Document Type] Specification
[Title of the Invention] HOT-STAMPED STEEL
[Technical Field of the Invention]
[0001]
The present invention relates to hot -stamped steel that is produced tlu·ough
hot-stamping.
Priority is claimed on Japanese Patent Application No. 2014-073814, filed on
March 31,2014, the content of which is incorporated herein by reference.
[Related Art]
[0002]
To realize high strength in a structural component used in automobiles, a
structural component, which is produced tlu·ough hot-stamping, may be used. The
hot-stamping is a method in which a steel sheet, which is heated to an Ac3 point or
higher, is rapidly cooled down by using a die while pressing the steel. That is, in the
hot-stamping, pressing and quenching are simultaneously performed. According to
the hot-stamping, it is possible to produce a structural component having high shape
accuracy and high strength. The steel (hot -stamped steel), which is produced by a
producing method including the hot -stamping, is disclosed in Patent Document 1,
Patent Document 2, and Patent Document 3. The hot-stamped steel, which is
disclosed in the Patent Documents, is steel that is produced by performing hotstamping
with respect to a steel sheet coated with a Zn coating layer so as to increase
corrosion resistance.
[0003]
As described above, in the hot-stamping, quenching is performed with respect
- 1 -
to the steel sheet simultaneously with pressing. In addition, the hot-stamping is
suitable to produce a structural component having high shape accuracy and high
strength. According to this, typically, the strength (tensile strength) of the hotstamped
steel is as high as approximately 1500 MPa or greater. However, recently,
the demand for collision safety in automobiles has increased, and thus a component for
automobiles may be required to have impact absorption properties in collision rather
than the strength. Typically a material having low strength is preferable so as to
increase the impact absorption properties. In the hot -stamped steel, it is known that
the strength can be changed to a certain degree by changing the amount of alloy
elements in the steel sheet or hot-stamping conditions. However, in a hot-stamping
process, it is not preferable to change the hot -stamping conditions in accordance with a
component when considering that an increase in pressing load may be caused.
According to this, there is a demand for hot-stamped steel that has the same chemical
composition as that of hot-stamped steel in which the tensile strength of approximately
1500 MPa or greater is obtained tln·ough quenching in the hot-stamping, has corrosion
resistance that is equal to or higher than the related art, and has a strength of
approximately 600 MPa to 1450 MPa.
[0004]
However, a method of reducing the strength of the hot -stamped steel is not
disclosed in Patent Document I to Patent Document 3.
[Prior Art Document]
[Patent Document]
[0005]
[Patent Document I] Japanese Unexamined Patent Application, First
Publication No. 2003-73774
- 2 -
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2003-129209
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2003-126921
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0006]
The present invention has been made in consideration of the above-described
problem. An object of the present invention is to provide hot-stamped steel that has
impact absorption properties higher than those of hot-stamped steel having the same
chemical composition in the related art, and includes a Zn coating layer excellent in
conosion resistance.
[Means for Solving the Problem]
[0007]
The gist of the present invention is as follows.
(I) According to an aspect of the present invention, hot-stamped steel
includes: a base metal that is a steel including a tempered pmiion having hardness
corresponding to 85% or less of the highest quenching hardness, the highest quenching
hardness being defined as a Vickers hardness at a depth position spaced away limn a
surface by 114 times a sheet thickness in a case of performing water quenching after
heating to a temperature equal to or higher than an Ac3 point and retaining for 30
minutes; and a Zn coating layer that is formed on the tempered pmiion of the base
metal, wherein the Zn coating layer includes a solid-solution layer including a solidsolution
phase that contains Fe and Zn that is solid-saluted in Fe, a lamella layer that
includes the solid-solution phase and a capital gamma phase, and an area ratio of the
- 3 -
lamella layer in the Zn coating layer is 20% or less.
(2) In the hot-stamped steel according to(!), the hardness of the tempered
portion may be 60% or less of the highest quenching hardness, and the area ratio of the
lamella layer in the Zn coating layer may be 5% to 20%.
(3) In the hot-stamped steel according to(!) or (2), the hardness of the
tempered portion may be 50% or less of the highest quenching hardness.
( 4) In the hot-stamped steel according to any one of (1) to (3), the hardness of
the tempered portion may be 180 Hv to 450 Hv.
(5) In the hot-stamped steel according to any one of(1) to (4), the hotstamped
steel may be produced by heating for a predetermined heating time so that the
highest heating temperature is the Ac3 point or higher, working and quenching
simultaneously through pressing by using a die, and tempering at a predetermined
tempering temperature, and when anAc1 point of the base metal is represented by Act,
the tempering temperature is represented by T in degrees °C, and a concentration of Zn
in an Fe-Zn solid-solution of a steel after the quenching and before the tempering is
represented by C in a unit of% by mass, the following Equation a may be satisfied.
Act~'I?:700-4.0x (35.0-C) (a)
(6) In the hot-stamped steel according to (5), the tempering temperature may
be 700°C to the Act point of the base metal.
(7) In the hot-stamped steel according to any one of(!) to (6), a part of the
base metal may be the tempered portion.
[Effects of the Invention]
[0008]
According to the aspect of the present invention, it is possible to provide a
hot-stamped steel having impact absorption properties higher than that of hot-stamped
- 4 -
steel having the same chemical composition in the related art, and including a Zn
coating layer excellent in corrosion resistance.
[Brief Description of the Drawings]
[0009]
FIG. I is a cross-sectional SEM image of a Zn coating layer and the periphery
thereof in a case where hot-stamped steel including the Zn coating layer is tempered at
400°C.
FIG. 2 is a cross-sectional SEM image of the Zn coating layer and the
periphery thereof in a case where the hot -stamped steel including the Zn coating layer
is tempered at 500°C.
FIG. 3 is a cross-sectional SEM image of the Zn coating layer and the
periphery thereof in a case where the hot-stamped steel including the Zn coating layer
is tempered at 700°C.
FIG. 4 is a view showing XRD measurement results of the Zn coating layer
shown in FIG. 1.
FIG. 5 is a view showing XRD measurement results ofthe Zn coating layer
shown in FIG. 2.
FIG. 6 is a view showing XRD measurement results of the Zn coating layer
shown in FIG. 3.
FIG. 7 is a view showing SST test results of the hot-stamped steel that are
tempered at tempering temperatures different from each other.
FIG. 8 is a Fe-Zn binary phase diagram.
[Embodiments of the Invention]
[0010]
The present inventors have made an investigation with respect to impact
- 5 -
absorption properties and corrosion resistance of hot-stamped steel. As a result, the
present inventors have obtained the following findings.
[00 11]
As described above, typically, as the strength (tensile strength) of hot-stamped
steel becomes lower, impact absorption properties become higher. When tempering is
performed with respect to the hot-stamped steel, it is possible to further lower the
tensile strength in comparison to hot -stamped steel having the same chemical
composition in the related mi. That is, it is possible to enhance the impact absorption
properties of the hot-stamped steel.
[0012]
However, when tempering is performed with respect to hot-stamped steel
including a Zn coating layer, a structure of the Zn coating layer changes. The change
in the structure of the Zn coating layer has an effect on corrosion resistance.
[0013]
In the related art, a change in the Zn coating layer when performing tempering
with respect to the hot-stamped steel including the Zn coating layer has not been
reported. According to this, the present inventors have made an investigation with
respect to an effect on the Zn coating layer by tempering conditions, and an effect on
corrosion resistance by a configuration of the Zn coating layer in the following manner.
[0014]
First, a plurality of steel sheets, which satisfy a preferred chemical
composition to be described later and have a sheet thickness of 1.6 mm, were prepared.
Then, the galvanized layer was formed on each of the steel sheets by using a hot dip
galvanizing method. A coating weight of galvanized layer was 60 g/m2
. Hotstamping
was performed with respect to the steel sheet on which the galvanized layer
- 6 -
was formed. Specifically, the steel sheet was charged into a heating fi.Jrnace in which
a fi.Jrnace temperature was set to 900°C that is a temperature equal to or higher than an
Ac3 point of the steel sheet, and was heated for 4 minutes. At this time, a temperature
ofthe steel sheet reached 900°C after approximately two minutes after being charged
into the fi.Jrnace. After the heating, the steel sheet was interposed by a flat die
equipped with a water-cooling jacket, and the hot-stamping was performed to produce
hot-stamped steel. A cooling rate during the hot-stamping was 50 °C/second or faster
up to a martensitic transformation start point even in a portion in which cooling rate is
slow.
Here, an Acl point and the AcJ point respectively represent an austenitic
transformation initiation temperature and an austenitic transformation termination
temperature during heating of the steel sheet. The Ac1 point and the Ac3 point can be
determined by measuring thermal expansion during heating the steel in a F ormaster
test and the like. Specifically, the Ac1 point and the Ac3 point can be determined by
observing volume constriction during transformation from ferrite to austenite. In
addition, the martensitic transformation start point can be determined by measuring
thermal expansion when rapidly cooling steel that is heated to an austenitizing
temperature. Specifically, the martensitic transformation statt point can be
determined by measuring volume expansion from austenite to mmtensite.
[0015]
Tempering was performed with respect to respective hot-stamped steel which
were produced. The tempering temperature was set to be different between the
respective hot-stamped steel in a range of 1 50°C to the ACI point of the base metal.
The heating time of the respective hot-stamped steel during tempering was set to 5
minutes.
- 7 -
[0016]
Micro-structure observation and XRD measurement were performed with
respect to the respective hot-stamped steel which was subjected to the tempering at
respective tempering temperatures. In addition, the structure of the Zn coating layer
was specified on the basis of results of the micro-structure observation and the XRD
measurement.
FIG. I is a cross-sectional image of the Zn coating layer of the hot-stamped
steel and the periphery thereof in a case where the tempering temperature is 400°C,
and FIG. 4 represents XRD measurement results from the surface. FIG. 2 is a crosssectional
image of the Zn coating layer of the hot -stamped steel and the periphery
thereof in a case where the tempering temperature is 500°C, and FIG. 5 is an XRD
measurement result from the surface. FIG. 3 is a cross-sectional image of the Zn
coating layer of the hot-stamped steel and the periphery thereof in a case where the
tempering temperature is 700°C, and FIG. 6 is an XRD measurement result from the
surface.
[0017]
The micro-structure observation of the cross-section was performed in the
following manner. Specifically, the cross-section was etched with 5% nita! for 20
seconds to 40 seconds, and after the etching, the micro-struchue was observed with an
SEM at a magnification of 2000 times. Whether or not an oxide layer is present has
hardly any effect on the strength or the corrosion resistance in comparison to the
configuration of a Zn coating layer. According to this, an investigation has been
made with focus given to the Zn coating layer.
The XRD measurement was performed by using a Co tubular bulb. In XRD,
typically, the intensity peak of a-Fe is shown at a diffraction angle of29=99.7°.
- 8 -
However, as the solid-solution amount of Zn increases, the intensity peak shifts toward
a small-angle side. In addition, the intensity peak of capital gamma (f), which is an
intermetallic compound ofFe3Zn10, is shown at a diffraction angle of29=94.0°. The
broken line L4 in FIG. 4 to FIG. 6 indicates the intensity peak position ofthe a-Fe
phase. A broken line L3 indicates the intensity peak position of a solid-solution phase
in which the solid-solution amount of Zn is small (the Zn content is 5% by mass to
25% by mass, and hereinafter, also may be referred to as "low Zn solid-solution
phase"). The broken line L2 indicates the intensity peak position of a solid-solution
phase in which the solid-solution amount of Zn is large (the Zn content is 25% by mass
to 40% by mass, and hereinafter, may also be referred to as "high Zn solid-solution
phase"). The broken line L1 indicates the intensity peak position of a f-phase. As
the intensity peak position shifts from the broken line L4 to the broken line L2, the
solid-solution amount of Zn in the solid-solution phase increases.
[0018]
In a case where the tempering temperature is equal to or higher than 150 oc
and lower than 500°C, as shown in FIG. 1 and FIG. 4, the Zn coating layer formed a
solid-solution layer 10. The solid-solution layer included the high Zn solid-solution
phase in which the intensity peak position is L2, and did not contain the r -phase. The
reference numeral 20 in FIG. 1 represents a tempered portion in the base metal, and a
reference numeral 30 represents a zinc oxide layer formed on the Zn coating layer.
[0019]
On the other hand, in a case where the tempering temperature is equal to or
higher than 500°C and lower than 700°C, as shown in FIG. 2, the solid-solution layer
10, and a lamella structure layer (hereinafter, referred to as "lamella layer") 40, which
included the f-phase and the low Zn solid-solution phase and was formed on the solid-
- 9 -
solution layer 10, was formed in the Zn coating layer. From results of the XRD
measurement, as shown in FIG. 5, the intensity peak (position of the broken line L3) of
the low Zn solid-solution phase, and the intensity peak (position of the broken line Ll)
of the f-phase are shown. That is, the lamella structure layer was a layer (lamella
layer) of a lamella structure mainly including the r -phase and the low Zn solidsolution
phase.
In a case where the tempering temperature is equal to or higher than 500°C
and lower than 700°C, the Zn coating layer included the solid-solution layer (including
the high Zn solid-solution phase) 10 in an area ratio of 0% to 70%, and the lamella
layer 40 in an area ratio of 30% or greater.
[0020]
In addition, in a case where the tempering temperature is 700°C to the Ac1
point of the base metal, as shown in FIG. 3, the Zn coating layer included a slight
amount of the lamella layer 40 on a surface layer side, and the solid-solution layer 10
on a lower side (on a steel side) of the lamella layer 40. The area ratio occupied by
the lamella layer 40 in the Zn coating layer was 5% to 20%. In addition, from results
of the XRD measurement, as shown in FIG. 6, the intensity peak of the solid-solution
phase, which was not detected in a case where the tempering temperate was 500°C to
lower than 700°C, was shown again at the position of the broken line L2, and the
intensity peak (position of the broken line Ll) of the r -phase was lowered in
comparison to the case where the tempering temperature was 500°C to lower than
[0021]
As described above, the structure of the Zn coaling layer changes depend on
the tempering conditions. Accordingly, the corrosion resistance of the hot-stamped
- I 0 -
steel, which was subjected to the tempering at each tempering temperature, was
investigated.
[0022]
The corrosion resistance was evaluated through an SST test (salt spray test).
The SST test was performed in the following manner.
A rear surface and an end surface of sheet -shaped hot -stamped steel tempered
at each tempering temperature were sealed with a polyester tape. Then, a surface of
each steel sheet was subjected to a test defined by JIS Z2371 "salt water spray test
. method" for 6 days (144 hours). A corrosion weight loss of the steel after the test was
obtained to draw FIG. 7. FIG. 7 is a view showing the corrosion weight loss (g/m2
) of
the hot-stamped steel after the SST test (salt spray test).
[0023]
The horizontal axis in FIG. 7 represents the tempering temperature CC), and
the veliical axis represents the corrosion weight loss (g/m2
). Referring to FIG. 7, the
corrosion weight loss of hot-stamped steel, in which the tempering temperature was set
to 200°C to 400°C, and 700°C, was in the same level as in hot-stamped steel that was
not subjected to tempering, and was 130 g/m2 or less. On the other hand, in hotstamped
steel in which the tempering temperature was set to 500°C to 600°C, the
corrosion weight loss of the Zn coating layer was significantly higher in comparison to
hot-stamped steel that was not subjected to tempering.
That is, in a Zn coating layer in which an area ratio of the lamella layer is 20%
or less, it is possible to secure the same corrosion resistance as in hot-stamped steel
that is not subjected to tempering.
[0024]
From the above-described results, it was proved that the corrosion resistance
- 11 -
can be retained when the area ratio of the lamella layer is 20% or less (including 0%)
in the Zn coating layer that includes the solid-solution layer and the lamella layer.
[0025]
In addition, a surface of hot -stamped steel, which is applied to a component
for automobiles, may be frequently subjected to painting. During the painting, a
surface with high chemical convetiibility has high film adhesiveness. Accordingly,
the chemical convertibilities were evaluated with respect to Zn coating layers which
are different in the area ratio of the lamella layer. As a result, the present inventors
have obtained the following finding. Specifically, when the Zn coating layer includes
the lamella layer in an area ratio of 5% or greater, the chemical convertibility is
improved.
[0026]
Next, hot-stamping was performed by using the same raw materials under the
same conditions except that the heating time during hot-stamping was set to 6 minutes
or 8 minutes. In addition, an effect on the Zn coating layer by the tempering
conditions was evaluated with respect to respective hot-stamped steel.
[0027]
Tempering was performed with respect to respective hot-stamped steel which
were produced. The tempering temperature was set to be different between the
respective hot-stamped steel in a range of 150°C to the Act point of the base metal.
The heating time of the respective hot-stamped steel during tempering was set to 5
minutes.
[0028]
As described above, in a case where the heating time during hot -stamping was
set to 4 minutes, when the tempering temperature was 500°C to 700°C, the area ratio
- 12 -
of the lamella layer was 30% or greater. However, in a case where the heating time
during hot -stamping was set to 6 minutes, even when the tempering temperature was
500°C or 690°C, the area ratio of the lamella layer in the Zn coating layer was 5% to
20%. In addition, in a case where the heating temperature during hot -stamping was 8
minutes, even when the tempering temperature was 520°C or 680°C, the area ratio of
the Zn coating layer was 5% to 20%.
As described above, even at the same tempering temperature, the area ratio of
the lamella layer changed depending on the heating time during hot-stamping. The
reason for the change is considered as follows. Specifically, during heating in hotstamping,
the degree of alloying of Zn in the Zn coating layer and Fe in steel as the
base metal (specifically, a ratio between Fe and Zn in a Fe-Zn solid-solution) changes
depending on the heating time. This is considered to be because a driving force for
two-phase separation from the solid-solution phase into the low Zn solid-solution
phase and the r -phase during tempering decreases depending on the degree of alloying.
The present inventors have made a fmiher investigation, and as a result, they
have obtained the following finding. Specifically, when a concentration(% by mass)
of Zn in the Fe-Zn solid-solution after hot-stamping and before tempering is set as C,
and the tempering temperature is set as T, in a case where the concentration C (% by
mass) ofZn in the Fe-Zn solid-solution after the hot-stamping, and the tempering
temperature T (°C) satisfY the following Equation 1 or Equation 2, the area ratio of the
lamella layer in Zn coating becomes 20% or less. In addition, in a case where the
following Equation 1 is satisfied, the area ratio of the lamella layer becomes 5% to
20%.
AcJ2:T2:700-4.0x (35.0-C) (1)
T:S500+8.0x (32.5-C) (2)
- 13 -
Provided that, in Equation 2, in a case where C is 32.5 or greater, C is set to
32.5.
Preferably, a relationship ofT:;. 700 or a relationship ofT<500 is satisfied.
With regard to the concentration(% by mass) ofZn in the Fe-Zn solidsolution
after hot-stamping and before tempering, arbitrary 5 sites on a cross-section of
Zn coating are measured with an EPMA, and the average of the Zn content at the 5
sites may be set as the concentration of Zn in the Fe-Zn solid-solution. In a case of
performing EPMA analysis with respect to the cross-section of Zn coating, it is
effective that a sample is embedded in a resin, the sample is polished, and the sample is
etched by using argon ions and the like.
[0029]
In order to realize impact absorption propeliies which are more excellent
those of hot-stamped steel having the same chemical composition in the related art, it
is necessary for the strength to be lower than the strength (tensile strength) after hotstamping.
The present inventors have evaluated hardness of a tempered poliion of the
base metal in the hot-stamped steel which is subjected to tempering at each tempering
temperature.
[0030]
During hot-stamping, a steel sheet is pressed and quenched by using a die
simultaneously. According to this, a structure of the hot-stamped steel becomes a
quenched structure. In this embodiment, the Vickers hardness, which is obtained by
heating steel at a temperature equal to or higher than austenitizing temperature (Ac3
point) for 30 minutes, and by subjecting the steel to water quenching, is defined as
"highest quenching hardness". It is considered that the highest quenching hardness is
approximately the same as hardness of steel after hot-stamping. According to this, in
- 14 -
a case where the hardness of the tempered pmtion of the hot-stamped steel is less than
the highest quenching hardness that is obtained by performing measurement by the
above-described method with respect to the steel having the same chemical component,
it can be said that the impact absorption propctties are improved.
Accordingly, the Vickers hardness of the tempered portion of the base metal in
the hot-stamped steel, which was subjected to tempering at each tempering temperature,
was measured. In addition, steel having the same chemical component was heated at
the austenitizing temperature or higher for 30 minutes and was subjected to water
quenching. Then, as the highest quenching hardness, the Vickers hardness was
measured at a depth spaced away from a surface by 1/4 times a sheet thickness.
As a result, it could be seen that if the tempering temperature is higher than
300°C, the hardness of the tempered portion is 85% or less of the highest quenching
hardness. In addition, it could be seen that if the tempering temperature satisfies
Equation 1, the hardness of the tempered pmtion becomes 60% or less of the highest
quenching hardness, and if the tempering temperature is 700°C or higher, the hardness
of the tempered portion becomes 50% or less of the highest quenching hardness.
[0031]
Accordingly, if the tempering temperature is higher than 300°C and lower
than 500°C, or satisfies Equation 1, the strength of the hot-stamped steel is lowered,
and the corrosion resistance is also retained. In addition, if the tempering temperature
satisfies Equation 1, the chemical convertibility is further improved. More preferably,
the tempering temperature is 700°C or higher.
[0032]
Hereinafter, description will be given of hot -stamped steel according to an
embodiment of the present invention (may also be referred to as "hot-stamped steel
- 15 -
according to this embodiment").
The hot -stamped steel according to this embodiment has the following
characteristics.
(a) The hot-stamped steel includes: a base metal that is steel including a
tempered portion having hardness corresponding to 85% or less of the highest
quenching hardness, the highest quenching hardness being defined as Vickers hardness
at a depth position spaced away from a surface by 114 times a sheet thickness in a case
of performing water quenching after heating at a temperature equal to or higher than
the AcJ point and retention for 30 minutes; and a Zn coating layer that is formed on the
tempered portion of the base metal. The hardness of the tempered portion is
preferably 60% or less of the highest quenching hardness, and is more preferably 50%
or less.
(b) The Zn coating layer includes a solid-solution layer including a solidsolution
phase that contains Fe and Zn that is solid-saluted in Fe, and a lamella layer
that includes the solid-solution phase and a capital gamma phase.
(c) An area ratio of the lamella layer in the Zn coating layer is 20% or less,
and is preferably 5% to 20%.
The characteristics are based on the above-described finding.
[0033]
[Base Metal]
The base metal is steel, and is formed, for example, by hot-stamping a steel
sheet. In addition, the base metal includes a tempered portion. The tempered
portion represents a portion having hardness (Vickers hardness) corresponding to 85%
or less of the highest quenching hardness of steel. The highest quenching hardness
represents Vickers hardness at a depth position spaced away from a surface by 1/4
- 16 -
times a sheet thickness in a case of performing water quenching after heating at a
temperature equal to or higher than an austenitizing temperature for 30 minutes. The
highest quenching hardness can be measured by using another steel (steel different
from the hot-stamped steel having the tempered pmiion) having the same chemical
component.
In the hot-stamped steel according to this embodiment, the base metal
includes the tempered portion having hardness corresponding to 85% or less of the
highest quenching hardness, and thus tensile strength is lower in comparison to hotstamped
steel which has the same chemical composition and is not subjected to
tempering. As a result, the impact absorption properties are excellent.
Matiensite is a structure in which hardness is high, and the hardness thereof is
greatly lowered through tempering. When the base metal has a chemical composition
in which martensitic transfonnation occurs when being subjected to water quenching,
it is easy for the base metal to have the tempered portion having hardness
corresponding to 85% or less of the highest quenching hardness. Accordingly, it is
preferable that the base metal has a chemical composition in which the martensitic
transformation occurs in a case of being subjected the water quenching from a
temperature equal to or higher than the Ac3 point. In addition, it is preferable that the
tempered portion includes 95% or greater of tempered martensite and less than 5% of
residual austenite in terms of% by volume.
[0034]
It is not necessary to limit the chemical composition of the base metal.
However, it is preferable that the base metal has, for example, the following chemical
composition. In a case where the base metal has the following chemical composition,
it is advantageous to obtain mechanical characteristics which are appropriate for usage
- 17 -
in a component for automobiles. In addition, it is advantageous to include the
tempered portion having hardness corresponding to 85% or less of the highest
quenching hardness. Hereinafte1~ "%"related to an element represents %by mass.
[0035]
C: 0.05% to 0.4%
Carbon (C) is an element that enhances the strength of steel (hot-stamped
steel) after hot-stamping. When the C content is too small, it is difficult to obtain the
above-described effect. According to this, it is preferable the lower limit of the C
content is set to 0.05% so as to obtain the effect, and is more preferably 0.1 %. On the
other hand, when the C content is too great, toughness of the steel sheet decreases.
Accordingly, it is preferable that the upper limit of the C content is set to 0.4%, and is
more preferably 0.35%.
[0036]
Si: 0.5% or less
Silicon (Si) is an element that is unavoidably contained in steel. In addition,
Si has an effect of deoxidizing steel. According to this, the Si content may be set to
0.05% or greater for deoxidation. However, when the Si content is great, Si in steel
diffuses during heating in the hot-stamping, and thus an oxide is formed on a surface of
a steel sheet. The oxide deteriorates phosphate treatability. Fmihermore, Si has a
function of raising the Ac3 point of the steel sheet. When the Ac3 point of the steel
sheet rises, there is a concern that a heating temperature during hot-stamping exceeds
an evaporation temperature of Zn coating. In a case where the Si content is greater
than 0.5%, the above-described problem becomes significant, and thus it is preferable
that the upper limit of the Si content is set to 0.5%, and is more preferably 0.3%.
[0037]
- 18 -
Mn: 0.5% to 2.5%
Manganese (Mn) is an element that enhances hardenability of steel and
enhances the strength of the hot-stamped steel. It is preferable that the lower limit of
the Mn content is set to 0.5% so as to obtain this effect, and is more preferably 0.6%.
On the other hand, even when the Mn content is greater than 2.5%, the effect is
saturated. Accordingly, it is preferable that the upper limit of the Mn content is set
to 2.5%, and is more preferably 2.4%.
[0038]
P: 0.03% or less
Phosphorus (P) is an impurity that is contained in steel. P is segregated to a
grain boundary, and deteriorates the toughness of and delayed fracture resistance of
steel. According to this, it is preferable that the P content is as low as possible.
However, in a case where the P content is greater than 0.03%, the effect ofP becomes
significant, and thus the P content may be set to 0.03% or less.
[0039]
S: 0.010% or less
Sulfur (S) is an impurity that is contained in steel. S forms a sulfide and
deteriorates toughness and delayed fracture resistance of steel. According to this, it is
preferable that the S content is as low as possible. However, in a case where the S
content is greater than 0.0 I 0%, the effect of S becomes significant, and thus the S
content may be set to 0.0 I 0% or less.
[0040]
sol. AI: 0.10% or less
Aluminum (AI) is an element that is effective for deoxidation of steel. To
obtain tins effect, the lower limit of the Al content may be set to 0.01 %. However,
- 19 -
when the AI content is too great, the Ac3 point of a steel sheet rises, and the heating
temperature necessary during hot-stamping may exceed the evaporation temperature of
Zn coating. Accordingly, it is preferable that the upper limit of the AI content is set to
0.1 0%, and more preferably 0.05%. The AI content in this embodiment is the sol. AI
(acid soluble AI) content.
[0041]
N: 0.010% or less
Nitrogen (N) is an impurity that is unavoidably contained in steel. N is an
element that forms a nitride and deteriorates toughness of steel. In addition, in a case
where B is contained, N is coupled to B, and reduces the solid-solution amount of B.
When the solid-solution amount of B is reduced, the hardenability deteriorates. From
the above-described reason, it is preferable that theN content be as low as possible.
However, when theN content is greater than 0.010%, the effect ofN becomes
significant, and thus theN content may be set to 0.010% or less.
[0042]
For example, the base metal portion of the hot -stamped steel according to this
embodiment may have a chemical composition including the above-described elements,
and Fe and impurities as the remainder. However, the base metal pmiion of the hotstamped
steel according to this embodiment may fmiher contain one or more kinds of
arbitrary elements selected from B, Ti, Cr, Mo, Nb, and Ni in place of a pmi of Fe in
the chemical composition in the following range so as to improve the strength or
toughness.
In this embodiment, the impurity represents a material that is mixed-in from
ore and scrap as a raw material during industrially manufacturing a steel material, or
due to the manufacturing environment and the like.
- 20 -
[0043]
B: 0.0001% to 0.0050%
Boron (B) enhances the hardenability of steel, and enhances the strength of
the hot-stamped steel. According to this, the preferable lower limit of the B content is
0.000 I% to obtain the effect. However, when the B content is too great, the effect is
saturated. Accordingly, even in a case where B is contained, it is preferable that the
upper limit of the B content is set to 0.0050%.
[0044]
Ti: 0.01% to 0.10%
Titanium (Ti) is coupled toN, and forms a nitride (TiN). As a result, binding
B with N is limited, and thus it is possible to limit the deterioration of hardenability
which is caused by formation ofBN. In addition, TiN makes an austenite grain size
fine during heating in hot-stamping due to a pinning effect, and enhances the toughness
of the steel and the like. To obtain this effect, the preferable lower limit of the Ti
content is 0.01%. However, when the Ti content is too great, the above-described
effect is saturated, and a Ti nitride excessively precipitates, and thus the toughness of
steel deteriorates. Accordingly, even when Ti is contained, it is preferable that the
upper limit of the Ti content is set to 0.10%.
[0045]
Cr: 0.1% to 0.5%
Chromium (Cr) enhances the hardenability of steel. To obtain this effect, the
preferable lower limit of the Cr content is 0.1 %. However, when the Cr content is too
great, Cr carbide is formed, and the carbide is less likely to be dissolved during heating
in hot-stamping. As a result, austenitizing of steel is less likely to progress, and thus
the hardenability deteriorates. Accordingly, even in a case where Cr is contained, it is
- 21 -
preferable that the upper limit of the Cr content is set to 0.5%.
[0046]
Mo: 0.05% to 0.50%
Molybdenum (Mo) enhances the hardenability of steel. To obtain this effect,
the preferable lower limit of the Mo content is 0.05%. However, when the Mo
content is too great, the above-described effect is saturated. Accordingly, even in a
case where Mo is contained, it is preferable that the upper limit of the Mo content is set
to 0.50%.
[0047]
Nb: 0.02% to 0.10%
Niobium (Nb) forms a carbide, and makes a grain size fine during hotstamping.
When the grain size becomes fine, the toughness of steel is improved. To
obtain this effect, the preferable lower limit of the Nb content is 0.02%. Howevet;
when the Nb content is too great, the above-described effect is saturated, and the
hardenability deteriorates. Accordingly, even in a case where Nb is contained, it is
preferable that the upper limit of the Nb content is set to 0.10%.
[0048]
Ni: 0.1% to 1.0%
Nickel (Ni) enhances the toughness of steel. In addition, Ni limits
embrittlement caused by molten Zn during heating in hot-stamping of galvanized steel.
To obtain this effect, the preferable lower limit of the Ni content is 0. I%. However,
when the Ni content is too great, the above-described effect is saturated, and an
increase in the cost is caused. Accordingly, even in a case where Ni is contained, it is
preferable that the upper limit of the Ni content be set to 1.0%.
[0049]
- 22 -
A part of the base metal may be the tempered portion, or the entirety of the
base metal may be the tempered portion. That is, a micro-structure of the entirety of
the base metal may be tempered martensite.
[0050]
Recently, a component, in which a demand for performance such as strength
and ductility is different in accordance with a position, has been required. The
performance is called a tailored prope1iy. For example, with regard to an automobile
component, in a fi·ame component called B pillar (center pillar), an upper portion,
which constitutes a getting-on area, is required to have high strength, and a lower
portion is required to have high impact absorption prope1iies.
In a case where only a part of the base metal in the hot-stamped steel
including the Zn coating layer is configured as the tempered portion, it is possible to
obtain a component which includes the high-strength portion and has impact
absorption properties. In addition, since the hot -stamped steel includes the Zn coating
layer, the corrosion resistance is also excellent.
[0051]
The tensile strength of the tempered portion is, for example, 600 MPa to 1450
MPa, and the Vickers hardness is 180 Hv to 450 Hv. In this case, the strength of the
tempered pmtion of the hot -stamped steel becomes lower in comparison to hotstamped
steel, which is not subjected to tempering, in the related art. According to
this, the impact absorption properties are more excellent in comparison to the hotstamped
steel of the related mt.
The Vickers hardness oftempered martensite is lower than Vickers hardness
of martensite. Accordingly, it is possible to determine whether or not a microstructure
of the base metal (tempered portion) is tempered martensite in accordance
- 23 -
with the Vickers hardness.
The Vickers hardness can be obtained through a Vickers hardness test in
conformity to JIS Z2244 (2009). The test force in the Vickers in the Vickers hardness
test is set to I 0 kgl'=98.07 N.
[0052]
[Zn coating Layer]
The hot -stamped steel according to this embodiment includes a Zn coating
layer at least on the tempered portion of the base metal. The Zn coating layer mainly
includes a solid-solution layer. Specifically, the Zn coating layer includes the solidsolution
layer, and a lamella layer in an area ratio of 0% to 20%.
The solid-solution layer includes a solid-solution phase. The solid-solution
phase contains Fe, and Zn that is solid-soluted in Fe. It is preferable that the Zn
content in the solid-solution layer is 25% by mass to 40% by mass, and is more
preferably 30% by mass to 40% by mass.
[0053]
The lamella layer has a lamella structure including a solid-solution phase and
a capital gamma (r) phase. As shown in FIG. 2, the lamella structure is a structure in
which different phases (the solid-solution phase and the r-phase in this embodiment)
are repetitively and alternately adjacent to each other in a layered shape. The r -phase
is an intermetallic compound (Fe3Zn10). The Zn content in the solid-solution phase of
the lamella layer is 5% by mass to 25% by mass, and is lower than the Zn content in
the solid-solution layer. The lamella layer is formed on a surface layer side of the Zn
coating layer.
When an area ratio of the lamella layer in the Zn coating layer is greater than
20%, corrosion resistance significantly deteriorates. The reason for the deterioration
- 24 -
is that the lamella layer has a lamella structure of the solid-solution phase (low Zn
solid-solution phase) and the !-phase as described above. A corrosion potential of
the solid-solution phase is different fi·om a corrosion potential of the !-phase.
Accordingly, it is considered that galvanic corrosion is likely to occur in the lamella
layer, and thus corrosion resistance becomes lower in comparison to the solid-solution
layer. According to this, the area ratio of the lamella layer in the Zn coating layer is
set to 20% or less.
[0054]
On the other hand, the lamella layer is more excellent in chemical
convertibility in comparison to the solid-solution layer. The reason for this is
considered as follows. As described above, the lamella layer has a lamella structure
of the solid-solution phase (low Zn solid-solution phase) and the !-phase .. In the
lamella structure, the solid-solution phase and the !-phase extend in a direction that is
approximately perpendicular to a surface of the base metal. In addition, as described
above, the lamella layer is formed on a surface layer side of the Zn coating layer.
Accordingly, when observing the surface ofthe Zn coating layer, both of the solidsolution
phase and the !-phase are observed. When chemical conversion treatment
(phosphate treatment) is performed with respect to the Zn coating layer having the
lamella structure as described above, the surface of the Zn coating layer, that is, the
lamella layer is etched. At this time, a portion, in which the concentration of Zn is
high, is preferentially etched. The concentration of Zn in the[' -phase in the lamella
layer is higher than the concentration of Zn in the solid-solution phase, and thus the['-
phase is preferentially etched in comparison to the solid-solution phase. As a result,
fine unevenness is formed on the surface of the Zn coating layer, and thus a phosphate
is likely to adhere to the surface. Accordingly, it is considered that the phosphate
- 25 -
treatability of the Zn coating layer, which includes the lamella layer on a surface layer
side, becomes higher in comparison to the Zn coating layer that includes only the
solid-solution layer on the surface layer side. When the area ratio of the lamella layer
in the Zn coating layer is 5% or greater, the phosphate treatability of the Zn coating
layer are enhanced, and thus it is preferable that the area ratio of the lamella layer in
the Zn coating layer is 5% or greater.
That is, when the area ratio of the lamella layer is 5% to 20%, not only the
corrosion resistance but also the chemical convertibility is excellent.
[0055]
The Zn content in the solid-solution phase (the high Zn solid-solution phase or
the low Zn solid-solution phase) can be measured by the following method. The Zn
content (% by mass) is measured at arbitrary 5 sites on the high Zn solid-solution
phase by using electron beam probe microanalyzer (EPMA), and the average of the Zn
content at the 5 sites may be defined as the Zn content in the high Zn solid-solution
phase. With regard to the low Zn solid-solution phase, the Zn content can be obtained
by the same method as in the high Zn solid-solution phase.
[0056]
The hot-stamped steel according to this embodiment includes the tempered
pmtion having hardness corresponding to 85% or less of the highest quenching
hardness. According to this, strength is lower in comparison to hot-stamped steel
which has the same chemical composition and is not subjected to tempering, and thus
the impact absorption resistance is excellent. In addition, in the Zn coating layer of
tllis embodiment, the proportion occupied by the lamella layer, in which the corrosion
resistance decreases, is small. According to this, it is possible to retain excellent
corrosion resistance that is approximately the same as corrosion resistance of hot-
- 26 -
stamped steel that is not subjected to tempering.
[0057]
[Method of Producing Hot-stamped steel]
The hot-stamped steel according to this embodiment can exhibit the effect
thereof without limitation to a producing method thereof as long as the base metal and
the Zn coating layer as described above are provided. For example, the hot-stamped
steel can be produced by the following producing method including a process of
preparing steel that is a base metal (process of preparing the base metal), a process of
forming a galvanized layer on the base metal (a galvanizing process), a process of
performing hot-stamping with respect to the base metal that includes a Zn coating layer
(hot -stamping process), and a process of performing tempering with respect to steel
after being subjected to hot-stamping (tempering process). Hereinafter, a description
will be given of a preferred example in the respective processes.
[0058]
[Process of Preparing Base Metal]
First, a steel sheet, which is used as the base metal, is prepared. For example,
molten steel having the above-described preferable chemical composition is prepared.
Slab is prepared by using the molten steel in accordance with a casting method such as
continuous casting. An ingot may be produced in place of the slab by using molten
steel in accordance with an ingot-making method. The slab or the ingot, which is
produced, is hot-rolled to produce a steel sheet (hot-rolled steel sheet). Pickling may
be additionally performed with respect to the hot-rolled steel sheet as necessary, and
cold-rolling may be performed with respect to the resultant hot-rolled steel sheet after
the pickling to obtain a steel sheet (cold-rolled steel sheet). The hot-rolling, the
pickling, and the cold-rolling may be performed by a known method in conformity to
- 27 -
characteristics which are required for a component to which the steel sheet is applied.
[0059]
[Galvanizing Process]
Galvanizing is performed with respect to the above-described steel sheet (the
hot-rolled steel sheet or the cold-rolled steel sheet) to form a galvanized layer on a
surface of the steel sheet. A method of forming the galvanized layer may be a hot-dip
galvanizing, galvallllealing, or electrogalvanizing without particular limitation.
[0060]
For example, formation of the galvanized layer through the hot-dip
galvanizing is performed in the following manner. Specifically, a steel sheet is
immersed in a galvanizing bath (hot-dip galvanizing bath) so as to allow galvanizing to
adhere to a smface of the steel sheet. The steel sheet, to which the galvanizing
coating adheres, is pnlled np from the galvanizing bath. Preferably, the coating
weight of galvanized layer on the surface of the steel sheet is adjusted to 20 g/m2 to
100 g/m2
. The coating weight of galvanized layer can be adjusted by adjusting the
pulling-up speed of the steel sheet or the flow rate of a wiping gas. The concentration
of AI in the hot-dip galvanizing bath is not particularly limited. Through the abovedescribed
processes, a steel sheet for hot-stamping (GI), which includes the galvanized
layer (hot -dip galvanized layer), is produced.
[0061]
For example, formation of the galvanized layer through the galvannealing
(hereinafter, also referred to "alloying process") is performed in the following mallller.
Specifically, the steel sheet, on which the hot-dip galvanized layer is formed, is heated
to 470°C to 600°C. After the heating, soaking is performed as necessary, and then the
steel sheet is cooled down. The soaking time is preferably 30 seconds or shorter, but
- 28 -
there is no limitation of the soaking time. In addition, immediately after heating to
the heating temperature, the steel sheet may be cooled down without performing the
soaking. The heating temperature and the soaking time are appropriately set in
accordance with a desired concentration of Fe in the resultant galvanized layer. The
preferable lower limit of the heating temperature in the alloying process is 540°C.
Through the above-described alloying process, a steel sheet for hot-stamping (GA),
which includes the galvanized layer (galvmmealed layer), is produced.
[0062]
For example, f01mation of the galvanized layer through the electrogalvanizing
is performed in the following manner. Specifically, as an electrogalvanizing bath, any
one of a sulfuric acid bath, a hydrochloric acid bath, a zincate bath, and a cyan bath,
which are known, is prepared. The above-described steel sheet is pickled, and the
steel sheet after the pickling is immersed in the electrogalvanizing bath. A current is
allowed to flow tin·ough the electrogalvanizing bath in a state in which the steel sheet
is set as a negative electrode. According to this, zinc precipitates to a surface of the
steel sheet, and thus the galvanized layer ( electrogalvanized layer) is formed.
Through the above-described processes, a steel sheet for hot-stamping (EG), which
includes the electrogalvanized layer, is produced.
[0063]
In a case where the galvanized layer is the galvannealed layer, and in a case
where the galvanized layer is the electro galvanized layer, a preferable coating weight
of the galvanized layer is the same as in the case of the hot-dip galvanized layer. That
is, the preferable coating weight of the galvanized layer is 20 g/m2 to 100 g/m2
, and is
more preferably 40 g/m2 to 80 g/m2
.
[0064]
- 29 -
These galvanized layers contain Zn. Specifically, the chemical composition
of the hot-dip galvanized layer and the electrogalvanized layer include Zn and
impurities. The chemical composition of the galvannealed layer contains 5% to 20%
of Fe, and the remainder includes Zn and impurities.
[0065]
[Hot -Stamping Process]
Hot-stamping is performed with respect to the above-described steel sheet for
hot -stamping. During heating before quenching in the hot -stamping process, it is
preferable to perform heating by mainly using radiant heat.
Specifically, first, a steel sheet for hot-stamping is charged into a heating
furnace (a gas furnace, an electrical furnace, an infrared furnace, and the like). In the
heating furnace, the steel sheet for hot-stamping is heated at the Ac3 point to 950°C (the
highest heating temperature), and is retained (soaked) at this temperature. Zn in a
galvanized layer is liquefied through the heating, and molten Zn and Fe in the
galvanized layer mutually diffhse and form a solid-solution phase (Fe-Zn solidsolution
phase). After the molten Zn in the galvanized layer is solid-saluted in Fe and
becomes a solid-solution phase, the steel sheet is taken out from the heating furnace.
Hot-stamping (pressing and quenching) is performed with respect to the steel sheet that
is taken out from the heating furnace, thereby obtaining the hot-stamped steel. A
preferable soaking time is 30 minutes or shorter. It is preferable that a heating time
be as shmt as possible from the viewpoint of productivity, and is more preferably 0
minutes to I 5 minutes.
[0066]
In the hot-stamping, the steel sheet is pressed by using a die in which a
cooling medium (for example, water) is circulated through the inside thereof. When
- 30 -
pressing the steel sheet, the steel sheet is quenched due to heat sink from the die.
Through the above-described processes, hot-stamped steel is produced.
[0067]
In the above description, the steel sheet for hot-stamping is heated by using
the heating furnace. However, the steel sheet for hot-stamping may be heated through
electrical heating. Even in this case, the steel sheet is soaked for a predetermined
time through the electrical heating to allow the molten Zn in the galvanized layer to be
a solid-solution phase. After the molten Zn in the galvanized layer becomes a solidsolution
phase, the steel sheet is pressed by using a die.
[0068]
[Tempering Process]
Tempering is performed with respect to the hot-stamped steel (steel after the
hot-stamping). When tempering is performed with respect to the hot-stamped steel, it
is possible to form a tempered portion in the base metal of the hot-stamped steel.
When the concentration(% by mass) ofZn in the Fe-Zn solid-solution after the hotstamping
and before the tempering is set as C, a tempering temperature is higher than
300°C and equal to or lower than 500+8.0x(32.5-C)°C (provided that, at this time, in a
case where Cis 32.5 or greater, Cis set to 32.5), or 700-4.0x(35.0-C)°C to the Ac1
point of the base metal. A preferable tempering temperature is higher than 300°C and
lower than 500°C, or 700°C to the Ac1 point of the base metal.
[0069]
In a case where the tempering temperature is in the above-described range, the
Zn coating layer after the tempering mainly includes a solid-solution layer, and an area
ratio of the lamella layer becomes 0% to 20%. In addition, the hardness of the
tempered portion of the base metal becomes 85% or less of the highest quenching
- 3 I -
hardness.
In addition, when the tempering temperature is set to 700-4.0x (35.0-C)°C to
the Aci point of the base metal, the area ratio of the lamella layer can be set to 5% to
20%. In addition, when the tempering temperature is set to 700°C or higher, the
hardness of the tempered portion of the base metal can be set to 50% or less of the
highest quenching hardness.
[0070]
The reason for the change in the area ratio of the lamella layer in accordance
with the tempering temperature is considered as follows.
FIG. 8 is the Fe-Zn binary phase diagram. The Zn coating layer of the hotstamped
steel produced through the hot-stamping includes a solid-solution phase in
which approximately 25% by mass to 40% by mass ofZn is solid-saluted in u-Fe.
However, a structure (that is, a lamella layer) including two phases, which includes the
low Zn solid-solution phase in which 5% by mass to 25% by mass of Zn is solidsaluted
in u-Fe, and the r-phase, is stable at room temperature in consideration of free
energy. That is, the solid-solution phase of the Zn coating layer after the hotstamping
is a solid-solution in which Zn is oversaturated.
[0071]
On the assumption that ihe concentration of Zn in the Zn coating layer is 35%
by mass in FIG. 8 (corresponds to a point A I in the drawing). In a case where a
temperature of the Zn coating layer is raised, a driving force for two-phase separation
from the solid-solution phase into the low Zn solid-solution phase and the r -phase is
generated on a lower temperature side in comparison to a point B on a boundary line
Ax, and becomes strong as it goes toward a low temperature side from the point B.
On the other hand, as a temperature becomes higher, the diffusion rate in the Zn
- 32 -
coating layer increases. Accordingly, whether or not the lamella layer is formed after
the tempering is determined from a relationship between the driving force for twophase
separation, and the diffusion rate. Specifically, as the driving force for twophase
separation is higher and the diffusion rate increases, the lamella layer is likely to
be formed.
[0072]
In a case where the temperature (tempering temperature) of the Zn coating
layer during the tempering is in a low-temperature region (higher than 300°C and
lower than 500°C) (for example, a point Al of310°C), it is sufficiently spaced away
fi·om the boundaty line Ax. In this case, the driving force for two-phase separation is
high. However, since a temperature is low, the diffusion rate is too slow. According
to this, even when performing the tempering, the Zn coating layer is not separated into
the two phases, and the lamella layer is not formed.
[0073]
In a case where the tempering temperature is 500°C to lower than 700°C, the
temperature region is close to the boundary line Ax, but a cettain degree of distance is
present (for example, a point A2 in the drawing). In this case, the driving force for
two-phase separation is present to a certain extent. In addition, the temperature
region increases, and thus the diffusion rate is fast. As a result, the Zn coating layer is
separated into the two phases to form the lamella layer. At A2 in FIG. 8, the Zn
coating layer is separated into the r-phase in which the Zn content is approximately
70% by mass (C2 in the drawing) and the solid-solution phase in which the Zn content
is approximately I 0% by mass (Cl in the drawing). As a result, the lamella layer is
formed.
[0074]
- 33 -
When the tempering temperature further rises and reaches 700°C or higher,
the temperature region approaches the vicinity of the boundary line Ax. In this case,
the diffusion rate becomes fast due to the temperature rise, but the driving force for
two-phase separation is very small. As a result, separation into the two phases is less
likely to occur. However, the temperature region does not exceed the boundary line
Ax, and thus a small amount of lamella layer is formed. According to this, the area
ratio of the lamella layer becomes 5% to 20%. When the tempering temperature
exceeds the boundary line Ax (when the tempering temperature exceeds the Ac1 point),
the driving force for two-phase separation is not generated, and thus the lamella layer
is not formed.
[0075]
In a case where the tempering temperature is 300°C or lower, the diffusion
rate is slow, and thus the area ratio of the lamella layer becomes 20% or less. On the
other hand, the strength of the tempered portion is less likely to decrease, and the
hardness of the tempered portion exceeds 85% of the highest quenching hardness.
[0076]
Accordingly, as described above, in a case where the concentration of Zn in
the Zn coating layer is 35% by mass in FIG. 8, when the tempering temperature is set
to be higher than 300°C and lower than 500°C, or 700°C to the Ac1 point of the base
metal, the area ratio of the lamella layer in the Zn coating layer can be set to 20% or
less, and the hardness of the tempered portion can be set to be 85% or less of the
highest quenching hardness.
[0077]
The tempering can be performed with respect to only a part of the hotstamped
steel. For example, the tempering can be performed with respect to a part of
- 34 -
the hot-stamped steel through induction heating by using a high frequency or electrical
heating.
When the tempering is performed with respect to only a part of the hotstamped
steel, strength can be made to change in the same component between a
portion for which the tempering is performed and a portion for which the tempering is
not performed. For example, a component as described above is applicable to a
component such as a B pillar of an automobile in which an upper portion is required to
have high strength and a lower portion is required to have high impact absorption
properties.
In addition, a tempered pmtion even in the partial tempering is the same as the
tempered portion in a case where the entirety is tempered.
[0078]
The hot-stamped steel is produced by performing quenching while being
pressed by using a die after heating, and by performing tempering in a temperature
range of higher than 300°C and equal to or lower than 500+8.0x(32.5-C)°C, or a
temperature range of700-4.0x(35.0-C)°C to theAc1 point of the base metal.
[0079]
Through the above-described processes, it is possible to produce a hotstamped
steel which includes the base metal that is steel including the tempered
portion having hardness corresponding to 85% or less of the highest quenching
hardness, and the Zn coating layer that is formed on the tempered portion of the base
metal and includes the solid-solution layer and the lamella layer, and in which the area
ratio of the lamella layer in the Zn coating layer is 20% or less.
[0080]
The method of producing the hot-stamped steel according to this embodiment
- 35 -
may further include the following processes.
[0081]
[Anti-Rust Oil Film Forming Process]
The above-described producing method may futther include an anti-rust oil
film forming process between the galvanizing process and the hot-stamping process.
[0082]
In the anti-rust oil film forming process, an anti-rust oil is applied to a surface
of the steel sheet for hot-stamping to form the anti-rust oil film. The steel sheet for
hot -stamping may be left for a long period of time before performing the hot -stamping
process after being rolled. In this case, the surface of the steel sheet for hot-stamping
may be oxidized. According to this process, the anti-rust oil film is fmmed on the
surface of the hot-stamped steel, and thus the surface of the steel sheet is less likely to
be oxidized. Accordingly, generation of scale is limited.
[0083]
[Blanking Process]
In addition, the above-described producing method may futther include a
blanking process between the anti-rust oil film forming process and the hot-stamping
process.
[0084]
In the blanking process, shearing and/or punching, and the like are performed
with respect to the steel sheet for hot-stamping for shaping (blanking) into a specific
shape. A shear plane of the steel sheet after the blanking is likely to be oxidized.
However, when the anti-rust oil film is formed on the surface ofthe steel sheet, an antirust
oil also spreads to the shear plane to a ce1tain extent. According to this,
oxidation of the steel sheet after the blanking is limited.
- 36 -
Examples
[0085]
Steel sheets of Steel Nos. A to G, which have chemical compositions shown in
Table 1, were prepared.
[0086]
[Table 1]
- 37 -
Kinds of
Sheet
Chemical composition (tmit is% by mass, and the remainder includes Fe and impurities) Highest quenching
thickness
steel hardness BO (HV)
(rum) c Si Mn p s soLA! N B Ti Cr Mo Nb Ni
A 1.6 0.2 0.2 1.3 0.01 0.005 O.Q2 0.002 0.002 0.02 0.2 - - - 514
B 1.6 0.2 0.5 1.3 0.01 0.005 0.02 0.002 0.002 0.02 0.2 - - - 512
c 1.6 0.2 0.5 1.3 0.01 0.005 0.02 0.002 0.002 O.Q2 0.2 - 0.05 - 519
D 1.6 0.2 0.5 1.3 0.01 0.005 0.02 0.002 0.002 0.02 0.2 - - 1.0 518
E 1.6 0.2 0.5 1.3 0.01 0.005 O.Q2 0.002 0.002 0.02 0.2 0.5 - - 519
F 1.6 0.2 0.2 1.3 0.01 0.005 O.Q2 0.002 - - - - - - 515
'--- G 1.6
~--
0.3 0.2 1.3 0.01 Q.002_ O.Q2_ Q0()_0~ 0.002 0.02_ 0.2 --'------ L_ --- 609 ~--::" __ - -- ----· ------------- - ··---
- 38 -
[0087]
Referring to Table l, it could be seen that a chemical composition of any steel
is in a range of a preferable chemical composition of the steel sheet of this embodiment.
[0088]
Slab was prepared by using molten steel having each of the above-described
chemical compositions in accordance with continuous casting method. The slab was
hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was pickled,
and after pickling, cold-rolling was performed to obtain a cold-rolled steel sheet having
a sheet thickness of 1.6 mm. The cold-rolled steel sheet, which was obtained, was set
as a steel sheet that is used to produce the hot-stamped steel.
[0089]
To investigate the highest quenching hardness, a pmt of a steel sheet having
each of the chemical compositions of Steel Nos. A toG was collected, and was heated
at a temperature of the Ac3 point or higher. Then, water quenching was performed
after retention for 30 minutes. In any kind of steel sheet, a structure after the water
quenching was full mmtensite.
The Vickers hardness was measured with respect to the steel sheet after the
water quenching, and the Vickers hardness that was obtained was defined as the
highest quenching hardness (HV) of each kind of steel. A Vickers hardness test was
performed in conformity to JIS Z2244 (2009), and the test force was set to 10
kgf=98.07 N.
[0090]
Galvanizing, hot-stamping, and tempering were performed by using each of
cooled-rolled steel sheets having the chemical compositions of Steel Nos. A to F under
conditions shown in Table 2, thereby producing hot-stamped steel in each of Test Nos.
- 39 -
1 to 23.
[0091]
Galvanizing was performed with respect to each of steel sheets of Test Nos. 1
to 23. In Test No. 6, a hot-dip galvanized layer (GI) was formed on the steel sheet
tlnough hot-dip galvanizing. In Test Numbers other than Test No. 6, an alloying
process was further performed with respect to the steel sheet including the hot-dip
galvanized layer to form a galvannealed layer (GA). In the alloying process, the
highest temperature was set to approximately 530°C, and after heating for
approximately 30 seconds, cooling was performed to room temperature.
[0092]
The Fe content in the galvannealed layer was 12% in terms of% by mass.
The Fe content was measured by the following measurement method. First, a sample
of a steel sheet including the galvannealed layer was collected. The Fe content(% by
mass) was measured at arbitrary 5 sites inside the galvannealed layer in the sample by
using electron probe micro analyzer (EPMA). The average of the resultant measured
values was defined as the Fe content (% by mass) of the galvannealed layer of a
corresponding test number.
[0093]
The coating weight of the galvanized layer (the hot-dip galvanized layer or the
galvmmealed layer) was measured by the following method. First, a sample including
a galvanized layer was collected from each of the steel sheets, and the galvanized layer
of the sample was dissolved in hydrochloric acid in conformity to JIS H0401. The
coating weight (g/m2
) of Zn coating was obtained on the basis of a sample weight
before dissolution, the sample weight after dissolution, and the galvanized layer
formed area. The measured results are shown in Table 2.
- 40 -
[0094]
After forming the galvanized layer, hot-stamping was performed with respect
to the steel sheet in each of the test numbers. Specifically, the steel sheet was charged
into a heating furnace in which the fhrnace temperature was set to 900°C, that is a
•
temperature equal to or higher than the Ac3 point of the steel sheet, and was heated at
900°C, that is a temperature equal to or higher than the AcJ point of each of the Steel
Nos. A to F by using radiant heat for 4 minutes to 8 minutes. At this time, the
temperature of the steel sheet reached 900°C after approximately 2 minutes after being
charged into the furnace, and the steel sheet was soaked at 900°C for 2 minutes to 6
minutes.
[0095]
After soaking, the steel sheet was interposed by a flat die equipped with a
water-coolingjacket to produce the hot-stamped steel (steel sheet). At this time, even
at a portion in which a cooling rate during the hot -stamping was slow, quenching was
performed in such a manner that a cooling rate up to a martensitic transformation start
point became 50 °C/second. After the hot -stamping, the concentration of Zn in the
Fe-Zn solid-solution was obtained by the EPMA.
[0096]
In addition, tempering was performed with respect to Test Nos. I to I 4, and
16 to 23 after hot -stamping. In this example, each steel was charged into a heat
treatment fi.u·nace. That is, tempering was performed with respect to entirety of each
of the steel sheets. The tempering temperature in each test number was set as shown
in Table 2, and the heating time was set to 5 minutes. Tempering was not performed
with respect to steel of Test No. 15. Through the above-described processes, hotstamped
steel in each of Test Nos. I to 23 was produced.
- 4 I -
A Vickers hardness test and micro-structure observation of the Zn coating
layer were performed with respect to the hot-stamped steel in each of Test Nos. 1 to 14.
In addition, phosphate treatability evaluation test was performed so as to evaluate
chemical convetiibility.
[0097]
[Vickers Hardness Test]
A sample was collected from the base metal of the steel (steel sheet) in each
of the test numbers at the center in a sheet thickness direction. The Vickers hardness
test conforming to JIS Z2244 (2009) was performed with respect to a surface
(corresponding to a surface perpendicular to a rolling direction of the steel sheet) of the
sample. The test force was set to 10 kgfoo98.07 N. BIIBOxlOO (%),which is a ratio
between Vickers hardness Bl (HVIO) that was obtained and the highest quenching
hardness BO, is shown in Table 2.
[0098]
[Micro-Structnre Observation of Zn coating Layer]
A sample including the Zn coating layer was collected from steel in each of
the test numbers. Among surfaces of the sample, a cross-section perpendicular to the
rolling direction was etched with 5% by mass of nita!. Across-section of the Zn
coating layer that was etched was observed with a SEM at a magnification of 2000
times to determine whether or not the solid-solution layer and the lamella layer were
present.
[0099]
In a case where the lamella layer was observed, the area ratio of the lamella
layer was further obtained by the following method. At 5 arbitrary visual fields (50
~unxSO JHn) on the cross-section, the area ratio(%) of the lamella layer with respect to
- 42 -
the entirety of the area of the Zn coating layer was obtained. At this time, a Zn oxide
layer (indicated by a reference numeral30 in FIG. 1), which floats to a surface, was not
included to the area of the Zn coating layer. Area ratios(%) of the solid-solution
layer and the lamella layer, which were obtained, are shown in Table 2.
[0100]
Measurement by the EPMA was performed with respect to the solid-solution
layer, which was observed through the micro-structure observation, by the abovedescribed
method. As a result, Zn in the solid-solution layer, which was observed,
was 25% by mass to 40% by mass in all cases.
[0101]
[SST Test (Salt Spray Test)]
The SST test was performed with respect to the hot-stamped steel in each of
the test numbers by the following method. A rear surface and an end surface of the
hot -stamped steel (steel sheet) in each of the test numbers were sealed with a polyester
tape. Then, a surface of each steel was subjected to a test defined by JIS Z2371 "salt
water spray test method" for 6 days (144 hours). The corrosion weight loss (g/m2
) of
the steel after the test was obtained. The corrosion weight loss, which was obtained,
is shown in Table 2.
[0102]
[Phosphate Treatability Property Evaluation Test]
Surface conditioning was performed with respect to the hot-stamped steel in
each of the test numbers at room temperature for 20 seconds using a surface
conditioning agent (PREP ALENE (product name), produced by Nihon Parkerizing Co.,
Ltd.). In addition, a phosphate treatment was performed using a zinc phosphate
treatment solution (PEARLBOND 3020 (product name), produced by Nihon
- 43 -
Parkerizing Co., Ltd.). The temperature of the treatment solution was set to 43°C,
and the hot-stamped steel was immersed in the treatment solution for 120 seconds.
[0103]
After the phosphate treatment, arbitrary 5 visual fields (125 J.tmx90 J.tm) of the
hot-stamped steel were observed with a scmming electron microscope (SEM) at a
magnification of 1000 times, and binarization processing was performed with respect
to the resultant SEM image. In a binarized image, a fine chemical crystal was formed
at a white pmtion. As the fine chemical crystal is much, the phosphate treatability is
high. According to this, the area ratio TR of a white portion was obtained by using
the binarized image. In a case where the area ratio TR was the same as in a case
where the tempering was not performed, this case was regarded as "OK", and in a case
where the area ratio TR was 30% or greater, it was determined that the phosphate
treatability was improved, and this case was regard as "GOOD". Results are shown
in Table 2.
- 44 -
:: tcm:::::: :IJ
[0104]
[Table 2)
after galvanizing
HS heating Tempering heating
Hot-.stamped steel
conditions
Concentration
conditions
of Zn in F e~Zn 700-4.0x 500+8.0x
Solid-
Lamella Vickers
Corrosion
Test Kinds of Acl Ac3 Coating solid-solution solution BilBO weight No. steel (OC) (OC) (35.0-C) (32.5-C) layer hardness
Temperature Time after hot-stamping Temperature Time layer loss chemical
Composition weight
(g!m')
(OC) (min) (mass%) (OC) (min) Area Area Hardness convertibility
ratio ratio Bl (%) (g!m')
(%) (%) (HV10)
I A Zn ~ 12%Fe 60 727 810 900 4 35.0 300 5 700 480 100 0 446 86.8 99.2 . OK
2 I 8 Zn ~ 12%Fc 60 727 823 900 4 35.0 350 5 700 480 100 0 422 82.4 100.1 OK
3 c Zn- l2%Fe 60 727 823 900 4 35.0 420 5 700 480 100 0 401 77.3 102.8 OK
4 D Zn- 12%Fe 60 727 823 900 4 35.0 420 5 700 480 100 0 397 76.6 99.7 OK
5 E Zn -12%Fe I 60 727 823 900 4 35.0 370 5 700 480 100 0 423 81.5 98.8 OK
6 F Zn 60 727 804 900 4 35.0 150 5 700 480 100 0 502 97.5 96.9 OK
7 F Zn -12%Fe 60 727 804 900 4 35.0 200 5 700 480 100 0 509 98.8 101.4 OK
8 F Zn- 12%Fe 60 727 804 900 4 35.0 300 5 700 480 100 0 445 86.4 98.9 OK
9 F Zn- 12%Fe 60 727 804 900 4 35.0 400 5 700 480 100 0 414 80.4 103.1 OK
10 F Zn- 12%Fe 60 727 804 900 4 35.0 460 5 700 480 80 20 361 70.1 108.9 GOOD
II F Zn~ 12%Fe 60 727 804 900 4 35.0 500 5 700 480 30 70 325 63.1 143.2 GOOD
12 F Zn- 12%Fc 60 727 804 900 4 35.0 600 5 700 480 10 90 275 53.4 139.6 GOOD
13 F Zn ~ J2%Fe 60 727 804 900 4 35.0 700 5 700 480 80 20 234 45.4 104.6 GOOD
14 F Zn- I2%Fe 60 727 804 900 4 35.0 720 5 700 480 90 10 214 41.6 101.5 GOOD
15 F Zn- 12%Fe 60 727 804 900 4 35.0 - - 700 480 100 0 484 94.0 97.1 OK
16 F Zn- 12%Fe 60 727 804 900 6 32.5 500 5 690 500 80 20 328 63.7 105.7 GOOD
17 F Zn- 12%Fe 60 727 804 900 6 32.5 520 5 690 500 60 40 309 60.0 130.4 GOOD
18 F Zn- 12%Fe 60 727 804 900 6 32.5 600 5 690 500 40 60 274 53.2 135.8 GOOD
19 F Zn- 12%Fe 60 727 804 900 6 32.5 690 5 690 500 80 20 253 49.1 102.8 GOOD
20 F Zn- 12%Fe 60 727 804 900 8 30.0 500 5 680 520 90 10 326 63.3 108.3 I GOOD
21 F Zn- 12%Fe 60 727 804 900 8 30.0 520 5 680 520 so 20 251 48.7 101.2 GOOD
22 F Zn- 12%Fc 60 727 804 900 8 30.0 600 5 680 520 50 50 278 54.0 124.2 GOOD
23 F Zn- 12%Fe 60 727 804 900 8 30.0 680 5 680 520 80 20 255 49.5 102.5 GOOD
- 45 -
[0 I 05]
[Test Result]
Referring to Table 2, in Test Nos. 2 to 5, 9, 10, 13, 14, 16, 19, 20, 21, and 23,
the tempering temperature was appropriate. According to this, the hardness B I of the
tempered portion was 85% or less of the highest quenching hardness BO.
[0106]
In addition, the area ratio of the solid-solution layer in the Zn coating layer
was 80% or greater, and the area ratio of the lamella layer was 20% or less. As a
result, the corrosion weight loss in the SST test was 130 g/m2 or less, and was
approximately the same as the corrosion weight loss of Test No. 15 in which the
tempering was not performed.
[0107]
On the other hand, in Test Nos. 1, and 6 to 8, the tempering temperature was
too low. According to this, the area ratio of the lamella layer was 20% or less, but the
hardness Bl of the tempered portion was higher than 85% of the highest quenching
hardness BO. In addition, in Test No. 15, since the tempering was not performed, Bl
became a value close to BO.
[01 08]
In Test Nos. 11, 12, 17, 18, and 22, the tempering temperature deviated from
the preferable range, and thus the area ratio of the lamella layer in the Zn coating layer
was greater than 20%. According to this, the corrosion weight loss greatly exceeded
[01 09]
In addition, in a case where the area ratio of the lamella layer in the Zn
coating layer was equal to or greater than 5% to 20%, the corrosion resistance did not
- 46 -
deteriorate, and the chemical conve1iibility was improved.
[0110]
Overall evaluation was performed in Table 3 by collecting the above
described results.
With regard to hardness, in a case where Bl/BOxlOO was 85 (%)or less,
evaluation was made as "GOOD", and in a case where B 1/BOx l 00 was greater than 85
(%),evaluation was made as "NG". In addition, with regard to the corrosion
resistance, in a case where the corrosion weight loss in the SST test was l 30 g/m2 or
less, evaluation was made as "GOOD", and in a case where the corrosion weight loss
was greater than 130 g/m2
, evaluation was made as "NG". In addition, in a case
where all of the hardness and the corrosion resistance were "GOOD", overall
evaluation was made as "GOOD", and in a case where any one of the hardness and the
corrosion resistance was "NG", overall evaluation was made as "NG".
[0 111]
[Table3]
- 47 -
Test Nos. Hardness
Corrosion
Overall
resistance
I NG GOOD NG
2 GOOD GOOD GOOD
3 GOOD GOOD GOOD
4 GOOD GOOD GOOD
5 GOOD GOOD GOOD
6 NG GOOD NG
7 NG GOOD NG
8 NG GOOD NG
9 GOOD GOOD GOOD
10 GOOD GOOD GOOD
II GOOD NG NG
12 GOOD NG NG
13 GOOD GOOD GOOD
14 GOOD GOOD GOOD
15 NG GOOD NG
16 GOOD GOOD GOOD
17 GOOD NG NG
18 GOOD NG NG
19 GOOD GOOD GOOD
20 GOOD GOOD GOOD
21 GOOD GOOD GOOD
22 GOOD NG NG
23 GOOD GOOD GOOD
[0112]
Hereinbefore, the embodiment of the present invention has been described.
However, the above-described embodiment is only illustrative examples of carryingout
the present invention. Accordingly, the present invention is not limited to the
above-described embodiment, and the present invention can be carried out by
appropriately modifying the above-described embodiment in a range not departing
from the gist of the present invention.
[Industrial Applicability]
[0113]
- 48 -
According to the present invention, it is possible to provide hot -stamped steel
that has impact absorption properties higher than those of hot -stamped steel having the
same chemical composition in the related mi, and includes a Zn coating layer excellent
in corrosion resistance.
[Brief Description of the Reference Symbols]
[0114]
10: SOLID-SOLUTION LAYER
20: TEMPERED PORTION
30: Zn OXIDE LAYER
40: LAMELLA LAYER

[Document Type] CLAIMS
What is claimed is:
1. A hot -stamped steel comprising:
a base metal that is a steel including a tempered portion having a hardness
corresponding to 85% or less of the highest quenching hardness, the highest quenching
hardness being defined as a Vickers hardness at a depth position spaced away from a
surface by 114 times a sheet thickness in a case of performing water quenching after
heating to a temperature equal to or higher than an Ac3 point and retaining for 30
minutes; and
a Zn coating layer that is formed on the tempered pmiion of the base metal,
wherein the Zn coating layer includes:
a solid-solution layer including a solid-solution phase that contains Fe and Zn
that is solid-soluted in Fe, and
a lamella layer that includes the solid-solution phase and a capital gamma
phase, and
an area ratio of the lamella layer in the Zn coating layer is 20% or less.
2. The hot -stamped steel according to claim 1, .
wherein the hardness of the tempered poition is 60% or less of the highest
quenching hardness, and the area ratio of the lamella layer in the Zn coating layer is
5%to 20%.
3. The hot -stamped steel according to claim I or 2,
wherein the hardness of the tempered portion is 50% or Jess of the highest
quenching hardness.
- 50 -
4. The hot-stamped steel according to any one of claims I to 3,
wherein the hardness of the tempered pmiion is 180 Hv to 450 Hv.
5. The hot-stamped steel according to any one of claims 1 to 4,
wherein the hot -stamped steel is produced by heating for a predetermined
heating time so that the highest heating temperature is the Ac3 point or higher, working
and 'quenching simultaneously through pressing by using a die, and tempering at a
predetermined tempering temperature, and ·
when an Aci point of the base metal is represented by Ach the tempering
temperature is represented by Tin degrees °C, and a concentration of Zn in a Fe-Zn
solid-solution of a steel after the quenching and before the tempering is represented by
C in a unit of% by mass, the following Equation 1 is satisfied.
Ac,~T2:700-4.0x(35.0-C) (I)
6. The hot-stamped steel according to claim 5,
wherein the tempering temperature is 700°C to the Ac1 point of the base metal.
7. The hot-stamped steel according to any one of claims· I to 6,
wherein a pmi of the base metal is the tempered portion.