EP2497840B1 - Oven system and process for partially heating steel blanks - Google Patents

Description

The invention relates to a furnace system and a method for targeted component zone-individual heat treatment of sheet metal components.

In the art, in many applications in different industries there is a desire for high strength sheet metal parts with low part weight. For example, in the automotive industry, efforts are being made to reduce the fuel consumption of motor vehicles and to reduce CO2 emissions, while at the same time increasing occupant safety. There is therefore a rapidly increasing demand for body components with a favorable strength to weight ratio. These components include, in particular, A and B pillars, side impact beams in doors, sills, frame members, bumper catches, floor and roof crossmembers, front and rear side members. In modern motor vehicles, the body shell with a safety cage usually consists of a hardened steel sheet with about 1500MPa strength. In many cases Al-Si-coated steel sheets are used. For the production of a component from hardened steel sheet the process of the so-called press hardening was developed. Steel sheets are first heated to austenite temperature between 850 ° C and 950 ° C, then placed in a press tool, quickly formed and quickly quenched by the water-cooled tool to martensite temperature of about 250 ° C. This results in hard, firm martensite with about 1.500MPa strength. However, such a hardened steel sheet has only about 6-8% elongation at break, which is disadvantageous in specific areas in the case of the collision of two vehicles, especially in a side impact. The kinetic energy of the penetrating vehicle can not be converted into deformation heat. Rather, in this case, the component will break brittle and also threatens to injure the occupants.

For the automotive industry, it is therefore desirable to obtain body components which have several different expansion and strength zones in the component, so that very strong areas on the one hand and very elastic areas on the other hand exist in one component. The general demands on a production plant should continue to be respected: so there should be no cycle time loss to the mold-hardening system, the entire system should be universally used without restrictions and can be quickly converted to customer-specific. The process should be robust and economical and the production plant need only minimal space. The shape and edge accuracy of the component should be so high that hard trimming can be largely eliminated in order to save material and labor.

The prior art describes corresponding methods and devices. These methods use partially heated tools, wherein a portion of the component is cooled above the martensite quenching rate. As usual, the remainder of the component is cooled abruptly and forms martensite. For example, the document describes EP 2 012 948 a forming tool for press hardening and tempered forming of a board made of higher and / or very high strength steels with means for temperature control of the forming tool and a method for press hardening and tempered forming of blanks of higher and / or very high strength steels in which the board is heated prior to forming is and then hot or semi-hot formed in a forming tool, wherein the forming tool has means for temperature control. In this case, a plurality of controllable means for temperature control are provided in the forming tool, whereby a plurality of temperature zones can be defined, wherein at least the contact surfaces of the Umformwerkzeugelemente used for forming individual temperature zones are assigned.

In the document DE 10 2005 032 113 discloses an apparatus and method for hot working and partially curing a component between two mold halves in a press. The tool halves are each divided into at least two segments, which are separated by a heat insulation. The two segments can be heated or cooled by a temperature control, so that different temperatures in different areas of the component and thus cooling curves are adjustable. This makes it possible to produce a component with regions of different hardness and ductility.

The document WO 2009/113 938 describes a press hardening process that can achieve soft sections in the finished product by reducing the cooling rate of these sections of material. This reduces the martensite in these areas and consequently increases the elongation at break of these areas.

In this case, all methods that use a partially heated tool, the disadvantage that the component is subject to delay, since the component with partially different temperatures of about 300 ° C to 500 ° C in the soft range and of about 100 ° C is removed from the tool in martensitic areas and continues to cool outside of the mold constraint. In addition, the cycle time of the process is prolonged because the rapid cooling is slowed down in favor of the perlite-ferrite formation, thus reducing the economy at the same time. In addition, such tools are very complex and therefore expensive and prone to failure.

In another, for example, in the prior art from the documents DE 10 350 885 . DE 10 240 675 . DE 10 2005 051 403 or DE 10 2007 012 180 known method, the component is heated in a two-zone oven in the soft range to a temperature below the material-dependent AC3 temperature, the area to be cured, however, to a temperature above the AC3 temperature. This results in stretchable soft perlite ferrite in one part and hard martensite in the other part of the part. The disadvantage of this process is that the oven can only be used to a limited extent and can no longer serve as a universal oven. Thus, this method loses economy. Another disadvantage is that the separation of the areas usually can not be permanently realized with sufficient accuracy. In addition, more than two different zones are not meaningfully achievable. Furthermore, when using Al-Si coated components, it is necessary to maintain the temperature at about 950 ° C for about 300 seconds to allow diffusion of the coating into the base material. At lower temperatures, this process takes much longer and the economy of the entire system is reduced.

In addition, another method is known in practice in which the soft areas are partially cooled slowly. In this case, the component is heated completely above the austenite temperature for the required diffusion time and temperature and then slowly cooled in a separate or the same furnace by partial hanging in air again below austenite temperature. Subsequently, when the mold hardening process is performed in the mold, the disadvantages with respect to the lack of molding accuracy and economy of the production furnace are eliminated. Disadvantage of this method is the slower cycle time through the additional step. Another disadvantage is the undefined cooling rate, which occasionally leads to martensite formation in components less than 1.2 mm thick. The cooling rate is undefined, since the cooling takes place in not exactly defined ambient temperature. The process can not therefore be described as robust. Furthermore, this process can only be displayed with two different hard zones.

The European patent application EP 2 143 808 A1 discloses a method for producing a molded component having at least two regions of different ductility from a component blank of hardenable steel, which is heated differently in areas and then formed in a thermoforming and hardening tool and partially cured, wherein the component blank in a heating device to a homogeneous temperature less AC3 is heated and then brought by means of an infrared lamp array in first areas to a temperature above AC3, to be subsequently cured in the thermoforming and curing areas in the first areas. To solve the problem of the temperature distribution in the component and, associated therewith, the exact setting of the hardness values in the finished component, bulkheads for separating the temperature fields are proposed. Due to the very good heat conduction of the material steel, it can not be avoided in the proposed method that there are relatively large temperature transition regions in which a temperature adjusts to the AC3 temperature without it being possible to precisely define where in the component a temperature is still below this temperature or already above this temperature.

Finally, different steels can be welded together so that non-hardenable steel is present in the soft and hardenable steel in the hard zones. In a subsequent hardening process, the desired hardness profile can be achieved over the component. The disadvantages of this method are the occasionally unsafe weld in an Al-Si-coated approximately 0.8-1.5mm thick sheet metal commonly used for body parts, the rugged hardness transition there and in the increased cost of the sheet due to the additional welding fabrication step , Occasionally, tests have failed due to breakage near the weld, so the process can not be said to be robust.

The object of the invention is therefore to provide a furnace system and a method for the targeted heat treatment of sheet metal components, which avoids the disadvantages described above.

According to the invention, this object is achieved by a furnace system having the features of independent claim 1. Advantageous developments of the furnace system will become apparent from the dependent claims 2-9.

Furthermore, the object is achieved by a method according to claim 10. Advantageous embodiments of the method emerge from the subclaims 11-13.

The furnace system according to the invention is suitable for partially heating components made of sheet steel to a temperature above the AC3 temperature. The furnace system includes a common, universal production furnace for heating the steel sheet parts to a temperature close to but below the AC3 temperature, the furnace system further comprising a profiling furnace having at least one plane. The at least one level has an upper and a lower part, as well as a product-specific intermediate flange introduced into a corresponding receptacle, the product-specific intermediate flange being designed to give the component a predetermined temperature profile with temperatures above the AC3 temperature for areas to be hardened and below Apply AC3 temperature for softer areas.

In a preferred embodiment, the furnace system further has a positioning system on which the component can be stored in a defined position after heating in the production furnace and / or after heating in the profiling furnace. This ensures that the component is in a predefined position after heating in the production furnace or after partial heating in the profiling furnace. Thus, the component can subsequently be inserted into a predefined position in the profiling oven or a press for the subsequent form hardening process. The more accurately the insertion position of the component can be maintained, the less trimming the finished, hard sheet metal part is required. The product-specific intermediate flange has means for active cooling of individual areas. In an advantageous embodiment, the cooling is designed as water cooling.

In a further particularly advantageous embodiment, the product-specific intermediate flange has means for heating individual regions, wherein in a particular embodiment, this is electrical heaters. This makes it possible to selectively heat and / or cool individual, product-specific areas, so that the temperatures of these areas can be kept within close tolerances. If individual areas are heated above the AC3 temperature, these areas will become particularly hard in the subsequent form hardening process. The other areas that are not intentionally heated above the AC3 temperature will be less hard in the subsequent mold hardening process and instead have a higher elongation at break. With the electric heaters a particularly accurate temperature control is possible.

It has proven to be advantageous to heat the production furnace with gas burners. As a result, a particularly economical heating of the components is possible. Since the method according to the invention provides for heating the components in the production furnace only to a temperature below the AC3 temperature and for introducing the heat for heating defined regions to a temperature above the AC3 temperature in a later process step in the profiling furnace, this is a very accurate Temperature control in the production furnace is not required, so that the disadvantage of the lower controllability of gas burners over that of electric heaters in favor of the economy for the cheaper energy gas can be accepted.

In a further advantageous embodiment, the furnace system to a production furnace, which has a continuous furnace via a transport system for passing the components through the production furnace. The cycle time for the heating of the components can thus be maintained at the level of conventional heating furnaces for the mold hardening process. If the subsequent process step of impressing a temperature profile on the component cycle time is determining, so that the Cycle time for the entire process threatens to extend, a profiling furnace can be used with multiple levels, in which the components are heated partially or partially parallel partially further. The parallel use of several Profilieröfen is conceivable.

In order to keep the temperature tolerances on the component in the targeted heating of individual areas particularly tight, it has proven to be advantageous to control the temperature control in a closed loop. For this purpose, the profiling oven in an advantageous embodiment means for temperature control in the closed loop. In this case, advantageously more than one control loop can be provided.

It has proved to be particularly advantageous if the furnace system further has a handling system for handling the components. The handling system can place the components specifically and quickly in the positioning system, remove them again from the positioning system and insert them into the product-specific intermediate flange in the profiling furnace and remove them again. Next, the handling system can then insert the components in a press tool for subsequent hardening. By using a handling system, the risk of injury to operating personnel is minimized by the hot components. A handling system executes the movements in defined and reproducible times, so that the components can be inserted with minimal temperature tolerances in the press tool for mold hardening, which has proven to be advantageous for the component quality.

• Erwärmen eines Bauteils im Produktionsofen bis nahe an seine AC3-Temperatur;
• Positionieren des erwärmten Bauteils mittels eines Positioniersystems;
• Einbringen des positionierten Bauteils in eine definierte Position in dem Profilierofen;
• Aufbringen eines Temperaturprofils auf das Bauteil im Profilierofen durch Erwärmung ausgewählter Bereiche auf eine Temperatur oberhalb der AC3-Temperatur, wobei andere Bereiche auf einer Temperatur unterhalb der AC3-Temperatur gehalten werden;
• Austragen des mit einem Temperaturprofil versehenen Bauteils aus dem Profilierofen.

The method according to the invention is characterized by the following process steps:

• Heating a component in the production furnace to near its AC3 temperature;
• Positioning the heated component by means of a positioning system;
• Introducing the positioned component into a defined position in the profiling oven;
• Applying a temperature profile to the component in the profiling oven by heating selected portions to a temperature above the AC3 temperature, keeping other areas at a temperature below the AC3 temperature;
• Discharge of the component provided with a temperature profile from the profiling oven.

It has proven to be advantageous if the heating of the component takes place in the production furnace by means of gas burners, which can be used as an energy source, for example, natural gas.

In a further advantageous embodiment, the introduction of the positioned component takes place in a defined position in the profiling furnace by a handling system. The advantages of this are that the risk of injury to the operator is minimized and the process becomes more robust due to the constant handling times. The advantage here is that such a system for existing systems can be retrofitted.

Advantageously, the application of a temperature profile is controlled on the component in the profiling oven via a closed loop. As a result, tightest temperature tolerances can be realized on the component, which has a positive effect on the quality of the form-hardened component. To apply the temperature profile, areas of the component to be hardened are selectively heated to a temperature above the AC3 temperature via a product-specific intermediate flange, while other areas which are to have a higher elasticity in the finished part are kept at a temperature below the AC3 temperature.

Further advantages, features and expedient developments of the invention will become apparent from the dependent claims and the following description of preferred embodiments with reference to the figure.

Fig. 1

das erfindungsgemäße Ofensystem in einer Draufsicht

Fig. 2

eine Detailansicht des Profilierofens

Fig. 3

Schnitt A-A aus Fig. 2

Show it:

Fig. 1

the furnace system according to the invention in a plan view

Fig. 2

a detailed view of the profiling furnace

Fig. 3

Cut AA off Fig. 2

Fig. 1 shows a furnace system according to the invention in a plan view. A first robot 61 positions a component 5 on a roller belt which transports the component 5 through the production furnace 10. The production furnace 10 is a conventional general-purpose furnace, which is heated with natural gas burners 9 to a temperature below the AC3 temperature of the respective component material. The transport speed of the components 5 through the production furnace 10 is selected so that the components 5 almost assume the temperature prevailing in the production furnace 10. In the transport direction behind the production furnace 10 is a positioning system 20 that brings each component 5 in a defined lying position. A handling system 22 receives the component 5 and brings it into a defined position in the profiling oven 40. In the profiling furnace 40 are an upper part 40 and a lower part 41, and a receptacle 44 for a product-specific intermediate flange 45, and the product-specific intermediate flange 45 itself The intermediate flange 45 has, on the one hand, regions with heaters 46 and, on the other hand, regions 48, which can be cooled. In addition, it is also possible to provide in the profiling oven 40 only means for targeted heating 46 or even only areas 48 which can be selectively cooled. In this case, such areas 48 have cooling holes through which a cooling medium, such as water or oil, flows. But it is also possible to use for very targeted cooling known means such as heat pipes or inserts made of highly heat-conductive materials such as copper alloys. As heaters 46, all known heaters such as electric heating cartridges or electric surface heaters can be used. Electric heaters have the advantage that they can be regulated very accurately and quickly. With the heater 46, the areas 30, which are to be particularly hard after a subsequent form hardening process, heated to a temperature above the AC3 temperature. Other areas 50, which should have a higher elongation at break after the subsequent form hardening process, are at a temperature below that by the targeted cooling 48 of these areas AC3 temperature maintained. The temperature control takes place in at least one closed loop. After the dwell time required for heating the areas 30 to the desired temperature above the AC3 temperature, the component 5, which is now provided with a temperature profile, is discharged from the profiling oven 40 by means of the handling system 22. The handling system 22 is executed in the illustrated embodiment as a rake. However, all other suitable handling systems can also be used. The handling system 22 places the component 5 again on the positioning system 20. However, it is also conceivable to deposit the component 5 after impressing a temperature profile on another transfer station. A second robot 60 then takes over the component 5 in order to insert it into the tool 70 of a press for molding. Usually, however, the component 5 can be inserted directly into the press tool 70 without repositioning, since no relative movement and thus no reorientation of the component 5 takes place in the profiling oven 40.

Fig. 2 shows the profiling oven 40 in a detailed view as a plan view. A further component 5 is located in the profiling oven 40. Areas 30 of the component 5, which should be particularly hard after the mold hardening process, are located on areas of the product-specific intermediate flange 45, which can be selectively heated by heaters 46. It is an electrical heating element, which is supplied via terminals 47 with electrical energy, which is provided by a control means (not shown). Another region 50 of the component 5, which after the mold hardening process is to have a greater elongation at break than the hard region 30, is located at a region 48 of the product-specific intermediate flange 45, which can be cooled in a targeted manner. For this purpose, cooling medium is conducted through the connections 49 into the region 48.

Fig. 3 is the section AA off Fig. 2 through the profiling oven 40. The Profiolierofen 40 has an upper part 41 and a lower part 42, and a receptacle 44 for a product-specific intermediate flange 45 and the product-specific intermediate flange 45 itself. In the product-specific intermediate flange 45 heaters 46 can be seen, which are powered via terminals 47 with energy. This can do that Component 5 are selectively heated in the range 30 to a temperature above the AC3 temperature. Furthermore, the handling system 22, which is located in front of the profiling furnace 40 can be seen. The arrows indicate that the handling system 22 can move a component 5 vertically and horizontally so that a component 5 (not shown) on the positioning system 20 can be inserted by means of the handling system 22 into the product-specific intermediate flange 45 within the profiling furnace 40.

Instead of the mentioned robot, any other suitable handling system can be used. In the embodiment shown in the figure, only one Profilierofen 40 is described with a plane. However, it is equally possible to provide more than one level in the profiling oven 40, wherein in each level an upper and lower part, and a receptacle for a product-specific intermediate flange is provided, so that a plurality of components 5 parallel or partially parallel, a temperature profile can be impressed. Likewise, to increase the capacity of the furnace system 1, it is possible to provide a plurality of profiling furnaces 40.

LIST OF REFERENCE NUMBERS

1

furnace system

5

Sheet steel part, component

9

gas burner

10

production furnace

20

positioning

22

handling system

30

hard area

40

Profilierofen

41

top

42

lower part

44

admission

45

product-specific intermediate flange

46

heating element

47

connection

48

refrigerated area

49

Cooling water connection

50

stretchable area

60

Second robot

61

First robot

70

press tool

Claims (13)

1. A furnace system (1) for partial heating of sheet steel parts (5) to a temperature above the Ac3 temperature with a production furnace (10) for heating the sheet steel parts (5) to a temperature close to but below the Ac3 temperature,
   characterized in that
   the furnace system (1) also has a profiling furnace (40) with at least one level, wherein the at least one level has an upper part (41) and a lower part (42), as well as a receptacle (44) for a product-specific intermediate flange (45) and the product-specific intermediate flange (45) introduced therein, and wherein the product-specific intermediate flange (45) is configured to impart a predetermined temperature profile to the component (5), with temperatures above Ac3 for areas (30) to be hardened and below Ac3 for softer areas (50), and means for active cooling of individual areas (48).
2. The furnace system (1) according to claim 1,
   characterized in that
   the furnace system (1) also has a positioning system (20) on which the component (5) can be placed in a defined position after heating in the production furnace (10) and/or after heating in the profiling furnace (40).
3. The furnace system (1) according to claim 1 or 2,
   characterized in that
   that the product-specific intermediate flange (45) has a water cooling system in individual areas (48).
4. The furnace system (1) according to one of the preceding claims,
   characterized in that
   that the product-specific intermediate flange (45) has means for heating (46) individual areas.
5. The furnace system (1) according to claim 4,
   characterized in that
   that the product-specific intermediate flange (45) has electrical heating systems (46) for heating individual areas.
6. The furnace system (1) according to one of the preceding claims,
   characterized in that
   that the production furnace (10) is heated by means of gas burners (9).
7. The furnace system (1) according to one of the preceding claims,
   characterized in that
   that the production furnace (10) also has a transport system for conveying the components (5) through the production furnace (10).
8. The furnace system (1) according to one of the preceding claims,
   characterized in that
   that the furnace system (1) also has a handling system (22, 60, 61) for handling the components (5).
9. The furnace system (1) according to one of the preceding claims,
   characterized in that
   the profiling furnace (40) has means for temperature control in a closed control circuit.
10. A method for partial heating of sheet steel parts (5) to a temperature above the Ac3 temperature, comprising the steps of
    heating a component (5) in the production furnace (10) to close to its Ac3 temperature;
    positioning the heated component (5) by means of a positioning system (20); introducing the positioned component (5) into a defined position in the profiling furnace (40);
    application of a temperature profile to the component (5) in the profiling furnace (40) by heating selected areas (30) to a temperature above the Ac3 temperature by means of a product-specific intermediate flange (45), wherein other areas (50) are kept at a temperature below the Ac3 temperature in a targeted manner by active cooling by means of the product-specific intermediate flange (45);
    discharging the component (5) provided with a temperature profile from the profiling furnace (40).
11. The method according to claim 10,
    characterized in that
    that the heating of the component (5) takes place in the production furnace (10) by means of gas burners (9).
12. The method according to one of claims 10 or 11,
    characterized in that
    the positioned component (5) is introduced into a defined position in the profiling furnace (40) by a handling system (22).
13. Method according to one of claims 10 to 12,
    characterized in that
    the application of a temperature profile to the component (5) in the profiling furnace (40) is controlled by means of a closed control circuit.

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