Painted steel material and method for manufacturing painted steel material

Abstract

To provide painted steel materials with high paint durability and a method for manufacturing painted steel materials. [Solution] The painted steel material according to the present invention comprises a primer layer formed on the surface of a chemical conversion treatment film formed on a zinc-plated steel material, and a topcoat layer formed on the surface of the primer layer, wherein the primer layer contains a weather-resistant resin and has a thickness of 20 μm to 150 μm, and the topcoat layer contains a weather-resistant resin and has a thickness of 20 μm to 150 μm.

Description

Technical Field

[0001] The present invention relates to a coated steel material and a method for manufacturing the coated steel material, and particularly to a coated steel material having excellent durability and a method for manufacturing the coated steel material.

Background Art

[0002] Conventionally, as weather-resistant steel materials, materials obtained by applying a coating to zinc-based plated steel materials have been widely used. These coated steel materials are used outdoors and require weather resistance, for example, for use in guardrails, fences, or building materials. For example, in Patent Document 1, a coated steel material is disclosed in which a hot-dip galvanized steel material is used as a base, a chemical conversion film is formed thereon, a primer layer is formed thereon, and a coating film is provided thereon. In the evaluation of the corrosion resistance of conventional coated steel materials, the time point when the surface coating film peels off is often used as the service life.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the coated steel material disclosed in Patent Document 1, the epoxy resin used in the primer layer has excellent adhesion properties with steel materials and the like, and thus is widely used as a primer paint. On the other hand, since the epoxy resin has low durability against ultraviolet rays, it is necessary to perform a topcoat when used outdoors. In the investigation by the present inventors, it was found that in an outdoor environment, delamination occurs between the topcoat film and the epoxy primer. Furthermore, when the epoxy primer is exposed due to delamination, it deteriorates due to ultraviolet rays, causing chalking or peeling of the coating film, and a problem occurs in that the corrosion resistance of the coated steel material is impaired.

[0005] The present invention aims to solve the above problems and to provide a painted steel material and a method for manufacturing a painted steel material that have high paint durability in outdoor environments with ultraviolet rays and salt damage environments containing a large amount of salt. [Means for solving the problem]

[0006] The painted steel material according to the present invention comprises a primer layer formed on the surface of a chemical conversion treatment film formed on a zinc-plated steel material, and a topcoat layer formed on the surface of the primer layer, wherein the primer layer contains a weather-resistant resin and has a thickness of 20 μm to 150 μm, and the topcoat layer contains a weather-resistant resin and has a thickness of 20 μm to 150 μm.

[0007] Furthermore, the method for manufacturing painted steel according to the present invention comprises a first step of forming an undercoat layer on the surface of a chemical conversion treatment film formed on a zinc-plated steel material, and a second step of forming a topcoat layer on the surface of the undercoat layer, wherein the undercoat layer contains a weather-resistant resin and has a thickness of 20 μm to 150 μm, and the topcoat layer contains a weather-resistant resin and has a thickness of 20 μm to 150 μm. [Effects of the Invention]

[0008] According to the present invention, by preventing deterioration of the undercoat film due to ultraviolet rays, delamination between the undercoat and topcoat layers is prevented, improving paint adhesion and corrosion resistance, and providing durable painted steel materials and a method for manufacturing painted steel materials in outdoor environments under ultraviolet rays or salt damage. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic diagram showing the cross-sectional structure of the painted steel material according to the embodiment. [Figure 2] This table summarizes the specifications and evaluation results of the painted steel material according to the embodiment. [Figure 3] This is a schematic diagram showing the cross-sectional structure of the painted steel material in the comparative example. [Figure 4]This is another schematic diagram showing the cross-sectional structure of the painted steel material in the comparative example. [Figure 5] This table summarizes the specifications and evaluation of the painted steel materials used in the comparative example. [Figure 6] This is a table showing the zinc plating layer used in evaluation samples of painted steel materials according to the embodiment and painted steel materials according to the comparative example. [Figure 7] This table shows the specifications of the paints used in the evaluation samples of painted steel materials according to the embodiment and the evaluation samples of painted steel materials according to the comparative example. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described below with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and their descriptions are omitted or simplified as appropriate. Furthermore, the shape, size, and arrangement of the configurations shown in each drawing can be modified as appropriate within the scope of the present invention. In addition, in this embodiment, terms indicating direction are used as appropriate to facilitate understanding, but these notations are for the convenience of explanation and do not limit the arrangement, direction, and orientation of the devices, instruments, or parts.

[0011] Embodiment. Figure 1 is a schematic diagram showing the cross-sectional structure of a painted steel material 100 according to an embodiment of the present invention. The painted steel material 100 is used, for example, in protective fences such as guardrails or guard pipes, or other building materials, which are used outdoors for long periods of time.

[0012] Prior to the invention of the painted steel material 100 according to the embodiment, the inventors investigated how to improve delamination between the topcoat and the epoxy primer. For example, to improve physical adhesion, they investigated by applying a primer to a zinc-plated steel material, roughening the surface of the coating, and then applying the topcoat. They also investigated how to improve chemical adhesion by examining baking conditions such as baking temperature and baking time. However, in neither investigation did they find any improvement in delamination between the topcoat and the epoxy primer. From the results of the investigation, they were able to find that ultraviolet rays that penetrate the topcoat reach the epoxy primer, degrade the epoxy primer resin, and thus deteriorate the adhesion between the coating and the topcoat, resulting in delamination between the topcoat and the epoxy primer. In other words, they concluded that the deterioration of adhesion between the topcoat and the epoxy primer is different from the physical or chemical adhesion factors that are usually considered.

[0013] Based on the above considerations, the painted steel material 100 according to the embodiment obtained has long-term durability, especially in environments exposed to ultraviolet rays or salt damage containing a large amount of salt.

[0014] As shown in Figure 1, the painted steel material 100 has a base coat layer 30 formed on a zinc-plated steel material composed of a steel material 10 and a zinc plating layer 11, and a top coat layer 40 formed on the surface of the base coat layer 30. The base coat layer 30 is formed on the surface of the chemical conversion treatment film 20 formed on the zinc-plated steel material.

[0015] (steel material 10) Zinc-plated steel is composed of a steel material 10 and a zinc plating layer 11. Zinc-plated steel is formed into various shapes such as plates or tubes, and its surface may be treated. By having a zinc plating layer 11, zinc-plated steel has higher corrosion resistance in the atmosphere than ordinary steel. The zinc plating layer 11 is formed, for example, by hot-dip galvanizing. The hot-dip galvanizing layer is, in particular, a hot-dip galvanizing bath as specified in JIS H 8641, consisting of Zn: 97.5% by weight or more, with the remainder being unavoidable impurities; a hot-dip zinc-aluminum alloy plating bath as specified in JIS H 8643, consisting of Al: 4.0-10.0% by weight, Mg: 3.0% by weight or less, iron, lead, cadmium and intentionally added elements: 0.3% by weight or less, with the remainder being Zn and unavoidable impurities; a hot-dip galvanizing bath for hot-dip galvanized steel materials and steel strips as specified in JIS G 3302, consisting of intentionally added elements other than Zn: 1.0% by weight or less, with the remainder being Zn and unavoidable impurities; a hot-dip zinc-5% aluminum alloy plating bath as specified in JIS G 3317, consisting of Al: 4.0-5.5% by weight, intentionally added elements other than Al and Zn: 1.0% by weight or less, with the remainder being Zn and unavoidable impurities; or JIS G The plating bath for hot-dip zinc-aluminum-magnesium alloy plated steel materials and steel strips, as specified in 3323, consists of Al: 5.0-13.0 wt%, Mg: 2.0-4.0 wt%, intentionally added elements other than Al, Mg, and Zn: 1.0 wt% or less, with the remainder being Zn and unavoidable impurities, and improves the corrosion resistance and appearance of the plated steel material 10. Alternatively, a plating bath for highly corrosion-resistant plated steel sheets consisting of Al: 15-23 wt%, Mg: 5-7 wt%, and Si: trace amounts may also be used.

[0016] (Chemical conversion coating 20) On the zinc plating layer 11 of the zinc-based plated steel material, a chemical conversion treatment such as zinc phosphate treatment is applied to the surface as a paint base treatment. The chemical conversion coating film 20 of the zinc plating layer 11 is formed by, for example, phosphate treatment in the chemical conversion treatment process. The chemical conversion coating film 20 forms a passivated metal layer on the surface of the zinc plating layer 11, and the passivated metal layer improves the rust prevention property. Further, an effect of suppressing the corrosion of the zinc plating layer 11 is obtained by the self-repair of the chemical conversion coating film 20. The chemical conversion coating film 20 is any one of a chromate coating film, a chromate-free coating film, or a phosphate coating film formed on the surface of the zinc plating layer 11.

[0017] (Undercoat layer 30) The undercoat layer 30 is directly applied to the surface of the chemical conversion coating film 20. The undercoat layer 30 contains a highly weather-resistant resin having durability against ultraviolet rays and takes the form of a powder paint or a liquid paint. Specifically, examples include resins such as acrylic, urethane, silicone, polyester, alkyd, and fluorine-based resins. Copolymers of these resins can also be used. Among them, polyester-based resins are preferably mentioned, and these can be used alone or in combination of two or more. When forming the undercoat layer 30 with a powder paint, the powder paint is adhered to the surface of the chemical conversion coating film 20 by static electricity and heated at a predetermined temperature for a predetermined time to be fixed on the surface of the chemical conversion coating film 20. In the case of a liquid paint, it is applied to the surface of the chemical conversion coating film 20 by spraying or the like and heated at a predetermined temperature for a predetermined time to be fixed on the surface of the chemical conversion coating film 20. The temperature and time for heating are appropriately set depending on the use of the paint. In the present invention, the step of forming the undercoat layer 30 is referred to as an undercoat painting step or a first step.

[0018] The undercoat layer 30 is composed of, for example, a paint containing a polyester resin. Examples of commercially available paints containing polyester resins include, for example, V-PET#4500 (trade name, manufactured by Dainippon Paint Co., Ltd.), Viryusha PL5600 (trade name, manufactured by Nippon Paint Industrial Coatings Co., Ltd.), and Porcelain 3000 (trade name, manufactured by Kawakami Paint Co., Ltd.). Since the polyester resin has light resistance compared to the epoxy resin, it suppresses deterioration due to sunlight.

[0019] (Topcoat layer 40) The topcoat layer 40 is applied to the surface of the undercoat layer 30 to form the appearance of the painted steel material 100. The topcoat layer 40 contains a highly weather-resistant resin having durability against ultraviolet rays and takes the form of a powder paint or a liquid paint. Specifically, resins such as acrylic, urethane, silicone, polyester, alkyd, and fluorine-based resins can be mentioned. Copolymers of these resins can also be used. Among them, polyester-based resins are preferably mentioned, and one of these can be used, or two or more can be used in combination. When forming the topcoat layer 40 with a powder paint, the powder paint is adhered to the surface of the undercoat layer 30 by static electricity and heated at a predetermined temperature for a predetermined time to be fixed on the surface of the undercoat layer 30. In the case of a liquid paint, it is applied to the surface of the undercoat layer 30 by spraying or the like and heated at a predetermined temperature for a predetermined time to be fixed on the surface of the undercoat layer 30. The temperature and time for heating are appropriately set depending on the use of the paint. In the present invention, the process of forming the topcoat layer 40 is called the topcoat painting process or the second process.

[0020] The topcoat layer 40 is composed of, for example, a paint containing a polyester resin. Examples of commercially available paints containing polyester resins include, for example, V-PET#4500 (trade name, manufactured by Dainippon Paint Co., Ltd.), Viryusha PL5600 (trade name, manufactured by Nippon Paint Industrial Coatings Co., Ltd.), and Porcelain 3000 (trade name, manufactured by Kawakami Paint Co., Ltd.). Since the polyester resin has light resistance compared to the epoxy resin, it suppresses deterioration due to sunlight, suppresses peeling of the paint film between the undercoat layer 30 and the topcoat layer 40, and improves the durability of the painted steel material 100.

[0021] Powder coating is performed by adhering powder coating using static electricity. However, due to electrostatic repulsion or differences in adhesion due to the shape of the steel material, the desired coating thickness may not be achieved. By applying both a primer coat and a topcoat, the influence of electrostatic repulsion or differences in adhesion due to the shape of the steel material on the coating thickness can be suppressed, improving the uniformity of the coating surface and allowing for the acquisition of a coated steel material 100 with a stable coating thickness. Furthermore, if the coating is completed in a single coat, coating defects such as pinholes or blemishes will remain on the coating surface. By applying two or more coats, coating defects that occurred in the primer coat are covered by the topcoat, effectively repairing the coating defects. In addition, by applying two or more coats, areas of thicker or thinner coatings will not occur, i.e., differences in coating thickness will not occur, thus preventing damage and corrosion from progressing from areas with thin coatings.

[0022] Furthermore, when applying two or more coats of paint, if the primer layer 30 and the topcoat layer 40 are from different paint manufacturers, even if they are both resin-based, the detailed resin components may differ, which can negatively impact paint adhesion. Therefore, using the same paint for both the primer and topcoat ensures stable paint adhesion.

[0023] Next, we will explain the evaluation results for the corrosion resistance and paint adhesion of painted steel material 100.

[0024] Figure 2 is a table summarizing the specifications and evaluation results of the painted steel material 100 according to the embodiment, showing the specifications of the zinc plating layer 11, the undercoat layer 30, and the topcoat layer 40 of each evaluation sample of the painted steel material 100 according to the embodiment. As shown in Figure 2, evaluation samples of the painted steel material 100 according to the embodiment were used, and evaluation results were obtained for outdoor exposure, salt spray, boiling water resistance, water resistance, and salt water immersion tests.

[0025] <Comparative Example> Figure 3 is a schematic diagram showing the cross-sectional structure of the painted steel material 101 according to the comparative example, showing the case in which the undercoat layer 30 is provided. Figure 4 is another schematic diagram showing the cross-sectional structure of the painted steel material 101 according to the comparative example, showing the case in which the undercoat layer 30 is not provided.

[0026] Figure 5 is a table summarizing the specifications and evaluation of the painted steel material 101 related to the comparative example, showing the specifications of the zinc plating layer 11, the undercoat layer 30, and the topcoat layer 40 of the evaluation sample of the painted steel material 101 related to the comparative example. As shown in Figure 5, evaluation results were obtained for outdoor exposure, salt spray, boiling water resistance, water resistance, and salt water immersion tests using the evaluation sample related to the comparative example.

[0027] Figure 6 is a table of the zinc plating layers 11 used in evaluation samples of painted steel material 100 according to the embodiment and painted steel material 101 according to the comparative example. A chemical conversion treatment film 20, which is either a chromate treatment film, a chromate-free treatment film, or a phosphate treatment film, is formed on the surface of the zinc plating layer 11. Figures 1 and 3 show the case where the zinc plating layer 11 is Z27 or ZAM.

[0028] Figure 7 is a table showing the specifications of the paints used in the evaluation sample of the painted steel material 100 according to the embodiment and the evaluation sample of the painted steel material 101 according to the comparative example. In Figure 7, A, B, and C are the paints that form the undercoat layer 30 of the painted steel material 100 according to the embodiment, the topcoat layer 40 of the painted steel material 100 according to the embodiment, and the topcoat layer 40 of the painted steel material 101 according to the comparative example. D in Figure 7 is the specification of the paint that forms the undercoat layer 30 of the painted steel material 101 according to the comparative example. In other words, in the comparative example, the undercoat layer 30 is formed with a paint containing epoxy.

[0029] The outlines of each test are described below, with reference to Figures 2 and 5.

[0030] (Outdoor exposure weathering test) Outdoor exposure weathering tests were conducted by taking test specimens from both the painted steel material 100 according to the embodiment and the painted steel material 101 according to the comparative example, and sealing the unpainted ends of the specimens to use as evaluation samples. Test specimens were prepared with and without cross-cuts on the painted portion. The test method was in accordance with JIS K 5600-7-6, and the specimens were exposed for a certain period, for example, 10 years, facing south (equator plane) at an angle of 30° to the horizontal plane in Okinawa.

[0031] The evaluation was performed using individual indicators for each test specimen, both with and without cross-cuts. For the test specimens with cross-cuts, the maximum peel width on one side from the cross-cut was measured by a tape peel test at the cross-cut area. The peel width measured was the peel width between the undercoat layer 30 and the chemical conversion coating 20, or the peel width between the topcoat layer 40 and the undercoat layer 30. The post-coating corrosion resistance of each evaluation sample was evaluated based on the measured peel width. In Figures 2 and 5, a maximum peel width of 3 mm or less is indicated by ◎, between 3 mm and 5 mm by ○, between 5 mm and 7 mm by △, and above 7 mm by ×.

[0032] Furthermore, for test specimens without cross-cutting, a tape peel test was performed on the entire painted surface, and each evaluation sample was measured based on the number of peeled areas on the painted surface. The number of peeled areas was the total number of peeled areas between the undercoat layer 30 and the chemical conversion coating 20, and between the topcoat layer 40 and the undercoat layer 30. In Figures 2 and 5, 0 peeled areas are indicated by ◎, 1 to 3 by ○, 4 to 10 by △, and more than 10 by ×.

[0033] The delamination location indicates which layer the paint film delamination originates from. Paint film delamination refers to delamination between the undercoat layer 30 and the chemical conversion treatment film 20, while interlayer delamination refers to delamination between the topcoat layer 40 and the undercoat layer 30.

[0034] (Salt spray test) Salt spray tests were conducted by taking test specimens from both the painted steel material 100 according to the embodiment and the painted steel material 101 according to the comparative example, and sealing the unpainted ends of the specimens to use as evaluation samples. Test specimens were prepared with and without cross-cuts on the painted portion. The test method followed JIS K 5600-7-1, in which a 5% NaCl aqueous solution was sprayed onto the painted surface of the test specimen in a constant temperature bath at 35°C. The test duration was 1000 hours.

[0035] The evaluation was performed on each test specimen, both with and without cross-cuts, using individual indicators. For the test specimens with cross-cuts, the maximum peel width on one side from the cross-cut was measured by a tape peel test at the cross-cut area. The peel width was measured as the width of the paint film peeling between the undercoat layer 30 and the chemical conversion treatment film 20. The post-painting corrosion resistance of each evaluation sample was evaluated based on the measured paint film peel width. In Figures 2 and 5, a maximum peel width of 3 mm or less is indicated by ◎, between 3 mm and 5 mm by ○, between 5 mm and 7 mm by △, and above 7 mm by ×.

[0036] For test specimens without cross-cutting, the surface was observed, and each evaluation sample was measured based on the number of blister areas on the painted surface. In Figures 2 and 5, 0 blister areas are indicated by ◎, 1-3 by ○, 4-10 by △, and more than 10 by ×.

[0037] (Boiling water resistance test) The boiling water resistance test was performed by taking test pieces from both the painted steel material 100 according to the embodiment and the painted steel material 101 according to the comparative example, sealing the unpainted ends, and using them as evaluation samples. The test method was in accordance with JIS K 5400-1990-8.20, in which the test pieces were immersed in boiling water for 1 hour. After the test, the evaluation was performed in accordance with the provisions of JIS K 5600-5-6, by conducting a cross-cut adhesion test and investigating the peeling condition. The peeling condition of the coating film was judged on a scale of 0 to 5 according to the classification of test results in JIS K 5400, and the secondary adhesion of the coating film was evaluated. In Figures 2 and 5, classification 0 or classification 1 is indicated by ◎, classification 2 by ○, classification 3 by △, and classification 4 or classification 5 by ×.

[0038] (Water resistance test) The water resistance test was conducted by taking test pieces from both the painted steel material 100 according to the embodiment and the painted steel material 101 according to the comparative example, and sealing the unpainted ends of the test pieces to use as evaluation samples. The test method was in accordance with JIS K 5600-6-1, in which the test pieces were immersed in water at a temperature of 23°C for 240 hours. After the test, the evaluation was performed by conducting a cross-cut adhesion test in accordance with the provisions of JIS K 5600-5-6 to investigate the degree of peeling. The degree of peeling of the coating film was judged on a scale of 0 to 5 according to the classification of test results in JIS K 5400, and the secondary adhesion of the coating film was evaluated. In Figures 2 and 5, classification 0 or classification 1 is indicated by ◎, classification 2 by ○, classification 3 by △, and classification 4 or classification 5 by ×.

[0039] (Saltwater immersion test) Salt resistance hot water immersion tests were conducted by taking test pieces from both the painted steel material 100 according to the embodiment and the painted steel material 101 according to the comparative example, and sealing the unpainted ends of the test pieces to use as evaluation samples. The test method involved making cross-cuts on the painted surface and immersing the samples in a 5% NaCl aqueous solution (55°C). The test duration was 240 hours.

[0040] The evaluation involved performing a tape peel test on the cross-cut section and measuring the maximum peel width on one side from the cross-cut. The peel width was measured as the width of the paint film peeling between the undercoat layer 30 and the chemical conversion treatment film 20. The post-painting corrosion resistance of each evaluation sample was evaluated based on the measured peel width. In Figures 2 and 5, a maximum peel width of 3 mm or less is indicated by ◎, between 3 mm and 5 mm by ○, between 5 mm and 7 mm by △, and above 7 mm by ×.

[0041] (Evaluation results) As shown in Figures 2 and 5, differences in evaluation results were observed for evaluation samples having a common zinc plating layer 11 and chemical conversion coating 20, due to differences in the specifications of the undercoat layer 30 and the topcoat layer 40. Examples 1 to 15 show the evaluation results when using the painted steel material 100 according to the embodiment, while Comparative Examples 1 to 10 show the evaluation results when using the painted steel material 101 according to the comparative examples. Looking at the results of the outdoor exposure weathering test, each evaluation sample shown in Examples 1 to 15 in Figure 2 received excellent results (◎), while each evaluation sample shown in Comparative Examples 1 to 10 in Figure 5 received evaluations (△ to ×). In particular, in Comparative Examples 2 to 8 and 10, where epoxy resin was used in the undercoat layer 30, delamination was observed, and long-term durability in an ultraviolet environment could not be obtained.

[0042] As shown in Figure 5, when the undercoat layer 30 is made of epoxy resin and the topcoat layer 40 is made of polyester resin, delamination occurs between the undercoat layer 30 and the topcoat layer 40. Because epoxy resin has high adhesion to the underlying layer, it can reduce the possibility of salt and other substances seeping between the undercoat and the underlying layer. For this reason, when epoxy resin is used, good results are obtained in all tests except the outdoor exposure weathering test.

[0043] However, epoxy resin has low resistance to ultraviolet light and is prone to damage. In outdoor exposure weathering tests, the epoxy resin deteriorates due to the effects of ultraviolet light, etc., which reduces the adhesion between the epoxy resin and the polyester resin, resulting in delamination between the epoxy resin undercoat layer 30 and the polyester resin topcoat layer 40. For this reason, in the outdoor exposure weathering tests, good test results were not obtained for any of Comparative Examples 1 to 10.

[0044] Furthermore, if the topcoat layer 40 peels off due to delamination, the epoxy resin of the undercoat layer 30 will be exposed. Since epoxy resin is not resistant to ultraviolet rays, the exposed epoxy resin will wear down and the damage will spread. If saltwater comes into contact with the damaged part of the epoxy resin, water will easily seep in from the damaged area, causing damage to the entire painted steel material 100.

[0045] Typically, accelerated testing in a laboratory involves specialized tests for a particular performance aspect, with observation and evaluation performed on each element. For example, for salt-resistant specifications, tests are conducted in a salt-damaged environment using salt spray tests or hot salt water immersion tests, but in these cases, the effects of ultraviolet light are not present. Similarly, for weather resistance, sunshine tests are performed, and in this case, the effects of salt damage are also not present.

[0046] However, in real-world environments, multiple factors, including the effects of ultraviolet light and salt damage, act synergistically on the coating. Therefore, even if a coating shows good results in salt damage tests and maintains a certain level of gloss in sunshine tests against ultraviolet light, long-term exposure may result in different outcomes from those in the individual tests due to the effects of salt damage, ultraviolet light, heat, or temperature fluctuations.

[0047] In contrast, in the painted steel material 100 according to the embodiment, as shown in Examples 1 to 15, test pieces having polyester resin instead of epoxy resin in the undercoat layer 30 are used. By using a highly weather-resistant polyester resin in the undercoat layer 30, delamination is not observed, and long-term durability in ultraviolet environments can be obtained. Thus, by providing the undercoat layer 30 according to the embodiment, delamination between the undercoat layer 30 and the topcoat layer 40 is prevented during outdoor exposure, and the paint adhesion and corrosion resistance of the painted steel material 100 are improved.

[0048] As shown in Figure 5, in Comparative Example 4, when the undercoat layer 30 is of specification D, the topcoat layer 40 is white, and the paint film thickness is 40 μm, the outdoor exposure test results are × for both with and without cuts. On the other hand, in Comparative Example 5, when the topcoat layer 40 is brown and the paint film thickness is 40 μm, the outdoor exposure test results are △ for both with and without cuts, showing a slight improvement. This is thought to be because white easily transmits ultraviolet rays, thus having a weak effect in preventing the deterioration of the epoxy resin in the undercoat layer 30, while brown does not transmit ultraviolet rays easily, thus preventing the deterioration of the epoxy resin in the undercoat layer 30.

[0049] In contrast, as shown in Figure 2, the painted steel material 100 provided with the undercoat layer 30 of the embodiment shows good test results in each test, regardless of the type of zinc plating layer 11, the type of topcoat layer 40, or their respective film thicknesses. Therefore, by providing the undercoat layer 30 according to the embodiment, painted steel material 100 with high corrosion resistance and paint adhesion can be obtained. Furthermore, if the paint film thickness of the topcoat layer 40 is 20 μm or more and 150 μm or less, and the total film thickness of the undercoat layer 30 and the topcoat layer 40 exceeds 60 μm, painted steel material 100 with high corrosion resistance and paint adhesion can be obtained.

[0050] In particular, in Examples 3 to 15 of Figure 2, when the paint film thickness of the undercoat layer 30 and the topcoat layer 40 is 40 μm or more, a painted steel material 100 with high corrosion resistance and paint adhesion is obtained regardless of whether the zinc plating layer 11 is Z27 or ZAM.

[0051] When forming the undercoat layer 30 according to specifications A, B, or C, the coating is applied using powder coating. By adhering the powder coating to the surface of the chemical conversion film 20 using static electricity, the film thickness can be increased in a single coating operation. In evaluation, the film thickness of the undercoat layer 30 is set to a range of 20 μm to 150 μm, and good results equivalent to those of the undercoat layers 30 according to specifications A, B, and C shown in Figure 2 can be obtained. However, in the actual manufacture of painted steel materials 100, it is necessary to ensure that the film thickness of the undercoat layer 30 does not fall below 20 μm at the lower limit of the film thickness to stabilize the corrosion resistance and paint adhesion of the painted steel materials 100. Therefore, in actual manufacture, it is preferable to set the film thickness of the undercoat layer 30 at the lower limit to 40 μm or more. Furthermore, even if the film thickness of the undercoat layer 30 is 150 μm, results equivalent to those of the undercoat layers 30 according to specifications A, B, or C shown in Figure 2 can be obtained. If the thickness of the undercoat layer 30 exceeds 150 μm, the surface strength of the painted steel material 100 decreases, making it more susceptible to scratches. In the actual manufacturing of painted steel material 100, it is desirable to set the upper limit thickness of the undercoat layer 30 to 100 μm or less, taking into account the amount of powder coating used to form the undercoat layer 30 and variations in product dimensions.

[0052] The topcoat layer 40 is coated with powder coating to ensure a film thickness of 20 μm or more. The powder coating process has the advantage of being able to increase the film thickness in a single step by using static electricity to adhere the powder coating to the surface of the undercoat layer 30.

[0053] In the actual manufacturing of the painted steel material 100, the topcoat layer 40 must be designed so that its thickness does not fall below 20 μm at the lower limit of the film thickness, thereby stabilizing the corrosion resistance and paint adhesion of the painted steel material 100. Therefore, it is desirable to set the lower limit of the film thickness of the topcoat layer 40 to 40 μm or more. Furthermore, it is desirable that the combined film thickness of the undercoat layer 30 and the topcoat layer 40 exceeds 60 μm. Moreover, if the film thickness of the topcoat layer 40 exceeds 150 μm, the surface strength of the painted steel material 100 will decrease, making it more susceptible to scratches. In the actual manufacturing of the painted steel material 100, it is desirable to set the upper limit of the film thickness of the topcoat layer 40 to 100 μm or less, taking into consideration the amount of powder coating used to form the topcoat layer 40 and variations in product dimensions.

[0054] Furthermore, at least one of the undercoat layer 30 or the topcoat layer 40 may consist of multiple layers. For example, if the undercoat layer 30 is made up of multiple layers, it is preferable that the multiple layers be made using the same type of paint. This helps maintain adhesion between the multiple layers. Also, for example, if the undercoat layer 30 is made up of multiple layers, it is sufficient that the total film thickness of the multiple layers is set to be in the range of 20 μm to 150 μm. In addition, a fluorine coating may be applied to the surface of the topcoat layer 40. By applying a fluorine coating, it is possible to obtain a painted steel material 100 that can withstand more severe ultraviolet radiation environments and has good weather resistance.

[0055] Furthermore, at least one of the undercoat layer 30 or the topcoat layer 40 can be formed by liquid coating. When powder coating is used, the powder adheres to the surface of the substrate until it reaches the capacitance due to electrostatic action, so a film thickness of about 30 to 40 μm can be formed in a single application, making film formation easy. On the other hand, when liquid coating is used, the viscosity is reduced in order to make the solvent into a mist, so it is difficult to adhere, and the film thickness that can be formed in a single application is thinner compared to powder coating. Therefore, when using liquid coating, one layer is formed, dried to the extent that the paint does not drip, and then the next layer is formed, and this process is repeated multiple times, so that for example, an undercoat layer 30 is formed by multiple layers, and an undercoat layer 30 with sufficient film thickness can be formed. When liquid coating is used, after the multiple films have been formed, a heating and fixing process is carried out.

[0056] For example, the undercoat layer 30 may be powder coated and the topcoat layer 40 may be liquid coated. In this case, the topcoat layer 40 may consist of, for example, three or four layers. Thus, first, the undercoat layer 30 is formed on the surface of the chemical conversion treated film 20 by powder coating, and then the undercoat layer 30 is fixed to the surface of the chemical conversion treated film 20 by heating at a predetermined temperature. This makes it possible to form an undercoat layer 30 with a certain thickness. Next, multiple topcoat layers 40 are formed on the surface of the undercoat layer 30 by liquid coating, and then the topcoat layers 40 are fixed to the surface of the undercoat layer 30 by heating at a predetermined temperature.

[0057] (Effects of the embodiment) (1) The painted steel material 100 according to the embodiment comprises a primer layer 30 formed on the surface of a chemical conversion coating 20 and a topcoat layer 40 formed on the surface of the primer layer 30. The chemical conversion coating 20 is formed on the surface of a zinc plating layer 11 that constitutes a zinc-plated steel material. The primer layer 30 contains a highly weather-resistant resin that is resistant to ultraviolet rays, for example, polyester resin, and has a thickness of 20 μm or more and 150 μm or less. The topcoat layer 40 contains a highly weather-resistant resin that is resistant to ultraviolet rays, for example, polyester resin, and has a thickness of 20 μm or more and 150 μm or less. The painted steel material 100 with this configuration can prevent delamination between the primer layer 30 and the topcoat layer 40 by preventing deterioration of the primer layer 30 due to ultraviolet rays, thereby obtaining high paint adhesion and high corrosion resistance. In particular, by exceeding a total film thickness of 60 μm between the undercoat layer 30 and the topcoat layer 40, resistance to ultraviolet rays is improved, as well as adhesion, allowing the appearance of the painted steel material 100 to be maintained for a long period of time.

[0058] (2) According to the painted steel material 100 of the embodiment, since the undercoat layer 30 and the topcoat layer 40 contain polyester resin, interlayer peeling of the coating due to ultraviolet rays is suppressed, and the durability of the painted steel material 100 can be improved.

[0059] (3) According to the painted steel material 100 of the embodiment, since the undercoat layer 30 and the topcoat layer 40 are made of the same paint, the adhesion between the layers is further improved and delamination between layers can be further suppressed.

[0060] (4) According to the painted steel material 100 of the embodiment, the thickness of the undercoat layer 30 and the topcoat layer 40 is the thickness after they have been heated and fixed to the surface of the chemical conversion treatment film 20 or the undercoat layer 30.

[0061] (5) According to the painted steel material 100 of the embodiment, the undercoat layer 30 and the topcoat layer 40 contain powder coating, so they can form thick and uniform layers.

[0062] (6) According to the painted steel material 100 of the embodiment, the undercoat layer 30 contains powder coating and the topcoat layer 40 contains liquid coating, so that a thick layer is formed with the powder coating and the painted steel material can be completed quickly with the fast-drying liquid coating.

[0063] (7) According to the painted steel material 100 of the embodiment, since both the undercoat layer 30 and the topcoat layer 40 contain a fast-drying liquid paint, the manufacturing period can be shortened.

[0064] (8) According to the painted steel material 100 of the embodiment, at least one of the undercoat layer 30 or the topcoat layer 40 is composed of multiple layers, so the film thickness can be increased even when liquid paint is used.

[0065] (9) According to the painted steel material 100 of the embodiment, the zinc plating layer 11 has a chemical conversion treatment film 20 on its surface.

[0066] (10) According to the painted steel material 100 of the embodiment, the chemical conversion coating 20 is a chromate coating, a chromate-free coating, or a phosphate coating.

[0067] (11) According to the method for manufacturing painted steel material 100 according to the embodiment, the method comprises a first step of forming an undercoat layer 30 and a second step of forming a topcoat layer 40, wherein the film thickness of the undercoat layer 30 and the topcoat layer 40 is 20 μm or more and 150 μm or less. Both the undercoat layer 30 and the topcoat layer 40 contain a weather-resistant resin.

[0068] (12) According to the method for manufacturing the painted steel material 100 according to the embodiment, the first step is to attach a powder coating containing a highly weather-resistant resin that is resistant to ultraviolet rays to the surface of the chemical conversion coating 20, and then heat it to fix it to the surface of the chemical conversion coating 20. With this configuration, a primer layer 30 of sufficient thickness can be formed in a single step, so that the thickness of the primer layer 30 can be formed within a predetermined range while keeping manufacturing costs down.

[0069] (13) According to the method for manufacturing painted steel material 100 according to the embodiment, the second step is to attach a powder coating containing polyester resin onto the undercoat layer 30 and heat it to fix it to the surface of the undercoat layer 30. With this configuration, a topcoat layer 40 of sufficient thickness can be formed in a single painting step, so that the thickness of the topcoat layer 40 can be formed within a predetermined range while keeping manufacturing costs down. Furthermore, if only the topcoat painting step is performed, there is a risk of unevenness in the paint film thickness, and moisture and salt may penetrate from areas with thin paint film thickness, causing corrosion of the zinc plating layer 11 beneath the paint film, which can lead to paint blistering or peeling. However, by performing the first step and the second step, a stable paint film thickness can be obtained, which slows down the penetration of moisture or salt compared to when only the second step for forming the topcoat layer 40 is performed, thus delaying paint blistering or peeling.

[0070] (14) According to the method for manufacturing painted steel material 100 according to the embodiment, the method includes a chemical conversion treatment step in which a chemical conversion treatment film 20 is formed on the surface of the zinc plating layer 11 before the undercoat layer painting step.

[0071] The painted steel material 100 and the method for manufacturing the painted steel material 100 described above may also include combinations of the features shown in the following appendices 1 to 14. These combinations are shown below.

[0072] [Note 1] A primer layer formed on the surface of a chemical conversion coating formed on zinc-plated steel, A topcoat layer formed on the surface of the aforementioned undercoat layer, Equipped with, The aforementioned undercoat layer is It contains a weather-resistant resin, and the thickness of the undercoat layer is 20 μm or more and 150 μm or less. The aforementioned topcoat layer is It contains a weather-resistant resin, and the thickness of the topcoat layer is 20 μm or more and 150 μm or less. Painted steel materials. [Note 2] The aforementioned undercoat layer and the aforementioned topcoat layer contain polyester resin. Painted steel materials as described in Appendix 1. [Note 3] The aforementioned undercoat layer and the aforementioned topcoat layer are made of the same paint. Painted steel materials as described in Appendix 1 or 2. [Note 4] The paint film thickness between the aforementioned undercoat layer and the aforementioned topcoat layer is Total size exceeds 60 μm Painted steel materials as described in any one of the appendices 1 to 3. [Note 5] The thickness of the aforementioned undercoat layer is the thickness after it has been heated and fixed to the surface of the chemical conversion treated film. Painted steel materials as described in any one of the appendices 1 to 4. [Note 6] The thickness of the topcoat layer is the thickness after it has been heated and fixed to the surface of the undercoat layer. Painted steel materials as described in any one of the appendices 1 to 5. [Note 7] The aforementioned undercoat layer includes powder coating, The aforementioned topcoat layer includes powder coating. Painted steel materials as described in any one of the appendices 1 to 6. [Note 8] The aforementioned undercoat layer includes a liquid paint, The aforementioned topcoat layer includes powder coating. Painted steel materials as described in any one of the appendices 1 to 6. [Note 9] The aforementioned undercoat layer includes a liquid paint, The aforementioned topcoat layer includes a liquid paint. Painted steel materials as described in any one of the appendices 1 to 6. [Note 10] At least one of the aforementioned undercoat layer or topcoat layer is composed of multiple layers. Painted steel materials as described in any one of the notes 1 to 9. [Note 11] The first step involves forming an undercoat layer on the surface of a chemical conversion coating formed on a zinc-plated steel material, A second step involves forming a topcoat layer on the surface of the aforementioned undercoat layer, Equipped with, The aforementioned undercoat layer is It contains a weather-resistant resin and has a thickness of 20 μm to 150 μm. The aforementioned topcoat layer is It contains a weather-resistant resin and has a thickness of 20 μm to 150 μm. A method for manufacturing painted steel materials. [Note 12] The aforementioned undercoat layer contains polyester resin The method for manufacturing painted steel materials as described in Appendix 11. [Note 13] The aforementioned undercoat layer and the aforementioned topcoat layer are made of the same paint. A method for manufacturing painted steel materials as described in Appendix 11 or 12. [Note 14] The total coating film thickness of the undercoat layer and the topcoat layer exceeds 60 μm. A method for manufacturing painted steel materials as described in any one of the appendices 11 to 13. [Explanation of Symbols]

[0073] 10 Steel material, 11 Zinc plating layer, 20 Chemical conversion coating, 30 Undercoat layer, 40 Topcoat layer, 100 Painted steel material, 101 Painted steel material.

Claims

1. A primer layer formed on the surface of a chemical conversion coating formed on zinc-plated steel, A topcoat layer formed on the surface of the aforementioned undercoat layer, Equipped with, The aforementioned undercoat layer is It contains a weather-resistant resin, and the thickness of the undercoat layer is 20 μm or more and 150 μm or less. The aforementioned topcoat layer is It contains a weather-resistant resin, and the thickness of the topcoat layer is 20 μm or more and 150 μm or less. Painted steel materials.

2. The aforementioned undercoat layer and the aforementioned topcoat layer contain polyester resin. The painted steel material according to claim 1.

3. The aforementioned undercoat layer and the aforementioned topcoat layer are made of the same paint. The painted steel material according to claim 1 or 2.

4. The paint film thickness between the aforementioned undercoat layer and the aforementioned topcoat layer is Total size exceeds 60 μm The painted steel material according to claim 1 or 2.

5. The thickness of the aforementioned undercoat layer is the thickness after it has been heated and fixed to the surface of the chemical conversion treated film. The painted steel material according to claim 1 or 2.

6. The thickness of the topcoat layer is the thickness after it has been heated and fixed to the surface of the undercoat layer. The painted steel material according to claim 1 or 2.

7. The aforementioned undercoat layer includes powder coating, The aforementioned topcoat layer includes powder coating. The painted steel material according to claim 1 or 2.

8. The aforementioned undercoat layer includes a liquid paint, The aforementioned topcoat layer includes powder coating. The painted steel material according to claim 1 or 2.

9. The aforementioned undercoat layer includes a liquid paint, The aforementioned topcoat layer includes a liquid paint. The painted steel material according to claim 1 or 2.

10. At least one of the aforementioned undercoat layer or topcoat layer is composed of multiple layers. The painted steel material according to claim 1 or 2.

11. A first step involves forming an undercoat layer on the surface of a chemical conversion treatment film formed on a zinc-plated steel material, A second step involves forming a topcoat layer on the surface of the aforementioned undercoat layer, Equipped with, The aforementioned undercoat layer is It contains a weather-resistant resin and has a thickness of 20 μm to 150 μm. The aforementioned topcoat layer is It contains a weather-resistant resin and has a thickness of 20 μm to 150 μm. A method for manufacturing painted steel materials.

12. The aforementioned undercoat layer contains polyester resin The method for manufacturing painted steel according to claim 11.

13. The aforementioned undercoat layer and the aforementioned topcoat layer are made of the same paint. A method for manufacturing painted steel according to claim 11 or 12.

14. The total coating film thickness of the undercoat layer and the topcoat layer exceeds 60 μm. A method for manufacturing painted steel according to claim 11 or 12.

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