Plated steel and manufacturing method thereof

Abstract

A plated steel according to the present invention comprises: a base steel; and a plating layer which is formed on the base steel and contains 1-4 wt% of aluminum (Al) and 1-3 wt% of magnesium (Mg), with the remainder comprising zinc (Zn) and inevitable impurities, wherein the weight ratio between aluminum and magnesium in the plating layer is 1.0-4.0, the plating layer includes a eutectic phase structure containing a Zn phase and a primary Zn phase structure, and the area fraction of the primary Zn phase structure and the Zn phase in the eutectic phase structure having matching crystal orientations is at least 30% in a cross section and the surface of the plating layer.

Description

Galvanized steel and its manufacturing method

[0001] The present invention relates to a plated steel and a method for manufacturing the same, and more particularly, to a plated steel having excellent corrosion resistance in a processed portion and a method for manufacturing the same.

[0002] Conventional hot-dip galvanized steel has excellent self-sacrificing properties, and is widely used in construction materials and home appliances. When hot-dip galvanized steel is exposed to a corrosive environment, zinc (Zn) acts as a sacrificial anode on the exposed iron portion, causing zinc loss from the plating layer. This sacrificial anode action of zinc plays an excellent role in suppressing rust of the base iron in a corrosive environment, but has the problem of somewhat low sacrificial anode efficiency. To solve this problem, recently in Japan and Europe, high-corrosion-resistant plating products are being produced by adding magnesium (Mg) to the zinc (Zn) plating bath. This creates dense corrosion products in corrosive environments and improves sacrificial anode efficiency, thereby exhibiting excellent corrosion resistance.

[0003] However, the plating bath process in which magnesium (Mg) is added to zinc (Zn) has the problem that as the magnesium (Mg) content increases, it reacts with oxygen in the air to form oxides, which degrades surface quality. To address this issue, an inert gas (nitrogen) is used in the wiping process to control the plating amount. Furthermore, an oxidation-free chamber is used to minimize oxygen concentration in the wiping process area, minimizing contact with oxygen.

[0004] In addition, although corrosion resistance improves as the magnesium (Mg) content increases, the magnesium rich phase created by the addition of magnesium (Mg) or the increase in the proportion of the lamellar structure in the process phase may act as a starting point for cracks in the plating layer during processing, which may result in poor corrosion resistance in the processed area. In addition, cracks in the plating layer may be transferred to the upper layer of the painted area during processing after painting, thereby lowering the processing quality.

[0005] Related prior art includes Japanese Patent Publication No. 2005-105367.

[0006] The technical problem to be achieved by the present invention is to provide a highly corrosion-resistant plated steel having excellent corrosion resistance and crack resistance in a processed part, and a method for manufacturing the same.

[0007] However, these tasks are exemplary and the technical idea of ​​the present invention is not limited thereto.

[0008] According to one aspect of the present invention for solving the above problem, a plated steel material comprises: a base steel; and a plating layer formed on the base steel, the plating layer being composed of 1 to 4 wt% of aluminum (Al), 1 to 3 wt% of magnesium (Mg), and the remainder being zinc (Zn) and other unavoidable impurities; wherein the weight ratio of aluminum and magnesium in the plating layer is 1.0 to 4.0, and the plating layer includes a eutectic structure containing a Zn phase and a primary Zn phase structure, wherein in a cross-section and a surface of the plating layer, the area fraction of the primary Zn phase structure and the Zn phase in the eutectic structure, in which crystal grain directions are identical to each other, is 30% or more.

[0009] In the above-mentioned plated steel, the thickness of the plated layer may be 5 ㎛ or more and less than 30 ㎛.

[0010] In the above-mentioned plated steel, the eutectic phase containing the Zn phase is the Zn phase and the MgZn2 phase. It may include a binary eutectic phase having a lamellar shape or a ternary eutectic phase including MgZn2 phase and Zn phase having a lamellar shape.

[0011] According to one aspect of the present invention for solving the above problem, a method for manufacturing a plated steel includes the steps of: (a) providing a base iron; (b) passing the base iron through a plating bath containing aluminum (Al), magnesium (Mg), and zinc (Zn), thereby forming a plated layer on the base iron, the plated layer being composed of 1 to 4 wt% of aluminum (Al), 1 to 3 wt% of magnesium (Mg), and the remainder being zinc (Zn) and other unavoidable impurities, wherein the weight ratio of aluminum to magnesium is 1.0 to 4.0; and (c) cooling the plated layer at a cooling rate of 5 to 15°C / sec; wherein the plated layer, which has undergone step (c), includes a eutectic structure containing a Zn phase and a primary Zn phase structure, wherein the primary Zn phase structure and the Zn phase within the eutectic structure, in which crystal grain directions coincide with each other in a cross-section and a surface of the plated layer, are at least 30% in area fraction.

[0012] In the above method for manufacturing the plated steel, the temperature of the plating bath may be 420 to 470°C.

[0013] In the manufacturing method of the above-mentioned plated steel, the area fraction (A) of the primary Zn phase structure and the Zn phase in the eutectic structure, in which the crystal grain directions are identical to each other, in the cross-section and surface of the plated layer can satisfy the following mathematical formula 1.

[0014] Formula 1: Area fraction (A) = 57 - 1.30 × [cooling rate] - 3.47 × [Mg] - 1.40 × [Al]

[0015] (However, the above [cooling rate] is the cooling rate (unit: ℃ / sec) in the cooling step, the above [Mg] is the content of magnesium in the above plating layer (unit: weight%), and the above [Al] is the content of aluminum in the above plating layer (unit: weight%).)

[0016] In the above method for manufacturing the plated steel, the area fraction (A, unit: %) may be 30% or more.

[0017] In the above method for manufacturing the plated steel, the thickness of the plated layer may be 5 ㎛ or more and less than 30 ㎛.

[0018] In the manufacturing method of the above-mentioned plated steel, the eutectic phase containing the Zn phase is a Zn phase and a MgZn2 phase. It may include a binary eutectic phase having a lamellar shape or a ternary eutectic phase including MgZn2 phase and Zn phase having a lamellar shape.

[0019] According to an embodiment of the present invention, a plated steel material having excellent corrosion resistance in a processed part and a method for manufacturing the same can be implemented.

[0020] Of course, the scope of the present invention is not limited by these effects.

[0021] FIG. 1 is a flowchart illustrating a method for manufacturing a plated steel according to one embodiment of the present invention.

[0022] FIG. 2 is a drawing showing the results of EBSD analysis for zinc (Zn) components in a cross-section of a plating layer of a plated steel according to Example 3 among experimental examples of the present invention.

[0023] FIG. 3 is a drawing showing the results of EBSD analysis for zinc (Zn) components on the surface of a plating layer of a plated steel according to Example 3 among experimental examples of the present invention.

[0024] FIG. 4 is a drawing showing the results of EBSD analysis for zinc (Zn) components in a cross-section of a plating layer of a plated steel according to Comparative Example 6 among experimental examples of the present invention.

[0025] FIG. 5 is a drawing showing the results of EBSD analysis for zinc (Zn) components on the surface of the plating layer of a plated steel according to Comparative Example 6 among the experimental examples of the present invention.

[0026] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. These embodiments are provided to more fully explain the technical concept of the present invention to those skilled in the art. The following embodiments may be modified in various ways, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely explain the present disclosure and to fully convey the technical concept of the present invention to those skilled in the art.

[0027] A plated steel material with excellent corrosion resistance at the machined portion and a method for manufacturing the same according to an embodiment of the present invention are described in detail. The terms described below have been appropriately selected in consideration of their functions in the present invention, and their definitions should be based on the contents of this specification.

[0028] FIG. 1 is a flowchart illustrating a method for manufacturing a plated steel according to one embodiment of the present invention.

[0029] Referring to FIG. 1, a method for manufacturing a plated steel according to an embodiment of the present invention includes (a) a step of providing a base iron (S10); (b) a step of passing the base iron through a plating bath containing aluminum (Al), magnesium (Mg), and zinc (Zn) to form a plated layer on the base iron, the plated layer being composed of 1 to 4 wt% of aluminum (Al), 1 to 3 wt% of magnesium (Mg), and the remainder being zinc (Zn) and other unavoidable impurities, wherein the weight ratio of aluminum to magnesium is 1.0 to 4.0; and (c) a step of cooling the plated layer at a cooling rate of 5 to 15°C / sec (S30).

[0030] The plating layer that has undergone the above step (c) (S30) includes a eutectic structure containing a Zn phase and a primary Zn phase structure, and in the cross-section and surface of the plating layer, the area fraction of the primary Zn phase structure and the Zn phase within the eutectic structure, in which the crystal grain directions are identical to each other, is 30% or more.

[0031] The thickness of the above plating layer may be 5 ㎛ or more and less than 30 ㎛.

[0032] In the above method for manufacturing the plated steel, the temperature of the plating bath may be 420 to 470°C, and in this case, in the cross-section and surface of the plating layer, the area fraction (A) of the primary Zn phase structure and the Zn phase in the eutectic structure, in which the crystal grain directions are identical to each other, may satisfy the following mathematical formula 1, and the area fraction (A, unit: %) may be 30% or more.

[0033] Formula 1: Area fraction (A) = 57 - 1.30 × [cooling rate] - 3.47 × [Mg] - 1.40 × [Al]

[0034] Here, the [cooling rate] is the cooling rate (unit: ℃ / sec) in the cooling step (S30), the [Mg] is the content of magnesium in the plating layer (unit: weight%), and the [Al] is the content of aluminum in the plating layer (unit: weight%).

[0035] In general, as the magnesium (Mg) content increases in high-corrosion-resistant plating steel, the corrosion resistance is improved, but the magnesium rich phase or the lamellar structure eutectic fraction generated by the addition of magnesium (Mg) may act as a crack initiation point in the plating layer during processing, which may result in poor corrosion resistance in the processed area. Here, the eutectic phase is a Zn phase and a MgZn2 phase. A binary eutectic phase having a lamellar shape, a MgZn2 phase containing Al, and a ternary eutectic phase having a Zn phase having a lamellar shape may be included. Meanwhile, during processing after painting, cracks in the plating layer may be transferred to the upper layer of the painted area, which may reduce the processing quality. In addition, as the Mg content increases, the amount of Mg dross adsorption increases, which may result in poor corrosion resistance.

[0036] On the other hand, if the magnesium (Mg) content is low and the fraction of the primary Zn phase in the plating structure increases, the processability of the plating layer can be improved. However, if the magnesium (Mg) content is too low, the primary Zn phase formed during the solidification of the plating layer is formed in a relatively coarse form, and the area where the coarse primary Zn phase is formed in a corrosive environment develops into an area with poor corrosion resistance. In addition, if the magnesium (Mg) content is low, the amount of the process phase containing magnesium (Mg) is reduced, making it difficult to exhibit high corrosion resistance.

[0037] The fraction of the primary Zn phase and the eutectic phase can be controlled by the aluminum (Al) and magnesium (Mg) contents, and additionally by adjusting the cooling rate during the solidification process of the plating layer after plating. However, in order to secure corrosion resistance and workability of the plating layer at the same time, the concentration of aluminum (Al) and magnesium (Mg) added to the zinc plating bath must be limited. It was confirmed that a plated steel with excellent workability can be produced by controlling the fraction of the Zn phase with the same grain orientation in the lamellar structure of the eutectic phase closest to the grain orientation of the primary Zn phase.

[0038] Depending on the content of aluminum (Al) and magnesium (Mg) added to the zinc plating solution to improve corrosion resistance, the plating layer upon solidification is composed of primary Zn phase, Mg rich phase, Zn phase, and MgZn2 phase. The tissue fractions produced are different because the binary process phase with a lamellar shape, the MgZn2 phase containing Al, and the ternary process phase with a Zn phase with a lamellar shape are distinguished.

[0039] The primary Zn phase is composed of more than 90% Zn and has a low hardness compared to the eutectic phase, which contributes to the crack resistance of the plating layer. The eutectic phase formed in the plating layer acts as a starting point for cracks during processing due to its high hardness. However, when the crystal direction of the primary Zn phase and the crystal direction of the eutectic phase (specifically, the crystal direction of the Zn phase existing in the eutectic phase) are parallel, the amount of cracks occurring during processing can be minimized.

[0040] In order to increase the probability that the crystal grain orientations of the primary Zn phase and the Zn phase existing in the eutectic phase are identical in the technology of the present invention, it is very important that solidification begins from the primary Zn phase during the solidification process and that the eutectic phase solidifies from the interface of the primary Zn phase at the end of solidification. In order to have such solidification behavior, if rapid cooling is performed in the solidification section of the eutectic phase after the primary Zn phase has solidified, the eutectic phase solidification is not generated from the interface with the primary Zn phase. If rapid cooling is performed in the solidification section of the eutectic phase, the eutectic phase starts to solidify from within the liquid phase, so the probability that the crystal grain orientation of the primary Zn phase and the crystal grain orientation of the Zn phase in the eutectic phase are identical is reduced.

[0041] In order to implement the technical idea of ​​the present invention, during the process of cooling the plating layer after controlling the plating amount during the production process, cooling must be performed so that the cooling rate satisfies the range of 5 to 15°C / sec. More preferably, it can be set to 7 to 13°C / sec.

[0042] When the fraction of the primary Zn phase and the Zn phase existing in the process phase are 30% or more and the grain direction of the cross-section and surface of the plating layer produced through these process conditions are identical, the plating layer has excellent crack resistance.

[0043] The fraction of the phase whose grain orientation matches that of the primary Zn phase and the eutectic phase is less than 30% when the cooling rate exceeds 15℃ / sec, and thus has a minimal effect on workability. In addition, when the cooling rate is less than 5℃ / sec, the plating layer does not solidify and flows down, resulting in flow patterns.

[0044] In order to implement the technical idea of ​​the present invention, the content of aluminum (Al) and magnesium (Mg) added to the zinc bath can be limited.

[0045] The appropriate range for the aluminum (Al) concentration is 1 to 4 wt%, and more strictly, it can be 1 to 3.5 wt%. The appropriate range for the magnesium (Mg) concentration is 1 to 3 wt%. In addition, the weight ratio of aluminum (Al) and magnesium (Mg) should be 1.0 to 4.0 in order to minimize Mg oxidation.

[0046] When the aluminum (Al) concentration is less than 1 wt%, the corrosion resistance is not sufficient when the magnesium (Mg) addition is less than 1 wt%, considering the addition ratio of aluminum (Al) and magnesium (Mg). When the aluminum (Al) concentration exceeds 4 wt%, the amount of iron (Fe) eluted from the base iron increases, which increases the probability of iron dross generation. On the other hand, when the weight ratio of aluminum (Al) to magnesium (Mg) exceeds 4, the content of iron (Fe) eluted from the base iron increases, which reduces the surface quality due to the generation of iron dross. When the magnesium (Mg) concentration exceeds 3.0 wt%, the Zn primary phase fraction decreases to less than 30% due to a rapid increase in the process, which rapidly reduces the workability.

[0047] In the above method for manufacturing a plated steel, the temperature of the plating bath is preferably 420 to 470°C. If the temperature of the plating bath is less than 420°C, cooling occurs in the plated steel during the process of controlling the plating amount. At this time, supercooling occurs to the temperature at which the eutectic phase solidifies, making it difficult to achieve a phase fraction of 30% or more in which the grain directions of the primary Zn and the Zn phase existing in the eutectic phase are identical. If the temperature of the plating bath exceeds 470°C, a problem arises in which ash defects increase on the surface of the plating layer due to an increase in the amount of zinc (Zn) vaporization caused by excessive temperature increase.

[0048] In the method for manufacturing the above-mentioned plated steel, when the temperature of the plating bath is 420 to 470°C, the area fraction (A) of the primary Zn phase structure and the Zn phase in the eutectic structure, in which the crystal grain directions are identical to each other, in the cross-section and surface of the plating layer can satisfy the following mathematical formula 1, and the area fraction (A, unit: %) can be 30% or more.

[0049] Formula 1: Area fraction (A) = 57 - 1.30 × [cooling rate] - 3.47 × [Mg] - 1.40 × [Al]

[0050] Here, the [cooling rate] is the cooling rate (unit: ℃ / sec) in the cooling step (S30), the [Mg] is the content of magnesium in the plating layer (unit: weight%), and the [Al] is the content of aluminum in the plating layer (unit: weight%).

[0051] A plated steel realized by the above-described manufacturing method comprises: a base steel; and a plating layer formed on the base steel, the plating layer being composed of 1 to 4 wt% of aluminum (Al), 1 to 3 wt% of magnesium (Mg), and the remainder being zinc (Zn) and other unavoidable impurities; wherein the weight ratio of aluminum and magnesium in the plating layer is 1.0 or more and 4.0 or less, and the plating layer includes a eutectic structure containing a Zn phase and a primary Zn phase structure, wherein the area fraction of the primary Zn phase structure and the Zn phase in the eutectic structure, in which crystal grain directions are identical to each other in a cross-section and a surface of the plating layer, is 30% or more.

[0052] The thickness of the above plating layer may be 5 ㎛ or more and less than 30 ㎛.

[0053] To aid in understanding the present invention, preferred experimental examples are presented below. However, the following experimental examples are provided solely to aid in understanding the present invention, and the present invention is not limited to the following experimental examples.

[0054] Experimental example

[0055] Prepare a 0.7 mm thick steel specimen by immersing it in a 50°C alkaline solution for 30 minutes, then washing it with water to remove any foreign matter and oil from the surface. This specimen is then annealed and plated. Annealing is performed in a reducing atmosphere consisting of 5–20% hydrogen and 80–95% nitrogen, and the annealing heat treatment temperature is 700–850°C.

[0056] Plating is performed by cooling the annealed heat-treated specimen to the plating bath temperature, immersing it in the plating bath for 2 seconds, and then pulling it up and adjusting the plating thickness to approximately 10㎛ using nitrogen wiping.

[0057] The cold rolled steel sheet constituting the above-mentioned base steel has a composition containing carbon: 0.15 wt%, manganese: 0.6 wt%, phosphorus: 0.05 wt%, sulfur: 0.05 wt%, and the remainder being iron (Fe).

[0058] Table 1 shows the composition and process conditions of the plated steel according to the experimental example of the present invention, and Table 2 shows the evaluation results for the processability of the plated layer, corrosion resistance of the processed part, and appearance quality of the plated surface according to the composition and process conditions of Table 1.

[0059]

[0060] ClassificationZnAlMgAl: MgArea fraction(%)Cooling speed(℃ / s)Plating bath temperature(℃)Formula satisfactionComparative example 1Bal.0.50.51:1517450○Comparative example 2Bal.10.52:1497450○Comparative example 3Bal.11.81: 1.8397450○Comparative example 4Bal.1.220.6:1387450○Comparative example 5Bal.1.220.6:1404450○Comparative example 6Bal.1.520.75:12517450×Comparative example 7Bal.21.51.3:12417450×Comparative example 8Bal.21.51.3:1277410○Comparative example 9Bal.1.51.51:1423450○Comparative Example 10Bal.61.25:1357450○Comparative Example 11Bal.441:1337450×Comparative Example 12Bal.515:1377450○Comparative Example 13Bal.21.21.6:13012480○Example 1Bal.1.511.5:1445450○Example 2Bal.1.51.51:13215450○Example 3Bal.212:1417450○Example 4Bal.21.21.6:13212450○Example 5Bal.2.51.51.6:13610450○Example 6Bal.31.81.6:1376450○Example 7Bal.2.51.21.6:13410460○Example 8Bal.2.51.21.6:13610470○

[0061] In Table 1, the Zn, Al, and Mg items represent the composition (unit: weight%) in the plating process, the cooling rate is the cooling rate in the cooling step (S30) illustrated in Fig. 1, and the plating bath temperature is the plating bath temperature in the plating layer forming step (S20) illustrated in Fig. 1. The area fraction item refers to the area fraction of the primary Zn phase structure and the Zn phase in the eutectic structure in which the crystal grain directions are identical to each other in the cross-section and surface of the plating layer. In addition, in the item on whether the formula is satisfied, if the value of the formula: 57 - 1.30 × [cooling rate] - 3.47 × [Mg] - 1.40 × [Al] is 30 or more, it is marked with ○, and if it is less than 30, it is marked with ×. In the above formula, [cooling rate] is a numerical value of the cooling rate (unit: ℃ / s) in the step (S30) of cooling the plating layer, [Mg] is a numerical value of the magnesium content (unit: weight%) in the plating layer, and [Al] is a numerical value of the aluminum content (unit: weight%) in the plating layer.

[0062] FIG. 2 is a drawing showing the results of EBSD analysis for zinc (Zn) components in a cross-section of a plating layer of a plating steel according to Example 3 among experimental examples of the present invention, FIG. 3 is a drawing showing the results of EBSD analysis for zinc (Zn) components in a surface of a plating layer of a plating steel according to Example 3 among experimental examples of the present invention, FIG. 4 is a drawing showing the results of EBSD analysis for zinc (Zn) components in a cross-section of a plating layer of a plating steel according to Comparative Example 6 among experimental examples of the present invention, and FIG. 5 is a drawing showing the results of EBSD analysis for zinc (Zn) components in a surface of a plating layer of a plating steel according to Comparative Example 6 among experimental examples of the present invention. In FIGS. 2 to 5, the part indicated as Zn phase corresponds to a primary Zn phase structure, and the part indicated as eutectic phase corresponds to a Zn phase in the eutectic phase structure.

[0063] ClassificationSurface Quality GradeFlow PatternFe Dross AdsorptionMg Dross AdsorptionProcessabilityCorrosion ResistanceComparative Example 1○Good Good Good◎△Comparative Example 2○Good Good Good◎△Comparative Example 3△Good Good Occurrence○○Comparative Example 4△Good Good Occurrence○○Comparative Example 5X Occurrence Good Occurrence○○Comparative Example 6△Good Good Occurrence△○Comparative Example 7○Good Good Good△○Comparative Example 8○Good Good Good△○Comparative Example 9△ Occurrence Good Good○○Comparative Example 10△Good Occurrence Good○◎Comparative Example 11○Good Good GoodX◎Comparative Example 12△Good Occurrence Good○◎Comparative Example 13○Good Good Poor○○Example 1○Good Good Good◎○Example 2○Good Good Good○○Example 3○Good Good Good◎○Example 4○GoodGoodGood○○Example 5○GoodGoodGood○○Example 6○GoodGoodGood○◎Example 7○GoodGoodGood○○Example 8○GoodGoodGood○○

[0064] In the evaluation of the plating layer processability in Table 2, for a plated steel sheet with a plating layer thickness of 10㎛, after 3T bending, an electron microscope was used to observe at 50x magnification, a line was drawn perpendicular to the bending direction of the processed area, and the number of cracks passing through the line was measured. The evaluation was conducted based on the following judgment criteria according to the degree of crack occurrence in the processed area. ◎ indicates that no cracks are observed, ○ indicates that the number of cracks is observed to be less than 20 on average, △ indicates that the number of cracks is observed to be 20 to 25 on average, and X indicates that the number of cracks is observed to be 26 or more on average.

[0065] In the corrosion resistance evaluation of the machined part, the time for red rust occurrence was evaluated using a salt spray test with a 5% NaCl solution at 35℃ for the machined part after 3T bending. ◎ item corresponds to a case where the time for red rust occurrence is 1200 hours or more, ○ item corresponds to a case where the time for red rust occurrence is approximately 1000 hours, △ item corresponds to a case where the time for red rust occurrence is approximately 800 hours, and X item corresponds to a case where the time for red rust occurrence is approximately 600 hours.

[0066] In the evaluation of the plating surface quality, the ○ item corresponds to a case where there is no visual flow pattern, no Fe dross adsorption, and no Mg dross adsorption; the △ item corresponds to a case where any one of the visual flow pattern, Fe dross adsorption, and Mg dross adsorption appears; and the X item corresponds to a case where two or more of the visual flow pattern, Fe dross adsorption, and Mg dross adsorption appear.

[0067] Referring to Tables 1 and 2, Examples 1 to 8 satisfy the composition range of aluminum (Al): 1 to 4 wt%, magnesium (Mg): 1 to 3 wt%, and the remainder being zinc (Zn), and the weight ratio of aluminum to magnesium being 1.0 to 4.0, and the base iron is immersed in a plating bath having a temperature of 420 to 470°C, and then a plating layer having a thickness of 5 μm to less than 30 μm is formed by nitrogen wiping, and cooling at a cooling rate of 5 to 15°C / sec, so that it can be confirmed that the area fraction of the primary Zn phase structure and the Zn phase in the eutectic structure, in which the crystal grain directions are identical to each other, is 30% or more in the cross-section and surface of the formed plating layer (see FIGS. 2 and 3).

[0068] In addition, according to Examples 1 to 8, when the plating bath temperature is 420 to 470°C, the area fraction (A) of the primary Zn phase structure and the Zn phase in the eutectic structure, in which the crystal grain directions are identical to each other, in the cross-section and surface of the plating layer satisfies the following mathematical formula 1, and it can be confirmed that the area fraction (A) is 30% or more.

[0069] Formula 1: Area fraction (A) = 57 - 1.30 × [cooling rate] - 3.47 × [Mg] - 1.40 × [Al]

[0070] (However, the above [cooling rate] is the cooling rate (unit: ℃ / sec) in the cooling step, the above [Mg] is the content of magnesium in the above plating layer (unit: weight%), and the above [Al] is the content of aluminum in the above plating layer (unit: weight%).)

[0071] In addition, according to Examples 1 to 8, no visible cracks were observed in the processed portion after 3T bending, indicating excellent plating layer processability. In addition, when the red rust occurrence time was evaluated by a salt spray test using a 5% NaCl solution at 35°C on the processed portion after 3T bending, the red rust occurrence time was 1000 hours or more, indicating excellent corrosion resistance in the processed portion, and furthermore, it was confirmed that the appearance quality of the plating surface was good.

[0072] In contrast, Comparative Example 1 is a plated steel having aluminum (Al): below the range of 1 to 4 wt% and magnesium (Mg): below the range of 1 to 3 wt%. As a result of evaluating the time for occurrence of red rust in a salt spray test using a 5% NaCl solution at 35°C on the processed area after 3T bending, the time for occurrence of red rust was measured to be approximately 800 hours, confirming that the corrosion resistance of the processed area is not good.

[0073] Comparative Example 2 is a plated steel having a magnesium (Mg) content below the range of 1 to 3 wt%. As a result of evaluating the time for occurrence of red rust in a salt spray test using a 5% NaCl solution at 35°C on the processed area after 3T bending, the time for occurrence of red rust was measured to be approximately 800 hours, confirming that the corrosion resistance of the processed area is not good.

[0074] Comparative Examples 3 and 4 are plating steels having a composition range of a weight ratio of aluminum and magnesium of 1.0 or more and 4.0 or less, and it can be confirmed that Mg dross adsorption occurs, resulting in poor plating surface quality.

[0075] Comparative Example 5 is a plating steel material having a composition range of 1.0 to 4.0 in weight ratio of aluminum and magnesium, and a cooling rate of 5 to 15°C / sec. It can be confirmed that Mg dross adsorption occurs and flow patterns occur, resulting in poor plating surface quality.

[0076] Comparative Example 6 is a plated steel material that falls below the composition range of the weight ratio of aluminum and magnesium: 1.0 or more and 4.0 or less, exceeds the range of the cooling rate: 5 to 15°C / sec, and does not satisfy the range of the area fraction of the Zn phase in the primary Zn phase structure and the eutectic structure in which the crystal grain directions are identical: 30% or more (see Figs. 4 and 5), and Mg dross adsorption occurs, so the plating surface quality is not good, and it can be confirmed that the processability is not good because the number of cracks in the processed area after 3T bending is observed to be 20 to 25 on average.

[0077] Comparative Example 7 is a plated steel material that exceeds the cooling rate range of 5 to 15°C / sec and does not satisfy the range of the area fraction of the Zn phase in the primary Zn phase structure and the eutectic structure in which the crystal grain directions are identical, but falls below 30%. After 3T bending, the number of cracks in the processed area is observed to be 20 to 25 on average, confirming poor workability.

[0078] Comparative Example 8 is a plated steel material that does not satisfy the range of the above-mentioned primary Zn phase structure and the above-mentioned eutectic structure in which the crystal grain directions are identical and the area fraction of the above-mentioned Zn phase is 30% or more, and after 3T bending, the number of cracks in the processed section is observed to be 20 to 25 on average, confirming that the processability is not good.

[0079] Comparative Example 9 is a plated steel with a cooling rate below the range of 5 to 15°C / sec, and it can be confirmed that flow patterns occur and the plating surface quality is not good.

[0080] Comparative Examples 10 and 12 are plating steels exceeding the composition range of aluminum (Al): 1 to 4 wt% and the weight ratio of aluminum and magnesium: 1.0 to 4.0, and it can be confirmed that Fe dross adsorption occurs and the plating surface quality is not good.

[0081] Comparative Example 11 is a plated steel with a magnesium (Mg) content exceeding the range of 1 to 3 wt%. After 3T bending, the number of cracks in the processed area was observed to be 26 or more on average, confirming poor workability.

[0082] Comparative Example 13 is a plating steel material exceeding the plating bath temperature range of 420 to 470℃, and it can be confirmed that Mg dross adsorption occurs and the plating surface quality is not good.

[0083] Meanwhile, referring to Comparative Examples 6, 7, and 11, it can be confirmed that the workability is not good when the plating bath temperature satisfies the range of 420 to 470°C, but the calculated value of 57 - 1.30 × [cooling rate] - 3.47 × [Mg] - 1.40 × [Al] is less than 30. In contrast, referring to Comparative Example 8 or Comparative Example 13, it can be confirmed that the workability is not good or the plating surface quality is not good when the plating bath temperature does not satisfy the range of 420 to 470°C even when the calculated value of 57 - 1.30 × [cooling rate] - 3.47 × [Mg] - 1.40 × [Al] exceeds 30. Here, the [cooling rate] is the cooling rate (unit: ℃ / sec) in the cooling step (S30), the [Mg] is the content of magnesium in the plating layer (unit: weight%), and the [Al] is the content of aluminum in the plating layer (unit: weight%).

[0084] While the above description focuses on specific embodiments of the present invention, those skilled in the art will appreciate that various modifications and variations can be made. As long as such modifications and variations do not depart from the scope of the present invention, they are considered to be within the scope of the present invention. Therefore, the scope of the present invention should be determined by the claims set forth below.

Claims

1. So Ji-cheol; and A plating layer formed on the above-mentioned base material, comprising 1 to 4 wt% of aluminum (Al), 1 to 3 wt% of magnesium (Mg), and the remainder being zinc (Zn) and other unavoidable impurities; The weight ratio of aluminum and magnesium in the above plating layer is 1.0 or more and 4.0 or less, The plating layer includes a eutectic structure containing a Zn phase and a primary Zn phase structure, and is characterized in that, in the cross-section and surface of the plating layer, the primary Zn phase structure and the area fraction of the Zn phase in the eutectic structure, in which the crystal grain directions are identical to each other, are 30% or more. Galvanized steel.

2. In paragraph 1, The thickness of the above plating layer is characterized by being 5 ㎛ or more and less than 30 ㎛. Galvanized steel.

3. In paragraph 1, The process phase containing the above Zn phase is composed of Zn phase and MgZn2 phase. A binary eutectic phase having a lamellar shape or a ternary eutectic phase including a MgZn2 phase containing Al and a Zn phase having a lamellar shape, Galvanized steel. 4.(a) Step of providing a possession iron; (b) a step of passing the base iron through a plating bath containing aluminum (Al), magnesium (Mg), and zinc (Zn), thereby forming a plating layer on the base iron, the plating layer being composed of 1 to 4 wt% of aluminum (Al), 1 to 3 wt% of magnesium (Mg), and the remainder being zinc (Zn) and other unavoidable impurities, wherein the weight ratio of aluminum to magnesium is 1.0 or more and 4.0 or less; and (c) a step of cooling the plating layer at a cooling rate of 5 to 15°C / sec; including, The plating layer that has performed the step (c) above includes a eutectic structure containing a Zn phase and a primary Zn phase structure, wherein the area fraction of the primary Zn phase structure and the Zn phase in the eutectic structure, in which the crystal grain directions are identical to each other in the cross-section and surface of the plating layer, is 30% or more. Method for manufacturing galvanized steel.

5. In paragraph 4, The above plating bath is characterized by a temperature of 420 to 470°C. Method for manufacturing galvanized steel.

6. In paragraph 5, In the cross-section and surface of the plating layer, the area fraction (A) of the Zn phase in the primary Zn phase structure and the eutectic phase structure, in which the crystal grain directions are identical to each other, satisfies the following mathematical formula 1. Method for manufacturing galvanized steel. Equation 1: Area fraction (A) = 57 - 1.30 × [cooling rate] - 3.47 × [Mg] - 1.40 × [Al] (However, the above [cooling speed] is the cooling speed (unit: ℃ / sec) in the cooling step, the above [Mg] is the content of magnesium in the above plating layer (unit: weight%), and the above [Al] is the content of aluminum in the above plating layer (unit: weight%).) 7. In paragraph 5, The above area fraction (A) is characterized by being 30% or more. Method for manufacturing galvanized steel.

8. In paragraph 4, The thickness of the above plating layer is characterized by being 5 ㎛ or more and less than 30 ㎛. Method for manufacturing galvanized steel.

9. In paragraph 4, The process phase containing the above Zn phase is composed of Zn phase and MgZn2 phase. A binary eutectic phase having a lamellar shape or a ternary eutectic phase including a MgZn2 phase containing Al and a Zn phase having a lamellar shape, Method for manufacturing galvanized steel.

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