The invention relates to a forming tool for hot forming and partially press hardening a workpiece made of sheet steel, comprising a die, a punch which can be inserted into a cavity of the die to form the workpiece, and a cooling device. In particular, the invention relates to a method for hot forming and partially press hardening a workpiece made of sheet steel, in which the workpiece is heated prior to forming and is subsequently hot formed in a forming tool comprising a die and a punch which can be inserted into a cavity of the die, the forming tool comprising a cooling device.
The hot forming of blanks made of higher strength and super high strength steels to produce press hardened components has become established in vehicle construction in recent years. In this context, numerous ideas for producing partially hardened components having different textures have also been developed, inter alia. An idea known from DE 10 2006 019 395 A1 is the heating of a blank consisting of suitable steel to a temperature higher than the austenitising temperature and the immediately subsequent forming in a hot forming tool which is provided in at least one region with a heating device for local setting of a weaker texture. However, this known idea has the drawback that at least one heating device must be provided, which results in considerable operating costs. In addition, the continuous exposure of the corresponding active surface of the forming tool to heat has a negative effect on the service life (lifetime) thereof.
The object of the present invention is to provide a method and forming tool for hot forming and partially press hardening sheet steel, which makes it possible, in a manner which is simple in terms of production, to produce mutually adjacent zones having different textures and thus different material properties in the metal component to be produced.
According to the invention, this object is achieved by the forming tool having the features of claim 1 and by the method having the features of claim 9.
The method according to the invention, in which the workpiece is heated prior to forming and is subsequently hot formed in a forming tool, the forming tool comprising a cooling device, is essentially characterised in that, in the closed state of the forming tool, the contact between the workpiece and the contact surfaces of the die and the punch is interrupted in regions by moving apart a movable die part and a movable punch part from a closed position to an opened position.
Accordingly, the forming tool according to the invention comprises a die, a punch which can be inserted into a cavity of the die, and a cooling device. According to the invention, the die is formed of a first die part and at least one second die part which is movable relative to the first die part, while the punch is formed of a first punch part and at least one second punch part which is movable relative to the first punch part, the at least one movable second die part and the at least one movable second punch part interacting with an opening device which causes the at least one second die part and the at least one second punch part to contact the workpiece with a shorter closing time than the first die part and the first punch part.
The method according to the invention and the forming tool according to the invention thus make it possible, in a simple manner in terms of production, to produce mutually adjacent zones having different textures and accordingly different material properties such as strength and ductility at the metal component to be produced.
An advantageous embodiment of the forming tool according to the invention consists in the die parts being movably connected to a die carrier and the punch parts being movably connected to a punch carrier, the die carrier and the punch carrier each being provided with a ram, and the rams causing the at least one second die part and the at least one second punch part to move apart in the closed state of the first die part and the first punch part owing to the die carrier and punch carrier being moved closer together. This embodiment can be operated without significant additional energy consumption in comparison to conventional forming tools for hot forming and partial press hardening of steel blanks. In particular, this embodiment does not require any cost-intensive additional drive means for moving apart the tool parts which are used to produce at least one zone having a relatively weak texture in the component. Rather, the press ram which is already present and is used to close the forming tool, that is to say to move the punch, can be used for this purpose. When the rams of the die carrier and the punch carrier are moved back, the at least one second die part and the at least one second punch part close again prior to the opening of the die and punch, whereby the region of the workpiece (component) in which the contact between workpiece and tool active surface was interrupted is restruck.
Another advantageous embodiment of the forming tool according to the invention is characterised in that the first die part is rigidly connected and the at least one second die part is movably connected to a die carrier, while the first punch part is rigidly connected and the at least one second punch part is movably connected to a punch carrier, the die carrier and the punch carrier each being provided with a drive means causing a forward and a backward movement, and, in the closed state of the first die part and first punch part, the drive means causing the at least one second die part and the at least one second punch part to move apart. This variant of the forming tool according to the invention can be operated such that production can take place as desired with and without restriking of the workpiece, that is to say with and without re-closure of the die part movably connected to the die carrier and the punch part movably connected to the punch carrier. The moment at which the die part movably connected to the die carrier and the punch part movably connected to the punch carrier are moved apart can be controlled in a variable manner and according to the strain to be set in the workpiece in the contact region of these tool parts. The drive means for moving apart the at least one second die part and the at least one second punch part are preferably formed of hydraulic, pneumatic or hydropneumatic working cylinders.
Another preferred embodiment of the forming tool according to the invention provides that the at least one second die part and/or the at least one second punch part, which are moved apart in the closed state of the forming tool, comprise a controllable temperature conditioning device, preferably a heater. As a result, it can not only be ensured with a high level of reliability that one or more non-hardened regions are produced in the workpiece in a targeted manner; the embodiment also provides the option of setting in a variable manner the material properties of the workpiece, such as strength and strain, in a particular region thereof according to the requirements for the component to be produced.
The at least one second die part and/or the at least one second punch part can advantageously be made of a material which is a poor conductor of heat or have a surface coating which is a poor conductor of heat, in order to counteract in a targeted manner loss of heat in the regions of the workpiece which are not hardened. For example, ceramic material could be used as a material having low thermal conductivity.
An advantageous embodiment of the method according to the invention is characterised in that the contact between the workpiece and the contact surfaces of the die and the punch of the forming tool is interrupted in a clocked manner in regions by moving apart the movable die part and the movable punch part from the closed position to the opened position repeatedly or a number of times and subsequently moving the movable die part and the movable punch part back from the opened position to the closed position. The cooling rate of the hot formed workpiece can thus be reduced or varied as desired within a wide range. If, for example, the cooling rate of the component is 100° C./s in the case of continuous, that is to say uninterrupted, contact between cooled forming tool and workpiece or component, then the average cooling rate of the component can be reduced to approximately 20° C./s in the case of a clocked contact time of 0.2 seconds per second (that is to say, there is no contact for 0.8 seconds per second).
The clocked contact interruption is preferably set such that the sum of the contact times is less than the sum of the contact interruption times. Alternatively, however, the clocked contact interruption can also be set such that the sum of the contact times is equal to or greater than the sum of the contact interruption times.
Other preferred and advantageous embodiments of the device according to the invention and of the method according to the invention are given in the dependent claims.
The invention will be described in detail below with reference to drawings showing a plurality of embodiments. In the drawings:
The drawings show different embodiments of a forming tool 1, 1′ according to the invention for hot forming and partially press hardening a blank made of higher strength or super high strength steel. The component 2 to be produced from the blank is, for example, a bumper, a B-pillar or another crash-related body component of a motor vehicle. The blank (workpiece) consists, for example, of a manganese-boron steel, in particular a manganese-boron steel of the alloy type 22MnB5.
Regions of the component (workpiece) 2 which are to be hardened must be cooled rapidly from the austenitising temperature, while regions of the component 2 which are not to be hardened must not undergo rapid cooling.
The forming tools 1, 1′ shown in the drawings provide the option of interrupting the contact between the component and the tool active surfaces in regions.
The forming tool 1, 1′ is designed in the manner of a deep-drawing device and comprises a die 3. A cavity (recess) 4 is provided in the die 3 and shows the outer shape of the three-dimensionally formed component 2 to be produced.
The forming tool 1, 1′ additionally comprises a punch 5 which determines the inner shape of the component 2 to be produced. The punch 5 can be moved by means of an adjustment device (press ram) (not shown) from a starting position at a distance from the die 3 to a closed position in which it is fully inserted into the cavity 4 of the die 3. The adjustment device comprises a control device which controls the speed at which the punch 5 moves into the cavity 4 of the die 3.
The die 3 is divided into at least two die parts 3.1, 3.2 which are held so as to be movable relative to one another on a stationary carrier, for example a plate 6 of a platen. The punch 5 is accordingly also divided into a corresponding number of punch parts 5.1, 5.2, each die part 3.1, 3.2 being allocated a respective punch part 5.1 or 5.2 which interacts therewith. The punch parts 5.1, 5.2 are likewise held so as to be movable relative to one another on a punch carrier 7, which for example is formed of a plate. The punch carrier or the plate 7 is mounted on the aforementioned adjustment device (not shown) by means of which the punch 5 can be moved from a starting position at a distance from the die 3 into the cavity 4 of the die 3.
In
The active surface of the punch part 5.1 shown on the left-hand side and/or the active surface of the associated die part 3.1 are cooled, while the active surfaces 3.21, 5.21 of the tool parts 3.2, 5.2 shown on the right-hand side, at which a different or relatively weak texture is to be set, are cooled and/or preferably heated and/or made of a material or a surface coating which is a poor conductor of heat. For this purpose, cooling conduits 8 are made in punch part 5.1 and the die part 3.1 near the active surfaces thereof. The cooling conduits 8 are part of a cooling device (not shown in more detail). Depending on the degree of cooling required in each case, water, ice water, a deep-cooled saline solution, liquid nitrogen or another cooling medium which is suitable for rapid removal of large amounts of heat flows through the cooling conduits.
Similarly, fluid conduits 9 of a temperature conditioning device (likewise not shown in more detail) are made in the second punch part 5.2 and the die part 3.2 near the active surfaces 3.21, 5.21 thereof. A cooling medium, for example a cooling oil, is conveyed through the conduits 9 of the temperature conditioning device and causes moderate cooling of the tool parts 3.2, 5.2 in this region. Alternatively, a heating fluid, for example superheated steam, can be conveyed at least through some of the conduits 9, in particular through the conduits 9.1 arranged adjacent to the first or left-hand tool parts 3.2, 5.2. Instead of fluid conduits 9 and 9.1, heating cartridges, heating spirals or heating wires can also be integrated in the tool parts 3.2, 5.2 at which a different or relatively weak texture is to be set.
To produce the component 2, a blank made of higher strength or super high strength steel (for example of 22MnB5) is initially heated to austenitising temperature in an oven (not shown). The workpiece (blank) is then placed in the opened forming tool 1, 1′ such that the edge of the workpiece is arranged on the upper side of the die 3. Blank holders 10 which hold the edge region of the workpiece down during the subsequent forming thereof are then applied. The holding force exerted by the blank holder 10 can be adjusted according to the forming speed in each case in order to allow optimised flow of the workpiece 2 into the cavity 4 of the die 3.
The punch 5 is then brought down onto the sheet steel at high speed such that the strongly cooled end face of the punch part 5.1 at which press hardening of the sheet steel 2 is to take place comes rapidly into intensive contact with the surface portion of the sheet steel 2 associated with said end face.
An opening device is allocated to the tool parts 3.2, 5.2 shown on the right-hand side in
In the embodiment shown in
In addition, the resiliently supported die parts 3.1, 3.2 and punch parts 5.1, 5.2 are each provided with guides (not shown in more detail) which define the direction of movement of the die parts and punch parts 3.1, 3.2, 5.1, 5.2 during closure and opening of the forming tool 1.
In addition, the plate 6 which serves as a die carrier and the plate-shaped punch carrier 7 are provided with rams 17, 18. The respective ram 17, 18 penetrates an opening (through-hole) 19, 20 formed in the punch part 5.2 and die part 3.2 respectively. The rams 17, 18 and accordingly the openings 19, 20 are axially offset from one another.
In addition, the plate-shaped die carrier 6 and the punch carrier 7 are provided with further stops 21, 22 which face the rear sides of the linearly displaceable punch parts 5.1, 5.2 and die parts 3.1, 3.2. In the closed position of the forming tool 1 shown in
When the forming tool 1 is fully closed, the rams 17, 18 cause the punch part 5.2 and the die part 3.2 to move apart, that is to say a partial opening of the forming tool 1, when the die carrier 6 and the punch part 7 are moved closer together, such that the contact between component 2 and tool active surface is interrupted in regions (cf.
The closing time of the punch part 5.2 and the die part 3.2 can be set via the ram speed, that is to say the speed at which the punch 5 is moved towards the plate 6, and the spacing of the stops 21, 22. When the ram 18 is moved back, the die part 3.2 and the punch part 5.2 close again before the opening of the die part 3.1 and the punch part 5.1, whereby the region 2.2 of the component 2 in which a relatively weak texture has been produced is again restruck.
In the embodiment shown in
The punch parts 5.1, 5.2 and the plate-shaped punch carrier 7 again comprise recesses (through-holes) 14, 15 which are penetrated with clearance by the pressure rods 16 carrying the blank holders 10.
The die carrier 6 and the punch carrier 7 are also provided with stops 13, 22 which limit the range of movement of the die part 3.2 and the punch part 5.2. The stops are formed on supports 12, at which the die part 3.2 and the punch part 5.2 are additionally supported via spring elements 11. The spring elements 11, which are preferably designed as helical springs, act on the side of the punch part 5.2 and the die part 3.2 respectively which is opposite the working cylinder 23.
The contact between tool active surface 3.21, 5.21 and component 2 at the bottom dead centre of the press is interrupted by means of the working cylinders 23, which can be operated pneumatically, hydraulically or hydropneumatically.
The forming tool 1′ according to
A valve 26, preferably a check valve, is arranged in the hydraulic line 25. In addition, a hydropneumatic pressure accumulator 27 is connected to the hydraulic line 25 between the check valve 26 and the working cylinder 23. The gas pressure in the pressure accumulator 27 is for example approximately 250 bar in the closed state of the punch part 5.2 and the die part 3.2. The hydraulic line 25 is also provided with a 3/2-way valve 28, at the third port of which a return line 29 bypassing the valve 26 is connected. Also arranged in the return line 29 is a valve 30, preferably a check valve, which acts in opposition to the valve 26. In addition, a hydropneumatic pressure accumulator 31 is again connected between the valve 30 and the 3/2-way valve 28. The gas pressure in this pressure accumulator 31 is for example approximately 5 bar in the closed state of the punch part 5.2 and the die part 3.2.
The degree of the average cooling rate reduction depends mainly on the ratio of the time at which there is (partially) no contact between forming tool 1′ and component 2 to the total clock time. For the aforementioned example, this means 0.8 s/1 s=0.8. The cooling rate thus decreases by approximately 80% from 100° C./s to 20° C./s. However, since, in the case of uniform contact pressure, the cooling of the component is not exactly dependent on the times at which there is partially contact and partially no contact between component and cooled forming tool, the above formula describes merely the basic trend. The times of contact and contact interruption can be varied as desired within a wide range in the clocked mode of operation according to the invention according to
The implementation of the invention is not limited to the embodiments described above. Rather, further variants are conceivable which make use of the invention specified in the appended claims, even in the case of configuration which deviates from the embodiments shown.
Number | Date | Country | Kind |
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10 2010 027 554.9 | Jul 2010 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP11/61399 | 7/6/2011 | WO | 00 | 4/3/2013 |