Document Type] Specification
[Title of the Invention] HOT-STAMPED STEEL
[Technical Field of the Invention]
[0001]
The present invention relates to hot -stamped steel.
Priority is claimed on Japanese Patent Application No. 2014-073811, 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 through hot-stamping, may be nsed. 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 using the die.
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, for example,
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 hot-stamping with respect to a steel sheet coated with a galvanized layer so
as to increase corrosion resistance.
[0003]
As described above, in the hot-stamping, quenching is performed
simultaneously with pressing. In addition, the hot-stamping is suitable to produce a
- 1 -
structural component having high shape accuracy and high strength. According to
this, typically, the strength (tensile strength) of the hot-stamped steel is 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
propetiies. 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 hotstamping
conditions. However, in a hot-stamping process, it is not preferable to
change the hot -stamping conditions in accordance with a componen:t 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 strength of approximately 1500 MPa or greater is obtained through
quenching in the hot-stamping, has cmmsion 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 without
decreasing the corrosion resistance is not disclosed in Patent Document l to Patent
Document 3.
[0005]
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,
surfaces with high chemical convertibility have high film adhesiveness. Accordingly,
in the hot-stamped steel, it is preferable that phosphate film which is formed by
phosphate treatment is likely to adhere (that is, phosphate treatability is high).
- 2 -
In general, it is known that phosphate treatability deteriorates when hot
stamping is performed with respect to steel (galvanized steel) having a galvanized layer.
A technique which can increase the phosphate treatability of the hot-stamped steel
which has a Zn coating layer has not been reported.
[0006]
Accordingly, hot-stamping steel, which has a Zn coating layer and has the
same chemical composition in the related mi and excellent phosphate treatability, has
not been provided.
[Prior Ali Document]
[Patent Document]
[0007]
[Patent Document I] Japanese Unexamined Patent Application, First
Publication No. 2003-73774
[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]
[0008]
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 mt, and includes a Zn coating layer excellent in
phosphate treatability.
- 3 -
[Means for Solving the Problem]
[0009]
The gist of the present invention is as follows.
( 1) According to an aspect of the present invention, hot -stamped steel
includes: 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 layer 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 portion 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-soluted in Fe, and a lamella layer
that includes the solid-solution phase and a capital gamma phase, and wherein in the
Zn coating layer, an area ratio of the lamella layer in the Zn coating layer is 30 to
1 00% and an area ratio of the solid-solution layer is 0 to 70%.
(2) In the hot-stamped steel according to (1), the area ratio of the lamella layer
in the Zn coating layer may be 80% or more.
(3) In the hot-stamped steel according to (1) or (2), a Vickers hardness of the
tempered pm1ion may be 180 Hv to 450 Hv.
(4) In the hot-stamped steel according to any one of(!) to (3), a hardness of
the tempered portion may be 65% or less of the highest quenching hardness.
(5) In the hot-stamped steel according to any one of(!) to (4), the hotstamped
steel may be produced by heating to the Ac3 point or higher, working and
quenching simultaneously through pressing by using a die, and then tempering at
500°C or more and less than 700°C.
- 4 -
(6) In the hot-stamped steel according to any one of(!) to (5), a part of the
base metal may be the tempered pmiion.
[Effects of the Invention]
[0010]
According to the aspect of the present invention, it is possible to provide a
hot-stamped steel having strength lower than that of hot-stamped steel having the same
chemical composition in the related art, and including a Zn coating layer excellent in
phosphate treatability.
[Brief Description of the Drawings]
[0011]
FIG. 1 is a cross-sectional SEM image of a Zn coating layer and the periphe1y
thereof in a case where hot -stamped steel including the galvanized 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 inclt1ding the galvanized 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 galvanized layer
is tempered at 700°C.
FIG. 4 is a view showing XRD measurement results of the Zn coating layer
shown in FIG. I.
FIG. 5 is a view showing XRD measurement results of the 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.
- 5 -
FIG. 7 is a Fe-Zn binary phase diagram.
FIG. 8 is a SEM image of the surface of the steel of Examples in a case where
hot-stamped steel tempered at 500°C is subjected to phosphate treatment.
FIG. 9 is a binarized image of the SEM image of FIG. 8.
FIG. 10 is a SEM image of the surface of the steel of Examples in a case
where hot-stamped steel tempered at 400°C is subjected to phosphate treatment.
FIG. II is a binarized image of the SEM image of FIG. 10.
FIG. 12 is a SEM image of the surface of the steel of Examples in a case
where hot-stamped steel tempered at 700°C is subjected to phosphate treatment.
FIG. !3 is a binarized image of the SEM image of FIG. 12.
[Embodiments of the Invention]
[0012]
The present inventor studied regarding a method for increasing impact
absorption prope1ties and phosphate treatability of hot-stamped steel including a Zn
coating layer. As a result, the present inventor obtained the following findings.
[00!3]
As described above, typically, as the strength (tensile strength) of hot-stamped
steel becomes lower, impact absorption prope1ties become higher. When tempering is
performed with respect to the hot-stamped steel, it is possible to fmther lower the
tensile strength in comparison to hot-stamped steel having the same chemical
composition in the related art. That is, it is possible to enhance the impact absorption
properties of the hot -stamped steel.
[0014]
However, when tempering is performed with respect to hot-stamped steel
including a Zn coating layer, a structure of the Zn coating layer varies. The variation
- 6 -
in the structure of the Zn coating layer has an effect on phosphate treatability.
[00 15]
The present inventors have made an investigation with respect to an effect on
the Zn coating layer by tempering conditions, and an effect on phosphate treatability
by the Zn coating layer, in the following manner.
[0016]
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 in which the coating weight of galvanized layer was 60
g/llllll
2 was formed on each of the steel sheets using a hot dip galvanizing method.
Then, hot -stamping was performed with respect to the steel sheet on which the
galvanized layer was formed. Specifically, the steel sheet was charged into a heating
furnace in which a furnace temperature was set to 900°C which is a temperature equal
to or higher than an Ac3 point of the steel sheet, and was heated for 4 minutes. At.' this
time, the temperature of the steel sheet reached 900°C approximately two minutes after
being charged into the furnace. After the heating, the steel sheet was interposed by a
flat die equipped with a water-cooling jacket, and the hot -stamping (working and
quenching) was performed to produce hot-stamped steel (steel sheet). The cooling
rate during the hot -stamping was 50 °C/second or faster up to a mmtensitic
transformation start point even in a portion in which cooling rate is slow.
The martensitic transformation start (Ms) point can be determined by
measuring thermal expansion when rapidly cooling steel that is heated to an
austenitizing temperature and measuring volume expansion from austenite to
mmtensite.
[0017]
- 7 -
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.
[00 18]
An Act point and the Ac3 point respectively represent an austenitic
transformation initiation temperature and an austenitic transformation termination
temperature during heating of the steel sheet. The Act point and the Ac3 point can be
determined by measuring thermal expansion during heating the steel in a Formaster
test and the like. Specifically, the Act point and the Ac3 point can be determined by
observing volume constriction during transformation from ferrite to austenite. In
addition, the martensitic transformation simi point can be determined by measuring
thermal expansion when rapidly cooling steel that is heated to an austenitizing
temperature. Specifically, the mmiensitic transformation start point can be
determined by measuring volume expansion from austenite to mmtensite.
[0019]
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. 1 is a cross-sectional SEM image of the Zn coating layer ofthe hotstamped
steel and the periphery thereof in a case where the tempering temperature is
400°C. FIG. 4 is an XRD measurement result from the surface
- 8 -
FIG. 2 is a cross-sectional SEM image of the Zn coating layer of the hotstamped
steel and the periphery thereof in a case where the tempering temperature is
500°C. FIG. 5 is an XRD measurement result from a snrface
FIG. 3 is a cross-sectional SEM image of the Zn coating layer of the hotstamped
steel and the periphery thereof in a case where the tempering temperature is
700°C. FIG. 6 is an XRD measurement result from the snrface.
[0020]
The micro-stmcture 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-structure was observed with an
SEM at a magnification of2000 times.
The XRD measurement was performed by using a Co tubular bulb. In XRD,
the intensity peak of u-Fe is shown at a difli-action angle of29=99.7° and as the solidsolution
amount of Zn increases, the intensity peak shifts toward a small-angle side.
The intensity peak of capital gamma (f), which is an intermetallic compound of
Fe3Zn10, is shown at a diffraction angle of29=94.0°. The broken line L4 in FIG. 4 to
FIG. 6 indicates the intensity peak position of the u-Fe phase. A broken line L3
indicates an intensity peak position of a solid-solution phase in which the solidsolution
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 great (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 an intensity peak position of a f-phase. As the intensity
peak position shifts fi·om the broken line L4 to the broken line L2, the solid-solution
- 9 -
amount of Zn in the solid-solution phase increases.
[0021]
In a case where the tempering temperature is 150 °C to lower than 500°C, as
shown in FIG. 1 and FIG. 4, the Zn coating layer formed a solid-solution layer I 0.
The solid-solution layer 10 was the high Zn solid-solution phase in which the intensity
peak position is L2. The reference numeral 20 in FIG. 1 represents a tempered
•
potiion in the base metal, and a reference numeral 30 represents a zinc oxide layer
formed on the Zn coating layer. The zinc oxide layer is not in a metallic state, and
thus is not a part of coating layer.
[0022] '
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 40 including the plural phases were observed 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 L1) of the r -phase are shown. That is,
the lamella structure layer was a layer (hereinafter, lamella layer) of a lamella stmcture
mainly including the r -phase and the low Zn solid-solution 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 lamella layer 40 in an area
ratio of30% or greater and the solid-solution layer (including the high Zn solidsolution
phase) I 0 in an area ratio of 0% to 70%. In addition, the lamella layer 40
was formed on the solid-solution layer 10. That is, the lamella layer 40 was formed
on the surface side of the Zn coating layer in comparison to the solid-solution layer.
In addition, in a ease where the tempering temperature is 600°C, the entirety of the Zn
- 10 -
coating layer essentially consists of the lamella layer.
[0023]
In addition, in a case where the tempering temperatnre is 700°C, as shown in
FIG. 3, the Zn coating layer included a slight amount of the lamella layer 40 in a
surface layer, 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 20% or less. In addition, from results of the XRD measurement, as shown
in FIG. 6, an intensity peak of the solid-solution phase (at the position of the broken
line L2), which was not detected in a case where the tempering temperate was 500°C
to lower than 700°C, was shown. On the other hand, the intensity peak (position of
the broken line L 1) of the r -phase was lowered in comparison to the case where the
· tempering temperatnre was 500°C to lower than 700°C.
[0024]
As described above, the structnre of the Zn coating layer varies depend on the
tempering conditions. Accordingly, the phosphate treatability of the hot -stamped
steel, which was subjected to the tempering at each tempering temperatnre, was
investigated. As the result, the present inventor found that when the Zn coating layer
includes the lamella layer 40 in an area ratio of 30% or greater, excellent phosphate
treatability is secured.
[0025]
The hot-stamped steel according to an embodiment of the present invention
(may also be referred to as "hot-stamped steel according to this embodiment") includes,
in a case that the highest quenching hardness is defined as a Vickers hardness at a
depth position spaced away from a surface by l/4 times a sheet thickness in a case of
performing water quenching after heating to a temperature equal to or higher than an
- II -
AcJ point and retaining for 30 minutes, a base metal that is a steel including a tempered
portion having a hardness corresponding to 85% or less of the highest quenching
hardness and a Zn coating layer that is formed on the tempered portion of the base
metal. 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, a lamella layer that includes
the solid-solution phase and a capital gamma phase. In addition, in the Zn coating
layer, an area ratio of the lamella layer is 30 to 100% and an area ratio of the solidsolution
layer is 0 to 70%.
Hereinafter, a description will be given of hot-stamped steel according to this
embodiment.
[0026]
[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 p01iion. The tempered
p01iion represents a portion having hardness (Vickers hardness) corresponding to 85%
or less of the highest quenching hardness of steel. The highest quenching hardness
represents the Vickers hardness at a depth position spaced away from a surface layer of
steel by a distance equal to 114 times a sheet thickness in a case of performing water
quenching after heating the steel to a temperature equal to or higher than Ac3 point and
holding for 30 minutes. The highest quenching hardness can be measured by using
another steel (steel different from the hot -stamped steel having the tempered portion)
having the same chemical component.
In the hot -stamped steel according to this embodiment, the base metal
includes the tempered p01iion having hardness corresponding to 85% or less of the
highest quenching hardness, and thus the tensile strength is lower and the impact
- 12 -
absorption properties are more excellent in comparison to hot-stamped steel which has
the same chemical composition and is not subjected to tempering. It is preferable that
the hardness of the tempered pm1ion is 65% or less of the highest quenching hardness.
In this case, the impact absorption properties are further excellent.
[0027]
Since martensite is a structure in which hardness is high, and the hardness
thereof is lowered tln-ough tempering, when the base metal has a chemical composition
in which martensitic transformation occurs when being subjected to water quenching,
it is easy for the base metal to have the tempered pm1ion 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 oftempered martensite and less than 5% of
residual austenite in terms of% by volume.
[0028]
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
in a component for automobiles. In addition, it is advantageous to include the
tempered pm1ion having hardness corresponding to 85% or less of the highest
quenching hardness. Hereinafter, "%" related to an element represents % by mass.
[0029]
C: 0.05% to 0.4%
- 13 -
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. I 0%. 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%.
[0030]
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 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 addition, 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 may deteriorate phosphate treatability. 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%.
[003 I]
Mn: 0.5% to 2.5%
Manganese (lv!n) is an element that enhances hardenability 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
- 14 -
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%.
[0032]
P: 0.03% or less
Phosphorus (P) is an impurity that is contained in steel. Pis segregated to a
grain boundary, and deteriorates the toughness of and delayed fracture resistance of
steel. According to tllis, 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 of P becomes
significant, and thus the P content may be set to 0.03% or less.
[0033]
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 0.010%, the effect ofS becomes significant, and thus the S
content may be set to 0.010% or less.
[0034]
sol. AI: 0.10% or less
Aluminum (AI) is an element that is effective for deoxidation of steel. To
obtain this effect, the lower limit of the AI content may be set to 0.01 %. However,
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
- 15 -
(acid soluble AI) content.
[0035]
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 touglmess 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.
[0036]
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 part 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 enviromnent and the like.
[0037]
B: 0.0001% to 0.0050%
Boron (B) enhances the hardenability of steel, and enhances the strength of
the hot-stamped steel. In order to obtain the effect, the preferable lower limit of the B
- 16 -
content is 0.0001%. 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%.
[0038]
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 ofhardenability
which is caused by formation of BN. In addition, Ti 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%.
[0039]
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
preferable that the upper limit of the Cr content is set to 0.5%.
[0040]
Mo: 0.05% to 0.50%
Molybdenum (Mo) enhances the hardenability of steel. To obtain this effect,
- 17 -
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%.
[0041]
Nb: 0.02% to 0.10%
Niobium (Nb) forms carbide, and makes a grain size fine during hot-stamping.
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%. However, 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%.
[0042]
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 0.1 %. 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%.
[0043]
A part of the base metal may be the tempered p01iion, or the entirety of the
base metal may be the tempered portion.
Recently, a component, in which a demand for performance such as strength
and ductility is difterent in accordance with a position, has been required. The
- 18 -
performance is called a tailored propetiy. For example, with regard to an automobile
component, in a frame component called B pillar (center pillar), an upper potiion,
which constitutes a getting-on area, is required to have high strength, and a lower
portion is required to have high impact absorption properties.
In a case where only a pati of the base metal in the hot-stamped steel
including the Zn coating layer is configured as the tempered potiion, it is possible to
obtain a component which includes the high-strength pmiion and has impact
absorption properties. In addition, since the hot-stamped steel includes the Zn coating
layet~ the corrosion resistance is also excellent.
[0044]
The tensile strength of the tempered portion of the base metal is, for example,
600 MPa to 1450 M.Pa, and the Vickers hardness is 180 Hv to 450 Hv. In this case,
the strength of the tempered pmiion of the hot-stamped steel becomes lower in
comparison to hot-stamped 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 hot-stamped steel of the related art.
The Vickers hardness of tempered matiensite is lower than Vickers hardness
of martensite. Accordingly, it is possible to determine whether or not a microstructure
of the tempered portion is tempered mmiensite in accordance 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 10 kgf=98.07 N.
[0045]
[Zn Coating Layer]
- 19 -
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
includes a lamella layer in an area ratio of 30% or more and a solid-solution layer of 0
to 70%.
[0046]
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-solutionlayer is 25% by mass to 40% by mass, and is more
preferably 30% by mass to 40% by mass.
The Zn coating layer is not required to include the solid-solution layer. That
is, the Zn coating layer may consist of the lamella layer and the area ratio of solidsolution
layer may be 0%.
[0047]
The lamella layer has a lamella structure including a solid-solution phase and
a capital gamma(!) 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. The r -phase is an intermetallic
compound (Fe3Znw). The Zn content in the solid-solu.tion 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.
That is, when the solid-solution layer is present, the lamella layer is formed on
the solid-solution layer.
[0048]
The lamella layer is more excellent in phosphate treatability in comparison to
the solid-solution layer. The reason for this is considered as follows. As described
- 20 -
above, the lamella layer has a lamella structure of the solid-solution phase (low Zn
solid-solution phase) and the r-phase. In the lamella structure, the solid-solution
phase and the r-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 obsmving the Zn
coating layer from the cross section, both of the solid-solution phase and the r-phase
are observed in the surface layer. When 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 r -phase in the lamella layer is higher than the concentration
of Zn in the solid-solution phase, and thus the r -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, the phosphate treatability of the Zn coating layer including the
lamella layer in surface layer is higher in comparison to the Zn coating layer including
only the solid-solution layer in surface layer. When the area ratio of the lamella layer
in the Zn coating layer is 30% or more, the phosphate treatability of the Zn coating
layer is improved. Therefore, it is necessary to include the area ratio of the 30% or
more of the lamella layer in Zn coating layer in the hot -stamped steel according to this
embodiment. It is preferable that the area ratio of the lamella layer is 80% or more.
When the area ratio of the lamella layer is 80% or more, the phosphate treatability is
more improved. In addition, it is expected that the chemical crystal becomes fine and
film adhesiveness is improved.
[0049]
- 21 -
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. In a case
of measuring the Zn content of the high Zn solid-solution phase, 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.
[0050]
[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 hotstamped
steel (tempering process). Hereinafter, a description will be given of a
preferred example in the respective processes.
[0051]
[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 range of chemical composition is
prepared. Slab is prepared by using the produced molten steel in accordance with a
- 22 -
casting method such as continuous casting. An ingot may be produced in place of the
slab by using produced 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 (hotrolled
steel sheet). Pickling may be additionally pe1fonned with respect to the hotrolled
steel sheet as necessmy, 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 characteristics which are required for a component to
which the steel sheet is applied.
[0052]
[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, galvannealing, or electrogalvanizing without particular limitation.
[0053)
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 coating to
adhere to a surface of the steel sheet. The steel sheet, to which the coating adheres, is
pulled up from the galvanizing bath. Preferably, the coating weight of galvanizing
layer on the surface of the steel sheet is adjusted to 20 g/m2 to 100 g/m2
. The coating
weight of galvanizing 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 pmiicularly limited.
- 23 -
[0053]
Through the above-described processes, a steel sheet for hot-stamping (GI), which
includes the galvanized layer (hot-dip galvanized layer), is produced.
[0054]
For example, formation of the galvanized layer through the galvmmealing
(hereinafter, also referred to "alloying process") is performed in the following mmmer.
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
there is no limitation of the soaking time. In addition, illllllediately after heating to
the heating temperature, the steel sheet may be cooled dO\m 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 coating 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 (galvannealed layer), is produced.
[0055]
For example, formation of the galvanized layer through the electrogalvanizing
is performed in the following mmmer. 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 electro galvanizing bath. A current is
allowed to flow tln·ough the electro galvanizing 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.
- 24 -
Through the above-described processes, a steel sheet for hot-stamping (EG),
which includes the electrogalvanized layer, is produced.
[0056]
In a case where the galvanized layer is the galvannealed layer, and in a case
where the galvanized layer is the electrogalvanized layet~ 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
.
[0057]
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.
[0058]
[Hot-Stamping Process]
Hot-stamping is performed with respect to the above-described steel sheet
including the galvanized layer 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
fiunace (a gas furnace, au electrical fimJace, 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,
and is retained (soaked) at this temperature. Zn in a coating layer is liquefied through
the heating, and molten Zn and Fe in the coating layer mutually diffuse and form a
solid-solution phase (Fe-Zn solid-solution phase) during soaking. After the molten
Zn in the coating layer is solid-soluted in Fe and becomes a solid-solution phase, the
- 25 -
steel sheet is taken out from the heating fiJrnace. 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
0 to I 0 minutes. It is preferable that the soaking time be 0 to 6 minutes, and is more
preferably 0 to 4 minutes.
[0059]
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
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.
[0060]
In the above description, the steel for hot-stamping is heated by using the
heating furnace. However, the steel 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.
[0061]
[Tempering Process]
Tempering is performed with respect to the hot-stamped steel (steel after the
hot-stamping). A tempering temperature is 500°C or more and less than 700°C.
[0062]
When the tempering temperature is 500°C or more and less than 700°C, the
Zn coating layer after tempering includes the lamella layer of30% or more, in area
ratio. Furthermore, in a case where the microstructure of the base metal before
- 26 -
tempering is martensite, the microstructure of the base metal after tempering becomes
tempered martensite and the tempered pmiion having hardness corresponding to 85%
or less of the highest quenching hardness can be obtained.
[0063]
The reason why the area ratio of the lamella layer is 30% or more when the
tempering temperature is 500°C or more and less than 700°C is considered to be as
follows.
FIG. 7 is a 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 f-phase, is stable at room temperature in consideration offi·ee
energy. That is, the solid-solution phase of the Zn coating layer after the hotstamping
is a solid-solution in which Zn is oversaturated.
[0064]
On the assumption that the concentration of Zn in the Zn coating layer is 35%
by mass in FIG. 7 (corresponds to a point AI in the drawing). A driving force for
two-phase separation from the solid-solution phase into the low Zn solid-solution
phase and the f-phase is generated on a lower temperature side from a point Bon a
boundary line Ax, and becomes strong as it goes toward a low temperature side fi·om
the point B. On the other hand, as a temperature becomes higher, the diffusion rate in
the Zn 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 two-phase separation, and the diffusion rate. Specifically, as the driving force for
- 27 -
two-phase separation is higher and the diffusion rate increases, the lamella layer is
likely to be formed.
[0065]
In a case where the temperature (tempering temperature) in the Zn coating
layer during the tempering is in a low-temperature region (150°C to lower than 500°C)
(for example, a pointAl of300°C), it is sufficiently spaced away from the bound;~ry
line Ax (point B). In this case, the driving force for two-phase separation is high.
However, since a temperature is low, the diffusion rate of Zn 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.
[0066]
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 (point B), but a certain 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 in comparison to the low-temperature region, 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. 7, 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 10% by mass
(C I in the drawing), and the lamella layer is formed.
[0067]
On the other hand, when the tempering temperature fmiher rises and reaches
700°C or higher, the temperature region approaches the vicinity of the boundary line
Ax or exceeds the boundary line Ax. In this case, the diffusion rate becomes fast due
- 28 -
-,
to the temperature rise, but the driving force for two-phase separation is very small or
the driving force does not occur. As a result, separation into the two phases is less
likely to occur and the area ratio of the lamella layer in the Zn coating layer becomes
30% or less.
[0068]
According to the above mechanism, when the tempering is performed with
respect to the hot-stamped steel including the Zn coating layer, the structure of the Zn
coating layer varies depending on the tempering temperature.
When the tempering temperature is set to be 500°C or more and less than
700°C, it is possible to form the lamella layer of30% or more, in area ratio, in the Zn
coating layer. In addition, in this case, it is possible to obtain excellent phosphate
treatability.
[0069]
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 pati of
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 pati of the hotstamped
steel, strength can be made to vary 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 pmiion is required to have high impact absorption
propetiies. In addition, a tempered portion even in the partial tempering is the same
as the tempered pmiion in a case where the entirety is tempered.
- 29 -
[0070]
Through the producing method including the above-described processes, it is
possible to produce a hot-stamped steel which includes the base metal including the
tempered portion having a hardness corresponding to 85% or less of the highest
quenching hardness, and the galvanized layer, and in which the area ratio of the
lamella layer in the galvanized layer is 3 0% or more.
[0071]
The method of producing the hot-stamped steel according to this embodiment
may further include the following processes.
[0072]
[Anti-Rust Oil Film Forming Process]
The above-described producing method may further include an anti-rust oil
film forming process between the galvanizing process and the hot-stamping process.
[0073]
In the anti-rust oil film forming process, an anti-rust oil is applied to a surface
of the steel for hot-stamping to form the anti-rust oil film. The steel 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 for hot-stamping may be oxidized.
According to this process, the anti-mst oil film is formed on the surface of the steel for
hot -stamping, and thus the surface of the steel sheet is less likely to be oxidized.
Accordingly, generation of scale is limited.
[0074]
[Blanking Process]
In addition, the above-described producing method may further include a
blanking process between the anti-rust oil film forming process and the hot-stamping
- 30 -
~ ---
process.
[0075]
In the blanking process, shearing and/or punching, and the like are performed
with respect to the steel for hot -stamping for shapi1ig (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 of the steel sheet, an anti-rust oil
also spreads to the shear plane to a ce1tain extent. According to this, oxidation of the
steel sheet after the blanking is limited.
Examples
[0076]
A description will be given of the present invention using examples.
[0077]
Slab was prepared by using molten steel having chemical compositions A to G
in accordance with continuous casting method, and the slab was hot-rolled to obtain a
hot-rolled steel sheet. The hot-rolled steel sheet was pickled, and after pickling, coldrolling
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.
[0078]
[Table 1]
- 31 -
L~.:1
Steel
Sheet Chemical composition (unit is% by mass, and the remainder includes Fe and impurities)
thickness
Highest quenching
lypC
(mrn) c Si Mn p s sol.Al N B Ti Cr Mo Nb Ni hardness BO (HV)
A 1.6 0.2 0.2 1.3 O.Ql 0.005 0.02 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 0.02 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 0.02 0.002 0.002 0.02 0.2 0.5 - - 519
F 1.6 0.2 0.2 1.3 0.01 0.005 0.02 0.002 - - - - - - 515
G 1.6 0.3 0.2 1.3 0.01 0.005 0.02 0.002 0.002 0.02 0.2 - - - 609
----- L .. - -- ---- ------
- 32 -
[0079]
To investigate the highest quenching hardness, a part of a steel sheet having
each of the chemical compositions of Steel type A to G 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 fhll martensite.
The Vickers hardness was measured with respect to the steel sheet at a depth
position spaced away from a surface by 1/4 times a sheet thickness after the water
quenching, and the Vickers hardness that was obtained was defined as the highest
quenching hardness BO (HV). A Vickers hardness test was performed in conformity
to JIS Z2244 (2009), and the test force was set to 10 kgF98.07 N.
[0080]
Galvanizing, hot -stamping, and tempering were performed by using each of
cooled-rolled steel sheets having the chemical compositions of Steel type A to G under
conditions in which the coating weight is as shown in Table 2, thereby producing hotstamped
steel in each of Test Nos. I to 14.
[0081]
[Table 2]
- 33 -
L t3E!l .:1
Zn coating layer
Hot-stamped steel
after galvanizing Tempering
temperature Solid-solution
Lamella layer Vickers Area ratio
Test Steel layer TR Hardness
phospahte Over
No. type coating weight ..
BilBO
treatability all
Composition
(gjm') Area Area (%)
Hardness Bl ec) ratio ratio
(HVIO)
(%) (%)
I A Zn-12%Fe 60 510 30 70 313 60.9 39.8 G G G
2 B Zn-12%Fe 60 550 20 80 294 57.4 38.4 G G G
3 c Zn-12%Fe 60 600 10 90 281 54.1 35.9 G G G
4 D Zn-12%Fe 60 600 10 90 264 51.0 37.3 G G G
5 E Zn-12%Fe 60 600 10 90 273 52.6 34.6 G G G
6 F Zn 60 500 30 70 331 64.3 39.7 G G G
7 F Zn-12%Fe 60 600 10 90 275 53.4 37.2 G G G
8 F Zn-12%Fe 60 650 60 40 250 48.5 30.9 G G G
9 F Zn-12%Fe 60 300 100 0 445 86.4 27.7 NG NG NG
10 F Zn-12%Fe 60 400 100 0 414 80.4 25.9 G NG NG
11 F Zn-12%Fe 60 460 80 20 347 67.4 25.9 G NG NG
12 F Zn-12%Fe 60 700 80 20 234 45.4 27.6 G NG NG
13 F Zn-J2%Fe 60 720 90 10 214 41.6 26.9 G NG NG
14 F Zn-12%Fe 60 - 100 0 484 94.0 24.8 NG NG NG
15 G Zn-12%Fe 60 600 10 90 334 54.8 35.7 G G G
'---16 F Zn-12%Fe 60 600 40 60 284 55.1 33.7 G G G
- 34 -
[0082]
In Test No.6, a hot-dip galvanized layer (GI) was formed on the steel sheet
through 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 in each case, and after heating for
approximately 30 seconds, cooling was performed to room temperature.
[0083]
The Fe content in the galvannealed layer was 12% in terms of% by mass.
The Fe content was obtained 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.
[0084]
The coating weight of galvanized layer (the hot-dip galvanized layer or the
galvannealed layer) was measured by the following method. First, a sample including
a coating layer was collected from each of the steel sheets, and the coating layer of the
sample was dissolved in hydrochloric acid in conformity to JIS H040 I. The coating
weight (g/nl) of galvanized layer 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 the column labeled "coating weight" in Table 2.
[0085]
After forming the coating layer, hot-stamping by heating was performed with
- 35 -
fl
respect to the steel sheet in each of the test numbers. Specifically, the steel sheet was
charged into a heating furnace in which the furnace 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 Ac3 point of each of the Steel
Nos. A toG by using radiant heat for 4 minutes. At this time, the temperature of the
steel sheet reached 900°C after approximately 2 to 2.5 minutes after being charged into
the furnace, and the steel sheet was soaked at 900°C for 1.5 to 2 minutes.
[0086]
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 stmi
point became 50 °C/second.
[0087]
In addition, tempering was performed with respect to Test Nos. 1 to 13, 15,
and 16after hot-stamping. In Test No.1 to 13, and 15, each of steel was charged into
a heat treatment furnace. That is, tempering was performed with respect to entirety of
the each of steel sheets. In Test No.16, the tempering was performed to a pati of the
steel by applying an electrical current to the part of steel tlu·ough electrical heating.
The tempering temperature in each test number was set as shown in Table 2, and the
heating time was set to 5 minutes when the steel was charged into a heating fiJrnace or
was set to 20 seconds when electrical heating is performed. Tempering was not
performed with respect to steel of Test No. 14. Through the above-described
processes, hot-stamped steel was produced in each of Test Nos. 1 to 16.
A Vickers hardness test, micro-structure observation of the galvanized layer,
- 36 -
[I
and the evaluation test for phosphate treatability were performed with respect to the
hot-stamped steel in each of Test Nos. 1 to 16. Regarding the hot-stamped steel of the
in which the tempering was performed to a part of the steel, evaluation of the tempered
portion was carried out.
[0088]
[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 (L
cross section)) of the sample. The test force was set to I 0 kgf=98.07 N. B liB Ox 100
(%),which is a ratio between Vickers hardness B1 (HVIO) that was obtained and the
highest quenching hardness BO, is shown in Table 2.
[0089]
[Micro-Structure 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!. A cross-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.
(0090]
In a case where the lamella layer was observed, the area ratio of the lamella
layer was fiuther obtained by the following method. At 5 arbitrary visual fields (50
jlmxSO 11111) on the cross-section, the area ratio(%) of the solid-solution layer and the
- 37 -
[j
area ratio(%) of the lamella layer with respect to the entirety of the area of the Zn
coating layer were obtained. At this time, a Zn oxide layer (indicated by a reference
numeral30 in FIG. 1), which floats to a surface, was 1iot included to the area of the Zn
coating layer since the zinc oxfde layer is not in a metallic state and is not coating
layer. Area ratios(%) of the solid-solution layer and the lamella layer, which were
obtained, are shown in Table 2.
[0091]
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 content in the solid-solution layer, which was
observed, was 25% by mass to 40% by mass in all cases.
[0092]
[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
Parkerizing Co., Ltd.). The temperature of the treatment solution was set to 43°C,
and the hot-stamped steel was inm1ersed in the treatment solution for 120 seconds.
[0093]
After the phosphate treatment, arbitrary 5 visual fields (125 f!mx90 f!m) of the
hot-stamped steel were observed with a scanning electron microscope (SEM) at a
magnification of 1000 times. FIG. 8 is a SEM image (at a magnification of 1000
times) of surface of the hot -stamped steel in which the phosphate treatment is
- 38 -
~I
performed with respect to the hot-stamped steel tempered at 500°C (Test No.6).
Binarization processing was performed with respect to the resultant SEM image. FIG.
9 is an image by binarizing the SEM image of FIG. 8. 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 30% or greater, it was determined that the phosphate treatability was good. The
area ratios TR of each Test Nos. are shown in Table 2. In the table "G" signifies
GOOD, and "NG" signifies NO GOOD.
[0094]
[Test Result]
FIG. 10 is a SEM image (at magnification of 1000 times) of surface of the hotstamped
steel in which the phosphate treatment is performed with respect to the hotstamped
steel tempered at 400°C (Test No.1 0). FIG. 11 is an image obtained by
binarizing the SEM image of FIG. 10. FIG. 12 is a SEM image (at a magnification of
1000 times) of surface of the hot-stamped steel in which the phosphate treatment was
performed with respect to the hot-stamped steel tempered at 700°C (Test No .I 0). FIG.
13 is an image obtained by binarizing the SEM image of FIG. 12.
[0095]
Referring to Table 2, the microstructure of the base metal of Test Nos. I to 8
which were subjected to tempering at 500 to 650°C were tempered martensite, and the
Vickers hardnesses thereof were 180 to 450HV and 85% or less of the highest
quenching hardness.
That is, the hardnesses of the hot-stamped steel of these Test Nos. were
hardness corresponding to a strength of 1450MPa or less. In addition, in these hot-
- 39 -
stamped steel, the area ratio of the lamella layer in the Zn coating layer is 30% or more,
and thus, the area ratios TR in the evaluation test for phosphate treatability were 30%
or more. That is, the hot-stamped steel of Test Nos. 1 to 8 indicated excellent impact
absorption prope1iies and phosphate treatability.
[0096]
On the other hand, in Test Nos.9 to 13, the tempering temperatures were less
than 500°C or 700°C or more. As a result, in the hot-stamped steel of Test Nos. 9 to
13, the area ratio of the lamella layer in the Zn coating layer was less than 30%.
Accordingly, the area ratios TR in the evaluation test for phosphate treatability were
less than 30% and phosphate treatability were low. In addition, in Test No. 9 since
the tempering temperature was low, the hardness of the base metal was not 85% or less
of the highest quenching hardness even after tempering
[0097]
Test No. 14 is an example which was not subjected to tempering. Therefore,
the microstructure of the base metal was martensite (fresh mmiensite). Accordingly,
the Vickers hardness was 450HV or more and thus exceeds 85% of the highest
quenching hardness. Moreover, the area ratio of the lamella layer in the Zn coating
layer was less than 30% and the phosphate treatability was low
[0098]
Hereinbefore, the embodiment of the present invention has been described.
Howevm~ 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 depmiing
from the gist of the present invention.
- 40 -
[Brief Description of the Reference Symbols]
[0099]
10: SOLID-SOLUTION LAYER
20: TEMPERED PORTION
30: ZINC OXIDE LAYER
40: LAMELLA LAYER
[Industrial Applicability]
[0100]
According to the present invention, it is possible to provide hot-stamped steel
that has strength lower than those of hot -stamped steel having the same chemical
composition in the related art, and includes a Zn coating layer excellent in phosphate
treatability.

[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 layer by 114 times a sheet thickness in a case of performing water quenching
after heating to a temperature equal to or higher than an AcJ point and retaining for 30
minutes; and
a Zn coating layer that is formed on the tempered pmtion 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
wherein in the Zn coating layer, an area ratio of the lamella layer is 30 to
100% and an area ratio of the solid-solution layer is 0 to 70%.
2. The hot-stamped steel according to claim I,
wherein the area ratio of the lamella layer in the Zn coating layer is 80% or
more.
3. The hot -stamped steel according to claim 1 or 2,
wherein a Vickers hardness of the tempered portion is 180 Hv to 450 Hv.
- 42 -
4. The hot-stamped steel according to any one of claims 1 to 3,
wherein a hardness of the tempered pmtion is 65% or less of the highest
quenching hardness.
5. The hot-stamped steel according to any one of claims 1 to 4,
wherein the hot-stamped steel is produced by heating to the Ac3 point or
higher, working and quenching simultaneously through pressing by using a die, and
then tempering at 500°C or more and less than 700°C.
6. The hot-stamped steel according to any one of claims 1 to 5,
wherein a patt of the base metal is the tempered portion.