EP3730664A1

EP3730664A1 - Molten aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability, and method for producing same

Info

Publication number: EP3730664A1
Application number: EP18892672.9A
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: 2018-12-17
Publication date: 2020-10-28
Also published as: CN111511955B; KR20190076129A; WO2019124901A1; KR102043519B1; EP3730664A4; CN111511955A

Abstract

The present invention relates to a molten aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability, and a method for producing same. A plated steel sheet according to one aspect of the present invention may be a molten aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability and comprising a molten aluminum alloy plating film including: an interface alloy layer present at the interface with the steel sheet; and a plating upper layer disposed on the interface alloy layer, wherein a phase having an atomic ratio between Fe and Al of between 1:2.8-1:3.3 constitutes at least 70% by area of the phases present within 1 µm in the interface alloy layer direction from the boundary between the interface alloy layer and a base steel sheet.

Description

[Technical Field]

The present invention relates to a hot-dip aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability.

[Background Art]

A molten Al-Zn-base plated steel sheet has both sacrificial corrosion resistance of Zn and high corrosion resistance of Al and thus has excellent corrosion resistance as compared to other hot-dip galvanized steel sheets.
In this regard, Patent Document 1 discloses a technology of improving corrosion resistance by alloy-plating formed of Al: 25 wt% to 75 wt%, Mg: 0.1 wt% to 10 wt%, Si: 1 wt% to 7.5 wt%, Cr: 0.05 wt% to 5 wt%, a remainder of Zn and inevitable impurities.
Due to a melting point of the plating bath increased by adding a high melting point metal, such as Cr, or the like, however, such plating composition system has problems of dross generation during plating bath and material deterioration when applied to a giga-pascal-level steel material for vehicles. Further, liquid metal embrittlement occurs at the time of welding due to non-formation of a high melting point alloy phase with Al.
Further, binding of a plating layer with a base steel sheet may not be sufficient depending on the plating bath, resulting in a possibility that a problem of poor plating adhesion, such as delamination of the plating layer, or the like, may arise.

[Prior Art Documents]

[Patent Documents]

(Patent Document 1) Korean Patent Laid-Open Publication No. 10-2011-0088573

[Disclosure]

[Technical Problem]

An aspect of the present invention is to provide a hot-dip aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability.
Another aspect of the present invention is to provide a hot-dip aluminum alloy-plated steel sheet having excellent plating adhesion.
The technical problems of the present invention are not limited to the above. Additional problems are described throughout the entire description of the specification, and one of ordinary skill in the art could understand the additional problems based on the description of the specification without any difficulty.

[Technical Solution]

According to an aspect of the present invention, a hot-dip aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability includes a hot-dip aluminum alloy plating film on a base steel sheet, and the plating film comprises an interface alloy layer present at an interface with the steel sheet and a plating upper layer present on the interface alloy layer, wherein a phase having an atomic ratio of Fe and Al of between 1:2.8 to 1:3.3 occupies 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 aspect, a hot-dip aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability includes a hot-dip aluminum alloy plating film on a base steel sheet, and the plating film comprises an interface alloy layer present at an interface with the steel sheet and a plating upper layer present on the interface alloy layer, wherein a phase having an atomic ratio of Fe and Al of between 1:2.2 to 1:2.7 occupies 10% or less by area of the interface alloy layer.
According to another aspect, a hot-dip aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability includes a hot-dip aluminum alloy plating film on a base steel sheet, wherein the interface alloy layer has a single-layer structure, and an atomic ratio of Fe and Al is between 1:2.8 to 1:3.3 when a composition of the interface alloy layer is analyzed in the central portion of a thickness direction.
According to another aspect, a method for manufacturing a hot-dip aluminum alloy-plated steel sheet, having excellent corrosion resistance and weldability, includes preparing a base steel sheet; dipping the prepared base steel sheet in a molten aluminum alloy-plating bath to plate; and cooling, wherein a temperature of the plating bath is a melting point thereof +30°C or less, and the cooling is performed such that a surface temperature of the base steel sheet released from the plating bath drops below the melting point of the plating bath in 5 seconds.

[Advantageous Effects]

As one of various effects of the present invention, a hot-dip aluminum alloy-plated steel sheet according to an embodiment of the present invention has an advantage of excellent weldability and corrosion resistance.

[Description of Drawings]

FIG. 1 is a photographic image of a cross-section of a plating film of Comparative Example 1.
FIG. 2 is a photographic image of a cross-section of a plating film of Inventive Example 2.
FIG. 3 is a photographic image of a cross-section of Inventive Example 4 after welding.
FIG. 4 is a photographic image of a cross-section of Comparative Example 7 after welding.

[Best Mode for Invention]

Hereinbelow, the present invention will be described in more detail. A hot-dip aluminum alloy-plated steel sheet having excellent weldability and corrosion resistance as an aspect of the present invention will be first described in detail.
As an aspect, a hot-dip aluminum alloy-plated steel sheet includes a base steel sheet and a hot-dip aluminum alloy-plating film (hereinafter, referred as a "plating film"). The plating film may be formed on one surface or both surfaces of the base steel sheet.
The plating film is formed of an interface alloy layer present at an interface with the steel sheet and a plating upper layer present on the interface alloy layer. Conventionally, a Fe₂Al₅ phase with a high Fe content is formed in a position close to a base steel sheet of an interface alloy layer. In one embodiment of the present invention, however, a FeAl₃ phase or a phase having a similar chemical composition ratio is formed in a position close to a base steel sheet of an interface alloy layer.
FIG. 1 illustrates an interface alloy layer of a conventional aluminum alloy-plated steel sheet, and is a photographic image of a cross-section of an interface alloy layer of Comparative Example 1 observed by a scanning electron microscope (SEM). In contrast, FIG. 2 is a photographic image of an interface alloy layer of an aluminum alloy-plated steel sheet of Inventive Example 2, observed by SEM.
An interface alloy layer of a conventional aluminum alloy-plated steel sheet illustrated in FIG. 1 is shown to be formed of multilayers. A lower portion of the interface alloy layer is formed of Fe₂Al₅, an Fe-Al-base hard alloy phase. Such a Fe-Al-base hard alloy phase may cause delamination of a plating layer or liquid metal embrittlement (LME) during spot welding.
In the case of an aluminum-based alloy plated steel sheet according to an embodiment of the present invention illustrated in FIG. 2, an interface alloy layer has a single layer structure. Such a single-layer interface alloy layer is mainly formed of FeAl₃. Accordingly, a Fe-Al-base hard alloy phase, such as Fe₂Al₅, does not substantially exist in a position close to a base steel sheet of the interface alloy layer. Further, occurrence of LME can be effectively prevented during spot welding.
In a first embodiment of the present invention, the expression that a FeAl₃ phase is formed at a position close to the base steel sheet in the interface alloy layer in a first embodiment of the present invention means that a FeAl₃ phase occupies at least 70% by area of phases present within 1 µm in a direction toward the interface alloy layer 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; 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 components derived from a plating bath, a base steel sheet, or the like, are included therein. Unlimited examples of the components, which can be additionally included in the FeAl₃ phase, are silicon (Si), manganese (Mn), or the like.
According to the second embodiment, 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 the third embodiment, the interface alloy layer may substantially be formed of a single layer. When a composition of the interface alloy layer in the central portion of a thickness direction is analyzed, Al content may correspond to the content in a FeAl₃ content. According to an embodiment of the present invention, the composition 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 component-analyzing the same with EDS followed by calculating an average value thereof.
The embodiments described above do not exclude each other. 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 example, the expression that an interface alloy layer has a single layer structure may mean 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 example, the plating film may include, by weight%, Al: 50% to 90%, Zn: 2% to 35%, Si: 3% to 15% and Fe: 0.1% to 5%. The plating film of the present invention may be analyzed by dissolving a plated upper layer and the interface alloy layer with hydrochloric acid and analyzing thus-obtained solution using an inductively coupled plasma (ICP) method. However, the analysis method is not necessarily limited thereto.
Al is added to form a high melting point alloy phase together with iron. When an added amount of Al is too small, a high melting point alloy phase is poorly formed, and welding liquid metal embrittlement may occur. Accordingly, an Al content may be 50 wt% or more, 55 wt% or more or 60 wt% or more. However, an excessive Al content may deteriorate cross-sectional corrosion resistance. In this regard, the Al content may be 90 wt% or less, 80 wt% or less, or 75 wt% or less.
In an embodiment of the present invention, Zn is added for sacrificed protection against corrosion in cross-section. When an added amount of Zn is too small, it may be difficult to secure sufficient cross-sectional corrosion resistance. Accordingly, it is preferable that a Zn content be 2 wt% or more; the Zn content may be 10 wt% or more, or 20 wt% or more. However, an excessive content thereof may cause LME during welding. In this regard, the Zn content may be 35 wt% or less, 30 wt% or less or 25 wt% or less.
In an embodiment of the present invention, Si is added to lower a melting point of a plating bath and improve plating adhesion. When an added amount of Si is too small, an Al-Fe alloy phase is excessively formed, thereby deteriorating plating adhesion. Accordingly, an Si content may be 3 wt% or more, 5 wt% or more or 6 wt% or more. However, an excessive amount thereof may increase a melting point of the plating bath and extremely prevent formation of an Al-Fe alloy phase. In this regard, an Si content may be 15 wt% or less, 12 wt% or less or 10 wt% or less.
Fe reacts with Al to prevent LME by forming a Fe-Al intermetallic compound having a high melting point. When a content of Fe is too small, the Al-Fe alloy phase is barely formed, thereby causing LME. In this regard, an Fe content may be 0.1 wt% or more, 1 wt% or more or 3 wt% or more. However, an excessive content thereof may cause excessive formation of the Al-Fe alloy phases, thereby deteriorating plating adhesion. In this regard, an Si content may be 5 wt% or less, 4.5% or less or 4 wt% or less.
The remaining part is inevitable impurities. In a conventional manufacturing method for a plated steel sheet, undesired impurities may be inevitably introduced from a raw material or surrounding environment and thus cannot be excluded. The impurities are well known to one of ordinary skill in the art and thus are not particularly mentioned in the present invention.
Meanwhile, effective components in addition to the above composition are not excluded; for example, the plating film may further contain Mg: 0.5 wt% to 5 wt%.
Mg is added to improve corrosion resistance of surfaces and cross-sections. To sufficiently obtain such an effect, 0.5 wt% or more, 1 wt% or more or 2 wt% or more of Mg may be added. However, an excessive content thereof may lead to LME during welding. In this regard, a Mn content may be 5 wt% or less, 4 wt% or less or 3 wt% or less.
According to an example, an 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. As previously described, 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 component-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 spot weldability and plating adhesion during processing is deteriorated. In this regard, it is preferable that such a region does not exist.
According to an example, an average Si content in the interface alloy layer may be twice an average Si content in the plating upper 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 smaller than twice that in the plating upper layer, alloy phases may be excessively formed.
Meanwhile, a specific method for measuring an average Si content in the plating upper layer and the interface alloy layer is not particularly limited, but may, for example, involve dissolving the plating upper 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).
As an example, it is preferable that the Si content in the plating upper layer be 0.7 wt% to 1 wt%, and that in the interface alloy layer be 7 wt% to 12 wt%.
According to an example, 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 in detail. 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, the following method may be employed.
A base steel sheet is prepared. A type of the base steel sheet is not particularly limited as long as it is acknowledged as being applied to the technical field to which the present invention pertains.
The base steel sheet is dipped in a hot-dip aluminum alloy-plating bath (hereinafter, referred as "plating bath") and plated. A composition of the plating bath may be, for example, Al: 50% to 90%, Zn: 2% to 35%, Si: 3% to 15% and Fe: 0.1∼5% by weight%.
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 properties 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, the temperature of the plating bath may be a melting point thereof +30°C or below, a melting point thereof +25°C or below, or a melting point thereof +20°C.
In addition, according to an embodiment of the present invention, an adhesion amount may be controlled by wiping using nitrogen gas during plating.
The plating layer is cooled after the plating is performed. 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 speed 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 plating upper layer is completely cooled. When the speed is less than 5°C/sec, the plating layer may be adsorbed 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 in 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)

A giga-level steel material (a steel material having strength of 1 GPa or more; the steel material used herein has strength of 1.18 GPa) 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 was 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 molten plating field, followed by dipping the same in a plating bath having the composition and temperature shown in Table 1 below to manufacture a hot-dip aluminum alloy-plated steel sheet.
Thus-prepared hot-dip aluminum alloy-plated steel sheet was cooled at a temperature equal to or below a melting point of the plating bath, and nitrogen gas was wiped to adjust plating adhesion to be 70 g/m² per side. The cooling was performed at a speed of 10°C/sec until the plating upper layer is completely solidified. Meanwhile, in each example, a time t_m taken for a temperature of the hot-dip aluminum alloy-plated steel sheet released in the plating bath to reach the melting point of the plating bath is indicated in Table 1 below.
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 measured by spot analysis using EDS to measure a maximum value of the Fe content, and an average Si content in the plating upper layer and that of the interface alloy layer were measured by the wet analysis (ICP). Results are shown in Table 2 below.
Each hot-dip aluminum alloy-plated steel sheet was measured in terms of properties thereof, and corrosion resistance, weldability and plating adhesion were evaluated. Results are shown in Table 3 below.
The corrosion resistance evaluation was measured by charging the hot-dip aluminum alloy-plated steel sheet in a salt spray tester, and 5% salt water (35°C, pH 6.8) was sprayed at 2 mL/80 cm² per hour. After 1200 hours from charging, corrosion products were removed, and a maximum depth of a hole formed on the base steel sheet was measured. As a result of the measurement, 300 µm or less was evaluated as "excellent", 300 µm to 600 µm was evaluated as "normal" while 600 µm was evaluated as "poor."
Welding was performed using a Cu-Cr electrode with a tip diameter of 6 mm to pass a welding current of 0.5 kA under a pressing force of 4.0 kN for evaluating weldability. After welding, a length of an LME crack formed on a cross-section thereof was measured by FE-SEM. As a result, it was evaluated as "excellent" when the LME crack length was 150 µm or less, "normal" when the LME crack length was greater than 150 µm and less than 500 µm, and "bad" when the LME crack length was larger than 500 µm.

Meanwhile, as for the plating adhesion evaluation, a sealer type D for vehicle structures was maintained at 175°C for 25 minutes and cured to perform a 90° bending test. A size of a sample was 30 mm × 75 mm, and a surface area of sealer application was 10 mm × 40 mm while a thickness was 10 mm. It was evaluated as "excellent" when shear adhesive strength was 24.5 MPa or more and "poor" in the case of shear adhesive strength less than 24.5 MPa.

[Table 1]

No.	Plating layer composition (%)					Plating bath melting point (°C)	Plating bath melting point (°C)	t_m (sec)	Plating layer composition (%)					Remark
No.	Al	Zn	Si	Fe	Etc.	Plating bath melting point (°C)	Plating bath melting point (°C)	t_m (sec)	Al	Zn	Si	Fe	Etc.	Remark
1	72	15	10	3	-	530	550	4	71	14	10	5	-	*IE1
2	84	1	10	5	-	620	650	7	81	2	10	7	-	**CE1
3	58	20	20	2	-	550	590	6	39	38	20	3	-	CE 2
4	64	25	7	4	-	535	550	2	65	24	7	4	-	IE 2
5	64	25	7	4	-	535	550	6	41	48	7	4	-	CE 3
6	56	30	10	4	-	535	550	4	58	29	10	3	-	IE 3
7	51	40	2	7	-	570	610	6	51	40	2	7	-	CE 4
8	71	20	5	4	-	540	600	6	71	20	5	4	-	CE 5
9	71	20	5	4	-	540	560	3	73	18	5	4	-	IE 4
10	71	20	5	4	-	540	560	7	71	20	5	4	-	CE 6
11	69	15	10	4	Mg 2	540	560	4	69	14	10	5	Mg 2	IE 5
12	63	15	10	2	Mg 10	530	570	7	61	15	10	4	Mg 10	CE 7
13	62	25	7	4	Mg 2	530	545	3	65	23	7	3	Mg 2	IE 6
14	62	25	7	4	Cr 2	580	620	5	63	24	7	4	Cr 2	CE 8

*IE: Inventive Example **CE: Comparative Example

[Table 2]

No.	Interface alloy layer						Plating upper layer	Remark
	Entire interface alloy layer					Within 1 µm from boundary with steel sheet
	Struc ture	Thick ness (µm)	Fe content (wt%)	Si content (wt%)	Fe₂Al₅ percentage (area %)	FeAl₃ percentage (area%)	Si content (wt%)
1	*SL	4	32	11	1	99	1	***IE 1
2	**ML	5	48	11	20	80	1	****CE1
3	ML	2	40	25	12	88	2	CE 2
4	SL	4	38	9	2	98	0.7	IE 2
5	ML	5	46	9	17	83	0.7	CE 3
6	SL	3	36	12	9	91	1	IE 3
7	ML	8	55	4	30	70	0.1	CE 4
8	ML	7	48	7	15	85	0.8	CE 5
9	SL	5	40	7	3	97	0.8	IE 4
10	ML	7	50	7	13	87	0.8	CE 6
11	SL	3	30	10	1	99	1	IE 5
12	ML	2	45	9	14	86	1	CE 7
13	SL	4	31	9	2	98	0.7	IE 6
14	ML	5	45	8	22	78	0.7	CE 8

*SL: Single layer **ML: Multilayer ***IE: Inventive Example ****CE: Comparative Example

[Table 3]

No.	Corrosion Resistance	Weldability	Plating adhesion	Remark
1	Excellent	Excellent	Excellent	*IE 1
2	Poor	Excellent	Poor	*CE 1
3	Excellent	Excellent	Poor	CE 2
4	Excellent	Excellent	Excellent	IE 2
5	Excellent	Excellent	Poor	CE 3
6	Excellent	Excellent	Excellent	IE 3
7	Excellent	Poor	Poor	CE 4
8	Excellent	Poor	Poor	CE 5
9	Excellent	Excellent	Excellent	IE 4
10	Excellent	Excellent	Poor	CE 6
11	Excellent	Excellent	Excellent	IE 5
12	Excellent	Poor	Poor	CE 7
13	Excellent	Excellent	Excellent	IE 6
14	Excellent	Excellent	Poor	CE 8

*IE: Inventive Example **CE: Comparative Example

FIGS. 1 and 2 are respective photographic images of cross-sections of plating films of Comparative Example 1 and Inventive Example 2, observed by SEM. As illustrated therein, Comparative Example 1 has a multilayer structure while Inventive Example 2 has a single layer structure.
Meanwhile, FIG. 3 is a photographic image of a cross-section of Inventive Example 4 after welding, observed by SEM, and FIG. 4 is a photographic image of a cross-section of Comparative Example 7 after welding, observed by SEM. Based on FIGS. 3 and 4, Inventive Example 4 satisfying the requirements suggested in the present disclosure shows improved weldability while LME is exhibited in Comparative Example 7.
As shown in Table 3, excellent corrosion resistance, weldability and plating adhesion were secured for the Inventive Examples satisfying the requirements of the present disclosure but not for the Comparative Examples beyond the scope of the present disclosure.
While embodiments have been illustrated and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.

Claims

A hot-dip aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability, comprising:
a hot-dip aluminum alloy plating film on a base steel sheet,

wherein the plating film comprises an interface alloy layer present at an interface with the steel sheet and a plating upper layer present on the interface alloy layer,

wherein a phase having an atomic ratio of Fe and Al of between 1:2.8 to 1:3.3 occupies 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.
A hot-dip aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability, comprising:
a hot-dip aluminum alloy plating film on a base steel sheet,

wherein the plating film comprises an interface alloy layer present at an interface with the steel sheet and a plating upper layer present on the interface alloy layer,

wherein a phase having an atomic ratio of Fe and Al of between 1:2.2 to 1:2.7 occupies 10% or less by area of the interface alloy layer.
The hot-dip aluminum alloy-plated steel sheet of claim 1 or 2, wherein the interface alloy layer is formed to have a single-layer structure.
The hot-dip aluminum alloy-plated steel sheet of claim 3, wherein distinction of layers is not 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.
The hot-dip aluminum alloy-plated steel sheet of claim 1 or 2, 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.
A hot-dip aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability, comprising:
a hot-dip aluminum alloy plating film on a base steel sheet,

wherein the interface alloy layer has a single-layer structure, and an atomic ratio of Fe and Al is between 1:2.8 to 1:3.3 when a composition of the interface alloy layer is analyzed in a central portion of a thickness direction.
The hot-dip aluminum alloy-plated steel sheet of claim 6, wherein distinction of layers is not 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.
The hot-dip aluminum alloy-plated steel sheet of any one of claims 1, 2 and 6, wherein the plating film comprises, by weight%, Al: 50% to 90%, Zn: 2% to 35%, Si: 3% to 15% and Fe: 0.1% to 5%.
The hot-dip aluminum alloy-plated steel sheet of claim 8, wherein the plating film further comprises Mg: 0.5% to 5%.
The hot-dip aluminum alloy-plated steel sheet of any one of claims 1, 2 and 6, wherein an amount of Fe in the interface alloy layer is 45 wt% or less.
The hot-dip aluminum alloy-plated steel sheet of any one of claims 1, 2 and 6, wherein an amount of Si included the interface alloy layer is at least twice an amount of Si included in the plating upper layer.
The hot-dip aluminum alloy-plated steel sheet of any one of claims 1, 2 and 6, wherein an average thickness of the interface alloy layer is 7 µm or less (excluding 0).
A method for manufacturing a hot-dip aluminum alloy-plated steel sheet having excellent corrosion resistance and weldability, comprising:
preparing a base steel sheet;

dipping the prepared base steel sheet in a molten aluminum alloy-plating bath to plate; and

cooling,

wherein a temperature of the plating bath is a melting point thereof +30°C or less, and the cooling is performed such that a surface temperature of the base steel sheet released from the plating bath drops below the melting point of the plating bath in 5 seconds.
The method for manufacturing hot-dip aluminum alloy-plated steel sheet of claim 13, wherein a cooling speed at a temperature below the melting point of the plating bath is 5°C/sec to 20°C/sec.
The method for manufacturing hot-dip aluminum alloy-plated steel sheet of claim 13, wherein the plating bath comprises, by weight%, Al: 50% to 90%, Zn: 2% to 35%, Si: 3% to 15% and Fe: 0.1% to 5%.
The method for manufacturing hot-dip aluminum alloy-plated steel sheet of claim 15, wherein the plating bath further comprises Mg: 0.5% to 5%.