Composite plated steel sheet having excellent corrosion resistance after processing and method for manufacturing the same

Abstract

An object of the present invention is to provide a composite plated steel sheet in which even if a crack is generated on a plated layer, the crack can be prevented from propagating in a resin layer, thereby having excellent corrosion resistance after processing.

Description

Technical Field

[0001] This invention relates to a coated steel sheet with excellent corrosion resistance after processing and its manufacturing method. Background Technology

[0002] Galvanized steel sheets possess the sacrificial characteristic that, when exposed to corrosive environments, zinc, with its lower oxidation-reduction potential than iron, corrodes first, thus inhibiting the corrosion of the steel. Furthermore, the zinc coating oxidizes, forming a dense network of corrosion products on the steel surface, isolating the steel from the oxidizing atmosphere and thereby improving its corrosion resistance. Due to these advantageous properties, the application of galvanized steel sheets has expanded in recent years to include building materials, household appliances, and automotive steel sheets.

[0003] However, due to increased air pollution caused by industrialization and the gradual deterioration of the corrosive environment, as well as due to strict regulations on resource and energy conservation, the demand for developing steel with superior corrosion resistance compared to existing galvanized steel sheets is increasing.

[0004] To address this issue, various studies have been conducted on manufacturing techniques for galvanized steel sheets that improve corrosion resistance by adding elements such as aluminum (Al) and magnesium (Mg) to the zinc plating bath. A representative example is Zn-Mg-Al based galvanized alloy steel sheets, in which Mg is further added to the Zn-Al plating matrix.

[0005] This type of Zn-Mg-Al based zinc-plated alloy steel sheet contains brittle phases such as MgZn2 and Zn-MgZn2 (binary phase) in the coating, making it prone to cracking during processing. These cracks propagate to the coating layer (resin layer) on top of the coating, causing the coating to burst and thus reducing the steel sheet's corrosion resistance.

[0006] Therefore, the inventors of this invention have studied a composite coated steel sheet that can suppress cracking in the coating while minimizing the propagation of cracks in the coating layer even if cracks occur on the coating layer.

[0007] [Existing Technical Documents]

[0008] [Patent Literature]

[0009] (Patent Document 1) Korean Patent Publication No. 10-2014-0083814 Summary of the Invention

[0010] Technical problems to be solved

[0011] The purpose of this invention is to provide a composite coated steel sheet in which even if cracks occur on the coating, the cracks can be prevented from propagating in the resin layer, thereby exhibiting excellent corrosion resistance after processing.

[0012] Another object of the present invention is to provide a method for manufacturing a composite coated steel sheet with excellent corrosion resistance after processing.

[0013] Technical solution

[0014] One aspect of the present invention provides a composite coated steel sheet comprising: a base steel sheet; a Zn-Mg-Al based coating disposed on at least one side of the base steel sheet; and a resin layer disposed on at least one side of the Zn-Mg-Al based coating, wherein the resin layer comprises a base resin and resin powder, and the hardness of the resin powder is 1.1-2 times that of the hardness of the base resin.

[0015] Another aspect of the present invention provides a method for manufacturing a composite coated steel sheet, comprising the following steps: preparing a base steel sheet; forming a Zn-Mg-Al based coating on at least one side of the base steel sheet; and forming a resin layer comprising a base resin and resin powder on at least one side of the Zn-Mg-Al based coating, wherein the hardness of the resin powder is 1.1-2 times that of the hardness of the base resin.

[0016] Beneficial effects

[0017] According to the present invention, a composite coated steel sheet is provided, wherein a resin powder with high hardness is mixed into the resin layer of the composite coated steel sheet to prevent cracks generated in the coating from propagating in the resin layer, thereby having excellent corrosion resistance after processing.

[0018] Furthermore, according to the present invention, a method for manufacturing a composite coated steel sheet with excellent corrosion resistance after processing is provided. Attached Figure Description

[0019] Figure 1 This is a diagram showing the bending line observed after bending a composite coated steel sheet specimen according to Embodiment 5 of one aspect of the present invention.

[0020] Figure 2 This is a diagram showing the bending line observed after bending the composite coated steel sheet specimen of Comparative Example 5.

[0021] Best practice

[0022] The preferred embodiments of the present invention will be described below. However, the embodiments of the present invention can be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0023] According to one aspect of the present invention, the composite coated steel sheet comprises: a base steel sheet; a Zn-Mg-Al based coating disposed on at least one side of the base steel sheet; and a resin layer disposed on at least one side of the Zn-Mg-Al based coating.

[0024] In this invention, there are no particular limitations on the type of base steel plate. For example, the base steel plate can be an Fe-based base steel plate, i.e., a hot-rolled steel plate or a cold-rolled steel plate, used as the base steel plate for conventional galvanized base steel plates, but is not limited thereto. Alternatively, the base steel plate can also be, for example, carbon steel, ultra-low carbon steel, or high manganese steel used as building materials, home appliance materials, automotive materials.

[0025] However, as a non-limiting example, the base steel plate may have a composition comprising, by weight %, less than 0.17% (excluding O), less than 1.5% (excluding O), Mn: 0.01-2.7%, P: less than 0.07% (excluding O), S: less than 0.015% (excluding O), Al: less than 0.5% (excluding O), Nb: less than 0.06% (excluding O), Cr: less than 1.1% (including O), Ti: less than 0.06% (excluding O), B: less than 0.03% (excluding O), and the balance being Fe and other unavoidable impurities.

[0026] According to one aspect of the invention, a Zn-Mg-Al based coating composed of a Zn-Mg-Al based alloy can be provided on at least one side of the base steel plate. The coating may be formed only on one side of the base steel plate, or it may be formed on both sides of the base steel plate. In this case, the Zn-Mg-Al based coating refers to a coating containing Mg and Al and containing more than 50% Zn.

[0027] Furthermore, according to one aspect of the invention, an Fe-Al based inhibition layer may be provided between the base steel plate and the Zn-Mg-Al based coating. The Fe-Al based inhibition layer is a layer containing an intermetallic compound of Fe and Al, such as FeAl, FeAl3, Fe2Al5, etc. It may also contain a portion of components derived from the coating, such as Zn and Mg, for example, less than 40%. The inhibition layer is formed by alloying Fe diffused from the base steel plate during the initial coating process with the plating bath composition. The inhibition layer improves the adhesion between the base steel plate and the coating, while simultaneously preventing Fe from diffusing from the base steel plate to the coating.

[0028] According to one aspect of the invention, the coating may contain, based on the composition other than iron (Fe) diffused from the base steel plate, in weight percent: Mg: 4-10%, Al: 5.1-25%, Si: less than 0.3% (except 0%) and the balance Zn and unavoidable impurities.

[0029] Mg: 1.0-6.0%

[0030] Mg is an element that provides corrosion resistance to galvanized alloy steel sheets. It effectively prevents corrosion of the galvanized steel sheet by forming a dense layer of zinc hydroxide-based corrosion products on the surface of the coating in a corrosive environment. To achieve this effect, the Mg content in the coating is controlled at 1.0% or higher in this invention. However, when the Mg content in the coating is too high, the effect of adding Mg to improve corrosion resistance will not be further enhanced. On the other hand, due to the oxidation of Mg in the plating bath, the generation of plating bath dross increases, thus requiring frequent dross removal. Therefore, in this invention, the Mg content is controlled at 6.0% or lower.

[0031] Al：1.5-13％

[0032] Al is the element that suppresses the formation of the aforementioned slag, and the Al content in the coating is preferably 1.5% or more. However, when too much Al is added to suppress slag formation, the melting point of the plating bath increases, resulting in excessively high operating temperatures. This can lead to problems such as corrosion of the plating bath structure and steel modification caused by high-temperature operation. Furthermore, when the Al content in the plating bath is too high, Al reacts with Fe in the base iron, thus failing to contribute to the formation of the Fe-Al suppression layer and rapidly undergoing reactions that promote the formation of outburst phases, resulting in excessive blocky outburst phases, which may worsen corrosion resistance. Therefore, the upper limit of the Al content in the coating is preferably controlled at 13%.

[0033] Si: less than 0.3% (except 0%)

[0034] Si, dissolved in Fe-Al compounds, contributes to ductility during the formation of the inhibition layer in galvanized alloy steel sheets. When the Si does not precipitate as Mg₂Si but accumulates in the inhibition layer, it improves the adhesion and fracture toughness of the coating. However, as the Si content in the coating increases, and given uneven surface roughness or shape of the steel sheet, brittle Mg₂Si precipitates can form coarsely at the interface between the coating and the base steel sheet, leading to cracking under external stress. Therefore, the Si content is preferably no more than 0.3%.

[0035] Balance Zn and other unavoidable impurities

[0036] In addition to the composition of the coating described above, the balance may include Zn and other unavoidable impurities. Unavoidable impurities are any impurities that are unavoidably introduced during the manufacturing process of galvanized alloy steel sheets, and their meaning will be readily understood by those skilled in the art.

[0037] In addition, a small amount of iron (Fe) can diffuse from the base steel plate and be contained in the coating. The amount of iron in the coating is so small that it is equivalent to the level of an impurity and therefore does not need to be defined separately.

[0038] According to one aspect of the invention, the average Fe content at a position halfway down the thickness direction of the coating can be 0.07% or less (including 0%). Herein, the thickness direction of the coating refers to the direction perpendicular to the rolling direction of the coated steel sheet.

[0039] In one aspect of the invention, it is necessary to control the Fe content that may be included in the plating bath. Specifically, in a Zn-Mg-Al based plating bath, a small amount of iron (Fe) introduced from the base iron may be included. When the Fe content in the plating bath increases, it reacts with Al to form fine FeAl crystals, which may cause plating defects when mixed into the coating. Therefore, as a management indicator, production management is required to ensure that the Fe content at the midpoint of the thickness direction of the Zn-Mg-Al based coating is within 0.007%. When the average Fe content at the halfway point of the thickness direction of the coating exceeds 0.07%, stress-induced coating cracks may increase.

[0040] According to one aspect of the invention, the Al to Mg ratio (Al / Mg) in the coating can be from 0.7 to 3.0. When Al / Mg is less than 0.7, excessive scum is generated on the surface of the plating bath, thus potentially increasing surface defects. On the other hand, when Al / Mg exceeds 3.0, the Al phase in the coating structure increases, thus potentially reducing the corrosion resistance of the coating.

[0041] In addition, the cracks in the coating mainly occur in the highly brittle MgZn2 phase and the large-scale Zn-MgZn2 phase. Therefore, the generation or propagation of cracks can be suppressed by appropriately adjusting the ratio of these phases.

[0042] According to one aspect of the invention, the surface of the coating, or the surface after polishing the coating surface to a thickness of 1 μm or more, may contain 10% or more but less than 80% of a region containing the MgZn2 phase, preferably 10-60% of such a region. The region containing the MgZn2 phase refers to a region where the MgZn2 phase exists alone, or in the form of an alloy phase such as Al-MgZn2, Zn-Al-MgZn2, or Zn-MgZn2. Various phases, such as Zn single phase, Al phase, and MgZn2 phase, may exist in the coating. Phases with a width of 3 μm or more are considered single phases, while phases with a width of less than 3 μm and adjacent to MgZn2 phases are considered alloy phases.

[0043] When the phase fraction of the region containing the MgZn2 phase is less than 10%, the corrosion resistance may decrease. On the other hand, when the phase fraction of the region containing the MgZn2 phase exceeds 80%, the brittleness of the coating increases, and cracks may occur during processing.

[0044] This phase present on the coating surface can be observed using a microscope, and the phase present on the polished surface can be measured using a microscope or image analyzer.

[0045] Additionally, as a separate method, the phase fraction of the MgZn2 phase can be measured by converting the relative weight ratio, which is obtained by calculating the integrated intensity of the diffraction peaks of each phase as measured by X-ray diffraction (XRD). The instrument used to measure the specimen of the present invention is a Rigaku D / Max2200. Furthermore, the relative weight ratio can be verified using the Rietveld measurement method and the phase diagram calculation program, which are accurate methods for measuring phase fraction.

[0046] The coating, by phase fraction, contains 10-60% MgZn2 phase, which is caused by the Al and Mg content in the plating bath. For example, when the Mg content is less than 1.0%, less than 10% MgZn2 phase can be generated. Compared with coatings composed of other alloy phases, the MgZn2 phase has a relatively higher hardness, so the hardness of the coating can be adjusted by controlling the phase fraction of the MgZn2 phase. When the phase fraction of the MgZn2 phase is less than 10%, the hardness of the coating is less than 220 Hv, which may lead to a significant increase in the coefficient of friction after more than 30 cycles.

[0047] On the other hand, when the phase fraction of MgZn2 exceeds 60%, the high-hardness MgZn2 phase in the coating is unevenly and coarsely aggregated. Therefore, since the Zn single phase and Zn-Al-MgZn2 ternary eutectic structure that ensure uniform processability cannot be uniformly distributed, cracks may be generated during processing. Corrosion can easily propagate through these cracks, and thus the corrosion resistance may decrease rapidly.

[0048] According to one aspect of the invention, the thickness of the Fe-Al based inhibition layer can be 0.02 μm or more and 2.5 μm or less. The inhibition layer serves to ensure corrosion resistance by preventing alloying, but due to its brittleness, it may adversely affect processability; therefore, the thickness of the inhibition layer can be controlled to be less than 2.5 μm. However, to achieve the function of an inhibition layer, it is preferable to control the thickness of the inhibition layer to be 0.02 μm or more. In this case, the thickness of the inhibition layer can refer to the minimum thickness measured using SEM or TEM equipment in the direction perpendicular to the interface.

[0049] According to one aspect of the present invention, a coating layer can be formed on the plating layer. The coating layer may include a resin layer, and in addition to the resin layer, may also include a chromium coating layer, an undercoat layer, etc. In the present invention, the resin layer is also referred to as an upper coating layer. As described above, in order to prevent cracks in the plating layer from propagating to the coating layer, it may contain resins of different hardness.

[0050] According to one aspect of the invention, the resin layer may comprise a base resin and resin powder. The base resin is a soft substance that absorbs deformation caused during the processing of the coated steel sheet. Preferably, the base resin is selected from one or more of polyester-based resins, urea-based resins, epoxy-based resins, urethane-based resins, and vinyl resins, but is not limited thereto.

[0051] The base resin may comprise 30-65% by weight relative to the total weight of the resin layer. When the base resin content is less than 40% by weight, the elasticity of the resin layer decreases, making it prone to breakage. On the other hand, when the base resin content exceeds 60% by weight, the resin may adhere to the mold during processing due to increased viscosity.

[0052] When processing coated steel sheets, if cracks occur in the coating exceeding the processing deformation, the cracks propagate through the base resin. When these cracks deform the outermost resin layer and exceed the crack critical point, cracks also form in the outermost resin layer. However, when the cracks reach the resin powder, which is a hard substance, the cracks stop or change their propagation direction, thus preventing further crack propagation.

[0053] To effectively prevent the propagation of cracks generated in the coating, the resin powder with a hardness 1.1 to 2 times that of the base resin can be used. When the hardness ratio is less than 1.1 times, the slight hardness difference between the base resin and the resin powder makes it difficult to sufficiently suppress crack propagation. On the other hand, when the hardness difference exceeds 2 times, the excessively high hardness difference causes stress to concentrate between the resin layer and the resin powder, which may actually lead to crack expansion, and is therefore not preferred.

[0054] In this invention, polyurethane-based resins, such as polyester urethane, polyether urethane, etc., can be used as resin powder, but are not limited thereto. As mentioned above, as long as they have a certain hardness ratio with the base resin, they can be used without restriction.

[0055] The resin powder content in the resin layer can be 4-25% by weight. When the resin powder content is less than 4% by weight, the resin powder content is too low and cannot fully exert the effect of inhibiting crack propagation. On the other hand, when the resin powder content exceeds 25% by weight, the ductility of the resin layer decreases, which may increase the generation of cracks.

[0056] The resin powder may contain 5-30% by volume relative to the volume of the resin layer, preferably 10-20% by volume. When the volume percentage of resin powder is less than 5% by volume, it may not be able to fully exert its effect of inhibiting crack propagation. On the other hand, when the content of resin powder exceeds 30% by volume, the ductility of the resin layer decreases, which may increase the occurrence of cracks.

[0057] Furthermore, the resin powder can have a particle size of 30-700 μm. 2 The average surface area is preferably 50-500 μm. 2 The average surface area. When the average surface area is less than 30 μm... 2 At times, the effect of crack suppression may decrease; on the other hand, when the average surface area exceeds 700 μm... 2 When this happens, the resin layer will have an uneven appearance, and due to this unevenness, the aesthetics may be reduced.

[0058] The shape of the resin powder is not particularly limited, and it can be round, angular granular, or cylindrical.

[0059] According to one aspect of the invention, in addition to the base resin and resin powder, the resin layer may also contain a crosslinking agent or a pigment.

[0060] Crosslinking agents are components that form crosslinks between base resins. For example, one or more of melamine resin and isocyanate resin can be selected, but are not limited to this.

[0061] The content of the crosslinking agent can be appropriately adjusted according to the content of the base resin, preferably 3-10% by weight relative to the total weight of the resin layer. When the content of the crosslinking agent is less than 3% by weight, the resin layer is not cured, so the hardness may be reduced. When the content of the crosslinking agent exceeds 10% by weight, the hardness of the resin layer is too high, and cracks may naturally occur even without the application of external stress.

[0062] Pigments are components included in the resin layer to provide color, and can be selected from one or more, but not limited to, titanium dioxide and rust-inhibiting pigments. The pigment content can be 10-35% by weight relative to the total weight of the resin layer. When the pigment content is less than 10% by weight, the rust-inhibiting performance of the resin layer may decrease, and it may be difficult to achieve a vibrant color. On the other hand, when the pigment content exceeds 35% by weight, the elasticity of the resin layer decreases, and therefore cracking may occur.

[0063] According to one aspect of the invention, the resin layer can be a single layer or two or more layers. The thickness of the resin layer can be 3-30 μm. When the thickness of the resin layer is less than 3 μm, the surface protection effect of the resin layer is reduced, and the surface quality may be reduced because bending or defects on the coating surface will transfer to the surface. On the other hand, when the thickness of the resin layer exceeds 30 μm, the coating and drying speed of the resin layer becomes slower, productivity decreases, and manufacturing costs increase, which is therefore not preferred.

[0064] Next, a method for manufacturing a composite coated steel sheet according to another aspect of the present invention will be described. However, this does not mean that the composite coated steel sheet of the present invention must be manufactured by the following method.

[0065] According to one aspect of the invention, the step of preparing a base steel plate may be included, wherein there is no particular limitation on the type of base steel plate. It may be an Fe-based base steel plate used as a base steel plate for conventional hot-dip galvanized steel plates, i.e., it may be a hot-rolled steel plate or a cold-rolled steel plate, but is not limited thereto. Furthermore, the base steel plate may be, for example, carbon steel, ultra-low carbon steel, or high-manganese steel used as materials for automobiles, but is not limited thereto.

[0066] According to one aspect of the invention, the process may include immersing a base steel plate in a plating bath for hot-dip galvanizing, wherein the plating bath, by weight percent, comprises: Mg: 1.0-6.0%, Al: 1.5-13%, Si: less than 0.3% (excluding 0%), and the balance Zn and other unavoidable impurities. To manufacture the plating bath with the above composition, composite steel ingots containing the specified Zn, Al, and Mg, or Zn-Mg or Zn-Al steel ingots containing individual components, can be used. Furthermore, the composition of the plating bath, except for the content of Fe introduced from the base steel plate, can be similarly described for the composition of the coating.

[0067] Furthermore, according to one aspect of the invention, the temperature of the plating bath is maintained at 440-520°C during dissolution. As the temperature of the plating bath increases, the fluidity of the plating bath can be ensured, a uniform composition can be formed, and the amount of scum generated can be reduced. When the temperature of the plating bath is below 440°C, the dissolution of the steel ingot is very slow, and the viscosity of the plating bath is high, making it difficult to ensure excellent coating surface quality. On the other hand, when the temperature of the plating bath exceeds 520°C, problems may occur with ash defects caused by Zn evaporation on the coated surface, and excessive Fe diffusion may lead to the formation of excessive protruding phases. The temperature of the plating bath can be maintained at a temperature 20-80°C higher than the melting point of the plating bath.

[0068] After immersing the base steel plate in the above-mentioned plating bath, the immersion time can be in the range of 1-6 seconds.

[0069] Furthermore, according to one aspect of the invention, the following steps may be included: cooling from the plating bath surface to the upper roller section using an inert gas at an average cooling rate of 3-30°C / s. In this case, when the cooling rate from the plating bath surface to the upper roller section is less than 3°C / s, the MgZn2 structure becomes excessively coarse, which may exacerbate surface curvature of the coating. Furthermore, the formation of a wide Zn-MgZn2 binary eutectic structure or an Al-Zn-MgZn2 ternary eutectic structure may be detrimental to ensuring uniform corrosion resistance and processability. On the other hand, when the cooling rate from the plating bath surface to the upper roller section exceeds 30°C / s, rapid solidification occurs in the temperature range during the hot-dip plating process, from the initial solidification from the liquid phase to the complete transformation of the liquid phase into the solid phase. Therefore, the MgZn2 structure becomes too small, potentially resulting in localized non-uniformity in corrosion resistance. Furthermore, due to insufficient uniform growth of the Fe-Zn-Al phase, which is concentrated at the interface between the coating and the base steel plate, the processability may be worse. In order to achieve an excessively fast cooling rate, the amount of nitrogen used increases, which may increase manufacturing costs.

[0070] According to one aspect of the invention, the inert gas may contain one or more of N2, Ar and He, with N2 or N2+Ar preferred for reducing manufacturing costs.

[0071] According to one aspect of the invention, surface oxides can be removed by shot peening the surface of the base steel plate before plating. This may include the following steps: the shot peening process gives the base steel plate a surface shape with Ra: 0.5-3.0 μm, Rz: 1-20 μm, and Rpc: 10-100 (counts / cm).

[0072] According to one aspect of the invention, shot peening controls the base steel plate to have a surface shape with Ra: 0.5-3.0 μm, Rz: 1-20 μm, and Rpc: 10-100 (counts / cm), activating the reactivity of the base steel plate surface, thereby enabling more uniform formation of solidification nuclei during coating solidification. Therefore, not only can a coated steel plate with excellent surface quality be obtained, but also excellent machinability can be ensured by preventing the formation of localized crack initiation points during processing through the formation of a uniform microstructure on the surface.

[0073] Furthermore, according to one aspect of the invention, the shot peening process can utilize metal balls with a diameter of 0.3-10 μm, or the running speed of the steel plate can be controlled at 50-150 meters per minute (mpm), or the metal balls can be controlled at 300-3000 kg / min to impact the surface of the steel plate.

[0074] That is, according to one aspect of the invention, the shot peening can be performed by impacting the surface of a steel plate moving at a speed of 50-150 mpm with a metal ball of diameter of 0.3-10 μm at a speed of 300-3000 kg / min.

[0075] According to one aspect of the invention, by shot peening the base steel plate before plating to meet the above conditions, mechanical dislocations are introduced before surface plating, thereby rapidly and uniformly forming an inhibition layer, or the surface of the base steel plate can be activated to form solidification nuclei more uniformly when the plating solidifies.

[0076] That is, by meeting the above conditions during shot peening, the following problems can be prevented: due to the harsh shot peening process, a rough structure is formed, resulting in poor machinability; or due to insufficient shot peening, the surface activation degree of the base steel plate before plating is low, thus reducing the surface uniformity.

[0077] Therefore, by shot peening the base steel plate before plating and optimizing the shot peening conditions, it is easy to manufacture a plated steel plate that meets one or more of the conditions of Ra, Rz, cross-sectional hardness and thickness of the coating within the above-mentioned range.

[0078] According to one aspect of the invention, in order to form a resin layer on the coating, the coating may first be washed with an alkaline degreasing agent to remove foreign matter adhering to the surface of the coating. Subsequently, a chromium coating layer may be formed by performing a chromate film treatment on the coating.

[0079] Next, an undercoat layer with a thickness of 3-30 μm can be formed by applying an undercoat layer over the chromium coating layer. The undercoat layer can be a coating that exhibits excellent adhesion and corrosion resistance to the plated steel sheet and excellent adhesion to the overcoat layer described below. For example, a coating comprising 40-80 wt% of a base resin such as polyester, 5-10 wt% of a melamine crosslinking agent, 10-30 wt% of TiO2, 10-40 wt% of an aromatic hydrocarbon or ester solvent, and 2-5 wt% of other additives can be used as the undercoat layer according to one aspect of the invention.

[0080] After applying the top coating to the undercoat layer formed by drying the undercoat at a temperature of 150-300°C, the top coating is dried at a temperature of 50-300°C to form a resin layer according to one aspect of the invention. At this time, as described above, the top coating may contain a base resin and resin powder with a hardness 1.1-2 times that of the base resin. For example, it may contain 30-65% by weight of the base resin, 3-8% by weight of a crosslinking agent, 5-25% by weight of pigment, 5-30% by weight of solvent, and 2-3% by weight of other additives. The resin layer formed as described above may contain 5-30% by volume of resin powder. Detailed Implementation

[0081] Example

[0082] The embodiments of the present invention will be described in detail below. These embodiments are for understanding the present invention only and do not limit the scope of the invention.

[0083] In Tables 1 to 6 below, apart from the base resin, resin powder, crosslinking agent, and pigment, the remaining components are solvents and other additives.

[0084] Experimental Example 1: Evaluation of the performance of composite coated steel sheets based on the hardness ratio of base resin and resin powder

[0085] For a base steel sheet containing 0.03% C, 0.7% Si, 0.5% Mn, 0.003% P, 0.002% S, 0.05% Al and the balance Fe and other unavoidable impurities by weight, a coating is formed containing 5.2% Mg, 12.5% ​​Al and 0.005% Si by weight.

[0086] A resin solution is coated onto the coating to give the resin layer the composition described in Table 1 below, and then dried to form the resin layer. After separately manufacturing plate-shaped test pieces of the base resin material and the resin powder material, the hardness of the base resin and the resin powder is measured using a Type D Shore hardness tester.

[0087] [Table 1]

[0088]

[0089] The composite coated steel sheets in Table 1 were cut transversely and longitudinally into 7cm×15cm pieces to manufacture composite coated steel sheet test pieces, and then the performance of the composite coated steel sheets was evaluated.

[0090] Evaluation of crack propagation suppression performance

[0091] The specimen was bent 180° relative to the centerline of the cut composite coated steel sheet, bringing both sides into contact. Cracks appearing in the coating at the bend line were then observed. The degree of crack initiation was evaluated according to the following criteria.

[0092] ◎: No cracks were found on the paint layer along the curve.

[0093] ○: Fewer than 5 defects with crack lengths less than 1 mm appearing on the coating layer along the curved line.

[0094] △: The formation of 5 to 10 microcracks with a length of less than 1 mm.

[0095] ×: Resulting in 10 or more defects with a length of less than 1 mm, or Resulting in 1 or more defects with a length of more than 1 mm.

[0096] <Evaluation of corrosion resistance after processing>

[0097] As described above, for the test pieces of the bent composite coated steel sheet, white rust was observed using a salt spray tester based on the test method of ISO 14993. The corrosion resistance after processing was evaluated according to the following evaluation criteria.

[0098] ◎: No white rust appeared on the surface of the curved line after more than 500 hours.

[0099] ○: White rust appears after 300 hours but less than 500 hours.

[0100] △: White rust appears after 100 hours but less than 300 hours.

[0101] ×: White rust appears within 100 hours.

[0102] [Table 2]

[0103] Hardness of resin powder / Hardness of base resin Performance in suppressing crack propagation Corrosion resistance after processing Example 1 1.7 ◎ ◎ Example 2 1.1 ◎ ◎ Example 3 1.5 ◎ ◎ Example 4 1.1 ◎ ◎ Example 5 1.2 ◎ ◎ Example 6 1.7 ◎ ◎ Comparative Example 1 2.2 X X Comparative Example 2 0.9 X X

[0104] As shown in Table 2, in Examples 1 to 6 where the hardness ratio of resin powder to base resin meets the scope of the present invention, it is evident that both crack propagation suppression performance and post-processing corrosion resistance are excellent. On the other hand, in Comparative Examples 1 and 2 where the hardness ratio of resin powder to base resin is outside the scope of the present invention, the crack propagation suppression performance and post-processing corrosion resistance are significantly worse.

[0105] Experimental Example 2: Evaluation of the performance of composite coated steel sheets based on the composition of the resin solution

[0106] A coating having the same composition as in Experimental Example 1 was formed on the same base steel plate as in Experimental Example 1. A resin solution having the composition described in Table 3 below was coated onto the coating and dried to form a resin layer.

[0107] [Table 3]

[0108]

[0109] The composite coated steel sheets in Table 3 were cut transversely and longitudinally into 7cm × 15cm pieces to manufacture composite coated steel sheet test pieces. The performance of the composite coated steel sheets was then evaluated and recorded in Table 4 below.

[0110] Furthermore, the composite coated steel sheet specimens of Example 5 and Comparative Example 5 were subjected to bending processing, and the results of observing the bending lines are shown below. Figure 1 and Figure 2 middle.

[0111] [Table 4]

[0112] Hardness of resin powder / Hardness of base resin Performance in suppressing crack propagation Corrosion resistance after processing Example 1 1.7 ◎ ◎ Example 2 1.1 ◎ ◎ Example 3 1.5 ◎ ◎ Example 4 1.1 ◎ ◎ Example 5 1.2 ◎ ◎ Example 6 1.7 ◎ ◎ Comparative Example 3 1.1 △ △ Comparative Example 4 1.2 △ △ Comparative Example 5 0.0 X X Comparative Example 6 1.4 △ X

[0113] Referring to Table 4, Examples 1 to 6, with resin layers having compositions according to the present invention, exhibited excellent crack propagation suppression performance and post-processing corrosion resistance. On the other hand, when the composition of the resin layer was not within the scope of the present invention, even if the hardness ratio of the resin powder to the base resin met the scope of the present invention, the crack propagation suppression performance and post-processing corrosion resistance showed poor results. In particular, it was confirmed that no cracks occurred on the composite coated steel plate of Example 5. Figure 1 However, in Comparative Example 5, numerous cracks appeared on the coating layer of the composite galvanized steel sheet. Figure 2 ).

[0114] Experimental Example 3: Evaluation of the performance of composite coated steel sheets based on the volume ratio of resin powder

[0115] A coating with the same composition as in Experimental Example 1 was formed on a base steel plate. A resin solution comprising polyester resin, polyurethane resin powder, melamine crosslinking agent, and TiO2 pigment was coated onto the coating, and then dried to form a resin layer, thereby manufacturing a composite coated steel plate. At this time, the hardness ratio of the resin powder to the base resin was 1.1 to 1.9.

[0116] The content (weight %) of each component of the resin layer and the volume ratio (volume %) of the resin powder are recorded in Table 5 below.

[0117] After fabricating cross-sectional specimens of the resin layer and performing mirror polishing, the distribution of resin powder contained in the resin layer was observed using a scanning electron microscope (SEM). Furthermore, after coloring the areas containing resin powder, the proportion of resin powder was measured using an image analyzer, and this value was determined as the volume ratio of the resin powder.

[0118] [Table 5]

[0119]

[0120] Referring to Table 5, Examples 7 to 12, where the volume ratio of resin powder meets the requirements of the present invention, exhibit excellent crack propagation suppression performance and post-processing corrosion resistance. However, Comparative Examples 7 to 12, where the volume ratio of resin powder does not meet the requirements of the present invention, show poorer crack propagation suppression performance and post-processing corrosion resistance.

[0121] Experimental Example 4: Evaluation of the performance of composite coated steel sheets based on the average surface area of ​​resin powder

[0122] A coating with the same composition as in Experimental Example 1 was formed on the same base steel plate as in Experimental Example 1. A resin solution comprising polyester resin, polyurethane resin powder, melamine crosslinking agent, and TiO2 pigment was coated onto the coating and then dried to form a resin layer with the composition described in Table 6. At this time, the hardness ratio of the resin powder to the base resin was 1.1 to 1.5.

[0123] The average surface area of ​​the resin powder was determined by taking photographs using a scanning electron microscope (SEM) at magnifications of 500 to 1000, and the resin powder was colored for differentiation. The average surface area of ​​the resin powder was then measured using an image analyzer. The measured values ​​are recorded in Table 6 below.

[0124] The composite coated steel sheets in Table 6 were cut transversely and longitudinally into 7cm×15cm pieces to manufacture composite coated steel sheet test pieces, and then the performance of the composite coated steel sheets was evaluated.

[0125] [Table 6]

[0126]

[0127] Referring to Table 6, the resin powders with average surface areas meeting the requirements of Examples 13 to 18 of the present invention exhibit excellent crack propagation suppression performance and post-processing corrosion resistance. However, Comparative Examples 13 to 18, whose average surface areas of the resin powders are not within the scope of the present invention, show poorer crack propagation suppression performance and post-processing corrosion resistance.

Claims

1. A composite coated steel sheet, comprising: Foundation steel plate; A Zn-Mg-Al based coating is disposed on at least one side of the base steel plate; as well as A resin layer is disposed on at least one side of the Zn-Mg-Al based coating. The resin layer comprises a base resin and resin powder. The Shore D hardness of the resin powder is 1.1-2 times that of the base resin. The Shore D hardness is measured using a Shore D hardness tester for plate-shaped specimens made from the resin powder and plate-shaped specimens made from the base resin, respectively. Relative to the total weight of the resin layer, the resin layer comprises 30-65% by weight of base resin and 4-25% by weight of resin powder. The resin powder has an average surface area of ​​30-700 μm. 2 .

2. The composite coated steel sheet according to claim 1, wherein, The base resin is selected from one or more of polyester-based resins, urea-based resins, epoxy-based resins, urethane-based resins, and vinyl resins.

3. The composite coated steel sheet according to claim 1, wherein, The resin powder comprises polyurethane-based resin powder.

4. The composite coated steel sheet according to claim 1, wherein, The amount of resin powder is 5-30% by volume relative to the volume of the resin layer.

5. The composite coated steel sheet according to claim 1, wherein, The resin layer further comprises at least one of a crosslinking agent and a pigment, wherein the crosslinking agent is selected from one or more melamine resins and isocyanate resins, and the pigment is selected from one or more titanium dioxide and anti-rust pigments.

6. The composite coated steel sheet according to claim 5, wherein, The resin layer contains 3-8% by weight of crosslinking agent and 5-25% by weight of pigment relative to the total weight of the resin layer.

7. The composite coated steel sheet according to claim 1, wherein, The coating, by weight percent, comprises: Mg: 1.0-6.0%, Al: 1.5-13%, Si: less than 0.3% and excluding 0%, with the balance being Zn and other unavoidable impurities. Furthermore, the average Fe content at a position halfway along the thickness direction of the coating is 0.07% or less and includes 0%. The ratio of Al to Mg is 0.7 to 3.

0.

8. The composite coated steel sheet according to claim 1, wherein, The phase fraction of MgZn2 phase and MgZn2 alloy phase in the coating is 10-80%.

9. The composite coated steel sheet according to claim 1, wherein, The composite coated steel sheet further includes an Fe-Al based inhibition layer disposed between the base steel sheet and the coating.

10. A method for manufacturing a composite coated steel sheet, comprising the following steps: Prepare the foundation steel plate; A Zn-Mg-Al based coating is formed on at least one side of the base steel plate; and A resin layer comprising a base resin and resin powder is formed on at least one side of the Zn-Mg-Al based coating. The Shore D hardness of the resin powder is 1.1-2 times that of the base resin. The Shore D hardness is measured using a Shore D hardness tester for plate-shaped specimens made from the resin powder and plate-shaped specimens made from the base resin, respectively. Relative to the total weight of the resin layer, the resin layer comprises 30-65% by weight of base resin and 4-25% by weight of resin powder. The resin powder has an average surface area of ​​30-700 μm. 2 .

11. The method for manufacturing composite coated steel sheet according to claim 10, wherein, The base resin is selected from one or more of polyester-based resins, urea-based resins, epoxy-based resins, urethane-based resins, and vinyl resins.

12. The method for manufacturing composite coated steel sheet according to claim 10, wherein, The resin powder comprises polyurethane-based resin powder.

13. The method for manufacturing composite coated steel sheet according to claim 10, wherein, The amount of resin powder is 5-30% by volume relative to the volume of the resin layer.

14. The method for manufacturing composite coated steel sheet according to claim 10, wherein, The resin layer further comprises at least one of a crosslinking agent and a pigment, wherein the crosslinking agent is selected from one or more melamine resins and isocyanate resins. The pigment is selected from one or more of titanium dioxide and anti-rust pigments.

15. The method for manufacturing composite coated steel sheet according to claim 14, wherein, The resin layer comprises, relative to the total weight of the resin layer, 3-8% by weight of crosslinking agent and 5-25% by weight of pigment.

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