.500% or less, Cr: 1.500% or less, Mo: 1.000% or less, Ti: 0.100% or less, V: 0.300% or less, B: 0.0100% or less, W: 0.500% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, and REM: 0.0500% or less.

[0080] [Cu: 2.000% or less]

[0081] Cu is an element that can increase the strength of the steel sheet 1 without significantly degrading its toughness. On the other hand, if the Cu content exceeds 2.000%, hot cracking caused by the Cu-rich layer formed directly below the oxide scale may become a problem. Therefore, when Cu is contained, it is preferably set to 2.000% or less. It should be noted that the more preferred Cu content is 0.010-1.500%.

[0082] [Ni: 2.500% or less]

[0083] Nickel (Ni) is an element that improves the hardenability and toughness of the steel plate 1. On the other hand, a Ni content exceeding 2.500% can increase manufacturing costs. Therefore, when Ni is included, the Ni content is preferably set to 2.500% or less. A more preferred Ni content is 0.010% to 2.000%.

[0084] [Cr: 1.500% or less]

[0085] Cr is an element that can improve the hardenability of the steel sheet 1 and thus increase its strength. On the other hand, a Cr content exceeding 1.500% may reduce weldability. Therefore, when Cr is included, it is preferably set to 1.500% or less. A more preferred Cr content is 0.010% to 1.200%.

[0086] [Mo: 1.000% or less]

[0087] Mo is an element that can improve the hardenability of the steel sheet 1 and thus increase its strength. On the other hand, a Mo content exceeding 1.000% may reduce weldability. Therefore, when Mo is included, it is preferably set to 1.000% or less. A more preferred Mo content is 0.010% to 0.800%.

[0088] [Ti: 0.100% or less]

[0089] Ti is an element that, by precipitating as TiN, pins grain boundaries and inhibits grain growth. On the other hand, a Ti content exceeding 0.100% reduces the cleanliness of the steel sheet 1's structure, potentially leading to reduced ductility and toughness. Therefore, when Ti is present, it is preferably kept at 0.100% or less. A more preferred Ti content is 0.001% to 0.080%.

[0090] [V: 0.300% or less]

[0091] V is an element that improves the hardenability of the steel sheet 1 and increases its strength by forming carbonitrides. On the other hand, a V content exceeding 0.300% may reduce weldability. Therefore, when V is included, it is preferably set to 0.300% or less. A more preferred V content is 0.010% to 0.250%.

[0092] [B: 0.0100% or less]

[0093] B is an element that, when added in extremely small amounts, improves the hardenability of steel, thereby increasing the strength of the steel plate 1. On the other hand, a B content exceeding 0.0100% may reduce weldability. Therefore, when B is included, it is preferably set to 0.0100% or less. A more preferred B content is 0.0001% to 0.0070%.

[0094] [W: 0.500% or less]

[0095] W is an element that can increase the strength of the steel plate 1 by improving the hardenability of the steel. On the other hand, a W content exceeding 0.500% may reduce weldability. Therefore, when W is included, it is preferably set to 0.500% or less. A more preferred W content is 0.010% to 0.400%.

[0096] [Ca: 0.0200% or less]

[0097] Ca is an element that improves weldability by forming oxysulfides with high stability at high temperatures. On the other hand, a Ca content exceeding 0.0200% may reduce cleanliness and impair the toughness of the steel plate 1. Therefore, when Ca is included, it is preferably set to 0.0200% or less. A more preferred Ca content is 0.0001% to 0.0180%.

[0098] [Mg: 0.0200% or less]

[0099] Mg is an element that improves the weldability of the steel plate 1 by forming oxysulfides with high stability at high temperatures. On the other hand, if the Mg content exceeds 0.0200%, the effect of Mg addition becomes saturated, and an effect commensurate with the content cannot be expected, which may be economically disadvantageous. Therefore, when Mg is included, it is preferably set to 0.0200% or less. A more preferred Mg content is 0.0001 to 0.0180%.

[0100] [REM: 0.0500% or less]

[0101] REM (rare earth metals) are elements that improve the weldability of the steel plate 1 by forming highly stable oxysulfides at high temperatures. On the other hand, if the REM content exceeds 0.0500%, the added effect of REM becomes saturated, and an effect commensurate with the content cannot be expected, potentially leading to economic disadvantages. Therefore, when REM is contained, the REM content is preferably set to 0.0500% or less. A more preferred REM content is 0.0001% to 0.0450%.

[0102] [Remainder of steel plate]

[0103] In the steel plate 1 of the welded joint of the present invention, the balance other than the above chemical composition is Fe and inevitable impurities. Examples of inevitable impurities include H, Zn, Re, Co, Sb, and Bi, and these are permitted as long as their total content is 0.0100% or less. Furthermore, as long as the above basic and optional compositions are met, elements other than these may be contained, and such embodiments are also within the technical scope of the present invention.

[0104] [Ceq: equivalent carbon content]

[0105] Furthermore, the chemical composition of the steel plate 1 further preferably satisfies the following conditions.

[0106] The relationship between Ceq and plate thickness T (mm) satisfies 0.0004T+0.25≤Ceq≤0.0004T+0.45 …(1).

[0107] Here, Ceq is defined by the following formula (2), and is an index of the hardenability of steel based on the elements contained.

[0108] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15…(2)

[0109] The symbol of each element in the above formula (2) represents the content (mass %) of the element, and is set to 0 when the element is not contained.

[0110] In the present invention, in order to obtain the target high-strength microstructure, it is necessary to control the cooling rate and the amount of alloy addition corresponding to the thickness T of a certain steel plate 1. If Ceq is less than (0.0004T + 0.25), the desired strength of the steel plate 1 may not be achieved. On the other hand, if Ceq is greater than (0.0004T + 0.45), the strength becomes too high at the surface of the steel plate 1, where the cooling rate is faster than the center of the thickness of the steel plate 1, and the low-temperature toughness of the surface of the steel plate 1 may decrease. Therefore, it is preferable to satisfy the above formula (1).

[0111] It should be noted that, more preferably, 0.0004T+0.27≤Ceq≤0.0004T+0.43.

[0112] [Chemical composition of weld metal]

[0113] Next, the chemical composition of the weld metal 2 of the weld joint of the present invention will be described. It should be noted that, unless otherwise specified, "%" regarding the chemical composition means "mass %."

[0114] [Basic composition of weld metal]

[0115] The chemical composition of the weld metal 2 of the weld joint of the present invention is basically as follows.

[0116] Preferably, the weld metal 2 has a chemical composition containing C: 0.04-0.14%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.020% or less, Nb: 0.001-0.100%, Al: 0.001-0.100%, Cu: 0.001-2.000%, Ni: 0.001-2.500%, Cr: 0.001-1.500%, Mo: 0.001-1.000%, Ti: 0.001-0.1000%, V: 0.001-0.300%, B: 0.001-0.020%, O: 0.050% or less, and N: 0.010% or less, with the balance being Fe and unavoidable impurities.

[0117] Therefore, the reason why the chemical composition of the weld metal 2 in the present invention is specified as above will be described.

[0118] It should be noted that each component may be added (included) in the weld metal 2 by any one of the steel plate 1 , the flux, or the welding wire as the welding material.

[0119] [C: 0.04~0.14%]

[0120] C is an element that increases the strength of the weld metal 2 through solid solution strengthening and contributes to the strengthening of the austenite grain boundaries. When the C content is less than 0.04%, the austenite grain boundary strength decreases, and the target strength of the present invention may not be achieved. On the other hand, when the C content exceeds 0.14%, the occurrence of weld cracks may be accelerated, and the low-temperature toughness of the weld metal 2 may also be reduced. Therefore, the C content is preferably set to 0.04-0.14%. It should be noted that the C content is more preferably set to 0.05-0.12%.

[0121] [Si: 0.03-0.70%]

[0122] Si acts as a deoxidizer and has the effect of increasing the viscosity of the weld metal 2 and stably maintaining the weld bead shape. To achieve this effect, the Si content is preferably 0.03% or more. However, if the Si content exceeds 0.70%, the low-temperature toughness of the weld metal 2 may be reduced. In addition, Si segregates during solidification, forming a liquid phase at the solidification cell interface, which may reduce weld crack resistance. Therefore, the Si content is preferably set to 0.03-0.70%. It should be noted that the Si content is more preferably set to 0.04-0.60%.

[0123] [Mn: 0.30-2.50%]

[0124] Mn is an element that can improve the hardenability and strength of the weld metal 2 at low cost. To achieve this effect, a Mn content of 0.30% or more is preferred. On the other hand, if the Mn content exceeds 2.50%, Mn segregation occurs during solidification, inducing high-temperature cracking, which may reduce the weldability of the steel plate 1. Therefore, the Mn content is preferably set to 0.30-2.50%. It should be noted that the Mn content is more preferably 0.50-2.20%.

[0125] [P: 0.030% or less]

[0126] P is an element that significantly embrittles grain boundaries. If present in large amounts, it can reduce the low-temperature toughness of the weld metal 2 and segregate at the interface during solidification, inducing high-temperature cracking. Therefore, the P content is preferably set to 0.030% or less. More preferably, the P content is set to 0.025% or less. On the other hand, the lower the P content, the better. Therefore, the lower limit of the P content is not particularly limited and can be 0%. However, P is an element that is inevitably contained in the weld metal 2 as an impurity. Excessive P content can increase refining time and refining costs. Therefore, the P content is preferably set to 0.001% or more.

[0127] [S: 0.020% or less]

[0128] S may reduce the low-temperature impact toughness of the weld metal 2 and may segregate at the interface during solidification, inducing high-temperature cracking. Therefore, the S content is preferably set to 0.020% or less. The S content is more preferably set to 0.010% or less. On the other hand, the lower the S content, the better. Therefore, the lower limit of the S content is not particularly limited and may be 0%. However, S is an element that is inevitably contained in the weld metal 2 as an impurity. If the S content is too low, it may lead to an increase in refining time and refining costs. Therefore, the S content is preferably set to 0.0001% or more.

[0129] [Nb: 0.001~0.100%]

[0130] Nb is a carbide-forming element that helps to precipitate carbides and increase the strength of the weld metal 2. In addition, Nb precipitates carbides at the solidification unit interface of the weld metal 2, which helps to suppress the occurrence of high-temperature cracks. In order to achieve this effect, Nb is preferably contained at 0.001% or more. On the other hand, when the Nb content exceeds 0.100%, the carbides are coarsened and become the starting point of fracture, which may lead to a decrease in toughness at extremely low temperatures. Therefore, the Nb content is preferably set to 0.001-0.100%. It should be noted that the Nb content is more preferably 0.005-0.075%. The Nb content is particularly preferably set to 0.005-0.050%.

[0131] [Al: 0.001~0.100%]

[0132] Al acts as a deoxidizer, playing an important role in increasing the viscosity of the weld metal 2 and stably maintaining the weld bead shape. To achieve this effect, the Al content is preferably set to 0.001% or more. On the other hand, when the Al content exceeds 0.100%, the cleanliness of the weld metal 2 decreases, resulting in a decrease in ductility and toughness. When ductility decreases, the possibility of ductility reduction cracks, a type of high-temperature cracking, occurring during welding increases. Furthermore, there is a possibility that the viscosity of the weld metal 2 becomes too high, the weld bead does not expand, and defects such as poor fusion may increase. Therefore, the Al content is preferably set to 0.001-0.100%. It should be noted that the Al content is more preferably set to 0.005-0.080%.

[0133] [Cu: 0.001~2.000%]

[0134] Cu is an element that can improve the strength of the weld metal 2 without significantly degrading its toughness. To achieve this effect, the Cu content is preferably set to 0.001% or more. On the other hand, if the Cu content exceeds 2.000%, it may segregate during solidification and induce high-temperature cracking. Therefore, when Cu is included, it is preferably set to 2.000% or less. It should be noted that the Cu content is more preferably 0.005-1.500%.

[0135] [Ni: 0.001~2.500%]

[0136] Nickel is an element that improves the hardenability and toughness of the weld metal 2. To achieve this effect, the Ni content is preferably set to 0.001% or more. However, nickel is an expensive element, and a Ni content exceeding 2.500% may be economically disadvantageous. Therefore, when nickel is contained, the Ni content is preferably set to 2.500% or less. It should be noted that a more preferred Ni content is 0.010% to 2.000%.

[0137] [Cr: 0.001~1.500%]

[0138] Cr is an element that can improve the strength of weld metal 2 by increasing its hardenability. To achieve this effect, the Cr content is preferably set to 0.001% or more. On the other hand, a Cr content exceeding 1.500% may reduce weldability. Furthermore, Cr carbides may form, leading to a decrease in low-temperature toughness. Therefore, when Cr is included, the Cr content is preferably set to 1.500% or less. It should be noted that a more preferred Cr content is 0.010% to 1.200%.

[0139] [Mo: 0.001~1.000%]

[0140] Mo is an element that can improve the strength of weld metal 2 by increasing its hardenability. To achieve this effect, the Mo content is preferably set to 0.001% or higher. On the other hand, a Mo content exceeding 1.000% may reduce weldability. Therefore, when Mo is included, it is preferably set to 1.000% or lower. A more preferred Mo content is 0.010% to 0.800%.

[0141] [Ti: 0.001~0.100%]

[0142] Ti is an element that can precipitate as fine carbonitrides in the weld metal 2, thereby increasing the strength of the weld metal 2. To achieve this effect, the Ti content is preferably set to 0.001% or greater. On the other hand, if the Ti content exceeds 0.100%, the cleanliness of the weld metal 2 structure decreases, resulting in a potential decrease in ductility and toughness. Therefore, when Ti is included, it is preferably set to 0.100% or less. A more preferred Ti content is 0.005% to 0.080%.

[0143] [V: 0.001~0.300%]

[0144] V is an element that improves the hardenability of the weld metal 2 and, through the formation of carbonitrides, increases the strength of the weld metal 2. To achieve this effect, the V content is preferably set to 0.001% or higher. On the other hand, a V content exceeding 0.300% may reduce weldability. Therefore, when V is included, it is preferably set to 0.300% or lower. A more preferred V content is 0.005% to 0.250%.

[0145] [B: 0.001~0.020%]

[0146] Boron (B) is an element that, when added in very small amounts, improves hardenability and thereby increases the strength of the weld metal 2. To achieve this effect, the B content is preferably set to 0.001% or higher. On the other hand, a B content exceeding 0.020% may reduce weldability. Therefore, when B is included, it is preferably set to 0.020% or lower. A more preferred B content is 0.005% to 0.018%.

[0147] [O: 0.050% or less]

[0148] O is an element that reduces the ductility and toughness of the weld metal 2, so the O content is preferably set to 0.050% or less. On the other hand, the lower the O content, the better, so the lower limit of the O content is not particularly limited and can be 0%. However, O is an element that is inevitably contained in the weld metal 2 as an impurity. If the O content is too low, it will increase the refining time and refining costs. Therefore, the O content is preferably set to 0.0005% or more.

[0149] [N: 0.010% or less]

[0150] Nitrogen is an element that reduces the ductility and toughness of the weld metal 2, so the N content is preferably set to 0.010% or less. On the other hand, the lower the N content, the better, so the lower limit of the N content is not particularly limited and can be 0%. However, since N is an element inevitably contained in the weld metal 2 as an impurity, it can be industrially greater than 0%. It should be noted that an excessively low N content increases refining time and refining costs, so the N content is preferably set to 0.0005% or more.

[0151] [Optional composition of weld metal]

[0152] The weld metal 2 of the present invention can achieve the target characteristics of the present invention by setting it to the above-mentioned basic composition, but in order to further improve the strength, weldability, that is, the toughness of the weld, welding workability, etc., it is preferable to contain the following optional components as needed in addition to the above-mentioned basic composition.

[0153] One or more selected from the group consisting of W: 0.500% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, and REM: 0.0500% or less.

[0154] [W: 0.500% or less]

[0155] W is an element that can improve the strength of the weld metal 2 by increasing its hardenability. On the other hand, a W content exceeding 0.500% may reduce weldability. Therefore, when W is included, it is preferably set to 0.500% or less. A more preferred W content is 0.010% to 0.400%.

[0156] [Ca: 0.0200% or less]

[0157] Ca is an element that improves the weldability of the weld metal 2 by forming oxysulfides that are highly stable at high temperatures. On the other hand, if the Ca content exceeds 0.0200%, the cleanliness of the weld metal 2 decreases, potentially impairing the toughness of the weld metal 2. Therefore, when Ca is included, it is preferably set to 0.0200% or less. A more preferred Ca content is 0.0001% to 0.0180%.

[0158] [Mg: 0.0200% or less]

[0159] Mg is an element that improves the weldability of the weld metal 2 by forming oxysulfides with high stability at high temperatures. On the other hand, if the Mg content exceeds 0.0200%, the effect of Mg addition becomes saturated, and an effect commensurate with the content cannot be expected, which may be economically disadvantageous. Therefore, when Mg is included, it is preferably set to 0.0200% or less. A more preferred Mg content is 0.0001 to 0.0180%.

[0160] [REM: 0.0500% or less]

[0161] REM (rare earth metals) are elements that improve the weldability of the weld metal 2 by forming highly stable oxysulfides at high temperatures. On the other hand, if the REM content exceeds 0.0500%, the added effect of REM becomes saturated, and an effect commensurate with the content cannot be expected, potentially leading to economic disadvantages. Therefore, when REM is contained, it is preferably set to 0.0500% or less. A more preferred REM content is 0.0001 to 0.0450%.

[0162] [Remainder of weld metal]

[0163] In the chemical composition of the weld metal 2 of the present invention, the chemical composition other than the above-mentioned chemical composition, i.e., the balance, is Fe and inevitable impurities. It should be noted that inevitable impurities refer to components that enter the weld metal 2 during welding from the welding wire, flux, steel plate 1, ambient atmosphere, etc., and are components not intentionally contained in the weld metal 2. Examples of inevitable impurities include H, Zn, Re, Co, Sb, and Bi, and these are permitted as long as their total content is 0.0100% or less. Furthermore, as long as the above-mentioned basic and optional compositions are met, elements other than these may be contained, and such embodiments are also included in the technical scope of the present invention.

[0164] [Steel Plate Characteristics]

[0165] Here, the characteristics of the steel plate 1 of the welded joint of the present invention are described. The dislocation density ρ (m -2 ) is preferably 4.0×10 14 m -2 Below. Dislocation density ρ(m -2 ) refers to the length of the dislocation line per unit volume in the metal crystal. The setting of the position 1 mm below the surface of the steel plate 1 is considered from the perspective of the bendability of the steel plate 1. The bendability of the steel plate 1 is determined by the ductility of the surface structure of the steel plate 1. If the dislocation density of the surface structure increases due to the processing strain during hot rolling, the deformation limit of the surface structure decreases, and thus the bendability decreases. Therefore, the dislocation density at the position 1 mm below the surface of the steel plate 1 is set to 4.0×10 14 m -2 It should be noted that, in general, dislocations are unavoidable in steel structures. Therefore, in order to reduce the dislocation density to less than 1.0×10 11 m -2 , which is very costly to manufacture. Therefore, the preferred dislocation density is 1.0×10 11 m -2 More preferably, the dislocation density is 3.0×10 14 m -2 the following.

[0166] In addition, it is preferable that the average grain size at a position 1 mm below the surface of the steel plate 1 is 15.0 μm or less, and the average grain size at a center position of the thickness of the steel plate 1 is 20.0 μm or less.

[0167] The average grain size refers to the average of the grain sizes of all grains at 1 mm below the surface and at the center of the thickness of the steel plate 1, respectively, when the area surrounded by the boundary with a crystal orientation difference of 15° or more is regarded as the grain. In addition, the average grain size can be measured by the method described in the examples described later. The finer the grain size of the surface structure of the steel plate 1, the higher the toughness of the surface layer of the steel plate 1. In order to achieve this effect, it is necessary to make the average grain size at 1 mm below the surface of the steel plate 1 less than 15.0 μm. Therefore, the average grain size at 1 mm below the surface of the steel plate 1 is set to less than 15.0 μm. It should be noted that the preferred average grain size is 13.0 μm or less. In addition, the finer the grain size of the steel structure at the center of the thickness of the steel plate 1, the higher the toughness of the steel plate 1 at the center of the thickness of the steel plate 1. In order to achieve this effect, it is necessary to make the average grain size at the center of the thickness of the steel plate 1 less than 20.0 μm. Therefore, the average grain size at the center of the steel plate 1's thickness is set to 20.0 μm or less. It should be noted that the average grain size is preferably 15.0 μm or less. In the present invention, "1 mm below the surface of the steel plate" refers to a depth of 1 mm from the surface of the steel plate 1 in the thickness direction. "Center of the thickness" refers to a position 1 / 2 of the thickness of the steel plate 1.

[0168] [Method for manufacturing steel sheet]

[0169] Next, the method for manufacturing the steel plate 1 will be described. The steel plate 1 of the present invention is obtained by heating, hot-rolling, and cooling a steel slab (steel raw material) having the above-described composition. After cooling, the steel plate 1 of the present invention may be further optionally tempered. Various preferred conditions for the method for manufacturing the steel plate 1 of the present invention will be described below. However, the method for manufacturing the steel plate 1 of the present invention is not limited to the method described below; any method having the characteristics described below may be used.

[0170] In the following description of the manufacturing method, unless otherwise specified, "°C" indicates the surface temperature of the billet or steel plate 1. The surface temperature can be measured using, for example, a radiation thermometer.

[0171] In the present invention, the method for melting the steel slab is not particularly limited; any known melting method, such as a converter, electric furnace, or vacuum melting furnace, is suitable. The steel slab is produced to the desired size, for example, by continuous casting. The molten steel may also be subjected to secondary refining, such as ladle refining.

[0172] As described above, the manufactured steel slab is preferably heated to a temperature of 1000-1200°C. If the heating temperature of the steel slab is lower than 1000°C, the coarse NbC precipitated within the steel slab during casting does not re-dissolve and remains. Consequently, the low-temperature effect of the pre-recrystallization temperature range, which is achieved by the solute Nb and the fine NbC re-precipitated during hot rolling, cannot be achieved. Consequently, the grain refinement effect of controlled rolling decreases, and the toughness of the steel plate 1 as the final product decreases. On the other hand, if the heating temperature of the steel slab exceeds 1200°C, the grain size at the start of hot rolling becomes coarser due to austenite grain growth. Consequently, the grain size of the final structure after hot rolling also becomes coarser, and the toughness of the steel plate 1 decreases. Therefore, the heating temperature of the steel slab is preferably set to 1000-1200°C. The more preferred heating temperature of the steel slab is 1030°C or higher, and even more preferably 1170°C or lower.

[0173] Next, the heated steel slab is preferably hot rolled. As described above, in the present invention, the dislocation density and average grain size 1 mm below the surface of the steel sheet 1, as well as the average grain size at the center of the thickness of the steel sheet 1, are important. To achieve various properties, the steel slab is preferably rolled under the following hot rolling conditions.

[0174] It should be noted that, herein, the temperature range in which the temperature measured at a position 1 mm below the surface of the steel plate 1 or at the center of the thickness of the steel plate 1 exceeds (8250 [Nb] + 770 ° C) is referred to as the recrystallization temperature range. In addition, the temperature range in which the temperature measured at a position 1 mm below the surface of the steel plate 1 or at the center of the thickness of the steel plate 1 is (8250 [Nb] + 770 ° C) to the Ar3 temperature is referred to as the non-recrystallization temperature range. The temperature at the center of the thickness of the steel plate 1 can be obtained, for example, by setting a 5 mm φ straight hole to the center of the thickness of the steel plate 1 and installing a thermocouple therefor for measurement, calculating the temperature distribution in the cross section of the steel plate 1 by heat transfer analysis, and correcting the result using the surface temperature of the steel plate 1 to obtain the temperature at the center of the thickness of the steel plate 1. It should be noted that the above-mentioned [Nb] represents the content (mass %) of the element.

[0175] First, the temperature of the steel plate 1 at a position 1 mm below the surface is temporarily cooled to below the Ar3 temperature, and then reheated to above the Ac3 temperature. Next, while the temperature at a position 1 mm below the surface of the steel plate 1 is within the temperature range of (8250 [Nb] + 770°C) to the Ar3 temperature, the steel plate 1 is rolled at a reduction ratio of 25% or more. Finally, while the temperature at a position 1 mm below the surface of the steel plate 1 is within the temperature range below the Ar3 temperature, the steel plate 1 is rolled at a total reduction ratio of 15% or less.

[0176] During hot rolling, the surface layer of the heated steel slab 1 is temporarily cooled to a temperature below the Ar3 temperature, thereby transforming from austenite to a low-temperature-generated structure such as ferrite. Subsequently, the surface layer of the steel slab 1 is reheated to a temperature above the Ac3 temperature, thereby transforming again to an austenite structure, resulting in a fine austenite structure on the surface layer of the steel slab 1.

[0177] It should be noted that the cooling of the steel plate 1 can be performed by methods such as water cooling and air cooling, and any method is not limited as long as the temperature can be controlled to the specified level. For example, cooling to a temperature below the Ar3 temperature is performed by water cooling, and the dwell time at a position 1 mm below the surface of the steel plate 1 to reach a temperature below the Ar3 temperature is preferably set to 5 seconds or more, preferably 300 seconds or less. After cooling, the steel plate 1 is held in the atmosphere for a holding time of 30 seconds or more, preferably 600 seconds or less.

[0178] Next, by applying a reduction of 25% or more to the steel plate 1 within the temperature range of (8250[Nb]+770°C) to the Ar3 temperature, the surface layer of the steel plate 1 is not recrystallized, thereby introducing processing strain into the austenite of the surface layer of the steel plate 1. This acts as a phase transformation nucleus during final cooling, thereby obtaining a fine structure with good toughness. It is preferred that the reduction rate within the above temperature range be set to 30% or more. It should be noted that, from the perspective of rolling efficiency, the reduction rate within this temperature range is preferably set to 80% or less, and more preferably to 70% or less.

[0179] It should be noted that the upper limit of the number of reduction passes within this temperature range is not particularly limited. In addition, as long as the above-mentioned reduction rate conditions are met, for example, it can be carried out in two or more passes.

[0180] [Rolling conditions]

[0181] If the total rolling reduction exceeds 15% within the temperature range where the surface temperature of the steel sheet 1 is lower than the Ar3 temperature, processing strain is introduced into the ferrite and pearlite structures in the surface layer of the steel sheet 1 that have completed phase transformation, resulting in a decrease in the ductility of the steel sheet 1 and deterioration in the bendability of the steel sheet 1. Therefore, the total rolling reduction within the temperature range where the surface temperature of the steel sheet 1 is lower than the Ar3 temperature is preferably set to 15% or less. More preferably, the total rolling reduction within this temperature range is set to 6% or less.

[0182] By controlling the structure of the surface layer of the steel sheet 1 according to the rolling conditions, the surface layer of the steel sheet 1 can have a structure with low dislocation density, small grain size, and excellent bending workability and toughness.

[0183] Next, when the temperature at the center of the thickness of the steel plate 1 is within the temperature range of (8250[Nb]+770°C) or lower and the Ar3 temperature or higher, the steel plate 1 may be reduced so that the total reduction ratio becomes 25% or higher.

[0184] As a result, working strain can be introduced into the austenite at the center of the thickness of the steel billet. It acts as a phase transformation nucleus during final cooling, thereby obtaining a fine structure with good toughness. Therefore, it is preferred to set the total reduction rate of the steel plate 1 to 25% or more when the center temperature of the thickness of the steel plate 1 is in a temperature range below (8250[Nb]+770°C). It is more preferred to set the total reduction rate of the steel plate 1 to 35% or more. From the perspective of rolling efficiency, the total reduction rate of the steel plate 1 in this temperature range is preferably set to 70% or less, and more preferably to 67% or less.

[0185] If the center temperature of the steel plate 1 is lower than the Ar3 temperature, processing strain is introduced into the generated ferrite, reducing the toughness of the steel plate 1. Therefore, the center temperature of the steel plate 1 is preferably set to be equal to or higher than the Ar3 temperature.

[0186] In the present invention, the temperature ranges of the two rolling processes, "rolling the steel plate surface" and "rolling the plate thickness center," within the aforementioned rolling conditions may overlap. In this case, the reductions in the overlapping temperature ranges are accumulated as the reductions for "rolling the steel plate surface" and "rolling the plate thickness center," and the accumulated reductions must be within the ranges for each rolling condition.

[0187] Here, the total reduction within the recrystallization temperature range at the surface layer and the center of the thickness of the steel plate 1 is calculated as follows: Total reduction = (r0 - r1) / r0 × 100 (%) within the temperature range exceeding (8250 [Nb] + 770°C). Note that r0 is the plate thickness at the start of initial rolling, and r1 is the plate thickness after final rolling. The Ar3 temperature and Ac3 temperature can be determined using methods such as the Formaster test.

[0188] By performing these structure controls based on the rolling conditions at the center of the thickness of the steel plate 1 , the center of the thickness of the steel plate 1 can be made into a structure having a small crystal grain size and excellent toughness.

[0189] [Cooling conditions, tempering conditions]

[0190] The steel plate 1 manufactured by hot rolling the steel slab is cooled. Specifically, the thickness of the steel plate 1 is set to t [mm], and the average cooling rate of the steel plate 1 when the temperature at the center of the thickness of the steel plate 1 is within the temperature range of 700 to 550°C is 2500×t -1.7The steel plate 1 is cooled at a rate of 0.1°C / second or more. Examples of cooling methods include water cooling by spraying water from a nozzle at a high flow rate. In the present invention, it is preferred that both surfaces of the steel plate 1 be cooled under the same conditions.

[0191] When the temperature at the center of the thickness of the steel plate 1 after hot rolling of the steel slab is between 700°C and 550°C, the average cooling rate of the steel plate 1 is less than 2500×t -1.7 ℃ / second, the cooling rate in the temperature range where the phase transformation from austenite to the low temperature phase transformation structure occurs is insufficient, and the strength of the steel plate 1 targeted by the present invention may not be obtained. In addition, due to the formation of coarse ferrite, the toughness of the steel plate 1 may be reduced. Therefore, the average cooling rate of the steel plate 1 when the temperature at the center of the plate thickness of the steel plate 1 is within the temperature range of 700 to 550℃ is preferably set to 2500×t -1.7 °C / second or more.

[0192] In the present invention, after cooling the steel plate 1, the steel plate 1 may be tempered as needed to further improve its strength and toughness. In this case, the steel plate 1 is tempered at a tempering temperature of 650°C or less after cooling. Tempering temperatures above 650°C may cause significant softening of the steel plate 1, making it impossible to ensure the required strength. Therefore, the tempering temperature is preferably set to 650°C or less. Meanwhile, the lower limit of the tempering temperature is not particularly limited, but is preferably set to 200°C or above. It should be noted that the tempering time can be adjusted appropriately. The tempering temperature herein refers to the surface temperature of the steel plate 1.

[0193] [Mechanical properties of weld metal]

[0194] Here, the preferred mechanical properties of the welded joint of the present invention are described. It is preferred that the yield strength (0.2% proof stress) of the welded metal 2 having the above-mentioned chemical composition, i.e., the welded metal 2 produced in accordance with the regulations of JIS Z 3111: 2005, in a tensile test at room temperature is 325 MPa or more, and its tensile strength is 520 MPa or more. In addition, the tensile strength of the welded joint at room temperature is preferably 520 MPa or more. It is further preferred that the absorbed energy ( v E -40 ) is 30 J or more. This is because if it is less than 30 J, the toughness of the structure having the weld joint may be reduced and fracture may occur easily.

[0195] [Method for manufacturing welded joint]

[0196] Next, the method for producing the welded joint of the present invention will be described.

[0197] First, preferably, a steel plate 1 having the above-described chemical composition and a thickness of 50 mm or greater is prepared. Then, the prepared steel plates 1 are beveled so that a predetermined groove shape is formed between the prepared steel plates 1. The groove shape formed on the steel plates 1 is not particularly limited, and examples thereof include conventional V-grooves, レ-grooves, X-grooves, and K-grooves for welded structures.

[0198] Next, the grooved steel plates 1 are welded together. Specifically, single-layer, single-pass welding is performed to form a multilayer weld metal 2, preferably three or more layers, as a weld joint. When welding the steel plates 1 together to form one or two layers of weld metal 2, there is a risk that heat from welding will diffuse unevenly within the groove, potentially causing welding defects. The welding materials used to weld the steel plates 1 together are not particularly limited, as long as they can form a weld metal 2 having the desired properties.

[0199] It should be noted that as welding methods, submerged arc welding, gas metal arc welding (also called gas shielded arc welding) and the like can be cited. Generally, submerged arc welding is preferably used from the perspective of being able to perform efficient welding construction. It should be noted that multi-electrode submerged arc welding is more preferred as a more efficient welding method. Further preferred is a welding method using a submerged arc welding machine with two electrodes. In this two-electrode submerged arc welding, an arc is generated by one welding wire passing through one welding torch in the leading first electrode, and an arc is generated by two welding wires passing through one welding torch in the trailing second electrode, thereby performing submerged arc welding. The bottom of the groove is melted by the arc generated by the front end of the welding wire of the first electrode, thereby suppressing the occurrence of poor fusion. By simultaneously generating arcs by two welding wires at the second electrode, the melting of the welding wires can be accelerated by the mutual arcs, thereby increasing the amount of deposited metal. Therefore, more efficient welding construction can be performed.

[0200] For two-electrode submerged arc welding, the preferred welding conditions are: a current of 600 to 1200 A and a voltage of 24 to 45 V for the first electrode, and a current of 450 to 1200 A and a voltage of 30 to 48 V for the second electrode. Furthermore, a welding speed of 50 to 130 cm / min and an energy input of 10 to 100 kJ / cm are preferred. It should be noted that these electrodes correspond to the welding electrodes of this embodiment.

[0201] The welding wire and welding flux used in double-electrode submerged arc welding are adjusted to achieve the aforementioned composition of weld metal 2, taking into account base metal dilution. It should be noted that the composition of weld metal 2 is not particularly limited as long as it falls within the aforementioned composition range.

[0202] Examples of welding wire include solid wire and flux-cored wire containing a flux coating the interior of the wire. In the present invention, either type of welding wire can be used. When using a flux-cored wire, it is manufactured so that the total composition of the steel sheath, metal powder, and flux powder used matches the target welding material composition.

[0203] Examples of the welding flux include molten flux and calcined flux. In the present invention, any of these welding fluxes can be used.

[0204] Example

[0205] The present invention will be further described below based on examples. However, the following examples are only for illustrating and explaining the present invention in more detail and do not limit the scope of the present invention.

[0206] First, molten steel having the chemical composition shown in Table 1 was melted and then continuous casting was performed to produce steel slabs (steel slabs). The compositions in Table 1 represent the chemical compositions of the steel plates. The "-" column in Table 1 indicates that the component indicated by "-" was not intentionally added to the molten steel. This includes not only the case where the molten steel does not contain the component (0%), but also the case where the molten steel inevitably contains the component.

[0207]

[0208] Next, the steel slab containing the above-mentioned components was subjected to the heating, hot rolling and cooling processes in sequence to obtain a steel plate having a thickness T (mm) as shown in Table 2. It should be noted that the rolling start temperature in the hot rolling process was in the range of 990 to 1140°C based on the surface of the steel plate, and the finishing rolling temperature was in the range of 670 to 830°C based on the surface of the steel plate. Regarding the above-mentioned steel plate temperatures, the surface temperature was measured using a radiation thermometer, and the temperature at the center of the plate thickness was measured by setting a 5mmφ straight hole to the center of the plate thickness of the steel plate and installing a thermocouple there. In addition, cooling after hot rolling was performed by spraying water at a large flow rate from the surface and back of the steel plate. The steel plate was subjected to groove processing. The steel type, plate thickness, groove shape, and the figure number showing the plate thickness and groove shape of the steel plate subjected to groove processing are summarized in Table 2. In addition, Table 2 shows whether Ceq, which is an indicator of the hardenability of steel, satisfies the above-mentioned formula (1). When Ceq satisfies the formula (1), "0" is recorded in the column determined by the formula (1) in Table 2. When it does not satisfy the formula (1), "×" is recorded in the column determined by the formula (1) in Table 2.

[0209]

[0210] Next, molten steel having the chemical composition (welding material composition) shown in Table 3 was melted in a vacuum melting furnace and cast to produce steel ingots. The resulting ingots were heated to 1200°C and then hot-rolled and then cold-rolled to produce 4.0 mm φ and 2.4 mm φ solid wires for submerged arc welding, and 1.2 mm φ solid wire for gas metal arc welding.

[0211] Separately, a steel sheath and flux-cored wire containing metal powder and flux powder encapsulated within the steel sheath were produced. A 0.5 mm thick thin steel plate with a composition of 0.1% C, 0.2% Si, 0.5% Mn, and the balance Fe was used as the raw material for the steel sheath. This steel sheath was then cold-bent in the width direction to form a U-shape. The resulting steel sheath was then filled with metal powder and flux powder, whose compositions were adjusted to achieve the chemical compositions shown in Table 3. This was then cold-drawn to produce flux-cored wires for welding (diameters: 1.2, 2.4, and 4.0 mm). The chemical compositions shown in Table 3 represent the combined values ​​of the steel sheath, metal powder, and flux powder.

[0212]

[0213] Next, the steel plates with groove shapes shown in Table 2 above were butted against each other to form Figures 2-4 The groove shown in Table 3 is welded. Using solid wire or flux-cored wire with the chemical composition shown in Table 3, the grooves of the butted steel plates are welded using submerged arc welding or gas metal arc welding. This forms weld metal within the groove, resulting in a welded joint. For submerged arc welding, a flux having a composition of 38% SiO₂, 11% MnO, 8% TiO₂, 16% Al₂O₃, and 27% MgO is used.

[0214] Furthermore, the welding experiments described below were conducted using the steel plates having the groove shapes shown in Table 2 and the welding wires having the chemical compositions shown in Table 3 in the combinations shown in Table 4.

[0215]

[0216] Specific experimental conditions for submerged arc welding included no preheating, flat welding, a current of 400 to 1000 A, a voltage of 26 to 44 V, a welding speed of 500 to 1200 mm / min, an interpass temperature of 200°C or less, and single-layer, single-pass welding. These experimental conditions are shown in Table 5.

[0217]

[0218] Specific experimental conditions for gas metal arc welding included no preheating, flat welding, a current of 200 to 400 A, a voltage of 26 to 40 V, a welding speed of 160 to 400 mm / min, an interpass temperature of 200°C or less, and single-layer, single-pass welding. These experimental conditions are shown in Table 6.

[0219]

[0220] [Evaluation of weld metal shape]

[0221] form Figure 1 The cross section of the welded joint along the vertical direction shown in the figure was taken at the 1 / 4L, 2 / 4L, and 3 / 4L positions along the weld line of the cross section for macroscopic observation. Here, L is the length of the weld line of the welded joint, and the 1 / 4L position indicates the position 1 / 4 of the plate width from the end of the longitudinal direction of the steel plate. Similarly, the 2 / 4L position indicates the position 2 / 4 of the plate width from the end of the longitudinal direction of the steel plate (the center of the weld line), and the 3 / 4L position indicates the position 3 / 4 of the plate width from the end of the longitudinal direction of the steel plate.

[0222] Then, the cross-sectional area A (mm) of the entire weld metal in the direction perpendicular to the weld line was recorded. 2 ) and divided by the number of weld layers p (layers), the first ratio [A / p] of the cross-sectional area A of the weld metal to the number of weld layers p.

[0223] Furthermore, the distance between the upper and lower portions of each weld metal layer was measured as the height H (mm) of each weld metal layer, and the width of the weld metal at the center line of each height H, i.e., the distance between fusion lines, was measured as the width W (mm) of the weld metal. The second ratio [H / W] was calculated for each layer, and the maximum and minimum values ​​of the second ratio [H / W] for the entire weld metal were recorded.

[0224] [Measurement of Dislocation Density of Steel Sheet]

[0225] Samples were cut so that the evaluation surface was 1 mm below the surface of the steel plate at the center of the longitudinal direction and the center of the width direction of each steel plate. The surface of the sample was mirror-polished by mechanical grinding and electrolytic polishing, and the dislocation density ρ was evaluated using an X-ray diffraction apparatus using the Williamson-Hall method (Reference 1).

[0226] (Reference 1) GK Williams and WH Hall: Acta Metall., 1 (1953), 22

[0227] [Average grain size]

[0228] In the same manner as above, samples were cut so that the longitudinal cross-section of the steel plate at the center position of the longitudinal direction and the center position of the width direction of each steel plate and the longitudinal cross-section of the steel plate at the center position of the plate thickness of the steel plate were used as evaluation surfaces. The surface of the obtained sample was mirror-polished by colloidal silica polishing, and the crystal orientation was measured by EBSP (backscattered electron diffraction) under the following conditions. The measurement area of ​​the crystal orientation was set to 300μm×400μm, and the measurement step was set to 1μm. In addition, based on the crystal orientation map obtained by automatically analyzing the measured crystal orientation, the equivalent circle diameter of the structure surrounded by large-angle grain boundaries with a crystal orientation difference of 15° or more from the adjacent grains was calculated, and the average value of the equivalent circle diameters in the above measurement area was taken as the average crystal grain size. It should be noted that the surface layer and the center of the plate thickness of the steel plate of the present invention are both structures with bainite and quasi-polygonal ferrite as the main body.

[0229] [Evaluation of high temperature crack resistance and poor fusion]

[0230] After the steel plates are welded to each other, a macro test piece with a thickness of 10 mm is cut from the center position of the weld line direction using a micro cutter. The cross section of the cut weld metal is observed with an optical microscope (30 times) to determine whether there are high-temperature cracks and poor fusion. If the occurrence of high-temperature cracks is confirmed, it is evaluated as "×" because the high-temperature crack resistance is reduced. If the occurrence of high-temperature cracks is not confirmed, it is evaluated as "○" because the high-temperature crack resistance is excellent. Similarly, if the occurrence of poor fusion is confirmed, it is evaluated as "×", and if the occurrence of poor fusion is not confirmed, it is evaluated as "○".

[0231] [Evaluation of mechanical properties of weld metal]

[0232] According to the provisions of JIS Z 3111:2005, tensile test pieces (parallel part diameter 6mm) and Charpy impact test pieces (V-notch) of the weld metal were cut from the weld joint and subjected to tensile test and impact test. The tensile test was carried out on 3 pieces at room temperature, and the average value of the obtained values ​​(0.2% proof stress, tensile strength) was used as the tensile properties of the weld metal of the weld joint. The Charpy impact test was also carried out on 3 pieces at the same time, and the absorbed energy ( v E -40 ), and the average value is taken as the cryogenic toughness of the weld metal of the weld joint.

[0233] Furthermore, tensile tests of welded joints at room temperature were conducted in accordance with JIS Z 3121: 2013. The test specimens used were No. 1A test specimens, cut perpendicular to the weld axis so that the weld axis was at the center of the parallel length of the test specimen, and the thickness of the specimen was the total thickness of the welded joint.

[0234] Charpy impact tests were also conducted on the heat-affected zones of welded joints in accordance with JIS Z 3128:2017. The V-notch of the test specimens was perpendicular to the steel plate surface, and the specimens were cut from the center of the plate thickness, at the center of the weld metal, at the fusion line, and at a position 1 mm above the fusion line.

[0235] As mentioned above, the target values ​​of the present invention are: the yield strength (0.2% proof stress) of the weld metal at room temperature is 325 MPa or more, the tensile strength is 520 MPa or more, and the tensile strength of the weld joint at room temperature is 520 MPa or more. In addition, the absorbed energy ( v E -40 ) is 30J or more.

[0236] [Deposition efficiency]

[0237] The diameter of the welding wire (mm) and the wire feed speed (mm / min) of each welding wire during welding were measured and divided by the unit welding time (min) to obtain the value recorded as the deposition efficiency (g / min).

[0238] Based on the above experimental results, the results of the chemical composition of the weld metal are shown in Table 7 (Table 7-1 and Table 7-2), and the results of the shape of the weld metal such as the cross-sectional area and the number of weld layers, defects in the weld metal, and various characteristic values ​​of the weld metal are shown in Table 8 (Table 8-1 and Table 8-2).

[0239]

[0240]

[0241]

[0242]

[0243] In the examples (hereinafter referred to as the examples of the present invention) within the appropriate range of the steel plates and weld metals of the present invention, all of them are sound weld joints without welding defects such as high temperature cracks and poor fusion. In addition, in all the examples of the present invention, the yield strength (0.2% proof stress) of the weld metal at room temperature is 325 MPa or more, the tensile strength thereof is 520 MPa or more, and the tensile strength of the weld joint at room temperature is 520 MPa or more. Moreover, in all the examples of the present invention, the absorbed energy ( v E -40 ) is 30J or more and has a welded joint with weld metal having both high strength and excellent low-temperature toughness.

[0244] On the other hand, in the examples outside the suitable range of the steel plate and weld metal of the present invention, high temperature cracking occurred, the high temperature cracking resistance decreased, or poor fusion occurred. In addition, the yield strength (0.2% proof stress) and tensile strength of the weld metal at room temperature, the tensile strength of the weld joint at room temperature, the absorbed energy of the Charpy impact test at a test temperature of -40°C for the weld metal or heat-affected zone ( v E -40 ) and other mechanical properties are at a slightly low level.

[0245] Explanation of symbols

[0246] 1 steel plate

[0247] 2 Weld metal

[0248] 3 Melt line

[0249] T plate thickness

[0250] A is the cross-sectional area of ​​the entire weld metal

[0251] L i Welding layer (i=1~p layer), (L1: initial layer, L p : The outermost layer)

[0252] H i Height of welding layer (i=1~p layer)

[0253] W i Width of the welding layer (i=1~p layer)

Claims

1. A welded joint of steel plates having a thickness T (mm) of 50 mm or more, wherein: The weld metal consists of multiple layers in a single pass. The cross-sectional area A (mm) of the weld metal in the direction perpendicular to the weld line 2 ) and the first ratio of the number of weld layers p (layers) [A / p] is 120.0 mm 2 / below the floor, A second ratio [H / W] of the height H (mm) to the width W (mm) of each layer of the weld metal is 1.00 or less.

2. The welded joint of steel plates according to claim 1, wherein: The weld metal is formed by multi-layer build-up welding using a narrow groove with a groove angle of 20° or less and a maximum groove width between the steel plates of 50% or less of the plate thickness T.

3. The welded joint of steel plates according to claim 1 or 2, wherein: The chemical composition of the steel plate has a chemical composition containing, in mass%, C: 0.04-0.14%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.020% or less, Nb: 0.001-0.100%, Al: 0.001-0.100%, O: 0.0100% or less, and N: 0.0100% or less, with the balance being Fe and unavoidable impurities, and Ceq defined by the following formula (2) and the plate thickness (T) satisfying 0.0004T+0.25≤Ceq≤0.0004T+0.45 ... (1), The dislocation density ρ at a position 1 mm below the surface of the steel plate is 4.0×10 14 m -2 the following, The average grain size of the steel plate at a position 1 mm below the surface is 15.0 μm or less, and the average grain size of the steel plate at the center of the thickness is 20.0 μm or less. Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Cu]+[Ni]) / 15…(2) The symbol of each element in the formula (2) represents the content (mass %) of the element, and is set to 0 when the element is not contained.

4. The welded joint of steel plates according to claim 3, wherein: The chemical composition of the steel plate further contains, in mass %, one or more selected from the group consisting of Cu: 2.000% or less, Ni: 2.500% or less, Cr: 1.500% or less, Mo: 1.000% or less, Ti: 0.100% or less, V: 0.300% or less, B: 0.0100% or less, W: 0.500% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, and REM: 0.0500% or less.

5. The welded joint of steel plates according to claim 1 or 2, wherein: The chemical composition of the weld metal contains, in mass%, C: 0.04-0.14%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.020% or less, Nb: 0.001-0.100%, Al: 0.001-0.100%, Cu: 0.001-2.000%, Ni: 0.001-2.500%, Cr: 0.001-1.500%, Mo: 0.001-1.000%, Ti: 0.001-0.100%, V: 0.001-0.300%, B: 0.001-0.020%, O: 0.050% or less, and N: 0.010% or less, with the balance being Fe and inevitable impurities.

6. The welded joint of steel plates according to claim 3, wherein: The chemical composition of the weld metal contains, in mass%, C: 0.04-0.14%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.020% or less, Nb: 0.001-0.100%, Al: 0.001-0.100%, Cu: 0.001-2.000%, Ni: 0.001-2.500%, Cr: 0.001-1.500%, Mo: 0.001-1.000%, Ti: 0.001-0.100%, V: 0.001-0.300%, B: 0.001-0.020%, O: 0.050% or less, and N: 0.010% or less, with the balance being Fe and inevitable impurities.

7. The welded joint of steel plates according to claim 4, wherein: The chemical composition of the weld metal contains, in mass%, C: 0.04-0.14%, Si: 0.03-0.70%, Mn: 0.30-2.50%, P: 0.030% or less, S: 0.020% or less, Nb: 0.001-0.100%, Al: 0.001-0.100%, Cu: 0.001-2.000%, Ni: 0.001-2.500%, Cr: 0.001-1.500%, Mo: 0.001-1.000%, Ti: 0.001-0.100%, V: 0.001-0.300%, B: 0.001-0.020%, O: 0.050% or less, and N: 0.010% or less, with the balance being Fe and inevitable impurities.

8. The welded joint of steel plates according to claim 5, wherein: The chemical composition of the weld metal further contains, in mass %, W: 0.500% or less, Ca: 0.0200% or less, Mg: One or more selected from the group consisting of 0.0200% or less and REM: 0.0500% or less.

9. The welded joint of steel plates according to claim 6, wherein: The chemical composition of the weld metal further contains, in mass %, W: 0.500% or less, Ca: 0.0200% or less, Mg: One or more selected from the group consisting of 0.0200% or less and REM: 0.0500% or less.

10. The welded joint of steel plates according to claim 7, wherein: The chemical composition of the weld metal further contains, in mass %, W: 0.500% or less, Ca: 0.0200% or less, Mg: One or more selected from the group consisting of 0.0200% or less and REM: 0.0500% or less.

11. A method for manufacturing a welded joint of a steel plate, which is the method for manufacturing a welded joint of a steel plate according to claim 1 or 2, wherein: Submerged arc welding is performed by generating an arc using two or more welding electrodes.

12. A method for manufacturing a welded joint of a steel plate, which is the method for manufacturing a welded joint of a steel plate according to claim 3, wherein: Submerged arc welding is performed by generating an arc using two or more welding electrodes.

13. A method for manufacturing a welded joint of a steel plate, which is the method for manufacturing a welded joint of a steel plate according to claim 4, wherein: Submerged arc welding is performed by generating an arc using two or more welding electrodes.

14. A method for manufacturing a welded joint of a steel plate, which is the method for manufacturing a welded joint of a steel plate according to claim 5, wherein: Submerged arc welding is performed by generating an arc using two or more welding electrodes.

15. A method for manufacturing a welded joint of a steel plate, which is the method for manufacturing a welded joint of a steel plate according to claim 6, wherein: Submerged arc welding is performed by generating an arc using two or more welding electrodes.

16. A method for manufacturing a welded joint of a steel plate, which is the method for manufacturing a welded joint of a steel plate according to claim 7, wherein: Submerged arc welding is performed by generating an arc using two or more welding electrodes.

17. A method for manufacturing a welded joint of a steel plate, which is the method for manufacturing a welded joint of a steel plate according to claim 8, wherein: Submerged arc welding is performed by generating an arc using two or more welding electrodes.

18. A method for manufacturing a welded joint of a steel plate, which is the method for manufacturing a welded joint of a steel plate according to claim 9, wherein: Submerged arc welding is performed by generating an arc using two or more welding electrodes.

19. A method for manufacturing a welded joint of a steel plate, which is the method for manufacturing a welded joint of a steel plate according to claim 10, wherein: Submerged arc welding is performed by generating an arc using two or more welding electrodes.

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