Plated steel sheet with excellent corrosion resistance and bendability, and method for manufacturing the same.

Abstract

The present invention provides a Zn-Mg-Al plated steel sheet with excellent corrosion resistance and bendability, and a method for manufacturing the same. [Solution] The plated steel sheet comprises a base steel sheet and a Zn-Mg-Al plating layer provided on at least one surface of the base steel sheet, wherein the plating layer comprises an MgZn2 phase and a Zn single phase formed along the outer edge of the MgZn2 phase.

Description

[Technical Field]

[0001] This invention relates to a highly corrosion-resistant plated steel sheet with excellent corrosion resistance and bendability, and a method for manufacturing the same. be. [Background technology]

[0002] In zinc-plated steel sheets, when exposed to a corrosive environment, zinc, which has a lower oxidation-reduction potential than iron, is the first to oxidize. It is corroded by the steel, and has the property of sacrificial corrosion protection, which suppresses the corrosion of the steel material. In addition, the zinc of the plating layer The process involves oxidizing the steel material and forming a dense layer of corrosion products on its surface, thereby shielding the steel from the oxidizing atmosphere. This improves the corrosion resistance of steel materials. Due to these advantageous properties, zinc-plated steel Recently, the application range of steel sheets has expanded to include building materials, home appliances, and automotive steel sheets.

[0003] However, due to the increase in air pollution accompanying industrial advancement, the corrosive environment is gradually deteriorating, and resources Furthermore, due to stringent energy conservation regulations, it offers even better corrosion resistance than conventional galvanized steel. There is a growing need for the development of steel materials that possess [certain properties].

[0004] To improve this problem, aluminum (Al) and magnesium are added to the zinc plating bath. Manufacturing technology for zinc alloy plated steel sheets, which improves the corrosion resistance of steel materials by adding elements such as (Mg). Various studies are underway on the technology. A typical example is the Zn-Al plating composition system. There is also a Zn-Mg-Al zinc alloy plated steel sheet in which Mg is further added.

[0005] On the other hand, in the case of Zn-Mg-Al zinc alloy plated steel sheets, they are often processed before use. However, it contains a large amount of intermetallic compounds with high hardness within the plating layer, and when bending, the hardness within the plating layer is lost. There is a disadvantage that bend workability such as rack forming is deteriorated.

[0006] Furthermore, in a Zn-Mg-Al based zinc alloy plated steel sheet, plating is performed in a plating bath During the plating process, oxides such as dross may adhere to the steel sheet or the reactivity with the steel sheet may be weakened , which also causes a problem that plating adhesion may be reduced.

[0007] Therefore, in reality, no technology has been developed so far that can meet the high-level demands for excellent performance not only in corrosion resistance but also in bendability and plating adhesion.

Prior Art Literature

Patent Literature

[0008]

Patent Literature 1

Summary of the Invention

Problem to be Solved by the Invention

[0009] According to one aspect of the present invention, there is provided a plated steel sheet excellent in corrosion resistance and bendability, and a method for producing the same .

[0010] According to another aspect of the present invention, there is provided a plated steel sheet excellent not only in corrosion resistance and bendability but also in plating adhesion, and a method for producing the same.

[0011] The problems of the present invention are not limited to the contents described above. Any person having ordinary knowledge in the technical field to which the present invention pertains can readily understand additional problems of the invention from the entire content of the specification of the present invention.

Means for Solving the Problem

[0012] One aspect of the present invention is a coated steel sheet comprising a base steel sheet and a Zn-Mg-Al based plating layer provided on at least one surface of the base steel sheet , wherein the plating layer comprises an MgZn₂ phase and Zn single phase formed along the outer contour of the MgZn₂ phase.

[0013] Another aspect of the present invention is a method for producing a coated steel sheet, comprising the steps of: dipping a base steel sheet into a plating bath containing, in % by mass, Mg: 4.0 to 7.0%, Al: 11.0 to 19.5%, with the balance being Zn and other unavoidable impurities, to perform hot-dip galvanizing such that a strip entry temperature satisfies T B +10°C to T B +50°C relative to the plating bath temperature (T B ); performing air wiping on the hot-dip galvanized steel sheet using inert gas so as to satisfy the following Relational Expression 1; and cooling the air-wiped steel sheet to a solidification end temperature at an average cooling rate of 2 to 5°C / s . [Relational Expression 1] 0.005 ≦ P air / (W air × T) (In Relational Expression 1, the W air represents an interval between air knives, and the unit thereof is mm . The P air represents a pressure of the air knives, and the unit thereof is kPa. The T represents a temperature of the supplied inert gas, and the unit thereof is °C.) [Effects of the Invention]

[0014] According to one aspect of the present invention, a coated steel sheet excellent in corrosion resistance and bendability and a method for producing the same can be pro vided.

[0015] ​​​​​Furthermore, according to another aspect of the present invention, not only corrosion resistance and flexibility, but also excellent plating adhesion is achieved. We can provide plated steel sheets and methods for manufacturing the same.

[0016] The diverse yet significant advantages and effects of this invention are not limited to those described above. This can be more easily understood in the process of describing specific embodiments of the invention. [Brief explanation of the drawing]

[0017] [Figure 1] This image shows a photograph of the surface of the plated steel sheet obtained from Example 1, magnified at 1,500x and taken with a field emission scanning electron microscope (FE-SEM). [Figure 2] This image shows a photograph of the surface of the plated steel sheet obtained from Example 3, magnified 5,000 times and taken with a field emission scanning electron microscope (FE-SEM). [Figure 3] This image shows a photograph of the surface of the plated steel sheet obtained from Example 4, magnified at 5,000x and taken with a field emission scanning electron microscope (FE-SEM). [Figure 4] This is a magnified view of the rectangular area shown in Figure 3 above. [Modes for carrying out the invention]

[0018] The terms used herein are for the purpose of describing specific embodiments and the present invention This is not intended to be limiting. Furthermore, the singular form used in this specification is used in relation to the relevant definition. This includes multiple forms unless they clearly indicate the opposite meaning.

[0019] In specifications, "includes" means to specify a configuration and exclude the existence or addition of other configurations. It's not something you should do.

[0020] Unless otherwise defined, this specification includes technical and scientific terms. All terms are used in a way that is generally understood by someone with ordinary skill in the art to which this invention pertains. It has the same meaning. Terms defined in the dictionary are those found in the relevant technical documents and currently disclosed content. It is interpreted as having a meaning that corresponds to that.

[0021] The following describes in detail a "plated steel sheet" relating to one aspect of the present invention. When expressing the content of each element, unless otherwise specified, it refers to weight percent.

[0022] In conventional Zn-Mg-Al zinc alloy plated steel sheet technologies, to improve corrosion resistance, Mg is added. However, if too much Mg is added, a large amount of floating dross will be generated in the plating bath. As a result, there is a problem that dross must be removed frequently, and the upper limit of Mg addition is 3% It was restricted to that.

[0023] Therefore, we conducted research to further improve corrosion resistance by increasing the amount of Mg added beyond 3%. However, as the amount of Mg added increases, a large amount of hard intermetallic compounds are produced within the plating layer. It contains and causes cracks in the plating layer during bending, resulting in poor bendability (or bending There was a problem with reduced processability.

[0024] In addition to these bending problems, the reason for the adhesion of Mg-based dross is the Zn-Mg-Al system Zinc alloy plated steel sheets also had the problem of reduced plating adhesion.

[0025] Therefore, conventional technology does not ensure corrosion resistance while also providing excellent flexibility and plating adhesion. Providing steel plates was technically difficult.

[0026] Therefore, the present inventors have solved the above-mentioned problems while simultaneously providing not only corrosion resistance but also flexibility And / or in order to provide plated steel sheets with excellent plating adhesion, we conducted thorough research and found that... In addition to the composition of the plating layer, the MgZn2 phase formed in the plating layer is formed along the outer line. We discovered that the single-phase Zn used in this invention is a crucial element, which led to the completion of this invention.

[0027] Therefore, the following properties are excellent in corrosion resistance, as well as weldability and / or chemical treatment properties. This section will specifically explain the composition of plated steel sheets, which also have excellent properties.

[0028] First, the plated steel sheet according to the present invention comprises a base steel sheet and a plate provided on at least one surface of the base steel sheet. It includes a Zn-Mg-Al plated layer.

[0029] In the present invention, the type of base steel sheet is not particularly limited. For example, the above base The base steel sheet is an Fe-based base steel sheet, which is used as the base steel sheet for ordinary zinc-plated steel sheets. It may be hot-rolled steel sheet or cold-rolled steel sheet, but is not limited to these. Alternatively, the above Raw steel sheets include carbon steel and ultra-low carbon steel, which are used as materials for construction, home appliances, and automobiles, for example. It can also be made of steel or high-manganese steel.

[0030] However, as an example, the above-mentioned base steel sheet has a C content exceeding 0% by weight (more preferably 0%). (0.001% or more) 0.18% or less, Si: greater than 0% (more preferably 0.001% or more) 1.5% or less, Mn: 0.01 to 2.7%, P: greater than 0% (more preferably 0.001%) % or more) 0.07% or less, S: exceeding 0% (more preferably 0.001% or more) 0.01 5% or less, Al: greater than 0% (more preferably 0.001% or more), 0.5% or less, Nb: More than 0% (more preferably 0.001% or more) 0.06% or less, Cr: more than 0% ( Preferably, 0.001% or more, 1.1% or less, Ti: greater than 0% (more preferably 0. (0.001% or more) 0.06% or less, B: greater than 0% (more preferably 0.001% or more) It can have a composition containing 0.03% or less, with the remainder being Fe and other unavoidable impurities. .

[0031] Although not particularly limited, according to one embodiment of the present invention, at least the above-mentioned base steel sheet On one side, it is equipped with a Zn-Mg-Al plating layer made of a Zn-Mg-Al alloy. This can be achieved. The above plating layer may be formed only on one surface of the base steel sheet, or on the base sheet. It may be formed on both sides of the steel plate. In this case, the Zn-Mg-Al plating layer is M This refers to a plating layer containing g and Al, and mainly containing Zn (i.e., containing 50% or more Zn). .

[0032] Although not particularly limited, according to one embodiment of the present invention, the Zn-Mg-Al system The thickness of the plating layer can be 5 to 100 μm, more preferably 10 to 90 μm. This is possible. If the thickness of the plating layer is less than 5 μm, it will result from the thickness deviation of the plating layer. Due to errors, the plating layer may become very thin in some areas, resulting in poor corrosion resistance. If the thickness of the molten plating layer exceeds 100 μm, the cooling of the molten plating layer may be delayed. As such, there is a possibility of solidification defects occurring on the surface of the plating layer, such as a drooping shape, and the plating layer solidifies This can lead to a decrease in the productivity of steel plate production.

[0033] On the other hand, although not particularly limited, according to one embodiment of the present invention, the above-mentioned base steel sheet and A Fe-Al-based suppression layer can be further included between the Zn-Mg-Al-based plating layer and the other layer. The above Fe-Al-based suppression layer mainly contains intermetallic compounds of Fe and Al (for example, 60%) The above layers are intermetallic compounds of Fe and Al, such as FeAl, FeAl3, and Fe2A. Examples include l5. In addition, some components originate from the plating layer, such as Zn and Mg. For example, it may be 40% or less, and may contain further. The above suppression layer is the base steel sheet in the initial plating stage. This layer is formed by alloying with diffused Fe and plating bath components. The above-mentioned suppression layer is It plays a role in improving the adhesion between the base steel sheet and the plating layer, and at the same time, it helps to improve the adhesion between the base steel sheet and the plating layer It can serve to prevent Fe diffusion into the base steel sheet. It may be formed continuously between the Zn-Mg-Al plating layer and the Zn-Mg-Al plating layer, or it may be formed discontinuously. This is also acceptable. Regarding the above-mentioned suppression layer, aside from the explanation above, it is not commonly known in the relevant technical field. The same content can be applied.

[0034] Although not particularly limited, according to one embodiment of the present invention, the thickness of the suppression layer is 0. The thickness can be 0.2 to 2.5 μm. The above-mentioned suppression layer prevents alloying and ensures corrosion resistance. It fulfills its role, but because it is a brittle, it can affect machinability, and its thickness is 2 It can be made 0.5 μm or less. However, in order to function as an inhibitory layer, its thickness It is preferable to control the thickness to 0.02 μm or more. From the viewpoint of further improving the effects described above... Preferably, the upper limit of the suppression layer thickness can be set to 1.8 μm. The lower limit of the suppression layer thickness can be set to 0.05 μm. In this case, the thickness of the suppression layer is This can mean the minimum thickness in the direction perpendicular to the interface of the base steel plate.

[0035] On the other hand, although not particularly limited, according to one embodiment of the present invention, the above Zn-Mg- The Al-based plating layer has a composition of Mg: 4.0-7.0% and Al: 11.0-19.5% by weight. The remainder may contain Zn and other unavoidable impurities. The following describes each component in detail. Explain it clearly.

[0036] (Mg:4.0% or more and 7.0% or less) Mg is an element that plays a role in improving the corrosion resistance of plated steel materials, and in this invention, the objective is To ensure excellent corrosion resistance, the Mg content in the plating layer is controlled to 4.0% or higher. On the other hand, if too much magnesium is added, not only can dross occur, but it can also cause problems. A large amount of hard, rigid MgZn2 phase is formed within the plating layer, causing cracks to form within the plating layer during bending. Because this can lead to a decrease in flexibility, such as causing [unclear], the Mg content should be controlled to 7.0% or less. It is possible.

[0037] (Al: 11.0% or more and 19.5% or less) Generally, when Mg is added at a concentration of 1% or more, the effect of improving corrosion resistance is observed, but when Mg is 2% When added in amounts exceeding %, the generation of floating dross in the plating bath due to the oxidation of Mg in the plating bath increases. However, there is a problem in that dross needs to be removed frequently.

[0038] Due to these problems, conventional technology uses Zn-Mg-Al zinc alloy plating with a ratio of 1. Adding 0% or more ensures corrosion resistance, but the upper limit for Mg content is set at 3.0% for regular use. However, in this invention, in order to further improve corrosion resistance, the Mg content is increased to 4% It is necessary to increase the amount above, but if the Mg in the plating layer contains more than 4%, the acid of the Mg in the plating bath There is a problem of dross formation due to galvanic filtration. In addition, oxides caused by the dross adhere to the surface. Alternatively, a problem arises where the adhesion of the plating is weakened due to a decrease in reactivity with the base steel sheet. Because this can sometimes occur, it is necessary to add 11.0% or more of Al. However, to suppress dross If aluminum is added excessively, the melting point of the plating bath will rise, and consequently the operating temperature will be too high. This can lead to problems caused by high-temperature work, such as erosion of the plating bath structure and deformation of the steel material. Problems may arise. Therefore, the upper limit of the Al content in the plating layer is controlled to 19.5%. It is preferable to do so.

[0039] (The remainder is zinc and other unavoidable impurities) In addition to the above-mentioned composition of the plating layer, the remainder can consist of Zn and other unavoidable impurities. It is possible. Inevitable impurities can be unintentionally introduced during the normal manufacturing process of hot-dip galvanized steel sheets. If so, it can include everything, and if you are an engineer in the relevant technical field, you can easily understand its meaning. It can be understood.

[0040] The plated steel sheet according to the present invention has a plating layer in which the microstructure includes an MgZn2 phase. In addition to the above, there are also single-phase Zn, Al-Zn binary eutectic phase, Zn-MgZn2-Al ternary eutectic phase, and A Various phases, such as single phases, may also be included in the plating layer. In this specification, the above MgZn The two-phase system refers to a phase mainly composed of MgZn2, while the single-phase Zn system is a phase mainly composed of Zn. This refers to a phase containing 85% or more Zn by weight. In this case, the above Zn single phase also contains other components besides Zn. Additional components of Al and Mg may be solid-dissolved in a range of 15% or less. The phase is primarily composed of Al, containing 85% or more Al by weight, but also contains other components besides Al. This refers to a phase in which components such as Zn and Mg are in solid solution. It also refers to the Zn-MgZn2-Al ternary system. The eutectic phase refers to a ternary eutectic phase in which the Zn phase, MgZn2 phase, and Al phase are all present together. The above Al-Zn binary eutectic phase is a mixture of Al and Zn phases that alternates in a lamellar or irregular manner. This refers to arrangement that displays its form. At this time, the Zn phase contained in the ternary eutectic phase. This is not included in the Zn single phase formed along the outer line of the MgZn2 phase described later in this specification. It is important to note this point.

[0041] On the other hand, according to the present invention, the plating layer is formed along the outer line of the MgZn2 phase. It contains a single phase of Zn. Conventional Zn-Mg-Al plated steel sheets have a large amount of Mg component, exceeding 4%. When included, a large amount of hard, rigid MgZn2 phase is formed within the plating layer, making it difficult to bend. There was a problem with reduced flexibility, such as the formation of cracks within the layer.

[0042] Therefore, after diligent research, the inventors determined that by precisely controlling the plating composition and manufacturing conditions... Along the outer edge of the hard MgZn2 phase within the plating layer, a soft Zn single phase is formed. This creates a buffer between the MgZn2 phase and the Zn-MgZn2-Al ternary eutectic phase. It functions as a buffer, improving corrosion resistance while also ensuring flexibility. They discovered that.

[0043] Furthermore, from the perspective of plating adhesion, the soft phase Zn is located along the outer line of the MgZn2 phase. By forming a single phase, the Zn-MgZn2-Al ternary eutectic phase and the MgZn2 phase are separated into a single Zn phase. We discovered that connecting these components can improve plating adhesion.

[0044] At this time, the single Zn phase formed along the outer line of the MgZn2 phase in the plating layer is This can be confirmed through photographs taken using FE-SEM of the surface of the plated steel sheet.

[0045] For example, to observe the surface structure of the plated steel sheet obtained from Example 1 of the present invention, FE-S Figure 1 shows a photograph taken using an EM (Electromagnetic Monitor) at a magnification of 1,500x. Furthermore, a single Zn phase is present along the outer edge of the MgZn2 phase, which is mainly composed of MgZn2. It can be confirmed that...

[0046] Furthermore, whether the phase formed along the outer line of the above MgZn2 phase corresponds to a single Zn phase is In addition, the cross-sectional images taken with the FE-SEM mentioned above were used, and the weight percentage content of Zn was also determined. EDS (Energy Dispersion) determines whether the phase is primarily composed of Zn (85% or more). Classification can be performed according to the criteria of (rsive spectroscopy). In this case, the single Zn phase formed along the outer line of the MgZn2 phase described above contains the following, This is defined as including both the first single-phase Zn and the second single-phase Zn.

[0047] Furthermore, although not particularly limited, according to one embodiment of the present invention, selectively, the above M The average thickness of the single Zn phase formed along the outer edge of the gZn2 phase can be 2 to 7 μm. Cut.

[0048] The average thickness of the single Zn phase formed along the outer edge of the MgZn2 phase is less than 2 μm. This involves a buffering role between the MgZn2 phase and the Zn-MgZn2-Al ternary eutectic phase. Due to a shortage of single-phase Zn capable of fulfilling this role, adhesion is weak, resulting in poor bendability and plating adhesion. Problems may occur with the properties. On the other hand, along the outer line of the above MgZn2 phase When the average thickness of the single-phase Zn exceeds 7 μm, the Zn phase increases excessively, leading to a deficiency of Mg. This can lead to localized corrosion resistance problems. From the perspective of further improving the effects mentioned above More preferably, the average thickness of the single Zn phase formed along the outer line of the MgZn2 phase The lower limit can be 5.0 μm, or formed along the outer line of the above MgZn2 phase. The upper limit of the average thickness of the single-phase Zn material can be set to 6.9 μm.

[0049] At this time, the average thickness of the single Zn phase formed along the outer line of the MgZn2 phase described above was measured. While there are no particular limitations on the method, images taken with FE-SEM and EDS are preferred. Based on surface photographs of the layer, and the length of the outer line of the above MgZn2 phase which is 5 μm or more, The average thickness of the single Zn phase formed along the outer edge of the MgZn2 phase can be measured. Cut.

[0050] On the other hand, according to the present invention, two types of single Zn phases are formed along the outer line of the MgZn2 phase. First, Z with a Mg solid solution content of less than 4 wt% is placed adjacent to the outer line of the MgZn2 phase. An n single phase (hereinafter referred to as the "first Zn single phase") is formed. Secondarily, the first phase described above is formed. Adjacent to the Zn single phase, but with a high Mg solid solution ratio of 4 wt% or more (hereinafter referred to as "the second Z") A single phase (called "n-phase") is formed. To confirm this, Figure 2 shows the result obtained from Example 3 of the present invention. The image shows a photograph of the surface of a plated steel sheet taken at 5,000x magnification. This corresponds to A in Figure 2. This corresponds to the first Zn single phase described above, which is formed so that the region is adjacent to the outer line of the MgZn2 phase. And a second Zn single phase is formed adjacent to the first Zn single phase corresponding to B in Figure 2. This applies.

[0051] Therefore, the average thickness of the single Zn phase formed along the outer line of the MgZn2 phase was measured. When doing so, use the images taken with the FE-SEM described above, and select the above MgZn particles that are 5 μm or larger. Based on the length of the outer line of the two phases, the Mg solid solution ratio from the outer line of the MgZn two phase is 4 wt% or more. and the average distance to the highest Zn single phase (for example, from the outer line of the MgZn2 phase in Figure 2 to region "B") The average distance to the region containing the Zn is defined as the "average thickness of the single-phase Zn" as described above.

[0052] On the other hand, although not particularly limited, according to one embodiment of the present invention, selectively, the above M The length of the Zn single phase formed along the outer edge of the gZn2 phase occupies the outer edge of the MgZn2 phase. The percentage can be between 30% and 98%.

[0053] The Zn single phase formed along the outer edge of the above MgZn2 phase is the outer edge of the above MgZn2 phase If the length ratio is less than 30%, the bendability and adhesion are not uniform across the entire steel plate. The problem of not having it can arise. On the other hand, it is formed along the outer line of the above MgZn2 phase. When the proportion of the length of the single Zn phase that occupies the outer edge of the MgZn2 phase exceeds 98%, In single-phase systems, corrosion spreads rapidly, leading to a problem of insufficient corrosion resistance. be.

[0054] To further improve the effects described above, more preferably, the outer line of the MgZn2 phase The lower limit of the ratio of the length of the single Zn phase formed along the outer line of the MgZn2 phase is 6 It can be set to 0%. Alternatively, Zn formed along the outer line of the above MgZn2 phase. The upper limit of the length that a single phase occupies on the outer edge of the MgZn2 phase can be set to 90%. .

[0055] The Zn single phase formed along the outer edge of the above MgZn2 phase is the outer edge of the above MgZn2 phase There are no particular limitations on the method for measuring the proportion of length occupied. However, as an example, FE-S Using photographs of the surface of the plated steel sheet taken with an EM, the outer surface of the above MgZn2 phase, which is 5 μm or thicker. Based on the length of the wire, the Zn single phase with a Zn content of 85 wt% or more is located outside the MgZn2 phase. This can be determined by measuring the proportion of the length that occupies the line. Thus, the above-mentioned Zn single phase includes both the first Zn single phase and the second Zn single phase described above.

[0056] On the other hand, although not particularly limited, the inventors have additionally improved the properties of the adhesion force between the phases. In order to further improve this, after diligent research, the above MgZn2 phase is formed along the outer line. The resulting Zn single phase includes a Zn single phase (first Zn single phase) with a Mg solid solution ratio of less than 4%, and Mg We confirmed the existence of two types of single-phase Zn (second single-phase Zn) with a solid solution ratio of 4% or more. However, through further research, the inventors have found that the shape along the outer line of the above MgZn2 phase Of the Zn single phases formed, 1) Zn single phases with a Mg solid solution ratio of 4% or more are Zn-MgZn2 -Contributes to improved adhesion with the Al-based ternary eutectic phase, and 2) Zn single phase with a Mg solid solution ratio of less than 4% We found a point that contributes to improved adhesion with the MgZn2 phase.

[0057] Specifically, although not limited thereto, according to one embodiment of the present invention, selectively, The area percentage of a single Zn phase (or first single Zn phase) with a Mg solid solution content of less than 4 wt% is: 10-90% of the total area of ​​the single Zn phase formed along the outer edge of the above MgZn2 phase This can be done. The proportion of Zn single phase with a Mg solid solution rate of less than 4 wt% is less than 10%. In this case, the effect of improving the bonding with MgZn2 may be slight. On the other hand, the above Mg solid solution If the proportion of single-phase Zn with a ratio of less than 4 wt% exceeds 1%, localized interphase coupling occurs. A problem can arise where there is only one. Further improvement to improve the adhesion between the single-phase Zn and the two-phase MgZn. To improve this, more preferably, the proportion of Zn single phase having a Mg solid solution ratio of less than 4 wt% The lower limit can be 12%, or the percentage of Zn single phase with a Mg solid solution ratio of less than 4 wt%. The upper limit for the combined percentage can be set at 75%.

[0058] Although not particularly limited, according to one embodiment of the present invention, selectively, Mg solid The area ratio of a single Zn phase (or a second single Zn phase) with a dissolution rate of 4 wt% or more is the same as the above MgZ The area of ​​the single Zn phase formed along the outer edge of the n2 phase should be 10-90% of the total area. This is possible. If the area ratio of the Zn single phase with a Mg solid solution rate of 4 wt% or more is less than 10%, The effect of improving connectivity with the Zn-MgZn2-Al ternary eutectic phase may be slight. On the other hand, if the area ratio of the Zn single phase with a Mg solid solution rate of 4 wt% or more exceeds 90%, localized A problem can arise where the coupling between phases is non-uniform. This occurs with single-phase Zn and Zn-MgZ To further improve adhesion with the n2-Al ternary eutectic phase, more preferably, the above Mg The lower limit of the area ratio of single-phase Zn with a solid solution content of 4 wt% or more can be set to 25%, or The upper limit for the area ratio of the Zn single phase with a Mg solid solution content of 4 wt% or more can be set to 88%. .

[0059] Therefore, according to an embodiment of the present invention, optionally, the formed single Zn phase along the outer contour of the MgZn 2 phase described above comprises, in area percentage, 10 to 90% of a first Zn single phase having an Mg solid solubility of less than 4 wt% and 10 to 90% of a second Zn single phase having an Mg solid solubility of 4 wt% or more . The first Zn single phase may be adjacent to the outer contour of the MgZn2 phase, or altern atively, the second Zn single phase may be adjacent to the first Zn single phase. The above description is al so equally applicable to the first Zn single phase and the second Zn single phase.

[0060] In this specification, the classification of a Zn single phase with an Mg solid solubility of 4 wt% or more and a Zn single phase with an Mg solid solubility of less than 4 wt% described above can be carried out by measuring the Mg weight percent content relative to the Zn single phase at each measurement point using FE-SEM and EDS.

[0061] On the other hand, the method for measuring the area ratio of the Zn single phase with an Mg solid solubility of 4 wt% or more and the Zn single phase with an Mg solid solubility of less than 4 wt% is not particularly limited. For example, the measurement can be performed by obtaining the area percentage of each Zn single phase relative to all phases 2 or more in the plating layer based on the surface of the plating layer, through images captured of the plating layer surface by FE-SEM and EDS .

[0062] However, in the present invention, all of the Zn single phases with an Mg solid solubility of 4 wt% or more may exist as a single connected shape, or may exist as island-shaped forms separated from each other.

[0063] Therefore, based on a surface area of the plating layer of 10 μm 2 or more, 1) as shown in Figure 2, M The Zn single phase (corresponding to region B), with a solid solubility of 4 wt% or more, exists as a single connected structure. In that case, within the MgZn 2-phase, from the outer line to the Zn single-phase with a Mg solid solution content of 4 wt% or more Viewing the region as "a single Zn phase formed along the outer line of the MgZn2 phase", each single Zn phase (the second The area ratio of the first Zn single-phase and the second Zn single-phase is measured.

[0064] On the other hand, 2) as shown in Figure 3, Zn single phases with a Mg solid solution ratio of 4 wt% or more are separated into island-like formations. If they exist, they will be located between two adjacent island-like structures as shown in the square area of ​​Figure 3. Draw the shortest distance line. Figure 4 shows an enlarged photograph of the area indicated by the rectangle above, and Figure 4 shows two The shortest distance line drawn between the islands is shown. Next, the single-phase Zn with a Mg solid solution content of 4 wt% or more... Draw the line connecting the outer line and the shortest distance between them, as shown in Figure 3. After that, the internal region from the outer line of the MgZn2 phase to the connected line mentioned above is called the "MgZn2 phase Considering each Zn single phase as "a Zn single phase formed along the outer line", each Zn single phase (the first Zn single phase and the second Z The area ratio of n (single phase) is measured. At this time, the shortest distance between the island-like structures is the same length. If there are two or more, the line closest to the outer line of the MgZn2 phase is used as the reference. Then, the area ratio of each Zn single phase is measured using the method described above.

[0065] Next, we will describe in detail another aspect of the present invention: "a method for manufacturing plated steel sheets." However, the plated steel sheet of the present invention does not necessarily have to be manufactured by the following manufacturing method. That does not mean that.

[0066] According to one embodiment of the present invention, the process may further include the step of first preparing a base steel sheet. The type of base steel sheet is not particularly limited. This can be an Fe-based steel sheet, that is, a hot-rolled steel sheet or a cold-rolled steel sheet, but It is not limited to these uses. Furthermore, the above-mentioned raw steel sheets are, for example, used in construction, home appliances, and automobiles. The material used can be carbon steel, very low carbon steel, or high manganese steel, but It is not limited to this. In this case, the above explanation applies to the raw steel sheet as described above. It can be applied.

[0067] Next, the prepared base steel sheet is plated at a plating bath temperature (T B ) vs T B +10℃~T B To meet the draw-in temperature of +50°C, the composition should be in weight percent: Mg: 4.0~7.0%, Al: Immerse in a plating bath containing 11.0-19.5% Zn and other unavoidable impurities and dissolve Perform hot-dip galvanizing.

[0068] At this time, the reasons for adding components to the plating bath and the reasons for limiting their content are as follows: Excluding the small amount of Fe content that can flow in from the base steel sheet, the components of the plating layer as described above. The explanation for this can be applied similarly.

[0069] On the other hand, in order to produce a plating bath with the above composition, a predetermined amount of Zn, Al, and Mg is required. Using composite ingots or Zn-Mg and Zn-Al ingots containing individual components This is possible. In order to replenish the plating bath consumed in molten plating, the above ingot can be used. Furthermore, it is dissolved and supplied. In this case, the method involves directly immersing the ingot in the plating bath and dissolving it. You can also choose to melt the ingot in a separate port, and then pour the molten metal into the plating bath. You can also choose how to replenish it.

[0070] Furthermore, during the hot-dip galvanizing process described above, the drawing temperature of the base steel sheet is the plating bath temperature (T B ) In contrast, T B +10℃~T B Control to satisfy the range of +50°C. At this time, there is no particular limitation It is not meant to be done, but the above plating bath temperature (T B ) is maintained in the range of 440-500°C This is possible. By setting the temperature at which the base steel is drawn into the plating bath as described above, This can improve the interfacial adhesion between the plate and the plating layer.

[0071] On the other hand, the drawing temperature of the above-mentioned base steel plate is T B Below +10℃, the interfacial adhesion improves. The amount is small, and dross adhesion problems may occur. On the other hand, the drawing temperature of the above-mentioned base steel sheet The degree is T B At temperatures exceeding +50°C, the generation of esch (Zn fumes) increases and adheres to the steel plate. This can sometimes lead to problems with the quality of the plated surface.

[0072] In this case, from the perspective of further improving the above-mentioned effects, more preferably the base steel sheet The lower limit of the draw-in temperature is T B It can be set to +20℃, and above the drawing temperature of the above-mentioned base steel sheet The limit is T B The temperature range can be set to +45℃.

[0073] Next, an inert gas is applied to the hot-dip galvanized steel sheet in such a manner that the following relational equation 1 is satisfied. Then, perform air wiping. Control the air wiping conditions so that the following relational equation 1 is satisfied. As a result, the Zn single-phase microstructure formed along the outer line of the MgZn2 phase as defined in this invention is formed. At the same time as ensuring a smooth and uniform surface, it also improves bendability. In this case, since the following relational equation 1 is an empirically obtained value, it is not necessary to specify a unit. Yes, the following defined W air The units are mm and P air The units are kPa and T It is sufficient if the unit, °C, is satisfied. [Relationship 1] 0.005≦P air / (W air ×T) (In the above relational expression 1, the above W air The interval between air knives is represented in millimeters. . The above P air The value of T represents the air knife pressure, and the unit is kPa. The above T is the supplied air pressure. This represents the temperature of the active gas, and the unit is °C.

[0074] On the other hand, although not particularly limited, according to one embodiment of the present invention, the inert gas and For example, argon (Ar) gas, nitrogen (N2) gas, or a mixture of argon and nitrogen gas. Any of these can be used, and the use of nitrogen gas is more preferable.

[0075] Furthermore, according to one embodiment of the present invention, in the air wiping step described above, the above The spacing between the knives should be in the range of 20 to 45 mm (more preferably 30 to 40 mm). This is possible. Alternatively, the pressure of the above air knife is 8-20 kPa (more preferably 10- It can be in the range of 18kPa. Alternatively, the temperature of the supplied gas can be 30~ The temperature range can be 100°C (more preferably 65-85°C). By adjusting the air wiping conditions to satisfy the requirements for corrosion resistance, flexibility, and plating density, It is possible to manufacture plated steel sheets with excellent adhesion properties.

[0076] After this, the air-wiped steel plate is subjected to the following process, with the surface temperature as the reference, until it reaches the solidification completion temperature: Cooling is performed at an average cooling rate of ~5°C / s during the cooling stage after the above air wiping, If the cooling rate is less than 2°C / s, problems may arise in the productivity of plated steel sheets, and 5°C If the value exceeds / s, the Zn single phase formed along the outer line of the MgZn2 phase as defined in this invention Organization may not develop.

[0077] On the other hand, although not particularly limited, according to one embodiment of the present invention, selectively, the above cold During the cooling process, the air-wiped steel plate is subjected to an average cooling temperature range of 420°C or less than 450°C. Primary cooling is performed at a cooling rate of 1.0~2.0°C / s, followed by a 420°C treatment on the steel plate that has undergone the primary cooling. Secondary cooling is performed to cool temperatures below 340°C or above at an average cooling rate of 2.1 to 4.0°C / s. The above-mentioned secondary-cooled steel plate is subjected to a temperature range of 150°C or higher but less than 340°C at an average cooling rate of 5.0 This can be carried out in three stages, including a tertiary cooling process that cools at ~7.0°C / s.

[0078] The inventors conducted thorough research and found that slow cooling is performed, but in the primary, secondary, and tertiary cooling stages... By dividing the material and gradually increasing the cooling rate in each section, the flexibility is further improved. Specifically, during the primary cooling described above, the average cooling rate is less than 1.0°C / s. This can lead to problems with the productivity of steel plates. On the other hand, during the primary cooling mentioned above, 2.0℃ / s If this limit is exceeded, problems will arise in ensuring uniform interfacial adhesion between the base steel sheet and the plating layer. Furthermore, if the average cooling rate during the above secondary cooling is less than 2.1°C / s, the raw steel plate will Productivity issues may arise. On the other hand, during the secondary cooling described above, the average cooling rate is 4.0°C / s. If this is exceeded, problems will arise in ensuring adhesion between MgZn2 and the single-phase Zn within the plating layer. Furthermore, if the average cooling rate during the above-mentioned tertiary cooling is less than 5.0°C / s, the steel plate will not cool down. Delays in the curing process could lead to the plating layer adhering to the cooling tower top roll. It has the property. On the other hand, if the average cooling rate exceeds 7.0°C / s during the above tertiary cooling, plating Problems arise with the adhesion between the single-phase Zn and the Zn-MgZn2-Al ternary eutectic phase within the layer. There is.

[0079] On the other hand, although not particularly limited, according to one embodiment of the present invention, selectively, the above cold At the time of rejection, the following relational equation 2 can be further satisfied: the average cooling rate during tertiary cooling and 1 By controlling the relationship between the average cooling rate during the first and second cooling phases as shown in relational equation 2 below, Furthermore, the adhesion between the MgZn2 phase and the Zn single phase, and the Zn-MgZn2-Al ternary eutectic phase This promotes improved adhesion with the Zn single phase, further enhancing the flexibility of the plated layer. Cut. [Relationship 2] C1 + C2 ≤ C3 ≤ 1.5 × (C1 + C2) (In the above relational equation 2, C1 represents the average cooling rate [°C / s] during primary cooling, and C2 is C3 represents the average cooling rate [°C / s] during secondary cooling, and C3 represents the average cooling rate [°C / s] during tertiary cooling. ] indicates.

[0080] As mentioned above, by precisely controlling the plating composition and manufacturing conditions, not only corrosion resistance but also Furthermore, it effectively provides plated steel sheets that are excellent in one or more of the following properties: bendability and plating adhesion. It is possible. [Examples]

[0081] (Examples) The present invention will be described more specifically below through examples. However, the following examples are illustrative. This is for the purpose of explaining the present invention and not to limit the scope of the rights of the present invention. It is important to note that the scope of the present invention is limited to the matters described in the claims and thereafter. This is because it is determined by factors that can be reasonably inferred.

[0082] (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 A substrate with a composition of 0.015% iron, the remainder being Fe and impurities, with a thickness of 2 mm and a width of 1300 mm. Prepare a steel plate.

[0083] The prepared base steel plates were immersed in the plating bath under the conditions shown in Table 1 below and hot-dip galvanized. Then, using nitrogen (N2) gas on the hot-dip galvanized steel plate described in Table 1 below... After performing an air wiping treatment, the material was cooled to the conditions shown in Table 2 below.

[0084] [Table 1]

[0085] [Table 2] C*: Average cooling rate until the solidification completion temperature [°C / s]

[0086] Test specimens of plated steel sheets obtained by the methods in Tables 1 and 2 above were prepared, and the plating layer was treated with hydrochloric acid solution. After dissolution, the dissolved liquid is analyzed using a wet chemical spectroscopy (ICP) method to measure the composition of the plating layer. The results are shown in Table 3 below (however, the remainder consists of Zn and impurities).

[0087] Furthermore, the interface between the plating layer and the base steel sheet can be observed in the thickness direction of the steel sheet (i.e., After preparing cross-sectional test specimens cut in a direction perpendicular to the rolling direction, the cross-sections were photographed using a scanning electron microscope (SEM). Between the above-mentioned base steel sheet and the Zn-Mg-Al plating layer, there is a 0.1-1 μm thick Fe-Al layer. We confirmed that an inhibitory layer was formed.

[0088] Furthermore, the surface of the steel plate was photographed at 1,500x magnification using FE-SEM and EDS, and the above M Table 3 below indicates the presence or absence of a single Zn phase formed along the outer edge of the gZn2 phase [○ / ×]. As shown.

[0089] At this time, whether or not it is a single-phase Zn is determined by imaging using FE-SEM and EDS. Using surface photographs, determine whether or not it is a single-phase Zn with a Zn content (wt%) of 85% or more. The classification was based on this criterion.

[0090] Furthermore, using FE-SEM, the outer line length of the above MgZn2 phase is 5 μm, and the above Mg The average thickness of the single Zn phase formed along the outer edge of the Zn2 phase was measured and is shown in Table 3 below. .

[0091] [Table 3] ○: A single Zn phase is present along the outer edge of the MgZn2 phase. ×: No single Zn phase formed along the outer edge of the MgZn2 phase exists.

[0092] Each example and comparative example was evaluated for its characteristics according to the following criteria, and the evaluation results for each characteristic are shown in the table below. As shown in 4.

[0093] <Corrosion resistance> To evaluate corrosion resistance, a salt spray tester is used. The following criteria were used to evaluate the product using a test method compliant with ISO 14993 (SST). . ○: The time required for red rust to form is 30 to 40 times longer than that of Zn plating of the same thickness. △: The time required for red rust to form is 20 to less than 30 times longer than that required for Zn plating of the same thickness. ×: The time required for red rust formation is less than 20 times longer than for Zn plating of the same thickness.

[0094] <Bendability> To evaluate cracks in the bent section, the material in question was cut into 30mm x 100mm pieces, The number of cracks occurring within a 10mm length after 3t bending was observed using FE-SEM. The flexibility was evaluated according to the following criteria. ◎: Less than 5 items ○: 5 or more but less than 10 △: 10 or more but less than 20 ×: 20 or more

[0095] <Plating adhesion> To evaluate the plating adhesion, the material in question was cut into 30mm x 100mm pieces and then each piece was measured. After bending the steel material 180° (OT bending), each bent test piece is... After taping, measure the area of ​​the peeled-off test piece and determine the plating adhesion according to the following criteria. I evaluated it. The evaluation criteria for plating adhesion are as follows: ◎: No peeling area ○: Peeling area exceeding 0% but less than 3% △: Peeling area 3-10% ×: Peeling area 10% or more

[0096] [Table 4]

[0097] As can be seen from the experimental results in Table 4 above, the plating composition and manufacturing conditions specified in this invention are met. In addition, in Examples 1 to 5, it was confirmed that the materials exhibited excellent corrosion resistance, flexibility, and plating adhesion.

[0098] On the other hand, Comparative Example 1 does not satisfy one or more of the plating composition and manufacturing conditions specified in the present invention. In the case of ~14, one or more of the properties of corrosion resistance, flexibility, and plating adhesion are found in Examples 1-5. I confirmed that it was inferior in comparison.

[0099] (Experimental Example 2) After performing hot-dip galvanizing and air wiping treatment under the conditions in Table 5 below, the following conditions in Table 6 below Aside from performing three stages of cooling by controlling the average cooling rate in the primary, secondary, and tertiary stages, Plated steel sheets were manufactured using the same method as in Experimental Example 1 described above.

[0100] At this time, analysis was performed using the same method as in Experimental Example 1, and the above-mentioned base steel sheet and Zn-Mg-Al system were analyzed. We confirmed that a 0.3 μm thick Fe-Al-based suppression layer is formed between the layer and the other layer.

[0101] On the other hand, regarding the plating layer composition, the presence of a single Zn phase formed along the outer line of the MgZn2 phase The presence or absence and average thickness were measured in the same manner as described above in Experimental Example 1 and are shown in Table 7 below.

[0102] Additionally, FE-SEM and EDS were used to form along the outer line of the above MgZn2 phase. The proportion of the single-phase Zn that occupies the outer line length of the MgZn 2-phase (10 μm) is as described above in the specification. The measurements were taken in accordance with the law and are shown in Table 7 below.

[0103] Additionally, an area of ​​25 μm was captured using FE-SEM and EDS. 2 The surface of the plating layer As a criterion, the area of ​​the Zn single phase with a Mg solid solution content of 4 wt% or more, similar to the method described herein. The area percentage of Zn single-phase with a Mg solid solution ratio of less than 4 wt% was measured, and these values ​​are shown in the table below. As shown in 7.

[0104] Furthermore, the plated steel sheets obtained from each example and comparative example were prepared using the same method as in Experimental Example 1. The corrosion resistance, flexibility, and plating adhesion were evaluated and are shown in Table 8 below.

[0105] [Table 5]

[0106] [Table 6] Primary cooling*: Cooling in a temperature range of 420°C or below 450°C. Secondary cooling*: Cooling in a temperature range of 340°C or higher but below 420°C. Tertiary cooling*: Cooling in a temperature range of 150°C or higher but below 340°C.

[0107] [Table 7]

[0108] [Table 8]

[0109] As can be seen from the experimental results in Table 8 above, the plating composition and manufacturing conditions specified in this invention are met. In addition, in Examples 6 to 10, it was confirmed that the materials exhibited excellent corrosion resistance, flexibility, and plating adhesion.

[0110] In particular, in the case of Examples 8 to 10 of the above embodiments, which perform the three-stage cooling specified in the present invention. Compared to Examples 6 and 7, it was confirmed that the bendability was further improved. Examples 8-10 show the occupancy ratio of the single Zn phase formed along the outer line of the MgZn2 phase and The average thickness is large, and a large amount of soft single-phase Zn is formed around the hard MgZn2 phase, causing bending. It is presumed to have even better properties. Also, a Zn single phase with a Mg solid solution ratio of 4% or more is Zn-MgZ It contributes to improved interphase adhesion with the n2-Al ternary eutectic phase, and the Mg solid solution ratio is less than 4% for Zn monophase. The phase contributes to improving interphase adhesion with the MgZn2 phase, ultimately resulting in improved plating adhesion. It is estimated to contribute to improvement.

[0111] On the other hand, Comparative Examples 15 and 16 do not satisfy all of the plating composition and manufacturing conditions specified in the present invention. In this case, one or more of the properties among corrosion resistance, flexibility, and plating adhesion are better than in Examples 6-10. It was confirmed that it was inferior.

Claims

1. Raw steel sheet, and The base steel sheet includes a Zn-Mg-Al plating layer provided on at least one surface, The aforementioned plating layer is MgZn ２ Phase and the aforementioned MgZn ２ Zn formed along the outer line of the phase Plated steel sheet, including single-phase.

2. The aforementioned plating layer consists of Mg: 4.0-7.0% and Al: 11.0-19.5% by weight. The plated steel sheet according to claim 1, comprising the remainder Zn and other unavoidable impurities.

3. Fe-Al suppression is provided between the base steel sheet and the Zn-Mg-Al plating layer. The plated steel sheet according to claim 1, further comprising a layer.

4. The aforementioned MgZn ２ The Zn single phase formed along the outer line of the phase is the MgZn ２ The outer line of the phase The plated steel sheet according to claim 1, wherein the proportion of the length occupied is 30 to 98%.

5. The aforementioned MgZn ２ The average thickness of the Zn single phase formed along the outer edge of the phase is 2 to 7 μm. The plated steel sheet according to claim 1.

6. The aforementioned MgZn ２ The Zn single phase formed along the outer edge of the phase has an area percentage and a Mg solid solubility of 4 A first Zn single phase with less than wt%: 10-90%, and a second Zn with a Mg solid solution ratio of 4 wt% or more. The plated steel sheet according to claim 5, comprising single-phase: 10-90%.

7. The first Zn single phase is the MgZn ２ The outer line of the phase, adjacent to the meter according to claim 6 Steel plate.

8. Plating bath temperature (T Ｂ ) with respect to T Ｂ +10°C to T Ｂ +50°C so as to satisfy the drawing-in temperature The raw steel sheet contains, by weight, Mg: 4.0-7.0%, Al: 11.0-19.5%. The step of immersing in a plating bath containing the remaining Zn and other unavoidable impurities to perform hot-dip zinc plating, The aforementioned hot-dip galvanized steel plate is subjected to an inert gas so as to satisfy the following relational equation 1. The stage of wiping, and The air-wiped steel plate is cooled to the solidification completion temperature at an average cooling rate of 2 to 5°C / s. The stage A method for manufacturing plated steel sheets, including the method described above. [Relationship 1] 0.005≦P ａｉｒ / (W ａｉｒ ×T) (In the above relational expression 1, the W ａｉｒ The interval between air knives is represented in millimeters. . P ａｉｒ represents the air knife pressure, and its unit is kPa. The aforementioned T is the supplied air. This represents the temperature of the active gas, and the unit is °C.

9. The aforementioned cooling step is, The air-wiped steel plate is subjected to a temperature range of 450°C to 420°C at an average cooling rate Primary cooling at 1.0–2.0°C / s, The aforementioned primary cooled steel plate is subjected to a temperature range of less than 420°C and above 340°C at an average cooling rate of 2.1 Secondary cooling to ~4.0°C / s, and The aforementioned secondary-cooled steel plate is subjected to a temperature range of 150°C or higher but less than 340°C at an average cooling rate of 5.0 Tertiary cooling at ~7.0°C / s A method for manufacturing a plated steel sheet according to claim 8, including the method described in claim 8.

10. The cooling step satisfies the following relational expression 2, wherein the method for manufacturing a plated steel sheet according to claim 9. 。 [Relationship Equation 2] C １ +C ２ ≦C ３ ≦1.5×(C １ +C ２ ) (In the above relational expression 2, C １ This represents the average cooling rate [°C / s] during primary cooling, C ２ teeth This represents the average cooling rate [°C / s] during secondary cooling, C ３ This is the average cooling rate during tertiary cooling [°C / s]. [This indicates...]

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