U.S. patent application number 12/787628 was filed with the patent office on 2011-04-28 for method and hot forming system for producing a hardened, hot formed workpiece.
This patent application is currently assigned to THYSSENKRUPP UMFORMTECHNIK GMBH. Invention is credited to Stephane Anquetil, Jens Overrath.
Application Number | 20110094282 12/787628 |
Document ID | / |
Family ID | 43480460 |
Filed Date | 2011-04-28 |
United States Patent
Application |
20110094282 |
Kind Code |
A1 |
Overrath; Jens ; et
al. |
April 28, 2011 |
Method and hot forming system for producing a hardened, hot formed
workpiece
Abstract
A method for producing a hardened, hot formed workpiece (2) of
sheet steel, comprising at least partially heating a workpiece
blank to a forming temperature in at least one furnace, introducing
the heated workpiece blank into at least one drawing press, and
forming the heated workpiece blank into a workpiece in the at least
one drawing press. To be able to produce particularly high
quantities of hardened, hot formed workpieces in a more
cost-efficient manner, provision is made for introducing the formed
workpiece into at least one hardening device, fixing the formed
workpiece in a receiver of the hardening device which is adapted to
the geometry of the formed workpiece, and at least partially
hardening the formed workpiece in the receiver by means of direct
heat exchange with a cooling medium.
Inventors: |
Overrath; Jens; (Wunstorf,
DE) ; Anquetil; Stephane; (Mareil Sur Mauldre,
FR) |
Assignee: |
THYSSENKRUPP UMFORMTECHNIK
GMBH
Ludwigsfelde
DE
THYSSENKRUPP SOFEDIT S.A.S.
Saint-Quentin-en-Yvelines
FR
|
Family ID: |
43480460 |
Appl. No.: |
12/787628 |
Filed: |
May 26, 2010 |
Current U.S.
Class: |
72/342.1 ;
72/364 |
Current CPC
Class: |
C21D 1/18 20130101; B21D
22/20 20130101; C21D 8/00 20130101; C21D 9/48 20130101; B21D 22/022
20130101; C21D 1/673 20130101 |
Class at
Publication: |
72/342.1 ;
72/364 |
International
Class: |
B21D 31/00 20060101
B21D031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 23, 2009 |
DE |
102009050533.4-14 |
Claims
1. A method for producing a hardened, hot formed workpiece from
sheet steel, comprising: at least partially heating a workpiece
blank in at least one furnace to a forming temperature: introducing
the heated workpiece blank into at least one drawing press: forming
the heated workpiece blank into a workpiece in the at least one
drawing press: introducing the formed workpiece into at least one
hardening device: fixing the formed workpiece in a receiver of the
hardening device, which is adapted to the geometry of the formed
workpiece: and at least partially hardening the formed workpiece in
the receiver by means of direct heat exchange with a cooling
medium.
2. The method according to claim 1, wherein the workpiece blank is
heated to a forming temperature which exceeds the austenitization
temperature of the workpiece blank.
3. The method according to claim 1, wherein the healed workpiece
blank is pre-cooled in air prior to the introduction into the
drawing press.
4. The method according to claim 1, wherein the workpiece blank is
gradually formed into a workpiece in a plurality of drawing
presses.
5. The method according to claim 1, wherein cooling medium flows at
least partially around the formed workpiece in the receiver of the
hardening device.
6. The method according to claim 1, wherein the formed and hardened
workpiece is introduced into a cutting device immediately after
removal from the hardening device and is at least partially cut
therein.
7. The method according to claim 1, wherein the workpiece is at
least partially cut during forming.
8. The method according to claim 6, wherein sections of the
workpiece which are to be cut are not hardened in the hardening
device.
9. The method according to claim 8, wherein sections of the
workpiece which are to be cut are kept away from the cooling medium
in the hardening device.
10. The method according to claim 6, wherein the sections of the
workpiece, which are to be cut, are at most slightly brought into
contact with drawing tools of the at least one drawing press.
11. A hot forming system for producing a hardened formed workpiece
of steel, comprising: a furnace for at least partially heating a
workpiece blank to a forming temperature; comprising a drawing
press for forming the workpiece blank; comprising a transfer device
for transferring the heated workpiece blank from the furnace to the
drawing press; a hardening device for hardening the workpiece; and
a transfer device for transferring the formed workpiece from the
drawing press to the hardening device, wherein the hardening device
comprises a receiver is adapted to the geometry of the formed
workpiece for fixing the formed workpiece, and for at least
partially hardening the formed workpiece by means of direct heat
exchange with a cooling medium.
12. The hot forming system according to claim 11, wherein the
receiver comprises ducts, bores, or a combination thereof for at
least partially hardening the formed workpiece by means of direct
heat exchange with the cooling medium.
13. The hot forming system according to claim 11, wherein the
receiver comprises a contact surface for the formed workpiece at
least on one side of the formed workpiece.
14. The hot forming system according to claim 11, wherein the
receiver comprises two halves comprising ducts, bores, or a
combination thereof and in areas of the workpiece provided for
hardening, contact surfaces for the formed workpiece of the one
half are arranged opposite to the ducts, bores, or combination
thereof of the other half.
Description
[0001] The invention relates to a method for producing a hardened,
hot formed workpiece of sheet steel, wherein a workpiece blank is
at least partially heated in at least one furnace to a forming
temperature, wherein the heated workpiece blank is introduced into
at least one drawing press and wherein the heated workpiece blank
is formed into a workpiece in the at least one drawing press. The
invention further relates to a hot forming system for producing a
hardened, formed workpiece of steel, comprising a furnace for at
least partially heating a workpiece blank to a forming temperature,
comprising a drawing press for forming the workpiece blank and
comprising a transfer device for transferring the heated workpiece
blank from the furnace to the drawing press.
[0002] Such systems serve the purpose of producing high-strength
body parts from sheet steel, for example. Hot forming systems
typically comprise a furnace system for heating the workpiece
blanks as well as a press system for forming and hardening the
workpiece blanks. After heating of the workpiece blanks to a
predetermined forming temperature, they are removed from the
furnace system and are placed into a drawing press. The tools of
the drawing press are cooled via a cooling medium, so that the
workpiece is quickly cooled down therein and is hardened thereby.
In this context, this is also referred to as press-hardening of the
workpieces.
[0003] Such a hot forming system and a method for producing hot
formed, hardened workpieces is known from DE 100 49 660 A1,for
example. The workpieces are here hardened in the drawing press by
quickly being cooled down below a predetermined limit temperature.
However, after leaving the drawing press, the temperature of the
workpieces is still too high to be able to immediately further
process the workpieces or to be able to handle them without
problems. After the press-hardening, the workpieces are thus
transferred to a fixing tool or gradually to a plurality of fixing
tools which are connected in a row. The formed workpieces are fixed
in the fixing tools and are thereby further cooled down. For this,
a cooling medium can be used for cooling the fixing tool or for
directly cooling the workpieces. When the temperature of the
workpieces has decreased sufficiently, they can be fed to a further
treatment.
[0004] In the case of the known method, the clock cycles of the
drawing press can be reduced by the at least one fixing tool which
is connected downstream from the drawing press. However, there is a
further need for optimization for reducing the production costs of
hot formed and hardened workpieces, for instance as vehicle body
component.
[0005] The invention is thus based on the object of embodying and
further developing a method and a hot forming system such that in
particular high quantities of hardened, hot formed workpieces can
be produced in a more cost-efficient manner.
[0006] This object is solved according to claim 1 by means of a
method, wherein the formed workpiece is introduced into at least
one hardening device, wherein the formed workpiece is fixed and
calibrated, if need be, in a receiver of the hardening device which
is adapted to the geometry of the formed workpiece, and wherein the
formed workpiece is at least partially hardened in the receiver by
means of a direct heat exchange with a cooling medium.
[0007] In the case of a hot forming system according to the
preamble of claim 10, the object is furthermore solved in that a
hardening device is provided for hardening the workpiece, in that a
transfer device is provided for transferring the formed workpiece
from the drawing press to the hardening device, in that the
hardening device comprises a receiver for fixing the formed
workpiece which is adapted to the geometry of the formed workpiece,
and in that the receiver is adapted for at least partially
hardening the formed workpiece by means of direct heat exchange
with a cooling medium.
[0008] To produce a hot formed and hardened workpiece, a workpiece
blank is thus initially brought to the required forming temperature
in a furnace. The forming temperature which is to be reached
depends on the desired structure composition of the workpiece prior
to and after the hardening. In particular a pre-cut piece of sheet
steel can also be considered as workpiece blank. Such a pre-cut
piece is also referred to as blank. The blank or the semifinished
part, respectively, can be embodied monolithically but also as a
tailored blank, which combines steels comprising different
thicknesses and/or qualities.
[0009] After it has been heated to the forming temperature, the
workpiece blank is transferred to a drawing press by means of a
transfer device, which is provided for this purpose. In the drawing
press, the workpiece blank is then formed in a manner known per se.
A so-called drawn component or molded component is created. For
this, the workpiece blank is preferably brought between two drawing
tools, which are pressed against the workpiece blank from both
sides under high pressure. The drawing tools are a female part and
a male part, for instance.
[0010] The hot formed workpiece can cool down to a certain degree
in the drawing press. The formed workpiece, however, is not
hardened in the drawing press. After the forming the workpiece is
removed from the drawing press and is transferred to a hardening
device by means of a further transfer device. In doing so, the
formed workpiece is received in a receiver of the hardening device,
the geometry of which is adapted to the geometry of the formed
workpiece, so that the formed workpiece is fixed in the receiver
during the hardening. For this purpose, the formed workpiece does
not have to have full surface contact with the receiver.
[0011] The workpiece is at least partially hardened in the
receiver, namely substantially by means of a direct heat exchange
with a cooling medium. For this purpose the receiver is embodied
such that the cooling medium can come into direct contact with the
formed workpiece. It is thus not so much the receiver as such,
which is cooled, but rather the workpiece.
[0012] According to the invention, a spatial and chronological
separation is provided between the forming of the hot workpiece
blank and the hardening of the formed workpiece. These two process
steps are carried out in different system parts by means of
different tools. Even though the system and procedural effort is
increased through this, shorter clock cycles can thus be realized
for the production of the hot formed and hardened workpieces as a
whole, whereby the cost effectiveness of the production of hot
formed and hardened workpieces as a whole improves.
[0013] The workpieces do not have to remain in the drawing press
any longer than necessary. The drawing press can thus be operated
at higher clock cycles in a more cost-efficient manner. As
compensation, the formed workpieces are transferred to a hardening
device for hardening. Due to the fact that a further forming of the
workpieces does not take place in the hardening device, the
hardening device can be embodied to be simpler and more
cost-efficient than the drawing press. The direct heat exchange
between the cooling medium and the formed workpiece furthermore
allows for the saving of time, preferably with reference to the
hardening and possibly to the further cooling down to a temperature
which does not impede the further handling of the workpieces.
[0014] If need be, for the heating of the workpiece blank to
forming temperature a furnace or a plurality of furnaces can be
provided through which the workpiece blank then runs successively.
The temperature increase of the workpiece blank during the heating
process can be adjusted better by means of a plurality of
furnaces.
[0015] In the alternative or in addition, the entire workpiece
blank can be brought to the desired forming temperature, whereby
the system and procedural effort decreases. However, it is also
possible to bring only the areas of the workpiece blank which are
to be hardened to the forming temperature. This can be advantageous
for the subsequent processing steps of the workpiece. For instance,
it is possible to bring to a forming temperature only the areas of
the workpieces which are also to be hardened later. For instance,
the areas of the workpiece which are not to be hardened are then
such areas which are punched and/or cut after the partial hardening
of the workpiece, for instance by means of a punching and/or
cutting tool.
[0016] To attain an efficient and uniform heating of the workpiece
blank, the workpiece blank can be heated in the furnace, if need
be, substantially by means of heat conduction, thermal radiation
and/or induction. In the alternative or additionally, provision can
be made for the workpiece blank to be heated to forming temperature
in at least one roller hearth furnace. The heating of the workpiece
blank can thus take place continuously. However, conductive heating
devices, which heat the blank, which is to be hardened and
tempered, by means of contact and by supplying a current, for
example like a "waffle iron", can also be considered.
[0017] The drawing tools of the drawing press can be cooled, for
instance via a cooling medium which flows through ducts arranged in
the drawing tools without contact to the workpiece. Heat is thus
additionally removed from the workpiece in response to the forming,
so that the time period required for the complete method can be
shortened. The workpiece, however, is not hardened by the cooling
in the drawing press.
[0018] The selection of a suitable cooling medium is dependent on
different factors. Additionally, different cooling mediums can be
used in the drawing press and in the hardening device. In
particular with reference to the hardening device, however, such
cooling mediums are preferred which can dissipate much heat per
time unit from the workpiece. In the alternative or in addition, it
is preferred when the cooling medium is suitable to purify the
surface of the workpiece, for instance from adhering substances.
Finally, water, oil, a water/oil mixture, a water/graphite mixture,
ice, air, steam and/or a soap solution, for example, can be
considered as cooling medium.
[0019] In a first preferred embodiment of the method, provision is
made for the workpiece blank to be heated to a forming temperature,
which lies above the austenitization temperature of the workpiece
blank or of the material thereof, respectively. This provides for
high strength values of the formed and hardened workpieces.
[0020] In a further embodiment of the method, the quality of the
material can be improved in that the workpiece blank is initially
pre-cooled in air after leaving the furnace and prior to the
introduction into the drawing press. The pre-cooling in air can
preferably last for approximately 4 to 10 seconds, in particular 5
to 7 seconds. Within the afore-mentioned limits, this pre-cooling
in air does not have a negative impact on the desired
characteristics to be set in the finished component.
[0021] If need be, the hot forming system can comprise a plurality
of drawing presses, wherein the workpiece blank is gradually formed
in the drawing presses. In so doing, workpieces comprising
extensive geometric structures can be produced as well. However, it
lends itself thereby when the workpiece is quickly transferred from
drawing press to drawing press and only remains in the drawing
presses for short clock cycles. The temperature of the formed
workpiece is then still high enough to be hardened in the hardening
device by means of a quick cooling. Preferably, the temperature of
a workpiece, which is to be hardened and which is to comprise a
substantially martensitic structure, lies at .gtoreq.M.sub.s+10K
(M.sub.s=martensite start). The quick cooling, which lies above the
critical cool-down rate, leads to high strengths in the component,
when cooling down to a temperature of below 250.degree. C.,
preferably M.sub.f (M.sub.f=martensite finish).
[0022] According to a particularly preferred embodiment of the
method, cooling medium flows at least partially around the formed
workpiece in the receiver of the hardening device. The cooling
medium is thus preferably conveyed in sections along the workpiece
located in the receiver. A large quantity of heat can thus be
removed from the workpiece within a short period of time.
[0023] With reference to the device, it is particularly preferred
in this context when the receiver of the hardening device comprises
ducts and/or bores, through which the cooling medium can flow.
These ducts and/or bores are here provided such that they provide
for the at least partial hardening of the formed workpiece by means
of direct heat exchange with the cooling medium. On the one hand,
the cooling medium is preferably guided through ducts and/or bores
to the workpiece or to certain sections of the workpiece,
respectively. On the other hand, the cooling medium is preferably
removed again through other ducts and/or bores after contact with
the workpiece.
[0024] The use of ducts and/or bores allows for a specific flow of
the cooling medium, wherein also only certain areas of the
workpiece can also be brought into contact with the cooling medium.
Only certain areas of the workpiece can thus be hardened, for
example, while other areas remain unhardened due to a lower and/or
slower cool-down.
[0025] In a structurally simple manner, the ducts and/or the bores
are embodied such that the workpiece located in the receiver closes
the ducts and/or bores at least on one side.
[0026] In a further preferred embodiment of the hot forming system,
the receiver is embodied such that a contact surface between
workpiece and receiver is always provided at least on one side of
the workpiece. The workpiece can thus be continuously fixed in the
receiver--even if more or less one-sidedly.
[0027] In the alternative or additionally, provision can be made
for a duct and/or a bore to always be provided in the receiver, at
least on one side of the workpiece. The workpiece can thus
analogously be cooled constantly--even if more or less
one-sidedly.
[0028] It is particularly advantageous in this context when a
contact surface of the receiver is in each case assigned to the
ducts and/or the bores of the receiver substantially opposite
thereto, that is, on the other side of the workpiece.
[0029] In the alternative or in addition, the receiver can here
comprise two halves, which comprise ducts and/or bores. Provision
can thus be made on each side of the workpiece for alternating
ducts and/or bores as well as for contact surfaces between the
workpiece and the receiver. This serves for an even cooling down
and/or an even fixing of the workpiece.
[0030] In the event that the workpiece is not to be cooled down
uniformly, provision can be made for only certain areas, preferably
the areas which are to be hardened, to be quickly cooled down by
means of direct heat exchange with the cooling medium. The
remaining areas, for instance the areas which are not to be
hardened preferably do not come into direct contact with the
cooling medium, whereby the workpiece cools down more slowly
there.
[0031] In this case, provision can only be made in the areas which
are to be cooled down or hardened quickly, respectively, at least
on one side of the workpiece for ducts and/or bores in the
receiver. A contact surface of the receiver can further preferably
be arranged so as to be located opposite to each duct and/or each
bore of the receiver, that is, on the other side of the workpiece.
In the remaining areas, i.e. at the sections of the workpiece which
do not cool down so quickly, the workpiece is preferably fixed from
both sides by means of contact surfaces between the receiver and
the workpiece.
[0032] A further chronological and cost-related optimization of the
method for producing hot formed and hardened workpieces can be
obtained, if need be, when the formed and hardened workpiece was
introduced into a cutting device and is cut therein at least
partially, immediately after it was removed from the hardening
device. This additional process step can thus preferably be
integrated into the method and can thus be carried out in-line,
because time is saved by means of the acceleration of the process
steps which precede the cutting. A spatially and chronologically
uncoupled method for cutting the workpiece is then not necessary.
The definition of the term "cut" can be viewed as round, partial
and/or hole cutting as a separating operation, for example.
[0033] In the alternative or in addition, provision can be made for
the workpiece to be at least partially cut during the forming, thus
in the at least one drawing press. In so doing, synergies can be
created or used, respectively. In so doing, the cutting can at
least partially also be carried out at a point in time in which the
workpiece is still unhardened.
[0034] Contrary to the remaining sections of the workpiece, the
sections of the workpiece which are to be cut may not be hardened
in the hardening device, if required. The cutting of the workpiece
after the hardening of the formed workpiece can thus be simplified
and a service live for a higher quantity of workpieces can be
attained in the cutting device.
[0035] For this purpose, the sections or areas which are to be cut
and which are not to be hardened, can be kept away from the cooling
medium in the hardening device. A direct heat exchange with the
cooling medium thus does not take place at the mentioned
sections.
[0036] To additionally or alternatively avoid an increased cooling
down of the sections which are to be cut and/or of the areas of the
workpiece which are not to be hardened during the forming thereof
it is possible to not bring these sections and/or areas into
contact with the drawing tools or to only bring them into a slight
contact therewith. In other words, it is possible for the sections
which are to be cut and/or the areas which are not to be hardened
to not be pressed between the drawing tools in the drawing press or
only slightly, thus preferably partially.
[0037] The invention aims at a hardening of the workpiece so as to
set high strengths in the finished component. The areas which are
to be hardened at least partially or the areas which are to be
hardened and tempered, respectively, do not necessarily have to
have a substantially martensitic structure, but another structure
can also be set, for example bainitic or a mixed structure, which
provides for an increase of the toughness in response to reduced
strength. The specific setting of the structure can be carried out
via the parameters of the hardening device, such as temperature and
flow rate of the cooling medium, for example.
[0038] The invention will be defined in more detail below by means
of a drawing, which only illustrates exemplary embodiments.
[0039] FIG. 1 shows an exemplary embodiment of a hot forming system
according to the invention,
[0040] FIG. 2 shows a temperature-time-diagram for a first
exemplary embodiment of the method according to the invention,
[0041] FIG. 3 shows a temperature-time-diagram for a method which
is known from the state of the art,
[0042] FIGS. 4a-4d show tools of the hot forming system according
to FIG. 1 in a schematic illustration, and
[0043] FIGS. 5a-5b show a schematic detailed illustration of the
receiver of the hardening device from FIG. 1.
[0044] FIG. 1 illustrates schematically a hot forming system 1 for
carrying out a method for producing a hot formed and hardened
workpiece 2. This workpiece 2 can be a vehicle body component, such
as a chassis arm, side impact beam, bumper bracket, B column, A
column or roof frame.
[0045] The hot forming system 1 comprises a punching press 3 for
separating sheet steel cuts, so-called blanks, from a sheet steel
strip. The sheet steel strip S is unwound from a coil C and
consists of manganese-boron steel, preferably (AlSi-22MnB5).
However, other sheet steels as well as other surface coatings which
are organic and/or metallic (zinc-based) are also possible.
[0046] A transfer device 4, which is embodied as a robotic arm in
the illustrated hot forming system 1, successively introduces the
workpiece blanks 2' in the form of blanks, which are buffered on a
stack, into a furnace 5, which is a roller hearth furnace. The
workpiece blanks 2' are heated in the furnace 5 to a forming
temperature, which is higher than the austenitization temperature
of the material of the workpiece blank 2'. In the case of the
illustrated method, the austenitization temperature is at least
approx. 730.degree. C. The conversion of the manganese-boron sheet
steel from ferrite to austenite takes place between approx.
730.degree. C. and approx. 830.degree. C. To ensure a reliable
austenitization of the workpiece blank 2' and a reliable hardening
of the formed workpiece 2, the workpiece blank 2' is heated in the
furnace 5 to a forming temperature of 880.degree. C.-950.degree.
C.
[0047] When heating the workpiece blank 2', it can be held at the
austenitization temperature for a certain time (holding time), so
as to ensure an even conversion into austenite. If need be, this
can also be attained by using a plurality of furnaces through which
the workpiece blank runs successively.
[0048] The workpiece blank 2' heated to the forming temperature is
transferred to a drawing press 7 by means of a further transfer
device 6. In the illustrated hot forming system 1, in turn, the
transfer device 6 is a robotic arm. However, other transfer devices
are also possible. During the transfer from the furnace 5 to the
drawing press 7, the workpiece blank 2' is pre-cooled for
approximately 6 seconds in air, wherein the temperature of the
workpiece blank 2' is decreased to approx. 820.degree. C.
[0049] The flat workpiece blank 2' is formed in the preferably
hydraulically operated drawing press 7, in that the workpiece blank
2' is pressed in the drawing press 7 between two drawing tools 8,
9, a male part and a female part. During the forming of the
workpiece 2 the temperature thereof can decrease to a more or less
considerable extent by heat dissipated to the drawing tools 8, 9.
In the illustrated hot forming system 1, the drawing tools 8, 9 are
not forcedly cooled via a cooling medium or the like.
[0050] After the forming the workpiece 2 is passed on to a
hardening device 10 by a transfer device (not illustrated). This
transfer device, in turn, can be embodied as a robotic arm or as
another device. The workpiece 2 is received in the hardening device
10 in a receiver 11 which consists of two halves 12, 13. The two
halves 12, 13 interact such that the workpiece 2 is fixed in the
closed receiver 11, so that the formed workpiece 2 cannot deform
during hardening.
[0051] The workpiece 2 is hardened in the receiver 11 by means of
direct contact and thus by means of direct heat exchange with a
cooling agent (not illustrated). Ducts 14 and bores 15, 15' are
introduced in the upper as well as in the lower half 12, 13 of the
receiver 11 so as to guide the cooling medium to the surface of the
workpiece 2 and to discharge it again after the heat exchange has
taken place. In the case of the illustrated method, the workpiece
is cooled down from approx. 520.degree. C. to below 200.degree. C.
in the hardening device 10 in approximately 4 seconds.
[0052] In the illustrated hot forming system 1, the hardened,
formed workpiece 2 is passed to a cutting device 16. Parts of the
workpiece 2 are cut there. The temperature of the workpiece 2
further decreases in the cutting device 16 by means of heat
dissipated to the air and the cutting device 16.
[0053] The temperature profile of the workpiece 2 is illustrated in
FIG. 2 as temperature time diagram during some of the
afore-described process steps. This temperature profile serves for
a better illustration. On principle, deviations from this
temperature profile are possible with reference to time as well as
with reference to the temperatures of the workpiece 2 during the
individual processing steps.
[0054] In the case of the temperature profile illustrated in FIG.
2, the workpiece blank 2' is initially heated to a temperature of
approx. 900.degree. C. This heating phase of the workpiece blank 2'
is not illustrated in the temperature time diagram. As required the
heating phase can be carried out with a steadily decreasing
temperature gradient or with a constant temperature gradient. It is
also possible for the temperature of the workpiece blank 2' to be
held over a certain period of time during the heating, for instance
in the transition range to the austenite of approx. 730.degree. C.
to 830.degree. C.
[0055] A pre-cooling phase I in which the workpiece blank 2' cools
down in air from approx. 900.degree. C. to approx. 820.degree. C.
in approximately 6 seconds, then follows the heating phase before
the workpiece blank 2' is formed in the drawing press 7. During the
forming phase II the temperature drops to approx. 520.degree. C. in
approx. 2 seconds. The transfer phase III of the workpiece 2 into
the hardening device 10 which lasts approximately 3 seconds only
slightly cools down the formed workpiece 2 further. In the
subsequent hardening phase IV, however, a quick cool-down of the
workpiece 2 to approx. 180.degree. C. takes place which is
completed in approximately 4 seconds. Subsequently, provision is
made for another phase V in which the workpiece 2 is transferred to
a cutting device 16 and is cut there. This phase lasts
approximately 4 more seconds. The temperature of the workpiece 2,
however, only changes marginally in this period. Approximately 19
seconds after the removal from the furnace 5, the workpiece 2 can
thus already be released to a processing system in a completely cut
manner.
[0056] FIG. 3 illustrates a temperature profile of a workpiece
which is processed in a conventional manner as temperature time
diagram. In the case of this method, the workpiece blank is heated
to a temperature of approx. 900.degree. C., so as to be formed
approximately 7 seconds after the removal from the furnace. The
forming and the hardening of the workpiece are here carried out in
parallel in a device for press-hardening by means of drawing tools,
which are cooled close to the surface by means of a cooling medium
and which thus dissipate heat from the workpiece. This process step
lasts approximately 12 seconds and is thus completed approximately
19 seconds after the removal of the workpiece from the furnace. A
cutting of the workpiece has not yet taken place in this period of
time. This requires a separate method.
[0057] FIGS. 4a to 4d illustrate the individual tools of the hot
forming system 1 illustrated in FIG. 1. The furnace 5, in the
furnace chamber 17 of which a flat workpiece blank 2' is introduced
so as to be heated to forming temperature, is schematically
illustrated in FIG. 4a. It is a sheet steel blank. FIG. 4b
illustrates the drawing tools 8, 9 of the drawing press 7. The
drawing tools 8, 9 are located in an open position. The male part
forms the workpiece blank 2' around the female part and thus
creates a workpiece 2 in the form of a molded component or drawn
component, respectively. FIG. 4c illustrates the two halves 12, 13
of a receiver 11 of a hardening device 10 which will be described
in more detail below. FIG. 2b illustrates a cutting device 16, in
which the two outer ends of the formed and hardened workpiece 2 are
cut.
[0058] The receiver 11 of the hardening device 10 is illustrated in
FIGS. 5a and 5b in an open and in a closed position. The receiver
11 consists of an upper and a lower half 12, 13. The receiver could
also consist of a plurality of parts or could be embodied in one
piece. On both halves 12, 13 of the receiver 11 ducts 14 are
provided which are open towards the outside. In the closed position
of the receiver 11 these ducts 14 are closed by means of the
workpiece 2 received therein, so that a cooling medium can
circulate in the ducts 14. The cooling medium is supplied and
discharged again via bores 15, 15' which are only illustrated
schematically and which are connected to the ducts 14. The sections
of the workpiece 2 which do not close a duct 14 provided in the
receiver 11 abut on contact surfaces 18 of the receiver 11 or on
one of the halves 12, 13 of the receiver 11.
[0059] The ducts are provided so as to be distributed only in an
area 19 of the workpiece 2 to be hardened. The edges 20 of the
workpiece 2 are not hardened. The workpiece 2 does not come into
contact with the cooling medium at that location. However, in the
area 19 which is to be hardened the workpiece 2 is in contact with
the cooling medium in the closed position of the receiver 11 and
with ducts 14 filled with cooling medium, at least from one side,
thus from the top or from below. In the area 19 which is to be
hardened the workpiece 2 is thus cooled everywhere either from the
top or from the bottom through direct heat exchange with the
cooling medium.
[0060] On the other half 13, 12 of the receiver 11, contact
surfaces 18 are located opposite to the ducts 14 of the one half
12, 13 of the receiver 11. In the area 19 to be hardened the
workpiece 2 is quasi fixed in the receiver 11 via the contact
surfaces 18 either from the top or from the bottom. The workpiece 2
rests in each case continuously against the contact surfaces 18 of
both halves 12, 13 of the receiver 11 at the edges 20, i.e. in the
areas of the workpiece 2 which are not to be hardened.
[0061] On principle, embodiments of the receiver, which deviate
from the receiver illustrated in FIGS. 5a and 5b and which were
described in detail above can also be considered.
* * * * *