EP4357478A1

EP4357478A1 - Highly corrosion-resistant plated steel sheet having excellent corrosion resistance and surface quality, and manufacturing method therefor

Info

Publication number: EP4357478A1
Application number: EP22825232.6A
Authority: EP
Inventors: Sung-Joo Kim; Il-Ryoung Sohn; Tae-Chul Kim; Kwang-Won Kim; Sang-Tae Han; Myung-Soo Kim; Yong-Kyun Cho
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2021-06-18
Filing date: 2022-06-10
Publication date: 2024-04-24
Also published as: KR20220169338A; CN117561347A; WO2022265307A1; KR102529740B1; KR20220169450A

Abstract

One aspect of the present invention provides a plated steel sheet and a manufacturing method therefor, the plated steel sheet comprising: a base steel sheet; a Zn-Mg-Al-based plated layer provided on at least one surface of the base steel sheet; and an Fe-Al-based inhibition layer provided between the base steel sheet and the Zn-Mg-Al-based plated layer, wherein, on the surface of the Zn-Mg-Al-based plated layer, the total area fraction of an Al single phase and MgZn2 phase is 45-60%, and the area ratio of the MgZn2 phase to the Al single phase is 1.2-3.3.

Description

Technical Field

The present invention relates to a highly corrosion-resistant plated steel sheet having excellent corrosion resistance and surface quality, and a manufacturing method therefor.

Background Art

When exposed to a corrosive environment, a zinc-based plated steel sheet has a characteristic of a sacrificial method in which zinc, which has a lower oxidation-reduction potential than iron, corrodes first, thereby suppressing corrosion of a steel material. In addition, as zinc in a plating layer oxidizes, dense corrosion products are formed on a surface of the steel material, thereby blocking the steel material from an oxidizing atmosphere, thereby improving corrosion resistance of the steel material. Thanks to these advantageous properties, the scope of application of the zinc-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, and the need for the development of steel materials with better corrosion resistance than conventional galvanized steel materials is increasing due to strict regulations on resource and energy conservation.
To improve such a problem, various studies are being conducted into a manufacturing technology for zinc alloy-based plated steel sheets improving the corrosion resistance of steel materials by adding elements such as aluminum (Al) and magnesium (Mg) to a zinc plating bath. A representative example is a Zn-Mg-Al-based zinc alloy plated steel sheet in which Mg is additionally added to a Zn-Al plating composition.
However, the Zn-Mg-Al-based zinc alloy plated steel sheet is often processed and used as a common zinc-based steel sheet, but because the plated steel sheet contains a large amount of intermetallic compounds with high hardness in a plating layer, there may be a disadvantage in bendability may deteriorate, causing cracks in the plating layer during bending.
Accordingly, attempts have been made to further improve the bendability of plated steel sheets. However, even if bendability is improved, a base steel sheet was exposed due to microcracks generated in the bending portion during bending, so that it was technically very difficult to secure not only corrosion resistance of a flat plate portion of the plate steel sheet, but also corrosion resistance of a bending portion.
Meanwhile, it is known that the corrosion resistance of the bending portion of a zinc alloy-plated steel sheet is usually caused by leaching of Mg and Al components in a moisture atmosphere, resulting in self-healing of the exposed portion of the base steel sheet. However, there may be a problem in that it may be difficult to secure the corrosion resistance of the bending portion to a desired level because the effect may be minimal.
In addition, a zinc-based plated steel sheet is often provided on an outside of a product, but the product is that a higher Mg content in the plating layer had a darker appearance, and surface quality was poor due to addition of surface damage factors caused by processing, so improvement in appearance quality was required.
However, until now, a level of technology that can meet the demand for high-end products having excellent corrosion resistance in the flat plate portion as well as corrosion resistance in the bending portion and appearance quality has not been developed.
(Patent Document 1) Korean Publication No. 2010-0073819

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide a plated steel sheet having excellent corrosion resistance in a flat plate portion as well as corrosion resistance in a bending portion and excellent appearance quality, and a method for manufacturing the same.
An object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.

Solution to Problem

According to an aspect of the present disclosure, provided is a plated steel sheet, the plated steel sheet including:

a base steel sheet;
a Zn-Mg-Al-based plating layer provided on at least one surface of the base steel sheet; and
an Fe-Al-based inhibition layer provided between the base steel sheet and the Zn-Mg-Al-based plating layer,
wherein on a surface of the Zn-Mg-Al-based plating layer, a total area fraction of an Al single phase and MgZn₂ phase is 45 to 60%, and an area ratio of the MgZn₂ phase to the Al single phase is 1.2 to 3.3.

According to another aspect of the present disclosure, provided is a manufacturing method of a plated steel sheet, the manufacturing method including operations of:

hot dip galvanizing a base steel sheet, including by weight: Mg: 4 to 6%, Al: 8.2 to 14.2%, with a balance Zn and other inevitable impurities, by immersing the base steel sheet in a plating bath maintained at a temperature of 20 to 80°C higher than a solidification initiation temperature in an equilibrium state; and
cooling the hot-dip galvanized steel sheet using an inert gas at an average cooling rate of 2 to 12°C/s from the solidification initiation temperature to a solidification end temperature,
wherein in the cooling operation, cooling is performed so that the following relations 1-1 and 1-2 are satisfied, and a ratio (De/Dc) of a damper opening rate De of an edge portion to a damper opening rate Dc of a center portion satisfies 60 to 99%.

A < \{(5 - 2 lnt) / (7 - 3 lnt)\} \times B

{15t}^{(- 0.8)} \leq B \leq 20 t^{(- 0.8)}

In the above relations 1-1 and 1-2, t is a thickness of the steel sheet (mm), A is an average cooling rate (°C/s) from a solidification initiation temperature to 375°C, and B is an average cooling rate (°C/s) from 375°C to 340°C.

Advantageous Effects of Invention

As set forth above, according to an aspect of the present disclosure, a plated steel sheet having excellent corrosion resistance of a flat plate as well as having corrosion resistance of a bending portion and excellent appearance quality, and a method for manufacturing the same, may be provided.
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 exemplary embodiments in the present disclosure.

Brief description of drawings

FIG. 1(a) is an image of a surface specimen illustrating a surface of the plated steel sheet of Example 13, wherein the surface specimen is magnified at 700x magnification and observed with a scanning electron microscope (Field Emission Scanning Electron Microscope, hereinafter referred to as 'FE-SEM'), and FIG. 1(b) illustrates a ratio of each phase measured for the photograph.
FIG. 2(a) is an image of a surface specimen illustrating a surface of the plated steel sheet of Example 13, the same as that in FIG. 1, being polished to a point of 1/2t, in which the surface specimen is magnified at 700x magnification and observed with a Field Emission Scanning Electron Microscope (FE-SEM), and FIG. 2(b) illustrates a ratio of each phase measured for the photograph.
FIG. 3(a) is an image of a surface specimen illustrating a surface of the plated steel sheet of Comparative Example 1, wherein the surface specimen is magnified at 700x magnification and observed with a Field Emission Scanning Electron Microscope (FE-SEM), and FIG. 3(b) illustrates a ratio of each phase measured for the photograph.
FIG. 4(a) is an image of a surface specimen illustrating a surface of the plated steel sheet of Comparative Example 1, the same as that in FIG. 3, being polished to a point of 1/2t, in which the surface specimen is magnified at 700x magnification and observed with a Field Emission Scanning Electron Microscope (FE-SEM), and FIG. 4(b) illustrates a ratio of each phase measured for the photograph.
FIG. 5 is a graph illustrating a fraction of elements dissolved in a microstructure observed with an energy dispersive spectrometer (EDS) in an Al single phase, a second Al single phase, and an Al-Zn based binary eutectic phase referred to in FIGS. 1 to 4.

Best Mode for Invention

Terms used in the present specification are for explaining specific exemplary embodiments rather than limiting the present disclosure. In addition, a singular form used in the present specification includes a plural form also, unless the relevant definition has a clearly opposite meaning thereto.
The meaning of "comprising" used in the specification is to embody the configuration and is not to exclude the presence or addition of other configurations.
Unless otherwise defined, all terms including technical terms and scientific terms used in the present specification have the same meaning as would be commonly understood by a person with ordinary skill in the art to which the present disclosure pertains. Pre-defined terms 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, when indicating a content of each element, measurements are weight %, unless specifically defined otherwise.
In a conventional technology related to Zn-Mg-Al-based zinc alloy plated steel sheet, Mg was added to improve corrosion resistance, but when Mg is added excessively, occurrence of floating dross in a plating bath increases, so that there was a problem that the dross is frequently removed. Therefore, an upper limit of Mg addition amount was limited to 3%. Accordingly, research was conducted to further improve corrosion resistance by increasing the amount of Mg added from 3%, but as the amount of Mg added increases, a large amount of intermetallic compounds with high hardness are generated, causing cracks in the plating layer during bending.
Therefore, studies have been conducted to ensure corrosion resistance and bendability, but even if corrosion resistance and bendability of the flat plate portion of the plated steel sheet are secured, a base steel sheet was exposed due to microcracks that inevitably occur during bending, so it was technically very difficult to even secure the corrosion resistance of such a bending portion. In addition, as more Mg was added, the appearance of the product became darker and surface damage elements were added, making it difficult to secure appearance quality.
Accordingly, the present inventors recognized that, as a result of conducting intensive studies to solve the above-mentioned problems and at the same time, to provide a plated steel sheet having excellent corrosion resistance in the flat plate portion, as well as corrosion resistance in the bending portion and having excellent appearance quality, LDH (Layered Double Hydroxide; (Zn,Mg)₆Al₂(OH)₁₆(CO₃)·4H₂O)) is uniformly formed as an initial corrosion product on a surface of the processed portion when maintained in a corrosive environment (or in an atmospheric environment for a long time), which is an important factor, thereby completing the present invention.
Therefore, hereinafter, a component of a plated steel sheet in which LDH is formed as an initial corrosion product on a surface of the bending portion, and at the same time, LDH is uniformly distributed throughout the surface of the bending portion over time to shield a corrosion active area will be described in detail.
First, a plated steel sheet according to an aspect of the present disclosure includes a base steel sheet; a Zn-Mg-Al-based plating layer provided on at least one surface of the base steel sheet; and a Fe-Al-based inhibition layer provided between the base steel sheet and the Zn-Mg-Al-based plating layer.
In the present disclosure, there may be no particular limitation on a type of base steel sheet. For example, the base steel sheet may be a Fe-based base steel sheet used as a base steel sheet for an ordinary zinc-based plated steel sheet, that is, a hot-rolled steel sheet or a cold-rolled steel sheet, but the present disclosure is not limited thereto. Alternatively, the base steel sheet may be, for example, carbon steel, ultra-low carbon steel, or high manganese steel used as materials for construction, home appliances, and automobiles. However, as an example, the base steel sheet may have a composition including, by weight: C: more than 0% to 0.18% or less, Si: more than 0% to 1.5% or less, Mn: 0.01 to 2.7%, P: more than 0% to 0.07% or less, S: more than 0% to 0.015% or less, Al: more than 0% to 0.5% or less, Nb: more than 0% to 0.06% or less, Cr: more than 0% to 1.1% or less, Ti: more than 0% to 0.06% or less, B: more than 0% to 0.03% or less, with a balance of Fe and other unavoidable impurities.
According to an aspect of the present disclosure, a Zn-Mg-Al-based plating layer formed of a Zn-Mg-Al-based alloy may be provided on at least one surface of the base steel sheet. 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 Zn-Mg-Al-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 thickness of the Zn-Mg-Al-based plating layer may be 5 to 100 pm, more preferably 5 to 90 um. If a thickness of the plating layer is less than 5um, the plating layer may become excessively thin locally due to errors resulting from a deviation in thickness of the plating layer, which may result in poor corrosion resistance. If the thickness of the plating layer exceeds 100 um, cooling of the hot-dip plating layer may be delayed, solidification defects such as flow patterns, for example, may occur on the surface of the plating layer, and productivity of the steel sheet may be reduced in order to solidify the plating layer.
In addition, according to an aspect of the present disclosure, an Fe-Al-based inhibition layer may be provided between the base steel sheet and the Zn-Mg-Al-based plating layer. The Fe-Al-based inhibition layer is a layer mainly containing intermetallic compounds of Fe and Al, and examples of the intermetallic compounds of Fe and Al include FeAl, FeAl₃, Fe₂Al₅, and the like. In addition thereto, some components derived from the plating layer, such as Zn and Mg, may be further included, for example, in an amount of 40% or less. The inhibition layer is a layer formed due to alloying of Fe diffused from the base steel sheet at the beginning of plating and plating bath components. The inhibition layer may serve to improve adhesion between the base steel sheet and the plating layer, and at the same time prevent Fe diffusion from the base steel sheet to the plating layer. In this case, the inhibition layer may be formed continuously between the base steel plate and the Zn-Mg-Al-based plating layer, or may be formed discontinuously. Except for the above-described description, information generally known in the art can be applied to the inhibition layer.
According to an aspect of the present disclosure, a thickness of the inhibition layer may be 0.02 to 2.5 um. The inhibition layer may serve to secure corrosion resistance by preventing alloying, but because it brittles, the inhibition layer may affect processability, so the thickness thereof may be 2.5 um or less. However, in order to function as an inhibition layer, it is preferable to control the thickness thereof to 0.02 um or more. In terms of further improving the above-mentioned effect, an upper limit of the thickness of the inhibition layer may be preferably 1.8 µm. In addition, a lower limit of the thickness of the inhibition layer may be 0.05 um. In this case, the thickness of the inhibition layer may mean a minimum thickness in a direction, perpendicular to an interface of the base steel sheet.
Meanwhile, according to an aspect of the present disclosure, the Zn-Mg-Al-based plating layer may include, by weight percent, Mg: 4 to 6%, Al: 8.2 to 14.2%, with a balance of Zn, and other inevitable impurities. Hereinafter, each component is described in detail.

Mg: 4% or more and 6% or less

Mg is an element serving to improve corrosion resistance of a plated steel material, and in the present disclosure, a Mg content in the plating layer is controlled to 4% or more to ensure the desired excellent corrosion resistance. Meanwhile, from a viewpoint of securing corrosion resistance, an effect of securing corrosion resistance improves as Mg is added, so an upper limit of the Mg content may not be particularly limited. However, as an example, when excessive Mg is added, dross may occur, so the Mg content can be controlled to 6% or less.

Al: 8.2% or more and 14.2% or less

In general, when Mg is added at 1% or more, an effect of improving corrosion resistance is exerted, but when Mg is added at 2% or more, generation of floating dross in a plating bath due to oxidation of Mg in the plating bath is increased, resulting in a problem that dross should be removed frequently. Due to this problem, in the prior art, Mg was added at 1.0% or more in Zn-Mg-Al-based zinc alloy plating to secure corrosion resistance, but an upper limit of the Mg content was set to be 3.0% for commercialization. However, as described above, in order to further improve corrosion resistance, it is necessary to increase the Mg content to 4% or more. However, if the plating layer contains 4% or more of Mg, there is a problem of dross occurring due to oxidation of Mg in the plating bath, so it is necessary to add Al. However, if excessive Al is added to suppress dross, a melting point of the plating bath increases and the resulting operating temperature becomes too high, causing problems caused by high-temperature work, such as erosion of a plating bath structure and deterioration of the steel material. In addition, if an Al content in the plating bath is excessive, Al reacts with Fe of base iron and does not contribute to formation of a Fe-Al inhibition layer, and a reaction between Al and Zn occurs rapidly, an excessive lump-shaped outburst phase is formed, which may worsen corrosion resistance. Therefore, an upper limit of the Al content in the plating layer is preferably controlled to 14.2%, and more preferably to 14.0%.

Balance of Zn and other inevitable impurities

In addition to the composition of the plating layer described above, a balance of Zn and other unavoidable impurities. Inevitable impurities may be all included, that may be unintentionally mixed in the manufacturing process of a normal hot-dip galvanized steel sheet, and the meaning may be easily understood by the person skilled in the art.
The Zn-Mg-Al-based plating layer may include a MgZn₂ phase and an Al single phase as a microstructure, and in addition thereto, various phases, such as an Al-Zn based binary eutectic phase, Zn-MgZn₂-Al-based ternary eutectic phase, Zn single phase, and the like can also be included in the plating layer.
In this case, in the present disclosure, the MgZn₂ phase refers to a phase mainly composed of MgZn₂, and the Al single phase refers to a phase mainly composed of Al, and specifically, a phase in which Zn is dissolved at less than 27% in atomic percentage, and a remainder thereof is composed of Al and other impurities. That is, in the Al single phase, in addition to the Al component, components such as Zn and Mg that can be included as plating layer components may be dissolved in solid solution, and in the present disclosure, the Al single phase refers to only a phase in which Zn is dissolved at less than 27 atomic%.
In addition, the Zn-MgZn₂-Al-based ternary eutectic phase refers to a ternary eutectic phase in which the Zn phase, MgZn₂ phase, and Al phase are all mixed, and the Al-Zn binary eutectic phase refers to an Al phase and a Zn phase arranged in an alternating lamellar or irregular mixed form. In this case, it is necessary to note that the Al phase in the Al-Zn based binary eutectic phase and the Zn-MgZn₂-Al-based ternary eutectic phase is not regarded as the Al single phase described above or the second Al single phase described later. Likewise, it is necessary to note that MgZn₂ in the Zn-MgZn₂-Al-based ternary eutectic phase is not regarded as a MgZn₂ phase mainly composed of the above-described MgZn₂.
In addition, the Zn-Mg-Al-based plating layer may additionally include a 'second Al single phase' that is distinguished from the Al single phase by a Zn solid solution ratio. The second Al single phase refers to a single phase in which 27% or more and 60% or less (27 to 60%) of Zn is dissolved in atomic percentage, and a remainder thereof is composed of Al and other impurities.
Meanwhile, a microstructure of the above-described Zn-Mg-Al-based plating layer may have a different distribution on a surface and cross-section thereof, and the microstructure on the surface and cross-section thereof may be confirmed using a scanning electron microscope (SEM) or transmission electron microscope (TEM) by enlarging magnification of the plating layer for each surface specimen or cross-section specimen.
As described above, the Zn-Mg-Al-based plating layer includes various phases depending on a composition and manufacturing conditions of the plating layer, but as a result of intensive studies to provide a plated steel sheet having excellent corrosion resistance in the bending portion and appearance quality, in addition to corrosion resistance of the conventional flat plate portion, the present inventors have discovered that when maintained in a corrosive environment (or in an atmospheric environment for a long time), uniform formation of LDH (Layered Double Hydroxide; (Zn,Mg)₆Al₂(OH)₁₆(CO₃)·4H₂O) as an initial corrosion product is an important factor on a surface of the steel sheet.
As described above, in order for LDH to be formed initially as a corrosion product on the surface of the plated steel sheet, the present inventors have confirmed that it is related to microstructural characteristics on a surface of the Zn-Mg-Al-based plating layer (i.e., an external surface, not a surface of base iron), thereby completing the present invention.
Specifically, according to an aspect of the present disclosure, on the surface of the Zn-Mg-Al-based plating layer, a total area fraction of an Al single phase and the MgZn₂ phase is 45 to 60%, and an area ratio of the MgZn₂ phase to the Al single phase is 1.2 to 3.3. In this case, on the surface of the Zn-Mg-Al-based plating layer, the total area fraction of the Al single phase and the MgZn₂ phase and the area ratio of the MgZn₂ phase to the Al single phase are measured based on a surface specimen having an area of 24,000 µm² or more.
In the present disclosure, in the case of a Zn-Mg-Al-based plating layer satisfying the above-described plating layer composition, the Zn-Mg-Al-based plating layer includes a microstructure in which a MgZn₂ phase and an Al single phase are adjacent. A form in which the MgZn₂ phase and the Al single phase are adjacent includes a case in which an Al single phase is completely contained within the MgZn₂ phase or a portion of the Al single phase is contained within the MgZn₂ phase, and additionally, a case in which an Al single phase is present to be in contact with the MgZn₂ phase.
In the present disclosure, the Zn-Mg-Al-based plating layer may include a Zn single phase and a Zn-MgZn₂-Al-based ternary eutectic phase, which are common phases in a highly corrosion-resistant plated steel sheet. In general, as Al and Mg contents in the plating layer decreases, an amount of the Zn single phase and Zn-MgZn₂-Al-based ternary eutectic phase generated in an entire plating layer tends to increase, and as the Al and Mg contents in the plating layer increases, an amount of the MgZn2 phase and the Al single phase generated tends to increase.
That is, in a plating composition system having Mg in amount of 4% or more such as in the present disclosure, as illustrated in FIG. 1 illustrating a surface of the plating layer, a MgZn₂ phase appears coarsely, and as a Mg content increases, an Al content should also increase simultaneously to suppress dross, accordingly, a coarse Al single phase also coexists. Accordingly, in order to secure the corrosion resistance of the bending portion described above, the present inventors have found that a total area fraction and area ratio of a MgZn₂ phase and an Al single phase adjacent to the MgZn₂ phase on a surface of the plating layer contribute to initial formation of LDH as a corrosion product when maintained under a corrosive environment (or under an atmospheric environment for a long time).
In other words, in order to ensure not only the corrosion resistance of the flat plate portion of the plated steel sheet but also the corrosion resistance of the bending portion, it may be important that the MgZn₂ phase and the Al single phase exist adjacent to each other, as a structure of the surface of the plating layer, which can promote rapid nucleation and crystallization of LDH. Therefore, as time passes after the initial rapid nucleation and crystallization of LDH, a corrosion active area may be effectively shielded by LDH formed uniformly throughout the surface, and secondarily, uniform formation of Simonkolleite; Zn₅(OH)₈Cl₂ and Hydrozincite; (Zn₅(OH)₆(CO₃)₂), which are secondary corrosion products may be induced.
Therefore, according to an aspect of the present disclosure, it is important to ensure that the MgZn₂ phase and the Al single phase are adjacent to each other on the surface of the plating layer by a certain amount or more. Specifically, on the surface of the Zn-Mg-Al-based plating layer, by satisfying the total area fractionn of the MgZn₂ phase and the Al single phase (adjacent to the MgZn₂ phase) to be 45 to 60%, and the area ratio of the MgZn₂ phase to the Al single phase to be 1.2 to 3.3, the MgZn₂ phase and the Al single phase may serve to form a sacrificial projection cell between the MgZn₂ phase and the Al single phase to secure excellent corrosion resistance. In this case, the corrosion resistance includes not only the corrosion resistance of the flat plate portion but also the corrosion resistance of the bending portion, and this corrosion resistance improves as the amount of MgZn₂ phase and the Al single phase present on the surface of the plating layer increases, compared to the inside of the plating layer.
On the surface of the Zn-Mg-Al-based plating layer, if the total area fraction of the MgZn₂ phase and the Al single phase is less than 45%, each phase forming an anode (MgZn₂) and a cathode (Al) of the sacrificial projection cell is insufficient, so that the corrosion resistance of the bending portion may be insufficient, and light scattering due to a phase present on the surface is also insufficient, so that there may be a risk in which appearance quality may deteriorate. On the other hand, if the total area fraction of the MgZn₂ phase and the Al single phase exceeds 60%, a brittle MgZn₂ phase may be excessively formed, causing a problem of excessive cracking in the plating layer during processing.
In addition, on the surface of the Zn-Mg-Al-based plating layer, if an area ratio of the MgZn₂ phase to the Al single phase is less than 1.2, an amount of the MgZn₂ phase (anode) forming the sacrificial projection cell can be dissolved is small, causing a problem, disadvantageous for corrosion resistance. If the area ratio of the MgZn₂ phase to the Al single phase exceeds 3.3, a rate of cathodic reaction (oxygen reduction reaction) occurring in Al on the surface is limited by accepting electrons transmitted by dissolution of MgZn₂, which may cause a problem, disadvantageous for corrosion resistance.
The corrosion resistance of the bending portion is determined by two mechanisms. First, a MgZn₂ phase and Al single phase present in the bending portion form an intact sacrificial projection cell, and corrosion products cover and obscure an exposed portion of the base steel sheet during bending. Second, it is a self-healing mechanism in which oxidation-friendly Mg and Al components are leached in a moisture atmosphere and move to the exposed portion of the base steel sheet in the bending portion to reform a plating layer. The greater an amount of Mg and Al components that are highly reactive with moisture which are present in a surface layer portion, the better the effect.
The sacrificial projection cell, acting as the first mechanism, secures a large potential difference as a potential of MgZn₂ is -1.2V above a hydrogen reduction potential and a potential of Al is -0.7V above the hydrogen reduction potential, thereby acting as an anode and a cathode, respectively, which means forming a galvanic cell between the MgZn₂ phase and Al single phase microstructure, adjacent to each other.
As a result of extensive research, the present inventors have confirmed that corrosion resistance in the bending portion by forming galvanic cells may be secured by securing a high potential difference between a MgZn₂ phase on a surface of the plating layer and the Al single phase, adjacent to the MgZn₂ phase, and have discovered that a phase capable of securing a high potential difference adjacent to the MgZn₂ phase is an Al single phase with a Zn solid-solution ratio of less than 27 atomic%.
That is, the Zn-Mg-Al-based plating layer according to an aspect of the present disclosure may include, among phases mainly composed of Al, ① an Al single phase with a Zn solid-solution ratio of less than 27 atomic%, and ② a second Al single phase with a high Zn solid-solution ratio of 27 to 60%. In addition, it was confirmed that, thereamong, a phase that can maintain a high potential difference by existing adjacent to the MgZn₂ phase is the Al single phase (corresponding to ①) with a low Zn solid-solution ratio.
In other words, according to an aspect of the present disclosure, in the Zn-Mg-Al-based plating layer, when a large amount of the second Al single phase having a Zn solid solution ratio of 27 atomic% or more are formed, the number of second Al single phases present around the MgZn₂ phase increases, and accordingly, a potential difference between the anode and cathode of the above-described galvanic cell may be reduced, which may impair excellent corrosion resistance and sacrificial projection of the galvanic cell.
Therefore, according to an aspect of the present disclosure, on the surface of the Zn-Mg-Al-based plating layer, an area fraction of the second Al single phase may be 2 to 9%. If the area fraction of the second Al single phase exceeds 9%, a second Al single phase may be excessively formed around the MgZn₂ phase, which may reduce the potential difference between the galvanic cells and worsen corrosion resistance in the bending portion. Therefore, in the present disclosure, on the surface of the Zn-Mg-Al-based plating layer, an area fraction of the second Al single phase is controlled to 9% or less, and as an amount of the second Al single phase present on the surface decreases, an effect of improving the corrosion resistance of the bending portion improves, so a lower limit thereof may not be specifically limited. However, considering that the second Al single phase is inevitably formed in a temperature range at which the second Al single phase is formed during a cooling process, after hot-dip plating, the lower limit may be set to 2%.
According to an aspect of the present disclosure, on the surface of the Zn-Mg-Al-based plating layer, the area fraction of the MgZn₂ phase may be 30 to 40%. A part of the plating layer, primarily in contact with atmospheric and chloride environments is a surface, and the higher the ratio of the MgZn₂ phase acting as an anode in a sacrificial method, the better reactivity in the galvanic cell. Therefore, by promoting the formation of the above-described galvanic cell, an area fraction of the MgZn₂ phase on a surface of the plating layer may be set to 30% or more, to ensure corrosion resistance of the bending portion. Therefore, if the area fraction of the MgZn₂ phase on the surface of the plating layer, the corrosion resistance of the bending portion may be insufficient. On the other hand, when a ratio of the MgZn₂ phase is excessively high, exceeding 40%, the plating layer may be brittle and cause cracks of the surface.
Alternatively, according to an aspect of the present disclosure, on the surface of the Zn-Mg-Al-based plating layer, an area fraction of the Al single phase (i.e., a phase in which, in atomic percentage, Zn is dissolved in less than 27%, and a balance thereof contains Al and other impurities) may be 15 to 20%. On the surface of the plating layer, if the area fraction of the Al single phase is 15% or more, as described above, it may act as a cathode along with MgZn₂, which acts as an anode in the galvanic cell, to help improve the corrosion resistance of the bending portion, and serve to maintain a skeleton of the MgZn₂ phase, so that the plating layer may contribute to a role as a physical protective barrier film. On the other hand, if the ratio of the Al single phase exceeds 20%, stability may deteriorate due to Al corrosion.
In addition, according to an aspect of the present disclosure, a total area fraction of the Zn single phase and a Zn-MgZn₂-Al-based ternary eutectic phase on a surface of the Zn-Mg-Al-based plating layer may be 20 to 30%. The Zn single phase and Zn-MgZn₂-Al based ternary eutectic phase present on the surface of the Zn-Mg-Al plating layer contribute to formation of Simonkolleite or hydrozinsite rather than LDH in an initial stage of corrosion. Therefore, by controlling a presence ratio of the Zn single phase and the Zn-MgZn₂-Al-based ternary eutectic phase on the surface of the Zn-Mg-Al-based plating layer, among corrosion products formed on the surface at the initial stage of corrosion, the corrosion resistance of the bending portion may be improved by increasing a formation ratio of LDH, rather than a formation ratio of Simonkolleite or hydrozinsite. Therefore, a total area fraction of the Zn single phase and the Zn-MgZn₂-Al-based ternary eutectic phase on a surface of the Zn-Mg-Al-based plating layer may be set to 20 to 30%. In this case, if the total area fraction of the Zn single phase and the Zn-MgZn₂-Al-based ternary eutectic phase on the surface of the Zn-Mg-Al-based plating layer is less than 20%, formation of Simonkolleite or hydrozinsite, which is secondarily generated after the formation of LDH and helps improve corrosion resistance may be insufficient, so that there may be a risk of a problem with corrosion resistance. On the other hand, if the total area fraction of the Zn single phase and the Zn-MgZn₂-Al-based ternary eutectic phase on the surface of the Zn-Mg-Al-based plating layer exceeds 30%, the formation of Simonkolleite or hydrozinsite is induced first, rather than the formation of LDH in the initial stage of corrosion, stable corrosion behavior as described above may not be formed, so that corrosion resistance may be inferior.
Meanwhile, as an aspect of the present disclosure, based on a cross-section of the Zn-Mg-Al-based plating layer cut in a thickness direction (i.e., in a direction perpendicular to a rolling direction of the steel sheet), an area fraction of the MgZn₂ phase is 20 to 40%, and an area fraction of the Al single phase may be 8 to 26%.
The characteristics of the plated steel sheet are related to the type and size of a crystal phase, and if the area fraction of the MgZn₂ phase is less than 20% or the area fraction of the Al single phase is less than 8%, the corrosion resistance of the plating layer may be weakened. Meanwhile, if the ratio of the MgZn₂ phase present in the plating layer exceeds 40%, it may be excessively brittle, which may have a side effect of excessive cracking in the plating layer occurring during processing. Based on the cross-section of the Zn-Mg-Al-based plating layer, the area fraction of the MgZn₂ phase and the Al single phase can be measured by observing an image taken by FE-SEM of a cross-sectional specimen of the plated steel sheet in the thickness direction.
In the present disclosure, even if the area fraction of the MgZn₂ phase and the Al single phase based on the cross-section of the Zn-Mg-Al-based plating layer described above is satisfied, and the corrosion resistance of the cross-sectional portion (cut-edge) of the steel sheet can be secured, but the area fraction of the MgZn₂ phase and the Al single phase, secured on the surface of the plating layer may be different. Therefore, the area fraction distribution of each phase on the surface of the plating layer may affect the degree of corrosion resistance of the bending portion during bending.
Accordingly, the present inventors have discovered that, even if the area fraction of the MgZn₂ phase and Al single phase described above is secured based on the cross-section of the plating layer in the thickness direction, securing a certain amount of more of the MgZn₂ phase and Al single phase on the surface of the plating layer is an important factor for securing corrosion resistance of the bending portion by promoting uniform formation of LDH on the surface of the plating layer in the initial stage of corrosion. That is, according to an aspect of the present disclosure, the present inventors have further discovered that, it is important to maintain a ratio of the total area fraction of the MgZn₂ phase and Al single phase in a center portion of the plating layer to the total area fraction of the MgZn₂ phase and Al single phase on the surface of the plating layer at an appropriate level.
Specifically, according to an aspect of the present disclosure, a ratio (S1/C1) of a total area fraction (S1) of the MgZn₂ phase and Al single phase on the surface of the Zn-Mg-Al-based plating layer to a total area fraction (C1) of the MgZn₂ phase and Al single phase on a surface at any point in a region from 1/4t to 3/4t of the Zn-Mg-Al-based plating layer in the thickness direction may be in a range of 0.8 to 1.2. If the S1/C1 is less than 0.8, a problem may occur in the corrosion resistance of the flat plate portion and the bending portion, due to a lack of microstructure forming LDH in the initial stage of corrosion in a surface layer portion of the plating layer, and if S1/C1 exceeds 1.2, a problem may occur in processability and corrosion resistance of bending portion, due to excessive coarsening of a brittle structure caused by the MgZn₂ phase in the surface layer portion of the plating layer.
According to an aspect of the present disclosure, on the surface of any point in regions from 1/4t to 3/4t in the thickness direction of the Zn-Mg-Al-based plating layer, the area fraction of the second Al single phase is 2 to 10%. If the value exceeds 10%, it may affect a structure of the surface layer portion, to adversely affect the corrosion resistance of the bending portion. In addition, considering that it passes through a temperature section in which a second Al single phase is formed, a lower limit thereof may be controlled to 2%.
In a region from 1/4t to 3/4t of the Zn-Mg-Al-based plating layer in a thickness direction, among plating specimens, a point at which a thickness of the plating layer is maximum is regarded as a total thickness t, wherein the region may refer to a region polished on a surface of the specimen to include any point in the region from 1/4t to 3/4t based on t.
Moreover, the present inventors have conducted additional research and found that, after the uniform formation of LDH on the surface of the plating layer, a ratio of the Zn phase and a Zn-MgZn₂-Al based ternary eutectic phase, which penetrates thereinto and promotes the formation of Simonkolleite and hydrozinsite, in the center portion to the surface thereof was also an important factor in further improving corrosion resistance.
That is, according to an aspect of the present disclosure, a ratio (S2/C2) of a total area fraction (S2) of the Zn phase and the Zn-MgZn₂-Al-based ternary eutectic phase on a surface of the Zn-Mg-Al-based plating layer to a total area fraction (C2) of the Zn phase and the Zn-MgZn₂-Al-based ternary eutectic phase on a surface at any point in a region from 1/4t to 3/4t of the Zn-Mg-Al-based plating layer in a thickness direction may be in a range of 0.6 to 1.2. If the S2/C2 is less than 0.6, problems with corrosion resistance may occur due to insufficient formation of Simonkolleite or hydrozinsite, which is generated secondarily after LDH formation in a surface layer portion of the plating layer, which helps improve corrosion resistance. In addition, if S2/C2 exceeds 1.2, there is a relative lack of an MgZn₂ phase and an Al single phase secured on a surface thereof, which may cause insufficient LDH formation on the surface, which may cause problems with corrosion resistance as described above in the present disclosure.
Meanwhile, definitions and dissolved atomic percentage for the above-described MgZn₂ phase, Al single phase, second Al single phase, Zn single phase, and Zn-MgZn₂-Al-basedAl-based ternary eutectic phase that can be derived from the surface of the Zn-Mg-Al-based plating layer satisfying the Mg and Al composition according to the present disclosure, were illustrated in FIGS. 1 to 5.
Specifically, after manufacturing a specimen so that a surface of the plating layer of the plated steel sheet can be observed using an SEM device, images taken using an SEM or EDS device on the surface of the Zn-Mg-Al-based plating layer may be distinguished by color and contrast differences for each microstructure, and each region may be calculated.
That is, from an image of a plane of the plated steel sheet shown in FIGS. 1 to 4, observed with a field emission scanning electron microscope (FE-SEM) at 700x magnification in a BEI (backscattered electron image) observation mode, 1280x960 pixel/254DPI resolution, and 8-bit properties, an Al single phase in which Zn was dissolved at less than 27 at%, and a second Al single phase in which Zn was dissolved at 27 at% or more and 60% or less, were distinguished by microstructure labelling of an SEM image.
For reference, a fraction of elements dissolved in each phase depending on a contrast of the SEM image can be obtained using an EDS (Energy Dispersive Spectrometer) commonly known in the art. In this case, as an example, an Al-based phase, excluding the MgZn₂ phase, Zn single phase, and Zn-MgZn₂-Al-based ternary eutectic phase, which are clearly distinguished by color, brightness, and shape for each microstructure as shown in FIG. 5, was distinguished into the following regions: region ① represents an Al region, in which Al is observed to be 73%, Zn is 26%, and a balance thereof is less than 1%, in atomic percentage on average, a region ② represents a second Al single phase, in which Al is observed to be 51%, Zn is 49%, and a balance thereof is less than 1%, in atomic percentage on average, and region ③ represents a Al-Zn based binary eutectic phase, in which Al is observed to be 43%, Zn is 57%, and a balance thereof is less than 1%, in atomic percentage on average (in this case, the balance being Mg or other unavoidable impurities). In the present disclosure, the Al single phase refers to a region ① in which Zn is dissolved at less than 27% in atomic%, and the second Al single phase refers to the region ② in which Zn is dissolved at 27% or more to 60% or less in atomic!, and the Al-Zn based binary eutectic phase refers to a region ③, and Fe and other components may be included as impurities in each phase.
In this case, microstructure labeling uses images derived under the above-described SEM measurement conditions using an automatic image generation software based on a super-pixel algorithm of RISA (microstructure phase fraction analysis software) of the Pohang Research Institute of Industrial Science and Technology (RIST). The super-pixel algorithm is a mechanism measuring similarity by dividing an entire image into thousands or tens of thousands of regions (superpixels) and comparing superpixels with similar patterns or features, and calculating a histogram for a brightness value of a pixel, and then automatically selecting a superpixel when the similarity is greater than a pre-defined threshold. As an example of specifying a pre-defined threshold. As an example of specifying a pre-defined threshold, a boundary between the Al single phase and the second Al single phase in the image derived under the above-described SEM measurement conditions defines each phase in advance based on a Zn solid solution ratio in 27 atomic %, dissolved in an Al structure using EDS, so that histrogramming and structure distinguishing is possible for the brightness value of a soft phase. The technical idea of the above-described RISA (microstructure fraction analysis software) can be confirmed through Korean Publication No. 2019-0078331 .
According to an aspect of the present disclosure, LDH may be formed on a surface of the plating layer before Simonkolleite and hydrozinsite under an atmospheric and chloride environment. The above-described plating layer undergoes rapid nucleation and crystallization of LDH, a dense corrosion product, on a surface in an initial stage of the corrosion environment due to the MgZn₂ phase present in large quantities in the surface layer portion and the Al single phase, adjacent thereto. Thereafter, over time, it can be uniformly distributed throughout the surface to shield a corrosion active region, and uniform formation of Simonkolleite and hydrozinsite, which are corrosion products, secondarily formed, may be induced.
According to an aspect of the present disclosure, a LDH corrosion product formed in a surface layer portion of the plating layer may be formed within 6 hours in an atmospheric environment, and within 5 minutes in a chloride environment (i.e., as measured by ISO14993).
According to an aspect of the present disclosure, as for the excellent corrosion resistance described above, a time taken for red rust to occur in a chloride environment including salt spray and immersion environments (i.e., as measured by ISO14993) may be 40 to 50 times longer in a flat plate portion; and 20 to 30 times in a 90° bending portion, compared to that of pure Zn plating of the same thickness. In this case, an evaluation of the time taken for red rust to occur may be comparatively evaluated using a test method in accordance with ISO14993 using a salt spray test device (SST).
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, a step of first preparing a base steel sheet may be further included, and the type of the bases steel sheet is not particularly limited. The base steel sheet may be a Fe-based steel sheet, which is used as a base steel sheet for ordinary hot-dip galvanized steel sheets, that is, a hot-rolled steel sheet or cold-rolled steel sheet, but the present disclosure is not limited thereto. In addition, the base steel sheet may be, for example, carbon steel, ultra-low carbon steel, or high manganese steel used as materials for construction, home appliances, and automobiles, but the present disclosure is not limited thereto. In this case, the above description can be equally applied to the base steel sheet.
Subsequently, according to an aspect of the present disclosure, an operation of hot-dip galvanizing a base steel sheet by immersing the base steel sheet in a plating bath containing, by wt%: Mg: 4 to 6%, Al: 8.2 to 14.2%, a balance of Zn, and other inevitable impurities. In this case, the explanation of the components of the plating layer described above can be applied in the same manner as to the reason for adding the components and limiting the content in the plating bath, except for a small amount of Fe that may flow from the base steel sheet.
To manufacture a plating bath of the above-described composition, a composite ingot containing predetermined Zn, Al, and Mg or a Zn-Mg and Zn-Al ingot containing individual components can be used. In order to replenish a plating bath consumed by hot dip plating, the ingot is additionally dissolved and supplied. In this case, a method of dissolving the ingot by directly immersing the same in a plating bath may be used, or dissolving the ingot in a separate pot and then replenishing molten metal in the plating bath may be used.
In addition, a temperature of the plating bath may be maintained at a temperature of 20 to 80°C higher than a solidification initiation temperature (Ts) in the equilibrium state. In this case, although not particularly limited, the solidification initiation temperature in the equilibrium state may be in a range of 390 to 460°C, or the temperature of the plating bath may be maintained in a range of 440 to 520°C. The higher the temperature of the plating bath, the more it is possible to secure fluidity and form a uniform composition within the plating bath, and to reduce an amount of floating dross generated. If the temperature of the plating bath is less than 20°C, compared to the solidification initiation temperature in the equilibrium state, the dissolution of the ingot is very slow and the viscosity of the plating bath is high, making it difficult to secure excellent plating layer surface quality. On the other hand, if the temperature of the plating bath exceeds 80°C, compared to the solidification initiation temperature in the equilibrium state, ash defects due to Zn evaporation may occur on the plating surface.
In addition, according to an aspect of the present disclosure, a step of cooling the hot-dip galvanized steel sheet using an inert gas at an average cooling rate of 2 to 12°C/s from the solidification initiation temperature to the solidification end temperature in the equilibrium state may be included. If the above-described average cooling rate is less than 2°C/s, the MgZn₂ structure develops too coarsely on the surface and a surface portion of the plating layer is brittle, so that occurrence of cracks may increase, which may be disadvantageous in ensuring uniform corrosion resistance and processibility. On the other hand, if the above-described average cooling rate exceeds 12°C/s, solidification begins from a liquid phase to a solid phase during the hot dip plating process, and rapid solidification occurs in a temperature section while the liquid phase all changes to the solid phase. Therefore, excessive coarsening and atomization of the MgZn₂ phase and the Al single phase may occur on the surface of the plating layer, which may cause phases, which are locally non-uniform, on the surface of the plating layer, resulting in a decrease in corrosion resistance.
In addition, according to an aspect of the present disclosure, in the cooling step, a cooling rate may be controlled to satisfy the following Relations 1-1 and 1-2. $A < \{(5 - 2 lnt) / (7 - 3 lnt)\} *B$
${15t}^{(- 0.8)} \leq B \leq 20 t^{(- 0.8)}$
In the above Relations 1-1 and 1-2, t is a thickness of the steel sheet (mm), A is an average cooling rate (°C/s) from a solidification initiation temperature to 375°C, and B is an average cooling rate (°C/s) from 375°C to 340°C.
That is, in the present disclosure, during cooling after hot dip galvanizing, an average cooling rate in each section according to a thickness of the steel sheet is controlled to satisfy the Relations 1-1 and 1-2, by dividing a first temperature section from a solidification initiation temperature to 375°C and a second temperature section from 375°C to 340°C. In the first temperature section from the solidification initiation temperature to 375°C, an Al single phase adjacent to a MgZn₂ phase in the plating layer formed in the content range of Mg and Al according to the present disclosure is cooled to form a binary eutectic phase, which corresponds to a section from a solidification initiation temperature to a solidification end temperature for an Al single phase including, Zn dissolved at less than 27%, in atomic percentage %, and a remainder thereof consists of Al and other impurities. In this case, it is necessary to note that the 'Al single phase' is indicated differently from a 'second Al single phase' described later. In addition, the second temperature section from 375°C to 340°C indicates a temperature section of forming the second Al single phase in which Zn is dissolved at 27% or more and 60% or less, in atomic percentage % (i.e., 27 to 60%) within the plating layer formed within a range of the contents of Mg and Al according to the present disclosure. Therefore, in the case in which the cooling conditions of the above-described Relations 1-1 and 1-2 are not satisfied, if an initial cooling rate is too fast, an area fraction of the MgZn₂-Al-based binary eutectic phase on a surface of the plating layer formed in the temperature section from the solidification initiation temperature to 375°C, so that a degree of formation of the Al single phase adjacent to the MgZn₂ phase may be insufficient. As a result, LDH may not be formed as an initial corrosive product on the surface of the plating layer, but Simonkolleite may be formed, and there may be a risk that corrosion resistance may further deteriorate.
Meanwhile, according to an aspect of the present disclosure, prior to hot-dip galvanizing, a step of performing pre-skin pass rolling treatment (SPM) by applying a roll reduction of 200 to 300 tons to the surface of the steel sheet using a BrightRoll with a surface roughness (Ra) of 0.2 to 0.4 um may be further included.
As described above, by performing a surface treatment on a base steel sheet before hot-dip galvanizing, a surface shape of the base steel sheet may be controlled to be uniform, and the thickness of the hot-dip coating layer formed by the subsequent plating process is controlled to be uniform, and at the same time, making the base steel sheet smooth, formation sites of solidification nuclei can be minimized. That is, by contributing to smooth nucleation of a surface layer portion of the plating layer rather than formation nuclei therein, in a thickness direction during cooling, microstructure formation in the surface layer portion generated in the first temperature section may be promoted, which may contribute to lowering a ratio of the second Al single phase generated in the second temperature section. Meanwhile, during the pre-skin pass rolling treatment, if the surface roughness of the roll is less than 0.2 um, problems with roll manufacturing and management may occur, and if the surface roughness exceeds 0.4 um, there may be a problem of facilitating internal nucleation rather than nucleation in the surface layer portion of the plating layer. In addition, if a roll reduction is less than 200 tons, an effect of controlling the shape of the base steel sheet is low, so it may be difficult to expect the effect of promoting solidification nucleation in the surface layer portion as described above, and if the roll reduction exceeds 300 tons, there may be a risk of inducing C bends, etc., and it may be difficult to expect an effect of contributing to uniform phase formation in the thickness direction of the plating layer. Meanwhile, in order to further improve the above-described effect, more preferably, the roll reduction of the pre-skin pass rolling treatment before hot-dip galvanizing can be set to 250 to 300 tons.
After the pre-skin pass rolling treatment, a step of heating the base steel sheet in a heating furnace with a dew point temperature of -60°C or higher and -15°C or lower, wherein a temperature of the base steel sheet is 20°C to 80°C higher than a temperature (Tb) of the plating bath to ensure plating wettability, in a last section of the heating furnace, may be included. The dew point temperature of the heating furnace is to prevent oxidation of the surface of the base steel sheet, and to ensure plating adhesion, the temperature of the heating furnace may be set to -60°C or higher to -15°C.
In addition, according to an aspect of the present disclosure, during the cooling, cooling may be performed so that a ratio (De/Dc) of a damper opening rate (De) of an edge portion to a damper opening rate (Dc) of a center portion in a width direction of the hot-dip galvanized steel sheet satisfies 60 to 99%. In this case, the 'width direction' of the steel sheet refers to a direction, perpendicular to a transport direction of the steel sheet, based on a surface excluding a thickness side surface of the hot-dip galvanized steel sheet (i.e., a surface where the thickness of the steel sheet is visible). In addition, the damper opening rate is a value that refers to an opening degree of a damper controlling a flow rate of cooling gas to be sent from a cooling device to a base steel sheet. In order to secure uniform cooling capacity according to a width of a steel sheet, to be described later, a damper is installed so that a total cooling gas input or controlled to the cooling device can be divided into the center portion and the edge portion in the width direction of the base steel sheet and injected. A boundary between the dampers may be divided into three sections according to the width of the base steel sheet, and a position thereof can be variably controlled so that a center is occupied by a center portion and two on an outer side thereof are occupied by an edge portion.
When cooling a conventional hot-dip galvanized steel sheet, there was a problem in that it was difficult to secure uniform microstructural characteristics on the surface of the plating layer by maintaining a constant flow rate of the cooling gas at the edge portion and the center portion, without using a method or device for controlling the ratio (De/Dc). On the other hand, in the present disclosure, contrary to typical cooling conditions, by controlling the damper opening rate of the edge portion to be lower than that of the center portion, with the ratio (De/Dc) in a range of 60 to 99%, uniform cooling capacity may be achieved in the width direction of the steel sheet. In other words, the present inventors have recognized that the edge portion in the width direction of the steel sheet has a larger area exposed to an external atmosphere than the center portion, so a rate at which a temperature of the steel sheet inevitably drops in a region corresponding to the edge portion is faster than in the center portion, and have found that uniform characteristics of the surface of the plating layer could be secured by artificially reducing the cooling rate in the edge portion. That is, the cooling gas incident on the center portion during the above-described cooling process naturally escapes from the center portion externally through the edge portion. However, since the edge portion receives the cooling gas after incident on the center portion in addition to the cooling gas incident on the edge portion, it may be overcooled compared to the center part and cause adverse effects. Therefore, since the cooling rate of the edge portion is faster even without applying artificial cooling gas, in order to realize uniform cooling performance in the width direction and increase corrosion resistance by forming LDH as an initial corrosion product, the damper opening rate of the edge portion needs to be controlled to be lower than that of the center portion.
In this case, if the ratio (De/Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion is less than 60%, the edge portion is cooled more slowly than the center portion, and if the ratio exceeds 99%, the edge portion may be overcooled compared to the center portion, which may be disadvantageous in realizing uniform cooling performance in the width direction of the steel sheet. As a result, a microstructure of the surface of the plating layer in the edge portion and the center portion becomes uneven, and the structural characteristics of the Al single phase and MgZn₂ phase may not be secured on the surface of the plating layer, so there may be a risk that the corrosion resistance of the flat plate portion and the corrosion resistance of the bending portion may deteriorate.
Alternatively, according to another aspect of the present disclosure, in the cooling step, cooling may be performed by changing the ratio (De/Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion, depending on the temperature section.
Specifically, during the cooling, the cooling may be performed so that the ratio (De/Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion satisfies 60 to 70% from a solidification initiation temperature to 375°C (corresponding to the `first temperature section'), and 90 to 99% from 375°C to 340°C (corresponding to the 'second temperature section').
By performing cooling by changing the ratio (De/Dc) depending on the temperature section, to satisfy the above conditions, slow cooling may be uniformly performed in a width direction of the steel sheet from the solidification initiation temperature at which the above-described MgZn₂-Al-based binary eutectic phase is formed to 375°C, so that corrosion resistance may be improved uniformly across the entire width. In addition, by controlling a second Al single phase with a high Zn solid solution ratio to be as low as possible, from 375°C at which the above-described second Al single phase is formed to a solidification end temperature, it may be controlled so as not to affect a galvanic cell between microstructures formed of the MgZn₂-Al-based binary eutectic phase, and as a result, not only the corrosion resistance of the flat plate portion, but also the corrosion resistance of the bending portion may be further improved.
In addition, according to an aspect of the present disclosure, after the cooling step, a step of improving a surface and shape of a base steel sheet by performing a skin pass rolling (SPM) treatment to improve surface quality of a final product may be further included. Thereby, appearance quality may be improved by ensuring a light scattering effect on a surface of a uniform plating layer in a width direction of the steel sheet.
Specifically, as an embodiment, the skin pass rolling treatment may be performed by applying a roll reduction of 50 to 300 tons to the surface of the steel sheet using a bright roll with a surface roughness (Ra) of 0.2 to 1.0 µm.
In this case, if surface roughness Ra of the bright roll is less than 0.2 um, the roughness of the roll is too low and a frictional force between the base steel sheet and the SPM roll is reduced, so that there may be a problem in which the base steel sheet is slipped, and if the surface roughness Ra of the bright roll exceeds 1.0 um, there may be a problem in which the microstructure of the surface of the plating layer is not preserved intact and excessive cracks occur. However, in terms of further improving the light scattering effect due to the structural characteristics of the plating layer, it is more preferable that the surface roughness Ra of the bright roll is in a range of 0.4 to 0.8 µm.
In addition, if the roll reduction is less than 50 tons, a problem may occur in uniformizing the shape of the base steel sheet in the width direction, and if the roll reduction exceeds 300 tons, a problem in which the microstructure of the surface of the plating layer is not preserved intact due to an excessive pressing force, and excessive cracks occur even in a surface roughness range of the bright roll described above, may occur. However, in terms of further improving the uniform light scattering effect, in the roll reduction, it is more preferable to apply a high reduction of 150 to 300 tons.
Therefore, by performing the above-described hot-dip galvanizing and cooling, and then performing a skin pass rolling (SPM) treatment on the cooled steel sheet, and optimizing the skin pass rolling conditions, it is possible to provide a plated steel sheet having a light scattering effect. Thereby, a plated steel sheet not only having both the corrosion resistance of the flat plate portion and the corrosion resistance of the processed portion excellent, but also having excellent surface quality, may be effectively provided.

Mode for Invention

Hereinafter, the present disclosure will be specifically described through the following Examples. However, it should be noted that the following examples are only for describing the present disclosure by illustration, and not intended to limit the right scope of the present disclosure. The reason is that the right scope of the present disclosure is determined by the matters described in the claims and reasonably inferred therefrom.

(Experimental Example 1)

A pre-SMP treatment was performed on a base steel sheet, including by weight: C: 0.018%, Si: 0.01%, Mn: 0.2%, P: 0.009%, S: 0.005%, Al: 0.1%, Nb: 0.02%, Cr: 0.2%, Ti: 0.02%, B: 0.015%, with a balance of Fe and other inevitable impurities, using a bright roll with a surface roughness (Ra) of 0.2 um under a condition of 100 tons. Subsequently, the base steel sheet was heated to a temperature, which is 20°C higher than a plating bath temperature (Tb) in a heating furnace with a dew point temperature of -15°C, and then immersed in a plating bath having a composition illustrated in Table 1 below to obtain a hot-dip galvanized steel sheet. The hot-dip galvanized steel sheet was cooled using one or more inert gases among N, Ar, and He in a portion of cooling sections to meet an average cooling rate (Vc) illustrated in Table 1 from a solidification initiation temperature to a solidification end temperature.

In this case, during the cooling, the average cooling rate for each temperature section was controlled as illustrated in Table 1 below, and at the same time, an average damper opening rate of an edge portion and a center portion of the steel sheet in a width direction was controlled as those illustrated in Table 2 below based on a surface of the hot-dip galvanized steel sheet. In addition, after the cooling, a skin pass rolling (SPM) treatment was performed with a roll reduction of 50 to 150 tons using a dull roll with a surface roughness of 2 um to improve the characteristics and shape of the surface of the steel sheet.

[Table 1]

No.	Plating bath composition [wt%] (Balance of Zn)		Ts*	Tb*	t*	A*	B*	Vc*
No.	Mg	Al	Ts*	Tb*	t*	A*	B*	Vc*
A1	5.0	11.9	417	470	1.2	7.7	12.0	9.7
A2	5.3	12.3	425	480	2.0	6.2	11.0	8.2
A3	5.4	12.6	428	460	2.4	5.5	9.0	6.9
B1	5.0	11.0	419	450	3.0	4.7	7.3	5.9
B2	6.0	14.0	446	500	3.5	3.1	7	3.7
B3	4.8	12.2	418	480	4.2	2.8	6.3	3.6
C	5.1	12.9	423	490	0.7	13.8	14.0	13.9
D	4.2	11.5	417	470	1.0	14.0	6.5	10.6
E	5.7	14.0	433	500	2.0	12.0	2.1	8.3
F	4.5	11.7	416	490	3.0	16.0	15.0	15.5
G	5.0	13.5	425	490	3.5	9.0	5.9	7.7
H	5.9	13.0	440	500	4.0	10.0	2.9	7.5
I	3.5	10.0	408	450	1.2	8.0	13.0	10.6
J	6.5	12.0	457	500	1.4	7.5	12.5	9.0
K	5.5	8.0	442	480	3.0	5.0	8.0	6.0
L	6.0	14.5	440	480	2.0	6.0	12.5	8.3
M	6.0	12.0	445	470	2.0	6.0	6.0	6.0

Ts*: Solidification initiation temperature in equilibrium state [°C]
Tb*: Temperature of plating bath [°C]
t*: Thickness of steel sheet [mm]
A*: Average cooling rate from solidification initiation temperature to 375°C[°C/s]
B*: Average cooling rate from 375°C to 340°C[°C/s]
Vc*: Average cooling rate from solidification initiation temperature to solidification end temperature [°C/s]

[Table 2]

Reference	No.	De*	Dc*	De/Dc
Example 1	A1	97	100	97%
Example 2	A2	90	94	96%
Example 3	A3	93	97	96%
Example 4	B1	92	93	99%
Example 5	B2	91	97	94%
Example 6	B3	98	99	99%
Comparative Example 1	C	100	93	108%
Comparative Example 2	D	96	97	99%
Comparative Example 3	E	100	92	109%
Comparative Example 4	F	95	93	102%
Comparative Example 5	G	98	93	105%
Comparative Example 6	H	97	97	100%
Comparative Example 7	I	99	100	99%
Comparative Example 8	J	91	98	93%
Comparative Example 9	K	98	96	102%
Comparative Example 10	L	91	94	97%
Comparative Example 11	M	97	100	97%

De*: Average damper opening rate of edge portion[%]
De*: Average damper opening rate of center portion [%]

A specimen of the above-described plated steel sheet was manufactured, a plating layer was dissolved in a hydrochloric acid solution, and the dissolved liquid was analyzed using a wet analysis (ICP) method to measure a composition of the plating layer, which was illustrated in Table 3 below. In addition, a cross-sectional specimen cut in a direction perpendicular to a rolling direction of the steel sheet was manufactured so that an interface between the plating layer and a base iron was observed, and then photographed with an SEM, so that it was confirmed that a base steel sheet; a Zn-Mg-Al-based plating layer; an Fe-Al-based inhibition layer was formed between the base steel sheet and the Zn-Mg-Al-based plating layer.
In addition, a specimen of a hot-dip galvanized steel sheet was manufactured so that a surface of an area of 24,000 µm² could be observed using an SEM device. Subsequently, on a surface of the Zn-Mg-Al-based plating layer, an area fraction of an MgZn₂ phase and an Al single phase in which Zn is dissolved at less than 27 at% in an MgZn₂-Al based binary eutectic phase were respectively determined using an image captured using an SEM device, and then a total area fraction and area ratio thereof were calculated and illustrated in Table 3 below.

In this case, among Al phases present in the MgZn₂-Al based binary eutectic phase, an Al single phase in which Zn is dissolved at less than 27 at% and a second Al single phase in which Zn is dissolved at 27 at% or more and 60% or less were identified by microstructure labeling using an SEM image. Specifically, regarding the SEM image taken with the following properties: observation mode (BEI), resolution (1280x960pixel/254DPI), magnification (700x), and bit (8), the microstructure labeling is based on a super-pixel algorithm of RISA (microstructure phase fraction analysis software) of the Pohang Research Institute of Industrial Science and Technology (RIST), and is distinguished by color and contrast differences for each microstructure using automatic image generation software, and an area% thereof was quantified.

[Table 3]

No.	Plating layer composition (Balance of Zn) [wt%]		Surface of Zn-Mg-Al-based plating layer
No.	Mg	Al	Total area fraction of Al single phase and MgZn₂ phase [%]	Area fraction of MgZn₂ phase [%]	Area fraction of Al single phase [%]	Area fraction of second Al single phase [%]	Area ratio of MgZn₂ phase to Al single phase
Example 1	5.0	11.9	45.5	35	10.5	5.5	3.3
Example 2	5.3	12.3	49.8	35	14.8	8	2.4
Example 3	5.4	12.6	45.8	30.1	15.7	9	1.9
Example 4	5.0	11.0	52.2	35	17.2	6.8	2.0
Example 5	6.0	14.0	53	35.2	17.8	5	2.0
Example 6	4.8	12.2	50.6	31.4	19.2	8	1.6
Comparative Example 1	5.1	12.9	38.3	20.7	17.6	2.9	1.2
Comparative Example 2	4.2	11.5	41.5	23.1	18.4	12.3	1.3
Comparative Example 3	5.7	14.0	27.5	14.7	12.8	10	1.1
Comparative Example 4	4.5	11.7	44.5	29.7	14.8	7.2	2.0
Comparative Example 5	5.0	13.5	42.7	22.6	20.1	10	1.1
Comparative Example 6	5.9	13.0	43.5	26.5	17	15	1.6
Comparative Example 7	3.5	10.0	29.5	12.5	17	3	0.7
Comparative Example 8	6.5	12.0	71.5	46.5	25	9	1.9
Comparative Example 9	5.5	8.0	41.7	31.5	10.2	9	3.1
Comparative Example 10	6.0	14.5	62	38	24	8	1.6
Comparative Example 11	6.0	12.0	65.2	43.2	22	14	2.0

For each Example and Comparative Example, the characteristics were evaluated based on the following criteria, and the evaluation results of the characteristics were illustrated in Table 4 below.

In order to evaluate corrosion resistance of a flat plate, it was evaluated according to the following criteria using a salt spray tester (SST) using a test method in accordance with ISO14993.

⊚: The time it takes for red rust to occur exceeds 40 times than that of Zn plating of the same thickness.
○: The time taken for red rust to occur is more than 30 times and less than 40 times compared to Zn plating of the same thickness.
△: The time taken for red rust to occur is more than 20 times and less than 30 times compared to Zn plating of the same thickness.
X: The time it takes for red rust to occur is less than 20 times that of Zn plating of the same thickness.

In order to evaluate corrosion resistance of a bending portion, it was evaluated using a salt spray test device (SST) using a test method in accordance with ISO14993. A specimen for evaluating the corrosion resistance was bend processed at 90° using the same material thickness and the same plating amount.

⊚: Time taken for red rust to occur is more than 30 times that of Zn plating of the same thickness.
○: Time taken for red rust to occur is more than 20 times and less than 30 times compared to Zn plating of the same thickness.
△: Time taken for red rust to occur is more than 10 times and less than 20 times compared to Zn plating of the same thickness.
X: Time taken for red rust to occur is less than 10 times that of Zn plating of the same thickness.

In order to evaluate an amount of light scattered and reflected compared to total reflection of specimens collected by distinguishing a position of a hot-dip galvanized steel sheet into 1/4 point, center, 3/4 point, and edge, in a width direction, light in a visible wavelength range (400 to 800 nm) was incident on an integrating sphere and evaluated using a test method in accordance with ISO9001 depending on the type of light reflected.

⊚: A ratio of scattering reflectance to average total reflectance in a width direction exceeds 80% and deviation in the scattering reflectance in the width direction is less than 10%.
○: A ratio of scattering reflectance to the average total reflectance in a width direction is 70% or more but less than 80% and deviation of scattering reflectance in the width direction is 10% or more.
△: A ratio of scattering reflectance to average total reflectance in a width direction: 60% or more, less than 70%, and deviation of scattering reflectance in the width direction: 10% or more.
X: A ratio of the scattering reflectance to average total reflectance in a width direction is less than 60% and deviation of the scattering reflectance in the width direction is more than 10%.

In addition, for the steel sheets obtained from each Example and Comparative Example, the types of corrosion products initially formed on the surface were evaluated using an EDS or XRD device, which are illustrated in Table 4 below.

[Table 4]

No.	Types of corrosion products first formed on a surface	Characteristic evaluation
No.	Types of corrosion products first formed on a surface	Corrosion resistance of flat plate portion	Corrosion resistance of bending portion	Scattering reflectance
Example 1	Layered Double Hydroxide	○	○	Δ
Example 2	Layered Double Hydroxide	○	○	Δ
Example 3	Layered Double Hydroxide	○	○	Δ
Example 4	Layered Double Hydroxide	○	○	Δ
Example 5	Layered Double Hydroxide	○	○	Δ
Example 6	Layered Double Hydroxide	○	○	Δ
Comparative Example 1	Simonkolleite	Δ	X	X
Comparative Example 2	Simonkolleite	Δ	Δ	X
Comparative Example 3	Simonkolleite	Δ	Δ	X
Comparative Example 4	Simonkolleite	Δ	X	X
Comparative Example 5	Simonkolleite	Δ	Δ	X
Comparative Example 6	Simonkolleite	Δ	Δ	X
Comparative Example 7	Simonkolleite	X	X	X
Comparative Example 8	LDH	○	X	Δ
Comparative Example 9	Simonkolleite	Δ	X	X
Comparative Example 10	LDH	○∘	X	Δ
Comparative Example 11	LDH	○∘	X	Δ

As can be seen in Table 1, in the case of Examples 1 to 6, which meets all of the requirements of the plating composition and manufacturing conditions of the present disclosure, it was confirmed that LDH was first formed on a surface of a plated steel sheet during a corrosion resistance evaluation test. As a result, it was confirmed that not only was the corrosion resistance improved in a flat plate, but also in a bending portion, and surface quality was excellent because scattering reflectance of a surface of the steel sheet was somewhat high.
On the other hand, in the case of Comparative Examples 1 to 6, which meets the requirements of the plating composition of the present disclosure, but does not satisfy one or more of the cooling conditions of the above-described Relations 1-1 and 1-2, it was confirmed that Simonkolleite was first formed on the surface of the plated steel sheet during the corrosion resistance evaluation test. Because of this, not only the corrosion resistance of the flat plate of the plated steel sheet, but also the corrosion resistance of the bending portion was somewhat inferior. In addition, the scattering reflectance was also somewhat low, confirming that the surface quality was inferior.
In addition, in the case of Comparative Examples 7 to 10, which did not meet the requirements of the plating composition of the present disclosure, it was confirmed that the corrosion resistance of the flat plate portion, the corrosion resistance of the bending portion, and the surface quality were inferior. Specifically, in the case of Comparative Example 7 in which a Mg content was insufficient, a MgZn₂ phase was not sufficiently formed on a surface of the plating layer. For this reason, a total area fraction of the Al single phase and the MgZn₂ phase and an area ratio of the MgZn₂ phase to the Al single phase of the present disclosure were not satisfied. Therefore, during a corrosion resistance evaluation experiment, Simonkolleite was first formed on a surface of the plated steel sheet, and not only was the corrosion resistance of the flat plate portion and the corrosion resistance of the bending portion inferior, but scattering reflection was also inferior.
In addition, in the case of Comparative Example 8, in which a Mg content was excessive, the corrosion resistance of the flat plat portion was secured by adding a large amount of Mg, but a MgZn₂ phase was formed too coarsely on the surface of the plating layer due to the excessive Mg content, which led to cracks during bending. In addition, LDH did not cover all surfaces of the bending portion, so that the corrosion resistance of the bending portion was inferior.
In addition, in the case of Comparative Example 9, in which an Al content was insufficient, a small amount of Al single phase was formed due to the insufficient amount of Al added, and LDH was not formed as an initial corrosion product on a surface of the plated steel sheet during a corrosion resistance evaluation test. Because of this, not only was the corrosion resistance of the flat plate portion and the corrosion resistance of the bending portion inferior, but also the scattering reflection was also inferior.
In addition, in the case of Comparative Example 10, in which an Al content was excessive, both the Al single phase and the MgZn₂ phase were excessively formed, and a total area fraction of the Al single phase and the MgZn₂ phase exceeded the range of the present disclosure. Therefore, in Comparative Example 10, even though the corrosion resistance of the flat part was secured by adding an appropriate amount of Mg, the corrosion resistance of the bending processed part was poor due to the side effect of excessive brittleness of the excessively formed MgZn₂ phase and excessive cracking in the plating layer during processing.
In addition, in the case of Comparative Example 11, which did not meet the cooling conditions of Relation 1-2 of the present disclosure, even if the plating composition and other manufacturing conditions of the present disclosure are satisfied, both the Al single phase and the MgZn₂ phase were excessively formed, so that the total area fraction of the Al single phase and the MgZn₂ phase exceeded the scope of the present disclosure. Therefore, even if the corrosion resistance of the flat plate portion was secured by adding an appropriate amount of Mg and Al, the corrosion resistance of the bending portion was inferior due to a side effect in which the excessively formed MgZn₂ phase brittled excessively, resulting in excessive cracks occurring in the plating layer during processing, the corrosion resistance in the bending portion was inferior.

(Experimental Example 2)

A plated steel sheet was manufacture in the same manner as in Experimental Example 1 described above, except a ratio of a damper opening rate was changed as follows, according to a temperature section divided based on a surface temperature of a steel sheet. In this case, it was confirmed that a base steel sheet, an Fe-Al-based inhibition layer, and a Zn-Al-Mg-based plating layer were formed sequentially using the same analysis method as in Experimental Example 1.

[Table 5]

Reference	No.	First temperature section*			Second temperature section*
Reference	No.	De*	De*	De/Dc	De*	De*	De/Dc
Example 7	A1	99	100	99%	98	100	98%
Example 8	A1	60	92	65%	91	94	97%
Example 9	A1	66	94	70%	95	97	98%
Example 10	B1	99	100	99%	91	92	99%
Example 11	B1	65	99	66%	90	100	90%
Example 12	B1	68	97	70%	90	92	98%
Comparative Example 12	I	95	92	103%	96	100	96%

First temperature section*: a section from a solidification initiation temperature to 375°C
Second temperature section*: a section from 375°C to 340°C

For the plated steel sheets obtained from each of the above-described Examples and Comparative Examples, after manufacturing a cross-sectional specimen cut in a thickness direction (a direction, perpendicular to a rolling direction of the steel sheet) in the same manner as in Experimental Example 1 described above so that an interface between a plating layer and base iron was observed, the specimen was magnified at 1000 magnification, and imaged with a SEM. For the cross-sectional specimen, an area fraction of the MgZn₂ phase and an area fraction of the Al phase were measured using the same method as in Experimental Example 1 described above, and were shown in Table 6 below.

Additionally, a surface specimen having a size of 5, 400 µm² was collected in the same manner as in Experimental Example 1 described above, and an area fraction of the MgZn₂ phase and the Al phase in which Zn was dissolved at less than 27 at% in the MgZn₂-Al based binary eutectic phase were measured, respectively, and was shown in Table 6 below. In addition, for the above-described surface specimen, an area fraction of the Zn phase and Zn-MgZn₂-Al based ternary eutectic phase was measured.

[Table 6]

Reference	Zn-Mg-Al-based plating layer cross-section		Zn-Mg-Al-based plating layer surface
Reference	Area fract ion of MgZn₂ phase [%]	Area fracti on of Al single phase [%]	Area fraction of MgZn₂ phase [%]	Area fracti on of Al single phase [%]	Total area fraction of Al single phase and MgZn₂ phase [%]	Area ratio of MgZn₂ phase to Al single phase	Total area fraction of Zn phase and Zn-MgZn₂-Al-based ternary eutectic phase [%]
Example 7	27	15	35	10.5	45.5	3.3	30.7
Example 8	40	18	37.8	15.1	52.9	2.5	25.4
Example 9	39	15	36.9	16.8	53.7	2.2	26.9
Example 10	35	11	30	25.6	55.6	1.2	23.7
Example 11	29	23	35.1	19.1	54.2	1.8	24.1
Comparative Example 12	22.4	25	22.5	10	32.5	2.3	40.2

In addition, for the steel sheets obtained from each Example and Comparative Example were comparatively evaluated using a salt spray test device (SST) using a test method in accordance with ISO14993. During the above evaluation, a time at which LDH corrosion products were formed on a surface of the plating layer of the plated steel sheet was measured over time using an EDS or XRD device, which was shown in Table 7 below. In addition, characteristic evaluation shown in Table 7 below was evaluated based on the same criteria as Experimental Example 1 described above.

[Table 7]

No.	Time at which LDH corrosion product is formed	Characteristic evaluation
No.	Time at which LDH corrosion product is formed	Corrosion resistance of flat plate portion	Corrosion resistance of bending portion	Scattering reflectance
Example 7	10min	○	○	Δ
Example 8	5min	⊚	⊚	Δ
Example 9	5min	⊚	⊚	Δ
Example 10	5min	⊚	⊚	Δ
Example 11	5min	⊚	⊚	Δ
Comparative Example 12	-	Δ	X	X

In the case of Comparative Example 12, not meeting requirements in which a plating composition of the present disclosure, and a ratio (De/Dc) of a damper opening rate (De) of an edge portion to a damper opening rate (Dc) of the center portion is 60 to 70% from a solidification initiation temperature to 375°C, and is 90 to 99% from 375°C to 340°C, during a corrosion resistance evaluation experiment, Simonkolleite was first formed on a surface of the plated steel sheet, and LDH was formed on the surface only after 12 hours. For this reason, it was confirmed that corrosion resistance of a flat plate, corrosion resistance of a bending portion, and scattering reflectance of Comparative Example 12 were all inferior.
On the other hand, in the case of Examples 7 to 11, meeting the requirements of the plating composition and manufacturing conditions of the present disclosure, LDH (Layered Double Hydroxide) was formed on a surface of the plated steel sheet within 10 minutes during a corrosion resistance evaluation test, and compared to Comparative Example 12, it was confirmed that at least one of the characteristics of corrosion resistance of a flat plate, corrosion resistance of a bending portion, and scattering reflectance was superior.
In particular, in the case of Examples 8 to 11, meeting the requirements of the conditions of the present disclosure in which a ratio (De/Dc) of a damper opening rate (De) of an edge portion to a damper opening rate (Dc) of the center portion is 60 to 70% from a solidification initiation temperature to 375°C, and is 90 to 99% from 375°C to a solidification end temperature, during a corrosion resistance evaluation experiment, it was confirmed that LDH (Layered Double Hydroxide) was formed on a surface of the plated steel sheet within 5 minutes, which is rather fast, during a corrosion resistance evaluation test. As a result, in Examples 8 to 11 of the present disclosure, compared to Examples 7 and 10, it was confirmed that corrosion resistance of a flat plate and corrosion resistance of a bending portion were further improved. This is presumed to be due to a MgZn₂ phase present in large quantities in a surface layer portion and an Al single phase in which a Zn solid solution ratio is less than 27%, adjacent to the MgZn₂ phase. In other words, due to rapid nucleation and crystallization of LDH, a dense corrosion product, on a surface in an early stage of a corrosive environment, it is because it is uniformly distributed throughout the surface over time, a corrosion active region is shielded and uniform formation of Simonkolleite and Hydrozincite, which are secondarily formed, is induced.

(Experimental Example 3)

For the hot-dip galvanized and cooled steel sheets were plated under the same conditions as in Experimental Examples 1 and 2 described above, a plated steel sheet was manufactured under the same conditions as in Experimental Examples 1 and 2 described above, except that a pre-skin pass rolling treatment, cooling, and a post-cooling skin pass rolling (SPM) treatment were performed under the conditions shown in Tables 8 and 9 below.

[Table 8]

Reference	No.	Before hot dip galvanizing, Pre-SPM processing conditions
Reference	No.	Type of roll	Roll surface roughness Ra [µm]	Roll reduction applied to surface of steel sheet [ton]
Example 12	A1	Bright	0.4	100.0
Example 13	A1	Bright	0.4	200.0
Example 14	A1	Bright	0.8	300.0
Example 15	B1	Bright	0.4	250.0
Example 16	B1	Bright	0.8	288.0
Comparative Example 13	J	Bright	0.4	50.0
Comparative Example 14	K	Bright	0.4	102.0
Comparative Example 15	M	Bright	0.8	250.0

[Table 9]

Reference	First temperature section*			Second temperature section*			SPM treatment condition
Reference	De*	De*	De/Dc	De*	De*	De/Dc	Type of roll	Roll surface rouhness Ra [um]	Roll reduction applied to surface of steel sheet [ton]
Example 12	66	97	68%	90	98	92%	Dull	2	138.0
Example 13	60	93	65%	98	100	98%	Bright	0.4	246.0
Example 14	63	97	65%	95	98	97%	Bright	0.8	299.0
Example 15	62	95	65%	94	96	98%	Bright	0.4	266.0
Example 16	60	93	65%	91	96	95%	Bright	0.8	248.0
Comparative Example 13	90	91	99%	96	96	100%	Dull	2	107.0
Comparative Example 14	92	98	94%	98	100	98%	Bright	0.4	218.0
Comparative Example 15	60	95	63%	97	100	97%	Bright	0.8	255.0

[First temperature section*: a section from a solidification initiation temperature to 375°C
Second temperature section*: a section from 375°C to 340°C

For a plated steel sheet obtained from each Example and Comparative Example, a specimen was manufactured in the same manner as in Experimental Example 1 described above, and then on a surface of the plating layer, an area fraction of a MgZn₂ phase and an Al single phase in which Zn was dissolved at less than 27 at% were measured, and an area fraction of a Zn single phase and a Zn-MgZn₂-Al-based ternary eutectic phase were measured, respectively, and were shown in Tables 10 and 11 below.
In addition, for a plated steel sheet obtained from each Example and Comparative Example, surface polishing was performed using the same standard to prepare a specimen illustrating a surface having an area of 24,000 µm² at any point from 1/4t to 3/4t in a thickness direction of the plating layer. The surface polishing was performed on a cold mounted specimen with the surface facing upwardly so that the surface could be observed in a depth direction. Surface polishing was performed at a speed of approximately 2um/min under the conditions of load of 30N AND 105RPM, and forward direction using an automatic polisher and silica suspension.

On the surface at any same point in Examples and Comparative Examples in the region from 1/4t to 3/4t of the plating layer in the thickness direction thus obtained, a total area fraction of the Al single phase and the MgZn₂ phase, and a total area fraction of the Zn single phase and Zn-MgZn₂-Al-based ternary eutectic phase was measured and shown in Table 10 below.

[Table 10]

Reference	Surface of Zn-Mg-Al-based plating layer
Reference	Area fract ion of MgZn₂ phase [%]	Area fracti on of Al single phase [%]	Total area fraction of Al single phase and MgZn₂ phase[%]	Area ratio of MgZn₂ phase to Al single phase	Total area fraction of Zn single phase and Zn-MgZn₂-Al-based ternary eutectic phase[%]	Area fraction of second Al single phase [%]
Example 12	35	10.5	45.5	3.3	30	7.3
Example 13	30.6	21.8	50	1.4	22.3	7.5
Example 14	32.2	15.8	48	2.0	30	5.4
Example 15	30.6	20.1	47.7	1.5	25.8	9.5
Example 16	32	17	49	1.9	23	6.5
Comparative Example 13	46.5	25	71.5	1.9	23.6	2.9
Comparative Example 14	31.5	10.2	41.7	3.1	42.1	7.2
Comparative Example 15	43.2	22	65.2	2.0	19.5	8.6

[Table 11]

Reference	Surface of Zn-Mg-Al-based plating layer		Surface in a region from 1/4t to 3/4t of a plating layer in a thickness direction			S1/C1	S2/C2
Reference	S1*	S2*	C1*	C2*	Area fraction of second Al single phase [%]	S1/C1	S2/C2
Example 12	45.5	30.0	46.9	33.9	9.9	0.97	0.88
Example 13	50	22.3	42.1	28.6	9.8	1.18	0.78
Example 14	48.0	30.0	44.0	31.0	8.2	1.09	0.97
Example 15	47.7	25.8	47	22.4	6.8	1.01	1.15
Example 16	49.0	23.0	47.0	24.0	9.6	1.04	0.96
Comparative Example 13	71.5	23.6	67.2	14	9.8	1.06	1.69
Comparative Example 14	41.7	42.1	56.9	12.7	6.5	0.73	3.31
Comparative Example 15	65.2	19.5	53.2	20.1	7.8	1.23	0.97

S1*: Total area fraction of Al single phase and MgZn₂ phase on a surface of Zn-Mg-Al plating layer [%]
S2*: Total area fraction of Zn phase and Zn-MgZn₂-Al ternary eutectic phase on a surface of Zn-Mg-Al-based plating layer [%]
C1*: Total area fraction of MgZn₂ phase and Al phase on a surface in a region from 1/4t to 3/4t in a thickness direction of a plating layer [%]
C2*: Total area fraction of Zn phase and Zn-MgZn₂-Al ternary phase on a surface in a region from 1/4t to 3/4t in a thickness direction of a plating layer [%]

In addition, the steel sheets obtained from each Example and Comparative Example were evaluated for the properties shown in Table 12 below based on the same criteria as in Experimental Example 1 described above.

[Table 12]

No.	Characteristic evaluation
No.	Corrosion resistance of flat plate portion	Corrosion resistance of bending portion	Scattering reflectance
Example 12	○	○	Δ
Example 13	⊚	⊚	⊚
Example 14	⊚	⊚	⊚
Example 15	⊚	⊚	⊚
Example 16	⊚	⊚	⊚
Comparative Example 13	○	X	X
Comparative Example 14	Δ	Δ	X
Comparative Example 15	○	X	X

As can be seen in Table 12, in the case of Comparative Example 13 in which an Mg content is insufficient in the plating composition of the present disclosure, a De/Dc condition in a first temperature section was not met, and a dull roll is used, an Al single phase and MgZn₂ phase were excessively formed on a surface of a plating layer, so that not only was corrosion resistance of a bending portion inferior, but also light scattering of a surface portion was also inferior.
In addition, in the case of Comparative Example 14, among plating compositions of the present disclosure, in which an Al content was insufficient and did not meet a De/Dc condition in a first temperature section, due to non-formation of initial LDH, not only was corrosion resistance of a flat plate and corrosion resistance of a bending portion inferior, but also scattering reflection was also low, resulting in inferior appearance quality.
In addition, in the case of Comparative Example 15, which met a plating composition and other manufacturing conditions of the present disclosure, but does not satisfy cooling conditions of Equation 1-2, although corrosion resistance of a flat plate was secured, corrosion resistance of a bending portion was inferior due to a side effect in which excessive brittle occurs due to an excessively formed MgZn₂ phase and cracks excessively occurred in a plating layer during processing.
On the other hand, in the case of Examples 12 to 16, which met a plating composition and manufacturing conditions of the present disclosure, it was confirmed that LDH was formed on a surface of a plated steel sheet within 5 minutes during a corrosion resistance evaluation experiment. As a result, it was confirmed that corrosion resistance was improved not only in a flat plate portion but also in a bending portion, surface quality was excellent scattering reflectance of the surface of the steel sheet was somewhat high.
In particular, in the case of Examples 13 to 16, which met SPM processing conditions of applying 50 to 300 tons of roll reduction to the surface of the steel sheet using a bright roll with a surface roughness (Ra) of 0.2 to 1.0 um, conditions of S1/C1 and S2/C2 were satisfied, and it was confirmed that not only corrosion resistance of a flat plate portion and corrosion resistance of a bending portion, but also scattering reflectance was the best.
While example embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present disclosure as defined by the appended claims.

Claims

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

a Zn-Mg-Al-based plating layer provided on at least one surface of the base steel sheet; and

an Fe-Al-based inhibition layer provided between the base steel sheet and the Zn-Mg-Al-based plating layer,

wherein on a surface of the Zn-Mg-Al-based plating layer, a total area fraction of an Al single phase and MgZn₂ phase is 45 to 60%, and an area ratio of the MgZn₂ phase to the Al single phase is 1.2 to 3.3.
The plated steel sheet of claim 1, wherein the Zn-Mg-Al-based plating layer comprises, by weight:
Mg: 4 to 6%, Al: 8.2 to 14.2%, with a balance of Zn and other inevitable impurities.
The plated steel sheet of claim 1, wherein with respect to a cross-section of the Zn-Mg-Al-based plating layer, an area fraction of the MgZn₂ phase is 20 to 40%, and an area fraction of the Al single phase is 8 to 26%.
The plated steel sheet of claim 1, wherein on the surface of the Zn-Mg-Al-based plating layer, the area fraction of the MgZn₂ phase is 30 to 40%.
The plated steel sheet of claim 1, wherein on the surface of the Zn-Mg-Al-based plating layer, the area fraction of the Al single phase is 15 to 20%,
wherein the Al single phase is a phase in which Zn is dissolved at less than 27%, in atomic percentage, the Al single phase including a balance of Al and other impurities.
The plated steel sheet of claim 1, wherein a ratio (S1/C1) of a total area fraction S1 of the MgZn₂ phase and the Al single phase on the surface of the Zn-Mg-Al-based plating layer to a total area fraction C1 of the MgZn₂ phase and the Al single phase on a surface of a point corresponding to any one of regions from 1/4t to 3/4t in a thickness direction of the Zn-Mg-Al plating layer is in a range of 0.8 to 1.2.
The plated steel sheet of claim 1, wherein a total area fraction of a Zn phase and a Zn-MgZn₂-Al-based ternary eutectic phase on the surface of the Zn-Mg-Al-based plating layer is 20 to 30%.
The plated steel sheet of claim 1, wherein a ratio (S2/C2) of a total area fraction S2 of the Zn phase and the Zn-MgZn₂-Al-based ternary eutectic phase on the surface of the Zn-Mg-Al-based plating layer to a total area fraction C2 of the Zn phase and the Zn-MgZn₂-Al-based ternary eutectic phase on a surface of any point of regions from 1/4t to 3/4t in a thickness direction of the Zn-Mg-Al plating layer is in a range of 0.6 to 1.2.
The plated steel sheet of claim 1, wherein on the surface of the Zn-Mg-Al-based plating layer, an area fraction of a second Al single phase in which 27 to 60% of Zn is dissolved, in atomic percentage, is 2 to 9%.
The plated steel sheet of claim 1, wherein under atmospheric environments and chloride environments of ISO14993, LDH((Zn,Mg)₆Al₂(OH)₁₆(CO₃)·4H₂O) is formed on the surface of the Zn-Mg-Al-based plating layer prior to Simonkolleite (Zn₅(OH)₈Cl₂) and hydrozinsite (Zn₅(OH)₆(CO₃)₂) formation.
The plated steel sheet of claim 1, wherein under atmospheric environments and chloride environments of ISO14993, LDH((Zn,Mg)₆Al₂(OH)₁₆(CO₃)·4H₂O) is formed on the surface of the Zn-Mg-Al-based plating layer is within 6 hours in the atmospheric environment and within 5 minutes in the chloride environment.
The plated steel sheet of claim 1, wherein under chloride environments of ISO14993 including salt spray and immersion environments, a time it takes for red rust to occur is 40 to 50 times longer in a flat plate portion; and 20 to 30 times in a 90 degree bending portion, compared to Zn plating of the same thickness.
A manufacturing method of a plated steel sheet, comprising operations of:
hot dip galvanizing a base steel sheet, including by weight: Mg: 4 to 6%, Al: 8.2 to 14.2%, with a balance Zn and other inevitable impurities, by immersing the base steel sheet in a plating bath maintained at a temperature of 20 to 80°C higher than a solidification initiation temperature in an equilibrium state; and

cooling the hot-dip galvanized steel sheet using an inert gas at an average cooling rate of 2 to 12°C/s from the solidification initiation temperature to a solidification end temperature,

wherein in the cooling operation, cooling is performed so that the following relations 1-1 and 1-2 are satisfied, and a ratio (De/Dc) of a damper opening rate De of an edge portion to a damper opening rate Dc of a center portion satisfies 60 to 99%. $A < \{(5 - 2 lnt) / (7 - 3 lnt)\} \times B$
${15t}^{(- 0.8)} \leq B \leq 20 t^{(- 0.8)}$

In the above relations 1-1 and 1-2, t is a thickness of the steel sheet (mm), A is an average cooling rate (°C/s) from a solidification initiation temperature to 375°C, and B is an average cooling rate (°C/s) from 375°C to 340°C.
The manufacturing method of claim 13, wherein in the cooling operation, cooling is performed by changing the ratio (De/Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion, depending on a temperature section, and
the ratio (De/Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion is 60 to 70% from the solidification initiation temperature to 375°C, and is 90 to 99% from 375°C to 340°C.
The manufacturing method of claim 14, further comprising an operation of, after the cooling operation:
improving a surface and shape of the base steel sheet by performing a skin pass rolling treatment,

wherein the skin pass rolling treatment is performed by applying a roll reduction of 50 to 300 tons to the surface of the steel sheet using a bright roll with surface roughness (Ra) of 0.2 to 1.0 um.
The manufacturing method of claim 13, further comprising an operation of, before the hot-dip galvanizing:
performing a pre-skin pass rolling treatment of applying a roll reduction of 200 to 300 tons to the surface of the steel sheet using a bright roll with surface roughness (Ra) of 0.2 to 0.4 µm.
The manufacturing method of claim 16, wherein during the pre-skin pass rolling treatment, the roll reduction is 250 to 300 tons.