Surface modification method of galvanized steel sheet and galvanized steel sheet obtained by surface modification

Abstract

According to one aspect of the present invention, a galvanized steel sheet may include: a base steel sheet; and a Zn-Al-Mg-based coating, wherein the Zn-Al-Mg-based coating is arranged on at least one side of the base steel sheet, wherein the Zn-Al-Mg-based coating may include a surface modified region enriched with cerium (Ce) on the surface side of the Zn-Al-Mg-based coating.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet for automobiles, and more particularly to a magnesium-containing galvanized steel sheet that effectively improves adhesion to a sealing adhesive. Background Art

[0002] Galvanized steel sheets or zinc alloy-plated steel sheets are used as automotive interior and exterior panel materials. In particular, galvanized steel sheets or zinc alloy-plated steel sheets with magnesium (Mg) added as a coating component are commercially available and widely used to improve corrosion resistance.

[0003] Typically, after forming, welding, and assembling coated steel sheets for automobiles, epoxy-based sealing adhesives are used to fill the gaps between the inner and outer panels to reduce vehicle body noise. During the post-coating curing process, epoxy-based sealing adhesives expand and develop porosity at temperatures above approximately 200°C. At this point, the epoxy-based sealing adhesive adheres to the coated steel sheets and effectively reduces vehicle body noise and vibration.

[0004] In conventional galvanized steel sheets or zinc alloy steel sheets, it is known that the sealing adhesion between epoxy-based sealing adhesives and the plated steel sheets will not have any special problems. However, in the galvanized steel sheets comprising magnesium (Mg) as a component of the coating, due to the magnesium oxide formed on the surface portion of the coating, there is a problem of deterioration of the adhesion between the sealing adhesive and the plated steel sheets. In order to solve the above problems, a method for increasing the contact area by giving roughness to the surface of the coating or a method for increasing the chemical bonding force of the interface by activating and modifying the outermost surface of the coating has been proposed, but these methods have not been evaluated as a practical method for improving the adhesion between the zinc alloy coating comprising magnesium (Mg) and the epoxy-based sealing adhesive.

[0005] (Prior art literature)

[0006] (Patent Document 1) Korean Patent Publication No. 10-2018-0073855 (published on July 3, 2018) Summary of the Invention

[0007] Technical problems to be solved

[0008] According to one aspect of the present invention, a surface modification method of a galvanized steel sheet for improving adhesion between a Mg-containing galvanized steel sheet and an adhesive and a Mg-containing galvanized steel sheet surface-modified thereby can be provided.

[0009] The technical problems of the present invention are not limited to the above contents. Those skilled in the art can easily understand the additional technical problems of the present invention from the entire content of this specification.

[0010] Technical Solution

[0011] According to one aspect of the present invention, a galvanized steel sheet may include: a base steel sheet; and a Zn-Al-Mg based coating, wherein the Zn-Al-Mg based coating is disposed on at least one side of the base steel sheet, wherein the Zn-Al-Mg based coating may include a surface modified region enriched with cerium (Ce) on the surface side of the Zn-Al-Mg based coating.

[0012] The surface modified region may include: a convex portion provided by zinc cerium nitrate (Zn-Ce-NO3-H2O) attached to the surface layer of the Zn-Al-Mg based plating layer; and a concave portion provided by removing magnesium (Mg) from the surface layer of the Zn-Al-Mg based plating layer.

[0013] The adhesion amount of the zinc cerium nitrate (Zn-Ce-NO3-H2O) can be 0.01-2.5g / m 2 .

[0014] Based on the centerline average roughness (Ra), the surface roughness of the surface modified area may be 1.2-1.6 μm.

[0015] In terms of weight %, the Zn-Al-Mg based plating layer may include 0.1-16% magnesium (Mg), 0.1-12% aluminum (Al), 0.0005-1.5% cerium (Ce), and the balance zinc (Zn) and inevitable impurities.

[0016] According to another aspect of the present invention, a surface modification method of a galvanized steel sheet may include the following steps: preparing a galvanized steel sheet having a Zn-Al-Mg based coating formed on at least one side of a base steel sheet; preparing an aqueous solution of cerium nitrate; immersing the galvanized steel sheet in the aqueous solution of cerium nitrate; and washing and drying the galvanized steel sheet immersed in the aqueous solution of cerium nitrate.

[0017] The step of preparing the galvanized steel sheet may include the following steps: preparing a Zn-Al-Mg based plating bath; and immersing the base steel sheet in the Zn-Al-Mg based plating bath to form a plating layer. In the step of preparing the Zn-Al-Mg based plating bath, a plating bath containing, in weight %, 0.1-16% magnesium (Mg), 0.1-12% aluminum (Al), the remainder zinc (Zn) and inevitable impurities may be prepared, wherein the content of cerium (Ce) inevitably added to the plating bath may be suppressed to 0.01% or less (including 0%).

[0018] In the step of preparing the cerium nitrate aqueous solution, 15-25 g of cerium nitrate may be dissolved in 1000 ml of water (H 2 O) and then titrated with nitric acid (HNO 3 ) to prepare the cerium nitrate aqueous solution with a pH of 3.2 to 3.8.

[0019] In the step of immersing the galvanized steel sheet in the cerium nitrate aqueous solution, the temperature of the cerium nitrate aqueous solution may be 20-80° C., and the immersion time of the galvanized steel sheet may be 10-150 seconds.

[0020] The above technical solutions do not list all the features of the present invention. The various features of the present invention and its advantages and effects may be understood in more detail by referring to the following specific embodiments.

[0021] Beneficial effects

[0022] According to one aspect of the present invention, a magnesium-containing galvanized steel sheet having an adhesive bond effectively improved by surface modification treatment can be provided.

[0023] According to another aspect of the present invention, a surface modification method of a galvanized steel sheet that can effectively improve adhesion between a magnesium-containing galvanized steel sheet and an adhesive can be provided.

[0024] The effects of the present invention are not limited to the above-described contents, and can be construed as including technical effects that can be derived from the contents described below by those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic diagram schematically illustrating the concept of the surface modification treatment of the present invention, Figure 1 (a) is a schematic diagram showing a cross section of the coating before surface modification treatment, Figure 1 (b) is a schematic diagram showing a cross section of the plating layer after the surface modification treatment.

[0026] Figure 2 (a) and (b) are schematic diagrams illustrating the concepts of cohesive failure and interfacial failure, respectively.

[0027] Figure 3 (a) and (b) are photographs of the bonding surface where cohesive failure was observed and the bonding surface where interfacial failure was observed, respectively.

[0028] Best Practice

[0029] The present invention relates to a surface modification method for a galvanized steel sheet and the galvanized steel sheet thus surface-modified. Preferred embodiments of the present invention are described below. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to further illustrate the present invention to those skilled in the art.

[0030] Hereinafter, a galvanized steel sheet according to a specific embodiment of the present invention will be described in detail.

[0031] The galvanized steel sheet of the present invention may include: a base steel sheet; and a Zn-Al-Mg based coating layer provided on at least one side of the base steel sheet.

[0032] The base steel plate is not particularly limited and can be interpreted as a concept including all steel plates that can be generally used to manufacture galvanized steel plates. As an example, the base steel plate of the present invention can be interpreted as a concept including not only cold-rolled steel plates, hot-rolled steel plates and heat-treated steel plates, but also wire rods and steel wires. In addition, the alloy composition, microstructure, etc. of the base steel plate of the present invention are not particularly limited. Preferably, the base steel plate of the present invention can be a low-carbon cold-rolled steel plate having a carbon (C) content of less than 0.05% by weight, but is not necessarily limited thereto.

[0033] The plating layer of the present invention will be described in more detail below. Unless otherwise specified, the percentages described below regarding the alloy composition of the plating layer represent weight percentages.

[0034] The method for forming the coating of the present invention is not particularly limited. As an example, the coating of the present invention can be formed by a hot-dip plating method, an electroplating method, a vacuum evaporation method, or the like, and can also be formed by a coating forming method that includes an alloying treatment. Preferably, the coating of the present invention can be a zinc-based hot-dip coating, and more preferably, the coating of the present invention can be a Zn-Al-Mg-based hot-dip coating.

[0035] The coating of the present invention may be a Zn-Al-Mg-based coating comprising, by weight, 0.1-16% magnesium (Mg), 0.1-12% aluminum (Al), the remainder zinc (Zn), and unavoidable impurities. Furthermore, the coating of the present invention may further comprise 0.0005-1.5% by weight of cerium (Ce), which is introduced into the coating through surface modification treatment rather than being artificially added to the plating bath.

[0036] Magnesium (Mg), aluminum (Al), and zinc (Zn) form a uniform liquid molten aluminum-magnesium alloy through a ternary eutectic reaction in a plating bath. The surface of a steel strip passing through the ternary plating bath is coated with the liquid ZnAluminum-Magnesium alloy. The liquid ZnAluminum-Magnesium alloy cools in the atmosphere and can form a single-phase Zn structure containing 1-4% aluminum (Al), a single-phase Mg structure containing 1-2% zinc (Zn), a single-phase MgZn2 structure, a binary Zn-MgZn2 eutectic structure, a binary Zn-Al eutectic structure, and a ternary MgZn2-Zn-Al eutectic structure. Depending on the composition range of the plating bath components, the plating bath temperature, and the cooling rate, the intermetallic compounds (single-phase, binary, and ternary eutectic structures) described above can form a combination of two or more in the coating.

[0037] When exposed to the atmosphere, the magnesium (Mg) in the coating reacts to form magnesium hydroxide earlier than zinc (Zn), thereby effectively improving the corrosion resistance of the coated steel sheet. In particular, magnesium (Mg) mainly exists in the form of a MgZn2 single-phase structure, but can also exist in a binary eutectic structure or a ternary eutectic structure, or can exist in the form of a Mg single-phase structure containing 1-2% zinc (Zn). On the surface side of the coating, the MgZn2 single-phase structure mainly exists in the form of a formation layer, thereby more effectively improving the corrosion resistance of the coated steel sheet. Therefore, the Zn-Al-Mg based coating of the present invention can contain more than 0.1% by weight of magnesium (Mg), preferably more than 0.5% by weight of magnesium (Mg), and more preferably more than 1.0% by weight of magnesium (Mg).

[0038] However, when the content of magnesium (Mg) is too much, not only the effect of improving corrosion resistance is saturated, but also scum associated with magnesium (Mg) oxide is formed in the plating bath, so the plating property may deteriorate, and excessive magnesium hydroxide is formed in the surface layer of the coating, so even if surface modification is performed, the adhesion to the epoxy-based sealing adhesive may also deteriorate. In addition, too much high-hardness Zn-Al-Mg-based intermetallic compound is formed as the fine structure of the coating, so the bending workability may be reduced. Therefore, the Zn-Al-Mg-based coating of the present invention can include less than 16 weight % of magnesium (Mg), preferably can include less than 12 weight % of magnesium (Mg), and more preferably can include less than 8 weight % of magnesium (Mg).

[0039] Aluminum (Al) promotes alloying by forming a eutectic reaction with zinc (Zn) and magnesium (Mg) in the plating bath. The intermetallic compound containing aluminum (Al) formed during solidification acts as a corrosion barrier, thus effectively improving corrosion resistance. Furthermore, aluminum (Al) suppresses the formation of magnesium (Mg)-based oxide scum, allowing sufficient amounts of Zn-Al-Mg-based intermetallic compounds to form in the coating, thus effectively improving the corrosion resistance of the plated steel sheet. Therefore, the Zn-Al-Mg-based coating of the present invention may contain at least 0.1% by weight of aluminum (Al), preferably at least 0.5% by weight, and more preferably at least 1.0% by weight. However, when the aluminum (Al) content is too high, not only is the effect of improving corrosion resistance saturated, but the plating bath temperature must be increased, and the resulting magnesium (Mg) and zinc (Zn) vapors may adversely affect the durability of the coating equipment. Furthermore, excessive formation of high-hardness Zn-Al-Mg-based intermetallic compounds as the fine structure of the coating may reduce bendability. Therefore, the Zn-Al-Mg-based plating layer of the present invention may contain 12 wt% or less of aluminum (Al), preferably 9 wt% or less of aluminum (Al), and more preferably 6 wt% or less of aluminum (Al).

[0040] The Zn-Al-Mg-based coating of the present invention may contain, in addition to the aforementioned magnesium (Mg) and aluminum (Al), a balance of zinc (Zn) and unavoidable impurities. The unavoidable impurities may refer to components that are unavoidably introduced during the manufacturing process of the plated steel sheet or components introduced from the base steel sheet. Although the unavoidably introduced components are not specifically limited, those skilled in the art can readily understand these components.

[0041] The Zn-Al-Mg based coating of the present invention can also include 0.0005-1.5 % by weight of cerium (Ce). The cerium (Ce) included in the Zn-Al-Mg based coating of the present invention is not a component artificially added as the composition of the plating bath, but can be introduced into the composition of the Zn-Al-Mg based coating by the surface modification treatment carried out after forming the coating as the Zn-Al-Mg based coating. In order to ensure desired sealing adhesion, the content of the cerium (Ce) introduced into the Zn-Al-Mg based coating of the present invention can be limited to more than 0.0005 % by weight, preferably the amount of introduction of cerium (Ce) can be limited to more than 0.001 % by weight, and more preferably the amount of introduction of cerium (Ce) can be limited to more than 0.005 % by weight. However, when the content of the cerium (Ce) introduced into the Zn-Al-Mg based coating is too much, the effect of improving sealing adhesion is saturated, and cerium (Ce) is precipitated at the grain boundary of the Zn-Al-Mg based intermetallic compound, so the mechanical and physical properties such as impact resistance and wear resistance may deteriorate. Therefore, the content of cerium (Ce) introduced into the Zn-Al-Mg based coating in the present invention can be limited to 1.5 wt% or less, preferably the amount of cerium (Ce) introduced can be limited to 1.2 wt% or less, and more preferably the amount of cerium (Ce) introduced can be limited to 1.0 wt% or less.

[0042] The Zn-Al-Mg-based coating of the present invention may have a fine structure contained in a conventional Zn-Al-Mg-based coating. As an example, the fine structure of the Zn-Al-Mg-based coating of the present invention may include one or more selected from the group consisting of a Zn single-phase structure containing 1-4% aluminum (Al), a Mg single-phase structure containing 1-2% zinc (Zn), a MgZn2 single-phase structure, a Zn-MgZn2 binary eutectic structure, a Zn-Al binary eutectic structure, and a MgZn2-Zn-Al ternary eutectic structure, as well as other structures inevitably introduced during the manufacturing process.

[0043] The surface layer of the Zn-Al-Mg-based coating of the present invention may include a surface-modified region introduced through a surface modification treatment, and a larger amount of cerium (Ce) may be concentrated in the surface-modified region compared to the central portion of the Zn-Al-Mg-based coating. As described below, the surface modification treatment of the coating of the present invention utilizes an aqueous solution of cerium nitrate, thereby allowing zinc-cerium nitrate (Zn-Ce-NO₃-H₂O) to adhere to the surface of the coating. Consequently, a surface-modified region in which a certain amount of cerium (Ce) is concentrated can be formed in the surface layer.

[0044] Figure 1 is a schematic diagram schematically illustrating the concept of the surface modification treatment of the present invention, Figure 1 (a) is a schematic diagram showing a cross section of the coating before surface modification treatment, Figure 1(b) is a schematic diagram showing the cross section of the coating after surface modification. Figure 1 In (b), in order to illustrate the concept of the surface modification treatment of the present invention, the shapes of the concave and convex parts are slightly exaggerated, but it should be noted that the shapes of the concave and convex parts of the present invention are not necessarily limited to Figure 1 The shape shown in (b).

[0045] like Figure 1 As shown in (a), a Zn single-phase structure 11, a Zn-Mg-Al-based intermetallic compound 12, and a MgZn2 single-phase structure 13 are mixed in the Zn-Al-Mg-based coating 10 formed on the base steel sheet 1. The MgZn2 single-phase structure 13 containing a large amount of highly oxidizing magnesium (Mg) is mainly distributed on the surface side of the coating 10. The magnesium (Mg) component of the MgZn2 single-phase structure 13 distributed in the surface layer of the Zn-Al-Mg-based coating 10 first reacts with moisture (H2O) in the atmosphere to form magnesium hydroxide, which may become a factor that reduces the adhesion between the coating 10 and the epoxy-based sealant.

[0046] like Figure 1 As shown in (b), a surface modified region 20 having a concavo-convex surface can be provided in the surface portion of the Zn-Al-Mg based plating layer 10 subjected to surface modification treatment using an aqueous cerium nitrate solution. Magnesium (Mg) on ​​the surface side of the plating layer 10 reacts with nitric acid in the aqueous cerium nitrate solution and dissolves into the aqueous solution, so that the corresponding region can be formed as a concave portion 21. On the other hand, zinc (Zn) on the surface side of the plating layer 10 reacts with cerium (Ce) in the aqueous cerium nitrate solution to form zinc cerium nitrate (Zn-Ce-NO3-H2O), so that the region to which zinc cerium nitrate (Zn-Ce-NO3-H2O) is attached can be formed as a convex portion 22. That is, a surface modified region 20 having a concavo-convex surface is formed in the surface portion of the Zn-Al-Mg based plating layer 10 subjected to surface modification treatment, so that the physical contact area can be increased when bonding with an epoxy-based sealing adhesive. Furthermore, zinc cerium nitrate (Zn—Ce—NO 3 —H 2 O) increases the chemical bonding force when bonding with the epoxy-based sealing adhesive, and thus can more effectively improve the bonding force between the plating layer 10 and the epoxy-based sealing adhesive.

[0047] The adhesion amount of zinc cerium nitrate (Zn-Ce-NO3-H2O) is preferably 0.01-2.5g / m 2 This is because when the adhesion amount of zinc cerium nitrate (Zn-Ce-NO3-H2O) is less than 0.01g / m 2 When the adhesion amount of zinc cerium nitrate (Zn-Ce-NO3-H2O) is less than 0.1 g / m 2Moreover, as the amount of zinc cerium nitrate (Zn-Ce-NO3-H2O) attached increases, the adhesion to the epoxy-based sealing adhesive shows an increasing trend. However, when the amount of zinc cerium nitrate (Zn-Ce-NO3-H2O) attached is above a certain level, not only is its effect saturated, but it may also cause coating defects such as bubbles and pinholes in the subsequent coating process. Therefore, the amount of zinc cerium nitrate (Zn-Ce-NO3-H2O) attached can be controlled at 2.5g / m 2 The following level.

[0048] The surface roughness of the surface modified region may be in the range of 1.2-1.6 μm, based on the centerline average roughness (Ra). When the surface roughness of the surface modified region does not reach a certain level, it means that the surface concavo-convex shape of the surface modified region is not formed to the desired level. Therefore, in the present invention, the surface roughness of the surface modified region may be limited to 1.2 μm or more, based on the centerline average roughness (Ra). In addition, when the surface roughness of the surface modified region exceeds a certain level, the adhesive force may deteriorate as the effective contact area decreases. Therefore, in the present invention, the surface roughness of the surface modified region may be limited to a range below 1.6 μm, based on the centerline average roughness (Ra).

[0049] Hereinafter, a surface modification method of a galvanized steel sheet according to another specific embodiment of the present invention will be described in more detail.

[0050] The surface modification method of the galvanized steel sheet of the present invention may include the following steps: preparing a galvanized steel sheet having a Zn-Al-Mg-based coating formed on at least one side of a base steel sheet; preparing an aqueous solution of cerium nitrate; immersing the galvanized steel sheet in the aqueous solution of cerium nitrate; and washing and drying the galvanized steel sheet immersed in the aqueous solution of cerium nitrate.

[0051] The step of preparing the galvanized steel sheet may include the following steps: preparing a Zn-Al-Mg based plating bath; and immersing the base steel sheet in the Zn-Al-Mg based plating bath to form a plating layer. In the step of preparing the Zn-Al-Mg based plating bath, a plating bath containing, in weight %, 0.1-16% magnesium (Mg), 0.1-12% aluminum (Al), the remainder zinc (Zn) and inevitable impurities may be prepared, wherein the content of cerium (Ce) inevitably added to the plating bath may be suppressed to 0.01% or less (including 0%).

[0052] The base steel sheet provided in the surface modification method of the present invention corresponds to the base steel sheet of the plated steel sheet described above, and thus the description of the base steel sheet provided in the surface modification method is substituted for the description of the base steel sheet of the plated steel sheet described above. Furthermore, the composition of the Zn-Al-Mg-based plating bath provided in the surface modification method of the present invention corresponds to the composition of the coating of the plated steel sheet described above, except that no cerium (Ce) is artificially added, and thus the description of the composition of the coating of the plated steel sheet described above is substituted for the description of the composition of the Zn-Al-Mg-based plating bath provided in the surface modification method. Furthermore, the plating conditions, such as the plating bath temperature and the temperature of the base steel sheet when immersed in the plating bath, apply conventional Zn-Al-Mg-based plating conditions. Therefore, even without further explanation, those skilled in the art can manufacture the Zn-Al-Mg-based plating bath of the present invention and form the desired coating without requiring any special technical means.

[0053] In the step of preparing the cerium nitrate aqueous solution, 15-25g of cerium nitrate can be dissolved in 1000ml of water (H o) and then titrated with nitric acid (HNO ) to prepare a cerium nitrate aqueous solution having a pH of 3.2 to 3.8. For the surface modification reactivity of the desired level, the dissolved amount of cerium nitrate in 1000ml of water (H o) can be more than 15g. However, when the dissolved amount of cerium nitrate is too much, sediment may be produced, so the dissolved amount of cerium nitrate in 1000ml of water (H o) may be limited to less than 25g. When the pH of the cerium nitrate aqueous solution is too low, the etching of coating is carried out faster than the formation of cerium (Ce) base coating, causing surface modification to be carried out slowly, so the pH of the cerium nitrate aqueous solution can be limited to more than 3.2. However, when the pH of the cerium nitrate aqueous solution is too high, the etching speed and surface modification speed of coating are slack, so the pH of the cerium nitrate aqueous solution can be limited to less than 3.8.

[0054] In addition, in the step of immersing the galvanized steel sheet in the cerium nitrate aqueous solution, the temperature of the cerium nitrate aqueous solution may be 20-80°C, and the immersion time of the galvanized steel sheet may be 10-150 seconds. When the temperature of the cerium nitrate aqueous solution is too low or the immersion time of the galvanized steel sheet is too short, the desired level of surface modification cannot be achieved, so the temperature of the cerium nitrate aqueous solution may be limited to above 20°C, and the immersion time of the galvanized steel sheet may be limited to above 10 seconds. When the temperature of the cerium nitrate aqueous solution is too high or the immersion time of the galvanized steel sheet is too long, it may be difficult to operate due to the generation of fume, so the temperature of the cerium nitrate aqueous solution may be limited to below 80°C, and the immersion time of the galvanized steel sheet may be limited to below 150 seconds.

[0055] The magnesium-containing galvanized steel sheet subjected to surface modification treatment using the surface modification method of the present invention can maintain the effect of improving corrosion resistance brought about by the addition of magnesium, and can effectively improve the adhesion with the epoxy-based sealing adhesive. DETAILED DESCRIPTION

[0056] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only used to more specifically illustrate the present invention, and the scope of the present invention is not limited to the following examples.

[0057] (Example)

[0058] Low-carbon steel with a carbon content of 0.05% by weight or less was cut into 75 x 150 mm (0.8 mm thick) pieces, degreased with acetone, and then immersed in a molten Zn-Al-Mg-based plating bath to prepare Zn-Al-Mg-based alloy-plated steel sheets (plating thickness: 20 μm) as test pieces. Two test pieces were prepared under the same conditions to form a pair, and the composition of the molten Zn-Al-Mg-based plating bath was applied to the conditions shown in Table 1.

[0059] 20g of cerium nitrate powder was dissolved in 1000ml of water (H2O) and titrated with nitric acid (HNO3) to prepare a cerium nitrate aqueous solution with a pH of 3.5. Each test piece was then immersed in the prepared cerium nitrate aqueous solution, rinsed with water, and dried to perform surface modification. The temperature of the cerium nitrate aqueous solution was maintained at 50°C, and the immersion time for each test piece was based on the conditions shown in Table 1.

[0060] After surface modification, the sealant adhesion of each test piece was evaluated. For each pair of test pieces, an epoxy sealant (766MD from Sunrise) was applied to an area of ​​15 x 15 mm and bonded together. The adhesive was then foamed and cured at 200°C for 2 minutes. A shear stress of 20 MPa was applied parallel to the sealant surfaces of the foamed and cured test pieces. The test pieces were then separated and the sealant adhesion was evaluated by observing the separated surfaces. Figure 2(a) and (b) are schematic diagrams showing the concepts of cohesive failure and interfacial failure, respectively. At this time, the sealing adhesion is evaluated based on the area ratio of the epoxy-based sealing adhesive remaining on the bonding surfaces of the two test pieces. That is, the case where the area coated with the epoxy-based sealing adhesive on all bonding surfaces of the separated test pieces is more than 95% is evaluated as cohesive failure (very excellent, ◎), the case where the area coated with the epoxy-based sealing adhesive on any bonding surface of the separated test piece is more than 80% is evaluated as quasi-cohesive failure (excellent, ○), and the case where the area coated with the epoxy-based sealing adhesive on any bonding surface of the separated test piece is less than 80% is evaluated as interfacial failure (insufficient, ×). The area ratio of the epoxy-based sealing adhesive recorded in Table 1 below refers to the coating area of ​​the epoxy-based sealing adhesive observed on the bonding surface of each test piece separated from a pair of test piece assemblies. Figure 3 (a) and (b) are photos of the bonding surface with cohesive failure and the bonding surface with interfacial failure, respectively. Figure 3 The relatively darkest area in (a) and (b) is the portion coated with the epoxy-based sealing adhesive, and the light gray area is the base steel plate portion.

[0061] [Table 1]

[0062]

[0063] As shown in Table 1, the sealing adhesion of test pieces No. 4 to No. 11, which met the surface modification conditions of the present invention, was rated as excellent or very excellent. On the other hand, the sealing adhesion of test pieces No. 2 and No. 3, which did not meet the surface modification conditions of the present invention, was rated as insufficient.

[0064] The present invention has been described in detail above through the embodiments, but it can also include other different forms of embodiments. Therefore, the technical concept and scope of the claims are not limited to the embodiments.

[0065] (Explanation of Reference Numerals)

[0066] 1: Base steel plate 10: Coating 11: Zn single phase structure

[0067] 12: Zn-Mg-Al based intermetallic compound 13: MgZn2 single phase structure

[0068] 20: Surface modified area 21: Concave part 22: Concave part

Claims

1. A galvanized steel sheet, comprising: foundation steel plate; as well as a Zn-Al-Mg based coating provided on at least one side of the base steel plate; The Zn-Al-Mg based coating comprises a surface modified region enriched with cerium (Ce) on the surface side of the Zn-Al-Mg based coating. The surface modified region has a concavoconvex surface, Wherein, the surface modified region is obtained by treatment with a cerium nitrate aqueous solution, and the surface modified region comprises: a convex portion provided by zinc cerium nitrate (Zn-Ce-NO3-H2O) attached to a surface layer of the Zn-Al-Mg based plating layer; and The recessed portion is provided by dissolving and removing magnesium (Mg) in a surface layer portion of the Zn-Al-Mg based plating layer.

2. The galvanized steel sheet according to claim 1, wherein The adhesion amount of the zinc cerium nitrate (Zn-Ce-NO3-H2O) is 0.01-2.5g / m 2 .

3. The galvanized steel sheet according to claim 1, wherein Taking the center line average roughness Ra as a benchmark, the surface roughness of the surface modified area is 1.2-1.6 μm.

4. The galvanized steel sheet according to claim 1, wherein In terms of weight %, the Zn-Al-Mg based coating layer comprises 0.1-16% of magnesium (Mg), 0.1-12% of aluminum (Al), 0.0005-1.5% of cerium (Ce), and the balance of zinc (Zn) and inevitable impurities.

5. A surface modification method for preparing the galvanized steel sheet according to claim 1, comprising the following steps: A galvanized steel sheet having a Zn-Al-Mg-based coating formed on at least one side of a base steel sheet is prepared; Prepare an aqueous solution of cerium nitrate; immersing the galvanized steel sheet in the cerium nitrate aqueous solution; as well as The galvanized steel sheet immersed in the cerium nitrate aqueous solution is washed with water and dried, In the step of preparing the cerium nitrate aqueous solution, 15-25 g of cerium nitrate is dissolved in every 1000 ml of water (H2O) and then titrated with nitric acid (HNO3) to prepare the cerium nitrate aqueous solution with a pH of 3.2 to 3.

8.

6. The surface modification method according to claim 5, wherein The steps of preparing the galvanized steel sheet include the following steps: preparing a Zn-Al-Mg based plating bath; and immersing the base steel sheet in the Zn-Al-Mg based plating bath to form a plating layer, In the step of preparing the Zn-Al-Mg based plating bath, a plating bath containing, in weight %, 0.1-16% of magnesium (Mg), 0.1-12% of aluminum (Al), the balance of zinc (Zn) and inevitable impurities is prepared, wherein the content of cerium (Ce) inevitably added to the plating bath is suppressed to 0.01% or less and inclusive of 0%.

7. The surface modification method according to claim 5, wherein In the step of immersing the galvanized steel sheet in the cerium nitrate aqueous solution, the temperature of the cerium nitrate aqueous solution is 20-80° C., and the immersion time of the galvanized steel sheet is 10-150 seconds.

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