Patent Application
20110094282
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 ≧Ms+10K (Ms=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° C., preferably Mf (Mf=martensite finish).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The invention will be defined in more detail below by means of a drawing, which only illustrates exemplary embodiments.
a-4d show tools of the hot forming system according to
a-5b show a schematic detailed illustration of the receiver of the hardening device from
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.
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° C. The conversion of the manganese-boron sheet steel from ferrite to austenite takes place between approx. 730° C. and approx. 830° 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° C.-950° C.
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.
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° C.
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.
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.
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° C. to below 200° C. in the hardening device 10 in approximately 4 seconds.
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.
The temperature profile of the workpiece 2 is illustrated in
In the case of the temperature profile illustrated in
A pre-cooling phase I in which the workpiece blank 2′ cools down in air from approx. 900° C. to approx. 820° 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° 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° 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.
a to 4d illustrate the individual tools of the hot forming system 1 illustrated in
The receiver 11 of the hardening device 10 is illustrated in
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.
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.
On principle, embodiments of the receiver, which deviate from the receiver illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 102009050533.4-14 | Oct 2009 | DE | national |
