Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
SYSTEM AND METHOD FOR HOT-FORMING BLANKS
The invention pertains to a system for hot-forming
blanks, as well as to a corresponding method for hot-
forming blanks.
PRIOR ART
The hot-forming of sheet metals is a relatively new
development trend in the manufacture of components,
particularly for car bodies. In the context of this
application, the sheet metals used in this case are in
accordance with the trade language used in the field of
forming technology also referred to as "blanks." A
blank usually consists of a sheet metal that is
correspondingly cut to size, punched out, joined and/or
preformed. However, the inventive measures not only can
be applied to correspondingly prepared sheet metals,
but also to the respective feedstock used. The
invention therefore concerns all workpieces and
semifinished products that can be shaped in a
corresponding forming operation, for example, by means
of pressing and/or deep-drawing.
Hot-forming allows a springback-free manufacture of
high-strength components with complex geometry and
makes it possible to significantly reduce the weight,
e.g., of car bodies produced thereof, as well as to
improve the safety, for example, of passengers in a
corresponding vehicle.
Due to more stringent requirements with respect to the
strength and rigidity of structural components,
particularly in a vehicle, high-strength and super
high-strength steels are increasingly utilized for
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these applications. An increased strength makes it
possible to reduce the vehicle weight such that, in
particular, the pollutant emission and the fuel
consumption can be reduced. In current vehicle models,
the utilization of hot-formed components makes it
possible to reduce the weight by more than 30 kg.
Hot-forming methods, in essence, are combined shaping
and hardening techniques. The utilization of
corresponding steels such as, for example, manganese-
boron steels makes it possible to achieve strengths up
to 1.500 MPa. In press-hardening methods, for example,
blanks are heated to a temperature that lies above the
complete austenitization temperature, e.g. above 850 C,
and the blank is subsequently quenched in the tool. The
desired martensite structure with the desired strength
is achieved in this way. The combination of the forming
process and the quenching process in a tool is
occasionally also referred to as press-hardening or
shape-hardening.
In the hot-forming of super high-strength materials for
automobile bodies, e.g., so-called roller hearth
furnaces are used for preheating the blanks. The
heating of such furnaces is usually realized by means
of radiant tubes that are heated electrically or by
means of gas burners. In order to realize the shortest
process cycle times possible, it is advantageous to
provide a certain "reserve" of preheated components in
the system. The heat treatment time for tempering the
steel represents a decisive parameter that defines the
cycle time of a corresponding press. Roller hearth
furnaces have a length of up to 40 meters and therefore
have corresponding structural requirements that include
the efficient removal of excess heat. Rotary drum-type
kilns that are used as an alternative to roller hearth
furnaces for preheating components also have
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corresponding disadvantages. They are also heated by
means of radiant tubes and rather unsatisfactory with
respect to their efficiency.
Press-hardened components are characterized by their
high strength and rigidity. As mentioned above, this
makes it possible to reduce the sheet metal thickness
and therefore to reduce the weight. However, one
problem can be seen in the low breaking elongation that
can lead to the formation of cracks during subsequent
production steps, e.g. when welding on other parts.
This is the reason why it is desirable to realize
certain regions, e.g. of a car body component, in a
press-hardened fashion and to realize other regions of
the same component such that they have a higher
ductility and therefore can absorb more energy due to
plastic deformation.
Prior attempts to produce such locally different
properties or so-called "tailored properties" include
purposefully influencing the alloying constituents of
corresponding semifinished products, the manufacture of
so-called "tailored welded blanks," i.e. blanks that
are joined of different materials, partial (local)
heating by means of inductive or conductive heating
technologies, partially tempering certain regions of
the press-hardening tools by means of local heating and
masking certain component regions in order to suppress
the heating (and therefore the austenitization) in a
corresponding roller hearth furnace. However, these
methods are elaborate and therefore often
unsatisfactory and very costly.
Consequently, there is a need for improved options for
making available blanks with locally different
properties.
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DISCLOSURE OF THE INVENTION
Based on these circumstances, the present invention
proposes a system for hot-forming blanks, as well as a
corresponding method for hot-forming blanks, with the
respective characteristics of the independent claims.
Preferred embodiments form the objects of the dependent
claims, as well as the following description.
Blanks and components with locally different properties
can be made available in a particularly effective
fashion due to the at least partial reheating operation
proposed in accordance with the invention, wherein said
reheating operation is carried out after forming or
press-hardening the blank in the pressing device.
According to the invention, it is particularly possible
to realize very complex shapes with the desired
material properties such as, e.g., an increased
ductility at any location.
ADVANTAGES OF THE INVENTION
As already mentioned above, the term "blanks" should be
interpreted comprehensively in the context of this
application. This term includes sheet metals,
semifinished products, joined and/or preformed
components that are hot-formed, particularly press-
hardened, in a corresponding system.
A particularly advantageous aspect of the invention
concerns the utilization of a premixing hydrogen-oxygen
burner or fuel gas-oxygen burner. Burners of this type
are basically known, for example, from DE 103 45 411
Al. For example, premixing fuel gas-oxygen burners are
used for the so-called fire-polishing of glass parts,
particularly parts of lead crystal or soda-lime glass.
In this case, at least part of the surface of the glass
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part is heated and fused with the burner. Corresponding
burners are also known as so-called Hydropox burners
and sold by the applicant under this brand name.
Premixing fuel gas-oxygen burners, particularly
hydrogen-oxygen burners, are characterized by a
particularly high heat transfer efficiency. In contrast
to so-called externally mixing burners, a fuel mixture
of fuel gas and oxygen is already fed to a burner head
of a premixing fuel gas-oxygen burner rather than
ultimately produced in a corresponding burner head.
Premixing burners produce particularly hard flames that
are suitable for fusing larger surface areas that may
also feature depressions or other irregularities.
According to the invention, it was determined that this
represents a decisive advantage in comparison with
externally mixing burners. Externally mixing burners
are only capable of producing a soft flame that cannot
penetrate, in particular, into corners, holes or
depressions of a surface. Consequently, the utilization
of a premixing burner makes it possible to locally heat
certain regions of corresponding blanks, particularly
regions that are shaped differently. Although prolonged
heating by means of an externally mixing burner would
also make it possible to achieve high temperatures, it
could occur that the entire blank is heated rather than
only the desired regions.
According to a particularly preferred embodiment of the
inventive system, the at least one reheating device is
realized such that it can be three-dimensionally
oriented and/or three-dimensionally displaced. The
reheating device used in accordance with the invention
may be mounted, for example, on an industrial robot.
This makes it possible to exactly guide and orient the
reheating device along or over the surface of the
formed blank such that it can be uniformly heated in
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the desired regions (partially) to a desired
temperature range, e.g., between 650 and 850 C,
particularly 700 C - 800 C, preferably about 750 C.
The heating device (particularly for completely heating
the blank prior to the forming operation in the
pressing device) is preferably realized in the form of
an austenitizing device. A complete austenitization is
preferred in this case. An austenitization delivers the
desired material properties that make it possible to
subsequently press and simultaneously cool or quench
the blank and then to at least partially heat (reheat)
the blank. A corresponding austenitizing device is
designed, in particular, for locally heating the blank
to a temperature of 750 - 1050 C, particularly 800 -
1000 C, for example 850 - 950 C. A corresponding
temperature depends on the respective materials and
lies above the austenitization temperature. For
example, the austenitization temperature of the
aforementioned manganese-boron steels lies at
approximately 850 C. If a corresponding blank is
preheated to a temperature slightly below the
austenitization temperature, the austenitization
temperature can be quickly reached or exceeded with a
corresponding burner, particularly in predefined
regions of the blank. In such a cooling process during
the pressing or forming operation, the blanks are
preferably cooled to temperatures of 100 C - 200 C,
wherein cooling to any temperature between room
temperature and 250 C would also be possible.
A corresponding system advantageously furthermore
features at least one loading device for loading the
system with blanks and/or at least one transfer device
for transferring the blanks into the at least one
pressing device of the system and/or at least one
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transfer device for transferring the blanks to the
reheating device.
The at least one heating device advantageously
comprises at least one paternoster furnace. For
example, it would be possible to utilize generally
known vertical paternoster furnaces that have an
improved energy efficiency and the advantage, in
particular, of being suitable replacements for
conventional roller hearth furnaces that, as mentioned
above, have a large structural size and therefore
corresponding structural requirements. Paternoster
furnaces can be heated, for example, electrically or
with fuel and operated in corresponding temperature
ranges such that an efficient and reliable heating
process is ensured.
The respective temperatures to be adjusted depend on
the respective material of the blanks. As mentioned
above, the complete austenitization temperature of
manganese-boron steels lies at approximately 850 C. A
person skilled in the art can easily derive
corresponding temperatures from available material
parameters.
It is advantageous to also realize the heating device
with at least one premixing hydrogen-oxygen burner or
fuel gas-oxygen burner. This likewise allows a very
effective and, in particular, also regional heating of
the blanks.
Although such a heating device, particularly
austenitizing device, is in the context of the present
invention preferably used for a complete
austenitization of a blank, it may also be realized for
partially heating blanks, particularly for
austenitizing blanks, i.e., for heating or
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austenitizing certain regions or local regions of
blanks. In this case, at least one burner flame of a
premixing hydrogen-oxygen burner may be directed at the
region(s) to be partially heated, particularly
austenitized. A corresponding burner arrangement
therefore makes it possible, in particular, to realize
a defined local austenitization of regions, in which a
high local strength can be subsequently be achieved,
for example, by means of press-hardening. However, a
sufficient ductility of the material is ensured in the
non-austenitized regions after the press-hardening
operation. In this way, it would be conceivable, e.g.,
to make available a desired ductility in first regions
of the blank with such a partial heating process by
means of the heating device, i.e. prior to the forming
operation, and in second regions of the blank with a
heating process by means of the reheating device, i.e.
subsequent to the forming operation.
In a corresponding system, a heating device,
particularly an austenitizing device, and a preheating
device are advantageously realized in the form of one
structural unit. This makes it possible to realize
compact and energy-efficient systems that have a small
structural size and, for example, merely require one
heat insulation or thermal insulation.
An inventive method comprises the steps of loading
blanks into an inventive system, heating or
austenitizing the blanks at least locally in a heating
device, particularly an austenitizing device, forming
the blanks by means of pressing in a pressing device
and subsequently heating the blanks at least partially
in a reheating device. As mentioned above, the pressing
operation may also concern a press-hardening process.
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The inventive system for hot-forming blanks and the
inventive method likewise benefit from the above-
described advantages.
It goes without saying that the above-described
characteristics, as well as the characteristics yet to
be described below, not only can be used in the
respectively described combination, but also in other
combinations or individually without deviating from the
scope of the present invention.
An exemplary embodiment of the invention is
schematically illustrated in the drawings and described
in greater detail below with reference to the drawings.
DESCRIPTION OF FIGURES
Figure 1 shows a schematic representation of a system
for hot-forming blanks according to a
preferred embodiment of the invention.
Figure 2 shows a schematic representation of a burner
head suitable for use in an embodiment of the
invention.
Figure 3 shows a flow chart of a method for hot-
forming blanks according to an embodiment of
the invention.
If applicable, elements that function or operate
identically are identified by the same reference
symbols in the figures and their description is not
repeated for reasons of simplicity.
Figure 1 shows a system for hot-forming blanks
according to a preferred embodiment of the invention.
The system as a whole is identified by the reference
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symbol 10. It features a loading device 3, in which
corresponding blanks P such as, for example, punched-
out sheet metal pieces can be loaded into a
corresponding system in the direction of the arrow
(lower horizontal arrow). A heating device 4 is
provided and features a schematically illustrated
paternoster furnace 4a. The blanks P are transported
into a lower region of the heating device 4 in the
direction of the arrow, raised (as indicated with the
vertical arrow) and continuously heated while being
raised. The blanks may be heated in the paternoster
furnace 4a in such a way that they austenitize.
However, Figure 1 shows an embodiment, in which the
heating device 4 features an austenitizing device 4b
that is arranged downstream of the paternoster furnace
4a. In this case, the paternoster furnace serves for
preheating the blanks.
The blanks P once again exit the paternoster furnace 4a
in an upper region thereof, namely in the direction of
the arrow (upper horizontal arrow). Subsequently, they
pass through the austenitizing device 4b that features
a burner 14 symbolized in the form of a three-flame
burner. The burner 14 may have an arbitrary number of
burner flames. The burner 14 may also be mobile and
successively act upon different regions of a blank P.
To this end, it would be possible to provide
corresponding moving devices that can also be actuated
in a fully automated fashion, for example, by utilizing
a corresponding control. The blanks P pass through the
austenitizing device 4b in the direction of the arrow
while being heated to a temperature (e.g. 900 C) that
lies above an austenitization temperature of the
corresponding material.
The blanks P subsequently reach a transfer device 5, by
means of which they are transferred to a pressing tool
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8. The pressing tool 8 forms the blanks in the desired
fashion, wherein the blanks are simultaneously cooled
to approximately 200 C or less during the forming
operation.
A martensitic or hard structure is created in the
austenitized regions of the blank due to this cooling
or quenching process that preferably takes place with a
rate in excess of 30 K/sec.
As mentioned above, the formed blanks have in this
state a temperature of approximately 200 C. In this
state, the formed blanks are now partially acted upon
with heat by means of a reheating device 16 that
features at least one premixing hydrogen-oxygen burner
18 or fuel gas-oxygen burner. In this way, the hard
structure is transformed into a mixed structure that
has improved properties, for example, with respect to
its ductility at the locations of the formed blank that
are acted upon with heat.
The reheating device 16 may be mounted, for example, on
a (not-shown) industrial robot such that the burner 18
can be three-dimensionally displaced and oriented. This
makes it possible to exactly guide the burner 18 along
a component surface such that it can be uniformly
heated to temperatures between approximately 650 and
850 C in the desired regions. The thusly achieved
structural change results, e.g., in a reduced hardness
and an increased elongation or ductility. In
experimental tests, e.g., the ductility values could be
improved by up to 18 percent.
The burners 18 may be realized with arbitrary
geometries (also with smaller diameters, for example,
for welding spot regions) and therefore are capable of
heating various regions of a component or of a formed
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blank P. In this case, the energy transfer is very
efficient and the treatment time can be reduced to a
few seconds.
The invention provides clear advantages in comparison
with other heating technologies such as, for example,
induction heating that is not suitable for three-
dimensional geometries or blank shapes, e.g. because
inside radii cannot be properly heated.
The inventive method also provides advantages in
comparison with conventional laser-assisted methods.
Although laser-assisted methods are generally capable
of performing similar tasks, the high energy density
and the relatively small focal surface require a
significantly higher effort, for example, for heating
larger coherent regions such that methods of this type
are relatively ineffective in practical applications.
The inventive method makes it possible to subsequently
heat partial regions of a blank, particularly of a
three-dimensionally formed blank such as, for example,
hardened blanks of UHS steel, in a highly variable and
effective fashion, wherein the ductility of the
material can be increased to a sufficient value for a
purposeful deformation.
The burners used in accordance with the invention make
it possible, for example, to realize focal surfaces
with a surface area up to 10 by 20 cm2. It is
particularly preferred to utilize burners that make it
possible to realize focal surfaces with a size of 2 cm
x 2 cm or 4 cm x 2 cm.
A preferred embodiment of an inventive burner head is
illustrated in Figure 2.
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In Figure 2, a suitable premixing burner head according
the invention is identified by the reference symbol 22.
A premixing hydrogen-oxygen burner used in accordance
with the invention features a channel 221, through
which a hydrogen-oxygen mixture can be fed to the
burner head 22, and is capable of producing a very hard
burner flame that ensures a very good energy transfer.
This makes it possible, in particular, to act upon
regions that have recesses or more complex contours
with the required heat in a more reliable fashion. In
this case, the corresponding gas mixture therefore
already exits the burner nozzles 223 in the form of a
mixture and is ignited at this location.
Figure 3 shows a flow chart of a method 100 according
to a particularly preferred embodiment of the invention
in the form of a schematic representation.
Corresponding blanks P are punched out of a sheet metal
in a first step 101. In a second step 102, these blanks
are loaded into an inventive hot-forming system, for
example, by means of a loading device. This loading
process may take place continuously. In step 103, the
blanks P are preheated in the system, wherein the
above-described means may be used for this purpose. An
austenitization of the above-described type then takes
place in step 104. After the austenitization, the
blanks P are transferred into a pressing tool by means
of a transfer device in step 105, wherein the blanks
are then formed or pressed and simultaneously quenched
in said pressing tool in step 106. After the quenching
process in the pressing tool, the press-hardened blanks
that may have complex three-dimensional shapes in this
state are partially heated in the desired fashion (step
107) by means of a reheating device, particularly a
premixing hydrogen-oxygen or fuel gas-oxygen burner,
such that a mixed structure with the desired properties
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=
(e.g. improved ductility) can be made available in the
heated regions.
14
