Manufacturing method for plated steel with excellent workability and corrosion resistance

Abstract

A method for producing a plated steel material with excellent workability and corrosion resistance according to one embodiment of the present invention includes the steps of: immersing base iron in a molten alloy plating bath; and withdrawing the immersed base iron from the molten alloy plating bath and performing a cooling step, thereby forming a molten alloy plating layer on the base iron, wherein a first average cooling rate in the cooling step varies depending on the difference between a first temperature, which is the temperature of the molten alloy plating bath, and a second temperature, which is the solidification start temperature of the MgZn2 phase that constitutes the molten alloy plating layer.

Description

[Technical Field]

[0001] The present invention relates to a steel material, and more particularly to a method for producing a plated steel material having excellent workability and corrosion resistance. [Background technology]

[0002] Hot-dip galvanized steel sheets have excellent sacrificial corrosion protection properties. When exposed to a corrosive environment, zinc, which has a lower potential, dissolves first, preventing corrosion of the steel. Due to these excellent corrosion properties, hot-dip galvanized steel sheets are used in home appliances, building materials, and automobiles. However, as technological advances and quality standards have led to higher expectations for corrosion resistance, there has been a growing need to develop products with better corrosion resistance than conventional hot-dip galvanized steel sheets. To address this issue, since the early 2000s, Europe and Japan have been producing highly corrosion-resistant coated steel sheets by adding aluminum (Al) and magnesium (Mg) to the zinc (Zn) plating bath. In addition to the sacrificial corrosion protection properties of Zn, the addition of Mg and Al forms dense corrosion products in corrosive environments, insulating the steel from oxidizing atmospheres and improving corrosion resistance. However, while Zn-Al-Mg coated steel sheets have superior corrosion resistance compared to galvanized steel sheets, they suffer from poor formability. Zn-Al-Mg intermetallic compounds have low crack resistance due to their high hardness, and such cracks can damage the appearance during processing or expose the base steel material, resulting in reduced corrosion resistance.

[0003] Related prior art includes Japanese Patent Publication No. 2005-105367. Summary of the Invention [Problem to be solved by the invention]

[0004] The technical problem to be achieved by the present invention is to provide a method for producing a plated steel product that is excellent in workability and corrosion resistance. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, one aspect of the present invention provides a method for manufacturing plated steel material with excellent workability and corrosion resistance, comprising the steps of: immersing base iron in a molten alloy plating bath; and withdrawing the immersed base iron from the molten alloy plating bath and performing a cooling step to form a molten alloy plating layer on the base iron, wherein a first average cooling rate in the cooling step varies depending on the difference between a first temperature, which is the temperature of the molten alloy plating bath, and a second temperature, which is the solidification start temperature of the MgZn2 phase that constitutes the molten alloy plating layer.

[0006] In the method for producing a plated steel material with excellent workability and corrosion resistance, when the difference between the first temperature and the second temperature is less than 50°C, the first average cooling rate may be 10 to 20°C / s, when the difference between the first temperature and the second temperature is 50°C or more and less than 100°C, the first average cooling rate may be 15 to 35°C / s, and when the difference between the first temperature and the second temperature is 100°C or more, the first average cooling rate may be 20 to 50°C / s.

[0007] In the method for producing a plated steel material having excellent workability and corrosion resistance, the first average cooling rate may be an average cooling rate from a time when the immersed base steel is withdrawn from the hot-dip alloy plating bath to a time when the MgZn2 phase starts to solidify.

[0008] In the method for producing a plated steel material having excellent workability and corrosion resistance, the second average cooling rate in the cooling step from the time when the MgZn2 phase starts to solidify to the time when solidification is completed can satisfy the relationship of the following mathematical formula 1: [Number 1] 0.0114×T-0.2841≦2nd average cooling rate≦0.025×T+10… <Mathematical formula 1> (where T is the solidification start temperature of the MgZn2 phase)

[0009] In the method for producing a plated steel material having excellent workability and corrosion resistance, the hot-dip alloy plating bath may be a Zn plating bath containing, by weight, 6 to 23% Al, 3 to 7% Mg, and other inevitable impurities.

[0010] In the method for producing a plated steel material having excellent workability and corrosion resistance, the hot-dip alloy plating layer formed on the base steel may have an area fraction of 70% or less of the MgZn2 phase at the surface, the area fraction having a ratio of an average minor axis length (a) to an average major axis length (b) of 0.5 or less among all the MgZn2 phases. [Effects of the Invention]

[0011] According to an embodiment of the present invention, a method for manufacturing a plated steel product having excellent workability and corrosion resistance can be realized.

[0012] It goes without saying that the scope of the present invention is not limited by such effects. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a photograph of the surface of a hot-dip alloy plating layer according to Example 6 in an experimental example. [Figure 2] 1 is a photograph of the surface of a hot-dip alloy plating layer according to Comparative Example 1 in an experimental example. [Figure 3] 10 is a photograph of a processed portion of a hot-dip alloy plating layer according to Example 6 in an experimental example, taken with an FE-SEM at a magnification of 200 times after evaluation of 3T bending workability. [Figure 4] 1 is a photograph of a processed portion of a hot-dip alloy plating layer according to Comparative Example 4 in an experimental example, taken with an FE-SEM at a magnification of 200 times after evaluation of 3T bending workability. [Figure 5] 1 is a photograph of a cross section of a hot-dip alloy plating layer according to Example 3 in an experimental example, taken with an FE-SEM at a magnification of 1000 times. [Figure 6] 1 is a photograph of a cross section of a hot-dip alloy plating layer according to Comparative Example 4 in an experimental example, taken with an FE-SEM at a magnification of 1000 times. DETAILED DESCRIPTION OF THE INVENTION

[0014] A method for manufacturing a plated steel sheet having excellent workability and corrosion resistance according to one 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 their definitions should be based on the overall content of this specification. Below, we will provide specific details of an ultra-high strength, highly corrosion-resistant plated steel sheet having excellent elongation and a method for manufacturing the same.

[0015] Zn-Al-Mg coated steel sheets have superior corrosion resistance compared to galvanized steel sheets, but suffer from poor workability. Zn-Al-Mg intermetallic compounds have low crack resistance due to their high hardness, and these cracks can damage the appearance during processing or expose the base steel, reducing corrosion resistance during processing. Because MgZn2 has the highest hardness among intermetallic compounds, technology to control the shape, distribution, and size of the MgZn2 phase is important.

[0016] The present invention relates to a method for producing a highly corrosion-resistant Zn-Al-Mg-based plated steel material containing, by weight, 6-23% Al, 3-7% Mg, the balance Zn, and other unavoidable impurities, and aims to control the microstructure of the high-hardness MgZn2 phase in order to improve workability and working corrosion resistance.

[0017] A method for producing a plated steel material with excellent workability and corrosion resistance according to one embodiment of the present invention includes the steps of immersing a base steel material in a molten alloy coating bath (S10); and withdrawing the immersed base steel material from the molten alloy coating bath and performing a cooling process to form a molten alloy coating layer on the base steel material (S20).

[0018] In the step (S10) of immersing the base steel, the hot-dip alloy plating bath may be a Zn plating bath containing, for example, by weight 6 to 23% Al, 3 to 7% Mg, and other inevitable impurities. Furthermore, the hot-dip alloy plating bath may further contain, by weight %, 0.05 to 10% Fe and more than 0 and less than 1% Si.

[0019] Mg and Al in the hot-dip alloy coating bath are elements that improve the corrosion resistance of the coating layer and further enhance corrosion resistance by forming denser corrosion products. When Mg is present in the coating bath at less than 1.0 wt%, its contribution to corrosion resistance is minimal. Conventionally, Mg was used at less than 2.0 wt% because production difficulties due to Mg oxide dross occurred when Mg was present at more than 2.0 wt%. However, in the present invention, Mg is added at 3.0 wt% or more to achieve better corrosion resistance. As mentioned above, production difficulties due to Mg oxide dross occur when Mg is added at more than 3.0 wt%, but the addition of Al at 6.0 wt% or more can reduce dross due to Mg oxidation in the molten metal. Furthermore, the addition of Al can improve corrosion resistance by forming primary Al crystals and a Zn-Al-Mg ternary eutectic phase. On the other hand, when the coating bath contains more than 7.0 wt.% Mg, rod- and needle-shaped MgZn2 or Al-containing MgZn2 phases grow in the coating layer, accounting for more than 70% of the total MgZn2 area, resulting in poor workability and reduced corrosion resistance due to cracks in the coating layer during processing, exposing the steel or the Fe-Al-Zn interfacial alloyed layer.On the other hand, when the coating bath contains more than 23 wt.% Al, the melting point of the coating bath increases, causing excessive growth of a discontinuous Fe-Al-Zn interfacial alloyed layer between the steel and the coating layer, potentially weakening interfacial adhesion during processing.

[0020] The shape and fraction of the MgZn2 phase can be precisely controlled by cooling. In a method for producing a plated steel material with excellent workability and corrosion resistance according to one embodiment of the present invention, in the step (S20) of forming a hot-dip alloy coating layer, a first average cooling rate in the cooling step varies depending on the difference between a first temperature, which is the temperature of the hot-dip alloy coating bath, and a second temperature, which is the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy coating layer. The first average cooling rate may be the average cooling rate from the time the immersed base steel is withdrawn from the hot-dip alloy coating bath to the time the MgZn2 phase starts to solidify.

[0021] Specifically, when the difference between the first temperature, which is the temperature of the hot-dip alloy coating bath, and the second temperature, which is the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy coating layer, is less than 50°C, the first average cooling rate may be 10 to 20°C / s. In this case, when the first average cooling rate is less than 10°C / s, constituent phases other than the MgZn2 phase, such as Al phase and Zn phase, are coarsely crystallized, making it difficult to favorably control the fraction of the MgZn2 domains, and the coating layer in a liquid phase may react with oxygen, which may act as a factor that impairs the appearance of the coating surface. On the other hand, when the first average cooling rate exceeds 20°C, it may be difficult to form coarse MgZn2 domains.

[0022] On the other hand, when the difference between the first temperature, which is the temperature of the hot-dip alloy coating bath, and the second temperature, which is the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy coating layer, is 50°C or more and less than 100°C, the first average cooling rate may be 15 to 35°C / s. In this case, when the first average cooling rate is less than 15°C / s, constituent phases other than the MgZn2 phase, such as Al phase and Zn phase, are coarsely crystallized, making it difficult to favorably control the fraction of the MgZn2 domains, and the coating layer in a liquid phase may react with oxygen, which may act as a factor that impairs the appearance of the coating surface. On the other hand, when the first average cooling rate exceeds 35°C, it may be difficult to form coarse MgZn2 domains.

[0023] Furthermore, when the difference between the first temperature, which is the temperature of the hot-dip alloy coating bath, and the second temperature, which is the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy coating layer, is 100°C or more, the first average cooling rate may be 20 to 50°C / s. In this case, when the first average cooling rate is less than 20°C / s, constituent phases other than the MgZn2 phase, such as Al phase and Zn phase, are coarsely crystallized, making it difficult to favorably control the fraction of the MgZn2 domains, and the coating layer in a liquid phase may react with oxygen, which may act as a factor that impairs the appearance of the coating surface. On the other hand, when the first average cooling rate exceeds 50°C, it may be difficult to form coarse MgZn2 domains.

[0024] Furthermore, in the method for producing a plated steel material with excellent workability and corrosion resistance according to an embodiment of the present invention, a second average cooling rate in the cooling step from when the MgZn2 phase starts to solidify to when the solidification is completed can satisfy the relationship of the following mathematical formula 1: [Number 2] 0.0114×T-0.2841≦2nd average cooling rate≦0.025×T+10… <Mathematical formula 1> (where T is the solidification start temperature of the MgZn2 phase)

[0025] When cooling is performed under conditions that do not satisfy the above mathematical formula 1, it is difficult to control the growth of the rod-type MgZn2 precipitate phase, resulting in poor workability and reduced productivity due to plate vibration.

[0026] The hot-dip alloy coating layer realized by the above-described method for producing a coated steel material having excellent workability and corrosion resistance may have an area fraction of 70% or less of the entire MgZn2 phase at the surface of the coating layer, the area fraction of which has an average minor axis length (a) to average major axis length (b) ratio of 0.5 or less. That is, the area fraction of rod- or needle-shaped MgZn2 phases may be 70% or less of the entire MgZn2 phase distributed at the surface of the realized coating layer. In this case, the area fraction of polygonal-shaped MgZn2 phases may be 30% or more of the entire MgZn2 phase distributed at the surface of the realized coating layer.

[0027] The zinc alloy plating layer of the present invention is composed of a primary Al phase (a single-phase structure of Al with Zn solid solution), an Al / Zn eutectoid phase, a Zn solid solution phase, MgZn2 (MgZn2 phase containing Al, Mg2Zn 11 In terms of the microstructure for improving workability and corrosion resistance during processing, the MgZn2 phase and the MgZn2 phase containing Al on the surface of the Zn-Al-Mg based coating layer may have polygonal, rod-like, and needle-like shapes.

[0028] The ratio of the average minor axis length (a) to the average major axis length (b) of the rod and needle-like shapes is 1:10≦a:b≦1:2. The rod and needle-shaped MgZn2 phase is distributed on the surface at an area fraction of less than 70%, more preferably less than 50%, of the total MgZn2, and the remaining MgZn2 is distributed in polygonal shapes.

[0029] In the present invention, an exemplary process for forming a hot-dip alloy plating layer on a base steel is as follows.

[0030] For example, the base steel annealed at 680 to 850°C is immersed in a plating bath at 440 to 530°C, and then passes through an air knife to coat the surface with 30 to 300 g / m² on one side. 2 However, the entry temperature of the annealed base steel must be adjusted so that it does not differ by more than ±20°C from the temperature of the plating bath.

[0031] A plated steel material according to one embodiment of the present invention is characterized in that the area fraction of the MgZn2 phase in a cross section (e.g., a vertical section) in the hot-dip alloy coating layer 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 a cross section (e.g., a vertical section) in the hot-dip alloy coating layer is 1 to 70%. Here, the Al-containing phase may be present separated from the MgZn2 phase or present within the MgZn2 phase in a cross section in the hot-dip alloy coating layer. In this embodiment, the Al-containing phase refers to i) a single Al phase, or ii) a phase containing 20% ​​or more Al, unavoidable impurities of 2% or less, and the remainder being Zn.

[0032] The hot-dip alloy coating layer may contain an MgZn2 phase at an area fraction of 20 to 70% in a cross section. That is, the proportion of the cross-sectional area (A2) occupied by the MgZn2 phase in the total cross-sectional area (A1) of the hot-dip alloy coating layer may be 20 to 70%, and the value of (A2 / A1) × 100 may be in the range of 20 to 70. Meanwhile, in the cross section of the hot-dip alloy coating layer, the sum of the cross-sectional area (B1) of the Al-containing phase present apart from the MgZn2 phase and the cross-sectional area (B2) of the Al-containing phase present inside the MgZn2 phase may be 1 to 70% of the cross-sectional area (B3) of the total MgZn2 phase. That is, the value of [(B1 + B2) / B3] × 100 may be in the range of 1 to 70. Such a structure may exhibit excellent crack resistance; specifically, the average crack width in bending tests (3T bending test, 1T bending test) may be 30 μm or less.

[0033] The hot-dip alloy plating layer of the plated steel material of the present invention may have an area fraction of the MgZn2 phase at the surface of 10 to 70%, and an area fraction of less than 10% is impossible to form, while an area fraction exceeding 70% reduces crack resistance. Here, the surface of the hot-dip alloy plating layer may refer to the upper surface that comes into contact with the outside.

[0034] In one embodiment of the plated steel material, the hot-dip alloy coating layer may have an area fraction of 70% or less of MgZn2 phases at the surface, the area fraction of which has a ratio of average minor axis length (a) to average major axis length (b) of 0.5 or less. For example, 70% or less of the MgZn2 phases at the surface of the hot-dip alloy coating layer may have a ratio of average minor axis length (a) to average major axis length (b) of 1:2 to 1:10, and 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 phases having a ratio of 0.5 or less may be 1 / 10 or more and 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, crack resistance is reduced. The average minor axis length (a) may be 1 to 20 μm, and the average major axis length (b) may be 2 to 200 μm. An average minor axis length (a) of less than 1 μm and an average major axis length (b) of less than 2 μm are impossible to form, and if the average minor axis length (a) exceeds 20 μm or the average major axis length (b) exceeds 200 μm, crack resistance decreases.

[0035] On the other hand, in the hot-dip alloy coating layer of the coated steel material according to another aspect of the present invention, the area fraction of Al-Zn dendrites composed of Al and Zn phases on the surface may be 30% or less. Since Al-Zn dendrites have an unfavorable effect on chemical conversion treatability and LME (liquid metal embrittlement) resistance, it is preferable that their area fraction is low. Therefore, in the coating layer according to this embodiment, the area fraction of Al-Zn dendrites is set to 30% or less.

[0036] As described above, in a plated steel material with excellent workability and corrosion resistance according to one embodiment of the present invention, the MgZn2 phase and the Al-containing MgZn2 phase on the surface of the Zn-Al-Mg-based coating layer are characterized in that they are composed of polygonal, rod-like, and needle-like shapes, and the ratio of the average minor axis length (a) to the average major axis length (b) of the rod-like and needle-like shapes is 1:2≦a:b≦1:10. Of the total MgZn2, the rod- and needle-like MgZn2 phase is distributed on the surface at an area fraction of 70% or less, more preferably at an area fraction of less than 50%, and the remainder MgZn2 is distributed in polygonal shapes.

[0037] The hot-dip alloy plating layer is characterized in that the area fraction of MgZn2 phases at the surface, in which the ratio of the average minor axis length (a) to the average major axis length (b) exceeds 0.5, is 30% or more. For example, the hot-dip alloy plating layer is characterized in that 30% or more of the MgZn2 phases at the surface, in which the ratio of the average minor axis length (a) to the average major axis length (b) exceeds 0.5, is 1:1.5, 1:1.2, or the like. The diameter (average diameter) of a virtual circle having the same area as the cross-sectional area of ​​the MgZn2 phases having the ratio of the average minor axis length (a) to the average major axis length (b) exceeding 0.5, may be 1 to 50 μm. An average diameter of less than 1 μm is impossible to form, and an average diameter exceeding 50 μm results in reduced crack resistance.

[0038] Preferred experimental examples are presented below to aid in understanding the present invention, but the following experimental examples are merely provided to aid in understanding the present invention and are not intended to limit the present invention.

[0039] Experimental example 1. Test specimen composition and process conditions A 1.2 mm cold-rolled steel sheet was prepared as the base steel sheet. Its composition was 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 remainder iron (Fe). After annealing at a temperature of 760°C in a nitrogen-5-10% hydrogen atmosphere within the range of 680-850°C, the annealed specimen was cooled to a temperature no more than 20°C different from the coating bath temperature and then immersed in the coating bath for 1-5 seconds. After immersion in a coating bath within the range of 440-530°C, specifically at 485°C, the coating thickness was adjusted by nitrogen wiping, and the specimen was cooled at a first average cooling rate and a second average cooling rate to obtain a Zn-Al-Mg-coated steel sheet.

[0040] 2. Evaluation of the composition and microstructure of the plating layer Table 1 shows the results of evaluating the composition (unit: weight %) of the hot-dip alloy coating layer, the microstructure due to the eutectic structure, and the bending workability of plated steel materials according to experimental examples of the present invention.

[0041] [Table 1]

[0042] In Table 1, the temperature difference column refers to the temperature difference between the first temperature, which is the temperature of the hot-dip alloy coating bath, and the second temperature, which is the solidification start temperature of the MgZn2 phase that constitutes the hot-dip alloy coating layer formed on the base steel immersed in the hot-dip alloy coating bath. The first average cooling rate refers to the average cooling rate in the cooling process from the time the immersed base steel is withdrawn from the hot-dip alloy coating bath to the time the MgZn2 phase starts to solidify. The second average cooling rate refers to the average cooling rate in the cooling process from the time the MgZn2 phase starts to solidify to the time it is completely solidified. In this experimental example, the solidification start temperature of the MgZn2 phase was calculated using a thermodynamic calculation program (FactSage 7.1). The difference between the first temperature, which is the temperature of the hot-dip alloy coating bath, and the second temperature, which is the solidification start temperature of the MgZn2 phase that constitutes the hot-dip alloy coating layer formed on the base steel immersed in the hot-dip alloy coating bath, varies depending on the coating composition. For the same coating composition, the difference was adjusted by adjusting the temperature of the coating bath, and the first average cooling rate was controlled by adjusting the height of the air knife. The MgZn2 area fraction was evaluated by observing the surface of each coated steel sheet with an FE-SEM at 500x magnification and measuring it using an image program. The area fractions disclosed in Table 1 represent the area ratio of rod- or needle-shaped MgZn2 phases among the total MgZn2 phase distributed on the surface of the coating layer.

[0043] The bending workability was evaluated by observing the bent area after 1T and 3T bending at 200x and 500x magnifications using a field emission scanning electron microscope (FE-SEM), measuring the width of bending cracks, and averaging the results. An "O" indicates that the average crack width in the bending evaluation was greater than 0 and less than 30 μm, and an "X" indicates that the average crack width in the bending evaluation exceeded 30 μm.

[0044] Referring to Table 1, in Examples 1 to 11, i) the composition of the hot-dip alloy plating bath satisfies the range of, by weight %, Al: 6 to 23%, Mg: 3 to 7%, and the balance being Zn, and ii) when the difference between the first temperature which is the temperature of the hot-dip alloy plating bath and the second temperature which is the solidification start temperature of the MgZn2 phase which constitutes the hot-dip alloy plating layer is less than 50°C, the first average cooling rate satisfies the range of 10 to 20°C / s (Examples 5, 6, and 7), and the difference between the first temperature which is the temperature of the hot-dip alloy plating bath and the second temperature which is the solidification start temperature of the MgZn2 phase which constitutes the hot-dip alloy plating layer is less than 50°C. When the difference between the first temperature, which is the temperature of the hot-dip alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn2 phase that constitutes the hot-dip alloy plating layer, is 50°C or more and less than 100°C, the first average cooling rate satisfies the range of 15 to 35°C / s (Examples 1, 3, 4, 9, and 10); when the difference between the first temperature, which is the temperature of the hot-dip alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn2 phase that constitutes the hot-dip alloy plating layer, is 100°C or more, the first average cooling rate satisfies the range of 20 to 50°C / s (Examples 2, 8, and 11); and iii) the second average cooling rate in the cooling step from the time when the MgZn2 phase starts to solidify to the time when solidification is completed satisfies the relationship of the following mathematical formula 1. [Number 3] 0.0114×T-0.2841≦2nd average cooling rate≦0.025×T+10… <Mathematical formula 1> (where T is the solidification start temperature of the MgZn2 phase)

[0045] In this case, it was confirmed that in Examples 1 to 11, the area fraction of rod- or needle-shaped MgZn2 phases among the total MgZn2 phases distributed on the surface of the realized coating layer was 70% or less, and the average crack width in bending evaluation was 30 μm or less (see FIGS. 1 and 3). Furthermore, it was confirmed that the growth of the Fe-Al interfacial alloy layer in the cross section of the coating layer could be controlled to less than 10 μm (see FIG. 5).

[0046] In contrast, in Comparative Examples 1 and 2, when the difference between the first temperature, which is the temperature of the hot-dip alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy plating layer, is 50°C or more and less than 100°C, the first average cooling rate does not satisfy the range of 15 to 35°C / s, and as a result, it can be confirmed that the area fraction of rod- or needle-shaped MgZn2 phase in the entire MgZn2 phase distributed on the surface of the realized plating layer exceeds 70%, and the average crack width in bending evaluation exceeds 30 μm.

[0047] In Comparative Example 3, when the difference between the first temperature, which is the temperature of the hot-dip alloy coating bath, and the second temperature, which is the solidification start temperature of the MgZn2 phase constituting the hot-dip alloy coating layer, is 50°C or more and less than 100°C, the first average cooling rate does not satisfy the range of 15 to 35°C / s, and the second average cooling rate in the cooling process from the time when the MgZn2 phase starts to solidify to the time when it is completed does not satisfy the relationship in Equation 1. As a result, it can be confirmed that the area fraction of rod- or needle-shaped MgZn2 phase exceeds 70% among all the MgZn2 phases distributed on the surface of the realized coating layer, and the average crack width in bending evaluation exceeds 30 μm.

[0048] In Comparative Examples 4 to 6, the composition of the hot-dip alloy plating bath did not satisfy the range of Al: 6 to 23% by weight, and in Comparative Examples 5 to 6, the composition of the hot-dip alloy plating bath did not satisfy the range of Mg: 3 to 7% by weight. However, it can be confirmed that the area fraction of rod- or needle-shaped MgZn2 phases in the total MgZn2 phase distributed on the surface of the realized plating layer exceeded 70%, and the average crack width in bending evaluation exceeded 30 μm (see FIGS. 2 and 4).

[0049] For example, in Examples 1 to 3, the formation and development of rod- and needle-shaped MgZn2 phases was relatively small, and the crack width was measured to be within 15 μm or 30 μm. In contrast, in Comparative Examples 1 and 2, which do not satisfy the first average cooling rate range of the present invention, when the area fraction of the rod- and needle-shaped MgZn2 phases exceeds 70%, cracks not only occur in the hard MgZn2 phase but also progress along the grain boundaries, resulting in an average crack width exceeding 30 μm.

[0050] Comparative Example 3 shows that when the ranges of the first average cooling rate and the second average cooling rate disclosed in the embodiment of the present invention are not satisfied, the bending workability is not good.

[0051] Comparative Example 4 did not satisfy the Al content range of the hot-dip alloy coating layer of the present invention, and Comparative Examples 5 and 6 did not satisfy the Al and Mg contents of the hot-dip alloy coating layer, and it was confirmed that the Fe-Al alloy layer was excessively formed and the area fraction of MgZn2 exceeded 70%, resulting in poor bending workability. In Comparative Example 4, it was confirmed that the growth of the Fe-Al interfacial alloy layer was thicker than 10 μm (see FIG. 6), and it was confirmed that the excessive formation of rod- and needle-shaped MgZn2 phases and the growth of the Fe-Al alloy layer resulted in non-directional cracks, resulting in poor average crack width and area (see FIG. 4).

[0052] According to the technical concept of the present invention described above, even if the MgZn2 phase, which has high hardness and is disadvantageous to formability, is formed, it is possible to suppress the growth of rod- and needle-shaped MgZn2 phases and adjust their area fractions to realize a plated steel sheet with excellent formability.

[0053] 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 following claims.

Claims

1. Immersing the base steel in a hot-dip alloy coating bath; and the immersed base steel is withdrawn from the hot-dip alloy coating bath and cooled to form a hot-dip alloy coating layer on the base steel; The first average cooling rate in the cooling step is a ratio of a first temperature, which is the temperature of the hot-dip alloy plating bath, to a MgZn alloy layer. ２ The temperature varies depending on the difference between the first and second temperatures, which are the solidification start temperatures of the phases, When the difference between the first temperature and the second temperature is less than 50°C, the first average cooling rate is 10 to 20°C / s; when the difference between the first temperature and the second temperature is 50°C or more and less than 100°C, the first average cooling rate is 15 to 35°C / s; When the difference between the first temperature and the second temperature is 100°C or more, the first average cooling rate is 20 to 50°C / s; the first average cooling rate is an average cooling rate from the time when the immersed base steel is withdrawn from the hot-dip alloy coating bath to the time when the MgZn 2 phase begins to solidify; The method for producing a plated steel product having excellent workability and corrosion resistance is characterized in that the hot-dip alloy plating layer formed on the base steel has an area fraction of MgZn 2 phases on the surface, the area fraction of which has a ratio of an average minor axis length (a) to an average major axis length (b) of 0.5 or less among all MgZn 2 phases.

2. The MgZn ２ 2. The method for producing a plated steel material having excellent workability and corrosion resistance according to claim 1, wherein the second average cooling rate in the cooling process from when the phase begins to solidify to when the phase completes solidification satisfies the relationship in the following mathematical formula 1: [Equation 1] (where T is MgZn ２ phase solidification start temperature)

3. 2. The method for producing a plated steel material excellent in workability and corrosion resistance according to claim 1, wherein the hot-dip alloy plating bath is a Zn plating bath containing, by weight %, 6 to 23% Al, 3 to 7% Mg, and other inevitable impurities.

4. A steel sheet comprising a base steel and a hot-dip alloy plating layer formed on the base steel, The hot-dip alloy plating layer contains, by weight %, Al: 6 to 23%, Mg: 3 to 7%, and the balance being Zn and other inevitable impurities, The hot-dip alloy plating layer is characterized in that the area fraction of MgZn 2 phases on the surface, in which the ratio of the average minor axis length (a) to the average major axis length (b) is 0.5 or less (excluding 0), in the entire MgZn 2 phase, is 40% or more and 70% or less. Plated steel with excellent workability and corrosion resistance.

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