EP2497840B2 - Oven system for partially heating steel blanks - Google Patents

Description

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

In technology, there is a desire for high-strength sheet metal parts with a low part weight in many applications in different industries. For example, in the automotive industry, efforts are being made to reduce the fuel consumption of motor vehicles and to reduce CO2 emissions, but at the same time to increase occupant safety. There is therefore a rapidly increasing need for body components with a favorable strength-to-weight ratio. These components include, in particular, A and B pillars, side impact protection beams in doors, sills, frame parts, bumper bumpers, cross beams for the floor and roof, front and rear longitudinal beams. In modern motor vehicles, the body-in-white with a safety cage usually consists of a hardened steel sheet with a strength of approximately 1500 MPa. Al-Si coated steel sheets are often used. The process of so-called press hardening was developed to produce a component from hardened steel sheet. Steel sheets are first heated to an 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 a martensite temperature of approx. 250 ° C. This creates a hard, firm martensite structure with a strength of approx.1,500MPa. A steel sheet hardened in this way has only about 6-8% elongation at break, which is disadvantageous in special areas in the event of a collision of two vehicles, particularly in the event of a side impact. The kinetic energy of the penetrating vehicle cannot be converted into heat of deformation. Rather, in this case the component will break brittle and also threaten to injure the occupants.

It is therefore desirable for the automotive industry to obtain body components which have several different expansion and strength zones in the component, so that there are very firm areas on the one hand and very elastic areas on the other hand in one component. The general requirements for a production plant should continue to be observed: there should be no loss of cycle times on the form hardening plant, the entire plant should be used without restriction in general and quickly be customized. The process should be robust and economical and the production plant should only take up minimal space. The shape and edge accuracy of the component should be so high that hard trimming can largely be omitted in order to save material and labor.

Corresponding methods and devices are described in the prior art. These processes use partially heated tools, with a region of the component being cooled above the quenching rate forming the martensite. The rest of the component is cooled as usual and forms martensite. For example, the publication describes EP 2 012 948 a forming tool for press hardening and tempered forming of a board made of high-strength and / or high-strength steels with means for tempering the forming tool as well as a method for press-hardening and temperature-controlled forming of boards made of high-strength and / or high-strength steels, in which the board heats up prior to forming is and then hot or semi-hot formed in a forming tool, the forming tool having means for tempering. A plurality of controllable means for temperature control are provided in the forming tool, as a result of which a plurality of temperature zones can be defined, at least the contact surfaces of the forming tool elements used for the forming being assigned to individual temperature zones.

In the document DE 10 2005 032 113 discloses an apparatus and a method for the hot forming and partial hardening of a component between two tool halves in a press. The tool halves are each divided into at least two segments that are separated from one another by thermal insulation. The two segments can be heated or cooled by a temperature control, so that different temperatures and thus cooling curves can be set in different areas of the component. This enables a component to be manufactured with areas of different hardness and ductility.

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

All methods that use a partially heated tool have the disadvantage that the component is distorted because the component has partially different temperatures of approximately 300 ° C to 500 ° C in the soft area and of approximately 100 ° C is removed from the tool in martensitic areas and cools further outside the form constraint. In addition, the cycle time of the process is increased, since the rapid cooling is slowed down in favor of pearlite-ferrite formation, which at the same time reduces the economy. In addition, such tools are very complex and therefore expensive and prone to failure.

Another, in the prior art, for example, from the documents DE 10 350 885 . DE 10 240 675 . DE 10 2005 051 403 or DE 10 2007 012 180 In known processes, the component is heated in a two-zone furnace in the soft area to a temperature below the material-dependent AC3 temperature, while the area to be hardened is heated to a temperature above the AC3 temperature. This creates expandable soft pearlite ferrite in one part and hard martensite in the other part of the component. 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. This makes the process less economical. Another disadvantage is that the separation of the areas can usually not be achieved with sufficient accuracy over the long term. In addition, more than two different zones cannot be reasonably reached. Furthermore, when using Al-Si-coated components, it is necessary to keep the temperature at about 950 ° C. for about 300 seconds so that the coating can diffuse 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 slowly cooled. The component is heated completely above the austenite temperature beyond the required diffusion time and temperature, and then slowly cooled again below the austenite temperature in a separate or the same furnace by partially hanging it out in air. If the mold hardening process is then carried out in the mold, the disadvantages with regard to the lack of dimensional accuracy and the economy of the production furnace are eliminated. The disadvantage of this method is the slower cycle time due to the additional work step. Another disadvantage is the undefined cooling rate, which sometimes leads to the formation of martensite in components less than 1.2 mm thick. The cooling rate is undefined because the cooling takes place in an ambient temperature that cannot be precisely defined. The process can therefore not be described as robust. Furthermore, this process can only be represented with two zones of different hardness.

The European patent application EP 2 143 808 A1 discloses a method for producing a molded component with at least two structural areas of different ductility from a component blank made of hardenable steel, which is heated differently in some areas and then shaped and hardened in areas in a hot-forming and hardening tool, the component blank being heated to a homogeneous temperature of less than AC3 in a heating device is heated and then brought to a temperature above AC3 in the first areas by means of an infrared lamp field, in order to be subsequently hardened in the hot areas and hardness areas in the first areas. In order to solve the problem of the temperature distribution in the component and the associated 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 steel material, it cannot be avoided with the proposed method that there are relatively large temperature transition ranges in which a temperature around the AC3 temperature is established without it being possible to define exactly where a temperature is still in the component prevails below this temperature or already above this temperature.

Finally, different steels can also 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 lie in the occasionally unsafe weld seam in the case of an Al-Si-coated sheet approximately 0.8-1.5 mm thick, which is usually used for body parts, the abrupt hardness transition there, and in the increased cost of the sheet due to the additional manufacturing step of welding , In tests, failures occasionally occurred due to breakage near the weld seam, so the process cannot be described as 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 with the features of independent claim 1. Advantageous further developments of the furnace system result from subclaims 2-7.

Furthermore, the object is achieved by a method according to claim 8. Advantageous embodiments of the method result from subclaims 9-11.

The furnace system according to the invention is suitable for partially heating sheet steel components to a temperature above the AC3 temperature. The furnace system has a conventional, universal production furnace for heating the sheet steel parts to a temperature close to, but below the AC3 temperature, the furnace system further comprising a profiling furnace with at least one level. The at least one level has an upper and a lower part, as well as a product-specific intermediate flange inserted 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 also has a positioning system on which the component can be placed 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. The component can then be directed into a pre-defined position in the profiling furnace or a press can be inserted for the subsequent mold hardening process. The more precisely the insertion position of the component can be maintained, the less trimming is required on the finished, hard sheet metal part. The product-specific intermediate flange has means for active cooling of individual areas. In an advantageous embodiment, the cooling is designed as water cooling.

The product-specific intermediate flange has means for heating individual areas, which are electrical heaters. This makes it possible to specifically heat and / or cool individual, product-specific areas, so that the temperatures of these areas can be kept within narrow tolerances. If individual areas are heated above the AC3 temperature, these areas will become particularly hard in the subsequent mold hardening process. The other areas, which are not specifically heated to the AC3 temperature, will become less hard in the subsequent mold hardening process and instead have a higher elongation at break. With the electric heaters, a particularly precise temperature control is possible.

It has proven advantageous to heat the production furnace with gas burners. This enables particularly economical heating of the components. Since the method according to the invention provides to heat the components in the production furnace only to a temperature below the AC3 temperature and to introduce the heat for heating defined areas to a temperature above the AC3 temperature in the profiling furnace in a later process step, this is a very precise one Temperature control in the production furnace is not necessary, so that the disadvantage of the poorer controllability of gas burners compared to that of electric heaters can be accepted in favor of the economic viability for the cheaper energy source gas.

In a further advantageous embodiment, the furnace system has a production furnace which, as a continuous furnace, has a transport system for passing the components through the production furnace. The cycle time for heating the components can thus be kept at the level of conventional heating ovens for the form hardening process. If the subsequent process step of stamping a temperature profile on the component determines the cycle time, so that the cycle time for the entire process threatens to be extended, a profiling furnace with multiple levels can be used, in which the components are partially or partially heated further in parallel. The parallel use of several profiling furnaces is also conceivable.

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

It has proven to be particularly advantageous if the furnace system continues to have a handling system for handling the components. The handling system can quickly and precisely place the components in the positioning system, remove them from the positioning system and insert and remove them in the product-specific intermediate flange in the profiling furnace. The handling system can then insert the components into a press tool for subsequent form hardening. Using a handling system minimizes the risk of injury to operating personnel from the hot components. A handling system executes the movements in defined and reproducible times so that the components can be inserted into the press tool for mold hardening with minimal temperature tolerances, 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 process according to the invention is characterized by the following process steps:

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

It has proven to be advantageous if the component is heated in the production furnace by means of gas burners, natural gas, for example, being used as the energy source.

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

The application of a temperature profile to the component in the profiling furnace is advantageously controlled via a closed control loop. This allows the tightest temperature tolerances to 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 The component is specifically heated to a temperature above the AC3 temperature via a product-specific intermediate flange, while other areas, which are supposed to have a higher elasticity in the finished part, are kept at a temperature below the AC3 temperature.

Further advantages, special features and expedient developments of the invention result from the subclaims and the following illustration of preferred exemplary 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 out AA 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 conveyor that transports the component 5 through the production furnace 10. The production furnace 10 is a conventional universal 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 such that the components 5 almost assume the temperature prevailing in the production furnace 10. In the transport direction behind the production furnace 10 there is a positioning system 20 that brings each component 5 into a defined lying position. A handling system 22 picks up the component 5 and brings it into a defined position in the profiling furnace 40. In the profiling furnace 40 there are an upper part 40 and a lower part 41, as well as 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 areas with electrical heaters 46 and on the other hand areas 48 which can be cooled. In addition, it is also possible to provide in the profiling furnace 40 only means for targeted heating 46 or only areas 48 that can be specifically cooled. Such areas 48 can have cooling bores through which a cooling medium, such as water or oil, flows. However, it is also possible to use known means such as heat pipes or inserts made of highly thermally conductive materials such as copper alloys for very targeted cooling. All known electrical heaters such as, for example, electrical heating cartridges or electrical surface heating systems can be used as electrical heaters 46. Electric heaters have the advantage that they can be regulated very precisely and quickly. The areas 30, which should be particularly hard after a subsequent mold hardening process, are heated to a temperature above the AC3 temperature by the electric heater 46. Other areas 50, which should have a higher elongation at break after the subsequent mold hardening process, are kept at a temperature below the AC3 temperature by the targeted cooling 48 of these areas. The temperature is controlled in at least one closed control loop. After the dwell time required to heat the regions 30 to the desired temperature above the AC3 temperature, the component 5 now provided with a temperature profile is removed from the profiling furnace 40 by means of the handling system 22. In the embodiment shown, the handling system 22 is designed 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 place component 5 on another transfer station after impressing a temperature profile. A second robot 60 then takes over the component 5 in order to insert it into the tool 70 of a press for die hardening. Typically, however, the component 5 can be inserted directly into the press tool 70 without repositioning, since there is no relative movement in the profiling furnace 40 and therefore no more reorientation of the component 5.

Fig. 2 shows the profiling furnace 40 in a detailed view as a plan view. A component 5 located in front of the profiling furnace 40 on the positioning system 20 can be seen. Another component 5 is located in the profiling furnace 40. Areas 30 of the component 5 which are supposed to be particularly hard after the shape hardening process are located on regions of the product-specific intermediate flange 45, which can be specifically heated by heaters 46. This is an electrical heating element which is supplied with electrical energy via connections 47, which is made available by a control means (not shown). Another area 50 of the component 5, which should have a greater elongation at break than the hard area 30 after the shape hardening process, is located on an area 48 of the product-specific intermediate flange 45, which can be specifically cooled. For this purpose, cooling medium is passed through the connections 49 into the area 48.

Fig. 3 is the section AA Fig. 2 through the profiling furnace 40. The profiling furnace 40 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 itself. In the product-specific intermediate flange 45, heaters 46 can be seen, which are supplied with energy via connections 47. As a result, the component 5 in the region 30 can be specifically heated 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 located on the positioning system 20 (not shown) by means of the handling system 22 can be inserted into the product-specific intermediate flange 45 within the profiling furnace 40.

Instead of the robots mentioned, any other suitable handling system can also be used. In the embodiment shown in the figure, only one profiling furnace 40 is described with one level. However, it is also possible to provide more than one level in the profiling furnace 40, an upper and lower part and a receptacle for a product-specific intermediate flange being provided in each level, so that a temperature profile can be impressed on a plurality of components 5 in parallel or partially parallel. Likewise, to increase the capacity of the furnace system 1, it is possible to provide several 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 (11)

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), wherein the product-specific intermediate flange (45) has electrical heating systems (46) for heating individual areas.
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 production furnace (10) is heated by means of gas burners (9).
5. 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).
6. 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).
7. 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.
8. 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).
9. The method according to claim 8,
   characterized in that
   that the heating of the component (5) takes place in the production furnace (10) by means of gas burners (9).
10. The method according to one of claims 8 or 9,
    characterized in that
    the positioned component (5) is introduced into a defined position in the profiling furnace (40) by a handling system (22).
11. Method according to one of claims 8 to 10,
    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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