US20160059295A1

US20160059295A1 - Method and press for producing sheet metal parts that are hardened at least in regions

Info

Publication number: US20160059295A1
Application number: US14/835,156
Authority: US
Inventors: Georg Frost
Original assignee: Benteler Automobiltechnik GmbH
Current assignee: Benteler Automobiltechnik GmbH
Priority date: 2014-08-26
Filing date: 2015-08-25
Publication date: 2016-03-03
Also published as: EP2993241A1; EP2993241B1; ES2828966T3; DE102014112244A1; CN105478559A

Abstract

A method for producing a sheet metal part with a press includes the steps of heating a sheet metal blank at least in regions to a above the austenitizing temperature Ac3, inserting the heated sheet metal blank into a forming tool station of the press, hot forming the sheet metal blank to the sheet metal part in the forming tool station, during which the press performs a closing movement, holding the forming tool station closed for a first holding time and cooling the formed sheet metal part during the first holding time, transferring the formed sheet metal part into a second tool station, hardening at least regions of the sheet metal part by cooling in the second tool station within the at least one second holding time, wherein the forming tool station during the closing movement of the press from an upper reversal point to a lower reversal point is moved relative to the press by at least one elastic actuating element so that the hot forming is terminated and the step of holding the forming tool station closed starts before the press reaches the lower reversal point.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of German Patent Application, Serial No. 10 2014 112 244.5, filed Aug. 26, 2014, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method and a press for producing sheet metal parts that are hardened at least in regions.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
In the forming technology it is known to form sheet metal parts from metal strip material, in particular made of steel, in a pressing tool, by a forming operation which involves at least one step. For this the sheet metal is uncoiled from the strip and sheet metal blanks of defined geometry are cut from the strip. Subsequently the sheet metal blank is formed in a press having at least one forming tool station.
Especially in the automobile industry it is common to produce complex geometries and along with the forming to also adjust defined mechanical properties. For this purpose hot forming, also known as press-hardening or forming-hardening, has been widely used for producing chassis- and structural parts of motor vehicles.
It would be desirable and advantageous to provide an improved reliable method that can be used in large-scale industrial applications, and a press for producing hardened sheet metal parts, which has an increased production throughput and a higher resulting product quality.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method for producing a sheet metal part which is hardened at least in regions in a press which has a press table, a press ram and multiple tool stations, includes the steps of heating at least regions of a sheet metal blank to a temperature above austenitizing temperature Ac3; inserting the heated sheet metal blank into a first one of the tool stations of the press, wherein the first tool station is configured as forming tool station; hot forming the sheet metal blank to the sheet metal part in the forming tool station, wherein the press during the hot forming step performs a closing movement from an upper reversal point to a lower reversal point; holding the forming tool station closed for a first holding time; cooling the formed sheet metal part during the first holding time; transferring the formed sheet metal part into a second tool station; and hardening the sheet metal part at least in regions by cooling the sheet metal part in the second tool station within a second holding time, wherein the forming tool station during the closing movement of the press is moved relative to the press by at least one elastic actuating element so that the hot forming is terminated and the step of holding the forming tool station closed starts before the press reaches the lower reversal point, wherein the upper die and the lower die have cooling channels through which a coolant is conducted.
This enables a highly economical production process, especially for large-scale industrial production, and at the same time improves the quality of the part. Concretely, the cycle time of the hot forming and hardening of the sheet metal part can be reduced, in that the forming is initiated and terminated very early and the holding time period for quenching the formed sheet metal part is maximally used in the individual tool stations. In addition inaccuracies regarding the positioning of the upper and lower dies of the forming tool station and unevenness in the surface properties and thickness of the sheet metal blank can be compensated by the elastic actuating elements.
The sheet metal blank is further processed by hot forming into the sheet metal part, wherein in the following description both terms are also used synonymously, when the properties caused by the forming are not the focus, but the method steps, in particular the heat treatment and the press are explained.
Within the framework of the invention, the sheet metal blank is heated at least in regions by known heating methods to a temperature above austenitizing temperature of the used steel alloy. It is useful within the framework of the invention to use heating devices with a high heating rate, for example heating devices that are based on contact heating with inductively or conductively heated contact masses, direct burner heating or furnace heating with over-temperature in the furnace chamber. Also a combination of these heating devices with each other or with other known furnace types is possible, for example when using sheet metal blanks with a metallic coating.
The austenitizing temperature Ac3 is also referred to as recrystallization temperature, wherein the degree of the austenitizing temperature depends on the exact alloy composition. For the method according to the invention the use of Manganese-Boron steels has proven advantageous, which after the heating are hardened throughout as a result of transformation of the austenitic microstructure into martensitic microstructure. Along with the hardness the mechanical properties yield strength Rp0.2 and tensile strength Rm increase, while the maximal bending angle and elongation at fracture A50 decrease.
The term holding time within the context of the present invention means the time period in which the upper die and the lower die of at least the forming tool station and the second tool station are closed, i.e., at least regions of the upper die and the lower die are in close contact with the formed sheet metal blank or with the sheet metal part.
The term transfer includes any handling operation that effects the transport of the sheet metal part from one tool station into the temporally following tool station, including the removal from the respective tool station and insertion into the respective tool station.
The definition of the term reversal point of the press according to the invention is that within the operating cycle the press reaches exactly one open position, the upper reversal point, and exactly one maximally closed position, the lower reversal point. From the foregoing a respective maximally open position of the press for maintenance and tool change purposes is to be distinguished, which depends on the type of press involved and which may be greater than the upper reversal point.
According to another advantageous feature of the invention, edge trimming and/or hole punching are performed during or after the hot forming in the forming tool station, in particular before the press is fully closed. This has the advantage that a subsequent hole-punching or edge trimming does not have to be performed when the sheet metal part is in the cold and hardened state, which reduces tool wear and avoids additional handling steps. Because the hole-punching or edge trimming occurs before the press is completely closed, the closing movement can be used for driving the tools that are required for the hole-punching or edge trimming. Subsequent thereto the trimmed steel sheet part is transferred into the second tool station for fast cooling of at least regions of the sheet metal part.
According to another advantageous feature of the invention, the second tool station is also moved relative to the press, at least in regions, during the closing movement of the press by at least one elastic actuating element, so that the step of holding the second tool station closed begins before the press has reached the lower reversal point. However, it is also possible that the step of holding the second tool station closed is terminated at least in regions by an elastic actuating element only during the upward movement of the press, after the press has completely traversed the lower reversal point. Most preferably, the second tool station is supported relative to the press by at least one elastic actuating element so that during a significant portion of time of the closing movement and during a significant portion of time during the upward movement, the upper die and the lower die remain closed. A significant portion of time in this context means more than 30 percent of the duration of the closing movement and/or the upward movement of the press.
The forming tool station, and also preferably the second tool station can be mechanically, hydraulically or pneumatically spring-supported by the elastic actuating element. The elastic actuating elements themselves can act either passively purely mechanically by permanently exerting a force on the upper or lower die, which acts in opposition to the pressing force. A simple example for this are helical springs or spring packages made of other mechanical springs. However, it is also possible that at least one elastic actuating element is actively controlled in order to adjust a course of an actuating force of the actuating element during the movement of the press in a differentiated manner. The latter makes it possible to differentiate between an actuating force level that is sufficient for the hot forming and an actuating force level that is required for holding the tool station closed, while at the same time reducing stress on the elastic actuating element and the actuating system associated therewith. This also allows reducing the overall holding time for performing the fast cooling and for the hardening of the sheet metal part.
According to another advantageous feature of the invention, the sheet metal part is transferred by a transfer system, in particular by a linearly guided transfer bar with grippers, within a cycle time of 1 to 4 seconds, preferably 2 to 3 seconds, between at least two tool stations. This makes it possible to minimize heat losses during the movement of the sheet metal part between the tool stations and ideally to dispense with complex multi-axes handling devices. Hereby it can be provided that the grippers of the transfer system are already moved close to the sheet metal part before the press has reached the upper reversal point. In particular for this purpose recesses can be provided in the tool stations for guiding the transfer system therethrough while avoiding collision or to move the grippers close to the sheet metal part so that the sheet metal part is immediately removed and further transported by the transfer system or the grippers of the forming tool station when the upper die and the lower die move apart, i.e., after expiration of the holding time.
The method according to the invention also enables in a particularly simple and reliable manner producing a sheet metal part that is only hardened in regions. Hardened in regions within the context of the present invention means that the sheet metal part has at least a first section with a relatively low strength and yield strength with a microstructure that is preferably non-hardened or only hardened to a low degree and at least one second section with high strength Rm and yield strength Rp0.2 but reduced elongation at fracture A50 and has essentially a martensitic microstructure. This means that the first section of the sheet metal part has a tensile strength between 400 and 800 Mega Pascal (MPa), in particular between 450 and 650 MPa, and predominantly ferrite-perlite microstructure components.
According to another advantageous feature of the invention, a cooling temperature of the sheet metal part is adjusted in the forming tool station, that is greater in a first section of the sheet metal part than the martensite start temperature Ms required for martensite transformation, and is smaller in a second section than Ms, wherein the sheet metal part in the first section is in particular cooled to a cooling temperature of between 540 and 660° C. Beside the cooling temperature it is important in the second section to also adhere to a high cooling rate above 25 Kelvin per second (K/s), in particular above 70K/s, in order to ensure a sufficient through-hardening of the microstructure in this section. Cooling the first section of the sheet metal part to a lesser degree and to a higher temperature compared with the cooling in the second section achieves that a microstructure transformation in the sheet metal part from the austenitic state to the martensitic state is prevented, however at the same time a microstructure transformation from austenite to ferrite and/or perlite is initiated.
In order to obtain a particularly stretchable sheet metal part that can also be reliably formed, in particular deformed, without crack-formation, a cooling temperature of the sheet metal part can be adjusted in the second tool station which at the end of the second holding period is between 350 and 500° C. in the first section and is smaller in the second section than the martensite finishing temperature Mf required for complete martensite microstructure transformation, wherein the sheet metal part in the second section is preferably cooled to below 200° C., in particular to room temperature. This ensures that the microstructure in the first section has sufficient time at a temperature that is significantly higher than the first section to transform the microstructure to ferrite and/or perlite, without significant formation of bainite or even martensite. On the other hand the temperature treatment of the second section is intended to achieve that a completely martensitic microstructure is generated, which results in a tensile strength between 1400 and 2100 MPa, preferably 1450 and 1800 MPa, depending on the steel alloy, in particular depending on the carbon and manganese content.
The sheet metal part may also be held for a first holding period of 2 to 8 seconds and for a second holding period between of 2 to 10 seconds. Thus the time period available for the cooling of the formed sheet metal part and for the associated microstructure transformation is almost doubled, while the cycle time of the press remains unchanged. The difference between the two holding times results in particular from the time period required for the hot forming in the forming tool station.
According to another advantageous feature of the invention, the press has a forming tool station and a second tool station that follows the forming tool station in temporal succession, and the cycle time of the press to move between its upper reversal point and its lower reversal point is between 3 and 11 seconds. This makes it possible to achieve a very high production rate or a high throughput and with this very low manufacturing costs. In combination with the production of sheet metal parts in which regions are hardened, the additional advantage is achieved that the properties and the quality of the sheet metal part are not adversely affected by the fast production cycle. The produced sheet metal parts have a high dimensional accuracy, in contrast to a one-step hot forming and press hardening process for producing sheet metal parts of which regions are press hardened with heated tool sections.
According to another advantageous feature of the invention, the hardening is only terminated in a third tool station, wherein after this the entire sheet metal part has a cooling temperature below 200° C. This has the advantage that the cycle time of the press can still further be reduced and at the end of the press a cold sheet metal part can be removed that is not critical with regard to touching. A residual heat deformation is in particular entirely prevented by adjusting the cooling temperatures of the two sections of the sheet metal part.
According to another advantageous feature of the invention, the cycle time of the press with forming tool station and a second tool station and a third tool station, i.e., in a process with three temporally successive stations, between its upper reversal point and its lower reversal point is between 3 and 9 seconds. In order to achieve this short cycle time of the press, preferably a mechanical crank press or eccentric press or a servo-electric press is used, wherein the mechanical press is capable of traveling through the reversal points without significant interruption of the movement of the press. A holding time as a result of standstill of the press is thus advantageously not required.
A further aspect of the invention relates to a press for producing a sheet metal part, which is hardened at least in regions. The press has multiple tool stations, a press ram and a press table and can in particular be used to implement the method described above. According to the invention it is provided that at least one tool station is a forming tool station for hot forming sheet metal blanks which can be cooled at least in regions and which has an upper die and a lower die which form a hollow mold space in the closed state.
The press is characterized in that between the press table and the lower die at least one elastic actuating element is arranged so that either the lower die can be lifted relative to the press table and/or the upper die can be impinged with pressure at a distance to the press ram in order to achieve the closed state of the forming tool station before the press is completely closed, wherein the upper die and the lower die have cooling channels through which a coolant can be conducted.
In other words it is possible that the lower die and/or the upper die are spaced apart from the press table or press ram at least until the press reaches the lower reversal point. The distance can be adjusted by the elastic actuating element and deceases during the hot forming and preferably also during a time period of the holding time when the upper die and the lower die are in the closed state. For compensating the position of the upper die and lower die relative to each other and possible uneven thickness of the sheet metal part, it is sufficient to arrange an elastic actuating element only on one side. However, it is also conceivable to provide corresponding actuating elements on both sides, i.e., between the upper die and the press ram and between the lower die and the press table. When a trimming or a hole punching is to be performed in the forming tool station simultaneously with or after the hot forming, it is also conceivable to additionally provide a trimming tool which can be supported on an elastic actuating element.
According to another advantageous feature of the invention, a vertical travel of the press is adjustable by the elastic actuating element, wherein the vertical travel is smaller than the maximal press stroke between the upper and the lower reversal points of the press, however, at least 100 mm. In combination with the speed that is predetermined by the press drive during the closing movement and during the upward movement of the press it is thus advantageously possible to extend the holding time by contact between the sheet metal blank, upper die and lower die.
According to another advantageous feature of the invention, the elastic actuating element has an actuating force which increases at least over a part of the travel from the upper reversal point to the lower reversal point of the press, in particular the actuating force in the closed state of the forming tool station is at least 20% greater, which increases the contact pressure between the tool station and the sheet metal part and with this enables a high heat transfer or respectively makes it possible to quickly reach at least one quenching temperature of the sheet metal part.
In order to produce the part that is hardened only in regions, it is further preferably provided that at least the forming tool station is partially heatable by a heating source, in order reduce the cooling rate in a first section of the sheet metal part, wherein unheated regions have cooling channels for conducting a cooling medium therethrough. Concretely this allows setting a temperature close to room temperature in the unheated region of the tool station, in any case however, below 200° C. The heat source allows setting a temperature of between 650 and 450° C. in the heated region. Thus the upper die and the lower die are configured to cool the formed sheet metal part during the holding time with different cooling rates and thereby different quenching temperatures or to hold the sheet metal part at these temperatures. As described above this causes a first section of the sheet metal part to have a smaller tensile strength and a second section of the sheet metal part to have a high strength.
As an alternative or preferably in addition it is also possible that the forming tool station and also the second subsequent tool station is heatable in regions in particular by a heating source, in order to prevent the first section of the sheet metal part to be completely hardened. This results in the same features and advantages as described with regard to the second tool station in the method for producing sheet metal parts only sections of which are hardened.
According to another advantageous feature of the invention, the unheated region of the second tool station can have an active cooling source, which at least corresponds to a transition region between the first section and the second section of the sheet metal part. The cooling source serves for reducing a heat transfer between the differently tempered sections of the sheet metal part and thereby ensures a most narrow transition section. This has the advantage that constructors have to take only a small surface into consideration when designing sheet metal parts for the vehicle industry, to which no mechanical characteristic values can directly be assigned or for which no mechanical characteristic values can be guarantied. This also ensures that the second region has a continuously uniform distribution of the microstructure and the mechanical properties. The cooling source can hereby be formed by the coolant itself and/or can include a heat exchanger which is arranged outside the press, and which is operatively connected with the coolant and the cooling channels of at least the forming tool station.
According to another advantageous feature of the invention, the first section and the transition section of the sheet metal part are form fittingly fixable at least in the second tool station by fixing elements. In the case of a three-station press with forming tool station, second tool station and third tool station, the sheet metal part is preferably also form fittingly fixable in the third tool station also by fixing elements. In the press according to the invention this results in an even distribution of the pressing force in all tool stations and in particular prevents that the press table and the press ram are oriented unevenly relative to each other.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:

FIG. 1 a first embodiment of the press according to the invention in cross section;

FIG. 2 a a cross sectional view of a second embodiment of a forming tool station of the press according to the invention;

FIG. 2 b a longitudinal sectional view of the second embodiment of FIG. 2 a;

FIG. 3 a third embodiment of a press according to the invention in longitudinal section;

FIGS. 4 a and 4 b a longitudinal section through the forming tool station illustrating the method according to the invention at different time points of the press cycle;

FIG. 5 an embodiment of a forming tool station of press according to the invention for producing sheet metal parts that are hardened in regions,

FIG. 6 an alternative embodiment of a forming tool station of a press according to the invention for producing sheet metal parts that are hardened in regions,

FIG. 7 a a longitudinal sectional view of an alternative embodiment of a press according to the invention for producing sheet metal parts that are hardened in regions;

FIG. 7 b a horizontal sectional view of the alternative embodiment of FIG. 7 a;

FIG. 8 a a longitudinal sectional view of an alternative embodiment of a press according to the invention for producing sheet metal parts that are hardened in regions;

FIG. 8 b a horizontal sectional view of the alternative embodiment of FIG. 8 a;

FIG. 9 a a time temperature course of the sheet metal blank for the method according to the invention in a two-station press and separate cooling station; and

FIG. 9 b a time-temperature course of the sheet metal blank for the method according to the invention in a three-station press.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.
Turning now to the drawing, and in particular to FIG. 1, there is shown a longitudinal section of a press 1 according to the invention with two tool stations, the forming tool station 2 and a second tool station 3. An initially unformed sheet metal blank 26 first passes through the forming tool station 2 to be formed into a sheet metal part 27 and then passes through the second tool station 3. The press 1 has a press ram 6 and a press table 5, wherein on the press table 5 two clamping plates 10 are arranged. Each clamping plate 10 further has multiple elastic actuating elements 7, which extend from the clamping plate 10 in the direction of the press ram 6, wherein on the ends of the press ram 6 that face away from the press table 5 a tool camping plate 9 is fixed. The clamping plates 10 are respectively fixed on the press table 5 via clamping elements 31. The tool clamping plates 9 are also fixed to the ends of the actuating elements 7 that face away from the press table 5.
On the tool clamping plates 9 two respective lower dies 12, i.e., the lower die of the forming tool station 2 and the lower die of the second tool station 3, are fixed via clamping elements 31′. Corresponding to the lower die 12, an upper die 11 is fixed on the press ram 6, wherein between the upper die 11 and the lower die 12 a sheet metal blank 26 can be arranged. The upper die 11 and the lower die 12 each have cooling channels 17, through which a coolant 18 can be conducted. The lower die 12 further has guide elements 32′, which are configured to be inserted into corresponding guide recesses 32 and enable the guiding of the upper die 12 and the lower die 11 relative to each other. As a result of the floating support of the lower dies 12 provided by the elastic actuating elements 7, possible positional deviations relative to the upper dies 11 transversely to the movement of the press can be compensated through the guide elements 21 and the guide recesses 32. The elastic actuating elements 7 in FIG. 1 are configured as pneumatic spring packages, i.e., they are actuating elements 7 that can be impinged with gas pressure, and which can be actively controlled. Shown is the state of the press in the upper reversal point OP, with the elastic actuating members 7 lifting the lower die 12 relative to the press table 5, whereby the travel of the closing movement Y of the forming tool station 2 and the second tool station 3 is shorter than the press lift travel between the upper reversal point OP and the lower reversal point UP of the press.
FIG. 2 a shows a further embodiment of the press according to the invention, wherein the press 1 has two tool stations, of which, however, only the dual forming tool station 2 is shown in the longitudinal section. Arranged behind the forming tool stage 2 is the not shown second tool station 3 which is also configured as dual tool station, which receive the sheet metal blanks 26 that were already hot formed in the forming tool stage 2 and further cool the sheet metal blanks. The term dual tool station means that in the tool two parts can be hot formed and cooled simultaneously within the press cycle. In contrast to the embodiment of FIG. 1, mechanical spring packages 8 are formed as elastic actuating elements 7, here as a plurality of helical springs.
In the second embodiment, the press 1 also has additional cutting means 33 on the upper die 11 and cutting means 33 on the lower die 12, which serve for trimming the formed sheet metal blank or the sheet metal part 27 in the still hot and unhardened state. In particular a border trimming is performed, wherein as exemplary shown the upper cutting means 33′ are fixed on the upper die 11 via elastic actuating elements 34. Similar to the actuating elements 7, the elastic actuating elements 34 can act actively or passively. The lower cutting means 33 on the other hand are connected with the lower die 12 in a fixed and immovable manner. However, the opposite arrangement of connecting the cutting means 33, 33′ on the forming tool station 2 is also possible. The left hand side in the image plane exemplary illustrates that the cutting tools 33 are fixedly but exchangeably connected to the upper die 11 and the lower die 12. Of course in praxis the same type of connection can be provided for both, for the right halves and also for the left halves of the forming tool station 2.
FIG. 2 b shows a cross sectional view of the second embodiment of the press 1 through the forming tool station 2, which also in this case is arranged on the mechanical spring package 8 so as to be movable relative the press table 5. It can be seen that the cooling channels 17 extend over the entire longitudinal extent of the upper die 11 and the lower die 12, including (here not shown) an inlet and outlet of the cooling channels 17 to a cooling source 19, which is situated for example in the form of a heat exchanger outside of the forming stage tool 2, preferably also outside of the press 1.
FIG. 3 shows a longitudinal sectional view of an alternative embodiment of the press 1 according to the invention with a forming tool station 2 and a second tool station 3, which follows the forming tool station 2 in temporal sequence. It can be seen that elastic actuating elements 7 in the form of mechanical spring packages 8 are arranged on a common press table 5 via a clamping plate 10. The ends of the elastic actuating elements 7, which face away from the press table 5, can be coupled with a tool clamping plate 9, which in turn is connected with the lower die 12 or is an integral component of the lower die 12. As in the preceding embodiments, the upper die 11 and also the lower die 12 have cooling channels 17 for conducting a coolant 18.
FIG. 4 a shows the respective operating position of the press 1 and the associated forming tool stage 2 at different time points of the press cycle. In the center of the image on the left hand side the press is shown in the upper reversal point OP, on the right hand side at the time point at which the forming tool station 2 completely encloses the sheet metal blank 26 in the hollow mold space 13 between the upper die 11 and the lower die 12 and the hot forming takes place. The lower die 12 is still completely raised at this time point by the elastic actuating elements 7. The closing movement Y of the press 1 continues uninterruptedly. The actuating travel W7 is the proportion of the press stroke by which the upper die 11 and the lower die 12 are moved toward each other starting from the upper reversal point UP, until the forming tool station 2 is completely closed, forming the hollow mold space 13.
FIG. 4 b again shows on the left hand side the time point at which the press is in the upper reversal point OP, however, the right hand side illustrates the press 1 at the time point at which the press passes through the lower reversal point UP or respectively the time point at which the distance A that can be adjusted by the actuating elements 7 between the lower die 12 and the press table 5 is minimal. The press stroke travel W1 is the distance that the press 1 travels from its upper reversal point OP to its lower reversal point UP.
FIG. 5 shows an embodiment of a forming tool station 2 of the press 1 according to the invention for producing sheet metal parts 27, which are hardened in regions. The forming tool station 2 includes an upper die 11 and a lower die 12, which each have an unheated region 22 and a heated region 21. Extending through the unheated region 22 are cooling channels 17 for conducting a coolant 18, wherein the cooling channels 17 are connected so that the coolant 18 can be supplied from the forming tool station 2 from outside in a not shown cooling source, for example a heat exchanger. In this embodiment the heated regions 21 are configured as mold inserts 15 and are fixedly but exchangeably connected with the unheated regions 22. In this case heating cartridges that are heated by the gas burner or electrical resistor serve as heating source 14. The forming tool station 2 can be coupled to the press ram 6 via the clamping plate 10, and additionally to the press table 5 via the mold clamping plate 9 and elastic actuating elements 7 or to the clamping plate 10 fixed on the press table 5.
In contrast to the embodiment of FIG. 5 described above, FIG. 6 shows a segmented upper die 11 and lower die 12 configured for producing sheet metal parts that are hardened in regions. A heated region 21 is arranged as separate tool segment 16 and separated by an insulation 20 from the unheated region 22 in the upper die 11 and in the lower die 12. This enables an energy-efficient use of the heating source 14 and cooling sources. After the mold closing time t₂and the first holding time t_2′ in the forming tool station 2 a temperature profile is established in the sheet metal part with a first section of the sheet metal part having a relatively high temperature and a second section having a relatively low temperature. This prepares the transformation of the microstructure into martensite in the second section and into ferrite and/or perlite in the first section. Between the first and second sections a small transition region 30 exists which later has relatively undefined mechanical properties.
Of course it is also possible to provide more than one heated region 21 in the forming tool station 2, as well as in the second tool station 3.
FIGS. 7 a and 7 b show a further embodiment of the entire press 1 for producing sheet metal parts 27 that are hardened in regions. FIG. 7 a shows a longitudinal section through the press 1. It includes the forming tool, station 2 according to FIG. 6 and a second tool station 3 of which regions are heated. The second tool station 3 has fixing elements 24 configured to in particular form fittingly receive the sheet metal part (not shown). The second tool station 3 also has regions 21 in form of a further tool segment 16 that are heated by heating sources 14. The tool segment 16 can be configured identical in the forming tool station 2, or can be configured stronger or more robust than in the second tool station 3 with regard to the material, the quality of the material, the heat capacity or the temperature resistance.
In addition transfer bars 25 are indicated with dashed lines and gripper recesses 25′ in the forming tool station 2, which recesses make it possible that grippers (not shown), which are connected with the transfer bars 25, can be moved close to the sheet metal part as soon as possible when the sheet metal part 27 is removed, without colliding with the lower die 12 during the upward movement Z.
Arranged subsequent in temporal order to the second tool station 3 is a further tool station 4, which here serves primarily for further cooling of the sheet metal part. Like the unheated region 23 of the second tool station 3 it has fixing elements 24 by which the sheet metal part can be accurately fixed in position for further cooling. The cooling itself can occur by not shown cooling sources for example by air ventilation, air or coolant shower or by immersion according to the German patent DE 10 2005 028 010 B3, in that a part of the third tool station 4 can be immersed with the sheet metal part 27 in the coolant.
As shown in FIG. 7 b, in this embodiment each tool station is configured dual action and has two molds in each of the forming tool station 2, the second tool station 3 and the third tool station 4. Not shown is the transfer bar for transporting the sheet metal blank or the sheet metal part into the tool stations 2, 3, 4 or remove them form these tool stations. A heating device 35 is indicated in which the sheet metal blanks 26 are at least partially heated to Ac3 temperature.
In FIGS. 8 a and 8 b an alternative embodiment of the entire press 1 for producing sheet metal parts 27 that are hardened in regions is shown, in FIG. 8 a as longitudinal section and in FIG. 8 b as horizontal section through the lower dies 12. In contrast to FIG. 7, the third tool station 4′ is here arranged separately in another press 36, which has the advantage that the final cooling can be decoupled from the fast press cycle and more space remains in the press 1 for the forming tool station 2 and the second tool station 3. In the present case a triple forming tool station 2 and a triple second tool station 3 and the heating device 35 are exemplary shown in front of the press 1.
In the third tool station 4′ one of the produced sheet metal parts 27 is shown, wherein the first section 28 and the second section 29 and the transition section 30 can be seen with the above described different mechanical properties and different microstructure adjusted to the application of the sheet metal part. Of course the sheet metal part 27 as shown here can also be produced as shown in the embodiment according to FIG. 7.
FIGS. 9 a and 9 b show time-temperature courses of the sheet metal blank 26 or sheet metal part 27 during the performance of the method according to the invention, which time-temperature courses are assigned to the last two embodiments.
Starting from a sheet metal blank 26 that has been heated at least in regions to austenitizing temperature Ac3, the sheet metal blank is transferred within 2 seconds from the heating device 35 into the forming tool station 2 of the press 1. Then the closing movement of the press and the mold begins until the forming tool station 2 is completely closed and the sheet metal blank 26 is hot formed to the sheet metal part 27. Then the first holding time t_2′ starts for cooling the sheet metal part 27 while the press 1 passes through the lower reversal point UP and the upward movement begins. During the upward movement the forming tool station 2 is opened with a delay, wherein the sheet metal part 27 due to the different temperatures and/or material properties of the tool has cooled to different cooling temperatures T_1.1and T_1.2. The sheet metal part 27 is then further transferred within the transfer time t₃from the forming tool station 2 into the second tool station 3. Thereafter the sheet metal part 27 is further cooled, while the second tool station 3 is closed and the sheet metal part 27 is held fixed in position. The temperature difference between the cooling temperature T_2.1and T_2.2in the sections 28, 29 of the sheet metal part 27 is again set by different cooling rates by the heated and unheated regions 21, 23 of the second tool station 3 (FIG. 8 b).
Subsequently the sheet metal part 27 is transferred into the third tool station 4, 4′ within the transfer time t₅, wherein FIGS. 9 a and 9 b differ from each other regarding this aspect. FIG. 9 a corresponds to the embodiment of the press according to FIG. 8, and FIG. 9 b corresponds to the embodiment according to FIGS. 7 a and 7 b. It can be seen that the cooling at the cooling time t₆in FIG. 9 a lasts longer than in FIG. 9 b, because the latter process occurs linked to the cycle time of the press 1. The cooling temperatures T_3.1and T_3.2have approached each other very closely and are in both sections below 200° C.
While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.
What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein:

Claims

What is claimed is:

1. A method for producing a sheet metal part which is hardened at least in regions in a press, which has a press table, a press ram and multiple tool stations, comprising the steps of:

heating at least regions of a sheet metal blank to a temperature above austenitizing temperature Ac3;

inserting the heated sheet metal blank into a first one of the tool stations of the press, said first tool station being configured as forming tool station;

hot forming the sheet metal blank to the sheet metal part in the forming tool station, wherein the press during the hot forming step performs a closing movement from an upper reversal point to a lower reversal point;

holding the forming tool station closed for a first holding time;

cooling the formed sheet metal part during the first holding time;

transferring the formed sheet metal part into a second one of the tool stations tool; and

hardening the sheet metal part at least in regions by cooling the sheet metal part in the second tool station within a second holding time,

wherein the forming tool station during the closing movement of the press is moved relative to the press by at least one elastic actuating element so that the hot forming is terminated and the step of holding the forming tool station closed starts before the press reaches the lower reversal point, wherein the upper die and the lower die have cooling channels through which a coolant is conducted.

2. The method of claim 1, wherein the step of holding the forming tool station closed is terminated by the at least one elastic actuating element during an upward movement of the press after the press has completely passed the lower reversal point.

3. The method of claim 1, further comprising performing a trimming and/or a hole punching in the forming tool station,

4. The method of claim 3, wherein the trimming and/or hole punching is performed before the press is completely closed.

5. The method of claim 1, wherein a second one of the multiple tool stations is moved at least in regions relative to the press during the closing movement of the press by at least one other actuating element, so that a closed state of the second tool station is initiated before the press has reached the lower reversal point.

6. The method of claim 5, wherein the closed state of the second tool station is terminated at least in regions by the at least one other elastic actuating element during an upward movement of the press after the press has completely passed the lower reversal point.

7. The method of claim 1, wherein the forming tool station and preferably also the second tool station are mechanically, hydraulically or pneumatically spring supported by the at least one elastic actuating element.

8. The method of claim 1, further comprising transporting the sheet metal part between two respective ones of the multiple tool stations with a transfer system, in particular with a linearly guided transfer bar with grippers, within a transfer time between 1 to 4 seconds, preferably between 2 to 3 seconds.

9. The method of claim 1, further comprising, in the forming tool station cooling a first section of the sheet metal part to a first cooling temperature that is greater than a martensite start temperature required for transformation of a martensite microstructure, and cooling a second section of the sheet metal part to a second cooling temperature that is smaller than the martensite start temperature, wherein the first cooling temperature is in particular between 540 to 660° C., thereby producing a sheet metal component that is hardened in regions.

10. The method of claim 9, further comprising adjusting a cooling temperature in the second tool station so that at the end of the second holding time a temperature of the first section of the sheet metal part is between 350 to 500° C. and a temperature of the second section of the sheet metal part is smaller than a martensite finishing temperature required for a complete transformation of the martensite microstructure, wherein the second section of the sheet metal part is preferably cooled in the second tool station to below 200° C., in particular to room temperature.

11. The method of claim 1, wherein the first holding time is between 2 and 8 seconds and the second holding time is between 2 and 10 seconds.

12. The method of claim 1, wherein a cycle time of the press to move between the upper reversal point and the lower reversal point is between 3 and 11 seconds.

13. The method of claim 1, wherein the hardening step is terminated in a third tool station, and wherein after the hardening step an entirety of the sheet metal part has a cooling temperature of below 200° C.

14. The method of claim 13, wherein the press comprises a third tool station and wherein a cycle time of the press to move between the upper reversal point and the lower reversal point is between 3 and 9 seconds.

15. A press for producing a sheet metal part that is hardened at last in regions, in particular for implementing the method of claim 1, said press comprising:

multiple tool stations, at least one of the multiple tool stations being constructed as a forming tool station that is coolable at least in regions and being configured for hot forming of sheet metal blanks, said forming tool station having an upper die and a lower die which in a closed state of the forming tool station form a hollow mold space;

a press ram; and

a press table;

at least one elastic actuating element arranged between the press table and the lower die so that either the lower die is liftable relative to the press table and/or the upper die is impingable with pressure at a distance relative to the press ram to cause the forming tool station to assume the closed state before the press is completely closed, wherein the upper die and the lower die have cooling channels through which a coolant is conductible.

16. The press of claim 15, wherein the at least one elastic actuating element is configured so that a vertical travel of the press is adjustable by the at least one elastic actuating element, said vertical travel being at least 100 mm and smaller than a maximal stroke traveled by the press between the upper and the lower reversal point of the press.

17. The press of claim 15, wherein the at least one elastic actuating element exerts an actuating force, which increases at least over a portion of a travel of the press from the upper reversal point to the lower reversal point of the press, in particular the actuating force is at least 20 percent greater in the closed state of the forming tool station.

18. The press of claim 12, wherein at least the forming tool station is heatable in regions by a heating source so as to effect a reduced cooling rate in a first section of the sheet metal part, wherein unheated regions of the forming tool station have cooling channels for conducting a coolant.

19. The press of claim 12, wherein the press comprises a second tool station following the forming tool station, and wherein the forming tool station and at least the second tool station are heatable at least in regions in particular by a heating source to prevent a complete hardening at least in a first section of the sheet metal part.

20. The press of claim 19, wherein an unheated region of the second tool station has an active cooling source that corresponds at least to a transition section between the first section and the second section of the sheet metal part.

21. The press of claim 20, wherein the first section and the transition section of the sheet metal part are form fittingly fixable at least in the second tool station by fixing elements.