TECHNICAL FIELD
[0001] The present invention relates to a plated
steel sheet including an Al-containing Zn-based
plating layer on at least a part of a surface of a
steel sheet.
BACKGROUND ART
[0002] A plated steel sheet has been used as a
structural member of an automobile from a viewpoint
of rust prevention. As a plated steel sheet for
automobile, there can be cited an alloyed galvanized
steel sheet and a hot-dip galvanized steel sheet, for
example.
[0003] The alloyed galvanized steel sheet has an
advantageous point that it is excellent in
weldability and corrosion resistance after coating.
One example of the alloyed galvanized steel sheet is
described in Patent Literature 1. However, a plating
layer of the alloyed galvanized steel sheet is
relatively hard due to diffusion of Fe which occurs
at a time of alloying treatment, so that it is easily
peeled off when compared to a plating layer of the
hot-dip galvanized steel sheet. Specifically, a
crack is likely to occur in the plating layer due to
an external pressure, the crack propagates up to an
interface between the plating layer and a base steel
sheet, and the plating layer is likely to peel off
from the interface as a starting point.
- 1 -
For this
reason, when the alloyed galvanized steel sheet is
used as an outer panel of an automobile, there is a
case where a collision of small stones (chipping) due
to stone splash with respect to a traveling vehicle
occurs, resulting in that a plating layer is peeled
off together with a coating, and a base steel sheet
is exposed and is likely to be corroded. Further,
the plating layer of the alloyed galvanized steel
sheet contains Fe, so that when the coating is peeled
off due to the chipping, the plating layer itself is
corroded, and a reddish-brown rust is sometimes
generated. There is also a case where powdering and
flaking occur in the plating layer of the alloyed
galvanized steel sheet.
[0004] The plaLlng layer of tl1e hot-dip galvanized
steel sheet which is not subjected to the alloying
treatment does not contain Fe, and thus is relatively
soft. For this reason, with the use of the hot-dip
galvanized steel sheet, it is possible to make it
difficult to cause corrosion accompanied by the
chipping, and it is alsd possible to suppress the
powdering and the flaking. One example of the hotdip
galvanized steel sheet is described in each of
Patent Literatures 2 to 5. However, because of a low
melting point of the plating layer of the hot-dip
galvanized steel sheet, seizing with respect to a
metal mold is likely to occur at a time of press
forming. Further, there is also a case where a crack
occurs in the plating layer at a time of the press
- 2 -
forming and bending.
[0005] As described above, in the conventional
plated steel sheets, it cannot be said that all of a
powdering resistance, a seizing resistance, a crack
resistance, and a chipping resistance are suitable
for the application of an automobile.
CITATION LIST
PATENT LITERATURE
[0006] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2003-253416
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2006-348332
Patent Literature 3: Japanese Laid-open Patent
Publication No. 2005-154856
Patent Literature 4: Japanese Laid-open Patent
Publication No. 2005-336546
Patent Literature 5: Japanese Laid-open Patent
Publication No. 2004-323974
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0007] The present invention has an object to
provide a plated steel sheet capable of obtaining an
excellent chipping resistance, and capable of
suppressing powdering and seizing with respect to a
metal mold at a time of press forming and an
occurrence of crack at a time of working.
SOLUTION TO PROBLEM
[0008] The present inventors conducted earnest
studies in order to solve the above-described
- 3 -
problems. As a result of this, they found out that
when a plating layer is provided with a predetermined
chemical composition and predetermined structures, it
is possible to obtain an excellent chipping
resistance, and it is possible to suppress powdering
and seizing with respect to a metal mold at a time of
press forming and an o~~llrrence of crack at a time of
working. Hereinafter, a plastic deformability, a
seizing resistance, and a powdering resistance are
sometimes named generically as workability. Further,
the present inventors also found out that the
aforementioned predetermined structures cannot be
obtained by a conventional manufacturing method of a
plated steel sheet, and the predetermined structures
can be obtained when a plated steel sheeL ls
manufactured through a method different from the
conventional method. Based on such findings, the
present inventors arrived at various embodiments of
the invention to be described below.
[0009] (1)
A plated steel sheet is characterized in that it
includes an Al-containing Zn-based plating layer on
at least a part .of a surface of a steel sheet, in
which an average chemical composition of the plating
layer and an intermetallic compound layer between the
plating layer and the steel sheet is represented by,
in terms of mass%, Al : 1 0% to 4 0%, S i : 0 . 0 5% to 4%,
Mg: 0% to 5%, and the balance: Zn and impurities, the
plating layer includes a first structure constituted
- 4 -
from Al phases containing Zn in solid solution and Zn
phases dispersed in the Al phases and having an
average chemical composition represented by, in terms
of mass%, Al: 25% to 50%, Zn: 50% to 75%, and
impurities: less than 2%, and a eutectoid structure
constituted from Al phases and Zn phases and having
an average chemical composition represented by, in
terms of mass%, Al: 10% to 24%, Zn: 76% to _90%, and
impurities:' less than 2%, in a cross section of the
plating layer, an area fraction of the first
structure is 5% to 40%, and a total area fraction of
the first structure and the eutectoid structure is
50% or more, an area fraction of Zn phases which are
structures containing 90% or more of Zn, contained in
the plating layer is 25% or less, a total area
fraction of intermetallic compound phases contained
in the plating layer is 9% or less, and a thickness
of the intermetallic compound layer is 2 ~m or less.
[0010] (2)
The plated steel sheet described in (1) is
characterized in that a number density of the, first
structure on a surface of the plating layer is 1.6
pieces/cm2 to 25.0 pieces/cm2
•
[0011] (3)
The plated steel sheet described in (1) or (2) is
characterized in that the first structure includes a
second structure having an average chemical
composition represented by, in terms of mass%, Al:
37% to 50%, Zn: 50% to 63%, and impurities: less than
- 5 -
2%, and a third structure having an average chemical
composition represented by, in terms of mass%, Al:
25% to 36%r Zn: 64% to 75%, and impurities: less than
2 9- 0 •
[0012] (4)
The plated steel sheet described in any of (1) to
(3) is characterized in that. the average chemical
composition of the plating layer and the
intermetallic compound layer is represented by, in
terms of mass%, Al: 20% to 40%, Si: 0.05% to 2.5%,
Mg: 0% to 2%, and the balance: Zn and impurities.
[0013] (5)
The plated steel sheet described in any of (1) to
(4) is characterized in that the thickness' of the
intermetallic compound layer is 100 nm to 1000 nm.
[0014] (6)
The plated steel sheet described in any of (1) to
(5) is characterized in that in the cross section of
the plating layer, the area fraction of the first
structure is 20% to 40%, the area fraction of the
eutectoid structure is 50% to 70%, and the'· total area
fraction' of the first structure and the eutectoid
structure is 90% or more.
[0015] (7)
The plated steel sheet described in any of (1) to
(6) is characterized in that in the cross section of
the plating layer, the area fraction of the first
structure is 30% to 40%, the area fraction of the
eutectoid structure is 55% to 65%, and the total area
- 6 -
frac.tion of the first structure and th,e eutectoid
structure is 95% or more.
[0016] (8)
The plated steel sheet described in any of (1) to
(7) is characterized in that in the average chemical
composition of the plating layer and the
intermetallic compound layer, the Mg concentration is
0.05% to 5%, when the Mg concentration is set to Mg%
and the Si concentration is set to Si%, a
relationship of "Mg% < 2 X Si%" is satisfied, and a
crystal of Mg 2Si which exists in the plating layer is
2 ~m or less in terms of maximum equivalent circle
diameter.
[0017] (9)
The plated steel sheet described in any of (1) to
(8) is characterized in that a volume fraction of the
Zn phases contained in the plating layer is 20% or
less.
ADVANTAGEOUS EFFECTS OF INVENTION
[0018] According to the present invention, a plating
layer is provided with predetermined chemical
composition and structures, and thus it is possible
to obtain an excellent chipping resistance, and
suppress powdering and seizing with respect to a
metal mold at a time of press forming and an
occurrence of crack at a time of working.
BRIEF DESCRIPTION OF DRAWINGS
[0019] [FIG. 1] FIG. 1 is a sectional view
illustrating one example of a plating layer included
- 7 -
in a plated steel sheet according to an embodiment of
the present invention;
[FIG. 2A] FIG. 2A is a view i~lustrating an
outline of a 2T bending test;
[FIG. 2B] FIG. 2B is a view illustrating an
outline of a lT bending test;
[FIG. 2C] FTG. 2C is a view illustrating an
outline of a OT bending test;
[FIG. 3] FIG. 3 is a view illustrating a change
of temperature (heat pattern) of a plated steel sheet
at a time of manufacturing a plated steel sheet of
test No. 16 being an invention example;
[FIG. 4] FIG. 4 is a view illustrating a BSE
image of the plated steel sheet of test No. 16;
[FIG. 5] FIG. 5 is a view illustrating a BSE
image of a plated steel sheet of test No. 92 being an
invention example;
[FIG. 6] FIG. 6 is a view illustrating a change
of temperature (heat pattern) of a plated steel sheet
:at a time of manufacturing a plated steel sheet of
test No. 20 being a comparative example; and
[FIG. 7] FIG. 7 is a view illustrating a BSE
image of the plated steel sheet of test No. 20.
DESCRIPTION OF EMBODIMENTS
[0020] Hereinafter, embodiments of the present
invention will be described. A plated steel sheet
according to the present embodiment relates to a
plated steel sheet including an Al-containing Znbased
plating layer on at least a part of a surface
- 8 -
of a steel sheet.
[0021] First, an average chemical composition of a
plating layer and an intermetallic compound layer
between the plating layer and a steel sheet will be
described. In the description hereinbelow, "%" being
a unit of concentration of each element means "mass%"
unless otherwise noted. The average chemical
composition of the plating layer and the
intermetallic compound layer included in the plated
steel sheet according to the present embodiment is
represented by Al: 10% to 40%, Si: 0.05% to 4%, Mg:
0% to 5%, and the balance: Zn and impurities.
[0022] (Al: 10% to 40%)
Al contributes to increase in a melting point and
improvement of hardness of an Al-containing Zn-based
plating layer. As the melting point of the plating
layer increases, seizing at a time of press forming
becomes difficult to occur. When an Al concentration
is less than 10%, the melting point of the plating
layer does not become higher than a melting point of
a plating layer composed of pure Zn, resulting in
that the seizing cannot be sufficiently suppressed.
Therefore, the Al concentration is set to 10% or more,
and preferably set to 20% or more. When the Al
concentration is 10% or more, the higher the Al
concentration, the higher a melting point of a Zn-Al
alloy, and a melting point of a Zn-Al alloy whose Al
concentration is about 40% is about 540°C.
[0023] Al can also contribute to improvement of
- 9 -
ductility of the Al-containing Zn-based plating layer.
By the studies conducted by the present inventors, it
has been clarified that the ductility of the Alcontaining
Zn-based plating layer is particularly
excellent when the Al concentration is 20% to 40%,
but, it is lower than the ductility of the plating
layer composed of pure Zn when the Al concentration
is less than 5% or greater than 40%. Therefore, Lhe
Al concentration is set to 40% or less.
[0024] (Si: 0.05% to 4%)
Si suppresses a reaction between Zn and Al
contained in a plating bath and Fe contained in a
steel sheet being a plating original sheet at a time
of forming a plating layer, to thereby suppress
generation of an intermetalllc compound layer at a
position between the plating layer and the steel
sheet. Although details will be described later, the
intermetallic compound layer contains an Al-Zn-Fe
compound, for example, and is also called as an
interface alloy layer, which reduces adhesiveness
between the plating layer and the steel sheet and
reduces the workability. When a concentration of Si
contained in the plating bath is less than 0.05%, the
intermetallic compound layer starts to grow
immediately after the plating original sheet is
immersed into the plating bath, resulting in that the
intermetallic compound layer is excessively formed,
and the reduction in the workability becomes
significant. Therefore, the Si concentration in the
- 10 -
plating bath is set to 0.05% or more, and an average
Si concentration in the plating layer and the
intermetallic compound layer is also set to 0.05% or
more. On the other hand, when the Si concentration
is greater than 4%, a Si phase to be a starting point
of fracture is likely to remain in the plating layer,
and it is sometimes impossible to obtain sufficient
ductility. Therefore, the Si concentration is set to
4% or less, and preferably set to 2% or less.
[0025] (Mg: 0% to 5%)
Mg contributes to improvement of corrosion
resistance after coating. For example, when Mg is
contained in the plating layer, even if there is a
cut in a coating film and the plating layer, it is
possible to suppress corrosion which occurs from the
cut. This is because, since Mg is eluted in
accordance with the corrosion, a corrosion product
containing Mg is generated around the cut, which
performs an action, such as a self-repair action, to
prevent further entrance of a corrosion factor such
as water and oxygen from the cut. The effect of
suppressing the corrosion is significant when a Mg
concentration is 0.05% or more. Therefore, the Mg
concentration is preferably 0.05% or more, and more
preferably 1% or more. On the other hand, Mg is
likely to form an intermetallic compound which is
poor in the workability such as MgZn2 or Mg2Si. When
Si is contained in the plating layer, Mg 2 Si tends to
precipitate more preferentially than MgZn2 •
- 11 -
As these
intermetallic compounds increase, the workability
decreases, and when the Mg concentration exceeds 5%,
the reduction in the ductility of the plating layer
is significant. Therefore, the Mg concentration is
set to 5% or less, and preferably set to 2% or less.
[0026] When a relationship of "Mg% > 2 X Si%" in
which the Mg concentration is set to ~Mg%" and the Si
concentration is set to ·''Si%" is satisfied, MgZn2
having the workability lower than that of Mg 2Si is
preferentially generated. Therefore, it is
preferable that even if the Mg concentration is 5% or
less, a relationship of ''Mg% < 2 X Si%,.. is satisfied4
A Mg 2Si phase and a MgZn2 phase are examples of other'
intermetallic compound phases.
[0027] (Balance: Zn and impurlLies)
Zn contributes to improvement of a sacrificial
corrosion-proof performance and the corrosion
resistance of the plating layer, and a performance of
a coating base. It is preferable that Al and Zn make
up most of the plating layer. As the impurities,
there can be cited Fe diffused from the steel sheet,
and elements which are inevitably contained in the
plating bath, for example.
[0028] Next, a structure of the plating layer will
be described. FIG. 1 is a sectional view
illustrating one example of a plating layer included
in a plated steel sheet according to an embodiment of
the present invention. A plating layer 11 included
in a plated steel sheet 10 according to the present
- 12 -
embodiment includes a first structure 11 constituted
from Al phases containing Zn in solid solution and Zn
phases dispersed in the Al phases and having an
average chemical composition represented by Al: 25%
to 50%, Zn: 50% to 75%, and impurities: less than 2%,
and a eutectoid structure 14 constituted from Al
phases and Zn phases and having an average chemical
composition represented by Al: 10% to 24%, Zn: 76% to
90%, and impurities: less than 2%. In a cross
section of the plating layer 10, an area fraction of
the first structure 11 is 5% to 40% and a total area
fraction of the first structure 11 and the eutectoid
structure 14 is 50% or more, an area fraction of Zn
phases 15 which are structures containing 90% or more
of Zn, contained in the plating layer 10 is 25% or
less, a total area fraction of intermetallic compound
phases contained in the plating layer 10 is 9% or
less, and a thickness of an intermetallic compound
layer 30 between the plating layer 10 and a steel
sheet 20 is 2 ,um or less.
[0029] (First structure)
The first structure is a structure constituted
from Al phases containing Zn in solid solution and Zn
phases dispersed in the Al phases and having an
average chemical composition represented by Al: 25%
to 50%, Zn: 50% to 75%, and impurities: less than 2%.
The first structure contributes to improvement of a
plastic deformability, workability, and a chipping
resistance. When an area fraction of the first
- 13 -
structure is less than 5% in a cross section of the
plating layer, sufficient workability cannot be
obtained. Therefore, the area fraction of the first
structure is set to 5% or more, more preferably set
to 20% or more, and still more preferably set to 30%
or more9 On the other hand, the area fraction of the
first strtJcture capable of being formed by· a method
to be described later is 40% at the maximum.
[0030] As illustrated in FIG. 1, the first structure
11 includes, for example, a second structur'e 12 and a
third structure 13. The second structure is a
structure having an average chemical composition
represented by Al: 37% to 50%, Zn: 50% to 63%, and
impurities:, less than 2%. The third structure is a
structure having an average chemical composition
represented by Al: 25% to 36%, Zn: 64% to 75%, and
impurities: less than 2%. Each of the second
structure and the third structure is constituted from
Al phases containing Zn in solid solution and Zn
phases dispersed in the Al phases. Although details
will be described later, a proportion of the second
structure and the third structure in the plating
layer can be determined from a backscattered electron
(BSE) image obtained by a scanning electron
microscope (SEMI, by utilizing image processing.
[0031] (Eutectoid structure)
The eutectoid structure is a structure
constituted from Al phases and Zn phases and having
an average chemical composition represented by Al:
- 14 -
10% to 24%, Zn: 76% to 90%, and impurities: less than
2%. The eutectoid structure also contributes to the
improvement of the plastic deformability. When an
area fraction of the eutectoid structure is less than
50% in the cross section of the plating layer, a
proportion of Zn phases becomes high, and there is a
case where sufficient press formability and corrosion
resistance after coating cannot be obtained.
Therefore, the area fraction of the eutectoid
structure is preferably set to 50% or more, and more
preferably set to 55% or more. On the other hand,
the area fraction of the eutectoid structure capable
of being formed by the method to be described later
is 75% -at the maximum. In order to obtain the first
structure, which is likely to contribute more than
the eutectoid structure to the improvement of the
workability, at a higher area fraction, the area
fraction of the eutectoid structure is preferably set
to 70% or less, and more preferably set to 65% or
less.
[0032] When a total area fraction of the first
structure and the eutectoid structure is less than
50% in .the cross section of the plating layer,
sufficient plastic deformability cannot be obtained.
For example, when complicated press forming is
performed, a lot of cracks sometimes occur.
Therefore, the total area fraction of the first
structure and the eutectoid structure is set to 50%
or more. Further, the first structure possesses a
- 15 -
plastic deformability which is better than that of
the eutectoid structure, so that the area fraction of
the first structure is preferably higher than the
area fraction of the eutectoid structure.
[0033] The total area fraction of the first
structure and the eutectoid structure is preferably
55% or more. When the total area f'raction is 55% or
more, further excellent workability can be obtained.
~or example, in a 2T bending test using a plated
steel sheet with a thickness of 0.8 mm, cracks do not
occur almost at all at a bent top portion. When the
total area fraction is 55% or more, the area fraction
of the eutectoid structure is 50% to 70% and the area
fraction of the first structure is 5% or more, for
example. An outline of the 2T bending tesL is
illustrated in FIG. 2A. As illustrated in FIG. 2A,
in the 2T bending test, a sample of a plated steel
sheet with a thickness of t is bent by 180° while
providing a space corresponding to 4t therebetween,
and a crack at a bent top portion 51 is observed.
LU034] The total area fraction of the first
structure and the eutectoid structure is more
preferably 90% or more. When the total area fraction
is 90% or more, still further excellent workability
can,.be obtained. For example, in a lT. bending test
using a plated steel sheet with a thickness of 0.8 mm,
cracks do not occur almost at all at a bent top
portion. When the total area fracti6n is 90% or more,
the area fraction of the eutectoid structure is 50%
- 16 -
to 70% and the area fraction of the first structure
is 20% or more and less than 30%, for example. An
outline of the lT bending test is illustrated in FIG.
2B. As illustrated in FIG. 2B, in the lT bending
test, a sample of a plated steel sheet with a
thickness of t is bent by 180° while providing a space
corresponding to 2t therebetween, and a crack at a
bent top portion 52 is observed.
[0035] The total area fraction of the first
structure and the eutectoid structure is still more
preferably 95% or more. When the total area fraction
.. is 95% or more, extremely excellent workability can
be obtained. For example, in a OT bending test using
a plated steel sheet with a thickness of 0.8 mm,
cracks do not occur almost at all at a bent top
portion. When the total area fraction is 95% or more,
the area fraction of the eutectoid structure is 50%
to 65% and the area fraction of the first structure
is 30% or more, for examplee An outline of the OT
bending test is illustrated in FIG. 2C. As
illustrated in FIG. 2C, in the OT., bending test, a
sample of a plated steel sheet with a thickness of t
is bent by 180° while providing no space therebetween,
and a crack at a bent top portion 53 is observed.
[0036] (Zn phases, intermetallic compound phases,
and the like)
The Zn phases being structures containing 90% or
more of Zn reduce the workability. The plating layer
may also contain phases other than the first
- 17 -
structure, the eutectoid structure, and the Zn phases,
such as Si phases and Mg2Si phases, for example, and
the plating layer may also contain the other
intermetallic compound phases (MgZn 2 phases and the
like), but, these also reduce the workability.
Therefore, it is preferable that the plating layer
does not contain the ~n phases and the intermetallic
compound phases. When an area fraction of the Zn
phases is greater than 25%, the workability reduces
significantly, and when a total area fraction of the
intermetallic compound phases is greater than 9%, the
workability reduces significantly. Therefore, the
area fraction of the Zn phases is set to 25% or less,
and the total area fraction of the intermetallic
compound phases is set to 9% or less. The area
fraction of the Zn phases is preferably 20% or less
also from a viewpoint of corrosion resistance.
Further, from a viewpoint of securing higher
ductility, the area fraction of the Si phases is
preferably 3% or less.
[0037] Although it is possible that an intermetallic
compound layer of an Al-Mn-Fe-based intermetallic
compound containing a slight amount of Si in solid
solution or the like, is provided between the plating
layer and the steel sheet, when a thickness of the
intermetallic compound layer is greater than 2 ~m,
the workability is likely to reduce. Therefore, the
thickness of the intermetallic compound layer is 2000
nm or less, and preferably 1000 nm or less. With the
- 18 -
use of the manufacturing method to be described later,
the thickness of the intermetallic compound layer
becomes 100 nm or more.
[0038] Next, a method of manufacturing the plated
steel sheet according to the embodiment of the
present invention will be described. In this method,
a surface of a steel sheet used as a plating original
sheet is reduced while performing annealing on the
steel sheet, the steel sheet is immersed into a ZnAl-
based plating bath, pulled out of the plating bath
and cooled under conditions to be described later.
[0039] A material of the steel sheet is not
particularly limited. For example, it is possible to
use an Al-killed steel, an ultralow carbon steel, a
high carbon steel, various high-tensile steels, a
steel containing Ni and Cr, and the like. The
strength of the steel is also not particularly
limited. Conditions at a time of manufacturing the
steel sheet in a steelmaking method, a hot-rolling
method, a pickling method, a cold-rolling method, and
the like are also not particularly limited. A
chemical composition of the steel which is, for
example, a C content and a Si content, is also not
particularly limited. The steel may also contain Ni,
Mn, Cr, Mo, Ti or B, or an arbitrary combination
thereof. An annealing temperature of the steel sheet
is set to about 800~, for example.
[0040] In formation of the plating layer, it is also
possible to employ a Sendzimir method or a pre-
19 -
plating method. When pre-plating of Ni is performed,
the intermetallic compound layer sometimes contains
Ni.
[0041] In the preparation of the Zn-Al-based plating
bath, for example, pure Zn, Al, Mg, and an Al-Si
alloy are used and mixed so that each component has a
predetermined concentratj_on, and are dissol_ved at
450°C to 650"C. The steel sheet having a
sufficiently-reduced surface is immersed into the
plating bath at 450°C to 600"C, and when this steel
sheet is pulled out of the plating bath, a molten
metal is adhered to the surface of the steel sheet.
By cooling the molten metal, the plating layer is
formed. It is preferable that an adhesion amount of
the plating layer is adjusted by performing wiping
with N2 gas before the molten metal is solidified. In
this manufacturing method, a cooling method is
differed in accordance with an Al concentration of
the plating bath.
[0042] (When Al concentration of plating bath is not
less than 20% nor more than 40%)
When the Al concentration is not less than 20%
nor more than 40%, cooling is performed at a first
cooling rate of lO"C/second or more from a plating
bath temperature to a first temperature within a
range of 360°C to 435°C, cooling is performed at a
second cooling rate of 0.02°C/second to 0.50°C/second
from the first temperature to a second temperature
within a range of 280°C to 310°C, and thereafter,
- 20 -
cooling is performed at a third cooling rate of 30°C
/second or more from the second temperature to a room
temperature.
[0043] By performing the cooling at the first
cooling rate of l0°C/second or more to the first
temperature corresponding to a solidus temperature in
a Zn-Al-based phase diagram, the molten metal is
turned into a super-cooled state. For this reason,
dendrites (crystals in dendritic form) being macro
solidification structures are finely generated, and a
number density thereof becomes 1.6 pieces/cm2 or more.
When an achievable cooling rate is taken into
consideration, the number density of the dendrites is
about 25.0 ·pieces/cm2 at the maximum. In the dendri,te,
the Al concentration is increased toward a center,
and the Zn concentration is increased as a distance
from the center increases. As the dendrite becomes
finer, a micro solidification segregation inside the
dendrite is further alleviated. At the first
temperature, a periphery of the dendrite is
substantially constituted from Zn phases. Under the
condition where the first cooling rate is lOt/second
or more, when the plating bath contains Mg, the Mg 2Si
phase being the intermetallic compound crystallized
as a primary crystal can be made finer to have an
equivalent circle diameter of 211m or less. For this
reason, it is easy to suppress the reduction in the
ductility caused by the formation of the
intermetallic compound. When the cooling at the
- 21 -
second cooling rate after that is taken into
consideration, the first cooling rate is preferably
set to 40°C/second or less.
[0044] During the cooling from the first temperature
to the second temperature, the Al phases containing
Zn in solid solution are generated in the dendrite at
a portion with reJ_ati.veJy high Al concentration, and
in the dendr~te at a portion with relatively low Al
concentration and at a portion containing Zn phases,
Al atoms and Zn atoms are mixed, resulting in that
the area fraction of the Zn phases is reduced. When
the second cooling rate is greater than 0.50°C/second,
the Zn atoms and the Al atoms cannot be sufficiently
diffused, and a lot of Zn phases are likely to be
remainede Therefore, the second cooling rate is set
to 0.50"C/ or less. On the other hand, when the
second cooling rate is less than 0.02"C/second, the
intermetallic compound layer is excessively formed,
resulting in that sufficient ductility cannot be
obtained. Therefore, the second cooling rate is set
to 0. 02°C/second or more. Further, a period of time
taken for performing the cooling from the first
temperature to the second temperature is set to not
less than 180 seconds nor more than 1000 seconds.
This is for realizing sufficient diffusion of the Zn
atoms and the Al atoms, and for suppressing the
excessive formation of the intermetallic compound
layer.
[0045] During the cooling from the second
- 22 -
temperature to the room temperature, Zn soliddissolved
in Al is finely precipitated, resulting in
that the first structure constituted from the Al
phases containing Zn in solid solution and the Zn
phases dispersed in the Al phases, and the eutectoid
structure constituted from the Al phases and the Zn
phases are obtained. Although Zn phases which are
independent from the first structure and the
eutectoid structure are sometimes precipitated, an
area fraction of the Zn phases becomes 20% o~ less.
Within the first structure, the second structure with
relatively high Al concentration (Al: 37% to 50%) is
generated, and the third structure with rela~ively
low Al concentration (Al: 25% to 36%) is generated
between the second structure and the eutectoid
structure. As the micro solidification segregation
inside the dendrite is further alleviated, the second
structure and the third structure are likely to be
generated. When the third cooling rate is less than
30°C/second, there is a case where the Zn phases are
precipitated, grown, and aggregated, resulting in
that the area fraction of the Zn phases in the
plating layer becomes 20% or more.. Therefore, the
third cooling rate is set to 30°C/second or more. The
first structure remains as the dendrite, so that a
number density of the first structure becomes 1.6
pieces/cm2 to 25.0 pieces/cm2
, for example.
[0046] (When Al concentration of plating bath is 10%
or more and less than 20%)
- 23 -
When the Al concentration is 10% or more and less
than 20%, cooling is performed at a first cooling
rate of lOt/second or more from a plating bath
temperature to a first temperature of 410t, cooling
is performed at a second cooling rate of 0.02°C/second
to O.llt/second from the first temperature to a
second temperature of 390t, and thereafter, cooling
is performed a L d LhLrd cooling rate of 30°C/second or
more from the second temperature to a rodm
temperature.
[0047] By performing the cooling at the first
cooling rate of lOt/second or more to the first
temperature, a molten metal is turned into a supercooled,
state. For this reason, dendrites (crystals
in dendritic form) being macro solidification
structures are finely generated, and a number density
thereof becomes 1.6 pieces/cm2 or more. When an
achievable cooling rate is taken into consideration,
the number density of the dendrites is about 25.0
pl' eces,J ern?- at the maximum. In the dendrite, the Al
concentration is increased toward a cent~r, and the
Zn concentration is increased as a distance from the
center•increases. As the dendrite becomes finer, a
micro ,solidification segregation inside the dendrite
is further alleviated. At the first tem~erature, a
periphery of the dendrite is substantially
constituted from Zn phases. Under the condition
where the first cooling rate is lOt/second or more,
when the plating bath contains Mg, the Mg 2Si phase
- 24 -
being the intermetallic compound crystallized as a
primary crystal can be made finer to have an
equ.ivalent circle diameter of 2 Jlm or less. For this
reason, it is easy to suppress the reduction in the
ductility caused by the formation of the
intermetallic compound. When the cooling at the
second cooling rate after that is taken into
consideration, the first cooling rate is preferably
set to 40"C/second or less.
[0048] During the cooling from the first temperature
to the second temperature, the Al phases containing
Zn in solid solution are generated in the dendrite at
a portion with relatively high Al concentration, and
in the dendrite at a portion with relatively low Al
concentration and at a portion containing Zn phases,
Al atoms and Zn atoms are mixed, resulting in that
the area fraction of the Zn phases is reduced. When
the second cooling rate is greater than O.ll"C/second,
the Zn atoms and the Al atoms cannot be sufficiently
diffused, and a lot of Zn phases are:likely to be
remained. Therefore, the second cool.ing rate is set
to O.ll°C/ or less. On the other hand, when the
second cooling rate is less than 0. 02°C/second, the
intermetallic compound layer is excessively formed,
resulting in that sufficient ductility cannot be
obtained. Therefore, the second cooling rate is set
to 0.02°C/second or more. Further, a period of time
taken for performing the cooling from the first
temperature to the second temperature is set to not
- 25 -
less than 180 seconds nor more than 1000 seconds.
This is for realizing sufficient diffusion of the Zn
atoms and the Al atoms, and for suppressing the
excessive formation of the intermetallic compound
layer.
[0049] During the cooling from the second
temperature to the room temperature, Zn soliddissolved
in Al is finely precipitated, resulting in
that the first structure constituted from the Al
phases containing Zn in solid solution and the Zn
phases dispersed in the Al phases, and the eutectoid
structure constituted from the Al phases and the Zn
phases are obtained. Although Zn phases which are
independent from the first structure and the
eutectoid structure are sometimes precipitated, an
area fraction of the Zn phases becomes 20% or less.
Within the first structure, the second structure with
relatively high Al concentration (Al: 37% to 50%) is
generated, and the third structure with relatively
low Al concentration (Al: 25% to 36%) is generated
between the second structure and the eutectoid
structure. As the micro solidification segregation
inside the dendrite is further alleviated, the second
structure and the third structure, are likely to be
generated. When the third cooling rate is less than
30°C/second, there is a case where the Zn phases are
precipitated, grown, and aggregated, resulting in
that the area fraction of the Zn phases in the
plating layer becomes 20% or more.
- 26 -
Therefore, the
third cooling rate is set to 30°C/second or more. The
first structure remains as the dendrite, so that a
number density of the first structure becomes 1.6
pieces/cm2 to 25.0 pieces/cm2
, for example.
[0050] With the use of this method, it is possible
to manufacture the plated steel sheet according to
the present embodiment, namely, the plated steel
sheet including the plating layer containing the
first structure and the eutectoid structure at
predetermined area fractions. Note that when the
second structure is generated, the third structure is
inevitably generated, but, it is possible to generate
the third structure without generating the second
structure~
[0051] In this method, the intermetallic compound
layer is inevitably formed between the plating layer
and the steel sheet. Due to the diffusion of Fe from
the steel sheet, a stack of the plating layer and the
intermetallic compound layer sometimes contains Fe of
about 3%. However, a large amount of Fe is
concentrated in the intermetallic compound layer, and
an amount of Fe contained in the plating layer is
extremely small, so that the characteristic of the
plating layer is not substantially affected by Fe.
[0052] Next, description will be made on an analysis
method of the chemical composition of the plating
layer and the intermetallic compound layer and the
phases of the plating layer. In the analysis thereof,
it is set that, in principle, a sample is obtained
- 27 -
from the vicinity of a center in a sheet width
direction of the plated steel sheet, and the sample
is not obtained from the plated steel sheet within a
range of 30 mm from end portions in a rolling
direction (longitudinal direction) and within a range
of 30 mm from end portions in a direction orthogonal
i·_o iche rolling direction (sh<"f't_ width di rect_i on), .i.n
particular.
[0053] In the analysis of the chemical composition
of the plating layer and the intermetallic compound
layer, the plated steel sheet is immersed into HCl to
which an inhibitor is added and having a
concentration of 10%, and'a peeling solution is
analyzed by using an inductively coupled plasma (ICP)
method. By this method, it is possible to understand
an average chemical composition of the plating layer
and the intermetallic compound layer.
[0054] The phases which constitute the plating layer
are analyzed by an X-ray diffraction method using a
Cu target with respect to a surface of the plating
layer. In the plating layer in the embodiment of the
present invention, peaks of Zn and Al are detected as
major peaks. Since an amount of Si is very small, a
peak of Si is not detected as a major peak. When Mg
is contained, a diffraction peak attributed to Mg 2Si
is also detected.
[0055] The area fractions of the respective
structures contained in the plating layer can be
calculated by performing image analysis on a BSE
- 28 -
image obtained by SEM and an element mapping image
obtained by energy dispersive X-ray spectrometry
( EDS) .
[0056] Next, evaluation methods for the performance
of the plating layer will be described. As the
performance of the plating layer, there can be cited
the corrosion resistance after coating, the plastic
deformability, the chipping resistance, the powdering
resistance, and the seizing resistance, for example.
[0057] In the evaluation of the corrosion resistance
after coating, a sample of the plated steel sheet is
subjected to zinc phosphate treatment and
electrodeposition coating, to thereby prepare a
coated plated steel sheet, and a cross-cut which
reaches a steel sheet being base iron of the coated
plated steel sheet is formed. Subsequently, the
coated plated steel sheet having the cross-cut formed
thereon is subjected to a combined cyclic corrosion
test, and a maximum swelling width around the crosscut
is measured. The combined cyclic corrosion test
is performed a plurality of times under the same
condition, and an average value of the maximum
swelling widths in the tests is calculated. It is
possible to evaluate the corrosion resistance after
coating based on the average value of the maximum
swelling widths. As the plating layer has further
excellent corrosion resistance after coating, it has
a smaller average value of the maximum swelling
widths. Further, a generation of red rust
- 29 -
significantly deteriorates an external appearance of
the coated plated steel sheet, so that normally, it
is evaluated such that the coated plated steel sheet
in which a period of time until when the red rust is
generated is longer has further excellent corrosion
resistance after coating.
[0058] In the evaluation of the plastic
deformability,
bent by 180°
bending test,
a sample of the plated steel sheet is
in a sheet width direction in the OT
the lT bending test, or the 2T bending
test, and the number of cracks at a bent top portion
is counted. The plastic deformability can be
evaluated based on the number of cracks. The number
of cracks is counted by using the SEM. The plated
steel sheet having further excellent plastic
deformability and better ductility has a smaller
number of cracks. It is also possible to evaluate
the corrosion resistance of the bent portion by
making the sample after being bent by 180° to be
directly subjected to an accelerated corrosion test.
[0059] In the evaluation of the chipping resistance,
a sample of the plated steel sheet is subjected to
zinc phosphate treatment and electrodeposition
coating, and then subjected to intermediate coating,
finish coating, and clear coating, to thereby form a
coating film with four-layer structure. Subsequently,
crushed stones are made to collide with the coating
film which is isothermally held to a predetermined
temperature, and a degree of peeling is visually
- 30 -
observed. It is possible to evaluate the chipping
resistance based on the degree of peeling. It is
also possible to classify the degree of peeling
through image processing.
(0060] In the evaluation of the powdering resistance,
a sample of the plated steel sheet is subjected to a
60° bending test in which a sheet width direction is
set to a bend axis direction. Subsequently, a width
of the plating layer peeled by an adhesive tape
(peeling width) is measured at a plurality of points.
It is possible to evaluate the powdering resistance
based on an average value of the peeling widths.
[0061] In the evaluation of the seizing resistpnce,
a sample of the plated steel sheet is subjected to
draw bead working to cause sliding among a surfpce of
the sample, a die shoulder portion and a bead portion
of a metal mold, and the plating layer adhered to the
metal mold is visually observed. It is possible to
evaluate the seizing resistance based on the
presence/absence of the adhesion of the plating layer
and based on the degree of adhesion when the adhesion
of the plating layer is occurred.
[0062] Note that each of the above-described

CLAIMS
[Claim 1] A plated steel sheet, comprising
an Al-containing Zn-based plating layer on at
least a part of a surface of a steel sheet, wherein:
an average chemical composition of the plating
layer and an intermetallic compound layer between the
plating layer and the steel sheet is represented by,
in terms of mass%, Al: 10% to 40%, Si: 0~05% to 4%,
Mg: 0% to 5%, and the balance: Zn and impurities;
the plating layer includes:
a first structure constituted from Al phases
containing Zn in solid solution and Zn phases
dispersed in the Al phases and having an average
chemical composition represented by, in terms of
mass%, Al: 25% to 50%, Zn: 50% to 75%, and
impurities: less than 2%; and
a eutectoid structure constituted from Al phases
and Zn phases and having an average chemical
composition represented by, in terms of mass%, Al:
10% to 24%, Zn: 76% to 90%, and impurities: less than
2 2- •
0 '
in a cross section of the plating layer, an area
fraction of the first structure is 5% to 40%, and a
total area fraction of the first structure and the
eutectoid structure is 50% or more;
an area fraction of Zn phases which are
structures containing 90% or more of Zn, contained in
the plating layer is 25% or less;
a total area fraction of intermetallic compound
- 63 -
phases contained in the plating layer is 9% or less;
and
a thickness of the intermetallic compound layer
is 2 flm or less.
[Claim 2] The plated steel sheet according to claim
1, wherein
a number density of the first structure on a
surface of the plating layer is 1.6 pieces/cm2 to 25.0
pieces/cm2
•
[Claim 3] The plated steel sheet according to claim
1 or 2, wherein
the first structure includes:
a second structure having an average chemical
composition represented by, in terms of mass%, Al:
37% to 50%, Zn: 50% to 63%, and impurities: less than
2%; and
a third structure having an average chemical
composition represented by, in terms of mass%, Al:
25% to 36%, Zn: 64% to 75%, and impurities: less than
2 "' 0 •
[Claim 4] The plated steel sheet according to any
one of claims 1 to 3, wherein
the average chemical composition of the plating
layer and the intermetallic compound layer is
represented by, in terms of mass%, Al: 20% to 40%,
Si.: 0.05% to 2.5%, Mg: 0% to 2%, and the balance: Zn
and impurities.
[Claim 5] The plated steel sheet according to any
one of claims l to 4, wherein
- 64 -
the thickness of the intermetallic compound layer
is 100 nm to 1000 nm.
[Claim 6] The plated steel sheet according to any
one of claims 1 to 5, wherein
in the cross section of the plating layer, the
area fraction of the first structure is 20% to 40%,
the area fraction of the eutectoid structure is 50%
to 70%, and the total area fraction of the first
structure and the eutectoid structure is 90% or more.
[Claim 7] The plated steel sheet according to any
one of claims l to 6, wherein
in the cross section of the plating layer, the
area fraction of the first structure is 30% to 40%,
the area fraction of the eutectoid structure is 55%
to 65%, and the total area fraction of the first
structure and the eutectoid structure is 95% or more.
[Claim 8] The plated steel sheet according to any
one of claims 1 to 7, wherein:
in the average chemical composition of the
plating layer and the intermetallic compound layer,the Mg concentration is 0.05% to 5%;
when the Mg concentration is set to Mg% and the
Si concentration is set to Si%, a relationship of
"Mg% <. 2 X Si%rr is satisfied; and
a crystal of Mg 2Si which exists in the plating
layer is 2 /liD or less in terms of maximum equivalent
circle diameter.
[Claim 9] The plated steel sheet according to any
one of claims 1 to 8, wherein