Vehicle bracket subjected to anti-corrosion treatment and preparation method thereof

Abstract

The vehicle support comprises a basic steel plate and a molten zinc-aluminum-magnesium plating layer formed on the basic steel plate, one or more cutting faces are formed on the basic steel plate, an anti-rust protective coating is formed on the cutting faces and the plating layer, and the anti-rust protective coating is formed by being soaked in anti-rust protective coating liquid. The anti-rust protective coating liquid comprises a metal coating liquid, wherein the metal coating liquid contains nitric acid metal and a surfactant in a mass ratio of (2-3): 1.

Description

[Technical field]

[0001] The present disclosure relates to a vehicle bracket and a method for preparing the same. More particularly, the present disclosure relates to a vehicle bracket having an improved corrosion-resistant cutting surface and a method for preventing the bracket from rusting. [Background technology]

[0002] Cold rolled steel sheets (SPCC) are widely used in components such as vehicle brackets because they have high formability and processability and can be easily bent, pressed, and stretched.

[0003] Cold rolled steel sheets have high formability and processability, but when the oxide film falls off, the cold rolled steel sheets are easily oxidized. And depending on the storage state, cold rolled steel sheets may rust. Therefore, cold rolled steel sheets are mainly galvanized and painted.

[0004] At the same time, the vehicle bracket is exposed to a humid environment and is affected by the outside air, so high corrosion resistance is required. Therefore, the galvanizing process and the painting process are carried out separately.

[0005] Vehicle brackets come in a variety of shapes. A wheel speed sensor (WSS) bracket used to accommodate an electronic parking brake system (EPB) cable includes two or more cut faces and curves, and is primarily prepared by galvanizing the surface of a cold-rolled steel sheet to form an electroplated layer, and then pressing the cold-rolled steel sheet into the bracket shape.

[0006] In this case, the bare part of the base steel plate formed on the complex cut surface by pressing processing is damaged, resulting in a decrease in the corrosion resistance of the entire bracket. This problem also affects the durability of the vehicle.

[0007] Therefore, there is an urgent need to develop a vehicle bracket with stronger corrosion resistance.

[0008] As a background art of the present disclosure, Korean Patent Application Publication No. 10-2005-0089568 discloses a fixing bracket for assembling a fender of a vehicle.

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Technical issues

[0009] An object of the present disclosure is to provide a vehicle bracket having a cut surface with improved corrosion resistance.

[0010] Another object of the present disclosure is to provide a method for preparing a vehicle bracket, which forms an anti-rust protective coating on the entire cut surface and the entire outer surface of the vehicle bracket having a cut surface through a stamping process, serving as a sealing film to improve corrosion resistance.

[0011] The above-mentioned objects and other objects of the present disclosure can be achieved through the present disclosure, which will be explained below.

Technical solution

[0012] 1. One aspect of the present disclosure relates to a vehicle bracket.

[0013] The vehicle support comprises: a base steel plate; and a molten zinc-aluminum-magnesium electroplating layer formed on the base steel plate, wherein one or more cutting surfaces are formed, and an anti-rust protective coating is formed on the cutting surfaces and the electroplating layer, wherein the anti-rust protective coating is formed by immersing in an anti-rust protective coating liquid, wherein the anti-rust protective coating liquid comprises a metal coating liquid, wherein the mass ratio of nitric acid metal and a surfactant is 2 to 3 to 1.

[0014] 2. In the specific example 1, the molten zinc-aluminum-magnesium coating may include 1.5 to 3.0 weight % zinc, 1.5 to 2.5 weight % aluminum, 1.5 to 2.5 weight % magnesium, and other inevitable impurities.

[0015] 3. In specific example 1 or 2, the metal coating solution may include 3% to 5% by weight of aluminum nitrate, The weight percentage is 5 to 10% silicon dioxide and the remainder is water.

[0016] 4. In any of the specific examples 1 to 3, the surfactant may include 3% to 5% by weight of sodium sulfate and residual amount of water.

[0017] 5. In any of the specific examples 1 to 4, when the salt water concentration is 5% and the salt water spraying rate is 1.5 ml / hour, continuous spraying for 740 hours or more in the salt water spray test, no red rust will appear.

[0018] 6. In any of the specific examples 1 to 5, no coating is formed on the rust-proof protective coating.

[0019] 7. Another aspect of the present disclosure relates to a method for preparing a vehicle bracket.

[0020] The method for preparing a vehicle bracket comprises: (a) preparing an alloy steel plate including a molten zinc-aluminum-magnesium coating; (b) The bracket shape is formed by pressing an alloy steel plate; and (c) an anti-rust protective coating is formed by immersing the bracket in an anti-rust protective coating liquid, wherein the anti-rust protective coating liquid includes a metal coating liquid containing nitric acid metal and a surfactant in a mass ratio of 2 to 3 to 1.

[0021] 8. In specific example 7, the metal coating liquid may include 3 to 5 weight % of aluminum nitrate, 5 to 10 weight % of silicon dioxide, and the remainder of water.

[0022] 9. In specific example 7 or 8, the surfactant may include 3 to 5 weight percent sodium sulfate and a residual amount of water.

[0023] 10. In any one of Specific Examples 7 to 9, in step (c), the temperature of the anti-rust protective coating liquid may be maintained at 40 to 60°C.

[0024] 11. In any one of specific examples 7 to 10, in step (c), the pH value of the anti-rust protective coating liquid may be maintained at 3.0 to 4.0.

[0025] 12. In any one of specific examples 7 to 10, in step (c), the soaking time may be 50 to 70 seconds.

Beneficial Effects

[0026] According to the vehicle bracket of the present disclosure, even in the case of a complex structure having one or more cut surfaces, An anti-rust protective coating can also be provided on the upper portion of the cutting surface so that the entire electroplating layer will not be corroded, which can significantly enhance the corrosion resistance.

[0027] According to the vehicle bracket disclosed in the present invention, a zinc-aluminum-magnesium ternary electroplated steel sheet with high corrosion resistance can be used to increase the surface hardness, and an additional anti-rust protective coating can be formed on the cut surface, thereby significantly improving the corrosion resistance.

[0028] According to another aspect of the present disclosure, the vehicle bracket manufacturing method does not need to separately form an electroplating layer and an additional coating layer. In addition, the single process of forming an anti-rust protective coating on the outer surface of the zinc-aluminum-magnesium ternary electroplated steel plate can improve corrosion resistance, thereby significantly improving the manufacturing efficiency of the vehicle bracket.

Brief Description of the Drawings

[0029] Figure 1 is a schematic diagram of a vehicle bracket according to a specific example of the present disclosure.

[0030] Figure 2 It is a process flow chart of a vehicle bracket preparation method according to another aspect of the present disclosure.

[0031] Figure 3 Scanning electron micrographs of a vehicle bracket having an anti-rust protective coating according to certain examples of the present disclosure and a vehicle bracket without an anti-rust protective coating are shown.

[0032] Figure 4 Photographs showing the vehicle stand according to certain examples of the present disclosure according to the timing of salt water spraying.

Publication method

[0033] The present disclosure will be described in more detail below with reference to the accompanying drawings. The following drawings are provided only to make it easier to understand the present disclosure, and the present disclosure is not limited to the following drawings. In addition, the shapes, sizes, proportions, angles, numbers, etc. described in the drawings are only exemplary, and the present disclosure is not limited to the contents described in the drawings.

[0034] Throughout the description, the same reference numerals represent the same constituent elements. In addition, in the description of the present disclosure, when it is determined that the specific description may unnecessarily obscure the subject matter of the present disclosure, the specific description of the related art known to the public will be omitted.

[0035] The terms "include", "have" or "comprises" used in this specification may mean that other constituent elements may be added. Unless these terms are used together with the term “only.” Unless otherwise specifically stated, singular expressions used in this specification also include plural expressions.

[0036] Analyses of constituent elements are to be interpreted as including margins of error unless individually and explicitly stated otherwise.

[0037] If terms such as "on", "above", "below", "side" etc. are used to describe the positional relationship between two components, one or more components may also be positioned between the two components without using terms such as "immediately next to" or "directly".

[0038] Positional relationships such as "upper part", "upper surface", "lower part" and "lower surface" are described based on the drawings, but do not illustrate absolute positional relationships. That is, the positions of "upper part", "upper surface", "lower part" and "lower surface" may change depending on the observation position.

[0039] In the present specification, "a to b" indicating a numerical range is defined as "≤a and ≥b".

[0040] Hereinafter, a vehicle bracket according to a specific example of the present disclosure will be described in detail with reference to the accompanying drawings.

[0041] Figure 1 is a schematic diagram of a vehicle bracket according to a specific example of the present disclosure.

[0042] Reference Figure 1 , a vehicle bracket includes a base steel plate 100 , an electroplating layer 200 and an anti-rust protective coating 300 .

[0043] The base steel plate 100 may include a steel plate that can be coated with a hot-dip galvanized layer. The base steel plate 100 may be a cold-rolled steel plate.

[0044] If the base steel plate 100 is a cold-rolled steel plate, the coating thickness is uniform and the surface is smooth, and it is easy to form a hot-dip galvanized coating.

[0045] If the base steel plate 100 is a hot-rolled steel plate, an oxide layer is formed by heat treatment, so it is difficult to form a separate electroplating layer on the upper surface of the base steel plate 100. In addition, since the oxide layer itself has corrosion resistance, an additional electroplating process is not required.

[0046] The molten zinc-aluminum-magnesium electroplating layer 200 may be formed by melting metal elements on the base steel plate 100 .

[0047] Specifically, a binary eutectic structure of Zn-MgZn2 and a ternary eutectic structure of Zn-Al-MgZn2 can be formed. In the process structure of Zn-MgZn2, MgZn2 is more actively / easily eluted to improve the corrosion resistance of the cut surface, so it is preferred to form a large amount of binary eutectic structure. In a specific example of the present disclosure, since the anti-rust protective coating is formed not only on the upper surface of the plating layer but also on the cut surface, it is not necessary to adjust the composition of zinc-aluminum-magnesium in the electroplating solution to form a large amount of binary eutectic structure.

[0048] In a specific example, the molten zinc-aluminum-magnesium coating 200 may include 1.5 to 3.0 weight percent zinc. 1.5 to 2.5 weight % aluminum, 1.5 to 2.5 weight % magnesium, and the remainder of inevitable impurities.

[0049] In this content range, a binary eutectic structure of Zn-MgZn2 and a ternary eutectic structure of Zn-Al-MgZn2 can be formed. If the coating is exposed to a corrosive environment, the corrosion potential of MgZn2 is lower than that of zinc, so MgZn2 is eluted first in the corrosive environment. The eluted magnesium forms Mg(OH)2 and reduces the alkalinity of the cathode area, thus forming a stable Zn5(OH)8Cl2H2O, thereby improving corrosion resistance.

[0050] The vehicle bracket has one or more cut surfaces, and the anti-rust coating is formed on the cut surfaces and the electroplating layer.

[0051] When forming the cut surface, the molten zinc-aluminum-magnesium electroplating layer 200 may be removed, so the corrosion resistance may be reduced. However, since the rust-proof protective coating 300 is formed on the upper part of the cut surface, the corrosion resistance of the cut surface may be greatly improved.

[0052] If the anti-rust protective coating 300 is not formed on the upper portion of the cutting surface, corrosion phenomena such as white rust or red rust may occur in a corrosive environment.

[0053] The anti-rust protective coating 300 is formed by immersing in an anti-rust protective coating liquid, which includes a metal coating liquid containing metal nitrate and a surfactant in a mass ratio of 2 to 3 to 1.

[0054] The anti-rust protective coating 300 is formed by immersing in an anti-rust protective coating liquid including a metal coating liquid and a surfactant and then drying.

[0055] If the rust-proof protective coating liquid includes a metal coating liquid containing metal nitrate and a surfactant within the above range, the dispersion of the metal nitrate in the rust-proof protective coating liquid may be increased, so that the rust-proof protective coating can be formed only by immersion. In addition, the rust-proof protective coating can have a uniform thickness, and the metal coating liquid can be permanently deposited on the electroplating layer.

[0056] In a specific example, the metal coating solution may include 3 to 5 weight percent aluminum nitrate, 5 to 10 weight percent silicon dioxide, and a residual amount of water.

[0057] If the aluminum nitrate content in the metal coating liquid is within the above range, a refined crystal structure can be formed, which can effectively improve not only the corrosion resistance of the anti-rust protective coating, but also the hardness of the anti-rust protective coating.

[0058] Silicon dioxide can effectively improve the corrosion resistance of the magnesium alloy layer. Within the above range, not only the corrosion resistance but also the wear resistance can be improved, thereby increasing the hardness of the rust-proof protective coating.

[0059] In a specific example, the surfactant may include 3 to 5 weight percent sodium sulfate and the balance water.

[0060] Sodium sulfate is an anionic surfactant. Within the above range, the crystal structure of aluminum nitrate can be stabilized, thereby improving corrosion resistance.

[0061] In a salt water spray test in which the salt water concentration is 5%, the salt water spraying rate is 1.5 ml / hour, and the spraying is continued for 740 hours or more, red rust is not formed on the vehicle bracket 1000.

[0062] The corrosion resistance of the vehicle bracket 1000 is significantly improved, and in a salt water spray test for 740 hours or more, for example, even if the salt water concentration is 5% and the salt water spraying rate is 1.5 ml / hour, red rust will not form even if the salt water is sprayed for 741 hours. Therefore, the corrosion resistance can be significantly improved.

[0063] In certain examples, the upper portion of the anti-rust protective coating 300 may not constitute a coating.

[0064] No separate coating is formed on the electroplating layer of the vehicle bracket 1000, and the anti-rust protective coating 300 may be replaced by anti-rust paint.

[0065] Compared with the electroplating layer, the color of the anti-rust protective coating layer 300 is dark gray, so it can be intuitively recognized whether the anti-rust protective coating layer is formed.

[0066] Another aspect of the present disclosure relates to a method for preparing a vehicle bracket.

[0067] Figure 2 It is a process flow chart of a vehicle bracket preparation method according to another aspect of the present disclosure.

[0068] Reference Figure 2 The method for preparing a vehicle bracket includes: (a) preparing an alloy steel plate including a molten zinc-aluminum-magnesium coating, (b) forming a bracket shape by pressing the alloy steel plate, and (c) forming an anti-rust protective coating by immersing the bracket in an anti-rust protective coating liquid.

[0069] First, a hot-dip galvanized steel sheet is prepared ( S100 ).

[0070] Hot dip galvanized steel sheet may be an alloy steel sheet which is produced by forming a molten zinc-aluminum-magnesium electroplating layer on top of a base steel sheet.

[0071] The bracket shape is made by pressing an alloy steel plate (S200).

[0072] The shape of the vehicle support can be determined by pressing and can form one or more cut surfaces.

[0073] The molten zinc-aluminum-magnesium electroplating layer can be removed from the cut surface. In this case, the iron content of the base steel plate may be exposed.

[0074] The method may include degreasing the vehicle bracket (S210).

[0075] Degreasing removes the oily substances remaining on the top of the electroplating layer, allowing the anti-rust protective coating to adhere firmly.

[0076] The method may include washing the vehicle bracket after the degreasing process ( S220 ).

[0077] When cleaning the vehicle bracket, foreign matter on the surface can be removed in advance.

[0078] The anti-rust protective coating can be formed by immersing the bracket in the anti-rust protective coating liquid (S300).

[0079] The anti-rust protective coating liquid may include a metal coating liquid containing metal nitrate and a surfactant, the mass ratio of the two being 2 to 3 to 1.

[0080] The anti-rust protective coating liquid can be prepared by mixing the metal coating liquid and the surfactant within the above range, and the anti-rust protective coating can be formed by adjusting the immersion conditions. The durability of the anti-rust protective coating can be improved by stabilizing the crystal structure of the anti-rust protective coating.

[0081] In a specific example, the metal coating solution may include 3 to 5 weight percent aluminum nitrate, 5 to 10 weight percent silicon dioxide, and a residual amount of water.

[0082] The metal coating liquid contains aluminum nitrate, which forms a delicate crystalline structure on the anti-rust protective coating and increases the corrosion resistance and hardness of the anti-rust protective layer.

[0083] In a specific example, the surfactant may include 3 to 5 weight percent sodium sulfate and the balance water.

[0084] The surfactant may include sodium sulfate for preparing the anti-rust protective coating liquid. The metal coating liquid may be dispersed to stabilize the crystalline structure.

[0085] In S300, the temperature of the anti-rust protective coating liquid may be maintained at 40 to 60°C.

[0086] For example, the temperature is preferably 50 to 60°C, particularly preferably 50°C.

[0087] Since the rust protection coating liquid is kept within this temperature range, the vehicle bracket can obtain a uniform coating by immersion.

[0088] In S300, the pH value of the anti-rust protective coating liquid may be maintained between 3.0 and 4.0.

[0089] Particularly, the pH value is preferably between 3.6 and 4.0, and the pH value is more preferably 4.0.

[0090] The rust-proof protective coating can be uniformly formed within the pH range, thereby improving the surface smoothness. If the pH value exceeds the above range, precipitation may occur in the metal coating liquid.

[0091] Within the pH range, the color of the rust protective coating may be darker. For example, the rust protective coating may be dark gray.

[0092] In S300 , the soaking time may be 50 to 70 seconds.

[0093] If the immersion is carried out within this time range, a uniform thickness of the rust protection coating can be achieved and the average illumination can be reduced, thereby achieving a smooth surface finish on the vehicle bracket.

[0094] After soaking, the vehicle bracket is recovered and dried at room temperature, thereby preparing a vehicle bracket with an anti-rust protective coating.

[0095] According to the vehicle bracket disclosed in the present invention, a basic steel plate with a highly corrosion-resistant molten zinc-aluminum-magnesium electroplating layer is pressed to form a bracket shape with one or more cut surfaces, and then an anti-rust protective coating is formed again on the upper surface of the cut surface and the electroplating layer surface, thereby significantly improving the corrosion resistance and also improving the hardness.

[0096] High corrosion resistance can be ensured by simply immersing the vehicle bracket in the anti-rust protective coating liquid to form the anti-rust protective coating, and then drying the vehicle bracket. Therefore, there is no need to conduct a separate coating process to deal with the color change of the vehicle bracket after coating, which greatly improves the production efficiency of the vehicle bracket.

[0097] Some examples are provided below to help understand the present disclosure. The following examples are only used to illustrate the present disclosure, and the scope of the present disclosure is not limited to the following embodiments.

[0098] Example 1. Preparation of vehicle bracket

[0099] Preparation of hot-dip galvanized steel sheet (the weight percentage of zinc in the alloy layer is 1.5%, the weight percentage of magnesium is 1.5%, The weight percentage of aluminum is 1.5%), and the hot-dip galvanized steel sheet is cut by a press / punch to form the vehicle bracket shape.

[0100] The vehicle bracket with the cut surface was degreased, cleaned twice, and then immersed in a bath containing a rust-proof protective coating liquid.

[0101] A metal coating liquid is prepared, which contains 5% by weight of aluminum nitrate, 10% by weight of silicon dioxide and 85% by weight of water. A surfactant is also prepared, which contains 5% by weight of sodium sulfate and 95% by weight of water.

[0102] An anti-rust protective coating liquid containing 25% by weight of a surfactant and 75% by weight of a metal plating liquid is prepared and introduced into an electroplating tank.

[0103] Determine whether the pH value of the anti-rust protective coating liquid is 4.0, adjust the temperature of the anti-rust protective coating liquid to 50°C, The vehicle bracket with the cut surface is then immersed in the anti-rust protective coating liquid of the electroplating tank for 60 seconds.

[0104] The vehicle bracket was recovered, washed twice, and dried at room temperature (25° C.) for 24 hours or more to recover the vehicle bracket having the rust-proof protective coating.

[0105] Experimental Example 1. Identification of Anti-Rust Protective Coating

[0106] Figure 3Scanning electron micrographs of a vehicle bracket having an anti-rust protective coating according to certain examples of the present disclosure and a vehicle bracket without an anti-rust protective coating are shown.

[0107] Reference Figure 3 ,like Figure 3 As shown in A, without the formation of anti-rust protective coating, the surface illumination increases and becomes uneven. Figure 3 As shown in B, when the upper surface of the electroplating layer also forms an anti-rust protective coating, it can be seen that the anti-rust protective coating is permanently attached to the upper surface of the electroplating layer, thereby making the surface smooth and stabilizing the crystal structure of the anti-rust protective coating.

[0108] Experimental Example 2. Anti-rust protective coating liquid composition

Table 1

[0109] Table 1 shows the shape of the anti-rust protective coating determined according to the composition of the anti-rust protective coating liquid.

[0110] Referring to Table 1, it can be seen that a smooth anti-rust protective coating with high smoothness is formed in Example 1. In Comparative Example 1, compared with the example, the anti-rust protective coating is hardly formed only by immersion when the content of the metal coating liquid is reduced. In Comparative Example 2, since there is a large amount of metal coating liquid after immersion, the thickness of the anti-rust protective coating is uneven and the smoothness is low, so an additional coating process is required.

[0111] Experimental Example 3. Salt spray test

[0112] In order to determine the corrosion resistance of the vehicle bracket prepared according to the present embodiment, a salt spray test was performed.

[0113] According to the conditions of KSD 9502, the salt concentration was 5%, the pH value was 6.7, and the temperature during the test was 35°C. The spray method was continuous spray, the spray pressure was 1.0 kg / cm2, and the salt water spray rate was adjusted to 1.5 ml / hour.

[0114] The vehicle bracket prepared according to Example 1 was sprayed with salt water at an angle of 15 to 30° using a salt water tester (VT ST-200).

[0115] Figure 4 Photographs showing a vehicle stand during a salt water spraying period according to certain examples of the present disclosure.

[0116] According to the result of the salt water spraying for a total of 741 hours, white rust appeared on the vehicle bracket according to the present embodiment after 429 hours. However, it can be seen that even if the salt water was sprayed for 741 hours or more, red rust did not appear, thereby showing extremely high corrosion resistance. In particular, it can be seen that red rust never appeared on the cut surface.

[0117] Therefore, according to the present disclosure, a rust-proof protective coating is formed on a vehicle bracket, and high corrosion resistance and surface hardness are significantly improved, so that the vehicle bracket can be used as a wheel speed sensor (WSS) bracket for supporting an electronic parking brake system (EPB) cable in a corrosive and vibrating environment. In particular, the corrosion resistance of the cut surface can be improved, so that the corrosion resistance and hardness of a wheel speed sensor (WSS) bracket with a complex shape can be significantly improved compared with a simple hot-dip galvanized steel plate.

[0118] The embodiments of the present disclosure have been described above. It is understandable that professionals in the technical field of the present disclosure can implement modifications without departing from the inherent characteristics of the present disclosure. Therefore, the disclosed embodiments need to be regarded as illustrative rather than restrictive. The scope of the present disclosure is defined by the claims rather than the above description, and all differences within the equivalent range should be understood to belong to the scope of the present disclosure. [Reference Number Description] 1000: Vehicle bracket 100: Basic steel plate 200: Electroplating layer 300: Anti-rust protective coating

Claims

1. A vehicle bracket, comprising: - Foundation steel plate; as well as A molten zinc-aluminum-magnesium electroplated layer formed on a base steel plate, Among them, one or more cutting surfaces are formed, Among them, an anti-rust protective coating is formed on the cutting surface and the electroplating layer. as well as The anti-rust protective coating is formed by immersing in an anti-rust protective coating liquid, which includes a metal coating liquid of metal nitrate and a surfactant in a mass ratio of 2 to 3 to 1.

2. The vehicle bracket according to claim 1, wherein: The molten zinc-aluminum-magnesium coating comprises 1.5 to 3.0 weight percent of zinc, 1.5 to 2.5 weight percent of aluminum, 1.5 to 2.5 weight percent of magnesium and other inevitable impurities.

3. The vehicle bracket according to claim 1, wherein: The metal coating liquid includes 3 to 5 weight percent of aluminum nitrate, 5 to 10 weight percent of silicon dioxide and the remainder of water.

4. The vehicle bracket according to claim 1, wherein: The surfactant includes 3 to 5 weight percent sodium sulfate and a residual amount of water.

5. The vehicle bracket according to claim 1, wherein: No red rust occurred in the salt water spray test at a salt water concentration of 5%, a salt water spraying rate of 1.5 ml / hour, and a continuous spraying condition of 740 hours or more.

6. The vehicle bracket according to claim 1, wherein: No paint layer is formed on the rust protection coating.

7. A method for preparing a vehicle bracket, the method comprising: (a) preparing alloy steel sheets, including molten zinc-aluminum-magnesium coatings; (b) preparing a bracket shape by pressing an alloy steel plate; as well as (c) forming an anti-rust protective coating by immersing the bracket in an anti-rust protective coating liquid, The anti-rust protective coating liquid comprises a metal coating liquid, which contains metal nitric acid and a surfactant in a mass ratio of 2 to 3 to 1.

8. The method according to claim 7, wherein: The metal coating liquid comprises 3 to 5 weight percent of aluminum nitrate, 5 to 10 weight percent of silicon dioxide and the remainder of water.

9. The method according to claim 7, wherein: The surfactant includes 3 to 5 weight percent sodium sulfate and a residual amount of water.

10. The method according to claim 7, wherein: In step (c), the temperature of the rust-proof protective coating liquid is maintained at 40 to 60°C.

11. The method according to claim 7, wherein: In step (c), the pH value of the rust-proof protective coating liquid is maintained at 3.0 to 4.

0.

12. The method according to claim 7, wherein: In step (c), the soaking time is 50 to 70 seconds.

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