Method for reducing orange peel defect of medium aluminum-zinc-aluminum-magnesium thick coating

Abstract

The invention discloses a method for reducing orange peel defects of a medium aluminum-zinc-aluminum-magnesium thick coating. The method comprises the following steps: adjusting the temperature of a zinc-aluminum-magnesium plating solution to be 455-465 DEG C, the height of an air knife to be 60-70mm, the distance between the air knife and strip steel to be 15-20mm, the pressure of the air knife to be 40-50mbar and the angle of the air knife to be 0-1 degree; a movable fan is arranged at the position 2-5 m above the air knife, and the cooling speed reaches 20-30 DEG C / s; the cooling speed of a fixed fan in the cooling tower to the strip steel is 15-20 DEG C / s, and the temperature of the strip steel is cooled to 150-190 DEG C when the strip steel reaches a turning roller at the top of the cooling tower; and the strip steel enters a water quenching tank to be continuously cooled to 40-50 DEG C, and a finished product is prepared after finishing, withdrawal and straightening and passivation. According to the method, the processes of cooling the coating by adding a movable fan and the like are adopted, so that orange-peel-texture-shaped surface defects caused by cracks generated in the processes of roll turning, finishing and withdrawal and straightening of the coating are reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of coating and plating, and relates to a method for preparing thick-specification and thick-coated medium-aluminum-zinc-aluminum-magnesium coated structural steel, specifically a method for reducing orange peel defects in medium-aluminum-zinc-aluminum-magnesium thick coatings. Background Art

[0002] Production of thick specifications (2-2.5mm) and thick zinc-aluminum-magnesium coatings (>250g / m 2 ) coated steel plate, due to the high heat in the middle of the strip, Fe diffusion continues after leaving the zinc pot until the strip temperature is lower than 300℃, and the Fe diffusion will drop to a very weak level. The diffusion of Fe into the zinc pot will increase FeAl 3 Bottom slag is formed and adheres to the surface of the sinking roller, increasing the zinc particles and zinc scar defects on the surface. Therefore, the residence time of the strip in the zinc pot should be reduced, and the temperature of the thick strip entering the zinc pot should be appropriately reduced. At the same time, the vertical cooling section fan should be turned on for enhanced cooling, the water quenching tank level should be increased, and the circulating water flow rate should be increased. However, if the medium aluminum zinc aluminum magnesium is produced by lowering the zinc pot temperature, the fluidity and wettability of the plating solution will be poor, resulting in uneven coating thickness and rough surface. After the strip on the continuous hot-dip galvanizing line comes out of the zinc pot, the air knife scrapes off the excess zinc liquid on the strip, and then enters the fixed vertical fan in the cooling tower, which is usually more than 10m away. Therefore, there is an air cooling process before entering the air cooling, which takes 10 to 15 seconds. Due to the high temperature and high heat in the center of thick strip steel, the running speed of the strip steel is slow, and the coating cools slowly, which leads to an increase in the grain size of the coating and poorer toughness of the coating. Due to the reduced number of grain boundaries, the ability of zinc-aluminum-magnesium coating with large grain size to hinder crack expansion is weakened. During the turning and straightening process of the turning roller, the strip steel will undergo a large deformation process, which will lead to the generation and expansion of cracks on the surface of the coating, resulting in orange peel defects. Compared with pure zinc, zinc-aluminum-magnesium plating has different requirements for the temperature and composition of the zinc pot, and the requirements for the cooling speed after plating are also more stringent. The mobile bellows are added to meet this requirement. The cooling process after plating is also very different due to different coating compositions. Therefore, by reasonably setting the production process of thick and medium aluminum-zinc-aluminum-magnesium coatings, the temperature of the strip steel entering the zinc pot and the lower plating solution temperature can be minimized while ensuring the fluidity of the plating solution. At the same time, increasing the process speed and cooling speed of the galvanizing process section can also effectively reduce orange peel defects.

[0003] In the prior art, patent CN 109280871 B provides a method for controlling the size of zinc flowers, by controlling the roughness of the substrate to 0.8-1.2 μm, the height of the air knife to 400 mm-450 mm, the spacing between the air knife and the galvanized sheet to 6.5 mm-7.5 mm, and a cooling air box that can move up and down is provided 3-4 m above the air knife after the galvanizing process, and the fan power opening is 80%, so as to increase the cooling speed after the zinc liquid solidifies and avoid the growth of zinc flowers due to slow cooling speed; the limitation of this method is that it is not reasonable to control the size of zinc flowers on pure galvanized steel plates by adopting a mobile fan air volume ratio of 4:2:1 from large to small. A large air volume in the early stage of cooling will cause the coating that has not been completely cooled to increase the edge defects under the action of the fan purge, such as uneven coating thickness and beard lines. Patent CN 113481455 A provides a method for producing high surface quality zinc-aluminum-magnesium coated steel strips / plates using an air knife. By adjusting the air knife pressure to 40-270 mbar, defects such as zinc cloud patterns, horseshoe prints, and beard patterns in the coating can be effectively suppressed. However, it can only produce zinc-aluminum-magnesium coated steel plates with relatively thin coatings and low Al and low Mg composition systems. For medium-aluminum zinc-aluminum-magnesium, if an air knife is used, it will cause severe oxidation in the zinc pot, increase zinc particles and zinc slag, and increase surface defects. Patent CN110438427 A provides a method for eliminating surface defects of thick-gauge zinc-aluminum-magnesium coated steel. By controlling the temperature of the strip entering the zinc pot to 445-455°C and the strip speed in the cooling tower section to ≤90m / min, the crystallization abnormality during the solidification process of the coating is effectively controlled; however, its zinc, aluminum and magnesium components belong to a low-aluminum zinc-aluminum-magnesium system with an Al content of 1-3%, and the relevant process parameters are not suitable for a medium-aluminum zinc-aluminum-magnesium coating system. If the running speed of thick-gauge strip is too high, it will bring a large load to the transmission system and various working rollers of the galvanizing unit. This process method cannot meet the requirements for rapid cooling of thick coatings. Summary of the invention

[0004] In order to overcome the defects of the above-mentioned prior art, the present invention arranges a mobile fan above the air knife, and then adjusts the zinc pot temperature, the air knife process, and the cooling tower fan parameters to obtain a zinc-aluminum-magnesium thick-coated surface without orange peel defects. By adding a mobile fan to cool the coating, the orange peel-like surface defects caused by large surface deformation and cracks caused by tensile stress during the turning roller, finishing, and straightening processes of the coating are reduced.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides a method for reducing orange peel defects in a thick aluminum-zinc-aluminum-magnesium coating, the method comprising the following steps:

[0006] ① The steel base of thick-coated aluminum-zinc-aluminum-magnesium coated steel plate is annealed after chemical degreasing and electrolytic degreasing;

[0007] ② After the annealing furnace cooling zone, the temperature of the steel plate in the hot tension chamber of the furnace nose before entering the zinc pot is 460℃~480℃, and the furnace nose atmosphere is 10% hydrogen, 85% nitrogen and 5% water vapor;

[0008] ③ hot-dip plating the zinc-aluminum-magnesium plating solution at a temperature of 455°C to 465°C on the upper and lower surfaces of the steel base of the steel plate;

[0009] ④ The steel plate is scraped and blown off the excess plating solution by an air knife;

[0010] ⑤ The coated steel plate is continuously cooled by a mobile fan;

[0011] ⑥The fixed fan in the cooling tower continues to cool the strip steel at a cooling rate of 15-20℃ / s. When the strip steel reaches the turning roller at the top of the cooling tower, its temperature is cooled to 150℃-190℃;

[0012] ⑦ The strip enters the water quenching tank and continues to cool to 40℃~50℃. After skin finishing, straightening and passivation, it is made into a finished product.

[0013] In the above technical scheme, further, in step ①, the temperature of the steel plate and steel base in the soaking section of the continuous annealing furnace RTF is 690°C~720°C, and the temperature of the strip in the furnace nose before entering the zinc pot is 480°C~490°C; the heating atmosphere of the continuous annealing furnace section is a nitrogen atmosphere, and contains 6%~8% volume fraction of hydrogen, and the dew point in the furnace is -35°C to -40°C.

[0014] Furthermore, in step ①, the steel base of the thick-coated aluminum-zinc-aluminum-magnesium coated steel plate is a structural hot-rolled pickled plate or a cold-rolled steel plate with a thickness of 1.8 to 2.5 mm, a width of 1000 to 1150 mm, and a yield strength of 250 to 550 MPa.

[0015] Furthermore, the chemical composition mass content of the steel base of the aluminum-zinc-aluminum-magnesium coated steel plate in the thick-gauge thick coating is C: 0.1%~0.2%, Si: 0.02%~0.05%, Mn: 0.2%~0.3%, P≤0.035%, S≤0.03%, and the rest is Fe and unavoidable impurities.

[0016] Furthermore, the roughness of the steel base of the aluminum-zinc-aluminum-magnesium coated steel plate in the thick-gauge thick coating is 0.8-1.2 μm, which can ensure that the steel strip has good wettability in zinc liquid, and the coating also has good adhesion.

[0017] In the above technical scheme, further, the coating of the thick-specification and thick-layer medium-aluminum-zinc-aluminum-magnesium coated steel plate is a medium-aluminum-zinc-aluminum-magnesium coating, the commercial name of which is Super dyma, and the coating composition is Al: 10%~12%, Mg: 2%~3%, Si: 0.1%~0.2%, impurity elements ≤0.1%, and the rest is Zn; the zinc-aluminum-magnesium plating solution temperature is 455℃~465℃, and setting this temperature is beneficial to increasing the fluidity and leveling of the zinc liquid, and the air knife can easily scrape and blow out a smooth thick coating surface.

[0018] Furthermore, in the production process, the air knife is nitrogen with a purity greater than 99.9%, the air knife is 60 to 70 mm away from the zinc pot coating liquid level, the air knife is 15 to 20 mm away from the strip, the air knife pressure is 40 to 50 mbar, the air knife angle is 0 to 1°, and the air knife lip gap is a wedge-shaped curve, narrow in the middle and wide on both sides, with a central gap of 1.10 to 1.15 mm and an edge gap of 1.45 to 1.50 mm.

[0019] Furthermore, in step ④, three mobile fans are arranged 2 to 5 m above the air knife (i.e., the next process direction of the strip from the air knife), such as Figure 1 As shown, the power (opening) of 1# mobile fan is set to 80%-90%, the power (opening) of 2# mobile fan is set to 50%-60%, and the power (opening) of 3# mobile fan is set to 50%-60%. The cooling speed achieved by the three mobile fans is 44-50℃ / s; 2# mobile fan and 3# mobile fan are single-sided outlet, symmetrically distributed on both sides of the strip, and are set to the same fan power to avoid abnormal shaking of the strip; 1# mobile fan and three-stage fixed fans are single-motor double-air outlets.

[0020] Furthermore, the fixed cooling in the cooling tower is divided into three sections, and the fixed fan in step ⑤ is a single-sided sweep, wherein the power (opening) of the cooling fan in the first fixed section is 40% to 50%, the power (opening) of the cooling fan in the second fixed section is set at 40% to 50%, and the power (opening) of the cooling fan in the third fixed section is set at 40% to 50%. The cooling speed of the fixed fan of the cooling tower to cool the strip steel meets 15℃ / s to 20℃ / s. When the strip steel reaches the turning roller at the top of the cooling tower, the temperature of the strip steel is cooled to 150℃ to 190℃. The vertical height from the cooling top roller to the zinc pot is 45m, which provides sufficient cooling distance for the strip steel. Then the strip steel enters the water quenching tank to continue cooling the strip steel to 40℃ to 50℃.

[0021] Furthermore, the running speed of the thick-gauge and thick-coated aluminum-zinc-aluminum-magnesium coated steel plate in each process section is 40-55 m / min to reduce the wear and load of the thick-gauge steel plate on the continuous plating unit.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention arranges a mobile fan above the air knife, and then adjusts the zinc pot temperature, the air knife process, and the cooling tower fan parameters to obtain a zinc-aluminum-magnesium thick-coated surface without orange peel defects. By adding a mobile fan to cool the coating, the orange peel-like surface defects caused by large surface deformation and cracks caused by tensile stress during the steering roller, finishing, and straightening processes can be reduced. The thick-coated aluminum-aluminum-magnesium coated steel with high surface quality obtained by this method can be used as structural steel in civil engineering, agricultural animal husbandry, road guardrails, wire tower infrastructure, construction and other fields, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the galvanizing cooling process section of the present invention;

[0025] Figure 2 This is a low-magnification picture of the zinc-aluminum-magnesium thick coating obtained after the mobile bellows was activated in Example 1 (without orange peel defects);

[0026] Figure 3 This is a low-magnification picture of the zinc-aluminum-magnesium thick coating obtained after the mobile bellows was closed in Comparative Example 1 (the orange peel defect is serious). DETAILED DESCRIPTION

[0027] The present invention is further described below in conjunction with specific examples, but the present invention is not limited in any way. To avoid redundant description, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0028] A method for reducing orange peel defects in a thick aluminum-zinc-aluminum-magnesium coating, the method comprising the following steps:

[0029] ① The steel base of thick-coated aluminum-zinc-aluminum-magnesium coated steel plate is annealed after chemical degreasing and electrolytic degreasing;

[0030] ② After the annealing furnace cooling zone, the temperature of the steel plate in the hot tension chamber of the furnace nose before entering the zinc pot is 460℃~480℃, and the furnace nose atmosphere is 10% hydrogen, 85% nitrogen and 5% water vapor;

[0031] ③ hot-dip plating the zinc-aluminum-magnesium plating solution at a temperature of 455°C to 465°C on the upper and lower surfaces of the steel base of the steel plate;

[0032] ④ The steel plate is scraped and blown off the excess plating solution by an air knife;

[0033] ⑤ The coated steel plate is continuously cooled by a mobile fan;

[0034] ⑥The fixed fan in the cooling tower continues to cool the strip steel at a cooling rate of 15-20℃ / s. When the strip steel reaches the turning roller at the top of the cooling tower, its temperature is cooled to 150℃-190℃;

[0035] ⑦ The strip enters the water quenching tank and continues to cool to 40℃~50℃. After skin finishing, straightening and passivation, it is made into a finished product.

[0036] The steel base of the thick-coated aluminum-zinc-aluminum-magnesium coated steel plate described in Examples 1 to 3 is a hot-rolled pickled steel plate or a cold-rolled steel plate for structure with a thickness of 1.8 to 2.5 mm, a width of 1000 to 1150 mm, and a yield strength of 250 to 550 MPa. The chemical composition mass content of the steel base of the thick-coated aluminum-zinc-aluminum-magnesium coated steel plate is C: 0.1% to 0.2%, Si: 0.02% to 0.05%, Mn: 0.2% to 0.3%, P≤0.035%, S≤0.03%, and the rest is Fe and unavoidable impurities; the roughness of the steel base of the steel plate is 0.8 to 1.2 μm.

[0037] The matters not described in the following embodiments are the same as the description of the above specific implementation methods.

[0038] Example 1

[0039] A method for reducing orange peel defects in thick aluminum-zinc-aluminum-magnesium coatings, the specific process flow is:

[0040] The cold-rolled steel sheet is annealed after chemical degreasing and electrolytic degreasing. The temperature of the steel plate base in the soaking section of the continuous annealing furnace RTF is 690°C, and the temperature of the strip in the furnace nose before entering the zinc pot is 485°C; the heating atmosphere of the continuous annealing furnace section is a nitrogen atmosphere, which contains 6% volume fraction of hydrogen, and the dew point in the furnace is -35°C. The molten plating solution is then hot-dip plated on the upper and lower surfaces of the cold-rolled steel plate at 465°C. After the excess plating solution is scraped off by an air knife, the plated layer is cooled continuously in multiple stages by moving fans and fixed fans during the cooling stage. The openings of 1#, 2#, and 3# moving fans (fan power) are set to 90%, 60%, and 60% respectively, and the openings of 1#, 2#, and 3# fixed fans (fan power) are set to 45%, 45%, and 90% respectively. The rapid cooling distance is 45m, and the strip runs at a speed of 55m / min in the cooling tower section. The temperature of the strip reaches the top roller and is cooled to 150°C~160°C. Finally, the steel plate is made into a finished product after finishing, straightening, and passivation.

[0041] The coating of the thick-coated medium aluminum-zinc-aluminum-magnesium coated steel plate described in Example 1 is a medium aluminum-zinc-aluminum-magnesium coating, and the coating composition is Al: 10%, Mg: 2%, Si: 0.1%, impurity elements ≤ 0.1%, and the rest is Zn; and there is no orange peel defect. Figure 2 .

[0042] Example 2

[0043] A method for reducing orange peel defects in thick aluminum-zinc-aluminum-magnesium coatings, the specific process flow is:

[0044] The cold-rolled steel sheet is annealed after chemical degreasing and electrolytic degreasing. The temperature of the steel plate base in the soaking section of the continuous annealing furnace RTF is 700°C, and the temperature of the strip in the furnace nose before entering the zinc pot is 485°C; the heating atmosphere of the continuous annealing furnace section is a nitrogen atmosphere, which contains 7% volume fraction of hydrogen, and the dew point in the furnace is -38°C. The molten plating solution is then hot-dip plated on the upper and lower surfaces of the cold-rolled steel plate at 460°C. After the excess plating solution is scraped off by an air knife, the plate is cooled continuously in multiple stages by moving fans and fixed fans during the cooling stage. The openings of 1#, 2#, and 3# moving fans (fan power) are set to 80%, 45%, and 45% respectively, and the openings of 1#, 2#, and 3# fixed fans are set to 40%, 40%, and 80% respectively. The rapid cooling distance is 45m, and the strip runs at a speed of 50m / min in the cooling tower section. The temperature of the strip reaches the top roller and is cooled to 150°C~160°C. Finally, the steel plate is made into a finished product after finishing, straightening, and passivation.

[0045] The coating of the thick-specification and thick-layer medium-aluminum-zinc-aluminum-magnesium coated steel plate described in Example 2 is a medium-aluminum-zinc-aluminum-magnesium coating, and the coating composition is Al: 11%, Mg: 3%, Si: 0.2%, impurity elements ≤0.1%, and the rest is Zn.

[0046] Example 3

[0047] A method for reducing orange peel defects in thick aluminum-zinc-aluminum-magnesium coatings, the specific process flow is:

[0048] The cold-rolled steel sheet is annealed after chemical degreasing and electrolytic degreasing. The temperature of the steel plate base in the soaking section of the continuous annealing furnace RTF is 720°C, and the temperature of the strip in the furnace nose before entering the zinc pot is 490°C; the heating atmosphere of the continuous annealing furnace section is a nitrogen atmosphere, which contains 8% volume fraction of hydrogen, and the dew point in the furnace is -40°C. The molten plating solution is then hot-dip plated on the upper and lower surfaces of the cold-rolled steel plate at 455°C. After the excess plating solution is scraped off by an air knife, the plate is cooled continuously in multiple stages by moving fans and fixed fans during the cooling stage. The openings of 1#, 2#, and 3# moving fans (fan power) are set to 80%, 40%, and 40% respectively, and the openings of 1#, 2#, and 3# fixed fans are set to 35%, 35%, and 85% respectively. The strip runs at a speed of 40m / min in the cooling tower section, with a rapid cooling distance of 45m. The temperature of the strip reaches the top roller and is cooled to 160°C~170°C. Finally, the steel plate is made into a finished product after finishing, straightening, and passivation.

[0049] The coating of the thick-gauge and thick-coated medium-aluminum-zinc-aluminum-magnesium coated steel plate described in Example 3 is a medium-aluminum-zinc-aluminum-magnesium coating, and the coating composition is Al: 12%, Mg: 3%, Si: 0.2%, impurity elements ≤0.1%, and the rest is Zn.

[0050] Comparative Example 1

[0051] A method for reducing orange peel defects in thick aluminum-zinc-aluminum-magnesium coatings, the specific process flow is:

[0052] The cold-rolled steel plate is annealed after chemical degreasing and electrolytic degreasing, and then the molten plating solution is hot-dip plated on the upper and lower surfaces of the cold-rolled steel plate at 465°C. After the excess plating solution is blown away by an air knife, the plated layer is cooled in the fixed fan cooling section. The opening (fan power) of 1#, 2#, and 3# fixed fans are set to 35%, 35%, and 80% respectively. After turning off the mobile fan, the air cooling distance is 10m, and the distance of the rapid cooling section is 35m. The strip runs at a speed of 50m / min in the cooling tower section and is cooled to 220°C~230°C when it reaches the top roller. Finally, the steel plate is made into a finished product after finishing, straightening, and passivation.

[0053] The low magnification picture of the zinc-aluminum-magnesium thick coating obtained after closing the mobile bellows in Comparative Example 1 is as follows: Figure 3 As shown, the cellulite defect is serious.

[0054] Comparative Example 2

[0055] A method for reducing orange peel defects in thick aluminum-zinc-aluminum-magnesium coatings, the specific process flow is:

[0056] After chemical degreasing and electrolytic degreasing, the cold-rolled steel sheet is annealed, and then the molten state plating solution is hot-dip plated on the upper and lower surfaces of the cold-rolled steel sheet at 460°C, and then the excess plating solution is blown off by an air knife, and the plated layer is cooled in the fixed fan cooling section. The opening (fan power) of the 1#, 2#, and 3# fixed fans is set to 45%, 45%, and 90% respectively. After the mobile fan is closed, the air cooling distance is 10m, the distance of the rapid cooling section is 35m, and the strip runs at a speed of 55m / min in the cooling tower section. The temperature of the top roller is cooled to 200°C to 210°C, and the steel sheet is finally made into a finished product after skin-finishing, straightening, and passivation. The finished product of Comparative Example 2 has serious orange peel defects.

[0057] Comparative Example 3

[0058] A method for reducing orange peel defects in thick aluminum-zinc-aluminum-magnesium coatings, the specific process flow is:

[0059] After chemical degreasing and electrolytic degreasing, the cold-rolled steel sheet is annealed, and then the molten state plating solution is hot-dip plated on the upper and lower surfaces of the cold-rolled steel sheet at 455°C, and then the excess plating solution is blown off by an air knife, and the plated layer is cooled in the fixed fan cooling section. The opening (fan power) of the 1#, 2#, and 3# fixed fans is set to 35%, 35%, and 85% respectively. After the mobile fan is turned off, the air cooling distance is 10m, the distance of the rapid cooling section is 35m, and the strip runs at a speed of 45m / min in the cooling tower section. The temperature of the top roller is cooled to 220°C to 230°C, and the steel sheet is finally made into a finished product after skin-finishing, straightening, and passivation. The finished product of Comparative Example 3 has serious orange peel defects.

[0060] The technical effects of the embodiments and comparative examples have proved that the orange peel defects of thick zinc-aluminum-magnesium coatings can be greatly reduced by lowering the plating solution temperature within a certain range, increasing the running speed of the strip in the cooling tower and reasonably setting the parameters of the mobile fan.

[0061] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0062] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0063] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for reducing orange peel defects in thick aluminum-zinc-aluminum-magnesium coatings, characterized in that: The method comprises the following steps: ① The steel base of thick-coated aluminum-zinc-aluminum-magnesium coated steel plate is annealed after chemical degreasing and electrolytic degreasing; ② After the annealing furnace cooling zone, the temperature of the steel plate in the hot tension chamber of the furnace nose before entering the zinc pot is 460℃~480℃, and the furnace nose atmosphere is 10% hydrogen, 85% nitrogen and 5% water vapor; ③ hot-dip plating the zinc-aluminum-magnesium plating solution at a temperature of 455°C to 465°C on the upper and lower surfaces of the steel base of the steel plate; ④ The steel plate is scraped and blown off the excess plating solution by an air knife; ⑤ The coated steel plate is continuously cooled by a mobile fan; ⑥The fixed fan in the cooling tower continues to cool the strip steel at a cooling rate of 15-20℃ / s. When the strip steel reaches the turning roller at the top of the cooling tower, its temperature is cooled to 150℃-190℃; ⑦ The strip enters the water quenching tank and continues to cool to 40℃~50℃. After skin finishing, straightening and passivation, it is made into a finished product.

2. The method according to claim 1, characterized in that Step ① The temperature of the steel plate and steel base in the soaking section of the continuous annealing furnace RTF is 690°C to 720°C, and the temperature of the strip in the furnace nose before entering the zinc pot is 480°C to 490°C; the heating atmosphere of the continuous annealing furnace section is a nitrogen atmosphere, and contains 6% to 8% volume fraction of hydrogen, and the dew point in the furnace is -35°C to -40°C.

3. The method according to claim 1, characterized in that The thick-coated aluminum-zinc-aluminum-magnesium coated steel plate has a steel base of 1.8-2.5 mm in thickness, 1000-1150 mm in width, and a yield strength of 250-550 MPa, and is a hot-rolled pickled plate or a cold-rolled steel plate for structure.

4. The method according to claim 1, characterized in that: The chemical composition mass content of the steel base of the aluminum-zinc-aluminum-magnesium coated steel plate in the thick-specification thick coating is C: 0.1%~0.2%, Si: 0.02%~0.05%, Mn: 0.2%~0.3%, P≤0.035%, S≤0.03%, and the rest is Fe and unavoidable impurities; the roughness of the steel base of the steel plate is 0.8~1.2μm.

5. The method according to claim 1, characterized in that The coating of the thick-specification and thick-coated medium-aluminum-zinc-aluminum-magnesium coated steel plate is a medium-aluminum-zinc-aluminum-magnesium coating, and the coating composition is Al: 10% to 12%, Mg: 2% to 3%, Si: 0.1% to 0.2%, impurity elements ≤0.1%, and the rest is Zn.

6. The method according to claim 1, characterized in that In step ③, the air knife is nitrogen gas with a purity greater than 99.9%. The air knife is 60 to 70 mm from the zinc pot coating liquid level, the air knife is 15 to 20 mm from the strip, the air knife pressure is 40 to 50 mbar, the air knife angle is 0 to 1°, and the air knife lip gap is a wedge-shaped curve, which is narrow in the middle and wide on both sides, with a middle gap of 1.10 to 1.15 mm and a side gap of 1.45 to 1.50 mm.

7. The method according to claim 1, characterized in that Step ④: three mobile fans are arranged 2 to 5 m above the air knife, wherein the 1# mobile fan is a single motor with dual air outlets, the 2# mobile fans and the 3# mobile fans are single-sided outlets, symmetrically distributed on both sides of the strip, and set to the same fan power; the power of the 1# mobile fan is 80% to 90%, the power of the 2# mobile fan is 50% to 60%, and the power of the 3# mobile fan is 50% to 60%; the cooling speed achieved by the three mobile fans is 44 to 50°C / s.

8. The method according to claim 1, characterized in that Step ⑤: The fixed fans in the cooling tower are single-sided blowing, wherein the power of 1# fixed fan is 40% to 50%, the power of 2# fixed fan is 40% to 50%, the power of 3# fixed fan is 40% to 50%, and the vertical height from the cooling top roller to the zinc pot is 45m.

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