Surface-treated steel sheet and arc welded joint

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

Surface-treated steel plates and arc-welded joints

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

[0002] In general, automotive parts (particularly suspension parts) are manufactured by arc welding raw steel sheets into a desired shape, followed by chemical conversion treatment and electrodeposition coating treatment.

[0003] 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 electrodeposition coating 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 is reduced, for example, by adjusting the amount of Si in the welding wire.

[0004] Meanwhile, 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 steel sheets, plated steel sheets, etc. For example, Patent Document 1 below proposes a composite-coat-treated zinc-containing plated steel material provided with a composite coating containing a basic zirconium compound, a vanadyl-containing compound, a phosphate compound, and a cobalt compound, with the intention of improving not only corrosion resistance but also blackening resistance, paint adhesion, and alkali resistance.

[0005] International Publication No. 2007 / 123276

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

[0007] 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 that can further improve the corrosion resistance of a weld bead portion formed when arc welding is performed, and an arc-welded joint using such a surface-treated steel sheet.

[0008] To solve the above problems, the present inventors conducted extensive research and came up with the idea that if an electrodeposition coating could be formed on a weld bead formed by arc welding, the corrosion resistance of the weld bead could be further improved. Based on this idea, the present inventors then discovered that in order to enable an electrodeposition coating to be formed on a weld bead, it would be necessary to generate conductive slag during arc welding, rather than non-conductive slag. Based on this discovery, the present inventors conducted further research and completed the present invention as described below. The gist of the present invention, which was completed based on the above findings, is as follows.

[0009] (1) A surface-treated steel sheet comprising a steel sheet serving as a substrate and a surface treatment layer located on at least a portion of the steel sheet, the surface treatment layer containing a Zr compound capable of forming a Zr-Mn-Si-based slag with elements Mn and Si, the Zr content in the surface treatment layer being 10 to 100 atomic %, and the average thickness of the surface treatment layer being 0.5 to 5.0 μm. (2) A surface-treated steel sheet according to (1), wherein the surface treatment layer contains the Zr compound capable of forming a slag and a resin, the resin being at least one of an epoxy resin and a urethane resin. (3) A surface-treated steel sheet according to (1), wherein the surface treatment layer contains the Zr compound capable of forming a slag, a resin, and a rust inhibitor excluding a Si compound, the content of the rust inhibitor being 10 to 30 mass %. (4) The surface-treated steel sheet according to (2) or (3), wherein the surface-treatment layer contains the Zr compound capable of forming slag, a resin, and a conductive pigment, and the content of the conductive pigment is 10 to 40 mass%. (5) The surface-treated steel sheet according to (1), wherein the Zr compound capable of forming slag contains a Zr compound other than Zr oxide. (6) The surface-treated steel sheet according to (5), wherein the Zr compound capable of forming 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, and 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 treatment layer. (9) The surface-treated steel sheet according to (4), wherein the conductive pigment is TiN. (10) The surface-treated steel sheet according to (1), wherein the Zr 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, comprising the surface-treated steel plate, a weld bead formed by the arc welding, slag present on at least a part of the surface of the weld bead, and an electrodeposition coating that covers the surface-treated steel plate, the weld bead, and the slag, wherein the surface-treated steel plate comprises a steel plate serving as a base material and a surface treatment layer located on at least a part of the steel plate, and the surface treatment layer forms a Zr-Mn-Si-based slag with elements Mn and Si. an arc welded joint comprising a surface treatment layer containing a Zr compound having a slag-forming ability, the Zr content in the surface treatment layer being 10 to 100 atomic %, the average thickness of the surface treatment layer being 0.5 to 5.0 μm, the slag containing at least a Zr-Mn-Si-based slag, and the average film thickness of the electrodeposition coating located on the surface of the surface-treated steel sheet in the flat portion of the arc welded joint being denoted as ts, and the average film thickness of the electrodeposition coating located on the surface of the weld bead portion being denoted as tb, satisfying the relationship represented by the following formula (1): tb / ts≧1.2...formula (1).

[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.

[0011] 1 is an explanatory diagram schematically showing the structure of a surface-treated steel sheet according to an embodiment of the present invention, and FIG. 2 is an explanatory diagram schematically showing the structure of an arc-welded joint using the surface-treated steel sheet according to the embodiment.

[0012] 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.

[0013] (Regarding Surface-Treated Steel Sheet) Hereinafter, the overall structure of a surface-treated steel sheet according to an embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is an explanatory view schematically showing an example of the configuration of a 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 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.

[0015] 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.

[0016] <Regarding the Base Steel Plate 10> The base steel plate 10 used as the base material 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 plates 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 that has been quenched during hot working and is generally referred to as hot-stamped steel. 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 contents thereof can also be appropriately adjusted.

[0017] Among the high-strength steels described above, for example, high-strength steels having a tensile strength of 780 MPa or more (so-called high-strength steels of 780 MPa class or more) are more preferable because they can 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.

[0018] Furthermore, 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. Furthermore, zinc-based plated steel sheets may contain 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, and arsenic, or may have inorganic substances, such as silica, alumina, and 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.

[0019] 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, approximately 1.0 to 25.0 mm.

[0020] <Regarding the Surface Treatment Layer 20> As schematically 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 Zr compound having the ability to form a Zr-Mn-Si-based slag together with elements Mn and Si.

[0021] FIG. 2 is an explanatory diagram schematically illustrating the structure of an arc-welded joint using a surface-treated steel sheet according to this embodiment. When the surface-treated steel sheet 1 according to this embodiment is subjected to arc welding, the Zr compounds present in the surface treatment layer 20 react with Mn and Si in the steel sheet and welding wire. As a result, as shown in FIG. 2, a Zr-Mn-Si-based slag 120 is formed on the weld bead 110. The Zr-Mn-Si-based slag in this embodiment (hereinafter sometimes referred to as slag 120) is an oxide containing Zr, Mn, and Si on the steel sheet surface. For convenience of explanation, a method for identifying the slag 120 will be described later. Because the Zr-Mn-Si-based slag 120 exhibits electrical conductivity, the electrical conductivity of the weld bead 110 is ensured. Therefore, when applying electrodeposition coating 130 to weld bead 110, poor film formation in the area where slag 120 exists is suppressed, and electrodeposition coating 130 can be formed on weld bead 110. This improves the corrosion resistance of weld bead 110 formed during arc welding.

[0022] Here, the Zr-Mn-Si slag being conductive means that the average resistance value of the Zr-Mn-Si slag is 150 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, measurements are taken at three arbitrary locations spaced at least 5 mm apart with the tester, and the 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.

[0023] The surface treatment layer 20 according to this embodiment may be composed solely of the Zr compound having the above-described slag-forming ability, or may be composed of such a Zr compound and other components (e.g., components such as resin). From this perspective, the Zr content in the surface treatment layer 20 according to this embodiment is within the range of 10 to 100 atomic %. For convenience of explanation, the slag-forming ability will be described in detail later.

[0024] If the Zr content in the surface treatment layer 20 is less than 10 atomic %, the Zr compound content in the surface treatment layer 20 is too low, resulting in a small amount of Zr-Mn-Si slag formed during arc welding. This makes it impossible to improve the corrosion resistance of the weld bead formed by arc welding. If the Zr content in the surface treatment layer 20 is 10 atomic % or more, it is possible to form Zr-Mn-Si slag by arc welding to an extent that allows electrodeposition coating, thereby improving the corrosion resistance of the weld bead. The Zr content in the surface treatment layer 20 is preferably 20 atomic % or more, and more preferably 40 atomic % or more.

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

[0026] On the other hand, when the surface treatment layer 20 according to the present embodiment further contains a component (e.g., a resin) other than the Zr compound having slag-forming ability as described above, it is preferable to fully exhibit the effects that can be obtained by including a component that can ensure electrical conductivity in addition to the Zr compound (e.g., improved adhesion between the electrodeposition coating and the surface treatment layer 20, improved spot weldability, corrosion resistance of the surface-treated steel sheet 1, etc.). From this perspective, when the surface treatment layer 20 according to the present embodiment further contains a component other than the Zr compound having slag-forming ability as 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 described below, 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 Zr content due to evaporation of components such as water in the treatment liquid. Therefore, the Zr content in the treatment liquid before heat drying may be less than 10 atomic %. In one embodiment, the Zr content in the surface treatment liquid before heat drying is set to 3 atomic % or more. This ensures that the Zr content in the surface treatment layer 20 after heat drying is 10 atomic % or more.

[0028] Here, the Zr 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 exists, and observing the sample using a scanning electron microscope (SEM) equipped with an electron probe microanalyzer (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 thickness-wise cross section of the surface treatment layer 20. Next, the cross section is embedded in resin so that the thickness-wise cross section is visible, and then polished. The resulting polished surface is then observed using SEM-EPMA. More specifically, EPMA mapping analysis of the thickness-wise cross section (polished surface) of the surface treatment layer 20 can be performed at a magnification of 5000x (acceleration voltage: 15 kV, beam diameter: 10 nm, measurement pitch: 1 nm). The boundary between the surface treatment layer 20 and the base steel sheet 10 can be clearly determined using SEM. In this case, Zr is selected as the element to be detected, and mapping images of the location and element concentration of the Zr element are taken. From the obtained results, the Zr content in the field of interest can be calculated. Such measurements are carried out 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 measured at each location may be taken as the Zr content in the surface treatment layer 20 .

[0030] [Regarding the average thickness of the surface treatment layer 20] The average thickness (thickness d in FIG. 1 ) of the surface treatment layer 20 containing the Zr compound 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 thickness of 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 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 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, 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 the surface of the surface treatment layer 20 and observing the cut surface from 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 from the cross-sectional direction using a microscope. Examples of methods for preparing a sample for observing the cut surface from 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 any multiple 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 by the number of measurement locations.

[0033] [Specific Examples of Zr Compounds Capable of Forming Zr—Mn—Si-Based Slag] Examples of Zr compounds capable of forming Zr—Mn—Si-based slag that are contained in the surface treatment layer 20 according to this embodiment include Zr compounds other than Zr oxides. Zr oxides, which are composed of Zr atoms and oxygen atoms, do not form conductive slag with Mn or Si during arc welding. For this reason, Zr oxides cannot be used as the Zr compound capable of forming slag according to this embodiment.

[0034] An example of a Zr compound capable of forming a Zr—Mn—Si-based slag is ammonium zirconium carbonate. Ammonium zirconium carbonate can be used alone to form the surface treatment layer 20. During arc welding, ammonium zirconium carbonate reacts with Mn and Si derived from the steel sheet and welding wire to form a conductive Zr—Mn—Si-based slag. The Zr compound capable of forming a Zr—Mn—Si-based slag is not limited to ammonium zirconium carbonate, as long as it is not a Zr oxide.

[0035] Examples of Zr compounds capable of forming Zr-Mn-Si slag include various Zr chelate compounds. In this embodiment, the term "Zr chelate compound" refers to a compound in which various ligands form complexes sandwiching Zr ions, forming a ring containing Zr atoms and coordinating atoms, and is different from Zr oxides (compounds consisting of Zr atoms and oxygen atoms).

[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 electrodeposition coating when the electrodeposition coating is applied.

[0037] Furthermore, among various Zr chelate compounds, the use of at least one of zirconium lactate ammonium salt, zirconyl chloride compound, zirconium lactate, zirconium triethanolamine, zirconium monoacetylacetonate, zirconium tetraacetylacetonate, and zirconium ethylacetoacetate can further improve the adhesion of electrodeposition coating. Among these, zirconium lactate ammonium salt and zirconyl chloride compound are water-soluble compounds, and therefore, when various resins are used as components of the surface treatment layer 20, they disperse well in the resin. As a result, the slag generated by arc welding is refined, further improving the adhesion of electrodeposition coating.

[0038] Examples of Zr compounds capable of forming Zr-Mn-Si slag include various alkoxides, such as normal propyl zirconate and normal butyl zirconate.

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

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

[0041] First, the surface-treated steel sheet 1 is cut in the thickness direction at any 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 Zr content of the surface treatment layer 20. EPMA mapping analysis is performed at several (e.g., three) arbitrary locations on the obtained cross-section, with Zr, Si, O, N, and C as the target elements. The magnification is 5000x (acceleration voltage: 15 kV). The presence or absence of Zr compounds can be confirmed by whether or not the presence of elemental Zr can be confirmed in the EPMA maps obtained for all Zr. Furthermore, if the presence of elemental Zr can be confirmed in all the obtained EPMA maps but the elemental Zr is not distributed in a particulate form, it can be determined that various Zr chelate compounds are used. The term "particulate distribution" refers to the observation at the above magnification showing visible aggregates of elemental Zr, with an average particle size of approximately 0.1 μm or more. Furthermore, whether or not an oxide of Zr is used can be determined by referring to the EPMA map of element Zr and the EPMA map of element O, based on whether or not element O is present at a position where element Zr can be confirmed.

[0042] Even if the element Zr is not distributed in a particulate form in the EPMA map, it is possible to obtain nano-zirconia (ZrO having a particle size on the nano-order). ２ Therefore, in order to distinguish between a Zr chelate compound and nano-zirconia, it is preferable to use measurement by X-ray photoelectron spectroscopy (XPS) in addition to the above-mentioned EPMA mapping analysis. In the results of such XPS measurement, ZrO ２ By focusing on whether or not a Zr-based peak other than the peak derived from the Zr—O—R bond (R represents an alkyl group)) is detected, it becomes easier to distinguish between a Zr chelate compound and nanozirconia.

[0043] [Components Other Than Zr Compound] The surface treatment layer 20 according to this embodiment may contain various components such as resin, conductive pigment, rust inhibitor, etc. in addition to the Zr compound.

[0044] <<Resin>> The surface treatment layer 20 according to this embodiment may contain various resins as a binder component for holding the Zr compound. Examples of such resins that can be used include polymer polyester resins, polyester resins, acrylic resins, epoxy resins, urethane resins, and fluororesins. 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 resins, methylated melamine resins, butylmethyl-mixed melamine resins, urea resins, isocyanate resins, or mixtures of these resins.

[0045] 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.

[0046] <Conductive Pigment> The surface treatment layer 20 according to this embodiment may further contain a conductive pigment in addition to the Zr compound and resin. When the surface treatment layer 20 according to this embodiment contains a specific amount of conductive pigment, 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.

[0047] Here, the content of the conductive pigment is preferably 10 to 40 mass % with respect to the total mass of the solid content of the surface treatment layer 20. By making the content of the conductive pigment 10 mass % or more, it is possible to make the electrical resistance value exhibited by the surface treatment layer 20 more preferable, and it is also possible to improve spot weldability. The content of the conductive pigment is more preferably 15 mass % or more.

[0048] On the other hand, by setting the content of the conductive pigment to 40% by mass or less, it is possible to make the electrical resistance value exhibited by the surface treatment layer 20 more preferable and improve spot weldability while suppressing a decrease in the corrosion resistance of the surface treatment layer 20. The content of the conductive pigment 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 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 FIG. 1).

[0050] 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.

[0051] 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.

[0052] Here, examples of the conductive pigments include TiN, ZnO, and VB. ２ Examples of suitable ceramic particles include 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 Si oxides, which are non-conductive during arc welding and reduce the electrodeposition paintability of arc-welded joints.

[0053] When a compound containing Ti, excluding titanium oxide, is used as the conductive pigment, it can react with Mn derived from the steel sheet or welding wire during arc welding to form a conductive Ti-Mn-based slag, similar to the Zr compound having slag-forming ability described above. Therefore, by using a compound containing Ti, excluding titanium oxide, as the conductive pigment, it becomes easier to form a film of electrodeposition coating at the weld bead, and the corrosion resistance of the weld bead can be further improved.

[0054] 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.

[0055] First, the surface treatment layer 20 is cut at an angle perpendicular to the surface of the surface treatment layer 20 of interest, and a sample for cross-sectional observation is collected. For a sample for identifying the type and average particle size of the conductive pigment, the obtained sample for cross-sectional 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. Here, the size of the observation field is set to 25 μm × 20 μm, and the magnification is set to 5000 times. Several particles (e.g., five particles) present in the observation field are randomly selected, and the circle-equivalent diameters of each particle are determined. The average value of these diameters is used as the average particle diameter. 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.

[0056] The content of the conductive pigment 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 3 or more, and the average number of particles in each field of view divided by the number of fields of view can be calculated.

[0057] 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 the pigments and the amount of paint attached to the base steel sheet. Furthermore, when the amounts of each conductive pigment are unknown, the average thickness of the surface treatment layer is calculated using the above-described method, 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 a 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 an apparatus such as a particle image analyzer Morphologi G3 ("MORPHOLOGI" is a registered trademark) manufactured by Malvern, particles in the surface treatment agent diluted to an appropriate concentration are individually identified and counted by image analysis, thereby calculating the amount of each particle in the calculated volume of the surface treatment layer. Next, the number of each particle in the surface treatment layer can 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.

[0058] <<Rust Inhibitor>> The surface treatment layer 20 according to this embodiment may further contain a rust inhibitor in addition to the Zr compound and resin 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.

[0059] 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 Si oxide, which exhibits non-conductivity, during arc welding, which reduces the electrodeposition paintability of the arc-welded joint, which is undesirable.

[0060] 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.

[0061] On the other hand, by setting the content of the rust inhibitor to 30 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 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, 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.

[0063] 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, when 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 welded joint in the same manner as for the conductive pigment described above. When 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 in 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.

[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 as described above.

[0065] Furthermore, the surface treatment layer 20 according to the present 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] The surface treatment layer 20 according to this embodiment has been described in detail above with reference to FIG.

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

[0068] (Regarding Arc-Welded Joint) 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. For convenience, the following description will be given appropriately using a coordinate system such as that shown in Fig. 2.

[0069] 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 with 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.

[0070] Here, the surface-treated steel sheets 1A and 1B constituting the arc welded joint 100 are the surface-treated steel sheets 1 having the surface treatment layer 20 as previously explained, and therefore detailed explanations thereof will be omitted below.

[0071] When taking a sample of the surface treatment layer 20 in order to identify the various characteristics (e.g., components, 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.

[0072] <Regarding the weld bead 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 sheets 1A and 1B. In Fig. 2, the joint 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 joint 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 joined by the weld bead 110.

[0073] Here, when identifying a portion of the arc-welded joint 100 of interest that corresponds to the weld bead 110, the portion can be easily visualized by etching using an etching solution. For example, the etching solution may be 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 (e.g., a common dishwashing detergent) with 2,400 cc of water.

[0074] <Regarding Slag 120> 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. This slag 120 is present on at least a portion of the surface of the weld bead portion 110.

[0075] The slag 120 in the arc welded joint 100 according to this embodiment contains conductive Zr-Mn-Si-based slag because the surface treatment layer 20 of the surface-treated steel sheet 1, which is the raw material for the joint, contains Zr compounds capable of forming Zr-Mn-Si-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.

[0076] In addition, the slag 120 according to this embodiment may contain, in addition to the Zr-Mn-Si-based slag, Si-based slag, Si-Mn-based slag, Mn-based slag, Ti-Mn-based slag, etc., as well as impurities.

[0077] Here, in the arc welded joint 100 according to this embodiment, the location of the slag 120 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 the 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 the 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.

[0078] Furthermore, 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, a portion of the weld bead 110 where the slag 120 is present is cut to an appropriate size, embedded in resin so that the cross-section in the thickness direction is visible, and the cross-section is polished. The resulting polished surface is then observed using a SEM-EPMA. More specifically, the resulting cross-section is subjected to EPMA mapping analysis at a magnification of 1000x (acceleration voltage: 15 kV). Zr, Mn, Si, O, and Ti 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.

[0079] <Regarding the Electrodeposition Coating 130 > The electrodeposition coating 130 is provided so as 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 sheet 1 serving as the material for the joint has a surface treatment layer 20 containing a Zr compound having slag-forming ability as described above, thereby realizing the following state. That is, in the arc welded joint 100 according to this embodiment, a relationship is established between the portion of the surface treatment layer 20 that remains after arc welding (hereinafter referred to as the "flat portion" of the arc welded joint) and the weld bead portion 110, as shown in the following formula (101) regarding electrical resistance. In this formula (101), the average electrical resistance value of the flat portion of the arc welded joint 100 is represented as Rs (unit: Ω), and the average electrical resistance value of the weld bead portion 110 is represented as Rb (unit: Ω).

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

[0082] Because this relationship holds, in the arc welded joint 100 according to this embodiment, the average thickness of the electrodeposition coating 130B formed on the surface of the weld bead 110 is thicker than the average thickness of the electrodeposition coating 130A formed on the surface of the flat portion, and the relationship shown in the following formula (103) holds: In the following formula (103), the average thickness of the electrodeposition coating 130A formed on the surface of the flat portion is represented as ts [unit: μm], and the average thickness of the electrodeposition coating 130B formed on the surface of the weld bead 110 is represented as tb [unit: μm].

[0083] tb / ts≧1.2...Formula (103)

[0084] In the arc welded joint 100 according to this embodiment, the relationship shown in the above formula (103) holds, and therefore 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 more, and even more preferably 1.5 or more.

[0086] On the other hand, the upper limit of the value of (tb / ts) is not particularly specified, and the larger the value, the better, but the upper limit is substantially about 2.0.

[0087] Here, the electrical resistance values ​​of the flat portion and the weld bead portion as described above can be calculated by measuring a plurality of arbitrary positions (e.g., three positions) belonging to each portion using a commercially available resistance meter and averaging the results obtained by the number of measurement positions.

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

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

[0090] 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.

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

[0092] (Regarding the manufacturing method of the surface-treated steel sheet) An example of a manufacturing method of the surface-treated steel sheet 1 according to this embodiment will be briefly described below. First, a steel sheet serving as a substrate is subjected to various pretreatments, including alkaline degreasing treatment, water washing treatment, pickling treatment, etc., to obtain a clean steel sheet surface. Thereafter, the surface of this steel sheet may be subjected to various plating treatments as necessary. In this manner, a substrate steel sheet 10 of the surface-treated steel sheet 1 according to this embodiment can be obtained.

[0093] 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 above-mentioned chemical conversion treatment agent 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.

[0094] 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 the chemical conversion coating 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 Zr compound described above (more preferably, the Zr compound described above 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.

[0095] 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.

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

[0097] (Regarding the manufacturing method of the arc-welded joint) 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.

[0098] Welding current: 140 A, welding voltage: 11.4 V, welding speed: 80 cm / min. Welding gas: 20% CO ２ +Ar, gas flow rate: 20 L / min Welding wire: mild steel wire (YM-24T, φ1.2 mm, manufactured by Nippon Steel Welding Co., Ltd. (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] 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.

[0100] An example of the method for manufacturing an arc welded joint according to this embodiment has been described above.

[0101] 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.

[0102] (Test Examples) 1. Sample Preparation Preparation 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 base 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.

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

[0104] As the rust inhibitor, calcium vanadate and aluminum tripolyphosphate (both general reagents) were used, and as the conductive pigment, ZnO and TiN (both general reagents) having the average particle sizes shown in Table 1 below were used.

[0105]

[0106] [Arc welding] 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 from the surface-treated steel sheets of each level, and these two test pieces were overlapped in the longitudinal direction with an overlap width of 10 mm, and lap fillet arc welding was performed. The welding wire used and the arc welding conditions were as follows: Mild steel wire (YM-24T, manufactured by Nippon Steel Welding Co., Ltd.) Shielding gas: Ar + 20% CO ２ , welding current 140A, welding voltage 11.4V, welding speed 80cm / min

[0107] [Electrodeposition Coating] Each arc-welded sample obtained as described above was degreased by immersion using an alkaline degreasing solution manufactured by Nippon Parkerizing Co., Ltd. for 2 minutes at 40°C. Surface conditioning was then performed using a surface conditioning treatment agent manufactured by Nippon Paint Co., Ltd. at 25°C for 15 seconds. 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, followed by rinsing with water and drying. Subsequently, electrodeposition coating was performed using a cationic electrodeposition paint manufactured by Nippon Paint at 35°C, with the voltage increased to 180V over 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] In this manner, a plurality of samples of arc-welded joints were prepared for each level.

[0109] 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 film thickness ratio tb / ts of the electrodeposition coating was measured using the method described above. The results are summarized in Table 1 above.

[0110] The arc-welded joints were evaluated for corrosion resistance after painting of the weld bead, adhesion of the coating to the flat portion, corrosion resistance after painting of the flat portion, and spot weldability. The evaluation methods are as follows. The results are summarized in Table 2.

[0111] [Corrosion Resistance of Weld Beads After Painting] For each arc-welded joint after electrodeposition painting, the end faces of the test specimens and both ends of the weld bead were sealed, and then corrosion resistance was evaluated. Corrosion resistance was evaluated using the JASO M609-91 test. Specifically, the corrosion resistance was evaluated after 120 cycles, with one cycle consisting of 2 hours of salt spray (5% NaCl) at 35°C, 4 hours of dry (25% humidity) at 60°C, and 2 hours of wet (95% humidity) at 50°C. Evaluation was performed according to the following evaluation criteria, and a rating 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 more than 30% and less than 40% Rating 3: Red rust area ratio is more than 40% and less than 50% Rating 2: Red rust area ratio is more than 50% and less than 70% Rating 1: Red rust area ratio is more than 70%

[0112] [Paint adhesion to flat area] A flat area (a portion 20 mm or more away from the toe of the weld bead) was cut out from each arc-welded joint after electrodeposition coating, and the corrosion resistance of the flat area 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 x 120 mm (area A = 7200 mm ２ A polyester tape was applied to the front surface of the area (A) and then the tape was peeled off. The area B of the coating film that peeled off when the tape was peeled off was determined, and the coating film peeling rate (%) was calculated based on the following formula (*): Coating film peeling rate (%) = (B / A) x 100 (formula (*))

[0113] [Corrosion resistance after painting of flat portion] For each arc-welded joint after electrodeposition painting, the corrosion resistance after painting was evaluated at the flat portion (a portion 20 mm or more away from the toe of the weld bead). The corrosion resistance was evaluated according to the JASO M609-91 test. Specifically, a cross-cut was made in the electrodeposition coating with a cutter at a position 20 mm or more away from the toe of the weld bead, and the blister width of the coating film from the cross-cut after 120 cycles of corrosion testing (unit: mm, average value of the maximum three points on one side) was evaluated.

[0114] [Spot Weldability] Before arc welding and before electrodeposition coating, the surface-treated steel sheets were sheared to a size of 30 × 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 × (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. <Measurement conditions> Current: DC Electrode: Chromium copper, DR (tip 6 mmφ, 40R) Pressure: 400 kgf (1 kgf is approximately 9.8 N) Current application time: 240 msec

[0115]

[0116] As is clear from Table 2 above, the levels corresponding to the examples of the present invention showed excellent corrosion resistance of the weld bead after painting, while the levels corresponding to the comparative examples 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 failed to achieve 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 failed to achieve 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, and as a result, weldability was reduced, resulting in increased spatter during arc welding and failing to achieve sufficient corrosion resistance. Comparative Example No. 19 contained zirconium oxide in the surface treatment layer, and therefore conductive slag was not formed during arc welding, resulting in many electrodeposition coating defects and failing to achieve sufficient corrosion resistance. Comparative Example No. 20 contained a Zr compound, but the content was insufficient, resulting in insufficient formation of conductive slag, and therefore many electrodeposition coating defects and failing to achieve sufficient corrosion resistance. Examples No. 21 and No. 22 differed in the content of Zr compound, and Example No. 22, which contained 20 atomic % or more of Zr compound, was different from Example No. 21 in that it contained 20 atomic % or more of Zr compound. The corrosion resistance was significantly superior to that of Example 21. Examples No. 23 to No. 27 used various Zr chelate compounds as the Zr compound, which further improved the adhesion of the electrodeposition coating and achieved favorable corrosion resistance after painting. In particular, Examples No. 23 and No. 24, which used aqueous Zr chelate compounds, zirconyl chloride compound and zirconium lactate, achieved better corrosion resistance after painting due to the fine slag formed by the well-dispersed Zr chelate compound in the resin. Examples No. 24 and No. 27 had average particle size to average thickness ratios within a preferred range, which prevented a decrease in adhesion between the surface treatment layer and the electrodeposition coating while improving spot weldability, resulting in favorable corrosion resistance after painting. Example No. 26 had average particle size to average thickness ratios within a more preferred range, which prevented a decrease in adhesion between the surface treatment layer 20 and the electrodeposition coating while improving spot weldability, resulting in favorable corrosion resistance after painting. In Example 25, the ratio of the average particle size to the average thickness was within the most preferable range, and therefore, while improving spot weldability, it was possible to further prevent a decrease in adhesion between the surface treatment layer 20 and the electrodeposition coating, and more preferable corrosion resistance after coating was obtained.

[0118] 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.

[0119] 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.

[0120] 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 are apparent to those skilled in the art from the description of this specification, in addition to or in place of the above-described effects.

[0121] The following configurations also fall within the technical scope of the present invention. (1) A surface-treated steel sheet comprising a steel sheet serving as a substrate and a surface treatment layer located at least partially on the steel sheet, wherein the surface treatment layer contains a Zr compound capable of forming a Zr-Mn-Si-based slag with elements Mn and Si, the Zr content in the surface treatment layer being 10 to 100 atomic %, and the average thickness of the surface treatment layer being 0.5 to 5.0 μm. (2) A surface-treated steel sheet according to (1), wherein the surface treatment layer contains the Zr compound capable of forming a slag and a resin, the resin being at least one of an epoxy resin and a urethane resin. (3) A surface-treated steel sheet according to (1) or (2), wherein the surface treatment layer contains the Zr compound capable of forming a slag, a resin, and a rust inhibitor excluding a Si compound, the content of the rust inhibitor being 10 to 30 mass %. (4) The surface-treated steel sheet according to any one of (1) to (3), wherein the surface-treatment layer contains the Zr compound capable of forming slag, a resin, and a conductive pigment, and the content of the conductive pigment is 10 to 40 mass%. (5) The surface-treated steel sheet according to any one of (1) to (4), wherein the Zr compound capable of forming slag is a Zr compound other than Zr oxide. (6) The surface-treated steel sheet according to any one of (1) to (5), wherein the Zr compound capable of forming slag is 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, and 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 treatment layer. (9) The surface-treated steel sheet according to (8) or (9), wherein the conductive pigment is TiN. (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 % or more.(11) An arc welded joint formed by arc welding at least two surface-treated steel plates, comprising: the surface-treated steel plate; a weld bead portion formed by the arc welding; slag present on at least a portion of the surface of the weld bead portion; and an electrodeposition coating covering the surface-treated steel plate, the weld bead portion, and the slag; the surface-treated steel plate comprises: a steel plate serving as a base material; and a surface treatment layer located on at least a portion of the steel plate; the surface treatment layer contains a Zr compound having a slag-forming ability to form Zr-Mn-Si-based slag with elements Mn and Si; the Zr content in the surface treatment layer is 10 to 100 atomic %; the average thickness of the surface treatment layer is 0.5 to 5.0 μm; and the slag contains at least Zr-Mn-Si-based slag. An arc welded joint that satisfies the relationship expressed by the following formula (1), where the average film thickness of the electrodeposition coating located on the surface of the coated steel sheet in the flat portion of the arc welded joint is denoted as ts and the average film thickness of the electrodeposition coating located on the surface of the weld bead portion is denoted as tb: tb / ts≧1.2...formula (1).

[0122] REFERENCE SIGNS LIST 1 Surface-treated steel sheet 10 Base steel sheet 20 Surface treatment layer 100 Arc-welded joint 110 Weld bead portion 120 Slag 130 Electrodeposition coating

Claims

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 between the elements Mn and Si. The Zr compound having slag-forming ability is at least one of zirconium lactate ammonium salt, zirconium lactate, zirconium triethanolamine, zirconium monoacetylacetonate, or zirconium ethylacetoacetate. The Zr content in the 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 a resin, The surface-treated steel sheet according to claim 1, wherein the Zr compound having slag-forming ability is a zirconium lactate ammonium salt.

3. A steel plate which serves as the base material, 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 having the ability to form a Zr-Mn-Si slag between the elements Mn and Si, and a conductive pigment. 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. A surface-treated steel sheet having a conductive pigment content of 10 to 40% by mass.

4. The surface-treated steel sheet according to claim 3, wherein the average particle size of the conductive pigment is 0.25 to 2.00 times the average thickness of the surface-treated layer.

5. The surface-treated steel sheet according to claim 3, wherein the conductive pigment is TiN.

6. The aforementioned surface treatment layer contains a resin, The surface-treated steel sheet according to any one of claims 1 to 5, wherein the resin is at least one of epoxy resin or urethane resin.

7. The aforementioned surface treatment layer contains a resin and a rust inhibitor excluding Si compounds. The surface-treated steel sheet according to any one of claims 1 to 5, wherein the content of the rust inhibitor is 10 to 30% by mass.

8. The aforementioned surface treatment layer contains a resin and a conductive pigment. The surface-treated steel sheet according to claim 1 or 2, wherein the content of the conductive pigment is 10 to 40% by mass.

9. The surface-treated steel sheet according to any one of claims 1 to 5, wherein the Zr content in the surface treatment layer is 20 atomic percent or more.

10. 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)