Surface-treated steel sheet

Abstract

A surface-treated steel sheet having a galvanized steel sheet and a coating film that is disposed on at least one main surface of the galvanized steel sheet, the average film thickness of the coating film being 0.5-3 μm, the coating film containing a binder resin and 0.5-2.0 mass% of a lubricant, and the ratio of the maximum film thickness of the coating film to the minimum film thickness being 10-100.

Description

[Technical Field]

[0001] The present disclosure relates to a surface-treated steel sheet. [Background technology]

[0002] The surface-treated steel sheet is, for example, press-formed and then assembled into a desired shape by spot welding or the like, and then electro-deposition coated, or if electro-deposition coating is omitted, an undercoat is applied, and then the sheet is used as an automobile part. Furthermore, the corrosion resistance of automotive components is often ensured by a chemical conversion coating applied in a chemical conversion treatment process and an electrodeposition coating applied in a subsequent electrodeposition coating process. However, at the joints (sheet joints) of formed materials of surface-treated steel sheets, particularly at the joints of the inner sheets of bag-shaped components and at folded hems, there are sometimes areas where the chemical conversion coating and electrodeposition coating do not wrap around. In such cases, the joints of formed materials are likely to be exposed to a corrosive environment in their bare state. For this reason, there is a demand for surface-treated steel sheets that can ensure corrosion resistance. Furthermore, such surface-treated steel sheets are assembled into a desired shape by, for example, press forming, spot welding, etc., and then electro-deposition coated, or if electro-deposition coating is omitted, an undercoat is applied. Therefore, it is necessary to improve press formability, make the coating conductive so that resistance welding or electro-deposition coating can be performed, and impart corrosion resistance.

[0003] Thus, surface-treated steel sheets must have an electrically conductive coating film to improve weldability so that resistance welding is possible, and must also be provided with corrosion resistance.

[0004] For example, Patent Document 1 describes a surface-treated steel sheet having a coating film on at least one side of the plated steel sheet, the coating film containing a binder resin, V-containing non-oxide ceramic particles (excluding VC particles), and doped zinc oxide particles, the content of the V-containing non-oxide ceramic particles and the doped zinc oxide particles relative to the coating film satisfying the following formula: C Zn ≧10.0 (1) CV ≦0.5 C Zn···(2) CV ≦70°C Zn ···(3) CV ≥ 0.125 C Zn ···(4) CV ≧2.0 (5) Here, CV means the content (mass%) of the non-oxide ceramic particles containing V, and CZn means the content (mass%) of the doped zinc oxide particles." has been proposed. According to Patent Document 1, it is described that "a surface-treated steel sheet having excellent corrosion resistance and weldability before electrodeposition coating can be provided."

[0005] Furthermore, automobile parts are manufactured by assembling a surface-treated steel sheet into a desired shape by, for example, press-forming it and then spot welding it, etc. Therefore, the surface-treated steel sheet needs to be provided with sliding properties in order to improve press-formability. [Prior art documents] [Patent documents]

[0006] Patent Document 1: International Publication No. 2018 / 092244 Summary of the Invention [Problem to be solved by the invention]

[0007] Surface-treated steel sheets are widely used for applications including automobile components, machine components, home appliance components, and building materials, and are required to have improved corrosion resistance, weldability, and sliding properties. However, although various research and development efforts have been made up to now, including the one described in Patent Document 1, there is currently a demand for further improvements in the corrosion resistance, weldability, and sliding properties of surface-treated steel sheets due to recent demands.

[0008] Therefore, an object of the present disclosure is to provide a surface-treated steel sheet that is excellent in corrosion resistance, weldability, and sliding properties. [Means for solving the problem]

[0009] Means for solving the above problems include the following aspects. <1> A zinc-plated steel sheet, a coating film disposed on at least one main surface of the zinc-based plated steel sheet; and The average thickness of the coating film is 0.5 μm or more and 3 μm or less, the coating film contains a binder resin and 0.5 to 2.0 mass % of a lubricant, The ratio of the maximum thickness to the minimum thickness of the coating film is 10 times or more and 100 times or less. Surface-treated steel sheet. <2> the zinc-based coated steel sheet is a galvannealed steel sheet, The ratio ζ / δ of the ζ phase to the δ phase in the galvannealed layer is 0.1 to 0.4. <1> The surface-treated steel sheet according to claim 1. <3> the coating film contains the binder resin, the conductive pigment, the lubricant, and the rust inhibitor; the conductive pigment is a doped oxide particle; The conductive pigment has an average particle size of 0.1 μm or more and 2 μm or less, The content of the conductive pigment is 2 to 30 mass % of the coating film. <1> or <2> The surface-treated steel sheet according to claim 1. <4> the doped oxide particles are doped zinc oxide particles; <3> The surface-treated steel sheet according to claim 1. <5> The lubricant is a polyolefin wax. <3> or <4> The surface-treated steel sheet according to claim 1. <6> The rust inhibitor is an organic acid. <3> ~ <5> The surface-treated steel sheet according to any one of the above items. <7> The coating film does not contain any inorganic substances other than the Zn-containing compound containing doped zinc oxide particles, or if it does contain any inorganic substances, the amount is 1% by mass relative to the coating film. below Including, <1> ~ <6> The surface-treated steel sheet according to any one of the above items. <8> The surface roughness Ra of the coating film is 0.1 to 10 μm. <1> ~ <7> The surface-treated steel sheet according to any one of the above items. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide a surface-treated steel sheet that is excellent in corrosion resistance, weldability, and sliding properties. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of a surface-treated steel sheet according to this embodiment. [Figure 2] FIG. 2 is a schematic cross-sectional view showing another example of the surface-treated steel sheet according to this embodiment. [Figure 3] FIG. 3 is a cross-sectional view showing an example of a coated member according to this embodiment. [Figure 4] FIG. 4 is a schematic plan view showing a bonded test piece produced in the corrosion resistance test 2 of the example. [Figure 5] FIG. 5 is a schematic side view showing a bonded test piece prepared in the corrosion resistance test 2 of the example. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment that is an example of the present disclosure will be described. In this specification, when the lower limit of the content of each element in the chemical composition is expressed as "0", this means that the element is an optional component and does not have to be contained. In a numerical range expressed using "to", when the numbers before and after "to" are not followed by "greater than" or "less than", it means a range that includes those numbers as the lower and upper limits. Furthermore, when the numbers before and after "to" are followed by "greater than" or "less than", it means a range that does not include those numbers as the lower or upper limits. In numerical ranges described in stages, the upper limit of a certain numerical range may be replaced by the upper limit of another numerical range described in stages, or may be replaced by a value shown in an example. Also, the lower limit of a certain numerical range may be replaced by the lower limit of another numerical range described in stages, or may be replaced by a value shown in an example. The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0013] <Surface-treated steel sheet> The surface-treated steel sheet according to this embodiment is A zinc-plated steel sheet, a coating film disposed on at least one main surface of a zinc-based plated steel sheet; (See Figures 1 and 2.) The average thickness of the coating film is 0.5 μm or more and 3 μm or less, The ratio of the maximum thickness to the minimum thickness of the coating film is 10 times or more and 100 times or less. 1 and 2, 10 denotes a surface-treated steel sheet, 11 denotes a base steel sheet in a zinc-based plated steel sheet (a steel sheet on which a zinc-based plated layer is to be formed), 12 denotes a zinc-based plated layer of the zinc-based plated steel sheet, and 13 denotes a coating film. The main surfaces of the zinc-based plated steel sheet refer to two surfaces that face each other in the thickness direction of the zinc-based plated steel sheet.

[0014] The surface-treated steel sheet according to this embodiment has the above-described configuration, and is excellent in corrosion resistance, weldability, and sliding properties. The surface-treated steel sheet according to this embodiment was discovered based on the following findings.

[0015] The inventors have investigated the corrosion resistance, weldability, and sliding properties of surface-treated steel sheets, and have obtained the following findings. To date, surface-treated steel sheets used for applications such as automotive parts have been provided with a coating film on a plated steel sheet, and corrosion resistance, weldability, and sliding properties have been ensured by adjusting the type and amount of conductive pigment in the coating film and the coating film thickness. However, further investigation is needed to improve corrosion resistance, weldability, and sliding properties. Therefore, the zinc-based plated steel sheet is a zinc-based plated steel sheet having a coating film with an average thickness of 0.5 to 3 μm on at least one main surface, and the ratio of the maximum thickness to the minimum thickness of the coating film is set to 10 to 100 times. That is, concaves and convexes are formed on the surface of the plating layer of the zinc-based plated steel sheet, and the coating material is applied to the surface, allowing the coating material to penetrate into the concaves and convexes. The coating is then dried and baked to form a coating film, and the ratio of the maximum thickness to the minimum thickness of the coating film can be set to 10 to 100 times. This configuration ensures corrosion resistance in the concave parts of the plating layer where the coating film is thick, while ensuring electrical conductivity in the convex parts of the plating layer where the coating film is thin, thereby improving the corrosion resistance and weldability of the steel sheet as a whole.

[0016] From the above findings, it has been found that the surface-treated steel sheet according to this embodiment is a surface-treated steel sheet that is excellent in all of corrosion resistance, weldability, and sliding properties.

[0017] Hereinafter, the surface-treated steel sheet according to this embodiment will be described in detail.

[0018] (Zinc-plated steel sheet) A zinc-based plated steel sheet is a plated steel sheet having a zinc-based plating layer containing zinc as the maximum component on one or both sides of a base steel sheet. Examples of zinc-based plated steel sheets include hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, zinc-aluminum-magnesium-based hot-dip galvanized steel sheets, zinc-aluminum-magnesium-silicon-based hot-dip galvanized steel sheets, zinc-aluminum hot-dip galvanized steel sheets, zinc-aluminum-silicon-based hot-dip galvanized layers, etc. Examples of zinc-based plated steel sheets also include electrogalvanized steel sheets such as electrogalvanized steel sheets and electrozinc-nickel alloy-plated steel sheets. Among these, from the viewpoint of improving corrosion resistance, a galvannealed hot-dip galvanized steel sheet is preferred as the zinc-based plated steel sheet.

[0019] Here, the base steel sheet of the zinc-based plated steel sheet (the steel sheet on which the zinc-based plated layer is to be formed) is not particularly limited. The base steel sheet may be a hot-rolled steel sheet obtained by hot-rolling a slab having an appropriate chemical composition for steel, or a cold-rolled steel sheet obtained by cold-rolling such a hot-rolled steel sheet. The strength of the base steel sheet may be, for example, 270 to 1470 MPa in tensile strength class. The thickness of the base steel plate is not particularly limited, but is preferably 0.4 to 3.2 mm, more preferably 0.5 to 2.4 mm. The thickness of the zinc-based coated steel sheet is measured using a micrometer (CPM-MX manufactured by Mitutoyo Corporation).

[0020] The coating weight per side of zinc-based coated steel sheet is 20 to 100 g / m 2 is preferable, and 25 to 80 g / m 2 More preferably, 30 to 60 g / m 2 is more preferable. Plating weight: 20g / m 2 On the other hand, if the coating weight is 100 g / m or more, the corrosion resistance is improved. 2 When the thickness is equal to or less than this, peeling at the interface between the zinc-based plating layer and the base steel sheet during pressing and deterioration of sliding properties due to cohesive failure inside the zinc-based plating layer are suppressed, and the contact area between the electrode and the surface-treated steel sheet during welding is reduced, thereby suppressing deterioration of weldability. Here, the plating weight can be measured by the following method. The coating weight of the plating layer can be measured by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel (for example, Asahi Chemical Industry's product name: Ivit 700A) has been added, and measuring the change in weight before and after pickling. If a coating is present, it can be measured by performing the above procedure after removing the coating with a chemical (remover, etc.) or resin shot.

[0021] The thickness of the zinc-based plating layer of the zinc-based plated steel sheet is preferably 2 to 20 μm, more preferably 3 to 15 μm, and even more preferably 4 to 10 μm. A zinc-based plating layer thickness of 2 μm or more improves corrosion resistance, while a zinc-based plating layer thickness of 20 μm or less suppresses peeling at the zinc-based plating layer / base steel sheet interface during pressing and deterioration of sliding properties due to cohesive failure within the zinc-based plating layer, while also suppressing deterioration of weldability due to a smaller contact area between the electrode and base steel sheet during welding. Here, the thickness of the zinc-based plating layer can be measured by observing the cross section.

[0022] When the zinc-based coated steel sheet is a galvannealed layer, the ratio ζ / δ (ratio value) of the ζ phase to the δ phase is preferably 0.1 to 0.4. When the ratio of the ζ phase to the δ phase (ζ / δ) is 0.1 or more, the unevenness of the galvannealed layer becomes large. This is because a relatively large amount of the ζ phase, which contains crystal grains with a relatively large aspect ratio, is generated on the surface side of the galvannealed layer of the zinc-based coated steel sheet (the side opposite the interface with the base steel sheet) at a relatively high level. As a result, a relatively large amount of unevenness due to the generation of the ζ phase is formed on the surface of the galvannealed layer of the zinc-based coated steel sheet (the side opposite the interface with the base steel sheet). When a coating film is formed on the surface due to the unevenness of the surface of the zinc-based coated steel sheet, a relatively thin portion of the coating film is formed, which makes it easier to ensure a conductive path during welding, thereby improving weldability. On the other hand, when the ratio of the ζ phase to the δ phase (ζ / δ) is 0.4 or less, the galvannealed layer becomes hard. This is because the generation of the relatively soft ζ phase in the galvannealed layer is limited to a certain amount or less. Because the ζ phase is relatively soft, it can adhere to the mold during press forming. By suppressing the amount of ζ phase produced on the surface side of the zinc-based plated steel sheet (the side opposite the interface with the base steel sheet) in the galvannealed layer to a certain level or less, the plating components are less likely to adhere to the mold during press forming, improving sliding properties. Therefore, when a galvannealed steel sheet is used, if the ratio of the ζ phase (ζ / δ) in the galvannealed layer is set within the above range, the surface irregularities of the galvannealed layer become large, the ratio of the maximum film thickness to the minimum film thickness of the coating film can be adjusted within the above range, and the corrosion resistance, weldability, and sliding properties are likely to be improved. In particular, from the viewpoint of corrosion resistance and inhibiting the progression of corrosion, the ratio ζ / δ of the ζ phase to the δ phase is preferably greater than 0.125 and may be 0.126 or greater. When the ratio ζ / δ of the ζ phase to the δ phase is 0.1 or greater, the effect of inhibiting the formation of red rust is observed, but by making the ratio ζ / δ greater than 0.125, in addition to inhibiting the formation of red rust, the progression of corrosion can also be more suitably inhibited.

[0023] Furthermore, when the zinc-based coated steel sheet is a galvannealed layer, the ratio of the ζ phase to the Γ phase, ζ / Γ, is preferably 1.0 to 20.0. When the ratio of the ζ phase to the Γ phase (ζ / Γ) is 1.0 or more, the unevenness of the galvannealed layer becomes large. This is because a relatively large amount of the ζ phase, which includes crystal grains with a relatively large aspect ratio, is generated on the surface side of the galvannealed layer of the zinc-based coated steel sheet (the side opposite the interface with the base steel sheet) at a relatively high level. As a result, a relatively large amount of unevenness due to the generation of the ζ phase is formed on the surface of the galvannealed layer of the zinc-based coated steel sheet (the side opposite the interface with the base steel sheet). When a coating film is formed on the surface due to the unevenness on the surface of the zinc-based coated steel sheet, a relatively thin portion of the coating film is formed, which makes it easier to ensure a conductive path during welding, thereby improving weldability. On the other hand, when the ratio of the ζ phase to the Γ phase (ζ / Γ) is 20.0 or less, the galvannealed layer becomes hard, which is preferable. Therefore, the ratio ζ / Γ of the ζ phase to the Γ phase is preferably in the above range, and more preferably in the range of 2.0 to 10.0.

[0024] The ratio of the ζ phase to the δ phase (ζ / δ) and the ratio of the ζ phase to the Γ phase (ζ / Γ) can be adjusted by the alloying temperature and alloying time.

[0025] In the galvannealed layer, the ζ phase is a phase of iron-zinc intermetallic compounds, mainly FeZn 13 It is a phase containing The δ phase is an iron-zinc intermetallic compound phase, mainly consisting of FeZn7 to FeZn 10 It is a phase containing The Γ phase is an iron-zinc intermetallic compound phase, mainly Fe3Zn 10 It is a phase containing The ζ, δ, and Γ phases can be identified by confirming the presence of diffraction peaks corresponding to the components of each phase using an X-ray diffraction (XRD) method.

[0026] The ratio of the ζ phase to the δ phase (ζ / δ) and the ratio of the ζ phase to the Γ phase (ζ / Γ) are measured as follows. First, the coating film is removed using methyl ethyl ketone (MEK), and after removal, the weight of the zinc-based plated steel sheet is measured. Then, constant-potential electrolysis is performed at -1030 mV against a saturated calomel electrode. After X-ray diffraction shows that the diffraction peak intensity corresponding to the ζ-phase component is below the background and the disappearance of the ζ-phase is confirmed, the weight is measured, and the ζ-phase content is determined. Subsequently, constant potential electrolysis is carried out at -940 mV against a saturated calomel electrode. After X-ray diffraction shows that the diffraction peak intensity corresponding to the δ-phase component is below the background and the disappearance of the δ-phase is confirmed, the weight is measured and the δ-phase content is determined. The remaining plating was then dissolved from the plated steel sheet using 5% hydrochloric acid containing 0.1 g / L of an inhibitor that suppresses iron dissolution (manufactured by Asahi Chemical Industry Co., Ltd., product name: Ivit 700A). After confirming the disappearance of the Γ phase by X-ray diffraction, the diffraction peak intensity corresponding to the Γ phase component became lower than the background, the sheet was weighed, and the Γ phase content was measured. The ζ-phase content, δ-phase content, and Γ-phase content obtained above were divided by the specific gravities of the ζ-phase, δ-phase, and Γ-phase, 7.15, 7.24, and 7.36, respectively, to obtain the volumes of the ζ-phase and δ-phase. The abundance ratios were calculated from these volumes, and the ratio (volume ratio) of the ζ-phase to the δ-phase (ζ / δ) and the ratio (volume ratio) of the ζ-phase to the Γ-phase (ζ / Γ) were obtained.

[0027] Here, the conditions for constant potential electrolysis are as follows: Working electrode: Zinc-plated steel sheet Counter electrode: Platinum electrode ·Reference electrode: saturated calomel electrode ·Measurement temperature: 23℃

[0028] X-ray diffraction was carried out by the θ / 2θ measurement method, and the conditions for X-ray diffraction were as follows: Measuring instrument: Rigaku Corporation RINT2200 ·Characteristic X-ray: CoKα ray Voltage: 40kV ·Current: 150mA X-ray diffraction measurement range: 30°≦2θ≦90° Step: 1° / min

[0029] A zinc-based plated steel sheet can be obtained, for example, by obtaining a slab by continuous casting, hot rolling the slab to obtain a hot-rolled sheet, coiling the hot-rolled sheet, cold rolling the hot-rolled sheet to obtain a cold-rolled sheet, annealing the cold-rolled sheet, and plating the annealed sheet, etc. The continuous casting conditions, hot rolling conditions, coiling conditions, cold rolling conditions, annealing conditions, and plating conditions may be conventionally known general conditions. Here, examples of methods for imparting irregularities to a zinc-based plated steel sheet include a method of grinding the surface of a zinc-based plated layer using an abrasive brush, and a method of imparting surface irregularities to the surface of a cold-rolled sheet or annealed sheet before plating using grit or the like, and then applying zinc-based electroplating. By the method for imparting irregularities to the zinc-based plated steel sheet, the ratio of the maximum film thickness to the minimum film thickness of the coating film can be controlled within the above range.

[0030] The ratio of the maximum film thickness to the minimum film thickness of the coating film can be controlled by adjusting, for example, the type, material, shape, size, pressure, speed, projection amount, etc. of the grit.

[0031] In the method for producing a zinc-based plated steel sheet, the plate may be subjected to temper rolling (that is, skin pass rolling) after the plating treatment. When temper rolling (that is, skin-pass rolling) is carried out, the area ratio of the skin-pass surface is preferably 20 to 80%, and more preferably 30 to 70%. By performing temper rolling (i.e., skin-pass rolling) so that the area ratio of the skin-pass surface falls within the above range, the convex portions of the plating surface become smooth, and when a coating film is formed on the upper layer, it becomes easier to control the ratio of the maximum film thickness to the minimum film thickness of the coating film to within a range of 10 to 100 times. The area ratio of the skin-pass surface can be measured by surface observation such as surface SEM analysis.

[0032] The method for forming the galvannealed layer of the galvannealed steel sheet is not particularly limited, except for adjusting the alloying temperature and alloying time for imparting irregularities to the galvannealed steel sheet. Here, the alloying temperature is preferably 450 to 600°C, more preferably 480 to 580°C. When the alloying temperature is 450°C or higher, it does not take an excessively long time to reach the desired alloying state. On the other hand, when the alloying temperature is 600°C or lower, it is possible to reduce the load on the alloying treatment equipment. The alloying time is preferably 10 to 180 seconds, more preferably 20 to 120 seconds. If the alloying time is 10 seconds or longer, it may be impossible to form a desired alloyed hot-dip galvanized layer. On the other hand, if the alloying time is 180 seconds or shorter, it is possible to form a desired alloyed hot-dip galvanized layer without reducing productivity. Therefore, when the alloying temperature and alloying time are within the above ranges, the ratio of the ζ phase to the δ phase (ζ / δ) and the ratio of the ζ phase to the Γ phase (ζ / Γ) can be controlled within the above ranges, which is preferable.

[0033] The formation of a zinc-based plating layer, including a galvannealed hot-dip plating layer, is carried out, for example, by hot-dip plating. The formation of the zinc-based plating layer may be carried out by either a continuous method or a batch method. After the formation of the zinc-based plating layer, treatments such as zero spangle treatment for uniform appearance, annealing treatment for modifying the zinc-based plating layer, and temper rolling for adjusting the surface condition or material properties may be carried out.

[0034] The zero-spangle treatment can be carried out, for example, by spraying mist onto molten zinc when the temperature of the molten zinc approaches its solidification temperature. When the zero-spangle treatment is carried out under the above conditions, a hot-dip galvanized steel sheet having a uniform surface appearance can be produced. Temper rolling (i.e., skin pass rolling) can be performed, for example, by a rolling mill equipped with a pair of work rolls and a stand that houses the pair of work rolls. The rolling mill used for temper rolling may be equipped with a plurality of pairs of work rolls arranged in a row. The rolling mill used for temper rolling may also be a multi-stage rolling mill. In particular, when temper rolling is performed under the above conditions, the convex portions of the coating surface are flattened. As a result, when a coating film is formed on the upper layer, the ratio of the maximum film thickness to the minimum film thickness of the coating film tends to be in the range of 10 to 100 times.

[0035] In the method for producing a zinc-based plated steel sheet, as described above, either hot-dip galvanizing or zinc-based electroplating may be adopted as the plating treatment. However, it is preferable to adopt hot-dip galvanizing, since it is relatively easy to achieve a thick coating thickness, and the thick coating thickness makes it easier to ensure corrosion resistance.

[0036] (paint film) The coating film contains at least a binder resin and a lubricant. Specifically, the coating film preferably contains a binder resin, a conductive pigment, a lubricant, and a rust inhibitor. If necessary, the coating film may contain other components.

[0037] -Binder resin- The binder resin may be either a water-soluble or water-dispersible aqueous resin that dissolves or disperses in water, or a solvent-based resin that dissolves or disperses in an organic solvent, but from the standpoints of production cost and environmental friendliness, a water-based resin is preferred.

[0038] Examples of the water-based resin include water-soluble or water-dispersible resins such as polyester resin, urethane resin, polyolefin resin, acrylic resin, epoxy resin, phenol resin, and mixed resins of two or more of these resins. When a polyester resin is used, the molecular weight is preferably 10,000 to 30,000. A molecular weight of 10,000 or more ensures sufficient processability. On the other hand, a molecular weight of 30,000 or less increases the number of bonding sites of the resin itself, ensuring excellent adhesion to the electrodeposition coating film. Furthermore, when crosslinking is performed using a curing agent such as melamine, the crosslinking reaction is carried out sufficiently, ensuring the performance of the coating film. Here, the molecular weight of the polyester resin is the weight average molecular weight. When a urethane resin is used, the urethane resin is preferably in the form of an emulsion with an emulsion particle size of 10 to 100 nm (preferably 20 to 60 nm). When the emulsion particle size is 10 nm or more, costs can be kept low. On the other hand, when the emulsion particle size is 100 nm or less, the gaps between the emulsion particles are small when the coating film is formed, ensuring the barrier properties of the coating film. Examples of urethane resin types include ether-based, polycarbonate-based, ester-based, and acrylic graphite-based types. These may be used alone or in combination.

[0039] Examples of solvent-based resins include polyester resins, urethane resins, epoxy resins, acrylic resins, and mixed resins of two or more of these resins.

[0040] Here, the binder resin may be a crosslinked resin having a crosslinked structure or a non-crosslinked resin not having a crosslinked structure, but a non-crosslinked resin is preferable from the viewpoint of low-temperature film formation of the coating film. As a crosslinking agent (curing agent) that imparts a crosslinked structure to the binder resin, a water-soluble crosslinking agent is preferable. Specific examples of the crosslinking agent include melamine, isocyanate, silane compounds, zirconium compounds, and titanium compounds.

[0041] The amount of crosslinking agent added is preferably 5 to 30 parts by mass per 100 parts by mass of resin solids. When the amount of crosslinking agent added is 5 parts by mass or more, the crosslinking reaction with the binder resin is ensured, resulting in sufficient performance as a coating film. On the other hand, when the amount of crosslinking agent added is 30 parts by mass or less, the crosslinking reaction proceeds moderately, preventing the coating film from becoming excessively hard. This ensures processability. In addition, the paint stability of silane compounds, zirconium compounds, and titanium compounds can be ensured.

[0042] The binder resin content is preferably 50.0 to 90.0% by mass relative to the coating film (total solids content of the coating film). When the binder resin content is 50.0% by mass or more, the binder function is easily exerted and the cohesive strength of the coating film is improved. As a result, internal failure of the coating film (cohesive failure of the coating film) is less likely to occur during adhesion tests and molding processing. On the other hand, when the binder resin content is 90.0% by mass or less, the proportion of pigment components contained in the coating film is reduced, making it easier to achieve both weldability, corrosion resistance, and adhesion to the electrodeposition coating film. The content of the binder resin is more preferably 55.0 to 85.0 mass % of the coating film (total solid content of the coating film) in order to achieve a binder function while also achieving weldability, corrosion resistance, and adhesion to the electrodeposition coating film.

[0043] -Conductive pigments- The inclusion of conductive pigments in the coating improves weldability. Examples of conductive pigments include oxide particles (doped zinc oxide particles, doped tin oxide particles, tin oxide-titanium oxide composite particles, tin oxide-barium sulfate composite particles, nickel oxide-aluminum composite particles, etc.), non-oxide ceramic particles (boride ceramics, carbide ceramics, nitride ceramics, silicide ceramics, etc.), iron alloy particles (ferrosilicon particles, etc.), stainless steel particles, particles other than iron alloys (particles of metals such as zinc, nickel, iron, aluminum, cobalt, manganese, copper, tin, etc., or alloys thereof, etc.), carbon particles (conductive carbon, graphite powder, etc.), and carbides (titanium carbide, silicon carbide, etc.).

[0044] Among these, doped oxide particles are preferred, and doped zinc oxide particles are more preferred, from the viewpoint of improving weldability.

[0045] The doped zinc oxide particles are electrically conductive zinc oxide particles. Examples of doped zinc oxide particles include particles that exhibit electrical conductivity by doping zinc oxide particles with at least one element (hereinafter also referred to as "dope element") selected from the group consisting of elements in Group 13 of the periodic table and elements in Group 15 of the periodic table.

[0046] Examples of Group 13 elements of the periodic table include B, Al, Ga, and In. Examples of Group 15 elements of the periodic table include P and As. Among these, from the viewpoint of improving electrical conductivity, the doping element is preferably Al or Ga. Furthermore, from the viewpoint of cost, the doping element is more preferably Al.

[0047] From the viewpoint of improving the electrical conductivity, the content of the doping element is preferably 0.05 to 5 atom %, more preferably 0.1 to 5 atom %, based on the undoped zinc oxide particles.

[0048] The average particle size of the conductive pigment is preferably from 0.1 to 2 μm, more preferably from 0.2 to 1.5 μm, and even more preferably from 0.4 to 1.0 μm. If the average particle size of the conductive pigment is 0.1 μm or more, the welding electrode is likely to come into contact with the zinc-based plating layer of the zinc-based plated steel sheet during welding, which tends to reduce weldability. On the other hand, if the average particle size of the conductive pigment is 2 μm or less, the conductive pigment is unlikely to come into uniform contact with the welding electrode (e.g., Cu electrode) during welding, which results in the welding electrode being more likely to come into contact with the zinc-based plating layer of the zinc-based plated steel sheet and reduced weldability.

[0049] The "average particle size" of the conductive pigment refers to the average primary particle size when the conductive pigment exists alone in the oil coating film, and refers to the average secondary particle size, which represents the particle size of the conductive pigment when aggregated, when the conductive pigment exists in aggregate.

[0050] The average particle size of the conductive pigment is determined by the following measurement method. First, the coated steel sheet is cut in the thickness direction to expose the cross section, which is then polished. The resulting cross section is observed with a scanning electron microscope to obtain an image of the cross section within the coating. Twenty-five doped zinc oxide particles present within the field of view of the observed image are randomly selected, and the long and short side lengths of each conductive pigment are measured. Finally, the particle diameter is calculated by arithmetically averaging the long and short side lengths of each doped zinc oxide particle. Furthermore, the average particle diameter is calculated by arithmetically averaging the particle diameters of the doped zinc oxide particles within the field of view. The doped zinc oxide particles present within the field of view of the observed image are identified as particles located in the region where Al is detected within the region where Zn is detected in EPMA analysis of the field of view. The observed image is analyzed using the image analysis software "Image J Ver. 1.54f," and the long and short side lengths of the doped zinc oxide particles are measured. The observation conditions for the scanning electron microscope are as follows: Measurement equipment: JEOL Ltd. JSM-7200F ·Magnification: 5000x Acceleration voltage: 10 keV

[0051] The content of the conductive pigment is preferably 2 to 30 mass % relative to the coating film (total solid content of the coating film), more preferably 3 to 25 mass %, and even more preferably 5 to 20 mass %. If the conductive pigment content is 2% by mass or more, the welding electrode is likely to come into contact with the zinc-based plating layer of the zinc-based plated steel sheet during welding, which tends to reduce weldability.On the other hand, if the conductive pigment content is 30% by mass or less, the proportion of the conductive pigment in the coating film is ensured, which tends to result in sufficient performance such as weldability and corrosion resistance.

[0052] -Lubricant- The lubricant can impart excellent lubricity to the coating film and improve the powdering resistance of the coating film. Examples of the lubricant include the organic lubricants (1) and (2) below. This is because, when used in combination with an organic lubricant, the material burns and carbonizes due to the heat during welding, making it difficult for the material to adhere to the welding electrode. Among these, polyolefin wax is preferred because it can impart high corrosion resistance.

[0053] (1) Polyolefin wax, paraffin wax: for example, polyethylene wax, synthetic paraffin, natural paraffin, microcrystalline wax, chlorinated hydrocarbons, etc. (2) Fluorine resin wax: for example, polyfluoroethylene resin (polytetrafluoroethylene resin, etc.), polyvinyl fluoride resin, polyvinylidene fluoride resin, etc.

[0054] When polyethylene wax is used as the lubricant, the average particle size is preferably 0.5 to 10 μm. When the average particle size of the polyethylene wax is 0.5 μm or more, surface thickening of the polyethylene wax is suppressed, and a decrease in adhesion between the coating film and the electrodeposition coating film can be suppressed. On the other hand, when the average particle size of the solid lubricant is 10 μm or less, peeling of the polyethylene wax from the coating film is suppressed, and lubricity and corrosion resistance can be ensured. The average particle size of the solid lubricant is more preferably 1 to 5 μm in order to obtain excellent adhesion between the coating film and the electrodeposition coating film, corrosion resistance, lubricity, and powdering resistance.

[0055] The softening point of the lubricant is preferably 100 to 135° C., more preferably 110 to 130° C. When the softening point of the lubricant is 100 to 135° C., the lubricity and powdering resistance of the coating film are further improved.

[0056] The content of the lubricant is 0.5 to 2 mass % relative to the coating film (total solid content of the coating film), preferably 0.75 to 1.75 mass %, more preferably 1.0 to 1.5 mass %. When the lubricant content is 0.5% by mass or more, sufficient lubricity is obtained. On the other hand, when the lubricant content is 2% by mass or less, a decrease in adhesion between the coating film and the electrodeposition coating film and a decrease in corrosion resistance can be suppressed. Therefore, the lubricant content is set to the above range.

[0057] -Rust inhibitor- As the rust inhibitor, organic rust inhibitors such as organic acids, organic acid salts, amine salts, and esters are used to impart high weldability. This is because when organic rust inhibitors are used, they burn and carbonize due to the heat during welding, making them less likely to adhere to the welding electrode. Among these, organic acids are preferred because they can impart high corrosion resistance. Examples of organic acids include carboxylic acids (tartaric acid, tannic acid, oleic acid, dimer acid, naphthalene acid, etc.), carboxylic acid metal soaps (lanolin Ca, naphthenate Zn, oxidized wax Ca, barium salt, etc.), and sulfonates (sodium sulfonate, calcium sulfonate, barium sulfonate). Examples of the esters include glycerin esters of higher fatty acids, sorbitan monoisostearate, and sorbitan norate. The content of the rust inhibitor is preferably 1 to 20 mass% of the coating film (total solid content of the coating film). When the content of the rust inhibitor is 1 mass% or more, corrosion resistance is sufficiently improved. On the other hand, when the content of the rust inhibitor is 20 mass% or less, a decrease in the liquid stability of the coating material is suppressed. From the viewpoint of corrosion resistance and paint stability, the content of the rust inhibitor is more preferably 5 to 15 mass % relative to the coating film (total solid content of the coating film).

[0058] (Other ingredients) Examples of other components include well-known additives such as lubricants, rust inhibitors, silane coupling agents, and leveling agents. However, inorganic substances other than Zn-containing compounds including doped zinc oxide particles tend to deteriorate weldability if they adhere to the welding electrode during welding. Therefore, it is preferable that the coating film does not contain any inorganic substances other than the Zn-containing compound containing the doped zinc oxide particles, or if it does contain any inorganic substances, it should be contained in an amount of 1 mass % or less relative to the coating film. Here, examples of inorganic substances other than the Zn-containing compound including the doped zinc oxide particles include silica, inorganic acids, and inorganic acid salts. Inorganic substances other than Zn-containing compounds including doped zinc oxide particles do not include metal salts of organic substances (such as metal salts of organic acids).

[0059] (average coating thickness) The average thickness of the coating film is preferably from 0.5 to 3 μm, more preferably from 0.8 to 2.2 μm, and even more preferably from 1.2 to 1.8 μm.

[0060] The average coating thickness is measured as follows. First, the surface-treated substrate with the coating film is cut along the thickness direction to expose the cross section, which is then embedded in resin and polished using a polishing machine. After polishing, Au is vapor-deposited onto the cross section to prepare a sample for cross-sectional observation. The cross section thus obtained is observed with a scanning electron microscope to obtain an observation image of the cross section of the coating film. In the observation image, the thickness of the coating film is measured at 10 points, and the arithmetic average value is calculated. When measuring the thickness of the coating film, the thickness of the coating film is measured at 10 points equally spaced at 40 μm intervals in the direction in which the coating film extends on the observation image. The observation conditions for the scanning electron microscope are as follows: Measurement equipment: JEOL Ltd. JSM-7200F ·Magnification: 3000x Acceleration voltage: 10 keV

[0061] The ratio of the maximum thickness to the minimum thickness of the coating film is 10 times or more and 100 times or less. A ratio of the maximum to the minimum coating thickness (maximum coating thickness / minimum coating thickness) of less than 10 means that the surface of the zinc-plated steel sheet is small in unevenness. As a result, the coating thickness is not sufficiently thin in the areas corresponding to the convex parts of the surface of the zinc-plated steel sheet. As a result, the coating becomes thicker in the areas corresponding to the convex parts of the surface of the zinc-plated steel sheet compared to when the ratio of the maximum to the minimum coating thickness (maximum coating thickness / minimum coating thickness) is 10 or more. This reduces the number of electrodes and current points during spot welding, thereby reducing weldability. A ratio of the maximum to minimum coating thickness (maximum coating thickness / minimum coating thickness) of more than 100 means that the surface of the zinc-plated steel sheet is highly uneven. As a result, the thickness of the coating film becomes significantly thinner near the areas that correspond to the protrusions on the surface of the zinc-plated steel sheet, making them more likely to become starting points for corrosion and reducing corrosion resistance. When the ratio of the maximum to minimum film thickness of a coating film (maximum film thickness / minimum film thickness) is between 10 and 100, this indicates that the surface of the zinc-based plated steel sheet is moderately uneven. Therefore, the coating film penetrates the unevenness appropriately, and the thickness of the coating film is appropriately controlled in the areas corresponding to the protrusions. During welding, the coating film functions as a coating film, ensuring electrical conductivity between the welding electrode and the zinc-based plated steel sheet while also ensuring corrosion resistance. Furthermore, the coating film's penetration into the recesses on the surface of the zinc-based plated steel sheet contributes to the adhesion of the coating film to the zinc-based plated steel sheet. The ratio of the maximum film thickness to the minimum film thickness of the coating film (maximum film thickness / minimum film thickness) is preferably 12 times or more, more preferably 15 times or more, 18 times or more, or 20 times or more. The ratio of the maximum film thickness to the minimum film thickness of the coating film (maximum film thickness / minimum film thickness) is preferably 80 times or less, more preferably 60 times or less, 50 times or less, or 40 times or less. The maximum thickness of the coating film is preferably 5.0 to 10.0 μm from the viewpoint of improving weldability and corrosion resistance, and the minimum thickness of the coating film is preferably 0.05 to 0.25 μm from the viewpoint of ensuring corrosion resistance.

[0062] The ratio of the maximum thickness to the minimum thickness of the coating film is measured as follows. A sample for cross-section observation is prepared in the same manner as in the above method for measuring the average film thickness of the coating film, and the obtained cross-section is observed with a scanning electron microscope to obtain an observation image of the cross-section of the coating film. Next, the positions where the coating film has the greatest thickness and the positions where the coating film has the smallest thickness in the observed image are identified, and the film thicknesses at these positions are measured as the maximum film thickness and the minimum film thickness in the observed image, respectively. The maximum and minimum film thicknesses of the coating film are determined for the 10 observation images, and the arithmetic mean values ​​of the maximum and minimum film thicknesses for each observation image are calculated. The ratio of the calculated arithmetic mean values ​​of the maximum film thickness and the minimum film thickness is defined as the "ratio of the maximum film thickness to the minimum film thickness of the coating film." The observed images of the cross section of the coating film to be measured may be images of different locations on the same cross section, or may be images of different cross sections. The locations to be photographed need not be arbitrarily selected, but may be 10 locations selected randomly. If necessary, the number of locations to be photographed may be more than 10.

[0063] (Surface properties of coating film) The surface of the coating film is preferably flat as shown in FIG.

[0064] Therefore, as a specific surface property of the coating film, the surface roughness Ra of the coating film is preferably 0.1 to 10 μm. The lower limit of the surface roughness Ra of the coating film may be 0.3 μm, 0.5 μm, or 0.7 μm, and more preferably 0.8 μm, 0.9 μm, or 1.0 μm. The upper limit of the surface roughness Ra of the coating film may be 2.0 μm or 1.4 μm, and more preferably 1.2 μm. By setting the surface roughness Ra of the coating film within the above range, it becomes easier to control the ratio between the maximum and minimum film thicknesses of the coating film. The surface roughness Ra of the coating film is measured as follows. Measurements were taken using a Surfcom manufactured by Tokyo Seimitsu in accordance with the JIS B 0601:2013 standard, with a cutoff value of 0.8 mm.

[0065] (Formation of coating film) The method for forming the coating film is not particularly limited, and well-known methods can be used. For example, a coating film-forming composition (paint) is obtained by mixing a binder resin, a conductive pigment, a lubricant, a rust inhibitor, and, if necessary, other components in a solvent. The solvent may be water or an organic solvent, but water is preferred from the standpoints of production cost and environmental friendliness. In other words, the coating film-forming composition is preferably an aqueous composition. The coating film is then formed by applying the coating film-forming composition to at least one surface of a zinc-based plated steel sheet and drying the applied film (coated film), or by drying and heating. Note that the surface of the zinc-based plated steel sheet is preferably degreased and washed with water before the coating film-forming composition is applied. The coating film-forming composition is applied using a coating method such as bar coating, roll coating, blade coating, or curtain coating. The drying temperature of the coating film is preferably 50 to 120°C, more preferably 60 to 100°C, in terms of the maximum temperature that can be reached on the surface of the zinc-based plated steel sheet. The heating temperature of the coating film is preferably 100 to 240°C, more preferably 120 to 210°C, in terms of the maximum temperature that can be reached on the surface of the zinc-based plated steel sheet.

[0066] <Other aspects of surface-treated steel sheets> The surface-treated steel sheet according to this embodiment may have an intermediate coating film (a functional coating film such as a chemical conversion coating film) between the zinc-based plated steel sheet and the coating film. The intermediate coating film preferably contains a binder resin, a silane coupling agent, silica fine particles, a phosphate compound, and a fluorine compound, and has an average film thickness of 0.1 to 0.5 μm. By having the intermediate coating film with the above composition and average film thickness, it is possible to improve corrosion resistance without compromising weldability. Relatively This is because a thin film has little effect on weldability and improves the adhesion between the plated steel sheet and the coating film, thereby improving corrosion resistance. Therefore, the surface-treated steel sheet according to this embodiment preferably has an intermediate coating film (functional coating film) with the above composition and thickness between the zinc-based plated steel sheet and the coating film.

[0067] The average thickness of the intermediate coating is measured in the same manner as the average thickness of the coating.

[0068] <Painting materials> The painted member of this embodiment comprises a formed material obtained by forming the surface-treated steel sheet of this embodiment, and an electrocoating film formed on the coating film of the formed material (i.e., the coating film of the surface-treated steel sheet) (see Figure 3). In Fig. 3, 100 denotes a coated member, 10A denotes a formed member, 11A denotes a base steel sheet in the formed member (the steel sheet on which a zinc-based plating layer is to be formed), 12A denotes the zinc-based plating layer of the formed member, 13A denotes a coating film on the formed member, and 14 denotes an electrodeposition coating film. Fig. 3 shows an example of a coated member having a hat-shaped cross section, but the overall shape and cross-sectional shape of the coated member are not particularly limited and may have any shape.

[0069] The coated member according to this embodiment is manufactured, for example, as follows: First, a surface-treated steel sheet is formed into a formed product of the desired shape using, for example, well-known forming techniques such as cutting and press forming. If necessary, the formed product may be assembled into the desired shape by welding (spot welding, etc.).

[0070] Next, the coating film of the molding material is subjected to an electrodeposition coating treatment. As a result, an electrodeposition coating film is formed on the coating film. The electrodeposition coating treatment may be either anionic electrodeposition coating or cationic electrodeposition coating, but cationic electrodeposition coating is preferred from the viewpoint of corrosion resistance.

[0071] In particular, when an electrodeposition coating film is formed by cationic electrodeposition coating using an aqueous paint containing a resin [for example, an aqueous resin (such as an acrylic resin, polyester resin, alkyd resin, epoxy resin, or polyurethane resin) having a hydrophilic group such as a carboxyl group, a hydroxyl group, a methylol group, an amino group, a sulfonic acid group, or a polyoxyethylene bond, and a functional group such as a hydroxyl group that reacts with a curing agent, and a curing agent (such as a melamine resin or a blocked polyisocyanate)], and other additives (such as a coloring pigment, an optical interference pigment, an extender pigment, a dispersant, an anti-settling agent, a reaction accelerator, an antifoaming agent, a thickener, an anti-rust agent, an ultraviolet absorber, or a surface conditioner), the adhesion between the coating film and the electrodeposition coating film is likely to be improved.

[0072] Thereafter, other coating films such as an intermediate coating film and a top coating film may be formed on the electrodeposition coating film of the molding material, if necessary.

[0073] Through these steps, the coated member according to this embodiment is manufactured.

[0074] Before electrodeposition coating, the molded material on which the coating film has been formed may be degreased, the surface adjusted, and then chemically treated (for example, phosphate treatment, Zr treatment, etc.). By performing chemical conversion treatment, a chemical conversion treatment film is less likely to form on the coating film, but a chemical conversion treatment film is formed in required areas other than the coating film. This improves the adhesion of the electrodeposition coating film to the entire molded material (coated member).

[0075] The coated members according to this embodiment are widely used in applications such as automobile members (automobile bodies, suspension members, etc.), machine members (casings, etc.), home appliance members (casings, etc.), and building materials (roofs, walls, etc.). [Example]

[0076] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to the following examples. The present disclosure allows various conditions to be adopted as long as they do not deviate from the gist of the disclosure and the object of the disclosure is achieved.

[0077] <Example> 1. Manufacturing of surface-treated steel sheets 1.1 Preparation of galvanized steel sheet The following five types of zinc-based plated steel sheets were prepared, and the surfaces were degreased by immersion in an aqueous solution (2.5 mass %, 40°C) of an aqueous alkaline degreasing agent (FC-301 manufactured by Nippon Parkerizing Co., Ltd.) for 2 minutes, followed by rinsing with water and drying to obtain zinc-based plated steel sheets for surface treatment. However, cold-rolled steel sheets were also prepared as steel sheets for surface treatment.

[0078] ZA: Galvannealed steel sheet (thickness 0.8 mm) ZB: Hot-dip galvanized steel sheet (thickness 0.8 mm) ZC: Zn-Al-Mg ternary hot-dip galvanized steel sheet (Zn-11%Al-3%Mg-0.2%Si) (sheet thickness 0.8 mm, coating weight 60 g / m 2 ) (Grit treatment on steel sheet before plating) ZD: Zinc-nickel alloy plated steel sheet (10% Ni by mass, thickness 0.8 mm) ZE: Zinc-nickel alloy plated steel sheet (grit-treated steel sheet before plating, 10% Ni by mass, sheet thickness 0.8 mm) C: Cold-rolled steel sheet (thickness 0.8 mm, unplated)

[0079] The coating weights are shown in Table 2. The coating weights were controlled by adjusting the plating pull-up speed and wiping amount during plating preparation. The ratio of the ζ phase to the δ phase (ζ / δ) and the ratio of the ζ phase to the Γ phase (ζ / Γ) in the coating layer of the galvannealed steel sheet are shown in Table 2. The ratio of the ζ phase to the δ phase (ζ / δ) and the ratio of the ζ phase to the Γ phase (ζ / Γ) were controlled by adjusting the alloying temperature and alloying time, as shown in Table 2.

[0080] 1.2 Formation of intermediate coating (chemical conversion coating) Next, the following chemical conversion treatment solution S was prepared and applied to the above-mentioned zinc-based plated steel sheet or cold-rolled steel sheet while changing the bar coating size so as to obtain the coating weight shown in Table 2. Thereafter, the zinc-based plated steel sheet or cold-rolled steel sheet was heated and dried in a hot air furnace so that the surface temperature reached 70°C, and then air-dried to form a chemical conversion treatment film on the surface of the zinc-based plated steel sheet or cold-rolled steel sheet. The average thickness of the chemical conversion treatment film was 0.2 μm. However, in some cases, no intermediate coating was formed.

[0081] S1: A chemical conversion treatment solution with a solids concentration of 10% consisting of silane coupling agents, silica particles, urethane resins, phosphoric acid compounds, and fluorine compounds. S2: A chemical conversion treatment solution with a solids concentration of 10% consisting of Zr compounds, silane coupling agents, phosphate compounds, and vanadium compounds.

[0082] 1.3 Coating formation Next, to form a coating film having the composition ratio (mass%) shown in Table 1, a solution of 20% solids was added. The components were mixed to obtain the % by mass paint to prepare a coating material for coating film formation. The coating material was applied to a zinc-plated steel sheet, a cold-rolled steel sheet, or a chemical conversion coating using a bar coater, while changing the bar coat size and dilution ratio to obtain the average film thickness shown in Table 2, and then dried in an oven under conditions that resulted in the maximum temperature reached (PMT) shown in Table 2, or by drying and heating, to form a coating material. In each test example, the zinc-based plated steel sheet after alloying (before forming a coating film) was subjected to temper rolling (skin pass rolling) at a reduction rate of 0.3%. The components contained in the paint are listed below.

[0083] (binder resin) J1: Epoxy resin (ADEKA ADEKA RESIN EM-0461N) J2: Polyester resin (Vylonal MD1480 manufactured by Byron) J3: Urethane resin (Superflex 150 manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) J4: Epoxy resin (ADEKA ADEKA RESIN EP-4100) J5: Polyester resin (Byron 200 manufactured by Byron) J6: Melamine resin (Allnex Cymel 325) J7: Silane coupling agent (KBE403 manufactured by Shin-Etsu Chemical Co., Ltd.)

[0084] (Conductive pigment) Z1: Doped zinc oxide particles (23-Kt, manufactured by Hakusui Tech Co., Ltd., average particle size 0.5 μm) Z2: Doped zinc oxide particles (23-K manufactured by Hakusui Tech Co., Ltd., average particle size 0.2 μm) Z3: Doped zinc oxide particles (Pazet CK, manufactured by Hakusui Tech Co., Ltd., average particle size 0.03 μm) Z4: Zinc oxide particles (general reagent, average particle size 0.5 μm) Z5: Titanium nitride particles (general reagent, average particle size 2.0 μm)

[0085] (rust inhibitor) B1: Tannic acid (general reagent) + zinc naphthenate (general reagent) mixed in a mass ratio of 1:1 B2: Tartaric acid (general reagent) + zinc naphthenate (general reagent) mixed in a mass ratio of 1:1 B3: Tannic acid (general reagent) + trizinc phosphate (general reagent) mixed in a mass ratio of 1:1 B4: Tartaric acid (general reagent) + trizinc phosphate (general reagent) mixed in a mass ratio of 1:1 B5: Colloidal silica (particle size 20 nm) B6: Aluminum dihydrogen triphosphate (particle size 2 μm) B7: Calcium ion-exchanged silica (Ca exchange rate 9%) (particle size 2 μm)

[0086] (lubricant) W1: Polyethylene wax (CHEMBASE S-394 MG) W2: Microcrystalline wax (Hi-Mic-1090, manufactured by Nippon Seiro Co., Ltd.) W2: Polytetrafluoroethylene (PTFE) particles (CHEMBASE SST-1MG-RC)

[0087] 2. Performance evaluation test The surface-treated steel sheets of each example were subjected to the following performance evaluation tests. 2.1 Spot weldability Each surface-treated steel sheet was spot-welded 1,000 times using a 5 mm tip diameter, R40 CF-type Cr-Cu electrode, with a pressure of 1.96 kN and a welding time of 12 cycles / 50 Hz. The welding current was varied to achieve a nugget diameter of 4√t (t = thickness of the surface-treated steel sheet). After 1,000 welds were performed, the nugget diameter and electrode condition were observed and the following ratings were assigned. Note that spot welds where no current was applied were judged as having no nugget formed. The presence or absence of nugget formation and nugget diameter were measured by observing a cross section of the sheet thickness including the center of the weld. In the weldability test, a specimen was judged to have excellent weldability when it was rated as "3," "4," or "5." The results are shown in Table 2. 1:1000 RBI cannot be implemented 2: 1000 dots can be performed, but there may be no current flowing at 10 dots or less (including no current flowing at all). When 1000 dots are completed, the diameter of the electrode contact surface using pressure-sensitive paper must be 1.7 times or more the diameter of the electrode contact surface at the first dot. 3: 1000 dots are performed, no power is turned off, and the diameter of the electrode contact surface using pressure-sensitive paper at the end of 1000 dots is 1.4 times or more but less than 1.7 times the diameter of the electrode contact surface at the first dot 4: 1000 dots are performed, no power is turned off, and the diameter of the electrode contact surface using pressure-sensitive paper at the end of 1000 dots is 1.1 times or more but less than 1.4 times the diameter of the electrode contact surface at the first dot. 5: 1000 dots are performed, no power is turned off, and the diameter of the electrode contact surface using pressure-sensitive paper at the end of 1000 dots is less than 1.1 times the diameter of the electrode contact surface at the first dot

[0088] 2.2 Sliding test The surface-treated steel sheets of each example were cut into 50 mm x 50 mm pieces. A sliding test was then conducted using a pin-on-disk friction and wear tester ("RD-100M" manufactured by Daiei Scientific Instruments Co., Ltd.). Specifically, the steel sheets were coated with rust-preventive oil ("Daphne Super Coat TW" manufactured by Idemitsu Kosan) and then placed on the stage of the pin-on-disk wear tester. The stage and steel sheet temperatures were then heated to 50°C, and the test was conducted at a load of 30 N, a speed of 1 rpm, and 10 revolutions. The change in the friction coefficient and the remaining condition of the coating and plating were then confirmed. In the sliding property test, a sample was judged to have excellent sliding property when it received a score of "3," "4," or "5." The results are shown in Table 2. 1: The maximum friction coefficient is more than 2.0 times the average friction coefficient of the first lap 2: The maximum friction coefficient is between 1.5 and 2.0 times the average friction coefficient of the first lap 3: The maximum friction coefficient is between 1.2 and 1.5 times the average friction coefficient of the first lap 4: The maximum friction coefficient is between 1.1 and 1.2 times the average friction coefficient of the first lap 5: The maximum friction coefficient is within 1.1 times the average friction coefficient of the first lap

[0089] 2.3 Corrosion resistance test (Advance preparation) For each example, the surface-treated steel sheet was subjected to surface conditioning at room temperature for 20 seconds using a surface conditioning treatment agent, Preparen X (trade name), manufactured by Nihon Parkerizing Co., Ltd. Furthermore, a chemical conversion treatment (phosphate treatment) was performed using Palbond 3020 (trade name), a chemical conversion treatment solution (zinc phosphate treatment solution) manufactured by Nihon Parkerizing Co., Ltd. The temperature of the chemical conversion treatment solution was set to 43°C, and the surface-treated steel sheet was immersed in the chemical conversion treatment solution for 120 seconds, then rinsed with water and dried. After the above chemical conversion treatment (phosphate treatment), a cationic electrodeposition paint manufactured by Nippon Paint Co., Ltd. was electrodeposited using a ramp current of 160V, and further baked at a baking temperature of 170°C for 20 minutes. The average thickness of the electrodeposition coating film after electrodeposition coating was 10 μm for all samples.

[0090] (Corrosion resistance test 1) After the electrodeposition coating, a cross-shaped cut was made with a cutter knife down to the base steel sheet of the surface-treated steel sheet, and a corrosion cycle test was performed. The corrosion cycle consisted of 2 hours of salt spray (SST: Salt Spray Test, 5% NaCl, 35°C atmosphere), 2 hours of dry (60°C), and 4 hours of wet (50°C, 98% RH) conditions, and 360 cycles were performed. In the corrosion resistance test, a sample was judged to have excellent corrosion resistance when it received a score of "3," "4," or "5." The results are shown in Table 2. 1: Red rust appears on the flat surface 2: Paint blister or white rust from flat area, or paint blister width from cross cut area exceeds 4mm 3: No rust on flat surfaces, paint blister width from cross-cut area is over 2mm and within 4mm 4: No rust on flat surfaces, paint blister width from cross-cut area is over 1mm and less than 2mm 5: No rust on flat surfaces, paint blister width within 1mm from cross-cut area Here, "paint blister width" refers to "the length of the blistered portion of the paint film (i.e., the protruding portion of the paint film) in the direction perpendicular to the extension direction of the cut scratch."

[0091] (Corrosion resistance test 2) Before the above-mentioned preparations were carried out, plate-shaped test pieces A cut to a size of 70 mm x 150 mm and plate-shaped test pieces B cut to a size of 30 mm x 100 mm were taken from the surface-treated steel sheets of each example. Next, as shown in Figures 4 and 5, test piece A and test piece B were overlapped with each other so that their centers overlapped and their longitudinal directions were parallel in a plan view, and then the test piece A and test piece B were joined by spot welding to prepare a joined test piece C. The spot welding locations were two locations 25 mm apart from the centers of test piece A and test piece B on both ends in the longitudinal direction. During spot welding, a spacer was used to adjust the clearance between test piece A and test piece B to approximately 200 µm. Thereafter, the above-mentioned preparatory chemical conversion treatment and electrodeposition coating were carried out. Next, a corrosion cycle test was conducted on the bonded specimen C. The corrosion cycle consisted of 2 hours of salt spray (SST, 0.5% NaCl, 35°C atmosphere), 2 hours of dry (60°C), and 4 hours of wet (50°C, 98% RH) exposure, and was conducted for 360 cycles. After the corrosion test, the spot weld of the bonded specimen C was drilled and specimen A was removed from specimen B. Next, test piece A was immersed in an ammonium citrate aqueous solution to remove the corrosion products, and then the corrosion depth of test piece A was measured. Specifically, test piece A after the corrosion products were removed was cut into eight equal pieces (in FIG. 4, two pieces were cut horizontally and four pieces were cut vertically, for a total of eight equal pieces), and the corrosion depth (mm) of each small piece was measured. The largest value of the corrosion depth (mm) of the eight small pieces was defined as the maximum corrosion depth (mm), and was evaluated as the result of corrosion resistance test 2 of this example. The corrosion depth was measured using a micrometer (CPM-MX manufactured by Mitutoyo Corporation). In the corrosion resistance test 2, a sample was judged to have excellent corrosion resistance when it was scored as "3," "4," "5," or "6." The results are shown in Table 2. 1: Maximum corrosion depth: over 0.4 mm, holes formed 2: Maximum corrosion depth: Over 0.2 mm and up to 0.4 mm 3: Maximum corrosion depth: Over 0.1mm and up to 0.2mm 4: Maximum corrosion depth: Over 0.05mm and within 0.1mm 5: Maximum corrosion depth: Over 0 mm, within 0.05 mm 6: Maximum corrosion depth: 0 mm 4 and 5, 20A indicates test piece A, 20B indicates test piece B, 20C indicates joined test piece C, 21 indicates the centers of test piece A and test piece B, and 22 indicates the spot welded portion.

[0092] [Table 1]

[0093] [Table 2-1]

[0094] [Table 2-2]

[0095] From the above results, it can be seen that the surface-treated steel sheets of the present example are superior in corrosion resistance, weldability, and sliding properties to the surface-treated steel sheets of the comparative example. In Table 1, the notation "inorganic substance*1" indicates the composition ratio of "Zn-containing compound including doped zinc oxide particles" to the coating film.

[0096] The disclosure of Japanese Patent Application No. 2023-056125 is incorporated herein by reference in its entirety. All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A zinc-plated steel sheet, a coating film disposed on at least one main surface of the zinc-based plated steel sheet; and The average thickness of the coating film is 0.5 μm or more and 3 μm or less, the coating film contains a binder resin and 0.5 to 2.0 mass % of an organic lubricant; The ratio of the maximum film thickness to the minimum film thickness of the coating film is 10 times or more and 100 times or less. Surface-treated steel sheet.

2. the zinc-based coated steel sheet is a galvannealed steel sheet, In the galvannealed layer, the ratio of ζ phase to δ phase, ζ / δ, is 0.1 to 0.

4. The surface-treated steel sheet according to claim 1.

3. the coating film contains the binder resin, the conductive pigment, the organic lubricant, and a rust inhibitor; the conductive pigment is a doped oxide particle; The conductive pigment has an average particle size of 0.1 μm or more and 2 μm or less, The content of the conductive pigment is 2 to 30 mass % of the coating film. The surface-treated steel sheet according to claim 1.

4. the doped oxide particles are doped zinc oxide particles; The surface-treated steel sheet according to claim 3.

5. The organic lubricant is a polyolefin wax. The surface-treated steel sheet according to claim 3.

6. The rust inhibitor is an organic acid. The surface-treated steel sheet according to claim 3.

7. the coating film does not contain any inorganic substances other than the Zn-containing compound containing the doped zinc oxide particles, or if it does contain any inorganic substances, the amount of such inorganic substances is 1 mass % or less relative to the coating film; The surface-treated steel sheet according to claim 1.

8. The surface roughness Ra of the coating film is 0.1 to 10 μm. The surface-treated steel sheet according to claim 1.

Images