KR102323645B1 - Plated steel sheet and method of manufacturing the same - Google Patents

Abstract

A method for manufacturing a plated steel sheet according to an embodiment of the present invention comprises the steps of: (a) providing a cold-rolled steel sheet; (b) annealing the cold-rolled steel sheet; (c) passing the annealed steel sheet through a plating bath containing aluminum (Al), magnesium (Mg) and zinc (Zn), on the steel sheet, aluminum (Al): 0.5 to 10% by weight, magnesium (Mg) ): forming a plating layer containing 0.5 to 4% by weight and the remainder zinc (Zn) and other unavoidable impurities; and (d) rapidly cooling the steel sheet on which the plating layer is formed in cooling water in _{which carbon dioxide (CO 2 ) is dissolved;} includes

Description

Coated steel sheet and its manufacturing method

The present invention relates to a galvanized steel sheet and a method for manufacturing the same, and more particularly, to a galvanized steel sheet having excellent blackening resistance and a method for manufacturing the same.

Existing hot-dip galvanized steel sheet has excellent self-sacrificing properties and is widely used in construction materials and home appliances. When a hot-dip galvanized steel sheet is exposed to a corrosive environment, zinc (Zn) acts as a sacrificial anode to the exposed iron portion, resulting in loss of zinc in the plating layer. The sacrificial anode action of zinc plays an excellent role in suppressing rust generation of base iron in a corrosive environment, but the sacrificial anode efficiency is somewhat lowered. In order to solve this problem, recently, in Japan and Europe, high corrosion resistance plating products that express excellent corrosion resistance by adding Mg to a Zn plating bath to generate a dense corrosion product in a corrosive environment to improve the efficiency of the sacrificial anode are being produced. However, in a plated steel sheet manufactured in a plating bath in which Mg is added to Zn, Mg pre-reacts in a corrosive environment to form an oxide, which causes the plating surface to turn black or brown. In order to improve this problem, lubrication treatment or post-treatment coating is performed for temporary rust prevention after plating. However, this improvement method has a limitation in having inferior properties compared to existing products because it serves as a complete barrier in a high temperature and high humidity atmosphere. In order to improve this limitation, it is necessary to minimize the exposure of Mg on the surface of the plating electrode to a metallic state in a corrosive environment in order to control the phenomenon that the Mg on the surface of the plating electrode continuously reacts and turns black or brown.

As a related prior art, there is Korean Patent Publication No. 10-20130053500.

In order to solve the above problems, the technical problem to be achieved by the present invention is to provide a galvanized steel sheet having excellent blackening resistance and a method for manufacturing the same.

A plated steel sheet according to an embodiment of the present invention for achieving the above object is a cold rolled steel sheet; and a plating layer formed on the cold-rolled steel sheet, containing aluminum (Al): 0.5 to 10% by weight, magnesium (Mg): 0.5 to 4% by weight, and the balance containing zinc (Zn) and other unavoidable impurities; Including, the surface portion of the plating layer is characterized in that the carbonate-based magnesium oxide is formed.

In the plated steel sheet, the plating layer may contain aluminum (Al): 1.0 to 6.0% by weight, magnesium (Mg): 1.0 to 3.0% by weight, and the balance zinc (Zn) and other unavoidable impurities.

In the plated steel sheet, the plating layer may have a plating adhesion amount of 20 to 300 g/m ² of single-sided plating.

A method for manufacturing a plated steel sheet according to an embodiment of the present invention for achieving the above object includes the steps of: (a) providing a cold-rolled steel sheet; (b) annealing the cold-rolled steel sheet; (c) passing the annealed steel sheet through a plating bath containing aluminum (Al), magnesium (Mg) and zinc (Zn), on the steel sheet, aluminum (Al): 0.5 to 10% by weight, magnesium (Mg) ): forming a plating layer containing 0.5 to 4% by weight and the remainder zinc (Zn) and other unavoidable impurities; and (d) rapidly cooling the steel sheet on which the plating layer is formed in cooling water in _{which carbon dioxide (CO 2 ) is dissolved;} includes

In the method for producing the coated steel sheet, the carbon dioxide (CO ₂₎ is the dissolved water is carbon dioxide (CO2) as the water (water) is carbon dioxide (CO ₂₎ the dissolved through the gas bubbling, the cooling water temperature: and 50 ~ 80 ℃ , cooling water pH: 4 to 6.5.

In the step (d) of the method for manufacturing the plated steel sheet, the quenching step includes immersing the steel sheet having the plating layer formed thereon in the cooling water for 3 seconds or more and cooling it to room temperature at a cooling rate of 5 to 30° C./sec. quenching) may be included.

In the method for manufacturing the plated steel sheet, step (d) may include generating a carbonate-based magnesium oxide on the surface of the plating layer.

In step (b) of the method for manufacturing the plated steel sheet, the annealing treatment is performed at a temperature of 700 to 850°C, and the temperature of the plating bath in step (c) may be 400 to 520°C.

According to an embodiment of the present invention, a galvanized steel sheet having excellent blackening resistance and a manufacturing method thereof can be implemented. Of course, the scope of the present invention is not limited by these effects.

1 is a flowchart schematically illustrating a method of manufacturing a plated steel sheet according to an embodiment of the present invention.
2 is a photograph of a surface of a plated layer of a plated steel sheet according to a comparative example of the present invention.
3 is a photograph taken of the plated layer surface of the plated steel sheet according to an embodiment of the present invention.

A plated steel sheet and a method for manufacturing the same according to an embodiment of the present invention will be described in detail. The terms described below are appropriately selected in consideration of their functions in the present invention, and definitions of these terms should be made based on the content throughout this specification. Hereinafter, a plated steel sheet having excellent blackening resistance and a manufacturing method thereof will be described.

plated steel sheet

A plated steel sheet according to an embodiment of the present invention is a cold rolled steel sheet; and a plating layer formed on the cold-rolled steel sheet, containing aluminum (Al): 0.5 to 10% by weight, magnesium (Mg): 0.5 to 4% by weight, and the balance containing zinc (Zn) and other unavoidable impurities; Including, the surface portion of the plating layer is characterized in that the carbonate-based magnesium oxide is formed.

Mg added to Zn to improve corrosion resistance forms Mg oxide in a corrosive environment, so the appearance of the plating surface turns black or brown, which is known to act as a cause of deterioration of appearance quality and physical properties.

This phenomenon can be controlled by controlling the microstructure of the surface of the plating layer. The structure of the plating layer plated with the plating bath in which Mg is added to Zn _{is composed of a Zn single phase, MgZn 2} , and a ternary eutectic phase consisting of Al+ MgZn _{2 .} The ternary eutectic phase means an eutectic phase containing Mg, Zn, and Al, and may be, for example, a ternary eutectic phase consisting of Al+MgZn2.

Among them, the main factor causing the color difference of the plating surface is MgZn ₂ containing Mg and Al+MgZn ₂ ternary process phase. For that reason, in order to lower the color difference on the plating surface, the less the MgZn ₂ phase and the Al+MgZn ₂ ternary eutectic phase, the less the color difference change. In order to control such a phase fraction, it is possible to control through Al and Mg content, but since Al and Mg content is possible in a limited input range, it is difficult to express excellent corrosion resistance.

On the other hand, Al: 0.5wt% ~ 10wt%, Mg: 0.5wt% ~ 4wt% with respect to the total weight of the plating composition capable of expressing corrosion resistance It is added within the range, so that the plated steel sheet with excellent plating surface and excellent corrosion resistance can be obtained. In the hot-dip galvanizing bath, aluminum (Al) forms a ternary process phase and plays a role in refining the structure.

When the Al content is 0.5wt% or more, a three-way process phase to improve corrosion resistance can be formed. More preferably, 1 wt% to 6 wt% of Al is added based on the total weight of the plating composition. The minimum amount of Al added must be 1wt% or more to reduce oxidation dross due to Mg oxidation in the molten metal. In addition, in order to minimize Fe dross due to iron (Fe) dissolved from the steel sheet during plating, it is preferable that the Al content be 6 wt% or less.

In addition, the content of Mg for improving corrosion resistance in the plating layer is preferably 1 wt% to 3 wt%. In order to generate a process contributing to corrosion resistance, Mg must be added at least 1 wt%, and when the Mg content exceeds 3 wt%, MgZn ₂ and Al+MgZn _{2 The} fraction of the ternary process increases rapidly _{, forming MgO, Mg(OH) 2} oxide on the plating surface in a corrosive environment, making it difficult to control the color difference change. That is, the most effective method for minimizing the color difference change of the plating surface layer in a corrosive environment is to minimize the formation of oxides such _{as MgO and Mg(OH) 2 in a corrosive environment.}

However, until now, improving the blackening resistance while expressing the corrosion resistance is a technically difficult part because it is a contradictory mechanism. In order to overcome this technical limitation, the present invention discloses a configuration for improving blackening resistance while maintaining corrosion resistance by generating carbonic acid-based oxides on the plating electrode surface layer through a water quenching process during a continuous plating process.

Method for manufacturing plated steel sheet

1 is a flowchart showing a method of manufacturing a plated steel sheet according to an embodiment of the present invention.

Referring to Figure 1, (a) providing a cold-rolled steel sheet (S100); (b) annealing the cold-rolled steel sheet (S200); (c) passing the annealed steel sheet through a plating bath containing aluminum (Al), magnesium (Mg) and zinc (Zn), on the steel sheet, aluminum (Al): 0.5 to 10% by weight, magnesium (Mg) ): forming a plating layer containing 0.5 to 4% by weight and the remainder zinc (Zn) and other unavoidable impurities (S300); and (d) rapidly cooling the steel sheet on which the plating layer is formed _{in cooling water in which carbon dioxide (CO 2} ) is dissolved (S400).

In the method for producing the coated steel sheet, the carbon dioxide (CO ₂₎ is the dissolved water is carbon dioxide (CO2) as the water (water) is carbon dioxide (CO ₂₎ the dissolved through the gas bubbling, the cooling water temperature: and 50 ~ 80 ℃ , cooling water pH: 4 to 6.5. The step of quenching in (d) step (S400) is a step of water quenching by immersing the steel sheet having the plating layer formed thereon in the cooling water for 3 seconds or more and cooling it to room temperature at a cooling rate of 5 to 30° C./sec. may include. The (d) step (S400) may include a step of generating a carbonate-based magnesium oxide on the surface of the plating layer.

In the (b) step (S200) of the method for manufacturing the plated steel sheet, the annealing treatment is performed at a temperature of 700 to 850°C, and the temperature of the plating bath in the (c) step (S300) is 400 to 520°C. can

The method for manufacturing a plated steel sheet according to an embodiment of the present invention can implement the configuration of the present invention through improvement of facilities of a water quenching tank that rapidly lowers the temperature of a steel sheet after plating in a continuous galvanizing line and change of process conditions. have.

Specifically, maintaining the water temperature of the water quenching tank at 50° C. or higher and increasing the amount of dissolved CO _{2 through bubbling CO 2} gas in the water quenching tank while maintaining a pH of 4 to 6.5 is disclosed. In this process, the plated steel sheet composed of Zn-Al-Mg is immersed in a water quenching tank for more than 3 seconds to change the Mg of the plated electrode surface layer into carbonic acid-based Mg oxide to realize the effect of blackening resistance.

When the water temperature of the water quenching tank is less than 50° C., the Mg of the plating surface layer has insufficient reactivity to form carbonate-based oxides, so there is a limit to the effect. In addition, when the immersion time is less than 3 seconds at a temperature of 50° C. or higher in the water quenching tank described above, carbonic acid-based Mg oxide is formed finely, and the effect is insignificant.

_{The amount of CO 2} gas input to the water quenching tank should be input within the allowable pH range. When _{the input amount of CO 2} gas is excessive, the pH is rapidly decreased, and the generated oxide is dissolved. On the other hand, when the _{amount of CO 2} gas input is small, the formation of carbonic acid-based oxides is small, so that it cannot serve as a sufficient protective film.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

1. Preparation of specimens

Prepare a cold-rolled steel sheet with a thickness of 0.7 mm. The composition of the cold-rolled steel sheet satisfies the composition range shown in Table 1.

C (wt%) Si (wt%) Mn (wt%) P(wt%) S (wt%) ≤0.15 - ≤0.60 ≤0.05 ≤0.05

For example, in the cold-rolled steel sheet, carbon (C): 0.07 wt%, manganese (Mn): 0.3 wt%, phosphorus (P): 0.05 wt%, sulfur (S): 0.005 wt%, and the balance iron (Fe) It may have a phosphorus composition. However, the composition of the cold-rolled steel sheet is exemplary, and is not necessarily required in implementing the technical idea of the present invention.

After immersing the cold-rolled steel sheet in an alkaline solution at 50° C. for 30 minutes, washing with water to remove foreign substances and oil on the surface is prepared.

Subsequently, the specimen is annealed and then plated. Annealing is carried out in a reducing atmosphere composed of hydrogen: 10 to 30%, nitrogen: 70 to 90%, and the annealing heat treatment temperature is 700 to 850 °C. For plating, the annealed specimen was cooled to a plating bath temperature (400 to 520° C.), immersed in the plating bath, and pulled up, and the plating thickness was adjusted to about 10 μm by nitrogen wiping. The plating adhesion amount was controlled to a level of ^{20 ~ 300g/m 2 for single-sided plating.} Subsequently, after the plating process, it is cooled and solidified by a water quenching process to room temperature at a cooling rate of 5°C/sec to 30°C/sec. The plating bath is a zinc (Zn) plating bath containing 0.5 to 10% by weight of aluminum (Al) and 0.5 to 4% by weight of magnesium (Mg).

2. Blackening resistance evaluation of plating surface

A plated steel sheet was prepared under the above conditions, and the blackening resistance of the plating surface was evaluated for each water quenching condition, and it is shown in Table 2. Black stool was evaluated by measuring the color difference on the surface of the steel sheet using a colorimeter after storage for 72 hours in a high-temperature and high-humidity environment with a relative humidity of 90% or more and a temperature of 50°C. The criteria for judging according to the color difference before and after the blackening resistance test shown in Table 2 are when the color difference (ΔE) is 7.0 or less, and the item '○' is when the color difference (ΔE) is greater than 7.0 and less than or equal to 9.0. 'Δ' corresponds to a case in which the color difference (ΔE) is greater than or equal to 9.0 and less than or equal to 11.0, and item 'X' corresponds to a case in which the color difference (ΔE) is 13.0 or more.

division Zn Al
(wt%) Mg
(wt%) cooling water
Temperature (℃) cooling water
pH immersion
time(s) Constant temperature and humidity evaluation result
Color difference (ΔE) Comparative Example 1 Bal. 1.5 One 30 4 10 X Comparative Example 2 Bal. 1.5 One 50 3 10 X Comparative Example 3 Bal. 1.5 One 50 7 10 X Comparative Example 4 Bal. 2 1.5 80 4 2 △ Example 1 Bal. 1.5 One 50 4 10 ○ Example 2 Bal. 1.5 One 50 5 10 ◎ Example 3 Bal. 1.5 One 50 6 10 ○ Example 4 Bal 2 1.5 80 4 10 ◎ Example 5 Bal. 2 1.5 80 6 15 ◎ Example 6 Bal. 5 3 80 4 10 ◎

Referring to Table 2, in the water quenching conditions of Examples 1, 2, 3, 4, 5 and 6, the cooling water is carbon dioxide through carbon dioxide (CO2) gas bubbling. (CO ₂ ) is dissolved water, cooling water temperature: 50 ~ 80 ℃, cooling water pH: 4 ~ 6.5, cooling water immersion time: 3 seconds or more process conditions were applied. In Example 1, Example 2, Example 3, Example 4, Example 5, and Example 6, the color difference (ΔE) was 9.0 or less as a result of constant temperature and humidity evaluation, confirming excellent blackening resistance.

On the other hand, in the water quenching conditions of Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4, carbon dioxide (CO2) gas bubbling was not applied to the cooling water. Furthermore, in Comparative Example 1, the cooling water temperature range was less than 50 to 80° C. and not satisfied, and in Comparative Examples 2 and 3, the cooling water pH range did not satisfy 4 to 6.5, Comparative Examples 1, 2 and As a result of the constant temperature and humidity evaluation in Comparative Example 3, the color difference (ΔE) was 11.0 or more, confirming that the blackening phenomenon was remarkably displayed. In Comparative Example 4, the cooling water immersion time was less than 3 seconds, which was not satisfactory. As a result of the constant temperature and humidity evaluation, it can be confirmed that the color difference (ΔE) exceeds 9.0, resulting in blackening.

2 is a photograph of the plated layer surface of the plated steel sheet according to Comparative Example 1 of the present invention, and FIG. 3 is a photograph of the plated layer surface of the plated steel sheet according to Example 6 of the present invention.

Referring to FIG. 2, when the plated steel sheet is quenched by a water quenching process at room temperature, carbonate-based oxides are not formed, whereas referring to FIG. In one case, it can be confirmed that a carbonic acid-based oxide is formed.

Although the above description has been focused on the embodiments of the present invention, various changes or modifications may be made at the level of those skilled in the art. Such changes and modifications can be said to belong to the present invention without departing from the scope of the present invention. Accordingly, the scope of the present invention should be judged by the claims described below.

Claims (8)

delete delete delete (a) providing a cold-rolled steel sheet;
(b) annealing the cold-rolled steel sheet;
(c) passing the annealed steel sheet through a plating bath containing aluminum (Al), magnesium (Mg) and zinc (Zn), on the steel sheet, aluminum (Al): 0.5 to 10% by weight, magnesium (Mg) ): forming a plating layer containing 0.5 to 4% by weight and the remainder zinc (Zn) and other unavoidable impurities; and
(d) rapidly cooling the steel sheet on which the plating layer is formed in cooling water in _{which carbon dioxide (CO 2 ) is dissolved;} A method for manufacturing a plated steel sheet comprising:
The carbon dioxide (CO ₂₎ is the dissolved water is carbon dioxide (CO2) as the water (water) is carbon dioxide (CO ₂₎ the dissolved through the gas bubbling, the cooling water temperature: and 50 ~ 80 ℃, cooling water pH: 4 ~ 6.5, and ,
The step of quenching in step (d) includes immersing the steel sheet having the plating layer formed thereon in the cooling water for 3 seconds or more and cooling it to room temperature at a cooling rate of 5 to 30°C/sec. A method of manufacturing a plated steel sheet, characterized in that.
delete delete 5. The method of claim 4,
The step (d) comprises the step of generating carbonate-based magnesium oxide on the surface of the plating layer,
A method for manufacturing a plated steel sheet. 5. The method of claim 4,
The annealing treatment in step (b) is performed at a temperature of 700 ~ 850 ℃,
The temperature of the plating bath in step (c) is characterized in that 400 ~ 520 ℃,
A method for manufacturing a plated steel sheet.

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