Surface treatment composition for hot-stamping plated steel sheet, aluminum-plated steel sheet surface-treated therewith, method for manufacturing the same, and hot-stamped member

Abstract

The present invention relates to a plated steel sheet for hot stamping suitable for use as a material for automobiles, and more particularly, to a composition for surface treatment of the plated steel sheet for hot stamping and a plated steel sheet surface-treated using the same and a method of manufacturing the same. Further, a hot stamped member using the surface-treated plated steel sheet is provided.

Description

Technical Field

[0001] The present invention relates to a hot-formed coated steel sheet suitable for use as an automotive material, and more specifically, to a composition for surface treatment of the hot-formed coated steel sheet, an aluminized steel sheet surface treated using the composition, a method for manufacturing the same, and a hot-formed component. Background Technology

[0002] In recent years, automakers have been continuously researching lightweight vehicle bodies to improve fuel efficiency and meet CO2 emission regulations. As part of this effort, to increase the strength of steel while maintaining the same weight, multiple alloying elements are incorporated into the steel alloy design. However, as mentioned above, including multiple alloying elements in steel can lead to problems such as increased hardenability, springback, and decreased machinability.

[0003] To address the problems mentioned above, hot pressing was proposed. Hot pressing involves processing steel of a certain strength in the austenitic single-phase region, followed by rapid cooling to a low temperature, thereby forming a low-temperature microstructure such as martensite within the steel. This method can dramatically increase the strength of products, thus minimizing the problem of reduced machinability when manufacturing high-strength (ultra-high-strength) components.

[0004] In addition, ongoing efforts are underway to achieve nationwide carbon neutrality, with automakers and steel companies jointly conducting various studies on technologies that can reduce CO2 emissions during hot forming.

[0005] As a result, the following solution was proposed. From a metallurgical point of view, this involves changing the composition of steel by adding Mn and other substances to lower the Ac3 temperature, thereby enabling forming (warm stamping) at lower temperatures compared to existing materials (Patent Document 1). This warm stamping does not require rapid cooling, thus offering advantages such as ensuring productivity and extending mold life. However, issues such as decreased corrosion resistance of the steel sheet due to increased Mn content, and productivity problems in continuous casting and hot rolling processes during manufacturing remain technical problems that need to be solved.

[0006] In addition, as another solution, to shorten the heating time to reach the target temperature during high-temperature heating for hot forming, a technique of coating the steel sheet surface with a polymer has been proposed (Patent Document 2). Coating the steel sheet surface with a hydrocarbon polymer can increase the heating rate during hot forming. However, when hot forming a coated steel sheet, residues may remain on the surface after forming, potentially causing reduced weldability.

[0007] To address the problems described above, surface treatment compositions combining Si-removed polymers with carbon have been developed (Patent Document 3), and P-free polymer coating agents have been developed (Patent Document 4). However, some Si-containing compounds, such as silane coupling agents, are important substances that affect the adhesion of the metal surface and the polymer coating, and complete removal is not preferred. Furthermore, the P component is necessary to ensure corrosion resistance, and therefore needs to be included within a range that does not impair weldability.

[0008] (Patent Document 1) Korean Patent Publication No. 10-2020-0051129 (Patent Document 2) Chinese Patent Publication No. 10616418 (Patent Document 3) Korean Patent Publication No. 2016-7026841 (Patent Document 4) Chinese Patent Publication No. 116219271 Summary of the Invention

[0009] (a) Technical problems to be solved One aspect of the present invention aims to provide a surface treatment composition for hot-formed coated steel sheets, which, when providing hot-formed coated steel sheets suitable for use as automotive materials, can shorten the heating time during high-temperature heating for hot forming of the coated steel sheets.

[0010] Furthermore, one aspect of the present invention aims to provide a surface-treated aluminized steel sheet and a method for manufacturing the same, wherein the aluminized steel sheet is surface-treated by the surface-treatment composition, thereby shortening the heating time during high-temperature heating.

[0011] As another aspect of the present invention, it is intended to provide a thermoformed component obtained by thermoforming a surface-treated aluminized steel sheet.

[0012] Furthermore, the technical problems to be solved in this invention are not limited to those mentioned above. Those skilled in the art to which this invention pertains can clearly understand other technical problems not mentioned from the following description.

[0013] (II) Technical Solution According to one aspect of the present invention, a composition for surface treatment of thermoforming coated steel sheet is provided, which is a composition in which graphite is dispersed in a polymer adhesive resin.

[0014] In one embodiment of the present invention, a composition having a polymer adhesive resin to graphite weight ratio of 1:0.25 or less can be provided.

[0015] In one embodiment of the present invention, the average particle size of the graphite may be 0.2-5.0 μm, and the composition may contain carbon (C), hydrogen (H), oxygen (O), nitrogen (N), silicon (Si) and phosphorus (P).

[0016] In one embodiment of the invention, the polymer adhesive resin may comprise, relative to 100% by weight, 6-20% acrylic resin, 0.01-1.00% Teflon-based wax, 0.01-0.50% silane coupling agent, 0.01-2.00% phosphate-based corrosion improver, and the balance being solvent.

[0017] According to another aspect of the present invention, a surface-treated aluminized steel sheet is provided, comprising: a base steel sheet; an aluminum-based coating formed on at least one side of the base steel sheet; and a surface-treated thin film layer formed on the aluminum-based coating.

[0018] In one embodiment of the invention, the surface-treated thin film layer may be formed from a composition according to one aspect of the invention.

[0019] In one embodiment of the present invention, the Si content of the surface-treated thin film layer can be less than 8.00% by weight (except 0%), and the adhesion amount can be 100.00 mg / m². 2 Furthermore, the phosphorus content of the surface-treated thin film layer can be less than 0.70% by weight (except 0%), and the adhesion amount can be 10.00 mg / m³. 2 the following.

[0020] In one embodiment of the present invention, the graphite content of the surface-treated thin film layer can be 0.1-60.0%.

[0021] In one embodiment of the present invention, the surface-treated thin film layer may have a thickness of less than 1.50 μm (dry thickness reference) and an emissivity of 0.8 or higher.

[0022] In one embodiment of the present invention, the aluminum-based coating may contain 0-27% zinc (Zn), 7.0-10.0% silicon (Si), the balance being Al and unavoidable impurities, by weight%.

[0023] According to another aspect of the present invention, a method for manufacturing a surface-treated aluminized steel sheet is provided, comprising the following steps: preparing an aluminized steel sheet having an aluminum coating formed on at least one side of a base steel sheet; applying a surface treatment composition to the aluminum coating; and drying and curing the applied composition to form a surface-treated thin film layer.

[0024] In one embodiment of the invention, the composition for surface treatment may be a composition according to one aspect of the invention.

[0025] In one embodiment of the invention, the coating process can be performed by any of the methods selected from bar coating, roller coating, spraying, dipping, jet extrusion, and dipping squeeze-drying.

[0026] In one embodiment of the invention, the drying and curing steps can be performed within a temperature range of 80-150°C, based on the final reach temperature (PMT) of the coated steel sheet.

[0027] According to another aspect of the invention, a thermoformed component is provided, which includes a thin film layer on the coating of a surface-treated aluminized steel sheet.

[0028] In one embodiment of the invention, the thin film layer may be formed from a composition according to one aspect of the invention.

[0029] (III) Beneficial Effects According to the present invention, when providing steel sheets suitable for hot forming, steel sheets with improved emissivity, particularly aluminized steel sheets, can be provided. As described above, when manufacturing parts using aluminized steel sheets with improved emissivity, the heating time during high-temperature heat treatment of the steel sheet can be shortened, resulting in economic and environmental benefits.

[0030] Furthermore, according to the present invention, it has the effect of providing a surface treatment composition that is beneficial to obtaining aluminized steel sheets with improved emissivity. Best practice

[0031] 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.

[0032] The inventors of this invention have conducted in-depth research on solutions for providing a hot-formed coated steel sheet that can meet the physical properties required by the automotive industry for which the hot-formed coated steel sheet is used, while reducing CO2 generation during its manufacturing process. In particular, solutions for improving the heating rate during the high-temperature heating process for forming the hot-formed coated steel sheet have been investigated.

[0033] Therefore, the aim is to provide a composition suitable for coating (surface treatment) the surface of a coated steel sheet for hot forming. In particular, it has been confirmed that a composition can be provided that, while ensuring the physical properties required as a surface treatment layer for coated steel sheets for hot forming, such as corrosion resistance and weldability, can promote improvements in productivity and reduce CO2 by shortening the heating time to the target temperature for high-temperature forming.

[0034] According to the present invention as described above, not only is a composition for surface treatment of coated steel sheet for hot forming provided, but also an aluminized steel sheet surface treated using the composition, a method for manufacturing the same, and a hot-formed component obtained from the surface-treated aluminized steel sheet.

[0035] The present invention will now be described in detail.

[0036] A composition for surface treatment of coated steel sheet for thermoforming according to one aspect of the present invention comprises a polymer binder resin as a main component, and may also contain graphite dispersed in the polymer binder resin.

[0037] In one embodiment of the present invention, the surface treatment composition further comprises graphite in addition to the polymer binder resin, thereby increasing the emissivity of the hot-formed coated steel sheet surface-treated with the composition. Emissivity refers to the ratio of energy absorbed by an object (e.g., heat, light, etc.) and then re-emitted or re-emitted after surface reflection. A high emissivity can be understood as an increased energy absorption rate. As an example, when the emissivity of the hot-formed coated steel sheet is high, the amount of radiant heat absorbed during high-temperature heating in a furnace for hot-forming the coated steel sheet is increased. Therefore, the time required to heat to the target temperature can be shortened.

[0038] In one embodiment of the invention, the average particle size of the graphite contained in the composition can be 0.2-5.0 μm. When the average particle size of the graphite is less than 0.2 μm, it may be necessary to excessively increase the content of graphite used to disperse in the polymer binder resin, in which case there is a possibility that the content of other components will be relatively reduced. On the other hand, when the average particle size of the graphite is greater than 5.0 μm, the particle size becomes too large and cannot be sufficiently dispersed in the polymer binder resin, and there is a possibility that it will easily precipitate.

[0039] In one embodiment of the present invention, the weight ratio of the polymer binder resin to graphite in the composition can be 1:0.25 or less. Here, weight ratio refers to the ratio of parts by weight (weight ratio). When the weight ratio of graphite relative to the polymer binder resin is greater than 0.25, the content of the polymer binder resin as a component of the composition becomes lower, and the physical properties of the surface-treated coated steel sheet, such as corrosion resistance and weldability, may deteriorate. Furthermore, there is no particular limitation on the lower limit of the weight ratio of graphite, but considering the effect of increasing emissivity brought by graphite, a weight ratio of graphite of 0.01 or more may be included.

[0040] In another embodiment of the invention, the polymer binder resin composition in which graphite is dispersed, as a non-limiting example, can be obtained by adding graphite powder to the polymer binder resin and then stirring.

[0041] The composition according to one embodiment of the present invention is a polymer composition that may contain a polymeric adhesive resin as a main component, said polymeric adhesive resin being a substance that contains various additives and solvents in the resin composition.

[0042] As an example, a polymer adhesive resin, based on a total weight of 100%, may contain 6-20% acrylic resin, 0.01-1.00% Teflon-based wax, 0.01-0.50% silane coupling agent, 0.01-2.00% phosphate-based corrosion improver, and the balance being solvent.

[0043] The acrylic resin acts as a binder in the composition. By adding it in a certain amount, the physical properties of the surface-treated layer, such as chemical resistance and alkali resistance, can be ensured. Based on 100% by weight of the total polymer binder resin, the content of this acrylic resin can be 6-20%. When the content of the acrylic resin is less than 6%, the viscosity of the composition becomes low, and it may become difficult to adjust the thickness of the surface-treated layer during the surface treatment of the coated steel sheet. On the other hand, when the content of the acrylic resin is greater than 20%, the storage stability of the composition in solution state may decrease. There is no particular limitation on the type of acrylic resin; substances commonly used in the field of steel sheet surface treatment can be used.

[0044] The Teflon-based wax can be added to impart lubricity to the surface treatment layer. Based on 100% by weight of the total polymer adhesive resin, the content of this Teflon-based wax can be 0.01-1.00%. When the content of the Teflon-based wax is less than 0.01%, the lubricity of the surface treatment layer becomes insufficient, and damage to the surface treatment layer and / or the material may occur during the processing of the surface-treated coated steel sheet. On the other hand, when the content of the Teflon-based wax is greater than 1.00%, excessive distribution of lubricant particles in the surface treatment layer may reduce corrosion resistance. While not limited to this, polyethylene-Teflon-based wax or polytetrafluoroethylene-based wax can also be used as the Teflon-based wax.

[0045] The silane coupling agent may be included to firmly maintain the bond between the organic resin and inorganic compound contained in the composition, thereby denaturing the organic resin to induce a coupling reaction. Based on 100% by weight of the total polymer adhesive resin, the content of this silane coupling agent can be 0.01-0.50%. When the content of the silane coupling agent is less than 0.01%, the adhesion of the surface treatment layer formed on the coated surface of the steel sheet may decrease. On the other hand, when the content of the silane coupling agent is greater than 0.50%, the above effect saturates, which may instead lead to an increase in manufacturing costs. The silane coupling agent may be selected from one or more of vinyltriethoxysilane, 3-glycidyl etheroxypropyltrimethoxysilane, 3-glycidyl etheroxypropyltriethoxysilane, 3-glycidyl etheroxypropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, and 2-perfluorooctylethyltrimethoxysilane.

[0046] The aforementioned phosphate-based corrosion resistance improver can be added to impart corrosion resistance to the surface treatment layer. Furthermore, when a certain amount of phosphorus (P) remains on the surface of the surface-treated coated steel sheet, it can prevent hydrogen absorption during subsequent heat treatment. Therefore, hydrogen-induced embrittlement and other defects can be suppressed. Based on 100% by weight of the total polymer adhesive resin, the content of this phosphate-based corrosion resistance improver can be 0.01-2.00%. When the content of the phosphate-based corrosion resistance improver is less than 0.01%, the corrosion resistance of the surface treatment layer may decrease, and the surface-treated coated steel sheet may become more brittle to hydrogen embrittlement after heat treatment. On the other hand, when the content of the phosphate-based corrosion resistance improver is greater than 2.00%, the P content in the surface treatment layer becomes excessive, and the weldability of the surface-treated coated steel sheet may deteriorate. The phosphate-based corrosion resistance improver can be one or more phosphate compounds selected from those having one or more of the elements Zn, Cr, Si, Al, Mo, Mg, and Ca as relative cations.

[0047] According to one embodiment of the present invention, the polymer binder resin in the composition may contain a solvent as the balance in addition to the above-mentioned components. In this case, a solvent may be added to disperse the components constituting the polymer binder resin and to manufacture a solution composition for surface treatment of coated steel sheets. The solvent may be a mixture of water and an organic solvent. The water may be deionized water or distilled water, and the organic solvent may be one or more selected from alcohols, ethers, and acetone. As an example, the mixed solvent may be a substance obtained by adding 0.1-10.0% of an organic solvent to water, where the organic solvent content refers to a content relative to 100% by weight of the total solvent.

[0048] As described above, the composition according to one embodiment of the present invention, comprising a polymer binder resin and graphite composed of the above-described components, may contain carbon (C), hydrogen (H), oxygen (O), nitrogen (N), silicon (Si), and phosphorus (P). In this case, the carbon (C) originates from the graphite and binder resin components, the silicon (Si) originates from a silane coupling agent that is a component of the polymer binder resin, and the phosphorus (P) may originate from a phosphate-based corrosion resistance improver. Furthermore, the hydrogen (H) and oxygen (O) in the composition are components that naturally exist within the composition.

[0049] In addition, according to one embodiment of the present invention, a composition comprising a polymer binder resin and graphite can be used in the surface treatment process of thermoforming coated steel sheets. The coated steel sheets surface treated with a composition according to one embodiment of the present invention will be described in detail below.

[0050] According to one embodiment of the present invention, the plated steel sheet used for surface treatment is a steel sheet that is thermoformed, such as a plated steel sheet suitable for being pressed and formed at high temperature by a mold or the like in order to obtain a part of a certain shape. As an example, it may be an aluminized steel sheet.

[0051] According to one embodiment of the present invention, an aluminized steel sheet may include a base steel sheet and an aluminum coating formed on at least one side of the base steel sheet.

[0052] In one embodiment of the invention, any material can be used as the base steel sheet, provided it is a high-strength steel suitable for use as an automotive material and also suitable for obtaining coated steel sheets through hot-dip galvanizing. Accordingly, there is no particular limitation on the alloy composition of the base steel sheet.

[0053] However, as a non-limiting example, the base steel plate is carbon steel containing a certain amount of carbon (C), for example, by weight percent, it may contain: carbon (C): 0.02-0.60%, silicon (Si): 0.001-2.000%, aluminum (Al): 0.001-1.000%, manganese (Mn): 0.1-4.0%, phosphorus (P): less than 0.05%, sulfur (S): less than 0.02%, nitrogen (N): less than 0.02%, titanium (Ti): 0-0.1%, boron (B): 0.0001-0.0100%, copper (Cu): 0-1.00%, molybdenum (Mo): 0-1.00%, Chromium (Cr): 0-1.00%, Nickel (Ni): 0-1.00%, Vanadium (V): 0-1.00%, Calcium (Ca): 0-0.01%, Niobium (Nb): 0-0.1%, Tin (Sn): 0-1.0%, Tungsten (W): 0-1.0%, Antimony (Sb): 0-1.0%, Magnesium (Mg): 0-1.0%, Cobalt (Co): 0-1.0%, Arsenic (As): 0-1.0%, Zirconium (Zr): 0-1.0%, Bismuth (Bi): 0-1.0%, Rare Earth Elements (REM): 0-0.3%, Balance Fe and unavoidable impurities.

[0054] The C and Mn elements in the above alloy composition can be added to ensure the strength of the steel, Si can make a significant contribution to strength stabilization, and Al has a deoxidizing effect. P, S, N, etc., are elements that inevitably flow into the steel during the manufacturing process, but it should be noted that they are not limited to these. In addition, considering the target physical properties of the final product, Ti, B, Cu, Mo, Cr, Ni, V, Ca, Nb, Sn, W, Sb, Mg, Co, As, Zr, Bi, REM, etc., can be further included in addition to the above composition, which is obvious to those skilled in the art.

[0055] According to one embodiment of the present invention, at least one side of the base steel plate may include an aluminum-based coating.

[0056] In one embodiment of the present invention, the aluminum-based coating may be a coating containing aluminum (Al) as the main element. As an example, by weight %, the aluminum-based coating may contain 0-27% zinc (Zn), 7.0-10.0% silicon (Si), the balance being aluminum (Al) and unavoidable impurities.

[0057] In one embodiment of the invention, the aluminum coating can be formed by immersing the base steel plate in an aluminum molten plating bath, as will be described in detail later. It is known that the composition of the aluminum coating is determined by the composition of the aluminum molten plating bath.

[0058] In one embodiment of the invention, zinc (Zn) in the alloy composition of the aluminum-based coating is an element that helps ensure the corrosion resistance of the coating, achieving a sacrificial corrosion protection effect due to Zn. As an example, when the Zn content is greater than 27%, during the hot forming of the base steel sheet with the coating, the Zn diffuses into the interior of the base steel sheet, potentially causing cracks due to low metallization (LME). Therefore, the Zn content can be at most 27%, and even 0% does not pose a problem in ensuring the physical properties of the aluminum-based coating.

[0059] In one embodiment of the present invention, the silicon (Si) in the alloy composition constituting the aluminum-based coating can suppress the diffusion of Al present in the plating bath into the interior of the base steel plate during plating. As an example, to achieve the above effect, the Si content can be 7% or more. However, when the Si content is greater than 10%, the melting point of the plating bath may increase sharply, leading to a problem of a sharp increase in the amount of ash generated in the plating bath.

[0060] In one embodiment of the present invention, the aluminum-based coating may include a surface-treated thin film layer. That is, a thin film layer with a certain thickness may be formed on the aluminum-based coating.

[0061] As mentioned above, coated steel sheets with a surface treatment film layer on an aluminum-based coating can have excellent corrosion resistance and high emissivity.

[0062] In one embodiment of the present invention, in order to obtain a coated steel sheet having the above-mentioned physical properties, a surface treatment film layer disposed on an aluminum-based coating may be formed from a surface treatment composition for a hot-formed coated steel sheet according to an embodiment of the present invention. That is, the film layer formed according to an embodiment of the present invention is formed from a composition in which a predetermined amount of graphite of a certain size is dispersed in a polymer binder resin, thereby providing a coated steel sheet having the desired physical properties.

[0063] According to one embodiment of the present invention, the surface-treated thin film layer may contain 0.1-60.0% graphite, where the content is based on 100% by weight of the total dried thin film layer. When the graphite content in the thin film layer is less than 0.1%, the emissivity of the surface-treated thin film layer cannot be improved. On the other hand, when the graphite content in the thin film layer is greater than 60.0%, the weldability may be reduced due to the residual carbon in the thin film layer after high-temperature heating for hot forming of surface-treated aluminized steel sheets.

[0064] In one embodiment of the invention, the thickness of the surface-treated thin film layer, based on the dried thickness, can be less than 1.50 μm. When the thickness of the thin film layer is greater than 1.50 μm, the processability and weldability may deteriorate due to the excessive thickness of the thin film layer. There is no particular limitation on the lower limit of the thickness of the surface-treated thin film layer, but a thickness of 0.30 μm or more is permissible in terms of imparting the effects brought about by the thin film layer.

[0065] In one embodiment of the present invention, the surface-treated thin film layer may be formed from a composition according to one embodiment of the present invention, and therefore the thin film layer may contain carbon (C), hydrogen (H), oxygen (O), nitrogen (N), silicon (Si), and phosphorus (P). The carbon (C) may primarily be derived from graphite, the silicon (Si) may be derived from a silane coupling agent, and the phosphorus (P) may be derived from a phosphate-based corrosion resistance improver.

[0066] In one embodiment of the invention, the Si content in the thin film layer may be 8.00% by weight or less (excluding 0%), and the P content may be 0.70% by weight or less (excluding 0%). The content is based on 100% by weight of the total dry thin film layer. When the Si content in the thin film layer is greater than 8.00%, there is a possibility that excessive amounts of unburned Si remain on the surface during the high-temperature heating (heat treatment) process for hot-forming surface-treated coated steel sheets. This residual Si tends to have high electrical resistance, which can hinder the weldability of the product (e.g., hot-formed components). Furthermore, when the P content in the thin film layer is greater than 0.70%, the amount of residual P in the thin film layer becomes excessive, which may seriously impair the weldability of components obtained by hot-forming surface-treated aluminized steel sheets.

[0067] Furthermore, in one embodiment of the present invention, the Si content of the surface-treated thin film layer containing Si and P as described above can be 100.00 mg / m³. 2 The following describes the adhesion amount of P as 10.00 mg / m³. 2 The following describes how the content of Si and P per unit area within the surface-treated thin film layer can be controlled. Here, the amount of each element refers to the relative proportion of each element within the thin film layer.

[0068] Even if the Si and P content in the thin film layer obtained after surface treatment is within a limited range, if the absolute amount of Si and P adhering to the film layer becomes too high due to excessive adhesion of the coating composition, there is a possibility of adverse effects on solderability and workability of subsequent processes. With this in mind, in one embodiment of the present invention, the amount of Si and P adhering to the thin film layer can be limited as described above.

[0069] In one embodiment of the present invention, when the Si deposition rate within the surface-treated thin film layer is greater than 100.00 mg / m², 2 In cases where surface-treated coated steel sheets undergo high-temperature heating during hot forming, excessive Si may remain on the surface. This increases the surface resistivity of the hot-formed component, ultimately hindering welding current or causing spatter and poor weldability. Furthermore, when the amount of P adhering to the surface-treated thin film layer exceeds 10.00 mg / m³... 2 In addition, it can also hinder the weldability of thermoformed components. In particular, P derived from phosphate compounds has a strong tendency to remain in a solid state, and therefore, when there is excessive residue on the surface, it becomes a cause of increased surface resistivity, similar to Si.

[0070] According to another embodiment of the present invention, the Si deposition rate within the surface-treated thin film layer can be 98.00 mg / m². 2 Below or 95.00 mg / m² 2 According to another embodiment, the amount of P adhering to the surface-treated thin film layer can be 9.00 mg / m². 2 Below or 8.50 mg / m 2 the following.

[0071] Furthermore, there is no particular limitation on the content of Si and P contained in the surface-treated thin film layer and the lower limit of the amount of these elements attached. It should be noted that it will be naturally determined based on the content of silane coupling agent and phosphate-based corrosion resistance improver contained in the composition used to form the thin film layer.

[0072] Furthermore, the content of Si and P in the surface-treated thin film layer and the amount of these elements attached can be measured using X-ray fluorescence analysis, wet resin coating adhesion measurement methods, non-destructive coating thickness measurement devices, etc. However, it is not limited to these methods.

[0073] X-ray fluorescence analysis is a method for quantitatively obtaining the content of each element using an X-ray fluorescence analyzer. Alternatively, the film layer can be dissolved in an acid solution, separated from a surface-treated coated steel sheet, and the resin coating adhesion per unit area can be obtained by measuring the weight of the separated coating as described above. Another method involves ensuring the film layer thickness using a non-destructive coating thickness gauge (e.g., Betascope®), and then weighting the known densities of resin and graphite according to their content ratios to obtain the film layer density. The film layer thickness and density are then multiplied to calculate the adhesion per unit area. Based on the above, the adhesion amount of each element can be calculated from the obtained element content and film layer adhesion amount.

[0074] Additionally, it should be noted that the content of graphite, Si, and P in the thin film layer is based on the content in the dried thin film layer.

[0075] As described above, according to one embodiment of the present invention, the surface-treated thin film layer disposed on the surface of the aluminum-based coating, by containing a certain amount of Si and P along with graphite, can improve the adhesion and corrosion resistance of the thin film layer, while also exhibiting excellent emissivity. As an example, the emissivity of the surface-treated thin film layer according to one embodiment of the present invention can be 0.8 or higher. The description of emissivity can be replaced by the previously mentioned content. Furthermore, considering that the emissivity of a typical aluminized steel sheet is around 0.5, the aluminized steel sheet having the surface-treated thin film layer according to one embodiment of the present invention achieves an emissivity increase of 0.8 or higher.

[0076] As described above, aluminized steel sheets with improved emissivity exhibit high thermal absorption during the process of heating to high temperatures for hot forming, thus shortening the time to reach the target temperature (heating time). Therefore, when manufacturing hot-formed components suitable for use as automotive materials, by utilizing aluminized steel sheets according to an embodiment of the present invention, not only can CO2 generation be effectively reduced, but productivity can also be improved.

[0077] The following describes a method for manufacturing a surface-treated aluminized steel sheet according to another aspect of the present invention. However, it should be noted that the following manufacturing method is an example for manufacturing a surface-treated aluminized steel sheet.

[0078] In one embodiment of the present invention, the surface-treated aluminum-coated steel sheet can be manufactured by the following steps: preparing a coated steel sheet having an aluminum coating formed on at least one side of a base steel sheet; applying a surface treatment composition to the aluminum coating of the coated steel sheet; and drying and curing the coated steel sheet after applying the composition to form a surface treatment film layer.

[0079] In one embodiment of the invention, the base steel sheet used to form the aluminum-based coating on at least one side is a high-strength steel suitable for use as an automotive material, and its alloy composition is not particularly limited. Alternatively, the aforementioned base steel sheet can be used instead.

[0080] In one embodiment of the invention, the aluminum-based coating can be formed by coating a base steel plate. As an example, this can be achieved by immersing the base steel plate in an aluminum-based plating bath containing unavoidable impurities of Si, Zn, and Fe, in addition to Al. As a non-limiting example, the aluminum-based plating bath may contain 7.0-10.0% silicon (Si). Adding Si can suppress the fluidity of the molten metal and the diffusion of Al into the base steel plate during plating. Furthermore, the aluminum-based plating bath may further contain: zinc (Zn): 0-27%, iron (Fe): up to 3%, with these contents replaced by the description above.

[0081] In one embodiment of the present invention, the plating can be performed in an aluminum plating bath at 600-680°C.

[0082] In one embodiment of the present invention, the step of coating the surface treatment composition on the aluminum coating can be performed using any coating method selected from bar coating, roller coating, spraying, dipping, jet extrusion and dipping squeeze.

[0083] In one embodiment of the present invention, the surface treatment composition coated on the aluminum-based coating may be a surface treatment composition for hot-formed coated steel sheet according to an embodiment of the present invention.

[0084] In one embodiment of the present invention, the composition coated on the aluminum-based coating may contain graphite dispersed in a polymer binder resin composed of organic resin, lubricant, silane coupling agent, rust inhibitor (corrosion improver), etc. The surface-treated thin film layer formed by surface-treating the aluminum-based coating using this composition has excellent physical properties such as corrosion resistance and weldability, and high emissivity.

[0085] When the above composition is coated onto aluminized steel sheet, as an example, a concentration of 400-1800 mg / m³ can be applied. 2 The adhesion amount is determined by the following process. After applying the composition with the adhesion amount described above, a surface-treated thin film layer with a thickness of 1.50 μm or less, preferably 0.30-1.50 μm, can be obtained after a subsequent drying process. The thickness of the thin film layer refers to the thickness after drying. In one embodiment of the invention, when the adhesion amount of the composition is less than 400 mg / m³... 2 When the composition is applied thinly to the rough, raised areas of the aluminum coating, reduced corrosion resistance may occur. On the other hand, when the thickness exceeds 1800 mg / m², corrosion resistance may also decrease. 2 If the film layer becomes too thick after drying, its physical properties, such as weldability and processability, may deteriorate.

[0086] In one embodiment of the present invention, after applying a surface treatment composition to the coating surface of an aluminized steel sheet, the coated steel sheet is dried and cured to obtain a surface treatment film layer. In one embodiment of the present invention, the drying and curing steps can be performed within a temperature range of 80-150°C, which is the final reach temperature of the coated steel sheet, i.e., the peak metal temperature (PMT) reference. When the drying temperature is lower than the PMT reference of 80°C, drying cannot be sufficiently performed, 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 higher than the PMT reference of 150°C, the organic components in the coated composition carbonize, making it impossible to ensure physical properties such as corrosion resistance and heat resistance. Furthermore, due to the excessive increase in hardness of the surface treatment layer, cracking of the surface treatment layer may occur during processing.

[0087] As described above, the aluminized steel sheet surface-treated using a surface treatment composition for thermoforming coated steel sheet according to an embodiment of the present invention exhibits excellent corrosion resistance and weldability, while also possessing higher emissivity compared to untreated aluminized steel sheet. This is due to the optimization of the types and contents of the components constituting the composition, particularly the inclusion of graphite in addition to the polymer binder resin, and the adjustment of the contents of Si and P present in the surface-treated thin film layer, etc.

[0088] Additionally, it should be noted that when surface-treating a coated steel sheet with a surface-treatment composition for hot-formed coated steel sheet according to an embodiment of the present invention, it does not mean that surface treatment must be performed only on aluminized steel sheets manufactured by hot-dip galvanizing. That is, the surface-treatment composition according to an embodiment of the present invention can be applied to any coated steel sheet having an aluminum coating on at least one side of a base steel sheet.

[0089] Hereinafter, a thermoformed component and its manufacturing method according to another aspect of the present invention will be described.

[0090] In one embodiment of the present invention, the hot-formed component can be obtained by hot-forming a hot-formed steel sheet, wherein the hot-formed steel sheet can be an aluminized steel sheet according to one embodiment of the present invention.

[0091] According to one embodiment of the present invention, an aluminized steel sheet comprises: a base steel sheet; an aluminum-based coating formed on at least one side of the base steel sheet; and a surface treatment film layer formed on the aluminum-based coating. In this case, the surface treatment film layer may be formed from a surface treatment composition according to one embodiment of the present invention.

[0092] By having a certain thin film layer on the coating of the aluminized steel sheet according to one embodiment of the present invention, the emissivity can be improved. As described above, when the aluminized steel sheet with improved emissivity is applied to the hot forming process, it can exhibit excellent properties in a variety of physical properties. As an example, compared with ordinary aluminized steel sheet without surface treatment, the heating time during high-temperature heating for hot forming can be shortened. Furthermore, in terms of weldability, equivalent or higher welding current can be ensured, and corrosion resistance can be further improved. Moreover, in terms of hydrogen absorption, hydrogen intrusion can be effectively suppressed, thus being effective in preventing hydrogen-induced embrittlement during automobile manufacturing.

[0093] Additionally, it should be noted that the base steel plate and aluminum coating that constitute the thermoformed components can be replaced by the above-mentioned components.

[0094] Furthermore, there is no particular limitation on the manufacturing method of the thermoformed component according to one embodiment of the present invention. It can be manufactured by heating the thermoforming steel sheet to a temperature above the austenitizing temperature, holding it, and then rapidly cooling it in a process that is known in the past.

[0095] As an example of the present invention, after obtaining a blank using a hot-formable steel sheet according to an embodiment of the present invention, the blank can be heated to a temperature range of 900-970°C and then held for 3-15 minutes. After hot-pressing the heated and held blank as described above, a desired hot-pressed component can be manufactured by cooling it at a cooling rate of 30°C / second or higher. As a non-limiting example, the cooling can be performed at a cooling rate of 30°C / second or higher.

[0096] However, according to one embodiment of the present invention, as the steel sheet for hot forming, an aluminized steel sheet having a certain thin film layer on the coating can be used, and the heating time in the heating process of the blank manufactured using it can be shortened. Therefore, compared with the manufacturing process used to obtain existing hot-formed components, a more economical manufacturing method can be provided. Detailed Implementation

[0097] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention for more detailed explanation and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the contents recorded in the claims and the contents reasonably deduced therefrom.

[0098] (Example) An aluminized steel sheet was prepared with an aluminum-based coating on both sides of a base steel sheet, consisting of 9% silicon (by weight), the balance Al, and unavoidable impurities. A surface treatment composition containing dispersed graphite was then applied to the aluminum-based coating using a roll coating method, followed by drying and curing at a PMT standard of 120°C. This resulted in a thin film layer with a dry coating thickness of 0.40-1.50 μm. For comparison, the aluminized steel sheet without surface treatment was used as a control steel.

[0099] At this point, the composition for surface treatment is a composition in which graphite is dispersed in a polymer binder resin solution and graphite at a weight ratio of 1:0.25 or less. The polymer binder resin, relative to 100% by weight, comprises 8% acrylic resin, 0.50% Teflon-based wax, silane coupling agent (contents in Table 1), and phosphate-based corrosion resistance improver (contents in Table 1), with the balance being solvent (water + 10% alcohol). After adding graphite powder with an average particle size of 0.2-5.0 μm to the polymer binder resin as described above at the respective weight ratios, the mixture is stirred at 450 rpm for at least 15 minutes using a stirrer to obtain the composition for final surface treatment.

[0100] Based on the above, the Si and P contents and adhesion amounts in each thin film layer formed on the coating are measured, calculated, and determined.

[0101] At this point, the content and adhesion amount of each element within the film layer are measured using X-ray fluorescence analysis and a non-destructive coating thickness gauge (Betascope®). First, the content of each element is quantitatively obtained using an X-ray fluorescence analyzer. Then, after ensuring the thickness of the film layer using a non-destructive coating thickness gauge, the density of the film layer is obtained. The coating thickness and film layer density are then multiplied to calculate the adhesion amount per unit area. Finally, the adhesion amount of each element is calculated from the previously determined content and adhesion amount of the film layer.

[0102] Furthermore, the emissivity of the thermoformed steel sheet with a thin film layer formed on the coating as described above was measured. The emissivity was then measured according to KS L 2514 within a wavelength range of 2.5-50.0 μm, and calculated according to KS L 2525.

[0103] In addition, each coated steel sheet with a thin film layer formed on the coating as described above was heated to 900°C, and then held for 5 minutes. It was then stamped (formed) using a die while being cooled to obtain a hot-formed component. The corrosion resistance, weldability, and hydrogen absorption of the hot-formed component were then measured. The results are shown in Table 2.

[0104] At this time, the time required to reach the target heating temperature is simultaneously measured. Specifically, the time required to heat up is defined as 100, which is the time required to heat the untreated aluminized steel sheet to the target temperature, and the value is expressed as a value converted from this 100.

[0105] Furthermore, weldability was expressed as a value converted from the maximum welding current of untreated aluminized steel sheets, set to 100, when tested according to ISO 18278-2 (2016) and RNES-B-00010v3 (2016) standards. Hydrogen absorption was expressed as a value converted from the hydrogen absorption of untreated aluminized steel sheets, set to 100, after each hot-formed component was placed in the atmosphere for one week. Corrosion resistance was evaluated according to ISO 14993:2018.

[0106] [Table 1] [Table 2] As shown in Tables 1 and 2, it can be confirmed that the aluminized steel sheet surface-treated with the surface treatment composition according to an embodiment of the present invention exhibits improved physical properties such as weldability, hydrogen absorption, and corrosion resistance compared to the untreated coated steel sheet (control example). In particular, the higher the graphite content present in the thin film layer, the better the physical properties are.

[0107] Furthermore, it can be confirmed that the heating time can be significantly shortened compared to the control example when using high-temperature heating for hot forming of the surface-treated aluminized steel sheet. This result suggests that the efficiency of the hot forming process can be improved, and environmental pollutants generated during the manufacture of parts by hot forming can be reduced.

[0108] In addition, in Comparative Examples 1 and 2, where the surface treatment composition does not contain graphite, although the time required to heat to the target temperature is reduced by forming a thin film layer, the hydrogen absorption capacity becomes worse.

[0109] Furthermore, it was confirmed that Comparative Examples 3 to 6, which contained graphite but no phosphorus, exhibited significantly worse hydrogen absorption and reduced corrosion resistance. Specifically, compared to Comparative Example 4, Comparative Example 3 had a higher Si content in its thin film layer, resulting in a further increase in surface resistivity and poorer weldability. Similar results were observed in Comparative Examples 5 and 6.

[0110] Furthermore, similarly, although the surface treatment compositions all contain graphite, Si, and P, the excessive Si adhesion in the dried film layer in Comparative Examples 7, 9, 11, and 13, and the excessive composition adhesion in Comparative Examples 5, 10, 12, and 14 resulted in excessive surface resistance and poor solderability.

[0111] As described above, it can be confirmed that the aluminized steel sheet surface-treated using the steel sheet surface treatment composition according to an embodiment of the present invention improves emissivity through the thin film layer. Therefore, the heating time to the target temperature can be shortened during the manufacturing process of hot-formed components, which is not only advantageous in terms of productivity and economy, but also reduces environmental pollutants such as CO2 generated during component manufacturing, thus providing an environmentally friendly product.

Claims

1. A composition for surface treatment of thermoformed coated steel sheets, comprising a polymer binder resin in which graphite is dispersed, wherein the weight ratio of the polymer binder resin to graphite is 1:0.25 or less. The average particle size of the graphite is 0.2-5.0 μm. The composition comprises carbon (C), hydrogen (H), oxygen (O), nitrogen (N), silicon (Si), and phosphorus (P).

2. The composition for surface treatment of thermoformed coated steel sheet according to claim 1, wherein, The composition comprises, relative to 100% by weight of the polymer adhesive resin, 6-20% acrylic resin, 0.01-1.00% Teflon-based wax, 0.01-0.50% silane coupling agent, 0.01-2.00% phosphate-based corrosion resistance improver, and the balance being solvent.

3. The composition for surface treatment of coated steel sheet for hot forming according to claim 2, wherein, The Teflon-based wax is a polyethylene-Teflon-based wax or a polyteflon-based wax.

4. The composition for surface treatment of coated steel sheet for hot forming according to claim 2, wherein, The silane coupling agent is selected from one or more of vinyltriethoxysilane, 3-glycidyl etheroxypropyltrimethoxysilane, 3-glycidyl etheroxypropyltriethoxysilane, 3-glycidyl etheroxypropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, and 2-perfluorooctylethyltrimethoxysilane.

5. The composition for surface treatment of coated steel sheet for hot forming according to claim 2, wherein, The solvent is a mixture of water and an organic solvent, wherein the water is deionized water or distilled water, and the organic solvent is one or more selected from alcohols, ethers, and acetone.

6. A surface-treated aluminized steel sheet, comprising: Foundation steel plate; An aluminum-based coating is formed on at least one side of the base steel plate; as well as A surface-treated thin film layer is formed on the aluminum-based coating. The surface-treated thin film layer is formed from the composition of any one of claims 1 to 5.

7. The surface-treated aluminized steel sheet according to claim 6, wherein, The Si content of the surface-treated thin film layer is less than 8.00% by weight and excluding 0%, and the adhesion amount is 100.00 mg / m³. 2 the following.

8. The surface-treated aluminized steel sheet according to claim 6, wherein, The phosphorus content of the surface-treated thin film layer is less than 0.70% by weight and excluding 0%, and the adhesion amount is 10.00 mg / m³. 2 the following.

9. The surface-treated aluminized steel sheet according to claim 6, wherein, The graphite content of the surface-treated thin film layer is 0.1-60.0%.

10. The surface-treated aluminized steel sheet according to claim 6, wherein, The surface-treated thin film layer has a dry thickness of less than 1.50 μm.

11. The surface-treated aluminized steel sheet according to claim 6, wherein, The emissivity of the surface-treated thin film layer is 0.8 or higher.

12. The surface-treated aluminized steel sheet according to claim 6, wherein, The base steel plate contains, by weight percent, carbon (C): 0.02-0.60% and silicon (Si): 0.001-2.000%, Aluminum (Al): 0.001-1.000%, Manganese (Mn): 0.1-4.0%, Phosphorus (P): below 0.05%, Sulfur (S): below 0.02%, Nitrogen (N): below 0.02%, Titanium (Ti): 0-0.1%, Boron (B): 0.0001-0.0100%, Copper (Cu): 0-1.00%, Molybdenum (Mo): 0-1.00%, Chromium (Cr): 0-1.00%, Nickel (Ni): 0-1.00%, Vanadium (V): 0-1.00%. 0%, Calcium (Ca): 0-0.01%, Niobium (Nb): 0-0.1%, Tin (Sn): 0-1.0%, Tungsten (W): 0-1.0%, Antimony (Sb): 0-1.0%, Magnesium (Mg): 0-1.0%, Cobalt (Co): 0-1.0%, Arsenic (As): 0-1.0%, Zirconium (Zr): 0-1.0%, Bismuth (Bi): 0-1.0%, Rare Earth Elements (REM): 0-0.3%, Balance Fe and unavoidable impurities in carbon steel.

13. The surface-treated aluminized steel sheet according to claim 6, wherein, The aluminum-based coating is an aluminum alloy coating containing 0-27% zinc (Zn), 7.0-10.0% silicon (Si), the balance Al, and unavoidable impurities by weight.

14. A method for manufacturing a surface-treated aluminized steel sheet, comprising the following steps: Prepare an aluminized steel sheet with an aluminum coating on at least one side of a base steel sheet; A surface treatment composition is applied to the aluminum-based coating. as well as After coating the composition, it is dried and cured to form a surface-treated thin film layer. The surface treatment composition is the composition according to any one of claims 1 to 5.

15. The method for manufacturing aluminized steel sheet with surface treatment according to claim 14, wherein, The coating process is performed by any one of the following methods: bar coating, roller coating, spraying, dipping, jet extrusion, and dipping squeeze.

16. The method for manufacturing aluminized steel sheet with surface treatment according to claim 14, wherein, The drying and curing steps are performed within a temperature range of 80-150°C, based on the final reach temperature (PMT) of the coated steel sheet.

17. A thermoformed component comprising a thin film layer on a coating of a surface-treated aluminized steel sheet as described in any one of claims 6 to 13.

18. The thermoformed component according to claim 17, wherein, The thin film layer is formed from the composition of any one of claims 1 to 5.

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