EP3730665A1

EP3730665A1 - Aluminum alloy-plated steel sheet having excellent resistance to welding liquation brittleness and excellent plating adhesion

Info

Publication number: EP3730665A1
Application number: EP18892684.4A
Authority: EP
Inventors: Suk-Kyu Lee; Il-Jeong Park; Myung-Soo Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2017-12-22
Filing date: 2018-12-18
Publication date: 2020-10-28
Also published as: KR102043522B1; CN111492087B; EP3730665A4; KR20190076796A; WO2019124927A1; CN111492087A

Abstract

The present invention relates to an aluminum alloy-plated steel sheet having excellent resistance to welding liquation brittleness and excellent plating adhesion. An aluminum alloy-plated steel sheet according to one aspect of the present invention may include: a base steel sheet; and an aluminum alloy plating film including, in wt%, 5-30% of Zn, 0.5-5% of Mg, and 0.01-3% of Mn.

Description

[Technical Field]

The present invention relates to an aluminum alloy-plated steel sheet having excellent resistance to liquid metal embrittlement during welding and excellent plating adhesion.

[Background Art]

A hot-dip aluminum (Al)-plated steel sheet is widely used in steel sheets for vehicles or other fields requiring corrosion resistance. However, due to weak sacrificial corrosion resistance, there are many cases in which Al plating layers have limited corrosion resistance.
To compensate for the above, an Al-Zn-based plated steel sheet in which Zn is added to an Al plating film has been proposed. A molten Al-Zn-based plated steel sheet has excellent corrosion resistance, as compared to other hot-dip galvanized steel sheets as it has both sacrificial corrosion resistance of Zn and high corrosion resistance of Al.
In the meantime, such plating components do not form a high melting point alloy phase with Al and thus may be problematic as there is a likelihood that liquid metal embrittlement (LME) may occur during welding(also known as "embrittlement caused by liquidizing during welding"). In this regard, there is a limitation of Zn addition, thereby creating limitations on securing sufficient corrosion resistance.
Besides, adhesion between a plating film and a base steel sheet may not be sufficient in the case of insufficient alloying therebetween, which might cause the plating film to be delaminated.

[Disclosure]

[Technical Problem]

An aspect of the present invention is to provide an aluminum alloy-plated steel sheet having a novel plating layer capable of suppressing liquid metal embrittlement and plating film delamination while having sufficient corrosion resistance, and a manufacturing method therefor.
The technical problem of the present invention is not limited to the problem mentioned above, and other problems not mentioned herein will be clearly understood by those skilled in the art from the following description.

[Technical Solution]

According to an aspect of the present invention, an aluminum alloy plated steel sheet may include a base steel sheet; and an aluminum alloy plating film by weight%, Zn: 5% to 30%, Mg: 0.5% to 5% and Mn: 0.01% to 3%.
The plating film further may further include Si: 5% to 12% and Fe: 0.1% to 5%.
According to an embodiment, the plating film may include an interface alloy layer formed at an interface between the base steel sheet and an upper plating layer disposed on the interface alloy layer, and a phase having an atomic ratio of Fe and Al of between 1:2.8 to 1:3.3 may account for at least 70% by area of phases present within 1 µm in an interface alloy layer direction from a boundary between the interface alloy layer and the base steel sheet.
According to another embodiment, the plating film may include an interface alloy layer formed at an interface between the base steel sheet and an upper plating layer disposed on the interface alloy layer, and a phase having an atomic ratio of Fe and Al of between 1:2.2 to 1:2.7 may account for 10% or less by area of the interface alloy layer.
The interface alloy layer according to the above embodiments may be formed to have a single layer structure.
Further, distinction of layers may not be observed in the interface alloy layer when the hot-dip aluminum alloy-plated steel sheet is cut in a thickness direction to observe a cross-section thereof using a field emission scanning electron microscope (FE-SEM) at 3,000× magnification.
When the interface alloy layer is formed of two layers or more, and Al may be included in all formed layers such that an atomic ratio of Fe and Al is greater than 1:2.8.
According to another embodiment, the plating film may include an interface alloy layer formed at an interface between the base steel sheet and an upper plating layer disposed on the interface alloy layer, and the interface alloy layer may be formed to have a single layer structure and may have an atomic ratio of Fe and Al of between 1:2.8 to 1:3.3 when a component of a central portion of the interface alloy layer is analyzed in a thickness direction.
In each embodiment relevant to the interface alloy layer of the present invention, distinction of layers may not be observed in the interface alloy layer when the hot-dip aluminum alloy-plated steel sheet is cut in a thickness direction to observe a cross-section thereof using a field emission scanning electron microscope (FE-SEM) at 3,000× magnification.
In each embodiment relevant to the interface alloy layer, an amount of Fe in the interface plating layer is 45 wt% or less.
According to another aspect, a method for manufacturing a hot-dip aluminum alloy-plated steel sheet may include preparing a base steel sheet; immersing the prepared base steel sheet in a plating bath comprising by weight%, Zn: 3% to 30%, Mg: 1% to 5%, Mn: 0.1% to 3%, a remainder of Al and inevitable impurities; and cooling the plating layer, wherein the cooling is performed such that a surface temperature of the base steel sheet released from the plating bath drops below a melting point of the plating bath in 5 seconds.
In an embodiment, a temperature of the plating bath may be melting point thereof +30°C or less.
In an embodiment, the melting point of the plating bath is 520°C to 560°C.

[Advantageous Effects]

According to the present invention, an aluminum alloy-plated steel sheet having not only high resistance to corrosion while suppressing liquid metal embrittlement by reducing a Zn content but also having excellent plating adhesion by facilitating alloying of a base steel sheet and a plating layer may be obtained.

[Description of Drawings]

FIG. 1 is a photographic image of an interface alloy layer of a conventional aluminum alloy-plated steel sheet, observed by a scanning electron microscope (SEM).
FIG. 2 is a photographic image of an interface alloy layer of an aluminum alloy-plated steel sheet according to an embodiment, observed by a scanning electron microscope (SEM).

[Best Mode]

Hereinbelow, the present invention will be described in detail.
As used herein, the expression "plated steel sheet" refers to a plated steel sheet, including a plating layer and a base steel sheet. The plating film is formed of an interface alloy layer and an upper plating layer and the interface alloy layer may be formed to be in direct contact with the base steel.
In the present invention, a content of each component is indicated based on a weight fraction (e.g., weight%, or the like) unless otherwise indicated.
The present inventors have discovered that it is necessary to add Mg while limiting a Zn content in an Al-plating film to a certain level or below such that LME is prevented during welding and high corrosion resistance is obtained. When a composition of the plating film is controlled as above, however, Mg suppresses the element contained in the steel sheet from spread to the plating film such that alloying does not easily occur. As a result, alloying between the plating film and the base steel sheet is not sufficient when Mg is contained in a certain amount, thereby leading to a problem of delamination of the plating film from the base steel sheet.
In the present invention, Mn is contained to resolve the problem of suppressing spread between the base steel sheet and the plating film. That is, Mn is an element facilitating formation of an alloy layer between the plating film and the plating steel plate without causing any particular problem relevant to corrosion resistance of the plated steel sheet and is thus added to the plating layer.
Accordingly, the plating film of the present invention may be an Al-based plating film containing certain amounts of Zn, Mg and Mn. The plating film of the present invention may further contain Si or Fe in addition to the said composition.
Hereinbelow, a composition of the plating film of the present invention will be described in more detail.

Zn: 5% to 30%

Zn is an element improving sacrificial corrosion resistance, and thus is added in an amount of at least 5% in the present invention. In an embodiment of the present invention, a Zn content may be 10% or more, or 15% in some cases. To prevent LME, the Zn content is limited to 30% or less in the present invention.

Mg: 0.5% to 5%

The sacrificial corrosion resistance may not be sufficient when the Zn content is limited to a certain level to prevent LME. Mg is added to compensate for the sacrificial corrosion resistance, and in an embodiment of the present invention, a content thereof is limited to 0.5% or more. In another embodiment, the Mg content may be limited to 0.5 wt% or more, or 1 wt% or more in some cases. To form an alloy layer properly between the base steel sheet and the plating film, the Mg content may be limited to 5% or below in an embodiment, or 4% or less in another embodiment, or 3% or less in some cases.

Mn: 0.01% to 3%

Mn is added to promote alloying. In an embodiment, a content thereof may be 0.01% or more. In another embodiment, the Mn content may be 0.05% or more, or 0.3% or more in some cases. However, as corrosion resistance may be deteriorated when the Mn content increases, the Mn content is limited to 3% or less to secure sufficient corrosion resistance. In another embodiment, the Mn content may be 2% or less, or 1% or less in some cases.
The plating film of the present invention may further include Si and Fe, in addition to the above essential elements, to appropriately control alloying.

Si: 5% to 12%

In the present invention, Si is added to control alloy in an appropriate range. That is, Si may be included in an amount of 5% or more to control excessive formation of alloy layers in accordance with Mn addition. In some cases, 7% or more or 9% or more may be added. To secure weldabiltiy, however, the Si content may be limited to 12% or less. In an embodiment, the Si content may be limited to 11% or less, or 10% or less in some cases.

Fe: 0.1% to 5%

Fe is added as a supplementary element facilitating alloy layer formation. Accordingly, in an embodiment, Fe may be added in an amount of 0.1% or more, and 0.5% or more in another embodiment. In some cases, 0.7% or more thereof may be added. To suppress generation of dross in a hot-dip plating bath, Fe may be added in an amount of 5% or less, or 4.5% or less in another embodiment. In some cases, 4.2% or less thereof may be added.
According to an embodiment, the remainder of the plating film, except the above elements, may be Al. In another embodiment, inevitable impurities may be partially contained in the plating film.
Examples of the impurities included in the plating layer may be Ca, Cr, Mo, Ni, and the like, but are not necessarily limited thereto. According to an embodiment, the content thereof may be limited to 0.03% or less.
The composition of the plating film may be analyzed by dissolving all the upper plating layer and the interface alloy layer in hydrochloric acid followed by analyzing thus-obtained solution with Inductively Coupled Plasma (ICP) method, but is not limited thereto.
According to an embodiment, the plating layer has the above-described composition and can thus regulate a melting point thereof to be 520°C to 560°C in a lower direction. As a result, a temperature of a steel sheet introduced into a plating bath can be lowered, thus decrease of tensile strength thereof can be effectively prevented. That is, the strength of a steel sheet decreases with a conventional plating bath composition in accordance with an increase in a temperature of the steel sheet despite a recent trend that the strength of the plated steel sheet increases. In the present invention, a plating layer composition enabling a low-melting point plating bath to be formed is used to minimize decrease of strength of the steel sheet.
Further, according to an embodiment, an interface alloy layer included in the plating film of the present invention may have the following characteristics.
According to an embodiment, the interface alloy layer of the present invention may have a thickness of 1 µm or more. That is, when the interface alloy layer has a thickness of a certain level or more in the plating composition system of the present invention, adhesion between the base steel sheet and the plating layer is improved, thereby reducing a possibility that the plating layer is delaminated from the base steel sheet. Accordingly, in an embodiment, the interface alloy layer may have a thickness of 1 µm or more. When the thickness excessively increases, however, delamination occurs in the interface alloy layer due to vulnerable characteristics of the interface alloy layer, thereby deteriorating adhesion of the plating layer during processing. Accordingly, in consideration of the above, the thickness of the alloy layer may be limited to be 7 µm or less in an embodiment, or 5 µm or less in another embodiment.
Further, the interface alloy layer of the present invention may be formed mainly of FeAl₃. FIG. 1 is a photographic image of an interface alloy layer of a conventional aluminum alloy-plated steel sheet, observed by a scanning electron microscope (SEM), while FIG. 2 is a photographic image of an interface alloy layer of an aluminum alloy-plated steel sheet according to an embodiment, observed by a scanning electron microscope (SEM).
In the case of the conventional aluminum alloy-plated steel sheet illustrated in FIG. 1, the interface alloy layer is shown to be formed of multilayers. A lower portion thereof is formed of Fe₂Al₅, an Fe-Al-based hard alloy phase. Such Fe-Al-based hard alloy phase may cause LME during spot welding or delamination of the plating layer.
In the case of the aluminum alloy-plated steel sheet according to an embodiment illustrated in FIG. 2, the interface alloy layer is shown to have a single layer structure. Such single-layer interface alloy layer is formed mainly of FeAl₃. Accordingly, the Fe-Al-based hard alloy phase, such as Fe₂Al₅, does not substantially exist in a location close the base steel sheet of the interface alloy layer and can thus effectively prevent generation of LME during welding.
In First Embodiment related to the interface alloy layer, formation of a FeAl₃ phase in a location closed to the base steel sheet of the interface alloy layer means that the FeAl₃ phase accounts for at least 70% by area of phases present within 1 µm in an interface alloy layer direction from a boundary between the interface alloy layer and the base steel sheet.
The interface alloy layer may have a single layer structure and may have a structure of two layers or more in some case; however, a FeAl₃ phase is formed in a position close to the base steel sheet. When the interface alloy layer has a structure of two layers or more, a content of Al may be higher in all formed layers as compared to that in the FeAl₃ phase.
As used herein, the expression "FeAl₃ phase" is not limited to a phase in which Fe and Al are necessarily combined at a ratio of 1:3, but refers to a phase in which an atomic ratio of Fe and Al (Fe content in weight/atomic weight of Fe:Al content in weight/atomic weight of Al) is 1:2.8 to 1:3.3. Further, the expression is to define a ratio between Fe and Al, and it should be noted that the expression does not exclude the fact that additional elements originated from a plating bath, a base steel sheet, or the like, are included therein. Unlimited examples of the elements, which can be additionally included in the FeAl₃ phase, are silicon (Si), manganese (Mn), or the like.
According to Second Embodiment relevant to the interface alloy layer, a percentage of Fe₂Al₅ contained in the interface alloy layer is limited to 10% or less, preferably 5% or less by area.
As used herein, the expression "Fe₂Al₅ phase" refers to a phase having an atomic ratio of Fe and Al of 1:2.2 to 1:2.7.
In this case, the interface alloy layer may have a single layer structure and may have a structure of two layers or more; however, a FeAl₃ phase is formed in a position close to the base steel sheet. When the interface alloy layer has a structure of two layers or more, a content of Al may be higher in all formed layers as compared to that in the FeAl₃ phase (that is, Al is included in all formed layers such that an atomic ratio of Fe and Al is greater than 1:2.8).
According to Third Embodiment relevant to the interface alloy layer, the interface alloy layer may substantially be formed of a single layer. When a composition of a central portion is analyzed in a thickness direction of the interface alloy layer, an Al content may correspond to a FeAl₃ content. According to an embodiment of the present invention, the component of the central portion in the thickness direction may be obtained by selecting 5 random points in the central portion in the thickness direction and calculating an average value thereof.
The embodiments above described in relation to the interface alloy layer of the present invention do not exclude the other embodiments. Each embodiment may have an overlapping range; however, it should be noted that requirements of all embodiments do not need to be simultaneously met to obtain advantageous effects of the present invention.
According to an embodiment, an interface alloy layer having a single layer structure may indicate that distinction of layers is not observed in the interface alloy layer when a hot-dip aluminum alloy-plated steel sheet is cut in a thickness direction to observe a cross-section thereof using a field emission scanning electron microscope (FE-SEM) at 3,000× magnification.
According to an embodiment, Fe content in the interface alloy layer is preferably 45 wt% or less. As an example of a method for measuring the Fe content of the interface alloy layer, spot analysis using energy dispersive spectroscopy (EDS) may be employed. According to an embodiment of the present invention, the spot analysis involves selecting 5 random points in a central portion in a thickness direction of the interface alloy layer and composition-analyzing the same with EDS followed by calculating an average value thereof. A value of the Fe content in the interface alloy layer exceeding 45 wt% indicates that an Fe-Al-base hard alloy phase is present in the interface alloy layer. As previously described, such an Fe-Al-base hard alloy phase is problematic in that plating adhesion during spot welding and processing is deteriorated. In this regard, it is preferable that such a region does not exist.
According to an embodiment, an average Si content in the interface alloy layer may be twice an average Si content in the upper plating layer or more, preferably three times or more, more preferably seven times or more, the most preferably ten times or more. When the Si content in the interface alloy layer is lower than twice that in the upper plating layer, alloy phases may be excessively formed.
Meanwhile, a specific method for measuring an average Si content in the upper plating layer and the interface alloy layer is not particularly limited, but may, for example, involve dissolving the upper plating layer in chromic acid and measuring by wet analysis (ICP), while an average Si content in the interface alloy layer may be measured by dissolving the interface alloy layer in hydrochloric acid followed by wet analysis (ICP).
In an embodiment, it is preferable that the Si content in the upper plating layer be 0.7 wt% to 1 wt%, and that in the interface alloy layer be 7 wt% to 12 wt%.
In an embodiment, the interface alloy layer may have an average thickness of 7 µm or less (excluding 0 µm), preferably 5 µm or less (excluding 0 µm). When the thickness exceeds 7 µm, plating adhesion may be deteriorated during processing. Meanwhile, there is no limitation on a lower limit of the average thickness of the interface alloy layer; however, when the thickness is too small, LME resistance may not be prevented during welding. In consideration thereof, the lower limit may be determined to 1 µm.
An example of a method for manufacturing a hot-dip aluminum alloy-plated steel sheet will now be described. The hot-dip aluminum alloy-plated steel sheet of the present invention may be manufactured by various methods, and the manufacturing methods are not particularly limited. However, as a preferable example for manufacturing an aluminum alloy-plated steel sheet having a layered structure according to several embodiments, the following method may be employed.
A base steel sheet is prepared. A type thereof is not particularly limited as long as the method is acknowledged as being applied to the technical field to which the present invention pertains.
The base steel sheet is immersed in a hot-dip aluminum alloy-plating bath (hereinafter, referred as "plating bath") and plated. A composition of the plating bath may contain, for example, Zn: 3% to 30%, Mg: 1% to 5%, Mn: 0.1% to 3%, by weight%, and a remainder of Al and inevitable impurities. In some cases, Si: 3% to 15% and Fe: 0.1% to 3% may be further added. When the plating is performed using a plating bath having said composition, an alloy layer is partially formed due to a reaction between the plating bath elements and the base steel sheet, and a composition of the entire plating film may be the composition according to an embodiment of the present invention.
Meanwhile, a temperature of the plating bath may affect not only characteristics of the base steel sheet but also a structure of the interface alloy layer. More specifically, when the plating bath temperature is higher than 30°C above a melting point of the plating bath, a structure of residual austenite and martensite is decomposed, thereby deteriorating a property of the base steel sheet. Further, formation of Fe₂Al₅ formed by alloying with molten aluminum on a surface of the base steel sheet introduced into the plating bath is facilitated, which may result in multilayer interface alloy layer. Accordingly, according to an embodiment of the present invention, the temperature of the plating bath may be controlled to be melting point thereof +30°C or below, melting point thereof +25°C or below, or melting point thereof +20°C. The temperature of the plating bath is not particularly limited as long as it is equal to or above the melting point of the plating bath. In an embodiment, however, a lower limit of the temperature of the plating bath may be determined to melting point thereof +10°C to prevent problems that drivability of a sink roll decreases due to the increase of plating bath.
The plating layer is cooled after the plating is performed. According to an embodiment, such cooling also has a great impact on a structure of the interface alloy layer. It is preferable that the cooling is performed such that a temperature of the steel sheet surface released from the plating bath drops below the melting point of the plating bath within 5 sec, 4 sec or 3 sec. When the plating layer is not solidified within a short period of time, a multilayer interface alloy layer may be obtained, or an Fe-Al alloy phase continues to grow, thereby deteriorating plating adhesion.
Meanwhile, a cooling rate at a temperature equivalent to or below the melting point of the plating bath is not particularly limited, but may be, for example, 5°C/sec to 20°C/sec until the upper plating layer is completely cooled. When the speed is less than 5°C/sec, the plating layer may be attached on a top roll, or the like, whereas the speed exceeding 20°C/sec may result in generation of a wave pattern on the surface thereof.

[Mode for Invention]

Hereinafter, embodiments of the present invention will be described in more detail. However, the description of these embodiments is only intended to illustrate the practice in the present invention, but embodiments are not limited thereto. The scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

(Embodiment)

Example 1

A giga-level steel material for vehicles, having a thickness of 1.4 mm and including C: 0.15%, Si: 1.5%, Mn: 2.5%, Cr: 0.4%, a remainder of Fe and inevitable impurities (a steel material having strength of 1 GPa or more; the steel material used herein has strength of 1.18 GPa) is prepared as a base steel sheet, and immersed and ultrasonic-cleaned to remove foreign substances, such as rolling oil, from a surface. A heat treatment was performed in a 750°C reduction environment to secure mechanical characteristics of the steel sheet in a general hot-dip plating site, followed by immersing the same in a plating bath having the composition and temperature shown in Table 1 below. A element not indicated in the table is Al. Further, t_m in Table 1 refers to a time taken for a surface temperature of the steel sheet released from the plating bath to reach a melting point of the plating bath or below.

[Table 1]

No.	composition of plating bath(%)					Melting Point of Plating Bath (°C)	Plating Bath Temperat ure (°C)	t_m (sec)	Type
No.	Zn	Mg	Mn	Si	Fe	Melting Point of Plating Bath (°C)	Plating Bath Temperat ure (°C)	t_m (sec)	Type
1	30	7	1	15	3	580	640	8	CE1
2	35	5	0	15	3	580	620	9	CE2
3	25	3	0	3	0.5	580	610	10	CE3
4	20	0	2	15	1.5	580	610	8	CE4
5	15	0.3	4	7	0	600	640	9	CE5
6	10	0	0	5	3	620	660	8	CE6
7	5	7	0	10	2	620	670	9	CE7
8	3	0.5	0	3	0.1	640	670	10	CE8
9	3	1	3	15	2	620	660	8	CE9
10	30	5	3	12	0.5	540	570	8	IE1
11	20	3	2	7	0.5	520	550	10	IE2
12	15	1	1	7	1.5	540	570	11	IE3
13	10	5	3	5	2	530	560	12	IE4
14	5	0.5	0.01	12	0.5	560	570	15	IE5

Thus-obtained plating upper layer and interface alloy layer of the plating film of the plated steel sheet are dissolved in hydrochloric acid. A solution obtained as a result was analyzed using an inductively coupled plasma method to measure a composition of an entire plating film. A result of the analysis is shown in Table 2 below.

[Table 2]

No.	Composition of plating layer (%)					Type
No.	Zn	Mg	Mn	Si	Fe	Type
1	30	7	1	15	2.9	CE1
2	35	5	0	15	3.0	CE2
3	25	3	0	3	0.7	CE3
4	20	0	2	15	2.2	CE4
5	15	0.3	4	7	0.9	CE5
6	10	0	0	5	3.5	CE6
7	5	7	0	10	1.8	CE7
8	3	0.5	0	3	0.6	CE8
9	3	1	3	15	2.7	CE9
10	30	5	3	12	3.8	IE1
11	20	3	2	7	3.9	IE2
12	15	1	1	7	4.5	IE3
13	10	5	3	5	5.3	IE4
14	5	0.5	0.01	12	6.0	IE5

*CE: Comparative Example, IE: Inventive Example

An Al-alloy-plated steel sheet obtained by the above method was measured with respect to properties thereof, and weldability, corrosion resistance (surface corrosion, cross-sectional corrosion) and plating adhesion were measured. Results are shown in Table 3 below.
To evaluate liquid metal embrittlement during welding, welding was performed using a Cu-Cr electrode having a tip radius of 6 mm at a welding current of 0.5 kA and a pressing force of 4.0 kN. After welding, a length of a LME crack formed on a cross-section was measured using an FE-SEM. As a result, a length of the LME crack of 150 µm or less was evaluated as "⊚" (excellent), and that greater than 150 µm and equal to or less than 500 µm was evaluated as "○" (fine), while that exceeding 500 µm was evaluated as "×" (poor).
The surface corrosion evaluation was performed by charging each hot-dip aluminum alloy-plated steel sheet into a salt spray tester and spraying 5% brine (35°C, pH 6.8) at 1 mL/80 cm² per hour, followed by determining whether or not read rust has occurred after 2,400 hours of charging. That is, "⊚" (excellent) was marked for the case in which red rust was not observed, and "○" (fine) was marked for the case in which red rust occurred in the steel sheet surface area of 50% or less, while "×" (poor) was marked for the case in which red rust occurred in the steel sheet surface area greater than 50%.
Further, the cross-sectional corrosion evaluation was performed 200 hours after the brine corrosion test by determining whether red rust was observed in a cross-section. "⊚" (Excellent) was marked for the case in which red rust was not observed, and "○" (fine) was marked for the case in which red rust occurred in the steel sheet surface area of 50% or less, while "×" (poor) was marked for the case in which red rust occurred in the steel sheet surface area greater than 50%.

Meanwhile, the plating adhesion evaluation was performed by applying a car structure sealer having an area and thickness of 10 mm×40 mm×5 mm (here, 5 mm is a thickness of the sealer) onto an area of 75 mm×150 mm, curing at 175°C for 25 minutes and bending at 90 degrees to observe delamination of the sealer with naked eye. "⊚" (Excellent) was marked for the case in which sealer was adhered to the base steel and no delamination was observed with the sealer, and "○" (fine) was marked for the case in which the plating layer is delaminated but a portion thereof still adhered to the sealer is 10% by area or less, while "×" (poor) was marked for the case in which a delaminated portion of the plating layer while being adhered to the sealer exceeds 10% by area.

[Table 3]

No.	LME	Surface Corrosion	Cross-sectional corrosion	Plating Adhesion	Type
1	Poor	Fine	Fine	Fine	CE 1
2	Poor	Fine	Fine	Poor	CE 2
3	Poor	Fine	Fine	Poor	CE 3
4	Poor	Poor	Poor	Fine	CE 4
5	Fine	Poor	Poor	Fine	CE 5
6	Fine	Poor	Poor	Fine	CE 6
7	Poor	Fine	Fine	Poor	CE 7
8	Fine	Fine	Poor	Poor	CE 8
9	Fine	Fine	Poor	Poor	CE 9
10	Fine	Fine	Fine	Fine	IE 1
11	Fine	Fine	Fine	Fine	IE 2
12	Fine	Fine	Fine	Fine	IE 3
13	Fine	Fine	Fine	Fine	IE 4
14	Fine	Fine	Fine	Fine	IE 5

Comparative Example 2 is the case in which a Zn content exceeds 30 wt% and thus has no interface alloy layer aimed in the present invention formed. Accordingly, a problem of welding brittleness may arise during welding. In contrast, Comparative Example 9 is the case in which the Zn content is merely 3%, which leads to problems of excessive formation of interface alloy layer and deteriorated plating adhesion. Meanwhile, Comparative Examples 1 and 7 are the cases in which a Mg content is 7%. When a large amount of Mg is added, formation of the interface alloy layer is suppressed, thereby deteriorating resistance to liquid metal embrittlement during welding. In contrast, Comparative Examples 4 to 6 and 8 are the cases in which the Mg content is less than 0.5% and resistance to surface corrosion or cross-sectional corrosion is deteriorated. Comparative Examples 2, 3 and 7 are the cases in which no Mn is added and showed deteriorated plating adhesion due to unsmooth formation of the interface alloy layer.
In contrast, Inventive Examples 1 to 7 satisfying the plating layer composition of the present invention showed fine properties. Accordingly, it is confirmed that when the plating layer composition defined in the present invention is satisfied, resistance to liquid metal embrittlement during welding and corrosion as well as plating adhesion may be improved.
As t_m is set to 8 sec or more in all Inventive and Comparative Examples, a multilayer structure is exhibited in spite of the interface alloy layer formed.

Example 2

The same method used in Example 1 was employed to manufacture a plated steel sheet except the description of Table 4 below. A composition of a plating film of the steel sheet was analyzed, and a result thereof is shown in Table 5.

Thus-prepared hot-dip aluminum alloy-plated steel sheet is then cut in a sheet thickness direction, and a cross-section thereof was observed using FE-SEM at 3,000× magnification to see whether the distinction of layers is observed in the interface alloy layer, followed by measuring a thickness. As previously described, an Fe content in the interface alloy layer was subject to spot analysis using EDS to measure a maximum value of the Fe content, and an average Si content in the upper plating layer and that of the interface alloy layer were measured by the wet analysis (ICP) . Results are shown in Table 6 below. In Table 6, thicknesses, contents, percentages of alloy phases such as Fe₂Al₅ or FeAl₃, are indicated in µm, weight%, and area%.

[Table 4]

NO.	Plating Bath Composition (%)					Plating Bath Melting Point (°C)	Plating Bath Temp (°C)	t_m (sec)	Type
NO.	Zn	Mg	Mn	Si	Fe	Plating Bath Melting Point (°C)	Plating Bath Temp (°C)	t_m (sec)	Type
16	30	5	3	12	0.5	540	570	6	IE 6
17	20	3	2	7	0.5	520	550	5	IE 7
18	15	1	1	7	1.5	540	570	6	IE 8
19	10	5	3	5	2	530	560	4	IE 9
20	5	0.5	0.01	12	0.5	560	570	3	IE 10
21	5	3	2	7	0.1	540	570	5	IE 11
22	3	5	3	5	2	530	550	4	IE 12

[Table 5]

No.	Plating Composition (%)					Type
No.	Zn	Mg	Mn	Si	Fe	Type
16	30	5	3	12	0.8	IE 6
17	20	3	2	7	0.9	IE 7
18	15	1	1	7	2.0	IE 8
19	10	5	3	5	2.3	IE 9
20	5	0.5	0.01	12	1.0	IE 10
21	5	3	2	7	0.5	IE 11
22	3	5	3	5	2.3	IE 12

[Table 6]

No.	interface alloy layer						upper plating layer	Type
	overall interface alloy layer				at a place within 1 µm in an interface alloy layer direction from a boundary between the interface alloy layer and the base steel sheet	a central portion of the interface alloy layer	upper plating layer
	Thickness (µm)	Fe content (wt%)	Si content (wt%)	Ratio of Fe₂Al₅ (Area %)	Ratio of FeAl₃ (Area %)	Atomic ratio of Al to Fe	Sicontent (wt%)
16	5	48	12	0	100	3.1	12	IE 6
17	5	47	7	1	100	2.9	7	IE 7
18	4	47	7	1	100	2.9	7	IE 8
19	5	45	5	2	100	3.1	5	IE 9
20	6	45	12	2	100	3.0	12	IE 10
21	5	45	7	2	100	3.0	7	IE 11
22	5	45	5	2	100	3.0	5	IE 12

The same method used in Example 1 was used to measure properties of the plated steel sheet, and a result thereof is shown in Table 7 below.

[Table 7]

No.	LME	Surface Corrosion	Cross-sectional corrosion	Plating Adhesion	Type
16	Excellent	Excellent	Excellent	Excellent	IE 6
17	Excellent	Excellent	Excellent	Excellent	IE 7
18	Excellent	Excellent	Excellent	Excellent	IE 8
19	Excellent	Excellent	Excellent	Excellent	IE 9
20	Excellent	Excellent	Excellent	Excellent	IE 10
21	Excellent	Excellent	Excellent	Excellent	IE 11
22	Excellent	Excellent	Excellent	Excellent	IE 12

As seen in Table 6, the above described Inventive Examples 6 to 12 satisfy the composition of the plating layer of the present invention. Further, due to the FeAl₃ single layer structure, it can be confirmed based on Table 7 that compared to Inventive Examples 1 to 5 having a multilayer structure, Inventive Examples 6 to 12 have more excellent resistance to liquid metal embrittlement during welding and corrosion as well as plating adhesion.

Claims

An aluminum alloy plated steel sheet, comprising:
a base steel sheet; and

an aluminum alloy plating film by weight%, Zn: 5% to 30%, Mg: 0.5% to 5% and Mn: 0.01% to 3%.
The aluminum alloy-plated steel sheet of claim 1, wherein the plating film further comprises Si: 5% to 12% and Fe: 0.1% to 5%.
The aluminum alloy-plated steel sheet of claim 1, wherein the plating film comprises an interface alloy layer formed at an interface between the base steel sheet and an upper plating layer disposed on the interface alloy layer,
and a phase having an atomic ratio of Fe and Al of between 1:2.8 to 1:3.3 accounts for at least 70% by area of phases present within 1 µm in an interface alloy layer direction from a boundary between the interface alloy layer and the base steel sheet.
The aluminum alloy-plated steel sheet of claim 1, wherein the plating film comprises an interface alloy layer formed at an interface between the base steel sheet and an upper plating layer disposed on the interface alloy layer,
and a phase having an atomic ratio of Fe and Al of between 1:2.2 to 1:2.7 accounts for 10% or less by area of the interface alloy layer.
The aluminum alloy-plated steel sheet of claim 3 or 4, wherein the interface alloy layer is formed to have a single layer structure.
The aluminum alloy-plated steel sheet of claim 5, wherein distinction of layers is not observed in the interface alloy layer when the aluminum alloy-plated steel sheet is cut in a thickness direction to observe a cross-section thereof using a field emission scanning electron microscope (FE-SEM) at 3,000× magnification.
The aluminum alloy-plated steel sheet of claim 4 or 5, wherein the interface alloy layer is formed of two layers or more, and Al is included in all formed layers such that an atomic ratio of Fe and Al is greater than 1:2.8.
The aluminum alloy-plated steel sheet of claim 1, wherein the plating film comprises an interface alloy layer formed at an interface between the base steel sheet and an upper plating layer disposed on the interface alloy layer,
and the interface alloy layer is formed to have a single layer structure and has an atomic ratio of Fe and Al of between 1:2.8 to 1:3.3 when a component of a central portion of the interface alloy layer is analyzed in a thickness direction.
The aluminum alloy-plated steel sheet of claim 8, wherein distinction of layers is not observed in the interface alloy layer when the luminum alloy-plated steel sheet is cut in a thickness direction to observe a cross-section thereof using a field emission scanning electron microscope (FE-SEM) at 3,000× magnification.
The hot-dip alloy-plated steel sheet of any one of claim 3, 4 and 8, wherein an amount of Fe in the interface plating layer is 45 wt% or less.
A method for manufacturing a hot-dip aluminum alloy-plated steel sheet, comprising:
preparing a base steel sheet;

immersing the prepared base steel sheet in a plating bath comprising by weight%, Zn: 3% to 30%, Mg: 1% to 5%, Mn: 0.1% to 3%, a remainder of Al and inevitable impurities; and

cooling the plating layer,

wherein the cooling is performed such that a surface temperature of the base steel sheet released from the plating bath drops below a melting point of the plating bath in 5 seconds .
The method of claim 11, wherein a temperature of the plating bath is melting point thereof +30°C or less.
The method of claim 12, wherein the melting point of the plating bath is 520°C to 560°C.