[Technical Field of the Invention]
[0001]
The present invention relates to a hot-stamping formed body.
Priority is claimed on Japanese Patent Application No. 2020-057272, filed
March 27, 2020, the content of which is incorporated herein by reference.
[Background Art]
[0002]
In recent years, a need for increasing the strength of vehicle members has
increased from the viewpoint of stricter collision safety criteria for vehicles and the
improvement of fuel efficiency. The application of hot stamps has been extended in
order to achieve an increase in the strength of vehicle members. Hot stamping is a
technique for pressing a blank that is heated to a temperature (Ac3 point), at which the
single-phase region of austenite is formed, or more (for example, heated to about
900°C) and then rapidly cooling the blank in a die at the same time as forming to
perform quenching. According to this technique, it is possible to manufacture a pressformed
product having high shape fixability and high strength.
[0003]
Since a zinc component remains on the surface layer of a formed product
obtained after hot stamping in a case where hot stamping is applied to a zinc-plated steel
sheet, an effect of improving corrosion resistance is obtained as compared to a formed
product obtained from the hot stamping of an unplated steel sheet. For this reason, the
application of hot stamping to a zinc-plated steel sheet is being extended.
[0004]
- 1 -
Patent Document 1 discloses a hot-press formed steel member manufactured by
a method which includes a heating step of heating a zinc-plated steel sheet to a
temperature equal to or higher than an Ac3 point and a hot press forming step of
performing hot press forming at least twice after the heating step and in which all the
hot press forming performed in the hot press forming step are performed to satisfy a
predetermined equation.
[0005]
In a case where the zinc-plated steel sheet is subjected to hot stamping,
electrode sticking (a phenomenon in which a copper electrode and plating provided on
the surface of the formed product are melted and adhered to each other) may occur
during spot welding in a formed product obtained after hot stamping. Electrode
sticking during spot welding is not preferable because it could cause a poor weld or it
will inevitably cause manufacturing downtime to replace the copper electrode.
Electrode sticking during spot welding is not considered in Patent Document 1.
[Prior Art Document]
[Patent Document]
[0006]
[Patent Document 1] PCT International Publication No. W02013/147228
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0007]
The present invention has been made in consideration of the above-mentioned
circumstances and an object of the present invention is to provide a hot-stamping
formed body that is excellent in spot weldability. Further, an object of the present
- 2 -
invention is to provide a hot-stamping formed body that has the above-mentioned
property and has strength generally required for a hot-stamping formed body.
[Means for Solving the Problem]
[0008]
The present inventor investigated the cause of electrode sticking during spot
welding. As a result, the present inventor found that electrode sticking during spot
welding is further suppressed as the number of voids present in a zinc-plated layer is
smaller since electrode sticking during spot welding is greatly affected by voids
(vacancy) present in the zinc-plated layer. The present inventor thought that an
overcurrent occurs by a narrow electric current path caused by the voids in the zincplated
layer, and the overcurrent causes overheating which makes electrode sticking
between an electrode and zinc plating easier.
[0009]
Further, although a detailed mechanism is uncertain, the present inventor
thought that the occurrence of voids is caused by a difference in thermal contraction
between a base metal and a zinc-plated layer and a different in thermal contraction
between different phases (a r phase and a Fe-Zn solid solution) present in the zincplated
layer during hot stamping forming. As a result of the investigation of a method
of suppressing the occurrence of voids, the present inventor found that a predetermined
contact pressure during hot stamping forming can crush voids (i.e. the pressure can
reduce the number density of voids present in the zinc-plated layer), and that results in
improving spot weldability. Excellent spot weldability means that electrode sticking
during spot welding can be suppressed. Further, tensile (maximum) strength generally
required for a hot-stamping formed body is in a range of 1500 MPa to 2500 MPa.
[0010]
- 3 -
The gist ofthe present invention made on the basis of the above-mentioned
knowledge is as follows.
[1] A hot-stamping formed body according to an aspect of the present
invention includes a steel sheet and a zinc-plated layer that is provided on the steel
sheet. The steel sheet has a chemical composition containing, by mass %, C: 0.18% to
0.50%, Si: 0.10% to 1.50%, Mn: 1.5% to 2.5%, sol.Al: 0.001% to 0.100%, Ti: 0.010%
to 0.100%, S: 0.0100% or less, P: 0.100% or less, N: 0.010% or less, Nb: 0% to 0.05%,
V: 0% to 0.50%, Cr: 0% to 0.50%, Mo: 0% to 0.50%, B: 0% to 0.010%, Ni: 0% to
2.00%, and a total of REM, Ca, Co, and Mg: 0% to 0.030%. A remainder consists of
Fe and impurities, an area % of martensite is 90% or more in microstructure at a
position corresponding to 1/4 of a sheet thickness of the steel sheet from a surface of the
steel sheet in a sheet thickness direction, the zinc-plated layer includes a r phase and a
Fe-Zn solid solution, and a cross-sectional area ratio of voids present in the zinc-plated
layer is 15.0% or less.
[2] In the hot-stamping formed body according to [1], the chemical
composition may contain, by mass %, one or two or more selected from the group
consisting ofNb: 0.02% to 0.05%, V: 0.005% to 0.50%, Cr: 0.10% to 0.50%, Mo:
0.005% to 0.50%, B: 0.0001% to 0.010%, Ni: 0.01% to 2.00%, and a total of REM, Ca,
Co, and Mg: 0.0003% to 0.030%.
[3] In the hot-stamping formed body according to [1] or [2], the chemical
composition may contain, by mass %, C: 0.24% to 0.50%.
[Effects ofthe Invention]
[0011]
- 4 -
According to the aspect of the present invention, it is possible to provide a hotstamping
formed body that is excellent in spot weldability and has strength generally
required for a hot-stamping formed body.
[Embodiments of the Invention]
[0012]
A hot-stamping formed body according to this embodiment will be described
in detail below. First, the reason why the chemical composition of a steel sheet of the
hot-stamping formed body according to this embodiment is to be limited will be
described. All percentages (%)related to the chemical composition mean mass %.
[0013]
A steel sheet of the hot-stamping formed body according to this embodiment
contains, by mass %: C: 0.18% to 0.50%; Si: 0.10% to 1.50%; Mn: 1.5% to 2.5%;
sol.Al: 0.001% to 0.100%; Ti: 0.010% to 0.100%; S: 0.0100% or les s; P: 0.100% or
less; N: 0.010% or less; and a remainder consisting of Fe and impurities. Each
element will be described below.
[0014]
C: 0.18% to 0.50%
Cis an element that improves the strength of the hot-stamping formed body.
The C content is set to 0.18% or more in order to obtain a desired strength. The C
content is preferably 0.20% or more and more preferably 0.24% or more. On the other
hand, in a case where the C content exceeds 0.50%, strength is excessively high, so that
the ductility and toughness of the hot -stamping formed body deteriorate. For this
reason, the C content is set to 0.50% or less. The C content is preferably 0.40% or
less.
[0015]
- 5 -
Si: 0.10% to 1.50%
Si is an element that improves a fatigue property. Further, Si is also an
element that improves a hot-dip galvanizing property, particularly plating wettability,
by forming a stable oxide film on the surface of the steel sheet during recrystallization
annealing. In order to obtain these effects, the Si content is set to 0.10% or more.
The Si content is preferably 0.15% or more. On the other hand, in a case where the Si
content is excessively high, Si contained in steel is diffused during heating at the time of
hot stamping and forms oxide on the surface of the steel sheet. The oxide formed on
the surface of the steel sheet deteriorates a phosphate treatment property. Further, Si is
also an element that raises the Ac3 point of the steel sheet. In a case where the Ac3
point is raised, a heating temperature needs to be raised in order to sufficiently
austenitize the steel sheet and a heating temperature during hot stamping exceeds the
evaporation temperature of zinc plating. For this reason, the Si content is set to 1.50%
or less. The Si content is preferably 1.40% or less.
[0016]
Mn: 1.5% to 2.5%
Mn is an element that improves the hardenability of steel. The Mn content is
set to 1.5% or more in order to improve hardenability and obtain a desired amount of
martensite. The Mn content is preferably 1.8% or more. On the other hand, even
though the Mn content exceeds 2.5%, an effect of improving hardenability is saturated
and steel is embrittled, so that quenching cracks are likely to occur during casting, hot
rolling, and cold rolling. For this reason, the Mn content is set to 2.5% or less. The
Mn content is preferably 2.1% or less.
[0017]
sol.Al: 0.001% to 0.100%
- 6 -
Al is an element that deoxidizes molten steel to suppress the formation of oxide
serving as the origin of fracture. Further, Al is also an element that suppresses an
alloying reaction between Zn and Fe and improves corrosion resistance. In order to
obtain these effects, the sol.Al content is set to 0.001% or more. The sol.Al content is
preferably 0.005% or more. On the other hand, in a case where the sol.Al content is
excessive, the Ac3 point of the steel sheet is raised, a heating temperature needs to be
raised in order to sufficiently austenitize the steel sheet, and a heating temperature
during hot stamping exceeds the evaporation temperature of zinc plating. For this
reason, the sol.Al content is set to 0.100% or less. The sol.Al content is preferably
0.090% or less.
In this embodiment, sol.Al means acid-soluble Al, and indicates solute Al that
is present in steel in the state of a solid solution.
[0018]
Ti: 0.010% to 0.100%
Ti is an element that increases oxidation resistance after galvanizing. Further,
Ti is also an element that improves hardenability by combining with N to form nitride
(TiN) and suppressing the formation of nitride (BN) from B. In order to obtain these
effects, the Ti content is set to 0.010% or more. The Ti content is preferably 0.020%
or more. On the other hand, in a case where the Ti content is excessive, the Ac3 point
is raised and a heating temperature during hot stamping is raised. For this reason,
productivity may deteriorate, and it may be difficult to secure a r phase since formation
into a Fe-Zn solid solution may be facilitated. Further, in a case where the Ti content
is excessive, a large amount of Ti carbide is formed and the amount of solute C is
reduced, so that strength is reduced. Furthermore, the wettability of plating may
deteriorate, and toughness may deteriorate due to the excessive precipitation of Ti
- 7 -
carbide. For this reason, the Ti content is set to 0.100% or les s. The Ti content is
preferably 0.070% or less.
[0019]
S: 0.0100% or less
S is an element that is contained as an impurity and is an element that forms
sulfide in steel to cause the deterioration of toughness and to deteriorate a delayed
fracture resistance property. For this reason, the S content is set to 0.0100% or les s.
The S content is preferably 0.0050% or less. It is preferable that the S content is 0%.
However, since cost required to remove S is increased in a case where the S content is
to be excessively reduced, the S content may be set to 0.0001% or more.
[0020]
P: 0.100% or less
P is an element that is included as an impurity, and is an element that
segregates at a grain boundary to deteriorate toughness and a delayed fracture resistance
property. For this reason, the P content is set to 0.100% or less. The P content is
preferably 0.050% or les s. It is preferable that the P content is 0%. However, since
cost required to remove Pis increased in a case where the P content is to be excessively
reduced, the P content may be set to 0.001% or more.
[0021]
N: 0.010% or less
N is an impurity element, and is an element that forms coarse nitride in steel to
deteriorate the toughness of steel. Further, N is also an element that facilitates the
occurrence of blow holes during spot welding. Furthermore, in a case where B is
contained, N combines with B to reduce the amount of solute B and deteriorates
hardenability. For this reason, theN content is set to 0.010% or less. TheN content
- 8 -
is preferably 0.007% or less. It is preferable that theN content is 0%. However,
since manufacturing cost is increased in a case where the N content is to be excessively
reduced, theN content may be set to 0.0001% or more.
[0022]
The remainder of the chemical composition of the steel sheet of the hotstamping
formed body according to this embodiment is Fe and impurities. Elements,
which are unavoidably mixed from a steel raw material or scrap and/or during the
manufacture of steel and are allowed in a range where the properties of the hot-stamping
formed body according to this embodiment do not deteriorate, are exemplified as the
impurities.
[0023]
The steel sheet of the hot-stamping formed body according to this embodiment
may contain the following elements as arbitrary elements instead of a part of Fe. The
contents of the following arbitrary elements in a case where the following arbitrary
elements are not contained are 0%.
[0024]
Nb: 0% to 0.05%
Nb is an element that forms carbide in steel to refine crystal grains during hot
stamping and improves the toughness of the hot-stamping formed body. In order to
reliably obtain this effect, it is preferable that the Nb content is set to 0.02% or more.
On the other hand, even though the Nb content exceeds 0.05%, the above-mentioned
effect is saturated and hardenability deteriorates. For this reason, the Nb content is set
to 0.05% or less.
[0025]
V: 0% to 0.50%
- 9 -
V is an element that finely forms carbonitride in steel to improve strength. In
order to reliably obtain this effect, it is preferable that the V content is set to 0.005% or
more. On the other hand, in a case where the V content exceeds 0.50%, the toughness
of steel deteriorates during spot welding and cracks are likely to occur. For this
reason, the V content is set to 0.50% or less.
[0026]
Cr: 0% to 0.50%
Cr is an element that improves the hardenability of steel. In order to reliably
obtain this effect, it is preferable that the Cr content is set to 0.10% or more. On the
other hand, in a case where the Cr content exceeds 0.50%, Cr carbide is formed in steel
and it is difficult for Cr carbide to be dissolved during heating of hot stamping, so that
hardenability deteriorates. For this reason, the Cr content is set to 0.50% or less.
[0027]
Mo: 0% to 0.50%
Moisan element that improves the hardenability of steel. In order to reliably
obtain this effect, it is preferable that the Mo content is set to 0.005% or more. On the
other hand, even though the Mo content exceeds 0.50%, an effect of improving
hardenability is saturated. For this reason, the Mo content is set to 0.50% or less.
[0028]
B: 0% to 0.010%
B is an element that improves the hardenability of steel. In order to reliably
obtain this effect, it is preferable that the B content is set to 0.0001% or more. On the
other hand, even though the B content exceeds 0.010%, an effect of improving
hardenability is saturated. For this reason, the B content is set to 0.010% or less.
[0029]
- 10 -
Ni: 0% to 2.00%
Ni is an element that has an effect of improving the toughness of steel, an
effect of suppressing the embrittlement of steel caused by liquid Zn during heating of
hot stamping, and an effect of improving the hardenability of steel. In order to reliably
obtain these effects, it is preferable that the Ni content is set to 0.01% or more. On the
other hand, even though the Ni content exceeds 2.00%, the above-mentioned effects are
saturated. For this reason, the Ni content is set to 2.00% or less.
[0030]
Total of REM, Ca, Co, and Mg: 0% to 0.030%
REM, Ca, Co, and Mg are elements that suppress the occurrence of cracks
during spot welding by controlling sulfide and oxide in a preferred shape and
suppressing the formation of coarse inclusions. In order to reliably obtain this effect, it
is preferable that the total content of REM, Ca, Co, and Mg is set to 0.0003% or more.
In order to reliably obtain the above-mentioned effect, the content of even any one of
REM, Ca, Co, and Mg may be 0.0003% or more. On the other hand, in a case where
the total content of REM, Ca, Co, and Mg exceeds 0.030%, inclusions are excessively
generated and cracks are likely to occur during spot welding. For this reason, the total
content of REM, Ca, Co, and Mg is set to 0.030% or less.
[0031]
In this embodiment, REM refers to a total of 17 elements that are composed of
Sc, Y, and lanthanoid and the REM content refers to the total content of these elements.
[0032]
The chemical composition of the steel sheet described above may be measured
by a general analysis method. For example, the chemical composition of the steel
sheet described above may be measured using inductively coupled plasma-atomic
- 11 -
emission spectrometry (ICP-AES). C and S may be measured using a combustioninfrared
absorption method and N may be measured using an inert gas fusion-thermal
conductivity method. Further, sol.Al may be measured by ICP-AES using a filtrate
that is obtained in a case where a sample is decomposed with an acid by heating. The
chemical composition may be analyzed after the zinc-plated layer provided on the
surface of the hot-stamping formed body may be removed by mechanical grinding.
[0033]
Next, the microstructure of the steel sheet of the hot-stamping formed body
according to this embodiment will be described.
In the microstructure of the steel sheet of the hot-stamping formed body
according to this embodiment at a position corresponding to 114 of a sheet thickness
from the surface of the steel sheet in a sheet thickness direction, the area % of
martensite is 90% or more.
In this embodiment, microstructure at a position corresponding to 1/4 of a sheet
thickness from the surface of the steel sheet in a sheet thickness direction (a region
between a position corresponding to 1/8 of a sheet thickness from the surface of the
steel sheet and a position corresponding to 3/8 of a sheet thickness from the surface of
the steel sheet) is specified. The reason for this is that this depth position is an
intermediate point between the surface of the steel sheet and a central position of the
sheet thickness and microstructure at this depth position typifies the steel structure of
the steel sheet (shows the average microstructure of the entire steel sheet).
[0034]
Martensite: 90% or more
Martensite is structure that improves the strength of the steel sheet. In a case
where the area ratio of martensite is less than 90%, a desired strength cannot be
- 12 -
obtained in the hot-stamping formed body. For this reason, the area ratio of martensite
is set to 90% or more. The area ratio of martensite is preferably 95% or more or 96%
or more. Since higher area ratio of martensite is more preferable, the upper limit of the
area ratio of martensite may be set to 100%.
[0035]
Ferrite, pearlite, bainite, and residual austenite are included in the
microstructure of the steel sheet of the hot -stamping formed body according to this
embodiment as a remainder in microstructure. Since a desired strength cannot be
obtained in a case where the area ratio of the remainder in microstructure is high, the
area ratio of the remainder in microstructure may be set to 10% or less. The area ratio
of the remainder in microstructure is preferably 5% or less, more preferably 4%, and
still more preferably 0%.
[0036]
The area ratio of martensite is measured by the following method.
A sample is cut out from an arbitrary position away from an end surface of the
hot-stamping formed body by a distance of 50 mm or more (a position that avoids an
end portion in a case where the sample cannot be taken from this position) so that a
cross section (sheet thickness-cross section) perpendicular to the surface can be
observed. The size of the sample also depends on a measurement device but is set to a
size that allows the size to be observed by about 10 mm in a rolling direction. In a
case where the hot-stamping formed body includes a welded portion, a sample is taken
from a position avoiding the welded portion and the vicinity of the welded portion.
[0037]
The cross section of the sample is etched using LePera reagent. A position
corresponding to t/4 (t denotes the sheet thickness) of the cross section etched using
- 13 -
LePera reagent (a region between a position corresponding to 118 of a sheet thickness
from the surface of the sample and a position corresponding to 3/8 of a sheet thickness
from the surface of the sample) is observed in 10 visual fields with a magnification of
500, and image analysis is performed on an obtained optical microscopy image by using
image analysis software of "Photoshop CS5" manufactured by Adobe Systems Inc. to
obtain the area ratio of martensite. As an image analysis method, the maximum
luminosity value Lmax and the minimum luminosity value Lmin of an image are acquired
from the image, a portion including pixels having luminosity in a range of Lmax-
0.3(Lmax-Lmin) to Lmax is defined as a white region, a portion including pixels having
luminosity in a range of Lmin to Lmin+0.3 (Lmax-Lmin) is defined as a black region, other
portions are defined as a gray region, and the area ratio of martensite, which is a white
region, is calculated. Image analysis is performed in the same manner as described
above for a total of 10 observed visual fields to measure the area ratios of martensite,
and these area ratios are averaged to calculate an average value. The obtained average
value is regarded as the area ratio of martensite. As result, the area ratio of martensite
is obtained.
Further, the area ratio of martensite is subtracted from 100% to obtain the area
ratio of the remainder in microstructure.
[0038]
Next, a zinc-plated layer forming the hot-stamping formed body according to
this embodiment will be described. The zinc-plated layer is provided on the abovementioned
steel sheet and includes a r phase and a Fe-Zn solid solution, and the crosssectional
area ratio of voids present in the zinc-plated layer is 15.0% or les s. The zincplated
layer may be provided on both surfaces or any one surface of the abovementioned
steel sheet. Further, the zinc-plated layer refers to a layer in which the r
- 14 -
phase and the Fe-Zn solid solution are present. The r phase and the Fe-Zn solid
solution will be described later.
The zinc-plated layer will be described below.
[0039]
Including r phase and Fe-Zn solid solution
The zinc-plated layer includes the r phase and the Fe-Zn solid solution. The
r phase is a layer that has a Zn concentration close to the Zn concentration of a zinc
bath. The Fe-Zn solid solution is a phase that is generated in a case where zinc present
in the zinc bath and Fe contained in the steel sheet are alloyed with each other. For
this reason, the Fe concentration of the r phase is lower than that of the Fe-Zn solid
solution. In this embodiment, a phase of which the Fe concentration is in a range of 10
mass % to 30 mass % is defined as the r phase, and a phase of which the Fe
concentration is in a range of 50 mass % to 80 mass % is defined as the Fe-Zn solid
solution. A ch phase and a~ phase may be included in the zinc-plated layer in addition
to the r phase and the Fe-Zn solid solution. The Fe concentration of each of the ch
phase and the~ phase is less than 10 mass %.
[0040]
In a case where the r phase is not included in the zinc-plated layer due to the
excessive alloying of the zinc-plated layer, corrosion resistance deteriorates as
compared to a zinc-plated layer including the r phase. Further, the fact that the r
phase is not included in the zinc-plated layer means that the zinc-plated layer is being
alloyed. As the zinc-plated layer is alloyed (in a case where the zinc-plated layer is an
alloyed zinc-plated layer), an oxide film (ZnO) is formed and grows on the surface of
plating, which increases contact resistance during spot welding. As a result, expulsion
- 15 -
is likely to occur. For this reason, it is important that the r phase is included in the
zinc-plated layer.
[0041]
Since the properties of the hot-stamping formed body according to this
embodiment can be exhibited as long as even a small amount of the r phase is included
in the zinc-plated layer, the ratio of the r phase in the zinc-plated layer is not
particularly limited. The adhesion amount of the zinc-plated layer depends on a
desired corrosion resistance target, but may be set in a range of, for example, 5 g/m2 to
150 g/m2 per side. However, for example, in order to secure corrosion resistance equal
to or higher than the corrosion resistance of a cold formed article of a zinc-plated steel
sheet, the amount of the r phase may be set to 30 g/m2 or more per side. The reason
for this is that, since the Fe-Zn solid solution generated by heating during quenching
causes Fe rust and increases in volume in a case where the amount of the r phase is less
than 30 g/m2
, Zn oxidized during corrosion does not form a dense protective film and
corrosion resistance equal to or higher than the corrosion resistance of a cold formed
article of the zinc-plated steel sheet may not be obtained.
[0042]
The analysis of a Fe concentration in the zinc-plated layer is performed using
the following method.
A sample is cut out from an arbitrary position away from an end surface of the
hot-stamping formed body by a distance of 50 mm or more (a position that avoids an
end portion in a case where the sample cannot be taken from this position) so that a
cross section (sheet thickness-cross section) perpendicular to the surface can be
observed. The size of the sample also depends on a measurement device but is set to a
size that allows the size to be observed by about 10 mm in a rolling direction. In a
- 16 -
case where the hot-stamping formed body includes a welded portion, a sample is taken
from a position avoiding the welded portion and the vicinity of the welded portion.
[0043]
After the sample is embedded in a resin and is polished, the layer structure of a
sheet thickness-cross section is observed with a scanning electron microscope (SEM).
Specifically, the layer structure of the sheet thickness-cross section is observed using
the SEM with a magnification in which the steel sheet and the zinc-plated layer are
included in an observed visual field. Then, linear analysis is performed in a sheet
thickness direction from the surface using an SEM-energy dispersive X-ray
spectroscopy (SEM-EDS) in order to specify each layer in the layer structure of the
sheet thickness-cross section, and the quantitative analysis of the Fe concentration of
each layer is performed. Linear analysis is performed in the observation cross section
of the sample at 100 points arranged at intervals of 0.1 J.lm in a direction parallel to the
surface. In the linear analysis, quantitative analysis is performed at intervals of 1 nm
in the sheet thickness direction by energy dispersive X-ray spectroscopy (EDS) in
which the diameter of an electron beam is set to 10 nm. A phase of which the Fe
concentration is in a range of 10 mass % to 30 mass % is defined as the r phase, and a
phase of which the Fe concentration is in a range of 50 mass % to 80 mass % is defined
as the Fe-Zn solid solution. Phases of which the Fe concentration is less than 10
mass% are defined as the ~h phase and the~ phase.
[0044]
Next, linear analysis is performed in the sheet thickness direction using an
SEM-energy dispersive X-ray spectroscopy (EDS), and the quantitative analysis of the
Fe concentration of each layer is performed. A device to be used is not particularly
limited, but, for example, SEM (NB5000 manufactured by Hitachi High-Tech
- 17 -
Corporation), EDS (XFlash(r)6130 manufactured by Bruker AXS Inc.), and EDS
analysis software (ESPRIT1.9 manufactured by Bruker AXS Inc.) may be used in this
embodiment. From the observation results of the COMPO image and the quantitative
analysis results of SEM-EDS described above, a region, which is present at the deepest
position in the sheet thickness direction and in which the Fe content exceeds 80 mass %
excluding measurement noise, is determined as a steel sheet. Further, a phase of which
the Fe content is in a range of 10 mass % to 30 mass % excluding measurement noise is
determined as the r phase, and a region of which the Fe concentration is in a range of
50 mass % to 80 mass % is determined as the Fe-Zn solid solution.
[0045]
Next, a method of measuring the r phase included in the zinc-plated layer will
be described. A test piece is taken from the hot-stamping formed body, and this test
piece is immersed in an aqueous solution ofNH4Cl:150g/L Constant-current
electrolysis is performed with 4 mA/cm2 using a saturated calomel electrode as a
reference electrode, and a range in which an electric potential is -800 mV vs.SCE or less
is regarded as the r phase. The reason for this is that this range can be regarded as the
r phase which contains Zn as a main component and in which the Fe content is 30
mass% or less. An electrolyte obtained from the electrolysis of the r phase is
measured using inductively coupled plasma (ICP), and the sum of the amount of Fe and
the amount of Zn is regarded as the amount of the r phase.
[0046]
Cross-sectional area ratio of voids present in zinc-plated layer: 15.0% or less
In a case where the cross-sectional area ratio of voids present in the zinc-plated
layer is set to 15.0% or less, it is possible to suppress electrode sticking during the spot
welding of the hot-stamping formed body. The present inventor thought that electrode
- 18 -
sticking is likely to occur in a case where the cross-sectional area ratio of the voids
exceeds 15.0%, which means that an electric current path is locally narrow enough to
cause an overcurrent leading to overheating during spot welding. For this reason, in
this embodiment, the cross-sectional area ratio of voids present in the zinc-plated layer
of at least a region, which serves as a portion to be welded, is set to 15.0% or less. The
cross-sectional area ratio of voids present in the zinc-plated layer is preferably 13.0% or
less, more preferably 10.0% or less, and still more preferably 5.0% or less. Since
lower cross-sectional area ratio of voids present in the zinc-plated layer is more
preferable, the lower limit of the cross-sectional area ratio of voids present in the zincplated
layer may be set to 0%.
[0047]
The cross-sectional area ratio of voids present in the zinc-plated layer is
measured using the following method.
First, a sample is cut out from an arbitrary position away from an end surface
of the hot-stamping formed body by a distance of 50 mm or more (a position that avoids
an end portion in a case where the sample cannot be taken from this position) so that a
cross section (sheet thickness-cross section) perpendicular to the surface can be
observed. The size of the sample also depends on a measurement device but is set to a
size that allows the size to be observed by about 10 mm in a rolling direction.
[0048]
Next, after an observation cross section is polished and is imaged using a
scanning electron microscope (SEM) with a magnification of 300, the cross-sectional
area ratio of voids is calculated using binarization image processing. A device used
for the calculation of the cross-sectional area ratio of voids is not particularly limited,
but, for example, the built-in software of a digital microscope VHX-5000 manufactured
- 19 -
by Keyence Corporation may be used to determine the voids using luminance and to
automatically measure the area of the voids.
The cross-sectional area ratio of voids present in the zinc-plated layer is
obtained using the above-mentioned method.
[0049]
Sheet thickness and tensile strength
The sheet thickness of the hot-stamping formed body according to this
embodiment is not particularly limited. However, in terms of reducing the weight of a
vehicle body, it is preferable that the sheet thickness of the hot-stamping formed body
according to this embodiment is set in a range of 0.5 mm to 3.5 mm.
Further, in terms of reducing the weight of a vehicle body, it is preferable that
the tensile strength of the hot-stamping formed body is set to 1500 MPa or more. On
the other hand, in a case where the tensile strength exceeds 2500 MPa, strength is
excessively high, so that the toughness and ductility of the hot-stamping formed body
may deteriorate. For this reason, it is preferable that the tensile strength is set to 2500
MPa or less.
[0050]
Next, a method of manufacturing the hot-stamping formed body according to
this embodiment will be described.
A steel sheet (zinc-plated steel sheet) including a zinc-plated layer on the
surface thereof is subjected to hot stamping to apply predetermined contact pressure and
is then cooled, so that the hot-stamping formed body according to this embodiment is
manufactured. Since the r phase included in a zinc-plated layer disappears (during the
alloying of zinc plating) in a case where a hot-dip galvannealed layer is used, an effect
- 20 -
of improving corrosion resistance is not obtained. For this reason, it is preferable that
the zinc-plated steel sheet may be a hot-dip zinc-plated steel sheet.
[0051]
First, a method of manufacturing the zinc-plated steel sheet will be described.
After a cast piece having cast is heated to 1200°C or more and is held for 20 minutes or
more, hot rolling is performed so that a finish rolling completion temperature is 81 oac
or more. Further, cold rolling is performed to manufacture a steel sheet having the
above-mentioned chemical composition and a zinc-plated layer is then formed on the
surface of the steel sheet by a continuous hot-dip galvanizing line, so that the zincplated
steel sheet is manufactured. A cumulative rolling reduction during the cold
rolling may be set in a range of 30% to 90%. In the method of manufacturing the zincplated
steel sheet, the annealing of a hot-rolled sheet may be performed between the hot
rolling and the cold rolling. Further, pickling may be performed. The cold rolling
may be omitted and a steel sheet subjected to the hot rolling may be introduced into the
continuous hot-dip galvanizing line. In a case where the cold rolling is omitted, the
annealing of a hot-rolled sheet and the pickling may be omitted.
[0052]
In the continuous hot-dip galvanizing line, the steel sheet is heated and held
and is then immersed in a hot-dip galvanizing bath, so that a zinc-plated layer is formed
on the surface of the steel sheet. The adhesion amount of the zinc-plated layer may be
set in a range of 5 g/m2 to 150 g/m2 per side. Since electrogalvanizing requires
additional elements for delaying alloying, which increases manufacturing cost,
electrogalvanizing is not desirable.
[0053]
- 21 -
Next, the zinc-plated steel sheet is heated so that a heating temperature is in a
range of higher one of "the Ac3 point and 800°C" to 950°C. Further, a heating time (a
time that has passed until the zinc-plated steel sheet is out of a heating furnace after
being put in the heating furnace and then held at the heating temperature (a time having
passed between carrying the zinc-plated steel sheet in the heating furnace and carrying
the zinc-plated steel sheet out the heating furnace)) is set in a range of 60 sec to 600 sec.
The Ac3 point is represented by the following equation (1). In a case where the
heating temperature is lower than higher one of "the Ac3 point and 800°C" or the
heating time is less than 60 sec, the zinc-plated steel sheet cannot be sufficiently
austenitized. As a result, a desired amount of martensite cannot be obtained. In a
case where the heating temperature exceeds 950°C or the heating time exceeds 600 sec,
the r phase included in a zinc-plated layer disappears due to the excessive alloying of
the zinc-plated steel sheet. An average heating rate during heating may be set in a
range of 0.1 ac/s to 200 oc/s. The average heating rate mentioned here is a value that
is obtained in a case where a temperature difference between the surface temperature of
the steel sheet at the time of start of the heating and the heating temperature is divided
by a time difference from the start of the heating to a time when a temperature reaches
the heating temperature. In a case where the steel sheet is held in a temperature range
of higher one of "the Ac3 point and 800°C" to 950°C, the temperature of the steel sheet
may be changed or constant.
[0054]
Examples of a heating method to be performed before the hot stamping include
heating using an electric furnace, a gas furnace, or the like, flame heating, electrical
resistance heating, high-frequency heating, induction heating, and the like.

CLAIMS
1. A hot-stamping formed body comprising:
a steel sheet; and
a zinc-plated layer that is provided on the steel sheet,
wherein the steel sheet has a chemical composition containing, by mass %,
C: 0.18% to 0.50%,
Si: 0.10% to 1.50%,
Mn: 1.5% to 2.5%,
sol.Al: 0.001% to 0.100%,
Ti: 0.010% to 0.100%,
S: 0.0100% or less,
P: 0.100% or less,
N: 0.010% or less,
Nb: 0% to 0.05%,
V: 0% to 0.50%,
Cr: 0% to 0.50%,
Mo: 0% to 0.50%,
B: 0% to 0.010%,
Ni: 0% to 2.00%, and
a total of REM, Ca, Co, and Mg: 0% to 0.030%,
a remainder consisting of Fe and impurities,
an area % of martensite is 90% or more in microstructure at a position
corresponding to 1/4 of a sheet thickness of the steel sheet from a surface of the steel
sheet in a sheet thickness direction,
the zinc-plated layer includes a r phase and a Fe-Zn solid solution, and
- 33 -
a cross-sectional area ratio of voids present in the zinc-plated layer is 15.0% or
less.
2. The hot-stamping formed body according to claim 1,
wherein the chemical composition contains, by mass%, one or two or more
selected from the group consisting of
Nb: 0.02% to 0.05%,
V: 0.005% to 0.50%,
Cr: 0.10% to 0.50%,
Mo: 0.005% to 0.50%,
B: 0.0001% to 0.010%,
Ni: 0.01% to 2.00%, and
a total of REM, Ca, Co, and Mg: 0.0003% to 0.030%.
3. The hot-stamping formed body according to claim 1 or 2,
wherein the chemical composition contains, by mass%,
C: 0.24% to 0.50%.