Plated steel sheet with excellent sealer adhesion and manufacturing method thereof

Abstract

The present invention relates to a plated steel sheet that can be used for automobiles, home appliances, building materials, etc., and a manufacturing method thereof, and more particularly to a zinc alloy plated steel sheet having excellent adhesion to adhesives and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a plated steel sheet having excellent sealer adhesion and a method for producing the same. [Background technology]

[0002] Galvanizing, which inhibits the corrosion of iron through cathodic protection, is excellent in corrosion prevention performance and economy, and is widely used to manufacture steel products with high corrosion resistance. In particular, hot-dip galvanized steel products, in which a coating layer is formed by immersing steel products in molten zinc, have a simpler manufacturing process and lower product prices than electrogalvanized steel products, and demand for hot-dip galvanized steel products is increasing across industries such as automobiles, home appliances, and building materials.

[0003] When hot-dip galvanized steel is exposed to a corrosive environment, zinc, which has a lower redox potential than iron, corrodes first, providing sacrificial corrosion protection. In addition, 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.

[0004] However, with the advancement of industry, air pollution and corrosive environments are on the rise, and strict regulations on resources and energy conservation are creating a growing need to develop steel materials with better corrosion resistance than conventional galvanized steel.

[0005] As part of this effort, various studies have been conducted on the manufacturing technology of zinc alloy-plated steel, which improves the corrosion resistance of steel by adding elements such as aluminum (Al) and magnesium (Mg) to the zinc plating bath. As a representative zinc alloy-plated material, active research is being conducted on the manufacturing technology of Zn-Al-Mg-plated steel sheet, which further adds Mg to the Zn-Al plating composition.

[0006] However, such Zn-Al-Mg plated steel sheets have the following drawbacks.

[0007] First, Zn-Al-Mg-plated steel sheets may have poorer adhesive (sealer) adhesion than standard galvanized steel sheets due to the formation of MgO-based oxides on the surface. Recently, automotive steel bonding is often achieved by combining spot welding with adhesive application. Therefore, ensuring adhesion between the adhesive and the steel sheet is becoming increasingly important. If adhesion between the adhesive and the steel sheet is not ensured, peeling occurs at the interface between the steel sheet and the adhesive (adhesive failure). If adhesion between the steel sheet and the adhesive is sufficient, fracture occurs within the adhesive (cohesive failure). There are two main approaches to improving adhesion: developing an adhesive suitable for highly corrosion-resistant plated steel sheets or modifying the surface structure of the highly corrosion-resistant plating layer.

[0008] The first method requires the application of a new adhesive to existing automotive joining processes, which requires additional R&D and process steps, increasing processing costs and potentially requiring a long development time. Therefore, there is a need for a technology that can ensure adhesion while using the adhesive currently used in hot-dip galvanizing. Various methods have been developed. One method involves adding a post-treatment process of resin coating after plating to prevent direct reaction between the MgO on the surface of the highly corrosion-resistant steel sheet and the adhesive, thereby modifying the interface characteristics and ensuring adhesion with the adhesive. This method has the disadvantages of increasing production costs due to the additional post-treatment process and requiring additional process costs and the hassle of removing the post-treatment material in the subsequent automobile manufacturing process. Another method involves weakly pickling the surface with hydrochloric acid after plating. This method involves removing the MgO oxide from the surface and then forming a metal plating layer on the surface, but it also has the disadvantage of requiring additional equipment and costs.

[0009] In addition to these chemical interface modification methods, there are also physical methods for modifying the surface of highly corrosion-resistant steel sheets. The first method involves using a skin-pass mill, which is heavily chrome-coated, to destroy the MgO-based oxides that form on the surface, allowing the adhesive to react with the metallic plating layer underneath the MgO oxide. However, this method carries the risk of introducing other surface defects, such as dents, due to excessive pressing with the skin-pass mill. Another method involves removing the MgO oxide on the surface after plating by mechanical brushing. However, this method has the disadvantages of potentially inducing scratches on the surface, making it difficult to remove the residue that forms after polishing, and requiring additional capital investment due to the need for additional abrasive brushes and rinsing equipment.

[0010] Therefore, it is extremely necessary to ensure excellent surface quality with excellent adhesion without any special process changes, but no technology has yet been developed that can meet such high demands. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Korean Patent Publication No. 2013-0133358 Summary of the Invention [Problem to be solved by the invention]

[0012] According to one aspect of the present invention, it is possible to provide a plated steel sheet having excellent adhesion to an adhesive and a method for producing the same.

[0013] The object of the present invention is not limited to the above content. Anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the entire content of the specification of the present invention. [Means for solving the problem]

[0014] One aspect of the present invention is With bare iron, a Zn-Mg-Al-based coating layer provided on at least one surface of the base steel; an Fe-Al-based suppression layer provided between the base steel and the Zn-Mg-Al-based coating layer; Including, The plating layer contains, by weight percent, 1.0 to 2.0% Mg, 1.0 to 3.0% Al, the remainder being Zn and other unavoidable impurities, The ratio of the Zn single phase on the surface of the plating layer is 50% or more in terms of area fraction, The plated steel sheet has an average diameter of the Zn single phase on the surface of the plated layer of 3 to 20 μm.

[0015] Furthermore, still another aspect of the present invention is After preparing the base steel, annealing the base steel at a temperature of 800°C or higher; a step of primarily cooling the annealed steel from 800°C to 600°C at an average cooling rate of 2 to 5°C / s; Secondary cooling of the primarily cooled base steel from 600°C to the coating bath temperature at an average cooling rate of 13 to 25°C / s; A step of immersing the second-cooled base steel in a coating bath containing, by weight, 1.0 to 2.0% Mg, 1.0 to 3.0% Al, the balance Zn and other unavoidable impurities to perform hot-dip galvanizing; After the hot dip galvanizing, cooling is started from the surface of the coating bath and cooled at an average cooling rate of 6 to 20 ° C. / s up to the top roll section; The present invention provides a method for producing a plated steel sheet, comprising: [Effects of the Invention]

[0016] According to one aspect of the present invention, it is possible to provide a plated steel sheet having excellent adhesion to an adhesive and a method for producing the same.

[0017] The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a comparison of heating and cooling temperature curves of steel plates of Example 1 and Comparative Example 1. [Figure 2] 1 is a 700x SEM image of the surface of the plated steel sheet according to Example 1. [Figure 3] 1 is a 700x SEM image of the surface of the plated steel sheet according to Comparative Example 1. [Figure 4] 1 is a 1,000x SEM image of a cross section of the plated steel sheet according to Example 1 in the thickness direction. [Figure 5] 1 is a 1,000x SEM image of a cross section of a plated steel sheet according to Comparative Example 1 in the thickness direction. DETAILED DESCRIPTION OF THE INVENTION

[0019] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the related definition clearly dictates otherwise.

[0020] The meaning of "comprises" as used herein is to specify features and does not exclude the presence or addition of other features.

[0021] 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 presently disclosed content.

[0022] 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.

[0023] Conventional Zn-Mg-Al-plated steel sheets have superior corrosion resistance compared to Zn-plated steel sheets, but have had the problem of poor adhesion to sealants and other coatings formed on the surface of the plating layer due to the formation of oxides of Mg and Al on the surface of the Zn-plated steel sheets.

[0024] Therefore, the present inventors have conducted extensive research to improve the adhesion between the plating layer and an adhesive such as a sealer while also improving corrosion resistance, and have found that the microstructure on the surface of the plating layer is a very important factor, leading to the completion of the present invention.

[0025] A plated steel sheet according to one aspect of the present invention includes a base steel and a Zn-Mg-Al-based coating layer provided on at least one surface of the base steel.

[0026] In the present invention, the type of the base iron is not particularly limited, and for example, the base iron may be an Fe-based base iron (i.e., a hot-rolled steel sheet or a cold-rolled steel sheet) that is used as a base iron for ordinary zinc-based or zinc alloy-based plated steel sheets.

[0027] Alternatively, the base steel may be, without limitation, carbon steel, extra-low carbon steel, or high manganese steel used for construction, home appliances, automobiles, or wire rods. Non-limiting examples include base steel having a composition, in weight percent, of C: more than 0.17% but not more than 0.17%, Si: more than 0% but not more than 1.5%, Mn: 0.01 to 2.7%, P: more than 0.07% but not more than 0.07%, S: more than 0% but not more than 0.015%, Al: more than 0.5% but not more than 0.06%, Nb: more than 0% but not more than 0.06%, Cr: 1.1% or less (including 0%), Ti: more than 0% but not more than 0.06%, B: more than 0% but not more than 0.03%, and the balance being Fe and other unavoidable impurities.

[0028] In the coated steel sheet, the Zn-Mg-Al-based coating layer may be formed on only one surface of the base steel, or may be formed on both surfaces of the base steel. In this case, the Zn-Mg-Al-based coating layer refers to a coating layer containing Mg and Al made of a Zn-Mg-Al-based alloy, with an excess amount of Zn.

[0029] Specifically, the Zn-Mg-Al-based coating layer contains, by weight, 1.0-2.0% Mg, 1.0-3.0% Al, and the balance being Zn and other inevitable impurities. The reasons for adding each component and limiting the content of each component in the Zn-Mg-Al-based coating layer are explained below.

[0030] "Mg: 1.0~2.0%" Mg in a zinc alloy plating layer is an element that improves the corrosion resistance of plated steel. If the Mg content is too low, the effect of improving corrosion resistance is minimal. Therefore, the lower limit of the Mg content in a zinc alloy plating layer is preferably 1.0 wt.%, more preferably 1.2 wt.%. However, if the Mg content is excessive, oxidation of Mg in the plating bath may induce the formation of a large coarse MgZn2 phase in the surface layer, resulting in the formation of a large Zn / MgZn2 binary phase distribution, which may result in the formation of a large amount of MgO-based oxides in the surface layer of the plating layer. Therefore, the upper limit of the Mg content in a zinc alloy plating layer is preferably 2.0 wt.%.

[0031] "Al: 1.0~3.0%" Al in the zinc alloy plating layer is an element that suppresses Mg oxidation. If the Al content is too low compared to the Mg content, the effect of preventing Mg oxidation in the plating bath is minimal. Therefore, the lower limit of the Al content in the zinc alloy plating layer is preferably 1.0 wt%. However, an excessive Al content poses the problem of requiring a high dissolution temperature for the plating bath. A high plating bath temperature can lead to corrosion of the plating tank and internal equipment, excessive ash generation, and other problems. Therefore, the upper limit of the Al content in the zinc alloy plating layer is preferably 3.0 wt%, which can induce the formation of a fine ternary eutectic structure of Zn / MgZn2 / Al in the surface layer of the plating layer. On the other hand, to further enhance the above-mentioned effects, the lower limit of the Al content may be 1.5%, or the upper limit of the Al content may be 2.5%.

[0032] "Mg / Al content ratio: 0.6~0.9" Furthermore, in order to form a large amount of ternary eutectic structure in the coating layer, the Al content must be higher than the Mg content. Only when the Mg / Al content ratio is adjusted to 0.6 or more and 0.9 or less can a beautiful coated product with a high distribution of ternary phases be obtained. On the other hand, from the viewpoint of further improving surface quality, the lower limit of the Mg / Al content ratio may be 0.67, or the upper limit of the Mg / Al content ratio may be 0.88.

[0033] "Remainder: Zn and other unavoidable impurities" In addition to the above-described 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 the normal manufacturing process of hot-dip zinc- or zinc-alloy-coated steel sheets, and those skilled in the art would easily understand the meaning of the term, so the present invention does not particularly limit the scope of the term. In this regard, the Zn-Mg-Al-based coating layer may contain a small amount of iron (Fe) that diffuses from the base steel. However, in the present invention, this content corresponds to an extremely small impurity level, so it is not separately defined.

[0034] On the other hand, the plated steel sheet has an Fe-Al-based inhibition layer (so-called "inhibition layer") formed between the base steel and the Zn-Mg-Al-based plating layer. The Fe-Al-based inhibition layer is a layer containing an intermetallic compound of Fe and Al (or a layer made of an intermetallic compound of Fe and Al), and examples of the intermetallic compound of Fe and Al include FeAl, FeAl3, and Fe2Al5.

[0035] The Fe-Al-based inhibiting layer may contain, by weight, 30 to 50% Fe and 50 to 70% Al. The explanations commonly used in the art are applicable to the Fe-Al-based inhibiting layer. In other words, the Fe-Al-based inhibiting layer may further contain, in addition to Fe and Al, a portion of components derived from the coating layer, such as Zn, Mg, or Si (e.g., 40% or less). Such an Fe-Al-based inhibiting layer is formed by alloying Fe diffused from the base steel during the initial coating process with the coating bath components. The Fe-Al-based inhibiting layer not only serves to improve adhesion between the base steel and the Zn-Mg-Al-based coating layer, but also serves as an inhibiting layer to prevent Fe diffusion from the base steel to the Zn-Mg-Al-based coating layer.

[0036] According to one aspect of the present invention, the thickness of the Fe-Al-based inhibiting layer may be 20 to 100 nm. Since the Fe-Al-based inhibiting layer serves to prevent alloying and ensure corrosion resistance, the lower limit of the thickness may be 20 nm. However, since the inhibiting layer is a brittle layer, it may adversely affect workability, so the upper limit of the thickness of the Fe-Al-based inhibiting layer may be 100 nm. In this case, the thickness of the Fe-Al-based inhibiting layer may refer to the minimum thickness from the interface with the base steel sheet in the thickness direction (direction perpendicular to the rolling direction).

[0037] In the present invention, the microstructure of the Zn-Mg-Al-based coating layer is not particularly limited, but may include a Zn single phase, a Zn-MgZn2 phase (i.e., a binary eutectic structure of Zn / MgZn2), and a Zn-MgZn2-Al phase (i.e., a ternary eutectic structure of Zn / MgZn2 / Al). In addition, the layer may further include other phases such as an MgZn2 phase and an Al-Zn phase.

[0038] As a method for confirming the microstructure of the Zn-Mg-Al-based plating layer, there is a method using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) at an enlarged cross-sectional magnification of the plating layer.

[0039] In the present invention, the term "Zn single phase" refers to a phase primarily composed of Zn, specifically a phase containing 95% or more by weight of Zn. In other words, the term "Zn single phase" refers to a phase in which Al, Mg, and other coating layer components other than Zn are dissolved in a solid solution at 5% or less (including 0%) or precipitated within the Zn single phase, with the remainder being Zn. The term "Zn-MgZn2-Al phase" refers to a ternary eutectic phase containing Zn, MgZn2, and Al phases. The term "MgZn2 phase" refers to a phase primarily composed of MgZn2. The term "Zn-MgZn2 phase" refers to a binary eutectic phase with a lamellar structure containing Zn and MgZn2 phases. The term "Al-Zn phase" refers to a binary eutectic phase with a lamellar structure containing Al and Zn phases, or a binary eutectic phase with a mixed structure of fine-diameter Al and Zn phases. Here, with respect to the above-mentioned Zn single phase and Zn-MgZn2-Al phase, matters generally known in the art can be similarly applied as long as they do not impair the object of the present invention, and the same applies to the above-mentioned MgZn2 phase, Zn-MgZn2 phase, and Al-Zn phase.

[0040] According to the present invention, the ratio of the single Zn phase on the surface of the plating layer may be 50% or more (excluding 100%), more preferably 50 to 90%, in terms of area fraction. If the ratio of the single Zn phase on the surface of the plating layer is less than 50%, the occupancy rate of the single Zn phase on the surface of the plating layer will be insufficient, which may result in poor adhesion to an adhesive due to MgO-based oxides. Meanwhile, with regard to the upper limit of the ratio of the single Zn phase on the surface of the plating layer, in the present invention, the higher the ratio of the single Zn phase, the more improved the adhesion to an adhesive, so there is no need to separately set the upper limit. However, as an example, the ratio of the single Zn phase on the surface of the plating layer may be 90% or less. To further improve the above-mentioned effects, the lower limit of the ratio of the single Zn phase on the surface of the plating layer may be 60%, or the upper limit of the ratio of the single Zn phase on the surface of the plating layer may be 81%.

[0041] The average diameter of the Zn single phase on the surface of the plating layer is preferably 3 to 20 μm. If the average diameter of the Zn single phase on the surface of the plating layer is less than 3 μm, the effect of improving adhesion may be insufficient, and if the average diameter of the Zn single phase exceeds 20 μm, corrosion resistance may be reduced.

[0042] Furthermore, although not particularly limited, most of the Zn single phases on the surface of the coating layer may have a spherical or elliptical shape. Here, spherical or elliptical does not mean only perfect spheres or ellipses, and does not mean that all of the Zn single phases on the surface of the coating layer are spherical or elliptical. In other words, it can mean that, based on a certain Zn single phase, 80% or more of the phases have a ratio of the longest diameter to the shortest diameter of 0.8 to 1.2.

[0043] Furthermore, although not particularly limited, according to one embodiment of the present invention, the ratio of the Zn-MgZn2 phase on the surface of the coating layer can be controlled to 10% or less (including 0%) in terms of area fraction. The Zn-MgZn2 phase, which is a binary eutectic structure on the surface of the coating layer, can act as a factor that deteriorates adhesion with adhesives due to its characteristic wrinkles. Therefore, in the present invention, by controlling the ratio of the Zn-MgZn2 phase on the surface of the coating layer to 10% or less, adhesion with adhesives can be further improved. Meanwhile, in the present invention, the smaller the ratio of the Zn-MgZn2 phase, which is a binary eutectic structure on the surface of the coating layer, the more improved the properties become, so there is no need to separately set a lower limit. However, as an example, the ratio of the Zn-MgZn2 phase on the surface of the coating layer may be 1% or more.

[0044] According to one aspect of the present invention, the ratio of the Zn-MgZn2-Al phase on the surface of the coating layer may be 10 to 40% in area fraction. The present inventors conducted extensive research into factors affecting adhesive adhesion resulting from the microstructure of the surface of the coating layer, and found that it is more preferable to control the ratio of the Zn-MgZn2-Al phase, which is a ternary eutectic structure, on the surface of the coating layer within an appropriate range in area fraction. Specifically, if the ratio of the Zn-MgZn2-Al phase on the surface of the coating layer is less than 10%, the proportion of the Zn-MgZn2 phase on the surface of the coating layer will be excessively high, which may result in poor sealer adhesion. Furthermore, if the ratio of the Zn-MgZn2-Al phase on the surface of the coating layer exceeds 40%, the proportion of the Zn single phase relative to the Zn-MgZn2-Al phase on the surface of the coating layer will be insufficient, which may result in a problem of not achieving the desired level of sealer adhesion.

[0045] Furthermore, according to one aspect of the present invention, the ratio of the area of the Zn-MgZn2 phase to the area of the Zn-MgZn2-Al phase on the surface of the coating layer may be 1 to 4, and more preferably 1.2 to 4. As described above, the microstructure on the surface of the coating layer is affected not only by the proportion and size of the Zn single phase, but also by binary and ternary eutectic structures. Therefore, in the present invention, it has been confirmed that by controlling the ratio of the binary eutectic structure to the ternary eutectic structure within an appropriate range, it is possible to further improve the corrosion resistance and sealer adhesion targeted by the present invention.

[0046] Furthermore, as a result of extensive investigations, the present inventors have found that the plating layer of the plated steel sheet according to one embodiment of the present invention may include a Zn single phase grown in a columnar morphology based on a cross section in the thickness direction (meaning a direction perpendicular to the rolling direction of the steel sheet).

[0047] Here, the Zn single phase grown in a columnar form refers to a Zn single phase that contacts the surface line of the coating layer but does not contact the interface line between the coating layer and the inhibitor layer, and may refer to a Zn single phase in which the ratio (Wa / Wb) of the maximum size (Wa) measured in the thickness direction (meaning the direction perpendicular to the rolling direction of the steel sheet) of a certain Zn single phase to the length (Wb) of the Zn single phase occupying the surface line of the coating layer exceeds 1.0 (preferably 1.02 or more, more preferably 1.2 or more).

[0048] Meanwhile, according to one embodiment of the present invention, 50% or more of the Zn single phase in the coating layer can satisfy the above-mentioned columnar morphology, but this does not mean that all of the Zn single phase grows in the form of columnar crystals.

[0049] Therefore, according to one aspect of the present invention, in the cross section of the plating layer, of all the Zn single phases, the proportion of Zn single phases that contact the surface line of the plating layer but do not contact the interface line between the plating layer and the suppression layer, in which the ratio (Wa / Wb) of the maximum length (Wa) of the Zn single phase in the thickness direction to the length (Wb) of the Zn single phase occupying the surface line of the plating layer, exceeds 1.0, may be 50% or more.

[0050] By controlling the growth morphology of the Zn single phase so as to satisfy the above conditions, the Zn single phase is uniformly distributed throughout the thickness of the coating layer, and binary and ternary eutectic structures are uniformly distributed between each Zn single phase, thereby enabling the surface quality aimed at in the present invention to exhibit uniform characteristics and ensuring uniform quality.

[0051] A method for producing a plated steel sheet according to yet another embodiment of the present invention will be described in detail below. However, this does not necessarily mean that the plated steel sheet of the present invention should be produced by the following production method.

[0052] First, a base iron is prepared. The above description can be similarly applied to the base iron. Next, the base iron can be annealed at a temperature of 800°C or higher (more preferably, 800°C or higher and 850°C or lower). If the annealing temperature does not satisfy the above temperature range, problems with material hardening may occur.

[0053] Next, the annealed base steel is primarily cooled from 800°C to 600°C (i.e., in the temperature range of 800°C to 600°C) at an average cooling rate of 2 to 5°C / s, based on the surface temperature of the base steel. By controlling the cooling rate of the base steel before coating as described above, the structure of the base layer can be uniformly controlled. On the other hand, from the viewpoint of further improving uniformity, the lower limit of the average cooling rate during the primary cooling may be 4.3°C / s, or the upper limit of the average cooling rate during the primary cooling may be 5.0°C / s.

[0054] Next, the primarily cooled base steel is secondarily cooled from 600°C to the coating bath temperature (i.e., in the temperature range from 600°C to the coating bath temperature (i.e., the same temperature as the coating bath temperature)) at an average cooling rate of 25°C / s, based on the surface temperature of the base steel. In this way, the secondary cooling can include a process of rapidly cooling the surface layer of the base steel by controlling the average cooling rate to be higher than that of the primary cooling conditions, thereby forming a dense fine ferrite phase on the surface of the base steel. This affects the formation of the solidification phase of the subsequent hot-dip coating layer, thereby effectively obtaining the solidification phase structure desired in the present invention. Meanwhile, from the viewpoint of further improving the above-mentioned effects, the lower limit of the average cooling rate during the secondary cooling may be 18°C / s, or the upper limit of the average cooling rate during the secondary cooling may be 20°C / s.

[0055] The secondarily cooled base steel is immersed in a coating bath containing, by weight, 1.0-2.0% Mg, 1.0-3.0% Al, the balance being Zn and other unavoidable impurities, to perform hot-dip galvanizing. The reasons for adding each component to the coating bath and for limiting the content thereof are the same as those explained above regarding the coating layer of the coated steel sheet.

[0056] Meanwhile, to prepare a coating bath having the above-mentioned composition, a composite ingot containing the specified Zn, Al, and Mg or a Zn-Mg or Zn-Al ingot containing individual components can be used. In this case, the above description of the coating layer can be applied to the components of the coating bath, except for Fe introduced from the base steel.

[0057] In order to replenish the coating bath consumed during hot dip coating, the ingots are melted and supplied. In this case, the ingots may be directly deposited in the coating bath and melted, or the ingots may be melted in a separate port and the molten metal may be replenished to the coating bath.

[0058] The temperature suitable for melting the ingot is 440 to 520°C. A higher bath temperature ensures fluidity within the bath, forming a uniform composition, and reducing the amount of floating dross. If the bath temperature is below 440°C, the ingot melts very slowly and the bath is highly viscous, making it difficult to ensure excellent surface quality of the coating layer. On the other hand, a bath temperature above 520°C is inappropriate because it induces ash-related defects on the coating surface due to Zn evaporation. Most preferably, the initial bath temperature is maintained at approximately 500 to 520°C to proceed with melting, and then the bath is stabilized at 440 to 480°C to prepare the coated product.

[0059] Furthermore, the temperature of the coating bath can be a normal coating bath temperature. Generally, as the Al content of the components in the coating bath increases, the melting point increases, which not only corrodes the internal equipment of the coating bath and shortens the equipment's lifespan, but also increases the amount of Fe alloy dross in the coating bath, potentially resulting in poor surface quality of the coated material. However, in the present invention, the Al content is controlled to a relatively low level of 1.0 to 3.0 wt %, eliminating the need to set the coating bath temperature high; for example, the temperature may be 440 to 480°C. Alternatively, to further improve the above-mentioned effects, the lower limit of the coating bath temperature may be 450°C, or the upper limit of the coating bath temperature may be 470°C.

[0060] According to one aspect of the present invention, the immersion time in the plating bath can be controlled to a range of 2 to 5 seconds, and the line speed can be set to 1 to 3 m / s. By controlling the plating so that the above conditions are satisfied, the problem of uneven plating surfaces can be suppressed.

[0061] The zinc alloy-plated steel material is then subjected to a gas wiping treatment to adjust the coating weight. The gas wiping treatment is for adjusting the coating weight, and the method is not particularly limited. The gas used here can be air, nitrogen, or argon, of which nitrogen is more preferable from the standpoints of economy and quality. This is because using air can cause Mg oxidation to occur preferentially on the surface of the coating layer, which can lead to surface defects in the coating layer, and using argon is expensive.

[0062] At this time, although not particularly limited, according to one embodiment of the present invention, after the hot dip galvanizing, nitrogen gas is used to adjust the coating weight of the plating layer on one side to 35 to 200 g / m 2 By performing the gas wiping process so as to satisfy this range, a beautiful surface without oxidation defects can be obtained. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the coating weight of the plating layer on one side is 120 g / m 2 Alternatively, the upper limit of the coating weight of the plating layer on one side may be 150 g / m 2 may be.

[0063] Next, the steel sheet having the hot-dip galvanized layer formed on its surface is removed from the coating bath and subjected to tertiary cooling, in which the steel sheet is cooled from below 460°C (i.e., in the temperature range of 460°C to 300°C) at an average cooling rate of 6 to 20°C / s. The cooling end temperature is not particularly limited, and the cooling can be performed under ordinary cooling conditions. The cooling method is also not particularly limited, and cooling can be performed using an air jet cooler, N2 wiping, water fog spraying, or the like. To further improve the above-mentioned effects, the lower limit of the average cooling rate during the tertiary cooling may be 10°C / s (more preferably 15°C / s), and the upper limit of the average cooling rate during the tertiary cooling may be 19°C / s. [Example]

[0064] (Example) The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are provided 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 claims and any reasonable inferences therefrom.

[0065] (Experimental Example 1) A low-carbon cold-rolled steel sheet measuring 0.8 mm in thickness, 100 mm in width, and 200 mm in length containing, by weight, 0.025% C, 0.03% Si, 0.15% Mn, 0.01% P, 0.003% S, 0.03% Al, with the balance being Fe and other unavoidable impurities, was prepared as the base steel sheet for plating. The base steel sheet was then immersed in acetone and ultrasonically cleaned to remove any residual rolling oil or other impurities from the surface. The steel sheet was then heat-treated in a reducing atmosphere at 800°C, a process commonly used in hot-dip galvanizing to ensure the mechanical properties of steel sheets. The steel sheet then underwent two cooling stages (primary and secondary cooling) before plating under the conditions listed in Table 1 below. The difference in heating and cooling curves between Example 1 and Comparative Example 1 is shown in Figure 1.

[0066] Next, after the above-mentioned two-stage cooling before plating, the base steel was immersed in a plating bath (bath temperature: 460°C) having the composition shown in Table 1 below to perform plating, and then cooled to produce a plated steel sheet. Thereafter, the zinc alloy plated steel material was gas wiped to reduce the coating weight to 140 g / m per side. 2 was adjusted to.

[0067] On the other hand, in the case of the inventive example, after the temperature was increased, primary and secondary cooling was performed, and then the slab was drawn into the plating bath and then tertiary cooling was performed.In the comparative example, after the temperature was increased, primary cooling was performed, and then the slab was drawn into the plating bath and then secondary cooling was performed.

[0068] For each of the plated steel sheets thus obtained, the plating layer was dissolved in a hydrochloric acid solution, and the resulting solution was analyzed by a wet analysis (ICP) method to measure the content (wt%) of each component, which is shown in Table 2 below.

[0069] Furthermore, by photographing the cross section of each of the produced coated steel sheets cut in the thickness direction (meaning the direction perpendicular to the rolling direction) with a scanning electron microscope (SEM) and then observing the photographs, it was confirmed that a Zn-Mg-Al-based coating layer was formed on the base iron, and that an Fe-Al-based inhibitor layer was formed between the base iron and the Zn-Mg-Al-based coating layer, satisfying a composition containing, by weight, 30 to 50% Fe and 50 to 70% Al. Furthermore, using the photographs, the thickness of the Fe-Al-based inhibitor layer was measured and confirmed to be in the range of 20 to 100 nm.

[0070] The surface structure of each manufactured coated steel sheet was then observed using a Field Emission-Scanning Electron Microscope (SUPRA-55VP, ZEISS) and the surface fraction of the Zn single phase and the diameter of the long axis of the cross-sectional structure were measured using an image analysis system (Analyser). FE-SEM analysis of the cross-sectional coating layer was used to observe the distribution of the Zn single phase within the coating layer and the columnar phase. Columnar crystals were defined as Zn single phases whose length in the thickness direction of the coating layer was longer than that of the surface of the Zn single phase.

[0071] Fig. 2 shows a 700x FE-SEM image of the surface of the plating layer of Example 1, and Fig. 3 shows a 700x FE-SEM image of the surface of the plating layer of Comparative Example 1. Fig. 4 shows a 1000x FE-SEM image of the cross section of the plating layer of Example 1, and Fig. 5 shows a 1000x FE-SEM image of the cross section of the plating layer of Comparative Example 1.

[0072] The adhesive adhesion of the manufactured zinc alloy-plated steel products was then evaluated, and the results are shown in Table 1 below. To evaluate adhesive adhesion, a lab shear test was performed using reinforced D-type structural adhesive #SA-1402D. For the lab shear test, the adhesive was applied to a thickness of 2 mm between two steel plates (adhesion area: 10 mm x 25 mm) and attached, and then the test was heated at 170°C for 20 minutes before a tensile test was performed. The fracture surface was then observed to evaluate whether the failure was adhesive or cohesive.

[0073] When the adhesive strength between the adhesive and the steel plate is strong, cohesive failure occurs between the adhesive, but when the adhesive strength between the adhesive and the steel plate is weak, adhesive failure occurs. In addition, the tensile strength was measured during the Lab shear test and is shown in the table.

[0074] The adhesion rating scale is as follows: 5: 90% or more cohesive failure 4: 70% or more cohesive failure + 30% or less adhesive failure 3: 50% or more cohesive failure + 50% or less adhesive failure 2: Cohesive failure of 30% or less + adhesive failure of 70% or more 1: 90% or more adhesive failure

[0075] Table 2 shows the results, including the ratio (%) of the Zn single phase structure on the surface, the average grain size (μm) of the Zn single phase structure on the surface, the Lab Shear Test results (grade), and the Lab shear tensile strength (MPa).

[0076] [Table 1]

[0077] [Table 2]

[0078] As shown in Table 2 above, Examples 1 to 4, which satisfy all of the plating layer compositions and manufacturing conditions of the present invention, had high Lab Shear Test grades, high Lab shear tensile strength, and excellent adhesion to adhesives.

[0079] On the other hand, in the case of Comparative Examples 1 to 12, which do not satisfy one or more of the plating layer composition and manufacturing conditions of the present invention, it was confirmed that one or more of the Lab Shear Test grade and Lab shear tensile strength were low, and the adhesion to the adhesive was poor.

[0080] (Experimental Example 2) Except for the experimental conditions changed to Tables 3 and 4 below, the line speed during hot dip galvanizing was 2 m / s, and the specimen was immersed for 3 seconds. After that, nitrogen gas was used to adjust the coating weight on one side of the coating layer to 130 g / m 2 A plated steel sheet was manufactured in the same manner as in Experimental Example 1, except that a gas wiping step was performed to ensure that the film thickness was within the range of 1.0 mm. The evaluation results are shown in Table 4 below.

[0081] [Table 3]

[0082] Furthermore, in order to evaluate the uniformity of the plated steel sheets obtained from each of the invention examples and comparative examples, they were evaluated according to the following criteria, and the results are shown in Table 4. ○: No unplated defects on the surface ×: Unplated defects on the surface

[0083] [Table 4]

[0084] As shown in Table 4 above, inventive examples 5 to 9, which fulfill all of the plating layer compositions and manufacturing conditions of the present invention, the Lab Shear Test grade was high, the Lab shear tensile strength was high, and the adhesion to the adhesive was excellent.

[0085] In this case, in the case of Examples 5 to 9, it was confirmed that the ratio of the Zn-MgZn2 phase on the surface of the coating layer was 10% or less in area fraction, the ratio of the Zn-MgZn2-Al phase on the surface of the coating layer was 10 to 40% in area fraction, and the ratio of the area of the Zn-MgZn2 phase to the area of the Zn-MgZn2-Al phase on the surface of the coating layer was 1.2 to 4.

[0086] Furthermore, through SEM photographs in which the cross section of the above-mentioned plated steel sheet was observed, it was confirmed that, in the cross section in the thickness direction of the plating layer (meaning the direction perpendicular to the rolling direction), of all the Zn single phases, the proportion of Zn single phases that contact the surface line of the plating layer but do not contact the interface line between the plating layer and the suppression layer, where the ratio (Wa / Wb) of the maximum length (Wa) of the Zn single phase in the thickness direction to the length (Wa) over which the Zn single phase occupies the surface of the plating layer, exceeds 1.0, was 50%.

[0087] On the other hand, in the case of Comparative Examples 9 to 12, which do not satisfy one or more of the plating layer composition and manufacturing conditions of the present invention, it was confirmed that one or more of the Lab Shear Test grade and Lab shear tensile strength were low, and the adhesion to the adhesive and plating uniformity were also poor.

Claims

1. With bare iron, a Zn—Mg—Al-based plating layer provided on at least one surface of the base steel; an Fe-Al-based suppression layer provided between the base steel and the Zn-Mg-Al-based plating layer, the plating layer contains, by weight %, 1.0 to 2.0% Mg, 1.0 to 3.0% Al, the balance being Zn and other inevitable impurities, and the ratio of Mg to Al (Mg / Al) satisfies 0.6 to 0.9; The ratio of the Zn single phase on the surface of the plating layer is 50% or more in terms of area fraction, the average diameter of the Zn single phase on the surface of the plating layer is 3 to 20 μm; The plated steel sheet includes a Zn single phase that is in contact with a surface line of the plated layer but not with an interface line between the plated layer and the suppression layer, based on a cross section in the thickness direction, and that has a ratio (Wa / Wb) of a maximum length (Wa) of the Zn single phase in the thickness direction to a length (Wb) of the Zn single phase occupying the surface of the plated layer that exceeds 1.

0.

2. Zn-MgZn on the surface of the plating layer ２ The plated steel sheet according to claim 1 , wherein the ratio of the phase is 10% or less (including 0%) in terms of area fraction.

3. Zn-MgZn on the surface of the plating layer ２ The plated steel sheet according to claim 1, wherein the ratio of the -Al phase is 10 to 40% in terms of area fraction.

4. Zn-MgZn on the surface of the plating layer ２ Zn-MgZn relative to the area of the phase ２ The plated steel sheet according to claim 1, wherein the area ratio of the -Al phase is 1 to 4.

5. After preparing the base steel, annealing the base steel at a temperature of 800°C or more; a step of primarily cooling the annealed steel from 800°C to 600°C at an average cooling rate of 2 to 5°C / s; Secondary cooling the primarily cooled raw steel from 600°C to the coating bath temperature at an average cooling rate of 13 to 25°C / s; immersing the second-cooled steel in a galvanizing bath containing, by weight, 1.0 to 2.0% Mg, 1.0 to 3.0% Al, the balance Zn and other inevitable impurities, with a ratio of Mg to Al (Mg / Al) of 0.6 to 0.9, to perform hot-dip galvanizing; After the hot dip galvanizing, cooling is started from the surface of the coating bath to the top roll section at an average cooling rate of 6 to 20 ° C. / s; A method for producing a plated steel sheet, comprising:

6. In the hot dip galvanizing step, the immersion time in the galvanizing bath is controlled to be in the range of 2 to 5 seconds, and the line speed of the steel sheet is controlled to be in the range of 1 to 3 m / s; After the hot dip galvanizing, nitrogen gas is used to reduce the coating weight of the plating layer on one side to 35 to 200 g / m ２ The method for producing a plated steel sheet according to claim 5 , further comprising the step of air wiping the steel sheet so that the thickness of the plated steel sheet is in the range of

7. The method for producing a plated steel sheet according to claim 5, wherein the temperature of the plating bath is controlled in the range of 440 to 480°C.

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