High-strength hot-dip galvanized steel sheet with excellent plating adhesion and weldability and method for manufacturing the same

Abstract

One embodiment of the present invention provides a high-strength hot-dip galvanized steel sheet having excellent adhesion and weldability and a method for manufacturing the same, the hot-dip galvanized steel sheet having a base steel sheet and a hot-dip galvanized layer formed on one side or both sides of the base steel sheet, the base steel sheet comprising, in terms of weight %, carbon (C): 0.1-0.3 %, silicon (Si): 0.1-2.0 %, aluminum (Al): 0.1-1.5 %, manganese (Mn): 1.5-3.0 %, the balance of Fe and inevitable impurities, the sum of the Si and Al satisfying 1.2-3.5 %, the ratio of the Al and Si (Al / Si) satisfying 0.5 to 2.0, an internal oxidation layer having a thickness of 1-5 µm being included immediately below the surface of the base steel sheet, and a decarburization rate in a region from immediately below the surface of the base steel sheet to 50 µm being 50 % or more.

Description

Technical Field

[0001] This invention relates to a high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability, and a method for manufacturing the same. Background Technology

[0002] The demand for ultra-high-strength steel sheets has surged due to recent environmental regulations aimed at addressing stricter fuel efficiency and crash stability controls on automobiles. Furthermore, countries need to improve fuel efficiency to achieve carbon reduction targets, while vehicle weight continues to increase due to higher performance and the addition of various convenience features. To address this issue, the demand for ultra-high-strength steel sheets is steadily rising. Therefore, steel companies are focusing their efforts on developing high-strength steel sheets such as dual-phase (DP) steel, transformation-induced plasticity (TRIP) steel, and complex-phase (CP) steel.

[0003] To increase the strength of steel sheets used in automobiles, large amounts of elements such as Si, Mn, and Al are typically added to the steel. However, during annealing heat treatment, these elements form oxides on the surface of the steel sheet, leading to poor plating properties when the sheet is immersed in a molten zinc plating bath, and potentially causing plating peeling. Furthermore, during subsequent spot welding, the molten metal in the liquid phase penetrates to the grain boundaries of the base metal, initiating liquid metal embrittlement and potentially resulting in poor weldability.

[0004] To improve the plating properties of steel sheets with a large amount of added Si, Mn, and Al, it is necessary to suppress the formation of oxides on the surface of the steel sheet. This requires reducing the amount of Si and Al added to the steel, but in this case, it is difficult to ensure the desired material.

[0005] A representative technology for solving this problem includes Patent Document 1. Patent Document 1 relates to a technique that preferentially enriches trace elements such as Sb in grain boundaries by adding them to steel to suppress the formation of Si oxides and the like on the surface.

[0006] However, when manufacturing steel plates, it is still necessary to develop a technology that can more reliably prevent the diffusion of alloying elements in steel.

[0007] [Existing technical documents]

[0008] (Patent Document 1) Japanese Patent Publication No. 6222040 Summary of the Invention

[0009] Technical problems to be solved

[0010] One aspect of the present invention is to provide a high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability, and a method for manufacturing the same.

[0011] Technical solution

[0012] One embodiment of the present invention provides a high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability, comprising a base steel sheet and a hot-dip galvanized layer formed on one or both sides of the base steel sheet. The base steel sheet, by weight percent, comprises: carbon (C): 0.1-0.3%, silicon (Si): 0.1-2.0%, aluminum (Al): 0.1-1.5%, manganese (Mn): 1.5-3.0%, the balance being Fe and unavoidable impurities. The sum of Si and Al satisfies 1.2-3.5%, and the Al / Si ratio satisfies 0.5 to 2.0. The base steel sheet has an internal oxide layer with a thickness of 1-5 μm directly beneath its surface, and the decarburization rate in the region from directly beneath the surface of the base steel sheet to 50 μm is 50% or more.

[0013] Another embodiment of the present invention provides a method for manufacturing a high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability, comprising the following steps: preparing a base steel sheet, which, by weight percent, comprises: carbon (C): 0.1-0.3%, silicon (Si): 0.1-2.0%, aluminum (Al): 0.1-1.5%, manganese (Mn): 1.5-3.0%, the balance being Fe and unavoidable impurities, wherein the sum of Si and Al is greater than 1.2%, and the ratio of Al to Si (Al / Si) is between 0.5 and 2.0; homogenizing the base steel sheet in a gaseous atmosphere at a temperature range of 750-900°C, wherein the dew point temperature of the gaseous atmosphere is 10-20°C, and the gaseous atmosphere, by volume percent, comprises 3-20% hydrogen, the balance being nitrogen and other unavoidable impurities; and immersing the homogenized base steel sheet in a molten zinc bath at 440-460°C to obtain a hot-dip galvanized steel sheet.

[0014] Beneficial effects

[0015] According to one aspect of the present invention, a high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability, and a method for manufacturing the same, can be provided. Attached Figure Description

[0016] Figure 1 These are photographs of Invention Example 1 according to an embodiment of the present invention, (a) being a photograph of the surface and (b) being a photograph after a coating adhesion test.

[0017] Figure 2These are photographs of Comparative Example 3 according to an embodiment of the present invention, (a) being a photograph of the surface and (b) being a photograph after a coating adhesion test.

[0018] Figure 3 The image is a photograph of Invention Example 1, which is an embodiment of the present invention, observed using SEM.

[0019] Best practice

[0020] The following describes a high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability according to an embodiment of the present invention. The hot-dip galvanized steel sheet of the present invention comprises a base steel sheet and a hot-dip galvanized layer formed on one or both sides of the base steel sheet. First, the alloy composition of the base steel sheet of the present invention will be described. Unless otherwise stated, the alloy composition content described below is expressed as weight %.

[0021] Carbon (C): 0.1-0.3%

[0022] The carbon (C) is an element that contributes to the stabilization of the austenitic structure; as the C content increases, it helps to ensure the austenitic structure. To achieve the above effect, the C content is preferably 0.1% or more. However, when the C content exceeds 0.3%, defects may occur in the cast billet, and weldability may also decrease. Therefore, the C content is preferably in the range of 0.1-0.3%. The lower limit of the C content is more preferably 0.15%. The upper limit of the C content is more preferably 0.25%.

[0023] Silicon (Si): 0.1-2.0%

[0024] Silicon (Si) is an element that helps stabilize retained austenite by inhibiting the precipitation of carbides in ferrite and promoting the diffusion of carbon from ferrite to austenite. To achieve these effects, the Si content is preferably 0.1% or more. When the Si content exceeds 2.0%, rollability deteriorates, and oxides form on the surface of the steel sheet during heat treatment, which may lead to poor plating and adhesion. Therefore, the Si content is preferably in the range of 0.1-2.0%. The lower limit of the Si content is more preferably 0.2%. The upper limit of the Si content is more preferably 1.8%.

[0025] Aluminum (Al): 0.1-1.5%

[0026] Aluminum (Al) is an element that combines with oxygen in steel to act as a deoxidizer. Furthermore, like Si, Al is an element that inhibits the formation of carbides in ferrite and helps stabilize retained austenite. To achieve these effects, Al is preferably added at 0.1% or more. When the Al content exceeds 1.5%, the slab's strength deteriorates, and Al is an element with a strong affinity for oxygen, thus forming oxides on the surface of the steel sheet, which may hinder plating and adhesion. Therefore, the Al content is preferably in the range of 0.1-1.5%. The lower limit of the Al content is more preferably 0.2%. The upper limit of the Al content is more preferably 1.4%.

[0027] Manganese (Mn): 1.5-3.0%

[0028] Mn is an element that, along with carbon, stabilizes the austenitic structure. When the Mn content is less than 1.5%, the target strength is difficult to ensure due to the ferrite phase transformation. When the Mn content exceeds 3.0%, martensite forms during the secondary annealing heat treatment due to the delayed phase transformation, making it difficult to ensure the desired ductility. Therefore, the Mn content is preferably in the range of 1.5-3.0%. The lower limit of the Mn content is more preferably 1.7%. The upper limit of the Mn content is more preferably 2.9%.

[0029] In addition to the steel composition described above, it may contain a balance of Fe and other unavoidable impurities. Undesirable impurities are inevitably introduced during conventional steel manufacturing processes, and therefore cannot be completely eliminated; this meaning is easily understood by those skilled in the art of steel manufacturing. Furthermore, this invention does not completely exclude the addition of other components besides the steel composition described above.

[0030] Furthermore, as mentioned above, both Si and Al are elements that contribute to stabilizing retained austenite. To effectively achieve this, the sum of the contents of Si and Al is preferably in the range of 1.2-3.5%. When the sum of the contents of Si and Al is less than 1.2%, it is difficult to sufficiently obtain the effect of increasing elongation. On the other hand, when the sum of the contents of Si and Al exceeds 3.5%, it may cause problems with poor castability and rollability. The lower limit of the sum of the contents of Si and Al is more preferably 1.3%. The upper limit of the sum of the contents of Si and Al is more preferably 3.4%.

[0031] Furthermore, the Al to Si ratio (Al / Si) is preferably between 0.5 and 2.0. When the Al to Si ratio is less than 0.5, plating peeling may occur because the Si-rich oxide is distributed in layers. On the other hand, when the Al to Si ratio exceeds 2.0, the Al-rich oxide is densely formed on the surface of the steel sheet. As external oxygen penetration into the steel sheet is suppressed, it is difficult to form an internal oxide layer directly below the surface of the base steel sheet, and plating performance and adhesion may deteriorate due to the Al-rich oxide forming in layers on the surface. The lower limit of the Al to Si ratio is more preferably 0.6. The upper limit of the Al to Si ratio is more preferably 1.9.

[0032] The hot-dip galvanized steel sheet of the present invention preferably includes an internal oxide layer with a thickness of 1-5 μm directly below the surface of the base steel sheet. A feature of the present invention is that by forming an internal oxide layer directly below the surface of the base steel sheet, it prevents the diffusion of Al or Si present in the base steel sheet to the surface portion of the steel sheet and prevents the formation of Al or Si oxides on the surface portion, thereby improving plating performance. However, when the thickness of the internal oxide layer is less than 1 μm, it may be difficult to fully obtain the above-mentioned effects. On the other hand, when the thickness of the internal oxide layer exceeds 5 μm, the oxides on the surface of the steel sheet are rolled up during the annealing heat treatment, thus having the disadvantage of potentially causing surface defects such as dents. Therefore, the thickness of the internal oxide layer is preferably 1-5 μm. The lower limit of the thickness of the internal oxide layer is more preferably 1.5 μm, and even more preferably 2 μm. The upper limit of the thickness of the internal oxide layer is more preferably 4.5 μm, even more preferably 4 μm, and most preferably 3.5 μm.

[0033] The internal oxide layer may comprise an oxide composed of a composite oxide of Al and Si. As described above, by making the oxide composed of a composite oxide of Al and Si, the oxide morphology is a discontinuous oxide morphology rather than a continuous morphology such as a layer, thereby achieving an effect that improves the adhesion of the plating.

[0034] The Al and Si composite oxides can exist within the grains or both at the grain boundaries. Preferably, the Al and Si composite oxides exist intermittently. As mentioned above, the intermittent presence of the Al and Si composite oxides is advantageous in ensuring coating adhesion compared to their continuous presence.

[0035] Furthermore, the decarburization rate of the hot-dip galvanized steel sheet of the present invention is preferably 50% or more in the region from directly below the surface of the base steel sheet to 50 μm. As proposed in the present invention, when the dew point temperature in the heat treatment furnace is high, internal oxides are formed in the surface layer of the steel sheet, and C in the steel reacts with oxygen adsorbed on the surface of the steel sheet to vaporize into CO or CO2, forming a C depletion region in the surface layer of the base material. When this decarburization reaction occurs, the resistance to LME cracking in spot welding becomes excellent. However, when the decarburization rate is less than 50%, the decarburization reaction is insufficient, and therefore the resistance to LME cracking may deteriorate.

[0036] The hot-dip galvanized steel sheet of the present invention, as described above, has a yield strength of 600 MPa or more, a tensile strength of 950 MPa or more, and an elongation of 20% or more, thereby ensuring excellent mechanical and physical properties. Furthermore, the hot-dip galvanized layer covers 95% or more of the total area relative to the base steel sheet while exhibiting good coating adhesion, thus providing excellent coating properties. In addition, the maximum length of LME cracks is 50 μm or less, thereby providing excellent resistance to LME cracking.

[0037] The following describes a method for manufacturing a high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability according to one embodiment of the present invention.

[0038] First, a base steel plate satisfying the above alloy composition is prepared. In this invention, there are no particular limitations on the method for preparing the base steel plate. However, as a preferred example, the steps for preparing the base steel plate include: reheating the slab at 1000-1300°C; hot-finishing the reheated slab at 800-950°C to obtain a hot-rolled steel plate; coiling the hot-rolled steel plate at 630-700°C; and pickling the coiled hot-rolled steel plate followed by cold rolling to obtain a cold-rolled steel plate.

[0039] The slab satisfying the above alloy composition is reheated at 1000-1300°C. When the reheating temperature of the slab is below 1000°C, a significant increase in rolling load may occur; when the reheating temperature exceeds 1300°C, excessive surface oxide scale may occur. Therefore, the reheating temperature of the slab is preferably in the range of 1000-1300°C. The lower limit of the reheating temperature is more preferably 1050°C. The upper limit of the reheating temperature is more preferably 1250°C.

[0040] Subsequently, the reheated slab is hot-rolled at 800-950°C to obtain a hot-rolled steel sheet. When the hot-rolling temperature is below 800°C, the rolling load increases, making rolling difficult. When the hot-rolling temperature exceeds 950°C, the increased thermal fatigue of the rolls leads to a shorter roll life. Therefore, the hot-rolling temperature is preferably in the range of 800-950°C. The lower limit of the hot-rolling temperature is more preferably 830°C. The upper limit of the hot-rolling temperature is more preferably 930°C.

[0041] The hot-rolled steel sheet is then coiled at 630-700°C. When the coiling temperature is below 630°C, the internal oxide layer is not formed, and oxide formation is promoted on the surface of the steel sheet during the annealing heat treatment, which may result in poor plating. When the coiling temperature exceeds 700°C, the depth of the internal oxide layer increases significantly, and the oxide is rolled up by the rollers during subsequent annealing heat treatment, which may cause surface defects such as dents. Therefore, the coiling temperature is preferably in the range of 630-700°C. The lower limit of the coiling temperature is more preferably 650°C. The upper limit of the coiling temperature is more preferably 680°C.

[0042] Subsequently, the coiled hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet. In this invention, the pickling and cold-rolling processes are not particularly limited, and all methods commonly used in this technical field can be employed.

[0043] In a gaseous atmosphere, the base steel plate prepared as described above is homogenized at a temperature range of 750-900°C. The dew point temperature of the gaseous atmosphere is 10-20°C, and the gaseous atmosphere contains 3-20% hydrogen by volume, with the balance being nitrogen and other unavoidable impurities. Homogenization refers to heating to the stated temperature range and then holding it there. When the dew point temperature is below 10°C, sufficient local internal oxidation cannot occur, limiting the effective suppression of surface enrichment of Si and Mn in the steel, and the decarburization effect is insufficient, thus potentially worsening resistance to LME cracking. On the other hand, when the dew point temperature exceeds 20°C, plating peeling may occur in the dew point region where Fe can be oxidized due to Fe oxides. When the hydrogen fraction in the gas is less than 3% by volume, Fe oxides and the like remain on the surface of the steel plate due to insufficient reducing power, potentially leading to unplated or plating peeling. When the hydrogen fraction in the gas exceeds 20% by volume, the increased cost due to the large-scale use of expensive hydrogen presents a disadvantage. When the soaking temperature is below 750°C, the recrystallization temperature above A3 cannot be guaranteed, which may lead to deviations in mechanical and physical properties caused by non-recrystallized areas. Furthermore, internal oxidation does not occur, and Si, Mn, etc., diffuse to the steel plate surface to form oxides, resulting in poor plating quality. On the other hand, when the soaking temperature exceeds 900°C, the limitations of the heat treatment equipment restrict the increase in steel plate temperature, and due to secondary recrystallization, it is difficult to obtain steel plates with excellent material properties.

[0044] Subsequently, the homogenized base steel sheet is immersed in a molten zinc bath at 440-460°C to obtain a hot-dip galvanized steel sheet. When the temperature of the molten zinc bath is below 440°C, the viscosity of the bath increases, reducing the mobility of the roll winding the steel sheet and causing slippage between the steel sheet and the roll. This can potentially lead to defects in the steel sheet. When the temperature of the molten zinc bath exceeds 460°C, it promotes the dissolution of the steel sheet in the bath and promotes the formation of dross in the form of Fe-Zn compounds, which may also lead to surface defects.

[0045] Furthermore, after obtaining the hot-dip galvanized steel sheet, a further step of alloying heat treatment at 480-600°C can be included. When the alloying heat treatment temperature is below 480°C, Fe in the base material cannot diffuse sufficiently into the coating, thus failing to adequately ensure the Fe content in the coating. When the alloying heat treatment temperature exceeds 600°C, the Fe content in the coating is excessive, which may lead to powdering and coating peeling during steel sheet processing. Detailed Implementation

[0046] The present invention will now be described in more detail through embodiments. However, the following embodiments are only used to illustrate the present invention in more detail and do not limit the scope of the invention.

[0047] (Example)

[0048] Molten metal with the alloy composition described in Table 1 was formed into ingots with a width of 175 mm and a thickness of 90 mm in a vacuum melting furnace. These ingots were then reheated at 1200 °C for 1 hour for homogenization, followed by hot finishing at 900 °C (above Ar3), and then held at 680 °C for 1 hour to simulate hot rolling coiling. The hot-rolled steel sheet was then immersed in a 15% HCl pickling solution for 40 seconds to simulate a pickling process. Afterward, it was cold-rolled with a cold rolling reduction of 50-60% to produce cold-rolled steel sheet. The cold-rolled steel sheet was homogenized in a reduction furnace at 800 °C in a gas atmosphere of 5 vol% H + 95 vol% N with dew point temperatures as described in Table 2, then cooled and immersed in a molten zinc plating bath at 460 °C for 5 seconds. The plating adhesion was then adjusted to 60 g / m² per side by air wiping. 2 This level of technology enables the manufacture of hot-dip galvanized steel sheets.

[0049] The mechanical and physical properties, thickness of the internal oxide layer, coating properties, decarburization rate, and maximum length of LME cracks of the hot-dip galvanized steel sheets manufactured as described above were measured, and the results are shown in Table 2 below. Furthermore, the oxides measured were composite oxides of Al and Si.

[0050] For mechanical and physical properties, hot-dip galvanized steel sheets were cut into 40mm×200mm sizes perpendicular to the rolling direction, the sides were milled, and tensile test pieces were manufactured according to JIS 5 standard. The yield strength (YS), tensile strength (TS), and elongation (EL) were measured using a tensile testing machine.

[0051] For the thickness of the internal oxide layer, 10 locations were randomly selected from the cross-sectional microstructure images obtained by scanning electron microscopy (SEM) and measured at 5000x magnification. The average value was then recorded.

[0052] The coating performance was evaluated by the following method: the area of ​​the hot-dip galvanized layer formed relative to the total area of ​​the hot-dip galvanized steel sheet was measured by image analysis and the score was measured. After the structural adhesive was applied to the hot-dip galvanized steel sheet, it was cured at 175°C for 20 minutes and then bent at 90° to confirm whether it adhered to the sealant (coating adhesion).

[0053] The decarburization rate is represented by measuring the fraction of the depleted area relative to the total area on the depth profile of the glow discharge optical emission spectroscopy (GDOES) up to approximately 50 μm along the depth direction of the base steel plate.

[0054] The maximum length of the LME crack was achieved by applying welding current to a Cu-Cr electrode with a front diameter of 6 mm after laminating the steel plate, and welding under the conditions of 16 cycles of energizing time and 15 cycles of holding time at a pressure of 2.6 kN. The cross-section of the inclined portion of the contact area between the electrode and the steel plate was then observed and measured using an optical microscope.

[0055] [Table 1]

[0056]

[0057] [Table 2]

[0058]

[0059]

[0060] As can be seen from Tables 1 and 2, in the case of Invention Examples 1 to 4, which meet the alloy composition and manufacturing conditions proposed in this invention, excellent plating properties and resistance to LME cracking are achieved by ensuring the thickness and decarburization rate of the internal oxide layer that is desired in this invention.

[0061] On the other hand, in Comparative Examples 1 and 3, it can be seen that not only is the Al / Si ratio lower than that proposed in this invention, but also the thickness and decarburization rate of the internal oxide layer proposed in this invention cannot be ensured due to the very low dew point temperature, thus the coating adhesion and resistance to LME cracking deteriorate.

[0062] In Comparative Example 2, it can be seen that not only does it exceed the Al / Si ratio proposed in this invention, but also due to the very low dew point temperature, it is impossible to ensure the thickness and decarburization rate of the internal oxide layer proposed in this invention, resulting in no coating, and the coating adhesion and resistance to LME cracking are also poor.

[0063] In Comparative Example 4, it can be seen that the elongation is low because the sum of Si and Al does not meet the requirements of the present invention. Due to the low dew point temperature, the decarburization rate proposed by the present invention cannot be ensured, and the resistance to LME cracking is poor.

[0064] In Comparative Example 5, it can be seen that the alloy composition proposed in this invention is satisfied, but due to the low dew point temperature, the decarburization rate proposed in this invention cannot be ensured, and the resistance to LME cracking is poor.

[0065] Figure 1 These are photographs of Invention Example 1, (a) is a photograph of the surface, and (b) is a photograph after the coating adhesion test. Figure 2 These are photographs of Comparative Example 3: (a) is a photograph of the surface, and (b) is a photograph after the coating adhesion test. Through... Figure 1 and Figure 2 It can be seen that Example 1 of the invention has almost no uncoated areas and no coating peeling occurs, so the coating quality is good. On the other hand, Comparative Example 3 has almost no uncoated areas, but the coating peels off, so the coating quality is poor.

[0066] Figure 3 The photograph of Invention Example 1 was observed using SEM. (Through...) Figure 3 It is understood that in Example 1 of the invention, an internal oxide layer of appropriate thickness containing intermittently existing oxides is formed.

Claims

1. A high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability, comprising a base steel sheet and a hot-dip galvanized layer formed on one or both sides of the base steel sheet. The base steel plate, by weight percent, comprises: carbon (C): 0.1-0.3%, silicon (Si): 0.1-2.0%, aluminum (Al): 0.1-1.5%, manganese (Mn): 1.5-3.0%, with the balance being Fe and unavoidable impurities. The sum of Si and Al is 1.2-3.5%, and the Al / Si ratio is 0.5 to 2.

0. The base steel plate contains an internal oxide layer with a thickness of 1-5 μm extending from its surface downwards, and the decarburization rate in the region extending 50 μm from directly below the surface of the base steel plate is greater than 50%. Regarding the thickness of the internal oxide layer, ten arbitrary locations were selected from cross-sectional microstructure images obtained by scanning electron microscopy (SEM) and measured at 5000x magnification. The average value was then calculated. The decarburization rate is represented by measuring the fraction of the depleted area relative to the total area on the glow discharge spectrum (GDOES) depth curve along the depth direction of the base steel plate up to about 50 μm.

2. The high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability according to claim 1, wherein, The inner oxide layer comprises an oxide composed of a composite oxide of Al and Si.

3. The high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability according to claim 2, wherein, The oxide exists intermittently.

4. The high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability according to claim 1, wherein, The hot-dip galvanized steel sheet has a yield strength of over 600 MPa, a tensile strength of over 950 MPa, and an elongation of over 20%.

5. The high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability according to claim 1, wherein, In the hot-dip galvanized steel sheet, the area of ​​the hot-dip galvanized layer relative to the total area of ​​the base steel sheet is more than 95%.

6. The high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability according to claim 1, wherein, The maximum length of the LME crack in the hot-dip galvanized steel sheet is less than 50 μm. The maximum length of the LME crack is obtained by the following method: after laminating the steel sheet, a Cu-Cr electrode with a front end diameter of 6 mm is used to apply a welding current, and welding is performed under the conditions of 16 cycles of energizing time and 15 cycles of holding time at a pressure of 2.6 kN. Then, the cross section of the inclined part of the boundary of the contact area between the welding electrode and the steel sheet is observed and measured using an optical microscope.

7. A method for manufacturing high-strength hot-dip galvanized steel sheets with excellent coating adhesion and weldability, comprising the following steps: Prepare a base steel plate, which, by weight percent, comprises: carbon (C): 0.1-0.3%, silicon (Si): 0.1-2.0%, aluminum (Al): 0.1-1.5%, manganese (Mn): 1.5-3.0%, with the balance being Fe and unavoidable impurities. The sum of Si and Al is greater than 1.2%, and the ratio of Al to Si, Al / Si, is between 0.5 and 2.

0. The base steel plate is homogenized in a gaseous atmosphere at a temperature range of 750-900°C, wherein the dew point temperature of the gaseous atmosphere is 10-20°C, and the gaseous atmosphere contains 3-20% hydrogen by volume, with the balance being nitrogen and other unavoidable impurities; and The homogenized base steel sheet is immersed in a molten zinc bath at 440-460°C to obtain hot-dip galvanized steel sheet. The steps for preparing the foundation steel plate include the following: The slab is reheated at 1000-1300℃; The reheated slab is hot-finished at 800-950°C to obtain hot-rolled steel sheet; The hot-rolled steel sheet is coiled at 630-700℃; and The hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet.

8. The method for manufacturing a high-strength hot-dip galvanized steel sheet with excellent coating adhesion and weldability according to claim 7, wherein after obtaining the hot-dip galvanized steel sheet, the method further includes a step of subjecting the hot-dip galvanized steel sheet to an alloying heat treatment at 480-600°C.

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