Zinc-aluminum-magnesium coated steel plate and cooling method thereof

Abstract

The invention discloses a zinc-aluminum-magnesium coated steel plate. The zinc-aluminum-magnesium coated steel plate comprises a base plate and a coating coated on the base plate. The coating of the steel plate does not contain an Mg2Zn11 phase. The invention further discloses a cooling method for controlling the black spot defect on the surface of the zinc-aluminum-magnesium coated steel plate, the cooling process of the method after plating is three-stage cooling, specifically, in the first-stage cooling, the initial cooling temperature of the first-stage cooling is controlled to be 420-440 DEG C, the final cooling temperature of the first-stage cooling is controlled to be 350-360 DEG C, and the speed of the first-stage cooling is controlled to be 4-30 DEG C / s; second-stage cooling is conducted, specifically, the final temperature of second-stage cooling is controlled to be 320-330 DEG C, and the speed of second-stage cooling is 0.5-5 DEG C / s; and third-stage cooling: controlling the final temperature of the third-stage cooling to be 150-250 DEG C, and controlling the speed of the third-stage cooling to be 10-40 DEG C / s.

Description

Technical Field

[0001] The invention relates to a coated steel plate and a manufacturing method thereof, and in particular to a zinc-aluminum-magnesium coated steel plate and a manufacturing method thereof. Background Art

[0002] In recent years, in order to further improve the corrosion resistance of the coating, the galvanizing material has gradually developed from pure zinc to zinc alloy. The use of hot-dip galvanizing technology for steel can effectively slow down the corrosion of steel, thereby extending its service life. Compared with other anti-corrosion methods, hot-dip galvanizing technology has the advantages of low production cost, simple process, firm coating and good appearance.

[0003] In the prior art, 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 the corrosion resistance or facilitate manufacturing and improve the surface quality.

[0004] For example: The Chinese patent document with announcement number CN108018514A, publication date May 11, 2018, and titled "A method for controlling surface defects of Zn-Al-Mg alloy coated sheet and Zn-Al-Mg alloy coated sheet", discloses a method for controlling surface defects of Zn-Al-Mg alloy coated sheet and Zn-Al-Mg alloy coated sheet, which improves a surface defect that looks like "frog skin" by controlling the composition of the plating solution, the galvanizing process and the oxygen content of the air knife scraping medium.

[0005] Another example: The Chinese patent document with the publication number CN114892114A and the publication date of 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 in a mobile cooling bellows and cooling at a set cooling rate to completely solidify the alloy coating on the surface of the strip; S3, forced cooling. This patent controls the streak defects caused by surface solidification shrinkage by reducing the volume change during the precipitation of the Zn-Al-Mg ternary eutectic.

[0006] However, the surface of low-aluminum-zinc-aluminum-magnesium coating products produced by existing technologies still has a surface defect of black spots, which becomes a serious obstacle for the product in applications with high surface quality requirements. Summary of the invention

[0007] One of the purposes of the present invention is to provide a zinc-aluminum-magnesium coated steel plate, which has excellent surface quality and can be used in applications with high requirements on surface quality, such as automotive inner and outer panels, etc., and has good promotion prospects and application value.

[0008] In order to achieve the above object, the present invention also provides a zinc-aluminum-magnesium coated steel plate, which includes a substrate and a coating coated on the substrate, wherein the coating does not contain Mg 2 Zn 11 Mutually.

[0009] Furthermore, in the zinc-aluminum-magnesium coated steel sheet described in 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 plate described in the present invention, the mass percentage content of each chemical element of the coating is: Mg: 1.0-2.0%, Al: 1.0-2.0%, and the remainder is Zn and unavoidable impurities.

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

[0012] In order to achieve the above object, the present invention provides a cooling method for a zinc-aluminum-magnesium coated steel plate, which controls the cooling process after plating to be a three-stage cooling, wherein:

[0013] First stage cooling: the first stage cooling start temperature is controlled at 420-440°C, the final cooling temperature is controlled at 350-360°C, and the first stage cooling rate is controlled at 4-30°C / s;

[0014] Second stage cooling: Control the end temperature of the second stage cooling to 320-330°C, and the cooling rate of the second stage cooling to 0.5-5°C / s;

[0015] The third stage cooling: the terminal temperature of the third stage cooling is controlled to be 150-250°C, and the cooling rate of the third stage cooling is 10-40°C / s.

[0016] The inventors have found that there is a black spot defect on the surface of the aluminum-zinc-aluminum-magnesium coating products produced by the prior art. 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 the present invention, the aluminum-zinc-aluminum-magnesium coating contains not only the primary Zn phase, but also the binary and ternary eutectic structures. Different cooling processes after plating will affect the proportion and chemical composition of each phase and structure, thereby changing the visual morphology and microscopic morphology of the coating surface. The inventors have found that different cooling rates, especially the cooling rates of binary and ternary eutectics, seriously affect the MgZn content in the eutectic structure.2 Phase and Mg 2 ZN 11 The generation of Mg 2 Zn 11 The appearance of the phase will cause black spot defects to form on the surface of the coating.

[0018] Based on the above research, in the cooling method for controlling the black spot defects on the surface of the zinc-aluminum-magnesium coated steel plate described in the present invention, the cooling rate of the solidification process after galvanizing is controlled in three stages to avoid the formation of Mg 2 Zn 11 phase, thereby avoiding black spot defects.

[0019] It should be noted that the start temperature of the second stage cooling is the end temperature of the first stage cooling, and similarly, the start temperature of the third stage cooling is the end temperature of the second stage cooling.

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

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

[0022] Furthermore, in the cooling method described in the present invention, the second stage cooling adopts air cooling or furnace cooling.

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

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

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

[0026] Furthermore, in the cooling method of the present invention, the rate of the second cooling stage is 0.5-3°C / s.

[0027] The zinc-aluminum-magnesium coated steel plate and the cooling method thereof described in the present invention have the following advantages and beneficial effects:

[0028] The cooling method of the present invention uses three-stage cooling control to avoid the presence of Mg in the coating. 2 Zn 11 Phase, can effectively improve the black spot defects on the coating surface.

[0029] The aluminum-zinc-aluminum-magnesium coated steel sheet of the present invention can obtain excellent surface quality. The coated steel sheet can be suitable for applications with high requirements on surface quality, such as automobile inner and outer panels, and has good promotion prospects and application value.

[0030] Instruction Manual

[0031] Figure 1 The XRD test results of the zinc-aluminum-magnesium coated steel plate 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 plate described in the present invention are shown.

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

[0033] The zinc-aluminum-magnesium coated steel plate and the cooling method thereof described in the present invention will be further explained and illustrated below in conjunction with specific embodiments and the accompanying drawings of the specification. However, such explanation and illustration do not constitute an improper 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 be prepared by the following process:

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

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

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

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

[0040] The third stage cooling: the terminal temperature of the third stage cooling is controlled to be 150-250°C, and the cooling rate of the third stage cooling is 10-40°C / s.

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

[0042] It should be noted that the present invention does not specifically limit the substrate used for the zinc-aluminum-magnesium coated steel plate. In actual application, technical personnel in this field can select CQ steel, IF steel, high-strength IF steel, bake-hardened steel or other high-strength steel according to needs, which has no direct correlation with the improvement in the surface quality of the coating obtained by the present invention.

[0043] The chemical composition of the zinc-aluminum-magnesium coating of the zinc-aluminum-magnesium coated steel sheet designed by the present invention is substantially consistent with the composition of the plating solution. Table 1 lists the plating solutions in Examples 1-9 and Comparative Examples 1-6 and the chemical element mass percentage ratios of the zinc-aluminum-magnesium coatings formed by the respective Examples and Comparative Examples obtained based on the plating solutions. Since the Examples and Comparative Examples of the present invention are intended to demonstrate the effect of the post-plating cooling process on the generation of surface black spots, the chemical element composition ratios of Comparative Examples 1-6 also meet the requirements of the present invention.

[0044] Table 1. (wt.%, the balance is 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 of the zinc-aluminum-magnesium coated steel sheets of Examples 1-9 and the comparative steels of Comparative Examples 1-6 in the first cooling stage, the second cooling stage and the third cooling stage of the above process steps.

[0047] Table 2.

[0048]

[0049]

[0050] The zinc-aluminum-magnesium coated steel sheets of Examples 1-9 and the comparative steels of Comparative Examples 1-6 were sampled respectively, and the surfaces of the steel sheets of each Example and Comparative Example were visually observed and subjected to XRD analysis of micro-areas.

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

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

[0053] Table 3.

[0054]

[0055]

[0056] It can be seen from Table 3 that the zinc-aluminum-magnesium coated steel sheets of Examples 1-9 obtained by the post-coating cooling process designed by the present invention have no black spot defects on their surfaces, are visually uniform, and have very excellent surface quality. XRD detection of any part shows that the phases in the coating are Zn, Al and MgZn. 2 , no Mg 2 Zn 11 The appearance of phase.

[0057] However, in the production of the comparative steel materials of Comparative Examples 1-6, the final cooling temperature and cooling rate of the first cooling section, the second cooling section and the third cooling section all deviate from the manufacturing process described in the present invention, and the surface of the steel plates finally prepared therefrom has black spot defects. XRD detection of the black spot parts shows that the phases in the coating are Zn, Al and Mg. 2 Zn 11 .

[0058] also, Figure 1 It also shows the XRD test results of the zinc-aluminum-magnesium coated steel plate coating of Example 1 obtained by using the cooling method of the present invention to control the black spot defects on the surface of the zinc-aluminum-magnesium coated steel plate. Figure 1 The three different symbols in the equation represent three different phases. Figure 1 It can also be seen that the phases in the coating are Zn, Al and MgZn 2 , without Mg 2 Zn 11 .

[0059] Figure 2 The XRD test results of the black spot area of ​​the zinc-aluminum-magnesium coated steel plate in comparative example 2 are shown. Figure 2 The three different symbols in the equation represent three different phases. Figure 2 It can be seen that the phases in the coating are Zn, Al and Mg 2 Zn 11 .

[0060] By comparing the above examples 1-9 with the comparative examples 1-6, it can be seen that the black spot defects on the surface of the coating are closely related to the cooling process after plating. The phase change process of the solidification process of the plating solution was measured by a differential thermal analyzer, and it was found that the zinc phase first solidified and precipitated between 440-350°C, and then as the temperature decreased, binary eutectic and ternary eutectic structures were successively generated. When the temperature was lower than 320°C, the plating solution was completely solidified. The cooling rate in different temperature ranges has an important influence on the organizational structure of the coating, thereby affecting the visual quality of the plated product.

[0061] Therefore, the manufacturing processes and parameters of the zinc-aluminum-magnesium coated steel sheets of Examples 1-9 all meet the design specification requirements of the present invention. However, the parameters of Comparative Examples 1-6 do not meet the design specification requirements of the present invention, which results in poor surface properties of the comparative steels finally prepared.

[0062] From the above, it can be seen that the zinc-aluminum-magnesium coated steel plate designed by the present invention adopts a three-stage cooling control between plating and the top roller, which effectively controls the organizational structure in the coating and improves the black spot defects existing in the prior art. The product can be suitable for applications with high requirements on surface quality, such as automotive inner and outer panels, and has good promotion prospects and application value.

[0063] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0064] It should also be noted that the above-listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therewith can be directly derived or easily associated with the contents disclosed by those skilled in the art from the present invention, and all should belong to the protection scope of the present invention.

Claims

1. A zinc-aluminum-magnesium coated steel plate, comprising a substrate and a coating coated on the substrate, characterized in that: The coating does not contain Mg2Zn 11 Mutually.

2. The zinc-aluminum-magnesium coated steel sheet according to claim 1, characterized in that: The coating contains Zn and the following chemical elements in percentage by mass: Mg: 1.0-2.0%, Al: 1.0-2.0%.

3. The zinc-aluminum-magnesium coated steel sheet according to claim 2, characterized in that: The mass percentage of each chemical element in the coating is: Mg: 1.0-2.0%, Al: 1.0-2.0%, and the remainder is Zn and inevitable impurities.

4. The cooling method of the zinc-aluminum-magnesium coated steel sheet according to any one of claims 1 to 3, characterized in that: The post-plating cooling process is controlled to be a three-stage cooling process, in which: First stage cooling: the first stage cooling start temperature is controlled at 420-440°C, the final cooling temperature is controlled at 350-360°C, and the first stage cooling rate is controlled at 4-30°C / s; Second stage cooling: Control the end temperature of the second stage cooling to 320-330°C, and the cooling rate of the second stage cooling to 0.5-5°C / s; The third stage cooling: the terminal temperature of the third stage cooling is controlled to be 150-250°C, and the cooling rate of the third stage cooling is 10-40°C / s.

5. The cooling method according to claim 4, characterized in that: The third stage cooling adopts jet cooling.

6. The cooling method according to claim 4, characterized in that: The third stage of cooling adopts water mist cooling.

7. The cooling method according to claim 4, characterized in that: The second stage cooling adopts air cooling or furnace cooling.

8. The cooling method according to claim 4, characterized in that: The cooling rate of the first stage is 5-20°C / s.

9. The cooling method according to claim 4, characterized in that: The cooling rate of the first stage is 10-20°C / s.

10. The cooling method according to claim 4, characterized in that: The cooling rate of the second stage is 0.5-3°C / s.

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