Welded joints, methods for manufacturing welded joints, and automotive parts

Abstract

This welded joint is obtained by stacking and welding two or more steel materials and includes a weld metal, a heat-affected part, and a steel material part. The steel material part has, on the surface thereof, a zinc-based plating layer that contains 85 mass% or more of zinc and has an average thickness of 3-30 μm. A ZnO compound is deposited to the welding back face along the heat-affected part. The proportion of the length of the ZnO compound in the welding direction with respect to the length of the heat-affected part in the welding direction of the welding metal is more than 0% but not more than 80%.

Description

[Technical Field]

[0001] This disclosure relates to welded joints, methods for manufacturing welded joints, and automotive parts. Regarding. [Background technology]

[0002] As climate change is recognized as a critical social issue, the automotive industry is seeking to improve fuel efficiency and electricity consumption by reducing vehicle weight. From the perspective of steel materials, it is possible to reduce the weight of conventional vehicle bodies by increasing the strength and thinning the steel plates used. On the other hand, it is not easy to reduce the weight of automotive parts such as lower arms and subframes (chassis components) that require long-term durability. Therefore, improving the corrosion resistance of welded joints is important from the perspective of achieving weight reduction in these automotive parts. In the automotive industry, corrosion resistance of welded joints is generally achieved by imparting rust prevention capabilities through chemical conversion treatment and electrodeposition coating, and the application of Zn plating materials is one method for further improving corrosion resistance.

[0003] For example, Japanese Patent Publication No. 2017-187260 discloses a surface treatment method for galvanized steel, comprising the steps of: preparing a galvanized steel material having a base material and a zinc-containing plating layer formed on the surface of the base material; and removing or altering a portion of the plating layer by heating with a flame generated by burning a flammable gas, wherein in the step of removing or altering a portion of the plating layer, a gas containing 25% to 44% by volume of ethylene, with the remainder being hydrogen and unavoidable impurities, is used as the flammable gas.

[0004] Furthermore, Japanese Patent Publication No. 2005-40806 discloses a laser irradiation arc welding method in which the arc welding method is a consumable electrode pulsed arc welding method, the laser irradiation position is adjusted to be near the boundary between the molten and unmolten parts in front of the molten pool, the focusing diameter of the laser irradiation part is set to a diameter slightly larger than the gap length present in the welded joint, and the energy density of the laser irradiation part is set to a value at which the zinc plating of the irradiation part evaporates.

[0005] Furthermore, Japanese Patent Publication No. 2007-38241 discloses a welding method for zinc-plated steel sheets, which involves forming a resin coating layer on the back surface of the area to be welded, and then arc welding or laser welding the area from the front surface.

[0006] Furthermore, Japanese Patent Publication No. 2004-1009 discloses a welding method for zinc-plated steel sheets, in which a cooling gas supply container with an open top is installed in close contact with the back side of the welding area of ​​the zinc-plated steel sheet to be welded, and cooling gas 7 is introduced from a gas inlet provided on one side of the container and discharged from a gas outlet provided on the other side, while arc welding the welding area. [Overview of the project] [Problems that the invention aims to solve]

[0007] In welded joints used in automotive parts and other applications, corrosion resistance at the weld is required. For example, to improve the corrosion resistance of the weld, rust-preventive coatings (such as electrodeposition coatings) are applied. Using steel materials with a zinc-based plating layer as the base material is also effective. However, the heat input during welding can cause evaporation of the zinc-based plating layer, generating ZnO compounds, which can adhere to the back surface of the welded part. These ZnO compounds can hinder the formation of the coating film and reduce corrosion resistance.

[0008] In view of the above circumstances, the object of this disclosure is to provide a welded joint with excellent corrosion resistance after painting, a method for manufacturing the welded joint, and an automobile part having the welded joint. [Means for solving the problem]

[0009] The means for solving the problem include the following aspects: <1> A welded joint formed by overlapping and welding two or more steel materials, It has a weld metal, a heat-affected zone, and a steel component. The steel material part has a zinc-based plating layer on its surface, which contains 85% by mass or more of zinc and has an average thickness of 3 μm or more and 30 μm or less. ZnO compounds adhere along the heat-affected zone on the back side of the weld. A welded joint in which the ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction of the weld metal is more than 0% and 80% or less. <2> The welded joint according to <1>, wherein the ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction of the weld metal is 10% or more and 60% or less. <3> The welded joint according to <1> or <2>, which has the ZnO compound, a chemical conversion coating, and an electrocoating film on at least the surface of the heat-affected zone. <4> A welding process for manufacturing a welded joint by overlapping two or more steel materials having a zinc-based plating layer containing 85% by mass or more of zinc and having an average thickness of 3 μm or more and 30 μm or less and welding them. In the welding process, the welding is performed while spraying at least one fluid selected from the group consisting of a gas and a liquid onto a portion that becomes the heat-affected zone on the back side of the weld at a flow rate of 0.3 m / s or more and 14.0 m / s or less. A method for manufacturing a welded joint. <5> The method for manufacturing a welded joint according to <4>, wherein the fluid is an inert gas. <6> An automotive part having the welded joint according to any one of <1> to <3>.

Advantages of the Invention

[0010] According to the present disclosure, there are provided a welded joint excellent in corrosion resistance after painting, a method for manufacturing the welded joint, and an automotive part having the welded joint.

Brief Description of the Drawings

[0011] [Figure 1] It is a plan view showing the back side of the weld of the welded joint according to an embodiment of the present disclosure. [Figure 2]It is a plan view showing the welded back surface of a conventional welded joint. [Figure 3] It is a schematic cross-sectional view showing a fillet welded joint as a welded joint according to an embodiment of the present disclosure. [Figure 4] It is a schematic cross-sectional view showing an overlapping fillet welded joint as a welded joint according to an embodiment of the present disclosure. [Figure 5] It is a schematic cross-sectional view showing a T-shaped fillet welded joint as a welded joint according to an embodiment of the present disclosure. [Figure 6] It is a schematic cross-sectional view for explaining a method of manufacturing a welded joint according to an embodiment of the present disclosure.

Mode for Carrying Out the Invention

[0012] An embodiment which is an example of the present disclosure will be described. These descriptions and examples are illustrative of the embodiments and do not limit the scope of the invention. In this specification, a numerical range represented by "~" means a range including these numerical values as the lower limit value and the upper limit value when "more than" and "less than" are not attached to the numerical values described before and after "~". Also, a numerical range in which "more than" or "less than" is attached to the numerical values described before and after "~" means a range not including these numerical values as the lower limit value or the upper limit value. In the numerical ranges described step by step in this specification, the upper limit value of a certain stepwise numerical range may be replaced with the upper limit value of the numerical range described in other stepwise descriptions, or may be replaced with the value shown in the examples. Also, the lower limit value of a certain stepwise numerical range may be replaced with the lower limit value of the numerical range described in other stepwise descriptions, or may be replaced with the value shown in the examples. Also, "% " for content means "% by mass" unless otherwise specified. "0~" as the content (%) means that the component is an optional component and may not be contained.

[0013] Each component may contain a plurality of corresponding substances. When referring to the amount of each component in a composition, if there are multiple substances corresponding to each component in the composition, unless otherwise specified, it refers to the total amount of those multiple substances present in the composition. The term "process" includes not only independent processes, but also any process that cannot be clearly distinguished from other processes, as long as its intended function is achieved.

[0014] <Welded joints> A welded joint according to the embodiment of this disclosure is a welded joint formed by welding two or more steel materials together. This welded joint comprises a weld metal, a heat-affected zone, and a steel material portion. The steel material portion has a zinc-based plating layer on its surface that contains 85% by mass or more of zinc and has an average thickness of 3 μm to 30 μm. In welded joints, a ZnO compound is deposited along the heat-affected zone on the back surface of the weld. Furthermore, the ratio of the length of this ZnO compound in the welding direction to the length of the heat-affected zone of the weld metal in the welding direction (hereinafter also referred to as the "ZnO length ratio") is greater than 0% and less than or equal to 80%.

[0015] The welded joint according to the embodiment of this disclosure, with the above configuration, provides a welded joint with excellent corrosion resistance after painting. The welded joint according to this disclosure was discovered by the following findings.

[0016] In welded joints used in automotive parts and other applications, corrosion resistance at the weld is required. For example, in automotive parts, a rust-preventive coating is formed on the surface to improve the corrosion resistance of the weld. Specifically, rust prevention is imparted through chemical conversion treatment and electrodeposition coating. Furthermore, the application of steel materials with a zinc-based plating layer (zinc-plated materials) can be used to further improve corrosion resistance. By utilizing zinc-plated materials, the corrosion of iron is prevented through sacrificial corrosion protection by zinc. However, the heat input during welding can cause evaporation of the zinc-based plating layer, conversely reducing the corrosion resistance of the welded area. Specifically, the zinc evaporated from the zinc-based plating layer reacts with oxygen in the atmosphere and burns, generating ZnO compounds along the heat-affected zone. These ZnO compounds hinder the formation of coatings (e.g., chemical conversion coatings and electrodeposition coatings), resulting in areas where corrosion is more likely to progress, i.e., areas with low corrosion resistance.

[0017] In contrast, in the welded joint according to the embodiment of this disclosure, a ZnO compound is deposited along the heat-affected zone on the back surface of the weld, and the length of this ZnO compound in the welding direction is greater than 0% and less than or equal to 80% in terms of the ratio of the length of the heat-affected zone in the welding direction (ZnO length ratio). Thus, in the welded joint according to the embodiment of this disclosure, the amount of ZnO compound deposited along the heat-affected zone is reduced. Therefore, the formation of coatings in the heat-affected zone (e.g., chemical conversion coatings and electrodeposited coatings) is suppressed, and as a result, a welded joint with excellent corrosion resistance after painting is obtained.

[0018] As described above, the welded joint according to the embodiment of this disclosure exhibits excellent corrosion resistance after painting. Furthermore, automotive parts (e.g., chassis members) having this welded joint can be expected to have an even longer lifespan. These effects improve the corrosion resistance of automotive parts after painting and can also promote weight reduction of the vehicle body.

[0019] Furthermore, a method for suppressing the amount of ZnO compounds generated along the heat-affected zone on the back surface of the welded joint according to the embodiments of this disclosure, specifically a method for controlling the ZnO length ratio to fall within the aforementioned range, will be described in detail later.

[0020] Next, a welded joint according to an embodiment of this disclosure will be described in detail.

[0021] ·ZnO compounds In the welded joint according to the embodiment of this disclosure, a ZnO compound is deposited along the heat-affected zone on the back surface of the weld.

[0022] Here, as an example of a conventional welded joint, a plan view showing the welded back surface of the conventional welded joint is shown in FIG. 2. As shown in FIG. 2, the conventional welded joint 100 has a base material 2 and a heat affected zone 4 that is long in the welding direction X formed in the base material 2. And ZnO compound 60 is attached along the heat affected zone 4. In this conventional welded joint 100, the length L of the welding metal in the welding direction X W is 100 mm, and the length L of ZnO compound 60 in the welding direction X on one side Z10 is 100 mm, and the length L of ZnO compound 60 in the welding direction X on the other side Z20 is 100 mm. And the length L of the welding metal in the welding direction X W The ratio ((L Z10 + L Z20 ) / 2) of the length of ZnO compound 60 in the welding direction X to L Z10 + L Z20 ) / 2) to L W × 100) is 100%.

[0023] Next, an example of a welded joint according to an embodiment of the present disclosure is shown in FIG. 1. FIG. 1 is a plan view showing the welded back surface of the welded joint according to the embodiment of the present disclosure. The welded joint 10 shown in FIG. 1 has a base material 2 and a heat affected zone 4 that is long in the welding direction X formed in the base material 2. Note that the region other than the heat affected zone 4 in the base material 2 is a steel material part. And ZnO compound 6 is attached along the heat affected zone 4. However, in this welded joint 10, the amount of ZnO compound 6 attached along the heat affected zone 4 is reduced. In the welded joint 10, the length L of the welding metal in the welding direction X W is 100 mm, and the length L of ZnO compound 6 in the welding direction X on one side Z1 is 25 mm, and the length L of ZnO compound 6 in the welding direction X on the other side Z2 is 50 mm. And the length L of the welding metal in the welding direction X W The ratio ((L Z1 + L Z2 ) / 2) of the length of ZnO compound 6 in the welding direction X to L Z1 + L Z2 ) / 2) to L WThe result of multiplying by 100 is 37.5%.

[0024] Thus, in the welded joint according to the embodiment of this disclosure, a ZnO compound is attached along the heat-affected zone on the back surface of the weld, and the length of this ZnO compound in the welding direction is greater than 0% and less than or equal to 80% in terms of the ratio of the length of the heat-affected zone in the welding direction (ZnO length ratio). As a result, it has excellent corrosion resistance after painting. From the viewpoint of corrosion resistance after painting, the ZnO length ratio is preferably 60% or less, and more preferably 50% or less. On the other hand, the lower limit of the ZnO length ratio may be 10% or more, or 20% or more, from the viewpoint of easily suppressing the generation of ZnO compounds.

[0025] The ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction (ZnO length ratio) is given by the length L of the weld metal in the welding direction X. W And, the length L in the welding direction X on one side of the ZnO compound. Z1 And, the length L in the welding direction X on the other side of the ZnO compound. Z2 And, it can be calculated using the following formula. ZnO length ratio = (((L Z1 +L Z2 ) / 2) / L W (x100)

[0026] On the back surface of the weld, if the ZnO compound attached along the heat-affected zone is continuous in the welding direction X (i.e., uninterrupted), the length of that continuous ZnO compound in the welding direction is measured. If the ZnO compound attached along the heat-affected zone is interrupted in the welding direction X (i.e., consists of two or more clumps), the length of the ZnO compound in each clump is measured in the welding direction, and the sum of these lengths is taken as the length of the ZnO compound. Furthermore, even if the weld line has curved sections (i.e., sections with curvature), the method for measuring the length of the heat-affected zone in the welding direction and the length of the ZnO compound in the welding direction remains the same. In other words, even in sections where the weld line is curved, the length of the heat-affected zone in the welding direction and the length of the ZnO compound in the welding direction are measured to determine the ZnO length ratio.

[0027] Whether or not ZnO compounds are adhering to the heat-affected zone on the back surface of a weld can be confirmed by performing electron probe microanalyzer (EPMA) measurements on a cross-section perpendicular to the welding direction.

[0028] Here, we will explain how to identify the heat-affected zone. The heat-affected zone generally refers to the area that arises as a result of the heat input to the steel material during welding, causing the metal structure of the steel to change. However, it is not easy to clearly define the boundary between the heat-affected zone and the area outside of it (i.e., the steel material that is not in the heat-affected zone) by visual inspection or microscopic observation. The range set based on the length of the weld metal on the weld surface of the welded joint can be considered to roughly coincide with the range of the actual heat-affected zone. Furthermore, the ZnO compound adheres along the heat-affected zone, and the range of the heat-affected zone set based on the length of the weld metal is considered to be an appropriate range for the adhesion of the ZnO compound. For the reasons stated above, the "length of the heat-affected zone" in this specification refers to the range that is 5 mm longer in the longitudinal direction, based on the length of the weld metal on the weld surface of the welded joint.

[0029] • Zinc-based plating layer The welded joint according to the embodiment of this disclosure has a zinc-based plating layer on at least the surface of the steel portion (i.e., a region other than the heat-affected zone of the base material) which contains 85% by mass or more of zinc and has an average thickness of 3 μm to 30 μm.

[0030] Here, we will explain the method for measuring the zinc content in the plating layer. First, the sample is embedded in resin so that the cross-section of the plating can be observed, and then polished. After polishing, energy dispersive x-ray spectroscopy (SEM-EDS) is performed using a scanning electron microscope (SEM) to quantify the composition ratio of various elements (Zn, Al, Mg, Fe, Cr, Ni, Ti, etc.) contained in the plating, thereby determining the zinc content. For the measurement location, it is sufficient to analyze the cross-section of the plating in the non-heat-affected zone with n=5 samples.

[0031] Examples of zinc-based plating layers containing 85% by mass or more of zinc include GA plating (alloyed hot-dip galvanizing) layers and GI plating (hot-dip galvanizing) layers. The GI plating layer can be formed, for example, by a reduction furnace plating process. The GA plating layer is formed by first forming a hot-dip galvanizing layer (GI plating layer) on a steel sheet (base material), and then performing an alloying treatment.

[0032] The zinc-based plating layer contains 85% by mass or more of zinc, and the other compositions are not particularly limited. However, the GI plating layer and the GA plating layer are preferably composed as follows.

[0033] The GI plating layer may consist of zinc, or it may contain small amounts of other dissimilar metal elements or impurities (for example, cobalt, molybdenum, tungsten, nickel, titanium, chromium, aluminum, manganese, iron, magnesium, lead, bismuth, antimony, tin, copper, cadmium, arsenic, etc.) in addition to zinc. It may also contain inorganic materials such as silica, alumina, and titania. A preferred composition of the GI plating layer is, for example, a plating layer containing 0.01 to 0.20% by mass of Al, with the remainder being Zn and impurities.

[0034] The composition of the GA plating layer may include, for example, a zinc-based alloy plating layer of zinc and another metal (at least one selected from the group consisting of iron, aluminum, cobalt, tin, nickel, chromium, titanium, magnesium, and manganese). In addition, it may also contain small amounts of other dissimilar metallic elements or impurities (for example, cobalt, molybdenum, tungsten, nickel, titanium, chromium, aluminum, manganese, iron, magnesium, lead, bismuth, antimony, tin, copper, cadmium, arsenic, etc.). Furthermore, it may also contain inorganic substances such as silica, alumina, and titania. A preferred composition of the GA plating layer is, for example, one containing, by mass%, Fe: 5-20% and Al: 0.01-0.20%, with the remainder being Zn and impurities.

[0035] The average thickness of the zinc-based plating layer is 3 μm or more and 30 μm or less. If the average thickness is 3 μm or more, ZnO compounds are more likely to be generated due to the evaporation of zinc during welding. However, in the welded joint according to the embodiment of this disclosure, the ZnO length ratio is within the aforementioned range, resulting in excellent corrosion resistance after painting. On the other hand, by having an average thickness of 30 μm or less, the amount of zinc evaporation during welding is suppressed, thereby improving corrosion resistance after painting. The average thickness of the zinc-based plating layer is preferably 5 μm to 25 μm, and more preferably 10 μm to 20 μm. The average thickness of the zinc-based plating layer is defined as the arithmetic mean of the plating thickness measured in 10 fields of view in electron microscope images of the cross-section of the steel material.

[0036] · Nurihime The welded joint according to the embodiment of this disclosure exhibits excellent corrosion resistance after painting. That is, to improve the corrosion resistance of the weld, a coating is formed on at least the heat-affected zone surface of the welded joint. Furthermore, the coating may be formed on the entire surface of the welded joint in addition to the heat-affected zone.

[0037] For welded joints according to the embodiments of this disclosure, an electrodeposited coating is preferred as the coating formed on at least the surface of the heat-affected zone, from the viewpoint of corrosion resistance of the welded joint. The electrodeposited coating can be a well-known type of electrodeposited coating, either an anionic or cationic type, but a cationic type is preferred from the standpoint of ease of application. An example of an electrodeposited coating is an electrodeposited coating film formed by an electrodeposition coating process using a water-based paint containing a resin, a hardener, and other additives. Examples of resins include aqueous resins having hydrophilic groups such as carboxyl groups, hydroxyl groups, methylol groups, amino groups, sulfonic acid groups, and polyoxyethylene bonds, as well as functional groups such as hydroxyl groups that react with the curing agent (known aqueous resins such as acrylic resins, polyester resins, alkyd resins, epoxy resins, and polyurethane resins). Examples of curing agents include melamine resin and blocked polyisocyanate. Other known additives include coloring pigments, light-interfering pigments, extender pigments, dispersants, anti-settling agents, reaction accelerators, defoaming agents, thickeners, rust inhibitors, UV absorbers, and surface modifiers.

[0038] The average thickness of the electrodeposited coating is preferably 1 to 40 μm, and more preferably 5 to 30 μm.

[0039] Furthermore, when a coating (e.g., an electrodeposited coating) is formed on the surface of a welded joint, a chemical conversion coating may be formed between the welded joint and the coating. In other words, at least the surface of the heat-affected zone may have a coating (e.g., an electrodeposited coating) via a chemical conversion coating, and even further, the coating (e.g., an electrodeposited coating) may be formed via a chemical conversion coating on the entire surface of the welded joint in addition to the heat-affected zone.

[0040] • Shape of welded joints The welded joint according to the embodiment of this disclosure is a welded joint formed by welding two or more steel materials together. Here, "welding together" means that parts of the steel materials are overlapped (for example, parts of two steel plates are stacked together) and then welded. Examples of these welded joints include lap welds, lap fillet welds, and T-fillet welds.

[0041] A lap welded joint is a joint formed by laminating the ends of two steel plates 21A and 22A, each having a zinc-based plating layer 8 on its surface, as shown in Figure 3, and welding the laminated area from one side (the upper side in Figure 3). In the lap welded joint shown in Figure 3, a heat-affected zone 4A is formed around the weld metal 3A, and the back side (the lower side in Figure 3) relative to the welded side becomes the weld back surface Y. A ZnO compound 6A is attached to the weld back surface Y. A lap fillet weld joint is a joint formed by laminating the ends of two steel plates 21B and 22B, each having a zinc-based plating layer 8 on its surface, as shown in Figure 4, and welding the end face of one of the laminated steel plates 21B to the flat surface of the other steel plate 22B (the upper surface in Figure 3). In the lap fillet weld joint shown in Figure 4, a heat-affected zone 4B is formed around the weld metal 3B, and the back side (the lower side in Figure 4) relative to the welded side becomes the weld back surface Y. A ZnO compound 6B is attached to the weld back surface Y. In addition, in a lap fillet weld joint, the end face of the other steel plate 22B and the flat surface of the one steel plate 21B (the lower surface in Figure 3) may be further welded. A T-shaped fillet welded joint is a joint formed using steel plates 21 and C22C, each having a zinc-based plating layer 8 on its surface, as shown in Figure 5. The steel plate 21C is placed upright in contact with the steel plate 22C, and the end face of the steel plate 21C contacts the plane of the steel plate 22C (the upper plane in Figure 5), creating a T-shaped joint. In this T-shaped fillet welded joint, one side of the area where the end face of the steel plate 21C and the plane of the steel plate 22C (the upper plane in Figure 3) contact is welded. In the T-shaped fillet welded joint shown in Figure 5, a heat-affected zone 4C is formed around the weld metal 3C, and the back side (lower side in Figure 5) relative to the welded side becomes the weld back surface YA and YB. ZnO compounds 6CA and 6CB are attached to the weld back surfaces YA and YB, respectively, but the ZnO compound 6CA attached to the weld back surface YA is used as the target for measuring the length of the ZnO compound in the welding direction. In other words, the ZnO compound 6CA attached to the flat surface of steel plate 22C (the lower surface in Figure 5) is the target of measurement, while the ZnO compound 6CB attached to the area where the end face of steel plate 21C and the flat surface of steel plate 22C (the upper surface in Figure 5) come into contact is not the target of measurement.

[0042] <Method for manufacturing welded joints> Next, a method for manufacturing a welded joint according to the embodiment of this disclosure will be described. Furthermore, the welded joint according to the embodiment of this disclosure can be obtained by the manufacturing method of the welded joint according to the embodiment of this disclosure. In other words, according to the manufacturing method of the welded joint according to the embodiment of this disclosure, a welded joint with excellent corrosion resistance after painting can be manufactured.

[0043] A method for manufacturing a welded joint according to the embodiment of this disclosure includes a welding step of welding together two or more steel materials having a zinc-based plating layer containing 85% by mass or more of zinc and having an average thickness of 3 μm to 30 μm. In the welding step, welding is performed while spraying at least one fluid selected from the group consisting of gases and liquids onto the heat-affected zone on the back surface of the weld at a flow velocity of 0.3 m / s to 14.0 m / s.

[0044] Here, we will describe a method for obtaining a welded joint according to the embodiments of the present disclosure described above, that is, a method for obtaining a welded joint in which the generation of ZnO compounds on the back surface of the weld is suppressed and the ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction (ZnO length ratio) is within the range described above.

[0045] The inventors observed the formation behavior of ZnO compounds on the welded back surface of zinc-plated material using a high-speed camera and obtained the following findings. First, depending on the temperature of the back surface of the weld, ZnO compounds were not generated, and only zinc plating vapor was produced. Furthermore, it was confirmed that ZnO compounds were not generated by the evaporation of zinc plating alone, but rather when the zinc vapor burned, ZnO compounds were formed and adhered to the back surface of the weld. The combustion phenomenon occurred when the zinc plating vapor came into contact with the high-temperature area on the back surface of the weld, and it was observed that the combustion propagated from the outermost layer that came into contact. Although manual wiping was performed to remove the adhered ZnO compounds, cross-sectional observation revealed that ZnO compounds of about 3 μm to 5 μm remained even after wiping, making complete removal difficult. Moreover, in areas where these ZnO compounds adhered, there was a tendency for coatings such as electrodeposited coatings not to form or to become thin, suggesting that they may become the starting point for corrosion.

[0046] Furthermore, combustion does not occur unless three elements are present: a combustible material, an oxygen supply, and an ignition source. Therefore, based on the above findings, it is thought that the adhesion of ZnO compounds can be reduced by performing welding in a situation where the three elements—zinc plating vapor (powder) as the combustible material, oxygen from the atmosphere as the oxygen supply, and the high-temperature area on the back of the weld (heat-affected zone) as the ignition source—are not present.

[0047] Therefore, in the method for manufacturing a welded joint according to the embodiment of this disclosure, welding is performed while spraying at least one fluid selected from the group consisting of gases and liquids onto the heat-affected zone on the back surface of the weld. This reduces the density of zinc plating vapor, decreases heat conduction from the outermost layer on the back surface of the weld into the vapor atmosphere, suppresses the combustion of zinc vapor, and reduces the adhesion of ZnO compounds.

[0048] Furthermore, it is possible to remove ZnO compounds adhering to the back surface of welds by applying strong removal treatments such as shot blasting. However, removal of ZnO compounds is difficult depending on the shape of the component; specifically, it is difficult to perform the removal treatment on components where the back surface of the weld faces a closed space. In addition, performing the removal treatment would lead to a decrease in production efficiency. For this reason, it is undesirable to implement a removal treatment. In contrast, the method for manufacturing a welded joint according to the embodiment of this disclosure can reduce the adhesion of ZnO compounds in the first place, so it is not necessary to perform a removal treatment on the ZnO compounds. Therefore, according to the method for manufacturing a welded joint according to the embodiment of this disclosure, the welded joint according to the embodiment of this disclosure can be obtained without performing any removal treatment. In other words, it is possible to obtain a method for manufacturing a welded joint according to the embodiment of this disclosure that has no traces of removal treatment on the back surface of the weld.

[0049] Here, an example of a method for manufacturing a welded joint according to the embodiment of this disclosure will be described with reference to the drawings. Figure 6 is a schematic cross-sectional view illustrating the method for manufacturing a welded joint according to the embodiment of this disclosure.

[0050] The manufacturing method for a welded joint according to the embodiment of this disclosure first involves a welding step, as shown in Figure 6, in which the ends of two steel plates 21B and 22B, each having a zinc-based plating layer 6 containing 85% by mass or more of zinc and having an average thickness of 3 μm to 30 μm, are overlapped (laminated), and the end face of one of the laminated steel plates 21B and the flat surface of the other steel plate 22B (the upper flat surface in Figure 6) are welded to form weld metal 3B. In this welding step, welding is performed while blowing gas Z onto the heat-affected zone 4B on the back surface Y of the weld (for example, while blowing gas using a nozzle 30 or the like).

[0051] Furthermore, during the welding process, the flow velocity of at least one fluid selected from the group consisting of gases and liquids, when spraying it onto the heat-affected zone on the back surface of the weld, shall be between 0.3 m / s and 14.0 m / s. By setting the flow velocity to 0.3 m / s or higher, the density of the zinc plating vapor can be reduced, thereby suppressing combustion and reducing the adhesion of ZnO compounds. On the other hand, by setting the flow velocity to 14.0 m / s or lower, gas leakage from the overlapping parts of the steel materials is suppressed, and the effect of preventing porosity defects from occurring inside the weld metal can be obtained. The flow velocity is preferably between 0.5 m / s and 12.0 m / s, more preferably between 1.0 m / s and 10.0 m / s, and even more preferably between 2.0 m / s and 8.0 m / s.

[0052] The fluid velocity is measured using a vane-type anemometer (AS ONE, part number: EA739AF-21). The vane probe tip for measuring the fluid velocity is fixed in a position that does not touch the heat-affected zone on the back side of the steel material (20 mm vertically away from the heat-affected zone), and the average of the values ​​measured for 10 seconds is defined as the fluid velocity.

[0053] The fluid to be sprayed may be a gas or a liquid. Examples of gases include air, inert gases (e.g., Ar), CO2 gas, oxygen gas, nitrogen gas, helium gas, neon gas and other noble gases, and mixtures thereof. Since oxygen gas is easily combustible, if oxygen gas is included, its content should preferably be less than or equal to that of air, and more preferably less than 10% oxygen. Examples of liquids include water, silicone oil, lubricating oil, and mixtures thereof, or mixtures containing compressed air. Furthermore, when spraying the liquid, it is preferable to spray it as a spray-type liquid. Among these, the fluid is preferably an inert gas, from the viewpoint of suppressing the formation of ZnO compounds (i.e., the reaction between Zn and O) and thereby reducing the adhesion of ZnO compounds.

[0054] In this disclosure, when the steel material (base material) is a steel plate, the plate thickness is not particularly limited, but for example, it can be 1 mm or more and 30 mm or less.

[0055] For example, a welded joint according to the embodiment of this disclosure is obtained by gas-shielded arc welding a steel material using a solid wire. In this case, the chemical composition of the weld metal includes components derived from the solid wire, which is the welding material, and the steel material, which is the base material.

[0056] (Automotive parts) The welded joints according to the embodiments of this disclosure can be used, for example, as automotive parts. Examples of automotive parts having welded joints according to the embodiments of this disclosure include chassis subframes, lower arms, upper arms, axle beams, and the like. [Examples]

[0057] Next, the feasibility and effects of this disclosure will be described in more detail with reference to examples of inventions and comparative examples. However, the following examples are not intended to limit this disclosure, and any design modifications made in accordance with the spirit of the preceding and following descriptions are all within the technical scope of this disclosure.

[0058] A welded joint was obtained by gas-shielded arc welding using solid wire.

[0059] (steel plate) A steel plate with a zinc-based plating on its surface was used as the base material. The zinc content in the plating and the average thickness of the plating are shown in Table 1.

[0060] (Manufacturing of welded joints) Using two base materials (steel plates), gas shielded arc welding was performed using the joint type shown in Table 1. The welding conditions for manufacturing the welded joint by arc welding were as follows, for example, when welding the first and second steel plates. Welding current: 200A, Welding voltage: 22.4V, Welding speed: 100cm / min Welding gas: 20% CO2 + Ar, Gas flow rate: 20 L / min Welding wire: YGW16, manufactured by Nippon Steel Welding Industries Co., Ltd., φ1.2mm (C: 0.1 mass%, Si: 0.80 mass%, Mn: 1.5 mass%, P: 0.015 mass%, S: 0.008 mass%, Cu: 0.36 mass%) Welding torch tilt angle: 45°

[0061] In the cases of No. 1 to 19 and No. 22, welding was performed while spraying the type of fluid listed in Table 1 onto the back surface of the weld at the flow rate listed in Table 1.

[0062] The ZnO compound adhering along the heat-affected zone on the back surface of the weld was identified using the method described above, and the length L of the ZnO compound in the welding direction was determined. Z1 and L Z2 and the length L of the weld metal in the welding direction. W The following measurements were taken. The results, as well as the ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction (ZnO length ratio), are shown in Table 1. In Table 1, items that do not meet the requirements of this disclosure are underlined.

[0063] [Table 1]

[0064] As shown in Table 1, in the examples (No. 1 to 19) where welding is performed while spraying fluid within the aforementioned flow velocity range, the length of the ZnO compound in the welding direction is reduced. Therefore, it is thought that the ability to form a coating film on the surface is also improved, as is the corrosion resistance. In No. 20 (Comparative Example), the zinc content of the zinc-based plating layer was low to begin with, which is thought to have suppressed the generation of ZnO compounds. In No. 21 (Comparative Example), the zinc-based plating layer was thin and the amount of zinc evaporated was small, which is thought to have suppressed the generation of ZnO compounds. In No. 22 (Comparative Example), the zinc-based plating layer was too thick, resulting in a large amount of zinc evaporation. Therefore, even when welding was performed while spraying fluid, the generation of ZnO compounds was not suppressed. Consequently, the film formation on the surface was poor, and the corrosion resistance was also poor. In No. 23 (Comparative Example), welding was performed without spraying a fluid, resulting in a large amount of ZnO compound adhesion. Therefore, it is thought that the ability to form a coating film on the surface was poor, and the corrosion resistance was also poor.

[0065] Furthermore, the disclosure of Japanese application 2023-223069 is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted to be incorporated by reference. [Explanation of Symbols]

[0066] 2 Base metal, 3A, 3B, 3C, 3D Weld metal, 4, 4A, 4B, 4C, 4D Heat-affected zone, 6, 60, 6A, 6B, 6CA, 6CB ZnO compound, 8 Zinc-based plating layer, 10, 100 Welded joint, 21A, 21B, 21C, 21D, 22A, 22B, 22C, 22D Steel plate, 30 Nozzle, Y Weld back surface, Z Fluid

Claims

1. A welded joint formed by overlapping and welding two or more steel materials, It has a weld metal, a heat-affected zone, and a steel component. The steel material portion has a zinc-based plating layer on its surface that contains 85% by mass or more of zinc and has an average thickness of 3 μm to 30 μm. On the back surface of the weld, a ZnO compound adheres along the heat-affected zone. A welded joint in which the ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone of the weld metal in the welding direction is greater than 0% and less than or equal to 80%.

2. The welded joint according to claim 1, wherein the ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone of the weld metal in the welding direction is 10% or more and 60% or less.

3. The welded joint according to claim 1, wherein at least the surface of the heat-affected zone has the ZnO compound, a chemical conversion coating, and an electrodeposited coating.

4. The process includes a welding step to manufacture a welded joint by welding together two or more steel materials having a zinc-based plating layer containing 85% or more by mass of zinc and having an average thickness of 3 μm to 30 μm. A method for manufacturing a welded joint, wherein the welding process involves performing the welding while spraying at least one fluid selected from the group consisting of gases and liquids onto the heat-affected zone on the back surface of the weld at a flow rate of 0.3 m / s to 14.0 m / s.

5. The method for manufacturing a welded joint according to claim 4, wherein the fluid is an inert gas.

6. An automobile part having a welded joint according to any one of claims 1 to 3.

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