WO2009131267A1

WO2009131267A1 - Hot-dip aluminum alloy plating composition and method for manufacturing hot-dip aluminum alloy plated steel using the same

Info

Publication number: WO2009131267A1
Application number: PCT/KR2008/002542
Authority: WO
Inventors: Yong Sug Chung; Jong Min Kim; Jae Min Lee
Original assignee: DONGBU STEEL Co Ltd
Current assignee: DONGBU STEEL Co Ltd
Priority date: 2008-04-25
Filing date: 2008-05-06
Publication date: 2009-10-29
Anticipated expiration: 2010-10-25
Also published as: CN101565806A

Abstract

Disclosed are a hot-dip aluminum alloy plating composition and a method of manufacturing hot-dip aluminum alloy-plated steel using the same. The hot-dip aluminum alloy plating composition includes 5~15 wt% of Si, 3-12 wt% of Mg, and 0.005-0.5 wt% of Sr, with the balance of Al, and forms an Mg₂Si phase in a plating layer when the surface of steel is plated therewith. The hot-dip aluminum alloy plating composition further includes Sr, in addition to Si and Mg, so that the [Mg₂Si] phase in the aluminum alloy plating layer becomes fine, the shape thereof is changed, and the plating structure becomes uniform, resulting in hot-dip aluminum alloy-plated steel having superior processability, corrosion resistance of a shear surface, and surface gloss.

Description

HOT-DIP ALUMINUM ALLOY PLATING COMPOSITION

AND METHOD FOR MANUFACTURING HOT-DIP ALUMINUM ALLOY PLATED

STEEL USING THE SAME

Technical Field

The present invention relates to a hot-dip aluminum alloy plating composition and a method of manufacturing hot-dip aluminum alloy-plated steel using the same, and more particularly, to a hot-dip aluminum alloy plating composition, which makes processability, the corrosion resistance of a shear surface, and the surface gloss of a plating layer superior, and to a method of manufacturing hot-dip aluminum alloy-plated steel using the same.

Background Art

Compared to hot-dip zinc-plated steel, conventional aluminum-plated steel has superior corrosion resistance (plane) and heat resistance, and is thus mainly applied to heat-resistant materials for automobile mufflers, home appliances, etc. Such aluminum-plated steel, which was first manufactured in a Sendzimir process in the year 1939, has begun to be generalized since automobile manufacturers in the United States of America applied it to a steel material for mufflers in the latter half of the 1950s.

Plated steel having a plating layer composed exclusively of pure aluminum has excellent corrosion resistance in an aqueous solution and in atmosphere, and is therefore utilized as construction material and corrosion- resistant steel material for a piping line. However, the plated steel having a plating layer composed exclusively of pure aluminum suffers because steel to be plated reacts rapidly with aluminum to thus produce a thick intermetallic compound layer, which functions to drastically deteriorate molding processability . For that reason, the use thereof is very limited.

Accordingly, aluminum alloy-plated steel, having an alloy layer which is controllably formed with the addition of about 5~11 wt% of Si, has been developed, and is mostly used for heat-resistant parts, such as automobile mufflers, water heaters, heaters, or inner coats of electric rice cookers. The aluminum alloy-plated steel added with Si exhibits superior corrosion resistance for a plain plate while manifesting inferior corrosion resistance for a shear surface and a processed surface, compared to zinc alloy- plated steel.

With the goal of solving the above problems, in the case of a zinc alloy plating bath, there has been reported an example in which Mg is added so that an [Mg₂Si] phase is formed in the plating layer, thereby improving corrosion resistance. In this case, however, the addition of Mg is problematic in that the surface hardness of the plating layer is increased and the Mg-based intermetallic compound (Mg₂Zn, Mg₂Znii) formed in the plating layer deteriorates the processability of the plated steel. Further, because Mg is an element having high affinity for oxygen, an Mg- based oxide film is formed on the surface in the course of wiping and cooling after a plating process. Such a film causes circular surface defects, resulting in reduced surface gloss.

In the case of the aluminum alloy plating bath, problems due to the formation of Mg₂Zn and Mg₂Zn_H do not occur, but circular surface defects should be eliminated in order to improve surface gloss. Further, plating processability varies depending on the ratio, shape and size of the [Mg₂Si] phase to be deposited, and consequently, corrosion resistance is greatly changed. Hence, the ratio, shape and size of the [Mg₂Si] phase should be appropriately adjusted in order to improve processability and corrosion resistance .

Technical Problem

Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and provides a hot-dip aluminum alloy plating composition, which makes processability, the corrosion resistance of a shear surface, and the surface gloss of a plating layer superior, and also provides a method of manufacturing hot-dip aluminum alloy-plated steel using the same .

Technical solution

According to the present invention, a hot-dip aluminum alloy plating composition may comprise 5-15 wt% of Si, 3-12 wt% of Mg, and 0.005-0.5 wt% of Sr, with the balance of Al, and may form an Mg₂Si phase in a plating layer when the surface of steel is plated therewith.

In the hot-dip aluminum alloy-plated steel according to the present invention, a plating structure and an intermetallic compound, in particular, an [Mg₂Si] phase, formed between a sheet to be plated and an aluminum plating layer, may be controlled in terms of size and shape, resulting in hot-dip aluminum alloy-plated steel having improved processability, corrosion resistance of a shear surface, and surface gloss of a plating layer.

In the plating bath of the present invention, Si is preferably added in an amount of 5-15 wt%. When the amount of Si is less than 5 wt%, effects of adding Si for inhibiting the growth of an alloy layer and improving the flowability of the plating bath to thus impart gloss are not manifested. In contrast, when the amount of Si exceeds 15 wt%, a planar silicon phase is deposited in the plating layer, thus remarkably degrading processability of the plating layer.

In the present invention, Mg is an important element for improving corrosion resistance. When hot-dip aluminum alloy-plated steel is exposed under corrosion conditions, the Mg component functions to coat the surface of the plating layer and the exposed portion of the steel with an Mg-containing corrosion product so that the inherent corrosion resistance of the hot-dip aluminum alloy-plated steel is further increased.

Further, the Mg component in the plating layer forms the [Mg₂Si] phase as the intermetallic compound along with Si. As such, the [Mg₂Si] phase is effective for improving the corrosion resistance of the shear surface and the processed portion. Such an intermetallic compound promotes the formation of a stable corrosion product under corrosion conditions, and serves as a supply source of the Mg component. Accordingly, the surface of the plating layer is rapidly coated with a corrosion product, and such a corrosion product acts as a stable protective film, thereby improving the corrosion resistance of the plated surface. Furthermore, Mg exhibits an effect of blocking the diffusion of oxygen via the reaction with aluminum, remarkably improving the corrosion resistance of the shear surface after processing.

Mg is present in the form of oxide in the outermost portion of the plating layer, and contributes to an increase in corrosion resistance. Even if the amount thereof is very small, the corrosion resistance effect is large. However, in order to ensure corrosion resistance which is apparently different from in conventional cases, it is preferred that Mg be added in an amount of at least 4.0 wt%. However, in the case where Mg having strong oxidation properties is contained in an excessive amount, the plating bath is saturated and the melting point is increased, thus making it difficult to handle the plating bath. Furthermore, an Mg oxide film is formed on the surface of the plating bath. Therefore, the upper limit of the amount of Mg is preferably set to 12 wt%.

The reason why the aluminum alloy plating composition according to the present invention is added with Sr is that the processability of the plating layer is improved, the generation of cracks in the plating layer is prevented, and the surface gloss of the plating layer becomes good. These effects are obtained by, thanks to the addition of Sr, transforming the bulky [Mg₂Si] phase to be fine, changing the shape thereof, and making the plating structure uniform. In this way, when the bulky [Mg₂Si] phase becomes fine and the plating structure becomes uniform, the generation of cracks is decreased upon processing, and the propagation of the generated cracks is retarded, generally inhibiting the generation of cracks. Also, the decrease in the generation of cracks is considered to be because the number of directions in which cracks are generated is reduced while the shape of the [Mg₂Si] phase is changed from a polygon to a quadrangle.

Through the addition of Sr, the plating structure is transformed to be fine and uniform, thereby reducing the occurrence of circular surface defects on the surface of the plating layer due to the addition of Mg, resulting in greatly increased surface gloss of the plating layer.

Moreover, the addition of Sr results in an effect exceeding the degree of improvement in the gloss which is to be expected through fineness and uniformity of the plating structure. This is considered to be because a plane of a specific orientation having very high reflectivity is formed on the surface while the shape of the [Mg₂Si] phase is changed from a polygon to a quadrangle.

Such Sr is preferably added in amount of 0.005~0.5 wt%. When the amount of Sr is less than 0.005 wt%, the above effects are not manifested. In contrast, when the amount thereof is greater than 0.5 wt%, the outer appearance becomes poor attributable to the adsorption of dross which is regarded as oxide of Sr after a plating process, undesirably resulting in an untidy outer appearance. The adsorption of such dross is shown little by little when the amount of Sr is greater than 0.3 wt%. Thus, it is more preferred that Sr be added in an amount of 0.005-0.3 wt%.

In addition, a method of manufacturing hot-dip aluminum alloy-plated steel may comprise preparing a hot- dip plating bath including 5-15 wt% of Si, 3-12 wt% of Mg, and 0.005-0.5 wt% of Sr, with the balance of Al; immersing steel in the hot-dip plating bath, thus plating the steel; and removing the plated steel from the plating bath and cooling it. When Sr is added in an amount greater than 0.3 wt%, the adsorption of dross occurs, and thus, Sr is preferably added in an amount ranging from 0.005 to 0.3 wt%.

In the method according to the present invention, cooling of the steel may be performed at a rate of 5-30 ^°C /sec. When cooling is conducted at a rate less than 5 ^°C /sec, the size of the [Mg₂Si] phase is too large and the processability of the plating layer is deteriorated. On the other hand, when cooling is conducted at a rate exceeding 30 ^°C /sec, the surface of the plating layer is roughened due to over-cooling, and the [Mg₂Si] phase is not formed. The cooling rate is the most important factor for adjusting the size and shape of the [Mg₂Si] phase and is also reguired to increase surface gloss. The cooling rate for increasing the surface gloss is in the range of 10~20 ^°C /sec.

Upon plating, the hot-dip plating bath preferably has a temperature of 600~700^°C When the plating temperature of the steel is lower than 600 ^°C , the outer appearance of the plating film becomes poor, and film adhesiveness is undesirably decreased. On the other hand, when the temperature is higher than 700^°C, the thermal diffusion of steel becomes fast, resulting in abnormal growth of the alloy layer. Accordingly, processability is deteriorated and the oxide layer in the melt is excessively formed. Further, the generation of circular defects on the surface of the plating layer attributable to the addition of Mg is increased in proportion to the increase in the temperature of the melt. More preferably, the temperature of the plating bath is set to 640~660^°C.

Advantageous Effect

According to the present invention, the hot-dip aluminum alloy plating composition comprises Si and Mg, and further comprises Sr, thereby realizing a fine [Mg_≥Si] phase in the aluminum alloy plating layer, with the changed shape, and making the plating structure uniform. Hence, the resulting plated steel can exhibit superior processability, corrosion resistance of a shear surface, and surface gloss. Description of Drawings

FIG. 1 is an electron micrograph illustrating the side-sectional structure of the plating layer and the alloy layer of the hot-dip aluminum alloy-plated steel according to the present invention;

FIG. 2 is a graph illustrating the gloss of the hot- dip aluminum alloy-plated steel depending on the added amount of Sr in the case where Sr is added to a plating bath composition of Al - 13wt% Si - 7.5wt% Mg according to the present invention;

FIG. 3 is electron micrographs illustrating the side- sectional structure of the plating layer and the alloy layer of the hot-dip aluminum alloy-plated steel depending on the added amount of Sr in the case where Sr is added to a plating bath composition of Al - 13wt% Si - 7.5wt% Mg according to the present invention; and

FIG. 4 is an electron micrograph illustrating the side-sectional structure of the plating layer and the alloy layer of conventional aluminum alloy-plated steel without Sr.

Mode for Invention

Hereinafter, a better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed as the limit of the present invention, without departing from the scope of the accompanying claims.

A cold rolled steel sheet having a size of 0.7 mm (thickness) x 180 mm (width) x 220 mm (length) was immersed in an alkaline solution at 50 ^°C for 30 min, and was then washed with water to remove surface impurities and oil, thus preparing a test sample.

A hot-dip plating test was conducted through annealing and plating using a hot-dip plating simulator. Specifically, as annealing conditions, the annealing atmosphere was a reducing atmosphere comprising 10~30% hydrogen and 70-90% nitrogen, and the annealing temperature was 750~850^°C in order to realize necessary mechanical properties. Thereafter, the plating process was conducted by subjecting the sample to cooling so that the temperature thereof was the same as the temperature of the plating bath, immersing the sample in the plating bath for about 3~5 sec, removing the sample therefrom, adjusting the plating amount to about 100~150 g/m² (for both surfaces) using an air wiper, and cooling the sample at a rate of 10~20 ^°C /sec to solidify it.

Using the plating bath including the composition of each of the examples and comparative examples as shown in Table 1 below, plating was conducted, thus obtaining hot- dip aluminum alloy-plated steel, the properties of which were then measured. The measurement items were ^λplated surface properties' , ^corrosion resistance of shear surface' , and ^Λprocessability' . The results are shown in Table 1 below.

1) Plated Surface Properties

Whether circular defects were generated on the plated surface for each type of composition was observed with the naked eye, and plated surface properties were evaluated.

In the following table, o indicates ^generation of no defect' , and * indicates ^generation of one or more defects' .

2) Corrosion Resistance of Shear surface

According to KS D 9502 (ASTM B-117), the corrosion resistance of a shear surface was evaluated using a 5%/35^°C NaCl salt spray test method. As such, the upper and lower portions of the shear surface of a test sample were coated, and neither of the side portions thereof was coated. After 1500 hours, the outer appearance was observed with the naked eye, and whether rust was generated was observed with the naked eye.

In the table, ©indicates ^Λrust generation of 5% or less' , o indicates ^λrust generation exceeding 5% but not more than 10%, Δ indicates ^Λrust generation exceeding 10% but not more than 30%' , and x indicates ^Λrust generation exceeding 30 % ' .

3) Processability

A test sample was subjected to a 180° 3T bending test, and the section thereof was observed with the microscope to determine the crack ratio generated per unit length, thus evaluating processability. As such, only cracks across the whole plating layer were observed.

In the following table, o indicates 'crack ratio of 5% or less' , Δ indicates ^Λcrack ratio exceeding 5% but not more than 12%' , and x indicates 'crack ratio exceeding 12%' .

TABLE 1

As is apparent from Table 1, in the case where Sr was added as in the examples of the present invention, plated surface properties were evaluated to be excellent, and as well, the corrosion resistance of the shear surface and processability were superior. In Comparative Examples 1, 2, and 3, as the amounts of Si and Mg were increased, corrosion resistance of the shear surface was good, but processability was poor. In the case where Sr was not added, when the amount of Mg was increased, the circular defects were generated on the plated surface. In Comparative Example 4, in the case where not Sr but Mn was added, corrosion resistance of the shear surface and processability were good, but circular defects were generated on the plated surface.

FIG. 1 is an electron micrograph showing the side- sectional structure of the plating layer and the alloy layer of the hot-dip aluminum alloy-plated steel according to the present invention, specifically, the side-sectional structure of the plating layer and the alloy layer using the plating bath composition of Al - 7.6 wt% Si - 7.3 wt% Mg - 0.5 wt% Sr of Example 1, as shown in Table 1. FIG. 4 shows the side-sectional structure of the plating layer and the alloy layer using the plating bath composition of Al - 6.6 wt% Si - 7.4 wt% Mg of Comparative Example 2, as shown in Table 1. From FIG. 1, it can be seen that, when Sr is added, the bulky [Mg₂Si] phase is transformed to be fine, and the plating structure is uniform. Further, the surface of the plating layer can be seen to be more even.

In the composition of Al - 13 wt% Si - 7.5 wt% Mg, the plating bath was prepared while adjusting the added amount of Sr. Further, the gloss of the plated steel manufactured using such a plating bath was measured using a gloss meter. The side-section thereof was observed using an electron microscope.

FIG. 2 is a graph showing the gloss of the hot-dip aluminum alloy-plated steel depending on the added amount of Sr, when Sr is added to the plating bath composition of Al - 13 wt% Si - 7.5 wt% Mg. As shown in FIG. 2, the gloss was remarkably increased in proportion to the increase in the added amount of Sr.

FIG. 3 is electron micrographs showing the side- sectional structure of the plating layer and the alloy layer of the hot-dip aluminum alloy-plated steel depending on the added amount of Sr, when Sr is added to the plating bath composition of Al - 13 wt% Si - 7.5 wt% Mg. From FIG. 3, it can be seen that, when Sr is added, a finely dispersed [Mg₂Si] phase, which is not bulky, is increased, and the surface of the plating layer is more even than when Sr is not added. In the case where Sr is not added, the [Mg₂Si] phase is present only in the bulky phase, whereas the addition of Sr results in the crystallization not only of the bulky [Mg₂Si] phase but also of the fine [Mg₂Si] phase.

Moreover, when Sr is added, the shape of the crystallized [Mg₂Si] phase is changed from a polygon to a quadrangle. In this way, when the shape thereof is changed, the number of directions in which cracks are generated is decreased, thereby reducing the crack ratio. Further, the change in the shape leads to the formation of the plane of a specific orientation having very high reflectivity on the surface, thereby greatly increasing surface gloss.

Claims

CLAIMS What is clamed is:

1. A hot-dip aluminum alloy plating composition, comprising 5-15 wt% of Si, 3-12 wt% of Mg, and 0.005-0.5 wt% of Sr, with a balance of Al, and forming an Mg₂Si phase in a plating layer when a surface of steel is plated therewith.

2. The hot-dip aluminum alloy plating composition according to claim 1, wherein the Sr is added in an amount ranging from 0.005 to 0.3 wt%.

3. A method of manufacturing hot-dip aluminum alloy- plated steel, comprising: preparing a hot-dip plating bath including 5-15 wt% of Si, 3-12 wt% of Mg, and 0.005-0.5 wt% of Sr, with a balance of Al; immersing steel in the hot-dip plating bath, thus plating the steel; and removing the plated steel from the plating bath, and cooling the steel.

4. The method according to claim 3, wherein the Sr of the hot-dip plating bath is added in an amount ranging from 0.005 to 0.3 wt%.

5. The method according to claim 3 or 4, wherein the cooling the steel is conducted at a rate of 5-30 ^°C/sec.

6. The method according to claim 3 or 4, wherein the cooling the steel is conducted at a rate of 10~20 ^°C/sec.

7. The method according to claim 3 or 4, wherein a temperature of the hot-dip plating bath is 600~700^°C.

8. The method according to claim 3 or 4, wherein a temperature of the hot-dip plating bath is 640~660^°C.