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

Abstract

The present invention relates to a method for manufacturing a zinc-free hot-dip aluminum-zinc coated steel plate. After hot-dip aluminum-zinc coated steel plate is hot-dip and cooled, it is subjected to micro-remelting treatment by reheating. The heating temperature of the coating outer plate surface is 530°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 3°C / s to 20°C / s. The present invention is intended to achieve zinc-free surface of aluminum-zinc coated steel plate. By reheating the surface of the hot-dip aluminum-zinc plate, the surface coating is micro-remelted, and it is rapidly cooled and solidified, so that the aluminum-zinc coated steel plate without zinc spangles is formed on the surface of the aluminum-zinc 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 coated steel plate. Background Art

[0002] Hot-dip aluminum-zinc coating is widely used in home appliances and construction due to its good corrosion resistance. The surface of the aluminum-zinc coating has obvious zinc flower morphology, which will bring a certain aesthetic feeling when used in home appliances, construction and other fields.

[0003] The process of aluminum-zinc coated steel plate is more complicated than hot-dip galvanizing, and the production line consumes more energy, so the price is more expensive than hot-dip galvanized products. When choosing, customers generally use aluminum-zinc coating for parts with more stringent use conditions, and choose cheaper hot-dip galvanized products for parts with good environment. However, the zinc spangle morphology of aluminum-zinc is incompatible with the hot-dip galvanized products without zinc spangles 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 spangle of the aluminum-zinc coating as much as possible, and even hope that the manufacturer can produce aluminum-zinc coated products without zinc spangles.

[0004] The size of zinc spangles in aluminum-zinc coating is closely related to the roughness of the plate surface before coating, the degree of cleanliness, and the cooling capacity after coating. 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 coating is becoming more and more refined. From the previous simple air cooling, aerosol cooling and aluminum powder nucleation promotion cooling methods have been developed. The design of post-cooling airflow nozzles is also becoming more and more diversified. In short, they are all for the purpose of quickly increasing the cooling speed and inhibiting the growth of zinc spangles in the coating.

[0005] Judging from the products currently on the market, the size of the zinc spangles of the aluminum-zinc 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 coated steel plate without zinc spangles.

[0006] The present invention aims to prepare an aluminum-zinc coated steel sheet without zinc spangles by changing the process means without affecting any usability. Summary of the invention

[0007] The present invention provides a method for manufacturing a zinc spangle-free hot-dip aluminum-zinc coated steel plate. The present invention aims to achieve zinc spangle-free surface of the aluminum-zinc coated steel plate. The surface of the hot-dip aluminum-zinc plate is reheated to cause a micro-remelting of the surface coating, which is then rapidly cooled and solidified to form a zinc spangle-free aluminum-zinc coated steel plate on the surface of the aluminum-zinc coating.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A method for manufacturing a zinc spangle-free hot-dip aluminum-zinc coated steel plate. After hot-dip aluminum-zinc coated steel plate is hot-dip, cooled, and then slightly remelted by reheating. The heating temperature of the coating outer plate surface is 530°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 3°C / s to 20°C / s.

[0010] After heating micro-remelting treatment, the coating structure includes ZnO-Al2O3 composite oxide layer, Zn-rich layer, and Al-Zn-Si dendrite phase layer. The organizational structure is in the order of ZnO-Al2O3 composite oxide layer, Zn-rich layer, and Al-Zn-Si dendrite phase layer from outside to inside. The thickness of the ZnO-Al2O3 composite oxide layer is less than 200nm, and the thickness of the Zn-rich layer is less than 0.5μm.

[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 contains 2wt% to 6wt% of Zn-Fe intermetallic compounds and 3wt% to 8wt% of Fe-Al metal compounds.

[0013] 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%, and the others are impurity elements.

[0014] The average cooling rate from the time the steel plate leaves the aluminum-zinc pot to the time the coating is fully solidified and zinc spangles are formed is above 6°C / s, ensuring that the size of the zinc spangles before micro-remelting treatment is less than 8mm.

[0015] After the hot-dip aluminum-zinc coated steel plate is remelted in an alloying furnace, micro-melting occurs on the surface, and it re-solidifies in the subsequent rapid cooling to eliminate zinc spangles. If the alloying temperature is too high, the coating melts severely and flows, and the surface becomes uneven. If the temperature is too low, the remelting effect cannot be achieved, especially when the zinc spangles are large. The present invention controls the remelting temperature to 530℃~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 present invention controls the heating time to 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 to 3℃ / 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 to reduce the O content. The dew point of the present invention is <-15℃.

[0016] In the hot-dip process, rapid cooling is used as much as possible to ensure that the original zinc spangle size is small. If the original zinc spangle 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 present invention sets the average cooling rate from the time the steel plate leaves the aluminum-zinc pot to the time the coating is fully solidified and the zinc spangles are formed to be above 6°C / s.

[0017] 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.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention aims to produce an excellent performance zinc-free aluminum-zinc coated steel sheet, the coating surface is zinc-free, can be used with zinc-free hot-dip galvanized sheet, and the corrosion resistance is better than that of traditional hot-dip aluminum-zinc steel sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a zinc flower morphology diagram before remelting treatment in Example 1 of the present invention.

[0021] Figure 2 This is a morphology diagram of zinc flowers after remelting treatment in Example 1 of the present invention. DETAILED DESCRIPTION

[0022] 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.

[0023] A method for manufacturing a zinc spangle-free hot-dip aluminum-zinc coated steel plate. After hot-dip aluminum-zinc coated steel plate is hot-dip, cooled, and then slightly remelted by reheating. The heating temperature of the coating outer plate surface is 530°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 3°C / s to 20°C / s.

[0024] After heating and remelting treatment, the organizational structure from outside to inside is ZnO-Al2O3 composite oxide layer, Zn-rich layer, Al-Zn-Si dendrite phase, among which the thickness of ZnO-Al2O3 composite oxide layer is less than 200nm, and the thickness of Zn-rich layer is less than 0.5μm.

[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 / m2 After heating and remelting treatment, the Fe content in the coating is less than 3wt%, and it contains 2wt% to 6wt% of Zn-Fe intermetallic compounds and 3wt% to 8wt% of Fe-Al metal compounds.

[0027] 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%, and the others are impurity elements.

[0028] 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 treatment is less than 8mm.

[0029] 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.

[0030] Table 1 Alloying process parameters

[0031]

[0032]

[0033] Table 2 Composition of coated steel plate after alloying treatment

[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 galvanized steel sheet, characterized in that: After hot-dip galvanized steel sheet is hot-dip and cooled, it is reheated for micro-remelting treatment. The heating temperature of the coating outer plate surface is 530℃~600℃, the dew point of the remelting zone dp<-15℃, the heating time is 10s~60s, and the cooling rate is 3℃ / s~20℃ / s; The composition of the plating solution is Al 45wt% to 60wt%, Zn 30wt% to 50wt%, RE 0.02wt% to 0.12wt%, Si 1.0wt% to 2.0wt%, and the others are impurity elements; After heating micro-remelting treatment, the coating structure includes ZnO-Al2O3 composite oxide layer, Zn-rich layer, Al-Zn-Si dendrite phase layer, and the organizational structure sequence from outside to inside is ZnO-Al2O3 composite oxide layer, Zn-rich layer, Al-Zn-Si dendrite phase layer, wherein the thickness of ZnO-Al2O3 composite oxide layer is less than 200nm, and the thickness of Zn-rich layer is less than 0.5µm; After heating, the surface micro-remelting thickness is less than 20% of the total coating thickness; 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 treatment is less than 8mm.

2. The method for manufacturing a zinc spangle-free hot-dip galvanized 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%, containing 2wt% to 6wt% of Zn-Fe intermetallic compounds and 3wt% to 8wt% of Fe-Al intermetallic compounds.

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