A method for manufacturing zinc-free hot-dip aluminum-zinc-magnesium coated steel plate

Abstract

The present invention relates to a method for manufacturing a zinc-spangle-free hot-dip aluminum-zinc-magnesium coated steel sheet. After hot-dip aluminum-zinc-magnesium coated steel sheet is hot-dip and cooled, it is subjected to micro-remelting treatment by reheating. The heating temperature of the coating outer plate surface is 510°C to 600°C, the dew point dp in the remelting zone is <-15°C, the heating time is 10s to 60s, the cooling rate is 6°C / s to 20°C / s, and the coating contains 0.5wt% to 3wt% of Mg-Zn intermetallic compounds after the heating and remelting treatment. The present invention heats and remelts the surface of the hot-dip aluminum-zinc-magnesium plate to cause micro-remelting of the surface coating, and rapidly cools and solidifies to form a zinc-spangle-free aluminum-zinc-magnesium coated steel sheet on the surface of the aluminum-zinc-magnesium coating.

Description

Technical Field

[0001] The invention belongs to the field of hot-dip galvanizing, and in particular relates to a method for manufacturing a zinc-flower-free hot-dip aluminum-zinc-magnesium coated steel plate. Background Art

[0002] On the basis of traditional hot-dip aluminum-zinc coating, adding a small amount of Mg element can further improve the corrosion resistance of the coated steel plate, especially the corrosion resistance of the cut edge can be increased by 1.5 to 2 times. In recent years, more and more aluminum-zinc coating products have begun to try to switch to aluminum-zinc-magnesium coating products, and related patents and literature have been continuously reported.

[0003] The magnesium content in aluminum-zinc-magnesium is very low, generally controlled at 1wt% to 3wt%, so it has little effect on the zinc spangle morphology brought by the main elements Al and Zn, and the plate surface basically continues the zinc spangle state in the aluminum-zinc coating.

[0004] This is incompatible with the hot-dip galvanized product without zinc spangles when used on the same finished product, resulting in inconsistent colors and seriously affecting the appearance. Therefore, in recent years, customers have continuously requested to reduce the zinc spangles of the aluminum-zinc-magnesium coating as much as possible, and even hope that manufacturers can produce aluminum-zinc-magnesium coating products without zinc spangles.

[0005] The size of zinc spangles in aluminum-zinc-magnesium coating is closely related to the roughness of the plate surface before plating, the composition of the plating solution, the cleanliness, the cooling capacity after plating and other parameters. Through strict control of parameters and improvement of hardware equipment capabilities, especially the improvement of post-cooling capacity, the zinc spangles have a significant trend of reduction. At present, the post-cooling control of aluminum-zinc-magnesium coating basically continues the process conditions of aluminum-zinc coating. From the previous simple air cooling, it has developed into aerosol cooling, aluminum powder to promote nucleation and other cooling methods. The design of the airflow nozzle of post-cooling is also becoming more and more diversified. In short, it is to quickly increase the cooling speed and inhibit the growth of zinc spangles in the coating.

[0006] Judging from the products currently on the market, the zinc spangle size of the aluminum-zinc-magnesium coating can be reduced to less than 2 mm, but the zinc spangles are still visible to the naked eye, and it is not yet possible to achieve an aluminum-zinc-magnesium coated steel plate without zinc spangles.

[0007] The present invention aims to prepare a zinc spangle-free aluminum-zinc-magnesium coated steel sheet by changing the process means without affecting any usability. Summary of the invention

[0008] The present invention provides a method for manufacturing a zinc spangle-free hot-dip aluminum-zinc-magnesium coated steel plate, wherein the surface of the hot-dip aluminum-zinc-magnesium plate is subjected to a heating and remelting treatment to cause a micro-remelting of the surface coating, and the surface coating is rapidly cooled and solidified to form a zinc spangle-free aluminum-zinc-magnesium coated steel plate on the surface of the aluminum-zinc-magnesium coating.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A method for manufacturing a zinc-free hot-dip aluminum-zinc-magnesium coated steel plate. After hot-dip aluminum-zinc-magnesium coated steel plate is hot-dip, cooled, and then slightly remelted by reheating. The heating temperature of the coating outer plate surface is 510°C to 600°C, the dew point dp in the remelting zone is <-15°C, the heating time is 10s to 60s, and the cooling rate is 6°C / s to 20°C / s. After the heating and slightly remelting treatment, the coating contains 0.5wt% to 3wt% of Mg-Zn intermetallic compounds.

[0011] After heating, the surface micro-remelting thickness is less than 20% of the total coating thickness.

[0012] Coating weight (double-sided) is 80g / m 2 ~300g / m 2 After heating and micro-remelting treatment, the Fe content in the coating is less than 3wt%, and it also contains 2wt% to 6wt% of Zn-Fe intermetallic compounds and 3wt% to 8wt% of Fe-Al intermetallic compounds.

[0013] After heating micro-remelting treatment, the coating structure includes ZnO-Al2O3 composite oxide layer, Zn-rich and Mg-Zn mixed phase, Al-Zn-Si dendrite phase and RE particles and Mg2Si intermetallic compounds dispersed between the dendrite phases; the coating structure order from outside to inside is ZnO-Al2O3 composite oxide layer, Zn-rich and Mg-Zn mixed phase layer, Al-Zn-Si dendrite phase layer; the thickness of the ZnO-Al2O3 composite oxide layer is less than 200nm.

[0014] The plating solution composition is Al 45wt% to 60wt%, Zn 30wt% to 50wt%, RE 0.02wt% to 0.12wt%, Si 1.0wt% to 2.0wt%, Mg 0.5wt% to 3.0wt%, and the others are impurity elements.

[0015] The average cooling rate from the time the steel plate leaves the aluminum-zinc pot to the time when the coating is fully solidified and zinc spangles are formed is above 6°C / s, and the size of the zinc spangles before micro-remelting is less than 8mm.

[0016] After the hot-dip aluminum-zinc coated steel plate is heated and remelted, the surface is slightly melted, and it re-solidifies in the subsequent rapid cooling to eliminate zinc spangles. If the remelting heating temperature is too high, the coating melts severely and flows, and the surface becomes uneven. The remelting temperature is too low and the remelting effect cannot be achieved, especially when the zinc spangles are large. The heating temperature of the coating outer plate surface of the present invention is controlled at 510℃~600℃; if the heating time is too long, the coating surface may flow, and the surface quality is seriously damaged. If the time is too short, it is difficult to complete the remelting process. The heating time of the present invention is controlled at 10s~60s; if the cooling speed is low, the micro-melting surface flows, and the surface quality is damaged. If the cooling speed is too fast and the air volume is too large, the coating surface that is initially in a semi-solid state is damaged. The present invention controls the cooling speed at 6℃ / s~20℃ / s; if the dew point is too high, the O content promotes the thickening of the surface oxide layer, causing powdering and shedding. The dew point is reduced as much as possible and the O content is reduced. The present invention controls the dew point to be less than -15℃.

[0017] In the hot-dip process, rapid cooling is used as much as possible for post-cooling to ensure that the original zinc flower size is small. If the original zinc flower size is large, the alloying process control becomes more difficult and requires a higher temperature or a longer heating time, which will lead to a serious decrease in the quality of the coating. The average cooling rate of the post-cooling of the present invention is above 6°C / s.

[0018] In the coating structure, the outermost ZnO-Al2O3 composite oxide layer can play a good role in corrosion resistance. The dense oxide layer can effectively block the invasion of external corrosive media. However, if the oxide layer is too thick, it will easily lead to embrittlement of the coating and even powdering and peeling. Therefore, the thickness of the composite oxide layer must be strictly controlled.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention provides a method for manufacturing a zinc-spangle-free aluminum-zinc-magnesium coated steel plate with excellent performance. The coating surface of the steel plate is zinc-spangle-free and can be used in conjunction with zinc-spangle-free hot-dip galvanized steel plates. The corrosion resistance is better than that of traditional hot-dip aluminum-zinc steel plates. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a zinc flower morphology diagram of the steel plate surface before remelting treatment in Example 1 of the present invention.

[0022] Figure 2 This is a morphology diagram of the surface of the steel plate without zinc flowers after remelting treatment in Example 1 of the present invention. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0024] A method for manufacturing a zinc-free hot-dip aluminum-zinc-magnesium coated steel plate. After hot-dip aluminum-zinc-magnesium coated steel plate is hot-dip, cooled, and then slightly remelted by reheating. The heating temperature of the coating outer plate surface is 510°C to 600°C, the dew point dp in the remelting zone is <-15°C, the heating time is 10s to 60s, and the cooling rate is 6°C / s to 20°C / s. After the heating and remelting treatment, the coating contains 0.5wt% to 3wt% of Mg-Zn intermetallic compounds.

[0025] After heating, the remelting thickness of the surface is less than 20% of the total thickness of the coating.

[0026] The coating weight is 80g / m 2 ~300g / m 2 After heating and remelting treatment, the Fe content in the coating is less than 3wt%, and it also contains 2wt% to 6wt% of Zn-Fe intermetallic compounds and 3wt% to 8wt% of Fe-Al intermetallic compounds.

[0027] After heating and remelting treatment, the coating structure is composed of ZnO-Al2O3 composite oxide layer, Zn-rich and Mg-Zn mixed phase, Al-Zn-Si dendrite phase and RE particles and Mg2Si intermetallic compounds interspersed between the dendrite phases from outside to inside; the thickness of the ZnO-Al2O3 composite oxide layer is less than 200nm.

[0028] The plating solution composition is Al 45wt% to 60wt%, Zn 30wt% to 50wt%, RE 0.02wt% to 0.12wt%, Si 1.0wt% to 2.0wt%, Mg 0.5wt% to 3.0wt%, and the others are impurity elements.

[0029] The average cooling rate from the time the steel plate leaves the aluminum-zinc pot to the time when the coating is fully solidified and zinc spangles are formed is above 6°C / s, and the size of the zinc spangles before micro-remelting and alloying is less than 8mm.

[0030] The remelting process of the embodiment of the present invention is shown in Table 1, and the composition of the plated steel plate after remelting is shown in Table 2.

[0031] Table 1 Remelting process parameters

[0032]

[0033] Table 2 Composition of coated steel sheet after remelting treatment (wt%)

[0034]

[0035] See Figure 1 This is a comparison of the surface morphology of the steel plate before and after the remelting treatment in Example 1. After the remelting treatment, basically no zinc spangles can be visually observed on the surface of the steel plate.

Claims

1. A method for manufacturing a zinc-free hot-dip aluminum-zinc-magnesium coated steel sheet, characterized in that: After hot-dip aluminum-zinc-magnesium coated steel plate is hot-dip coated and cooled, it is reheated for micro-remelting treatment. The heating temperature of the coating outer plate surface is 510℃~600℃, the dew point dp of the remelting zone is <-15℃, the heating time is 10s~60s, and the cooling rate is 6℃ / s~20℃ / s. After the heating micro-remelting treatment, the coating contains 0.5wt%~3wt% of Mg-Zn intermetallic compounds; The plating solution composition is Al 45wt%~60wt%, Zn 30wt%~50wt%, RE 0.02wt%~0.12wt%, Si 1.0wt%~2.0wt%, Mg 0.5wt%~3.0wt%, and the others are impurity elements; After heating, the surface micro-remelting thickness is less than 20% of the total coating thickness; After heating micro-remelting treatment, the coating structure includes ZnO-Al2O3 composite oxide layer, Zn-rich and Mg-Zn mixed phase, Al-Zn-Si dendrite phase and RE particles and Mg2Si intermetallic compounds dispersed between the dendrite phases; the coating structure sequence from outside to inside is ZnO-Al2O3 composite oxide layer, Zn-rich and Mg-Zn mixed phase layer, Al-Zn-Si dendrite phase layer; the thickness of the ZnO-Al2O3 composite oxide layer is less than 200nm; The average cooling rate from the time the steel plate leaves the aluminum-zinc pot to the time when the coating is fully solidified and zinc spangles are formed is above 6°C / s, and the size of the zinc spangles before micro-remelting is less than 8mm.

2. The method for manufacturing a zinc spangle-free hot-dip aluminum-zinc-magnesium coated steel sheet according to claim 1, characterized in that: The coating weight is 80g / m 2 ~300g / m 2 After heating and micro-remelting treatment, the Fe content in the coating is less than 3wt%, and it also contains 2wt% to 6wt% of Zn-Fe intermetallic compounds and 3wt% to 8wt% of Fe-Al intermetallic compounds.

3. The method for manufacturing a zinc spangle-free hot-dip aluminum-zinc-magnesium coated steel sheet according to claim 1, characterized in that: The thickness of the Zn-rich layer is less than 0.4µm.

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