TECHNICAL FIELD
[0001] The present invention relates to a hot press apparatus 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
apparatus 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 apparatus according to an embodiment of the
present invention includes: a first die part; a second die part capable of
moving relative to the first die part in a direction of pressing; and a
control unit adapted to control relative movement of 5 the first die part
and the second die part. At least one of the first die part and the
second die part includes a recess in a surface facing the other die part in
the direction of pressing. A movable die part is provided in the recess,
the movable die part movable in a direction crossing the surface facing
10 the other part. The control unit controls the movable die part such
that a heated and press formed metal sheet is held between the first die
part and the second die part and a bottom-dead-center holding period
for which the first die part and the second die part are at a bottom-dead
center includes an abutment period for which the movable die part
15 abuts the metal sheet and a non-abutment period for which the
movable die part does not abut the metal sheet.
EFFECTS OF THE INVENTION
[0009] The present disclosure ensures the shape accuracy of a formed
20 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
25 [0010] [FIG. 1] FIG. 1 shows a cross-sectional view of a press
apparatus according to an embodiment, illustrating an exemplary
construction.
[FIG. 2] FIG. 2 shows the press apparatus shown in FIG. 1 as
being at its bottom-dead center.
30 [FIG. 3] FIG. 3 illustrates how the movable die part 4 abuts the
metal sheet B while the die is at its bottom-dead center.
[FIG. 4] FIG. 4 is a graph illustrating an implementation where
an abutment period is provided in an early stage of the
bottom-dead-center holding period.
35 [FIG. 5] FIG. 5 is a graph illustrating an implementation where
5
an abutment period is provided in a late stage of the
bottom-dead-center holding period.
[FIG. 6] FIG. 6 shows a variation of the clearance portion,
modified in construction.
[FIG. 7] FIG. 7 shows another variation of the 5 clearance portion,
modified in construction.
[FIG. 8] FIG. 8 shows a variation of the movable die part 4.
[FIG. 9] FIG. 9 shows yet another variation of the clearance
portion, modified in construction.
10 [FIG. 10] FIG. 10 shows the position on the formed product of an
embodiment at which shape accuracy is evaluated.
[FIG. 11] FIG. 11 is a graph showing the results of measurement
of the hardness distributions of formed products.
[FIG. 12] FIG. 12 is a graph showing the results of
15 measurement of the torsion angles of formed products.
[FIG. 13] FIG. 13 is a graph showing the results of
measurement of the out-of-plane deformations of formed products.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
20 [0011] (Arrangement 1)
A hot press apparatus according to an embodiment of the
present invention includes: a first die part; a second die part capable of
moving relative to the first die part in a direction of pressing; and a
control unit adapted to control relative movement of the first die part
25 and the second die part. At least one of the first die part and the
second die part includes a recess in a surface facing the other die part in
the direction of pressing. A movable die part is provided in the recess,
the movable die part movable in a direction crossing the surface facing
the other part. The control unit controls a position of the movable die
30 part such that a heated and press formed metal sheet is held between
the first die part and the second die part and a bottom-dead-center
holding period for which the first die part and the second die part are at
a bottom-dead center includes an abutment period for which the
movable die part is at an abutting position where it abuts the metal
35 sheet and a non-abutment period for which the movable die part is at a
6
retracted position where it does not abut the metal sheet.
[0012] In Arrangement 1 described above, during the
bottom-dead-center holding period, cooling rate can be decreased, i.e.,
gradual cooling can be performed, for the non-abutment period for
which the movable die part does not abut the metal 5 sheet. Further,
cooling rate can be increased, i.e., rapid cooling can be performed, for
the abutment period in the bottom-dead-center holding time for which
the movable die part abuts the metal sheet. This will achieve a
close-to-uniform temperature distribution of the metal sheet while
10 some of the portions of the formed metal sheet contacted by the movable
die part have cooling conditions different from those of the other
portions. This provides the formed metal sheet removed from the die
with a property distribution derived from the differences in cooling
conditions and, at the same time, reduces a decrease in the shape
15 accuracy of the formed product due to temperature 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
20 clearance or by means of thermal conductivities of the die surface, the
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
25 from the die also depend on the construction of the die. Changing
these features requires adjusting the construction of the die or
re-fabricating a die. In contrast, in Arrangement 1 described above,
cooling conditions can be controlled by adjusting the length of the
abutment period in the bottom-dead-center holding period. Thus, in a
30 pressing process using a hot press apparatus, the cooling conditions
under which some portions of the formed metal sheet are gradually
cooled can be easily changed.
[0014] In Arrangement 1, the movable die part is constructed to be
movable between an abutting position and a retracted position.
35 During the bottom-dead-center holding period, the control unit switches
7
the movable die part between a state in which it is at the abutting
position and a state in which it is at the retracted position. When the
movable die part is at the abutting position, the abutment surface of the
movable die part against the metal sheet is at a position equivalent to
the forming surface. When the movable die part is 5 at the retracted
position, the movable die part is pulled deep into the recess, positioned
to be separated from the forming surface. The retracted position is set,
for example, to a position at which the metal sheet does not contact the
movable die part even when the portion of the metal sheet
10 corresponding to the recess experiences out-of-plane deformation while
the die is at the bottom-dead center. A retracted position is decided
upon depending on at least one of the surface area of the recess and the
sheet thickness of the metal sheet to be processed, for example.
[0015] (Arrangement 2)
15 Starting from Arrangement 1 described above, the control unit
may cause the movable die part to abut the metal sheet in an early
stage of the bottom-dead-center holding period and then cause the
movable die part to be separated from the metal sheet in a late stage of
the bottom-dead-center holding period. That is, the control unit may
20 move the movable die part from the abutting position to the retracted
position within the bottom-dead-center holding period. This enables
forming the metal sheet with the movable die part in a sub-period of the
bottom-dead-center holding period in which the metal sheet has a
relatively high temperature and is thus easy to form. This makes it
25 easier to ensure local shape accuracy, i.e., the shape accuracy of a
portion of the formed metal sheet that corresponds to the movable die
part.
[0016] (Arrangement 3)
Starting from Arrangement 1 described above, the control unit
30 may cause the movable die part to be separated from the metal sheet in
an early stage of the bottom-dead-center holding period and then cause
the movable die part to abut the metal sheet in a late stage of the
bottom-dead-center holding period. That is, the control unit may move
the movable die part from the retracted position to the abutting position
35 within the bottom-dead-center holding period. This makes it possible
8
to cause the movable die part to contact the metal sheet for rapid
cooling in a sub-period of the bottom-dead-center holding period in
which the temperature of the metal sheet is relatively low. This also
makes it easier to achieve a close-to-uniform temperature distribution
in the metal sheet. This makes it easier to ensure 5 the shape accuracy
of the entire formed metal sheet. Further, it facilitates adjustment of
cooling conditions by controlling the abutment period.
[0017] Starting from Arrangement 1 described above, the control unit
may cause the movable die part to abut the metal sheet in an early
10 stage of the bottom-dead-center holding period and then cause it to be
separated and then again cause the movable die part to abut the metal
sheet in a late stage of the bottom-dead-center holding period. That is,
the control unit may cause the movable die part to move from the
abutting position to the retracted position and then again cause it to
15 move to the abutting position within the bottom-dead-center holding
period. This makes it possible to rapidly cool the movable die part in a
sub-period of the bottom-dead-center holding period with a relatively
high temperature and a sub-period with a low temperature. This
facilitates control of shape accuracy.
20 [0018] The control unit may control the movable die part such that the
abutment period accounts for 10 to 90 % of the entire
bottom-dead-center holding period, for example. In such
implementations, the abutment period is preferably not longer than
80 % of the entire bottom-dead-center holding period, more preferably
25 not longer than 70 %, and yet more preferably not longer than 50 %.
In other words, the control unit may control the movable die part such
that the non-abutment period is not shorter than 10 % of the entire
bottom-dead-center holding period. This makes it easier to ensure
that the cooling rate of some portions of the metal sheet during the
30 bottom-dead-center holding period is different from that of the other
portions and some portions of the formed product have a different metal
structure composition from that of the other portions. Thus, the
control unit is capable of controlling the lengths of the abutment period
and non-abutment period within the bottom-dead-center holding
35 period.
9
[0019] (Arrangement 4)
Starting from any one of Arrangements 1 to 3 described above,
the recess may include a first recess provided in the first die part and a
second recess provided in the second die part and positioned to face the
first recess. In such implementations, the movable die 5 part is located
in at least one of the first recess and the second recess that face each
other. Thus, a clearance is present on each side of the metal sheet
during the non-abutment period for which the movable die part does
not abut the metal sheet. This increases the robustness of the cooling
10 conditions.
[0020] (Arrangement 5)
Starting from Arrangement 4 described above, the movable die
part may be provided in each of the first recess and the second recess
facing each other. This further increases the robustness of the cooling
15 conditions.
[0021] (Arrangement 6)
Starting from any one of Arrangements 1 to 5 described above, a
surface of the movable die part to be in contact with the metal sheet
when the movable die part abuts the metal sheet while the die parts are
20 at the bottom-dead center may be positioned to be flush with a die
surface surrounding the recess provided with the movable die part.
This provides flat portions of the formed metal sheet with a property
distribution.
[0022] (Arrangement 7)
25 Starting from any one of Arrangements 1 to 6 described above, a
top surface of the movable die part may include a protrusion adapted to
abut the metal sheet and a recess adapted not to abut the metal sheet
when the movable die part abuts the metal sheet while the die parts are
at the bottom-dead center. A clearance is created by the recess in the
30 movable die part even during the abutment period for which the
movable die part is in contact with the metal sheet. This enables
gradual cooling. Further, cooling conditions can be changed by
changing the shapes of the protrusion and recess on the top surface of
the movable die part. As used herein, top surface of the movable die
35 part is the surface of that one of the ends, as determined along the
10
direction of movement, of the movable die part in the recess which is
closer to the position at which the metal sheet is to be located.
[0023] (Arrangement 8)
Starting from any one of Arrangements 1 to 7 described above, a
top surface of one movable die part in one of the first 5 die part and the
second die part may include a recess or protrusion recessed or
protruding in the direction of pressing. A surface of the other die part
or movable die part facing the one movable die part in the direction of
pressing may have a shape corresponding to the recess or protrusion on
10 the top surface of the one movable die part. When the one movable die
part abuts the metal sheet while the die parts are at the bottom-dead
center, the surface of the other die part or movable die part facing the
one movable die part may be constructed to abut the metal sheet.
Thus, the metal sheet can be formed to a shape corresponding to the
15 recessed/protruding shape of the movable die part.
[0024] (Arrangement 9)
Starting from any one of Arrangements 1 to 8 described above,
the control unit may adjust a distance between the metal sheet and the
movable die part found when the movable die part is at the retracted
20 position. This makes it possible to set an appropriate retracted
position that makes it less likely that the metal sheet at the retracted
position and the movable die part contact each other. When the die is
at the bottom-dead center, a portion of the metal sheet corresponding to
the recess may experience out-of-plane deformation. The degree of
25 such out-of-plane deformation varies depending on the surface area of
the recess and the thickness of the metal sheet. In view of this, the
control unit may, for example, adjust the retracted position of the
movable die part depending on the thickness of the metal sheet to make
it less likely that, when the die is at the bottom-dead center, the metal
30 sheet with out-of-plane deformation and the movable die part at the
retracted position contact each other.
[0025] Starting from any one of Arrangements 1 to 9, the hot press
apparatus may include a cooling mechanism adapted to cool the first
die part and the second die part. For example, at least one of the first
35 and second die parts may include a tube or groove for allowing a cooling
11
medium to pass through.
[0026] (Manufacturing Method 1)
A method of manufacturing a hot press-formed product
according to an embodiment of the present invention includes:
positioning a heated metal sheet between a first die 5 part and a second
die part; press forming the metal sheet by moving the first die part and
the second die part closer to each other in a direction of pressing;
holding the metal sheet while the first die part and the second die part
are at a bottom-dead center; and moving, to the metal sheet, a movable
10 die part provided in a recess in at least one of the first die part and the
second die part during a bottom-dead-center holding period for which
the metal sheet is held at the bottom-dead center. The
bottom-dead-center holding period includes an abutment period for
which the movable die part is at an abutting position where it abuts the
15 metal sheet and a non-abutment period for which the movable die part
is at a retracted position where it does not abut the metal sheet.
[0027] In Manufacturing Method 1 described above, cooling rate can be
reduced during the non-abutment period in the bottom-dead-center
holding period. Further, cooling rate can be increased during the
20 abutment period in the bottom-dead-center holding period. This will
achieve a close-to-uniform temperature distribution of the metal sheet
while the portions of the formed metal sheet contacted by the movable
die part have different cooling conditions from those of the other
portions. This ensures the shape accuracy of a formed product
25 provided with a property distribution without prolonging the
bottom-dead-center holding time for the formed product in the die.
[0028] (Manufacturing Method 2)
Starting from Manufacturing Method 1 described above, the
movable die part may abut the metal sheet in an early stage of the
30 bottom-dead-center holding period and then the movable die part may
be separated from the metal sheet in a late stage of the
bottom-dead-center holding period. That is, the movable die part may
be controlled to move from the abutting position to the retracted
position during the bottom-dead-center holding period.
35 [0029] (Manufacturing Method 3)
12
Starting from Manufacturing Method 1 described above, the
movable die part may be separated from the metal sheet in an early
stage of the bottom-dead-center holding period and then the movable
die part may abut the metal sheet in a late stage of the
bottom-dead-center holding period. That is, the movable 5 die part may
be controlled to move from the retracted position to the abutting
position during the bottom-dead-center holding period.
[0030] (Manufacturing Method 4)
Starting from any one of Manufacturing Methods 1 to 3
10 described above, a surface of the movable die part to be in contact with
the metal sheet when the movable die part abuts the metal sheet while
the die parts are at the bottom-dead center may be positioned to be
flush with a die surface to be in contact with the metal sheet
surrounding the recess provided with the movable die part.
15 [0031] (Manufacturing Method 5)
Starting from any one of Manufacturing Methods 1 to 4
described above, a top surface of the movable die part may include a
protrusion adapted to abut the metal sheet and a recess adapted not to
abut the metal sheet when the movable die part abuts the metal sheet
20 while the die parts are at the bottom-dead center.
[0032] (Manufacturing Method 6)
Starting from any one of Manufacturing Methods 1 to 5
described above, a top surface of one movable die part in one of the first
die part and the second die part may include a recess or protrusion
25 recessed or protruding in the direction of pressing. A surface of the
other die part or movable die part facing the one movable die part in the
direction of pressing may have a shape corresponding to the recess or
protrusion on the top surface of the one movable die part. When the
one movable die part abuts the metal sheet while the die parts are at
30 the bottom-dead center, the surface of the other die part or movable die
part facing the one movable die part may abut the metal sheet.
[0033] Starting from any one of Manufacturing Methods 1 to 6
described above, the movable die part may be controlled such that the
non-abutment period is not shorter than 10 % of the entire
35 bottom-dead-center holding period. It should be noted that a method
13
of manufacturing a hot press-formed product using the hot press
apparatus of any one of Arrangements 1 to 9 described above by any one
of Manufacturing Methods 1 to 6 described above is included in the
embodiments of the present invention.
[0034] Now, embodiments of the present invention will 5 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 be made below show
10 components in a simplified or schematic manner, or omit some
components.
[0035] (Exemplary Construction of Press Apparatus)
FIG. 1 is a cross-sectional view of a hot press apparatus
according to an embodiment, illustrating an exemplary construction.
15 FIG. 2 shows the die of the press apparatus shown in FIG. 1 as being at
its bottom-dead center. The hot press apparatus 1 press forms a metal
sheet B to a press-formed product. The hot press apparatus 1 includes,
as its die parts, a die block 2, a punch 3, a die pad 5, a movable die part
4, and a control unit 9. The die block 2 is movable in the direction of
20 pressing PD relative to the punch 3. That is, the die block 2 and punch
3 are movable relative to each other. The directions of such relative
movements are referred to as direction of pressing.
[0036] The die block 2 is movable by a lift mechanism (i.e., actuator) 8
in the direction of pressing relative to the punch 3. The lift mechanism
25 8 may include, for example, a hydraulic cylinder, air cylinder, air
cushion or cam. In the present implementation, the die block 2 moves
relative to the punch 3; in some arrangements, the punch 3 may move
relative to the die block 2. In other arrangements, both the die block 2
and punch 3 may be constructed to move.
30 [0037] In the hot press apparatus 1, the metal sheet B is positioned
between the die block 2 and punch 3 and the metal sheet B is pushed by
both the die block 2 and punch 3 to press form the metal sheet B. The
die block 2 and punch 3 exemplify the first and second die parts.
[0038] The die block 2 has a shape recessed inwardly and
35 corresponding to the shape of the product to be press formed. The
14
punch 3 has a protruding shape corresponding to the recessed shape of
the die block 2. The surface of the die block 2 that faces the punch 3
includes a pressing surface that contacts the metal sheet B to press it.
The surface of the die block 2 that faces the punch 3 has recesses 2a.
The recesses 2a do not abut the metal sheet B even 5 when the die is at
the bottom-dead center. That is, each recess 2a forms a clearance
between it and the metal sheet B when the die is at the bottom-dead
center (see FIG. 2).
[0039] The die pad 5 is movable by a lift mechanism 6, such as a
10 hydraulic cylinder, in the direction of pressing relative to the die block 2.
The die pad 5 is movable in the top–bottom direction together with the
punch 3 with the top surface of the die pad being pressed against the
metal sheet B. The die pad 5 is located to face the top surface of the
punch 3. The top surface of the die pad 5 and the top surface of the
15 punch 3 face each other in the direction of pressing. In some
implementations, the die pad 5 may be omitted.
[0040] The surface of the punch 3 facing the die block 2 includes a
pressing surface that contacts the metal sheet B to press it. The
surface of the punch 3 facing the die block 2 includes recesses 3a. A
20 movable die part 4 is provided in each recess 3a. The recess 3a of the
punch 3 is located to face the respective recess 2a of the die block 2 in
the direction of pressing. That is, when viewed in the direction of
pressing, at least part of the recess 3a of the punch 3 overlaps the recess
2a of the die block 2.
25 [0041] The movable die part 4 is provided in the recess 3a of the punch
3 and movable relative to the punch 3 in a direction crossing the surface
of the punch 3 facing the die block 2. The movable die part 4 is
movable by a lift mechanism (i.e., actuator) 7 in the depth direction of
the recess 3a. In the implementation shown in FIG. 1, the movable die
30 part 4 may be in a state in which it is pushed into the recess 3a of the
punch 3. That is, the top surface of the movable die part 4 can enter
deep into the recess 3a away from the edge of the opening of the recess
3a. In the implementation shown in FIG. 2, with the die being at the
bottom-dead center, the top surface 4u of the movable die part 4 has
35 been pulled deep into the recess 3a. In this case, the movable die part
15
4 does not abut the metal sheet B between the die block 2 and punch 3.
Thus, the movable die part 4 is constructed to be movable between a
position where it abuts the metal sheet B held by the die at the
bottom-dead center, i.e., abutting position, and a position where a
clearance is present between it and the metal sheet 5 B, i.e., retracted
position. In the implementation shown in FIG. 2, the movable die part
4 is movable in the direction of pressing; however, the movement of the
movable die part 4 is not limited to the direction of pressing. The lift
mechanism 7 may be, for example, a hydraulic cylinder, air cylinder,
10 cam, or gas cushion.
[0042] FIG. 3 illustrates how the movable die part 4 abuts the metal
sheet B while the die is at its bottom-dead center. In this manner, the
movable die part 4 is also movable from a state in which it sits deep in
the recess 3a to a state where at least the top surface 4u of the movable
15 die part 4 is located as high as the pressing surface 3u surrounding the
recess 3a. In the present implementation, the top surface 4u of the
movable die part 4 is shaped such that it can be flush with the pressing
surface 3u surrounding the recess 3a. The entire top surface 4u of the
movable die part 4 abuts the metal sheet B.
20 [0043] The control unit 9 controls the die block 2, punch 3 and movable
die part 4. In the implementation shown in FIGS. 1 to 3, the control
unit 9 controls the lift mechanism 8 for the die block 2 to control
relative movement of the die block 2 and punch 3. The control unit 9
controls the lift mechanism 7 to control movement of the movable die
25 parts 4. The control unit 9 supplies the lift mechanisms (i.e.,
actuators) 8 and 7 with control signals to control driving of these
mechanisms.
[0044] Beginning with the state in which the heated metal sheet B is
positioned between the die block 2 and punch 3 as separated from each
30 other, the control unit 9 moves the die block 2 and punch 3 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 9 holds the die block 2 and punch 3 at the bottom-dead
center. Thus, during the bottom-dead-center holding period, the
35 portions of the formed metal sheet B that are in contact with the die
16
block 2 and punch 3 are rapidly cooled and hardened.
[0045] The control unit 9 controls each movable die part 4 in such a
manner that the bottom-dead-center holding period includes an
abutment period for which the movable die part 4 abuts the metal sheet
B and a non-abutment period for which the movable die 5 part 4 does not
abut the metal sheet B. That is, the control unit 9 controls operation
of the movable die part so as to hold the movable die part 4 at a position
where it abuts the metal sheet B for an abutment period that is shorter
than the bottom-dead-center holding period. The control unit 9
10 switches the movable die part 4 from the abutting position to the
retracted position, or from the retracted position to the abutting
position, during the bottom-dead-center holding period.
[0046] In the non-abutment period of the bottom-dead-center holding
period, for example, the control unit 9 causes each movable die part 4 to
15 stay deep in its recess 3a, as shown in FIG. 2. That is, the top surface
4u of the movable die part 4 is controlled to be deep within the recess 3a
away from the edge of its opening, that is, at the retracted position
where the top surface 4u is not in contact with the metal sheet B. This
creates a clearance between the movable die part 4 and metal sheet B.
20 It will be understood that the control unit 9 may control the amount of
clearance. For example, the control unit 9 may control the associated
lift mechanism 7 to adjust the retracted position of the top surface 4u of
the movable die part 4.
[0047] In the abutment period in the bottom-dead-center holding
25 period, for example, the control unit 9 moves the movable die part 4 so
it reaches at least the opening of the recess 3a, as shown in FIG. 3.
That is, the top surface 4u of the movable die part 4 is controlled to be
at the opening of the recess 3a, that is, such that the top surface 4u is in
contact with the metal sheet B, i.e., at the abutting position. Thus, the
30 movable die part 4 abuts the metal sheet B.
[0048] During the bottom-dead-center holding period, the control unit
9 causes at least one of two operations to occur, namely, causing the
movable die part 4 as abutting the metal sheet B to be separated
therefrom, or causing the movable die part 4 as separated from the
35 metal sheet B to abut it. Further, the control unit 9 controls the
17
movable die part 4 such that the abutment period and non-abutment
period of the bottom-dead-center holding period have predetermined
lengths. Data specifying an abutment period or non-abutment period
in the bottom-dead-center holding period may be stored on a storage
device accessible for the control unit 9, for example. 5 By way of
example, the storage device may store data specifying at least one of a
period of time (i.e., timing) where the movable die part 4 is to abut the
metal sheet B and a period of time where the movable die part 4 is
separated from the metal sheet B, with respect to the point of time at
10 which the die begins to be at the bottom-dead center. The control unit
9 uses the stored data to control the abutment period and
non-abutment period of the bottom-dead-center holding period.
[0049] The control unit 9 may be constituted by, for example, a
computer including a processor and a storage device (i.e., memory).
15 The processor executes a program stored on the storage device to
implement the function of supplying control information to the lift
mechanisms 7 and 8 for the die block 2 and punch 3 (i.e., first and
second die parts) as well as the movable die parts 4. By way of
example, based on input from the outside and/or data stored in advance
20 on the memory, the control unit 9 decides on times where the die block 2,
punch 3 and movable die parts 4 are to be moved as well as amounts of
movement (or directions of movement), and determines the control
information necessary for these movements. The control unit 9
outputs the control information to the lift mechanism 7.
25 [0050] (Exemplary Manufacturing Process)
Now, an exemplary process of manufacturing a hot press-formed
product using the hot press apparatus 1 will be described. First, a
material, i.e., a metal sheet B, is heated. The metal sheet B may be,
for example, a flat sheet, or may be an intermediate formed product
30 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 metallic microstructure. The heated
metal sheet B is transported and positioned between the die block 2 and
punch 3 of the hot press apparatus 1.
35 [0051] In the hot press apparatus 1, the heated metal sheet B is
18
positioned between the die block 2 and punch 3, and at least one of the
die block 2 and punch 3 is moved to the bottom-dead center. Thus, the
metal sheet B is hot press formed. The formed metal sheet B is held
between the die block 2 and punch 3 at the bottom-dead center.
During this bottom-dead-center holding period, the 5 metal sheet B in
contact with the die block 2 and punch 3 is rapidly cooled. Some
portions of the die parts of the hot press apparatus 1 provide clearance
portions constituted by recesses 2a in the die block 2 and recesses 3a in
the punch 3. Each recess 3a is provided with a movable die part 4.
10 When the die is at the bottom-dead center, the recesses 2a are not in
contact with the metal sheet B. Each movable die part 4, when pulled
deep into the recess 3a i.e. at the retracted position, is not in contact
with the metal sheet B. Thus, portions of the metal sheet B that
correspond to the clearance portions constituted by the recesses 2a and
15 3a experience lower cooling rates than the portions that are in contact
with the die block 2 and punch 3. This achieves gradual cooling of
some portions of the metal sheet B.
[0052] Halfway through the bottom-dead-center holding period, the
control unit 9 operates the movable die parts 4 so as to abut the metal
20 sheet B. This switches cooling rate from gradual cooling to rapid
cooling. Alternatively, the control unit 9 ensures that the movable die
parts 4 already abut the metal sheet B at the beginning of the
bottom-dead-center holding period, and then, halfway through the
bottom-dead-center holding period, operates the movable die parts 4 to
25 cause the movable die parts 4 to be separated from the metal sheet B.
This switches cooling rate from rapid cooling to gradual cooling. This
achieves a close-to-uniform temperature distribution of the metal sheet
B at the end of the bottom-dead-center holding period while gradually
cooling some portions of the formed metal sheet B to change cooling
30 conditions. Further, controlling the operation of the movable die parts
4 while the die is in the bottom-dead-center holding period enables
controlling cooling conditions of the gradually cooled portions.
[0053] Upon completion of the bottom-dead-center holding period, the
formed metal sheet B (i.e., formed product) is removed from the die
35 parts (i.e., die block 2 and punch 3). The formed product thus obtained
19
has been provided with a strength distribution, and has good shape
accuracy.
[0054] 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 that 5 correspond to the
clearance portions i.e. recesses 2a and 3a, 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 cooling rates at the clearance portions are lower than those
10 from heat conduction from the metal sheet B to the die due to the
sheet’s contact 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 soft metallic microstructure of ferrite and/or bainite, for
15 example. On the other hand, the portions in contact with the die
experience non-diffusion transformation, providing a hard metallic
microstructure mainly composed of martensite. That is, reducing
cooling rate for some portions of the metal sheet enables manufacturing
of a press-formed product with some softened portions.
20 [0055] 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 bottom-dead-center holding period includes
25 an abutment period for which the movable die part 4 abuts the metal
sheet B and a non-abutment period for which a clearance is present
between the movable die part 4 and metal sheet B. This results in a
close-to-uniform temperature, rather than temperature differences, in
the formed product upon completion of the bottom-dead-center holding
30 period. This makes it easier to ensure the shape accuracy of the entire
formed product. Further, during the abutment period in the
bottom-dead-center holding period, the metal sheet is cooled while
being gripped by the movable die parts 4. This makes it easier to
ensure shape accuracy, due to the portions gripped by the movable die
35 parts 4 than in arrangements where the metal sheet is not gripped
20
throughout the bottom-dead-center holding period.
[0056] The movable die parts 4 are caused to abut the metal sheet B in
an early stage of the bottom-dead-center holding period and, thereafter,
the movable die parts 4 are caused to be separated from the metal sheet
B in a late stage of the bottom-dead-center holding 5 period. That is,
during the bottom-dead-center holding period, there is an operation of
causing the movable die parts 4 as abutting the metal sheet B to be
separated therefrom.
[0057] FIG. 4 is a graph illustrating an implementation where an
10 abutment period is provided in an early stage of the bottom-dead-center
holding period. In FIG. 4, line L1 indicates the temperature of the
portions of the metal sheet B being press formed that correspond to the
movable die parts 4. Line L2 indicates the temperature of the portions
of the metal sheet B that abut the die throughout the
15 bottom-dead-center holding period (i.e., other portions). In the
implementation shown in FIG. 4, at the beginning of the
bottom-dead-center holding period, the movable die parts 4 abut the
metal sheet B. That is, at this time, the clearance CL between the
movable die parts 4 and metal sheet B is 0 mm. Thereafter, the
20 movable die parts 4 are separated from the metal sheet B, and still
remain separated at the end of the bottom-dead-center holding period.
At its time, the clearance is, by way of example, 13 mm. That is, in the
implementation of FIG. 4, when the die starts to be at the bottom-dead
center, there is an abutment period for which the movable die parts 4
25 abut the metal sheet B in an early stage, and the period of time after
the abutment period until the die ceases to be at the bottom-dead center
constitutes the non-abutment period. During the abutment period, the
portions of the metal sheet B corresponding to the movable die parts 4
are gripped by the movable die parts 4 such that their shape is
30 maintained, and at the same time rapidly cooled as is the case with the
other portions, as indicated by line L1. Thus, the difference between
the temperature of the portions of the metal sheet B corresponding to
the movable die parts 4 and the temperature of the other portions does
not widen. During the non-abutment period, the portions of the metal
35 sheet B corresponding to the movable die parts 4 experience a cooling
21
rate lower than during the abutment period, and are gradually cooled.
[0058] Thus, the bottom-dead-center holding period includes a period
where some portions of the metal sheet are rapidly cooled and a period
where they are gradually cooled, which reduces the difference between
the temperature of the gradually cooled portions and 5 that of the other
portions. This makes it easier to ensure the shape accuracy of the
entire formed product. Further, the gradually cooled portions are
gripped by the die for part of the bottom-dead-center holding period,
which makes it easier to ensure the shape accuracy of the gradually
10 cooled portions.
[0059] In the implementation shown in FIG. 4, the portions of the
metal sheet B corresponding to the movable die parts 4, rapidly cooled
during the abutment period, are separated from the movable die parts 4
before the portions reach the Ms point (martensitic transformation
15 starting point), and gradual cooling begins. 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 portions
corresponding to the movable die parts 4 continue to abut the die part
and are rapidly cooled throughout the bottom-dead-center holding
20 period. The other portions are cooled to the Mf point (martensitic
transformation finishing point) and lower during the
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 movable die parts 4 and
25 the other portions have different properties (i.e., strengths in the
present implementation).
[0060] In the implementation of FIG. 4, in an early stage of the
bottom-dead-center holding period where the temperature is relatively
high, the portions of interest of the metal sheet B are in the abutment
30 period and are thus rapidly cooled. Since the metal sheet B is gripped
by the movable die part 4 for a period where the metal sheet has a high
temperature and is soft, this makes it easier to ensure shape accuracy,
due to the gripped portions.
[0061] During the bottom-dead-center holding period, the movable die
35 part 4 are caused to abut the metal sheet B in an early stage and, then,
22
the movable die parts 4 are separated from the metal sheet B in a late
stage of the bottom-dead-center holding period. That is, during the
bottom-dead-center holding period, there is an operation of causing the
movable die parts 4 as abutting the metal sheet B to be separated
5 therefrom.
[0062] FIG. 5 is a graph illustrating an implementation where an
abutment period is provided in a late stage of the bottom-dead-center
holding period. In FIG. 5, line L3 indicates the temperature of the
portions of the metal sheet B being press formed that correspond to the
10 movable die parts 4. Line L2 indicates the temperature of the portions
of the metal sheet B that abut the die throughout the
bottom-dead-center holding period (i.e., other portions). In the
implementation shown in FIG. 5, at the beginning of the
bottom-dead-center holding period, the movable die parts 4 are
15 separated from the metal sheet B. At this time, the clearance CL is, by
way of example, 13 mm. Thereafter, the movable die parts 4 abut the
metal sheet B such that the clearance is 0 mm, and still abut it at the
end of the bottom-dead-center holding period. That is, in the
implementation of FIG. 5, when the die starts to be at the bottom-dead
20 center, there is a non-abutment period in an early stage, and the period
of time after the non-abutment period until the die ceases to be at the
bottom-dead center constitutes the abutment period. Since the
bottom-dead-center holding period includes a non-abutment period and
an abutment period, it is easier to ensure the shape accuracy of the
25 formed product.
[0063] In the implementation shown in FIG. 5, the abutment period for
the portions of the metal sheet B corresponding to the movable die parts
4 ends before the temperature drops down to the Ms point, as indicated
by line L3. This produces a soft metallic microstructure. On the
30 other hand, as indicated by line L2, the other portions of the metal
sheet B other than the portions corresponding to the movable die parts
4 are rapidly cooled during the bottom-dead-center holding period, and
cooled to the Mf point and below. This produces a hard metallic
microstructure mainly composed of martensite. Thus, the portions of
35 the formed metal sheet B corresponding to the movable die parts 4 and
23
the other portions have different properties (e.g., strengths).
[0064] In the implementation of FIG. 5, in a late stage of the
bottom-dead-center holding period where the temperature is relatively
low and cooling rate has become low, the portions of interest of the
metal sheet B are in the abutment period and are thus 5 rapidly cooled.
In this implementation, the temperature differences caused by rapid
cooling are small, which facilitates temperature control. Further, the
metal sheet B is gripped and rapidly cooled when the temperature has
become low and the metal sheet has become somewhat hard, which
10 make it easier to ensure shape accuracy.
[0065] Early stage of the bottom-dead-center holding period means a
period including at least part of the first half of the bottom-dead-center
holding period, and may include, or may not include, the beginning of
the bottom-dead-center holding period. Late stage of the
15 bottom-dead-center holding period means a period including at least
part of the second half of the bottom-dead-center holding period, and
may include, or may not include, the end of the bottom-dead-center
holding period.
[0066] The abutment period and non-abutment period of the
20 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
and a late stage of the bottom-dead-center holding period, and a
25 non-abutment period may be provided in an intermediate period
between the early and late stages.
[0067] Although not limiting, the bottom-dead-center holding period
may be 2 to 90 seconds, for example. The longer the
bottom-dead-center holding period, the better from the viewpoint of the
30 uniformity of the temperature distribution of the formed product upon
completion of the 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 bottom-dead-center holding period is preferably 10 seconds, and
35 more preferably 15 seconds. An upper limit for the bottom-dead-center
24
holding period is preferably 90 seconds, and more preferably 30 seconds.
In the present embodiment, the 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 bottom-dead-center holding can 5 easily be achieved
even when the bottom-dead-center holding period is not longer than 30
seconds, for example.
[0068] The clearance CL of the non-abutment period, that is, the
distance between the movable die parts 4 at the retracted position and
10 the metal sheet B is not limited to the above-mentioned example of 13
mm. The distance between the movable die parts 4 at the retracted
position and the metal sheet B may be, for example, not smaller than 2
mm, preferably not smaller than 4 mm, and more preferably not
smaller than 6 mm. A distance between the movable die parts 4 at the
15 retracted position and the metal sheet B may decided upon depending
on the surface area of a recess 3a as viewed in the direction of pressing
and the thickness of the metal sheet B. For example, if the surface
area of a recess 3a is 1800 mm2 (60 by 30 mm) and the thickness of the
metal sheet is 2.6 mm, then, the distance between the movable die
20 parts at the retracted position and the metal sheet is preferably not
smaller than 2 mm. This makes it less likely that the metal sheet
contacts a movable die part at the retracted position even when the
metal sheet experiences out-of-plane deformation. The control unit 9
may adjust the clearance CL by adjusting the retracted position of the
25 movable die parts 4. By way of example, the control unit 9 may decide
on a retracted position for the movable die parts 4 in response to input
by an operator. The control unit 9 may decide on a retracted position
for the movable die parts 4 depending on the clearance CL, the
thickness of the metal sheet B or other values that have been input.
30 [0069] The position of the movable die parts 4 for the period between
the initiation of pressing and the point of time at which the die reaches
the bottom-dead center is not limited to any particular one, and they
may be at the abutting position or may be at the retracted position.
For example, if an abutment period is provided in an early stage of the
35 bottom-dead-center holding period as shown in FIG. 4, the movable die
25
parts 4 may be at the abutting position from the initiation of pressing,
even before the die reaches the bottom-dead center. This eliminates
the necessity to move the movable die part 4 at the beginning of the
bottom-dead-center holding period. For the same reason, if a
non-abutment period is provided in an 5 early stage of the
bottom-dead-center holding period as shown in FIG. 5, the movable die
parts 4 may be at the retracted position from the initiation of pressing
i.e. before the die reaches the bottom-dead center.
[0070] (Variations of Movable Die Parts)
10 FIG. 6 shows a variation of the clearance portion, modified in
construction. In the implementation shown in FIG. 6, a recess 2a in
the die block 2 is provided, positioned to face a recess 3a in the punch 3.
A movable die part 4 is provided in the recess 3a of the punch 3, and a
movable die part 21 is provided in the recess 2a of the die block 2. The
15 movable die parts 4 and 21 are positioned to face each other. That is,
as viewed in the direction of movement of the movable die part 4 in the
recess 3a, at least part of the top surface 4u of the movable die part 4 is
positioned to overlap at least part of the top surface 21u of the movable
die part 21.
20 [0071] In the implementation shown in FIG. 6, during the abutment
period of the bottom-dead-center holding period, the top surface 4u of
the movable die part 4 abuts one side of the metal sheet B, while the top
surface 21u of the movable die part 21 abuts the other side (i.e.,
opposite side) of the metal sheet B. Thus, during the abutment period,
25 both sides of the metal sheet B can be cooled and both sides can be
gripped by movable die parts. This makes it easier to ensure cooling
rate and shape accuracy.
[0072] FIG. 7 shows another variation of the clearance portion,
modified in construction. In the implementation shown in FIG. 7, no
30 recess is provided in the die block 2 at a position facing a recess 3a in
the punch 3. A movable die part 4 is provided in the recess 3a of the
punch 3. During the abutment period of the bottom-dead-center
holding period, the top surface 4u of the movable die part 4 abuts one
side of the metal sheet B, while the die block 2 abuts the other side (i.e.,
35 opposite side) of the metal sheet B. During the non-abutment period, a
26
clearance is present between the movable die part 4 and metal sheet B,
while no clearance is present between the metal sheet B and the portion
of the die block 2 facing the movable die part 4. In this arrangement,
too, the presence of an abutment period and a non-abutment period in
the bottom-dead-center holding period produces the 5 effect of ensuring
shape accuracy.
[0073] FIG. 8 shows a variation of the movable die part 4. In the
implementation shown in FIG. 8, the top surface 4u of the movable die
part 4 has protrusions protruding in the direction of pressing. During
10 the abutment period of the bottom-dead-center holding period, the
protrusions of the top surface 4u of the movable die part 4 abut the
metal sheet B. The portions of the top surface 4u other than the
protrusions, i.e., portions providing recesses, do not abut the metal
sheet B. Thus, the movable die part 4 may be constructed such that,
15 during the abutment period, some portions of the top surface 4u of the
movable die part 4 abut the metal sheet B and the other portions do not.
[0074] In the implementation shown in FIG. 8, during the abutment
period, the movable die part 4 abuts the metal sheet B and a clearance
is present between the top surface 4u of the movable die part 4 and the
20 metal sheet 4. For example, the proportion of the portions of the top
surface 4u of the movable die part 4 that are in contact with the metal
sheet B during the abutment period may be changed to change cooling
conditions. That is, cooling conditions may be controlled by means of
the shape of the movable die part 4.
25 [0075] FIG. 9 shows yet another variation of the clearance portion,
modified in construction. In the implementation shown in FIG. 9, a
recessed/protruding shape of the top surface 4u of the movable die part
4 is complementary with a recessed/protruding shape of the surface 2f
of the die block 2 facing the top surface 4u. More specifically, the top
30 surfaces are shaped such that the protrusion of the top surface 4u is
fitted into the recess of the surface 2f facing it. During the abutment
period of the bottom-dead-center holding period, while the movable die
part 4 abuts the metal sheet B, the surface 2f of the die block 2 that
faces the movable die part also abuts the metal sheet B. In this
35 arrangement, the metal sheet B is formed into a shape corresponding to
27
the shape of the top surface 4u of the movable die part 4. In the
implementation shown in FIG. 9, the surface of the die block 2 faces the
movable die part 4; alternatively, a movable die part may also be
provided in the die block 2, positioned to face the movable die part 4.
[0076] Although not limiting, the hot press apparatus 5 and the method
of manufacturing the hot press-formed product according to the
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
10 shape accuracy. The embodiments may suitably be applied to such
structural members for vehicles. For example, the hot press
apparatus 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
15 have been softened in order to reduce the weight of the vehicle or
achieve high performance, for example. Examples of such structural
members for 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
20 impact.
[0077] (Examples)
B-pillar dies that enabled clearance control (hereinafter
referred to as controllable-clearance die) were fabricated and tested.
The construction of a controllable-clearance die was the same as the
25 construction shown in FIG. 1. The controllable-clearance dies featured
the presence of clearance portions at positions corresponding to the
flanges of a B-pillar. A clearance portion included a recess (i.e., blank)
in a die block 2 and a recess in a punch 3 facing that recess, a movable
die part being positioned in the recess of the punch. The movable die
30 part allowed the amount of clearance for the recess of that punch to be
changed to 0 mm or 13 mm. The clearance portion was not cooled by
the die and thus cooled gradually such that the metallic microstructure
of the corresponding portion of the metal sheet was softened. For the
testing, the metal sheet used was a hot-rolled sheet to be hot stamped
35 (hereinafter HS) (thickness: 2.6 mm). The metal sheet was heated for
28
5 minutes in a furnace set to 900 °C, transported to the
controllable-clearance die, and, after a bottom-dead-center holding
period of 30 seconds, removed from the die and left to cool. The four
sets of clearance conditions during bottom-dead-center holding shown
in Table 5 1 below were used.
[0078] [Table 1]
[0079] In Table 1, condition set (a) means that a die without a
clearance was used for press forming, and represents typical HS
10 conditions, where the entire surface of the metal sheet including the
flanges contacts the die. Condition set (b) means that a die having
fixed clearances at positions corresponding to the flanges was used for
press forming. Clearances were provided in both the die block and
punch. The amount of clearance was constant throughout the
15 bottom-dead-center holding period. Upon completion of the
bottom-dead-center holding period, the metal sheet was removed from
the die while the portions corresponding to the flanges were still at high
temperatures. Condition sets (c) and (d) mean that the amount of
clearance for the punch was changed during bottom-dead-center
20 holding. The thermal history of condition set (c) was the same as that
of the implementation shown in FIG. 4, and the thermal history of
condition set (d) was the same that of as the implementation shown in
FIG. 5. Under condition set (c), the amount of clearance between the
movable die part and metal sheet at the beginning of
25 bottom-dead-center holding was 0 mm and, five seconds after the
beginning of bottom-dead center holding, the amount of clearance was
changed from 0 mm to 13 mm to switch to gradual cooling. Under
29
condition set (d), the amount of clearance at the beginning of
bottom-dead-center holding was 13 mm and, 25 seconds after the
beginning of bottom-dead-center holding, the amount of clearance was
changed from 13 mm to 0 mm to switch to die cooling. A movable die
part was provided in a recess in the punch. A 5 recess without a
movable die part was provided in the surface of the die block facing the
movable die part. That is, the amount of clearance for the die block
was constant throughout the bottom-dead-center holding period.
[0080] The formed products after hot press forming were evaluated
10 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
shown in FIG. 10. The shape accuracy for each of condition sets (b), (c)
15 and (d) was evaluated with respect to the data from condition set (a).
[0081] FIG. 11 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
condition sets (b) and (c) had low hardnesses at the clearance portions.
20 Clearance portion means the portion of the formed product
corresponding to a clearance of the die. The results shown in FIG. 11
demonstrate the partial softening effect produced by the clearance
portions of the fixed-clearance and controllable-clearance dies.
[0082] FIG. 12 is a graph showing the results of measurement of the
25 torsion angles of the formed products. Torsion angle in the graph of
FIG. 12 indicates to what degree the torsion-evaluation cross section C1
was twisted relative to that in the formed product from condition set (a),
as found when the formed products from condition sets (a) to (d) were
aligned in position with respect to the torsion-alignment surface W1
30 shown in FIG. 10.
[0083] The results shown in FIG. 12 demonstrate that the formed
product formed by the fixed-clearance die of condition set (b) had larger
twisting, i.e., lower shape accuracy, than that from condition set (a)
without a clearance. On the other hand, the formed products press
35 formed by the controllable-clearance dies of condition sets (c) and (d)
30
had torsion angles not higher than a half of that from condition set (b),
demonstrating improvements in shape accuracy.
[0084] FIG. 13 is a graph showing the results of measurement of the
out-of-plane deformations of the formed products. An amount of
out-of-plane deformation shown in the graph of FIG. 5 13 indicates the
amount of deformation of the surface at the out-of-plane deformation
evaluation position F1 shown in FIG. 10 relative to that of the formed
product from condition set (a). The out-of-plane deformation
evaluation position F1 for each of condition sets (b) to (d) was a portion
10 including a position on a flange corresponding to a clearance in the die.
The examples shown in FIG. 13 demonstrate that the
controllable-clearance dies from condition sets (c) and (d) also improved
local shape accuracy in the flanges corresponding to the clearance
portions.
15 [0085] 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,
and the above-described embodiments, when carried out, may be
20 modified as appropriate without departing from the spirit of the
invention.
EXPLANATION OF CHARACTERS
[0086] 1: press apparatus
25 2: die block
3: punch
4: movable die part
9: control unit.

We claim:
1. A hot press apparatus comprising:
a first die part;
a second die part capable of moving relative to 5 the first die part
in a direction of pressing; and
a control unit adapted to control relative movement of the first
die part and the second die part,
wherein at least one of the first die part and the second die part
10 includes a recess in a surface facing the other die part in the direction of
pressing,
a movable die part is provided in the recess, the movable die
part movable in a direction crossing the surface facing the other part,
and
15 the control unit controls a position of the movable die part such
that a heated and press formed metal sheet is held between the first die
part and the second die part and a bottom-dead-center holding period
for which the first die part and the second die part are at a bottom-dead
center includes an abutment period for which the movable die part is at
20 an abutment position where the movable die part abuts the metal sheet
and a non-abutment period for which the movable die part is at a
retracted position where it does not abut the metal sheet.
2. The hot press apparatus according to claim 1, wherein the
25 control unit causes the movable die part to abut the metal sheet in an
early stage of the bottom-dead-center holding period and then causes
the movable die part to be separated from the metal sheet in a late
stage of the bottom-dead-center holding period.
30 3. The hot press apparatus according to claim 1, wherein the
control unit causes the movable die part to be separated from the metal
sheet in an early stage of the bottom-dead-center holding period and
then causes the movable die part to abut the metal sheet in a late stage
of the bottom-dead-center holding period.
35
32
4. The hot press apparatus according to any one of claims 1 to 3,
wherein
the recess includes a first recess provided in the first die part
and a second recess provided in the second die part and positioned to
face the 5 first recess, and
the movable die part is located in at least one of the first recess
and the second recess.
5. The hot press apparatus according to claim 4, wherein the
10 movable die part is provided in each of the first recess and the second
recess.
6. The hot press apparatus according to any one of claims 1 to 5,
wherein a surface of the movable die part to be in contact with the
15 metal sheet when the movable die part abuts the metal sheet while the
die parts are at the bottom-dead center is positioned to be flush with a
die surface to be in contact with the metal sheet surrounding the recess
provided with the movable die part.
20 7. The hot press apparatus according to any one of claims 1 to 6,
wherein a top surface of the movable die part includes a protrusion
adapted to abut the metal sheet and a recess adapted not to abut the
metal sheet when the movable die part abuts the metal sheet while the
die parts are at the bottom-dead center.
25
8. The hot press apparatus according to any one of claims 1 to 7,
wherein a top surface of one movable die part in one of the first die part
and the second die part includes a recess or protrusion recessed or
protruding in the direction of pressing; a surface of the other die part or
30 movable die part facing the one movable die part in the direction of
pressing has a shape corresponding to the recess or protrusion on the
top surface of the one movable die part; and, when the one movable die
part abuts the metal sheet while the die parts are at the bottom-dead
center, the surface of the other die part or movable die part abuts the
35 metal sheet facing the one movable die part.
33
9. The hot press apparatus according to any one of claims 1 to 8,
wherein the control unit adjusts a distance between the metal sheet
and the movable die part found when the movable die part is at the
retracted 5 position.
10. A method of manufacturing a hot press-formed product,
comprising:
positioning a heated metal sheet between a first die part and a
10 second die part;
press forming the metal sheet by moving the first die part and
the second die part closer to each other in a direction of pressing;
holding the metal sheet while the first die part and the second
die part are at a bottom-dead center; and
15 moving, to the metal sheet, a movable die part provided in a
recess in at least one of the first die part and the second die part during
a bottom-dead-center holding period for which the metal sheet is held at
the bottom-dead center,
wherein the bottom-dead-center holding period includes an
20 abutment period for which the movable die part is at an abutment
position where the movable die part abuts the metal sheet and a
non-abutment period for which the movable die part is at a retracted
position where it does not abut the metal sheet.
25 11. The method of manufacturing a hot press-formed product
according to claim 10, wherein the movable die part abuts the metal
sheet in an early stage of the bottom-dead-center holding period and
then the movable die part is separated from the metal sheet in a late
stage of the bottom-dead-center holding period.
30
12. The method of manufacturing a hot press-formed product
according to claim 10, wherein the movable die part is separated from
the metal sheet in an early stage of the bottom-dead-center holding
period and then the movable die part abuts the metal sheet in a late
35 stage of the bottom-dead-center holding period.
34
13. The method of manufacturing a hot press-formed product
according to any one of claims 10 to 12, wherein a surface of the
movable die part to be in contact with the metal sheet when the
movable die part abuts the metal sheet while the die 5 parts are at the
bottom-dead center is positioned to be flush with a die surface to be in
contact with the metal sheet surrounding the recess provided with the
movable die part.
10 14. The method of manufacturing a hot press-formed product
according to any one of claims 10 to 13, wherein a top surface of the
movable die part includes a protrusion adapted to abut the metal sheet
and a recess adapted not to abut the metal sheet when the movable die
part abuts the metal sheet while the die parts are at the bottom-dead
15 center.
15. The method of manufacturing a hot press-formed product
according to any one of claims 10 to 14, wherein a top surface of one
movable die part in one of the first die part and the second die part
20 includes a recess or protrusion recessed or protruding in the direction of
pressing; a surface of the other die part or movable die part facing the
one movable die part in the direction of pressing has a shape
corresponding to the recess or protrusion on the top surface of the one
movable die part; and, when the one movable die part abuts the metal
25 sheet while the die parts are at the bottom-dead center, the surface of
the other die part or movable die part facing the one movable die part
abuts the metal sheet.