DE102008063985B4 - Method and device for producing partially hardened sheet steel components - Google Patents

Abstract

A method of producing a hardened component from a steel sheet, wherein a steel sheet or a preformed or finished sheet steel component is heated to a temperature necessary for curing and then placed in a tool in which the board or the sheet steel component is cured, wherein to achieve areas a lesser or no cure in these areas the tool has gas flushable recesses (4), wherein a gas purging can be made so that arise in these areas gas cushions, which is a cooling at a speed which is above the critical hardness rate, diminished or excludes.

Description

The invention relates to a method and an apparatus for producing partially hardened sheet steel components with adjusted ductilities.

In order to meet the increasing requirements in vehicle construction with regard to the protection of passenger compartments in the event of a crash and weight savings, increasingly high strength steels are used in motor vehicles, which have a higher hardness than conventional motor vehicle steels at a lower weight.

For the production of such components, it is known to heat boards and reshape in a corresponding tool and cool rapidly to thereby achieve a press hardening. In addition, a variety of methods are known in which the board partially or completely preformed to final geometry, then heated and either finished in a suitable tool or only cured by rapid cooling.

The press-hardening of the components makes it possible, after heating and subsequent rapid cooling, to produce ultra-high-strength components with very narrow cutting and shape tolerances.

From the WO 2006/015849 A2 a method is known in which a virtually finished on final geometry component is placed in a tool, said component is clamped firmly in the region of the edges and is preferably supported only in the region of the positive radii. This component is placed in the heated state and is rapidly cooled in the mold to harden it. During this cooling, the component forms in the area of the positive radii to the mold, whereby it is dimensionally stable cooled. In other areas, the air-gap shapes may be located away from the component so as not to hinder the shrinkage of the component.

From the WO 2006/038868 A1 For example, there is known a method of hot working and tempering a metal sheet in which cutouts are provided in the mold halves so that retaining webs that hold the sheet metal component during cooling remain. In the areas where the mold halves are not adjacent to the sheet, softer zones of the final product should arise.

Due to the high brittleness and the low elongation values of press-hardened components, there is a risk in the event of a crash that the components break or break due to the high voltages that occur. It is therefore necessary to deliberately relieve areas which could lead to a component failure in a vehicle crash by areas with different mechanical material properties. In this case, it is not sufficient to produce targeted attenuation ranges, because high demands are placed on such components, so that it is imperative to adjust the mechanical characteristics in all areas of the component and predict them reliably.

The object of the invention is to provide a method for producing partially hardened sheet steel components, with which it is possible to achieve hardened sheet steel components with zones of different predictable mechanical properties.

The object is achieved by a method having the features of claim 1.

Advantageous developments are characterized in the dependent claims.

In addition, it is an object to provide an apparatus for carrying out the method.

The object is achieved with a device having the features of claim 4.

Advantageous developments are characterized in the dependent claims.

According to the invention, in the case of a component made of sheet steel, a subregion of the yield strength, the tensile strength, the elongation and the ductility is changed during rapid deformation in the event of a crash during production. This allows the stresses occurring during the crash to be distributed to the component in a targeted manner.

In this case, according to the invention, the mechanical characteristics and the ductility of press-hardened or hot-formed components can be influenced in precisely defined and precisely restricted subregions without weakening the entire part cross section and, in addition, with certain changes, also certain desired values.

According to the invention, the press hardening tool used for this purpose is provided with milled air cushions in those areas for which greater ductility is required. The air cushions are chosen in their geometry so that there is a partial installation of the components in the active radii, so that a deformation of the components is prevented during cooling, but also these areas can be specifically influenced in the ductility.

In order to cool the components also in the area of the air cushion with each for the ductility To achieve the required best cooling time, the air cushions are flushed with compressed air, which is cooled or heated in temperature depending on the application, or other gases.

Hereby, a cooling rate is achieved, which is usually lower than the cooling rate of the particular water-cooled mold halves.

In this case, the air cushions are generally realized by milling grooves and, in particular, flat grooves, circular-shaped recesses or even rectangular flat recesses in a mold half or mold surface shell, these recesses being equipped with supply air and exhaust air ducts, wherein preferably one edge side is provided for a number of grooves Groove is formed with a Zuluftbohrung and then the grooves are provided with each other with connecting holes through which the gas, for example compressed air passes from one groove to the other, while the opposite edge-side groove has a corresponding outlet opening.

In the invention, it is advantageous that a dimensional distortion of the components is prevented, since the geometry of the air cushion is selected so that the components abut in regions on cooling at the Anlageradien and thus the onset during cooling shrinkage process and resulting distortion of parts is positively influenced. In particular, the geometry of the Luftpolsternuten is chosen so that the component is held in the form of a kind of lattice structure, which results from lying between the grooves and these limiting webs.

The invention will be explained by way of example with reference to a drawing, in which:

1 strongly schematized and in an oblique top view a partially cut mold shell with milled air-cushion grooves and a component lying thereon;

2 a section through a closed mold according to the invention;

3 : a detail section according to 2 ;

4 an arrangement and geometry of possible air-cushion grooves on a sample mold half,

5 : a rear view of the mold half behind 4 ;

6 : a cut of the mold half behind 4 ;

7 : Cooling curve in the area of an air cushion groove;

8th : another cooling curve;

9 another cooling-cooling curve;

10 : Hardness results on the sample board at 1.5 mm sheet thickness;

11 : Hardness results on the sample board at 2 mm sheet thickness;

12 : the cooling curve without air cushion;

13 : the cooling curve at 3 mm air cushion thickness;

14 : an overview of cooling curves at different air cushion depths;

15 a tool according to the invention in cross-section;

16 a further embodiment of the tool according to the invention;

17 a renewed embodiment of the tool according to the invention;

18 a further cross section through a tool according to the invention with integrated ceramic pins;

19 : an enlarged detail of a tool according to the invention;

20 a further embodiment of the tool according to the invention with visible connection holes;

21 a cross section through an inventive tool with integrated in the air cushion induction coils;

22 : a cooling curve with a tool with a non-heated ceramic insert;

23 : a cooling curve of a tool with heated ceramic insert;

24 : the temperature profile in a two-part press hardening process;

25 : a comparison curve of the cooling of a sheet of 500 ° C at 30 ° C ambient temperature.

In 1 is the cooling shell according to the invention 1 shown in a partially sectioned view at that of a part of a component to be formed 2 is covered. One recognizes the supply air lines 3 , the interconnected Luftpolsternuten 4 and the exhaust pipes 5 , In this case, the Luftpolsternuten are circular disc-shaped recesses.

Seen on average one recognizes a tool shell 6 and a tool base 7 , wherein on the upper tool 6 and on the lower tool part 7 one cooling bowl each 1 is arranged. The cooling shells 1 essentially have a contour that the finished component 2 equivalent. In the cooling cups 1 can be recognized by the component 2 facing surfaces 8th the introduced Luftpolsternuten 4 , which have an air supply line 3 can be acted upon with a corresponding gas flow and the exhaust air lines 5 equipped with which the gas stream is derived.

In 3 , which represents a corresponding enlargement of the section, you can see that between the cooling shells 1 clamped workpiece, being in the cooling shells 1 the supply air lines 3 to the nearest diametrically opposed Luftpolsternuten 4 reach. It also recognizes the connection holes V of Luftpolsternuten 4 to allow a continuous gas flow.

The number of feed holes or lines 3 and drainage holes or pipes 5 is chosen so that an optimal gas flow is achieved, wherein it is important that when closing the tool in all Luftpolsternuten 4 a sufficient gas pressure to build the corresponding air cushion is present. In this respect, the gas flow must be adjusted so that a repeatable and process-safe delayed cooling during press hardening is possible.

In the first cooling phase (from component insert) can by cold air supply, which flows through the air cushion, the temperature quickly to a desired temperature, eg. B. 500 ° C are brought. In order to keep this desired temperature for a certain time, hot air is now blown through the air cushion in a second cooling phase. This causes the component to remain at the desired temperature and thus a homogeneous bainitic microstructure transformation can take place.

The 4 to 6 show the arrangement and geometry of Luftpolsternuten invention 4 in a test refrigerator.

The air cushion grooves shown 4 are flat, in the surface 8th the cooling shell 1 introduced recesses, which may have different shapes and diameters. It can be seen here, for example, circular disk-shaped Luftpolsternuten 4a , which are each arranged in groups, wherein the Luftpolsternuten 4a circular or cylindrical recesses are different from the surface 8th the cooling shell 1 extend into it.

In a further advantageous embodiment, the Luftpolsternut 4 as approximately square large air cushion grooves 4b formed, wherein the corner areas are rounded and the Luftpolsternuten 4b essentially the same depth into the interior of the cooling shell 1 extend, like the grooves 4a ,

In addition, it is possible the Luftpolsternuten 4 as rectangular, parallel to the longitudinal extent of the cooling shell 1 running grooves 4c train.

In a further advantageous embodiment, a plurality of small cylindrical grooves 4d formed as Nutenfeld, said Nutenfeld of grooves 4d , In particular, in order to be able to make a comparison, a same base area covered as the square groove 4b , Here are the remaining between the cylindrical grooves 4d remaining residual surfaces of the surface 8th almost as a grid pattern, which acts on a board to be placed.

In a further advantageous embodiment, the Luftpolsternut 4 as longitudinally extending grooves 4e formed, in the example shown, a plurality of longitudinally extending Luftpolsternuten 4e transverse to the longitudinal extent of the cooling shell 1 is arranged, wherein the Luftpolsternuten 4e have a small distance from each other, so that the remaining parts of the surface 8th as narrow strips are present.

In a further advantageous embodiment, the grooves 4 as Luftpolsternuten 4f elongated rectangular analogous to the grooves 4c formed, but these grooves with their longitudinal extent transverse to the longitudinal extent of the cooling shell 1 are arranged.

According to the invention, one or more of the grooves shown can also be present in the regions in which the component is to be more ductile, corresponding to the contour of the component; in particular, combinations of the grooves or groove fields shown can also be present.

Cooling tests were carried out with a mirror-symmetrical pair of the test vessel shell described above.

The corresponding experimental arrangements and cooling curves that could be achieved are in the 7 to 12 shown.

The tests were carried out on flat 600 × 300 mm flat boards using three sheet thicknesses, namely 1 mm, 1.5 mm and 2 mm. The boards were in this case heated in an oven, the furnace temperature was 910 ° C. The boards were taken out manually after reaching the target temperature from the oven and on one of the cooling shells 1 or mold halves 1 stored.

The board tray in the tool is done on so-called Sankyo pens.

The temperature is detected during the pressing process by means of a thermocouple T and recorded in the diagram, wherein the thermocouple T are each welded to the board.

Due to the air cushion, the cooling of the component in the air cushion area is significantly delayed. This is because air has a very low thermal conductivity and heats up during the cooling process to form a hot / hot air cushion ( 13 ). This causes the material does not convert martensitic over the original press hardening process, but a mixed structure is created. This mixed structure is significantly softer, more elastic and plastically deformable ( 12 . 13 ).

It is possible to produce different cooling rates by a variation of the air cushion shapes. Here, small air cushions dissipate the energy of the component much faster than large air cushions, which of course affects the component properties ( 7 . 8th . 9 ). The problem here is the bearing surfaces (holding areas / active radii) in which the component rests directly on the tool shell. These bearing surfaces are imperative to keep the component in shape and to size. Depending on the size of the bearing surface, these bearing surfaces can produce hard (actually undesirable) areas, since martensitic transformation takes place here, as in the original press hardening process on the cold tool.

Basically, it can be said that the larger the contact surface, the lower the reduction in the hardness of the air cushion. Different air cushion depths produce different cooling rates with the same contact surface and air cushion shape ( 14 ). The size, depth, shape, placement and number of air cushions is variable and also different depending on the component geometry.

Through the connection holes 5 it is possible during the pressing process hot or cold air to blow through the air cushion and thus the component temperature to a certain level, eg. B. 500 ° C for a certain time to keep ( 1 - 3 . 16 . 20 ). In addition, it is advantageous that the bearing surface on the tool shells (holding regions of the component) assume the temperature of the hot air flowing through. This can be ensured by directly below the surface extending communication holes and by the adjacent hot air cushion, so that the previously mentioned possibly hard holding areas are avoided.

According to the invention, a nearly homogeneous bainitic microstructure can be achieved in the problematic holding areas.

Similar to the variability of the air cushions, the connection bores are variable in size, depth, shape, placement and number and vary depending on the component geometry.

A further embodiment according to the invention provides for the bearing surfaces to be formed in the holding regions of ceramic with low thermal conductivity ( 15 . 16 . 19 ), these ceramic bearing surfaces 5a or these ceramic inserts 5a can also be heated. Here are the air cushions, as in 15 or 16 shown, maintained.

The size, shape, placement and number of ceramic bearing surfaces is variable and is essentially determined by the geometry of the component, as these the positive radii and support / resp. Defined contact surfaces.

In a further advantageous embodiment ( 17 ) a full ceramic tool shell is used, which makes it possible to set a homogeneous structure. If such a ceramic tool shell is formed from a ceramic or a ceramic casting which is not heated, there is still a relatively hard but low-stress martensite ( 22 ). Is advantageously the ceramic or ceramic casting accordingly by a heating circuit 11 ( 17 ), a softer bainitic structure forms ( 23 ).

It is advantageous to select the ceramic so that the thermal conductivity is very low and the heat capacity as high as possible. When using a ceramic, the temperature of the ceramic to be held is between 200 ° C and 300 ° C to form a bainitic structure. If a cast ceramic is used, the temperature of the ceramic to be held is about 100 ° C.

In a further embodiment of the invention many small ceramic pins 14 arranged in the air cushion to provide sufficient support surface for the component ( 18 ). The small ceramic bearing surfaces through the pins 14 and the surrounding air cushion cause the energy to be slowly removed from the component in these areas, even at the support points. If necessary, there is the option of either the air cushion again 12 or the individual ceramic pins 13 ( 4 ) to heat. Again, of course, the size, shape, placement and number of ceramic bearing surfaces variable and also different depending on the component geometry.

In a further advantageous embodiment ( 21 ) are compared to the holding areas (active radii) in the tool and the holding area of the dividing induction coils 15 integrated or attached ( 21 ). The induction coils thus counteract the flow of energy. The inductors supply as much energy in the component as energy is dissipated via the contact surfaces. This allows a constant temperature of z. B. 500 ° C are generated in the holding areas. It is also possible in the air cushion, the induction coils 15 evenly distributed and so in the component for a certain time to produce a constant homogeneous temperature, whereby the microstructure can transform homogeneously bainitic.

Advantage of the integrated in the air cushion induction coils 15 is that the induction coil 15 do not have to work against the otherwise cooled directly on the component tool surfaces. Since the tool is cooled, the induction coils also do not heat because they are embedded in the tool.

Again, the air pads act as an insulating layer between the cooled tool and the hot component, with the tools not being heated by the induction coils since they are not ferromagnetic. Against the radiant heat of the component, the induction coils can be protected by additional insulating material or by deep embedding in the tool.

If a heater is used to keep the component at a certain temperature for a certain time, it is sometimes necessary to carry out a two-part process ( 24 ). This is necessary in order to realize a cycle time reduction, wherein the process shown is a two-part press hardening process, which is suitable for partial areas of a component, as well as for complete components.

In the press are two different pressing tools per component side or following the press, a device for step (cooling to room temperature). In the first, the temperature is brought to a desired level by heating the tool shell (air cushion or ceramic or induction, as already described) in certain component areas, and this temperature is maintained for a certain time. After the holding time has ended, the component, which is still hot in certain areas, is placed by robot in a second pressing tool, with the component cooling further in air during the transfer. Care should be taken here that the temperature does not drop below the martensite start temperature.

In the second pressing tool, the still hot component area is then rapidly cooled to room temperature, in accordance with the usual, previous press hardening process. This two-part process allows a significant cycle time reduction compared to a single press tool.

In the invention it is advantageous that the hardness of a quench-hardened component can be adjusted very sensitively by the number, size and distribution of the air-cushion grooves and the gas flow.

LIST OF REFERENCE NUMBERS

1

cooling tray

2

component

3

supply air

4

Luftpolsternuten

4a

circular disk-shaped Luftpolsternuten

4b

approximately square large air cushion grooves

4c

rectangular grooves parallel to the longitudinal extent of 1

4d

cylindrical grooves, forming a Nutenfeld

4e

longitudinally extending grooves

4f

elongated rectangular grooves transverse to the longitudinal extent of 1

5

exhaust pipes

6

Upper die

7

Tool part

8th

surface

11

heating circuit

12

bubble

13

pencils

14

ceramic pins

15

inductors

V

connecting bores

T

thermocouple

Claims (11)

A method for producing a hardened component from a steel sheet, wherein a sheet steel plate or a preformed or finish-formed sheet steel component to a necessary for curing Temperature is heated and then inserted into a tool in which the board or the sheet steel component is cured, wherein to achieve areas of lesser or no cure in these areas, the tool via gas flushable recesses ( 4 ), wherein a gas purging can be made so as to result in these areas gas cushions, which reduces or eliminates cooling at a rate that is above the critical hardness speed. A method according to claim 1, characterized in that for the gas purging air or inert gas, such as CO ₂ or N is used. A method according to claim 1 or 2, characterized in that the recesses are rinsed with a preheated, in particular cooled or heated gas. Apparatus for hardening steel components, wherein at least two mold halves ( 6 . 7 ) are present, which at least partially cool down a heated steel component or a heated board on at least one cooling shell ( 1 ) per mold half ( 6 . 7 in particular apparatus for carrying out the method according to one of the preceding claims, wherein in the cooling shells ( 1 ) on the workpiece side in the areas in which a reduced or no hardness of the component is desired, recesses ( 4 ) in the surface ( 8th ) are present, wherein the recesses ( 4 ) for generating a gas cushion during the pressing of the board or the component via gas supply and gas discharge holes and also a device is provided, with the gas through the feed holes in the recesses can be introduced. Device according to claim 4, characterized in that the recesses ( 4 ) or Luftpolsternuten ( 4 ) occupy a contour or surface corresponding to the surface which is to have a reduced or no hardness. Apparatus according to claim 5, characterized in that in areas in which a reduced or no hardness is desired, a plurality of recesses ( 4 ) or Luftpolsternuten ( 4 ), which cover the area of reduced or non-existent hardness, leaving ridges between the air cushion grooves. Device according to one of the preceding claims, characterized in that the bearing surfaces or holding areas are arranged in the active radii of the component by extending below the surface connecting holes heated. Device according to one of the preceding claims, characterized in that relative to the holding areas or active radii in the tool induction coils ( 15 ) are integrated, which are arranged counteracting the energy termination of the sheet in the tool. Device according to one of the preceding claims, characterized in that the tool shell is formed from a ceramic or a ceramic casting. Device according to one of claims 4 to 8, characterized in that the holding regions in the region of the active radii, in which the component rests, are formed of ceramic. Device according to one of the preceding claims, characterized in that ceramic pins are present as additional bearing surfaces for the component in the Luftpolsternute.

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