A method for controlling dot defects on the surface of zinc-aluminum-magnesium coated plates

Abstract

This invention provides a method for controlling surface dot defects on zinc-aluminum-magnesium coated steel sheets, belonging to the field of steel rolling. The method includes: hot-dip galvanizing a steel substrate and cooling it to obtain a steel sheet with a zinc-aluminum-magnesium coating. During the cooling process, the temperature of the zinc-aluminum-magnesium coated steel sheet at the top roller of the cooling tower is controlled at 150℃~250℃, the fan power of the cooling tower is controlled at 100kW~150kW, and the tension of the cooling tower is controlled. The temperature at the top of the cooling tower ensures the position of the solidification line in the upper section of the fan. Controlling the fan power reduces the proportion of binary phases in the coating, rapidly suppressing the nucleation tendency of binary phases. Controlling the cooling tower tension reduces strip swaying, improves surface quality uniformity and stability, and avoids local binary phase nucleation in the strip due to uneven cooling. This improves the coating's resistance to dot defects, thereby reducing surface dot defects on the zinc-aluminum-magnesium coated steel sheet.

Description

Technical Field

[0001] This application relates to the field of steel rolling technology, and in particular to a method for reducing dot defects on the surface of zinc-aluminum-magnesium coated steel plates. Background Technology

[0002] Hot-rolled galvanizing is a process of coating hot-rolled steel sheets after pickling. The resulting product combines the formability of hot-rolled steel with the corrosion resistance of coated steel. The surface coatings of hot-rolled galvanized sheets mainly include pure zinc coatings and zinc-aluminum-magnesium (ZAM) coatings. ZAM is a new type of alloy coating formed by adding small amounts of Al and Mg elements to traditional pure zinc coatings, exhibiting higher corrosion resistance than pure zinc coatings. Applying hot-rolled ZAM steel sheets to automotive parts can significantly extend the service life of parts and reduce energy consumption and carbon emissions associated with parts repair, replacement, and remanufacturing.

[0003] Currently, zinc-aluminum-magnesium coated steel sheets produced domestically and internationally, while exhibiting excellent resistance to red rust due to the addition of highly reactive magnesium in the coating, suffer from surface dot defects due to a lack of targeted process optimization. This negatively impacts surface quality, resulting in a difference compared to pure zinc products. During the cooling process after galvanizing, the nucleation and growth of binary phase grains in zinc-aluminum-magnesium steel strips leads to severe surface dot defects, resulting in black spots that do not meet user requirements. This significantly limits the application of low-aluminum zinc-aluminum-magnesium coated steel sheets in automotive panels, particularly exterior panels. Therefore, developing a dot defect control method suitable for the characteristics of low-aluminum zinc-aluminum-magnesium products is of great significance. Summary of the Invention

[0004] This application provides a method for controlling dot defects on the surface of zinc-aluminum-magnesium coated plates, in order to solve the technical problem of numerous dot defects on the surface of existing low-aluminum zinc-aluminum-magnesium coated plates.

[0005] In a first aspect, this application provides a method for controlling dot defects on the surface of a zinc-aluminum-magnesium coated plate, the method comprising:

[0006] The steel substrate is hot-dip galvanized and cooled to obtain a steel plate with a zinc-aluminum-magnesium coating. During the cooling process, the temperature of the steel plate with the zinc-aluminum-magnesium coating at the top roller of the cooling tower is 150℃~250℃, the fan power of the cooling tower is 100kW~150kW, the insertion amount of the correction roller is adjusted, and the tension of the cooling tower is controlled.

[0007] Optionally, the fan power of the cooling tower is 125kW to 150kW.

[0008] Optionally, the cooling tower has a fan power of 130kW.

[0009] Optionally, the tension of the cooling tower is ≥40KN.

[0010] Optionally, the tension of the cooling tower is 42KN to 50KN.

[0011] Optionally, the temperature of the plating solution for hot-dip plating is 465℃~475℃.

[0012] Optionally, the chemical composition of the plating solution, by mass fraction, is: Al: 1.5%–1.7%, Mg: 1.0%–1.2%, with the remainder being Zn and unavoidable impurities.

[0013] Optionally, the temperature of the zinc-aluminum-magnesium coated steel plate at the top roller of the cooling tower is 200°C.

[0014] Secondly, this application provides a zinc-aluminum-magnesium coated plate, which is prepared by the method described in any one embodiment of the present application.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art:

[0016] This application provides a method for controlling surface dot defects on zinc-aluminum-magnesium coated steel sheets. The method utilizes the temperature at the top of a cooling tower to ensure the position of the solidification line in the upward section of the fan. By controlling the fan power, the proportion of binary phases in the coating is reduced, rapidly suppressing the nucleation tendency of binary phases. By controlling the cooling tower tension, strip swaying is reduced, improving surface quality uniformity and stability, and preventing uneven cooling that could lead to localized binary phase nucleation in the strip. This improves the coating's resistance to dot defects, thereby reducing surface dot defects on zinc-aluminum-magnesium coated steel sheets. Attached Figure Description

[0017] 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.

[0018] 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.

[0019] Figure 1 This is a flowchart illustrating a method for controlling dot defects on the surface of a zinc-aluminum-magnesium coated plate, as provided in an embodiment of this application. Detailed Implementation

[0020] 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.

[0021] 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.

[0022] Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0023] 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.

[0024] Firstly, this application provides a method for controlling dot defects on the surface of zinc-aluminum-magnesium coated plates. Please refer to [link to relevant documentation]. Figure 1 The method includes:

[0025] S1. The steel substrate is hot-dip coated and cooled to obtain a steel plate with a zinc-aluminum-magnesium coating. During the cooling process, the temperature of the steel plate with the zinc-aluminum-magnesium coating at the top roller of the cooling tower is controlled to be 150℃~250℃, the fan power of the cooling tower is 100kW~150kW, and the tension of the cooling tower is controlled.

[0026] Controlling the temperature of the zinc-aluminum-magnesium coated steel plate at the top roller of the cooling tower between 150℃ and 250℃ has a positive effect on ensuring the solidification line position of the fan's upward section: if the tower top temperature is higher than 250℃, the zinc-aluminum-magnesium coating cannot solidify, resulting in color difference defects; if the tower top temperature is lower than 150℃, uneven cooling occurs. For example, the temperature of the zinc-aluminum-magnesium coated steel plate at the top roller can be 150℃, 170℃, 200℃, 220℃, 250℃, etc.

[0027] Controlling the cooling tower's fan power to 100kW-150kW has a positive effect on reducing the proportion of binary phases in the coating and rapidly suppressing the nucleation tendency of binary phases: when the fan power is below 100kW, the cooling rate is slow, the nucleation of binary phases increases, and the number of dot defects increases; when the fan power is above 150kW, the strip steel becomes unstable, and the cooling effect is poor. For example, the fan power of this cooling tower can be 100kW, 110kW, 120kW, 140kW, 150kW, etc.

[0028] In some embodiments, the temperature of the plating solution for hot-dip plating is 465°C to 475°C.

[0029] Controlling the plating bath temperature to 465℃~475℃ has the positive effect of ensuring effective cooling of the coating: if the zinc bath temperature is below 465℃, the required fan power is high, which does not benefit cooling uniformity; if the zinc bath temperature is above 475℃, the cooling rate is slow, and the high temperature at the top of the tower can easily lead to color difference defects. For example, the plating bath temperature can be 465℃, 470℃, 475℃, etc.

[0030] In some embodiments, the chemical composition of the plating solution, by mass fraction, is: Al: 1.5% to 1.7%, Mg: 1.0% to 1.2%, with the remainder being Zn and unavoidable impurities.

[0031] Controlling the Al content to 1.5%–1.7% has the positive effect of improving corrosion resistance: If the Al mass fraction is below 1.5%, it will affect the corrosion resistance of low-aluminum-zinc-aluminum-magnesium alloys, leading to a decrease in resistance to red rust. If the Al mass fraction is above 1.7%, it will lead to an increase in aluminum-rich phases on the surface, resulting in poorer resistance to blackening. For example, the Al content can be 1.5%, 1.6%, 1.7%, etc.

[0032] Controlling the Mg content to 1.0%–1.2% has the positive effect of improving corrosion resistance: If the Mg mass fraction is below 1.0%, it will affect the corrosion resistance of low-aluminum-zinc-aluminum-magnesium alloys, leading to a decrease in resistance to red rust. If the Mg mass fraction is above 1.2%, it will lead to an increase in binary phases on the plate surface, resulting in a poorer resistance to blackening. For example, the Mg content can be 1.0%, 1.1%, 1.2%, etc.

[0033] In some embodiments, the temperature of the zinc-aluminum-magnesium coated steel plate at the top roller of the cooling tower is 200°C.

[0034] In some implementations, the cooling tower fan power is 125kW to 150kW.

[0035] For example, the fan power of the cooling tower can be 125kW, 130kW, 135kW, 145kW, 150kW, etc.

[0036] In some embodiments, the fan power of the post-plating cooling tower is 130kW.

[0037] In some embodiments, the tension of the post-plating cooling tower is ≥40KN.

[0038] The positive effects of controlling the tension of the post-plating cooling tower to ≥40KN include reducing strip sway and improving the uniformity and stability of surface quality; if the tension of the post-plating cooling tower is <40KN, the strip stability is insufficient and the cooling uniformity of the strip is poor. For example, the tension of the post-plating cooling tower can be 40KN, 42KN, 44KN, 46KN, 48KN, etc.

[0039] In some embodiments, the tension of the post-plating cooling tower is 42KN to 50KN.

[0040] For example, the tension of the post-plating cooling tower can be 42KN, 44KN, 46KN, 48KN, etc.

[0041] Secondly, this application provides a zinc-aluminum-magnesium coated plate, which is prepared by the method described in any one embodiment of the present application.

[0042] In some embodiments, the number of dots per 100 meters of the zinc-aluminum-magnesium coated plate is ≤17.

[0043] By controlling the temperature at the top of the cooling tower, the position of the solidification line in the upward section of the fan is ensured. Fan power control reduces the proportion of binary phases in the coating, rapidly suppressing their nucleation tendency. Controlling the cooling tower tension reduces strip swaying, improves surface quality uniformity and stability, and prevents uneven cooling that could lead to localized binary phase nucleation. This also improves the coating's resistance to surface defects, reducing the number of surface defects on the zinc-aluminum-magnesium coated sheet. The number of surface defects per 100 meters on this zinc-aluminum-magnesium coated sheet can be 6, 8, 12, 14, or 17.

[0044] 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.

[0045] This application provides a method for controlling dot defects on the surface of zinc-aluminum-magnesium coated plates, the method comprising the following steps:

[0046] The steel substrate is hot-dip galvanized and cooled to obtain a steel plate with a zinc-aluminum-magnesium coating. During the cooling process, the temperature of the steel plate with the zinc-aluminum-magnesium coating at the top roller of the cooling tower is 150℃~250℃, the fan power of the cooling tower is 100kW~150kW, the insertion amount of the correction roller is adjusted, and the tension of the cooling tower is controlled.

[0047] Table 1. Main process parameters of the surface dot defect control methods in the embodiments and comparative examples.

[0048]

[0049] The zinc-aluminum-magnesium coated plates obtained by the methods of Examples 1-7 and Comparative Examples 1-4 were tested for surface dot defects, and the results are shown in Table 2.

[0050]

[0051]

[0052] Table 2. Number of round dot defects per 100 meters on zinc-aluminum-magnesium coated steel sheets

[0053] The zinc-aluminum-magnesium coated plates obtained in the examples and comparative examples in Table 2 have the same width.

[0054] Data from Examples 1-7 and Comparative Examples 1-4 show that:

[0055] By controlling the temperature at the top of the cooling tower, the position of the solidification line in the upward section of the fan is ensured. Fan power control reduces the proportion of binary phases in the coating, rapidly suppressing the nucleation tendency of binary phases. Controlling the cooling tower tension reduces strip swaying, improves surface quality uniformity and stability, and avoids localized binary phase nucleation in the strip due to uneven cooling. This improves the coating's resistance to spot defects, thereby reducing surface spot defects in zinc-aluminum-magnesium coated sheets. The method of this application yields steel sheets with fewer spots, achieving 60% or less of the defects found in conventional production methods.

[0056] 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 method for controlling dot defects on the surface of zinc-aluminum-magnesium coated plates, characterized in that, The method includes: The steel substrate is hot-dip galvanized and cooled to obtain a steel plate with a zinc-aluminum-magnesium coating. During the cooling process, the temperature of the aluminum-magnesium coated steel plate at the top roller of the cooling tower is controlled at 150℃~250℃, the fan power of the cooling tower is 100kW~150kW, and the tension of the cooling tower is controlled at ≥40KN. The temperature of the hot-dip plating solution is 465℃~475℃, and the chemical composition of the plating solution, by mass fraction, is: Al: 1.5%~1.7%, Mg: 1.0%~1.2%, with the remainder being Zn and unavoidable impurities.

2. The method according to claim 1, characterized in that, The cooling tower has a fan power of 125kW~150kW.

3. The method according to claim 2, characterized in that, The cooling tower has a fan power of 130kW.

4. The method according to claim 1, characterized in that, The tension of the cooling tower is 42KN~50KN.

5. The method according to claim 1, characterized in that, The temperature of the steel plate with zinc-aluminum-magnesium coating at the top roller of the cooling tower is 200°C.

6. A zinc-aluminum-magnesium coated plate, characterized in that, The zinc-aluminum-magnesium coated plate is prepared by the method described in any one of claims 1-5.

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