Zinc-aluminum-magnesium coated steel plate and manufacturing method therefor

Abstract

A zinc-aluminum-magnesium coated steel plate, comprising a cold-rolled substrate and a coating coated on the cold-rolled substrate, the coating containing Zn and the following chemical elements in percentage by mass: 1-30.0% of Al, 1-10.0% of Mg, 0.01-0.5% of Ca, 0.01-0.5% of Sr, 0<B≤0.05%, 0<Cr≤0.3%, 0.001-0.3% of Ti, and 0.001-1.0% of Ni. The manufacturing method for the zinc-aluminum-magnesium coated steel plate comprises the steps: performing continuous annealing and hot dipping on a cold-rolled substrate; using jet cooling; heating; air-cooling strip steel to below 320℃; further cooling the strip steel to below 100℃, and then cooling in a quenching tank; and leveling and straightening.

Description

Zinc-aluminum-magnesium coated steel plate and manufacturing method thereof Technical Field

[0001] The present invention relates to a coated steel plate and a manufacturing method thereof, in particular to a zinc-aluminum-magnesium coated steel plate and a manufacturing method thereof. Background Art

[0002] Zinc-aluminum-magnesium (Zn-Aluminum-Magnesium) coated steel sheets are gaining widespread application due to their excellent corrosion resistance. The widespread use of these coatings for paint-free exterior parts makes controlling the coating's appearance quality crucial. However, due to the rich and complex microstructure of Zn-aluminum-magnesium coatings, controlling their appearance quality remains challenging. In particular, when the Al, Mg, and Zn contents of the coating fall near the eutectic point of the ternary equilibrium phase diagram, black circular speckle defects may form.

[0003] The formation of black spot defects is attributed to the abnormal crystallization of the zinc-aluminum-magnesium coating in a local area, which is rich in Mg2Zn 11 The eutectic structure of the phase has a different visible light reflectivity from its surrounding area, so it appears visually black.

[0004] In the prior art, there have been some solutions that attempt to control black spot defects by adjusting the process or coating composition.

[0005] For example, the Chinese patent document with publication number CN1193113C and publication date of March 16, 2005, entitled “Melted Zn-Al-Mg Electroplated Steel Sheet with Good Corrosion Resistance and Surface Appearance and Preparation Method Thereof” discloses that by optimizing the plating bath temperature range and cooling rate, adjusting the composition of the plating bath, selecting the composition range of the hypereutectic and adding trace elements to suppress the Mg2Zn 11 The formation of Mg2Zn 11 The spot size of the phase-affected area becomes minute.

[0006] In addition, a Chinese patent document with publication number CN110760774B, publication date February 1, 2022, and titled "Preparation method for zinc-aluminum-magnesium steel plate and effective control of black spots on the surface of hot-dip zinc-aluminum-magnesium steel plate in CSP process" discloses that the occurrence of black spot defects can be curbed by controlling the chemical composition of the coating and the substrate, optimizing the immersion plating process, air knife parameters, post-plating cooling process and the purity of the gas source.

[0007] However, in the face of the actual production environment, even at the same zinc pot temperature, the actual cooling rates of the strip steel are different for different coating thicknesses, running speeds, and coating adhesion amounts, resulting in the need to adopt different post - plating cooling air volume controls for strip steels of different specifications, making the control of cooling parameters more complex. In addition, under conditions of different cooling rates, for strip steels of different specifications, the morphology of zinc alloy crystals on the surface is different, and color differences between specifications may also occur. Moreover, in the actual production of galvanizing, it is also difficult to completely remove impurity particles in the post - plating cooling channel line. Therefore, in the actual production environment, avoiding black - dot defects on the coating surface is still a major challenge. Summary of the Invention

[0008] One of the objectives of the present invention is to provide a zinc - aluminum - magnesium coated steel plate, which eliminates the black - dot defects on the surface of the coated steel plate, thereby enabling good appearance and surface quality to be obtained.

[0009] To achieve the above objective, the present invention provides a zinc - aluminum - magnesium coated steel plate, which comprises a cold - rolled substrate and a coating plated on the cold - rolled substrate. The coating contains Zn and chemical elements with the following mass percentages:

[0010] Al: 1.0 - 30.0%, Mg: 1.0 - 10.0%, and at least one of Ca: 0.01 - 0.5%, Sr: 0.01 - 0.5%, 0 < B ≤ 0.05%, 0 < Cr ≤ 0.3%, Ti: 0.001 - 0.3%, Ni: 0.001 - 1.0%.

[0011] Further, in the zinc - aluminum - magnesium coated steel plate of the present invention, the mass percentages of the chemical elements in the coating are: Al: 1.0 - 30.0%, Mg: 1.0 - 10.0%, and at least one of Ca: 0.01 - 0.5%, Sr: 0.01 - 0.5%, 0 < B ≤ 0.05%, 0 < Cr ≤ 0.3%, Ti: 0.001 - 0.3%, Ni: 0.001 - 1.0%, and the balance is Zn and inevitable impurities.

[0012] In the zinc - aluminum - magnesium coated steel plate of the present invention, the design principles of the chemical elements are specifically as follows:

[0013] Al: The present invention adds Al to the coating to improve the corrosion resistance of the coating, but too high an Al content will lead to a decrease in the Zn content in the coating, weakening the sacrificial protection (notch corrosion resistance) of the Fe substrate, and when the Al content is increased to about 25%, the plane corrosion resistance of the Zn-based coating will also begin to gradually decrease. With regard to black spot defects in the coating, in coatings with an Al content of ≤30.0%, the eutectic phase occupies a considerable proportion under equilibrium conditions, and a large number of black spot defects may occur if the process is not properly controlled. When the Al content is >30.0%, the plating solution temperature will increase significantly, which can easily cause a large number of surface zinc slag defects, while enhancing the reaction between the coating and the substrate, and significantly improving the thickness and hardness of the interface alloy layer. Therefore, the Al content range in the coating applicable to the present invention is limited to 1.0 to 30.0%.

[0014] Mg: The reason why zinc-aluminum-magnesium coatings have better corrosion resistance than zinc-aluminum coatings is that the Mg in the coating can evenly form stable and dense corrosion products with a certain fluidity. The presence of these corrosion products enables the steel plate processing cuts to have a "self-healing" mechanism. When the Mg added to the coating is ≥1.0%, a coating with significantly improved corrosion resistance can be obtained. When the Mg content is >10%, it is not only easy to cause a large amount of oxidized slag on the zinc pot liquid surface, but also easy to cause oxidation defects on the coating surface during the cooling and purging process. It is also easy to cause the coating itself to become more brittle, prone to cracking and shedding powder slag. Moreover, further increasing the Mg content will no longer have a further effect on improving the corrosion resistance of the coating. Therefore, the Mg content in the coating of the present invention should not be too high, and its content is controlled within the range of 1.0 to 10.0%.

[0015] Ca: The present inventors discovered that the addition of Ca and Mg has a similar effect on stabilizing corrosion products. During the solidification process of the plating solution, intermetallic compounds containing Ca and Mg are introduced into the Zn phase. In a corrosive environment, the Zn phase corrodes first, releasing Ca and Mg elements simultaneously, thereby more effectively stabilizing corrosion products in the early stages of corrosion. The inventors also discovered that granular intermetallic compounds formed by Mg and Ca, such as MgZn2, Mg2Si, Al2Ca, Al4Ca, and Al2CaSi2, can, when properly controlled, increase the hardness of the coating, thereby improving the scratch resistance of the coating. Furthermore, when the Al and Mg content increases, the plating bath temperature needs to be raised due to the solidification line, which increases oxidation at the bath surface. Adding a certain amount of Ca can better control Mg oxidation in the plating bath and on the coating surface. However, if the Ca content exceeds 0.5%, it can easily cause scum and form coating defects. Therefore, in the present invention, when Ca is added, the Ca content in the coating can be controlled to 0.01-0.5%, achieving better results.

[0016] Sr: In the present invention, the addition of 0.01-0.5% Sr can inhibit the oxidation reaction of zinc-aluminum-magnesium coatings. During the cooling process of the coating, the zinc, aluminum, and magnesium metal elements in the hot dip bath react with oxygen in the air at high temperatures, easily forming oxides, which results in an oxide film covering the surface of the coating. In the plating solution with the addition of Sr, the Sr element preferentially reacts with oxygen to form stable Sr oxides that are adsorbed onto the surface of the plating solution, thereby reducing the concentration of free oxygen in the plating solution and inhibiting the oxidation reaction of the zinc, aluminum, and magnesium metal elements. This reduces surface wrinkle and folding oxidation and maintains a smooth surface appearance. Therefore, in the present invention, when Sr is added, the Sr content in the coating can be controlled to 0.01-0.5% to achieve better results.

[0017] Ti and B: When the plating solution contains Ti and B, the coating structure can be further refined. The two elements can exist independently in the plating solution or in combination. When the Ti and B content is too high, Ti-Al, Al-B, and Ti-B system precipitates will be generated in the coating, resulting in fine particles on the coating and causing appearance defects of the coated steel sheet. Therefore, when Ti is added, the present invention controls the Ti content in the coating to be 0.001-0.3%. When B is added, the coating can be controlled to be 0.001-0.3%. <B≤0.05%。

[0018] Cr: Adding Cr to the zinc-aluminum-magnesium plating solution promotes the formation of a dense oxide, resulting in a coating with improved corrosion resistance. Cr also improves the coating's crystal structure, making the grain boundaries more uniform and compact, thereby enhancing the coating's density and corrosion resistance. However, excessive Cr addition is not recommended, as it can alter coating properties and react with Al to form zinc slag, which can negatively impact coating performance. Therefore, the Cr content in the coating of the present invention is controlled to 0-0.3%.

[0019] Ni: Ni has excellent corrosion resistance. Adding Ni to the plating solution can provide a Ni-O composite oxide film with excellent corrosion resistance. This oxide film can form a protective layer on the metal surface, preventing oxygen from entering the coating, thereby improving the corrosion resistance of the coating. Furthermore, the addition of Ni can form a Ni-Al phase with high thermal stability and a uniform grain structure, thereby improving the deformation and crack resistance of the coating. Therefore, in the present invention, when Ni is added, the Ni content in the coating can be controlled to 0.001-1.0% to achieve better corrosion and crack resistance.

[0020] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the inevitable impurities in the coating layer include Fe≤2.0%.

[0021] Fe: The Fe in the zinc pot originates from the dissolution of Fe on the steel plate by the reaction with the plating solution. It is an impurity in the plating solution. When the Fe content is greater than 2.0%, the impurities in the coating increase and the corrosion resistance deteriorates. Therefore, it is preferred to control the Fe content of the coating to ≤2.0%.

[0022] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the coating also contains 0 <Si≤2.0%。

[0023] In some embodiments of the present invention, Si is added to enhance adhesion at low zinc bath temperatures near the eutectic composition, while also suppressing interfacial Fe-Al reactions when high Al content is added. Conventional experience suggests that Si addition inhibits the growth of Fe2Al5 intermetallic compounds by blocking the diffusion pathways of Al atoms in the Fe2Al5 phase. However, the present invention discovers that, in addition to these effects, Si addition can also promote the dissolution of matrix Fe in the early stages of the Fe-Al reaction, facilitating the formation of continuous, dense Fe2Al5 intermetallic compounds at the substrate / plating bath interface. This is particularly true for hot-dip galvanized products near the eutectic point with an Al content between 5-7 wt.%. The addition of trace amounts of Si allows for production at lower zinc bath temperatures while ensuring good bonding between the coating and the substrate.

[0024] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the coating does not contain Mg2Zn 11 Mutually.

[0025] The zinc-aluminum-magnesium coated steel plate of the present invention can avoid the generation of black spot defects on the surface of the steel plate. The Mg2Zn 11 Very little, preferably no, Mg2Zn 11 .

[0026] Furthermore, in the zinc-aluminum-magnesium coated steel sheet of the present invention, the weight of the coating is 100 to 400 g / m 2 .

[0027] It should be noted that the present invention does not impose any particular restrictions on the substrate used for the zinc-aluminum-magnesium coated steel sheet. In practical applications, those skilled in the art may select CQ steel, IF steel, high-strength IF steel, bake-hardened steel, or other high-strength steel as the substrate for this application, as needed. This has no direct bearing on the improved surface quality of the coating obtained by the present invention. As described herein, the substrate is obtained by cold rolling. In some embodiments, the thickness of the substrate is 0.8 to 3.0 mm.

[0028] Another object of the present invention is to provide a method for manufacturing zinc-aluminum-magnesium coated steel plates, which eliminates black spot defects on the surface of the coated steel plates by adopting a specific post-plating cooling process. The method can be used to produce hot-dip zinc-aluminum-magnesium coated steel plates with good appearance quality and has good industrial application value.

[0029] In order to achieve the above object, the present invention also provides a method for manufacturing a zinc-aluminum-magnesium coated steel plate, comprising the steps of:

[0030] Continuous annealing and hot-dip coating of cold-rolled substrates;

[0031] Use jet cooling: control the cooling rate to 10-50℃ / s and cool to 320-340℃;

[0032] Heating: Heat the strip to 345-360℃;

[0033] Air-cool the strip to below 320°C;

[0034] Continue cooling the strip to below 100°C, then cool it in a quenching tank;

[0035] Leveling and straightening.

[0036] In some embodiments of the present invention, the cold-rolled steel strip may be degreased and cleaned before continuous annealing and hot-dip plating. If rolling oil, iron powder, and non-metallic solid particles remain on the surface of the cold-rolled steel strip, they can be removed through chemical and electrolytic degreasing. After exiting the degreasing tank, the strip is squeezed dry with a squeeze roller, then the degreasing solution is scrubbed and rinsed with hot water, and then blown dry with hot air.

[0037] In the present invention, during jet cooling, the cooling gas can be derived from compressed air drawn into a bellows. However, since clean management of the channel lines in the post-plating cooling section is extremely difficult, the greater the jet flow, the higher the probability of particles being ejected onto the strip surface. The cooled particles can disturb the molten coating, causing localized crystallization anomalies and potentially forming black spot defects. Therefore, during the jet cooling step of the present invention, a relatively high cooling rate of 10-50°C / s is used to ensure a fine coating structure.

[0038] In the present invention, when the steel strip is cooled to between 320 and 340°C, it is easily disturbed by foreign particles, thus forming black spot defects. However, the inventors have found through research and experiments that when the steel strip with black spot defects is reheated, the Mg2Zn 11 The phase will reconvert and decompose into the Zn phase + MgZn2 phase. Therefore, the present invention creatively adds a heating step after the air jet cooling step. However, the inventors have also discovered that the higher the strip heating temperature, the better. Excessively high heating temperatures will further coarsen the coating structure obtained by the rapid cooling during the early air jet cooling. Therefore, in the heating step of the present invention, the heating temperature needs to be controlled between 345 and 360°C.

[0039] In the present invention, the heated steel strip is not cooled by air jet from a cooling bellows, but is air-cooled to below 320°C. Since the probability of particles being sprayed onto the surface of the steel strip during the natural air cooling process is very low, black spot defects can be effectively controlled.

[0040] Furthermore, in the hot-dip coating step of the method for manufacturing the zinc-aluminum-magnesium coated steel sheet of the present invention, the zinc pot temperature is controlled to be 390-550° C., and the temperature of the steel strip entering the zinc pot is controlled to be 400-550° C. In some embodiments, during the hot-dip coating, the steel strip travels at a speed of 50-120 mpm.

[0041] Furthermore, in the heating step of the method for manufacturing the zinc-aluminum-magnesium coated steel plate described in the present invention, the heating time (i.e., the time for heat treatment while controlling the temperature between 345 and 360° C.) is 2 to 20 seconds.

[0042] In the above embodiment of the present invention, in order to further prevent the coating structure obtained by rapid cooling due to air jet cooling from being further coarsened, the heating time is preferably controlled to be 2 to 20 seconds.

[0043] Furthermore, in the manufacturing method of the zinc-aluminum-magnesium coated steel plate described in the present invention, in the step of continuing to cool the strip to below 100°C, the strip is first air-cooled to below 200°C (such as 130-200°C), and then a cooling bellows is used to cool the strip to below 100°C (such as 75-95°C).

[0044] Furthermore, in the manufacturing method of the zinc-aluminum-magnesium coated steel plate described in the present invention, in the step of continuing to cool the strip to below 100°C, the strip is first jet-cooled to below 200°C (such as 130-200°C), and then a cooling bellows is used to cool the strip to below 100°C (such as 75-95°C).

[0045] After the strip is air-cooled to below 320°C (e.g., 310-320°C), the coating on the strip surface has solidified. Therefore, the strip can be air-cooled or jet-cooled again using cooling bellows. However, both require that the strip temperature be ≤ 200°C when it reaches the tower top rollers. After passing the tower top rollers, the zinc-aluminum-magnesium strip enters the post-plating cooling down section, passes through several stages of cooling bellows, and continues to cool to below 100°C before entering the quenching tank for cooling.

[0046] Subsequently, the production process of the zinc-aluminum-magnesium coated steel sheet of the present invention is exactly the same as the process of conventional hot-dip galvanized products, and the film is processed after leveling and coating with a roller coater, and then it can be rolled up after drying.

[0047] In some embodiments, the method for manufacturing the zinc-aluminum-magnesium coated steel sheet of the present invention comprises:

[0048] (1) Uncoiling of cold rolled strip;

[0049] (2) Degreasing;

[0050] (3) Annealing;

[0051] (4) Immersion in the plating solution in the zinc pot: Control the temperature of the zinc pot at 390 - 550 °C, the temperature of the strip entering the zinc pot at 400 - 550 °C, the immersion time of the strip in the plating solution for 1 - 10 s, and after exiting the zinc pot, control the plating amount by an air knife to scrape off the excess zinc solution;

[0052] (5) Post - plating cooling:

[0053] First, use a post - plating cooling air box to jet - cool the strip, control the cooling rate at 10 - 50 °C / s, and cool the strip to 320 - 340 °C;

[0054] Then heat the strip to 345 - 360 °C and control the heating time for 2 - 20 s;

[0055] Then naturally air - cool the strip to ≤320 °C (such as 310 - 320 °C);

[0056] Continue to use air cooling or use an air box to jet - cool the strip so that the strip temperature when reaching the top - tower roll is ≤200 °C (such as 130 - 200 °C);

[0057] Continue to use an air box to cool the strip to below 100 °C (such as 75 - 95 °C);

[0058] Then enter the quenching water tank for cooling;

[0059] (6) Skin pass and tension leveling, then coat with a post - treatment film by a roll coater, and wind up after drying.

[0060] In some embodiments, the annealing is a conventional annealing in the art. In an exemplary annealing step, the strip after degreasing and cleaning is fully annealed in a N2 + H2 mixed gas.

[0061] In some embodiments, the plating solution contains Zn and chemical elements with the following mass percentages: Al: 1.0 - 30.0%, Mg: 1.0 - 10.0%, and at least one of Ca: 0.01 - 0.5%, Sr: 0.01 - 0.5%, 0 < B ≤ 0.05%, 0 < Cr ≤ 0.3%, Ti: 0.001 - 0.3%, Ni: 0.001 - 1.0%.

[0062] In some embodiments, the mass percentage of each chemical element in the plating solution is: Al: 1.0-30.0%, Mg: 1.0-10.0%, and at least one of Ca: 0.01-0.5%, Sr: 0.01-0.5%, 0<B≤0.05%, 0<Cr≤0.3%, Ti: 0.001-0.3%, Ni: 0.001-1.0%, and the balance is Zn and unavoidable impurities.

[0063] In some embodiments, Fe is an unavoidable impurity in the plating solution, and the Fe content is ≤ 2.0%, and the Fe in the plating solution originates from the dissolution of Fe on the steel sheet.

[0064] In some embodiments, the plating solution contains 0 <Si≤2.0%。

[0065] The zinc-aluminum-magnesium coated steel plate prepared by the present invention eliminates black spot defects on the surface of the coated steel plate, has excellent appearance and surface quality, and has good industrial application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] FIG1 schematically shows a flow chart of steps of a method for manufacturing a zinc-aluminum-magnesium coated steel sheet according to some embodiments of the present invention.

[0067] FIG2 is a macroscopic photograph of the coated steel plate obtained in Example 1 with no black spot defects on the surface.

[0068] FIG3 is a macroscopic photograph of the coated steel plate in Comparative Example 1, on the surface of which black spot defects are easily generated. DETAILED DESCRIPTION

[0069] The zinc-aluminum-magnesium coated steel plate and the manufacturing method thereof described in the present invention will be further explained and illustrated below in conjunction with specific embodiments and the accompanying drawings. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0070] Examples 1-10 and Comparative Examples 1-4

[0071] FIG1 schematically shows a flow chart of the steps of the method for manufacturing the zinc-aluminum-magnesium coated steel sheet according to the present invention.

[0072] As shown in FIG1 , the zinc-aluminum-magnesium coated steel sheets of Examples 1-10 and the comparative steels of Comparative Examples 1-4 can be prepared by the following steps:

[0073] (1) Uncoiling of cold rolled strip;

[0074] (2) Degreasing: Chemical degreasing and electrolytic degreasing are used to remove the rolling oil, iron powder and non-metallic solid particles remaining on the surface of the cold-rolled strip. After the strip leaves the degreasing tank, it is squeezed dry by a squeezing roller, and then the degreasing liquid is brushed and rinsed with hot water, and then dried with hot air;

[0075] (3) Annealing: The degreased and cleaned strip steel is fully annealed in a mixture of N2+H2 gas;

[0076] (4) Immersing in the zinc pot: The zinc pot temperature is controlled at 390-550°C, the strip temperature of the zinc pot is controlled at 400-550°C, the strip is immersed in the plating solution for 1-10 seconds, and the amount of plating is controlled by an air knife after exiting the zinc pot, and the excess zinc liquid is scraped off;

[0077] (5) Cooling after plating:

[0078] First, the strip is cooled by air jet using a post-plating cooling bellows, with the cooling rate controlled at 10-50°C / s, until the strip is cooled to 320-340°C.

[0079] Then heat the strip to 345-360°C and control the heating time to 2-20s;

[0080] Then the strip is naturally air-cooled to ≤320℃;

[0081] Continue to use air cooling or cooling bellows to jet-cool the strip so that the strip temperature is ≤200℃ when it reaches the top roller.

[0082] Continue to use the cooling bellows to cool the strip steel to below 100°C;

[0083] Then it enters the quenching tank for cooling;

[0084] (6) The strip is leveled and straightened using the same process as the conventional process, and then coated with a post-treatment film by a roller coater, and can be rolled up after drying.

[0085] The substrates used in the various embodiments and comparative examples are shown in Table 2-1, and these substrates are all commercially available products.

[0086] Table 1 lists the mass percentages of chemical elements in the plating solutions of Examples 1-10 and Comparative Examples 1-4 of the present invention and the zinc-aluminum-magnesium coatings formed in the corresponding Examples and Comparative Examples based on the plating solutions.

[0087] Table 1 (wt.%, the balance is Zn and other inevitable impurities except Fe)

[0088] Table 2-1 and Table 2-2 list the specific process parameters of the zinc-aluminum-magnesium coated steel sheets of Examples 1-10 and the comparative steels of Comparative Examples 1-4 in the above process steps.

[0089] Table 2-1

[0090] Table 2-2

[0091] The zinc-aluminum-magnesium coated steel plates of Examples 1-10 and the comparative steel materials of Comparative Examples 1-4 were sampled respectively, and the surfaces of the steel plates of each Example and Comparative Example were visually observed. The number of black spots was monitored by the detection results of a surface defect recorder. X-ray diffraction was used to compare the physical phase composition of the black spot sample and the normal sample. The relevant detection and observation results are listed in the following Table 3.

[0092] Table 3

[0093] It can be seen from Table 3 above that the zinc-aluminum-magnesium coatings of Examples 1-10 obtained by the technical solution of the present invention do not contain Mg2Zn 11 In terms of surface quality, there are no black spot defects on the surface of the steel plate, and it is visually uniform, with very excellent surface quality.

[0094] However, the comparative steels of Comparative Examples 1-4 deviated from the manufacturing process of the present invention during production and preparation, and black spot defects existed on the surfaces of the steel plates finally prepared therefrom.

[0095] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0096] 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 therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A zinc-aluminum-magnesium coated steel sheet, comprising a cold-rolled substrate and a coating coated on the cold-rolled substrate, characterized in that: The coating contains Zn and the following chemical elements in percentage by mass: Al: 1.0-30.0%, Mg: 1.0-10.0%, and at least one of Ca: 0.01-0.5%, Sr: 0.01-0.5%, 0<B≤0.05%, 0<Cr≤0.3%, Ti: 0.001-0.3%, and Ni: 0.001-1.0%.

2. The zinc-aluminum-magnesium coated steel sheet according to claim 1, characterized in that: The mass percentage of each chemical element of the coating is: Al: 1-30.0%, Mg: 1-10.0%, and at least one of Ca: 0.01-0.5%, Sr: 0.01-0.5%, 0<B≤0.05%, 0<Cr≤0.3%, Ti: 0.001-0.3%, Ni: 0.001-1.0%, and the balance is Zn and unavoidable impurities.

3. The zinc-aluminum-magnesium coated steel sheet according to claim 1 or 2, characterized in that: Among the inevitable impurities in the coating: Fe≤2.0%.

4. The zinc-aluminum-magnesium coated steel sheet according to claim 1 or 2, characterized in that: The coating also contains 0<Si≤2.0%.

5. The zinc-aluminum-magnesium coated steel sheet according to claim 1 or 2, characterized in that: The coating does not contain Mg2Zn 11 Mutually.

6. The zinc-aluminum-magnesium coated steel sheet according to claim 1 or 2, characterized in that: The weight of the coating is 100 to 400 g / m 2 .

7. The zinc-aluminum-magnesium coated steel sheet according to claim 1 or 2, characterized in that: The substrate is CQ steel, IF steel, high-strength IF steel or bake-hardened steel.

8. The method for manufacturing a zinc-aluminum-magnesium coated steel sheet according to any one of claims 1 to 6, characterized in that: Includes steps: Continuous annealing and hot-dip coating of cold-rolled substrates; Use jet cooling: control the cooling rate to 10-50℃ / s and cool to 320-340℃; Heating: Heat the strip to 345-360°C; Air cool the strip to below 320°C; Continue cooling the strip to below 100°C, then cool in a quenching tank; Leveling and straightening.

9. The manufacturing method according to claim 8, characterized in that: In the hot-dip galvanizing step, the temperature of the zinc pot is controlled at 390-550°C, and the temperature of the strip entering the zinc pot is controlled at 400-550°C.

10. The manufacturing method according to claim 8, characterized in that: In the heating step, the heating time is 2 to 20 seconds.

11. The manufacturing method according to claim 8, characterized in that: In the step of continuing to cool the steel strip to below 100° C., the steel strip is firstly air-cooled or jet-cooled to below 200° C., and then cooled to below 100° C. by a cooling bellows.

12. The manufacturing method according to claim 11, characterized in that: First, the strip is air-cooled or jet-cooled to 130-200°C, and then a cooling bellows is used to cool the strip to 75-95°C.

13. The manufacturing method according to claim 7, characterized in that: The step of air cooling the steel strip to below 320°C is to air cool the steel strip to 310-320°C.

14. The manufacturing method according to claim 8, characterized in that the method include: (1) Uncoiling of cold rolled strip; (2) Degreasing; (3) Annealing; (4) Immersing in the zinc pot: The temperature of the zinc pot is controlled at 390-550°C, the temperature of the strip entering the zinc pot is controlled at 400-550°C, the strip is immersed in the plating solution for 1-10 seconds, and the amount of plating is controlled by an air knife after exiting the zinc pot, and the excess zinc liquid is scraped off; (5) Cooling after plating: First, the strip is cooled by jet cooling with a post-plating cooling bellows, and the cooling rate is controlled at 10-50°C / s to cool the strip to 320-340°C. Then the strip is heated to 345-360°C and the heating time is controlled to be 2-20s; Then the strip is naturally air-cooled to ≤320℃; Continue to use air cooling or cooling bellows to jet-cool the strip so that the strip temperature is ≤200℃ when it reaches the tower top roller; Continue to use the cooling bellows to cool the strip steel to below 100°C; Then it enters the quenching tank for cooling; (6) Smoothing, straightening, and then coating the film with a roller coater, and then rolling it up after drying.

15. The manufacturing method according to claim 8, characterized in that: The plating solution used for hot-dip plating contains Zn and the following chemical elements in mass percentage: Al: 1.0-30.0%, Mg: 1.0-10.0%, and at least one of Ca: 0.01-0.5%, Sr: 0.01-0.5%, 0<B≤0.05%, 0<Cr≤0.3%, Ti: 0.001-0.3% and Ni: 0.001-1.0%.

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