Welded joint, manufacturing method of welded joint, automobile parts and building materials

Abstract

To improve an electro deposition property of a welding portion not depending on a chemical composition or welding conditions of a steel material and a welding wire to be used, without greatly reducing productivity.SOLUTION: A weld joint according to the present invention has a plurality of steel materials and weld beads. In the weld joint, at least part of a weld slag, a spatter, and a surface of a heat affected zone has a conductive coating film having a volume resistivity of 3.00×108 Ω m or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a welded joint, a method for manufacturing a welded joint, an automobile component, and a building material. [Background technology]

[0002] Automobile components, including chassis components, and various building materials are often manufactured using welded joints in which multiple steel materials are welded together. In order to ensure long-term strength reliability in salt-damaged areas, the thickness of the steel materials used in the welded joints of these automobile and building materials is designed taking corrosion into consideration, and as a result, the lower limit of the thickness of the steel materials used is limited. In order to reduce the weight of these automobile and building materials, it is necessary to improve the post-painting corrosion resistance of the welded joints used. Since there is room for improvement in the electrodeposition paintability of the welded parts of the welded joints, in order to improve the post-painting corrosion resistance of the welded joints, it is effective to improve the electrodeposition paintability of the welded parts of the welded joints.

[0003] One of the factors that leaves room for improvement in the electrocoatability of welded joints is that non-conductive silicon-based slag generated during welding can cause poor paintability. In areas where poor paintability occurs, red rust can develop and grow quickly, and the plate thickness can decrease due to the progression of corrosion, which can lead to a decline in component performance. Therefore, various technologies have been proposed to improve the electrocoatability of welds.

[0004] For example, the following Patent Documents 1 and 2 propose methods for physically or chemically removing non-conductive Si-based slag. Specifically, the following Patent Document 1 proposes a technique for removing slag by pickling, and the following Patent Document 2 proposes a technique for removing slag by shot blasting.

[0005] Furthermore, the following Patent Document 3 proposes a technology for reducing the amount of slag produced by reducing deoxidizing elements (elements that have a high affinity with oxygen that constitutes slag) in the steel material and welding wire, and by reducing the proportion of oxidizing gas in the shielding gas.

[0006] Furthermore, the following Patent Document 4 proposes a technology for improving electrodeposition paintability by changing non-conductive slag, which causes painting defects, into conductive slag. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 9-20994 [Patent Document 2] JP 2013-226585 A [Patent Document 3] Japanese Patent Application Publication No. 8-33982 [Patent Document 4] JP 2019-81195 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, in the case of the techniques for physically or chemically removing slag as proposed in the above Patent Documents 1 and 2, it is necessary to add a process for removing the slag, which results in a significant increase in production costs and a decrease in production efficiency.

[0009] In addition, in the case of the technology for reducing slag as proposed in the above Patent Document 3, it is expected that the slag that causes poor electrodeposition coating can be reduced, but the material properties and welding workability become new problems. For example, from the viewpoint of steel, the amount of alloy elements added increases with the increase in the strength of steel, and the amount of deoxidizing elements that constitute the slag increases, so it is difficult to reduce the slag that causes poor electrodeposition coating while achieving high strength and thinning of the steel. In addition, the deoxidizing elements in the welding wire cause insufficient deoxidization of the weld metal (weld bead), and there is a concern that the performance of the welded joint will decrease. Furthermore, from the viewpoint of shielding gas, it is considered that the reduction in oxidizing gas makes the droplet transfer phenomenon unstable, increasing spatter and decreasing the stability of the arc. In this way, there is a trade-off between the material properties and welding workability and the reduction in slag, and there is a limit to the reduction in deoxidizing elements or oxidizing gas.

[0010] Furthermore, to modify the slag as proposed in Patent Document 4, it is necessary to increase or decrease the content of a specific deoxidizing element, and for steel materials such as TRIP steel that contain a large amount of silicon (Si), the effect of reducing electrodeposition coating defects is limited.

[0011] From the above viewpoints, there is a demand for a technology that can improve the electrodeposition paintability of a weld without significantly impairing productivity, regardless of the chemical composition of the steel material and welding wire used or the welding conditions.

[0012] Therefore, the present invention has been made in consideration of the above problems, and an object of the present invention is to provide a welded joint and a method for manufacturing a welded joint that are capable of improving the electrodeposition paintability of a welded portion without significantly impairing productivity, regardless of the chemical composition of the steel material and welding wire used or the welding conditions, and an automotive component and a building material having such a welded joint. [Means for solving the problem]

[0013] In order to solve the above problems, the present inventors conducted intensive research and came up with the idea of ​​providing a conductive coating film exhibiting a specific electrical conductivity on at least a portion of the surface of the heat-affected zone and on the welding slag and spatter that cause poor coating quality of the welded zone.

[0014] Conventionally, materials for welded joints (especially steel thickness) have been studied on the assumption that the welded portion will corrode, so no attempt has been made to provide a conductive coating on the welded portion, which would increase costs. However, as steel becomes thinner in the trend toward reducing the weight of automobile components, it becomes difficult to satisfy the desired mechanical strength if corrosion of the welded portion is allowed. The inventors of the present invention thought that a conductive coating can be realized regardless of the chemical composition of the steel. Furthermore, the inventors of the present invention thought that a painting process for providing a conductive coating can be applied to the current welded joint manufacturing line without significant expansion or improvement of the line.

[0015] Also, for example, in the case of automobile components, the strength of the steel used for the undercarriage components (chassis components) and the body components differ. For example, chassis components often use steel plates with strengths in the 440 to 780 MPa range, while body components use higher strength steel plates of 980 MPa or 1180 MPa range.

[0016] When improving the strength of steel plates for chassis components, the hardenability of the steel is important. In order to improve the hardenability of steel, Si is generally added to the steel. The addition of Si to steel has the effect of suppressing the carbon (C) in the steel from becoming carbide, increasing the ratio of martensite formation and increasing the hardness of martensite. As a result, the hardenability of the steel is improved. In order to realize such an effect, it is preferable that the amount of Si in the steel is 0.8 to 1.7 mass% in the production of high-strength steel. On the other hand, an increase in the amount of Si in the steel generates non-conductive Si-based slag that leads to poor electrocoating, so from the viewpoint of corrosion resistance, it is desirable to reduce the amount of Si in the steel to a very small amount. From this viewpoint, it is considered that the addition of Si to steel is unavoidable in promoting the weight reduction of automobile components (higher strength and thinner chassis components).

[0017] In addition, with regard to the welding wire used in manufacturing a welded joint, the amount of Si in the welding wire components affects the workability during welding and the corrosion resistance of the weld. From the viewpoint of workability, a Si content of 0.1 to 0.9 mass% in the welding wire is expected to stabilize droplet transfer and reduce spatter. On the other hand, from the viewpoint of corrosion resistance, it is desirable to reduce the amount of Si in the welding wire to an extremely small amount. Thus, when manufacturing a welded joint, it is not possible to easily reduce the amount of Si in the steel material and the welding wire. The gist of the present invention, which has been completed based on the above-mentioned concept, is as follows.

[0018] (1) A welded joint having a plurality of steel materials and a weld bead located between the plurality of steel materials, in which welding slag, spatter, and at least a part of the surface of the heat-affected zone have a volume resistivity of 3.00×10 8 Conductive coating with a resistance of Ω·m or less A chemical conversion coating (excluding cases where the chemical conversion coating contains iron phosphate) is formed on the conductive coating. , welded joint. (2) The welded joint described in (1), wherein the thickness of at least one of the plurality of steel materials is 2.0 mm or less. (3) The welded joint according to (1) or (2), wherein the Si content of at least one of the plurality of steel materials is 0.8 to 1.7 mass %. (4) The volume resistivity of the conductive coating is 1.00×10 6 The welded joint according to any one of (1) to (3), having a resistance to deformation of tensile strength (SSS) of Ω·m or less. (5) The welded joint according to any one of (1) to (4), wherein the conductive coating has a thickness of 10 μm or more. (6) The welded joint according to any one of (1) to (5), wherein the conductive coating has a thickness of 20 μm or less. (7) The welded joint according to any one of (1) to (6), wherein the conductive coating film is made of a polyester-based AgCu conductive paint. (8) The welded joint according to any one of (1) to (6), wherein the conductive coating film is made of zinc paint. (9) The welded joint according to any one of (1) to (8), wherein the conductive coating is provided over the entire surfaces of the welding slag, the spatter, and the heat-affected zone. (10) The welded joint according to any one of (1) to (9), wherein the conductive coating is provided over the entire surface of the weld bead. (11) The welded joint according to any one of (1) to (10), further comprising an electrodeposition coating on a surface of the conductive coating. ( 12 ) A chemical conversion coating is further provided between the plurality of steel materials and the conductive coating. 11 ) A welded joint described in any one of the above. ( 13 ) The welded joint according to any one of (1) to (12), wherein the chemical conversion coating contains one or more selected from the group consisting of a resin, a silane coupling agent, a zirconium compound, silica, phosphoric acid and its salts (excluding iron phosphate), a fluoride, a vanadium compound, and tannin or tannic acid. (14) A method for manufacturing a welded joint having a plurality of steel materials and a weld bead located between the plurality of steel materials, the method comprising the steps of: welding the plurality of steel materials together using a welding wire; and forming a weld slag, spatter, and a weld bead having a volume resistivity of 3.00×10 on at least a part of a surface of a heat-affected zone. 8 a deposition process for depositing a conductive coating film having a resistance of Ω·m or less; a chemical conversion treatment step of forming a chemical conversion film on the conductive coating film (excluding cases where the chemical conversion film is a chemical conversion film containing iron phosphate); A method for manufacturing a welded joint comprising the steps of: (15) The method for manufacturing a welded joint according to (14), wherein in the film-forming step, the conductive coating film is formed by immersion in a conductive paint or by applying the conductive paint by brushing or spraying. (16) A method for manufacturing a welded joint as described in (14) or (15), wherein in the film-forming process, the welding slag is identified as non-conductive, and a conductive paint is applied to at least a portion of the non-conductive welding slag to form the conductive coating film. (17) The method for producing a welded joint according to any one of (14) to (16), further comprising at least one of a degreasing step, a surface conditioning step, or a chemical conversion treatment step. (18) An automobile component having the welded joint according to any one of (1) to (13). (19) A building material having the welded joint according to any one of (1) to (13). Effect of the Invention

[0019] As described above, according to the present invention, it is possible to improve the electrodeposition paintability of a welded portion without significantly impairing productivity, regardless of the chemical compositions of the steel material and welding wire used or the welding conditions. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Preferred embodiments of the present invention will be described in detail below.

[0021] (About welded joints) As described above, the welded joint according to the embodiment of the present invention is provided with a conductive coating film that exhibits electrical conductivity at a portion that would cause poor electrodeposition coating. Such a welded joint can be suitably used as a material for an automobile component or a building material.

[0022] In this embodiment, the term "a coating film exhibits electrical conductivity" means that the electrical resistivity (more specifically, the volume resistivity at room temperature (20° C.)) of the coating film of interest is 3.00×10 8 This means that the volume resistivity is Ω·m or less. In the following, unless otherwise specified, the volume resistivity value is the value at room temperature (20°C).

[0023] More specifically, the weld joint according to the present embodiment has a plurality of steel materials and a weld bead located between the plurality of steel materials. The weld slag located on the surface of the weld bead, the spatter scattered on the steel material surface, and at least a part of the surface of the heat-affected zone present around the weld bead have a volume resistivity of 3.00×10 8 It has a conductive coating with a resistance of Ω·m or less.

[0024] The welded joint according to this embodiment has a volume resistivity (more specifically, the volume resistivity at room temperature (20° C.)) of 3.00×10 8 By having a conductive coating that exhibits a specific volume resistivity of Ω·m or less, it is possible to impart conductivity to areas that cause electrodeposition coating defects (i.e., welding slag, spatter, and heat-affected zones). As a result, it becomes possible to apply electrodeposition coating to these areas, and regardless of the chemical composition of the steel and welding wire used or the welding conditions, it is possible to improve the electrodeposition paintability of welds without significantly impairing productivity, and ultimately contribute to improving corrosion resistance after painting. Hereinafter, the members constituting the welded joint according to this embodiment will be described in detail.

[0025] <About steel materials> The welded joint according to the present embodiment is manufactured by welding a plurality of steel materials using a welding wire. Here, the chemical composition of the steel material used for the welded joint is not particularly limited, and may be selected according to the mechanical strength (for example, tensile strength, etc.) required for the welded joint, and various steel materials including TRIP steel containing a large amount of Si can be used. However, the present invention is particularly useful when a steel material having a Si content of 0.8 to 1.7 mass% is used. By using the present invention, it is possible to improve the electrodeposition paintability of the welded portion even in the case of a steel material having the above-mentioned Si content. The steel material used for the welded joint according to the present embodiment may be a plated steel material in which various zinc-based plating, aluminum-based plating, etc. are formed on the surface of the base steel material.

[0026] In addition, the thickness of the steel material is not particularly limited, but the present invention is particularly useful when the thickness of the steel material is 2.0 mm or less. By using the present invention, it is possible to improve the electrodeposition paintability of the welded portion even when the thickness of the steel material is 2.0 mm or less.

[0027] <About weld beads> The welded joint according to the present embodiment is manufactured by welding a plurality of steel materials using a welding wire, and a weld metal (weld bead) is formed at the portion where the steel materials are welded. Here, the weld bead refers to a weld formed as a result of a part of the steel material and the welding wire melting and mixing with each other. Here, the welding wire used for welding is not particularly limited, and various welding wires can be used. However, the present invention is particularly useful when a welding wire having a Si content of 0.1 to 0.9 mass% is used. By using the present invention, it is possible to improve the electrodeposition paintability of the welded portion even when a welding wire having the above-mentioned Si content is used.

[0028] Welding slag is present on the surface of such weld beads, and scattered spatter is present on the weld bead surface and the steel surface. Welding slag is a metal oxide produced during welding, and often has poor electrical conductivity required for electrodeposition coating, resulting in poor electrodeposition coating. Spatter refers to molten metal produced during welding that is scattered onto steel materials and solidified. In addition, a heat affected zone (HAZ) is formed around the weld bead. The weld bead itself is electrically conductive, while the spatter and the heat affected zone are less electrically conductive than the weld bead. Therefore, these spatter and the heat affected zone cause the coating film to become thinner in the electrodeposition coating that is performed later, and there is a concern that the electrodeposition coating properties will be reduced.

[0029] The locations of the welding slag, spatter, and heat-affected zone can be easily identified based on the captured image obtained by capturing an image of the appearance around the weld bead using an imaging device.

[0030] <About conductive coating> In the welded joint according to the present embodiment, the volume resistivity is 3.00×10 at least on a part of the surface of the welding slag, the spatter, and the heat-affected zone. 8 A conductive coating film having a volume resistivity of 3.00×10 Ω·m or less is provided. 8 If the volume resistivity exceeds Ω·m, the electrical conductivity required for the subsequent electrodeposition coating cannot be achieved, and the electrodeposition coating properties of the welded joint cannot be improved. 8 By keeping the resistivity below Ω·m, it becomes possible to apply electrodeposition coating to areas where poorly conductive welding slag, spatter, and heat-affected zones are present. There is no particular lower limit for the volume resistivity of the conductive coating, and the smaller the better.

[0031] The volume resistivity of the conductive coating is preferably 1.00×10 6 Ω·m or less, and more preferably 1.00×10 3 When the volume resistivity of the conductive coating falls within the above range, the coating exhibits a level of conductivity that allows electrodeposition coating to be applied sufficiently even in areas where welding slag, spatter, and heat-affected zones, which have poor electrical conductivity, are present, and the electrodeposition paintability of the welded joint can be further improved.

[0032] Here, the volume resistivity refers to the volume resistivity of the conductive paint for forming the conductive coating film in a completely solid state. The volume resistivity of the conductive coating film can be measured by a four-terminal method. More specifically, a pressing vessel is prepared, which is composed of a cylinder having a hollow portion and a pair of punches inserted from above and below into the hollow portion of the cylinder, and a weighed sample (conductive paint in a solid state) is placed in the hollow portion of the cylinder. Then, a hydraulic hand pump is used to press the pair of punches to prepare a powder sample of a predetermined size. The volume resistivity of the obtained powder sample can then be measured by a four-terminal method.

[0033] The conductive paint constituting such a conductive coating film may be a conductive paint in which particles of a material exhibiting conductivity are held in a binder resin, or a conductive paint containing metal particles exhibiting conductivity, so long as it can realize the volume resistivity value described above. Examples of such conductive paints include polyester-based AgCu conductive paints in which AgCu is held in a polyester resin, carbon-based conductive paints in which carbon black or the like is held in various resins, zinc-based paints, nickel-based paints, etc. These conductive paints can be used in any form, such as an organic solvent-based, water-based, or powder-based paint.

[0034] Here, in a cross section of the conductive coating film cut in the thickness direction, the area ratio of the conductive paint components (i.e., for example, the conductive particles as described above) in the cross section is preferably 30% or more. In addition, in consideration of the uniformity of the conductive coating film, the content of the paint components (conductive particles) in the conductive coating film can be said to be 30% or more by volume with respect to the total solid content of the coating film. When the area ratio of the paint components in the cross section is 30% or more, 8 This makes it possible to more reliably achieve a volume resistivity of Ω·m or less. The area ratio of the coating component in the cross section is more preferably 50% or more, and even more preferably 70%. Note that since the higher the area ratio of the coating component in the cross section, the better, the upper limit is not specified and may be 100%.

[0035] In the welded joint according to the present embodiment, the thickness of the conductive coating is preferably 3 μm or more even at the thinnest point. By making the thickness of the conductive coating 3 μm or more even at the thinnest point, it is possible to reliably ensure the flatness of the surface of the conductive coating, and it is possible to provide a uniform thickness to the conductive coating. The thickness of the conductive coating is more preferably 10 μm or more, and even more preferably 12 μm or more.

[0036] In the welded joint according to the present embodiment, for example, an electrodeposition coating is further formed on the conductive coating, so that very good coating adhesion between the weld bead and the conductive coating is not required. However, if the thickness of the conductive coating exceeds 40 μm, not only will the cost increase too much, but the thickness of the conductive coating may become too thick, and the adhesion between the weld bead and the conductive coating may decrease. Therefore, the thickness of the conductive coating is preferably 40 μm or less even at the thickest point. By making the thickness of the conductive coating 40 μm or less, it is possible to improve the electrodeposition paintability of the welded joint while ensuring the adhesion between the weld bead and the conductive coating. The thickness of the conductive coating is more preferably 20 μm or less, and even more preferably 16 μm or less.

[0037] The thickness of the conductive coating can be measured by observing the cross section of the conductive coating using an optical microscope. More specifically, the weld joint of interest is embedded in a thermosetting resin such as an epoxy resin, and the sample is cut parallel to the thickness direction at the location to be observed using a cutting machine such as a precision cutter, and the resulting cross section is observed using an optical microscope. The shortest distance from any of a plurality of positions (e.g., five positions) on the interface between the embedding resin and the conductive coating to the interface between the conductive coating and the weld bead is measured (i.e., the distance is measured in a direction perpendicular to the interface), and the measured values ​​obtained are averaged. The average thickness of the conductive coating obtained in this manner can be regarded as the thickness of the conductive coating.

[0038] The area ratio of the coating component in the cross section of the conductive coating film is also measured by observing the cross section image using a microscope in the same manner as above. This measurement is performed in multiple arbitrary fields of view, as in the thickness measurement above, and the average of the obtained measurements is taken as the area ratio of the coating component in the cross section.

[0039] By providing such a conductive coating film on at least a portion of the surfaces of the welding slag, spatter, and heat-affected zone, it is possible to improve the electrodeposition paintability of the welded joint. Also, by providing such a conductive coating film over the entire surfaces of the welding slag, spatter, and heat-affected zone, it is possible to more reliably improve the electrodeposition paintability of the welded joint. Furthermore, by providing such a conductive coating film over the entire weld bead, not just the welding slag, spatter, and heat-affected zone, it is possible to more reliably ensure the electrodeposition paintability of the welded joint.

[0040] The conductive coating film according to the present embodiment may contain various additives within the range that does not impair the above-mentioned conductivity. Examples of such additives include various extender pigments, colorants, rust inhibitors, etc.

[0041] The components that may be contained in the conductive coating film according to this embodiment have been described above by listing the paint compositions. Usually, when these paint compositions are applied to a desired location, the components of these paint compositions may differ from the component composition of the film that is formed. For example, when an organic solvent-based or water-based conductive paint is used as the conductive paint, the composition of the paint composition and the conductive coating film after application change due to the volatilization of volatile components in the paint composition, and it is usually technically difficult to specify the composition of the conductive coating film that is formed. In addition, it is also technically difficult to specify the composition of such a conductive coating film by instrumental analysis or the like. Therefore, in this embodiment, the conductive coating film that is formed is specified by specifying the components that may be contained in the paint composition.

[0042] <About electrodeposition coating> In the welded joint according to the present embodiment, it is preferable that an electrodeposition coating is further provided on the surface of the conductive coating as described above. By providing such an electrodeposition coating, it is possible to ensure the corrosion resistance of the welded joint. It is more preferable that the electrodeposition coating is provided not only on the surface of the conductive coating as described above, but also on the surface of the portion other than the conductive coating (i.e., the steel portion).

[0043] Here, the detailed components of the electrodeposition coating film are not particularly limited, and various known electrodeposition coating films can be appropriately formed.

[0044] <About chemical coating> In the welded joint according to this embodiment, a chemical conversion coating may be further provided between the conductive coating and the electrodeposition coating, or between the steel material and the conductive coating.

[0045] The detailed composition of the chemical conversion coating according to this embodiment is not particularly limited, and may contain one or more selected from the group consisting of resin, silane coupling agent, zirconium compound, silica, phosphoric acid and its salt, fluoride, vanadium compound, and tannin or tannic acid. The inclusion of these substances further improves the film-forming properties after application of the chemical conversion treatment solution, the barrier properties (density) of the coating against corrosive factors such as moisture and corrosive ions, and the coating adhesion, thereby contributing to raising the corrosion resistance of the coating.

[0046] The chemical conversion coating agent containing the above-mentioned various components is applied to the surface of a steel material or a conductive coating, and then dried to form a chemical conversion coating. In the welded joint according to this embodiment, the chemical conversion coating is applied to the surface of the steel material or the conductive coating in an amount of 10 to 1000 mg / m2 per side. 2 It is preferable to form a chemical conversion coating on the weld joint. The coating weight of the chemical conversion coating is more preferably 20 to 800 mg / m 2 and most preferably 50 to 600 mg / m 2 It is.

[0047] The welded joint according to this embodiment has been described in detail above.

[0048] By using the welded joint as described above, it is possible to manufacture automobile components and building materials having the above-mentioned welded joint.

[0049] (About the manufacturing method of welded joints) Next, a method for manufacturing a welded joint according to this embodiment will be described. The manufacturing method is a manufacturing method for a welded joint having a plurality of steel materials and a weld bead located between the plurality of steel materials, and includes a welding step of welding the plurality of steel materials using a welding wire, and a welding slag, spatter, and a weld bead having a volume resistivity of 3.00×10 on at least a part of a surface of a heat-affected zone. 8 and a film forming process for forming a conductive coating film having a resistance of Ω·m or less.

[0050] Here, the welding process is not particularly limited, and various known welding methods such as arc welding, laser welding, electron beam welding, plasma arc welding, etc. can be used as appropriate. The welding conditions when performing such welding are also not particularly limited, and appropriate welding conditions may be adopted. For example, by performing welding under the following welding conditions, it is possible to reliably weld multiple steel materials. Welding method: Pulse MAG welding, Welding current: 160A, Welding voltage: 22.4V, Welding speed: 100cm / min, Shielding gas type: 80%Ar+20%CO2, Flow rate: 20L / min, Welding torch forward / reverse angle: 0°, Welding torch tilt angle: 60°, Wire extension length: 15mm

[0051] The film-forming step is to form a film having a volume resistivity of 3.00×10 on at least a part of the surface of the welding slag, spatter, and heat-affected zone generated by welding. 8 This is the process of forming a conductive coating film with a resistance of Ω·m or less.

[0052] Here, the conductive paint used to form the conductive coating film is as described above, and therefore a detailed description thereof will be omitted below.

[0053] In the film-forming step, the conductive coating film is preferably formed by immersion in the conductive paint or by applying the conductive paint with a brush or spray. The amount of the conductive paint to be applied is preferably adjusted appropriately so that the thickness of the conductive coating film after drying is as described above.

[0054] The conductive paint is applied to at least a part of the surfaces of the welding slag, spatter, and heat-affected zone. As mentioned above, it is preferable to apply the conductive paint to the entire surfaces of the welding slag, spatter, and heat-affected zone, and it is more preferable to apply the conductive paint not only to the above-mentioned parts but also to the entire surface of the weld bead.

[0055] In addition, when applying the conductive paint, non-conductive welding slag may be identified, and the conductive paint may be applied to at least a portion of the non-conductive welding slag. By applying the paint in this manner, the conductive paint can be used efficiently, and the cost required for forming the conductive coating film can be reduced. Non-conductive welding slag can be identified by a known method, and for example, the evaluation method disclosed in International Publication No. 2017 / 126657 can be applied.

[0056] In addition, in the manufacturing method of the welded joint according to the present embodiment, in addition to the above-mentioned welding step and film formation step, at least one of a degreasing step, a surface conditioning step, and a chemical conversion treatment step may be further included. By further including at least one of these steps, it is possible to further improve the product characteristics of the welded joint to be manufactured. The degreasing step is a step of removing fumes and oily stains generated during welding. The surface conditioning step is a step of promoting the nucleation of chemical conversion coating crystals on the steel surface and promoting the formation of a good chemical conversion coating. The chemical conversion treatment step is a step of immersing the steel in a chemical conversion treatment solution adjusted under specific conditions to promote a chemical reaction on the steel surface, form an insoluble product on the steel surface, and prepare a paint base with excellent rust prevention and adhesion.

[0057] The method for manufacturing a welded joint according to this embodiment has been described above.

[0058] As described above, according to the embodiment of the present invention, it is possible to render non-conductive slag on the surface of the weld bead harmless, improve the electrodeposition paintability of the weld, and make it possible to produce a welded joint with excellent electrodeposition paintability. Furthermore, by improving the electrodeposition paintability of the weld, it is expected that the post-painting corrosion resistance of automotive parts and building materials will be improved. These effects further improve the post-painting corrosion resistance of automotive parts and building materials, and the technology promotes weight reduction of automobiles and longer life of building materials, and therefore will make a significant contribution to industry. EXAMPLES

[0059] The welded joint and the method for manufacturing the welded joint according to the present invention will be specifically described below while showing examples and comparative examples. Note that the examples shown below are merely examples of the welded joint and the method for manufacturing the welded joint according to the present invention, and the welded joint and the method for manufacturing the welded joint according to the present invention are not limited to the examples below.

[0060] In the following, welded joint specimens were manufactured using steel plates and welding wires having the chemical compositions shown in Table 1. In Table 1, the units of components are mass%, with the remainder being Fe and impurities. The steel plates were manufactured by Nippon Steel Corporation and had a thickness of 1.6 mm, and the welding wire was also manufactured by Nippon Steel Corporation and had a wire diameter of 1.2 mm.

[0061] [Table 1]

[0062] Using the above steel plates and welding wire, lap fillet welded joints were produced by welding under the following welding conditions. Welding method: Pulse MAG welding, Welding current: 160A, Welding voltage: 22.4V, Welding speed: 100cm / min, Shielding gas type: 80%Ar+20%CO2, Flow rate: 20L / min, Welding torch forward / reverse angle: 0°, Welding torch tilt angle: 60°, Wire extension length: 15mm

[0063] A conductive coating was formed on each of the obtained welded joints using conductive paints A to E shown in Table 2 below. The areas where the conductive coating was formed are indicated with an "O" mark in Table 2 below, and the conductive coating was applied by spraying and brushing over the entire area marked with an "O". Details of the conductive paints A to E used are as follows:

[0064] Paint A: Polyester-based AgCu conductive paint, manufactured by Pluscoat Co., Ltd., Polycarm conductive paint touch-up pen Paint B: Zinc paint, manufactured by Ichinen Chemicals Co., Ltd., Spot Zinc Paint C: Nickel paint, manufactured by Pluscoat Co., Ltd., Polycarm PCS-104n Ni Paint D: Carbon paint, manufactured by Pluscoat Co., Ltd., Polycarm water-soluble conductive paint touch-up pen black Paint E: Silicone resin paint, manufactured by Shin-Etsu Chemical Co., Ltd., one-component condensation type RTV rubber KE-44

[0065] For each test material, Fine Cleaner E2083 (manufactured by Nippon Parkerizing Co., Ltd.) was used for degreasing, and Preparelan X series PL-X (manufactured by Nippon Parkerizing Co., Ltd.) was used for surface preparation. Palbond 3054 PB3054 (manufactured by Nippon Parkerizing Co., Ltd.) was used as the chemical conversion treatment solution to form a chemical conversion coating on the areas where the weld beads, spatters, and heat-affected zones were to be formed. Then, Power Float 510 (manufactured by Nippon Paint Co., Ltd.) was used as the electrocoating paint to form an electrocoating film.

[0066] For each of the obtained test materials, the thickness of the conductive coating film and the proportion of the coating film components were measured for any five visual fields using an optical microscope by the above-mentioned method, and the obtained average values ​​are shown in Table 2 below as the thickness of the conductive coating film and the proportion of the coating film components.

[0067] The electrodeposition paintability was calculated as the ratio of the projected area of ​​the electrodeposition paint defect to the projected area of ​​the weld bead surface, which was defined as the paint defect area rate. The evaluation criteria were as follows. Scores A to C were considered to be acceptable. The results are summarized in Table 2 below. A: 0% paint defect area rate B: Painting defect area rate more than 0% and less than 3% C: Painting defect area rate more than 3% but less than 5% D: Paint defect area rate exceeds 5%

[0068] [Table 2]

[0069] As is clear from Table 2 above, the welded joints corresponding to the examples of the present invention exhibit excellent electrocoatability, whereas the welded joints corresponding to the comparative examples of the present invention exhibit unacceptable electrocoatability.

[0070] Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to such examples. It is clear that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various modified or amended examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention.

Claims

1. A welded joint having a plurality of steel materials and a weld bead located between the plurality of steel materials, At least a portion of the surface of the welding slag, spatter, and heat-affected zone has a volume resistivity of 3.00×10 ８ It has a conductive coating film with a resistance of Ω·m or less, A welded joint having a chemical conversion coating on the conductive coating (excluding the case where the chemical conversion coating is a chemical conversion coating containing iron phosphate).

2. The welded joint according to claim 1 , wherein the thickness of at least one of the plurality of steel materials is 2.0 mm or less.

3. The welded joint according to claim 1 or 2, wherein the Si content of at least one of the plurality of steel materials is 0.8 to 1.7 mass%.

4. The volume resistivity of the conductive coating film is 1.00×10 ６ The welded joint according to any one of claims 1 to 3, wherein the welded joint has a strength of Ω·m or less.

5. The welded joint according to any one of claims 1 to 4, wherein the conductive coating has a thickness of 10 µm or more.

6. The welded joint according to any one of claims 1 to 5, wherein the conductive coating has a thickness of 20 µm or less.

7. The welded joint according to any one of claims 1 to 6, wherein the conductive coating is made of a polyester-based AgCu conductive paint.

8. The welded joint according to any one of claims 1 to 6, wherein the conductive coating is made of zinc paint.

9. The welded joint according to any one of claims 1 to 8, wherein the conductive coating is provided over the entire surfaces of the welding slag, the spatter, and the heat-affected zone.

10. The welded joint according to any one of claims 1 to 9, wherein the conductive coating is provided over the entire surface of the weld bead.

11. The welded joint according to any one of claims 1 to 10, further comprising an electrodeposition coating on a surface of the conductive coating.

12. The welded joint according to any one of claims 1 to 11, further comprising a chemical conversion coating between the plurality of steel materials and the conductive coating.

13. A welded joint as described in any one of claims 1 to 12, wherein the chemical coating contains one or more selected from the group consisting of resin, silane coupling agent, zirconium compound, silica, phosphoric acid and its salts (excluding iron phosphate), fluoride, vanadium compound, and tannin or tannic acid.

14. A method for manufacturing a welded joint having a plurality of steel materials and a weld bead located between the plurality of steel materials, a welding step of welding the plurality of steel materials together using a welding wire; At least a part of the surface of the welding slag, spatter, and heat-affected zone produced by welding has a volume resistivity of 3.00×10 ８ a film forming step of forming a conductive coating film having a resistivity of Ω·m or less; a chemical conversion treatment step of forming a chemical conversion film on the conductive coating film (excluding cases where the chemical conversion film is a chemical conversion film containing iron phosphate); A method for manufacturing a welded joint comprising the steps of:

15. The method for manufacturing a welded joint according to claim 14, wherein in the film-forming step, the conductive coating film is formed by immersion in a conductive paint or by applying the conductive paint with a brush or by spraying.

16. 16. The method for manufacturing a welded joint according to claim 14 or 15, wherein in the film forming process, the welding slag is identified as being non-conductive, and a conductive paint is applied to at least a portion of the non-conductive welding slag to form the conductive coating film.

17. The method for manufacturing a welded joint according to any one of claims 14 to 16, further comprising at least one of a degreasing step, a surface conditioning step, or a chemical conversion treatment step.

18. An automobile component having the welded joint according to any one of claims 1 to 13.

19. A building material member having the welded joint according to any one of claims 1 to 13.

Images