KR20230153716A - Method of fabricating a galvanizing steel having excellent bendability and corrosion resistance - Google Patents

Abstract

A method of manufacturing plated steel with excellent processability and corrosion resistance according to an 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 process, thereby forming a molten alloy plating layer on the base iron; It includes, and the first average cooling rate in the cooling process is different depending on the difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer. .

Description

Method for manufacturing plated steel with excellent processability and corrosion resistance {METHOD OF FABRICATING A GALVANIZING STEEL HAVING EXCELLENT BENDABILITY AND CORROSION RESISTANCE}

The present invention relates to steel materials, and more specifically, to a method of manufacturing plated steel materials with excellent processability and corrosion resistance.

Hot-dip galvanized steel sheets have excellent sacrificial corrosion resistance, so when exposed to a corrosive environment, zinc with a low potential is preemptively eluted to prevent corrosion of the steel material. Thanks to these excellent corrosion properties, hot-dip galvanized steel sheets are used as steel sheets for home appliances, construction materials, and automobiles. However, as expectations for corrosion resistance are increasing due to technological advancements and improved quality standards, the need for the development of products with better corrosion resistance than conventional hot-dip galvanized steel sheets is increasing. To solve this problem, since the early 2000s, highly corrosion-resistant plated steel sheets have been produced in Europe and Japan to improve corrosion resistance by adding aluminum (Al) and magnesium (Mg) to zinc (Zn) plating baths. In addition to the sacrificial corrosion resistance of Zn, high-corrosion-resistant plated steel sheets form dense corrosion products in a corrosive environment due to the addition of Mg and Al, thereby blocking the steel from the oxidizing atmosphere and improving corrosion resistance. However, Zn-Al-Mg plated steel sheets have superior corrosion resistance compared to galvanized steel sheets, but have the disadvantage of inferior processability. The intermetallic compound of Zn-Al-Mg has high hardness and low crack resistance, and these cracks have the problem of damaging the appearance during the processing process or exposing the base steel material, thereby reducing corrosion resistance.

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

The technical problem to be achieved by the present invention is to provide a method for manufacturing plated steel materials with excellent processability and corrosion resistance.

In order to solve the above problems, a method of manufacturing a plated steel material with excellent processability and corrosion resistance according to an aspect 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 process, thereby forming a molten alloy plating layer on the base iron; It includes, and the first average cooling rate in the cooling process is different depending on the difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer. .

In the method of manufacturing 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 is 10 to 20°C/s, and the first temperature and the If the difference between the second temperatures is 50°C or more and less than 100°C, the first average cooling rate is 15 to 35°C/s, and if the difference between the first temperature and the second temperature is 100°C or more, the first average cooling rate is 100°C or more. The speed may be 20 to 50°C/s.

In the method of manufacturing a plated steel material with excellent workability and corrosion resistance, the first average cooling rate may be the average cooling rate from the time the immersed base iron is withdrawn from the molten alloy plating bath to the time the MgZn ₂ phase begins to solidify. there is.

In the method of manufacturing a plated steel material with excellent workability and corrosion resistance, the second average cooling rate in the cooling process from the time when the MgZn ₂ phase begins to solidify to the time when solidification is completed may satisfy the relationship of Equation 1 below.

<Equation 1>

0.0114 x T - 0.2841 ≤ _2nd average cooling rate ≤ 0.025

In the method of manufacturing a plated steel material with excellent processability and corrosion resistance, the molten alloy plating bath may be a Zn plating bath containing Al: 6 to 23%, Mg: 3 to 7% and other inevitable impurities in weight percent.

In the method of manufacturing a plated steel material with excellent workability and corrosion resistance, the molten alloy plating layer _formed on the base iron is MgZn 2 having a ratio of the average minor axis length (a) and the average major axis length (b) of 0.5 or less among the entire MgZn ₂ phase on the surface. The area fraction of the phase may be 70% or less.

According to an embodiment of the present invention, a method for manufacturing plated steel with excellent processability and corrosion resistance can be implemented.

Of course, the scope of the present invention is not limited by this effect.

Figure 1 is a photograph of the surface of a molten alloy plating layer according to Example 6 of the experimental examples.
Figure 2 is a photograph taken of the surface of the molten alloy plating layer according to Comparative Example 1 among the experimental examples.
Figure 3 is a photograph taken at 200x FE-SEM of the processed part after evaluating the 3T bending processability of the molten alloy plating layer according to Example 6 among the experimental examples.
Figure 4 is a photograph taken at 200x FE-SEM of the processed part after evaluating the 3T bending processability of the molten alloy plating layer according to Comparative Example 4 among the experimental examples.
Figure 5 is a photograph taken at 1000x FE-SEM of the cross section of the molten alloy plating layer according to Example 3 among the experimental examples.
Figure 6 is a photograph of the cross section of the molten alloy plating layer according to Comparative Example 4 among the experimental examples taken at 1000 times FE-SEM.

A method for manufacturing a plated steel material with excellent processability and corrosion resistance according to an embodiment of the present invention will be described in detail. The terms described below are terms appropriately selected in consideration of their functions in the present invention, and definitions of these terms should be made based on the content throughout the present specification. Below, we will provide specific details on ultra-high strength, high corrosion resistance plated steel sheets with excellent elongation and their manufacturing methods.

Zn-Al-Mg plated steel sheets have superior corrosion resistance compared to galvanized steel sheets, but have the disadvantage of inferior processability. The intermetallic compound of Zn-Al-Mg has high hardness and low crack resistance, and these cracks damage the appearance during processing or expose the base steel, reducing corrosion resistance during processing. Among intermetallic compounds, MgZn ₂ has the highest hardness, so technology to control the shape, distribution and size of the MgZn ₂ phase is important.

The present invention relates to a method for manufacturing a Zn-Al-Mg-based highly corrosion-resistant plated steel containing, in weight percent, Al: 6 to 23%, Mg: 3 to 7%, the balance Zn and other unavoidable impurities, and its processability and processing corrosion resistance. The purpose is to control the microstructure of the MgZn ₂ phase with high hardness in order to improve.

A method for manufacturing plated steel with excellent processability and corrosion resistance according to an embodiment of the present invention includes the step of immersing base iron in a molten alloy plating bath (S10); And forming a molten alloy plating layer on the base iron by withdrawing the immersed base iron from the molten alloy plating bath and performing a cooling process (S20); Includes.

In the step of immersing the base iron (S10), the molten alloy plating bath may be, for example, a Zn plating bath containing, in weight percent, Al: 6 to 23%, Mg: 3 to 7% and other unavoidable impurities. there is. Furthermore, the molten alloy plating bath may further contain 0.05 to 10% of Fe and more than 0 to less than 1% of Si in weight percent.

Mg and Al in the molten alloy plating bath are one of the elements that improve the corrosion resistance of the plating layer, and improve corrosion resistance by forming corrosion products more densely. When Mg in the plating bath is less than 1.0% by weight, its contribution to corrosion resistance is minimal, and in the past, when it exceeds 2.0% by weight, Mg is used at less than 2.0% by weight due to difficulties in production due to Mg oxidation dross. However, in the present invention, Mg is added to the plating bath in an amount of 3.0% by weight or more to achieve better corrosion resistance. As mentioned above, when more than 3.0% by weight of Mg is added, there is difficulty in production due to oxidized dross, but when more than 6.0% by weight of Al is added, dross caused by oxidation of Mg in the molten metal can be reduced. In addition, when adding Al, it can play a role in improving corrosion resistance by generating primary Al and forming a Zn-Al-Mg ternary eutectic phase. On the other hand, when more than 7.0% by weight of Mg is added in the plating bath, MgZn ₂ phases in the form of rods and needles or Al containing MgZn ₂ in the plating layer grow to exceed 70% of the area fraction of the total MgZn ₂ , reducing the processability of the plating layer. The corrosion resistance deteriorates due to exposure of the steel or Fe-Al-Zn interface alloy layer due to cracks in the plating layer during processing. Meanwhile, when more than 23% by weight of Al is added in the plating bath, the discontinuous Fe-Al-Zn interfacial alloy layer between the steel and the plating layer grows excessively due to an increase in the melting point of the plating bath, which may result in poor interfacial adhesion during processing.

The form and fraction of the MgZn ₂ phase can be closely controlled through cooling. In the method of manufacturing a plated steel material with excellent processability and corrosion resistance according to an embodiment of the present invention, in the process of performing the step (S20) of forming a molten alloy plating layer, the first average cooling rate in the cooling process is the molten alloy plating bath. It is different depending on the difference between the first temperature, which is the temperature, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer. The first average cooling rate may be an average cooling rate from the time the immersed base iron is withdrawn from the molten alloy plating bath to the time the MgZn ₂ phase begins to solidify.

Specifically, when the difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer, is less than 50°C, the first average cooling rate is 10 ~ It may be 20℃/s. In this case, when the first average cooling rate is less than 10°C/s, the constituent phases other than the MgZn ₂ phase, such as Al phase and Zn phase, are coarsely crystallized, making it difficult to preferably control the fraction of the MgZn ₂ region, and the plating layer in the liquid state This reacts with oxygen and can act as a factor that impairs the appearance of the plating surface. On the other hand, when the first average cooling rate exceeds 20°C, it may be difficult for a coarse region of MgZn ₂ to be formed.

Meanwhile, when the difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating 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, the constituent phases other than the MgZn ₂ phase, such as Al phase and Zn phase, are coarsely crystallized, making it difficult to preferably control the fraction of the MgZn ₂ region, and the plating layer in the liquid state This reacts with oxygen and can act as a factor that impairs the appearance of the plating surface. Meanwhile, when the first average cooling rate exceeds 35°C, it may be difficult to form a coarse region of MgZn ₂ .

In addition, when the difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer, is 100°C or more, the first average cooling rate is 20 to 50°C. It may be ℃/s. In this case, when the first average cooling rate is less than 20°C/s, the constituent phases other than the MgZn ₂ phase, such as Al phase and Zn phase, are coarsely crystallized, making it difficult to control the fraction of the MgZn ₂ region appropriately, and the plating layer in the liquid state This reacts with oxygen and can act as a factor that impairs the appearance of the plating surface. Meanwhile, when the first average cooling rate exceeds 50°C, it may be difficult to form a coarse region of MgZn ₂ .

Furthermore, in the method of manufacturing a plated steel material with excellent processability and corrosion resistance according to an embodiment of the present invention, the second average cooling rate in the cooling process from the time when the MgZn ₂ phase begins to solidify to the time when solidification is completed is calculated using the following equation: The relationship in Equation 1 can be satisfied.

<Equation 1>

0.0114 x T - 0.2841 ≤ _2nd average cooling rate ≤ 0.025

When cooling under conditions that do not satisfy the above equation 1, it is difficult to control the growth of the rod-shaped MgZn ₂ precipitated phase, which results in poor processability, and has the disadvantage of lowering productivity due to vibration of the plate.

The molten alloy plating layer realized by the above-described method of manufacturing a plated steel material with excellent processability and corrosion resistance is an area fraction of the MgZn ₂ phase whose ratio of the average minor axis length (a) to the average major axis length (b) is 0.5 or less among the total MgZn ₂ phases on the surface of the plating layer. This may be less than 70%. That is, the area fraction of the MgZn ₂ phase in the form of rods or needles among the total MgZn ₂ phases distributed on the surface of the implemented plating layer may be 70% or less. In this case, the area fraction of the polygon-shaped MgZn ₂ phase among the total MgZn ₂ phases distributed on the surface of the implemented plating layer may be 30% or more.

The zinc alloy plating layer of the present invention is composed of primary Al phase (Al single phase structure with Zn dissolved in solid solution), Al/Zn eutectoid phase, Zn solid solution phase, MgZn ₂ (MgZn ₂ phase containing Al, Mg ₂ Zn ₁₁ phase included), and Al/ It may be composed of a Zn/Mg eutectic structure or a combination thereof. In terms of microstructure for improving processability and processing corrosion resistance, the MgZn ₂ phase and the MgZn ₂ phase containing Al on the surface of the Zn-Al-Mg-based plating layer may be formed in the form of polygons, rods, and needles. .

The ratio of the average minor axis length (a) and the average major axis length (b) of the rod and needle shapes is characterized as 1:10 ≤ a:b ≤ 1:2. Among the total MgZn ₂ , the rod and needle-shaped MgZn ₂ phases are distributed on the surface with an area fraction of less than 70%, more preferably with an area fraction of less than 50%, and the remaining MgZn ₂ is in the shape of a polygon. It is characterized by being distributed.

In the present invention, an exemplary process for forming a molten alloy plating layer on base iron is as follows.

For example, base iron annealed at 680~850℃ is immersed in a plating bath at 440~530℃ and then passed through an air knife to satisfy the single side standard of 30~300g/ ^m2 . However, the entry temperature of the base iron after annealing is adjusted so that it does not differ more than ±20℃ from the plating bath temperature.

In the plated steel material according to an embodiment of the present invention, the area fraction of the MgZn ₂ phase in the cross section (e.g., longitudinal section) within the molten alloy plating layer is 20 to 70%, and the cross section (e.g., longitudinal section) within the molten alloy plating layer is 20 to 70%. ), the ratio of the area fraction of the Al-containing phase to the area fraction of the MgZn ₂ phase is characterized in that it is 1 to 70%. Here, the Al-containing phase may exist spaced apart from the MgZn ₂ phase or may exist inside the MgZn ₂ phase in a cross section within the molten alloy plating layer. Additionally, in this embodiment, the Al-containing phase refers to i) a single Al phase and ii) a phase containing more than 20% Al, with unavoidable impurities within 2% and the remainder being Zn.

The molten alloy plating layer may contain 20 to 70% of the MgZn ₂ phase as an area fraction in the cross section. That is, the ratio of the cross-sectional area (A2) occupied by the MgZn ₂ phase among the total cross-sectional area (A1) of the molten alloy plating layer is 20 to 70%, and the value of (A2 / A1) Meanwhile, in the cross section of the molten alloy plating layer, the sum of the cross-sectional area (B1) of the Al-containing phase present separately from the MgZn ₂ phase and the cross-sectional area (B2) of the Al-containing phase present inside the MgZn ₂ phase is the cross-sectional area of the entire MgZn ₂ phase (B3). It can have a ratio of 1 to 70%. That is, the value of [(B1 + B2) / B3] According to this structure, crack resistance is excellent, and specifically, the average crack width in bending evaluation (3T bending evaluation, 1T bending evaluation) may be 30㎛ or less.

The molten alloy plating layer of the plated steel of the present invention may have an area fraction of the MgZn ₂ phase on the surface of 10 to 70%. If the area fraction is less than 10%, it cannot be formed, and if it exceeds 70%, crack resistance deteriorates. . Here, the surface of the molten alloy plating layer may refer to the upper surface in contact with the outside.

In the plated steel material according to an embodiment of the present invention, the molten alloy plating layer may have an area fraction of the MgZn ₂ phase having a ratio of the average minor axis length (a) and the average major axis length (b) of 0.5 or less among the total MgZn ₂ phases on the surface of 70% or less. there is. For example, the MgZn ₂ phase, which accounts for less than 70% of the total MgZn ₂ phase on the surface of the molten alloy plating layer, has a ratio of the average minor axis length (a) and the average major axis length (b) of 1:2 to 1:10, a value of 0.5 or less. You can have it. In this case, the ratio of the average minor axis length (a) and the average major axis length (b) of the MgZn ₂ phase is 0.5 or less. The ratio of the average minor axis length (a) and the average major axis length (b) may be 1/10 or more and 1/2 or less. there is. When the ratio between the average minor axis length (a) and the average major axis length (b) is less than 0.5, crack resistance deteriorates. The average minor axis length (a) may be 1 to 20 ㎛, and the average major axis length (b) may be 2 to 200 ㎛. It is impossible to form an average minor axis length (a) of less than 1㎛ and an average major axis length (b) of less than 2㎛, and the average minor axis length (a) exceeds 20㎛ or the average major axis length (b) is 200㎛. If it exceeds ㎛, crack resistance decreases.

Meanwhile, the molten alloy plating layer of a plated steel material according to another aspect of the present invention may have an area fraction of Al-Zn dendrites composed of Al and Zn phases of 30% or less on the surface. Since Al-Zn dendrites do not have a desirable effect on chemical conversion processability or LME (Liquid Metal Embrittlement) resistance, it is preferable that the area fraction is low. Therefore, in the plating layer according to this embodiment, the area fraction of Al-Zn dendrites is set to 30% or less.

As seen above, in the plated steel material with excellent processability and corrosion resistance according to an embodiment of the present invention, the MgZn ₂ phase and the MgZn ₂ phase containing Al on the surface of the Zn-Al-Mg-based plating layer are in the form of a polygon. It consists of a rod and needle shape, and the ratio of the average minor axis length (a) and the average major axis length (b) of the rod and needle shape is 1:2 ≤ a:b ≤ 1:10. do. Among the total MgZn ₂ , the MgZn ₂ phase in the form of rods and needles is distributed on the surface with an area fraction of 70% or less, and more preferably with an area fraction of less than 50%, and the remaining MgZn ₂ is in the shape of a polygon. It is distributed.

The molten alloy plating layer is characterized in that the area fraction of the MgZn ₂ phase whose ratio between the average minor axis length (a) and the average major axis length (b) exceeds 0.5 among all MgZn ₂ phases on the surface is 30% or more. For example, in the molten alloy plating layer, more than 30% of the total MgZn ₂ phase on the surface has a ratio of the average minor axis length (a) and the average major axis length (b) exceeding 0.5, _such as 1:1.5, 1:1.2, etc. It can be characterized as having. The diameter (average diameter) of a virtual circle having an area equal to the cross-sectional area of the MgZn ₂ phase in which the ratio of the average minor axis length (a) and the average major axis length (b) exceeds 0.5 may be 1 to 50 μm. If the average diameter is less than 1㎛, it is impossible to form, and if it exceeds 50㎛, crack resistance deteriorates.

Below, preferred experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are only intended to aid understanding of the present invention, and the present invention is not limited by the following experimental examples.

Experiment example

1. Composition and process conditions of specimens

A 1.2 mm cold-rolled material was prepared from a base steel plate, and the ingredients were carbon (C): 0.15% by weight, silicon (Si): 0.01% by weight, manganese (Mn): 0.6% by weight, phosphorus (P): 0.05% by weight, and sulfur. (S): It has a composition of 0.05% by weight and the remainder is iron (Fe). Nitrogen - After annealing to a temperature of 760°C within the range of 680 to 850°C, specifically, 760°C in a 5-10% hydrogen atmosphere gas, the annealed specimen is cooled to a temperature that does not differ by more than 20°C from the plating bath, and then placed in the plating bath at 1~20°C. It was immersed for 5 seconds. Within the range of 440 to 530°C, specifically, the plating thickness was adjusted by nitrogen wiping after immersion in the plating bath at a temperature of 485°C, and cooling was performed at the first and second average cooling rates to form a Zn-Al-Mg base. A plated steel sheet was obtained.

2. Plating layer composition and microstructure evaluation

Table 1 shows the results of evaluating the composition (unit: weight %) of the molten alloy plating layer in the plated steel material according to the experimental example of the present invention and the microstructure and bending workability according to the process structure.

division Zn
(wt%) Al
(wt%) Mg
(wt%) temperature difference
(℃) first average
Cooling speed
(℃/s) second average
Cooling speed
(℃/s) _MgZn2
Area fraction
(area%) flex
Processability Example 1 Bal. 10 3.2 70 20 13 45 ○ Example 2 Bal. 10 3 100 22 13 44 ○ Example 3 Bal. 10 3.1 90 30 13 40 ○ Example 4 Bal. 10 3.2 70 20 13 42 ○ Example 5 Bal. 10.2 5 40 15 13 55 ○ Example 6 Bal. 10.1 5.3 30 13 13 52 ○ Example 7 Bal. 10.3 5.4 10 12 13 51 ○ Example 8 Bal. 15 5 100 35 15 54 ○ Example 9 Bal. 10 7 70 30 15 62 ○ Example 10 Bal. 15 5.3 95 34 13 53 ○ Example 11 Bal. 15 5.1 100 45 11 55 ○ Comparative Example 1 Bal. 10.2 7 50 10 13 71 X Comparative example 2 Bal. 10.3 7 70 7 13 73 X Comparative Example 3 Bal. 15 5.3 120 10 3 71 X Comparative example 4 Bal. 23.5 7 60 25 13 75 X Comparative Example 5 Bal. 24 7.5 60 26 15 72 X Comparative Example 6 Bal. 24 7.3 40 15 11 80 X

In Table 1, the temperature difference item refers to the temperature difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer formed on the base iron immersed in the molten alloy plating bath. The first average cooling rate refers to the average cooling rate in the cooling process from the time the immersed base iron is withdrawn from the molten alloy plating bath to the time the MgZn ₂ phase begins to solidify, and the second average cooling rate is This refers to the average cooling rate in the cooling process from the time when the MgZn ₂ phase begins to solidify to the time when solidification is completed. In this experimental example, the solidification initiation temperature of MgZn ₂ phase was derived using a thermodynamic calculation program (FactSage 7.1). The temperature difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer formed on the base iron immersed in the molten alloy plating bath, varies depending on the plating component. However, the same plating component was controlled by adjusting the plating bath temperature, and the first average cooling rate was controlled by adjusting the air knife height.

The evaluation of the MgZn ₂ area fraction was measured using an image program after observing the surface at 500x magnification using FE-SEM for each manufactured plated steel sheet. The area fraction disclosed in Table 1 represents the fraction of the rod or needle-shaped MgZn ₂ phase among the total MgZn ₂ phases distributed on the surface of the plating layer as an area ratio.

In the bending processability evaluation, the bending part after 1T and 3T bending was observed 200 and 500 times with a FE-SEM (Field Emission Scanning Electron Microscope), and the width of the bending crack was measured and averaged for evaluation. The '○' item refers to cases where the average crack width in the bending evaluation exceeds 0 and is less than 30㎛, and the 'X' item refers to the case where the average crack width exceeds 30㎛ in the bending evaluation.

Referring to Table 1, in Examples 1 to 11, i) the composition of the molten alloy plating bath satisfies the range of Al: 6 to 23%, Mg: 3 to 7%, and the balance being Zn, in weight percent. , ii) When the difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer, is less than 50°C, the first average cooling rate is 10 ~ Satisfies the range of 20°C/s (Examples 5, 6, and 7), and the first temperature is the temperature of the molten alloy plating bath and the second temperature is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer. If the difference 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), and the temperature of the molten alloy plating bath When the difference between the first temperature and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten 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, 11), iii) The second average cooling rate in the cooling process from the time when the MgZn ₂ phase begins to solidify to the time when solidification is completed satisfies the relationship of Equation 1 below.

<Equation 1>

0.0114 x T - 0.2841 ≤ _2nd average cooling rate ≤ 0.025

In this case, Examples 1 to 11 can confirm that the area fraction of the MgZn 2 phase in the form of a rod or needle among the total MgZn ₂ phases distributed on the surface of the implemented plating layer is 70% or less, and no _cracks are observed in the bending evaluation. It can be confirmed that the average width is 30㎛ or less (see Figures 1 and 3). Furthermore, it can be confirmed that the growth of the Fe-Al interface alloy layer in the cross section of the plating layer can be controlled to less than 10㎛ (see Figure 5).

On the other hand, in Comparative Examples 1 and 2, the difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer, is 50° C. or more and 100° C. If it is less than ℃, the first average cooling rate does not satisfy the range of 15 to 35 ℃/s, and accordingly, among the total MgZn ₂ phases distributed on the surface of the implemented plating layer, the MgZn ₂ phase in the form of a rod or needle is It can be seen that the area fraction exceeds 70%, and the average crack width exceeds 30㎛ in bending evaluation.

In Comparative Example 3, when the difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer, is 50°C or more and less than 100°C, the first average The 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 MgZn ₂ phase starts to solidify to the time when solidification is completed does not satisfy the relationship in Equation 1. Accordingly, the area fraction of the MgZn ₂ phase in the form of a rod or needle among the total MgZn ₂ phases distributed on the surface of the implemented plating layer exceeds 70%, and the average crack width in the bending evaluation exceeds 30㎛. can confirm.

In Comparative Examples 4 to 6, the composition of the molten alloy plating bath in weight% did not satisfy the range of 6 to 23% Al, and in Comparative Examples 5 to 6, the composition of the molten alloy plating bath in weight% was not satisfied. , Mg: does not satisfy the range of 3 to 7%, and the area fraction of the rod or needle-shaped MgZn ₂ phase among the total MgZn ₂ phases distributed on the surface of the implemented plating layer exceeds 70%, and bending evaluation It can be seen that the average crack width exceeds 30㎛ (see Figures 2 and 4).

For example, in Examples 1 to 3, the formation of rod and needle-like MgZn ₂ phases was developed relatively little, and the crack width was measured to be within 15㎛ or 30㎛. On the other hand, Comparative Examples 1 and 2 do not satisfy the first average cooling rate range of the present invention, and when the area fraction of the rod and needle-like MgZn ₂ phase exceeds 70%, cracks occur on the MgZn ₂ phase with high hardness. In addition, cracks progress along grain boundaries, and the crack width exceeds 30㎛ on average.

Comparative Example 3 shows that bending workability is not good when the range of the first average cooling rate and the second average cooling rate disclosed in the embodiment of the present invention is not satisfied.

Comparative Example 4 does not satisfy the Al content range of the molten alloy plating layer of the present invention, and Comparative Examples 5 and 6 do not satisfy the Al and Mg contents of the molten alloy plating layer, resulting in excessive production of the Fe-Al alloy layer. And it can be confirmed that the bending workability is inferior because the area fraction of MgZn ₂ exceeds 70%. In the case of Comparative Example 4, it can be seen that the growth of the Fe-Al interface alloy layer was formed thicker than 10㎛ (see Figure 6), and the excessive formation of the rod and needle-shaped MgZn ₂ phase and the growth of the Fe-Al alloy layer resulted in the formation of cracks. It can be seen that there is no directionality, and the average crack width and area are inferior (see Figure 4).

According to the technical idea of the present invention described above, even if a MgZn ₂ phase with high hardness that is unfavorable for processability is formed, it is possible to suppress the growth of rod and needle-shaped MgZn ₂ phases and control the area fraction to implement a plated steel sheet with excellent processability.

Although the above description focuses on the embodiments of the present invention, various changes and modifications can be made at the level of those skilled in the art. These changes and modifications can be said to belong to the present invention as long as they do not depart from the scope of the present invention. Therefore, the scope of rights of the present invention should be determined by the claims described below.

Claims (6)

Immersing base iron in a molten alloy plating bath; and
forming a molten alloy plating layer on the base iron by withdrawing the immersed base iron from the molten alloy plating bath and performing a cooling process; Includes,
The first average cooling rate in the cooling process is characterized in that it is different depending on the difference between the first temperature, which is the temperature of the molten alloy plating bath, and the second temperature, which is the solidification start temperature of the MgZn ₂ phase constituting the molten alloy plating layer.
Method for manufacturing plated steel with excellent processability and corrosion resistance. According to claim 1,
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,
Method for manufacturing plated steel with excellent processability and corrosion resistance. According to claim 1,
The first average cooling rate is the average cooling rate from the time the immersed base iron is withdrawn from the molten alloy plating bath to the time the MgZn ₂ phase begins to solidify,
Method for manufacturing plated steel with excellent processability and corrosion resistance. According to claim 1,
The second average cooling rate in the cooling process from the time when the MgZn ₂ phase begins to solidify to the time when solidification is completed satisfies the relationship in Equation 1 below,
Method for manufacturing plated steel with excellent processability and corrosion resistance.
<Equation 1>
0.0114 x T - 0.2841 ≤ _2nd average cooling rate ≤ 0.025 According to claim 1,
The molten alloy plating bath is characterized in that it is a Zn plating bath containing Al: 6 to 23%, Mg: 3 to 7% and other inevitable impurities in weight percent.
Method for manufacturing plated steel with excellent processability and corrosion resistance. According to claim 1,
The molten alloy plating layer formed on the base iron is characterized in that the area fraction of the MgZn ₂ phase having a ratio of the average minor axis length (a) and the average major axis length (b) of 0.5 or less among all MgZn ₂ phases on the surface is 70% or less,
Method for manufacturing plated steel with excellent processability and corrosion resistance.