EP0946311B1 - Method for the production of a sheet metal part by forming - Google Patents

Description

The invention relates to a method for producing a according to the strength or rigidity requirements sheet metal part having different material thicknesses Deep drawing.

The generic manufacturing process serves in particular Lightweight and takes place in a special way in the manufacture of Vehicle bodies application. While it was common until now, a sheet metal part in its thickness according to the area / section of the highest In the meantime, one has to interpret mechanical requirements passed, regarding the material thicknesses with regard to the local differentiate between different stiffness requirements.

A corresponding method is, for example, in DE 43 07 563 C2 described. The patent explains a method of manufacturing one Sheet metal structure part, consisting of a multiple sheet metal structure a base plate and in some cases an associated one Reinforcing plate or several reinforcing plates, wherein the base plate and the reinforcement plate or the reinforcement plates be deep-drawn together. The reinforcement plate or the reinforcement plates before deep drawing together at least partially attached to the base plate and after deep drawing permanently connected to the base plate.

A corresponding procedure is described in patent application DE 42 28 396 A1, in which in addition to a partial increase in stiffness Another goal is pursued, the vibratory mass level or low to reduce deformed sheet metal part areas in order to reduce the natural frequencies to increase.

The above-mentioned prior art suffers from the disadvantage that with such an approach to manufacturing and logistics high effort must be operated, which is correspondingly expensive.

EP 0 486 093 B1 describes a method in which the amplification of Areas of a molded part by means of a reinforcing structural member takes place, which is deformed separately by a plate body to be covered and only then with the plate body, which is also fully formed is connected. This is the process of shaping complicated and expensive.

For the sake of completeness, reference is also made to DE 41 04 256 A1. There is particularly the example of body parts for people and Trucks explained how heavily used local areas (Hinge mounts, lock reinforcements, attachment areas for bars or other load-bearing parts) can be effectively reinforced. As a result This process creates molded parts, which are also known as so-called "tailored blanks" have become known (see also VDI report no. 1002, 1993, pages 45-51). In the latter reference especially shown on the example of a door inner panel, as by larger Sheet thicknesses in the area of the hinge and lock attachment one sufficient rigidity can be generated. The weight saving yields a thin sheet inserted between thicker sheets. A disadvantage of these sheets is the fact that they are basically taken can only be used for molded parts that are finished Product are not visible. Both of the aforementioned publications make explicit on moldings that are either the inner part of a group of parts or by separate molded parts on an inner partial surface with a fault the outer surface are reinforced.

The invention has for its object a further method for Manufacture one according to the strength or Stiffness requirements having different material thicknesses Provide molded sheet metal part, which is inexpensive and in terms of Shaping process is easily carried out.

The solution according to the invention is in accordance with the process features to see claim 1. Such a way manufactured sheet metal part requires only a relatively small additional manufacturing and logistical effort. The sheet metal parts are characterized by a high surface quality and smooth transitions between areas of different material thicknesses.

As such, it is already known from DE 23 32 287 B2 for deep drawing Steel sheets, especially sheets made of austenitic steels, one Provide heating in the area of shaping. In the field of On the other hand, power transmission is cooled. The Overall, heat treatment serves the purpose of deep drawing shape that austenitic steel sheets are formable. One about the Board surface different heat treatment with that of underlying the present invention, according to the Strength requirements to obtain different material thicknesses, does not take place.

DE 44 25 033 A1 also describes a method and a method Device for pressure forming workpieces described, wherein a Workpiece clamped in a clamping device and by at least one Press tool is formed. One is in particular Laser beam device is provided through which the workpiece with a Laser beam is applied and heated to increase the yield stress lower and improve the formability. The Forming temperature is adaptable to different materials and adjustable. This can cause local heating of the workpiece in the Areas of high degrees of deformation. With various Embodiments there is also the possibility of the wall thickness of the To reduce the workpiece without going into why such a reduction should be aimed for.

DE 43 16 829 A1 also describes a method for Material processing with diode radiation described, one Adjustment of the beam profile to the machining process can take place. As Possible applications are given: forming and bending one Workpiece, laser flame cutting, welding of workpieces, Removal of impurities or coatings on workpieces, local warming to support a exciting Workpiece processing and soldering of workpieces.

By using high, high and high strength steel sheets achieved a reduction in the weight of parts, for example in body construction become. However, since such steels only have a limited formability own, their use is often required due to their function Degrees of deformation in certain areas of the deep-drawn part excluded. According to the invention, this defect is eliminated and a further reduction in part weight due to different material thicknesses achieved by a local temperature increase before or during the forming, in particular deep drawing, the yield point locally lowered and the strengthening and reshaping ability changed becomes. This can be achieved in the areas where functionally high strengths / stiffness are required, none or there is only a slight reduction in material thickness when forming, while, on the other hand, in areas where there are no or less strength / stiffness requirements be put the sheet thickness during of the forming relatively strong, d. H. to the technically permissible level can be reduced.

Depending on the sheet material used, sheet thickness and part geometry a different flow behavior z. B. by the following achieve the variants listed.

Option A:

• bei gewalzten Stahlblechen der Qualität St 15 erfolgt eine lokale Reduzierung der Fließgrenze durch lokale Rekristallisation bzw. Erholung des Blechwerkstoffes,
• bei Dualphasenstählen (DP 500) erfolgt eine lokale Reduzierung der Fließgrenze durch lokale Veränderung des Martensit-, Ferritanteiles oder durch Änderung der Martensithärte der martensitischen Phasenbestandteile,
• bei ausscheidungsgehärteten Blechwerkstoffen erfolgt eine lokale Reduzierung der Fließgrenze durch lokale Überalterung bzw. Homogenisierung des Blechwerkstoffes

Local heating takes place after one of the last rolling steps; this creates a coil or sheet metal sheet which has a shape change behavior which is adapted to the subsequent forming process. The local change in flow behavior is achieved depending on the material, namely

• in the case of rolled steel sheets of quality St 15, the yield point is locally reduced by local recrystallization or recovery of the sheet material,
• in the case of dual-phase steels (DP 500), the yield point is reduced locally by changing the martensite or ferrite content locally or by changing the martensite hardness of the martensitic phase components,
• In the case of precipitation hardened sheet materials, the yield point is locally reduced by local aging or homogenization of the sheet material

Variant B:

A local temperature change occurs during or immediately before Forming the sheet metal to form a sheet metal part z. B. in the deep-drawing tool or in a heating or Cooling device. This will result in the temperature changing range the yield point due to its temperature dependence and that Forming behavior changed locally.

Variant C:

Here the sheet is formed in two steps. After a first one Deformation with a small degree of deformation follows Final shaping - according to the properties of the sheet to be deformed were changed accordingly by a local temperature increase - with a high degree of deformation especially in the areas where a Material thickness reduction is sought.

Tests have already been carried out on various types of steel the following:

Variant A - Local heating after one of the last rolling steps

The aim of this variant is to change the yield point locally Heat after one of the last rolling steps, but before the Sheet finishing is introduced into a coil or sheet metal. At high-strength steels, steels ZStE 180 BH (bake hardening) and DP 500 in a temperature range between 200 ° C to 400 ° C an increase in Yield strengths can be determined by approx. 25% based on the initial values. While the bake hardening steel has no more Strength changes occur at higher temperatures, the fall Values for the dual-phase steel from temperatures above 550 ° C again by up to 25% of the highest yield strength values.

For the highest strength steels, e.g. B. TRIP 800 and CP 1000 fluctuate Strength parameters. Overall there are comparatively small differences in strength of approx. 10% compared to room temperature resistance.

Conclusion: It is possible with the previously discussed steels according to variant A. Coils or sheet metal plates change local yield strength accordingly adjust a suitable molding. The heat input can, for. B. done with a laser.

Variant B - Local heating immediately before or during the Reshaping

The aim of this variant is to change the yield point locally Heat that is immediately before or during the forming into the (black) Sheet metal is introduced.

The introduction of temperature can very quickly, i. H. in seconds respectively. For the steels tested (ZStE 180 BH, DP 500) notice that when heated, certain areas are strong, others again weakly deform.

With bake hardening steel, the temperature is - around a local fluidity to effect - preferably choose between 100 ° C and 200 ° C, with a strength reduction of up to 8% compared to the initial state is expected. The moderate temperatures required in this case can can be tolerated by the forming tool. In contrast to temperatures in the dual-phase steel are around 200 ° C or around which needs 500 ° C or more to make sense in certain areas To achieve an increase in local elongation. At temperatures around 200 ° C is a strength reduction of at least 10% from 550 ° C by at least Make up 20% of the initial state.

The highest strength sheet metal qualities TRIP 800 and CP 1000 require one Temperature of approx. 500 ° C in order to achieve a sensible local expansion. This leads to a reduction in strength of approx. 22% or 28% compared to the initial state.

Conclusion: Variant B is definitely for the bake hardening steel and for the Dual-phase steel can be used to a limited extent. For the highest strength variants TRIP 800 and CP 1000, which are used to lower the strength values temperatures variant B is less suitable than 500 ° C. The necessities Temperatures are too high for use in the forming tool. As a heat input source (Temperature range: approx. 100 ° C to 250 ° C) is the following imaginable: oil bath, hair dryer.

It seems to be of no importance for a local strain change for all of the above Steel qualities in the temperature range between approx. 350 ° C to To be 450 ° C. There is no reduction in strength in this area, but to a maximum strength that with the bake hardening effect can be justified.

Variant C - forming in two steps

The goal of variant C is to form the sheet in two steps. After the first forming step, you can proceed as in variant B.

In connection with the application of the method according to the invention the following considerations are worth noting:

Usually it is done in a simple tensile test at local Heating a tensile specimen prefers a constriction in the described range because the yield point is increased due to the Temperature drops, and so an area of particularly strong flow arises.

The following relationships apply to tensile tests: σ = F S 0 R m = F Max S 0 ε = ΔL L 0 = L - L 0 L 0

σ :

Nennspannung

S₀ :

Ausgangsquerschnitt

F :

Zugkraft

R_m :

Zugfestigkeit

F_max :

maximale Zugkraft

R_p :

Dehngrenze, z.B. R_P0,2

ε :

Dehnung

ΔL :

Verlängerung

L₀ :

Anfangsmeßlänge

L :

jeweilige Meßlänge

Here is:

σ:

nominal voltage

S ₀ :

Output cross section

F:

traction

R _m :

tensile strenght

F _max :

maximum traction

R _p :

_Yield strength, _e.g. R _P0.2

ε:

strain

ΔL:

renewal

L ₀ :

Initial measuring length

L:

respective measuring length

Because the mechanical work done in the constriction in heat is converted, the temperature continues to rise, with the result that the Solidification cannot increase as the yield stress drops and the sample eventually fails. If you put in different areas certain differences in the yield point - e.g. B. 20% / 10% / 5% -, which is achieved by staggered temperature increase can be, this "normal" behavior described above be avoided. The aim of variants A, B and C is to set slight differences in the yield strengths with different Hardening behavior in the different areas.

A practical implementation can consist in that the sheet material after one of the last rolling steps by local heating in one Furnace with different heating zones by means of a burner arrangement, inductive heating or by high-energy radiation sources locally in the Yield point is lowered. By appropriate markings on the Surface of the sheet can be the areas where the strength was lowered, can be recognized by the deep drawing press or presses, so that appropriate positioning of the sheet in the forming tool becomes possible.

A practical version of variants B and C can consist of that the sheet metal material by local heating immediately before Forming in a furnace with different heating zones using a Burner arrangement inductive or by high-energy radiation sources locally in the Yield point is changed, or by this during the Forming happens through the action of appropriate heat sources.

The application of measures (e.g. Diode radiation) according to the status of the Technology.

The possibilities associated with the method according to the invention for the design of different sheet metal part strengths and material thicknesses are still expandable if they become a sheet metal part to be formed from z. B. two joined (welded) Partial sheets of different steel materials and / or different Sheet thicknesses.

1a shows a sheet metal plate 1 with an initial sheet thickness d ₀ and with areas 1.1, 1.2, 1.3 that have been heat-treated with different intensities, while FIG. from which it can be seen that regions with different sheet thicknesses d1, d2 and d3 have also arisen.

2 shows two examples using FIGS. 2a and 2b Yield strengths with different hardening behavior, as shown by the corresponding heat treatment in different areas of the Sheet 1 occur. Areas where both material areas can flow plastically are hatched. This is illustrated using σ / ε diagrams.

Initial situation: In a material there are two different material states 1 and 2 with the different tensile strengths R _m ¹ and R _m ² and the yield strengths R _p ¹ and R _p ² .

In a material that has different material areas Tensile strengths / yield limits, the following conditions must be met so that both material areas at a given tension can flow plastically without breaking:

zu 1) Die aufgebrachte Spannung muß größer sein als die höhere der beiden Dehngrenzen, damit sich beide Werkstoffbereiche plastisch verformen,
muß aber gleichzeitig

zu 2) kleiner sein, als die kleinere der beiden Zugfestigkeiten, damit es nicht zum Versagen des Werkstoffs kommen kann.

Conditions: 1) R p 2nd <R p 1 <σ 2) σ <minimum (R m 2nd , R m 1 )

to 1) The applied tension must be greater than the higher of the two yield strengths, so that both material areas deform plastically,
but must at the same time

to 2) be smaller than the smaller of the two tensile strengths, so that the material cannot fail.

:

Bereiche möglicher auftretender Dehnungen bei Vorgabe einer Spannung innerhalb des zulässigen Bereichens Δσ

1, 2 :

Werkstoffzustand 1 bzw. 2 mit R_m ¹ bzw. R_m ² und R_p ¹ bzw. R_p ²

Δσ :

Zulässiger Spannungsbereich, in dem beide Werkstoffbereiche plastisch fließen, ohne daß Werkstoffversagen eintritt

Δε_1,2:

Zum zulässigen Spannungsbereich Δσ gehörender Dehnungsbereich für Werkstoffbereich 1 bzw. 2

In FIGS. 2a) and 2b):

:

Areas of possible strains that occur when specifying a stress within the permissible range Δσ

1, 2:

Material condition 1 or 2 with R _m ¹ or R _m ² and R _p ¹ or R _p ²

Δσ:

Permissible stress range in which both material areas flow plastically without material failure occurring

Δε _1.2 :

Elongation range for material range 1 or 2 belonging to the permissible stress range Δσ

Claims (6)

1. Method of manufacturing a shaped sheet-metal part, having different thicknesses of material according to the need for strength or rigidity, by deep-drawing, characterized in that a sheet-metal flat (1) is heated in the region to be shaped, in which case, with the objective of achieving a variable coefficient of elongation of the material over the surface extension of the sheet-metal flat (1) and thus a variable level of elongation with the concomitant reduction in thickness of the sheet-metal flat (1) during deep-drawing, the heating is carried out only in parts or with varying intensity.
2. Method according to Claim 1, characterized by the use of a sheet-metal flat (1) consisting of at least two joined partial sheets of different steel materials.
3. Method according to Claim 1, characterized by the use of a sheet-metal flat (1) consisting of at least two joined partial sheets of different thicknesses.
4. Method according to Claim 1, characterized in that the sheet-metal flat (1) is shaped in at least two stages.
5. Method according to Claim 1, characterized in that the heating is carried out between two shaping stages.
6. Method according to Claim 1, characterized in that the heating is carried out after one of the final rolling stages during manufacture of the sheet-metal and before further processing of the sheet-metal material.

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