Hot-dip aluminum-zinc-magnesium plated steel sheet with excellent bendability and method for manufacturing the same

Abstract

The present invention provides a hot-dip aluminum-zinc-magnesium plated steel sheet that exhibits excellent corrosion resistance while dramatically suppressing crack formation in the plating film, even during particularly severe bending processes, and showing excellent bendability, as well as a method for manufacturing the same. [Solution] A molten aluminum-zinc-magnesium plated steel sheet containing a plating film comprising, by mass%, Zn: 35-55%, Si: 0.5-3.0%, Mg: 0.01-3.0%, with the remainder being Al and unavoidable impurities, wherein the plating film consists of an α-Al phase, a Zn phase, a Mg phase, a Mg2Si phase, a MgZn2 phase, and a Mg2Zn 11 Disclosed is a molten aluminum-zinc-magnesium plated steel sheet having a structure in which the phases are complexly mixed, wherein the α-Al phase forms a matrix that penetrates the entire plating layer in a dendrite-like manner, and Mg-containing precipitates are dispersed inside the matrix, with the average particle size of the precipitates being 0.01 to 0.1 μm, and a method for producing the same.

Description

[Technical Field]

[0001] The present invention relates to a hot-dip aluminum-zinc-magnesium plated steel sheet with excellent bendability and a method for producing the same. [Background technology]

[0002] Hot-dip galvanized steel sheets are widely used to prevent corrosion of steel sheets. In particular, hot-dip aluminum-zinc (Al-Zn) plated steel sheets, which have aluminum (Al) added, combine the excellent corrosion resistance of aluminum with the sacrificial corrosion protection of zinc (Zn), exhibiting superior corrosion resistance compared to general hot-dip galvanized steel sheets. Among these, 55% Al-Zn (also known as Galvalume) alloy plated steel sheets were proposed in the late 1960s and are still used in various industrial fields such as building exterior materials, home appliances, and automotive parts.

[0003] In recent years, molten aluminum-zinc-magnesium (Al-Zn-Mg) plated steel sheets, which have added magnesium (Mg) to further improve corrosion resistance, have been developed and are attracting attention. Magnesium promotes the formation of stable and dense corrosion products on the surface of the plated layer, and has the effect of dramatically improving the corrosion resistance of weak areas such as cut surfaces and processed parts.

[0004] However, such Al-Zn or Al-Zn-Mg plated steel sheets generally have the problem of having high hardness in the plating film, resulting in poor workability, especially in terms of bending. When the steel sheet is bent, cracks easily occur in the plating film, which not only damages the appearance but also induces corrosion of the base steel sheet at the cracked areas, shortening the product's lifespan.

[0005] To address these bending problems, various attempts have been made to control the microstructure of the plating film through post-plating heat treatment. For example, techniques have been proposed to soften the plating film by performing heat treatment within a specific temperature range, or to improve flexibility by adjusting the distribution of alloying elements. Patent Document 1 attempts to improve flexibility by subjecting an Al-Zn-Si plated steel sheet to a specific thermal history (heating at 150-277°C followed by relative rapid cooling) and controlling the Zn content in the α-Al matrix, the size of precipitates, and the Al-Zn eutectic structure. Patent Document 2 describes a Zn-Al-Mg plated steel sheet that is heat-treated at a high temperature of 260°C or higher for a long time to soften the hard phase of Zn2Mg. 11 The attempt was made to improve flexibility by transforming the material into an Mg2-like state.

[0006] However, these are mainly limited to specific binary or ternary alloy systems, or they use different temperature ranges or cooling methods than those in the present invention, and the microstructural factors being controlled are also different. In particular, attempts to precisely control the microstructure of quaternary alloy systems by actively utilizing Si in addition to Al, Zn, and Mg, and thereby maximize bendability, have been insufficient. Therefore, there is a real need for the development of innovative Al-Zn-Mg plated steel sheets and their manufacturing technologies that can withstand severe bending while maintaining excellent corrosion resistance. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2023-143890 [Patent Document 2] Japanese Patent Publication No. 4542468 [Overview of the project] [Problems that the invention aims to solve]

[0008] The present invention is for solving the problems of the above-described prior art. One of various objects of the present invention is to provide a hot-dip aluminum-zinc-magnesium plated steel sheet that has excellent corrosion resistance and, in particular, can remarkably suppress the generation of cracks in the plating film even during severe bending, and exhibits excellent bending workability.

[0009] Another object of the present invention is to provide a new manufacturing method capable of stably and efficiently manufacturing the plated steel sheet having the above excellent bending workability.

Means for Solving the Problems

[0010] According to one aspect, there is provided a hot-dip aluminum-zinc-magnesium plated steel sheet including a plating film containing, in mass%, Zn: 35 to 55%, Si: 0.5 to 3.0%, Mg: 0.01 to 3.0%, and the balance being Al and inevitable impurities. The plating film has a structure in which α-Al phase, Zn phase, Mg phase, Mg2Si phase, MgZn2 phase, and Mg2Zn phase are mixed in a composite manner. The α-Al phase forms a matrix penetrating the entire plating layer in a dendritic shape, and precipitates containing Mg are dispersed inside the matrix. The average particle diameter of the precipitates is 0.01 to 0.1 μm. 11 There is provided a hot-dip aluminum-zinc-magnesium plated steel sheet.

[0011] In one embodiment, in the cumulative particle size distribution based on the number of the precipitates, when the 10% particle diameter of the precipitates is D 10 and the 90% particle diameter of the precipitates is D 90 in the cumulative particle size distribution based on the number of the precipitates, D 90 / D 10 may be 15 or less.

[0012] In one embodiment, the mass ratio of Mg to Si in the plating film may be 0.2:1 to 2.0:1.

[0013] In one embodiment, the mass ratio of Zn to Al in the plating film may be 0.7:1 to 1.2:1.

[0014] In one embodiment, the Mg content in the α-Al phase may be lower than the Mg content in the precipitate, with the Mg content in the α-Al phase being 0.3 to 0.4% by mass and the Mg content in the precipitate being 0.4 to 0.6% by mass.

[0015] In one embodiment, the area fraction of the MgZn2 phase is a, and the Mg2Zn 11 When the surface fraction of the phase is denoted as b, b / a may be between 2 and 10.

[0016] In one embodiment, the area fraction of the MgZn2 phase is a, and the Mg2Zn 11 When the area fraction of the phase is b and the area fraction of the Mg2Si phase is c, (a+b) / c may be between 1 and 5.

[0017] In another embodiment, a method for producing a molten aluminum-zinc-magnesium plated steel sheet is provided, comprising the steps of: forming a plating film on the surface of a steel sheet using a plating bath containing, by mass%, Zn: 35-55%, Si: 0.5-3.0%, Mg: 0.01-3.0%, with the remainder being Al and unavoidable impurities; primary cooling the steel sheet on which the plating film has been formed at a temperature of 150°C or less; reheating the primary cooled steel sheet at a temperature of 200°C or more and less than 260°C; and secondary cooling the reheated steel sheet.

[0018] In one embodiment, the average cooling rate in the primary cooling step may be 10°C / sec or higher.

[0019] In one embodiment, the reheating temperature may be 220°C or higher and 250°C or lower.

[0020] In one embodiment, the reheating time may be 1 hour or more and 12 hours or less.

[0021] In one embodiment, the average rate of heating from 150°C to the reheating temperature in the reheating step may be 3 to 8°C / h.

[0022] In one embodiment, the average cooling rate in the secondary cooling step may be 10-12°C / h from the reheating temperature to 150°C, 5-6°C / h from 150°C to 80°C, and 1-2°C / h from 80°C to room temperature. [Effects of the Invention]

[0023] The molten aluminum-zinc-magnesium plated steel sheet and its manufacturing method according to the present invention have the following effects.

[0024] (1) By precisely controlling the microstructure of the plating film, the bendability of the plated steel sheet can be dramatically improved. The plated steel sheet of the present invention has the advantages of significantly suppressing crack formation in the plating film even during severe bending, being easy to process into complex shapes, and having excellent appearance quality after processing.

[0025] (2) By suppressing crack initiation in the bent section, the problem of reduced corrosion resistance in the processed section can be effectively prevented. This contributes to improving the reliability and durability of the product.

[0026] (3) The manufacturing method of the present invention includes specific low-temperature reheating and stepwise slow cooling steps, which can stably and reproducibly realize the target microstructure and have advantages in terms of energy efficiency compared to the prior art.

[0027] The effects of the present invention are not limited to those described herein, but should be understood to include all effects that can be inferred from the detailed description or claims herein. [Brief explanation of the drawing]

[0028] [Figure 1] These are SEM images of the cross-section of the plating film of a plated steel sheet (plated steel sheet before heat treatment) according to Comparative Example 1, observed at 2,000x or 5,000x magnification. [Figure 2]These are SEM images of the cross-section of the plating film on the plated steel sheet according to Example 9, observed at 2,000x or 5,000x magnification. [Figure 3] These are images showing the surface condition of typical processed parts that correspond to the bending processability evaluation criteria (◎, ○, △, and X). [Modes for carrying out the invention]

[0029] One aspect of the present invention will be described below. However, the provisions described herein may be embodied in various different forms and are therefore not limited to the embodiments described herein.

[0030] When a part of the specification "includes" a certain component, unless otherwise stated, this does not exclude other components, but rather means that other components may be further included.

[0031] When a range of numerical values ​​is described herein, unless otherwise specified, the value shall have the precision of significant figures provided by the standard rules for significant figures in chemistry. For example, 10 includes the range 5.0 to 14.9, and the figure 10.0 includes the range 9.50 to 10.49.

[0032] Hereinafter, one embodiment of the present invention, a hot-dip aluminum-zinc-magnesium plated steel sheet with excellent bendability, will be described in detail.

[0033] Hot-dip aluminum-zinc-magnesium plated steel sheet The molten aluminum-zinc-magnesium plated steel sheet of the present invention comprises a base steel sheet and a plating film formed on the surface of the base steel sheet, the plating film containing Zn: 35-55% by mass, Si: 0.5-3.0% by mass, Mg: 0.01-3.0% by mass, the remainder being Al and unavoidable impurities.

[0034] Zn is one of the main constituent elements of the plating film, and it plays a role in effectively suppressing corrosion of the steel sheet by providing excellent sacrificial corrosion protection to the base steel sheet. If the Zn content is too low, the temperature of the plating bath will rise, which will increase top dross and cause operational problems, resulting in poor workability. In addition, the sacrificial corrosion protection effect may be insufficient, reducing the protective capacity of the steel sheet, and corrosion of the base steel sheet may be accelerated, especially at cut surfaces and scratched areas. Furthermore, if the Al content becomes relatively high, the plating film may be composed of an Al-centered phase, increasing brittleness, which increases the possibility of crack initiation during bending. The presence of a sufficient amount of Zn contributes to ensuring the overall ductility necessary for bending. Conversely, if the Zn content is too high, the cost will increase due to the rise in the specific gravity of the plated steel sheet, and economic efficiency will rapidly decline. Also, if the Al content becomes relatively low, the corrosion resistance improvement effect of the Al base, i.e., the effect of the stable Al2O3 protective film formed on the surface, may decrease, and the overall durability of the plating film may decline. Furthermore, if the Zn content is too high, it may become difficult to control the target microstructure. Therefore, in this invention, it is preferable to limit the Zn content to 35 to 55% by mass.

[0035] Si plays a crucial role in the plating process by suppressing excessive reactions between the base steel sheet and the molten metal, improving the fluidity of the plating bath, and imparting luster. Specifically, Si reacts with Fe in the base steel sheet to form an Fe-Al-Si intermetallic layer (IML). This alloy layer grows slower than the Fe-Al alloy layer, which is advantageous for controlling the thickness of the intermetallic layer and ensuring adhesion between the plating film and the base steel sheet. This is essential for preventing delamination of the plating layer during bending. Furthermore, in this invention, Si reacts with Mg within the plating film to form an Mg2Si phase. As will be described later, this Mg2Si phase is an important component that affects the microstructure, mechanical properties, and corrosion behavior of the plating film. If the Si content is too low, the growth-suppressing effect of the intermetallic layer is insufficient, leading to excessive Fe-Al reactions and the formation of a thick, brittle alloy layer. This reduces the adhesion of the plating, potentially causing delamination during bending. Furthermore, if a sufficient amount of Si is not supplied for the formation of the Mg2Si phase, it becomes difficult to achieve the intended balance between the Mg2Si phase and the Mg-Zn phase. Conversely, if the Si content is too high, the growth-inhibiting effect of the interfacial alloy layer saturates and cannot be improved further, and instead, the excess Si may form in the plating film as coarse single-phase Si or an excess amount of coarse Mg2Si phase. These coarse phases act as stress concentration points, increasing the brittleness of the plating film and reducing its flexibility. In addition, the excessive generation and growth of the Mg2Si phase on the surface roughens the surface, causing premature surface discoloration and reducing the post-treatment coating properties. Therefore, in the present invention, it is preferable to limit the Si content to 0.5 to 3.0 mass%.

[0036] Mg combines with oxygen in the air in contact with the plating layer to form a passive film, preventing oxygen from diffusing into the interior of the alloy layer, blocking additional corrosion phenomena, and improving corrosion resistance. The presence of the Mg2Si phase formed by the reaction of magnesium and silicon components in the plating layer and the MgZn2 phase formed by the reaction of magnesium and zinc plays a role in reducing the corrosion rate through the sacrificial corrosion prevention ability of zinc and the formation of local batteries during the progress of corrosion. It also has the effect of reacting with aluminum to block the diffusion of oxygen, significantly improving the corrosion resistance of the cross-section after processing. If the Mg content is too low, the effect of improving corrosion resistance is small, and the formation of Mg-containing precipitates and Mg-related intermetallic compounds (MgZn2, Mg2Zn 11 , Mg2Si) phases that contribute to the improvement of bendability is insufficient, and it may be difficult to obtain the effect of improving bendability targeted by the present invention. Conversely, if the Mg content is too high, the effect of improving corrosion resistance may saturate, rather increasing the generation of Mg oxide-based dross in the plating bath, deteriorating operability, and inducing surface defects. In addition, the addition of excessive Mg induces the formation of coarse or non-uniform Mg-related phases (e.g., coarse Mg2Si, coarse MgZn2, or at very high Mg contents, Al3Mg2, etc.) in the plating film, rather decreasing bendability or increasing brittleness. The excessive and non-uniform Mg phase may also induce local corrosion. Therefore, in the present invention, it is preferable to limit the Mg content to 0.01 to 3.0% by mass.

[0037] Al is a fundamental constituent element of the plating layer and plays a key role in improving corrosion resistance. The higher the Al content, the more a dense alumina (Al2O3) protective film is formed on the surface of the plating layer, which delays the corrosion of the base metal in the atmosphere and also improves heat resistance. In this invention, it is recommended that the Al content be at least about 30% by weight or more, because if the Al content is lower than that, the contribution of aluminum to corrosion resistance will be small, and in fact there will be no significant difference from general zinc plating. On the other hand, if the Al content is excessively high, the proportion of zinc in the plating layer will be relatively low, which may reduce the performance of sacrificial corrosion protection, and the alloy composition may be biased towards Al, which may lead to problems such as an increase in the formation of unnecessary Fe-Al alloys in the hot-dip plating process or the plating layer becoming excessively brittle. Considering this, the upper limit of the Al content may be limited to 65%.

[0038] The remainder consists of unavoidable impurities. These unavoidable impurities may include trace amounts of Fe, Cr, Ni, Cu, etc., and their total content is usually controlled to be 3.0% by mass or less, preferably 1.0% by mass or less.

[0039] For example, the mass ratio of Mg to Si in the plating film may be between 0.2:1 and 2.0:1. If the mass ratio of Mg to Si is less than 0.2:1, there may be insufficient Mg to form the Mg2Si phase, or there may be excess Si, leading to the formation of a coarse Si phase, or excessive growth on the Fe-Al-Si interface alloy may be induced, increasing brittleness and reducing flexibility. Conversely, if the mass ratio of Mg to Si exceeds 2.0:1, there may be insufficient Si to form the Mg2Si phase, making it difficult to obtain the corrosion behavior regulating effect of Mg2Si phase formation, or excess Mg may lead to the excessive formation of coarse MgZn2 phases, which may actually reduce flexibility or result in non-uniformity in corrosion resistance. Therefore, by controlling this ratio within the range of 0.2:1 to 2.0:1, an optimal balance can be achieved between the Mg2Si phase and other Mg-related phases (Mg-Zn phases, precipitates), ensuring both excellent bendability and corrosion resistance.

[0040] For example, the mass ratio of Zn to Al may be between 0.7:1 and 1.2:1. If the mass ratio of Zn to Al is less than 0.7:1, the plating film will be excessively Al-centered, weakening the sacrificial corrosion protection effect of Zn. This can lead to insufficient ductility and reduced bendability due to the properties of the Al matrix itself and a lack of Zn-containing phase. Conversely, if the mass ratio of Zn to Al exceeds 1.2:1, the formation of a continuous dendrite network of the α-Al phase may be inhibited, or the corrosion resistance improvement effect of the Al base may decrease. Therefore, by controlling this ratio within the range of 0.7:1 to 1.2:1, an appropriate balance between the contribution of Al for corrosion resistance / durability and the contribution of Zn for sacrificial corrosion protection / ductility can be maintained, optimizing overall performance.

[0041] The plating film consists of α-Al phase, Zn phase, Mg phase, Mg2Si phase, MgZn2 phase, and Mg2Zn 11 The structure has a complex mixture of phases, and the α-Al phase forms a matrix that penetrates the entire plating layer in a dendritic manner. In this invention, the dendritic α-Al phase continuously connects the entire plating layer, providing a foundation for ensuring the mechanical ductility of the plating layer. During bending, local stress does not concentrate in one place but is distributed throughout the aluminum matrix, suppressing crack initiation. Within this α-Al matrix are Mg-containing precipitates and various intermetallic compound phases (Zn, MgZn2, Mg2Zn) described later. 11 The excellent bendability of the present invention is achieved when Mg2Si is appropriately distributed in the dendritic gap region and other areas.

[0042] For example, precipitates containing Mg are dispersed within the matrix, and the average particle size of these precipitates may be 0.01 to 0.1 μm. These precipitates are fine particles formed through a specific low-temperature reheating and slow cooling process, and are generated by the diffusion of Mg (and potentially other elements) that were supersaturated and dissolved in the α-Al matrix during the initial rapid cooling. This size range is important for the following reasons: If the average particle size of the precipitates is too small, it means that precipitation may not occur sufficiently during the heat treatment process, or that precipitates that are too fine and dense may be formed, resulting in an excessive precipitation strengthening effect that maintains high hardness in the α-Al matrix, reducing ductility and decreasing bendability. Conversely, if the average particle size of the precipitates is too large, it means that the precipitates are coarsening, and such coarse precipitates can induce stress concentration during deformation, act as initiation points for the generation and propagation of fine cracks, and can actually reduce ductility and toughness, worsening bendability. Therefore, the fine Mg-containing precipitates in the 0.01-0.1 μm range controlled in this invention are optimized in size to appropriately adjust the potential shift during bending deformation without excessively inhibiting the ductility of the α-Al matrix, effectively suppressing the generation and propagation of fine cracks, and simultaneously improving the ductility and toughness of the plating film.

[0043] For example, in the cumulative particle size distribution based on the number of precipitates, the 10% particle size of the precipitate is D 10 Assuming that the cumulative particle size distribution based on the number of precipitates is D 90 In that case, D 90 / D 10The coefficient of precipitation may be 15 or less, preferably 12 or less, and more preferably 10 or less. This maintains a more homogeneous and softened state throughout the plating layer, allowing for consistent mechanical behavior during bending. If very large precipitates are present in some areas and only very small precipitates in other areas, the stress distribution becomes uneven during deformation, leading to localized weak points (e.g., around large precipitates) or over-hardened areas (areas of densely packed fine precipitates), which can promote crack initiation during bending. A uniform size precipitate distribution helps maintain a more uniformly softened state throughout the plating layer, resulting in consistent deformation behavior during bending and suppressing localized fracture initiation points.

[0044] For example, the Mg content in the α-Al phase may be lower than the Mg content in the precipitate, with the Mg content in the α-Al phase being 0.3-0.4% by mass and the Mg content in the precipitate being 0.4-0.6% by mass. In a normal rapidly cooled structure (immediately after plating or after primary cooling), the Mg in the α-Al matrix is ​​dissolved in a supersaturated state above the equilibrium solid solution limit, and the matrix is ​​relatively hard due to the solid solution strengthening effect. However, in this invention, diffusion of Mg atoms is induced through a reheating and slow cooling process at a specific low temperature (200-260°C), effectively reducing the amount of Mg solid solution in the α-Al matrix to a level of 0.3-0.4%. This means that some Mg has leached out of the matrix as precipitate, mitigating the solid solution strengthening effect and making the matrix itself softer and more ductile. At the same time, the Mg that leached out of the matrix participates in the formation of precipitates, and the Mg content in the precipitates becomes relatively higher to a level of 0.4-0.6%. As a result, the matrix softens and can readily accept bending deformation, allowing for other positive effects from the fine precipitates (e.g., deformation homogenization) without excessive hardening or brittleness. If the Mg content in the matrix remains high (e.g., over 0.4%), it means that the matrix softening is insufficient. Conversely, if the Mg content in the matrix is ​​too low (e.g., less than 0.3%), or if the Mg content in the precipitate falls outside this range, this suggests that the intended precipitation process did not occur correctly, or that other types of phases may have formed.

[0045] For example, the area fraction of the MgZn2 phase is a, and the Mg2Zn 11 When the area fraction of the phase is denoted as b, b / a may be between 2 and 10. The Al-Zn-Mg plating layer contains MgZn2 and Mg2Zn 11 Two main Mg-Zn intermetallic compounds may be formed. Generally, the MgZn2 phase is Mg2Zn 11 Because it has higher hardness and greater brittleness than the other phases, it can negatively affect flexibility. On the other hand, Mg2Zn 11 The phase is known to be relatively ductile. The post-heat treatment process of the present invention, in particular the specific low temperature maintenance and slow cooling conditions of 200-260°C, is thermodynamically or kinetically Mg2Zn 11To promote the formation of the MgZn2 phase, or to induce the transformation from the already formed MgZn2 phase to Mg2n 11 This can provide favorable conditions for stabilizing the phase. Therefore, Mg2Zn 11 Controlling the area fraction of phase (b) to be significantly higher than the area fraction of the MgZn2 phase (a) (b / a ≥ 2) acts as an important strategy to improve the overall ductility of the plating film and enhance its bendability. If the b / a ratio is less than 2, the influence of the hard phase MgZn2 becomes large, and the bendability may not be sufficiently improved. However, Mg2Zn 11 If an excess of one phase exists (e.g., b / a exceeds 10), the balance with other phases in the plating layer (such as α-Al and Mg2Si) is disrupted, potentially negatively affecting overall mechanical properties and other characteristics such as corrosion resistance. Therefore, the upper limit is restricted to 10 to achieve the optimal balance between corrosion resistance and ductility.

[0046] For example, let a be the area fraction of the MgZn2 phase, and let Mg2Zn 11 When the area fraction of the phase is b and the area fraction of the Mg2Si phase is c, (a+b) / c may be 1 to 5. This ratio is due to the total Mg-Zn system phase (MgZn2+Mg2Zn) formed when Mg atoms bond with Zn within the plating film. 11 This shows the relative balance between the amount of Mg-Zn and the amount of Mg2Si phase formed by bonding with Si. The Mg2Si phase is generally a hard phase, but when finely dispersed, it can contribute to improved strength or affect corrosion behavior. This ratio represents a balance point between ensuring sufficient Mg-Zn phase, which is favorable for ductility, and suppressing the excessive formation of the Mg2Si phase, which can induce brittleness. By properly dispersing the Mg2Si phase within this range, localized Si segregation within the plating layer is prevented, and MgZn2·Mg2Zn 11 (Mg2Zn 11 Along with the dominant phase, it exhibits a complex strengthening and softening effect, simultaneously achieving high bendability and corrosion resistance. This optimal balance of phase fractions is achieved through a combination of the specific quaternary system composition and precise heat treatment process of the present invention.

[0047] The molten aluminum-zinc-magnesium and plated steel sheets described above may be manufactured by a variety of methods, and the manufacturing method is not particularly limited. However, as a preferred example, they may be manufactured by the following method.

[0048] Manufacturing method for hot-dip aluminum-zinc-magnesium plated steel sheets First, the steel sheet to be surface-treated is immersed in an aluminum-zinc-magnesium plating bath to form a plating film on its surface. The type and composition of the plating bath are as described above (by mass%, Zn: 35-55%, Si: 0.5-3.0%, Mg: 0.01-3.0%, the remainder being Al and unavoidable impurities). On the other hand, the present invention does not particularly limit the method of forming the plating film on the surface of the steel sheet. For example, the steel sheet can be plated in a continuous line using a conventional continuous galvanizing line (CGL), or the steel sheet can be deposited in batches for plating.

[0049] Next, the plated steel sheet is first cooled to a temperature of 150°C or lower. Immediately after being removed from the plating bath, the steel sheet is at a high temperature (e.g., 450°C or higher). If post-heat treatment is performed immediately or if it is cooled gradually at this temperature, undesirable coarse phases (e.g., coarse intermetallic compounds) may be formed during the solidification process, or the microstructure may become non-uniform. Therefore, by first cooling relatively quickly to a temperature of 150°C or lower, preferably 100°C or lower, the microstructure immediately after plating (mainly a non-equilibrium supersaturated solid solution or fine solidification structure) is stabilized to some extent and frozen. This is important to ensure a uniform and reproducible starting state for inducing controlled diffusion and phase transformation in the subsequent reheating process. Furthermore, cooling to this temperature makes it easier to control the starting temperature of the subsequent reheating process. This step can be considered a microstructure "preparation" step for the subsequent heat treatment.

[0050] For example, the average cooling rate in the primary cooling step may be 10°C / sec or higher. A relatively fast cooling rate of 10°C / sec or higher effectively suppresses the growth of coarse intermetallic compounds that can form during the plating solidification process and the solid-phase diffusion process at high temperatures, and is advantageous for dissolving alloying elements (especially Mg and Zn) in the α-Al matrix in a supersaturated state above the equilibrium solid solubility limit. The fine and non-equilibrium microstructure thus formed provides a driving force for controlled precipitation and phase transformation in the subsequent reheating step, contributing to obtaining the fine, uniform, and optimized final microstructure (softened matrix, fine precipitates, high b / a ratio, etc.) targeted by the present invention. A slow cooling rate reduces this driving force and can lead to the formation of undesirable coarse structures.

[0051] Next, the primary cooled steel sheet is reheated at a temperature of 200°C to less than 260°C, preferably at a temperature of 220°C to 250°C. This reheating step, as a core step of the present invention, supplies controlled thermal energy to the fine and supersaturated microstructure stabilized by primary cooling, thereby inducing the target microstructural change, i.e., the optimal state for improved bendability. The specific low temperature range of 200°C to less than 260°C is optimized to simultaneously achieve the following combined objectives in the Al-Zn-Mg-Si quaternary alloy system of the present invention: Specifically, it allows sufficient diffusion of alloy atoms such as Mg, Zn, and Si, effectively reduces the concentration of supersaturated elements in the α-Al matrix (matrix softening), induces the uniform formation and growth of Mg-containing precipitates to the target fine size (average particle size 0.01 to 0.1 μm), and promotes the formation and growth of Mg2Zn, which is relatively ductile among Mg-Zn intermetallic compounds. 11The process induces a phase equilibrium or metastable state (b / a = 2~10) in which the phase predominantly forms or stabilizes relative to the hard MgZn2 phase, and controls the formation of the Mg2Si phase in an appropriate amount and form without excessive coarsening, contributing to the achievement of the target (a+b) / c ratio (1~5). If the reheating temperature is below 200°C, atomic diffusion may be too slow for the above change to occur sufficiently, or it may take a very long time, resulting in a minimal or uneconomical improvement in bendability. Conversely, if the opening heating temperature is 260°C or higher, Mg2Zn 11 The phase stability may decrease or decompose, leading to the formation of other undesirable phases (e.g., coarse Mg2Si or MgZn2 phases), or the precipitate may become excessively coarse, potentially reducing flexibility.

[0052] For example, the reheating time in the reheating step may be between 1 hour and 12 hours. At the relatively low reheating temperature of 200-260°C adopted in this invention, the atomic diffusion rate is relatively slow. Therefore, the target microstructural changes (diffusion of Mg from the α-Al matrix and decrease in solid solubility, nucleation of fine Mg-containing precipitates and growth to the target size, Mg2Zn 11 To fully complete the phase formation / stabilization and ensure dominance over the MgZn2 phase, and to achieve a uniform state throughout the entire plating layer, a relatively long reheating time of 1 hour or more is necessary. If the reheating time is less than 1 hour, this diffusion control process will not proceed sufficiently, the microstructural change will remain incomplete, and as a result, the bendability improvement effect may not be fully realized. On the other hand, if the reheating time exceeds 12 hours, the target microstructural change effect will almost reach a saturated state, and the additional improvement effect will be minimal, while the possibility of unwanted grain growth or excessive coarsening of precipitates may slightly increase, which is also disadvantageous in terms of productivity. Therefore, controlling the time in the range of 1 to 12 hours is preferable in terms of achieving the target microstructure and process efficiency, and controlling it in the range of 4 to 10 hours is more preferable.

[0053] For example, the average heating rate from 150°C to the reheating temperature in the reheating step may be 3-8°C / h. Applying a slow average heating rate of 3-8°C / h during the process of heating from the primary cooled state (≤150°C) to the target reheating temperature (200-260°C) is to induce microstructural changes gradually and uniformly. If the heating rate is too fast, a temperature gradient will be generated inside the steel sheet, causing the degree of microstructural change to vary from location to location, or passing through a particular temperature range too quickly, resulting in uncontrolled abrupt phase transformation or non-uniform precipitation, which can impair the uniformity of the final microstructure. On the other hand, if the heating rate is too slow, the overall process time will be excessively long, which can reduce productivity. Therefore, a controlled slow heating rate of 3-8°C / h minimizes temperature deviations throughout the steel sheet, induces gradual and uniform microstructural changes (e.g., initial precipitation nucleation) caused by the temperature rise, and contributes to the uniform formation of the final target microstructure.

[0054] Next, the reheated steel sheet is secondarily cooled to complete the production of the molten aluminum-zinc-magnesium plated steel sheet. This secondarily cooling step is not simply a process of returning to room temperature, but is an important process that stably maintains and preserves the optimal microstructure (softened α-Al matrix, fine precipitates of specific sizes, Mg-Zn phase with a high b / a ratio, appropriate (a+b) / c ratio, etc.) that was painstakingly obtained through the reheating process, down to room temperature. It also plays a role in minimizing undesirable phase transformations that may occur during the cooling process (e.g., decomposition of the high-temperature stable phase or formation of a new brittle phase) and the formation of excessive residual stress, thereby ensuring the performance and reliability of the final product. In particular, this invention adopts a controlled, stepwise slow cooling method to stabilize the microstructure and ensure the final properties.

[0055] For example, in the secondary cooling step, the average cooling rate may be 10-12°C / h from the reheating temperature to 150°C, 5-6°C / h from 150°C to 80°C, and 1-2°C / h from 80°C to room temperature. This stepwise control of the slow cooling rate is a strategy for precisely controlling the cooling process, taking into account that the microstructural change behavior and stress generation possibility differ for each temperature interval.

[0056] In the section from the reheating temperature down to 150°C, a relatively faster cooling rate (10-12°C / h) is applied compared to the subsequent steps. This is because the microstructure stabilized at the reheating temperature (200-260°C), especially the relatively less stable phase (e.g., Mg2Zn), is not affected. 11 This is to relatively effectively fix the microstructure obtained at high temperatures while suppressing the decomposition of the microstructure and further coarsening of already formed fine precipitates. However, since it is not rapid cooling, it is necessary to avoid rapid phase transformation and the generation of large thermal shock stresses.

[0057] In the temperature range from 150°C to 80°C, a relatively slow cooling rate (5-6°C / h) is applied. This temperature range is where several low-temperature phase transformations may occur, or where adjustment of interphase interfacial energy or relaxation of internal stresses in already formed phases is important. Such a relatively low cooling rate provides time for these processes to proceed near equilibrium or helps to alleviate stress accumulation due to temperature changes.

[0058] In the temperature range from 80°C to room temperature, a very slow cooling rate (1-2°C / h) is applied. In this temperature range, thermal contraction stress due to the difference in thermal expansion coefficients between the plating layer and the base steel sheet can be a problem. Very slow cooling minimizes the generation and accumulation of such thermal contraction stress, ensuring the dimensional stability (flatness, etc.) of the final product and preventing potential problems due to internal residual stress (e.g., stress concentration during bending, long-term reliability degradation).

[0059] This precisely controlled multi-stage slow cooling process is a unique feature of the present invention and plays a crucial role in effectively maintaining the optimized microstructure through the reheating process to the final product, minimizing residual stress, and stably exhibiting excellent bendability.

[0060] The embodiments of the present invention will be described in more detail below. However, the following experimental results represent only representative results from the embodiments described above, and the scope and content of the present invention cannot be narrowed or limited by these embodiments. The effects of various embodiments of the present invention that are not explicitly presented below will be described in detail in the relevant sections.

[0061] Examples A degreased cold-rolled steel sheet with a thickness of 0.8 mm, a width of 120 mm, and a length of 250 mm was plated using a hot-dip galvanizing simulator. The composition of the plating bath was kept constant at Zn: 41.9%, Si: 1.6%, Mg: 1.5%, with the remainder being Al and unavoidable impurities. The plated steel sheet was then first cooled to room temperature at an average cooling rate of 12°C / sec, and then heated to the reheating temperature shown in Table 1 below, where it was maintained for the reheating time shown in Table 1 below. In the reheating step, the average heating rate from 150°C to the reheating temperature was kept constant at 5°C / h. Subsequently, the sheet was secondarily cooled from the reheating temperature to 150°C at an average cooling rate of 10°C / h, from 150°C to 80°C at an average cooling rate of 5°C / h, and from 80°C to room temperature at an average cooling rate of 1°C / h.

[0062] [Table 1]

[0063] Experimental Example 1: Observation of the cross-section of the plated film The cross-sections of the plated films in Examples 1 and 9 were observed using a field emission scanning electron microscope (FE-SEM, Hitachi). The acceleration voltage was set to 15kV, and the observation magnifications were set to 2000x and 5000x. Figure 1 is an SEM image of the cross-section of the plated film of the plated steel sheet (plated steel sheet before heat treatment) according to Comparative Example 1, observed at 2000x or 5000x magnification, and Figure 2 is an SEM image of the cross-section of the plated film of the plated steel sheet according to Example 9, observed at 2000x or 5000x magnification. In the specimen before heat treatment in Figure 1, a relatively coarse solidification structure is observed, but in the sample after heat treatment in Figure 2, it can be clearly confirmed that fine precipitates are uniformly dispersed and formed inside the α-Al phase matrix. Such fine precipitates are distributed inside the α-Al phase dendrite matrix.

[0064] Experimental Example 2: Measurement of Magnesium Content in the α-Al Phase Based on the microstructure of the plating film observed in Experimental Example 1, particularly the presence and distribution of precipitates within the α-Al phase, quantitative analysis was performed as follows to confirm the local differences in magnesium content in each region. The magnesium content within the α-Al phase was measured using energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments Ultim Extreme). In Comparative Example 1, as shown in Figure 1, no clear precipitates were observed within the α-Al phase, and the Mg content was measured at 10 arbitrary locations within the α-Al phase. On the other hand, in Example 9, as shown in Figure 2, clear precipitates were observed within the α-Al phase, so the Mg content was measured at 10 arbitrary locations within the precipitates and at 10 arbitrary locations within the peripheral α-Al phase matrix (periphery) where no precipitates were present. The results are shown in Table 2 below.

[0065] [Table 2]

[0066] The average Mg content in the α-Al phase of the specimen before heat treatment (Comparative Example 1) was measured at 0.73% by weight. The average Mg content in the fine precipitates formed in the α-Al phase of the specimen after heat treatment (Example 9) was measured at 0.49% by weight, and the average Mg content in the peripheral area of ​​the same specimen was measured at 0.34% by weight. This suggests that through heat treatment, supersaturated Mg in the α-Al matrix participated in precipitate formation, while the Mg concentration of the matrix decreased and softened.

[0067] Experimental Example 3: Measurement of Precipitate Size After the heat treatment in Example 9, the fine precipitates in the α-Al phase matrix observed on the test specimen (see Figure 2) were analyzed using a field emission scanning electron microscope (FE-SEM, Hitachi). The precipitate image (10,000x magnification) was used to measure the overall precipitate size in the image using an image processing-based object extraction method. A technique was employed to distinguish the object of interest from the background based on the brightness difference of pixels in the image and identify the contour of the object. As a result, the average particle size was approximately 0.029 μm, and D 10 and D 90 These were 0.008 μm and 0.080 μm, respectively. These results quantitatively support the fact that the precipitates observed in the specimen of Example 9 in Experimental Example 1 actually have an extremely fine size on the level of tens of nanometers. Also, D 10 and D 90 The range of values ​​indicates that the size of the precipitates is distributed relatively uniformly, which is consistent with the uniformly dispersed precipitates observed in the SEM image of Experimental Example 1. The formation of fine and uniformly sized precipitates within the α-Al phase matrix, along with the decrease in Mg concentration (softening) of the matrix confirmed in Experimental Example 2, is judged to be a major factor contributing to improved bendability by improving the overall ductility of the plating film and mitigating stress concentration during bending deformation.

[0068] Experimental Example 4: Analysis of the phase fraction of Mg-based intermetallic compounds in plated films To analyze the phase composition of Mg-based intermetallic compounds within the plating layer after heat treatment following plating, an X-ray diffraction (XRD) analyzer from Rigaku was used. A Cu K-alpha beam was used as the incident beam, and the analysis was performed on Comparative Example 1 (before heat treatment) and Example 9 (after heat treatment), respectively. Mg2Si, MgZn2, and Mg2Zn were analyzed through XRD pattern analysis. 11 The phase fraction (area %) was quantified, and the results are shown in Table 3 below.

[0069] [Table 3]

[0070] Table 3 shows the results of the XRD analysis, confirming that the composition of intermetallic compounds in the plating layer changed significantly through the heat treatment according to the present invention (Example 9). In particular, the fraction of MgZn2, known as the hard phase, decreased significantly from 9% in Comparative Example 1 to 2.9% in Example 9, and the Mg2Zn that imparts relative ductility was reduced. 11 While the Mg2Si phase was absent in Comparative Example 1 (0%), it was newly formed in a large quantity (14.0%) in Example 9. Furthermore, the fraction of the Mg2Si phase decreased from 21% to 7%.

[0071] Such changes in phase fraction, namely the decrease in MgZn2 and Mg2Zn 11 The formation of this layer is the main factor that lowers the overall hardness of the plating layer and improves its ductility. Example 9 is Mg2Zn 11 The ratio of phase (b) to MgZn2 phase (a) (b / a) is approximately 4.83, and the ratio of the total amount of Mg-Zn compounds to Mg2Si phase (c) ((a+b) / c) is approximately 2.41, both of which are quantitatively confirmed to be within the target range of the present invention. This supports the idea that the specific heat treatment process according to the present invention can dramatically improve the bendability of plated steel sheets by effectively controlling the types and fractions of intermetallic compounds in the plating film.

[0072] Experimental Example 5: Evaluation of Bendability The adhesion and bendability of the plating film on the test pieces of Comparative Example 1 and Examples 1-9 were evaluated in accordance with the plating adhesion test described in JIS G 3321 (2019). Each test piece was bent 180° with an inner spacing nt (where t: thickness of the plated steel sheet (0.8 mm in this experiment), and n: number of plated steel sheets corresponding to 0, 1, 2, and 3), and the outer surface of the bent portion was observed with a 10x magnifying glass. The evaluation criteria are as follows, and the surface condition of representative processed portions corresponding to each criterion (◎, ○, △, and ×) was photographed and is shown in Figure 3. The evaluation results of the bendability for each test piece are summarized in Table 4 below.

[0073] ◎: No cracks ○: Microcrack formation △: Cracks occur throughout the entire surface (discontinuous) ×: Cracks occur throughout the entire area (continuous)

[0074] [Table 4]

[0075] As shown in Table 4, the bendability varied significantly depending on the heat treatment conditions. In Comparative Example 1, which was not heat-treated, continuous overall cracks (X) occurred not only in the most severe 0T bend but also in 1T and 2T bends, and discontinuous overall cracks (△) were observed in the 3T bend, indicating that the bendability was extremely weak.

[0076] Examples 1-6 involved heat treatment at 200°C or 220°C, and showed a slight improvement compared to Comparative Example 1. For example, Example 3, which was heat-treated at 200°C for 5 hours, and Example 6, which was heat-treated at 220°C for 5 hours, showed a level of fine cracking (○) in 2T and 3T bending, confirming a certain level of improvement in workability. However, under even more severe bending conditions such as 0T or 1T, overall cracking (△ or X) still occurred, which was somewhat insufficient for application in fields requiring high workability.

[0077] On the other hand, in Examples 7 and 8, where the reheating temperature was increased to 240°C, the bendability was significantly improved. In these test specimens, discontinuous overall cracks (△) were observed under 0T bending, but under 1T, 2T, and 3T bending conditions, they all showed good workability at the level of fine cracks (○), suggesting that these are desirable heat treatment conditions to consider in the present invention.

[0078] In particular, Example 9, which was heat-treated at 240°C for 5 hours, showed the best bendability in this experiment. Even under extremely harsh conditions such as 0T and 1T bending, Example 9 remained at the level of fine cracks (○), and no cracks were observed under 2T and 3T bending conditions (◎), demonstrating perfect crack resistance. This is judged to be a result of realizing the optimal bendability targeted by the present invention.

[0079] These differences in bendability are directly related to the changes in microstructure and phase fraction observed in Experimental Examples 1-4. Specifically, the brittle workability of Comparative Example 1 is thought to be due to its coarse solidification structure and high MgZn2 fraction. On the other hand, in the Examples, particularly Examples 7-9, which underwent sufficient heat treatment at 240°C, the formation of fine and uniform precipitates within the α-Al phase matrix, matrix softening, and a decrease in MgZn2 and Mg2Zn2 were observed. 11 It is interpreted that the combined action of advantageous phase fraction control of intermetallic compounds, such as an increase in [specific compound], significantly improved the ductility of the plating film and maximized bendability by effectively mitigating stress concentration. Among these, Example 9 is judged to be the result of the most optimized control of such microstructure.

[0080] The descriptions herein provided herein are illustrative, and those with ordinary skill in the art to which one aspect of the present invention belongs will understand that the invention can be readily modified into other specific forms without altering the technical idea or essential features described herein. Therefore, the embodiments described herein should be understood to be illustrative and not limiting in all respects. For example, each component described as a single type may be implemented in a distributed manner, and similarly, components described as distributed may be implemented in a combined manner.

[0081] The scope of the present invention is defined by the claims set forth below, and all modifications or alterations derived from the meaning and scope of the claims and the concept of equivalents thereof should be construed as being included within the scope of the present invention.

Claims

1. A hot-dip aluminum-zinc-magnesium plated steel sheet containing a plating film that, by mass%, comprises Zn: 35-55%, Si: 0.5-3.0%, Mg: 0.01-3.0%, with the remainder being Al and unavoidable impurities, The aforementioned plating film consists of a dendritic α-Al phase matrix and Zn phase, Mg phase, and Mg present in the interdendritic regions. ２ Si phase, MgZn ２ Phase, and Mg ２ Zn １１ Having an organization that includes a phase, The α-Al phase forms a matrix that penetrates the entire plating layer in a dendritic manner. A precipitate containing Mg is dispersed within the matrix. The average particle size of the precipitates is 0.01 to 0.1 μm, the mass ratio of Mg to Si in the plating film is 0.2:1 to 2.0:1, and the mass ratio of Zn to Al in the plating film is 0.7:1 to 1.2:

1. A hot-dip aluminum-zinc-magnesium plated steel sheet, wherein when the area fraction of the MgZn 2 phase is a, the area fraction of the Mg2Zn 11 phase is b, and the area fraction of the Mg2Si phase is c, b / a is 2 to 10 and (a + b) / c is 1 to 5.

2. In the cumulative particle size distribution based on the number of precipitates, the 10% particle size of the precipitate is D １０ In the cumulative particle size distribution based on the number of precipitates, the 90% particle size of the precipitate is D ９０ In that case, D ９０ / D １０ The hot-dip aluminum-zinc-magnesium plated steel sheet according to claim 1, wherein the ratio is 15 or less.

3. The hot-dip aluminum-zinc-magnesium plated steel sheet according to claim 1, wherein the Mg content in the α-Al phase is 0.3 to 0.4% by mass.

4. The process involves forming a plating film on the surface of a steel sheet using a plating bath containing, by mass%, Zn: 35-55%, Si: 0.5-3.0%, Mg: 0.01-3.0%, with the remainder being Al and unavoidable impurities. The steps include: primary cooling of the steel sheet on which the plating film is formed at a temperature of 150°C or lower, with an average cooling rate of 10°C / sec or higher; The first step is to heat the steel plate that has been cooled in the first step at a temperature of 200°C or higher but less than 260°C, such that the average rate of heating from 150°C to the reheating temperature is 3 to 8°C / h, and maintain it for 1 hour or more but not more than 12 hours. The reheated steel plate is subjected to a secondary cooling step such that the average cooling rate from the reheating temperature to 150°C is 10 to 12°C / h, the average cooling rate from 150°C to 80°C is 5 to 6°C / h, and the average cooling rate from 80°C to room temperature is 1 to 2°C / h. A method for manufacturing a molten aluminum-zinc-magnesium plated steel sheet, including the following:

5. The method for producing a molten aluminum-zinc-magnesium plated steel sheet according to claim 4, wherein the reheating temperature is 220°C or higher and 250°C or lower.

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