A low-aluminum zinc-aluminum-magnesium plated steel sheet for automobile bodies having excellent adhesive properties and a method for producing the same

Abstract

The application discloses a low-aluminum-zinc-aluminum-magnesium plated steel sheet with excellent adhesive property for automobile body and a production method thereof. The low-aluminum-zinc-aluminum-magnesium plated steel sheet comprises a base plate and a low-aluminum-zinc-aluminum-magnesium plating layer. The surface of the low-aluminum-zinc-aluminum-magnesium plating layer is covered with a nascent Mg and Al oxide film with a proportion of less than or equal to 30%. The volume proportion of nascent Zn phase in the plating layer is 80% to 90%. The surface roughness of the plating layer is 1.0 to 2.0 microns, and the RPC is greater than or equal to 130. The low-aluminum-zinc-aluminum-magnesium plated steel sheet is produced by using a hot-dip plating and finishing process. The low-aluminum-zinc-aluminum-magnesium plated steel sheet has excellent adhesive property, and the cohesive fracture area of the adhesive joint accounts for more than 95% of the joint area.

Description

Technical Field

[0001] This invention belongs to the field of coated steel sheet technology, specifically relating to a low-aluminum-zinc-aluminum-magnesium coated steel sheet for automobile bodies with excellent adhesive properties and its production method. Background Technology

[0002] Using low-aluminum-zinc-aluminum-magnesium coated steel sheets in automobile body manufacturing helps reduce vehicle weight and costs, and also reduces stamping interruptions and die wear caused by zinc powder contamination in stamping equipment, improving the stamping efficiency of automotive parts. Furthermore, it offers superior corrosion resistance compared to pure zinc coatings. Some automakers have already completed comprehensive evaluations of low-aluminum-zinc-aluminum-magnesium coated products and are gradually beginning to apply them in mass production for passenger vehicle bodies.

[0003] Compared to pure zinc coatings, low-aluminum zinc-aluminum-magnesium (ZN-Mg) coatings have a more complex composition and phase structure, with the addition of Al and Mg elements. The coating phase structure consists of primary Zn phase, binary eutectic (Zn-MgZn2), and ternary eutectic (Zn-Al-MgZn2). According to traditional production methods, Mg and Al oxides inevitably exist on the coating surface. Compared to Zn oxides on pure zinc coatings, Mg and Al oxides have poor compatibility with adhesives, resulting in a significantly higher rate of poor adhesion for ZN-Mg coatings compared to pure zinc coatings.

[0004] Chinese patent CN113025939A discloses a zinc-aluminum-magnesium coated steel and its preparation method. The method controls the post-plating cooling, particularly by using water mist cooling to increase the cooling rate, thereby controlling the grain size of the coating to ≤300μm. Furthermore, water mist cooling promotes the formation of more hydroxides on the coating surface, thus improving the coating's adhesive properties. However, this method requires modification of existing post-plating cooling equipment.

[0005] Chinese patent CN113512691A discloses a zinc-aluminum-magnesium coated steel and its preparation method. The method aims to form Mg2Zn on the coating surface. 11 Phase, one is Mg2Zn 11 Compared to traditional processes for preparing zinc-aluminum-magnesium coated steel, this method consumes more Mg (Mg) element (MgZn2), thus reducing the magnesium oxide content. Secondly, Mg2Zn... 11 Compared to MgZn2, which is more irregular and discontinuous, this method can improve adhesive properties. However, using this method makes the coating structure more complex and harder to control, increasing the difficulty of production.

[0006] Chinese patent CN113122790A discloses a zinc-aluminum-magnesium coated steel sheet with excellent adhesive properties and its manufacturing method. The process route is steel sheet pretreatment - hot-dip galvanizing - post-galvanizing cooling - coating surface treatment. The post-galvanizing surface treatment involves alkaline activation of the coating surface using an alkaline aqueous solution. This method removes Mg and Al oxides from the surface and subsurface layers of the coating, thereby improving the adhesive properties of the zinc-aluminum-magnesium coating. However, this method adds an extra step compared to traditional methods, increasing production costs and hindering the control of coating surface quality. Summary of the Invention

[0007] To solve the above-mentioned technical problems, the present invention provides a low-aluminum-zinc-aluminum-magnesium coated steel sheet for automobile body with excellent adhesive properties and its production method. The steel sheet has a small thickness and proportion of Mg and Al oxide films on the coating surface, and has excellent adhesive properties. The cohesive fracture area of ​​the adhesive joint accounts for ≥95% of the joint area.

[0008] The technical solution adopted in this invention is as follows:

[0009] A low-aluminum-zinc-aluminum-magnesium coated steel sheet for automobile bodies with excellent adhesive properties includes a substrate and a low-aluminum-zinc-aluminum-magnesium coating. The primary Mg and Al oxide films on the surface of the low-aluminum-zinc-aluminum-magnesium coating account for ≤30%, and the volume percentage of the primary Zn phase in the coating is 80%-90%. The surface roughness of the coating is 1.0-2.0 μm, and the RPC is ≥130.

[0010] The coating structure of the low-aluminum zinc-aluminum-magnesium coating consists of a primary Zn phase, a Zn-MgZn2 binary eutectic phase, and a Zn-Al-MgZn2 ternary eutectic phase. The proportions of each phase in the coating satisfy the following formula:

[0011]

[0012] In the formula, V Zn V represents the volume ratio of the primary Zn phase in the coating. 二元 V represents the volume percentage of the binary eutectic layer in the coating. 三元 This represents the volume percentage of the ternary eutectic phase in the coating.

[0013] The composition and mass fraction of the low-aluminum zinc-aluminum-magnesium coating are: Mg 0.65%–0.95%, Al 0.85%–1.25%, with the remainder being Zn and unavoidable impurity elements.

[0014] The thickness of the single-sided coating is 4.5μm to 6.5μm.

[0015] The substrate thickness is 0.6–1.2 mm. The substrate is made of low-carbon mild steel or IF steel.

[0016] The present invention also provides a method for producing the low-aluminum-zinc-aluminum-magnesium coated steel sheet for automobile body with excellent adhesive properties, comprising the following steps: hot-dip galvanizing the substrate in a zinc pot, achieving the preset coating thickness by using an air knife, performing three-stage cooling after galvanizing, and finally finishing.

[0017] During the hot-dip galvanizing process, the zinc pot temperature is controlled between 425℃ and 435℃. If the zinc pot temperature is lower than 425℃, the zinc pot composition is prone to unevenness due to the low temperature. If the zinc pot temperature is too high, the Mg and Al oxides on the coating surface will increase after the strip exits the zinc pot. The zinc pot temperature should be ≤ the temperature of the substrate when it enters the zinc pot ≤ the zinc pot temperature + 5℃ to avoid changes in the content of Al and Mg in the coating caused by fluctuations in the zinc pot temperature.

[0018] In the hot-dip galvanizing process, the air knife height is controlled between 450mm and 550mm. The air knife height is the height of the air knife from the zinc bath surface. During this section, the strip steel carries a large amount of zinc liquid out of the zinc bath. Compared with traditional production, this method uses a higher air knife height control, which can provide more cooling time for the zinc liquid on the strip steel surface. After passing through the air knife, the temperature of the coating surface is low, which is conducive to reducing the formation of Mg and Al oxides. Therefore, the lower limit of the air knife height is set at 450mm. However, if the air knife height is too high, the temperature of the zinc liquid on the strip steel surface will be low, the fluidity will be poor, and it will not be conducive to the control of the zinc layer thickness. Therefore, the upper limit of the air knife height is set at 550mm.

[0019] The air knife pressure is controlled between 400 mbar and 500 mbar. For the air knife purging system, when the blade lip gap is constant, the air knife pressure is determined by the air knife N2 flow rate. The higher the air knife pressure, the more N2 is purged per unit time. On the one hand, it can cool the plating solution on the strip surface, and on the other hand, it can clean the Mg and Al oxides generated in the plating solution between the zinc pot and the air knife section. This helps to reduce the generation of Mg and Al oxides after the plating solution exits the air knife. Therefore, the lower limit of the air knife pressure is set at 400 mbar. The air knife pressure should not be too high, as too high pressure will cause the strip to vibrate, resulting in poor uniformity of the strip coating thickness. Therefore, the upper limit of the air knife pressure is set at 500 mbar.

[0020] The air source for the air knife is N2.

[0021] After plating, a three-stage controlled cooling system of slow cooling, fast cooling, and medium cooling is adopted.

[0022] Furthermore, during post-plating cooling, the power of the first-stage cooling fan is controlled at 5%-15%, with an outlet strip temperature of 365-380℃; the power of the second-stage cooling fan is controlled at 70%-80%, with an outlet strip temperature ≤250℃; and the power of the third-stage cooling fan is controlled at 20%-60%, with an outlet strip temperature of 150-200℃. The low power control in the first stage ensures sufficient time for the primary Zn phase to grow during the initial solidification phase of the zinc layer, and that the primary Zn phase occupies ≥80% of the coating volume. Simultaneously, since the zinc liquid is not yet fully solidified, if the fan power exceeds 15%, it will cause deformation and wrinkles on the zinc layer surface, as shown in the attached image. Figure 2 As shown, excessive fan power in this section can lead to an increase in eutectic phase, especially a decrease in the proportion of ternary eutectic and an increase in the proportion of binary eutectic. This is detrimental to the adhesion of the coating because the ternary eutectic contains Al phase. When the proportion of ternary eutectic satisfies formula (2), Al and Mg elements in the primary Zn phase are fully segregated into the eutectic structure during solidification, resulting in a purer primary Zn phase structure. The second section uses medium-high power control, with a lower power limit of not less than 70%. By increasing the cooling rate of the coating in the second section, the generation of Mg and Al oxides is reduced. The upper power limit is set at 80%, as excessive fan power can cause strip vibration, increasing the difficulty of production control. The third section uses wide-range power control to further regulate the temperature of the strip and ensure that the strip temperature is 150℃~200℃ when passing through the top roller of the tower.

[0023] In the finishing process, the surface roughness of the finishing work roll is 1.4 μm to 2.0 μm. Using a low-roughness finishing work roll, under the same conditions, results in more and deeper finishing pits on the coated surface. This means that when the strip passes through the finishing work roll, the axial pressure and friction between the work roll and the strip surface can break up more Mg and Al primary oxide films. However, the roughness cannot be too low; when it is below 1.4 μm, the finished strip surface cannot achieve sufficient roughness. Therefore, the roughness of the work roll is controlled within the range of 1.4 to 2.0 μm.

[0024] In the finishing process, the finishing elongation is controlled between 1.2% and 1.6%. When the finishing elongation is less than 1.2%, the unit rolling force is too small, resulting in low axial pressure and friction between the work roll and the strip surface, which cannot fully break the Mg and Al oxide film. When the finishing elongation is greater than 1.6%, the stamping performance of the strip deteriorates.

[0025] The present invention provides a method for producing low-aluminum-zinc-aluminum-magnesium coated steel sheets for automotive bodies. By rationally matching the hot-dip galvanizing process and post-galvanizing cooling process, and in conjunction with a finishing process, a low-aluminum-zinc-aluminum-magnesium coated steel sheet for automotive bodies with excellent adhesive properties is obtained. In the hot-dip galvanizing process, a relatively low zinc pot temperature is controlled to prevent an increase in Mg and Al oxides on the coating surface after the substrate exits the zinc pot. The temperature difference between the substrate entering the zinc pot and the zinc pot temperature is controlled to be less than 5°C to avoid changes in the Al and Mg content in the coating due to zinc pot temperature fluctuations. Furthermore, during hot-dip galvanizing, a higher air knife height and a larger air knife flow rate are used, with N2 as the air source. The increased air knife height significantly lowers the temperature of the strip and its surface plating solution when the air knife blows through the strip, and increases the viscosity of the plating solution. This provides a basis for a larger air knife flow rate. Under a larger air knife flow rate, the air knife can effectively cool the plating solution on the strip surface, reducing the formation of primary Mg and Al oxides.

[0026] In the post-plating cooling process section, a three-stage post-plating cooling fan power control system ensures that the volume percentage of the primary Zn phase is ≥80%, and the Mg and Al content in the primary Zn is extremely low, as shown in the attached figure. Figure 6 As shown, the combined content of Al and Mg is 0.7%. There are relatively few Mg and Al oxides formed on the surface, while the volume of the primary Zn phase increases. According to the principle of solute redistribution during solidification, Mg and Al elements can accumulate in the relatively small binary and ternary eutectic phases, as shown in the attached figure. Figure 7 , 8 As shown, the combined Al and Mg contents are 8.7% and 9.5%, respectively. Limited experiments indicate that when the volume percentage of the primary Zn phase is ≥80%, the grain size of the primary Zn phase is larger, and the influence of the eutectic phase on the compatibility of the primary Zn phase with the adhesive is weakened. Individual primary Zn phase grains can bond with the adhesive. In this grain region, the zinc-aluminum-magnesium (ZAM) coating exhibits good adhesive compatibility, similar to the GI coating. The various primary Zn phases with good adhesive compatibility on the surface of the ZAM coating encapsulate the eutectic phase, mitigating the poor adhesive compatibility of the eutectic phase due to Al and Mg enrichment. By controlling the power of the second-stage fan, the coating is rapidly cooled to below 250°C, minimizing the residence time of the coating in the high-temperature zone, reducing the oxidation rate of Al and Mg on the coating surface, and thus reducing the amount of Al and Mg oxides generated. Ultimately, this results in a thinner primary Mg and Al oxide film on the coating surface, fundamentally reducing the amount of Al and Mg oxides generated.

[0027] In the finishing process, a low-roughness finishing roll and a suitable finishing elongation are used. On the one hand, the low-roughness, high-RPC finishing roll increases the contact area between the finishing roll and the coating surface. On the other hand, a larger finishing elongation provides a larger unit rolling force, which increases the axial force and friction when the coating contacts the finishing roll. This maximizes the breakage and removal of the nascent Mg and Al oxide films on the coating surface, as shown in the attached figure. Figure 9 , 10 As shown, attached Figure 9 The oxygen content within 50nm of the coating surface is 15%, with... Figure 10 The oxygen content within 50nm of the coating surface is 34%. Compared with traditional processes, the finishing process of this invention reduces the oxide content on the coating surface by more than half, ensuring that the proportion of the initial Mg and Al oxide film in the final product is ≤30%. Simultaneously, the use of low-roughness finishing rollers and a larger finishing elongation in the finishing stage can generate more and deeper finishing pits on the steel plate coating surface. During adhesive bonding, these finishing pits on the coating surface provide mechanical locking force between the coating and the adhesive. The increased proportion and depth of finishing pits on the coating surface enhance the mechanical locking force, thereby further improving the adhesive performance of the coating.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] This invention does not require modification of the existing production line. By precisely controlling the hot-dip galvanizing and subsequent processes, it is possible to obtain low-aluminum-zinc-aluminum-magnesium coated steel sheets for automotive bodies with excellent adhesive properties. In these low-aluminum-zinc-aluminum-magnesium coated steel sheets, the coverage rate of primary Mg and Al oxide films is ≤30%, the volume percentage of primary Zn phase in the coating is 80%-90%, the surface roughness Ra of the coating is 1.0-2.0μm, RPC≥130, and the cohesive fracture area of ​​the adhesive joint accounts for ≥95% of the joint area. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the surface phase structure of the coating in the low-aluminum-zinc-aluminum-magnesium coated steel sheet in Example 1;

[0031] Figure 2 This is a schematic diagram of the surface phase structure of the coating in the low-aluminum-zinc-aluminum-magnesium coated steel sheet in Comparative Example 4.

[0032] Figure 3 This is a schematic diagram showing the proportion of cohesive fracture area in the adhesive joint of the low-aluminum-zinc-aluminum-magnesium coated steel plate in Example 1.

[0033] Figure 4 This is a schematic diagram showing the proportion of cohesive fracture area in the adhesive joint of the low-aluminum-zinc-aluminum-magnesium coated steel plate in Comparative Example 4.

[0034] Figure 5This is a schematic diagram of the phase microstructure of the coating cross section in the low-aluminum-zinc-aluminum-magnesium coated steel sheet in Example 2;

[0035] Figure 6 The energy spectrum and elemental composition of the primary Zn phase in the low-aluminum-zinc-aluminum-magnesium coated steel sheet in Example 1 are shown.

[0036] Figure 7 The energy spectrum and elemental composition of the binary eutectic phase in the low-aluminum-zinc-aluminum-magnesium coated steel sheet in Example 1 are shown.

[0037] Figure 8 The energy spectrum and elemental composition of the ternary eutectic phase in the low-aluminum-zinc-aluminum-magnesium coated steel sheet in Example 1 are shown.

[0038] Figure 9 The results of GDMS testing of the O element content in the coating in Example 4;

[0039] Figure 10 The results of GDMS testing of O element content in the coating in Comparative Example 3;

[0040] Figure 11 This is a schematic diagram of the surface finishing pits on the low-aluminum-zinc-aluminum-magnesium coated steel plate in Example 2.

[0041] Figure 12 This is a schematic diagram of the surface finishing pits on the low-aluminum-zinc-aluminum-magnesium coated steel plate in Comparative Example 3. Detailed Implementation

[0042] This invention provides a low-aluminum-zinc-aluminum-magnesium (LAM) coated steel sheet for automotive bodies with excellent adhesive properties, comprising a substrate and a LAM coating. The primary Mg and Al oxide films on the surface of the LAM coating account for ≤30%, and the volume percentage of the primary Zn phase in the coating is 80%-90%. The surface roughness of the coating is 1.0-2.0 μm, R0 PC ≥130.

[0043] The thickness of the substrate is 0.6 to 1.2 mm; the substrate is low-carbon mild steel or IF steel, produced by conventional steelmaking-continuous casting-hot rolling-pickling+cold rolling-continuous annealing steps.

[0044] The composition of low-aluminum zinc-aluminum-magnesium coating is as follows: Mg content is 0.65% to 0.95% by mass fraction, Al content is 0.85% to 1.25% by mass fraction, and the remainder is Zn and unavoidable impurity elements.

[0045] The thickness of the low-aluminum zinc-aluminum-magnesium coating is 4.5μm to 6.5μm.

[0046] A method for producing low-aluminum-zinc-aluminum-magnesium coated steel sheets for automotive bodies with excellent adhesive properties includes the following steps: hot-dip galvanizing of the substrate in a zinc bath, followed by finishing.

[0047] During hot-dip galvanizing, the zinc pot temperature is controlled between 425℃ and 435℃, and the zinc pot temperature is less than or equal to the temperature of the substrate when it enters the zinc pot and less than or equal to the zinc pot temperature + 5℃.

[0048] The air knife height is controlled between 450mm and 550mm, and the air knife pressure is controlled between 400mbar and 500mbar.

[0049] After plating, a three-stage controlled cooling system is adopted: slow cooling, fast cooling, and medium cooling. The power of the cooling fan in the first stage is controlled at 5%-15%, and the outlet strip temperature is 365-380℃; the power of the cooling fan in the second stage is controlled at 70%-80%, and the outlet strip temperature is ≤250℃; the power of the cooling fan in the third stage is controlled at 20%-60%, and the outlet strip temperature is 150-200℃.

[0050] In the finishing process, the surface roughness of the finishing roller is 1.4μm to 2.0μm; the finishing elongation is 1.2% to 1.6%.

[0051] The present invention will now be described in detail with reference to the embodiments.

[0052] Table 1 shows the substrate types and thicknesses of the low-aluminum-zinc-aluminum-magnesium coated steel sheets for automotive bodies in the examples and comparative examples. Example 1 uses IF steel grade DC54D with a substrate thickness of 0.65 mm; Example 2 uses IF steel grade DC56D with a substrate thickness of 0.6 mm; Example 3 uses IF steel grade DC53D with a substrate thickness of 0.8 mm; Example 4 uses low-carbon mild steel grade DC51D with a substrate thickness of 1.2 mm; Example 5 uses IF steel grade DC56D with a substrate thickness of 0.65 mm. Comparative Example 1 uses IF steel grade DC56D with a substrate thickness of 0.65 mm; Comparative Example 2 uses IF steel grade DC54D with a substrate thickness of 0.7 mm; Comparative Example 3 uses IF steel grade DC54D with a substrate thickness of 0.7 mm.

[0053] Table 1

[0054] steel grades Substrate thickness (mm) Example 1 DC54D 0.65 Example 2 DC56D 0.6 Example 3 DC53D 0.8 Example 4 DC51D 1.2 Example 5 DC56D 0.65 Comparative Example 1 DC51D 1.2 Comparative Example 2 DC54D 0.7 Comparative Example 3 DC54D 0.7 Comparative Example 4 DC56D 0.8 Comparative Example 5 DC56D 0.8

[0055] During hot-dip galvanizing, the composition and mass fraction of the plating solution are: Mg 0.65%–0.95%, Al 0.85%–1.25%, with the remainder being Zn and unavoidable impurity elements. The same plating solution is used in all examples and comparative examples. Table 2 shows the relevant processes for the examples and comparative examples, specifically the strip entry temperature, zinc pot temperature, air knife height, air knife pressure, power of the first-stage cooling fan, power of the second-stage cooling fan, power of the third-stage cooling fan, surface roughness of the finishing roll, and rolling force of the finishing roll. The parameters in Examples 1-5 are all within the range of this invention. The strip entry temperature, zinc pot temperature, and air knife pressure in Comparative Example 1 are not within the range of this invention. The air knife height, second-stage fan power, and finishing elongation in Comparative Example 2 are not within the range of this invention. The surface roughness and finishing elongation of the finishing roll in Comparative Example 3 are not within the range required by this invention. The power of the first-stage cooling fan in Comparative Example 4 is not within the range of this invention. The power of the second-stage cooling fan in Comparative Example 5 is not within the range required by this invention.

[0056] Table 2

[0057]

[0058] Under the process described in Table 2, the evaluation results of coating thickness, primary Mg and Al oxide coverage ratio, primary Zn phase volume ratio, coating roughness, coating RPC, and cohesive fracture area ratio of adhesive joints for the steel plates of the present invention examples and comparative examples are shown in Table 3.

[0059] Table 3 Performance Indicators of Steel Plates

[0060]

[0061]

[0062] Examples 1-5, implemented according to the method of the present invention, all show a cohesive fracture area ratio of ≥95% for the adhesive joints, meeting the requirements. Comparative Examples 1-5 fail to meet the method of the present invention due to one or more of the following: Comparative Example 1 suffers from excessively high strip entry temperature and zinc pot temperature, resulting in excessive formation of primary Mg and Al oxides; simultaneously, insufficient air knife pressure leads to increased coating thickness, resulting in a coating thickness of 7.5 μm. The final cohesive fracture area ratio of the adhesive joint is 40%, failing to meet the requirements. Comparative Example 2 suffers from excessively low air knife height, insufficient power of the second-stage cooling fan, and low finishing elongation, resulting in an excessively high coverage of primary Mg and Al oxides, ultimately leading to a cohesive fracture area ratio of 30% for the adhesive joint, failing to meet the requirements. Comparative Example 3 suffers from excessively rough finishing rollers and insufficient finishing elongation, resulting in an excessively high coverage of primary Mg and Al oxide films, low coating PRC, and a final cohesive fracture area ratio of 50% for the adhesive joint, failing to meet the requirements. In Comparative Example 4, the power of the first-stage cooling fan was too high, and the power of the second-stage cooling fan was too low, resulting in an excessively low proportion of the primary Zn phase. Consequently, the cohesive fracture area of ​​the adhesive joint was 30%, which did not meet the requirements. In Comparative Example 5, the power of the second-stage fan was zero, with only the first and third stages operating. This resulted in an excessively high coverage of the primary Mg and Al oxide films, and the cohesive fracture area of ​​the adhesive joint was 40%, which also did not meet the requirements.

[0063] The above detailed description of a low-aluminum-zinc-aluminum-magnesium coated steel sheet for automobile body with excellent adhesive properties and its production method is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, changes and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A low-aluminum-zinc-aluminum-magnesium coated steel sheet for automotive bodies with excellent adhesive properties, characterized in that, The coating includes a substrate and a low-aluminum-zinc-aluminum-magnesium (LAM) plating layer. The primary Mg and Al oxide films on the surface of the LAM plating layer account for ≤30%, and the volume percentage of the primary Zn phase in the plating layer is 80%-90%. The surface roughness of the plating layer is 1.0-2.0 μm. RPC ≥ 130; The low-aluminum zinc-aluminum-magnesium coating structure consists of a primary Zn phase, a Zn-MgZn2 binary eutectic phase, and a Zn-Al-MgZn2 ternary eutectic phase. The proportions of each phase in the coating satisfy the following formula: In the formula, V Zn V represents the volume ratio of the primary Zn phase in the coating. 二元 V represents the volume percentage of the binary eutectic layer in the coating. 三元 This represents the volume percentage of the ternary eutectic phase in the coating.

2. The low-aluminum-zinc-aluminum-magnesium coated steel sheet for automotive bodies with excellent adhesive properties according to claim 1, characterized in that, The composition and mass fraction of the low-aluminum zinc-aluminum-magnesium coating are: Mg 0.65%–0.95%, Al 0.85%–1.25%, with the remainder being Zn and unavoidable impurity elements.

3. The low-aluminum-zinc-aluminum-magnesium coated steel sheet for automotive bodies with excellent adhesive properties according to claim 1, characterized in that, The thickness of the single-sided coating is 4.5μm to 6.5μm.

4. The low-aluminum-zinc-aluminum-magnesium coated steel sheet for automotive bodies with excellent adhesive properties according to claim 1, characterized in that, The substrate thickness is 0.6–1.2 mm; the substrate is low-carbon mild steel or IF steel.

5. The method for producing a low-aluminum-zinc-aluminum-magnesium coated steel sheet for automotive bodies with excellent adhesive properties as described in any one of claims 1-4, characterized in that, The production method includes the following steps: the substrate is placed in a zinc pot for hot-dip galvanizing, the preset coating thickness is achieved by air knife, the substrate is cooled in three stages after galvanizing, and finally it is finished. The air knife height is controlled between 450mm and 550mm, and the air knife pressure is controlled between 400mbar and 500mbar; after plating, a three-stage cooling system of slow cooling, fast cooling, and medium cooling is adopted.

6. The production method according to claim 5, characterized in that, During post-plating cooling, the power of the first cooling fan is controlled at 5%-15%, and the outlet strip temperature is 365-380℃; the power of the second cooling fan is controlled at 70%-80%, and the outlet strip temperature is ≤250℃; the power of the third cooling fan is controlled at 20%-60%, and the outlet strip temperature is 150-200℃.

7. The production method according to claim 5, characterized in that, During hot-dip galvanizing, the zinc pot temperature is controlled between 425℃ and 435℃; the zinc pot temperature ≤ the temperature of the substrate when it enters the zinc pot ≤ the zinc pot temperature + 5℃.

8. The production method according to claim 5, characterized in that, In the finishing process, the surface roughness of the finishing work roll is 1.4μm to 2.0μm.

9. The production method according to claim 5, characterized in that, During the finishing process, the finishing elongation is controlled between 1.2% and 1.6%.

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