EP2561108A2

EP2561108A2 - Method and device for hot forming and heat treating of coated metal sheet

Info

Publication number: EP2561108A2
Application number: EP11743181A
Authority: EP
Inventors: Hans-Joachim Schwiese; Torge Behrens; Gerd Hintz
Original assignee: Huettinger Elektronik GmbH and Co KG
Current assignee: Trumpf Huettinger GmbH and Co KG
Priority date: 2010-04-21
Filing date: 2011-04-21
Publication date: 2013-02-27
Also published as: WO2011131174A3; DE102010017905B4; DE102010017905A1; WO2011131174A2

Abstract

In a method for hot sheet metal forming, a sheet metal is heated to a temperature less than or equal to the Curie temperature in a first step by induction heating by means of a first induction heating device, and in a second step the sheet metal is heated to temperatures greater than 800 °C by means of conventional heating in a furnace or by means of induction heating in a second induction heating device different from the first induction heating device.

Description

Method and apparatus for hot sheet metal forming

Technical field

In a method for hot sheet metal forming a pre-cut or danced sheet is heated to a temperature greater than 800 ° C, especially between 800 ° C and 1000 ° C and then formed and simultaneously cooled and cured. This makes it possible to form very hard and stable steels which are used, for example, in the automobile body. Especially in this use, they are of particular interest, because with such shaped steel sheets, the thickness and the number of sheets in load-bearing body parts can be reduced. This makes it easier to build automobiles, which saves fuel and CO ₂ emissions and improves crash performance. The sheets are usually made of steel. The sheet may already be coated before heating, for example with an A-silicon silicon layer (AISi). Such a layer prevents scaling during hot sheet forming and forms the basis for painting. The sheet usually has thicknesses less than 3 mm. The thickness of the sheets is usually homogeneous before the hot sheet metal forming, but this is not mandatory. The sheet may already be pre-cut or punched in the mold, so one represent any shape. It has different areas, which are usually between 0.2 m ^J and 2 m *. For heating conventional ovens are usually used, which transfer the heat by means of heat radiation or hot air to the sheets. When heated, the coating melts and it arrives

Diffusion process, which has been little studied.

When heating in conventional ovens much energy is released to the environment, which makes the type of heating uneconomical. There is therefore a desire to make heating more economical, faster and cheaper.

Description of the invention

The object of the invention is to provide a method and a device which enables fast, energy-efficient and space-saving hot sheet metal forming.

The object is achieved by a method for hot sheet metal forming, in which a pre-cut or danced sheet is heated to a temperature greater than 800 ° C and then reformed and simultaneously cooled and cured, and wherein the heating of the sheet in a first step by inductive heating by means of a first induction heating device to a first temperature is less than or equal to the Curie temperature and in a second step, the heating to temperatures greater than 800 ° C by conventional heating in an oven or by means of inductive heating in a second, different from the first

Induktionsewärmungsvorrichtung takes place wherein the sheet with a protective layer is coated, which is melted after heating to the first temperature and wherein the melting point of the protective layer is below the first temperature.

At the Curie temperature, a sheet loses its magnetization properties almost completely. Thus, the induction heating process above the Curie temperature can not be economically performed by the same induction heating apparatus as below the Curie temperature. A

Induction heating device consists essentially at least of a medium or high frequency generating power unit, an output resonant circuit and an inductor, which may be part of the output resonant circuit. Of the

Output resonant circuit may be a parallel or series resonant circuit or a

Combination of both oscillating circuits. The inductor generates an alternating

Magnetic field, which generates eddy currents and magnetic reversals in the sheet, which lead to heating of the sheet. If the sheet has a high permeability, the coupling is better and the sheet heats up faster. The induced eddy currents are forced by electromagnetic effects on the surface of the sheet. This effect is frequency dependent. For up to 3 mm thick sheet, a penetration depth of up to 1.5 mm is desirable. The frequency must be kept comparatively low, it is in tests between 5 and 30 kHz, in particular at 15 kHz. The inductance of the inductor must not be too large at these frequencies. Otherwise, the voltage at the output resonant circuit can increase to very high values. Too high voltages increase the risk of flashovers z. B. on the inductor or in capacities of the Alisgangsschwingkreises. The sheet has to be heated homogeneously. Inhomogeneities in the warming lead to a different rapid melting of the

Protective layer on the sheet. This different melting can be one have different strong reaction during the diffusion process result. Both can have disadvantages in the further processing of the hot-formed sheet metal parts and negative effects on the stability of the sheet metal parts.

The melting point of the protective layer may be below the Curie temperature. For the diffusion process, a temperature below the Curie temperature may also be required.

The diffusion process for AISi is between 600 ° C and 650 ° C. The Curie temperature for the steel sheets used is 710 ° C to 770 ° C.

In the method or in a method independent of the second step, the heating in the first step can take place in two substeps. In a first sub-step, the sheet can be heated to a first temperature and in a second substep, the sheet can be kept at a temperature more than 70 K lower than the first temperature. Thus, a very homogeneous melting of a protective layer can be achieved, and the Diffussionsvorgang can also be very homogeneous. In one of the methods mentioned, a protective layer can be completely melted on the metal sheet in the first sub-step. In particular, the first temperature can be suitably selected so that the protective layer is completely and homogeneously melted on the sheet.

The first temperature may be close to the Curie temperature, that is 710 ° C to 770 ° C. In one of the methods mentioned, a diffusion process can take place in the second substep. This can now be done very homogeneously, since the protective layer has already melted particularly homogeneous. The diffusion process for AISi is between 600 ° C and 650 ° C. For other materials, a suitable different temperature range may be chosen.

In the method can be provided that the sheet is not moved during the first step. This can improve the homogeneity. In one of the methods mentioned, the sheet can also be deliberately moved during the first step. This may be useful if the structure of the inductor leads to a still insufficient homogeneity of the heating of the sheet. Direct under a conductor portion of an inductor, the heating may be higher than between two conductor portions of the inductor. The sheet can now be moved perpendicular to the conductor sections to achieve improved homogeneity of the heating of the sheet.

The object of the invention is also achieved by an induction heating device with a medium- or high-frequency generating power unit, a

An output resonant circuit for homogeneously heating a precut or danced steel sheet by means of an inductor arrangement comprising a first inductor and a second inductor, the first inductor being electrically connectable in parallel with the second inductor and having a substantially comparable inductance and shape, the first inductor being mounted on the first inductor Is arranged on top of the sheet and the second inductor is arranged on the underside of the sheet, and that the first inductor is formed so that at least in the region of the sheet-like extent of the sheet, the current at a time in several arranged in a first plane

Induktorabschnitten flows in only one direction and the current in the second inductor is formed so that at least in the area of the flat extent of the sheet metal Current at the same time in the said inductor portions of the first inductor facing in a plane parallel to the first plane level inductor sections also flows only in one direction. Thus, an inductor with sufficiently low inductance can be realized, which ensures at the same time a sufficiently homogeneous heating. An inductor formed in this way can be, for example, a so-called butterfly inductor. He leads all his Induktorabschnitte in the region of the sheet to be heated in one direction only. The return of the inductor winding is outside the range of the sheet to be heated.

In the induction heating apparatus, the two inductors may drive the current in opposite directions in each of the inductor sections. Thus, an inductor with sufficiently low inductance can be realized, which ensures at the same time a sufficiently homogeneous heating.

In the induction heating device, additional steel components may be arranged at least partially at the edges of the steel sheet. This improves the homogeneity in the edge areas of the steel sheet. The additional steel components may be arranged at least at the edges of the steel sheet, which run parallel to the direction of flow in the inductors, since in these areas inhomogeneities in the

Warming without such additional steel components can train particularly.

In the induction heating apparatus, said inductor portions of the first inductor may be offset from the inductor portions of the second inductor. This leads to an improved homogeneity of the heating.

The induction heating device may be used to carry out any of the above

described methods are used. Brief description of the figures

Fig. 1 heating curve according to the inventive method

Fig. 2 process steps according to the inventive method

Fig. 3 Induction heating device according to the invention Fig. 4a Side view of an inductor arrangement of the induction heating device according to the invention

4b shows a side view of the inductor arrangement of FIG. 4a with the first inductor removed, FIG. 5a shows a section through an inductor arrangement with non-offset inductor sections FIG. 5b shows a section through an inductor arrangement with offset inductor sections FIG. 6 shows a perspective drawing of an inductor arrangement according to the invention

Amfilhrungsbeispiele for carrying out the invention

In Fig. 1, a heating curve is shown by the inventive method. To the right is the course of time t and up the temperature T of the sheet to be heated applied. At time to the inductive heating begins after the first step 1 and after the first sub-step la. At time ti, the first temperature T _{t is} reached. This may be the Curie temperature of the sheet or a temperature very close to it. Between the times ti and t ₂ , the sheet cools and is then held until the time t ₃ at a constant temperature T ₂ . The second sub-step 1 b is therefore between the times and 13. After the time t3, the second step 2 can take place in which the sheet is heated to temperatures greater than 800 ° C.

From time to until time ti, a temperature gradient of greater than 30 K / s can be achieved. This allows plates to be heated from 10 ° C to over 700 ° C in less than 25 s. The diffusion process can be chosen larger than 50 s. Good results can be achieved by ending the diffusion process after 60 seconds to 90 seconds.

FIG. 2 shows the method steps according to the method according to the invention. In the method for baking sheet blanketing, the heating of a sheet in a first step 1 by inductive heating by means of a first

Induktionssewärmungsvorrichtung to a temperature less than or equal to the Curie temperature and carried out in a second step 2, the heating to temperatures greater than 800 ° C by conventional heating in an oven or by inductive heating in a second, different from the first induction heating. The first step 1 can be subdivided into a first

Sub-step la, in which the sheet is heated to a first temperature, and in a second substep lb, in which the sheet is kept at a temperature, the second temperature should be higher than the melting temperature of the coating. For the result, a temperature has proved to be advantageous, which is more than 70 K lower than the first temperature.

In Fig. 3, an induction heating device 30 according to the invention is shown. It has a medium or high frequency generating power unit 31, a

Output resonant circuit 32 and an inductor 34. The inductor 34 is conventional Part of the output resonant circuit 32, so here too. The output resonant circuit 32 has additional capacitances 33 in series or parallel to the inductor 34, which is indicated schematically here. In the inductor 34 is shown a butterfly shape, which is characterized in that in a predetermined region 35, in which, for example, the sheet to be heated, the current in all inductor portions 36 at a time always flows in the same direction, indicated by arrows is indicated. Parallel to the inductor 34, a second inductor (not shown) may be connected.

In Fig. 4a is a side view of an inductor of the

Induction heating device according to the invention shown. The bends 41 and 42 show the connection bridges of the inductor arrangement to the rest of the

Output resonant circuit 32. The inductor arrangement has a first and a second inductor. The first inductor 44 is disposed on the upper surface of the steel sheet 45, and the second inductor 43 is disposed on the lower surface of the steel sheet 45. Electrically insulating and heat resistant spacers 46 hold the sheet in a constant position in two parallel planes to each other.

FIG. 4b shows a second side view of the inductor arrangement of FIG. 4a with the first inductor 44 removed. The same reference numbers were used for the same components. In order that the steel sheet 45 may be placed between and taken out of the inductors 43, 44 with a robot gripper (not shown), one of the inductors is constructed so that it can be lifted and the

Robot gripper provides enough space to grab the sheet metal.

In Fig. 5a is a section through an inductor arrangement with unequipped

Inductor sections shown. In contrast, in Fig. 5b is a section through a Inductor shown with offset inductor sections. As in FIG. 4 a, the first inductor 44 is disposed on the upper side of the steel sheet 45, and the second inductor 43 is disposed on the lower surface of the steel sheet 45. The inductor sections 44a of the first inductor 44 lying in a first plane show all the currents pointing in the same direction in the region of the steel sheet 45. In this case, a point means a stream flowing towards the observer, a cross a stream flowing away from the observer. The inductor sections 43a of the second inductor 43 lying in a second plane likewise show the currents pointing in the same direction in the region of the steel sheet 45 but pointing in the opposite direction to the current direction of the inductor sections 44a. Left and right of the steel sheet 45, two additional steel components 47a, 47b are arranged. This results in a more homogeneous heat distribution in the steel sheet 45.

In the outer inductor portions 44b, 44c, 43b, 43c, the current becomes

recycled. These are not in the range of the sheet to be heated 45.

FIG. 6 shows a perspective drawing of an inductor arrangement according to the invention. The connection bridges 41 from FIGS. 4 a and 4 b can also be seen here, as well as the inductor sections 43 a, 43 b, 43 c, 44 a, 44 b, 44 c from FIG. 5. The connection 62 can be connected to the remainder of the outer oscillating circuit 32. Since the upper inductor 44 is liftable for receiving and removing a sheet, not shown here, it has contact jaws 63 a and 63 b, with which it can be connected electrically parallel to the lower inductor 43. The electrically insulating brackets 61 and 62 are for adjustment when folding down the upper inductor 44. The sheet, which is not shown, can be moved in the direction of the arrow 64 during all steps and substeps to increase the homogeneity.

Claims

claims

1. A process for the sheet-metal forming in which a pre-cut or danced sheet is heated to a temperature greater than 800 ° C and then formed and simultaneously cooled and cured, and wherein the heating of the sheet in a first step by inductive heating by means of a first

Induktionserwärmungsvorrichtung to a first temperature is less than or equal to the Curie temperature and in a second step, the heating on

Temperatures greater than 800 ° C by conventional heating in one furnace or by inductive heating in a second, different from the first

Induktionserwärmungsvorrichtung takes place wherein the sheet is coated with a protective layer which is melted after heating to the first temperature and wherein the melting point of the protective layer is below the first temperature.

2. The method according to claim 1, characterized in that the heating in the first step takes place in two substeps, wherein in a first substep, the sheet is heated to a first temperature and in a second substep, the sheet at a temperature by more than 70 K lower than the first temperature is maintained.

3. The method according to claim 2, characterized in that in the first substep, a protective layer is completely melted on the sheet.

4. The method according to any one of claims 2 or 3, characterized in that in

second substep a diffusion process takes place.

5. The method according to any one of the preceding claims, characterized in that the sheet is not moved during the first step

6. The method according to any one of the preceding claims, characterized in that the sheet is moved during the first step.

An induction heating apparatus comprising a medium or high frequency generating power unit, an output resonant circuit for homogeneously heating a precut or danced steel sheet by means of an inductor arrangement comprising a first inductor and a second inductor, the first inductor being electrically connectable in parallel to the second inductor, and a first inductor has substantially comparable inductance and shape, wherein the first inductor is arranged on the upper side of the sheet and the second inductor is arranged on the underside of the sheet, and that the first inductor is formed so that at least in the area of the sheet-like extent of the sheet Current in a plurality of arranged in a first level inductor sections at a time in one direction only flows and the current in the second inductor is formed so that at least in the area of the sheet-like extent of the sheet at the same time in the genan NEN inductor portions of the first inductor facing in a plane parallel to the first plane level inductor sections also flows in only one direction.

8. induction heating device according to claim 7, characterized in that the two inductors each carry the current in said inductor portions of opposite direction.

9. induction heating device according to claim 7 or 8, characterized in that at least partially at the edges of the steel sheet additional steel components are arranged.

10. Induktionsemämungsvorrichtung according to one of claims 7 to 9, characterized in that said inductor portions of the first inductor are arranged offset from the inductor portions of the second inductor.

11. induction heating device according to one of claims 7 to 9, characterized in that it for carrying out the method according to one of the claims

1 to 6 is used.