METHOD FOR HEATING STEEL SHEETS AND DEVICE FOR CARRYING OUT THE METHOD

Abstract

A method for heating a blank or a preformed steel sheet component for hot forming and/or quench hardening purposes. In at least some regions, the heating is carried out to a temperature above AC3; the heating of the Hank is embodied as a rapid heating and to this end, the blank is heated in a first zone at an average heating rate of >25 K/s up to about 600° C. and above this temperature, is heated at an average heating rate of >10 K/s up to a maximum of the AC3 temperature and then is transferred to a second zone in which the blank that has been preheated in the first zone is heated in at least some regions to temperatures greater than AC3, in particular >850° C., with the heating rate in the second zone being >10 K/s. The invention also relates to a device for carrying out the method.

Claims

1. A method for heating a blank or a preformed steel sheet component for hot forming and/or quench hardening purposes, comprising: heating the blank, in at least some regions, to a temperature above AC3; rapidly heating the blank, by heating the blank in a first zone at an average heating, rate of >25 K/s up to about 600° C. and above this temperature, heating the blank at an average heating rate of >10 K/s up to a maximum of the AC3 temperature; and then transferring the blank to a second zone in which the blank that has been preheated in the first zone is heated in at least some regions to temperatures greater than AC3, with the heating rate in the second zone being >10 K/s.

2. The method according to claim 1, further comprising, after zone 2, a third zone, which is a homogenizing and holding zone; wherein the heating rate in the third zone is <10 K/s or the heating capacity is dimensioned so as to ensure a controlled temperature decrease of the blank, but without undershooting transformation points of austenitized regions, or so as to ensure maintenance of a temperature and temperature profile a the blank.

3. The method according to claim 1, comprising heating the first zone with jacketed radiant tubes or open flames.

4. The method according to claim 3, wherein the zone temperature in the first zone is approx. 1,100° C to 1,300° C.

5. The method according to claim 1, wherein the zone temperature in the second zone is 1,000° C to 1,400° C.

6. The method according to claim 1, wherein in the first zone, the blanks are heated using a longitudinal field inductor.

7. The method according to claim 1, wherein the heating rates in the first zone and second zone are matched to each other so that despite a different ΔT of the blanks from an inlet to an outlet of the zones, the heating respectively takes place at the same time in order to enable a synchronization of cycles.

8. The method according to claim 2, wherein the heating of a blank or of a preformed steel sheet component for hot forming and/or quench hardening purposes takes place in at least one of the group consisting of the first zone, the second zone, and the third zone, using at least two contact plates.

9. The method according to claim 8, comprising placing a blank to be treated or a preformed steel sheet component between two contact plates in order to bring the blank or the preformed steel sheet component to a necessary temperature.

10. A device for carrying out a method for heating a blank or a preformed steel sheet component for purposes of a subsequent hot forming and hardening. or hardening through a placement of cooler components against the blank or preformed steel sheet component, the device comprising: at least two heating zones positioned one after another; including a preheating zone, with a zone temperature 1,000° C. to 1,400° C. in which the blank or preformed steel sheet component is heated, and a subsequent second zone in which the blank or preformed steel sheet component can be heated to an austenitizing temperature in at least some regions.

11. The device according to claim 10, wherein the second zone is followed by a third zone into which the blank or preformed steel sheet component is brought after leaving the second zone and in which the blank or preformed steel sheet component can be kept and/or homogenized while a temperature profile of the blank or preformed steel sheet component is maintained.

12. The device according to claim 10, further comprising: jacketed radiant tubes or burners in the preheating zone for heating the preheating zone; a plurality of electric heating modules in the second zone, which are adapted to a form of the blank or preformed steel sheet component, for additionally heating the blank or preformed steel sheet component; and cooling elements or shielding elements in regions in which a temperature of the blank or preformed steel sheet component should not exceed a temperature at which the blank or preformed steel sheet component has left the preheating zone.

13. The device according to claim 10, wherein in the first zone and/or the second zone, at least two contact plates are provided for purposes of hot forming and: or quench hardening a blank or a preformed steel sheet component.

14. The device according to claim 13, wherein the first and/or second zones are then embodied in the form of radiation furnaces and the at least two contact plates are placed on peaks made of firebricks.

15. The device according to claim 13, wherein upper and lower contact plates correspond approximately in shape and geometry to the part that is to be heated.

16. The device according to claim 13, wherein the contact plates are composed of a hot-working steel.

17. The device according to claim 11, further comprising: jacketed radiant tubes or burners in the preheating zone for heating the preheating zone; a plurality of electric heating modules in the second zone and, optionally, in the third zone, which are adapted to a form of the blank or preformed steel sheet component, for additionally heating the blank or preformed steel sheet component; and cooling elements or shielding elements in regions in which a temperature of the blank or preformed steel sheet component should not exceed a temperature at which the blank or preformed steel sheet component has left the preheating. zone.

18. The device according to claim 11, wherein in the first zone and/or the second zone and/or the third zone, at least two contact plates are provided for purposes of hot forming and/or quench hardening a blank or a preformed steel sheet component.

19. The device according to claim 18, wherein the first, second, and/or third zones are then embodied in the form of radiation furnaces and the at least two contact plates are placed on peaks made of firebricks.

20. The device according to claim 18, wherein upper and lower contact plates correspond approximately in shape and geometry to the pan that is to be heated,

Description

BRIEF DESCRIPTION OF THE DRAWING

[0082] The invention will be explained based on a drawing; the sole FIGURE provides a very schematic cross-sectional depiction of the device for heating steel sheet blanks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0083] The device 1 according to the invention is a heating device, which makes it possible to heat sheets and maintain their temperature in three different zones. In this case, the zones can be produced in a furnace that structurally forum a single unit, but it is also possible for the zones to be positioned structurally separate from one another. The transport from the zones to one another or into zone 1 or out of zone 3—is usually carried out with blank supports, which are inserted into the respective zones with grippers tar are transported within the zones with intrinsically known drives.

[0084] Regardless of whether the blank supports are inserted or driven, a cyclical advancing that is adapted to the press cycle is preferable.

[0085] Zone 1 is a preheating zone with rapid heating; in the preheating zone, jacketed radiant tubes 2 (or open flames) act on the blank 3. The zone temperature in this case is approx. 1,300° C. since such a high zone temperature ensures a rapid heating of the blanks to about 600° C. in 10 seconds.

[0086] Alternatively, the zone temperature can be selected to be somewhat lower so that a blank temperature of 600° C. is achieved after 20 seconds; but two blanks are heated simultaneously in zone 1. After the blanks have been heated in zone 1, they are transferred to zone 2.

[0087] Zone 2 is the austenitization zone; in the austenitization zone, electric heating modules 4, which are adapted to the blank form, act on the blank and bring about the temperature change from the existing temperature of the blank of at least 600° C. up to 850° C. at a heating rate of >10 K/s, The zone temperature in this case is set to approx. 1,200° C.

[0088] By means of the number and density of, the meanders, the electric heating modules 4 can be adapted to the blank, in particular to existing laser, welding seams, sheet thickness differences, and different emissivities of the surfaces of the blanks.

[0089] In regions in which the austenitizing temperature should not be achieved, in order to ensure that the blank does not heat further, cooling inserts or cooling elements 5 are provided. The cooling inserts and cooling elements are positioned so that the regions that should remain more ductile are efficiently prevented from reaching the austenitizing temperature. In lieu of cooling inserts, this can also be ensured by means of likewise provided shields, which can, for example, be made of fiber composite components, or shielding plates.

[0090] Because of the high zone temperatures in zone 1 and zone 2. It is particularly preferable for the blanks to only spend the time in these zones that is required to reach the target temperature of the blank and for them not to remain in these zones any longer since otherwise, an unwanted overheating can take place. This can be reduced in zone 2, however, by regulating the electric heating modules as needed.

[0091] Zone 3, which can be optionally provided, is a holding zone, which is comparable or identical to zone 2 in design, but has a significantly lower zone temperature (approx. 950° C.), which is suitable for maintaining the preset temperatures with which the blank has left zone 2 and/or the temperature profile with which the blank has left zone 2, taking into account the heat transfer between the electric heating modules on the one hand and the cooling elements on the other to and from the blank via the furnace atmosphere.

[0092] In this case, particularly with delayed-transformation hardenable steels in zone 3, a controlled cooling of the blank can also take place, provided that a preset temperature profile is maintained and no transformation takes place.

[0093] In this connection, zone 3 can be suitable for compensating for cycle differences between zone 1 and zone 2 on the one hand and the press on the other. To this end, zone 3 can in particular also have a plurality of positions in which the blanks can be kept until the shaping takes place.

[0094] Zones 1, 2, and 3 can have a coupled transport system or individual transport systems; the coupling can be both of a mechanical type and of a control-based type.

[0095] It has turned out to be particularly advantageous to work with very high heating rates in the two stations; the heating rates can theoretically be the same, preferably the heating rate in the first zone is >25 K/s up to about 600° C. and is >10 K/s at above 600° C. so that the basic heating of the blank takes place very quickly. The beating rate of the blank in the second zone at which the austenitizing temperature is reached is usually somewhat less and amounts to >10 K/s.

[0096] In the optional third zone, the heating rate is <10 K/s and in particular, the heating rate there is virtually nonexistent and the blanks are just thermally homogenized and held.

[0097] In order to bring the steel sheets to the necessary temperature for purposes of hot forming and hardening, as an alternative to the approach described above, it is possible for the device to respectively have at least two contact plates in the first zone and/or in the second zone and/or in the third zone.

[0098] In this case, a sheet metal part that is to be treated is placed between two contact plates in order to bring it to the necessary temperature.

[0099] The first, second, and/or third zone are then embodied in the form of radiation furnaces. The two contact plates can then be placed, for example, on peaks made of firebricks, which are usually provided for accommodating sheet metal parts.

[0100] Consequently, an upper and lower contact plate can be positioned in such a furnace so that they are heated to the desired temperature.

[0101] After the upper and lower contact plates have reached the desired temperature, then a sheet metal part is placed in the region between the two plates.

[0102] According; to an advantageous embodiment, the upper and lower plates correspond approximately in shape and geometry to the sheet metal part that is to be heated.

[0103] The steel plates are composed of a hot-working steel. Naturally, the contact plates can also be positioned outside of the radiation furnace.