EP4640910A1

EP4640910A1 - Plated steel sheet having excellent corrosion resistance, and manufacturing method therefor

Info

Publication number: EP4640910A1
Application number: EP23907646.6A
Authority: EP
Inventors: Myung-Soo Kim; Tae-Chul Kim; Sung-Joo Kim; Seul-Gi SO
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-19
Filing date: 2023-12-18
Publication date: 2025-10-29
Also published as: WO2024136361A1; JP2026500353A; KR20250126778A; EP4640910A4; CN120344708A

Abstract

The present invention provides a plated steel sheet having excellent corrosion resistance even under complex corrosive conditions, and a manufacturing method therefor.

Description

Technical Field

The present disclosure relates to a plated steel sheet having excellent corrosion resistance even under a complex corrosive environment and a method for manufacturing the same.

Background Art

A zinc-based plated steel sheet has a property of sacrificial corrosion, in which zinc having a lower oxidation-reduction potential than iron corrodes first to suppress corrosion of a steel material, when exposed to a corrosive environment. In addition, as zinc in a plating layer oxidizes, dense corrosion products are formed on a surface of the steel material, which blocks the steel material from an oxidizing atmosphere, thereby improving corrosion resistance of the steel material. Due to these advantageous properties, the scope of applications of the zinc-based plated steel sheet has recently been expanded to steel sheets for construction materials, home appliances, and automobiles.
However, a corrosive environment is gradually worsening due to an increase in air pollution due to industrial advancement. In addition, in accordance with industrial diversification, steel sheets are exposed to more complex corrosive environments. Accordingly, there is a need to develop a steel material having better corrosion resistance in various corrosive environments than conventional zinc-based plated steel sheets.
For example, steel sheets used for the interior of buildings may be placed inside the building or in portions in contact with cement, while steel sheets used for the exterior of buildings may be placed in an acidic corrosive atmosphere due to acid rain. As described above, in a complex corrosive environment, a plated steel sheet having excellent corrosion resistance in both acidic and alkaline atmospheres is required, but in the technique related to conventional zinc-based plated steel sheets, a technique having sufficient corrosion resistance in both acidic and alkaline atmospheres has not been developed.

[Related Art Document]

[Patent Document]

(Patent Document 1) Korean Patent Publication No. 2013-0133358

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide a plated steel sheet having excellent corrosion resistance even under a complex corrosive environment and a method for manufacturing the same.
Another aspect of the present disclosure is to provide a plated steel sheet having excellent corrosion resistance even in an acidic environment and an alkaline environment and a method for manufacturing the same.
Still another aspect of the present disclosure is to provide a plated steel sheet having excellent corrosion resistance not only in a neutral environment but also in an acidic environment and an alkaline environment, and a method for manufacturing the same.
An object of the present disclosure is not limited to the above description. Those skilled in the art to which the present disclosure pertains may have no difficulty in understanding an additional object of the present disclosure from the entire content of the present specification.

Solution to Problem

According to an aspect of the present disclosure, a plated steel sheet includes:

a base steel sheet; and
an Al-Mg-Zn-based plating layer provided on at least one surface of the base steel sheet,
wherein in the plating layer, a number density of Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase is 500 to 4,000/0.1 mm².

According to another aspect of the present disclosure, a method for manufacturing a plated steel sheet includes:

hot-dip plating a base steel sheet by immersing the base steel sheet in a plating bath containing, by weight%, 4.00 to 7.00% of Mg, 8.000 to 20.000% of Al, 0.002 to 0.050% of Fe, a balance of Zn and other unavoidable impurities, and having a temperature of 430 to 520°C and a flow velocity of 0.03 to 0.20 m/s;
wiping the hot-dip plated steel sheet; and
cooling the wiped steel sheet so that an average cooling rate satisfies 6.0°C/s or more in a range from an Al phase crystallization temperature to 330°C.

Advantageous Effects of Invention

According to an aspect of the present disclosure, it is possible to provide a plated steel sheet having excellent corrosion resistance even under a complex corrosive environment and a method for manufacturing the same.
According to another aspect of the present disclosure, it is possible to provide a plated steel sheet having excellent corrosion resistance even in an acidic environment and an alkaline environment and a method for manufacturing the same.
According to still another aspect of the present disclosure, it is possible to provide a plated steel sheet having excellent corrosion resistance not only in a neutral environment but also in an acidic environment and an alkaline environment and a method for manufacturing the same.
Various and beneficial merits and effects of the present disclosure are not limited to the descriptions above, and may be more easily understood in a process of describing specific example embodiments in the present disclosure.

Brief Description of Drawings

FIG. 1 is an image obtained by observing a cross-sectional specimen of a plated steel sheet obtained from Inventive Example B4 of the present disclosure using a scanning electron microscope (SEM).
FIG. 2 is an image obtained by observing a cross-sectional specimen of a plated steel sheet obtained from Comparative Example B14 of the present disclosure using a scanning electron microscope (SEM).

Best Mode for Invention

The terms used in the present specification are for explaining specific example embodiments and are not intended to limit the present disclosure. In addition, a singular form used in the present specification also includes a plural form unless the context clearly indicates otherwise.
The meaning of "comprising" used in the specification is to specify the configuration and does not exclude the presence or addition of other configurations.
Unless otherwise defined, all the terms including the technical terms and scientific terms used in the present specification have the same meanings as commonly understood by those skilled in the art to which the present disclosure pertains. The terms defined in the preamble are interpreted as being consistent with the relevant technical literature and the disclosure herein.
Hereinafter, a plated steel sheet according to an aspect of the present disclosure will be described in detail. In the present disclosure, a content of each element means weight(wt)%, unless otherwise defined.
Recently, in accordance with industrial diversification, steel sheets are exposed to more complex corrosive environments, and due to these complex corrosive environments, there is a need for a plated steel sheet having excellent corrosion resistance in both acidic and alkaline atmospheres.
However, in the technique related to conventional zinc-based plated steel sheets, a technique having sufficient corrosion resistance in both acidic and alkaline atmospheres has not been developed.
In addition to the zinc-based plated steel sheets, there are aluminum-based plated steel sheets such as Al plating and Al-Zn plating (Galvalume); however, aluminum-based plated steel sheets have excellent corrosion resistance in an acidic atmosphere, but cannot be applied in complex corrosive environments because aluminum corrodes severely in an alkaline atmosphere.
In order to solve these problems, various studies have been conducted on a technique for manufacturing a zinc alloy-based plated steel sheet that improves corrosion resistance of a steel sheet by adding elements such as aluminum (Al) and magnesium (Mg) to a zinc plating bath. A representative example is an Al-Mg-Zn-based zinc alloy plated steel sheet obtained by additionally adding Mg to an Al-Zn plating composition. When Mg and Al are appropriately added to zinc, excellent corrosion resistance may be ensured even in acidic and alkaline atmospheres. In the Al-Mg-Zn-based zinc alloy plated steel sheet, when the amount of Mg added to the plating layer increases, corrosion is easily caused by rapid elution of Mg in an acidic atmosphere, but corrosion is suppressed in neutral (including a salt water environment) and alkaline atmospheres due to the formation of stable Mg-based hydroxide on the surface of the steel sheet.
On the other hand, when the amount of Al added to the plating layer increases, the corrosion resistance increases because the aluminum oxide on the surface is stable in acid and neutral environments, but, conversely, in an alkaline atmosphere, the corrosion resistance decreases because elution of Al becomes severe; thus, it is necessary to appropriately adjust the components of Mg and Al in the plating layer, and it is necessary to uniformly adjust the distribution of the components of Mg and Al in the plating layer.
First, a plated steel sheet according to an aspect of the present disclosure includes: a base steel sheet; and a plating layer provided on at least one surface of the base steel sheet.
In the present disclosure, the type of the base steel sheet may not be particularly limited. For example, the base steel sheet may be an Fe-based base steel sheet, that is, a hot-rolled steel sheet or a cold-rolled steel sheet, used as a base steel sheet of a conventional zinc-based plated steel sheet, but is not limited thereto. Alternatively, the base steel sheet may be carbon steel, ultra-low carbon steel, or high manganese steel used for construction, home appliances, or automobiles, or may be stainless steel or the like. Meanwhile, in the case of the carbon steel, ultra-low carbon steel, medium-low carbon steel, low carbon steel, and carbon steel all exhibit similar effects, and there is no need to specifically limit the steel components; thus, it is hardly affected by components such as Mn, Si, Ti, Nb, and B, which are added in large quantities to high-strength steel and ultra-high-strength steel. Meanwhile, examples of the base steel sheet include a base steel sheet containing, by wt%, more than 0% and 0.18% or less of C, more than 0% and 1.5% or less of Si, 0.01 to 2.7% of Mn, more than 0% and 0.07% or less of P, more than 0% and 0.015% or less of S, more than 0% and 0.5% or less of Al, 0.06% or less (including 0%) of Nb, 1.1% or less (including 0%) of Cr, 0.06% or less (including 0%) of Ti, 0.03% or less (including 0%) of B, and a balance of Fe and other unavoidable impurities. Examples of the base steel sheet include a base steel sheet containing, by wt%, more than 0% and 0.18% or less of C, more than 0% and 1.5% or less of Si, 0.01 to 2.7% of Mn, more than 0% and 0.07% or less of P, more than 0% and 0.015% or less of S, more than 0% and 0.5% or less of Al, more than 0% and 0.06% or less of Nb, more than 0% and 1.1% or less of Cr, more than 0% and 0.06% or less of Ti, more than 0% and 0.03% or less of B, and a balance of Fe and other unavoidable impurities.
According to an aspect of the present disclosure, an Al-Mg-Zn-based plating layer formed of an Al-Mg-Zn-based alloy may be provided on at least one surface of the base steel sheet as a plating layer. The plating layer may be formed on only one surface of the base steel sheet, or may be formed on both surfaces of the base steel sheet. In this case, the Al-Mg-Zn-based plating layer refers to a plating layer containing Mg and Al and mainly containing Zn (that is, containing 50% or more of Zn).
According to an aspect of the present disclosure, the plating layer may contain, by wt%, 4.00 to 7.00% of Mg, 8.000 to 20.000% of Al, 0.002 to 0.050% of Fe, and a balance of Zn and other unavoidable impurities. Meanwhile, although not specifically limited, the plating layer may optionally further contain, by wt%, one or more selected from 0.20% or less (including 0%) of Si and 0.200% or less (including 0%) of Ca. Hereinafter, each component will be described in detail.

Mg: 4.00 to 7.00%

Mg is an element serving to improve corrosion resistance of a plated steel sheet, and in the present disclosure, a content of Mg in the plating layer is controlled to 4.00% or more to ensure the desired excellent corrosion resistance. According to an example embodiment of the present disclosure, the content of Mg may be 4.10% or more.
When the Al-Mg-Zn-based plated steel sheet is exposed to a neutral, weakly acidic, or weakly alkaline corrosive environment such as salt water or rainwater, Mg within the plating layer is eluted, and as a result, layered double hydroxide (LDH) ((Zn,Mg)₆Al₂(OH)₁₆(CO₃)·4H₂O)) is uniformly formed, and corrosion resistance is improved. Therefore, a higher content of Mg is preferable. However, in an acidic corrosive environment, since Mg elution is too fast, the greater the amount of Mg added, the worse the corrosion resistance in an acidic environment. In addition, when Mg is excessively added, since dross in the form of MgO may be generated in a plating bath, in the present disclosure, the content of Mg may be set to be 7.00% or less. According to an example embodiment of the present disclosure, the content of Mg may be 6.90% or less.

Al: 8.000 to 20.000%

Al is a component that improves corrosion resistance together with Mg, and since Al is resistant to acid, corrosion resistance in an acid environment increases significantly as a content of Al increases. Therefore, in order to ensure the effects described above, in the present disclosure, the content of Al may be set to be 8.000% or more. According to an example embodiment of the present disclosure, the content of Al may be 8.200% or more. According to an example embodiment of the present disclosure, the content of Al may be 8.500% or more. On the other hand, as the amount of Al added increases, corrosion increases in an alkaline environment. In addition, Al in the plating bath has the effect of inhibiting the oxidation of Mg. Therefore, as the amount of Al added increases, the generation of MgO-based dross is suppressed. However, when the amount of Al added is too high, since a melting point of the plating bath increases, a high plating bath temperature should be maintained, which causes severe erosion of the structure within the plating bath. Therefore, the content of Al in the plating layer may be set to be 20.000% or less. According to an example embodiment of the present disclosure, the content of Al may be 19.800% or less.

Fe: 0.002 to 0.050%

Fe is a component that plays an important role in the present disclosure, and in some cases, Fe is directly added to the plating bath, and may also be eluted and present from the steel sheet. Fe is contained in an ingot that is inserted when the plating bath is first manufactured and the component is adjusted, but as a plating process progresses, Fe is eluted from the steel sheet. Therefore, Fe is periodically analyzed, and when a content of Fe is low, an additional ingot is replenished, and when the content of Fe is excessive, the content is adjusted through a dilution or removal process.
In a plating bath to which Al is added, Fe is usually present in the form of Fe₂Al₅. Since Fe₂Al₅ has a lighter specific gravity than the plating bath, Fe₂Al₅ floats to the surface of the plating bath and agglomerates together to grow into large dross. Such dross adheres to the steel sheet during the plating process, which causes dross adhesion defects. In addition, when a plating bath flow is large, coarse dross flows within the plating bath, which causes dross imprinting defects on the steel sheet; thus, Fe is periodically removed. However, in a case in which Fe₂Al₅ is fine with a size (diameter) of 0.05 µm or less, when a flow is applied to the plating bath, Fe₂Al₅ does not float to the surface, but flows in the plating bath, and some of Fe₂Al₅ adheres to the steel sheet and is present in the plating layer. In the present disclosure, since Fe present in the plating layer before being solidified acts as a crystal nucleation site during a solidification process, Fe needs to be added at least 0.002% or more. On the other hand, when the content of Fe exceeds 0.050%, the amount of dross generated on the surface of the plating bath increases, which may cause a problem in which dross adhesion defects increase. According to an example embodiment of the present disclosure, the content of Fe may be 0.045% or less.

Si: 0.20% or less (including 0%)

When Si is added in an amount of 0.2% or less, a thick Fe-Al-based alloy layer is prevented from forming at an interface between the base iron and the plating layer, such that a decrease in the interface strength between the plating layer and the base iron may be prevented. Therefore, although it is advantageous to add Si, even when Si is not added, it has little effect on the corrosion resistance in the present disclosure, and therefore, a lower limit thereof is set to be 0%.
However, even when the amount of Si added exceeds 0.20%, the effect of suppressing the Fe-Al-based alloy layer at the interface reaches saturation. As the amount of Si added increases, the melting point of the plating bath increases, and a high plating bath temperature should be maintained, which is not preferable from the viewpoint of equipment protection. According to an example embodiment of the present disclosure, the content of Si may be 0.18% or less.

Ca: 0.200% or less (including 0%)

Ca does not necessarily need to be added, and when added up to 0.200%, MgO oxide formation in the plating bath may be suppressed. In addition, since Ca is added in a small amount for convenience of work when manufacturing ingots for manufacturing a plating bath, Ca may be present in a small amount in the plating bath manufactured using the ingots. However, when the amount of Ca added exceeds 0.200%, the color of the steel sheet may darken, which is not preferable. According to an example embodiment of the present disclosure, the content of Ca may be 0.180% or less.

Balance of Zn and other unavoidable impurities

In addition to the components described above, the components eluted during the ingot manufacturing process or from the steel sheet are comprised of unavoidable impurities and zinc components present in the plating bath. The unavoidable impurities may include Sb, Sn, Pb, Sr, Cu, and the like, which are components that are unavoidably mixed in trace amounts during the manufacturing of ingots for manufacturing a plating solution. Meanwhile, as the steel sheet is immersed in the plating bath, the components that are unavoidably eluted and present in trace amounts in the plating bath may include Mn, Ti, Ni, B, Nb, and the like, and other components may also be present depending on the components of the steel sheet. However, even when added unavoidably, it is not preferable for each component to be present in an amount of 0.1% or more.
According to an example embodiment of the present disclosure, the plating layer may further contain one or more of the following groups (a) to (h).
However, since the elements of each group below are not essential elements for achieving the object of the present disclosure, a lower limit of the content of each element is not limited. Therefore, even when not specifically mentioned below, the lower limit of the content of each element may be 0%.

(a) 0.5% or less of Ni
(b) One or more of 0.1% or less of La, 0.1% or less of Ce, 0.1% or less of Y, and 1.0% or less of Sr
(c) 0.1% or less of Ti
(d) 0.5% or less of W
(e) 2.0% or less of Cu
(f) One or more of 0.5% or less of Cr, 0.5% or less of Mn, and 0.5% or less of V
(g) One or more of 0.1% or less of B and 0.1% or less of P
(h) One or more of 1.0% or less of Sn, 1.0% or less of Sb, and 1.0% or less of Bi

(a) 0.5% or less of Ni

Ni has the effect of preventing Fe diffusion by forming an Al-Ni alloy phase, but when a content of Ni exceeds 0.5%, the cost of secondary raw materials may increase excessively.

(b) One or more of 0.1% or less of La, 0.1% or less of Ce, 0.1% or less of Y, and 1.0% or less of Sr

La, Ce, Y, and Sr have the effect of preventing oxidation of Mg in the plating bath by forming an oxide film, but when contents of these elements exceed 0.1%, 0.1%, 0.1%, and 1.0%, respectively, a viscosity of the plating bath may increase, which may deteriorate platability.

(c) 0.1% or less of Ti

Ti has the effect of refining grains (spangles) because a Ti-Al intermetallic compound acts as a nucleation site, but when a content of Ti exceeds 0.1%, the melting point of the plating bath may increase and dross may increase.

(d) 0.5% or less of W

W forms W oxide on the surface, which improves corrosion resistance, but when a content of W exceeds 0.5%, the melting point of the plating bath may increase.

(e) 2.0% or less of Cu

Cu has the effect of forming an Al-Cu eutectic structure and lowering the hardness of the plating layer, but when a content of Cu exceeds 2.0%, the spangles may coarsen.

(f) One or more of 0.5% or less of Cr, 0.5% or less of Mn, and 0.5% or less of V

Cr, Mn, and V have the effect of preventing electrode deterioration by suppressing alloying between zinc and a welding electrode due to rapid liquid loss, but when a content of each element exceeds 0.5%, the melting point of the plating bath may increase excessively.

(g) One or more of 0.1% or less of B and 0.1% or less of P

B and P have the effect of suppressing welding LME cracks, but when a content of each element exceeds 0.1%, dross generation may increase.

(h) One or more of 1.0% or less of Sn, 1.0% or less of Sb, and 1.0% or less of Bi

Sn, Sb, and Bi have the effect of implementing uniform spangles and improving pot durability by lowering the plating bath temperature, but when a content of each element exceeds 1.0%, the spangles may coarsen.
Hereinafter, an alloy phase in the plating layer according to an aspect of the present disclosure will be described. The plating layer according to an aspect of the present disclosure may contain various phases, such as an MgZn₂ phase, an Al phase, an Al-Zn-based binary eutectic phase, a Zn-MgZn₂-Al, and a Zn phase. In particular, according to an aspect of the present disclosure, the plating layer may essentially contain an Al phase and an MgZn₂ phase, and may further contain one or more selected from an Al-Zn-based binary eutectic phase, a Zn-MgZn₂-Al-based ternary eutectic phase, and a Zn phase.
In this case, in the present disclosure, the MgZn₂ phase means a phase mainly comprised of MgZn₂, and components other than Mg and Zn may be contained in an amount of 5% or less (including 0%) in terms of atomic%. In addition, the Al phase means a phase mainly comprised of Al, and specifically, a phase in which Zn is solid-dissolved in an amount of less than 27% (including 0%) in terms of atomic% and a balance is comprised of Al and other impurities (the total amount of impurities is 2 atomic% or less (including 0%)). That is, in addition to the Al component, the Al phase may also contain solid-dissolved components such as Zn and Mg that may be contained as plating layer components, and in the present disclosure, it should be noted that the Al phase refers only to a phase in which Zn is solid-dissolved in an amount of less than 27 atomic% (including 0%).
In addition, the Zn-MgZn₂-Al-based ternary eutectic phase refers to a ternary eutectic phase in which a Zn phase, a MgZn₂ phase, and an Al phase are all mixed, and the Al-Zn-based binary eutectic phase refers to a phase in which an Al phase and a Zn phase are arranged alternately in a lamellar or irregularly mixed form.
In this case, it should be noted that the Al phase within the Al-Zn-based binary eutectic phase and the Zn-MgZn₂-Al-based ternary eutectic phase is not regarded as the Al phase described above. Similarly, it should be noted that MgZn₂ within the Zn-MgZn₂-Al-based ternary eutectic phase is not regarded as the MgZn₂ phase mainly comprised of MgZn₂ described above.
Meanwhile, a microstructure of the plating layer described above may have different distributions on the surface and cross section. The microstructure on the surface and cross section may be confirmed by magnifying the magnification of the plating layer for each surface specimen or cross-sectional specimen using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).
As such, the Zn-Mg-Al-based plating layer contains various phases depending on the composition and manufacturing conditions of the plating layer. However, as a result of conducting extensive studies on the distribution of phases within the plating layer in order to ensure that an Al-Mg-Zn-based plated steel sheet has excellent corrosion resistance not only in a corrosive environment near neutrality but also in acidic and alkaline regions, the present inventors have confirmed that the MgZn₂ phase exhibits excellent corrosion resistance in an alkaline range and the Al phase exhibits corrosion resistance in an acidic range, and discovered that when these two phase are appropriately distributed, excellent corrosion resistance may be ensured not only in a neutral atmosphere but also in all atmospheres including acidic and alkaline atmospheres, thereby completing the present disclosure.
Although not particularly limited, according to an example embodiment of the present disclosure, a Zn-Mg-Al-based plating layer satisfying the plating layer composition described above includes a microstructure in a form in which an Al phase is present inside a MgZn₂ phase or present in contact with a MgZn₂ phase. The form in which the Al phase is present inside the MgZn₂ phase means a form in which one Al phase is completely contained inside the MgZn₂ phase. Alternatively, the form in which the Al phase is present in contact with the MgZn₂ phase includes a form in which only a part of one Al phase is contained inside the MgZn₂ phase, or a form in which the Al phase is present in contact with the MgZn₂ phase.
Meanwhile, although not particularly limited, according to an aspect of the present disclosure, the plating layer may contain, by area%, 15.0 to 60.0% of a MgZn₂ phase and 3.0 to 25.0% of an Al phase based on a cross section in a thickness direction (meaning a direction perpendicular to a rolling direction). Corrosion resistance under a complex corrosive environment may be further improved by controlling fractions of the two phases to satisfy the above ranges.
The plating layer according to an aspect of the present disclosure contains the MgZn₂ phase, Al phase, Al-Zn-based binary eutectic phase, Zn-MgZn₂-Al, and Zn phase described above, and when a plated steel sheet containing these phases is used in an acidic corrosive atmosphere, the MgZn₂ phase corrodes first, whereas the Al phase corrodes slowly in an acid. On the other hand, when the plated steel sheet is used in an alkaline corrosive environment, the MgZn₂ phase is relatively resistant to corrosion compared to other phases, whereas the Al phase corrodes at a rapid rate. Therefore, in order to have excellent corrosion resistance under complex corrosive environments including acidic and alkaline environments, it is necessary to appropriately control the ratio, size, or distribution of these two phases. Hereinafter, the characteristics of these two phases, such as ratio, size, and distribution, will be described in detail.
According to an aspect of the present disclosure, the plating layer may have a ratio of the number of Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase to the total number of Al phases of 85% or more and 100% or less. When the ratio of the number of Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase to the total number of Al phases is less than 85%, corrosion resistance in acidic and alkaline environments may decrease. In this case, in the present disclosure, it should be noted that an Al phase, which is at least 0.5 µm or more based on an equivalent circular diameter, is targeted, and the same applies to the description below.
Although not particularly limited, according to an aspect of the present disclosure, even when a ratio of Al phases satisfies 3.0 to 25.0% in terms of area% based on the cross section in the thickness direction of the plated steel sheet, as illustrated in FIG. 2, in a case in which the size of the Al phase is coarse based on the cross section in the thickness direction of the plated steel sheet, when the Al phase is exposed in an alkaline corrosive environment, corrosion occurs preferentially, and the Al phase may act as a passage for movement of corrosion substances from the surface of the plating layer to the base steel sheet. Due to this, corrosion may occur in the base steel sheet even when the remaining phases, including the MgZn₂ phase, are intact. Meanwhile, as in FIG. 1 according to the present disclosure, in a case in which the Al phase is fine based on the cross section in the thickness direction of the plated steel sheet, even if the Al phase corrodes when exposed to an alkaline corrosive environment, another phase such as the MgZn₂ phase having excellent alkaline corrosion resistance is present with the Al phase, such that the movement of corrosion substances in a depth direction (thickness direction) of the plating layer is blocked, thereby improving the corrosion resistance. Although not particularly limited, in particular, when a large number of fine Al phases of 5 µm or less that are stable in an acidic environment are present, the effect of inhibiting penetration of corrosion substances toward the steel sheet may be better in an acidic environment. Therefore, although not particularly limited, according to an aspect of the present disclosure, an average longitudinal straight length of the Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase may be 5 µm or less (excluding 0 um). In addition, the more fine Al phases described above are present, the more corrosion resistance may be improved under a complex corrosive environment. Therefore, according to an aspect of the present disclosure, a ratio of the number of Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase and having a longitudinal straight length of 5 µm or less (excluding 0 µm) to the total number of Al phases may be 90.0 to 100.0%, and is not particularly limited thereto.
In addition, although not particularly limited, according to an aspect of the present disclosure, in the plating layer, a number density (D_al) of the Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase may be 500 to 4,000/0.1 mm². In the present disclosure, the number density described above means the number of Al phases present inside the MgZn₂ phase or preset in contact with the MgZn₂ phase per unit area of 0.1 mm² based on the cross section in the thickness direction of the plating layer. In this case, when the value of D_al is less than 500/0.1 mm², since a gap between the Al phases is too far, corrosion factors may easily propagate between the Al phases, which may cause a problem in that the effect of blocking the corrosion factors in an acidic atmosphere is reduced. In addition, when the value of D_al exceeds 4,000/0.1 mm², since the plating layer is mainly comprised of an Al phase and a MgZn₂ phase, a ratio of Al-Zn-based binary eutectic phase and/or Zn-MgZn₂-Al-based ternary eutectic phase, which has relatively excellent corrosion resistance in a neutral environment, decreases, which may cause a problem in ensuring corrosion resistance in a neutral environment.
Meanwhile, in the present disclosure, there are no particular limitations on the measurement methods for the area ratio of each phase, the number ratio of the Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase described above, the average longitudinal straight length, the number density, and the like, and the measurements may be performed using conventional methods known in the related art.
For example, the area ratio of each phase, the number ratio of the Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase described above, the average longitudinal straight length, the number density (D_al), and the like may be measured through component analysis using energy dispersive X-ray spectroscopy (EDS) to distinguish and define each phase after capturing a structural image of a cross-sectional specimen in the thickness direction of the plated steel sheet using a field emission scanning electron microscope (FE-SEM). Thereafter, the respective phases are labeled and distinguished from the captured image, and the area ratio, the number ratio, the average longitudinal straight length, the number density, and the like of the respective phases may be measured using an image analyzer. In this case, considering a deviation by region of each specimen, 20 points of each specimen are photographed at a magnification of 2,000 times and then an average value of each specimen is calculated, such that the parameters such as the area ratio, the number ratio, the average longitudinal straight length, and the number density of each phase described above may be measured.
Next, a method for manufacturing a plated steel sheet according to another aspect of the present disclosure will be described in detail. However, this does not mean that the plated steel sheet of the present disclosure should be manufactured by the following manufacturing method.
According to an aspect of the present disclosure, first, preparing a base steel sheet may be further included, and the type of the base steel sheet is not particularly limited. In this case, the same description described above may be applied to the base steel sheet.
Next, the base steel sheet is immersed in a plating bath containing, by wt%, 4.00 to 7.00% of Mg, 8.000 to 20.000% of Al, 0.002 to 0.050% of Fe, and a balance of Zn and other unavoidable impurities for hot-dip plating. In this case, the same description for the components of the plating layer described above may be applied to the reason for adding components and limiting the contents in the plating bath described above except for the small amount of impurities that may be introduced from the base steel sheet. Therefore, according to an aspect of the present disclosure, the plating bath may optionally further contain, by wt%, one or more selected from 0.20% or less (including 0%) of Si and 0.200% or less (including 0%) of Ca.
In order to manufacture the plating bath having the composition described above, a composite ingot containing predetermined Zn, Al, and Mg or a Zn-Mg or Zn-Al ingot containing individual components may be used. In order to replenish the plating bath consumed by hot-dip plating, the ingot is additionally melted and supplied. In this case, a method of melting an ingot by directly immersing the ingot in a plating bath may be adopted, or a method of melting an ingot in a separate pot and then replenishing molten metal in a plating bath may be adopted.
Meanwhile, when manufacturing the plating bath, 0.002 to 0.050% of Fe is contained as an essential component other than Zn, Al, and Mg. When manufacturing the initial plating bath, Fe in the plating bath may be manufactured by alloying Fe during the manufacture of the ingot, or Fe may be separately added during the manufacture of the plating bath. However, once the plating process begins, a portion of Fe may be eluted from the steel sheet. Therefore, it is necessary to analyze Fe periodically and manage Fe so that Fe is in a range of 0.002 to 0.050% specified in the present disclosure.
Fe in the plating bath reacts with Al to form an Fe₂Al₅ phase. The Fe₂Al₅ phase has a light specific gravity and floats to the surface of the plating bath, but when the size (diameter) is 0.05 µm or less, the Fe₂Al₅ phase does not float to the surface when there is a flow in the plating bath and flows within the plating bath. Therefore, according to an aspect of the present disclosure, fine Fe₂Al₅ dross having a size of 0.05 µm or less (excluding 0 um) flowing in the plating bath is a key factor affecting the number of alloy phase sizes of a final plating product in the present disclosure, and this will be described in detail. Meanwhile, coarse dross present on the surface of the plating bath does not correspond to 0.002 to 0.050% of Fe in the plating bath specified in the present disclosure. When analyzing the plating bath components, a sample collected at a depth of 30 cm in the thickness direction from the surface is analyzed to determine a content of Fe.
In addition, although not particularly limited, according to an aspect of the present disclosure, fine Fe₂Al₅ of 0.05 µm or less flowing in the plating bath adheres to the steel sheet together with the plating bath while the steel sheet passes through the plating bath. When the plating layer is cooled during the subsequent cooling process, Fe becomes a nucleation site. In the present disclosure, the phase that crystallizes first when the plating layer is solidified is an Al phase. Therefore, since the fine dross present in the plating layer before cooling after plating becomes an Al phase crystal nucleation site during the cooling process, the more floating dross there is, the more Al nucleation occurs and the finer the Al phase becomes. Therefore, in the present disclosure, the size (average diameter) of the floating dross in the plating bath may be present as fine dross of 0.05 µm or less (excluding 0 µm). In this case, as one of several factors that refine the size of the floating dross in the plating bath, there is control of the content of Fe, and when the content of Fe present in the plating bath is less than 0.002%, there is no or too little Fe₂Al₅ floating dross, which may cause a problem in that the Al phase becomes coarse due to low Al phase nucleation. On the other hand, when the content of Fe in the plating bath exceeds 0.05%, as the generation of Fe₂Al₅ dross increases, the dross may agglomerate together and coarsen, such that the coarse dross may float to the surface of the plating bath, which may interfere with the plating process, or the coarse dross may be mixed into the plating layer, which may cause dross imprinting defects on the steel sheet.
In addition, although not particularly limited, according to an aspect of the present disclosure, the temperature of the plating bath may be maintained at a temperature higher than a solidification initiation temperature (Ts) by 20 to 100°C. In this case, since the solidification initiation temperature may vary depending on the plating bath composition, the plating bath temperature may also be adjusted accordingly. Meanwhile, the temperature of the plating bath may be maintained in a range of 430 to 520°C.
In addition, according to an aspect of the present disclosure, a flow velocity of the plating bath may be controlled to 0.03 to 0.20 m/s. Even if Fe₂Al₅ dross of 0.05 µm or less is generated in the plating bath according to the present disclosure, when there is no flow in the plating bath, the dross is easy to float to the surface of the plating bath, and a dross deviation may also occur locally in the plating bath. Therefore, the flow velocity of the plating bath should be at least 0.03 m/s or more. On the other hand, when the flow velocity is too fast, the floating dross becomes too fine and adheres to the plating layer, and the number of Al phases becomes too excessive; thus, the effect of improving corrosion resistance in an acidic environment by the Al phase disappears.
In addition, according to an aspect of the present disclosure, in order to achieve a target coating weight after plating is completed, the hot-dip plated steel sheet may be wiped, and for example, a nitrogen (N₂) or air knife may be used. In this case, the coating weight is not particularly limited and is usually set to a level of 20 to 400 g/m² per surface.
In addition, according to an aspect of the present disclosure, cooling is performed after controlling the coating weight. In this case, the cooling is controlled so that the temperature of the steel sheet satisfies an average cooling rate of 6.0°C/s or more in a range from the Al phase crystallization temperature to 330°C. In the range from the Al phase crystallization temperature to 330°C, the faster the cooling, the slower the nucleus growth and the finer the crystals become. Therefore, the cooling rate is controlled as described above. There is no need to specifically limit an upper limit of the cooling rate in the corresponding range, and the cooling rate may be managed at an appropriate level that is typical in the related art, taking into account limitations of the cooling equipment, vibration, economic efficiency, and the like. In the present disclosure, the Al phase crystallization nucleation site is fine dross present in the plating layer before the plating layer solidification, but when the cooling rate is slow after the Al phase crystallization, the Al phase may grow and coarsen. Therefore, although several process factors are involved in controlling the number density of the Al phase and the size of the Al phase as described above, as one of the control factors, cooling at an average cooling rate of 6.0°C/sec or more in the range from the Al phase crystallization temperature to 330°C is required to ensure the number density and size of the Al phase desired in the present disclosure.
Meanwhile, since solidification of the plating layer is completed when the steel sheet temperature is lower than 330°C, a subsequent cooling rate is not significantly important, but considering productivity, a faster cooling rate is preferable.

Mode for Invention

Hereinafter, the present disclosure will be described more specifically with reference to Examples. However, it should be noted that the following Examples are only for illustrating the present disclosure in more detail and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Examples)

A plating bath was manufactured and used so that the composition of the components was as shown in Table 1. After manufacturing the plating bath, the final component confirmation was performed using inductively coupled plasma mass spectrometry (ICP). Meanwhile, the presence of fine dross in the plating bath was determined by collecting a plating bath specimen at a depth of 300 mm (in a thickness direction) from a plating bath surface, rapidly cooling the specimen in water, and confirming the specimen using a transmission electron microscope (TEM) at a magnification of 300,000X.
As base steel sheets, a hot-rolled steel sheet and a cold-rolled steel sheet were used, and the components in the base steel sheets were 0.017% of C, 0.014% of Si, 0.25% of Mn, 0.008% of P, 0.005% of S, 0.03% of Al, 0.02% of Nb, 0.1% of Cr, 0.02% of Ti, 0.015% of B, and a balance of Fe and other unavoidable impurities.
The hot-rolled steel sheet was first pickled with a hydrochloric acid aqueous solution to remove iron oxide formed on a surface of the steel sheet during a hot rolling process, heated to 650°C in a furnace under reducing conditions at a dew point temperature of -20°C, and then plated under the conditions of manufacturing process A in Table 2. The cold-rolled steel sheet was first subjected to alkaline immersion and electrolytic degreasing to remove rolling oil, iron, and other foreign substances on the surface of the steel sheet, annealed at 840°C in an annealing furnace in a reducing atmosphere, and then plated under the conditions of manufacturing process B in Table 2. After plating, a coating weight was adjusted to 150 g/m² per surface through N₂ wiping.
The plated steel sheet was cut into a predetermined size for analysis, and then, for a cross-sectional specimen in a thickness direction of the steel sheet (meaning a direction perpendicular to a rolling direction), an image of the plating layer was captured using FE-SEM, and the components were confirmed using EDS for accurate phase confirmation.
The images were captured at a magnification of 2,000x, and in order to ensure representativeness, 20 points on the specimen at a length of 20 mm were measured and analyzed to confirm phases on each image, and then, the area ratio, number, and the like of each phase were measured using an image analyzer. The average value of the 20 points is shown in Table 3.
In order to evaluate corrosion resistance, the specimens were cut to a size of 80 mm x 150 mm, three tests were performed for each specimen and experimental condition, and then, the average was calculated and evaluated according to the following criteria. The results are shown in Table 3.

⊚: A case in which the weight loss before and after the experiment is 1 g/m² or less is evaluated as very good.
∘: A case in which the weight loss before and after the experiment is more than 1 g/m² and less than 2 g/m² is evaluated as good.
△: A case in which the weight loss before and after the experiment is more than 2 g/m² and less than 3 g/m² is evaluated as poor.
×: A case in which the weight loss before and after the experiment is more than 3 g/m² is evaluated as very poor.

⊚: A case in which the red rust occurrence time is 4,200 Hr or longer is evaluated as very good.
∘: A case in which the red rust occurrence time is longer than 3,500 Hr and shorter than 4,200 Hr is evaluated as good.
△: A case in which the red rust occurrence time is longer than 2,000 Hr and shorter than 3,500 Hr is evaluated as poor.
×: A case in which the red rust occurrence time is shorter than 2,000 Hr is evaluated as very poor.

⊚: A case in which the weight loss before and after the experiment is 2 g/m² or less is evaluated as very good.
∘: A case in which the weight loss before and after the experiment is more than 2 g/m² and less than 4 g/m² is evaluated as good.
△: A case in which the weight loss before and after the experiment is more than 4 g/m² and less than 7 g/m² is evaluated as poor.
×: A case in which the weight loss before and after the experiment is more than 7 g/m² is evaluated as very poor.

In addition, for the corrosion resistance test in an acidic environment, a sulfuric acid aqueous solution with a pH of 3.5 was used, for the corrosion resistance test in a neutral environment, a 3.5% NaCl aqueous solution with a pH of 6.7 was used, and for the corrosion resistance test in an alkaline environment, ammonium water with a pH of 12.5 was used.
In the acidic and alkaline experiments, the specimens were immersed in the solution for 48 hours and then taken out, and the corrosion loss before and after the experiment was measured. A case in which the corrosion loss was small was evaluated as very good, and a case in which the corrosion loss was large was evaluated as very poor.

In the neutral test, the longer the time until red rust (corrosion of the base steel sheet) occurred after salt water was sprayed on the steel sheet, the better the corrosion resistance was evaluated.

[Table 1]

Classification	Al	Mg	Fe	Si	Ca	Sn	Sb	Mn
C1	9.820	4.81	0.003	-	-	-	-	-
C2	13.580	4.95	0.014	-
C3	12.530	5.27	0.008	0.12	-	-	-	-
C4	16.150	5.46	0.038	-	0.060	0.01	-	0.040
C5	19.380	6.10	0.026	0.15	0.090	-	0.06	-
C6	14.200	5.26	0.120	-	0.010	-	-	-
C7	4.360	4.23	0.003	-	-	-	-	-
C8	10.410	4.92	-	-	-	-	-	-
C9	19.070	2.23	-	0.15	0.010	-	-	-
C10	18.400	3.02	0.007	-	-	-	-	-
C11	0.251	-	0.021	-	-	-	-	-
C12	31.190	4.14	-	0.08	0.002			0.008

(In Table 1, '-' means 0 wt%.)

[Table 2]

Classification	Manufacturing process	Plating bath composition (Table 1)	Steel sheet introduction temperature (°C)	Plating bath temperature (°C)	Plating bath flow velocity (m/sec)	Al phase crystallization temperature to 330°C average cooling rate (°C/sec)
B1	A	C1	475	465	0.04	14.1
B2		C1	485	439	0.10	8.3
B3		C2	470	455	0.08	15.2
B4		C2	455	450	0.13	11.2
B5		C2	470	461	0	8.4
B6		C2	470	470	0.05	2.7
B7		C2	480	460	0.01	4.1
B8		C3	470	455	0.10	12.6
B9		C3	470	460	0.06	6.7
B10		C3	460	460	0.01	3.0
B11		C4	490	470	0.08	10.1
B12		C4	487	465	0.45	10.7
B13		C5	500	490	0.03	8.1
B14		C6	499	472	0.04	9.4
B15		C7	470	460	0.05	10.2
B16		C8	475	455	0.05	11.8
B17		C9	490	485	0.05	10.7
B18		C11	490	460	0.05	9.1
B19	B	C1	480	455	0.09	7.3
B20		C2	466	455	0.13	8.4
B21		C2	487	445	0	6.8
B22		C3	472	451	0.17	11.7
B23		C3	481	453	0.01	2.3
B24		C7	482	451	0.09	9.2
B25		C9	480	451	0.04	2.3
B26		C10	480	455	0	7.2
B27		C12	510	500	0.03	6.7

[Table 3]

Classification	MgZn₂ area ratio (%)	Al phase area ratio (%)	Al phase number density (/0.1 mm²)	Occupation ratio of Al phase having longitudinal straight length of 5 µm or less (%)	Ratio of Al phases present inside MgZn₂ phase or in contact with MgZn₂ phase (%)
B1	25.2	6.6	705	98.4	96
B2	21.3	5.1	575	90.1	100
B3	26.2	17.2	2720	97.7	100
B4	27.8	17.3	4000	96.4	100
B5	27.3	16.8	490	65.6	97
B6	28.8	18.5	299	72.8	100
B7	27.8	17.3	355	57.0	94
B8	31.7	16.3	2359	97.3	92
B9	32.0	13.8	1983	92.1	100
B10	36.2	12.7	332	51.4	99
B11	32.8	20.9	2925	93.9	91
B12	34.2	21.4	6300	100	89
B13	42.4	24.4	950	86.4	92
B14	26.9	16.4	1042	91.5	99
B15	20.1	1.1	48	100.0	100
B16	24.7	9.7	344	72.6	97
B17	4.8	24.5	299	45.1	99
B18	0	0	-	-	-
B19	22.8	5.2	500	89.7	100
B20	27.2	16.8	2936	92.3	100
B21	25.7	16.7	198	33.2	95
B22	29.1	16.1	3008	95.0	100
B23	30.9	15.8	399	29.8	94
B24	21.5	0.9	39	100.0	92
B25	1.7	24.1	207	25.3	97
B26	2.2	23.7	1860	74.2	56
B27	19.8	37.0	403	35.1	95

[Table 4]

Classification	Presence or absence of dross defects	Acidic corrosion resistance (pH 3.5)	Neutral corrosion resistance (pH 6.7)	Alkaline corrosion resistance (pH 12.5)
B1	Absence	○	○	⊚
B2	Absence	○	○	○
B3	Absence	⊚	○	○
B4	Absence	⊚	○	○
B5	Absence	⊚	○	Δ
B6	Absence	⊚	○	Δ
B7	Absence	⊚	○	X
B8	Absence	⊚	⊚	⊚
B9	Absence	⊚	⊚	⊚
B10	Absence	○	⊚	X
B11	Absence	⊚	⊚	⊚
B12	Absence	Δ	⊚	⊚
B13	Absence	⊚	⊚	○
B14	Presence	⊚	⊚	○
B15	Absence	X	○	○
B16	Absence	○	○	Δ
B17	Absence	⊚	Δ	X
B18	Absence	X	X	Δ
B19	Absence	○	○	○
B20	Absence	⊚	○	○
B21	Absence	⊚	○	Δ
B22	Absence	⊚	⊚	○
B23	Absence	⊚	⊚	X
B24	Absence	Δ	⊚	⊚
B25	Absence	⊚	Δ	X
B26	Absence	Δ	○	Δ
B27	Absence	⊚	⊚	X

As can be seen from the experimental results in the tables, in the cases of Examples B1 to B4, B8, B9, B11, B13, B19, B20, and B22 satisfying the plating composition and manufacturing conditions of the present disclosure, the corrosion resistance was excellent or significantly excellent under all conditions of acidic, neutral, and alkaline environments. In addition, since the content of Fe in the plating bath also did not exceed the upper limit of 0.05% specified in the present disclosure, no dross adhesion defects occurred.
Meanwhile, in the cases of Comparative Examples B5 and B21 in which the plating bath components and other manufacturing conditions were in accordance with the present disclosure, but since there was no flow in the plating bath, the Al crystallization nucleation sites were small, the number density of the Al phases was less than 500/0.1 mm², which is the lower limit specified in the present disclosure. Therefore, since there was a relatively large amount of coarse Al phases, the corrosion resistance was significantly excellent in an acidic environment and also excellent in a neutral environment, but the corrosion resistance was poor in an alkaline environment.
In the case of Comparative Example B6 in which the cooling rate from the Al phase crystallization temperature to 330°C after plating was slower than 6.0°C/sec specified in the present disclosure, although there were a large number of Al nucleus crystallization sites according to the present disclosure, it took time for the Zn-Al binary and ternary eutectic phases other than the Al phase and MgZn₂ phase to crystallize; thus, the number density of the Al phases was less than the lower limit of 500/0.1 mm² specified in the present disclosure, and the corrosion resistance was poor in an alkaline environment.
In the cases of Comparative Examples B7, B10, and B23 in which the plating bath flow velocity was lower than the range specified in the present disclosure and the cooling rate from the Al phase crystallization temperature to 330°C was slower than 6°C/s specified in the present disclosure, the number density of the Al phases was less than the lower limit of 500/0.1 mm² specified in the present disclosure, and the corrosion resistance in an alkaline environment was very poor.
Meanwhile, in the case of Comparative Example B12 in which the flow velocity of the plating bath was 0.45 m/sec, which exceeded the range specified in the present disclosure, when the flow velocity was too fast, the floating dross became too fine and adhered to the plating layer, and the number density of the Al phases exceeded the upper limit of 4,000/0.1 mm² specified in the present disclosure; thus, the effect of improving corrosion resistance in an acidic environment by the Al phase disappeared, resulting in poor corrosion resistance in an acidic environment.
In the case of Comparative Example B14 in which the content of Fe in the plating bath exceeded the range specified in the present disclosure, and coarse dross was generated on the surface of the plating bath, resulting in dross adhesion defects on the plated steel sheet.
In the cases of Comparative Examples B15 and B24 in which the content of Al in the plating bath components was lower than the range specified in the present disclosure, the number density of the Al phases was outside the range specified in the present disclosure, resulting in very poor corrosion resistance in an acidic environment.
In the case of Comparative Example B16 in which Fe was not present in the plating bath, the number of Al phases was smaller than the range specified in the present disclosure, and the number of Al phases which were coarse and had a size of 5 µm or less was smaller than 80.0% specified in the present disclosure, resulting in poor corrosion resistance in an alkaline environment.
In the case of Comparative Example B17 in which the content of Mg in the plating bath was lower than the range specified in the present disclosure and Fe was not added, the number density of the Al phases was low, resulting in very poor corrosion resistance in an alkaline atmosphere and poor corrosion resistance in a neutral atmosphere.
Meanwhile, in the case of Comparative Example B18 in which Mg was not added to the plating bath and Al was lower than the range specified in the present disclosure, a MgZn₂ phase and an Al phase were not crystallized in the plating layer, resulting in poor or very poor corrosion resistance in all acidic, neutral, and alkaline atmospheres.
In the case of Comparative Example B25 in which the content of Mg in the plating bath was lower than the range specified in the present disclosure and the cooling rate from the Al phase crystallization temperature to 330°C after plating in the plating bath in which Fe was not added was slower than 6.0°C/sec specified in the present disclosure, the number density of the Al phases in the plating layer was lower than the range specified in the present disclosure, and the number ratio of Al phases of 5 µm or less was also lower than the range specified in the present disclosure, resulting in very poor corrosion resistance in an alkaline atmosphere and poor corrosion resistance in a neutral atmosphere.
In the case of Comparative Example B26 in which the content of Mg in the plating bath was lower than the range specified in the present disclosure and there was no flow in the plating bath, the ratio of Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase exceeded the lower limit of the present disclosure, resulting in very poor corrosion resistance in an alkaline atmosphere and poor corrosion resistance in an acidic atmosphere.
In the case of comparative example B27 in which the content of Al in the plating bath was higher than the range specified in the present disclosure and Fe was not added, the number density of the Al phases was outside the range specified in the present disclosure, resulting in very poor corrosion resistance in an alkaline atmosphere.

Claims

A plated steel sheet comprising:
a base steel sheet; and

an Al-Mg-Zn-based plating layer provided on at least one surface of the base steel sheet,

wherein in the plating layer, a number density of Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase is 500 to 4,000/0.1 mm².
The plated steel sheet of claim 1, wherein the plating layer contains, by weight%, 4.00 to 7.00% of Mg, 8.000 to 20.000% of Al, 0.002 to 0.050% of Fe, and a balance of Zn and other unavoidable impurities.
The plated steel sheet of claim 2, wherein the plating layer further contains, by weight%, one or more selected from 0.2% or less (including 0%) of Si and 0.2% or less (including 0%) of Ca.
The plated steel sheet of claim 2, wherein the plating layer further contains one or more of the following groups (a) to (h):
(a) 0.5% or less of Ni,

(b) one or more of 0.1% or less of La, 0.1% or less of Ce, 0.1% or less of Y, and 1.0% or less of Sr,

(c) 0.1% or less of Ti,

(d) 0.5% or less of W,

(e) 2.0% or less of Cu,

(f) one or more of 0.5% or less of Cr, 0.5% or less of Mn, and 0.5% or less of V,

(g) one or more of 0.1% or less of B and 0.1% or less of P, and

(h) one or more of 1.0% or less of Sn, 1.0% or less of Sb, and 1.0% or less of Bi.
The plated steel sheet of claim 1, wherein in the plating layer, a ratio of the number of Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase to the total number of Al phases is 85% or more and 100% or less.
The plated steel sheet of claim 1, wherein an average longitudinal straight length of Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase is 5 µm or less (excluding 0 µm).
The plated steel sheet of claim 1, wherein a ratio of the number of Al phases present inside the MgZn₂ phase or present in contact with the MgZn₂ phase and having a longitudinal straight length of 5 µm or less (excluding 0 µm) to the total number of Al phases is 90.0 to 100.0%.
A method for manufacturing a plated steel sheet, the method comprising:
hot-dip plating a base steel sheet by immersing the base steel sheet in a plating bath containing, by weight%, 4.00 to 7.00% of Mg, 8.000 to 20.000% of Al, 0.002 to 0.050% of Fe, and a balance of Zn and other unavoidable impurities, and having a temperature of 430 to 520°C and a flow velocity of 0.03 to 0.20 m/s;

wiping the hot-dip plated steel sheet; and

cooling the wiped steel sheet so that an average cooling rate satisfies 6.0°C/s or more in a range from an Al phase crystallization temperature to 330°C.
The method of claim 8, wherein the plating bath further contains, by weight%, one or more selected from 0.20% or less (including 0%) of Si and 0.200% or less (including 0%) of Ca.
The method of claim 8, wherein the plating bath further contains one or more of the following groups (a) to (h):
(a) 0.5% or less of Ni,

(b) one or more of 0.1% or less of La, 0.1% or less of Ce, 0.1% or less of Y, and 1.0% or less of Sr,

(c) 0.1% or less of Ti,

(d) 0.5% or less of W,

(e) 2.0% or less of Cu,

(f) one or more of 0.5% or less of Cr, 0.5% or less of Mn, and 0.5% or less of V,

(g) one or more of 0.1% or less of B and 0.1% or less of P, and

(h) one or more of 1.0% or less of Sn, 1.0% or less of Sb, and 1.0% or less of Bi.
The method of claim 8, wherein a temperature of the plating bath is maintained at a temperature higher than a solidification initiation temperature (Ts) by 20 to 100°C.
The method of claim 8, wherein an average diameter of floating dross in the plating bath is controlled to be 0.05 µm or less (excluding 0 µm).