Preparation method of high-corrosion-resistance coated steel plate

Abstract

The invention discloses a preparation method of a high-corrosion-resistance coated steel plate, and belongs to the technical field of corrosion prevention of metal materials. The core of the method is that specific zinc pot plating solution components are adopted and comprise, by mass, larger than or equal to 15% and smaller than 21% of Al, larger than or equal to 5% and smaller than or equal to 11% of Mg, larger than or equal to 0.05% and smaller than or equal to 0.5% of Si, larger than or equal to 0.05% and smaller than or equal to 2.0% of CeO is creatively added to serve as a grain refiner, and the balance is Zn and inevitable impurities; wherein CeO is added in the form of oxide. Key parameters of the hot dipping process are as follows: the temperature of a zinc pot is 10-20 DEG C higher than the melting point of a plating solution, the temperature of a plate in the pot is 5-20 DEG C higher than the temperature of the zinc pot, and the cooling rate after plating is not lower than 9 DEG C / s. By adding CeO, the microstructure of the coating is effectively refined, the successfully prepared coating has excellent plate corrosion resistance, sacrificial anode protection capability and good processing formability, the process is stable, and industrial continuous production is easy to realize.

Description

Technical Field

[0001] This invention relates to a method for preparing a highly corrosion-resistant coated steel sheet, belonging to the field of metal material corrosion protection technology. Background Technology

[0002] Hot-dip galvanizing is a classic corrosion protection process that involves immersing metal workpieces in a bath of molten metal to form a protective coating on their surface. Hot-dip galvanized (GI) steel sheets are widely used in construction, automotive, and home appliance industries due to their excellent sacrificial anode protection performance and cost advantages. With increasingly stringent requirements for material lifespan, developing coated steel sheets with higher corrosion resistance has become an important research direction.

[0003] To improve corrosion resistance, the industry has introduced aluminum (Al) into the zinc plating process. Galfan (Zn-5%Al) and Galvalume (Zn-55%Al-1.6%Si) coatings have been developed. The addition of aluminum significantly improves the corrosion resistance of the coating on flat surfaces. However, with increasing aluminum content, the sacrificial anodic protection capability of the coating decreases significantly, resulting in weakened electrochemical protection of the steel substrate at scratched or cut edges, thus limiting its application range.

[0004] To compensate for the shortcomings of sacrificial anode protection while maintaining high corrosion resistance, zinc-aluminum-magnesium (Zn-Al-Mg) ternary alloy coatings have been developed. The addition of magnesium (Mg) promotes the formation of the MgZn2 phase, which is beneficial for sacrificial anode protection, making the overall corrosion resistance and edge protection of Zn-Al-Mg coatings superior to traditional GI, Galfan, and Galvalume coatings.

[0005] Currently, numerous patents disclose technical solutions for Zn-Al-Mg coatings. For example, patent US6235410B1 discloses a hot-dip galvanized steel sheet with a Mg content of 1.0~4.0% in the coating composition, but its flat plate corrosion resistance still has room for improvement. While patent US6465114B1 appears to increase the Al and Mg content, it sets a limit of no more than 20% for the sum of Al and Mg content, restricting further optimization of its corrosion resistance. Patent WO2018 / 139620A1 proposes a scheme with a higher Mg content (5~20%) and the addition of various rare earth elements (such as Y, La, Ce, etc.) to balance the performance degradation caused by high Al content. However, this scheme directly adds chemically active rare earth elements to the zinc pot, resulting in complex process control, easy element burn-off, and poor plating solution stability, making it difficult to achieve high-quality and stable industrial production.

[0006] In summary, existing Zn-Al-Mg coatings either suffer from suboptimal corrosion resistance due to compositional limitations or are difficult to industrialize due to complex additive processes. Currently, there is no effective solution to simultaneously optimize the corrosion resistance, sacrificial anode protection, and processability of coated steel sheets by adding metal oxides (such as CeO) to refine the coating microstructure. Therefore, developing a method for preparing highly corrosion-resistant coated steel sheets with a rationally designed composition, stable process, excellent comprehensive performance, and ease of industrial production is of significant practical importance. Summary of the Invention

[0007] To address the shortcomings of existing Zn-Al-Mg coating technologies, namely, either the corrosion resistance is not optimal due to compositional limitations, or the process control is difficult and industrialization is challenging due to the addition of reactive elemental substances, this invention aims to provide a high corrosion-resistant coated steel sheet with a reasonable composition design, stable process, and easy industrial production, as well as its preparation method.

[0008] Another objective of this invention is to optimize the microstructure of the coating by adding a specific metal oxide (CeO) as a grain refiner, thereby simultaneously improving the flat corrosion resistance, sacrificial anode protection performance, and processability of the coated steel sheet.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a highly corrosion-resistant coated steel sheet includes the following steps: Zinc pot plating solution composition control: The composition of the plating solution in the zinc pot, by mass percentage, is as follows: Al: ≥15% to <21%, Mg: ≥5% to ≤11%, CeO: ≥0.05% to ≤2.0%, Si: ≥0.05% to ≤0.5%, with the remainder being Zn and unavoidable impurities; wherein, CeO is added in the form of oxide and exists in the zinc pot.

[0010] Hot-dip galvanizing process control: The temperature of the zinc pot is controlled at 10~20℃ higher than the melting point of the galvanizing solution; the temperature of the strip before immersion in the zinc pot is controlled at 5~20℃ higher than the temperature of the galvanizing solution; after immersion, the cooling rate is not less than 9℃ / s.

[0011] The high corrosion-resistant coated steel sheet obtained by the preparation method of the present invention has the above-mentioned chemical composition in its coating, and its cross-sectional microstructure is characterized by: Al dendrites account for 5-10% (aluminum dendrites are composed of α-Al and Al-Zn eutectoid structures, of which α-Al accounts for no less than 60% and Al-Zn eutectoid structure accounts for no more than 40%); Al-MgZn2 binary eutectic structure accounts for 50-65%; Zn-MgZn2-Al ternary eutectic structure accounts for 35-45%.

[0012] The roles and synergistic effects of each element in the coating of this invention: Al (Al) is fundamental for forming highly corrosion-resistant coatings. Its main function is to improve the flat surface corrosion resistance of the coating and promote the formation of the Fe-Al alloy layer, ensuring coating adhesion. This invention controls the Al content at 15%~21% to achieve high corrosion resistance while avoiding weakening the sacrificial anolyte protection capability due to excessive Al content reducing the Zn phase.

[0013] Mg (Mg) is a key element for improving the protective performance of sacrificial anodes, and it can form the MgZn2 phase. This invention increases the Mg content to 5%~11%, significantly enhancing the edge protection performance of the coating. Simultaneously, the process challenge of spontaneous combustion due to high Mg content is solved through process control (such as a low-oxygen environment).

[0014] CeO (cerium oxide): This is the key innovation of the present invention. With its high melting point and non-aggregating properties, CeO is dispersed in the plating solution as a heterogeneous nucleation core, effectively refining the coating grains, thereby improving the density, uniformity and formability of the coating.

[0015] Si element: Its main function is to inhibit the excessive growth of Fe-Al alloy layer and ensure the flexibility and adhesion of the coating.

[0016] Process synergy: The combination of specific component design and precise control of the pot entry temperature and post-plating cooling rate ensures the formation of the above-mentioned ideal microstructure, so that the overall performance of the coating reaches the optimal level.

[0017] Compared with the prior art, the present invention has the following significant advantages: 1. Excellent corrosion resistance: Through the high Al and high Mg composition ratio, and by using CeO to refine the grains to make the structure more compact, the corrosion resistance of the coating on flat plates far exceeds that of traditional GI coatings and ordinary Zn-Al-Mg coatings. 2. Strong sacrificial anode protection capability: The high Mg content ensures that the coating has excellent edge protection performance, overcoming the defects of high-aluminum coatings in this regard; 3. Good processability and formability: The addition of CeO refines the grains and effectively improves the brittleness problem that may be caused by high Mg content, making the coating less prone to cracking or peeling during bending, stamping and other processing. 4. Stable process and easy to industrialize: The use of chemically stable CeO oxides instead of reactive rare earth elements avoids element burn-off and zinc pot management problems. The process window is wide and it is very suitable for large-scale continuous production. 5. High coating quality: Through strict process control, the surface quality of the coating can reach FB level, with excellent adhesion. Attached Figure Description

[0018] Figure 1This is a scanning electron microscope (SEM) image of the surface microstructure of the high corrosion-resistant coated steel plate prepared in Example 1 of the present invention.

[0019] Figure 2 This is a scanning electron microscope (SEM) image of the cross-sectional microstructure of the high corrosion-resistant coated steel plate prepared in Example 1 of this invention.

[0020] Figure 3 yes Figure 2 The magnified image of the Al dendritic region more clearly shows the morphology of the α-Al and Al-Zn co-eutectoid structure.

[0021] Figure 4 This is an interface morphology diagram of the Al-Fe alloy layer between the highly corrosion-resistant coating prepared in Example 1 of the present invention and the substrate.

[0022] Figure 5 The image shows the X-ray diffraction (XRD) phase analysis results of the high corrosion-resistant coating prepared in Example 1 of this invention.

[0023] Figure 6 This is a comparison diagram of the neutral salt spray test of the high corrosion resistant coating and the ordinary coating prepared in Example 1 and the comparative example of the present invention, where (a): GI, (b): Zn-19%Al-6%Mg.

[0024] Figure 7 This is a comparison diagram of cyclic corrosion tests of the high corrosion-resistant coating and the ordinary coating prepared in Example 1 and the comparative example of the present invention, where (a): GI, (b): Zn-19%Al-6%Mg. Detailed Implementation

[0025] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. These embodiments are only for explaining the present invention and are not intended to limit the scope of protection of the present invention. Any modifications or variations made by those skilled in the art within the scope of the claims fall within the scope of protection of the present invention.

[0026] Example 1 This embodiment provides a method for preparing a highly corrosion-resistant coated steel sheet, the specific steps of which are as follows: (1) Zinc pot composition: The zinc pot plating solution is prepared with the following composition by mass percentage: Al content 19.30%, Mg content 5.90%, Si content 0.18%, CeO content 0.18%, and the remainder is Zn and unavoidable impurity elements. Among them, CeO is added directly to the zinc pot in the form of oxide.

[0027] (2) Substrate preparation and pretreatment: A cold-rolled steel plate with dimensions of 220mm×110mm×0.8mm was selected as the substrate. Continuous surface pretreatment was performed on it, including hot water alkaline washing, clean water rinsing, rinsing and drying, to ensure that the surface is clean and free of grease and oxides.

[0028] (3) Hot-dip plating process: A laboratory hot-dip plating simulator was used for the experiment. First, high-purity nitrogen gas was introduced into the simulator to strictly control the oxygen concentration in the system to below 20 ppm to prevent the oxidation of Mg. To accurately monitor the temperature, a thermocouple was welded to the back of the substrate 35 mm from the top. The key process parameters were set as follows: Annealing temperature: 750℃; Zinc pot temperature: 500℃ (approximately 12℃ higher than the melting point of the plating solution); Temperature of the sheet before it enters the zinc pot: 520℃ (20℃ higher than the temperature of the zinc pot); Immersion time: 3 seconds; Zinc pot unloading speed: 150 mm / s; Post-plating cooling rate: 10℃ / s (using an air knife device, the coating thickness is controlled at 20-30μm by dynamically adjusting the nitrogen flow rate and pressure).

[0029] (4) Results and characterization: The composition of the obtained galvanized sheet was analyzed, and the results were: Al 18.92%, Mg 5.83%, Si 0.17%, CeO 0.17%, which is basically consistent with the composition of the zinc pot.

[0030] The coating surface is smooth and meets FB grade standards. Cupping tests show that there is no coating peeling on either side of the galvanized sheet, indicating excellent adhesion.

[0031] like Figure 1 As shown, the coating surface has a uniform and dense structure, with visible fine eutectic structures.

[0032] like Figure 2 As shown, the coating cross-section has a uniform thickness, exhibiting a typical multiphase composite structure.

[0033] right Figure 2 The area within the Chinese border is enlarged to obtain... Figure 3 The Al dendritic structure (bright white part in the figure) can be clearly observed, and its interior is composed of α-Al and Al-Zn co-depositional structures.

[0034] like Figure 4 As shown, a continuous, flat, and moderately thick Al-Fe alloy layer is formed between the coating and the steel substrate, which is the key to the coating's excellent adhesion.

[0035] like Figure 5The XRD pattern shows that the main phases in the coating are Zn phase, α-Al phase, and MgZn2 phase, confirming that the present invention has successfully formed the expected Zn-Al-Mg ternary alloy coating.

[0036] Example 2 The only difference between this embodiment and Embodiment 1 is the fine-tuning of the hot-dip plating process parameters, in order to illustrate the stability of the process window: Zinc pot temperature: 501℃; Temperature of the sheet before it enters the zinc pot: 519℃; The remaining steps and zinc pot composition are exactly the same as in Example 1.

[0037] The composition of the resulting coating was: Al 19.00%, Mg 5.70%, Si 0.18%, CeO 0.16%. The surface quality of the coating also reached FB grade, and the cupping test showed excellent adhesion, proving that high-performance coatings can be stably prepared under these process parameters.

[0038] Example 3 The only difference between this embodiment and Embodiment 1 is the fine-tuning of the hot-dip plating process parameters: Zinc pot temperature: 498℃; Temperature of the sheet before it enters the zinc pot: 520℃; The remaining steps and zinc pot composition are exactly the same as in Example 1.

[0039] The composition of the resulting coating was: Al 18.70%, Mg 5.85%, Si 0.17%, CeO 0.17%. The coating exhibited excellent surface quality and adhesion, further validating the good repeatability and stability of the method within a given process window.

[0040] Comparative Example In comparison, the same substrate and pretreatment process as in Example 1 were used, but the zinc pot composition was only a conventional Zn-0.2%Al plating solution for hot-dip galvanizing. Corrosion resistance tests (such as neutral salt spray tests) of the resulting ordinary galvanized sheet (GI sheet) showed that the time to red rust was much shorter than that of the coated steel sheet prepared in Example 1 of this invention, demonstrating the significant advantage of the high corrosion resistance of this invention.

[0041] The specific experiment is as follows: Neutral Salt Spray Test Comparison: The accelerated corrosion assessment of coated steel sheets was conducted using the neutral salt spray test (NSS) specified in ISO 9227 standard. Results are as follows... Figure 6 As shown: GI developed red rust after 496 hours; the high corrosion-resistant coating Zn-19%Al-6%Mg prepared according to this patent scheme did not develop red rust after 2400 hours.

[0042] Cyclic Corrosion Test: To further compare the corrosion resistance of the two coatings, the samples underwent a cyclic corrosion test in a 5 wt% NaCl solution. This cyclic corrosion test consisted of three consecutive stages: first, a 2-hour salt spray test was conducted using a 5% sodium chloride solution at a temperature maintained at 35±1℃; followed by a 4-hour drying treatment at a temperature of 60±1℃ and a relative humidity controlled at 20~30%RH; finally, a 2-hour wetting treatment was performed at a temperature of 50±1℃ and a relative humidity maintained above 95%RH. The results are as follows: Figure 7 As shown: After 53 cycles of cyclic corrosion, GI showed obvious red rust; the high corrosion-resistant coating Zn-19%Al-6%Mg did not show red rust after 300 cycles.

[0043] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for producing a high corrosion resistant plated steel sheet, characterized by, The method comprises the following steps: 1) providing a steel plate substrate; 2) performing surface pretreatment on the steel plate substrate; 3) immersing the pretreated steel plate substrate into a zinc pot for hot-dip plating, the plating solution in the zinc pot comprises, by mass percentage: Al: ≥15% to <21%, Mg: ≥5% to ≤11%, CeO: ≥0.05% to ≤2.0%, Si: ≥0.05% to ≤0.5%, and the rest is Zn and unavoidable impurities, wherein the CeO is added in the form of oxide; 4) controlling the temperature of the zinc pot to be 10-20℃ higher than the melting point of the plating solution; 5) controlling the temperature of the plate strip before being immersed into the zinc pot to be 5-20℃ higher than the temperature of the plating solution; 6) after the immersion plating, cooling at a cooling rate of not less than 9℃ / s.

2. The production method according to claim 1, characterized by, The mass percentage of Mg in the plating solution is ≥7% to ≤11%.

3. The preparation method according to claim 1, characterized in that, The mass percentage of CeO in the plating solution is ≥0.1% to ≤0.5%.

4. The production method according to claim 1, characterized by, The post-plating cooling rate is 10-50℃ / s.

5. The method of claim 1, wherein, During the hot-dip plating process, the oxygen concentration in the zinc pot is strictly controlled to be below 20ppm.

6. The method of claim 1, wherein, The plating layer formed after the hot-dip plating has the following microstructure: Al dendrites account for 5%-10% of the cross-sectional microstructure of the plating layer, wherein the Al dendrites are composed of α-Al and Al-Zn eutectoid structure, and the mass percentage of α-Al in the Al dendrites is not less than 60%; Al-MgZn2 binary eutectic structure accounts for 50%-65% of the cross-sectional microstructure of the plating layer; Zn-MgZn2-Al ternary eutectic structure accounts for 35%-45% of the cross-sectional microstructure of the plating layer.

7. The preparation method according to claim 1, characterized in that, The surface pretreatment comprises hot water alkali washing, cleaning, rinsing and drying.

8. The method of claim 1, wherein, The thickness of the plating layer is controlled to be 20-30μm.

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