Surface-treated metal sheets and automotive components

Abstract

This surface-treated metal sheet comprises a metal sheet and a coating film formed on a surface of the metal sheet and having a thickness of 0.5-5.0 μm. The coating film comprises a binder resin, doped oxide particles, and a rust-preventive pigment. The content of the doped oxide particles is 5-20 vol% with respect to the coating film. The content of the rust-preventive pigment is 20-50 vol% with respect to the coating film. The ratio of the average particle diameter (B) of the doped oxide particles to the average particle diameter (A) of the rust-preventive pigment, B / A, is 0.75-4.00.

Description

[Technical Field]

[0001] The present invention relates to a surface-treated metal sheet and an automobile component. [Background technology]

[0002] Many automotive components are made from metal sheets such as steel sheets and are manufactured through a number of processes, including (1) cutting the metal sheets to a specified size, (2) cleaning the cut metal sheets, (3) press-forming the cleaned metal sheets, (4) joining the formed materials by spot welding or adhesive bonding, (5) degreasing and cleaning the press oil from the surfaces of the joined components, (6) chemical conversion treatment, and (7) painting. Furthermore, automotive components used as exterior panels typically undergo further painting processes, including (8) a primer coat and (9) a top coat. Therefore, there is a strong demand in the automotive industry for cost reductions through simplification of manufacturing processes, particularly the chemical conversion treatment and painting processes.

[0003] Furthermore, the corrosion resistance of automotive components is often ensured by a chemical conversion treatment layer formed through a chemical conversion treatment process and an electrodeposition coating film formed through a subsequent electrodeposition coating process. However, the joints of molded materials (particularly the inner plate joints and folded hems of bag-shaped components) lack the electrodeposition coating and are likely to be exposed to a corrosive environment in their bare state. For this reason, secondary rust-preventive materials such as sealers and rust-preventive waxes are used to supplement the corrosion resistance of the joints of molded materials. The use of these secondary rust-preventive materials not only increases automobile manufacturing costs, but also reduces productivity and increases vehicle weight. For this reason, there is a high demand for automotive components that can maintain corrosion resistance even while reducing the use of these secondary rust-preventive materials.

[0004] In response to these needs, research and development of surface-treated metal sheets that can simultaneously simplify the chemical conversion coating and electrodeposition coating processes during automobile manufacturing and reduce the amount of secondary rust-preventive materials has been actively pursued. Such surface-treated metal sheets are assembled into the desired shape, for example, by press forming, spot welding, or the like, and then electrodeposition coated. Therefore, it is necessary to impart electrical conductivity and corrosion resistance to the coating film so that the surface-treated metal sheets can be resistance-welded and / or electrodeposition coated.

[0005] For example, Patent Document 1 discloses a surface-treated metal sheet that meets the above needs, in which a coating film containing a binder resin, oxide particles, non-oxide ceramic particles, and an anti-rust pigment is formed on the surface of the metal sheet. The oxide particles dissolve during the chemical conversion treatment process, imparting an anchoring effect to the chemical conversion treatment layer, thereby improving the adhesion of the subsequently formed electrodeposition coating film to the surface-treated metal sheet. The non-oxide ceramic particles function as conductive particles, imparting conductivity to the coating film. The anti-rust pigment imparts corrosion resistance to the surface-treated metal sheet. As a result, the surface-treated metal sheet described in Patent Document 1 has excellent adhesion, weldability, and corrosion resistance of the electrodeposition coating film. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2016 / 159138 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the surface-treated metal sheet described in Patent Document 1 has room for improvement in terms of manufacturing costs. Specifically, the surface-treated metal sheet described in Patent Document 1 uses expensive non-oxide ceramic particles as conductive particles. This significantly increases the manufacturing cost of the surface-treated metal sheet, undermining the cost benefits of reducing the use of secondary rust-preventing materials.

[0008] An object of the present invention is to provide a surface-treated metal sheet that can be resistance welded and electrodeposition coated, has excellent corrosion resistance, and can be produced without using non-oxide ceramic particles, i.e., does not contain non-oxide ceramic particles. Another object of the present invention is to provide an automotive component that includes the surface-treated metal sheet, i.e., an automotive component that includes a surface-treated metal sheet that can be resistance welded and electrodeposition coated, has excellent corrosion resistance, and does not contain non-oxide ceramic particles. [Means for solving the problem]

[0009] The present invention relates to the following surface-treated metal sheet and automotive component.

[0010] [1] A surface-treated metal sheet comprising a metal sheet and a coating film having a thickness of 0.5 to 5.0 μm disposed on the surface of the metal sheet, the coating film containing a binder resin, doped oxide particles, and an anti-rust pigment, the content of the doped oxide particles being 5 to 20% by volume of the coating film, the content of the anti-rust pigment being 20 to 50% by volume of the coating film, and the ratio (B / A) of the average particle size (B) of the doped oxide particles to the average particle size (A) of the anti-rust pigment being 0.75 to 4.00. [2] The surface-treated metal sheet according to [1], wherein the doped oxide particles have an average particle size of 1.0 to 4.0 μm. [3] The surface-treated metal sheet according to [1] or [2], wherein the doped oxide particles comprise at least one selected from the group consisting of zinc oxide particles doped with Al, Ga, or In, tin oxide particles doped with P, Sb, or As, indium oxide particles doped with Sn or Ge, titanium oxide particles coated with zinc oxide doped with Al, Ga, or In, titanium oxide particles coated with tin oxide doped with P, Sb, or As, and titanium oxide particles coated with indium oxide doped with Sn or Ge. [4] The surface-treated metal sheet according to any one of [1] to [3], wherein the anti-rust pigment has an average particle size of 0.5 to 4.0 μm. [5] The surface-treated metal sheet according to any one of [1] to [4], wherein the anti-rust pigment comprises at least one selected from the group consisting of phosphate compounds, silicate compounds, amorphous silica, and vanadate compounds. [6] The surface-treated metal sheet according to any one of [1] to [5], wherein the binder resin is a water-soluble or water-dispersible aqueous resin. [7] The surface-treated metal sheet according to [6], wherein the binder resin is an epoxy resin. [8] The surface-treated metal sheet according to any one of [1] to [7], wherein the coating film does not contain any of non-oxide ceramic particles, iron alloy particles, and stainless steel particles. [9] An automobile component including a painted metal sheet, the painted metal sheet having the surface-treated metal sheet according to any one of [1] to [8], a chemical conversion coating layer disposed on the surface of the coating film, and an electrodeposition coating film disposed on the surface of the chemical conversion coating layer. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a surface-treated metal sheet that can be resistance welded and electrodeposited, has excellent corrosion resistance, and can be produced more cheaply than conventional methods, and an automobile member having the same. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram showing a cross section of a surface-treated metal sheet according to one embodiment of the present invention. [Figure 2] FIG. 2A is a schematic diagram showing the cross section of a coating film when the ratio of average particle diameters is within a predetermined range, and FIG. 2B is a schematic diagram showing the cross section of a coating film when the ratio of average particle diameters is outside the predetermined range. [Figure 3] FIG. 3A is a backscattered electron image (composition image) of a cross section of a coating film of a surface-treated metal sheet according to one embodiment of the present invention, and FIGS. 3B to 3G are element mapping images of various elements in the same field of view. [Figure 4]FIG. 4A is a backscattered electron image (composition image) of the cross section of the coating film of a surface-treated metal sheet for comparison, and FIGS. 4B to 4G are element mapping images of various elements in the same field of view. [Figure 5] FIG. 5 is a schematic diagram showing a cross section of a coated metal sheet according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment of the present invention will be described in detail below with reference to the drawings, although the present invention is not limited thereto.

[0014] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In numerical ranges described in stages in this specification, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages.

[0015] <Surface-treated metal sheet> A surface-treated metal sheet according to one embodiment of the present invention includes a metal sheet and a coating film disposed on the surface of the metal sheet. The coating film is disposed on at least one main surface of the metal sheet and includes a binder resin, doped oxide particles, and an anti-corrosion pigment.

[0016] Fig. 1 is a schematic diagram showing a cross section of a surface-treated metal sheet according to one embodiment of the present invention. In the example shown in Fig. 1, the surface-treated metal sheet 100 includes a metal sheet 110 and a coating film 120 disposed on one surface of the metal sheet 110. The coating film 120 includes a binder resin 122, doped oxide particles 124, and an anti-rust pigment 126.

[0017] Each component of the surface-treated metal sheet will be described below.

[0018] [Metal plate] The type of metal plate is not particularly limited and can be appropriately selected depending on the application. Examples of materials for the metal plate include steel (iron-based alloy), aluminum, aluminum alloy, magnesium, and magnesium alloy.

[0019] The steel sheet may be an ordinary steel sheet or a special steel sheet containing additive elements such as chromium. However, when press-forming is performed, it is preferable that the steel sheet be one in which the types and amounts of additive elements and the metal structure are appropriately controlled so as to have the desired formability. The steel sheet may also be a plated steel sheet. Examples of plated steel sheets include zinc-based plated steel sheets and aluminum-based plated steel sheets.

[0020] Examples of zinc-based plating layers on zinc-based plated steel sheets include zinc plating layers; alloy plating layers of zinc and at least one selected from the group consisting of aluminum, cobalt, tin, nickel, iron, chromium, titanium, magnesium, and manganese; and various zinc-based alloy plating layers containing other metallic or non-metallic elements (e.g., quaternary alloy plating layers of zinc, aluminum, magnesium, and silicon). However, the alloy components other than zinc in the zinc-based plating layer are not particularly limited. These zinc-based plating layers may further contain small amounts of dissimilar metallic 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 contain inorganic substances such as silica, alumina, and titania.

[0021] Examples of the aluminum-based plating layer of the aluminum-based plated steel sheet include a plating layer made of aluminum; and an alloy plating layer of aluminum and at least one selected from the group consisting of silicon, zinc, and magnesium (for example, an aluminum-silicon alloy plating layer, an aluminum-zinc alloy plating layer, or an aluminum, silicon, and magnesium ternary alloy plating layer).

[0022] The zinc-based plated steel sheet and the aluminum-based plated steel sheet may be a multi-layer plated steel sheet in which they are combined with other types of plating layers (for example, an iron plating layer, an iron and phosphorus alloy plating layer, a nickel plating layer, a cobalt plating layer, etc.).

[0023] The method for forming the plating layer is not particularly limited. For example, electroplating, electroless plating, hot dip plating, vapor deposition plating, dispersion plating, etc. can be used to form the plating layer. The plating layer can be formed by either a continuous system or a batch system. After the plating layer is formed, it may be subjected to treatments such as zero spangle treatment for uniform appearance, annealing treatment for modifying the plating layer, and temper rolling for adjusting the surface condition or material properties.

[0024] [Coating] The coating film is disposed on at least one main surface (i.e., on at least one surface) of the metal plate. "Main surfaces" refers to the two relatively large surfaces (the front surface and the back surface) of the metal plate. Depending on the application, the coating film may be formed on both sides (both main surfaces) of the metal plate, or on only one side (one main surface) of the metal plate. Furthermore, the coating film may be formed on only a portion of the surface of the metal plate, or on the entire surface of the metal plate.

[0025] The coating film contains a binder resin, doped oxide particles, and an anti-rust pigment. The coating film may contain other additives, etc., as necessary.

[0026] (binder resin) The binder resin functions as a binder that binds the various components in the coating film. The binder resin may be 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. From the viewpoints of production cost and environmental friendliness, the binder resin is preferably an aqueous resin.

[0027] The type of aqueous resin is not particularly limited. Examples of the aqueous resin include water-soluble or water-dispersible resins such as epoxy resins, urethane resins, polyester resins, acrylic resins, phenolic resins, and mixed resins of two or more of these resins.

[0028] When an epoxy resin is used as the aqueous resin, the number-average molecular weight (Mn) of the epoxy resin is not particularly limited, but is preferably 1,400 to 20,000, more preferably 2,000 to 10,000, and particularly preferably 2,000 to 4,000. That is, the lower limit of the number-average molecular weight (Mn) of the epoxy resin is preferably 1,400 or more, more preferably 2,000 or more. The upper limit of the number-average molecular weight (Mn) of the epoxy resin is preferably 20,000 or less, more preferably 10,000 or less, and particularly preferably 4,000 or less. By having the number-average molecular weight of the epoxy resin be 1,400 to 20,000, the crosslinking reaction can proceed sufficiently when the epoxy resin is crosslinked, thereby improving the corrosion resistance of the surface-treated metal sheet. Furthermore, excessive crosslink density of the coating film can be prevented, thereby maintaining the processability of the coating film. In this specification, the term "epoxy resin" simply refers to at least one selected from the group consisting of epoxy resins and modified epoxy resins. In this specification, the number average molecular weight (Mn) of a resin means the number average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC).

[0029] The glass transition temperature (Tg) of the epoxy resin is not particularly limited, but is, for example, 120°C or lower, and may be 115°C or lower, or may be 110°C or lower. The glass transition temperature (Tg) of the epoxy resin is, for example, 50°C or higher, and may be 55°C or higher. For example, the glass transition temperature (Tg) of the epoxy resin may be 50 to 120°C. When the glass transition temperature (Tg) of the epoxy resin is 50 to 120°C, the corrosion resistance of the surface-treated metal sheet can be improved without excessively increasing the moisture permeability of the coating film. The glass transition temperature (Tg) can be measured, for example, using a thermal analyzer TMA7100 (Hitachi High-Tech Science Corporation).

[0030] The acid value of the epoxy resin (including modified products thereof) is not particularly limited, but is, for example, 0 to 30 mgKOH / g. In this specification, the acid value means the acid value of the solid content, and can be measured in accordance with the provisions of JIS K 0070.

[0031] The epoxy resin is preferably in the form of an emulsion with an emulsion particle size of 10 to 100 nm (preferably 20 to 60 nm). If the emulsion particle size is too small, the production cost may increase. On the other hand, if the emulsion particle size is too large, the gaps between the emulsion particles may become large when the emulsion is formed into a coating film, which may reduce the barrier properties of the coating film.

[0032] The type of epoxy resin is not particularly limited. The epoxy resin may be a hydroxyl group-containing epoxy resin (including modified hydroxyl group-containing epoxy resins). Examples of epoxy resins include resins obtained by condensing epichlorohydrin and bisphenol to a high molecular weight, optionally in the presence of a catalyst such as an alkali catalyst; bisphenol-type epoxy resins such as bisphenol A and bisphenol F; and novolac-type epoxy resins. Examples of modified epoxy resins include modified epoxy resins such as acrylic-modified epoxy resins, urethane-modified epoxy resins, and amine-modified epoxy resins. For example, acrylic-modified epoxy resins can be prepared by reacting the bisphenol-type epoxy resin or the novolac-type epoxy resin with a polymerizable unsaturated monomer component containing acrylic acid, methacrylic acid, or the like. Furthermore, urethane-modified epoxy resins can be prepared by reacting the bisphenol-type epoxy resin or the novolac-type epoxy resin with a polyisocyanate compound. These epoxies may be used alone or in combination.

[0033] When a polyester resin is used as the aqueous resin, the number-average molecular weight (Mn) of the polyester resin is not particularly limited, but is preferably 10,000 to 30,000. If the number-average molecular weight of the polyester resin is less than 10,000, it may be difficult to ensure sufficient processability. On the other hand, if the number-average molecular weight of the polyester resin exceeds 30,000, the adhesion between the coating film and the electrodeposition coating film formed thereon may decrease. Furthermore, when crosslinking is performed using a curing agent such as melamine, the crosslinking reaction may not proceed sufficiently, resulting in a decrease in the performance of the coating film.

[0034] When a urethane resin is used as the aqueous resin, 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). If the emulsion particle size is excessively small, the cost may be high. On the other hand, if the emulsion particle size is excessively large, the gaps between the emulsion particles may become large when the coating film is formed, and the barrier properties of the coating film may be reduced. The type of urethane resin is not particularly limited. Examples of urethane resins include polyether-based urethane resins, polycarbonate-based urethane resins, and polyester-based urethane resins. These may be used alone or in combination.

[0035] On the other hand, examples of solvent-based resins include epoxy resins, polyester resins, urethane resins, acrylic resins, and mixed resins of two or more of these resins.

[0036] The binder resin may be a crosslinked resin having a crosslinked structure or a non-crosslinked resin having no crosslinked structure. From the viewpoint of low-temperature film formation of the coating film, the binder resin is preferably a non-crosslinked resin.

[0037] A water-soluble crosslinking agent is preferred as the crosslinking agent (curing agent) that imparts a crosslinked structure to the binder resin. Specifically, the crosslinking agent is preferably melamine, isocyanate, or the like. The amount of crosslinking agent added is not particularly limited, but is preferably 5 to 30 parts by mass per 100 parts by mass of resin solids. If the amount of crosslinking agent added is less than 5 parts by mass, the crosslinking reaction with the resin may not proceed sufficiently, resulting in insufficient performance of the coating film. On the other hand, if the amount of crosslinking agent added is more than 30 parts by mass, the crosslinking reaction may proceed too far, resulting in the coating film becoming excessively hard and reducing processability.

[0038] The binder resin content is not particularly limited, but is preferably 20 to 80% by mass relative to the coating film (total solid content of the coating film). If the binder resin content is less than 20% by mass, the binder function will not be exhibited, the cohesive strength of the coating film will decrease, and the coating film may become more susceptible to cohesive failure. If the binder resin content exceeds 80% by mass, the proportion of pigment components (e.g., doped oxide particles and rust-preventive pigments) contained in the coating film will decrease, making it difficult to achieve electrical conductivity, corrosion resistance, and adhesion. In order to exhibit the binder function and achieve electrical conductivity, corrosion resistance, and adhesion, the binder resin content is more preferably 25 to 70% by mass relative to the coating film (total solid content of the coating film), and even more preferably 30 to 60% by mass. That is, the lower limit of the binder resin content relative to the coating film (total solid content of the coating film) is 20% by mass or more, preferably 25% by mass or more, and more preferably 30% by mass or more. The upper limit of the binder resin content is 80% by mass or less, preferably 70% by mass or less, and more preferably 60% by mass or less, based on the coating film (total solid content of the coating film).

[0039] The binder resin content is calculated using the following mass spectrometry method. A sample is prepared by scraping off the coating from the surface-treated metal plate, and the resulting sample is analyzed using pyrolysis gas chromatography-mass spectrometry (GC-MS). Specifically, a fixed amount of polystyrene is added to the resulting sample as a standard substance, and the sample is then heated to 600°C in a GC-MS device (e.g., an Agilent GC system "7890B") for thermal decomposition. The decomposition products obtained by thermal decomposition are analyzed by GC-MS and their type and amount are identified through peak analysis. The amount of binder resin is determined by comparing the peak area of ​​the styrene monomer corresponding to the known mass (fixed amount) of polystyrene added as the standard substance with the peak area of ​​all remaining peaks corresponding to the binder resin. The binder resin content (mass%) in the coating is determined by calculating the ratio of the amount of binder resin to the mass of the scraped sample (coating).

[0040] When the binder resin contains an epoxy resin, a urethane resin, and / or a polyester resin, the total amount of the epoxy resin, the urethane resin, and the polyester resin relative to the entire binder resin is preferably 50% or more, more preferably more than 50%, and particularly preferably 60% by volume or more, from the viewpoints of adhesion to the metal plate and retention of the pigment component. The binder resin may be composed of a combination of an epoxy resin, a urethane resin, and a polyester resin. In other words, the total content of the epoxy resin, the urethane resin, and the polyester resin in the binder resin may be 100%.

[0041] (Doped oxide particles) Doped oxide particles are particles whose surfaces are composed of metal oxides containing doping elements (impurities). The doped oxide particles are electrically conductive and impart electrical conductivity to the coating film. The surface-treated steel sheet according to this embodiment contains doped oxide particles in the coating film, and therefore can be subjected to resistance welding and electrodeposition coating even if the coating film does not contain other electrically conductive particles such as non-oxide ceramic particles, iron alloy particles, or stainless steel particles.

[0042] Furthermore, when the surface-treated steel sheet according to this embodiment is subjected to painting such as electrodeposition coating, a chemical conversion treatment is often performed before painting to form a chemical conversion treatment layer. A typical example of such a chemical conversion treatment layer is an acid chloride layer such as a phosphate coating. The chemical conversion treatment solution for forming such an acid chloride layer is acidic (e.g., pH 2 to 3), and doped oxide particles have the property of dissolving in an acidic chemical conversion treatment solution. Therefore, when the surface-treated steel sheet according to this embodiment is chemically treated using an acidic chemical conversion treatment solution, the doped oxide particles in the coating surface dissolve, and the pH in the vicinity increases, causing components of the chemical conversion treatment solution (e.g., acid chlorides such as phosphates) to precipitate and grow in the areas where the doped oxide particles have dissolved. As a result, it is believed that the components of the chemical conversion treatment solution grow in a wedge shape, protruding from the interior of the coating surface layer to the surface. When a paint film is formed by electrodeposition coating or other methods on a chemical conversion treatment layer with protruding crystals of the chemical conversion treatment solution components, the adhesion between the paint film and the coating is further enhanced by the anchoring effect of the wedge-shaped crystals (for example, crystals of acid chlorides such as phosphates) in addition to the high adhesion provided by the chemical properties of the chemical conversion treatment solution components (see Figure 5).

[0043] As mentioned above, the coating film also contains an anti-rust pigment. Depending on the type, this anti-rust pigment can be dissolved in an acidic chemical conversion treatment solution. However, if doped oxide particles are contained in the coating film together with the anti-rust pigment, the doped oxide particles are actively dissolved by the acidic chemical conversion treatment solution, making the anti-rust pigment less likely to dissolve. This further improves the corrosion resistance of the surface-treated steel sheet according to this embodiment.

[0044] The metal oxide (metal oxide doped with a doping element) that constitutes the doped oxide particles is not particularly limited as long as it can impart conductivity to the coating film, and examples include zinc oxide (ZnO), tin oxide (SnO2), and indium oxide (In2O3).

[0045] The doped oxide particles may be composed entirely of a metal oxide containing a doping element, or only the surface layer of the particle may be composed of a metal oxide containing a doping element. For example, the doped oxide particles may be zinc oxide particles doped with a doping element, tin oxide particles doped with a doping element, or indium oxide particles doped with a doping element. The doped oxide particles may also be particles composed of other metal oxides coated with zinc oxide, tin oxide, or indium oxide doped with a doping element. Examples of other metal oxides include titanium oxide (TiO).

[0046] The doping element is an element having a different number of valence electrons from the metal element (e.g., Zn, Sn, or In) contained in the metal oxide. For example, for zinc oxide (ZnO, valence 2), it is preferable to select at least one element selected from the group consisting of Group 13 elements (valence 3) or Group 15 elements (valence 5) of the periodic table as the doping element. Among these, Al, Ga, and In, which are Group 13 elements with similar valences, are more preferable, with Al and Ga being particularly preferable. Furthermore, for tin oxide (SnO2, valence 4), it is preferable to select at least one element selected from Group 15 elements (valence 5) of the periodic table as the doping element. Among these, P, Sb, and As are more preferable, with P and Sb being particularly preferable. Furthermore, for indium oxide (In2O3, valence 3), it is preferable to select at least one element selected from Group 14 elements (valence 4) of the periodic table as the doping element. Among these, Sn and Ge are more preferable, with Sn being particularly preferable. By combining a metal oxide with a doping element as described above, the metal oxide becomes an n-type semiconductor and conducts electricity.

[0047] For example, the doped oxide particles include at least one selected from the group consisting of zinc oxide particles doped with Al, Ga, or In, tin oxide particles doped with P, Sb, or As, indium oxide particles doped with Sn or Ge, titanium oxide particles coated with zinc oxide doped with Al, Ga, or In, titanium oxide particles coated with tin oxide doped with P, Sb, or As, and titanium oxide particles coated with indium oxide doped with Sn or Ge.

[0048] 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 metal oxide.

[0049] The shape of the doped oxide particles is not particularly limited, but is preferably a nearly spherical shape, such as a sphere, pseudo-sphere (e.g., an oblong sphere, an ellipsoid, an egg shape, a rugby ball shape, etc.), or a polyhedron (e.g., a soccer ball shape (truncated icosahedron), a cube shape (cube), or the brilliant-cut shapes of various gemstones, etc.). Doped oxide particles having a nearly spherical shape tend to be easily dispersed uniformly throughout the coating film, making it easier to form effective electrical paths that penetrate the coating film in the thickness direction uniformly throughout the entire coating film, further improving the conductivity of the coating film.

[0050] As will be explained later, the ratio (B / A) of the average particle size (B) of the doped oxide particles to the average particle size (A) of the anti-rust pigment is preferably 0.75 to 4.00. By keeping the average particle size ratio within the above range, it becomes easier to form an effective electrical path that penetrates the coating film in the thickness direction, and sufficient conductivity can be imparted to the coating film.

[0051] The average particle size of the doped oxide particles is not particularly limited as long as the above average particle size ratio is satisfied, but the lower limit is preferably 0.1 μm or more, more preferably 0.4 μm or more, and particularly preferably 1.0 μm or more. The upper limit of the average particle size of the doped oxide particles is preferably 7.0 μm or less, more preferably 5.0 μm or less, and particularly preferably 4.0 μm or less. By setting the average particle size of the doped oxide particles within the above range, the conductivity of the coating film and the anchoring effect of the chemical conversion coating layer can be more easily improved.

[0052] In this specification, the "average particle size" of doped oxide particles refers to the average primary particle size when the doped oxide particles are present alone in the coating film, and refers to the average secondary particle size, which represents the particle size of the doped oxide particles when they are aggregated together in the coating film. The average particle size of doped oxide particles is calculated by the following cross-sectional observation. The cross-section of the thickness direction of the surface-treated metal sheet is mirror-polished, and the cross-section of the coating film is photographed at 5000x magnification (field of view: 24 μm × 18 μm) using a scanning electron microscope (JEOL Ltd., "JSM-7100F", acceleration voltage: 15 kV). From the obtained backscattered electron image, 10 doped oxide particles with relatively large particle sizes are randomly selected. For each particle, the average values ​​of the long and short side lengths measured using image processing software (Image J Ver. 1.54h) are calculated to determine the particle size. The arithmetic mean of the particle sizes of the 10 particles is calculated to determine the average particle size. The same measurement is carried out for 10 backscattered electron images, and the arithmetic mean value of the backscattered electron images is taken as the average particle size of the doped oxide particles.

[0053] The content of the doped oxide particles is 5 to 20% by volume, preferably 10 to 15% by volume, based on the total solid content of the coating film. That is, the lower limit of the content of the doped oxide particles is 5% by volume or more, preferably 10% by volume or more, based on the total solid content of the coating film. The upper limit of the content of the doped oxide particles is 20% by volume or less, preferably 15% by volume or less, based on the total solid content of the coating film. If the content of the doped oxide particles is less than 5% by volume, the coating film may not be sufficiently conductive. Furthermore, the adhesiveness-improving effect of the anchoring effect of the chemical conversion coating layer formed thereon may also be difficult to achieve. On the other hand, if the content of the doped oxide particles exceeds 20% by volume, the cohesive strength of the coating film may decrease, resulting in a decrease in the adhesiveness between the coating film and the electrodeposition coating film formed thereon. For example, the content of the doped oxide particles is preferably 15% by volume or less, and may be less than 15% by volume.

[0054] The content of doped oxide particles in the coating film is calculated by the following cross-sectional observation. For a backscattered electron image obtained in the same manner as in the above-mentioned method for measuring the average particle size of doped oxide particles, the area occupied by the coating film and the area occupied by the doped oxide particles are measured using image processing software (Image J). Since the volumetric proportion of each component in the coating film roughly corresponds to the area proportion of each component when the coating film is observed in cross-section, the measured area proportion of the doped oxide particles is taken as the volume proportion of the doped oxide particles. Similar measurements are performed on 10 backscattered electron images, and the arithmetic mean value is taken as the content (volume %) of doped oxide particles.

[0055] (Anti-rust pigment) The anti-rust pigment improves the corrosion resistance of the surface-treated metal sheet, particularly the metal sheet surface adjacent to the coating film. The anti-rust pigment is not particularly limited, but preferably contains at least one selected from the group consisting of phosphate compounds, silicate compounds, amorphous silica, and vanadate compounds.

[0056] Phosphate compounds, silicate compounds, amorphous silica, and vanadate compounds can release silicate ions, phosphate ions, vanadate ions, or countercations of these anions (e.g., alkaline earth metal ions, Zn ions, Al ions, etc.) in aqueous coating compositions or coating films in response to changes in the surrounding environment. Among these ions, for example, phosphate ions react with metal ions on metal sheets to form a sparingly soluble film, thereby inhibiting corrosion. Silicate ions form an inorganic polymer layer (barrier layer) on the surface of the metal sheet, protecting the metal sheet from corrosion factors. Like phosphate ions, vanadate ions form a film on the surface of the metal sheet, thereby inhibiting corrosion. Oxidizing ions such as vanadate ions are thought to promote the formation of the sparingly soluble film or inorganic polymer layer, thereby further inhibiting corrosion. While the addition of these rust-preventive pigments (ions) alone can improve corrosion resistance, the addition of multiple types in combination can achieve a combined effect, further improving corrosion resistance.

[0057] Examples of phosphate compounds include metal salts of orthophosphoric acid, polyphosphoric acid (linear polymers of orthophosphoric acid with a degree of polymerization of up to 6, or mixtures of two or more of these), metaphosphoric acid (cyclic polymers of orthophosphoric acid with a degree of polymerization of 3 to 6, or mixtures of two or more of these), tetrametaphosphate, and hexametaphosphate; phosphorus pentoxide; phosphate minerals such as monetite, torfilite, whitlockite, xenotime, stuccolite, strubite, and orthorite; commercially available complex phosphate pigments such as silica polyphosphate and tripolyphosphate; metal salts of phytic acid, phosphonic acid (phosphorous acid), and phosphinic acid (hypophosphorous acid); and mixtures of two or more of these. Orthophosphates as used herein include their monohydrogen salts (HPO4 2- ), dihydrogen salt (H2PO4 -) are included. Polyphosphates also include hydrogen salts. The cationic species forming the phosphate is not particularly limited, and examples include metal ions such as Co, Cu, Fe, Mn, Nb, Ni, Sn, Ti, V, Y, Zr, Al, Ba, Ca, Mg, Sr, and Zn; and oxocations such as vanadyl, titanyl, and zirconyl. Of these, preferred cationic species forming the phosphate are Al, Ca, Mg, Mn, and Ni. The phosphate compounds may be used alone or in combination of two or more.

[0058] It is not preferable to use a large amount of alkali metal as a cationic species forming a phosphate. When an alkali metal phosphate is used, the product obtained by calcination in an industrial production process tends to be too soluble in water. However, when an alkali metal phosphate is used, if the solubility in water can be controlled during the production of the anti-rust pigment, the production of an aqueous composition for forming a coating film, the formation of a coating film on a metal plate, or the use of a surface-treated metal plate, a slightly larger amount of alkali metal may be used. Examples of such control include controlling the dissolution rate in water by coexisting the anti-rust pigment with other additives that suppress its solubility in water, or by coexisting it with a highly crosslinked resin or inorganic polymer.

[0059] Examples of silicate compounds include alkaline earth metal silicates such as magnesium silicate and calcium silicate; alkali metal silicates such as lithium silicate, sodium silicate, and potassium silicate; and aluminum silicate. Among these, lithium silicate, sodium silicate, and potassium silicate include, for example, lithium silicate in which the molar ratio of silicon oxide (SiO) to lithium oxide (LiO) is 0.5≦(SiO / LiO)≦8, sodium silicate in which the molar ratio of silicon oxide (SiO) to sodium oxide (NaO) is 0.5≦(SiO / NaO)≦4, potassium silicate in which the molar ratio of silicon oxide (SiO) to potassium oxide (KO) is 0.5≦(SiO / KO)≦4, and hydrates of these silicates. Specific examples of these include lithium orthosilicate (Li4SiO4; 2Li2O·SiO2), hexalithium orthodisilicate (Li6Si2O7; 3Li2O·2SiO2), lithium metasilicate (Li2SiO3; Li2O·SiO2), lithium disilicate (Li2Si2O5; Li2O·2SiO2), tetralithium heptasilicate (2Li2O·7SiO2), lithium tetrasilicate (Li2Si4O9; Li2O·4SiO2), tetralithium nonasilicate (2Li2O·9SiO2), tetralithium hexapesilicate (2Li2O·15SiO2), and sodium orthosilicate. Examples of silicate compounds include silicate silicate (NaSiO; 2NaO·SiO), sodium metasilicate (NaSiO; NaO·SiO), sodium disilicate (NaSiO; NaO·2SiO), sodium tetrasilicate (NaSiO; NaO·4SiO), potassium orthosilicate (KSiO; 2KO·SiO), potassium metasilicate (KSiO; KO·SiO), potassium disilicate (KSiO; KO·2SiO), potassium tetrasilicate (KSiO; KO·4SiO), and hydrates of these silicates. Many of these silicate hydrates readily gel while in a hydrated state due to environmental changes such as pH and temperature, and some may polymerize to form polysilicates. Such polysilicates are also included in the silicate compounds applicable to the present invention.

[0060] Examples of amorphous silica include those with an oil absorption of 100 to 1000 ml / 100 g and a specific surface area of ​​200 to 1000 m 2 The oil absorption of silica can be measured according to JIS K 5101-13-2. The specific surface area of ​​silica can be measured by the BET method.

[0061] Vanadate compounds are composite compounds in which vanadium has a valence of 0, 2, 3, 4, or 5, or two or more valencies. Examples include oxides, hydroxides, oxyacid salts of various metals, vanadyl compounds, halides, sulfates, and metal powders. These decompose when heated or in the presence of water and react with coexisting oxygen. For example, vanadium metal powder or divalent compounds ultimately convert to trivalent, tetravalent, or pentavalent compounds. While zero-valent compounds, such as vanadium metal powder, can be used for the reasons mentioned above, they are not practically preferred due to problems such as insufficient oxidation reactions. Pentavalent vanadium compounds contain vanadate ions and are prone to react with phosphate ions during heating to form heteropolymers that contribute to rust prevention. Therefore, it is preferable to include a pentavalent vanadium compound as a component. Specific examples of vanadium compounds include vanadium(II) compounds such as vanadium(II) oxide and vanadium(II) hydroxide, vanadium(III) compounds such as vanadium(III) oxide, vanadium(IV) compounds such as vanadium(IV) oxide and vanadyl halides, vanadium(V) oxide, vanadium(V) compounds such as vanadates (orthovanadates, metavanadates, pyrovanadates, etc. of various metals), and mixtures thereof. Preferred metal species constituting vanadates are the same as those listed for phosphates.

[0062] When alkali metal vanadates are used, the product obtained by calcination in the industrial production process tends to dissolve too much in water, so as with phosphates, using large amounts of alkali metal vanadates is not recommended. However, as with alkali metal phosphates, if the solubility in water can be controlled, their use is acceptable. The same applies to vanadium halides and sulfates.

[0063] As will be explained later, the ratio (B / A) of the average particle size (B) of the doped oxide particles to the average particle size (A) of the anti-rust pigment is preferably 0.75 to 4.00. By keeping the average particle size ratio within the above range, it becomes easier to form an effective electrical path that penetrates the coating film in the thickness direction, and sufficient conductivity can be imparted to the coating film.

[0064] The average particle size of the anti-rust pigment is not particularly limited as long as it satisfies the above average particle size ratio, but the lower limit is preferably 0.2 μm or more, more preferably 0.5 μm or more, and particularly preferably 1.0 μm or more. The upper limit of the average particle size of the anti-rust pigment is preferably 5.0 μm or less, more preferably 4.0 μm or less, and particularly preferably 2.0 μm or less. By setting the average particle size of the anti-rust pigment within the above range, the conductivity of the coating film can be more easily improved.

[0065] In this specification, the "average particle size" of an anti-rust pigment refers to the average primary particle size when the anti-rust pigment is present alone in the coating film, and refers to the average secondary particle size, which represents the particle size of the anti-rust pigment when aggregated together in the coating film. The average particle size of the anti-rust pigment is calculated by the following cross-sectional observation. Ten anti-rust pigment particles with relatively large particle sizes are randomly selected from backscattered electron images obtained using the same method as for measuring the average particle size of doped oxide particles described above. For each particle, the average values ​​of the long and short side lengths measured using image processing software (Image J) are calculated to determine the particle size of the particle. The arithmetic mean of the particle sizes of the 10 particles is calculated to determine the average particle size. Similar measurements are performed on 10 backscattered electron images, and the arithmetic mean of the backscattered electron images is used to determine the average particle size of the anti-rust pigment.

[0066] As will be explained later, in the surface-treated metal sheet according to this embodiment, the thickness of the coating film is reduced in order to ensure the conductivity of the coating film. To provide the surface-treated metal sheet with sufficient corrosion resistance even with such a thin coating film, the content of the rust-preventive pigment is 20 to 50% by volume of the coating film (total solid content of the coating film). If the content of the rust-preventive pigment is less than 20% by volume, the corrosion resistance may be insufficient. If the content of the rust-preventive pigment exceeds 50% by volume, the workability and cohesive strength of the coating film may decrease. For example, the content of the rust-preventive pigment is preferably 25% by volume or more, and may be 30% by volume or more, or may exceed 30% by volume. The content of the rust-preventive pigment is preferably 45% by volume or less.

[0067] The content of the anti-rust pigment in the coating film is calculated by the following cross-sectional observation. For a backscattered electron image obtained in the same manner as the above-mentioned method for measuring the average particle size of doped oxide particles, the area occupied by the coating film and the area occupied by the anti-rust pigment are measured using image processing software (Image J). Since the volumetric proportion of each component in the coating film roughly corresponds to the area proportion of each component when the coating film is observed in cross-section, the measured area proportion of the anti-rust pigment is taken as the volume proportion of the anti-rust pigment. Similar measurements are performed on 10 backscattered electron images, and the arithmetic mean value is taken as the content (volume %) of the anti-rust pigment.

[0068] (ratio of average particle size of doped oxide particles and anti-corrosion pigment) As described above, the coating film contains doped oxide particles for imparting electrical conductivity and an anti-rust pigment for imparting corrosion resistance. If the doped oxide particles in the coating film cannot form an effective electrical path that penetrates the coating film in the thickness direction, the coating film will not be able to be imparted with appropriate electrical conductivity. From this perspective, in the surface-treated metal sheet according to this embodiment, the ratio (B / A) of the average particle size (B) of the doped oxide particles to the average particle size (A) of the anti-rust pigment is 0.75 to 4.00, preferably 1.00 or more, and preferably 3.00 or less.

[0069] Fig. 2A is a schematic diagram showing the cross section of a coating film when the ratio of average particle diameters (B / A) is 2.40, and Fig. 2B is a schematic diagram showing the cross section of a coating film when the ratio of average particle diameters (B / A) is 0.42. As shown in Fig. 2A, when the ratio of average particle diameters (B / A) is within the range of 0.75 to 4, that is, when the doped oxide particles 124 are somewhat larger than the anticorrosive pigment 126, the doped oxide particles 124 can efficiently form effective electrical paths E that penetrate the coating film 120 in the thickness direction. 2B, when the ratio of the average particle sizes (B / A) is less than 0.75, i.e., when the doped oxide particles 124 are significantly smaller than the anti-rust pigment 126 (smaller than a certain ratio), the anti-rust pigment 126 interferes with the arrangement of the doped oxide particles 124 in the thickness direction of the coating film 120, and an effective current path E penetrating through the thickness direction of the coating film 120 cannot be efficiently formed. Note that when the ratio of the average particle sizes (B / A) exceeds 4.00, the doped oxide particles tend to fall off, which may result in a decrease in corrosion resistance.

[0070] 3A to 4G show the results of EPMA analysis of a coating film formed on the surface of a galvannealed layer (Zn-10 mass% Fe) containing doped oxide particles (zinc oxide particles doped with Al) and anti-rust pigments (amorphous silica, Mg-containing phosphate, and Al-containing phosphate). In Figs. 3A to 4G, reference numeral 112 denotes the coating layer (galvannealed layer), and reference numeral 120 denotes the coating film.

[0071] Figures 3A to 3G are EPMA images of the cross section of the coating film of a surface-treated metal sheet when the ratio of average particle sizes (B / A) is 2.00 (corresponding to Figure 2A). Figure 3A is a backscattered electron image (composition image) of the cross section of the coating film, Figure 3B is an elemental mapping image of Fe, Figure 3C is an elemental mapping image of Zn, Figure 3D is an elemental mapping image of P, Figure 3E is an elemental mapping image of Mg, Figure 3F is an elemental mapping image of Si, and Figure 3G is an elemental mapping image of Al.

[0072] Figures 4A to 4G are EPMA images of the cross section of the coating film of a surface-treated metal sheet when the ratio of average particle sizes (B / A) is 0.50 (corresponding to Figure 2B). Figure 4A is a backscattered electron image (composition image) of the cross section of the coating film, Figure 4B is an elemental mapping image of Fe, Figure 4C is an elemental mapping image of Zn, Figure 4D is an elemental mapping image of P, Figure 4E is an elemental mapping image of Mg, Figure 4F is an elemental mapping image of Si, and Figure 4G is an elemental mapping image of Al.

[0073] As is clear from FIGS. 3A, 3C, and 3G, in the surface-treated metal sheets shown in FIGS. 3A-G, oxide (ZnO) particles 124 doped with a doping element (Al) are found to be close to both surfaces (both the upper and lower surfaces) of the coating film 120. These doped oxide particles 124 are believed to function as electrical paths. On the other hand, as is clear from FIGS. 4A, 4C, and 4G, in the surface-treated metal sheets shown in FIGS. 4A-G, the oxide (ZnO) particles 124 doped with a doping element (Al) are found to be far away from at least one of the surfaces (top and bottom surfaces) of the coating film 120. These oxide particles 124 are unlikely to function as electrical paths.

[0074] As can be seen by comparing Figures 3A, 3C, and 3G with Figures 4A, 4C, and 4G, it is believed that by appropriately controlling the ratio of average particle sizes (B / A), it becomes easier to form an effective electrical path that penetrates the coating film in the thickness direction, and sufficient conductivity can be imparted to the coating film.

[0075] The EPMA images were obtained by the following measurement method. As in the SEM measurement described above, the cross section of the coating film was observed using an SEM attached to an EPMA device (JEOL Ltd., "JXA-iHP200F," measurement conditions: accelerating voltage 15 kV, probe current: 3 × 10 A), and elemental analysis of the elements Fe, Zn, P, Mg, Si, and Al was performed to obtain EPMA images. Figures 3A and 4A show the analysis results of each element superimposed on each other.

[0076] (Other additives) The coating film may further contain other additives, such as well-known additives such as extender pigments, solid lubricants, and leveling agents.

[0077] Examples of the extender pigment include titania and zirconia.

[0078] The solid lubricant can impart excellent lubricity to the coating film and improve the powdering resistance. Examples of the solid lubricant include polyolefin waxes or paraffin waxes such as polyethylene wax, synthetic paraffin, natural paraffin, microcrystalline wax, and chlorinated hydrocarbons; and fluororesin waxes such as polyfluoroethylene resins (e.g., polyethylene tetrafluoride resins), polyvinyl fluoride resins, and polyvinylidene fluoride resins.

[0079] The average particle size of the solid lubricant is not particularly limited, but is preferably 0.05 to 4 μm. If the average particle size of the solid lubricant is less than 0.05 μm, the lubricant will tend to thicken on the surface, increasing the area occupied by the lubricant on the surface layer of the coating film, which may result in reduced adhesion between the coating film and the coating film formed thereon. On the other hand, if the average particle size of the solid lubricant exceeds 4 μm, the lubricant will tend to fall off from the resin coating film, making it difficult to achieve the desired lubricity and reducing corrosion resistance. From the viewpoint of obtaining paint adhesion, corrosion resistance, lubricity, and powdering resistance, the average particle size of the solid lubricant is more preferably 0.1 to 3 μm, and even more preferably 0.3 to 2 μm.

[0080] The softening point of the solid lubricant is preferably 100° C. to 135° C., more preferably 110 to 130° C. When the softening point of the solid lubricant is 100° C. to 135° C., the lubricity and powdering resistance are further improved.

[0081] The content of the solid lubricant is preferably 0.1 to 10% by mass of the coating film (total solid content of the coating film). If the content of the solid lubricant is less than 0.1% by mass, sufficient lubrication may not be obtained. If the content of the solid lubricant is more than 10% by mass, the adhesion and corrosion resistance between the coating film and the coating film formed thereon may decrease.

[0082] From the viewpoint of adhesion between the coating film, lubricity and corrosion resistance, the content of the solid lubricant is more preferably 0.2 to 5 mass % of the coating film (total solid content of the coating film), and even more preferably 0.5 to 2.5 mass %.

[0083] (Coating thickness) In the surface-treated metal sheet according to this embodiment, the thickness of the coating film is 0.5 to 5.0 μm from the viewpoint of enabling resistance welding and electrodeposition coating while maintaining corrosion resistance. Here, when the doped oxide particles and / or anti-rust pigments protrude from the surface of the coating film, the thickness of the coating film refers to the thickness of the portion where the doped oxide particles and / or anti-rust pigments do not protrude from the surface of the coating film. If the thickness of the coating film is less than 0.5 μm, sufficient adhesion and corrosion resistance between the coating film and the electrodeposition coating film formed thereon may be insufficient. On the other hand, if the thickness of the coating film exceeds 5 μm, the conductivity of the coating film may decrease, making resistance welding and electrodeposition coating difficult, and the cohesive strength of the coating film may decrease. For example, the thickness of the coating film is preferably 3.0 μm or less, and may be less than 3.0 μm.

[0084] The coating thickness is measured by the following cross-sectional observation. From the multiple images (backscattered electron images) obtained by the SEM measurement described above, 10 locations are arbitrarily selected for measuring the coating thickness, with an interval of 5 μm or more between each location. The 10 locations may be selected not only from a single image, but also from multiple images measured in different observation regions (fields of view). The average value of the coating thickness measured at the 10 locations is calculated using image processing software (Image J), ​​and this is used as the coating thickness. The coating thickness is the length of the coating film in a direction perpendicular to the surface of the surface-treated metal sheet. The surface of the surface-treated metal sheet referred to here does not refer to a microscopic range on the order of a few μm, but rather to the surface (main surface) extending across the entire surface-treated metal sheet being measured.

[0085] (Method of forming coating film) The method for forming a coating film is not particularly limited, and well-known methods can be used. For example, a coating film-forming composition is prepared by mixing a binder resin, doped oxide particles, an anti-rust pigment, and, if necessary, other additives 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 a water-based composition. The coating film can then be formed by applying the coating film-forming composition to at least one surface of a metal plate, followed by drying and heating.

[0086] [Application] The uses of the surface-treated metal sheet according to the present embodiment (and the painted metal sheet obtained by painting the surface-treated metal sheet) are not particularly limited. For example, the surface-treated metal sheet and painted metal sheet according to the present embodiment can be widely used for automobile parts (automobile bodies, undercarriage parts, etc.), machine parts (casings, etc.), home appliance parts (casings, etc.), building materials (roofs, walls, etc.), etc.

[0087] [effect] In the surface-treated metal sheet according to the present embodiment, conductive doped oxide particles are blended into the coating film, the ratio (B / A) of the average particle size (B) of the doped oxide particles to the average particle size (A) of the anti-rust pigment blended into the coating film is set to 0.75 to 4.00, and the coating film thickness is reduced. As a result, the conductivity of the coating film is improved in the surface-treated metal sheet according to the present embodiment. Therefore, the surface-treated metal sheet according to the present embodiment is suitable not only for resistance welding but also for electrodeposition coating. Meanwhile, while reducing the coating film thickness in this manner tends to reduce corrosion resistance, the amount of anti-rust pigment blended into the coating film in the surface-treated metal sheet according to the present embodiment is increased. As a result, the surface-treated metal sheet according to the present embodiment maintains sufficient corrosion resistance while ensuring the conductivity of the coating film.

[0088] Furthermore, the surface-treated metal sheet according to this embodiment uses doped oxide particles as conductive particles in the coating. That is, the surface-treated metal sheet according to this embodiment does not require the use of expensive non-oxide ceramic particles as conductive particles. Therefore, the surface-treated metal sheet according to this embodiment can be manufactured at a lower cost than conventional surface-treated metal sheets containing non-oxide ceramic particles.

[0089] <Automotive parts> Next, an automotive member according to an embodiment of the present invention will be described.

[0090] The automobile component according to the present embodiment is an automobile component including a painted metal sheet, the painted metal sheet having the surface-treated metal sheet according to the above embodiment, a chemical conversion coating layer disposed on the surface of the coating film of the surface-treated metal sheet, and an electrodeposition coating film disposed on the surface of the chemical conversion coating layer. The automobile component is, for example, a component of an automobile body or a component of an undercarriage.

[0091] [Painted metal plate] Figure 5 is a schematic diagram (partially enlarged cross-sectional view of an automotive component) showing a cross section of a coated metal sheet according to one embodiment of the present invention. In the example shown in Figure 5, the coated metal sheet 200 has a metal sheet 110, a coating film 120 disposed on the surface of the metal sheet 110, a chemical conversion treatment layer 210 disposed on the surface of the coating film 120, and an electrodeposition coating film 220 disposed on the surface of the chemical conversion treatment layer 210. In this example, the chemical conversion treatment layer 210 is a discontinuous layer composed of numerous acid chloride crystals 212 precipitated on the surface of the coating film 120.

[0092] The surface-treated metal sheet may be formed into a predetermined shape depending on the application. Alternatively, a plurality of surface-treated metal sheets may be joined by welding, adhesive, etc. The surface-treated metal sheet according to the present embodiment has excellent electrical conductivity of the coating film, so that defects such as cracks due to welding are unlikely to occur, and a coating film can be formed by electrodeposition coating.

[0093] (chemical conversion coating layer) The chemical conversion layer is located on the surface of the coating film and is formed by chemically treating the coating film surface. The chemical conversion layer may be a continuous layer that seamlessly covers the surface of the surface-treated steel sheet, or a discontinuous layer that intermittently covers the surface of the surface-treated steel sheet. A typical example of such a chemical conversion layer is an acid chloride layer such as a phosphate layer. The chemical conversion solution for forming such an acid chloride layer is acidic (e.g., pH 2 to 3), and doped oxide particles have the property of dissolving in an acidic chemical conversion solution. Therefore, when the surface-treated steel sheet according to this embodiment is chemically treated using an acidic chemical conversion solution, the doped oxide particles in the coating film surface dissolve, and the pH in the vicinity increases, causing components of the chemical conversion solution (e.g., acid chlorides such as phosphates) to precipitate and grow in the areas where the doped oxide particles have dissolved. As a result, it is believed that the components of the chemical conversion solution grow in a wedge shape, protruding from the interior of the coating film surface layer to the surface. As shown in Figure 5, when an electrodeposition coating film 220 is formed by electrodeposition coating on the surface of the chemical conversion treatment layer 210 (a discontinuous layer composed of acid chloride crystals 212) in this state, in addition to the high adhesion due to the chemical properties of the acid chloride, the anchor effect of the acid chloride crystals 212 that have grown in a wedge shape further enhances the adhesion between the coating film and the electrodeposition coating film 220.

[0094] Examples of phosphates include crystalline phosphates and amorphous phosphates. From the viewpoint of having wedge-shaped phosphates present in the chemical conversion coating layer, crystalline phosphates are preferred. Examples of crystalline phosphates include zinc phosphate (hopite: Zn3(PO4)2·4H2O), zinc iron phosphate (phosphophyllite: Zn2Fe(PO4)2·4H2O), manganese phosphate (heuriolite: Mn5(PO3(OH))2(PO4)2·4H2O), manganese iron phosphate ((Mn 1-x Fe x)5H2(PO4)4·4H2O, where x represents that the iron-based metal material is etched during the chemical conversion treatment and its iron component is contained in the film. 0 < x < 1. ), calcium zinc phosphate (shoarzite: CaZn2(PO4)2·2H2O) is included. Examples of amorphous phosphates also include iron phosphate, tin phosphate, zirconium phosphate, titanium phosphate, and hafnium phosphate.

[0095] Also, the chemical conversion treatment layer may be composed of components other than phosphates. For example, the chemical conversion treatment layer may contain a salt of at least one selected from iron, titanium, zirconium, hafnium, indium, tin, bismuth, vanadium, nickel, cerium, molybdenum, and tungsten, and nitrate ions, sulfate ions, fluoride ions, complex fluoride ions, or carbonate ions. Examples of such salts include titanium oxide, zirconium oxide, hafnium oxide, indium oxide, tin oxide, bismuth oxide, vanadium oxide, nickel oxide, cerium oxide, molybdenum oxide, tungsten oxide, iron sulfide, zirconium fluoride, titanium fluoride, hafnium fluoride, and indium fluoride.

[0096] The thickness of the chemical conversion treatment layer is not particularly limited, but is preferably 0.01 μm to 3 μm, more preferably 0.03 μm to 2 μm, and even more preferably 0.05 μm to 1 μm. When the chemical conversion treatment layer contains a crystalline salt, it is not appropriate to discuss the thickness because the chemical conversion treatment layer has irregularities due to crystals. On the other hand, attention can be paid to the crystal diameter of the chemical conversion treatment layer. The crystal diameter of the crystalline phosphate is preferably 0.10 to 5 μm, more preferably 0.30 to 4 μm, and even more preferably 0.50 to 3 μm.

[0097] (Electrocoating film) The electrodeposition coating film is disposed on the surface-treated metal sheet via a chemical conversion treatment layer. The electrodeposition coating film is a film formed by electrodeposition coating. The coating film may be a single layer or a multi-layer (for example, a coating film consisting of an undercoat layer, an intermediate coat layer, and a top coat layer). Note that the coated metal sheet according to this embodiment can also be coated by other coating processes such as powder coating and solvent coating in addition to electrodeposition coating.

[0098] [Manufacturing methods for automotive components] The automotive component according to this embodiment can be manufactured, for example, by a step (first step) of forming a chemical conversion coating layer on the surface of a coating film of a surface-treated metal sheet, and a step (second step) of forming an electrodeposition coating film on the surface of the chemical conversion coating layer.

[0099] First, prior to the first step, the above-mentioned surface-treated metal sheet is prepared. The surface-treated metal sheet may be formed into a predetermined shape. The surface-treated metal sheet may be formed using a known forming technique such as cutting or press forming. Alternatively, a plurality of surface-treated metal sheets or sheets may be joined by welding (e.g., spot welding) or the like. The surface-treated metal sheet may be subjected to known pretreatments such as degreasing and surface conditioning.

[0100] In the first step, the surface-treated metal sheet is subjected to a chemical conversion treatment to form a chemical conversion layer on the surface of the coating film. The chemical conversion treatment solution and treatment conditions used for the chemical conversion treatment can be appropriately selected depending on the state and composition of the chemical conversion layer to be formed.

[0101] For example, when the chemical conversion treatment layer contains crystalline phosphate, the chemical conversion treatment solution can be an acidic aqueous solution containing phosphate ions as anions and at least one selected from zinc, calcium, and manganese as cations. To increase the reaction rate, the acidic aqueous solution preferably further contains transition metal ions such as nickel and cobalt; oxidizing agents such as nitric acid and nitrous acid; and etching components such as fluoride ions and complex fluoride ions. Commercially available acidic aqueous solutions for phosphate treatment, which appropriately combine the types and contents of the above-mentioned anions and cations, can be used as is. Examples of commercially available acidic aqueous solutions for phosphate treatment include "Palbond 860," "Palbond L3020," "Palphos M1A," "Palphos M5," "Palbond 880," "Palbond SX35," "Palbond L47," and "Ferricote 7," manufactured by Nihon Parkerizing Co., Ltd. The pH of the above chemical conversion treatment solution is not particularly limited, but is preferably 1.0 to 5.0, more preferably 2.0 to 4.0.

[0102] The temperature of the chemical conversion treatment solution during the chemical conversion treatment is not particularly limited and is preferably 30° C. to 120° C., more preferably 35° C. to 110° C., and even more preferably 40° C. to 100° C. The time for the chemical conversion treatment is not particularly limited and can be selected appropriately depending on the desired deposition amount of the chemical conversion treatment layer to be formed.

[0103] Furthermore, when the chemical conversion treatment layer contains amorphous phosphate, the chemical conversion treatment solution can be, for example, an acidic aqueous solution containing phosphate ions as anions and at least one selected from iron, tin, zirconium, titanium, and hafnium as cations. To increase the reaction rate, the acidic aqueous solution preferably further contains transition metal ions such as nickel and cobalt; oxidizing agents such as nitric acid and nitrous acid; and etching components such as fluoride ions and complex fluoride ions. Commercially available acidic aqueous solutions for phosphating, which appropriately combine the types and contents of the above-mentioned anions and cations, can be used as is. Examples of commercially available acidic aqueous solutions for phosphating include "Palphos 1077," "Palphos 525T," and "Palphos K5100" manufactured by Nihon Parkerizing Co., Ltd. The pH of the above-mentioned chemical conversion treatment solution is not particularly limited, but is preferably 1.0 to 5.0, more preferably 2.0 to 4.0.

[0104] In addition, as a chemical conversion treatment solution for forming another chemical conversion treatment layer, an acidic aqueous solution containing at least one anion selected from nitrate ions, sulfate ions, fluoride ions, complex fluoride ions, and carbonate ions, and at least one cation selected from iron, titanium, zirconium, hafnium, indium, tin, bismuth, vanadium, nickel, cerium, molybdenum, and tungsten, can be used. Such an acidic aqueous solution can be prepared by appropriately combining the types and contents of compounds corresponding to the above anions and cations, or a commercially available solution can be used as is. The pH of the above chemical conversion treatment solution is not particularly limited, but is preferably 1.0 to 5.0, and more preferably 2.0 to 4.0.

[0105] When forming a chemical conversion treatment layer containing an amorphous phosphate or other chemical conversion treatment layer, the temperature of the chemical conversion treatment solution during the chemical conversion treatment is not particularly limited and is preferably 10° C. to 100° C., more preferably 15° C. to 80° C., and even more preferably 20° C. to 60° C. The time for the chemical conversion treatment is not particularly limited and can be selected appropriately depending on the desired coating weight of the chemical conversion treatment layer to be formed.

[0106] In the second step, the surface-treated metal sheet that has been subjected to the chemical conversion treatment is subjected to electrodeposition coating. The conditions for electrodeposition coating are not particularly limited, but for example, a cationic electrodeposition paint is electrodeposited at a voltage of 160 V and baked and cured at a baking temperature of 170°C for 20 minutes to form an electrodeposition coating film on the coating film of the surface-treated metal sheet. Multiple layers of electrodeposition coating films may be formed by performing electrodeposition coating multiple times.

[0107] Through these steps, the automotive component according to this embodiment can be manufactured.

[0108] [effect] In the automotive component according to this embodiment, the surface-treated metal sheet has a conductive coating film, and the coating film is appropriately formed by electrodeposition coating, resulting in excellent adhesion between the coating film and the electrodeposition coating film. Furthermore, since the surface-treated metal sheet according to this embodiment is also suitable for resistance welding, defects such as cracks due to welding are prevented even when the surface-treated metal sheet or the coated metal sheet is welded.

[0109] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. [Example]

[0110] [Manufacturing surface-treated metal sheets] 1. Metal Plate Preparation The following two types of zinc-based plated steel sheets were prepared and immersed in a 2.5% by mass aqueous solution of an aqueous alkaline degreasing agent (FC-301, Nihon Parkerizing Co., Ltd.) at 40°C for 2 minutes to degrease the surface, then rinsed with water and dried to prepare the coating substrate. GA: Galvannealed steel sheet (Zn-10% Fe by mass) (sheet thickness 0.8 mm, coating weight 45 g / m 2 ) GI: Hot-dip galvanized steel sheet (sheet thickness 0.8 mm, coating weight 60 g / m 2 )

[0111] 2. Preparation of aqueous composition To form coating films having the compositions shown in Tables 1 to 4, the components were mixed to give solid concentrations similar to those in Tables 1 to 4 to prepare aqueous compositions for forming coating films.

[0112] In Tables 1 to 4, the solid content of each component is shown as the ratio (unit: volume %) of the solid content (non-volatile content) of each component to the solid content (non-volatile content) of the entire aqueous composition.

[0113] Details of each component (symbol) in Tables 1 to 4 are as follows.

[0114] (A) Binder resin E: Epoxy resin emulsion (ADEKA RESIN (registered trademark) EM-0718, ADEKA Corporation) U: Urethane resin emulsion (Superflex (registered trademark) E-2000, Daiichi Kogyo Seiyaku Co., Ltd.) P: Polyester resin emulsion (Vylonal (registered trademark) MD-2000, Toyobo Co., Ltd.)

[0115] (B) Doped oxide particles ZnO particles doped with Zn1:Al (23-K, Hakusui Tech Co., Ltd.) Zn2:Ga-doped zinc oxide (ZnO) particles (Pazet GK-40, Hakusui Tech Co., Ltd.) Sn1: P-doped tin oxide (SnO) particles (SP-2, Mitsubishi Materials Electronic Chemicals Co., Ltd.) Sn2:Sb-doped tin oxide (SnO) particles (SN-100P, Ishihara Sangyo Kaisha, Ltd.) In:Sn-doped indium oxide (In2O3) particles (E-ITO, Mitsubishi Materials Electronic Chemicals Co., Ltd.) ·Ti:Sb-doped tin oxide (SnO2) coated titanium oxide (TiO2) particles (EC-210, Titanium Industry)

[0116] (C) Anti-rust pigment PAM: Mg-containing aluminum dihydrogen tripolyphosphate (K-WHITE G105, Teika Corporation) PA: Aluminum dihydrogen tripolyphosphate (K-WHITE K105, Teika Corporation) Si: silicon dioxide (amorphous silica) (Snowtex® ZL, Nissan Chemical Industries, Ltd.) V: Vanadium pentoxide (Kanto Chemical Co., Ltd.)

[0117] The various doped oxide particles were dispersed in water containing resin and then pulverized in a ball mill. The average particle size of the doped oxide particles was adjusted by controlling the pulverization time. The average particle size of the doped oxide particles in the coating was measured using the following procedure after producing a surface-treated metal sheet, as described below. The cross section in the thickness direction of the surface-treated metal sheet was mirror-polished, and the cross section of the coating was photographed at 5000x magnification using a scanning electron microscope (JEOL Ltd., "JSM-7100F", accelerating voltage: 15 kV) (field of view: 24 μm × 18 μm). From the obtained backscattered electron image, 10 doped oxide particles with relatively large particle sizes were selected. For each particle, the average values ​​of the long and short side lengths measured using image processing software (Image J Ver. 1.54h) were calculated to determine the particle size of the particle. The arithmetic mean of the particle sizes of the 10 particles was calculated to determine the average particle size. The same measurement was carried out for 10 backscattered electron images, and the arithmetic mean of the backscattered electron images was taken as the average particle size of the doped oxide particles. The average particle size of the anti-corrosion pigment in the coating was also measured in the same manner.

[0118] Furthermore, after producing a surface-treated metal sheet as described below, the contents (volume %) of doped oxide particles and anti-rust pigment in the coating film were calculated using the following procedure. The results were essentially the same as the concentrations (volume %) of doped oxide particles and anti-rust pigment shown in Tables 1 to 4. For backscattered electron images obtained using the same method as for measuring the average particle size of doped oxide particles described above, the areas occupied by the coating film, the doped oxide particles, and the anti-rust pigment were measured using image processing software (Image J). Since the volumetric proportions of each component in the coating film roughly correspond to the area proportions of each component when the coating film is observed cross-sectionally, the measured area proportions of doped oxide particles and the anti-rust pigment were taken as the volume proportions of doped oxide particles and the volume proportions of anti-rust pigment, respectively. Similar measurements were performed on 10 backscattered electron images, and the arithmetic mean values ​​were taken as the content (volume %) of doped oxide particles and the content (volume %) of anti-rust pigment.

[0119] 3. Manufacturing of surface-treated metal sheets The aqueous compositions were applied to metal plates using a bar coater so as to conform to the compositions shown in Tables 1 to 4, and then dried in an oven under conditions where the maximum temperature reached was 140°C and maintained for 8 seconds, thereby forming coating films. The thickness of the coating film was adjusted by diluting the aqueous composition and adjusting the grit of the bar coater to obtain the values ​​shown in Tables 1 to 4. The thickness of the coating film was measured using the following procedure. From the multiple images (backscattered electron images) obtained by the SEM measurement described above, 10 locations were selected for measuring the film thickness, with an interval of 5 μm or more between each location. The average value of the film thickness measured at the 10 locations was calculated using image processing software (Image J), ​​and this was taken as the film thickness of the coating film.

[0120] [Evaluation test] 1. Evaluation of the appearance and adhesion of the electrodeposition coating Each surface-treated metal plate was electrodeposited with a cationic electrodeposition paint (Nippon Paint Co., Ltd.) using a slope current of 160 V, and baked for 20 minutes at 170° C. The average thickness of the coating film after electrodeposition was 10.0 μm for all samples.

[0121] After electrodeposition coating, the appearance of the electrodeposition coating film was visually observed and evaluated according to the following criteria: When the evaluation result was "C," it was determined that the material was unsuitable for electrodeposition coating. A: A uniform electrodeposition coating film was formed. B: An uneven electrodeposition coating film was formed. C: No electrodeposition coating film was formed. When the appearance (surface) of the electrodeposition coating film was visually observed, if any areas were visible that differed from the other areas in terms of color tone, gloss, etc., it was judged as "B: An uneven electrodeposition coating film was formed." If no areas were visible that differed from the other areas in terms of color tone, gloss, etc., it was judged as "A: A uniform electrodeposition coating film was formed." Furthermore, if no electrodeposition coating film was formed, the surface of the underlying metal plate was visible. The surface of the metal plate can be identified as a metal plate by its metallic luster, spangle pattern, etc.

[0122] The electrodeposition coating was then subjected to 80 hours of salt spray, after which an adhesion test was conducted using the cross-cut method in accordance with JIS K 5600-5-6 (2018). The ratio of the area where the electrodeposition coating had peeled to the test area (paint film peeling rate (%)) was then measured for each surface-treated metal plate. The adhesion of the electrodeposition coating was evaluated based on the paint film peeling rate according to the following criteria. Evaluation results of "C" were deemed unsuitable for electrodeposition coating. A: The peeling rate was less than 5%. B: The peeling rate was 5% or more and less than 15%. C: The peeling rate was 15% or more.

[0123] 2. Evaluation of corrosion resistance Each surface-treated metal sheet that had not been electrocoated was subjected to 90 corrosion cycles in accordance with JASO M 609. After that, the occurrence of red rust on each surface-treated metal sheet was measured, and the area ratio (%) of red rust was calculated. The corrosion resistance of the surface-treated metal sheet was evaluated based on the area ratio of red rust according to the following criteria. An evaluation result of "C" was determined to be insufficient corrosion resistance. AA: No red rust occurred. A: The area ratio of red rust was more than 0% and less than 10%. B: The area ratio of red rust was 10% or more and less than 20%. C: The area ratio of red rust was 20% or more.

[0124] Details of the examples are listed below in Tables 1 to 4. In addition, samples with blank "Remarks" in Tables 1 to 4 correspond to examples.

[0125] [Table 1]

[0126] [Table 2]

[0127] [Table 3]

[0128] [Table 4]

[0129] The above results show that the surface-treated metal sheets of Samples 1 to 44 corresponding to the Examples have superior appearance and adhesion of the electrodeposition coating film, as well as corrosion resistance, compared to the surface-treated metal sheets of Samples 45 to 53 corresponding to the Comparative Examples. Furthermore, the surface-treated metal sheets of Samples 1 to 44 were able to be properly resistance-welded.

[0130] This application claims priority from Japanese Patent Application No. 2023-67823, filed April 18, 2023. The contents of the specification and drawings of that application are incorporated herein by reference in their entirety. [Industrial Applicability]

[0131] The surface-treated metal sheet and the painted metal sheet according to the present invention are useful in a variety of applications, such as automobile parts. [Explanation of symbols]

[0132] 100 Surface-treated metal plate 110 Metal plate 112 plating layer 120 Paint film 122 Binder resin 124 Doped oxide particles 126 Anti-rust pigments 200 painted metal plates 210 Chemical conversion coating layer 212 Acid chloride crystals 220 Electrodeposition coating film

Claims

1. A metal plate; a coating film having a thickness of 0.5 to 5.0 μm disposed on the surface of the metal plate; and the coating film comprises a binder resin, doped oxide particles, and an anti-corrosion pigment; the content of the doped oxide particles is 5 to 20% by volume of the coating film; The content of the anti-rust pigment is 20 to 50% by volume of the coating film, the ratio (B / A) of the average particle size (B) of the doped oxide particles to the average particle size (A) of the anti-rust pigment is 0.75 to 4.00; the coating does not contain any of non-oxide ceramic particles, iron alloy particles, and stainless steel particles; Surface-treated metal plate.

2. 2. The surface-treated metal sheet according to claim 1, wherein the doped oxide particles have an average particle size of 1.0 to 4.0 μm.

3. 2. The surface-treated metal sheet according to claim 1, wherein the doped oxide particles comprise at least one selected from the group consisting of zinc oxide particles doped with Al, Ga, or In, tin oxide particles doped with P, Sb, or As, indium oxide particles doped with Sn or Ge, titanium oxide particles coated with zinc oxide doped with Al, Ga, or In, titanium oxide particles coated with tin oxide doped with P, Sb, or As, and titanium oxide particles coated with indium oxide doped with Sn or Ge.

4. 2. The surface-treated metal sheet according to claim 1, wherein the anti-rust pigment has an average particle size of 0.5 to 4.0 μm.

5. 2. The surface-treated metal sheet according to claim 1, wherein the anti-rust pigment comprises at least one selected from the group consisting of a phosphate compound, a silicate compound, amorphous silica, and a vanadate compound.

6. The surface-treated metal sheet according to claim 1 , wherein the binder resin is a water-soluble or water-dispersible water-based resin.

7. The surface-treated metal sheet according to claim 6 , wherein the binder resin is an epoxy resin.

8. An automotive component including a painted metal plate, The coated metal plate is The surface-treated metal sheet according to any one of claims 1 to 7, a chemical conversion coating layer disposed on the surface of the coating film; an electrodeposition coating film disposed on the surface of the chemical conversion treatment layer; having Automotive parts.

Images