Surface-treated steel plates and arc-welded joints

Abstract

A surface-treated steel sheet according to the present invention comprises: a steel sheet as a base material; and a surface treatment layer located on at least a part of the steel sheet. The surface treatment layer contains a Zr compound having slag-forming ability to form Zr-Mn-Si-based slag between elemental Mn and Si. The Zr content in the surface treatment layer is 10 to 100 at% and the average thickness of the surface treatment layer is 0.5 to 5.0 μm.

Description

Technical Field

[0001] The present invention relates to a surface-treated steel sheet and an arc weld joint.

Background Art

[0002] In the manufacture of automotive parts (particularly, underbody parts), it is common to perform chemical conversion treatment and electrodeposition coating treatment after arc-welding a steel sheet used as a material into a desired shape.

[0003] Here, during arc welding, Si and Mn contained in the steel sheet or welding wire used as a material react with oxygen in the shielding gas to form slag composed of Si-Mn-based oxides. Since Si-Mn-based slag is non-conductive, electrodeposition coating cannot form a film thereafter, and the portion where the slag exists becomes a coating defect, so the vicinity of the weld bead portion is likely to be corroded. Therefore, in order to prevent such coating defects and ensure the corrosion resistance of the weld bead portion, for example, the amount of Si-Mn-based slag formed during arc welding is suppressed by adjusting the amount of Si in the welding wire.

[0004] On the other hand, conventionally, various surface-treated steel sheets provided with a film layer having corrosion resistance on the surface of a steel sheet or a plated steel sheet have been proposed in order to improve the corrosion resistance of the steel sheet or the plated steel sheet. For example, in Patent Document 1 below, a composite coating-treated zinc-containing plated steel material provided with a composite coating containing a basic zirconium compound, a vanadyl-containing compound, a phosphoric acid compound, and a cobalt compound is proposed with the intention of improving not only corrosion resistance but also blackening resistance, coating adhesion, and alkali resistance.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, Patent Document 1 mentioned above does not mention corrosion resistance during arc welding, and even when using a composite coating as described in Patent Document 1, there was still room for further improvement in terms of improving the corrosion resistance of the weld bead during arc welding.

[0007] Therefore, the present invention has been made in view of the above problems, and the object of the present invention is to provide a surface-treated steel sheet that can further improve the corrosion resistance of the weld bead formed when arc welding is performed, and an arc-welded joint using such a surface-treated steel sheet. [Means for solving the problem]

[0008] To solve the above problems, the inventors conducted diligent studies and came up with the idea that if electrodeposition coating could be applied to the weld bead formed by arc welding, the corrosion resistance of the weld bead could be further improved. Based on this idea, the inventors found that in order to apply electrodeposition coating to the weld bead, it is necessary to generate conductive slag instead of non-conductive slag during arc welding. Based on this finding, the inventors conducted further studies and came to complete the present invention as shown below. Based on the findings described above, the gist of this invention is as follows:

[0009] [1] An arc-welded joint in which at least two surface-treated steel plates are arc-welded, The surface-treated steel sheet and, The weld bead portion formed by the aforementioned arc welding, Slag present on at least a portion of the surface of the weld bead, The surface-treated steel plate, the weld bead portion, and the slag are covered with electrodeposition coating, It has, The surface-treated steel sheet is The base material is a steel plate, A surface treatment layer located in at least a portion of the steel plate, It has, The aforementioned surface treatment layer contains a Zr compound that has the ability to form a Zr-Mn-Si system slag with the elements Mn and Si. The Zr content in the aforementioned surface treatment layer is 10 to 100 atomic percent. The average thickness of the surface treatment layer is 0.5 to 5.0 μm. The slag contains at least Zr-Mn-Si slag, An arc-welded joint that satisfies the relationship expressed by the following equation (1), where ts is the average film thickness of the electrodeposited coating located on the surface of the surface-treated steel plate in the flat portion of the arc-welded joint, and tb is the average film thickness of the electrodeposited coating located on the surface of the weld bead portion. tb / ts≧1.2...Equation (1) [2] The arc welding joint according to [1] above, wherein the Zr compound having slag-forming ability is at least one of zirconium lactate ammonium salt, zirconium lactate, zirconium triethanolamine, zirconium monoacetylacetonate, and zirconium ethylacetoacetate. [3] The surface treatment layer contains a resin, The arc welded joint according to [1] above, wherein the Zr compound having slag-forming ability is a zirconium lactate ammonium salt. [4] The surface treatment layer contains a conductive pigment, The arc welding joint according to [1] above, wherein the content of the conductive pigment is 10 to 40% by mass. [5] The arc welding joint according to [4] above, wherein the average particle size of the conductive pigment is 0.25 to 2.00 times the average thickness of the surface treatment layer. [6] The arc welding joint according to [4] above, wherein the conductive pigment is TiN. [7] The surface treatment layer contains a resin, The arc welding joint according to any one of [1] to [6] above, wherein the resin is at least one of epoxy resin or urethane resin. [8] The surface treatment layer contains a resin and a rust inhibitor other than a Si compound, The arc welding joint according to any one of the above [1] to [6], wherein the content of the rust inhibitor is 10 to 30% by mass. [9] The surface treatment layer contains a resin and a conductive pigment, The arc welding joint according to [1] or [2] above, wherein the content of the conductive pigment is 10 to 40% by mass.

[10] The arc welded joint according to any one of [1] to [6] above, wherein the Zr content in the surface treatment layer is 20 atomic percent or more.

Advantages of the Invention

[0010] As described above, according to the present invention, it is possible to further improve the corrosion resistance of the weld bead portion formed when arc welding is performed.

Brief Description of the Drawings

[0011] [Figure 1] It is an explanatory view schematically showing the structure of the surface-treated steel sheet according to an embodiment of the present invention. [Figure 2] It is an explanatory view schematically showing the structure of an arc weld joint using the surface-treated steel sheet in the same embodiment.

Modes for Carrying Out the Invention

[0012] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant descriptions will be omitted.

[0013] (Regarding surface-treated steel sheets) The overall structure of the surface-treated steel sheet according to an embodiment of the present invention will be described below with reference to Figure 1. Figure 1 is a schematic explanatory diagram showing an example of the configuration of the surface-treated steel sheet according to this embodiment.

[0014] The surface-treated steel sheet according to this embodiment is particularly suitable as a material for arc-welded joints, which are manufactured by joining multiple parts processed into a desired shape to each other by arc welding. It is also suitable as a material for various automobile parts, such as undercarriage components.

[0015] As schematically shown in Figure 1, the surface-treated steel sheet 1 according to this embodiment comprises a base steel sheet 10 and a surface treatment layer 20 formed over the entire surface of both surfaces of the base steel sheet 10. Although Figure 1 illustrates the case where the surface treatment layer 20 is formed over the entire surface of both surfaces of the base steel sheet 10, in this embodiment, the surface treatment layer 20 only needs to be provided on at least a portion of the surface of the base steel sheet 10.

[0016] <Regarding the base steel sheet 10> The base steel sheet 10 used as the base material for the surface-treated steel sheet 1 according to this embodiment is not particularly limited, and various types of steel sheets can be used depending on the mechanical strength (e.g., tensile strength) required for the surface-treated steel sheet 1. Examples of such base steel sheets 10 include steel materials standardized by Japanese Industrial Standards (JIS), such as carbon steel, alloy steel, and high-tensile steel used for general structural and machine structural purposes. Specific examples of such steel materials include cold-rolled steel, hot-rolled steel, hot-rolled steel sheets for automobile structures, hot-rolled high-tensile steel sheets for automobile processing, cold-rolled steel sheets for automobile structures, cold-rolled high-tensile steel sheets for automobile processing, and high-tensile steel materials generally called hot-stamped materials that have been quenched during hot working. The composition of such steel materials is not particularly limited, but in addition to Fe and C, it may contain one or more of the following: Si, Mn, S, P, Al, N, Cr, Mo, Ni, Cu, Ca, Mg, Ce, Hf, La, Zr, and Sb. These optional additive elements can be selected individually or in combination of two or more as appropriate to obtain the desired material strength and moldability, and their content can also be adjusted accordingly.

[0017] Among the high-strength steels described above, it is preferable to use high-strength steel with a tensile strength of 780 MPa or higher (so-called high-strength steel of 780 MPa class or higher) because it is possible to further improve the robustness of the manufactured articles. Here, the tensile strength of the base steel plate 10 can be measured by a known method. For example, a test piece can be prepared from a part of the steel plate whose tensile strength is to be measured using the method specified in JIS Z 2241:2011, and the tensile strength of the obtained test piece can be measured according to the above specification.

[0018] Furthermore, as the base steel sheet 10 according to this embodiment, a zinc-plated steel sheet may be used, which has various zinc-based plating layers containing at least zinc provided on the surface of the steel sheet. Examples of zinc-plated steel sheets include zinc-plated steel sheets, zinc-nickel plated steel sheets, zinc-iron plated steel sheets, zinc-chromium plated steel sheets, zinc-aluminum plated steel sheets, zinc-titanium plated steel sheets, zinc-magnesium plated steel sheets, zinc-manganese plated steel sheets, zinc-aluminum-magnesium plated steel sheets, and zinc-aluminum-magnesium-silicon plated steel sheets. In addition, as a zinc-plated steel sheet, one may be used which contains a small amount of dissimilar metal elements or impurities in the plating, such as cobalt, molybdenum, tungsten, nickel, titanium, chromium, aluminum, manganese, iron, magnesium, lead, bismuth, antimony, tin, copper, cadmium, arsenic, etc., or which has inorganic substances such as silica, alumina, and titania dispersed in it. Furthermore, as zinc-plated steel sheets, steel sheets having a multi-layer plating combining the above-mentioned plating with other types of plating (for example, iron plating, iron-phosphorus plating, nickel plating, cobalt plating, etc.) may be used. The plating method is not particularly limited, and various known plating methods such as electroplating, hot-dip plating, vapor deposition, dispersion plating, and vacuum plating may be used.

[0019] Here, the thickness of the base steel sheet 10 is not particularly limited and can be set appropriately according to the mechanical strength required for the surface-treated steel sheet 1, but for example it can be about 1.0 to 25.0 mm.

[0020] <About the surface treatment layer 20> As schematically shown in Figure 1, a surface treatment layer 20 is formed on both the front and back surfaces of the base steel sheet 10 as described above. The surface treatment layer 20 according to this embodiment is a film layer containing a Zr compound that has the ability to form a Zr-Mn-Si system slag with the elements Mn and Si, as will be described in detail below.

[0021] Figure 2 is a schematic diagram illustrating the structure of an arc-welded joint using surface-treated steel plates in this embodiment. When the surface-treated steel sheet 1 according to this embodiment is subjected to arc welding, the Zr compound present in the surface treatment layer 20 reacts with Mn and Si in the steel sheet and welding wire. As a result, as shown in Figure 2, a Zr-Mn-Si slag 120 is formed on the weld bead portion 110. The Zr-Mn-Si slag in this embodiment (hereinafter sometimes referred to as slag 120) is an oxide containing Zr, Mn, and Si on the surface of the steel sheet. For the sake of explanation, the method for identifying the slag 120 will be described later. Since such a Zr-Mn-Si slag 120 is a conductive slag, the conductivity of the weld bead portion 110 is ensured. Therefore, when applying electrodeposition coating 130 to the weld bead portion 110, film formation defects in the areas where slag 120 is present are suppressed, and electrodeposition coating 130 can be formed on the weld bead portion 110. This makes it possible to improve the corrosion resistance of the weld bead portion 110 formed during arc welding.

[0022] Here, for a Zr-Mn-Si slag to be conductive means that the average resistance of the Zr-Mn-Si slag is 150 MΩ or less. The method for identifying the location of slag in an arc-welded joint manufactured by arc welding a surface-treated steel plate 1 will be explained in detail below. The resistance value of the slag can be measured using a commercially available tester. At the location where the slag is present, measure three arbitrary locations at least 5 mm apart using the tester and obtain the resistance value for each measurement location. Then, by averaging the obtained resistance values ​​over the number of measurement locations, the average resistance value of the slag of interest can be obtained.

[0023] The surface treatment layer 20 according to this embodiment may consist only of a Zr compound having the slag-forming ability described above, or it may consist of such a Zr compound and other components (for example, components such as resin). From this viewpoint, the Zr content in the surface treatment layer 20 according to this embodiment is in the range of 10 to 100 atomic percent. For the sake of explanation, the details of the slag-forming ability will be described later.

[0024] If the Zr content in the surface treatment layer 20 is less than 10 atomic percent, the amount of Zr compound in the surface treatment layer 20 is too low, resulting in a small amount of Zr-Mn-Si slag being formed during arc welding. Therefore, it is not possible to improve the corrosion resistance of the weld bead formed by arc welding. By having a Zr content of 10 atomic percent or more in the surface treatment layer 20, it is possible to form a Zr-Mn-Si slag by arc welding sufficient for electrodeposition coating, thereby improving the corrosion resistance of the weld bead. The Zr content in the surface treatment layer 20 is preferably 20 atomic percent or more, and more preferably 40 atomic percent or more.

[0025] Since the surface treatment layer 20 according to this embodiment may be composed solely of a Zr compound having the slag-forming ability described above, the upper limit of the Zr content in the surface treatment layer 20 is 100 atomic percent.

[0026] On the other hand, if the surface treatment layer 20 according to this embodiment further contains components other than the Zr compound having the slag-forming ability described above (for example, resin, etc.), it is preferable to fully exhibit the effects that can be obtained by including components other than the Zr compound that can ensure conductivity (for example, improved adhesion between the electrodeposition coating and the surface treatment layer 20, improved spot weldability, corrosion resistance as a surface-treated steel sheet 1, etc.). From this viewpoint, if the surface treatment layer 20 according to this embodiment further contains components other than the Zr compound having the slag-forming ability described above, the Zr content in the surface treatment layer 20 is preferably 70 atomic% or less, and more preferably 40 atomic% or less.

[0027] As will be described later, in the manufacturing process of the surface-treated steel sheet 1 according to one embodiment, the surface treatment layer 20 can be formed by applying a treatment solution containing a surface treatment agent and then performing heat drying. With such heat drying, the Zr content may increase due to the evaporation of components such as water in the treatment solution. Therefore, the Zr content in the treatment solution before heat drying may be less than 10 atomic percent. In one embodiment, the Zr content in the surface treatment solution before heat drying is 3 atomic percent or more. This ensures that the Zr content in the surface treatment layer 20 after heat drying is 10 atomic percent or more.

[0028] Here, the Zr content of the surface treatment layer 20 described above can be measured by cutting out a sample for cross-sectional observation from any position where the surface treatment layer 20 exists on the surface-treated steel sheet 1, and observing the sample using a scanning electron microscope (SEM) equipped with an electron probe microanalyzer (EPMA).

[0029] More specifically, the surface treatment layer 20 is cut to an appropriate size in a direction perpendicular to the surface of the surface treatment layer 20 (thickness direction) at any location where the surface treatment layer 20 exists, exposing the thickness-direction cross-section of the surface treatment layer 20. Then, with the cross-section embedded in resin so that the thickness-direction cross-section is visible, the cross-section is polished. After that, the resulting polished surface is observed using SEM-EPMA. More specifically, EPMA mapping analysis of the surface treatment layer 20 in the thickness-direction cross-section (polished surface) can be performed at a magnification of 5000x (acceleration voltage: 15kV, beam diameter: 10nm, measurement pitch: 1nm). The boundary between the surface treatment layer 20 and the base steel plate 10 can be clearly determined by SEM. In this case, Zr is selected as the element to be detected, and mapping of the location and concentration of the Zr element is performed, and the Zr content in the field of view of interest can be calculated from the obtained results. Such measurements can be performed at three arbitrary locations on the obtained cross-section that are at least 100 nm apart from each other, and the average value of the Zr content obtained at each location can be taken as the Zr content of the surface treatment layer 20.

[0030] [Regarding the average thickness of the surface treatment layer 20] The average thickness (thickness d in Figure 1) of the surface treatment layer 20 containing the Zr compound described above is in the range of 0.5 to 5.0 μm. If the average thickness of the surface treatment layer 20 is less than 0.5 μm, the thickness of the surface treatment layer 20 is too thin, and the corrosion resistance of the surface-treated steel sheet 1 cannot be guaranteed. By making the average thickness of the surface treatment layer 20 0.5 μm or more, the corrosion resistance required for the surface-treated steel sheet 1 can be guaranteed. The average thickness of the surface treatment layer 20 is preferably 1.0 μm or more, and more preferably 2.0 μm or more.

[0031] On the other hand, if the average thickness of the surface treatment layer 20 exceeds 5.0 μm, the weldability of the surface-treated steel sheet 1 will decrease as a result of the surface treatment layer 20 being too thick. By making the average thickness of the surface treatment layer 20 5.0 μm or less, it is possible to ensure the corrosion resistance required for the surface-treated steel sheet 1 while suppressing the decrease in weldability. The average thickness of the surface treatment layer 20 is preferably 4.5 μm or less, and more preferably 4.0 μm or less.

[0032] Here, the average thickness of the surface treatment layer 20 can be determined by cutting the surface treatment layer 20 at an angle perpendicular to its surface and observing it from a direction perpendicular to the cut surface (cross-sectional direction). Specifically, it can be measured by observing the cut surface of the surface treatment layer 20 from the cross-sectional direction with a microscope. As a method for preparing a sample for observation of the cut surface from the cross-sectional direction, known methods such as embedding a small piece of surface-treated steel plate 1 with the surface treatment layer 20 in resin and polishing the cut surface to be observed, processing with a focused ion beam (FIB), or microtome method can be used. In addition, a scanning electron microscope (SEM) can be used as the type of microscope. The thickness of the surface treatment layer 20 is measured at any multiple locations (for example, 3 locations) of the obtained sample using the length measuring function implemented in the microscope, using the method described above. The average thickness of the surface treatment layer 20 can be determined by averaging the obtained multiple measurement values ​​by the number of measurement locations.

[0033] [Specific examples of Zr compounds that have the ability to form Zr-Mn-Si slags] Examples of Zr compounds that have the ability to form Zr-Mn-Si slag and are contained in the surface treatment layer 20 according to this embodiment include Zr compounds other than Zr oxides. Here, Zr oxides, which consist of Zr atoms and oxygen atoms, do not form conductive slags with Mn and Si when arc welding is performed. For this reason, Zr oxides cannot be used as Zr compounds with slag-forming ability according to this embodiment.

[0034] An example of a Zr compound capable of forming Zr-Mn-Si slag is ammonium zirconium carbonate. Ammonium zirconium carbonate can alone constitute the surface treatment layer 20. During arc welding, ammonium zirconium carbonate reacts with Mn and Si derived from the steel plate and welding wire to form a conductive Zr-Mn-Si slag. Note that any Zr compound capable of forming Zr-Mn-Si slag is acceptable, and is not limited to ammonium zirconium carbonate.

[0035] Furthermore, examples of Zr compounds that have the ability to form Zr-Mn-Si slags include various Zr chelate compounds. In this embodiment, a Zr chelate compound refers to a compound in which various ligands form a complex with a Zr ion in between, and a ring containing a Zr atom and a coordinating atom is formed, and is a compound different from a Zr oxide (a compound consisting of a Zr atom and an oxygen atom).

[0036] In the surface treatment layer 20 according to this embodiment, by using a Zr chelate compound as the Zr compound having slag-forming ability, it is possible to further improve the adhesion of the electrodeposited coating when electrodeposition coating is applied.

[0037] Furthermore, by using at least one of the following Zr chelate compounds—zirconium lactate ammonium salt, zirconyl chloride compound, zirconium lactate, zirconium triethanolamine, zirconium monoacetylacetonate, zirconium tetraacetylacetonate, or zirconium ethylacetoacetate—the adhesion of electrodeposition coating can be further improved. In particular, zirconium lactate ammonium salt and zirconyl chloride compound are water-soluble compounds, so when various resins are used as components of the surface treatment layer 20, for example, they disperse well in the resin. As a result, the slag generated by arc welding becomes finer, further improving the adhesion of electrodeposition coating.

[0038] Furthermore, various alkoxides can be cited as Zr compounds that have the ability to form Zr-Mn-Si slags. Examples of alkoxides include n-propyl zirconate and n-butyl zirconate.

[0039] In the surface treatment layer 20 according to this embodiment, the various Zr compounds described above may be used individually or in combination of multiple Zr compounds as appropriate.

[0040] Furthermore, when identifying the Zr compound contained in the surface treatment layer 20 from the surface-treated steel sheet or the arc-welded joint formed by the surface-treated steel sheet, the following method may be used.

[0041] First, a sample for cross-sectional observation is cut from the surface-treated steel sheet 1 at an arbitrary position in the thickness direction where the surface treatment layer 20 exists. Next, the cross-section of the sample is polished in the same manner as the measurement method for the Zr content of the surface treatment layer 20, and EPMA mapping analysis is performed at several arbitrary locations (for example, 3 locations) of the obtained cross-section, with the elements to be detected being Zr, Si, O, N, and C. At this time, the magnification is 5000x (acceleration voltage: 15kV). The presence or absence of a Zr compound can be confirmed by whether or not the presence of element Zr can be confirmed in the EPMA map for all obtained Zr. Furthermore, if the presence of element Zr can be confirmed in all obtained EPMA maps, but element Zr is not distributed in a particulate manner, it can be determined that various Zr chelate compounds are being used. Note that "distributed in a particulate manner" refers to the case where, in the observation results at the above magnification, particles in which element Zr has aggregated to be visible to the naked eye are observed, and particles with an average particle size of approximately 0.1 μm or larger are observed. Furthermore, whether or not a Zr oxide is used can be determined by referring to the EPMA maps of element Zr and element O, and based on whether or not element O is present at the locations where element Zr is detected.

[0042] Even if element Zr is not distributed in particulate form in the EPMA map, nanozirconia (ZrO2 with nano-order particle size) may still be present. Therefore, in order to distinguish between Zr chelate compounds and nanozirconia, it is preferable to use X-ray photoelectron spectroscopy (XPS) in addition to the EPMA mapping analysis described above. By focusing on whether or not Zr-related peaks other than those derived from ZrO2 (for example, peaks derived from Zr-OR bonds (R represents alkyl groups)) are detected in the XPS measurement results, it becomes easier to distinguish between Zr chelate compounds and nanozirconia.

[0043] [Regarding components other than Zr compounds] The surface treatment layer 20 according to this embodiment may contain various components other than the Zr compound described above, such as resin, conductive pigment, and rust inhibitor.

[0044] ≪Resin≫ The surface treatment layer 20 according to this embodiment may contain various resins as binder components for holding the Zr compound as described above. Here, examples of such resins can be high-molecular-weight polyester resins, polyester resins, acrylic resins, epoxy resins, urethane resins, fluororesins, etc. Furthermore, it is also possible to use film-forming resin components such as modified resins of these resins, which are crosslinked with butylated melamine resins, methylated melamine resins, butylmethyl mixed melamine resins, urea resins, isocyanate resins, or crosslinking agent components of mixed systems of these resins.

[0045] Furthermore, among the resins described above, it is more preferable that at least one of epoxy resin or urethane resin is used. By using at least one of epoxy resin or urethane resin, it is possible to further improve the adhesion between the electrodeposition coating and the surface treatment layer 20.

[0046] Conductive Pigments The surface treatment layer 20 according to this embodiment may further contain a conductive pigment in addition to the Zr compound and resin described above. By including a conductive pigment in a specific amount in the surface treatment layer 20 according to this embodiment, the electrical resistance value exhibited by the surface treatment layer 20 can be made more desirable. This makes it possible to further improve the corrosion resistance of the weld bead formed by arc welding. Furthermore, the presence of a conductive pigment in the surface treatment layer 20 makes it possible to improve spot weldability.

[0047] Here, the content of the conductive pigment is preferably 10 to 40% by mass relative to the total mass of the solid content of the surface treatment layer 20. By setting the content of the conductive pigment to 10% by mass or more, it is possible to make the electrical resistance value exhibited by the surface treatment layer 20 more desirable, and spot weldability can also be improved. The content of the conductive pigment is more preferably 15% by mass or more.

[0048] On the other hand, by limiting the conductive pigment content to 40% by mass or less, it becomes possible to suppress a decrease in the corrosion resistance of the surface treatment layer 20, while also improving the electrical resistance value exhibited by the surface treatment layer 20 and enhancing spot weldability. The conductive pigment content is more preferably 30% by mass or less.

[0049] Furthermore, in the surface treatment layer 20 according to this embodiment, the conductive pigment is preferably in particulate form, and the average particle size of such particulate conductive pigment is preferably 0.25 to 2.00 times the average thickness of the surface treatment layer 20 (thickness d in Figure 1).

[0050] By making the average particle size of the particulate conductive pigment 0.25 times or more the average thickness of the surface treatment layer 20, it becomes possible to disperse the conductive pigment more uniformly in the surface treatment layer 20, making it possible to make the electrical resistance value exhibited by the surface treatment layer 20 uniform throughout the entire surface treatment layer 20, and to further improve spot weldability. Preferably, the average particle size of the particulate conductive pigment is 0.35 times or more the average thickness of the surface treatment layer 20, and even more preferably 0.50 times or more the average thickness of the surface treatment layer 20.

[0051] On the other hand, by making the average particle size of the particulate conductive pigment 2.00 times or less the average thickness of the surface treatment layer 20, it becomes possible to more uniformly disperse the conductive pigment in the surface treatment layer 20 while preventing a decrease in adhesion between the surface treatment layer 20 and the electrodeposition coating, thereby further improving spot weldability. More preferably, the average particle size of the particulate conductive pigment is 1.70 times or less the average thickness of the surface treatment layer 20, and even more preferably, it is 1.50 times or less the average thickness of the surface treatment layer 20.

[0052] Examples of conductive pigments mentioned above include TiN, ZnO, VB2, SUS powder, iron phosphide, carbon black, non-oxide ceramic particles excluding Si compounds, and metal powders such as Zn, Zn-Mg, and Fe alloys. Here, Si compounds are excluded from the non-oxide ceramic particles because they become non-conductive Si oxides during arc welding, which reduces the electrodeposition coating properties of arc-welded joints.

[0053] When a compound containing Ti (excluding titanium dioxide) is used as a conductive pigment, it can react with Mn derived from steel plates and welding wires during arc welding to form a conductive Ti-Mn slag, similar to the Zr compounds that have slag-forming ability described above. Therefore, by using a compound containing Ti (excluding titanium dioxide) as a conductive pigment, electrodeposition coating can be more easily formed on the weld bead, thereby improving the corrosion resistance of the weld bead.

[0054] Furthermore, when determining the type and content of conductive pigment used in the surface treatment layer 20, as well as the average particle size, from the surface-treated steel sheet or the arc-welded joint formed by the surface-treated steel sheet, the following method can be used.

[0055] First, the surface treatment layer 20 of interest is cut at an angle perpendicular to the surface of the surface treatment layer 20, and a sample for cross-sectional observation is taken. For samples used to identify the type of conductive pigment and average particle size, the acquired cross-sectional observation sample is embedded in resin, and the cross-sectional surface, which will serve as the observation surface, is polished. Then, the resulting observation surface is observed using a scanning electron microscope (SEM) to obtain a backscattered electron compositional image (BSE). Here, the size of the observation field should be 25 μm × 20 μm, and the magnification should be 5000x. Several particles (for example, 5) are arbitrarily selected from the particles present in the observation field, the equivalent circle diameter of each particle is determined, and the average value of these values ​​is taken as the average particle size. Furthermore, whether or not the particles present in the observation field are conductive pigments, and the type of conductive pigment used, can be easily determined by performing elemental analysis on the particles of interest.

[0056] Furthermore, the conductive pigment content can be calculated by observing the cross-section of the surface treatment layer 20 cut in the thickness direction using a scanning electron microscope (SEM), identifying each particle, counting the number of particles per cross-section, and converting this to the number of particles per volume of the surface treatment layer 20. If necessary, an EDX spectrometer or similar device can be used to identify each particle. In this case, the observation field size should be 25 μm × 20 μm, and the magnification should be 5000x. The number of fields of view should be 3 or more, and the average number of particles in each field of view can be obtained by dividing the number of particles in each field of view by the number of fields of view.

[0057] If multiple types of conductive pigments are included, the amount of each particle in the surface treatment layer 20 can be calculated from the amount of conductive pigment contained in the surface treatment agent for forming the surface treatment layer 20 and the amount of surface treatment agent adhering to the base steel plate 10. If the amount of each conductive pigment added to the surface treatment agent before painting is known, the amount of each particle in the film can be calculated from the amount added and the amount of paint adhering to the base steel plate. If the amount of each conductive pigment added is unknown, the average thickness of the surface treatment layer is calculated using the method described above, and then the surface treatment layer in a plan view of, for example, 20 mm x 20 mm is melted. The melting method does not need to be specified as long as it does not melt the conductive particles. As a result, the surface treatment layer is melted in an area of ​​20 mm x 20 mm and in a volume equal to the average thickness of the surface treatment layer. As an example of a method for melting the surface treatment layer, an organic solvent can be used. Next, using a device such as the Morphologi G3 particle image analysis system manufactured by Malvern ("MORPHOLOGI" is a registered trademark), the particles in the surface treatment agent diluted to an appropriate concentration are individually identified and counted through image analysis, thereby calculating the amount of each particle in the calculated volume of the surface treatment layer. Then, from the number of each particle and the calculated volume as described above, it is possible to calculate the number of each particle in the surface treatment layer. This method can also be used when dissolving the surface treatment layer 20 attached to the base steel plate 10 and counting the number of particles.

[0058] Rust Inhibitor In addition to the Zr compound and resin described above, the surface treatment layer 20 according to this embodiment may further contain a rust inhibitor. By including a rust inhibitor in the surface treatment layer 20, the corrosion resistance of the surface-treated steel sheet 1 can be further improved.

[0059] As such a rust inhibitor, it is preferable to use a rust inhibitor other than a Si compound. When a Si compound is used as a rust inhibitor, it becomes a non-conductive Si oxide during arc welding, which reduces the electrodeposition coating properties of the arc-welded joint, and is therefore undesirable.

[0060] Here, the content of the rust inhibitor is preferably 10 to 30% by mass relative to the total mass of the solids in the surface treatment layer 20. By setting the rust inhibitor content to 10% by mass or more, it is possible to further improve the corrosion resistance of the surface-treated steel sheet 1. The rust inhibitor content is more preferably 15% by mass or more.

[0061] On the other hand, by limiting the rust inhibitor content to 30% by mass or less, it is possible to further improve the corrosion resistance of the surface-treated steel sheet 1 while suppressing a decrease in the adhesion of the surface treatment layer 20. More preferably, the rust inhibitor content is 25% by mass or less.

[0062] Examples of rust inhibitors other than Si compounds include aluminum tripolyphosphate, zinc phosphate, iron phosphate, aluminum phosphate, magnesium phosphate, magnesium oxide, calcium vanadate, calcium molybdate, aluminum molybdate, barium molybdate, orthophosphate, pyrophosphate, metaphosphate, subphosphate, phosphorous acid, hypophosphate, and their salts. These rust inhibitors may exist as particles or as ions. The state in which they exist is determined by the type of resin contained in the surface treatment layer.

[0063] Furthermore, to determine the type and content of rust inhibitor used in the surface treatment layer 20 from the condition of the surface-treated steel plate and arc-welded joint, the following procedure can be followed. Specifically, if the resin contained in the surface treatment layer 20 is a solvent-based resin, the type and content of rust inhibitor can be measured from the condition of the surface-treated steel plate and welded joint in the same manner as the conductive pigment described above. Also, if the resin contained in the surface treatment layer 20 is a water-based resin, the type and content of rust inhibitor can be measured by dissolving the surface treatment layer 20 in the same manner as when multiple types of conductive pigment are contained, and then subjecting the resulting solution to inductively coupled plasma (ICP) emission spectroscopy.

[0064] Furthermore, the surface treatment layer 20 according to this embodiment may further contain both a conductive pigment and a rust inhibitor, in addition to the Zr compound and resin described above.

[0065] Furthermore, the surface treatment layer 20 according to this embodiment may contain, as necessary, additives such as extender pigments, coloring pigments, colorants, viscosity modifiers, leveling agents, defoamers, and ultraviolet absorbers, in addition to the conductive pigments and rust inhibitors described above.

[0066] The surface treatment layer 20 according to this embodiment has been described in detail above with reference to Figure 1.

[0067] In addition, in the surface-treated steel sheet 1 according to this embodiment, a chemical conversion treatment layer (not shown) using various chemical conversion treatment agents may be provided to further improve the adhesion between the base steel sheet 10 and the surface treatment layer 20.

[0068] (Regarding arc welded joints) Next, with reference to Figure 2, the overall configuration of the arc-welded joint 100 using the surface-treated steel plate 1 according to this embodiment will be described. For convenience, the coordinate system shown in Figure 2 will be used in the following explanation as appropriate.

[0069] Figure 2 schematically shows the overall structure of an arc-welded joint 100 obtained by overlapping a portion of surface-treated steel sheet 1A and a portion of surface-treated steel sheet 1B and then performing fillet welding by arc welding. In Figure 2, a cross-section of the arc-welded joint 100 perpendicular to the extension direction of the weld bead is schematically shown. As schematically shown in Figure 2, the arc-welded joint 100 according to this embodiment includes surface-treated steel sheets 1A and 1B, a weld bead portion 110, slag 120, and electrodeposited coating 130.

[0070] Here, the surface-treated steel plates 1A and 1B that constitute the arc-welded joint 100 are the same surface-treated steel plates 1 having the surface-treated layer 20 as described earlier, so a detailed explanation will be omitted below.

[0071] Furthermore, in order to determine the various characteristics (e.g., composition and thickness) of the surface treatment layer 20 on the surface-treated steel plates 1A and 1B constituting the arc-welded joint 100 from the state of the arc-welded joint 100, when taking a sample of the surface treatment layer 20, the sample shall be taken from a location at least 20 mm away from the weld bead portion 110 in the direction away from the toe (the "toe" as defined in JIS Z3001:2018, the position of point T in Figure 2) of the arc-welded joint 100.

[0072] <Regarding the weld bead portion 110> The weld bead 110 is a portion formed by arc welding, and interdiffusion of constituent elements occurs between the welding wire, which is used as needed during welding, and the surface-treated steel plates 1A and 1B. In Figure 2, the joint interface between the weld bead 110 and the surface-treated steel plates 1A and 1B is shown as a smooth curve or straight line for illustrative purposes, but the actual joint interface is a complex curved surface due to the oscillation of the molten metal during welding by the arc plasma, etc. Furthermore, this weld bead 110 extends along the Y-axis direction in the figure, and the surface-treated steel plate 1A and the surface-treated steel plate 1B are joined by this weld bead 110.

[0073] Here, to identify the area corresponding to the weld bead 110 in the arc welded joint 100 of interest, it can be easily visualized by etching using an etching solution. For example, as an etching solution, it is possible to use Nital (composition: 95% ethanol, 5% sulfuric acid) or an etching solution prepared by mixing 60g of sodium dodecylbenzenesulfonate, 36g of picric acid, 60cc of ethanol, and 60cc of household detergent solution (for example, a common one such as dish soap) with 2400cc of water.

[0074] <About Slag 120> The slag 120 is formed by arc welding and is generated by a reaction between the surface-treated steel plate 1, the welding wire used as needed during welding, and the oxygen source in the shielding gas used during arc welding. This slag 120 is present on at least a portion of the surface of the weld bead 110.

[0075] In the arc-welded joint 100 according to this embodiment, the slag 120 contains conductive Zr-Mn-Si slag because the surface treatment layer 20 of the surface-treated steel plate 1, which is the material of the joint, contains a Zr compound that has the ability to form Zr-Mn-Si slag. As a result, in the arc-welded joint 100 according to this embodiment, it is possible to form the electrodeposited coating 130, which will be described later, even in the vicinity of the weld bead portion 110, thereby improving corrosion resistance.

[0076] In addition, the slag 120 according to this embodiment may contain Si-based slag, Si-Mn-based slag, Mn-based slag, Ti-Mn-based slag, and other impurities, in addition to the Zr-Mn-Si-based slag described above.

[0077] In this embodiment, the location where the slag 120 described above exists in the arc welded joint 100 can be identified as follows. First, the electrodeposited coating 130 on the surface of the arc-welded joint 100 is removed in advance using a coating remover (for example, Neoriver manufactured by Sansai Chemical Co., Ltd.) so that the weld bead 110 and slag 120 are exposed on the surface. The location of the slag 120 can be determined by observing the backscattered electron composition image when the surface where the weld bead 110 and slag 120 are located is observed using a SEM. More specifically, the location of the slag 120 can be easily identified by observing the area around the weld bead 110 at a magnification of 50x so that the field of view is 2mm x 2mm in size.

[0078] Furthermore, to identify the type of slag constituting the slag 120, a sample for cross-sectional observation can be cut from the weld bead 110 where the slag 120 is present, and this sample can be observed using SEM-EPMA and EPMA mapping analysis can be performed. More specifically, an appropriate size section can be cut from an arbitrary location in the weld bead 110 where the slag 120 is present, embedded in resin so that the cross-section in the thickness direction is visible, and the cross-section can be polished. Then, the resulting polished surface can be observed in cross-section using SEM-EPMA. More specifically, EPMA mapping analysis can be performed on the obtained cross-section at a magnification of 1000x (acceleration voltage: 15kV). In this case, Zr, Mn, Si, O, and Ti can be selected as the elements to be detected, and mapping images of the location and concentration of these elements can be taken, and the type of slag can be identified from the results obtained.

[0079] <About Electrodeposition Coating 130> The electrodeposited coating 130 is applied to cover the surface-treated steel plates 1A and 1B, the weld bead portion 110, and the slag 120 in order to improve the corrosion resistance of the arc-welded joint 100.

[0080] In the arc-welded joint 100 according to this embodiment, the surface-treated steel plate 1 used as the joint material has a surface treatment layer 20 containing a Zr compound having the slag-forming ability described above, thereby achieving the following state. That is, in the arc-welded joint 100 according to this embodiment, the following relationship regarding electrical resistance values, as shown in equation (101), holds between the portion where the surface treatment layer 20 remains without being destroyed by arc welding (hereinafter referred to as the "flat portion" of the arc-welded joint) and the weld bead portion 110. Here, in equation (101) below, the average value of the electrical resistance of the flat portion of the arc-welded joint 100 is denoted as Rs [unit: Ω], and the average value of the electrical resistance of the weld bead portion 110 is denoted as Rb [unit: Ω].

[0081] 1.5 ≦ (Rs / Rb) ≦ 4.0...Equation (101)

[0082] Because of this relationship, in the arc-welded joint 100 according to this embodiment, the average thickness of the electrodeposited coating 130B formed on the surface of the weld bead portion 110 is greater than the average thickness of the electrodeposited coating 130A formed on the surface of the flat portion, and the relationship shown in the following equation (103) holds true. Here, in the following equation (103), the average thickness of the electrodeposited coating 130A formed on the surface of the flat portion is denoted as ts [unit: μm], and the average thickness of the electrodeposited coating 130B formed on the surface of the weld bead portion 110 is denoted as tb [unit: μm].

[0083] tb / ts≧1.2...Equation (103)

[0084] In the arc-welded joint 100 according to this embodiment, the relationship shown in formula (103) above holds true, so that the weld bead portion 110 of the arc-welded joint 100 according to this embodiment exhibits superior corrosion resistance.

[0085] Here, the value of (tb / ts) is more preferably 1.3 or greater, and even more preferably 1.5 or greater.

[0086] On the other hand, there is no specific upper limit to the above (tb / ts) value; the larger the value, the better, but in practice, around 2.0 is the upper limit.

[0087] Here, the electrical resistance values ​​of the flat sections and weld bead sections described above can be calculated by measuring the resistance at any multiple locations (for example, three locations) belonging to each section using a commercially available resistance meter, and then averaging the results obtained over the number of measurement locations.

[0088] The average thickness ts of the electrodeposited coating 130A formed on the flat surface can be measured by taking a sample for cross-sectional observation from any position at least 20 mm away from the weld bead 110, starting from the toe of the weld bead 110 (the "toe" as defined in JIS Z3001:2018, the position of point T in Figure 2), and measuring it in the same manner as the measurement method for the average thickness of the surface treatment layer 20.

[0089] Furthermore, to determine the average thickness tb of the electrodeposited coating 130B formed on the surface of the weld bead portion 110, the weld bead portion 110 is cut in a direction perpendicular to the weld bead portion 110, and a sample for cross-sectional observation is taken. Here, "cutting in a direction perpendicular to the weld bead portion 110" means cutting the weld bead portion 110 in a direction perpendicular to the tangent to the weld bead portion 110 at the cutting start position on the surface of the weld bead portion 110. The cross-sectional observation sample obtained in this way can be measured in the same manner as the measurement method for the average thickness of the surface treatment layer 20.

[0090] In this embodiment, the details of the electrodeposited coating 130 in the arc welding joint 100 are not particularly limited, and various known electrodeposited coatings can be used.

[0091] The arc welding joint 100 according to this embodiment has been described in detail above with reference to Figure 2.

[0092] (Regarding the manufacturing method of surface-treated steel sheets) Below, an example of a method for manufacturing the surface-treated steel sheet 1 according to this embodiment will be briefly described. First, the base steel sheet is subjected to various pretreatments, including alkaline degreasing, water washing, and pickling, to obtain a clean steel sheet surface. Subsequently, various plating treatments may be applied to the surface of the steel sheet as needed. In this way, the base steel sheet 10 of the surface-treated steel sheet 1 according to this embodiment can be obtained.

[0093] Various known chemical treatments are performed on the base steel sheet 10 as needed to form a chemical treatment layer. For example, a chemical treatment agent containing the desired components can be prepared and applied to the surface of the base steel sheet 10. Here, the application of the chemical treatment agent can be carried out by generally known application methods, such as roll coating, curtain flow coating, air spray, airless spray, immersion, bar coating, or brush application.

[0094] For example, a surface treatment agent for forming a surface treatment layer 20 is applied to the surface of the base steel sheet 10 (or chemical conversion treatment layer) formed as described above, and the surface treatment layer 20 is formed by heating and drying. Here, such a surface treatment agent is prepared by containing the above-mentioned Zr compound (more preferably the above-mentioned Zr compound and a resin as a film-forming component, and optionally various additives such as conductive pigments and rust inhibitors) in a solvent. The prepared surface treatment agent can be applied in the same manner as described above by generally known application methods, such as roll coating, curtain flow coating, air spray, airless spray, immersion, bar coating, and brush application.

[0095] Subsequently, the coating solution can be heated and dried using any method, such as hot air, near-infrared radiation, far-infrared radiation, induction heating, or a combination thereof. This allows the surface treatment layer 20 according to this embodiment to be formed.

[0096] The manufacturing method of the surface-treated steel sheet 1 according to this embodiment has been briefly described above.

[0097] (Regarding the manufacturing method of arc welded joints) The welded joint according to this embodiment is manufactured by using surface-treated steel sheets manufactured as described above as a material, arranging the surface-treated steel sheets to form the shape required for the arc-welded joint, and welding the surface-treated steel sheets to each other. Here, for welding the surface-treated steel sheets, an arc welding method that satisfies the following conditions, for example, is used.

[0098] Welding current: 140A, Welding voltage: 11.4V, Welding speed: 80cm / min Welding gas: 20% CO2 + Ar, Gas flow rate: 20 L / min Welding wire: Mild steel wire (YM-24T, manufactured by Nippon Steel Welding Industries Co., Ltd., φ1.2mm) (C: 0.07 mass%, Si: 0.61 mass%, Mn: 1.21 mass%, P: 0.008 mass%, S: 0.005 mass%) Welding torch tilt angle: 45°

[0099] Furthermore, electrodeposition coating is performed on the entire joint after arc welding using various known electrodeposition paints. This results in electrodeposition coating being formed over the entire arc-welded joint. The type of electrodeposition paint used and the drying conditions are not particularly limited.

[0100] The above describes an example of a method for manufacturing an arc-welded joint according to this embodiment. [Examples]

[0101] The surface-treated steel sheet and arc-welded joint according to this embodiment will be described in detail below with reference to examples and comparative examples. Note that the following examples are merely examples of the surface-treated steel sheet and arc-welded joint according to this embodiment, and the surface-treated steel sheet and arc-welded joint according to the present invention are not limited to the examples below.

[0102] (Example test) <1. Sample Preparation> [Manufacturing of surface-treated steel sheets] As the base material, cold-rolled steel sheets with a thickness of 1.6 mm and tensile strengths of 780 MPa, 980 MPa, and 1180 MPa (all manufactured by Nippon Steel Corporation) were used. In addition, surface treatment solutions containing the components shown in Table 1 below were prepared and applied using a bar coater so that the average thickness of the surface treatment layer after drying was the value shown in Table 1 below. The sheets were baked in a hot air oven under conditions that the surface temperature of the steel sheets reached 140°C, and then air-cooled to room temperature. Multiple samples of surface-treated steel sheets were prepared for each level in the manner described above.

[0103] Here, epoxy resin (EM series, manufactured by ADEKA Corporation) and urethane resin (HUX series, manufactured by ADEKA Corporation) were used as film-forming resins. In addition, as Zr compounds, zirconium ammonium carbonate, zirconium alkoxide, zirconium monoacetylacetonate, zirconium lactate ammonium salt, zirconium oxide, zirconyl chloride compounds, zirconium lactate, zirconium triethanolamine, zirconium tetraacetylacetonate, and zirconium ethylacetoacetate (all of which are common reagents) were used.

[0104] Furthermore, calcium vanadate and aluminum tripolyphosphate (both common reagents) were used as rust inhibitors, and ZnO and TiN (both common reagents) with the average particle sizes shown in Table 1 below were used as conductive pigments.

[0105] [Table 1]

[0106] [arc welding] From surface-treated steel plates of each level, test pieces measuring 50 mm x 150 mm for the upper plate and 30 mm x 150 mm for the lower plate were cut out. These two test pieces were overlapped longitudinally with a lap width of 10 mm, and fillet arc welding was performed. The welding wire used and the arc welding conditions are as follows. Wire for mild steel (manufactured by Nippon Steel Welding Industries, YM-24T) Shielding gas: Ar + 20% CO2, Welding current 140A, welding voltage 11.4V, welding speed 80cm / min

[0107] [Electrodeposition coating] Each arc welding sample obtained as described above was degreased by immersion in an alkaline degreasing solution manufactured by Nippon Parkerizing Co., Ltd. at 40°C for 2 minutes. Subsequently, surface conditioning was performed using a surface conditioning agent manufactured by Nippon Paint Co., Ltd. at 25°C for 15 seconds. Furthermore, zinc phosphate treatment was performed using a zinc phosphate treatment solution manufactured by Nippon Paint Co., Ltd. The temperature of the chemical treatment solution was set to 35°C, and the arc welding samples were immersed in the chemical treatment solution for 120 seconds, followed by rinsing with water and drying. Then, using a cationic electrodeposition paint manufactured by Nippon Paint, electrodeposition coating was performed at 35°C, with the voltage increased to 180V in 30 seconds and current applied for 150 seconds. During this process, baking was performed at a baking temperature of 170°C for 25 minutes.

[0108] As described above, multiple samples of arc-welded joints were prepared for each level.

[0109] <2. Evaluation Method> For each arc-welded joint obtained as described above, the slag composition in the weld bead was identified using the method previously explained, and the average film thickness ratio tb / ts of the electrodeposited coating was measured. The results obtained are summarized in Table 1 above.

[0110] Furthermore, each obtained arc-welded joint was evaluated from the perspectives of post-painting corrosion resistance of the weld bead, paint adhesion to the flat surface, post-painting corrosion resistance of the flat surface, and spot weldability. The evaluation methods are as follows. The results obtained are summarized in Table 2 below.

[0111] [Corrosion resistance of the weld bead after painting] For each arc-welded joint after electrodeposition coating, the end face of the test piece and both ends of the weld bead were sealed, and then the corrosion resistance was evaluated. Corrosion resistance was evaluated according to the JASO M609-91 test. Specifically, one cycle consisted of salt spray (5% NaCl) at 35°C for 2 hours, drying (25% humidity) at 60°C for 4 hours, and wetting (95% humidity) at 50°C for 2 hours. The occurrence of red rust after 120 cycles was evaluated. The evaluation was conducted according to the evaluation criteria shown below, and a score of 2 or higher was considered a pass. [Evaluation Criteria] Rating 5: Red rust area ratio is 30% or less Rating 4: Red rust area ratio is between 30% and 40%. Rating 3: Red rust area ratio is between 40% and 50%. Rating 2: Red rust area ratio is between 50% and 70%. Rating 1: Red rust area ratio exceeds 70%

[0112] [Coating adhesion to flat surfaces] For each arc-welded joint after electrodeposition coating, a flat section (a portion at least 20 mm away from the toe of the weld bead) was cut out, and the corrosion resistance of the flat section after coating was evaluated. More specifically, the end faces of the cut-out test pieces were sealed and immersed in a 5% NaCl aqueous solution at 50°C for 500 hours. After immersion, the test surface was 60 mm × 120 mm (area A = 7200 mm²). 2 A polyester tape was applied to the front surface of the area, and then the tape was peeled off. The area B of the paint film that was peeled off by peeling off the tape was determined, and the paint film peeling rate (%) was calculated based on the following formula (*). Coating peel rate (%) = (B / A) × 100 …Formula (*)

[0113] [Corrosion resistance of flat surfaces after painting] For each arc-welded joint after electrodeposition coating, the corrosion resistance after coating was evaluated in the flat area (the part at least 20 mm away from the toe of the weld bead). Corrosion resistance was evaluated using the JASO M609-91 test. Specifically, cross-cuts were made in the electrodeposited coating with a cutter at a distance of 20 mm or more from the weld bead toe, and the width of the coating blister from the cross-cut after 120 corrosion cycles (unit: mm, average of the maximum 3 points on one side) was evaluated.

[0114] [Spot weldability] Surface-treated steel plates, before arc welding and electrodeposition coating, were sheared to a size of 30 x 50 mm, and the appropriate spot welding current range (upper limit current - lower limit current) was measured. The measurement conditions are as follows: The lower limit current was calculated as nugget diameter 4 x (plate thickness). 0.5 Specifically, the current value was set to result in a nugget diameter of 5.1 mm, and the upper limit current was defined as the scattering current. <Measurement conditions> ·Current: DC • Electrode: Made of chromium copper, DR (6mm diameter tip with 40R radius) • Pressurization: 400 kgf (1 kgf is approximately 9.8 N.) ·Electrification time: 240msec

[0115] [Table 2]

[0116] As is clear from Table 2 above, the level corresponding to the embodiment of the present invention showed excellent corrosion resistance of the weld bead after painting, while the level corresponding to the comparative example of the present invention showed insufficient corrosion resistance of the weld bead after painting.

[0117] For example, Comparative Example No. 2 did not contain a Zr compound and therefore could not obtain sufficient corrosion resistance. Comparative Example No. 12 had an average thickness of the surface treatment layer that was thinner than the range of the present invention and therefore could not obtain sufficient corrosion resistance. Comparative Example No. 14 had an average thickness of the surface treatment layer that was thicker than the range of the present invention, resulting in reduced weldability, increased spatter during arc welding, and insufficient corrosion resistance. Comparative Example No. 19 contained zirconium oxide in the surface treatment layer, which prevented the formation of conductive slag during arc welding, resulting in many defects in electrodeposition coating and insufficient corrosion resistance. Comparative Example No. 20 contained a Zr compound, but the content was insufficient, resulting in insufficient formation of conductive slag, many defects in electrodeposition coating, and insufficient corrosion resistance. Examples No. 21 and No. 22 differed in the content of the Zr compound, and Example No. 22, which contained 20 atomic percent or more of the Zr compound, showed significantly better corrosion resistance than Example No. 21. In Examples No. 23 to No. 27, using various Zr chelate compounds as the Zr compound made it possible to further improve the adhesion of the electrodeposition coating and obtain desirable post-coating corrosion resistance. In particular, in Examples No. 23 and No. 24, which used water-based Zr chelate compounds, zirconyl chloride and zirconium lactate, the Zr chelate compound was well dispersed in the resin, resulting in finer slag and thus superior post-coating corrosion resistance. In Examples No. 24 and No. 27, the ratio of average particle size to average thickness was within a favorable range, which improved spot weldability while preventing a decrease in adhesion between the surface treatment layer and the electrodeposition coating, resulting in desirable post-coating corrosion resistance. In Example No. 26, the ratio of average particle size to average thickness was within an even more favorable range, which improved spot weldability while further preventing a decrease in adhesion between the surface treatment layer 20 and the electrodeposition coating, resulting in desirable post-coating corrosion resistance. In Example No. 25, the ratio of the average particle size to the average thickness was within the most preferable range, which allowed for improved spot weldability while further preventing a decrease in adhesion between the surface treatment layer 20 and the electrodeposited coating, resulting in more favorable post-coating corrosion resistance.

[0118] Although preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention belongs that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these are also understood to fall within the technical scope of the present invention.

[0119] The embodiments disclosed herein are illustrative and not restrictive in all respects. The embodiments described above may be omitted, replaced, or modified in various ways without departing from the appended claims, the technical scope of the invention as described later, and the spirit thereof. For example, the constituent elements of the embodiments described above can be combined in any way without impairing their effects. Furthermore, such any combination will naturally yield the effects and benefits of each constituent element in the combination, as well as other effects and benefits that will be obvious to those skilled in the art from the description herein.

[0120] Furthermore, the effects described herein are merely descriptive or illustrative, and not limiting. In other words, the technology according to the present invention may produce other effects that will be apparent to those skilled in the art from the description herein, in addition to or instead of the effects described above.

[0121] Furthermore, the following configurations also fall within the technical scope of the present invention. (1) The base material is a steel plate, A surface treatment layer located in at least a portion of the steel plate, It has, The aforementioned surface treatment layer contains a Zr compound that has the ability to form a Zr-Mn-Si system slag with the elements Mn and Si. The Zr content in the aforementioned surface treatment layer is 10 to 100 atomic percent. A surface-treated steel sheet having an average thickness of 0.5 to 5.0 μm for the surface treatment layer. (2) The surface treatment layer contains the Zr compound having the ability to form slag and a resin. The surface-treated steel sheet according to (1), wherein the resin is at least one of epoxy resin or urethane resin. (3) The surface treatment layer contains the Zr compound having slag-forming ability, a resin, and a rust inhibitor excluding the Si compound. The surface-treated steel sheet according to (1) or (2), wherein the content of the rust inhibitor is 10 to 30% by mass. (4) The surface treatment layer contains the Zr compound having slag-forming ability, a resin, and a conductive pigment. The surface-treated steel sheet according to any one of (1) to (3), wherein the content of the conductive pigment is 10 to 40% by mass. (5) A surface-treated steel sheet according to any one of (1) to (4), wherein the Zr compound having the ability to form slag contains a Zr compound other than Zr oxide. (6) A surface-treated steel sheet according to any one of (1) to (5), wherein the Zr compound having the ability to form slag contains a Zr chelate compound. (7) The surface-treated steel sheet according to (6), wherein the Zr chelate compound is at least one of zirconium lactate ammonium salt, zirconyl chloride compound, zirconium lactate, zirconium triethanolamine, zirconium monoacetylacetonate, zirconium tetraacetylacetonate, or zirconium ethylacetoacetate. (8) The surface-treated steel sheet according to (4), wherein the average particle size of the conductive pigment is 0.25 to 2.00 times the average thickness of the surface-treated layer. (9) The conductive pigment is TiN, the surface-treated steel sheet according to (8) or (9). (10) The surface-treated steel sheet according to any one of (1) to (9), wherein the Zr content in the surface treatment layer is 20 atomic percent or more. (11) An arc-welded joint in which at least two surface-treated steel plates are arc-welded, The surface-treated steel sheet and, The weld bead portion formed by the aforementioned arc welding, Slag present on at least a portion of the surface of the weld bead, The surface-treated steel plate, the weld bead portion, and the slag are covered with electrodeposition coating, It has, The surface-treated steel sheet is The base material is a steel plate, A surface treatment layer located in at least a portion of the steel plate, It has, The aforementioned surface treatment layer contains a Zr compound that has the ability to form a Zr-Mn-Si system slag with the elements Mn and Si. The Zr content in the aforementioned surface treatment layer is 10 to 100 atomic percent. The average thickness of the surface treatment layer is 0.5 to 5.0 μm. The slag contains at least Zr-Mn-Si slag, An arc-welded joint that satisfies the relationship expressed by the following equation (1), where ts is the average film thickness of the electrodeposited coating located on the surface of the surface-treated steel plate in the flat portion of the arc-welded joint, and tb is the average film thickness of the electrodeposited coating located on the surface of the weld bead portion. tb / ts≧1.2...Equation (1) [Explanation of Symbols]

[0122] 1. Surface-treated steel sheet 10 Base steel plate 20 Surface treatment layer 100 Arc Welded Joints 110 Weld bead section 120 slag 130 Electrodeposition coating

Claims

1. An arc-welded joint in which at least two surface-treated steel plates are arc-welded, The surface-treated steel sheet and, The weld bead portion formed by the aforementioned arc welding, Slag present on at least a portion of the surface of the weld bead, The surface-treated steel plate, the weld bead portion, and the slag are covered with electrodeposition coating, It has, The surface-treated steel sheet is The base material is a steel plate, A surface treatment layer located in at least a portion of the steel plate, It has, The aforementioned surface treatment layer contains a Zr compound that has the ability to form a Zr-Mn-Si system slag between the elements Mn and Si. The Zr content in the surface treatment layer is 10 to 100 atomic percent. The average thickness of the surface treatment layer is 0.5 to 5.0 μm. The slag contains at least Zr-Mn-Si slag, An arc-welded joint that satisfies the relationship expressed by the following formula (1), where ts is the average film thickness of the electrodeposited coating located on the surface of the surface-treated steel plate in the flat portion of the arc-welded joint, and tb is the average film thickness of the electrodeposited coating located on the surface of the weld bead portion. tb / ts≧1.2...Formula (1)

2. The arc welding joint according to claim 1, wherein the Zr compound having slag-forming ability is at least one of zirconium lactate ammonium salt, zirconium lactate, zirconium triethanolamine, zirconium monoacetylacetonate, and zirconium ethylacetoacetate.

3. The surface treatment layer contains a resin, The arc welding joint according to claim 1, wherein the Zr compound having slag-forming ability is a zirconium lactate ammonium salt.

4. The surface treatment layer contains a conductive pigment, The arc welding joint according to claim 1, wherein the content of the conductive pigment is 10 to 40% by mass.

5. The arc welding joint according to claim 4, wherein the average particle size of the conductive pigment is 0.25 to 2.00 times the average thickness of the surface treatment layer.

6. The arc welding joint according to claim 4, wherein the conductive pigment is TiN.

7. The surface treatment layer contains a resin, The arc welding joint according to any one of claims 1 to 6, wherein the resin is at least one of epoxy resin or urethane resin.

8. The surface treatment layer contains a resin and a rust inhibitor other than a Si compound, The arc welding joint according to any one of claims 1 to 6, wherein the content of the rust inhibitor is 10 to 30% by mass.

9. The surface treatment layer contains a resin and a conductive pigment, The arc welding joint according to claim 1 or 2, wherein the content of the conductive pigment is 10 to 40% by mass.

10. The arc welding joint according to any one of claims 1 to 6, wherein the Zr content in the surface treatment layer is 20 atomic percent or more.

Images