High-adhesion nano-black film hot-dip galvanized steel sheet and method for manufacturing the same

Abstract

The application relates to the field of metal surface treatment, and particularly discloses a high-adhesion nano black film hot galvanized steel plate and a preparation method thereof. The high-adhesion nano black film hot galvanized steel plate comprises a steel base body, a pure zinc layer formed on the surface of the steel base body, and a nano black film layer covering the surface of the pure zinc layer; the interface between the pure zinc layer and the nano black film layer has a micro-nano hierarchical anchoring structure, which is composed of micron-level pits distributed on the surface of the pure zinc layer and nano-level rough textures embedded in the inner wall of the micron-level pits and the surface of the pure zinc layer. The high-adhesion nano black film hot galvanized steel plate has the advantages of high interface adhesion between the hot galvanized layer and the nano black film and good durability.

Description

Technical Field

[0001] This application relates to the field of metal surface treatment, and more specifically, it relates to a hot-dip galvanized steel sheet with high adhesion nano black film and its preparation method. Background Technology

[0002] Hot-dip galvanized steel sheets are widely used in building curtain walls, appliance housings, and automobile manufacturing due to their excellent corrosion resistance. To meet aesthetic requirements or specific heat absorption and light-shielding functions, a black organic coating, namely a nano black film, is usually applied to the surface of the hot-dip galvanized layer.

[0003] However, after galvanizing, the zinc layer of traditional hot-dip galvanized steel sheets exhibits a microscopically smooth surface and low chemical activity. In existing coating processes, the bonding between the organic black film and the zinc layer relies primarily on intermolecular forces (van der Waals forces), lacking mechanical interlocking points. This makes the black film highly susceptible to brittle cracking or large-area peeling during subsequent secondary processing such as stamping, bending, or deep suction, severely impacting the product's lifespan and visual appearance.

[0004] Therefore, how to significantly improve the interfacial adhesion between the hot-dip galvanized layer and the nano black film is a core technical problem that urgently needs to be solved in the field of high-performance coated steel sheets. Summary of the Invention

[0005] To address the aforementioned issues, this application provides a hot-dip galvanized steel sheet with high adhesion nano black film and its preparation method.

[0006] The first part of this application provides a high-adhesion nano-black film hot-dip galvanized steel sheet using the following technical solution: A high-adhesion nano-black film hot-dip galvanized steel sheet includes a steel substrate, a pure zinc layer formed on the surface of the steel substrate, and a nano-black film layer covering the surface of the pure zinc layer; wherein, the interface between the pure zinc layer and the nano-black film layer has a micro-nano hierarchical anchoring structure, the micro-nano hierarchical anchoring structure being composed of micron-level pits distributed on the surface of the pure zinc layer and nano-level rough textures embedded in the inner walls of the micron-level pits and the surface of the pure zinc layer.

[0007] By employing the above technical solution, a hierarchical anchoring structure with "micron-level pits embedded with nano-level rough textures" is constructed at the interface between the pure zinc layer and the nano-black film. This fundamentally changes the shortcomings of traditional hot-dip galvanized sheets, which have excessively smooth surfaces and lack effective anchoring points. This hierarchical structure significantly increases the specific surface area of ​​the interface, allowing the organic black film coating to deeply penetrate and fill the micro- and nano-pores during the curing process. This "mechanical interlocking" effect transforms the originally fragile intermolecular forces into a strong physical interlocking force. This enables the black film layer to deform synchronously with the substrate without brittle cracking or large-area peeling when the steel sheet is subjected to strong mechanical stresses such as stamping, bending, or shearing, greatly improving the product's processing resistance.

[0008] Optionally, the pure zinc layer and the nano black film layer may further contain a chemical chelation conversion film generated by the in-situ reaction of an organic chelating agent with zinc ions, the thickness of which is 10 nm to 100 nm.

[0009] By employing the above technical solution, the multidentate organic chelating agent forms a chemically stable chelate with zinc ions in situ through its active functional groups. One end of the chelate is tightly anchored to the zinc substrate, while the exposed polar groups at the other end can form covalent bonds or strong hydrogen bonds with the resin matrix in the black film. The extremely thin thickness of 10nm-100nm ensures sufficient modification of the interfacial polarity while avoiding increased internal stress or brittle fracture caused by excessively thick conversion films, thereby significantly enhancing the long-term chemical stability of the interface and its resistance to the spread of electrochemical corrosion.

[0010] Secondly, this application provides a method for preparing a hot-dip galvanized steel sheet with high adhesion nano black film as follows: A method for preparing a hot-dip galvanized steel sheet with high adhesion nano black film includes the following steps: (1) Degreasing, pickling, washing and drying are performed on cold-rolled steel sheets; (2) The pretreated steel plate is immersed in molten zinc liquid for hot-dip galvanizing. After being drawn out, the zinc layer thickness is controlled by air knife and cooled to form a pure zinc layer on the surface of the steel substrate. (3) The surface treatment liquid is sprayed onto the surface of the pure zinc layer after being nano-atomized. After reacting for 5 to 15 seconds, it is washed with water and dried. The micro-nano hierarchical anchoring structure is constructed in situ on the surface of the pure zinc layer using the surface treatment liquid. (4) A nano black film coating is applied to the surface of the pure zinc layer after step (3) using a roller coating process, so that the nano black film coating fills into the gaps of the micro-nano graded anchoring structure; (5) Heating and curing to make the nano black film layer tightly bonded to the pure zinc layer.

[0011] By employing the above-mentioned technical solution, the treatment solution is uniformly dispersed on the surface of the pure zinc layer in the form of tiny droplets through nano-atomization spraying. This enables controllable local micro-reactions to be achieved in a very short time. Nano-atomization ensures that the surface treatment solution covers the zinc layer surface with a highly uniform amount of fine droplets, avoiding the localized excessive corrosion or "leakage" that easily occurs in traditional immersion processes. This method can precisely induce the formation of a graded anchoring structure in situ without damaging the overall anti-corrosion thickness of the zinc layer, providing an ideal morphological basis for the subsequent stable bonding of the nano-black film.

[0012] Optionally, the surface treatment liquid in step (3) comprises the following components by mass percentage: A mixture of inorganic acids, ranging from 0.5% to 2.0%. 0.1%~0.5% of multidentate organic chelating agents; 0.05%~0.1% of nanocrystal seed inducer; 0.01%~0.03% aqueous surfactant; The remainder is deionized water.

[0013] By employing the above technical solution, the inorganic acid is responsible for opening the oxide layer and initiating etching, the multi-toothed organic chelating agent is responsible for surface polarity modification and reaction rate control, the nanocrystal seeds are pre-implanted as "anchor points," and the surfactant ensures that the droplets can completely wet the micron-level grain boundary gaps. This compositional formula forms a dynamically balanced micro-reaction field on the zinc layer surface, ensuring that the etching depth is sufficient to form mechanical interlocking while maintaining the integrity of the zinc layer surface through chemical inhibition. Thus, it improves adhesion without sacrificing the original anti-corrosion performance of the steel plate.

[0014] Optionally, the inorganic acid mixture is composed of nitric acid and phosphoric acid mixed in a mass ratio of 1:2 to 1:4; the multidentate organic chelating agent is at least one of phytic acid, sodium phytate, and ethylenediaminetetraacetic acid; and the nanocrystal seed inducer is colloidal silica with an average particle size of 20 nm to 50 nm.

[0015] By employing the above technical solution, the nitric acid and phosphoric acid compound system utilizes the strong oxidizing properties of nitric acid to rapidly initiate the reaction, and the corrosion-inhibiting properties of phosphoric acid to form a preliminary phosphate microstructure, preventing excessive thinning of the zinc layer. Organic chelating agents such as phytic acid possess multi-coordinating atoms, enabling the formation of more stable chelate rings than ordinary monodentate ligands, significantly enhancing interfacial bonding energy. Colloidal silica, acting as a nanocrystal seed, is deposited on the inner wall of the micropits, increasing the geometric complexity and frictional resistance of the interface, allowing the black film to solidify as if rooted within the zinc layer.

[0016] Optionally, in step (3), the median diameter D50 of the surface treatment liquid droplets after nano-atomization is 10μm~50μm, and the spraying pressure is 0.2MPa~0.5MPa.

[0017] By employing the above technical solution, micro-droplets of 10-50 μm possess an extremely large specific surface area, enabling rapid reaction on the pure zinc layer surface with uniform heat and mass transfer. This avoids the uneven deep corrosion pits caused by excessively large droplets leading to localized liquid accumulation. Simultaneously, droplets of this size, under a spraying pressure of 0.2-0.5 MPa, possess sufficient impact kinetic energy to overcome surface tension and spread, without causing mechanical damage to the newly formed micro / nano structures. This combination of parameters ensures the formation of a uniform and controllable micro / nano hierarchical anchoring structure within 5-15 seconds, while consuming minimal processing solution, easily removing residues through water washing, and improving production efficiency.

[0018] Optionally, the nano-black film coating in step (4) comprises, by mass percentage, the following components: 30%~50% of film-forming resin, wherein the film-forming resin is a modified polyurethane resin or an epoxy resin; 5%~15% of nano carbon black, and the average particle size of the nano carbon black is ≤100nm; 1%~3% silane coupling agent; The remainder is solvent.

[0019] By employing the above technical solution, the nano-carbon black particles are extremely small, enabling them to easily penetrate and fill the nanoscale rough texture of the inner walls of micron-sized pits, forming a continuous, void-free interfacial contact and avoiding adhesion degradation caused by interfacial porosity. One end of the silane coupling agent molecule can hydrolyze and react with the hydroxyl groups on the zinc layer or chemical conversion film to form a covalent bond (Si-O-Zn), while the other end's organic functional groups crosslink with the film-forming resin, significantly increasing the contribution of chemical bonding. The film-forming resin provides the coating with the main mechanical properties, corrosion resistance, and black appearance. The combination of these three elements allows the coating to achieve strong chemical bonding while physically filling and anchoring the structure, ultimately obtaining a nano-black film layer with extremely high adhesion.

[0020] In summary, this application has the following beneficial effects: 1. Due to the technical feature of constructing a micro-nano hierarchical anchoring structure between the pure zinc layer and the nano black film layer, this structure significantly increases the interfacial specific surface area through micron-level pits and embedded nano-level rough textures, so that the nano black film coating forms a deep mechanical interlock after curing, and the interfacial bonding force is upgraded from intermolecular force to physical interlocking force. This effectively solves the problem of easy cracking and peeling of the black film in traditional hot-dip galvanized steel sheets during stamping, bending and other processing, and greatly improves the processing tolerance of the product.

[0021] 2. In this application, a chemical chelation conversion film with a thickness of 10nm to 100nm is preferably set between the pure zinc layer and the nano black film layer. The conversion film is generated by the in-situ reaction of an organic chelating agent with zinc ions. One end of the film is anchored to the zinc layer, and the other end forms a covalent bond or hydrogen bond with the nano black film resin matrix. This further introduces strong chemical bonding on the basis of physical interlocking, which significantly enhances the long-term chemical stability of the interface and the ability to resist the spread of electrochemical corrosion.

[0022] 3. The method of this application sprays a surface treatment liquid of specific components onto the surface of a pure zinc layer after nano-atomization. It utilizes micro-droplets (D50 of 10μm~50μm) to achieve uniform and controllable local micro-reactions, inducing the formation of a uniform micro-nano hierarchical anchoring structure in situ within 5~15 seconds. This avoids the problem of local over-corrosion in traditional immersion processes, and the treatment liquid consumption is low and easy to clean, providing an ideal adhesion morphology basis for the nano black film and achieving a balance between high adhesion and high production efficiency. Detailed Implementation

[0023] The following detailed description of this application is provided in conjunction with the embodiments. It should be noted that: unless otherwise specified, the conditions in the following embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Unless otherwise specified, the raw materials used in the following embodiments are all from commercially available sources.

[0024] Example 1 This embodiment provides a method for preparing a hot-dip galvanized steel sheet with high adhesion nano black film: (1) Substrate pretreatment: Select DC01 cold-rolled steel plate with a thickness of 0.8mm, and perform alkaline degreasing at 60℃ (the degreasing agent is a 3%-5% sodium hydroxide solution containing surfactant), pickling with 15% hydrochloric acid to remove surface oxide scale, three-stage rinsing with deionized water and hot air drying in sequence.

[0025] (2) Hot-dip galvanizing: The pretreated steel plate is immersed in molten pure zinc liquid at 450℃ (zinc content ≥99.995%). When it is taken out, the zinc layer thickness is controlled to be 15-20μm by air knife. Then, it is cooled to room temperature by air cooling combined with water mist.

[0026] (3) Interface micro / nano structuring treatment: Surface treatment solution preparation: The inorganic acid mixture is premixed with 68% industrial nitric acid and 85% industrial phosphoric acid at a mass ratio of 1:3. The multi-toothed organic chelating agent is a 50% (w / w) aqueous solution of phytic acid (analytical grade). The nanocrystal seed inducer is a weakly basic colloidal silica (30% solid content, average particle size 35nm, stable type). The aqueous surfactant is a perfluoroalkyl polyoxyethylene ether (nonionic, 100% active). The proportions (by mass percentage of active components) are: inorganic acid mixture 1.25%, phytic acid 0.3%, colloidal silica 0.075%, surfactant 0.02%, and the balance being deionized water.

[0027] Using a high-pressure ultrasonic atomizing nozzle, the treatment liquid is nano-atomized (the median droplet diameter D50 is 30μm) at a pressure of 0.35MPa and sprayed evenly onto the zinc layer surface. After reacting for 10 seconds, it is quickly rinsed with deionized water and dried with hot air.

[0028] (4) Coating with Nano Black Film: A nano black film coating is applied to the treated surface using a roller coating method. The components are: blocked isocyanate-modified polyurethane resin (solid content 40%, hydroxyl value 50-80); high-pigment nano carbon black (average particle size 50nm, surface oleophilic modification); γ−(2,3−epoxypropoxy)propyltrimethoxysilane (coupling agent KH-560); and propylene glycol methyl ether acetate (PMA) as the solvent. The ratio by mass percentage is: modified polyurethane resin 40%, nano carbon black 10%, silane coupling agent 2%, and solvent 48%.

[0029] (5) Place it in a 180°C oven for 60 seconds to cure. The plate temperature reaches PMT160°C, so that the coating is fully cross-linked.

[0030] Example 2 The only difference between this embodiment and embodiment 1 is that the content of the surface treatment liquid components in step (3) is the lower limit of the claims: 0.5% inorganic acid mixture, 0.1% multidentate organic chelating agent, 0.05% nanocrystal seed inducer, and 0.01% aqueous surfactant.

[0031] Example 3 The difference between this embodiment and embodiment 1 is only that the content of the surface treatment liquid components in step (3) is the upper limit of the claims: 2.0% inorganic acid mixture, 0.5% multidentate organic chelating agent, 0.1% nanocrystal seed inducer, and 0.03% aqueous surfactant.

[0032] Example 4 The only difference between this embodiment and Embodiment 1 is that the mass ratio of nitric acid to phosphoric acid in the inorganic acid mixture is 1:2.

[0033] Example 5 The only difference between this embodiment and Embodiment 1 is that the mass ratio of nitric acid to phosphoric acid in the inorganic acid mixture is 1:4.

[0034] Example 6 The only difference between this embodiment and Example 1 is that the multidentate organic chelating agent is replaced with ethylenediaminetetraacetic acid (EDTA).

[0035] Example 7 The only difference between this embodiment and embodiment 1 is that the median diameter D50 of the atomized droplets in step (3) is 10 μm and the spraying pressure is 0.2 MPa.

[0036] Example 8 The only difference between this embodiment and embodiment 1 is that the median diameter D50 of the atomized droplets in step (3) is 50 μm and the spraying pressure is 0.5 MPa.

[0037] Example 9 The only difference between this embodiment and embodiment 1 is that the nano black film coating in step (4) uses bisphenol A type epoxy resin (molecular weight 900-1200) as the film-forming resin, with a mass percentage of 30% and a mass percentage of 15% for nano carbon black.

[0038] Comparative Example 1 The only difference between this comparative example and Example 1 is that step (3) is omitted, that is, after hot-dip galvanizing and cooling, the nano black film coating of step (4) is carried out directly.

[0039] Comparative Example 2 The only difference between this comparative example and Example 1 is that the surface treatment solution in step (3) is replaced with a 5% dilute sulfuric acid solution, and the traditional immersion pickling is used.

[0040] Comparative Example 3 The only difference between this comparative example and Example 1 is that: in step (3), nano-atomized spraying is not used, but the surface treatment liquid is directly dipped onto the zinc layer surface, and the reaction time is also 10 seconds.

[0041] Comparative Example 4 The only difference between this comparative example and Example 1 is that the black film coating in step (4) is a conventional industrial black paint (the average particle size of carbon black is about 5-10 μm).

[0042] Comparative Example 5 The only difference between this comparative example and Example 1 is that the surface treatment solution in step (3) does not contain phytic acid (organic chelating agent) and colloidal silica (nanocrystalline seed), but only retains inorganic acid mixture and surfactant.

[0043] 1. Coating Adhesion Test (T-Bend Test): The test is conducted according to the national standard GB / T12754 "Color Coated Steel Sheets and Strips". The sample is bent 180°. A bending radius of 0 is recorded as 0T, and a radius equal to one time the sheet thickness is recorded as 1T. Transparent tape is applied to the bend and quickly pulled up; observe for coating peeling or cracking. The minimum bending radius (0T or 1T) without peeling or cracking is used as the evaluation index.

[0044] 2. Coating adhesion test (cross-cut test): The test shall be conducted in accordance with the national standard GB / T9286 "Cross-cut test for paints and paint films". A 1mm×1mm grid shall be cut on the surface of the black film using a multi-bladed cutter. Standard tape shall be used to adhere the coating and then pull it off. The grade shall be determined according to the peeling area (grade 0 is the strongest and grade 5 is the worst).

[0045] 3. Neutral Salt Spray Resistance Test (NSS): Conducted according to the national standard GB / T10125 "Artificial Atmosphere Corrosion Test - Salt Spray Test". After sealing the edges and back of the steel plate, place it in a salt spray chamber and spray continuously for 120 hours in a 5% NaCl solution at 35℃. After removal, observe the black film surface for bubbles, rust, or coating peeling distance starting from the edge.

[0046] Table 1 Test Results Example 1 0T Level 0 No bubbles / no peeling Excellent Example 2 1T Level 0 No bubbles / no peeling excellent Example 3 0T Level 0 No bubbles / no peeling excellent Example 4 0T Level 0 No bubbles / no peeling excellent Example 5 0T Level 0 No bubbles / no peeling excellent Example 6 1T Level 1 No bubbles / no peeling good Example 7 0T Level 0 No bubbles / no peeling excellent Example 8 0T Level 0 No bubbles / no peeling excellent Example 9 0T Level 0 No bubbles / no peeling excellent Comparative Example 1 4T Level 3 Severe blistering / large-area peeling Difference Comparative Example 2 2T Level 1 Peeling at the edges Poor Comparative Example 3 2T Level 1 Obvious localized blistering Poor Comparative Example 4 2T Level 2 interlayer peeling Poor Comparative Example 5 1T Level 1 Tiny bubbles appear generally Combining Example 1 and Comparative Examples 1-2 with Table 1, it can be seen that Comparative Example 1, which did not undergo interface structuring treatment, only achieved a T-bend test result of 4T and exhibited severe peeling in the salt spray test. While Comparative Example 2, which used conventional acid washing, showed improved adhesion (2T), it was still far below the 0T level of Example 1. This demonstrates that the hierarchical anchoring structure constructed in this application, with micron-level pits embedded with nano-level textures, significantly increases the interfacial mechanical interlocking force compared to a flat surface or a single micron-level roughened surface, enabling the black film layer to maintain extremely high stability even under severe deformation.

[0047] Combining Example 1 and Comparative Example 3 with Table 1, it can be seen that, under completely identical chemical composition, Comparative Example 3, which uses a traditional dip-coating process without nano-atomization, exhibits significantly inferior T-bend performance (2T) and salt spray resistance (localized blistering) compared to Example 1. This demonstrates that the micro-droplets generated by the nano-atomization process can achieve highly uniform spreading of the treatment solution on the zinc layer surface and a gentle micro-reaction, avoiding localized "leakage" or excessive corrosion caused by uneven reaction in traditional processes, thereby ensuring consistent adhesion across the entire board.

[0048] Combining Examples 1, 5, and 6 with Table 1, it can be seen that Comparative Example 5, which lacks both phytic acid chelating agent and nanocrystal seeds, exhibited physical etching pits but showed microbubbles in the 120-hour salt spray test, resulting in a only average adhesion rating. Example 6, which used a different chelating agent, showed better overall performance than the comparative example but was slightly inferior to Example 1, which used phytic acid. This indicates that multi-toothed organic chelating agents (especially phytic acid) generate a strong synergistic effect with the physically anchored structure through in-situ chemical chelation conversion film, providing chemical bonding while blocking interfacial corrosion channels, thus greatly enhancing the long-term wet adhesion of the coating.

[0049] Combining Example 1 and Comparative Example 4 with Table 1, it can be seen that when the carbon black particle size in the black film coating increases from the nanometer level to the micrometer level of conventional industrial grade (Comparative Example 4), its cross-cut test grade drops to level 2, accompanied by interlayer delamination. This verifies that the core technology of this application lies in "scale matching", that is, only nanometer-sized pigment particles can truly fill and anchor into the nano-texture gaps constructed in step (3), thereby achieving true seamless cross-linking between the coating and the substrate.

[0050] As can be seen from Examples 2-5 and 7-9, and in conjunction with Table 1, under the component content, acid ratio range, atomization parameters, and different resin systems defined in the claims of this application, the T-bending performance of the prepared hot-dip galvanized steel sheets remains between 0T and 1T, and the salt spray resistance performance is excellent. This demonstrates that the technical solution provided in this application has good process stability and a wide range of applications, and can achieve high adhesion within the defined parameter range.

[0051] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A high-adhesion nano black film hot-dip galvanized steel sheet, characterized in that, It includes a steel substrate, a pure zinc layer formed on the surface of the steel substrate, and a nano black film layer covering the surface of the pure zinc layer; wherein, the interface between the pure zinc layer and the nano black film layer has a micro-nano hierarchical anchoring structure, the micro-nano hierarchical anchoring structure is composed of micron-level pits distributed on the surface of the pure zinc layer and nano-level rough textures embedded in the inner wall of the micron-level pits and the surface of the pure zinc layer.

2. The high-adhesion nano black film hot-dip galvanized steel sheet according to claim 1, characterized in that: Between the pure zinc layer and the nano black film layer, there is also a chemical chelation conversion film generated by the in-situ reaction of an organic chelating agent with zinc ions, and the thickness of the chemical chelation conversion film is 10nm~100nm.

3. A method for preparing a high-adhesion nano-black film hot-dip galvanized steel sheet as described in any one of claims 1-2, characterized in that, Includes the following steps: (1) Degreasing, pickling, washing and drying are performed on cold-rolled steel sheets; (2) The pretreated steel plate is immersed in molten zinc liquid for hot-dip galvanizing. After being drawn out, the zinc layer thickness is controlled by air knife and cooled to form a pure zinc layer on the surface of the steel substrate. (3) The surface treatment liquid is sprayed onto the surface of the pure zinc layer after being nano-atomized. After reacting for 5 to 15 seconds, it is washed with water and dried. The micro-nano hierarchical anchoring structure is constructed in situ on the surface of the pure zinc layer using the surface treatment liquid. (4) A nano black film coating is applied to the surface of the pure zinc layer after step (3) using a roller coating process, so that the nano black film coating fills into the gaps of the micro-nano graded anchoring structure; (5) Heating and curing to make the nano black film layer tightly bonded to the pure zinc layer.

4. The method for preparing a high-adhesion nano-black film hot-dip galvanized steel sheet according to claim 3, characterized in that, The surface treatment liquid in step (3) comprises the following components by mass percentage: A mixture of inorganic acids, ranging from 0.5% to 2.0%. 0.1%~0.5% of multidentate organic chelating agents; 0.05%~0.1% of nanocrystal seed inducer; 0.01%~0.03% aqueous surfactant; The remainder is deionized water.

5. The method for preparing a high-adhesion nano-black film hot-dip galvanized steel sheet according to claim 4, characterized in that: The inorganic acid mixture is composed of nitric acid and phosphoric acid mixed in a mass ratio of 1:2 to 1:4; the multidentate organic chelating agent is at least one of phytic acid, sodium phytate, and ethylenediaminetetraacetic acid; and the nanocrystal seed inducer is colloidal silica with an average particle size of 20 nm to 50 nm.

6. The method for preparing a high-adhesion nano-black film hot-dip galvanized steel sheet according to claim 3, characterized in that: In step (3), the median diameter D50 of the surface treatment liquid droplets after nano-atomization is 10μm~50μm, and the spraying pressure is 0.2MPa~0.5MPa.

7. The method for preparing a high-adhesion nano-black film hot-dip galvanized steel sheet according to claim 3, characterized in that, The nano black film coating in step (4) comprises the following components by mass percentage: 30%~50% of film-forming resin, wherein the film-forming resin is a modified polyurethane resin or an epoxy resin; 5%~15% of nano carbon black, and the average particle size of the nano carbon black is ≤100nm; 1%~3% silane coupling agent; The remainder is solvent.