Zinc-based coated steel sheet and its manufacturing method

Abstract

The purpose of the present disclosure is to provide a galvanized steel sheet subjected to complicated molding for which it is difficult to adopt press molding, the galvanized steel sheet having a lubricating film which has: a small sliding resistance at a site having the risk of being cracked during press molding; and excellent press moldability at a site which has high surface pressure and in which the occurrence of mold galling is estimated.　The present invention involves forming, on the surface of a galvanized steel sheet, a film containing: an acrylic resin having a glass transition point (Tg) of at least 100ºC and an acid value ratio R=acid value (mg-KOH / g) / Tg (ºC) of at least 1.50; and at least 5 mass% of a polyolefin wax having a melting point of 100-145ºC and an average particle diameter of at most 3.0 μm, so that the adhesion amount thereof per surface is in the range of 0.2 g / m2 to 2.5 g / m2.

Description

[Technical Field]

[0001] The present invention relates to a zinc-based plated steel sheet having excellent sliding properties in press forming, and a method for manufacturing the same. In particular, the present invention relates to a zinc-based plated steel sheet having a lubricating film that provides excellent formability even during severe drawing, and a method for manufacturing the same. [Background technology]

[0002] Zinc-based plated steel sheets are widely used in a wide range of fields, primarily for automobile bodies. For such applications, they are generally press-formed before use. However, zinc-based plated steel sheets have the disadvantage of being inferior in press formability compared to cold-rolled steel sheets. This is due to the fact that the sliding resistance of zinc-based plated steel sheets in press dies is greater than that of cold-rolled steel sheets. That is, during drawing, zinc-based plated steel sheets, which have a high sliding resistance with the bead, have difficulty flowing into the press die, making them prone to fracture. Even 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 method of applying a high-viscosity lubricant is widely used to improve the press formability of zinc-based plated steel sheets. However, because the lubricant is highly viscous, this method can result in coating defects due to insufficient degreasing during the coating process. Furthermore, problems such as instability in press performance due to oil shortage during pressing can occur. Another method for improving press formability is surface treatment of the mold. While surface treatment of the mold is widely used, this method does not allow adjustment of the mold after surface treatment. Another problem is the high cost. Therefore, there is a demand for an improvement in the press formability of zinc-based plated steel sheets themselves that does not rely on high-viscosity lubricants or surface treatment of the mold.

[0004] Therefore, various types of lubricant surface-treated steel sheets have been investigated as a method for solving the above problems.

[0005] Patent Document 1 describes a technique for forming a lubricating film, which is an acrylic resin film containing synthetic resin powder, on a zinc-based plated steel sheet.

[0006] Patent Document 2 describes a metal plate coated with a lubricating film in which a solid lubricant protrudes from the surface of a resin film by 0.01 to 1.5 μm.

[0007] Patent Document 3 describes a lubricating surface-treated metal product that is excellent in press formability and is coated with a 0.5 to 5 μm thick film made of polyurethane resin containing a lubricant.

[0008] Patent Document 4 describes a technique for forming an alkali-soluble organic coating on a steel sheet, the organic coating being an epoxy resin containing a lubricant. [Prior art documents] [Patent documents]

[0009] [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 Application Laid-Open No. 2000-309747 [Patent Document 4] Japanese Patent Application Laid-Open No. 2000-167981 Summary of the Invention [Problem to be solved by the invention]

[0010] However, in Patent Documents 1 to 4, although lubricity is achieved by the lubricating effect of the contained lubricant or the like, press formability is not necessarily sufficient in complex forming.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a zinc-based plated steel sheet that is subjected to difficult press forming and has low sliding resistance during press forming and excellent press formability, and a method for manufacturing the same.

[0012] Furthermore, when zinc or zinc alloy plated steel sheets are used for automobile bodies, they are required to have excellent adhesiveness and weldability in the assembly process, and also to have sufficient film removal properties in the alkaline degreasing step of the painting process.

[0013] Here, zinc-based plated steel sheet refers to a steel sheet on which a zinc-based plating film is formed. Examples of zinc-based plating films include zinc plating consisting of zinc and inevitable impurities without intentionally adding any other components than zinc, and zinc plating consisting mainly of zinc with the addition of alloying elements such as iron, nickel, aluminum, and magnesium, and compounds such as alumina and silica. The coating weight of the zinc-based plating on zinc-based plated steel sheet is not particularly limited, but is preferably 3 to 120 g / m per side. 2 is preferably used. [Means for solving the problem]

[0014] The present inventors have conducted extensive research to solve the above problems, and as a result, have discovered an organic resin coating having a coating weight W of 0.2 g / m per side, the organic resin coating containing an acrylic resin having a glass transition temperature (Tg) of 100°C or higher and an acid value ratio R = acid value (mg-KOH / g) / Tg (°C) of 1.50 or higher and 5 mass% or more of polyolefin wax having a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or less. 2 More than 2.5g / m 2 It has been found that the above problems can be solved by forming the coating on the surface of a zinc-based plated steel sheet within the following ranges.

[0015] The present invention has been completed based on the above findings, and the gist of the present invention is as follows. [1] A zinc-based plated steel sheet having a coating containing an acrylic resin and wax formed 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 the acid value to the glass transition point (acid value (mg-KOH / g) / Tg (°C)) of 1.50 or higher, the wax is a polyolefin wax having a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or less, the proportion of wax in the coating being 5% by mass or higher, and the coating weight W per side of the coating being 0.2 g / m 2 More than 2.5g / m 2 Zinc-plated steel sheet as follows: [2] The zinc-based plated steel sheet 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-based plated steel sheet 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-based plated steel sheet according to any one of [1] to [3], wherein the coating contains 30% by mass or more of the acrylic resin and 50% by mass or less of the wax. [5] The zinc-based plated steel sheet according to any one of [1] to [4], wherein the acrylic resin has a mass average molecular weight of 5,000 or more and 30,000 or less. [6] The zinc-plated steel sheet according to any one of [1] to [5], wherein the acrylic resin is a styrene-acrylic resin. [7] The zinc-based plated steel sheet according to any one of [1] to [6], wherein the arithmetic mean roughness Ra of the surface of the zinc-based plated steel sheet before the coating is formed is 0.4 μm or more and 2.5 μm or less. [8] The zinc-based plated steel sheet according to any one of [1] to [7], wherein the coating contains 1 mass % or more and 30 mass % or less of a rust inhibitor. [9] The zinc-based plated steel sheet according to [8], wherein the rust inhibitor is at least one selected from the group consisting of aluminum salts of phosphoric acids, zinc salts, and zinc oxide.

[10] The zinc-based plated steel sheet according to any one of [1] to [9], wherein the wax has an average particle size of 0.01 μm or more and 0.5 μm or less.

[11] The zinc-based plated steel sheet according to any one of [1] to

[10] , wherein the coating contains silica in an amount of 1% by mass or more and 10% by mass or less.

[12] A method for producing a zinc-based plated steel sheet according to any one of [1] to

[11] , which comprises applying a coating material containing the acrylic resin and wax according to any one of [1] to

[11] to at least one surface of a zinc-based plated steel sheet, and drying the coating material.

[13] The method for producing a zinc-based plated steel sheet according to

[12] , wherein the maximum temperature of the zinc-based plated steel sheet during drying is 60°C or higher and is lower than the melting point of the wax.

[14] The method for producing a zinc-based plated steel sheet according to

[12] or

[13] , wherein the proportion of the total solid content in the paint is 1% by mass or more and 30% by mass or less. [Effects of the Invention]

[0016] According to the present invention, a zinc-based coated steel sheet having excellent press formability due to a significantly reduced coefficient of friction with a die or the like can be obtained. Therefore, the zinc-based coated steel sheet subjected to complex forming has stable and excellent press formability. Furthermore, the steel sheet also has excellent weldability. Furthermore, the zinc-based plated steel sheet provided with a suitable coating has good adhesion and removability, allowing adhesives to be used in the same manner as for conventional zinc-based plated steel sheets, and the excellent removability by alkaline degreasing does not interfere with the painting process. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic front view showing a friction coefficient measuring device. [Figure 2] FIG. 2 is a schematic perspective view showing the shape and dimensions of the bead in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the present invention will be described.

[0019] The zinc-based plated steel sheet of the present invention is a zinc-based plated steel sheet having a coating containing an acrylic resin and wax formed 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 the acid value to the glass transition point, R = acid value (mg-KOH / g) / Tg (°C), of 1.50 or higher, the wax is a polyolefin wax having a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or less, the proportion of wax in the coating is 5 mass% or higher, and the coating is applied in a coating weight W of 0.2 g / m per side. 2 More than 2.5g / m 2 The present invention is characterized by the following:

[0020] Hereinafter, the ratio of the acid value to the glass transition point, R=acid value (mg-KOH / g) / Tg (°C), will be expressed as R=acid value / Tg.

[0021] The glass transition point of the acrylic resin in the coating of the present invention is set to 100°C or higher in order to obtain good lubricity. If the glass transition point is lower than 100°C, the resin softens during sliding, reducing the wax retention and the ability to prevent direct contact between the zinc-plated steel sheet and the mold, making it difficult to obtain good sliding properties. The glass transition point is preferably 110°C or higher and 150°C or lower. If the glass transition point exceeds 150°C, the resin may become too hard and brittle during sliding, making it difficult to obtain good lubricity.

[0022] Here, the glass transition point is the intermediate glass transition temperature measured in accordance with JIS K 7121 "Method for measuring transition temperature of plastics."

[0023] The ratio of the acid value to the glass transition temperature of the acrylic resin (R = acid value / Tg) must be 1.50 or higher. Even if the glass transition temperature is above 100°C, a low acid value (R < 1.50) will not provide excellent lubrication. While the reason for this is unclear, it is believed that the carboxyl groups in the acrylic resin have a high affinity with the mold, effectively transferring the polyolefin wax in the coating to the mold during sliding. When the acrylic resin component containing the polyolefin wax transfers to the mold during sliding, the mold surface is protected by the polyolefin wax, which effectively prevents direct contact with the zinc-plated steel sheet and improves sliding properties. Therefore, when the acid value is low (R < 1.50), sliding properties are poor due to a lack of carboxyl groups. As the glass transition temperature of the acrylic resin increases, the resin becomes less likely to soften during sliding, making it less likely to transfer to the mold. Therefore, to achieve excellent sliding properties when the glass transition temperature increases, the acid value must also be increased. In other words, the ratio of the acid value to the glass transition temperature (R = acid value / Tg) must be 1.50 or higher. R is preferably 1.80 or more. There is no particular upper limit to R, but it is preferably 2.05 or less. The reason for this is that if R exceeds 2.05, the rust prevention properties may deteriorate.

[0024] The acid value of the acrylic resin is preferably 180 mg-KOH / g or more and 350 mg-KOH / g or less. If it is less than 180 mg-KOH / g, the film may be poorly removable with alkali, and sufficient adhesive strength may not be obtained. If it exceeds 350 mg-KOH / g, the rust prevention properties may be deteriorated.

[0025] Here, the acid value refers to the number of milligrams of potassium hydroxide required to neutralize the carboxyl groups contained in 1 g of resin, and is measured based on JIS K 0070 "Testing methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products." In the present invention, the unit is expressed as mg-KOH / g.

[0026] The wax used in the present invention may be any polyolefin wax having a melting point of 100° C. or higher and 145° C. or lower and an average particle size of 3.0 μm or less.

[0027] Polyolefin wax is used as the wax because it has low surface energy and self-lubricating properties, which provide good lubrication. In addition, it is relatively easy to adjust the melting point of polyolefin to between 100°C and 145°C by controlling the density and molecular weight.

[0028] When the melting point of the polyolefin wax is between 100°C and 145°C, in addition to the self-lubricating properties of the polyolefin wax itself, the polyolefin wax becomes semi-molten due to sliding during press molding, allowing the lubricating coating components mixed with the acrylic resin to coat the mold surface. This prevents direct contact between the mold and the zinc-plated steel sheet, resulting in excellent lubrication. When the melting point of the polyolefin wax is less than 100°C, the polyolefin wax completely melts due to frictional heat caused by sliding during press molding, resulting in insufficient lubrication by the polyolefin wax itself and the aforementioned mold coating effect. Furthermore, when the melting point of the polyolefin wax exceeds 145°C, the polyolefin wax does not melt during sliding, resulting in insufficient lubrication and mold coating effect. Furthermore, the melting point of the polyolefin wax is preferably between 120°C and 140°C.

[0029] Here, the melting point of the polyolefin wax is the melting temperature measured in accordance with JIS K 7121 "Method for measuring transition temperature of plastics."

[0030] If the average particle size of the polyolefin wax exceeds 3.0 μm, it becomes difficult to mix with the acrylic resin during sliding, and the aforementioned mold coating effect cannot be obtained, resulting in insufficient lubrication. The average particle size of the polyolefin wax is preferably 0.5 μm or less, and even more preferably 0.3 μm or less.

[0031] The average particle size of the polyolefin wax is preferably 0.01 μm or more. If the average particle size of the polyolefin wax is less than 0.01 μm, it will be easily dissolved in the lubricating oil during sliding, which may result in insufficient lubrication improvement, and it will be prone to agglomeration in the coating material used to form the film, resulting in low coating material stability. The average particle size of the polyolefin wax is more preferably 0.03 μm or more. Taking into consideration the compatibility with the acrylic resin, the average particle size of the polyolefin wax is preferably 0.01 μm or more and 0.5 μm or less.

[0032] The average particle size is the median diameter of the volume average diameter, and can be determined by a laser diffraction / scattering method, for example, by measuring a sample diluted with pure water using a laser diffraction / scattering particle size distribution analyzer, Partica (registered trademark) LA-960V2 (manufactured by Horiba, Ltd.).

[0033] Among polyolefin waxes, polyethylene wax is preferred because it provides the greatest lubricating effect.

[0034] The mass percentage of polyolefin wax in the coating is 5% by mass or more. If the mass percentage of polyolefin wax in the coating is less than 5% by mass, a sufficient lubricating effect cannot be obtained. If the mass percentage of polyolefin wax in the coating is 10% by mass or more, a particularly good lubricating effect can be obtained. Furthermore, the mass percentage of polyolefin wax in the coating is preferably 50% by mass or less. If the mass percentage of polyolefin wax in the coating exceeds 50% by mass, the polyolefin wax is likely to fall off due to a lack of base resin components, resulting in poor adhesion to the steel sheet and an inability to form a stable coating, which may result in poor adhesion. Furthermore, when used on automobile bodies, sufficient degreasing may not be achieved in the alkaline degreasing step of the painting process, and the coating may not be sufficiently removed in the alkaline degreasing step, resulting in residual coating and impaired paintability. The mass percentage of polyolefin wax in the coating is more preferably 30% by mass or less.

[0035] Here, the mass ratio of the polyolefin wax in the coating is the mass ratio of the solid content of the polyolefin wax to the mass of the total solid content in the coating material.

[0036] The coating of the present invention preferably contains 30% by mass or more of the acrylic resin. When the mass proportion of the acrylic resin in the coating is 30% by mass or more, the properties influenced by the physical properties of the acrylic resin component, such as the effect of improving lubrication due to transfer to the mold during sliding, film removal, and adhesiveness, can be sufficiently obtained. When the mass proportion of the 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 achieved.

[0037] The mass average molecular weight of the acrylic resin is preferably 5000 or more and 30000 or less. If the mass average molecular weight of the acrylic resin is less than 5000, the rust prevention properties may be poor, and if it exceeds 30000, the adhesiveness may be deteriorated.

[0038] Here, the mass average molecular weight is the mass average molecular weight measured in accordance with JIS K 7252 "Plastics - Determination of average molecular weight and molecular weight distribution of polymers by size exclusion chromatography."

[0039] Furthermore, the acrylic resin is preferably a styrene-acrylic resin. By including styrene in the monomer of the resin, water resistance is improved, resulting in good rust prevention. Furthermore, the effect of obtaining better sliding properties compared to when styrene is not included is also exhibited.

[0040] The coating of the present invention preferably contains a rust inhibitor in an amount of 1% by mass or more and 30% by mass or less. Even without the rust inhibitor, rust and discoloration do not occur under normal storage conditions. However, if the rust inhibitor content is less than 1% by mass, rust may occur under unfavorable storage conditions. In particular, when the steel strip is stacked in a coil and stored in a humid environment, moisture absorption may cause discoloration. If the rust inhibitor content exceeds 30% by mass, adhesion may deteriorate, and the rust inhibitor may precipitate in the paint, resulting in deterioration of paint stability. It is preferable to use at least one rust inhibitor selected from the group consisting of aluminum salts of phosphates, zinc salts, and zinc oxide. Here, phosphoric acids include orthophosphoric acid as well as condensed phosphoric acids such as pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, and metaphosphoric acid. The use of these rust inhibitors provides sufficient rust prevention effects while minimizing deterioration of paint stability.

[0041] Furthermore, the coating of the present invention preferably contains silica in an amount of 1% by mass or more and 10% by mass or less. The inclusion of silica increases the water repellency of the coating and improves rust prevention. The inclusion of silica also makes it possible to suppress precipitation of the rust inhibitor, improving paint stability. Furthermore, if the content is less than 1% by mass, it is difficult to achieve the above-mentioned effects, and if it exceeds 10% by mass, adhesion may deteriorate. When silica is contained in the coating of the present invention, it is preferable to use colloidal silica with a particle size of 5 nm to 200 nm.

[0042] In the present invention, the coating composition may contain, as components other than the acrylic resin, wax, rust inhibitor, and silica, surface conditioners, antifoaming agents, dispersants, and the like that are generally added to coating materials.

[0043] The surface roughness of the zinc-based plated steel sheet used in the present invention before coating formation is preferably 0.4 μm or more and 2.5 μm or less in terms of arithmetic mean roughness Ra. If Ra is 2.5 μm or less, the lubricating effect of the coating can be stably obtained. If Ra is less than 0.4 μm, fine scratches that may occur during press forming may be more noticeable, and mold galling may occur during press forming. If Ra exceeds 2.5 μm, the steel sheet's unevenness becomes large, making it difficult for the coating in the recesses to function effectively during sliding, and the lubricating effect of the coating may be reduced. The arithmetic mean roughness Ra (μm) of the steel sheet can be measured in accordance with JIS B 0633:2001 (ISO 4288:1996). For example, if Ra is greater than 0.1 but less than 2, the roughness can be determined from a roughness curve measured with a cutoff value and a reference length of 0.8 mm and an evaluation length of 4 mm. When Ra is greater than 2 and less than 10, the cutoff value and reference length are set to 2.5 mm, and the evaluation length is set to 12.5 mm, and the roughness is calculated from the measured roughness curve. The dry mass of the coating on one side of the zinc-based coated steel sheet is 0.2 g / m 2 More than 2.5g / m 2 Form it so that it is 0.2g / m 2 If the thickness is less than 2.5g / m, sufficient sliding properties may not be obtained. 2 If it exceeds this value, alkali weldability, film removal, and adhesiveness may deteriorate.

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

[0045] The method for producing a zinc-based plated steel sheet of the present invention is a method for producing a zinc-based plated steel sheet having an acrylic resin coating formed on at least one side thereof, the acrylic resin having a glass transition point (Tg) of 100°C or higher and a ratio of acid value to glass transition point (R = acid value / Tg) of 1.50 or higher, and 5 mass% or more of polyolefin wax having a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or less. First, a method for manufacturing a zinc-based plated steel sheet according to the present invention will be described. A steel sheet such as a cold-rolled steel sheet or a hot-rolled steel sheet is subjected to a zinc-based plating treatment. The method for the zinc-based plating treatment is not particularly limited, and the steel sheet can be formed by applying a zinc-based plating to the steel sheet using various manufacturing methods, such as hot-dip plating, electroplating, vapor deposition plating, and thermal spraying. It is also possible to use a hot-dip galvannealed steel sheet that is subjected to an alloying treatment after the zinc plating treatment. Furthermore, a zinc-based plating containing a metal other than zinc, such as a zinc-aluminum alloy plating, a zinc-aluminum-magnesium alloy plating, or a zinc-nickel alloy plating, may be applied. Next, a method for forming an acrylic resin film is described. A paint containing wax added to an acrylic resin solution or emulsion, in which an acrylic resin is dissolved or dispersed in a solvent, is applied to at least one side of a zinc-plated steel sheet and then dried. Water or an organic solvent can be used as the paint solvent, but water is preferred. The total solids concentration in the paint is preferably 1% by mass or more and 30% by mass or less. A total solids concentration of less than 1% by mass or more than 30% by mass may result in uneven coating. The application method is not particularly limited, but examples include the use of a roll coater or bar coater, as well as spraying, immersion, and brush application. The steel sheet can be dried using a conventional method after application. Examples include drying with hot air, drying with an induction heater, and infrared heating. The maximum temperature reached by the zinc-plated steel sheet during drying is preferably 60°C or higher and below the melting point of the wax used. If the maximum temperature reached by the zinc-plated steel sheet is less than 60°C, drying takes a long time and rust prevention properties may be poor. If the maximum temperature of the zinc-based coated steel sheet exceeds the melting point of the wax, the wax will melt and coalesce, causing the particle size to become coarse, which may result in a deterioration of lubricity. Also, if the coating weight per side of the zinc-based coated steel sheet is 0.2 g / m2 in dry mass, 2 More than 2.5g / m 2The coating weight can be determined by dividing the difference in weight of the zinc-based plated steel sheet before and after coating by its area, or by completely removing the coating from the zinc-based plated steel sheet after coating with an alkaline aqueous solution or organic solvent having a pH of 10 to 13, and then dividing the difference in weight of the zinc-based plated steel sheet before and after removing the coating by its area. [Example]

[0046] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples. Galvannealed steel sheets (GA), electrogalvanized steel sheets (EG), and hot-dip galvanized steel sheets (GI) with a thickness of 0.8 mm and arithmetic mean roughness Ra shown in Table 1 were used to prepare zinc-based plated steel sheets of the present invention as test materials. Coatings with the compositions shown in Table 2 were applied using a bar coater, and the steel sheets were then dried using an induction heater so that the maximum temperature reached by the steel sheets reached 80°C. Note that steel sheets A to G were all zinc-based plated steel sheets prepared by applying a zinc-based coating to an SPCD (JIS G 3141) substrate with a tensile strength of 270 MPa. Colloidal silica with a volume average particle size of 9 nm was used as the silica. The total solids concentration in the coating materials used ranged from 3% to 25%.

[0047] The coating weight was determined by removing the coating from the zinc-based plated steel sheet with a sodium hydroxide solution at pH 12, and dividing the difference in mass between the zinc-based plated steel sheet before and after coating removal by the area.

[0048] [Table 1]

[0049] [Table 2]

[0050] (1) Evaluation method for press formability (sliding characteristics) To evaluate press formability, the coefficient of friction of each test material was measured as follows.

[0051] FIG. 1 is a schematic front view of a friction coefficient measurement device. As shown in the figure, a friction coefficient measurement sample 1 taken from a test material is fixed to a sample stage 2, which is fixed to the upper surface of a horizontally movable slide table 3. A vertically movable slide table support 5 having a roller 4 in contact with the slide table 3 is provided on the underside of the slide table 3. A first load cell 7 is attached to the slide table support 5 to measure the pressing load N applied to the friction coefficient measurement sample 1 by a bead 6 when the slide table 3 is pushed up. A second load cell 8 is attached to one end of the slide table 3 to measure the sliding resistance force F caused by moving the slide table 3 horizontally while the pressing force is applied. The test was conducted using a press cleaning oil, Pleton® R352L, manufactured by Sugimura Chemical Industry Co., Ltd., applied to the surface of the sample 1 as a lubricant.

[0052] Figure 2 is a schematic perspective view showing the shape and dimensions of the bead used. The underside of the bead 6 slides while being 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, and the lower ends of both ends in the sliding direction are curved surfaces with a curvature of 4.5 mmR, and the underside of the bead against which the sample is pressed has a flat surface with a width of 10 mm and a length of 50 mm in the sliding direction.

[0053] The friction coefficient measurement test was carried out using the bead shown in Figure 2, with a pressing load N of 400 kgf and a sample pull-out speed (horizontal movement speed of the slide table 3) of 20 cm / min. The friction coefficient μ between the test material and the bead was calculated using the formula μ = F / N.

[0054] The sliding properties were evaluated as follows: when the friction coefficient was 0.119 or less, it was evaluated as being particularly excellent sliding properties, ⊚; when it was more than 0.119 and 0.130 or less, it was evaluated as being good sliding properties, ◯; when it was more than 0.130, it was evaluated as being insufficient, and ×.

[0055] (2) Weldability evaluation method For each test piece, a welding test for continuous weldability was conducted under the following conditions: electrode: DR-type Cr-Cu electrode, pressure: 150 kgf, current application time: 10 cycles / 60 Hz, welding current: 7.5 kA, and the test was evaluated based on the number of continuous welds. If the number of continuous welds was 90% or more compared to the zinc-based plated steel sheet without the acrylic coating, the weldability was evaluated as good (○), and if it was less than 90%, the weldability was evaluated as insufficient (×).

[0056] (3) Evaluation method for film removal Assuming that the steel sheet according to the present invention will be used in automobile applications, the removability of the coating during degreasing was evaluated. To determine the removability of the coating, each test piece was first degreased with an alkaline degreaser, Fine Cleaner (registered trademark) E6403 (manufactured by Nihon Parkerizing Co., Ltd.). The degreasing treatment was performed by immersing the test piece in a degreasing solution with a degreaser concentration of 20 g / L and a temperature of 40°C for a predetermined period of time, followed by rinsing with tap water. The surface carbon intensity of the degreased test piece was measured using an X-ray fluorescence analyzer, and the coating peeling rate was calculated using the measured value, the previously measured surface carbon intensity before degreasing, and the measured surface carbon intensity of an untreated metal sheet, according to the following formula:

[0057] Film peeling rate (%) = [(carbon strength before degreasing - carbon strength after degreasing) / (carbon strength before degreasing - carbon strength of untreated steel plate)] x 100 The removability of the coating was evaluated according to the criteria shown below, based on the immersion time in the alkaline degreasing solution at which the coating removal rate was 98% or more. A time of 120 seconds or less was evaluated as good removability and rated as ◯, and a time of more than 120 seconds was evaluated as insufficient removability and △.

[0058] (4) Evaluation method for rust prevention The rust prevention properties of the zinc-based plated steel sheets according to the present invention in a stacked state were evaluated, assuming that they were stored in a coiled state as steel strips. Each test piece of the test material was cut into a size of 150 mm x 70 mm, and rust prevention oil was applied at 1.0 g / m per side. 2 The two test pieces were then placed one on top of the other and the surface pressure was 0.02 kgf / mm 2The test was carried out in an environment with a temperature of 50°C and a humidity of 95% RH with a load applied so that the temperature was as high as possible. The rust prevention properties were evaluated by checking the inside surfaces of the overlapped pieces every seven days and counting the number of days until discoloration occurred. A period of 56 days or more was evaluated as excellent rust prevention, with a rating of ◎, a period of 35 days or more as good rust prevention, and a rating of △, meaning insufficient rust prevention, when the period was less than 35 days.

[0059] (5) Evaluation method for adhesiveness Each test piece was cut to a size of 100 x 25.4 mm, immersed in rust-preventive oil, and then placed vertically for 24 hours to remove excess oil. Two pieces were used, and an epoxy adhesive was applied evenly to a thickness of 0.2 mm on the 25.4 mm x 13 mm area. The pieces were then overlapped and clamped with clips 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 good adhesion with an adhesive strength of 20 MPa or more, rated as ○, and poor adhesion with an adhesive strength of less than 20 MPa, rated as △.

[0060] [Table 3-1]

[0061] [Table 3-2]

[0062] [Table 3-3]

[0063] According to Tables 3-1 to 3-3, the zinc-based plated steel sheets of the examples of the present invention all have excellent press formability and weldability. In contrast, the zinc-based plated steel sheets of the comparative examples, which do not have the technical features of the present invention, all have poor press formability. Furthermore, by selecting an appropriate technical range, the zinc-based plated steel sheets of the examples of the present invention can be provided with good coating removal properties, rust prevention properties, and adhesion properties. [Industrial Applicability]

[0064] The zinc or zinc alloy plated steel sheet of the present invention has excellent sliding properties and weldability during press forming. Furthermore, by selecting an appropriate range of techniques, it can be provided with good coating removal properties, rust prevention properties, and adhesion properties. Because of these excellent properties, it can be used in a wide range of fields, primarily for automobile bodies. [Explanation of symbols]

[0065] 1. Sample for measuring friction coefficient 2 Sample stage 3 Sliding table 4. Laura 5 Slide table support 6 beads 7. First load cell 8 Second load cell 9 Rail

Claims

1. A zinc-based plated steel sheet having a coating containing an acrylic resin and wax formed on at least one surface thereof, wherein the acrylic resin has a glass transition point (Tg) of 100°C or higher and a ratio R of the acid value to the glass transition point, R = acid value (mg-KOH / g) / Tg (°C), of 1.80 to 2.05, the wax is a polyolefin wax having a melting point of 100°C to 145°C and an average particle size of 3.0 μm or less, the proportion of wax in the coating being 5% by mass or higher, and the coating weight W per one surface being 0.2 g / m ２ 2.5g / m or more ２ Zinc-plated steel sheet as follows:

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

3. 3. The zinc-based plated steel sheet according to claim 1, wherein the coating contains 1% by mass or more and 30% by mass or less of a rust inhibitor that is at least one selected from the group consisting of aluminum salts of phosphates, zinc salts, and zinc oxide.

4. 3. The zinc-based plated steel sheet according to claim 1, wherein the coating contains silica in an amount of 1% by mass to 10% by mass.

5. The zinc-based plated steel sheet according to claim 3, wherein the coating contains silica in an amount of 1% by mass to 10% by mass.

6. 3. The method for producing a zinc-based plated steel sheet according to claim 1, wherein the paint containing the acrylic resin and the wax is applied to at least one surface of the zinc-based plated steel sheet and then dried.

7. 4. The method for producing a zinc-based plated steel sheet according to claim 3, wherein the paint containing the acrylic resin and the wax is applied to at least one surface of the zinc-based plated steel sheet and then dried.

8. 5. The method for producing a zinc-based plated steel sheet according to claim 4, wherein the paint containing the acrylic resin and the wax is applied to at least one surface of the zinc-based plated steel sheet and then dried.

9. 6. The method for producing a zinc-based plated steel sheet according to claim 5, wherein the paint containing the acrylic resin and the wax is applied to at least one surface of the zinc-based plated steel sheet and then dried.

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