Highly corrosion-resistant plated steel sheet with excellent corrosion resistance and surface quality, and its manufacturing method

Abstract

One embodiment of the present invention provides a plated steel sheet and a manufacturing method thereof, the plated steel sheet including: a base steel sheet; a Zn-Mg-Al-based plating layer provided on at least one surface of the base steel sheet; and an Fe-Al-based inhibition layer provided between the base steel sheet and the Zn-Mg-Al-based plating layer; wherein, on a surface of the Zn-Mg-Al-based plating layer, a total area ratio of an Al single phase and an MgZn2 phase is 45-60%, and an area ratio of the MgZn2 phase to the Al single phase is 1.2-3.3.

Description

[Technical Field]

[0001] The present invention relates to a highly corrosion-resistant plated steel sheet having excellent corrosion resistance and surface quality, and a method for producing the same. [Background technology]

[0002] When exposed to a corrosive environment, zinc-based coated steel sheets exhibit sacrificial corrosion protection properties, in which zinc, which has a lower redox potential than iron, corrodes first, inhibiting corrosion of the steel. Furthermore, as the zinc in the coating layer oxidizes, it forms dense corrosion products on the surface of the steel, insulating it from the oxidizing atmosphere and improving the corrosion resistance of the steel. Thanks to these advantageous properties, zinc-based coated steel sheets have recently been widely used in building materials, home appliances, and automobiles.

[0003] However, the corrosive environment is gradually worsening due to increased air pollution caused by industrial advancement, and strict regulations on resources and energy conservation are increasing the need to develop steel materials with better corrosion resistance than conventional galvanized steel materials.

[0004] To address these issues, various studies have been conducted on manufacturing technologies for zinc alloy-coated steel sheets that improve the corrosion resistance of steel by adding elements such as aluminum (Al) and magnesium (Mg) to the zinc plating bath. A typical example is Zn-Mg-Al zinc alloy-coated steel sheets, which further contain Mg in the Zn-Al plating composition.

[0005] However, Zn-Mg-Al zinc alloy plated steel sheets are usually processed into zinc alloys for use, but they have the drawback of poor bending workability, such as the induction of cracks in the plating layer, because they contain a large amount of intermetallic compounds with high hardness in the plating layer.

[0006] Therefore, attempts have been made to further improve the bending workability of plated steel sheets. However, even if the bending workability is improved, fine cracks that occur in the bent portion during bending expose the base steel sheet. Therefore, it has been technically very difficult to ensure corrosion resistance not only in the flat portion of the plated steel sheet but also in the bent portion.

[0007] On the other hand, it is known that the corrosion resistance of the bent portion of zinc alloy-plated steel sheet is usually achieved by self-healing the exposed portion of the base steel sheet due to the leaching of Mg and Al components in a moisture atmosphere. However, this effect is small, and it has been difficult to ensure the desired level of corrosion resistance in the bent portion.

[0008] Furthermore, zinc-based plated steel sheets are often used on the exterior of products, but the higher the magnesium content in the plating layer, the darker the appearance of the product becomes. This also leads to surface damage during processing, which reduces the surface quality, making it necessary to improve the appearance quality.

[0009] However, no technology has yet been developed that can meet the high level of demand for excellent corrosion resistance in both flat plate parts and bent parts, as well as excellent corrosion resistance and appearance quality. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Korean Patent Publication No. 2010-0073819 Summary of the Invention [Problem to be solved by the invention]

[0011] An object of one embodiment of the present invention is to provide a plated steel sheet that not only has excellent corrosion resistance in the flat portion but also has excellent corrosion resistance and appearance quality in the bent portion, and a method for manufacturing the same.

[0012] The object of the present invention is not limited to the above-mentioned content, and anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the invention from the entire content of the specification of the present invention. [Means for solving the problem]

[0013] One embodiment of the present invention comprises: Base steel sheet; a Zn-Mg-Al-based coating layer provided on at least one surface of the base steel sheet; and an Fe-Al-based suppression layer provided between the base steel sheet and the Zn-Mg-Al-based coating layer; The present invention provides a plated steel sheet in which, on the surface of a Zn-Mg-Al-based plating layer, the total area ratio of the Al single phase and the MgZn2 phase is 45 to 60%, and the area ratio of the MgZn2 phase to the Al single phase is 1.2 to 3.3.

[0014] Yet another embodiment of the present invention is A step of immersing the base steel sheet in a coating bath containing, by weight, 4 to 6% Mg, 8.2 to 14.2% Al, the balance Zn and other unavoidable impurities, and maintained at a temperature 20 to 80°C higher than the solidification start temperature on the equilibrium diagram, to perform hot-dip galvanizing; and cooling the hot-dip galvanized steel sheet from a solidification start temperature to a solidification finish temperature using an inert gas at an average cooling rate of 2 to 12°C / s; The present invention provides a method for producing a plated steel sheet, wherein the cooling step satisfies the following relational expressions 1-1 and 1-2, and cooling is performed so that the ratio (De / Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion satisfies 60 to 99%.

[0015] [Equation 1-1] A<{(5-2lnt) / (7-3lnt)}×B

[0016] [Equation 1-2] 15t (-0.8) ≦B≦20t (-0.8) (In the above relational expressions 1-1 and 1-2, t is the thickness (mm) of the steel plate, A is the average cooling rate (°C / s) from the solidification start temperature to 375°C, and B is the average cooling rate (°C / s) from 375°C to 340°C.) [Effects of the Invention]

[0017] According to one embodiment of the present invention, it is possible to provide a plated steel sheet that not only has excellent corrosion resistance as a flat sheet, but also has excellent corrosion resistance and appearance quality in a processed portion, and a method for manufacturing the same.

[0018] The various beneficial advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0019] [Figure 1] 1(a) is a photograph of a surface test piece prepared to allow observation of the surface of the plated steel sheet of Example 13, the surface test piece being observed with a field emission scanning electron microscope (hereinafter referred to as "FE-SEM") at a magnification of 700. 1(b) shows the measured ratios of each phase in the above photograph. [Figure 2] 1. The plated steel sheet of Example 13, similar to that shown in Figure 1, was polished up to the 1 / 2t point, and then a 1 / 2t surface test piece was prepared to observe the polished surface. Photograph (a) shows the test piece observed with a scanning electron microscope (FE-SEM) at a magnification of 700. Photograph (b) shows the ratio of each phase measured in the above photograph. [Figure 3] 1(a) is a photograph of a surface test piece prepared to observe the surface of the plated steel sheet of Comparative Example 1, the surface test piece being observed with a field emission scanning electron microscope (hereinafter referred to as "FE-SEM") at a magnification of 700. 1(b) shows the ratio of each phase measured in the photograph. [Figure 4]Photograph (a) shows a photograph of a 1 / 2t surface test piece prepared by polishing the plated steel sheet of Comparative Example 1 similar to that shown in Figure 3 up to the 1 / 2t point, allowing observation of the polished surface, and the test piece was observed with a scanning electron microscope (FE-SEM) at a magnification of 700. Photograph (b) shows the ratio of each phase measured in the photograph. [Figure 5] 5 is a graph illustrating the fractions of elements dissolved in the microstructures of the Al single phase, the second Al single phase, and the Al-Zn binary eutectic phase shown in FIGS. 1 to 4, observed with an EDS (Energy Dispersive Spectrometer). DETAILED DESCRIPTION OF THE INVENTION

[0020] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the related definition clearly dictates otherwise.

[0021] As used herein, the meaning of "comprises" embodies features and does not exclude the presence or addition of other features.

[0022] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted as having a meaning consistent with the relevant technical literature and the present disclosure.

[0023] Hereinafter, a "plated steel sheet" according to one embodiment of the present invention will be described in detail. In the present invention, when the content of each element is indicated, it means % by weight unless otherwise specified.

[0024] In conventional technology for Zn-Mg-Al zinc alloy coated steel sheets, magnesium is added to improve corrosion resistance, but adding too much magnesium increases the amount of floating dross in the coating bath, necessitating frequent removal of the dross, so the upper limit for magnesium addition was set at 3%. Research was therefore conducted to further improve corrosion resistance by increasing the amount of magnesium added beyond 3%, but as the amount of magnesium added increases, a large amount of highly hard intermetallic compounds is formed, which causes cracks in the coating layer during bending.

[0025] In response to this, research has been conducted to ensure corrosion resistance and bendability, but even if corrosion resistance and bendability are ensured in the flat portion of a coated steel sheet, it is technically very difficult to ensure corrosion resistance in the bent portion because the base steel sheet is exposed due to microcracks that inevitably occur during bending. Furthermore, the more Mg is added, the darker the appearance of the product becomes, and the more susceptible it is to surface damage, making it difficult to ensure appearance quality.

[0026] Therefore, the present inventors have conducted extensive research to solve the above-mentioned problems and to provide a plated steel sheet that not only has excellent corrosion resistance in the flat portion but also has excellent corrosion resistance and appearance quality in the bent portion. As a result, they have found that when the plated steel sheet is kept in a corrosive environment (or in an atmospheric environment for a long period of time), LDH (Layered Double Hydroxide; (Zn,Mg)Al(OH) 16 The inventors have found that the uniform formation of (CO3)·4H2O) is an important factor, and have completed the present invention.

[0027] Therefore, the following describes in detail the configuration of a plated steel sheet that forms LDHs as initial corrosion products on the surface of the bent portion, and that distributes the LDHs uniformly over the entire surface of the bent portion over time, thereby shielding the corrosion-active region.

[0028] First, a plated steel sheet according to one embodiment of the present invention includes a base steel sheet; a Zn-Mg-Al-based coating layer provided on at least one surface of the base steel sheet; and an Fe-Al-based inhibition layer provided between the base steel sheet and the Zn-Mg-Al-based coating layer.

[0029] In the present invention, the type of base steel sheet is not particularly limited. For example, the base steel sheet may be an Fe-based base steel sheet used as a base steel sheet for ordinary zinc-based plated steel sheets, i.e., a hot-rolled steel sheet or a cold-rolled steel sheet, but is not limited thereto. Alternatively, the base steel sheet may be, for example, a carbon steel, an ultra-low carbon steel, or a high manganese steel used as a material for construction, home appliances, or automobiles. However, as an example, the base steel sheet may have a composition, in weight percent, of C: more than 0% and not more than 0.18%, Si: more than 0% and not more than 1.5%, Mn: 0.01 to 2.7%, P: more than 0% and not more than 0.07%, S: more than 0% and not more than 0.015%, Al: more than 0% and not more than 0.5%, Nb: more than 0% and not more than 0.06%, Cr: more than 0% and not more than 1.1%, Ti: more than 0% and not more than 0.06%, B: more than 0% and not more than 0.03%, and the balance being Fe and other unavoidable impurities.

[0030] According to one embodiment of the present invention, at least one surface of the base steel sheet may be provided with a Zn-Mg-Al-based plating layer made of a Zn-Mg-Al-based alloy. The plating layer may be formed on only one surface of the base steel sheet, or may be formed on both surfaces of the base steel sheet. In this case, the Zn-Mg-Al-based plating layer refers to a plating layer that contains Mg and Al and primarily contains Zn (i.e., contains 50% or more Zn).

[0031] According to one embodiment of the present invention, the thickness of the Zn-Mg-Al-based coating layer may be 5 to 100 μm, and more preferably 5 to 90 μm. If the coating layer thickness is less than 5 μm, errors resulting from thickness variations in the coating layer may cause the coating layer to become too thin locally, resulting in poor corrosion resistance. If the coating layer thickness exceeds 100 μm, the cooling of the hot-dip coating layer may be delayed, which may lead to solidification defects, such as flow patterns, on the surface of the coating layer, and the productivity of the steel sheet may be reduced due to the need to solidify the coating layer.

[0032] According to one embodiment of the present invention, an Fe-Al-based inhibition layer may be provided between the base steel sheet and the Zn-Mg-Al-based coating layer. The Fe-Al-based inhibition layer is a layer primarily containing an intermetallic compound of Fe and Al, such as FeAl, FeAl3, or Fe2Al5. Other components derived from the coating layer, such as Zn or Mg, may also be included, for example, in an amount of 40% or less. The inhibition layer is formed by alloying Fe diffused from the base steel sheet in the initial coating stage with components of the coating bath. The inhibition layer serves to improve adhesion between the base steel sheet and the coating layer and to prevent diffusion of Fe from the base steel sheet to the coating layer. In this case, the inhibition layer may be formed continuously or discontinuously between the base steel sheet and the Zn-Mg-Al-based coating layer. Except for the above description, the inhibition layer may be similarly described in the art.

[0033] According to one embodiment of the present invention, the thickness of the inhibitor layer may be 0.02 to 2.5 μm. The inhibitor layer prevents alloying and ensures corrosion resistance. However, since it is brittle and may affect workability, the thickness may be set to 2.5 μm or less. However, in order to fulfill its role as an inhibitor layer, it is preferable to control the thickness to 0.02 μm or more. From the viewpoint of further improving the above-mentioned effects, the upper limit of the thickness of the inhibitor layer may preferably be 1.8 μm. The lower limit of the thickness of the inhibitor layer may preferably be 0.05 μm. In this case, the thickness of the inhibitor layer may refer to the minimum thickness in a direction perpendicular to the interface of the base steel sheet.

[0034] Meanwhile, according to one embodiment of the present invention, the Zn-Mg-Al-based coating layer may contain, by weight %, 4 to 6% Mg, 8.2 to 14.2% Al, the balance being Zn and other inevitable impurities. Each component will be specifically described below.

[0035] Mg: 4% or more and 6% or less Mg is an element that plays a role in improving the corrosion resistance of plated steel materials, and in the present invention, the Mg content in the coating layer is controlled to 4% or more to ensure the desired excellent corrosion resistance. On the other hand, from the viewpoint of ensuring corrosion resistance, the effect improves as the amount of Mg added increases, so there is no need to particularly limit the upper limit of the Mg content. However, as an example, if excessive Mg is added, dross may be generated, so the Mg content can be controlled to 6% or less.

[0036] Al: 8.2% or more and 14.2% or less Generally, the addition of 1% or more of magnesium improves corrosion resistance. However, the addition of 2% or more of magnesium increases the generation of floating dross in the coating bath due to oxidation of magnesium, requiring frequent removal of the dross. To address this issue, conventional Zn-Mg-Al zinc alloy coatings contain 1.0% or more magnesium to ensure corrosion resistance and set the upper limit of the magnesium content at 3.0% for compatibility. However, as mentioned above, to further improve corrosion resistance, the magnesium content must be increased to 4% or more. However, if the magnesium content in the coating layer exceeds 4%, dross will be generated due to oxidation of magnesium in the coating bath, necessitating the addition of aluminum. However, adding excessive aluminum to suppress dross increases the melting point of the coating bath, which in turn increases the operating temperature, potentially resulting in problems associated with high-temperature operation, such as corrosion of the coating bath structure and deterioration of the steel material. Furthermore, if the Al content in the coating bath becomes excessive, the Al will react with the Fe in the base steel, not contributing to the formation of the Fe-Al inhibitory layer, but instead the Al will react rapidly with Zn, resulting in the formation of an excessive amount of blocky outburst phases, which may actually worsen corrosion resistance. Therefore, the upper limit of the Al content in the coating layer is preferably controlled to 14.2%, and more preferably to 14.0%.

[0037] The balance is Zn and other unavoidable impurities In addition to the above-mentioned composition of the coating layer, the balance may be Zn and other inevitable impurities. The inevitable impurities may include any impurities that may be unintentionally mixed in during a normal manufacturing process of a hot-dip galvanized steel sheet, and the meaning thereof will be easily understood by a person skilled in the art.

[0038] The Zn-Mg-Al-based coating layer may contain an MgZn2 phase and an Al single phase as a microstructure, and may also contain various other phases, such as an Al-Zn binary eutectic phase, a Zn-MgZn2-Al ternary eutectic phase, and a Zn single phase.

[0039] In this regard, in the present invention, the MgZn2 phase refers to a phase mainly composed of MgZn2, and the Al single phase refers to a phase mainly composed of Al, specifically a phase in which Zn is solid-dissolved at less than 27 atomic % and the remainder is composed of Al and other impurities. That is, the Al single phase can contain, in addition to the Al component, solid solutions of components such as Zn and Mg that can be contained as coating layer components, and in the present invention, the Al single phase specifically refers to only a phase in which Zn is solid-dissolved at less than 27 atomic %.

[0040] The Zn-MgZn2-Al ternary eutectic phase refers to a ternary eutectic phase in which the Zn phase, MgZn2 phase, and Al phase are all mixed, and the Al-Zn binary eutectic phase refers to a phase in which the Al phase and the Zn phase are alternately arranged in a lamellar or irregular mixed form. It should be noted that the Al phase in the Al-Zn binary eutectic phase and the Zn-MgZn2-Al ternary eutectic phase is not considered to be the aforementioned Al single phase or the second Al single phase described below. Similarly, it should be noted that the MgZn2 in the Zn-MgZn2-Al ternary eutectic phase is not considered to be the aforementioned MgZn2 phase mainly composed of MgZn2.

[0041] The Zn-Mg-Al-based coating layer may further include a "second Al single phase" that is distinguished from the Al single phase by the Zn solid solution rate. The second Al single phase refers to a single phase in which Zn is dissolved in an amount of 27% to 60% (27-60 atomic %), with the remainder being Al and other impurities.

[0042] Meanwhile, the microstructure of the above-mentioned Zn-Mg-Al-based coating layer may have different distributions on the surface and in the cross section, and such microstructures on the surface and in the cross section can be confirmed by using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) at an increased magnification of the coating layer for each surface test piece or cross section test piece.

[0043] As described above, Zn-Mg-Al-based coating layers contain various phases depending on the coating layer composition and manufacturing conditions. However, the present inventors have conducted extensive research to provide a coated steel sheet that not only has the conventional corrosion resistance of flat parts, but also has excellent corrosion resistance and appearance quality in bent parts. As a result, they have found that when a steel sheet is kept in a corrosive environment (or in an atmospheric environment for a long period of time), LDH (Layered Double Hydroxide; (Zn,Mg)Al(OH)) forms as an initial corrosion product on the surface of the steel sheet. 16 We found that the uniform formation of (CO3) 4H2O) is an important factor.

[0044] As described above, the inventors have confirmed that the initial formation of LDHs as corrosion products on the surface of a plated steel sheet is related to the microstructural characteristics of the surface of the Zn-Mg-Al-based plating layer (i.e., meaning the outer surface, not the surface on the base steel side), and have completed the present invention.

[0045] Specifically, according to one embodiment of the present invention, the total area ratio of the Al single phase and the MgZn2 phase on the surface of the Zn-Mg-Al based coating layer is 45 to 60%, and the area ratio of the MgZn2 phase to the Al single phase is 1.2 to 3.3. At this time, the total area ratio of the Al single phase and the MgZn2 phase and the area ratio of the MgZn2 phase to the Al single phase on the surface of the Zn-Mg-Al based coating layer are such that the area is 24,000 μm 2 Measurement is carried out using the surface test piece having the above properties as a reference.

[0046] In the present invention, a Zn-Mg-Al-based coating layer having the above-described coating layer composition includes a microstructure in which an MgZn2 phase and an Al single phase are adjacent to each other. The adjacent structure of an MgZn2 phase and an Al single phase includes a case in which the Al single phase is completely contained within the MgZn2 phase, a case in which the Al single phase is partially contained within the MgZn2 phase, and a case in which the Al single phase is present in contact with the MgZn2 phase.

[0047] In the present invention, the Zn-Mg-Al-based coating layer can contain a Zn single phase and a Zn-MgZn2-Al ternary eutectic phase, which are phases commonly found in highly corrosion-resistant coated steel sheets. Generally, the lower the Al and Mg contents in the coating layer, the greater the amounts of the Zn single phase and the Zn-MgZn2-Al ternary eutectic phase formed in the entire coating layer, and the higher the Al and Mg contents in the coating layer, the greater the amounts of the MgZn2 phase and Al single phase formed.

[0048] That is, in a coating composition system such as that of the present invention, in which the Mg content is 4% or more, the MgZn2 phase becomes coarse, and as the Mg content increases, the Al content must also increase simultaneously to suppress dross, resulting in the coexistence of a coarse Al single phase, as shown in Figure 1, which shows the surface of the coating layer. Therefore, the present inventors have discovered that, in order to ensure the corrosion resistance of the bent portion described above, the total area fraction and area ratio of the MgZn2 phase and the Al single phase adjacent to the MgZn2 phase on the surface of the coating layer contribute to the early formation of LDHs as corrosion products when the coating layer is maintained in a corrosive environment (or in an atmospheric environment for a long period of time).

[0049] That is, to ensure corrosion resistance in both flat and bent sections of coated steel sheets, it is important that the MgZn2 phase and the Al single phase exist adjacently in the surface structure of the coating layer, which can promote the rapid nucleation and crystallization of LDHs. Therefore, after the initial rapid nucleation and crystallization of LDHs, the uniformly formed LDHs over time effectively shield the corrosion-active areas and induce the secondary uniform formation of the corrosion products simonkolleite (Zn5(OH)8Cl2) and hydrozincite (Zn5(OH)6(CO3)2).

[0050] Therefore, according to one embodiment of the present invention, it is important to ensure that the MgZn2 phase and the Al single phase are adjacent to each other in a specific amount or more on the surface of the coating layer. Specifically, on the surface of the Zn-Mg-Al-based coating layer, the total area ratio of the MgZn2 phase and the Al single phase (adjacent to the MgZn2 phase) satisfies 45 to 60%, and the area ratio of the MgZn2 phase to the Al single phase satisfies 1.2 to 3.3, thereby forming sacrificial corrosion protection cells between the MgZn2 phase and the Al single phase and ensuring excellent corrosion resistance. Here, the corrosion resistance includes not only the corrosion resistance of the flat portion but also the corrosion resistance of the bent portion, and such corrosion resistance improves as the amount of the MgZn2 phase and the Al single phase present on the surface of the coating layer increases compared to the interior of the coating layer.

[0051] If the total area ratio of the MgZn2 phase and the single Al phase on the surface of the Zn-Mg-Al-based plating layer is less than 45%, the phases that form the anode (MgZn2) and cathode (Al) of the sacrificial corrosion protection cell will be insufficient, which may result in insufficient corrosion resistance at the bent portion, and there will also be insufficient light scattering due to the phases present on the surface, which may result in a deterioration in appearance quality.On the other hand, if the total area ratio of the MgZn2 phase and the single Al phase exceeds 60%, the brittle MgZn2 phase will be formed in excess, which will result in the problem of excessive cracking in the plating layer during processing.

[0052] Furthermore, if the area ratio of the MgZn2 phase to the Al single phase on the surface of the Zn-Mg-Al-based plating layer is less than 1.2, the amount of MgZn2 anode that forms the sacrificial corrosion protection cell described above that can dissolve is small, which may result in a problem of being disadvantageous in corrosion resistance. On the other hand, if the area ratio exceeds 3.3, the MgZn2 dissolves and accommodates the transferred electrons, which limits the speed of the cathodic reaction (oxygen reduction reaction) that occurs in the Al on the surface, which may result in a problem of being disadvantageous in corrosion resistance.

[0053] The corrosion resistance of bent sections is achieved through two mechanisms. First, the MgZn2 phase and single Al phase present in the bent section form a complete sacrificial corrosion protection cell, and corrosion products cover the areas of the base steel sheet exposed during bending. Second, there is a self-healing mechanism in which oxidant-loving Mg and Al elements leach out in a humid atmosphere and migrate to the exposed areas of the base steel sheet in the bent section, reforming the coating layer. The greater the amount of Mg and Al elements, which are highly reactive with moisture, present in the surface layer, the greater the effect.

[0054] The sacrificial protection cell acting as the first mechanism is characterized by the large potential difference between the MgZn2 phase and the Al phase, which is -1.2 V above the hydrogen reduction potential and -0.7 V above the hydrogen reduction potential, respectively acting as an anode and a cathode, forming a galvanic cell between the adjacent MgZn2 phase and Al single-phase microstructures.

[0055] As a result of extensive research, the present inventors have confirmed that it is possible to ensure a high potential difference between the MgZn2 phase and the single Al phase adjacent to the MgZn2 phase on the surface of the coating layer, and to ensure corrosion resistance in the bent portion due to the formation of a galvanic cell. They have also found that the phase that ensures a high potential difference adjacent to the MgZn2 phase is a single Al phase with a Zn solid solution rate of less than 27 atomic %.

[0056] That is, the Zn-Mg-Al-based coating layer according to one embodiment of the present invention can have two types of Al-based phases: (1) an Al single phase with a Zn solid solubility of less than 27 atomic % and (2) a second Al single phase with a Zn solid solubility of 27 to 60%. Furthermore, it was confirmed that, of these, the phase that can maintain a high potential difference by existing adjacent to the MgZn2 phase is the Al single phase with a low Zn solid solubility (corresponding to (1)).

[0057] In other words, according to one embodiment of the present invention, when a large amount of second Al single phase having a high Zn solid solution rate of 27 atomic % or more is formed in the Zn-Mg-Al-based coating layer, the amount of second Al single phase present around the MgZn2 phase increases, which may reduce the anode-cathode potential difference of the galvanic cell described above and impair the excellent corrosion resistance and sacrificial corrosion protection of the galvanic cell.

[0058] Therefore, according to one embodiment of the present invention, the area ratio of the second Al single phase on the surface of the Zn-Mg-Al-based coating layer may be 2 to 9%. If the area ratio of the second Al single phase exceeds 9%, the second Al single phase may be excessively formed around the MgZn2 phase, reducing the galvanic cell potential difference and potentially deteriorating the corrosion resistance of the bent portion. Therefore, in the present invention, the area ratio of the second Al single phase on the surface of the Zn-Mg-Al-based coating layer is controlled to 9% or less. The smaller the amount of the second Al single phase present on the surface, the better the effect of improving the corrosion resistance of the bent portion. Therefore, the lower limit does not need to be separately set. However, considering that the second Al single phase is necessarily formed in the temperature range where the second Al single phase is formed during the cooling process after hot-dip coating, the lower limit may be set to 2%.

[0059] According to one embodiment of the present invention, the area ratio of the MgZn2 phase on the surface of the Zn-Mg-Al-based coating layer may be 30 to 40%. The surface is the portion of the coating layer that is primarily in contact with the atmosphere and chloride environments, and the higher the ratio of the MgZn2 phase, which acts as an anode in sacrificial corrosion protection, the more improved the reactivity in the galvanic cell. Therefore, to ensure corrosion resistance in the bent portion by promoting the formation of the galvanic cell, the area ratio of the MgZn2 phase on the surface of the coating layer can be set to 30% or more. Therefore, if the area ratio of the MgZn2 phase on the surface of the coating layer is less than 30%, the corrosion resistance of the bent portion may be insufficient. On the other hand, if the ratio of the MgZn2 phase is excessively high, exceeding 40%, the coating layer may be brittle, potentially inducing cracks on the surface.

[0060] Alternatively, according to one embodiment of the present invention, the area ratio of the single Al phase (i.e., a phase in which, in atomic %, Zn is dissolved at less than 27%, with the remainder containing Al and other impurities) on the surface of the Zn-Mg-Al-based plating layer may be 15 to 20%. When the area ratio of the single Al phase on the surface of the plating layer is 15% or more, as described above, the single Al phase acts as a cathode together with MgZn2, which acts as an anode in the galvanic cell, thereby contributing to improving the corrosion resistance of the bent portion. Furthermore, by maintaining the skeleton of the MgZn2 phase, the plating layer can contribute to its role as a physical protective barrier. On the other hand, if the ratio of the single Al phase exceeds 20%, there is a possibility that stability may be deteriorated due to corrosion of Al.

[0061] According to one embodiment of the present invention, the total area ratio of the Zn single phase and the Zn-MgZn2-Al ternary eutectic phase on the surface of the Zn-Mg-Al-based coating layer may be 20 to 30%. The Zn single phase and the Zn-MgZn2-Al ternary eutectic phase present on the surface of the Zn-Mg-Al-based coating layer contribute more to the formation of simonkollite and hydrozinsite than LDHs in the early stage of corrosion. Therefore, by controlling the abundance ratio of the Zn single phase and the Zn-MgZn2-Al ternary eutectic phase on the surface of the Zn-Mg-Al-based coating layer, the formation ratio of LDHs can be increased compared to the formation ratio of simonkollite and hydrozinsite among the corrosion products formed on the surface in the early stage of corrosion, thereby further improving the corrosion resistance of the bent portion. Therefore, the total area ratio of the Zn single phase and the Zn-MgZn2-Al ternary eutectic phase on the surface of the Zn-Mg-Al-based coating layer can be set to 20 to 30%. If the total area ratio of the Zn single phase and the Zn-MgZn2-Al ternary eutectic phase on the surface of the Zn-Mg-Al-based coating layer is less than 20%, the formation of simonkollite or hydzinsite, which are generated secondarily after the formation of LDHs and help improve corrosion resistance, will be insufficient, potentially resulting in problems with corrosion resistance.On the other hand, if the total area ratio of the Zn single phase and the Zn-MgZn2-Al ternary eutectic phase on the surface of the Zn-Mg-Al-based coating layer is more than 30%, the formation of simonkollite and hydzinsite will be induced before the formation of LDHs in the early stages of corrosion, preventing the stable corrosion behavior described above and potentially resulting in poor corrosion resistance.

[0062] On the other hand, according to one embodiment of the present invention, the area ratio of the MgZn two-phase may be 20 to 40% and the area ratio of the Al single phase may be 8 to 26% based on a cross section of the Zn-Mg-Al-based coating layer cut in the thickness direction (i.e., the direction perpendicular to the rolling direction of the steel sheet).

[0063] The properties of coated steel sheets are related to the type and size of the crystalline phases. If the area ratio of the MgZn2 phase is less than 20% or the area ratio of the Al single phase is less than 8%, the corrosion resistance of the coating layer may be weakened. On the other hand, if the ratio of the MgZn2 phase in the coating layer exceeds 40%, the coating layer may become excessively brittle, which may result in the side effect of excessive cracking during processing. The area ratios of the MgZn2 phase and the Al single phase can be measured by observing FE-SEM photographs of a cross-section test piece of the coated steel sheet in the thickness direction, based on the cross-section of the Zn-Mg-Al coating layer.

[0064] Even if the area ratios of the MgZn2 phase and the Al single phase based on the cross section of the Zn-Mg-Al-based coating layer described above are satisfied in the present invention and corrosion resistance at the cross section (cut edge) of the steel sheet is ensured, the area ratios of the MgZn2 phase and the Al single phase ensured from the surface of the coating layer may differ. Therefore, the degree of corrosion resistance of the processed part during bending may be affected depending on the distribution of the area ratios of each phase on the surface of the coating layer.

[0065] Therefore, the present inventors have found that even if the above-mentioned area ratios of the MgZn2 phase and the single Al phase are ensured based on the cross section in the thickness direction of the coating layer, ensuring a specific amount or more of the MgZn2 phase and the single Al phase on the surface of the coating layer is an important means for promoting the uniform formation of LDHs on the surface of the coating layer in the early stage of corrosion and ensuring the corrosion resistance of the processed part. That is, according to one embodiment of the present invention, it has further been found that it is important to maintain an appropriate level for the ratio of the total area ratio of the MgZn2 phase and the single Al phase at the center of the coating layer to the total area ratio of the MgZn2 phase and the single Al phase on the surface of the coating layer.

[0066] Specifically, according to one embodiment of the present invention, the ratio (S1 / C1) of the total area fraction (S1) of the MgZn2 phase and the Al single phase at the surface of the Zn-Mg-Al-based coating layer to the total area fraction (C1) of the MgZn2 phase and the Al single phase at the surface at any point within the region from 1 / 4t to 3 / 4t in the thickness direction of the Zn-Mg-Al-based coating layer may be in the range of 0.8 to 1.2. If the S1 / C1 is less than 0.8, problems may arise in the corrosion resistance of the flat part and the processed part due to a lack of microstructure that forms LDHs in the surface layer of the coating layer at the early stage of corrosion. If the S1 / C1 is more than 1.2, problems may arise in the formability and the corrosion resistance of the processed part due to excessive coarsening of the brittle structure caused by the MgZn2 phase at the surface layer of the coating layer.

[0067] According to one embodiment of the present invention, the area ratio of the second Al single phase may be 2 to 10% at any point on the surface of the Zn-Mg-Al-based coating layer in the thickness direction within the region from 1 / 4t to 3 / 4t. If this value exceeds 10%, it may affect the structure of the surface layer and adversely affect the corrosion resistance of the bent portion. Furthermore, taking into consideration the point passing through the temperature range where the second Al single phase occurs, the lower limit can be controlled to 2%.

[0068] The region from 1 / 4t to 3 / 4t in the thickness direction of the Zn-Mg-Al-based coating layer may refer to a region polished on the surface of the test piece so as to include any point within the region from 1 / 4t to 3 / 4t, with the point where the thickness of the coating layer is greatest being the total thickness t of the coating test piece.

[0069] Furthermore, the inventors conducted further research and found that the ratio of the Zn phase and the Zn-MgZn2-Al ternary eutectic phase at the center to the surface, which penetrate into the interior and promote the formation of simoncolite and hydrozinsite after the uniform formation of LDH on the surface of the coating layer, is also an important factor for further improving corrosion resistance.

[0070] That is, according to one embodiment of the present invention, the ratio (S2 / C2) of the total area ratio (S2) of the Zn phase and the Zn-MgZn2-Al ternary eutectic phase at the surface of the Zn-Mg-Al coating layer to the total area ratio (C2) of the Zn phase and the Zn-MgZn2-Al ternary eutectic phase at the surface of the Zn-Mg-Al coating layer at a point corresponding to any one of the 1 / 4t to 3 / 4t regions in the thickness direction of the Zn-Mg-Al coating layer may be in the range of 0.6 to 1.2. If the S2 / C2 ratio is less than 0.6, the formation of simoncolite or hydrozinsite, which are formed secondarily after the formation of LDHs in the surface layer of the coating layer and help improve corrosion resistance, may be insufficient, resulting in poor corrosion resistance. Furthermore, if the S2 / C2 ratio exceeds 1.2, the relative amount of MgZn2 and Al single phases secured at the surface may be insufficient, leading to insufficient LDH formation at the surface, which may result in poor corrosion resistance as described above.

[0071] Meanwhile, the definitions of each phase and the atomic percentages of the dissolved phases for the aforementioned MgZn2 phase, Al single phase, second Al single phase, Zn single phase and Zn-MgZn2-Al ternary phase, which may be derived from the surface of the Zn-Mg-Al based coating layer satisfying the Mg and Al composition according to the present invention, are shown in Figs.

[0072] Specifically, a test piece is prepared so that the surface of the coating layer of the coated steel sheet can be observed using an SEM device, and then images of the surface of the Zn-Mg-Al-based coating layer taken using an SEM or EDS device are distinguished by color and brightness differences for each microstructure, and each region can be calculated.

[0073] Specifically, a plane of the plated steel sheet shown in Figures 1 to 4 was observed with a field emission scanning electron microscope (FE-SEM) in backscattered electron image (BEI) observation mode at a resolution of 1280 × 960 pixels / 254 DPI and 8-bit attributes, magnified 700 times. From the photographs, the SEM images were classified by structural labeling into an Al single phase in which Zn was dissolved to less than 27 at% and a second Al single phase in which Zn was dissolved to 27 at% or more and 60% or less.

[0074] For reference, the fraction of elements dissolved in each phase can be determined by distinguishing between light and dark in the SEM image using an EDS (Energy Dispersive Spectrometer) commonly known in the art. As an example, as shown in Figure 5, the Al-based phase, excluding the MgZn2 phase, Zn single phase, and Zn-MgZn2-Al ternary phase, which are clearly distinguished by color, light and dark, and shape according to their microstructure, can be divided into the following regions: (1) indicates an Al region observed to have an average atomic percentage of 73% Al, 26% Zn, and the remainder being less than 1%; (2) indicates a single Al phase observed to have an average atomic percentage of 51% Al, 49% Zn, and the remainder being less than 1%; and (3) indicates an Al-Zn binary eutectic phase observed to have an average atomic percentage of 43% Al, 57% Zn, and the remainder being less than 1% (the remainder may be Mg or other unavoidable impurities). In the present invention, the Al single phase refers to the region (1) where Zn is dissolved in an amount of less than 27 atomic %, the second Al single phase refers to the region (2) where Zn is dissolved in an amount of 27 atomic % or more and 60 atomic % or less, and the Al-Zn binary eutectic phase refers to the region (3), and each phase may contain Fe and other components as impurities.

[0075] For this purpose, the image generated under the SEM measurement conditions described above was subjected to automatic image generation software based on the Super-pixel algorithm of RISA (Microstructure Phase Fraction Analysis Software) from the Pohang Research Institute of Industrial Science and Technology (RIST). The Super-pixel algorithm divides the entire image into thousands to tens of thousands of regions (super-pixels), compares super-pixels with similar patterns and features to measure similarity, calculates a histogram of pixel brightness values, and automatically selects super-pixels when the similarity exceeds a predefined critical value. As an example of specifying a predefined critical value, the boundary between the single Al phase and the second single Al phase in the image generated under the SEM measurement conditions described above was defined in advance using EDS based on the 27 atomic % Zn solid solution in the Al phase, enabling the software to generate a histogram of brightness values ​​and distinguish the structures. The technical concept of the above-mentioned RISA (microstructure phase fraction analysis software) can be found in Korean Patent Publication No. 2019-0078331.

[0076] According to one embodiment of the present invention, in atmospheric and chloride environments, LDHs can be formed on the surface of the plating layer before simoncollite and hydrozinsite. The presence of a large amount of MgZn2 phase in the surface layer and an adjacent Al single phase allows rapid nucleation and crystallization of LDHs, which are dense corrosion products, on the surface of the plating layer in the initial corrosive environment. Subsequently, over time, LDHs are uniformly distributed across the entire surface, shielding the corrosion-active region and inducing the uniform formation of secondary corrosion products, simoncollite and hydrozinsite.

[0077] According to one embodiment of the present invention, the LDH corrosion product formed on the surface of the plating layer can be formed within 6 hours in an atmospheric environment and within 5 minutes in a chloride environment (i.e., when measured according to ISO 14993).

[0078] According to one embodiment of the present invention, the excellent corrosion resistance can be demonstrated by the fact that the time it takes for red rust to develop in a chloride environment (i.e., when measured according to ISO 14993), including salt spray and immersion environments, is 40 to 50 times longer in a flat plate portion and 20 to 30 times longer in a 90° bent portion than that of a pure Zn plating of the same thickness. Here, the evaluation of the red rust development time can be carried out by a comparative evaluation using a salt spray tester (SST) in accordance with a test method in accordance with ISO 14993.

[0079] Next, a method for producing a plated steel sheet according to yet another embodiment of the present invention will be described in detail. However, this does not necessarily mean that the plated steel sheet of the present invention should be produced by the following production method.

[0080] According to an embodiment of the present invention, the method may further include a step of preparing a base steel sheet. The type of the base steel sheet is not particularly limited. It may be, but is not limited to, an Fe-based base steel sheet used as a base steel sheet for conventional hot-dip galvanized steel sheets, i.e., a hot-rolled steel sheet or a cold-rolled steel sheet. Furthermore, the base steel sheet may be, but is not limited to, a carbon steel, an ultra-low carbon steel, or a high manganese steel used as a material for construction, home appliances, or automobiles. In this case, the above-mentioned description is equally applicable to the base steel sheet.

[0081] Next, according to one embodiment of the present invention, the method may include a step of immersing the base steel sheet in a coating bath containing, by weight, 4-6% Mg, 8.2-14.2% Al, the balance being Zn and other unavoidable impurities to perform hot-dip galvanizing. Here, the reasons for adding the components in the coating bath and for limiting their contents are the same as those described above for the components of the coating layer, except for the small amount of Fe that may flow in from the base steel sheet.

[0082] To prepare the coating bath having the above-described composition, a composite ingot containing the desired Zn, Al, and Mg or a Zn-Mg or Zn-Al ingot containing individual components can be used. To replenish the coating bath consumed during hot dip coating, the ingot is melted and supplied. In this case, the ingot may be directly deposited in the coating bath and melted, or the ingot may be melted in a separate pot and the molten metal may be added to the coating bath.

[0083] The temperature of the coating bath may be maintained at a temperature 20 to 80°C higher than the solidification start temperature (Ts) on the equilibrium diagram. While not particularly limited, the solidification start temperature on the equilibrium diagram may be in the range of 390 to 460°C, or the temperature of the coating bath may be maintained in the range of 440 to 520°C. A higher coating bath temperature can ensure fluidity within the coating bath, form a more uniform composition, and reduce the amount of floating dross. If the coating bath temperature is 20°C lower than the solidification start temperature on the equilibrium diagram, the ingot dissolution is very slow, the viscosity of the coating bath is high, and it may be difficult to ensure excellent surface quality of the coating layer. On the other hand, if the coating bath temperature exceeds 80°C higher than the solidification start temperature on the equilibrium diagram, there is a risk of ash defects being induced on the coating surface due to Zn evaporation.

[0084] According to one embodiment of the present invention, the hot-dip galvanized steel sheet may be cooled using an inert gas at an average cooling rate of 2 to 12°C / s from the solidification start temperature to the solidification finish temperature on the equilibrium diagram. If the average cooling rate is less than 2°C / s, the MgZn2 structure may develop excessively coarsely on the surface, making the surface of the coating layer brittle and increasing the risk of cracking, which may be detrimental to ensuring uniform corrosion resistance and workability. On the other hand, if the average cooling rate exceeds 12°C / s, rapid solidification occurs during the hot-dip galvanizing process, during which the liquid phase begins to solidify and the liquid phase completely transforms into a solid. Therefore, excessive coarsening and refinement of the MgZn2 and Al single phases may occur on the surface of the coating layer, which may result in the formation of locally inhomogeneous phases on the surface of the coating layer, potentially resulting in reduced corrosion resistance.

[0085] According to an embodiment of the present invention, the cooling step may control the cooling rate so as to satisfy the following relations 1-1 and 1-2.

[0086] [Equation 1-1] A<{(5-2lnt) / (7-3lnt)}*B

[0087] [Equation 1-2] 15t (-0.8) ≦B≦20t (-0.8) (In the above relational expressions 1-1 and 1-2, t is the thickness (mm) of the steel plate, A is the average cooling rate (°C / s) from the solidification start temperature to 375°C, and B is the average cooling rate (°C / s) from 375°C to 340°C.)

[0088] That is, the present invention is characterized in that the cooling process after hot-dip galvanizing is divided into a first temperature section from the solidification start temperature to 375°C and a second temperature section from 375°C to 340°C, and the average cooling rate in each section according to the steel sheet thickness is controlled to satisfy the above-mentioned Relational Expressions 1-1 and 1-2. The first temperature section from the solidification start temperature to 375°C corresponds to the section from the solidification start temperature to the solidification finish temperature of the Al single phase, in which the Al single phase adjacent to the MgZn2 phase in the coating layer formed with the Mg and Al content ranges according to the present invention is cooled to form a binary eutectic, with Zn solidifying at less than 27% by atomic % and the remainder being Al and other impurities. It should be noted that the "Al single phase" is distinguished from the "second Al single phase" described below. Furthermore, the second temperature range from 375°C to 340°C indicates the temperature range for the formation of a second Al single phase, in which Zn is dissolved in a solid solution at a concentration of 27% to 60% (i.e., 27-60%), in atomic percent, in a coating layer formed with the Mg and Al content range according to the present invention. Therefore, if the cooling conditions of the aforementioned Relational Formulas 1-1 and 1-2 are not satisfied and the initial cooling rate is too fast, the area ratio of the MgZn2-Al binary eutectic phase formed on the coating layer surface in the temperature range from the solidification start temperature to 375°C will be too low, potentially resulting in insufficient formation of an Al single phase adjacent to the MgZn2 phase. This may result in the formation of Simonkolite instead of LDH as an initial corrosion product on the coating layer surface, further deteriorating corrosion resistance.

[0089] Meanwhile, according to one embodiment of the present invention, the method may further include a step of performing a pre-qualify rolling (SPM) treatment on the surface of the steel sheet using a bright roll having a surface roughness (Ra) of 0.2 to 0.4 μm, applying a roll pressure of 200 to 300 tons to the surface of the steel sheet, before the hot dip galvanizing.

[0090] In this way, by performing a surface treatment on the base steel sheet before hot-dip galvanizing, it is possible to uniformly control the surface shape of the base steel sheet, thereby controlling the thickness of the hot-dip galvanized layer formed in the subsequent galvanizing process and minimizing the number of solidification nuclei formation sites by smoothing the base steel sheet. That is, by contributing to smoother nucleation in the surface layer of the galvanized layer rather than nucleation in the interior in the thickness direction during cooling, it is possible to promote the formation of a microstructure in the surface layer formed in the first temperature range and contribute to reducing the proportion of the second Al single phase formed in the second temperature range. Meanwhile, during the pre-temper rolling process, if the surface roughness of the roll is less than 0.2 μm, problems with roll production and management may occur, while if it exceeds 0.4 μm, problems may occur in that smoother nucleation in the interior is more likely than nucleation in the surface layer of the galvanized layer. Furthermore, if the roll reduction is less than 200 tons, the effect of controlling the shape of the base steel sheet is low, and it may be difficult to expect the aforementioned effect of promoting solidification nucleation in the surface layer, while if it exceeds 300 tons, there is a risk of inducing C-bending, etc., and it may be difficult to expect the effect of contributing to uniform phase formation in the thickness direction of the coating layer. On the other hand, in order to further improve the aforementioned effect, it is more preferable to set the roll reduction in the pre-temper rolling treatment before the above-mentioned hot-dip galvanizing to 250 to 300 tons.

[0091] After the preliminary temper rolling, the method may include heating the base steel sheet in a heating furnace having a dew point temperature of -60°C to -15°C, where the temperature of the base steel sheet in the final section of the heating furnace is 20°C to 80°C higher than the coating bath temperature (Tb) to ensure wettability of the coating. The dew point temperature of the heating furnace is intended to prevent oxidation of the surface of the base steel sheet, and the temperature of the heating furnace may be -60°C to -15°C to ensure coating adhesion.

[0092] According to one embodiment of the present invention, the cooling may be performed such that the ratio (De / Dc) of the damper opening ratio (De) at the edge to the damper opening ratio (Dc) at the center in the width direction of the hot-dip galvanized steel sheet is 60 to 99%. Here, the "width direction" of the steel sheet refers to a direction perpendicular to the conveyance direction of the steel sheet, based on the surface of the hot-dip galvanized steel sheet excluding the thickness-side surface (i.e., the surface where the thickness of the steel sheet is visible). The damper opening ratio is a value indicating the degree of opening of an adjusting plate that controls the flow rate of cooling gas to be conveyed from a cooling device to the base steel sheet. To ensure uniform cooling performance according to the width of the steel sheet, as described below, dampers are installed so that the total cooling gas input or controlled to the cooling device can be divided into a center portion and an edge portion according to the width direction of the base steel sheet. The boundaries between the dampers may be divided into three sections according to the width of the base steel sheet, with the center portion being the center portion and the two outer portions being the edge portions, and their positions may be variably controlled.

[0093] Conventional cooling of hot-dip galvanized steel sheets involves maintaining a constant flow rate of cooling gas at the edge and center without using a method or device for adjusting the ratio (De / Dc), making it difficult to ensure uniform microstructural characteristics on the surface of the coating layer. In contrast, the present invention achieves uniform cooling performance across the width of the steel sheet by controlling the ratio (De / Dc) to a range of 60-99%, which is the opposite of conventional cooling conditions, and by lowering the damper opening rate at the edge compared to the center. Specifically, the inventors recognized that the edge has a larger area exposed to the external atmosphere across the width of the steel sheet than the center, and therefore the temperature of the steel sheet decreases faster in the region corresponding to the edge than in the center. They then discovered that uniform characteristics on the surface of the coating layer can be achieved by artificially reducing the cooling rate at the edge. In other words, during the cooling process, the cooling gas injected into the center naturally exits from the center via the edge to the outer shell. However, the edge portion receives both the cooling gas injected into the edge portion and the cooling gas injected into the center portion, which may result in overcooling compared to the center portion, causing adverse effects. Therefore, since the cooling rate of the edge portion is faster without the need for artificial cooling gas, in order to achieve uniform cooling performance in the width direction and to form LDH as an initial corrosion product to increase corrosion resistance, it is necessary to control the damper opening rate of the edge portion to be lower than that of the center portion.

[0094] If the ratio (De / Dc) of the damper opening rate (De) at the edge to the damper opening rate (Dc) at the center is less than 60%, the edge will be cooled more slowly than the center, and if it exceeds 99%, the edge will be overcooled compared to the center, which may hinder the realization of uniform cooling capacity in the width direction of the steel sheet. This will result in a non-uniform structure of the coating layer surface at the edge and center, making it impossible to ensure the structural characteristics of a single Al phase and two MgZn phases on the surface of the coating layer, which may deteriorate the corrosion resistance of the flat sheet portion and the bent portion.

[0095] Alternatively, according to yet another embodiment of the present invention, the cooling step may be performed by varying the ratio (De / Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion according to the temperature range.

[0096] Specifically, during the cooling, the ratio (De / Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion can be 60 to 70% from the solidification start temperature to 375°C (corresponding to the above-mentioned "first temperature range"), and 90 to 99% from 375°C to 340°C (corresponding to the above-mentioned "second temperature range").

[0097] By varying the ratio (De / Dc) according to the temperature range to satisfy the above conditions, the steel sheet can be cooled uniformly in the width direction from the solidification start temperature at which the MgZn2-Al binary eutectic phase is formed to 375°C, thereby improving corrosion resistance uniformly across the entire width. Furthermore, by controlling the formation of the second Al single phase, which has a high Zn solid solution rate, as little as possible from 375°C at which the second Al single phase is formed to the solidification end temperature, it is possible to control the formation of the second Al single phase, which has a high Zn solid solution rate, so as not to affect the galvanic cell between the microstructures consisting of the MgZn2-Al binary eutectic phase, thereby improving the corrosion resistance not only of the flat plate portion but also of the bent portion.

[0098] Furthermore, according to one embodiment of the present invention, after the cooling step, a step of improving the surface and shape of the base steel sheet by performing a skin pass mill (SPM) treatment may be further included to improve the surface quality of the final product, thereby ensuring a uniform light scattering effect on the coating layer surface in the width direction of the steel sheet and improving the appearance quality.

[0099] Specifically, in one embodiment, the temper rolling treatment can be carried out using bright rolls with a surface roughness (Ra) of 0.2 to 1.0 μm, applying a roll pressure of 50 to 300 tons to the steel sheet surface.

[0100] If the surface roughness Ra of the bright roll is less than 0.2 μm, the roll roughness is too low, reducing the friction between the base steel sheet and the SPM roll, which can lead to the problem of the base steel sheet slipping, while if it exceeds 1.0 μm, the microstructure of the coating layer surface is not fully preserved, which can lead to the problem of excessive cracking. However, from the perspective of further improving the light scattering effect resulting from the structural characteristics of the coating layer, it is more preferable that the surface roughness Ra of the bright roll be in the range of 0.4 to 0.8 μm.

[0101] Furthermore, if the roll reduction is less than 50 tons, problems may arise in uniforming the shape of the base steel sheet in the width direction, while if it exceeds 300 tons, the excessive rolling force may not completely preserve the microstructure of the coating layer surface, resulting in problems such as excessive cracking even within the above-mentioned range of surface roughness of the bright roll. However, from the viewpoint of further improving the uniform light scattering effect, it is more preferable to apply a high roll reduction of 150 to 300 tons.

[0102] Therefore, by subjecting the cooled steel sheet to a temper pass rolling (SPM) treatment after the above-described hot dip galvanizing and cooling and optimizing the temper pass rolling conditions, a coated steel sheet having a light scattering effect can be provided. This effectively provides a coated steel sheet that not only has excellent corrosion resistance in both the flat portion and the processed portion, but also has excellent surface quality. [Example]

[0103] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0104] (Experimental Example 1) A base steel sheet containing 0.018% C, 0.01% Si, 0.2% Mn, 0.009% P, 0.005% S, 0.1% Al, 0.02% Nb, 0.2% Cr, 0.02% Ti, 0.015% B, with the balance being Fe and other unavoidable impurities, was subjected to preliminary SPM treatment at 100 ton load using a bright roll with a surface roughness (Ra) of 0.2 μm. The base steel sheet was then heated in a furnace with a dew point temperature of −15°C to a temperature 20°C higher than the coating bath temperature (Tb) and then immersed in a coating bath with the composition shown in Table 1 below to obtain a hot-dip galvanized steel sheet. The hot-dip galvanized steel sheet was then cooled from the solidification start temperature to the solidification finish temperature using one or more inert gases selected from N, Ar, and He in part of the cooling zone to satisfy the average cooling rate (Vc) listed in Table 1.

[0105] During the cooling, the average cooling rate for each temperature section was controlled as shown in Table 1 below, and the average damper opening ratios of the edge and center parts in the width direction of the steel sheet based on the surface of the hot-dip galvanized steel sheet were controlled as shown in Table 2 below. After the cooling, the steel sheet was subjected to skin pass milling (SPM) treatment under a roll pressure of 50 to 150 tons using a dull roll with a surface roughness of 2 μm to improve the surface properties and shape.

[0106] [Table 1]

[0107] Ts*: solidification start temperature on the equilibrium phase diagram [℃] Tb*: Plating bath temperature [℃] t*: thickness of steel plate [mm] A*: Average cooling rate from the solidification start temperature to 375°C [°C / s] B*: Average cooling rate from 375°C to 340°C [°C / s] Vc*: Average cooling rate from the solidification start temperature to the solidification end temperature [℃ / s]

[0108] [Table 2]

[0109] De*: Average damper opening rate at the edge [%] Dc*: Average damper opening rate in the center [%]

[0110] Test pieces of the above-described coated steel sheets were prepared, and the coating layer was dissolved in a hydrochloric acid solution. The resulting solution was then analyzed by inductively coupled plasma (ICP) to determine the composition of the coating layer, which is shown in Table 3 below. In addition, cross-sectional test pieces were prepared by cutting the steel sheets perpendicular to the rolling direction so that the interface between the coating layer and the base steel could be observed, and images were taken with an SEM. It was confirmed that the base steel sheet, the Zn-Mg-Al-based coating layer, and an Fe-Al-based inhibitor layer were formed between the base steel sheet and the Zn-Mg-Al-based coating layer.

[0111] In addition, the 24,000 μm test piece of hot-dip galvanized steel sheet 2 Test pieces were prepared so that the surface area could be observed using an SEM device. Next, on the surface of the Zn-Mg-Al-based coating layer, the area fractions of the MgZn2 phase in the MgZn2-Al-based binary eutectic phase and the Al single phase in which Zn was dissolved at less than 27 at% were determined using images taken with the SEM device, and then the total area fraction and area ratio were calculated and are shown in Table 3 below.

[0112] The SEM images were then texture-labeled to distinguish between the Al single phase, in which Zn is dissolved at less than 27 at% in the Al phase present in the MgZn2-Al binary eutectic phase, and the second Al single phase, in which Zn is dissolved at 27 at% to 60%. Specifically, the texture labeling was performed on the SEM images taken with the observation mode (BEI), resolution (1280 x 960 pixels / 254 DPI), magnification (700x), and bit (8) attributes. The SEM images were then classified by color and brightness using automatic image generation software based on the Super-pixel algorithm of RISA (microstructure phase fraction analysis software) from the Pohang Research Institute of Industrial Science and Technology (RIST). The area percentages were then quantified.

[0113] [Table 3]

[0114] The properties of each of the Examples and Comparative Examples were evaluated according to the following criteria, and the evaluation results are shown in Table 4 below.

[0115] <Renowned corrosion resistance> To evaluate the corrosion resistance of the flat plates, a salt spray tester (SST) was used in accordance with the test method of ISO14993, and evaluation was performed according to the following criteria.

[0116] ◎: The time it takes for red rust to appear is over 40 times longer than that of Zn plating of the same thickness ○: The time it takes for red rust to appear is 30 to 40 times longer than that of Zn plating of the same thickness. △: The time it takes for red rust to appear is 20 to 30 times longer than that of Zn plating of the same thickness. X: The time it takes for red rust to appear is less than 20 times longer than that of Zn plating of the same thickness.

[0117] <Corrosion resistance of bent parts> To evaluate the corrosion resistance of the bent portion, a salt spray tester (SST) was used and a test method based on ISO 14993 was used. The test pieces for the corrosion resistance evaluation were made of the same material thickness and the same plating amount and were bent 90 degrees.

[0118] ◎: The time it takes for red rust to appear is more than 30 times longer than that of Zn plating of the same thickness ○: The time it takes for red rust to form is 20 to 30 times longer than that of Zn plating of the same thickness. △: The time it takes for red rust to appear is 10 to 20 times longer than that of Zn plating of the same thickness. X: The time it takes for red rust to appear is less than 10 times longer than that of Zn plating of the same thickness.

[0119] <Scattered reflectance> In order to evaluate the amount of scattered reflected light relative to the total reflection of test specimens taken at the 1 / 4 point, center, 3 / 4 point, and edge positions in the width direction of hot-dip galvanized steel sheets, light in the visible wavelength range (400-800 nm) was incident on an integrating sphere and evaluated according to the type of reflected light using a test method in accordance with ISO 9001.

[0120] ◎: The ratio of diffuse reflectance to the average total reflectance in the width direction is over 80%, and the deviation of diffuse reflectance in the width direction is less than 10% ○: The ratio of diffuse reflectance to the average total reflectance in the width direction is 70% or more but less than 80%, and the deviation of diffuse reflectance in the width direction is 10% or more △: The ratio of scattered reflectance to the average total reflectance in the width direction is 60% or more but less than 70%, and the deviation of scattered reflectance in the width direction is 10% or more X: The ratio of diffuse reflectance to the average total reflectance in the width direction is less than 60%, and the deviation of diffuse reflectance in the width direction is 10% or more

[0121] Furthermore, the steel plates obtained from each of the Examples and Comparative Examples were evaluated for the type of corrosion product that was initially formed on the surface using an EDS or XRD device, and the results are shown in Table 4 below.

[0122] [Table 4]

[0123] As shown in Table 1 above, in Examples 1 to 6, which all satisfied the coating composition and manufacturing conditions of the present invention, it was confirmed that LDHs were first formed on the surface of the coated steel sheet during corrosion resistance evaluation experiments. This not only improved corrosion resistance not only in flat sheets but also in bent sections, but also confirmed that the steel sheet surface had a slightly higher degree of scattered reflectance and excellent surface quality.

[0124] On the other hand, in Comparative Examples 1 to 6, which satisfied the coating composition of the present invention but did not satisfy one or more of the cooling conditions of Relational Expressions 1-1 and 1-2 described above, it was confirmed that simonkolite was initially formed on the surface of the coated steel sheet during corrosion resistance evaluation experiments. As a result, not only the flat corrosion resistance of the coated steel sheet but also the corrosion resistance of the bent portion was somewhat inferior. Furthermore, the degree of scattered reflectance was somewhat low, confirming inferior surface quality.

[0125] Furthermore, it was confirmed that Comparative Examples 7 to 10, which do not satisfy the coating composition of the present invention, exhibited inferior corrosion resistance in the flat portion, corrosion resistance in the bent portion, and surface quality. Specifically, in Comparative Example 7, which had an insufficient Mg content, the MgZn2 phase was not sufficiently formed on the surface of the coating layer. As a result, the total area ratio of the Al single phase and the MgZn2 phase and the area ratio of the MgZn2 phase to the Al single phase of the present invention were not satisfied. Therefore, during corrosion resistance evaluation experiments, simonkolite was initially formed on the surface of the coated steel sheet, resulting in inferior corrosion resistance in the flat portion and the bent portion, as well as inferior scattered reflectance.

[0126] In Comparative Example 8, which contained an excessive amount of Mg, the corrosion resistance of the flat plate portion was ensured by adding a large amount of Mg, but the excessive Mg content caused the MgZn2 phase to form excessively coarse on the surface of the coating layer, which resulted in excessive cracking during bending. Furthermore, the LDH was unable to cover the entire surface of the bent portion, resulting in poor corrosion resistance of the bent portion.

[0127] Furthermore, in the case of Comparative Example 9, in which the Al content was insufficient, the Al single phase was formed in a small amount due to the insufficient amount of Al added, and LDH was not formed as an initial corrosion product on the surface of the plated steel sheet during the corrosion resistance evaluation experiment. As a result, not only was the corrosion resistance of the flat plate portion and the bent portion poor, but the degree of scattered reflectance was also poor.

[0128] In addition, in the case of Comparative Example 10, which contained an excessive amount of Al, both the Al single phase and the MgZn2 phase were formed in excess, and the total area ratio of the Al single phase and the MgZn2 phase exceeded the range of the present invention. Therefore, in Comparative Example 10, although the corrosion resistance of the flat plate portion was ensured by adding an appropriate amount of Mg, the excessively formed MgZn2 phase was excessively brittle, which resulted in excessive cracking in the coating layer during processing, resulting in poor corrosion resistance in the bent portion.

[0129] Furthermore, in the case of Comparative Example 11, which does not satisfy the cooling condition of Relational Formula 1-2 of the present invention, even though the coating composition and other manufacturing conditions of the present invention were all met, both the Al single phase and the MgZn2 phase were formed in excess, and the total area ratio of the Al single phase and the MgZn2 phase exceeded the range of the present invention. Therefore, even though the corrosion resistance of the flat plate portion was ensured by adding appropriate amounts of Mg and Al, the excessively formed MgZn2 phase was excessively brittle, which resulted in excessive cracking in the coating layer during processing, resulting in poor corrosion resistance in the bent portion.

[0130] (Experimental Example 2) Coated steel sheets were manufactured in the same manner as in Experimental Example 1, except that the damper opening rate ratios were changed as follows according to the temperature ranges divided based on the surface temperature of the steel sheet. Using the same analytical methods as in Experimental Example 1, it was confirmed that the base steel sheet, Fe-Al-based inhibition layer, and Zn-Al-Mg-based coating layer were formed in this order.

[0131] [Table 5]

[0132] First temperature section*: From the solidification start temperature to 375°C Second temperature section*: section from 375℃ to 340℃

[0133] For the plated steel sheets obtained in the above-mentioned respective Examples and Comparative Examples, cross-sectional test pieces were prepared by cutting them in the thickness direction (direction perpendicular to the rolling direction of the steel sheet) so as to observe the interface between the plated layer and the base steel, using the same method as in the above-mentioned Experimental Example 1, and then photographed with an SEM at a magnification of 1000. For the above-mentioned cross-sectional test pieces, the area ratios of the MgZn2 phase and the Al phase were measured using the same method as in the above-mentioned Experimental Example 1, and the results are shown in Table 6 below.

[0134] Furthermore, in the same manner as in Experimental Example 1, 2 Surface test pieces of various sizes were collected, and the area ratios of the MgZn2 phase and the Al phase in which Zn was dissolved at less than 27 at% in the MgZn2-Al binary eutectic phase were measured, and the results are shown in Table 6. In addition, the area ratios of the Zn phase and the Zn-MgZn2-Al ternary eutectic phase were measured for the surface test pieces described above.

[0135] [Table 6]

[0136] Furthermore, the steel sheets obtained from each of the Examples and Comparative Examples were comparatively evaluated using a salt spray tester (SST) in accordance with a test method in accordance with ISO 14993. During the evaluation, the time it took for LDH corrosion products to form on the coating layer surface of the plated steel sheets was measured over time using an EDS or XRD device, and the results are shown in Table 7 below. The property evaluations shown in Table 7 below were also performed using the same criteria as in Experimental Example 1 described above.

[0137] [Table 7]

[0138] In the case of Comparative Example 12, which does not satisfy the coating composition of the present invention and the condition that "the ratio (De / Dc) of the damper opening rate (De) at the edge to the damper opening rate (Dc) at the center is 60-70% from the solidification start temperature to 375°C, and 90-99% from 375°C to 340°C," during a corrosion resistance evaluation experiment, Simonkolleite was formed first on the surface of the coated steel sheet, and LDH was formed on the surface 12 hours later. This confirmed that Comparative Example 12 was inferior in all of the corrosion resistance of the flat plate, the corrosion resistance of the bent portion, and the scattered reflectance.

[0139] Meanwhile, in the case of Examples 7 to 11, which satisfied the coating composition and manufacturing conditions of the present invention, layered double hydroxide (LDH) was formed on the surface of the plated steel sheet within 10 minutes during a corrosion resistance evaluation experiment, and it was confirmed that the plated steel sheet had superior corrosion resistance of the flat sheet, corrosion resistance of the bent portion, and diffuse reflectance compared to Comparative Example 12.

[0140] In particular, in the case of Examples 8 to 11, which also satisfied the condition of the present invention that "the ratio (De / Dc) of the damper opening rate (De) at the edge to the damper opening rate (Dc) at the center is 60 to 70% from the solidification start temperature to 375°C, and 90 to 99% from 375°C to the solidification end temperature," it was confirmed that layered double hydroxides (LDHs) were formed on the surface of the plated steel sheet within 5 minutes during corrosion resistance evaluation experiments. This confirmed that Examples 8 to 11 of the present application exhibited improved flat sheet corrosion resistance and corrosion resistance in bent portions compared to Examples 7 and 10. This is presumably due to the presence of a large amount of MgZn2 phase in the surface layer and a low Al single phase, with a Zn solid solution rate of less than 27% adjacent to the MgZn2 phase. That is, the rapid nucleation and crystallization of LDH, a dense corrosion product, on the surface in the early stages of a corrosive environment leads to uniform distribution over the entire surface over time, shielding the corrosion-active area and inducing the uniform formation of secondarily formed simoncolite and hydrozinsite.

[0141] (Experimental Example 3) Plated steel sheets were produced under the same conditions as those of Experimental Examples 1 and 2 described above, except that the hot-dip galvanized and cooled steel sheets were subjected to pre-temper rolling treatment, cooling, and post-cooling temper rolling (SPM) treatment under the conditions shown in Tables 8 and 9 below.

[0142] [Table 8]

[0143] [Table 9]

[0144] First temperature section*: From the solidification start temperature to 375°C Second temperature section*: section from 375℃ to 340℃

[0145] For the plated steel sheets obtained in each of the Examples and Comparative Examples, test pieces were produced in the same manner as in Experimental Example 1 described above, and then the area fractions of the MgZn2 phase and the single Al phase containing less than 27 at% Zn as a solid solution were measured on the surface of the plated layer, and the area fractions of the single Zn phase and the Zn-MgZn2-Al ternary eutectic phase were also measured. The results are shown in Tables 10 and 11 below.

[0146] In addition, the plated steel sheets obtained in each of the Examples and Comparative Examples were subjected to surface polishing according to the same standard, and the thickness of the plated layer was measured at any point in the region from 1 / 4t to 3 / 4t by 24,000 μm. 2 A test specimen was prepared that allowed observation of the surface area. The surface polishing was performed on a cold-mounted test specimen with the surface facing up so that the surface could be observed along the depth direction. The surface polishing was performed using an automatic polishing machine and silica suspension under conditions of a load of 30 N, 105 RPM, and forward rotation at a speed of approximately 2 μm / min.

[0147] The total area ratio of the Al single phase and the MgZn2 phase, and the total area ratio of the Zn single phase and the Zn-MgZn2-Al ternary eutectic phase were measured at any one point on the surface of the coating layer obtained in this way in the region from 1 / 4t to 3 / 4t in the thickness direction for the examples and comparative examples. The results are shown in Table 10 below.

[0148] [Table 10]

[0149] [Table 11]

[0150] S1*: Total area ratio of the Al single phase and the MZn2 phase on the surface of the Zn-Mg-Al coating layer [%] S2*: Total area ratio of Zn phase and Zn-MgZn2-Al ternary eutectic phase on the surface of the Zn-Mg-Al coating layer [%] C1*: Total area ratio of MgZn2 phase and Al phase on the surface in the region from 1 / 4th to 3 / 4th of the thickness of the coating layer [%] C2*: Total area ratio [%] of the Zn phase and Zn-MgZn2-Al ternary phase on the surface in the region from 1 / 4 to 3 / 4 of the thickness of the coating layer

[0151] The steel sheets obtained from each of the examples and comparative examples were evaluated for the properties shown in Table 12 below according to the same criteria as in Experimental Example 1 described above.

[0152] [Table 12]

[0153] As shown in Table 12 above, the coating composition of the present invention had an insufficient Mg content, did not satisfy the De / Dc condition in the first temperature range, and in Comparative Example 13, in which a dull roll was used, an Al single phase and an MgZn2 phase were excessively formed on the surface of the coating layer, resulting in poor corrosion resistance in the bent portion as well as poor light scattering in the surface.

[0154] Furthermore, in the case of Comparative Example 14, which had an insufficient Al content among the coating compositions of the present invention and did not satisfy the De / Dc condition in the first temperature range, not only were the corrosion resistance of the flat plate and the corrosion resistance of the bent part poor due to the unformed initial LDH, but the degree of scattered reflectance was also low, resulting in poor appearance quality.

[0155] Furthermore, in the case of Comparative Example 15, which satisfied the coating composition and other manufacturing conditions of the present invention but did not satisfy the cooling condition of Relational Formula 1-2, flat plate corrosion resistance could be ensured, but the coating layer was excessively brittle due to the excessively formed MgZn2 phase, which had the side effect of excessively cracking during processing, resulting in poor corrosion resistance in the bent portion.

[0156] On the other hand, in the case of Examples 12 to 16, which satisfied the coating composition and manufacturing conditions of the present invention, it was confirmed that LDHs were formed on the surface of the plated steel sheet within 5 minutes during a corrosion resistance evaluation experiment. This not only improved corrosion resistance not only in the flat sheet portion but also in the bent portion, but also confirmed that the degree of scattered reflectance on the steel sheet surface was somewhat high, resulting in excellent surface quality.

[0157] In particular, in Examples 13 to 16, which satisfy the SPM treatment conditions of applying a roll pressure of 50 to 300 tons to the steel sheet surface using a bright roll with a surface roughness (Ra) of 0.2 to 1.0 μm, the conditions of S1 / C1 and S2 / C2 can be satisfied, and it was confirmed that this not only provides the best corrosion resistance in the flat plate portion and the bent portion, but also the best degree of scattered reflectance.

Claims

1. Base steel sheet; a Zn—Mg—Al-based plating layer provided on at least one surface of the base steel sheet and containing, by weight, 4 to 6% Mg, 8.2 to 14.2% Al, the balance being Zn and other inevitable impurities; and an Fe—Al-based suppression layer provided between the base steel sheet and the Zn—Mg—Al-based coating layer; On the surface of the Zn-Mg-Al based plating layer, there is a single phase of Al and MgZn. ２ The total area ratio of the MgZn phase to the Al single phase is 45 to 60%. ２ The area ratio of the phase is 1.2 to 3.3, The Al single phase contains, in atomic %, less than 27% Zn as a solid solution, and the remainder is a phase containing Al and other impurities.

2. Based on the cross section of the Zn-Mg-Al-based plating layer, MgZn ２ 2. The plated steel sheet according to claim 1, wherein the area ratio of the Al single phase is 20 to 40%, and the area ratio of the Al single phase is 8 to 26%.

3. On the surface of the Zn-Mg-Al-based plating layer, the MgZn ２ The plated steel sheet according to claim 1, wherein the area ratio of the phase is 30 to 40%.

4. 2. The plated steel sheet according to claim 1, wherein an area ratio of the Al single phase on the surface of the Zn—Mg—Al-based plating layer is 15 to 20%.

5. MgZn on the surface of the Zn-Mg-Al-based plating layer at a point corresponding to any one of the regions from 1 / 4t to 3 / 4t in the thickness direction. ２ The ratio of the total area ratio (C1) of the MgZn phase and the Al single phase to the total area ratio (C2) of the MgZn phase and the Al single phase on the surface of the Zn-Mg-Al based plating layer ２ The plated steel sheet according to claim 1, wherein the ratio (S1 / C1) of the total area ratio (S1) of the Al phase to the Al single phase is in the range of 0.8 to 1.

2.

6. The Zn phase and Zn—MgZn on the surface of the Zn—Mg—Al-based plating layer ２ The plated steel sheet according to claim 1, wherein the total area ratio of the -Al-based ternary eutectic phase is 20 to 30%.

7. The Zn phase and the Zn-MgZn phase on the surface at any point in the region from 1 / 4t to 3 / 4t in the thickness direction of the Zn-Mg-Al based plating layer ２ The ratio of the Zn phase and the Zn-MgZn phase on the surface of the Zn-Mg-Al-based plating layer to the total area ratio (C2) of the Zn-Mg-Al-based ternary eutectic phase ２ The plated steel sheet according to claim 1, wherein the ratio (S2 / C2) of the total area ratio (S2) of the -Al-based ternary eutectic phase is in the range of 0.6 to 1.

2.

8. 2. The plated steel sheet according to claim 1, wherein an area ratio of a second Al single phase in which Zn is solid-dissolved at 27 to 60 atomic % in the surface of the Zn-Mg-Al-based plating layer is 2 to 9 atomic %.

9. In an air environment and a chloride environment according to ISO 14993, LDH ((Zn, Mg) ６ Al ２ (OH) １６ (CO ３ ) 4H ２ O) is simoncollite (Zn ５ (OH) ８ Cl ２ ) and hydrozinsite (Zn ５ (OH) ６ (CO ３ ) ２ The plated steel sheet according to claim 1, wherein the plated steel sheet is formed prior to the first and second metal layers.

10. In an air environment and a chloride environment according to ISO 14993, LDH ((Zn, Mg) ６ Al ２ (OH) １６ (CO ３ ) 4H ２ 2. The plated steel sheet according to claim 1, wherein the oxidized steel sheet is formed within 6 hours in an air environment and within 5 minutes in a chloride environment.

11. 2. The plated steel sheet according to claim 1, wherein the time required for red rust to develop in a chloride environment of ISO 14993, including a salt spray and immersion environment, is 40 to 50 times longer in a flat portion and 20 to 30 times longer in a 90-degree bent portion than that of a Zn-plated steel sheet of the same thickness.

12. A step of immersing the base steel sheet in a coating bath containing, by weight, 4 to 6% Mg, 8.2 to 14.2% Al, the balance Zn and other unavoidable impurities, and maintained at a temperature 20 to 80°C higher than the solidification start temperature on the equilibrium diagram to perform hot-dip galvanizing; and cooling the hot-dip galvanized steel sheet from a solidification start temperature to a solidification finish temperature using an inert gas at an average cooling rate of 2 to 12°C / s; The cooling step is performed so that the following relational expressions 1-1 and 1-2 are satisfied, and a ratio (De / Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion satisfies 60 to 99%. [Relationship 1-1] A<{(5-2lnt) / (7-3lnt)}×B [Relationship 1-2] 15t （－０．８） ≦B≦20t （－０．８） (In the relational expressions 1-1 and 1-2, t represents the thickness (mm) of the steel plate, A represents the average cooling rate (°C / s) from the solidification start temperature to 375°C, and B represents the average cooling rate (°C / s) from 375°C to 340°C.)

13. The cooling step is performed by changing a ratio (De / Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion according to a temperature range, 13. The method for producing a plated steel sheet according to claim 12, wherein a ratio (De / Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion is 60 to 70% from the solidification start temperature to 375°C, and is 90 to 99% from 375°C to 340°C.

14. The method further includes a step of improving the surface and shape of the base steel sheet by performing a temper rolling treatment after the cooling step, The method for producing a plated steel sheet according to claim 13, wherein the temper rolling is carried out using bright rolls having a surface roughness (Ra) of 0.2 to 1.0 μm, so as to apply a roll pressure of 50 to 300 tons to the steel sheet surface.

15. The method for producing a plated steel sheet according to claim 12, further comprising, before the hot-dip galvanizing, performing a pre-temper rolling treatment in which a roll reduction of 200 to 300 tons is applied to the surface of the steel sheet using a bright roll having a surface roughness (Ra) of 0.2 to 0.4 μm.

16. The method for producing a plated steel sheet according to claim 15, wherein the roll reduction during the preliminary temper rolling treatment is 250 to 300 tons.

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