Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
PLATED STEEL SHEET FOR HOT STAMPING, METHOD OF MANUFACTURING
PLATED STEEL SHEET FOR HOT STAMPING, METHOD OF MANUFACTURING
HOT-STAMPED COMPONENT, AND METHOD OF MANUFACTURING VEHICLE
Technical Field
[0001] The present disclosure relates to a plated steel sheet for hot
stamping, a method of
manufacturing a plated steel sheet for hot stamping, a method of manufacturing
a hot-stamped
component, and a method of manufacturing a vehicle.
Background Art
[0002] In recent years, there has been a growing demand for suppressing
consumption of
chemical fuels in order to protect the environment and to prevent global
warming, and this
demand has influence on a variety of manufacturing industries. For example,
automobiles
that are indispensable to daily life and activities as a means of
transportation are no exception,
and improvements in fuel economy, such as weight reduction of the car body, or
the like is
required. However, in automobiles, simply realizing weight reduction of
vehicle body is not
permitted due to product quality, and it is necessary to ensure adequate
safety.
[0003] Many of the structures of automobiles are formed of iron, in particular
steel sheets,
and it is important for reducing the weight of car body to reduce the weight
of the steel sheet.
However, as mentioned above, simply reducing the weight of the steel sheet is
not permitted,
and it is also demanded to ensure the mechanical strength of the steel sheet.
Such demands
for steel sheets exist not only in the automobile manufacturing industry but
also in a variety of
manufacturing industries as well. Therefore, research and development is being
conducted
on steel sheets that can maintain or increase the mechanical strength even
when the steel sheets
are made thinner than the steel sheets previously used by increasing the
mechanical strength of
the steel sheets.
[0004] In general, materials having high mechanical strength tend to have
lower shape
freezing properties in forming such as bending processing, and when such
materials are
processed into complicated shapes, processing itself becomes difficult. As one
means for
solving the problem concerning the formability, a so-called "hot stamping
method (hot
stamping method, high temperature stamping method, die quenching method)" can
be
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mentioned. In this hot stamping method, a material to be formed is temporarily
heated to a
high temperature, and the material softened by heating is stamped, molded, and
then cooled.
[0005] According to this hot stamping method, a material is once heated to a
high temperature
and softened, and therefore, the material can be easily stamped. Therefore, by
this hot
stamping, a molded component satisfying both favorable shape freezing property
and high
mechanical strength can be obtained. Particularly when the material is steel,
the mechanical
strength of the stamped component can be enhanced by quenching effect by
cooling after
forming.
[0006] However, when this hot stamping method is applied to a steel sheet, for
example, by
heating to a high temperature of 800 C or higher, a scale (oxide) is generated
by iron oxidation
on the surface. Therefore, it is necessary to perform a process (descaling
process) for
removing this scale after performing hot stamping, resulting in a decrease in
productivity. For
a member or like that requires corrosion resistance, it is necessary to
subject the surface thereof
to rust prevention treatment or metal coating after processing, which requires
a surface cleaning
step and a surface treatment step, reducing the productivity as well.
[0007] Examples of a method of suppressing such a decrease in productivity
include a method
of applying coating to a steel sheet. In general, a variety of materials such
as an organic
material and an inorganic material are used as coating on a steel sheet. Among
other things,
a zinc-based plated steel sheet having sacrificial and corrosion-proof action
against a steel sheet
is widely used for an automobile steel sheet or the like from the viewpoint of
corrosion
protection performance and steel sheet production technology. The heating
temperature in a
hot stamping aims at a temperature higher than the Ac3 transformation point of
steel in order
to obtain quenching effect. In other words, the heating temperature is about
from 700 to
1000 C. However, this heating temperature is higher than the decomposition
temperature of
an organic material and the boiling point of a metal material such as a Zn-
based material.
Therefore, when heated for hot stamping, a plating layer on the surface
evaporates, which may
be a cause of remarkable deterioration of surface properties.
[0008] Therefore, it is preferable to use, for example, an Al-based metal
coated steel sheet
having higher boiling point than organic material coating or Zn-based metal
coating, so-called
an aluminum-plated steel sheet for a steel sheet subjected to hot stamping to
be heated to high
temperature.
[0009] By applying Al-based metal coating, it is possible to prevent a scale
from adhering to
the surface of a steel sheet, and a process such as a descaling step becomes
unnecessary, by
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which the productivity is improved. Al-based metal coating also has anti-rust
effect, and
therefore, corrosion resistance after painting is also improved. A method of
using an
aluminum-plated steel sheet obtained by applying an Al-based metal coating to
steel having a
predetermined steel component for hot stamping is described in Patent Document
1.
[0010] However, when Al-based metal coating is applied, Al coating first melts
depending on
preheating conditions prior to stamping in a hot stamping method, and
thereafter, an Al-Fe
compound layer is formed by Fe diffusion from the steel sheet. The Al-Fe
compound layer
grows to the surface of a steel sheet as an Al-Fe compound layer in some
cases. This
compound layer is hereinafter referred to as an alloy layer. Since this alloy
layer is extremely
hard, a processed scratch is formed by contact with a mold during stamping.
[0011] In order to address this problem, Patent Document 2 discloses a method
of forming a
film of a wurtzite type compound such as a film of ZnO on the surface of an
aluminum-plated
steel sheet for the purpose of improving the hot lubricity, the chemical
conversion treatment
property, and corrosion resistance for preventing occurrence of processing
damage.
[0012] On the other hand, Patent Document 3 discloses a method of forming a
film of one or
more Zn compounds selected from the group consisting of Zn hydroxide, Zn
phosphate, and
organic acid Zn on the surface of an Al-plated steel sheet for the purpose of
enhancing adhesion
of a film of ZnO during stamping. In the method of Patent Document 2, it is
possible to
improve hot lubricity, film adhesion, spot weldability, corrosion resistance
after coating by
forming a film of ZnO by heat generated by hot stamping an aluminum-plated
steel sheet on
which a film of a Zn compound is formed and forming a ZnO film having
excellent adhesion.
[0013] Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No.
2000-38640
Patent Document 2: W02009/131233
Patent Document 3: JP-A No. 2014-139350
SUMMARY OF INVENTION
Technical Problem
[0014] Here, each of the plated steel sheets described in Patent Documents 2
to 3 is excellent
in hot lubricity, and occurrence of processing flaws can be suppressed. By the
way, generally,
when hot stamping is performed using a non-plated material or a plated steel
sheet, abrasion
occurs on a sliding surface of a mold for hot stamping in which a plated steel
sheet slides, such
as a portion to be a vertical wall portion and a flange portion of a stamped
component. For
this reason, in a high surface pressure portion in hot stamping, it is
necessary to perform mold
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maintenance as a countermeasure against wear occurring on a sliding surface of
a mold.
Although the plated steel sheets of Patent Documents 2 to 3 were expected to
reduce die wear,
even with Patent Documents 2 to 3, mold wear was not solved as with other non-
plated
materials or plated steel sheets.
[0015] In view of the above, an object of one embodiment of the disclosure is
to provide a
plated steel sheet for hot stamping that suppresses occurrence of wear of a
sliding surface of a
mold for hot stamping, and a method of manufacturing such a sheet.
Another object of one embodiment of the disclosure is to provide a method of
manufacturing a hot stamped product for suppressing occurrence of wear of a
sliding surface
of a mold for hot stamping and a method of manufacturing a vehicle using a
stamped
component manufactured with a method of manufacturing a hot-stamped component
by using
the plated steel sheet for hot stamping.
Solution to Problem
[0016] The inventors studied and found the following. When a zinc oxide film
layer (ZnO
film) is formed on the surface of an aluminum plating layer, the surface
properties of the
aluminum plating layer having a convex portion on the surface is reflected on
the surface
properties of a zinc oxide film. When an aluminum plated steel sheet with a
zinc oxide film
layer formed on the surface slides on the surface of a mold for hot stamping,
local pressure is
applied to a convex portion of the zinc oxide film, causing wear on a sliding
surface of the
mold for hot stamping. Therefore, the inventors found that if a zinc oxide
film having high
smoothness could be formed, the occurrence of wear of the sliding surface of a
mold for hot
stamping could be suppressed.
[0017] The gists of the disclosure are as follows.
[0018] <1> A plated steel sheet for hot stamping including:
a plated steel sheet body including a steel sheet and an aluminum plating
layer
provided on one side or both sides of the steel sheet; and
a zinc-based metal soap film provided on a surface of the plated steel sheet
body on a
side of the aluminum plating layer and having an adhesion amount of an adhered
portion of
from 7.1 to 19.8 g/m2 based on a Zn amount.
<2> A plated steel sheet for hot stamping, including:
a plated steel sheet body including a steel sheet, an aluminum plating layer
provided
on one side or both sides of the steel sheet, and a zinc oxide film provided
on a surface of the
aluminum plating layer; and
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a zinc-based metal soap film provided on a surface of the zinc oxide film of
the plated
steel sheet body,
a total the adhesion amount of adhered portions of the zinc oxide film and the
zinc-
based metal soap film being from 7.1 to 19.8 g/m2 based on a Zn amount.
<3> The plated steel sheet for hot stamping according to <2>, wherein at least
half of the total
adhesion amount of adhered portions of the zinc oxide film and the zinc-based
metal soap film
is an adhesion amount of an adhered portion of the zinc-based metal soap film.
<4> The plated steel sheet for hot stamping according to any one of <1> to
<3>, wherein the
zinc-based metal soap film is a film of at least one zinc-based metal soap
selected from the
group consisting of zinc bis-octanoate, zinc octylate, zinc laurate, and zinc
stearate.
<5> A plated steel sheet for hot stamping including:
a plated steel sheet body including a steel sheet and an aluminum plating
layer
provided on one side or both sides of the steel sheet; and
a zinc oxide film provided on a surface of the plated steel sheet body on an
aluminum
plating layer side,
wherein a maximum value of a skewness Rsk of a roughness curve of a surface of
the
zinc oxide film is less than 0.
<6> A method of manufacturing a plated steel sheet for hot stamping, the
method including the
forming a zinc-based metal soap film on a surface of a aluminum plating layer
side of a plated
steel sheet body, including a steel sheet and an aluminum plating layer
provided on one side or
both sides of the steel sheet, in such a manner that an adhesion amount of an
adhered portion
is from 7.1 to 19.8 g/m2 based on a Zn amount.
<7> A method of manufacturing a plated steel sheet for hot stamping, the
method including a
forming a zinc-based metal soap film on a surface of a zinc oxide film of a
plated steel sheet
body having a steel sheet, an aluminum plating layer provided on one side or
both sides of the
steel sheet, and the zinc oxide film provided on the aluminum plating layer,
in such a manner
that a total adhesion amount of an adhered portion of the zinc-based metal
soap film together
with an adhesion amount of an adhered portion of the zinc oxide film is from
7.1 to 19.8 g/m2
based on a Zn amount.
<8> The method of manufacturing a plated steel sheet for hot stamping
according to <7>,
wherein, in the forming the zinc-based metal soap film, at least half of the
total adhesion
amount of the adhered portions of the zinc oxide film and the zinc-based metal
soap film is the
adhesion amount of the adhered portion of the zinc-based metal soap film.
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<9> The method of manufacturing a plated steel sheet for hot stamping
according to any one
of <6> to <8>, including a heating the zinc-based metal soap film at 300 C or
higher and
obtaining a zinc oxide film.
<10> The method of manufacturing a plated steel sheet for hot stamping
according to <9>,
wherein a maximum value of a skewness Rsk of a surface roughness curve of the
zinc oxide
film formed by heating the zinc-based metal soap film is less than 0.
<11> The method of manufacturing a plated steel sheet for hot stamping
according to any one
of <6> to <10>, wherein the zinc-based metal soap film is a film of at least
one zinc-based
metal soap selected from the group consisting of zinc bis-octanoate, zinc
octylate, zinc laurate,
and zinc stearate.
<12> A method of manufacturing a hot-stamped component, the method including:
a manufacturing a plated steel sheet for hot stamping according to the method
of
manufacturing a plated steel sheet for hot stamping according to any one of
<6> to <11>,
wherein, in the forming the zinc-based metal soap film, at least the zinc-
based metal soap film
is formed on a surface, which is to be in contact with a sliding surface of a
mold for hot
stamping in a later hot stamping in the surface of the plated steel sheet body
on the aluminum
plating layer side; and
a hot stamping of subjecting the plated steel sheet for hot stamping to hot
stamping.
<13> A method of manufacturing a hot-stamped component by hot stamping a
plated steel
sheet for hot stamping manufactured by the method of manufacturing a plated
steel sheet for
hot stamping according to <10>.
<14> A method of manufacturing a vehicle, wherein a stamped component
manufactured by
the method of manufacturing a hot-stamped component according to <12> or <13>
is attached
with the surface of the zinc oxide film facing an outer side of the vehicle.
Advantageous Effects of Invention
[0019] According to one embodiment of the disclosure, it is possible to
provide a plated steel
sheet for hot stamping for suppressing occurrence of wear of a sliding surface
of a mold for hot
stamping, and a method of manufacturing such a sheet.
According to one aspect of the disclosure, it is possible to provide a method
of
manufacturing a hot stamped product for suppressing occurrence of a flaw of a
sliding surface
of a mold for hot stamping and a method of manufacturing a vehicle using a
stamped
component manufactured with a method of manufacturing a hot-stamped component
by using
the plated steel sheet for hot stamping.
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BRIEF DESCRIPTION OF DRAWINGS
[0020] Fig. lA is a schematic sectional view illustrating an example of a
plated steel sheet for
hot stamping according to the embodiment.
Fig. 1B is a schematic sectional view illustrating a state where a plated
steel sheet for
hot stamping according to the embodiment and a mold are in contact with each
other.
Fig. 2A is a schematic sectional view illustrating an example of a
conventional plated
steel sheet for hot stamping.
Fig. 2B is a schematic sectional view illustrating a state where a
conventional plated
steel sheet for hot stamping and a mold are in contact with each other.
Fig. 3 is a process chart illustrating an example of a normal process from
manufacturing of a plated steel sheet to hot stamping.
Fig. 4 is a schematic configuration diagram illustrating an apparatus for
evaluating hot
lubricity.
DESCRIPTION OF EMBODIMENTS
[0021] Next, an embodiment which is an example of the disclosure will be
described in detail.
Preferred embodiments of the disclosure will be described in detail below with
reference to the accompanying drawings.
In the specification and the drawings, the same reference numerals are
attached to
constituent elements having substantially the same functional configuration,
and redundant
explanation may be omitted in some cases.
Herein, the numerical range expressed by using "to" means a range including
numerical values described before and after "to" as a lower limit value and an
upper limit value.
The term "step" herein encompasses not only an independent step but also a
step of
which the desired object is achieved even in a case in which the step is
incapable of being
definitely distinguished from another step.
[0022] < Plated Steel Sheet >
A plated steel sheet according to one embodiment of the disclosure will be
described.
The plated steel sheet for hot stamping (hereinafter, also referred to as
"plated steel sheet")
according to the embodiment includes: a plated steel sheet body including a
steel sheet and an
aluminum plating layer (hereinafter, also referred to as "Al plating layer")
provided on one side
or both sides of the steel sheet; and a zinc-based metal soap film provided on
the Al plating
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layer side surface of the plated steel sheet body and having an adhesion
amount of an adhered
portion of from 7.1 to 19.8 g/m2 based on Zn amount.
In the plated steel sheet according to the embodiment, the plated steel sheet
body may
include a zinc oxide film (hereinafter, also referred to as "ZnO film")
provided on the Al plating
layer. It is noted that, when the plated steel sheet body includes a ZnO film,
the total adhesion
amount of adhered portions of the ZnO film and the zinc-based metal soap film
is set to from
7.1 to 19.8 g/m2 based on Zn amount.
[0023] With the above structure, the plated steel sheet according to the
embodiment
suppresses occurrence of wear of a sliding surface of a mold for hot stamping
(hereinafter, also
referred to as "mold") when hot stamped. The plated steel sheet according to
the embodiment
was found by the following findings.
[0024] First, the stamped component obtained by hot stamping the plated steel
sheet (plated
steel sheet on which a ZnO film was formed on the Al plating layer) and the
mold were analyzed,
and the following was confirmed. In the plated steel sheet of Patent Document
2, the ZnO
film had a convex portion conforming to the surface properties of the Al
plating layer (see Fig.
2A: in Fig. 2A, 12 denotes a steel sheet, 14 denotes an Al plating layer, and
16 denotes a ZnO
film). When a plated steel sheet is hot-stamped, a local surface pressure is
applied to a convex
portion of a ZnO film from a sliding surface sliding on a mold, and as a
result, a top portion of
the convex portion of the ZnO film peels off, and an Al plating layer was
exposed. A mold-
derived substance was adhered to the vicinity of the exposed Al plating layer.
This revealed
that Al in the exposed Al plating layer reacted with Fe of the mold to form an
intermetallic
compound and wear the sliding surface of the mold.
That is, the following was found out: 1) In the plated steel sheet of Patent
Document
2, since a thin ZnO film is formed on an Al plating layer, the maximum value
of the skewness
Rsk of the surface roughness curve exceeds 0, and a protruding convex portion
is formed on
the surface; 2) The protruding convex portion of the surface is in point
contact with a mold;
and 3) When the stamping pressure of hot stamping increases, a high surface
pressure is
generated at the convex portion of the surface, and a sliding surface of the
mold is worn (see
to Fig. 2B: in Fig. 2B, 12 denotes a steel sheet, 14 denotes an Al plating
layer, 16 denotes a
ZnO film, and 26 denotes a mold).
[0025] The inventors then found that the following is effective for
suppressing wear of the
sliding surface of the mold. 1) Applying lubricant to improve the smoothness
of the ZnO film.
2) Using zinc-based metal soap containing Zn as a lubricant in consideration
of chemical
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conversion treatment properties after hot stamping and corrosion resistance.
Specifically, the
inventors found the following.
[0026] Since zinc-based metal soap is used for lubricant applications, when
the adhesion
amount of the zinc-based metal soap is increased, it is difficult to be
affected by the surface
properties of an underlying plated steel sheet body (Al plating layer or ZnO
film), and a zinc-
based metal soap film having high smoothness can be formed (see Fig. 1A: 10
denotes a plated
steel sheet, 10A denotes a plated steel sheet body, 12 denotes a steel sheet,
14 denotes an Al
plating layer, 16 denotes a ZnO film, and 18 denotes a zinc-based metal soap
film.). In this
zinc-based metal soap film, zinc is oxidized by heating before stamping of hot
stamping, and
an organic substance (a fatty acid or the like) other than zinc is decomposed,
resulting in a ZnO
film. In other words, during stamping of hot stamping, a ZnO film having a
high smoothness
(for example, a ZnO film having a maximum value of the skewness Rsk of the
surface
roughness curve less than 0) is formed on the surface of a plated steel sheet.
Before hot
stamping, the zinc-based metal soap film may be heated to form a ZnO film.
[0027] Since the ZnO film on the outermost surface of the plated steel sheet
is smooth, when
hot stamping the plated steel sheet, the surface pressure applied to the ZnO
film from the sliding
surface sliding on the mold is reduced. In other words, the ZnO film of the
plated steel sheet
and the sliding surface on the mold are in surface contact, the true contact
area between the
ZnO film of the plated steel sheet and the sliding surface on the mold
increases, and the contact
surface pressure decreases. Therefore, peeling of the ZnO film is suppressed
(see Fig. 1B: In
Fig. 1B, 10 denotes a plated steel sheet, 10A denotes a plated steel sheet
body, 12 denotes a
steel sheet, 14 denotes an Al plating layer, 16 denotes a ZnO film, 18 A
denotes a ZnO film
formed of a zinc-based metal soap film, and 26 denotes a metal mold.). Since
peeling of the
ZnO film is suppressed, it is possible to suppress the mold and the Al plating
layer from
contacting and reacting to form an intermetallic compound. As a result, since
formation of
an intermetallic compound which causes wear of the mold is suppressed, wear of
the sliding
face of the mold on which a plated steel sheet slides is suppressed.
[0028] Based on the above findings, the inventors found that the plated steel
sheet according
to the embodiment suppresses occurrence of wear of a sliding surface of a mold
for hot
stamping when hot stamped by the above configuration.
Then, the inventors also found the following. In the plated steel sheet
according to
the embodiment, a ZnO film formed from a zinc-based metal soap film is
difficult to peel off
during hot stamping and after molding, and wear of a mold hardly occurs.
Therefore, a
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molded component having high mass productivity, high chemical conversion
processability,
excellent adhesion between an Al plating layer and a ZnO film after molding,
and high
corrosion resistance (or corrosion resistance after coating) is obtained.
[0029] Hereinafter, details of the plated steel sheet according to the
embodiment will be
described.
[0030] < Plated Steel Sheet Body >
The plated steel sheet body includes a steel sheet and an Al plating layer
provided on
one side or both sides of the steel sheet. The plated steel sheet body may
have a ZnO film
provided on the Al plating layer.
[0031] (Steel Sheet)
As the steel sheet (steel sheet before plating), for example, it is preferable
to use a steel
sheet formed to have high mechanical strength (meaning properties related to
mechanical
deformation and fracture such as tensile strength, breakdown point,
elongation, drawing,
hardness, impact value, fatigue strength, or creep strength). An example of a
steel sheet (steel
sheet before plating) that realizes high mechanical strength used for the
plated steel sheet
according to the embodiment is as follows. The notation of % means % by mass
unless
otherwise specified.
[0032] A steel sheet preferably contains at least one of C: from 0.1 to 0.6%,
Si: from 0.01 to
0.6%, Mn: from 0.5 to 3%, Ti: from 0.01 to 0.1%, and B: from 0.0001 to 0.1%
based on % by
mass, the balance being Fe and impurities.
[0033] C is included for securing an intended mechanical strength. When C is
less than
0.1%, sufficient improvement in mechanical strength can not be obtained and
the effect of
containing C is poor. On the other hand, when C exceeds 0.6%, although a steel
sheet can be
further cured, melt cracking is likely to occur. Therefore, the C content is
preferably from
0.1% to 0.6%.
[0034] Si is one of strength improving elements for improving the mechanical
strength, and
is contained in order to ensure an intended mechanical strength like C. When
Si is less than
0.01%, a strength improving effect is hardly exerted, and sufficient
improvement in mechanical
strength can not be obtained. On the other hand, Si is also an easily
oxidizable element.
Therefore, when Si exceeds 0.6%, the wettability decreases and non-plating may
occur when
molten aluminum plating is performed. Therefore, the Si content is preferably
from 0.01% to
0.6%.
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[0035] Mn is one of the strengthening elements for strengthening steel, and is
also one of
elements which increase the hardenability. Further, Mn is also effective for
preventing hot
embrittlement caused by S which is one of impurities. When Mn is less than
0.5%, these
effects can not be obtained, and when the Mn is 0.5% or more, the above
effects are exerted.
On the other hand, when Mn exceeds 3%, there is a fear that the residual 7
phase becomes too
much and the strength decreases. Therefore, the Mn content is preferably from
0.5% to 3%.
[0036] Ti is one of the strength strengthening elements, and is also an
element for improving
the heat resistance of an Al plating layer. When Ti is less than 0.01%, a
strength improving
effect or an oxidation resistance improving effect can not be obtained, and
these effects are
exerted at 0.01% or more. On the other hand, when too much Ti is contained in
a steel, there
is a risk of, for example, forming a carbide or a nitride, and softening the
steel. In particular,
when Ti exceeds 0.1%, there is a high possibility that an intended mechanical
strength can not
be obtained. Therefore, the Ti content is preferably from 0.01% to 0.1%.
[0037] B acts during quenching and has an effect of improving the strength.
When B is less
than 0.0001%, such a strength improving effect is low. On the other hand, when
B exceeds
0.1%, there is a risk of forming inclusions and embrittling and lowering the
fatigue strength.
Therefore, the B content is preferably from 0.0001% to 0.1%.
[0038] This steel sheet may contain impurities that may be mixed in other
manufacturing
processes or the like.
[0039] A steel sheet formed from such a chemical component can be quenched by
heating by
a hot stamping method or the like, and have a mechanical strength of about
1,500 MPa or higher.
Although this steel sheet has such a high mechanical strength, when the steel
sheet is processed
by a hot stamping method, since a hot stamping can be performed with the steel
sheet being
softened by heating, the steel sheet can be easily molded. The steel sheet can
realize high
mechanical strength, and as a result, even when the sheet is thinned for
weight reduction,
mechanical strength can be maintained or improved.
[0040] (Al Plating Layer)
An Al plating layer is formed on one side or both sides of a steel sheet
before plating.
The Al plating layer is formed on one side or both sides of the steel sheet
by, for example, a
hot plating method, but the forming method is not limited thereto.
[0041] The component composition of the Al plating layer may be 50% or more of
Al. The
element other than Al is not particularly limited, and Si may be positively
contained for the
following reason.
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[0042] When Si is contained, an Al-Fe-Si alloy layer is formed at the
interface between
plating and base steel, and formation of a brittle Al-Fe alloy layer generated
at the time of hot
plating can be suppressed. When Si is less than 3%, the Al-Fe alloy layer
grows thickly at the
stage of aluminum plating, which may promote cracking of the plating layer at
the time of
processing and adversely affect corrosion resistance. On the other hand, when
Si exceeds
15%, on the contrary, the volume fraction of a Si-containing layer increases,
possibly reducing
the workability and corrosion resistance of the plating layer. Therefore, the
Si content in the
Al plating layer is preferably from 3 to 15%.
[0043] An Al plating layer prevents corrosion of a steel sheet. Further, when
the plated steel
sheet is processed by a hot stamp method, the Al plating layer does not
oxidize the surface and
does not generate a scale (iron oxide) even when heated to a high temperature.
By preventing
generation of a scale with the Al plating layer, it is possible to omit a step
of removing the scale,
a surface cleaning step, a surface treatment step, or the like, and the
productivity of a molded
component is improved. The Al plating layer has a higher boiling point and
melting point
than a plating layer of an organic material or a plating layer of another
metal-based material
(for example, a Zn-based material). Therefore, when forming by hot stamping is
performed,
since the plating layer does not evaporate, hot stamping at a high temperature
becomes possible.
Therefore, the formability in hot stamping can be further enhanced, and
molding can be easily
performed.
[0044] The Al plating layer can be alloyed with Fe in the steel sheet by
heating during hot
plating and hot stamping. Therefore, the Al plating layer is not necessarily
formed of a single
layer having a constant component composition, and includes a partially
alloyed layer (alloy
layer).
[0045] (ZnO Film)
A ZnO film (coating containing ZnO) is formed on the surface of the Al plating
layer
of the plated steel sheet body, if necessary. In particular, when a zinc-based
metal soap film
is formed on a part of the plated steel sheet body on the Al plating layer
side surface, it is
preferable that the ZnO film is formed on the entire surface of the Al plating
layer of the plated
steel sheet body. In a region where the ZnO film serves as the outermost
surface layer of the
plated steel sheet, the ZnO film gives the hot lubricity, the chemical
conversion property, and
the corrosion resistance to the plated steel sheet.
[0046] The method of foiming the ZnO film is not particularly limited, and can
be formed on
the Al plating layer by the methods described in Patent Documents 1 and 2, for
example.
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[0047] The adhesion amount of an adhered portion of the ZnO film (hereinafter,
also simply
referred to as "adhesion amount") is preferably from 0.5 to 7 g/m2 based on Zn
amount per one
side of a steel sheet. When the adhesion amount of ZnO film is 0,5 g/m2 or
more based on
the Zn amount, a lubrication improving effect can be effectively exerted in a
region in contact
with a part other than the sliding surface of a mold in hot stamping. On the
other hand, when
the adhesion amount of ZnO film exceeds 7 g/m2 based on the Zn amount, the
thickness of the
Al plating layer and the ZnO film becomes too thick, and the weldability and
paint adhesion
may decrease.
The adhesion amount of the ZnO film is particularly preferably about from 1 to
4 g/m2
based on the Zn amount per one side of a steel sheet, lubricity at the time of
hot stamping can
also be secured in a region which is in contact with a part other than the
sliding surface of a
mold during hot stamping, and weldability and paint adhesion are also
favorable.
As a method of measuring the adhesion amount of the ZnO film, a fluorescent X-
ray
method is used. Specifically, a calibration curve is prepared by using
fluorescent X-ray
method using several kinds of standard samples whose adhesion amount of the
ZnO film (based
on Zn amount) is known, the Zn intensity of a sample to be measured is
converted into the
adhesion amount of ZnO film, and the adhesion amount of the ZnO film is
determined.
[0048] < Zinc-based Metal Soap Film >
A zinc-based metal soap film (coating containing a zinc-based metal soap) is
provided
on the surface of the plated steel sheet body on the Al plating layer side.
Specifically, when
a ZnO film is not provided on the Al plating layer of the plated steel sheet
body, a zinc-based
metal soap film is provided on the surface (entire surface) of the Al plating
layer. On the other
hand, when a ZnO film is provided on the Al plating layer of the plated steel
sheet body, a zinc-
based metal soap film is provided on at least a part of the surface of the ZnO
film.
[0049] Examples of the metal soap of the zinc-based metal soap film include a
metal salt
(fatty acid zinc salt) of a fatty acid (for example, a fatty acid having from
7 to 20 carbon atoms)
and zinc. The fatty acid may be either a saturated fatty acid or an
unsaturated fatty acid.
In particular, from the viewpoint of forming a zinc-based metal soap film
having high
smoothness, the metal soap of the zinc-based metallic soap coating is
preferably a liquid metal
soap at room temperature (25 C).
Specific examples of the zinc-based metal soap film include a film of at least
one zinc-
based metal soap selected from the group consisting of zinc bis-octanoate,
zinc octylate, zinc
laurate, and zinc stearate.
13
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[0050] Since the zinc-based metal soap film is formed using a zinc-based metal
soap for
lubricant application, it becomes a film with high smoothness. On the other
hand, in the zinc-
based metal soap film, for example, zinc is oxidized by heating at 300 C or
higher (heating
before stamping by hot stamping or preheating before hot stamping), and an
organic substance
(a fatty acid or the like) other than zinc is decomposed to obtain a ZnO film.
In other words,
a region where the zinc-based metal soap film of the plated steel sheet body
is provided
becomes a region covered with the ZnO film derived from the zinc-based metal
soap film by
heating.
[0051] By heating the zinc-based metal soap film having high smoothness, a ZnO
film having
high smoothness (for example, a ZnO film satisfying the maximum value of the
skewness Rsk
of the surface roughness curve satisfying Rsk < 0) can be formed. When a ZnO
film having
high smoothness is used to hot stamp a plated steel sheet, wear of the sliding
surface of a mold
on which the plated steel sheet slides is suppressed.
[0052] Here, when the plated steel sheet body does not have a ZnO film (or
when a zinc-
based metal soap film is formed on the surface of the Al plating layer of the
plated steel sheet
body), the adhesion amount of the adhered portion of the zinc-based metal soap
film is affected
by the surface properties of an underlying plated steel sheet body (Al plating
layer) when the
amount is too small or too large, and the smoothness of the zinc-based metal
soap film and the
smoothness of the ZnO film formed from a zinc-based metal soap film decrease.
Therefore,
the adhesion amount (hereinafter also simply referred to as "adhesion amount")
of an adhered
portion of the zinc-based metal soap film is, based on Zn amount, from 7.1 to
19.8 g/m2, and
preferably from 8.82 to 16.3 g/m2. The adhesion amount may be in the range of,
based on Zn
amount, from 8.9 to 19.8 g/m2, 9.2 to 19.8 g/m2, or from 9.5 to 19.8 g/m2.
[0053] On the other hand, when the plated steel sheet body has a ZnO film (or
when a zinc-
based metal soap film is formed on the surface of the ZnO film of the plated
steel sheet body),
the adhesion amount of the zinc-based metal soap film is needed to be
considered such that
both the surface of the ZnO film of the underlying plated steel sheet body and
the surface of
the ZnO film formed of the zinc-based metal soap film are smooth. Accordingly,
the total of
the adhesion amount of a zinc-based metal soap film with the adhesion amount
of a ZnO film
of a plated steel sheet body (the total adhered amount of the ZnO film of the
plated steel sheet
body and the zinc-based metal soap film) is, based on Zn amount, from 7.1 to
19.8 g/m2, and
preferably from 8.82 to 16.3 g/m2. The adhesion amount may be in the range of,
based on Zn
amount, from 8.9 to 16.3 g/m2, from 9.2 to 16.3 g/m2, or from 9.5 to 16.3
g/m2.
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[0054] When a plated steel sheet body has a ZnO film (or when a zinc-based
metal soap film
is formed on the surface of a ZnO film of a plated steel sheet body), the
adhesion amount of
the zinc-based metal soap film to the total adhesion amount of the ZnO film
and the zinc-based
metal soap film is not less than half, from the viewpoint of enhancing the
smoothness of the
surface of a ZnO film formed from the zinc-based metal soap film.
[0055] As a method of measuring the adhesion amount of the zinc-based metal
soap film, for
example, a fluorescent X-ray method is used. Specifically, a calibration curve
is prepared by
using a fluorescent X-ray method using several types of standard samples with
known amounts
of zinc-based metal soap film (based on Zn amount), the Zn intensity of a
sample to be
measured is converted into the adhesion amount of the zinc-based metal soap
film, and the
adhesion amount of the zinc-based metal soap film is determined.
[0056] Here, the zinc-based metal soap film is preferably formed at least on
the surface of the
plated steel sheet body on the Al plating layer side surface which is in
contact with a sliding
surface of a mold for hot stamping.
Specifically, for example, in cases in which a ZnO film is formed on the Al
plating
layer of the plated steel sheet body, when the plated steel sheet is hot
stamped by a mold for
hot stamping, the zinc-based metal soap film is preferably formed at least on
the surface of the
plated steel sheet (the Al plating layer or the ZnO film of the plated steel
sheet main body)
which is to be a vertical wall portion and a flange portion of a stamped
component to be
obtained. This is because since a plated steel sheet at a portion to be a
vertical wall portion
and a flange portion of a stamped component is a site where the surface is
formed while being
slid on a mold (for example, "a holder portion and a shoulder portion for
holding a steel sheet"
in an upper mold, "a holder portion and a shoulder portion for holding a steel
sheet" in a lower
mold) (see Fig. 3 (8)), the portion is a region where wear is likely to occur
in the mold.
On the other hand, when a ZnO film is not formed on the Al plating layer of
the plated
steel sheet body, a zinc-based metal soap film is preferably formed on the
entire surface of the
Al plating layer.
[0057] The plated steel sheet according to the embodiment as described above
is used for hot
stamping in a state having a zinc-based metal soap film, and may be used for
hot stamping in
a state in which the zinc-based metal soap film was heated in advance and a
ZnO film was
formed.
In other words, the plated steel sheet according to the embodiment may be used
for
hot stamping as a plated steel sheet for hot stamping including: a plated
steel sheet body
CA 03048362 2019-06-25
including a steel sheet and an Al plating layer provided on one side or both
sides of the steel
sheet; and a ZnO film provided on the surface of the plated steel sheet body
on the Al plating
layer side, wherein the maximum value of the skewness Rsk of the roughness
curve of the
surface of the ZnO film is less than 0.
[0058] Here, the skewness Rsk of the roughness curve is measured in accordance
with JIS B
0601 (2001). Specifically, the skewness Rsk of the roughness curve is measured
in
accordance with IS B 0601 (2001) under the following measurement conditions.
[0059] - Measurement Conditions -
Measuring device: "Surface roughness/Profile shape measuring machine Form
Tracer"
manufactured by Mitutoyo Corporation
Measurement length L: 9.6 mm
Cutoff wavelength kc: 0.8 mm
Stylus tip shape: Tip angle 60 cone
Stylus tip radius: 2 pm
Measurement speed: 1 mm/sec
[0060] Here, the skewness Rsk of the roughness curve is defined in JIS B 0601
(2001) and is
an index indicating the symmetry of ridges and valleys with respect to the
average line of the
roughness curve. When Rsk is positive (0 < Rsk), the peaks and valleys are
unevenly
distributed downward from the average line of the roughness curve. On the
other hand, when
Rsk is negative (Rsk < 0), the peaks and valleys are unevenly distributed
upward from the
average line of the roughness curve. In other words, when Rsk is negative (Rsk
< 0), the
number of ridges protruding on the surface is small and the smoothness is
high.
When the value of Rsk is partly positive, there is a convex portion protruding
on a part
of the surface of a plated steel sheet for hot stamping. In other words, the
surface pressure
between the convex portion of the surface and a sliding surface of a mold is
relatively high,
and the sliding surface of the mold is likely to wear. Therefore, the maximum
value of Rsk
on the surface of the ZnO film is preferably less than 0. By setting the
maximum value of
Rsk of the ZnO film surface to less than 0, uniform surface contact between
the ZnO film
surface and a sliding surface of a mold is realized, the effective surface
pressure when the mold
slides can be reduced, and wear of the sliding surface of the mold can be
suppressed.
16
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[0061] < Manufacturing Method of Plated Steel Sheet for Hot Stamping >
The method of manufacturing a plated steel sheet according to the embodiment
includes a step of forming a zinc-based metal soap film on the surface of a
plated steel sheet
body on the Al plating layer side.
Specifically, for example, when a ZnO film is not provided on an Al plating
layer for
a plated steel sheet body, the method of manufacturing the plated steel sheet
includes a step of
forming a zinc-based metal soap film on the surface of the Al plating layer.
On the other hand,
when a ZnO film is provided on an Al plating layer of a plated steel sheet
body, the method of
manufacturing a plated steel sheet includes a step of forming a zinc-based
metal soap film on
at least a part of the surface of the ZnO film.
[0062] When a plated steel sheet body does not include a ZnO film (or when a
zinc-based
metal soap film is formed on the surface of an Al plating layer of a plated
steel sheet body), the
adhesion amount of a zinc-based metal soap film is, based on Zn amount, from
7.1 to 19.8 g/m2,
and preferably from 8.82 to 16.3 g/m2.
On the other hand, when a plated steel sheet body includes a ZnO film (or when
a
zinc-based metal soap film is formed on the surface of a ZnO film of a plated
steel sheet body),
the total adhesion amount of the zinc-based metal soap film with the adhesion
amount of the
ZnO film on the plated steel sheet body (total adhesion amount of the ZnO film
and the zinc-
based metal soap film of the plated steel sheet body) is set to, based on Zn
amount, from 7.1 to
19.8 g/m2, and preferably from 8.82 to 16.3 g/m2. It is noted that the
adhesion amount of the
zinc-based metal soap film to the total adhesion amount of the ZnO film and
the zinc-based
metal soap film is not less than half, from the viewpoint of enhancing the
smoothness of the
surface of a ZnO film formed from the zinc-based metal soap film.
[0063] In the step of forming a zinc-based metal soap film, a zinc-based metal
soap film is
formed by applying a zinc-based metal soap itself utilizing a well-known
coating apparatus
such as a spray coater, a roll coater, or a die coater. In addition, a zinc-
based metal soap film
may be formed by utilizing a sponge, an electrostatic lubricating device, or
the like. At the
time of coating, the viscosity of the zinc-based metal soap may be adjusted
with an organic
solvent. Then, after applying the zinc-based metal soap, a zinc-based metal
soap film is
formed by drying the coating film of the zinc-based metal soap, if necessary,
for example at
300 C or higher for 2 minutes or more.
[0064] Here, in the step of forming a zinc-based metal soap film, the type of
the zinc-based
metal soap and a formation region of the zinc-based metal soap film are as
described above.
17
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[0065] The method of manufacturing a plated steel sheet according to the
embodiment may
include a step of heating a zinc-based metal soap film at 300 C or higher to
form a ZnO film.
In other words, by heating, zinc in the zinc-based metal soap film is
oxidized, an organic
substance (a fatty acid or the like) other than zinc is decomposed, and a ZnO
film (for example,
a ZnO film whose surface roughness curve skewness Rsk satisfies Rsk < 0) is
obtained, and
then, the obtained plated steel sheet may be used for hot stamping.
Heating to convert a zinc-based metal soap film to a ZnO film is preferably
performed
under conditions of 300 C or higher and 2 minutes or more.
[0066] < Method of Manufacturing Hot-stamped Component >
The method of manufacturing a hot-stamped component according to the
embodiment
is a method of manufacturing a formed component by hot stamping the plated
steel sheet
according to the embodiment.
[0067] Specifically, for example, in a method of manufacturing a hot-stamped
component, in
the method of manufacturing a plated steel sheet according to the embodiment,
in a step of
forming a zinc-based metal soap film, at least a zinc-based metal soap film is
formed on the
surface of the plated steel sheet body on the Al plating layer side surface
which is in contact
with a sliding surface of a mold for hot stamping, and then, the manufactured
plated steel sheet
for hot stamping is hot stamped. In this case, after the zinc-based metal soap
film of the plated
steel sheet becomes a ZnO film by heating before stamping, the plated steel
sheet is stamped.
[0068] For example, in a method of manufacturing a hot-stamped component, in
the method
of manufacturing a plated steel sheet according to the embodiment, a plated
steel sheet
manufactured through a step of heating a zinc-based metal soap film at 300 C
or higher and
obtaining a ZnO film whose skewness Rsk of the surface roughness curve
satisfies 0 < Rsk
may be hot-stamped.
[0069] In the method of manufacturing a hot-stamped component according to the
embodiment, in a hot stamping method of example, after blanking (punching) if
necessary,
heating is performed at a high temperature and a plated steel sheet is
softened. Then, using a
mold, softened plated steel sheet is stamped and formed, and then cooled. In
this way, in hot
stamping, subsequent stamping can be easily performed by once softening the
plated steel sheet.
The stamped component hot stamped is quenched by heating and cooling to obtain
a formed
component having a high tensile strength of about 1500 MPa or higher.
18
CA 03048362 2019-06-25
[0070] As a heating method of hot stamping, other than a normal electric
furnace and a radiant
tube furnace, a heating method by infrared heating, electrification heating,
induction heating,
or the like may be employed.
[0071] The Al plating layer of the plated steel sheet melts when heated above
the melting
point, and at the same time, the Al phase changes to Al-Fe alloy phase, Al-Fe-
Si alloy phase
due to mutual diffusion with Fe. The melting points of the Al-Fe alloy phase
and the Al-Fe-
Si alloy phase are high and about 1150 C. There are a plurality of types of
intermetallic
compounds contained in the Al-Fe alloy phase and the Al-Fe-Si alloy phase.
When heated at
high temperature or heated for a long time, the alloy phase changes to an
alloy phase having a
higher Fe concentration.
[0072] The state of the Al plating layer preferable as a stamped component is
a state in which
the layer is alloyed to the surface and in which the Fe concentration in the
alloy phase is not
high. When unalloyed Al remains, only this portion is rapidly corroded,
corrosion resistance
after coating deteriorates, and coating film blistering tends to occur very
easily, which is not
preferable. On the other hand, when the Fe concentration in the alloy phase
becomes too high,
the corrosion resistance of the alloy phase itself decreases, corrosion
resistance after coating
deteriorates, and coating film blistering is likely to occur. In other words,
the corrosion
resistance of the alloy phase depends on the Al concentration in the alloy
phase. Therefore, in
order to improve the corrosion resistance after coating, the state of alloying
is controlled by the
Al adhesion amount and heating conditions.
[0073] In the heating method of hot stamping, the average temperature rising
rate in a
temperature range from 50 C to the temperature 10 C lower than the highest
attainable sheet
temperature is preferably set to from 10 to 300 C/s. The average temperature
rising rate
affects productivity in hot stamping of a plated steel sheet. When the average
temperature
rising rate is less than 10 C/s, it takes time to soften a plated steel sheet
for hot stamping. On
the other hand, when the temperature exceeds 300 C, although softening is
rapid, alloying of
the Al plating layer is considerable, which may cause powdering. The average
temperature
rising rate is about 5 C/sec in the case of atmosphere heating. An average
temperature rising
rate of 100 C/s or more can be achieved by electric heating or high frequency
induction heating.
[0074] On the other hand, since it is necessary to perform hot stamping in the
austenite single
phase region, a temperature of about from 900 to 950 C is usually employed as
the maximum
attainable temperature in many cases. In the hot stamping, the maximum
attainable
temperature is not particularly limited, and when the temperature is less than
850 C, sufficient
19
CA 03048362 2019-06-25
quench hardness can not be obtained, which is not preferable. The Al plating
layer also needs
to be made of an Al-Fe alloy phase. From these viewpoints, the maximum
attainable
temperature is preferably 850 C or higher. On the other hand, when the maximum
attainable
temperature exceeds 1000 C, alloying progresses too much, and the Fe
concentration in the Al-
Fe alloy phase increases, which may lead to a reduction in corrosion
resistance after coating.
From these viewpoints, although it can not be said unconditionally since the
limit depends on
the temperature rising rate and the Al adhesion amount, the upper limit of the
maximum
attainable temperature is preferably 1100 C or lower in consideration of
economic efficiency.
[0075] In hot stamping, a plated steel sheet heated to a high temperature is
stamped with a
mold. Then, by cooling, a stamped component having a desired shape can be
obtained.
[0076] Here, an example of a normal process from manufacturing of a plated
steel sheet to
hot stamping is as follows.
First, an Al plating layer is formed on one side or both sides of a steel
sheet (Fig. 3
(1): 12 in Fig. 3 denotes a steel sheet) (Fig. 3(2): 14 in Fig.3 denotes an Al
plating layer).
Next, a ZnO film is formed on the surface of the Al plating layer (Fig. 3 (3):
16 in Fig.
3 denotes a ZnO film).
Next, the obtained plated steel sheet is wound into a coil shape (Fig. 3 (4):
20 in Fig.
3 denotes a plated steel sheet wound in a coil shape (plated steel sheet body
in the embodiment)).
Next, a plated steel sheet wound in a coil shape is drawn out and blanking
(blanking
processing) is performed (Figs. 3(5) to 3(6): 22 in Fig. 3 denotes a blank).
Next, in a heating furnace, the blank is heated (Fig. 3(7): 24 in Fig. 3
denotes a heating
furnace).
Next, the heated blank is stamped with a pair of upper and lower molds and
formed
and quenched (Fig. 3 (8): 26A in Fig. 3 denotes an upper mold and 26B denotes
a lower mold).
Then, by removing the blank from the mold, a desired stamped component can be
obtained (Fig. 3(9): 28 in Fig. 3 denotes a stamped component).
[0077] On the other hand, in a normal process from manufacturing of a plated
steel sheet to
hot stamping, a zinc-based metal soap film is formed in each step or between
steps after
formation of Al plating layer before a plated steel sheet (blank material) is
heated.
Specifically, a zinc-based metal soap film 1) is formed on the surface of an
Al plating layer of
a plated steel sheet main body (blank after blanking or the like) after
forming the Al plating
layer (when a zinc-based metal soap film is formed on the entire surface of an
Al plating layer,
formation of a ZnO film may be omitted), or, 2) a zinc-based metal soap film
is formed on the
CA 03048362 2019-06-25
surface of a ZnO film of a plated steel sheet body (blank after blanking or
the like) after
formation of a ZnO film.
[0078] A site where a zinc-based metal soap film is formed is preferably on
the entire surface
of an Al plating layer or a ZnO film, and may be on a surface in contact with
a sliding surface
of a mold for hot stamping. A surface of the plated steel sheet in contact
with the sliding
surface of the mold for hot stamping is, for example, the surface of the
plated steel sheet (the
Al plating layer or the ZnO film of the plated steel sheet main body) to be a
vertical wall portion
and a flange portion of a stamped component to be obtained. Specifically, for
example, the
surface of a plated steel sheet which is in contact with a sliding surface of
a mold for hot
stamping is the surface of the plated steel sheet (plated steel sheet body)
which is in contact
with ''a holder portion and a shoulder portion for holding a steel sheet" in
an upper mold and
"a holder portion and a shoulder portion for holding a steel sheet" in a lower
mold (see Fig.
3(8): in Fig. 3, 26A1 denotes a holder portion of the upper mold, 26A2 denotes
a shoulder
portion of the upper mold, 26B1 denotes a holder portion of the lower mold,
and 26B2 denotes
a shoulder portion of the lower mold).
[0079] Heating of a plated steel sheet before stamping may be performed with a
zinc-based
metal soap film as it is, or performed after the zinc-based metal soap film is
formed into a ZnO
film.
[0080] < Method of Manufacturing Vehicle >
In the hot stamping method according to the embodiment, a variety of stamped
components can be manufactured. In the manufactured stamped component, a
surface on
which a ZnO film is formed is particularly excellent in corrosion resistance
(or coating
corrosion resistance). For this
reason, when a stamped component for a vehicle is
manufactured, it is preferable to manufacture a vehicle by attaching the
manufactured stamped
component with the surface of a ZnO film facing the outer side of the vehicle.
[0081] Specifically, for example, a stamped component (such as a center pillar
outer, a door
outer, a roof rail outer, a side panel, or a fender) that is exposed when
attached to a vehicle is
manufactured by the hot stamping method according to the embodiment. When
these
stamped components are attached to a vehicle, the stamped components are
attached to the
vehicle in such a manner that the "surface on which a ZnO film is formed"
faces the outer side
of the vehicle (for example, in such a manner to be exposed from the vehicle).
21
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EXAMPLES
[0082] The disclosure will be further described with reference to Examples.
The disclosure is
not limited to the following embodiments.
[0083] < Comparative Examples 1 to 4>
Both sides of a cold-rolled steel sheet having a thickness shown in Table 1
(based on %
by mass, C: 0.21%, Si: 0.12%, Mn: 1.21%, P: 0.02%, S: 0.012%, Ti: 0.02%, B:
0.03%, Al:
0.04%, and the balance: Fe and impurities) were Al plated by a Sendzimir
process. The
annealing temperature was about 800 C, an Al plating bath contained 9% Si and
further
contained Fe eluted from the cold-rolled steel sheet. The Al basis weight
after plating was
adjusted by a gas wiping method, the Al basis weight per one side shown in
Table I was
obtained, and the sheet was then cooled. Thereafter, a chemical solution
(nanotek slurry
manufactured by C. I. Kasei Co., Ltd., particle size of zinc oxide particles =
70 nm) was coated
on the Al plating layer thus formed with a roll coater and baked at about 80
C, and a ZnO film
having an adhesion amount shown in Table 1 was formed. In this way, a test
material of a
plated steel sheet was obtained.
[0084] < Examples 1 to 4, Comparative Examples 5 to 6>
Both sides of a cold-rolled steel sheet having a thickness shown in Table 1
(based on %
by mass, C: 0.21%, Si: 0.12%, Mn: 1.21%, P: 0.02%, S: 0.012%, Ti: 0.02%, B:
0.03%, Al:
0.04%, and the balance: Fe and impurities) were Al plated by a Sendzimir
process. The
annealing temperature was about 800 C, an Al plating bath contained 9% Si and
further
contained Fe eluted from the cold-rolled steel sheet. The Al basis weight
after plating was
adjusted by a gas wiping method, the Al basis weight per one side shown in
Table 1 was
obtained, and the sheet was then cooled. Thereafter, a chemical solution
(nanotek slurry
manufactured by C. I. Kasei Co., Ltd., particle size of zinc oxide particles =
70 nm) was coated
on the Al plating layer thus formed with a roll coater and baked at about 80
C, and a ZnO film
having an adhesion amount (based on Zn amount) shown in Table 1 was formed.
Next, zinc
bis-octanoate (Zn-OCTOATE 22% solvent-free "manufactured by DIC Corporation)
as a zinc-
based metal soap was coated on the ZnO film with a roll coater, and a zinc-
based metal soap
film having an adhesion amount shown in Table 1 was formed. In this way, a
test material of
a plated steel sheet was obtained.
[0085] < Examples 5 to 8, Comparative Examples 7 to 8>
Both sides of a cold-rolled steel sheet having a thickness shown in Table 1
(based on %
by mass, C: 0.21%, Si: 0.12%, Mn: 1.21%, P: 0.02%, S:0.012%, Ti: 0.02%, B:
0.03%, Al:
22
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0.04%, and the balance: Fe and impurities) were Al plated by a Sendzimir
process. The
annealing temperature was about 800 C, an Al plating bath contained 9% Si and
further
contained Fe eluted from the cold-rolled steel sheet. The Al basis weight
after plating was
adjusted by a gas wiping method, the Al basis weight per one side shown in
Table I was
obtained, and the sheet was then cooled. Thereafter, zinc bis-octanoate ("Zn-
OCTOATE 22%
solvent-free" manufactured by DIC Corporation) as zinc-based metal soap was
coated on the
Al plating layer thus formed with a roll coater, and a zinc-based metal soap
film having an
adhesion amount shown in Table 1 was formed. In this way, a test material of a
plated steel
sheet was obtained.
[0086] < Evaluation >
Characteristics of the test material of the plated steel sheet manufactured as
described
above were evaluated by the following method. The average temperature rising
rate during
heating to 920 C was 7.5 C/s.
[0087] (1) Hot Lubricity
Using the apparatus for evaluating hot lubricity illustrated in Fig. 4, the
hot lubricity
of the test material of the plated steel sheet was evaluated. The apparatus
for evaluating hot
lubricity illustrated in Fig. 4 includes a near infrared heating furnace 100
and a mold including
an upper mold 102A and a lower mold 102B. The upper mold 102A and the lower
mold 102B
include convex portions with a width of 10 mm extending in a direction
orthogonal to the
drawing direction of a plated steel sheet, and a predetermined spressing load
is applied by
sandwiching the test material between top surfaces of the convex portions. The
apparatus for
evaluating hot lubricity also includes a plated steel sheet heated in a near
infrared heating
furnace 100 and a thermocouple (not shown) for measuring the temperature of
the plated steel
sheet when the sheet is sandwiched between molds. In Fig. 4, 10 denotes a test
material of a
plated steel sheet.
Using the apparatus for evaluating hot lubricity illustrated in Fig. 4, a test
material of
30 mm > 500 mm was heated at 920 C in a nitrogen atmosphere by the near
infrared heating
furnace 100, and then, the specimen which had reached about 700 C was drawn
out while
applying a pressing load of 3 kN (or while sliding the test material on the
mold) with a mold
composed of an upper mold 102A and a lower mold 102B, and the drawing load was
measured.
The drawing length was 100 mm, and the drawing speed was 40 mm/s. Then, the
hot friction
coefficient (= (drawing load)/(pressing load)) was determined.
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CA 03048362 2019-06-25
[0088] (2) Mold Wear Amount
The mold wear amount was measured by analyzing the surface shape difference of
"mold of apparatus for measuring hot lubricity" before and after the
evaluation test of (1) Hot
Lubricity. Specifically, using a contact-type shape measuring machine,
profiles of the mold
surface at a sliding portion before and after sliding were measured, and the
mold wear amount
was measured. The mold wear amount was the average value of the wear amounts
of the
upper mold and the lower mold.
[0089] (3) Surface Properties of Test Material
Regarding the surface properties of the test material (ZnO film) after the
evaluation
test of (1) Hot Lubricity, the maximum value of the skewness Rsk of the
roughness curve was
evaluated with positive and negative signs. The skewness Rsk of the roughness
curve was
measured in two directions in the rolling direction of the material and in a
direction
perpendicular to the rolling direction by the above-described method. The
maximum value
of the values therein was used as the evaluation value. In the table, the
notation "+" indicates
"0 < Rsk", and the notation "-" indicates "Rsk < 0".
[0090] (4) Adhesion Amount of ZnO Film of Test Material
The adhesion amount (based on Zn amount) of the ZnO film on the surface of the
test
material after the evaluation test of (1) Hot Lubricity was measured by the
above-described
method.
[0091] The details of Examples 1 to 8 and Comparative Examples 1 to 8 are
listed below in
Table 1.
In Table 1, the total amount adhered on the surface based on Zn amount means
"adhesion amount (based on Zn amount) of ZnO film and zinc-based metal soap
film".
24
[0092] [Table 1]
Test material of plated steel sheet
Evaluation
Adhesion amount
Adhesion amount Total amount
Adhesion amount of Surface properties
of zinc-based
Hot
Sheet Al basis mass of ZnO film
adhered on the ZnO film of test Mold wear of test material
Steel type metal soap film
friction
thickness per side (based on Zn surface based
on Zn material (after heating) amount (maximum value
of
(based on Zn
coefficient
amount) amount (based on Zn amount) skewness Rsk)
amount)
mm g/m2 g/m2 g/m2 g/m2 g/m2 _ Pm
Comparative
1.4 80 0.70 0.00 0.70 0.70 0.46
1.70 +
Example 1
Comparative
1.6 40 0.80 0.00 0.80 0.80 0.53
2.73 +
Example 2
Comparative
1.8 60 2.00 0.00 2,00 2.00 0.51
1.47 +
Example 3
Comparative
2.0 40 1.60 0.00 1.60 1.60 0.52
2.40 + 0
Example 4
.
Comparative
.
' 1.4 40 0.70 2.2 2.9 0.88 0.51
1.33 +
N.) Example 5
.
ui
Example 1 1.6 80 0.80 6.6 7.4
1.21 0.41 0.28 -
Example 2 1.4 40 0.70 8.82 9.5
1.06 0.39 0.15 -
,
Example 3 1.8 60 2.00 14.3 16.3
3.17 0.38 0.18 - T
.
0,
Example 4 2.0 40 1.60 17.6 19.2
3.05 0.45 0.44 -
Comparative
1.4 40 0.40 22.0 22.4 2.21 0.53
2.89 +
Example 6
Comparative
1.4 40 0.00 2.2 2.2 0.18 0.46
1.53 +
Example 7
Example 5 1.6 80 0.00 7.7 7.7
0.65 0.44 0.48 -
Example 6 1.6 40 , 0.00 8.82 8.82
0.72 0.40 0.47 -
Example 7 1.8 60 0.00 14.3 14.3
1.17 0.38 0.24 -
Example 8 2.0 40 0.00 19.8 19.8
1.63 0.44 0.37 -
Comparative 22.0
1.4 40 0.00 22.0 1.80 0.49
2.44 +
Example 8
,
CA 03048362 2019-06-25
[0093] It was confirmed from Table 1 that in Examples 1 to 8, by forming an
appropriate
amount of a zinc-based metal soap film, a ZnO film having high smoothness was
formed from
the zinc-based metal soap film, and wear of a sliding surface of a mold can be
reduced together
with an improvement in hot lubricity.
It was also confirmed that in Examples 1 to 8, peeling of the ZnO film on the
surface
of the test material was not observed after the evaluation test of hot
lubricity, and the chemical
conversion processability and corrosion resistance of an obtained formed
component can be
improved.
It was confirmed that, in Examples 2, 3, 6, and 7, the total adhesion amount
of zinc-
based metal soap film with the adhesion amount of ZnO film was larger than 8.8
g/m2 (not
including 8.8) based on Zn amount, and when the amount is 16.3 g/m2 or less,
mold wear is
suppressed and the hot friction coefficient is 0.4 or less, and as a result,
the formability of a
material in hot stamping (hot stamp) can be improved.
[0094] < Examples 9 to 16, Comparative Examples 9, and Reference Example 1>
During the period from manufacturing of a plated steel sheet to hot stamping
(hot
stamp) (see Fig. 3), a target (period) and conditions (formation method,
adhesion amount,
presence or absence of a heating step in which a zinc-based metal soap film is
made into a ZnO
film, site to be formed) for forming a zinc-based metal soap film were changed
as shown in
Table 2, and stamped components (dish-like shaped components) were
manufactured. When
the surface of the Al plating layer was a target for forming a zinc-based
metal soap film, no
ZnO film was formed.
In the hot stamping (hot stamp), as in the test for evaluating hot lubricity,
the plated
steel sheet was heated to 900 C, and then stamped under conditions of 700 C
and 3 kN pressing
load.
The type of the steel sheet, conditions for foiming the Al plating layer,
condition for
forming the ZnO film, and the type of the zinc-based metal soap were the same
as in Example
1.
[0095] Then, the following evaluation was carried out.
1) A test material of a plated steel sheet having the same conditions as those
of the
obtained formed component was manufactured, and the hot lubricity was
evaluated by using
the test materials.
2) The wear amount of a mold ("a holder portion and a shoulder portion for
holding
the steel sheet" in the upper mold, "a holder portion and a shoulder portion
for holding a steel
26
CA 03048362 2019-06-25
sheet'' in the lower mold))) was measured in the same manner as the evaluation
of the mold
wear amount.
3) The surface properties of the formed component (vertical wall portion and
flange
portion) were evaluated in the same manner as the evaluation of the surface
properties of the
test material.
[0096] The details of Examples 9 to 16, Comparative Example 9, and Reference
Example 1
are listed below in Table 2.
In Table 2, the column adhesion amount (based on Zn amount) of Zinc-based
aliphatic
metal soap film indicates, for Example 9 in which a zinc-based aliphatic metal
soap film is
formed on the surface of the Al plating layer, "Adhesion amount (based on Zn
amount) of the
zinc-based aliphatic metal soap film itself', and indicates, for Examples 10
to 16 in which the
zinc-based aliphatic metal soap film is formed on the surface of the ZnO film,
"the total
adhesion amount (based on Zn amount) of the ZnO film and the zinc-based
aliphatic metal
soap film".
27
[0097] [Table 2]
Zinc-based metal soap film
Evaluation
Adhesion Presence
Hot
Mold wear Surface properties of
amount Or
Target (period) Formation method
Formation site friction amount formed component
(based on Zn absence
coefficient
[pm] (skewness Rsk)
amount) of heating
Al plating layer (immediately after Application with Present
Example 9 14.3 g/m2 Entire surface on both
sides 0.39 0.22 .. -
formation of Al plating layer) roll coater (400 C)
_
ZnO film (immediately after Application with Present
Example 10 8.82 g/m2 Entire surface on both
sides 0.40 0.15 .. -
formation of ZnO film) , roll coater (400 C)
ZnO film Application with
Example 11 8.82 g/m2 Absent
Entire surface on both sides 0.40 0.15 -
(immediately after blanking) roll coater _
ZnO film Application with
Example 12 electrostatic 8.82 g/m2
Absent Entire surface on both sides 0.40 0.22 -
(immediately after blanking)
lubricating device
ZnO film Application with
Example 13 8.82 g/m2 Absent
Entire surface on both sides 0.40 0.17 0
(immediately after blanking) sponge
.
Some part: portions
.
ZnO film 82 g Application with
corresponding to vertical wall 0.40 0.15 -
IN-) /m2 * Absent
Example 14
.
00 (immediately after blanking) roll coater 8.
portion/flange portion of formed
component, Both sides
.
i-,
. .
.
Some part: portions
.
Application with
ZnO film to vertical wall corresponding Example 15 electrostatic
8.82 g/m2* Absent corresp 0.40 0.22 -
(immediately after blanking) lubricating device
portion/flange portion of formed
component, Both sides
Some part: portions
ZnO film Application with
corresponding to vertical wall
Example 16 8.82 g/m2 *
Absent 0.40 0.17 -
(immediately after blanking) sponge
portion/flange portion of formed
component, Both sides
Some part: Surface of shoulder
Comparative One application
Mold 22 g/m2* Absent
portion and holder portion of 0.49 1.70 +
Example 9 before stamping
upper mold and lower mold
Reference Continuous Surface of shoulder
portion and
Example Mold provision on mold 8.82 g/m2.sec
Absent holder portion of upper mold and 0.30 0.13 -
1 surface
lower mold
*:Adhesion amount of adhered portion
,
[0098] From Table 2, as shown in Examples 9 to 16, it was confirmed that, by
forming a zinc-
based metal soap film on the surface of the Al plating layer or the ZnO film
during the period
from manufacturing of the plated steel sheet to stamping of the hot stamping
(hot stamp), wear
on the sliding surface of the mold can be reduced as well as improving the hot
lubricity.
As shown in Examples 9 to 10, it was also confirmed that even when a ZnO film
was
formed on the zinc-based metal soap film by heating, wear on the sliding
surface of the mold
can be reduced as well as improving the hot lubricity.
As shown in Examples 14 to 16, it was confirmed that by forming at least a
zinc-based
metal soap film on the surface of the plated steel sheet (the Al plating layer
or the ZnO film of
the plated steel sheet main body) which is the vertical wall portion and the
flange portion of
the formed component, wear on the sliding surface of the mold can be reduced.
In Examples 9 to 16, it was confirmed that no peeling of the ZnO film on the
surface
of the formed component was observed, and the chemical conversion property and
corrosion
resistance of the obtained formed component can be improved.
[0099] As shown in Comparative Example 9, it was confirmed that, even when a
zinc-based
metal soap film is formed on the surface of a mold ("a shoulder portion and a
holder portion of
the upper mold and the lower mold" where the plated steel sheet slides at the
time of stamping),
improvement in hot lubricity and reduction in wear of the sliding surface of
the mold were not
observed.
However, as shown in Reference Example 1, by continuously providing the metal
soap
in such a manner not to cause film breakage, improvement in hot lubricity and
reduction in
wear on the sliding surface of the mold were observed.
[0100] Although suitable embodiments according to the invention have been
described in
detail above with reference to the accompanying drawings, it goes without
saying the invention
is not limited to such examples. It is obvious that various changes or
modifications can be
reached at by those having ordinary knowledge in the technical field to which
the invention
pertains within the scope of the technical idea described in the claims, and
it is understood that
these are naturally also within the technical scope of the invention.
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