Welded joint and method for manufacturing same

Abstract

Provided is a welded joint capable of achieving excellent electrodeposition coatability even when weld slag is formed on the surface of a weld metal. A welded joint (10) has a weld metal (3) and a weld slag (4) formed on at least a portion of the surface of the weld metal (3). The value A1 calculated by equation (1): A1=[Mn]s＋[Fe]s＋[Ti]s is 40 or more, where [Mn]s, [Fe]s, and [Ti]s represent the contents of Mn, Fe, and Ti in the weld slag (4), respectively, in mass% relative to the total mass of the weld slag. The weld slag (4) contains a highly electrically conductive phase. The occupancy of the highly electrically conductive phase in a cross section of the weld slag (4) is 4.5% or more. The highly electrically conductive phase contains 30 mass% or more of Mn, 5 mass% or more of Fe, 30 mass% or less of Ti, 10 mass% or less of Si, 10 mass% or less of Al, and 30 mass% or less of O (oxygen), relative to the total mass of the highly electrically conductive phase.

Description

Welded joint and its manufacturing method

[0001] The present invention relates to a welded joint and a method for manufacturing the same.

[0002] Undercarriage parts of automobiles and other vehicles are exposed to a corrosive environment due to moisture from the road surface and salt damage contained in snow-melting agents, so technology to prevent rusting of undercarriage parts is needed. Generally, when manufacturing undercarriage parts, the parts are arc-welded and then electro-deposition-coated to form an anti-rust coating on the surface, thereby preventing rusting of the undercarriage parts. However, after welding, welding slag (hereinafter simply referred to as "slag"), which has low electrical conductivity, is generated on the bead surface, and depending on the condition of the slag, it can inhibit the formation of the anti-rust coating (hereinafter simply referred to as "coating" or "electro-deposition coating").

[0003] Generally, electrodeposition coating is performed after mechanically removing the slag remaining on the bead surface after welding, or while leaving the slag on the bead surface. Mechanically removing the slag increases the burden on the worker. Electrodeposition coating performed while leaving the slag on the bead surface makes it difficult to form a sufficient coating film, resulting in variations in the quality of the rust prevention properties of the finished parts. Thus, electrodeposition coating performance is a very important performance factor that affects the quality and workload of automotive parts during manufacturing. Therefore, from the perspectives of quality and workload, there is a need for the development of welded joints that have stable rust prevention properties without inhibiting the formation of an electrodeposition coating film even when slag remains on the bead surface.

[0004] For example, Patent Document 1 proposes a welded joint provided with welding slag containing a conductive oxide phase. Patent Document 1 describes that electrodeposition coatability can be improved by specifying the chemical composition of the conductive oxide phase and specifying the area ratio of the conductive oxide phase in the cross section of the welding slag.

[0005] Japanese Patent Application Publication No. 2021-178354

[0006] However, in the welded joint described in Patent Document 1, the welding slag is composed of a conductive oxide phase and other regions, but no consideration is given to the regions other than the conductive oxide phase or the composition of the entire slag. The composition of the slag in these regions varies depending on the welding method, including the type of shielding gas used, the welding wire, etc. Therefore, even if the chemical composition of the conductive oxide phase is specified within a predetermined range, the electrodeposition paintability may be reduced depending on the composition of the entire slag.

[0007] It is also known that the type of steel plate, wire, shielding gas, etc. greatly affect the composition of the slag. However, in the welded joint described in the above Patent Document 1, the slag is made of 80% by volume Ar-20% by volume CO ２ The study only examined the use of CO shielding gas, and did not examine the electrodeposition coating properties when using various types of shielding gas. ２ Ya O ２ When the content of oxidizing gases such as argon, argon, and argon increases, the slag composition changes, the amount of slag generated increases, and the thickness of the slag itself increases, deteriorating the electrodeposition paintability. As a result, depending on the type of shielding gas used, a sufficient current path may not be secured during electrodeposition painting, resulting in partial coating defects.

[0008] The present invention has been made in view of the above-mentioned problems, and has an object to provide a welded joint that can obtain excellent electrodeposition paintability even when welding slag is formed on the surface of the weld metal, and a method for manufacturing a welded joint that can easily manufacture the welded joint.

[0009] The above object of the present invention is achieved by the following configuration [1] relating to a welded joint.

[0010] [1] A welded joint having a weld metal and welding slag formed on at least a portion of a surface of the weld metal, wherein, when the Mn content in the welding slag is expressed as [Mn]s in mass % relative to the total mass of the welding slag, the Fe content in the welding slag is expressed as [Fe]s in mass % relative to the total mass of the welding slag, and the Ti content in the welding slag is expressed as [Ti]s in mass % relative to the total mass of the welding slag, the value A1 calculated by formula (1): A1 = [Mn]s + [Fe]s + [Ti]s is 40 or more, the welding slag includes a highly conductive phase, and an occupancy rate of the highly conductive phase in a cross section of the welding slag is 4.5% or more, and the highly conductive phase contains, relative to the total mass of the highly conductive phase: Mn: 30% by mass or more, Fe: 5% by mass or more, Ti: 30% by mass or less, Si: 10% by mass or less, Al: 10% by mass or less, O (oxygen): 30 mass% or less.

[0011] Furthermore, a preferred embodiment of the present invention relating to a welded joint relates to the following [2].

[0012] [2] The welded joint according to [1], characterized in that it has an electrodeposition coating formed on at least a portion of the surface of the welding slag.

[0013] The above object of the present invention is achieved by the following configuration [3] relating to a method for manufacturing a welded joint.

[0014] [3] A method for manufacturing a welded joint, the method comprising a step of performing gas-shielded arc welding using a welding wire on a steel base material, wherein the welded joint has a weld metal and welding slag formed on at least a portion of a surface of the weld metal, wherein the value A1 calculated by formula (1): A1 = [Mn]s + [Fe]s + [Ti]s is 40 or more, where [Mn]s is the manganese content in the welding slag in mass % relative to the total mass of the welding slag, [Fe]s is the iron content in the welding slag in mass % relative to the total mass of the welding slag, and [Ti]s is the titanium content in the welding slag in mass % relative to the total mass of the welding slag, the value A1 is 40 or more, the welding slag includes a highly electrically conductive phase, and an occupancy rate of the highly electrically conductive phase in a cross section of the welding slag is 4.5% or more, and the highly electrically conductive phase contains, with respect to the total mass of the highly electrically conductive phase, Mn: 30% by mass or more, and Fe: 5% by mass or more. A method for manufacturing a welded joint, characterized in that Ti: 30 mass % or less, Si: 10 mass % or less, Al: 10 mass % or less, and O (oxygen): 30 mass % or less.

[0015] Furthermore, preferred embodiments of the present invention relating to a method for manufacturing a welded joint relate to the following [4] to [5].

[0016] [4] The method for manufacturing a welded joint according to [3], characterized in that after the step of performing the gas-shielded arc welding, a step of forming an electrodeposition coating on at least a portion of the surface of the welding slag is included.

[0017] [5] In the step of performing the gas-shielded arc welding, CO ２ Gas and O ２ The method for manufacturing a welded joint according to [3] or [4], characterized in that a shielding gas containing at least one gas selected from the group consisting of:

[0018] According to the present invention, it is possible to provide a welded joint that can obtain excellent electrodeposition coatability even when welding slag has formed on the surface of the weld metal, when electrodeposition coating is performed while the welding slag is left on the surface of the weld metal, and a method for manufacturing a welded joint that can easily manufacture the above welded joint.

[0019] FIG. 1 is a schematic cross-sectional view showing a welded joint according to an embodiment of the present invention.

[0020] As a result of intensive research to solve the above problems, the inventors of the present invention have found that the electrical conductivity of welding slag varies greatly depending on the composition of the welding slag, and that the composition of the welding slag is particularly greatly affected by the type of shielding gas. ２ Or O ２ The inventors have found that the presence of a large amount of oxidizing gases, such as argon, increases the amount of welding slag produced and results in the formation of a thick welding slag. In this specification, "welding slag" refers to a non-metallic substance that solidifies after welding to form multiple oxide phases. The term "oxide" also includes composite oxides. Furthermore, the inventors measured the electrical resistance of various regions of welding slag using a scanning probe microscope (SPM). As a result, they found that a region with particularly high electrical conductivity sometimes forms in a portion of the welding slag, and identified the composition of this region. In this specification, this region with a specific composition is referred to as a "highly electrically conductive phase."

[0021] The present invention was made based on these findings. That is, when electrodeposition coating is performed while welding slag is left on the surface of the weld metal, a welded joint with extremely good electrodeposition coatability can be obtained by appropriately controlling both the composition of the entire welding slag and the composition and occupancy rate of the highly electrically conductive phase.

[0022] Hereinafter, an embodiment of the present invention will be described in detail. Note that the present invention is not limited to the embodiment described below, and can be implemented with any modifications within the scope of the gist of the present invention. Below, a welded joint according to this embodiment will be described.

[0023] [Welded Joint] Fig. 1 is a schematic cross-sectional view showing a welded joint according to an embodiment of the present invention. For example, as shown in Fig. 1, a steel material 1 serving as a lower plate and a steel material 2 serving as an upper plate are stacked so that their main surfaces face each other, and a weld metal 3 is formed at a corner formed by an end face of the steel material 2 and the upper surface of the steel material 1, thereby forming a welded joint 10. Furthermore, welding slag 4 is formed on at least a portion of the surface of the welded metal 3, and the welding slag 4 contains a highly electrically conductive phase (not shown) having a specific composition. Although not shown, the welded joint 10 according to an embodiment of the present invention may have an electrodeposition coating formed on at least a portion of the surface of the welding slag 4.

[0024] <High Electrical Conductivity Phase> Observation of a backscattered electron image of the cross section of welding slag 4 and a concentration map image of each element in the cross section of welding slag 4 reveals the formation of multiple regions, each with a uniform composition. In this embodiment, of the multiple regions, a region and composition with particularly high electrical conductivity are identified, and this region is designated as the high electrical conductivity phase. In other words, while it is difficult to stably form an electrodeposition coating on ordinary welding slag, in this embodiment, because the welding slag 4 contains a highly electrically conductive phase with a specific composition, an electrodeposition coating can be formed in good condition.

[0025] The welding slag 4 does not need to cover the entire surface of the weld metal 3. High electrical conductivity can be ensured in the areas of the surface of the weld metal 3 that are not covered with the welding slag 4. Furthermore, if the electrical conductivity of the welding slag 4 can be improved, a good electrodeposition coating can be formed on the entire surface of the weld metal 3.

[0026] The method for analyzing the composition of the entire welding slag and the identification and composition of the highly electrically conductive phase, as well as the method for calculating the area ratio of the highly electrically conductive phase to the entire welding slag, will be described below.

[0027] <Method for Identifying and Analyzing the Overall Composition of Welding Slag and the High Electrical Conductivity Phase> First, the region of a welded joint where welding slag is formed is cut perpendicular to the welding direction, and the joint is embedded in resin so that the cut surface serves as the observation surface. The joint is then observed with a scanning electron microscope (SEM). In the SEM observation, a backscattered electron (BSE) image is used to select and adjust an observation field that includes the entire welding slag, a field that includes a portion of the welding slag and a portion of the weld metal, or a field that includes only a portion of the welding slag. The high electrical conductivity phase can be identified by performing elemental analysis and elemental mapping on a composition image obtained using energy dispersive X-ray spectroscopy (EDX) for this observation field. Furthermore, the composition of the entire welding slag and the highly electrically conductive phase can be measured by area analysis using EDX on the composition image of the above observation field.

[0028] <Method for calculating the area ratio of the highly electrically conductive phase in the cross section of the welding slag> The area of ​​the highly electrically conductive phase and other regions can be measured by binarizing the entire image of the composition image of the observation field using image analysis software. The area ratio can then be obtained by calculating the area of ​​the highly electrically conductive phase relative to the entire area of ​​the welding slag as a percentage. In this specification, the area ratio is referred to as the "occupancy ratio of the highly electrically conductive phase." As long as binarization is possible, the threshold for binarization is not particularly limited, and any threshold can be set.

[0029] The components of the welding slag 4 and the highly electrically conductive phase and the reasons for limiting the numerical values ​​of the welded joint 10 according to this embodiment will be described in detail below.

[0030] (Value A1 calculated based on the content of a predetermined element in the welding slag: 40 or more) Welding slag containing oxides containing Fe, Mn, and Ti has the effect of improving electrical conductivity. In this embodiment, in order to improve the electrical conductivity of the entire welding slag, the total amount of Fe, Mn, and Ti in the welding slag is set to a predetermined amount or more. If the value A1 calculated using the following formula (1), which represents the total amount of these elements, is less than 40, a sufficient current path cannot be secured within the welding slag, and the desired electrodeposition paintability cannot be obtained. Therefore, the value A1 calculated using the following formula (1) is set to 40 or more, preferably 50 or more, more preferably 61 or more, and even more preferably 65 or more. Although the upper limit of the value A1 is not particularly limited, it is practically 80 or less.

[0031] Formula (1): A1 = [Mn]s + [Fe]s + [Ti]s In the above formula (1), [Mn]s is the Mn content in the welding slag expressed as a mass % relative to the total mass of the welding slag, [Fe]s is the Fe content in the welding slag expressed as a mass % relative to the total mass of the welding slag, and [Ti]s is the Ti content in the welding slag expressed as a mass % relative to the total mass of the welding slag.

[0032] In the present specification, the term "desired electrodeposition coating properties" refers to a coating area in which the area of ​​the electrodeposition coating is poorly coated is 2.5 mm ２ This refers to the following cases:

[0033] (Mn in the High Electrical Conductivity Phase: 30% by Mass or More) Mn is an element that improves the strength and toughness of weld metal. Welding slag typically contains Mn. The inventors of the present application have also discovered that the high electrical conductivity phase, which is a region of welding slag with a high content of Mn and Fe oxides, significantly affects the electrical conductivity of the entire welding slag. Specifically, specifying the Mn content in the high electrical conductivity phase can improve the electrical conductivity of the welding slag. If the Mn content in the high electrical conductivity phase is less than 30% by mass, a sufficient current path cannot be secured within the welding slag, resulting in poor electrodeposition coatability. Therefore, the Mn content in the high electrical conductivity phase is set to 30% by mass or more, preferably 32% by mass or more, more preferably 35% by mass or more, and even more preferably 38% by mass or more, based on the total mass of the high electrical conductivity phase. While the upper limit of the Mn content is not particularly limited, a value of 70% by mass or less is practically preferred.

[0034] (Fe in the High Electrical Conductivity Phase: 5% by Mass or More) Fe is a major component of weld metal, and therefore is also present in welding slag. Furthermore, as described above, the high electrical conductivity phase, which is a region in welding slag with a high content of Mn and Fe oxides, significantly affects electrical conductivity. Therefore, specifying the Fe content in the high electrical conductivity phase can improve the electrical conductivity of the welding slag. If the Fe content in the high electrical conductivity phase is less than 5% by mass, a sufficient current path cannot be secured within the welding slag, and the desired electrodeposition coatability cannot be achieved. Therefore, the Fe content in the high electrical conductivity phase is set to 5% by mass or more, preferably 15% by mass or more, more preferably 20% by mass or more, and even more preferably 25% by mass or more, based on the total mass of the high electrical conductivity phase. While the upper limit of the Fe content is not particularly limited, a value of 70% by mass or less is practically preferred.

[0035] (Ti in the High Electrical Conductivity Phase: 30% by Mass or Less) Ti is an element that improves the strength and toughness of the weld metal. However, Ti does not necessarily need to be contained in the high electrical conductivity phase; it may be 0% by mass. However, if the high electrical conductivity phase contains an oxide containing Ti, the electrical conductivity of the welding slag can be further improved. On the other hand, if the Ti content in the high electrical conductivity phase exceeds 30% by mass, Fe and Mn, which control the electrical conductivity of the high electrical conductivity phase, are prevented from appearing in the high electrical conductivity phase in a predetermined proportion. Therefore, the Ti content in the high electrical conductivity phase is set to 30% by mass or less, preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, based on the total mass of the high electrical conductivity phase.

[0036] (Si in the High Electrical Conductivity Phase: 10% by Mass or Less) Si is an element that has the effect of deoxidizing the molten metal and adjusting the viscosity of the molten metal. Si is typically contained in the high electrical conductivity phase. Oxides containing Si are insulators, and the presence of Si in the high electrical conductivity phase reduces the electrical conductivity of the welding slag. Therefore, if the Si content in the high electrical conductivity phase exceeds 10% by mass, the desired electrodeposition coatability cannot be achieved. Therefore, the Si content in the high electrical conductivity phase is set to 10% by mass or less, preferably 8% by mass or less, more preferably 6% by mass or less, and even more preferably 5% by mass or less, based on the total mass of the high electrical conductivity phase. While the lower limit of the Si content is not particularly limited, a value of 0.1% by mass or more is practically acceptable.

[0037] (Al in the High Electrical Conductivity Phase: 10% by Mass or Less) Al is an element that has the effect of deoxidizing the molten metal and adjusting the viscosity of the molten metal, and Al may be contained in the high electrical conductivity phase. Oxides containing Al are insulators, and the presence of Al in the high electrical conductivity phase reduces the electrical conductivity of the welding slag. Therefore, if the Al content in the high electrical conductivity phase exceeds 10% by mass, the desired electrodeposition coatability cannot be obtained. Therefore, the Al content in the high electrical conductivity phase is set to 10% by mass or less, preferably 8% by mass or less, more preferably 6% by mass or less, and even more preferably 4% by mass or less, based on the total mass of the high electrical conductivity phase. While the lower limit of the Al content is not particularly limited, a value of 0.1% by mass or more is practically acceptable.

[0038] (O (oxygen) in the high electrical conductivity phase: 30 mass% or less) O ​​(oxygen) is an element inevitably contained in welding slag. If the O (oxygen) content in the high electrical conductivity phase exceeds 30 mass%, the desired electrodeposition coatability cannot be obtained. Therefore, the O (oxygen) content in the high electrical conductivity phase is set to 30 mass% or less, preferably 27 mass% or less, more preferably 24 mass% or less, and even more preferably 20 mass% or less, relative to the total mass of the high electrical conductivity phase. Although the lower limit of the O (oxygen) content is not particularly limited, it is practically 5 mass% or more.

[0039] (Value A2 Calculated Based on the Content of a Predetermined Element in the High Electrical Conductivity Phase) The electrical conductivity of welding slag is dominated by the Fe and Mn contained in the welding slag. The higher the Fe content in the electrically conductive phase, the better the electrical conductivity of the welding slag. The inventors of the present application discovered that the higher the Fe content relative to the Mn content in the highly conductive phase and the lower the O (oxygen) content in the highly conductive phase, the better the electrical conductivity of the slag. When the value A2 calculated using the following formula (2) based on the contents of Mn, O (oxygen), and Fe in the highly conductive phase is 70 or less, the electrical conductivity of the slag can be further improved. Therefore, the value A2 calculated using the following formula (2) is preferably 70 or less, more preferably 50 or less, and even more preferably 30 or less. While the lower limit of the value A2 is not particularly limited, a value of 5 or more is practically preferable.

[0040] Formula (2): A2 = [Mn]e × [O]e / [Fe]e In the above formula (2), [Mn]e is the Mn content in the highly electrically conductive phase expressed as mass % relative to the total mass of the highly electrically conductive phase, [O]e is the O (oxygen) content in the highly electrically conductive phase expressed as mass % relative to the total mass of the highly electrically conductive phase, and [Fe]e is the Fe content in the highly electrically conductive phase expressed as mass % relative to the total mass of the highly electrically conductive phase.

[0041] (Other Components in the High Electrical Conductivity Phase) The high electrical conductivity phase may contain elements other than the above-mentioned Mn, Fe, Ti, Si, Al, and O (oxygen). For example, automotive steel sheets and welding wires may contain elements such as Zr, Mg, Cr, Mo, Ni, B, Cu, Sn, and Sb. Therefore, the high electrical conductivity phase in welding slag may also contain such elements. However, as long as the value A1 calculated by the above-mentioned formula (1) and the content of each element in the high electrical conductivity phase satisfy the ranges specified in the present invention and the occupancy rate of the high electrical conductivity phase, described below, is within a predetermined range, desired electrodeposition coatability can be obtained. Therefore, for example, at least one of the elements in the high electrical conductivity phase, such as Zr, Mg, Cr, Mo, Ni, B, Cu, Sn, and Sb, may each be contained in an amount of 0.05 mass% or more relative to the total mass of the high electrical conductivity phase. However, from the viewpoint of maintaining good welding workability, the content of each element in the high electrical conductivity phase relative to the total mass of the high electrical conductivity phase is preferably 1.0 mass% or less for Zr, 1.0 mass% or less for Mg, 5.0 mass% or less for Cr, 5.0 mass% or less for Mo, 1.0 mass% or less for Ni, 1.0 mass% or less for B, 1.0 mass% or less for Cu, 1.0 mass% or less for Sn, and 1.0 mass% or less for Sb. Of course, these elements may be 0 mass%.

[0042] (Remainder in High Electrical Conductivity Phase) The remainder in the high electrical conductivity phase other than the above elements is impurities. Examples of impurities include Na, K, and Ca. The total amount of impurities contained in the high electrical conductivity phase according to this embodiment is preferably 1.0 mass% or less with respect to the total mass of the high electrical conductivity phase. Of course, the total amount of these elements may be 0 mass%.

[0043] (High Electrical Conductivity Phase Occupancy: 4.5% or More) In this embodiment, the electrical conductivity of the entire slag is improved by controlling the value A1 calculated by the above formula (1) and the content of each element in the high electrical conductivity phase. However, because welding slag is composed of a conductive oxide phase and other regions, the occupancy of the high electrical conductivity phase must also be controlled. That is, if the occupancy of the high electrical conductivity phase is less than 4.5%, the effect of improving the electrical conductivity of the welding slag due to the presence of the high electrical conductivity phase cannot be fully obtained, and the desired electrodeposition coatability cannot be achieved. That is, if the value A1 calculated by the above formula (1) and the content of each element in the high electrical conductivity phase are controlled within the above specified ranges and the occupancy of the high electrical conductivity phase is 4.5% or more, the desired electrical conductivity can be achieved. Therefore, the occupancy of the high electrical conductivity phase relative to the cross section of the welding slag is set to 4.5% or more. Since the higher the occupancy of the highly electrically conductive phase, the more improved the electrodeposition coatability, the occupancy of the highly electrically conductive phase is preferably 8% or more, more preferably 13% or more, even more preferably 18% or more, and may even be 100%. Improved electrodeposition coatability means not only that the desired electrodeposition coatability is obtained, but also that a stable electrodeposition coating film is formed even when thick slag is formed.

[0044] The inventors of the present application have discovered that the electrical conductivity of the entire welding slag can be further improved by setting the content of each element in not only the highly electrically conductive phase but also the entire welding slag within the ranges shown below.

[0045] (Mn Content in Welding Slag) Mn is an element that improves the strength and toughness of weld metal, and is typically contained in welding slag. The inventors of the present application have also discovered that oxides containing Mn in welding slag further improve the electrical conductivity of the welding slag. Specifically, this can more fully ensure a current path within the welding slag during electrodeposition coating. Therefore, the Mn content in the welding slag is preferably 30% by mass or more, more preferably 32% by mass or more, and even more preferably 35% by mass or more, based on the total mass of the welding slag. While the upper limit of the Mn content is not particularly limited, a value of 70% by mass or less is practically acceptable.

[0046] (Fe Content in Welding Slag) Fe is the main component of weld metal, and therefore Fe is also contained in welding slag. Furthermore, as described above, it has been discovered that the higher the content of oxides containing Fe in welding slag, the more improved the electrical conductivity of the welding slag. Specifically, a current path can be more sufficiently secured within the welding slag during electrodeposition coating. Therefore, the Fe content in welding slag is preferably 5% by mass or more, more preferably 8% by mass or more, and even more preferably 12% by mass or more, based on the total mass of the welding slag. While the upper limit of the Fe content is not particularly limited, a value of 60% by mass or less is practically acceptable.

[0047] (Ti Content in Welding Slag) Ti is an element that improves the strength and toughness of the weld metal, but Ti does not necessarily need to be contained in the welding slag and may be 0 mass%. However, if the welding slag contains an oxide containing Ti, the electrical conductivity of the welding slag can be further improved. Therefore, the Ti content in the welding slag is preferably 0.1 mass% or more, more preferably 0.5 mass% or more, and even more preferably 0.8 mass% or more, based on the total mass of the welding slag. Although there is no particular upper limit for the Ti content, it is preferably 10 mass% or less because the amount of slag generated increases.

[0048] (Si Content in Welding Slag) Si is an element that has the effect of deoxidizing molten metal and adjusting the viscosity of molten metal, and Si is usually contained in welding slag. Oxides containing Si are insulators, and if welding slag contains excessive Si, the conductivity of the welding slag decreases. Therefore, the Si content in welding slag is preferably 20 mass% or less, more preferably 16 mass% or less, and even more preferably 12 mass% or less, based on the total mass of the welding slag. Although the lower limit of the Si content is not particularly limited, it is practically 0.1 mass% or more.

[0049] (Al Content in Welding Slag) Al is an element that has the effect of deoxidizing molten metal and adjusting the viscosity of molten metal, and Al may be contained in welding slag. Oxides containing Al are insulators, and if welding slag contains excessive Al, the conductivity of the welding slag decreases. Therefore, the Al content in welding slag is preferably 20 mass% or less, more preferably 15 mass% or less, and even more preferably 10 mass% or less, based on the total mass of the welding slag. Although the lower limit of the Al content is not particularly limited, it is practically 0.1 mass% or more.

[0050] (O (oxygen) content in welding slag) O (oxygen) is an element inevitably contained in welding slag. If the welding slag contains an excessive amount of O (oxygen), the electrical conductivity of the welding slag decreases. Therefore, the O (oxygen) content in the welding slag is preferably 35 mass% or less, more preferably 30 mass% or less, and even more preferably 25 mass% or less, based on the total mass of the welding slag. Although the lower limit of the O (oxygen) content is not particularly limited, it is practically 10 mass% or more.

[0051] (Value A3 Calculated Based on the Contents of Predetermined Elements in Welding Slag) The electrical conductivity of welding slag is significantly affected by the Fe content in the welding slag. As described above, the higher the content of oxides containing Fe in the welding slag, the better the electrical conductivity of the welding slag. The inventors of the present application have discovered that the higher the Fe content relative to the total content of Mn, Si, Ti, and Al in the welding slag and the lower the O (oxygen) content, the better the electrical conductivity of the slag. That is, when the value A3 calculated using the following formula (3) based on the contents of the above elements in the welding slag is 135 or less, the electrical conductivity of the slag can be further improved. Therefore, the value A3 calculated using the following formula (3) is preferably 135 or less, more preferably 100 or less, and even more preferably 70 or less. Although the lower limit of the value A3 is not particularly limited, a value of 5 or more is practically preferable.

[0052] Equation (3): A3 = ([Mn]s + [Si]s + [Ti]s + [Al]s) × [O]s / [Fe]s In the above equation (3), [Mn]s is the Mn content in the welding slag expressed as a mass % relative to the total mass of the welding slag, [Si]s is the Si content in the welding slag expressed as a mass % relative to the total mass of the welding slag, [Ti]s is the Ti content in the welding slag expressed as a mass % relative to the total mass of the welding slag, [Al]s is the Al content in the welding slag expressed as a mass % relative to the total mass of the welding slag, [O]s is the O (oxygen) content in the welding slag expressed as a mass % relative to the total mass of the welding slag, and [Fe]s is the Fe content in the welding slag expressed as a mass % relative to the total mass of the welding slag.

[0053] (Other Components in Welding Slag) The welding slag as a whole may contain elements other than the above-mentioned Mn, Fe, Ti, Si, Al, and O (oxygen). For example, automotive steel sheets and welding wires may contain elements such as Zr, Mg, Cr, Mo, Ni, B, Cu, Sn, and Sb. Therefore, although the welding slag as a whole may contain such elements, as long as the value A1 calculated by the above-mentioned formula (1) and the content of each element in the high electrical conductivity phase satisfy the ranges specified in the present invention and the occupancy rate of the high electrical conductivity phase is within a predetermined range, the desired electrodeposition coatability can be obtained. Therefore, for example, the welding slag may contain at least one of the elements Zr, Mg, Cr, Mo, Ni, B, Cu, Sn, and Sb in an amount of 0.05 mass% or more relative to the total mass of the welding slag. However, from the viewpoint of maintaining a good bead appearance, the content of each element in the welding slag relative to the total mass of the welding slag is preferably 1.0 mass% or less for Zr, 1.0 mass% or less for Mg, 5.0 mass% or less for Cr, 5.0 mass% or less for Mo, 1.0 mass% or less for Ni, 1.0 mass% or less for B, 1.0 mass% or less for Cu, 1.0 mass% or less for Sn, and 1.0 mass% or less for Sb. Of course, these elements may be 0 mass%.

[0054] (Remainder in Welding Slag) The remainder in the entire welding slag other than the above elements is impurities. Examples of impurities include K, Na, and Ca. The total amount of impurities contained in the entire welding slag according to this embodiment is preferably 1.0 mass% or less relative to the total mass of the welding slag. Of course, the total amount of these elements may be 0 mass%.

[0055] (Steel material constituting the welded joint and joint shape) In this embodiment, the shape of the welded joint and the steel material contained in the welded joint are not particularly limited. For example, if the welded joint is an automobile suspension part, a steel material with a tensile strength of 440 to 980 MPa is included. Furthermore, the shape of the welded joint can be a lap fillet joint or the like.

[0056] [Method for manufacturing a welded joint] The method for manufacturing a welded joint according to this embodiment includes a step of performing gas-shielded arc welding using a welding wire on a steel base material, and is a method for manufacturing the welded joint described above in the [Welded joint] section. In this embodiment, as long as a welded joint having the welding slag is formed, there are no particular limitations on the conditions, etc. Preferred conditions, etc. for the method for manufacturing a welded joint according to this embodiment are described below.

[0057] <Shielding gas> There are no particular restrictions on the shielding gas used, but the composition of the welding slag depends not only on the composition of the welding base material and welding wire, but also on the type of shielding gas used during welding. ２ Gas and O ２ It has been found that it is preferable to use a shielding gas containing at least one oxidizing gas selected from the group consisting of oxidizing gases. Furthermore, the higher the volumetric content of this oxidizing gas, the more likely it is that a highly electrically conductive phase will be formed in the welding slag. As a result, the proportion of the highly electrically conductive phase in the welding slag can be increased, and electrodeposition paintability can be improved.

[0058] The reason why a high electrical conductivity phase is more likely to be formed when the volume content of the oxidizing gas is higher is believed to be as follows. As described above, in order to improve the electrical conductivity of the welding slag, it is preferable to increase the proportion of the high electrical conductivity phase whose main constituent elements are Fe and Mn. Ti, Al, and Si form complex oxides preferentially over Fe and Mn. Therefore, by increasing the volume content of the oxidizing gas in the shielding gas, the oxygen partial pressure directly below the arc is increased, making it easier to form complex oxides of Fe and Mn. Therefore, the shielding gas used in gas-shielded arc welding is preferably CO ２ Gas and O ２ It is preferable that the gas contains at least one gas selected from the group consisting of:

[0059] In addition, the shielding gas is CO ２ If gas is included, CO ２It is preferable to use a mixed gas having a gas content of, for example, 20% by volume or more, more preferably 30% by volume or more, even more preferably 40% by volume or more, and even more preferably 99.8% by volume or more, and it is particularly preferable to use carbon dioxide gas. ２ It refers to a gas composed of gas and impurities, and the content of impurities is preferably, for example, 0.5% by volume or less, and more preferably about 0.2% by volume.

[0060] Shielding gas is O ２ If gas is included, O ２ It is preferable to use a mixed gas having a gas content of, for example, 0.1% by volume or more, more preferably 0.5% by volume or more, and even more preferably 2.0% by volume or more.

[0061] When using mixed gases, CO ２ Gas and O ２ The gas other than the gas may be selected appropriately, and specifically, Ar gas may be used from the viewpoint of easy availability on the market, and for example, a gas containing 15% or more Ar gas by volume may be used.

[0062] <Welding Conditions> In this embodiment, there are no particular limitations on the steel material to be welded or the welding position. As described above, automobile undercarriage parts (welded joints) can be manufactured by lap fillet welding using a steel plate with a tensile strength of 440 to 980 MPa as the base material. The welding conditions in this case are not particularly limited, and general gas-shielded arc welding conditions can be applied.

[0063] <Feed Control Method> In the present embodiment, the specific welding method for gas-shielded arc welding is not particularly limited. However, it is preferable to use, for example, a feed control method. The feed control method is a method of welding steel plates while alternately switching the feed of the welding wire between a forward feed period and a reverse feed period. The feed control method can be categorized into a "short-circuit feed control method" and a "short-circuit suppression feed control method." The short-circuit feed control method is a type of welding based on a short-circuit transition mode in which the feed rate of the welding wire is alternately switched between a forward feed period and a reverse feed period, thereby generating a short-circuit period and an arc period. The short-circuit suppression feed control method is a type of welding based on a globule transition mode in which the feed rate of the welding wire is alternately switched between a forward feed period and a reverse feed period, thereby suppressing the occurrence of a short-circuit period. The short-circuit suppression feed control method has a pulse waveform in which a welding current alternates between a high-current period and a low-current period based on a phase related to the wire tip position (hereinafter referred to as the "wire position phase"). Here, the wire position phase is preferably 0 to 360°, with the forward feed period and reverse feed period being one cycle, with 0° being closest to the tip side and 180° being closest to the base material side.

[0064] In the method for manufacturing a welded joint according to this embodiment, the use of a feed control method is not essential. However, since an increase in the content of oxidizing gas in the shielding gas tends to increase spatter, it is preferable to use a feed control method when improving welding workability, etc. If a feed control method is used, it is preferable to use a short-circuit suppression type feed control method, and it is more preferable to select the wire position phase at which the high current period ends from the range of 100 to 150° and the wire position phase at which the high current period starts from the range of 300 to 360°. By using the short-circuit suppression type feed control method and applying this condition, it is possible to obtain a weld metal with a better appearance and improve welding workability. It is also possible to suppress the occurrence of porosity defects.

[0065] Furthermore, although the use of a feed control method is not essential, when an oxidizing gas is contained in the shielding gas, the use of a feed control method can further improve the electrical conductivity of the welding slag. As described above, in order to improve the electrical conductivity of the welding slag, it is preferable to increase the proportion of the highly conductive phase, which is mainly composed of Fe and Mn. Ti, Al, and Si form complex oxides preferentially over Fe and Mn. Therefore, by promoting the reaction between the droplets formed at the tip of the welding wire and the oxidizing gas in the welding shielding gas, the Fe-Mn complex oxide is more likely to be formed. This is because welding steel sheets while alternately switching the welding wire feed between a forward feed period and a reverse feed period makes the droplets formed at the tip of the welding wire more likely to be exposed to an oxygen atmosphere, which makes it easier for the Fe-Mn complex oxide to be formed, thereby improving the proportion of the highly conductive phase.

[0066] <Welding Wire> In the method for manufacturing a welded joint according to this embodiment, the type, composition, and wire diameter (diameter) of the welding wire used are not particularly limited. Regarding the wire diameter, welding wire specified in welding material standards such as AWS or JIS can be used, such as JIS Z 3312:2009. Furthermore, in general, in the welding of automotive suspension components, welding materials that can reduce tip wear are required from the viewpoint of improving quality and work efficiency. Therefore, it is preferable to use a solid wire as the welding wire.

[0067] (Method for Manufacturing Welding Wire) The welding wire according to this embodiment is not particularly limited in its manufacturing method, and no special manufacturing conditions are required, and it can be manufactured by a conventional method. Specifically, in the case of solid wire, first, steel of an appropriately selected composition is melted to obtain an ingot. Next, the ingot is subjected to hot forging or the like as needed, followed by hot rolling and cold wire drawing to form a wire. Thereafter, the obtained wire is annealed at a temperature of about 500 to 900°C as needed, pickled, copper plated as needed, and further subjected to finish wire drawing as needed to obtain a target wire diameter. Thereafter, a lubricant is applied as needed, and the welding wire can be manufactured.

[0068] The effects of the present invention will be specifically explained below with reference to examples and comparative examples, but the present invention is not limited thereto.

[0069] [Gas-shielded arc welding] Welding wires having various compositions shown in Table 1 and a wire diameter of 1.2 mm were prepared, and welded joints were produced by gas-shielded arc welding using these welding wires. The welding conditions used were a welding method without a feed control method and a welding method with a feed control method. Table 2 shows detailed welding conditions when a feed control method was not used, and Table 3 shows detailed welding conditions when a feed control method was used. Table 4 below shows the welding conditions used to produce welded joints for each of the invention examples and comparative examples. The pulse waveform control shown in Tables 2 and 4 refers to output control in which the output current and voltage waveforms are changed in a pulsed form. In the column for the shielding gas composition in Table 4, "-" indicates that the corresponding gas was not contained in the shielding gas.

[0070] [Identification and Composition Measurement of Each Component in the Welding Slag and Each Component in the Highly Electrically Conductive Phase] The region of the resulting welded joint where the welding slag was formed was cut perpendicular to the welding direction. The joint was embedded in resin so that the cut surface served as the observation surface, and the observation surface was observed using a SEM. For SEM observation, the magnification was 2000x, the beam current was 2 nA, and the acceleration voltage was 15 kV. A field of view (60 × 45 μm, 640 × 480 pixels) was selected from the BSE image, capturing only the welding slag. Elemental analysis and element mapping were performed on this observation field using EDX composition images to identify the highly electrically conductive phase. The content of each component in the welding slag (the entire welding slag) and the highly electrically conductive phase was measured using area analysis of the composition image of the observation field using EDX.

[0071] [Measurement of the Occupancy Rate of the Highly Electrically Conductive Phase] Furthermore, the entire image of the composition image of the above observation field was binarized to 640 × 480 pixels using image analysis software ImageJ (version 1.50i, developed by the National Institutes of Health, USA) to measure the area of ​​the highly electrically conductive phase and other regions. The area of ​​the highly electrically conductive phase relative to the entire area of ​​the welding slag was then calculated as the occupancy rate of the highly electrically conductive phase. When using the above image analysis software, the binarization threshold for distinguishing the highly electrically conductive phase from other regions was set to a range of 140 to 200.

[0072] [Evaluation of electrodeposition paintability] Electrodeposition paint was applied to the weld metal of the obtained welded joint, and the area of ​​poor paint was measured to evaluate the electrodeposition paintability. The evaluation criteria for electrodeposition paintability were as follows: the area of ​​poor paint in the area where electrodeposition paint was applied was 2.5 mm ２ If the area of ​​the region with poor coating was 2.5 mm or less, it was judged that the desired electrodeposition coating properties were obtained and was evaluated as ○ (good). ２ Those exceeding this were judged to have not achieved the desired electrodeposition coating properties and were evaluated as x (poor).

[0073] The values ​​A1 and A3 calculated by formula (1) and formula (3) using the content of each component in the welding slag and the content of a specific component in the welding slag are shown in Table 5 below. The values ​​A2 calculated by formula (2) using the content of each component in the highly conductive phase and the content of a specific component in the highly conductive phase, and the occupancy rate of the highly conductive phase are shown in Table 6 below. The evaluation results of electrodeposition coatability are shown in Table 7 below. In the welding mode column in Table 2 and the welding method column in Table 4, CV stands for constant voltage (CV) mode, indicating that no wire feed control method was used. Formulas (1), (2), and (3) are as follows:

[0074] Formula (1): A1 = [Mn]s + [Fe]s + [Ti]s In the above formula (1), [Mn]s is the Mn content in the welding slag expressed as a mass % relative to the total mass of the welding slag, [Fe]s is the Fe content in the welding slag expressed as a mass % relative to the total mass of the welding slag, and [Ti]s is the Ti content in the welding slag expressed as a mass % relative to the total mass of the welding slag.

[0075] Formula (2): A2 = [Mn]e × [O]e / [Fe]e In the above formula (2), [Mn]e is the Mn content in the highly electrically conductive phase expressed as mass % relative to the total mass of the highly electrically conductive phase, [O]e is the O (oxygen) content in the highly electrically conductive phase expressed as mass % relative to the total mass of the highly electrically conductive phase, and [Fe]e is the Fe content in the highly electrically conductive phase expressed as mass % relative to the total mass of the highly electrically conductive phase.

[0076] Equation (3): A3 = ([Mn]s + [Si]s + [Ti]s + [Al]s) × [O]s / [Fe]s In the above equation (3), [Mn]s is the Mn content in the welding slag expressed as a mass % relative to the total mass of the welding slag, [Si]s is the Si content in the welding slag expressed as a mass % relative to the total mass of the welding slag, [Ti]s is the Ti content in the welding slag expressed as a mass % relative to the total mass of the welding slag, [Al]s is the Al content in the welding slag expressed as a mass % relative to the total mass of the welding slag, [O]s is the O (oxygen) content in the welding slag expressed as a mass % relative to the total mass of the welding slag, and [Fe]s is the Fe content in the welding slag expressed as a mass % relative to the total mass of the welding slag.

[0077]

[0078]

[0079]

[0080]

[0081]

[0082]

[0083]

[0084] As shown in Tables 5 to 7, for Inventive Examples 1 to 21, the value A1 calculated using formula (1) based on the content of specific components in the welding slag was within the range specified by the present invention. Furthermore, a highly electrically conductive phase having the composition specified by the present invention was formed in the welding slag, and the proportion of the highly electrically conductive phase was also within the range specified by the present invention. Therefore, no areas of poor coating were observed during electrodeposition coating, and a satisfactory electrodeposition coating film was formed. Furthermore, the higher the proportion of the highly electrically conductive phase, the more stable the electrodeposition coating film could be formed, even when the slag became thicker.

[0085] On the other hand, in Comparative Examples 1 to 6, a highly electrically conductive phase having the composition specified in the present invention was formed in the welding slag, and the occupancy rate of the highly electrically conductive phase was less than 4.5%, which resulted in poor coating of the electrodeposition coating film and poor evaluation results for electrodeposition coatability.

[0086] As described above in detail, according to the welded joint of the embodiment of the present invention, an electrodeposition coating can be formed even on the surface of the welding slag, and a welded joint that can obtain excellent electrodeposition paintability can be obtained.

[0087] Although various embodiments have been described above, it goes without saying that the present invention is not limited to such examples. It is clear that a person skilled in the art can conceive of various modifications or alterations within the scope of the claims, and it is understood that these also naturally fall within the technical scope of the present invention. Furthermore, the components of the above embodiments may be combined in any manner as long as they do not deviate from the spirit of the invention.

[0088] This application is based on a Japanese patent application filed on July 4, 2024 (Patent Application No. 2024-108257) and a Japanese patent application filed on May 30, 2025 (Patent Application No. 2025-091171), the contents of which are incorporated by reference into this application.

[0089] 1, 2 Steel material 3 Weld metal 4 Welding slag 10 Weld joint

Claims

1. A welded joint having a weld metal and welding slag formed on at least a portion of a surface of the weld metal, wherein the Mn content in the welding slag is expressed as [Mn]s in mass % relative to the total mass of the welding slag, the Fe content in the welding slag is expressed as [Fe]s in mass % relative to the total mass of the welding slag, and the Ti content in the welding slag is expressed as [Ti]s in mass % relative to the total mass of the welding slag, the value A1 calculated by formula (1): A1 = [Mn]s + [Fe]s + [Ti]s is 40 or more, the welding slag contains a highly conductive phase, and the occupancy rate of the highly conductive phase in a cross section of the welding slag is 4.5% or more, and the highly conductive phase contains, relative to the total mass of the highly conductive phase: Mn: 30% by mass or more, Fe: 5% by mass or more, Ti: 30% by mass or less, Si: 10% by mass or less, Al: 10% by mass or less, O (oxygen): 30 mass% or less.

2. The welded joint according to claim 1, characterized in that it has an electrocoating coating formed on at least a portion of the surface of the welding slag.

3. A method for manufacturing a welded joint, the method comprising a step of performing gas-shielded arc welding using a welding wire on a steel base material, wherein the welded joint has weld metal and welding slag formed on at least a portion of a surface of the weld metal, wherein the value A1 calculated by formula (1): A1 = [Mn]s + [Fe]s + [Ti]s is 40 or more, where [Mn]s is the manganese content in the welding slag in mass % relative to the total mass of the welding slag, [Fe]s is the iron content in the welding slag in mass % relative to the total mass of the welding slag, and [Ti]s is the titanium content in the welding slag in mass % relative to the total mass of the welding slag, the value A1 is 40 or more, the welding slag contains a highly conductive phase, and the occupancy rate of the highly conductive phase in a cross section of the welding slag is 4.5% or more, and the highly conductive phase contains, relative to the total mass of the highly conductive phase, Mn: 30% by mass or more, and Fe: 5% by mass or more. A method for manufacturing a welded joint, characterized in that Ti: 30 mass % or less, Si: 10 mass % or less, Al: 10 mass % or less, and O (oxygen): 30 mass % or less.

4. The method for manufacturing a welded joint according to claim 3, further comprising the step of forming an electrodeposition coating on at least a portion of the surface of the welding slag after the step of performing the gas-shielded arc welding.

5. In the step of performing the gas shielded arc welding, CO ２ Gas and O ２ 5. The method for manufacturing a welded joint according to claim 3, wherein a shielding gas containing at least one gas selected from the group consisting of:

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