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

Abstract

The invention discloses a zinc-aluminum-magnesium coated steel plate which comprises a cold-rolled substrate and a coating coated on the cold-rolled substrate, the coating contains Zn and at least one of 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 invention also discloses a zinc-aluminum-magnesium coated steel plate and a manufacturing method of the zinc-aluminum-magnesium coated steel plate. The manufacturing method comprises the following steps: carrying out continuous annealing and hot dipping on a cold-rolled substrate; air injection cooling is adopted; heating is performed; the strip steel is air-cooled to 320 DEG C or below; continuously cooling the strip steel to below 100 DEG C, and then cooling in a quenching tank; and flattening and straightening.

Description

Technical Field

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

[0002] Zinc-aluminum-magnesium coated steel sheets are gradually being used more widely due to their excellent corrosion resistance. Among them, the large-scale use of them as paint-free appearance parts makes the appearance quality control of the coating very important. Due to the rich and complex phase structure of the zinc-aluminum-magnesium coating itself, there are still some problems to be solved in its appearance quality control. 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 spot defects may be formed.

[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 Mg 2 Zn 11 The eutectic structure of the phase has a different visible light reflectivity from its surrounding area, so it appears black visually.

[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 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 interval of the hypereutectic and adding trace elements to inhibit Mg 2 Zn 11 The formation of the phase makes Mg 2 Zn 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 galvanized 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, when faced with an actual production environment, even at the same zinc pot temperature, the actual cooling rate of the strip varies with different strip thicknesses, operating speeds, and coating adhesion amounts, resulting in the need to use different post-plating cooling air volume controls for strips of different specifications, making the control of cooling parameters more complicated. In addition, under conditions of different cooling rates, the zinc alloy crystal morphology on the surface of strips of different specifications is different, and color differences between specifications may also occur. Furthermore, in the actual production of galvanizing, it is also difficult to completely remove impurity particles in the post-plating cooling channel line. Therefore, in an actual production environment, avoiding black spot defects on the coating surface is still a major challenge. Summary of the invention

[0008] One of the purposes of the present invention is to provide a zinc-aluminum-magnesium coated steel sheet, which eliminates the black spot defects on the surface of the coated steel sheet, thereby achieving good appearance and surface quality.

[0009] In order to achieve the above object, the present invention provides a zinc-aluminum-magnesium coated steel sheet, which includes a cold-rolled substrate and a coating coated on the cold-rolled substrate, wherein the coating contains Zn and the following chemical elements in mass percentage:

[0010] 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%, and Ni: 0.001-1.0%.

[0011] Furthermore, in the zinc-aluminum-magnesium coated steel plate described in the present invention, 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.

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

[0013] Al: The present invention adds Al to the coating to improve the corrosion resistance of the coating, but too high Al content will lead to a decrease in the Zn content in the coating, weakening the sacrificial protection effect (notch corrosion resistance) on 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. For black spot defects in the coating, in the coating with an Al content of ≤30%, the eutectic phase occupies a considerable proportion under equilibrium conditions, and a large number of black spot defects may occur if the process is improperly controlled. When the Al content is greater than 30%, the plating solution temperature will increase significantly, which is likely to 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 to 30%.

[0014] Mg: The reason why zinc-aluminum-magnesium coating has better corrosion resistance than zinc-aluminum coating is that Mg in the coating can evenly form stable and dense corrosion products with certain fluidity. The presence of the corrosion products enables the steel plate processing cut to have a "self-healing" mechanism. When the Mg added to the coating is ≥1%, a coating with significantly improved corrosion resistance can be obtained. When the Mg content is greater than 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 and easy to crack and drop powder slag. Moreover, further increase in Mg content will no longer have a further improvement effect on the corrosion resistance of the coating. Therefore, the Mg content in the coating of the present invention is not easy to be too high, and its content is controlled in the range of 1 to 10.0%.

[0015] Ca: The present invention finds that the addition of Ca and Mg has a similar effect of stabilizing corrosion products. During the solidification process of the plating solution, the intermetallic compounds containing Ca and Mg are introduced into the Zn phase. In the corrosive environment, the Zn phase corrodes first and releases Ca and Mg elements at the same time, thereby more effectively stabilizing the corrosion products in the early stage of corrosion. The inventors also found through research that the granular intermetallic compounds formed by Mg and Ca elements, such as MgZn 2 Mg 2 Si、Al 2 Ca、Al 4 Ca、Al 2 CaSi 2 , under the premise of proper control, the hardness of the coating can be improved, thereby improving the scratch resistance of the coating. Moreover, when the Al and Mg contents are increased, the temperature of the plating solution needs to be increased due to the influence of the solidification line, and the oxidation of the liquid surface increases. By adding a certain amount of Ca element, the oxidation of Mg in the plating bath and the coating surface can be better controlled. However, if the Ca content is added in an amount greater than 0.5%, it is easy to cause scum and form coating defects. Therefore, in the present invention, when Ca is added, the Ca content in the coating can be controlled at 0.01-0.5% to obtain better results.

[0016] Sr: In the present invention, adding 0.01-0.5% Sr can inhibit the occurrence of oxidation reaction of zinc-aluminum-magnesium coating. During the cooling process of the coating, due to the reaction of metal elements such as zinc, aluminum, and magnesium in the hot dip solution at high temperature with oxygen in the air, oxides are easily generated, resulting in a layer of oxide film covering the surface of the coating. In the plating solution to which the Sr element is added, the Sr element reacts with oxygen first to generate stable Sr oxides and adsorbs to the surface of the plating solution, thereby reducing the concentration of free oxygen in the plating solution, inhibiting the occurrence of oxidation reactions of metal elements such as zinc, aluminum, and magnesium, thereby reducing the oxidation of surface wrinkles and folds, and maintaining the smooth appearance of the surface. Therefore, in the present invention, when Sr is added, the Sr content in the coating can be controlled at 0.01-0.5% to obtain better effects.

[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 content of Ti and B is too high, Ti-Al, Al-B, and Ti-B precipitates will be generated in the coating, and fine particles will be generated on the coating, causing appearance defects of the coated steel plate. Therefore, when Ti is added, the present invention controls the Ti content in the coating to 0.001-0.3%, and when B is added, the coating can be controlled to 0<B≤0.05%.

[0018] Cr: Adding Cr to the zinc-aluminum-magnesium plating solution can promote the formation of dense oxides, so that the coating formed has better corrosion resistance. In addition, adding Cr can improve the crystal structure of the coating, making the grain boundaries of the coating more uniform and tight, thereby improving the density and corrosion resistance of the coating. However, it is not advisable to add too much Cr, as too much will change the coating performance and react with Al to form a certain amount of zinc slag, thereby affecting the coating performance. Therefore, the Cr element in the coating of the present invention is controlled to be 0-0.3%.

[0019] Ni: Ni element has good corrosion resistance. Adding Ni element to the plating solution can provide a Ni-O composite oxide film with good corrosion resistance. The oxide film can form a protective layer that protects the metal surface, prevents oxygen from entering the coating, and improves the corrosion resistance of the coating. In addition, the addition of Ni element can form a Ni-Al phase with high thermal stability and 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%, in order to obtain better corrosion resistance and crack resistance.

[0020] Furthermore, in the zinc-aluminum-magnesium coated steel sheet described in the present invention, among the inevitable impurities in the coating, Fe≤2%.

[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, and 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 described in 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 liquid temperatures near the eutectic composition, while inhibiting the interfacial Fe-Al reaction when high Al content is added. In general experience, Si addition is believed to block the Al atoms in the Fe 2 Al 5 Diffusion channels in the phase inhibit Fe 2 Al 5 However, the present invention finds that in addition to the above effects, the addition of Si can also promote the dissolution of the matrix Fe in the early stage of the Fe-Al reaction and promote the continuous and dense Fe on the substrate / plating solution interface. 2 Al 5 The formation of intermetallic compounds, especially for hot-dip galvanized products close to the eutectic point with an Al content of 5-7wt.%, can be produced at a lower zinc pot temperature by adding a small amount of Si 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 Mg 2 Zn 11 Mutually.

[0025] The zinc-aluminum-magnesium coated steel sheet of the present invention can avoid the generation of black spot defects on the surface of the steel sheet. 2 Zn 11 Very little, preferably no Mg 2 Zn 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] 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.

[0028] 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:

[0029] Continuous annealing and hot-dip coating of cold-rolled substrates;

[0030] Use jet cooling: control the cooling rate to 10-50℃ / s and cool to 320-340℃;

[0031] Heating: Heat the strip to 345-360°C;

[0032] Air cool the strip to below 320°C;

[0033] Continue cooling the strip to below 100°C, then cool in a quenching tank;

[0034] Leveling and straightening.

[0035] In some embodiments of the present invention, the cold-rolled steel strip may be degreased and cleaned before continuous annealing and hot-dip plating of the cold-rolled substrate. If rolling oil, iron powder and non-metallic solid particles remain on the surface of the cold-rolled steel strip, they can be removed by chemical degreasing and electrolytic degreasing. After the steel strip leaves the degreasing tank, it is squeezed dry by a squeeze roller, and then the degreasing liquid is brushed and rinsed clean with hot water, and then dried by hot air.

[0036] In the present invention, when jet cooling is performed, the cooling gas can be derived from the compressed air sucked into the bellows. Since the channel line clean management of the post-plating cooling section is extremely difficult, the larger the jet airflow, the higher the probability of particles being sprayed onto the surface of the strip. The cooled particles disturb the molten coating, causing abnormal crystallization in local areas, which may form black spot defects. Therefore, in the step of jet cooling of the present invention, a higher cooling rate of 10 to 50°C / s is used to ensure a fine coating structure.

[0037] In the present invention, when the strip is cooled to between 320 and 340°C, the strip is easily disturbed by foreign particles, thereby forming black spot defects. However, the inventors have found through research and experiments that when the strip with black spot defects is reheated, the Mg 2 Zn 11 The phase will be transformed and decomposed into Zn phase + MgZn 2 Therefore, the present invention creatively adds a heating step after the jet cooling step. However, the inventors also found that the higher the strip heating temperature is, the better. Too high a heating temperature will further coarsen the coating structure obtained by rapid cooling in the early jet cooling. Therefore, in the heating step of the present invention, the heating temperature needs to be controlled between 345 and 360°C.

[0038] In the present invention, the heated steel strip is not cooled by cooling bellows jet, but 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.

[0039] Furthermore, in the hot-dip plating step of the method for manufacturing zinc-aluminum-magnesium coated steel plate described in the present invention, 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.

[0040] 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 is 2 to 20 seconds.

[0041] In the above embodiment of the present invention, in order to further prevent the rapid cooling of the coating obtained by jet cooling from further coarsening, the heating time is preferably controlled to be 2 to 20 seconds.

[0042] 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, and then a cooling bellows is used to cool the strip to below 100°C.

[0043] Furthermore, in the manufacturing method of 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, and then a cooling bellows is used to cool the strip to below 100°C.

[0044] After the strip is air-cooled to below 320℃, the coating layer on the strip surface has solidified, so the strip can be air-cooled or jet-cooled with cooling bellows again, but it is necessary to ensure that the strip temperature is ≤200℃ when it reaches the tower top roller. After passing the tower top roller, 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℃ before entering the quenching tank for cooling.

[0045] 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.

[0046] The zinc-aluminum-magnesium coated steel plate prepared by the present invention eliminates the 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

[0047] Figure 1 The flowchart schematically shows the steps of the method for manufacturing the zinc-aluminum-magnesium coated steel sheet according to some embodiments of the present invention. DETAILED DESCRIPTION

[0048] The zinc-aluminum-magnesium coated steel sheet and the method for manufacturing the same described in the present invention will be further explained and illustrated below in conjunction with specific embodiments and the accompanying drawings of the specification. However, such explanation and illustration do not constitute an improper limitation on the technical solution of the present invention.

[0049] Examples 1-10 and Comparative Examples 1-4

[0050] Figure 1 The flowchart schematically shows the steps of the method for manufacturing the zinc-aluminum-magnesium coated steel plate according to the present invention.

[0051] like Figure 1 As shown, 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:

[0052] (1) Open book;

[0053] (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 squeeze roller, and then the degreasing liquid is brushed and rinsed with hot water, and then dried with hot air;

[0054] (3) Annealing: The degreased and cleaned strip is annealed in N 2 +H 2 Fully annealed in mixed gas;

[0055] (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;

[0056] (5) Cooling after plating:

[0057] 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.

[0058] Then the strip is heated to 345-360°C and the heating time is controlled to be 2-20s;

[0059] Then the strip is naturally air-cooled to ≤320℃;

[0060] 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;

[0061] Continue to use the cooling bellows to cool the strip steel to below 100°C;

[0062] Then it enters the quenching tank for cooling;

[0063] (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.

[0064] It should be noted that the present invention does not specifically limit the substrate used for the zinc-aluminum-magnesium coated steel plate. In actual application, technical personnel in this field can select CQ steel, IF steel, high-strength IF steel, bake-hardened steel or other high-strength steel according to needs, which has no direct correlation with the improvement in the surface quality of the coating obtained by the present invention.

[0065] Table 1 lists the mass percentages of chemical elements of the plating solutions in 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.

[0066] Table 1. (wt.%, the balance is Zn and other inevitable impurities except Fe)

[0067] serial number Al Mg Si Ca Sr B Cr Ti Ni Fe Example 1 1.5 1.3 0.001 - 0.02 - - - - 0.1 Example 2 6.2 2.9 0.1 0.2 0.01 - - - - 0.2 Example 3 11.4 3.2 0.12 0.01 - 0.002 - - 0.002 0.3 Example 4 11.4 3.2 0.12 - 0.5 - - 0.3 - 0.4 Example 5 2.7 2.5 - - - - - - - 0.5 Example 6 15.2 6.1 1.2 0.4 - 0.005 0.15 - 0.8 1 Example 7 15.2 6.1 1.2 - - - - - - 0.6 Example 8 18.6 4.5 0.7 - - 0.05 - - - 0.7 Example 9 25.7 5.7 0.6 0.5- - - - 0.001 - 1 Example 10 29.5 9.8 2.0 - - - 0.3 - - 1.9 Comparative Example 1 6.2 2.9 0.1 0.025 - - - - - 1.1 Comparative Example 2 11.4 3.2 0.12 - - - - - 0.6 0.5 Comparative Example 3 11.4 3.2 0.12 - - 0.01 - - - 0.3 Comparative Example 4 25.7 5.7 0.6 - - - - - - 2.3

[0068] 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.

[0069] Table 2-1.

[0070]

[0071] Table 2-2.

[0072]

[0073]

[0074] The zinc-aluminum-magnesium coated steel plates of Examples 1-10 and the comparative steels 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, and the physical phase composition of the black spot sample and the normal sample was compared by X-ray diffraction. The relevant detection and observation results are listed in the following Table 3.

[0075] Table 3.

[0076]

[0077]

[0078] It can be seen from Table 3 that the zinc-aluminum-magnesium coatings of Examples 1-10 obtained by the technical solution of the present invention do not contain Mg 2 Zn 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.

[0079] However, the comparative steel materials of Comparative Examples 1-4 deviate from the manufacturing process described in the present invention during production and preparation, and black spot defects exist on the surfaces of the steel plates finally prepared therefrom.

[0080] 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.

[0081] 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 similar changes or modifications made therewith can be directly derived or easily associated with by those skilled in the art from the contents disclosed in the present invention, and all should belong to the protection scope 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, It is characterized in that The coating contains Zn and the following chemical elements in percentage by mass: 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%, and Ni: 0.001-1.0%.

2. The zinc-aluminum-magnesium coated steel sheet according to claim 1, It is 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, It is 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, It is 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, It is characterized in that The coating does not contain Mg 2 Zn 11 Mutually.

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

7. The method for manufacturing the zinc-aluminum-magnesium coated steel sheet according to any one of claims 1 to 6, It is 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.

8. The manufacturing method according to claim 7, It is 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.

9. The manufacturing method according to claim 7, It is characterized in that In the heating step, the heating time is 2 to 20 seconds.

10. The manufacturing method according to claim 7, It is characterized in that In the step of continuing to cool the steel strip to below 100° C., the steel strip is first air-cooled to below 200° C., and then cooled to below 100° C. using a cooling bellows.

11. The manufacturing method according to claim 7, It is characterized in that In the step of continuing to cool the steel strip to below 100° C., the steel strip is firstly jet-cooled to below 200° C., and then the steel strip is cooled to below 100° C. by a cooling bellows.

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