CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of Korean Patent Application No. 10-2014-0173734 filed on December 5, 2014, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND
[0002] The present disclosure relates a plated steel sheet, a steel sheet coated with a composite resin, and methods for manufacturing thereof.
[0003] Examples of steel sheets widely used to form automotive fuel tanks or the like include hot-dip Al-Si-plated steel sheets, Zn-Ni electro-galvanized steel sheets, and Zn-Sn electro-galvanized steel sheets. Such plated steel sheets have high quality but incur high manufacturing costs due to relatively expensive metals used to plate the steel sheets.
[0004] In addition, steel sheets which are galvanized and then coated mainly with polymer resins have high resistance to fuels and corrosion but have poor weldability, due to insulating coatings formed thereon. In the related art, methods of mixing a polymer resin composition with metal particles have been tried, in order to improve conductivity and thus, weldability. However, such techniques increase coating thicknesses, manufacturing costs, and difficulties in terms of working, and lead to the generation of dust and defects during processes.
[0005] Although techniques of using relatively expensive hot-dip Zn-Sn alloy plated steel sheets have been commercialized, these techniques incur high costs and lead to poor resistance to fuels and corrosion in portions formed by a deep drawing method, due to the absence of sacrificial protection for anti-corrosion. Moreover, various fuels such as biofuels have recently been used, and thus the environments in which fuel tanks are used have been diversified. Therefore, the development of steel sheets having higher resistance to fuels is required.

SUMMARY
[0006] Aspects of the present disclosure may provide a plated steel sheet having a high degree of resistance to fuels and corrosion and a high degree of seam weldability, a method for manufacturing the plated steel sheet, a composite resin coated steel sheet manufactured by coating the plated steel sheet with a composite resin containing a urethane-acrylic dendrimer resin, and a method for manufacturing the composite resin coated steel sheet.
[0007] According to an aspect of the present disclosure, a plated steel sheet may include a plating layer having a crater fraction of 15% or less, a plating amount of 20 g/m2 to 60 g/m2, and an alloying degree of 7% to 15%.
[0008] According to another aspect of the present disclosure, a composite resin coated steel sheet may include: a steel sheet; a plating layer having a crater fraction of 15% or less, a plating amount of 20 g/m2 to 60 g/m2, and an alloying degree of 7% to 15%; and a coating layer formed on the plating layer, wherein the coating layer may be a cured product of a steel sheet coating composition.
[0009] The steel sheet coating composition may include a urethane-acrylic dendrimer resin, 15 parts by weight to 100 parts by weight of a hardener, 10 parts by weight to 70 parts by weight of a metal silicate, and 5 parts by weight to 50 parts by weight of a titanium compound, based on 100 parts by weight of the urethane-acrylic dendrimer resin.
[0010] The hardener may include at least one of a melamine-containing hardener and an aziridine-containing hardener.
[0011] The melamine-containing hardener may include at least one of melamine, butoxymethyl melamine, hexamethoxymethyl melamine, and trimethoxymethyl melamine.
[0012] The metal silicate may include at least one of lithium polysilicate, sodium polysilicate, potassium polysilicate, and colloidal silica.
[0013] The titanium compound may include at least one of titanium carbonate, isopropyl ditriethanolamino titanate, lactic acid titanium chelate, and titanium acetylacetonate.
[0014] The coating layer may have a thickness of 0.1 µm to 2.0 µm in a dried state.
[0015] The coating layer may have a density of 0.1 g/m2 to 2.0 g/m2 in a dried state.
[0016] According to another aspect of the present disclosure, a method for manufacturing a plated steel sheet may include: dipping a steel sheet into a plating bath; adjusting a plating amount within a range of 20 g/m2 to 60 g/m2; and adjusting an alloying temperature within a range of 460°C to 520°C to manufacture an alloy-plated steel sheet having an alloying degree of 7% to 15%, wherein the alloy-plated steel sheet may have a crater faction of 15% or less.
[0017] According to another aspect of the present disclosure, a method for manufacturing a composite resin coated steel sheet may include: dipping a steel sheet into a plating bath; adjusting a plating amount within a range of 20 g/m2 to 60 g/m2; adjusting an alloying temperature within a range of 460°C to 520°C to manufacture an alloy-plated steel sheet having an alloying degree of 7% to 15%; coating the alloy-plated steel sheet with a steel sheet coating composition; and performing a curing process on the alloy-plated steel sheet coated with the steel sheet coating composition so as to form a coating layer, wherein the alloy-plated steel sheet may have a crater faction of 15% or less.
[0018] The steel sheet coating composition may include a urethane-acrylic dendrimer resin, 15 parts by weight to 100 parts by weight of a hardener, 10 parts by weight to 70 parts by weight of a metal silicate, and 5 parts by weight to 50 parts by weight of a titanium compound, based on 100 parts by weight of the urethane-acrylic dendrimer resin.
[0019] The coating of the alloy-plated steel sheet may be performed using a method selected from a roll coating method, a casting method, a screen printing method, a spraying method, a gravure coating method, and a dipping method.
[0020] The coating layer may be formed while maintaining the alloy-plated steel sheet at a temperature of 80°C to 200°C.
[0021] The coating layer may have a thickness of 0.1 µm to 2.0 µm in a dried state.
[0022] The coating layer may have a density of 0.1 g/m2 to 2.0 g/m2 in a dried state.

BRIEF DESCRIPTION OF DRAWINGS
[0023] The above and other aspects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 illustrates the chemical structural formula of a urethane-acrylic dendrimer;
FIG. 2 illustrates the chemical structural formula of an aziridine hardener;
FIG. 3 illustrates scanning electron microscope (SEM) images of cross-sections of plating layers of Example 6 and Comparative Example 1;
FIG. 4 is surface images taken from hot-dip galvanized steel sheets of Example 6 and Comparative Example 1 after a corrosion resistance test; and
FIG. 5 is surface images of specimen cups formed in Example 6 and Comparative Example 1, photographed after a fuel resistance test.

DETAILED DESCRIPTION
[0024] Exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings.
[0025] The disclosure may, however, be exemplified in many different forms and should not be construed as being limited to the specific embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
[0026] In the drawings, the shapes and dimensions of elements may be exaggerated for clarity, and the same reference numerals will be used throughout to designate the same or like elements.
[0027] Exemplary embodiments of the present disclosure provide a plated steel sheet and a composite resin coated steel sheet that are relatively inexpensive and have a high degree of weldability and a high degree of resistance to fuels, and methods for manufacturing the steel sheets.
[0028] An exemplary embodiment of the present disclosure may provide a plated steel sheet including a plating layer having a crater fraction of 15% or less, a plating amount of 20 g/m2 to 60 g/m2, and an alloying degree of 7% to 15%.
[0029] If the crater faction of the plated steel sheet is greater than 15%, the curing temperature of a coating layer may increase, and thus energy efficiency may decrease. In addition, if the plating amount is less than 20 g/m2, the plating layer may not have a sufficient degree of resistance to corrosion and fuel, and if the plating amount is greater than 60 g/m2, the insulating ability of the plating layer may increase, and thus the weldability of the plated steel sheet may decrease. In addition, if the alloying degree of the plated steel sheet is less than 7%, it may be difficult to obtain a sufficient degree of resistance to corrosion and fuel. If the alloying degree of the plated steel sheet is greater than 15%, although the corrosion and fuel resistance of the plated steel sheet increases, the weldability of the plated steel sheet may decrease because of an increase in insulating ability.
[0030] Another exemplary embodiment of the present disclosure may provide a composite resin coated steel sheet including a steel sheet; a plating layer formed on at least one surface of the steel sheet and having a crater fraction of 15% or less, a plating amount of 20 g/m2 to 60 g/m2, and an alloying degree of 7% to 15%; and a coating layer formed on the plating layer. The coating layer may be a cured product of a steel sheet coating composition.
[0031] The steel sheet coating composition may include a urethane-acrylic dendrimer resin, 15 parts by weight to 100 parts by weight of a hardener, 10 parts by weight to 70 parts by weight of a metal silicate, and 5 to 50 parts by weight of a titanium compound, based on 100 parts by weight of the urethane-acrylic dendrimer resin.
[0032] Since the urethane-acrylic dendrimer resin has many reactive groups, as illustrated in the chemical structural formula of FIG. 1, the urethane-acrylic dendrimer resin may be cured at a relatively low temperature. In addition, since the urethane-acrylic dendrimer resin reacts with the hardener and forms a dense 3D network structure, the steel sheet coated with the coating layer including the urethane-acrylic dendrimer resin may have a high degree of fuel and corrosion resistance.
[0033] Preferably, the content of the urethane-acrylic dendrimer resin may be 5 wt% to 20 wt% based on the total weight of the steel sheet coating composition. If the content of the urethane-acrylic dendrimer resin is less than 5 wt%, the coating layer may have poor fuel resistance. If the content of the urethane-acrylic dendrimer resin is greater than 20 wt%, it may be difficult to cure the coating layer at a low temperature and obtain a high degree of corrosion resistance.
[0034] The hardener may include at least one selected from an aziridine hardener and a melamine hardener having a high degree of reactivity. The melamine hardener may include at least one of melamine, butoxymethyl melamine, hexamethoxymethyl melamine, and trimethoxymethyl melamine. The aziridine hardener may have the chemical structural formula illustrated in FIG. 2 where R may refers to -H, -CH3, -CH2CH3, -CH2CH2CH3, -CH2OH, or -CH2CH2OH.
[0035] Preferably, the content of the hardener may be 15 parts by weight to 100 parts by weight based on 100 parts by weight of the urethane-acrylic dendrimer resin. If the content of the hardener is less than 15 parts by weight, the coating layer may not completely undergo a curing reaction in a dried state and thus may have undesirable properties, and if the content of the hardener is greater than 100 parts by weight, since the amount of the hardener is excessive, side reactions may occur and the processability of the coating layer may deteriorate.
[0036] Although the urethane-acrylic dendrimer resin and the hardener are organic compositions, the metal silicate is an inorganic composition and thus may improve the corrosion resistance of the dried coating layer. For example, the metal silicate may include, but is not limited to, at least one selected from lithium polysilicate, sodium polysilicate, potassium polysilicate, and colloidal silica.
[0037] Preferably, the content of the metal silicate may be 10 parts by weight to 70 parts by weight based on 100 parts by weight of the urethane-acrylic dendrimer resin. If the content of the metal silicate is less than 10 parts by weight, the effect of improving corrosion resistance may be insufficient because the content of the metal silicate is too low, and if the content of the metal silicate is greater than 70 parts by weight, the coating layer may have a tough surface even though the corrosion resistance of the coating layer may be improved.
[0038] The titanium compound being an inorganic composition may improve the corrosion resistance of the dried coating layer, and even though the titanium compound is added in small amounts, the titanium compound may improve the corrosion resistance of machined portions of the steel sheet. The titanium compound may include at least one selected from titanium carbonate, isopropyl ditriethanolamino titanate, lactic acid titanium chelate, and titanium acetylacetonate.
[0039] Preferably, the content of the titanium compound may be 5 parts by weight to 50 parts by weight based on 100 parts by weight of the urethane-acrylic dendrimer resin. If the content of the titanium compound is less than 5 parts by weight, corrosion resistance may not be sufficiently improved, and if the content of the titanium compound is greater than 50 parts by weight, manufacturing costs may markedly increase even though corrosion resistance may improve.
[0040] Preferably, the coating layer may have a thickness within the range of 0.1 µm to 2.0 µm in a dried state. If the thickness of the coating layer in a dried state is less than 0.1 µm, it may be difficult to obtain sufficient resistance to corrosion and fuels because the thickness of the coating layer in a dried state is too low. If the thickness of the coating layer in a dried state is greater than 2.0 µm, even though the corrosion and fuel resistance of the coating layer increases, the insulating ability of the coating layer increases, and thus the weldability of the composite resin coated steel sheet decreases.
[0041] Preferably, when the coating layer is dried, the amount of the coating layer may be 0.1 g/m2 to 2.0 g/m2. If the amount of the coating layer is less than 0.1 g/m2, it may be difficult to obtain sufficient resistance to corrosion and fuels. If the amount of the coating layer is greater than 2.0 g/m2, the insulating ability of the coating layer may increase, and thus the weldability of the composite resin coated steel sheet may decrease.
[0042] Another exemplary embodiment of the present disclosure may provide a method for manufacturing a plated steel sheet. The method may include: dipping a steel sheet into a plating bath; adjusting a plating amount to be within the range of 20 g/m2 to 60 g/m2; and adjusting an alloying temperature to be within the range of 460°C to 520°C so as to manufacture an alloy-plated steel sheet having an alloying degree of 7% to 15%. The alloy-plated steel sheet may have a crater faction of 15% or less.
[0043] After the steel sheet is dipped into the plating bath to plate the steel sheet, the amount of a plating layer on the steel sheet may be adjusted using an air knife to be within the range of 20 g/m2 to 60 g/m2. Thereafter, an alloying process may be performed on the plated steel sheet within an alloying temperature range of 460°C to 520°C, and then the alloy-plated steel sheet may have a Zn-Fe alloying degree of 7% to 15%. If the alloy-plated steel sheet is passed through a skin pass mill, the surface roughness of the alloy-plated steel sheet may become uniform. The alloy-plated steel sheet manufactured in this manner may have a low crater fraction, for example, within the range of 15% or less.
[0044] Another exemplary embodiment of the present disclosure may provide a method for manufacturing a composite resin coated steel sheet. The method may include: dipping a steel sheet into a plating bath; adjusting a plating amount to be within the range of 20 g/m2 to 60 g/m2; adjusting an alloying temperature to be within the range of 460°C to 520°C so as to manufacture an alloy-plated steel sheet having an alloying degree of 7% to 15%; coating the alloy-plated steel sheet with a steel sheet coating composition; and performing a curing process on the alloy-plated steel sheet coated with the steel sheet coating composition so as to form a coating layer. The alloy-plated steel sheet may have a crater faction of 15% or less.
[0045] If the crater faction of the alloy-plated steel sheet is greater than 15%, the curing temperature of the coating layer may increase, and thus energy efficiency may decrease. In addition, if the plating amount is less than 20 g/m2, it may be difficult to obtain a sufficient degree of corrosion and fuel resistance, and if the plating amount is greater than 60 g/m2, insulation properties may increase, and thus weldability may decrease. In addition, if the alloying degree of the alloy-plated steel sheet is less than 7%, it may be difficult to obtain a sufficient degree of corrosion and fuel resistance. If the alloying degree of the alloy-plated steel sheet is greater than 15%, although corrosion and fuel resistance increases, weldability may decrease because of an increase in insulating ability.
[0046] The steel sheet coating composition may include a urethane-acrylic dendrimer resin, 15 parts by weight to 100 parts by weight of a hardener, 10 parts by weight to 70 parts by weight of a metal silicate, and 5 parts by weight to 50 parts by weight of a titanium compound, based on 100 parts by weight of the urethane-acrylic dendrimer resin.
[0047] The coating of the alloy-plated steel sheet may be performed using any method. For example, the coating of the alloy-plated steel sheet may be performed using a method selected from a roll coating method, a casting method, a screen printing method, a spraying method, a gravure coating method, and a dipping method. For example, the roll coating method may be used, among the listed coating methods.
[0048] The coating layer may be formed while maintaining the alloy-plated steel sheet at a temperature of 80°C to 200°C. If the coating layer is cured/dried at a temperature of less than 80°C, the curing reaction between organic and inorganic compositions of the coating layer may occur insufficiently, and thus a high degree of corrosion and fuel resistance may not be obtained. If the coating layer is cured/dried at a temperature greater than 200°C, the crosslinking reaction of the hardener may occur excessively, thereby excessively curing the coating layer and deteriorating the workability of the composite resin coated steel sheet.
[0049] The coating layer may be adjusted so that when dried, the coating layer may have a thickness within the range of 0.1 µm to 2.0 µm and a density within the range of 0.1 g/m2 to 2.0 g/m2.
[0050] Hereinafter, the embodiments of the present disclosure will be described more specifically through examples. However, the examples are for clearly explaining the embodiments of the present disclosure and are not intended to limit the scope of the present invention.
[0051] After an alkaline cleaning process, a pickling process, and an annealing process were sequentially performed on cold-rolled steel sheets, the steel sheets were hot-dip galvanized by dipping the steel sheets into a zinc pot. The plating amounts of the hot-dip galvanized steel sheets were adjusted using an air knife, and the temperatures (alloying temperature) of the hot-dip galvanized steel sheets was adjusted in an alloying furnace. The amount of a plating layer, the alloying temperature, the degree of alloying, and the crater faction of each of the hot-dip galvanized steel sheets are illustrated in Table 1.
[0052] Thereafter, a steel sheet coating composition was prepared, which included 10 wt% urethane-acrylic dendrimer, 5 wt% aziridine hardener, 5 wt% lithium polysilicate, and 2 wt% isopropyl ditriethanolamino titanate. An alcohol, a leveling agent, and a defoamer were added to the steel sheet coating composition in small amounts to improve the properties of the steel sheet coating composition.
[0053] The steel sheets were coated with the steel sheet coating composition by a roll coating method while adjusting the amounts of dried coating layers on the steel sheets to be 0.7 g/m2 and the thicknesses of the dried coating layers to be 1.0 µm. The coating layers formed on the steel sheets were cured/dried while maintaining the temperature of the steel sheets at 100°C, and then the steel sheets were cooled. In this manner, composite resin coated steel sheets were manufactured.
[0054] The corrosion resistance, fuel resistance, and seam weldability of the composite resin coated steel sheets were evaluated. Results of the evaluation are illustrated in Table 1.

[0055] The corrosion resistance of the steel sheets was tested by maintaining the steel sheets under the conditions of a salt content of 5%, a temperature of 35°C, and a spraying pressure of 1 kg/cm2 for 1,000 hours, and subsequently measuring the areas of red rust on the steel sheets.
[0056] FIG. 4 is surface images taken from the hot-dip galvanized steel sheets of Example 6 and Comparative Example 1 after the corrosion resistance test.
?: corroded area ratio 0%
?: corroded area ratio 5% or less
?: corroded area ratio 5% to 30%
×: corroded area ratio 30% or greater

[0057] The fuel resistance of the steel sheets was evaluated through an accelerated fuel resistance test using degraded gasoline, degraded diesel, bioethanol, and biodiesel. Specimen cups were formed of the steel sheets, and then the cups were filled with fuels and covered using glass plates and O-rings. In detail, cups were filled with gasoline containing 5% pure water and 20 ppm formic acid, and the cups were maintained at 60°C for 1,000 hours while shaking the cups at a rate of 60 cycles per minute. Thereafter, corrosion of the cups (steel sheets) was evaluated. In addition, cups were filled with diesel containing 5% pure water and 20 ppm formic acid, and the cups were maintained at 60°C for 8 weeks while shaking the cups at a rate of 60 cycles per minute. Thereafter, corrosion of the cups (steel sheets) was evaluated. The cups were evaluated as follows.
[0058] FIG. 5 is surface images of cups of Example 6 and Comparative Example 1, photographed after the fuel resistance test.
?: corroded area ratio 0%
?: corroded area ratio 5% or less
?: corroded area ratio 5% to 30%
×: corroded area ratio 30% or greater

[0059] The seam weldability of the steel sheets was evaluated by welding the steel sheets using an Ironman welding machine (Inverter DC seam welding machine) under the conditions of pressing force of 4 kN, a welding rate of 6 mpm, and a current application time of 33 ms and a rest time of 10 ms, and determining whether weld zones have a constant degree of strength without spatters. Welding current ranges in which the steel sheets could be welded as described above were used as evaluation criteria.
? : 1.5kA or greater
? : 1.0 to 1.5kA
? : 1.0kA or less
[0060] The crater faction (%) of each of the plating layers was calculated from values measured from a microscopic image of a cross section of the plating layer. The crater fraction was calculated by measuring the areas of craters observed from a 5-mm wide cross section, and dividing the total area of the cross section by the areas of the craters.
[0061] FIG. 3 illustrates scanning electron microscope (SEM) images of cross-sections of plating layers of Example 6 and Comparative Example 1.
[Table 1]
No Plating layer properties Fuel resistance CR SW
Plating amount (g/m2) Alloying Temp. (°C) Alloying degree (%) Crater faction (%) Degraded gasoline Degraded diesel Bio-
ethanol Bio-
diesel
E1 30.0 465 8.5 8.5 ? ? ? ? ? ?
E2 30.0 475 10.3 9.5 ? ? ? ? ? ?
E3 30.0 495 12.5 10.8 ? ? ? ? ? ?
E4 30.0 515 13.7 12.5 ? ? ? ? ? ?
E5 40.0 465 7.2 7.6 ? ? ? ? ? ?
E6 40.0 475 9.4 8.4 ? ? ? ? ? ?
E7 40.0 495 11.5 10.5 ? ? ? ? ? ?
E8 40.0 515 13.1 11.4 ? ? ? ? ? ?
E9 50.0 465 6.8 7.4 ? ? ? ? ? ?
E10 50.0 475 9.2 8.5 ? ? ? ? ? ?
E11 50.0 495 11.1 9.2 ? ? ? ? ? ?
E12 50.0 515 13.2 9.5 ? ? ? ? ? ?
CE1 30.0 500 11.5 15.3 ? ? ? ? ? ?
CE2 40.0 500 12.3 16.5 ? ? ? ? ? ?
CE3 50.0 500 13.2 18.6 ? ? ? ? ? ?
E: Example
CE: Comparative Example
CR: Corrosion resistance
SW: Seam weldability

[0062] As illustrated in Table 1, the steel sheets of Examples 1 to 12 having a crater fraction of 15% or less had a high degree of fuel resistance and a high degree of corrosion resistance when compared to the steel sheets of Comparative Examples 1 to 3.
[0063] As set forth above, according to the exemplary embodiments of the present disclosure, the plated steel sheet, and the composite resin coated steel sheet manufactured by coating the plated steel sheet with a composite resin have a high degree of fuel resistance, a high degree of corrosion resistance, and a high degree of seam weldability.
[0064] While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.

Claims:
We Claim:
1. A plated steel sheet comprising a plating layer having a crater fraction of 15% or less, a plating amount of 20 g/m2 to 60 g/m2, and an alloying degree of 7% to 15%.

2. A composite resin coated steel sheet comprising:
a steel sheet;
a plating layer having a crater fraction of 15% or less, a plating amount of 20 g/m2 to 60 g/m2, and an alloying degree of 7% to 15%; and
a coating layer formed on the plating layer,
wherein the coating layer is a cured product of a steel sheet coating composition.

3. The composite resin coated steel sheet of claim 2, wherein the steel sheet coating composition comprises a urethane-acrylic dendrimer resin, 15 parts by weight to 100 parts by weight of a hardener, 10 parts by weight to 70 parts by weight of a metal silicate, and 5 parts by weight to 50 parts by weight of a titanium compound, based on 100 parts by weight of the urethane-acrylic dendrimer resin.

4. The composite resin coated steel sheet of claim 3, wherein the hardener comprises at least one of a melamine-containing hardener and an aziridine-containing hardener.

5. The composite resin coated steel sheet of claim 4, wherein the melamine-containing hardener comprises at least one of melamine, butoxymethyl melamine, hexamethoxymethyl melamine, and trimethoxymethyl melamine.

6. The composite resin coated steel sheet of claim 3, wherein the metal silicate comprises at least one of lithium polysilicate, sodium polysilicate, potassium polysilicate, and colloidal silica.

7. The composite resin coated steel sheet of claim 3, wherein the titanium compound comprises at least one of titanium carbonate, isopropyl ditriethanolamino titanate, lactic acid titanium chelate, and titanium acetylacetonate.

8. The composite resin coated steel sheet of claim 2, wherein the coating layer has a thickness of 0.1 µm to 2.0 µm in a dried state.

9. The composite resin coated steel sheet of claim 2, wherein the coating layer has a density of 0.1 g/m2 to 2.0 g/m2 in a dried state.

10. A method for manufacturing a plated steel sheet, the method comprising:
dipping a steel sheet into a plating bath;
adjusting a plating amount within a range of 20 g/m2 to 60 g/m2; and
adjusting an alloying temperature within a range of 460°C to 520°C to manufacture an alloy-plated steel sheet having an alloying degree of 7% to 15%,
wherein the alloy-plated steel sheet has a crater faction of 15% or less.
11. A method for manufacturing a composite resin coated steel sheet, the method comprising:
dipping a steel sheet into a plating bath;
adjusting a plating amount within a range of 20 g/m2 to 60 g/m2;
adjusting an alloying temperature within a range of 460°C to 520°C to manufacture an alloy-plated steel sheet having an alloying degree of 7% to 15%;
coating the alloy-plated steel sheet with a steel sheet coating composition; and
performing a curing process on the alloy-plated steel sheet coated with the steel sheet coating composition so as to form a coating layer,
wherein the alloy-plated steel sheet has a crater faction of 15% or less.
12. The method of claim 11, wherein the steel sheet coating composition comprises a urethane-acrylic dendrimer resin, 15 parts by weight to 100 parts by weight of a hardener, 10 parts by weight to 70 parts by weight of a metal silicate, and 5 parts by weight to 50 parts by weight of a titanium compound, based on 100 parts by weight of the urethane-acrylic dendrimer resin.
13. The method of claim 11, wherein the coating of the alloy-plated steel sheet is performed using a method selected from a roll coating method, a casting method, a screen printing method, a spraying method, a gravure coating method, and a dipping method.
14. The method of claim 11, wherein the coating layer is formed while maintaining the alloy-plated steel sheet at a temperature of 80°C to 200°C.
15. The method of claim 11, wherein the coating layer has a thickness of 0.1 µm to 2.0 µm in a dried state.
16. The method of claim 11, wherein the coating layer has a density of 0.1 g/m2 to 2.0 g/m2 in a dried state.