Thin-coating galvanized hot-stamping formed steel and preparation method thereof

Abstract

The invention provides thin-coating galvanized hot-stamping formed steel and a preparation method thereof, and relates to the technical field of metal material processing. The hot stamping forming steel comprises a steel substrate and a zinc coating, and the steel substrate comprises the following chemical components in percentage by mass: less than or equal to 0.5 wt% of C, 0.1-1.0 wt% of Si, 0.5-3 wt% of Mn, 0.1-0.8 wt% of Al, less than or equal to 1.5 wt% of Cr, less than or equal to 0.1 wt% of Mo, less than or equal to 0.1 wt% of Ni, 0.05-0.07 wt% of N, less than or equal to 0.01 wt% of O and the balance of Fe and inevitable impurity elements; the zinc coating is composed of an eta (Zn) phase, a delta (Zn) phase and other inevitable impurity phases, and a compact Fe-Al inhibition layer with the thickness smaller than or equal to 1 micrometer exists between the steel base body and the zinc coating. After hot stamping, a thinner zinc plating layer enables a smaller amount of liquid zinc to permeate into a steel substrate, and through obstruction of the Fe-Al inhibition layer, the thin plating layer zinc plating hot stamping forming steel inhibiting formation of LME cracks is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material processing, and in particular to a thin-layer galvanized hot stamping steel and a preparation method thereof. Background Art

[0002] Galvanized hot-stamped steel is a high-strength steel commonly used in the automotive industry. In recent years, with increasing demands for safety and energy conservation and emission reduction, lightweighting vehicles has become a research priority. Galvanized hot-stamped steel can improve vehicle safety while reducing overall weight. The zinc coating also offers excellent corrosion resistance and ductility, and is more cost-effective than aluminum coatings. Consequently, galvanized hot-stamped steel has become a research hotspot.

[0003] For conventional continuous hot-dip galvanized steel, the coating thickness is typically 20-25 μm. However, thicker coatings can negatively impact subsequent processing steps, such as welding, stamping, and bending. Thick coatings are prone to cracking or flaking during processing, affecting the precision and appearance of components. Furthermore, thicker coatings require more zinc solution and longer galvanizing times, increasing production costs. Furthermore, for hot-stamped galvanized steel, thicker zinc coatings can also lead to more severe liquid metal embrittlement cracking.

[0004] Liquid metal embrittlement cracking occurs when the galvanized hot-stamped steel is deformed at a temperature higher than the melting point of zinc (419.5°C) during the hot stamping process. This causes the coating to melt and penetrate into the steel matrix along the iron grain boundaries, destroying the grain boundary bonding and ultimately causing microcracks under the action of tensile stress. These microcracks significantly reduce the ductility and toughness of the hot-stamped steel.

[0005] Currently, zinc coating thickness is primarily controlled by modifying the composition and content of the zinc bath. For example, rare earth elements such as lanthanum and cerium are added to the bath to reduce its surface tension and improve its fluidity. Alternatively, an appropriate amount of nickel is added to the bath to form an iron-zinc-nickel ternary compound, mitigating the "Sanderin effect" of Si, which segregates and causes uneven or excessively thick coatings.

[0006] However, these added elements are expensive, difficult to control, and improper content can lead to unstable coating properties. Finding a low-cost, accurate control method for thin galvanized layers in existing galvanizing processes is an urgent problem to be solved.

[0007] Currently, liquid metal embrittlement of galvanized hot-formed steel is mainly achieved by changing the composition and content of the steel matrix and the existing process technology. For example, 0.7wt% to 2.5wt% of Si is added to the steel matrix, so that during the hot stamping process, Si selectively oxidizes to form fine SiO2 on the iron grain boundaries, thereby hindering the penetration of zinc atoms into the steel matrix and suppressing the occurrence of liquid metal embrittlement cracks. Alternatively, the galvanized steel is pre-cooled after annealing and before hot stamping, with a cooling rate of ≥25°C to rapidly cool the coating, thereby reducing the liquid zinc content and suppressing the occurrence of liquid metal embrittlement cracks.

[0008] In summary, the existing technology is unable to combine thin coating with the suppression of liquid metal embrittlement to produce thin coating galvanized hot stamping steel that can effectively suppress liquid metal embrittlement. Summary of the Invention

[0009] In view of this, the present invention provides a thin-coated galvanized hot-stamped steel and a preparation method thereof. First, by controlling the surface roughness of the cold-rolled steel strip, the surface roughness of the cold-rolled steel strip can be appropriately increased, thereby increasing the contact area between the zinc liquid and the steel strip surface, allowing the zinc liquid to more fully wet the steel surface, allowing the Al element in the zinc liquid to react with the Fe element to form a dense Fe-Al inhibition layer, and increasing the adhesion of the zinc layer. Secondly, the air knife process parameters are optimized, and the zinc coating thickness is ultimately controlled to be 4-8μm. At the same time, the aluminum content in the steel substrate and the zinc liquid is appropriately increased, so that a certain thickness of fine-grained Fe-Al inhibition layer is retained between the substrate and the coating during the hot stamping process, thereby inhibiting the penetration of liquid zinc into the steel substrate. The thin coating reduces the generation of liquid zinc in the galvanized steel during the hot stamping process. The dense Fe-Al inhibition layer not only strengthens the bonding between the coating and the substrate, but also hinders the penetration of liquid zinc into the steel substrate. The combined effect of these two factors suppresses the formation of LME cracks.

[0010] To this end, the technical solution adopted in the present invention is as follows:

[0011] The present invention provides a thin-layer galvanized hot-stamped steel, comprising a steel substrate and a galvanized layer, wherein the chemical composition of the steel substrate comprises, by mass percentage, C≤0.5wt%, Si: 0.1-1.0wt%, Mn: 0.5-3wt%, Al: 0.1-0.8wt%, Cr: ≤1.5wt%, Mo: ≤0.1wt%, Ni: ≤0.1wt%, N: 0.05-0.07wt%, O: ≤0.01wt%, and the balance is Fe and unavoidable impurity elements;

[0012] The galvanized layer is composed of η(Zn) phase, δ(Zn) phase, and other inevitable impurity phases. A dense Fe-Al inhibition layer with a thickness of ≤1μm exists between the steel substrate and the galvanized layer. The phases in the Fe-Al layer are calculated by percentage as follows: Fe2Al5 phase (30%-70%), FeAl phase (20%-30%), Fe3Al phase (5%-15%), α-Fe and γ-Fe (≤10wt%), and the balance is other inevitable impurity phases. The Fe2Al5 phase and FeAl phase satisfy the requirement of 60wt%≤Fe3Al+FeAl≤80wt%.

[0013] Preferably, the liquid metal embrittlement crack depth of the hot stamping steel is less than 100 μm.

[0014] The present invention also provides a method for preparing the hot stamping steel, comprising the following steps:

[0015] S1. Smelting, continuous casting, hot rolling and pickling the steel plate raw material in sequence to obtain a pickled hot-rolled slab;

[0016] S2, cold rolling the pickled hot-rolled slab to obtain a cold-rolled steel strip;

[0017] S3, annealing and hot-dip galvanizing the cold-rolled steel strip to obtain a thin-layer galvanized steel coil;

[0018] S4. Hot stamping the thin-layer galvanized steel coil to obtain the thin-layer galvanized hot stamped steel.

[0019] Preferably, the process involved in S1 is well known to those skilled in the art. Specific continuous casting parameters include a casting speed of 0.5-2.5 m / min and a slab drawing speed of 1.0-3.0 m / min. Hot rolling parameters include a heating furnace temperature of 1150-1250°C, a finishing rolling temperature of 800-950°C, and a coiling temperature of 500-700°C. Pickling is performed using a 5% HCl solution at a temperature of 50-70°C for 60-100 seconds. The resulting pickled hot-rolled slab has no surface defects.

[0020] Preferably, in S2, the cold rolling process is well known to those skilled in the art, and the specific cold rolling parameters are: rolling speed of 300-1000m / min, total reduction rate of 65-90%, and roller temperature of 25-50°C. The difference from the prior art is that the present application defines the surface roughness of the roller of the end stand of the rolling mill treated by the cold rolling process as Ra1.8~Ra3.2, and the surface roughness of the strip is uniform and is Ra1.2~Ra1.8. When it does not affect the surface quality requirements in the subsequent processing process, the surface roughness of the strip is appropriately increased. If the surface roughness of the strip after rolling is higher than Ra1.8, the steel plate is flattened once or multiple times until the surface roughness meets the requirements. Studies have found that reasonable surface roughness can increase the contact area between the zinc liquid and the surface of the strip, so that the zinc liquid can more fully wet the surface of the steel, allowing the Al element in the zinc liquid to react with the Fe element to form a dense Fe-Al inhibition layer, thereby increasing the adhesion of the zinc layer.

[0021] Preferably, in S3, the annealing process is well known to those skilled in the art, specifically, an annealing temperature of 750-850°C, a holding time of 20-60 seconds, an atmosphere of 5-20% H₂ + 80-95% N₂, and a dew point above -30°C. Unlike the prior art, this application specifies that during hot-dip galvanizing, the aluminum content in the zinc bath is 0.2wt% to 0.4wt%. More preferably, when the surface roughness of the cold-rolled sheet is Ra1.5 to Ra1.8, the aluminum content is 0.2wt% to 0.3wt%, and when the surface roughness is Ra1.2 to Ra1.5, the aluminum content is 0.3wt% to 0.4wt%. This is because the aluminum content in the zinc bath affects its viscosity. Higher viscosity results in stronger zinc bonding, resulting in a relatively lower surface roughness for the strip.

[0022] Preferably, in S3, when the hot-dip galvanizing process is performed, the temperature of the zinc bath during the galvanizing process is 450-550°C. More preferably, when the aluminum content of the zinc bath is 0.2wt%-0.3wt%, the temperature of the zinc bath is 450-500°C, and when the aluminum content of the zinc bath is 0.3wt%-0.4wt%, the temperature of the zinc bath is 500-550°C. Since higher zinc bath temperatures reduce viscosity, and higher aluminum content in the zinc bath increases viscosity, it is necessary to select different zinc bath temperatures based on the aluminum content in the zinc bath to effectively control the viscosity of zinc baths with different aluminum contents within the required range.

[0023] Preferably, in S3, when the hot-dip galvanizing process is performed, the viscosity of the zinc solution during the galvanizing process is 0.1-0.5 Pa·s.

[0024] Preferably, in S3, when the hot-dip galvanizing process is performed, the hot-dip time of the steel sheet during the galvanizing process is 1 to 6 seconds. More preferably, when the viscosity of the zinc solution is 0.1 to 0.3 Pa·s, the hot-dip time is 4 to 6 seconds; when the viscosity of the zinc solution is 0.3 to 0.5 Pa·s, the hot-dip time is 1 to 4 seconds. The higher the viscosity of the zinc solution, the shorter the time it takes to ensure that the coating content meets the requirements.

[0025] Preferably, in S3, during the hot-dip galvanizing process, the air knife pressure during the galvanizing process is 1-3 bar, the nozzle angle is 30°-45°, the distance between the nozzle and the steel plate is 3-20 mm, and the air flow temperature is 120-150°C. More preferably, when the final zinc layer thickness is 4-6 μm, the air knife pressure is 2.5-3.0 bar, the nozzle angle is 30°-35°, the distance between the nozzle and the steel plate is 3-12 mm, and the air flow temperature is 135-150°C; when the final zinc layer thickness is 6-8 μm, the air knife pressure is 1-2.5 bar, the nozzle angle is 35°-45°, the distance between the nozzle and the steel plate is 12-20 mm, and the air flow temperature is 120-135°C. Studies have found that appropriately increasing the air knife pressure and decreasing the nozzle angle and the distance between the nozzle and the steel plate can increase the force of the airflow on the zinc solution, making the zinc solution flow more evenly and achieving a thinner thickness. A reasonable air knife temperature can slow down the solidification of the zinc solution and increase its fluidity. A reasonable combination of these parameters can ultimately achieve the ideal zinc coating thickness.

[0026] Preferably, in S3, the zinc layer of the thin-layer galvanized steel coil has a thickness of 4-8 μm, and the zinc layer is mainly composed of η(Zn) phase, δ(Zn) phase and other inevitable impurity phases.

[0027] Preferably, in S4, the hot stamping process is well known to those skilled in the art, and the specific parameters are: heating temperature of 870-930°C, holding time of 5-10 min, mold temperature of 20-60°C, and stamping speed of 10-20 mm / s.

[0028] Preferably, in S4, the zinc layer of the thin-layer galvanized hot stamping steel mainly consists of about 50% to 75% of α-Fe(Zn) phase and the balance of Γ(Fe3Zn 10 ) phase, a very small amount of η(Zn) phase and inevitable impurity phases.

[0029] The present invention has at least the following beneficial effects:

[0030] Based on the steel substrate composition and preparation process, this invention increases the surface roughness of the steel substrate to enhance the adhesion of the zinc coating. By appropriately increasing the aluminum content in the steel substrate and the zinc bath, a dense Fe-Al inhibition layer with a thickness of ≤1 μm is formed between the zinc coating and the steel substrate. Furthermore, by optimizing the air knife parameters, the invention ultimately produces thin-coated galvanized steel coils with a zinc coating thickness ranging from 4 to 8 μm. After hot stamping, the thinner zinc coating allows less liquid zinc to penetrate the steel substrate, where it is blocked by the Fe-Al inhibition layer, resulting in thin-coated galvanized hot-stamped steel that suppresses LME crack formation. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0032] Figure 1 Schematic diagram of the coating structure before and after stamping;

[0033] Figure 2 Schematic diagram of suppressing LME crack generation. DETAILED DESCRIPTION

[0034] In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0035] The present invention provides a thin-layer galvanized hot-stamped steel, comprising a steel substrate and a galvanized layer, wherein the chemical composition of the steel substrate comprises, by mass percentage, C≤0.5wt%, Si: 0.1-1.0wt%, Mn: 0.5-3wt%, Al: 0.1-0.8wt%, Cr: ≤1.5wt%, Mo: ≤0.1wt%, Ni: ≤0.1wt%, N: 0.05-0.07wt%, O: ≤0.01wt%, and the balance is Fe and unavoidable impurity elements;

[0036] The galvanized layer consists of η(Zn) phase, δ(Zn) phase and other inevitable impurity phases. A dense Fe-Al inhibition layer with a thickness of ≤1 μm exists between the steel substrate and the galvanized layer.

[0037] The preparation method of the thin-layer galvanized hot stamping steel includes smelting, continuous casting, hot rolling, pickling, cold rolling, annealing, hot-dip galvanizing and hot stamping. The specific process parameters are as follows:

[0038] (1) Continuous casting process: casting speed is 0.5-2.5m / min, and billet drawing speed is 1.0-3.0m / min.

[0039] (2) Hot rolling process: heating furnace temperature is 1150-1250℃, finishing rolling temperature is 800-950℃, and coiling temperature is 500-700℃. Pickling uses 5% HCl solution at a temperature of 50-70℃ and a pickling time of 60-100s.

[0040] (3) Cold rolling process: rolling speed is 300-1000 m / min, total reduction is 65-90%, and roll temperature is 25-50°C. The surface roughness of the end stand roll of the cold-rolled mill is Ra1.8-Ra3.2, and the surface roughness of the strip is uniform and is Ra1.2-Ra1.8.

[0041] (4) Annealing process: heating temperature is 750-850℃, holding time is 20-60s, atmosphere control is 5-20%H2+80-95%N2, and dew point is controlled above -30℃.

[0042] (5) Hot-dip galvanizing process: the aluminum content in the zinc solution is 0.2-0.4wt%, the hot-dip time is 1-6s, the air knife pressure is 1-3bar, the nozzle angle is 30-45°, the distance between the nozzle and the steel plate is 3-20mm, and the air flow temperature is 120-150℃. The obtained zinc coating thickness is 4-8μm.

[0043] (6) Hot stamping process: heating temperature is 870-930℃, holding time is 5-10min, mold temperature is 20-60℃, and stamping speed is 10-20mm / s. Figure 1 Schematic diagram of the coating structure before and after stamping. Before stamping, an Fe-Al inhibition layer exists at the interface between the steel substrate and the galvanized layer. After stamping, due to the increase in temperature, Al atoms in the Fe-Al inhibition layer diffuse toward the surface, reacting with oxygen to form Al₂O₃. A certain amount of Al remains at the interface between the galvanized layer and the steel substrate, forming a new, fine-grained Fe-Al inhibition layer with the Fe substrate.

[0044] The liquid metal embrittlement crack depth of the product obtained after hot stamping is less than 100 μm. Figure 2 Schematic diagram of LME crack suppression. The galvanized steel coil is mainly composed of zinc layer, Fe-Al inhibition layer and steel matrix. During the annealing heating process, the zinc liquid in the zinc layer melts and mainly consists of liquid zinc and α-Fe(Zn) phase. A certain Fe-Al inhibition layer is still retained at the interface. During the cooling process, due to the peritectic reaction, Γ(Fe3Zn 10 ) phase, mainly composed of α-Fe(Zn), Γ(Fe3Zn 10) and a small amount of liquid zinc. Zinc atoms penetrate along the iron grain boundaries. Ultimately, during the stamping process, liquid metal embrittlement cracks form due to stress. However, aluminum atoms in the inhibition layer react with the iron matrix to form fine Fe2Al5 particles at the grain boundaries, hindering the penetration of zinc atoms and ensuring that the crack depth meets surface quality requirements.

[0045] In Examples 1-8, the continuous casting speed was 2 m / min and the drawing speed was 1.5 m / min. The hot rolling temperature was 1250°C, the finishing temperature was 950°C, and the coiling temperature was 650°C. Pickling was performed using a 5% HCl solution at 60°C for 90 seconds. The cold rolling reduction was 70%, and the surface roughness was controlled within Ra 1.5-1.8. Annealing was performed using continuous annealing (CAL) at 850°C for 60 seconds in an atmosphere of 5% H₂ + 95% N₂ with a dew point of -30°C. Hot stamping was performed at 900°C for 5 minutes, the die temperature was 25°C, and the stamping speed was 15 mm / s.

[0046] The surface roughness of the end stand rolling mill roll during the cold rolling process in Comparative Example 1 was Ra 0.8-1.0, and the other parameters were the same as in Example 7. The air knife pressure in Comparative Example 2 exceeded 1-3 bar, and the other parameters were the same as in Example 7. The aluminum content in the galvanizing bath in Comparative Example 3 exceeded 0.2-0.4 wt%, and the other parameters were the same as in Example 7.

[0047] The chemical composition and mass percentage of the steel substrates of Examples 1-8 and Comparative Examples 1-3 are shown in Table 1, the hot-dip galvanizing process parameters and coating thickness are shown in Table 2, and the phase composition and LME crack depth of the galvanized layer of each example after hot stamping deformation are shown in Table 3.

[0048] Table 1 Chemical composition and mass percentage of steel substrates in various examples

[0049]

[0050] Table 2 Hot dip galvanizing process parameters and coating thickness of each embodiment

[0051]

[0052] Table 3 Phase composition of the galvanized layer and LME crack depth after hot stamping in various examples

[0053]

[0054] As shown in Table 1-3, in Examples 1-8, as the aluminum content of the steel substrate gradually increased, the zinc coating thickness was significantly reduced by increasing the zinc bath temperature, air knife pressure, and airflow temperature, while reducing the hot dip time, nozzle angle, and nozzle-to-steel distance. After hot stamping, the thinner the zinc layer, the shallower the liquid metal embrittlement crack depth, meeting the liquid metal embrittlement crack depth requirements for automotive galvanized steel.

[0055] Compared with Example 7, Comparative Example 1 reduces the surface roughness of the rolling mill, which correspondingly reduces the surface roughness of the slab and reduces the adhesion of the zinc liquid. As a result, under the same parameters as in Example 7, the zinc liquid has difficulty adhering and the thickness is less than the requirement. The air knife pressure in Comparative Example 2 is higher than that in Example 7, causing the air knife to blow away more zinc liquid, resulting in a thinner zinc layer and a thickness less than the requirement. In Comparative Example 3, while other parameters are the same as in Example 7, the aluminum content in the zinc liquid is increased, which increases the viscosity of the zinc liquid, causes more zinc liquid to adhere to the surface, and increases the thickness of the zinc layer. As a result, more zinc liquid penetrates deeper into the steel matrix during the stamping process, causing a significant increase in the crack length.

[0056] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A thin-layer galvanized hot stamping steel, characterized in that: The invention comprises a steel substrate and a galvanized layer, wherein the chemical composition of the steel substrate comprises, by mass percentage, C≤0.5wt%, Si: 0.1-1.0wt%, Mn: 0.5-3wt%, Al: 0.1-0.8wt%, Cr: ≤1.5wt%, Mo: ≤0.1wt%, Ni: ≤0.1wt%, N: 0.05-0.07wt%, O: ≤0.01wt%, and the balance is Fe and unavoidable impurity elements; The galvanized layer consists of η(Zn) phase, δ(Zn) phase and other inevitable impurity phases. A dense Fe-Al inhibition layer with a thickness of ≤1 μm exists between the steel substrate and the galvanized layer.

2. The thin-layer galvanized hot stamping steel according to claim 1, characterized in that: The liquid metal embrittlement crack depth of the hot stamping steel is less than 100 μm.

3. The method for preparing hot stamping steel according to claim 1 or 2, characterized in that: The following steps are involved: S1. Smelting, continuous casting, hot rolling and pickling the steel plate raw material in sequence to obtain a pickled hot-rolled slab; S2, cold rolling the pickled hot-rolled slab to obtain a cold-rolled steel strip; S3, annealing and hot-dip galvanizing the cold-rolled steel strip to obtain a thin-layer galvanized steel coil; S4. Hot stamping the thin-layer galvanized steel coil to obtain the thin-layer galvanized hot stamped steel.

4. The method for preparing hot stamping steel according to claim 3, characterized in that: In S2, the surface roughness of the rollers of the end stand of the rolling mill subjected to the cold rolling treatment is Ra1.8-Ra3.2, and the surface roughness of the strip steel is uniform and is Ra1.2-Ra1.

8.

5. The method for preparing hot stamping steel according to claim 4, characterized in that: When hot-dip galvanizing is carried out, the aluminum content in the zinc solution is 0.2wt%~0.4wt%; Preferably, when the surface roughness of the cold-rolled sheet is Ra1.5~Ra1.8, the aluminum content in the zinc liquid is 0.2wt%~0.3wt%; when the surface roughness of the cold-rolled sheet is Ra1.2~Ra1.5, the aluminum content in the zinc liquid is 0.3wt%~0.4wt%.

6. The method for preparing hot stamping steel according to claim 5, characterized in that: In S3, when hot-dip galvanizing is performed, the temperature of the zinc solution during the galvanizing process is 450~550℃; Preferably, when the aluminum content in the zinc liquid is 0.2wt%~0.3wt%, the temperature of the zinc liquid is 450~500℃; when the aluminum content in the zinc liquid is 0.3wt%~0.4wt%, the temperature of the zinc liquid is 500~550℃.

7. The method for preparing hot stamping steel according to claim 6, characterized in that: In S3, when hot-dip galvanizing is performed, the viscosity of the zinc solution during the galvanizing process is 0.1~0.5Pa·s.

8. The method for preparing hot stamping steel according to claim 7, characterized in that: In S3, when the hot-dip galvanizing process is performed, the hot-dip time of the steel plate during the galvanizing process is 1 to 6 seconds; Preferably, when the viscosity of the zinc liquid is 0.1-0.3 Pa·s, the hot dipping time is 4-6 s; when the viscosity of the zinc liquid is 0.3-0.5 Pa·s, the hot dipping time is 1-4 s.

9. The method for preparing hot stamping steel according to claim 3, characterized in that: In S3, when hot-dip galvanizing is performed, the air knife pressure during the galvanizing process is 1~3 bar, the nozzle angle is 30°~45°, the distance between the nozzle and the steel plate is 3~20 mm, and the air flow temperature is 120~150℃.

10. The method for preparing hot stamping steel according to claim 3, characterized in that: In S3, the zinc layer of the thin-coated galvanized steel coil has a thickness of 4 to 8 μm, and the zinc layer is composed of η(Zn) phase, δ(Zn) phase and other inevitable impurity phases; Preferably, when the zinc layer thickness is 4 ~ 6μm, the air knife pressure is 2.5 ~ 3.0bar, the nozzle angle is 30° ~ 35°, the distance between the nozzle and the steel plate is 3 ~ 12mm, and the air flow temperature is 135 ~ 150℃; when the zinc layer thickness is 6 ~ 8μm, the air knife pressure is 1 ~ 2.5bar, the nozzle angle is 35° ~ 45°, the distance between the nozzle and the steel plate is 12 ~ 20mm, and the air flow temperature is 120 ~ 135℃.

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