Zinc-plated steel sheet with coating and method for manufacturing the same

Abstract

The present invention provides a zinc-plated steel sheet with a coating that exhibits excellent sliding properties during press forming and excellent spot weldability, as well as a method for manufacturing the same. A zinc-plated steel sheet has a coating on at least one side containing an acrylic resin and graphite, wherein the acrylic resin has a glass transition temperature (Tg) of 100°C or higher, an acid value-to-glass transition temperature ratio R (acid value (mg-KOH / g) / Tg (°C)) of 1.50 or higher, the average particle size of the graphite is 10 μm or less, the mass percentage of the graphite in the coating is 5% by mass or higher, and the amount of the coating W attached to one side of the zinc-plated steel sheet is 0.2 g / m². 2 More than 2.0g / m 2 The following are zinc-plated steel sheets with a coating.

Description

[Technical Field]

[0001] This invention relates to a coated zinc-plated steel sheet and a method for manufacturing the same, and more particularly to a coated zinc-plated steel sheet with excellent sliding properties and spot weldability in press forming, and a method for manufacturing the same. In particular, it relates to a coated zinc-plated steel sheet with excellent formability even during severe deep drawing, and a method for manufacturing the same. [Background technology]

[0002] Zinc-plated steel sheets are widely used in a broad range of fields, primarily for automobile body applications. In such applications, zinc-plated steel sheets are generally press-formed before use. However, zinc-plated steel sheets have the disadvantage of being less press-formable than cold-rolled steel sheets. This is because the sliding resistance of zinc-plated steel sheets in the press die is greater than that of cold-rolled steel sheets. In other words, during deep drawing, zinc-plated steel sheets with high sliding resistance to the bead have difficulty flowing into the press die, making the steel sheet prone to fracture. Similarly, during stretch forming, if the sliding resistance with the die is high, strain becomes localized, making the steel sheet prone to fracture.

[0003] Therefore, a widely used method to improve the press formability when using zinc-plated steel sheets is to apply a high-viscosity lubricant. However, because the lubricant is highly viscous, this method can lead to painting defects due to insufficient degreasing during the painting process. In addition, problems such as unstable press performance due to lack of lubrication during pressing can occur.

[0004] In addition to the method using the lubricating oils mentioned above, another method for improving press formability is surface treatment of the mold. Although surface treatment of the mold is a widely used method, it does not allow for adjustment of the mold after surface treatment. Furthermore, it is costly. Therefore, there is a need to improve the press formability of the zinc-plated steel sheet itself without relying on high-viscosity lubricating oils or mold surface treatments.

[0005] Therefore, various types of lubricated surface-treated steel sheets are being considered as a way to solve the above problems.

[0006] Patent Document 1 describes a technique for forming a resin film containing synthetic resin powder in an acrylic resin film on the surface of a zinc-plated steel sheet.

[0007] Patent Document 2 describes a resin-coated metal plate covered with a lubricating film in which a solid lubricant protrudes from the surface of the resin film with a protrusion height of 0.01 to 1.5 μm.

[0008] Patent Document 3 describes a lubricated surface-treated metal product with excellent press-formability, which is coated with a 0.5 to 5 μm thick film containing a lubricant in polyurethane resin.

[0009] Patent document 4 describes a technique for forming an alkali-soluble organic film on a steel plate by adding a lubricant to an epoxy resin. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 9-170059 [Patent Document 2] Japanese Patent Application Publication No. 10-52881 [Patent Document 3] Japanese Patent Publication No. 2000-309747 [Patent Document 4] Japanese Patent Publication No. 2000-167981 [Overview of the project] [Problems that the invention aims to solve]

[0011] However, in Patent Documents 1-4, although lubrication was achieved through the lubricating effect of lubricants contained in the film, the press formability was not always sufficient in complex molding processes such as severe deep drawing. In addition, spot weldability in the assembly process was sometimes reduced.

[0012] The present invention has been made in view of these circumstances, and aims to provide a coated zinc-plated steel sheet and a method for manufacturing the same, which have excellent sliding properties during press forming and excellent spot weldability.

[0013] Furthermore, when zinc-plated steel sheets are used as automobile bodies, they must have excellent adhesion during the assembly process. In addition, they must have sufficient degreasing properties during the alkaline degreasing process in the painting process.

[0014] Here, a zinc-plated steel sheet is a steel sheet on which a zinc-based plating film (zinc plating) has been formed. Examples of zinc-based plating films include zinc plating consisting of zinc and unavoidable impurities, with no other components intentionally added. Other examples of zinc-based plating films include zinc plating with zinc as the main component, to which one or more alloying elements such as iron, nickel, aluminum, and magnesium, or compounds such as alumina and silica are added. Note that "main component" may, for example, be a content exceeding 50% by mass. The amount of plating film (zinc plating) adhering to a zinc-plated steel sheet is not particularly limited, but for a zinc-plated steel sheet, it is typically 3 to 120 g / m² per side of the steel sheet. 2 A plating with the specified amount of adhesion is preferably used. [Means for solving the problem]

[0015] The inventors diligently conducted research to solve the above problems. As a result, they developed a coating containing an acrylic resin with a glass transition temperature (Tg) of 100°C or higher and an acid value-to-glass transition temperature ratio R = acid value (mg-KOH / g) / Tg (°C) of 1.50 or higher, and graphite with an average particle size of 10 μm or less, with an adhesion amount W of 0.2 g / m² per side of a steel plate. 2 More than 2.0g / m 2 We found that the above problem can be solved by forming the following on the surface of the zinc-plated steel sheet.

[0016] The present invention has been completed based on the above findings, and the gist thereof is as follows. [1] A coated zinc-plated steel sheet having a film containing an acrylic resin and graphite on at least one side thereof, wherein the acrylic resin has a glass transition point (Tg) of 100°C or higher and a ratio R of acid value to glass transition point (acid value (mg-KOH / g) / Tg (°C)) of 1.50 or higher, the average particle size of the graphite is 10 μm or less, and the mass ratio of the graphite in the film is 5% by mass or more, and the coating amount W per one side of the zinc-plated steel sheet of the film is 0.2 g / m 2 or more and 2.0 g / m 2 or less, a zinc-plated steel sheet with a film. [2] The zinc-plated steel sheet with a film according to [1], wherein the acid value of the acrylic resin is 180 mg-KOH / g or more and 350 mg-KOH / g or less. [3] The zinc-plated steel sheet with a film according to [1] or [2], wherein the ratio R of the acid value to the glass transition point of the acrylic resin is 2.05 or less. [4] The zinc-plated steel sheet with a film according to any one of [1] to [3], wherein the film contains 30% by mass or more of the acrylic resin and the mass ratio of the graphite is 50% by mass or less. [5] The zinc-plated steel sheet with a film according to any one of [1] to [4], wherein the mass average molecular weight of the acrylic resin is 5000 or more and 30000 or less. [6] The zinc-plated steel sheet with a film according to any one of [1] to [5], wherein the acrylic resin is a styrene-acrylic resin. [7] The zinc-plated steel sheet with a film according to any one of [1] to [6], wherein the film contains 1% by mass or more and 30% by mass or less of a rust inhibitor. [8] The zinc-plated steel sheet with a film according to [7], wherein the rust inhibitor is at least one selected from the group consisting of aluminum salts of phosphoric acids, zinc salts of phosphoric acids, and zinc oxide. [9] The zinc-plated steel sheet with a film according to any one of [1] to [8], wherein the average particle size of the graphite is 0.1 μm or more and 6.0 μm or less.

[10] A zinc-plated steel sheet with a coating according to any one of [1] to [9], wherein the coating contains 1% by mass or more and 10% by mass or less of silica.

[11] The zinc-plated steel sheet with a coating according to any one of [1] to

[10] , wherein the thickness of the zinc-plated steel sheet is 0.10 mm or more and 0.70 mm or less.

[12] A method for manufacturing a coated zinc-plated steel sheet according to any of the above [1] to

[11] , A method for manufacturing a zinc-plated steel sheet with a coating, comprising applying a coating containing the acrylic resin and the graphite to at least one side of the zinc-plated steel sheet and drying it.

[13] The method for manufacturing a zinc-plated steel sheet with a coating according to

[12] , wherein the maximum temperature reached by the zinc-plated steel sheet during drying is 60°C or more and 150°C or less.

[14] A method for manufacturing a coated zinc-plated steel sheet according to

[12] or

[13] , wherein the mass ratio of the total solids in the coating is 1% by mass or more and 30% by mass or less. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a zinc-plated steel sheet with a coating that has excellent sliding properties during press forming and excellent spot weldability.

[0018] According to the present invention, a zinc-plated steel sheet with a coating is obtained in which the coefficient of friction with molds and the like is significantly reduced, resulting in excellent press formability. Therefore, even in complex forming processes such as severe deep drawing, stable and excellent press formability can be obtained. Furthermore, the zinc-plated steel sheet with a coating of the present invention also has excellent weldability. The zinc-plated steel sheet with a coating of the present invention is suitable for use, for example, in automobile bodies.

[0019] Furthermore, in a preferred embodiment of the present invention, a zinc-plated steel sheet with a coating exhibiting excellent adhesion and defilm removal properties can be obtained. Adhesives can be used in the same manner as with conventional zinc-plated steel sheets, and defilm removal by alkaline degreasing is excellent. Therefore, deterioration of paintability due to insufficient defilm removal can be suppressed in the painting process. [Brief explanation of the drawing]

[0020] [Figure 1] Figure 1 is a schematic front view showing a friction coefficient measuring device. [Figure 2] Figure 2 is a schematic perspective view showing the shape and dimensions of the bead in Figure 1. [Modes for carrying out the invention]

[0021] Embodiments of the present invention will be described below. However, the present invention is not limited to the following embodiments.

[0022] The zinc-plated steel sheet with a coating of the present invention has a coating containing an acrylic resin and graphite on at least one side of the zinc-plated steel sheet. The acrylic resin has a glass transition temperature (Tg) of 100°C or higher, and a ratio R (acid value (mg-KOH / g) / Tg (°C)) of 1.50 or higher. The graphite has an average particle size of 10 μm or less. The mass percentage of the graphite in the coating is 5% by mass or higher. The amount W of the coating attached to one side of the zinc-plated steel sheet is 0.2 g / m². 2 More than 2.0g / m 2 The following applies:

[0023] Hereafter, the ratio R (acid value (mg-KOH / g) / Tg(°C)) between the acid value and the glass transition temperature will also be simply referred to as R.

[0024] (Acrylic resin) The glass transition temperature of acrylic resins is 100°C or higher. Setting the glass transition temperature of acrylic resins to 100°C or higher is necessary to obtain good sliding properties (lubricity). If the glass transition temperature of acrylic resins is below 100°C, the resin softens during sliding, reducing the graphite's holding power and decreasing its ability to prevent direct contact between the zinc-plated steel sheet and the mold, thus preventing good sliding properties. The glass transition temperature of acrylic resins is preferably 110°C or higher. Furthermore, a glass transition temperature of acrylic resins is preferably 150°C or lower. A glass transition temperature of 150°C or lower helps to prevent the resin from becoming too hard and thus reduces its lubricity during sliding.

[0025] Here, the glass transition temperature (Tg) is the intermediate glass transition temperature measured according to JIS K 7121:2012 "Method for Measuring Transition Temperatures of Plastics".

[0026] The ratio R of the acid value to the glass transition temperature of the acrylic resin should be 1.50 or higher. Even if the glass transition temperature is 100°C or higher, if the acid value is low (R < 1.50), excellent lubricity cannot be obtained. The reason for this is not clear, but it is thought that the carboxyl groups in the acrylic resin have a high affinity for the mold, and that this has the effect of transferring graphite in the film to the mold during sliding. When the acrylic resin component containing graphite is transferred to the mold during sliding, the mold surface is protected by graphite, which increases the effect of preventing direct contact between the mold and the zinc-plated steel sheet, and improves sliding performance. Therefore, when the acid value is low (R < 1.50), there is a lack of carboxyl groups, resulting in poor sliding performance. When the glass transition temperature of the acrylic resin rises, the resin does not soften easily due to sliding, making it difficult to transfer to the mold. Therefore, in order to obtain excellent sliding performance when the glass transition temperature rises, the acid value also needs to be increased. That is, the ratio R of the acid value to the glass transition temperature needs to be 1.50 or higher. R is preferably 1.80 or higher. There is no particular upper limit to R. For example, R may be 3.50 or less. It is preferable that R be 2.05 or less. This is because if R exceeds 2.05, the rust prevention performance may deteriorate.

[0027] Furthermore, the acid value of the acrylic resin is preferably between 180 mg-KOH / g and 350 mg-KOH / g. If the acid value is less than 180 mg-KOH / g, the defilm-removing properties with alkali may be poor, and sufficient adhesive strength may not be obtained with adhesives. If the acid value exceeds 350 mg-KOH / g, the rust-preventive properties may deteriorate.

[0028] Here, the acid value is the number of milligrams of potassium hydroxide required to neutralize the carboxyl groups contained in 1 gram of resin, and is measured according to JIS K 0070:1992 "Test methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products". In this invention, the unit of acid value is expressed as mg-KOH / g.

[0029] Furthermore, the mass-average molecular weight of the acrylic resin is preferably between 5,000 and 30,000. If the mass-average molecular weight of the acrylic resin is less than 5,000, the rust-preventive properties may be poor. Also, if the mass-average molecular weight of the acrylic resin exceeds 30,000, the adhesive properties may deteriorate.

[0030] Here, the mass-average molecular weight is the mass-average molecular weight measured according to JIS K 7252:2016 "Plastics - Method for determining the average molecular weight and molecular weight distribution of polymers by size exclusion chromatography".

[0031] Furthermore, the acrylic resin is preferably a styrene-acrylic resin. Styrene-acrylic resin contains styrene as a monomer. Including styrene as a monomer in the resin improves water resistance, resulting in good rust prevention. Moreover, styrene-acrylic resin also exhibits the effect of providing better sliding properties compared to acrylic resins that do not contain styrene as a monomer.

[0032] (Graphite) The average particle size of the graphite is 10 μm or less. In this invention, graphite is included in the coating because graphite has a layered structure, and the layers are bonded together by weak intermolecular forces, giving it the property of peeling in layers and providing excellent lubricity. In addition, because graphite is conductive, when included in an organic coating, it acts as a conductive point, resulting in an effect of improving weldability.

[0033] If the average particle size of the graphite exceeds 10 μm, it becomes difficult to mix with the acrylic resin, and the aforementioned transfer effect to the mold during sliding is not obtained, resulting in insufficient lubrication. The average particle size of the graphite is preferably 6.0 μm or less, more preferably 3.0 μm or less.

[0034] Furthermore, it is preferable that the average particle size of the graphite is 0.1 μm or larger. An average particle size of 0.1 μm or larger of graphite makes it easier to obtain sufficient lubrication during sliding. In addition, aggregation is more easily suppressed in the paint used to form the film, as described later, and the paint stability is improved. The average particle size of the graphite is more preferably 0.3 μm or larger. Considering the miscibility with the acrylic resin mentioned above, it is preferable that the average particle size of the graphite is between 0.1 μm and 6.0 μm.

[0035] The aforementioned average particle size is the median diameter based on volume and can be determined by laser diffraction / scattering. For example, it can be determined by measuring a sample diluted with pure water using a laser diffraction / scattering particle size distribution analyzer partica® LA-960V2 (manufactured by Horiba, Ltd.).

[0036] The mass percentage of graphite in the coating should be 5% by mass or more. If the mass percentage of graphite in the coating is less than 5% by mass, sufficient lubrication and weldability cannot be obtained. From the viewpoint of obtaining better lubrication and weldability, the mass percentage of graphite in the coating is preferably 10% by mass or more, and more preferably 18% by mass or more. Furthermore, it is preferable that the mass percentage of graphite in the coating is 50% by mass or less. If the mass percentage of graphite in the coating exceeds 50% by mass, the graphite is prone to falling off due to insufficient base resin components, resulting in poor adhesion to the steel plate, and the coating may not exist stably, leading to poor adhesion. Also, if the mass percentage of graphite in the coating exceeds 50% by mass, when used as an automobile body, sufficient defilm removal may not be obtained in the alkaline degreasing step of the painting process, and the coating may remain without sufficient defilm removal in the alkaline degreasing step, degrading the paintability. The mass percentage of graphite in the coating is more preferably 40% by mass or less.

[0037] Here, the mass ratio of graphite in the coating corresponds to the ratio of the mass of graphite solids to the total mass of solids in the paint, which will be described later.

[0038] The coating on the zinc-plated steel sheet with a coating according to the present invention preferably contains 30% by mass or more of the acrylic resin. When the mass percentage of acrylic resin in the coating is 30% by mass or more, properties influenced by the physical properties of the acrylic resin component, such as improved lubricity due to transfer to the mold during sliding, defilmability, and adhesion, are more easily obtained. If the mass percentage of acrylic resin in the coating is less than 30% by mass, the influence of other components becomes greater, and the target performance may not be obtained. The mass percentage of acrylic resin in the coating is 95% by mass or less.

[0039] Furthermore, the coating on the zinc-plated steel sheet with a coating of the present invention may contain a rust inhibitor. When the coating contains a rust inhibitor, for example, the coating may contain 40% by mass or less of the rust inhibitor. Preferably, the coating contains 1% by mass or more and 30% by mass or less of the rust inhibitor. Even if the coating does not contain a rust inhibitor, rust and discoloration will not occur on the steel sheet under normal storage conditions. However, if the rust inhibitor content in the coating is less than 1% by mass, rust may occur on the steel sheet under unfavorable storage conditions. In particular, if the steel sheet is stored in a high-humidity environment with the sheet stacked in a coil shape, it may absorb moisture and discolor. On the other hand, if the rust inhibitor content in the coating exceeds 30% by mass, the adhesiveness may deteriorate, and the rust inhibitor may precipitate in the paint state described later, which may deteriorate the paint stability. It is preferable to use at least one selected from the group consisting of aluminum salts of phosphates, zinc salts of phosphates, and zinc oxide as the rust inhibitor. Here, phosphoric acids include orthophosphoric acid as well as condensed phosphoric acids such as pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, and metaphosphoric acid. Using these rust inhibitors provides sufficient rust prevention and also minimizes deterioration of paint stability.

[0040] Furthermore, the coating on the zinc-plated steel sheet with a coating of the present invention may contain silica. When the coating contains silica, for example, it may contain 15% by mass or less of silica. Preferably, the coating contains 1% by mass or more and 10% by mass or less of silica. The inclusion of silica in the coating enhances the water repellency of the coating and improves rust prevention. Also, in the state of the paint described later, the inclusion of silica makes it possible to suppress the precipitation of rust inhibitors, thereby improving paint stability. Furthermore, if the silica content in the coating is less than 1% by mass, the above effects are difficult to obtain, and if it exceeds 10% by mass, the adhesion may deteriorate. Also, when the coating contains silica, it is preferable to use colloidal silica with an average particle size of 5 nm or more and 200 nm or less. The average particle size of silica is defined in the same way as the average particle size of graphite described above.

[0041] In addition, the film may contain components other than the above acrylic resin, graphite, rust inhibitor, and silica. Examples of such components include surface conditioners, defoamers, and dispersants that are generally added to paints.

[0042] In the zinc-based plated steel sheet with a film of the present invention, the surface roughness of the zinc-based plated steel sheet before film formation is preferably 0.4 μm or more in terms of arithmetic mean roughness Ra. Further, the Ra is preferably 2.5 μm or less. More preferably, the Ra is 0.4 μm or more and 2.5 μm or less. If the Ra is 2.5 μm or less, the lubricating effect by the film can be stably obtained easily. When the Ra is less than 0.4 μm, fine scratches that may occur during press forming may be conspicuous, and die chatter may occur during press forming. When the Ra exceeds 2.5 μm, the unevenness on the steel sheet surface becomes large, so that the film formed in the recesses hardly acts effectively during sliding, and the lubricating effect by the film may become small. The arithmetic mean roughness Ra (μm) of the zinc-based plated steel sheet surface can be measured in accordance with JIS B 0633:2001 (ISO 4288:1996). For example, when the Ra (μm) is greater than 0.1 and less than or equal to 2, it is obtained from the measured roughness curve with a cut-off value and a reference length of 0.8 mm and an evaluation length of 4 mm. When the Ra (μm) exceeds 2 and is less than or equal to 10, it is obtained from the measured roughness curve with a cut-off value and a reference length of 2.5 mm and an evaluation length of 12.5 mm.

[0043] The film adhesion amount W per one side of the steel sheet of the zinc-based plated steel sheet with a film of the present invention is 0.2 g / m 2 or more and 2.0 g / m 2 or less. The film adhesion amount W corresponds to the dry mass after applying a paint described later to the steel sheet surface and drying. If the film adhesion amount W is less than 0.2 g / m 2 , sufficient slidability cannot be obtained, and if it exceeds 2.0 g / m 2 , the weldability deteriorates. Furthermore, the film removal property and adhesiveness by alkali may deteriorate. The film adhesion amount W is preferably 1.5 g / m 2 or less, and more preferably 1.0 g / m 2The following applies:

[0044] Next, the method for manufacturing a zinc-plated steel sheet with a coating according to the present invention will be described.

[0045] First, the manufacturing method of the zinc-plated steel sheet that serves as the base material will be described. The zinc-plated steel sheet is obtained by applying a zinc-plated treatment to a steel sheet such as a cold-rolled steel sheet or a hot-rolled steel sheet. The method of zinc-plated treatment is not particularly limited. Zinc plating can be formed by applying zinc plating to a steel sheet using various manufacturing methods such as hot-dip galvanizing, electroplating, vapor deposition, and thermal spraying. In addition, alloyed hot-dip galvanized steel sheets, which undergo alloying treatment after zinc plating, can also be used. Furthermore, zinc-plated steel sheets containing metals other than zinc, such as zinc-aluminum alloy plating, zinc-aluminum-magnesium alloy plating, and zinc-nickel alloy plating, may be applied. In particular, when the thickness of the zinc-plated steel sheet is 0.70 mm or less, applying a film to the surface of the zinc-plated steel sheet tends to reduce spot weldability. However, with the film according to the present invention described above, even when the thickness of the zinc-plated steel sheet is 0.70 mm or less, the reduction in spot weldability can be suppressed. Therefore, the thickness of the zinc-plated steel sheet may be 0.70 mm or less as an example. The lower limit of the thickness of the zinc-plated steel sheet is not particularly limited, but it is preferable that the thickness be 0.10 mm or more, which is feasible by conventional manufacturing methods.

[0046] Next, a method for forming the coating will be described. The coating can be formed by applying a coating containing the acrylic resin and graphite described above (a coating for forming the coating) to at least one side of a zinc-plated steel sheet and drying it. The coating may contain any components such as rust inhibitors and silica. The coating can be prepared by adding graphite to an acrylic resin solution or emulsion obtained by dissolving or dispersing the acrylic resin in a solvent. Water or an organic solvent can be used as the solvent, but water is preferred. Furthermore, the concentration (mass percentage) of total solids in the coating is preferably 1% by mass or more and 30% by mass or less. If the concentration of total solids in the coating is less than 1% by mass or more than 30% by mass, uneven coating may occur. The method of applying the coating is not particularly limited, but examples include using a roll coater or bar coater, or applying by spraying, dipping, or brushing. The steel sheet after coating can be dried by general methods. For example, drying by hot air, drying by an IH heater, or infrared heating can be used. Preferably, the maximum temperature reached by the zinc-plated steel sheet during drying is between 60°C and 150°C. If the maximum temperature reached by the zinc-plated steel sheet is below 60°C, drying will take longer and the rust prevention properties may be inferior. If the maximum temperature reached by the zinc-plated steel sheet exceeds 150°C, the acrylic resin may oxidize, which may degrade the lubricity and alkali-based defilm removal properties. Furthermore, the amount of film adhering to one side of the zinc-plated steel sheet should be 0.2 g / m² by dry mass. 2 More than 2.0g / m 2 The following conditions shall be met. The amount of film adhesion is determined by dividing the difference in mass of the steel sheet before and after film formation by the surface area of ​​the steel sheet. Alternatively, after film formation, the film may be completely removed with an alkaline aqueous solution or organic solvent with a pH of 10 to 13, and the amount of film adhesion may be determined by dividing the difference in mass of the steel sheet before and after film removal by the surface area of ​​the steel sheet. [Examples]

[0047] The present invention will be described below with reference to examples. However, the present invention is not limited to the following examples.

[0048] As zinc-plated steel sheets, alloyed hot-dip galvanized steel sheets (GA), electro-galvanized steel sheets (EG), and hot-dip galvanized steel sheets (GI) with the arithmetic mean roughness Ra and sheet thickness shown in Table 1 were used. Coatings with the compositions shown in Table 2 were applied to the surface of these steel sheets using a bar coater, and the sheets were dried on an IH heater to a maximum temperature of 80°C to create coated zinc-plated steel sheets for use as test materials. Some samples were tested without forming a coating. Furthermore, steel sheets A to H shown in Table 1 are all zinc-plated steel sheets obtained by applying zinc plating to SPCD (JIS G 3141) substrates with a tensile strength of 270 MPa. Colloidal silica with an average particle size of 9 nm was used as silica, and the total solid content concentration in the coatings used ranged from 3% to 25% by mass.

[0049] The amount of film adhesion was determined by removing the film with a pH 12 sodium hydroxide aqueous solution after film formation, and then dividing the difference in mass of the steel plate before and after film removal by the surface area of ​​the steel plate where the film was formed.

[0050] [Table 1]

[0051] [Table 2]

[0052] (1) Method for evaluating press formability (sliding properties) To evaluate the press formability, the coefficient of friction of each test material was measured as follows. Figure 1 is a schematic front view showing the coefficient of friction measuring apparatus. As shown in the figure, a sample 1 for measuring the coefficient of friction (hereinafter simply referred to as sample 1) taken from each test material is fixed to the sample stand 2. The sample stand 2 is fixed to the upper surface of a slide table 3 which is horizontally movable by rails 9. A slide table support base 5 that is vertically movable and has rollers 4 in contact with the slide table is provided on the lower surface of the slide table 3. A first load cell 7 is attached to the slide table support base 5. The first load cell 7 measures the pressing load N applied to the sample 1 by the bead 6 by pushing up the slide table support base 5. A second load cell 8 is attached to one end of the slide table 3. The second load cell 8 measures the sliding resistance force F required to move the slide table 3 horizontally while the above pressing load is applied. Furthermore, the test was conducted by applying Preton® R352L, a press cleaning oil manufactured by Sugimura Chemical Industry Co., Ltd., to the surface of Sample 1 as a lubricant.

[0053] Figure 2 is a schematic perspective view showing the shape and dimensions of the bead 6 used. The lower surface of the bead 6 slides while pressed against the surface of the sample 1. The shape of the bead 6 shown in Figure 2 is 10 mm wide, 59 mm long in the sliding direction of the sample 1, and the lower parts of both ends in the sliding direction are composed of curved surfaces with a curvature of 4.5 mmR, while the lower surface of the bead 6 against which the sample 1 is pressed has a flat surface with a width of 10 mm and a length of 50 mm in the sliding direction.

[0054] The friction coefficient measurement test was performed using the bead shown in Figure 2, with a pressing load N: 400 kgf and a sample withdrawal speed (horizontal movement speed of slide table 3): 20 cm / min. The friction coefficient μ between the test material (sample 1) and bead 6 was calculated using the formula: μ = F / N.

[0055] The sliding properties were evaluated as follows: a friction coefficient of 0.119 or less was marked as particularly excellent sliding properties (◎), a value between 0.119 and 0.130 was marked as good sliding properties (〇), and a value above 0.130 was marked as insufficient sliding properties (×). ◎ and 〇 in the above evaluation were considered passing grades. If the friction coefficient is 0.130 or less in this evaluation, it can be judged that the sliding properties during press forming are excellent, and that the material has consistently excellent press formability even in complex forming processes such as severe deep drawing.

[0056] (2) Method for evaluating weldability For each test specimen taken from the sample material, a continuous spot welding test was performed under the following conditions: electrode: DR type Cr-Cu electrode, pressure: 200 kgf, energizing time: 10 cycles / 60 Hz, welding current: 8.5 kA. Weldability was evaluated by the number of continuous spots. If the number of continuous spots was 90% or more compared to a zinc-plated steel sheet without a coating, it was evaluated as particularly excellent weldability (◎). If it was 80% or more but less than 90%, it was evaluated as good weldability (○). If it was less than 80%, it was evaluated as insufficient weldability (×). ◎ and ○ were considered passing grades in the above evaluation. If the evaluation is 80% or more, it can be judged that the material has excellent spot weldability.

[0057] (3) Method for evaluating demembranosis The degreasing properties during degreasing were evaluated, assuming use in automotive applications. To determine the degreasing properties of the coating, test pieces taken from each test material were first degreased with the alkaline degreasing agent Fine Cleaner (registered trademark) E6403 (manufactured by Nippon Parkerizing Co., Ltd.). Degreasing was performed by immersing the test pieces in a degreasing solution with a degreasing agent concentration of 20 g / L and a temperature of 40°C for a predetermined time, and then washing them with tap water. The surface carbon intensity (carbon intensity after degreasing) of the degreased test pieces was measured using an X-ray fluorescence analyzer. The coating peeling rate was then calculated using the following formula, along with the surface carbon intensity of the test pieces before degreasing (carbon intensity before degreasing) and the surface carbon intensity of untreated steel sheets (zinc-plated steel sheets that have not undergone coating treatment), which were measured in advance.

[0058] Coating peel rate (%) = [(Carbon strength before degreasing - Carbon strength after degreasing) / (Carbon strength before degreasing - Carbon strength of untreated steel sheet)] × 100

[0059] The defilm removal properties of the coating were evaluated according to the following criteria based on the immersion time in an alkaline degreasing solution that resulted in a coating removal rate of 98% or more. A immersion time of 120 seconds or less was considered good defilm removal (○), while a time exceeding 120 seconds was considered insufficient (△). For test materials that did not form a coating, defilm removal was not evaluated and is indicated by "-" in the table.

[0060] (4) Method for evaluating rust prevention Assuming that coated zinc-plated steel sheets are stored in a coil state as steel strips, the rust prevention properties of the steel sheets in a stacked state were evaluated. Test pieces taken from each test material were processed to a size of 150 mm x 70 mm, and rust-preventive oil was applied to each side at a rate of 1.0 g / m². 2 The coating was applied to both sides in this manner. Then, the two test pieces were placed on top of each other, and a surface pressure of 0.02 kgf / mm² was applied. 2 The tests were conducted in an environment with a temperature of 50°C and a humidity of 95% RH under load. The rust prevention performance was evaluated by checking the inner surface of two stacked test pieces every 7 days and counting the number of days until discoloration occurred. If the number of days was 56 days or more, it was evaluated as particularly good rust prevention (◎), if it was 35 days or more but less than 56 days, it was evaluated as good rust prevention (○), and if it was less than 35 days, it was evaluated as insufficient rust prevention (△).

[0061] (5) Method for evaluating adhesion Test specimens taken from each test material were processed to a size of 100 mm x 25.4 mm, immersed in rust-preventive oil, and then stood upright for 24 hours to remove excess oil. Using two of these oil-removed test specimens, epoxy adhesive was uniformly applied to a 25.4 mm x 13 mm area to a thickness of 0.2 mm, then the two specimens were overlapped and clamped together, and baked at 180°C for 20 minutes to dry and harden. After cooling, a shear tensile test was performed using an autograph testing machine to measure the shear adhesive strength. Adhesion was evaluated as follows: a shear adhesive strength of 20 MPa or more was rated as good adhesion (○), and less than 20 MPa was rated as insufficient adhesion (△).

[0062] [Table 3]

[0063] [Table 4]

[0064] [Table 5]

[0065] According to Tables 3 to 5, the zinc-plated steel sheets with coatings of the present invention all exhibit excellent sliding properties during press forming and excellent spot weldability. In contrast, the comparative examples that do not meet the requirements of the present invention fail to achieve both excellent sliding properties during press forming and excellent spot weldability. Furthermore, by meeting the suitable requirements, the present invention can provide not only excellent formability and weldability, but also good defilmability, corrosion resistance, and adhesion. [Industrial applicability]

[0066] The zinc-plated steel sheet with a coating of the present invention exhibits excellent sliding properties during press forming and excellent weldability. Furthermore, it can possess good defilm removal properties, corrosion resistance, and adhesion. Because of these excellent properties, the zinc-plated steel sheet with a coating of the present invention can be applied to a wide range of fields, mainly in automobile body applications. [Explanation of symbols]

[0067] 1. Sample for measuring the coefficient of friction 2 Sample stage 3. Slide Table 4 Laura 5. Slide Table Support Base 6 beads 7. First load cell 8. Second load cell 9 rails

Claims

1. A zinc-plated steel sheet has a coating on at least one side containing an acrylic resin and graphite. The aforementioned acrylic resin has a glass transition temperature (Tg) of 100°C or higher, and a ratio R (acid value (mg-KOH / g) / Tg (°C)) of 1.50 or higher. The average particle size of the graphite is 10 μm or less, and the mass percentage of the graphite in the coating is 5% by mass or more. The amount W of the coating applied to one side of the zinc-plated steel sheet is 0.2 g / m². ２ 2.0g / m or more ２ The following are zinc-plated steel sheets with a coating.

2. The zinc-plated steel sheet with a coating according to claim 1, wherein the acid value of the acrylic resin is 180 mg-KOH / g or more and 350 mg-KOH / g or less.

3. The zinc-plated steel sheet with a coating according to claim 1 or 2, wherein the ratio R of the acid value to the glass transition temperature of the acrylic resin is 2.05 or less.

4. The zinc-plated steel sheet with a coating according to claim 1 or 2, wherein the coating contains 30% by mass or more of the acrylic resin and the mass ratio of the graphite is 50% by mass or less.

5. The zinc-plated steel sheet with a coating according to claim 1 or 2, wherein the mass-average molecular weight of the acrylic resin is 5,000 or more and 30,000 or less.

6. The zinc-plated steel sheet with a coating according to claim 1 or 2, wherein the acrylic resin is styrene-acrylic resin.

7. The zinc-plated steel sheet with a coating according to claim 1 or 2, wherein the coating contains 1% by mass or more and 30% by mass or less of a rust inhibitor.

8. The zinc-plated steel sheet with a coating according to claim 7, wherein the rust inhibitor is at least one selected from the group consisting of aluminum salts of phosphates, zinc salts of phosphates, and zinc oxide.

9. The zinc-plated steel sheet with a coating according to claim 1 or 2, wherein the average particle size of the graphite is 0.1 μm or more and 6.0 μm or less.

10. The zinc-plated steel sheet with a coating according to claim 1 or 2, wherein the coating contains 1% by mass or more and 10% by mass or less of silica.

11. The zinc-plated steel sheet with a coating according to claim 1 or 2, wherein the thickness of the zinc-plated steel sheet is 0.10 mm or more and 0.70 mm or less.

12. A method for manufacturing a coated zinc-plated steel sheet according to claim 1, A method for manufacturing a zinc-plated steel sheet with a coating, comprising applying a coating containing the acrylic resin and the graphite to at least one side of the zinc-plated steel sheet and drying it.

13. The method for manufacturing a coated zinc-plated steel sheet according to claim 12, wherein the maximum temperature reached by the zinc-plated steel sheet during drying is 60°C or more and 150°C or less.

14. A method for manufacturing a coated zinc-plated steel sheet according to claim 12 or 13, wherein the mass percentage of total solids in the coating is 1% by mass or more and 30% by mass or less.

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