Zinc-aluminum-magnesium coated steel sheet and its manufacturing method

Abstract

A zinc-aluminum-magnesium plated steel sheet and a method for manufacturing the same. The steel sheet includes a steel substrate and a plating layer provided on the steel substrate, the surface of which is coated with an anti-fingerprint film, and the plating layer contains the following components in mass percent: Al: 45% to 65%, Mg: 0.2% to 5.0%, Si: 0.1% to 3.0%, Zr: 0.001% to 0.15%, Ca: 0.001% to 0.15%, and the balance being Zn and unavoidable impurities.

Description

[Technical Field]

[0001] The present invention relates to steel and methods for producing the same, and in particular to plated steel sheets and methods for producing the same. [Background technology]

[0002] background Currently, in order to prepare steel sheet products with longer service lives, the market and users have increasingly higher requirements for the corrosion resistance of steel materials. Zinc-aluminum-magnesium coated steel sheets have been recognized by the market for their excellent corrosion resistance.

[0003] In recent years, zinc-aluminum-magnesium coated steel sheet products have appeared on the market, which have a high aluminum content in the coating layer and offer improved corrosion resistance. However, such zinc-aluminum-magnesium coated steel sheet products are prone to "blackening," i.e., the surface of the steel sheet is prone to blackening during production, storage, transportation, or use, which affects the appearance and use of the steel sheet products.

[0004] Currently, there are differences between domestic and foreign manufacturers in the technological paths used to produce zinc-aluminum-magnesium-coated steel sheets. Overseas manufacturers mainly use a two-step process for post-treatment coatings on the surface of zinc-aluminum-magnesium-coated steel sheets, i.e., first passivating the surface of the coating layer and then coating the required coating on the surface of the coating layer. The final zinc-aluminum-magnesium-coated steel sheets have better blackening resistance. In contrast, domestic manufacturers of zinc-aluminum-magnesium-coated steel sheets use a one-step process for post-treatment coatings, i.e., coating the required coating on the surface of the coating layer in a single step. This one-step coating process is highly efficient, which not only reduces capital investment but also saves costs, and has good economic advantages and application prospects.

[0005] However, the development of post-treatment films and the control of the coating process currently face practical challenges. Zinc-aluminum-magnesium coated steel sheets produced by the above-mentioned one-step coating process are prone to blackening, especially under conditions such as humid heat, stacking, and rain washing. Summary of the Invention

[0006] overview To solve the above-mentioned problems existing in the prior art, the present invention provides a zinc-aluminum-magnesium coated steel sheet having excellent resistance to blackening, particularly remarkable resistance to blackening under high temperature and humidity conditions, which can overcome the problem that existing zinc-aluminum-magnesium coated steel sheet products are prone to blackening, ensure good appearance during long-term use, and meet the usage needs of users.

[0007] A first aspect of the present invention provides a zinc-aluminum-magnesium plated steel sheet comprising a steel substrate and a plating layer provided on the steel substrate, wherein the surface of the plating layer is coated with an anti-fingerprint film; and the plating layer contains the following components in mass percent: Al: 45% to 65%, Mg: 0.2% to 5.0%, Si: 0.1% to 3.0%, Zr: 0.001% to 0.15%, Ca: 0.001% to 0.15%, and the balance being Zn and unavoidable impurities.

[0008] It is known that the mass of the plating layer on the surface of the steel substrate directly affects the blackening resistance of the plated steel sheet. Therefore, in the present invention, the inventors have optimized and designed the chemical composition of the plating layer.

[0009] The design principles of each chemical element in the coating layer of the zinc-aluminum-magnesium coated steel sheet of the present invention are as follows:

[0010] Al: Al element is easily oxidized, and the oxide film is very dense. The formed oxide film of the aluminum-rich phase can provide the plating layer with excellent protective function, thus imparting excellent corrosion resistance to the plating layer. Therefore, to achieve the beneficial effects of Al element, the mass percent content of Al element in the plating layer is controlled to be between 45% and 65% in the present invention.

[0011] Mg: Mg element can corrode together with Zn to form corrosion products, and the corrosion products of Mg are denser and have better protection than the corrosion products of pure Zn, which can improve the corrosion resistance of the coating layer. Therefore, to ensure the corrosion resistance of the coating layer, the mass percent content of Mg element in the coating layer is controlled to be between 0.2% and 5.0% in the present invention.

[0012] Si: In the coating layer of the zinc-aluminum-magnesium-coated steel sheet of the present invention, adding an appropriate amount of Si can reduce the reaction between the substrate and Al in the liquid hot-dip coating bath during the formation of the coating layer, thereby ensuring the flatness of the substrate and the uniformity of the coating layer. Based on this, in the present invention, the mass percent content of Si element in the coating layer is controlled to be between 0.1% and 3.0%.

[0013] Zr: An appropriate amount of Zr element needs to be added to the coating layer of the zinc-aluminum-magnesium-plated steel sheet of the present invention. During the formation of the coating layer, when the molten coating layer alloy cools, Zr element can cooperate with Al element to form AlZr particles, which participate in the peritectic reaction to refine the aluminum-rich phase on the surface of the steel sheet, increase the aluminum-rich phase on the surface of the steel sheet, and enhance the Al / Zn ratio on the surface of the coating layer, thereby improving the corrosion resistance of the coating layer. Therefore, to achieve the beneficial effects of Zr element, the mass percent content of Zr in the coating layer in the present invention is controlled to be between 0.001% and 0.15%.

[0014] Ca: The role of the Ca element is to inhibit the formation of oxides on the surface of the liquid hot-dip coating bath, reduce the amount of slag produced, and thereby reduce surface quality defects of the coated steel sheet caused by the oxides and slag in the coating bath. Therefore, to achieve the beneficial effects of the Ca element, the mass percent content of Ca in the coating layer is controlled to be between 0.001% and 0.15% in the present invention.

[0015] Preferably, in the zinc-aluminum-magnesium plated steel sheet of the present invention, the mass percent contents of each component in the plating layer are as follows: Al: 50%~60%, Mg: 1.0%~3.0%, Si: 1.0% to 2.0% Zr: 0.01% to 0.1%, Ca: 0.01% to 0.1% At least one of the following is satisfied.

[0016] Preferably, in the zinc-aluminum-magnesium plated steel sheet of the present invention, the plated layer has a thickness of 8 to 38 μm.

[0017] In the above-mentioned technical solution of the present invention, the thickness of the coating layer is controlled to be 8 to 38 μm. When the thickness of the coating layer is less than 8 μm, the corrosion resistance of the coating layer is poor, the protection of the substrate is insufficient, and the uniformity of the coating layer covering the substrate is reduced. To obtain a coated steel sheet with excellent surface quality, the requirements for the substrate and the coating layer thickness control equipment are very high, which leads to manufacturing difficulties. When the thickness of the coating layer is more than 38 μm, the requirements for the coating layer thickness control equipment become extremely high, and the uniformity of the coating layer is difficult to ensure, making it difficult to manufacture a zinc-aluminum-magnesium coated steel sheet with good surface quality.

[0018] Preferably, in the zinc-aluminum-magnesium-plated steel sheet of the present invention, the anti-fingerprint film has a thickness of 0.5 to 2.5 μm. In the solution of the present invention, the composition of the anti-fingerprint film is not particularly limited and can be a water-soluble resin commonly used in the prior art, such as an acrylic resin or a polyurethane resin. The coating method of the anti-fingerprint film is also not particularly limited and can be formed by spraying, roll coating, electrophoresis, and other methods. The anti-fingerprint film can improve the corrosion resistance of the plated steel sheet and enhance the appearance of the steel sheet.

[0019] In the above-described technical solution of the present invention, the thickness of the anti-fingerprint film coated on the plating layer surface is controlled to be between 0.5 and 2.5 μm. When the thickness of the anti-fingerprint film is less than 0.5 μm, the coverage of the anti-fingerprint film on the plated steel sheet is reduced, which causes localized chipping of the coating, affecting the corrosion resistance, blackening resistance, formability, and other properties of the plated steel sheet. When the thickness of the anti-fingerprint film is more than 2.5 μm, the requirements for film thickness control equipment become extremely high, and the uniformity of the anti-fingerprint film is difficult to ensure. The requirements for the subsequent heating, baking, and curing processes are also very high, and the electrical conductivity and other properties of the anti-fingerprint film will also deteriorate.

[0020] More preferably, in the zinc-aluminum-magnesium plated steel sheet of the present invention, the fingerprint-resistant film has a thickness of 1.0 to 2.0 μm.

[0021] Preferably, in the zinc-aluminum-magnesium plated steel sheet of the present invention, the surface color difference ΔE of the zinc-aluminum-magnesium plated steel sheet before and after 24 hours of lamination and moist heat treatment is <3, where ΔE is the difference between the surface lightness value of the zinc-aluminum-magnesium plated steel sheet before the lamination and moist heat treatment and the surface lightness value of the zinc-aluminum-magnesium plated steel sheet after 24 hours of the moist heat treatment.

[0022] In the technical solution of the present invention, the laminated heat-moisture treatment involves stacking steel sheets under conditions of relatively high temperature (50°C) and relatively high humidity (95%). Under these conditions, there are small gaps between the stacked steel sheets. Under the combined action of water vapor and oxygen, the steel sheets are subject to electrochemical corrosion. If the steel sheet has poor blackening resistance, the surface layer of the steel sheet will corrode, which will cause localized or overall blackening of the steel sheet, i.e., blackening problems.

[0023] The inventors have found that if a steel sheet has poor blackening resistance, the steel sheet will also undergo blackening under conditions of water vapor, rainwater, or dew during storage or use, but this is a long and slow process. It takes a long time to detect the blackening problem of the steel sheet. Furthermore, the inventors have also found that the laminating moist heat conditions are relatively harsh conditions with relatively high temperatures, high humidity, water vapor, and small gaps created by the laminating process, which create conditions for electrochemical corrosion and lead to accelerated corrosion of the surface layer of the steel sheet. If the steel sheet has poor blackening resistance, the blackening problem of the steel sheet can be detected within 24 hours under the laminating moist heat conditions, and the blackening risk of the steel sheet can be monitored.

[0024] Furthermore, the inventors have also found that when a steel sheet undergoes blackening, the lightness value of the steel sheet surface decreases. In the present invention, the color difference ΔE is the difference between the surface lightness value of the steel sheet before lamination and moist heat treatment and the surface lightness value after 24 hours of moist heat treatment. ΔE≧3 indicates that the blackening phenomenon is visible to the naked eye, especially when the steel sheet undergoes localized blackening.

[0025] The zinc-aluminum-magnesium coated steel sheet designed according to the present invention has excellent resistance to blackening under moist heat conditions: the surface color difference ΔE of the zinc-aluminum-magnesium coated steel sheet before and after lamination and treatment under moist heat conditions for 24 hours is <3, i.e., no blackening phenomenon is visible to the naked eye.

[0026] Correspondingly, another object of the present invention is to provide a method for producing the above-mentioned zinc-aluminum-magnesium coated steel sheet, which is simple and can be used to efficiently prepare the above-mentioned zinc-aluminum-magnesium coated steel sheet.

[0027] The present invention relates to a method for producing a method for manufacturing a semiconductor device comprising the steps of: (1) Pretreatment of the steel substrate; (2) supplying the pretreated steel substrate into a plating bath and performing hot-dip galvanizing in the plating bath to obtain a plated steel sheet; (3) After the plated steel sheet leaves the plating bath, the following stepwise cooling is carried out: the first step is air cooling, in which the steel sheet is cooled to 300°C or less, and the cooling rate of the first step is 10-60°C / sec; the second step is roll cooling or roll cooling + air cooling to cool the steel sheet to 100°C or less, and the cooling rate of roll cooling is 60-100°C / sec; the third step is water cooling to room temperature in a quenching water bath, and the sheet temperature is lower than 100°C when the plated steel sheet enters the quenching water bath; optionally, squeezing and drying the steel sheet thereafter; (4) coating an anti-fingerprint film on the surface of the plated steel sheet, and heating and baking at 100 to 160°C to harden the film; (5) cooling the plated steel sheet to room temperature; (6) Winding the plated steel sheet to form a finished coil. The present invention relates to a method for producing a zinc-aluminum-magnesium plated steel sheet, comprising:

[0028] In the above-mentioned manufacturing method, the pretreatment in step (1) may include: cleaning the steel substrate to remove dirt and grease from the surface of the steel substrate, and then performing reduction annealing on the cleaned and degreased steel substrate to remove oxides from the surface of the steel substrate.

[0029] In the above-mentioned step (2), the steel substrate is supplied into a hot-dip galvanizing bath, and the chemical composition of the coating bath is substantially the same as that of the steel substrate coating layer. The chemical elements of the steel substrate coating layer can be controlled by controlling the chemical elements of the coating bath.

[0030] In the above-mentioned step (3), after the plated steel sheet leaves the plating bath, it first passes through an air knife, and the thickness of the plating layer on the steel sheet is controlled within the range of production requirements by blowing nitrogen, and the plating layer is cooled to a certain extent. Thereafter, the plated steel sheet needs to undergo stepwise cooling, where: The first stage is air cooling through an air box to cool the steel plate to below 300°C, and the cooling rate is 10-60°C / s; The second stage employs roll cooling or roll cooling plus air cooling, where the cooling rate of roll cooling is 60-100°C / sec, and the steel plate is cooled to 100°C or less in the second stage so that the plate temperature is lower than 100°C when the steel plate enters the quenching water tank.

[0031] After the steel plate has cooled to below 100°C, it undergoes the third stage of cooling, i.e., it enters a quenching water bath for water cooling.

[0032] If the temperature of the steel sheet is above 100°C when it enters the quenching water bath, the plating layer on the surface of the steel sheet will react with water, causing corrosion of the surface of the plating layer, which will affect the coating effect of the subsequent anti-fingerprint film, and the finished steel sheet will be more prone to blackening problems under moist and hot conditions. Therefore, the temperature of the steel sheet should be controlled below 100°C when it enters the quenching water bath.

[0033] Preferably, the steel plate is squeezed and dried after the stepwise cooling.

[0034] Furthermore, in step (4) of the manufacturing method of the present invention, the anti-fingerprint film can be coated on the surface of the plated steel sheet by roll coating, and the thickness of the anti-fingerprint film can be specifically controlled according to manufacturing requirements. After the anti-fingerprint film is coated on the surface of the steel sheet, it can be heated and baked at 100 to 160°C using electromagnetic induction heating to harden the anti-fingerprint film.

[0035] In the present invention, the heating and baking temperature should be controlled between 100 and 160°C. If the heating and baking temperature is lower than 100°C, the anti-fingerprint film cannot be baked and cured sufficiently, the internal crosslinking and curing degree of the anti-fingerprint film will be incomplete, and the waterproof performance and other properties will be deteriorated. External water vapor is more likely to pass through the anti-fingerprint film and come into contact with and react with the plating layer underneath, causing corrosion of the plating layer surface and blackening of the plated steel sheet. This is particularly prone to blackening problems under conditions such as water infiltration into steel coils, condensation, water infiltration during stacking of pressed sheets, and humid and hot lamination conditions. If the heating and baking temperature is higher than 160°C, the anti-fingerprint film will be over-baked, which will result in deterioration of the anti-fingerprint film's performance. Furthermore, excessively high heating and baking temperatures impose high demands on the equipment for the cooling process after heating and baking, which leads to high energy consumption and increased production costs.

[0036] Preferably, in the production method of the present invention, in step (2), the temperature of the plating bath is 570 to 610°C. As an implementation method, in step (3), the second stage is roll cooling + air cooling, where the cooling rate of the roll cooling is 60 to 100°C / sec, and the cooling rate of the air cooling is 20 to 30°C / sec.

[0037] When the temperature of the coating bath is higher than 610°C, the reaction between the Fe in the steel substrate and the Al in the coating bath becomes severe after the steel substrate enters the coating bath, leading to a thickening of the alloy layer, mainly composed of Al and Fe, formed at the interface between the coating layer and the steel substrate, which will result in a deterioration of the workability of the coated steel sheet. Furthermore, when the temperature of the coating bath is too high, the dissolution of Fe in the steel substrate in the coating bath becomes severe, and the evaporation of Zn in the neck also becomes severe, which results in an increase in bottom slag and surface slag in the coating bath and an increase in zinc ash in the neck, which leads to a decrease in the surface quality of the coated steel sheet, increased zinc consumption, and higher production costs. Furthermore, when the temperature of the coating bath is too high, the corrosion of the cooling rollers by the coating bath also becomes severe, which will shorten the service life of the roller system equipment and increase production risks.

[0038] When the temperature of the plating bath is lower than 570°C, the fluidity of the plating bath decreases, making it difficult to control the thickness of the plating layer and thereby ensuring the uniformity and surface quality of the plating layer. Furthermore, if the temperature of the plating bath is too low, after the steel substrate enters the plating bath, the alloy layer formed at the plating layer-substrate interface by the reaction between Fe in the substrate and Al in the plating bath will be incomplete or thin, thereby affecting the adhesion of the plating layer and the formability of the plated steel sheet. Therefore, taking into account the influence of the temperature of the plating bath, in the manufacturing method of the present invention, the temperature of the plating bath is preferably controlled to be between 570 and 610°C.

[0039] In the production method of the present invention, the heating and baking are preferably carried out at 110 to 140°C in step (4).

[0040] Preferably, in the production method of the present invention, in step (4), the heating and baking are carried out by electromagnetic induction heating.

[0041] Preferably, in the production method of the present invention, in step (5), the cooling is carried out by air cooling, and the cooling rate of the plated steel sheet is 5 to 20° C. / second.

[0042] In the cooling step of step (5), the cooling rate of the steel sheet is preferably controlled to be 5 to 20°C / s. If the cooling rate is higher than 20°C / s, the requirements for the cooling device are very high and manufacturing is difficult to achieve. Furthermore, if the cooling rate of the air cooling is too high, the fan flow is very large and the steel strip vibrates significantly, which makes it impossible to accurately measure the thickness of the anti-fingerprint film and does not lead to stable control of the film thickness. If the cooling rate is lower than 5°C / s, the anti-fingerprint film cannot be cooled sufficiently, which leads to aging of the film and deterioration of the performance of the anti-fingerprint film.

[0043] Compared with the prior art, the zinc-aluminum-magnesium coated steel sheet and its manufacturing method have the following advantages and beneficial effects:

[0044] The present invention provides a new zinc-aluminum-magnesium-plated steel sheet with excellent blackening resistance. The steel substrate surface of the zinc-aluminum-magnesium-plated steel sheet is coated with a plating layer having an optimized and designed composition, and the plating layer surface is also coated with an anti-fingerprint film. The surface color difference ΔE before and after 24 hours of treatment under moist heat conditions is <3, demonstrating excellent blackening resistance and overcoming the blackening problem of existing zinc-aluminum-magnesium-plated steel sheet products. The zinc-aluminum-magnesium-plated steel sheet of the present invention also has good appearance after long-term use, which can enhance the user experience and promote the healthy development of zinc-aluminum-magnesium-plated steel sheet products.

[0045] In addition to the advantages mentioned above, the present invention also optimizes and designs a manufacturing method for zinc-aluminum-magnesium coated steel sheet, which is simple and can effectively improve the blackening resistance of the steel sheet through the control of the post-treatment coating process, heating and baking process, and cooling process, ensuring that the prepared zinc-aluminum-magnesium coated steel sheet has excellent blackening resistance. DETAILED DESCRIPTION OF THE INVENTION

[0046] Detailed Description The zinc-aluminum-magnesium coated steel sheet and its manufacturing method according to the present invention will be further described and illustrated in conjunction with specific embodiments below, but the descriptions and illustrations do not unduly construe limitations on the technical solutions of the present invention.

[0047] Examples A1 to A9 and Comparative Examples B1 to B4 The zinc-aluminum-magnesium plated steel sheets of Examples A1 to A9 were produced using the following steps (1) to (6). (1) Pretreatment of the steel substrate: A cold-rolled hard steel plate with a thickness of 0.6 mm was used as the substrate. After cleaning and degreasing, it was annealed at 770°C for 2 minutes under a protective atmosphere of N2-5vol%H2. (2) Pretreated steel substrate immersed in a plating bath and hot-dip galvanized in the plating bath: The steel substrate was immersed in a plating bath for hot-dip galvanization, and the temperature of the plating bath was controlled at 570-610°C. After immersion plating for 3 seconds, a plated steel sheet was obtained. The chemical compositions of the plating baths used in each example and comparative example are shown in Table 1. (3) After the coated steel sheet left the coating bath, it passed through an air knife, and the thickness of the coating layer was controlled by adjusting the air knife's blowing strength. Then, staged cooling was performed: the first stage was air cooling via an air box to cool the steel sheet to below 300°C, and the cooling rate of the steel sheet was 10-60°C / s; the second stage was roll cooling or roll cooling plus air cooling to cool the steel sheet to below 100°C, with a roll cooling rate of 60-100°C / s and an air cooling rate of 20-30°C / s; and the third stage was water cooling in a quenching water bath. When the steel sheet entered the quenching water bath, the sheet temperature was below 100°C, and the final cooling temperature was room temperature. It should be noted that Comparative Examples B1-B3 only used air cooling and water cooling. The manufacturing process of Comparative Example B4 complied with the limitations of the present invention. (4) Coating a plated steel sheet with an anti-fingerprint film by a roll coating machine, controlling the thickness of the anti-fingerprint film by adjusting the rotation speed and roller gap of the roll coating machine; after the anti-fingerprint film is coated on the surface of the plated steel sheet, the steel sheet is passed through an induction heating device and heated at 100 to 160°C, so that the anti-fingerprint film is heated, baked, and hardened by electromagnetic induction heating. (5) The steel plate is air-cooled to room temperature via a fan while controlling the cooling rate of the steel plate to be 5 to 20°C / second. (6) Winding the steel sheet into a finished coil when it has cooled to or near room temperature.

[0048] The specific parameters of the above process are listed in Table 2.

[0049] It should be noted that the type of steel substrate used is not particularly limited in the present invention. In practical applications, those skilled in the art can select according to requirements, such as commonly used DC51 steel sheet and S350 steel sheet. The specific type of steel substrate does not have any direct correlation with the excellent blackening resistance of the plating layer formed on its surface.

[0050] Table 1 lists the plating baths used and the mass percent ratios of chemical elements in the formed plating layers for the zinc-aluminum-magnesium plated steel sheets of Examples A1 to A9 and the comparative steels of Comparative Examples B1 to B4. In the present invention, the plating layer is formed by cooling the plating solution on the substrate, and the composition of the plating layer substantially matches the composition of the plating bath.

[0051] [Table 1]

[0052] Table 2 lists the specific process parameters used in the above process steps (1) to (6) for the zinc-aluminum-magnesium plated steel sheets of Examples A1 to A9 and the comparative steel sheets of Comparative Examples B1 to B4.

[0053] [Table 2]

[0054] The finished steel coils obtained in Examples A1 to A9 and Comparative Examples B1 to B4 were sampled, and the steel coil samples of each Example and Comparative Example were subjected to moist heat resistance tests simultaneously under the same conditions. The steel sheets were stacked and treated under moist heat conditions for 24 hours. The surface lightness values ​​and color difference ΔE of the plated steel sheets of each Example and Comparative Example before and after the test are listed in Table 3.

[0055] The specific procedure for the moist heat resistance test was as follows: before the test, the surface brightness of the steel sheets to be tested was measured, and then the steel sheets were stacked together. A preload of 30 N·m was applied to the clamps, and the stacked steel sheets were fixed. The fixed steel sheets were placed in a moist heat box at 50°C and 95% humidity. The fixed steel sheets were kept in the moist heat box for 24 hours, removed from the box, and remeasured for surface brightness. This test can effectively detect changes in the surface brightness of the steel sheets of Examples A1 to A9 and Comparative Examples B1 to B4 before and after the 24-hour laminate moist heat treatment test.

[0056] Furthermore, after the above test, the steel plate surface of each example and comparative example was further visually observed to confirm whether the steel plate surface had turned black, i.e., whether "blackening" had occurred. "Yes" was recorded when "blackening" had occurred; "No" was recorded when no "blackening" had occurred, which indicated that the steel plate still had good blackening resistance performance under moist heat conditions. The relevant results are listed in Table 3 below.

[0057] Table 3 lists the results of the moist heat resistance test of the finished steel coils of Examples A1 to A9 and Comparative Examples B1 to B4.

[0058] [Table 3]

[0059] Under the same conditions, after the moist heat resistance test, the zinc-aluminum-magnesium-plated steel sheets of the present invention had higher brightness values ​​than the comparative steel sheets, and the surface brightness values ​​of the steel sheets after the moist heat resistance test were between 68.49 and 72.08, as can be seen from Table 3. Furthermore, the surface color differences ΔE of the zinc-aluminum-magnesium-plated steel sheets of Examples A1 to A9 before and after 24 hours of treatment under laminated moist heat conditions were 0.16 to 2.35, all <3, and no blackening was observed on the surface.

[0060] In contrast, the surface color difference ΔE of Comparative Examples B1 to B4 before and after 24 hours of treatment under moist heat conditions was >3. Furthermore, the surfaces of the steel sheets of Comparative Examples B1 to B4 developed blackening and blackening problems after the moist heat resistance test. Comparative Examples B1 to B4 did not use the plating bath of the present invention to form the steel sheet plating layer, which resulted in poor moist heat resistance and blackening resistance.

[0061] In summary, the zinc-aluminum-magnesium coated steel sheet prepared by the technical solution of the present invention has excellent resistance to blackening, especially under laminated moist heat conditions. The zinc-aluminum-magnesium coated steel sheet of the present invention has a wide range of applications, can effectively meet the current market and user needs, and has good promotion prospects and application value.

[0062] It should be noted that the combination of technical features in this case is not limited to the combinations recorded in the claims or the combinations described in the specific embodiments. All technical features recorded in this case can be freely combined or related in any way as long as there is no contradiction between them.

[0063] It should also be noted that the above listed embodiments are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and all similar changes or modifications that can be directly derived or easily envisioned by those skilled in the art from the content disclosed by the present invention should fall within the protection scope of the present invention.

Claims

1. A zinc-aluminum-magnesium plated steel sheet comprising: a steel substrate; and a plating layer provided on the steel substrate, wherein the surface of the plating layer is coated with an anti-fingerprint film; and the plating layer contains the following components in mass percent: Al: 45% to 65%, Mg: 0.2% to 5.0%, Si: 0.1% to 3.0%, Zr: 0.001% to 0.15%, Ca: 0.001% to 0.15%, and the balance being Zn and unavoidable impurities.

2. The mass percent content of each component in the plating layer is as follows: Al: 50% to 60%, Mg: 1.0% to 3.0%, Si: 1.0% to 2.0%, Zr: 0.01% to 0.1%, Ca: 0.01% to 0.1% The zinc-aluminum-magnesium plated steel sheet according to claim 1, which satisfies at least one of the following conditions.

3. The zinc-aluminum-magnesium plated steel sheet according to claim 1, wherein the plating layer has a thickness of 8 to 38 μm.

4. 2. The zinc-aluminum-magnesium plated steel sheet according to claim 1, wherein the anti-fingerprint film has a thickness of 0.5 to 2.5 μm.

5. The zinc-aluminum-magnesium plated steel sheet according to claim 4, wherein the anti-fingerprint film has a thickness of 1.0 to 2.0 μm.

6. 6. The zinc-aluminum-magnesium-plated steel sheet according to claim 1, wherein the zinc-aluminum-magnesium-plated steel sheet has a surface color difference ΔE of <3 before and after 24 hours of treatment under laminating moist heat conditions, wherein ΔE is the difference between the surface lightness value before the laminating moist heat treatment and the surface lightness value after the 24 hours of the moist heat treatment.

7. 7. A method for producing a zinc-aluminum-magnesium plated steel sheet according to any one of claims 1 to 6, wherein the method comprises the following steps, carried out in sequence: (1) Pre-treating the steel substrate; (2) supplying the pretreated steel substrate into a plating bath and performing hot-dip galvanizing in the plating bath to obtain a plated steel sheet; (3) After the plated steel sheet leaves the plating bath, stepwise cooling is carried out as follows: the first step is air cooling, in which the steel sheet is cooled to 300°C or less, and the cooling rate of the first step is 10-60°C / sec; the second step is roll cooling or roll cooling + air cooling, in which the cooling rate of roll cooling is 60-100°C / sec; the third step is water cooling in a quenching water tank, and the sheet temperature is lower than 100°C when the plated steel sheet enters the quenching water tank; (4) Coating an anti-fingerprint film on the surface of the plated steel sheet, and heating and baking at 100-160°C to harden the film; (5) cooling the plated steel sheet to room temperature; (6) Winding the plated steel sheet to form a finished coil. A method comprising:

8. The method according to claim 7, wherein in step (2), the temperature of the coating bath is 570 to 610°C; and / or in step (3), the second stage is roll cooling plus air cooling, wherein the cooling rate of the roll cooling is 60 to 100°C / sec and the cooling rate of the air cooling is 20 to 30°C / sec.

9. 8. The method according to claim 7, wherein in step (4), the heating and baking are carried out at 110 to 140°C.

10. 8. The method according to claim 7, wherein in step (4), the heating and baking are carried out by electromagnetic induction heating.

11. The method according to claim 7, wherein in step (5), the cooling is carried out by air cooling, and the cooling rate of the plated steel sheet is 5 to 20°C / sec.

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