阅读说明：本技术 耐蚀性和加工性优异的热浸镀钢板及其制造方法 (Hot-dip plated steel sheet having excellent corrosion resistance and workability, and method for producing same ) 是由 金洙永 权纹栽 于 2018-12-10 设计创作，主要内容包括：本发明涉及一种可以用于建筑材料、家电、汽车等的各种目的的镀覆钢板,更详细地涉及一种耐蚀性和加工性优异的热浸镀钢板及其制造方法。(The present invention relates to a plated steel sheet that can be used for various purposes such as building materials, home appliances, and automobiles, and more particularly, to a hot-dip plated steel sheet having excellent corrosion resistance and workability, and a method for producing the same.)

1. A hot-dip coated steel sheet excellent in corrosion resistance and workability, comprising a base steel sheet and a Zn-Al-Mg alloy plating layer on at least one surface of the base steel sheet,

the Zn-Al-Mg alloy plating layer comprises, in wt%: aluminum (Al): 20-30%, magnesium (Mg): 3-5% by mass, and the balance of Zn and other unavoidable impurities, wherein the surface structure of the Zn-Al-Mg alloy plating layer comprises a Zn phase, a Zn-Al phase, and MgZn₂Phase and Zn-Al-MgZn₂A phase composition, and an internal structure including an Fe-Al alloy phase.

2. The hot-dip coated steel sheet excellent in corrosion resistance and workability according to claim 1, wherein the Zn-Al-Mg based alloy plating layer contains aluminum (Al) and magnesium (Mg) in a total content of 25 wt% or more.

3. The hot-dip coated steel sheet excellent in corrosion resistance and workability according to claim 1, wherein the Zn-Al-Mg based alloy plating layer has a thickness of 3 to 40 μm.

4. The hot-dip coated steel sheet excellent in corrosion resistance and workability according to claim 1, wherein a surface structure of the Zn-Al-Mg based alloy plating layer is a region from 1/3t to 1/2t in a thickness direction from a surface of the alloy plating layer, where t represents a thickness (μm) of the alloy plating layer.

5. A method of manufacturing a hot-dip coated steel sheet excellent in corrosion resistance and workability, comprising the steps of:

preparing an alloy plating bath comprising, in weight%: aluminum (Al): 20-30%, magnesium (Mg): 3-5%, the balance of Zn and other inevitable impurities;

dipping a base steel sheet into the alloy plating bath and plating to produce a plated steel sheet; and

cooling the plated steel sheet at a cooling rate of 8-30 ℃/sec,

wherein, the plating is carried out by adjusting the temperature of the plating bath to 500-550 ℃, adjusting the temperature of the plating bath to 480-550 ℃ and passing through the plating tank.

6. The method of manufacturing a hot-dip coated steel sheet excellent in corrosion resistance and workability according to claim 5, wherein the cooling is performed to 300 ℃ or less.

7. The method of producing a hot-dip coated steel sheet excellent in corrosion resistance and workability according to claim 5, wherein the cooling is from 4% by volume or less and includes 0% hydrogen gas (H)₂) And the balance of nitrogen (N)₂) The composition is carried out in a gas atmosphere.

8. The method of producing a hot dip plated steel sheet excellent in corrosion resistance and workability according to claim 5, further comprising a step of subjecting the plated steel sheet to a gas wiping treatment before the cooling.

9. The method of manufacturing a hot dip coated steel sheet excellent in corrosion resistance and workability according to claim 5, wherein the base steel sheet is a cold rolled material degreased and subjected to annealing heat treatment in a temperature range of 700-850 ℃.

Technical Field

The present invention relates to a plated steel sheet that can be used for various purposes such as building materials, home appliances, and automobiles, and more particularly, to a hot-dip plated steel sheet having excellent corrosion resistance and workability, and a method for producing the same.

Background

A process of plating a steel sheet by a continuous hot dip plating process has a low manufacturing cost and can secure excellent quality as compared with a process such as electroplating or dry plating, and thus the range of use as a material for buildings, ships, home electric appliances, automobile inner and outer panels, and the like is expanding.

In addition, in recent years, due to the rapid increase in the price of materials containing zinc, a new plating system having a small plating adhesion amount but excellent corrosion resistance has been actively developed to replace the conventional galvanized steel sheet.

In this connection, there is a trend toward alloy-plated steel sheets having excellent corrosion resistance even when the amount of adhesion is small by adding aluminum or magnesium to conventional zinc plating systems.

Since the zinc-aluminum-magnesium plating bath has low wettability with a steel sheet and a wide solidification range as compared with a zinc plating system, a technique of controlling cooling after plating is important.

Conventionally, when an aluminum structure and a magnesium structure which are easily oxidized are formed on the surface of a zinc-aluminum-magnesium alloy plated steel sheet (patent document 1 and patent document 2) developed mainly in japan, the surface quality is remarkably reduced because the surface is easily discolored due to uneven oxidation and the surface is blackened with the passage of time. In order to improve these problems, there is a method of adding high-quality alloying elements to the plating bath, but there is a disadvantage in that problems such as an increase in cost and an increase in side reaction products of the plating bath, etc. are caused.

A zinc-aluminum-magnesium alloy plated steel sheet recently developed mainly in europe (patent document 3) is a final object for application to automobiles, and has a problem that the total amount of aluminum and magnesium added is less than that of a zinc-aluminum-magnesium alloy plated steel sheet developed in japan, and thus sufficient corrosion resistance cannot be ensured.

(patent document 1) Japanese laid-open publication No. 1999-140615

(patent document 2) Japanese laid-open publication No. 2000-104154

(patent document 3) European patent publication 1621645A1

Disclosure of Invention

Technical problem to be solved

An object of an aspect of the present invention is to provide a hot dip plated steel sheet having not only high corrosion resistance but also good surface appearance and excellent workability by optimizing the composition of an alloy plating system and optimizing the cooling process of a plated steel material, and a method of manufacturing the same.

The technical problem to be solved by the present invention is not limited to the above. The technical problems added to the present invention are described in the entire contents of the present specification, and those skilled in the art to which the present invention pertains will not have any difficulty in understanding the technical problems added to the present invention from the contents described in the present specification.

Technical scheme

An aspect of the present invention provides a hot-dip coated steel sheet excellent in corrosion resistance and workability, including a base steel sheet and a Zn-Al-Mg-based alloy plating layer on at least one side of the base steel sheet, the Zn-Al-Mg-based alloy plating layer including, in weight%: aluminum (Al): 20-30%, magnesium (Mg): 3-5% by mass, and the balance of Zn and other unavoidable impurities, wherein the surface structure of the Zn-Al-Mg alloy plating layer comprises a Zn phase, a Zn-Al phase, and MgZn₂Phase and Zn-Al-MgZn₂A phase composition, and an internal structure including an Fe-Al alloy phase.

Another aspect of the present invention provides a method of manufacturing a hot-dip coated steel sheet excellent in corrosion resistance and workability, the method including the steps of: preparing an alloy plating bath comprising, in weight%: aluminum (Al): 20-30%, magnesium (Mg): 3-5%, the balance of Zn and other inevitable impurities; dipping a base steel sheet into the alloy plating bath and plating to produce a plated steel sheet; and cooling the plated steel sheet at a cooling rate of 8-30 ℃/sec, wherein the plating is performed by adjusting the temperature of the plating bath to 500-550 ℃, adjusting the temperature of the plating bath to 480-550 ℃ and passing through the plating tank.

Advantageous effects

According to the present invention, it is possible to provide an alloy-plated steel sheet which can improve corrosion resistance by improving the adhesiveness of an alloy plating layer by finely and uniformly forming the cross-sectional structure of the alloy plating layer, and which has good surface quality and excellent workability.

Drawings

Fig. 1 is a photograph showing the appearance of plating of a plated steel sheet in which one embodiment of the present invention is observed.

Fig. 2 is a photograph showing a cross section of the alloy plating layer of the plated steel sheet of fig. 1 as described above, which is observed by SEM.

Fig. 3 is a photograph showing the structure in the alloy plating layers of comparative example 1 (left side) and invention example 2 (right side) in one embodiment of the present invention observed by SEM.

Best mode for carrying out the invention

In order to obtain a Zn-Al-Mg-based alloy-plated steel sheet, when a Zn-Al-Mg alloy is plated, the solidification of the alloy plating layer starts depending on the contents of aluminum and magnesium, and finally a eutectic phase containing Zn-Al-Mg solidifies at the end of solidification and completes the solidification reaction of the plating layer. However, before solidification of the Zn — Al — Mg ternary eutectic structure is completed, flow mark-shaped surface defects due to strong oxidation of Mg are likely to occur, and therefore use of Mg is limited.

Nevertheless, in order to improve the corrosion resistance of Zn — Al — Mg alloy plated steel sheets, it is necessary to add a certain amount or more of Mg, and the present inventors have confirmed the problem that flow mark shape defects due to oxidation increase with such an increase in Mg content, and have conducted intensive studies on a method capable of ensuring corrosion resistance and ensuring good surface quality by a certain amount or more of Mg.

Specifically, it was confirmed that in the case of the Zn — Al — Mg alloy plating system, the corrosiveness in the plating bath is strong, the solidification interval is wide, and the structure of the alloy plating layer is controlled by optimizing the alloy composition and cooling process of the Zn — Al — Mg alloy plating system.

As a result, it was confirmed that a Zn — Al — Mg alloy-plated steel sheet having corrosion resistance, good surface quality, and excellent workability could be provided, and the present invention was completed.

The present invention will be described in detail below.

A hot-dip coated steel sheet according to an aspect of the present invention includes a base steel sheet and a Zn-Al-Mg based alloy coating layer on at least one side of the base steel sheet, and the Zn-Al-Mg based alloy coating layer may include, in wt%: aluminum (Al): 20-30%, magnesium (Mg): 3-5%, and the balance of Zn and other inevitable impurities.

The Zn — Al — Mg alloy plating layer has the alloy composition described above, and may be formed from an alloy plating bath containing Al, Mg, and Zn.

Specifically, Al in the alloy plating bath is a main element that ensures high corrosion resistance of the hot-dip plated steel sheet, and when the content of Al is less than 20%, it is difficult to ensure sufficient corrosion resistance of the Zn-Al-Mg-based alloy plating layer. On the other hand, when the content of Al exceeds 30%, generation of dross in the plating bath increases, and thus the surface quality of the final product deteriorates.

Mg in the alloy plating bath is an element added to improve corrosion resistance of the alloy plating layer and the cross section of the plating layer, and when the Mg content is less than 3%, the effect of improving corrosion resistance is very small, while when the Mg content exceeds 5%, generation of dross due to oxidation of the plating bath is greatly increased.

Since Al and Mg are elements that improve the corrosion resistance of the plating layer and the corrosion resistance can be further improved as the sum of these elements increases, the sum of the Al and Mg contents can be limited to 25% or more in the present invention.

In addition, the components in the alloy plating bath are substantially the same as the components in the alloy plating layer.

The Zn-Al-Mg alloy plating layer having the above alloy composition may have a fine and uniform structure, and specifically, the surface layer structure of the Zn-Al-Mg alloy plating layer is preferably composed of a Zn phase, a Zn-Al phase, and MgZn₂Phase and Zn-Al-MgZn₂The phase composition, the internal structure preferably comprises an Fe-Al alloy phase.

In the invention, through the use of a metal in the alloyFine Zn-Al phase and MgZn are formed on the surface (surface layer) of the plating layer₂Phase and Zn-Al-MgZn₂The phase is used as a plating structure of the Zn-Al-Mg alloy plating layer, and corrosion resistance can be improved.

In the present invention, the Fe — Al intermetallic compound (Fe — Al alloy phase) formed at the interface of the conventional Zn — Al — Mg alloy-plated steel sheet (interface between the base steel sheet and the plating layer) is uniformly formed in the interior (internal structure) of the alloy plating layer, and thus, a good surface appearance is ensured.

In addition, the Zn — Al — Mg based alloy plating layer may have a thickness of 3 to 40 μm, and the surface structure in such a Zn — Al — Mg based alloy plating layer may correspond to a region from the surface of the alloy plating layer to 1/3t to 1/2t (where t represents the thickness (μm) of the alloy plating layer) in the thickness direction. The remaining portion other than the superficial tissues corresponding to the above-described region may be referred to as internal tissues.

The hot-dip galvanized steel sheet according to the present invention has the advantages of remarkably excellent corrosion resistance and fine surface appearance compared to conventional hot-dip galvanized steel sheets because the surface layer structure and the internal structure of the Zn — Al — Mg alloy plating layer are formed differently from each other, and in this case, the surface layer structure is formed as described above.

Hereinafter, a method for producing a hot-dip plated steel sheet excellent in corrosion resistance and workability according to another aspect of the present invention will be described in detail.

The method of manufacturing a hot-dip coated steel sheet excellent in corrosion resistance and workability according to the present invention may include the steps of: preparing an alloy plating bath having the alloy composition described above; dipping a base steel sheet into the alloy plating bath and plating to produce a plated steel sheet; and cooling the plated steel sheet.

When the base steel sheet is immersed in an alloy plating bath satisfying the alloy composition proposed in the present invention and plated, it is preferable to adjust the temperature entering the plating bath to 500-550 ℃, adjust the temperature of the plating bath to 480-550 ℃, and pass through the plating tank.

When the temperature of the entry into the plating bath is less than 500 ℃, non-plating or deterioration in adhesion of the plating layer occurs on the surface of the formed alloy plating layer, and on the other hand, when the temperature of the entry into the plating bath exceeds 550 ℃, there is a problem that adhesion of the plating layer is deteriorated.

Preferably, the plated steel sheet obtained by plating as described above is cooled, preferably at a cooling rate of 8-30 ℃/sec to 300 ℃ or less.

In the cooling, when the cooling rate is less than 8 ℃/sec, the coating layer cannot be uniformly solidified, while when the cooling rate exceeds 30 ℃/sec, flow mark shape defects are generated on the coating layer surface.

Further, the cooling may be from 4% by volume or less (including 0%) of hydrogen (H)₂) And the balance of nitrogen (N)₂) The composition is carried out in a gas atmosphere, and as an example, an air-jet cooler may be used.

By controlling the cooling process as described above, it is possible to form a primary Zn phase, a Zn-Al phase, on the surface of the alloy plating layer and form a fine Zn-Al phase, MgZn phase, in the region under the surface (i.e., the region between the surface and the internal alloy layer (internal structure))₂Phase and Zn-Al-MgZn₂And (4) phase(s).

Finally, a uniform structure is formed on the surface layer of the alloy plating layer, thereby greatly improving the corrosion resistance of the plane part, and particularly, a Zn phase and MgZn are obtained₂The effect of the sacrificial corrosion resistance of the cross-sectional portion is improved.

A step of gas wiping treatment may be further included before cooling the plated steel sheet, and the plating adhesion amount may be adjusted by the gas wiping treatment.

The gas used in the gas wiping process may be air or nitrogen, and among them, nitrogen is more preferably used. This is because oxidation of Mg occurs preferentially on the surface of the plating layer when air is used, and thus surface defects of the plating layer may be induced.

In addition, the base steel sheet may be a cold rolled material such as a plain carbon steel (including a low carbon steel), a stainless steel, and the like, and is not particularly limited.

After the cold rolled material is subjected to a degreasing process for washing rolling oil on the surface of the rolled steel sheet, annealing heat treatment may be performed to restore the rolled structure and ensure the material quality.

The annealing heat treatment may be performed at a temperature in the range of 700-850 deg.c, but in general, in the case of low-carbon steel, the annealing heat treatment may be performed at a temperature in the range of 700-800 deg.c, and in the case of ultra-low-carbon steel or high-strength steel, the annealing heat treatment may be performed at a temperature in the range of 800-850 deg.c.

The cold rolled material may have a thickness of 0.3 to 1mm, but is not limited thereto.

The present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are only for illustrating the present invention to perform a more detailed description, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the contents recited in the claims and reasonably derived therefrom.

Detailed Description

(example 1)

A cold-rolled test piece (0.7 mm in thickness) of low carbon steel (0.003% C, 0.15% Mn, and the balance Fe and other inevitable impurities) was degreased and then annealed at 780 ℃. During the annealing heat treatment, the gas atmosphere in the furnace is a reducing gas atmosphere, and the content of H is controlled to be 4-20%₂And the remainder of N₂And a dew point temperature of-40 ℃ or lower.

Thereafter, the test piece was subjected to alloy hot dip plating according to the conditions shown in table 1 below. At this time, an alloy plating bath having a composition of 22.77% of Al, 3.6% of Mg, and the balance of Zn (0.027% of Fe) was used in wt%.

[ Table 1]

The results of observing the plating appearance of each plated steel sheet plated as described above are shown in fig. 1, and the photographs of the alloy plating layer observed by SEM in cross section are shown in fig. 2.

As shown in fig. 2, it was confirmed that the surface layer structure and the internal structure were separated and uniformly formed only in the test pieces 4 and 5 corresponding to inventive examples 1 and 2. In particular, it was confirmed that the alloy phase was uniformly formed in the interior of the plating layer.

On the other hand, it was confirmed that the test pieces of comparative examples 1 to 13 were formed by mixing the Fe-Al alloy phase with the Zn phase and the Zn-Al phase.

Further, fig. 3 shows a photograph obtained by observing the alloy plating structures of comparative example 1 and invention example 2 by SEM.

As shown in fig. 3, it was confirmed that in the case of comparative example 1, the alloy plating layer having the desired plated structure of the present invention was not uniformly formed on the surface of the plated steel sheet, while in inventive example 2, the Fe — Al alloy layer was formed inside the alloy plating layer and the plated structure was uniformly formed on the alloy layer.

(example 2)

The same test pieces as in example 1 were degreased, that is, cold rolled test pieces (thickness of 0.7mm) of carbon steel were degreased and then subjected to annealing heat treatment at 750 ℃. During the annealing heat treatment, the gas atmosphere in the furnace is a reducing gas atmosphere, and the content of H is controlled to be 4-20%₂And the remainder of N₂And a dew point temperature of-40 ℃ or lower.

Thereafter, the test piece was subjected to alloy hot dip plating according to the conditions shown in table 2 below.

[ Table 2]

In order to evaluate the corrosion resistance of the plated steel sheet which was plated according to each condition, an accelerated corrosion test was performed by a salt water spray test (salt water spray standard test conforming to KS-C-0223), and then the time elapsed until the area of red rust generated on the surface of the plated layer reached 5% was measured.

In order to evaluate plating adhesion, a 180-degree bending test was performed, and then the bent surface was visually observed to confirm whether or not cracks were generated.

The results are shown in table 3 below. As shown in table 3 below, it was confirmed that the inventive example a was excellent in corrosion resistance and no non-plating phenomenon was observed, compared to the comparative example a. In particular, the bending adhesion of invention example a was on the same level as that of the hot-dip galvanized material.

[ Table 3]

Categories	Salt spray test	Crack generation in curved surface
			Comparative example A	Red rust is generated after 7-8 weeks	×
Inventive example A	Red rust is generated after 11-12 weeks	×