A method for refining the surface layer grains of a plated layer, zinc-aluminum-magnesium plated steel sheet

Abstract

This invention provides a method for refining the surface grains of a coating, and a zinc-aluminum-magnesium coated steel sheet, belonging to the field of coating. The method includes: hot-dip galvanizing a steel substrate to obtain a coated steel sheet; cooling the coated steel sheet to a set temperature; and applying a grain refiner to at least a portion of the surface of the coating, wherein the set temperature is 390℃~420℃, and the grain refiner includes potassium fluorotitanate and a mixed salt of potassium fluorotitanate and potassium fluoroborate. By spraying a grain refiner that increases the supercooling of the coating surface, the coating surface is promoted to solidify first, thereby refining the grain structure of the coating. By controlling the temperature at which the grain refiner is sprayed onto the coating surface and the equivalent diameter of the grain refiner, the resulting coating surface has a fine eutectic structure with good atmospheric corrosion resistance and good resistance to under-film filamentary corrosion, making it less prone to under-film filamentary corrosion problems in the atmosphere.

Description

Technical Field

[0001] This application relates to the field of coating technology, and in particular to a method for refining the grain size of a coating surface and a zinc-aluminum-magnesium coated steel sheet. Background Technology

[0002] Zinc-aluminum-magnesium coated steel sheet is a new type of high corrosion-resistant alloy coated steel sheet. This coating was developed based on the traditional pure zinc coating, with the addition of magnesium and aluminum elements, which significantly improves the corrosion resistance of the coating on both the surface and the cut edges. It can be widely used in the manufacture of automobiles, home appliances, building exterior walls, etc.

[0003] However, when the steel plate and coating are thick, the zinc-aluminum-magnesium alloy coating produced by hot-dip galvanizing results in a larger grain size on the surface, especially in the eutectic structure. This larger grain size leads to a decrease in the mechanical properties of the coating, making it prone to bending cracks and reducing the corrosion resistance of the formed parts. Simultaneously, the larger grains result in more coarse eutectic particles in the coating, causing severe localized corrosion, exhibiting problems similar to pitting and filiform corrosion. Therefore, there is an urgent need to develop a method to refine the surface grains of the zinc-aluminum-magnesium alloy coating to improve the corrosion resistance of zinc-aluminum-magnesium alloy coated steel plates. Summary of the Invention

[0004] This application provides a method for refining the surface grains of a coating and a zinc-aluminum-magnesium coated steel sheet. By refining the surface grains of the zinc-aluminum-magnesium alloy coating, the technical problem of poor corrosion resistance of existing zinc-aluminum-magnesium coated steel sheets can be solved.

[0005] In a first aspect, this application provides a method for refining the grain size of a coating surface layer, the method comprising:

[0006] A steel substrate is hot-dip coated to obtain a steel sheet with a coating.

[0007] The steel plate containing the coating is cooled to a set temperature, and a refining agent is applied to at least a portion of the surface of the coating. The set temperature is 390°C to 420°C. The refining agent includes potassium fluorotitanate and a mixed salt of potassium fluorotitanate and potassium fluoroborate.

[0008] Optionally, the particle equivalent diameter of the refining agent is ≤10μm.

[0009] Optionally, the atomic ratio of B to Ti in the mixed salt of potassium fluorotitanate and potassium fluoroborate is ≤1:10.

[0010] Optional, relative to an area of ​​1m 2 The coating has a refining agent with a mass of 0.005 g to 0.1 g.

[0011] Optionally, the hot-dip galvanizing temperature is 430℃~500℃.

[0012] Secondly, this application provides a zinc-aluminum-magnesium coated steel sheet, which is obtained by the method described in any embodiment of the first aspect, wherein the mass fraction of aluminum in the coating of the zinc-aluminum-magnesium coated steel sheet is 1% to 15%.

[0013] Optionally, the mass ratio of magnesium to aluminum in the coating is (3-10):15.

[0014] Optionally, the equivalent grain diameter of the coating is ≤400μm.

[0015] Optionally, the corrosion weight loss rate of the zinc-aluminum-magnesium coated steel sheet is ≤0.5g / m³. 2 / week.

[0016] The technical solutions provided in this application have the following advantages compared with the prior art:

[0017] This application utilizes a grain refiner sprayed onto the coating surface to enhance the undercooling of the coating. Potassium fluoroborate and potassium fluorotitanate are excellent grain refiners for zinc-aluminum-magnesium coatings, significantly improving the nucleation ability and undercooling of the coating, thus promoting surface solidification and refining the grain structure. By controlling the spraying temperature and equivalent diameter of the grain refiner, a fine-grained eutectic structure is obtained on the coating surface, exhibiting excellent atmospheric corrosion resistance and resistance to underfilm filamentary corrosion, making it less prone to underfilm filamentary corrosion problems in the atmosphere. Attached Figure Description

[0018] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A flowchart illustrating a method for refining the grain size of a coating surface layer, provided in an embodiment of this application;

[0021] Figure 2 The surface microstructure of the zinc-aluminum-magnesium coated steel sheet provided in Example 1 of this application;

[0022] Figure 3 The microstructure of the zinc-aluminum-magnesium coated steel sheet provided in Comparative Example 1 of this application. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0024] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0025] Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0026] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0027] Firstly, this application provides a method for refining the grain size of a coating surface layer; please refer to [link to relevant documentation]. Figure 1 The method includes:

[0028] S1. The steel substrate is hot-dip galvanized to obtain a steel plate with a coating.

[0029] In some embodiments, the hot-dip galvanizing temperature is 430°C to 500°C.

[0030] The positive effects of controlling the hot-dip galvanizing temperature to 430℃~500℃: If the hot-dip galvanizing temperature of zinc-aluminum-magnesium coatings is too high, the coating will overheat excessively, resulting in slow grain growth during solidification and preventing the formation of fine grains. However, if the temperature is too low, the grain refiner will not decompose at high temperatures. Suitable hot-dip galvanizing temperatures include 430℃, 440℃, 450℃, 470℃, 480℃, 490℃, and 500℃.

[0031] S2. Cool the steel plate containing the coating to a set temperature and apply a refining agent to at least a portion of the surface of the coating. The set temperature is 390°C to 420°C. The refining agent includes potassium fluorotitanate and a mixed salt of potassium fluorotitanate and potassium fluoroborate.

[0032] During the cooling process, this application sprays a refining agent on the coating surface that can improve the supercooling of the coating, causing the coating surface to solidify first, thereby refining the grain structure of the coating.

[0033] Furthermore, potassium fluoroborate and potassium fluorotitanate are excellent grain refiners for zinc-aluminum-magnesium coatings, significantly improving the nucleation ability and undercooling of the coating. Typically, grain refiners are added to the alloy, but this results in excessively high dosages. During hot-dip plating, cooling after plating is crucial for refining the coating. The surface grain size of the coating is more important than the overall grain size. Corrosion begins at the surface. Therefore, the key issue is how to refine the surface grains of the coating. This application therefore selects to spray a grain refiner onto the coating surface during the cooling stage after hot-dip plating.

[0034] The positive effects of controlling the coating surface temperature to 390℃~420℃: When spraying the grain refiner, the coating surface temperature should not be too high, otherwise the grain refiner will easily volatilize and decompose at high temperatures, losing its ability to refine grains. However, it should not be too low either, otherwise the coating will have completely solidified and become ineffective. The coating surface temperature can be 390℃, 395℃, 400℃, 410℃, 415℃, 420℃, etc.

[0035] In some embodiments, the particle equivalent diameter of the refining agent is ≤10 μm.

[0036] The positive effects of controlling the particle equivalent diameter of the refining agent to ≤10μm: The refining agent first undergoes a chemical reaction with the high-temperature coating, decomposing to form titanium and / or boron compounds. These compounds react with aluminum in the coating, causing aluminum to precipitate around the titanium and / or boron compounds, preferentially forming fine aluminum-rich phases. Then, fine eutectic grains form between these aluminum-rich phases. By refining the aluminum-rich phase, the eutectic grains are refined. Therefore, the size of the refining agent itself cannot be too large; otherwise, the aluminum-rich phase precipitated around the refining agent particles will be large and uneven. The equivalent diameter of the refining agent particles can be 2μm, 4μm, 6μm, 8μm, 9μm, 10μm, etc.

[0037] In some embodiments, the atomic ratio of B to Ti in the potassium fluorotitanate and potassium fluoroborate mixed salt is ≤1:10.

[0038] The positive effects of controlling the atomic ratio of B to Ti in a mixed salt of potassium fluorotitanate and potassium fluoroborate to ≤1:10: The refining agent includes potassium fluoroborate and potassium fluorotitanate. However, using too much potassium fluoroborate will result in excessive Al-B compounds formed by the combination of boron and aluminum. These Al-B compounds tend to aggregate and are difficult to disperse, thus reducing the refining effect. In this application, potassium fluoroborate is not used alone. The atomic ratio of B to Ti can be 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, etc.

[0039] In some implementations, relative to an area of ​​1m 2 The coating has a refining agent with a mass of 0.005 g to 0.1 g.

[0040] Control relative to an area of ​​1m 2 The coating described above, with a refining agent mass of 0.005g to 0.1g, has the following positive effects: too little refining agent will not achieve the desired refining effect. However, too much refining agent will cause the particles to aggregate to a size exceeding 10 micrometers, resulting in a decrease in the refining effect. The amount of refining agent can be 0.005g, 0.01g, 0.03g, 0.05g, 0.07g, 0.09g, 0.1g, etc.

[0041] Secondly, this application provides a zinc-aluminum-magnesium coated steel sheet, which is obtained by the method described in any embodiment of the first aspect, wherein the mass fraction of aluminum in the coating of the zinc-aluminum-magnesium coated steel sheet is 1% to 15%.

[0042] The positive effects of controlling the aluminum content in the coating to be between 1% and 15% by mass: Aluminum in zinc-aluminum-magnesium (ZAM) coatings is a key element in refining grain size. If the aluminum content is too low, the grain refiner will not be effective. At the same time, a low aluminum content will also lead to a decrease in the corrosion resistance of the ZAM coating. Therefore, the aluminum content should not be less than 1%. However, if too much aluminum is added to the coating, it will cause the formation of coarse, aluminum-rich phases during solidification. Because of the high aluminum content, the aluminum precipitation and growth rate is too fast, making it difficult for the grain refiner to refine the grains. Therefore, the aluminum content should not exceed 15%. The aluminum content in this coating can be 1%, 3%, 5%, 9%, 11%, 13%, 15%, etc.

[0043] In some embodiments, the mass ratio of magnesium to aluminum in the coating is (3-10):15.

[0044] The positive effects of controlling the mass ratio of magnesium to aluminum in the coating to (3-10):15 are as follows: Mg significantly improves the coating's resistance to atmospheric corrosion, especially when Mg and Al are added simultaneously. This allows for the formation of a dense double-layer hydroxide during corrosion, further enhancing the coating's corrosion resistance. Therefore, the amount of Mg added should be no less than one-fifth of the Al content. However, if too much Mg is added, the coating will preferentially form Mg-Zn compounds or zinc-rich compounds during solidification, rather than aluminum-rich phases. In this case, the refining agent will not react with the Mg-Zn compounds or zinc-rich compounds, thus losing its effect. Therefore, the Mg content should not exceed two-thirds of the Al content. The mass ratio of magnesium to aluminum in this coating can be 3:15, 4:15, 6:15, 7:15, 8:15, 9:15, 10:15, etc.

[0045] In some embodiments, the equivalent grain diameter of the coating is ≤400μm.

[0046] The positive effects of controlling the equivalent grain diameter of the eutectic structure in the coating to ≤400μm: The eutectic structure in the coating is the preferential corrosion site during the corrosion process. If the size of the eutectic structure is too large, the coating will be prone to localized corrosion, such as pitting and filiform corrosion. The equivalent grain diameter of the eutectic structure in this coating can be 150μm, 200μm, 250μm, 300μm, 350μm, 400μm, etc.

[0047] In some embodiments, the corrosion weight loss rate of the zinc-aluminum-magnesium coated steel sheet is ≤0.5 g / m³. 2 / week.

[0048] The zinc-aluminum-magnesium coated steel sheet of this application exhibits a fine-grained eutectic structure on its coating surface, resulting in excellent atmospheric corrosion resistance and resistance to under-film filamentary corrosion. Under-film filamentary corrosion is not easily observed in the atmosphere. The corrosion weight loss rate can be 0.1 g / m³.2 / week, 0.2g / m 2 / week0, 0.3g / m 2 / week, 0.4g / m 2 / week, 0.5g / m 2 / week, etc.

[0049] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0050] This application provides a method for preparing zinc-aluminum-magnesium coated steel sheet, comprising:

[0051] S11. Hot-dip galvanizing is performed on the steel substrate to obtain a steel sheet with a coating.

[0052] S21. Cool the steel plate containing the coating to a set temperature, and apply a refining agent to at least a portion of the surface of the coating. After cooling, a zinc-aluminum-magnesium coated steel plate is obtained. For specific process parameters, please refer to Table 1.

[0053] Table 1. Process parameters for preparing zinc-aluminum-magnesium coated steel sheets

[0054]

[0055]

[0056] The coating characteristics of the zinc-aluminum-magnesium coated steel sheets of the examples and comparative examples are shown in Table 2. The equivalent diameter of the eutectic structure was measured using EBSD technology; different grains exhibit different orientations and show different colors in the inverse pole figure of EBSD. The eutectic structure of Example 1 is shown in the attached figure. Figure 2 As shown. The eutectic structure of Comparative Example 1 is shown in the attached figure. Figure 3 As shown.

[0057] Table 2. Coating characteristics of zinc-aluminum-magnesium coated steel sheets in the examples and comparative examples.

[0058] Group Aluminum content of coating Magnesium content / aluminum content of coating Equivalent diameter of eutectic structure (μm) Example 1 1％ 0.40 250 Example 2 1％ 0.20 300 Example 3 2％ 0.20 150 Example 4 2％ 0.20 350 Example 5 2％ 0.50 350 Example 6 2％ 0.50 200 Example 7 2％ 0.50 240 Example 8 3％ 0.50 400 Example 9 10％ 0.60 320 Example 10 15％ 0.60 230 Example 11 15％ 0.60 160 Example 12 6％ 0.67 180 Example 13 6％ 0.67 200 Comparative Example 1 0.3％ 0.00 500 Comparative Example 2 0.3％ 0.10 600 Comparative Example 3 0.3％ 0.10 600 Comparative Example 4 20％ 0.70 450 Comparative Example 5 20％ 0.80 500

[0059] The zinc-aluminum-magnesium coated steel sheets prepared according to the process parameters in the above embodiments and comparative examples were subjected to corrosion evaluation. Specific results are shown in Table 3. The corrosion evaluation method involved placing the galvanized steel sheet in a cyclic corrosion test chamber and conducting 18 cycles of cyclic corrosion testing. The cyclic corrosion test met the requirements of Annex A of ISO 1 1997-1:2017. The mass loss of the coating before and after the test was measured, and the corrosion resistance of the coating was evaluated using the mass loss per unit area. Lower mass loss indicates better corrosion resistance. The zinc-aluminum-magnesium coating was evaluated for resistance to filamentous corrosion. A 20-micron-thick PVB organic film was coated onto the surface of the zinc-aluminum-magnesium coating. Scratches were then made on the surface of the organic film, with a scratch width of 1 mm and a scratch depth reaching the steel substrate. 5 μL of acetic acid solution with a concentration of 1 mol / dm³ was injected into the scratch location. 3 The samples were then placed in a constant temperature and humidity environment (22℃, 86% RH) and stored for 4 weeks. The growth length of filamentary corrosion on the sample surface was then evaluated according to GB / T 30789.9. Longer growth indicates a greater susceptibility to filamentary corrosion.

[0060] Table 3 Corrosion evaluation of zinc-aluminum-magnesium coated steel sheets in the examples and comparative examples.

[0061] serial number <![CDATA[Corrosion weight loss rate (g / m 2 / week)]]> Corrosion wire length (mm) Example 1 0.25 0 Example 2 0.12 0 Example 3 0.45 0 Example 4 0.34 0 Example 5 0.23 0 Example 6 0.24 0 Example 7 0.24 0 Example 8 0.45 0 Example 9 0.03 0 Example 10 0.04 0 Example 11 0.12 0 Example 12 0.11 0 Example 13 0.21 0 Comparative Example 1 4.56 2 Comparative Example 2 3.45 3 Comparative Example 3 4.56 1.5 Comparative Example 4 5.43 4 Comparative Example 5 5.60 4

[0062] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method of refining the surface layer grains of a plated layer, characterized by, The method includes: A steel substrate is hot-dip coated to obtain a steel sheet with a coating. The steel plate containing the coating is cooled to a set temperature, and a refining agent is applied to at least a portion of the surface of the coating. The set temperature is 390°C to 420°C, and the refining agent includes potassium fluorotitanate or a mixed salt of potassium fluorotitanate and potassium fluoroborate. The particle equivalent diameter of the refining agent is ≤10 μm, and the atomic ratio of B to Ti in the mixed salt of potassium fluorotitanate and potassium fluoroborate is ≤1:10, relative to an area of ​​1 m². 2 The coating has a refining agent weighing 0.005g to 0.1g, and the hot-dip coating temperature is 430℃ to 500℃. The aluminum content in the coating is 1% to 15% by mass, the magnesium to aluminum ratio in the coating is (3 to 10):15, and the equivalent grain diameter of the coating is ≤400 μm.

2. A zinc-aluminum-magnesium plated steel sheet, characterized by, The zinc-aluminum-magnesium coated steel sheet is obtained by the method described in claim 1, and the corrosion weight loss rate of the zinc-aluminum-magnesium coated steel sheet is ≤0.5 g / m³. 2 / week.

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