Filamentous-corrosion-resistant hot-dip zinc-aluminum-magnesium coated steel plate and preparation method thereof

Abstract

The invention relates to a filamentous-corrosion-resistant hot-dip zinc-aluminum-magnesium plated steel plate and a preparation method thereof, and belongs to the technical field of coating and plating. The coated steel plate comprises a steel substrate, a zinc-aluminum-magnesium coating and a Fe-Al alloy layer, the Fe-Al alloy layer is located between the steel substrate and the zinc-aluminum-magnesium coating, and the zinc-aluminum-magnesium coating comprises the following chemical components in percentage by mass: 1.5%-3% of Mg, 2.5%-4% of Al and a substrate element Zn. According to the hot-dip galvanized aluminum magnesium coating steel plate, through the measures of optimizing coating components, controlling the coating microstructure, optimizing the Fe-Al compound layer and the like, the filiform corrosion resistance of the hot-dip galvanized aluminum magnesium coating steel plate is remarkably improved, so that the coating has higher corrosion resistance in a severe environment, and the service life of the steel plate is prolonged.

Description

Technical Field

[0001] The present application relates to the field of coating technology, and in particular to a hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion and a preparation method thereof. Background Art

[0002] Hot-dip galvanizing is a process in which steel is immersed in molten zinc liquid to form a zinc-iron alloy layer on the surface of the steel through a chemical reaction. This alloy layer has extremely high adhesion and corrosion resistance, and can effectively protect the steel from corrosion. Hot-dip galvanized steel has the advantages of uniform coating, strong adhesion, long service life, simple manufacturing process and low product price, so it has been widely used in the fields of automobiles and home appliances. In order to further improve the corrosion resistance of hot-dip galvanized coating, especially the protective effect at the cut position of steel plates, people began to add an appropriate amount of Mg to the hot-dip galvanized coating to obtain a zinc-aluminum-magnesium coating. On the basis of retaining the advantages of hot-dip galvanized steel, the zinc-aluminum-magnesium coating can improve the corrosion resistance by more than 20%, and at the same time has the corrosion resistance of the processed cut position.

[0003] However, after adding magnesium to the hot-dip galvanized coating, the zinc-aluminum-magnesium coating will undergo filamentary corrosion under the organic film under certain conditions. Filiform corrosion is a specific corrosion form that expands along a specific direction under the organic film to form a continuous corrosion channel. These corrosion channels often have a long and thin filamentous morphology, hence the name "filamentary corrosion". This corrosion form not only seriously affects the appearance quality of the organic film and reduces the aesthetics of the product, but more importantly, in the process of continuous expansion of the corrosion channel, it will gradually weaken the protective ability of the coating, thereby threatening the corrosion resistance of the entire coating system. Summary of the invention

[0004] The present application provides a hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion, in order to solve the following technical problem: how to improve the filiform corrosion resistance of the hot-dip galvanized aluminum-magnesium coated steel plate.

[0005] In the first aspect, the present application provides a hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion, wherein the coated steel plate comprises a steel substrate, a zinc-aluminum-magnesium coating and a Fe-Al alloy layer, wherein the Fe-Al alloy layer is located between the steel substrate and the zinc-aluminum-magnesium coating, and the chemical composition of the zinc-aluminum-magnesium coating comprises, by mass fraction: Mg: 1.5% to 3%, Al: 2.5% to 4% and a matrix element Zn.

[0006] Optionally, the Fe-Al alloy layer has a thickness of 500 nm to 2 μm.

[0007] Optionally, the chemical composition of the zinc-aluminum-magnesium coating satisfies: [Al]=(1.2-2.0)×[Mg], wherein [Mg] represents the mass fraction of Mg, and [Al] represents the mass fraction of Al.

[0008] Optionally, the area percentage of the binary eutectic structure on the surface of the zinc-aluminum-magnesium coating is ≤10%.

[0009] Optionally, the area percentage of the surface ternary eutectic structure of the zinc-aluminum-magnesium coating is ≥50%.

[0010] Optionally, the equivalent diameter of the initial solidified zinc-rich phase of the zinc-aluminum-magnesium coating is ≤100 μm.

[0011] In a second aspect, the present application provides a method for preparing the hot-dip galvanized aluminum-magnesium coated steel sheet resistant to filiform corrosion as described in the first aspect, the method comprising:

[0012] Hot-dip galvanizing the steel substrate to obtain a hot-dip galvanized aluminum-magnesium coated steel plate;

[0013] The hot-dip galvanized aluminum-magnesium coated steel sheet is cooled at a set cooling rate.

[0014] Optionally, the hot-dip coating time is 5s to 10s.

[0015] Optionally, the plating solution temperature of the hot dip plating is 400°C to 480°C.

[0016] Optionally, the set cooling rate is ≥10K / s.

[0017] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0018] The present application provides a hot-dip galvanized aluminum-magnesium coated steel sheet resistant to filiform corrosion, the coated steel sheet comprising a steel substrate, a zinc-aluminum-magnesium coating and an Fe-Al alloy layer, the Fe-Al alloy layer being between the steel substrate and the zinc-aluminum-magnesium coating, and the chemical composition of the zinc-aluminum-magnesium coating comprising, by mass fraction, Mg: 1.5% to 3%, Al: 2.5% to 4% and matrix element Zn. By optimizing the coating composition, the eutectic structure in the coating is mainly a ternary eutectic structure, and the proportion of the binary eutectic structure is reduced, thereby reducing the risk of filiform corrosion. By setting the cooling rate, the size of the initial solidified zinc-rich phase in the coating is controlled to enhance the connection between the ternary eutectic structures and improve the uniformity of corrosion. By precisely controlling the temperature and time of the hot-dip coating process to form an Fe-Al compound layer of appropriate thickness, the spread of filiform corrosion is slowed down. These measures work together to significantly improve the filiform corrosion resistance of the hot-dip galvanized aluminum-magnesium coated steel sheet, so that the coating has stronger corrosion resistance in harsh environments and prolongs the service life of the steel sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0020] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0021] Figure 1 A schematic flow chart of a method for preparing a hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion provided in an embodiment of the present application. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0023] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a rigid limitation on the scope of the present application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the 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., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which applies regardless of the range; in addition, whenever a numerical range is indicated in this document, it is meant to include any cited numbers (fractions or integers) within the indicated range.

[0024] In this document, the terms including "including" and "including" mean "including but not limited to". Relational terms such as "first" and "second" are only used 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. "And / or" describes the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone; where A and B can be singular or plural. "At least one" means one or more, and "plurality" 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 all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. "Parts" such as parts by weight and parts by mass indicate the proportional relationship between the components. In the proportional relationship involved in this article, the parameters that need to be described by proportion should be understood as the first term of the proportional formula in the order of description, and the proportional numbers should be understood as the second term of the proportional formula. For example, if the weight ratio of substance A, substance B and substance C is 1:2:3, then substance A, substance B and substance C should correspond to the proportional numbers in the proportional formula in the order of description, that is, the weight of substance A: the weight of substance B: the weight of substance C = 1:2:3.

[0025] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.

[0026] In the first aspect, the present application provides a hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion, wherein the coated steel plate comprises a steel substrate, a zinc-aluminum-magnesium coating and a Fe-Al alloy layer, wherein the Fe-Al alloy layer is located between the steel substrate and the zinc-aluminum-magnesium coating, and the chemical composition of the zinc-aluminum-magnesium coating comprises, by mass fraction: Mg: 1.5% to 3%, Al: 2.5% to 4% and a matrix element Zn.

[0027] The hot-dip galvanized aluminum-magnesium coated steel sheet of the embodiment of the present application comprises a steel substrate, a zinc-aluminum-magnesium coating layer, and a Fe-Al alloy layer between the steel substrate and the zinc-aluminum-magnesium coating layer. There is no special limitation on the steel substrate, and ordinary steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets can be used. There is no special limitation on the type of steel, and for example, aluminum-killed steel, ultra-low carbon steel, and high-strength steel can be used.

[0028] The Al 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 bonding between the coating and the steel plate will be very poor, resulting in the coating being unusable and reducing corrosion resistance. Exemplarily, the mass fraction of Al in the coating can be: 2.5%, 2.8%, 3.1%, 3.4%, 3.7%, 4%, etc. Mg in the coating can significantly improve the atmospheric corrosion resistance of the coating. The mechanism is that the Mg in the coating will preferentially dissolve into the water film on the surface of the coating in the atmospheric environment, react with dissolved carbon dioxide in the water film, and precipitate a dense protective film. This protective film can exist stably in neutral and weak alkaline environments, and can also cause the electrolyte solution on the surface of the coating to become a weak alkaline solution, thereby improving the corrosion resistance of the coating. Exemplarily, the mass fraction of Mg in the coating can be: 1.5%, 1.8%, 2.1%, 2.4%, 2.7%, 3%, etc.

[0029] In the atmosphere, especially in an atmosphere with high humidity, a thin liquid film will adhere to the surface of the zinc-aluminum-magnesium coating. The oxygen enriched in the thin liquid film preferentially reacts electrochemically with the magnesium in the coating to quickly form magnesium compounds, while the pH value in the liquid film gradually increases. In this case, the liquid film can dissolve Al, causing the aluminum-rich phase in the coating to react with the oxygen, hydroxide and carbonate in the liquid film to form aluminum-rich complex compounds.

[0030] In some embodiments, the chemical composition of the zinc-aluminum-magnesium coating satisfies: [Al]=(1.2-2.0)×[Mg], where [Mg] represents the mass fraction of Mg and [Al] represents the mass fraction of Al.

[0031] When the mass fraction of Al in the zinc-aluminum-magnesium coating is 3.7% and the mass fraction of Mg is 2.5%, the coating is almost entirely eutectic structure. Therefore, the embodiment of the present application limits the mass fraction of Al in the coating to 2.5% to 4%, and the mass fraction of Mg to 1.5% to 3%. Furthermore, when the Al content is slightly higher than the Mg content, no obvious binary eutectic structure will appear in the solidification process of the coating. Therefore, the mass fractions of Al and Mg in the coating are limited to satisfy: the mass fraction of aluminum is 1.2 to 2.0 times the mass fraction of magnesium. When the mass fractions of Al and Mg are within this range, the eutectic structure in the coating can be mainly a ternary eutectic structure.

[0032] Zn is a matrix element. The specific content / content range of Zn can be obtained by the upper and lower limit formula of the component, that is:

[0033] The sum of the percentages of the contents of each component in a composition should be equal to 100%, and the content ranges of several components should meet the following conditions: the upper limit value of a component + the lower limit value of other components ≤ 100; the lower limit value of a component + the upper limit value of other components ≥ 100.

[0034] In some embodiments, the Fe—Al alloy layer has a thickness of 500 nm to 2 μm.

[0035] The thickness of the Fe-Al compound layer between the coating and the steel substrate has a significant effect on controlling the filamentary corrosion of the zinc-aluminum-magnesium coating. This is because after the acidic solution corrodes the eutectic structure, if it continues to corrode the steel substrate, it will cause the iron to dissolve to form ferrous ions, and the dissolved ferrous ions will accelerate the spread of the filamentary corrosion morphology. This is because the ferrous ions can diffuse to the location where the magnesium compound is precipitated, and then be oxidized into trivalent iron ions, thus forming another chemical reaction between the corrosion location and the non-corrosion location. If the Fe-Al compound layer between the coating and the steel substrate is thick enough, it can significantly delay the corrosion of the steel substrate by the acid solution. Although the Fe-Al compound layer also contains Fe, the Fe content in the Fe-Al compound layer is significantly lower than that of the steel substrate, only about 50%, so the Fe-Al compound layer does not have a significant effect on promoting filamentary corrosion. Therefore, in the embodiment of the present application, the thickness of the Fe-Al compound layer between the coating and the steel substrate is controlled to be ≥500nm, but if the Fe-Al compound layer is too thick, the corrosion resistance of the zinc-aluminum-magnesium coating will be deteriorated, so that the corrosion potential of the coating is increased and the corrosion current is increased. Therefore, the thickness of the Fe-Al compound layer is controlled to be ≤2 μm. For example, the thickness of the Fe-Al alloy layer can be 500 nm, 800 nm, 1.1 μm, 1.4 μm, 1.7 μm, 2 μm, etc.

[0036] In some embodiments, the area percentage of the binary eutectic structure on the surface of the zinc-aluminum-magnesium coating is ≤10%.

[0037] In some embodiments, the area percentage of the surface ternary eutectic structure of the zinc-aluminum-magnesium coating is ≥50%.

[0038] Eutectic structures are divided into ternary eutectic structures and binary eutectic structures. The Mg in the zinc-aluminum-magnesium coating is mainly concentrated in the eutectic structure. The ternary eutectic structure is a special structure formed by the coexistence of three crystals of different components at a specific ratio at a eutectic point. The ternary eutectic structure includes aluminum-rich phase, zinc-rich phase and magnesium-zinc compounds, such as Mg2Zn 11, MgZn2, etc., while the binary eutectic structure usually only contains magnesium-zinc compounds and zinc-rich phases or aluminum-rich phases. In an acidic environment, the coarse binary eutectic structure will dissolve first, followed by the dissolution of the ternary eutectic structure. As the eutectic structure dissolves, the acid ions combine with the magnesium element to form a compound with low solubility and precipitate. If the reaction occurs under the organic film, the location where the acid dissolves is not the same as the location where the compound precipitates, and the corrosion presents a filamentary corrosion morphology. In order to inhibit the occurrence of this filamentary corrosion, the most effective way is to eliminate the acidic environment. However, in some cases, the acidic environment is difficult to avoid, so it is necessary to optimize the zinc-aluminum-magnesium coating material. Since the binary eutectic structure is most sensitive to acid solutions, if the proportion of the binary eutectic structure is reduced, the risk of filamentary corrosion can be significantly reduced. Therefore, the proportion of binary eutectic structure in the coating is controlled to be ≤10%. For example, the area percentage of the binary eutectic structure on the surface of the zinc-aluminum-magnesium coating can be 2%, 4%, 6%, 8%, 10%, etc.

[0039] For the ternary eutectic structure, it is difficult to completely eliminate the zinc-aluminum-magnesium coating, because as long as aluminum and magnesium are added, the eutectic structure will be formed. However, if the area ratio of the ternary eutectic structure is increased, although the acid solution will corrode the eutectic structure, the position of the precipitate can be close to the position where the eutectic structure is corroded by the acid solution, which can also avoid the appearance of filamentary corrosion morphology. Therefore, the proportion of the ternary eutectic structure in the coating can be controlled to be ≥50%. Exemplarily, the area percentage of the surface ternary eutectic structure of the zinc-aluminum-magnesium coating can be 50%, 52%, 54%, 56%, 58%, 60%, etc.

[0040] In some embodiments, the equivalent diameter of the initial solidified zinc-rich phase of the zinc-aluminum-magnesium coating is ≤100 μm.

[0041] The initial solidified zinc-rich phase refers to the zinc-rich phase that solidifies first during the solidification process of the zinc-aluminum-magnesium coating due to the relatively low melting point of zinc. During the solidification process of the zinc-aluminum-magnesium coating, due to the low melting point of zinc, it begins to solidify first, forming the initial zinc-rich phase. As the solidification process proceeds, other alloying elements such as aluminum and magnesium gradually solidify to form a more complex phase structure. The initial solidified zinc-rich phase is rich in zinc, has high electrochemical activity and corrosion resistance, plays a major role in corrosion protection in the coating, and can corrode first, thereby protecting the substrate from corrosion.

[0042] The equivalent diameter is usually used to describe the equivalent size of particles, droplets or phases under certain specific conditions. Here, it refers to the average size of the zinc-rich phase in the initial solidification stage. The smaller the equivalent diameter, the more uniform the distribution of the zinc-rich phase in the coating, and the size of each phase is relatively small. The equivalent diameter of the initial solidified zinc-rich phase of the zinc-aluminum-magnesium coating is ≤100μm. This limitation ensures that the size of the zinc-rich phase will not be too large, thereby avoiding the formation of excessive stress concentration points or defects in the coating. On the other hand, the size of the initial solidified zinc-rich phase in the coating is reduced, which can make the connection between the ternary eutectic structures closer, and can improve the uniformity of the ternary eutectic structure during acid solution corrosion. Exemplarily, the equivalent diameter of the initial solidified zinc-rich phase of the zinc-aluminum-magnesium coating can be 10μm, 20μm, 30μm, 40μm, 50μm, 60μm, 70μm, 80μm, 90μm, 100μm, etc.

[0043] Figure 1 A schematic flow chart of a method for preparing a hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion provided in an embodiment of the present application.

[0044] See also Figure 1 In a second aspect, the present application provides a method for preparing the hot-dip galvanized aluminum-magnesium coated steel sheet resistant to filiform corrosion as described in the first aspect, the method comprising:

[0045] S1, hot-dip galvanizing the steel substrate to obtain a hot-dip galvanized aluminum-magnesium coated steel plate;

[0046] S2. Cooling the hot-dip galvanized aluminum-magnesium coated steel plate at a set cooling speed.

[0047] In some embodiments, the hot-dip coating time is 5s to 10s.

[0048] In some embodiments, the bath temperature of the hot-dip plating is 400°C to 480°C.

[0049] In some embodiments, the set cooling rate is ≥10K / s.

[0050] In the manufacturing process of zinc-aluminum-magnesium coated steel sheets, hot-dip coating and the cooling process after hot-dip coating play an important role in the coating structure. During the hot-dip coating process, a Fe-Al compound layer is formed between the steel substrate and the coating. The thickness of the Fe-Al compound layer is related to the hot-dip coating time and temperature. The longer the hot-dip coating time and the higher the temperature, the thicker the Fe-Al compound layer.

[0051] Controlling the hot-dip plating time within 5s to 10s can ensure the formation of a Fe-Al compound layer of a certain thickness but not too thick. This can effectively isolate the direct corrosion of the acid solution on the steel substrate, delay the formation and diffusion of ferrous ions, and thus slow down the spread of filamentary corrosion; it can also avoid the deterioration of the corrosion resistance of the coating caused by the excessive thickness of the Fe-Al compound layer, and maintain the good protective performance of the coating. In addition, the hot-dip plating time of 5s to 10s not only affects the formation of the Fe-Al compound layer, but also has an important impact on the overall quality of the coating. A hot-dip plating time of more than 10s may cause the coating to be too thick and the structure to be coarsened, affecting the adhesion and corrosion resistance of the coating; while a time of less than 5s may cause the coating to be incomplete and poor in bonding. Controlling the hot-dip plating time within 5s to 10s can ensure that the coating has an appropriate thickness, uniform microstructure and good adhesion, and provide the coating with excellent resistance to filamentary corrosion. Exemplarily, the hot-dip plating time can be 5s, 6s, 7s, 8s, 9s, 10s, etc.

[0052] Hot-dip plating in the temperature range of 400℃ to 480℃ can ensure that a Fe-Al compound layer with a certain thickness and dense structure is formed between the steel plate and the coating. A temperature below 400℃ may result in insufficient formation of the Fe-Al compound layer, insufficient thickness, and inability to provide sufficient protection; while a temperature above 480℃ may make the Fe-Al compound layer too thick, deteriorating the corrosion resistance of the coating. In addition, a bath temperature of 400℃ to 480℃ not only helps to obtain a high-quality coating, but also improves production efficiency. Too high or too low a temperature may lead to a decrease in production efficiency, increase energy consumption and production costs. Hot-dip plating in the temperature range of 400℃ to 480℃ can ensure a good balance between production efficiency and energy consumption. Exemplarily, the bath temperature of hot-dip plating can be 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, 460℃, 470℃, 480℃, etc.

[0053] After hot-dip plating, the liquid zinc-aluminum-magnesium plating solution will solidify and form various microstructures as it cools. Initially, an initial solidification structure will be formed at a higher temperature, and then a eutectic structure will be formed at a lower temperature. A cooling rate of ≥10K / s can allow the liquid plating solution to form a finer initial solidification structure during the solidification process. These fine structures have higher density and uniformity, and can more effectively block the penetration of corrosive media, thereby improving the corrosion resistance of the coating. In addition, a cooling rate of ≥10K / s can also effectively avoid the formation of too much binary eutectic structure. The binary eutectic structure is more sensitive to acid solutions, is easy to dissolve and cause filamentous corrosion, and reducing the proportion of binary eutectic structures can significantly reduce the risk of filamentous corrosion. Exemplarily, the cooling rate can be 10K / s, 15K / s, 20K / s, 25K / s, 30K / s, 35K / s, etc.

[0054] The product prepared by the method for preparing hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion is the above-mentioned hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion. Since the method for preparing hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion adopts part or all of the technical solutions of the embodiment of the hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion, it at least has all the beneficial effects brought by the technical solutions of the hot-dip galvanized aluminum-magnesium coated steel plate resistant to filiform corrosion, which will not be repeated here one by one.

[0055] The present application is further described below in conjunction with specific embodiments. The experimental methods in the following embodiments that do not specify specific conditions are usually measured in accordance with national standards / industry standards; if there are no corresponding national standards / industry standards, they are measured in accordance with common international standards, conventional conditions, or conditions recommended by the manufacturer.

[0056] A steel plate with a thickness of 1.0 mm and a width of 1500 mm is used as the substrate, and the coating weight is 100 g / m2 on both sides. The present application sets Examples 1 to 13 and Comparative Examples 1 to 5. The chemical composition (wt%), surface characteristics and thickness of the zinc-aluminum-magnesium coating are shown in Table 1.

[0057] Table 1 Chemical composition (wt%), surface characteristics and thickness of Fe-Al alloy layer (μm) of Zn-Al-Mg coating

[0058]

[0059]

[0060] The preparation process conditions of Examples 1 to 13 and Comparative Examples 1 to 5 set in this application are shown in Table 2.

[0061] The zinc-aluminum-magnesium coated steel sheets in the above-mentioned Examples 1 to 13 and Comparative Examples 1 to 5 were subjected to a filiform corrosion resistance evaluation test:

[0062] A 20 μm thick PVB organic film is coated on the surface of the zinc-aluminum-magnesium coating, and then a scratch is made on the surface of the organic film. The scratch width is 1 mm and the scratch depth reaches the steel substrate. 5 μL of acetic acid solution is injected into the scratch position. The concentration of the acetic acid solution is 1 mol / dm 3 , and then placed in a constant temperature and humidity environment (22°C, 86% RH) for 4 weeks. Then the growth length of the filiform corrosion on the sample surface was evaluated according to GB / T 30789.9. The longer the growth, the easier it is to have filiform corrosion. The experimental evaluation results are shown in Table 2.

[0063] Table 2 Preparation process parameters and filiform corrosion resistance evaluation test results of the embodiments and comparative examples

[0064]

[0065]

[0066] It can be seen from Tables 1 to 2 that in Examples 1 to 13, the chemical composition and surface characteristics of the zinc-aluminum-magnesium coatings are within the required range of the present invention, and the corrosion resistance is good.

[0067] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0068] The method for preparing the hot-dip galvanized aluminum-magnesium coated steel plate provided by the embodiment of the present invention is simple and easy to operate, has low production cost, and is easy to promote and use.

[0069] The above is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. It will be apparent to those skilled in the art that various modifications to these embodiments are possible, and the general principles defined in the present application can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown in the present application, but will conform to the widest scope consistent with the principles and novel features applied for by the present application.

Claims

1. A hot-dip galvanized aluminum-magnesium coated steel sheet resistant to filiform corrosion, the coated steel sheet comprising a steel substrate, a zinc-aluminum-magnesium coating and a Fe-Al alloy layer, the Fe-Al alloy layer being between the steel substrate and the zinc-aluminum-magnesium coating, and the chemical composition of the zinc-aluminum-magnesium coating comprising, by mass fraction: Mg: 1.5%~3%, Al: 2.5%~4% and matrix element Zn.

2. The coated steel sheet according to claim 1, characterized in that: The thickness of the Fe-Al alloy layer is 500nm-2μm.

3. The coated steel sheet according to claim 1, characterized in that: The chemical composition of the zinc-aluminum-magnesium coating satisfies: [Al] = (1.2-2.0) × [Mg], Wherein, [Mg] represents the mass fraction of Mg, and [Al] represents the mass fraction of Al.

4. The coated steel sheet according to claim 1, characterized in that: The area percentage of the binary eutectic structure on the surface of the zinc-aluminum-magnesium coating is ≤10%.

5. The coated steel sheet according to claim 1, characterized in that: The area percentage of the surface ternary eutectic structure of the zinc-aluminum-magnesium coating is ≥50%.

6. The coated steel sheet according to claim 1, characterized in that: The equivalent diameter of the initial solidified zinc-rich phase of the zinc-aluminum-magnesium coating is ≤100 μm.

7. A method for preparing a hot-dip galvanized aluminum-magnesium coated steel sheet resistant to filiform corrosion according to any one of claims 1 to 5, characterized in that: The method comprises: Hot-dip galvanizing the steel substrate to obtain a hot-dip galvanized aluminum-magnesium coated steel plate; The hot-dip galvanized aluminum-magnesium coated steel sheet is cooled at a set cooling rate.

8. The method according to claim 6, characterized in that The hot-dip coating time is 5s to 10s.

9. The method according to claim 6, characterized in that The bath temperature of the hot-dip plating is 400°C to 480°C.

10. The method according to claim 6, characterized in that The set cooling rate is ≥10K / s.

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