Hot-dip galvanized steel sheet with excellent bending workability and corrosion resistance and its manufacturing method

Abstract

The present invention provides a hot-dip galvanized steel sheet having excellent bending workability and corrosion resistance, and a method for manufacturing the same. The hot-dip galvanized steel sheet 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, the coating layer containing, by weight, 5.1 to 25% Al, 4.0 to 10% Mg, and the remainder Zn and other inevitable impurities, excluding iron (Fe) diffused from the base steel sheet; and an interfacial alloy layer having an Fe-Al-Zn composition formed between the base steel sheet and the coating layer, the interfacial alloy layer having a thickness of 0.5 to 2 μm and a dendritic morphology, and the Zn-Mg-Al-based coating layer has a structure including one or more of a Zn-Al-MgZn2 ternary eutectic structure, a Zn-MgZn2 binary eutectic structure, an Al single-phase structure containing Zn as a solid solution, and a Zn single-phase structure, with Al agglomerated within the MgZn2 structure.

Description

[Technical Field]

[0001] The present invention relates to a hot-dip galvanized steel sheet having excellent bending workability and corrosion resistance, and a method for producing the same. [Background technology]

[0002] Hot-dip galvanized steel sheets, which are coated with zinc, have the property of sacrificial corrosion protection, in that when exposed to a corrosive environment, zinc, which has a lower redox potential than iron, corrodes first, thereby 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, hot-dip galvanized steel sheets have recently been widely used in 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 for the development of steel materials with better corrosion resistance than conventional galvanized steel materials.

[0004] To address this issue, 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. A typical example is the active research into the manufacturing technology of Zn-Mg-Al zinc alloy-plated steel sheet, which is a zinc alloy-plated material that further adds Mg to the Zn-Al plating composition.

[0005] However, compared to zinc-coated steel sheets, Zn-Mg-Al zinc alloy-coated steel sheets have superior corrosion resistance but suffer from the drawback of poor bending workability. Specifically, zinc alloy-coated steel sheets contain a large amount of Zn-Al-Mg intermetallic compounds formed by thermodynamic interactions between Zn, Al, and Mg in the coating layer. These intermetallic compounds have high hardness, which can cause cracks in the coating layer during bending, resulting in poor bending workability. These cracks can impair the appearance of bent sections and reduce corrosion resistance.

[0006] There are various factors that affect crack formation during processing, but in terms of the material's physical properties, the hardness of the coating layer and the interfacial alloy layer are known to have an effect. For this reason, various process modifications have been attempted to suppress local coarsening of the MgZn2 phase, which has the highest hardness among Zn-Al-Mg intermetallic compounds, and to distribute it uniformly within the coating layer.

[0007] However, even now, localized coarsening of the MgZn2 phase is fundamentally unavoidable in Mg-Al-Zn alloys containing large amounts of Mg. To minimize crack formation during bending, attempts have been made to suppress the growth of the Fe-Al interfacial alloy phase between the coating layer and the base steel by adding trace amounts of Si to Zn-Mg-Al coating systems. This Fe-Al interfacial alloy phase typically grows into the Fe2Al5 phase, which is highly hard and known to be vulnerable to interfacial fracture during processing if it grows coarsely in a layer-like form. However, while the Si addition technique is typically finely tuned to 0.1–0.2 wt%, excessive addition can lead to the coarse formation of an additional alloy phase in the form of Mg2Si within the coating layer. Therefore, ensuring sustained bendability is considered a critical characteristic related to forming stability and corrosion resistance after processing. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2003-155549 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention is intended to solve the above-mentioned problems of the conventional art, and provides a hot-dip galvanized steel sheet having a Zn-Al-Mg-based coating layer that can reduce cracks in the coating layer during bending and ensure excellent bending workability and corrosion resistance, and a manufacturing method thereof.

[0010] The object of the present invention is not limited to the above-mentioned content, and a person 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 overall matters of the specification of the present invention. [Means for solving the problem]

[0011] One aspect of the present invention is Base steel sheet; a Zn-Mg-Al-based coating layer provided on at least one surface of the base steel sheet, the remaining components excluding iron (Fe) diffused from the base steel sheet being, in weight percent, 5.1 to 25% Al, 4.0 to 10% Mg, with the remainder being Zn and other unavoidable impurities; and an interfacial alloy layer of Fe-Al-Zn composition formed between the base steel sheet and the coating layer, The interfacial alloy layer has a thickness of 0.5 to 2 μm and a dendritic morphology; and The Zn-Mg-Al-based coating layer has a structure containing one or more of a Zn-Al-MgZn2 ternary eutectic structure, a Zn-MgZn2 binary eutectic structure, an Al single-phase structure containing Zn as a solid solution, and a Zn single-phase structure, and relates to a hot-dip galvanized steel sheet with excellent bending workability and corrosion resistance, characterized in that the MZn2 structure contains coagulated Al.

[0012] In the present invention, the Al and Mg contents can be determined so as to be located on the dieutectic line of MgZn2 and Al in the ternary phase diagram of Mg-Al-Zn.

[0013] The widths of bending cracks generated during 90-degree bending and OT bending of the coating layer including the base steel sheet may be 30 μm or less and 100 μm or less, respectively.

[0014] The present invention also provides Preparing the base steel sheet; hot-dip galvanizing the base steel sheet in a coating bath containing, by weight, 5.1 to 25% Al, 4.0 to 10% Mg, the remainder Zn and other unavoidable impurities; and cooling the galvanized steel sheet using an inert gas at a cooling rate of 5 to 30°C / s from the surface of the galvanizing bath to a top roll section, thereby manufacturing a hot-dip galvanized steel sheet having an interface alloy layer and a Zn-Mg-Al-based coating layer sequentially formed on the base steel sheet; The present invention relates to a method for producing a hot-dip galvanized steel sheet having excellent bending workability and corrosion resistance, characterized in that the base steel sheet is immersed in a coating bath having a temperature (T) such that the thickness (H) of the interface alloy layer defined by the following relational expression 1 satisfies the range of 0.5 to 2 μm. [Equation 1] H(μm)=170.53+0.0008T 2 -0.7376T

[0015] The Zn-Mg-Al-based coating layer has a structure containing one or more of a Zn-Al-MgZn2 ternary eutectic structure, a Zn-MgZn2 binary eutectic structure, an Al single-phase structure containing Zn as a solid solution, and a Zn single-phase structure, and may contain Al aggregated in the MgZn2 structure.

[0016] The temperature of the plating bath can be maintained in the range of 470 to 520°C.

[0017] The Al and Mg contents in the plating bath can be determined so as to be located on the dieutectic line of MgZn2 and Al in the Mg-Al-Zn ternary phase diagram.

[0018] The bathing time for which the base steel sheet is immersed in the coating bath may be 1 to 5 seconds.

[0019] The inert gas can be one of N, Ar and He. [Effects of the Invention]

[0020] The hot-dip galvanized steel sheet according to the present invention has the advantages of excellent bending workability and corrosion resistance. Furthermore, the present invention can provide a hot-dip galvanized steel sheet having a coating layer containing no Si and containing 5.1 to 25 wt % Al and 4.0 to 10 wt % Mg, in which the crack width measured in a 90-degree bending test is as low as 30 μm or less, and which has excellent bending workability and excellent corrosion resistance even after bending. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a photograph of a cross section of a hot-dip galvanized steel sheet according to Example 1, which is a preferred embodiment of the present invention, observed with a field emission scanning electron microscope (hereinafter referred to as "FE-SEM") (magnification: 2,000 times). [Figure 2] 1 is a photograph of the surface of the interface alloy layer of the hot-dip galvanized steel sheet of Invention Example 1, which is one preferred embodiment of the present invention, observed with an FE-SEM (magnification: ×10,000). [Figure 3] FIG. 1 is a photograph of the Fe, Al, and Zn components of a replica sample obtained by cross-sectional polishing in the interface alloy layer of a hot-dip galvanized steel sheet according to Example 1, which is a preferred embodiment of the present invention, observed by TEM-EDS. [Figure 4]This shows the weight percentage (Wt%) detected along the yellow line shown in FIG. 3 for the interface alloy layer of the hot-dip galvanized steel sheet of Invention Example 1, which is a preferred embodiment of the present invention. [Figure 5] 1A and 1B are photographs of a hot-dip galvanized steel sheet according to Example 1, which is a preferred embodiment of the present invention, bent by 90 degrees, and then observed with an FE-SEM (magnifications of ×100, ×200, and ×300) to show cracks formed at the top of the bent steel sheet. [Figure 6] 1 shows photographs of the hot-dip galvanized steel sheet of Comparative Example 1, which was bent 90 degrees, and then the cracks generated at the top of the bent part were observed with an FE-SEM (magnifications of ×100, ×200, ×300). [Figure 7] 1 is a photograph of a cross section of Comparative Example 4 observed with an FE-SEM (magnification: ×2,000). [Figure 8] 1 is a photograph of a cross section of Comparative Example 5 observed with an FE-SEM (magnification: ×2,000). [Figure 9] 1 is a photograph of a cross section of Comparative Example 6 observed with an FE-SEM (magnification: ×2,000). DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention will be described below.

[0023] As mentioned above, a method for minimizing crack formation during bending of Mg-Al-Zn coated steel sheets containing a large amount of Mg has been proposed, which involves adding a small amount of Si to the coating bath to suppress the coarsening of the high-hardness interfacial alloy phase formed between the coating layer and the steel substrate. However, this method has limitations because it can lead to the coarse formation of an additional alloy phase in the form of Mg2Si within the coating layer.

[0024] Here, the present inventors have conducted detailed studies on the composition and shape of the interfacial alloy phase and cracks that occur during bending depending on whether or not Si is added, in order to maintain the excellent corrosion resistance that is an advantage of zinc-based alloy-plated steel sheets containing a large amount of Mg, while minimizing cracks that occur during bending, and have completed the present invention based on the results of these studies.

[0025] The hot-dip galvanized steel sheet of the present invention comprises: a base steel sheet; a Zn-Mg-Al-based coating layer provided on at least one surface of the base steel sheet, the coating layer containing, in weight percent, 5.1 to 25% Al, 4.0 to 10% Mg, with the remainder being Zn and other unavoidable impurities, excluding iron (Fe) diffused from the base steel sheet; and an interfacial alloy layer having an Fe-Al-Zn composition formed between the base steel sheet and the coating layer.

[0026] The interface alloy layer has a thickness of 0.5 to 2 μm and a dendritic morphology, and the Zn-Mg-Al-based coating layer has a structure including one or more of a Zn-Al-MgZn2 ternary eutectic structure, a Zn-MgZn2 binary eutectic structure, an Al single-phase structure with Zn as a solid solution, and a Zn single-phase structure, and contains Al aggregated within the MgZn2 structure.

[0027] First, a hot-dip galvanized steel sheet according to one aspect 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-Zn interfacial alloy layer formed between the base steel sheet and the coating layer.

[0028] At least one surface of the base steel sheet may be provided with a plating layer made of a Zn-Mg-Al alloy. The plating layer may be formed on only one surface of the base steel sheet, or on both surfaces of the base steel sheet.

[0029] In the present invention, the Zn-Mg-Al-based coating layer contains, in weight percent, 5.1 to 25% Al, 4.0 to 10% Mg, and the remainder Zn and other unavoidable impurities, relative to the remaining components excluding a small amount of iron (Fe) diffused from the base steel sheet.

[0030] Al: 5.1-25%, Mg: 4.0-10% Mg in the plating layer is an element that plays a role in improving the corrosion resistance of plated steel materials. It uniformly generates corrosion products, and the corrosion products thus formed prevent further corrosion, ultimately improving corrosion resistance.

[0031] Generally, when Mg is added in an amount less than 1.0%, the effect of improving corrosion resistance is slight, and when Mg exceeds 2.0%, the oxidation of Mg in the plating bath increases the generation of floating dross in the plating bath, which requires frequent dross removal, resulting in a problem of poor operability. For this reason, in the case of Zn-Mg-Al zinc alloy plating, Mg is added in an amount of 1.0% or more, but the upper limit of the Mg content has been set at around 3.0% in the prior art.

[0032] However, in the present invention, the Mg content in the coating layer is increased to 4.0% or more, and in addition, 5.1% or more of Al can be added to suppress Mg oxide dross in the zinc alloy coating bath. Furthermore, Al can also play a role in improving the corrosion resistance of the coated steel sheet in combination with Zn and Mg.

[0033] On the other hand, as mentioned above, in Zn-Mg-Al ternary alloy plating, Mg is known to play an auxiliary role in further stabilizing the formation of Zn corrosion products. However, if the Mg content exceeds 10%, the rate at which Mg itself corrodes becomes faster than the rate at which Mg stabilizes the Zn corrosion products, and the corrosion resistance of the plated steel sheet deteriorates. Therefore, the upper limit of the Mg content in the plating layer can be limited to 10% or less.

[0034] Furthermore, when the Mg content is 4.0 to 10% and Al is added in an amount exceeding 25%, the melting point will be 480°C or higher. However, considering that the temperature of the coating bath is generally set 40 to 60°C higher than the melting point of the coating composition, an excessively high temperature of the coating bath can cause problems such as erosion of the coating bath structure and modification of the steel material. Taking this into consideration, the Al content can be limited to 25% or less.

[0035] The Al and Mg contents may be determined so as to be positioned on the bieutectic line of MgZn2 and Al in the Mg-Al-Zn ternary phase diagram. Here, determining to be positioned on the bieutectic line includes determining to be positioned exactly on the bieutectic line as well as determining to be positioned slightly off the bieutectic line and in the vicinity of the bieutectic line.

[0036] The remainder of the coating layer other than the above-mentioned composition may be Zn and other inevitable impurities. Since inevitable impurities may be unintentionally mixed in during the normal manufacturing process of hot-dip galvanized steel sheets, they cannot be completely excluded, and those skilled in the art will easily understand the meaning of this.

[0037] Meanwhile, hot-dip galvanized steel sheets are prone to developing a unique coating structure called spangle. This spangle is due to the characteristics of the zinc solidification reaction. Specifically, as zinc solidifies, dendrites grow from solidification nuclei, forming the skeleton of the coating structure. The pool of unsolidified molten zinc remaining between the dendrites eventually solidifies, completing the solidification of the coating layer. When aluminum separates from MgZn2 and forms a primary crystal structure, the primary aluminum crystal structure grows in the form of dendrites. This Al dendrite morphology becomes more pronounced with a higher coating weight or a slower solidification rate. If this dendrite-shaped primary aluminum crystal structure grows excessively, it can cause severe curvature in the coating layer, adversely affecting the surface appearance, and can also cause excessive exposure of aluminum, which has a high oxidation reactivity, on the surface, potentially reducing the oxidation stability of the galvanized steel sheet.

[0038] However, in the present invention, the Al and Mg contents are determined so as to be located on the dieutectic line of MgZn2 and Al in the Mg-Al-Zn ternary phase diagram, thereby solving such problems.

[0039] In addition, in the present invention, the Zn-Mg-Al based coating layer has a microstructure that includes one or more of a Zn-Al-MgZn2 ternary eutectic structure, a Zn-MgZn2 binary eutectic structure, an Al single-phase structure containing Zn as a solid solution, and a Zn single-phase structure.Al aggregated in the MgZn2 structure is contained, which can solve the problem of spangle generation as described above.

[0040] Furthermore, in the hot-dip galvanized steel sheet of the present invention, an Fe-Al-Zn interfacial alloy layer composed of an Fe-Al-Zn intermetallic compound can be formed between the base steel sheet and the coating layer. The interfacial alloy layer can be formed by Fe diffused from the initial base steel sheet and Al and Zn from the coating bath, and can serve to improve the adhesion between the base steel sheet and the coating layer, as well as to act as an inhibitor layer that prevents further diffusion of Fe from the base steel sheet to the coating layer.

[0041] In the present invention, the interfacial alloy layer has a dendritic morphology with an Fe-Al-Zn composition, and this dendritic interfacial alloy phase induces an anchoring effect, which is very advantageous in reducing cracks during bending.

[0042] In the present invention, the thickness of the Fe-Al-Zn interfacial alloy phase is limited to a range of 0.5 μm to 2 μm. If the thickness is less than 0.5 μm, the interfacial alloy phase does not grow sufficiently into a dendritic shape, which prevents the anchoring effect between the base steel sheet and the coating layer and results in poor bending workability. On the other hand, if the thickness exceeds 2 μm, the lower part of the interfacial alloy phase in the direction of the base steel sheet grows thickly into a layer, which may make the interfacial fracture vulnerable during processing.

[0043] The hot-dip galvanized steel sheet of the present invention having the above-described configuration has a coating layer composed of the above-described alloy composition and microstructure, and therefore can provide superior corrosion resistance compared to conventional zinc-based alloy coated steel sheets containing up to about 3.0% Mg, and can minimize the width of cracks during bending.

[0044] Next, a method for producing a hot-dip galvanized steel sheet according to another aspect of the present invention will be described in detail.

[0045] A method for producing a hot-dip galvanized steel sheet according to the present invention includes the steps of: preparing a base steel sheet; hot-dip galvanizing the base steel sheet in a coating bath containing, by weight, 5.1 to 25% Al, 4.0 to 10% Mg, with the remainder being Zn and other inevitable impurities; and cooling the coated steel sheet using an inert gas at a cooling rate of 5 to 30°C / s starting from the surface of the coating bath up to the top roll section, thereby producing a hot-dip galvanized steel sheet having an interfacial alloy layer and a Zn-Mg-Al-based coating layer sequentially formed on the base steel sheet. The base steel sheet is then immersed in a coating bath having a temperature (T) such that the thickness (H) of the interfacial alloy layer, as defined by Relation 1, falls within the range of 0.5 to 2 μm.

[0046] First, in the present invention, a base steel sheet is prepared, and the present invention is not limited to a specific type of base steel sheet. For example, a cold-rolled steel sheet or a hot-rolled steel sheet, which are general carbon steels, can be used without any restrictions.

[0047] Next, in the present invention, the base steel sheet is dipped in a plating bath containing, by weight, 5.1 to 25% Al, 4.0 to 10% Mg, the remainder Zn and other unavoidable impurities to be hot-dip galvanized.

[0048] The coating bath of the present invention is prepared by manufacturing a coating bath that does not contain Si and contains, by weight, 5.1-25% Al, 4.0-10% Mg, and the remainder Zn and other unavoidable impurities. To manufacture a coating bath of the above 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. Furthermore, the Al and Mg contents in the coating bath can be determined so that they lie on the MgZn2 and Al dieutectic line in the Mg-Al-Zn ternary phase diagram.

[0049] In the present invention, hot-dip plating is carried out by immersing the base steel sheet in a plating bath having the above-mentioned composition.

[0050] In this case, the present invention is characterized in that the temperature of the coating bath is determined in consideration of the thickness of the interface alloy layer that constitutes the final coated steel sheet. Specifically, the coating is performed by immersing the steel sheet in a coating bath having a temperature (T) that satisfies the range of 0.5 to 2 μm for the thickness (H) of the interface alloy layer, as defined by the following relational expression 1: As a result, the interface alloy layer having an Fe-Al-Zn composition that constitutes the hot-dip coated steel sheet to be produced can be formed in a dendritic form. As described above, this dendritic form of the interface alloy phase induces an anchoring effect, which is very advantageous in reducing cracks during bending. [Equation 1] H(μm)=170.53+0.0008T 2 -0.7376T

[0051] If the plating bath temperature is too low, the ingot dissolution is very slow and the plating bath becomes too viscous, making it difficult to ensure good plating surface quality. On the other hand, if the temperature is too high, there is a risk of ash defects being induced on the plating surface due to zinc evaporation. Above all, an excessively high plating bath temperature without Si addition can induce excessive iron elution from the base steel sheet into the plating layer, resulting in outbursts that cause the plating layer to peel off.

[0052] In order to prevent this phenomenon while simultaneously forming a dendritic interfacial alloy phase of Fe-Al-Zn composition, the temperature (T) of the plating bath must be set within the range of 470 to 520°C, and the relationship with the thickness (H) of the interfacial alloy phase satisfies the above-mentioned relational expression 1.

[0053] Furthermore, the temperature of the base steel sheet when immersed in the coating bath is preferably in the range of 5°C to 10°C higher than the temperature of the coating bath, and the immersion time in the coating bath is preferably 1 to 5 seconds.

[0054] Thereafter, in the present invention, cooling is started from the surface of the coating bath and the coated steel sheet is cooled using an inert gas at a cooling rate of 5 to 30°C / s up to the top roll section, thereby producing a hot-dip galvanized steel sheet in which an interface alloy layer and a Zn-Mg-Al-based coating layer are sequentially formed on the base steel sheet.

[0055] That is, the plated steel sheet is pulled up and cooling starts from the molten metal surface, and is then cooled using an inert gas at a rate of 5 to 30°C / s up to the top roll section. The inert gas may be any one of N, Ar, and He, and it is more preferable to use N in terms of reducing production costs.

[0056] If the cooling rate from the molten metal surface to the top roll section is less than 5°C / sec, the MgZn2 structure may become excessively coarse, resulting in severe surface curvature of the coating layer. Furthermore, the Zn-MgZn2 binary eutectic structure may be widely formed, which may hinder uniform corrosion resistance and workability. On the other hand, if the cooling rate exceeds 30°C / sec, rapid solidification occurs in the solid-liquid range of 60–100°C, where the liquid phase begins to solidify during the hot-dip coating process, resulting in a non-uniform formation of the alloy structure and locally inconsistent corrosion resistance. Furthermore, insufficient diffusion of the Fe-Al-Zn phase may result in the formation of an interfacial alloy phase that fails to grow into a dendritic morphology and is therefore excessively suppressed, resulting in poor workability. Furthermore, an excessive cooling rate may increase the amount of nitrogen used, potentially increasing manufacturing costs. [Example]

[0057] The present invention will be described in more detail below with reference to examples. It should be noted that the following examples are intended to aid in understanding the present invention and are not intended to limit the scope of the present invention.

[0058] (Example) First, a cold-rolled steel sheet containing 0.018% C, 0.2% Mn, 0.001% Si, 0.009% P, 0.022% Al, and the balance being Fe and unavoidable impurities, was prepared as a base steel sheet. Then, a coating weight of 140 g / m2 was applied to one side of the cold-rolled steel sheet. 2 After hot dip galvanizing was performed so that the temperature reached the top roll, the steel sheet was cooled at a rate of 15°C / s from the molten metal surface to the top roll to obtain a Zn-Mg-Al alloy plated steel sheet.

[0059] In this case, the composition of the plating solution was varied, in weight percent, from 2.8 to 13% Al and from 2.2 to 5.1% Mg. The remainder, excluding components inevitably present in the plating bath, was Zn. The composition of the plating layer was analyzed by depositing the plating layer in 5% hydrochloric acid and completely dissolving it, and then conducting wet analysis of the resulting solution, with the results shown in Table 1. For reference, the Fe in the plating layer components in Table 1 below was diffused from the base steel sheet during hot-dip galvanizing.

[0060] Meanwhile, a steel sheet having a coating layer composition shown in Table 1 below was immersed in a coating bath at a temperature shown in Table 2 to produce a galvanized steel sheet, and the thickness of the interface alloy layer of the final hot-dip galvanized steel sheet was measured and shown in Table 2 below. For comparison, the target interface alloy layer thickness calculated according to Relation 1 depending on the temperature of the coating bath is also shown in Table 2 below.

[0061] [Table 1]

[0062] [Table 2]

[0063] As can be seen from Table 1-2 above, Example 1-2 represents a case where plating was performed under conditions that satisfied all of the plating composition system and plating bath temperature range proposed by the present invention. Comparative Example 1-2 represents a case where the Mg content was less than 4.0%, the range proposed by the present invention, and Si was added. Comparative Example 3 represents a case where a plated steel sheet was produced using a plating bath with low Mg and Al contents. Note that, unlike Example 1-2, Comparative Examples 4-6 satisfy the plating Al and Mg composition system proposed by the present invention, but unlike Example 1-2, the temperature range of the plating bath was higher than 530°C, causing excessive alloying between the coating layer and the base steel sheet, resulting in an Fe content of 7.1 to 8.7% in the coating layer.

[0064] Table 2 above shows the thickness of the interface alloy layer calculated using Relational Formula 1 and the average thickness of the final interface alloy layer as a function of the temperature of the coating bath. Inventive Examples 1-2 satisfy the coating bath temperature range of 470 to 520°C proposed by the present invention, and it can be confirmed that the thickness of the interface alloy layer calculated using Relational Formula 1 and the thickness of the final interface alloy layer are similar. Meanwhile, Comparative Examples 1-3 are cases that deviate from the coating composition system proposed by the present invention. In Comparative Example 1-2, the composition system contains Si, an element that suppresses alloying between the base steel sheet and the coating layer, and it can be confirmed that the thickness of the interface alloy layer is less than 0.1 μm. Meanwhile, Comparative Examples 4-6 were performed at coating bath temperatures of 530°C, 540°C, and 570°C, respectively, and therefore deviate from the coating bath temperature range applicable to Relational Formula 1.

[0065] Thereafter, each of the produced coated steel sheets was cut lengthwise and perpendicularly, and the cross sections were photographed at 2,000x magnification using an FE-SEM, with the results shown in Figure 1 below. Figure 1 shows a photograph of the cross section of the hot-dip galvanized steel sheet of Invention Example 1, which is a preferred embodiment of the present invention, observed with a field emission scanning electron microscope (hereinafter referred to as "FE-SEM") at 2,000x magnification. Figure 2 is a photograph of the surface of the interface alloy layer of the hot-dip galvanized steel sheet of Invention Example 1, which is a preferred embodiment of the present invention, observed with an FE-SEM at 10,000x magnification.

[0066] As shown in Figure 1, the coating layer of Example 1 contains a Zn-Al-MgZn2 ternary eutectic structure and a Zn-MgZn2 binary eutectic structure, as well as a single-phase Al structure with Zn dissolved therein, a single-phase Zn structure, and an MgZn2 structure. It can also be seen that a dark Al structure is located within the MgZn2 structure. Furthermore, as shown in Figure 2, it can also be seen that an interfacial alloy layer made of Fe-Al-Zn is formed in a dendritic form with a thickness of 0.5 μm to 2 μm.

[0067] FIG. 3 is a photograph of the mapping of Fe, Al, and Zn components observed by TEM-EDS of a replica sample obtained by cross-sectional polishing for the interface alloy layer of the hot-dip galvanized steel sheet of Example 1, which is a preferred embodiment of the present invention. FIG. 4 shows the weight percentage (wt%) detected along the yellow line shown in FIG. 3 for the interface alloy layer of the hot-dip galvanized steel sheet of Example 1, which is a preferred embodiment of the present invention.

[0068] As shown in Figure 3-4, the components of the interface alloy layer do not contain Si, and it can be confirmed that an alloy phase is formed in the composition range of Fe: 20-35 wt%, Al: 15-30 wt%, and Zn: 30-36 wt%.

[0069] Meanwhile, for the above-mentioned Inventive Examples 1-2 and Comparative Examples 1-3, physical property evaluations were carried out according to the following criteria to evaluate the bending workability and corrosion resistance of the steel sheets depending on the coating layer composition. At this time, for Comparative Examples 4-6, the entire coating layer was alloyed, and physical property evaluations were carried out excluding evaluation of corrosion resistance.

[0070] (1) Corrosion resistance evaluation Each plated steel sheet was placed in a salt spray tester and the optimum time for the test was measured according to the international standard (ASTM B117-11). At this time, 5% salt water (temperature 35°C, pH 6.8) was used, and the test was carried out at a rate of 2 ml / 80 cm per hour. 2 In order to eliminate the influence of differences in coating weight, the time (hr) that elapsed until red rust appeared was used as the coating weight (g / m2 ) and expressed as an index of corrosion resistance for evaluation. ○: If it is over 50 △: 10~50 ×: Less than 10

[0071] (2) Bending workability Same material thickness (1.2t) and same plating weight (275-285g / m 2 After bending the specimens 90 degrees (bending diameter: 3R), 3T bending, 1T bending, and OT bending, a 1mm length of the bent tip was observed using an SEM, and the width of the bending cracks was observed and averaged for evaluation. ◎: Bending crack width 30 μm or less ○: Bending crack width is over 30 μm and 60 μm or less △: Bending crack width is over 60 μm and 100 μm or less ×: Bending crack width exceeds 100 μm

[0072] [Table 3]

[0073] As can be seen from Table 3 above, Comparative Example 1-3, in which the Mg and Al compositions in the coating layer did not satisfy the requirements of the present invention, exhibited inferior corrosion resistance compared to Inventive Example 1-2. In particular, Comparative Example 1-2 exhibited inferior bending workability, with the widths of bending cracks typically generated during 90-degree bending and OT bending failing to meet the requirements of 30 μm or less and 100 μm or less, respectively. It can also be seen that the width of bending cracks during 3T bending, which is even more severe than 90-degree bending, was significantly reduced compared to Inventive Example 1-2. The superior bending workability of Inventive Example 1-2 is believed to be due to the anchoring effect of the dendritic interfacial alloy phase, which favorably reduced cracking during bending. In addition, in Comparative Examples 4-6, in which the composition of the coating bath was within the range of the present invention but the temperature of the coating bath was outside the range of the present invention, it was confirmed that the interfacial alloy layer did not have a dendritic form as proposed in the present invention, but alloying progressed throughout the entire coating layer, resulting in bending cracks with widths exceeding 100 μm, and bending workability was significantly deteriorated.

[0074] FIG. 5 is a photograph of a hot-dip galvanized steel sheet according to Example 1, which is a preferred embodiment of the present invention, bent at 90 degrees, and then observed with an FE-SEM (magnifications of ×100, ×200, and ×300) to show cracks formed at the top of the bent part. FIG. 6 is a photograph of a hot-dip galvanized steel sheet according to Comparative Example 1 bent at 90 degrees, and then observed with an FE-SEM (magnifications of ×100, ×200, and ×300).

[0075] As can be seen from Figures 5 and 6, the bending workability of Example 1 is superior to that of Comparative Example 1, and it is clear that there are no coarse Al and MgZn2 structures in the coating layer, and that the bending workability is at the same level as Comparative Example 3, which has excellent bending workability.

[0076] On the other hand, Figure 7 is a photograph of the cross section of Comparative Example 4 observed with an FE-SEM (magnification x 2,000), Figure 8 is a photograph of the cross section of Comparative Example 5 observed with an FE-SEM (magnification x 2,000), and Figure 9 is a photograph of the cross section of Comparative Example 6 observed with an FE-SEM (magnification x 2,000).

[0077] As shown in Figures 7 to 9, in all of Comparative Examples 4 to 6, alloying with the base steel sheet progressed throughout the entire coating layer, and an interfacial alloy layer consisting of Fe-Al-Zn was formed with a thickness of more than 2 μm.

[0078] The present invention is not limited to the above-described embodiments, and can be manufactured in various different forms, and a person skilled in the art to which the present invention pertains can understand that the present invention can be embodied in other specific forms without changing the technical concept or essential features of the present invention. Therefore, the above-described embodiments are to be understood as illustrative in all respects and not restrictive.

Claims

1. Base steel sheet; a Zn-Mg-Al-based coating layer provided on at least one surface of the base steel sheet, the coating layer comprising, in weight percent, 5.1 to 25% Al, 4.0 to 10% Mg, the remainder Zn, and other unavoidable impurities, excluding iron (Fe) diffused from the base steel sheet; and an interface alloy layer having an Fe-Al-Zn composition formed between the base steel sheet and the coating layer, the interfacial alloy layer has a thickness of 0.5 to 2 μm and a dendritic morphology; and The Zn-Mg-Al-based plating layer is Zn-Al-MgZn ２ Zn-MgZn ternary eutectic structure ２ a Zn-Mg-Al-based coating layer further includes an MgZn 2 structure containing agglomerated Al.

2. The Al and Mg contents are determined based on the MgZn content in the Mg-Al-Zn ternary phase diagram. ２ The hot-dip galvanized steel sheet excellent in bending workability and corrosion resistance according to claim 1 , wherein the Zn and Al bieutectic line is determined to be located on the eutectic line of Zn and Al.

3. 2. The hot-dip galvanized steel sheet excellent in bending workability and corrosion resistance according to claim 1, wherein widths of bending cracks generated during 90-degree bending and OT bending of a coating layer including the base steel sheet are 30 μm or less and 100 μm or less, respectively.

4. preparing a base steel sheet; A step of immersing the base steel sheet in a coating bath containing, by weight, Al: 5.1 to 25%, Mg: 4.0 to 10%, the remainder Zn and other unavoidable impurities to perform hot-dip galvanizing; and cooling the galvanized steel sheet using an inert gas at a cooling rate of 5 to 30°C / s starting from the surface of the galvanizing bath up to a top roll section, thereby manufacturing a hot-dip galvanized steel sheet having an interface alloy layer and a Zn-Mg-Al-based coating layer sequentially formed on the base steel sheet; A method for producing a hot-dip galvanized steel sheet excellent in bending workability and corrosion resistance, comprising immersing the base steel sheet in a coating bath having a temperature (T (°C)) in the range of 470 to 520°C so that the thickness (H) of the interface alloy layer defined by the following relational expression 1 satisfies the range of 0.5 to 2 μm, The Zn-Mg-Al-based plating layer is Zn-Al-MgZn ２ Zn-MgZn ternary eutectic structure ２ a Zn single-phase structure containing Zn as a solid solution, and / or a Zn single-phase structure; and the Zn-Mg-Al-based coating layer further includes an MgZn 2 structure containing agglomerated Al. [Relationship 1] H(μm)=170.53+0.0008T ２ -0.7376T

5. The Al and Mg contents in the plating bath are determined as follows: MgZn in the Mg-Al-Zn ternary phase diagram ２ 5. The method for producing a hot-dip galvanized steel sheet excellent in bending workability and corrosion resistance according to claim 4, wherein the Al content is determined to be located on a dieutectic line of Al and Zn.

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