TECHNICAL FIELD
[0001] The present invention relates to a hot press line and a method of
manufacturing a hot-press-formed product.
5 BACKGROUND ART
[0002] In some metallic structural members, properties such as
strength may be locally varied. For example, when a high-strength
member is used as a vehicle-skeleton member, some low-strength
portions may be provided in the member, rather than providing high
10 strength to all the portions. There are several reasons for doing this.
For example, machining such as drilling may be performed in
low-strength portions. In other applications, the deformation behavior
of a member may be controlled by providing low-strength portions that
are to be deformed early during deformation of the member.
15 [0003] One method for manufacturing a member with low-strength
portions involves welding steels with different properties to provide a
tailor-welded blank, followed by hot working (i.e., hot stamping). For
example, Japanese Patent No. 5864414 describes a method of hot press
forming a steel sheet blank composed of separate sheets that have been
20 welded together. In this method, a steel sheet blank is heated and
then hot press formed inside a pair of cooled tools, and, while the blank
is still inside the pair of tools, the formed product is hardened. The
welded portions of the two sheets are cooled at lower cooling rates with
respect to portions on both sides of each welded portion. This forms
25 portions with low martensite contents along the welded portions. The
cooling rate is lowered by keeping a gap between the pair of tools and
the end product.
[0004] JP 2015-226936 A discloses a manufacturing method that
enables local adjustment of the construction of a metal structure
30 component. In this manufacturing method, a steel member is hot
formed and then at least several sections are hardened through contact
with the tool surface. At least one of two sections of the tool surface
has a surface coating that decreases or increases thermal conductivity.
Sections of the tool surface with different thermal conductivities lead to
35 different cooling rates. The sub-regions of the steel member with
3
different cooling rates have different microscopic structures after
hardening.
PRIOR ART DOCUMENTS
5 PATENT DOCUMENTS
[0005] Patent Document 1: Japanese Patent No.5864414
Patent Document 2: JP 2015-226936 A
SUMMARY OF THE INVENTION
10 PROBLEMS TO BE SOLVED BY THE INVENTION
[0006] The above-described conventional techniques achieve a local
decrease in the cooling rate of a metal sheet by virtue of a gap (or
clearance) between the formed product and the die, or a
thermal-conductivity distribution in the die surface. However, when
15 the formed product has been removed from the die, the temperatures in
portions with lower cooling rates are still high. Then, as these
portions experience thermal contraction during cooling, the formed
product may develop defects of shape. Further, if there are large
temperature differences within the formed product when the formed
20 product is removed from the die, the formed product may deform due to
thermal contraction, leading to defects of shape. To reduce the
temperature of the formed product upon removal from the die as well as
temperature differences within the formed product, the formed product
must be kept in the die until a uniform temperature is reached inside
25 the formed product. On the other hand, from the viewpoint of
manufacture cost, for example, it is preferable to minimize the period of
time for which the formed product is held in the die (i.e.,
bottom-dead-center holding period). That is, it is difficult to achieve
both productivity and shape accuracy with conventional methods.
30 [0007] In view of this, the present disclosure provides a hot press line
and a method of manufacturing a hot press-formed product that ensure
the shape accuracy of a formed product provided with a property
distribution without prolonging the bottom-dead-center holding time
for the formed product in the die during hot pressing.
35
4
MEANS FOR SOLVING THE PROBLEMS
[0008] A hot press line according to an embodiment of the present
invention includes: a heating device adapted to heat a metal sheet; a
first press device including a pair of first die parts movable relative to
each other in a direction of pressing and adapted 5 to press form the
heated metal sheet by moving the first die parts closer to each other in
the direction of pressing and, at a bottom-dead center, hold the metal
sheet; a second press device including a pair of second die parts
movable relative to each other in the direction of pressing and adapted,
10 at a bottom-dead center of the second die parts, to hold the metal sheet
press formed by the first press device; a first transportation device
adapted to transport the metal sheet from the heating device to the first
press device; and a second transportation device adapted to transport
the metal sheet from the first press device to the second press device.
15 At least one of the pair of first pairs and the pair of second die parts
includes a clearance portion recessed inwardly to create a clearance
with the metal sheet while the die parts are at the bottom-dead center,
and the other pair of die parts includes an abutment surface located in
at least part of a portion corresponding to the clearance portion of the
20 one pair of die parts and adapted to abut the metal sheet while the die
parts are at the bottom-dead center.
EFFECTS OF THE INVENTION
[0009] The present disclosure ensures the shape accuracy of a formed
25 product provided with a property distribution without prolonging the
bottom-dead-center holding time for the formed product in the die
during hot pressing.
BRIEF DESCRIPTION OF THE DRAWINGS
30 [0010] [FIG. 1] FIG. 1 shows an exemplary construction of a hot press
line according to an embodiment.
[FIG. 2] FIG. 2 is a cross-sectional view of a first press device
according to an embodiment, illustrating its construction.
[FIG. 3] FIG. 3 illustrates the first press device shown in FIG. 2
35 as being at its bottom-dead center.
5
[FIG. 4] FIG. 4 is a cross-sectional view of a second press device
according to an embodiment, illustrating its construction.
[FIG. 5] FIG. 5 shows the second press device shown in FIG. 4
as being at its bottom-dead center.
[FIG. 6] FIG. 6 shows a variation of the first 5 and second die
parts, modified in construction.
[FIG. 7] FIG. 7 is a graph illustrating an implementation where
an abutment period is provided in the first bottom-dead-center holding
period and a non-abutment period is provided in the second
10 bottom-dead-center holding period.
[FIG. 8] FIG. 8 is a graph illustrating an implementation where
a non-abutment period is provided in the first bottom-dead-center
holding period and an abutment period is provided in the second
bottom-dead-center holding period.
15 [FIG. 9] FIG. 9 shows a variation of the first and second die
parts, modified in construction.
[FIG. 10] FIG. 10 shows a variation of the first and second die
parts, modified in construction.
[FIG. 11] FIG. 11 shows a variation of the first and second die
20 parts, modified in construction.
[FIG. 12] FIG. 12 shows a variation of the first and second die
parts, modified in construction.
[FIG. 13] FIG. 13 shows a variation of the first and second die
parts, modified in construction.
25 [FIG. 14] FIG. 14 shows the position on the formed product of an
embodiment at which shape accuracy is evaluated.
[FIG. 15] FIG. 15 is a graph showing the results of
measurement of the hardness distributions of formed products.
[FIG. 16] FIG. 16 is a graph showing the results of
30 measurement of the torsion angles of formed products.
[FIG. 17] FIG. 17 is a graph showing the results of
measurement of the out-of-plane deformations of formed products.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
35 [0011] (Arrangement 1)
6
A hot press line according to an embodiment of the present
invention includes: a heating device adapted to heat a metal sheet; a
first press device including a pair of first die parts movable relative to
each other in a direction of pressing and adapted to press form the
heated metal sheet by moving the first die parts closer 5 to each other in
the direction of pressing and, at a bottom-dead center, hold the metal
sheet; a second press device including a pair of second die parts
movable relative to each other in the direction of pressing and adapted,
at a bottom-dead center of the second die parts, to hold the metal sheet
10 press formed by the first press device; a first transportation device
adapted to transport the metal sheet from the heating device to the first
press device; and a second transportation device adapted to transport
the metal sheet from the first press device to the second press device.
At least one of the pair of first die parts and the pair of second die parts
15 includes a clearance portion recessed inwardly to create a clearance
with the metal sheet while the die parts are at the bottom-dead center,
and the other pair of die parts includes an abutment surface located in
at least part of a portion corresponding to the clearance portion of the
one pair of die parts and adapted to abut the metal sheet while the die
20 parts are at the bottom-dead center.
[0012] In Arrangement 1 described above, the formed metal sheet is
rapidly cooled in a bottom-dead-center holding period, which is
represented by the sum of the period of time for which the first die parts
of the first press device hold the metal sheet at their bottom-dead center
25 and the period of time for which the second die parts of the second press
device hold the metal sheet at their bottom-dead center. At least one of
the pair of first die parts and the pair of second die parts includes a
clearance portion, and the portion of the other pair of die parts
corresponding to the clearance portion is provided with an abutment
30 surface for abutting the metal sheet while the die is at the bottom-dead
center. Thus, the bottom-dead-center holding period represented by
the sum of the bottom-dead-center holding period for the first die parts
and the bottom-dead-center holding period for the second die parts
includes a non-abutment period for which some portions of the metal
35 sheet are not in contact with a die due to the clearance portion, as well
7
as an abutment period for which a die is in contact with those portions
of the metal sheet. During the non-abutment period, cooling rate can
be reduced, i.e. gradual cooling can be performed. Further, during the
abutment period in the bottom-dead-center holding time for which a die
abuts the above-mentioned portions of the metal sheet, 5 cooling rate can
be increased, i.e., rapid cooling can be performed. This will achieve a
close-to-uniform temperature distribution of the metal sheet while a
portion of the formed metal sheet corresponding to the clearance
portion has different cooling conditions from those of portions with
10 which a die is in contact throughout the bottom-dead-center holding
period. This provides the formed metal sheet removed from the second
die parts with a property distribution derived from the differences in
cooling conditions and, at the same time, reduces a decrease in the
shape accuracy of the formed product due to the temperature
15 differences. This ensures the shape accuracy of a formed product
provided with a property distribution without prolonging the
bottom-dead-center holding time for the formed product in the die.
[0013] In conventional methods in which cooling rate is reduced by a
clearance or by means of thermal conductivities of the die surface, the
20 cooling conditions under which part of the member is gradually cooled
are predetermined cooling conditions based on the construction of the
die. Thus, the metal structure composition obtained by gradual
cooling and the temperature distribution of the member as removed
from the die also depend on the construction of the die. Changing
25 these features requires adjusting the construction of the die or
re-fabricating a die. In contrast, in Arrangement 1 described above,
cooling conditions can easily be adjusted by changing press conditions
or transportation conditions. For example, the cooling conditions can
be controlled by adjusting the lengths of the time for which the first die
30 parts hold the metal sheet at the bottom-dead center and the time for
which the second die parts hold the metal sheet at the bottom-dead
center. Thus, in a pressing process using a hot press line, the cooling
conditions under which some portions of the formed metal sheet are
gradually cooled can be easily changed.
35 [0014] The construction of the clearance-related portions of the first die
8
parts and that for the second die parts are different from each other. A
clearance portion is provided in at least one of the pair of first die parts
and the second die parts. The geometry of the pair of surfaces of the
pair of first die parts that face each other in the direction of pressing
(i.e., forming surfaces) and the geometry of the pair 5 of surfaces of the
pair of second die parts that face each other in the direction of pressing
(i.e., forming surfaces) may be the same except for the clearance portion.
Thus, the metal sheet formed by the first die parts can be held by the
second die parts at the bottom-dead center while the portions of the
10 metal sheet other than the portions corresponding to the clearance
portion of the first die parts can maintain their shape. In some
implementations, the second die parts may be constructed, at the
bottom-dead center, to hold the metal sheet that has been press formed
by the first die parts while maintaining the shape of the metal sheet.
15 [0015] (Arrangement 2)
Starting from Arrangement 1 described above, the first die
parts may include the clearance portion, and the second die parts may
include the abutment surface in at least part of a portion corresponding
to the clearance portion of the first die parts. Thus, the second die
20 parts can contact the metal sheet for rapid cooling in a sub-period of the
bottom-dead-center holding period for the second die parts for which
the temperature of the metal sheet is relatively low. This will make it
easier to achieve a close-to-uniform temperature distribution of the
metal sheet. That is, it will be easier to ensure the shape accuracy of
25 the entire formed metal sheet. Further, it will be easier to adjust the
cooling conditions by controlling the abutment period.
[0016] (Arrangement 3)
Starting from Arrangement 1 or 2 described above, the second
die parts may include the clearance portion, and the first die parts may
30 include the abutment surface in at least part of a portion corresponding
to the clearance portion of the second die parts. Thus, the first die
parts can form the metal sheet in a sub-period of the
bottom-dead-center holding period for the first die for which the metal
sheet has a relatively high temperature and easy to form. This will
35 make it easier to ensure local shape accuracy, i.e., the shape accuracy of
9
a portion of a formed metal sheet that corresponds to the clearance
portion of the die.
[0017] (Arrangement 4)
Starting from any one of Arrangements 1 to 3 described above,
the clearance portion of the one pair of die parts may 5 include a pair of
clearance portions facing each other with the metal sheet positioned
therebetween. In such implementations, the abutment surface of the
other pair of die parts may include a pair of abutment surfaces facing
each other with the metal sheet positioned therebetween, the pair of
10 abutment surfaces located in at least part of portions corresponding to
the pair of clearance portions of the one pair of die parts. At the
clearance portions of the one pair of die parts, clearances are present on
both sides of the metal sheet when the die is at the bottom-dead center,
and, in the other pair of die parts, both sides of the metal sheet abut the
15 die at the bottom-dead center. This will increase the robustness of the
cooling conditions.
[0018] (Arrangement 5)
Starting from any one of Arrangements 1 to 4 described above,
the abutment surface of the other pair of die parts includes a pair of
20 abutment surfaces facing each other in the direction of pressing, and
the pair of abutment surfaces are shaped to bend the metal sheet in the
direction of pressing. This will enable forming the portion of the metal
sheet corresponding to the clearance portion of the one pair of die parts
to a shape corresponding to the pair of abutment surfaces of the other
25 pair of die parts.
[0019] For example, one of the abutment surfaces of the other pair of
die parts may have a recess or protrusion recessed or protruding in the
direction of pressing. In such implementations, the other abutment
surface facing the one abutment surface may have a shape
30 corresponding to the recess or protrusion of the one abutment surface.
[0020] Starting from any one of Arrangements 1 to 4, the abutment
surface of the other pair of die parts corresponding to the clearance
portion of the one pair of die parts may be a flat surface. Thus, a
flat-surface portion of the formed metal sheet is provided with a
35 property distribution.
10
[0021] (Arrangement 6)
Starting from any one of Arrangements 1 to 5 described above, a
die portion of the one pair of die parts facing the clearance portion of the
one pair may be provided with the abutment surface for abutting the
metal sheet when the die parts are at the bottom-dead 5 center, and a die
portion of the other pair of die parts facing the abutment surface of the
other pair may be provided with the clearance portion recessed
inwardly for creating a clearance with the metal sheet when the die
parts at the bottom-dead center.
10 [0022] The area of the clearance portion in the one pair of dies is
preferably not larger than a half of the area that abuts the metal sheet
when the die is at the bottom-dead center, more preferably not larger
than 30 %, and yet more preferably not larger than 20 %. An
excessively high proportion of the clearance portion reduces the
15 proportion of the area that grips the formed metal sheet when the die is
at the bottom-dead center, making it difficult to achieve high shape
accuracy.
[0023] The edge of the recess forming the clearance portion in the one
pair of die parts may be surrounded by the pressing surface of the die.
20 The pressing surface is the surface of the die that abuts the metal sheet
when the die is at the bottom-dead center. That is, the clearance
portion may be located in a region surrounded by the surface that abuts
and presses the metal sheet B when the die is at the bottom-dead center.
Thus, when the die is at the bottom-dead center, the portions of the
25 formed metal sheet B surrounding the clearance portion are gripped by
the die. This makes it easier to ensure the shape accuracy of the
formed metal sheet B.
[0024] The second transportation device preferably transports the
metal sheet such that the period of time from the point at which the
30 metal sheet is removed from the first die parts to the point at which the
metal sheet is positioned at the second die parts is not longer than 30
seconds, more preferably not longer than 15 seconds, and yet more
preferably not longer than 10 seconds. This will reduce the time from
the end of bottom-dead-center holding by the first die parts to the
35 beginning of bottom-dead-center holding by the second die parts,
11
thereby reducing a temperature decrease in this time.
[0025] Starting from any one of Arrangements 1 to 6 described above,
the first press device and the second press device may include a cooling
mechanism adapted to cool the first die parts and the second die parts.
For example, at least one of the pair of first die parts 5 and the pair of
second die parts may include a tube or a groove for allowing a cooling
medium to pass therethrough.
[0026] Starting from any one of Arrangements 1 to 6 described above,
the hot press line may include a control unit adapted to control the first
10 press device and the second press device. The control unit is capable of
controlling, for example, a holding time for the metal sheet by the first
die parts in the first press device at the bottom-dead center, and a
holding time for the metal sheet by the second die parts in the second
press device at the bottom-dead center. This enables adjusting the
15 non-abutment period and abutment period in the entire
bottom-dead-center holding period. That is, the cooling conditions for
a portion of the metal sheet corresponding to the clearance portion can
be adjusted.
[0027] For example, the control unit may control the first die parts and
20 the second die parts such that the abutment period accounts for 20 to
90 % of the entire bottom-dead-center holding period. In such
implementations,, the abutment period is preferably not longer than
70 % of the entire bottom-dead-center holding period, and more
preferably not longer than 50 %.
25 [0028] (Manufacturing Method 1)
A method of manufacturing a hot press-formed product
according to an embodiment of the present invention includes: heating
a metal sheet; positioning the heated metal sheet between a pair of first
die parts of a first press device; press forming the metal sheet by
30 moving the first die parts closer to each other in a direction of pressing;
a first bottom-dead-center holding step for holding the metal sheet
while the pair of first die parts are at a bottom-dead center; after the
first bottom-dead-center holding step, transporting the press-formed
metal sheet to a pair of second die parts of a second press device and
35 positioning the metal sheet therebetween; and a second bottom-dead
12
center holding step for holding the metal sheet press formed by the first
press device while the pair of second die parts are at a bottom-dead
center. During one of the first bottom-dead-center holding step and
the second bottom-dead-center holding step, a surface of the metal
sheet has a non-abutment region that does not 5 contact a die at a
bottom-dead center, and at least part of the non-abutment region
contacts a die at a bottom-dead center during the other
bottom-dead-center holding step.
[0029] In the Manufacturing Method 1 described above, the
10 bottom-dead-center holding period represented by the sum of the times
of the first and second bottom-dead-center holding steps includes a
non-abutment period for which the non-abutment region of the surface
of the metal sheet does not abut a die at its bottom-dead center, as well
as an abutment period for which that surface abuts a die at its
15 bottom-dead center. Cooling rate can be reduced during the
non-abutment period of the bottom-dead-center holding period.
Further, cooling rate can be increased during the abutment period of
the bottom-dead-center holding period. This will achieve a
close-to-uniform temperature distribution of the metal sheet while the
20 non-abutment region of the formed metal sheet has different cooling
conditions from those of the other portions. This ensures the shape
accuracy of a formed product provided with a property distribution
without prolonging the bottom-dead-center holding time for the formed
product in the die.
25 [0030] (Manufacturing Method 2)
Starting from Manufacturing Method 1 described above, at least
part of the non-abutment region of the metal sheet for the first
bottom-dead-center holding step may abut at least one of the pair of the
second parts die during the second bottom-dead-center holding step.
30 Thus, in the entire bottom-dead-center holding period represented by
the sum of the times of the first and second bottom-dead-center holding
steps, the die may be in contact with the metal sheet for rapid cooling
for a sub-period for which the temperature of the metal sheet is
relatively low. This will make it easier to ensure the shape accuracy of
35 the entire formed metal sheet. Further, it will be easier to adjust the
13
cooling conditions by controlling the abutment period.
[0031] (Manufacturing Method 3)
Starting from Manufacturing Method 1 or 2 described above, at
least part of the non-abutment region of the metal sheet for the second
bottom-dead-center holding step may abut at least 5 one of the pair of
first die parts during the first bottom-dead-center holding step. Thus,
in the entire bottom-dead-center holding period, the die may be in
contact with the metal sheet for rapid cooling for a sub-period for which
the temperature of the metal sheet is relatively high. This will make it
10 easier to ensure local shape accuracy, i.e., the shape accuracy of a
portion of a formed metal sheet that corresponds to the clearance
portion.
[0032] (Manufacturing Method 4)
Starting from Manufacturing Method 1 or 2 described above,
15 the non-abutment region of the metal sheet during the one
bottom-dead-center holding step may include a pair of regions, facing
each other, of both sides of the metal sheet, and at least part of each
region of the pair of regions of the non-abutment region may contact a
die part at the bottom-dead center during the other bottom-dead-center
20 holding step. This increases the robustness of the cooling conditions.
[0033] (Manufacturing Method 5)
Starting from any one of Manufacturing Methods 1 to 4
described above, during the other bottom-dead-center holding step, at
least part of the non-abutment region of the metal sheet for the one
25 bottom-dead-center holding step may contact a die part at the
bottom-dead center and may be formed to bend in the direction of
pressing.
[0034] (Manufacturing Method 6)
Starting from any one of Manufacturing Methods 1 to 5
30 described above, during the one bottom-dead-center holding step, at
least part of a back side region of the metal sheet for the non-abutment
region may be abutted by a die part at the bottom-dead center and,
during the other bottom-dead-center holding step, at least part of the
back side region for the non-abutment region may not be abutted by a
35 die part.
14
[0035] Now, embodiments of the present invention will be described in
detail with reference to the drawings. The same or corresponding
elements in the drawings are labeled with the same reference
characters and their description will not be repeated. For ease of
explanation, the drawings to which reference will 5 be made below show
components in a simplified or schematic manner, or omit some
components.
[0036] (Exemplary Construction of Hot Press Line)
FIG. 1 shows an exemplary construction of a hot press line
10 according to an embodiment. The hot press line 100 includes a heating
device 30, a first transportation device 41, a first press device 10, a
second transportation device 42, a second press device 20, and a control
unit 5.
[0037] The heating device 30 heats an object to be heated. The
15 heating device 30 may be, for example, a gas heating furnace, a
far-infrared heating furnace or a near-infrared heating furnace. The
heating device 30 is not limited to a heating furnace, and may be, for
example, a high-frequency induction heater, a low-frequency induction
heater, or an electrical heater that heats the object to be heated by
20 passing electricity therethrough. The heating device 30 may include a
heating chamber. The heating device 30 may include, inside the
heating chamber, a plurality of in-chamber rollers 31 that are driven by
a driving mechanism, not shown, to rotate. As the in-chamber rollers
31 are rotated, the object to be heated on the in-chamber rollers 31 (in
25 the present implementation, metal sheet B to be pressed) is transported.
Next to the heating device 30 are positioned a transportation rollers 26.
The metal sheet B heated by the heating device 30 is transported by the
transportation rollers 26 out of the heating device 30.
[0038] The first transportation device 41 transports the metal sheet B
30 from the heating device 30 to the first press device 10. The first
transportation device 41 may be a manipulator, for example.
Operations by the first transportation device 41 include lifting, holding
and transporting, and putting of the metal sheet B. The first
transportation device 41 is not limited to a manipulator. The first
35 transportation device 41 may be, for example, a forklift or a roller
15
conveyor.
[0039] The first press device 10 includes a pair of first die parts 1A and
1B movable relative to each other in the direction of pressing. The
first transportation device 41 places the metal sheet B between the first
die parts 1A and 1B of the first press device 10. The 5 first press device
10 press forms the heated metal sheet B by moving the first die parts
1A and 2A closer to each other in the direction of pressing and holds the
sheet between the die parts at the bottom-dead center.
[0040] The second transportation device 42 transports the metal sheet
10 B from the first press device 10 to the second press device 20. Similar
to the first transportation device 41, the second transportation device
42 may be constituted by a manipulator, a forklift, or a roller conveyor.
[0041] The second press device 20 includes a pair of second die parts 2A
and 2b movable relative to each other in the direction of pressing. The
15 second transportation device 42 places, between the second die parts 2A
and 2, the metal sheet B that has been press formed by the first press
device 10. The second press device 20 holds, between the second die
parts 2A and 2B at the bottom-dead center, the metal sheet B that has
been press formed by the first press device 10.
20 [0042] At least one of the pair of first die parts 1A and 1B and the pair
of second die parts 2A and 2B includes a clearance portion 1Ac. In the
implementation shown in FIG. 1, the pair of first die parts 1A and 1B
includes a clearance portion. A clearance portion is provided in at
least one of the two surfaces of the pair of die parts that face each other
25 in the direction of pressing. A clearance portion is a recess in a die
part, recessed inwardly. With one pair of die parts (i.e., first die parts
1A and 1B in the implementation of FIG. 1) including a clearance
portion, the other pair of die parts (i.e., second die parts 2A and 2B in
the implementation of FIG. 1) includes an abutment surface 2At. The
30 abutment surface 2At is constituted by at least part of the portion of a
surface of the other pair of die parts that corresponds to the clearance
portion of the one pair of die parts. The abutment surface 2At abuts
the metal sheet when the die is at the bottom-dead center. Thus, the
first die parts 1A and 1B are different from the second die parts 2A and
35 2B in the construction of the clearance-related portions. Except for the
16
clearance-related portions, the forming surfaces of the first die parts 1A
and 1B have the same constructions as the forming surfaces of the
second die parts 2A and 2B.
[0043] The implementation shown in FIG. 1 is an exemplary
implementation where the one pair of die parts with a 5 clearance portion
is constituted by the pair of first die parts 1A and 1B while the other
pair of die parts is constituted by the pair of second die parts 2A and 2B.
In other implementations, conversely, the pair of second die parts 2A
and 2B may constitute the one pair of die parts with a clearance portion
10 while the pair of first die parts 1A and 1B may constitute the other pair
of die parts with an abutment surface.
[0044] The control unit 5 controls the hot press line 100. The control
unit 5 may be configured to control at least one of the heating device 30,
first transportation device 41, first press device 10, second
15 transportation device 42 and second press device 20. The control unit
5 may be constituted by one or more computers including a processor
and memory.
[0045] The processor of the control unit 5 executes a program stored on
the memory to implement the function of supplying control information
20 to at least one of the heating device 30, first transportation device 41,
first press device 10, second transportation device 42 and second press
device 20 (i.e., device to be controlled). By way of example, based on
input from the outside and/or data stored in advance on the memory,
the control unit 5 decides on times where the device to be controlled is
25 operated and amounts of operation (or directions of operation), and
determines the control information necessary for the relevant
movements. The control unit 5 outputs the control information to the
device to be controlled.
[0046] In the hot press line 100, the metal sheet B heated by the
30 heating device 30 is press formed by the first press device 10 and held
by the first die parts 1A and 1B at the bottom-dead center. Thus, the
metal sheet B, while maintaining the shape resulting from the press
forming, is gripped by the die parts and rapidly cooled. The portions of
the surfaces of the metal sheet B that correspond to the clearance
35 portion 1Ac of the first die parts 1A and 1B at the bottom-dead center
17
provide a non-abutment region that does not contact the die. The
non-abutment region of the metal sheet B is gradually cooled. The
non-abutment region has different cooling conditions from those of the
other regions. In the second press device 20, the metal sheet B that
has been press formed by the first press device 10 is 5 held between the
second die parts 2A and 2B at the bottom-dead center. Thus, the
formed metal sheet B is gripped by the second die parts 2A and 2B and
cooled rapidly. At this time, the second die parts 2A and 2B abut at
least part of the non-abutment region, too. This rapidly cools the
10 non-abutment region. The formed metal sheet B is cooled and
quenched for the sum of the bottom-dead-center holding period of the
first die parts 1A and 1B and the bottom-dead-center holding period of
the second die parts 2A and 2B, i.e., the total bottom-dead-center
holding period.
15 [0047] In the implementation shown in FIG. 1, the pair of first die
parts 1A and 1B of the first press device 10, on the one hand, and the
pair of second die parts 2A and 2B of the second press device, on the
other hand, are configured to operate independently from each other.
More specifically, the first press device 10 includes a pair of supports
20 (e.g., slides and bolsters, not shown) that support the respective first die
parts 1A and 1B, and an actuator (not shown) that moves at least one of
these supports in the direction of pressing. The second press device 20,
independently from the first press device 10, includes a pair of supports
that support the respective second die parts 2A and 2B and an actuator
25 that moves at least one of these supports.
[0048] The first and second press devices 10 and 20 are not limited to
this arrangement. For example, the first die parts 1A and 1B and the
second die parts 2A and 2B may share supports. More specifically, it is
possible to provide a common support (for example, slider) that
30 supports one first die part 1A and one second die part 2A, a common
support (for example, bolster) that supports the other first die part 1B
and the other second die part 2B, and a common actuator that moves at
least one of these supports. In such implementations, the first and
second press devices 10 and 20 are constructed to share supports and
35 an actuator. By way of example, the first and second press devices 10
18
and 20 may be constituted by a single press device that performs
transfer pressing with first die parts 1A and 1B and second die parts 2A
and 2B.
[0049] (Exemplary Construction of First Press Device)
FIG. 2 is a cross-sectional view of the first 5 press device 10
shown in FIG. 1, illustrating its construction. FIG. 3 illustrates the
first press device 10 shown in FIG. 2 as being at its bottom-dead center.
In the implementation shown in FIGS. 2 and 3, the first press device 10
includes a die block 1B and a punch 1A that exemplify the pair of first
10 die parts 1A and 1B. The die block 1B is movable relative to the punch
1A in the direction of pressing PD. That is, the die block 1B and punch
1A are movable relative to each other. The directions of such relative
movements are referred to as direction of pressing.
[0050] The die block 1B is movable by a lift mechanism (i.e., actuator)
15 81 in the direction of pressing relative to the punch 1A. The lift
mechanism 81 may include, for example, a hydraulic cylinder, air
cylinder, air cushion or cam. In the present implementation, the die
block 1B moves relative to the punch 1A; in some arrangements, the
punch 1A may move relative to the die block 1B. In other
20 arrangements, both the die block 1B and punch 1A may move.
[0051] The control unit 5 controls the die block 1B and punch 1A. In
the implementation shown in FIGS. 2 and 3, the control unit 5 controls
the lift mechanism 8 for the die block 1B to control relative movement
of the die block 1B and punch 1A. The control unit 5 supplies the lift
25 mechanisms (i.e., actuators) 8 and 7 with control signals to control such
driving.
[0052] The first press device 10 press forms the metal sheet B by
positioning the metal sheet B between the die block 1B and punch 1A
and pushing the metal sheet B with both the die block 1B and punch 1A.
30 The die block 1B has, in its interior, a recessed shape that corresponds
to the shape of the product to be press shaped. The punch 1A has a
protruding shape that corresponds to the recessed shape of the die block
1B.
[0053] The surface of the die block 1B facing the punch 1A includes a
35 pressing surface 1Bu that contacts and presses the metal sheet B. The
19
die block 1B includes recessed portions, i.e., clearance portions 1Bc, in
the surface thereof that faces the punch 1A. The clearance portions
1Bc do not abut the metal sheet B even when the die is at the
bottom-dead center. That is, when the die is at the bottom-dead center,
the clearance portions 1Bc form a clearance with the 5 metal sheet B. A
portion of the surface of the metal sheet B held by the die at the
bottom-dead center that corresponds to each clearance portion 1Bc
constitutes a non-abutment region.
[0054] The surface of the punch 1A facing the die block 1B includes a
10 pressing surface 1Au that contacts and presses the metal sheet B. The
punch 1A includes clearance portions 1Ac positioned to face the
clearance portions 1Bc of the die block 1B. Each clearance portion 1Bc
of the die block 1B and the associated clearance portion 1Ac of the
punch 1A are positioned to face each other. As viewed in the direction
15 of pressing, at least part of the clearance portion 1Ac of the punch 1A
overlaps the clearance portion 1Bc of the die block 1B.
[0055] As shown in FIG. 3, when the die is at the bottom-dead center,
the surfaces of the metal sheet B abut the pressing surface 1Bu of the
die block 1B and the pressing surface 1Au of the punch 1A. At the
20 clearance portions 1Bc and 1Ac, the surfaces of the metal sheet B do not
abut the die. A portion of a surface of the metal sheet B that
corresponds to a clearance portion 1Bc, 1Ac constitutes a non-abutment
region Bc. In the implementation shown in FIG. 3, the clearance
portions 1Bc and 1Ac are positioned to face each other, and thus
25 non-abutment regions Bc are produced in the opposite regions of both
sides of the metal sheet B.
[0056] Beginning with a state where the heated metal sheet B is
positioned between the die block 1B and punch 1A as separated from
each other, the control unit 5 causes the die block 1B and punch 1A to
30 move closer to each other in the direction of pressing until they reach
the bottom-dead center. The metal sheet B is thus press formed.
Thereafter, the control unit 5 holds the die block 1B and punch 1A at
the bottom-dead center. Thus, during the bottom-dead-center holding
period of the first press device 10, the portions of the formed metal
35 sheet B that are in contact with the die block 1B and punch 1A are
20
rapidly cooled and hardened. The non-abutment regions Bc of the
metal sheet B at the clearance portions 1Bc and 1Ac are gradually
cooled.
[0057] In the implementation shown in FIG. 2, each of the die parts 1A
and 1B of the first press device 10 includes a tube 5 11 that works as a
channel for allowing a cooling medium to pass therethrough. The tube
11 exemplifies the cooling device. The tube 11 is constituted by, for
example, a through-hole in the die part 1A, 1B. The amount of cooling
medium flowing through the tube 11 is controlled by a valve 21, for
10 example. The channel is not limited to a tube 11, and may be a groove
in the surface of the die part 1A, 1B, for example. The cooling medium
flowing through the channel cools the die part 1A, 1B. Such cooling
keeps the die part 1A, 1B not higher than the Mf point (about 300 °C),
for example. The cooling devices are not shown in the other drawings
15 showing the die parts 1A and 1B.
[0058] (Exemplary Construction of Second Press Device)
FIG. 4 is a cross-sectional view of the second press device 20
shown in FIG. 1, illustrating its construction. FIG. 5 illustrates the
second press device 20 shown in FIG. 4 as being at its bottom-dead
20 center. In the implementation shown in FIGS. 4 and 5, the second
press device 20 includes a die block 2Bdie block 2B and a punch 2A that
exemplify the pair of second die parts 2A and 2B. The die block 2Bdie
block 2B is movable relative to the punch 2A in the direction of pressing
PD.
25 [0059] The die block 2B has the same shape as the die block 1B of the
first press device 10 except for the clearance portions 1Bc. The punch
2A has the same shape as the punch 1A of the first press device 10
except for the clearance portions 1Ac. The lift mechanism (i.e.,
actuator) 82 that moves the die block 2B and punch 2A relative to each
30 other and the control unit 5 may have the same configurations as those
of the first press device 10.
[0060] The surface of the die block 2B facing the punch 2A includes a
pressing surface that contacts and presses the metal sheet B. The
pressing surface of the die block 2B includes abutment surfaces 2Bt
35 that correspond to the clearance portions 1Bc of the first press device 10.
21
The abutment surfaces 2Bt abut the metal sheet B when the die is at
the bottom-dead center. That is, when the die is at the bottom-dead
center, the non-abutment regions Bc of the metal sheet B are positioned
at those locations on the die block 2B of the second press device 20
which correspond to the clearance 5 portions 1Bc.
[0061] The surface of the punch 2A facing the die block 2B includes a
pressing surface that contacts and presses the metal sheet B. The
pressing surface of the punch 2A includes abutment surfaces 2At that
correspond to the clearance portions 1Ac of the first press device 10.
10 The abutment surfaces 2At abut the metal sheet B when the die is at
the bottom-dead center. When the die is at the bottom-dead center, the
non-abutment regions Bc of the metal sheet B are positioned at those
locations on the punch 2A of the second press device 20 which
correspond to the clearance portions 1Ac.
15 [0062] As shown in FIG. 5, when the die is at the bottom-dead center,
the surfaces of the metal sheet B abut the pressing surface of the die
block 2B and the pressing surface of the punch 2A. The pressing
surfaces also include the abutment surfaces 2Bt and 2At corresponding
to the clearance portions 1Bc and 1Ac. The non-abutment regions Bc
20 of the metal sheet B, which did not abut the die in the first press device
10 at the bottom-dead center, are now abutted by the die block 2B and
punch 2A. In the implementation shown in FIG. 5, the die, i.e., die
block 2B and punch 2A, abuts both non-abutment regions of the metal
sheet B in the opposite regions of both sides.
25 [0063] Beginning with a state where the metal sheet B formed by the
first press device 10 is positioned between the die block 2B and punch
2A as separated from each other, the control unit 5 causes the die block
2B and punch 2A to move closer to each other in the direction of
pressing until they reach the bottom-dead center. Thereafter, the
30 control unit 5 holds the die block 2B and punch 2A at the bottom-dead
center. Thus, during the bottom-dead-center holding period of the
second press device 20, the portions of the formed metal sheet B that
are in contact with the die block 2B and punch 2A are rapidly cooled
and hardened.
35 [0064] In the implementation shown in FIG. 4, each of the die parts 2A
22
and 2B of the second press device 20 includes a tube 12 that works as a
channel for allowing a cooling medium to pass therethrough. The tube
12 exemplifies the cooling device. The tube 12 is constituted by, for
example, a through-hole in the die part 2A, 2B. The amount of cooling
medium flowing through the tube 12 is controlled 5 by a valve 22, for
example. The channel is not limited to a tube 22, and may be a groove
in the surface of the die part 2A, 2B, for example. The cooling medium
flowing through the channel cools the die part 2A, 2B. Such cooling
keeps the die part 2A, 2B not higher than the Mf point (about 300 °C),
10 for example. The cooling devices are not shown in the other drawings
showing the die parts 2A and 2B.
[0065] In the implementation shown in FIGS. 2 to 5, the metal sheet B,
having an as-formed shape, is gripped by a die and cooled in the sum of
the bottom-dead-center holding period of the first press device 10
15 (hereinafter referred to as first bottom-dead-center holding period) and
the bottom-dead-center holding period of the second press device 20
(hereinafter referred to as second bottom-dead-center holding period),
i.e., total bottom-dead-center holding period. In the total
bottom-dead-center holding period, the portions of the of the metal
20 sheet B that correspond to the clearance portions 1Ac and 1Bc, i.e.,
non-abutment regions Bc, experience a non-abutment period and an
abutment period. Thus, the metal sheet B is locally gradually cooled,
that is, the portions defined by the non-abutment regions Bc are
gradually cooled and have different cooling conditions from those of the
25 other portions. Thus, the properties of the portions of the metal sheet
B defined by the non-abutment regions Bc are different from the
properties of the other portions. Further, for the portions of the metal
sheet B defined by the non-abutment regions Bc, the total
bottom-dead-center holding period includes a non-abutment period with
30 gradual cooling and an abutment period with rapid cooling; as such,
while these portions are gradually cooled, their temperature decreases
to some degree as they are gripped by the die. This reduces the
difference between the temperature of the portions defined by the
non-abutment regions Bc and those of the other portions i.e. portions
35 that abut a die and are rapidly cooled in the total bottom-dead-center
23
holding period. This makes it easier to ensure shape accuracy.
[0066] (Exemplary Manufacturing Process)
Now, an exemplary process of manufacturing a hot press-formed
product using the hot press line 100 will be described. First, a
material, i.e., a metal sheet B, is heated by the heating 5 device 30. The
metal sheet B may be, for example, a flat sheet, or may be an
intermediate formed product that has been press formed. By way of
example, the metal sheet B is a steel sheet. At the heating step, the
metal sheet B is heated to the Ac3 point or above to austenitize the
10 metallic microstructure. The heated metal sheet B is transported by
the first transportation device 41 and positioned between the die block
1B and punch 1A of the first press device 10.
[0067] In the first press device 10, the heated metal sheet B is
positioned between the die block 1B and punch 1A, and at least one of
15 the die block 1B and punch 1A is moved to the bottom-dead center.
The metal sheet B is thus hot press formed. The formed metal sheet B
is held between the die block 1B and punch 1A at the bottom-dead
center. During this first bottom-dead-center holding period, the metal
sheet B in contact with the die block 1B and punch 1A is rapidly cooled.
20 Some portions of the die of the first press device 10 provide clearance
portions, constituted by clearance portions 1Bc recessed in the die block
1B and clearance portions 1Ac recessed in the punch 1A. When the die
is at the bottom-dead center, the metal sheet B does not abut the
clearance portions 1Ac and 1Bc. Thus, the portions of the metal sheet
25 B corresponding to the clearance portions 1Ac and 1Bc, i.e., the portions
defined by the non-abutment regions Bc, are cooled at a lower rate than
the portions that are in contact with the die block 1B and punch 1A.
This achieves gradual cooling of some portions of the metal sheet B.
[0068] Upon completion of the first bottom-dead-center holding period,
30 the formed metal sheet B is positioned by the second transportation
device 42 between the die block 2B and punch 2A of the second press
device 20. The second press device 20 moves at least one of the die
block 2B and punch 2A to the bottom-dead center. The formed metal
sheet B is held between the die block 2B and punch 2A at the
35 bottom-dead center. The die block 2B and punch 2A have no clearance
24
portions. Thus, both sides of the entire metal sheet B abut the die.
During this second bottom-dead-center holding period, the metal sheet
B in contact with the die block 2B and punch 2A are rapidly cooled.
[0069] The non-abutment regions Bc of the metal sheet B, which were
gradually cooled in the first press device 10, now abut 5 the second die
parts of the second press device 20, i.e., die block 2B and punch 2A,
during the second bottom-dead-center holding period. During the
second bottom-dead-center holding period, the non-abutment regions Bc
of the metal sheet B are rapidly cooled. This achieves a
10 close-to-uniform temperature distribution of the metal sheet B upon
completion of the second bottom-dead-center holding period.
[0070] Upon completion of the second bottom-dead-center holding
period, the formed metal sheet B (i.e., formed product) is removed from
the die (i.e., die block 2B and punch 2A). The formed product thus
15 obtained has been provided with a strength distribution, and has good
shape accuracy.
[0071] Details of the mechanism with which a strength distribution is
provided are as follows: there are three types of cooling of portions of
the metal sheet B being hot press formed by the first press device 10
20 that correspond to the clearance portions 1Bc and 1Ac of the first die
parts (i.e., die block 1B and punch 1A), i.e., portions defined by the
non-abutment regions Bc, namely: (1) heat conduction within the metal
sheet B; (2) heat conduction from the metal sheet B to the atmosphere;
and (3) radiation from the metal sheet B to the die. As such, the
25 cooling rates at the clearance portions are lower than those from heat
conduction from the metal sheet B to the die due to the sheet’s
abutment with the die. If the cooling rate from austenite is lower than
the critical cooling rate which depends on the steel sheet serving as a
material, diffusion transformation occurs within the steel, producing a
30 soft metallic microstructure of ferrite and/or bainite, for example. On
the other hand, the portions in contact with the die experience
non-diffusion transformation, producing a hard metallic microstructure
mainly composed of martensite. That is, reducing cooling rate for
some portions of the metal sheet enables manufacturing of a
35 press-formed product with some softened portions.
25
[0072] If there are large temperature differences within the formed
metal sheet (i.e., formed product) when the formed product is removed
from the die, thermal contraction may cause the formed product to
deform, leading to defects of shape. In contrast, according to the
present embodiment, the second die parts (i.e., die 5 block 2B and punch
2A) abut the non-abutment regions Bc of the metal sheet B during the
second bottom-dead-center holding period of the second press device 20.
This results in a close-to-uniform temperature, rather than
temperature differences, in the formed product upon completion of the
10 second bottom-dead-center holding period. This makes it easier to
ensure the shape accuracy of the entire formed product. Further,
during the abutment period in the second bottom-dead-center holding
period, the metal sheet is cooled while being gripped by the die. This
makes it easier to ensure shape accuracy, due to the portions gripped by
15 the die, than in arrangements where the metal sheet is not gripped
throughout the bottom-dead-center holding period.
[0073] In the above-described implementation, some portions of a die
are caused to be separated from the metal sheet B in an early stage of
the total bottom-dead-center holding period, i.e., during the first
20 bottom-dead-center holding period, to provide non-abutment regions
and, thereafter, a die is caused to abut the non-abutment regions of the
metal sheet B in a late stage of the total bottom-dead-center holding
period, i.e., during the second bottom-dead-center holding period.
[0074] In the above-described implementation, the non-abutment
25 regions of the metal sheet B do not abut a die in an early stage of the
total bottom-dead-center holding period, and abut a die in a late stage.
In other implementations, conversely, the non-abutment regions of the
metal sheet B may abut a die in an early stage of the total
bottom-dead-center holding period and not abut a die in a late stage.
30 FIG. 6 shows such a variation of the first and second die parts, modified
in construction. In the implementation shown in FIG. 6, the first die
parts (i.e., die block 1B and punch 1A) of the first press device 10 have
no clearance portions. The second die parts (i.e., die block 2B and
punch 2A) of the second press device 20 include clearance portions 2Bc
35 and 2Ac. The portions of the surfaces of the metal sheet B that do not
26
abut a die when the second press device 20 is at the bottom-dead center,
i.e., non-abutment regions Bc, abut the abutment surfaces 1At and 1Bt
of the die when the first press direction 10 is at the bottom-dead center.
That is, in the total bottom-dead-center holding period, the
non-abutment regions Bc of the metal sheet B 5 abut a die and are
rapidly cooled during the first bottom-dead-center holding period, and
the non-abutment regions Bc of the metal sheet B do not abut a die and
are gradually cooled during the second bottom-dead-center holding
period. In such implementations, too, the properties of the portions of
10 the formed metal sheet B defined by the non-abutment regions Bc are
different from those of the other portions. Further, a close-to-uniform
temperature, rather than temperature differences, in the formed
product results upon completion of the second bottom-dead-center
holding period.
15 [0075] FIG. 7 is a graph illustrating an implementation where an
abutment period is provided in the first bottom-dead-center holding
period and a non-abutment period is provided in the second
bottom-dead-center holding period. FIG. 7 illustrates an exemplary
implementation where press forming is performed by the first die parts
20 and, then, by the second die parts, both shown in FIG. 6. In FIG. 7,
line L1 indicates the temperature of the portions of the metal sheet B
being press formed that correspond to the clearance portions. Line L2
indicates the temperature of the portions of the metal sheet B that abut
a die throughout the bottom-dead-center holding period (i.e., other
25 portions).
[0076] In the implementation shown in FIG. 7, from the beginning of
the first bottom-dead-center holding period, the first die parts which
have no clearance portions abut the metal sheet B. At this time, the
clearance CL between the die and the metal sheet B is 0 mm. During
30 the first bottom-dead-center holding period, the metal sheet B is rapidly
cooled. Upon completion of the first bottom-dead-center holding period,
the metal sheet B is removed from the first die, transported, and then
positioned between the second die parts. The second die parts press
form the metal sheet B and, after reaching the bottom-dead center, hold
35 the metal sheet B while being at the bottom-dead center. The second
27
die parts include clearance portions. When the die is at the
bottom-dead center, the portions of the metal sheet B that correspond to
the clearance portions of the second die parts do not abut the die. That
is, the portions of the metal sheet B that correspond to the clearance
portions are separated from the second die parts 5 during the second
bottom-dead-center holding period, and remain separated even at the
end of the second bottom-dead-center holding period. Thus, the
portions of the metal sheet B corresponding to the clearance portions,
i.e., portions defined by the non-abutment regions, do not abut a die
10 after completion of the first bottom-dead-center holding period i.e.
abutment period. The period of time after the abutment period until
the second die parts cease to be at the bottom-dead center constitutes
the non-abutment period. During the second bottom-dead-center
holding period, the portions of the metal sheet B defined by the
15 non-abutment regions experience a lower cooling rate than the other
portions, and are gradually cooled.
[0077] Thus, the first and second bottom-dead-center holding periods
include an abutment period with rapid cooling and a non-abutment
period with gradual cooling of some portions of the metal sheet. This
20 reduces the difference between the temperature of the gradually cooled
portions of the metal sheet B and that of the other portions. This
makes it easier to ensure the shape accuracy of the entire formed
product. Further, since the gradually cooled portions are gripped by a
die during the first bottom-dead-center holding period, it is easier to
25 ensure the shape accuracy of the gradually cooled portions.
[0078] In the implementation shown in FIG. 7, during the first
bottom-dead-center holding period i.e. abutment period, the portions of
the metal sheet B defined by the non-abutment regions are rapidly
cooled, as indicated by line L1, and then separated from the first die
30 parts before the Ms point (martensitic transformation starting point) is
reached, and gradual cooling begins. Thereafter, the portions of the
metal sheet B defined by the non-abutment regions abut no die and are
thus gradually cooled during transportation, press forming by the
second die parts and second bottom-dead-center holding. This
35 produces a soft metallic microstructure. On the other hand, as
28
indicated by line L2, the other portions of the metal sheet B other than
the non-abutment regions Bc abut a die and are thus rapidly cooled
during both the first and second bottom-dead-center holding periods.
These other portions are cooled to the Mf point (martensitic
transformation finishing point) and below 5 during the second
bottom-dead-center holding period. This produces a hard metallic
microstructure mainly composed of martensite. Thus, the portions of
the formed metal sheet B corresponding to the clearance portions, on
the one hand, and the other portions, on the other, have different
10 properties (i.e., strengths in the present implementation).
[0079] In the implementation of FIG. 7, the portions of interest of the
metal sheet B are in the abutment period and thus rapidly cooled
during the first bottom-dead-center holding period, i.e., period where
the temperature is relatively high. Since the metal sheet B is gripped
15 by a die in a period where the metal sheet B has a high temperature
and is soft, it is easier to ensure shape accuracy, due to the gripped
portions.
[0080] FIG. 8 is a graph illustrating an implementation where a
non-abutment period is provided in the first bottom-dead-center
20 holding period and an abutment period is provided in the second
bottom-dead-center holding period. FIG. 8 illustrates an
implementation where press forming is performed by the first die parts
shown in FIG. 2 and, then, the second die parts shown in FIG. 4. In
FIG. 8, line L3 indicates the temperature of the portions of the metal
25 sheet B being press formed that correspond to the clearance portions.
Line L2 indicates the temperature of portions of the metal sheet B that
abut a die throughout the bottom-dead-center holding period (i.e., other
portions). In the implementation shown in FIG. 8, the first die parts
include clearance portions. At the beginning of the first
30 bottom-dead-center holding period, the clearance portions of the first
die parts are separated from the metal sheet B. During the first
bottom-dead-center holding period, the portions of the metal sheet B
corresponding to the clearance portions, i.e., the portions defined by the
non-abutment regions Bc, do not about the first die parts and are
35 separated therefrom. Upon completion of the first bottom-dead-center
29
holding period, the formed metal sheet B is removed from the first die
parts, transported, and positioned between the second die parts. The
second die parts press form the metal sheet B and, at the bottom-dead
center, hold the sheet. During the second bottom-dead-center holding
period, the portions of the metal sheet B corresponding 5 to the clearance
portions abut the second die parts. Thus, in the implementation
shown in FIG. 8, there is a non-abutment period from the beginning of
the first bottom-dead-center holding period. The transportation period
is also part of the non-abutment period. The period of time from the
10 beginning of the second bottom-dead-center holding period until the end
of the second bottom-dead-center holding period constitutes the
abutment period.
[0081] Thus, the first and second bottom-dead-center holding periods
include a non-abutment period with gradual cooling and an abutment
15 period with rapid cooling of some portions of the formed metal sheet B.
This reduces the difference between the temperature of the gradually
cooled portions of the metal sheet B and that of the other portions.
This makes it easier to ensure the shape accuracy of the entire formed
product. Further, since the gradually cooled portions are gripped by a
20 die during the second bottom-dead-center holding period, it is easier to
ensure the shape accuracy of the gradually cooled portions.
[0082] In the implementation shown in FIG. 8, for the portions of the
metal sheet B of the non-abutment regions Bc, the abutment period, i.e.,
second bottom-dead-center holding period, ends before the temperature
25 drops to the Ms point, as indicated by line L3. This produces a soft
metallic microstructure. On the other hand, as indicated by line L2,
the other portions of the metal sheet B other than the non-abutment
regions Bc are rapidly cooled during the first and second
bottom-dead-center holding periods, and thus cooled such that
30 temperature drops to the Mf point or below at the end of the second
bottom-dead-center holding period. This produces a hard metallic
microstructure mainly composed of martensite. Thus, the portions of
the formed metal sheet B corresponding to the clearance portions, on
the one hand, and the other portions, on the other, have different
35 properties (e.g., strengths).
30
[0083] In the implementation of FIG. 8, the portions of the metal sheet
B corresponding to the clearance portions are in the abutment period
and are rapidly cooled during the second bottom-dead-center holding
period, i.e., a period where the temperature is relatively low and cooling
rate has become low. In this implementation, 5 the temperature
difference caused by rapid cooling is small, making temperature control
easier. Further, the metal sheet B is gripped and rapidly cooled when
its temperature has decreased and the sheet has become somewhat
hard, which makes it yet easier to ensure shape accuracy.
10 [0084] The abutment period and non-abutment period of the
bottom-dead-center holding period are not limited to these exemplary
implementations. For example, there may be two or more separate
abutment periods in the bottom-dead-center holding period. By way of
example, an abutment period may be provided in each of an early stage
15 and a late stage of the bottom-dead-center holding period, and a
non-abutment period may be provided in an intermediate period
between the early and late stages. For example, after the first die
parts having no clearance portions press form the sheet and hold it
while being at the bottom-dead center, the second die parts having
20 clearance portions may hold the sheet while being at the bottom-dead
center and then, again, the first die parts (or third die parts with no
clearance portions) may hold the sheet while being at the bottom-dead
center.
[0085] Although not limiting, the length of the total
25 bottom-dead-center holding period, i.e. sum of the first and second
bottom-dead-center holding periods, may be 2 to 90 seconds, for
example. The longer the total bottom-dead-center holding period, the
better from the viewpoint of the uniformity of the temperature
distribution of the formed product upon completion of the
30 bottom-dead-center holding period; on the other hand, the shorter the
bottom-dead-center holding period, the better from the viewpoint of
manufacture efficiency. In view of this, a lower limit for the total
bottom-dead-center holding period is preferably 10 seconds, and more
preferably 15 seconds. An upper limit for the total bottom-dead-center
35 holding period is preferably 90 seconds, and more preferably 30 seconds.
31
In the present embodiment, the total bottom-dead-center holding period
includes an abutment period and a non-abutment period; as such, a
uniform temperature distribution in the formed product after
completion of the total bottom-dead center holding period can easily be
achieved even when the total bottom-dead-center holding 5 period is not
longer than 30 seconds, for example.
[0086] In the implementations shown in FIGS. 7 and 8, the portions of
the sheet B other than the non-abutment region Bc are cooled to the Mf
point and below in the total bottom-dead-center holding period. This
10 enables quenching. All of the die parts 1A and 1B of the first press
device 10 and the die parts 2A and 2B of the second press device 20 can
be held at temperatures not higher than the Mf point by the cooling
devices.
[0087] The clearance of the clearance portions 1Ac and 1Bc of the first
15 die parts 1A and 1B, or the clearance portions 2Ac and 2Bc of the second
die parts 2A and 2B, i.e., distance between a die and the metal sheet is
not limited to any particular value; for example, it may be not smaller
than 2 mm, preferably not smaller than 4 mm, and more preferably not
smaller than 6 mm.
20 [0088] (Variations of Die Parts)
FIG. 9 shows a variation of the first and second die parts,
modified in construction. In the implementation shown in FIG. 9, both
the first die parts and the second die parts include clearance portions
and abutment surfaces. A non-abutment region Bc of the metal sheet
25 B corresponding to the pair of clearance portions 1Bc and 1Ac of the die
block 1B and punch 1A constituting the first die parts is abutted by the
abutment surfaces 2Bt and 2At of the die block 2B and punch 2A
constituting the second die parts at the bottom-dead center. A
non-abutment region Bc of the metal sheet B corresponding to the pair
30 of clearance portions 2Ac and 2Bc constituting the second die parts is
abutted by the abutment surfaces 1Bt and 1At of the die block 1B and
punch 1A constituting the first die parts.
[0089] As shown in FIG. 9, at least part of the portions of the second
die parts corresponding to the clearance portions of the first die parts
35 may include abutment surfaces that abut the metal sheet while being at
32
the bottom-dead center, and at least part of the portions of the first die
parts corresponding to the clearance portions of the second die parts
may include abutment surfaces that abut the metal sheet while being at
the bottom-dead center. In this implementation, the metal sheet B
includes both a portion that abuts a die 5 during the first
bottom-dead-center holding period and does not abut a die during the
second bottom-dead-center holding period and a portion that does not
abut a die during the first bottom-dead-center holding period and abuts
a die during a second bottom-dead-center holding period.
10 [0090] FIG. 10 shows another variation of the first and second die parts,
modified in construction. In the implementation shown in FIG. 10, out
of a pair of first die parts 1A and 1B, one die part 1B includes a
clearance portion 1Bc constituted by a recess. The portion of the other
die part 1A facing the clearance portion 1Bc has no clearance portion,
15 i.e., recess, and is part of the pressing surface 1Au that abuts the metal
sheet B. In this implementation, a non-abutment region Bc of the
metal sheet B is created on the side associated with the clearance
portion 1Bc, and the backside, i.e. side opposite to the non-abutment
region Bc, has no non-abutment region Bc.
20 [0091] In the implementation of FIG. 10, the portion of the pair of the
second die parts 2A and 2B corresponding to the clearance portion 1Bc
of the first die part 1B, i.e., portion on which the non-abutment region
Bc of the metal sheet B is positioned, represents an abutment surface
2Bt with which the second die part 2B abuts the metal sheet. Thus, a
25 clearance portion, even if provided on one side only of the metal sheet B,
produces the effect of ensuring the shape accuracy of a formed product
provided with a property distribution. Starting from the
implementation shown in FIG. 10, the first die parts and the second die
parts may be reversed. That is, the second die parts may include a
30 clearance portion on one side of the metal sheet B, whereas the first die
parts may include an abutment surface at the position corresponding to
the clearance portion of the second die parts.
[0092] FIG. 11 shows yet another variation of the first and second die
parts, modified in construction. In the implementation shown in FIG.
35 11, out of a pair of first die parts 1A and 1B, one die part 1B includes a
33
clearance portion 1Bc constituted by a recess. The portion of the other
die part 1A facing the clearance portion 1Bc has no clearance, i.e.,
recess, and abuts the metal sheet B. The portion of the second die part
2B corresponding to the clearance portion 1Bc of the first die part 1B
represents an abutment surface 2Bt that abuts the metal 5 sheet B. The
portion of the second die part 2A facing the abutment surface 2Bt
provides a clearance portion 2Ac constituted by a recess. The portion
of the first die part 1A that corresponds to the clearance portion 2Ac of
the second die part 2A represents an abutment surface 1At that abuts
10 the metal sheet B. This construction also produces the effect of
ensuring the shape accuracy of a formed product provided with a
property distribution.
[0093] FIG. 12 shows still another variation of the first and second die
parts, modified in construction. In the implementation shown in FIG.
15 12, the first die parts 1A and 1B include clearance portions 1Ac and 1Bc.
In the second die parts 2A and 2B, part of each of the portions
corresponding to the clearance portions 1Ac and 1Bc of the first die
parts 1A and 1B provides a clearance portion 2Ac, 2Bc, and the other
portions abut the metal sheet B. In other words, in the second die
20 parts 2A and 2B, part of each of the portions on which the
non-abutment region Bc of the metal sheet B is positioned represents
an abutment surface, and the other portions provide clearance portions
2Ac and 2Bc. In this implementation, both the first die parts and
second die parts include clearance portions. The clearance portions of
25 the first die parts correspond in position to the clearance portions of the
second die, but have a different surface area. Thus, cooling conditions
can be set by modifying the areas of the clearance portions, for example.
[0094] FIG. 13 shows yet another variation of the first and second die
parts, modified in construction. In the implementation shown in FIG.
30 13, the first die parts 1A and 1B include clearance portions 1Ac and 1Bc.
The portions of the second die parts 2A and 2B corresponding to the
clearance portions 1Ac and 1Bc of the first die parts 1A and 1B
represent a pair of abutment surfaces 2Ap and 2Bp facing each other
that abut both sides of the metal sheet B. The pair of abutment
35 surfaces 2Ap and 2Bp are shaped to bend the metal sheet B in the
34
direction of pressing. One of the abutment surfaces, 2Ap, has a
protruding shape, and the other has a recessed shape corresponding to
the protruding shape. Thus, if each of the abutment surfaces 2Ap and
2Bp of the second die parts 2A and 2B has a recessed/protruding shape
protruding or recessed in the direction of pressing, it is 5 possible to form
the associated portion of the metal sheet B into a shape that
corresponds to that recessed/protruding shape.
[0095] Although not limiting, the hot press line and the method of
manufacturing the hot press-formed product according to the
10 embodiments may be applied to, for example, manufacturing of
structural members for vehicles. Structural members for vehicles are
often required to be provided with a strength distribution and provide
shape accuracy. The embodiments may suitably be applied to such
structural members for vehicles. For example, the hot press line
15 according to an embodiment may manufacture a structural member for
a vehicle constituted by a hot press-formed product (i.e., hot-stamped
member) having some portions, within a single part, that have been
softened in order to reduce the weight of the vehicle or achieve high
performance, for example. Examples of such structural members for
20 vehicles include high-strength center pillars having soft flanges or rear
side members or bumper beams in which softened portions are
positioned so as to control sharp-bend mode upon an impact.
[0096] (Examples)
B-pillar dies having clearance portions (hereinafter referred to
25 as clearance dies) and dies having no clearance portions, i.e.,
clearance-less dies, were fabricated and tested. A clearance-less die is
an example of a pair of first die parts, and had the same construction as
the first die parts 1A and 1B shown in FIG. 6. A clearance die is an
example of a pair of second die parts, and had the same construction as
30 the second die parts 2A and 2B shown in FIG. 6. The first die parts
had no clearance portions. The second die parts had clearance
portions at positions corresponding to the flanges of the B-pillar. A
clearance portion of the second die parts included a recessed portion
(i.e., gap) 2Bc in the die block 2B and a recessed portion 2Ac in the
35 punch 2A, facing that recessed portion. The portions of the metal
35
sheet B corresponding to the clearance portions of the second die parts
were not cooled by the die at the bottom-dead center and were thus
gradually cooled, producing a soft metallic microstructure.
[0097] For the testing, the metal sheet used was a hot-rolled sheet to
be hot stamped (hereinafter HS) (thickness: 2.6 mm). 5 The metal sheet
was heated for 5 minutes in a furnace set to 900 °C, and formed by the
first die parts and/or second die parts, held while the die was at the
bottom-dead center, removed from the die, and left to cool. The three
sets of clearance conditions applied while the sheet was held by the first
10 die parts and/or second die parts at the bottom-dead center, i.e.,
condition sets (a) to (c) shown in Table 1 below, were used.
[0098] [Table 1]
15 [0099] In Table 1, condition set (a) means that a die without a
clearance was used for press forming, and represents typical HS
conditions, where the entire surface of the metal sheet including the
flanges are in contact with the first die parts. In condition set (a), the
metal sheet was transported to the first die after heating, held by the
20 die parts at the bottom-dead center for 10 seconds, and then removed
from the die and left to cool. Condition set (b) means that second die
parts having clearance portions at positions corresponding to the
flanges were used to press forming. In condition set (b), the metal
sheet was transported to the second die parts after heating, held by the
25 die at the bottom-dead center for 10 seconds, and then removed from
the die and left to cool. The amount of clearance was constant
throughout the bottom-dead-center holding period. Upon completion
of the bottom-dead-center holding period, the sheet was removed from
the die while the portions corresponding to the flanges were still at high
30 temperatures. In condition set (c), the metal sheet was transported to
36
the first die parts after heating, removed from the die immediately after
the bottom-dead center was reached, transported to the second die
parts, held by the die at the bottom-dead center for 30 seconds, and
then removed from the die and left to cool.
[0100] The formed products after hot press forming 5 were evaluated
with respect to the hardness and shape accuracy of the flanges. Shape
accuracy was evaluated based on the twisting of the formed product and
the out-of-plane deformation of the flanges. The position on the
formed product of an example at which shape accuracy was evaluated is
10 shown in FIG. 14. The shape accuracy for each of condition sets (b)
and (c) was evaluated with respect to the data from condition set (a).
[0101] FIG. 15 is a graph showing the results of measurement of the
hardness distributions of the formed products. Compared with the
formed product from condition set (a), the formed products from
15 condition sets (b) and (c) had low hardnesses for the clearance portions.
Clearance portion means a portion of the formed product corresponding
to a clearance portion of a die. The results shown in FIG. 15
demonstrate the partial softening effect produced by the clearance
portions of the clearance die with one step and the clearance dies with
20 two steps.
[0102] FIG. 16 is a graph showing the results of measurement of the
torsion angles of the formed products. Torsion angle in the graph of
FIG. 16 indicates to what degree the torsion-evaluation cross section C1
was twisted relative to that in the formed product from condition set (a),
25 as found when the formed products from condition sets (a) to (c) were
aligned in position with respect to the torsion-alignment surface W1
shown in FIG. 14.
[0103] The results shown in FIG. 16 demonstrate that the formed
product formed by the clearance die of condition set (b) (with one step)
30 had larger twisting, i.e., lower shape accuracy, than condition set (a)
without a clearance. On the other hand, the formed product press
formed by the clearance dies with two steps of condition set (c) had a
torsion angle not higher than a half of that from condition set (b),
demonstrating an improvement in shape accuracy.
35 [0104] FIG. 17 is a graph showing the results of measurement of the
37
out-of-plane deformations of the formed products. An amount of
out-of-plane deformation shown in the graph of FIG. 17 indicates the
amount of deformation of the surface at the out-of-plane deformation
evaluation position F1 shown in FIG. 14 relative to that of the formed
product from condition set (a). The out-of-5 plane deformation
evaluation position F1 for condition sets (b) and (c) was a portion
including a position on a flange corresponding to a clearance portion in
a die. The examples shown in FIG. 17 demonstrate that the clearance
dies with two steps from condition set (c) also improved local shape
10 accuracy in the flanges corresponding to the clearance portions.
[0105] Although embodiments of the present invention have been
described, the above-described embodiments are merely illustrative
examples useful for carrying out the present invention. Thus, the
present invention is not limited to the above-described embodiments,
15 and the above-described embodiments, when carried out, may be
modified as appropriate without departing from the spirit of the
invention.
EXPLANATION OF CHARACTERS
20 [0106] 1A, 1B: first die parts
1Ac, 1Bc: clearance portions
1At, 1Bt: abutment surfaces
10: first press device
100: hot press line
25 2A, 2B: second die parts
2Ac, 2Bc: clearance portions
2At, 2Bt: abutment surfaces
20: second press device
30: heating device
30 41: first transportation device
42: second transportation device
5: control unit
B: metal sheet
Bc: non-abutment regions.

We claim:
1. A hot press line comprising:
a heating device adapted to heat a metal sheet;
a first press device including a pair of first 5 die parts movable
relative to each other in a direction of pressing and adapted to press
form the heated metal sheet by moving the first die parts closer to each
other in the direction of pressing and, at a bottom-dead center, hold the
metal sheet;
10 a second press device including a pair of second die parts
movable relative to each other in the direction of pressing and adapted,
at a bottom-dead center of the second die parts, to hold the metal sheet
press formed by the first press device;
a first transportation device adapted to transport the metal
15 sheet from the heating device to the first press device; and
a second transportation device adapted to transport the metal
sheet from the first press device to the second press device,
wherein at least one of the pair of first die parts and the pair of
second die parts includes a clearance portion recessed inwardly to
20 create a clearance with the metal sheet while the die parts are at the
bottom-dead center, and the other pair of die parts includes an
abutment surface located in at least part of a portion corresponding to
the clearance portion of the one pair of die parts and adapted to abut
the metal sheet while the die parts are at the bottom-dead center.
25
2. The hot press line according to claim 1, wherein the first die
parts include the clearance portion, and the second die parts include the
abutment surface in at least part of a portion corresponding to the
clearance portion of the first die parts.
30
3. The hot press line according to claim 1 or 2, wherein the
second die parts include the clearance portion, and the first die parts
include the abutment surface in at least part of a portion corresponding
to the clearance portion of the second die parts.
35
39
4. The hot press line according to any one of claims 1 to 3,
wherein the clearance portion of the one pair of die parts includes a pair
of clearance portions facing each other with the metal sheet positioned
therebetween, and the abutment surface of the other pair of die parts
includes a pair of abutment surfaces facing each other 5 with the metal
sheet positioned therebetween, the pair of abutment surfaces located in
at least part of portions corresponding to the pair of clearance portions
of the one pair of die parts.
10 5. The hot press line according to any one of claims 1 to 4,
wherein the abutment surface of the other pair of die parts includes a
pair of abutment surfaces facing each other in the direction of pressing,
and the pair of abutment surfaces are shaped to bend the metal sheet in
the direction of pressing.
15
6. The hot press line according to any one of claims 1 to 5,
wherein a die portion of the one pair of die parts facing the clearance
portion of the one pair is provided with the abutment surface for
abutting the metal sheet when the die parts are at the bottom-dead
20 center, and a die portion of the other pair of die parts facing the
abutment surface of the other pair is provided with the clearance
portion recessed inwardly for creating a clearance with the metal sheet
when the die parts are at the bottom-dead center.
25 7. A method of manufacturing a hot press-formed product,
comprising:
heating a metal sheet;
positioning the heated metal sheet between a pair of first die
parts of a first press device;
30 press forming the metal sheet by moving the first die parts
closer to each other in a direction of pressing;
a first bottom-dead-center holding step for holding the metal
sheet while the pair of first die parts are at a bottom-dead center;
after the first bottom-dead-center holding step, transporting the
35 press-formed metal sheet to a pair of second die parts of a second press
40
device and positioning the metal sheet therebetween; and
a second bottom-dead center holding step for holding the metal
sheet press formed by the first press device while the pair of second die
parts are at a bottom-dead center,
wherein, during one of the first bottom-dead-5 center holding step
and the second bottom-dead-center holding step, a surface of the metal
sheet has a non-abutment region that does not contact a die at a
bottom-dead center, and at least part of the non-abutment region
contacts a die at a bottom-dead center during the other
10 bottom-dead-center holding step.
8. The method of manufacturing a hot press-formed product
according to claim 7, wherein at least part of the non-abutment region
of the metal sheet for the first bottom-dead-center holding step abuts at
15 least one of the pair of second die parts during the second
bottom-dead-center holding step.
9. The method of manufacturing a hot press-formed product
according to claim 7 or 8, wherein at least part of the non-abutment
20 region of the metal sheet for the second bottom-dead-center holding
step abuts at least one of the pair of first die parts during the first
bottom-dead-center holding step.
10. The method of manufacturing a hot press-formed product
25 according to any one of claims 7 to 9, wherein the non-abutment region
of the metal sheet during the one bottom-dead-center holding step
includes a pair of regions, facing each other, of both sides of the metal
sheet, and at least part of each region of the pair of regions of the
non-abutment region contacts a die part at the bottom-dead center
30 during the other bottom-dead-center holding step.
11. The method of manufacturing a hot press-formed product
according to any one of claims 7 to 10, wherein, during the other
bottom-dead-center holding step, at least part of the non-abutment
35 region of the metal sheet for the one bottom-dead-center holding step
41
contacts a die part at the bottom-dead center and is formed to bend in
the direction of pressing.
12. The method of manufacturing a hot press-formed product
according to any one of claims 7 to 11, wherein, 5 during the one
bottom-dead-center holding step, at least part of a back side region of
the metal sheet for the non-abutment region is abutted by a die part at
the bottom-dead center, during the other bottom-dead-center holding
step, and at least part of the back side region for the non-abutment
10 region is not abutted by a die part.