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 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 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.
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 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.
[0016] 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
[0017] 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.
[0018] The gists of the disclosure are as follows.
- <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
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
oxide film is the adhesion amount of the adhered portion of the zinc-based metal soap
film.
- <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.
[0020] 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.
BRIEF DESCRIPTION OF DRAWINGS
[0021]
Fig. 1A 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
[0022] Next, an embodiment which is an example of the disclosure will be described in detail.
[0023] Preferred embodiments of the disclosure will be described in detail below with reference
to the accompanying drawings.
[0024] 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.
[0025] 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.
[0026] 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.
< Plated Steel Sheet >
[0027] 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 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/m
2 based on Zn amount.
[0028] 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/m
2 based on Zn amount.
[0029] 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.
[0030] 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.
[0031] 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).
[0032] 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 conversion treatment properties after hot stamping and corrosion resistance.
Specifically, the inventors found the following.
[0033] 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.
[0034] 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 a metal tube 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.
[0035] 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.
[0036] 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 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.
[0037] Hereinafter, details of the plated steel sheet according to the embodiment will be
described.
< Plated Steel Sheet Body >
[0038] 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.
(Steel Sheet)
[0039] 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.
[0040] 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.
[0041] 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%.
[0042] 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%.
[0043] 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
γ phase becomes too much and the strength decreases. Therefore, the Mn content is
preferably from 0.5% to 3%.
[0044] 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%.
[0045] 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%.
[0046] This steel sheet may contain impurities that may be mixed in other manufacturing
processes or the like.
[0047] 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.
(Al Plating Layer)
[0048] 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.
[0049] 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.
[0050] 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%.
[0051] 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.
[0052] 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).
(ZnO Film)
[0053] 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.
[0054] The method of forming 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.
[0055] 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/m
2 based on Zn amount per one side of a steel sheet. When the adhesion amount of ZnO
film is 0.5 g/m
2 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/m
2 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.
[0056] The adhesion amount of the ZnO film is particularly preferably about from 1 to 4
g/m
2 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.
[0057] 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.
< Zinc-based Metal Soap Film >
[0058] 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.
[0059] 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.
[0060] 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).
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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/m
2, and preferably from 8.82 to 16.3 g/m
2. The adhesion amount may be in the range of, based on Zn amount, from 8.9 to 19.8
g/m
2, 9.2 to 19.8 g/m
2, or from 9.5 to 19.8 g/m
2.
[0065] 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/m
2, and preferably from 8.82 to 16.3 g/m
2. The adhesion amount may be in the range of, based on Zn amount, from 8.9 to 16.3
g/m
2, from 9.2 to 16.3 g/m
2, or from 9.5 to 16.3 g/m
2.
[0066] 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 oxide 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.
[0067] 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.
[0068] 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.
[0069] 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.
[0070] 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.
[0071] 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.
[0072] 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 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.
[0073] 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.
- Measurement Conditions -
[0074] Measuring device: "Surface roughness/Profile shape measuring machine Form Tracer"
manufactured by Mitutoyo Corporation
Measurement length L: 9.6 mm
Cutoff wavelength λc : 0.8 mm
Stylus tip shape: Tip angle 60° cone
Stylus tip radius: 2 µm
Measurement speed: 1 mm/sec
[0075] 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.
[0076] 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.
< Manufacturing Method of Plated Steel Sheet for Hot Stamping >
[0077] 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.
[0078] 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.
[0079] 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/m
2, and preferably from 8.82 to 16.3 g/m
2.
[0080] 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/m
2, and preferably from 8.82 to 16.3 g/m
2. 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 oxide 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.
[0081] 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.
[0082] 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.
[0083] 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.
[0084] 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.
< Method of Manufacturing Hot-stamped Component >
[0085] 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.
[0086] 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.
[0087] 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.
[0088] 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.
[0089] 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.
[0090] 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.
[0091] 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.
[0092] 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.
[0093] 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 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.
[0094] 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.
[0095] Here, an example of a normal process from manufacturing of a plated steel sheet to
hot stamping is as follows.
[0096] 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).
[0097] 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).
[0098] 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)).
[0099] 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).
[0100] Next, in a heating furnace, the blank is heated (Fig. 3(7): 24 in Fig. 3 denotes
a heating furnace).
[0101] 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).
[0102] 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).
[0103] 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 surface of a ZnO film of a plated
steel sheet body (blank after blanking or the like) after formation of a ZnO film.
[0104] 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).
[0105] 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.
< Method of Manufacturing Vehicle >
[0106] 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.
[0107] 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).
EXAMPLES
[0108] The disclosure will be further described with reference to Examples. The disclosure
is not limited to the following embodiments.
< Comparative Examples 1 to 4 >
[0109] 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 shown in
Table 1 was formed. In this way, a test material of a plated steel sheet was obtained.
< Examples 1 to 4, Comparative Examples 5 to 6>
[0110] 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 I 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.
< Examples 5 to 8, Comparative Examples 7 to 8 >
[0111] 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, 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.
< Evaluation >
[0112] 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.
(1) Hot Lubricity
[0113] 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.
[0114] 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.
(2) Mold Wear Amount
[0115] 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.
(3) Surface Properties of Test Material
[0116] 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".
(4) Adhesion Amount of ZnO Film of Test Material
[0117] 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.
[0118] The details of Examples 1 to 8 and Comparative Examples 1 to 8 are listed below in
Table 1.
[0119] 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".
[Table 1]
Steel type |
Test material of plated steel sheet |
Evaluation |
Sheet thickness |
Al basis mass per side |
Adhesion amount of ZnO film (based on Zn amount) |
Adhesion amount of zinc-based metal soap film (based on Zn amount) |
Total amount adhered on the surface based on Zn amount |
Adhesion amount of ZnO film of test material (after heating) (based on Zn amount) |
Hot friction coefficient |
Mold wear amount |
Surface properties of test material (maximum value of skewness Rsk) |
mm |
g/m2 |
g/m2 |
g/m2 |
g/m2 |
g/m2 |
- |
µm |
|
Comparative Example 1 |
1.4 |
80 |
0.70 |
0.00 |
0.70 |
0.70 |
0.46 |
1.70 |
+ |
Comparative Example 2 |
1.6 |
40 |
0.80 |
0.00 |
0.80 |
0.80 |
0.53 |
2.73 |
+ |
Comparative Example 3 |
1.8 |
60 |
2.00 |
0.00 |
2.00 |
2.00 |
0.51 |
1.47 |
+ |
Comparative Example 4 |
2.0 |
40 |
1.60 |
0.00 |
1.60 |
1.60 |
0.52 |
2.40 |
+ |
Comparative Example 5 |
1.4 |
40 |
0.70 |
2.2 |
2.9 |
0.88 |
0.51 |
1.33 |
+ |
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 |
- |
Example 4 |
2.0 |
40 |
1.60 |
17.6 |
19.2 |
3.05 |
0.45 |
0.44 |
- |
Comparative Example 6 |
1.4 |
40 |
0.40 |
22.0 |
22.4 |
2.21 |
0.53 |
2.89 |
+ |
Comparative Example 7 |
1.4 |
40 |
0.00 |
2.2 |
2.2 |
0.18 |
0.46 |
1.53 |
+ |
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 Example 8 |
1.4 |
40 |
0.00 |
22.0 |
22.0 |
1.80 |
0.49 |
2.44 |
+ |
[0120] 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.
[0121] 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.
[0122] 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/m
2 (not including 8.8) based on Zn amount, and when the amount is 16.3 g/m
2 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.
< Examples 9 to 16, Comparative Examples 9, and Reference Example 1 >
[0123] 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.
[0124] 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.
[0125] The type of the steel sheet, conditions for forming 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.
[0126] 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 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.
[0127] The details of Examples 9 to 16, Comparative Example 9, and Reference Example 1 are
listed below in Table 2.
[0128] 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".
[Table 2]
|
Zinc-based metal soap film |
Evaluation |
|
Target (period) |
Formation method |
Adhesion amount (based on Zn amount) |
Presence absence of heating |
Formation site |
Hot friction coefficient |
Mold wear amount [µm] |
Surface properties of formed component (skewness Rsk) |
Example 9 |
Al plating layer (immediately after formation of Al plating layer) |
Application with roll coater |
14.3 g/m2 |
Present (400°C) |
Entire surface on both sides |
0.39 |
0.22 |
- |
Example 10 |
ZnO film (immediately after formation of ZnO film) |
Application with roll coater |
8.82 g/m2 |
Present (400°C) |
Entire surface on both sides |
0.40 |
0.15 |
- |
Example 11 |
ZnO film (immediately after blanking) |
Application with roll coater |
8.82 g/m2 |
Absent |
Entire surface on both sides |
0.40 |
0.15 |
- |
Example 12 |
ZnO film (immediately after blanking) |
Application with electrostatic lubricating device |
8.82 g/m2 |
Absent |
Entire surface on both sides |
0.40 |
0.22 |
- |
Example 13 |
ZnO film (immediately after blanking) |
Application with sponge |
8.82 g/m2 |
Absent |
Entire surface on both sides |
0.40 |
0.17 |
- |
Example 14 |
ZnO film (immediately after blanking) |
Application with roll coater |
8.82 g/m2 * |
Absent |
Some part: portions corresponding to vertical wall portion/flange portion of formed
component, Both sides |
0.40 |
0.15 |
- |
Example 15 |
ZnO film (immediately after blanking) |
Application with electrostatic lubricating device |
8.82 g/m2 * |
Absent |
Some part: portions corresponding to vertical wall portion/flange portion of formed
component, Both sides |
0.40 |
0.22 |
- |
Example 16 |
ZnO film (immediately after blanking) |
Application with sponge |
8.82 g/m2 * |
Absent |
Some part: portions corresponding to vertical wall portion/flange portion of formed
component, Both sides |
0.40 |
0.17 |
- |
Comparative Example 9 |
Mold |
One application before stamping |
2 g/m2 * |
Absent |
Some part: Surface of shoulder portion and holder portion of upper mold and lower
mold |
0.49 |
1.70 |
+ |
Reference Example 1 |
Mold |
Continuous provision on mold surface |
8.82 g/m2·sec |
Absent |
Surface of shoulder portion and holder portion of upper mold and lower mold |
0.30 |
0.13 |
- |
Adhesion amount of adhered portion |
[0129] 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.
[0130] 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.
[0131] 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.
[0132] 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.
[0133] 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.
[0134] 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.
[0135] 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.
[0136] The disclosure of Japanese Patent Application No.
2016-256016 is hereby incorporated by reference in its entirety.
[0137] All the literature, patent applications, and technical standards cited herein are
also herein incorporated to the same extent as provided for specifically and severally
with respect to an individual literature, patent application, and technical standard
to the effect that the same should be so incorporated by reference.