DE102011051458B3 - Preparing press-hardened form conservations e.g. body or structure conservations of motor cars, comprises heating a blank in a liquid bath, and press-hardening the blank in a pressing tool for forming a hot-formed mold component - Google Patents

Abstract

The method for preparing press-hardened form conservations such as body or structure conservations of motor cars from blanks (4) consisting of a hot-forming steel, comprises heating the blank in a liquid bath at a temperature above the AC3-temperatur of a steel material of the blank for 0.25-0.75 minutes and press-hardening in a pressing tool (7) for forming a hot-formed mold component, where the blank is arranged vertically in a liquid bath such as metal melt. The blanks are arranged parallel to each other in the liquid bath and position-orientedly held in the liquid bath. The method for preparing press-hardened form conservations such as body or structure conservations of motor cars from blanks (4) consisting of a hot-forming steel, comprises heating the blank in a liquid bath at a temperature above the AC3-temperatur of a steel material of the blank for 0.25-0.75 minutes and press-hardening in a pressing tool (7) for forming a hot-formed mold component, where the blank is arranged vertically in a liquid bath such as metal melt. The blanks are arranged parallel to each other in the liquid bath and position-orientedly held in the liquid bath. The blank: is continuously moved in the liquid bath; comprises a surface coating in the liquid bath, where the surface coating is made of a metal alloy layer with a layer thickness of 20 mu m; and is cut before heating in such a manner that their geometry corresponds to the development of a finished form conservation.

Description

The invention relates to a method for the production of press-hardened molded components, in particular body or structural components of motor vehicles, from a blank of uncured, hot-forming steel sheet.

Sheet metal or metal alloy sheets are generally hot or cold formed. To reduce weight and increase crash resistance, high-strength boron-alloyed steel sheets are increasingly being used in the automotive industry, which are hot-formed into hot-formed components and press-hardened. The hot working and press hardening of metal sheets as such is known, for example, by the DE 24 52 486 A1 , In this case, a metal plate is heated to a temperature in the specific Austenitisierungstemperatur the material, ie at a temperature above the transition temperature AC1, preferably greater than AC3, the iron-carbon diagram of the alloy in a heat treatment plant, then inserted into a press tool and reshaped. Clamped in the press tool, the molded components are hardened by cooling.

By means of this procedure, the values of the mechanical properties of the molded components can be influenced in a targeted manner. In addition to the uniform heating of the entire cross section, other methods are currently being pursued which make it possible to produce partial component properties of the later molded components by different process strategies.

In this context, the DE 102 56 621 B3 a method for hot forming with subsequent press hardening process in which the boards are heated in zones in a continuous furnace to different temperature levels, so that set in a subsequent hardening process at least two structural areas with different ductility.

Current heating methods of hot forming or press hardening of coated or uncoated boards differ in the generation and transfer of the amount of heat into the board by direct or indirect methods.

As direct method special conductivity or induction systems can be used for heating. In these methods, the required amount of heat to increase the temperature is generated directly in the board, so that relatively small heating systems can be designed (piece heating). A disadvantage of this heating technique is the strong geometry dependence of the heating result of the board.

In indirect heating methods, conventional roller or rotary hearth furnaces are predominantly used. Furthermore, high-power radiators can be used, which heat the component surface by means of radiant heat to the desired temperature. In gas-fired roller or rotary hearth furnaces, the efficiency is about 40% and is limited by the relatively poor heat transfer. Furthermore, the process time of the heating is relatively long, so that a large furnace distance or a large space requirement for the furnaces must be planned to ensure the production cycle time.

Both for the thermoforming process and the curing process, the heating of the starting materials is essential. Here, the known heating processes, as explained above, in particular with regard to a homogeneous temperature profile, but also with regard to the efficiency not the best conditions with it.

The energy balance of these systems is also degraded by a frequently used protective fumigation of the furnace, that nitrogen must be kept or -absaugvorrichtungen.

An essential aspect in the production of body and structural components of motor vehicles is still the corrosion protection.

The EP 1 143 029 B1 discloses a method for producing a molded component by deep drawing a particularly hot-rolled and coated steel sheet. Also the WO 2010/085983 A1 describes a method for producing thermoformed sheet steel components from precoated boards.

By the DE 102 20 323 C1 includes a method for producing a structural component for a vehicle according to the prior art, in which first a structural component is preformed from a boron steel circuit board to near its final shape under ambient temperature, whereupon the preformed structural component in a melt of aluminum or an aluminum alloy on a Temperature above the upper conversion line in the ZTU graph (time-temperature conversion diagram) is brought, coated in the melt and then configured with simultaneous hardening in a tool in the final shape.

The disadvantage of this is that the cold preformed semi-finished a comparatively large space claim. Consequently, it is not possible to achieve a reduction in the heating or production area while maintaining normal cycle times. In addition, the preforming process can represent a time-consuming additional effort.

The invention is therefore, starting from the prior art, the object of demonstrating a more efficient and energy-saving and space-saving process for the production of high-strength press-hardened molded parts made of sheet steel, preferably for the production of provided with a surface coating moldings.

According to the invention the object is achieved by a method according to claim 1.

Thereafter, it is provided that a printed circuit board of hot-forming steel is heated in a liquid bath and then hot-formed in a pressing tool for the molded component and press-hardened. The board is heated undeformed in the flat state in the liquid bath and immediately afterwards thermoformed in a pressing tool and press-hardened. Due to the heating of the planar board is an efficient, fast and homogeneous or even temperature distribution on a small footprint and significantly reduced energy consumption.

The board is placed vertically in the liquid bath. This optimizes space requirements and handling. In particular, a plurality of circuit boards can be arranged vertically next to one another and heated vertically in the liquid bath.

Advantageous embodiments of the method according to the invention show the dependent claims 2 to 12.

The heating of the board in the liquid bath is preferably carried out to a temperature above the AC3 temperature of the steel material. The AC3 temperature is at a high-strength steel at about 900 ° C and above, for example at about 930 ° C. After reaching the forming temperature, the board is transferred to the press tool and formed there and press hardened.

The heating in the liquid bath can take place in a time of less than 5 minutes, in particular in a time of 1 minute to 3 minutes. This time window makes it possible to provide the board in the liquid bath, in particular in a liquid bath consisting of a molten metal, with a surface coating. In particular, the surface coating is formed by forming an alloy layer on the surface of the board. Also, a pre-coated sheet steel plate can be used. This coating is alloyed in the liquid bath. Particularly preferred in this context is a circuit board which has a pre-coating of an aluminum-silicon alloy. This pre-coating is then alloyed in the liquid bath, whereby a surface coating of an alloy of iron, aluminum and silicon is formed.

If a surface coating is not in the foreground, the heating in the liquid bath can take place in a time of less than 1 minute, in particular in a time of 0.25 to 0.75 minutes, preferably in less than 30 seconds.

In particular, the inventive method is also suitable for thick boards, especially sheet steel boards with a thickness greater than 2 mm. Furthermore, the method is advantageous for the production of press-hardened molded components made of high-strength and ultra-high-strength steel materials, in particular also tank steels.

The board or the boards are moved in the liquid bath, preferably continuously passed through the liquid bath. This can optimize the process flow.

A procedure which is particularly advantageous in practice provides for dipping the boards in a liquid bath, wherein the boards are statically positioned in the liquid bath. In the liquid bath, the boards are position-oriented by means of suitable fixing or holding means. For example, a bottom comb is possible, in which the blanks are fixed in the manner of a fan tray at the bottom of the basin.

In the context of the invention, different liquid baths or bath compositions are possible. In particular, metal melts come into consideration. But also oil or salt baths with appropriate formulation or composition are possible. Before dipping the boards in the liquid bath, the boards can be cleaned by flaming or brushing to remove oxides, impurities and / or oil.

The liquid bath is particularly preferably a molten metal. Here, in particular, a molten metal based on aluminum is used. Preferably, the molten metal contains 80 to 96 wt .-% aluminum (Al) and 3 to 12 wt .-% silicon (Si), in particular 7 to 10 wt .-% silicon. Optionally, up to 5% by weight of magnesium (Mg) and up to 5% by weight of zinc (Zn) may be included, as well as unavoidable impurities.

A particularly advantageous aspect of the invention is to provide the board in the liquid bath with a surface coating, in particular with a surface coating of a metal alloy layer. The surface coating may have a layer thickness of 3.0 μm to 50 μm, preferably of approximately 20 μm. The adjustment of the layer thickness takes place over the residence time of the board in the molten metal.

Homogenization of the liquid bath can advantageously take place in a static arrangement by flowing through the liquid bath with a gaseous medium. This is particularly preferably done from the bottom to the surface of the molten metal. For this purpose, gas openings or nozzles are provided within the basin.

A possible variant of the method envisages setting zones with different high temperatures in the liquid bath. The board can then be heated in the liquid bath in different areas to different temperatures. It is considered advantageous that the temperature of the board in a first zone of the liquid bath to a temperature below the AC1 temperature of the material of the board and the temperature of the board in a second zone above the AC3 temperature of the material of the board is adjusted. As a result, specific microstructural adjustments with different mechanical properties can take place on the finished molded component. Thus, motor vehicle components can be produced with at least two areas that have mutually different ductilities and strengths.

It is also possible to effect locally different temperatures through a gaseous medium, which is passed as an insulator at certain areas of the board surface in order to achieve a locally lower end temperature, in particular a temperature less than AC3. Within the liquid bath, such a gassing with guided nozzles or by a sequential control of several gas openings in the direction of movement of the board can be done.

It is particularly expedient if the board is cut before heating in such a way that its geometry substantially corresponds to the development of the finished molded component. Also, the board can be provided with openings. As a result, subsequent trimming and / or machining operations can be avoided, but at least greatly reduced.

A possible procedure according to the invention is described below:
An uncoated steel sheet is unrolled from the belt or coil and cut into blanks by means of one or more shearing or punching devices. The blanks are then fed by means of a handling device of a conveyor. The conveyor continuously conveys the boards through a temperature-controlled liquid bath, in particular a molten metal. In this case, a plurality of circuit boards are arranged at a distance next to each other parallel and vertically to the pelvic floor. The boards are completely submerged in the molten metal. Until the exit from the liquid bath, the board or boards take on the one hand to a temperature which allows austenitization or at least partial austenitization of the steel structure. On the other hand, the surface of the boards is coated with a metallic coating or a surface coating. After the dwell time of the boards in the molten metal, an alloy layer has formed on the board surface. The surface coating essentially corresponds to the composition of the molten metal. In particular, the surface coating consists of an alloy layer of iron (Fe), aluminum (Al) and silicon (Si). The production of an artificial protective gas atmosphere is not required despite the heating of the boards, since they are completely enclosed by the molten metal and so can not take place a surface decarburization. In addition, a protective effect is maintained even after exiting the boards from the molten metal, which brings additional benefits for the subsequent processing steps. It should be noted as particularly advantageous that in the inventive method a Stickstoffabscheidebzw. -absaugvorrichtung can be saved because, as I said, the boards are immersed in the heating completely in the liquid bath.

After completion of the heating in the liquid bath, the blanks are in a partially austenitized or austenitized state, depending on the temperature setting. The warm blanks are fed with a handling device of a press and placed in an actively coolable mold. As an intermediate step before or during the transfer to the pressing tool, excess molten metal may be removed from the surface of the board, for example by means of mechanical scrapers or the like. It can also be carried out by additional heat application, for example by means of a burner or other heat source. Also conceivable is a combination of non-contact removal and readjustment of the residual melt by means of a fan heater.

Subsequently, a hot forming of the coated board takes place to a mold component, which quenched while remaining in the closed mold and thereby hardened.

Preferably, the molten metal is an aluminum alloy with constituents of silicon, wherein the proportion of silicon is advantageously more than 7%.

The temperature of the sheet steel plate should be maintained above an austenitizing temperature AC3 for so long that a preferably complete austenite structure is formed. Accordingly, the molten metal preferably has a higher temperature than the transformation temperature AC3 of the steel material during the entire immersion time of the board.

The transfer from the liquid bath or the molten metal to the pressing tool is advantageously kept short in order to avoid heat loss or to avoid unnecessary heating of the boards in the molten metal. At the beginning of the hot forming of the boards their temperature should still be largely completely above the transition temperature AC1.

Alternatively, it is possible to realize the transfer from the liquid bath to the pressing tool isothermally, for example, with a small amount of heat to keep the boards to temperature.

The hot forming takes place in a pressing tool consisting of upper and lower tools and press punches. By thermoforming the board is brought by pulling in a three-dimensional shape.

It is possible to make openings or trimming operations on the hot board directly in the mold, since lower pressing forces than in the cold forming and shorter cycle times for the overall process are possible. Subsequent trimming after hot forming and hardening of the molded components is also possible. This is done, for example, in complicated trim operations or geometries and tight tolerance specifications on the finished mold component or even if a trim in the mold for other reasons is not possible.

Hardening itself takes place by rapid cooling or quenching of at least part of the mold component within the cooled die, at a cooling rate above the critical cooling rate. This leads to the desired martensite formation in the sufficiently rapidly cooled component areas, which sets the desired increase in strength.

Alternatively, a subsequent partial hardening is possible by a targeted flow of component areas of the boards by means of a gas due to the hereby received insulating effect.

It is also conceivable that only a partial austenitization of the board is made by a differentiated temperature control of the liquid bath, for example by zoning or locally enhanced heating in the liquid bath. Consequently, then only a partial hardening of the finished molded component takes place during the subsequent press hardening process. For a molten metal, this presupposes that the melt temperature is everywhere above the solidus temperature of the melt alloy, but partially below the AC1 temperature of the board.

If there is no surface coating of the sinkers in the liquid bath or if a second surface coating, in particular an anticorrosive coating, is to be carried out for permanent protection of the mold component, the finished mold components may additionally be subsequently surface-coated individually or in combination with other components, for example by dip-coating Hot dip galvanizing or powdering as well as spraying or painting.

It should be emphasized once again that in the process according to the invention, the usual fumigation with nitrogen which is otherwise customary in the case of conventional heating in a continuous furnace can be dispensed with. Thus, the nitrogen reprocessing can be omitted.

As already mentioned, a continuous guidance of the boards can be effected by the liquid bath. Preferably, a sequential or clocked passage of the boards is performed by the liquid bath. The boards are held in a position-oriented manner in the liquid bath for this purpose. The boards can be set or introduced, for example, by means of a handling device for heating and optionally for surface coating at fixed positions in the liquid bath. For this purpose, appropriate fixing or positioning means are provided in the basin of the liquid bath. Here, too, a zone division is possible by a targeted temperature control of the liquid bath, so that the boards are heated to different degrees by at least two different temperature zones. Accordingly, in the subsequent thermoforming and press-hardening process, a molding component having locally different structural properties corresponding to different strength and ductility results.

As a starting material, a pre-coated hot-forming steel can be used. For example, a thin pre-coating is possible, preferably based on aluminum, which does not oxidize in air. The precoating may also consist of a higher melting alloy. The precoating causes better adhesion of a formed during heating in the molten metal surface coating. The pre-coating can also be higher melting and, for example, be formed as a by-product of the strip rolling process on the steel strip.

In principle, it is also possible to operate processes according to the invention under an inert gas atmosphere, in particular before or during the heating of the sinkers in the liquid bath. This can be done in particular if a fumigation of the liquid bath is provided for homogenization or for a partial insulation of the dipped boards.

The invention is described below with reference to exemplary embodiments illustrated in the drawings. Show it:

1 schematically a production line with the representation of the basic principle of a method sequence according to the invention;

2 an alternative procedure with the presentation of handling equipment;

3 another alternative procedure and

4 the loading scheme of a liquid bath in plan view.

In the 1 to 4 carry corresponding components or component components the same reference numerals. The illustrations are schematic and not to scale.

The 1 to 3 show three possible processes for the production of press-hardened molded components, wherein the 1 the basic principle shows and the 2 and 3 possible process variants, which differ essentially by the handling of the boards during transfer into or out of the liquid bath or within the liquid bath.

A tape on a coil 1 provided steel sheet 2 Made of a hardenable, hot-workable steel (thermoformed steel) is made by the coil 1 withdrawn and in a cutting station 3 in boards 4 divided. The boards 4 can in the cutting station 3 be trimmed such that their geometry substantially corresponds to the development of the finished mold component. The boards 4 become a basin 5 transferred, in which a liquid bath 6 located. The liquid bath 6 is heated to a temperature above the transition temperature AC1 of the steel material of the boards 4 , in particular to a lying above the AC3 temperature of the steel material forming and hardening temperature.

The liquid bath 6 consists of a molten metal, in particular of a molten metal based on aluminum, the 80 to 96 wt .-% aluminum (Al) and 3 to 12 wt .-% silicon (Si), in particular 7 to 10 wt .-% silicon (Si ) contains. Optionally, the molten metal may each contain up to 5% by weight of magnesium (Mg) and / or zinc (Zn).

Based on 4 it becomes clear that the boards 4 within the basin 5 with the liquid bath 6 vertically (z-direction) and are arranged parallel to each other. This allows the heating of a variety of boards 4 at the same time with a small footprint and optimized energy use. The arrangement and spacing of the boards 4 to each other are chosen so that a process-optimized heat transfer from the liquid bath 6 on the boards 4 he follows.

After the boards 4 in the liquid bath 6 heated to forming temperature, the boards are 4 in a pressing tool 7 transferred and formed there to the mold component warm and by cooling by means of a cooling device 8th and in the pressing tool 7 integrated cooling circuit in the pressing tool 7 Press-hardened clamped.

When heating the boards 4 in the liquid bath 6 For example, a surface coating in the form of an alloy layer, which essentially consists of the material of the molten metal of the liquid bath, can be formed on the circuit board surface 6 and the steel material of the boards 4 , ie in particular of iron (Fe), aluminum (Al) and silicon (Si). The layer thickness of the surface coating depends on the dwell time of the boards 4 in the liquid bath 6 and may be between 3.0 μm and 50 μm, preferably about 20 μm. The heating of the boards 4 in the liquid bath 6 then takes place in a time of less than 5 minutes, in particular in a time of 1 minute to 3 minutes.

It is also possible, the boards 4 in the liquid bath 6 to heat without formation of a surface coating and to heat to thermoforming or Presshärttemperatur. This takes place in a time of less than 1 minute, preferably in about 30 seconds.

2 shows a procedure in which the boards 4 from the cutting station 3 by means of a transfer robot 9 to a gripper unit 10 be transferred. The boards 4 be from the gripper unit 10 taken up and in the liquid bath 6 on mounts 11 positioned. After heating and optionally forming the coating, the boards 4 through the gripper unit 10 recorded again and for a downstream transfer robot 12 provided the boards 4 in the pressing tool 7 transferred.

In the in the 3 process shown are the cut boards 4 on a roller conveyor 13 to the pelvis 5 with the liquid bath therein 6 promoted. There they are by a gripper unit 10 taken up and in the liquid bath 6 on mounts 11 positioned. The brackets 11 are on a conveyor 14 arranged so that the boards 4 through the liquid bath 6 moves, preferably continuously through the liquid bath 6 be moved through in the direction of arrow P. At the end of the conveyor line are the boards 4 from the gripper unit 10 taken as well as from the liquid bath 6 taken and by the transfer robot 12 the pressing tool 7 fed. In the pressing tool 7 become the boards 4 then hot formed and press hardened.

LIST OF REFERENCE NUMBERS

1

coil

2

sheet steel

3

cutting station

4

circuit board

5

pool

6

liquid bath

7

press tool

8th

cooling device

9

transfer robot

10

gripper unit

11

bracket

12

transfer robot

13

roller conveyors

14

Conveyor

P

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Claims (12)

Method for producing press-hardened molded components, in particular bodywork or structural components of motor vehicles, from a printed circuit board ( 4 ) of hot-forming steel, the board ( 4 ) in a liquid bath ( 6 ) and then in a pressing tool ( 7 ) is thermoformed and press-hardened to the mold component; characterized in that the board ( 4 ) in the liquid bath ( 6 ) is arranged vertically. Method according to Claim 1, characterized in that a plurality of circuit boards ( 4 ) arranged parallel to one another in the liquid bath ( 6 ) are heated. A method according to claim 1 or 2, characterized in that the heating of the board ( 4 ) in the liquid bath ( 6 ) to a temperature above the AC3 temperature of the steel material of the board ( 4 ) is carried out. Method according to at least one of claims 1 to 3, characterized in that the heating in the liquid bath ( 6 ) in a time of less than 5 minutes, in particular in a time of 1 minute to 3 minutes. Method according to at least one of claims 1 to 4, characterized in that the heating in the liquid bath ( 6 ) takes place in a time of less than 1 min, in particular in a time of 0.25 min to 0.75 min. Method according to at least one of claims 1 to 5, characterized in that the board ( 4 ) in the liquid bath ( 6 ) is held in a situation-oriented manner. Method according to at least one of claims 1 to 6, characterized in that the board ( 4 ) in the liquid bath ( 6 ) is moved, preferably continuously through the liquid bath ( 6 ) is passed through. Method according to one of claims 1 to 7, characterized in that as liquid bath ( 6 ) a molten metal is used. A method according to claim 8, characterized in that a molten metal based on aluminum is used, preferably a molten metal, the 80 to 96 wt .-% of aluminum (Al) and 3 to 12 wt .-% silicon (Si), in particular 7 to 10 wt .-% silicon (Si) and optionally up to 5 wt .-% magnesium (Mg) and up to 5 wt .-% zinc (Zn). Method according to at least one of claims 1 to 9, characterized in that the board ( 4 ) in the liquid bath ( 6 ) receives a surface coating, in particular a surface coating of a metal alloy layer and a layer thickness of 3.0 .mu.m to 50 .mu.m, preferably of about 20 microns. Method according to at least one of claims 1 to 10, characterized in that in the liquid bath zones are set with different high temperatures and the board in the liquid bath is partially heated to different degrees. Method according to at least one of claims 1 to 11, characterized in that the board ( 4 ) is pruned before heating, that their geometry substantially corresponds to the development of the finished mold component.

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