Composition for surface treatment of plated steel sheet, plated steel sheet surface-treated using the composition, and method for manufacturing the same

Abstract

The present invention relates to a composition for surface treatment of a steel sheet, and more particularly, to a trivalent chromium-containing surface treatment solution composition, a molten galvanized steel sheet surface-treated using the same, and a method of manufacturing the same.

Description

Technical Field

[0001] This invention relates to a composition for surface treatment of steel sheets, and more specifically, to a composition for surface treatment of coated steel sheets containing trivalent chromium, coated steel sheets surface treated using the composition, and a method for manufacturing the same. Background Technology

[0002] Hot-dip galvanized materials with a zinc (Zn) coating achieve excellent corrosion resistance by sacrificing the protective effect of the base iron. However, in general corrosive environments, especially when exposed to humid atmospheres, these hot-dip galvanized materials are prone to developing white rust, which is a zinc oxide, on the surface, thus deteriorating the material's quality characteristics. Furthermore, when hot-dip galvanized materials are exposed to high temperature and high humidity environments, they are prone to a blackening phenomenon, known as tarnishing.

[0003] To address this issue, hot-dip galvanized steel sheets have traditionally undergone hexavalent chromate treatment to ensure corrosion resistance and resistance to blackening. However, because hexavalent chromium is designated as an environmentally harmful substance, its use is currently subject to stricter regulations. Furthermore, the use of hexavalent chromium as a surface treatment agent for hot-dip galvanized steel sheets has resulted in defects such as the steel surface turning black or developing black spots.

[0004] In recent years, to address the environmental hazards of hexavalent chromium, methods are being applied to ensure the corrosion resistance and anti-blackening properties of galvanized steel sheets by coating them with a surface treatment solution composition containing trivalent chromium. For example, Patent Documents 1 and 2 employ a chemical conversion treatment method involving immersing the steel sheet in a composition containing trivalent chromium to ensure corrosion resistance and anti-blackening properties. However, this immersion time is long, making it unsuitable for continuous processes in steel companies, and the chemical conversion treatment method suffers from reduced fingerprint resistance.

[0005] Furthermore, Patent Documents 3 and 4 disclose methods for applying a composition containing trivalent chromium to galvanized steel sheets by spraying or roller coating. These methods can be applied to continuous production lines in steel companies and also help ensure fingerprint resistance. However, hot-dip galvanized steel sheets coated with the compositions provided in these documents suffer from a problem of rapid discoloration in humid atmospheres.

[0006] (Patent Document 1) Korean Patent Publication No. 10-2006-0123628 (Patent Document 2) Korean Patent Publication No. 10-2009-0024450 (Patent Document 3) Korean Patent Publication No. 10-2004-0046347 (Patent Document 4) Japanese Patent Publication No. 2002-069660 Summary of the Invention

[0007] (a) Technical problems to be solved One aspect of the present invention is to provide a composition for surface treatment of plated steel sheets, wherein the composition, as a composition for surface treatment of steel sheets, excludes hexavalent chromium and contains trivalent chromium and inorganic compounds as main components that are harmless to the human body.

[0008] Furthermore, a coated steel sheet surface-treated using a composition for coated steel sheet surface treatment according to one aspect of the present invention is provided, as well as a method for manufacturing the coated steel sheet.

[0009] Furthermore, the technical problem to be solved by the present invention is not limited to the technical problems mentioned above. Those skilled in the art to which the present invention pertains can clearly understand other technical problems not mentioned through the following description.

[0010] (II) Technical Solution According to one aspect of the present invention, a composition for surface treatment of coated steel sheet is provided, comprising: (a) a trivalent chromium compound combined with an organic acid: 15-45% by weight, (b) an antioxidant: 0.10-2.00% by weight, (c) a silane compound: 1.00-25.00% by weight, (d) a polysiloxane copolymer: 15-50% by weight, (e) a cobalt-based rust and corrosion inhibitor: 1.0-15.0% by weight, (f) a lubricant: 0.10-3.00% by weight, and (g) a defoamer: 0.10-2.00% by weight.

[0011] In one embodiment of the present invention, the trivalent chromium compound bound to the organic acid may be one or more selected from chromium acetylacetonate (III), chromium acetate (III), chromium 2-ethylhexanoate (III), chromium acetate hydroxide, and potassium chromium oxalate (III) trihydrate.

[0012] As described above, the composition for surface treatment of plated steel sheets contains a trivalent chromium compound. Furthermore, by using this trivalent chromium compound as a compound that combines with organic acids, problems such as surface etching of the plated steel sheet that may occur when using a composition containing an existing trivalent chromium compound for surface treatment can be prevented.

[0013] In one embodiment of the present invention, the composition for surface treatment of coated steel sheet may be a solution composition having a solid content of 5-20% by weight and containing the balance solvent, wherein the pH of the solution composition may be 4 to 6.

[0014] According to another aspect of the present invention, a surface-treated coated steel sheet is provided, comprising: a base steel sheet; a zinc-based coating formed on at least one side of the base steel sheet; and a surface-treated thin film layer formed on the coating.

[0015] In one embodiment of the invention, the surface treatment thin film layer formed on the coating can be formed by a composition according to one embodiment of the invention.

[0016] In one embodiment of the present invention, the surface treatment thin film layer formed on the coating may have a thickness of 0.3-1.5 μm.

[0017] According to another aspect of the present invention, a method for manufacturing a surface-treated coated steel sheet is provided, comprising the steps of: preparing a coated steel sheet on which a zinc-based coating is formed on at least one side of a base steel sheet; applying a surface treatment composition to the zinc-based coating; and drying the coated steel sheet with the surface treatment composition applied thereto.

[0018] In one embodiment of the invention, the step of coating the coating with a surface treatment composition can be performed using a solution composition according to one embodiment of the invention.

[0019] In one embodiment of the invention, the coating process can be performed by any one of bar coating, roller coating, spraying, dipping, jet extrusion and dipping extrusion.

[0020] In one embodiment of the invention, the drying step after coating is based on the final reached temperature (PMT) of the base steel plate and can be carried out in a temperature range of 40-200°C.

[0021] (III) Beneficial Effects According to the present invention, a composition for surface treatment of plated steel sheets composed of trivalent chromium compounds and inorganic compounds that are harmless to the human body can be provided.

[0022] In particular, when the composition provided in this invention is used to surface treat the plated steel sheet, the corrosion resistance and blackening resistance of the surface-treated plated steel sheet can be ensured, while the surface uniformity during coating and the appearance characteristics of the surface-treated plated steel sheet can also be excellently ensured. Best practice

[0023] Hereinafter, preferred embodiments of the present invention will be described with reference to various implementation schemes. However, the embodiments of the present invention can be modified into various other embodiments, and the scope of the present invention is not limited to the embodiments described below.

[0024] The present invention provides a composition for surface treatment of coated steel sheet containing trivalent chromium, a coated steel sheet surface treated using the composition, and a method for manufacturing the steel sheet. The present invention will be specifically described below.

[0025] According to one aspect of the present invention, a composition for surface treatment of coated steel sheet is provided, comprising: (a) a trivalent chromium compound combined with an organic acid: 15-45% by weight, (b) an antioxidant: 0.10-2.00% by weight, (c) a silane compound: 1.00-25.00% by weight, (d) a polysiloxane copolymer: 15-50% by weight, (e) a cobalt-based rust and corrosion inhibitor: 1.0-15.0% by weight, (f) a lubricant: 0.10-3.00% by weight, and (g) a defoamer: 0.10-2.00% by weight.

[0026] This composition for surface treatment of steel plates according to one aspect of the present invention contains trivalent chromium, which is harmless to the human body, as the main component, and does not contain hexavalent chromium, which is harmful to the environment. Therefore, it can prevent problems such as harm to the human body and environmental pollution. Moreover, when the coated steel plate is surface treated using the composition for surface treatment according to one aspect of the present invention, the surface-treated coated steel plate can have excellent corrosion resistance, and when used for coating for surface treatment, it can also ensure surface uniformity and appearance characteristics.

[0027] The components constituting a composition for surface treatment of coated steel sheet according to one aspect of the present invention will be described in detail below. It is hereby clarified that the content of each component shown below is expressed on a basis of 100% by weight.

[0028] (a) Trivalent chromium compounds bound to organic acids: 15-45% by weight Compositions containing trivalent chromium compounds as the main component of a surface treatment for clad steel sheets play an advantageous role in ensuring corrosion resistance and resistance to blackening when clad steel sheets are surface treated with such compositions.

[0029] In one embodiment of the present invention, the trivalent chromium compound is a compound bound to an organic acid. Any trivalent chromium compound bound to an organic acid can be used, and therefore its type is not particularly limited. However, among the trivalent chromium compounds bound to organic acids, those with a dissolved pH of 4 or higher can be trivalent chromium compounds. As a non-limiting example, it can be one or more selected from chromium acetylacetonate (III), chromium acetate (III), chromium 2-ethylhexanoate (III), chromium acetate hydroxide, and potassium chromium (III) oxalate trihydrate.

[0030] When chromium compounds such as chromium phosphate and chromium nitrate are used as chromium compounds in compositions for surface treatment of coated steel sheets, the pH of the solution composition becomes strongly acidic when the composition containing such chromium compounds is dissolved in a solvent (e.g., water). The inventors of this invention have discovered that when this solution composition is used to coat (spray) the surface of coated steel sheets (e.g., hot-dip galvanized steel sheets), the surface is etched, resulting in streak-like defects on the surface of the surface-treated coated steel sheet, thus deteriorating its appearance. The technical significance of the composition according to one embodiment of the present invention lies in the fact that the chromium compound includes a trivalent chromium compound that combines with an organic acid, rather than the previously mainly used compounds such as chromium phosphate and chromium nitrate, which lower the pH of the solution composition.

[0031] In one embodiment of the invention, the content of the trivalent chromium compound bound to the organic acid can be 15-45% by weight relative to 100% of the total composition. When the content of the trivalent chromium compound is less than 15% by weight, the robust, insoluble film layer becomes thinner when the composition is applied to the surface of the coated steel sheet, making it unable to effectively block moisture penetration on the surface of the coated steel sheet requiring corrosion resistance, thereby causing blackening and reducing corrosion resistance. Furthermore, when the content of the trivalent chromium compound is greater than 45% by weight, the content of corrosion-resistant agents or silane compounds acting as binders added to improve corrosion resistance is relatively reduced, which may make it difficult to ensure the corrosion resistance and blackening resistance of the surface-treated coated steel sheet.

[0032] (b) Antioxidants: 0.10-2.00% by weight In one embodiment of the invention, when an acidic solution composition is applied to the surface of a plated steel sheet, an antioxidant may be added to prevent spot-like uneven coloring caused by oxidation of the plated steel sheet surface before the solvent evaporates and dries.

[0033] In one embodiment of the present invention, the antioxidant content can be 0.10-2.00% by weight relative to 100% of the total composition. When the antioxidant content is less than 0.10% by weight, the above-mentioned effects cannot be sufficiently ensured, and the surface uniformity of the surface-treated coated steel sheet may deteriorate. On the other hand, when the antioxidant content is greater than 2.00% by weight, the stability of the solution composition deteriorates, and the corrosion resistance of the surface-treated coated steel sheet may deteriorate.

[0034] In one embodiment of the invention, the antioxidant may be a triazole derivative. As a non-limiting example, the triazole derivative may be one or more selected from 1,3,4-thiadiazolidine-2,5-dithione, 1,2,3-triazoles, 3-amino-1,2,4-triazole, and 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole.

[0035] (c) Silane compounds: 1.00-25.00% by weight In one embodiment of the invention, a silane compound, namely a silane coupling agent, promotes the drying of the composition by crosslinking the inorganic and organic components, and the silane compound can be added to ensure high corrosion resistance of the thin film layer.

[0036] In one embodiment of the present invention, the content of the silane compound relative to 100% by weight of the total composition can be 1.00-25.00% by weight. When the content of the silane compound is less than 1.00% by weight, the corrosion resistance and resistance to foreign matter adhesion of the film layer may deteriorate. On the other hand, when the content of the silane compound is greater than 25.00% by weight, the dryness of the film layer increases, forming a film with excessively high hardness. When the coated steel sheet having such a film layer is processed, the corrosion resistance and resistance to blackening of the processed part will deteriorate.

[0037] In one embodiment of the invention, the silane compound may be any compound known as a silane coupling agent, and therefore its type is not particularly limited. However, as a non-limiting example, it may be selected from one or more of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-epoxypropoxypropyltrimethoxysilane, 3-epoxypropoxypropylmethyldiethoxysilane, 3-epoxypropoxypropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldiethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-ureopropyltrimethoxysilane, and tetraethyl orthosilicate.

[0038] (d) Polysiloxane copolymers: 15-50% by weight In one embodiment of the invention, a polysiloxane copolymer forms a dense film on the surface of a surface-treated steel sheet using an organic-inorganic bond, imparting flexibility to the film and thereby improving the corrosion resistance of the flat and processed portions of the surface-treated steel sheet. Furthermore, the polysiloxane copolymer prevents the penetration of corrosive agents, not only improving the blackening resistance of the surface-treated steel sheet but also enhancing processability by forming a flexible film.

[0039] In one embodiment of the invention, the content of the polysiloxane copolymer relative to 100% by weight of the total composition can be 15-50% by weight. When the content of the polysiloxane copolymer is less than 15% by weight, the corrosion resistance of the flat and processed portions of the surface-treated coated steel sheet cannot be guaranteed, and the resistance to blackening and processability will also deteriorate. On the other hand, when the content of the polysiloxane copolymer is greater than 50% by weight, the alkali resistance of the film layer may deteriorate.

[0040] In one embodiment of the invention, the polysiloxane copolymer may be a copolymer comprising one or more compounds selected from groups i) and ii) below and an acid catalyst with a molecular weight between 300 and 1500.

[0041] i) A siloxane compound selected from one or more of polydimethylsiloxane, polyvinylsiloxane, polyphenylmethylsiloxane, and hexamethylsiloxane. ii) One or more silane compounds selected from methyltrimethoxysilane, ethyltrimethoxysilane, hexamethyldisilane, ethoxytriethylsilane, vinyltrimethoxysilane, vinyltriethoxysilane, γ-epoxypropoxytriethylsilane, and γ-epoxypropoxytrimethylsilane. iii) One or more acid catalysts selected from phosphoric acid and organic phosphoric acid, oxalic acid, citric acid, and formic acid. (e) Cobalt-based rust and corrosion inhibitors: 1.0-15.0% by weight In one embodiment of the present invention, the cobalt-based rust and corrosion inhibitor acts to shield the coating cracks that occur during processing of the surface-treated steel sheet using the surface-treatment composition. When the surface-treated steel sheet is surface-treated using a surface-treatment composition containing this cobalt-based rust and corrosion inhibitor, the corrosion resistance of the processed portion can be improved.

[0042] In one embodiment of the present invention, the content of the cobalt-based rust and corrosion inhibitor can be 1.0-15.0% by weight relative to 100% of the total composition. When the content of the cobalt-based rust and corrosion inhibitor is less than 1.0% by weight, it is difficult to fully obtain the above-mentioned effects, and excessive cracking in the coating during the processing of surface-treated coated steel sheets can lead to a decrease in corrosion resistance. On the other hand, when the content of the cobalt-based rust and corrosion inhibitor is greater than 15.0% by weight, there is a problem of decreased resistance to blackening and alkali resistance of the thin film layer.

[0043] In one embodiment of the present invention, the cobalt-based rust and corrosion inhibitor may be one or more selected from cobalt(II) nitrate, cobalt(II) sulfate, cobalt(II) acetate, cobalt(II) oxalate, cobalt(III) nitrate, cobalt(III) acetate, cobalt(III) oxalate, cobalt(IV) chloride, cobalt(III) oxide, and cobalt(IV) oxide.

[0044] (f) Lubricant: 0.10-3.00% by weight In one embodiment of the invention, the lubricant enhances the lubricity of the solution composition prepared by dissolving the surface treatment composition in a solvent. Specifically, in one embodiment of the invention, it prevents the viscosity of the organosilane compounds contained in the composition from increasing under high temperature and high humidity conditions, thus preventing the coated steel sheet from adhering to the roller conveyor and generating foreign matter as it moves along the roller conveyor.

[0045] In one embodiment of the present invention, the lubricant content can be 0.10-3.00% by weight relative to 100% of the total composition. When the lubricant content is less than 0.10% by weight, it is difficult to fully obtain the above-mentioned effects, and the adhesion of the composition to the roller conveyor is aggravated under high temperature and high humidity conditions, which may lead to excessive foreign matter defects. On the other hand, when the lubricant content is greater than 3.00% by weight, the shielding effect of the film layer is insufficient, resulting in a decrease in the corrosion resistance of the coated steel sheet.

[0046] In one embodiment of the invention, the lubricant may be a polyethylene wax dispersed with a nonionic dispersant, suitable for compositions containing trivalent chromium compounds as the main component and exhibiting acidity.

[0047] (g) Defoamer: 0.10-2.00% by weight In one embodiment of the invention, an antifoaming agent may be included to prevent the coating properties of the solution composition from decreasing due to the generation of air bubbles in the solution during stirring when dissolving the surface treatment composition in a solvent to prepare the solution composition.

[0048] In one embodiment of the present invention, the content of the defoamer relative to 100% by weight of the total composition can be 0.10-2.00% by weight. When the content of the defoamer is less than 0.10% by weight, the above-mentioned effect cannot be sufficiently ensured, and when the solution composition is applied to the surface of the coated steel sheet, not only does the workability decrease, but physical properties such as corrosion resistance may also decrease. On the other hand, when the content of the defoamer is greater than 2.00% by weight, the solution stability of the composition decreases, and surface defects may occur when coating is performed after the thin film layer is formed by the solution composition.

[0049] In one embodiment of the present invention, an organosilicone defoamer may be used as the defoamer.

[0050] Furthermore, according to one embodiment of the present invention, the solid content of the surface treatment composition may be 5-20% by weight, with the balance being a solvent. That is, the surface treatment composition in solid form can be dissolved in a solvent to obtain a solution composition.

[0051] In one embodiment of the invention, water can be used as the solvent, which can be used to dilute the components added to the surface treatment composition of one embodiment of the invention. Herein, water refers to deionized water or distilled water.

[0052] In one embodiment of the invention, the solvent is added as the balance in addition to the solids content of the composition, and its content can be 80-95% by weight. When the solvent content is less than 80% by weight, the spreadability may be insufficient when the solution composition is coated onto the plated steel sheet. On the other hand, when the solvent content is greater than 95%, the adhesion of the film layer obtained after coating and drying the solution composition may not be guaranteed.

[0053] The solution composition obtained by dissolving the surface treatment composition according to one embodiment of the present invention in the above solvent can have a pH range of 4 to 6.

[0054] As described above, the surface treatment composition according to one embodiment of the present invention contains a trivalent chromium compound as the main component, and uses a compound combined with an organic acid as this trivalent chromium compound, thus exhibiting a higher pH value compared to compositions containing existing trivalent chromium compounds. In this way, by increasing the pH of the solution composition used for surface treatment of coated steel sheets, problems such as surface appearance arising when using existing low-pH solution compositions for surface treatment of coated steel sheets can be fundamentally solved.

[0055] In one embodiment of the invention, when the pH of the solution composition is less than 4, the surface of the surface-treated coated steel sheet is etched, resulting in a deteriorated surface appearance, which may worsen the corrosion resistance and blackening resistance of the coated steel sheet. On the other hand, when the pH is greater than 6, the solution stability decreases, and the corrosion resistance and blackening resistance of the processed parts of the surface-treated coated steel sheet may worsen.

[0056] The coated steel sheet according to another aspect of the present invention will now be described in detail.

[0057] According to one aspect of the invention, a surface-treated coated steel sheet may include: a base steel sheet; a zinc-based coating formed on at least one side of the steel sheet; and a surface-treated thin film layer formed on the coating.

[0058] In one embodiment of the present invention, any steel plate can be used as long as it can be obtained by plating, and therefore the type is not particularly limited. As a non-limiting example, it can be carbon steel containing a certain amount of carbon (C), manganese (Mn), silicon (Si), phosphorus (P), sulfur (S), etc., which is well known in the art, and therefore the composition of alloying elements is not particularly limited.

[0059] In one embodiment of the invention, a zinc-based coating may be included on at least one side of the base steel plate. The zinc-based coating contains zinc (Zn) as the main component, for example, a coating containing 50% by weight or more of Zn. Furthermore, the coating may be a zinc-based alloy coating that, in addition to zinc (Zn), further contains one or more elements such as aluminum (Al), magnesium (Mg), and silicon (Si). This coating can be applied not only to one side of the base steel plate, but also to both sides.

[0060] In one embodiment of the present invention, a surface treatment thin film layer may be included on the zinc-based coating. That is, a thin film layer of a certain thickness may be formed on the zinc-based coating. Thus, the plated steel sheet with a surface treatment thin film layer on the zinc-based coating can have excellent physical properties such as corrosion resistance, blackening resistance, alkali resistance, and chemical resistance.

[0061] In one embodiment of the present invention, in order to obtain a coated steel sheet having the above-mentioned physical properties, the surface treatment thin film layer disposed on the zinc-based coating can be formed by a surface treatment composition for coated steel sheets according to one embodiment of the present invention. That is, the thin film layer formed by the composition according to one embodiment of the present invention contains a trivalent chromium compound combined with an organic acid as the main component, and includes antioxidants, silane compounds, corrosion inhibitors, copolymers, lubricants, defoamers, etc. in appropriate amounts, thereby providing a coated steel sheet having the target physical properties.

[0062] In one embodiment of the present invention, the surface-treated thin film layer can have a thickness of 0.3-1.5 μm. This thickness is based on the thickness after drying. When the thickness of the thin film layer is less than 0.3 μm, the solution composition coating on the roughness protrusions of the coated steel sheet is too thin, resulting in poor corrosion resistance. On the other hand, when the thickness of the thin film layer is greater than 1.5 μm, the thin film layer is formed too thick, leading to poor processability and increased processing costs of the solution composition, which is economically disadvantageous.

[0063] The following describes a method for manufacturing a surface-treated coated steel sheet according to another aspect of the present invention. However, it is to be clear that the following manufacturing method is merely an example for manufacturing a surface-treated coated steel sheet.

[0064] In one embodiment of the present invention, the surface-treated galvanized steel sheet can be manufactured by the following steps: preparing a galvanized steel sheet having a zinc-based coating formed on at least one side of a base steel sheet; applying a surface-treatment composition to the zinc-based coating of the galvanized steel sheet; and drying the galvanized steel sheet after the coating.

[0065] In one embodiment of the invention, any steel sheet can be used for the base steel sheet on which a zinc-based coating is formed on at least one side, as long as it can be manufactured by a coating process; therefore, its type is not particularly limited. Furthermore, the aforementioned base steel sheet can be used instead.

[0066] In one embodiment of the present invention, a zinc-based coating is a coating containing zinc (Zn) as the main component, without excluding the addition of other elements besides zinc. As an example, the elements other than zinc may be one or more selected from aluminum (Al), magnesium (Mg), and silicon (Si). A coating containing these elements mixed with zinc can be called a zinc-based alloy coating. Furthermore, the aforementioned content regarding zinc-based coatings can be used instead.

[0067] In one embodiment of the invention, the step of coating the zinc-based coating with a surface treatment composition can be performed using any coating method selected from bar coating, roller coating, spray coating, dip coating, jet extrusion and dip extrusion.

[0068] In one embodiment of the present invention, the surface treatment composition coated on a zinc-based plating may be a surface treatment composition for plated steel sheet according to an embodiment of the present invention. In this case, the composition may be a solution composition, and as an example of such a solution composition, it may be prepared by mixing a solvent into the surface treatment composition at 5-20% by weight based on the solids content. Here, the solvent may be water such as deionized water or distilled water.

[0069] In one embodiment of the invention, the solution composition applied to the zinc-based coating may contain a trivalent chromium compound bound to an organic acid as the main component, thereby allowing the pH of the solution composition to be in the range of 4 to 6. Thus, by using a solution composition with a specific pH range to surface-treat the plated steel sheet, surface appearance defects that may occur when using existing strongly acidic, i.e., low-pH solution compositions to surface-treat the plated steel sheet can be prevented.

[0070] When coating the above-mentioned solution composition, as an example, the coating thickness can be set to the range of 2.5-12.5 μm. By coating the solution composition with the thickness described above, and then performing a subsequent drying process, a thin film layer with a thickness of 0.3-1.5 μm can be obtained. Here, the thickness of the thin film layer refers to the thickness after drying. In one embodiment of the present invention, when the coating thickness of the solution composition is less than 2.5 μm, the composition coating on the roughness of the protrusions of the coating is too thin, which may lead to a decrease in corrosion resistance. On the other hand, when the coating thickness is greater than 12.5 μm, the dried thin film layer is too thick, and physical properties such as weldability and processability may deteriorate.

[0071] In one embodiment of the present invention, the drying step of the coated steel sheet with the surface treatment composition can be performed within a temperature range of 40-200°C, based on the final reached temperature (Peak Metal Temperature, PMT) of the base steel sheet. When the drying temperature is below 40°C based on the PMT, drying cannot be completed completely, and the physical properties of the surface-treated coated steel sheet, such as corrosion resistance and alkali resistance, may deteriorate. On the other hand, when the drying temperature is above 200°C based on the PMT, the surface-treated coated steel sheet cannot be sufficiently cooled during the air cooling process (air cooling process) after drying. In this case, if the coated steel sheet is packaged or otherwise not fully cooled, condensation will occur, ultimately leading to a decrease in resistance to blackening.

[0072] In another embodiment of the invention, the drying process can be carried out in a hot air drying oven or an induction heating furnace. As an example, when drying a solution composition coated on a plated steel sheet using a hot air drying oven, the internal temperature of the hot air drying oven can be 100-300°C. As another example, when drying a surface-treated solution composition using an induction heating furnace, the current applied to the induction heating furnace can be 1000-5000A.

[0073] When the internal temperature of the hot air drying oven is less than 100°C or the current applied to the induction heating furnace is less than 1000A, the drying of the surface-treated solution composition cannot be completed, and the physical properties of the surface-treated coated steel sheet, such as its resistance to alkali and pipe-making oil corrosion, will deteriorate. Furthermore, when the internal temperature of the hot air drying oven is higher than 200°C or the current applied to the induction heating furnace is greater than 5000A, during the air cooling process (air cooling), the surface-treated coated steel sheet may be packaged in a state of insufficient cooling. In this case, condensation will lead to a decrease in resistance to blackening.

[0074] In one embodiment of the present invention, a step of cooling the galvanized steel sheet after the drying process may be further included. In this case, the cooling may be the aforementioned air cooling process or a water cooling process that can be given a certain cooling rate as needed.

[0075] In one embodiment of the present invention, when the cooling process is a water cooling process, the cooling rate is not particularly limited. It is hereby clarified that those skilled in the art can set it appropriately according to the cooling equipment and the desired thickness of the dried film layer.

[0076] In one embodiment of the invention, the process of manufacturing the surface-treated coated steel sheet can be carried out by a continuous process. As an example, the speed of the continuous process can be 50-120 meters per minute (mpm). When the speed of the continuous process is less than 50 m / min, the productivity may decrease. On the other hand, when the speed of the continuous process is greater than 120 m / min, the solution-state composition coated on the surface of the coated steel sheet may scatter during the drying process, potentially causing surface defects.

[0077] The present invention will now be described in more detail through embodiments. However, these embodiments are merely illustrative of implementation of the invention, and the invention is not limited to these embodiments. The scope of the invention is determined by the matters set forth in the claims and those reasonably deduced therefrom. Detailed Implementation

[0078] (Example) [Basic Composition of the Surface Treatment Composition] Based on 100% by weight of the composition, a surface treatment composition for coated steel sheets was prepared, comprising 37% by weight of a trivalent chromium compound bound to an organic acid, 1.00% by weight of an antioxidant, 12.00% by weight of a silane coupling agent, 40% by weight of a polysiloxane copolymer, 7% by weight of a cobalt-based rust and corrosion inhibitor, 2.00% by weight of a lubricant, and 1.00% by weight of a defoamer. The composition was prepared in a solid state.

[0079] Next, 14% by weight of the solids of the composition prepared above is prepared, and then solvent (water) is added to it until the total reaches 100% by weight to prepare a solution composition, the pH of which is adjusted to 5.

[0080] In the composition, chromium acetate (III) is used as the trivalent chromium compound bound to the organic acid, 1,2,3-triazole is used as the antioxidant, tetraethyl orthosilicate is used as the silane coupling agent, and a polysiloxane copolymer with a molecular weight of 650 synthesized from polyvinylsiloxane, vinyltrimethoxysilane, and phosphoric acid as an acid catalyst is used. Furthermore, cobalt nitrate (II) is used as the cobalt-based rust and corrosion inhibitor, polyethylene wax is used as the lubricant, and an organosilicon defoamer is used.

[0081] [Preparation criteria for test specimens] Hot-dip galvanized steel sheets with a zinc coating (pure zinc coating: composed of Zn and other unavoidable impurities) were cut into 7cm (transverse) × 15cm (longitudinal) dimensions and grease was removed. The prepared composition was then sprayed onto the cut hot-dip galvanized steel sheet surface through a nozzle for coating, and then cured at a PMT standard of 60±20℃ for the base steel sheet to produce test specimens.

[0082] [Experimental and Evaluation Methods] For the test specimens prepared according to the above content, the corrosion resistance of the plate, the corrosion resistance of the processed part, the resistance to blackening, the alkali resistance, the resistance to foreign matter adhesion, the solution stability and the surface uniformity were evaluated by the following methods, and the results are shown in the following tables.

[0083] <Plate Corrosion Resistance> Following the method specified in ASTM B117, the rate of white rust formation on the coated steel sheet was measured over time after the specimens were treated. The evaluation criteria are as follows.

[0084] ◎: The time elapsed until white rust appears is more than 144 hours. ○: The time elapsed until white rust appears is more than 96 hours but less than 144 hours. △: The time elapsed until white rust appears is more than 55 hours but less than 96 hours. ×: The time elapsed until white rust appears is less than 55 hours. <Corrosion Resistance of Machining Sections> The specimen was pushed to a height of 6 mm using an Erichsen tester, and the degree of white rust formation was measured after 24 hours. The evaluation criteria at this point are as follows.

[0085] ◎: The area of ​​white rust that appears after 48 hours is less than 5%. △: The area of ​​white rust that appears after 48 hours is more than 5% but less than 7%. ×: The area of ​​white rust that appeared after 48 hours was over 7%. <Blackening Resistance> The test piece was placed in a constant temperature and humidity chamber at 50°C and 95% relative humidity for 120 hours, and the color change of the test piece before and after the test (color difference: ΔE) was observed. The evaluation criteria are as follows.

[0086] ◎:ΔE≤2 ○:2<ΔE≤3 △:3<ΔE≤4 ×:ΔE>4 <Alkali resistance> After immersing the test pieces in an alkaline degreasing solution at 60°C for 2 minutes, they were washed with water and air-blown. The color difference (ΔE) before and after immersion was then measured. The alkaline degreasing solution used was Finecleaner L 4460A (20 g / 2.4 L) + L 4460 B (12 g / 2.4 L) from DAEHAN PARKERIZING (pH=12). The evaluation criteria are as follows.

[0087] ◎:ΔE≤2 ○:2<ΔE≤3 △:3<ΔE≤4 ×:ΔE>4 <Anti-foreign body adhesion> With a 2.5 kg load applied to a metal probe covered with white gauze, the test piece was rubbed, and the change in whiteness of the gauze before and after the test (whiteness color difference: ΔL) was observed. The evaluation criteria are as follows.

[0088] ◎:ΔE≤1.0 ○: 1.0 < ΔE ≤ 2.0 △: 2.0 < ΔL ≤ 2.5 ×:ΔL>2.5 Solution stability Each surface treatment composition was placed in a container and stored in a 50°C constant temperature oven for 7 days. The presence of precipitates was then visually observed, and viscosity changes were measured. The evaluation criteria at this point are as follows.

[0089] ○: No precipitation occurred, and the viscosity change was less than 1 CP. △: No precipitation occurred, viscosity changed by 1-5 CP ×: Precipitation or viscosity change greater than 5 CP occurs. <Surface Uniformity> Visually inspect the surface of the test piece for defects such as streaks and / or blotches, and evaluate them. The evaluation criteria are as follows.

[0090] ○ (Good): No defects in surface streaks and / or blotches were observed. × (Defect): Defects observed in surface streaks and / or blotches. Experimental Example 1. Based on the changes in physical properties caused by the content of trivalent chromium compounds. The types of components used to prepare the surface treatment compositions are based on the above-mentioned baseline composition, and the content of each component is as shown in Table 1 below, to prepare each composition. Then, solvent (water) is added to 14% by weight of the solids of each composition until the total reaches 100% by weight, to prepare solution compositions with a pH of 5 respectively.

[0091] Using the various solution compositions, test specimens were prepared according to the above description, and their physical properties were evaluated. The results are shown in Table 1 below.

[0092] [Table 1] As shown in Table 1, when a trivalent chromium compound combined with an organic acid is used as the trivalent chromium compound, and the surface is treated with a composition containing each component according to an embodiment of the present invention (Examples 1 to 3 of the invention), good or better results are shown in all physical properties.

[0093] On the other hand, in Comparative Example 1, where the content of trivalent chromium compounds was insufficient, the corrosion resistance of the plate, the corrosion resistance of the processed part, the resistance to blackening, and the surface uniformity showed poor results. In Comparative Example 2, where the content of trivalent chromium compounds was excessive, the corrosion resistance of the plate, the corrosion resistance of the processed part, and the resistance to blackening also showed poor results.

[0094] Experimental Example 2. Based on the physical property changes of the pH of the solution composition. The composition for surface treatment was prepared based on the above-described baseline composition, with the types and amounts of each component. Then, solvent (water) was added to 14% by weight of the solids in the composition until the total reached 100% by weight to prepare a solution composition. The pH of the solution composition was then adjusted using an aqueous solution of nitric acid and sodium hydroxide. At this point, solution compositions with the pH values ​​shown in Table 2 below were prepared.

[0095] Using the various solution compositions, test specimens were prepared according to the above description, and their physical properties were evaluated. The results are shown in Table 2 below.

[0096] [Table 2] As shown in Table 2, when surface treatment is performed with a composition whose pH meets the range proposed in one embodiment of the present invention (Examples 4 to 6 of the invention), good or better results are shown in all physical properties.

[0097] On the other hand, in Comparative Example 3 where the pH of the solution composition was too low, the plate corrosion resistance, the corrosion resistance of the processed part, the blackening resistance and the surface uniformity showed poor results. In Comparative Example 4 where the pH of the solution composition was too high, the processed part corrosion resistance, the solution stability and the surface uniformity also showed poor results.

[0098] Experimental Example 3. Based on the changes in physical properties caused by the content of antioxidants. The types of components used to prepare the surface treatment compositions are based on the above-mentioned baseline composition, and the content of each component is as shown in Table 3 below, to prepare each composition. Then, solvent (water) is added to 14% by weight of the solids of each composition until the total reaches 100% by weight, to prepare solution compositions with a pH of 5 respectively.

[0099] Using the various solution compositions, test specimens were prepared according to the above description, and their physical properties of corrosion resistance, solution stability, and surface uniformity were evaluated. The results are shown in Table 3 below.

[0100] [Table 3] As shown in Table 3, when surface treatment was performed with a composition containing antioxidants at contents that meet the range proposed in one embodiment of the present invention (Examples 7 to 10 of the invention), good or better results were observed in all physical properties.

[0101] On the other hand, in Comparative Examples 5 and 6 where the antioxidant content was insufficient, the surface uniformity showed poor results, while in Comparative Example 7 where the antioxidant content was excessive, the plate corrosion resistance, the corrosion resistance of the processed part, and the solution stability showed poor results.

[0102] Experimental Example 4. Based on the changes in physical properties caused by the content of silane compounds (silane coupling agents). The types of each component used to prepare the surface treatment composition are based on the above-mentioned baseline composition, and the content of each component is as shown in Table 4 below, to prepare each composition. Then, solvent (water) is added to 14% by weight of the solids of each composition until the total reaches 100% by weight, to prepare solution compositions with a pH of 5 respectively.

[0103] Using the various solution compositions, test specimens were prepared according to the above description, and the physical properties of corrosion resistance, blackening resistance, and foreign matter adhesion were evaluated. The results are shown in Table 4 below.

[0104] [Table 4] As shown in Table 4, when surface treatment is performed with a composition containing silane coupling agent at contents that meet the range proposed in one embodiment of the present invention (Examples 11 to 14 of the invention), good or better results are shown in all physical properties.

[0105] On the other hand, in Comparative Example 8, where the content of silane coupling agent was insufficient, the corrosion resistance of the plate, the corrosion resistance of the processed part, and the resistance to foreign matter adhesion showed poor results. In Comparative Example 9, where the content of silane coupling agent was excessive, the corrosion resistance of the processed part and the resistance to blackening showed poor results.

[0106] Experimental Example 5. Based on the changes in physical properties caused by the content of polysiloxane copolymers. The types of components used to prepare the surface treatment compositions are based on the above-mentioned baseline composition, and the content of each component is as shown in Table 5 below, to prepare each composition. Then, solvent (water) is added to 14% by weight of the solids of each composition until the total reaches 100% by weight, to prepare solution compositions with a pH of 5 respectively.

[0107] Using the various solution compositions, test specimens were prepared according to the above description, and their physical properties of corrosion resistance, blackening resistance, and alkali resistance were evaluated. The results are shown in Table 5 below.

[0108] [Table 5] As shown in Table 5, when surface treatment is performed with a composition containing the polysiloxane copolymer in amounts that meet the ranges proposed in one embodiment of the present invention (Examples 15 to 17 of the invention), good or better results are observed in all physical properties.

[0109] On the other hand, in Comparative Example 10, where the content of polysiloxane copolymer was insufficient, the corrosion resistance of the flat plate, the corrosion resistance of the processed part, and the resistance to blackening showed poor results. In Comparative Example 11, where the content of polysiloxane copolymer was excessive, the physical properties of alkali resistance showed poor results.

[0110] Experimental Example 6. Based on the changes in physical properties of cobalt-based rust and corrosion inhibitors with varying content. The types of components used to prepare the surface treatment compositions are based on the above-mentioned baseline composition, and the content of each component is as shown in Table 6 below, to prepare each composition. Then, solvent (water) is added to 14% by weight of the solids of each composition until the total reaches 100% by weight, to prepare solution compositions with a pH of 5 respectively.

[0111] Using the various solution compositions, test specimens were prepared according to the above description, and their physical properties of corrosion resistance, blackening resistance, and alkali resistance were evaluated. The results are shown in Table 6 below.

[0112] [Table 6] As shown in Table 6, when surface treatment is performed with a composition containing cobalt-based rust and corrosion inhibitors at contents that meet the range proposed in one embodiment of the present invention (Examples 18 to 20 of the invention), good or better results are observed in all physical properties.

[0113] On the other hand, in Comparative Example 12, where the content of cobalt-based rust and corrosion inhibitor was insufficient, the corrosion resistance of the flat plate and the processed parts showed poor results. In addition, in Comparative Example 13, where a large amount of cobalt-based rust and corrosion inhibitor was added, the alkali resistance deteriorated. In Comparative Example 14, where the content of cobalt-based rust and corrosion inhibitor was seriously excessive, not only the alkali resistance but also the resistance to blackening showed poor results.

[0114] Experimental Example 7. Based on the changes in physical properties of lubricant content. The types of each component used to prepare the surface treatment composition are based on the above-mentioned baseline composition, and the content of each component is as shown in Table 7 below, to prepare each composition. Then, solvent (water) is added to 14% by weight of the solids of each composition until the total reaches 100% by weight, to prepare solution compositions with a pH of 5 respectively.

[0115] Using the various solution compositions, test specimens were prepared according to the above description, and the physical properties of corrosion resistance, blackening resistance, and foreign matter adhesion were evaluated. The results are shown in Table 7 below.

[0116] [Table 7] As shown in Table 7, when surface treatment is performed with a composition containing lubricant components at contents that meet the range proposed in one embodiment of the present invention (Examples 21 to 24 of the invention), good or better results are observed in all physical properties.

[0117] On the other hand, in Comparative Examples 15 and 16 where the lubricant content was insufficient, the resistance to foreign matter adhesion showed poor results, while in Comparative Example 17 where the lubricant content was excessive, the corrosion resistance of the plate and the corrosion resistance of the machined part showed poor results.

[0118] Experimental Example 8. Based on the changes in physical properties caused by the content of defoamer The types of each component used to prepare the surface treatment composition are based on the above-mentioned baseline composition, and the content of each component is as shown in Table 8 below, to prepare each composition. Then, solvent (water) is added to 14% by weight of the solids of each composition until the total reaches 100% by weight, to prepare solution compositions with a pH of 5 respectively.

[0119] Using the various solution compositions, test specimens were prepared according to the above description, and the physical properties of corrosion resistance, blackening resistance, and solution stability were evaluated. The results are shown in Table 8 below.

[0120] [Table 8] As shown in Table 8, when surface treatment was performed with a composition containing the defoamer at the range set forth in one embodiment of the present invention (Examples 25 to 28 of the invention), good or better results were observed in all physical properties.

[0121] On the other hand, in Comparative Example 18, where no defoamer was added, the plate corrosion resistance, the corrosion resistance of the processed area, and the resistance to blackening showed poor results. In Comparative Example 19, where the defoamer content was insufficient, the plate corrosion resistance and the corrosion resistance of the processed area deteriorated. In addition, in Comparative Example 20, where the defoamer content was excessive, the solution stability showed poor results.

[0122] Experimental Example 9. Based on the changes in physical properties due to drying temperature and film thickness. The composition for surface treatment is prepared based on the above-mentioned baseline composition, with the types and amounts of each component. Then, solvent (water) is added to 14% by weight of the solids in the composition until the total reaches 100% by weight, to prepare a solution composition.

[0123] Hot-dip galvanized steel sheets were cut into 7cm (transverse) × 15cm (longitudinal) dimensions. After removing grease, the prepared solution composition was sprayed onto the sheets and then dried in an induction heating furnace (2000A) to produce surface-treated test pieces. The thickness of the thin film layer (drying thickness) and the PMT reference drying temperature were controlled to be different from each other as shown in Table 9 below. The physical properties of each test piece were evaluated accordingly, and the results are presented in Table 9 below.

[0124] [Table 9] As shown in Table 9, in Invention Examples 29 to 32, where the thickness of the dried film layer was formed to be 0.3-1.5 μm and dried at 60°C (PMT standard), good or better results were observed in all physical properties. Furthermore, in Invention Examples 34 and 45, where the film layer was formed to be 0.8 μm thick and dried at 40-200°C (PMT standard), good or better results were also observed.

[0125] On the other hand, in Comparative Example 21, where the thickness of the film layer after drying was as thin as 0.2 μm, the corrosion resistance deteriorated, and the resistance to blackening and alkali also deteriorated slightly. In Comparative Example 22, where the film layer after drying was too thick, the corrosion resistance of the processed part showed poor results.

[0126] Furthermore, in Comparative Example 23, where the drying temperature was below 40°C based on PMT, the coated solution composition did not dry sufficiently, resulting in poor performance in all physical properties. In Comparative Example 24, where the drying temperature was above 200°C based on PMT, poor resistance to blackening was observed. This is because, during the final cooling (water cooling) process after the evening drying process, water vapor generated by the test piece caused condensation on the upper part of the drying equipment, resulting in fumes falling onto the test piece surface.

Claims

1. A composition for surface treatment of coated steel sheet, comprising: (a) Trivalent chromium compounds bound to organic acids: 15-45% by weight (b) Antioxidants: 0.10-2.00% by weight (c) Silane compounds: 1.00-25.00% by weight (d) Polysiloxane copolymer: 15-50% by weight (e) Cobalt-based rust and corrosion inhibitors: 1.0-15.0% by weight (f) Lubricant: 0.10-3.00% by weight, and (g) Defoamer: 0.10-2.00% by weight.

2. The composition for surface treatment of coated steel sheet according to claim 1, wherein, The trivalent chromium compound that is bound to the organic acid is selected from one or more of chromium acetylacetonate (III), chromium acetate (III), chromium 2-ethylhexanoate (III), chromium acetate hydroxide, and potassium chromium oxalate (III) trihydrate.

3. The composition for surface treatment of coated steel sheet according to claim 1, wherein, The antioxidant is a triazole derivative, which is selected from one or more of 1,3,4-thiadiazolidine-2,5-dithione, 1,2,3-triazole, 3-amino-1,2,4-triazole and 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole.

4. The composition for surface treatment of coated steel sheet according to claim 1, wherein, The silane compound is selected from one or more of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-epoxypropoxypropyltrimethoxysilane, 3-epoxypropoxypropylmethyldiethoxysilane, 3-epoxypropoxypropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldiethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-ureopropyltrimethoxysilane, and tetraethyl orthosilicate.

5. The composition for surface treatment of coated steel sheet according to claim 1, wherein, The polysiloxane copolymer is a copolymer containing one or more compounds selected from groups i) and ii) below and an acid catalyst, with a molecular weight between 300 and 1500. i) A siloxane compound selected from one or more of polydimethylsiloxane, polyvinylsiloxane, polyphenylmethylsiloxane, and hexamethylsiloxane. ii) One or more silane compounds selected from methyltrimethoxysilane, ethyltrimethoxysilane, hexamethyldisilane, ethoxytriethylsilane, vinyltrimethoxysilane, vinyltriethoxysilane, γ-epoxypropoxytriethylsilane, and γ-epoxypropoxytrimethylsilane. iii) An acid catalyst selected from one or more of phosphoric acid and organic phosphoric acid, oxalic acid, citric acid, and formic acid.

6. The composition for surface treatment of coated steel sheet according to claim 1, wherein, The cobalt-based rust and corrosion inhibitor is selected from one or more of cobalt nitrate (II), cobalt sulfate (II), cobalt acetate (II), cobalt oxalate (II), cobalt nitrate (III), cobalt acetate (III), cobalt oxalate (III), cobalt chloride (IV), cobalt oxide (III), and cobalt oxide (IV).

7. The composition for surface treatment of coated steel sheet according to claim 1, wherein, The lubricant is a polyethylene wax dispersed with a nonionic dispersant.

8. The composition for surface treatment of coated steel sheet according to claim 1, wherein, The defoamer is an organosilicon-based defoamer.

9. The composition for surface treatment of coated steel sheet according to claim 1, wherein, The composition is a solution composition having a solid content of 5-20% by weight and containing the balance solvent, and the pH of the solution composition is 4 to 6.

10. A surface-treated coated steel sheet, comprising: Foundation steel plate; A zinc-based coating is formed on at least one side of the base steel plate; as well as A surface treatment thin film layer is formed on the coating. The surface-treated thin film layer is formed from the composition of any one of claims 1 to 9.

11. The surface-treated plated steel sheet according to claim 10, wherein, The surface-treated thin film layer has a thickness of 0.3-1.5 μm.

12. A method for manufacturing a surface-treated coated steel sheet, comprising the following steps: Prepare to form a zinc-plated steel sheet on at least one side of a base steel sheet; A surface treatment composition is applied to the zinc-based plating; as well as The plated steel sheet coated with the surface treatment composition is dried. The surface treatment composition is the composition according to any one of claims 1 to 9.

13. The method for manufacturing a surface-treated coated steel sheet according to claim 12, wherein, The coating process is performed by any one of the following methods: bar coating, roller coating, spray coating, dip coating, jet extrusion, and dip extrusion.

14. The method for manufacturing a surface-treated coated steel sheet according to claim 12, wherein, The coating step involves applying the surface treatment composition with a coating thickness of 2.5-12.5 μm.

15. The method for manufacturing a surface-treated coated steel sheet according to claim 12, wherein, The drying step is carried out in a temperature range of 40-200°C, based on the final reached temperature (PMT) of the base steel plate.

16. The method for manufacturing a surface-treated coated steel sheet according to claim 12, wherein, The drying step is carried out in a hot air drying oven or an induction heating oven.

17. The method for manufacturing a surface-treated coated steel sheet according to claim 12, wherein, The manufacturing method further includes a step of air cooling or water cooling of the galvanized steel sheet after the drying step.

18. The method for manufacturing a surface-treated coated steel sheet according to claim 12, wherein, The method for manufacturing the galvanized steel sheet is a continuous process, and the speed of the continuous process is 50-120 meters per minute.

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