Pure zinc substrate coating, pure zinc color-coated sheet and preparation method of pure zinc color-coated sheet

Abstract

The invention discloses a pure zinc substrate plating layer, a pure zinc color-coated sheet and a preparation method of the pure zinc color-coated sheet. The pure zinc substrate plating layer comprises the following chemical components in percentage by weight: 0.18-0.32% of Al, 0.04-0.20% of Mg and the balance of Zn and inevitable impurities. The pure zinc color-coated sheet comprises a steel substrate, a pure zinc substrate plating layer, a passivation layer and a coating, wherein the pure zinc substrate coating is attached to at least part of the surface of the steel substrate; the passivation layer is attached to at least part of the surface of the pure zinc substrate coating; the coating comprises a primary coating and a fine coating, and the coating is attached to at least part of the surface of the passivation layer. By optimizing the hot dipping process and the color coating process, the surface activity of the plating layer of the pure zinc substrate is enhanced, the coating adhesive force of the color-coated sheet is improved, and the durability and attractiveness of the color-coated sheet in the using process are ensured.

Description

Technical Field

[0001] The invention relates to the field of hot-dip metal materials, and in particular to a pure zinc substrate coating, a pure zinc color-coated plate and a preparation method thereof. Background Art

[0002] Color-coated steel sheet is a pre-coated steel sheet. It is produced by continuous production units with galvanized steel sheet, galvanized steel sheet or cold-rolled steel sheet as the base material, and a certain thickness of organic coating is applied on its surface after pretreatment. This pre-coated steel sheet has been widely used in many industries such as construction, home appliances, transportation, etc. due to its good corrosion resistance and beautiful appearance. Color-coated steel sheet can not only reduce the user's subsequent processing procedures, but also reduce processing costs, making installation more convenient and faster.

[0003] Coating adhesion is one of the important indicators to measure the quality of color-coated sheets. Good coating adhesion means that the coating can be stably attached to the surface of the substrate and is not easy to peel or fall off. This is crucial to ensure the durability and reliability of color-coated sheets in actual use. Different fields of application have different requirements for the coating adhesion of color-coated sheets, but generally they need to ensure that the coating can maintain good adhesion in various environments.

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

[0005] (1) Coating properties: The type, composition and physical and chemical 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 of the coating. (2) Substrate surface pretreatment process: The cleanliness, roughness and whether the substrate surface has been subjected to chemical conversion treatment (such as phosphating, passivation, etc.) will have a significant impact on the adhesion of the coating. Proper pretreatment can improve the bonding between the substrate and the coating. (3) Coating curing process: 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) Substrate surface characteristics: 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 effect on the adhesion of the coating. (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 Causes Affecting T-bend Performance of Color-coated 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-bend performance of color-coated sheets. The article believes that the performance and surface quality of the substrate itself have relatively little effect on the T-bend performance of color-coated sheets, emphasizing the importance of process control. The paper "The Effect of Substrate Surface Characteristics on the Adhesion of Color-coated Sheet Coatings" studies the effect of substrate surface characteristics on the adhesion of color-coated sheet coatings, and points out that the electrochemical activity of the substrate surface has a significant effect on the adhesion of color-coated sheet coatings, especially the enrichment of specific alloy elements (such as aluminum) on the substrate surface. A patent with patent number ZL202211159099.0 discloses a method for analyzing the poor adhesion defects of hot-dip galvanized color-coated steel sheets. This method can specifically and accurately determine the cause of the poor adhesion defect problem of color-coated sheets, which helps manufacturers to promptly discover and solve possible quality problems during the production process, but the patent does not point out how to solve the problem of poor coating adhesion. Another patent with the patent number ZL201380053903.X introduces a method for manufacturing a metal-coated steel strip, which suppresses the alkalinity of the cooling water by controlling the pH value and temperature of the cooling water, reduces the formation of precipitates, and thus reduces the effect of the cooling water on the corrosion resistance of the Al-Zn-Si-Mg alloy coating. The patent focuses on improving the corrosion resistance of the coated product. Another patent with the patent number ZL202180076021.X introduces a Zn-based coated steel sheet. The invention can provide a Zn-based coated steel sheet containing Al that has a high-grade golden appearance and improved corrosion resistance. The patent focuses on improving the aesthetic appearance and corrosion resistance of the coated product.

[0007] The above literature mainly discusses the corrosion resistance and adhesion performance of alloy coatings and color-coated plates, while there is less content related to the adhesion performance of pure zinc substrate coatings and pure zinc color-coated plates. Therefore, how to improve the surface activity of pure zinc substrate coatings and the coating adhesion of pure zinc color-coated plates is one of the technical problems that urgently need to be solved. Summary of the invention

[0008] In view of the deficiencies in the prior art, the purpose of the present invention is to propose a pure zinc substrate coating, a pure zinc color-coated plate and a preparation method thereof, and to enhance the surface activity of the pure zinc substrate coating, improve the coating adhesion of the color-coated plate, and ensure the durability and aesthetics of the color-coated plate during use by optimizing the hot-dip plating process and the color-coating process.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] The 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 rest is Zn and inevitable impurities.

[0011] The present invention also provides a pure zinc color-coated plate, the pure zinc color-coated plate comprises a steel substrate, the pure zinc substrate coating, a passivation layer and a coating; 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 coating;

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

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

[0016] The present invention also provides a method for preparing a pure zinc color-coated sheet, which is used to prepare the pure zinc color-coated sheet. The method comprises:

[0017] Pre-treating the steel strip so that the surface properties of the steel strip meet the set requirements, thereby obtaining a steel matrix;

[0018] Hot-dip galvanizing the steel substrate, and after cooling, obtain a first steel plate having a pure zinc substrate coating;

[0019] Performing a passivation treatment and a first curing on the first steel plate to obtain a second steel plate having a passivation layer;

[0020] The second steel plate is subjected to primary coating and secondary curing, and then to fine coating and tertiary curing to obtain a pure zinc color-coated plate.

[0021] As a further optimization scheme of the present invention, the pre-treatment includes: performing a skin-finishing treatment on the steel strip to control the surface roughness Rpc value of the steel substrate to be greater than 100 pieces / cm.

[0022] As a further optimization scheme of the present invention, the conditions for hot-dip galvanizing are: controlling the immersion temperature to 460±5°C, the air knife height to the liquid level to be 300-360mm, the angle to be -0.7°, the distance between the knife lip and the strip to be 7-9mm, and the air knife pressure to be 300-360mbar.

[0023] As a further optimization scheme of the present invention, the cooling method after hot-dip galvanizing includes air cooling, and the air cooling adopts a cooling fan to cool in four sections. The height of the first cooling fan from the plating solution level is controlled to be 1.5-2.0m, and the cooling rate of the air cooling is controlled to be 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 scheme of the present invention, water cooling is performed after air cooling, and the conditions of water cooling are: the total content of metal impurities in 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° C. The metal impurities include Mg, Ca and Si.

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

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

[0027] As a further optimization scheme of the present invention, before the first steel plate is passivated, it needs to be cleaned by 2 alkaline washes and 3 water washes, and after water washing and drying, the surface is subjected to hexavalent chromium passivation treatment.

[0028] The present invention relates to a method for improving the adhesion of color-coated plates, specifically, controlling the composition of the plating solution, cooling parameters, water quality of the quenching tank, cooling temperature and roughness of the strip steel during the hot-dip plating process, as well as the passivation film weight, passivation drying temperature, and heating temperature of the color-coated coating in the color-coating process, so as to improve the surface activity of the pure zinc substrate coating, improve the adhesion of the color-coated product, and ensure that the passivation film and coating do not fall off in the 2T bending test. The following is a summary of the main contents, advantages and effects of the invention:

[0029] Fine control of chemical composition: The present invention controls the Al content to 0.18-0.32% and the Mg content to 0.06-0.20%. This is because when the Al content is too low, the inhibition layer is not fully formed and the coating adhesion is insufficient. When the Al content is too high, the fluidity of the plating solution is affected, which restricts the control of the surface quality of the strip coating. When the Mg content is too low, the alloy phase cannot be formed, which affects the adhesion of the color coating and the substrate. When the Mg content is too high, the surface quality of the coating product is affected. The present invention introduces trace amounts of Al and Mg into the pure zinc substrate coating. The appropriate amount of Al can promote the formation of the inhibition layer and ensure the adhesion of the coating. The appropriate amount of Mg helps to form a MgZn2 alloy phase on the coating surface, thereby improving the reaction activity of the coating surface.

[0030] Optimization of production process:

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

[0032] (2) Dipping temperature: When the dipping temperature is too high, the amount of zinc slag increases, the fluidity of the zinc solution increases, and the control of the coating thickness is affected, resulting in a decrease in the coating thickness. When the dipping temperature is too low, the fluidity of the plating solution decreases, and the coating thickness increases. The preferred temperature is 460±5°C.

[0033] (3) Air knife working mode: The air knife adopts a working mode of high height, negative angle, small distance and small pressure. When the air knife is too high from the liquid level, the temperature of the steel strip before the air knife drops too much, the viscosity of the zinc liquid increases, and the thickness of the coating increases; when the air knife is too low from the liquid level, the airflow of the air knife is too large, which can easily cause the zinc liquid to splash, affecting the surface quality control of the strip, preferably 300-360mm. The air knife angle refers to the angle between the nozzle's blowing airflow and the vertical line of the steel strip. The blowing airflow can only tilt downward, and the angle is negative. Within a certain angle, the larger the angle, the smaller the resulting coating thickness, preferably -0.7°. The greater the distance between the blade lip and the strip, the greater the coating thickness, and the greater the air knife pressure required to achieve the same coating, resulting in increased energy consumption; in general, the closer the blade lip is to the strip, the better. Considering that the strip tension is small or the steel strip has a poor plate shape and collides with the air knife lip, the distance needs to be increased, preferably 7-9mm. Generally speaking, the greater the air knife pressure, the smaller the coating thickness; when the strip speed is low, the air knife pressure increases, the cooling caused by the blown gas, and the coating thickness also increases; when the air knife pressure is small, the gas volume is small, the cooling effect is small, and the wiping effect is not good, preferably 300-360mbar.

[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 the adhesion can be improved. By controlling the impurity content and conductivity in the quenching tank, the formation of precipitates can be prevented, which can block some nozzles and cause cooling color stripes on the plate surface, thereby avoiding affecting the quality of the coating.

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

[0036] (6) Color coating process: Segmented heating can more accurately control the temperature of each stage, so that the coating can be gradually cured at different temperatures, which helps to evenly distribute the internal stress of the coating and avoid coating defects caused by temperature changes. The design of the temperature gradient can reduce bubbles and pinholes in the coating, which helps to improve the density and adhesion of the coating. In addition, the use of segmented heating can reduce energy consumption, because the temperature does not need to be too high during the initial curing process of the coating. As the coating gradually cures, the temperature is gradually increased, which can effectively utilize thermal energy. BRIEF DESCRIPTION OF THE DRAWINGS

[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 detection results of Example 1 and Comparative Example 1. DETAILED DESCRIPTION

[0039] In order to enable those skilled in the art to better understand the technical solution of the present invention, the preferred implementation scheme of the present invention is described below in conjunction with specific embodiments, but it should not be understood as limiting the present invention and is only used as an example.

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

[0041] The present invention is further described in detail below in conjunction with specific embodiments and comparative examples. The present invention controls the composition of the plating solution, cooling parameters, water quality of the quenching tank, cooling temperature and roughness of the strip steel in the galvanizing process, and the passivation film weight, passivation drying temperature and heating temperature of the color coating process, so that the product produced generates a MgZn2 alloy phase at the grain boundary of the coating surface, improves the reaction activity, and the coating surface is slightly damaged, has Al element enrichment, and has high surface activity.

[0042] The chemical composition and content of the pure zinc substrate coating of Examples 1-3 and Comparative Examples 1-4 of the present invention, and the production process of the pure zinc color-coated plate are shown in Table 1 and Table 2.

[0043] Table 1 Related process parameters

[0044]

[0045] Table 2 Related process parameters

[0046]

[0047]

[0048] By analyzing Examples 1-3 in the above table, it can be seen that the coating surface quality produced by adopting the chemical composition and production process of the present invention is excellent, the adhesion between the substrate and the passivation film and between the passivation film and the coating is excellent, and there is no fall-off of the passivation film and the coating in the 2T bending test.

[0049] By analyzing Example 1 and Comparative Example 1 in the above table, it can be seen that under the same process conditions, the lack of Mg element in Comparative Example 1 relative to Example 1 will cause 50% shedding of the coating, which indicates that by adding an appropriate amount of Mg element, the adhesion of the coating can be enhanced and the phenomenon of coating shedding can be avoided.

[0050] Through the analysis of Example 2 and Comparative Example 2 in the above table, it can be seen that under the same chemical composition conditions, the height of the cooling fan from the liquid surface in Comparative Example 2 is increased relative to that in Example 2, resulting in a reduction in the cooling rate after plating from 18°C / s to 5°C / s, and a 30% shedding phenomenon occurs in its 2T bending test. This indicates that by reasonably controlling the cooling rate after plating, the adhesion can be improved and the coating shedding phenomenon can be avoided.

[0051] By analyzing Example 3 and Comparative Example 3 in the above table, it can be seen that under the same chemical composition conditions, compared with Example 3, the temperature of the strip entering the quenching tank is increased from 80°C to 200°C, the impurity content of the water quality is increased from 0.6μg / ml to 1.6μg / ml, the water quality conductivity is increased from 6μs / cm to 16μs / cm, and the water temperature is increased from 50°C to 80°C. In the 2T bending test, 80% of the coating falls off, which indicates that water cooling has a greater effect on the adhesion of the coating. By reasonably controlling the cooling temperature of the strip and the water quality of the quenching tank, the adhesion can be significantly improved and the coating can be avoided.

[0052] By analyzing Example 3 and Comparative Example 4 in the above table, it can be seen that under the same chemical composition conditions, Comparative Example 4 has lower roughness Rpc value, passivation film weight, curing temperature, initial coating temperature and fine coating temperature than Example 3, and its 2T bending test shows 40% shedding phenomenon, which shows that by reasonably controlling the passivation and color coating process, the adhesion can be improved and the coating shedding phenomenon can be avoided.

[0053] from Figure 1 and Figure 2 It can be seen that Figure 1 This is the SEM image of the surface structure of the coating in Example 1. There are alloy phases such as MgZn2 at the grain boundaries, which have lower potential than pure Zn and higher surface activity; Figure 2 There is an enrichment peak of aluminum on the surface of Example 1. 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 only provides a specific exemplary description of the present invention. It should be noted that the specific implementation of the present invention is not limited to the above method. As long as various non-substantial improvements are made using the technical concept and technical solution of the present invention, or the technical concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. A pure zinc substrate coating, characterized in that: The chemical composition of the pure zinc substrate coating is calculated by weight percentage as follows: Al: 0.18-0.32%, Mg: 0.04-0.20%, and the rest is Zn and unavoidable impurities.

2. A pure zinc color-coated sheet, characterized in that: The pure zinc color-coated sheet comprises a steel substrate, a pure zinc substrate coating as claimed in claim 1, a passivation layer and a coating; 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 coating; The coating layer comprises a primary coating layer and a finishing coating layer, and the coating layer 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 / m 2 .

4. A method for preparing a pure zinc color-coated sheet, characterized in that: For preparing the pure zinc color-coated sheet according to claim 2 or 3, the preparation method comprises: Pre-treating the steel strip so that the surface properties of the steel strip meet the set requirements, thereby obtaining a steel matrix; Hot-dip galvanizing the steel substrate, and after cooling, obtain a first steel plate having a pure zinc substrate coating; Performing a passivation treatment and a first curing on the first steel plate to obtain a second steel plate having a passivation layer; The second steel plate is subjected to primary coating and secondary curing, and then to fine coating and tertiary curing to obtain a pure zinc color-coated plate.

5. The method for preparing the pure zinc color-coated sheet according to claim 4, characterized in that: Pre-treatment includes: The strip steel is subjected to skin finishing treatment to control the surface roughness Rpc value of the steel substrate to be greater than 100 pieces / cm.

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

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

8. The method for preparing the pure zinc color-coated sheet according to claim 7, characterized in that: The air cooling is followed by water cooling, and the conditions for the 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°C.

9. The method for preparing the pure zinc color-coated sheet according to claim 4, characterized in that: The temperature of the first curing is 120-140°C.

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

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