Zinc-aluminum-magnesium plated layer, zinc-aluminum-magnesium plated steel sheet and method for producing the same, automobile outer panel

Abstract

The application relates to a zinc-aluminum-magnesium coating, a zinc-aluminum-magnesium coated steel plate and a preparation method thereof, and an automobile outer plate, wherein the chemical composition of the zinc-aluminum-magnesium coating comprises Al, Mg, Ti, B, RE and Zn; the content of the Al is 0.8-2.5% in mass fraction, the content of the Mg is 0.8-2.5% in mass fraction, and the following relation is satisfied: [Al]-[Mg] >= 0, [Ti]+[B]+[RE] <= 50ppm; and the equivalent diameter of intermetallic compound particles on the surface of the zinc-aluminum-magnesium coating is not more than 300 microns. The application solves the technical problem that the existing zinc-aluminum-magnesium coated steel plate is prone to local high points and stamping bright spots on the surface during stamping.

Description

Technical Field

[0001] This application relates to the field of coating technology, and in particular to a zinc-aluminum-magnesium coating, a zinc-aluminum-magnesium coated steel plate and its preparation method, and an automotive outer panel. Background Technology

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

[0003] Zinc-aluminum-magnesium (ZAM) coatings are widely used in automobile body manufacturing. However, ZAM coatings are prone to inclusion defects, which can cause localized high points and stamping bright spots on the surface of automobile outer panels during stamping, failing to meet the actual application requirements of users.

[0004] Compared to traditional pure zinc coatings and zinc-iron alloy coatings, zinc-aluminum-magnesium coatings contain more alloying elements. The addition of these alloying elements can introduce unexpected impurities. These impurities may form inclusions in the coating that affect stamping quality. Summary of the Invention

[0005] This application provides a zinc-aluminum-magnesium coating, a zinc-aluminum-magnesium coated steel sheet and its preparation method, and an automotive outer panel, to solve the technical problem that local high points and stamping bright spots easily occur on the surface of existing zinc-aluminum-magnesium coated steel sheets during stamping.

[0006] In a first aspect, this application provides a zinc-aluminum-magnesium coating, the chemical composition of which includes:

[0007] Al, Mg, Ti, B, RE, and Zn; where, by mass fraction,

[0008] The content of Al is 0.8% to 2.5%, and the content of Mg is 0.8% to 2.5%.

[0009] And it satisfies the following relationships: [Al]-[Mg]≥0, [Ti]+[B]+[RE]≤50ppm;

[0010] The equivalent diameter of the intermetallic compound particles on the surface of the zinc-aluminum-magnesium coating does not exceed 300 micrometers;

[0011] In the formula, [Al] represents the mass fraction of Al, [Mg] represents the mass fraction of Mg, [Ti] represents the mass fraction of Ti, [B] represents the mass fraction of B, and [RE] represents the mass fraction of RE.

[0012] Optionally, the equivalent diameter of the intermetallic compound particles on the surface of the zinc-aluminum-magnesium coating does not exceed 100 micrometers.

[0013] Optionally, the content of Al and the content of Mg satisfy the following relationship:

[0014] [Al]-[Mg]≥0.5%

[0015] In the formula, [Al] represents the mass fraction of Al, and [Mg] represents the mass fraction of Mg.

[0016] Optionally, the content of RE does not exceed 10 ppm.

[0017] Optionally, the types of intermetallic compounds include Fe-Zn compounds, Zn-RE compounds, Fe-Zn-Al compounds, Al-Ti compounds, Al-B compounds, Al-Ti-B compounds, and Mg-Zn compounds.

[0018] Secondly, this application provides a zinc-aluminum-magnesium coated steel sheet, the zinc-aluminum-magnesium coated steel sheet comprising a steel substrate and a zinc-aluminum-magnesium coating as described in the first aspect, which is attached to at least a portion of the surface of the steel substrate.

[0019] Thirdly, this application provides a method for preparing the zinc-aluminum-magnesium coated steel sheet described in the second aspect, the method comprising:

[0020] A steel matrix is ​​obtained;

[0021] The steel substrate is hot-dip galvanized and then surface-treated to obtain a zinc-aluminum-magnesium coated steel sheet.

[0022] Optionally, the surface treatment includes light rolling.

[0023] Optionally, the elongation of the slight rolling is 0.6% to 1.5%.

[0024] Fourthly, this application provides an automotive outer panel, the raw material of which includes the zinc-aluminum-magnesium coated steel sheet described in the second aspect.

[0025] The technical solutions provided in this application have the following advantages compared with the prior art:

[0026] The zinc-aluminum-magnesium coating provided in this application embodiment limits the Al content to provide high-quality atmospheric corrosion resistance and prevent surface cracking that could lead to dense polishing highlights. It also limits the Mg content to improve atmospheric corrosion resistance and prevent excessively large Mg-Zn compound sizes from causing polishing highlights after stamping. Furthermore, it limits the Al content to be no less than the Mg content to reduce the formation of large Mg-Zn compounds. The content of Ti, B, and RE (rare earth elements) in the coating is also limited to reduce the formation of non-hard intermetallic compounds. Finally, the size of the intermetallic compounds on the coating surface is limited to significantly reduce the formation of polishing highlights on the zinc-aluminum-magnesium coating substrate after stamping. In summary, this solves the technical problem of localized high points and stamping highlights easily occurring on the surface of existing zinc-aluminum-magnesium coated steel sheets during stamping. 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 The surface morphology of the zinc-aluminum-magnesium coated steel sheet in Example 1 of this application;

[0030] Figure 2 The surface morphology of the zinc-aluminum-magnesium coated steel sheet is shown in Comparative Example 1 of this application.

[0031] Figure 3 This is a schematic flowchart illustrating a method for preparing zinc-aluminum-magnesium coated steel sheets, provided in an embodiment of this application. Detailed Implementation

[0032] 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.

[0033] 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.

[0034] In this application, and in the description of this application specification, the terms "comprising," "including," etc., mean "including but not limited to."

[0035] 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.

[0036] In a first aspect, this application provides a zinc-aluminum-magnesium coating, the chemical composition of which includes:

[0037] Al, Mg, Ti, B, RE (rare earth elements), and Zn; where, by mass fraction,

[0038] The content of Al is 0.8% to 2.5%, and the content of Mg is 0.8% to 2.5%.

[0039] And it satisfies the following relationships: [Al]-[Mg]≥0, [Ti]+[B]+[RE]≤50ppm;

[0040] The equivalent diameter of the intermetallic compound particles on the surface of the zinc-aluminum-magnesium coating does not exceed 300 micrometers;

[0041] In the formula, [Al] represents the mass fraction of Al, [Mg] represents the mass fraction of Mg, [Ti] represents the mass fraction of Ti, [B] represents the mass fraction of B, and [RE] represents the mass fraction of RE.

[0042] In some embodiments, the equivalent diameter of the intermetallic compound particles on the surface of the zinc-aluminum-magnesium coating does not exceed 100 micrometers.

[0043] In some embodiments, the content of Al and the content of Mg satisfy the following relationship:

[0044] [Al]-[Mg]≥0.5%

[0045] In the formula, [Al] represents the mass fraction of Al, and [Mg] represents the mass fraction of Mg.

[0046] In some embodiments, the content of RE does not exceed 10 ppm.

[0047] In some embodiments, the types of intermetallic compounds include Fe-Zn compounds, Zn-RE compounds, Fe-Zn-Al compounds, Al-Ti compounds, Al-B compounds, Al-Ti-B compounds, and Mg-Zn compounds.

[0048] In this embodiment, the Al element in the coating provides high-quality atmospheric corrosion resistance because, during corrosion, Al can form dense oxides and hydroxides on the surface. If there is no Al in the coating, the adhesion between the coating and the steel sheet will be poor, rendering the coating unusable and reducing corrosion resistance. When the Al content in the coating is too high, a large number of dendritic aluminum-rich crystals will appear in the coating, making the coated steel sheet prone to surface cracking during deep drawing deformation, such as when manufacturing automotive parts, resulting in dense polishing highlights on the coating surface. For example, the Al content can be 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.5%, etc.

[0049] Mg (Mg) in the coating significantly improves its atmospheric corrosion resistance. The mechanism is that Mg preferentially dissolves into the water film on the coating surface in the atmosphere, reacting with dissolved carbon dioxide to precipitate a dense protective film. This film is stable in neutral and weakly alkaline environments and also helps the electrolyte solution on the coating surface become weakly alkaline, thus enhancing the coating's corrosion resistance. When the Mg content in the coating is within the aforementioned range, the Mg will preferentially corrode during corrosion, allowing it to dissolve into the water film on the coating surface and form a dense protective film. However, if the Mg content is too high, it will cause the formation of coarse Mg-Zn compounds on the coating surface. Mg-Zn compounds are also intermetallic compounds; if they are too large, they will also cause polishing highlights after stamping. For example, the content of Mg can be 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.5%, etc.

[0050] To control the growth of coarse Mg-Zn compounds in the coating, the Al content in the coating must not be lower than the Mg content. This is because when the Al content is low, Mg-Zn compounds precipitate first during coating solidification. At this time, there is still a large amount of liquid phase in the coating, so the Mg-Zn compounds tend to grow sufficiently. However, if the Al content is not lower than the Mg content, the coating tends to precipitate both Mg-Zn compounds and aluminum-rich phases simultaneously during solidification. The aluminum-rich phase can inhibit the growth of Mg-Zn compounds. For example, the [Al]-[Mg] value can be 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, etc. Optionally, the [Al]-[Mg] value can be 0.5% or higher, with the most significant inhibition effect.

[0051] During the stamping process of zinc-aluminum-magnesium coated steel sheets, if the coating contains intermetallic compounds with a significantly different hardness from the coating itself, these compounds may be exposed during stamping, forming localized high points. After polishing with materials such as an oilstone or wire mesh, these high points will appear as localized bright spot defects. Whether these intermetallic compounds will form polishing bright spots after stamping depends primarily on their size. Experimental studies have shown that if the size of the inclusions is greater than 500 micrometers, polishing bright spots will definitely form after stamping. If the size of the inclusions is greater than 300 micrometers, there is a 30% chance that polishing bright spots will form after stamping. For example, the equivalent diameter of the intermetallic compound particles on the surface of the zinc-aluminum-magnesium coating can be 300 micrometers, 290 micrometers, 280 micrometers, 270 micrometers, 260 micrometers, 250 micrometers, etc.; when the size of the inclusions does not exceed 100 micrometers, it can be guaranteed that no visible bright spots will appear after polishing. This is because stamping itself not only highlights the inclusions but also carries away a portion of the coating around the inclusions. Therefore, the size of the bright spot is larger than the size of the particle.

[0052] The hardness of intermetallic compounds is also an important influencing factor. If the hardness is very low, the intermetallic compounds will break directly during stamping and will not be prominent. In zinc-aluminum-magnesium (ZAM) coatings, the most common hard intermetallic compounds are Fe-Zn compounds and Fe-Al-Zn compounds, which are also common in traditional pure zinc coatings. However, intermetallic compounds containing rare earth elements, Ti, and B are also easily found in ZAM coatings. This is because the raw materials for Al in ZAM coatings are prone to contain these elements. Rare earth elements can form various Zn-RE compounds with Zn, and Ti and B can form typical intermetallic compounds with Al. These compounds are to be avoided in ZAM coatings for automotive exterior panels. Therefore, the total content of Ti, B, and RE in the coating is controlled. For example, the values ​​of [Ti]+[B]+[RE] can be 50ppm, 48ppm, 45ppm, 43ppm, 42ppm, etc.

[0053] Furthermore, rare earth elements are particularly prone to forming intermetallic compounds with Zn and precipitating out. Therefore, for example, the content of RE mentioned above can be 10 ppm, 9 ppm, 8 ppm, 7 ppm, etc.

[0054] Secondly, this application provides a zinc-aluminum-magnesium coated steel sheet, the zinc-aluminum-magnesium coated steel sheet comprising a steel substrate and a zinc-aluminum-magnesium coating as described in the first aspect, which is attached to at least a portion of the surface of the steel substrate.

[0055] The zinc-aluminum-magnesium coated steel sheet is based on the above-mentioned zinc-aluminum-magnesium coating. The specific chemical composition of the zinc-aluminum-magnesium coating can be referred to the above embodiments. Since the zinc-aluminum-magnesium 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.

[0056] Thirdly, Figure 3 A schematic flowchart illustrating a method for preparing zinc-aluminum-magnesium coated steel sheet provided in this application embodiment; please refer to [link / reference]. Figure 3 This application provides a method for preparing the zinc-aluminum-magnesium coated steel sheet described in the second aspect, the method comprising:

[0057] S1. Obtain the steel matrix;

[0058] S2. The steel substrate is hot-dip galvanized and then surface-treated to obtain a zinc-aluminum-magnesium coated steel sheet.

[0059] In this embodiment of the application, step S1 includes obtaining a slab after smelting molten steel, heating the slab, and then performing rough rolling, finish rolling, cooling, and cold rolling to obtain a steel matrix.

[0060] Furthermore, the composition of the hot-dip galvanizing solution is: Al: 0.8–2.5%, Mg: 0.8–2.5%, with the remainder being Zn and unavoidable impurity elements. The total content of Ti, B, and rare earth elements in the solution does not exceed 50 ppm, and the difference between the contents of Al and Mg is not less than 0. Furthermore, the content of rare earth elements in the solution does not exceed 10 ppm. Furthermore, the difference between the Al and Mg contents in the solution is not less than 0.5%.

[0061] In some embodiments, the surface treatment includes light rolling.

[0062] In some embodiments, the elongation of the slight rolling is 0.6% to 1.5%.

[0063] In this embodiment, after hot-dip galvanizing, the zinc-aluminum-magnesium coated automotive exterior panel undergoes slight rolling, which helps to break up the intermetallic compounds in the zinc-aluminum-magnesium coating in advance. During rolling, the zinc-aluminum-magnesium coating undergoes compressive deformation in the thickness direction, and the intermetallic compounds also undergo compressive deformation. Due to the high hardness and low compressibility of the intermetallic compounds, they are easily compressed and broken during rolling. This differs from the deformation state during stamping. During stamping, the coating surface often undergoes tensile and bending deformation, at which point the intermetallic compounds are easily torn from the coating and exposed. Slight rolling can break up the intermetallic compounds. However, if the rolling is too intense, it will cause severe work hardening of the zinc-aluminum-magnesium coated automotive panel, which is not conducive to stamping. For example, the elongation rate of the above-mentioned slight rolling can be 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.5%, etc. In addition, the surface treatment also includes coiling.

[0064] The method for preparing zinc-aluminum-magnesium coated steel sheet is based on the above-mentioned zinc-aluminum-magnesium coated steel sheet. The specific structure of the zinc-aluminum-magnesium coated steel sheet can be referred to the above embodiments. Since the method for preparing zinc-aluminum-magnesium 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.

[0065] Fourthly, this application provides an automotive outer panel, the raw material of which includes the zinc-aluminum-magnesium coated steel sheet described in the second aspect.

[0066] The car exterior panel is based on the zinc-aluminum-magnesium coated steel sheet described above. The specific structure of the zinc-aluminum-magnesium coated steel sheet can be referred to in the above embodiments. Since the car exterior panel 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.

[0067] 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.

[0068] The characteristics of a zinc-aluminum-magnesium coating are shown in Table 1.

[0069] Table 1 Characteristics of Zinc-Aluminum-Magnesium Coatings

[0070]

[0071]

[0072] The process parameters for a method of preparing zinc-aluminum-magnesium coated steel sheet are shown in Table 2.

[0073] Table 2. Process parameters for preparing zinc-aluminum-magnesium coated steel sheets

[0074]

[0075] Simulated stamping tests were conducted on the zinc-aluminum-magnesium coated steel sheets prepared according to the process parameters in Examples 1-13 and Comparative Examples 1-5. The simulated stamping tests adopted the flat-head bulging test method in the China Society of Automotive Engineers' standard "Test Method for the Ultimate Thinning Rate of Thin Steel Sheets for Automobiles" (T / CSAE 201-2021), with a bulging height of 10 mm. After the test, the bulging outer surface of the coating was polished with an oilstone, and the number of bright spots in the bulging area was evaluated. The corrosion evaluation method was to place the galvanized steel sheet in a cyclic corrosion test chamber and conduct 18 cycles of cyclic corrosion testing. The cyclic corrosion test met the requirements of Annex A of ISO 1 1997-1:2017. Then, the mass loss of the coating before and after the test was measured, and the corrosion resistance of the coating was evaluated by the mass loss per unit area. The less the mass loss, the better the corrosion resistance. The surface quality evaluation results of a zinc-aluminum-magnesium coated steel sheet are shown in Table 3.

[0076] Table 3 Surface Quality Evaluation Results of Zinc-Aluminum-Magnesium Coated Steel Sheets

[0077]

[0078]

[0079] Tables 1 to 3 show that the zinc-aluminum-magnesium coated steel sheet provided in the embodiments of this application has good surface quality, with fewer local high points and stamping bright spots on the surface during stamping, and the steel sheet has strong surface corrosion resistance. Figure 1 This is the surface morphology of the zinc-aluminum-magnesium coated steel sheet of Example 1 of this application; please refer to... Figure 1 In contrast, Comparative Examples 1-5 did not employ the scheme described in this application, and the resulting zinc-aluminum-magnesium coated steel sheets exhibited numerous localized high points and stamping highlights on their surfaces. Figure 2 The surface morphology of the zinc-aluminum-magnesium coated steel sheet is shown in Comparative Example 1 of this application; please refer to [link to Comparative Example 1]. Figure 2 .

[0080] 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 coating, characterized in that, The chemical composition of the zinc-aluminum-magnesium coating includes: Al, Mg, Ti, B, RE, and Zn; where, by mass fraction, The content of Al is 0.8%~2.5%, and the content of Mg is 0.8%~2.5%. And it satisfies the following relationship: [Al]-[Mg]≥0, [Ti]+[B]+[RE]≤50 ppm and the content of RE does not exceed 10 ppm; The equivalent diameter of the intermetallic compound particles on the surface of the zinc-aluminum-magnesium coating does not exceed 300 micrometers; In the formula, [Al] represents the mass fraction of Al, [Mg] represents the mass fraction of Mg, [Ti] represents the mass fraction of Ti, [B] represents the mass fraction of B, and [RE] represents the mass fraction of RE.

2. The zinc-aluminum-magnesium coating according to claim 1, characterized in that, The equivalent diameter of the intermetallic compound particles on the surface of the zinc-aluminum-magnesium coating does not exceed 100 micrometers.

3. The zinc-aluminum-magnesium coating according to claim 1, characterized in that, The content of Al and the content of Mg satisfy the following relationship: [Al]-[Mg]≥0.5% In the formula, [Al] represents the mass fraction of Al, and [Mg] represents the mass fraction of Mg.

4. The zinc-aluminum-magnesium coating according to claim 1, characterized in that, The intermetallic compounds include one or more of the following: Fe-Zn compounds, Zn-RE compounds, Fe-Zn-Al compounds, Al-Ti compounds, Al-B compounds, Al-Ti-B compounds, and Mg-Zn compounds.

5. A zinc-aluminum-magnesium coated steel sheet, characterized in that, The zinc-aluminum-magnesium coated steel sheet includes a steel substrate and a zinc-aluminum-magnesium coating as described in any one of claims 1 to 4, which is attached to at least a portion of the surface of the steel substrate.

6. A method for preparing the zinc-aluminum-magnesium coated steel sheet of claim 5, characterized in that, The method includes: A steel matrix is ​​obtained; The steel substrate is hot-dip galvanized and then surface-treated to obtain a zinc-aluminum-magnesium coated steel sheet.

7. The method according to claim 6, characterized in that, The surface treatment includes light rolling.

8. The method according to claim 7, characterized in that, The elongation of the slight rolling is 0.6% to 1.5%.

9. An automotive outer panel, characterized in that, The raw material for the automobile outer panel includes the zinc-aluminum-magnesium coated steel sheet as described in claim 5.

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