A hot-dip galvanized product and a method for reducing the occurrence of zinc flow lines in a hot-dip galvanized product

Abstract

This invention discloses a hot-dip galvanized product and a method for reducing the zinc flow rate of the hot-dip galvanized product, belonging to the field of metallurgical technology. The method of this invention is applicable to coatings with a thickness ≥1.8mm and a coating strength ≥275g / m². 2 Hot-dip galvanized products, such as steel strips, undergo galvanizing in a zinc pot after passing through a cooling section, a hot tension roller section, and a grate section. Before entering the zinc pot, an induction heating device, such as a microwave, is added to raise the surface temperature of the hot-dip galvanized product to 455–465°C. This induction heating device utilizes the skin effect to generate a high-density induced current on the surface of the product, raising the surface temperature of the substrate, such as the steel strip, to 455–465°C through the current heating effect. Furthermore, by optimizing key manufacturing process parameters such as the temperature of the steel plate in the hot tension roller section, the composition of the zinc bath, and the post-galvanizing cooling rate, the incidence of surface zinc flow marks on thick-gauge hot-dip galvanized products is effectively reduced.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, and more specifically, relates to a hot-dip galvanized product and a method for reducing the zinc flow rate of the hot-dip galvanized product. Background Technology

[0002] When producing hot-dip galvanized products with a thickness of ≥1.8mm, continuous hot-dip galvanizing lines are highly susceptible to zinc flow defects on the surface. Slight zinc flow defects typically appear at the edges of the strip, while severe defects can cover the entire width of the steel sheet, significantly impacting the surface quality. The affected area is thicker than the normal coating thickness, resulting in an uneven zinc layer on the strip surface. The raised areas have a slight tactile feel when touched. Furthermore, this defect exhibits a certain degree of color difference compared to a normal galvanized surface, and it is not easily eliminated even after finishing and post-treatment. This is currently the most common and difficult problem to solve in continuous hot-dip galvanizing lines.

[0003] In existing hot-dip galvanizing processes, to save energy, most galvanized products are typically produced using a substrate entry temperature slightly higher than the zinc bath temperature. However, for thicker galvanized products, due to the significant amount of heat carried by the substrate, the coating temperature cannot be reduced below the zinc bath's solidification point (419°C) after the steel plate is blown by the air knife. The incompletely solidified coating flows downwards under gravity, resulting in surface quality defects such as zinc flow marks. Some companies have attempted to reduce the core heat carried by adjusting the substrate entry temperature, hoping to decrease the incidence of zinc flow marks in thicker products. However, based on past experience, if the substrate entry temperature is too low, the Fe-Al reaction on the substrate surface is insufficient, which is extremely detrimental to the integrity of the inhibition layer; if it is too high, the zinc bath temperature rises, leading to increased zinc dross. Therefore, most companies currently use a temperature close to or slightly lower than the zinc bath temperature. However, this method still cannot solve the technical limitation of both significantly reducing the core heat of the steel plate and ensuring the integrity of the coating inhibition layer. Existing patented technologies typically employ methods such as improving the cooling capacity of the air knife's jet stream and adjusting air knife parameters to address the issue, but the actual effects are not significant, and the incidence of zinc flow marks remains high.

[0004] A search revealed Chinese patent application publication number CN103966537B, which discloses a method for controlling zinc flow marks in thick-material, thick-coated hot-dip galvanized products. This method utilizes an air conditioner installed in the fan room to control the temperature of the blowing medium within the range of 0–10°C, thereby improving the cooling capacity of the blowing medium on the strip steel. Simultaneously, it optimizes the distance between the air knife lip and the liquid surface to 600–1000 mm, the distance between the air knife lip and the strip steel to 15–30 mm, and the air knife pressure to the range of 100–300 mbar, aiming to control zinc flow mark defects. However, because this method only significantly cools the surface temperature of the coating, it cannot effectively reduce the core heat carried within the steel plate, especially when the thickness exceeds 2.5 mm. This can easily cause the coating temperature to rise again above the solidification point, forming zinc flow marks. Therefore, this invention cannot significantly reduce the incidence of zinc flow marks in thick-material hot-dip galvanized products.

[0005] Chinese patent application publication number CN112899600A discloses a galvanizing method for high-strength hot-dip galvanized steel strip with an ultra-thick zinc layer. This method optimizes the hot-dip galvanizing furnace control unit, zinc pot control unit, zinc layer control unit, and cooling control unit, controlling the temperature of the steel substrate entering the zinc pot at 425–440°C and the temperature of the molten zinc at 445–455°C. While this temperature control reduces some of the core heat, it is still insufficient to quickly lower the coating temperature below the solidification point, causing the zinc flow lines to appear as the coating flows downwards under gravity.

[0006] Chinese patent application publication number CN112899600A discloses a method to avoid zinc flow marks on the surface of galvanized steel strip. This method involves adjusting the temperature of the molten zinc in the zinc bath before the strip enters the bath, and adjusting the strip's entry temperature according to its thickness; thereby preventing zinc flow marks on the galvanized steel strip surface. However, this method controls the entry temperature of the thick steel plate substrate into the zinc bath at 435–455°C, and the molten zinc temperature at 450–460°C. This temperature control results in the substrate retaining a significant amount of heat, and the cooling device is insufficient to quickly lower the coating temperature below the solidification point, easily leading to zinc flow mark defects. Summary of the Invention

[0007] 1. The problem to be solved

[0008] To address the surface quality issues such as zinc flow marks that easily occur when producing thick-gauge hot-dip galvanized products using existing technologies, this invention provides a hot-dip galvanized product and a method for reducing the occurrence rate of zinc flow marks in hot-dip galvanized products. By controlling and optimizing key manufacturing process parameters such as the temperature of the hot tension roller section steel plate, the composition of the zinc bath, and the post-galvanizing cooling rate of the thick-gauge hot-dip galvanized product, and by adding a high-frequency induction heating device before entering the pot, the surface temperature range of the substrate is controlled, effectively reducing the occurrence rate of surface zinc flow mark defects in thick-gauge (≥1.8mm) hot-dip galvanized products.

[0009] 2. Technical Solution

[0010] To solve the above problems, the present invention adopts the following technical solution:

[0011] A method for reducing the occurrence rate of zinc flow marks in hot-dip galvanized products, applicable to coating thickness ≥1.8mm and coating strength ≥275g / m². 2 Hot-dip galvanized products, such as steel strips, undergo galvanizing in a zinc pot after passing through a cooling section, a hot tension roller section, and a grate section. Before entering the zinc pot, an induction heating device, such as a microwave, is installed to raise the surface temperature of the hot-dip galvanized product to 455–465°C. The induction heating device utilizes the skin effect to generate a high-density induced current on the surface of the product. By adjusting the power of the heating device and relying on the current heating effect, the surface temperature of the substrate, such as steel strip, is raised to 455-465°C before entering the pot. This temperature control ensures the integrity and continuity of the inhibition layer during the galvanizing process.

[0012] A further technical solution involves controlling the temperature of the hot-dip galvanized product in the hot tension roller section at 350–380°C, thereby controlling the product substrate temperature in the hot tension roller area within the range of 350–380°C, laying the foundation for the skin effect that can be utilized by the induction heating device.

[0013] A further technical solution involves the following chemical composition by weight percentage in the zinc plating solution: Al: 0.30–0.45%, Sb: 0.04–0.06%, Co: 0.07–0.09%, with the balance being Zn and unavoidable impurities. By adjusting this composition, the fluidity of the zinc solution can be reduced to a certain extent, decreasing the activity of the zinc solution flowing downwards under gravity, thereby effectively reducing the occurrence rate of zinc flow marks.

[0014] A further technical solution involves a three-stage progressive cooling process after hot-dip galvanizing to avoid excessively fast or slow cooling, which could affect the reduction of zinc flow marks.

[0015] A further technical solution involves a three-stage progressive cooling method: the upper section of the cooling tower includes three sets of fixed cooling fans. The cooling rate of the first cooling fan is 5-8℃ / s; the cooling rate of the second cooling fan is 7-9℃ / s; and the cooling rate of the third cooling fan is approximately 11-13℃ / s. The temperature of the hot-dip galvanized product is controlled at 200-210℃ when it reaches the top roller of the cooling tower. This progressive three-stage cooling method, with lower temperatures at the bottom and higher temperatures at the top, optimizes the post-galvanizing cooling process and achieves the goal of controlling zinc flow marks on thick-gauge hot-dip galvanized products.

[0016] A hot-dip galvanized product is prepared by the above method of reducing the zinc flow rate of hot-dip galvanized products.

[0017] 3. Beneficial effects

[0018] The hot-dip galvanized product of the present invention and the method for reducing the occurrence rate of zinc flow marks in hot-dip galvanized products are achieved by controlling and optimizing key manufacturing process parameters such as the temperature of the steel plate of the hot tension roller section, the composition of the zinc liquid, and the cooling rate after galvanizing of the thick-gauge hot-dip galvanized product, and by adding a high-frequency induction heating device before entering the pot to control the surface temperature range of the substrate. This method can effectively reduce the occurrence rate of defects such as zinc flow marks in existing thick-gauge hot-dip galvanized products, reducing the zinc flow mark occurrence rate from the previous 83% to below 5%. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the process flow for a method to reduce the zinc flow mark occurrence rate of hot-dip galvanized products according to a specific embodiment.

[0020] Figure 2 For specific comparison, macroscopic images of the surface of hot-dip galvanized products that did not employ methods to reduce the incidence of zinc flow marks in hot-dip galvanized products are provided.

[0021] Figure 3 The image shows a macroscopic view of the surface of a hot-dip galvanized product, illustrating a specific embodiment of a method for reducing the occurrence of zinc flow marks in hot-dip galvanized products.

[0022] In the diagram: 1. Zinc pot; 2. Induction heating device; 3. Grate section; 4. Hot tension roller section; 5. Cooling section; 6. Strip steel. Detailed Implementation

[0023] The present invention will now be further described with reference to specific embodiments and accompanying drawings.

[0024] Basic Implementation

[0025] The method for reducing the zinc flow rate of hot-dip galvanized products in this embodiment is applicable to coatings with a thickness ≥1.8mm and a coating strength ≥275g / m². 2 Hot-dip galvanized products, such as Figure 1As shown, for example, strip steel 6 passes through cooling section 5, hot tension roller section 4, and grate section 3 in sequence before undergoing galvanizing in zinc pot 1. The temperature of strip steel 6 in hot tension roller section 4 is controlled at 350–380℃. After passing through grate section 3 and before entering zinc pot 1 for galvanizing, an induction heating device 2, such as a microwave, is added to raise the surface temperature of strip steel 6 to 455–465℃. The chemical composition (weight percentage) of the galvanizing solution in zinc pot 1 is: Al: 0.30–0.45%, Sb: 0.04–0.06%, Co: 0.07–0.09%, with the balance being Zn and unavoidable impurities. This composition adjustment can reduce the fluidity of the galvanizing solution to a certain extent, decreasing its downward flow under gravity, thereby effectively reducing the incidence of zinc flow marks. After galvanizing, hot-dip galvanized products undergo a three-stage progressive cooling process to avoid excessively fast or slow cooling, which could negatively impact the reduction of zinc flow marks. The three-stage progressive cooling process consists of three sets of fixed cooling fans in the upper section of the cooling tower. The cooling rate of the first fan is 5–8℃ / s; the second fan's cooling rate is 7–9℃ / s; and the third fan's cooling rate is approximately 11–13℃ / s. The temperature of the hot-dip galvanized product is controlled at 200–210℃ at the top roller of the cooling tower. This progressive three-stage cooling method, with lower temperatures at the bottom and higher temperatures at the top, optimizes the post-galvanizing cooling process, thereby controlling zinc flow marks in thick-gauge hot-dip galvanized products and ultimately producing hot-dip galvanized products with virtually no zinc flow marks.

[0026] In this embodiment, the hot-dip galvanized product and the method for reducing the zinc flow rate of the hot-dip galvanized product are described. The induction heating device 2 can utilize the skin effect to generate a high-density induced current on the surface layer of the product to be hot-dip galvanized. By adjusting the power of the heating device, the surface temperature of the product substrate, such as strip steel 6, is raised to 455-465°C before entering the pot by relying on the current heating effect. This temperature control combined with the skin effect can ensure that the inhibition layer in the galvanizing process of the steel plate is complete and continuous.

[0027] The hot-dip galvanized product and the method for reducing the zinc flow rate of hot-dip galvanized products in this embodiment control and optimize key manufacturing process parameters such as the temperature of the hot tensioning roller section 4 of the strip steel 6, the composition of the zinc liquid, and the cooling rate after galvanizing for thick-gauge hot-dip galvanized products, and add a high-frequency induction heating device 2 before entering the pot to control the surface temperature range of the substrate, which can effectively reduce the existing thick-gauge hot-dip galvanized products.

[0028] Specific embodiments 1 to 7 and comparative example 1

[0029] The basic methods of Examples 1 to 7 below are the same as those of the basic examples above. The specific process parameters of Examples 1 to 7 and Comparative Example 1 are shown in Table 1:

[0030] Table 1: Thickness ≥ 1.8 mm, coating ≥ 275 g / m 2 Strip galvanizing process parameters:

[0031]

[0032] The hot-dip galvanized steel strip products obtained in Examples 1 to 7 and Comparative Example 1 are respectively as follows: Figure 3 and 2 As shown, the hot-dip galvanized steel strip products prepared in Examples 1 to 7 have no obvious zinc flow defects, while the hot-dip galvanized steel strip products prepared in the comparative examples have obvious and irregular zinc flow defects.

[0033] Based on statistics from the batch production of galvanized steel strip on the applicant's own production line, the hot-dip galvanized steel strip products produced by the methods described in the above embodiments have a thickness ≥1.8mm and a coating thickness ≥275g / m². 2 The zinc flow rate was reduced from 83% to below 5%, achieving an unexpected technical effect.

[0034] The examples described herein are merely preferred embodiments of the invention and are not intended to limit the concept and scope of the invention. Any modifications and improvements made by those skilled in the art to the technical solutions of the invention without departing from the design concept of the invention should fall within the protection scope of the invention.

Claims

1. A method for reducing the occurrence rate of zinc flow marks in hot-dip galvanized products, wherein the hot-dip galvanized products pass sequentially through a cooling section, a hot tension roller section, and a grate section before undergoing galvanizing in a zinc pot, characterized in that: After passing through the grate section and before entering the zinc pot for galvanizing, hot-dip galvanized products are equipped with an induction heating device to raise the surface temperature of the hot-dip galvanized products to 455~465℃. The temperature of hot-dip galvanized products is controlled at 350~380℃ in the hot tension roller section; The chemical composition of the zinc plating solution in the zinc pot, by weight percentage, is: Al: 0.30~0.45%, Sb: 0.04~0.06%, Co: 0.07~0.09%, with the balance being Zn and unavoidable impurities; After hot-dip galvanizing, the product undergoes a three-stage progressive cooling process. The three-stage progressive cooling method is as follows: the upper section of the cooling tower includes three sets of fixed cooling fans. The cooling rate of the first cooling fan is 5~8℃ / s; the cooling rate of the second cooling fan is 7~9℃ / s; and the cooling rate of the third cooling fan is 11~13℃ / s. The temperature of the hot-dip galvanized product is controlled at 200~210℃ when it reaches the top roller of the cooling tower. Suitable for thickness ≥1.8mm and coating ≥275g / m² 2 Hot-dip galvanized products.

2. A hot-dip galvanized product, characterized in that: It is prepared by the method for reducing the zinc flow rate of hot-dip galvanized products as described in claim 1.

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