KR20220169338A - Plated steel sheet having excellent corrosion resistance and surface property and method for manufacturing the same - Google Patents

Abstract

One aspect of the present invention provides a plated steel sheet and a method for manufacturing the same, wherein the plated steel sheet comprises 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 suppression layer provided between the base steel sheet and the Zn-Mg-Al-based plating layer, wherein the total area ratio of the Al single phase and the MgZn_2 phase on the surface of the Zn-Mg-Al-based plating layer is 45 to 60 %, and the area ratio of the MgZn_2 phase to the Al single phase is 1.2 to 3.3.

Description

Highly corrosion-resistant plated steel sheet with excellent corrosion resistance and surface quality and its manufacturing method

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

When exposed to a corrosive environment, the zinc-based coated steel sheet has a characteristic of a sacrificial method in which zinc, which has a lower oxidation-reduction potential than iron, is corroded first and corrosion of the steel is suppressed. In addition, while zinc in the plating layer oxidizes, a dense corrosion product is formed on the surface of the steel material to block the steel material from the oxidizing atmosphere, thereby improving the corrosion resistance of the steel material. Thanks to these advantageous characteristics, the range of application of zinc-based coated steel sheets has recently been expanding to steel sheets for construction materials, home appliances, and automobiles.

However, the corrosive environment is gradually deteriorating due to the increase in air pollution due to industrial advancement, and the need for the development of steel materials having better corrosion resistance than conventional galvanized steel materials is increasing due to strict regulations on resource and energy saving. .

In order to improve these problems, various studies are being conducted on manufacturing techniques for zinc alloy-based coated steel sheets that improve corrosion resistance of steel materials by adding elements such as aluminum (Al) and magnesium (Mg) to a zinc plating bath. As a representative example, there is a Zn-Mg-Al based zinc alloy plated steel sheet in which Mg is additionally added to the Zn-Al plating composition system.

However, Zn-Mg-Al-based zinc alloy-coated steel sheets are often used after being processed in a zinc-based manner. Since they contain a large amount of intermetallic compounds with high hardness in the plating layer, bending workability such as causing cracks in the plating layer during bending is poor. The downside is that it gets worse.

Accordingly, attempts have been made to further improve the bending workability of the plated steel sheet, but even if the bending workability is improved, the base steel sheet is exposed due to the micro cracks generated in the processing portion during bending, so that not only the corrosion resistance of the plate portion of the plated steel sheet but also the bending workability is improved. It was technically very difficult to secure even the corrosion resistance of the negative.

On the other hand, it is known that the corrosion resistance of the bent part of the zinc-alloy plated steel sheet self-healing the exposed part of the base steel sheet by leaching Mg and Al components in a moisture atmosphere, but the effect is insignificant, There was a problem that it was difficult to secure the corrosion resistance of the bending processing part to the level.

In addition, zinc-based coated steel sheets are often provided on the outer shell of a product, but the product with a higher Mg content in the plating layer has a darker appearance, and due to the addition of surface damage factors due to processing, the surface quality is poor, resulting in poor appearance quality. Improvement was needed.

However, a level of technology capable of meeting the high-grade demand for excellent corrosion resistance and appearance quality of a bent part as well as corrosion resistance of a flat part has not been developed.

Korean Publication No. 2010-0073819

According to one aspect of the present invention, it is intended to provide a coated steel sheet excellent in corrosion resistance and appearance quality in a bent portion as well as corrosion resistance in a flat plate portion and a method for manufacturing the same.

The object of the present invention is not limited to the foregoing. Anyone with ordinary knowledge in the technical field to which the present invention belongs will have no difficulty in understanding additional tasks of the invention from the content throughout the present specification.

One aspect of the present invention,

base steel plate;

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

Including; Fe-Al-based suppression layer provided between the base steel sheet and the Zn-Mg-Al-based plating layer,

On the surface of the Zn-Mg-Al-based plating layer, the total area ratio of the Al single phase and the MgZn ₂ phase is 45 to 60%, and the area ratio of the MgZn ₂ phase to the Al single phase is 1.2 to 3.3.

Another aspect of the present invention is,

In a plating bath containing steel sheet by weight, Mg: 4-6%, Al: 8.2-14.2%, balance Zn and other unavoidable impurities, maintained at a temperature 20-80°C higher than the solidification initiation temperature in equilibrium. immersion and hot-dip galvanizing; and

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

The cooling step satisfies the following relational expressions 1-1 and 1-2, 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 satisfies 60 to 99% It provides a manufacturing method of a plated steel sheet in which cooling is performed so as to

[Relationship 1-1]

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

[Relationship 1-2]

15t ^(-0.8) ≤ B ≤ 20t ^(-0.8)

(In the relational expressions 1-1 and 1-2, t is the thickness of the steel sheet (mm), A is the average cooling rate (° C./s) from the solidification start temperature to 375° C., and B is at 375° C. Average cooling rate (°C/s) up to 340°C is shown.)

According to one aspect of the present invention, it is possible to provide a plated steel sheet excellent in corrosion resistance and appearance quality of a processed part, as well as plate corrosion resistance, and a manufacturing method thereof.

Various and beneficial advantages and effects of the present invention are not limited to the above description, and will be more easily understood in the process of describing specific embodiments of the present invention.

1(a) shows a surface specimen capable of observing the surface of the plated steel sheet of Example 13, and magnifying the surface specimen at a magnification of 700 to obtain a Field Emission Scanning Electron Microscope (FE-SEM). It is a photograph observed by), and FIG. 1 (b) shows the measured ratio of each phase with respect to the photograph.
FIG. 2(a) shows the coated steel sheet of Example 13, the same as that of FIG. 1, after polishing to the 1/2t point, and then making a 1/2t surface specimen from which the polished surface can be observed. It is a photograph observed with a scanning electron microscope (FE-SEM) by magnifying 700 times, and FIG. 2 (b) shows the measured ratio of each phase with respect to the photograph.
3(a) shows a surface specimen for observing the surface of the plated steel sheet of Comparative Example 1, and magnifying the surface specimen at a magnification of 700 to obtain a Field Emission Scanning Electron Microscope (FE-SEM). It is a photograph observed by), and FIG. 3 (b) shows the measured ratio of each phase with respect to the photograph.
FIG. 4(a) shows the coated steel sheet of Comparative Example 1, the same as that of FIG. 3, after polishing to the 1/2t point, and then making a 1/2t surface specimen from which the polished surface can be observed. It is a photograph observed with a scanning electron microscope (FE-SEM) by magnifying 700 times, and FIG. 4 (b) shows the measured ratio of each phase with respect to the photograph.
Figure 5 is observed with EDS (Energy Dispersive Spectrometer) on the Al single phase, the second Al single phase, and the Al-Zn-based binary process referred to in FIGS. 1 to 4, and the fraction of the elements dissolved in the microstructure It is an urbanized graph.

Terms used herein are for describing specific embodiments and are not intended to limit the present invention. Also, the singular forms used herein include the plural forms unless the related definition clearly dictates the contrary.

The meaning of "comprising" as used in the specification specifies a component, and does not exclude the presence or addition of other components.

Unless otherwise defined, all terms including technical terms and scientific terms used in this specification have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. The terms defined in the dictionary are interpreted to have a meaning consistent with the related technical literature and the currently disclosed content.

Hereinafter, a [coated steel sheet] according to an aspect of the present invention will be described in detail. In the present invention, when indicating the content of each element, it means weight% unless otherwise specifically defined.

In the conventional technology related to Zn-Mg-Al-based zinc alloy coated steel sheet, Mg was added to improve corrosion resistance, but when Mg is added excessively, floating dross in the plating bath increases, so the dross must be removed frequently. , the upper limit of the amount of Mg added was limited to 3%. Accordingly, studies have been conducted to further improve corrosion resistance by increasing the amount of Mg added to more than 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. There is a problem.

Therefore, there have been studies to secure corrosion resistance and bending workability, but even if corrosion resistance and bending workability of the flat plate portion of the plated steel sheet are secured, the base steel sheet is exposed due to microcracks that inevitably occur during bending. It was technically very difficult to secure even corrosion resistance. In addition, the more Mg is added, the darker the product's appearance and the more surface damage factors are added, making it difficult to secure the appearance quality.

Therefore, the inventors of the present invention, as a result of intensive examination to solve the above-mentioned problems, as well as corrosion resistance of the flat plate part, corrosion resistance of the bending part and excellent appearance quality, as a result of intensive examination, under a corrosive environment (or in an atmospheric environment) When maintained for a long time), it is important to uniformly form LDH (Layered Double Hydroxide; (Zn,Mg) ₆ Al ₂ (OH) ₁₆ (CO ₃ ) 4H ₂ O)) as an initial corrosion product on the surface of the processing part. discovered and completed the present invention.

Therefore, in the following, LDH is formed as an initial corrosion product on the surface of the bending workpiece, and at the same time, LDH is uniformly distributed over the entire surface of the workpiece over time, so that the corrosion active area can be shielded. Explain.

First, a coated steel sheet according to an aspect of the present invention includes a holding 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 suppression layer provided between the base steel sheet and the Zn-Mg-Al-based plating layer.

In the present invention, the type of the base steel sheet may not be particularly limited. For example, the holding steel sheet may be a Fe-based holding steel sheet, that is, a hot-rolled steel sheet or a cold-rolled steel sheet, which is used as a holding steel sheet of a general zinc-based coated steel sheet, but is not limited thereto. Alternatively, the base steel plate may be, for example, carbon steel, ultra-low carbon steel, or high manganese steel used as a material for construction, home appliances, and automobiles. However, as an example, the base steel plate, in weight%, C: 0% More than 0.18%, Si: More than 0% and less than 1.5%, Mn: 0.01 to 2.7%, P: More than 0% and less than 0.07%, S: More than 0% and less than 0.015%, Al: More than 0% and less than 0.5%, Nb: It may have a composition including more than 0% and 0.06% or less, Cr: more than 0% and 1.1% or less, Ti: more than 0% and 0.06% or less, B: more than 0% and 0.03% or less, and the balance Fe and other unavoidable impurities.

According to one aspect of the present invention, a Zn-Mg-Al-based plating layer made 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 side of the base steel sheet, or may be formed on both sides of the base steel sheet. At this time, the Zn-Mg-Al-based plating layer refers to a plating layer containing Mg and Al and mainly containing Zn (ie, containing 50% or more of Zn).

According to one aspect of the present invention, the thickness of the Zn-Mg-Al-based plating layer may be 5 to 100 μm, more preferably 5 to 90 μm. If the thickness of the plating layer is less than 5 μm, the plating layer may locally become too thin due to errors due to variations in the thickness of the plating layer, and thus corrosion resistance may be deteriorated. If the thickness of the plating layer exceeds 100 μm, cooling of the hot-dip plating layer may be delayed, for example, solidification defects such as flow patterns may occur on the surface of the plating layer, and productivity of the steel sheet may decrease in order to solidify the plating layer.

In addition, according to one aspect of the present invention, a Fe-Al-based suppression layer may be provided between the base steel sheet and the Zn-Mg-Al-based plating layer. The Fe-Al-based suppression layer is a layer mainly containing an intermetallic compound of Fe and Al, and examples of the Fe and Al intermetallic compound include FeAl, FeAl ₃ , Fe ₂ Al ₅ , and the like. In addition, some components derived from the plating layer, such as Zn and Mg, may be further included, for example, 40% or less. The suppression layer is a layer formed due to alloying by Fe diffused from the base steel sheet in the initial stage of plating and plating bath components. The suppression layer may serve to improve adhesion between the base steel sheet and the plating layer, and at the same time prevent diffusion of Fe from the base steel sheet to the plating layer. At this time, the suppression layer may be formed continuously or discontinuously between the base steel sheet and the Zn-Mg-Al-based plating layer. With respect to the suppression layer, except for the above description, contents commonly known in the art may be equally applied.

According to one aspect of the present invention, the thickness of the suppression layer may be 0.02 ~ 2.5㎛. The suppression layer serves to secure corrosion resistance by preventing alloying, but may affect workability due to brittle, so its thickness may be 2.5 μm or less. However, in order to serve as a suppression layer, it is preferable to control the thickness to 0.02 μm or more. In terms of further improving the above effect, the upper limit of the thickness of the suppression layer may be preferably 1.8 μm. In addition, the lower limit of the thickness of the suppression layer may be 0.05 μm. In this case, the thickness of the suppression layer may mean a minimum thickness in a direction perpendicular to the interface of the base steel plate.

Meanwhile, according to one aspect of the present invention, the Zn-Mg-Al-based plating layer may include Mg: 4 to 6%, Al: 8.2 to 14.2%, the balance Zn and other unavoidable impurities in weight%. Hereinafter, each component will be described in detail.

Mg: 4% or more and 6% or less

Mg is an element that serves to improve the corrosion resistance of coated steel materials, and in the present invention, the Mg content in the plating layer is controlled to 4% or more to secure the desired excellent corrosion resistance. On the other hand, since the effect is improved as Mg is added from the viewpoint of securing corrosion resistance, the upper limit of the Mg content may not be particularly limited. However, as an example, when Mg is excessively added, dross may be generated, so the Mg content may 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, the effect of improving corrosion resistance is exhibited, but when Mg is added at 2% or more, the generation of floating dross in the plating bath due to oxidation of Mg in the plating bath increases, and the dross is frequently There is a problem that needs to be eliminated. 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 the upper limit of the Mg content was set at 3.0% to commercialize it. However, as described above, , In order to further improve corrosion resistance, it is necessary to increase the Mg content to 4% or more, but if Mg is included in the plating layer by 4% or more, there is a problem in that dross is generated due to oxidation of Mg in the plating bath, so it is necessary to add Al. there is However, if Al is added excessively to suppress dross, the melting point of the plating bath increases and the operating temperature becomes too high, causing problems due to high-temperature work such as erosion of the plating bath structure and deterioration of steel materials. It can be. In addition, if the Al content in the plating bath is excessive, Al reacts with Fe in the base iron and does not contribute to the formation of the Fe-Al suppression layer, and the reaction between Al and Zn occurs rapidly, resulting in a lumpy outburst phase. Corrosion resistance may be rather deteriorated due to excessive formation. Therefore, the upper limit of the Al content in the plating layer is preferably controlled to 14.2%, more preferably 14.0%.

balance Zn and other unavoidable impurities

In addition to the composition of the plating layer described above, the remainder may be Zn and other unavoidable impurities. Any unavoidable impurities may be included as long as they can be unintentionally mixed in the manufacturing process of a typical hot-dip galvanized steel sheet, and those skilled in the art can easily understand their meaning.

The Zn-Mg-Al-based plating layer may include MgZn ₂ -phase and Al single-phase as a microstructure, and in addition, Al-Zn-based binary eutectic phase, Zn-MgZn ₂ -Al-based ternary eutectic phase, Zn single-phase It may be included in various top coat plating layers, such as the like.

At this time, in the present invention, the MgZn ₂ phase refers to a phase mainly composed of MgZn ₂ , and the Al single phase refers to a phase mainly composed of Al, and specifically, Zn is dissolved in an atomic % of less than 27%. , the balance of which 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 components of the plating layer may be solidified.

In addition, the Zn-MgZn ₂ -Al-based ternary eutectic phase refers to a ternary eutectic phase in which all of the Zn phase, the MgZn ₂ -phase and the Al phase are mixed, and the Al-Zn-based binary eutectic phase refers to an Al phase and Zn phases are alternately arranged in lamella or irregular mixed form. At this time, it is necessary to note that the Al phase in the Al-Zn-based binary eutectic phase and the Zn-MgZn ₂ -Al-based ternary eutectic phase is not regarded as the above-mentioned Al single phase or the second Al single phase described later. Similarly, it is also worth noting that MgZn ₂ in the Zn-MgZn ₂ -Al-based ternary eutectic phase is not regarded as the above-mentioned MgZn ₂ phase mainly composed of MgZn ₂ .

In addition, the Zn-Mg-Al-based plating layer may further include a 'second Al single phase', which is distinguished from the Al single phase by a Zn solid solution rate. 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 percent, and the balance is composed of Al and other impurities.

On the other hand, the microstructure of the above-described Zn-Mg-Al-based plating layer may have a different distribution on the surface and cross-section, and the microstructure on the surface and cross-section is scanned by expanding the magnification of the plating layer for each surface specimen or cross-section specimen. It can be confirmed using an electron microscope (SEM) or a transmission electron microscope (TEM).

As such, the Zn-Mg-Al-based plating layer includes various phases depending on the composition and manufacturing conditions of the plating layer, but the inventors of the present invention, in addition to the conventional corrosion resistance of the flat plate part, the corrosion resistance and appearance quality of the bending part are excellent in both the coated steel sheet As a result of careful examination to provide LDH (Layered Double Hydroxide; (Zn,Mg) ₆ Al ₂ (OH) ₁₆ (CO It was found that the uniform formation of ₃ )·4H ₂ O) was an important factor.

As such, the initial formation of LDH as a corrosion product on the surface of the plated steel sheet is related to the microstructural features on the surface of the Zn-Mg-Al-based plating layer (ie, the surface of the exterior rather than the surface of the base iron). confirmed and came to complete the present invention.

Specifically, according to one aspect of the present invention, on the surface of the Zn-Mg-Al-based plating layer, the total area ratio of the Al single phase and the MgZn ₂ phase is 45 to 60%, and the area ratio of the MgZn ₂ phase to the Al single phase is It is characterized by being 1.2 to 3.3. At this time, on the surface of the Zn-Mg-Al-based plating layer, the total area ratio of the Al single phase and the MgZn ₂ phase and the area ratio of the MgZn ₂ phase to the Al single phase are measured based on a surface specimen having an area of 24,000 μm ² or more. .

In the present invention, in the case of a Zn-Mg-Al-based plating layer satisfying the above-described plating layer composition, the MgZn ₂ phase and the Al single phase include an adjacent microstructure. The form in which the MgZn ₂ phase and the Al single phase are adjacent includes the case where the Al single phase is completely included in the MgZn ₂ phase or the Al single phase is partially included in the MgZn ₂ phase, and additionally, the Al single phase is in contact with the MgZn ₂ phase. Including the case where this exists.

In the present invention, the Zn-Mg-Al-based plating layer may include a Zn single phase and a Zn-MgZn ₂ -Al-based ternary eutectic phase, which are common in high corrosion-resistant plated steel sheets. In general, as the Al and Mg content in the plating layer decreases, the amount of the Zn single phase and Zn-MgZn ₂ -Al-based ternary eutectic phase generated in the entire plating layer increases, and as the Al and Mg content in the plating layer increases, the MgZn ₂ phase and The amount of Al single phase produced tends to increase.

That is, in a plating component system such as the present invention in which the Mg content is 4% or more, the MgZn ₂ phase appears coarse as shown in FIG. Accordingly, a coarse Al single phase also coexists. Accordingly, the inventors of the present invention, in order to secure the corrosion resistance of the above-mentioned bending process, the total area ratio and area ratio of the MgZn _{2 phase and the Al single phase adjacent to the MgZn 2 phase} on the surface of the plating layer under a corrosive environment (or under an atmospheric environment for a long time) It was found that it contributes to the formation of LDH as a corrosion product initially during maintenance.

That is, in order to secure not only the corrosion resistance of the flat plate part of the plated steel sheet, but also the corrosion resistance of the bent part, it is important that the MgZn ₂ phase and the Al single phase exist in an adjacent form as the structure of the surface of the plated layer, and rapid nucleation-crystallization of LDH can promote Therefore, as time elapses after the initial rapid nucleation and crystallization of LDH, the corrosion active region is effectively shielded due to the uniformly formed LDH across the surface, and secondarily, Simonkolleite (Zn ₅ (OH), a corrosion product) ) ₈ Cl ₂ ) and hydrozincite; (Zn ₅ (OH) ₆ (CO ₃ ) ₂ ) can be induced to form uniformly.

Therefore, according to one aspect of the present invention, it is important to ensure that the MgZn ₂ phase and the Al single phase are adjacent to each other on the surface of the plating layer by a specific amount or more. Specifically, on the surface of the Zn-Mg-Al-based plating layer, the total area ratio of the MgZn ₂ phase and the Al single phase (adjacent to the MgZn ₂ phase) satisfies 45 to 60%, and the MgZn ₂ to the Al single phase By satisfying the area ratio of 1.2 to 3.3, it is possible to secure excellent corrosion resistance by performing a role of forming a sacrificial cell between the MgZn ₂ phase and the Al single phase. At this time, the corrosion resistance includes not only the corrosion resistance of the flat plate part but also the corrosion resistance of the bent part, and this corrosion resistance is improved as the amount of MgZn ₂ phase and Al single phase present on the surface of the plating layer is higher than that of the inside of the plating layer.

On the surface of the Zn-Mg-Al-based plating layer, when the total area ratio of the MgZn ₂ phase and the Al single phase is less than 45%, the respective phases forming the anode (MgZn ₂ ) and the cathode (Al) of the sacrificial cell are insufficient for bending processing. Secondary corrosion resistance may be insufficient, and light scattering due to phases present on the surface may also be insufficient, resulting in deterioration in appearance quality. On the other hand, if the total area ratio of the MgZn ₂ phase and the Al single phase exceeds 60%, brittle MgZn ₂ phase is excessively formed, resulting in excessive cracking of the plating layer during processing.

In addition, on the surface of the Zn-Mg-Al-based plating layer, if the area ratio of the MgZn ₂ phase to the Al single phase is less than 1.2, the amount of the MgZn ₂ anode forming the sacrificial cell can be dissolved is small, resulting in corrosion resistance. Disadvantageous problems may occur, and if MgZn ₂ is dissolved and transferred, there may be a limit to the rate of cathode reaction (oxygen reduction reaction) occurring in Al on the surface by accepting electrons that are dissolved and transferred.

The corrosion resistance of the bending part is shaped by two mechanisms. First, the MgZn ₂ phase and Al single phase present in the bending part form a complete sacrificial cell, and the corrosion product covers and hides the exposed portion of the steel sheet during bending. Second, as a self-healing mechanism that reforms the plating layer by leaching oxidatively friendly Mg and Al components in a moisture atmosphere and moving to the exposed portion of the steel sheet during bending, Mg and Al components with high reactivity with moisture The effect is improved so that a large amount exists in this surface layer part.

In the sacrificial type cell serving as the first mechanism, the potential of MgZn ₂ is -1.2V on the hydrogen reduction potential and the potential of Al is -0.7V on the hydrogen reduction potential, securing a large potential difference, thereby acting as an anode and a cathode, respectively. Thus, it means to form a galvanic cell between the adjacent MgZn ₂ -phase and Al single-phase microstructures.

As a result of intensive research, the inventors of the present invention have secured a high potential difference between the MgZn ₂ phase and the Al single phase adjacent to the MgZn ₂ phase on the surface of the plating layer, thereby ensuring corrosion resistance at the bending part by forming a galvanic cell. was confirmed, and it was found that the phase enabling a high potential difference to be secured adjacent to the MgZn ₂ phase was an Al single phase having a Zn solid solution ratio of less than 27 atomic%.

That is, in the Zn-Mg-Al-based plating layer according to one aspect of the present invention, among the phases mainly composed of Al, ① a single phase of Al having a Zn solid solution of less than 27 atomic%, and ② a second phase having a high Zn solid solution of 27 to 60%. Two types of Al single phase can exist. In addition, among them, it was confirmed that the phase capable of maintaining a high potential difference by existing adjacent to the MgZn ₂ phase is an Al single phase (corresponding to ①) having a low Zn solid solution.

In other words, according to one aspect of the present invention, in the Zn-Mg-Al-based plating layer, when a large number of second Al single phases having a high Zn solid solution rate of 27 atomic% or more are formed, the second Al existing around the MgZn ₂ phase The number of single phases increases, and as a result, the potential difference between the anode and the cathode of the galvanic cell is reduced, which may impair excellent corrosion resistance and sacrificial corrosion resistance of the galvanic cell.

Therefore, according to one aspect of the present invention, the area ratio of the second Al single phase on the surface of the Zn-Mg-Al-based plating layer may be 2 to 9%. If the area ratio of the second Al single phase exceeds 9%, the second Al single phase is excessively formed around the MgZn ₂ phase, reducing the potential difference of the galvanic cell and deteriorating corrosion resistance at the bending part. Therefore, in the present invention, on the surface of the Zn-Mg-Al-based plating layer, the area ratio of the second Al single phase is controlled to 9% or less, and the smaller the amount of the second Al single phase present on the surface, the better the corrosion resistance of the bending part. Since the effect is improved, the lower limit may not be separately limited. However, considering that the second Al single phase is necessarily formed in the temperature range in which the second Al single phase is formed during the cooling process after hot-dip plating, the lower limit may be set to 2%.

According to one aspect of the present invention, on the surface of the Zn-Mg-Al-based plating layer, the area ratio of the MgZn ₂ phase may be 30 to 40%. The portion of the plating layer that is primarily in contact with the air and chloride environment is the surface, and the higher the ratio of the MgZn ₂ phase acting as an anode in the sacrificial method, the higher the reactivity in the galvanic cell. Therefore, by promoting the formation of the aforementioned galvanic cell, the area ratio of the MgZn ₂ phase on the surface of the plating layer may be set to 30% or more in order to secure corrosion resistance of the bending portion. Therefore, if the area ratio of the MgZn ₂ phase on the surface of the plating layer is less than 30%, the corrosion resistance of the bent portion may be insufficient. On the other hand, when the MgZn ₂ phase ratio exceeds 40% and is excessively high, the plating layer may become brittle and cause cracks on the surface.

Alternatively, according to one aspect of the present invention, or according to one aspect of the present invention, on the surface of the Zn-Mg-Al-based plating layer, the Al single phase (ie, in atomic %, Zn is dissolved at less than 27%, , the balance of which includes Al and other impurities) may have an area ratio of 15 to 20%. On the surface of the plating layer, if the area ratio of the Al single phase is 15% or more, as described above, MgZn ₂ acting as an anode in the galvanic cell can act as a cathode to help improve the corrosion resistance of the bending part, and the framework for the MgZn ₂ phase By performing a holding function, the plating layer can contribute to a role as a physical protective barrier. On the other hand, if the ratio of the Al single phase exceeds 20%, there is room for deterioration in stability due to Al corrosion.

In addition, according to one aspect of the present invention, the total area ratio of the Zn single phase and the Zn-MgZn ₂ -Al-based ternary phase on the surface of the Zn-Mg-Al-based plating layer may be 20 to 30%. The Zn single phase and the Zn-MgZn ₂ -Al-based ternary eutectic phase present on the surface of the Zn-Mg-Al-based plating layer contribute to the formation of Simoncolite or hydrozincite rather than LDH at the initial stage of corrosion. Therefore, by controlling the existence ratio of the Zn single phase and the Zn-MgZn ₂ -Al-based ternary eutectic phase on the surface of the Zn-Mg-Al-based plating layer, among corrosion products formed on the initial surface of corrosion, Simon collite or hydrozincite It is possible to further improve the corrosion resistance of the bent portion by increasing the formation rate of LDH rather than the formation rate. Therefore, the total area ratio of the Zn single phase and the Zn-MgZn ₂ -Al-based ternary eutectic phase on the surface of the Zn-Mg-Al-based plating layer may be 20 to 30%. At this time, if the total area ratio of the Zn single phase and the Zn-MgZn ₂ -Al-based ternary phase on the surface of the Zn-Mg-Al-based plating layer is less than 20%, it is generated secondarily after LDH formation to help improve corrosion resistance. Simoncol There is a concern that the formation of light or hytrozinsite becomes insufficient and a problem arises in corrosion resistance. On the other hand, when the total area ratio of the Zn single phase and the Zn-MgZn ₂ -Al-based ternary phase on the surface of the Zn-Mg-Al-based plating layer exceeds 30%, the formation of Simoncolite and hydrozincite rather than the formation of LDH at the initial stage of corrosion Since this is induced first, stable corrosion behavior as described above may not be formed, and corrosion resistance may be inferior.

On the other hand, according to one aspect of the present invention, based on the cross section of the Zn-Mg-Al-based plating layer cut in the thickness direction (ie, the direction perpendicular to the rolling direction of the steel sheet), the area ratio of the MgZn ₂ phase is 20 to 40%, , the area ratio of the Al single phase may be 8 to 26%.

The properties of the plated steel sheet are related to the type and size of the crystal phase, and when the area ratio of the MgZn ₂ phase is less than 20% or the area ratio of the Al single phase is less than 8%, the corrosion resistance of the plating layer may be weakened. On the other hand, when the ratio of the MgZn ₂ phase present in the plating layer exceeds 40%, it may be too brittle, resulting in excessive cracking in the plating layer during processing. The area ratio of the MgZn ₂ phase and the Al single phase based on the cross-section of the Zn-Mg-Al-based plating layer can be measured by observing a photograph of a cross-sectional specimen in the thickness direction of the coated steel sheet with FE-SEM.

In the present invention, even if the corrosion resistance of the cut-edge of the steel sheet can be secured by satisfying the area ratio of the MgZn ₂ phase and the Al single phase based on the cross section of the Zn-Mg-Al-based plating layer, the surface of the plating layer Area ratios of the MgZn ₂ phase and the Al single phase secured may be different. Therefore, according to the area ratio distribution of each phase on the surface of the plating layer, the degree of corrosion resistance of the processed portion may be affected during bending.

Therefore, the inventors of the present invention found that, even if the above-mentioned MgZn ₂ phase and Al single phase area ratio is secured on a cross-sectional basis in the thickness direction of the plating layer, securing a specific amount or more of the MgZn ₂ phase and Al single phase on the surface of the plating layer is the surface of the plating layer in the initial stage of corrosion. found that it is an important means to secure corrosion resistance of the processed part by promoting the uniform formation of LDH. That is, according to one aspect of the present invention, it is important to maintain the ratio of the total area ratio of the MgZn ₂ phase and the Al single phase at the center of the plating layer to the total area ratio of the MgZn ₂ phase and the Al single phase on the surface of the plating layer at an appropriate level. additionally found.

Specifically, according to one aspect of the present invention, the total area ratio of the MgZn ₂ phase and the Al single phase (C1 ), the ratio (S1/C1) of the total area ratio (S1) of the MgZn ₂ phase and the Al single phase on the surface of the Zn-Mg-Al-based plating layer may be in the range of 0.8 to 1.2. If S1/C1 is less than 0.8, problems may arise in the corrosion resistance of the flat plate and processed parts due to the lack of microstructure forming LDH in the initial stage of corrosion on _the surface layer of the plating layer. Excessive coarsening of the resulting brittle tissue may cause problems in workability and corrosion resistance of the processed part.

According to one aspect of the present invention, the area ratio of the second Al single phase is 2 to 10% on the surface of any point in the region from 1/4t to 3/4t in the thickness direction of the Zn-Mg-Al-based plating layer. can If the value exceeds 10%, it may affect the structure of the surface layer portion and adversely affect the corrosion resistance of the bent portion. In addition, the lower limit may be controlled to 2% considering that the second Al single phase passes through the temperature range.

In the area from 1/4t to 3/4t in the thickness direction of the Zn-Mg-Al-based plating layer, the point where the plating layer thickness is the maximum in the plated specimen is regarded as the total thickness t, and based on the t, from 1/4t It may mean a region polished with respect to the surface of the specimen to include an arbitrary point in the region of 3/4t.

Furthermore, the inventors of the present invention conducted additional studies to determine the Zn phase and the Zn-MgZn ₂ -Al-based ternary process phase that penetrates into the inside and promotes the formation of Simoncolite and hydrozincite after the uniform formation of LDH on the surface of the plating layer. It was found that the ratio of the center to the surface is also an important factor to further improve the corrosion resistance.

That is, according to one aspect of the present invention, the Zn phase and the Zn-MgZn ₂ - The ratio of the total area ratio (S2) of the Zn phase and the Zn-MgZn ₂ -Al-based ternary phase on the surface of the Zn-Mg-Al-based plating layer to the total area ratio (C2) of the Al-based ternary eutectic phase (S2/C2) may be in the range of 0.6 to 1.2. When the S2/C2 is less than 0.6, a problem may arise in corrosion resistance due to lack of formation of simonicolite or hydrozincite, which is secondarily generated after the formation of LDH on the surface layer of the plating layer and helps to improve corrosion resistance. In addition, when the S2/C2 exceeds 1.2, the MgZn ₂ phase and the Al single phase secured on the surface are relatively insufficient, causing a lack of LDH formation on the surface, which may cause a problem in the corrosion resistance described in the present invention.

On the other hand, the above-mentioned MgZn ₂ phase, Al single phase, 2nd Al single phase, Zn single phase and Zn-MgZn ₂ -Al system 3 that can be derived from the surface of the Zn-Mg-Al-based plating layer satisfying the Mg and Al compositions according to the present invention The definition of each phase and the atomic percent employed for the original phase are shown in FIGS. 1 to 5.

Specifically, after manufacturing a specimen so that the surface of the plated layer of the plated steel sheet can be observed using an SEM device, images taken using an SEM or EDS device are taken on the surface of the Zn-Mg-Al-based plated layer for each microstructure. Each region can be calculated by distinguishing by color and contrast difference.

That is, the plane of the plated steel sheet as shown in FIGS. 1 to 4 is enlarged to 700 times in BEI (backscattered electron image) observation mode, 1280x960 pixel / 254 DPI resolution, and 8-bit properties, and observed with a field emission scanning electron microscope (FE-SEM). From one photograph, 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 tissue labeling of SEM images.

For reference, the fraction of elements dissolved in different phases for each light and shade of the SEM image can be obtained using an energy dispersive spectrometer (EDS) commonly known in the art. At this time, as an example, as shown in FIG. 5, the Al-based phase, except for the MgZn ₂ phase, Zn single phase, and Zn-MgZn ₂ -Al-based ternary phase, which are clearly distinguished by color, contrast, and shape for each microstructure, is averaged in the region of ①. In atomic%, Al is 73%, Zn is 26%, and the balance is less than 1%. 2 represents an Al single phase, and ③ is distinguished by an Al-Zn-based binary eutectic region in which Al is 43%, Zn is 57%, and the remainder is less than 1% in atomic percent (at this time, the remainder is Mg). or other unavoidable impurities). The Al single phase referred to in the present invention means the region ① in which Zn is dissolved at less than 27% in atomic %, and the second Al single phase means the region in ② in which Zn is dissolved at 27% or more and 60% or less in atomic %, , the Al-Zn-based binary eutectic phase refers to the region of ③, and Fe and other components may be included as impurities in each phase.

At this time, the tissue labeling uses the image derived from the above-mentioned SEM measurement conditions, based on the Super-pixel algorithm of the Pohang Institute of Industrial Science and Technology (RIST)'s RISA (microtissue phase fraction analysis software), image automatic generation software. . The superpixel algorithm divides the entire image into thousands or tens of thousands of regions (superpixels), compares superpixels with similar patterns or features to measure similarity, calculates a histogram of brightness values of pixels, and then calculates the similarity. It is a mechanism that automatically selects a superpixel when is greater than a predefined threshold. As an example of specifying a predefined threshold value, the boundaries of the Al single phase and the 2nd Al single phase of the image derived from the above-mentioned SEM measurement conditions are each based on the Zn employment rate of 27 atomic% employed in Al tissue using EDS. By defining the phase in advance, histogram of the brightness value of the soft phase and tissue discrimination are possible. The technical idea of the above-mentioned RISA (microstructure phase fraction analysis software) can be confirmed through Korean Publication No. 2019-0078331.

According to one aspect of the present invention, LDH may be formed on the surface of the plating layer before _Simoncolite and hydrozincite are formed on the surface of the plating layer in an air environment and a chloride environment. Rapid nucleation-crystallization of LDH, a dense corrosion product, proceeds on the initial surface of the corrosive environment by the single phase. Thereafter, as time elapses, it is uniformly distributed over the entire surface to shield the corrosion active area, and induces uniform formation of secondarily formed corrosion products, simonicolite and hydrozincite.

According to one aspect of the present invention, the LDH corrosion product formed on the surface layer portion of the plating layer can be formed within 6 hours in an air environment and within 5 minutes in a chloride environment (ie, as measured by ISO14993).

According to one aspect of the present invention, the above-mentioned excellent corrosion resistance is that the time required for red rust to occur in a chloride environment including salt spray and immersion environments (i.e., as measured by ISO14993) is 40 to 40 to 40 in the flat plate compared to pure Zn plating of the same thickness. 50 times; And it may be 20 to 30 times in the 90 ° bending part. At this time, the evaluation of the red rust generation time performed can be comparatively evaluated by a test method conforming to ISO14993 using a salt spray tester (SST).

Next, a [method of manufacturing a coated steel sheet] according to another aspect of the present invention will be described in detail. However, this does not mean that the plated steel sheet of the present invention must be manufactured by the following manufacturing method.

According to one aspect of the present invention, first, a step of preparing a steel sheet may be further included, and the type of the steel sheet is not particularly limited. It may be a Fe-based steel sheet, that is, a hot-rolled steel sheet or a cold-rolled steel sheet, which is used as a holding steel sheet of a conventional hot-dip galvanized steel sheet, but is not limited thereto. In addition, the holding steel sheet may be, for example, carbon steel, ultra-low carbon steel, or high manganese steel used as a material for construction, home appliances, and automobiles, but is not limited thereto. At this time, the above description can be equally applied to the holding steel plate.

Then, according to one aspect of the present invention, the step of immersing the base steel sheet in a plating bath containing, by weight, Mg: 4-6%, Al: 8.2-14.2%, the balance Zn and other unavoidable impurities, to perform hot-dip galvanizing can include At this time, the description of the components of the plating layer described above can be equally applied to the reason for adding the components and the reason for limiting the content in the plating bath, except for the content of a small amount of Fe that may flow from the base steel sheet. .

In order to manufacture a plating bath having the above-described composition, a composite ingot containing predetermined Zn, Al, and Mg or a Zn-Mg or Zn-Al ingot containing individual components may be used. In order to supplement the plating bath consumed by hot-dip plating, the ingot is additionally melted and supplied. In this case, a method of directly immersing and dissolving the ingot in the plating bath may be selected, or a method of dissolving the ingot in a separate pot and then replenishing the molten metal in the plating bath may be adopted.

In addition, the temperature of the plating bath may be maintained at a temperature 20 to 80 ° C higher than the solidification start temperature (Ts) in the equilibrium state. At this time, although not particularly limited, the solidification initiation temperature on the equilibrium phase may be in the range of 390 to 460 ° C, or the temperature of the plating bath may be maintained in the range of 440 to 520 ° C. As the temperature of the plating bath increases, it is possible to secure fluidity in the plating bath and form a uniform composition, and to reduce the amount of floating dross generated. If the temperature of the plating bath is less than 20° C. compared to the solidification initiation temperature on the equilibrium diagram, 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 on the equilibrium diagram, a problem of ash defects caused by Zn evaporation may occur on the plating surface.

In addition, according to one aspect of the present invention, the 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 start temperature to the solidification end temperature on the equilibrium diagram. can do. If the average cooling rate described above is less than 2°C/s, the MgZn ₂ structure develops too coarsely on the surface, and the surface of the plating layer becomes brittle, resulting in severe cracking, and it may be disadvantageous in securing uniform corrosion resistance and workability. On the other hand, if the above-mentioned average cooling rate exceeds 12 ° C. / s, the liquid phase starts to solidify during the hot dip plating process, and rapid solidification occurs in the temperature range while the liquid phase is all changed to the solid phase. Therefore, excessive coarsening and atomization of the MgZn ₂ phase and the single phase of Al may occur on the surface of the plating layer, and as a result, non-uniform phases may be locally formed on the surface of the plating layer, resulting in deterioration in corrosion resistance.

In addition, according to one aspect of the present invention, the cooling may control the cooling rate to satisfy the following relational expressions 1-1 and 1-2.

[Relationship 1-1]

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

[Relationship 1-2]

15t ^(-0.8) ≤ B ≤ 20t ^(-0.8)

(In the relational expressions 1-1 and 1-2, t is the thickness (mm) of the steel sheet, A is the average cooling rate (° C./s) from the solidification start temperature to 375° C., and B is at 375° C. Average cooling rate (°C/s) up to 340°C is shown.)

That is, the present invention divides the first temperature range from the solidification start temperature to 375°C and the second temperature range from 375°C to 340°C when cooling after hot-dip galvanizing, and averages the average in each interval according to the thickness of the steel sheet. It is characterized in that the cooling rate is controlled to satisfy the above relational expressions 1-1 and 1-2. In the first temperature range from the solidification initiation temperature to 375° C., the Al single phase adjacent to the MgZn ₂ phase in the plating layer formed in the range of Mg and Al according to the present invention is cooled to form a binary process, and Zn is reduced by atomic %. , corresponding to the interval from the solidification start temperature to the solidification end temperature for the Al single phase in which less than 27% of the solid solution is dissolved and the balance is composed of Al and other impurities. At this time, it should be noted that the 'Al single phase' is distinguished from the 'second Al single phase' to be described later. In addition, in the second temperature range from 375 ° C to 340 ° C, in the plating layer formed in the content range of Mg and Al according to the present invention, in atomic%, Zn is 27% or more and 60% or less (ie, 27 to 60%). %) represents the formation temperature range of the second Al single phase. Therefore, as a case where the cooling conditions of the above-described relational expressions 1-1 and 1-2 are not satisfied, if the initial cooling rate is too fast, from the solidification start temperature The area ratio of the MgZn ₂ -Al-based binary eutectic phase on the surface of the plating layer formed in the temperature range up to 375 °C is too low, and the degree of formation of the MgZn ₂ phase and the adjacent Al single phase may be insufficient. For this reason, LDH is not formed as an initial corrosive product on the surface of the plating layer, but Simoncolite may be formed, and there is a fear that corrosion resistance is further deteriorated.

On the other hand, according to one aspect of the present invention, before the hot-dip galvanizing, using a bright roll having a surface roughness (Ra) of 0.2 to 0.4 μm, 200 to 300 tons of roll pressure is applied to the surface of the steel sheet, pre-temperature rolling (SPM) processing may be further included.

In this way, by subjecting the base steel sheet to surface treatment before hot-dip galvanizing, the surface shape of the base steel sheet is uniformly controlled so that the thickness of the hot-dip plating layer formed by the subsequent plating process is uniform, and at the same time, By making the base steel sheet smooth, it is possible to minimize the generation sites of solidification nuclei. That is, by contributing more smoothly to nucleation in the surface layer portion of the plating layer than internal nucleation in the thickness direction during cooling, formation of tissue in the surface layer portion generated in the first temperature range is promoted, and the second Al single phase generated in the second temperature range can contribute to lowering the percentage of On the other hand, during the pre-temperature rolling treatment, if the surface roughness of the roll is less than 0.2 μm, problems in roll manufacturing and management may occur, and if it exceeds 0.4 μm, there is a problem in that nucleation of the inside rather than nucleation of the surface layer of the coating layer can be facilitated. can happen In addition, if the roll reduction is less than 200 ton, the effect of controlling the shape of the base steel sheet is low, and it may be difficult to expect the above-mentioned effect of promoting solidification nucleation in the surface layer portion. It may be difficult to expect the effect of uniform image formation contribution in the thickness direction. On the other hand, in order to further improve the above effect, more preferably, the roll rolling of the preliminary temper rolling treatment before the hot-dip galvanizing may be set to 250 to 300 tons.

After the pre-temperature rolling treatment, the base steel plate is placed in a heating furnace having a dew point temperature of -60 ° C or more and -15 ° C or less, and the temperature of the base steel plate in the last section of the heating furnace is compared to the plating bath temperature (Tb) to secure wettability of plating. It may include heating to a high temperature of 20 ° C to 80 ° C. The dew point temperature of the heating furnace is for preventing oxidation of the surface of the base steel sheet, and the temperature of the heating furnace may be -60°C or more -15°C in order to secure plating adhesion.

In addition, according to one aspect of the present invention, the ratio of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion in the width direction of the hot-dip galvanized steel sheet during cooling ( De/Dc) can be cooled to meet 60 to 99%. At this time, the 'width direction' of the steel sheet refers to a direction perpendicular to the conveying direction of the steel sheet, based on the surface excluding the thickness-side surface of the hot-dip galvanized steel sheet (ie, the surface where the thickness of the steel sheet is visible). In addition, the damper opening rate is a numerical value referring to the opening degree of the control plate that controls the flow rate of the cooling gas to be sent from the cooling device to the steel sheet. In order to secure a uniform cooling capacity according to the width of the steel plate, which will be described later, a damper is installed so that the total cooling gas input or controlled to the cooling device can be divided into the center part and the edge part according to the width direction of the steel plate and injected. The boundary between the dampers can be divided into three sections according to the width of the holding steel plate, and the position can be variably controlled so that the center portion is occupied by the center portion and the edge portion is occupied by two portions existing on the outer side.

In the cooling of the conventional hot-dip galvanized steel sheet, in the cooling of the conventional hot-dip galvanized steel sheet, the edge portion and the center portion are cooled without using a method or device for adjusting the ratio of the ratio (De/Dc). There was a problem in that it was difficult to ensure uniform microstructural characteristics on the surface of the plating layer by keeping the gas flow rate constant. In contrast, in the present invention, contrary to conventional cooling conditions, the ratio (De/Dc) was set to 60 to 60 Uniform cooling performance in the width direction of the steel sheet can be realized by controlling the opening rate of the edge portion to be lower than that of the center portion within the range of 99%. That is, the present inventors found that the edge portion in the width direction of the steel sheet has a larger area exposed to the external atmosphere than the center portion, so that the temperature of the steel sheet in the region corresponding to the edge portion is inevitably lowered at a faster rate than the center portion. and it was found that uniform characteristics of the surface of the plating layer can be secured by artificially reducing the cooling rate at the edge portion. That is, the cooling gas incident on the center portion in the above cooling process It passes through the edge and exits to the outer shell. However, since the edge part receives the cooling gas incident on the edge part and the cooling gas after entering the center part in an overlapping manner, the cooling gas may be overcooled compared to the center part, thereby adversely affecting the cooling gas. 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 to increase corrosion resistance by forming LDH as an initial corrosion product, the damper opening rate of the edge portion It needs to be controlled in a direction lower than the center portion.

At this time, when 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 rather slowly cooled than the center portion, and 99% If it is exceeded, the edge portion is 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, the structure of the surface of the plating layer in the edge portion and the center portion becomes non-uniform, and the structural characteristics of the Al single phase and the MgZn ₂ phase cannot be secured on the surface of the plating layer, thereby deteriorating the corrosion resistance of the flat part and the corrosion resistance of the bending part. .

Alternatively, according to another aspect of the present invention, in the cooling step, the ratio (De / Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion according to the temperature range Cooling can be performed by giving a change.

Specifically, during the cooling, 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 from the solidification start temperature to 375 ° C. (in the 'first temperature section' Correspondence) may be performed to satisfy 60 to 70%, and to satisfy 90 to 99% from 375 ° C to 340 ° C (corresponding to the 'second temperature range').

By cooling by varying the ratio (De/Dc) according to the temperature range to satisfy the above conditions, the aforementioned MgZn ₂ -Al-based binary eutectic phase is formed uniformly in the width direction of the steel sheet from the solidification initiation temperature to 375 ° C. By performing slow cooling, corrosion resistance can be improved uniformly over the entire width. In addition, from 375 ° C. to the solidification end temperature at which the above-described second Al single phase is formed, by controlling to form the second Al single phase having a high Zn solid solution rate as much as possible, galvanic between microstructures composed of MgZn ₂ -Al-based binary eutectic phase It can be controlled so as not to affect the cell, and thus, not only the corrosion resistance of the flat plate part, but also the corrosion resistance of the bent part can be further improved.

In addition, according to one aspect of the present invention, after the cooling step, in order to improve the surface quality of the final product, it may further include the step of improving the surface and shape of the steel sheet by performing temper rolling (SPM) treatment. . Through this, it is possible to improve the appearance quality by securing a light scattering effect on the surface of the plating layer uniform in the width direction of the steel sheet.

Specifically, as an embodiment, the temper 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 having a surface roughness (Ra) of 0.2 to 1.0 μm. .

At this time, if the surface roughness Ra of the bright roll is less than 0.2 μm, the roughness of the roll is too low, and the frictional force between the steel sheet and the SPM roll is reduced, resulting in a problem of slipping of the steel sheet, and exceeding 1.0 μm. The microstructure of the surface of the backside plating layer may not be completely preserved, and excessive cracking may occur. However, it is more preferable that the surface roughness Ra of the bright roll is in the range of 0.4 to 0.8 μm in terms of further improving the light scattering effect due to the textural characteristics of the plating layer.

In addition, if the roll reduction is less than 50 tons, a problem may arise in uniformizing the shape of the steel sheet in the width direction, and if it exceeds 300 tons, the microstructure of the surface of the plating layer cannot be completely preserved due to excessive reduction force, and the above-described bright roll Even in the surface roughness range of , excessive cracking may occur. However, in terms of further improving the uniform light scattering effect, it is more preferable to apply a high pressure of 150 to 300 ton for the roll pressing.

Therefore, after hot-dip galvanizing and cooling, the cooled steel sheet is subjected to temper rolling (SPM) treatment, and by optimizing conditions for temper rolling, it is possible to provide a coated steel sheet having a light scattering effect. Through this, it is possible to effectively provide even a plated steel sheet having excellent surface quality as well as excellent corrosion resistance of the flat plate part and corrosion resistance of the processed part.

(Example)

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for explaining the present invention through examples, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Experimental Example 1)

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%, balance Fe and other unavoidable impurities. A pre-SPM treatment was performed on the steel sheet under the condition of 100 ton using a bright roll having a surface roughness (Ra) of 0.2 μm. Subsequently, the base steel sheet was heated at a temperature 20° C. higher than the plating bath temperature (Tb) in a heating furnace having a dew point temperature of -15° C., and then immersed in a plating bath having the composition shown in Table 1 below to obtain a hot-dip plated steel sheet. The hot-dipped steel sheet was cooled using at least one inert gas of N, Ar, and He in a part of the cooling section so as to satisfy the average cooling rate (Vc) shown in Table 1 from the solidification start temperature to the solidification end temperature.

At this time, during the cooling, the average cooling rate for each temperature section is controlled as shown in Table 1 below, and the average damper opening rate of the edge portion and the center portion in the width direction of the steel sheet based on the surface of the hot-dipped steel sheet is In addition, after the cooling, a temper rolling (SPM) treatment is performed at a roll reduction of 50 to 150 tons using a dumb roll having a surface roughness of 2 μm to improve the characteristics and shape of the surface of the steel sheet. treatment was performed.

No. Plating bath composition [wt%]
(balance Zn) Ts* Tb* t* A* B* Vc* Mg Al 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 on equilibrium diagram [°C]

Tb*: plating bath temperature [°C]

t*: Thickness of steel plate [mm]

A*: average cooling rate from solidification onset temperature to 375°C [°C/s]

B*: average cooling rate from 375°C to 340°C [°C/s]

Vc*: Average cooling rate from solidification start temperature to solidification end temperature [°C/s]

note 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 degree of the edge [%]

Dc*: average damper opening in the center section [%]

Specimens of the above-described plated steel sheet were prepared, the plated layer was dissolved in a hydrochloric acid solution, and the dissolved liquid was analyzed by wet analysis (ICP) to measure the composition of the plated layer, and the results are shown in Table 3 below. In addition, after preparing a cross-sectional specimen cut in a direction perpendicular to the rolling direction of the steel sheet so that the interface between the plating layer and the base iron can be observed, photographed by SEM, the base steel plate; Zn-Mg-Al-based plating layer; It was confirmed that a Fe-Al-based suppression layer was formed between the base steel sheet and the Zn-Mg-Al-based plating layer.

In addition, specimens of the hot-dipped steel sheet were prepared so that the surface of the specimen having an area of 24,000 μm ² could be observed using an SEM device. Subsequently, on the surface of the Zn-Mg-Al-based plating layer, the area ratio of the MgZn ₂ phase and the Al single phase in which Zn was dissolved at less than 27 at% in the MgZn ₂ -Al-based binary process phase was determined by using an image taken using an SEM device. After obtaining each, the total area ratio and area ratio were calculated and shown in Table 3 below.

At this time, among the Al phases present in the MgZn ₂ -Al-based binary eutectic phase, a single Al 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 are SEM images was separated by tissue labeling. Specifically, the tissue labeling is performed by the Pohang Institute of Industrial Science and Technology (RIST)'s RISA ( Using the super-pixel algorithm-based image generation software of microstructure phase fraction analysis software), each microstructure was distinguished by color and contrast difference, and area% was quantified.

No. plating layer composition
(balance Zn) [wt%] Surface of Zn-Mg-Al-based plating layer Mg Al Total area ratio of Al single phase and MgZn ₂ phase [%] MgZn ₂ phase area ratio [%] Al single phase area ratio [%] 2nd Al single phase area ratio [%] Area ratio of MgZn ₂ 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 properties were evaluated based on the following criteria, and the evaluation results of the properties are shown in Table 4 below.

<Platform corrosion resistance>

In order to evaluate the corrosion resistance of the flat plate, the salt spray tester (Salt Spray Tester, SST) was used to evaluate according to the following criteria in accordance with ISO14993 test method.

◎: Time taken for occurrence of red rust exceeds 40 times compared to Zn plating of the same thickness

○: Time required for occurrence of red rust is 30 times or more and less than 40 times compared to Zn plating of the same thickness

△: The time required for occurrence of red rust is 20 times or more and less than 30 times compared to Zn plating of the same thickness

Х: The time required for occurrence of red rust is less than 20 times that of Zn plating of the same thickness

<Corrosion resistance of bending parts>

In order to evaluate the corrosion resistance of the bent portion, it was evaluated by a test method according to ISO14993 using a salt spray tester (SST). The corrosion resistance evaluation specimen was subjected to 90° bending with the same material thickness and the same coating weight.

◎: The time required for occurrence of red rust is more than 30 times that of Zn plating of the same thickness

○: The time required for occurrence of red rust is 20 times or more and less than 30 times compared to Zn plating of the same thickness

△: Time required for occurrence of red rust is 10 times or more and less than 20 times compared to Zn plating of the same thickness

Х: Less than 10 times the time required for occurrence of red rust compared to Zn plating of the same thickness

<Scattering reflectance>

In order to evaluate the amount of scattered and reflected light compared to the total reflection of the sample collected by dividing the position into 1/4 point, center, 3/4 point, and edge in the width direction of the hot-dipped steel sheet, the visible ray wavelength band was placed in the integrating sphere. (400 ~ 800nm) light was incident and evaluated according to the test method according to ISO9001 according to the type of reflected light.

◎: ratio of scattering reflectivity to average total reflectance in the width direction exceeding 80% and deviation of scattering reflectivity in the width direction less than 10%

○: Ratio of scattering reflectivity to average total reflectance in the width direction of 70% or more and less than 80% and deviation of scattering reflectivity in the width direction of 10% or more

△: Ratio of scattering reflectivity to average total reflectance in the width direction of 60% or more and less than 70% and deviation of scattering reflectivity in the width direction of 10% or more

Х: ratio of scattering reflectivity to average total reflectance in the width direction less than 60% and scattering reflectivity deviation in the width direction greater than 10%

In addition, with respect to the steel sheets obtained from each Example and Comparative Example, the type of corrosion product initially formed on the surface was evaluated using an EDS or XRD device, and these are shown in Table 4 below.

No. Types of corrosion products that first form on the surface Characteristic evaluation reputation department
corrosion resistance bending part
corrosion resistance spawning
reflectivity Example 1 Layered Double Hydroxide ○ ○ △ Example 2 Layered Double Hydroxide ○ ○ △ Example 3 Layered Double Hydroxide ○ ○ △ Example 4 Layered Double Hydroxide ○ ○ △ Example 5 Layered Double Hydroxide ○ ○ △ Example 6 Layered Double Hydroxide ○ ○ △ Comparative Example 1 Simonkolleite △ Х Х Comparative Example 2 Simonkolleite △ △ Х Comparative Example 3 Simonkolleite △ △ Х Comparative Example 4 Simonkolleite △ Х Х Comparative Example 5 Simonkolleite △ △ Х Comparative Example 6 Simonkolleite △ △ Х Comparative Example 7 Simonkolleite Х Х Х Comparative Example 8 LDH ○ Х △ Comparative Example 9 Simonkolleite △ Х Х Comparative Example 10 LDH ○ Х △ Comparative Example 11 LDH ○ Х △

As can be seen in Table 1, in the case of Examples 1 to 6 satisfying both the plating composition and manufacturing conditions of the present invention, it was confirmed that LDH was first formed on the surface of the plated steel sheet during the corrosion resistance evaluation experiment. As a result, it was confirmed that not only the flat plate but also the corrosion resistance was further improved in the bending part, and the surface quality was excellent because the scattering reflectance of the steel plate surface was somewhat high.

On the other hand, in the case of Comparative Examples 1 to 6, which satisfy the plating composition of the present invention but do not satisfy at least one cooling condition of the above-described relational expressions 1-1 and 1-2, Simon Cole was first applied to the surface of the coated steel sheet during the corrosion resistance evaluation experiment. It was confirmed that light was formed. For this reason, not only the flat plate corrosion resistance of the plated steel sheet but also the corrosion resistance of the bent portion was somewhat inferior. In addition, it was confirmed that the surface quality was inferior because the scattering reflectance was somewhat low.

In addition, in the case of Comparative Examples 7 to 10, which do not satisfy the plating composition of the present invention, it was confirmed that the corrosion resistance of the flat plate part, the corrosion resistance of the bending part, and the surface quality were inferior. Specifically, in the case of Comparative Example 7 having an insufficient Mg content, the MgZn ₂ phase was not sufficiently formed on the surface of the plating layer. For this reason, the total area ratio of the Al single phase and the MgZn ₂ phase and the area ratio of the MgZn ₂ phase to the Al single phase of the present invention could not be satisfied. Therefore, during the corrosion resistance evaluation experiment, Simoncolite was first formed on the surface of the plated steel sheet, resulting in poor corrosion resistance of the flat plate part and corrosion resistance of the bent part, as well as inferior scattering reflectance.

In addition, in the case of Comparative Example 8 having an excessive Mg content, the corrosion resistance of the flat plate portion was secured by adding a large amount of Mg, but the MgZn ₂ phase was too coarsely formed on the surface of the plating layer due to the excessive Mg content, and thus during bending. Cracks occurred excessively. In addition, since the LDH did not cover all of the surface of the bent portion, the corrosion resistance of the bent portion was inferior.

In addition, in the case of Comparative Example 9 having an insufficient Al content, a small amount of Al single phase was formed due to the insufficient amount of Al added, so that LDH was not formed as an initial corrosion product on the surface of the plated steel sheet during the corrosion resistance evaluation experiment. Due to this, not only the corrosion resistance of the flat plate part and the corrosion resistance of the bent part were inferior, but also the scattering reflectance was inferior.

In addition, in the case of Comparative Example 10 having an excess Al content, both the Al single phase and the MgZn ₂ phase were excessively formed, so that the total area ratio of the Al single phase and the MgZn ₂ phase exceeded the scope of the present invention. Therefore, in Comparative Example 10, even if the corrosion resistance of the plate part is secured by adding an appropriate amount of Mg, the excessively formed MgZn ₂ phase is excessively brittle, resulting in excessive cracking in the plating layer during processing. was inferior

In addition, in the case of Comparative Example 11, which does not satisfy the cooling conditions of the relational expression 1-2 of the present invention, even if both the plating composition and other manufacturing conditions of the present invention are satisfied, both the Al single phase and the MgZn ₂ phase are excessively formed, and the Al single phase and The total area ratio of the MgZn ₂ phase exceeded the scope of the present invention. Therefore, even if the corrosion resistance of the plate part is secured by adding appropriate amounts of Mg and Al, the excessively formed MgZn ₂ phase is excessively brittle, resulting in excessive cracking in the plating layer during processing. The corrosion resistance of the bending part was inferior. .

(Experimental Example 2)

A coated steel sheet was manufactured in the same manner as in Experimental Example 1 described above, except that the open rate ratio of the damper was changed as follows according to the temperature range divided based on the surface temperature of the steel sheet. At this time, it was confirmed that the base steel sheet, the Fe-Al-based suppression layer, and the Zn-Al-Mg-based plating layer were sequentially formed using the same analysis method as in Experimental Example 1.

note No. 1st temperature zone* Second temperature zone* De* Dc* De/Dc De* Dc* 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%

1st temperature range*: range from solidification onset temperature to 375°C

Second temperature range*: range from 375°C to 340°C

With respect to the coated steel sheets obtained in each of the above-described Examples and Comparative Examples, in the same manner as in Experimental Example 1 described above, cross-sectional specimens were cut in the thickness direction (direction perpendicular to the rolling direction of the steel sheet) so that the interface between the plated layer and the base iron was observed. After manufacturing, it was magnified at 1000 times and photographed with SEM. With respect to the cross-section specimen, the area ratio of the MgZn ₂ phase and the area ratio of the Al phase were measured using the same method as in Experimental Example 1 described above, and are shown in Table 6 below.

Additionally, a surface specimen having a size of 5,400 μm _{2 was taken in the same manner as in Experimental Example 1 described above, and the area ratios of the MgZn 2} ^phase and the Al phase in which Zn was dissolved at less than 27 at% in the MgZn ₂ -Al-based binary process were measured, respectively. It is shown in Table 6. In addition, with respect to the surface specimens described above, the area ratios of the Zn phase and the Zn-MgZn ₂ -Al-based ternary phase were measured.

note Cross section of Zn-Mg-Al-based plating layer Surface of Zn-Mg-Al-based plating layer MgZn ₂ phase area ratio [%] Al single phase area ratio [%] MgZn ₂ phase area ratio [%] Al single phase area ratio [%] Total area ratio of Al single phase and MgZn ₂ phase [%] Area ratio of MgZn ₂ phase to Al single phase Total area ratio of Zn phase and Zn-MgZn ₂ -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, with respect to the steel sheets obtained from each Example and Comparative Example, comparative evaluation was performed by a test method conforming to ISO14993 using a salt spray tester (SST). In the above evaluation, the time for LDH corrosion products to form on the surface of the coated layer of the plated steel sheet was measured over time using an EDS or XRD device, and is shown in Table 8 below. In addition, the evaluation of properties described in Table 8 below was evaluated according to the same criteria as in Experimental Example 1 described above.

No. Time for LDH corrosion products to form Characteristic evaluation Plate corrosion resistance Corrosion resistance of bending parts scattering reflectance Example 7 10min ○ ○ △ Example 8 5min ◎ ◎ △ Example 9 5min ◎ ◎ △ Example 10 5min ◎ ◎ △ Example 11 5min ◎ ◎ △ Comparative Example 12 - △ Х Х

The plating composition of the present invention and the ratio (De / Dc) of the damper open rate (De) of the edge portion to the damper open rate (Dc) of the center portion are 60 to 70% from the solidification start temperature to 375 ° C, and 375 ° C In the case of Comparative Example 12, which does not satisfy the condition of 90 to 99% from 340 ° C., Simonkolleite was first formed on the surface of the coated steel sheet during the corrosion resistance evaluation experiment, and LDH did not appear on the surface until after 12 hours. was formed For this reason, it was confirmed that the plate corrosion resistance, bending corrosion resistance, and scattering reflectance of Comparative Example 12 were all inferior.

On the other hand, in the case of Examples 7 to 11 satisfying the plating composition and manufacturing conditions of the present invention, LDH (Layered Double Hydroxide) was formed on the surface of the coated steel sheet within 10 minutes during the corrosion resistance evaluation experiment, compared to Comparative Example 12, flat plate It was confirmed that at least one of the corrosion resistance, bending corrosion resistance and scattering reflectance was superior.

In particular, the ratio (De / Dc) of the damper opening rate (De) of the edge portion to the damper opening rate (Dc) of the center portion of the present invention is 60 to 70% from the solidification initiation temperature to 375 ° C., solidification at 375 ° C. In the case of Examples 8 to 11, which also satisfies the condition of 90 to 99% up to the end temperature, it was confirmed that LDH (Layered Double Hydroxide) was first formed on the surface of the coated steel sheet within 5 minutes faster during the corrosion resistance evaluation experiment. For this reason, it was confirmed that Examples ₈ to 11 of the present application have improved plate corrosion resistance and corrosion resistance _of bent parts compared to Examples 7 and 10. It is estimated that it is due to the Al single phase as low as less than 27%. In other words, due to rapid nucleation-crystallization of LDH, a dense corrosion product, on the initial surface of the corrosive environment, it is uniformly distributed over the surface over time to shield the corrosive active area, and secondarily formed Simoncolite and Hydrozin This is because it induces the uniform formation of sites.

(Experimental Example 3)

The hot-dip galvanized and cooled steel sheet was plated under the same conditions as in Experimental Examples 1 and 2, except that the photo temper rolling treatment, cooling, and temper rolling (SPM) treatment after cooling were performed under the conditions shown in Tables 9 and 10 below. A steel plate was prepared.

note No. Before hot dip galvanizing,
Pre-SPM processing conditions roll type Roll surface roughness Ra [㎛] Roll pressure applied to the steel plate surface [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

note 1st temperature zone* Second temperature zone* SPM processing conditions De* Dc* De/Dc De* Dc* De/Dc roll type Roll surface roughness Ra [㎛] Roll pressure applied to the steel plate surface [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

1st temperature range*: range from solidification onset temperature to 375°C

Second temperature range*: range from 375°C to 340°C

For the plated steel sheets obtained from each example and comparative example, specimens were prepared in the same manner as in Experimental Example 1 described above, and then the area ratio of the MgZn ₂ phase and the Al single phase in which Zn was dissolved at less than 27 at% was determined on the surface of the plating layer. After measurement, the area ratios of the Zn single phase and the Zn-MgZn ₂ -Al-based ternary eutectic phase were measured, respectively, and are shown in Tables 11 and 12 below.

In addition, surface polishing was performed on the plated steel sheets obtained in each Example and Comparative Example on the same basis to obtain a surface area of 24,000 μm ² at any point in the region from 1/4t to 3/4t in the thickness direction of the plated layer. Observable specimens were prepared. The surface polishing was performed on a cold-mounted specimen with the surface facing upward so as to observe the surface along the depth direction. Surface polishing was performed at a speed of about 2 μm/min using an automatic polishing machine and a silica suspension under conditions of a load of 30 N, 105 RPM, and forward rotation.

In the area from 1/4t to 3/4t in the thickness direction of the plated layer thus obtained, the total area ratio of the Al single phase and the MgZn ₂ phase, the Zn single phase and the Zn-MgZn phase, on the surface at any same point for Examples and Comparative Examples The total area ratio of the ₂ -Al-based ternary process phase was measured and shown in Table 11 below.

note Surface of Zn-Mg-Al-based plating layer MgZn ₂ phase area ratio [%] Al single phase area ratio [%] Total area ratio of Al single phase and MgZn ₂ phase [%] Area ratio of MgZn ₂ phase to Al single phase Total area ratio of Zn single phase and Zn-MgZn ₂ -Al ternary eutectic phase [%] 2nd Al single phase area ratio
[%] 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

note Surface of Zn-Mg-Al-based plating layer Surface in the region from 1/4t to 3/4t in the thickness direction of the plating layer S1/C1 S2/C2 S1* S2* C1* C2* 2nd Al single phase area ratio [%] 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 ratio of Al single phase and MgZn ₂ phase on the surface of the Zn-Mg-Al-based plating layer [%]

S2*: Total area ratio of Zn phase and Zn-MgZn ₂ -Al-based ternary process phase on the surface of the Zn-Mg-Al-based plating layer [%]

C1*: Total area ratio of MgZn ₂ phase and Al phase on the surface in the area from 1/4t to 3/4t in the thickness direction of the plating layer [%]

[% _]

In addition, with respect to the steel sheets obtained from each Example and Comparative Example, the characteristics described in Table 13 were evaluated according to the same criteria as in Experimental Example 1 described above.

No. characteristic evaluation Plate corrosion resistance Corrosion resistance of bending parts scattering reflectance Example 12 ○ ○ △ Example 13 ◎ ◎ ◎ Example 14 ◎ ◎ ◎ Example 15 ◎ ◎ ◎ Example 16 ◎ ◎ ◎ Comparative Example 13 ○ Х Х Comparative Example 14 △ △ Х Comparative Example 15 ○ Х Х

As can be seen in Table 13, in the case of Comparative Example 13, in which the Mg content in the plating composition of the present invention is insufficient, the De / Dc condition is not satisfied in the first temperature range, and a less roll is used, a single phase of Al is formed on the surface of the plating layer. The MgZn ₂ phase was excessively formed, resulting in poor corrosion resistance at the bend and also poor light scattering at the surface.

In addition, in the case of Comparative Example 14, which has an insufficient Al content and does not satisfy the De / Dc condition in the first temperature range, among the plating compositions of the present invention, plate corrosion resistance and bending corrosion resistance are inferior due to the non-formation of initial LDH In addition, the scattering reflectance was also low, and the appearance quality was inferior.

In addition, in the case of Comparative Example 15, which satisfies the plating composition and other manufacturing conditions of the present invention but does not satisfy the cooling conditions of the relational expression 1-2, the plate corrosion resistance was secured, but was excessively brittle due to the excessively formed MgZn ₂ phase. (Brittle), the corrosion resistance of the bending part was inferior due to the side effect of excessive cracking in the plating layer during processing.

On the other hand, in the case of Examples 12 to 16 satisfying the plating composition and manufacturing conditions of the present invention, it was confirmed that LDH was formed on the surface of the coated steel sheet within 5 minutes during the corrosion resistance evaluation experiment. As a result, it was confirmed that not only corrosion resistance was further improved not only in the flat plate part but also in the bending part, and the surface quality was excellent because the scattering reflectance of the steel plate surface was somewhat high.

In particular, in the case of Examples 13 to 16 satisfying the SPM treatment condition in which a roll reduction of 50 to 300 tons is applied to the surface of the steel sheet using a bright roll having a surface roughness (Ra) of 0.2 to 1.0 μm, S1 / C1 and S2 / It was possible to satisfy the condition of C2, and thus, it was confirmed that not only the corrosion resistance of the flat plate part and the corrosion resistance of the bent part, but also the scattering reflectivity was the best.

Claims (17)

base steel plate;
a Zn-Mg-Al-based plating layer provided on at least one surface of the base steel sheet; and
Including; Fe-Al-based suppression layer provided between the base steel sheet and the Zn-Mg-Al-based plating layer,
On the surface of the Zn-Mg-Al-based plating layer, the total area ratio of the Al single phase and the MgZn ₂ phase is 45 to 60%, and the area ratio of the MgZn ₂ phase to the Al single phase is 1.2 to 3.3.
According to claim 1,
The plated steel sheet, wherein the Zn-Mg-Al-based plating layer contains, in weight percent, Mg: 4 to 6%, Al: 8.2 to 14.2%, the balance Zn and other unavoidable impurities.
According to claim 1,
Based on the cross section of the Zn-Mg-Al-based plating layer, the area ratio of the MgZn ₂ phase is 20 to 40%, and the area ratio of the Al single phase is 8 to 26%.
According to claim 1,
On the surface of the Zn-Mg-Al-based plating layer, the area ratio of the MgZn ₂ phase is 30 to 40%, the plated steel sheet.
According to claim 1,
On the surface of the Zn-Mg-Al-based plating layer, the area ratio of the Al single phase is 15 to 20%,
The Al single phase is a phase in which Zn is dissolved by less than 27% in atomic percent, and the balance includes Al and other impurities, the coated steel sheet.
According to claim 1,
The Zn-Mg with respect to the total area ratio (C1) of the MgZn ₂ phase and the Al single phase on the surface of a point corresponding to any one of the regions from 1/4t to 3/4t in the thickness direction of the Zn-Mg-Al-based plating layer. -A plated steel sheet in which the ratio (S1/C1) of the total area ratio (S1) of the MgZn ₂ phase and the Al single phase on the surface of the Al-based plating layer is in the range of 0.8 to 1.2.
According to claim 1,
The total area ratio of the Zn phase and the Zn-MgZn ₂ -Al-based ternary process phase on the surface of the Zn-Mg-Al-based plating layer is 20 to 30%, the plated steel sheet.
According to claim 1,
The total area ratio (C2) of the Zn phase and the Zn-MgZn ₂ -Al-based ternary process phase on the surface of any point in the region from 1/4t to 3/4t in the thickness direction of the Zn-Mg-Al-based plating layer The ratio (S2/C2) of the total area ratio (S2) of the Zn phase and the Zn-MgZn ₂ -Al-based ternary eutectic phase on the surface of the Zn-Mg-Al-based plating layer is in the range of 0.6 to 1.2, the plated steel sheet.
According to claim 1,
On the surface of the Zn-Mg-Al-based plating layer, the area ratio of the second Al single phase in which 27 to 60% of Zn is dissolved in atomic percent is 2 to 9%.
According to claim 1,
Under the atmospheric environment and the chloride environment of ISO14993, LDH ((Zn,Mg) ₆ Al ₂ (OH) ₁₆ (CO ₃ ) 4H ₂ O) on the surface of the Zn _- Mg-Al-based plating layer is (OH) ₈ Cl ₂ ) and hydrozincite (Zn ₅ (OH) ₆ (CO ₃ ) ₂ ), which is formed before the plated steel sheet.
According to claim 1,
LDH ((Zn,Mg) ₆ Al ₂ (OH) ₁₆ (CO ₃ )·4H ₂ O) is applied to the surface of the Zn-Mg-Al-based plating layer in an atmospheric environment and a chloride environment of ISO14993 for 6 hours in an atmospheric environment, Plated steel, formed in less than 5 minutes in a chloride environment.
According to claim 1,
In the chloride environment of ISO14993, including salt spray and immersion environments, the time required for red rust to occur is 40 to 50 times higher than that of Zn plating of the same thickness in flat parts; and coated steel sheet, which is 20 to 30 times in the 90 degree bending processing part.
In a plating bath containing steel sheet by weight, Mg: 4-6%, Al: 8.2-14.2%, balance Zn and other unavoidable impurities, maintained at a temperature 20-80°C higher than the solidification initiation temperature in equilibrium. immersion and hot-dip galvanizing; and
cooling the hot-dip galvanized steel sheet from a solidification start temperature to a solidification end temperature using an inert gas at an average cooling rate of 2 to 12° C./s;
The cooling step satisfies the following relational expressions 1-1 and 1-2, 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 satisfies 60 to 99% A method for manufacturing a plated steel sheet, wherein cooling is performed so as to do so.
[Relationship 1-1]
A < {(5-2lnt)/(7-3lnt)}*B
[Relationship 1-2]
15t ^(-0.8) ≤ B ≤ 20t ^(-0.8)
(In the relational expressions 1-1 and 1-2, t is the thickness of the steel sheet (mm), A is the average cooling rate (° C./s) from the solidification start temperature to 375° C., and B is at 375° C. Average cooling rate (°C/s) up to 340°C is shown.)
According to claim 13,
In the cooling step, 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 according to the temperature range,
The ratio (De / Dc) of the damper open rate (De) of the edge portion to the damper open rate (Dc) of the center portion is 60 to 70% from the solidification start temperature to 375 ° C, and 90 to 99% from 375 ° C to 340 ° C A method for manufacturing phosphorus and plated steel sheets.
15. The method of claim 14,
After the cooling step, further comprising the step of improving the surface and shape of the steel sheet by performing a temper rolling treatment,
The temper rolling treatment is performed by using a bright roll having a surface roughness (Ra) of 0.2 to 1.0 μm to apply a roll reduction of 50 to 300 tons to the surface of the steel sheet.
According to claim 13,
Before the hot-dip galvanizing, using a bright roll having a surface roughness (Ra) of 0.2 to 0.4 μm, a pre-temperature rolling treatment of applying a roll reduction of 200 to 300 tons to the surface of the steel sheet Further comprising the step of manufacturing a plated steel sheet method.
The method of claim 16
During the pre-temperature rolling treatment, the roll rolling is 250 to 300 ton, a method of manufacturing a coated steel sheet.

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