Welded joint and method for manufacturing welded joint

Abstract

Provided is a welded joint having a tensile strength of 780 MPa or more and excellent toughness even after PWHT. This welded joint (1) is characterized in that a welded part (2) contains a welding metal having a specific chemical composition, and in that a reheated part region (32) is included at a structure ratio of 34% or more in a region between a position at a depth of 2 mm and a position at a depth of 12 mm from the surface in the center of the welded part (2).

Description

TECHNICAL FIELD

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

[0002] Gas shielded arc welding and the like using a welding metal are widely used in various fields of industrial equipment, structures, and the like. A welded joint formed using such a welding technique is required to have excellent strength and toughness in the welded portion or the welding metal contained in the welded portion.

[0003] For example, Patent Literature 1 discloses a solid wire for gas shielded arc welding containing, by mass%, C: 0.02 to 0.14%, Si: 0.4 to 1.5%, Mn: 1.0 to 2.5%, Ti: 0.05 to 0.4%, Mg: 0.0003 to 0.010%, and B: 0.0005 to 0.010%, and containing one or two or more elements selected from Al and Zr in a total of 0.005 to 0.050%, limiting N to 0.005% or less, the remainder consisting of Fe and unavoidable impurities, the total content of Si and Mn being 1.5 to 3.5%, and the ratio (Mn / Si) of the Mn content (%) to the Si content (%) being 0.85 or more. According to the solid wire for gas shielded arc welding disclosed in Patent Literature 1, when gas shielded arc welding is performed on a high-tension steel of Grade 490 to 780 N / mm 2 2, excellent toughness and strength of the welding metal can be stably ensured even when welding is performed under high-efficiency welding conditions.

[0004] In addition, Patent Literature 2 discloses a flux-coated welding wire for gas shielded arc welding, which adds C: 0.01 to 0.3%, Si: 0.01 to 1.5%, Mn: 0.8 to 8.5%, Mo: 0.2 to 1.5%, Nb: 0.005 to 0.05%, and V: 0.005 to 0.05% in either or both of a metal sheath or a flux, and on the other hand, the flux containing Ti02: 1.8 to 7.5%, iron powder of less than 15%, and the remainder being a slag forming agent containing an arc stabilizer is filled in the metal sheath at a filling rate of 10 to 25%. According to the flux-coated welding wire for gas shielded arc welding disclosed in Patent Literature 2, even after long-time post-weld heat treatment (PWHT), the degree of coarsening of ferrite grains and the generation of ferrite bands in the welding metal is small, and a welding metal having excellent mechanical properties can be obtained.

[0005] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: Japanese Patent Application Publication No. 2007-253163 Patent Literature 2: Japanese Patent Application Laid-Open No. 5-77086 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION The welded joint formed using the above-described welding technique is sometimes subjected to post-weld heat treatment (PWHT) for the purpose of removing stress. However, the welded metal after PWHT can be deteriorated by the P segregated at the original γ grain boundaries during the heat treatment causing grain boundary breakage. If the welded metal is deteriorated like this, the toughness of the welded portion is reduced, and the required characteristics can not be obtained.

[0007] Therefore, an object of the present application is to provide a welded joint having a tensile strength of 780 MPa or more and excellent toughness even after PWHT.

[0008] MEANS FOR SOLVING THE PROBLEMS The present application includes each aspect described below.

[0009] (Aspect 1) A welded joint characterized by comprising a welded metal in a welded portion, the chemical composition of the welded metal contains, in mass %: C: 0.020 to 0.080 %, Si: 0.10 to 0.70 %, Cr: 0.10 to 0.60 %, Mo: 0.10 to 0.70 %, Ti: 0.010 to 0.050 %, P: 0.020 % or less, S: 0.0200 % or less, N: 0.0200 % or less, O: 0.0400 % or less, Mn: 0 to 1.50 %, Ni: 0 to 5.00 %, Nb: 0 to 0.010 %, V: 0 to 0.010 %, Al: 0 to 0.010 %, W: 0 to 0.70 %, Ta: 0 to 0.0050, Cu: 0 to 1.00 %, Co: 0 to 0.50 %, Pb: 0 to 0.100 %, Sn: 0 to 0.100 %, B: 0.0001 to 0.0030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.0100%, Hf: 0 to 0.0050%, REM: 0 to 0.0050%, the remainder consisting of Fe and impurities, and a carbon equivalent Ceq represented by the following formula 1 is 0.50 to 0.64 mass%, the weld metal contains a reheated portion region at a structure ratio of 34% or more in a region between a depth position of 2 mm from the surface and a depth position of 12 mm in the central portion of the weld.

[0010] Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (Formula 1), In the above formula 1, [C], [Si], [Mn], [Ni], [Cr], [Mo], and [V] represent the contents of C, Si, Mn, Ni, Cr, Mo, and V, respectively, in mass%, and elements not contained in the weld metal are 0 mass%.

[0011] (Scheme 2) The welded joint according to the above-described scheme 1, characterized in that the chemical composition contains, in mass%, one or two or more kinds of elements selected from the following: Mn: 0.01 to 1.50%, Ni: 0.001 to 5.00%, Nb: 0.0001 to 0.010%, V: 0.0001 to 0.010%, Al: 0.0001 to 0.010%, W: 0.01 to 0.70%, Ta: 0.0001 to 0.0050%, Cu: 0.01 to 1.00%, Co: 0.001 to 0.50%, Pb: 0.001 to 0.100%, Sn: 0.001 to 0.100%, B: 0.0001 to 0.0030%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Zr: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0050%, and REM: 0.0001 to 0.0050%.

[0012] (Scheme 3) A method for manufacturing a welded joint, characterized by manufacturing the welded joint by multi-layer build-up welding, comprising: a step (a) of performing welding of an n-th layer; a step (b) of reducing an upper surface portion of a welded metal portion formed by the welding of the n-th layer; and a step (c) of further forming a welded metal portion on the welded metal portion of the n-th layer after the reduction of the upper surface portion, and forming a reheated portion region in at least a part of the welded metal portion of the n-th layer, the n is a natural number.

[0013] (Scheme 4) The method for manufacturing a welded joint according to the above-described scheme 3, characterized by repeatedly performing the step (b) and the step (c), thereby reducing an upper surface portion of all the welded metal portions which are not exposed to a surface of the welded joint, and forming the reheated portion region in at least a part of the all the not-exposed welded metal portions.

[0014] Effects of the Invention According to the present application, it is possible to provide a welded joint having a tensile strength of 780 MPa or more and excellent toughness even after PWHT. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is a view schematically showing a cross section of a welded joint 1 of one embodiment of the present application with a weld 2 as the center.

[0016] Figure 2 is an optical microscope photograph for explaining a reheated portion region. In Figure 2 , the left and right photographs are the same photograph, but in the right photograph, the original γ grain boundaries and the molten boundaries are indicated by dotted lines.

[0017] Figure 3 is a schematic view for explaining a method of manufacturing a welded joint of one embodiment of the present application by multi-layer build-up welding of gas shielded arc welding. DETAILED DESCRIPTION

[0018] The following describes a preferred embodiment of the welded joint of the present application in detail with reference to the drawings. Note that in this specification, various numerical ranges refer to ranges including the upper and lower limits thereof unless otherwise specified.

[0019] The inventors of the present application have conducted intensive research in order to achieve the above-mentioned object, focusing on reducing the amount of P segregated at the original γ grain boundaries at the time of PWHT. As a result, the inventors of the present application have found that when using a welding metal having a specific chemical composition, performing multi-layer surfacing in a manner that stacks the multi-layer welding metal portions, and applying heat from the subsequent passes to each of the welding metal portions, ferrite structures are formed on the grain boundaries of the welding metal, and the proportion of the ferrite structures is increased to an amount of or more than a certain amount, thereby ensuring a tensile strength of 780 MPa or more and excellent toughness even after PWHT. The present application was completed based on this insight.

[0020] < Welded joint > Figure 1 is a view schematically showing a cross section of the welded joint 1 of one embodiment of the present application with the weld 2 as the center. As shown in Figure 1 , the welded joint 1 of the present embodiment is composed of the weld 2 including a welding metal and the steel material 4 as a base material.

[0021] Note that in Figure 1 , the up-and-down direction of the Figure 1 is a direction corresponding to the plate thickness direction of the welded joint 1, and the left-and-right direction of the Figure 1 is a direction orthogonal to the direction in which the weld 2 extends, i.e., a direction corresponding to the width direction of the weld 2. In addition, the direction orthogonal to the cross section of the welded joint 1 shown in Figure 1 , i.e., the direction orthogonal to both the up-and-down direction and the left-and-right direction of the Figure 1 is a direction corresponding to the direction in which the weld 2 extends.

[0022] In the welded joint 1 of the present embodiment, the weld 2 is formed by multi-layer surfacing such as gas shielded arc welding, in a manner that stacks the multi-layer welding metal portions 31 including the welding metal. Furthermore, the molten line (thick line portion in Figure 1 ) at the boundary portion of the multi-layer welding metal portions 31 becomes a reheated portion region 32 that is reheated to the Ac1 transformation point or more by the subsequent passes at the time of multi-layer surfacing. The reheated portion region 32 is a region in which the structure of the welding metal is transformed from the grain boundaries to austenite (γ) by reheating of the welding metal portions 31, and is further transformed to ferrite in the subsequent cooling process.

[0023] Furthermore, the welded joint 1 of the present embodiment is such thatFigure 1 As shown, it has the following unique configuration: at the center of the welded part 2, the distance surface S A In region A, which is between a depth of 2 mm and a depth of 12 mm, there is a reheated region 32 formed by reheating to above the Ac1 phase transition point, with a tissue ratio of 34% or more.

[0024] (The area between the central part of the weld and the depth position of 2mm and 12mm from the surface) It should be noted that "the central part of the welded section" refers to the section along... Figure 1 The imaginary center line C along the width direction of the welded part 2 in the cross-section shown is shown in the figure. L The straight section. Furthermore, "the area between a depth of 2mm and a depth of 12mm from the center of the welded portion" in... Figure 1 The center refers to the distance S from the surface of the welded part 2 in the central part. A ( Figure 1 The upper surface of the plate (at a depth of 2 mm in the thickness direction) is located at a distance S from the weld portion 2. A Region A is located at a depth of 12 mm in the thickness direction of the plate. This region is the central part of the welded part 2, that is, the imaginary center line C along the width direction of the welded part 2. L The region A is a linear segment, therefore region A was originally also a linear region, but... Figure 1 In order to easily understand and represent region A, it is represented by a region with a certain width.

[0025] (Contains a reheat region with a tissue ratio of over 34%) In addition, "containing a reheated region with a tissue ratio of 34% or more" means that the tissue ratio of the reheated region, as measured in accordance with the following <Method for Determining the Tissue Ratio of the Reheated Region>, is 34% or more.

[0026] If the microstructure ratio in the reheat region is 34% or higher, then even after a PWHT at 600°C for 2 hours, an absorbed energy of over 50 J can be ensured in a Charpy impact test at -40°C. In other words, if the microstructure ratio in the reheat region is 34% or higher, excellent toughness can be ensured even after PWHT. It should be noted that the absorbed energy value in the Charpy impact test at -40°C is used as an evaluation index for toughness at low temperatures.

[0027] The welded joint 1 of the present embodiment can suppress the segregation of P at the grain boundary because the welded joint 1 uses a welding metal having a specific chemical composition described later, and the structure ratio of the reheated portion region 32 in the region A is 34% or more, so that a certain amount or more of ferrite structure is formed at the grain boundary where P segregation originally occurs, and as a result, the region where P segregation can occur at the time of PWHT is reduced. As a result, the welded joint 1 of the present embodiment can ensure a tensile strength of 780 MPa or more and excellent toughness even after PWHT. The structure ratio of the reheated portion region 32 in the region A can also be 35% or more, 37% or more, or 39% or more. In addition, the structure ratio of the reheated portion region 32 in the region A can also be 95% or less, 90% or less, 85% or less, or 80% or less.

[0028] The reheated portion region 32 can also have a structure such as martensite and retained austenite depending on the welding conditions, but regardless of this, a certain amount or more of other structure is formed at the original γ grain boundary, so that the region where P segregation can occur at the time of PWHT is reduced, and as a result, the region where P segregation can be suppressed at the grain boundary.

[0029] In the welded portion of the welded joint, the reheated portion region can also be identified by observing the cross section of the welded portion using an optical microscope, similarly to the method for measuring the structure ratio of the reheated portion region described later. Specifically, as shown in FIG. 6, the original γ grain boundary becomes clear in the un-reheated portion, and the original γ grain boundary becomes unclear in the reheated portion. Figure 2

[0030] The structure ratio of the reheated portion region can be measured by the following measurement method.

[0031] <Measurement method of structure ratio of reheated portion region> (1) The welded portion of the welded joint is cut along the width direction orthogonal to the direction in which the welded portion extends.

[0032] (2) The cut surface is etched with a 3% nitric acid ethanol etchant.

[0033] (3) The cut surface after etching is observed using an optical microscope. At this time, the region A between the depth position of 2 mm and the depth position of 12 mm from the surface of the central portion of the welded portion is observed, and the reheated portion region divided by the portion (i.e., the fusion line) in which the color is relatively dark is confirmed by visual observation.

[0034] ​(4) The plate thickness direction length (mm) of the portion of the reheated zone along the imaginary center line in the width direction of the weld portion is measured. The measurement is performed for all the reheated zones within the above-described region A, and the total value is taken as the total plate thickness direction length (mm) of the reheated zone.

[0035] (5) The structure ratio (%) of the reheated zone is calculated by dividing the total plate thickness direction length of the reheated zone by the plate thickness direction length (10 mm) of the above-described region A and multiplying by 100.

[0036] In order to adjust the reheated zone to have a structure ratio of 34% or more, when performing multi-layer deposition welding in a manner that the multi-layer weld metal portions are stacked, it is necessary to reduce the upper surface portion of the weld metal portion of each bead, on which the subsequent bead is performed, thereby reducing the thickness of the weld metal portion of each bead and relatively increasing the proportion of the reheated zone.

[0037] The welding method that can be used in the production of the welded joint 1 of the present embodiment is a gas shielded arc welding method using a welding wire that becomes a weld metal and a shielding gas. Note that the shielding gas used in this gas shielded arc welding can use, for example, 100 vol% Ar gas (pure Ar gas), 100 vol% carbon dioxide gas (pure carbon dioxide gas), a mixed gas of Ar and 3 to 30 vol% CO2, or the like. The gas flow rate is 20 to 25 L / min.

[0038] The kind of base material, welding voltage, welding current, welding posture, and the like are the usual welding conditions corresponding to the desired product form, the kind of welding method, and the like. For example, the welding current is 50 to 500 A, and the welding voltage is 10 to 40 V. In addition, the welding speed is 20 to 40 cm / min, and the linear energy is 15 to 25 kJ / cm. Furthermore, the preheating / interpass temperature is 80 to 200°C.

[0039] (Weld Metal) The weld metal used in the welded joint 1 of the present embodiment has the following specific chemical composition.

[0040] That is, the weld metal has the following chemical composition: contains, in mass%: C: 0.020 to 0.080%, Si: 0.10 to 0.70%, Mn: 0 to 1.50%, P: 0.020% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0400% or less, Ni: 0 to 5.00%, Cr: 0.10 to 0.60%, Mo: 0.10 to 0.70%, Nb: 0 to 0.010%, V: 0 to 0.010%, Ti: 0.010 to 0.050%, Al: 0 to 0.010%, W: 0 to 0.70%, Ta: 0 to 0.0050, Cu: 0 to 1.00%, Co: 0 to 0.50%, Pb: 0 to 0.100%, Sn: 0 to 0.100%, B: 0 to 0.0030%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.0100%, Hf: 0 to 0.0050%, REM: 0 to 0.0050%, the remainder consisting of Fe and impurities, and a carbon equivalent Ceq represented by the following formula 1 is 0.50 to 0.64 mass% or more.

[0041] Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (Formula 1) In the above formula 1, [C], [Si], [Mn], [Ni], [Cr], [Mo], and [V] respectively represent the contents of C, Si, Mn, Ni, Cr, Mo, and V in mass%, and the elements not contained in the weld metal are 0 mass%.

[0042] Hereinafter, each component constituting the weld metal will be described in detail. Note that in the following description, "%" means "mass%" unless otherwise specified, and the content of each component means the mass ratio of each component with respect to the total mass of the weld metal.

[0043] (C: 0.020 to 0.080%) C is an element that improves the strength of the weld metal. In order to improve the strength of the weld metal, the C content is set to 0.020% or more. In order to further improve the strength of the weld metal, the lower limit of the C content can also be 0.040%, 0.050%, or 0.060%. In addition, in order to improve the toughness of the weld metal and suppress the susceptibility to both high-temperature cracking and low-temperature cracking, the C content is set to 0.080% or less. In order to ensure stable low-temperature toughness, the upper limit of the C content can also be 0.075% or 0.070%.

[0044] (Si: 0.10 to 0.70%) Si is a deoxidizing element, and is an element that reduces the amount of O in the weld metal to improve cleanliness. In order to obtain this effect, the Si content is set to 0.10% or more. In order to sufficiently reduce the amount of O in the weld metal, the lower limit of the Si content can also be 0.15%, 0.20%, or 0.25%. In addition, in order to further improve the toughness of the weld metal, the Si content is set to 0.70% or less. In order to stably ensure the toughness of the weld metal, the upper limit of the Si content can also be 0.60%, 0.50%, or 0.40%.

[0045] (Cr: 0.10 to 0.60%) Cr is an element that is effective for high-strength of the weld metal because it improves the hardenability of the weld metal. In order to improve the strength of the weld metal, the Cr content is set to 0.10% or more. In order to further improve the strength of the weld metal, the lower limit of the Cr content can also be 0.15%, 0.20%, 0.25%, or 0.30%. In addition, in order to uniformly harden the bainite structure of the weld metal and further improve the toughness of the weld metal, the Cr content is set to 0.60% or less. In order to further improve the toughness of the weld metal, the upper limit of the Cr content can also be 0.58%, 0.57%, 0.56%, or 0.55%.

[0046] (Mo: 0.10 to 0.70%) Mo is an element that improves the hardenability of the weld metal, and is an element that is effective for ensuring the tensile strength due to precipitation strengthening by forming fine carbides. In addition, Mo has the effect of suppressing the reduction in strength at the time of re-heating due to subsequent passes at the time of multi-layer deposition welding, making it easier to ensure the toughness of the weld metal. In order to sufficiently obtain such an effect, the Mo content is set to 0.10% or more. In order to more reliably ensure the strength and toughness of the weld metal, the lower limit of the Mo content can also be 0.15%, 0.20%, or 0.25%. In addition, in order to ensure stable toughness, the Mo content is set to 0.70% or less. In order to ensure more stable toughness, the upper limit of the Mo content can also be 0.60%, 0.55%, or 0.50%.

[0047] (Ti: 0.010~0.050%) Ti is an effective deoxidizing element, reducing the amount of oxygen (O) in the weld metal. It is also effective in fixing dissolved nitrogen (N) and mitigating its adverse effects on toughness. To fully realize these effects, the Ti content is set at 0.010% or higher. To further enhance these effects, the lower limit of the Ti content can also be 0.011% or 0.012%. Furthermore, to further improve the toughness of the weld metal, the Ti content is set at 0.050% or lower. To more reliably improve the toughness of the weld metal, the upper limit of the Ti content can also be 0.045%, 0.040%, 0.035%, or 0.030%.

[0048] (P: below 0.020%) Phosphorus (P) is an impurity element. Since it reduces both the toughness and ductility of the weld metal, its content is preferably minimized. The P content is set to below 0.020%. To more reliably prevent the reduction in the toughness and ductility of the weld metal, the upper limit of the P content can also be 0.018%, 0.016%, or 0.015%. It should be noted that the lower limit of the P content can also be 0%, 0.0001%, 0.0005%, or 0.0010%.

[0049] (S: below 0.0200%) Sulfur (S) is an impurity element. Since it reduces both the toughness and ductility of the weld metal, its content is preferably minimized. The S content is set to below 0.0200%. To more reliably prevent the reduction in the toughness and ductility of the weld metal, the upper limit of the S content can also be 0.0180%, 0.0160%, 0.0140%, or 0.0120%. It should be noted that the lower limit of the S content can also be 0%, 0.0001%, 0.0005%, or 0.0010%.

[0050] (N: below 0.0200%) Nitrogen (N) is a component that excessively increases the strength of the weld metal but can cause a decrease in low-temperature toughness. Furthermore, N is an unavoidable component in the weld metal. To prevent excessive strength increase in the weld metal and ensure low-temperature toughness, the N content is set to 0.0200% or less. To more reliably achieve these effects, the upper limit of the N content can also be 0.0180%, 0.0170%, or 0.0160%. It should be noted that a lower N content is preferred. The lower limit of the N content can also be 0%, but setting it to 0% is difficult industrially; therefore, it is practically set to 0.0010% or 0.0020%.

[0051] (O: below 0.0400%) O (oxide) is an oxide that forms during welding, serving as the starting point for the formation of fine microstructures and facilitating the improvement of weld metal toughness. Furthermore, O is an unavoidable component in weld metal. From the viewpoint of preventing oxide coarsening and further improving weld metal toughness, the O content is set to 0.0100% or less. To more reliably achieve these effects, the upper limit of the O content can also be 0.0080% or 0.0060%. Alternatively, the O content can be 0%, but from the viewpoint of improving weld metal toughness, 0.0050% or more is preferred. To more reliably improve weld metal toughness, the lower limit of the O content can also be 0.0060%, 0.0070%, 0.0080%, or 0.0090%.

[0052] (Mn: 0~1.50%) Mn is an element that ensures the hardenability of the weld metal to improve its strength. The lower limit for Mn content is 0%. To more reliably improve the strength of the weld metal, the Mn content is preferably 0.01% or more. To more stably improve the strength of the weld metal, the lower limit for Mn content can also be 0.05%, 0.10%, or 0.50%. Furthermore, to suppress the susceptibility to grain boundary embrittlement and further improve the toughness of the weld metal, the Mn content is set to 1.50% or less. To further improve the toughness of the weld metal, the upper limit for Mn content can also be 1.48%, 1.46%, or 1.44%.

[0053] (Ni: 0~5.00%) Ni is an element that, through solid solution treatment, enhances toughness, allowing for further improvement of weld metal toughness regardless of microstructure and composition. In particular, Ni is an effective element for further improving toughness in high-strength weld metals with a tensile strength of 780 MPa or higher. The lower limit for Ni content is 0%. To more reliably achieve the aforementioned effect, a Ni content of 0.001% or higher is preferred. To further improve the toughness of the weld metal, the lower limit for Ni content can also be 0.10%, 0.50%, or 1.00%. Furthermore, to ensure resistance to weld cracking, the Ni content is set to 5.00% or less. To more reliably ensure resistance to weld cracking, the upper limit for Ni content can also be 4.00%, 3.50%, or 3.00%.

[0054] (Nb: 0~0.010%) Nitrogen (Nb) forms fine carbides in weld metal, making it an effective element for ensuring the tensile strength of weld metal due to precipitation strengthening. The lower limit for Nb content is 0%. To improve the tensile strength of the weld metal, the Nb content can also be 0.0001% or higher. The lower limit for Nb content can also be 0.0005% or 0.0010%. Furthermore, to further improve the toughness of the weld metal, the Nb content is set to 0.010% or less. To more reliably improve the toughness of the weld metal, the upper limit for Nb content can also be 0.008%, 0.007%, 0.006%, or 0.005%.

[0055] (V: 0~0.010%) V (V) is an effective element for increasing the strength of weld metal because it improves the hardenability of the weld metal. The lower limit of V content is 0%. To further improve the strength of the weld metal, the V content can also be 0.0001% or more. The lower limit of V content can also be 0.0005% or 0.0010%. In addition, to suppress the hardening of the weld metal and further improve its toughness, the V content is set to 0.010% or less. To reliably obtain these effects, the upper limit of V content can also be 0.008%, 0.007%, 0.006%, or 0.005%.

[0056] (Al: 0~0.010%) Al is a deoxidizing element that reduces the oxygen content in weld metal and improves its cleanliness. The lower limit for Al content is 0%. To fully realize these effects, the Al content can also be 0.0001% or higher. The lower limit for Al content can also be 0.0005% or 0.0010%. Furthermore, to suppress the formation of nitrides and oxides and further improve the toughness of the weld metal, the Al content is set to 0.010% or lower. To more reliably improve the toughness of the weld metal, the upper limit for Al content can also be 0.008%, 0.007%, 0.006%, or 0.005%.

[0057] [W: 0~0.70%, Ta: 0~0.0050%, Cu: 0~1.00%, Co: 0~0.50%, Pb: 0~0.100%, Sn: 0~0.100%, B: 0~ 0.0030%, Ca: 0~0.0050%, Mg: 0~0.0050%, Zr: 0~0.0100%, Hf: 0~0.0050%, REM: 0~0.0050%] In this embodiment, the welding metal may optionally contain W, Ta, Cu, Co, Pb, Sn, B, Ca, Mg, Zr, Hf, and REM in specified amounts. For example, it may contain W at 0-0.70%, Ta, Ca, and Mg at 0-0.0050%, Cu at 0-1.00%, Co at 0-0.50%, Pb and Sn at 0-0.100%, B at 0-0.0030%, Zr at 0-0.0100%, and Hf and REM at 0-0.0050%. These components may be incorporated due to various factors in the raw materials and manufacturing process, or they may be intentionally included to achieve effects such as high strength. The total content of these components may also be 0.100% or less.

[0058] It should be noted that, in this specification, REM refers to the collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are lanthanide elements. REM content is the total content of these elements.

[0059] In this embodiment, the chemical composition of the welding metal may also contain one or more elements selected from the following, expressed as a percentage by mass: Mn: 0.01~1.50%, Ni: 0.001~5.00% Nb: 0.0001~0.010% V: 0.0001~0.010% Al: 0.0001~0.010% W: 0.01~0.70%, Ta: 0.0001~0.0050%, Cu: 0.01~1.00%, Co: 0.001~0.50%, Pb: 0.001~0.100% Sn: 0.001~0.100% B: 0.0001~0.0030% Ca: 0.0001~0.0050%, Mg: 0.0001~0.0050%, Zr: 0.0001~0.0100% Hf: 0.0001~0.0050%, and REM: 0.0001~0.0050%.

[0060] (The rest of the text) The remaining components of the welding metal besides the aforementioned elements are Fe and impurities. Examples of impurities include Sb, As, and Bi.

[0061] (Ceq: 0.50–0.64% by mass) In addition, in this embodiment, in order to improve tensile strength, the contents of C, Si, Mn, Ni, Cr, Mo and V in the weld metal need to be adjusted so that the carbon equivalent Ceq specified by the Japan Welding Society (WES) as expressed by the following formula 1 is 0.50 to 0.64% by mass.

[0062] Ceq=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (Formula 1) In Equation 1 above, [C], [Si], [Mn], [Ni], [Cr], [Mo], and [V] represent the content (mass%) of C, Si, Mn, Ni, Cr, Mo, and V, respectively. It should be noted that elements not present in the weld metal are represented by 0% by mass.

[0063] To more reliably improve the tensile strength of the weld metal, the lower limit of the Ceq value can also be 0.52% by mass or 0.54% by mass. Alternatively, to balance the tensile strength and toughness of the weld metal, the Ceq value should be below 0.64% by mass. To more effectively balance the tensile strength and toughness of the weld metal, the upper limit of the Ceq value can also be 0.62% by mass, 0.60% by mass, or 0.58% by mass.

[0064] It should be noted that the C, Si, Mn, Ni, Cr, Mo and V mentioned above do not necessarily need to be pure substances, and can also be included in the weld metal in the form of Cu-Ni alloys.

[0065] It should be noted that the chemical composition of the weld metal can be determined using general analytical methods. For example, the chemical composition of the weld metal can be determined using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Specifically, a 20mm square test piece is collected from approximately 7mm below the surface at the center of the weld, and the chemical composition of the weld metal can be determined using a measuring device such as the Shimadzu ICPS-8100, under conditions based on pre-made calibration lines. C and S, which cannot be measured using ICP-AES, can be measured using the combustion-infrared absorption method; N can be measured using the inert gas melting-thermal conductivity method; and O can be measured using the inert gas melting-non-dispersive infrared absorption method.

[0066] As described above, the welded joint of this embodiment exhibits a tensile strength of 780 MPa or more even after PWHT. Here, the tensile strength of the welded joint refers to the tensile strength in the central portion of the weld, measured by the method described later. Such a tensile strength of 780 MPa or more can be obtained by using a weld metal having the specific chemical composition described above.

[0067] (Method for determining the tensile strength of welded joints) The tensile strength of the welded joint can be determined as the tensile strength of the weld metal by a tensile test according to JIS Z 3111:2005. Test specimen A0 is used in this tensile test. Test specimen A0 samples are taken from the central part of the weld, specifically from the center of the test specimen, which is considered the weld centerline.

[0068] It should be noted that when it is not possible to collect A0 test pieces from the welded joint, the Vickers hardness of the central part of the weld can also be measured. Using the measured value of Vickers hardness, the tensile strength of the welded joint can be derived from the following correlation ("Correlation between static strength parameters", Norihiko Hasegawa, Junichi Arai, Michishichi Tanaka, "Materials" Vol. 39 No. 442, pp. 859-863).

[0069] Hv = 0.301 × TS + 5.701 In the above formula, Hv represents Vickers hardness and TS represents tensile strength (MPa).

[0070] The Vickers hardness of the central part of the weld can be measured according to JIS Z 2244:2009. Specifically, the Vickers hardness of the central part of the weld can be measured 10 times at a depth of 7 mm from the surface of the central part of the weld with a load of 1 kgf (approximately 9.80 N), and the arithmetic mean of the 10 measured values ​​is obtained.

[0071] It should be noted that the tensile strength of the welded joint can also be, for example, below 1000 MPa.

[0072] (Steel) In the welded joint of this embodiment, the steel used as the welded material is, as described above, designed for a tensile strength of 780 MPa or higher. Generally, the tensile strength of the steel is also 780 MPa or higher. Examples of such steel include steel plates with a tensile strength of 780 MPa or higher. It should be noted that the tensile strength of the steel can also be, for example, 1000 MPa or lower. Furthermore, the steel used as the welded material is steel with a thickness corresponding to the desired joint strength, application, etc. Examples of such steel include steel plates with a thickness of 10 mm or more, and steel plates with a thickness of 100 mm or less.

[0073] It should be noted that the tensile strength (TS) of steel can be determined as follows: First, a test piece No. 2 according to JIS Z 2241:2011 is collected from the center of the width of the steel to be tested, with the length direction perpendicular to the rolling direction. Then, a tensile test is performed on this test piece according to JIS Z 2241:2011, thereby determining the tensile strength TS (MPa).

[0074] In addition, when it is difficult to collect test pieces from the steel to be tested, the Vickers hardness of the steel can be determined in the same way as the tensile strength of the welded joint mentioned above. Using the measured value of Vickers hardness, the value of tensile strength can be derived from the following correlation formula ("Correlation between static strength parameters", Norihiko Hasegawa, Junichi Arai, Michishichi Tanaka, "Materials" Vol. 39 No. 442, pp. 859-863).

[0075] Hv = 0.301 × TS + 5.701 In the above formula, Hv represents Vickers hardness and TS represents tensile strength (MPa).

[0076] The Vickers hardness of steel can be determined according to JIS Z 2244:2009. Specifically, the Vickers hardness of steel can be measured 10 times at a depth of 1 / 4 of the steel thickness with a load of 1 kgf (approximately 9.80 N), and the average value of the 10 measurements is obtained.

[0077] In this invention, the form of the welded joint corresponds to the desired joint strength, application, etc. Examples of such welded joint forms include butt joints or T-joints obtained by beveling, T-joints or cross joints obtained by fillet welds, and corner joints.

[0078] (Application example) The welded joint of the present invention has a tensile strength of over 780 MPa and excellent toughness even after PWHT, and therefore can be applied to various structural components in various fields such as various structures of buildings, various structural components of transportation machinery such as automobiles or airplanes, and various structural components of industrial machinery.

[0079] (Manufacturing method of welded joint) Next, regarding Figure 1 The preferred manufacturing method of the welded joint 1, an embodiment of the present invention, will be described below. It should be noted that the following description is intended to illustrate a characteristic method for manufacturing an embodiment of the present invention, namely the welded joint 1, and is not intended to limit the manufacture of the welded joint 1 to the manufacturing method described below.

[0080] The method for manufacturing the welded joint 1 in this embodiment is a method of manufacturing the welded joint 1 by multi-layer surfacing welding using gas-shielded arc welding. Here, Figure 3 This is a schematic diagram illustrating a method for manufacturing a welded joint according to one embodiment of the present invention using multi-layer surfacing welding via gas-shielded arc welding. Figure 3 In the text, (1) indicates a conventional method of manufacturing a welded joint by multi-layer welding, and (2) indicates a method of manufacturing a welded joint according to an embodiment of the present invention.

[0081] like Figure 3 As shown in (1), the conventional method includes the following steps: step (1-a), which forms a predetermined position at the weld portion of the steel 4, and performs welding of the nth layer using welding metal; and step (1-b), which involves welding the weld metal portion 31 formed by the welding of the aforementioned nth layer. n On top of this, another (n+1)th layer of welded metal portion 31 is further formed. n+1 In the aforementioned nth layer of welded metal portion 31 n A reheat region 32 is formed in at least a portion of it. Here, n is a natural number.

[0082] In this conventional method, the reheating regions 32 are widely spaced, making it impossible to increase the size of the reheating region 32 in the aforementioned region A.

[0083] On the other hand, in the manufacturing method of the welded joint 1 in this embodiment, such as Figure 3 As shown in (2), it includes the following steps: step (2-a), which forms a predetermined part in the weld portion of the steel 4, and performs welding of the nth layer using welding metal; step (2-b), which forms the weld metal portion 31 formed by the welding of the aforementioned nth layer. n The upper surface portion is cut off; and process (2-c), which involves cutting off the upper surface portion of the above-mentioned nth layer of welded metal portion 31. n On top of this, another (n+1)th layer of welded metal portion 31 is further formed. n+1 A reheating region 32 is formed in at least a portion of the weld metal portion 31n of the nth layer. Here, n is a natural number.

[0084] Through these processes, a welded portion 2 is formed at a predetermined location in the welded portion of the steel 4, consisting of a multi-layer (i.e., (n+1) or more) welded metal portion 31 containing the aforementioned welded metal.

[0085] In this embodiment, by stacking additional weld metal portions 31 of subsequent weld passes on top of the weld metal portions 31 after the upper surface portion has been cut in the above-described process (B), a reheated region 32 is formed in the boundary portion of the stacked plurality of weld metal portions 31, which has been reheated by the heat of the subsequent weld passes.

[0086] Therefore, in this embodiment, in the above-described process (B), by reducing the upper surface portion of the weld metal portion 31 of each weld pass, the thickness of the weld metal portion 31 of each weld pass can be reduced, thereby relatively increasing the proportion of the reheat region 32. At this time, it is necessary to reduce the upper surface portion of the weld metal portion 31 of each weld pass so that the reheat region 32 is contained in region A between a depth of 2 mm and a depth of 12 mm from the surface at the center of the weld portion 2, with a microstructure ratio of 34% or more.

[0087] Here, in order to set the microstructure ratio of the reheated region 32 in region A to 34% or more, the upper surface portion of the weld metal portion 31 of each weld pass is cut as follows: Specifically, in a cross-section perpendicular to the weld line, the upper surface portion of the weld metal portion 31 is cut such that the thickness of the thickest portion of the weld metal portion 31 of each weld pass is 60% or less relative to the thickness before the weld metal portion 31 is cut. It should be noted that the thickness of the thickest portion of the weld metal portion 31 of each weld pass after the upper surface portion is cut is more preferably 50% or less, and even more preferably 40% or less, relative to the thickness before the weld metal portion 31 is cut.

[0088] However, if the weld metal portion 31 is excessively reduced, the number of weld passes increases, resulting in increased time and cost. Therefore, the thickness of the thickest portion of the weld metal portion 31 in each weld pass after the upper surface portion is reduced is set to be 10% or more compared to the thickness before the reduction. It should be noted that the thickness of the thickest portion of the weld metal portion 31 in each weld pass after the upper surface portion is reduced is more preferably 20% or more, and even more preferably 30% or more. Specifically, the cutting amount is 0.8 mm to 3.5 mm, preferably 1.0 to 3.0 mm. Furthermore, regarding the above-described process (B), the upper surface portion of the weld metal portion 31 of all weld passes can be reduced, or only the upper surface portion of the weld metal portion 31 of a portion of the weld passes can be reduced.

[0089] In particular, in this embodiment, by repeatedly performing the above-described steps (b) and (c), the upper surface portion of all weld metal parts 31 not exposed on the surface of the weld joint 1 is cut off, and the reheat region 32 is formed in at least a portion of the unexposed weld metal parts 31. By manufacturing the weld joint 1 in this manner, the distance surface S at the center of the weld part 2 is... A In region A, between a depth of 2 mm and a depth of 12 mm, the reheated region 32 can be easily controlled to have a tissue ratio of 34% or more.

[0090] Furthermore, in this embodiment, the reduction amount of the upper surface portion of the weld metal portion 31 is adjusted such that the weld metal contains a reheated portion region 32 with a microstructure ratio of 34% or more in region A between a depth of 2 mm and a depth of 12 mm from the center of the weld portion 2. The upper surface portion of the weld metal portion 31 can be reduced using known grinding or cutting methods such as a grinding machine.

[0091] The welding joint of the present invention is not limited to the above-described embodiments or the following examples. Suitable combinations, substitutions, and modifications can be made without departing from the purpose and spirit of the present invention.

[0092] Example The present invention will be described in more detail below with reference to embodiments, but the present invention is not limited to such embodiments.

[0093] (Fabrication of welded joints) To verify the effectiveness of the present invention, gas-shielded arc welding was performed using steel with an X-groove and welding wire under specific welding conditions to produce welded joints of the present invention examples No. 1 to 4, 9, 11 to 16 and welded joints of the comparative examples No. 5 to 8, 10. The chemical composition of samples collected from the weld metal of each welded joint was analyzed, and the results are shown in Table 1. Furthermore, the gas-shielded arc welding conditions are shown in Table 2. In the welded joints of the present invention examples No. 1 to 4, 9, 11 to 16, by performing multi-layer welding in a manner that stacks multiple layers of weld metal, the upper surface portion of the weld metal portion of each weld pass is reduced while subsequent weld passes are performed, thereby reducing the thickness of the weld metal portion of each weld pass and relatively increasing the proportion of the reheated area.

[0094] For welded joints No.1 to No.16 obtained by such operation, post-weld heat treatment (PWHT) is performed at 600°C for 2 hours.

[0095] For weld joints No. 1 to 16, the microstructure ratio of the reheated region was measured, and the absorbed energy of the Charpy impact test at -40°C was determined according to the <Charpy Impact Test Method at -40°C> described below. Additionally, the tensile strength of the weld joint was measured according to the aforementioned method. These test results are shown in Table 2 below. It should be noted that underlines in Tables 1 and 2 indicate values ​​outside the scope of this invention, weld joints with tensile strength below 780 MPa, or Charpy impact test absorbed energy at -40°C that do not meet the toughness evaluation criteria.

[0096] Charpy Impact Test Method at -40℃ Charpy test pieces (V-notch test pieces) according to JIS Z3111-2005 (Tension and Impact Test Methods for Deposited Metal) were collected from the weld metal of the weld joint. The Charpy test pieces were collected such that the area between a depth of 2 mm and 12 mm from the center of the weld portion of the weld joint corresponded to the notch position. Then, a Charpy impact test was performed at -40°C using these Charpy test pieces, and the absorbed energy was measured. The absorbed energy was measured using three Charpy test pieces, and the average value of their measurements was used.

[0097] It should be noted that welded joints with an absorbed energy of over 50 J in the Charpy impact test at -40℃ are evaluated as having excellent toughness.

[0098] [Table 1] [Table 2] As shown in Table 2, it can be seen that the welded joints of Examples No. 1 to 4, 9, and 11 to 16 of the present invention, which have the specific chemical composition described above and a microstructure ratio of 34% or more in the reheated region, all possess a tensile strength of 780 MPa or more and an absorbed energy of 50 J or more in a Charpy impact test at -40°C. That is, it can be seen that the welded joints of Examples No. 1 to 4, 9, and 11 to 16 of the present invention possess a tensile strength of 780 MPa or more and excellent toughness even after PWHT.

[0099] In contrast, it can be seen that the welded joints of Comparative Examples No. 5 to 7, whose microstructure ratio in the reheated region is less than 34%, all exhibited absorbed energy of less than 50 J in the Charpy impact test at -40°C, indicating poor toughness. Furthermore, the welded joint of Comparative Example 8, which has a low Ti content in the weld metal and a chemical composition outside the aforementioned specific range, has a microstructure ratio in the reheated region of 34% or more, exhibiting excellent toughness, but its tensile strength is only 727 MPa, resulting in low strength. Moreover, the welded joint of Comparative Example 10, which has a high C, Ni, Cr, and V content in the weld metal and a chemical composition outside the aforementioned specific range, has a microstructure ratio in the reheated region of 34% or more and a tensile strength of 780 MPa, but its absorbed energy in the Charpy impact test at -40°C is only 28 J, resulting in poor toughness.

[0100] Industrial availability The welded joint of the present invention has a tensile strength of over 780 MPa and excellent toughness even after PWHT, and therefore can be suitable for structural components in various fields such as various structures of buildings, various structural components of transportation machinery such as automobiles or airplanes, and various structural components of industrial machinery.

[0101] Symbol Explanation 1 Welded joint 2 Welding section 31 Welded metal parts 32 Reheated area 4. Steel

Claims

1. A welded joint, characterized in that containing a weld metal, the chemical composition of the weld metal contains, in mass%: C：0.020～0.080%、 Si: 0.10 to 0.70%, Cr：0.10～0.60%、 Mo: 0.10 to 0.70%, Ti: 0.010 to 0.050%, P: 0.020% or less, S: 0.0200% or less, N: 0.0200% or less, O: 0.0400% or less, Mn: 0 to 1.50%, Ni: 0 to 5.00%, Nb: 0 to 0.010%, V：0～0.010%、 Al：0～0.010%、 W：0～0.70%、 Ta: 0 to 0.0050%, Cu: 0 to 1.00%, Co: 0 to 0.50%, Pb: 0 to 0.100%, Sn: 0 to 0.100%, B：0～0.0030%、 Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr：0～0.0100%、 Hf: 0 to 0.0050%, REM: 0 to 0.0050%, the remainder being constituted by Fe and impurities, and a carbon equivalent Ceq represented by the following formula 1 is 0.50 to 0.64 mass%: the weld metal contains a reheated portion region at a structure ratio of 34% or more in a region between a depth position of 2 mm and a depth position of 12 mm from a surface in a central portion of the weld portion, Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 (Formula 1), in the formula 1, [C], [Si], [Mn], [Ni], [Cr], [Mo], and [V] respectively represent contents of C, Si, Mn, Ni, Cr, Mo, and V in mass%, and elements not contained in the weld metal are 0 mass%.

2. The welded joint of claim 1, wherein, the chemical composition contains, in mass%, one or two or more kinds selected from the following elements: Mn: 0.01 to 1.50%, Ni: 0.001 to 5.00%, Nb: 0.0001 to 0.010%, V：0.0001～0.010%、 Al：0.0001～0.010%、 W：0.01～0.70%、 Ta: 0.0001 to 0.0050%, Cu: 0.01 to 1.00%, Co: 0.001 to 0.50%, Pb: 0.001 to 0.100%, Sn: 0.001 to 0.100%, B：0.0001～0.0030%、 Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Zr：0.0001～0.0100%、 Hf: 0.0001 to 0.0050%, and REM: 0.0001 to 0.0050%.

3. A method of manufacturing a welded joint, characterized by a welded joint is manufactured by multi-layer build-up welding, comprising: a step (a) of performing welding of an n-th layer; a step (b) of reducing an upper surface portion of a weld metal portion formed by the welding of the n-th layer; and a step (c) of further forming a weld metal portion on the weld metal portion of the n-th layer after the reduction of the upper surface portion, a reheated portion region being formed in at least a part of the weld metal portion of the n-th layer, the n being a natural number.

4. The method of producing a welded joint according to claim 3, characterized in that, by repeatedly performing the step (b) and the step (c), an upper surface portion of all of the weld metal portions not exposed to a surface of the welded joint is reduced, and the reheated portion region is formed in at least a part of the all of the weld metal portions not exposed.

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