Method for manufacturing plated steel having excellent workability and corrosion resistance

Abstract

A method of manufacturing plated steel having excellent workability and corrosion resistance according to one embodiment of the present invention includes the steps of immersing an iron base into a molten alloy plating bath, and pulling out the immersed iron base from the molten alloy plating bath and performing a cooling process, thereby forming a molten alloy plating layer on the iron base, wherein in the cooling process, a first average cooling rate varies depending on a difference between a first temperature (i.e., a temperature of the molten alloy plating bath) and a second temperature (i.e., a solidification start temperature of a MgZn2 phase constituting the molten alloy plating layer).

Description

Technical Field

[0001] This invention relates to a type of steel, and more specifically, to a method for manufacturing a coated steel with excellent workability and corrosion resistance. Background Technology

[0002] Hot-dip galvanized steel sheets exhibit excellent sacrificial corrosion resistance; when exposed to corrosive environments, the low-potential zinc is preferentially leached away to prevent corrosion. Due to this superior corrosion resistance, hot-dip galvanized steel sheets have been used in household appliances, building materials, and automotive applications. However, with increasing expectations and demands for corrosion resistance due to technological advancements and improved quality levels, the need to develop products with even better corrosion resistance than existing hot-dip galvanized steel sheets is also growing. To address these issues, since the early 2000s, Europe and Japan have begun producing high-corrosion-resistant coated steel sheets by adding aluminum (Al) and magnesium (Mg) to the zinc (Zn) plating bath to enhance corrosion resistance. In addition to the sacrificial corrosion resistance of Zn, high-corrosion-resistant coated steel sheets can form dense corrosion products in corrosive environments due to the addition of Mg and Al, which protect the steel from oxidizing atmospheres, thus improving corrosion resistance. However, while Zn-Al-Mg coated steel sheets offer superior corrosion resistance compared to galvanized steel sheets, they suffer from reduced processability. Zn-Al-Mg intermetallic compounds have high hardness and low crack resistance, and cracks can lead to problems such as appearance deterioration or reduced corrosion resistance of the base steel during processing.

[0003] The relevant prior art is Japanese Patent Application Publication No. 2005-105367. Summary of the Invention

[0004] Technical issues

[0005] The technical problem to be solved by the present invention is to provide a method for manufacturing coated steel with excellent processability and corrosion resistance.

[0006] Technical solution

[0007] A method for manufacturing coated steel with excellent workability and corrosion resistance according to one aspect of the present invention, for solving the above-mentioned problems, comprises the following steps: immersing a base steel in a hot-dip alloying bath; and pulling the immersed base steel out of the hot-dip alloying bath and subjecting it to a cooling treatment, thereby forming a hot-dip alloying coating on the base steel. The first average cooling rate during the cooling treatment varies according to the difference between a first temperature (i.e., the temperature of the hot-dip alloying bath) and a second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloying coating).

[0008] In the method described herein, when the difference between the first temperature and the second temperature is less than 50°C, the first average cooling rate can be from 10°C / s to 20°C / s; when the difference between the first temperature and the second temperature is greater than or equal to 50°C and less than 100°C, the first average cooling rate can be from 15°C / s to 35°C / s; and when the difference between the first temperature and the second temperature is greater than or equal to 100°C, the first average cooling rate can be from 20°C / s to 50°C / s.

[0009] In the method described herein, the first average cooling rate can be the average cooling rate from the time point when the immersed base steel is pulled out of the hot-dip alloy bath to the time point when the MgZn2 phase begins to solidify.

[0010] In the method described herein, the second average cooling rate from the start of solidification of the MgZn2 phase to the completion of solidification during the cooling process can satisfy the following relationship: Equation 1:

[0011] <Formula 1>

[0012] 0.0114×T-0.2841≤Second average cooling rate≤0.025×T+10 (T: Solidification start temperature of MgZn2 phase)

[0013] In the method described herein, the hot-dip alloy plating bath may be a Zn plating bath containing 6% to 23% Al, 3% to 7% Mg, and unavoidable impurities.

[0014] In the method described herein, on the surface of the hot-dip alloy coating formed on the base steel, the area fraction of the MgZn2 phase with an average short axis length (a) to average long axis length (b) ratio of 0.5 or less in the entire MgZn2 phase can be 70% or less.

[0015] Beneficial effects

[0016] According to an exemplary embodiment of the present invention, a method for manufacturing coated steel with excellent processability and corrosion resistance can be provided.

[0017] It should be noted that the scope of the present invention is not limited by these effects. Attached Figure Description

[0018] Figure 1 This is a photograph of the surface of the hot-dip alloy coating in Example 6 of the experimental case.

[0019] Figure 2 The photograph shows the surface of the hot-dip alloy coating in Comparative Example 1 of the experimental examples.

[0020] Figure 3This is a 200x magnified FE-SEM image of the processed part after the hot-dip alloy coating of Example 6 in the experimental example was evaluated for 3T bending workability.

[0021] Figure 4 This is a 200x magnified FE-SEM image of the processed part after evaluating the 3T bending workability of the hot-dip alloy coating according to Comparative Example 4 in the experimental examples.

[0022] Figure 5 It is a 1000x magnified FE-SEM image of the cross-section of the hot-dip alloy coating in Example 3 of the experimental case.

[0023] Figure 6 The image is a 1000x magnified FE-SEM photograph of the cross-section of the hot-dip alloy coating in Comparative Example 4 of the experimental examples. Detailed Implementation

[0024] A method for manufacturing coated steel with excellent processability and corrosion resistance according to an exemplary embodiment of the present invention will be described in detail. The terminology used herein is appropriately selected in consideration of its function in the invention. Therefore, the definitions of the terminology should be based on the entire contents of this specification. Specific details of ultra-high strength, highly corrosion-resistant coated steel sheets with excellent elongation and methods for manufacturing the same will be provided below.

[0025] Compared to galvanized steel, Zn-Al-Mg coated steel exhibits superior corrosion resistance, but suffers from deterioration in processability. The Zn-Al-Mg intermetallic compound possesses high hardness and low crack resistance, and cracks can lead to surface degradation during processing or reduce the corrosion resistance of the base steel during machining. Among the intermetallic compounds, MgZn2 exhibits the highest hardness; therefore, techniques for controlling the shape, distribution, and size of the MgZn2 phase are crucial.

[0026] The purpose of this invention is to provide a method for manufacturing a Zn-Al-Mg based high corrosion-resistant coated steel containing 6% to 23% Al, 3% to 7% Mg, the balance Zn and unavoidable impurities, and to control the microstructure of the high-hardness MgZn2 phase to improve processability and corrosion resistance during processing.

[0027] A method for manufacturing coated steel with excellent processability and corrosion resistance according to an exemplary embodiment of the present invention includes the following steps: immersing a base steel in a hot-dip alloy plating bath (S10); and pulling the immersed base steel out of the hot-dip alloy plating bath and cooling it to form a hot-dip alloy plating layer on the base steel (S20).

[0028] In the step of immersing the base steel (S10), the hot-dip alloying bath can be, for example, a Zn bath containing 6% to 23% Al, 3% to 7% Mg, and unavoidable impurities. Furthermore, the hot-dip alloying bath may also contain 0.05% to 10% Fe and more than 0% and less than 1% Si.

[0029] Both Mg and Al in hot-dip alloy plating baths are elements that improve the corrosion resistance of the coating by forming more dense corrosion products. If the Mg content in the plating bath is less than 1.0 wt%, its contribution to corrosion resistance is negligible. In related technologies, the amount of Mg used is less than 2.0 wt%, because when its amount exceeds 2.0 wt%, production becomes difficult due to Mg oxide scum. However, in this invention, to obtain better corrosion resistance, the amount of Mg added to the plating bath is 3.0 wt% or higher. As mentioned above, when the amount of Mg added is greater than 3.0 wt%, production becomes difficult due to oxide scum. However, when the amount of Al added is 6.0 wt% or higher, scum caused by Mg oxidation in the hot-dip metal can be reduced. Furthermore, when Al is added, it can improve corrosion resistance by forming a primary Al and Zn-Al-Mg ternary eutectic phase. On the other hand, if the amount of Mg added in the plating bath exceeds 7.0% by weight, the growth of rod-shaped and needle-shaped MgZn2 or Al-containing MgZn2 phases in the coating exceeds 70% of the total MgZn2 area fraction. Therefore, the processability of the coating deteriorates, and cracks are generated in the coating during processing, thereby reducing the corrosion resistance of the steel or the Fe-Al-Zn interfacial alloy layer. Conversely, if the amount of Al added in the plating bath exceeds 23% by weight, the discontinuous Fe-Al-Zn interfacial alloy layer between the steel and the coating grows excessively due to the increased melting point of the plating bath, resulting in poor interfacial adhesion during processing.

[0030] The shape and proportion of the MgZn2 phase can be precisely controlled by cooling. In a method for manufacturing coated steel with excellent workability and corrosion resistance according to an exemplary embodiment of the present invention, in the step (S20) of forming a hot-dip alloy coating, the first average cooling rate in the cooling process varies according to the difference between a first temperature (i.e., the temperature of the hot-dip alloy bath) and a second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy coating). The first average cooling rate can be the average cooling rate from the time point when the immersed base steel is pulled out of the hot-dip alloy bath to the time point when the MgZn2 phase begins to solidify.

[0031] Specifically, when the difference between the first temperature (i.e., the temperature of the hot-dip alloy plating bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating layer) is less than 50°C, the first average cooling rate can be 10°C / s to 20°C / s. In this case, if the first average cooling rate is less than 10°C / s, the Al phase, Zn phase, and other constitutive phases besides the MgZn2 phase will crystallize coarsely, making it difficult to advantageously control the area fraction of MgZn2, and the liquid plating layer may react with oxygen, becoming a factor that damages the appearance of the plating layer. On the other hand, if the first average cooling rate is greater than 20°C / s, it may be difficult to form rough regions of MgZn2.

[0032] Meanwhile, when the difference between the first temperature (i.e., the temperature of the hot-dip alloy plating bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating layer) is greater than or equal to 50°C and less than 100°C, the first average cooling rate can be from 15°C / s to 35°C / s. In this case, if the first average cooling rate is less than 15°C / s, the Al phase, Zn phase, and other constitutive phases besides the MgZn2 phase will crystallize coarsely, making it difficult to advantageously control the area fraction of MgZn2, and the liquid plating layer may react with oxygen, becoming a factor that damages the appearance of the plating layer. On the other hand, if the first average cooling rate is greater than 35°C / s, it may be difficult to form rough regions of MgZn2.

[0033] Furthermore, when the difference between the first temperature (i.e., the temperature of the hot-dip alloy plating bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating layer) is 100°C or greater, the first average cooling rate can be from 20°C / s to 50°C / s. In this case, if the first average cooling rate is less than 20°C / s, the Al phase, Zn phase, and other constitutive phases besides the MgZn2 phase will crystallize coarsely, making it difficult to advantageously control the area fraction of MgZn2, and the liquid plating layer may react with oxygen, becoming a factor that damages the appearance of the plating layer. On the other hand, if the first average cooling rate is greater than 50°C / s, it may be difficult to form rough regions of MgZn2.

[0034] Furthermore, in the method for manufacturing coated steel with excellent processability and corrosion resistance according to an exemplary embodiment of the present invention, the second average cooling rate from the time point when the MgZn2 phase begins to solidify to the time point when solidification is completed during the cooling process can satisfy the relationship of Equation 1 below.

[0035] <Formula 1>

[0036] 0.0114×T-0.2841≤Second average cooling rate≤0.025×T+10 (T: Solidification start temperature of MgZn2 phase)

[0037] If cooling is performed under conditions that do not satisfy Equation 1, it is difficult to control the growth of rod-shaped MgZn2 precipitates during the cooling process, resulting in poor machinability and reduced productivity due to steel plate vibration.

[0038] In a hot-dip alloy coating achieved by a method for manufacturing coated steel with excellent workability and corrosion resistance, the area fraction of the MgZn2 phase with an average minor axis length (a) to average major axis length (b) ratio of 0.5 or less in the entire MgZn2 phase on the surface of the applied coating can be 70% or less. That is, the area fraction of rod-shaped or needle-shaped MgZn2 phases distributed throughout the MgZn2 phase on the surface of the applied coating can be 70% or less. In this case, the area fraction of polygonal MgZn2 phases distributed throughout the MgZn2 phase on the surface of the applied coating can be 30% or more.

[0039] The zinc alloy coating of the present invention can be composed of a primary Al phase (a single-phase Al structure with Zn solid solution), an Al-Zn eutectic phase, a Zn solid solution phase, and MgZn2 (including Al-containing MgZn2 phase and Mg2Zn). 11 The MgZn2 phase and Al-containing MgZn2 phase on the surface of the Zn-Al-Mg based coating can have polygonal, rod-shaped and needle-shaped shapes in terms of microstructure to improve processability and corrosion resistance.

[0040] The ratio of the average minor axis length (a) to the average major axis length (b) of the rod-shaped and needle-shaped MgZn2 phases is 1:10 ≤ a:b ≤ 1:2. The rod-shaped and needle-shaped MgZn2 phases in the entire MgZn2 phase are distributed on the surface with an area fraction of less than 70%, more preferably less than 50%, and the remaining MgZn2 is distributed in a polygonal shape.

[0041] An exemplary process for forming a hot-dip alloy coating on base steel in this invention is as follows.

[0042] For example, base steel annealed at 680°C to 850°C is immersed in a plating bath at 440°C to 530°C, and one side of the base steel is coated with 30 g / m using an air knife. 2 Up to 300g / m 2 However, by controlling the entry temperature of the base steel after annealing, there is no difference of ±20℃ or more from the plating bath temperature.

[0043] In the coated steel according to an exemplary embodiment of the present invention, the area fraction of the MgZn2 phase in the cross section (e.g., longitudinal section) of the hot-dip alloy coating is 20% to 70%, and the ratio of the area fraction of the Al-containing phase to the area fraction of the MgZn2 phase in the cross section (e.g., longitudinal section) of the hot-dip alloy coating is 1% to 70%. Here, the Al-containing phase may exist outside or within the MgZn2 phase in the cross section of the hot-dip alloy coating. Furthermore, in this exemplary embodiment, the Al-containing phase refers to i) a single Al phase, and ii) a phase containing 20% ​​or more Al, but containing less than 2% unavoidable impurities and the balance Zn.

[0044] The hot-dip alloy coating can contain 20% to 70% MgZn2 phase in its cross-section by area. That is, the cross-sectional area (A2) of the MgZn2 phase is 20% to 70% of the total cross-sectional area (A1) of the hot-dip alloy coating, and the value of (A2 / A1)×100 is in the range of 20 to 70. On the other hand, in the cross-section of the hot-dip alloy coating, the sum of the cross-sectional area (B1) of the Al-containing phase existing outside the MgZn2 phase and the cross-sectional area (B2) of the Al-containing phase existing within the MgZn2 phase, relative to the total cross-sectional area (B3) of the MgZn2 phase, can be 1% to 70%. That is, the value of [(B1+B2) / B3]×100 can be in the range of 1 to 70. Based on this structure, excellent crack resistance is achieved; specifically, in bending evaluation (3T bending evaluation, 1T bending evaluation), the average crack width can be 30 μm or less.

[0045] On the surface of the hot-dip alloy coating of the coated steel of the present invention, the area fraction of the MgZn2 phase can be from 10% to 70%. If the area fraction is less than 10%, it is impossible for it to form, and if the area fraction exceeds 70%, the crack resistance is reduced. Here, the surface of the hot-dip alloy coating can refer to the upper surface in contact with the outside.

[0046] In the coated steel according to an exemplary embodiment of the present invention, on the surface of the hot-dip alloy coating, the area fraction of the MgZn2 phase with an average minor axis length (a) to average major axis length (b) ratio of 0.5 or less in the entire MgZn2 phase can be 70% or less. For example, in the entire MgZn2 phase on the surface of the hot-dip alloy coating, the ratio of the average minor axis length (a) to the average major axis length (b) of 70% or less of the MgZn2 phase can be from 1:2 to 1:10, i.e., a value of 0.5 or less. In this case, the ratio of the average minor axis length (a) to the average major axis length (b) of the MgZn2 phase with an average minor axis length (a) to average major axis length (b) ratio of 0.5 or less can be 1 / 10 or greater, and can be 1 / 2 or less. If the ratio of the average minor axis length (a) to the average major axis length (b) is less than 0.5, the crack resistance decreases. The average minor axis length (a) can range from 1 μm to 20 μm, and the average major axis length (b) can range from 2 μm to 200 μm. It is impossible to form an average minor axis length (a) of less than 1 μm and an average major axis length (b) of less than 2 μm, and crack resistance decreases if the average minor axis length (a) is greater than 20 μm or the average major axis length (b) exceeds 200 μm.

[0047] Meanwhile, on the surface of the hot-dip alloy coating of the coated steel according to another aspect of the present invention, the area fraction of Al-Zn dendrites composed of Al and Zn phases can be 30% or less. Al-Zn dendrites preferably have a low area fraction because they adversely affect chemical conversion treatment performance or resistance to liquid metal embrittlement (LME). Therefore, in the coating according to this exemplary embodiment, the area fraction of Al-Zn dendrites is set to 30% or less.

[0048] As described above, in the coated steel with excellent processability and corrosion resistance according to an exemplary embodiment of the present invention, the MgZn2 phase and the Al-containing MgZn2 phase on the Zn-Al-Mg-based coating surface have polygonal shapes, as well as rod-shaped and needle-shaped shapes, and the ratio of the average minor axis length (a) to the average major axis length (b) of the rod-shaped and needle-shaped shapes is 1:2 ≤ a:b ≤ 1:10. The rod-shaped and needle-shaped MgZn2 phases in the entire MgZn2 are distributed on the surface with an area fraction of 70% or less, more preferably less than 50%, and the remaining MgZn2 is distributed in a polygonal shape.

[0049] On the surface of a hot-dip alloy coating, the area fraction of the MgZn2 phase with an average minor axis length (a) to average major axis length (b) ratio greater than 0.5 is 30% or greater in the entire MgZn2 phase. For example, in the entire MgZn2 phase on the surface of the hot-dip alloy coating, 30% or more of the MgZn2 phase has an average minor axis length (a) to average major axis length (b) ratio greater than 0.5, for example, 1:1.5 or 1:1.2. The diameter (average diameter) of the imaginary circle having the same cross-sectional area as the MgZn2 phase with an average minor axis length (a) to average major axis length (b) ratio greater than 0.5 can be from 1 μm to 50 μm. If the average diameter is less than 1 μm, it is impossible for it to form; if the average diameter is greater than 50 μm, the crack resistance decreases.

[0050] Preferred experimental examples are presented below to better understand the invention. However, these examples are only for illustrative purposes and are not intended to limit the invention.

[0051] Experimental example:

[0052] 1. Sample composition and processing conditions

[0053] A 1.2 mm cold-rolled material was prepared as the base steel sheet, with the following composition: 0.15 wt% carbon (C), 0.01 wt% silicon (Si), 0.6 wt% manganese (Mn), 0.05 wt% phosphorus (P), 0.05 wt% sulfur (S), and the balance iron (Fe). After annealing in a nitrogen-5% to 10% hydrogen atmosphere at a temperature ranging from 680°C to 850°C, specifically at 760°C, the annealed sample was cooled to a temperature not exceeding 20°C from the coating bath temperature and then immersed in the coating bath for 1 to 5 seconds. After immersion in a coating bath at a temperature ranging from 440°C to 530°C (specifically 485°C), the coating thickness was adjusted by nitrogen purging, and the sample was cooled at a first average cooling rate and a second average cooling rate to obtain a Zn-Al-Mg based coated steel sheet.

[0054] 2. Evaluation of coating composition and microstructure

[0055] Table 1 shows the composition (unit: wt%) of the hot-dip alloy coating in the coated steel according to the experimental examples of the present invention, as well as the microstructure and the evaluation results of bending workability based on the eutectic structure.

[0056] [Table 1]

[0057]

[0058]

[0059] In Table 1, "temperature difference" refers to the temperature difference between the first temperature (i.e., the temperature of the hot-dip alloying bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase in the hot-dip alloy coating formed on the base steel immersed in the hot-dip alloying bath). "First average cooling rate" refers to the average cooling rate from the time the immersed base steel is pulled out of the hot-dip alloying bath to the time when the MgZn2 phase begins to solidify. "Second average cooling rate" refers to the average cooling rate from the time when the MgZn2 phase begins to solidify to the time when solidification is complete during the cooling process. In this experimental example, the solidification start temperature of the MgZn2 phase was obtained using the thermodynamic calculation program (FactSage 7.1). The temperature difference between the first temperature (i.e., the temperature of the hot-dip alloying bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase in the hot-dip alloy coating formed on the base steel immersed in the hot-dip alloying bath) varies depending on the coating composition. However, for the same coating composition, the temperature difference is adjusted by adjusting the bath temperature, and the first average cooling rate is controlled by adjusting the height of the air knife. After 500 observations of the surface of each manufactured coated steel sheet using FE-SEM, the area fraction of MgZn2 was measured using an image processing program. The area fraction shown in Table 1 is the proportion (i.e., area ratio) of rod-shaped or needle-shaped MgZn2 phases distributed on the coating surface in the entire MgZn2 phase.

[0060] In the bending workability evaluation, after bending at 1T and 3T, the bent processed parts were observed 200 and 500 times using field emission scanning electron microscopy (FE-SEM), respectively. The width of the bending crack was then measured and averaged for evaluation. "○" indicates that the average crack width in the bending evaluation is greater than 0 and less than or equal to 30 μm, and "X" indicates that the average crack width in the bending evaluation is greater than 30 μm.

[0061] Referring to Table 1, in Examples 1 to 11, i) the composition of the hot-dip alloy plating bath satisfies the following range: 6% to 23% Al, 3% to 7% Mg, and the balance Zn; ii) when the difference between the first temperature (i.e., the temperature of the hot-dip alloy plating bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating layer) is less than 50°C, the first average cooling rate is 10°C / s to 20°C / s (Examples 5, 6, and 7), and when the difference between the first temperature (i.e., the temperature of the hot-dip alloy plating bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating layer) is less than 50°C, the first average cooling rate is 10°C / s to 20°C / s. When the difference between the first temperature (i.e., the temperature of the hot-dip alloy plating bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating) is greater than or equal to 50°C and less than 100°C, the first average cooling rate is 15°C / s to 35°C / s (Examples 1, 3, 4, 9, 10), and when the difference between the first temperature (i.e., the temperature of the hot-dip alloy plating bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating) is 100°C or greater, the first average cooling rate is 20°C / s to 50°C / s (Examples 2, 8 and 11), and the second average cooling rate from the time point when the MgZn2 phase begins to solidify to the time point when solidification is completed in the cooling process satisfies the relationship of Equation 1.

[0062] <Formula 1>

[0063] 0.0114×T-0.2841≤Second average cooling rate≤0.025×T+10 (T: Solidification start temperature of MgZn2 phase)

[0064] In this case, it can be confirmed that in Examples 1 to 11, the area fraction of rod-shaped or needle-shaped MgZn2 phases distributed throughout the MgZn2 phase on the surface of the applied coating is 70% or less, and the average crack width in the bending evaluation is 30 μm or less (refer to...). Figure 1 and Figure 3 Furthermore, it can be confirmed that the growth of the Fe-Al interfacial alloy layer in the cross-section of the coating can be controlled to be less than 10 μm (see [reference]). Figure 5 ).

[0065] In contrast, it can be confirmed that in Comparative Examples 1 and 2, the following conditions are not met: when the difference between the first temperature (i.e., the temperature of the hot-dip alloy plating bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating layer) is greater than or equal to 50°C and less than 100°C, the first average cooling rate is 15°C / s to 35°C / s, thereby the area fraction of rod-shaped or needle-shaped MgZn2 phases in the entire MgZn2 phase distributed on the surface of the applied plating layer is greater than 70%, and the average crack width in the bending evaluation is greater than 30 μm.

[0066] It can be confirmed that in Comparative Example 3, the following conditions are not met: when the difference between the first temperature (i.e., the temperature of the hot-dip alloy plating bath) and the second temperature (i.e., the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating layer) is greater than or equal to 50°C and less than 100°C, the first average cooling rate is 15°C / s to 35°C / s, and the second average cooling rate from the time point when the MgZn2 phase begins to solidify to the time point when solidification is completed in the cooling process does not satisfy the relationship of Equation 1. Accordingly, the area fraction of rod-shaped or needle-shaped MgZn2 phases in the entire MgZn2 phase distributed on the surface of the implemented plating layer is greater than 70%, and the average crack width in the bending evaluation is greater than 30 μm.

[0067] It can be confirmed that in Comparative Examples 4 to 6, the composition of the hot-dip alloy plating bath did not meet the requirement of containing 6% to 23% Al; in Comparative Examples 5 and 6, the composition of the hot-dip alloy plating bath did not meet the requirement of containing 3% to 7% Mg; the area fraction of rod-shaped or needle-shaped MgZn2 phase in the entire MgZn2 phase distributed on the surface of the applied coating was greater than 70%, and the average crack width in the bending evaluation was greater than 30 μm (see...). Figure 2 and Figure 4 ).

[0068] For example, in Examples 1 to 3, the formation of rod-shaped and acicular MgZn2 phases was relatively small, and the measured crack widths were less than 15 μm or 30 μm. On the other hand, in Comparative Examples 1 and 2, the first average cooling rate range of the present invention was not met; when the area fraction of rod-shaped and acicular MgZn2 phases was greater than 70%, cracks propagated along grain boundaries within the MgZn2 phase, which has high hardness, and the average crack width was greater than 30 μm.

[0069] Comparative Example 3 shows that when the range of the first average cooling rate and the second average cooling rate disclosed in the embodiments of the present invention is not met, the bending processability is poor.

[0070] It can be confirmed that in Comparative Example 4, the Al content range of the hot-dip alloy coating of the present invention is not met; in Comparative Examples 5 and 6, the Al and Mg content ranges of the hot-dip alloy coating are not met; the Fe-Al alloy layer is excessively grown, with the area fraction of MgZn2 exceeding 70%, thus deteriorating the bending workability. It can be confirmed that in Comparative Example 4, the Fe-Al interface alloy layer is grown thicker, for example, 10 μm or more (see...). Figure 6 The cracks are non-directional, and the average crack width and area are degraded due to the excessive formation of rod-shaped and acicular MgZn2 phases and the growth of the Fe-Al alloy layer (see [link]). Figure 4 ).

[0071] According to the above-described technical aspects of the present invention, even when a MgZn2 phase with high hardness that is detrimental to processability is formed, a coated steel sheet with excellent processability can be achieved by suppressing the growth of rod-shaped and needle-shaped MgZn2 phases and controlling their area fraction.

[0072] Although exemplary embodiments of the invention have been described, various changes or modifications can be made by those skilled in the art. Such changes and modifications fall within the scope of the invention as long as they do not depart from it. Therefore, the scope of the invention should be determined by the claims.

Claims

1. A method for manufacturing coated steel with excellent processability and corrosion resistance, the method comprising: The base steel is immersed in a hot-dip alloy plating bath; as well as The immersed base steel is pulled out of the hot-dip alloy plating bath and cooled to form a hot-dip alloy coating on the base steel. The first average cooling rate in the cooling process varies according to the difference between a first temperature and a second temperature, wherein the first temperature is the temperature of the hot-dip alloy plating bath and the second temperature is the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating layer. When the difference between the first temperature and the second temperature is less than 50°C, the first average cooling rate is 10°C / s to 20°C / s. When the difference between the first temperature and the second temperature is greater than or equal to 50°C and less than 100°C, the first average cooling rate is 15°C / s to 35°C / s, and When the difference between the first temperature and the second temperature is 100°C or greater, the first average cooling rate is 20°C / s to 50°C / s. The first average cooling rate is the average cooling rate from the time point when the base steel is pulled out of the hot-dip alloy bath to the time point when the MgZn2 phase begins to solidify. as well as In the entire MgZn2 phase on the surface of the hot-dip alloy coating formed on the base steel, the area fraction of the MgZn2 phase with an average minor axis length (a) to an average major axis length (b) ratio of 0.5 or less and excluding 0 is 70%.

2. The manufacturing method according to claim 1, wherein, The second average cooling rate during the cooling process, from the start of solidification of the MgZn2 phase to the completion of solidification, satisfies the following relationship: Equation 1: <Formula 1> 0.0114×T-0.2841 ≤ Second average cooling rate ≤ 0.025×T+10, where T is the solidification start temperature of the MgZn2 phase.

3. The manufacturing method according to claim 1, wherein, The hot-dip alloy plating bath is a Zn plating bath containing 6% to 23% Al, 3% to 7% Mg, and unavoidable impurities.

4. A coated steel with excellent processability and corrosion resistance, obtained by the manufacturing method according to any one of claims 1 to 3, comprising a base steel and a hot-dip alloy coating formed on said base steel. in, The hot-dip alloy coating contains 6% to 23% Al, 3% to 7% Mg, and unavoidable impurities. as well as In the entire MgZn2 phase on the surface of the hot-dip alloy coating, the area fraction of the MgZn2 phase with an average minor axis length (a) to average major axis length (b) ratio of 0.5 or less and excluding 0 is 40% or more and 70% or less.

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