Plated steel sheet and method for manufacturing same

Abstract

The invention provides a coated steel sheet. The coated steel sheet comprises a cold-rolled steel sheet, and a plating layer formed on the cold-rolled steel sheet, the plating layer consisting of 0.5 wt% to 3 wt% of aluminum (Al), 1 wt% to 2 wt% of magnesium (Mg), 0.005 wt% to 0.1 wt% of silicon (Si), 0.01 wt% to 0.1 wt% of titanium (Ti), the remainder being zinc (Zn) and other unavoidable impurities, in which the content ratio of titanium to silicon in the plating layer is 1.0 or more, and the content ratio of titanium to silicon in the plating layer is 1.0 or more. And the plating layer includes a titanium-containing intermetallic compound.

Description

Technical Field

[0001] The present invention relates to a coated steel sheet and a method for manufacturing the same, and more particularly, to a coated steel sheet having excellent surface quality and a method for manufacturing the same. Background Art

[0002] Existing hot-dip galvanized steel sheets are widely used in building materials, household appliances, etc. due to their excellent sacrificial properties. When exposed to a corrosive environment, zinc (Zn) acts as a sacrificial anode for the exposed iron (Fe), and thus zinc loss occurs in the coating. As described above, zinc, as a sacrificial anode, plays an important role in suppressing the rusting of the base steel in a corrosive environment, but exhibits a relatively low sacrificial anode efficiency. To solve this problem, currently, high-corrosion-resistant coating products are being produced, which improve the sacrificial anode efficiency by adding magnesium (Mg) to the Zn plating bath and forming a dense corrosion product in a corrosive environment, thereby enabling excellent corrosion resistance.

[0003] Adding magnesium to zinc improves the corrosion resistance, but also sharply increases the initial surface activity in a corrosive environment. As a result, due to the formation of magnesium oxide, the coating surface quickly turns black and deteriorates. In addition, in high-corrosion-resistant coated steel sheets, when external foreign substances that cause supercooling are adsorbed during the immediate solidification after electroplating, a eutectic phase with a layer spacing of several nanometers is formed on the coating surface. When exposed to an external corrosive environment, the eutectic phase corrodes in a short time, forming circular black dot defects and resulting in a quality decline. To alleviate this problem, oiling or post-coating is performed after electroplating to temporarily prevent rusting. However, the above methods all have limitations in controlling darkening.

[0004] Related art includes Japanese Patent Publication No. 2005-105367. Summary of the Invention

[0005] Technical Problem

[0006] The present invention provides a coated steel sheet and a method for manufacturing the same, wherein the coated steel sheet minimizes darkening defects and black dot defects caused by excessive increase in the surface activity of the coating by controlling the microstructure of the coating, thereby enabling excellent surface quality.

[0007] However, the above description is only an example, and the scope of the present invention is not limited thereto.

[0008] Technical Solution

[0009] According to one aspect of the present invention, there is provided a coated steel sheet, which comprises a cold-rolled steel sheet; and a coating applied on the cold-rolled steel sheet, and the coating is composed of the following components: 0.5 wt% to 3 wt% of aluminum (Al), 1 wt% to 2 wt% of magnesium (Mg), 0.005 wt% to 0.1 wt% of silicon (Si), 0.01 wt% to 0.1 wt% of titanium (Ti), the balance of zinc (Zn) and other inevitable impurities, wherein the Ti / Si content ratio of the coating is 1.0 or greater, and wherein the coating comprises a Ti-containing intermetallic compound.

[0010] In the coated steel sheet, the intermetallic compound may contain 3 wt% to 40 wt% of Ti, and may further contain two or more of the following components: 3 wt% to 30 wt% of Si, 1 wt% to 40 wt% of iron (Fe), and 1 wt% to 50 wt% of Al.

[0011] In the coated steel sheet, the intermetallic compound may contain Ti and Fe, and contains 3 wt% to 40 wt% of Ti and 1 wt% to 40 wt% of Fe.

[0012] Advantageous Effects

[0013] According to an embodiment of the present invention, a coated steel sheet and a manufacturing method thereof can be realized, and the coated steel sheet minimizes darkening defects and black spot defects caused by excessive increase in the surface activity of the coating by controlling the microstructure of the coating, thereby enabling excellent surface quality.

[0014] However, the scope of the present invention is not limited to the above effects. Brief Description of the Drawings

[0015] Figure 1 is a flowchart of a manufacturing method of a coated steel sheet according to an embodiment of the present invention.

[0016] Figure 2 is a micrograph showing the coating surface of the coated steel sheet according to Test Example 2 of the present invention.

[0017] Figure 3 is a micrograph showing the coating surface of the coated steel sheet according to Test Example 9 of the present invention. Detailed Description of the Embodiments

[0018] Hereinafter, the present invention will be described in detail by explaining embodiments of the present invention with reference to the drawings. However, the present invention can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the present invention to those skilled in the art.

[0019] Now, a coated steel sheet with excellent surface quality and a method for manufacturing the same according to an embodiment of the present invention will be described in detail. The terms used herein are selected according to their functions in the present invention, and the definitions of these terms should be combined with the context of the entire specification.

[0020] Figure 1 is a flowchart of a method for manufacturing a coated steel sheet according to an embodiment of the present invention.

[0021] Referring to Figure 1 , the method for manufacturing a coated steel sheet according to an embodiment of the present invention includes: (a) providing a cold-rolled steel sheet (S10); (b) annealing the cold-rolled steel sheet (S20); (c) passing the steel sheet through a plating bath containing Al, Mg, Si, Ti, and Zn to form a coating on the annealed steel sheet, the coating consisting of the following components: 0.5 wt% to 3 wt% of aluminum (Al), 1 wt% to 2 wt% of magnesium (Mg), 0.005 wt% to 0.1 wt% of silicon (Si), 0.01 wt% to 0.1 wt% of titanium (Ti), the balance of zinc (Zn), and other inevitable impurities (S30); and (d) cooling the steel sheet coated with the coating (S40).

[0022] The coated steel sheet prepared by the above method includes a cold-rolled steel sheet; and a coating coated on the cold-rolled steel sheet, the coating consisting of the following components: 0.5 wt% to 3 wt% of Al, 1 wt% to 2 wt% of Mg, 0.005 wt% to 0.1 wt% of Si, 0.01 wt% to 0.1 wt% of Ti, the balance of Zn, and other inevitable impurities, wherein the Ti / Si content ratio of the coating is 1.0 or greater, and wherein the coating includes a Ti-containing intermetallic compound.

[0023] The coating may further contain 0.002 wt% to 0.02 wt% of boron (B).

[0024] The intermetallic compound may contain 3 wt% to 40 wt% of Ti and may further contain two or more of the following components: 3 wt% to 30 wt% of Si, 1 wt% to 40 wt% of iron (Fe), and 1 wt% to 50 wt% of Al. The intermetallic compound may contain Ti and Fe and contain 3 wt% to 40 wt% of Ti and 1 wt% to 40 wt% of Fe.

[0025] The coating layer may include an Mg-rich phase, a primary Zn phase, a binary eutectic phase having a layered structure of a Zn phase and an MgZn2 phase, and a ternary eutectic phase having a layered structure of an MgZn2 phase, a Zn phase, and Al. In the coating layer, the area fraction of the ternary eutectic phase may be 30% or greater, and the area fraction of the binary eutectic phase may be less than 30%.

[0026] The Al / Mg ratio of the coating layer may be 1.0 or greater and 2.0 or less.

[0027] The coating layer may include an Mg-rich phase, a primary Zn phase, a binary eutectic phase having a layered structure of a Zn phase and an MgZn2 phase, and a ternary eutectic phase having a layered structure of an MgZn2 phase, a Zn phase, and Al. The Mg-rich phase may include: 5 wt% to 15 wt% of Mg, 85 wt% to 95 wt% of Zn, and less than 1 wt% of Al. An intermetallic compound containing Ti may be present in the Mg-rich phase.

[0028] A method for manufacturing a coated steel sheet according to an embodiment of the present invention includes: (a) providing a cold-rolled steel sheet; (b) annealing the cold-rolled steel sheet; and (c) passing the steel sheet through a plating bath containing Al, Mg, Si, Ti, and Zn, thereby forming a coating layer on the annealed steel sheet, the coating layer being composed of the following components: 0.5 wt% to 3 wt% of Al, 1 wt% to 2 wt% of Mg, 0.005 wt% to 0.1 wt% of Si, 0.01 wt% to 0.1 wt% of Ti, the balance being Zn and other inevitable impurities, wherein forming the coating layer includes cooling the steel sheet at a cooling rate of 5 °C / second to 30 °C / second, wherein the Ti / Si content ratio of the coating layer is 1.0 or greater, wherein the coating layer contains an intermetallic compound, and wherein the intermetallic compound contains 3 wt% to 40 wt% of Ti and further contains two or more of the following components: 3 wt% to 30 wt% of Si, 1 wt% to 40 wt% of Fe, and 1 wt% to 50 wt% of Al.

[0029] The coating layer may be formed by sequential solidification of a primary Zn phase, a binary eutectic phase, and a ternary eutectic phase, and the cooling rate after the formation of the binary eutectic phase during cooling may be greater than the cooling rate before the formation of the binary eutectic phase.

[0030] When the difference between the temperature at which the binary eutectic phase is formed and the temperature at which the ternary eutectic phase is formed is 5 °C or less, the cooling rate during coating layer formation may be 5 °C / second or greater. When the difference between the temperature at which the binary eutectic phase is formed and the temperature at which the ternary eutectic phase is formed is 10 °C or greater, the cooling rate during coating layer formation may be 10 °C / second or greater.

[0031] Adding Mg to Zn improves corrosion resistance, but darkens the coating surface because Mg in the coating forms oxides in the corrosive environment. Therefore, it is necessary to control the surface structure of the coating to minimize darkening. The microstructure of the coating applied using a plating bath of Zn and Mg can consist of a Zn single phase, a binary eutectic phase (Zn + MgZn2), a ternary eutectic phase (Zn + Al + MgZn2), and a Mg-rich phase. The binary eutectic phase can have a layered structure of a Zn phase and a MgZn2 phase, and the ternary eutectic phase can have a layered structure of a MgZn2 phase, a Zn phase, and Al. The Mg-rich phase can contain: 5 wt% to 15 wt% of Mg, 85 wt% to 95 wt% of Zn, and less than 1 wt% of Al.

[0032] In the microstructure of the coating deposited using a plating bath of Zn and Mg, the lamellar eutectic phase with a large number of reaction interfaces accelerates the darkening of the coating in a corrosive environment. Among these eutectic phases, the ternary eutectic phase has a significant effect on darkening. The smaller the layer spacing of the eutectic phase, the faster the acceleration of darkening. For these reasons, it is necessary to control the fractions of the eutectic phase and the primary Zn phase. Although the fractions of these phases can be controlled by adjusting the contents of Al and Mg, the limitations in the contents of Al and Mg make it difficult to achieve excellent corrosion resistance. At the same time, when the surface of the coating is made to have excellent quality by adding 0.5 wt% to 3 wt% of Al and 1 wt% to 2 wt% of Mg relative to the total weight of the coating composition capable of exhibiting corrosion resistance, a coated steel sheet with excellent corrosion resistance can be obtained. The content of Al needs to be at least 0.5 wt% to prevent the oxidation of Mg. When the Al content exceeds 3 wt%, the ternary eutectic phase develops excessively, accelerating the darkening of the coating surface. The content of Mg directly involved in the formation of the active phase on the coating surface can be at least 1 wt%. To form a eutectic phase contributing to corrosion resistance, it is necessary to add 1 wt% or more of Mg. When the Mg content exceeds 2 wt%, the fraction of the Mg-related phase increases rapidly, whereby the activation of the coating surface becomes uncontrollable. At the same time, in order to more effectively reduce the activity of the coating surface within the limited ranges of the Al and Mg contents as desired in the present invention, trace elements are added. Since the highly corrosion-resistant plating bath contains a higher content of Al compared with the existing Zn plating bath, Fe is easily dissolved out from the substrate steel sheet during the electroplating process. To control this situation, a small amount of Si is added. The Si added for this purpose reduces the amount of Fe dissolved out from the base steel, significantly reduces the formation of Fe scum, and ensures the provision of a coated steel sheet with excellent surface quality. The content of Si can be 0.005 wt% to 0.1 wt%. To control the alloy layer, the content of Si needs to be 0.005 wt% or more. When the content of Si exceeds 0.1 wt%, the Mg2Si phase is formed, thereby hindering the workability of the coating. In addition, Si acts as a nucleation site and a grain growth barrier to refine the microstructure of the coating and improve the corrosion resistance. However, since a large number of fine eutectic phases are formed on the coating surface by Si, the surface activation may increase and the darkening of the coating surface is accelerated. Therefore, it is necessary to appropriately control the content of Si.

[0033] As described above, while ensuring the effect of reducing Fe dissolution by adding Si, Ti and / or titanium boride (TiB) can also be added to inhibit the refinement of eutectic phases, thereby reducing the acceleration of darkening on the surface of the coating in a corrosive environment. Ti and / or TiB combine with Fe and Si acting as nucleation sites in the plating bath to form intermetallic compounds. This reduces the refinement of primary phases and the lamellar spacing of eutectic phases during solidification after electroplating. In addition, the Ti-containing intermetallic compound itself acts as a nucleation site and forms a Mg-rich phase in the coating. This reduces the Mg content for forming eutectic phases and increases the lamellar spacing of eutectic phases. Therefore, the reaction interface on the surface of the coating is reduced, and the acceleration of darkening is decreased.

[0034] The above-mentioned Ti-based intermetallic compound necessarily contains 3 wt% to 40 wt% of Ti and may further contain two or more of the following components: 3 wt% to 30 wt% of Si, 1 wt% to 40 wt% of Fe, and 1 wt% to 50 wt% of Al. In addition, the intermetallic compound containing Ti and Fe also combines with Si, which reduces Fe acting as a nucleation site and inhibits grain growth, thereby making the microstructure of the coating rough. Therefore, due to the roughening of the microstructure of the coating, the active interface is reduced and the darkening resistance is improved.

[0035] Now, the reduction of black dot defects will be described. Black dot defects are characterized by having a very fine lamellar structure, which is usually caused by supercooling due to the adsorption of foreign substances. As described above, when TiB is added, it makes the microstructure rough. Therefore, the very fine lamellar structure that is usually formed due to supercooling caused by the adsorption of foreign substances is not easily formed, thereby reducing the formation of black dot defects. At the same time, the content of Ti and / or TiB can be 0.01 wt% to 0.1 wt%. In order to control darkening and black dot defects through microstructure control, the content of Ti and / or TiB needs to be 0.01 wt% or more. When the content of Ti and / or TiB exceeds 0.1 wt%, the intermetallic compound combined with Ti and / or TiB acts as a nucleation site, which will excessively increase the fraction of the Mg-rich phase and reduce the workability. When Si and Ti and / or TiB are added simultaneously, the effect of adding Ti and / or TiB cancels out the effect of adding Si. Therefore, it is necessary to adjust the content ratio between them considering phase control and appearance improvement. It is necessary to control the ratio of Ti and / or TiB to Si such that when Si is added at a ratio of 1.0, Ti and / or TiB are added at a ratio of 1.0 or higher.

[0036] The Al-5%Ti-1%B alloy ingot is adopted and TiB is added, and its content is determined based on the content of Ti. It is known that when TiB is added, intermetallic compounds such as TiB2 and Al3Ti are formed and serve as nucleation sites during the solidification of the coating. B is generally not added alone but is added at a Ti / B ratio of 5:1 or greater. That is, when the Ti content is 0.01 wt% to 0.1 wt%, the B content is 0.002 wt% to 0.02 wt%.

[0037] The coating according to an embodiment of the present invention includes a primary Zn phase, a binary eutectic phase having a layered structure of a Zn phase and a MgZn2 phase, and a ternary eutectic phase having a layered structure of a MgZn2 phase, a Zn phase, and Al, and the solidification temperature difference between the primary Zn phase and the binary eutectic phase is not greater than 30°C. The area ratio of the primary Zn phase (where the ratio of the average width to the average height is 3 or greater and the average height is 80% or greater of the coating thickness) is less than 50% of the total primary Zn phase. The Al / Mg ratio of the coating can be 1.0 or greater and 2.0 or less.

[0038] In step (b) (S20), annealing can be carried out at a temperature of 700°C to 850°C. In step (c) (S30), the temperature of the plating bath can be 400°C to 520°C.

[0039] During the above electroplating process, the coating has different fractions of a primary Zn phase, a binary eutectic phase having a layered structure of a Zn phase and a MgZn2 phase, and a ternary eutectic phase having a layered structure of a MgZn2 phase, a Zn phase, and Al (which are respectively formed during solidification depending on the contents of Al, Mg, Si, Ti, and TiB added to Zn) to improve corrosion resistance. Depending on the fractions of the respective phases, the coating exhibits different mechanical properties and corrosion resistance.

[0040] When the fraction of the primary Zn phase is high, the coating exhibits reduced hardness and corrosion resistance, but the darkening of the coating surface may be reduced when exposed to the external environment. Controlling the fraction of the primary Zn phase is a key issue in preventing darkening.

[0041] Although the binary eutectic phase can improve corrosion resistance, when the binary eutectic phase is formed roughly, cracks will appear in the binary eutectic phase during the processing, thereby reducing the workability. Therefore, it is necessary to appropriately control the fraction of the binary eutectic phase.

[0042] The ternary eutectic phase includes a layered structure in which Zn layers and MgZn2 layers alternate with each other, and Al is incorporated into the layered structure. Like the binary eutectic phase, the ternary eutectic phase can also improve corrosion resistance. However, since the ternary eutectic phase has a smaller layer spacing compared to the binary eutectic phase, the ternary eutectic phase has more active interfaces per unit area. Therefore, when the fraction of the ternary eutectic phase on the surface is high, darkening is likely to occur in a corrosive environment. By controlling the fraction of the phase having the above characteristics, corrosion resistance can be improved and surface quality can be controlled.

[0043] When the primary Zn phase is formed roughly, the darkening resistance in a corrosive environment is excellent, but the corrosion resistance decreases because Zn is directly exposed to the corrosive environment. In particular, when the area ratio of the primary Zn phase (where the ratio of the average width to the average height is 3 or more and the average height is 80% or more of the total thickness of the coating) is 50% or more of the total primary Zn phase in the cross-sectional view of the coating, the corrosion resistance decreases rapidly. The reason for this phenomenon is that the corrosion products of the eutectic phase near the primary Zn phase aimed at improving corrosion resistance may not be able to form sufficient passivating oxides to prevent the corrosion of the primary Zn phase.

[0044] To construct the microstructure of the present invention, the composition ratio of Al / Mg and the proportion of the solidified phase can be controlled by controlling the cooling rate. When the composition ratio of Al / Mg is 1:2 or less, the solidification sequence is as follows: first, the primary Zn phase solidifies, then the binary eutectic phase solidifies, and finally the ternary eutectic phase solidifies. However, when the composition ratio exceeds 1:2, the solidification sequence may change. To construct an embodiment of the present invention, after coating a composition (in which the temperature difference between the formation of the primary Zn phase and the formation of the binary eutectic phase is 30°C or less), the coating needs to be cooled at a cooling rate of 5°C / second or more, more specifically 7°C / second or more. However, when the cooling rate exceeds 30°C / second, excessive cooling pressure may cause uneven solidification on the surface of the coating.

[0045] During electroplating, Al and Mg need to be added to Zn at a ratio of 1:1 or higher (more specifically 1.2:1 or higher) to minimize the oxidation of Mg. Al in the molten metal reacts with oxygen in the air to form a dense oxide layer, which prevents the supply of oxygen to the surface layer of the molten metal, thereby preventing the oxidation of Mg. However, when the Al / Mg ratio exceeds 2:1, the probability of first forming the ternary eutectic phase increases. Therefore, the total fraction of the ternary eutectic phase increases, which reduces the darkening resistance when exposed to the outside.

[0046] In the electroplating process of the present invention, the content range of Al is 0.5 wt% to 3 wt%. When the content of Al is less than 0.5 wt%, the effect of preventing Mg oxidation is insufficient. When the content of Al exceeds 3 wt%, a large amount of Fe slag generated due to the dissolution of Fe from the base steel may cause surface quality problems.

[0047] The content range of Mg is 1 wt% to 2 wt%. When the content of Mg is less than 1 wt%, the formation of eutectic phases that contribute to corrosion resistance is insufficient. When the content of Mg exceeds 2 wt%, although the corrosion resistance is improved, due to the rapid increase in the fraction of eutectic phases, cracks on the surface of the coating layer increase during the processing, and due to the corrosion of eutectic phases on the surface of the coating layer in a corrosive environment, the change in color difference becomes uncontrollable.

[0048] The freezing point difference between the primary Zn phase and the binary eutectic phase is related to the duration of the growth of the primary Zn phase. The greater the freezing temperature difference between the primary Zn phase and the binary eutectic phase, the longer the time during which the primary Zn phase can grow during cooling. Therefore, the possibility that the ratio of the average width to the average height of the coating layer is 3 or more and the average height of the primary Zn phase is 80% or more of the total thickness of the coating layer increases.

[0049] Now, test examples will be described to better understand the present invention. However, the following test examples are only used to facilitate the understanding of the present invention, and the present invention is not limited thereto.

[0050] Test Example

[0051] Cold-rolled steel sheets with a thickness of 0.7 mm were immersed in an alkaline solution at 50 °C for 30 minutes, and then washed with water to remove foreign substances and oil on the surface, thereby preparing various samples. The samples were annealed and then electroplated. The annealing was carried out in a reducing atmosphere composed of 10% to 30% hydrogen and 70% to 90% nitrogen at a temperature of 700 °C to 750 °C.

[0052] The electroplating included cooling the annealed samples to the plating bath temperature, immersing the samples in the plating bath for 2 seconds, taking out the samples, and performing nitrogen purging to adjust the coating layer thickness to about 10 μm. In this case, the temperature of the plating bath was 400 °C to 520 °C. The composition of the cold-rolled steel sheet was as follows: 0.15 wt% of carbon (C), 0.6 wt% of manganese (Mn), 0.05 wt% of phosphorus (P), 0.005 wt% of sulfur (S), and the balance of Fe.

[0053] Table 1 shows the process conditions, as well as the microstructure and properties of the coating layer of the coated steel sheets according to the test examples of the present invention.

[0054] In Table 1, Zn, Al, Mg, Si, and TiB represent the composition of the plating bath (unit: wt%) during the electroplating process.

[0055] Each sample was stored in a damp heat environment with a relative humidity of 90% or higher and a temperature of 50°C for 1 hour, and then the color difference of the steel plate surface was measured using a color difference meter to evaluate the darkening resistance. The evaluation criteria based on the color difference before and after the darkening resistance test are as follows: ◎ indicates that the color difference ΔE is 6.0 or less, ○ indicates that the color difference ΔE is 8.0 or less, Δ indicates that the color difference ΔE is 12.0 or less, and X indicates that the color difference ΔE is greater than 12.0.

[0056] The black dot defects were evaluated by measuring the number of black dot defects per unit area (10×10 cm). ◎ indicates no black dot defects, ○ indicates one black dot defect, Δ indicates two or three black dot defects, and X indicates more than three black dot defects.

[0057] The coating cracks indicate the workability of the coating, and they were evaluated according to the following criteria based on the crack formation on the processed part after 3T-bending. ○ indicates the case where thin and fine cracks can be observed with the naked eye, Δ indicates the case where thick and a few cracks can be observed with the naked eye, but due to the presence of a large number of cracks, it is expected that the coating will peel off, and X indicates the case where thick and many cracks can be observed with the naked eye.

[0058] [Table 1]

[0059]

[0060]

[0061] Referring to Table 1, Test Examples 1, 2, and 3 correspond to the following cases: the plating bath composition (unit: wt%) during the electroplating process satisfies Al: 0.5 wt% to 3 wt%, Mg: 1 wt% to 2 wt%, Si: 0.005 wt% to 0.1 wt%, but Ti and / or TiB are not added to the plating bath. The obtained coating shows that no Ti-based intermetallic compound is formed, two or more black dot defects, and a color difference ΔE greater than 8.0 before and after the anti-darkening test.

[0062] Referring to Figure 2 (which shows the microscopic image of the coating of the coated steel plate of Test Example 2 according to the present invention), it can be seen that the coating includes a Mg-rich phase, a primary Zn phase, a binary eutectic phase with a layered structure of Zn phase and MgZn2 phase, and a ternary eutectic phase with a layered structure of MgZn2 phase, Zn phase, and Al, and no Ti-containing intermetallic compound is formed.

[0063] Test Example 4 corresponds to the following situation: During electroplating, the plating bath composition (unit: wt%) satisfies Al: 0.5 wt% to 3 wt%, Mg: 1 wt% to 2 wt%, Si: 0.005 wt% to 0.1 wt%, Ti or TiB: 0.01 wt% to 0.1 wt%, and the balance Zn, but the Ti / Si content ratio is less than 1.0. The obtained coating shows that no Ti-based intermetallic compound is formed, there are three or more black dot defects, and the color difference △E is greater than 8.0 before and after the anti-darkening test.

[0064] Test Example 5 corresponds to the following situation: During electroplating, the plating bath composition (unit: wt%) satisfies Al: 0.5 wt% to 3 wt%, Mg: 1 wt% to 2 wt%, Si: 0.005 wt% to 0.1 wt%, Ti or TiB: 0.01 wt% to 0.1 wt%, and the balance Zn, but the Ti / Si content ratio is less than 1.0. The obtained coating shows that there are three or more black dot defects, and the color difference △E is greater than 12.0 before and after the anti-darkening test.

[0065] Test Example 6 corresponds to the following situation: During electroplating, the plating bath composition (unit: wt%) satisfies Al: 0.5 wt% to 3 wt%, Mg: 1 wt% to 2 wt%, Ti or TiB: 0.01 wt% to 0.1 wt%, but the range of Si exceeds 0.005 wt% to 0.1 wt%, and the Ti / Si content ratio is less than 1.0. The obtained coating shows that thicker and more cracks can be observed with the naked eye on the processed part after 3T bending, three or more black dot defects appear, and the color difference △E before and after the anti-darkening test is 12.0 or less.

[0066] Test Example 7 corresponds to the following situation: During electroplating, the plating bath composition (unit: wt%) satisfies Al: 0.5 wt% to 3 wt%, Mg: 1 wt% to 2 wt%, Si: 0.005 wt% to 0.1 wt%, but the range of Ti or TiB exceeds 0.01 wt% to 0.1 wt%. The obtained coating shows that thicker and fewer cracks can be observed with the naked eye on the processed part after 3T bending, but due to the presence of a large number of cracks, it is expected that the coating will peel off, no black dot defects appear, and the color difference △E before and after the anti-darkening test is 6.0 or less.

[0067] Test Examples 8 to 12 correspond to the following cases: during the electroplating process, the plating bath composition (unit: wt%) satisfies Al: 0.5 wt% to 3 wt%, Mg: 1 wt% to 2 wt%, Si: 0.005 wt% to 0.1 wt%, Ti or TiB: 0.01 wt% to 0.1 wt%, and the balance Zn, and the Ti / Si content ratio is 1.0 or more. The obtained coating shows that: Ti-based intermetallic compounds are formed, thinner and finer cracks can be observed with the naked eye on the processed part after 3T bending, there are no black spot defects or there is one black spot defect, and the color difference ΔE before and after the anti-darkening test is 8.0 or less.

[0068] Referring to Figure 3 (which shows the microscopic image of the coating of the coated steel sheet according to Test Example 9 of the present invention), it can be seen that the coating includes a Mg-rich phase, a primary Zn phase, a binary eutectic phase having a layered structure of a Zn phase and a MgZn2 phase, a ternary eutectic phase having a layered structure of a MgZn2 phase, a Zn phase and Al, and Ti-containing intermetallic compounds appear in the Mg-rich phase.

[0069] Although the present invention has been specifically shown and described with reference to the embodiments of the present invention, those skilled in the art will understand that various changes in form and details can be made without departing from the scope of the present invention defined by the following claims.

Claims

1. A coated steel sheet, comprising: A cold-rolled steel sheet; And A coating applied to the cold-rolled steel sheet, the coating consisting of the following components: 0.5 wt% to 3 wt% of aluminum (Al), 1 wt% to 2 wt% of magnesium (Mg), 0.005 wt% to 0.1 wt% of silicon (Si), 0.01 wt% to 0.1 wt% of titanium (Ti), the balance being zinc (Zn) and other unavoidable impurities, Wherein the Ti / Si content ratio of the coating is 1.0 or greater, and Wherein the coating includes a Ti-containing intermetallic compound.

2. The coated steel sheet according to claim 1, wherein, The coating further contains 0.002 wt% to 0.02 wt% of boron (B).

3. The coated steel sheet according to claim 1, wherein, The intermetallic compound contains 3 wt% to 40 wt% of Ti and further contains two or more of the following components: 3 wt% to 30 wt% of Si, 1 wt% to 40 wt% of iron (Fe), and 1 wt% to 50 wt% of Al.

4. The coated steel sheet according to claim 3, wherein, The intermetallic compound contains Ti and Fe and contains 3 wt% to 40 wt% of Ti and 1 wt% to 40 wt% of Fe.

5. The coated steel sheet according to claim 1, wherein, The coating contains a Mg-rich phase, a primary Zn phase, a binary eutectic phase having a layered structure of Zn phase and MgZn2 phase, and a ternary eutectic phase having a layered structure of MgZn2 phase, Zn phase and Al, and Wherein, in the coating, the area fraction of the ternary eutectic phase is 30% or greater, and the area fraction of the binary eutectic phase is less than 30%.

6. The coated steel sheet according to claim 1, wherein the Al / Mg ratio of the coating is 1.0 or greater and 2.0 or less.

7. The coated steel sheet according to claim 1, wherein, The coating contains a Mg-rich phase, a primary Zn phase, a binary eutectic phase having a layered structure of Zn phase and MgZn2 phase, and a ternary eutectic phase having a layered structure of MgZn2 phase, Zn phase and Al, and Wherein, the Mg-rich phase contains: 5 wt% to 15 wt% of Mg, 85 wt% to 95 wt% of Zn, and less than 1 wt% of Al.

8. The coated steel sheet according to claim 7, wherein, A Ti-containing intermetallic compound is present in the Mg-rich phase.

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