Zinc-aluminum-magnesium plated steel sheet having excellent degreasing property and method for manufacturing the same

Abstract

This invention discloses a zinc-aluminum-magnesium coated steel sheet with excellent degreasing properties, comprising a substrate and a zinc-aluminum-magnesium coating. The surface of the zinc-aluminum-magnesium coating has a hydrophilic chemical conversion film, which fills nanoscale trenches in the eutectic structure of the zinc-aluminum-magnesium coating and passivates Al particles in the ternary eutectic structure. The chemical conversion film contains salts of Al, Mg, and Zn, and its thickness is ≥20 nm. Accordingly, this invention also discloses a method for manufacturing this zinc-aluminum-magnesium coated steel sheet. The zinc-aluminum-magnesium coated steel sheet of this invention exhibits excellent degreasing properties under harsh alkaline degreasing conditions, thus making it suitable for automotive structural parts and inner / outer panels where high surface quality and formability are required.

Description

Technical Field

[0001] This invention relates to a steel plate and a method for manufacturing the same, and more particularly to a coated steel plate and a method for manufacturing the same. Background Technology

[0002] Zinc-aluminum-magnesium coated steel sheet is an upgraded product of hot-dip galvanized steel sheet. By adding specific amounts of Al and Mg to the zinc plating solution, its corrosion resistance is significantly improved, while also achieving better formability and maintaining good weldability. In recent years, zinc-aluminum-magnesium coated steel sheet has been increasingly used in car bodies, where its zinc layer thinning method can replace continuously hot-dip galvanized (GI) car body materials.

[0003] Painting, as one of the four major manufacturing processes in automobiles, plays a crucial role in protecting the vehicle body from corrosion and enhancing its appearance. The first step in the painting process is degreasing, which removes the rust-preventive oil from the steel sheet surface, allowing a continuous water film to form. This process significantly impacts subsequent phosphating crystallization or zirconium leaching film formation.

[0004] Inadequate degreasing can cause discontinuous water film on the steel plate surface, which may allow floating dirt to directly adhere to the steel plate surface and cause electrophoretic defects. On the other hand, it may cause poor phosphating crystallization and significantly lower zirconium film weight, thereby weakening the adhesion of the paint film and the resistance to under-film corrosion of the subsequent electrophoretic layer.

[0005] Current research indicates that the degreasing properties of zinc-aluminum-magnesium coated steel sheets are significantly weaker than those of GI and GA coated steel sheets commonly used in automotive manufacturing. This suggests that residual oil on the zinc-aluminum-magnesium coating surface may further hinder subsequent phosphating crystallization or zirconium coating formation. Further research revealed that inadequate degreasing of zinc-aluminum-magnesium coated steel sheets has a relatively small impact on phosphating crystallization, but it significantly reduces the zirconium coating weight, thereby severely weakening the adhesion of the electrophoretic coating and its resistance to under-film corrosion. Summary of the Invention

[0006] One of the objectives of this invention is to provide a zinc-aluminum-magnesium coated steel sheet with excellent degreasing properties. This steel sheet exhibits excellent degreasing properties under harsh alkaline degreasing conditions, thus making it suitable for automotive structural parts and inner and outer panels that require high surface quality and formability.

[0007] To achieve the above objectives, the present invention provides a zinc-aluminum-magnesium coated steel sheet with excellent degreasing properties, comprising a substrate and a zinc-aluminum-magnesium coating, wherein:

[0008] The surface of the zinc-aluminum-magnesium coating has a hydrophilic chemical conversion film that fills the nanoscale trenches in the eutectic structure of the zinc-aluminum-magnesium coating and passivates (in particular, completely passivates) the Al particles in the ternary eutectic structure of the zinc-aluminum-magnesium coating.

[0009] The chemical conversion membrane contains salts of Al, Mg, and Zn;

[0010] The thickness of the chemical conversion film is ≥20nm.

[0011] The Al, Mg, and Zn salts contained in the chemical conversion membrane of this invention can be either inorganic or organic salts.

[0012] Through extensive research, the inventors discovered that under harsh alkaline degreasing conditions (similar to those in automotive painting production lines), where the degreasing solution contains rust-preventive oil (e.g., 2-4 g / L of rust-preventive oil in the alkaline degreasing solution), the rust-preventive oil consumes a large amount of surfactant, significantly reducing the degreasing ability of the degreasing solution. The degreasing properties of zinc-aluminum-magnesium coated steel sheets are noticeably weaker and inferior to those of GI and GA coated steel sheets.

[0013] Inadequate degreasing of zinc-aluminum-magnesium coated steel sheets can lead to discontinuous water films on the steel sheet surface, increasing the probability of floating dirt adhering directly to the steel sheet surface and causing electrophoretic defects. In addition, it can result in a significantly lower zirconium coating weight, which in turn severely weakens the adhesion of the electrophoretic coating and the resistance to under-film corrosion.

[0014] The inventors discovered through research that the weak degreasing properties of zinc-aluminum-magnesium coated steel sheets are closely related to the multiphase microstructure of the coating surface. For zinc-aluminum-magnesium coated steel sheets, the surface of the coating consists of: primary zinc phase + binary eutectic structure (Zn / MgZn2) + ternary eutectic structure (Zn / MgZn2 / Al). The ternary eutectic structure is relatively dense, with fine nanoscale Al particles deeply embedded in the nanoscale microgrooves of the eutectic. These Al particles and their surrounding tiny gaps strongly adsorb oil particles, especially exhibiting strong van der Waals adsorption forces between Al particles and oil particles. When the surfactant content in the degreasing solution is low, on the one hand, the surfactant has difficulty penetrating the tiny gaps in the ternary eutectic to capture oil particles; on the other hand, the surfactant that reaches these gaps is insufficient to remove the oil droplets strongly adsorbed by the Al particles, resulting in inadequate degreasing, macroscopically manifested as a discontinuous water film on the sheet surface.

[0015] Based on the above research findings, the inventors of this case, by immersing steel plates in acidic or alkaline electrolyte solutions and then drying them, formed a chemical conversion film with a thickness ≥20nm on the surface of the zinc-aluminum-magnesium coating through the reaction between the zinc-aluminum-magnesium coating and the electrolyte. This chemical conversion film contains inorganic or organic salts of Al, Mg, and Zn, and is hydrophilic. Microscopically, the chemical conversion film partially fills the nanoscale grooves in the eutectic structure of the coating surface. More importantly, Al particles are partially dissolved in the electrolyte or completely passivated to form inorganic or organic Al salt particles, significantly reducing their van der Waals adsorption. These three effects significantly reduce the accumulation and adsorption of oil particles on the surface of the zinc-aluminum-magnesium coating, significantly improving its degreasing properties to the same level as GI or GA coatings.

[0016] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the thickness of the chemical conversion film is 20-500 nm.

[0017] As mentioned above, when the thickness of the chemical conversion film is too low, it cannot improve the degreasing performance. However, the inventors have found through experiments and research that when the thickness of the chemical conversion film is too high, it will also affect the subsequent phosphating crystallization and zirconium film formation. Based on this, in the preferred embodiment of the present invention, the thickness of the chemical conversion film is controlled to be 20-500 nm.

[0018] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the surface roughness Ra value of the zinc-aluminum-magnesium coating is 0.6-1.8 μm.

[0019] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the zinc-aluminum-magnesium coating contains Mg: 0.8-4.0wt% and Al: 1.0-7.0wt%.

[0020] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the zinc-aluminum-magnesium coating contains Mg: 2.0-4.0 wt% and Al: 5.0-7.0 wt%.

[0021] It can be seen that the present invention is applicable to aluminum-zinc-aluminum-magnesium coated steel sheets.

[0022] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the zinc-aluminum-magnesium coating contains Mg: 0.8-3.5wt% and Al: 1.0-5.0wt%.

[0023] It can be seen that the present invention is also applicable to low-aluminum-zinc-aluminum-magnesium coated steel sheets.

[0024] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the chemical conversion film also has a hydrophilic organic film layer, which has polar functional groups.

[0025] In this embodiment, an organic film layer can be formed on the surface of the chemical conversion film. The surface of the organic film layer has a large number of polar functional groups, which allows it to be well adsorbed with water after degreasing and oil removal, thereby further ensuring the continuity and integrity of the water film on the surface of the coated steel plate.

[0026] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the polar functional groups include at least 3 carboxyl functional groups or 2 hydroxyl functional groups.

[0027] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the organic film layer contains acrylic resin or epoxy resin.

[0028] To achieve the technical effects of this invention, the organic membrane layer needs to possess two properties: firstly, a high degree of cross-linking to ensure density; and secondly, a large number of polar functional groups to ensure hydrophilicity. Acrylic resin or epoxy resin can simultaneously meet the aforementioned two requirements. Among them, the acrylic resin monomer contains carboxyl groups, which can undergo self-cross-linking reactions to form a dense cross-linked structure. At the same time, the resin contains a large number of carboxyl groups, and the considerable number of strongly polar functional groups ensures the hydrophilicity of the membrane layer. Epoxy resin is derived from the condensation polymerization of epoxy propane and bisphenol A. It has a high degree of cross-linking and a large number of polar functional groups OH, ensuring good adsorption of water on the membrane surface.

[0029] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the thickness of the organic film layer is 20-200 nm.

[0030] Through experiments and research, the inventors discovered that when the organic film layer is too thin, the effect is not good, and when the thickness is too large, it will affect the subsequent welding, phosphating crystallization and zirconium film formation. Based on this, in the preferred embodiment of the present invention, the thickness of the chemical conversion film is controlled to be 20-200 nm.

[0031] Furthermore, the zinc-aluminum-magnesium coated steel sheet of the present invention has a degreasing and water retention rate of >95% after degreasing and washing under harsh alkaline degreasing conditions, wherein the harsh alkaline degreasing conditions include the alkaline degreasing solution containing 2-4 g / L of rust-preventive oil.

[0032] Another objective of this invention is to provide a method for manufacturing zinc-aluminum-magnesium coated steel sheet with excellent degreasing properties. The zinc-aluminum-magnesium coated steel sheet obtained by this method has excellent degreasing properties under harsh alkaline degreasing conditions, thus making it suitable for automotive structural parts and inner and outer panels that have high requirements for surface quality and formability.

[0033] Based on the above-mentioned objectives, the present invention also provides a method for manufacturing zinc-aluminum-magnesium coated steel sheet as described above, comprising the following steps:

[0034] The substrate is hot-dip coated to obtain zinc-aluminum-magnesium coated steel sheet;

[0035] Zinc-aluminum-magnesium coated steel sheets are immersed in acidic or alkaline electrolyte solutions for surface treatment, and then dried to form a hydrophilic chemical conversion film on the surface of the zinc-aluminum-magnesium coating.

[0036] Furthermore, in the manufacturing method described in this invention, when an acidic electrolyte solution is used, its pH value is 2-6; when an alkaline electrolyte solution is used, its pH value is 8-13.

[0037] If the pH of the acidic electrolyte solution used is too low, the acidity will be too strong, the reaction will be too violent, and it may cause excessive erosion of the coating, which may lead to the risk of microscopic pores on the coating surface. If the pH is too high, the reactivity will be insufficient, and it will be difficult to form an effective film. The pH range of the alkaline electrolyte solution is also based on similar considerations.

[0038] Furthermore, in the manufacturing method described in this invention, the acidic electrolyte solution contains at least one of the following: permanganic acid, hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, selenic acid, hydrobromic acid, hydroiodic acid, chloric acid, carbonic acid, acetic acid, sulfurous acid, hypochlorous acid, hydrosulfuric acid, silicic acid, aluminic acid, formic acid, acetic acid, oxalic acid, tartaric acid, citric acid, and salicylic acid.

[0039] Furthermore, in the manufacturing method described in this invention, the alkaline electrolyte solution contains at least one of the following: sodium hydroxide, potassium hydroxide, barium hydroxide, sodium pyrophosphate, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, potassium citrate, and ammonia.

[0040] Furthermore, in the manufacturing method described in this invention, the step of applying oil is included after the surface treatment step.

[0041] Furthermore, in the oiling step, the amount of oil applied to a single side can be 300-1800 mg / m². 2 .

[0042] The present invention also provides another method for manufacturing zinc-aluminum-magnesium coated steel sheet as described above, which includes the following steps:

[0043] The substrate is hot-dip coated to obtain zinc-aluminum-magnesium coated steel sheet;

[0044] Zinc-aluminum-magnesium coated steel sheets are immersed in acidic or alkaline electrolyte solutions and then dried to form a hydrophilic chemical conversion film on the surface of the zinc-aluminum-magnesium coating.

[0045] The water-based treatment agent is coated onto the surface of the zinc-aluminum-magnesium coating obtained in the previous step and then heated to cure, so as to form a hydrophilic organic film layer on the surface of the chemical conversion film.

[0046] Furthermore, in the manufacturing method described in this invention, when an acidic electrolyte solution is used, its pH value is 2-6; when an alkaline electrolyte solution is used, its pH value is 8-13.

[0047] Furthermore, in the manufacturing method described in this invention, the water-based treatment agent contains:

[0048] 25–50 wt% of water-soluble acrylic resin or water-soluble epoxy resin;

[0049] 5-15 wt% nano-silica;

[0050] 5-10 wt% of silane coupling agent.

[0051] Furthermore, in the manufacturing method described in this invention, the mass percentage ratio of each component of the water-based treatment agent is as follows:

[0052] 25–50 wt% of water-soluble acrylic resin or water-soluble epoxy resin;

[0053] 5-15 wt% nano-silica;

[0054] 5-10 wt% silane coupling agent;

[0055] The remainder consists of water and other unavoidable impurities.

[0056] In this embodiment: water-soluble acrylic resin or water-soluble epoxy resin is the main film-forming agent. If the content is too low, the number of polar functional groups in the film layer is insufficient, resulting in poor hydrophilicity. If the content is too high, the solution homogeneity is insufficient, and precipitation is easily generated. Therefore, its content is preferably controlled at 25-50 wt%. Nano-silica is an additive, mainly used to enhance the density of the film layer, prevent water penetration into the film layer, and improve the corrosion resistance of the coating. It can also improve the viscosity of the surface treatment solution to a certain extent. If the content is too low, its effect is limited; if the content is too high, it is difficult to dissolve completely. Its content is preferably controlled in the range of 5-15 wt%. Silane coupling agent is a film-forming auxiliary agent, which can further improve the crosslinking degree of the film layer and enhance the adhesion between the film layer and the coating. If the content is too low, its effect is limited; if the content is too high, it leads to poor stability of the formulation system. Its content is preferably controlled in the range of 5-10 wt%.

[0057] Furthermore, in the manufacturing method described in this invention, the heating and curing temperature is 100-140℃, and the heating and curing time is 5-15s.

[0058] Furthermore, in the manufacturing method described in this invention, the step of applying oil is included after the heating and curing step.

[0059] Furthermore, in the oiling step, the amount of oil applied to a single side can be 300-1800 mg / m². 2 .

[0060] The zinc-aluminum-magnesium coated steel sheet with excellent degreasing properties and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0061] The zinc-aluminum-magnesium coated steel sheet with excellent degreasing properties described in this invention uses an electrolyte to treat the steel sheet surface, causing the zinc-aluminum-magnesium coating to react with the electrolyte to form a chemical conversion film with a thickness of ≥20nm on the surface. This chemical conversion film contains inorganic or organic salts of Al, Mg, and Zn, and is hydrophilic. Microscopically, this chemical conversion film fills the nanoscale grooves in the eutectic structure of the coating surface. More importantly, Al particles are partially dissolved in the electrolyte or completely passivated to form inorganic or organic Al salt particles, significantly reducing their van der Waals adsorption capacity. These three effects significantly reduce the accumulation and adsorption of oil particles on the coating surface, thus significantly improving its degreasing properties.

[0062] The zinc-aluminum-magnesium coated steel sheet of the present invention, under the harsh alkaline degreasing conditions containing 2-4 g / L of rust-preventive oil in the alkaline degreasing solution, has a degreasing water retention rate of >95% after degreasing and washing, and its degreasing performance is significantly improved, reaching the same level as GI or GA coatings. Therefore, it can be used for automotive structural parts and inner and outer panels with high requirements for surface quality and formability.

[0063] In some embodiments, an organic film layer is also formed on the chemical conversion film, which isolates the penetration of the anti-rust oil film, avoids contact between the coating and the oil, and completely eliminates the possibility of oil particles being adsorbed by the nanogrooves and Al particles in the eutectic structure, thereby making the surface oil film easy to remove. Furthermore, the surface of the organic film layer has a large number of polar functional groups, which allows it to readily adsorb water after degreasing, thus ensuring the continuity and integrity of the water film on the surface of the coated steel plate. Attached Figure Description

[0064] Figure 1 This is a schematic diagram of the cross-sectional structure of the zinc-aluminum-magnesium coated steel plate with excellent degreasing properties described in this invention.

[0065] Figure 2 The surface microstructure of the zinc-aluminum-magnesium coating in Comparative Example 1 is shown.

[0066] Figure 3 This is a magnified view of a portion of the ternary eutectic structure on the surface of the zinc-aluminum-magnesium coating in Comparative Example 1. Detailed Implementation

[0067] The zinc-aluminum-magnesium coated steel sheet with excellent degreasing properties and its manufacturing method described in this invention will be further explained and illustrated below with reference to specific embodiments. However, this explanation and illustration do not constitute an undue limitation on the technical solution of this invention.

[0068] Examples 1-15

[0069] The zinc-aluminum-magnesium coated steel sheets in Examples 1-5 were all prepared using the following steps:

[0070] The substrate is sequentially subjected to hot-dip galvanizing, flattening, surface treatment, oiling, and rolling to obtain zinc-aluminum-magnesium coated steel sheet.

[0071] Figure 1 This is a schematic diagram of the cross-sectional structure of the zinc-aluminum-magnesium coated steel plate with excellent degreasing properties described in this invention.

[0072] like Figure 1 As shown, the zinc-aluminum-magnesium coated steel sheet of the present invention includes a substrate 1 and a zinc-aluminum-magnesium coating 2. Furthermore, the improvement of the present invention is that a chemical conversion film 3 is formed on the surface of the zinc-aluminum-magnesium coating 2.

[0073] In embodiments 1-5 of this invention, the specific steps of the surface treatment are as follows: The steel plate is immersed in an acidic electrolyte solution with a pH of 2-6 and a temperature of 35-40°C for 5-40 seconds, then removed and dried at a temperature ≥60°C to form a hydrophilic chemical conversion film on the surface of the zinc-aluminum-magnesium coating. This chemical conversion film fills the nanoscale trenches in the eutectic structure of the zinc-aluminum-magnesium coating and completely passivates the Al particles in the ternary eutectic structure of the zinc-aluminum-magnesium coating. The chemical conversion film contains salts of Al, Mg, and Zn, and its thickness is ≥20 nm.

[0074] The zinc-aluminum-magnesium coated steel sheets in Examples 6-10 were all prepared using the following steps:

[0075] The substrate is sequentially subjected to hot-dip galvanizing, flattening, surface treatment, oiling, and rolling to obtain zinc-aluminum-magnesium coated steel sheet.

[0076] The specific steps of the surface treatment are as follows: The steel plate is immersed in an alkaline electrolyte solution with a pH of 8-13 and a temperature of 35-40℃ for 5-40 seconds, then removed and dried at a temperature ≥60℃ to form a hydrophilic chemical conversion film on the surface of the zinc-aluminum-magnesium coating. This chemical conversion film fills the nanoscale trenches in the eutectic structure of the zinc-aluminum-magnesium coating and completely passivates the Al particles in the ternary eutectic structure of the zinc-aluminum-magnesium coating. The chemical conversion film contains salts of Al, Mg, and Zn, and its thickness is ≥20nm.

[0077] The zinc-aluminum-magnesium coated steel sheets in Examples 11-13 were all prepared using the following steps:

[0078] The substrate is sequentially subjected to hot-dip galvanizing, flattening, surface treatment, coating with water-based treatment agent, heat curing, oiling and rolling to obtain zinc-aluminum-magnesium coated steel sheet.

[0079] The specific steps of the surface treatment are as follows: The steel plate is immersed in an acidic electrolyte solution with a pH of 2-6 and a temperature of 25-50℃ for 5-40 seconds, then removed and dried at a temperature ≥60℃ to form a hydrophilic chemical conversion film on the surface of the zinc-aluminum-magnesium coating. This chemical conversion film fills the nanoscale trenches in the eutectic structure of the zinc-aluminum-magnesium coating and completely passivates the Al particles in the ternary eutectic structure of the zinc-aluminum-magnesium coating. The chemical conversion film contains salts of Al, Mg, and Zn, and its thickness is ≥20nm.

[0080] The specific steps for coating the water-based treatment agent are as follows: the water-based treatment agent is coated onto the surface of the chemical conversion membrane using a roller coater, and cured by hot air or infrared induction heating to form a hydrophilic organic film layer on the surface of the chemical conversion membrane. The organic film layer has polar functional groups and a thickness of ≥20nm.

[0081] The water-based treatment agent has the following component ratio: 5-15% nano-silica, 5-10% silane coupling agent, 25-50% water-soluble acrylic resin or water-soluble epoxy resin, and the remainder is water and unavoidable impurities.

[0082] In some implementations, during heat curing, the plate temperature can be controlled at 100–140°C, and the heating time at 5–15 seconds.

[0083] The zinc-aluminum-magnesium coated steel sheets in Examples 14-15 were all prepared using the following steps:

[0084] The substrate is sequentially subjected to hot-dip galvanizing, flattening, surface treatment, coating with water-based treatment agent, heat curing, oiling and rolling to obtain zinc-aluminum-magnesium coated steel sheet.

[0085] The specific steps of the surface treatment are as follows: The steel plate is immersed in an alkaline electrolyte solution with a pH of 8-13 and a temperature of 25-50℃ for 2-15 seconds, then removed and dried at a temperature ≥60℃ to form a hydrophilic chemical conversion film on the surface of the zinc-aluminum-magnesium coating. This chemical conversion film fills the nanoscale trenches in the eutectic structure of the zinc-aluminum-magnesium coating and completely passivates the Al particles in the ternary eutectic structure of the zinc-aluminum-magnesium coating. The chemical conversion film contains salts of Al, Mg, and Zn, and its thickness is ≥20nm.

[0086] The specific steps for coating the water-based treatment agent are as follows: the water-based treatment agent is coated onto the surface of the chemical conversion membrane using a roller coater, and cured by hot air or infrared induction heating to form a hydrophilic organic film layer on the surface of the chemical conversion membrane. The organic film layer has polar functional groups and a thickness of ≥20nm.

[0087] The water-based treatment agent has the following component ratio: 5-15% nano-silica, 5-10% silane coupling agent, 25-50% water-soluble acrylic resin or water-soluble epoxy resin, and the remainder is water and unavoidable impurities.

[0088] In some implementations, during heat curing, the plate temperature can be controlled at 100–140°C, and the heating time at 5–15 seconds.

[0089] In some embodiments, during the oiling step, the amount of oil applied to a single side can be controlled to be 300-1800 mg / m². 2 .

[0090] Comparative Examples 1-5

[0091] The zinc-aluminum-magnesium coated steel sheets in Comparative Examples 1-5 were obtained by the following method:

[0092] The substrate was sequentially subjected to hot-dip galvanizing, planing, oiling, and winding to obtain comparative zinc-aluminum-magnesium coated steel sheets. It can be seen that comparative examples 1-5 did not undergo surface treatment, and therefore the coating surface did not have a chemical conversion film.

[0093] Table 1 lists the chemical element content of the zinc-aluminum-magnesium coated steel sheets of Examples 1-15 and Comparative Examples 1-5.

[0094] Table 1. (wt%, balance Zn and other unavoidable impurities)

[0095] Example 1 0.8 1.0 Example 2 1.1 1.4 Example 3 1.2 1.7 Example 4 1.4 2.2 Example 5 1.6 2.5 Example 6 2.0 3.2 Example 7 2.2 3.5 Example 8 2.4 4.0 Example 9 2.5 4.2 Example 10 2.5 4.7 Example 11 2.8 5.1 Example 12 3.2 5.6 Example 13 3.5 6.2 Example 14 3.8 6.5 Example 15 4.0 7.0 Comparative Example 1 0.8 1.0 Comparative Example 2 1.6 2.5 Comparative Example 3 2.4 4.0 Comparative Example 4 3.2 5.6 Comparative Example 5 4.0 7.0

[0096] Table 2 lists the specific process parameters for the surface treatment steps in Examples 1-15.

[0097] Table 2.

[0098]

[0099]

[0100] Table 3 lists the specific process parameters for coating the water-based treatment agent, heating and curing, and oiling in Examples 1-15.

[0101] Table 3.

[0102]

[0103] Note: In Table 2, Examples 12, 14, and 15 used water-soluble acrylic resin, while Examples 11 and 13 used water-soluble epoxy resin.

[0104] In addition, the "types of polar functional groups" in Table 2 were determined using an infrared spectrometer, and the "number of polar functional groups" were determined using X-ray photoelectron spectroscopy.

[0105] To verify the degreasing performance of the zinc-aluminum-magnesium coated steel sheet described in this invention, the zinc-aluminum-magnesium coated steel sheets of Examples 1-15 and Comparative Examples 1-5 were degreased and then washed with water. The specific degreasing and washing processes are listed in Table 4.

[0106] Table 4. Degreasing and washing process parameters

[0107]

[0108] After degreasing and washing Examples 1-15 and Comparative Examples 1-5, the water retention rate (i.e., the percentage of water-retaining area to the total board area) was calculated, and the results are shown in Table 5.

[0109] Table 5.

[0110] Example 1 100％ Example 2 100％ Example 3 100％ Example 4 100％ Example 5 100％ Example 6 100％ Example 7 100％ Example 8 100％ Example 9 100％ Example 10 100％ Example 11 100％ Example 12 100％ Example 13 100％ Example 14 100％ Example 15 100％ Comparative Example 1 0％ Comparative Example 2 5％ Comparative Example 3 5％ Comparative Example 4 10％ Comparative Example 5 10％

[0111] As shown in Table 5, under the degreasing process shown in Table 4, the water-retention rate of zinc-aluminum-magnesium coated steel sheets in Examples 1-15 of the present invention is 100%, which meets the degreasing and water-retention rate of GI and GA coated steel sheets currently used in automobile factories.

[0112] The degreasing and water-retention rates of the zinc-aluminum-magnesium coated steel sheets in Comparative Examples 1-5 were only 0-10%, indicating that the degreasing properties of the zinc-aluminum-magnesium coated steel sheets in Comparative Examples 1-5, which do not have a chemical conversion film, are significantly weaker.

[0113] Figure 2 The surface microstructure of the zinc-aluminum-magnesium coating in Comparative Example 1 is shown.

[0114] like Figure 2 As shown, the surface microstructure of the zinc-aluminum-magnesium coating consists of: primary zinc phase + binary eutectic structure (Zn / MgZn2) + ternary eutectic structure (Zn / MgZn2 / Al).

[0115] Figure 3 This is a partially enlarged view of the ternary eutectic structure on the surface of the zinc-aluminum-magnesium coating of Comparative Example 1 of the present invention.

[0116] like Figure 3 As shown, the ternary eutectic structure is relatively fine, with tiny nano-sized Al particles deeply embedded in the nano-scale micro-grooves of the eutectic. These Al particles and the surrounding tiny gaps have a strong adsorption effect on oil particles. In particular, the Al particles and oil particles have a strong van der Waals adsorption force. When the surfactant content in the degreasing solution is low, on the one hand, it is difficult for the surfactant to penetrate into the tiny gaps of the ternary eutectic to capture oil particles. On the other hand, the surfactant that reaches the gap is too few to remove the oil droplets strongly adsorbed by the Al particles. Therefore, the degreasing is unqualified, which is macroscopically manifested as a discontinuous water film on the plate surface.

[0117] The zinc-aluminum-magnesium coated steel sheet with excellent degreasing properties described in this invention uses an electrolyte to treat the steel sheet surface, allowing the zinc-aluminum-magnesium coating to react with the electrolyte and form a chemical conversion film with a thickness of ≥20nm on the surface. This chemical conversion film partially fills the nanoscale grooves in the eutectic structure of the coating surface at the microscopic level. More importantly, Al particles are partially dissolved in the electrolyte or completely passivated to form inorganic or organic Al salt particles, significantly reducing their van der Waals adsorption capacity. These three effects significantly reduce the accumulation and adsorption of oil particles on the coating surface, thus significantly improving its degreasing properties.

[0118] In embodiments 11-15 of this invention, an organic film layer is also formed on the chemical conversion film, which further isolates the penetration of the anti-rust oil film, avoids contact between the coating and the oil, and completely eliminates the possibility of oil particles being adsorbed by the nanogrooves and Al particles in the eutectic structure, thereby making the surface oil film easy to degrease and remove. Furthermore, the surface of the organic film layer has a large number of polar functional groups, which allows it to adsorb water well after degreasing, thereby further ensuring the continuity and integrity of the water film on the surface of the coated steel plate.

[0119] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0120] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A zinc-aluminum-magnesium coated steel sheet with excellent degreasing properties, comprising a substrate and a zinc-aluminum-magnesium coating, characterized in that: The surface of the zinc-aluminum-magnesium coating has a hydrophilic chemical conversion film, which fills the nanoscale trenches in the eutectic structure of the zinc-aluminum-magnesium coating and passivates the Al particles in the ternary eutectic structure of the zinc-aluminum-magnesium coating. The chemical conversion membrane contains salts of Al, Mg, and Zn; The thickness of the chemical conversion film is ≥20 nm; The zinc-aluminum-magnesium coated steel sheet is prepared by the following steps: The substrate is hot-dip coated to obtain zinc-aluminum-magnesium coated steel sheet; Zinc-aluminum-magnesium coated steel sheets are immersed in acidic or alkaline electrolyte solutions for surface treatment, and then dried to form a hydrophilic chemical conversion film on the surface of the zinc-aluminum-magnesium coating. When using an acidic electrolyte solution, the pH value is 2-6 and the immersion time is 5-40 seconds. When using an alkaline electrolyte solution, the pH value is 8-13 and the immersion time is 5-40 seconds.

2. The zinc-aluminum-magnesium coated steel sheet as described in claim 1, characterized in that, The thickness of the chemical conversion film is 20-500 nm.

3. The zinc-aluminum-magnesium coated steel sheet as described in claim 1, characterized in that, The surface roughness Ra value of the zinc-aluminum-magnesium coating is 0.6-1.8 μm.

4. The zinc-aluminum-magnesium coated steel sheet as described in claim 1, characterized in that, The zinc-aluminum-magnesium coating contains Mg: 0.8-4.0wt% and Al: 1.0-7.0wt%.

5. The zinc-aluminum-magnesium coated steel sheet as described in claim 4, characterized in that, The zinc-aluminum-magnesium coating contains Mg: 2.0-4.0 wt% and Al: 5.0-7.0 wt%.

6. The zinc-aluminum-magnesium coated steel sheet as described in claim 4, characterized in that, The zinc-aluminum-magnesium coating contains Mg: 0.8-3.5wt% and Al: 1.0-5.0wt%.

7. The zinc-aluminum-magnesium coated steel sheet as described in claim 1, characterized in that, The chemical conversion membrane also has a hydrophilic organic membrane layer, which has polar functional groups.

8. The zinc-aluminum-magnesium coated steel sheet as described in claim 7, characterized in that, The polar functional groups include at least three carboxyl functional groups or two hydroxyl functional groups.

9. The zinc-aluminum-magnesium coated steel sheet as described in claim 7, characterized in that, The organic film layer contains acrylic resin or epoxy resin.

10. The zinc-aluminum-magnesium coated steel sheet as described in claim 7, characterized in that, The thickness of the organic film layer is 20-200 nm.

11. The zinc-aluminum-magnesium coated steel sheet as described in claim 1, characterized in that, Under harsh alkaline degreasing conditions, the degreasing and water retention rate after degreasing and washing is >95%, wherein the harsh alkaline degreasing conditions include the presence of 2-4 g / L of rust-preventive oil in the alkaline degreasing solution.

12. The method for manufacturing zinc-aluminum-magnesium coated steel sheet according to any one of claims 1-6, 11, characterized in that, Including the following steps: The substrate is hot-dip coated to obtain zinc-aluminum-magnesium coated steel sheet; Zinc-aluminum-magnesium coated steel sheets are immersed in acidic or alkaline electrolyte solutions for surface treatment, and then dried to form a hydrophilic chemical conversion film on the surface of the zinc-aluminum-magnesium coating. When using an acidic electrolyte solution, the pH value is 2-6 and the immersion time is 5-40 seconds. When using an alkaline electrolyte solution, the pH value is 8-13 and the immersion time is 5-40 seconds.

13. The manufacturing method as described in claim 12, characterized in that, The acidic electrolyte solution contains at least one of the following: permanganic acid, hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, selenic acid, hydrobromic acid, hydroiodic acid, chloric acid, carbonic acid, acetic acid, sulfurous acid, hypochlorous acid, hydrosulfuric acid, silicic acid, aluminic acid, formic acid, acetic acid, oxalic acid, tartaric acid, citric acid, and salicylic acid.

14. The manufacturing method as described in claim 12, characterized in that, The alkaline electrolyte solution contains at least one of the following: sodium hydroxide, potassium hydroxide, barium hydroxide, sodium pyrophosphate, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, potassium citrate, and ammonia.

15. The manufacturing method as described in claim 12, characterized in that, The surface treatment step is followed by an oiling step.

16. The manufacturing method as described in claim 15, characterized in that, During the oiling process, the amount of oil applied to one side is 300-1800 mg / m². 2 .

17. The method for manufacturing zinc-aluminum-magnesium coated steel sheet according to any one of claims 7-10, characterized in that, Including the following steps: The substrate is hot-dip coated to obtain zinc-aluminum-magnesium coated steel sheet; Zinc-aluminum-magnesium coated steel sheets are immersed in acidic or alkaline electrolyte solutions and then dried to form a hydrophilic chemical conversion film on the surface of the zinc-aluminum-magnesium coating. When using an acidic electrolyte solution, the pH value is 2-6 and the immersion time is 5-40 seconds. When using an alkaline electrolyte solution, the pH value is 8-13 and the immersion time is 5-40 seconds. The water-based treatment agent is coated onto the surface of the zinc-aluminum-magnesium coating obtained in the previous step and then heated to cure, so as to form a hydrophilic organic film layer on the surface of the chemical conversion film.

18. The manufacturing method as described in claim 17, characterized in that, The water-based treatment agent contains: Water-soluble acrylic resin or water-soluble epoxy resin 25~50wt%; 5-15 wt% nano-silica 5-10 wt% silane coupling agent.

19. The manufacturing method as described in claim 18, characterized in that, The mass percentage ratio of each component in the water-based treatment agent is as follows: Water-soluble acrylic resin or water-soluble epoxy resin 25~50wt%; 5-15 wt% nano-silica 5-10 wt% silane coupling agent; The remainder consists of water and other unavoidable impurities.

20. The manufacturing method as described in claim 17, characterized in that, The curing temperature is 100-140℃, and the curing time is 5-15 seconds.

21. The manufacturing method as described in claim 17, characterized in that, The process includes an oiling step following the heat curing step.

22. The manufacturing method as described in claim 21, characterized in that, During the oiling process, the amount of oil applied to one side is 300-1800 mg / m². 2 .

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