Zinc-aluminum-magnesium plated steel sheet and cooling method thereof

Abstract

The present application discloses a zinc-aluminum-magnesium plated steel sheet, which comprises a substrate and a plated layer plated on the substrate. The plated layer of the steel sheet does not contain Mg2Zn 11 phase. The present application also discloses a cooling method for controlling the black spot defects on the surface of the zinc-aluminum-magnesium plated steel sheet, which is a three-stage cooling method in the post-plating cooling process, wherein: in the first stage cooling, the opening cooling temperature is controlled to be 420-440 DEG C, the final cooling temperature is controlled to be 350-360 DEG C, and the rate of the first stage cooling is controlled to be 4-30 DEG C / s; in the second stage cooling, the end temperature is controlled to be 320-330 DEG C, and the rate of the second stage cooling is controlled to be 0.5-5 DEG C / s; in the third stage cooling, the end temperature is controlled to be 150-250 DEG C, and the rate of the third stage cooling is controlled to be 10-40 DEG C / s.

Description

Technical Field

[0001] This invention relates to a coated steel sheet and its manufacturing method, and more particularly to a zinc-aluminum-magnesium coated steel sheet and its manufacturing method. Background Technology

[0002] In recent years, to further improve the corrosion resistance of coatings, galvanized materials have gradually shifted from pure zinc to zinc alloys. Hot-dip galvanizing technology can effectively slow down steel corrosion, thereby extending its service life. Compared with other anti-corrosion methods, hot-dip galvanizing technology has advantages such as low production cost, simple process, strong coating, and good appearance.

[0003] In existing technologies, one or more aluminum, magnesium, silicon, copper, boron, manganese, strontium, rare earth elements are usually added or production process parameters are controlled to further improve its corrosion resistance or make it easier to manufacture, and to improve its surface quality.

[0004] For example, Chinese patent document CN108018514A, published on May 11, 2018, entitled "A Method for Controlling Surface Defects of Zn-Al-Mg Alloy Coated Sheets and Zn-Al-Mg Alloy Coated Sheets", discloses a method for controlling surface defects of Zn-Al-Mg alloy coated sheets and Zn-Al-Mg alloy coated sheets. It improves a surface defect that looks like "frog skin" by controlling the composition of the plating solution, the zinc plating process, and the oxygen content of the air knife scraping medium.

[0005] For example, Chinese patent document CN114892114A, published on August 12, 2022, entitled "Cooling Method for Hot-Dip Galvanized Aluminum-Magnesium Products," discloses a new cooling method for hot-dip galvanized aluminum-magnesium products, including the steps of: S1, natural cooling; S2, placing the strip steel in a movable cooling fan box and cooling it at a set cooling rate to completely solidify the alloy coating on the surface of the strip steel; and S3, forced cooling. This patent controls the stripe defects caused by surface solidification shrinkage by reducing the volume change during the precipitation of the Zn-Al-Mg ternary eutectic.

[0006] However, low-aluminum-zinc-aluminum-magnesium coating products produced using existing technologies still have a surface defect of black spots, which becomes a serious obstacle for the product in applications requiring high surface quality. Summary of the Invention

[0007] One of the objectives of this invention is to provide a zinc-aluminum-magnesium coated steel sheet, which has excellent surface quality and is suitable for applications with high surface quality requirements, such as automotive interior and exterior panels, and has good prospects for promotion and application value.

[0008] To achieve the above objectives, the present invention also provides a zinc-aluminum-magnesium coated steel sheet, comprising a substrate and a coating applied to the substrate, wherein the coating does not contain Mg2Zn. 11 Mutually.

[0009] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the coating contains Zn and chemical elements with the following mass percentages: Mg: 1.0-2.0%, Al: 1.0-2.0%.

[0010] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the mass percentage content of each chemical element in the coating is: Mg: 1.0-2.0%, Al: 1.0-2.0%, with the balance being Zn and unavoidable impurities.

[0011] Another objective of this invention is to provide a method for controlling the cooling of zinc-aluminum-magnesium coated steel sheets. This method, through optimized design of the post-plating cooling process, can greatly improve the black spot defects existing on the surface of existing zinc-aluminum-magnesium coated steel sheets, ensuring that the prepared zinc-aluminum-magnesium coated steel sheets are suitable for applications with high surface quality requirements, such as automotive inner and outer panels, and has very good prospects for promotion and application value.

[0012] To achieve the above objectives, the present invention provides a cooling method for zinc-aluminum-magnesium coated steel sheets, wherein the post-coating cooling process is controlled as a three-stage cooling process, wherein:

[0013] First stage cooling: The starting temperature of the first stage cooling is controlled at 420-440℃, the final cooling temperature is 350-360℃, and the cooling rate of the first stage is 4-30℃ / s.

[0014] Second stage cooling: The final temperature of the second stage cooling is controlled at 320-330℃, and the cooling rate of the second stage cooling is 0.5-5℃ / s;

[0015] Third-stage cooling: The final temperature of the third-stage cooling is controlled at 150-250℃, and the cooling rate of the third stage is 10-40℃ / s.

[0016] The inventors discovered that the aluminum-zinc-aluminum-magnesium coating products produced by existing technologies have a black spot defect on their surface. If this defect cannot be effectively controlled, it will become a serious obstacle for the coating products in applications with high surface quality requirements.

[0017] In this invention, the aluminum-zinc-aluminum-magnesium coating not only contains the primary Zn phase but also binary and ternary eutectic structures. Different post-plating cooling processes affect the proportions and chemical composition of each phase and structure, thereby altering the visual and microscopic morphology of the coating surface. The inventors have discovered that different cooling rates, especially those for binary and ternary eutectic structures, significantly influence the MgZn2 and Mg2ZN phases within the eutectic structure.11 The formation of Mg2Zn 11 The presence of this phase can cause black spot defects to form on the coating surface.

[0018] Based on the above research, the cooling method for controlling black spot defects on the surface of zinc-aluminum-magnesium coated steel plates described in this invention, by controlling the cooling rate of the solidification process after galvanizing in three stages, can avoid the formation of Mg2Zn. 11 This phase helps avoid black spot defects.

[0019] It should be noted that the starting temperature of the second cooling stage is the same as the ending temperature of the first cooling stage, and similarly, the starting temperature of the third cooling stage is the same as the ending temperature of the second cooling stage.

[0020] Furthermore, in the cooling method described in this invention, the third stage of cooling employs jet cooling.

[0021] Furthermore, in the cooling method described in this invention, the third stage of cooling employs water mist cooling.

[0022] Furthermore, in the cooling method described in this invention, the second stage of cooling employs air cooling or furnace cooling.

[0023] Furthermore, in the cooling method described in this invention, the cooling rate of the first stage can be 4-25°C / s.

[0024] Furthermore, in the cooling method described in this invention, the cooling rate of the first stage can be 5-20°C / s.

[0025] Furthermore, in the cooling method described in this invention, the cooling rate of the first stage can be 10-20°C / s.

[0026] Furthermore, in the cooling method described in this invention, the cooling rate of the second stage is 0.5-3℃ / s.

[0027] The zinc-aluminum-magnesium coated steel sheet and its cooling method described in this invention have the following advantages and beneficial effects:

[0028] The cooling method described in this invention, through three-stage cooling control, avoids the formation of Mg2Zn in the coating. 11 This phase can effectively improve black spot defects on the coating surface.

[0029] The aluminum-zinc-aluminum-magnesium coated steel sheet of the present invention can achieve excellent surface quality. This coated steel sheet is suitable for applications with high requirements for surface quality, such as automotive interior and exterior panels, and has good prospects for promotion and application value.

[0030] Instruction manual illustrations

[0031] Figure 1The XRD test results of the zinc-aluminum-magnesium coated steel sheet coating of Example 1 are shown, obtained by using the cooling method for controlling black spot defects on the surface of zinc-aluminum-magnesium coated steel sheet as described in this invention.

[0032] Figure 2 The XRD test results of the black spot area of ​​the coated steel plate in Comparative Example 2 are shown. Detailed Implementation

[0033] The zinc-aluminum-magnesium coated steel sheet and its cooling method according to the present invention will be further explained and described below with reference to specific embodiments and accompanying drawings. However, such explanation and description do not constitute an undue limitation on the technical solution of the present invention.

[0034] Examples 1-9 and Comparative Examples 1-6

[0035] The zinc-aluminum-magnesium coated steel sheets of Examples 1-9 and the comparative steels of Comparative Examples 1-6 can all be prepared using the following process:

[0036] (1) The pretreated substrate is immersed in the plating solution for hot-dip plating. The surface of the plated zinc-aluminum-magnesium coating is sprayed with an air knife to obtain a steel plate with a zinc-aluminum-magnesium coating. Table 1 lists the mass percentage of the coating chemical composition of the zinc-aluminum-magnesium coated steel plates of Examples 1-9 and Comparative Examples 1-6.

[0037] (2) Three-stage cooling control after plating, that is, the post-plating cooling process of strip steel from air knife to top roll includes the following steps:

[0038] First stage cooling: The starting temperature of the first stage cooling is controlled at 420-440℃, the final cooling temperature is controlled at 350-360℃, and the cooling rate of the first stage cooling is 4-30℃ / s; in some embodiments, the cooling rate of the first stage cooling can be further controlled at 4-25℃ / s, or 5-20℃ / s, or 10-20℃ / s.

[0039] Second-stage cooling: The endpoint temperature of the second-stage cooling is controlled at 320-330℃, and the cooling rate is 0.5-5℃ / s; in some embodiments, the cooling rate can be further controlled at 0.5-3℃ / s. In some embodiments, the second-stage cooling can be air cooling or furnace cooling.

[0040] Third-stage cooling: The final temperature of the third-stage cooling is controlled at 150-250℃, and the cooling rate of the third stage is 10-40℃ / s.

[0041] The third stage of cooling can be achieved using either jet cooling or water mist cooling.

[0042] It should be noted that the present invention does not impose any special limitations on the substrate used for zinc-aluminum-magnesium coated steel sheets. In practical applications, those skilled in the art can select CQ steel, IF steel, high-strength IF steel, bake-hardening steel or other high-strength steel according to their needs, which is not directly related to the improvement in surface quality of the coating obtained by the present invention.

[0043] The chemical composition of the zinc-aluminum-magnesium coated steel sheet designed in this invention is basically consistent with the composition of the plating solution. Table 1 lists the plating solutions in Examples 1-9 and Comparative Examples 1-6, as well as the chemical element mass percentage of the zinc-aluminum-magnesium coated layers formed in each example and comparative example based on the plating solution. Since the examples and comparative examples of this invention aim to demonstrate the influence of the post-plating cooling process on the generation of surface black spots, the chemical element composition of Comparative Examples 1-6 also satisfies the requirements of this invention.

[0044] Table 1. (wt.%, balance Zn and other unavoidable impurities)

[0045] serial number Mg Al Example 1 1.0 1.0 Example 2 1.5 1.0 Example 3 1.8 1.0 Example 4 2.0 1.0 Example 5 1.5 1.3 Example 6 1.5 1.5 Example 7 1.5 1.8 Example 8 1.5 2.0 Example 9 2.0 2.0 Comparative Example 1 1.0 1.0 Comparative Example 2 1.5 1.0 Comparative Example 3 1.5 1.3 Comparative Example 4 1.5 1.5 Comparative Example 5 1.5 1.8 Comparative Example 6 2.0 2.0

[0046] Table 2 lists the specific process parameters for the zinc-aluminum-magnesium coated steel sheets of Examples 1-9 and the comparative steels of Comparative Examples 1-6 in the first, second, and third stages of cooling in the above process steps.

[0047] Table 2.

[0048]

[0049]

[0050] Samples were taken from the zinc-aluminum-magnesium coated steel sheets of Examples 1-9 and the comparative steels of Comparative Examples 1-6, and the surfaces of the steel sheets of each example and comparative example were visually observed and subjected to micro-area XRD analysis.

[0051] The parameters for the XRD analysis experiment can be: Co radiation of 35kV-40mA, scanning range of 15-105°, scanning speed of 0.15 steps / s, and scanning step of 0.01°. The relevant detection and observation results are listed in Table 3 below.

[0052] Table 3 lists the frequency of visual black spot occurrence, the diameter of the black spots, and the phase results detected by XRD for the zinc-aluminum-magnesium coated steels of Examples 1-9 and the control steels of Comparative Examples 1-6.

[0053] Table 3.

[0054]

[0055]

[0056] As can be seen from Table 3 above, the zinc-aluminum-magnesium coated steel sheets of Examples 1-9 obtained according to the post-plating cooling process designed in this invention have no black spot defects on their surface, appear uniform, and have excellent surface quality. XRD testing of any part shows that the phases in the coating are Zn, Al, and MgZn2, with no Mg2Zn. 11 The appearance of phases.

[0057] In contrast, the comparative steels in Examples 1-6 deviated from the manufacturing process described in this invention in terms of the final cooling temperature and cooling rate of the first, second, and third cooling stages during production. This resulted in black spot defects on the surface of the final steel plates. XRD analysis of the black spot areas revealed that the phases in the coating were Zn, Al, and Mg2Zn. 11 .

[0058] also, Figure 1 The XRD test results of the zinc-aluminum-magnesium coated steel sheet coating of Example 1, obtained by using the cooling method for controlling black spot defects on the surface of the zinc-aluminum-magnesium coated steel sheet described in this invention, are also shown. Figure 1 The three different symbols in the text represent three different phases, from Figure 1 It can also be seen from the image that the phases in the coating are Zn, Al, and MgZn2, but not Mg2Zn. 11 .

[0059] Figure 2 The XRD test results of the black spot areas on the zinc-aluminum-magnesium coated steel sheet of Comparative Example 2 are shown. Figure 2 The three different symbols in the text represent three different phases, from Figure 2 It can be seen that the phases in the coating are Zn, Al and Mg2Zn. 11 .

[0060] By comparing Examples 1-9 with Comparative Examples 1-6, it can be seen that the black spot defects on the coating surface are highly related to the cooling process after plating. Differential thermal analysis (DTA) was used to determine the phase transition process during the solidification of the plating solution. It was found that a zinc phase first solidifies and precipitates between 440-350℃, and then, as the temperature decreases, binary eutectic and ternary eutectic structures are formed successively. Below 320℃, the plating solution completely solidifies. The cooling rate in different temperature ranges has a significant impact on the microstructure of the coating, thus affecting the visual quality of the post-plating product.

[0061] Therefore, the manufacturing processes and parameters of the zinc-aluminum-magnesium coated steel sheets in Examples 1-9 all meet the design specifications of this invention. However, Comparative Examples 1-6 contain parameters that do not meet the design specifications of this invention, resulting in poor surface properties of the final comparative steels.

[0062] In summary, the zinc-aluminum-magnesium coated steel sheet designed in this invention adopts a three-stage cooling control between the coating and the top roller, which effectively controls the microstructure of the coating and improves the black spot defects existing in the prior art. This product is suitable for applications with high requirements for surface quality, such as automotive inner and outer panels, and has good prospects for promotion and application value.

[0063] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0064] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A zinc-aluminum-magnesium coated steel sheet, comprising a substrate and a coating applied to the substrate, characterized in that, The mass percentage of each chemical element in the coating is: Mg: 1.0-2.0%, Al: 1.0-2.0%, with the balance being Zn and unavoidable impurities; The controlled post-plating cooling process of the zinc-aluminum-magnesium coated steel sheet is a three-stage cooling process, in which: First stage cooling: The starting temperature of the first stage cooling is controlled at 420-440℃, the final cooling temperature is 350-360℃, and the cooling rate of the first stage is 10-30℃ / s. Second stage cooling: The final temperature of the second stage cooling is controlled at 320-330℃, and the cooling rate of the second stage cooling is 0.5-5℃ / s; Third-stage cooling: The final temperature of the third-stage cooling is controlled at 150-250℃, and the cooling rate of the third stage is 10-40℃ / s.

2. The cooling method for zinc-aluminum-magnesium coated steel sheet as described in claim 1, characterized in that, The post-plating cooling process is controlled as a three-stage cooling process, in which: First stage cooling: The starting temperature of the first stage cooling is controlled at 420-440℃, the final cooling temperature is 350-360℃, and the cooling rate of the first stage is 10-30℃ / s. Second stage cooling: The final temperature of the second stage cooling is controlled at 320-330℃, and the cooling rate of the second stage cooling is 0.5-5℃ / s; Third-stage cooling: The final temperature of the third-stage cooling is controlled at 150-250℃, and the cooling rate of the third stage is 10-40℃ / s.

3. The cooling method as described in claim 2, characterized in that, The third stage of cooling uses jet cooling.

4. The cooling method as described in claim 2, characterized in that, The third stage of cooling uses water mist cooling.

5. The cooling method as described in claim 2, characterized in that, The second stage of cooling uses either air cooling or furnace cooling.

6. The cooling method as described in claim 2, characterized in that, The cooling rate for the first stage is 10-20℃ / s.

7. The cooling method as described in claim 2, characterized in that, The cooling rate for the second stage is 0.5-3℃ / s.

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