Zinc-aluminum-magnesium alloy plated layer, zinc-aluminum-magnesium alloy plated steel sheet, and method for producing same

Abstract

The application relates to a zinc-aluminum-magnesium alloy coating, a zinc-aluminum-magnesium alloy coated steel sheet and a preparation method thereof. The chemical composition of the zinc-aluminum-magnesium alloy coating comprises Mg, Al and Zn; wherein the content of Mg is 1.5-3% in mass fraction, and the content of Al is 3-4% in mass fraction; and the area fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium alloy coating is not less than 80%. The application improves the resistance to filamentous corrosion of the zinc-aluminum-magnesium alloy coating.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to a zinc-aluminum-magnesium alloy coating, a zinc-aluminum-magnesium alloy coated steel plate, and a method for preparing the same. Background Technology

[0002] Hot-dip galvanizing is a process in which molten metal reacts with an iron substrate to create an alloy layer, thus bonding the substrate and the coating together. Hot-dip galvanized steel has advantages such as uniform coating, strong adhesion, long service life, simple manufacturing process, and low product price, and is widely used in the manufacture of automobile bodies, home appliances, etc.

[0003] Currently, to improve the protective effect of hot-dip galvanized coatings on steel plate cut edges and enhance planar corrosion resistance, an appropriate amount of magnesium is added to the hot-dip galvanized coating to obtain a zinc-aluminum-magnesium alloy coating. This can further improve corrosion resistance by more than 20%, while also providing corrosion resistance at machined cut edges. However, the addition of magnesium to the hot-dip galvanized coating causes filamentary corrosion under organic film conditions. This filamentary corrosion forms a corrosion morphology that extends in a specific direction, affecting the appearance quality of the organic film and, in severe cases, even impacting the coating's corrosion resistance. Summary of the Invention

[0004] This application provides a zinc-aluminum-magnesium alloy coating, a zinc-aluminum-magnesium alloy coated steel plate, and a method for preparing the same, to solve the following technical problem: how to improve the resistance of zinc-aluminum-magnesium alloy coating to filamentous corrosion.

[0005] In a first aspect, this application provides a zinc-aluminum-magnesium alloy coating, the chemical composition of which includes:

[0006] Mg, Al, and Zn; wherein, by mass fraction,

[0007] The content of Mg is 1.5% to 3%, and the content of Al is 3% to 4%;

[0008] The area fraction of the surface eutectic structure of the zinc-aluminum-magnesium alloy coating is not less than 80%.

[0009] Optionally, the Mg content is 2.5% and the Al content is 3.7%.

[0010] Optionally, the content of Mg and the content of Al satisfy the following relationship:

[0011] -0.5≤[Mg]-[Al]*0.7≤0.5

[0012] In the formula, [Mg] represents the mass fraction of Mg, and [Al] represents the mass fraction of Al.

[0013] Optionally, the surface eutectic structure includes a ternary eutectic structure, and the area fraction of the ternary eutectic structure is not less than 40%.

[0014] In a second aspect, the present application provides a zinc-aluminum-magnesium alloy coated steel sheet, which includes a steel substrate and the zinc-aluminum-magnesium alloy coating as described in the first aspect attached to at least a part of the surface of the steel substrate.

[0015] In a third aspect, the present application provides a method for preparing the zinc-aluminum-magnesium alloy coated steel sheet as described in the second aspect, and the method includes:

[0016] Hot-dip galvanizing the steel substrate with a plating solution, and then performing first-stage cooling and second-stage cooling in sequence to obtain a zinc-aluminum-magnesium alloy coated steel sheet; wherein, according to the content of Mg and the content of Al in the plating solution, the end temperature of the first-stage cooling is obtained.

[0017] Optionally, the obtaining of the end temperature of the first-stage cooling according to the content of Mg and the content of Al in the plating solution includes:

[0018] When 3% ≤ content of Al ≤ 3.5% and 1.5% ≤ content of Mg ≤ 2.5% in the plating solution, the end temperature of the first-stage cooling satisfies the following relational expression: T0 = 400 - 10 * [Al] - 8 * [Mg];

[0019] In the formula, T0 represents the end temperature of the first-stage cooling, [Mg] represents the mass fraction of Mg, and [Al] represents the mass fraction of Al.

[0020] Optionally, the obtaining of the end temperature of the first-stage cooling according to the content of Mg and the content of Al in the plating solution includes:

[0021] When 3.5% < content of Al ≤ 4% and 2.5% < content of Mg ≤ 3% in the plating solution, the end temperature of the first-stage cooling satisfies the following relational expression: T0 = 400 - 5 * [Al] - 4 * [Mg];

[0022] In the formula, T0 represents the end temperature of the first-stage cooling, [Mg] represents the mass fraction of Mg, and [Al] represents the mass fraction of Al.

[0023] Optionally, the cooling rate of the first-stage cooling is 5 K / s to 20 K / s.

[0024] Optionally, the cooling rate of the second-stage cooling is 0.1 K / s to 5 K / s.

[0025] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art:

[0026] The zinc-aluminum-magnesium alloy coating provided in this application embodiment has the following chemical composition: Mg, Al, and Zn; wherein, by mass fraction, the content of Mg is 1.5%–3%, and the content of Al is 3%–4%; the area fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium alloy coating is not less than 80%. Adding Mg and Al to the zinc coating can provide high-quality atmospheric corrosion resistance; and with the synergistic effect of Mg content of 1.5%–3% and Al content of 3%–4%, the eutectic structure content on the surface of the zinc-aluminum-magnesium alloy coating can be significantly increased, thereby avoiding insufficient eutectic structure on the surface of the zinc-aluminum-magnesium alloy coating, which would cause corrosion of the zinc-aluminum-magnesium alloy coating to be confined to the grain boundaries, ultimately extending along the grain boundary to form a filamentous corrosion morphology; the area fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium alloy coating being not less than 80% allows the eutectic structure to form a cluster state, thereby avoiding the formation of filamentous corrosion morphology, ultimately improving the resistance of the zinc-aluminum-magnesium alloy coating to filamentous corrosion. Attached Figure Description

[0027] 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.

[0028] 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.

[0029] Figure 1 This is a schematic flowchart illustrating a method for preparing a zinc-aluminum-magnesium alloy coated steel sheet according to some embodiments of this application. Detailed Implementation

[0030] 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.

[0031] 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.

[0032] In this application, the terms "comprising," "including," etc., mean "including but not limited to." Relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. 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 multiple 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 each be single or multiple.

[0033] 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.

[0034] In a first aspect, this application provides a zinc-aluminum-magnesium alloy coating, the chemical composition of which includes: Mg, Al, and Zn; wherein, by mass fraction,

[0035] The content of Mg is 1.5% to 3%, and the content of Al is 3% to 4%;

[0036] The area fraction of the surface eutectic structure of the zinc-aluminum-magnesium alloy coating is not less than 80%.

[0037] In some embodiments, the Mg content is 2.5% and the Al content is 3.7%.

[0038] In the embodiments of this application, the Al element in the coating provides the coating with high-quality atmospheric corrosion resistance because, during the corrosion process, Al can form dense oxides and dense hydroxides on the surface. If there is too little Al in the coating, the adhesion between the coating and the steel plate will be poor, rendering the coating unusable and reducing corrosion resistance.

[0039] The Mg element in the coating can significantly improve the coating's resistance to atmospheric corrosion. The mechanism is that the Mg in the coating will preferentially dissolve into the water film on the coating surface in the atmospheric environment. In the water film, it will react with dissolved carbon dioxide to precipitate a dense protective film. This protective film can exist stably in neutral and weakly alkaline environments. At the same time, it can also promote the electrolyte solution on the coating surface to become a weakly alkaline solution, thereby improving the corrosion resistance of the coating.

[0040] In the atmosphere, especially in high-humidity environments, a thin liquid film adheres to the surface of zinc-aluminum-magnesium alloy coatings. The oxygen enriched in this film preferentially reacts with the magnesium in the coating, rapidly forming magnesium compounds, while the pH value of the liquid film gradually increases. Under these conditions, the liquid film also allows Al to dissolve. This leads to the reaction between the aluminum-rich phase in the coating and oxygen, hydroxide, and carbonate ions in the liquid film, forming complex aluminum-rich compounds.

[0041] Eutectic structures in zinc-aluminum-magnesium alloy coatings preferentially lead to localized corrosion. If the eutectic structure on the surface of the zinc-aluminum-magnesium alloy coating is sparse, this corrosion will be confined to the vicinity of the grain boundaries. When an organic film is applied to the coating surface, localized corrosion can propagate along the grain boundaries, forming filamentary corrosion morphologies.

[0042] Therefore, by appropriately adjusting the Al and Mg content in the zinc-aluminum-magnesium alloy coating, significantly increasing the proportion of eutectic structure in the coating, the eutectic structure can form clusters, thereby preventing the formation of filamentous corrosion. For example, the Mg content can be 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, etc., and the Al content can be 3%, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4%, etc., and the area fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium alloy coating can be 80%, 81%, 82%, 83%, 84%, 85%, etc. Furthermore, when the Al content in the zinc-aluminum-magnesium alloy coating is 3.7% and the Mg content is 2.5%, the coating is almost entirely composed of eutectic structure.

[0043] In some embodiments, the surface eutectic structure includes a ternary eutectic structure, wherein the area fraction of the ternary eutectic structure is not less than 40%.

[0044] In the embodiments of this application, the aforementioned surface eutectic structure includes a ternary eutectic structure. This ternary eutectic structure has fine grains, which allows the anodic active sites and cathodic active sites of localized corrosion to be closer together, thereby making it less likely for localized corrosion to propagate into filamentous corrosion morphology. For example, the area fraction of the aforementioned ternary eutectic structure can be 40%, 41%, 42%, 43%, 44%, 45%, etc.

[0045] In some embodiments, the content of Mg and the content of Al satisfy the following relationship:

[0046] -0.5≤[Mg]-[Al]*0.7≤0.5

[0047] In the formula, [Mg] represents the mass fraction of Mg, [Al] represents the mass fraction of Al, and x takes the value of -0.5 to 0.5.

[0048] In the embodiments of this application, when the content of Mg and the content of Al satisfy the following relationship: -0.5≤[Mg]-[Al]*0.7≤0.5, the content of Al and Mg elements is located near the ternary eutectic reaction boundary, thus making the eutectic structure in the coating mainly a ternary eutectic structure.

[0049] Secondly, this application provides a zinc-aluminum-magnesium alloy coated steel sheet, the zinc-aluminum-magnesium alloy coated steel sheet comprising a steel substrate and a zinc-aluminum-magnesium alloy coating as described in the first aspect, which is attached to at least a portion of the surface of the steel substrate.

[0050] In this embodiment, the steel substrate is not specifically limited; ordinary steel plates such as hot-rolled steel plates and cold-rolled steel plates can be used. The type of steel is also not specifically limited; for example, aluminum-killed steel, ultra-low carbon steel, and high-strength steel can be used. Through the above-mentioned design of the coating alloy composition and the chemical composition of the coating surface, the resistance to filamentous corrosion of the zinc-aluminum-magnesium alloy coating can be significantly improved, thereby giving the zinc-aluminum-magnesium alloy coated steel plate good resistance to filamentous corrosion.

[0051] The zinc-aluminum-magnesium alloy coated steel sheet is based on the above-mentioned zinc-aluminum-magnesium alloy coating. The specific chemical composition of the zinc-aluminum-magnesium alloy coating can be referred to the above embodiments. Since the zinc-aluminum-magnesium alloy coated steel sheet adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0052] Thirdly, this application provides a method for preparing the zinc-aluminum-magnesium alloy coated steel sheet described in the second aspect. Figure 1 For a flowchart illustrating a method for preparing a zinc-aluminum-magnesium alloy coated steel sheet according to some embodiments of this application, please refer to [link / reference]. Figure 1, the method includes:

[0053] S1. Hot-dip galvanize the steel substrate with a plating solution, and then perform first-stage cooling and second-stage cooling in sequence to obtain a zinc-aluminum-magnesium alloy coated steel sheet; wherein, according to the content of Mg and the content of Al in the plating solution, the end temperature of the first-stage cooling is obtained.

[0054] In some embodiments, obtaining the end temperature of the first-stage cooling according to the content of Mg and the content of Al in the plating solution includes:

[0055] When 3% ≤ the content of Al ≤ 3.5% and 1.5% ≤ the content of Mg ≤ 2.5% in the plating solution, the end temperature of the first-stage cooling satisfies the following relationship: T0 = 400 - 10 * [Al] - 8 * [Mg];

[0056] In the formula, T0 represents the end temperature of the first-stage cooling, [Mg] represents the mass fraction of Mg, and [Al] represents the mass fraction of Al.

[0057] In some embodiments, obtaining the end temperature of the first-stage cooling according to the content of Mg and the content of Al in the plating solution includes:

[0058] When 3.5% < the content of Al ≤ 4% and 2.5% < the content of Mg ≤ 3% in the plating solution, the end temperature of the first-stage cooling satisfies the following relationship: T0 = 400 - 5 * [Al] - 4 * [Mg];

[0059] In the formula, T0 represents the end temperature of the first-stage cooling, [Mg] represents the mass fraction of Mg, and [Al] represents the mass fraction of Al.

[0060] In the embodiments of the present application, during the preparation of the zinc-aluminum-magnesium alloy coated steel sheet, the cooling process after hot-dip plating plays an important role in the surface structure of the coating. After hot-dip plating, the liquid zinc-aluminum-magnesium plating solution will solidify with cooling to form various microstructures. At the beginning, an initial solidification structure will be formed at a relatively high temperature, and then a eutectic structure will be formed at a relatively low temperature. When forming the initial solidification, a relatively high cooling rate is required to minimize the formation ratio of the initial solidification structure. During the formation of the eutectic structure, a relatively low cooling rate is required to ensure that the eutectic structure can be fully precipitated. Therefore, a two-stage cooling process needs to be designed. The first stage cools from the hot-dip plating temperature to the temperature T0, and the second stage cools from the temperature T0 and can be cooled to 300°C. After 300°C, the coating has completely solidified and no longer changes.

[0061] In some embodiments, the cooling rate of the first-stage cooling is 5 K / s to 20 K / s.

[0062] In some embodiments, the cooling rate of the second stage of cooling is 0.1 K / s to 5 K / s.

[0063] In this embodiment, the cooling rate of the first stage is limited. If the cooling rate of this stage is too fast, fine cracks caused by thermal stress will appear in the coating, making the crack locations more prone to filamentous corrosion morphology. If the cooling rate of this stage is too slow, the proportion of eutectic structure will decrease. For example, the cooling rate of the first stage can be 5K / s, 10K / s, 15K / s, 20K / s, etc.

[0064] The cooling rate of the second stage is limited. If the cooling rate of this stage is too slow, it will result in more binary eutectic structures instead of ternary eutectic structures; if the cooling rate of this stage is too fast, it will result in a decrease in the proportion of eutectic structures on the coating surface. For example, the cooling rate of the second stage can be 0.1 K / s, 0.5 K / s, 1 K / s, 2 K / s, 3 K / s, 4 K / s, 5 K / s, etc.

[0065] The preparation method of the zinc-aluminum-magnesium alloy coated steel sheet is based on the above-mentioned zinc-aluminum-magnesium alloy coated steel sheet. The specific structure of the zinc-aluminum-magnesium alloy coating can be referred to the above embodiments. Since the preparation method of the zinc-aluminum-magnesium alloy coated steel sheet adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0066] 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.

[0067] This application provides a zinc-aluminum-magnesium alloy coating, the characteristics of which are shown in Table 1.

[0068] Table 1 Characteristics of zinc-aluminum-magnesium alloy coatings

[0069]

[0070] This application provides a zinc-aluminum-magnesium alloy coated steel sheet and its preparation method. A steel sheet with a thickness of 1.0 mm and a width of 1500 mm is used as the substrate, and the zinc-aluminum-magnesium alloy coating weighs 100 grams per square meter on both sides. The preparation process parameters for this zinc-aluminum-magnesium alloy coated steel sheet are shown in Table 2.

[0071] Table 2. Preparation process parameters of zinc-aluminum-magnesium alloy coated steel sheets

[0072]

[0073] Filament corrosion resistance assessment test: The zinc-aluminum-magnesium alloy coated steel sheets obtained according to Examples 1-13 and Comparative Examples 1-5 above were subjected to filament corrosion resistance assessment test.

[0074] A 20-micron-thick PVB organic film was coated on the zinc-aluminum-magnesium plating surface. Then, scratches were 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 scratched areas. 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. Table 3 shows the experimental results of the resistance to filamentary corrosion of zinc-aluminum-magnesium alloy coated steel sheets.

[0075] Table 3. Experimental Results of Resistance to Filament Corrosion Assessment for Zinc-Aluminum-Magnesium Alloy Coated Steel Sheets

[0076] serial number Corrosion wire length (mm) Example 1 0 Example 2 0 Example 3 0 Example 4 0 Example 5 0 Example 6 0 Example 7 0 Example 8 0 Example 9 0 Example 10 0 Example 11 0 Example 12 0 Example 13 0 Comparative Example 1 5.6 Comparative Example 2 10.3 Comparative Example 3 11.4 Comparative Example 4 14.5 Comparative Example 5 6.7

[0077] As shown in Table 1, the zinc-aluminum-magnesium alloy coating surface of this application embodiment has excellent resistance to filamentous corrosion, and thus the zinc-aluminum-magnesium alloy coated steel plate surface has excellent resistance to filamentous corrosion.

[0078] 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 zinc-aluminum-magnesium alloy coating, characterized in that, The chemical composition of the zinc-aluminum-magnesium alloy coating includes: Mg, Al, and Zn; wherein, in terms of mass fraction, the content of Mg is 1.5% - 3%, and the content of Al is 3% - 3.9%; the area fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium alloy coating is not less than 80% to avoid the formation of filiform corrosion morphology; the content of Mg and the content of Al satisfy the following relationship: -0.5 ≤ [Mg] - [Al] * 0.7 ≤ 0.5; In the formula, [Mg] represents the mass fraction of Mg, and [Al] represents the mass fraction of Al; the surface eutectic structure includes a ternary eutectic structure, and the area fraction of the ternary eutectic structure is not less than 40%.

2. The zinc-aluminum-magnesium alloy coating according to claim 1, characterized in that, The content of Mg is 2.5%, and the content of Al is 3.7%.

3. A zinc-aluminum-magnesium alloy coated steel sheet, characterized in that, The zinc-aluminum-magnesium alloy coated steel plate includes a steel substrate and the zinc-aluminum-magnesium alloy coating according to any one of claims 1 - 2 attached to at least a part of the surface of the steel substrate.

4. A method for preparing a zinc-aluminum-magnesium alloy coated steel sheet as described in claim 3, characterized in that, The method includes: hot-dip galvanizing the steel substrate with a plating solution, and then successively performing the first-stage cooling and the second-stage cooling to obtain a zinc-aluminum-magnesium alloy coated steel plate; wherein, according to the content of Mg and the content of Al in the plating solution, the end temperature of the first-stage cooling is obtained; the cooling rate of the first-stage cooling is 10 K / s - 20 K / s, and the cooling rate of the second-stage cooling is 0.1 K / s - 5 K / s.

5. The preparation method according to claim 4, characterized in that, The obtaining of the end temperature of the first-stage cooling according to the content of Mg and the content of Al in the plating solution includes: when 3% ≤ the content of Al in the plating solution ≤ 3.5%, and 1.5% ≤ the content of Mg ≤ 2.5%, the end temperature of the first-stage cooling satisfies the following relationship: T0 = 400 - 10 * [Al] - 8 * [Mg]; In the formula, T0 represents the end temperature of the first-stage cooling, [Mg] represents the mass fraction of Mg, and [Al] represents the mass fraction of Al.

6. The preparation method according to claim 4 or 5, characterized in that, The obtaining of the end temperature of the first-stage cooling according to the content of Mg and the content of Al in the plating solution includes: when 3.5% < the content of Al in the plating solution ≤ 4%, and 2.5% < the content of Mg ≤ 3%, the end temperature of the first-stage cooling satisfies the following relationship: T0 = 400 - 5 * [Al] - 4 * [Mg]; In the formula, T0 represents the end temperature of the first-stage cooling, [Mg] represents the mass fraction of Mg, and [Al] represents the mass fraction of Al.

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