Coated steel sheet and its manufacturing method

Abstract

The present invention provides a plated steel sheet comprising: a cold-rolled steel sheet; and a plating layer formed on the cold-rolled steel sheet, the plating layer consisting of 0.5 to 3 wt% aluminum (Al), 1 to 2 wt% magnesium (Mg), 0.005 to 0.1 wt% silicon (Si), 0.01 to 0.1 wt% titanium (Ti), and the balance being zinc (Zn) and other unavoidable impurities; wherein the plating layer has a titanium to silicon content ratio of 1.0 or more, and the plating layer contains an intermetallic compound containing titanium.

Description

[Technical Field]

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

[0002] Conventional hot-dip galvanized steel sheets are widely used in building materials and home appliances due to their excellent self-sacrificial properties. When hot-dip galvanized steel sheets are exposed to a corrosive environment, zinc (Zn) acts as a sacrificial anode on the exposed iron, causing zinc loss from the coating layer. While this sacrificial anode function of zinc plays an important role in preventing rust from forming on the base steel in a corrosive environment, it also somewhat reduces the efficiency of the sacrificial anode. To address this issue, magnesium (Mg) has recently been added to zinc (Zn) plating baths to form dense corrosion products in a corrosive environment, improving the efficiency of the sacrificial anode and resulting in the production of highly corrosion-resistant plated products that exhibit excellent corrosion resistance.

[0003] The addition of magnesium (Mg) to zinc (Zn) can improve corrosion resistance, but the addition of magnesium (Mg) has the drawback of rapidly increasing the initial surface activity in a corrosive environment, causing the coating surface to rapidly blacken with magnesium oxide, resulting in inferior coating surface quality. Furthermore, in the case of highly corrosion-resistant coated steel sheets, when external foreign matter that induces supercooling is adsorbed during the solidification process immediately after coating, a eutectic phase with nanometer-sized lamellar gaps is formed on the coating surface. When exposed to an external corrosive environment, this eutectic phase corrodes within a short time, causing circular black spot defects and reducing quality. To address these issues, temporary rust prevention oiling or post-treatment coatings are often used after coating, but these have limitations in terms of controlling the blackening.

[0004] Related prior art includes Japanese Patent Publication No. 2005-105367. Summary of the Invention [Problem to be solved by the invention]

[0005] The technical problem to be solved by the present invention is to provide a plated steel sheet having excellent surface quality, in which the microstructure of the plating layer is controlled in order to minimize black discoloration defects and black spot defects caused by excessive activity increase on the plating surface, and a manufacturing method thereof.

[0006] However, these problems are merely examples, and the technical idea of ​​the present invention is not limited to these. [Means for solving the problem]

[0007] A plated steel sheet according to one aspect of the present invention for solving the above-mentioned problems includes: a cold-rolled steel sheet; and a plating layer formed on the cold-rolled steel sheet, the plating layer comprising 0.5 to 3 wt % of aluminum (Al), 1 to 2 wt % of magnesium (Mg), 0.005 to 0.1 wt % of silicon (Si), 0.01 to 0.1 wt % of titanium (Ti), and the balance being zinc (Zn) and other unavoidable impurities; wherein the plating layer has a titanium to silicon content ratio of 1.0 or more, and the plating layer contains an intermetallic compound containing titanium.

[0008] In the plated steel sheet, the intermetallic compound contains 3 to 40% by weight of titanium (Ti), and may further contain at least two or more components selected from the group consisting of 3 to 30% by weight of silicon (Si), 1 to 40% by weight of iron (Fe), and 1 to 50% by weight of aluminum (Al).

[0009] In the plated steel sheet, the intermetallic compound is an intermetallic compound containing titanium and iron, but may be an intermetallic compound containing 3 to 40% by weight of titanium (Ti) and 1 to 40% by weight of iron (Fe). [Effects of the Invention]

[0010] According to embodiments of the present invention, it is possible to realize a plated steel sheet with excellent surface quality, in which the microstructure of the plating layer is controlled to minimize black discoloration defects and black spot defects caused by excessive activity increase on the plating surface, and a manufacturing method thereof.

[0011] Of course, the scope of the present invention is not limited to such effects. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a flowchart illustrating a method for manufacturing a plated steel sheet according to one embodiment of the present invention.

[0013] [Figure 2] 1 is a photograph of the surface of the plating layer of a plated steel sheet according to Experimental Example 2 of the present invention.

[0014] [Figure 3] 10 is a photograph of the surface of the plating layer of a plated steel sheet according to Experimental Example 9 of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

[0015] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical concept of the present invention to those skilled in the art, and the following embodiments may be modified into various other forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more complete and complete, and to completely convey the technical concept of the present invention to those skilled in the art.

[0016] A plated steel sheet with excellent surface quality and a method for manufacturing the same according to an embodiment of the present invention will now be described in detail. The terms used below have been appropriately selected in consideration of the functions of the present invention, and the definitions of such terms should be based on the content of this specification as a whole.

[0017] FIG. 1 is a flow chart illustrating a method for manufacturing a plated steel sheet according to one embodiment of the present invention.

[0018] Referring to FIG. 1 , a method for producing a plated steel sheet according to one 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 annealed steel sheet through a coating bath containing aluminum (Al), magnesium (Mg), silicon (Si), titanium (Ti), and zinc (Zn) to form a coating layer on the steel sheet (S30) containing 0.5 to 3 wt. % aluminum (Al), 1 to 2 wt. % magnesium (Mg), 0.005 to 0.1 wt. % silicon (Si), 0.01 to 0.1 wt. % titanium (Ti), and the balance being zinc (Zn) and other unavoidable impurities; and (d) cooling the steel sheet with the coating layer formed thereon (S40).

[0019] The plated steel sheet realized by the above-described manufacturing method includes a cold-rolled steel sheet and a plating layer formed on the cold-rolled steel sheet, the plating layer consisting of 0.5 to 3 wt% aluminum (Al), 1 to 2 wt% magnesium (Mg), 0.005 to 0.1 wt% silicon (Si), 0.01 to 0.1 wt% titanium (Ti), and the balance being zinc (Zn) and other unavoidable impurities, wherein the content ratio of titanium to silicon in the plating layer is 1.0 or more, and the plating layer contains an intermetallic compound containing titanium.

[0020] The plating layer may further contain boron (B): 0.002 to 0.02% by weight.

[0021] The intermetallic compound contains 3 to 40% by weight of titanium (Ti) and may further contain at least two or more components selected from the group consisting of 3 to 30% by weight of silicon (Si), 1 to 40% by weight of iron (Fe), and 1 to 50% by weight of aluminum (Al).The intermetallic compound is an intermetallic compound containing titanium and iron, but may also be an intermetallic compound containing 3 to 40% by weight of titanium (Ti) and 1 to 40% by weight of iron (Fe).

[0022] The plating layer includes an Mg-rich phase, a primary Zn phase, a binary eutectic phase in which the Zn phase and the MgZn2 phase are in a lamellar shape, and a ternary eutectic phase in which the Al-containing MgZn2 phase and the Zn phase are in a lamellar shape, and in the plating layer, the area fraction of the ternary eutectic phase may be 30% or more, and the area fraction of the binary eutectic phase may be less than 30%.

[0023] In the plating layer, the ratio of aluminum (Al) to magnesium (Mg) may be 1.0 or more and 2.0 or less.

[0024] The coating layer includes an Mg-rich phase, a primary Zn phase, a binary eutectic phase in which the Zn phase and the MgZn2 phase are in a lamellar shape, and a ternary eutectic phase in which the Al-containing MgZn2 phase and the Zn phase are in a lamellar shape, and the Mg-rich phase can include 5 to 15 wt % of magnesium (Mg), 85 to 95 wt % of zinc (Zn), and less than 1 wt % of aluminum (Al). The titanium-containing intermetallic compound can be present in the Mg-rich phase.

[0025] The method for producing a plated steel sheet according to one embodiment of the present invention includes the steps of: (a) providing a cold-rolled steel sheet; (b) annealing the cold-rolled steel sheet; and (c) passing the annealed steel sheet through a plating bath containing aluminum (Al), magnesium (Mg), silicon (Si), titanium (Ti), and zinc (Zn) to coat the steel sheet with a plating bath containing 0.5 to 3 wt% of aluminum (Al), 1 to 2 wt% of magnesium (Mg), 0.005 to 0.1 wt% of silicon (Si), 0.01 to 0.1 wt% of titanium (Ti), and the balance being zinc (Zn). and other unavoidable impurities, wherein the step of forming the plating layer includes a step of cooling the steel sheet at a cooling rate of 5 to 30°C / sec, wherein the plating layer has a titanium to silicon content ratio of 1.0 or more, and the plating layer contains an intermetallic compound, and the intermetallic compound contains 3 to 40 wt% titanium (Ti), and may further contain at least two or more components selected from the group consisting of 3 to 30 wt% silicon (Si), 1 to 40 wt% iron (Fe), and 1 to 50 wt% aluminum (Al).

[0026] The plating layer is formed by sequential solidification of the primary Zn phase, the binary eutectic phase, and the ternary eutectic phase, and the cooling rate after the binary eutectic phase is formed in the cooling step may be higher than the cooling rate before the binary eutectic phase is formed.

[0027] 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 in the step of forming the plating layer may be 5°C / sec or more, and 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 more, the cooling rate in the step of forming the plating layer may be 10°C / sec or more.

[0028] Magnesium (Mg), added to zinc (Zn) to improve corrosion resistance, can form oxides in the coating layer in corrosive environments, causing the coating surface to blacken. Therefore, controlling the surface structure of the coating layer to minimize blackening is important. The structure of a coating layer formed using a zinc (Zn)-added magnesium (Mg) bath can consist of a single Zn phase, a binary eutectic phase (Zn + MgZn2), a ternary eutectic phase (Zn + Al + MgZn2), or an Mg-rich phase. The binary eutectic phase can be a binary eutectic phase in which the Zn phase and the MgZn2 phase form a lamellar structure, while the ternary eutectic phase can be a ternary eutectic phase in which the Zn phase and the MgZn2 phase, including Al, form a lamellar structure. The Mg-rich phase can contain 5 to 15 wt. % magnesium (Mg), 85 to 95 wt. % zinc (Zn), and less than 1 wt. % aluminum (Al).

[0029] In the structure of a coating layer plated using a zinc (Zn)-added magnesium (Mg) plating bath, the phase that accelerates the progression of blackening in a corrosive environment is a lamellar eutectic phase with a large number of reaction interfaces. Among eutectic phases, the ternary eutectic phase contributes significantly to the progression of blackening, and the smaller the spacing between the lamellar structures of the eutectic phase, the more accelerated the blackening tends to be. For this reason, it is important to control the fraction of the eutectic phase and the fraction of the primary Zn phase. The fraction of these phases can be controlled by adjusting the content of aluminum (Al) and magnesium (Mg). However, since the content of aluminum (Al) and magnesium (Mg) is limited, it is difficult to achieve excellent corrosion resistance. On the other hand, when aluminum (Al) and magnesium (Mg) are added within the range of 0.5 to 3 wt. % and 1 to 2 wt. % of the total weight of the coating composition that can achieve corrosion resistance, excellent corrosion-resistant coated steel sheets can be obtained. An aluminum (Al) content of 0.5 wt% or higher can prevent magnesium (Mg) oxidation. However, an aluminum (Al) content exceeding 3 wt% leads to excessive development of a ternary eutectic phase, accelerating the blackening of the coating surface. Furthermore, the minimum magnesium (Mg) content, which is directly involved in the formation of the activated phase on the coating surface, is preferably 1 wt%. To form the eutectic phase that contributes to corrosion resistance, magnesium (Mg) must be added at a concentration of 1 wt% or more. However, if the magnesium (Mg) content exceeds 2 wt%, the fraction of magnesium (Mg)-related phases increases sharply, making it difficult to control the activation of the coating surface. Meanwhile, to more effectively reduce the activation of the coating surface, which is the objective of the present invention, within the limited aluminum (Al) and magnesium (Mg) content range, trace elements are added. High-corrosion-resistant coating baths have a higher aluminum (Al) content than conventional zinc coating baths, which facilitates the dissolution of iron (Fe) from the base steel sheet during coating. To prevent this, trace amounts of silicon (Si) are added. Silicon (Si), which is added for this purpose, reduces the amount of iron (Fe) eluted from the base steel, thereby significantly reducing the amount of iron (Fe) dross generated, making it possible to provide a coated steel sheet with excellent surface appearance.The amount of silicon (Si) added is preferably 0.005 to 0.1 wt%, but to achieve the alloy layer control effect, the amount of silicon (Si) must be 0.005 wt% or more; if the amount of silicon (Si) added exceeds 0.1 wt%, an Mg2Si phase is generated, hindering the workability of the coating layer. Furthermore, the addition of silicon (Si) has the advantage of improving corrosion resistance by acting as a nucleation site and a grain growth barrier, thereby refining the coating structure. However, it also has the disadvantage of generating a large amount of fine eutectic phase on the coating surface, increasing surface activation and accelerating the blackening of the coating surface. Therefore, it is important to properly control the amount of silicon (Si) added.

[0030] As mentioned above, the addition of silicon (Si) reduces the dissolution of iron while also reducing the accelerated blackening of the coating surface in a corrosive environment by adding titanium (Ti) and / or titanium boride (TiB), which inhibits the refinement of the eutectic phase. Titanium (Ti) and / or titanium boride (TiB) combine with silicon (Si) and iron (Fe), which act as nucleation sites in the coating bath, to form intermetallic compounds, reducing the refinement of the spacing between the lamellar structures of the primary and eutectic phases during solidification after coating. Furthermore, the titanium-containing intermetallic compounds themselves act as nucleation sites, forming a magnesium-rich phase in the coating layer, thereby reducing the magnesium (Mg) content used to form the eutectic phase and widening the spacing between the lamellar structures of the eutectic phase. This reduces the number of reaction interfaces on the coating surface and reduces the accelerated blackening.

[0031] The Ti-based intermetallic compound composition described above must contain 3-40 wt% titanium (Ti) and may further contain at least two of the following components: 3-30 wt% silicon (Si), 1-40 wt% iron (Fe), and 1-50 wt% aluminum (Al). Furthermore, the formation of intermetallic compounds containing titanium and iron reduces iron (Fe), which acts as a nucleation site, and also combines with silicon (Si), which inhibits grain growth, thereby coarsening the coating structure. As a result, the coating layer structure becomes coarse, reducing the active interface and improving blackening resistance.

[0032] On the other hand, let's consider the reduction of black spot defects. Black spot defects are characterized by a very fine lamellar structure, and are usually generated by supercooling due to the adsorption of foreign matter. When titanium boride (TiB) is added, it acts to coarsen the structure as mentioned above, making it relatively difficult to create a fine lamellar structure due to supercooling caused by the adsorption of foreign matter, thereby reducing the occurrence of black spot defects. The amount of titanium (Ti) and / or titanium boride (TiB) added is preferably 0.01-0.1 wt%, but to control black discoloration and black spot defects through microstructure control, the amount of titanium (Ti) and / or titanium boride (TiB) added must be at least 0.01 wt%. If the amount of titanium (Ti) and / or titanium boride (TiB) added exceeds 0.1 wt%, intermetallic compounds bonded with titanium (Ti) and / or titanium boride (TiB) act as nucleation sites, resulting in an excessive increase in the Mg-rich phase and reduced workability. When silicon (Si) and titanium (Ti) and / or titanium boride (TiB) are added simultaneously, the addition of titanium (Ti) and / or titanium boride (TiB) cancels out the effects of the addition of silicon (Si), so the addition ratio must be adjusted to take into account phase control and improved appearance. The addition ratio of titanium (Ti) and / or titanium boride (TiB) to silicon (Si) must be 1.0 or more when the addition ratio of silicon (Si) is 1.0.

[0033] TiB is added using an Al-5%Ti-1%B alloy ingot, with the weight of Ti determined based on the amount of Ti. When TiB is added, an intermetallic compound of TiB2 and Al3Ti is formed during solidification of the coating layer, which is known to act as a nucleation site. B is generally not added alone, but in a Ti to B ratio of 5:1 or greater. In other words, when Ti is 0.01 to 0.1 wt%, B is added in the range of 0.002 to 0.02 wt%.

[0034] The coating layer according to one embodiment of the present invention includes a primary Zn phase, a binary eutectic phase in which the Zn phase and the MgZn2 phase are in a lamellar shape, and a ternary eutectic phase in which the MgZn2 phase containing Al and the Zn phase are in a lamellar shape, and the difference between the solidification temperature of the primary Zn phase and the binary eutectic phase is 30°C or less. The ratio of the average width to the average vertical height of the entire primary Zn phase is 3 or more, and the area ratio of the primary Zn phase whose average vertical height is 80% or more of the thickness of the coating layer is less than 50%. In the coating layer, the ratio of aluminum (Al) to magnesium (Mg) may be 1.0 or more and 2.0 or less.

[0035] In the step (b) (S20), the annealing treatment may be carried out at a temperature of 700 to 850°C, and in the step (c) (S30), the temperature of the plating bath may be 400 to 520°C.

[0036] During the above-mentioned coating process, depending on the content of aluminum (Al), magnesium (Mg), silicon (Si), titanium (Ti), and titanium boride (TiB) added to zinc (Zn) to improve corrosion resistance, the coating layer upon solidification is divided into a primary Zn phase, a binary eutectic phase in which the Zn phase and MgZn2 phase are in a lamellar configuration, and a ternary eutectic phase in which the MgZn2 phase containing Al and the Zn phase are in a lamellar configuration, resulting in different phase fractions of the resulting structure. The mechanical properties and corrosion resistance of the coating layer vary depending on the fraction of each phase.

[0037] When the fraction of the primary Zn phase is high, the hardness of the coating layer decreases, and corrosion resistance tends to decrease, but it can reduce the phenomenon of the coating surface darkening when exposed to the outside. From the perspective of preventing darkening, controlling the fraction of the primary Zn phase is an important technology.

[0038] The binary eutectic phase plays a role in improving corrosion resistance, but if the binary eutectic phase is formed coarsely, cracks tend to occur in the binary eutectic phase during processing, which tends to deteriorate workability. Therefore, it is important to maintain an appropriate fraction of the binary eutectic phase.

[0039] The ternary eutectic phase has a lamellar structure in which Zn layers and MgZn2 layers are alternately stacked, with aluminum (Al) formed between these lamellar structures. Like the binary eutectic phase, the ternary eutectic phase also improves corrosion resistance. However, compared to the binary eutectic phase, the ternary eutectic phase has a denser lamellar structure, resulting in more active interfaces per unit area. Therefore, if the proportion of the ternary eutectic phase on the surface is high, it tends to easily turn black in a corrosive environment. By controlling the proportion of phases with these properties, it is possible to improve corrosion resistance and control surface quality.

[0040] When the primary Zn phase is coarse, it has good resistance to blackening in a corrosive environment, but zinc (Zn) is directly exposed to the corrosive environment, which tends to reduce corrosion resistance. In particular, when viewed from the cross section of the coating layer, if the ratio of the major axis (average width) to minor axis (average vertical height) of the entire primary Zn phase is 3 or more, and the area ratio of primary Zn phase whose minor axis (average vertical height) is 80% or more of the total thickness of the coating layer is 50% or more, corrosion resistance decreases rapidly. This phenomenon occurs because the corrosion products of the eutectic phase, which improve corrosion resistance, are unable to form a passive oxide around the primary Zn phase to prevent corrosion of the primary Zn phase.

[0041] To create the microstructure of the present invention, the aluminum (Al) to magnesium (Mg) component ratio and the resulting solidification phase ratio can be adjusted by controlling the cooling rate. When the aluminum (Al) to magnesium (Mg) component ratio is 1:2 or less, the solidification sequence is as follows: the primary Zn phase is formed first, followed by the binary eutectic phase, and finally the ternary eutectic phase. However, when the component ratio is 1:2 or more, the solidification sequence of the eutectic phases may be changed. To create an embodiment of the present invention, 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 can be achieved by cooling the coating layer after coating at a cooling rate of 5°C / sec or more. Preferably, it should be 7°C / sec or more. However, if the cooling rate exceeds 30°C / sec, excessive cooling pressure may cause uneven solidification of the coating surface.

[0042] The ratio of aluminum (Al) and magnesium (Mg) added to zinc (Zn) during the plating process should be at least 1:1, preferably 1.2:1 or greater, to minimize oxidation of the magnesium (Mg). Aluminum (Al) present in the molten metal reacts with oxygen in the air to form a dense oxide film, preventing oxygen from reaching the surface of the molten metal and thereby preventing oxidation of the magnesium (Mg). However, if the aluminum to magnesium ratio exceeds 2:1, the probability of the ternary eutectic phase forming first increases, resulting in an overall increase in the proportion of the ternary eutectic phase and reduced resistance to blackening when exposed to the outside.

[0043] In the plating process of the present invention, the concentration range of aluminum (Al) is 0.5 to 3 wt%. If aluminum (Al) is added at less than 0.5 wt%, it does not sufficiently prevent the oxidation of the added magnesium (Mg), and if aluminum (Al) is added at more than 3 wt%, iron (Fe) eluted from the material generates a large amount of iron dross, which can cause problems with surface quality.

[0044] The concentration range of magnesium (Mg) is 1 to 2 wt%. If the amount of magnesium (Mg) added is less than 1 wt%, the eutectic phase that contributes to corrosion resistance is not sufficiently generated, and if it is added in excess of 2 wt%, although corrosion resistance is improved, the fraction of the eutectic phase increases sharply, which not only increases the number of cracks on the plating surface during processing, but also makes it impossible to control color difference changes due to corrosion of the eutectic phase on the plating surface in a corrosive environment.

[0045] The difference between the solidification points of the primary Zn phase and the binary eutectic phase is related to the growth time of the primary Zn phase. The greater the difference between the solidification temperatures of the primary Zn phase and the binary eutectic phase, the longer the time available for the primary Zn phase to grow during the cooling process, which increases the probability that the ratio of the major axis (average width) to the minor axis (average vertical height) of the primary Zn phase will be 3 or more, and that the length of the minor axis (average vertical height) of the primary Zn phase will be 80% or more of the total thickness of the coating layer.

[0046] In the following, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are merely provided to aid in understanding the present invention, and the present invention is not limited to the following experimental examples.

[0047] Experimental example

[0048] A 0.7 mm thick cold-rolled steel sheet was immersed in an alkaline solution at 50°C for 30 minutes, then washed with water to remove any impurities or oil from the surface to prepare a test specimen. The test specimen was then annealed and plated. Annealing was performed in a reducing atmosphere consisting of 10-30% hydrogen and 70-90% nitrogen, with the annealing heat treatment temperature at 700-750°C.

[0049] For plating, the annealed heat-treated test piece is cooled to the temperature of the plating bath, immersed in the plating bath for 2 seconds, then pulled out, and the plating thickness is adjusted to about 10 μm by nitrogen wiping, with the temperature of the plating bath being 400° C. to 520° C. The cold-rolled steel sheet has a composition of 0.15 wt % carbon, 0.6 wt % manganese, 0.05 wt % phosphorus, 0.005 wt % sulfur, and the balance being iron (Fe).

[0050] Table 1 shows the process conditions for plated steel sheets according to the experimental examples of the present invention, and the results of evaluation of the structure and physical properties of the plated layer.

[0051] In Table 1, the items Zn, Al, Mg, Si, and TiB indicate the composition (unit: weight %) of the plating bath in the plating process.

[0052] Blackening resistance was evaluated by measuring the color difference of the steel sheet surface using a colorimeter after storing it for one hour in a hot and humid environment with a relative humidity of 90% or more and a temperature of 50°C. The criteria for judging the color difference before and after the blackening resistance test are as follows: ◎ indicates a color difference (△E) of 6.0 or less, ○ indicates a color difference (△E) of 8.0 or less, △ indicates a color difference (△E) of 12.0 or less, and × indicates a color difference (△E) of 12.0 or more.

[0053] In the item of black dot defects, the number of black dot defects per unit area (10cm x 10cm) was measured relatively and evaluated. ◎ indicates that the number of black dot defects is 0, ○ indicates that the number of black dot defects is 1, △ indicates that the number of black dot defects is 2 to 3, and × indicates that the number of black dot defects is 3 or more.

[0054] The item for cracks in the plating layer indicates an evaluation of the workability of the plating layer, and was evaluated according to the following criteria depending on the degree of cracking in the processed area after 3T bending: ○ indicates a case where relatively thin and minute cracks are observed visually, △ indicates a case where relatively thick and few cracks are observed visually, but there are so many cracks that there is concern about the plating layer falling off, and × indicates a case where relatively thick and many cracks are observed visually.

[0055] [Table 1]

[0056] Referring to Table 1, Experimental Examples 1, 2, and 3 correspond to cases in which the plating bath composition (unit: wt%) in the plating process satisfies the ranges of aluminum (Al): 0.5 to 3 wt%, magnesium (Mg): 1 to 2 wt%, and silicon (Si): 0.005 to 0.1 wt%, but titanium (Ti) and / or titanium boride (TiB) are not added to the plating bath composition. It can be confirmed that the plating layer formed in this way does not form titanium-based intermetallic compounds, has two or more black spot defects, and has a color difference (△E) before and after the blackening resistance test of more than 8.0.

[0057] Referring to FIG. 2 , which is a photograph of the coating layer of the coated steel sheet according to Experimental Example 2 of the present invention, it can be seen that the coating layer contains an Mg-rich phase, a primary Zn phase, a binary eutectic phase in which the Zn phase and the MgZn2 phase have a lamellar structure, and a ternary eutectic phase in which the Al-containing MgZn2 phase and the Zn phase have a lamellar structure, but no intermetallic compounds containing titanium are formed.

[0058] Experimental Example 4 corresponds to a case where the composition (unit: wt%) of the plating bath in the plating process satisfies the range of aluminum (Al): 0.5 to 3 wt%, magnesium (Mg): 1 to 2 wt%, silicon (Si): 0.005 to 0.1 wt%, titanium (Ti) or titanium boride (TiB): 0.01 to 0.1 wt%, and the remainder is zinc (Zn), but the titanium to silicon content ratio is less than 1.0.It can be confirmed that the plating layer formed thereby does not form titanium-based intermetallic compounds, has three or more black spot defects, and has a color difference (△E) before and after the blackening resistance test of more than 8.0.

[0059] Experimental Example 5 corresponds to a case where the composition (unit: weight %) of the plating bath in the plating process satisfies the range of aluminum (Al): 0.5 to 3 weight %, magnesium (Mg): 1 to 2 weight %, silicon (Si): 0.005 to 0.1 weight %, titanium (Ti) or titanium boride (TiB): 0.01 to 0.1 weight %, and the balance is zinc (Zn), but the titanium to silicon content ratio is less than 1.0.It can be confirmed that the plating layer formed thereby has three or more black spot defects and a color difference (△E) before and after the blackening resistance test exceeds 12.0.

[0060] Experimental Example 6 corresponds to a case where the composition (unit: wt%) of the plating bath in the plating process satisfies the range of aluminum (Al): 0.5 to 3 wt%, magnesium (Mg): 1 to 2 wt%, titanium (Ti) or titanium boride (TiB): 0.01 to 0.1 wt%, but exceeds the range of silicon (Si): 0.005 to 0.1 wt%, and the titanium to silicon content ratio is less than 1.0.In the plating layer formed thereby, after 3T bending, relatively thick and numerous cracks were visually observed in the processed area, the number of black spot defects was 3 or more, and it was confirmed that the color difference (△E) before and after the blackening resistance test was 12.0 or less.

[0061] Experimental Example 7 corresponds to a case where the composition (unit: wt%) of the plating bath in the plating process satisfies the range of aluminum (Al): 0.5 to 3 wt%, magnesium (Mg): 1 to 2 wt%, and silicon (Si): 0.005 to 0.1 wt%, but exceeds the range of titanium (Ti) or titanium boride (TiB): 0.01 to 0.1 wt%. In the plating layer formed thereby, relatively thick and small cracks are visually observed in the processed area after 3T bending, but because there are so many cracks, there is concern about the plating layer falling off. It can be confirmed that the number of black spot defects is 0, and the color difference (△E) before and after the blackening resistance test is 6.0 or less.

[0062] In Experimental Examples 8 to 12, the composition of the plating bath in the plating process (unit: wt%) was 0.5 to 3 wt% aluminum (Al), 1 to 2 wt% magnesium (Mg), 0.005 to 0.1 wt% silicon (Si), 0.01 to 0.1 wt% titanium (Ti) or titanium boride (TiB), and the remainder was zinc (Zn), and the titanium to silicon content ratio was 1.0 or more. In the plating layer formed thereby, titanium-based intermetallic compounds were formed, and after 3T bending, only relatively thin and fine cracks were visually observed in the processed area, and the number of black spot defects was limited to 0 or 1. It was confirmed that the color difference (△E) before and after the blackening resistance test was 8.0 or less.

[0063] Referring to FIG. 3 , which is a photograph of the coating layer of the coated steel sheet according to Experimental Example 9 of the present invention, it can be seen that the coating layer includes an Mg-rich phase, a primary Zn phase, a binary eutectic phase in which the Zn phase and the MgZn2 phase have a lamellar structure, and a ternary eutectic phase in which the Al-containing MgZn2 phase and the Zn phase have a lamellar structure, and that the titanium-containing intermetallic compound is present in the Mg-rich phase.

[0064] Although the present invention has been described above with reference to the preferred embodiments, various modifications and variations may be made by those skilled in the art. Such modifications and variations are within the scope of the present invention. Therefore, the scope of the present invention should be determined by the appended claims.

Claims

1. Cold-rolled steel sheet; a plating layer formed on the cold-rolled steel sheet, the plating layer comprising 0.5 to 3 wt % of aluminum (Al), 1 to 2 wt % of magnesium (Mg), 0.005 to 0.1 wt % of silicon (Si), 0.01 to 0.1 wt % of titanium (Ti), and the balance being zinc (Zn) and other unavoidable impurities; The plating layer has a titanium to silicon content ratio of 1.0 or more, The plated steel sheet, wherein the plating layer contains an intermetallic compound containing titanium.

2. The plated steel sheet according to claim 1, wherein the plating layer further contains 0.002 to 0.02 wt % of boron (B).

3. 2. The plated steel sheet according to claim 1, wherein the intermetallic compound contains 3 to 40% by weight of titanium (Ti), and further contains at least two or more components selected from the group consisting of 3 to 30% by weight of silicon (Si), 1 to 40% by weight of iron (Fe), and 1 to 50% by weight of aluminum (Al).

4. The plated steel sheet according to claim 3, wherein the intermetallic compound is an intermetallic compound containing titanium and iron, but is an intermetallic compound containing 3 to 40 wt% of titanium (Ti) and 1 to 40 wt% of iron (Fe).

5. The plating layer is composed of a Mg-rich phase, a primary Zn phase, a Zn phase and an MgZn phase. ２ The binary eutectic phase has a lamellar structure, and MgZn containing Al ２ The ternary eutectic phase includes a Zn phase and a Zn phase having a lamellar shape, The plated steel sheet according to claim 1 , wherein in the plating layer, an area fraction of the ternary eutectic phase is 30% or more, and an area fraction of the binary eutectic phase is less than 30%.

6. The plated steel sheet according to claim 1 , wherein the ratio of aluminum (Al) to magnesium (Mg) in the plating layer is 1.0 or more and 2.0 or less.

7. The plating layer is composed of a Mg-rich phase, a primary Zn phase, a Zn phase and an MgZn phase. ２ The binary eutectic phase has a lamellar structure, and MgZn containing Al ２ 2. The plated steel sheet according to claim 1, wherein the Mg-rich phase comprises a ternary eutectic phase having a lamellar shape, and the Mg-rich phase comprises 5 to 15 wt. % of magnesium (Mg), 85 to 95 wt. % of zinc (Zn), and less than 1 wt. % of aluminum (Al).

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

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