A method for controlling tear marks on the surface of a hot-dip galvannealed steel sheet and a hot-dip galvanized steel sheet

Abstract

This invention belongs to the field of steel surface treatment. It discloses a method for controlling tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheets. The method includes: degreasing and cleaning the steel sheet; annealing the cleaned steel sheet in a reducing atmosphere, wherein the steel sheet is heated to the annealing temperature in a two-stage heating process; hot-dip galvanizing the annealed steel sheet; and cooling the hot-dip galvanized steel sheet to obtain a hot-dip galvanized aluminum-magnesium steel sheet. This invention also discloses hot-dip galvanized aluminum-magnesium steel sheets prepared using this method. In the technical solution of this invention, by optimizing the annealing process and the composition of the plating solution, the wettability of the plating solution to the steel sheet is improved, reducing the probability of tear marks caused by oxide particles on the steel sheet surface.

Description

Technical Field

[0001] This invention relates to the field of steel surface treatment technology, specifically to a method for controlling tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheets and the hot-dip galvanized steel sheets themselves. Background Technology

[0002] Hot-dip galvanized aluminum-magnesium products possess excellent corrosion resistance, which is 3 to 15 times that of ordinary hot-dip galvanized materials, meeting the national high-quality development requirements of "dual carbon, clean, and energy-saving." Hot-dip galvanized aluminum-magnesium steel sheets are an upgraded material for the automotive, home appliance, construction, highway guardrail, solar panel, photovoltaic, and livestock industries, with broad development prospects. Due to the differences in standards and functional requirements for formed parts in different industries, the performance requirements of zinc-aluminum-magnesium coated steel sheets vary. Highway guardrails, solar panel, and photovoltaic panels have higher requirements for corrosion resistance and processability; the construction, home appliance, and automotive industries not only require high corrosion resistance but also higher requirements for the appearance quality and stamping properties of raw materials; while the livestock industry has high requirements for the corrosion resistance and ammonia resistance of raw steel sheets. Tear marks are a persistent tactile defect in zinc-aluminum-magnesium production. Tear marks not only greatly affect the product's appearance but also significantly reduce the corrosion resistance and service life of the zinc-aluminum-magnesium steel sheet due to the thinning of the coating. Furthermore, tear-mark defects can cause stress concentration during processing, leading to the peeling off of the plating or zinc powder, which in turn affects the corrosion resistance and processing quality of the parts. Therefore, the presence of tear-mark defects cannot meet the requirements of the automotive, home appliance, construction, highway guardrail, solar panel, photovoltaic bracket, and livestock industries for the appearance, machinability, and corrosion resistance of parts.

[0003] Some methods for producing zinc-aluminum-magnesium coated steel sheets are disclosed in the prior art.

[0004] Patent application CN106222593A discloses a high corrosion-resistant hot-dip galvanized aluminum-magnesium-nickel rare earth alloy coated steel sheet and its production method. The coating alloy composition is (5-25% Al)-(1-5% Mg)-(0.1-0.3% Si)-(0.01-0.1% Ni)-(0.01-0.1% Ce)-Zn. This method is used to improve the oxidation resistance of the steel sheet coating surface, and to improve the anti-blackening performance and corrosion resistance of the steel sheet coating.

[0005] Patent CN105420653B discloses a hot-dip galvanizing method for a Zn-Al-Mg alloy layer on the surface of bridge steel wires, and its preparation method. The alloy layer composition, by mass percentage, is Zn: 30-97%, Al: 5-60%, and Mg: 0.02-11%, with a total content of 100%. This method is used to produce coatings with corrosion resistance superior to zinc plating and zinc-aluminum alloy plating.

[0006] Patent application CN110760774A discloses a zinc-aluminum-magnesium steel sheet and a method for effectively controlling black spots on the surface of hot-dip galvanized aluminum-magnesium steel sheets processed by CSP. The coating alloy composition is Al: 10-12%, Mg: 2-4%, Si: 0.01-0.3%, Ni: 0.01-0.1%, Ce: 0.01-0.15%, with the remainder being Zn and unavoidable impurities. This method ensures good surface quality of the zinc-aluminum-magnesium steel sheet without black spot defects.

[0007] Patent application CN103507324A discloses an alloyed zinc-aluminum-magnesium coated steel sheet and its production method. The alloy composition of the coating is: Al: 1.0wt%–11wt%, Mg: 0.5wt%–5wt%, mixed rare earth content 0.01wt%–0.10wt%, with the remainder being Zn and unavoidable impurities. The alloying temperature is 450–650℃, and the alloying time is 3–20s. The Fe content in the alloyed coating does not exceed 5%, ensuring that the coating does not peel off during complex forming processes. After forming, the zinc-aluminum-magnesium coating can exert its excellent corrosion resistance and extend the service life of the components.

[0008] Patent application CN110777290A discloses a method for preparing hot-dip galvanized aluminum-magnesium high-strength steel. The alloy composition of the coating is: Al: 9.0-13.0%, Mg: 2.0-4.0%, Si: 0.02-0.1%, Ni: 0.01-0.05%, RE: 0.01-0.2%, with the balance being Zn and unavoidable impurity elements. Through the process of smelting, hot rolling, cold rolling, and annealing, and the control of core production technologies, hot-dip galvanized aluminum-magnesium high-strength steel with a yield strength greater than 550MPa and an elongation greater than 17% is produced.

[0009] While the aforementioned patents have improved the surface quality, corrosion resistance, coating adhesion, and steel base strength of zinc-aluminum-magnesium steel sheets, none of them have proposed how to improve tear marks. As a result, they cannot effectively improve the surface quality, corrosion resistance, and processability of zinc-aluminum-magnesium steel sheets, and thus cannot meet the high demands of the construction, home appliance, and other industries for corrosion resistance, processability, and aesthetics.

[0010] Therefore, there is an urgent need in this field for a method to control tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheets. Summary of the Invention

[0011] To address the aforementioned problems in the prior art, this invention provides a method for controlling tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheets and a hot-dip galvanized aluminum-magnesium steel sheet prepared using this method.

[0012] According to one aspect of the present invention, a method for controlling tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheet is provided, comprising the following steps:

[0013] The steel plate is degreased and cleaned.

[0014] The cleaned steel plate is subjected to annealing heat treatment in a reducing atmosphere. The steel plate is heated to the annealing temperature in a two-stage heating method. The heating rate of the first stage is 4-6℃ / s, and the heating rate of the second stage is 1.5-2.5℃ / s. The volume content of hydrogen in the reducing atmosphere is 16%-18%.

[0015] The annealed steel sheet is hot-dip galvanized, wherein the galvanizing solution comprises the following components by weight percentage: 7.0%–11.0% Al, 0.15%–0.35% Si, 2.0%–3.0% Mg, 0.1%–0.6% La and Ce together, 0.01%–0.10% Ge, with the balance being Zn and unavoidable impurities;

[0016] The hot-dip galvanized steel sheet is cooled to obtain hot-dip galvanized aluminum-magnesium steel sheet.

[0017] According to one embodiment of the present invention, the temperature of the degreasing cleaning bath is controlled at 75±5℃, the free alkalinity of the alkaline solution in the degreasing bath is 75±10Pt, and the electrolytic degreasing current intensity is 8000~10000A.

[0018] According to one embodiment of the present invention, the annealing temperature of the annealing heat treatment is 700-800°C, and the holding time is 40-60 seconds.

[0019] According to one embodiment of the present invention, the temperature range of the first stage of heating is below 550°C, and the temperature range of the second stage of heating is from 550°C to the annealing temperature.

[0020] According to one embodiment of the present invention, the temperature of the steel sheet entering the zinc pot during hot-dip galvanizing is 470-500°C.

[0021] According to one embodiment of the present invention, the temperature of the plating solution in the zinc pot during hot-dip galvanizing is 465-470°C.

[0022] According to one embodiment of the present invention, the aluminum and magnesium content in the plating solution satisfies Al / Mg≤4.

[0023] According to one embodiment of the present invention, the coating thickness of the hot-dip galvanized steel sheet is controlled by a nitrogen gas knife.

[0024] According to one embodiment of the present invention, the maximum span temperature during cooling of the steel plate is ≤250°C.

[0025] According to another aspect of the present invention, a hot-dip galvanized steel sheet is provided, which is prepared by a method for controlling tear marks on the surface of a hot-dip galvanized aluminum-magnesium steel sheet as described above.

[0026] In the technical solution of this invention, by optimizing the annealing process and the composition of the plating solution, the wettability of the plating solution on the steel plate is improved, and the probability of tear-mark defects caused by oxide particles on the steel plate surface is reduced. This eliminates the adverse effects on the appearance, processability, and corrosion resistance of the coating, resulting in a continuous hot-dip galvanized aluminum-magnesium alloy coated steel plate with excellent corrosion resistance, processability, and aesthetics, meeting the needs of high-quality construction, home appliance, and automotive manufacturing industries. The method for controlling tear-mark defects on the surface of hot-dip galvanized aluminum-magnesium steel plates described in this invention has good reproducibility and repeatability, and can stably and continuously produce high-quality zinc-aluminum-magnesium alloy coated steel plates / strips for construction and home appliances. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a flowchart illustrating a method for controlling tear marks on the surface of a hot-dip galvanized aluminum-magnesium steel sheet according to an embodiment of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and specific examples.

[0030] It should be noted that all uses of "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two entities or parameters with the same name but different names. It is clear that "first" and "second" are only for the convenience of expression and should not be construed as limiting the embodiments of the present invention. Subsequent embodiments will not explain this in detail.

[0031] It should be understood that the embodiments of the invention shown in the exemplary embodiments are merely illustrative. Although only a few embodiments have been described in detail in this invention, those skilled in the art will readily recognize that various modifications are possible without substantially departing from the teachings of the invention. Accordingly, all such modifications should be included within the scope of the invention. Other substitutions, modifications, variations, and deletions can be made to the design, operating conditions, and parameters of the following exemplary embodiments without departing from the spirit of the invention.

[0032] Tear marks are a persistent tactile defect in the production of hot-dip galvanized aluminum-magnesium steel products. Essentially, they result in uneven coating, with areas of thinning. In severe cases, this manifests as noticeable pitted thinning of the coating and may even include spot-like incomplete coating. Tear marks not only significantly affect the product's appearance, but the thinning and incomplete coating also drastically reduce the corrosion resistance and service life of the zinc-aluminum-magnesium steel sheet. Furthermore, during processing, stress concentration caused by tear marks can lead to the peeling off of the coating or zinc powder, further impacting the corrosion resistance and processing quality of the parts. Therefore, eliminating tear marks is crucial for improving the surface quality of hot-dip galvanized aluminum-magnesium steel sheets.

[0033] One object of the present invention is to provide a method for controlling tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheets, such as... Figure 1 As shown, the method generally includes the following steps:

[0034] Step S1: Degrease and clean the steel plate;

[0035] Step S2: The cleaned steel plate is subjected to annealing heat treatment in a reducing atmosphere, wherein the steel plate is heated to the annealing temperature in a two-stage heating method. The heating rate of the first stage is 4-6℃ / s, and the heating rate of the second stage is 1.5-2.5℃ / s. The volume content of hydrogen in the reducing atmosphere is 16%-18%.

[0036] Step S3: The annealed steel sheet is hot-dip galvanized, wherein the plating solution comprises the following components by weight percentage: 7.0% to 11.0% Al, 0.15% to 0.35% Si, 2.0% to 3.0% Mg, 0.1% to 0.6% La and Ce together, 0.01% to 0.10% Ge, and the balance being Zn and unavoidable impurities;

[0037] Step S4: Cool the hot-dip galvanized steel sheet to obtain hot-dip galvanized aluminum-magnesium steel sheet.

[0038] In step S1, the steel plate is degreased and cleaned. Degreasing and cleaning is used to remove oil stains from the surface of the steel plate. The quality of the degreasing process will directly affect the subsequent processes and the quality of hot-dip galvanizing. This is mainly reflected in the following two aspects: (1) Oil stains will hinder or reduce the dissolution and peeling effect of pickling solution on the rust layer on the workpiece surface, causing pollution to the pickling solution, and thus affecting the quality of the next pickling; (2) Grease will pollute the fluxing solution, delay the activation effect of the solvent on the workpiece, and affect the formation of a good solvent thin layer on the workpiece surface. Therefore, the steel plate needs to be degreased and cleaned before hot-dip galvanizing.

[0039] In step S2, the cleaned steel plate undergoes annealing heat treatment in a reducing atmosphere with a hydrogen volume content of 16%–18%. The steel plate is heated to the annealing temperature in a two-stage heating process, with a heating rate of 4–6 °C / s in the first stage and 1.5–2.5 °C / s in the second stage. This heating method, combined with the set H2 content, allows the steel plate to quickly reach the oxide film reduction temperature, prolonging the reaction time between the steel plate and H2. Under a high-hydrogen protective atmosphere, the surface of the steel plate is reduced to spongy pure iron, improving the wettability of the plating solution and reducing the probability of surface oxide droplet defects such as tear marks.

[0040] In step S3, the annealed steel sheet is hot-dip coated, wherein the coating solution comprises the following components by weight percentage: 7.0% to 11.0% Al, 0.15% to 0.35% Si, 2.0% to 3.0% Mg, 0.1% to 0.6% La and Ce, 0.01% to 0.10% Ge, with the balance being Zn and unavoidable impurities.

[0041] Al (Al) is added to the plating bath as a beneficial element, and it is an important component of the coating for aluminum-zinc and zinc-aluminum plating products. Al's influence on the galvanizing process includes two aspects: First, Al inhibits the reaction between iron and zinc. After the hot-dip galvanized steel strip enters the plating bath, the more reactive Al reacts with iron first, forming a dense iron-aluminum compound film (mainly Fe2Al5), which inhibits iron diffusion and slows down the growth rate of the zinc-iron intermetallic compound, hindering the formation and thickening of the iron-zinc compound layer. Second, Al facilitates the flotation of zinc dross. Because Al is more reactive than Zn, it can displace iron from the zinc dross, forming compounds that float to the surface of the plating bath, making it easier to remove the floating zinc dross. The addition of Al must be appropriate. Too high an aluminum content in the hot-dip galvanizing bath will accelerate the corrosion rate of the steel strip in the zinc pot and steel equipment, leading to a large amount of iron being carried into the plating bath, causing adverse effects. For zinc-iron alloy products that require iron atom diffusion annealing after galvanizing, Al inhibits the inward diffusion of iron atoms, hindering the annealing transformation. Furthermore, hot-dip galvanized products with excessively high aluminum content exhibit reduced corrosion resistance to copper sulfate.

[0042] Adding Si to a Zn-Al solution significantly inhibits diffusion and chemical reactions between Al and Fe, improving the alloy's fluidity, enhancing the wettability of the plating solution and the steel substrate, and reducing iron loss and zinc dross. Si also inhibits alloy layer growth, but Si-containing coatings are more brittle. Controlling the Si content in the plating solution is crucial for controlling zinc dross and ensuring coating quality.

[0043] The amount of magnesium (Mg) added to the plating bath has a significant impact on the performance of the coating, and its dosage must be precisely controlled. Adding magnesium can control the growth of active steel coatings, increase the fluidity of the plating bath, and allow for lower galvanizing temperatures without reducing productivity. Adding magnesium to the plating bath can significantly improve the corrosion resistance of the galvanized layer. The addition of magnesium greatly enhances the zinc alloy's resistance to intergranular corrosion, thus improving the corrosion resistance of the galvanized layer. Magnesium reacts with silicon to form stable magnesium-silicon compounds, replacing the original iron-silicon compounds and directly inhibiting the zinc-iron reaction. It can also indirectly inhibit the reaction by lowering the alloy's melting point. However, when the magnesium content in the plating bath is too high, it will cause the alloy layer in the galvanized layer to thicken and coarsen, increase its hardness, and reduce the adhesion of the coating to the substrate.

[0044] Adding 0.1%–0.6% La+Ce to the plating bath refines the grains and purifies the bath, thereby improving its fluidity and reducing tear marks. Furthermore, adding 0.01%–0.10% Ge enhances the bath's oxidation resistance and wettability, further reducing tear marks, thanks to Ge's high melting point and stable chemical properties. Ge also combines with La and Ce, utilizing its high melting point to generate numerous heterogeneous nuclei during bath condensation, refining the grains and improving the coating's corrosion resistance and processability.

[0045] By designing the above-mentioned plating solution composition, the formation of the condensed phase is controlled, the grain size of the coating is refined, tear marks are reduced, and the corrosion resistance and processability of the coating are improved.

[0046] In step S4, the hot-dip galvanized steel sheet is cooled to obtain a hot-dip galvanized aluminum-magnesium steel sheet.

[0047] Optionally, in some embodiments, the temperature of the degreasing bath is controlled at 75±5℃, the free alkalinity of the alkaline solution in the degreasing bath is 75±10Pt, and the electrolytic degreasing current intensity is 8000~10000A. Using high-current electrolytic degreasing can electrolyze water into H2 and O2, deeply cleaning the microstructure of the steel plate surface, improving the wettability of the steel plate to be plated with the plating solution, eliminating tear-like defects caused by wetting factors, and improving product quality.

[0048] Optionally, in some embodiments, the annealing temperature of the annealing heat treatment is 700–800°C, and the holding time is 40–60 seconds. The first stage of heating is below 550°C, and the second stage is from 550°C to the annealing temperature. For the reduction of iron oxide scale on steel plates, when reacting at high temperatures (approximately 800°C), the reaction rate decreases significantly in the later stages due to the formation of dense reduction products. Under simple annealing conditions, it is difficult to completely reduce the iron oxide scale in a short time. Therefore, this invention employs a segmented heating method. Before reaching 550°C, a heating rate of 4–6°C / s is used to allow the steel plate to reach the oxide film reduction temperature more quickly. When the temperature reaches 550°C, a heating rate of 1.5–2.5°C / s is used to pre-reduce the steel plate in the low-temperature stage (before 600°C). The main purpose of this is to obtain a loose oxide scale structure, thereby providing a larger reaction area for the high-temperature reduction stage.

[0049] Optionally, in some embodiments, the temperature of the steel sheet entering the zinc bath during hot-dip galvanizing is 470–500°C, which is higher than the temperature of the galvanizing solution. This can significantly improve the fluidity of the zinc solution, suppress tear marks, and improve the surface quality of the product.

[0050] Optionally, in some embodiments, the temperature of the zinc plating bath during hot-dip galvanizing is 465–470°C. A plating bath temperature higher than 470°C or lower than 460°C can easily reduce the fluidity of the plating bath, leading to tear-mark defects. Furthermore, the cooling of the coating can easily cause the precipitation of harmful Mg-Zn phases, resulting in black spots and color difference defects.

[0051] Optionally, in some embodiments, the aluminum and magnesium content in the plating solution meets the requirement of Al / Mg ≤ 4. An aluminum to magnesium content ratio higher than 4 can easily lead to coarsening of the primary Al-rich phase, which is detrimental to the corrosion resistance of the coating. At the same time, this component ratio effectively reduces plating solution oxidation, improves fluidity, and prevents tear marks.

[0052] Optionally, in some embodiments, the hot-dip galvanized steel sheet uses a nitrogen gas knife to control the coating thickness. The nitrogen gas knife can reduce the oxidation of the plating solution and improve the fluidity of the zinc liquid, eliminating tear marks defects.

[0053] Alternatively, in some embodiments, the maximum span temperature during cooling of the steel plate is ≤250°C.

[0054] Another object of the present invention is to provide a hot-dip galvanized steel sheet prepared by a method for controlling tear marks on the surface of a hot-dip galvanized aluminum-magnesium steel sheet as described above.

[0055] The technical solution of the present invention will be further illustrated by specific embodiments below. Unless otherwise stated, the raw materials, equipment, consumables, etc. used in the following embodiments can all be obtained through conventional commercial means.

[0056] Example 1

[0057] The steel plates were degreased and cleaned in a bath at 75°C. The free alkalinity of the alkaline solution in the degreased bath was 72 Pt, and the electrolytic degreasing current intensity was 10000 A. The cleaned steel plates were then subjected to annealing heat treatment in a reducing atmosphere. The annealing temperature of the strip was set at 700°C. The strip temperature was rapidly heated at a rate of 6°C / s up to 550°C, and then at a rate of 1.5°C / s between 550°C and 700°C. After reaching 700°C, the strip was held for 42 seconds and then cooled. The H2 content in the nitrogen-hydrogen protective gas during annealing was 16%, and the strip temperature upon entering the zinc bath was 490°C. The plating bath composition was 7.5% Al, 0.16% Si, 2.48% Mg, 0.58% La+Ce, and 0.04% Ge, with the balance being Zn and unavoidable impurities. The Al / Mg ratio was 3.02, and the plating bath temperature was 468℃. After immersion plating, an N2 air knife was used to control the coating thickness, resulting in a coating thickness of 300 g / m. 2 The maximum cross temperature during cooling is 240℃.

[0058] Testing revealed that the coated steel sheet was free of tear marks and had a uniform appearance. The Al-rich phase in the coating ranged in size from 10 to 18 μm. Under neutral salt spray testing, red rust appeared after 5200 hours, demonstrating excellent corrosion resistance. No abnormalities were observed during the product processing. The product exhibits superior surface quality, corrosion resistance, and machinability, meeting the requirements of high-quality construction, home appliance, and automotive manufacturing industries.

[0059] Example 2

[0060] The steel plates were degreased and cleaned in a bath at 70°C. The free alkalinity of the alkaline solution in the degreased bath was 65 Pt, and the electrolytic degreasing current intensity was 8000 A. The cleaned steel plates were then subjected to annealing heat treatment in a reducing atmosphere. The annealing temperature of the strip was set at 740°C. The strip temperature was rapidly heated at a rate of 4°C / s up to 550°C, and then heated at a rate of 1.8°C / s between 550°C and 740°C. After reaching 740°C, the strip was held for 51 seconds and then cooled. The H2 content in the nitrogen-hydrogen protective gas during annealing was 17%, and the strip temperature upon entering the zinc bath was 470°C. The plating bath composition was 9.8% Al, 0.33% Si, 2.93% Mg, 0.33% La+Ce, and 0.08% Ge, with the balance being Zn and unavoidable impurities. The Al / Mg ratio was 3.34, and the plating bath temperature was 465℃. After immersion plating, an N2 air knife was used to control the coating thickness, resulting in a coating thickness of 275 g / m. 2 The maximum cross temperature during cooling is 235℃.

[0061] Testing revealed that the coated steel sheet was free of tear marks and had a uniform appearance. The Al-rich phase in the coating ranged in size from 10 to 18 μm. Under neutral salt spray testing, red rust appeared in 4700 hours, demonstrating excellent corrosion resistance. The product processing was also smooth. The product exhibits superior surface quality, corrosion resistance, and machinability, meeting the requirements of high-quality construction, home appliance, and automotive manufacturing industries.

[0062] Example 3

[0063] The steel plates were degreased and cleaned in a bath at 80°C. The free alkalinity of the alkaline solution in the degreased bath was 85 Pt, and the electrolytic degreasing current intensity was 9000 A. The cleaned steel plates were then subjected to annealing heat treatment in a reducing atmosphere. The annealing temperature of the strip was set at 800°C. The strip temperature was rapidly heated at a rate of 5°C / s up to 550°C, and then heated at a rate of 2.5°C / s between 550°C and 800°C. After reaching 800°C, the strip was held for 45 seconds and then cooled. The H2 content in the nitrogen-hydrogen protective gas during annealing was 17%, and the strip temperature upon entering the zinc bath was 480°C. The plating bath composition was 7% Al, 0.19% Si, 2% Mg, 0.45% La+Ce, and 0.06% Ge, with the balance being Zn and unavoidable impurities. The Al / Mg ratio was 3.5, and the plating bath temperature was 468℃. After immersion plating, an N2 air knife was used to control the coating thickness, resulting in a coating thickness of 250 g / m. 2 The maximum cross temperature during cooling is 250℃.

[0064] Testing revealed that the coated steel sheet was free of tear marks and had a uniform appearance. The Al-rich phase in the coating ranged in size from 10 to 18 μm. Under neutral salt spray testing, red rust appeared after 4200 hours, demonstrating excellent corrosion resistance. The product processing was also smooth. The product exhibits superior surface quality, corrosion resistance, and machinability, meeting the requirements of high-quality construction, home appliance, and automotive manufacturing industries.

[0065] Example 4

[0066] The steel plates were degreased and cleaned in a bath at 77°C. The free alkalinity of the alkaline solution in the degreased bath was 75 Pt, and the electrolytic degreasing current intensity was 8700 A. The cleaned steel plates were then subjected to annealing heat treatment in a reducing atmosphere. The strip annealing temperature was set at 760°C. The strip temperature was rapidly heated at a rate of 5.5°C / s up to 550°C, and then at a rate of 2°C / s between 550°C and 760°C. After reaching 760°C, the strip was held for 44 seconds and then cooled. The H2 content in the nitrogen-hydrogen protective gas during annealing was 18%, and the strip temperature upon entering the zinc bath was 500°C. The plating bath composition was 11% Al, 0.28% Si, 3% Mg, 0.13% La+Ce, and 0.05% Ge, with the balance being Zn and unavoidable impurities. The Al / Mg ratio was 3.67, and the plating bath temperature was 470℃. After immersion plating, an N2 air knife was used to control the coating thickness, resulting in a coating thickness of 280 g / m. 2 The maximum cross temperature during cooling is 225℃.

[0067] Testing revealed that the coated steel sheet was free of tear marks and had a uniform appearance. The Al-rich phase in the coating ranged in size from 10 to 18 μm. Under neutral salt spray testing, red rust appeared after 4600 hours, demonstrating excellent corrosion resistance. The product processing was also smooth. The product exhibits superior surface quality, corrosion resistance, and machinability, meeting the requirements of high-quality construction, home appliance, and automotive manufacturing industries.

[0068] Comparative Example 1

[0069] The steel plates were degreased and cleaned in a bath at 77°C. The free alkalinity of the alkaline solution in the degreased bath was 85 Pt, and the electrolytic degreasing current intensity was 8500 A. The cleaned steel plates were then subjected to annealing heat treatment in a reducing atmosphere. The strip annealing temperature was set at 720°C. The strip temperature was rapidly heated at a rate of 3.5°C / s up to 550°C, and then heated at a rate of 3°C / s between 550°C and 720°C. After reaching 720°C, the strip was held for 46 seconds and then cooled. The H2 content in the nitrogen-hydrogen protective gas during annealing was 17%, and the strip temperature upon entering the zinc bath was 485°C. The plating bath composition was 11% Al, 0.28% Si, 3% Mg, 0.62% La+Ce, and 0.04% Ge, with the balance being Zn and unavoidable impurities. The Al / Mg ratio was 3.67, and the plating bath temperature was 465℃. After immersion plating, an N2 air knife was used to control the coating thickness, resulting in a coating thickness of 275 g / m. 2 The maximum cross temperature during cooling is 230℃.

[0070] Testing revealed that the coated steel sheet had no tear marks defects, a uniform appearance, and an Al-rich phase size of 10–18 μm. Under neutral salt spray testing conditions, red rust appeared in 4300 hours, indicating good corrosion resistance. However, the coating surface had tear marks defects, resulting in a generally poor surface quality that could not meet the requirements of high-quality construction, home appliances, and automobile manufacturing.

[0071] Comparative Example 2

[0072] The steel plates were degreased and cleaned in a bath at 80°C. The free alkalinity of the alkaline solution in the degreasing bath was 83 Pt, and the electrolytic degreasing current intensity was 8700 A. The cleaned steel plates were then subjected to annealing heat treatment in a reducing atmosphere. The annealing temperature of the strip was set to 750°C. The strip temperature was rapidly heated at a rate of 4.5°C / s up to 550°C, and then heated at a rate of 1.8°C / s between 550°C and 750°C. After reaching 750°C, the strip was held for 50 seconds and then cooled. The H2 content in the nitrogen-hydrogen protective gas during annealing was 17%, and the strip temperature upon entering the zinc bath was 490°C. The plating bath composition was 10.8% Al, 0.25% Si, 2.19% Mg, 0.45% La+Ce, and 0.06% Ge, with the balance being Zn and unavoidable impurities. The Al / Mg ratio was 4.93, and the plating bath temperature was 470℃. After immersion plating, an N2 air knife was used to control the coating thickness, resulting in a coating thickness of 275 g / m. 2 The maximum cross temperature during cooling is 235℃.

[0073] Testing revealed that the coated steel plate had no tear marks defects, had a uniform appearance, and the size of the Al-rich phase in the coating ranged from 25 to 40 μm. Under neutral salt spray testing conditions, the time for red rust to appear was 3500 hours. The coating exhibited moderate corrosion resistance, but the surface of the coating showed significant tear marks defects, resulting in a generally poor surface quality. This does not meet the requirements for high-quality applications in fields such as construction, home appliances, and automobile manufacturing.

[0074] Comparative Example 3

[0075] The steel plates were degreased and cleaned in a bath at 75°C. The free alkalinity of the alkaline solution in the degreased bath was 80 Pt, and the electrolytic degreasing current intensity was 8500 A. The cleaned steel plates were then subjected to annealing heat treatment in a reducing atmosphere. The annealing temperature of the strip was set at 730°C. The strip temperature was rapidly heated at a rate of 5.2°C / s up to 550°C, and then heated at a rate of 1.6°C / s between 550°C and 730°C. After reaching 730°C, the strip was held for 47 seconds and then cooled. The H2 content in the nitrogen-hydrogen protective gas during annealing was 18%, and the strip temperature upon entering the zinc bath was 480°C. The plating bath composition was 10.2% Al, 0.21% Si, 2.6% Mg, and 0.36% La+Ce, with the balance being Zn and unavoidable impurities. The Al / Mg ratio was 3.92, and the plating bath temperature was 465℃. After immersion plating, an N2 air knife was used to control the coating thickness, resulting in a coating thickness of 295 g / m. 2 The maximum cross temperature during cooling is 240℃.

[0076] Testing revealed that the coated steel plate had no tear marks defects, had a uniform appearance, and the size of the Al-rich phase in the coating ranged from 10 to 18 μm. Under neutral salt spray testing conditions, the time for red rust to appear was 4000 hours. The coating exhibited moderate corrosion resistance, but the surface of the coating showed significant tear marks defects, resulting in a generally poor surface quality. This does not meet the requirements for high-quality applications in fields such as construction, home appliances, and automobile manufacturing.

[0077] The process parameters and performance parameters of each embodiment and comparative example are shown in Tables 1-5 below:

[0078] Table 1. Process parameters for hot-dip galvanizing and cleaning of zinc-aluminum-magnesium alloys.

[0079]

[0080] Table 2. Zinc-aluminum-magnesium hot-dip annealing process parameters

[0081]

[0082] Table 3 Composition parameters of zinc-aluminum-magnesium hot-dip galvanizing solution

[0083] Example 1 7.50 0.16 2.48 0.58 0.04 margin 3.02 Example 2 9.8 0.33 2.93 0.33 0.08 margin 3.34 Example 3 7 0.19 2 0.45 0.06 margin 3.5 Example 4 11 0.28 3 0.13 0.05 margin 3.67 Comparative Example 1 11 0.28 3 0.62 0.04 margin 3.67 Comparative Example 2 10.8 0.25 2.19 0.45 0.06 margin 4.93 Comparative Example 3 10.2 0.21 2.6 0.36 - margin 3.92

[0084] Table 4. Zinc-aluminum-magnesium hot-dip galvanizing pot and high-span process parameters

[0085]

[0086] Table 5 Corrosion Resistance and Defects of Zinc-Aluminum-Magnesium Hot-Dip Steel Sheets

[0087] Example 1 10～18 5200 excellent none excellent Example 2 10～18 4700 excellent none excellent Example 3 10～18 4200 excellent none excellent Example 4 10～18 4600 excellent none excellent Comparative Example 1 10～18 4300 excellent have generally Comparative Example 2 25～40 3500 generally have generally Comparative Example 3 10～18 4000 generally have generally

[0088] The experimental results of the above examples and comparative examples show that the products obtained using the method of the present invention in Examples 1-4 exhibit excellent corrosion resistance, no tear-mark defects, and excellent coating quality, meeting the application requirements of high-quality construction, home appliances, and automobile manufacturing. The annealing heating rate used in Comparative Example 1 was too high, resulting in tear-mark defects in its product. The Al / Mg ratio of the plating solution used in Comparative Example 2 was high, making it prone to tear-mark defects and exhibiting poor corrosion resistance. The plating solution in Comparative Example 3 did not contain added Ge, making it not only prone to tear-mark defects but also exhibiting only average corrosion resistance.

[0089] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the disclosed embodiments of the present invention is limited to these examples. Within the framework of the present invention, technical features of the above embodiments or different embodiments can also be combined, and many other variations of different aspects of the present invention as described above exist, which are not provided in detail for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of controlling the surface tear marks of a hot-dip galvannealed steel sheet, characterized in that, Includes the following steps: The steel plate is degreased and cleaned. The cleaned steel plate is subjected to annealing heat treatment in a reducing atmosphere, wherein the steel plate is heated to the annealing temperature in two stages. The heating rate of the first stage is 4~6℃ / s, and the heating rate of the second stage is 1.5~2.5℃ / s. The volume content of hydrogen in the reducing atmosphere is 16%~18%. The annealed steel sheet is hot-dip galvanized, wherein the galvanizing solution comprises the following components by weight percentage: 7.0%~11.0% Al, 0.15%~0.35% Si, 2.0%~3.0% Mg, 0.1%~0.6% La and Ce together, 0.01%~0.10% Ge, with the balance being Zn and unavoidable impurities; The hot-dip galvanized steel sheet is cooled to obtain hot-dip galvanized aluminum-magnesium steel sheet. The annealing heat treatment is performed at an annealing temperature of 700~800℃ and a holding time of 40~60s. The first stage of heating is below 550℃, and the second stage of heating is from 550℃ to the annealing temperature. The temperature of the zinc plating solution in the hot-dip galvanizing pot is 465~470℃, and the aluminum and magnesium content in the plating solution meets the requirement of Al / Mg≤4.

2. The method for controlling tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheet according to claim 1, characterized in that, The temperature of the degreasing cleaning tank is controlled at 75±5℃, the free alkalinity of the alkaline solution in the degreasing tank is 75±10Pt, and the electrolytic degreasing current intensity is 8000~10000A.

3. The method for controlling tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheet according to claim 1, characterized in that, During hot-dip galvanizing, the temperature of the steel sheet entering the zinc pot is 470~500℃.

4. The method for controlling tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheet according to claim 1, characterized in that, The thickness of the coating on hot-dip galvanized steel sheets is controlled by a nitrogen gas knife.

5. The method for controlling tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheet according to claim 1, characterized in that, The maximum span temperature during cooling of the steel plate is ≤250℃.

6. A hot-dip galvanized steel sheet, characterized in that, The hot-dip galvanized steel sheet is prepared by the method for controlling tear marks on the surface of hot-dip galvanized aluminum-magnesium steel sheet according to any one of claims 1-5.

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