Plated steel sheet and method for manufacturing the same

Abstract

An aspect of the present application is to provide a Zn-Mg-Al-based zinc alloy plated steel sheet having excellent surface appearance and anti-glare properties and a method for manufacturing the same.

Description

Technical Field

[0001] This invention relates to a coated steel sheet and its manufacturing method. More specifically, this invention relates to a Zn-Mg-Al zinc alloy coated steel sheet with excellent surface appearance and anti-glare properties. Background Technology

[0002] When galvanized steel sheets are exposed to corrosive environments, zinc, with a lower oxidation-reduction potential than iron, corrodes first, thus inhibiting the corrosion of the steel and exhibiting sacrificial corrosion protection. Furthermore, as the zinc coating oxidizes, it forms a dense corrosion product on the steel surface, isolating the steel from the oxidizing atmosphere and thereby improving its corrosion resistance. Due to these excellent properties, the application of galvanized steel sheets in building materials, home appliances, and automotive steel sheets is continuously expanding.

[0003] However, with the increasing air pollution caused by industrialization, the corrosive environment is gradually deteriorating. Furthermore, due to strict regulations related to resource and energy conservation, the demand for steel with superior corrosion resistance compared to existing galvanized steel is constantly increasing.

[0004] To address these issues, various studies are underway on manufacturing technologies for zinc alloy-coated steel sheets that incorporate elements such as aluminum (Al) and magnesium (Mg) in the galvanizing bath to enhance corrosion resistance. A representative example is the Zn-Mg-Al zinc alloy-coated steel sheet, which incorporates Mg as an additional element in the Zn-Al coating system.

[0005] However, in the case of Zn-Mg-Al zinc alloy coated steel sheets, due to the large amount of Mg and Al components, the coating surface is prone to oxidation, and Mg and Al oxides adhere to the steel sheet surface, which has the disadvantage of deteriorating the surface quality.

[0006] In addition, although Zn-Mg-Al zinc alloy coated steel sheets containing large amounts of Mg and Al have excellent corrosion resistance, they may produce glare when used in infrastructure applications such as road noise barriers and guardrails, thus obstructing the driver's vision.

[0007] Therefore, there is a need to develop technologies that can meet the requirements of Zn-Mg-Al galvanized steel sheets, which can simultaneously meet the requirements of excellent corrosion resistance, surface appearance and anti-glare properties.

[0008] [Existing Technical Documents] [Patent Literature] (Patent Document 1) Korean Patent Publication No. 2010-0073819 Summary of the Invention

[0009] (a) Technical problems to be solved According to one aspect of the present invention, it is intended to provide a coated steel sheet with excellent surface appearance and anti-glare properties, and a method thereof for manufacturing the same.

[0010] The technical problems of this invention are not limited to those described above. Those skilled in the art can readily understand the additional technical problems of this invention from the entire contents of this specification.

[0011] (II) Technical Solution According to one aspect of the invention, a coated steel sheet may include: a base steel sheet; and a Zn-Mg-Al coating disposed on at least one side of the base steel sheet, wherein, by weight percent, the coating comprises: Mg: 4.0-6.3%, Al: 11.0-19.5%, with the balance being Zn and other unavoidable impurities, and the surface of the coated steel sheet can satisfy an average roughness Ra: 2.0-3.0 μm, a peak count RPc: 25 to 60 (1 / 10 mm), and a skewness Rsk: -0.5 to 0.5.

[0012] The surface gloss B of the above-mentioned coated steel sheet can be above 25 and below 60, and the surface whiteness L can be above 85.

[0013] By weight percent, the aforementioned base steel plate may contain: C: greater than 0% and less than 0.18%, Si: greater than 0% and less than 1.5%, Mn: 0.010-2.7%, P: greater than 0% and less than 0.0700%, S: greater than 0% and less than 0.0150%, Al: greater than 0% and less than 0.50%, Nb: greater than 0% and less than 0.060%, Cr: greater than 0% and less than 1.1%, Ti: greater than 0% and less than 0.060%, and B: greater than 0% and less than 0.03000%, and may consist of the balance Fe and other unavoidable impurities.

[0014] In weight percent, the above-mentioned Zn-Mg-Al coating may further contain one or more of Si: less than 0.2% (inclusive) and Ca: less than 0.2% (inclusive).

[0015] The aforementioned Zn-Mg-Al coating may further include any one or more of the following groups (a) to (h).

[0016] (a) Ni: less than 0.5%, (b) One or more of the following: La: less than 0.1%, Ce: less than 0.1%, Y: less than 0.1%, and Sr: less than 1.0%. (c)Ti: less than 0.1%, (d)W: below 0.5%, (e)Cu: below 2.0%, (f) One or more of the following: Cr: less than 0.5%, Mn: less than 0.5%, and V: less than 0.5%. (g)B: less than 0.1%, P: less than 0.1% or more of these two concentrations. (h) Sn: less than 1.0%, Sb: less than 1.0%, Bi: less than 1.0% or more of the following:

[0017] The thickness of the Zn-Mg-Al coating can be 5-100 μm.

[0018] A method for manufacturing a coated steel sheet according to another aspect of the present invention may include the following steps: preparing a base steel sheet; immersing the base steel sheet in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet; and wiping the hot-dip galvanized steel sheet with a mixed gas comprising an inert gas and air, wherein the mixed gas may contain 5% to 20% by volume of air based on the total volume of the mixed gas, and the hot-dip galvanizing bath may contain: Mg: 4.0-6.3%, Al: 11.0-19.5%, with the balance being Zn and other unavoidable impurities, wherein in the step of obtaining the hot-dip galvanized steel sheet, the introduction temperature A of the base steel sheet may satisfy the following relationship 1.

[0019] [Relation 1](T) B -40)℃≤A≤(T B -10℃ (In relation 1, T) B This indicates the temperature (°C) of the plating bath. By weight percent, the aforementioned base steel plate may contain: C: greater than 0% and less than 0.18%, Si: greater than 0% and less than 1.5%, Mn: 0.010-2.7%, P: greater than 0% and less than 0.0700%, S: greater than 0% and less than 0.0150%, Al: greater than 0% and less than 0.50%, Nb: greater than 0% and less than 0.060%, Cr: greater than 0% and less than 1.1%, Ti: greater than 0% and less than 0.060%, and B: greater than 0% and less than 0.03000%, and may consist of the balance Fe and other unavoidable impurities.

[0020] By weight percent, the hot-dip galvanizing bath may further contain one or more of Si: less than 0.2% (inclusive) and Ca: less than 0.2% (inclusive).

[0021] The hot-dip galvanizing bath described above may further include any one or more of groups (a) to (h) below.

[0022] (a) Ni: less than 0.5%, (b) One or more of the following: La: less than 0.1%, Ce: less than 0.1%, Y: less than 0.1%, and Sr: less than 1.0%. (c)Ti: less than 0.1%, (d)W: below 0.5%, (e)Cu: below 2.0%, (f) One or more of the following: Cr: less than 0.5%, Mn: less than 0.5%, and V: less than 0.5%. (g)B: less than 0.1%, P: less than 0.1% or more of these two concentrations. (h) Sn: less than 1.0%, Sb: less than 1.0%, Bi: less than 1.0% or more of the following:

[0023] The temperature T of the above plating bath B It can be above 440℃ and below 500℃.

[0024] The aforementioned inert gas may include at least one of argon and nitrogen.

[0025] (III) Beneficial Effects According to one aspect of the present invention, a coated steel sheet with excellent surface appearance and anti-glare properties and a method thereof can be provided.

[0026] The various and beneficial advantages and effects of this invention are not limited to those described above, and can be more easily understood in the process of explaining the specific embodiments of this invention. Attached Figure Description

[0027] Figure 1 These are photographic images showing the surfaces of (a) Invention Example 4 and (b) Comparative Example 4, respectively.

[0028] Figure 2 The results are comparative evaluations of the surface shape characteristics of (a) Invention Example 4 and (b) Comparative Example 4, respectively. Detailed Implementation

[0029] The preferred embodiments of the present invention will be described below. However, the embodiments of the present invention can be modified into many other forms, and the scope of the present invention is not limited to the embodiments described below.

[0030] Unless otherwise stated, the terms “comprising” or “including” in this specification are used to indicate that other constituent elements are not excluded and may further include other constituent elements.

[0031] Furthermore, unless otherwise specified, the percentage units in this specification refer to weight.

[0032] In existing technologies related to Zn-Mg-Al zinc alloy coated steel sheets, Mg is added to improve corrosion resistance. However, when Mg is added in excess, the amount of suspended dross in the plating bath increases, which requires frequent dross removal. Therefore, the upper limit of Mg addition is limited to 3.0%.

[0033] In order to improve corrosion resistance by increasing the Mg content to over 3.0%, research was conducted. However, with the increase of Mg and Al content, surface quality could not be guaranteed due to the adhesion of scum.

[0034] In addition to these surface quality issues, as mentioned above, when the amount of Mg and Al added increases, the gloss of the coated steel sheet is high, which leads to the problem of glare that cannot be prevented when it is used in road infrastructure such as sound barriers and guardrails.

[0035] Therefore, in the prior art, it is difficult to provide a coated steel sheet that simultaneously ensures corrosion resistance, has an excellent surface appearance, and possesses anti-glare properties by reducing gloss.

[0036] Therefore, the inventors conducted in-depth research to solve the above problems and found that not only the composition of the coating, but also ensuring the surface roughness characteristics of the coated steel plate are very important.

[0037] Furthermore, the inventors have discovered that, as a way to ensure the above-mentioned characteristics, controlling the introduction temperature of the base steel plate and the composition of the mixed gas supplied in the hot-dip galvanizing wiping step at an appropriate level has a significant effect.

[0038] The coated steel sheet of the present invention obtained according to the above has an appropriate level of surface gloss and surface whiteness, thus exhibiting excellent surface quality and anti-glare properties.

[0039] Based on the above viewpoints, a coated steel sheet according to an embodiment of the present invention may include: a base steel sheet; and a Zn-Mg-Al coating disposed on at least one side of the base steel sheet, wherein, by weight percent, the coating may contain: Mg: 4.0-6.3%, Al: 11.0-19.5%, with the balance being Zn and other unavoidable impurities, and the surface of the coated steel sheet may satisfy an average roughness Ra: 2.0-3.0 μm, a peak count RPc: 25 to 60 (1 / 10 mm), and a skewness Rsk: -0.5 to 0.5. The various components will be described in detail below.

[0040] First, according to an example of the present invention, the coated steel sheet includes: a base steel sheet; and a Zn-Mg-Al coating disposed on at least one side of the base steel sheet.

[0041] In this invention, the type of base steel plate is not particularly limited. For example, the base steel plate can be an Fe-based base steel plate used as the base steel plate for conventionally applicable galvanized steel plates, i.e., hot-rolled steel plate or cold-rolled steel plate, but is not limited thereto. Alternatively, the base steel plate can be, for example, carbon steel, ultra-low carbon steel or high manganese steel used in building materials, home appliance materials, and automotive materials.

[0042] However, as an example, the base steel plate may contain, by weight percent: C: greater than 0% and less than 0.18%, Si: greater than 0% and less than 1.5%, Mn: 0.010-2.7%, P: greater than 0% and less than 0.0700%, S: greater than 0% and less than 0.0150%, Al: greater than 0% and less than 0.50%, Nb: greater than 0% and less than 0.060%, Cr: greater than 0% and less than 1.1%, Ti: greater than 0% and less than 0.060%, and B: greater than 0% and less than 0.03000%, and may consist of the balance Fe and other unavoidable impurities.

[0043] Although not specifically limited, according to one embodiment of the present invention, at least one side of the base steel plate may be provided with a Zn-Mg-Al based coating composed of a Zn-Mg-Al alloy. The coating may be formed only on one side of the base steel plate or on both sides. In this case, the Zn-Mg-Al based coating refers to a coating containing Mg and Al, with Zn as the main component (i.e., Zn content of 50% or more).

[0044] Although not specifically limited, according to one embodiment of the present invention, the thickness of the Zn-Mg-Al coating can be 5-100 μm, more preferably 7-90 μm. When the coating thickness is less than 5 μm, errors due to coating thickness deviations may result in locally excessively thin coatings, leading to reduced corrosion resistance. When the coating thickness exceeds 100 μm, the cooling of the molten coating may be delayed, potentially causing solidification defects such as flow marks on the coating surface. Furthermore, the increased time required for coating solidification may reduce the productivity of the steel sheet.

[0045] Furthermore, according to the present invention, the Zn-Mg-Al coating may contain, by weight%, Mg: 4.0-6.3%, Al: 11.0-19.5%, with the balance being Zn and other unavoidable impurities. Additionally, without particular limitation, the coating may optionally further contain, by weight%, one or more components selected from Si: 0.20% or less (inclusive) and Ca: 0.200% or less (inclusive). The reasons for adding each component and the reasons for limiting its content are explained in detail below.

[0046] Mg: 4.0-6.3% Mg is an element that enhances the corrosion resistance of coated steel sheets. In this invention, to ensure excellent corrosion resistance, the Mg content in the coating is controlled to be above 4.0%. Furthermore, excessive Mg addition may produce scum, therefore the Mg content can be controlled to below 6.3%. As another example, the Mg content can be 4.5-6.0% by weight; as yet another example, the Mg content can be 5.0-5.5% by weight.

[0047] Al: 11.0-19.5% Generally, when the amount of Mg added is above 1.0%, it can improve corrosion resistance. However, when the amount of Mg added is above 2.0%, the amount of suspended dross in the plating bath increases due to the oxidation of Mg in the plating bath, which requires frequent dross removal.

[0048] Due to these issues, in existing technologies, more than 1.0% Mg is added to Zn-Mg-Al zinc alloy coatings to ensure corrosion resistance, while the upper limit of Mg content is set at 3.0% for commercialization.

[0049] However, in order to further improve corrosion resistance, the Mg content needs to be increased to more than 4.0%. But when the coating contains more than 4.0% Mg, there is a problem of slag formation due to the oxidation of Mg in the plating bath. Therefore, in order to suppress the formation of such slag, according to an example of the Zn-Mg-Al coating of the present invention, more than 11.0% of the Al can be added.

[0050] However, when excessive Al is added to suppress scum, the melting point of the plating bath increases, leading to excessively high operating temperatures. This can cause problems such as corrosion of plating components and steel deformation due to high-temperature operation. Furthermore, when the Al content in the plating bath is excessive, Al reacts with Fe in the base steel plate, failing to contribute to the formation of the Fe-Al inhibitory layer. Instead, it causes a rapid reaction between Al and Zn, excessively forming agglomerated outburst phases, which may actually degrade corrosion resistance. Therefore, the upper limit of Al content in the coating is preferably controlled at 19.5%. As another example, the Al content can be 13.0-17.5% by weight, and as yet another example, it can be 14.0-16.0% by weight.

[0051] Si: 0.20% or less (including 0%) Adding less than 0.20% Si can prevent the formation of an excessively thick Fe-Al alloy layer at the interface between the base iron and the coating, thus avoiding a decrease in the interfacial strength between the coating and the base iron. Therefore, adding this element has advantages, but even without its addition, it has almost no effect on corrosion resistance in this invention, so the lower limit is set to 0%.

[0052] However, even when the Si addition exceeds 0.20%, the inhibition effect of the Fe-Al alloy layer at the interface reaches saturation. Furthermore, the higher the Si addition, the higher the melting point of the plating bath, requiring the plating bath temperature to be maintained at a higher level, which has adverse effects from the perspective of equipment protection. According to one embodiment of the present invention, the Si addition can be below 0.18%.

[0053] Ca: 0.200% or less (including 0%) Ca is not an essential element, but its addition to 0.200% can suppress the formation of MgO oxides in the plating bath. Furthermore, a small amount may be added during the manufacture of the plating bath ingot for operational convenience, so a small amount of Ca may be present in the plating bath produced using this ingot. However, when the Ca addition exceeds 0.200%, the steel plate may darken, which is therefore undesirable. According to one embodiment of the invention, the Ca content can be 0.180% or less.

[0054] Balance Zn and other unavoidable impurities In addition to the components mentioned above, the components dissolved during the ingot manufacturing process or from the steel plate, along with unavoidable impurities and zinc components present in the plating bath, constitute the composition. Examples of unavoidable impurities include trace components such as Sb, Sn, Pb, Sr, and Cu that are inevitably introduced during the manufacture of the ingot for the plating solution. Furthermore, components inevitably dissolved from the steel plate during immersion in the plating bath, present in trace amounts in the plating bath, may include Mn, Ti, Ni, B, and Nb; other components may also be present depending on the composition of the steel plate. However, even for unavoidable additions, an addition amount of 0.01% or more for each component is not preferred.

[0055] According to one embodiment of the present invention, the coating may further comprise any one or more of the following groups (a) to (h).

[0056] However, the elements in the following groups are not essential to achieving the technical problem of this invention, and therefore there is no lower limit to their content. Therefore, even if not specifically mentioned below, the lower limit for the content of each element can be 0%.

[0057] (a) Ni: less than 0.5%, (b) One or more of the following: La: less than 0.1%, Ce: less than 0.1%, Y: less than 0.1%, and Sr: less than 1.0%. (c)Ti: less than 0.1%, (d)W: below 0.5%, (e)Cu: below 2.0%, (f) One or more of the following: Cr: less than 0.5%, Mn: less than 0.5%, and V: less than 0.5%. (g)B: less than 0.1%, P: less than 0.1% or more of these two concentrations. (h) Sn: less than 1.0%, Sb: less than 1.0%, Bi: less than 1.0% or more of the following:

[0058] The reasons for adding each ingredient and the reasons for limiting its content are explained in detail below.

[0059] (a) Ni: less than 0.5% Ni can prevent Fe diffusion by forming an Al-Ni alloy phase, but when the Ni content exceeds 0.5%, the cost of auxiliary materials may increase excessively.

[0060] (b) One or more of the following: La: less than 0.1%, Ce: less than 0.1%, Y: less than 0.1%, and Sr: less than 1.0%. La, Ce, Y, and Sr form an oxide film that prevents Mg oxidation in the plating bath. However, when the contents of La, Ce, Y, and Sr exceed 0.1%, 0.1%, 0.1%, and 1.0%, respectively, the viscosity of the plating bath increases, which may lead to a decrease in plating performance.

[0061] (c)Ti: less than 0.1% Ti-Al intermetallic compounds can serve as nucleation sites, resulting in grain (zinc flower) refinement. However, when the Ti content exceeds 0.1%, the melting point of the plating bath increases, which may lead to an increase in slag.

[0062] (d)W: below 0.5% W forms W oxide on the surface, which improves corrosion resistance. However, when the W content exceeds 0.5%, the melting point of the plating bath may increase.

[0063] (e)Cu: below 2.0% Cu can form an Al-Cu eutectic structure, which can reduce the hardness of the coating. However, when the Cu content exceeds 2.0%, the zinc spangle may become coarse.

[0064] (f) One or more of the following: Cr: less than 0.5%, Mn: less than 0.5%, and V: less than 0.5%. Cr, Mn and V disappear from the liquid phase quickly, which can prevent electrode deterioration by inhibiting the alloying of zinc with the welding electrode. However, when the contents of Cr, Mn and V exceed 0.5% respectively, the melting point of the plating bath may rise excessively.

[0065] (g)B: less than 0.1%, P: less than 0.1% or more of these two types. B and P have the effect of suppressing liquid metal embrittlement (LME) cracks in welds, but when the contents of B and P exceed 0.1% respectively, the amount of slag produced may increase.

[0066] (h) One or more of Sn: less than 1.0%, Sb: less than 1.0%, and Bi: less than 1.0%. Sn, Sb, and Bi can homogenize zinc spangles and improve the durability of the plating bath by lowering the bath temperature. However, when the contents of Sn, Sb, and Bi exceed 1.0%, the zinc spangles may become coarse.

[0067] In addition, the surface of the plated steel sheet according to another embodiment of the present invention satisfies the following conditions: average surface roughness Ra: 2.0-3.0 μm, peak count RPc: 25 to 60 (1 / 10 mm), and skewness Rsk: -0.5 or more and 0.5 or less.

[0068] Average surface roughness Ra: 2.0-3.0 μm In one example of the present invention, to ensure a glare-free surface with a milky white, high-quality texture, the average surface roughness is set to 2.0 μm or higher. That is, when the average surface roughness is less than 2.0 μm, the presence of flat portions can cause light reflection, potentially leading to glare. On the other hand, when the average surface roughness exceeds 3.0 μm, the surface roughness is too high, which may adversely affect subsequent processing steps such as bending and roll forming. As another example, the average surface roughness can be 2.3-2.8 μm, and as yet another example, it can be 2.5-2.6 μm.

[0069] Peak count RPc: 25 to 60 (1 / 10 mm) Throughout this specification, the peak count refers to the number of peaks present on the surface of the coated steel sheet per 1 cm length. That is, the peak count can be used as an indicator of the surface roughness of the coated steel sheet. When the peak count is less than 25 (1 / 10 mm), the surface cannot exhibit sufficient curvature, which may adversely affect the anti-glare properties. When the peak count exceeds 60 (1 / 10 mm), the surface grooves are too dense, and slippage may occur during processing. As another example, the peak count can be 35 to 50 (1 / 10 mm), or 40 to 45 (1 / 10 mm).

[0070] Skewness (Rsk): Above -0.5 and below 0.5 Throughout this specification, the skewness Rsk value represents the degree of oil retention in the coating, and this value can affect the processing characteristics of the coated steel sheet. When the skewness value is less than -0.5, it is difficult to uniformly retain rust-preventive oil or lubricating oil in the width direction of the steel sheet, and subsequent degreasing processes may result in localized poor degreasing. On the other hand, when the skewness value exceeds 0.5, there are too many upward-protruding probe-like peaks in the surface shape, and the surface shape may be crushed and deformed during processing. As another example, the skewness value of the coated steel sheet surface can be from -0.3 to 0.3, or from -0.1 to 0.1.

[0071] Furthermore, according to one embodiment of the present invention, the coated steel sheet can ensure excellent surface appearance and anti-glare properties at a target level while ensuring corrosion resistance. More specifically, according to an example of the coated steel sheet of the present invention, the surface gloss B can be 25 or higher and 60 or lower, and the surface whiteness L can be 85 or higher.

[0072] Surface gloss B: 25 or higher and 60 or lower To prevent the coated steel sheet from appearing dark due to excessively low gloss, one embodiment of the present invention sets the lower limit of the surface gloss level to 25 or higher. On the other hand, when the surface gloss level exceeds 60, it is highly likely to cause glare to drivers when applied to road guardrails; therefore, one embodiment of the present invention sets the surface gloss level to 60 or lower. That is, according to an example of the coated steel sheet of the present invention, by setting the surface gloss level to 25 or higher and 60 or lower, glare can be prevented while ensuring a target level of gloss. While not strictly limited to this, as an example, the surface gloss level B is measured using a REF-260 from SHEEN Corporation, with an incident angle of 60°. According to another example, the surface gloss level can be 35 or higher and 55 or lower; as yet another example, it can be 40 or higher and 50 or lower.

[0073] Whiteness L: 85 or higher In a non-limiting embodiment of the present invention, to ensure the aesthetic appearance of the surface quality, the whiteness is set to 85 or higher. Higher whiteness is more beneficial to achieving the objectives of the present invention; therefore, one example of the present invention does not specifically limit the upper limit of the whiteness. However, considering that achieving a whiteness exceeding 95 would require excessive time and effort in actual processing, a non-limiting embodiment of the present invention may set the upper limit of the whiteness to 95. While not necessarily limited to this, as an example, the whiteness is measured using a Minolta CM-3700A, where the whiteness is displayed in the range of 0 to 100. According to another example, the whiteness can be 90 or higher, and as yet another example, it can be 94 or higher.

[0074] Next, a method for 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 coated steel sheet of the present invention must be manufactured by the following method.

[0075] According to one embodiment of the present invention, the process may first include a step of preparing a base steel plate, and the type of base steel plate is not particularly limited. The base steel plate may be an Fe-based base steel plate, such as that used for conventional hot-dip galvanized steel plates, i.e., hot-rolled steel plate or cold-rolled steel plate, but is not limited thereto. Furthermore, the base steel plate may be, for example, carbon steel, ultra-low carbon steel, or high-manganese steel used in construction, home appliances, and automotive materials, but is not limited thereto. In this case, the above description can be equally applied to the base steel plate.

[0076] Next, the base steel plate prepared according to the above description is immersed in a hot-dip galvanizing bath, which, by weight percent, contains: Mg: 4.0-6.3%, Al: 11.0-19.5%, and the balance being Zn and other unavoidable impurities, thereby obtaining a hot-dip galvanized steel plate. At this time, the reasons for adding and limiting the content of the components in the above-mentioned plating bath, except for the content of trace impurities that may flow in from the base steel plate, are equally applicable to the above description regarding the coating composition. Therefore, according to one aspect of the invention, the plating bath may optionally further contain, by weight percent, one or more components selected from Si: 0.20% or less (inclusive) and Ca: 0.200% or less (inclusive).

[0077] At this point, the reasons for adding components and limiting their content in the plating bath mentioned above can be applied equally to the above explanations regarding the plating composition, except for the trace amounts of Fe that may flow in from the base steel plate.

[0078] In addition, to manufacture the plating bath with the above composition, a composite ingot containing the specified Zn, Al, and Mg, or a Zn-Mg or Zn-Al ingot containing each individual component, can be used. To replenish the plating bath consumed during molten plating, the ingot needs to be additionally dissolved and supplied. In this case, one can choose to dissolve the ingot directly in the plating bath, or one can choose to dissolve the ingot in a separate pot and then replenish the plating bath with molten metal.

[0079] Furthermore, according to a non-limiting example of the present invention, when obtaining the hot-dip galvanized steel sheet, the introduction temperature A of the base steel sheet satisfies the following relationship 1.

[0080] [Relation 1](T) B -40)℃≤A≤(T B -10℃ (In relation 1, T) BThis indicates the temperature (°C) of the plating bath. That is, when the temperature A introduced into the base steel plate according to the relationship 1 is less than T B At -40℃, the crystalline phase may not be dense enough. When the introduction temperature A of the base steel plate exceeds T... B At -10℃, issues with coating adhesion may arise. As another example, the introduction temperature A of the base steel plate can be (T... B -30)℃ to (T B -10)℃, as another example, can be (T) B -20)℃ to (T B -10)℃.

[0081] At this point, without specific limitations, as an example, the above-mentioned plating bath temperature (T) B The temperature can be in the range of 440-500℃. Furthermore, a more preferred plating bath temperature (T) is... B The lower limit of the temperature can be 455°C, or the plating bath temperature (T) can be... B The upper limit can be 490℃.

[0082] Next, a method for manufacturing coated steel sheet according to a non-limiting example of the present invention includes the step of wiping the hot-dip galvanized steel sheet by supplying a mixture of inert gas and air.

[0083] Furthermore, according to one embodiment of the method for manufacturing coated steel sheet, the mixed gas contains 5% to 20% air based on the total volume of the mixed gas.

[0084] That is, through in-depth research, the inventors discovered that, unlike conventional methods, when wiping with a mixture of 5% to 20% by volume of air and an inert gas, the surface of the plated steel sheet can exhibit an extremely excellent whiteness with a milky appearance through weak oxidation, and due to the disappearance of gloss, the anti-glare properties become excellent.

[0085] When the air content is less than 5% by volume, based on the total proportion of the mixed gas, the aforementioned weak oxidation effect may be insufficient, resulting in difficulty in improving the whiteness of the steel sheet. On the other hand, when the air content exceeds 20% by volume, based on the total proportion of the mixed gas, severe over-oxidation defects may occur on the surface of the steel sheet. Therefore, in a non-limiting embodiment of the present invention, the air content in the mixed gas is maintained at 5-20% by volume. As another example, the air content in the mixed gas can be 5-15% by volume, and as yet another example, it can be 5-10% by volume.

[0086] In addition, without particular limitation, according to one embodiment of the present invention, the inert gas can be argon (Ar), nitrogen (N2) or a mixture of argon and nitrogen, etc., and from an economic point of view, the use of nitrogen is more preferred.

[0087] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention and are not intended to limit the scope of the invention. The scope of the invention is determined by the matters set forth in the claims and by reasonable analogy thereof. Detailed Implementation

[0088] (Example) Prepare a base steel plate, which, by weight percent, comprises: C: 0.018%, Si: 0.010%, Mn: 0.20%, P: 0.009%, S: 0.005%, Al: 0.1%, Nb: 0.02%, Cr: 0.20%, Ti: 0.020%, B: 0.015%, with the balance being Fe and impurities. The base steel plate has a thickness of 1.5 mm and a width of 1200 mm. Immerse the base steel plate in a hot-dip galvanizing bath under the conditions specified in Table 1 below to obtain a hot-dip galvanized steel sheet. Table 1 shows the composition of the hot-dip galvanizing bath, excluding trace amounts of Fe that may have seeped in from the base steel plate.

[0089] Next, a wiping process was performed using a mixture of air and nitrogen (N2). The air content in this mixture is shown in Table 1. Subsequently, the surface gloss, whiteness, surface roughness, number of surface peaks, and skewness of the manufactured coated steel sheet were measured, and the measured values ​​are shown in Table 2. The thickness of the Zn-Mg-Al coating on the manufactured coated steel sheet was 5-100 μm.

[0090] At this time, the surface gloss B is measured using SHEEN's REF-260, and the measured surface gloss is the gloss at an incident angle of 60°.

[0091] Furthermore, the L (lightness) value in the whiteness was measured using a Minolta CM-3700A.

[0092] Surface roughness, surface peak count, and skewness Rsk were measured using a KOSAKA contact roughness measuring instrument that conforms to the JIS 2013 standard.

[0093] [Table 1] [Table 2] In the cases of Comparative Examples 1 to 16, any one or more of the plating composition and manufacturing conditions specified in this invention were not met, resulting in surface roughness, surface peak count, or skewness exceeding the range proposed by this invention. Consequently, the plated steel sheets of Comparative Examples 1 to 16 exhibited poor performance in terms of surface appearance quality and anti-glare properties. On the other hand, in the cases of Invention Examples 1 to 9, the plating composition and manufacturing conditions specified in this invention were met, thus exhibiting excellent surface appearance quality and anti-glare properties.

[0094] Figure 1 These are photographic images showing the surfaces of (a) Invention Example 4 and (b) Comparative Example 4, respectively. Observe the... Figure 1 It can be confirmed that, compared with Comparative Example 4, the Invention Example 4 exhibits a higher whiteness value and a more aesthetically pleasing surface quality.

[0095] also, Figure 2 These are the comparative evaluation results of the surface shape characteristics of (a) Invention Example 4 and (b) Comparative Example 4, respectively. Observe the... Figure 2 It can be confirmed that Comparative Example 4 has a lower surface roughness than Invention Example 4. As a result, Comparative Example 4, due to the presence of the flat portion, is more likely to produce glare than Invention Example 4. Furthermore, through the... Figure 2 It can be confirmed that, compared to Comparative Example 4, the surface peak count of Invention Example 4 is lower. Therefore, Invention Example 4 can ensure no glare while presenting a milky white, high-quality surface quality.

Claims

1. A galvanized steel sheet, comprising: Foundation steel plate; as well as A Zn-Mg-Al coating is provided on at least one side of the base steel plate. The coating, by weight percent, comprises: Mg: 4.0-6.3%, Al: 11.0-19.5%, with the balance being Zn and other unavoidable impurities. The surface of the galvanized steel sheet meets the following requirements: average roughness Ra: 2.0-3.0 μm, peak count RPc: 25 to 60 (1 / 10 mm), and skewness Rsk: -0.5 to 0.

5.

2. The galvanized steel sheet according to claim 1, wherein, The surface gloss B is 25 or higher and 60 or lower, and the surface whiteness L is 85 or higher.

3. The galvanized steel sheet according to claim 1, wherein, The base steel plate, by weight percent, comprises: C: greater than 0% and less than 0.18%, Si: greater than 0% and less than 1.5%, Mn: 0.010-2.7%, P: greater than 0% and less than 0.0700%, S: greater than 0% and less than 0.0150%, Al: greater than 0% and less than 0.50%, Nb: greater than 0% and less than 0.060%, Cr: greater than 0% and less than 1.1%, Ti: greater than 0% and less than 0.060%, and B: greater than 0% and less than 0.03000%, and consists of the balance Fe and other unavoidable impurities.

4. The galvanized steel sheet according to claim 1, wherein, In weight percent, the Zn-Mg-Al coating further comprises one or more of Si: less than 0.2% and less than 0% and Ca: less than 0.2% and less than 0%.

5. The coated steel sheet according to claim 1, wherein, The Zn-Mg-Al coating further comprises one or more of the following groups (a) to (h): (a) Ni: less than 0.5%, (b) One or more of the following: La: less than 0.1%, Ce: less than 0.1%, Y: less than 0.1%, and Sr: less than 1.0%. (c)Ti: less than 0.1%, (d)W: below 0.5%, (e)Cu: below 2.0%, (f) One or more of the following: Cr: less than 0.5%, Mn: less than 0.5%, and V: less than 0.5%. (g)B: less than 0.1%, P: less than 0.1% or more of these two concentrations. (h) Sn: less than 1.0%, Sb: less than 1.0%, Bi: less than 1.0% or more of the following:

6. The galvanized steel sheet according to claim 1, wherein, The thickness of the Zn-Mg-Al coating is 5-100 μm.

7. A method for manufacturing a galvanized steel sheet, comprising the following steps: Prepare the foundation steel plate; The base steel plate is immersed in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel plate. as well as A mixture of inert gas and air is supplied to the hot-dip galvanized steel sheet for wiping. The gas mixture contains, based on its total volume, 5% to 20% air. The hot-dip galvanizing bath, by weight percent, contains: Mg: 4.0-6.3%, Al: 11.0-19.5%, with the balance being Zn and other unavoidable impurities. In the step of obtaining the hot-dip galvanized steel sheet, the introduction temperature A of the base steel sheet satisfies the following relationship 1. [Relation 1](T) B -40)℃≤A≤(T B -10℃ In relation 1, T B The temperature of the plating bath is indicated in °C.

8. The method for manufacturing galvanized steel sheet according to claim 7, wherein, The base steel plate, by weight percent, comprises: C: greater than 0% and less than 0.18%, Si: greater than 0% and less than 1.5%, Mn: 0.010-2.7%, P: greater than 0% and less than 0.0700%, S: greater than 0% and less than 0.0150%, Al: greater than 0% and less than 0.50%, Nb: greater than 0% and less than 0.060%, Cr: greater than 0% and less than 1.1%, Ti: greater than 0% and less than 0.060%, and B: greater than 0% and less than 0.03000%, and consists of the balance Fe and other unavoidable impurities.

9. The method for manufacturing galvanized steel sheet according to claim 7, wherein, The hot-dip galvanizing bath further comprises, by weight percent, one or more of Si: less than 0.2% and including 0% and Ca: less than 0.2% and including 0%.

10. The method for manufacturing galvanized steel sheet according to claim 7, wherein, The hot-dip galvanizing bath further comprises one or more of groups (a) to (h) below. (a) Ni: less than 0.5%, (b) One or more of the following: La: less than 0.1%, Ce: less than 0.1%, Y: less than 0.1%, and Sr: less than 1.0%. (c)Ti: less than 0.1%, (d)W: below 0.5%, (e)Cu: below 2.0%, (f) One or more of the following: Cr: less than 0.5%, Mn: less than 0.5%, and V: less than 0.5%. (g)B: less than 0.1%, P: less than 0.1% or more of these two concentrations. (h) Sn: less than 1.0%, Sb: less than 1.0%, Bi: less than 1.0% or more of the following:

11. The method for manufacturing galvanized steel sheet according to claim 7, wherein, The temperature T of the plating bath B The temperature is above 440℃ and below 500℃.

12. The method for manufacturing galvanized steel sheet according to claim 7, wherein, The inert gas includes at least one of argon and nitrogen.

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