KR100544646B1 - Surface Treated Steel Sheet Having Excellent Corrosion Resistance And Manufacturing Method Thereof - Google Patents

Abstract

A galvanized steel sheet having excellent corrosion resistance and a method of manufacturing the same, the magnesium plated layer and the galvanized layer is formed in multiple layers on the steel sheet, the outermost layer is a galvanized layer of at least 2g / ㎡ or more, the entire plating layer is excellent in corrosion resistance of 5g / ㎡ or more Provided is a method for producing a surface-treated steel sheet which is galvanized in a surface-treated steel sheet and a zinc plating solution having a minimum pH of 4 or more to form a zinc plating layer on a magnesium plating layer, and a magnesium plating layer is formed by a cathodic electrolytic method in an aqueous plating solution. The surface-treated steel sheet of the present invention prevents the loss of magnesium in the phosphate solution during phosphate treatment, and the color of the steel sheet is almost the same as that of galvanized steel sheet. Can be.

Magnesium Plating Layer, Zinc Plating Layer, Corrosion Resistance

Description

Surface Treated Steel Sheet Having Excellent Corrosion Resistance And Manufacturing Method Thereof}

The present invention relates to a galvanized steel sheet excellent in corrosion resistance and a method of manufacturing the same, and more particularly to a surface-treated steel sheet having a magnesium plating layer and a galvanized layer in multiple layers and excellent surface treatment steel sheet and a method of manufacturing the same.

Galvanized steel sheet among rust-proof surface treated steel sheets is widely used for automobiles, home appliances and building materials because of its excellent corrosion resistance. However, in recent years, a new plated steel sheet is required to secure high corrosion resistance with thin plating in terms of energy and resource saving. have.

  In response to these demands, zinc-iron and zinc-nickel alloy electroplated steel sheets have been developed and put into practice, and recently, zinc-chromium alloy plated steel sheets have been developed. However, the zinc-iron alloy plated steel sheet contains iron in the plating layer, but when the steel plate is exposed to the corrosion atmosphere, the plating layer protects the steel plate by sacrificial anticorrosive action, but as the plating layer is dissolved, the iron in the plating layer is oxidized and reddish. The use of zinc-iron alloy plated steel sheet is often avoided because customers who use the final product make the steel plate rust because it produces corrosion products. In addition, when manufacturing the zinc-iron alloy plated steel sheet, since the first iron ions in the plating solution is oxidized to ferric ions to produce sludge, plating workability is bad.

Zinc-nickel alloy plated steel sheet is widely used as a automotive material because of its excellent corrosion resistance. However, since nickel causes an allergy reaction to the human body, the use of nickel plated products is prohibited in Europe. It is spreading all over the world. In addition, since 10% or more of alloying expensive nickel in the plating layer is disadvantageous economically disadvantage.

  In addition, the surface treated steel sheet used for home appliance case is important to the surface appearance, so the surface color changes even with the slight change of alloy content. Therefore, it is difficult to use the alloy coated steel sheet. The improved steel sheet is mainly used. However, since chromium is harmful to the environment, it is recently required to develop a steel sheet having excellent corrosion resistance without using chromium.

 Surface treatment steel sheet developed for the purpose of securing high corrosion resistance while being less harmful to the human body, as disclosed in Korean Patent Publication No. 2000-62855, includes Zn-Mg alloy electroplating steel sheet. If Mg is alloyed on the Zn plating layer, the corrosion resistance is greatly increased. Therefore, Zn-Mg alloy plated steel sheet produced by the hot-dip plating method has been put into practical use.

However, precipitation of Mg is very difficult by electroplating method because the standard electrode potential of Mg is very low in aqueous solution. Therefore, in the case of the patent, Zn-Mg alloy plating is performed by further adding a nonionic or cationic surfactant to the plating solution. However, when surfactant is added to the plating solution, it is physically adsorbed to the negative electrode during the plating process, which prevents the electrodeposition of metallic materials. Therefore, plating is not performed at low current density. Therefore, the patent also limits the plating current density to 100 ~ 800A / dm ² . In the continuous plating process in which the steel plate is plated while passing continuously through the plating bath, it is difficult to conduct a large current to the steel plate because of high contact resistance between the current roll and the steel plate, and is usually plated at a current density of 30 to 100 A / dm ² , It is difficult to apply it in continuous plating equipment because an arc may be generated between the conductive roll and the steel sheet if more current is applied.

Accordingly, an object of the present invention is to provide a galvanized steel sheet excellent in corrosion resistance which can be plated at a low current density and does not lose magnesium in the plated layer by post treatment.

Another object of the present invention is to provide a method for producing a galvanized steel sheet which is capable of plating at a low current density and which does not lose magnesium in the plating layer by post treatment.

According to the first aspect of the present invention,

A magnesium plated layer and a galvanized layer are formed on the steel sheet, and the outermost layer is a galvanized layer of at least 2 g / m 2 and the entire plated layer is provided at 5 g / m 2 or more.

According to the second aspect of the present invention,

Provided is a method for producing a surface-treated steel sheet according to the present invention, characterized by forming a zinc plating layer on a magnesium plating layer by galvanizing in a zinc plating solution having a minimum pH of 4 or more.

According to the third aspect of the present invention,

Provided is a method for producing a surface-treated steel sheet according to the present invention, characterized in that a magnesium plated layer is formed by a cathodic electrolysis method in a water-soluble plating solution.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In a salt water corrosive environment, the galvanized steel sheet is slowly corroded from the surface layer so that the surface layer is covered with zinc hydroxide, a corrosion product of zinc. However, since zinc hydroxide is porous, water or salt easily penetrates into the interior, and corrosion of the plating layer proceeds relatively quickly. In the case of magnesium, magnesium hydroxide forms a membrane in the form of a dense gel to prevent the penetration of moisture or salt into the inside. However, since Mg hydroxide is easily dissolved in colloidal form by a large amount of water, the Mg hydroxide must be thick in order to continuously protect the steel sheet.

However, if a zinc plating layer is present on the magnesium oxide layer, the zinc plating layer preferentially corrodes to form a porous zinc hydroxide, and the magnesium oxide in the lower layer is dissolved and gelated by water and salt penetrated through the zinc hydroxide to form pores of the porous zinc hydroxide. It prevents the ingress of moisture and salt to prevent corrosion.

In addition, since the zinc hydroxide in the outermost layer inhibits the dissolution of magnesium hydroxide, the corrosion barrier effect caused by magnesium oxide lasts for a long time, thereby greatly improving corrosion resistance. Therefore, the galvanized layer and the Mg plated layer exist in two or more layers, and the presence of the galvanized layer in the outermost layer can most effectively block the penetration of the corrosive material into the plated layer, thereby showing excellent corrosion resistance.

That is, when Mg is present only in the surface layer of the plated steel sheet, Mg of the surface layer is dissolved in most or a substantial portion of the acidic phosphate solution during post-treatment such as phosphate treatment. In the case of phosphate treatment on a galvanized steel sheet, zinc phosphate coating is formed when the zinc plated layer of the surface layer is dissolved about 1 to 3 g / m ² when phosphate is treated in solution.

Therefore, when the zinc plating layer having at least 2 g / m ² is formed as the outermost layer on the Mg plating layer, a zinc phosphate film is formed while the zinc plating layer existing in the outermost layer is dissolved in the phosphate treatment so that the existing Mg plating layer is not damaged. It is excellent in corrosion resistance even in phosphated steel. In the outermost layer, there is no problem that corrosion resistance decreases even if a part of the plating layer is dissolved by post-treatment treatment such as phosphate treatment, which is mandatory in automobile companies, when the zinc plating layer is present at least 2g / m ² or more. Can be obtained.

The galvanizing method is not particularly limited, but the pH of the plating solution for galvanizing on the Mg plating layer should be at least 4 or higher. If the pH of the zinc plating solution is lower than 4, a problem occurs that some or all of the first plated magnesium plating layer is dissolved in an acidic zinc plating solution.

Mg plating layer to the zinc plated layer and the Mg plating layer makes up a layer of the steel sheet consisting of a multi-layer is in the form of magnesium oxide, magnesium hydroxide or a mixture thereof, in terms of the metal magnesium is less than 0.01g / m ² the corrosion resistance improving effect is slight and the adhesion amount Corrosion resistance increases with increasing, but if it exceeds 0.7g / m ² , the adhesion strength with the zinc plating layer decreases and the plating layer is peeled off. Therefore, the adhesion amount of magnesium oxide electrodeposited on the Mg plating layer is 0.01-0.7 in terms of metal magnesium. It is preferable to limit to g / m ² .

In the galvanized layer and the Mg plated layer exist in multiple layers, the galvanized layer and the Mg plated layer must exist alternately, and the outermost layer must have a galvanized layer, but the number of layers does not need to be particularly limited. When the total amount of zinc and magnesium plated on the surface is 5g / m ² or more, proper corrosion resistance can be secured as a surface-treated steel sheet.

Furthermore, a modified zinc phosphate system in which one, two or more kinds of Fe, Ni, Mn, Co, Cu, Mg, or Mo are added to the zinc phosphate chemical conversion coating or the zinc phosphate chemical conversion coating on the multilayer plated steel sheet of the present invention. The chemical conversion treatment film is formed in 0.1 ~ 3g / m ² can be used as the uncoated steel sheet of the automobile company. These chemical conversion treatment film formation methods need not be particularly limited and may be carried out by the same method as the conventional chemical conversion treatment method for zinc and zinc-based alloy plated steel sheets.

In the present invention, the method of forming the zinc and magnesium plating layers in multiple layers is performed by sequentially plating the steel plate through a plurality of plating baths containing zinc plating solution and Mg plating solution, respectively. Plating degree can be either horizontal or vertical.

The method for electrodepositing the Mg oxide described in the present invention is basically based on the cathodic electrolytic method in aqueous solution. The supply of Mg in the plating solution can be in any form as long as the pH of the solution can exist in the form of magnesium ions or colloids, but in the form of magnesium acetate, magnesium sulfate, magnesium hydrochloride, and magnesium nitrate, which are relatively solubilized in aqueous solutions. Since the solution is easy to prepare, these are used alone or in combination.

If the concentration of Mg ion in the solution is less than 10g / l, it will take a long time to obtain the target amount of adhesion. Therefore, the productivity decreases, and there is no problem even if it exceeds 100g / l. The concentration of ions is preferably limited to 10 ~ 100g / l. Electrolysis temperature is possible at room temperature (about 20-25 ℃), and as the temperature increases, the amount of oxide adhesion slightly increases, but if it exceeds 60 ℃, the concentration of water is easily changed due to the increased evaporation of water, making it difficult to manage the concentration. If the pH of the Mg plating solution is less than 3, it is not economical because some of the electrodeposited oxides are re-dissolved and inferior in adhesion.If the pH is above 10, a considerable amount of Mg ions in the plating solution precipitates as hydroxide, The pH is preferably limited to 3 to 10.

In Mg plating, the plating current density is related to the deposition rate of the plating material. If the current density is low, the plating reaction takes a long time, and the productivity decreases. If the current density is too high, the plating layer becomes rough, resulting in poor adhesion to the zinc plating layer. It is preferable to limit to 100A / dm ² .

  Hereinafter, the present invention will be described in more detail with reference to Examples.

 Example 1

In a typical cold rolled steel sheet having a thickness of 0.8 mm, zinc plating was performed by sulfate-based zinc plating.

Zinc Plating Conditions

ZnSO ₄ : 200g / l, Na ₂ SO ₄ : 50g / l

Current density: 60A / dm ²

Plating solution temperature: 60 ℃

pH: 5

Flow rate: 1.5m / sec

Electrode: IrO ₂

Magnesium plating was carried out under the composition and plating conditions as shown in Table 1 below, and then quantitatively analyzed the metal components attached thereto. In addition, in order to analyze the film of the Mg plated layer, the X-ray diffraction analysis of the plated layer without performing zinc plating of the outermost layer confirmed that magnesium is composed of metal oxide and hydroxide. Some of the plated steel sheets were subjected to zinc phosphate chemical conversion, and their weights were measured before and after attachment.

Table 2 shows the quality evaluation of the plated steel sheet and the chemically treated steel sheet. Corrosion resistance evaluation was performed by performing alkali degreasing of the specimen and performing a salt spray test to measure the time until the red blue color of the steel sheet was 5%.

Plating adhesion evaluation was evaluated as the degree of plating material on the tape when the tape was peeled off by 180 ° bend after attaching the vinyl tape to the steel sheet is completed, the whiteness of the steel sheet was used to measure the whiteness.

TABLE 1

Number of plated layers; 1 layer: Mg plating-> Zn plating

               1.5 layer: Zn plating-> Mg plating-> Zn plating

               2 layer: Mg plating-> Zn plating-> Mg plating-> Zn plating

               2.5 layer: Zn plating-> Mg plating-> Zn plating-> Mg plating-> Zn plating

TABLE 2

* Plating Adhesion: ◎ Excellent, × Poor

* Whiteness difference = whiteness of pure galvanized steel sheet (whiteness of Comparative Example 1)

              -Whiteness of the plated steel sheet

As shown in Tables 1 and 2, in the case of Inventive Examples (1 to 11) according to the present invention, after performing Mg plating and zinc plating on the surface of the steel sheet in multiple layers, the results of the quality evaluation showed that the corrosion resistance time was 70 hours. It was relatively long, the plating adhesion was excellent, and there was little white difference. On the other hand, Examples 12 and 13 of the present invention were the case where the phosphate treatment was applied to the multilayer coated steel sheet as a post-treatment, which was excellent in corrosion resistance and plating adhesion even after the post-treatment.

On the other hand, in the case of Comparative Example 1 was evaluated as the quality of the galvanized steel sheet as it was, the red-blue occurrence time was extremely bad (20 hours), but the plating adhesion was excellent.

In the case of Comparative Example 2, when the phosphate treatment was performed on the galvanized steel sheet, the red blue development time was as short as 22 hours.

Comparative Example 3 is a case where the pH of the Mg plating bath is 2, which is lower than the range limited in the present invention, and the Mg attached to the steel sheet is dissolved in the plating solution so that the amount of Mg attached to the steel sheet is lower than the range defined in the present invention. The red blue development time was very short (17 hours). In the case of Comparative Example 4, even if the Mg deposition amount is within the range defined in the present invention, the current density in the Mg plating exceeds the range defined in the present invention, but the corrosion resistance is excellent, but the plating layer is rough, so that the adhesion to the zinc plated layer is reduced. As a result, the plating adhesion was inferior.

On the other hand, in the case of Comparative Example 5, the plating conditions satisfy the scope of the present invention, but the Mg deposition amount of the Mg plated layer was less than the range defined in the present invention, and the effect of improving corrosion resistance was small. In Comparative Example 6, when the Mg deposition amount per layer of the Mg plating layer exceeded the range defined in the present invention, the corrosion resistance was extremely excellent, but the plating adhesion was poor.

In Comparative Example 7, the Mg plating conditions, the number of plating layers and the Mg deposition amount per layer of the Mg plating layer satisfy the range defined in the present invention, but the zinc plating deposition of the outermost layer is lower than the range defined in the present invention. It is relatively large at 5.5 and is difficult to use for steel plate. In the case of Comparative Example 8, the pH of the Mg plating solution exceeded the range defined in the present invention. Since Mg forms a hydroxide in the plating solution and precipitates, Mg adhesion is difficult and the amount of Mg deposition per plated layer is limited in the present invention. Lower than the range, the effect of improving the corrosion resistance was weak.

According to the present invention, a galvanized layer and a magnesium plated layer are formed on the surface of the steel sheet in multiple layers, but the galvanized layer is present at least 2 g / m ² or more in the outermost layer to prevent the loss of magnesium in the phosphate solution during phosphate treatment and also the color of the steel sheet. There is little difference between the galvanized steel sheet and can produce economically treated surface treated steel sheet that can be used as a steel plate.

The Mg plated layer and the Zn plated layer of the present invention are formed in multiple layers, and the outermost layer is a surface-treated steel sheet made of a zinc plated layer of 2 g / m ² , even after performing a post-treatment such as phosphate treatment. This is more excellent, and the color of steel sheet is the same as galvanized steel sheet, so it can be used for home appliances where color is important.

In addition, when used as a case of home appliances where the design is important, many are used in the non-painted steel plate coated by coating a transparent anti-fingerprint resin on the galvanized steel sheet. Therefore, the color of the anti-fingerprint steel sheet is required to be the same color as the galvanized steel sheet. If Mg plating is performed on the outermost layer, the color of the steel sheet is dark and gloss, unlike the galvanized steel sheet, and thus it cannot be used for the production of the anti-fingerprint steel sheet used as the case of home appliances. However, when the outermost layer is galvanized at least about ² g / m ² in multiple layers, the color of the steel sheet is the same as that of the galvanized steel sheet.

Claims (8)

A magnesium plated layer and a zinc plated layer are formed on the steel plate, and the outermost layer is a zinc plated layer of at least 2 g / m 2 and the entire plated layer is 5 g / m 2 or more. The surface-treated steel sheet according to claim 1, wherein the magnesium plating layer is present in the form of magnesium oxide, magnesium hydroxide or a mixture thereof. The surface-treated steel sheet according to claim 1, wherein the magnesium plating layer has a magnesium plating amount of 0.01 to 0.7 g / m ² in terms of metal Mg. The surface-treated steel sheet according to claim 1, wherein the magnesium plating layer and the zinc plating layer formed of the multilayer layer are alternately present, and the outermost layer is a zinc plating layer. According to claim 1, Further, one or two or more of Fe, Ni, Mn, Co, Cu, Mg or Mo is added to the zinc phosphate chemical conversion coating or zinc phosphate chemical conversion coating on the surface-treated steel sheet Surface-treated steel sheet, characterized in that the modified zinc phosphate chemical conversion coating is formed in 0.1 ~ 3g / m ² . The method for producing a surface-treated steel sheet according to any one of claims 1 to 5, wherein a zinc plating layer is formed on the magnesium plating layer by zinc plating in a zinc plating solution having a minimum pH of 4 or more. The method for producing a surface-treated steel sheet according to any one of claims 1 to 5, wherein a magnesium plated layer is formed by a cathode electrolytic method in a water-soluble plating solution. 8. The plating solution according to claim 7, wherein Mg in the plating solution is added alone or mixed in the form of magnesium acetate, magnesium sulfate, magnesium hydrochloride, magnesium nitrate, and Mg ion in a range of 10 to 100 g / l, at room temperature to 60 ° C, pH. Method for producing a surface-treated steel sheet, characterized in that to form a magnesium plated layer under the conditions of 3 ~ 10, current density of 5 ~ 100A / ㎡