[0001]The present invention relates to a plated steel sheet.
Priority is claimed on Japanese Patent Application No. 2019-080286, filed on
April 19, 2019, the content of which is incorporated herein by reference.
[Related Art]
[0002]
Recently, as a vehicle structural member, a plated steel sheet is used from the
viewpoint of corrosion resistance, and a hot-dip galvanized steel sheet such as a
galvannealed steel sheet is mainly used in the Japanese market. The galvannealed steel
sheet is a plated steel sheet in which weldability and post-coating corrosion resistance
are improved by performing an alloying heat treatment on a steel sheet after hot-dip
galvanizing and diffusing Fe from the steel sheet (base steel sheet) into a plating layer.
However, further improvement of corrosion resistance such as post-coating corrosion
resistance or red rust resistance is required for the hot-dip galvanized steel sheet.
[0003]
Examples of a method of improving corrosion resistance of the hot-dip
galvanized steel sheet include addition of Al to Zn. For example, in the building
material field, as a high corrosion-resistant plated steel sheet, a hot-dip Al-Zn alloy
plated steel sheet has been widely put into practice. A plating layer of the hot-dip Al-
Zn alloy plated steel sheet is formed of: a dendritic α-(Zn,Al) phase that is initially
crystallized from a molten state (Al primary crystal phase: α-(Zn,Al) phase crystallized
as a primary phase in the Al-Zn binary phase diagram or the like; this phase is not
- 2 -
necessarily an Al-rich phase and is crystallized as a solid solution of Zn and Al); and a
structure (Zn/Al mixed phase structure) formed of a Zn phase and an Al phase that is
formed in a gap of the dendritic Al primary crystal phase. The Al primary crystal
phase is passivated, and the Zn/Al mixed phase structure has a higher Zn concentration
than the Al primary crystal phase. Therefore, corrosion concentrates on the Zn/Al
mixed phase structure. As a result, the corrosion propagates in a worm-eaten state in
the Zn/Al mixed phase structure, and a corrosion propagation path is complicated.
Therefore, the corrosion is not likely to easily reach the base steel sheet. As a result,
the hot-dip Al-Zn alloy plated steel sheet has higher corrosion resistance than a hot-dip
galvanized steel sheet where a plating layer has the same thickness.
[0004]
When this hot-dip Al-Zn alloy plated steel sheet is used as a vehicle exterior
panel, the plated steel sheet is provided to automobile manufacturers in a state where it
is plated in a continuous hot-dip plating facility, is processed into a panel component
shape, and subsequently undergoes automotive general coating of chemical conversion
treatment, further electrodeposition coating, intermediate coating, and top coating in the
automobile manufacturers. However, in the exterior panel formed of the hot-dip Al-Zn
alloy plated steel sheet, when a coating film is damaged, due to the unique plating phase
structure formed of the two phases including the Al primary crystal phase and the Zn/Al
mixed phase structure, preferential dissolution of Zn (selective corrosion of the Zn/Al
mixed phase structure) initiates from a damaged part at an interface between the coating
film and the plating. This corrosion propagates deep into a coating defect-free part,
severe coating blistering occurs, and thus there is a problem in that sufficient corrosion
resistance (post-coating corrosion resistance) cannot be secured.
[0005]
- 3 -
In order to improve corrosion resistance, addition of an element such as Mg to
the Al-Zn plating is also considered. However, it is presumed that, even when Mg is
added, an Al primary crystal phase including a passivation film is still formed in the
hot-dip Al-Zn alloy plated steel sheet. Therefore, the problem with respect to
corrosion resistance caused when the coating film is damaged after coating (postcoating
corrosion resistance) is not solved.
[0006]
In order to solve this problem, Patent Document 1 discloses a hot-dip Zn-based
plated steel sheet having excellent post-coating corrosion resistance. Patent Document
1 discloses that a plating layer includes Zn, Al, Mg, and Si, and when a total area ratio
of a lamellar structure in which a layered Zn phase and a layered Al phase are
alternately arranged in the plating layer is 5% or more, coating blistering in the coated
state is suppressed.
However, in Patent Document 1, post-coating corrosion resistance is improved
to some extent. However, the effect is limited, and sufficient post-coating corrosion
resistance cannot be secured. In addition, in the technique disclosed in Patent
Document 1, it is necessary to perform a complex thermal history process for
microstructure control, and there is a problem in that manufacturing costs increase.
[0007]
Patent Document 2 discloses a hot-dip Al-Zn based plated steel sheet including
a plating layer, in which the plating layer includes, by mass%, Al: 25% to 90% and Sn:
0.01% to 10% and further includes 0.01% to 10% of one kind or more selected from the
group consisting of Mg, Ca, and Sr. Patent Document 2 discloses that, since an Al
oxide film formed around the α-Al phase is fractured by Sn and the solubility of the α-
Al phase increases, uniform corrosion of the plating layer in which the α-Al phase and a
- 4 -
Zn-rich phase are dissolved occurs. As a result, selective corrosion of the Zn-rich
phase can be suppressed, and post-coating corrosion resistance is improved.
However, the plated steel sheet disclosed in Patent Document 2 has a
disadvantageous effect in that adhesion with an electrodeposition film for a vehicle is
poor. In addition, in Patent Document 2, Sn addition is essential. Therefore, alloy
costs increase, and there is a problem in that it is difficult to manage a plating bath.
[0008]
In addition, Patent Document 3 discloses a chemical conversion steel sheet
including, a hot-dip Zn-Al-Mg alloy plated steel sheet where a proportion of
[Al/Zn/Zn2Mg ternary eutectic structure] in an outermost surface of a plating layer is 60
area% or more is a substrate, in which a surface of the plating layer is covered with a
chemical conversion film. However, in Patent Document 3, corrosion resistance is
improved by controlling a configuration of the chemical conversion film. In addition,
in order to improve the reactivity with the chemical conversion film, the Al/Zn/Zn2Mg
ternary eutectic structure is set as a primary phase in the plating layer. Therefore,
when a typical chemical conversion treatment is performed, chemical convertibility is
improved, but the microstructure control of the plating layer is not sufficient.
Therefore, it is presumed that post-coating corrosion resistance required for a vehicle
cannot be sufficiently obtained.
[0009]
Accordingly, in the related art, a hot-dip zinc-plated steel sheet that can secure
sufficient post-coating corrosion resistance required for a recent vehicle structural
member is not disclosed.
[Prior Art Document]
[Patent Document]
- 5 -
[0010]
[Patent Document 1] Japanese Patent No. 6350780
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2015-214747
[Patent Document 3] Japanese Patent No. 4579715
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0011]
The present invention has been made in consideration of the above-described
problems. An object of the present invention is to provide a plated steel sheet having
excellent post-coating corrosion resistance on the premise of a plated steel sheet is a
hot-dip zinc-plated steel sheet.
[Means for Solving the Problem]
[0012]
The present inventors conducted an investigation on post-coating corrosion
resistance of a hot-dip zinc-based plated steel sheet containing Al and Mg. As a result,
it was found that post-coating corrosion resistance can be improved by adjusting the Al
content and the Mg content in the plating layer to be in appropriate ranges, controlling
the solidification of the plating layer, and controlling configurations and area ratios of
phases in the plating layer.
[0013]
The present invention has been made based on the above-described findings.
The summary of the present invention is as follows.
(1) According to one aspect of the present invention, a plated steel sheet
includes: a steel sheet; and a plating layer that is formed on at least a part of a surface of
- 6 -
the steel sheet, in which a chemical composition of the plating layer includes, by
mass%, Al: more than 5.00% and 35.00% or less, Mg: 3.00% to 15.00%, Si: 0% to
2.00%, Ca: 0% to 2.00%, and a remainder of Zn and impurities, in which in a cross
section of the plating layer in a thickness direction, an area ratio of a lamellar structure
in which an (Al-Zn) phase and a MgZn2 phase are arranged in layers is 10% to 90%, a
lamellar spacing of the lamellar structure is 2.5 μm or less, and an area ratio of an (Al-
Zn) dendrite is 35% or less.
(2) In the plated steel sheet according to (1), the chemical composition of the
plating layer may include, by mass%, one or more kinds of Al: 11.00% to 30.00%, Mg:
3.00% to 11.00%, and Ca: 0.03% to 1.00%.
(3) The plated steel sheet according to (1) or (2) may further include, by
mass%, Sb: 0.50% or less, Pb: 0.50% or less, Cu: 1.00% or less, Sn: 1.00% or less, Ti:
1.00% or less, Sr: 0.50% or less, Ni: 1.00% or less, Mn: 1.00% or less, and Fe: 2.00% or
less.
(4) In the plated steel sheet according to any one of (1) to (3), in the cross
section of the plating layer in the thickness direction, the area ratio of the lamellar
structure may be 50% to 90%.
(5) In the plated steel sheet according to (4), in the cross section of the plating
layer in the thickness direction, the area ratio of the lamellar structure may be 70% to
90%.
(6) The plated steel sheet according to any one of (1) to (5) may further include
an alloy layer that is provided between the plating layer and the steel sheet and is
formed of an Al-Fe-based intermetallic compound having an average thickness of 0.05
μm to 3.0 μm.
(7) In the plated steel sheet according to any one of (1) to (5), the steel sheet
- 7 -
may include an internally oxidized layer that is provided in a surface layer area on an
interface side between the steel sheet and the plating layer.
(8) In the plated steel sheet according to (6), the steel sheet may include an
internally oxidized layer that is provided in a surface layer area on an interface side
between the steel sheet and the alloy layer.
[Effects of the Invention]
[0014]
According to the aspect of the present invention, a plated steel sheet having
excellent post-coating corrosion resistance can be obtained. The plated steel sheet
according to the present invention is suitable for a vehicle structural member.
Therefore, the present invention improves collision safety of a plated steel sheet for a
vehicle structural member and increases the lifetime thereof, thereby contributing to the
development of the industry.
[Brief Description of the Drawings]
[0015]
FIG. 1 is a diagram showing an example of a plating layer in a plated steel
sheet (No. 15 of Example) according to an embodiment.
FIG. 2 is a diagram showing an example of a plating layer in a plated steel
sheet in the related art.
[Embodiments of the Invention]
[0016]
As shown in FIG. 1, a plated steel sheet according to an embodiment of the
present invention (the plated steel sheet according to the embodiment) include: a steel
sheet 1; and a plating layer 2 that is formed on at least a part of a surface of the steel
sheet 1. In addition, in the plated steel sheet according to the embodiment, the
- 8 -
chemical composition of the plating layer 2 includes, by mass%, Al: more than 5.00%
and 35.00% or less, Mg: 3.00% to 15.00%, Si: 0% to 2.00%, and Ca: 0% to 2.00%, and
optionally further includes one or more kinds of Sb: 0.50% or less, Pb: 0.50% or less,
Cu: 1.00% or less, Sn: 1.00% or less, Ti: 1.00% or less, Sr: 0.50% or less, Ni: 1.00% or
less, Mn: 1.00% or less, Fe: 2.00% or less, and a remainder of Zn and impurities. In
addition, in the plated steel sheet according to the embodiment, in a cross section of the
plating layer 2 in a thickness direction, the area ratio of a lamellar structure 21 in which
an (Al-Zn) phase and a MgZn2 phase are arranged in layers is 10% to 90%, a lamellar
spacing of the lamellar structure 21 is 2.5 μm or less, and the area ratio of an (Al-Zn)
dendrite 23 is 35% or less.
The plated steel sheet according to the embodiment may include an alloy layer
3 that is provided between the steel sheet 1 and the plating layer 2 and is formed of an
intermetallic compound containing Fe and Al.
[0017]

In the plated steel sheet according to the embodiment, the plating layer 2 is
important, and the kind of the steel sheet 1 is not particularly limited. The kind of the
steel sheet 1 may be determined depending on the product to be applied and the
strength, sheet thickness, and the like that are required. For example, a hot-rolled steel
sheet defined in JIS G3193: 2008 or a cold-rolled steel sheet defined in JIS G3141:
2017 can be used.
[0018]
It is preferable that the steel sheet 1 includes an internally oxidized layer 11
that is provided in a surface layer area on an interface side between the steel sheet 1 and
the plating layer 2 (when the alloy layer 3 described is formed between the steel sheet 1
- 9 -
and the plating layer 2, in a surface layer area on an interface side between the steel
sheet 1 and the alloy layer 3).
The internally oxidized layer 11 is formed by annealing the steel sheet before
plating in a predetermined atmosphere. By causing the internally oxidized layer 11 to
be present in the steel sheet 1, when hot-dip plating is performed on the steel sheet 1,
the lamellar structure 21 in which the (Al-Zn) phase and the MgZn2 phase are arranged
in layers is likely to be formed in the plating layer 2. In order to obtain the effect, the
thickness of the internally oxidized layer 11 is preferably 0.10 μm to 8.00 μm.
[0019]
[Alloy layer]
In the plated steel sheet according to the embodiment, the alloy layer 3 may be
formed between the steel sheet 1 and the plating layer 2. By forming the alloy layer 3,
adhesion between the steel sheet 1 and the plating layer 2 is improved, which is
preferable. In order to obtain the effect, the average thickness of the alloy layer 3 is
preferably 0.05 μm to 3.00 μm.
The alloy layer is formed of the Al-Fe-based intermetallic compound (for
example, when the Al-Fe alloy layer or the plating layer 2 includes Si, an Al-Fe-Si alloy
layer).
[0020]
Whether or not the alloy layer 3 is present and the thickness of the alloy layer 3
can be obtained by measuring the thickness of the Al-Fe-based intermetallic compound
from an element distribution image obtained by EDS spectroscopy.
[0021]

In the plated steel sheet according to the embodiment, the plating layer 2 is
- 10 -
provided on at least a part of the surface of the steel sheet 1. The plating layer 2 may
be formed on a single surface or both surfaces of the steel sheet 1.
The adhesion amount of the plating layer is preferably 15 g/m2 to 250 g/m2.
[0022]
[Chemical Composition]
The chemical composition of the plating layer 2 in the plated steel sheet
according to the embodiment will be described.
[0023]
Al: more than 5.00% and 35.00% or less
Al is an element that is effective for securing post-coating corrosion resistance
in the plating layer 2 containing aluminum (Al), zinc (Zn), and magnesium (Mg). In
addition, Al is an element necessary to form the lamellar structure 21 in the plating layer
2 of the plated steel sheet according to the embodiment. In addition, Al is an element
that contributes to the formation of the alloy layer 3 (Al-Fe alloy layer) and is effective
for securing plating adhesion. In order to sufficiently obtain the effects, the Al content
is more than 5.00%. The Al content is preferably 11.00% or more.
On the other hand, when the Al content is more than 35.00%, the area ratio of
the (Al-Zn) dendrite increases, and the post-coating corrosion resistance and the
corrosion resistance of a cut end surface of the plating layer decrease. Therefore, the
Al content is 35.00% or less. The Al content is preferably 30.00% or less.
[0024]
Mg: 3.00% to 15.00%
Mg is an element having an effect of improving the post-coating corrosion
resistance of the plating layer 2. In addition, Mg is an element necessary to form the
lamellar structure 21 in the plating layer 2 of the plated steel sheet according to the
- 11 -
embodiment. In order to sufficiently obtain the effects, the Mg content is preferably
3.00% or more.
On the other hand, when the Mg content is more than 15.00%, the lamellar
structure 21 is not sufficiently formed, post-coating corrosion resistance deteriorates,
and the workability of the plating layer deteriorates. In addition, there is a
manufacturing problem in that, for example, the amount of dross formed in a plating
bath increases. Therefore, the Mg content is 15.00% or less. The Mg content is
preferably 11.00% or less.
[0025]
Si: 0% to 2.00%
Si is an element that forms a compound together with Mg and contributes to
improvement of post-coating corrosion resistance. In addition, Si is an element having
an effect of improving adhesion between the steel sheet 1 and the plating layer 2 by
suppressing the formation of an excessively thick alloy layer between the steel sheet 1
and the plating layer 2 when the plating layer 2 is formed on the steel sheet 1.
Therefore, Si may be contained. In order to obtain the effects, the Si content is
preferably 0.10% or more. The Si content is more preferably 0.20% or more.
On the other hand, when the Si content is more than 2.00%, an excess amount
of Si is crystallized in the plating layer, the lamellar structure 21 is not sufficiently
formed, and post-coating corrosion resistance deteriorates. In addition, the workability
of the plating layer decreases. Accordingly, the Si content is 2.00% or less. The Si
content is more preferably 1.50% or less. Si does not need to be contained, and the
lower limit thereof is 0%.
[0026]
Ca: 0% to 2.00%
- 12 -
When Ca is contained in the plating layer, the amount of dross that is more
likely to be formed during a plating operation along with an increase in Mg content
decreases, and plating manufacturability is improved. Therefore, Ca may be
contained. Ca does not need to be contained, and the lower limit thereof is 0%. In
order to obtain the effect, the Ca content is preferably 0.03% or more and more
preferably 0.10% or more.
On the other hand, when the Ca content is high, the lamellar structure 21 is not
sufficiently formed, the area ratio of Ca-based intermetallic compounds such as a
CaZn11 phase and the like formed as other intermetallic compound phases is 10% or
more, and post-coating corrosion resistance deteriorates. In addition, the post-coating
corrosion resistance of a flat portion of the plating layer itself tends to deteriorate, and
the corrosion resistance of the vicinity of a welded part may also deteriorate.
Therefore, when Ca is contained, the Ca content is 2.00% or less. The Ca content is
preferably 1.00% or less.
[0027]
The chemical composition of the plating layer 2 in the plated steel sheet
according to the embodiment basically includes the above-described chemical
composition and a remainder of Zn and impurities. The amount of the impurities is
preferably 5.0% or less and more preferably 3.0% or less.
However, in the embodiment, the plating layer 2 in the plated steel sheet
according to the embodiment may further include, for example, Sb, Pb, Cu, Sn, Ti, Sr,
Ni, Mn, and Fe in the following ranges instead of a part of Zn. Since these elements
do not need to be contained, the lower limit of the amount of each of the elements is
0%. In addition, even when these elements are contained at an impurity level, there
are substantially no effects on the characteristics of the plating layer.
- 13 -
[0028]
Sb: 0.50% or less
Sr: 0.50% or less
Pb: 0.50% or less
Even when Sr, Sb, and Pb are contained in the plating layer 2, the external
appearance of the plating layer 2 changes, spangle is formed, and improvement of
metallic gloss is verified. However, when the amount of each of these elements is
more than 0.50%, various intermetallic compound phases are formed, and workability
and corrosion resistance deteriorate. In addition, when the amount of each of these
elements is excessive, the viscosity of the plating bath increases, it is difficult to prepare
the plating bath, and a plated steel sheet having excellent plating characteristics cannot
be manufactured. Therefore, it is preferable that the Sr content is 0.50% or less, the Sb
content is 0.50% or less, and the Pb content is 0.50% or less.
[0029]
Sn: 1.00% or less,
Sn is an element that increases a dissolution rate of Mg in the plating layer 2
containing Zn, Al, and Mg. When the dissolution rate of Mg increases, flat portion
corrosion resistance deteriorates. Therefore, the Sn content is preferably 1.00% or
less.
[0030]
Cu: 1.00% or less
Ti: 1.00% or less
Ni: 1.00% or less
Mn: 1.00% or less
These elements are elements contributing to improvement of corrosion
- 14 -
resistance. On the other hand, when the amount of each of these elements is excessive,
the viscosity of the plating bath increases, it is difficult to prepare the plating bath, and a
plated steel sheet having excellent plating characteristics cannot be manufactured.
Therefore, the amount of each of the elements is preferably 1.00% or less.
[0031]
Fe: 2.00% or less
Fe is mixed into the plating layer as an impurity when the plating layer is
manufactured. Although about 2.00% of Fe may be contained, when the Fe content is
in this range, there is little adverse effect on characteristics of the plated steel sheet
according to the embodiment. Therefore, the Fe content is preferably 2.00% or less.
The Fe content is more preferably 1.50% or less and still more preferably 1.00% or less.
[0032]
The chemical composition of the plating layer 2 is measured using the
following method.
First, an acid solution is obtained by peeling and dissolving the plating layer
with an acid containing an inhibitor that suppresses the corrosion of the base metal
(steel). Next, by measuring the obtained acid solution by ICP analysis, the chemical
composition of the plating layer 2 can be obtained (when the alloy layer 3 is formed
between the plating layer 2 and the steel sheet 1, the total chemical composition of the
plating layer 2 and the alloy layer 3 is obtained. However, since the alloy layer 3 is
thin, there is little influence). The kind of the acid is not particularly limited as long as
it is an acid that can dissolve the plating layer. The chemical composition is measured
as an average chemical composition.
[0033]
When it is desired to obtain each of the chemical compositions of the alloy
- 15 -
layer 3 and the plating layer 2, a calibration curve of quantitative analysis of each
element is obtained by glow discharge spectrometry (GDS). Next, the chemical
composition of the plating layer as a target in a depth direction may be measured. For
example, a plurality of samples of 30 mm × 30 mm are collected from the prepared
plated steel sheet as samples for GDS. Argon ion sputtering is performed from the
surface layer of the plating layer to obtain an element intensity plot in the depth
direction. Further, by preparing a standard sample such as each element pure metal
sheet to obtain an element intensity plot in advance, the concentration can be converted
from the intensity plot. When GDS is used for analysis of the chemical composition, it
is preferable that the analysis area is φ4 mm or more, the measurement is performed 10
or more times, and an average value of a component at each of positions is adopted.
The sputter rate is preferably in a range of about 0.04 μm/sec to 0.10 μm/sec.
[0034]
[Structures (Phases) in Plating Layer]
As shown in FIG. 1, the plating layer 2 in the plated steel sheet according to the
embodiment includes the lamellar structure 21 in which the (Al-Zn) phase and the
MgZn2 phase are arranged in layers, in which the area ratio of the lamellar structure 21
is 10% to 90%. In addition, in the plating layer 2 of the plated steel sheet according to
the embodiment, the area ratio of the (Al-Zn) dendrite 23 is 35% or less.
In general, when the steel sheet dipped in the plating bath containing Zn, Mg,
and Al is cooled, as shown in FIG. 2, the (Al-Zn) dendrite 23 as a primary phase and the
Zn/Al/MgZn2 ternary eutectic structure 24 are formed in the plating layer. The (Al-Zn)
dendrite 23 has low corrosion resistance. Therefore, even in a case where the steel
sheet is coated, for example, when defects occur in the coating film, corrosion
propagates into the plating layer and coating blistering occurs.
- 16 -
On the other hand, the lamellar structure 21 in which the (Al-Zn) phase and the
MgZn2 phase are arranged in layers has high corrosion resistance. Therefore, even
when defects occur in the coating film, the propagation of corrosion in the plating layer
is suppressed.
That is, in the plating layer 2 of the plated steel sheet according to the
embodiment, the area ratio of the lamellar structure 21 in which the (Al-Zn) phase and
the MgZn2 phase are arranged in layers in the plating layer and contributes to
improvement of post-coating corrosion resistance, is 10% or more. The area ratio of
the lamellar structure is preferably 50% or more and more preferably 70% or more.
The lamellar structure 21 has an effect of improving not only post-coating corrosion
resistance but also LME resistance. On the other hand, under the pre-condition of the
above-described chemical composition, it is difficult to adjust the area ratio of the
lamellar structure 21 to be more than 90% from the viewpoint of industrial application.
Therefore, the area ratio of the lamellar structure is 90% or less.
In addition, in the plating layer 2 of the plated steel sheet according to the
embodiment, the area ratio of the (Al-Zn) dendrite 23 that deteriorates post-coating
corrosion resistance is 35% or less. The area ratio of the (Al-Zn) dendrite 23 is
preferably as low as possible and may be 0%.
In the lamellar structure 21, the (Al-Zn) phase and the MgZn2 phase are
arranged in layers. As the lamellar spacing of the lamellar structure 21 decreases, the
effect of improving post-coating corrosion resistance and LME resistance increases.
The lamellar spacing for obtaining sufficient performance is 2.5 μm or less (2500 nm or
less) and preferably 500 nm or less. This lamellar structure is also called “feathery
structure”.
The Zn/Al/MgZn2 ternary eutectic structure includes, by area%, Zn phase: 45%
- 17 -
to 60%, MgZn2 phase: 35% to 45%, and Al phase: 3% to 10%. On the other hand, the
fractions of the respective phases in the lamellar structure 21 include, by area%, Zn
phase: 0% to 10%, MgZn2 phase: 40% to 65%, and Al phase: 30% to 45%.
[0035]
The plating layer 2 in the plated steel sheet according to the embodiment may
include a massive MgZn2 phase, the Zn/Al/MgZn2 ternary eutectic structure 24, and
other intermetallic compounds as phases other than the lamellar structure 21 and the
(Al-Zn) dendrite 23 described above. The amount of the remainder is preferably 50%
or less and more preferably 30% or less.
[0036]
The massive MgZn2 phase contributes to improvement of post-coating
corrosion resistance. In order to sufficiently obtain the effect, the area ratio of the
massive MgZn2 phase is preferably 5% or more. On the other hand, from the
viewpoint of workability, the area ratio of the massive MgZn2 phase is preferably 40%
or less.
[0037]
The area ratio of the Zn/Al/MgZn2 ternary eutectic structure 24 is preferably
45% or less. When the area ratio of the Zn/Al/MgZn2 ternary eutectic structure 24 is
more than 45%, post-coating corrosion resistance may deteriorate.
In addition, the MgSi2 phase or other intermetallic compounds deteriorate postcoating
corrosion resistance. Therefore, the area ratio of each of the MgSi2 phase or
other intermetallic compounds is preferably 10% or less. The total area ratio is more
preferably 10% or less. Examples of the other intermetallic compound phases include
a CaZn11 phase, an Al2CaSi2 phase, and an Al2CaZn2 phase.
[0038]
- 18 -
The structures of the plating layer (the area ratios of the respective phases, the
lamellar spacing) are measured using the following method.
A sample having a size of 25 mm in a direction perpendicular to a rolling
direction and 15 mm in the rolling direction is collected from the plated steel sheet
according to the embodiment, and this sample is embedded in a resin and polished such
that a thickness direction of the plating layer is an observed section. Next, a cross
sectional SEM image of the plating layer and an element distribution image by EDS are
obtained. Regarding the area ratios of the lamellar structure, the massive MgZn2
phase, the Zn/Al/MgZn2 ternary eutectic structure, the (Al-Zn) dendrite, the other
intermetallic compounds in the plating layer, one visual field is imaged from each of
five samples having different cross sectional EDS mapping images of the plating layers,
that is, five visual fields (magnification: 1500-fold, 60 μm × 50 μm/One Field) in total
are imaged, and the area ratio of each of the structures is measured from the image.
At this time, when the lamellar spacing between an α phase formed of Al and
Zn and a MgZn2 phase is 2.5 μm or less, this structure is determined as the lamellar
structure. When the minor axis is more than 2.5 μm, this structure is determined as the
massive MgZn2 phase. A lamellar structure of a Zn phase, an α phase, and MgZn2
having a lamellar spacing of 2.5 μm or less is determined as the Zn/Al/MgZn2 ternary
eutectic structure. When the minor axis is 2.5 μm or more, the structure is determined
as the (Al-Zn) dendrite. When 10% or more of metals other than Zn, Al, Mg, and Si
are contained in the phase, the structure is determined as the other intermetallic
compound.
In addition, the lamellar spacing of the lamellar structure is obtained by
measuring the spacing between a phase having the smallest area ratio and a phase
adjacent thereto among phases forming the lamellar structure in the SEM observation
- 19 -
and calculating the average value of spacings measured at 10 positions.
In addition, the area ratio of the lamellar structure and the area ratios of the Zn
phase, the MgZn2 phase, and the Al phase forming the Zn/Al/MgZn2 ternary eutectic
structure can be obtained using a method in which a region where the corresponding
structure is present on the cross sectional SEM image is surrounded by a line using
image processing software or the like to calculate the area of the region surrounded by
the line.
[0039]

Next, a preferable method for manufacturing the plated steel sheet according to
the embodiment will be described. As long as the plated steel sheet according to the
embodiment has the above-described characteristics, the effects can be obtained
irrespective of the manufacturing method. However, with the following method, the
steel sheet can be stably manufactured, which is preferable.
[0040]
Specifically, the steel sheet according to the embodiment can be manufactured
with a manufacturing method including the following processes (I) to (IV).
(I) An annealing process of performing reduction annealing to the steel sheet
(II) A plating process of dipping the steel sheet in a plating bath containing Al,
Mg, and Zn to prepare a plating base sheet
(III) A controlled cooling process of cooling the plating base sheet to a cooling
stop temperature which is range from (Al-Zn/MgZn2 binary eutectic temperature -
30)°C to (Al-Zn/MgZn2 binary eutectic temperature - 10)°C at an average cooling rate
of 15 °C/sec or faster
(IV) A slow cooling process of cooling the plated steel sheet to 335°C or lower
- 20 -
after the controlled cooling process such that an average cooling rate is 5 °C/sec or
slower
[0041]
[Annealing Process]
In the annealing process, a steel sheet (hot-rolled steel sheet or cold-rolled steel
sheet) obtained using a well-known method is annealed before the plating process.
Annealing conditions may be well-known conditions. For example, the steel sheet is
heated to 750°C to 900°C in a 5% H2-N2 gas atmosphere having a dew point - 10°C or
higher and is held at this state for 30 seconds to 240 seconds.
However, in order to form an internally oxidized layer in the steel sheet, it is
preferable that the annealing temperature is 800°C to 870°C and the annealing time is
60 seconds to 130 seconds. When the internally oxidized layer is formed, the
formation of the lamellar structure 21 is promoted by controlled cooling and slow
cooling.
[0042]
[Plating Process]
In the plating process, in the middle of process of temperature decrease after
annealing, the steel sheet 1 is dipped in the plating bath to form the plating layer 2. As
a result, the plating base sheet is formed.
It is preferable that the plating bath includes Al: more than 5.00% and 35.00%
or less, Mg: 3.00% to 15.00%, Si: 0% to 2.00%, Ca: 0% to 2.00%, and a remainder of
Zn and impurities. The composition of the plating bath is substantially the same as the
composition of the plating layer to be formed.
[0043]
[Controlled Cooling Process]
- 21 -
In the controlled cooling process, the plating adhesion amount of the plating
base sheet after the plating process (the plating base sheet that is pulled from the plating
bath) is adjusted by wiping gas such as N2 and is cooled. During cooling, the plating
base sheet is cooled to a cooling stop temperature which is range from (Al-Zn/MgZn2
binary eutectic temperature - 30)°C to (Al-Zn/MgZn2 binary eutectic temperature -
10)°C such that the average cooling rate is 15 °C/sec or faster.
By performing cooling under the above-described conditions, formation of the
(Al-Zn) dendrite is suppressed, a product nucleus of the lamellar structure is formed,
and the lamellar structure is formed in the next slow cooling process.
When the average cooling rate is slower than 15 °C/sec, the (Al-Zn) phase and
the MgZn2 phase do not form the lamellar structure, a large amount of the (Al-Zn)
dendrite is formed, and post-coating corrosion resistance deteriorates.
In addition, when the cooling stop temperature is lower than (Al-Zn/MgZn2
binary eutectic temperature - 30)°C, it is difficult to a sufficient amount of the lamellar
structure in the next slow cooling process. In addition, when the cooling stop
temperature is higher than (Al-Zn/MgZn2 binary eutectic temperature - 10)°C, the α
phase and the MgZn2 phase do not satisfy conditions of eutectic solidification, and thus
a large amount of an (Al-Mg) dendrite is formed.
The upper limit of the average cooling rate does not need to be limited but may
be 40 °C/sec or slower due to restrictions in facility or the like.
The Al-Zn/MgZn2 binary eutectic temperature can be obtained from a
projection of liquidus of Zn-Al-Mg ternary phase diagram.
[0044]
[Slow Cooling Process]
In the slow cooling process, the plating base sheet after the completion of
- 22 -
controlled cooling is cooled to 335°C or lower such that the average cooling rate is
5 °C/sec or slower.
In this slow cooling process, the product nucleus of the lamellar structure
formed in the controlled cooling process grows, and a predetermined area ratio of the
lamellar structure is obtained.
When the average cooling rate during cooling to 335°C or lower is faster than
5 °C/sec, the nucleus growth is insufficient, and the area ratio of the lamellar structure is
insufficient.
[0045]
With the above-described manufacturing method, the plated steel sheet
according to the embodiment can be obtained.
[Examples]
[0046]
(Example 1)
As a steel sheet to be subjected to annealing and plating, a cold-rolled steel
sheet (0.2% C-2.0% Si-2.3% Mn) having a sheet thickness of 1.6 mm was prepared.
This steel sheet was cut into 100 mm × 200 mmm, and subsequently annealing
and hot-dip plating were performed using a batch type hot dip plating test apparatus.
During annealing, in a furnace having an oxygen concentration of 20 ppm or
lower, annealing was performed at 860°C for 120 seconds in an atmosphere formed of
gas containing 5% of H2 gas and a remainder of N2 and having a dew point of 0°C.
After annealing, the steel sheet was air-cooled with N2 gas such that the steel
sheet temperature reached the bath temperature + 20°C, and was dipped in the plating
bath having a bath temperature shown in Table 1 for about 3 seconds. The
composition of the plating bath and the composition of the formed plating layer are as
- 23 -
shown in Table 1.
The plating base sheet on which the plating layer was formed was cooled to
room temperature by controlled cooling and slow cooling under conditions shown in
Table 1.
The temperature of the steel sheet was measured using a thermocouple spotwelded
to a central part of the plating base sheet.
[0047]
Regarding the obtained plated steel sheet, the area ratios of the respective
phases in the plating layer and the lamellar spacing of the lamellar structure were
measured using the following method.
A sample having a size of 25 mm in a direction perpendicular to a rolling
direction and 15 mm in the rolling direction was collected from the obtained plated steel
sheet, and this sample was embedded in a resin and polished such that a thickness
direction of the plating layer was an observed section. Next, a cross sectional SEM
image of the plating layer and an element distribution image by EDS were obtained.
Regarding the area ratios of the lamellar structure, the massive MgZn2 phase, the
Zn/Al/MgZn2 ternary eutectic structure, the (Al-Zn) dendrite, the other intermetallic
compounds in the plating layer, one visual field was imaged from each of five samples
having different cross sectional EDS mapping images of the plating layers, that is, five
visual fields (magnification: 1500-fold, 60 μm × 50 μm/One Field) in total were
imaged, and the area ratio of each of the structures was measured by image analysis.
In addition, the lamellar spacing of the lamellar structure was obtained by
measuring the spacing between a phase having the smallest area ratio and a phase
adjacent thereto among phases forming the lamellar structure in the SEM observation
and calculating the average value of spacings measured at 10 positions.
- 24 -
In addition, when the alloy layer was formed between the steel sheet and the
plating layer, the thickness of the alloy layer was obtained by measuring the thickness of
the Al-Fe-based intermetallic compound from an element distribution image obtained
by EDS spectroscopy.
FIG. 1 is a SEM image (BSE image) of No. 15 in Tables 1 and 2. The plating
layer included the lamellar structure (the feathery structure-21 in FIG. 1) and the
massive MgZn2 phase (22 in FIG. 1), in which the alloy layer (3 in FIG. 1) was formed
at an interface between the plating layer and the steel sheet. Further, the internally
oxidized layer (11 in FIG. 1) was formed in the surface layer area of the steel sheet. In
addition, the lamellar structure included, by area%, Zn phase: 0% to 10%, MgZn2
phase: 40% to 65%, and Al phase: 30% to 45%.
[0048]
In addition, the obtained plated steel sheet, a commercially available
galvanized steel sheet (No. 52), an alloyed galvanized steel sheet (No. 53), and an
electrogalvanized steel sheet (No. 54) were evaluated for post-coating corrosion
resistance.
Specifically, a sample of 50 mm × 100 mm was collected from the plated steel
sheet, and a zinc phosphating (SD5350 system, manufactured by Nippon Paint
Industrial Coatings Co., Ltd.) was performed thereon. Next, an electrodeposition
coating (PN110 POWERNICS (registered tradename) GREY, manufactured by Nippon
Paint Industrial Coatings Co., Ltd.) having a thickness of 20 μm was formed and was
baked at a baking temperature of 150°C for 20 minutes. Next, crosscut damages (40
×√2, two damages) reaching the steel sheet (base metal) were formed. The painted
plated steel sheet was provided for a combined cyclic corrosion test according to JASO
(M609-91), the maximum blister width was measured at eight positions around the
- 25 -
crosscuts, and the average value thereof was obtained to evaluate post-coating corrosion
resistance.
When the number of cycles of JASO (M609-91) was 180, a case where the
blister width from the crosscut damage was less than 1.5 mm was evaluated as “AA”, a
case where the blister width from the crosscut damage was 1.5 mm to 2.5 mm was
evaluated as “A”, and a case where the blister width from the crosscut damage was
more than 2.5 mm was evaluated as “B”.
In addition, not only coating blistering but also red rust resistance were
evaluated according to JASO (M609-91). A case where red rust occurred from the
crosscut damages when the number of cycles was 240 was evaluated as “AA”, a case
where red rust occurred from the crosscut damages when the number of cycles was 120
or more and less than 240 was evaluated as “A”, a case where red rust occurred from the
crosscut damages when the number of cycles was 60 or more and less than 120 was
evaluated as “B”, and a case where red rust occurred from the crosscut damages when
the number of cycles was less than 60 was evaluated as “C”.
When both coating blistering and red rust resistance were evaluated as A or
AA, post-coating corrosion resistance was determined to be excellent.
The results are shown in Table 2.
[0049]
[Table 1]
Classification No.
Plating Bath
Temperature
(°C)
Plating Layer Composition
(mass%)
Controlled Cooling Slow Cooling
Average
Cooling Rate
from Bath
Temperature
to Controlled
Cooling Stop
Temperature
(°C/sec)
Binary
Eutectic
Temperature
(ºC)
Controlled
Cooling
Stop
Temperature
Average
Cooling Rate
from
Controlled
Cooling Stop
Temperature
to 335ºC
(°C/sec)
Zn Al Mg Ca Si
Other Elements
Main
Element
(Content %)
Total%
Comparative
Example
1 440 91.6 5.00 3.00 0.20 0.20 - 0.00 15 355 335 5
Comparative
Example
2 455 91.6 6.00 2.00 0.20 0.20 - 0.00 15 430 410 5
- 26 -
Example 3 430 91.0 6.00 3.00 0.00 0.00 - 0.00 15 380 360 2
Example 4 450 84.9 10.00 5.00 0.10 0.00 - 0.00 15 420 400 5
Example 5 470 82.8 10.00 7.00 0.10 0.00 Ni:0.10 0.10 15 430 410 5
Example 6 500 79.0 10.00 10.00 1.00 0.00 Sb:0.05 0.05 15 430 410 5
Comparative
Example
7 460 83.0 10.00 7.00 0.00 0.00 - 0.00 15 430 335 5
Example 8 460 85.8 11.00 3.00 0.00 0.20 Ti:0.01 0.01 15 400 380 5
Example 9 480 80.4 12.00 7.50 0.10 0.00 - 0.00 15 440 420 5
Example 10 540 71.5 12.00 15.00 1.50 0.00 - 0.00 20 440 420 5
Example 11 470 82.7 14.00 3.00 0.10 0.00 Mn:0.20 0.20 20 430 410 5
Example 12 470 78.9 14.00 7.00 0.10 0.00 - 0.00 20 430 410 5
Example 13 470 70.9 14.00 15.00 0.10 0.00 - 0.00 20 430 410 5
Example 14 480 76.4 16.00 7.50 0.10 0.00 - 0.00 20 440 420 5
Example 15 480 76.0 16.00 7.50 0.10 0.00 Ni:0.40 0.40 20 440 420 5
Comparative
Example
16 480 74.9 18.00 7.00 0.10 0.00 - 0.00 20 440 335 5
Comparative
Example
17 480 74.9 18.00 7.00 0.10 0.00 - 0.00 5 440 420 5
Comparative
Example
18 480 74.9 18.00 7.00 0.10 0.00 - 0.00 20 440 420 20
Example 19 480 78.7 18.00 3.00 0.10 0.20 - 0.00 20 440 420 5
Example 20 480 74.4 18.00 7.50 0.10 0.00 Ti:0.01 0.01 20 440 420 5
Example 21 480 66.9 18.00 15.00 0.10 0.00 - 0.00 20 440 420 5
Example 22 500 76.4 20.00 3.00 0.10 0.00 Fe:0.50 0.50 20 415 395 5
Example 23 500 72.5 20.00 7.50 0.00 0.00 - 0.00 20 460 440 5
Example 24 540 72.4 20.00 7.50 0.10 0.00 Pb:0.02 0.02 20 460 440 5
Example 25 510 64.8 20.00 15.00 0.10 0.10 - 0.00 20 470 450 5
Example 26 510 69.9 22.00 8.00 0.10 0.00 Sn:0.02 0.02 20 470 450 5
Example 27 510 69.6 22.00 8.00 0.20 0.20 - 0.00 20 470 450 5
Comparative
Example
28 510 69.8 22.00 8.00 0.20 0.00 - 0.00 20 470 335 5
Comparative
Example
29 510 69.8 22.00 8.00 0.20 0.00 - 0.00 5 470 450 5
Comparative
Example
30 510 69.8 22.00 8.00 0.20 0.00 - 0.00 20 470 450 20
Comparative
Example
31 510 67.5 22.00 8.00 2.50 0.00 - 0.00 20 470 450 5
Comparative
Example
32 510 67.6 22.00 8.00 0.20 2.20 - 0.00 20 470 450 5
Example 33 515 67.9 24.00 8.00 0.10 0.00 - 0.00 20 470 450 5
Example 34 515 67.8 24.00 8.00 0.10 0.00 Mn:0.10 0.10 20 470 450 5
Example 35 510 65.9 26.00 8.00 0.10 0.00 - 0.00 20 475 455 5
Comparative
Example
36 510 56.8 27.00 16.00 0.20 0.00 - 0.00 20 475 455 5
Comparative
Example
37 510 64.8 27.00 8.00 0.20 0.00 - 0.00 20 475 455 20
Example 38 510 63.8 28.00 8.00 0.10 0.10 - 0.00 20 475 455 5
Example 39 510 63.7 28.00 8.00 0.10 0.20 - 0.00 20 475 455 5
Example 40 510 66.6 30.00 3.00 0.10 0.30 - 0.00 20 435 415 5
Example 41 510 61.6 30.00 7.00 0.10 0.30 Cu:1.00 1.00 20 480 460 5
Example 42 510 54.3 30.00 15.00 0.10 0.60 - 0.00 20 480 460 5
Example 43 520 60.3 32.00 7.00 0.10 0.60 - 0.00 20 490 470 5
Example 44 520 60.3 32.00 7.00 0.10 0.60 Sr:0.02 0.02 20 490 470 5
Example 45 520 62.1 34.00 3.00 0.10 0.80 - 0.00 20 490 470 5
Example 46 520 58.1 34.00 7.00 0.10 0.80 - 0.00 20 490 470 5
Example 47 540 49.9 34.00 15.00 0.10 1.00 - 0.00 20 490 470 5
Example 48 540 57.4 34.00 7.00 0.10 1.50 - 0.00 20 500 480 5
Example 49 540 55.9 35.00 7.00 0.10 2.00 - 0.00 20 500 480 5
Comparative
Example
50 580 55.8 37.00 7.00 0.20 0.00 - 0.00 20 510 490 5
Comparative
Example
51 480 74.9 18.00 4.00 0.10 0.00 - 0.00 20 440 300 -
Comparative
Example
52 Commercially Available Galvanized Steel Sheet
53 Commercially Available Alloyed Galvanized Steel Sheet
54 Commercially Available Electrogalvanized Steel Sheet
[0050]
- 27 -
[Table 2]
Classification No.
Plating Layer
Alloy
Layer
Internally
Oxidized
Layer
Evaluation
Result
(Al-Zn)/MgZn2
Lamellar
Structure
Massive
MgZn2
Phase
(Al-Zn)
Dendrite
Zn/Al/MgZn2
Ternary
Eutectic
Structure
Mg2Si Phase
Other
Intermetallic
Compounds
Post-Coating
Corrosion
Resistance
Area%
Lamellar
Spacing
(nm)
Area% Area% Area%
Circle
Equivalent
Diameter
(μm)
Area% Area%
Thickness
(μm)
Thickness
(μm)
Red
Rust
Coating
Blistering
Comparative
Example
1 7 480 15 10 68 - 0.0 0.0 0.30 1.10 A B
Comparative
Example
2 8 480 13 12 67 - 0.0 0.0 0.30 1.20 A B
Example 3 10 500 11 16 63 - 0.0 0.0 0.10 1.50 A A
Example 4 11 500 12 0 77 - 0.0 0.0 0.10 2.00 A A
Example 5 13 480 11 0 76 - 0.0 0.0 0.10 2.00 A A
Example 6 15 480 22 0 63 - 0.0 0.0 0.20 1.50 A A
Comparative
Example
7 2 450 20 11 67 - 0.0 0.0 0.20 2.00 B B
Example 8 15 500 12 28 45 - 0.0 0.4 0.20 1.00 AA A
Example 9 26 480 15 4 53 - 0.0 2.1 0.20 1.90 AA A
Example 10 24 470 32 0 44 - 0.0 0.0 0.10 2.00 AA A
Example 11 25 300 10 29 36 - 0.0 0.2 0.20 1.40 AA A
Example 12 41 480 10 2 47 - 0.0 0.0 0.20 2.00 AA A
Example 13 43 490 29 4 24 - 0.0 0.0 0.30 2.00 AA A
Example 14 70 500 10 2 18 - 0.0 0.0 0.20 1.50 AA AA
Example 15 71 480 8 0 21 - 0.0 0.5 0.20 2.10 AA AA
Comparative
Example
16 4 450 30 36 30 - 0.0 0.0 0.20 2.10 B B
Comparative
Example
17 3 480 29 37 31 - 0.0 0.0 0.20 2.20 B B
Comparative
Example
18 4 410 30 34 32 - 0.0 0.0 0.20 2.00 A B
Example 19 33 490 5 35 23 1.9 4.0 0.0 0.20 2.00 A A
Example 20 83 470 16 0 0 - 0.0 0.7 0.20 2.00 AA AA
Example 21 48 430 33 15 4 - 0.0 0.0 0.20 2.00 A A
Example 22 44 410 0 33 23 - 0.0 0.0 0.20 1.90 A A
Example 23 68 380 18 14 0 - 0.0 0.0 0.20 1.20 AA A
Example 24 82 490 13 5 0 - 0.0 0.2 0.20 1.10 AA AA
Example 25 72 450 20 8 0 - 0.0 0.0 0.20 1.30 A A
Example 26 82 430 15 3 0 - 0.0 0.2 0.20 1.40 AA AA
Example 27 80 410 20 0 0 - 0.0 0.0 0.20 1.50 AA AA
Comparative
Example
28 5 410 29 40 26 - 0.0 0.0 0.20 2.20 B B
Comparative
Example
29 3 450 29 42 26 - 0.0 0.0 0.20 1.90 B B
Comparative
Example
30 5 420 28 36 31 - 0.0 0.0 0.20 1.50 B B
Comparative
Example
31 9 430 29 39 12 - 0.0 11.0 0.20 1.60 B B
Comparative
Example
32 8 440 30 44 5 2.1 13.0 0.0 0.20 1.70 B B
Example 33 81 430 15 4 0 - 0.0 0.0 0.20 1.20 AA AA
Example 34 82 480 11 7 0 - 0.0 0.4 0.20 1.30 AA AA
Example 35 83 420 13 4 0 - 0.0 0.0 0.20 1.90 AA AA
Comparative
Example
36 8 420 39 36 6 - 0.0 11.0 0.20 1.70 B B
Comparative
Example
37 2 410 29 44 25 - 0.0 0.0 0.20 1.80 B B
Example 38 44 460 28 28 0 - 0.0 0.0 0.20 2.10 A A
Example 39 70 480 19 11 0 - 0.0 0.0 0.20 2.50 AA AA
Example 40 51 410 0 30 19 - 0.0 0.1 0.20 3.40 A AA
Example 41 72 420 18 10 0 - 0.0 0.0 0.20 2.10 AA AA
Example 42 62 480 38 0 0 - 0.0 0.0 0.20 2.50 AA A
- 28 -
Example 43 64 430 11 25 0 - 0.0 0.0 0.20 2.10 A A
Example 44 63 480 13 24 0 - 0.0 0.1 0.20 2.60 A A
Example 45 48 410 0 34 18 - 0.0 0.0 0.20 2.10 A A
Example 46 49 410 0 35 16 - 0.0 0.0 0.20 1.10 A A
Example 47 49 400 39 12 0 - 0.0 0.0 0.20 0.70 A A
Example 48 48 200 13 35 0 1.1 4.0 0.0 0.20 1.50 A A
Example 49 45 210 15 32 0 1.6 8.0 0.0 0.20 2.10 A A
Comparative
Example
50 32 200 8 37 23 - 0.0 0.0 0.20 1.30 A B
Comparative
Example
51 3 410 29 38 30 - 0.0 0.0 0.20 2.00 A B
Comparative
Example
52 Commercially Available Galvanized Steel Sheet B B
53 Commercially Available Alloyed Galvanized Steel Sheet C B
54 Commercially Available Electrogalvanized Steel Sheet B B
[0051]
As can be seen from the results of Tables 1 and 2, in Examples prepared from
the predetermined plating bath composition under appropriate cooling conditions, the
area ratio of the feathery structure formed of the lamellar structure in which the (Al-Zn)
phase and the MgZn2 phase were arranged in layers was 10% to 90%, the lamellar
spacing of the lamellar structure was 2.5 μm (2500 nm) or less, and the area ratio of the
(Al-Zn) dendrite was 35% or less. In addition, the results show that post-coating
corrosion resistance was excellent.
On the other hand, in Comparative Examples prepared with an inappropriate
plating bath composition or under inappropriate cooling conditions, the composition of
the plating layer was outside of the present invention, and post-coating corrosion
resistance was poor.
[0052]
(Example 2)
Regarding Tests No. 3 to 6, 8 to 15, 19 to 27, 33 to 35, and 38 to 49, LME
resistance was evaluated.
Specifically, a sample of 200 mm × 20 mm was collected from the plated steel
sheet and was provided for a hot tensile test, and a stress-strain curve at 800°C was
measured. In the hot tensile test, a tension rate was 5 mm/min, the chuck-to-chuck
distance was 112.5 mm, and the strain amount was measured until the maximum stress
- 29 -
was reached in the obtained stress-strain curve. A case where the strain amount was
85% or more with respect to a steel sheet sample on which plating was not performed
was evaluated as “AA”, and a case where the strain amount was 70% to 80% with
respect to a steel sheet sample on which plating was not performed was evaluated as
“A”.
The results are shown in Table 3.
[0053]
As can be seen from Table 3, although all of Examples have a certain level of
LME resistance, in particular, in Examples where the area ratio of the lamellar structure
was 70% or more, LME resistance was higher.
[0054]
[Table 3]
Classification No.
LME
Resistance
Example 3 A
Example 4 A
Example 5 A
Example 6 A
Example 8 A
Example 9 A
Example 10 A
Example 11 A
Example 12 A
Example 13 A
Example 14 AA
Example 15 AA
Example 19 A
Example 20 AA
Example 21 A
Example 22 AA
Example 23 A
Example 24 AA
Example 25 AA
Example 26 AA
Example 27 AA
Example 33 AA
Example 34 AA
Example 35 AA
Example 38 A
Example 39 AA
- 30 -
Example 40 A
Example 41 AA
Example 42 A
Example 43 A
Example 44 A
Example 45 A
Example 46 A
Example 47 A
Example 48 A
Example 49 A
[Brief Description of the Reference Symbols]
[0055]
1: steel sheet
2: plating layer
3: alloy layer
11: internally oxidized layer
21: lamellar structure of (Al-Zn) Phase (in the drawing, black) and MgZn2
phase
22: massive MgZn2 phase
23: (Al-Zn) dendrite
24: Zn/Al/MgZn2 ternary eutectic structure

WE CLAIMS

1. A plated steel sheet comprising:
a steel sheet; and
a plating layer that is formed on at least a part of a surface of the steel sheet,
wherein a chemical composition of the plating layer includes, by mass%,
Al: more than 5.00% and 35.00% or less,
Mg: 3.00% to 15.00%,
Si: 0% to 2.00%,
Ca: 0% to 2.00%, and
a remainder of Zn and impurities,
in a cross section of the plating layer in a thickness direction,
an area ratio of a lamellar structure in which an (Al-Zn) phase and a MgZn2
phase are arranged in layers is 10% to 90%,
a lamellar spacing of the lamellar structure is 2.5 μm or less, and
an area ratio of an (Al-Zn) dendrite is 35% or less.
2. The plated steel sheet according to claim 1,
wherein the chemical composition of the plating layer includes, by mass%, one
or more kinds of
Al: 11.00% to 30.00%,
Mg: 3.00% to 11.00%, and
Ca: 0.03% to 1.00%.
3. The plated steel sheet according to claim 1 or 2, further comprising, by mass%:
Sb: 0.50% or less;
Pb: 0.50% or less;
- 32 -
Cu: 1.00% or less;
Sn: 1.00% or less;
Ti: 1.00% or less;
Sr: 0.50% or less;
Ni: 1.00% or less;
Mn: 1.00% or less; and
Fe: 2.00% or less.
4. The plated steel sheet according to any one of claims 1 to 3,
wherein in the cross section of the plating layer in the thickness direction, the
area ratio of the lamellar structure is 50% to 90%.
5. The plated steel sheet according to claim 4,
wherein in the cross section of the plating layer in the thickness direction, the
area ratio of the lamellar structure is 70% to 90%.
6. The plated steel sheet according to any one of claims 1 to 5, further comprising an
alloy layer that is provided between the plating layer and the steel sheet and is formed of
an Al-Fe-based intermetallic compound having an average thickness of 0.05 μm to 3.0
μm.
7. The plated steel sheet according to any one of claims 1 to 5,
wherein the steel sheet includes an internally oxidized layer that is provided in
a surface layer area on an interface side between the steel sheet and the plating layer.
8. The plated steel sheet according to claim 6,
wherein the steel sheet includes an internally oxidized layer that is provided in
a surface layer area on an interface side between the steel sheet and the alloy layer.