Surface-treated steel plates and arc-welded joints

Abstract

The 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 containing a Ti compound having a slag-forming ability to form a Ti-Mn-based slag with element Mn, a resin, and a conductive pigment, the Ti content in the surface treatment layer being 10 to 40 atomic %, the conductive pigment content in the surface treatment layer being 10 to 40 mass %, and the average thickness of the surface treatment layer being 0.5 to 5.0 μm.

Description

[Technical Field]

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

[0002] In the manufacture of automotive parts (particularly suspension parts), raw steel sheets are typically arc-welded to a desired shape, followed by chemical conversion treatment and electrodeposition coating. Meanwhile, in recent years, automobile manufacturers have been seeking to reduce the weight of vehicle bodies in order to improve fuel efficiency and reduce carbon dioxide emissions. While weight reduction is often achieved by reducing the thickness of raw steel sheets, it is important to achieve weight reduction in suspension parts, which frequently use arc welding. Specifically, when raw steel sheets are arc-welded, the thickness of the steel sheets is reduced during arc welding, so thicker steel sheets are prepared to ensure corrosion resistance. Therefore, it is important to achieve both thinning for weight reduction and setting the thickness of the steel sheets to ensure corrosion resistance, taking into account the thinning during arc welding.

[0003] Furthermore, during arc welding, Si and Mn contained in the steel sheet and welding wire react with oxygen in the shielding gas to form slag composed of Si-Mn oxides. Because Si-Mn slag is non-conductive, subsequent electrocoating does not form a film. This slag causes coating defects in the areas where the slag is present, making the area around the weld bead more susceptible to corrosion. Therefore, to prevent such coating defects and ensure the corrosion resistance of the weld bead, the amount of Si-Mn slag formed during arc welding has been reduced, for example, by adjusting the amount of Si in the welding wire.

[0004] On the other hand, various surface-treated steel sheets have been proposed in which a corrosion-resistant coating layer is provided on the surface of a steel sheet or plated steel sheet in order to improve the corrosion resistance of the steel sheet, plated steel sheet, or the like. For example, Patent Document 1 below discloses a coated metal sheet having excellent electrical conductivity and corrosion resistance, in which a coating film containing an organic resin having a specific functional group and metal particles exhibiting a predetermined standard electrode potential in a specific ratio is provided on at least one side of the metal sheet.

[0005] Furthermore, Patent Document 2 below discloses a coated metal sheet for automobiles that has excellent resistance weldability, corrosion resistance, and formability. The coated metal sheet has a coating film on at least one surface of the metal sheet that contains an organic resin having a specific functional group, non-oxide ceramic particles that exhibit a predetermined electrical resistivity, and an anti-rust pigment. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-71596 [Patent Document 2] International Publication No. 2013 / 133284 Summary of the Invention [Problem to be solved by the invention]

[0007] However, even when a coated metal plate such as those described in Patent Documents 1 and 2 is used, there is still room for further improvement in terms of achieving both improved corrosion resistance of the weld bead during arc welding and a lighter arc-welded joint.

[0008] Therefore, the present invention has been made in consideration of the above problems, and an object of the present invention is to provide a surface-treated steel sheet and an arc-welded joint that can achieve both improved corrosion resistance of the weld bead during arc welding and a reduced weight as an arc-welded joint. [Means for solving the problem]

[0009] In order to solve the above problems, the present inventors conducted extensive research and came up with the idea that if it were possible to more easily form an electrodeposition coating on a weld bead formed by arc welding, the corrosion resistance of the weld bead could be further improved and weight reduction of the arc-welded joint could also be realized. Based on this idea, the present inventors discovered that in order to enable the formation of an electrodeposition coating on a weld bead, it is necessary to generate conductive slag during arc welding, rather than non-conductive slag. Based on this discovery, the present inventors conducted further research and have completed the present invention, as described below. The gist of the present invention, which has been completed based on the above findings, is as follows.

[0010] (1) A surface-treated steel sheet comprising a steel sheet as a substrate and a surface-treated layer located on at least a part of the steel sheet, the surface-treated layer containing a Ti compound having a slag-forming ability to form a Ti-Mn-based slag with elemental Mn, a resin, and a conductive pigment, the Ti content in the surface-treated layer being 10 to 40 atomic %, the conductive pigment content in the surface-treated layer being 10 to 40 mass %, and the average thickness of the surface-treated layer being 0.5 to 5.0 μm. (2) The surface-treated steel sheet according to (1), wherein the Ti compound having slag-forming ability contains a Ti compound other than Ti oxide. (3) The surface-treated steel sheet according to (2), wherein the Ti compound capable of forming slag contains at least one of a Ti chelate compound and TiN. (4) The surface-treated steel sheet according to (3), wherein the Ti chelate compound is at least one of titanium lactate ammonium salt, titanium lactate, titanium triethanolamine, titanium acetylacetonate, titanium tetraacetylacetonate, titanium ethylacetoacetate, titanium phosphate ester complex, and titanium octylene glycolate. (5) The surface-treated steel sheet according to (3), wherein the content of Ti derived from the Ti chelate in the surface treatment layer is 5 to 40 atomic %. (6) The surface-treated steel sheet according to (1), wherein the average particle size of the conductive pigment is 0.25 to 2.00 times the average film thickness of the surface treatment layer. (7) The surface-treated steel sheet according to (1), wherein the conductive pigment is TiN. (8) The surface-treated steel sheet according to (1), wherein the surface treatment layer further contains a rust inhibitor other than a Si compound, and the content of the rust inhibitor is 10 to 30 mass %. (9) The surface-treated steel sheet according to (1), wherein the resin is at least one resin selected from the group consisting of epoxy resins and urethane resins. (10) The surface-treated steel sheet according to (1), wherein the Ti content in the surface treatment layer is 20 atomic % or more. (11) An arc-welded joint formed by arc-welding at least two surface-treated steel sheets, the arc-welded joint comprising: the surface-treated steel sheet; a weld bead formed by the arc welding; slag present on at least a portion of the surface of the weld bead; and an electrodeposition coating covering the surface-treated steel sheet, the weld bead, and the slag; the surface-treated steel sheet comprises a steel sheet serving as a base material and a surface treatment layer located on at least a portion of the steel sheet; the surface treatment layer contains a Ti compound having a slag-forming ability to form a Ti-Mn-based slag with element Mn, a resin, and a conductive pigment; the Ti content in the surface treatment layer is 10 to 40 atomic %; the conductive pigment content in the surface treatment layer is 10 to 40 mass %; the surface treatment layer has an average thickness of 0.5 to 5.0 μm; the slag contains at least Ti-Mn-Si-based slag; and the arc-welded joint has an average resistance of 200 MΩ or less. (12) The arc welded joint according to (11), wherein the average particle size of the slag is 1.0 mm or less. [Effects of the Invention]

[0011] As described above, according to the present invention, it is possible to improve the corrosion resistance of the weld bead during arc welding and reduce the weight of the arc welded joint. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is an explanatory diagram schematically illustrating a structure of a surface-treated steel sheet according to an embodiment of the present invention. [Figure 2] 2 is an explanatory diagram schematically illustrating the structure of an arc-welded joint using a surface-treated steel plate in the same embodiment. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant explanations will be omitted.

[0014] (About surface-treated steel sheets) The overall structure of a surface-treated steel sheet according to an embodiment of the present invention will be described below with reference to Fig. 1. Fig. 1 is an explanatory diagram that schematically shows an example of the configuration of a surface-treated steel sheet according to this embodiment.

[0015] The surface-treated steel sheet according to this embodiment is particularly suitable as a material for arc-welded joints, which are produced by joining together multiple parts processed into a desired shape by arc welding, and is also suitable as a material for various automobile parts, such as suspension components.

[0016] As shown schematically in Fig. 1, a surface-treated steel sheet 1 according to this embodiment includes a base steel sheet 10 and a surface treatment layer 20 formed over the entire surfaces of both surfaces of the base steel sheet 10. While Fig. 1 illustrates a case in which the surface treatment layer 20 is formed over the entire surfaces of both surfaces of the base steel sheet 10, the surface treatment layer 20 in this embodiment may be provided on at least a portion of the surface of the base steel sheet 10.

[0017] <About base steel sheet 10> The base steel plate 10 used as the substrate of the surface-treated steel plate 1 according to this embodiment is not particularly limited, and various steel plates can be used depending on the mechanical strength (e.g., tensile strength) required of the surface-treated steel plate 1. Examples of such base steel plate 10 include steel materials standardized by the Japanese Industrial Standards (JIS) and the like, such as carbon steel, alloy steel, and high-tensile steel used for general structures and machine structures. Specific examples of such steel materials include cold-rolled steel, hot-rolled steel, hot-rolled steel for automotive structures, hot-rolled high-tensile steel for automotive processing, cold-rolled steel for automotive structures, cold-rolled high-tensile steel for automotive processing, and high-tensile steel, commonly referred to as hot-stamped steel, which is quenched during hot processing. The components of such steel materials are not particularly limited, but may contain one or more of Si, Mn, S, P, Al, N, Cr, Mo, Ni, Cu, Ca, Mg, Ce, Hf, La, Zr, and Sb in addition to Fe and C. One or more of these optional added elements can be appropriately selected to obtain the desired material strength and formability, and the content can also be adjusted appropriately.

[0018] Among the high-strength steels described above, it is more preferable to use a high-strength steel having a tensile strength of 780 MPa or more (so-called high-strength steel of 780 MPa class or more), as this makes it possible to further improve the robustness of the manufactured article. Here, the tensile strength of the base steel plate 10 can be measured by a known method. As an example, a test piece may be prepared from a part of the steel plate whose tensile strength is to be measured by the method specified in JIS Z 2241:2011, and the tensile strength of the obtained test piece may be measured in accordance with the above-mentioned specification.

[0019] The base steel sheet 10 according to the present embodiment may be a zinc-based plated steel sheet having a surface coated with various zinc-based plating layers containing at least zinc. Examples of zinc-based 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. Zinc-based plated steel sheets may also be those containing small amounts of different metal elements or impurities, such as cobalt, molybdenum, tungsten, nickel, titanium, chromium, aluminum, manganese, iron, magnesium, lead, bismuth, antimony, tin, copper, cadmium, or arsenic, or those having inorganic substances, such as silica, alumina, or titania, dispersed therein. Furthermore, the zinc-based plated steel sheet may be a steel sheet having a multi-layer plating that combines the above-mentioned plating with other types of plating (e.g., iron plating, iron-phosphorus plating, nickel plating, cobalt plating, etc.) The plating method is not particularly limited, and various known plating methods such as electroplating, hot-dip plating, vapor deposition plating, dispersion plating, vacuum plating, etc. may be used.

[0020] Here, the thickness of the base steel sheet 10 is not particularly limited and may be set appropriately depending on the mechanical strength required of the surface-treated steel sheet 1, and may be, for example, about 1.0 to 25.0 mm.

[0021] <Surface treatment layer 20> As shown in Fig. 1, a surface treatment layer 20 is formed on the front and back surfaces of the above-described base steel sheet 10. As will be described in detail below, the surface treatment layer 20 according to this embodiment is a coating layer containing a Ti compound having a slag-forming ability to form a Ti-Mn-based slag with element Mn, a resin, and a conductive pigment.

[0022] FIG. 2 is an explanatory diagram that schematically shows the structure of an arc-welded joint using a surface-treated steel sheet in this embodiment. When the surface-treated steel sheet 1 according to this embodiment is subjected to arc welding, the Ti compounds present in the surface treatment layer 20 react with Mn in the steel sheet and welding wire. As a result, Ti-Mn-based slag 120 is formed on the weld bead 110, as shown in FIG. 2. Because the Ti-Mn-based slag 120 is electrically conductive, the electrical conductivity of the weld bead 110 is ensured, allowing the electrodeposition coating 130 to be formed on the weld bead 110. This improves the corrosion resistance of the weld bead 110 formed during arc welding.

[0023] Here, the Ti-Mn-based slag exhibiting electrical conductivity means that the average resistance value of the Ti-Mn-based slag is 200 MΩ or less. A method for identifying the location of slag in an arc-welded joint produced by arc welding a surface-treated steel sheet 1 will be described 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, a plurality of arbitrary locations (for example, three locations) are measured using the tester, and a resistance value is obtained for each measurement location. The obtained resistance values ​​are then averaged by the number of measurement locations to obtain the average resistance value of the slag of interest. For convenience of explanation, details of the slag formation ability will be described later.

[0024] The surface treatment layer 20 according to this embodiment contains a resin and a conductive pigment in addition to the Ti compound having slag-forming ability as described above. From this viewpoint, the Ti content in the surface treatment layer 20 according to this embodiment is within the range of 10 to 40 atomic %.

[0025] If the Ti content in the surface treatment layer 20 is less than 10 atomic %, the content of Ti compounds in the surface treatment layer 20 is too low, resulting in a small amount of Ti-Mn-based slag formed during arc welding. As a result, the corrosion resistance of the weld bead formed by arc welding cannot be improved. If the Ti content in the surface treatment layer 20 is 10 atomic % or more, it is possible to form Ti-Mn-based slag by arc welding to an extent that allows electrodeposition coating, thereby improving the corrosion resistance of the weld bead. The Ti content in the surface treatment layer 20 is preferably 20 atomic % or more, and more preferably 25 atomic % or more.

[0026] On the other hand, if the Ti content in the surface treatment layer 20 according to this embodiment exceeds 40 atomic %, the resin and conductive pigment contents in the surface treatment layer 20 become too small, and the effects that can be obtained by including the resin and conductive pigment cannot be fully exhibited. From this viewpoint, the Ti content in the surface treatment layer 20 according to this embodiment is set to 40 atomic % or less. The Ti content in the surface treatment layer 20 is preferably 35 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 liquid containing a surface treatment agent and then performing heat drying. Such heat drying can increase the Ti content due to evaporation of components such as water in the treatment liquid. Therefore, the Ti content in the treatment liquid before heat drying may be less than 10 atomic %. In one embodiment, the Ti content in the surface treatment liquid before heat drying is set to 3 atomic % or more. This ensures that the Ti content in the surface treatment layer 20 after heat drying is 10 atomic % or more.

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

[0029] More specifically, an arbitrary position where the surface treatment layer 20 is present is cut to an appropriate size in a direction perpendicular to the surface of the surface treatment layer 20 (thickness direction) to expose a cross section of the surface treatment layer 20 in the thickness direction. Next, the cross section is embedded in resin so that the cross section in the thickness direction can be seen, and then polished. The resulting polished surface is then observed using a 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: 15 kV). In this case, Ti is selected as the detection target element, and mapping images of the location and element concentration of Ti element are taken. From the obtained results, the Ti content in the field of view of interest can be calculated. This measurement can be performed at any number of locations (e.g., three locations) on the obtained cross section, and the average of the Ti contents obtained at each location can be used as the Ti content in the surface treatment layer 20.

[0030] [Specific examples of Ti compounds capable of forming Ti-Mn slag] Examples of Ti compounds capable of forming Ti-Mn-based slag contained in the surface treatment layer 20 according to this embodiment include Ti compounds other than Ti oxides. Here, Ti oxides (e.g., TiO2, etc.) consisting of Ti atoms and oxygen atoms do not form conductive slag with Mn during arc welding. For this reason, Ti oxides cannot be used as Ti compounds capable of forming slag according to this embodiment.

[0031] An example of a Ti compound capable of forming a Ti-Mn-based slag is TiN. During arc welding, TiN reacts with Mn derived from the steel sheet or welding wire to form a Ti-Mn-based slag that exhibits electrical conductivity. Note that the Ti compound capable of forming a Ti-Mn-based slag is not limited to TiN, and may be any Ti oxide.

[0032] Examples of Ti compounds capable of forming Ti-Mn-based slag include various Ti chelate compounds. In this embodiment, the term "Ti chelate compound" refers to a compound in which various ligands form a complex sandwiching a Ti ion, forming a ring containing a Ti atom and a coordinating atom, and is different from Ti oxides (compounds consisting of Ti atoms and oxygen atoms).

[0033] In the surface treatment layer 20 according to this embodiment, by using a Ti chelate compound as the Ti compound having slag-forming ability, it is possible to further improve the adhesion of the electrodeposition coating when the electrodeposition coating is applied.

[0034] Furthermore, among various Ti chelate compounds, the use of at least one of titanium lactate ammonium salt, titanium lactate, titanium triethanolamine, titanium diethanolamine, titanium aminoethylaminoethanolate, titanium acetylacetonate, titanium tetraacetylacetonate, titanium ethylacetoacetate, titanium phosphate ester complex, and titanium octylene glycolate can further improve the adhesion of electrodeposition coating. Because these Ti chelate compounds are water-soluble, they disperse well in various resins, for example, when used as a component of the surface treatment layer 20. As a result, the slag generated by arc welding is refined, further improving the adhesion of electrodeposition coating.

[0035] Examples of Ti compounds capable of forming Ti-Mn slag include various alkoxides, such as tetraisopropyl titanate, tetra-n-butyl titanate, butyl titanate dimer, tetraoctyl titanate, tetra-tert-butyl titanate, and tetrastearyl titanate.

[0036] In the surface treatment layer 20 according to this embodiment, the above-mentioned various Ti compounds may be used alone, or a plurality of Ti compounds may be used in appropriate combination.

[0037] In the surface treatment layer 20 according to this embodiment, the content of Ti derived from the Ti chelate compound is preferably within the range of 5 to 40 atomic %. When the content of Ti derived from the Ti chelate compound is 5 atomic % or more, it becomes possible to improve the adhesion of electrodeposition coating by miniaturizing the slag as described above. The content of Ti derived from the Ti chelate compound is more preferably 10 atomic % or more, and even more preferably 20 atomic % or more.

[0038] On the other hand, if the content of Ti derived from the Ti chelate compound is 40 atomic % or less, it is possible to fully realize the effects that can be obtained by adding resin and conductive pigment, while improving the adhesion of electrodeposition coating by miniaturizing the slag as described above. The content of Ti derived from the Ti chelate compound is more preferably 35 atomic % or less.

[0039] When identifying the Ti compounds contained in the surface treatment layer 20 from a surface-treated steel sheet or an arc-welded joint formed from the surface-treated steel sheet, an EPMA mapping analysis can be performed on the cross section (polished surface) of the surface treatment layer 20 at a magnification of 5000 (acceleration voltage: 15 kV) in the same manner as in the measurement of the Ti content in the surface treatment layer 20. In this case, Ti is selected as the element to be detected, and mapping photography is performed to capture the location and element concentration of the Ti element, and the Ti content in the field of interest can be calculated from the obtained results.

[0040] Furthermore, when the Ti content derived from the Ti chelate compound in the surface treatment layer 20 is to be determined afterward, the following method may be used.

[0041] First, the surface-treated steel sheet 1 is cut in the thickness direction at an arbitrary position where the surface treatment layer 20 is present to obtain a sample for cross-sectional observation. Next, the cross-section of the sample is polished in the same manner as in the measurement method for the Ti content of the surface treatment layer 20, and EPMA mapping analysis is performed at several arbitrary locations (e.g., three locations) on the obtained cross-section, with the target elements being Ti, Si, O, N, and C. The magnification is 5000x (acceleration voltage: 15 kV). The presence or absence of Ti compounds can be confirmed by whether or not the presence of elemental Ti can be confirmed in all the obtained EPMA maps. If the presence of elemental Ti can be confirmed in all the obtained EPMA maps but the elemental Ti is not distributed in a particulate form, it can be determined that various Ti chelate compounds are used. Note that "particulate distribution" refers to the observation at the above magnification showing visible aggregates of elemental Ti with an average particle size of approximately 0.1 μm or more. From the results obtained in this way, the Ti content derived from Ti chelate compounds in the field of interest can be calculated.

[0042] Even when elemental Ti is not distributed in a particulate form in the EPMA map, nanotitania (TiO2 having nano-order particle sizes) may be contained. Therefore, in order to distinguish between Ti chelate compounds and nanotitania, it is preferable to use X-ray photoelectron spectroscopy (XPS) measurement in addition to the above-mentioned EPMA mapping analysis. By focusing on whether or not Ti-based peaks other than those derived from TiO2 (for example, peaks derived from Ti-OR bonds (R represents an alkyl group)) are detected in the XPS measurement results, it becomes easier to distinguish between nanotitania and Ti chelate compounds.

[0043] Furthermore, whether or not an oxide of Ti is used can be determined by referring to the EPMA maps of element Ti and element O, based on whether or not element O is present at the position where element Ti is confirmed.

[0044] [Ingredients other than Ti compounds] The surface treatment layer 20 according to this embodiment further contains a resin and a conductive pigment in addition to the Ti compound. The surface treatment layer 20 according to this embodiment may also contain various other components such as a rust inhibitor in addition to the Ti compound, resin, and conductive pigment.

[0045] <Resin> The surface treatment layer 20 according to this embodiment contains various resins as binder components for holding the Ti compound. Examples of such resins include polymer polyester resin, polyester resin, acrylic resin, epoxy resin, urethane resin, and fluororesin. It is also possible to use film-forming resin components, such as modified versions of these resins, crosslinked with a crosslinking agent component, such as butylated melamine resin, methylated melamine resin, butylmethyl-mixed melamine resin, urea resin, and isocyanate resin, or a mixture of these resins.

[0046] Among the resins mentioned above, at least one of epoxy resin and urethane resin is more preferable. By using at least one of epoxy resin and urethane resin, it is possible to further improve the adhesion between the electrodeposition coating and the surface treatment layer 20.

[0047] <Conductive pigment> The surface treatment layer 20 according to this embodiment further contains a conductive pigment in addition to the Ti compound and resin. By including a specific amount of conductive pigment 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 preferable. This can further improve the corrosion resistance of the weld bead formed by arc welding. Furthermore, the presence of the conductive pigment in the surface treatment layer 20 can also improve spot weldability.

[0048] The content of the conductive pigment is 10 to 40% by mass relative to the total mass of the solid content of the surface treatment layer 20. If the content of the conductive pigment is less than 10% by mass, the effect of the conductive pigment in improving corrosion resistance is insufficient, and the desired corrosion resistance cannot be achieved in the weld bead portion. If the content of the conductive pigment is 10% by mass or more, not only can the desired corrosion resistance be achieved in the weld bead portion, but also spot weldability can be improved. The content of the conductive pigment is preferably 15% by mass or more, and more preferably 20% by mass or more.

[0049] On the other hand, if the content of the conductive pigment exceeds 40% by mass, the content of the Ti compound and resin in the surface treatment layer 20 becomes too low, and the effects that can be obtained by including the Ti compound and resin cannot be fully realized. By keeping the content of the conductive pigment at 40% by mass or less, not only can the desired corrosion resistance be achieved in the weld bead portion, but also spot weldability can be improved. The content of the conductive pigment is preferably 30% by mass or less.

[0050] Furthermore, in the surface treatment layer 20 according to this embodiment, the conductive pigment is preferably particulate, 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).

[0051] When the average particle size of the particulate conductive pigment is 0.25 times or more 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, making it possible to make the electrical resistance value exhibited by the surface treatment layer 20 uniform throughout the surface treatment layer 20 and further improving the spot weldability. The average particle size of the particulate conductive pigment is more preferably 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.

[0052] On the other hand, by making the average particle size of the particulate conductive pigment 20 2.00 times or less the average thickness of the surface treatment layer 20, it is possible to prevent a decrease in adhesion between the surface treatment layer 20 and the electrodeposition coating, while more uniformly dispersing the conductive pigment in the surface treatment layer 20 and further improving spot weldability. The average particle size of the particulate conductive pigment is more preferably 1.70 times or less the average thickness of the surface treatment layer 20, and even more preferably 1.50 times or less the average thickness of the surface treatment layer 20.

[0053] Examples of the conductive pigments 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. Si compounds are excluded from the non-oxide ceramic particles because they become Si oxides, which are non-conductive during arc welding and reduce the electrodeposition paintability of arc-welded joints.

[0054] These conductive pigments can react with Mn and Si derived from the steel sheet and welding wire during arc welding to form conductive slag. Therefore, by using such compounds as conductive pigments, it becomes easier to form an electrodeposition coating film at the weld bead, and it becomes possible to further improve the corrosion resistance of the weld bead.

[0055] In addition, when the type of conductive pigment used in the surface treatment layer 20, its content, and average particle size are identified from the surface-treated steel sheet or the arc-welded joint formed from the surface-treated steel sheet, the following procedure can be used.

[0056] First, the surface treatment layer 20 of interest is cut at an angle perpendicular to the surface of the surface treatment layer 20 to obtain a sample for cross-sectional observation. To identify the type and average particle size of the conductive pigment, the sample for cross-section observation is embedded in resin and the cut surface to be observed is polished. The obtained observation surface is then observed using an SEM to obtain a backscattered electron (BSE) compositional image. The observation field size is 25 μm × 20 μm, and the magnification is 5000x. Several particles (e.g., five) are randomly selected from the particles present in the observation field, and the circle-equivalent diameter of each particle is calculated. The average of these diameters is used as the average particle size. Furthermore, whether 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 of the particles of interest.

[0057] The conductive pigment content can be calculated by observing a cross section of the surface treatment layer 20 cut in the thickness direction using an SEM to identify each particle, counting the number of particles per cross section, and converting this into the number per volume of the surface treatment layer 20. If necessary, each particle can be identified using an EDX spectrometer or the like. In this case, the size of the observation field is 25 μm × 20 μm, and the magnification is 5000 times. The number of fields of view should be three or more, and the average number of particles in each field of view can be calculated by dividing the number of particles in each field of view by the number of fields of view.

[0058] When multiple types of conductive pigments are contained, 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 used to form the surface treatment layer 20 and the amount of the surface treatment agent attached to the base steel sheet 10. If the amounts of each conductive pigment in the surface treatment agent before coating are known, the amount of each particle in the coating can be calculated from the amounts of each pigment and the amount of paint attached to the base steel sheet. Alternatively, if the amounts of each conductive pigment are unknown, the average thickness of the surface treatment layer is calculated using the method described above, and then a portion of the surface treatment layer measuring, for example, 20 mm x 20 mm in plan view is melted. This melting method is not limited as long as it does not melt the conductive particles. This melts the surface treatment layer with an area of ​​20 mm x 20 mm and a volume equivalent to the average thickness of the surface treatment layer. Examples of methods for melting the surface treatment layer include those using organic solvents. Next, using a particle image analyzer such as Malvern's Morphologi G3 ("MORPHOLOGI" is a registered trademark), particles in the surface treatment agent diluted to an appropriate concentration are individually identified and counted by image analysis, and the amount of each particle in the calculated volume of the surface treatment layer is calculated. The number of each particle in the surface treatment layer can then be calculated from the number of each particle and the calculated volume. This method can also be used when dissolving the surface treatment layer 20 attached to the base steel sheet 10 and counting the number of particles.

[0059] <Rust inhibitor> The surface treatment layer 20 according to this embodiment may further contain a rust inhibitor in addition to the Ti compound, resin, and conductive pigment described above. When the surface treatment layer 20 contains a rust inhibitor, the corrosion resistance of the surface-treated steel sheet 1 can be further improved.

[0060] As such a rust inhibitor, it is preferable to use, for example, a rust inhibitor other than a Si compound. If a Si compound is used as a rust inhibitor, the Si compound becomes a non-conductive Si oxide, which reduces the electrodeposition coating property, and is therefore undesirable.

[0061] Here, the content of the rust inhibitor is preferably 10 to 30 mass % with respect to the total mass of the solid content of the surface treatment layer 20. By making the content of the rust inhibitor 10 mass % or more, it is possible to further improve the corrosion resistance of the surface-treated steel sheet 1. The content of the rust inhibitor is more preferably 15 mass % or more.

[0062] On the other hand, by setting the content of the rust inhibitor 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. The content of the rust inhibitor is more preferably 25% by mass or less.

[0063] 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, orthophosphoric acid, pyrophosphoric acid, metaphosphoric acid, hypophosphoric acid, phosphorous acid, hypophosphorous acid, and salts thereof. These rust inhibitors may exist as particles or in the form of ions. The state of these compounds is determined by the type of resin contained in the surface treatment layer.

[0064] The type and content of the rust inhibitor used in the surface treatment layer 20 can be identified from the state of the surface-treated steel sheet or the arc-welded joint as follows. That is, if the resin contained in the surface treatment layer 20 is a solvent-based resin, the type and content of the rust inhibitor can be measured from the state of the surface-treated steel sheet or the arc-welded joint in the same manner as for the conductive pigment described above. Furthermore, if the resin contained in the surface treatment layer 20 is a water-based resin, the surface treatment layer 20 can be dissolved in the same manner as for the case where multiple types of conductive pigments are contained, and the resulting solution can be subjected to inductively coupled plasma (ICP) emission spectroscopy to measure the type and content of the rust inhibitor.

[0065] Furthermore, the surface treatment layer 20 according to this embodiment may contain, in addition to the conductive pigment and the rust inhibitor as described above, additives such as an extender pigment, a coloring pigment, a colorant, a viscosity adjuster, a leveling agent, an antifoaming agent, and an ultraviolet absorber, as necessary.

[0066] [Average thickness of surface treatment layer 20] The average thickness (thickness d in FIG. 1) of the surface treatment layer 20 containing at least the Ti compound, resin, and conductive pigment as described above is within 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 surface treatment layer 20 is too thin to ensure the corrosion resistance of the surface-treated steel sheet 1. By setting the average thickness of the surface treatment layer 20 to 0.5 μm or more, the corrosion resistance required of the surface-treated steel sheet 1 can be ensured. The average thickness of the surface treatment layer 20 is preferably 2.0 μm or more, and more preferably 3.0 μm or more.

[0067] On the other hand, if the average thickness of the surface treatment layer 20 exceeds 5.0 μm, the surface treatment layer 20 will be too thick, resulting in a decrease in the weldability of the surface-treated steel sheet 1. By setting the average thickness of the surface treatment layer 20 to 5.0 μm or less, it is possible to suppress a decrease in weldability while ensuring the corrosion resistance required of the surface-treated steel sheet 1. The average thickness of the surface treatment layer 20 is preferably 4.5 μm or less.

[0068] 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 the surface of the surface treatment layer 20 and observing the cut surface in a direction perpendicular to the cut surface (cross-sectional direction). Specifically, the average thickness can be measured by observing the cut surface of the surface treatment layer 20 in the cross-sectional direction using a microscope. Examples of methods for preparing a sample for observing the cut surface in the cross-sectional direction include embedding a small piece of the surface-treated steel sheet 1 provided with the surface treatment layer 20 in resin and polishing the cut surface to be observed, processing using a focused ion beam (FIB), and microtome. Furthermore, a SEM can be used as the microscope. The thickness of the surface treatment layer 20 is measured at multiple arbitrary locations (e.g., three locations) of the obtained sample using the above-described method, using a length measurement function or the like implemented in the microscope. The average thickness of the surface treatment layer 20 can be determined by averaging the multiple measured values ​​obtained for the number of measurement locations.

[0069] The surface treatment layer 20 according to this embodiment has been described in detail above with reference to FIG.

[0070] 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 in order to further improve the adhesion between the base steel sheet 10 and the surface treatment layer 20.

[0071] (About arc welded joints) Next, the overall configuration of an arc-welded joint 100 using the surface-treated steel sheet 1 according to this embodiment as a material will be described with reference to Fig. 2. Fig. 2 is an explanatory diagram that schematically shows an example of the structure of an arc-welded joint according to this embodiment. Note that, for convenience, the following description will be made appropriately using a coordinate system such as that shown in Fig. 2.

[0072] Fig. 2 is a schematic diagram showing the overall configuration of an arc-welded joint 100 obtained by overlapping a portion of a surface-treated steel sheet 1A and a portion of a surface-treated steel sheet 1B and then performing lap fillet welding by arc welding. Fig. 2 is a schematic diagram showing a cross section of the arc-welded joint 100 perpendicular to the extension direction of the weld bead. As shown in Fig. 2, the arc-welded joint 100 according to this embodiment has surface-treated steel sheets 1A and 1B, a weld bead 110, slag 120, and an electrodeposition coating 130.

[0073] Here, the surface-treated steel sheets 1A, 1B constituting the arc-welded joint 100 are the surface-treated steel sheets 1 having the surface-treated layer 20 as explained above, and therefore detailed explanations thereof will be omitted below.

[0074] In addition, when taking a sample of the surface treatment layer 20 in order to identify the various characteristics (e.g., composition, thickness, etc.) of the surface treatment layer 20 in the surface-treated steel plates 1A, 1B that constitute the arc-welded joint 100 from the state of the arc-welded joint 100, the sample is taken from a location that is 20 mm or more away from the outer edge of the toe of the arc-welded joint 100 (the "toe" defined in JIS Z3001:2018, the position of point T in Figure 2) in the direction away from the weld bead portion 110.

[0075] <Regarding the weld bead portion 110> The weld bead 110 is a region 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 sheets 1A and 1B. In Fig. 2, the bonding interface between the weld bead 110 and the surface-treated steel sheets 1A and 1B is shown as a smooth curve or line for convenience of illustration, but the actual bonding interface has a complex curved surface due to the oscillation of the molten metal during welding caused by arc plasma, etc. Furthermore, the weld bead 110 extends along the Y-axis direction in the figure, and the surface-treated steel sheet 1A and the surface-treated steel sheet 1B are bonded together by the weld bead 110.

[0076] Here, when identifying a portion corresponding to the weld bead portion 110 in the arc-welded joint 100 of interest, the portion can be easily visualized by etching using an etching solution. For example, the etching solution that can be used is nital (composition: 95% ethanol, 5% sulfuric acid) or an etching solution obtained by mixing 60 g of sodium dodecylbenzenesulfonate, 36 g of picric acid, 60 cc of ethanol, and 60 cc of household detergent solution (for example, a common one such as dishwashing detergent) with 2400 cc of water.

[0077] <About Slag 120> The slag 120 is a portion formed by arc welding and is generated by a reaction between the surface-treated steel sheet 1, a welding wire used as needed during welding, and an oxygen source in the shielding gas used during arc welding. The slag 120 is present on at least a portion of the surface of the weld bead portion 110.

[0078] The slag 120 in the arc welded joint 100 according to this embodiment contains conductive Ti-Mn-based slag because the surface treatment layer 20 of the surface-treated steel sheet 1, which is the raw material for the joint, contains Ti compounds capable of forming Ti-Mn-based slag. This makes it possible to form an electrodeposition coating 130 (described later) in the vicinity of the weld bead 110 of the arc welded joint 100 according to this embodiment, thereby improving corrosion resistance.

[0079] In the arc welded joint 100 according to this embodiment, the average electrical resistance (average resistance) of the slag 120 is 200 MΩ or less due to the inclusion of the conductive slag described above. This makes it possible to perform electrodeposition coating on the weld bead portion 110 of the arc welded joint 100 according to this embodiment. The average resistance of the slag 120 is preferably 150 MΩ or less.

[0080] Here, to measure the average resistance value of the slag 120 as described above from the state of the arc-welded joint 100, a commercially available tester may be used. At the position where the slag 120 exists, a plurality of arbitrary locations (e.g., three locations) are measured with the tester, and a resistance value is obtained for each measurement location. Then, the obtained resistance values ​​are averaged by the number of measurement locations to obtain the average resistance value of the slag 120 of interest.

[0081] Here, the average particle size of the slag 120 according to this embodiment is more preferably 1.0 mm or less. When the slag 120 according to this embodiment is a finer slag having an average particle size of 1.0 mm or less, electrodeposition coating becomes easier, and the weld bead portion 110 exhibits better corrosion resistance.

[0082] The slag 120 according to this embodiment may contain, in addition to the Ti--Mn-based slag, Si--Mn-based slag, Mn-based slag, etc., as well as impurities.

[0083] Here, in the arc welded joint 100 according to this embodiment, the position where the above-described slag 120 exists can be identified as follows. First, the electrodeposition coating 130 present on the surface of the arc-welded joint 100 is removed in advance using a paint remover (e.g., Neoliver manufactured by Sansai Kako Co., Ltd.), leaving the weld bead 110 and slag 120 exposed on the surface. The location of the slag 120 can be determined by focusing on a backscattered electron composition image when observing the surface on which the weld bead 110 and slag 120 are present using an SEM. More specifically, the location of the slag 120 can be easily identified by observing the periphery of the weld bead 110 at 50x magnification to obtain a field of view of 2 mm x 2 mm.

[0084] To identify the type of slag constituting the slag 120, a sample for cross-sectional observation is cut from the portion of the weld bead 110 where the slag 120 is present, and the sample is observed using a SEM-EPMA and subjected to EPMA mapping analysis. More specifically, the weld bead 110 where the slag 120 is present is cut to an appropriate size at an arbitrary position, embedded in resin so that the cross-section in the thickness direction is visible, and the cross-section is polished. The polished surface is then observed using a SEM-EPMA. More specifically, the cross-section is subjected to EPMA mapping analysis at a magnification of 1000x (acceleration voltage: 15 kV). In this case, Ti, Mn, Si, and O are selected as the elements to be detected, and mapping images of the locations and concentrations of these elements are taken. The type of slag can be identified from the results obtained.

[0085] Furthermore, when the average particle size of the slag 120 is determined from the state of the arc-welded joint 100, the location of the slag 120 can be determined by SEM observation of the surface as described above, and then a backscattered electron image (BSE) can be obtained. Here, the size of the observation field can be set to 25 μm × 20 μm, and the magnification can be set to 5000 times. Several (e.g., five) pieces of slag present in the observation field can be randomly selected, and the circle-equivalent diameters of each slag can be determined, and the average value can be used as the average particle size.

[0086] <About Electrodeposition Coating 130> The electrodeposition coating 130 is provided 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.

[0087] In the arc welded joint 100 according to this embodiment, the surface-treated steel sheet 1 used as the material for the joint has a surface treatment layer 20 containing a Ti compound having the above-described slag-forming ability. As a result, the average resistance of the slag 120 is 200 MΩ or less, making it possible to easily form the electrodeposition coating 130, and the weld bead portion 110 exhibits excellent corrosion resistance.

[0088] Furthermore, the improved film-forming properties of the electrodeposition coating 120 in the weld bead portion 110 and the improved corrosion resistance make it easier to reduce the thickness of the surface-treated steel sheet 1 used in arc welding. As a result, the arc-welded joint 100 according to this embodiment can achieve both improved corrosion resistance and weight reduction.

[0089] In the arc welded joint 100 according to this embodiment, the details of the electrodeposition coating 130 are not particularly limited, and various known electrodeposition coatings can be used.

[0090] The arc welded joint 100 according to this embodiment has been described in detail above with reference to FIG.

[0091] (Regarding the manufacturing method of surface-treated steel sheets) An example of a method for manufacturing the surface-treated steel sheet 1 according to this embodiment will be briefly described below. First, various pretreatments such as alkaline degreasing, water washing, and pickling are performed on the steel sheet serving as the substrate to obtain a clean steel sheet surface. Thereafter, various plating treatments may be performed on the surface of the steel sheet as needed. In this manner, the substrate steel sheet 10 of the surface-treated steel sheet 1 according to this embodiment can be obtained.

[0092] If necessary, various known chemical conversion treatments are performed on the base steel sheet 10 to form a chemical conversion treatment layer. For example, a chemical conversion treatment agent containing desired components may be prepared and applied to the surface of the base steel sheet 10. Here, the application of the chemical conversion treatment agent as described above can be performed by a commonly known application method, such as roll coating, curtain flow coating, air spraying, airless spraying, immersion, bar coating, or brush coating.

[0093] For example, a surface treatment agent for forming the 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 then heated and dried to form the surface treatment layer 20. Here, such a surface treatment agent is prepared by adding the above-mentioned Ti compound (more preferably the above-mentioned Ti compound and a resin as a film-forming component, and, if necessary, various additives such as a conductive pigment and a rust inhibitor) to a solvent. The prepared surface treatment agent can be applied by a commonly known application method, such as roll coating, curtain flow coating, air spraying, airless spraying, dipping, bar coating, or brush coating, as described above.

[0094] Thereafter, the coating liquid may be heated and dried by any method such as hot air, near infrared rays, far infrared rays, induction heating, or a combination of these, thereby forming the surface treatment layer 20 according to this embodiment.

[0095] The method for manufacturing the surface-treated steel sheet 1 according to this embodiment has been briefly described above.

[0096] (Regarding the manufacturing method of arc welded joints) The welded joint according to this embodiment is manufactured by using the surface-treated steel sheets manufactured as described above as raw materials, arranging the surface-treated steel sheets so as to form the desired shape of the arc-welded joint, and welding the surface-treated steel sheets together. Here, for example, an arc welding method that satisfies the following conditions is used to weld the surface-treated steel sheets.

[0097] 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, φ1.2 mm, manufactured by Nippon Steel Welding Industry Co., Ltd.) (C: 0.07% by mass, Si: 0.61% by mass, Mn: 1.21% by mass, P: 0.008% by mass, S: 0.005% by mass) Welding torch tilt angle: 45°

[0098] In addition, the entire joint after arc welding is subjected to electrodeposition coating using various known electrodeposition paints. As a result, the entire arc-welded joint is coated with an electrodeposition paint. The type of electrodeposition paint used and the drying conditions are not particularly limited.

[0099] An example of the method for manufacturing an arc welded joint according to this embodiment has been described above. [Example]

[0100] The surface-treated steel sheet and arc-welded joint according to the present embodiment will be specifically described below with reference to examples and comparative examples. Note that the examples shown below are merely examples of the surface-treated steel sheet and arc-welded joint according to the present embodiment, and the surface-treated steel sheet and arc-welded joint according to the present invention are not limited to the examples below.

[0101] (Test example) <1. Sample preparation> [Production of surface-treated steel sheets] Cold-rolled steel sheets (all manufactured by Nippon Steel Corporation) with a thickness of 1.6 mm and tensile strengths of 780 MPa, 980 MPa, and 1180 MPa were used as the base steel sheets. Surface treatment solutions containing the components shown in Table 1 below were prepared and applied with a bar coater so that the average thickness of the surface treatment layer after drying would be the value shown in Table 1 below. The steel sheets were baked in a hot air furnace under conditions such that the steel sheet surface reached a temperature of 140°C, and then air-cooled to room temperature. In this manner, multiple samples of surface-treated steel sheets were prepared for each level.

[0102] Here, epoxy resin (EM series manufactured by ADEKA Corporation) and urethane resin (HUX series manufactured by ADEKA Corporation) were used as the resins used as film-forming components. In addition, titanium alkoxide, titanium acetylacetonate, titanium lactate ammonium salt, titanium oxide, TiN, titanium lactate ammonium salt, titanium lactate, titanium triethanolamine, titanium acetylacetonate, titanium tetraacetylacetonate, titanium ethylacetoacetate, phosphate ester titanium complex, and titanium octylene glycolate (all general reagents) were used as the Ti compounds.

[0103] Furthermore, aluminum tripolyphosphate (general reagent) was used as the rust inhibitor, and ZnO and TiN (both general reagents) having the average particle sizes shown in Table 1 below were used as the conductive pigment.

[0104] [Table 1]

[0105] [Arc welding] Test pieces measuring 50mm x 150mm for the upper plate and 30mm x 150mm for the lower plate were cut out from each level of surface-treated steel sheet, and these two test pieces were overlapped in the longitudinal direction with an overlap width of 10mm and lap fillet arc welding was carried out. The welding wire and arc welding conditions used were as follows: Mild steel wire (Nippon Steel Welding Industry, YM-24T) Shielding gas: Ar+20%CO2, Welding current 140A, welding voltage 11.4V, welding speed 80cm / min

[0106] [Electrodeposition coating] Each arc-welded sample obtained as described above was degreased by immersion in an alkaline degreasing solution manufactured by Nippon Parkerizing Co., Ltd. for 2 minutes at 40°C. Surface conditioning was then performed for 15 seconds at 25°C using a surface conditioning treatment agent manufactured by Nippon Paint Co., Ltd. Furthermore, a zinc phosphate treatment was performed using a zinc phosphate treatment solution manufactured by Nippon Paint Co., Ltd. The temperature of the chemical conversion treatment solution was set to 35°C, and the arc-welded sample was immersed in the chemical conversion treatment solution for 120 seconds, then rinsed with water and dried. Subsequently, the sample was electrodeposited using a cationic electrodeposition paint manufactured by Nippon Paint at 35°C, with the voltage increased to 180V over 30 seconds and the current applied for 150 seconds. The sample was then baked at a baking temperature of 170°C for 25 minutes.

[0107] In this manner, a plurality of samples of arc-welded joints were prepared for each level.

[0108] <2. Evaluation Method> For each of the arc-welded joints obtained as described above, the components of the slag in the weld bead were identified and the average resistivity and average particle size of the slag were measured using the method described above. The results are summarized in Table 1.

[0109] The weld beads were evaluated for corrosion resistance after painting, spot weldability, and corrosion resistance after painting on the flat surface. The evaluation methods were as follows. The results are summarized in Table 2.

[0110] [Corrosion resistance of weld bead after painting] After the electrodeposition coating, the end faces of the test pieces and both ends of the weld bead were sealed for each arc-welded joint, and then corrosion resistance was evaluated. Corrosion resistance was evaluated according to the JASO M609-91 test. Specifically, one cycle consisted of two hours of salt spray (5% NaCl) at 35°C, four hours of dry (25% humidity) at 60°C, and two hours of wet (95% humidity) at 50°C. After 120 cycles, the occurrence of red rust was evaluated. Evaluation was based on the following criteria, with a score of 2 or higher considered a pass. [Evaluation criteria] Grade 5: Red rust area ratio is 30% or less Grade 4: Red rust area ratio is over 30% and up to 40% Grade 3: Red rust area ratio is over 40% and up to 50% Grade 2: Red rust area ratio is over 50% and up to 70% Grade 1: Red rust area ratio is over 70%

[0111] [Spot weldability] Before arc welding and before electrodeposition coating, the surface-treated steel sheet was sheared to a size of 30 x 50 mm, and the optimum spot welding current range (upper limit current - lower limit current) was measured. The measurement conditions are as follows: The lower limit current is nugget diameter 4 x (sheet thickness) 0.5 Specifically, the current value at which the nugget diameter becomes 5.1 mm was set, and the upper limit current was set as the current at which expulsion occurs. Evaluation was performed according to the following evaluation criteria, and a score of 2 or higher was considered to be acceptable. <Measurement conditions> ·Current: DC Electrode: Chromium copper, DR (tip 6mmφ 40R) Pressure: 400 kgf (1 kgf is approximately 9.8 N) ·Electrification time: 240msec [Evaluation criteria] Rating 3: Spot welding optimum current range is over 1.5kA Grade 2: Spot welding appropriate current range is over 1.0kA and 1.5kA or less Rating 1: Spot welding appropriate current range is 1.0kA or less

[0112] [Corrosion resistance after painting on flat surfaces] For each arc-welded joint after electrodeposition coating, the corrosion resistance after coating was evaluated on the flat part (the part 20 mm or more away from the weld bead, starting from the toe of the weld bead (point T in Figure 2)). Corrosion resistance was evaluated according to JASO M609-91. Specifically, a cross-cut was made in the electrodeposition coating at a position at least 20 mm away from the toe of the weld bead with a cutter, and the coating blister width (unit: mm, average of the maximum three points on one side) from the cross-cut after 120 cycles of corrosion testing was evaluated. A coating blister width of less than 5.0 mm was considered to have passed.

[0113] [Table 2]

[0114] As is clear from Table 2 above, the levels corresponding to the examples of the present invention showed excellent corrosion resistance and spot weldability of the weld bead after painting, while the levels corresponding to the comparative examples of the present invention showed insufficient at least one of the corrosion resistance and spot weldability of the weld bead after painting.

[0115] For example, in Comparative Example No. 1, the surface treatment layer did not contain a Ti compound, so conductive slag was not formed during welding. As a result, there were many electrodeposition coating defects, and sufficient corrosion resistance after painting was not achieved. Furthermore, the surface treatment layer did not contain a conductive pigment, so sufficient spot weldability was not achieved. In Comparative Example No. 2, the surface treatment layer did not contain a conductive pigment, so sufficient spot weldability was not achieved. In Comparative Example No. 14, the average thickness of the surface treatment layer was below the range of the present invention, so sufficient corrosion resistance after painting was not achieved. In Comparative Example No. 15, the average thickness of the surface treatment layer exceeded the range of the present invention, so sufficient spot weldability was not achieved. In Comparative Example No. 16, the content of the conductive pigment was below the range of the present invention, so sufficient spot weldability was not achieved. In Comparative Example No. 17, the content of the conductive pigment exceeded the range of the present invention, so the corrosion resistance after painting on flat areas was insufficient. In Comparative Example No. 18, the surface treatment layer contained titanium oxide, so conductive slag was not formed during welding. As a result, there were many electrodeposition coating defects, and sufficient corrosion resistance after painting was not achieved. In Comparative Example No. 20, the Ti compound content was below the range specified by the present invention, and therefore insufficient conductive slag was formed during welding. As a result, electrodeposition coating was often defective, and sufficient corrosion resistance after painting was not achieved. Examples No. 21 and No. 22 differed in Ti content, and Example No. 22, which contained 20 atomic % or more of Ti compound, exhibited significantly better corrosion resistance than Example No. 21. Examples No. 23 to No. 28 used various Ti chelate compounds as Ti compounds, which further improved the adhesion of electrodeposition coating and achieved favorable corrosion resistance after painting. In particular, Examples No. 23 and No. 24, which used aqueous Ti chelate compounds titanium lactate and titanium triethanolamine, exhibited better corrosion resistance after painting due to the fine slag formed by the well-dispersed Ti chelate compounds in the resin. In Examples No. 25 and No. 28, the ratio of the average particle size to the average thickness was within a preferable range, and therefore, while improving spot weldability, it was possible to prevent a decrease in adhesion between the surface treatment layer and the electrodeposition coating, and good corrosion resistance after coating was obtained.In Example No. 27, the ratio of the average particle size to the average thickness was in a more preferable range, so that it was possible to prevent a decrease in adhesion between the surface treatment layer 20 and the electrodeposition coating while improving spot weldability, and more preferable corrosion resistance after painting was obtained.In Example No. 26, the ratio of the average particle size to the average thickness was in the most preferable range, so that it was possible to prevent a decrease in adhesion between the surface treatment layer 20 and the electrodeposition coating while improving spot weldability, and more preferable corrosion resistance after painting was obtained.

[0116] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

[0117] The embodiments disclosed herein are illustrative in all respects and are not limiting. The above-described embodiments may be omitted, substituted, or modified in various ways without departing from the scope of the appended claims, the technical scope of the present invention as described below, and the spirit thereof. For example, the components of the above-described embodiments may be arbitrarily combined within the scope that does not impair the effects of the components. Furthermore, such an arbitrary combination naturally provides the functions and effects of each of the components involved in the combination, and also provides other functions and effects that are apparent to those skilled in the art from the description of this specification.

[0118] Furthermore, the effects described in this specification are merely descriptive or exemplary, and are not limiting. In other words, the technology according to the present invention may achieve other effects that will be apparent to those skilled in the art from the description of this specification, in addition to or in place of the above-described effects.

[0119] The following configurations also fall within the technical scope of the present invention. (1) A steel plate as a base material; a surface treatment layer located on at least a portion of the steel sheet; It has the surface treatment layer contains a Ti compound having a slag-forming ability to form a Ti-Mn-based slag with elemental Mn, a resin, and a conductive pigment; The Ti content in the surface treatment layer is 10 to 40 atomic %; the content of the conductive pigment in the surface treatment layer is 10 to 40 mass %, The average thickness of the surface treatment layer is 0.5 to 5.0 μm. (2) The surface-treated steel sheet according to (1), wherein the Ti compound having slag-forming ability contains a Ti compound other than Ti oxide. (3) The surface-treated steel sheet according to (2), wherein the Ti compound capable of forming slag contains at least one of a Ti chelate compound and TiN. (4) The surface-treated steel sheet according to (3), wherein the Ti chelate compound is at least one of titanium lactate ammonium salt, titanium lactate, titanium triethanolamine, titanium acetylacetonate, titanium tetraacetylacetonate, titanium ethylacetoacetate, titanium phosphate ester complex, and titanium octylene glycolate. (5) The surface-treated steel sheet according to (3) or (4), wherein the content of Ti derived from the Ti chelate in the surface treatment layer is 5 to 40 atomic %. (6) The surface-treated steel sheet according to any one of (1) to (5), wherein the average particle size of the conductive pigment is 0.25 to 2.00 times the average film thickness of the surface treatment layer. (7) The surface-treated steel sheet according to any one of (1) to (6), wherein the conductive pigment is TiN. (8) the surface treatment layer further contains a rust inhibitor excluding a Si compound, The surface-treated steel sheet according to any one of (1) to (7), wherein the content of the rust inhibitor is 10 to 30 mass %. (9) The surface-treated steel sheet according to any one of (1) to (8), wherein the resin is at least one resin selected from epoxy resins and urethane resins. (10) The surface-treated steel sheet according to any one of (1) to (9), wherein the Ti content in the surface treatment layer is 20 atomic % or more. (11) An arc welded joint in which at least two surface-treated steel plates are arc-welded, The surface-treated steel sheet; a weld bead portion formed by the arc welding; slag present on at least a portion of the surface of the weld bead; an electrodeposition coating that covers the surface-treated steel sheet and the weld bead portion; It has The surface-treated steel sheet is A steel plate as a base material; a surface treatment layer located on at least a portion of the steel sheet; It has the surface treatment layer contains a Ti compound having a slag-forming ability to form a Ti-Mn-based slag with elemental Mn, a resin, and a conductive pigment; The Ti content in the surface treatment layer is 10 to 40 atomic %; the content of the conductive pigment in the surface treatment layer is 10 to 40 mass %, The average thickness of the surface treatment layer is 0.5 to 5.0 μm, The arc welded joint, wherein the slag contains at least a Ti-Mn based slag and has an average resistance of 200 MΩ or less. (12) The arc welded joint according to (11), wherein the average particle size of the slag is 1.0 mm or less. [Explanation of symbols]

[0120] 1. Surface-treated steel sheet 10 Base steel plate 20 Surface treatment layer 100 Arc welded joints 110 Weld bead 120 Slag 130 Electrodeposition coating

Claims

1. A steel plate as a base material; a surface treatment layer located on at least a portion of the steel sheet; It has the surface treatment layer contains a Ti compound having a slag-forming ability to form a Ti-Mn-based slag together with elemental Mn, a resin, and a conductive pigment; The Ti content in the surface treatment layer is 10 to 40 atomic %; the content of the conductive pigment in the surface treatment layer is 10 to 40 mass %, The surface-treated steel sheet, wherein the average thickness of the surface treatment layer is 0.5 to 5.0 μm.

2. The surface-treated steel sheet according to claim 1 , wherein the Ti compound having slag-forming ability contains a Ti compound other than Ti oxide.

3. The surface-treated steel sheet according to claim 2 , wherein the Ti compound having slag-forming ability contains at least one of a Ti chelate compound and TiN.

4. 4. The surface-treated steel sheet according to claim 3, wherein the Ti chelate compound is at least one of titanium lactate ammonium salt, titanium lactate, titanium triethanolamine, titanium acetylacetonate, titanium tetraacetylacetonate, titanium ethylacetoacetate, titanium phosphate ester complex, and titanium octylene glycolate.

5. 4. The surface-treated steel sheet according to claim 3, wherein the content of Ti derived from the Ti chelate in the surface treatment layer is 5 to 40 atomic %.

6. 2. The surface-treated steel sheet according to claim 1, wherein the average particle size of the conductive pigment is 0.25 to 2.00 times the average film thickness of the surface treatment layer.

7. The surface-treated steel sheet according to claim 1 , wherein the conductive pigment is TiN.

8. the surface treatment layer further contains a rust inhibitor excluding Si compounds, The surface-treated steel sheet according to claim 1, wherein the content of the rust inhibitor is 10 to 30 mass %.

9. The surface-treated steel sheet according to claim 1 , wherein the resin is at least one resin selected from the group consisting of an epoxy resin and a urethane resin.

10. The surface-treated steel sheet according to claim 1 , wherein the Ti content in the surface treatment layer is 20 atomic % or more.

11. An arc welded joint in which at least two surface-treated steel plates are arc-welded, The surface-treated steel sheet; a weld bead portion formed by the arc welding; slag present on at least a portion of the surface of the weld bead; an electrodeposition coating that covers the surface-treated steel sheet, the weld bead portion, and the slag; It has The surface-treated steel sheet is A steel plate as a base material; a surface treatment layer located on at least a portion of the steel sheet; It has the surface treatment layer contains a Ti compound having a slag-forming ability to form a Ti-Mn-based slag together with elemental Mn, a resin, and a conductive pigment; The Ti content in the surface treatment layer is 10 to 40 atomic %; the content of the conductive pigment in the surface treatment layer is 10 to 40 mass %, the average thickness of the surface treatment layer is 0.5 to 5.0 μm; The arc welded joint, wherein the slag contains at least a Ti-Mn based slag and has an average resistance value of 200 MΩ or less.

12. The arc welded joint according to claim 11, wherein the average particle size of the slag is 1.0 mm or less.

Images