A method for controlling surface plating defects of a thick plating hot-formed aluminum silicon steel plate

Abstract

The present application relates to the technical field of cold rolling, in particular to a method for controlling surface plating defects of thick plating layer hot forming aluminum silicon steel plate. A cold hard base plate with specific components is provided, and hot aluminum silicon plating treatment is performed. According to the thickness of the steel strip, the aluminum pot temperature, the aluminum liquid temperature and the hot dipping time are finely matched. Specifically, when the thickness is less than or equal to 1.50 mm, the temperature is controlled at 650-660 DEG C, and the hot dipping time is less than or equal to 4.5 min; when the thickness is greater than 1.50 mm, the pot temperature is 660-670 DEG C, the aluminum liquid temperature is 665-680 DEG C, and the hot dipping time is 5-5.5 min. The method can further include the steps of cleaning the base plate, continuous annealing and air knife blowing after plating. Through the synergistic control of key parameters in the whole process, the surface plating defects of the thick plating layer aluminum silicon steel plate are effectively eliminated without adding equipment, and stable production of high-quality products is realized.

Description

Technical Field

[0001] This invention relates to the field of cold rolling technology, specifically to a method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets. Background Technology

[0002] Hot-dip aluminized silicon steel sheets (typically referring to products with an Al-10wt%Si coating) are widely used in automotive lightweighting, safety components (such as A / B pillars and crash beams), and high-end home appliance manufacturing due to the excellent heat resistance, corrosion resistance, heat reflectivity, and good post-stamping performance of their coating. In particular, with the industry's increasing demands for material strength and lightweighting, the demand for hot-formed aluminized silicon steel sheets that combine ultra-high strength with long-term corrosion resistance is growing rapidly.

[0003] In the production of hot-dip aluminized silicon steel sheets, to achieve a longer corrosion resistance life, it is often necessary to increase the coating thickness, i.e., to produce so-called "thick coating" products (e.g., double-sided coating weight ≥ 150g / m²). 2 However, compared to conventional thin-coated products, the production of thick-coated aluminum-silicon steel sheets faces significantly greater technical challenges. The aluminum-silicon alloy melt itself has high viscosity and unique surface tension characteristics. Under thick-coating conditions, the flowability, uniformity, and solidification behavior of the plating solution on the steel strip surface become more complex. Surface plating defects are highly likely to occur during production, meaning that the coating is discontinuous or completely missing in localized areas, exposing the steel substrate. Such defects not only severely affect the product's appearance but also significantly reduce the corrosion resistance of that area, becoming a weak point that leads to premature failure of the entire component, resulting in product downgrading or even scrapping, and failing to meet the stringent requirements of the automotive, home appliance, and other industries for high surface quality and high reliability.

[0004] Currently, there is some research and practice in the industry regarding improving the surface quality of coated products. For example, some technical solutions improve the problem of incomplete plating in galvanized steel by optimizing substrate roughness, adjusting the dew point of the annealing atmosphere, or performing pre-nickel plating before galvanizing. However, these methods mainly target zinc or zinc-based alloy coatings, whose physicochemical properties are fundamentally different from those of aluminum-silicon alloy coatings. Aluminum-silicon coatings are more sensitive to thermal regimes (especially temperature and cooling rate), and simple parameter transfer is unlikely to be effective. Other technical solutions focus on the post-defect detection and root cause analysis of incomplete plating defects. While these methods help to understand the causes of defects, they do not provide a complete process solution for proactively preventing and systematically controlling incomplete plating defects in thick-coated aluminum-silicon products on continuous production lines. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention provides a method for controlling surface plating defects of thick-coated hot-formed aluminum-silicon steel plates. This method can stably and effectively suppress or eliminate surface plating defects by systematically optimizing key process parameters throughout the entire process without significantly increasing equipment investment.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets includes the following steps: S1: Provide a cold-hardened substrate that meets the following composition requirements; The chemical composition of the cold-hardened substrate, by weight percentage, is as follows: C: 0.20%~0.23%, Si: 0.228%~0.27%, Mn: 1.23%~1.33%, P≤0.02%, S≤0.005%, Al: 0.02%~0.05%, Ti≤0.05%, B≤0.003%, Cr: 0.11%~0.17%; Mo: 0.001%~0.03%, Cu≤0.01%, Ni: 0.004%~0.04%, Nb≤0.003%, with the remainder being Fe and unavoidable impurities; S2: Clean the chilled substrate; S3: Continuous annealing of the cleaned chilled substrate; S4: The cold hard substrate is hot-dip aluminum-silicon coated to form an aluminum-silicon coating. The chemical composition of the aluminum-silicon coating, by weight percentage, is: Al: 85%~90%, Si: 7%~11%, Fe: 2.0%~4.0%, with the remainder being unavoidable impurities; The following relationships must be satisfied when controlling the temperature of the strip entering the aluminum pot, the temperature of the molten aluminum, and the hot-dip galvanizing time: When the strip thickness is ≤1.50mm, the temperature at which the strip enters the aluminum pot is ≤660℃, the temperature at which the aluminum liquid is ≤665℃, and the hot-dip galvanizing time is ≤4.5min. When the strip thickness is greater than 1.50 mm, the following conditions apply: 660℃ < temperature in aluminum pot ≤ 670℃, 665℃ ≤ aluminum liquid temperature ≤ 680℃, and 5 min ≤ hot-dip galvanizing time ≤ 5.5 min. The double-sided coating weight of the aluminum-silicon coating is 150~160g / m². 2 ; S5: Use an air knife to blow clean the surface of the galvanized steel plate.

[0007] Furthermore, the microstructure of the cold-hardened substrate after thermoforming and quenching is mainly martensite, with a volume fraction of martensite of not less than 90%.

[0008] Furthermore, in step S1, the surface quality of the cold-hardened substrate meets the following requirements: no visible color difference, width or narrow printing, roller printing, rust, or residual emulsion defects; the shape of the cold-hardened substrate meets the following requirements: center wave height ≤ 5mm, and edge wave steepness ≤ 1%.

[0009] Furthermore, in step S2, the cleaning process parameters are: alkali concentration 2%~7%, alkali conductivity 60~70ms / cm.

[0010] Furthermore, in step S3, the continuous annealing process parameters meet the following requirements: the outlet plate temperature of the heating section of the annealing furnace is 650℃~800℃, the dew point inside the annealing furnace is ≤-20℃, and the oxygen content is ≤40ppm.

[0011] Furthermore, in step S4, when the strip thickness is less than 1.50 mm, the hot-dip galvanizing time is 3 min ≤ 4.5 min.

[0012] Furthermore, in step S5, the air knife process parameters are: blade lip opening ≤ 1.0 mm, air knife angle tilted 0~5° relative to the normal direction of the strip forward, air knife pressure 70~85 mbar, and air knife height 200~600 mm.

[0013] Compared with existing methods, the beneficial effects of the present invention are: 1. It achieves systematic and full-process collaborative control, fundamentally reducing the incidence of plating defects in thick coatings.

[0014] This invention does not involve adjusting a single process parameter in isolation, but rather constructs a comprehensive precision control system covering the entire chain from "substrate composition and quality → surface cleaning → annealing and reduction → plating and forming." The parameters at each stage are coupled and work synergistically. In particular, the strict design of the substrate composition (such as specific ranges for C, Mn, and Cr) lays a metallurgical foundation for the coating adhesion. Furthermore, through optimized pretreatment and matching of core plating parameters, thick-coated products without plating defects can be stably produced on existing production lines without relying on complex and expensive additional processes such as pre-plating nickel.

[0015] 2. Through strict source and process control, stable, clean and active substrate surface conditions are created for high-quality plating.

[0016] Raw material surface and shape control: The substrate must be free of visual defects and have a straight shape (center waviness ≤ 5mm, edge waviness ≤ 1%). Surface defects (such as rust and residue) directly hinder the wetting of the plating solution; while poor shape (wavy pattern) leads to uneven contact between the strip and the rollers in the annealing furnace, resulting in scratches or localized oxidation. In different locations in the plating pan, the gravity and friction forces on the liquid coating are uneven, severely damaging the coating uniformity and becoming a major cause of incomplete plating of thick layers. This invention eliminates these variables at the source, ensuring that all subsequent processes are carried out on a physically stable substrate.

[0017] Cleaning and annealing process optimization: Specific alkaline solution concentrations, conductivity, and precise air-fuel ratio, plate temperature, hydrogen flow rate, low dew point, and low oxygen content are set for the annealing furnace. Reasoning: Proper alkaline cleaning effectively removes rolling oil and iron powder without excessively corroding the substrate; while a precisely controlled annealing atmosphere (strong reducing properties, low oxidation potential) ensures the strip surface is fully reduced before entering the aluminum bath, forming a uniform, clean, active iron surface, greatly improving the wettability of the aluminum-silicon plating solution on the steel substrate. This is a prerequisite for obtaining good coating adhesion and avoiding "rejection" (a type of missed plating) caused by surface oxides or contamination. Through these two process controls, an almost "idealized" substrate surface state is provided for the core step of "plating," significantly reducing random missed plating caused by improper pretreatment.

[0018] 3. A refined temperature-time matching model based on strip thickness was established to ensure the uniformity and integrity of the coating for all specifications of products, from thin to thick.

[0019] (1) When the strip thickness is ≤1.50mm: use a relatively low temperature for entering the pot and the aluminum liquid (650~660℃) and a shorter time (≤4.5min). Thin plates have a small heat capacity, so using a lower temperature can avoid overheating, and a shorter time can ensure production efficiency. At the same time, the coating is thinner and easier to flow and solidify quickly.

[0020] (2) When the strip thickness is >1.50mm: further increase the inlet temperature (660~670℃) and the aluminum liquid temperature (665~680℃), and significantly increase the time (5~5.5min). Thick plates have a large heat capacity and require a higher inlet temperature to reduce the temperature difference with the aluminum liquid and avoid poor wetting of the plating solution due to rapid cooling; the higher aluminum liquid temperature and the longer heating time together provide sufficient low shear leveling time and more moderate solidification conditions for high viscosity aluminum-silicon alloys. This is the key to preventing the formation of "shrinkage holes" or "exposed iron" defects in the thick coating due to the retraction of liquid metal and premature local solidification.

[0021] 4. Optimized air knife process parameters ensure the uniformity and density of the final thick coating.

[0022] This invention employs strict parameters for the air knife step, particularly the blade lip opening (≤1.0 mm) and specific angles and pressures. The air knife is used to remove excess plating solution and control the thickness and uniformity of the coating. For viscous, thick aluminum-silicon coatings, excessively large air knife openings or improper pressure can easily lead to uneven coating thinning or streaks. This invention limits the opening size (≤1.0 mm) and specific angle (tilted forward), combined with appropriate pressure (70~85 mbar), to generate a concentrated, stable, and uniformly applied airflow. This airflow can precisely control the final coating thickness without disturbing the already leveled plating solution and promotes further homogenization of the coating surface before solidification. This avoids microscopic inhomogeneities such as excessively thin edges, thick centers, or airflow patterns caused by improper air knife control, which can easily develop into macroscopic plating defects in thick coatings. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets according to this invention will be further described in detail below with reference to embodiments and appendices. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention. Any equivalent substitutions or modifications made based on the spirit and principles of this invention should be included within the scope of protection of this invention.

[0024] The chemical composition (weight percentage, wt%) of the cold-hardening substrate of this invention is shown in Table 1, the key process parameter control is shown in Table 2, and the final performance and surface quality evaluation results of the obtained steel plate are shown in Table 3. Meanwhile, comparative examples are provided to highlight the effects of this invention.

[0025] Table 1. Main chemical composition (wt%) of the cold-hardened substrates of the embodiments and comparative examples of the present invention. Table 2 Key process parameters for examples and comparative examples Table 3. Results of Examples and Comparisons Note 1: The "Yes" in "Whether to perform cleaning / annealing" and "Whether to perform air knife control" indicates that the process parameters respectively meet the scope of the claims.

[0026] Note 2: The core plating parameters of Comparative Example D1 deviate significantly from the window set by this invention for a thickness of 1.5mm (660℃, 660~665℃, ≤4.5min).

[0027] Detailed analysis of the examples: Examples 1-6 demonstrate the successful application of the method of the present invention on different thickness specifications (1.2 mm to 2.0 mm). All examples follow a complete collaborative process of "raw material composition control → cleaning → annealing → core plating (temperature and time matched according to thickness) → air knife control". The key is that the core plating parameters (aluminum pot temperature, aluminum liquid temperature, hot-dip plating time) are strictly set according to the thickness-based functional relationship in claim 1.

[0028] Example 3 (1.5mm critical thickness): The combination of a bath entry temperature of 660℃, an aluminum melt temperature of 663℃, and a hot-dip plating time of 4.4min was precisely adopted. This combination ensured the thermal balance between the strip and the aluminum melt at this thickness, as well as sufficient leveling time for the aluminum-silicon plating solution, thereby obtaining a defect-free, uniform, thick coating.

[0029] Examples 1 and 2 (thickness < 1.5 mm) and Examples 4, 5, and 6 (thickness > 1.5 mm): The parameters were systematically adjusted towards lower temperature / shorter time and higher temperature / longer time, respectively, reflecting the core logic of this invention that "increased thickness requires higher temperature and longer time" to adapt to the characteristics of the aluminum-silicon plating solution. All examples achieved the excellent effect of "no plating defects".

[0030] Comparative analysis (negative verification): Comparative examples D1 and D2 are used to demonstrate from the opposite perspective that deviating from the core control logic of this invention will inevitably lead to plating defects.

[0031] Comparative Example D1: The substrate thickness was 1.5 mm, but a low-temperature, short-time parameter combination (640℃, 645℃, 3.5 min) suitable for thin plates was intentionally used. Analysis: The excessively low temperature caused the viscosity of the aluminum-silicon plating solution to increase further, resulting in poor fluidity; the excessively short time severely shortened the leveling and wetting time of the plating solution on the strip surface. The combined effect prevented the plating solution from uniformly covering the entire strip surface before solidification, resulting in "severe plating omissions." This demonstrates the necessity of finely matching parameters to the thickness; a simple "thin plate process" cannot be used to produce thick-coated products.

[0032] Comparative Example D2: In the production of 1.8mm thick plates, the cleaning and annealing steps were omitted, and only the core plating and air knife parameters of Example 5 were imitated. Effect Analysis: Although the subsequent plating parameters were correct, the substrate surface contained residual rolling oil, oxide film, and other contaminants, which were not activated by the annealing reducing atmosphere, resulting in extremely poor wettability of the aluminum-silicon plating solution on the substrate. Even with suitable plating solution temperature and time, a continuous plating layer could not be formed on the contaminated substrate, ultimately resulting in "localized plating omissions." This demonstrates the importance of coordinated control throughout the entire process in this invention; the core plating steps must be based on good pretreatment.

[0033] in conclusion: This invention, through comparisons of six examples covering major thickness specifications and two targeted comparative examples, fully verifies the effectiveness and innovation of the control method described herein. This invention systematically solves the problem of surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets through a full-chain, collaborative process control: "specific component substrate ensures basic performance → cleaning and annealing provide a clean, active surface → precise matching of temperature and time based on thickness to optimize plating solution leveling and solidification → air knife precisely controls coating formation." Any deviation from this systematic control approach, especially deviations in the core "temperature-time-thickness" matching relationship, will lead to plating defects and prevent high-quality, stable production.

[0034] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets, characterized in that, Includes the following steps: S1: Provide a cold-hardened substrate that meets the composition requirements; S2: Clean the chilled substrate; S3: Continuous annealing of the cleaned chilled substrate; S4: The cold hard substrate is hot-dip aluminum-silicon coated to form an aluminum-silicon coating. The following relationships must be satisfied when controlling the temperature of the strip entering the aluminum pot, the temperature of the molten aluminum, and the hot-dip galvanizing time: When the strip thickness is ≤1.50mm, the temperature at which the strip enters the aluminum pot is ≤660℃, the temperature at which the aluminum liquid is ≤665℃, and the hot-dip galvanizing time is ≤4.5min. When the strip thickness is greater than 1.50 mm, the following conditions apply: 660℃ < temperature in aluminum pot ≤ 670℃, 665℃ ≤ aluminum liquid temperature ≤ 680℃, and 5 min ≤ hot-dip galvanizing time ≤ 5.5 min. The double-sided coating weight of the aluminum-silicon coating is 150~160g / m². 2 ; S5: Use an air knife to blow clean the surface of the galvanized steel plate.

2. The method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets according to claim 1, characterized in that, The microstructure of the cold-hardened substrate after thermoforming and quenching is mainly martensite, with a volume fraction of martensite of not less than 90%.

3. The method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets according to claim 1, characterized in that, In step S1, the chemical composition of the cold-hardened substrate, by weight percentage, is as follows: C: 0.20%~0.23%, Si: 0.228%~0.27%, Mn: 1.23%~1.33%, P≤0.02%, S≤0.005%, Al: 0.02%~0.05%, Ti≤0.05%, B≤0.003%, Cr: 0.11%~0.17%; Mo: 0.001%~0.03%, Cu≤0.01%, Ni: 0.004%~0.04%, Nb≤0.003%, the remainder being Fe and unavoidable impurities.

4. The method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets according to claim 1, characterized in that, In step S1, the surface quality of the cold-hardened substrate meets the following requirements: no visible color difference, width or narrow printing, roller printing, rust, or residual emulsion defects; the shape of the cold-hardened substrate meets the following requirements: center wave height ≤ 5mm, and edge wave steepness ≤ 1%.

5. The method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets according to claim 1, characterized in that, In step S2, the cleaning process parameters are: alkali concentration 2%~7%, alkali conductivity 60~70ms / cm.

6. The method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets according to claim 1, characterized in that, In step S3, the continuous annealing process parameters meet the following requirements: the outlet plate temperature of the heating section of the annealing furnace is 650~800℃, the dew point inside the annealing furnace is ≤-20℃, and the oxygen content is ≤40ppm.

7. The method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets according to claim 1, characterized in that, In step S4, the chemical composition of the aluminum-silicon coating by weight percentage is: Al: 85%~90%, Si: 7%~11%, Fe: 2.0%~4.0%, with the remainder being unavoidable impurities.

8. The method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets according to claim 1, characterized in that, In step S4, when the strip thickness is less than 1.50 mm, the hot-dip galvanizing time is 3 min ≤ 4.5 min.

9. The method for controlling surface plating defects in thick-coated hot-formed aluminum-silicon steel sheets according to claim 1, characterized in that, In step S5, the air knife process parameters are: blade lip opening ≤ 1.0 mm, air knife angle tilted 0°~5° relative to the normal direction of the strip and forward, air knife pressure 70~85 mbar, and air knife height 200~600 mm.

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