Cooling composition for zinc alloy plated steel sheet, method for manufacturing zinc alloy plated steel sheet using same, and zinc alloy plated steel sheet produced by same

Abstract

One aspect of the present invention provides a method for manufacturing a zinc alloy plated steel sheet. The method comprises the steps of: preparing a base steel sheet; plating the base steel sheet by dipping same in a Zn-based plating bath comprising Al and Mg; adjusting the plating adhesion amount of the base steel sheet; and spraying droplets of a cooling composition onto the steel sheet having the plating adhesion amount adjusted, thereby cooling the steel sheet, wherein the cooling composition may comprise 0.1 to 10.0 wt% of urea and the balance of water.

Description

Composition for cooling zinc alloy-plated steel sheet, method for manufacturing zinc alloy-plated steel sheet using the same, and zinc alloy-plated steel sheet manufactured thereby

[0001] The present invention relates to a composition for cooling a zinc alloy-plated steel sheet, a method for manufacturing a zinc alloy-plated steel sheet using the same, and a zinc alloy-plated steel sheet manufactured thereby.

[0002] Typically, zinc-plated steel sheets are manufactured by immersing high-temperature steel sheets that have passed through an annealing furnace into a molten zinc plating bath, then solidifying the molten zinc applied to the steel sheet. As the molten zinc cools, spangles, which are dendritic crystals, form on the surface of the steel sheet.

[0003] Specifically, during the initial solidification of molten zinc, dendrites grow very rapidly by solidification nuclei, forming a coarse spangle pattern. In other words, the size of the spangles can be determined in the early stage of solidification.

[0004] To minimize the formation of spangles as described above, a ternary galvanized steel sheet having a Zn-Al-Mg composition was developed, which is manufactured by immersing the steel sheet in a molten zinc plating bath containing Al and Mg. This ternary galvanized steel sheet has the advantage of preventing the formation of spangles because it contains aluminum and magnesium, which have high melting points.

[0005] However, when manufacturing ternary galvanized steel sheets, there is a disadvantage in that the cooling range between the liquid and solid phases of the plating layer applied to the steel sheet (△T > 70℃) is much wider than that of pure zinc (△T ≒ 30℃). As a result, irregular over-plating areas are formed on the surface of the steel sheet where solidification has not been completely performed, and there is a problem in that dot-shaped defects occur due to pressing when passing through a subsequent roll (Pass line). Accordingly, the quality of the surface appearance of the ternary galvanized steel sheet deteriorates, and there is also a problem in that the defect rate increases as the dot-shaped defects oxidize and turn black over time.

[0006] To address these issues and improve surface quality, methods for rapidly cooling ternary galvanized steel sheets have been developed. For example, conventional methods involve spraying water or a phosphate solution onto the steel sheet before the pure zinc coating solidifies.

[0007] However, this method not only had an insufficient cooling effect, but also had the side effect of causing phosphate components to remain on the surface of the plating layer, which lowered the plating quality in further processes.

[0008] Therefore, when manufacturing ternary galvanized steel sheets, a method is required to improve surface appearance and quality without surface defects while rapidly cooling the molten plating layer.

[0009] (Patent Document 1) Korean Patent Publication No. 10-2010-0076817

[0010] One aspect of the present invention is to provide a cooling composition for minimizing the occurrence of surface defects in a zinc alloy-plated steel sheet.

[0011] Another aspect of the present invention is to provide a method for manufacturing a zinc alloy-plated steel sheet by rapidly cooling a molten plating layer applied to the surface of a base steel sheet using the cooling composition.

[0012] Another aspect of the present invention is to provide a zinc alloy-plated steel sheet having excellent physical properties such as surface appearance and quality, and paint adhesion.

[0013] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0014] One aspect of the present invention provides a composition for cooling a zinc alloy-plated steel sheet. The composition may include 0.1 to 10.0 wt% of urea and the remainder of water.

[0015] Additionally, in the aforementioned composition, the cooling composition may further include 0.1 to 10.0 wt% of alcohol.

[0016] Additionally, in one of the aforementioned compositions, the alcohol may include at least one selected from the group consisting of ethanol, propanol, isopropanol, butanol, and isobutanol.

[0017] Another aspect of the present invention provides a method for manufacturing a zinc alloy-plated steel sheet. The method comprises the steps of: preparing a base steel sheet; plating the base steel sheet by immersing it in a Zn-based plating bath containing Al and Mg; adjusting the plating adhesion amount of the base steel sheet; and cooling the steel sheet with the adjusted plating adhesion amount by spraying droplets of a cooling composition, wherein the cooling composition may include 0.1 to 10.0 wt% of urea; and a balance of water.

[0018] Additionally, in one of the aforementioned methods, the cooling composition may further comprise 0.1 to 10.0 wt% of alcohol.

[0019] Additionally, in one of the aforementioned methods, the alcohol may include at least one selected from the group consisting of ethanol, propanol, isopropanol, butanol, and isobutanol.

[0020] Additionally, in one of the above-described methods, in the cooling step, air may be sprayed onto the steel plate at a pressure of 1.0 to 5.0 kgf / ㎠.

[0021] Additionally, in one of the above-described methods, in the cooling step, the cooling composition can be sprayed at a pressure of 0.5 to 2.5 kgf / cm2.

[0022] Additionally, in one of the above-described methods, in the cooling step, the injection pressure ratio of the cooling composition and the air may be 1:0.3 to 0.9.

[0023] Additionally, in one of the above-described methods, in the cooling step, the cooling composition may be sprayed onto the steel plate after passing through the charging electrode.

[0024] Additionally, in one of the aforementioned methods, a voltage of -5 to -50 kV may be applied to the cooling composition from the charging electrode.

[0025] Additionally, in one of the aforementioned methods, in the cooling step, the temperature of the steel plate may be 350 to 460°C.

[0026] Additionally, in one of the aforementioned methods, the temperature of the plating bath may be 430 to 500°C.

[0027] Another aspect of the present invention provides a zinc alloy-plated steel sheet. The zinc alloy-plated steel sheet can be manufactured according to one of the methods described above.

[0028] According to the present invention, by spraying a cooling composition including an element onto the surface of a steel plate, the solidification speed of a molten plating layer can be increased and solidification of the surface of the steel plate can be promoted.

[0029] In addition, according to the present invention, by rapidly cooling the plating layer, defects such as the formation of dot-like shapes in the plating steel sheet can be minimized, and the surface appearance of the steel sheet can be made beautiful.

[0030] In addition, according to the present invention, a cooling composition charged with high voltage is sprayed as ultra-fine droplets onto the surface of a steel plate, so that a larger amount of fine-sized droplets can be attached to the surface of the steel plate.

[0031] The various advantageous and beneficial advantages and effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0032] In order to more fully understand the drawings cited in the detailed description of the present invention, a brief description of each drawing is provided.

[0033] Figure 1 is a photograph showing the EPMA surface analysis results for Invention Example 10, Invention Example 16, and Comparative Example 7.

[0034] Figure 2 is a graph showing the results of performing a profile on the surface temperature for Invention Example 4, Invention Example 10, Invention Example 16, Comparative Example 1, Comparative Example 4, and Comparative Example 7.

[0035] Figure 3 is a graph showing the results of performing a profile on the surface temperature for invention examples 10 to 12.

[0036] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified in various other forms, and the terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include plural forms, unless the relevant definition clearly indicates a contrary meaning.

[0037] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0038] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0039] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0040] Hereinafter, a cooling composition for a zinc alloy-plated steel sheet according to one embodiment of the present invention will be described. The cooling composition acts as a solidification nucleus by spraying droplets onto the molten plating layer of the zinc alloy-plated steel sheet, thereby increasing the solidification speed of the plating layer and minimizing the occurrence of surface defects.

[0041] A cooling composition according to one embodiment of the present invention may include an element.

[0042] Conventionally, a method of spraying a phosphate aqueous solution on the surface of a steel sheet has been used to promote the formation of solidification nuclei on the surface of the steel sheet. However, when using a phosphate aqueous solution, there was a problem in that not only did the dried powder contaminate surrounding equipment, but also the phosphate remained as a glassy film on the surface of the steel sheet, causing secondary deterioration of physical properties. A cooling composition according to one embodiment of the present invention contains urea, and by spraying such a cooling composition on the surface of a steel sheet, the cooling rate of a molten plating layer can be increased. In addition, after drying of the plating layer, no residue is formed on the surface of the steel sheet, thereby resolving the above-mentioned problem.

[0043] The element may be included in an amount of 0.1 to 10.0 wt% based on the total weight of the cooling composition. If the content of the element is less than 0.1 wt%, the above-described effect (i.e., improvement in the cooling speed of the steel plate through the formation of solidification nuclei) cannot be sufficiently obtained. In addition, if the content of the element exceeds 10.0 wt%, it may become a factor in increasing the manufacturing cost, or the element may be dried and powdered, thereby contaminating surrounding equipment. That is, the content of the element may be 0.1 to 10.0 wt%, more specifically 0.3 to 7.0 wt%, and even more specifically 0.5 to 5.0 wt%.

[0044] For example, the cooling composition may further include alcohol. When spraying the cooling composition as droplets onto a molten plating layer, the alcohol reduces the surface tension of the cooling composition, thereby further minimizing the droplets, thereby forming a greater number of solidification nuclei and allowing the plating layer to cool more quickly and solidify.

[0045] The alcohol may include at least one selected from the group consisting of ethanol, propanol, isopropanol, butanol, and isobutanol. However, the present invention is not limited thereto, and various types of alcohol commonly used in the industry may be used.

[0046] Alcohol may be included in an amount of 0.1 to 10.0 wt% based on the total weight of the cooling composition. If the alcohol content is less than 0.1 wt%, droplet atomization may not be sufficiently achieved. In addition, if the alcohol content exceeds 10.0 wt%, it may be a factor in increasing the manufacturing cost or there may be a problem of odor generation due to volatility. That is, the alcohol content may be 0.1 to 10.0 wt%, more specifically 0.3 to 7.0 wt%, and even more specifically 0.5 to 5.0 wt%.

[0047] Hereinafter, a method for manufacturing a zinc alloy-plated steel sheet according to one embodiment of the present invention will be described. The method for manufacturing a zinc alloy-plated steel sheet according to one embodiment of the present invention may include the steps of preparing a base steel sheet; plating the base steel sheet; controlling the amount of plating applied; and cooling the steel sheet.

[0048] Below, each step is explained in detail.

[0049] [Preparation of the steel plate]

[0050] A base steel sheet that serves as the base for a zinc alloy-plated steel sheet can be prepared. The type of base steel sheet, alloy composition, etc. are not particularly limited, and any steel sheet commonly used for metal plating in the relevant technical field can be used. For example, hot-rolled steel sheets, hot-rolled fine steel sheets, or cold-rolled steel sheets can be used as the base steel sheet. Furthermore, carbon steel, aluminum, aluminum alloy steel, stainless steel, or copper can be used as the base steel sheet.

[0051] [Plating for steel plates]

[0052] The steel sheet prepared in the aforementioned steps can be plated. Specifically, the plating step can be performed by immersing the steel sheet in a zinc-based plating bath. By forming a plating layer on the surface of the steel sheet through the plating step, the corrosion resistance of the steel sheet can be improved.

[0053] Zn-based plating baths may contain Al and Mg. Since Al and Mg have high melting points, they can rapidly cool the molten plating layer on the steel plate surface, thereby minimizing surface defects and improving surface appearance and quality.

[0054] For example, Al may be included in an amount of 0.1 to 30 wt% based on the total weight of the plating bath. If the content of Al is less than 0.1 wt%, the aforementioned effect may not be sufficiently obtained, or problems may arise in that it is difficult to secure high corrosion resistance. In addition, if the content of Al exceeds 30 wt%, the temperature of the plating bath may become excessively high and operability may deteriorate. That is, the content of Al may be 0.1 to 30 wt%, specifically 0.1 to 30.0 wt%, more specifically 0.5 to 20.0 wt%, and even more specifically 1.0 to 15.0 wt%.

[0055] For example, Mg may be included in an amount of 0.1 to 30 wt% based on the total weight of the plating bath. If the content of Mg is less than 0.1 wt%, the aforementioned effect may not be sufficiently obtained, or problems may arise in that it is difficult to secure high corrosion resistance. In addition, if the content of Mg exceeds 30 wt%, dross may be formed on the surface of the plating bath. That is, the content of Mg may be 0.1 to 30 wt%, specifically 0.1 to 30.0 wt%, more specifically 0.5 to 20.0 wt%, and even more specifically 1.0 to 15.0 wt%.

[0056] In addition to the composition described above, the plating bath may contain a remainder of zinc and other unavoidable impurities. For example, during the normal manufacturing process, unintended impurities (e.g., iron (Fe)) may inevitably be mixed into the plating bath from raw materials or the surrounding environment, and this cannot be ruled out.

[0057] For example, the temperature of the plating bath may be 430 to 500°C. If the temperature of the plating bath is lower than 430°C, the fluidity of the plating bath may deteriorate, or dross may be excessively formed on the surface of the plating bath. In addition, if the temperature of the plating bath exceeds 500°C, energy costs may increase excessively due to excessive electricity use, and the temperature of the steel sheet may increase, thereby reducing the efficiency of cooling in subsequent processes. That is, the temperature of the plating bath may be 430 to 500°C, more specifically 440 to 480°C, and even more specifically 450 to 470°C. However, the present invention is not limited thereto, and the temperature of the plating bath may be adjusted depending on the type, ratio, etc. of the alloy elements constituting the base steel sheet and / or the plating bath.

[0058] [Adjusting the amount of plating]

[0059] The plating amount of the base steel sheet, on which plating has been performed in the aforementioned steps, can be controlled. By removing excess plating metal attached to the surface of the base steel sheet, the plating amount can be adjusted to an appropriate level, thereby ensuring performance such as corrosion resistance and plating adhesion.

[0060] For example, the plating adhesion amount can be controlled by wiping the surface of the base steel sheet. Specifically, air wiping can be performed using air or gas. For example, the gas can be argon (Ar), nitrogen (N2), or a mixture of argon and nitrogen, but is not limited thereto. Any gas commonly used in the art for controlling the plating adhesion amount can be used.

[0061] For example, the plating adhesion amount can be adjusted to 20 to 1000 g / ㎡ on one side of the steel sheet. If the plating adhesion amount is less than 20 g / ㎡, not only is it difficult to precisely control, but it can also be difficult to secure sufficient corrosion resistance. In addition, if the plating adhesion amount exceeds 1000 g / ㎡, defects in the surface appearance can occur. That is, the plating adhesion amount can be 20 to 1000 g / ㎡, more specifically 50 to 600 g / ㎡, and even more specifically 60 to 500 g / ㎡. However, the present invention is not limited thereto, and the plating adhesion amount can be appropriately adjusted depending on the intended use of the steel sheet.

[0062] [Cooling of the steel plate]

[0063] In the aforementioned step, the steel plate with the controlled coating amount can be cooled by spraying a cooling composition as droplets. Specifically, when a cooling composition containing a component is sprayed as droplets on the surface of the steel plate, the cooling composition can act as a solidification nucleus and increase the solidification speed of the molten plating layer.

[0064] When a cooling composition containing a urea is sprayed onto the surface of a steel plate, a decomposition reaction of the urea represented by the following reaction formula 1 and evaporation of water represented by the following reaction formula 2 may occur. Generally, a Zn-Al-Mg plating layer in a molten state has a wide cooling range, but the latent heat of the steel plate is taken away by the endothermic reaction, and the cooling composition sprayed as droplets temporarily remains on the surface and acts as solidification nuclei, so that the plating layer solidifies around the solidification nuclei, thereby increasing the cooling rate.

[0065] [Reaction Formula 1]

[0066] (NH2)2CO + H2O → 2NH3+ CO2△H°(heat of reaction) = 185.5 kJ / mol

[0067] [Reaction Formula 2]

[0068] H2O(l) → H2O(g) △H°(heat of vaporization) = 43.1 kJ / mol

[0069] The element may be included in an amount of 0.1 to 10.0 wt% based on the total weight of the cooling composition. If the content of the element is less than 0.1 wt%, the above-described effect (i.e., improvement in the cooling speed of the steel plate through the formation of solidification nuclei) cannot be sufficiently obtained. In addition, if the content of the element exceeds 10.0 wt%, it may become a factor in increasing the manufacturing cost, or the element may be dried and powdered, thereby contaminating surrounding equipment. That is, the content of the element may be 0.1 to 10.0 wt%, more specifically 0.3 to 7.0 wt%, and even more specifically 0.5 to 5.0 wt%.

[0070] For example, the cooling composition may further include alcohol. In this case, the alcohol (e.g., ethanol, etc.) evaporates, as shown in the following reaction formula 3, thereby removing latent heat from the steel plate and increasing the cooling rate. Furthermore, the alcohol reduces the surface tension of the cooling composition, further miniaturizing the droplets, thereby forming a greater number of solidification nuclei and allowing the plating layer to cool more quickly and solidify.

[0071] [Reaction Formula 3]

[0072] C2H5OH(l) → C2H5OH(g) △H°(heat of vaporization) = 39.3 kJ / mol

[0073] The alcohol may include at least one selected from the group consisting of ethanol, propanol, isopropanol, butanol, and iso-butanol. However, the present invention is not limited thereto, and various types of alcohol commonly used in the industry may be used.

[0074] Alcohol may be included in an amount of 0.1 to 10.0 wt% based on the total weight of the cooling composition. If the alcohol content is less than 0.1 wt%, droplet atomization may not be sufficiently achieved. In addition, if the alcohol content exceeds 10.0 wt%, it may be a factor in increasing the manufacturing cost or there may be a problem of odor generation due to volatility. That is, the alcohol content may be 0.1 to 10.0 wt%, more specifically 0.3 to 7.0 wt%, and even more specifically 0.5 to 5.0 wt%.

[0075] For example, in the cooling step, the cooling composition may be sprayed onto the steel sheet after passing through the charged electrode. In this case, since the cooling composition passes through the charged electrode and generates static electricity, it may adhere to the surface of the steel sheet by electrical attraction. If the size of the sprayed droplets is excessively large, a pitting phenomenon may occur due to the collision of the droplets with the molten plating layer. In the present invention, since the charged cooling composition is sprayed, the droplets are sprayed in a smaller size than when the cooling composition is not charged, and therefore, a greater number of droplets can be sprayed onto the surface of the steel sheet based on the same spray amount. Therefore, a greater number of droplets per unit area of ​​the steel sheet may adhere to the steel sheet, thereby preventing the pitting phenomenon and increasing the adhesion efficiency.

[0076] A high voltage can be applied to the cooling composition using a mesh-shaped charging electrode. In this case, the electric field formed by the charging electrode is uniform, so the effect of the high voltage can be enhanced. Specifically, when the cooling composition passes through the mesh-shaped high-voltage charging electrode, electrostatic atomization occurs, causing large droplets to separate into smaller droplets, reducing the average droplet size and increasing the number of droplets.

[0077] A voltage of -5 to -50 kV may be applied to the cooling composition from the charging electrode. If the voltage is less than -5 kV, the droplet refinement effect may not be sufficient. In addition, if the voltage exceeds -50 kV, electric sparks may occur between the charging electrode and the steel plate. That is, the voltage may be -5 to -50 kV, more specifically -10 to -40 kV, and even more specifically -10 to -30 kV.

[0078] In the above cooling step, air may be sprayed onto the steel plate together with the cooling composition. Specifically, by spraying air together with the cooling composition onto the steel plate at high pressure, the droplets of the cooling composition can be more efficiently attached to the surface of the steel plate.

[0079] For example, a two-fluid spray nozzle can be used to spray the cooling composition and air at high pressure, respectively. For example, the air can be sprayed at a pressure of 1.0 to 5.0 kgf / ㎠. If the spray pressure of the air is less than 1.0 kgf / ㎠, the efficiency of the composition droplets being refined and the efficiency of the droplets being attached to the steel plate may be reduced. In addition, if the spray pressure of the air exceeds 5.0 kgf / ㎠, the kinetic energy of the droplets becomes very large, and the droplets may cause a pitting phenomenon on the surface of the plating layer, thereby damaging the surface appearance. That is, the spray pressure of the air can be 1.0 to 5.0 kgf / ㎠, more specifically, 1.2 to 4.0 kgf / ㎠, and even more specifically, 1.4 to 3.0 kgf / ㎠.

[0080] The cooling composition can be sprayed at a pressure of 0.5 to 2.5 kgf / cm2. If the spray pressure of the cooling composition is less than 0.5 kgf / cm2, the droplets may not be sufficiently finely divided. In addition, if the spray pressure of the cooling composition exceeds 2.5 kgf / cm2, a pitting phenomenon may occur due to the collision of the droplets with the plating layer. That is, the spray pressure of the cooling composition can be 0.5 to 2.5 kgf / cm2, more specifically 0.6 to 2.0 kgf / cm2, and even more specifically 0.7 to 1.5 kgf / cm2.

[0081] For example, the spray pressure ratio of the cooling composition and air may be 1:0.3 to 0.9. If the spray pressure ratio of the cooling composition and air is less than 1:0.3, the aqueous solution droplets may not properly adhere to the surface of the steel sheet. In addition, if the spray pressure ratio of the cooling composition and air exceeds 1:0.9, the size of the droplets may be too large, causing pitting on the surface of the plating layer. That is, the spray pressure ratio of the cooling composition and air may be 1:0.3 to 0.9, more specifically 1:0.4 to 0.8, and even more specifically 1:0.5 to 0.7.

[0082] In the above cooling step, the temperature of the steel plate may be 350 to 460°C. If the temperature of the steel plate is lower than 350°C, the aqueous solution droplets may have difficulty functioning as solidification nuclei, and thus the cooling rate may not increase sufficiently. In addition, if the temperature of the steel plate exceeds 460°C, limitations may arise in the manufacturing process and equipment structure. That is, the temperature of the steel plate may be 350 to 460°C, more specifically 370 to 450°C, and even more specifically 380 to 440°C.

[0083] For example, an additional cooling process may be performed on the steel sheet cooled in the aforementioned step. Specifically, after the step of spraying a cooling composition to cool the steel sheet for the first time, the steel sheet that has been cooled for the first time may be cooled for the second time. The method of the second cooling is not particularly limited, and various methods commonly used in the relevant technical field may be utilized.

[0084] Hereinafter, a zinc alloy-plated steel sheet according to one embodiment of the present invention will be described. The zinc alloy-plated steel sheet according to one embodiment of the present invention may be manufactured using the manufacturing method described above.

[0085] When manufacturing zinc alloy-plated steel sheets, the cooling rate of the plating layer can be increased by spraying droplets of a cooling composition containing urea onto the surface of the molten plating layer. This promotes surface solidification of the steel sheet, minimizing defects such as the formation of dot-like patterns, and enhancing the surface appearance of the steel sheet.

[0086] Conventionally, spraying a phosphate solution onto the surface of steel sheets has been used to promote the formation of coagulation nuclei. However, the use of phosphate solutions has presented problems: not only can the dried powder contaminate surrounding equipment, but the phosphate remains on the surface of the steel sheet as a glassy film, causing secondary deterioration of physical properties. In particular, the glassy phosphate remaining on the surface of the steel sheet reduces paint adhesion during painting and can cause low-temperature bonding brittleness.

[0087] The present invention is characterized in that, by spraying a cooling composition containing elements onto the surface of a steel sheet, the cooling rate of a molten plating layer is increased, and no residue is formed on the surface of the steel sheet even after drying, thereby solving the above-mentioned problems. A zinc alloy-plated steel sheet having such excellent physical properties (i.e., surface appearance and quality, paint adhesion, low-temperature bonding brittleness, etc.) can be used as a material for steel sheets for interior and exterior plating and painting in automobiles, home appliances, and building materials.

[0088] (Example)

[0089] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0090] (1) Manufacture of invention examples

[0091] A hot-rolled steel coil having a thickness of 2.0 mm (C: 0.0013%, Mn: 0.091%, P: 0.0092%, S: 0.005%, the remainder: Fe and unavoidable impurities) was immersed at a moving speed of 60 m / min in a plating bath having the composition shown in Table 1 below, and plated at 140 g / ㎡ per side of the steel sheet by air wiping. A cooling composition containing urea [(NH2)2CO] and / or ethanol was sprayed as droplets on the surface of the steel sheet with a two-fluid spray nozzle together with air for 3 seconds under the conditions shown in Table 1 below, and the steel sheet was cooled to manufacture a zinc alloy-plated steel sheet.

[0092] At this time, a mesh-shaped high-voltage charging electrode was placed between the two-fluid injection nozzle and the steel plate, so that the cooling composition passing through the nozzle passed through the charging electrode (voltage: -30 kV). The spray pressure of the cooling composition was 0.9 kgf / ㎠, the spray pressure of the air was 1.5 kgf / ㎠, and the ratio of the spray pressure of the cooling composition and the air was applied equally at 1:0.6. In addition, the temperature of the steel plate when the cooling composition was sprayed was controlled under the conditions of Table 1 below.

[0093] Distinguishing factor (wt%)Ethanol (wt%)Whether electrostatic sprayingPlating composition (Zn-xAl-yMg)Material thickness (mm)Plating bath temperature (℃)Temperature of steel sheet during spraying (℃)Invention example 111○x=1.5±0.5wt%, y=2.0±0.5wt%2.0460±5440Invention example 211○420Invention example 311○400Invention example 413○440Invention example 513○420Invention example 613○400Invention example 731○440Invention example 831○420Invention example 931○400Invention example 1033○440Invention example 1133○420shot Honor 1233○400 Invention Honor 1351○440 Invention Honor 1451○420 Invention Honor 1551○400 Invention Honor 1653○440 Invention Honor 1753○420 Invention Honor 1853○400 Invention Honor 1910○440 Invention Honor 2030○440 Invention Honor 2150○440 Invention Honor 2211○x=12±3wt%, y=5.0±2wt%2.0460±5440Invention Honor 2311○420Invention Honor 2411○400Invention Honor 2513○440Invention Honor 2613○420Invention Honor 2713○400Invention Honor 2831○440Invention Honor 2931○420Invention Honor 3031○400Invention Honor 3133○440Invention Honor 3133○420Invention Honor 3333○400Invention Honor 3451○440Invention Honor 3551○420Invention Honor 3651○400Invention Honor 3753○440Invention Honor 3853○420Invention Honor 3953○400Invention Honor 4010○440Invention Honor 4130○440Invention Honor 4250○440

[0094] (2) Manufacturing of comparative examples

[0095] In the manufacturing method of the above invention examples, comparative examples were manufactured in the same manner as above, except for the material to be sprayed. Specifically, Comparative Examples 1 to 3 and Comparative Examples 10 to 12 were manufactured by spraying only air, excluding the cooling composition, Comparative Examples 4 to 6 and Comparative Examples 13 to 15 were manufactured by spraying pure water instead of the cooling composition, and Comparative Examples 7 to 9 and Comparative Examples 16 to 18 were manufactured by spraying an aqueous phosphate solution instead of the cooling composition.

[0096] ClassificationSpray materialElectrostatic sprayingPlating composition(Zn-xAl-yMg)Material thickness(mm)Plating bath temperature(℃)Steel plate temperature during spraying(℃)Comparative example 1AirХx=1.5±0.5wt%, y=2.0±0.5wt%2.0460±5440Comparative example 2AirХ420Comparative example 3AirХ400Comparative example 4Air + H2O○440Comparative example 5Air + H2O○420Comparative example 6Air + H2O○400Comparative example 7Air + phosphate aqueous solution (concentration: 0.5%)○440Comparative example 8Air + phosphate aqueous solution (concentration: 0.5%)○420Comparative example 9Air + phosphate aqueous solution (concentration: 0.5%)○400Comparative example 10AirХx=12±3wt%, y=5.0±2wt%2.0460±5440Comparative Example 11 AirX420Comparative Example 12 AirX400Comparative Example 13 Air + H2O○440Comparative Example 14 Air + H2O○420Comparative Example 15 Air + H2O○400Comparative Example 16 Air + phosphate aqueous solution (concentration: 0.5%)○440Comparative Example 17 Air + phosphate aqueous solution (concentration: 0.5%)○420Comparative Example 18 Air + phosphate aqueous solution (concentration: 0.5%)○400

[0097] (3) Property evaluation

[0098] For the above-mentioned manufactured invention examples and comparative examples, the presence or absence of residual elements on the surface of the steel sheet, change in surface temperature of the steel sheet, gloss of the steel sheet, surface roughness of the steel sheet, whiteness of the steel sheet, and surface quality of the steel sheet were evaluated, and are shown in Table 3 below.

[0099] EPMA (Electron Probe Micro-Analysis) surface analysis was performed to evaluate the presence or absence of residual elements on the steel plate surface. Specifically, the distribution amounts of C, N, and O, which are constituent elements of elements, and the distribution amount of P, which is a constituent element of phosphoric acid, were evaluated, and the results are shown in Fig. 1. In the red-yellow-green-blue graph, the larger the red area, the higher the content of residual elements, and the larger the blue area, the lower the content of residual elements.

[0100] The surface temperature of the steel plate was measured using a thermocouple (0.5 mmΦ, INC600, SENTECH ENG.). Specifically, a hole of 1.0 mmΦ was drilled in the steel plate (thickness: 2.0 mm, width direction Х rolling direction: 300 mm Х 120 mm) to the center of the specimen, and a thermocouple was inserted to measure the temperature of the steel plate before and after cooling. At this time, in the case of Comparative Examples 1 to 3, since no cooling composition was used, the temperature before and after cooling could not be measured.

[0101] The gloss of the steel plate was measured at an incident angle of 60° using a portable gloss meter (TQC-Sheen, model number: SH260C).

[0102] The surface roughness of the steel plate was measured using a 3D roughness meter (Leica, model number: DCM8).

[0103] The whiteness of the steel plate was measured using a whiteness meter (X-Rite Ci7860, X-Rite, USA).

[0104] The surface quality of steel sheets was evaluated by measuring the number of point defects that occurred on the surface of the steel sheet after plating, air wiping, spraying, and passing through a pass roll. Specifically, if there was less than 1 point defect per 1 m2 of the steel sheet, it was evaluated as good (○). If there were 1 or more but less than 10 point defects per 1 m2 of the steel sheet, it was evaluated as poor (△). If there were 10 or more point defects per 1 m2 of the steel sheet, it was evaluated as poor (Х).

[0105] Presence or absence of residual elements on the surface Surface temperature deviation (△T, ℃) Surface roughness (Ra) Gloss Whiteness (L) Surface Quality invention example 1 Х183.0±290±593±1○Invention example 2 Х163.0±290±593±1○Invention example 3 Х133.0±290±593±1○Invention example 4 Х193.0±290±593±1○Invention example 5 Х173.0±290±593±1○Invention example 6 Х133.0±290±593±1○Invention example 7 Х203.0±290±593±1○Invention example 8 Х183.0±290±593±1○Invention example 9 Х143.0±290±593±1○Invention example 10 Х203.0±290±593±1○Invention example 11 Х173.0±290±593±1○ Honor 12Х143.0±290±593±1○Invention Example 13Х203.0±290±593±1○Invention Example 14Х183.0±290±593±1○Invention Example 15Х143.0±290±593±1○Invention Example 16Х203.0±290±593±1○Invention Example 17Х183.0±290±593±1○Invention Example 18Х143.0±290±593±1○Invention Example 19Х173.0±290±593±1○Invention Example 20Х183.0±290±593±1○Invention Example 21Х193.0±290±593±1○Invention Example 22Х183.0±290±5 93±1○Invention Example 23Х163.0±290±593±1○Invention Example 24Х133.0±290±593±1○Invention Example 25Х193.0±290±593±1○Invention Example 26Х173.0±290±593±1○Invention Example 27Х133.0±290±593±1○Invention Example 28Х203.0±290±593±1○Invention Example 29Х183.0±290±593±1○Invention Example 30Х143.0±290±593±1○Invention Example 31Х203.0±290±593±1○Invention Example 31Х183.0±290±593±1○Invention Example 33Х143.0 ±290±593±1○Invention Example 34Х203.0±290±593±1○Invention Example 35Х183.0±290±593±1○Invention Example 36Х143.0±290±593±1○Invention Example 37Х203.0±290±593±1○Invention Example 38Х183.0±290±593±1○Invention Example 39Х143.0±290±593±1○Invention Example 40Х173.0±290±593±1○Invention Example 41Х183.0±290±593±1○Invention Example 42Х193.0±290±593±1○Comparative Example 1--6.0±260±592±1ХComparative Example 2--6.0±260±592±1XComparative Example 3--5.0±260±592±1XComparative Example 4-116.0±270±592±1△Comparative Example 5-85.0±270±592±1△Comparative Example 6-65.0±270±592±1△Comparative Example 7○193.0±2115±591±1△Comparative Example 8○173.0±2115±591±1△Comparative Example 9○133.0±2115±591±1△Comparative Example 10--6.0±260±592± 1XComparative Example 11--6.0±260±592±1XComparative Example 12--5.0±260±592±1XComparative Example 13-116.0±270±592±1△Comparative Example 14-85.0±270±592±1△Comparative Example 15-65.0±270±592±1△Comparative Example 16○193.0±2115±591±1△Comparative Example 17○173.0±2115±591±1△Comparative Example 18○133.0±2115±591±1△.

[0106] As shown in Tables 1 and 3 above, inventive examples 1 to 42, which all satisfy the manufacturing conditions proposed in the present invention, no residual elements (C, N, or O) were measured on the surface of the steel sheet. Specifically, Figs. 1 (a) and (b) are photographs showing the EPMA surface analysis results of inventive examples 10 and 16, which represent the inventive examples. Referring to Figs. 1 (a) and (b), it was confirmed that the area of ​​the blue portion was formed very wide, and thus no residual elements were measured on the surface of the steel sheet. It was found that this was because the elements were vaporized by the decomposition reaction and did not remain on the surface of the steel sheet.

[0107] That is, it was found that invention examples 1 to 42 did not form residues of urea or alcohol on the surface of the steel sheet even after cooling and drying. Accordingly, it was expected that no secondary deterioration of physical properties would occur even if further processing of the steel sheet was performed.

[0108] In addition, it was found that Invention Examples 1 to 42 exhibited a large temperature deviation on the surface, which increased the cooling rate of the molten plating layer, thereby promoting surface solidification of the steel sheet. Accordingly, the surface roughness, gloss, and whiteness of the steel sheet were also measured to be at appropriate levels, and it was found that no point defects were formed, resulting in excellent surface quality and a pleasing surface appearance.

[0109] Meanwhile, Comparative Examples 1 to 3 and Comparative Examples 10 to 12, in which only air was sprayed without the cooling composition, showed more than 10 point-like defects on the surface of the steel plate, indicating that the surface quality was very poor. This was because, since the cooling composition was not sprayed, solidification nuclei were not formed on the molten plating layer, and thus surface solidification was not promoted.

[0110] In addition, Comparative Examples 4 to 6 and Comparative Examples 13 to 15, in which only pure water was sprayed instead of a cooling composition, were measured to have a small temperature deviation on the surface, indicating that the effect of increasing the cooling rate of the molten plating layer was insufficient and that the effect of promoting surface solidification of the steel sheet was also insufficient. Accordingly, it was found that the surface quality was poor, as the number of point-like defects on the surface of the steel sheet was measured to be 1 to 10.

[0111] In addition, Comparative Examples 7 to 9 and Comparative Examples 16 to 18, in which a phosphate aqueous solution was sprayed instead of a cooling composition, residual elements (P) were measured on the surface of the steel plate. Specifically, Fig. 1 (c) is a photograph showing the EPMA surface analysis results of Comparative Example 7, representing the comparative example. Referring to Fig. 1 (c), it was confirmed that the area of ​​the blue portion was formed very narrowly, and residual elements (P) were measured on the surface of the steel plate.

[0112] That is, Comparative Examples 7 to 9 and Comparative Examples 16 to 18 showed that phosphate residues formed on the surface of the steel sheet after cooling and drying. Accordingly, it was found that the surface quality of the steel sheet was poor, as the number of point-like defects measured on the surface of the steel sheet was 1 to 10. In addition, it was expected that secondary deterioration of the physical properties of the steel sheet could also occur.

[0113] (4) Confirmation of the effect of increasing cooling speed according to the type of spray material

[0114] In order to confirm the effect of increasing the cooling rate for the invention examples 4, 10, 16, comparative examples 1, 4, and 7, a profile of the surface temperature of the steel sheets was performed. Specifically, for each steel sheet, the surface temperature of the steel sheet after immersion in the plating bath and air wiping was measured, and the surface temperature of the steel sheet after spraying and passing through a pass roll was measured, and a graph of the surface temperature versus time is shown in Fig. 2.

[0115] Referring to Fig. 2, in the case of Comparative Examples 1 and 4, the temperature deviation values ​​of the steel plate were measured to be very small, so it was found that the effect of increasing the cooling rate of the molten plating layer was insufficient, and thus the effect of promoting surface solidification of the steel plate was also insufficient.

[0116] In addition, in the case of Invention Examples 4, 10, 16, and Comparative Example 7, it was confirmed that the temperature deviation values ​​of the steel plates were measured to be similar values. That is, it was found that the cooling composition containing urea had an endothermic reactivity at almost the same level as that of a conventional phosphate aqueous solution.

[0117] (5) Confirmation of the effect of increasing cooling rate according to steel plate temperature in the spraying process

[0118] In order to confirm the effect of increasing the cooling rate for the above invention examples 10 to 12, a profile of the surface temperature of the steel sheet was performed. Specifically, for each steel sheet, the surface temperature of the steel sheet after immersion in the plating bath and air wiping was measured, and the surface temperature of the steel sheet after spraying and passing through a pass roll was measured. A graph of the surface temperature versus time is shown in Fig. 3.

[0119] Referring to Fig. 3, it was confirmed that the temperature deviation value of the steel plate of Invention Example 10 was measured to be the largest value, and the temperature deviation value of the steel plate of Invention Example 12 was measured to be the smallest value. In other words, it was found that when spraying aqueous solution droplets, the higher the surface temperature of the steel plate, the better the effect of increasing the cooling rate of the plating layer and promoting the surface solidification of the steel plate.

[0120] The above embodiments are merely examples and the present invention is not limited thereto. Anything that has substantially the same configuration and achieves the same operational effects as the technical concepts described in the claims of the present invention is included within the technical scope of the present invention.

Claims

1. 0.1 to 10.0 wt% of the element; and A composition for cooling a zinc alloy-plated steel sheet containing water as a residue.

2. In paragraph 1, The above cooling composition is a cooling composition for a zinc alloy-plated steel sheet further containing 0.1 to 10.0 wt% of alcohol.

3. In paragraph 2, A composition for cooling a zinc alloy-plated steel sheet, wherein the alcohol comprises at least one selected from the group consisting of ethanol, propanol, isopropanol, butanol and isobutanol.

4. Step for preparing the steel plate; A step of plating by immersing the base steel sheet in a Zn-based plating bath containing Al and Mg; A step for controlling the amount of plating adhesion of the above steel plate; and It comprises a step of cooling by spraying a cooling composition as droplets on the steel plate with the adjusted plating adhesion amount, A method for manufacturing a zinc alloy-plated steel sheet, wherein the cooling composition comprises 0.1 to 10.0 wt% of the element and the remainder water.

5. In paragraph 4, A method for manufacturing a zinc alloy-plated steel sheet, wherein the above cooling composition further contains 0.1 to 10.0 wt% of alcohol.

6. In paragraph 5, A method for manufacturing a zinc alloy-plated steel sheet, wherein the alcohol comprises at least one selected from the group consisting of ethanol, propanol, isopropanol, butanol, and isobutanol.

7. In paragraph 4, A method for manufacturing a zinc alloy-plated steel sheet, wherein, in the cooling step, air is sprayed onto the steel sheet at a pressure of 1.0 to 5.0 kgf / cm2.

8. In paragraph 7, A method for manufacturing a zinc alloy-plated steel sheet, wherein in the cooling step, the cooling composition is sprayed at a pressure of 0.5 to 2.5 kgf / cm2.

9. In paragraph 7, A method for manufacturing a zinc alloy-plated steel sheet, wherein in the cooling step, the injection pressure ratio of the cooling composition and the air is 1:0.3 to 0.

9.

10. In paragraph 4, A method for manufacturing a zinc alloy-plated steel sheet, wherein in the cooling step, the cooling composition is sprayed onto the steel sheet after passing through a charging electrode.

11. In Article 10, A method for manufacturing a zinc alloy-plated steel sheet, wherein a voltage of -5 to -50 kV is applied to the cooling composition from the above-mentioned electrode.

12. In paragraph 4, A method for manufacturing a zinc alloy-plated steel sheet, wherein in the cooling step, the temperature of the steel sheet is 350 to 460°C.

13. In paragraph 4, A method for manufacturing a zinc alloy-plated steel sheet, wherein the temperature of the plating bath is 430 to 500°C.

14. A zinc alloy-plated steel sheet manufactured by a method according to any one of clauses 4 to 13.

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