KR102250323B1 - Plated steel sheet and method of manufacturing the same - Google Patents

Abstract

The present invention is a cold rolled steel sheet; And a plating layer formed on the cold-rolled steel sheet, aluminum (Al): 0.5 to 10% by weight, magnesium (Mg): 0.5 to 4% by weight, and the balance being zinc (Zn) and other unavoidable impurities, and the plating layer. Silver Zn single phase, MgZn ₂ phase and Mg, Zn and It provides a plated steel sheet including a three-way process containing Al.

Description

Plated steel sheet and its manufacturing method {PLATED STEEL SHEET AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a plated steel sheet and a method for manufacturing the same, and more particularly, to a galvanized steel sheet having excellent post-treatment corrosion resistance and a method for manufacturing the same.

Existing hot-dip galvanized steel sheets have excellent self-sacrificing properties and are widely applied to construction materials and home appliances. When the hot-dip galvanized steel sheet is exposed to a corrosive environment, zinc (Zn) acts as a sacrificial anode to a portion exposed to iron, resulting in loss of zinc in the plating layer. The sacrificial anode action of zinc plays an excellent role in suppressing the rust generation of base iron in a corrosive environment, but the anode efficiency is somewhat lowered. In order to solve this problem, recently, in Japan and Europe, high corrosion-resistant plating products have been produced in Japan and Europe that show excellent corrosion resistance by increasing the anode efficiency by adding Mg to Zn to generate a dense corrosion product in a corrosive environment. As Mg is added to Zn, it also acts to increase corrosion resistance, but the initial surface activity increases rapidly in a corrosive environment due to the addition of Mg, resulting in a phenomenon in which the alkalinity of the surface increases. This phenomenon acts as a role of inhibiting corrosion resistance during post-treatment. When exposed to a corrosive environment after post-treatment, the alkalinity of the surface of the plating layer rises, and as a result, the post-treatment coating does not act as a complete barrier. In order to control this phenomenon, it is very important to control the texture of the surface.

Related prior art is the Republic of Korea Patent Publication No. 10-20130053500 No.

In order to solve the above problems, the technical problem to be achieved by the present invention is to provide a plated steel sheet capable of reducing the alkalinity activation of a plated surface and a method of manufacturing the same.

The 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, aluminum (Al): 0.5 to 10% by weight, magnesium (Mg): 0.5 to 4% by weight, and the balance being zinc (Zn) and other unavoidable impurities. Including, the plating layer is a Zn single phase, MgZn ₂ phase and Mg, Zn and A three-way process phase containing Al may be included.

In the plated steel sheet, in order to reduce the surface alkalinity of the plated layer, the _{phase fraction of the MgZn 2} phase and the Mg, Zn and The sum of the phase fractions of the three-way process including Al may be 80% or less, and the phase fraction of the Zn single phase may be 20% or more.

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

A method of manufacturing a plated steel sheet according to an embodiment of the present invention for achieving the above object comprises the steps of: (a) providing a cold rolled steel sheet; (b) annealing the cold-rolled steel sheet; (c) The annealed steel sheet was passed through a plating bath containing aluminum (Al), magnesium (Mg) and zinc (Zn), and aluminum (Al) on the steel sheet: 0.5 to 10% by weight, magnesium (Mg) ): forming a plating layer made of 0.5 to 4% by weight and the balance of zinc (Zn) and other inevitable impurities; And (d) quenching the steel sheet on which the plating layer is formed to room temperature at a cooling rate of 5 to 30° C./sec.

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

In the step (d) of the manufacturing method of the plated steel sheet, the rapid cooling may include water quenching by immersing the plated steel sheet in water at 60 to 100°C for 5 to 60 seconds. have.

The step (d) of the method of manufacturing the plated steel sheet may include a step of generating a hydroxide on the surface of the plated layer.

According to an embodiment of the present invention, it is possible to implement a plated steel sheet capable of reducing the alkalinity activation of a plated surface and a method of manufacturing the same. Of course, the scope of the present invention is not limited by these effects.

1 is a flow chart schematically showing a method of manufacturing a plated steel sheet according to an embodiment of the present invention.
2 is a photograph of the surface of a plated steel sheet according to a comparative example of the present invention.
3 is a photograph of the surface of a plated steel sheet according to an embodiment of the present invention.

A plated steel sheet and a manufacturing method thereof according to an embodiment of the present invention will be described in detail. Terms to be described later are terms appropriately selected in consideration of functions in the present invention, and definitions of these terms should be made based on the contents throughout the present specification. Hereinafter, a plated steel sheet capable of reducing the alkalinity activation on a plated surface and a method of manufacturing the same will be described.

Plated steel sheet

A plated steel sheet according to an embodiment of the present invention includes a cold rolled steel sheet; And a plating layer formed on the cold-rolled steel sheet, aluminum (Al): 0.5 to 10% by weight, magnesium (Mg): 0.5 to 4% by weight, and the balance being zinc (Zn) and other unavoidable impurities. Including, the plating layer is a Zn single phase, MgZn ₂ phase and Mg, Zn and A three-way process phase containing Al may be included. Strictly, the plating layer may be made of aluminum (Al): 1.0 to 6.0% by weight, magnesium (Mg): 1.0 to 3.0% by weight, and the balance is zinc (Zn) and other unavoidable impurities.

In the plated steel sheet, in order to reduce the surface alkalinity of the plated layer, the _{phase fraction of the MgZn 2} phase and the Mg, Zn and The sum of the phase fractions of the three-way process including Al may be 80% or less, and the phase fraction of the Zn single phase may be 20% or more.

Magnesium (Mg) added to improve corrosion resistance to zinc (Zn) increases the alkalinity of the surface of the plating layer in a corrosive environment, and acts as a cause of the occurrence of physical property inferiority of the coating layer during post-treatment coating after plating. The post-treatment coating may be a post-treatment coating including an inorganic film organic film, an organic-inorganic composite film, and a phosphate film. The increase in alkalinity of the surface of the plating layer has a close relationship with the structure of the surface layer of plating, and can be controlled through the control of some microstructures.

The structure of the plating layer plated with a plating bath in which magnesium (Mg) is added to zinc (Zn) is composed of a single phase of Zn, a phase of MgZn ₂ , and a three-way process including Mg, Zn and Al. In the hot-dip galvanizing bath, aluminum (Al) forms a three-way process to refine the structure. The ternary process phase refers to a process phase including Mg, Zn, and Al, and may be, for example, a ternary process phase consisting of _{Al+MgZn 2.} Among them, the phases that increase the surface activation of the surface are the MgZn ₂ phase containing magnesium (Mg) and the three-way process phase containing Mg, Zn, and Al. In order to lower the surface alkalinity, _{the surface phase fraction of the MgZn 2} phase and the ternary process should be formed to be 80% or less, and the Zn single phase to be formed to be 20% or more, thereby preventing an increase in alkalinity of the surface of the plating layer. In order to control this phase fraction, it can be controlled by adjusting the content of aluminum (Al) and magnesium (Mg), but it is difficult to express excellent corrosion resistance because the content of aluminum (Al) and magnesium (Mg) is possible within a limited input range. .

On the other hand, with respect to the total weight of the plating composition capable of expressing corrosion resistance, aluminum (Al): 0.5 to 10% by weight, magnesium (Mg): 0.5 to 4% by weight, and the balance of zinc (Zn) has excellent plating surface and excellent corrosion resistance. A plated steel sheet can be obtained. When the aluminum (Al) content is added at least 0.5% by weight, a three-way process phase that improves corrosion resistance can be formed, and when the aluminum (Al) content exceeds 10% by weight, the aluminum (Al) single phase increases on the surface, resulting in the appearance of the plating surface. Will decrease. More preferably, based on the total weight of the plating composition, aluminum (Al): It is preferably 1.0 to 6.0% by weight. When the minimum amount of aluminum (Al) is added at least 1% by weight, it is possible to reduce the oxide dross due to the oxidation of magnesium (Mg) in the molten metal, and the aluminum (Al) content must be designed to be higher than the magnesium (Mg) content. It is easy to suppress the oxidation of magnesium (Mg). In addition, in order to minimize iron dross due to iron (Fe) dissolved from the steel sheet during plating, it is preferable to control the aluminum (Al) content to 6% by weight or less. In addition, the content of magnesium (Mg) that improves corrosion resistance in the plating layer is preferably 1.0 to 3.0% by weight.In order to generate a process contributing to corrosion resistance, magnesium (Mg) must be added at least 1% by weight, and magnesium (Mg ) If the content exceeds 3% by weight, the fraction of the MgZn ₂ phase and the ternary process phase including Mg, Zn, and Al increases rapidly, making it impossible to control the activation of alkalinity on the plating surface. In order to more effectively reduce the alkalinity of the plating surface in a limited range of aluminum (Al) and magnesium (Mg) content, reducing the amount of magnesium (Mg) that contributes to the reaction in a corrosive environment is the most effective method. In order to overcome these technical limitations, the present invention reduces the activated magnesium (Mg) content through a water quenching process during the continuous plating process for magnesium (Mg) present in the surface layer of the plating layer, thereby reducing the alkalinity activation of the plating surface. I want to implement it.

Manufacturing method of plated steel sheet

1 is a flow chart 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) The annealed steel sheet was passed through a plating bath containing aluminum (Al), magnesium (Mg) and zinc (Zn), and aluminum (Al) on the steel sheet: 0.5 to 10% by weight, magnesium (Mg) ): forming a plating layer made of 0.5 to 4% by weight and the balance of zinc (Zn) and other inevitable impurities (S300); And (d) rapidly cooling the steel sheet on which the plating layer is formed to room temperature at a cooling rate of 5 to 30° C./sec (S400).

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

In the (d) step (S400) of the manufacturing method of the plated steel sheet, the quenching step includes water quenching by immersing the steel sheet on which the plating layer is formed in water at 60 to 100° C. for 5 to 60 seconds. Can include.

The (d) step (S400) of the method for manufacturing the plated steel sheet may include a step of generating a hydroxide on the surface of the plated layer.

The manufacturing method of a plated steel sheet according to an embodiment of the present invention can be implemented by improving the equipment and changing the fixing conditions of a water quenching tank that rapidly lowers the temperature of the steel sheet after plating in a continuous zinc plating line. Specifically, the water temperature of the water quenching tank is maintained at 60° C. or higher, and the plated steel sheet composed of Zn-Al-Mg is immersed for 5 to 60 seconds to lower the activity of the plated surface layer. When the water temperature of the water quenching tank is set to less than 60°C, it may take a long time to generate the hydroxide on the surface of the plating layer, so there may be a limit to application to the line. In addition, in order to generate a hydroxide on the surface of the plating layer at a water temperature of 60°C, the immersion time must be at least 5 seconds to generate a hydroxide capable of reducing plating surface activity. On the other hand, when the immersion time exceeds 60 seconds, excessive hydroxide is generated on the plating surface, and the surface of the plating layer turns black, thereby impairing the appearance.

Experimental example

Hereinafter, a preferred experimental example is presented to aid the 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.

1. Preparation of specimen

Prepare a cold rolled steel sheet with a thickness of 0.7mm. The cold rolled steel sheet is carbon (C):0.07% by weight, manganese (Mn):0.3% by weight, phosphorus (P):0.05% by weight, sulfur (S):0.005% by weight, chromium (Cr):0.2% by weight, molybdenum (Mo): 0.05% by weight and the balance has a composition of iron (Fe). However, the composition of the cold-rolled steel sheet is exemplary, and is not necessarily required to implement the technical idea of the present invention. The cold-rolled steel sheet is immersed in an alkaline solution at 50° C. for 30 minutes, and then washed with water to prepare a specimen from which foreign substances and oil are removed from the surface.

Subsequently, the specimen is annealed and then plated. Annealing is carried out in a reducing atmosphere consisting of hydrogen: 10 ~ 30%, nitrogen: 70 ~ 90%, annealing heat treatment temperature is 700 ~ 850 ℃. For plating, the annealing heat-treated specimen was cooled to a plating bath temperature (400 to 520°C), immersed in the plating bath for 2 seconds, and then pulled up to adjust the plating thickness to around 10 μm by nitrogen wiping. The amount of plating was adjusted to a level of 20 to 300 g/m ^{2 of plating on one side.} 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. Measurement of plating surface alkali activity and evaluation of post-treatment corrosion resistance

A plated steel sheet was manufactured under the above conditions, and the alkali activity of the plating surface by water quenching condition was measured and the post-treatment corrosion resistance was evaluated, and the results are shown in Table 1.

division Zn Al Mg Water quenching temperature (℃) Water quenching time (sec) Plating surface before post-treatment
Alkalinity (pH) Exterior Corrosion resistance after treatment Comparative Example 1 Bal. One One 30 10 11 ◎ X Comparative Example 2 Bal. 1.7 1.5 50 20 10 ○ △ Comparative Example 3 Bal. 2 1.5 80 2 10 ○ △ Comparative Example 4 Bal. 5 3 100 70 8 △ ◎ Example 1 Bal. One One 80 5 8 ◎ ○ Example 2 Bal 1.7 1.5 90 20 8 ○ ◎ Example 3 Bal. 2 1.5 80 10 8 ○ ◎ Example 4 Bal. 5 3 100 20 8 ○ ◎

The plating layer formed on the plated steel sheets according to Comparative Examples 1 and 1 consisted of aluminum (Al): 1% by weight, magnesium (Mg): 1% by weight, and the balance zinc (Zn), and Comparative Examples 2 and 2 The plating layer formed on the plated steel sheet according to the above is made of aluminum (Al): 1.7% by weight, magnesium (Mg): 1.5% by weight, and the balance is zinc (Zn), and the plating layer formed on the plated steel sheet according to Comparative Examples 3 and 3 Silver aluminum (Al): 2% by weight, magnesium (Mg): 1.5% by weight, and the balance consists of zinc (Zn), and the plating layer formed on the plated steel sheets according to Comparative Examples 4 and 4 is aluminum (Al): 5 % By weight, magnesium (Mg): 3% by weight and the balance consists of zinc (Zn). On the other hand, in cooling the steel sheet on which the plated layer was formed by a water quenching process, Comparative Example 1 applied the condition of immersion in water at 30°C for 10 seconds, and Comparative Example 2 was applied in water at 50°C for 20 seconds. The conditions of immersion were applied, and in Comparative Example 3, the conditions of immersion in water at 80°C for 2 seconds were applied, and in Comparative Example 4, the conditions of immersion in water at 100°C for 70 seconds were applied. Comparative Example 4 did not apply the condition of water quenching by immersing the steel sheet on which the plated layer was formed in water at 60 to 100° C. for 5 to 60 seconds.

In contrast, in Example 1, the condition of immersion in water at 80°C for 5 seconds was applied, in Example 2, the condition of immersion in water at 90°C for 20 seconds was applied, and in Example 3, in water at 80°C. The condition of immersion for 10 seconds was applied, and in Example 4, the condition of immersion in water at 100° C. for 20 seconds was applied. In Examples 1 to 4, the steel sheet on which the plated layer was formed was applied in water at 60 to 100°C. The condition of water quenching was applied by immersion for 5 to 60 seconds.

To measure the alkalinity of the plating surface, the pH paper was placed on the plated steel sheet prepared for each water quenching condition, and 1 mg of water was dropped on the pH paper to observe the color changing in the pH paper over time to observe the alkali activity.

The appearance items in Table 1 represent the results of observation of the plating surface color through visual observation, and the'◎' item indicates that there is no difference in color of the plating surface before and after, and the'○' item indicates that the surface color before and after the plating changes slightly, and'△' The item indicates that the color difference of the surface of the plating before and after is severe.

In addition, the post-treatment corrosion resistance item in Table 1 is that the salt spray test was conducted for 72 hours in accordance with JIS Z 2371, and the corrosion resistance was evaluated from the white rust generation area over the entire area of the test piece. %, the'○' item indicates that the white rust generation area is less than 5%, the'△' item indicates that the white rust generation area is less than 10%, and the'X' item indicates that the white rust generation area is more than 10%.

Looking at the results of Table 1, in Comparative Example 4, the appearance of the front and rear plating surface color difference was severe, and in Comparative Example 1, the white rust generation area was 10% or more, indicating poor corrosion resistance, and Comparative Examples 2 and 3 In, it can be seen that the appearance of finely changing the surface color of the plating before and after, and the white rust generation area is 5 to 10%, indicating that the corrosion resistance is not good. On the other hand, in Examples 1 to 4, it can be confirmed that the alkalinity of the plating surface before the post-treatment is relatively low, the appearance is good, and the post-treatment corrosion resistance is also excellent.

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

2, while the ratio of the ternary process phase (A) including magnesium (Mg) hydroxide is relatively low, referring to FIG. 3, the ratio of the ternary process phase (B) including magnesium (Mg) hydroxide is It can be understood that it is relatively high. In the three-way process including magnesium (Mg) hydroxide, the alkalinity of the plating surface is lowered and the diffusion of magnesium inside the plating layer to the surface can be prevented, thereby improving corrosion resistance.

In the existing application to the high corrosion resistance plated steel sheet market, it is widely used because of its excellent corrosion resistance in the construction material market, but the surface alkalinity in the corrosive environment increases due to magnesium (Mg) present in the plating layer, and the post-treatment performance decreases during post-treatment. It has a problem. In the present invention, in order to overcome these technical limitations, the surface of the plated surface is passed through the steel plate by maintaining the water temperature of the water quenching tank at 60 to 100°C during the plating process for magnesium (Mg) present in the surface layer of the plating layer. By reducing the activated magnesium (Mg) content, it is possible to reduce the alkalinity of the plating surface. Accordingly, it is possible to improve a phenomenon in which the post-treatment coating is inferior due to the increase in alkali activation of the plating surface in a corrosive environment after post-treatment.

In the above, the embodiments of the present invention have been mainly described, but various changes or modifications may 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 the present invention should be determined by the claims set forth below.

Claims (7)

delete delete delete (a) providing a cold rolled steel sheet;
(b) annealing the cold-rolled steel sheet;
(c) The annealed steel sheet is passed through a plating bath containing aluminum (Al), magnesium (Mg) and zinc (Zn), and aluminum (Al) on the steel sheet: 1 to 2% by weight, magnesium (Mg) ): forming a plating layer made of 1 to 1.5% by weight and the balance of zinc (Zn) and other unavoidable impurities; And
(d) quenching the steel sheet on which the plating layer is formed to room temperature at a cooling rate of 5 to 30° C./sec; including,
After passing through the plating bath in step (c), the cold-rolled steel sheet was subjected to nitrogen wiping to adjust the plating thickness to 10 µm,
In the step of (d), the quenching of the plating layer includes the step of water quenching by immersing the steel sheet on which the plating layer is formed in water at 80 to 90° C. for 5 to 20 seconds, and the alkalinity of the plating surface is 8. Characterized in that,
Manufacturing method of plated steel sheet. The method of claim 4,
In the step (b), the annealing treatment is performed at a temperature of 700 to 850°C,
In the step (c), characterized in that the temperature of the plating bath is 400 ~ 520 ℃,
Manufacturing method of plated steel sheet. delete The method of claim 4,
The step (d) comprises the step of generating a hydroxide on the surface of the plating layer,
Manufacturing method of plated steel sheet.

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