A blackening-resistant aluminized zinc-plated steel sheet and a control method

Abstract

The application discloses a blackening-resistant aluminized zinc-plated steel plate and a control method, and relates to the technical field of metal plate surface protection. The steel plate comprises a low-carbon steel base body, and the base body is provided with a zinc-aluminum-magnesium alloy plating layer, a chromium-free passivation conversion layer and a chromium-free fingerprint-resistant coating layer in sequence on both sides. The zinc-aluminum-magnesium alloy plating layer comprises 53-55% of aluminum, 43-44% of zinc, 1.5-1.6% of silicon and 1.6-2.0% of magnesium in percentage by mass. The precise proportioning realizes the micro-uniform distribution of multiple phases, and the whole-process process regulation and control and the two-coating and two-baking surface treatment system are combined, so that the problems of poor blackening resistance and insufficient corrosion resistance of the existing aluminized zinc-plated steel plate are solved, and the strict requirements on surface stability of household appliance backlights and high-end building outer facades can be met.

Description

Technical Field

[0001] This invention relates to the field of metal sheet surface protection technology, specifically to a blackening-resistant aluminum-zinc coated steel sheet and a control method thereon. Background Technology

[0002] Aluminized zinc-coated steel sheets, possessing both the passivation protection properties of aluminum and the sacrificial anode protection properties of zinc, are widely used in building envelopes, home appliance manufacturing, photovoltaic brackets, and other fields. However, as high-end applications increasingly demand longer service life, higher surface quality, and greater resistance to environmental aging, existing aluminized zinc-coated steel sheets are gradually revealing technical shortcomings: Insufficient coating control precision: The coating uniformity deviation of existing similar products is up to 15g / ㎡, and the coating thickness deviation is ±0.1mm. Uneven zinc spangle size leads to poor coating protection performance and localized early corrosion. Poor resistance to blackening: Under high temperature, high humidity and ultraviolet radiation, the surface of the existing aluminized zinc steel sheet is prone to oxidation and blackening. At the same time, the passivation layer and the fingerprint-resistant layer are not fully cured, and blackening and peeling are easy to occur during bending. It cannot meet the stringent requirements for surface stability of home appliance backlights, high-end building exteriors, etc. Currently, most research on the modification of aluminized zinc steel sheets focuses on optimizing single coating components or improving surface treatment processes. It fails to achieve synergistic optimization of coating components, microstructure, preparation processes, and surface treatment systems, and cannot simultaneously ensure the steel sheet's resistance to blackening and high corrosion resistance, making it difficult to meet the needs of high-end applications. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a blackening-resistant aluminized zinc-coated steel sheet and a control method therefor. The steel sheet includes a low-carbon steel substrate, on both sides of which are sequentially coated with a zinc-aluminum-magnesium alloy coating, a chromium-free passivation conversion layer, and a chromium-free fingerprint-resistant coating. The zinc-aluminum-magnesium alloy coating consists of 53%-55% aluminum, 43%-44% zinc, 1.5%-1.6% silicon, and 1.6%-2.0% magnesium by mass percentage. Through precise proportioning, a microscopic uniform distribution of the multi-phase components is achieved. Combined with full-process process control and a two-coat, two-bake surface treatment system, this invention solves the problems of poor blackening resistance and insufficient corrosion resistance of existing aluminized zinc-coated steel sheets, and can meet the stringent requirements for surface stability in home appliance backlights and high-end building exterior surfaces.

[0004] To solve the above-mentioned technical problems, the present invention provides the following technical solution: On one hand, a blackening-resistant aluminum-zinc coated steel sheet, comprising a low-carbon steel substrate, wherein the low-carbon steel substrate is sequentially coated with a zinc-aluminum-magnesium alloy coating, a chromium-free passivation conversion layer, and a chromium-free fingerprint-resistant coating on both sides, wherein: The chemical composition of the zinc-aluminum-magnesium alloy coating, by mass percentage, is: aluminum 53%-55%, zinc 43%-44%, silicon 1.5%-1.6%, magnesium 1.6%-2.0%, with the balance being impurities. Among them, aluminum provides long-term passivation protection, zinc provides sacrificial anode protection, and magnesium works synergistically with zinc and aluminum to reduce the coating consumption rate. The MgZn2 phase formed in the coating can form an insulating protective layer at the cut and cross-section, and the Mg2Si phase provides a barrier protection inside the coating to prevent moisture from penetrating the substrate. Silicon can refine the coating grains and improve the adhesion between the coating and the substrate. The uniformity deviation of the zinc-aluminum-magnesium alloy coating is controlled to be 0-3 g / m², and the coating thickness deviation is controlled to be ±0.03 mm.

[0005] Furthermore, the zinc-aluminum-magnesium alloy coating also contains trace elements with a total mass percentage of ≤0.1%, wherein the trace elements are at least one of strontium and vanadium, which are used to further optimize the microstructure of the coating, inhibit coating oxidation, and improve corrosion resistance and anti-blackening performance.

[0006] Furthermore, the low-carbon steel substrate is an ultra-low-carbon steel substrate, in which the mass percentage of carbon is ≤0.008%, the mass percentage of silicon is ≤0.03%, and the mass percentage of manganese is ≤0.2%. The content of the substrate elements can ensure the deep drawing performance of the substrate and the adhesion of the coating, and ensure that there is no cracking during processing and stamping.

[0007] Furthermore, the chromium-free passivation conversion layer is a titanium-zirconium-based chromium-free conversion layer with a dry film thickness of 0.1μm-0.3μm. The film-forming solution of the titanium-zirconium-based chromium-free conversion layer, by mass parts, includes: 5-15 parts of total content of fluorotitanic acid and fluorozirconic acid, 2-8 parts of nano-silica sol, 0.5-3 parts of rare earth cerium salt anti-blackening agent, and the balance of deionized water. The rare earth cerium salt anti-blackening agent is at least one of cerium nitrate, cerium sulfate, and cerium acetate. The titanium-zirconium chromium-free conversion layer can form a dense conversion film on the coating surface. Nano-silica sol fills the pores of the film layer, improving the film density. Rare earth cerium salts can inhibit the oxidation of the active phase of the coating, significantly improving the blackening resistance of the steel plate under high temperature and high humidity conditions.

[0008] Furthermore, the chromium-free fingerprint-resistant coating is a single-component water-based acrylic resin-based chromium-free fingerprint-resistant coating with a single-sided dry film thickness of 1.5μm-1.8μm. The film-forming liquid of the chromium-free fingerprint-resistant coating, by mass parts, includes: 35-45 parts of hydroxyl acrylic resin emulsion with a glass transition temperature Tg of 15-25℃ and a hydroxyl value of 50-70mgKOH / g, 8-10 parts of water-based amino curing agent, 1.5-2.5 parts of γ-aminopropyltriethoxysilane coupling agent, 1-1.5 parts of polyethylene wax lubricant, and the balance being deionized water.

[0009] Furthermore, the chemical composition of the zinc-aluminum-magnesium alloy coating, by mass percentage, is: aluminum 53%, zinc 43.4%, silicon 1.6%, magnesium 2%, with the balance being impurities.

[0010] On the other hand, a method for controlling blackening of galvanized steel sheets includes the following steps: The steps of this control method are as follows: S1 matrix pretreatment: Select a low-carbon steel hot-rolled plate, and obtain a low-carbon steel matrix strip of a predetermined thickness after pickling and cold rolling; S2 Electrolytic Cleaning: The low-carbon steel substrate strip is sent into a 9-segment cleaning section for deep cleaning, sequentially undergoing alkaline rinsing, alkaline brushing, electrolytic cleaning, alkaline brushing, electrolytic cleaning, alkaline brushing, water brushing, water rinsing, and water rinsing. During the cleaning process, three sets of magnetic filtration systems continuously remove iron powder from the cleaning solution to avoid secondary contamination of the board surface. The hydrogen and oxygen generated by electrolysis deeply peel off the oil and iron powder adsorbed on the board surface, ensuring the cleanliness of the board surface and providing a foundation for the adhesion of the coating. S3 Annealing Treatment: The deeply cleaned low-carbon steel substrate strip is sent into a horizontal annealing furnace for continuous annealing. The annealing furnace is equipped with an open flame section and a reduction section. The open flame section fully burns off the trace oil stains remaining on the plate surface, while the reduction section is purged with a nitrogen-hydrogen protective atmosphere to complete the recrystallization annealing of the steel strip and the reduction of the surface oxide layer, ensuring the substrate's forming performance and the coating's adhesion. During the annealing process, a zinc ash blocking device and a zinc ash filtering device are used to effectively prevent zinc ash formed when zinc vapor cools from adhering to the plate surface and causing quality defects. S4 hot-dip galvanizing treatment: The annealed low-carbon steel base strip is sent into a zinc pot with a pre-melting pot for hot-dip galvanizing. The zinc liquid in the zinc pot is a zinc-aluminum-magnesium alloy melt with a preset composition. After galvanizing, the coating thickness and uniformity are controlled by an air knife system controlled by the edge follower baffle and negative pressure zone to obtain a coated steel strip. S5 Post-plating cooling: The coated steel strip is rapidly cooled using a cooling system, with the cooling rate controlled at ≥15℃ / s, to refine the size of the coating grains and zinc flowers, while reducing the dendrite spacing, further improving the corrosion resistance of the coating. S6 Finishing and Straightening: The cooled coated steel strip is sequentially fed into a wet finishing machine and a wet straightening machine for processing. The finishing force of the wet finishing machine is controlled at 3000-5000kN, and the wet straightening machine adopts a two-bending and two-straightening process, with the elongation controlled at 0.5%-2%. S7 Two-coat Two-bake Surface Treatment: The plated steel strip after smoothing and straightening is sent into the two-coat two-bake treatment line. First, a chromium-free passivation coating is applied, and then the strip is dried and cured in the first oven to form a chromium-free passivation conversion layer. Then, a chromium-free fingerprint-resistant coating is applied, and the strip is dried and cured in the second oven to form a chromium-free fingerprint-resistant coating. S8 Finished Product Coiling: After surface treatment, the steel strip is electrostatically coated with oil and inspected on both sides, then coiled to obtain the finished product of aluminum zinc-coated steel sheet resistant to blackening.

[0011] Furthermore, in the S2 electrolytic cleaning process, two sets of high-efficiency electrolytic cleaning tanks connected in series are used, with the electrolytic current density controlled at 15-25 A / dm² and the cleaning solution temperature controlled at 55-65℃.

[0012] Furthermore, in the S4 hot-dip galvanizing process, the zinc pot temperature is controlled at 590-610℃, the steel strip temperature is controlled at 580-600℃, the immersion time is controlled at 3-8s, the air knife pressure in the air knife system is controlled at 0.03-0.08MPa, and the distance between the air knife and the steel strip is controlled at 8-15mm.

[0013] Furthermore, in the S7 two-coat two-bake surface treatment, the chromium-free passivation coating is applied by roller coating, the wet film thickness is controlled to be 1.0-2.0μm, and the drying and curing temperature of the first oven is controlled to be 80-100℃. The chromium-free fingerprint-resistant coating is applied by roller coating, with the wet film thickness controlled at 4-6μm, the drying and curing time controlled at 20-40s, and the drying and curing temperature of the second oven controlled at 110℃-130℃.

[0014] Compared with existing technologies, the new resistant aluminum-zinc coated steel sheet and its control method have the following advantages: This invention achieves a breakthrough in anti-blackening performance through three synergistic aspects: First, the coating composition is precisely optimized by synergistically regulating magnesium, silicon, and trace elements to inhibit the oxidation of the active phase in the coating and prevent the coating body from blackening. Second, rare earth cerium salt anti-blackening additives are added to the chromium-free passivation conversion layer to form a dense and stable passivation film that blocks the intrusion of water vapor and oxygen and inhibits interface oxidation and blackening. Third, a two-coat, two-bake process is adopted to ensure that the fingerprint-resistant resin is fully cured, avoiding blackening during bending during processing, while improving the coating's resistance to ultraviolet aging. It does not show obvious discoloration after long-term use and can meet the stringent requirements for surface stability in home appliance backlights, high-end building exteriors, etc.

[0015] This invention achieves a uniform distribution of aluminum-rich phase, zinc-rich phase, MgZn2 phase, and Mg2Si phase at the microscale through precise proportioning of multiple components in the coating. It constructs a four-fold protection system consisting of sacrificial protection of zinc, passivation protection of aluminum, synergistic enhancement of magnesium, and barrier protection of silicon. Corrosion products at the coating cut can form a dense self-healing layer, effectively protecting the cut edges, welds, and damaged parts of the coating. It is also suitable for the harsh corrosive environments of marine and livestock industries.

[0016] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0018] Figure 1 A flowchart illustrating the steps of a method for controlling blackening of galvanized steel sheets; Figure 2 This is a comparison image of the appearance of the blackening-resistant aluminum-zinc coated steel sheet prepared in Example 1 of the present invention and the conventional aluminum-zinc coated steel sheet in Comparative Example 1. Figure 3 This is a comparison diagram of the morphology of the corrosion products of Comparative Example 1 and Example 2 after cyclic corrosion test of the present invention. Detailed Implementation

[0019] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0020] To address the limitations of existing aluminized zinc-coated steel sheets, such as insufficient coating control precision, poor resistance to blackening, poor corrosion resistance, and inability to meet the stringent requirements of high-end applications, this invention proposes a blackening-resistant aluminized zinc-coated steel sheet and its control method. The steel sheet comprises a low-carbon steel substrate, with a zinc-aluminum-magnesium alloy coating, a chromium-free passivation conversion layer, and a chromium-free fingerprint-resistant coating sequentially laminated on both sides of the substrate. Figure 1 As shown, the control method steps are as follows: S1 matrix pretreatment: Select a low-carbon steel hot-rolled plate, and obtain a low-carbon steel matrix strip of a predetermined thickness after pickling and cold rolling; S2 Electrolytic Cleaning: The low-carbon steel base strip is sent into a 9-section cleaning section for deep cleaning, and then sequentially undergoes alkaline rinsing, alkaline brushing, electrolytic cleaning, alkaline brushing, electrolytic cleaning, alkaline brushing, water brushing, water rinsing, and water rinsing. S3 Annealing: The deep-cleaned low-carbon steel base strip is sent into a horizontal annealing furnace for continuous annealing. The annealing furnace is equipped with an open flame section and a reduction section. The open flame section fully burns off the trace oil stains remaining on the plate surface, and the reduction section is introduced with a nitrogen-hydrogen protective atmosphere to complete the recrystallization annealing of the steel strip and the reduction of the surface oxide layer. S4 hot-dip galvanizing treatment: The annealed low-carbon steel base strip is sent into a zinc pot with a pre-melting pot for hot-dip galvanizing. The zinc liquid in the zinc pot is a zinc-aluminum-magnesium alloy melt with a preset composition. After galvanizing, the coating thickness and uniformity are controlled by an air knife system controlled by the edge follower baffle and negative pressure zone to obtain a coated steel strip. S5 Post-plating cooling: The coated steel strip is rapidly cooled using a cooling system, with the cooling rate controlled at ≥15℃ / s; S6 Finishing and Straightening: The cooled coated steel strip is sequentially fed into a wet finishing machine and a wet straightening machine for processing. The finishing force of the wet finishing machine is controlled at 3000-5000kN, and the wet straightening machine adopts a two-bending and two-straightening process, with the elongation controlled at 0.5%-2%. S7 Two-coat Two-bake Surface Treatment: The plated steel strip after smoothing and straightening is sent into the two-coat two-bake treatment line. First, a chromium-free passivation coating is applied, and then the strip is dried and cured in the first oven to form a chromium-free passivation conversion layer. Then, a chromium-free fingerprint-resistant coating is applied, and the strip is dried and cured in the second oven to form a chromium-free fingerprint-resistant coating. S8 Finished Product Coiling: After surface treatment, the steel strip is electrostatically coated with oil and inspected on both sides, then coiled to obtain the finished product of aluminum zinc-coated steel sheet resistant to blackening.

[0021] This invention achieves a uniform distribution of microphases in the coating and multiple layers of protection through a precise ratio of zinc-aluminum-magnesium alloy (53%-55% aluminum, 43%-44% zinc, 1.5%-1.6% silicon, and 1.6%-2.0% magnesium), combined with a synergistic design of titanium-zirconium chromium-free passivation and water-based acrylic fingerprint-resistant coating, and precise control of the entire process. This results in a solution for blackening-resistant aluminum-zinc coated steel sheets that combines high resistance to blackening, high corrosion resistance, uniform coating, and excellent formability.

[0022] This invention is mainly applied to the field of metal sheet surface protection. It addresses the problems of existing aluminized zinc-coated steel sheets, such as easy blackening under high temperature and humidity, easy blackening and peeling of the coating during bending, and insufficient corrosion resistance consistency, making it difficult to meet the requirements of harsh environments such as home appliance backlights, high-end building facades, photovoltaic brackets, and marine and livestock industries. This invention significantly improves the blackening resistance and corrosion resistance of steel sheets through synergistic optimization of coating composition, rare earth cerium salt anti-blackening modification, a two-coat, two-bake curing process, and precise control of all process parameters. The coating uniformity deviation is 0-3g / ㎡, and the thickness deviation is ±0.03mm. There is no obvious discoloration after long-term use, and the cut edges and damaged areas can self-repair and protect. It is suitable for industrial production and application in high-end home appliances, building facades, photovoltaics, and harsh corrosion scenarios.

[0023] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments, comparative examples, and performance test verifications.

[0024] General basic conditions and unified testing standards Substrate material: All the examples and comparative examples below use ultra-low carbon steel substrate, whose chemical composition by mass percentage is: carbon ≤0.008%, silicon ≤0.03%, manganese ≤0.2%, phosphorus ≤0.015%, sulfur ≤0.008%, with the balance being iron and unavoidable impurities, to ensure the deep drawing performance of the substrate and the adhesion of the coating.

[0025] Production equipment: The production equipment used is a continuous hot-dip aluminized zinc galvanizing production line, equipped with a multi-stage electrolytic cleaning section, a horizontal annealing furnace, a zinc pot with a pre-melting pot, an air knife system with edge following baffles and negative pressure zone control, a wet finishing machine, a two-bending and two-straightening wet tension straightening machine, and a two-coating and two-drying roller coating production line.

[0026] Raw materials: All metal auxiliaries and chemical raw materials used are commercially available industrial-grade products that meet the mass production standards of the hot-dip galvanizing and metal sheet surface treatment industries.

[0027] Unified Explanation of Performance Testing and Evaluation Standards All samples in the examples and comparative examples were tested using completely consistent methods and evaluation criteria to ensure that the test data could be directly compared horizontally, as detailed below: Coating uniformity test: Using an X-ray coating thickness gauge, 12 test points are evenly selected on the steel plate surface to test the coating weight and thickness at each point, and calculate the uniformity deviation of coating weight and coating thickness deviation on the plate surface. Resistance to blackening test: High temperature and high humidity blackening test: Place the standard size sample in a constant temperature and humidity chamber at 85℃ and 85% relative humidity, and let it stand for 720 hours. Use a colorimeter to test the color difference ΔE on the sample surface before and after the test. The smaller the ΔE value, the better the blackening resistance. Blackening test of bending process: The sample is bent at 180° with the inner radius of the bend being the same as the thickness of the sample plate. Visually observe whether blackening or coating peeling occurs at the bend. UV aging and blackening test: A UVB-313 UV lamp was used, with an irradiance of 0.71 W / m². 2 The sample surface color difference ΔE was measured using a 4-hour irradiation and 4-hour condensation cycle after 1000 hours of testing. Corrosion resistance test: Neutral salt spray test: Performed in accordance with GB / T10125-2021 standard, record the time when 5% of the sample surface area of ​​white rust appears and the time when red rust first appears; Cutting corrosion resistance test: After mechanically cutting the sample, a neutral salt spray test is carried out. The time when red rust first appears at the cut edge is recorded to evaluate the cut self-healing protection performance of the coating. Forming performance and bonding strength test: Perform a 180° bending test according to GB / T31935-2015 standard, and visually observe whether the coating and plating show cracks or peeling; use a universal testing machine to test the elongation and cupping value of the specimen to evaluate the deep drawing performance of the specimen. Example 1

[0028] This embodiment provides a blackening-resistant aluminum-zinc coated steel sheet and its preparation method. By precisely setting the chemical composition of the zinc-aluminum-magnesium alloy coating, the formulation and thickness of the titanium-zirconium chromium-free passivation conversion layer and the water-based acrylic resin-based chromium-free fingerprint-resistant coating, and with a fully controlled preparation process, an aluminum-zinc coated steel sheet with excellent blackening resistance, high corrosion resistance, good formability and coating adhesion is prepared to verify the comprehensive performance of the present invention.

[0029] The blackening-resistant aluminum-zinc coated steel sheet described in this embodiment includes an ultra-low carbon steel substrate. The ultra-low carbon steel substrate has a zinc-aluminum-magnesium alloy coating, a titanium-zirconium chromium-free passivation conversion layer, and a single-component water-based acrylic resin-based chromium-free fingerprint-resistant coating sequentially laminated on both sides. The specific parameters of each layer are as follows: Zinc-aluminum-magnesium alloy coating: The chemical composition by mass percentage is: aluminum 53%, zinc 43.4%, silicon 1.6%, magnesium 2.0%, with the balance being unavoidable impurities; the coating weight per side is 75g / ㎡, the uniformity deviation of the plate surface is 2.1g / ㎡, and the coating thickness deviation is ±0.02mm.

[0030] Titanium-zirconium chromium-free passivation conversion layer: dry film thickness is 0.2μm; its film-forming solution, by mass parts, is composed of: 8 parts fluorotitanic acid, 4 parts fluorozirconic acid, 5 parts nano silica sol, 2 parts cerium nitrate, and the balance being deionized water.

[0031] One-component waterborne acrylic resin-based chromium-free fingerprint-resistant coating: single-sided dry film thickness is 1.6μm; the film-forming liquid, by mass parts, is composed of: 40 parts of hydroxyl acrylic resin emulsion with a glass transition temperature Tg of 20℃ and a hydroxyl value of 60mgKOH / g, 9 parts of waterborne amino curing agent, 2 parts of γ-aminopropyltriethoxysilane coupling agent, 1.2 parts of polyethylene wax lubricant, and the balance being deionized water.

[0032] The preparation method of the blackening-resistant aluminum-zinc coated steel sheet described in this embodiment includes the following steps: S1 matrix pretreatment: Select ultra-low carbon steel hot-rolled plate, and successively pickle to remove iron oxide scale and cold rolling to obtain a steel strip matrix with a thickness of 0.5mm; S2 Electrolytic Cleaning: The cold-rolled steel strip substrate is sent into a 9-section cleaning section for deep cleaning. The cleaning process is as follows: alkaline rinsing, alkaline brushing, electrolytic cleaning, alkaline brushing, electrolytic cleaning, alkaline brushing, water brushing, water rinsing, and water rinsing. During the cleaning process, three sets of magnetic filtration systems continuously circulate to remove iron powder from the cleaning solution, avoiding secondary contamination of the plate surface. Electrolytic cleaning uses two sets of electrolytic cleaning tanks connected in series, controlling the electrolytic current density at 20A / dm² and the cleaning solution temperature at 60℃. The hydrogen and oxygen generated by electrolysis deeply peel off the oil and iron powder adsorbed on the plate surface, ensuring the cleanliness of the plate surface. S3 Annealing Treatment: The deeply cleaned steel strip is sent to a horizontal annealing furnace for continuous annealing. The annealing furnace is equipped with an open flame section and a reduction section. The temperature of the open flame section is controlled at 850℃ to fully burn off the trace amounts of residual oil on the plate surface. The reduction section is purged with a nitrogen-hydrogen protective atmosphere, in which the hydrogen volume fraction is 5%. The temperature of the reduction section is controlled at 800℃ to complete the recrystallization annealing of the steel strip and the reduction of the surface oxide layer, ensuring the forming performance of the substrate and the adhesion of the coating. During the annealing process, the zinc ash blocking device and the zinc ash filtering device are activated simultaneously to prevent zinc ash formed when zinc vapor cools from adhering to the plate surface and causing quality defects. S4 Hot-dip galvanizing treatment: The annealed steel strip is fed into a zinc pot with a pre-melting pot for hot-dip galvanizing. The molten liquid in the zinc pot is a zinc-aluminum-magnesium alloy molten liquid with the above-mentioned proportions. The temperature of the zinc pot is controlled at 600℃, the temperature of the steel strip entering the pot is 590℃, and the immersion time of the steel strip in the zinc liquid is 5s. After the immersion is completed, the coating thickness and uniformity are adjusted by an air knife system controlled by a strip edge follower baffle and a negative pressure zone. The air knife pressure is controlled at 0.05MPa, and the distance between the air knife and the steel strip is 10mm, resulting in a coated steel strip. S5 Post-plating Cooling: The hot-dip galvanized steel strip with coating is rapidly cooled using a large air box cooling system, with the cooling rate controlled at 20℃ / s. The steel strip is rapidly cooled to room temperature, refining the size of the coating grains and zinc flowers, reducing the dendrite spacing, and improving the corrosion resistance of the coating. S6 finishing and straightening: The cooled coated steel strip is sequentially fed into a wet finishing machine and a wet straightening machine for processing. The finishing force of the wet finishing machine is controlled at 4000kN. The wet straightening machine adopts a two-bending and two-straightening process to control the elongation rate at 1.0%, thereby improving the shape and surface condition of the steel strip. S7 Two-coat, two-bake surface treatment: The steel strip after smoothing and straightening is sent to the two-coat, two-bake treatment line for surface treatment. First, a chromium-free passivation liquid is coated by roller coating, and the wet film thickness is controlled to be 1.5μm. It is then sent to the first oven, and the drying and curing temperature is controlled to be 85℃. After drying and curing, a titanium-zirconium chromium-free passivation conversion layer is formed. Then, a chromium-free fingerprint-resistant coating liquid is coated by roller coating, and the wet film thickness is controlled to be 5μm. It is then sent to the second oven, and the drying and curing temperature is controlled to be 120℃. The drying and curing time is 30s. After curing, a single-component water-based acrylic resin-based chromium-free fingerprint-resistant coating is formed. S8 Finished product winding: After the surface treatment, the steel strip is electrostatically oiled and inspected on both sides. Then, it is wound up to obtain the finished aluminum-zinc coated steel sheet resistant to blackening.

[0033] This embodiment achieves a uniform distribution of aluminum-rich phase, zinc-rich phase, MgZn2 phase, and Mg2Si phase at the microscale through precise component ratio in the zinc-aluminum-magnesium alloy coating, constructing a multi-layered protective system. Combined with a chromium-free passivation conversion layer containing rare-earth cerium salts and a fully cured chromium-free fingerprint-resistant coating, triple protection against blackening is achieved for the coating body, interface, and coating layer. Precise control of the entire preparation process ensures coating uniformity, adhesion, and formability. The blackening-resistant zinc-aluminum steel sheet prepared in this embodiment exhibits excellent comprehensive performance, such as… Figure 2 The appearance comparison between the blackening-resistant aluminized zinc-coated steel sheet and the control example is shown in Figure 1. Figure 2 (a) is the zinc-aluminum-magnesium steel plate prepared in this embodiment. Figure 2 (b) As a comparative example, the aluminum-zinc coated steel sheet has finer zinc flowers and better surface uniformity, which can meet the stringent requirements for surface stability and long service life in high-end home appliances, building exteriors and other scenarios. Example 2

[0034] This embodiment provides a blackening-resistant aluminum-zinc coated steel sheet and its preparation method. By adjusting the chemical composition, coating thickness and preparation process parameters of the zinc-aluminum-magnesium alloy coating, a blackening-resistant aluminum-zinc coated steel sheet is prepared to verify the performance stability and applicability of the technical solution of the present invention under different component ratios and process parameters.

[0035] The blackening-resistant aluminum-zinc coated steel sheet described in this embodiment includes an ultra-low carbon steel substrate. The ultra-low carbon steel substrate has a zinc-aluminum-magnesium alloy coating, a titanium-zirconium chromium-free passivation conversion layer, and a single-component water-based acrylic resin-based chromium-free fingerprint-resistant coating sequentially laminated on both sides. The specific parameters of each layer are as follows: Zinc-aluminum-magnesium alloy coating: The chemical composition by mass percentage is: aluminum 55%, zinc 41.9%, silicon 1.5%, magnesium 1.6%, with the balance being unavoidable impurities; the coating weight per side is 75g / ㎡, the uniformity deviation of the plate surface is 2.8g / ㎡, and the coating thickness deviation is ±0.025mm.

[0036] Titanium-zirconium chromium-free passivation conversion layer: dry film thickness is 0.1μm; its film-forming solution, by mass parts, is composed of: 3 parts fluorotitanic acid, 2 parts fluorozirconic acid, 2 parts nano silica sol, 0.5 parts cerium sulfate, and the balance being deionized water.

[0037] One-component waterborne acrylic resin-based chromium-free fingerprint-resistant coating: single-sided dry film thickness is 1.5μm; its film-forming liquid, by mass parts, is composed of: 35 parts of hydroxyl acrylic resin emulsion with a glass transition temperature Tg of 15℃ and a hydroxyl value of 50mgKOH / g, 8 parts of waterborne amino curing agent, 1.5 parts of γ-aminopropyltriethoxysilane coupling agent, 1 part of polyethylene wax lubricant, and the balance being deionized water.

[0038] The preparation method of the blackening-resistant aluminum-zinc coated steel sheet described in this embodiment includes the following steps: S1 matrix pretreatment: Select ultra-low carbon steel hot-rolled plate, and successively pickle to remove iron oxide scale and cold rolling to obtain a steel strip matrix with a thickness of 0.5mm; S2 Electrolytic Cleaning: The cold-rolled steel strip substrate is sent into a 9-section cleaning section for deep cleaning. The cleaning process is as follows: alkaline rinsing, alkaline brushing, electrolytic cleaning, alkaline brushing, electrolytic cleaning, alkaline brushing, water brushing, water rinsing, and water rinsing. During the cleaning process, three sets of magnetic filtration systems continuously circulate to remove iron powder from the cleaning solution, avoiding secondary contamination of the plate surface. The electrolytic cleaning uses two sets of electrolytic cleaning tanks connected in series, controlling the electrolytic current density at 15A / dm² and the cleaning solution temperature at 55℃. S3 Annealing Treatment: The deeply cleaned steel strip is sent to a horizontal annealing furnace for continuous annealing. The annealing furnace is equipped with an open flame section and a reduction section. The temperature of the open flame section is controlled at 850℃ to fully burn off the trace amounts of residual oil on the plate surface. The reduction section is purged with a nitrogen-hydrogen protective atmosphere, in which the hydrogen volume fraction is 5%. The temperature of the reduction section is controlled at 800℃ to complete the recrystallization annealing of the steel strip and the reduction of the surface oxide layer. During the annealing process, the zinc ash blocking device and the zinc ash filtering device are activated simultaneously. S4 Hot-dip galvanizing treatment: The annealed steel strip is fed into a zinc pot with a pre-melting pot for hot-dip galvanizing. The molten liquid in the zinc pot is a zinc-aluminum-magnesium alloy molten liquid with the above-mentioned proportions. The temperature of the zinc pot is controlled at 590℃, the temperature of the steel strip entering the pot is 580℃, and the immersion time of the steel strip in the zinc liquid is 8s. After the immersion is completed, the coating thickness and uniformity are adjusted by an air knife system controlled by a strip edge follower baffle and a negative pressure zone. The air knife pressure is controlled at 0.03MPa, and the distance between the air knife and the steel strip is 15mm, resulting in a coated steel strip. S5 Post-plating cooling: The hot-dip galvanized steel strip with coating is rapidly cooled using a large air box cooling system, with the cooling rate controlled at 15℃ / s, and the steel strip is rapidly cooled to room temperature. S6 Finishing and Straightening: The cooled coated steel strip is sequentially fed into a wet finishing machine and a wet straightening machine for processing. The finishing force of the wet finishing machine is controlled at 3000kN, and the wet straightening machine adopts a two-bending and two-straightening process, controlling the elongation rate at 0.5%. S7 Two-Coat Two-Baking Surface Treatment: The smoothed and straightened steel strip is sent to the two-coat two-baking treatment line for surface treatment. First, a chromium-free passivation liquid is coated by roller coating, and the wet film thickness is controlled to be 1.0 μm. It is then sent to the first oven, and the drying and curing temperature is controlled to be 80℃. After drying and curing, a titanium-zirconium chromium-free passivation conversion layer is formed. Then, a chromium-free fingerprint-resistant coating liquid is coated by roller coating, and the wet film thickness is controlled to be 4 μm. It is then sent to the second oven, and the drying and curing temperature is controlled to be 110℃. The drying and curing time is 40s. After curing, a single-component water-based acrylic resin-based chromium-free fingerprint-resistant coating is formed. S8 Finished Product Coiling: After surface treatment, the steel strip is electrostatically coated with oil, inspected on both sides, and then coiled to obtain the finished product of blackening-resistant aluminum-zinc coated steel sheet.

[0039] This embodiment adjusts the chemical composition of the coating, the coating thickness, and the preparation process parameters. The resulting aluminum-zinc coated steel sheet still maintains excellent coating uniformity, blackening resistance, corrosion resistance, and formability. This verifies the performance stability of the technical solution of the present invention under different parameter settings and can be adapted to the production needs of different mass production conditions. Example 3

[0040] This embodiment provides a blackening-resistant aluminum-zinc coated steel sheet and its preparation method. Strontium and vanadium trace elements are added to the zinc-aluminum-magnesium alloy coating. The remaining components, coating formula and preparation process are consistent with those in Example 1. This is used to verify the optimization and improvement effect of trace elements on the microstructure, blackening resistance and corrosion resistance of the coating.

[0041] The blackening-resistant aluminum-zinc coated steel sheet described in this embodiment includes an ultra-low carbon steel substrate. The ultra-low carbon steel substrate has a zinc-aluminum-magnesium alloy coating, a titanium-zirconium chromium-free passivation conversion layer, and a single-component water-based acrylic resin-based chromium-free fingerprint-resistant coating sequentially laminated on both sides. The specific parameters of each layer are as follows: Zinc-aluminum-magnesium alloy coating: The chemical composition by mass percentage is: aluminum 54%, zinc 42.8%, silicon 1.55%, magnesium 1.8%, strontium 0.05%, vanadium 0.05%, with the balance being unavoidable impurities; the coating weight per side is 75g / ㎡, the uniformity deviation of the plate surface is 1.8g / ㎡, and the coating thickness deviation is ±0.018mm.

[0042] Titanium-zirconium chromium-free passivation conversion layer: The dry film thickness and film-forming solution formulation are completely consistent with those in Example 1.

[0043] Single-component water-based acrylic resin chromium-free fingerprint-resistant coating: The single-sided dry film thickness and film-forming liquid formulation are completely consistent with those of Example 1.

[0044] The preparation method of the blackening-resistant aluminum-zinc coated steel sheet described in this embodiment is completely consistent with that in Embodiment 1 in all steps and process parameters.

[0045] In this embodiment, strontium and vanadium trace elements were added to the zinc-aluminum-magnesium alloy coating. The resulting aluminum-zinc coated steel sheet exhibited better coating uniformity, and its resistance to blackening under high temperature and humidity and ultraviolet aging environments, as well as its corrosion resistance under neutral salt spray environments, were further improved. This verifies that trace elements can optimize the microstructure of the coating, inhibit coating oxidation, and further enhance the comprehensive protective performance of the steel sheet. Example 4

[0046] This embodiment provides a blackening-resistant aluminum-zinc coated steel sheet and its preparation method. The process parameters of electrolytic cleaning, hot-dip galvanizing, cooling, finishing and straightening, and two coating and two baking processes in the preparation process are adjusted. The steel sheet structure, coating chemical composition, coating formula and thickness are consistent with those in Embodiment 1. This is used to verify the performance and mass production adaptability of the technical solution of the present invention under different process parameters.

[0047] The blackening-resistant aluminum-zinc coated steel sheet described in this embodiment has the same steel sheet structure, zinc-aluminum-magnesium alloy coating chemical composition, titanium-zirconium chromium-free passivation conversion layer and chromium-free fingerprint-resistant coating formulation and thickness as in Embodiment 1.

[0048] The preparation method of the blackening-resistant aluminum-zinc coated steel sheet described in this embodiment includes the following steps: S1 matrix pretreatment: completely consistent with Example 1; S2 Electrolytic Cleaning: The cold-rolled steel strip substrate is sent into a 9-section cleaning section for deep cleaning. The cleaning process and auxiliary equipment are the same as in Example 1. The electrolytic cleaning current density is controlled at 25A / dm² and the cleaning solution temperature is 65℃. S3 annealing treatment: completely consistent with Example 1; S4 Hot-dip galvanizing treatment: The annealed steel strip is fed into a zinc pot with a pre-melting pot for hot-dip galvanizing treatment. The molten liquid in the zinc pot is the same as in Example 1. The temperature of the zinc pot is controlled at 610°C, the temperature of the steel strip entering the pot is 600°C, and the immersion time of the steel strip in the zinc liquid is 3s. After the immersion is completed, the coating is adjusted by the air knife system. The air knife pressure is controlled at 0.08MPa, and the distance between the air knife and the steel strip is 8mm, to obtain a coated steel strip. S5 Post-plating cooling: The hot-dip galvanized steel strip with coating is rapidly cooled using a large air box cooling system, with the cooling rate controlled at 25℃ / s, and the steel strip is rapidly cooled to room temperature. S6 Finishing and Straightening: The cooled coated steel strip is sequentially fed into a wet finishing machine and a wet straightening machine for processing. The finishing force of the wet finishing machine is controlled at 5000kN, and the wet straightening machine adopts a two-bending and two-straightening process, controlling the elongation rate at 2.0%. S7 Two-Coat Two-Baking Surface Treatment: The smoothed and straightened steel strip is fed into the two-coat two-baking treatment line for surface treatment. First, a chromium-free passivation liquid is coated by roller coating, controlling the wet film thickness to 2.0 μm. It is then sent to the first oven, where the drying and curing temperature is controlled at 100℃. After drying and curing, a passivation conversion layer is formed. Next, a chromium-free fingerprint-resistant coating liquid is coated by roller coating, controlling the wet film thickness to 6 μm. It is then sent to the second oven, where the drying and curing temperature is controlled at 130℃, and the drying and curing time is 20 seconds. After curing, a fingerprint-resistant coating is formed. S8 Finished product winding: Completely consistent with Example 1.

[0049] This embodiment adjusts the process parameters of key steps in the preparation process. The resulting aluminized zinc steel sheet still maintains excellent comprehensive performance. The coating uniformity, blackening resistance, corrosion resistance and formability all meet the requirements of high-end applications. This verifies that the preparation method of the present invention can be adapted to a wide range of process parameter adjustments and has good mass production adaptability and process stability. Comparative Example 1

[0050] This comparative example provides a conventional aluminized zinc-coated steel sheet and its preparation method. It uses a conventional aluminized zinc coating without magnesium. The remaining coating formulations, preparation processes and parameters are completely consistent with those in Example 1. It is used to compare and verify the effect of zinc-aluminum-magnesium multi-element alloy coating on the steel sheet's resistance to blackening and corrosion.

[0051] The aluminum-zinc coated steel sheet described in this comparative example includes an ultra-low carbon steel substrate. The ultra-low carbon steel substrate has, sequentially, an aluminum-zinc coating, a titanium-zirconium chromium-free passivation conversion layer, and a single-component water-based acrylic resin chromium-free fingerprint-resistant coating on both sides. Specific parameters for each layer are as follows: Aluminized zinc coating: The chemical composition by mass percentage is: aluminum 55%, zinc 43.5%, silicon 1.5%, no magnesium, and the balance is unavoidable impurities; the coating weight per side is 75g / ㎡.

[0052] Titanium-zirconium chromium-free passivation conversion layer: The dry film thickness and film-forming solution formulation are completely consistent with those in Example 1.

[0053] Single-component water-based acrylic resin chromium-free fingerprint-resistant coating: The single-sided dry film thickness and film-forming liquid formulation are completely consistent with those of Example 1.

[0054] The preparation method of the aluminum-zinc coated steel sheet described in this comparative example is completely consistent with that in Example 1 in all steps and process parameters.

[0055] This comparative example uses a conventional aluminum-zinc coating without magnesium. The resulting steel plate coating exhibits significantly increased uniformity deviation. The color difference ΔE under high temperature, high humidity, and UV aging conditions is much higher than in Example 1. The time for white rust and red rust resistance under neutral salt spray conditions is significantly shortened, and the corrosion resistance of the cut surface is significantly reduced. This verifies the synergistic effect of magnesium with zinc, aluminum, and silicon in the zinc-aluminum-magnesium multi-element alloy coating, which can significantly improve the steel plate's resistance to blackening, corrosion resistance, and self-healing protection capability of the cut surface. Comparative Example 2

[0056] This comparative example provides an aluminized zinc steel sheet and its preparation method. A zinc-aluminum-magnesium coating with a magnesium content of 1.0% is used, which is lower than the set value in Example 1. The remaining components, coating formulation, preparation process and parameters are completely consistent with Example 1, and are used to compare and verify the effect of magnesium content on the protective performance of the coating.

[0057] The aluminum-zinc coated steel sheet described in this comparative example includes an ultra-low carbon steel substrate. The ultra-low carbon steel substrate has a zinc-aluminum-magnesium alloy coating, a titanium-zirconium chromium-free passivation conversion layer, and a single-component water-based acrylic resin-based chromium-free fingerprint-resistant coating sequentially laminated on both sides. The specific parameters of each layer are as follows: Zinc-aluminum-magnesium alloy coating: The chemical composition by mass percentage is: aluminum 53%, zinc 44.4%, silicon 1.6%, magnesium 1.0%, with the balance being unavoidable impurities; the coating weight per side is 75g / ㎡.

[0058] Titanium-zirconium chromium-free passivation conversion layer: The dry film thickness and film-forming solution formulation are completely consistent with those in Example 1.

[0059] Single-component water-based acrylic resin chromium-free fingerprint-resistant coating: The single-sided dry film thickness and film-forming liquid formulation are completely consistent with those of Example 1.

[0060] The preparation method of the aluminum-zinc coated steel sheet described in this comparative example is completely consistent with that in Example 1 in all steps and process parameters.

[0061] The zinc-aluminum-magnesium coating used in this comparative example has a magnesium content of 1.0%. The steel plate prepared with this coating showed a significant decrease in blackening resistance and corrosion resistance compared to Example 1. The color difference ΔE under high temperature and humidity and ultraviolet aging increased, and the time for resistance to white rust and red rust under neutral salt spray and the time for corrosion resistance at the cut surface were significantly shortened. This verifies that the magnesium content range set in this invention can fully utilize the synergistic protective effect of magnesium, zinc, aluminum and silicon to ensure the blackening resistance and corrosion resistance of the steel plate. Comparative Example 3

[0062] This comparative example provides an aluminum-zinc coated steel sheet and its preparation method. The chromium-free passivation conversion layer used does not contain rare earth cerium salt anti-blackening additives. The remaining coating components, coating formulations, preparation processes and parameters are completely consistent with those in Example 1. This is used to compare and verify the effect of rare earth cerium salts on improving the blackening resistance of the steel sheet.

[0063] The aluminum-zinc coated steel sheet described in this comparative example includes an ultra-low carbon steel substrate. The ultra-low carbon steel substrate has a zinc-aluminum-magnesium alloy coating, a titanium-zirconium chromium-free passivation conversion layer, and a single-component water-based acrylic resin-based chromium-free fingerprint-resistant coating sequentially laminated on both sides. The specific parameters of each layer are as follows: Zinc-aluminum-magnesium alloy coating: The chemical composition, thickness, and weight parameters are completely consistent with those of Example 1.

[0064] Titanium-zirconium chromium-free passivation conversion layer: dry film thickness is 0.2μm; its film-forming solution, by mass parts, is composed of: 8 parts fluorotitanic acid, 4 parts fluorozirconic acid, 5 parts nano silica sol, with deionized water as the balance, and no rare earth cerium salt anti-blackening additives added.

[0065] Single-component water-based acrylic resin chromium-free fingerprint-resistant coating: The single-sided dry film thickness and film-forming liquid formulation are completely consistent with those of Example 1.

[0066] The preparation method of the aluminum-zinc coated steel sheet described in this comparative example is completely consistent with that in Example 1 in all steps and process parameters.

[0067] The chromium-free passivation conversion layer used in this comparative example did not contain rare earth cerium salt anti-blackening additives. The resulting steel plate showed a significant decrease in blackening resistance under high temperature, high humidity, and ultraviolet aging conditions, with a color difference ΔE much higher than that in Example 1. This verifies that rare earth cerium salts can effectively inhibit the oxidation of the active phase of the coating, block water vapor and oxygen from penetrating the coating interface, and significantly improve the blackening resistance of the steel plate under harsh environments. Comparative Example 4

[0068] This comparative example provides an aluminum-zinc coated steel sheet and its preparation method. It adopts a single-coat, single-bake surface treatment process and does not use a two-coat, two-bake step-by-step curing process. The other coating components, total coating formula, preparation process and parameters are completely consistent with those in Example 1. It is used to compare and verify the effect of the two-coat, two-bake process on the steel sheet's resistance to blackening and the coating adhesion.

[0069] The aluminum-zinc coated steel sheet described in this comparative example includes an ultra-low carbon steel substrate, on both sides of which are sequentially coated with a zinc-aluminum-magnesium alloy coating and a surface composite coating. The specific parameters of each layer are as follows: Zinc-aluminum-magnesium alloy coating: The chemical composition, thickness, and weight parameters are completely consistent with those of Example 1.

[0070] Surface composite coating: The chromium-free passivation conversion layer film-forming liquid and the chromium-free fingerprint-resistant coating film-forming liquid in Example 1 are mixed according to the ratio, coated and cured in one step to form a total dry film thickness of 1.8μm.

[0071] The preparation method of the aluminum-zinc coated steel sheet described in this comparative example is completely consistent with that in Example 1, except for the surface treatment process. The surface treatment process adopts a single coating and baking process, specifically: the steel strip after smoothing and straightening is coated with a mixed coating liquid by roller coating, the wet film thickness is controlled to be 6.5 μm, and it is sent into the oven. The drying and curing temperature is controlled to be 120℃ and the drying and curing time is 30s. After curing, a surface composite coating is formed.

[0072] This comparative example uses a single-coat, single-bake surface treatment process. The steel plate prepared in this example shows obvious blackening and localized coating peeling after bending. The blackening resistance and corrosion resistance under high temperature, high humidity and ultraviolet aging environments are significantly lower than those in Example 1. This verifies that the two-coat, two-bake step-by-step curing process used in this invention can ensure that the passivation layer and the fingerprint-resistant layer are fully cured, significantly improving the coating's adhesion, processing resistance and blackening resistance, and avoiding the problems of blackening and coating peeling during bending. Comparative Example 5

[0073] This comparative example provides an aluminum-zinc coated steel sheet and its preparation method. The cooling rate of the post-coating cooling process is 10℃ / s, which is lower than the set value of Example 1. The other coating components, coating formulations, preparation processes and parameters are completely consistent with Example 1. This is used to compare and verify the influence of the post-coating cooling rate on the microstructure and protective performance of the coating.

[0074] The aluminum-zinc coated steel sheet described in this comparative example has the same steel sheet structure, zinc-aluminum-magnesium alloy coating chemical composition, titanium-zirconium chromium-free passivation conversion layer and chromium-free fingerprint-resistant coating formulation and thickness as in Example 1.

[0075] The preparation method of the aluminum-zinc coated steel sheet described in this comparative example is completely consistent with that of Example 1, except for the post-coating cooling process. The post-plating cooling process is as follows: the hot-dip galvanized steel strip with coating is cooled using a large air box cooling system, with the cooling rate controlled at 10℃ / s, until the steel strip is cooled to room temperature.

[0076] The comparative example used a post-plating cooling rate of 10℃ / s. The resulting steel plate had coarse grain and zinc flower size, increased coating uniformity deviation, and decreased corrosion resistance and formability compared to Example 1. Slight cracking occurred in the coating after bending. This verifies that the cooling rate set in this invention can effectively refine the coating grains, reduce dendrite spacing, and improve the coating's corrosion resistance, formability, and adhesion.

[0077] Performance Test Results and Analysis The steel plate samples prepared in all the above embodiments and comparative examples were subjected to performance tests according to a unified testing standard. Through horizontal comparison of multiple core indicators, the comprehensive performance and technical advantages of the present invention were systematically verified. The test results are as follows: (1) To verify the coating control accuracy of each embodiment and comparative example of the present invention, the uniformity deviation of the coating weight and the coating thickness deviation of all samples were systematically tested according to a unified standard. The test results are shown in Table 1 below:

[0078] Table 1: Results of Coating Uniformity Test (2) The blackening resistance directly determines the surface stability and appearance retention of the steel plate under high temperature and humidity storage, outdoor service under ultraviolet irradiation, bending and stamping processing, etc. It is also the core assessment requirement for high-end home appliances, building exteriors and other scenarios. In order to systematically verify the blackening resistance effect of the technical solution of this invention, three blackening resistance tests were carried out simultaneously on all samples: high temperature and humidity static placement, ultraviolet aging cycle, and bending processing, which fully covered the core blackening scenarios in actual use. The test results are shown in Table 2 below:

[0079] Table 2: Test Results of Resistance to Blackening (3) Corrosion resistance directly determines the service life, environmental adaptability, and long-term reliability of the product. Among them, the corrosion resistance of the cut surface is used to verify the self-repair capability of the coating. In order to fully verify the overall corrosion resistance and self-repair protection capability of the damaged parts of the technical solution of this invention, neutral salt spray test and cut surface corrosion resistance test were carried out on all samples. The key time points of 5% white rust and the first appearance of red rust on the sample surface were accurately recorded. The test results are shown in Table 3 below:

[0080] Table 3: Results of Corrosion Resistance Tests As shown in Table 3, the cut corrosion resistance of the embodiments of the present invention is far superior to that of the comparative aluminized zinc products. The core reason for this is the self-healing ability of the coating of the present invention. After cyclic corrosion testing, the morphology of the cut corrosion products of the zinc-aluminum-magnesium coatings of Comparative Example 1 and Example 1 was compared. Figure 3 As shown, where, Figure 3 (a) is the edge corrosion product of a conventional aluminum-zinc coating, a comparative example. It exhibits a coarse and loose structure, failing to effectively prevent the intrusion of corrosive media; (attached) Figure 3 (b) is the edge corrosion product of the zinc-aluminum-magnesium coating in Embodiment 1 of the present invention. It has a fine and dense structure, which can cover the exposed steel base of the cut, slow down the corrosion process, and achieve excellent self-repair protection effect of the cut.

[0081] (4) Forming performance and coating adhesion are key indicators to ensure that the steel plate does not crack or peel off during complex processing such as stamping, bending, and rolling. They directly determine the processing adaptability and application range of the product. To verify the actual processing application performance of the steel plate of this invention, the elongation and cupping value of all samples, as well as the coating adhesion under 180° bending conditions, were systematically tested. The test results are shown in Table 4 below:

[0082] Table 4: Test Results of Molding Performance and Bonding Strength Test Result Analysis Regarding coating uniformity: the coating surface uniformity deviation of all embodiments is ≤3g / ㎡, and the coating thickness deviation is ≤±0.03mm, which is far superior to the conventional aluminum-zinc coated products of Comparative Example 1, effectively solving the problems of poor coating uniformity and insufficient protective performance consistency of existing products.

[0083] Regarding the resistance to blackening: the color difference ΔE under high temperature and high humidity and ultraviolet aging in all embodiments is much lower than that in the comparative examples, and there is no blackening or peeling during bending. Through the comparison of comparative examples one, three and four, it can be verified that the zinc-aluminum-magnesium multi-element coating, rare earth cerium salt passivation layer and two-coat two-baking curing process work together to achieve the excellent resistance to blackening of the present invention, and solve the technical pain points of easy oxidation and blackening and bending blackening of existing products.

[0084] In terms of corrosion resistance: the neutral salt spray resistance time to white rust, red rust, and the time to appearance of red rust at the cut were all more than 3 times longer than those of conventional aluminum-zinc coated products. This verifies that the multi-layer protection system constructed by the MgZn2 phase and Mg2Si phase in the zinc-aluminum-magnesium alloy coating can achieve excellent overall corrosion resistance and self-healing protection at the cut, making it suitable for use in harsh corrosive environments.

[0085] In terms of forming performance and bonding strength: all embodiments have excellent deep drawing forming performance, with no coating cracking or peeling during 180° bending, which verifies that the ultra-low carbon steel substrate, silicon grain refinement, and precise control of the entire process can effectively ensure the forming performance and coating bonding strength of the steel plate, meeting the needs of complex stamping processing.

[0086] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A blackening-resistant aluminized zinc-coated steel sheet, characterized in that, The aluminized zinc-coated steel sheet comprises a low-carbon steel substrate, on both sides of which are sequentially coated with a zinc-aluminum-magnesium alloy coating, a chromium-free passivation conversion layer, and a chromium-free fingerprint-resistant coating, wherein: The chemical composition of the zinc-aluminum-magnesium alloy coating, by mass percentage, is: aluminum 53%-55%, zinc 43%-44%, silicon 1.5%-1.6%, magnesium 1.6%-2.0%, with the balance being impurities; The uniformity deviation of the zinc-aluminum-magnesium alloy coating is controlled to be 0-3 g / m², and the coating thickness deviation is controlled to be ±0.03 mm.

2. The aluminized zinc-coated steel sheet resistant to blackening according to claim 1, characterized in that, The zinc-aluminum-magnesium alloy coating also contains trace elements with a total mass percentage of ≤0.1%, wherein the trace elements are at least one of strontium and vanadium.

3. The aluminized zinc-coated steel sheet resistant to blackening according to claim 1, characterized in that, The low-carbon steel matrix is ​​an ultra-low-carbon steel matrix, in which the mass percentage of carbon is ≤0.008%, the mass percentage of silicon is ≤0.03%, and the mass percentage of manganese is ≤0.2%.

4. The aluminized zinc-coated steel sheet resistant to blackening according to claim 1, characterized in that, The chromium-free passivation conversion layer is a titanium-zirconium chromium-free conversion layer with a dry film thickness of 0.1μm-0.3μm. The film-forming solution of the titanium-zirconium chromium-free conversion layer, by mass parts, includes: 5-15 parts of total content of fluorotitanic acid and fluorozirconic acid, 2-8 parts of nano silica sol, 0.5-3 parts of rare earth cerium salt anti-blackening agent, and the balance of deionized water. The rare earth cerium salt anti-blackening agent is at least one of cerium nitrate, cerium sulfate, and cerium acetate.

5. The aluminized zinc-coated steel sheet resistant to blackening according to claim 1, characterized in that, The chromium-free fingerprint-resistant coating is a single-component water-based acrylic resin-based chromium-free fingerprint-resistant coating with a single-sided dry film thickness of 1.5μm-1.8μm. The film-forming liquid of the chromium-free fingerprint-resistant coating, by mass parts, includes: 35-45 parts of hydroxyl acrylic resin emulsion with a glass transition temperature Tg of 15-25℃ and a hydroxyl value of 50-70mgKOH / g, 8-10 parts of water-based amino curing agent, 1.5-2.5 parts of γ-aminopropyltriethoxysilane coupling agent, 1-1.5 parts of polyethylene wax lubricant, and the balance being deionized water.

6. The aluminized zinc-coated steel sheet resistant to blackening according to claim 1, characterized in that, The chemical composition of the zinc-aluminum-magnesium alloy coating, by mass percentage, is: aluminum 53%, zinc 43.4%, silicon 1.6%, magnesium 2%, with the balance being impurities.

7. A method for controlling blackening of galvanized steel sheet, applicable to the galvanized steel sheet with blackening resistance as described in any one of claims 1-6, characterized in that, The steps of this control method are as follows: S1 matrix pretreatment: Select a low-carbon steel hot-rolled plate, and obtain a low-carbon steel matrix strip of a predetermined thickness after pickling and cold rolling; S2 Electrolytic Cleaning: The low-carbon steel base strip is sent into a 9-section cleaning section for deep cleaning, and then sequentially undergoes alkaline rinsing, alkaline brushing, electrolytic cleaning, alkaline brushing, electrolytic cleaning, alkaline brushing, water brushing, water rinsing, and water rinsing. S3 Annealing: The deep-cleaned low-carbon steel base strip is sent into a horizontal annealing furnace for continuous annealing. The annealing furnace is equipped with an open flame section and a reduction section. The open flame section burns off the trace oil stains remaining on the plate surface, and the reduction section is introduced with a nitrogen and hydrogen protective atmosphere to complete the recrystallization annealing of the steel strip and the reduction of the surface oxide layer. S4 hot-dip galvanizing treatment: The annealed low-carbon steel base strip is sent into a zinc pot with a pre-melting pot for hot-dip galvanizing. The zinc liquid in the zinc pot is a zinc-aluminum-magnesium alloy melt with a preset composition. After galvanizing, the coating thickness and uniformity are controlled by an air knife system controlled by the edge follower baffle and negative pressure zone to obtain a coated steel strip. S5 Post-plating cooling: The coated steel strip is rapidly cooled using a cooling system, with the cooling rate controlled at ≥15℃ / s; S6 Finishing and Straightening: The cooled coated steel strip is sequentially fed into a wet finishing machine and a wet straightening machine for processing. The finishing force of the wet finishing machine is controlled at 3000-5000kN, and the wet straightening machine adopts a two-bending and two-straightening process, with the elongation controlled at 0.5%-2%. S7 Two-coat Two-bake Surface Treatment: The plated steel strip after smoothing and straightening is sent into the two-coat two-bake treatment line. First, a chromium-free passivation coating is applied, and then the strip is dried and cured in the first oven to form a chromium-free passivation conversion layer. Then, a chromium-free fingerprint-resistant coating is applied, and the strip is dried and cured in the second oven to form a chromium-free fingerprint-resistant coating. S8 Finished Product Coiling: After surface treatment, the steel strip is electrostatically coated with oil and inspected on both sides, then coiled to obtain the finished product of aluminum zinc-coated steel sheet resistant to blackening.

8. The method for controlling blackening of galvanized steel sheet according to claim 7, characterized in that, In the S2 electrolytic cleaning process, two sets of high-efficiency electrolytic cleaning tanks are connected in series. The electrolytic current density is controlled at 15-25A / dm², and the cleaning solution temperature is controlled at 55-65℃.

9. The method for controlling blackening of galvanized steel sheet according to claim 7, characterized in that, In the S4 hot-dip galvanizing process, the zinc pot temperature is controlled at 590-610℃, the steel strip temperature is controlled at 580-600℃, the immersion time is controlled at 3-8s, the air knife pressure in the air knife system is controlled at 0.03-0.08MPa, and the distance between the air knife and the steel strip is controlled at 8-15mm.

10. The method for controlling blackening of galvanized steel sheet according to claim 7, characterized in that, In the S7 two-coat two-bake surface treatment, the chromium-free passivation coating is applied by roller coating, the wet film thickness is controlled to be 1.0-2.0μm, and the drying and curing temperature of the first oven is controlled to be 80-100℃. The chromium-free fingerprint-resistant coating is applied by roller coating, with the wet film thickness controlled at 4-6μm, the drying and curing time controlled at 20-40s, and the drying and curing temperature of the second oven controlled at 110℃-130℃.

Images