Lubricating film coated zinc-based plated steel sheet and its manufacturing method

Abstract

To provide a galvanized steel sheet including a lubricant film that has a low slide resistance in a crack potential part at the time of press molding, has excellent press molding property in a portion where occurrence of molding seizure due to high surface pressure is predicted, and especially, has excellent press molding property with respect to a steel sheet with a wide range surface roughness, in a galvanized steel sheet to be subject to complicated molding of difficult-to-press molding, and to provide a method for manufacturing the same.SOLUTION: An organic resin that is at least any one of the acrylic resin, epoxy resin, urethane resin, phenolic resin, vinyl acetate resin, and polyester resin containing 10 mass% or more of a polyolefin wax having a melting point of 120°C or higher and 140°C or lower and an average grain size of 3.0 μm or less is formed at a surface of a galvanized steel sheet including a zinc metal layer having a coating weight of 5 g / m2 or more so that a relationship between an organic resin film coating weight W(g / m2) per single side and an arithmetic average roughness Ra of the steel sheet becomes within a range of W≥0.12×Ra2+0.1.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] Zinc-plated steel sheets are widely used in a wide range of fields, primarily for automobile bodies. Usually, zinc-plated steel sheets are used after press forming. However, zinc-plated steel sheets have the disadvantage of being inferior in press formability compared to cold-rolled steel sheets. This is because the sliding resistance of zinc-plated steel sheets in press dies is greater than that of cold-rolled steel sheets. That is, in areas where the sliding resistance between the die and the bead is high, the zinc-plated steel sheet, which has a high sliding resistance, is less likely to flow into the press die, and the steel sheet is more likely to break.

[0003] Therefore, a method of applying a highly viscous lubricant is widely used to improve the press formability of zinc-based plated steel sheets. However, this method has problems such as coating defects due to poor degreasing during the coating process due to the high viscosity of the lubricant. In addition, oil shortage during pressing can cause unstable press performance. For this reason, there is a demand for improving the press formability of zinc-based plated steel sheets themselves.

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

[0005] Patent Document 1 describes a metal sheet 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.

[0006] Patent Document 2 describes a lubricating surface-treated metal product having excellent press formability, which is coated with a 0.5 to 5 μm thick film made of polyurethane resin containing a lubricant.

[0007] Patent Document 3 describes a technique for forming an alkali-soluble organic film, which is an epoxy resin containing a lubricant added thereto, on a steel sheet. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 10-52881 [Patent Document 2] JP 2000-309747 A [Patent Document 3] JP 2000-167981 A Summary of the Invention [Problem to be solved by the invention]

[0009] However, in Patent Documents 1 to 3, although lubricity is achieved by the lubricating effect of the contained lubricant, etc., the press formability is not necessarily sufficient in complex forming. In particular, when the surface roughness of the steel sheet changes, stable and good press formability cannot be obtained.

[0010] The present invention has been made in view of the above circumstances, and has an object to provide a lubricating film-coated zinc-based plated steel sheet that is provided with a lubricating film that has low sliding resistance in areas at risk of cracking during press forming, excellent press formability in areas where the contact pressure is high and die galling is likely to occur, and in particular has excellent press formability for steel sheets with a wide range of surface roughness, in zinc-based plated steel sheets that are subjected to complex forming that makes press forming difficult, and a method for manufacturing the same.

[0011] Furthermore, when used as a steel sheet for automobiles, sufficient film-removal property is also required in the alkaline degreasing step in the painting process, and in such applications, it is a further object to have good film-removal property as well.

[0012] Here, the zinc-based plated steel sheet is a steel sheet having a zinc-based plating film formed on the steel sheet. Examples of zinc-based plating films include zinc plating consisting of zinc and unavoidable impurities without intentionally adding any other components than zinc, and zinc plating consisting mainly of zinc with the addition of alloy elements such as iron, nickel, aluminum, and magnesium, and compounds such as alumina and silica. [Means for solving the problem]

[0013] The present inventors have conducted extensive research to solve the above problems. As a result, they have found that a coating amount of 5 g / m2 on the surface of a steel sheet is necessary to dramatically improve press formability. 2 It has been discovered that the above problems can be solved by forming an organic resin coating containing polyolefin wax having a melting point of 120°C or more and 140°C or less and an average particle size of 3.0 μm or less on the top layer of a zinc-based plated steel sheet having the above-mentioned metallic zinc layer, and by controlling the surface roughness and coating weight of the steel sheet.

[0014] The present invention has been completed based on the above findings, and the gist of the present invention is as follows. [1] At least one surface has a coating weight of 5 g / m 2 A lubricating film-coated zinc-based plated steel sheet having the above metallic zinc layer and an upper layer formed with a lubricating film containing an organic resin and wax, wherein the organic resin is at least one of acrylic resin, epoxy resin, urethane resin, phenol resin, vinyl acetate resin, and polyester resin, the wax is a polyolefin wax having a melting point of 120°C or higher and 140°C or lower and an average particle size of 3.0 μm or less, the wax ratio in the film is 10 mass% or higher, and the coating has a coating weight W (g / m 2 The relationship between the surface roughness R (μm) and the arithmetic mean roughness Ra (μm) of the metallic zinc layer surface is within the range of the following formula (1). W≧0.12×Ra 2 +0.1···(1) [2] The coating weight W (g / m 2The lubricating coating-coated zinc-based plated steel sheet according to [1], characterized in that the relationship between the surface roughness R (μm) and the arithmetic mean roughness Ra (μm) of the metallic zinc layer surface is within the range of the following formula (2). W≧0.25×Ra 2 +0.1···(2) [3] The coating weight W per side is 2.0 g / m 2 A lubricating coating-coated zinc-based plated steel sheet according to the following [1] or [2]. [4] The lubricating coating-coated zinc-based plated steel sheet according to any one of [1] to [3], wherein the arithmetic mean roughness Ra of the surface of the metallic zinc layer is 0.4 μm or more and 2.5 μm or less. [5] The lubricating coating-coated zinc-based plated steel sheet according to any one of [1] to [4], wherein the wax has an average particle size of 0.01 μm or more and 0.5 μm or less. [6] A zinc-based plated steel sheet having a lubricating coating according to any one of [1] to [5], wherein the proportion of wax in the coating is less than 50 mass %. [7] The coating weight W per side is 0.9 g / m 2 The lubricating coating-coated zinc-based plated steel sheet according to any one of [3] to [6] below. [8] The lubricating coating-coated zinc-based plated steel sheet according to any one of [1] to [7], wherein the organic resin is an alkali-soluble resin. [9] A method for producing a lubricating film-coated zinc-based plated steel sheet according to any one of [1] to [8], comprising applying a coating material containing the organic resin and wax according to any one of [1] to [8] to at least one side of a zinc-based plated steel sheet, and drying the coating material.

[10] The method for producing a lubricating coating-coated zinc-based plated steel sheet according to [9], wherein the maximum temperature of the steel sheet during drying is 60°C or higher and 140°C or lower.

[11] The method for producing a lubricating coating-coated zinc-based plated steel sheet according to [9] or

[10] , wherein the ratio of the organic resin and wax in the paint is 1 mass% or more and 25 mass% or less.

[12] When the formula (1) is not satisfied, the coating amount W (g / m 2) or the arithmetic mean roughness Ra (μm) of the steel sheet.

[0015] In the present invention, the metallic zinc layer is a plating layer formed on a cold-rolled steel sheet or a hot-rolled steel sheet. Effect of the Invention

[0016] According to the present invention, the friction coefficient between the lubricating coating-coated zinc-based plated steel sheet and a die or the like is significantly reduced, and a zinc-based plated steel sheet excellent in press formability can be obtained. Therefore, when the present invention is applied to a zinc-based plated steel sheet having a relatively low strength that is subjected to complex forming, the sheet has stable and excellent press formability.

[0017] Furthermore, even in high-strength galvanized steel sheets in which the surface pressure during press forming increases, a lubricating film-coated galvanized steel sheet can be obtained which has low sliding resistance in areas at risk of cracking during press forming and excellent press formability in areas where the surface pressure is high and die galling is likely to occur.

[0018] In the above, high strength is assumed to have a tensile strength (TS) of 440 MPa or more, and relatively low strength is assumed to have a TS of less than 440 MPa. [Brief description of the drawings]

[0019] [Figure 1] FIG. 2 is a schematic front view showing a friction coefficient measuring device. [Diagram 2] FIG. 2 is a schematic perspective view showing the shape and dimensions of the bead in FIG. [Diagram 3] FIG. 2 is a diagram showing the friction coefficients of various lubricating-film-coated zinc-based plated steel sheet surface roughnesses and various coating thicknesses for the paint that is the subject of the present invention. [Figure 4] FIG. 2 is a graph showing the friction coefficient at various lubricating film-coated zinc-based plated steel sheet surface roughnesses and various coating thicknesses for paints not covered by the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment of the present invention will be described.

[0021] The present invention provides a coating of 5 g / m2 as a lower layer on at least one side. 2 The lubricating film-coated zinc-based plated steel sheet has the above-mentioned metallic zinc layer and an upper layer formed with a film containing an organic resin and wax, and is characterized in that the wax is a polyolefin wax having a melting point of 120°C to 140°C and an average particle size of 3.0 μm or less, and the wax ratio in the film is 10 mass% or more. Furthermore, the coating weight W (g / m 2 The relationship between the surface roughness (R) and the arithmetic mean roughness Ra (μm) of the steel sheet is within the range of the following formula (1). W≧0.12×Ra 2 +0.1···(1) The method for producing the metallic zinc layer in the present invention is not limited, and it can be formed by applying a zinc-based plating to a steel sheet by 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 zinc plating. Furthermore, even if a zinc-based plated steel sheet containing metals other than zinc, such as zinc-aluminum alloy plating, zinc-aluminum-magnesium alloy plating, or zinc-nickel alloy plating, is used as the metallic zinc layer, the coating weight of the metallic zinc can be as low as 5 g / m. 2 It is sufficient if the thickness is 5 g / m or more. The presence of the metallic zinc layer as the lower layer of the organic lubricating film promotes adhesion between the die and the wax component in the organic lubricating film during sliding, and the die is more likely to be covered with the wax component, resulting in a superior lubrication effect compared to when there is no metallic zinc layer. In addition, since the metallic zinc layer is softer than a steel sheet, the deformation resistance during sliding is small and the friction coefficient is likely to be low. 2 If the thickness is less than this, the lubricating effect of the zinc layer may not be sufficient, and corrosion resistance may be poor.

[0022] The wax used in the present invention is not limited as long as it is a polyolefin wax having a melting point of 120° C. or higher and 140° C. or lower and an average particle size of 3.0 μm or less.

[0023] The reason why polyolefin wax is used as the wax is that it has low surface energy and self-lubricating properties, and therefore provides good lubrication. In addition, it is relatively easy to adjust the melting point of polyolefin to 120°C or higher and 140°C or lower by controlling the density and molecular weight.

[0024] When the melting point is 120°C or more and 140°C or less, in addition to the self-lubricating properties of the polyolefin wax itself, the wax becomes semi-molten due to sliding during press molding, and the lubricating film components mixed with the organic resin can cover the surface of the mold, and excellent lubrication effects can be obtained by suppressing direct contact between the mold and the steel plate. When the melting point is less than 120°C, the wax completely melts due to frictional heat caused by sliding during press molding, and the wax itself does not provide sufficient lubrication effect, and the above-mentioned mold coating effect cannot be obtained. Also, when the melting point exceeds 140°C, it does not melt during sliding, and sufficient lubrication effect cannot be obtained, and the mold coating effect cannot be obtained.

[0025] When the melting point of the wax is 120°C or more and 140°C or less, the wax in the film efficiently adheres to the die in the sliding state during press molding, and is less likely to fall off, which is thought to provide a high lubrication effect. When the melting point is less than 120°C, even if the film adheres to the die, the adhesion is weak, and the film is likely to fall off when slid. When the melting point is more than 140°C, the film is less likely to adhere to the die. Furthermore, it is preferable that the melting point is 125°C or more and 135°C or less.

[0026] The melting point of the wax herein refers to the melting temperature measured based on JIS K 7121:1987 "Method of measuring transition temperature of plastics".

[0027] If the average particle size of the wax exceeds 3.0 μm, it becomes difficult to mix with the organic resin during sliding, and the above-mentioned coating effect of the mold cannot be obtained, and sufficient lubrication cannot be obtained. The average particle size of the wax is preferably 1.5 μm or less, more preferably 0.5 μm or less, and even more preferably 0.3 μm or less.

[0028] The average particle size of the wax is preferably 0.01 μm or more. If it is less than 0.01 μm, it will be easily dissolved in the lubricating oil during sliding, and may not be able to fully improve the lubrication, and it will be easily aggregated in the paint for forming the film, resulting in low paint stability. More preferably, it is 0.03 μm or more.

[0029] 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, it can be determined by measuring a sample diluted with pure water using a laser diffraction / scattering type particle size distribution measuring device Partica LA-960V2 (manufactured by Horiba, Ltd.).

[0030] Among the polyolefin waxes, the use of polyethylene wax is preferred because the highest lubricating effect is obtained when the polyethylene wax is used.

[0031] The mass percentage of wax in the film is 10% by mass or more. If it is less than 10% by mass, sufficient lubrication effect cannot be obtained. If the mass percentage of wax in the film is 15% by mass or more, a particularly good lubrication effect can be obtained. In addition, the mass percentage of wax in the film is preferably less than 50% by mass. If it is 50% by mass or more, the wax is likely to fall off due to a shortage of the base resin component, and the adhesion to the zinc-plated steel sheet may be poor, and the film may not exist stably. In addition, when used as a steel sheet for automobiles, sufficient degreasing may not be obtained in the alkaline degreasing step in the painting process, and even if an alkali-soluble organic resin is used, the film may not be sufficiently removed in the alkaline degreasing step, and the film may remain, causing deterioration of paintability. More preferably, it is 30% by mass or less.

[0032] Here, the mass proportion of the wax in the coating refers to the ratio of the mass of the solid content of the wax in the coating to the total mass of the solid content of the resin in the coating and the solid content of the wax in the coating.

[0033] As a specific measurement method, a test piece with a known amount of resin and wax attached to a zinc-based plated steel sheet is prepared, an infrared absorption spectrum is measured using an FT-IR measurement device, and a calibration curve for the amount of resin and wax attached is created from the peak intensity attributable to the resin and wax. Next, the infrared absorption spectrum of the zinc-based plated steel sheet to be measured, which is the subject of the measurement, is measured, and the amount of resin and wax attached is calculated from the calibration curve, thereby making it possible to determine the mass ratio of wax in the coating.

[0034] In the present invention, the organic resin plays the role of a binder that holds the wax on the surface of the zinc-based plated steel sheet. The sliding effect caused by the mold coating with the mixture of wax and organic resin formed during sliding as described above is not exhibited with inorganic binders because of their low affinity with polyolefin. As the organic resin, acrylic resins, epoxy resins, urethane resins, phenolic resins, vinyl acetate resins, and polyester resins can be used.

[0035] The acrylic resin used in the present invention is a polymer or copolymer of one or more selected from unsaturated monocarboxylic acids having one carboxy group in the molecule, such as acrylic acid or methacrylic acid, esters of the above unsaturated monocarboxylic acids, and styrene, or a derivative thereof, such as a sodium salt, potassium salt, ammonium salt, or amine salt thereof.

[0036] Acrylic resins in which the monomer is a fatty acid with two or more carboxy groups in the molecule may have poor paint stability, so in the present invention, an acrylic resin in which the monomer is a fatty acid or its fatty acid ester having one carboxy group in the molecule is used.

[0037] The epoxy resin is not particularly limited, but examples thereof include bisphenol A type epoxy resin, bisphenol F type epoxy resin, novolac type epoxy resin, and the like.

[0038] The urethane resin is not particularly limited, but preferably has a carboxy group in the molecule.

[0039] The phenolic resin is not particularly limited, but a resol-based phenolic resin that can be dissolved or dispersed in an aqueous solvent is preferred.

[0040] The vinyl acetate resin is not particularly limited, but it is preferable to use polyvinyl acetate.

[0041] The polyester resin is not particularly limited, but a polyester resin containing a monomer having a carboxy group as a constituent is preferred.

[0042] These resins can also be used in combination of two or more kinds. Furthermore, by using an alkali-soluble organic resin, it becomes possible to remove the film by alkaline degreasing in the painting process, and the subsequent paintability becomes good.

[0043] In the present invention, a surface conditioner, a defoamer, a dispersant, etc. may be contained as components other than the organic resin and the wax. Also, a rust inhibitor for improving rust prevention may be added.

[0044] In the present invention, when the above-mentioned coating is formed on the surface of a zinc-based plated steel sheet, a wide range of coating weights are formed on zinc-based plated steel sheets with a wide range of surface roughnesses, and press formability is evaluated. It has been found that stable and good press formability is achieved only in a region that satisfies a limited relationship between the surface roughness of the zinc-based plated steel sheet and the coating weight. In general, the greater the surface roughness of a zinc-based plated steel sheet, the thinner the lubricating coating is at the protruding parts of the steel sheet, and when press-formed, the coating is scraped off by sliding with the die, and the underlying zinc-based plated steel sheet is easily exposed, making it difficult to obtain a lubricating effect. However, by using the technology of the present invention, adhesion of the lubricating coating components to the die during sliding is promoted, and even if the roughness of the zinc-based plated steel sheet is large, the die side is protected, and lubrication is not impaired.

[0045] The range of coating weight that exhibits good press formability is as follows:

[0046] Coating weight per side W (g / m2 The relationship between the surface roughness W and the arithmetic mean roughness Ra (μm) of zinc-based coated steel sheet is given by the formula (1): W ≧ 0.12 × Ra 2 Good press formability can be obtained within the range of +0.1. W<0.12×Ra 2 In the case of +0.1, the amount of coating is insufficient, the protective effect on the die side is insufficient, and good press formability cannot be obtained.

[0047] More preferably, the formula (2): W≧0.25×Ra 2 The range should be +0.1.

[0048] The film components that mainly contribute to the sliding properties are those present on the protruding parts of the zinc-based plated steel sheet that come into contact with the die during press working. The area of ​​the protruding parts of the steel sheet that come into contact with the die is Ra 2 It is considered that the tendency is to decrease in proportion to Ra. 2 It is believed that by increasing the coating weight in proportion to the amount of coating components that contribute to the sliding properties, a sufficient amount of coating components can be ensured.

[0049] The amount of adhesion W is 2.0g / m 2 It is preferable that the thickness is 2.0 g / m or less. 2 If it exceeds this value, the coating may be difficult to remove and may be poorly welded. 2 It is particularly preferable that the coating weight W is 0.9 g / m or less. 2 If it is equal to or less than this, the film removal property is particularly good.

[0050] The coating weight can be determined by a method in which the difference in weight of the zinc-based plated steel sheet before and after coating is divided by the area, or by a method in which the coating of the zinc-based plated steel sheet after coating is completely removed with an alkaline aqueous solution or an organic solvent, and then the difference in weight of the zinc-based plated steel sheet before and after coating removal is divided by the area.

[0051] The arithmetic mean roughness Ra of the zinc-based plated steel sheet is preferably 0.4 μm or more and 2.5 μm or less.

[0052] If it is smaller than 0.4 μm, fine scratches that may occur during press molding may be easily noticeable, and galling may occur during press molding. If it exceeds 2.5 μm, the required coating weight may become large, increasing manufacturing costs and deteriorating sharpness after painting.

[0053] The arithmetic mean roughness Ra (μm) of zinc-based coated steel sheets can be measured in accordance with JIS B 0633:2001 (ISO 4288:1996). For example, when Ra is greater than 0.1 and less than or equal to 2, the cutoff value and reference length are set to 0.8 mm, the evaluation length is set to 4 mm, and the roughness is determined from the measured roughness curve. When Ra is greater than 2 and less than or equal to 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 determined from the measured roughness curve.

[0054] Next, a method for forming a coating containing an organic resin and wax on the metallic zinc layer of a zinc-based plated steel sheet will be described.

[0055] An applicable method for forming an organic resin coating containing polyolefin wax with a melting point of 120°C to 140°C and an average particle size of 3.0 μm or less on the upper layer of the metallic zinc layer of a zinc-based plated steel sheet is to apply a paint made by adding wax to an organic resin solution or emulsion in which an organic resin is dissolved or dispersed in a solvent, to the surface of the steel sheet, and then drying the paint. Here, the paint is adjusted so that C is 10 mass% or more when the mass of the organic resin as solid content (MA) and the mass of the wax as solid content (MB) are C={MB / (MA+MB)}×100.

[0056] The mass ratio of the coating components (organic resin and polyolefin wax) in the coating material is preferably 1 mass % or more and 25 mass % or less.

[0057] The solvent for the paint is water or an organic solvent.

[0058] If the mass ratio of the coating component in the paint is less than 1 mass% or more than 25 mass%, unevenness may occur during application. The application method is not particularly limited, but examples include a method using a roll coater or a bar coater, and a method of application by spraying, immersion, or brushing. The method of drying the zinc-based plated steel sheet after application can be performed by a general method. For example, drying by hot air, drying by an IH heater, or a method by infrared heating may be included. The maximum temperature reached by the zinc-based plated steel sheet during drying is preferably 60°C or higher and 140°C or lower. If the temperature is lower than 60°C, it will take a long time to dry and the rust prevention properties may be poor. If the temperature exceeds 140°C, the wax may melt and combine, and the particle size may become coarse, resulting in deterioration of lubricity.

[0059] In the method for producing a zinc-based coated steel sheet according to the present invention, the coating weight W (g / m 2 The relationship between the surface roughness W and the arithmetic mean roughness Ra (μm) of zinc-based coated steel sheet is given by the formula (1): W ≧ 0.12 × Ra 2 If +0.1 is ultimately satisfied, a lubricating film-coated zinc-based plated steel sheet having good press formability can be manufactured.

[0060] In another embodiment of the manufacturing method, when the formula (1) is not satisfied, the coating amount W (g / m) is changed so as to satisfy the formula (1). 2 ) or the arithmetic mean roughness Ra (μm) of the zinc-based plated steel sheet. This makes it possible to more reliably produce a zinc-based plated steel sheet having good press formability.

[0061] As an example of carrying out this step during operation, the coating weight W (g / m) per side of the coating is adjusted so as to satisfy the above formula (1) according to the target or measured value of the arithmetic mean roughness Ra (μm) of the zinc-based coated steel sheet. 2) and in the coating step, the coating amount is adjusted so as to obtain the determined coating amount W. More specifically, the arithmetic mean roughness Ra (μm) of the zinc-based plated steel sheet is substituted into the formula (1), and the coating amount W (g / m) per one side of the coating is determined so as to satisfy the formula (1) substituted therein. 2 ) is determined. Here, "substituting the value of the arithmetic mean roughness Ra (μm) of the zinc-based plated steel sheet into the formula (1)" is not limited to a case where the value is strictly substituted into the same formula as the formula (1), but also includes a case where the value is substituted into an inequality in a narrower range that always satisfies the formula (1). By carrying out such management, even when the arithmetic mean roughness Ra (μm) of the zinc-based plated steel sheet changes significantly due to, for example, a change in the zinc-based plated steel sheet that is continuously passed through, or wear of the rolling rolls, and thus the formula is not satisfied (when the formula (1) is actually no longer satisfied, or when a situation occurs in which the formula (1) is no longer satisfied), the formula (1) can be managed to be satisfied.

[0062] The surface roughness of the zinc-based plated steel sheet coated with a lubricating film may be measured before the lubricating film is applied, or after the lubricating film has been completely removed with an alkaline aqueous solution or an organic solvent.

[0063] When adjusting the arithmetic mean roughness Ra (μm) of the zinc-based plated steel sheet so as to satisfy the above formula (1), a known method can be applied as appropriate. When the base steel sheet for zinc-based plating is a hot-rolled steel sheet, the surface roughness of the steel sheet after removing the scale by pickling can be adjusted by adjusting the scale thickness during hot rolling by the amount of Si added in the steel sheet. In addition, the surface roughness of the steel sheet can be adjusted by adjusting the intensity of descaling (a process of crushing and removing the scale by water pressure) during hot rolling. When the base steel sheet for zinc-based plating is a cold-rolled steel sheet, the surface roughness of the steel sheet can be adjusted by changing the roll load or roll surface roughness during temper rolling. In addition, the surface roughness of the zinc-based plating layer can be adjusted by changing the roll load or roll surface roughness during temper rolling of the steel sheet after zinc-based plating.

[0064] The above describes an example of an embodiment in which the formula (1) is satisfied during operation. However, the coating weight W (g / m 2 ) and the arithmetic mean roughness Ra (μm) of the zinc-based coated steel sheet satisfy the above formula. If they do not satisfy the formula, the coating weight W (g / m 2 The manufacturing condition determination method may be implemented as a method for determining manufacturing conditions for a zinc-based plated steel sheet, in which either one of the surface roughness (surface roughness) of the surface roughness (surface roughness) and the arithmetic mean roughness Ra (μm) of the zinc-based plated steel sheet is changed in advance. Such a manufacturing condition determination method may be implemented as a part of a manufacturing method for a lubricating coating-coated zinc-based plated steel sheet, or may be implemented as an independent step. EXAMPLES

[0065] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

[0066] Using galvannealed steel sheets (GA), electrogalvanized steel sheets (EG), and hot-dip galvanized steel sheets (GI) with a thickness of 0.8 mm and the arithmetic mean roughness Ra and metal zinc coating amount shown in Table 1, A lubricated steel sheet was obtained by applying a coating material having the composition shown in Table 2 with a bar coater and drying with an IH heater so that the maximum temperature of the steel sheet reached was 80° C. All of the steel sheets A to G have a tensile strength of 270 MPa.

[0067] (1) Method for evaluating press formability (sliding characteristics) In order to evaluate the press formability, the friction coefficient of each test material was measured as follows.

[0068] FIG. 1 is a schematic front view showing a friction coefficient measuring device. As shown in the figure, a friction coefficient measuring sample 1 taken from a test material is fixed to a sample stage 2, and the sample stage 2 is fixed to the upper surface of a horizontally movable slide table 3. A slide table support base 5 that can move up and down and has a roller 4 in contact with the slide table 3 is provided on the lower surface of the slide table 3, and a first load cell 7 is attached to the slide table support base 5 to measure the pressing load N applied by a bead 6 to the friction coefficient measuring sample 1 by pushing up the slide table 3. A second load cell 8 is attached to one end of the slide table 3 to measure the sliding resistance force F for moving the slide table 3 in the horizontal direction while the pressing force is applied. The test was performed by applying press cleaning oil Pleton R352L manufactured by Sugimura Chemical Industry Co., Ltd. as a lubricant to the surface of the sample 1.

[0069] Figure 2 is a schematic perspective view showing the shape and dimensions of the bead used. The bead 6 slides while its underside is 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 parts 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.

[0070] The friction coefficient measurement test was performed 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 by the formula: μ = F / N.

[0071] A friction coefficient of 0.125 or less was evaluated as being particularly excellent in terms of sliding properties and rated as ⊚, a friction coefficient of more than 0.125 but not exceeding 0.150 was evaluated as being good in terms of sliding properties and rated as x when it exceeded 0.150.

[0072] (2) Evaluation method for film removal Assuming that the lubricating film-coated zinc-plated steel sheet according to the present invention is used in automobile applications, the film-removability during degreasing was evaluated. In order to determine the film-removability of the steel sheet, first, each test piece was degreased with an alkaline degreaser Fine Cleaner E6403 (manufactured by Nihon Parkerizing Co., Ltd.). In this treatment, the test piece was immersed in a degreasing solution with a degreaser concentration of 20 g / L and a temperature of 40° C. for a predetermined period of time, and then washed with tap water. The surface carbon intensity of the test piece after this treatment was measured using an X-ray fluorescence analyzer, and the film peeling rate was calculated according to the following formula using this measured value, the previously measured surface carbon intensity before degreasing, and the measured value of the surface carbon intensity of the untreated steel sheet.

[0073] 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 lubricating film-coated zinc-based plated steel sheet was evaluated according to the following criteria based on the immersion time in the alkaline degreasing solution at which the film peeling rate was 98% or more. The following cases were judged to be good removability: ◎ and ◯. ◎ (particularly good): Within 30 seconds ○ (Good): More than 30 seconds, less than 60 seconds △ (Insufficient): More than 60 seconds but less than 120 seconds × (bad): Over 120 seconds

[0074] [Table 1]

[0075] [Table 2]

[0076] [Table 3]

[0077] [Table 4]

[0078] The results are shown in Tables 3 and 4. In Tables 3 and 4, the judgments based on formula (1) and formula (2) were marked with ◯ when formula (1) and formula (2) were satisfied, and × when they were not satisfied.

[0079] According to Tables 3 and 4, the lubricating coating-coated zinc-based plated steel sheets of the examples of the present invention all have excellent press formability, whereas the zinc-based plated steel sheets of the comparative examples not having the technical features of the present invention all have poor press formability.

[0080] Furthermore, in the lubricating film-coated zinc-based plated steel sheet of the present invention, the coating weight W (g / m 2 ) is W ≧ 0.25 × Ra 2 It is clear that when the melting point of the wax in the lubricating film is 125°C or higher and 135°C or lower, and the average particle size of the wax in the lubricating film is 0.01 μm or higher and 0.5 μm or lower, the press formability of the lubricating film-coated zinc-based plated steel sheet is particularly good.

[0081] As shown in FIG. 4, the zinc-based plated steel sheets coated with a paint not targeted by the present invention tended to exhibit a higher coefficient of friction as the surface roughness of the zinc-based plated steel sheets increased.

[0082] On the other hand, as shown in FIG. 3, a lubricating coating-coated zinc-based plated steel sheet that is coated with the paint of the present invention and satisfies formula (1) exhibits a stable low friction coefficient even when the surface roughness of the zinc-based plated steel sheet changes, and therefore exhibits stable and good press formability even when the surface roughness of the zinc-based plated steel sheet changes during production.

[0083] The removability of the lubricating film-coated zinc-based plated steel sheets of the present invention was excellent when the mass ratio of wax in the film was 50% or more and / or the amount of adhesion W of the lubricating film per side was 0.9 g / m 2 Except for cases where it was extremely poor, the rating was ◎ (particularly good). [Industrial Applicability]

[0084] The lubricating film-coated zinc-based plated steel sheet of the present invention has excellent press formability and can be used in a wide range of fields, primarily in automobile body applications. [Explanation of symbols]

[0085] 1. Sample for measuring friction coefficient 2 Sample stage 3 Slide table 4. Lola 5 Slide table support 6 Beads 7 First load cell 8 Second Load Cell 9. Rail

Claims

1. At least one surface is coated with a lower layer with a coating weight of 5 g / m ２ A lubricating film-coated zinc-based plated steel sheet having the above metallic zinc layer and an upper layer formed with a film containing an organic resin and wax, wherein the organic resin is at least one of acrylic resin, epoxy resin, urethane resin, phenol resin, vinyl acetate resin, and polyester resin, the wax is a polyolefin wax having a melting point of 120°C or higher and 140°C or lower and an average particle size of 3.0 μm or less, the proportion of wax in the film is 10 mass% or higher, and the coating weight W (g / m) per one side of the film is 100 g / m. ２ and the arithmetic mean roughness Ra (μm) of the surface of the metallic zinc layer, the relationship of which is within the range of the following formula (1): W ≧0.12×Ra ２ +0.1・・・(1)

2. The coating weight W (g / m ２ 2. The lubricating coating-coated zinc-based plated steel sheet according to claim 1, characterized in that the relationship between the surface roughness R (μm) of the metallic zinc layer and the surface roughness R (μm) of the metallic zinc layer is within the range of the following formula (2). W≧0.25×Ra ２ +0.1・・・(2)

3. The coating weight W per side is 2.0 g / m ２ The lubricating coating-coated zinc-based plated steel sheet according to claim 1 or 2, wherein:

4. The lubricating coating-coated zinc-based plated steel sheet according to any one of claims 1 to 3, wherein the arithmetic mean roughness Ra of the surface of the metallic zinc layer is 0.4 µm or more and 2.5 µm or less.

5. The lubricating coating-coated zinc-based plated steel sheet according to any one of claims 1 to 4, wherein the wax has an average particle size of 0.01 µm or more and 0.5 µm or less.

6. The zinc-based plated steel sheet having a lubricating coating according to any one of claims 1 to 5, wherein the proportion of wax in the coating is less than 50 mass %.

7. The coating weight W per side is 0.9 g / m ２ The lubricating coating-coated zinc-based plated steel sheet according to any one of claims 3 to 6, wherein:

8. The lubricating coating-coated zinc-based plated steel sheet according to any one of claims 1 to 7, wherein the organic resin is an alkali-soluble resin.

9. A method for producing a lubricating film-coated zinc-based plated steel sheet according to any one of claims 1 to 8, comprising applying a coating material containing the organic resin and wax according to any one of claims 1 to 8 to at least one side of a zinc-based plated steel sheet and drying the coating material.

10. The method for producing a lubricating coating-coated zinc-based plated steel sheet according to claim 9, wherein the maximum temperature of the steel sheet during the drying is 60°C or higher and 140°C or lower.

11. The method for producing a lubricating coating-coated zinc-based plated steel sheet according to claim 9 or 10, wherein the ratio of the organic resin and the wax in the paint is from 1 mass % to 25 mass %.

12. The adhesion amount W (g / m) is set to satisfy the above formula (1). ２ 12. The method for producing a lubricating coating-coated zinc-based plated steel sheet according to claim 9, further comprising a step of changing at least one of the surface roughness, surface roughness ratio, surface roughness ...

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