Pure zinc base plate coating, pure zinc color coated plate and preparation method thereof

Abstract

The application discloses a pure zinc substrate plating layer, a pure zinc color-coated plate and a preparation method of the pure zinc color-coated plate. The chemical composition of the pure zinc substrate plating layer is as follows in terms of percentage by weight: Al: 0.18-0.32%, Mg: 0.04-0.20%, and the rest is Zn and inevitable impurities. The pure zinc color-coated plate comprises a steel base body, a pure zinc substrate plating layer, a passivation layer and a coating layer. The pure zinc substrate plating layer is attached to at least part of the surface of the steel base body. The passivation layer is attached to at least part of the surface of the pure zinc substrate plating layer. The coating layer comprises a primary coating layer and a fine coating layer, and the coating layer is attached to at least part of the surface of the passivation layer. The application optimizes the hot-dip plating process and the color-coating process, enhances the surface activity of the pure zinc substrate plating layer, improves the coating adhesion of the color-coated plate, and ensures the durability and aesthetic appearance of the color-coated plate in use.

Description

Technical Field

[0001] This invention relates to the field of hot-dip galvanized metal materials, specifically to a pure zinc substrate coating, a pure zinc color-coated sheet, and a method for preparing the same. Background Technology

[0002] Color-coated steel sheet is a pre-coated steel sheet produced by continuous production lines using galvanized steel sheet, aluminized zinc steel sheet, or cold-rolled steel sheet as the base material. After pretreatment, an organic coating of a certain thickness is applied to its surface. This pre-coated steel sheet is widely used in various industries such as construction, home appliances, and transportation due to its excellent corrosion resistance and aesthetically pleasing appearance. Color-coated steel sheets not only reduce subsequent processing steps for users but also lower processing costs, making installation more convenient and faster.

[0003] Coating adhesion is one of the key indicators for evaluating the quality of pre-coated steel sheets. Good coating adhesion means that the coating can adhere stably to the substrate surface and is not easily peeled or detached. This is crucial for ensuring the durability and reliability of pre-coated steel sheets in actual use. While the requirements for coating adhesion vary across different applications, they generally require ensuring that the coating maintains good adhesion under various conditions.

[0004] Coating adhesion is affected by a variety of factors, mainly including the following aspects:

[0005] (1) Coating performance: The type, composition, and physicochemical properties of the coating have a direct impact on the adhesion of the coating. For example, different types of resin systems, pigments, and additives will affect the adhesion performance of the coating. (2) Substrate surface pretreatment process: The cleanliness, roughness, and whether the substrate surface has undergone chemical conversion treatment (such as phosphating, passivation, etc.) will have a significant impact on the adhesion of the coating. Appropriate pretreatment can improve the bonding force between the substrate and the coating. (3) Coating curing process: The coating curing conditions (such as temperature, time, etc.) will affect the degree of cross-linking of the coating, and thus affect the adhesion between the coating and the substrate. Correct curing conditions help to form a stronger coating. (4) Surface characteristics of the substrate: The substrate material itself and its surface microstructure will also affect the adhesion of the coating. For example, the enrichment of specific alloying elements (such as aluminum) on the substrate surface will have a positive impact on the coating adhesion. (5) Other factors: External conditions such as ambient humidity and temperature changes may also indirectly affect the performance of coating adhesion.

[0006] The paper "Analysis and Improvement of Factors Affecting the T-Bending Performance of Color-Coated Steel Sheets" points out that factors such as cleaning quality, passivation treatment, primer type and curing temperature, and topcoat curing temperature have a significant impact on the T-bending performance of color-coated steel sheets. The paper argues that the substrate's own properties and surface quality have a relatively small impact on the T-bending performance of color-coated steel sheets, emphasizing the importance of process control. The paper "Influence of Substrate Surface Characteristics on the Adhesion of Color-Coated Steel Sheet Coatings" studies the influence of substrate surface characteristics on the adhesion of color-coated steel sheet coatings, pointing out that the electrochemical activity of the substrate surface has a significant impact on the adhesion of the color-coated steel sheet coating, especially the enrichment degree of specific alloying elements (such as aluminum) on the substrate surface. A patent with patent number ZL202211159099.0 discloses a method for analyzing poor adhesion defects in hot-dip galvanized color-coated steel sheets. This method can specifically and accurately determine the causes of poor adhesion defects in color-coated steel sheets, which helps manufacturers to promptly identify and resolve potential quality problems during production. However, this patent does not specify how to solve the problem of poor coating adhesion. Another patent, ZL201380053903.X, describes a method for manufacturing metal-coated steel strip. By controlling the pH and temperature of the cooling water, the alkalinity of the cooling water is suppressed, reducing the formation of precipitates and thus mitigating the impact of the cooling water on the corrosion resistance of the Al-Zn-Si-Mg alloy coating. This patent primarily focuses on improving the corrosion resistance of the coated product. Another patent, ZL202180076021.X, describes a Zn-based coated steel sheet. This invention provides a Zn-based coated steel sheet with a high-end golden appearance and improved corrosion resistance, primarily focusing on enhancing the aesthetics and corrosion resistance of the coated product.

[0007] The aforementioned literature mainly discusses the corrosion resistance and adhesion properties of alloy coatings and color-coated steel sheets, while there is relatively little content on the adhesion properties of pure zinc substrate coatings and pure zinc color-coated steel sheets. Therefore, how to improve the surface activity of pure zinc substrate coatings and the coating adhesion of pure zinc color-coated steel sheets is one of the technical problems that urgently need to be solved. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a pure zinc substrate coating, a pure zinc color-coated sheet, and a method for preparing the same. By optimizing the hot-dip galvanizing process and the color-coating process, the surface activity of the pure zinc substrate coating is enhanced, the coating adhesion of the color-coated sheet is improved, and the durability and aesthetics of the color-coated sheet are ensured during use.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] The present invention provides a pure zinc substrate coating, wherein the chemical composition of the pure zinc substrate coating is as follows by weight percentage: Al: 0.18-0.32%, Mg: 0.04-0.20%, and the remainder is Zn and unavoidable impurities.

[0011] The present invention also provides a pure zinc color-coated steel sheet, the pure zinc color-coated steel sheet comprising a steel substrate, a pure zinc substrate plating layer, a passivation layer, and a coating layer; wherein,

[0012] The pure zinc substrate coating is attached to at least a portion of the surface of the steel substrate;

[0013] The passivation layer is attached to at least a portion of the surface of the pure zinc substrate plating;

[0014] The coating comprises a primary coating and a secondary coating, and the coating is attached to at least a portion of the surface of the passivation layer.

[0015] As a further optimization of the present invention, the content of the passivation layer is controlled at 20-50 mg / m³. 2 .

[0016] This invention also provides a method for preparing pure zinc color-coated steel sheet, the method comprising:

[0017] The strip steel is pretreated to ensure that the surface properties of the strip steel meet the set requirements, thereby obtaining a steel matrix;

[0018] The steel substrate is hot-dip galvanized and cooled to obtain a first steel plate with a pure zinc substrate coating.

[0019] The first steel plate is subjected to passivation treatment and first curing to obtain a second steel plate with a passivation layer;

[0020] The second steel plate is subjected to initial coating and second curing, followed by fine coating and third curing to obtain pure zinc color-coated sheet.

[0021] As a further optimization of the present invention, the pretreatment includes: performing a finishing treatment on the strip steel to control the surface roughness Rpc value of the steel substrate to be >100 particles / cm.

[0022] As a further optimization of the present invention, the conditions for hot-dip galvanizing are as follows: the galvanizing temperature is controlled at 460±5℃, the height of the air knife from the liquid surface is 300-360mm, the angle is -0.7°, the distance between the blade lip and the strip is 7-9mm, and the air knife pressure is 300-360mbar.

[0023] As a further optimization of the present invention, the cooling method after hot-dip galvanizing includes air cooling. The air cooling is carried out in four sections by a cooling fan. The height of the first cooling fan from the surface of the galvanizing solution is controlled at 1.5-2.0m, the cooling rate is controlled at 15-20℃ / s, and the temperature is cooled to less than 340℃. The cooling fans of the second, third and fourth sections are controlled to cool the strip temperature to less than 100℃.

[0024] As a further optimization of the present invention, the air cooling is followed by water cooling, and the conditions for water cooling are: the total content of metal impurities in the cooling water is less than 1 μg / ml, the water conductivity is less than 10 μs / cm, and the water temperature is less than 50℃. The metal impurities include Mg, Ca, and Si.

[0025] As a further optimization of the present invention, the first curing temperature is 120-140℃.

[0026] As a further optimization of the present invention, both the second curing and the third curing are divided into 5 stages for heating. The heating temperatures of the 1st to 5th stages of the second curing are 130-180℃, 160-220℃, 210-270℃, 230-290℃ and 220-280℃, respectively. The heating temperatures of the 1st to 5th stages of the third curing are 140-190℃, 170-230℃, 220-280℃, 245-310℃ and 240-320℃, respectively.

[0027] As a further optimization of the present invention, before the first steel plate is passivated, it needs to be cleaned by two alkaline washings and three water washings. After water washing and drying, the surface is subjected to hexavalent chromium passivation treatment.

[0028] This invention relates to a method for improving the adhesion of color-coated steel sheets. Specifically, it involves controlling parameters such as the composition of the plating solution, cooling parameters, water quality in the quenching tank, strip cooling temperature, and surface roughness during the hot-dip galvanizing process, as well as parameters such as the passivation film weight, passivation drying temperature, and color coating heating temperature in the color coating process. This improves the surface activity of the coating on the pure zinc substrate, thereby enhancing the adhesion of the color-coated products. A 2T bending test showed no passivation film or coating peeling. The following is a summary of the main contents, advantages, and effects of this invention:

[0029] Precise control of chemical composition: This invention controls the Al content to 0.18-0.32% and the Mg content to 0.06-0.20%. This is because if the Al content is too low, the inhibition layer will not form completely, resulting in insufficient coating adhesion. If the Al content is too high, it will affect the fluidity of the plating solution and restrict the control of the surface quality of the strip coating. If the Mg content is too low, an alloy phase cannot be formed, affecting the adhesion between the color coating and the substrate. If the Mg content is too high, it will affect the surface quality of the coated product. This invention introduces trace amounts of Al and Mg into the pure zinc substrate coating. An appropriate amount of Al can promote the formation of the inhibition layer and ensure the adhesion of the coating. An appropriate amount of Mg helps to form a MgZn2 alloy phase on the coating surface, improving the reactivity of the coating surface.

[0030] Optimization of production processes:

[0031] (1) Strip surface treatment: The surface roughness of the strip is increased by the finishing process, which increases the contact area between the coating and the substrate and is beneficial to improve the adhesion.

[0032] (2) Immersion temperature: When the immersion temperature is too high, the amount of zinc dross increases, the fluidity of the zinc liquid increases, which affects the control of the coating thickness and reduces the coating thickness; when the immersion temperature is too low, the fluidity of the plating solution decreases and the coating thickness increases. The preferred temperature is 460±5℃.

[0033] (3) Air Knife Working Mode: The air knife adopts a working mode of high height, negative angle, small distance, and low pressure. When the air knife is too high above the liquid surface, the steel strip cools down too much before reaching the air knife, increasing the viscosity of the zinc liquid and the coating thickness. When the air knife is too low above the liquid surface, the airflow from the air knife is too large and easily causes zinc liquid to splash, affecting the surface quality control of the strip. The preferred distance is 300-360 mm. The air knife angle refers to the angle between the airflow from the nozzle and the vertical line of the steel strip. The airflow can only be inclined downwards, and the angle is negative. Within a certain angle range, the larger the angle, the smaller the coating thickness. The preferred angle is -0.7°. The greater the distance between the blade lip and the strip, the greater the coating thickness. The greater the air knife pressure required to achieve the same coating, the greater the energy consumption. Generally, the closer the blade lip is to the strip, the better. Considering that the strip tension is small or the strip shape is poor, the blade lip may collide with the air knife. The preferred distance is 7-9 mm. Generally, the higher the air knife pressure, the thinner the coating thickness. When the strip speed is low, the air knife pressure increases, and the cooling effect caused by the blown gas also increases the coating thickness. When the air knife pressure is low, the gas volume is small, the cooling effect is small, and the blowing effect is poor. The preferred pressure is 300-360 mbar.

[0034] (4) Cooling: By controlling the control parameters of the first-stage cooling fan, the grain size of the coating can be reduced, the number of grain boundaries can be increased, and thus the adhesion can be improved. By controlling the impurity content and conductivity in the quenching tank, the formation of precipitates can be prevented, which may block some nozzles and cause cooling color difference streaks on the board surface, thus avoiding affecting the coating quality.

[0035] (5) Passivation: Hexavalent chromium passivation treatment is adopted, and the content of passivation film and curing temperature are controlled to ensure that the passivation film is completely cured, which enhances the adhesion between the coating and the substrate.

[0036] (6) Color Coating Process: Segmented heating allows for more precise temperature control at each stage, enabling the coating to cure gradually at different temperatures. This helps to achieve a uniform distribution of internal stress within the coating and avoids coating defects caused by sudden temperature changes. The temperature gradient design can reduce bubbles and pinholes in the coating, contributing to improved coating density and adhesion. Furthermore, segmented heating reduces energy consumption because the initial curing process does not require excessively high temperatures. As the coating gradually cures, the temperature is gradually increased, effectively utilizing thermal energy. Attached Figure Description

[0037] Figure 1 It is the alloy phase at the grain boundary on the surface of the coating in Example 1.

[0038] Figure 2 These are the surface Al element glow discharge detection results for Example 1 and Comparative Example 1. Detailed Implementation

[0039] To enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, these should not be construed as limiting the present invention and are merely examples.

[0040] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are obtained from conventional commercial sources or prepared by conventional methods.

[0041] The present invention will be further described in detail below with reference to specific embodiments and comparative examples. The present invention controls the composition of the plating solution, cooling parameters, water quality of the quenching tank, strip cooling temperature, and roughness in the galvanizing process, as well as the passivation film weight, passivation drying temperature, and color coating heating temperature in the color coating process. The produced product has a MgZn2 alloy phase generated at the grain boundaries on the coating surface, which improves the reactivity. Moreover, the coating surface is less damaged, and there is an enrichment of Al element, resulting in high surface activity.

[0042] The chemical composition and content of the pure zinc substrate coatings in Examples 1-3 and Comparative Examples 1-4 of this invention, as well as the production process of the pure zinc color-coated sheets, are shown in Tables 1 and 2.

[0043] Table 1 Relevant process parameters

[0044]

[0045] Table 2 Relevant process parameters

[0046]

[0047]

[0048] Analysis of Examples 1-3 in the table above shows that the coating produced using the chemical composition and production process of the present invention has excellent surface quality, excellent adhesion between the substrate and the passivation film and between the passivation film and the coating, and no passivation film or coating peels off in the 2T bending test.

[0049] Analysis of Example 1 and Comparative Example 1 in the table above shows that, under the same process conditions, the lack of Mg element in Comparative Example 1 compared to Example 1 leads to a 50% coating peeling phenomenon. This indicates that adding an appropriate amount of Mg element can enhance the adhesion of the coating and prevent the coating from peeling off.

[0050] Analysis of Example 2 and Comparative Example 2 in the table above shows that, under the same chemical composition conditions, Comparative Example 2 increased the height of the cooling fan from the liquid surface compared to Example 2, resulting in a decrease in the post-plating cooling rate from 18℃ / s to 5℃ / s. In the 2T bending test, 30% of the coating peeled off. This indicates that by reasonably controlling the post-plating air cooling rate, the adhesion can be improved and the coating peeling can be avoided.

[0051] Analysis of Example 3 and Comparative Example 3 in the table above shows that, under the same chemical composition conditions, compared to Example 3, Comparative Example 3 exhibits the following characteristics: the strip temperature in the quenching tank is increased from 80℃ to 200℃, the water impurity content increases from 0.6μg / ml to 1.6μg / ml, the water conductivity increases from 6μs / cm to 16μs / cm, and the water temperature increases from 50℃ to 80℃. Furthermore, 80% of the strip in the 2T bending test shows peeling. This indicates that water cooling has a significant impact on coating adhesion. By reasonably controlling the strip cooling temperature and the water quality in the quenching tank, adhesion can be significantly improved, and coating peeling can be avoided.

[0052] Analysis of Example 3 and Comparative Example 4 in the table above shows that, under the same chemical composition conditions, Comparative Example 4, compared to Example 3, reduced the roughness Rpc value, passivation film weight, curing temperature, initial coating temperature, and finishing coating temperature. Its 2T bending test showed a 40% peeling phenomenon, indicating that by reasonably controlling the passivation and color coating processes, the adhesion can be improved and the phenomenon of coating peeling can be avoided.

[0053] from Figure 1 and Figure 2 It can be seen from this that Figure 1 The SEM image of the surface microstructure of the coating in Example 1 shows that alloy phases such as MgZn2 exist at the grain boundaries, which have a lower potential than pure Zn and higher surface activity. Figure 2 In Example 1, there is an enrichment peak of aluminum on the surface. The surface activity of the coating is related to the enrichment of aluminum on the surface of the coating. The higher the surface aluminum content, the higher the surface activity and the better the adhesion of the color-coated plate coating.

[0054] The above description is only a specific example of the present invention. It should be noted that the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the technical concept and technical solution of the present invention, or the direct application of the technical concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A coating on a pure zinc substrate, characterized in that, The chemical composition of the pure zinc substrate coating, by weight percentage, is: Al: 0.18-0.32%, Mg: 0.04-0.20%, with the remainder being Zn and unavoidable impurities; a MgZn2 alloy phase is formed on the surface of the pure zinc substrate coating.

2. A pure zinc color-coated steel sheet, characterized in that, The pure zinc color-coated steel sheet comprises a steel substrate, a pure zinc substrate plating layer as described in claim 1, a passivation layer, and a coating layer; wherein... The pure zinc substrate coating is attached to at least a portion of the surface of the steel substrate; The passivation layer is attached to at least a portion of the surface of the pure zinc substrate plating; The coating comprises a primary coating and a secondary coating, and the coating is attached to at least a portion of the surface of the passivation layer.

3. The pure zinc color-coated sheet according to claim 2, characterized in that, The content of the passivation layer is controlled at 20-50 mg / m³. 2 .

4. A method for preparing pure zinc color-coated steel sheet, characterized in that, The method for preparing the pure zinc color-coated sheet according to claim 2 or 3 includes: The strip steel is pretreated to ensure that the surface properties of the strip steel meet the set requirements, thereby obtaining a steel matrix; The steel substrate is hot-dip galvanized and cooled to obtain a first steel plate with a pure zinc substrate coating. The first steel plate is subjected to passivation treatment and first curing to obtain a second steel plate with a passivation layer; The second steel plate is subjected to initial coating and second curing, followed by fine coating and third curing to obtain pure zinc color-coated sheet.

5. The method for preparing pure zinc color-coated steel sheet according to claim 4, characterized in that, Preprocessing includes: The strip steel is subjected to a finishing process to control the surface roughness Rpc value of the steel substrate to be >100 particles / cm.

6. The method for preparing pure zinc color-coated steel sheet according to claim 4, characterized in that, The conditions for hot-dip galvanizing are as follows: the immersion temperature is controlled at 460±5℃, the height of the air knife from the liquid surface is 300-360mm, the angle is -0.7°, the distance between the blade lip and the strip is 7-9mm, and the air knife pressure is 300-360mbar.

7. The method for preparing pure zinc color-coated steel sheet according to claim 4, characterized in that, The cooling method after hot-dip galvanizing includes air cooling. The air cooling is carried out in four sections by a cooling fan. The height of the first cooling fan from the surface of the galvanizing solution is controlled at 1.5-2.0m, and the cooling rate is controlled at 15-20℃ / s. The strip steel is cooled to less than 340℃ after the first cooling fan. The second, third and fourth cooling fans are controlled to cool the strip steel temperature to less than 100℃.

8. The method for preparing pure zinc color-coated steel sheet according to claim 7, characterized in that, After air cooling, water cooling is performed. The conditions for water cooling are: the total content of metal impurities in the cooling water is less than 1 μg / ml, the water conductivity is less than 10 μs / cm, and the water temperature is less than 50℃.

9. The method for preparing pure zinc color-coated steel sheet according to claim 4, characterized in that, The first curing temperature is 120-140℃.

10. The method for preparing pure zinc color-coated steel sheet according to claim 4, characterized in that, Both the second curing and the third curing are divided into 5 stages for heating. The heating temperatures of stages 1 to 5 of the second curing are 130-180℃, 160-220℃, 210-270℃, 230-290℃ and 220-280℃, respectively. The heating temperatures of stages 1 to 5 of the third curing are 140-190℃, 170-230℃, 220-280℃, 245-310℃ and 240-320℃, respectively.

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