KR20180069371A - A zinc plated steel sheet having corrosion-resistant processed part, and manufacturing method thereof - Google Patents

Abstract

A steel sheet having excellent corrosion resistance of a processed part and a manufacturing method thereof are provided. The steel sheet of the present invention comprises: a basis material steel plate; a Zn-Mg-Al-based alloy plating layer formed on one side or both sides of the basis material steel sheet; a lower coating film layer comprising 55 to 75 wt% of a polyester resin on the Zn-Mg-Al-based alloy plating layer; and an upper coating film layer formed on the lower coating film layer, wherein the polyester resin constituting the lower coating film layer comprises 25 to 35 wt% of the same having the molecular weight (Mw) in the range of 25,000 to 30,000 and 30 to 40 wt% of the same having the molecular weight (Mw) in the range of 20,000 to 25,000.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coated steel sheet having excellent corrosion resistance,

The present invention relates to a color coated steel sheet using a Zn-Mg-Al based high corrosion resistant coated steel sheet as a raw material, and to a coated steel sheet having improved processability and cross-sectional corrosion resistance, and a method for manufacturing the same.

Generally, galvanized steel sheets are widely used as a base material for coated steel sheets. The zinc-plated steel sheet is made of zinc-plated steel sheet. When the galvanized steel sheet is used as the base of the coated steel sheet, the zinc-plated steel sheet is excellent in corrosion resistance and surface appearance.

However, the coated steel sheet using the galvanized steel sheet as such has a problem of corrosion resistance because the coated layer and the plated layer protect the base steel in the normal state. However, the cut surface of the coated steel sheet is exposed to the outside due to processing or breakage There is a problem in that a fundamental method is not guaranteed only by the function of the sacrificial method of zinc because the iron as the base material is directly exposed to the outside.

Recently, the increase of the air pollution and the corrosion environment due to the advancement of the industry are getting worse, and the need for resource and energy saving is increasing. As a part of this, zinc alloy plating products which improve the corrosion resistance of steel by adding elements such as Al and Mg to the zinc plating bath are being developed. An alloying element which is added for corrosion resistance Mg reacts with Zn thereby forming a Zn-Mg-based compound (MgZn ₂₎ a coating layer inside and the surface, thus formed MgZn ₂ Since the hardness, toughness inferior to high, whereas when bending Thereby causing a crack in the plating layer. Particularly, in the case of a coated steel sheet in which a zinc alloy plating material is used as a base, a crack in the bending portion formed from the inside of the alloy plating layer and from the surface is propagated to the color coating layer at the upper portion, and white rust is formed in the crack portion.

Korean Patent Publication No. 2015-0114086 (published October 12, 2015)

DISCLOSURE Technical Problem Accordingly, the present invention has been made to overcome the above-mentioned problems of the prior art, and it is an object of the present invention to provide a coated steel sheet excellent in corrosion resistance at a machined portion by minimizing occurrence of cracks in a coated steel sheet using a Zn- And the like.

Further, the technical problems to be solved by the present invention are not limited to the technical problems mentioned above, and other technical problems which are not mentioned can be understood from the following description in order to clearly understand those skilled in the art to which the present invention belongs .

According to an aspect of the present invention,

Base steel sheet;

A Zn-Mg-Al-based alloy plating layer formed on one side or both sides of the base steel sheet;

An undercoating film layer comprising 55 to 75% by weight of a polyester resin on the Zn-Mg-Al-based alloy plating layer; And

And an upper coating film layer formed on the lower coating film layer,

Wherein the polyester resin constituting the undercoat layer has a molecular weight Mw of 25,000 to 30,000 in an amount of 25 to 35% by weight and a molecular weight Mw of 20,000 to 25,000 in an amount of 30 to 40% by weight. The present invention relates to an excellent coated steel sheet.

Further, according to the present invention,

Preparing a Zn-Mg-Al alloy-plated steel sheet on which a Zn-Mg-Al alloy plating layer is formed on one side or both sides of a base steel sheet;

The undercoating composition containing 55 to 75% by weight of polyester resin is applied to one side of the Zn-Mg-Al-based alloy coated steel sheet and then dried under conditions of PMT (Peak Metal Temperature) of 160 to 280 ° C, Forming a layer; And

Coating a top coat composition on one side of a Zn-Mg-Al alloy-plated steel sheet having the undercoating layer formed thereon, and drying the coated composition at a temperature of 160 to 280 ° C to form a top coat layer; Lt; / RTI >

Wherein the polyester resin constituting the undercoat layer has a molecular weight Mw of 25,000 to 30,000 in an amount of 25 to 35% by weight and a molecular weight Mw of 20,000 to 25,000 in an amount of 30 to 40% by weight. And a method of manufacturing a coated steel sheet.

The present invention according to the above-described constitution has an advantage that the corrosion resistance of the processed portion is very good in the painted steel sheet, and the occurrence of cracks in the processed portion is minimized even in a region where the processing is severe,

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a photograph showing the degree of occurrence of a machined portion of crack in Inventive Example 4 in the embodiment of the present invention. Fig.
Fig. 2 is a photograph showing the degree of occurrence of the crushed part of the comparative example 1 in the embodiment of the present invention. Fig.

Hereinafter, the present invention will be described in detail.

The coated steel sheet excellent in corrosion resistance of the processed portion of the present invention comprises: a base steel sheet; A Zn-Mg-Al-based alloy plating layer; Undercoating layer; And an upper coating layer.

In the present invention, the kind of the above-mentioned base steel sheet is not particularly limited. It may be a hot-rolled steel sheet or a cold-rolled steel sheet used as a base of an ordinary coated steel sheet. However, in the case of a hot-rolled steel sheet, a large amount of oxide scale is present on its surface. Such an oxide scale has a problem of deteriorating the plating adhesion and deteriorating the quality of the plating, so it is desirable to have a hot-rolled steel sheet with the oxide scale removed.

The Zn-Mg-Al based alloy plating layer may be formed on one side or both sides of the base steel sheet. The alloy plating layer according to an embodiment of the present invention may contain 0.5 to 10% of Mg, 0.5 to 10% %, The remainder Zn and unavoidable impurities.

The Zn-Mg-Al based alloy plating layer of the present invention contains Mg in a range of 0.5 to 10 wt%. The Mg is for imparting corrosion resistance to the coated steel sheet together with Al. Mg is a metal having a higher ionization tendency than Zn. It is ionized earlier than Zn in the corrosive environment to release electrons, and the released electrons are continuously supplied to Zn and Fe to prevent Zn and Fe ionization, . If the content of Mg is less than 0.5% by weight, the effect of improving the corrosion resistance is not sufficient. If it exceeds 10% by weight, the occurrence of dross in the plating bath increases due to the oxidizing property of magnesium, May be increased to cause peeling and cracking of the plating layer.

The Al also plays an important role in improving the corrosion resistance of the coated steel sheet. Moreover, Al also helps to prevent plating bath dross from being oxidized by magnesium. When the concentration of Al is less than 0.5% by weight, the corrosion resistance and the effect of suppressing the generation of dross are not sufficient. When the concentration of Al is more than 10% by weight, the workability decreases due to the formation of intermetallic compounds reacted with iron (Fe) And a problem that iron is eluted from the structure in the plating bath.

The rest of the composition is Zn. Impurities that are not intended from the raw material or the surrounding environment may be inevitably incorporated in a conventional manufacturing process, so that this can not be excluded. These impurities are not specifically referred to in this specification, as they are known to one of ordinary skill in the art.

According to an embodiment of the present invention, the coating amount of the Zn-Mg-Al based alloy plating layer may be 20 to 200 g / m 2. If the amount of plating is less than 20 g / m 2, it may be difficult to secure sufficient corrosion resistance. On the other hand, if the amount exceeds 200 g / m 2, plating layer peeling and cracking may occur.

The undercoating layer is formed on the Zn-Mg-Al-based alloy plating layer, and provides adhesion to the overcoat layer, which will be described later, and improves the corrosion resistance and workability of the coated steel sheet. According to an embodiment of the present invention, the undercoat layer may include polyester resin as a main component and may be included in the range of 55 to 75% by weight based on the total weight of the undercoat layer.

However, in one embodiment of the present invention, the polyester resin forming the undercoat layer has a molecular weight of 20,000 to 30,000 (Mw If the molecular weight is less than 20,000 (Mw), the quality such as workability and adhesion may be deteriorated, while if it exceeds 30,000 (Mw), the cost may increase to be.

At this time, in the present invention, it is preferable that the mixing ratio is different according to the molecular weight of the polyester resin. Specifically, in the present invention, the polyester resin constituting the undercoat layer has a molecular weight in the range of 25,000 to 30,000 (Mw) in the range of 5 to 35% by weight based on the total weight of the coat layer and a molecular weight in the range of 20,000 to 25,000 (Mw) 30 to 40% by weight is desirable.

If the molecular weight is 25,000 to 30,000 (Mw) is less than 10%, the processability of the plating layer may be deteriorated. If the molecular weight is more than 35%, the paint cost may increase and the surface hardness may be lowered. If the molecular weight is 20,000 to 25,000 (Mw) less than 30%, there is a fear that the adhesion between the material and the interlayer may be deteriorated. If the molecular weight is more than 40%, there is a problem that the cost of the coating material is increased without improving the physical properties required for the paint properties.

In the present invention, it is possible to simultaneously improve the coating film adhesion and the workability by controlling the content ratio of such a polyester resin by the molecular weight

The remainder of the resin may be a pigment and other additives.

In the present invention, the thickness of the undercoat layer is preferably in the range of 2.0 to 20.0 mu m. If the thickness is less than 2.0 탆, the corrosion resistance of the coated steel sheet may be deteriorated. On the other hand, if the thickness exceeds 20.0 탆, the processability and adhesion of the coated steel sheet may be deteriorated.

On the other hand, the top coat layer is formed on the undercoat layer, realizes a desired color, and imparts functionalities such as chemical resistance, corrosion resistance, moisture resistance and weather resistance to the coated steel sheet.

According to one embodiment of the present invention, the top coat layer may contain 20 to 70 wt% of at least one selected from the group consisting of polyester resin, urethane resin, acrylic resin, epoxy resin and fluororesin. The remainder of the resin may be a pigment and other additives. In the present invention, the kind of the additive is not particularly limited, and for example, it may be a defoaming agent, a smoothing agent, and a quencher.

According to an embodiment of the present invention, the thickness of the top coat layer may be 2.0 to 80.0 mu m. If the thickness is less than 2.0 占 퐉, there is a concern that the water resistance such as chemical resistance, corrosion resistance, moisture resistance, weather resistance, chemical resistance and weather resistance may be lowered, while if it exceeds 80.0 占 퐉, the workability of the coated steel sheet may decrease.

According to an embodiment of the present invention, a clear coating layer may be further formed on the top coat layer. The clear coating layer may contain 20.0 to 70.0% by weight of at least one selected from the group consisting of an acrylic resin, a melamine resin, a polyester resin and a fluororesin. The remainder of the resin may be a pigment and other additives. The clear coating layer can provide the color and gloss of the final product and protect the appearance of the final product from external environment such as chemical and mechanical friction.

The thickness of the clear coating film layer may be 1.0 to 20.0 mu m. If the thickness is less than 1.0 탆, the function of protecting the appearance of the coated steel sheet may deteriorate, while if it exceeds 20.0 탆, the gloss of the coated steel sheet may be affected.

Hereinafter, a method for producing a coated steel sheet having excellent corrosion resistance at a machining portion of the present invention will be described.

First, in the present invention, a Zn-Mg-Al alloy-plated steel sheet in which a Zn-Mg-Al alloy plating layer is formed on one side or both sides of a base steel sheet is prepared.

In the present invention, the step of preparing the Zn-Mg-Al-based alloy coated steel sheet is not particularly limited, and can be prepared by conventional hot-dip coating or electroplating.

Next, in the present invention, a polyester-based undercoating composition is applied to one side of the Zn-Mg-Al-based alloy coated steel sheet, and then the PMT (Peak Metal Temperature) is dried at a temperature of 160 to 280 ° C, .

If the PMT is lower than 160 ° C, the undercoat layer may not be completely dried, and the appearance of the product may be adversely affected. If the temperature is higher than 280 ° C, the resin component of the undercoating layer may be deformed, thereby causing appearance defects of the product.

In the present invention, the top coat composition is coated on one side of the Zn-Mg-Al alloy coated steel sheet having the undercoat layer formed thereon, and then the PMT (Peak Metal Temperature) is dried at 160 to 280 ° C to form an upper coat layer do.

If the PMT is lower than 160 캜, the top coat layer may not be completely dried and adversely affect the appearance of the product. If it exceeds 280 ° C, the resin component of the top coat layer may be deformed, which may cause appearance defects of the product.

According to an embodiment of the present invention, after forming the upper coat layer, a clear paint is applied on the upper coat layer, and then the PMT (Peak Metal Temperature) is dried at 160 to 280 ° C to form a clear coat layer You may.

If the PMT is lower than 160 ° C, the clear coating film layer is not completely dried, and the appearance of the product may be adversely affected. If it exceeds 280 ° C, the clearness may cause deformation of the resin component of the coating film layer, which may cause appearance defects of the product.

Hereinafter, the present invention will be described in detail by way of examples.

(Example)

A Zn-Mg-Al alloy-plated steel sheet on which a Zn-Mg-Al based alloy plating layer having 1.0 to 5.0 wt% of Mg and 1.0 to 5.0 wt% of Al was formed on the surface of the cold- ). Then, a coating film composition containing a polyester resin, a pigment, a solvent and other additives was applied to the surface of the alloy-plated steel sheet. At this time, as shown in the following Table 1, the content of the resin added by the molecular weight of the polyester resin was set differently Respectively.

The undercoat layer thus coated was dried under the condition of PMT 230 DEG C to form a undercoat layer having a dry film thickness of 5.0 to 6.0 mu m. Then, an upper coat composition (55 wt% of polyester resin, 15 wt% of pigment, 20 wt% of solvent and other additives) was coated on the surface of the alloy coated steel sheet on which the undercoat layer was formed, , And an upper coat layer having a dry film thickness of 20.0 탆 was formed to prepare a final coated steel sheet.

On the other hand, for comparison, a coated steel sheet was prepared by preparing a conventional zinc-plated steel sheet (single-side coating adhesion amount: 60 g / m 2) and applying a conventional undercoating film and forming a top coat layer .

division Plating bath composition (%) Composition by composition of molecular weight (%) Zn Mg Al Less than 20,000
(Mw) 20,000 ~
25,000 (Mw) 25,000 to 30,000 (Mw) More than 30,000
(Mw) Comparative Example 1 94 3 3 - 60 (%) 5 (%) - Comparative Example 2 97 1.5 1.5 40 (%) 30 (%) - - Comparative Example 3 97 1.5 1.5 - 60 (%) 5 (%) - Comparative Example 4 94 3 3 - 55 (%) 20 (%) - Comparative Example 5 97 1.5 1.5 - 55 (%) 20 (%) - Comparative Example 6 97 1.5 1.5 - 30 (%) - 40 (%) Inventory 1 94 3 3 - 30 (%) 25 (%) - Inventory 2 97 1.5 1.5 - 30 (%) 25 (%) - Inventory 3 94 3 3 - 40 (%) 35 (%) - Honorable 4 97 1.5 1.5 - 40 (%) 35 (%) -

The corrosion resistance of the coated steel sheet thus produced was evaluated. The results are shown in Table 2 below. Evaluation criteria are as follows.

(Processability evaluation)

Each prepared steel sheet was evaluated by 4T bending evaluation and the area of cracks generated on the surface.

(Corrosion resistance evaluation)

In order to evaluate the corrosion resistance of the processed steel plates, corrosion specimens having a width of 7.0 mm (width) x 15.0 mm (width) were cut and subjected to a corrosion promotion test for 2000 hours by a salt spray standard test conforming to ASTM D B1117 And then the depth of the coating film blister propagation in the processing portion was measured.

division Crack occurrence (%) Coating depth (mm) Comparative Example 1 45 to 50 3.0 mm Comparative Example 2 45 to 50 3.0 mm Comparative Example 3 35 to 40 2.5 mm Comparative Example 4 30 to 35 2.5 mm Comparative Example 5 20-25 2.5 mm Comparative Example 6 1 to 10 1.0 mm Inventory 1 15-20 2.0mm Inventory 2 10 to 15 1.5mm Inventory 3 10 to 15 1.5mm Honorable 4 1 to 10 1.0 mm

As shown in Table 1-2, in the case of Examples 1-4 in which the content of the resin was different by molecular weight when forming the undercoat layer, the degree of occurrence of crreaks in the processed portion was 20% or less, 2.0 mm or less. Particularly, it can be seen that the content of the plating layer is low, and Example 4 using the improved high-work-rate product has the best cracking rate and the excellent corrosion resistance at the processed portion. On the other hand, FIG. 1 is a photograph showing the degree of occurrence of the crushed part of the inventive example 4.

On the other hand, in the case of Comparative Example 1-5, the cracking rate of the processed portion was at least 30% as the product using the conventional undercoat layer, and the depth of the coating film swelling at the processed portion was 2 mm or more. In the case of Comparative Example 6, the cracking rate in the processed portion and the depth of the coating film swelling are good, but the cost increases due to the use of a resin having a high molecular weight. Fig. 2 is a photograph showing the degree of occurrence of the crushed part of the comparative example 1, and it can be seen that the degree of occurrence of crake is excessive.

While the present invention has been particularly shown and described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, Of course, this is possible. Accordingly, the scope of the present invention should not be limited to the described embodiments, but should be defined by the following claims as well as equivalents thereof

Claims (5)

Base steel sheet;
A Zn-Mg-Al-based alloy plating layer formed on one side or both sides of the base steel sheet;
An undercoating film layer comprising 55 to 75% by weight of a polyester resin on the Zn-Mg-Al-based alloy plating layer; And
And an upper coating film layer formed on the lower coating film layer,
Wherein the polyester resin constituting the undercoat layer has a molecular weight Mw of 25,000 to 30,000 in an amount of 25 to 35% by weight and a molecular weight Mw of 20,000 to 25,000 in an amount of 30 to 40% by weight. Excellent coated steel plate.
The coated steel sheet according to claim 1, further comprising a chemical conversion coating layer formed between the Zn-Mg-Al-based alloy plating layer and the undercoating layer.
The coated steel sheet according to claim 1, further comprising a clear coating film layer formed on the top coat layer.
Preparing a Zn-Mg-Al alloy-plated steel sheet on which a Zn-Mg-Al alloy plating layer is formed on one side or both sides of a base steel sheet;
The undercoating composition containing 55 to 75% by weight of polyester resin is applied to one side of the Zn-Mg-Al-based alloy coated steel sheet and then dried under conditions of PMT (Peak Metal Temperature) of 160 to 280 ° C, Forming a layer; And
Coating a top coat composition on one side of a Zn-Mg-Al alloy-plated steel sheet having the undercoating layer formed thereon, and drying the coated composition at a temperature of 160 to 280 ° C to form a top coat layer; Lt; / RTI >
Wherein the polyester resin constituting the undercoat layer has a molecular weight Mw of 25,000 to 30,000 in an amount of 25 to 35% by weight and a molecular weight Mw of 20,000 to 25,000 in an amount of 30 to 40% by weight. (Method for manufacturing a coated steel sheet).
The method as claimed in claim 4, wherein a clear paint is applied to one surface of the Zn-Mg-Al-based alloy coated steel sheet on which the top coat layer is formed, and then dried under conditions of PMT (Peak Metal Temperature) Wherein the step of forming the coated steel sheet comprises the steps of:

Images