CA2851920A1 - System and method for hot-forming blanks - Google Patents

Abstract

The present invention pertains to a system for hot-forming blanks (P) with at least one heating device (4) and at least one pressing device (2) that is arranged downstream of the at least one heating device (4), wherein at least one reheating device (6) for at least partially acting upon the blanks (P) formed in the pressing device with heat is provided downstream of the pressing device (2).

Description

DESCRIPTION
SYSTEM AND METHOD FOR HOT-FORMING BLANKS
The invention pertains to a system for hot-forming blanks, as well as to a corresponding method for hot-forming blanks.
PRIOR ART
The hot-forming of sheet metals is a relatively new development trend in the manufacture of components, particularly for car bodies. In the context of this application, the sheet metals used in this case are in accordance with the trade language used in the field of forming technology also referred to as "blanks." A
blank usually consists of a sheet metal that is correspondingly cut to size, punched out, joined and/or preformed. However, the inventive measures not only can be applied to correspondingly prepared sheet metals, but also to the respective feedstock used. The invention therefore concerns all workpieces and semifinished products that can be shaped in a corresponding forming operation, for example, by means of pressing and/or deep-drawing.
Hot-forming allows a springback-free manufacture of high-strength components with complex geometry and makes it possible to significantly reduce the weight, e.g., of car bodies produced thereof, as well as to improve the safety, for example, of passengers in a corresponding vehicle.
Due to more stringent requirements with respect to the strength and rigidity of structural components, particularly in a vehicle, high-strength and super high-strength steels are increasingly utilized for these applications. An increased strength makes it possible to reduce the vehicle weight such that, in particular, the pollutant emission and the fuel consumption can be reduced. In current vehicle models, the utilization of hot-formed components makes it possible to reduce the weight by more than 30 kg.
Hot-forming methods, in essence, are combined shaping and hardening techniques. The utilization of corresponding steels such as, for example, manganese-boron steels makes it possible to achieve strengths up to 1.500 MPa. In press-hardening methods, for example, blanks are heated to a temperature that lies above the complete austenitization temperature, e.g. above 850 C, and the blank is subsequently quenched in the tool. The desired martensite structure with the desired strength is achieved in this way. The combination of the forming process and the quenching process in a tool is occasionally also referred to as press-hardening or shape-hardening.
In the hot-forming of super high-strength materials for automobile bodies, e.g., so-called roller hearth furnaces are used for preheating the blanks. The heating of such furnaces is usually realized by means of radiant tubes that are heated electrically or by means of gas burners. In order to realize the shortest process cycle times possible, it is advantageous to provide a certain "reserve" of preheated components in the system. The heat treatment time for tempering the steel represents a decisive parameter that defines the cycle time of a corresponding press. Roller hearth furnaces have a length of up to 40 meters and therefore have corresponding structural requirements that include the efficient removal of excess heat. Rotary drum-type kilns that are used as an alternative to roller hearth furnaces for preheating components also have corresponding disadvantages. They are also heated by means of radiant tubes and rather unsatisfactory with respect to their efficiency.
Press-hardened components are characterized by their high strength and rigidity. As mentioned above, this makes it possible to reduce the sheet metal thickness and therefore to reduce the weight. However, one problem can be seen in the low breaking elongation that can lead to the formation of cracks during subsequent production steps, e.g. when welding on other parts.
This is the reason why it is desirable to realize certain regions, e.g. of a car body component, in a press-hardened fashion and to realize other regions of the same component such that they have a higher ductility and therefore can absorb more energy due to plastic deformation.
Prior attempts to produce such locally different properties or so-called "tailored properties" include purposefully influencing the alloying constituents of corresponding semifinished products, the manufacture of so-called "tailored welded blanks," i.e. blanks that are joined of different materials, partial (local) heating by means of inductive or conductive heating technologies, partially tempering certain regions of the press-hardening tools by means of local heating and masking certain component regions in order to suppress the heating (and therefore the austenitization) in a corresponding roller hearth furnace. However, these methods are elaborate and therefore often unsatisfactory and very costly.
Consequently, there is a need for improved options for making available blanks with locally different properties.

DISCLOSURE OF THE INVENTION
Based on these circumstances, the present invention proposes a system for hot-forming blanks, as well as a corresponding method for hot-forming blanks, with the respective characteristics of the independent claims.
Preferred embodiments form the objects of the dependent claims, as well as the following description.
Blanks and components with locally different properties can be made available in a particularly effective fashion due to the at least partial reheating operation proposed in accordance with the invention, wherein said reheating operation is carried out after forming or press-hardening the blank in the pressing device.
According to the invention, it is particularly possible to realize very complex shapes with the desired material properties such as, e.g., an increased ductility at any location.
ADVANTAGES OF THE INVENTION
As already mentioned above, the term "blanks" should be interpreted comprehensively in the context of this application. This term includes sheet metals, semifinished products, joined and/or preformed components that are hot-formed, particularly press-hardened, in a corresponding system.
A particularly advantageous aspect of the invention concerns the utilization of a premixing hydrogen-oxygen burner or fuel gas-oxygen burner. Burners of this type are basically known, for example, from DE 103 45 411 Al. For example, premixing fuel gas-oxygen burners are used for the so-called fire-polishing of glass parts, particularly parts of lead crystal or soda-lime glass.
In this case, at least part of the surface of the glass part is heated and fused with the burner. Corresponding burners are also known as so-called Hydropox burners and sold by the applicant under this brand name.
Premixing fuel gas-oxygen burners, particularly hydrogen-oxygen burners, are characterized by a particularly high heat transfer efficiency. In contrast to so-called externally mixing burners, a fuel mixture of fuel gas and oxygen is already fed to a burner head of a premixing fuel gas-oxygen burner rather than ultimately produced in a corresponding burner head.
Premixing burners produce particularly hard flames that are suitable for fusing larger surface areas that may also feature depressions or other irregularities.
According to the invention, it was determined that this represents a decisive advantage in comparison with externally mixing burners. Externally mixing burners are only capable of producing a soft flame that cannot penetrate, in particular, into corners, holes or depressions of a surface. Consequently, the utilization of a premixing burner makes it possible to locally heat certain regions of corresponding blanks, particularly regions that are shaped differently. Although prolonged heating by means of an externally mixing burner would also make it possible to achieve high temperatures, it could occur that the entire blank is heated rather than only the desired regions.
According to a particularly preferred embodiment of the inventive system, the at least one reheating device is realized such that it can be three-dimensionally oriented and/or three-dimensionally displaced. The reheating device used in accordance with the invention may be mounted, for example, on an industrial robot.
This makes it possible to exactly guide and orient the reheating device along or over the surface of the formed blank such that it can be uniformly heated in the desired regions (partially) to a desired temperature range, e.g., between 650 and 850 C, particularly 700 C - 800 C, preferably about 750 C.
The heating device (particularly for completely heating the blank prior to the forming operation in the pressing device) is preferably realized in the form of an austenitizing device. A complete austenitization is preferred in this case. An austenitization delivers the desired material properties that make it possible to subsequently press and simultaneously cool or quench the blank and then to at least partially heat (reheat) the blank. A corresponding austenitizing device is designed, in particular, for locally heating the blank to a temperature of 750 - 1050 C, particularly 800 -1000 C, for example 850 - 950 C. A corresponding temperature depends on the respective materials and lies above the austenitization temperature. For example, the austenitization temperature of the aforementioned manganese-boron steels lies at approximately 850 C. If a corresponding blank is preheated to a temperature slightly below the austenitization temperature, the austenitization temperature can be quickly reached or exceeded with a corresponding burner, particularly in predefined regions of the blank. In such a cooling process during the pressing or forming operation, the blanks are preferably cooled to temperatures of 100 C - 200 C, wherein cooling to any temperature between room temperature and 250 C would also be possible.
A corresponding system advantageously furthermore features at least one loading device for loading the system with blanks and/or at least one transfer device for transferring the blanks into the at least one pressing device of the system and/or at least one transfer device for transferring the blanks to the reheating device.
The at least one heating device advantageously comprises at least one paternoster furnace. For example, it would be possible to utilize generally known vertical paternoster furnaces that have an improved energy efficiency and the advantage, in particular, of being suitable replacements for conventional roller hearth furnaces that, as mentioned above, have a large structural size and therefore corresponding structural requirements. Paternoster furnaces can be heated, for example, electrically or with fuel and operated in corresponding temperature ranges such that an efficient and reliable heating process is ensured.
The respective temperatures to be adjusted depend on the respective material of the blanks. As mentioned above, the complete austenitization temperature of manganese-boron steels lies at approximately 850 C. A
person skilled in the art can easily derive corresponding temperatures from available material parameters.
It is advantageous to also realize the heating device with at least one premixing hydrogen-oxygen burner or fuel gas-oxygen burner. This likewise allows a very effective and, in particular, also regional heating of the blanks.
Although such a heating device, particularly austenitizing device, is in the context of the present invention preferably used for a complete austenitization of a blank, it may also be realized for partially heating blanks, particularly for austenitizing blanks, i.e., for heating or austenitizing certain regions or local regions of blanks. In this case, at least one burner flame of a premixing hydrogen-oxygen burner may be directed at the region(s) to be partially heated, particularly austenitized. A corresponding burner arrangement therefore makes it possible, in particular, to realize a defined local austenitization of regions, in which a high local strength can be subsequently be achieved, for example, by means of press-hardening. However, a sufficient ductility of the material is ensured in the non-austenitized regions after the press-hardening operation. In this way, it would be conceivable, e.g., to make available a desired ductility in first regions of the blank with such a partial heating process by means of the heating device, i.e. prior to the forming operation, and in second regions of the blank with a heating process by means of the reheating device, i.e.
subsequent to the forming operation.
In a corresponding system, a heating device, particularly an austenitizing device, and a preheating device are advantageously realized in the form of one structural unit. This makes it possible to realize compact and energy-efficient systems that have a small structural size and, for example, merely require one heat insulation or thermal insulation.
An inventive method comprises the steps of loading blanks into an inventive system, heating or austenitizing the blanks at least locally in a heating device, particularly an austenitizing device, forming the blanks by means of pressing in a pressing device and subsequently heating the blanks at least partially in a reheating device. As mentioned above, the pressing operation may also concern a press-hardening process.

The inventive system for hot-forming blanks and the inventive method likewise benefit from the above-described advantages.
It goes without saying that the above-described characteristics, as well as the characteristics yet to be described below, not only can be used in the respectively described combination, but also in other combinations or individually without deviating from the scope of the present invention.
An exemplary embodiment of the invention is schematically illustrated in the drawings and described in greater detail below with reference to the drawings.
DESCRIPTION OF FIGURES
Figure 1 shows a schematic representation of a system for hot-forming blanks according to a preferred embodiment of the invention.
Figure 2 shows a schematic representation of a burner head suitable for use in an embodiment of the invention.
Figure 3 shows a flow chart of a method for hot-forming blanks according to an embodiment of the invention.
If applicable, elements that function or operate identically are identified by the same reference symbols in the figures and their description is not repeated for reasons of simplicity.
Figure 1 shows a system for hot-forming blanks according to a preferred embodiment of the invention.
The system as a whole is identified by the reference symbol 10. It features a loading device 3, in which corresponding blanks P such as, for example, punched-out sheet metal pieces can be loaded into a corresponding system in the direction of the arrow (lower horizontal arrow). A heating device 4 is provided and features a schematically illustrated paternoster furnace 4a. The blanks P are transported into a lower region of the heating device 4 in the direction of the arrow, raised (as indicated with the vertical arrow) and continuously heated while being raised. The blanks may be heated in the paternoster furnace 4a in such a way that they austenitize.
However, Figure 1 shows an embodiment, in which the heating device 4 features an austenitizing device 4b that is arranged downstream of the paternoster furnace 4a. In this case, the paternoster furnace serves for preheating the blanks.
The blanks P once again exit the paternoster furnace 4a in an upper region thereof, namely in the direction of the arrow (upper horizontal arrow). Subsequently, they pass through the austenitizing device 4b that features a burner 14 symbolized in the form of a three-flame burner. The burner 14 may have an arbitrary number of burner flames. The burner 14 may also be mobile and successively act upon different regions of a blank P.
To this end, it would be possible to provide corresponding moving devices that can also be actuated in a fully automated fashion, for example, by utilizing a corresponding control. The blanks P pass through the austenitizing device 4b in the direction of the arrow while being heated to a temperature (e.g. 900 C) that lies above an austenitization temperature of the corresponding material.
The blanks P subsequently reach a transfer device 5, by means of which they are transferred to a pressing tool 8. The pressing tool 8 forms the blanks in the desired fashion, wherein the blanks are simultaneously cooled to approximately 200 C or less during the forming operation.
A martensitic or hard structure is created in the austenitized regions of the blank due to this cooling or quenching process that preferably takes place with a rate in excess of 30 K/sec.
As mentioned above, the formed blanks have in this state a temperature of approximately 200 C. In this state, the formed blanks are now partially acted upon with heat by means of a reheating device 16 that features at least one premixing hydrogen-oxygen burner 18 or fuel gas-oxygen burner. In this way, the hard structure is transformed into a mixed structure that has improved properties, for example, with respect to its ductility at the locations of the formed blank that are acted upon with heat.
The reheating device 16 may be mounted, for example, on a (not-shown) industrial robot such that the burner 18 can be three-dimensionally displaced and oriented. This makes it possible to exactly guide the burner 18 along a component surface such that it can be uniformly heated to temperatures between approximately 650 and 850 C in the desired regions. The thusly achieved structural change results, e.g., in a reduced hardness and an increased elongation or ductility. In experimental tests, e.g., the ductility values could be improved by up to 18 percent.
The burners 18 may be realized with arbitrary geometries (also with smaller diameters, for example, for welding spot regions) and therefore are capable of heating various regions of a component or of a formed blank P. In this case, the energy transfer is very efficient and the treatment time can be reduced to a few seconds.
The invention provides clear advantages in comparison with other heating technologies such as, for example, induction heating that is not suitable for three-dimensional geometries or blank shapes, e.g. because inside radii cannot be properly heated.
The inventive method also provides advantages in comparison with conventional laser-assisted methods.
Although laser-assisted methods are generally capable of performing similar tasks, the high energy density and the relatively small focal surface require a significantly higher effort, for example, for heating larger coherent regions such that methods of this type are relatively ineffective in practical applications.
The inventive method makes it possible to subsequently heat partial regions of a blank, particularly of a three-dimensionally formed blank such as, for example, hardened blanks of UHS steel, in a highly variable and effective fashion, wherein the ductility of the material can be increased to a sufficient value for a purposeful deformation.
The burners used in accordance with the invention make it possible, for example, to realize focal surfaces with a surface area up to 10 by 20 cm2. It is particularly preferred to utilize burners that make it possible to realize focal surfaces with a size of 2 cm x 2 cm or 4 cm x 2 cm.
A preferred embodiment of an inventive burner head is illustrated in Figure 2.

In Figure 2, a suitable premixing burner head according the invention is identified by the reference symbol 22.
A premixing hydrogen-oxygen burner used in accordance with the invention features a channel 221, through which a hydrogen-oxygen mixture can be fed to the burner head 22, and is capable of producing a very hard burner flame that ensures a very good energy transfer.
This makes it possible, in particular, to act upon regions that have recesses or more complex contours with the required heat in a more reliable fashion. In this case, the corresponding gas mixture therefore already exits the burner nozzles 223 in the form of a mixture and is ignited at this location.
Figure 3 shows a flow chart of a method 100 according to a particularly preferred embodiment of the invention in the form of a schematic representation.
Corresponding blanks P are punched out of a sheet metal in a first step 101. In a second step 102, these blanks are loaded into an inventive hot-forming system, for example, by means of a loading device. This loading process may take place continuously. In step 103, the blanks P are preheated in the system, wherein the above-described means may be used for this purpose. An austenitization of the above-described type then takes place in step 104. After the austenitization, the blanks P are transferred into a pressing tool by means of a transfer device in step 105, wherein the blanks are then formed or pressed and simultaneously quenched in said pressing tool in step 106. After the quenching process in the pressing tool, the press-hardened blanks that may have complex three-dimensional shapes in this state are partially heated in the desired fashion (step 107) by means of a reheating device, particularly a premixing hydrogen-oxygen or fuel gas-oxygen burner, such that a mixed structure with the desired properties =
(e.g. improved ductility) can be made available in the heated regions.

Claims (11)

1. A system for hot-forming blanks (P) with at least one heating device (4) and at least one pressing device (2) that is arranged downstream of the at least one heating device (4) and serves for forming the blanks, characterized in that at least one reheating device (16) for at least partially acting upon the blanks (P) formed in the pressing device with heat is provided downstream of the pressing device (2).

2. The system according to claim 1, characterized in that the reheating device (16) features at least one premixing hydrogen-oxygen burner or a premixing fuel gas-oxygen burner (18).

3. The system according to one of the preceding claims, characterized in that the reheating device (16) can be three-dimensionally displaced and/or oriented.

4. The system according to one of the preceding claims, in which the heating device (4) is realized in the form of an austenitizing device.

5. The system according to one of the preceding claims, furthermore featuring a loading device (3) for loading the system with blanks and/or at least one transfer device for transferring the blanks into the at least one pressing device and/or at least one transfer device for transferring the formed blanks to the at least one reheating device (16).

6. The system according to one of the preceding claims, characterized in that the heating device (4) comprises at least one paternoster furnace.

7. The system according to one of the preceding claims, characterized in that the heating device features at least one premixing hydrogen-oxygen burner or a premixing fuel gas-oxygen burner.

8. The system according to one of the preceding claims, characterized in that the pressing device (8) is realized in such a way that the blanks are cooled to a temperature between room temperature and 300°C, particularly 150 to 250°C or less than 200°C, while they are formed.

9. The system according to one of the preceding claims, characterized in that regions of the formed blanks are heated to temperatures of approximately 650 to 850°C, particularly 700 to 800°C, preferably about 750°C, in the reheating device (16).

10. A method for hot-forming blanks, in which the blanks are loaded into a system according to one of the preceding claims, heated to an austenitization temperature in a heating device of the system, formed and simultaneously cooled in a pressing device and then at least partially heated to a temperature of approximately 650 to 850°C, particularly 700 to 800°C, preferably about 750°C, in a reheating device.

11. The method according to claim 10, characterized in that coated blanks are hot-formed, particularly that blanks coated with aluminum silicone or zinc are hot-formed.