A hot-dip galvannealed aluminum magnesium product and a method of controlling zinc flow pattern defects in the production thereof

Abstract

The present application belongs to the technical field of hot-dip plating, and particularly relates to a hot-base galvannealed aluminum magnesium product and a method for controlling zinc flow pattern defects of the product. The method comprises the following steps: immersing a steel plate substrate to be plated with a zinc-aluminum-magnesium coating into a zinc pot in a plating solution for hot-dip plating, wherein the temperature at which the steel plate substrate is immersed into the zinc pot is related to the plating solution temperature and the cross-sectional width and thickness of the steel plate substrate; performing air-knife scraping treatment on the steel plate after hot-dip plating; and performing cooling treatment on the molten plated steel plate through a post-plating cooling section to obtain the hot-base galvannealed aluminum magnesium product, wherein the post-plating cooling section comprises a fixed air-cooling device and a movable air-cooling device, the molten plated steel plate sequentially passes through the movable air-cooling device and the fixed air-cooling device, and the cooling rate of the movable air-cooling device is less than that of the fixed air-cooling device. The method can effectively avoid the occurrence of zinc flow pattern defects of the thick-coated hot-base galvannealed aluminum magnesium product when producing the thick-coated hot-base galvannealed aluminum magnesium product.

Description

Technical Field

[0001] This invention belongs to the field of hot-dip galvanizing technology, specifically relating to a hot-dip zinc-aluminum-magnesium galvanized product and a method for controlling zinc flow defects in it. Background Technology

[0002] Hot-dip galvanized aluminum-magnesium products are alloy coatings obtained by adding appropriate amounts of Al, Mg, and other trace alloying elements to the plating bath, based on traditional hot-dip galvanizing. Hot-dip galvanized aluminum-magnesium products are directly coated using pickled hot-rolled steel sheets as raw materials. Compared to traditional cold-dip galvanized products, they can produce thicker, heavier coatings. Due to the reduction of the cold rolling process and the adjustment of the plating bath composition, hot-dip galvanized aluminum-magnesium products have significant advantages in terms of shorter production processes, lower costs, stronger corrosion resistance, and energy efficiency. They are gradually replacing more expensive cold-rolled galvanized products of the same specifications and are widely used in photovoltaic manufacturing, highway guardrails, construction, and warehousing. Thick-coated hot-dip galvanized aluminum-magnesium products typically refer to coatings with a weight of approximately 275–350 g / m². 2 Steel plates with a thickness of approximately 3.0 to 6.0 mm.

[0003] In the existing process of producing thick-coated hot-dip zinc-aluminum-magnesium products, the air knife pressure is too low, resulting in insufficient cooling capacity for the strip steel. In addition, the thick material carries a lot of core heat energy, and the incompletely solidified coating flows downward under the influence of gravity, which easily leads to surface quality defects such as zinc flow lines. This defect will cause a significant thickness difference of the coating along the longitudinal direction on the surface of the strip steel, making the strip steel surface uneven and the protruding parts feel tactile to the touch. This not only affects the appearance of the product, but also significantly reduces the corrosion resistance of the coating, greatly limiting its application fields.

[0004] Chinese patent CN103966537B discloses a method for controlling zinc flow marks in thick-material, thick-coated hot-dip galvanized products. This method controls the temperature of the blowing medium within the range of 0-10℃ by installing an air conditioner in the fan room, thereby improving the cooling capacity of the blowing medium on the strip steel. At the same time, it optimizes the distance between the air knife lip and the liquid surface to 600-1000mm, the distance between the air knife lip and the strip steel to 15-30mm, and the air knife pressure to the range of 100-300mbar, in order to control zinc flow mark defects. However, since the types of coating media are different, and the solidification temperatures of pure zinc and zinc-aluminum-magnesium plating solutions are also different, this method is not applicable to the product of this invention. Chinese patent CN111534774A discloses a method for reducing zinc flow defects in hot-dip galvanized steel. This method adjusts the strip running speed to a range of 65–95 m / min and employs a dual-purging mode of air and nitrogen using an air knife, depending on the annealing temperature, to control zinc flow defects. However, this method lacks parameter adjustments for key processes such as air knife purging and post-galvanizing cooling, and therefore cannot effectively control zinc flow defects in thick-coated hot-dip galvanized aluminum-magnesium products. Chinese patent CN112899600A discloses a galvanizing method for ultra-thick zinc-coated high-strength hot-dip galvanized steel strip. This method optimizes the hot-dip galvanizing furnace control unit, zinc pot control unit, zinc layer control unit, and cooling control unit to obtain high-strength hot-dip galvanized steel strip with excellent performance and surface quality. However, this method only addresses hot-dip galvanized products with a thickness of 2.5–3.0 mm, and cannot effectively control defects such as zinc flow for thicker hot-dip galvanized aluminum-magnesium products. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies in effectively controlling zinc flow defects in thick hot-dip galvanized aluminum-magnesium products. This invention proposes a method for controlling zinc flow defects in hot-dip galvanized aluminum-magnesium products. The method first reduces the core heat carried within the steel plate by precisely controlling the hot-dip galvanizing process parameters, preventing the plating solution from solidifying and then remelting, thus avoiding zinc flow defects. Finally, during the post-plating cooling process, a combination of movable cooling followed by fixed cooling is employed, with the cooling rates of both controlled from low to high. This effectively reduces the agitation effect of airflow on the incompletely solidified plating layer, preventing zinc flow defects. Using this method in the production of thick-coated hot-dip galvanized aluminum-magnesium products, zinc flow defects can be effectively avoided, and the surface quality is improved to meet the FB (higher-grade finishing surface) and FC (high-grade finishing surface) surface grade requirements in GB2518, reducing the surface defect rate to below 15%.

[0006] The first aspect of this invention provides a method for controlling zinc flow defects in hot-dip galvanized aluminum-magnesium products, the method comprising the following steps:

[0007] S1. The steel plate substrate to be coated with zinc-aluminum-magnesium plating is immersed in a zinc pot and hot-dip coated in a plating solution containing zinc, aluminum and magnesium elements to obtain the hot-dip coated steel plate.

[0008] S2. The hot-dip galvanized steel sheet is subjected to air knife blowing treatment to obtain a steel sheet with a molten coating.

[0009] S3. The steel plate with the molten coating is cooled in the post-coating cooling section to obtain a hot-dip zinc-aluminum-magnesium product. The post-coating cooling section includes a fixed air-cooling device and a movable air-cooling device. The steel plate with the molten coating passes through the movable air-cooling device and the fixed air-cooling device in sequence, and the cooling rate of the movable air-cooling device is less than that of the fixed air-cooling device.

[0010] In step S1, the temperature at which the steel substrate to be coated with zinc-aluminum-magnesium alloy is immersed in the zinc bath is shown in the following formula:

[0011] t1 = T - 2.2 * (w * h) * 10 -3 ;

[0012] Where t1 is the temperature at which the steel plate substrate to be coated with zinc aluminum magnesium is immersed in the zinc pot, T is the temperature of the plating solution, w is the cross-sectional width of the steel plate substrate to be coated with zinc aluminum magnesium, and h is the cross-sectional thickness of the steel plate substrate to be coated with zinc aluminum magnesium.

[0013] Preferably, in step S1, the temperature of the plating solution is 410-420°C; in the plating solution, the Al content is 1.2-3 wt%, the Mg content is 1.0-1.5 wt%, the Sb content is 0.07-0.075 wt%, and the remainder is Zn, and the ratio of Al content to Mg content is 1.2-2:1.

[0014] More preferably, the cross-sectional width of the steel plate substrate to be coated with zinc-aluminum-magnesium coating is 900-1450 mm, and the cross-sectional thickness of the steel plate substrate to be coated with zinc-aluminum-magnesium coating is 3.0-6.0 mm.

[0015] Preferably, in step S2, the conditions for the air knife blowing treatment include: the blowing medium is nitrogen, the temperature of the blowing medium is -10 to 0°C, the blowing angle is -0.8 to -0.9°, the height of the air knife lip from the liquid surface is 180 to 220 mm, the distance of the air knife lip from the strip is 10 to 20 mm, and the air knife pressure is 100 to 120 mbar.

[0016] Preferably, in step S3, the post-plating cooling section includes three fixed air-cooling devices and one movable air-cooling device. The steel plate with the molten plating layer passes sequentially through the movable air-cooling device, the first fixed air-cooling device, the second fixed air-cooling device, and the third fixed air-cooling device. The cooling rate of the movable air-cooling device is 10-15℃ / s, the cooling rate of the first fixed air-cooling device is 15-18℃ / s, the cooling rate of the second fixed air-cooling device is 18-20℃ / s, and the cooling rate of the third fixed air-cooling device is >25℃ / s. The cooling rate of the movable air-cooling device is < the cooling rate of the first fixed air-cooling device < the cooling rate of the second fixed air-cooling device < the cooling rate of the third fixed air-cooling device.

[0017] More preferably, the height of the movable air-cooling device from the plating liquid surface is 5 to 10 m, the temperature of the strip steel plate after exiting the movable air-cooling device is 315 to 335°C, and the temperature of the strip steel plate when it reaches the top roller of the tower is <205°C.

[0018] Preferably, in step S1, the tension of the strip steel in the zinc pot section is controlled to be about 30-50KN, and the running speed of the steel plate substrate to be coated with zinc-aluminum-magnesium coating is 40-60m / min.

[0019] Preferably, in step S2, the blowing medium for the hot-dip galvanized steel plate undergoing air knife blowing treatment is nitrogen gas, which is provided by a nitrogen mixing system. The nitrogen mixing system includes a liquid nitrogen tank, a flow control valve, a temperature detector, a first delivery pipe, a second delivery pipe, a third delivery pipe, a blower, and a mixing tank. The input end of the first delivery pipe is connected to the liquid nitrogen outlet of the liquid nitrogen tank, and the output end of the first delivery pipe is connected to the mixing tank. A flow control valve and a temperature detector are sequentially arranged on the first delivery pipe along the liquid nitrogen flow direction. The second delivery pipe delivers room temperature nitrogen gas to the mixing tank. The blower is installed inside the mixing tank to mix the liquid nitrogen and room temperature nitrogen gas input into the mixing tank to achieve the temperature and pressure required for the blowing medium. The mixing tank is connected to the third delivery pipe to provide the air source for the air knife with the blowing medium after mixing the liquid nitrogen and room temperature nitrogen gas. A temperature detector and a flow control valve are sequentially arranged on the third delivery pipe along the blowing medium flow direction.

[0020] A second aspect of the present invention provides a hot-dip zinc-aluminum-magnesium product prepared by the method described above.

[0021] Preferably, the coating weight of the hot-dip zinc-aluminum-magnesium product is 275–350 g / m³. 2 The coating thickness is 3.0 to 6.0 mm.

[0022] The hot-dip galvanized aluminum-magnesium product and the method for controlling zinc flow defects described in this invention have at least the following beneficial effects:

[0023] (1) In this invention, the core heat carried inside the steel plate is reduced by precisely controlling the temperature of the substrate entering the pot during the hot-dip galvanizing process, preventing the plating solution from solidifying and then melting back, thus avoiding zinc flow defects. Finally, in the post-plating cooling process, a combination of movable cooling and fixed cooling is adopted, and the cooling rates of movable cooling and fixed cooling are controlled from small to large. This can effectively reduce the agitation effect of airflow on the incompletely solidified plating layer, avoid the occurrence of zinc flow defects, improve the surface quality of the product, meet the surface grade requirements of FB and FC in GB2518, and reduce the surface defect ratio to less than 15%.

[0024] (2) In the preferred embodiment, by controlling the temperature of the plating solution and the content and proportion of different components in the plating solution, it is possible to ensure that the steel plate obtains excellent corrosion resistance while increasing the viscosity of the plating solution and reducing its fluidity, thereby reducing the generation of slag and zinc flow defects, and improving the surface quality of the coating. At the same time, during the air knife blowing process, by using low air knife pressure in conjunction with air knife height and angle within a certain range, it is possible to effectively avoid thick coatings (275~350g / m²). 2 This method reduces the occurrence of zinc flow marks in hot-dip galvanized products with specifications (3.0mm~6.0mm) and improves the surface quality of the products, meeting the FC surface grade requirements in GB2518, and reducing the surface defect rate to less than 3%. Attached Figure Description

[0025] Figure 1 This is a nitrogen mixing system for the blowing medium used in the air knife blowing process of the present invention.

[0026] Figure Labels

[0027] 1. Liquid nitrogen tank; 2. Flow control valve; 3. Temperature detector; 4. First delivery pipe; 5. Second delivery pipe; 6. Third delivery pipe; 7. Fan; 8. Mixing tank. Detailed Implementation

[0028] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0029] The first aspect of this invention provides a method for controlling zinc flow defects in thick-coated hot-dip zinc-aluminum-magnesium products, the method comprising the following steps:

[0030] S1. The steel plate substrate to be coated with zinc-aluminum-magnesium plating is immersed in a zinc pot and hot-dip coated in a plating solution containing zinc, aluminum and magnesium elements to obtain the hot-dip coated steel plate.

[0031] S2. The hot-dip galvanized steel sheet is subjected to air knife blowing treatment to obtain a steel sheet with a molten coating.

[0032] S3. The steel plate with the molten coating is cooled in the post-coating cooling section to obtain a hot-dip zinc-aluminum-magnesium product. The post-coating cooling section includes a fixed air-cooling device and a movable air-cooling device. The steel plate with the molten coating passes through the movable air-cooling device and the fixed air-cooling device in sequence, and the cooling rate of the movable air-cooling device is less than that of the fixed air-cooling device.

[0033] In step S1, the temperature at which the steel substrate to be coated with zinc-aluminum-magnesium alloy is immersed in the zinc bath is shown in the following formula:

[0034] t1 = T - 2.2 * (w * h) * 10 -3 ;

[0035] Where t1 is the temperature at which the steel plate substrate to be coated with zinc aluminum magnesium is immersed in the zinc pot, T is the temperature of the plating solution, w is the cross-sectional width of the steel plate substrate to be coated with zinc aluminum magnesium, and h is the cross-sectional thickness of the steel plate substrate to be coated with zinc aluminum magnesium.

[0036] In a specific embodiment of the method described in this invention, the steel plate substrate to be coated with zinc-aluminum-magnesium coating is obtained by degreasing, rinsing, drying and continuous annealing of a substrate.

[0037] In a preferred embodiment of the method of the present invention, the cross-sectional width of the steel plate substrate to be coated with zinc-aluminum-magnesium oxide is 900–1450 mm, for example, 900 mm, 1100 mm, 1250 mm, 1300 mm, or 1450 mm; the cross-sectional thickness of the steel plate substrate to be coated with zinc-aluminum-magnesium oxide is 3.0–6.0 mm, for example, 3.0 mm, 4.0 mm, 5.0 mm, or 6.0 mm. In a more preferred embodiment, the cross-sectional width of the steel plate substrate to be coated with zinc-aluminum-magnesium oxide is 900–1450 mm, and the cross-sectional thickness of the steel plate substrate to be coated with zinc-aluminum-magnesium oxide is 4.0–5.0 mm.

[0038] In a specific embodiment of the method described in this invention, in order to reduce the core heat carried inside the steel plate and prevent the plating solution from solidifying and then remelting, thereby generating zinc flow defects, the temperature of the plating solution is controlled at 410–420°C in step S1, for example, 410°C, 415°C, 418°C, or 420°C. In a preferred embodiment, the plating solution temperature is 415–418°C.

[0039] In a specific embodiment of the method described in this invention, by controlling the content and proportion of different components in the plating solution, it is possible to ensure that the steel plate achieves excellent corrosion resistance while increasing the viscosity of the plating solution and reducing its fluidity. This reduces the generation of slag and zinc flow defects, thereby improving the surface quality of the coating. Specifically, in step S1, the Al content in the plating solution is 1.2–3 wt%, for example, it can be 1.2 wt%, 1.4 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, 2.2 wt%, 2.7 wt%, or 3 wt%; the Mg content is… The content is 1.0 to 1.5 wt%, for example, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, or 1.5 wt%; the Sb content is 0.07 to 0.075 wt%, for example, 0.07 wt%, 0.072 wt%, 0.074 wt%, or 0.075 wt%; the remainder is Zn; and the ratio of Al content to Mg content is 1.2 to 2:1, for example, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, or 2:1.

[0040] In a preferred embodiment, the plating solution contains 1.4–2.2 wt% Al, 1.1–1.4 wt% Mg, 0.075 wt% Sb, with the remainder being Zn; and the ratio of Al content to Mg content is 1.3–1.6:1.

[0041] In a specific embodiment of the method described in this invention, tension control and running speed can effectively prevent the steel plate from shaking during operation, and improve the uniformity and stability of the coating. In step S1, the tension of the strip steel in the zinc pot section is controlled to be about 30 to 50 kN, specifically, for example, 30 kN, 35 kN, 38 kN, 40 kN, 42 kN, 43 kN, 45 kN, 48 kN or 50 kN; the running speed of the steel plate substrate to be coated with zinc-aluminum-magnesium coating is 40 to 60 m / min, specifically, for example, 40 m / min, 45 m / min, 52 m / min or 60 m / min.

[0042] Since the formation of zinc flow marks is closely related to the air knife pressure and the distance between the air knife and the steel plate, using a close-range, low-pressure method in the strip steel production process, along with a certain air knife height and angle, can effectively avoid the generation of zinc flow mark defects in thick-coated hot-dip galvanized products. In a specific embodiment of the method described in this invention, in step S2, the conditions for the air knife blowing treatment include: the blowing medium is nitrogen gas; the temperature of the blowing medium is -10 to 0°C, specifically, for example, -10°C, -8°C, -5°C, or 0°C; the blowing angle is -0.8 to -0.9°, specifically, for example, -0.8° or -0.9°; the height of the air knife lip from the plating bath surface is 180 to 220 mm, specifically, for example, 180 mm, 1... The air knife nozzle angle is 90mm, 200mm, or 220mm; the vertical distance between the air knife lip and the strip surface is 10-20mm, specifically, for example, 10mm, 11mm, 12mm, 13mm, 14mm, 15mm, 16mm, 17mm, 18mm, 19mm, or 20mm; the air knife pressure is 100-120mbar, specifically, for example, 100mbar, 110mbar, 115mbar, or 120mbar. In this article, the blowing angle refers to the air knife nozzle angle.

[0043] In a preferred embodiment, in step S2, the conditions for the air knife blowing treatment include: the blowing medium is nitrogen, the temperature of the blowing medium is -5 to -8°C, the blowing angle is -0.8 to -0.9°, the height of the air knife lip from the surface of the plating solution is 190 to 200 mm, the vertical distance of the air knife lip from the surface of the strip is 13 to 17 mm, and the air knife pressure is 110 to 115 mbar.

[0044] In this invention, the temperature of the nitrogen gas being injected is achieved by cooling it using a self-developed nitrogen mixing system, as described in the invention. Figure 1 As shown, the system includes a liquid nitrogen tank 1, a flow control valve 2, a temperature detector 3, a first delivery pipe 4, a second delivery pipe 5, a third delivery pipe 6, a blower 7, and a mixing tank 8. The input end of the first delivery pipe 4 is connected to the liquid nitrogen outlet of the liquid nitrogen tank 1, and the output end of the first delivery pipe 4 is connected to the mixing tank 8. The flow control valve 2 and the temperature detector 3 are sequentially arranged on the first delivery pipe 4 along the liquid nitrogen flow direction. The second delivery pipe 5 delivers room temperature nitrogen gas to the mixing tank 8. The blower 7 is installed inside the mixing tank 8 and is used to mix the liquid nitrogen and room temperature nitrogen gas input into the mixing tank 8 to achieve the temperature and pressure required for the spraying medium. The mixing tank 8 is connected to the third delivery pipe and is used to provide a gas source for the air knife with the spraying medium after mixing liquid nitrogen and room temperature nitrogen gas. The temperature detector 3 and the flow control valve 2 are sequentially arranged on the third delivery pipe along the spraying medium flow direction.

[0045] Specifically, the first conveying pipe 4 is a copper alloy conveying pipe, the second conveying pipe is a room temperature nitrogen conveying pipe, the third conveying pipe is a mixed gas conveying pipe, and the fan 7 is a stirring fan.

[0046] In the specific operation process, liquid nitrogen in liquid nitrogen tank 1 is extracted into the interior of mixing tank 8 through the first delivery pipe 4. At the same time, room temperature nitrogen gas is introduced into the tank and fully mixed by the blower 7. Finally, the gas source is provided to the air knife through the third delivery pipe 6. The pressure and temperature of the gas source flow rate can be controlled and adjusted in real time by the flow control valve 2 and temperature detector 3 in the mixer system.

[0047] By controlling the cooling rates of mobile and stationary cooling fans and adopting a gradual cooling mode with cooling rates increasing from small to large, the agitation effect of airflow on the incompletely solidified coating can be effectively reduced, avoiding the occurrence of zinc flow marks, while obtaining a coating with a dense phase structure and good corrosion resistance.

[0048] In a specific embodiment of the method described in this invention, in step S3, the post-plating cooling section includes three fixed air-cooling devices and one movable air-cooling device. The steel plate with the molten plating layer sequentially passes through the movable air-cooling device, the first fixed air-cooling device, the second fixed air-cooling device, and the third fixed air-cooling device. The cooling rate of the movable air-cooling device is 10-15℃ / s, specifically, for example, 10℃ / s, 12℃ / s, 13℃ / s, or 15℃ / s. The cooling rate of the first fixed air-cooling device is 15-18℃ / s, specifically, for example, 10℃ / s, 12℃ / s, 13℃ / s, or 15℃ / s. The cooling rate of the first fixed air-cooling device is 15℃ / s, 16℃ / s, 17℃ / s, or 18℃ / s; the cooling rate of the second fixed air-cooling device is 18-20℃ / s, specifically, for example, 18℃ / s, 19℃ / s, or 20℃ / s; the cooling rate of the third fixed air-cooling device is >25℃ / s, specifically, for example, 25℃ / s, 26℃ / s, 27℃ / s, or 28℃ / s; and the cooling rate of the movable air-cooling device is < the cooling rate of the first fixed air-cooling device < the cooling rate of the second fixed air-cooling device < the cooling rate of the third fixed air-cooling device.

[0049] In a preferred embodiment of the method of the present invention, in step S3, the cooling rate of the movable air-cooling device is 12-13°C / s, the cooling rate of the first fixed air-cooling device is 16-17°C / s, the cooling rate of the second fixed air-cooling device is 19°C / s, and the cooling rate of the third fixed air-cooling device is 26-27°C / s.

[0050] The temperature of the strip after exiting the mobile rapid cooling section is controlled to be below 335°C. This is mainly because, according to the phase diagram calculation, a eutectic reaction will occur in the coating structure before complete solidification under this plating solution composition system. The process of transforming from a liquid phase to a ternary eutectic phase is completed at a temperature of 335.8°C. In a specific embodiment, the mobile air-cooling device is installed at a height of 5–10 m above the plating solution surface. The strip temperature after exiting the mobile air-cooling device is 315–335°C, specifically, for example, 315°C, 325°C, 330°C, or 335°C; the strip temperature upon reaching the top roller is <205°C, specifically, for example, 195°C, 198°C, 202°C, or 205°C.

[0051] A second aspect of the present invention provides a hot-dip zinc-aluminum-magnesium product prepared by the method described above.

[0052] In a specific embodiment of the hot-dip galvanized aluminum-magnesium product of the present invention, the coating mass of the hot-dip galvanized aluminum-magnesium product is 275-350 g / m. 2 Specifically, for example, it can be 275g / m 2 300g / m 2 Or 350g / m 2 The coating thickness is 3.0 to 6.0 mm, specifically, for example, it can be 3.0 mm, 4.0 mm, 5.0 mm or 6.0 mm.

[0053] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are obtained from conventional commercial sources or prepared by conventional methods.

[0054] The preparation process of the steel plate substrate to be coated with zinc, aluminum and magnesium is as follows: the substrate is obtained by degreasing, rinsing, drying and continuous annealing.

[0055] A method for controlling zinc flow marks in hot-dip galvanized aluminum-magnesium products, the method comprising the following steps:

[0056] S1. The steel plate substrate to be coated with zinc-aluminum-magnesium plating is immersed in a zinc pot and hot-dip coated in a plating solution containing zinc, aluminum and magnesium elements to obtain the hot-dip coated steel plate.

[0057] S2. The hot-dip galvanized steel sheet is subjected to air knife blowing treatment to obtain a steel sheet with a molten coating.

[0058] S3. The steel plate with the molten coating is cooled in the post-coating cooling section to obtain a hot-dip zinc-aluminum-magnesium coated product. The post-coating cooling section includes three fixed air-cooling devices and one movable air-cooling device. The steel plate with the molten coating passes through the first fixed air-cooling device, the second fixed air-cooling device and the third fixed air-cooling device in sequence.

[0059] Examples 1-14 use the above method to control zinc flow marks in hot-dip galvanized aluminum-magnesium products. The hot-dip galvanizing process parameters are shown in Table 1.

[0060] Table 1 Hot-dip galvanizing process parameters

[0061]

[0062]

[0063] Examples 1-14 used the above method to control zinc flow marks in hot-dip galvanized aluminum-magnesium products. The key parameters for the air knife scraping treatment are shown in Table 2.

[0064] Table 2 Key parameters for air knife scraping treatment

[0065]

[0066]

[0067] Examples 1-14 used the above method to control zinc flow marks in hot-dip galvanized aluminum-magnesium products. The key parameters for the cooling process are shown in Table 3.

[0068] Table 3 Key parameters for cooling treatment

[0069]

[0070] Comparative Example 1: The process was carried out in accordance with Example 1, except that the immersion temperature in the hot-dip galvanizing process parameters was adjusted. In this comparative example, the immersion temperature was 405°C.

[0071] Comparative Example 2: Implemented in accordance with Example 1, except that the cooling rate of the portable air-cooling device during the cooling process is 15°C / s.

[0072] The surface quality grade and effect of the hot-dip galvanized aluminum-magnesium products prepared in Examples 1-14 and Comparative Examples 1-2 were measured. The results are shown in Table 4. In this table, FA refers to ordinary finished surface, FB refers to higher-grade finished surface, and FC refers to high-grade finished surface. The defect ratio was determined by dynamically detecting the defect type and length of the hot-dip galvanized aluminum-magnesium products using a surface inspection instrument. The defect ratio is calculated as: defect steel plate length / total steel plate length.

[0073] Table 4

[0074] Surface quality grade Defect ratio Example 1 FB 8％ Example 2 FB 5％ Example 3 FB 7％ Example 4 FB 7％ Example 5 FC 3％ Example 6 FB 5％ Example 7 FC 3％ Example 8 FB 6％ Example 9 FC 2％ Example 10 FB 7％ Example 11 FB 6％ Example 12 FB 7％ Example 13 FB 10％ Example 14 FB 15％ Comparative Example 1 FA 32％ Comparative Example 2 FA 24％

[0075] As shown in Table 4, by optimizing key manufacturing processes such as hot-dip plating, air knife parameters, and post-plating cooling, and by adopting a self-developed nitrogen mixer system to improve the cooling capacity of the spraying medium, the problem of thick coatings (275–350 g / m³) can be effectively avoided. 2 The occurrence of zinc flow marks in hot-dip galvanized aluminum-magnesium products with specifications (3.0mm~6.0mm) was reduced. Meanwhile, Examples 5, 7 and 9 improved the surface quality of the products by controlling the temperature of the plating solution and the content and proportion of different components in the plating solution, and by using low pressure air knife in conjunction with the height and angle of the air knife within a certain range, thus meeting the FC surface grade requirements in GB2518 and reducing the surface defect rate to less than 3%.

[0076] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for controlling zinc flow marks defects in hot-dip galvanized aluminum-magnesium products, characterized in that, The method includes the following steps: S1. The steel plate substrate to be coated with zinc-aluminum-magnesium plating is immersed in a zinc pot and hot-dip coated in a plating solution containing zinc, aluminum and magnesium elements to obtain the hot-dip coated steel plate. S2. The hot-dip galvanized steel sheet is subjected to air knife blowing treatment to obtain a steel sheet with a molten coating. S3. The steel plate with the molten coating is cooled in the post-coating cooling section to obtain a hot-dip zinc-aluminum-magnesium product. The post-coating cooling section includes a fixed air-cooling device and a movable air-cooling device. The steel plate with the molten coating passes through the movable air-cooling device and the fixed air-cooling device in sequence, and the cooling rate of the movable air-cooling device is less than that of the fixed air-cooling device. In step S1, the temperature at which the steel substrate to be coated with zinc-aluminum-magnesium alloy is immersed in the zinc bath is shown in the following formula: Where t1 is the temperature at which the steel plate substrate to be coated with zinc aluminum magnesium is immersed in the zinc pot, T is the temperature of the plating solution, w is the cross-sectional width of the steel plate substrate to be coated with zinc aluminum magnesium, and h is the cross-sectional thickness of the steel plate substrate to be coated with zinc aluminum magnesium.

2. The method according to claim 1, characterized in that, In step S1, the temperature of the plating solution is 410-420℃; in the plating solution, the Al content is 1.2-3wt%, the Mg content is 1.0-1.5wt%, the Sb content is 0.07-0.075wt%, and the remainder is Zn, and the ratio of Al content to Mg content is 1.2-2:

1.

3. The method according to claim 2, characterized in that, The cross-sectional width of the steel plate substrate to be coated with zinc-aluminum-magnesium coating is 900-1450 mm, and the cross-sectional thickness of the steel plate substrate to be coated with zinc-aluminum-magnesium coating is 3.0-6.0 mm.

4. The method according to claim 1, characterized in that, In step S2, the conditions for the air knife blowing treatment include: the blowing medium is nitrogen, the temperature of the blowing medium is -10 to 0°C, the blowing angle is -0.8 to -0.9°, the height of the air knife lip from the liquid surface is 180 to 220 mm, the distance of the air knife lip from the strip is 10 to 20 mm, and the air knife pressure is 100 to 120 mbar.

5. The method according to claim 1, characterized in that, In step S3, the post-plating cooling section includes three fixed air-cooling devices and one movable air-cooling device. The steel plate with the molten coating passes through the movable air-cooling device, the first fixed air-cooling device, the second fixed air-cooling device, and the third fixed air-cooling device in sequence. The cooling rate of the movable air-cooling device is 10-15℃ / s, the cooling rate of the first fixed air-cooling device is 15-18℃ / s, the cooling rate of the second fixed air-cooling device is 18-20℃ / s, and the cooling rate of the third fixed air-cooling device is >25℃ / s. The cooling rate of the movable air-cooling device is less than the cooling rate of the first fixed air-cooling device, which is less than the cooling rate of the second fixed air-cooling device, which is less than the cooling rate of the third fixed air-cooling device.

6. The method according to claim 5, characterized in that, The movable air-cooling device is installed at a height of 5 to 10 meters above the plating solution. The temperature of the strip steel after exiting the movable air-cooling device is 315 to 335°C, and the temperature of the strip steel when it reaches the top roller of the tower is <205°C.

7. The method according to claim 1, characterized in that, In step S1, the tension of the strip steel in the zinc pot section is controlled at 30-50KN, and the running speed of the steel plate substrate to be coated with zinc-aluminum-magnesium coating is 40-60m / min.

8. The method according to claim 1, characterized in that, In step S2, the blowing medium for the hot-dip galvanized steel plate to be blown by air knife is nitrogen. The nitrogen is provided by a nitrogen mixing system, which includes a liquid nitrogen tank (1), a flow control valve (2), a temperature detector (3), a first delivery pipe (4), a second delivery pipe (5), a third delivery pipe (6), a blower (7), and a mixing tank (8). The input end of the first delivery pipe (4) is connected to the liquid nitrogen outlet of the liquid nitrogen tank (1), and the output end of the first delivery pipe (4) is connected to the mixing tank (8). A flow control valve (2) and a temperature detector (3) are sequentially arranged on the first delivery pipe (4) along the liquid nitrogen flow direction. The second delivery pipe (5) delivers room temperature nitrogen to the mixing tank (8). The blower (7) is installed inside the mixing tank (8) to mix the liquid nitrogen and room temperature nitrogen gas input into the mixing tank (8) to achieve the temperature and pressure required for the spraying medium; the mixing tank (8) is connected to the third delivery pipe to provide the gas source for the air knife after the spraying medium after the liquid nitrogen and room temperature nitrogen gas are mixed; a temperature detector (3) and a flow control valve (2) are sequentially arranged on the third delivery pipe along the flow direction of the spraying medium.

9. A hot-dip zinc-aluminum magnesium product prepared by the method according to any one of claims 1-8.

10. The hot-dip galvanized aluminum-magnesium product according to claim 9, characterized in that, The coating weight of the hot-dip zinc-aluminum-magnesium product is 275-350 g / m². 2 The coating thickness is 3.0 to 6.0 mm.

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