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

Abstract

The invention relates to a filiform corrosion resistant zinc-aluminum-magnesium coated steel plate and a preparation method thereof, and belongs to the technical field of coating. The coated steel plate comprises a steel substrate and a zinc-aluminum-magnesium coating attached to at least part of the surface of the steel substrate, and the zinc-aluminum-magnesium coating comprises the following chemical components in percentage by mass: 1-4% of Mg, 1-4% of Al, 0.5-2% of Bi and a substrate element Zn; and the zinc-aluminum-magnesium plating layer contains an Mg-Bi compound. Bi is added into the plating layer, the Bi and Mg in the plating layer form an Mg-Bi compound, the Mg-Bi compound exists in the plating layer in a granular or ellipsoidal shape, on one hand, part of magnesium is consumed, the number of eutectic structure Mg-Zn compounds in the plating layer is reduced, and therefore the sensitivity of filiform corrosion is reduced; and on the other hand, the Mg-Bi compound also has certain corrosion resistance, so that the overall corrosion resistance of the coating can be further enhanced.

Description

Technical Field

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

[0002] Hot-dip galvanizing is to make the molten metal react with the iron matrix to produce an alloy layer, thereby combining the matrix and the coating. Hot-dip galvanized steel has the advantages of uniform coating, strong adhesion, long service life, simple manufacturing process, and low product price. It is widely used in the manufacture of automobile bodies, home appliances, etc. In order to improve the protective effect of the hot-dip galvanizing coating on the incision position of the steel plate and improve the flat corrosion resistance, an appropriate amount of Mg is added to the hot-dip coating to obtain a zinc-aluminum-magnesium coating. The corrosion resistance can be further improved by more than 20%, and it also has the corrosion resistance of the processing incision position.

[0003] However, after adding magnesium to the hot-dip galvanized coating, the zinc-aluminum-magnesium coating will undergo filiform corrosion under the organic film under certain conditions. This filiform corrosion will form a corrosion morphology that extends in a specific direction, affecting the appearance quality of the organic film and, in severe cases, the corrosion resistance of the coating. Summary of the invention

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

[0005] In the first aspect, an embodiment of the present application provides a zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion, wherein the coated steel plate comprises a steel substrate and a zinc-aluminum-magnesium coating attached to at least a portion of the surface of the steel substrate, wherein the chemical composition of the zinc-aluminum-magnesium coating comprises, by mass fraction: Mg: 1% to 4%, Al: 1% to 4%, Bi: 0.5% to 2%, and a matrix element Zn; the zinc-aluminum-magnesium coating contains a Mg-Bi compound.

[0006] Optionally, the volume fraction of the Mg-Bi compound is ≥0.05%.

[0007] Optionally, the volume fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium coating is ≤20%.

[0008] Optionally, the volume fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium coating is ≥70%.

[0009] Optionally, the chemical composition of the zinc-aluminum-magnesium coating satisfies: [Mg]=(2-10)×[Bi], wherein [Mg] represents the mass fraction of Mg, and [Bi] represents the mass fraction of Bi.

[0010] Optionally, the Mg-Bi compound is in a granular and / or ellipsoidal shape, and the diameter of the circumscribed circle of the Mg-Bi compound is 10 μm to 500 μm.

[0011] Optionally, the chemical composition of the zinc-aluminum-magnesium coating satisfies: Al+Mg≤2.5%, wherein Al+Mg represents the sum of the mass fractions of Al and Mg.

[0012] Optionally, the chemical composition of the zinc-aluminum-magnesium coating satisfies: Al+Mg≥5%, wherein Al+Mg represents the sum of the mass fractions of Al and Mg.

[0013] In a second aspect, an embodiment of the present application provides a method for preparing the zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion as described in the first aspect, the method comprising:

[0014] Hot-dip coating the steel substrate to obtain a zinc-aluminum-magnesium coated steel plate;

[0015] The zinc-aluminum-magnesium coated steel sheet is cooled.

[0016] Optionally, the plating solution temperature of the hot dip plating is 450°C to 500°C.

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

[0018] The embodiment of the present application provides a zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion, the coated steel sheet comprising a steel substrate and a zinc-aluminum-magnesium coating attached to at least a portion of the surface of the steel substrate, the chemical composition of the zinc-aluminum-magnesium coating comprising, by mass fraction: Mg: 1% to 4%, Al: 1% to 4%, Bi: 0.5% to 2%, and a matrix element Zn; the zinc-aluminum-magnesium coating contains a Mg-Bi compound. By adding Bi to the coating, a Mg-Bi compound is formed with the Mg in the coating, and the Mg-Bi compound exists in the coating in a granular or ellipsoidal shape, which consumes part of the magnesium, reduces the amount of Mg-Zn compounds in the eutectic structure of the coating, and thus reduces the sensitivity to filiform corrosion; on the other hand, the Mg-Bi compound itself also has certain anti-corrosion properties, which can further enhance the overall corrosion resistance of the coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings herein are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present application, and together with the description, are used to explain the principles of the embodiments 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 zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion provided in an embodiment of the present application;

[0022] Figure 2 The microstructure diagram provided for Example 3 of the present application;

[0023] Figure 3 This is a microstructure diagram provided for comparative example 5 of the present application. DETAILED DESCRIPTION

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] In the first aspect, an embodiment of the present application provides a zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion, wherein the coated steel plate comprises a steel substrate and a zinc-aluminum-magnesium coating attached to at least a portion of the surface of the steel substrate, wherein the chemical composition of the zinc-aluminum-magnesium coating comprises, by mass fraction: Mg: 1% to 4%, Al: 1% to 4%, Bi: 0.5% to 2%, and a matrix element Zn; the zinc-aluminum-magnesium coating contains a Mg-Bi compound.

[0029] The zinc-aluminum-magnesium coated steel sheet of the embodiment of the present application comprises a steel substrate and a zinc-aluminum-magnesium coating layer attached to at least a portion of the surface of the steel substrate. There is no particular limitation on the steel substrate, and ordinary steel plates such as hot-rolled steel plates and cold-rolled steel plates can be used. There is no particular limitation on the type of steel, and for example, aluminum-killed steel, ultra-low carbon steel, and high-strength steel can be used.

[0030] Mg is one of the key elements in zinc-aluminum-magnesium coatings to improve atmospheric corrosion resistance. When the mass fraction of Mg is in the range of 1% to 4%, the Mg in the coating can be preferentially dissolved into the water film on the surface of the coating in the atmospheric environment, react with dissolved carbon dioxide, and precipitate a dense protective film. This protective film can exist stably in neutral and weakly alkaline environments, thereby significantly improving the corrosion resistance of the coating, especially in atmospheric environments with high humidity, where the protective effect of Mg is particularly significant. In the deep drawing process of zinc-aluminum-magnesium coated automotive sheets, the brittleness of the coating is an issue that needs to be considered. Too high or too low a mass fraction of Mg may increase the brittleness of the coating, thereby affecting the processing performance of the automotive sheet. When the mass fraction of Mg is controlled at 1% to 4%, the strength and toughness of the coating can be effectively balanced, so that it can meet the corrosion resistance requirements while also having good processing performance, which helps to reduce the cracking and peeling of the coating during processing and improve the yield and quality of automotive sheets. Exemplarily, the mass fraction of Mg in the coating may be: 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, etc.

[0031] In a corrosive environment, Al can form a dense oxide and hydroxide protective film on the surface of the coating. This protective film has high stability and density, and can effectively isolate corrosive media in the environment, such as oxygen, moisture and carbon dioxide, thereby slowing down or preventing the occurrence of the corrosion process. When the mass fraction of Al is in the range of 1% to 4%, it can not only ensure the formation of a protective film of sufficient quantity and quality, but also avoid other negative effects that may be caused by excessive Al content. Secondly, the addition of 1% to 4% Al can also help improve the bonding between the coating and the steel substrate. If the mass fraction of Al in the coating is too low, the bonding between the coating and the steel plate may be very poor, resulting in the coating falling off or peeling off during processing or use, thereby reducing the corrosion resistance of the coating. In addition, limiting the mass fraction of Al to 1% to 4% can also help control the physical properties and processing properties of the coating. Within this range, the physical properties of the coating, such as hardness, toughness and ductility, can be well balanced, which not only enables the coating to maintain good formability during processing, but also reduces defects such as cracks and fractures that occur during deep processing. At the same time, 1% to 4% Al can also help improve the fatigue resistance of the coating and extend the service life of the coating. In addition, it is worth noting that although Al has a positive effect on the corrosion resistance, bonding strength and physical properties of the coating, if the content is higher than 4%, it may cause excessive aluminum-rich phases to form in the coating, which can easily cause cracking and peeling of the coating during processing. Therefore, limiting the mass fraction of the Al element to 1% to 4% can not only make full use of its positive effects, but also avoid its possible negative effects. Exemplarily, the mass fraction of Al in the coating can be: 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, etc.

[0032] The addition of bismuth can form Mg-Bi compounds with Mg in the coating. The presence of this compound in the coating can effectively consume part of the magnesium, thereby reducing the number of Mg-Zn compounds in the eutectic structure. Eutectic structures, especially Mg-Zn compounds, are prone to preferential dissolution in acidic environments, thereby inducing filiform corrosion. Therefore, the formation of Mg-Bi compounds is an effective corrosion protection mechanism that can reduce the risk of filiform corrosion. The amount of bismuth added has an important influence on the overall performance of the coating. If the mass fraction of bismuth is higher than 2%, large blocks of Mg-Bi compounds will be formed in the coating instead of granules, making the Mg-Bi compounds greatly destroy the filiform corrosion resistance of the coating. At the same time, the mass fraction of bismuth higher than 2% will also increase the brittleness of the coating and reduce its forming performance. In addition, a mass fraction of bismuth higher than 2% will cause obvious galvanic corrosion effects in the coating, and the corrosion resistance of the coating will be significantly reduced. Therefore, limiting the mass fraction of bismuth to between 0.5% and 2% can ensure the corrosion resistance of the coating while taking into account its forming performance and toughness. Exemplarily, the mass fraction of Bi in the coating may be: 0.5%, 0.8%, 1.1%, 1.4%, 1.7%, 2%, etc.

[0033] 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:

[0034] 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.

[0035] In some embodiments, the volume fraction of the Mg—Bi compound is ≥ 0.05%.

[0036] Filiform corrosion is a form of local corrosion that may occur in zinc-aluminum-magnesium coatings under specific environments (such as acidic environments), which will cause the coating to gradually fail, thereby affecting the overall performance of the automotive plate. The formation of Mg-Bi compounds can consume part of the Mg element in the coating and reduce the content of Mg-Zn compounds in the eutectic structure. Since the eutectic structure is a prone area for filiform corrosion, reducing the proportion of Mg-Zn compounds helps to reduce the risk of filiform corrosion. By limiting the volume fraction of Mg-Bi compounds to ≥0.05%, it can be ensured that a sufficient amount of Mg-Bi compounds are formed, thereby effectively inhibiting the occurrence of filiform corrosion.

[0037] In some embodiments, the volume fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium coating is ≤20%.

[0038] In some embodiments, the volume fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium coating is ≥70%.

[0039] The eutectic structure is divided into ternary eutectic structure and binary eutectic structure. The Mg in the zinc-aluminum-magnesium coating is mainly concentrated in the eutectic structure. The ternary eutectic structure includes aluminum-rich phase, zinc-rich phase and magnesium-zinc compounds, such as Mg2Zn 11 , MgZn2, etc. The binary eutectic structure usually only contains magnesium-zinc compounds and zinc-rich phases or aluminum-rich phases. In an acidic environment, the Mg-Zn compounds in the eutectic structure will dissolve preferentially. 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 filamentous corrosion morphology. In order to inhibit the occurrence of this filamentous corrosion, the most effective way is to eliminate the acidic environment. For the eutectic structure, the zinc-aluminum-magnesium coating is difficult to completely eliminate, because as long as aluminum and magnesium are added, a eutectic structure will be formed. Because the eutectic structure is prone to local corrosion in an acidic environment, it will evolve into filamentous corrosion. Therefore, it is beneficial to reduce the proportion of eutectic structure. The embodiment of the present application can control the proportion of eutectic structure in the coating to less than 20%. For example, the volume fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium coating can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, etc.

[0040] However, if the content of eutectic structure is high, although the eutectic structure is prone to local corrosion, the local corrosion will not evolve into filiform corrosion, so it is also beneficial to further increase the proportion of eutectic structure. Studies have found that if the proportion of eutectic structure in the coating is not less than 70%, the possibility of filiform corrosion is also low. Exemplarily, the volume fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium coating can be 70%, 72%, 74%, 76%, 78%, 80%, 82%, 84%, 86%, 88%, 90%, etc.

[0041] In some embodiments, the chemical composition of the zinc-aluminum-magnesium coating satisfies: [Mg]=(2-10)×[Bi], where [Mg] represents the mass fraction of Mg, and [Bi] represents the mass fraction of Bi.

[0042] The purpose of adding Bi is to obtain Mg-Bi compound, so there should be a certain relationship between the contents of Mg and Bi so that Mg and Bi can be formed to the maximum extent. Therefore, the amount of Bi added is required to reach one-tenth to one-half of Mg.

[0043] In some embodiments, the Mg-Bi compound is in a granular and / or ellipsoidal shape, and the diameter of the circumscribed circle of the Mg-Bi compound is 10 μm to 500 μm.

[0044] The morphology and size of the Mg-Bi compound are crucial to its corrosion protection effect. When the mass fraction of bismuth is controlled at 0.5% to 2%, a granular or ellipsoidal Mg-Bi compound can be formed. If the Mg-Bi compound is too small, it cannot prevent the extension of filamentary corrosion. However, if the Mg-Bi compound is too large, the brittleness of the coating will increase, and the coating will easily crack and peel off during deep drawing of automobile plates. Therefore, the circumscribed circle diameter of the Mg-Bi compound is limited to between 10 μm and 500 μm. Mg-Bi compounds of this size can effectively prevent the extension of filamentary corrosion without making the coating too brittle, thereby avoiding the problem of cracking and peeling of the coating during deep drawing. Exemplarily, the circumscribed circle diameter of the Mg-Bi compound can be 10 μm, 50 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 450 μm, 500 μm, etc.

[0045] In some embodiments, the chemical composition of the zinc-aluminum-magnesium coating satisfies: Al+Mg≤2.5%, where Al+Mg represents the sum of the mass fractions of Al and Mg.

[0046] In order to control the proportion of eutectic structure in the coating to ≤20%, the sum of the contents of Al and Mg in the coating needs to be controlled to ≤2.5%, and of course the contents of Al and Mg cannot be less than 1%. For example, the sum of the mass fractions of Al and Mg in the coating can be 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, etc.

[0047] In some embodiments, the chemical composition of the zinc-aluminum-magnesium coating satisfies: Al+Mg≥5%, where Al+Mg represents the sum of the mass fractions of Al and Mg.

[0048] In order to control the eutectic structure in the coating to ≥70%, the sum of the Al and Mg contents in the coating needs to be controlled to 5% or more, and of course the Al and Mg contents cannot exceed 4%. For example, the sum of the mass fractions of Al and Mg in the coating can be 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, etc.

[0049] In summary, the embodiments of the present application provide a zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion, which has the following technical advantages:

[0050] This zinc-aluminum-magnesium coated steel plate with filamentary corrosion resistance has a wide range of application prospects in many fields such as construction, automobiles, and home appliances due to its excellent corrosion resistance, hardness, and wear resistance. Especially in environments that require high corrosion resistance, such as seaside and industrial areas, this coated steel plate will show more outstanding performance advantages.

[0051] The zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion provided in the embodiment of the present application significantly improves the filiform corrosion resistance of the coating by introducing Mg-Bi compounds and optimizing the coating composition and structure. This innovative achievement has brought new breakthroughs in the field of coating technology and also provided related industries with a better quality and high performance material selection.

[0052] Figure 1 A schematic flow chart of a method for preparing a zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion provided in an embodiment of the present application.

[0053] See also Figure 1 In a second aspect, an embodiment of the present application provides a method for preparing the zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion as described in the first aspect, the method comprising:

[0054] Hot-dip coating the steel substrate to obtain a zinc-aluminum-magnesium coated steel plate;

[0055] The zinc-aluminum-magnesium coated steel sheet is cooled.

[0056] In some embodiments, the bath temperature of the hot-dip plating is 450°C to 500°C.

[0057] During the hot-dip plating process, the temperature of the plating solution has an important influence on the structure and performance of the coating. When the plating solution temperature is too low, it will make it difficult to form Mg-Bi compounds in the coating; when the plating solution temperature is too high, a coarse, blocky Mg-Bi phase will be formed. The size of the Mg-Bi phase is too large, the corrosion protection effect is weakened, and the coating becomes brittle. Therefore, it is necessary to perform hot-dip plating at a plating solution temperature of 450°C to 500°C to ensure that the content and morphology of the Mg-Bi compound in the coating reach the optimal state, thereby improving the quality and performance of the coating. Exemplarily, the plating solution temperature can be 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, etc.

[0058] The product prepared by the method for preparing zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion is the above-mentioned zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion. Since the method for preparing zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion adopts part or all of the technical solutions of the embodiment of the zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion, it at least has all the beneficial effects brought by the technical solutions of the zinc-aluminum-magnesium coated steel plate resistant to filiform corrosion, which will not be repeated here one by one.

[0059] 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.

[0060] A steel plate with a thickness of 1.0 mm and a width of 1500 mm was used as the substrate, and the coating weight was 100 g / m2 on both sides. The characteristics of the inventive examples 1-13 and the comparative examples 1-5 are shown in Table 1.

[0061] Table 1

[0062]

[0063] The preparation process conditions of Examples 1-13 of the present invention and Comparative Examples 1-5 are shown in Table 2.

[0064] The zinc-aluminum-magnesium coated steel sheets prepared in the above-mentioned examples 1-13 and comparative examples 1-5 were subjected to a filiform corrosion resistance evaluation experiment: a layer of 20 micron thick PVB organic film was coated on the surface of the zinc-aluminum-magnesium coating, and then a scratch was made on the surface of the organic film, the scratch width was 1 mm, and the scratch depth reached the steel substrate, 5 μL of acetic acid solution was injected into the scratch position, and the pH of the acetic acid solution was 3, and then placed in a constant temperature and humidity environment (40°C, 86% RH) and stored for 4 weeks. Then the maximum length of filiform corrosion on the surface of the sample was evaluated according to GB / T 30789.9. The longer the corrosion filaments, the easier it is to cause filiform corrosion. If the length of the filiform corrosion is not greater than 2 mm, the anti-filiform corrosion performance is excellent (□). If the length of the filiform corrosion exceeds 2 mm but is less than 4 mm, the anti-filiform corrosion performance is good (▲), and if the length of the filiform corrosion exceeds 4 mm, the anti-filiform corrosion performance is poor (×).

[0065] The zinc-aluminum-magnesium coated steel sheets prepared in the above examples 1-13 and comparative examples 1-5 were evaluated for coating brittleness: the samples were bent 180° by a bending method, and then the coating cracking and peeling at the bending position were observed. If the coating cracked or peeled at the bending position, the coating brittleness was poor (×), otherwise the coating brittleness was excellent (□).

[0066] The zinc-aluminum-magnesium coated steel plates prepared in the above-mentioned Examples 1-13 and Comparative Examples 1-5 were subjected to a neutral salt spray test, and the average thickness of the coating before corrosion and the average thickness of the coating after corrosion of the zinc-aluminum-magnesium coated steel were measured. The difference between the two was calculated and then divided by the corrosion time to obtain the average corrosion rate of the coating.

[0067] Average corrosion rate = (average thickness of coating before corrosion - average thickness of coating after corrosion) / corrosion time (μm / h).

[0068] If the average corrosion rate of the plating layer does not exceed 0.03 μm / hour, the corrosion resistance of the plating layer is excellent (□), and if the average corrosion rate of the plating layer exceeds 0.03 μm / hour, the corrosion resistance of the plating layer is poor (×).

[0069] The experimental evaluation results are shown in Table 2.

[0070] Table 2

[0071] Group Bath temperature / ℃ Filiform corrosion resistance of coating Corrosion resistance of coating Brittleness of coating Example 1 500 ▲ Example 2 480 Example 3 480 Example 4 450 Example 5 460 Example 6 460 Example 7 480 Example 8 470 Example 9 480 Example 10 460 Embodiment 11 500 Example 12 500 Embodiment 13 490 Comparative Example 1 430 × × × Comparative Example 2 410 × × × Comparative Example 3 540 × Comparative Example 4 580 × × × Comparative Example 5 510 ×

[0072] It can be seen from Tables 1 to 2 that in Examples 1 to 13, the chemical composition and characteristics of the zinc-aluminum-magnesium coatings are all within the required range of the present invention, and the corrosion resistance is good.

[0073] Attached Figure 2-3 Detailed description:

[0074] Figure 2 The microstructure diagram provided in Example 3 of the present application; Figure 2 As shown, the bright white dots are compounds formed by Mg and Bi, the light gray is the Zn phase, and the rest are eutectic structures.

[0075] Figure 3 The microstructure diagram provided for comparative example 5 of the present application embodiment; Figure 3 As shown, the light grey is the Zn phase and the rest is the eutectic structure.

[0076] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0077] The method for preparing the zinc-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.

[0078] The above description 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 the embodiments shown in the present application, but will conform to the widest range consistent with the principles and novel features applied for by the present application.

Claims

1. A zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion, the coated steel sheet comprising a steel substrate and a zinc-aluminum-magnesium coating attached to at least a portion of the surface of the steel substrate; The chemical composition of the zinc-aluminum-magnesium coating includes, by mass fraction, Mg: 1% to 4%, Al: 1% to 4%, Bi: 0.5% to 2%, and matrix element Zn; The zinc-aluminum-magnesium coating contains a Mg-Bi compound.

2. The coated steel sheet according to claim 1, characterized in that: The volume fraction of the Mg-Bi compound is ≥0.05%.

3. The coated steel sheet according to claim 1, characterized in that: The volume fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium coating is ≤20%.

4. The coated steel sheet according to claim 1, characterized in that: The volume fraction of the eutectic structure on the surface of the zinc-aluminum-magnesium coating is ≥70%.

5. The coated steel sheet according to claim 1, characterized in that: The chemical composition of the zinc-aluminum-magnesium coating satisfies: [Mg] = (2-10) × [Bi], In the formula, [Mg] represents the mass fraction of Mg, and [Bi] represents the mass fraction of Bi.

6. The coated steel sheet according to claim 1, characterized in that: The Mg-Bi compound is in a granular and / or ellipsoidal shape, and the diameter of the circumscribed circle of the Mg-Bi compound is 10 μm to 500 μm.

7. The coated steel sheet according to claim 1, characterized in that: The chemical composition of the zinc-aluminum-magnesium coating satisfies: Al+Mg≤2.5%, wherein Al+Mg represents the sum of the mass fractions of Al and Mg.

8. The coated steel sheet according to claim 1, characterized in that: The chemical composition of the zinc-aluminum-magnesium coating satisfies: Al+Mg≥5%, wherein Al+Mg represents the sum of the mass fractions of Al and Mg.

9. A method for preparing a zinc-aluminum-magnesium coated steel sheet resistant to filiform corrosion according to any one of claims 1 to 8, characterized in that: The method comprises: Hot-dip coating the steel substrate to obtain a zinc-aluminum-magnesium coated steel plate; The zinc-aluminum-magnesium coated steel sheet is cooled.

10. The method according to claim 9, characterized in that The bath temperature of the hot-dip plating is 450°C to 500°C.

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