Plated steel sheet

Abstract

According to exemplary embodiments of the present invention, a plated steel sheet may be provided, the plated steel sheet having excellent workability while having a level of ultra-high corrosion resistance suitable for use even in harsh corrosive environments such as marine environments.

Description

galvanized steel sheet

[0001] The present invention relates to plated steel sheets.

[0002] Galvanized steel sheets possess a so-called sacrificial protection characteristic, in which zinc (Zn), having a lower oxidation-reduction potential than iron (Fe), corrodes first when exposed to a corrosive environment, thereby inhibiting the corrosion of the steel. Furthermore, as Zn, the main component of the plating layer, oxidizes, it forms dense corrosion products on the surface of the steel; these products protect the steel from oxidizing atmospheres, thereby improving the steel's corrosion resistance. Due to these advantageous characteristics, the application range of zinc-based galvanized steel has recently been expanding to include construction materials, home appliances, and automotive materials.

[0003] However, corrosive environments are gradually deteriorating due to increased air pollution resulting from the diversification and advancement of industries. As we face increasingly complex corrosive conditions, there is a growing need to develop steel materials that possess superior corrosion resistance in various environments compared to conventional galvanized steel sheets. Furthermore, with the recent strengthening of carbon neutrality policies leading to an increase in solar power generation as an eco-friendly energy source, power facilities are being constructed not only on land but also on coastlines and at sea; consequently, the corrosion resistance of steel materials used in these facilities is becoming even more critical.

[0004] [Prior Art Literature]

[0005] [Patent Literature]

[0006] (Patent Document 1) Japanese Published Patent Application No. 2000-219950

[0007] The problem that the technical concept of the present invention aims to solve is to provide a plated steel sheet having excellent processability while possessing ultra-high corrosion resistance suitable for use even in harsh corrosive environments such as marine environments, and a method for manufacturing the same.

[0008] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall details of the specification.

[0009] According to exemplary embodiments for solving the problem of the present invention, a plated steel sheet is provided. The plated steel sheet comprises a base steel sheet; and a Zn-Al-Mg-based plating layer disposed on the surface of the base steel sheet, wherein the Zn-Al-Mg-based plating layer comprises an Al phase and a MgZn2 phase, and the area fraction of a lamellar structure in which a first Al phase having an aspect ratio of 5 or more is arranged in layers on the surface of the Zn-Al-Mg-based plating layer is 10.0 to 80.0%.

[0010] In the above lamellar structure, when the spacing between the first Al phases is D (μm) and the average aspect ratio of the first Al phases is A, the following relationship 1 can be satisfied.

[0011] [Relationship 1]

[0012] D / A ≤ 0.25

[0013] The D / A value derived by the above relationship 1 may be 0.20 or less.

[0014] The above Zn-Al-Mg plating layer may comprise, on its surface, an Al phase of 15.0 to 80.0%, an MgZn2 phase of 15.0 to 75.0%, and a remainder of 40.0% or less (including 0%) in terms of area %.

[0015] The above Zn-Al-Mg-based plating layer may contain the above residual phase in an amount of 5.0% or less (including 0%).

[0016] The above Zn-Al-Mg plating layer may contain, in weight percent, aluminum (Al): 20.0~50.0%, magnesium (Mg): 5.0~22.0%, the remainder being zinc (Zn) and unavoidable impurities.

[0017] The weight ratio ([Mg] / [Al]) of the Mg content ([Mg]) and Al content ([Al]) of the Zn-Al-Mg plating layer may be 0.20 or more and less than 0.70.

[0018] The above Zn-Al-Mg-based plating layer may further include one or more elements selected from the following groups i) to iii).

[0019] i) Total content of elements derived from the base steel sheet: 1.000% or less

[0020] ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1.000% or less

[0021] iii) Total content of elements added to control the surface quality of galvanized steel sheets: 1,000% or less

[0022] The elements belonging to i) above are one or more of Si, Cr, Mn, Co, Ti, Ni, Fe, Cu, V, Nb, Mo, P, W, and B, the elements belonging to ii) above are one or more of Y, Zr, La, Ce, Ca, Sr, and Be, and the elements belonging to iii) above may be one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In.

[0023] The above-mentioned plated steel sheet may further include an interfacial alloy layer interposed between the Zn-Al-Mg-based plating layer and the base steel sheet.

[0024] The above interface alloy layer may include one or more alloy phases among FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al.

[0025] The thickness of the above interface alloy layer may be 0.001 to 3.000 μm.

[0026] The thickness of the above Zn-Al-Mg plating layer may be 5 to 60 μm.

[0027] According to exemplary embodiments of the present invention, a plated steel sheet having excellent processability and ultra-high corrosion resistance suitable for use even in harsh corrosive environments such as marine environments can be provided.

[0028] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0029] Figure 1 is an image of the surface of a plated steel sheet according to Invention Example 5, taken by setting the magnification of a scanning electron microscope (SEM) to approximately 2000 times.

[0030] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Prior to this, terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings. Instead, based on the principle that the inventor can appropriately define the concepts of terms to best describe his invention, they should be interpreted in a meaning and concept consistent with the technical spirit of the present invention.

[0031] In the following descriptions with reference to the drawings, identical or corresponding components are assigned the same reference numerals, and redundant descriptions thereof will be omitted.

[0032] In the following embodiments, the terms first, second, etc. are used not in a limiting sense, but for the purpose of distinguishing one component from another component.

[0033] In the following embodiments, the singular expression includes the plural expression unless the context clearly indicates otherwise.

[0034] In the following embodiments, terms such as "include" or "have" mean that the features or components described in the specification are present, and do not preclude the possibility that one or more other features or components may be added.

[0035] In the present invention, when indicating the concentration (content) of each element, it means weight % unless specifically otherwise determined.

[0036] In the drawings, the size of components may be exaggerated or reduced for convenience of explanation. For example, the size and thickness of each component shown in the drawings are depicted arbitrarily for convenience of explanation, so the present invention is not necessarily limited to what is illustrated.

[0037] Where an embodiment can be implemented differently, a specific process sequence may be performed differently from the order described. For example, two processes described consecutively may be performed substantially simultaneously or proceed in the reverse order of the description.

[0038] In addition, in describing the present invention, if it is determined that a detailed description of related known components or functions may obscure the essence of the invention, such detailed description is omitted.

[0039] The present invention will be described in detail below through each embodiment. It should be noted that each embodiment described in this specification is not limited to a single embodiment but may also be combined with other embodiments. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0040] The present invention will be described in detail below through examples. However, it should be noted that the following examples are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0041] [Plated Steel Sheet]

[0042] According to exemplary embodiments, the plated steel sheet may comprise a base steel sheet and a Zn-Al-Mg-based plating layer.

[0043] Any steel suitable for manufacturing galvanized steel sheets may be used as the base steel sheet. To give a non-limiting example, the base steel sheet may be carbon steel containing a certain amount of carbon (C), and various steels such as stainless steel and aluminum sheets may be applied. As an example, if the base steel sheet is carbon steel, it may be ultra-low carbon steel, medium-low carbon steel, low carbon steel, general carbon steel, high carbon steel, etc., which are well known in the steel industry, and all of these can exhibit similar effects when used as the base steel sheet for galvanized steel sheets. Therefore, the alloy composition of the carbon steel is not specifically limited. In particular, since there is almost no influence from elements such as manganese (Mn), silicon (Si), titanium (Ti), niobium (Nb), and boron (B), which are actively added to obtain carbon steel with high or ultra-high strength, no restrictions are placed on the alloy composition of the carbon steel.

[0044] The base steel sheet may be a hot-rolled steel sheet manufactured through a series of hot rolling processes, or a cold-rolled steel sheet manufactured through a series of cold rolling processes on the hot-rolled steel sheet. Additionally, the cold-rolled steel sheet may be an annealed steel sheet that has undergone an annealing process performed after the cold rolling, or an unanesthetized steel sheet that has not undergone the annealing process.

[0045] A Zn-Al-Mg-based plating layer can be disposed on the surface of a base steel sheet. A Zn-Al-Mg-based plating layer can be disposed on at least one surface of the base steel sheet. A Zn-Al-Mg-based plating layer can be disposed on both opposing surfaces of the base steel sheet.

[0046] According to exemplary embodiments, the thickness of the Zn-Al-Mg-based plating layer may be 5 to 60 μm. If the thickness of one side of the plating layer is less than 5 μm, it may be difficult to obtain the intended ultra-high corrosion resistance, and if it exceeds 60 μm, the adhesion of the plating to the substrate steel sheet is reduced, and problems such as the plating layer peeling off during processing may occur.

[0047] As the corrosive environments in which conventional galvanized steel sheets have been used have become increasingly harsh and complex, leading to a growing demand for galvanized steel sheets with ultra-high corrosion resistance, the inventors of the present invention have conducted in-depth research to provide galvanized steel sheets suitable for such needs. As a result, by optimizing the metal structure of the plating layer, it was confirmed that it is possible to provide a galvanized steel sheet capable of securing corrosion resistance not only in a general neutral atmosphere but also in acidic and alkaline atmospheres, leading to the completion of the present invention.

[0048] A Zn-Al-Mg plating layer can be formed using a molten plating bath containing elements such as aluminum (Al) and magnesium (Mg) in addition to zinc (Zn). As a result, the plated steel sheet can be provided with enhanced corrosion resistance in addition to the sacrificial protection provided by Zn by forming a unique microstructure. The Zn-Al-Mg plating layer may include an Al phase and a MgZn2 phase. The Al phase exhibits strong corrosion resistance in acidic and neutral environments, but its corrosion resistance may be relatively inferior in alkaline environments. Conversely, the MgZn2 phase exhibits strong corrosion resistance in alkaline and neutral environments, but its corrosion resistance may be relatively inferior in acidic environments. However, according to exemplary embodiments, the Zn-Al-Mg plating layer includes both the Al phase and the MgZn2 phase, thereby mutually compensating for the lack of corrosion resistance in various corrosive environments. Consequently, excellent corrosion resistance can be secured even in harsh corrosive environments such as marine environments.

[0049] In the present invention, the Al phase includes all forms of Al phases, such as primary Al phases and binary eutectic structures. That is, the Al phase may exist alone or may be provided in a form precipitated within another alloy phase. The MgZn2 phase is a phase of an intermetallic compound of Mg and Zn, and may exist alone or be provided in a form such as a eutectic structure with other phases.

[0050] When observed on the surface of a Zn-Al-Mg plating layer, the Al phase may include a first Al phase with an aspect ratio of 5 or more and a second Al phase with an aspect ratio of less than 5. As another example, when observed on the surface of a Zn-Al-Mg plating layer, the Al phase may include a first Al phase with an aspect ratio of 15 or more and a second Al phase with an aspect ratio of less than 15. Among these, the first Al phase may be arranged in layers on the matrix of the MgZn2 phase to form a lamellar structure.

[0051] According to exemplary embodiments, the area fraction of a lamellar structure, in which a first Al phase is arranged in layers on a matrix of MgZn2 on the surface of a Zn-Al-Mg plating layer, may be 10.0 to 80.0%. In this way, the slender first Al phase is arranged in layers, thereby preventing the penetration of corrosion agents and increasing diffusion paths, which can further improve corrosion resistance. To fully exhibit this effect, the area fraction of the lamellar structure may be 10.0% or more. More specifically, the area fraction of the lamellar structure may be 20.0% or more. To improve corrosion resistance in various environments, particularly in seawater atmospheres, it is desirable for the area fraction of the lamellar structure to be higher. However, since there is a large difference in hardness between the Al phase and the MgZn2 phase, there is a risk that cracks may easily propagate along the boundary between the Al phase and the MgZn2 phase during processing, potentially causing damage to the plated steel sheet. Therefore, the area fraction of the lamellar structure may be 80.0% or less. More specifically, the area fraction of the lamellar structure may be 71% or less. The remainder other than the lamellar structure may be a second Al phase, an MgZn2 phase, a first Al phase not arranged in layers, and a remainder phase, but the present invention is not necessarily limited thereto. The description of the remainder phase is replaced by the description given below.

[0052] In the present invention, the statement that the first Al phase is arranged in layers means that the first Al phases having an extremely elongated shape with an aspect ratio of 5 or more are arranged substantially parallel to each other. As a non-limiting example, the statement that the first Al phase is arranged in layers may mean that the angle formed by the major axes of two adjacent first Al phases on the surface of the Zn-Al-Mg plating layer is 0 to 45°.

[0053] In the present invention, the aspect ratio of the Al surface refers to the ratio of the major axis to the minor axis of the Al surface (major axis length / minor axis length). The major axis and minor axis of the Al surface can be represented by drawing an ellipse that circumscribes the Al surface, and the major axis and minor axis of said ellipse can be considered as the major axis and minor axis of the Al surface.

[0054] According to exemplary embodiments, in a lamellar structure, when the average spacing between the first Al phases is D (μm) and the average aspect ratio of the first Al phases is A, the following relationship 1 can be satisfied.

[0055] [Relationship 1]

[0056] D / A ≤ 0.25

[0057] If the average spacing of the first Al phase becomes excessively large or the average aspect ratio of the first Al phase becomes excessively small, excessive cracking may occur at the interface between the first Al phase and the MgZn2 phase within the lamellar structure during processing of the plated steel sheet. As a result, the corrosion resistance of the processed part may be reduced during processing of the plated steel sheet. Furthermore, if the above-described relationship Equation 1 is not satisfied, there is a risk that the characteristics of the lamellar structure will disappear and the corrosion resistance will become inferior. As another example, the D / A value derived by the above relationship Equation 1 may be 0.20 or less.

[0058] As a non-limiting example, the average spacing of the first Al may be 4.00 μm or less, but the present invention is not necessarily limited thereto.

[0059] According to exemplary embodiments, the surface of the Zn-Al-Mg plating layer may contain, in area %, an Al phase: 15.0 to 80.0%, a MgZn2 phase: 15.0 to 75.0%, and a remainder phase: 40.0% or less (including 0%). In this case, the area fractions of the Al phase and the MgZn2 phase are fractions that appear when observing the surface of the Zn-Al-Mg plating layer as a whole, and include the Al phase and the MgZn2 phase constituting the lamellar structure described above.

[0060] Al phase: 15.0~80.0%

[0061] Since the Al phase exhibits excellent corrosion resistance in the region where the pH of the corrosive medium is neutral to weakly acidic, the area fraction of the Al phase may be 15.0% or more. More specifically, the area fraction of the Al phase may be 30.0% or more or 34.0% or more. However, since the Al phase corrodes relatively quickly in an alkaline atmosphere, there is a concern that if the area fraction of the Al phase is excessively high, it may be difficult to exhibit corrosion resistance in an alkaline environment. To prevent this, the area fraction of the Al phase may be 80.0% or less. More specifically, the area fraction of the Al phase may be 70% or less. Even more specifically, the area fraction of the Al phase may be 65.0% or less.

[0062] MgZn2 phase: 15.0~75.0%

[0063] Since the MgZn2 phase can exhibit excellent corrosion resistance in regions where the pH of the corrosive medium is neutral or alkaline, the area fraction of the MgZn2 phase may be 15.0% or more. More specifically, the area fraction of the MgZn2 phase may be 20% or more or 30% or more. Even more specifically, the area fraction of the MgZn2 phase may be 40% or more. As such, according to exemplary embodiments, the corrosion resistance of each metal structure can be mutually complemented by including both the MgZn2 phase and the Al phase. In particular, corrosion resistance can be further improved by forming a lamellar structure, which is a dense layered structure with the Al phase, within the aforementioned range. As a result, the Zn-Al-Mg plated steel sheet can possess excellent corrosion resistance in various environments. On the other hand, the MgZn2 phase can corrode relatively easily in acidic environments. Therefore, there is a concern that if the area fraction of the MgZn2 phase is excessively high, sufficient corrosion resistance may not be secured in acidic environments. To prevent this, the area fraction of the MgZn2 phase may be 75.0% or less. More specifically, the area fraction of the MgZn2 phase may be 66% or less.

[0064] Remaining balance: 40.0% or less (including 0%)

[0065] The Zn-Al-Mg plating layer may further include alloy phases resulting from the combination of Zn, Al, Mg, and other metal components within the Zn-Al-Mg plating layer as residual phases, in addition to the Al phase and the MgZn2 phase. The residual phase may be a Zn phase or a Zn-Al phase, but the present invention is not necessarily limited thereto. Depending on the composition of the Zn-Al-Mg plating layer, the residual phase may further include any one of the Mg-Si phase, the Al-Zn-Ca phase, or the Al-Si-Ca-Zn phase. That is, the residual phase may be one or more of the Zn phase, the Zn-Al phase, the Mg-Si phase, the Al-Zn-Ca phase, and the Al-Si-Ca-Zn phase, but the present invention is not necessarily limited thereto. The Mg-Si phase may typically be the Mg2Si phase, and the Al-Zn-Ca phase may typically be Al 2.94 CaZn 1.06 It can be an award.

[0066] The residual phase not only fails to significantly contribute to the improvement of corrosion resistance, but the Zn phase, Zn-Al phase, etc., can actually reduce corrosion resistance in various corrosive environments. The Mg2Si phase is a brittle phase that is mainly distributed in the surface layer of the Zn-Al-Mg plating layer and can act as a site for crack initiation. Therefore, it is desirable to suppress the residual phase, and its fraction may be 0%. As another example, on the surface of the Zn-Al-Mg plating layer, the residual phase may be included in an amount of 5.0% or less (including 0%).

[0067] The metallographic phases described above in the present invention can be identified using X-ray diffraction patterns. As one example, the composition of the Al phase may be pure Al, but may include less than 30% Zn and less than 10% Mg in terms of atomic ratio (At%). Even if components within the above content range are included, the metallurgical crystal characteristics of the Al phase are not lost, so it is detected as the Al phase in the X-ray diffraction pattern and conforms to the quality characteristics of the Al phase of the present invention. As another example, less than 5% Al in terms of atomic ratio may be included within the MgZn2 phase, but because it possesses the unique metallurgical crystal characteristics of the MgZn2 phase, it is detected as the MgZn2 phase in the X-ray diffraction pattern and conforms to the quality characteristics of the MgZn2 phase of the present invention.

[0068] According to exemplary embodiments, the plated steel sheet may further include an interfacial alloy layer interposed between the Zn-Al-Mg-based plating layer and the substrate steel sheet. The interfacial alloy layer may be formed as Fe leached from the substrate steel sheet reacts with Al in the plating bath during the plating layer formation process. This can further increase the adhesion between the substrate steel sheet and the Zn-Al-Mg-based plating layer.

[0069] According to exemplary embodiments, the thickness of the interfacial alloy layer may be 0.001 to 3.000 μm. If the thickness of the interfacial alloy layer is less than 0.001 μm, the adhesion between the substrate steel sheet and the Zn-Al-Mg plating layer is not sufficiently high, which may cause problems such as the plating layer detaching during processing of the plated steel sheet. Therefore, the thickness of the interfacial alloy layer may be 0.001 μm or more. More specifically, it may be 0.005 μm or more. If the thickness of the interfacial alloy layer exceeds 3.000 μm, the adhesion between the substrate steel sheet and the Zn-Al-Mg plating layer may be excellent, but due to the high hardness of the interfacial alloy layer, the likelihood of powdering occurring during processing may increase. Therefore, the thickness of the interfacial alloy layer may be 3.000 μm or less. More specifically, it may be 2.500 μm or less or 2.00 μm or less.

[0070] There are no specific limitations on the alloy phases constituting the interfacial alloy layer. As a non-limiting example, the interfacial alloy layer may include one or more alloy phases among FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al. Additionally, depending on the alloy composition of the Zn-Al-Mg plating layer, one or more of Zn, Mg, and metal components within the Zn-Al-Mg plating layer may be additionally contained within the alloy phases, and even in such cases, there is no difficulty in ensuring adhesion between the substrate steel sheet and the alloy plating layer.

[0071] According to exemplary embodiments, the Zn-Al-Mg plating layer may comprise, in weight percent, aluminum (Al): 20.0 to 50.0%, magnesium (Mg): 5.0 to 20.0%, the remainder being zinc (Zn) and unavoidable impurities.

[0072] Aluminum (Al): 20.0~50.0%

[0073] Aluminum (Al) is an element that contributes to improving corrosion resistance and possesses particularly strong properties against acids. Additionally, Al can contribute to suppressing the formation of MgO-based dross by inhibiting the oxidation of Mg within the plating bath. Therefore, the Al content may be 20.0% or higher. More specifically, the Al content may be 22.0%. However, if the Al content is excessively high, there is a concern that corrosion in alkaline environments may increase, and it may raise the melting point of the plating bath. In this case, the temperature of the plating bath must be maintained high during the plating process, which may intensify the erosion of structures within the plating bath. Furthermore, the aforementioned interfacial alloy layer may be formed excessively thickly, leading to powdering where the plating layer detaches during processing. Therefore, the Al content may be 50.0% or lower. More specifically, the Al content may be 40.0% or lower.

[0074] Magnesium (Mg): 5.0~22.0%

[0075] Magnesium (Mg) is an element that contributes to improving corrosion resistance and possesses strong properties, particularly in neutral and alkaline environments. Consequently, it can improve the corrosion resistance of galvanized steel sheets in neutral seawater environments, such as marine environments, and exhibits excellent corrosion resistance even in alkaline corrosive environments like concrete. If the Mg content is less than 5.0%, it may be difficult to secure the level of ultra-high corrosion resistance required in marine environments. Therefore, the Mg content may be 5.0% or higher. More specifically, the Mg content may be 6.0% or higher, or 7.0% or higher. However, if the Mg content exceeds 22.0%, it becomes difficult to use in structures in industrial areas where acid rain occurs, as Mg is susceptible to acid corrosion. Furthermore, as Mg is an element with a high affinity for oxygen, it can oxidize to MgO on the surface of the plating bath in contact with air, forming dross and impairing plating performance. Therefore, the Mg content may be 22.0% or lower. More specifically, the Mg content may be 15.0% or less.

[0076] Ratio of Mg content ([Mg]) to Al content ([Al]) ([Mg] / [Al]): 0.20 or more and less than 0.70

[0077] Since Mg and Al in the Zn-Al-Mg plating layer possess mutually complementary corrosion resistance characteristics, uniform corrosion resistance can be imparted in all corrosive environments, including acidic, neutral, and alkaline conditions, by controlling the ratio of Mg content ([Mg]) to Al content ([Al]) ([Mg] / [Al]; hereinafter Mg / Al). Even if the Mg and Al content satisfy the ranges proposed in this invention, if Mg / Al is excessively low, there is a concern that corrosion resistance may be reduced in alkaline corrosive environments. Conversely, if Mg / Al is excessively high, there is a concern that corrosion resistance may be inferior in acidic corrosive environments. Furthermore, if Mg / Al is excessively high, the MgZn2 fraction in the Zn-Al-Mg plating layer increases excessively, leading to an increase in the hardness of the Zn-Al-Mg plating layer, which causes cracks to occur during processing and consequently reduces processing corrosion resistance. Therefore, Mg / Al may be between 0.20 and less than 0.70. More specifically, Mg / Al can be in the range of 0.25 to 0.50 or 0.2 to 0.58.

[0078] The Zn-Al-Mg plating layer contains Zn and unavoidable impurities as residual components in addition to the aforementioned elements. The Zn is an important element for ensuring the corrosion resistance of the alloy-plated steel sheet, along with the Al and Mg, and the Zn content can be adjusted to match the content of the aforementioned Al and Mg. The Zn-Al-Mg plating layer may contain other unavoidable impurities in addition to the aforementioned alloy composition. Since such impurities can be unintentionally incorporated from raw materials or the surrounding environment during the conventional steel manufacturing process, they cannot be completely eliminated. As any skilled technician in the conventional steel manufacturing process would be aware of these impurities, the present invention does not specifically mention all such details.

[0079] Optionally, the plating layer of the present invention may further include calcium (Ca): 0.001 to 0.500%.

[0080] Optionally, the Zn-Al-Mg-based plating layer may further include one or more elements selected from the following groups i) to iii). However, since the elements of each of the following groups are not essential for achieving the objectives of the present invention, the lower limit of their content is not restricted. Accordingly, the lower limit of the content of each element may be 0%, even if not specifically mentioned below. The elements of each of the following groups may be intentionally added to the plating bath or derived from the base steel sheet, but the present invention is not necessarily limited thereto.

[0081] i) Total content of elements derived from the base steel sheet: 1.000% or less

[0082] ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1.000% or less

[0083] iii) Total content of elements added to control the surface quality of galvanized steel sheets: 1,000% or less

[0084] The elements of i) above may contribute to the formation of corrosion resistance or contribute to grain stabilization. However, if the total content of these elements exceeds 1.000%, the surface properties of the plated steel sheet may deteriorate. As a specific example, the elements belonging to group i) above may be one or more of Si, Cr, Mn, Co, Ti, Ni, Fe, Cu, V, Nb, Mo, P, W, and B.

[0085] The elements of ii) above can suppress the formation of Mg oxide in the plating bath and contribute to the formation of corrosion resistance of the plated steel sheet. However, if the total content of these elements exceeds 1.000%, the brittleness of the plating layer may increase. As a specific example, the elements belonging to group ii) above may be one or more of Y, Zr, La, Ce, Ca, Sr, and Be.

[0086] The elements of iii) above are elements that affect the size of spangles, which are solidification bodies of the Zn-Al-Mg plating layer. However, if the total content of these elements exceeds 1.000%, there is a risk of discoloration of the Zn-Al-Mg plating layer in a humid environment. As a specific example, the elements belonging to group iii) above may be one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In.

[0087] Furthermore, the Zn-Al-Mg-based plating layer may include additional elements other than the alloying elements described above within the scope of the technical concept of the present invention, and the route of addition of these elements (addition to the plating bath, origin from the base steel sheet, etc.) is not particularly limited.

[0088] [Method for manufacturing galvanized steel sheets]

[0089] According to exemplary embodiments, a method for manufacturing a plated steel sheet may include the steps of temper rolling; plating; controlling the amount of plating; and cooling.

[0090] However, it should be noted that the following method is included in exemplary embodiments for manufacturing Zn-Al-Mg plated steel sheets, and that the Zn-Al-Mg plated steel sheets according to the exemplary embodiments must not necessarily be manufactured by the present manufacturing method, and that any manufacturing method that satisfies the claims of the present invention may be used to implement each embodiment of the present invention without any problem.

[0091] Temper rolling stage

[0092] By performing pre-plating temper rolling (SPM, Skin Pass mill, hereinafter referred to as SPM) on the substrate steel sheet, the surface shape of the substrate steel sheet can be uniformly controlled, thereby enabling uniform control of the thickness of the molten plating layer formed by the subsequent plating process. At the same time, the surface roughness (R) of the substrate steel sheet maxBy reducing ), the flat surface of the substrate steel sheet can be made smooth. As a result, during the solidification process of the plating melt, solidification nucleation sites are homogenized, causing the Al phase and MgZn2 phase to precipitate right next to each other, which can contribute to forming the surface lamellar structure of the Zn-Al-Mg plating layer proposed in the present invention.

[0093] Refer to the detailed description regarding the base steel plate. That is, the type and method of providing the base steel plate can be appropriately selected within the scope that does not deviate from the technical concept of the present invention.

[0094] Temper rolling of the base steel sheet can be performed so that the surface roughness (Rmax, μm) of the temper-rolled base steel sheet is 2.00 μm or less. If the surface roughness of the base steel sheet exceeds 2.00 μm, the plating layer is not formed uniformly in the subsequent plating process, making it difficult to provide the surface lamellar structure of the Zn-Al-Mg-based plating layer proposed in the present invention. Although the lower limit of the surface roughness of the temper-rolled base steel sheet is not particularly limited, the temper rolling can be performed so that the surface roughness of the temper-rolled base steel sheet is 0.80 μm or more.

[0095] As one example, when the base steel sheet is a hot-rolled steel sheet, SPM treatment can be performed to control the surface roughness immediately after the pickling and rinsing processes to remove oxide scale present on the surface of the hot-rolled steel sheet. As another example, when the base steel sheet is a cold-rolled steel sheet, SPM treatment can be performed during the final stage of cold rolling, specifically by performing temper rolling. That is, for hot-rolled steel sheets, the surface roughness and rolling force of the SPM roll are controlled in a separate SPM process after pickling and rinsing, whereas for cold-rolled steel sheets, the surface roughness of the base steel sheet is controlled by adjusting the surface roughness and rolling force of the final roll of cold rolling.

[0096] When performing SPM processing, if the surface roughness of the SPM roll is less than 0.20㎛, problems may arise in the production and management of the roll. If it exceeds 0.40㎛, it is difficult to sufficiently lower the surface roughness of the substrate steel sheet, which inhibits the formation of lamellar structures during the solidification process after plating, thereby making it difficult to secure the target lamellar structure fraction. Therefore, according to exemplary embodiments, the surface roughness of the SPM roll may be 0.20 to 0.40㎛.

[0097] In addition, if the roll rolling force is less than 150 tons, the effect of controlling the surface roughness of the base steel sheet is low, and the effect of homogenizing the surface roughness after pickling hot-rolled steel sheets or after cold-rolling cold-rolled steel sheets may be reduced. If the roll rolling force exceeds 300 tons, there is a risk of inducing C-curvature, etc. Therefore, according to exemplary embodiments, the rolling force of the SPM roll may be 150 to 300 tons.

[0098] According to exemplary embodiments, the temper rolling step may have a Q value defined by the following Equation 2 in the range of 0 to 0.85.

[0099] [Relationship 2]

[0100] Q = ((0.4-R) / 0.2)*((P-150) / 150)

[0101] In the above Equation 2, R represents the surface roughness (μm) of the SPM roll, and P represents the rolling force (ton) of the SPM roll. In this way, by appropriately controlling the rolling force according to the surface roughness of the SPM roll, the surface control of the substrate steel sheet can be performed more precisely. Consequently, the plating layer can be formed more uniformly in the subsequent plating process, allowing the average spacing and average aspect ratio of the first Al phase within the lamellar structure to be controlled within an appropriate range. As a result, the corrosion resistance of the processed part of the plated steel sheet can be further improved. This can contribute to obtaining the surface structure of the Zn-Al-Mg-based plating layer proposed in the present invention. Since the above Equation 2 is an empirically obtained value, a separate unit may not be defined, and it is sufficient to satisfy only the unit of each variable. As another example, the above Q value may be in the range of 0.100 to 0.850.

[0102] plating step

[0103] A temper-rolled steel sheet can be plated by immersing it in a Zn-Al-Mg-based plating bath.

[0104] The Zn-Al-Mg plating bath may contain, in weight percent, aluminum (Al): 20.0~50.0%, magnesium (Mg): 5.0~22.0%, and the remainder being zinc (Zn) and unavoidable impurities. For the content of each component of the Zn-Al-Mg plating bath, one may refer to the description of the Zn-Al-Mg plating layer described above. Additionally, during the process of immersing the substrate steel plate in the Zn-Al-Mg plating bath to perform plating, Fe leached from the substrate steel plate may be present in the plating bath. At this time, the content of Fe present in the plating bath is not specifically limited, and it is acceptable as long as the Fe is present in the formed Zn-Mg-Al plating layer at a level of 0.300% or less.

[0105] Meanwhile, the temperature of the plating bath is determined by the composition within the plating bath. If the temperature of the plating bath is maintained at a level similar to the melting point, the plating layer may solidify during the wiping process to control the plating adhesion amount after plating, making it difficult to control the adhesion amount. On the other hand, the higher the temperature of the plating bath is above the melting point, the better the fluidity of the plating bath becomes, allowing for a smooth plating layer to be obtained after plating. However, if the temperature becomes excessively high, components within the substrate steel sheet may leach into the plating bath during the plating process, resulting in a large amount of dross. Furthermore, as the interfacial alloy layer formed between the substrate steel sheet and the plating layer becomes too thick, plating layer powdering may easily occur during subsequent processing. Moreover, the higher the temperature of the plating bath, the more severe the erosion of structures such as sink rolls provided within the plating bath may become. Therefore, according to exemplary embodiments, the temperature of the Zn-Al-Mg plating bath may be +70°C or lower relative to the melting point depending on the plating bath components. More specifically, the temperature of the Zn-Al-Mg plating bath can be 450 to 650°C.

[0106] Optionally, according to exemplary embodiments, the SPM-treated steel sheet may undergo heat treatment prior to plating. The process of heating the steel sheet may be carried out by passing it through a heat treatment furnace (e.g., a non-oxidizing furnace or a reducing furnace). The temperature at which the steel sheet is heated may be equal to the temperature of the Zn-Al-Mg plating bath or set to a temperature higher than the temperature of the plating bath, and the temperature is not specifically limited. As one example, it may be set at least 10°C higher than the temperature of the plating bath. As another example, the steel sheet may be heated to about 600°C or higher and then cooled to an appropriate temperature before immersion in the plating bath. As yet another example, if the steel sheet is an un-tendered cold-rolled steel sheet, the steel sheet may be degreased, washed, and dried, then loaded into an annealing furnace controlled by a reducing atmosphere to perform annealing heat treatment, and then cooled to an appropriate temperature. The annealing heat treatment conditions at this time are not specifically limited and may be conditions for annealing conventional cold-rolled steel sheets, and may be an annealing furnace maintained in a reducing atmosphere by controlling the dew point temperature.

[0107] Step to control the plating amount

[0108] The plating amount can be controlled by performing a gas wiping treatment after plating. The gas wiping treatment can be performed immediately after the plated substrate steel sheet exits the plating bath, and can be carried out using commonly used air or nitrogen gas (N2 gas). However, if gas wiping is performed using air on a plating layer containing regions of unsolidified state with high Mg content, Mg and Al within the unsolidified regions may oxidize, causing the surface of the plating layer to become rough, increasing surface roughness, and forming a thick oxide layer on the surface of the plating layer. In this case, the corrosion resistance of the plated steel sheet may be reduced. Therefore, according to exemplary embodiments, the gas wiping treatment can be performed using nitrogen gas. By using nitrogen gas, the formation of a thick oxide layer on the surface of the Zn-Al-Mg plating layer can be suppressed even if it inevitably comes into contact with air with a high oxygen concentration. When performing gas wiping treatment using the above nitrogen gas, nitrogen gas at room temperature may be used, or if the surface quality of the plating layer is to be controlled more strictly, the nitrogen gas may be heated and used.

[0109] Meanwhile, when a large amount of Mg is contained in the Zn-Al-Mg plating bath, even if a wiping treatment using nitrogen gas is performed after plating, a thick oxide layer may form on the surface of the plated steel sheet if it comes into contact with air with a high oxygen concentration. Accordingly, according to exemplary embodiments, the gas wiping treatment using nitrogen gas can be performed in a sealing box.

[0110] According to exemplary embodiments, the sealing box may be positioned from the surface of the Zn-Al-Mg plating bath (where the bottom of the sealing box is in contact with the surface of the plating bath) to the point where the gas wiping means for gas wiping treatment ends. That is, the steel sheet plated in the Zn-Al-Mg plating bath can be loaded into the sealing box without coming into contact with air when withdrawn from the plating bath. From this, contact with air with a high oxygen concentration can be prevented. Furthermore, by performing gas wiping treatment inside the sealing box, contact between the plated steel sheet and air with a high oxygen concentration before and after the gas wiping treatment can be fundamentally prevented. More specifically, when gas wiping treatment using nitrogen gas is performed inside the sealing box, the interior of the sealing box becomes a static pressure state (a state where the pressure inside is higher than the pressure outside the sealing box), thereby suppressing the inflow of external air.

[0111] Meanwhile, although air can be blocked from the surface of the Zn-Al-Mg plating bath to the point where gas wiping ends by using a sealing box as described above, it is not possible to completely block the inflow of air from the outside due to situations such as the need to open and close the sealing box after gas wiping. In this case, the oxygen concentration inside the sealing box may increase, and consequently, there is a concern that it may be difficult to obtain the effect of the sealing box. Accordingly, according to exemplary embodiments, the interior of the sealing box can be controlled to a non-oxidizing atmosphere. More specifically, the oxygen concentration inside the sealing box may be 3.00% or less. At this time, the control of the oxygen concentration can be achieved by supplying a non-oxidizing gas (e.g., N2 gas, etc.) into the sealing box through a nozzle provided in the sealing box. If the oxygen concentration inside the sealing box exceeds 3.0%, a thick oxide layer is formed on the surface of the steel plate, increasing the surface roughness, and the corrosion resistance may also be reduced due to the oxide. The oxygen concentration inside the sealing box may be, more specifically, 2.00% or less, or 1.00% or less.

[0112] The adhesion amount controlled by gas wiping treatment is not specifically limited, and it is sufficient if the thickness of the Zn-Al-Mg plating layer on the final Zn-Al-Mg plated steel sheet satisfies 5 to 60 µm. As one example, the said adhesion amount is 25 to 350 g / m² on a single side. 2 It could be.

[0113] Cooling stage

[0114] Cooling can be performed in four stages as follows, based on the solidification start temperature (A, °C) and solidification end temperature (B, °C) of the Zn-Al-Mg plating layer. Hereinafter, the cooling rate mentioned in each stage refers to the average cooling rate in the corresponding temperature range. Meanwhile, the method for measuring the solidification start temperature (A) and solidification end temperature (B) of the Zn-Al-Mg plating layer is not specifically limited. However, as a non-limiting example, the solidification start temperature and solidification end temperature of the plating composition can be determined from the differential scanning calorimetry method using a differential scanning calorimeter (DSC). As one example, when a plating bath sample is placed in a differential scanning calorimeter (DSC), heated above the melting point of the plating bath, and then cooled to near room temperature at a cooling rate of 5 °C / min, a differential heat peak occurs according to the latent heat of solidification, and at this time, the first peak corresponds to the solidification start temperature and the last peak corresponds to the solidification end temperature.

[0115] Stage 1: Cooling rate of 6.0℃ / s or higher until A(℃)

[0116] Phase 2: From A(°C) to before B(°C), cooling rate of 5.0–14.0°C / s

[0117] Stage 3: From B(°C) to before water cooling, cooling rate of 5.0°C / s or higher

[0118] Phase 4: Initiate water cooling at 250℃ or below

[0119] According to exemplary embodiments, the first stage of cooling may be performed immediately after the gas wiping process. It may be performed from the temperature immediately after the end of the gas wiping process to the solidification start temperature (A, °C) of the Zn-Al-Mg plating layer. In this section, it is necessary to perform cooling at a rapid rate so that the Zn-Al-Mg plating layer, which is still in a liquid state, does not flow down, and considering this, it may be performed at a cooling rate of at least 6.0 °C / s. If the cooling rate of the first stage is less than 6.0 °C / s, the Zn-Al-Mg plating layer in a liquid state may flow down, and flow pattern defects may occur on the surface of the final Zn-Al-Mg plated steel sheet. The upper limit of the cooling rate of the first stage is not specifically limited, but it may be performed at a maximum of 30.0 °C / s considering the specifications of the cooling equipment, etc.

[0120] According to exemplary embodiments, the temperature range in which the second stage of cooling is performed is from the solidification start temperature (A) of the Zn-Al-Mg plating layer to the solidification end temperature (B), and an alloy phase is formed in this range. If the cooling rate is too fast in the temperature range where the alloy phase is formed, it may be difficult to satisfy the alloy phase fraction proposed in the present invention. More specifically, in the present invention, the Zn phase is limited to a maximum of 2% or less because it does not exhibit excellent corrosion resistance in a weakly acidic atmosphere and a seawater atmosphere (neutral); however, if the second stage cooling rate exceeds 14.0°C / s, the fraction of the MgZn2 phase decreases, and the fraction of the Zn phase may exceed 2%. If the second stage cooling rate is less than 5.0°C / s, there is no difficulty in forming the alloy phase fraction proposed in the present invention, but productivity may be disadvantageous, such as due to cooling being performed for a long time. Therefore, the second stage cooling rate can be limited to 5.0 to 14.0°C / s.

[0121] After the cooling of the second stage, that is, after the solidification of the molten Zn-Al-Mg plating layer is complete, it may be advantageous to have a faster cooling rate. Generally, immediately after the plating layer solidifies, it is in a solid state but has low hardness, so it is prone to indentations, etc., forming on the surface of the steel plate—that is, on the surface of the solidified plating layer—due to the rolls passing through it during the process of transporting the steel plate for subsequent processes. Therefore, it may be advantageous to increase the hardness of the plating layer by cooling it relatively quickly after the solidification of the plating layer is complete. Accordingly, according to exemplary embodiments, the cooling of the third stage can be performed at a cooling rate of 5.0°C / s or higher. At this time, to more advantageously increase the cooling rate, air mist, etc., may be used. Although the upper limit of the cooling rate of the third stage is not specifically limited, it may be performed at a maximum of 70.0°C / s considering the specifications of the cooling equipment, etc.

[0122] According to exemplary embodiments, the cooling of the third stage may be performed from the solidification end temperature (B) of the Zn-Al-Mg plating layer, which is the end temperature of the second stage, and may be performed up to the point immediately before the water cooling of the fourth stage is performed. Accordingly, the end temperature of the cooling of the third stage is not specifically set. As one example, the cooling of the fourth stage described below may be started at 250°C or lower, and the cooling of the third stage may be performed up to that starting temperature.

[0123] According to exemplary embodiments, a fourth stage of cooling, corresponding to water cooling, may be performed after the third stage of cooling. The water cooling may be performed by passing the Zn-Al-Mg plated steel sheet, which has completed the third stage of cooling, through a water cooling tank containing water (e.g., tap water), or by spraying the water or purified water onto the plated steel sheet. Through the water cooling process, the temperature of the Zn-Al-Mg plated steel sheet can be rapidly cooled to near room temperature, thereby achieving the effect of reducing the manufacturing time of the final plating material, i.e., the final Zn-Al-Mg plated steel sheet. The cooling rate of the fourth stage of water cooling is not specifically limited, but as a non-limiting example, it may be performed at a cooling rate of 30 to 100°C / s.

[0124] If the temperature of the Zn-Al-Mg plated steel sheet is too high at the time when the fourth stage of cooling begins, defects such as warping of the plated steel sheet may occur. Taking this into consideration, the temperature of the plated steel sheet at the time when the fourth stage of cooling begins may be 250°C or lower.

[0125] [Example]

[0126] Test Example 1: Verification of corrosion resistance according to the area fraction of the lamellar structure

[0127] First, the composition of the plating bath was designed to have a plating layer composition as shown in Table 1 below, and then the prepared steel sheets were treated with plating using each plating bath. As for the steel sheets, a hot-rolled steel sheet composed of, in weight%, C: about 0.011%, Si: about 0.018%, Mn: about 0.45%, P: about 0.010%, S: about 0.005%, Al: about 0.022%, Nb: about 0.01%, Cr: about 0.005%, Ti: about 0.02%, B: about 0.007%, and the remainder being Fe and other unavoidable impurities, and a cold-rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet were used.

[0128] At this time, when using hot-rolled steel sheets, pickling treatment using an aqueous hydrochloric acid solution was performed before plating treatment to remove iron oxides remaining on the surface of the hot-rolled steel sheets, and then temper rolling treatment was performed under the conditions shown in Table 2 below, and specimens of the base steel sheets were taken to measure the surface roughness of the base steel sheets. Subsequently, the sheets were charged into a heating furnace with a dew point temperature of -35℃ and heated to 650℃, after which a subsequent alloy plating process was carried out.

[0129] Meanwhile, when using cold-rolled steel sheets, the final roll roughness and rolling force of the cold rolling process were carried out under the conditions shown in Table 2, and then, in order to remove rolling oil, iron powder, and other foreign substances remaining on the surface before plating treatment, the cold-rolled steel sheets were immersed in an alkaline electrolyte and subjected to electrolytic degreasing, then charged into an annealing furnace in a reducing atmosphere with a dew point temperature of -40℃ and heat-treated at 820℃, after which a subsequent plating process was carried out.

[0130] In performing the alloy plating process on each of the above-mentioned substrate steel sheets, the temperature of the substrate steel sheet immersed in the plating bath was standardized to +10℃ relative to the temperature of the plating bath, and the adhesion amount during wiping treatment after the completion of alloy plating was 150g / m² per side. 2 It was matched identically.

[0131] In addition, the temperature of the base steel plate at the time when the fourth stage of cooling (water cooling) begins in the cooling process after the above wiping treatment was controlled to be 250℃ or lower.

[0132] Plating Bath Number Plating Layer Composition (Wet%, remainder Zn and unavoidable impurities) AlMgMg / Al Ratio SiCaSnSrCeMn 131 11.4 0.3 70.01 1.5 00--0.002-226 12.5 0.4 80.1 0.1 00 0.010--0.03 330 9.2 0.3 10.02 0.2 00---0.010 425 13.8 0.5 50.2 0.1 00-0.001--537 7.3 0.2 0.1 20.5 00 0.010-0.010-616.3 5.1 0.3 10.2 90.2 00-0.015--731 11.4 0.3 70.01 1.5 00--0.002-

[0133] Temper Rolling Conditions Steel Sheet Surface Roughness After Temper Rolling (Rmax, μm) Plating Bath Conditions Wiping Sealing Box Cooling Conditions Classification Roll Roughness (Ra, μm) Reduction Force (ton) Composition (Table 1) Temperature (°C) Gas Type Oxygen / Oxygen Concentration (Vol%) 1st Stage Speed ​​(°C / s) 2nd Stage Speed ​​(°C / s) 3rd Stage Speed ​​(°C / s) 0.38 16 71.45 15 40 N2 Oil 0.94 11.19.3 15.2 Invention Example 10.2 22 870.99 25 20 N2 Oil 1.10 8.38.17.9 Invention Example 20.2 8 210 1.25 35 21 N2 Oil 0.25 9.67.3 15.5 Invention Example 30.282651.044509N2U0.7410.411.715.2 Inventive Example 40.282651.083521N2U0.118.49.217.1 Inventive Example 50.381072.985533N2U1.469.78.215.1 Comparative Example 1--3.356469N2U0.697.59.413.9 Comparative Example 20.681912.717518N2U0.7510.77.918.4 Comparative Example 3--3.293521N2U0.2412.811.017.7 Comparative Example 4

[0134] Subsequently, each plated steel sheet was cut to a predetermined size, the plating layer was dissolved, and the composition of the plating layer was confirmed by analyzing each component using ICP (Ion Coupled Plasma Spectrometry). It was confirmed that the plating layer compositions of each inventive example and comparative example were substantially identical to the composition of the plating bath. To confirm the surface structure of the plating layer of the plated steel sheet, specimens were prepared by cutting each plated steel sheet in the thickness direction to a size suitable for measuring XRD diffraction patterns, and XRD analysis was performed on each specimen to determine which phases (alloy phases) were present.

[0135] In addition, the surface of the same specimen was photographed using a Field Emission Scanning Electron Microscope (FE-SEM), and a compositional analysis was performed using Energy Dispersive X-ray Spectroscopy (EDS) to confirm how the phases identified in the XRD were distributed in the FE-SEM image.

[0136] At this time, for each Al phase observed in the FE-SEM image, an ellipse circumscribed around the contour of the Al phase was set, and the ratio of the major axis to the minor axis of the ellipse (hereinafter referred to as the "aspect ratio") was calculated. Based on the calculated aspect ratio, Al phases with an aspect ratio of 5 or more were classified as the first Al phase, and Al phases with an aspect ratio of less than 5 were classified as the second Al phase.

[0137] Next, the lamellar structure was defined as follows. More specifically, the lamellar structure was defined by setting two endpoints on each first Al with an aspect ratio of 5 or more, and then sequentially connecting the outermost points among these two endpoints along the lamellar extension direction with a straight line. Here, the "two endpoints" on each first Al were set as the two points among multiple points located inside the first Al that have the maximum distance from each other. In addition, when sequentially connecting the two endpoints, connections were not considered to form a connecting line that is in the reverse direction (retreating direction) relative to the already formed direction of progress. Here, the reverse direction refers to a direction in which the angle formed with the direction of progress of the previous connecting line is within 90 degrees. In other words, when drawing a connecting line (new connecting line) from the last endpoint where a connecting line was formed to the next endpoint (new endpoint), if the new connecting line is formed in the reverse direction, it is not connected to that new endpoint, but rather connected to the next closest endpoint to form the connecting line. If the connecting line to the next nearest endpoint is also formed in the reverse direction, it naturally connects to the next endpoint.

[0138] Meanwhile, FIG. 1 is an image of the surface of a plated steel sheet according to Invention Example 5, taken by setting the magnification of the FE-SEM to approximately 2000x. As can be seen in FIG. 1, in the present invention, for first Al phases with an aspect ratio of 5 or more, the two endpoints are calculated in the manner described above, and then the lamellar structure is specified by selecting the points located at the outermost position in the lamellar extension direction among these two endpoints and connecting them with a straight line. Since a person skilled in the art can easily determine the selection criteria for the "points located at the outermost position" and the lamellar extension direction based on ordinary technical common sense, this specification does not further limit the description thereof.

[0139] The area fraction of the lamellar structure specified in this way was measured through image analysis. In addition, the fractions of the Al phase and MgZn2 phase were measured through image analysis of the same FE-SEM image.

[0140] In order to provide representativeness to each phase (alloy phase) during the phase analysis above, 20 random points were measured within a square area of ​​20 mm by 20 mm on each observation specimen, and the average value was applied as the representative value. Image analysis was performed using an image analysis program (Image analyzer, Clemex).

[0141] All of the above results are shown in Table 3 below.

[0142] Classification Zn-Al-Mg System Plating Layer Surface Structure (Area %) Lamellar Structure Fraction (Area %) Al Phase MgZn2 Phase Remainder Invention Example 1 35.5 62.2 2.3 28.3 Invention Example 2 24.3 72.3 3.4 66.5 Invention Example 3 48.8 50.8 0.4 45.1 Invention Example 4 36.8 61.7 1.5 69.1 Invention Example 5 47.6 51.9 0.5 43.8 Comparative Example 1 54.8 450.2 7.8 Comparative Example 2 13.2 14.1 72.70 Comparative Example 3 22.7 4829.3 9.1 Comparative Example 4 48.8 29.7 21.50

[0143] Referring to Fig. 1, it was confirmed that a first Al phase (A1), a second Al phase (A2), and a MgZn2 phase (M) are formed on the surface of the Zn-Al-Mg plating layer, and in particular, a lamellar structure (L) can be appropriately formed on the surface of the plated steel sheet by satisfying the conditions proposed in the present invention. To evaluate the corrosion resistance of each alloy-plated steel sheet, specimens were prepared with dimensions of 80 mm x 150 mm. Subsequently, a Salt Spray Test (SST, 5% NaCl) was performed on each specimen, and the corrosion resistance was evaluated by measuring the time until red rust occurred. At this time, to provide an acidic environment, sulfuric acid was added to a neutral 5% NaCl solution to adjust the pH to 4.0 before the test was conducted, and to provide an alkaline environment, ammonia water was added to a neutral 5% NaCl solution to adjust the pH to 11.0 before the test was conducted.

[0144] The evaluation criteria for the above corrosion resistance were determined as follows, and each result is shown in Table 4 below.

[0145] <Criteria for Evaluating Corrosion Resistance in Acidic Atmospheres>

[0146] ◎: Time required for red-blue discoloration to occur in 5% of the area is 480 hours or more

[0147] ○: Time required for red-blue discoloration to occur in 5% of the area is 360 hours or more but less than 480 hours

[0148] △: Time required for red-blue discoloration to occur in 5% of the area is 200 hours or more but less than 360 hours

[0149] X: Time required for red-blue to occur in 5% of the area is less than 200 hours

[0150] Evaluation Criteria for Corrosion Resistance in Neutral Atmospheres

[0151] ◎: Time required for red-blue discoloration to occur in 5% of the area is 3,000 hours or more

[0152] ○: Time required for 5% of the area to develop red-blue is 2,400 hours or more ~ ​​less than 3,000 hours

[0153] △: Time required for 5% of the area to develop red-blue is 1,500 hours or more to less than 2,400 hours

[0154] X: Time required for red-blue to occur in 5% of the area is less than 1,500 hours

[0155] Evaluation Criteria for Corrosion Resistance in Alkaline Atmospheres

[0156] ◎: Time required for red-blue discoloration to occur in 5% of the area is 1,500 hours or more

[0157] ○: Time required for red-blue to occur in 5% of the area is 1,000 hours or more ~ ​​less than 1,500 hours

[0158] △: Time required for red-blue discoloration to occur in 5% of the area is 600 hours or more to less than 1,000 hours

[0159] X: Time required for red-blue to occur in 5% of the area is less than 600 hours

[0160] Classification Plate Corrosion Resistance Acidic (pH 4.0) Neutral (pH 6.7) Alkaline (pH 11.0) Invention Example 1 ○○◎ Invention Example 2 ○◎◎ Invention Example 3 ◎◎◎ Invention Example 4 ◎◎◎ Invention Example 5 ◎◎◎ Comparative Example 1 ○△△ Comparative Example 2 ××× Comparative Example 3 △△△ Comparative Example 4 ○△×

[0161] Referring to Tables 1 to 4, Invention Examples 1 to 5, which appropriately contain a lamellar structure, exhibited excellent corrosion resistance in acidic, neutral, and alkaline environments. However, in Comparative Examples 1 to 4, which do not satisfy one or more of the conditions proposed in the present invention, it was confirmed that inferior corrosion resistance was observed in one or more corrosive environments.

[0162] Test Example 2: Confirmation of characteristics of plated steel sheets according to the ratio of the first Al phase spacing and the average aspect ratio

[0163] In addition to Table 1 above, plating baths having the compositions shown in Table 5 below were further prepared. Subsequently, plated steel sheets were prepared under the process conditions shown in Table 6 below. At this time, plating baths 1 to 4 are the same plating baths as those shown in Table 1. In addition, the Q value was additionally considered during SPM treatment. Other details were prepared in the same manner as described in Test Example 1 above.

[0164] Plating Bath Number Plating Layer Composition (Wet%, remainder Zn and unavoidable impurities) AlMgMg / Al Ratio SiCaSnSrCeMn 8377.30.200.120.5000.010-0.010-9229.90.450.200.005----

[0165] Temper Rolling Condition Q Value Steel Sheet Surface Roughness After Temper Rolling (Rmax, μm) Plating Bath Condition Wiping Sealing Box Cooling Condition Classification Roll Roughness (Ra, μm) Reduction Force (ton) Composition (Table 1 and Table 5) Temperature (°C) Gas Type Oil / Oxygen Concentration (Vol%) 1st Stage Speed ​​(°C / s) 2nd Stage Speed ​​(°C / s) 3rd Stage Speed ​​(°C / s) 0.38 16 70.01 11.45 15 40 N2 Oil 0.94 11.19.3 15.2 Invention Example 10.2 22 870.8 220.99 25 20 N2 Oil 1.10 8.38.17.9 Invention Example 20.2 8 2100.24 1.25 35 21 N2 Oil 0.25 9.67.3 15.5 Invention Example 30.282650.461.044509N2U0.7410.411.715.2 Invention Example 40.282650.461.083521N2U0.118.49.217.1 Invention Example 50.282100.241.223521N2U0.4810.111.913.7 Invention Example 60.312650.3451.108533N2U0.377.58.412.8 Invention Example 70.221670.1021.499489N2U2.247.46.917.0 Invention Example 80.292200.2570.983521N2Yu0.8811.210.217.1 Invention Example 90.292200.2571.014509N2Yu0.4510.48.715.0 Invention Example 100.292200.2571.004509N2Yu0.4510.910.016.3 Invention Example 110.292110.2241.099489N2Yu0.4912.79.817.2 Invention Example 120.292080.2131.149489N2Yu0.9811.18.614.8 Invention Example 13

[0166] Subsequently, for each plated steel sheet, the spacing between first Al phases within the lamellar structure and the average aspect ratio were measured and are shown in Table 7 below. In this case, the spacing between first Al phases was measured as the shortest distance between adjacent first Al phases. The average aspect ratio of the first Al phases was measured as the average aspect ratio of the first Al phases distributed within the lamellar structure in the FE-SEM image. Other than that, the method for identifying the phases and measuring the area fraction was performed in the same manner as described in Test Example 1 above. The measured results are shown in Table 7 below.

[0167] Classification Zn-Al-Mg plating layer 1st Al phase spacing, D (㎛) 1st Al phase average aspect ratio, A Relationship 1D / A Surface structure (Area %) Lamellar structure fraction (Area %) Al phase Mg Zn 2 phase remainder Invention Example 1 35.5 6 2.2 2.3 28.3 3.5 15.2 0.23 Invention Example 2 24.3 7 2.3 3.4 6 6.5 1.1 18.6 0.06 Invention Example 3 48.8 50.8 0.4 45.1 2.3 916.2 0.15 Invention Example 4 36.8 61.7 1.5 69.1 0.1 417.3 0.01 Invention Example 5 47.6 51.9 0.5 43.8 0.2 215.4 0.01 Invention Example 647.3520.748.60.9116.30.06 Invention Example 755.744.20.132.41.3518.20.07 Invention Example 842.353.64.133.70.4416.50.03 Invention Example 949.749.70.644.10.4720.30.02 Invention Example 1036.661.51.964.80.2224.80.01 Invention Example 1136.9612.166.30.3226.80.01 Invention Example 1242.153.9433.10.4219.90.02 Invention Example 1342.353.64.133.70.3521.10.02

[0168] The corrosion resistance of the flat section and the corrosion resistance of the processed section of the above-mentioned plated steel sheets were evaluated. For this purpose, two specimens measuring 80 mm x 150 mm were prepared. Among these, the specimen for evaluating the corrosion resistance of the processed section was bent at a 90° angle so that the radius of curvature of the processed section was 0.5 mm. The evaluation criteria for the corrosion resistance of the flat section were carried out in the same manner as in Test Example 1 described above. The corrosion resistance of the processed section was evaluated according to the following criteria. The evaluation results are shown in Table 8 below.

[0169] <Criteria for Evaluating Corrosion Resistance of Processed Parts>

[0170] ◎: Time required for red-blue discoloration to occur in 5% of the area is 2,000 hours or more

[0171] ○: Time required for 5% of the area to develop red-blue is 1,500 hours or more to less than 2,000 hours

[0172] △: Time required for 5% of the area to develop red-blue is 1,000 hours or more to less than 1,500 hours

[0173] X: Time required for red-blue to occur in 5% of the area is less than 1,000 hours

[0174] Classification Plate Corrosion Resistance Processed Part Corrosion Resistance Acidic (pH 4.0) Neutral (pH 6.7) Alkaline (pH 11.0) Neutral (pH 6.7) Invention Example 1 ○○◎△ Invention Example 2 ○◎◎◎ Invention Example 3 ◎◎◎◎ Invention Example 4 ◎◎◎◎ Invention Example 5 ◎◎◎◎ Invention Example 6 ◎◎◎◎ Invention Example 7 ◎◎◎◎ Invention Example 8 ○○◎○ Invention Example 9 ◎◎◎◎ Invention Example 10 ◎◎◎◎ Invention Example 11 ◎◎◎◎ Invention Example 12 ◎◎◎◎ Invention Example 13 ◎◎◎◎

[0175] Referring to Tables 5 through 8, it was confirmed that the corrosion resistance of the processed part can be further improved when the D / A value derived by Equation 1 proposed in the present invention is 0.20 or less. Although the invention has been described with reference to the above examples, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the spirit and scope of the invention as described in the following claims.

Claims

1. Base steel plate; and It includes a Zn-Al-Mg-based plating layer disposed on the surface of the above-mentioned base steel plate, and The above Zn-Al-Mg-based plating layer comprises an Al phase and a MgZn2 phase, and A plated steel sheet having an area fraction of 10.0 to 80.0% of a lamellar structure in which a first Al phase with an aspect ratio of 5 or more is arranged layerwise on the surface of the Zn-Al-Mg-based plating layer.

2. In Paragraph 1, A plated steel sheet satisfying the following equation 1, wherein in the above lamellar structure, the spacing between the first Al phases is D (μm) and the average aspect ratio of the first Al phases is A. [Relationship 1] D / A ≤ 0.25 3. In Paragraph 1, A plated steel sheet having a D / A value of 0.20 or less derived by the above relationship 1.

4. In Paragraph 1, A plated steel sheet comprising, on its surface, a Zn-Al-Mg-based plating layer comprising, in area %, the Al phase: 15.0~80.0%, the MgZn2 phase: 15.0~75.0%, and the remainder phase: 40.0% or less (including 0%).

5. In Paragraph 4, A plated steel sheet containing the above residual phase at 5.0% or less (including 0%).

6. In Paragraph 1, The above-mentioned Zn-Al-Mg plating layer comprises, in weight percent, aluminum (Al): 20.0~50.0%, magnesium (Mg): 5.0~22.0%, the remainder being zinc (Zn) and unavoidable impurities, in a plated steel sheet.

7. In Paragraph 6, A plated steel sheet in which the weight ratio ([Mg] / [Al]) of the Mg content ([Mg]) and Al content ([Al]) of the Zn-Al-Mg-based plating layer is 0.20 or higher and less than 0.

70.

8. In Paragraph 6, The above Zn-Al-Mg-based plating layer is a plated steel sheet further comprising one or more elements selected from the following groups i) to iii). i) Total content of elements derived from the base steel sheet: 1.000% or less ii) Total content of elements added to inhibit the formation of Mg oxide in the plating bath: 1,000% or less iii) Total content of elements added to control the surface quality of galvanized steel sheets: 1,000% or less 9. In Paragraph 8, The elements belonging to i) above are one or more of Si, Cr, Mn, Co, Ti, Ni, Fe, Cu, V, Nb, Mo, P, W, and B, and The elements belonging to ii) above are one or more of Y, Zr, La, Ce, Ca, Sr, and Be, and A plated steel sheet in which the element belonging to iii) above is one or more of Sb, Sn, Bi, Pb, Ga, Ge, and In.

10. In Paragraph 1, The above-mentioned plated steel sheet is, A plated steel sheet further comprising an interfacial alloy layer interposed between the above Zn-Al-Mg-based plating layer and the above base steel sheet.

11. In Paragraph 10, The above-mentioned interfacial alloy layer comprises one or more alloy phases selected from FeAl, FeAl2, FeAl3, FeAl4, Fe2Al5, Fe2Al, and Fe3Al, forming a plated steel sheet.

12. In Paragraph 10, A plated steel sheet having a thickness of 0.001 to 3.000 μm of the interface alloy layer.

13. In Paragraph 1, A plated steel sheet having a Zn-Al-Mg-based plating layer with a thickness of 5 to 60 μm.

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