EP1536898B1 - Method for the production of a press-hardened part - Google Patents

Description

The invention relates to a method for producing a metallic mold component, in particular a body component, from a semifinished product of thermoformable sheet steel.

Many components, in particular body parts in the automotive industry, must meet high requirements in terms of rigidity and strength. At the same time, the components should have the lowest possible material thickness in the interest of weight reduction. In order to meet these two requirements, increasingly high-strength and ultra-high-strength steel materials are used, which - depending on the composition and heat treatment - have very high strengths. The production of body components from these high-strength steel sheets is preferably carried out in a hot forming process, in which - as described for example in DE 100 49 660 A1 or DE 101 49 221 C1 - heated a board and then molded and cured in a special mold. By a suitable choice of the process parameters during hot forming while the strength and toughness values of the component can be adjusted specifically.

To produce such a component by means of hot forming, a blank is first cut out of a coil, which then above the microstructure transformation temperature of the steel material, above which the material structure is in the austenitic state, heated, placed in the heated state in a forming tool and formed into the desired component shape and cooled with mechanical fixation of the desired Umformzustands, wherein a compensation or curing of the component takes place.

In order to cut a component produced in this way dimensionally stable, however, a high expenditure on equipment is required: In particular, very high cutting forces are required for cold cutting of hardened materials, which leads to rapid tool wear and high maintenance costs. Furthermore, the cold trimming of such high-strength components is problematic because, for example, the trimmed in the cold state component edges have more or less large burrs, which can lead to rapid cracking in the component due to the high notch sensitivity of high-strength materials.

To avoid this occurring during mechanical trimming of the cured components difficulties many alternative cutting methods are used, such as laser cutting or water jet cutting. Although a high-quality trimming of the component edge can be achieved with the aid of these methods, these cutting methods work comparatively slowly, since the cycle times here depend directly on the length of the cut edge and on the tolerances to be maintained. The final clipping process thus creates a bottleneck in the manufacture of hot formed components which limits the number of components to be produced per unit of time. Although the total cycle time of the component production can be reduced if - depending on the length of the cutting edge - several parallel laser or water jet cutting systems are provided, but this is associated with high additional investments and logistics and therefore disadvantageous.

The invention is therefore based on the object to improve the process flow in the manufacture of components from hot-forming sheets to the effect that the cycle time - regardless of the length of the component outer contour - can be reduced.

The object is achieved by the features of claim 1.

The basic idea of the invention lies in the consideration that the component manufacturing process should be designed in such a way that it is possible to dispense with the procedurally complex and costly final trimming of the hardened component. The edge regions are therefore according to the invention already cut in the uncured state of the component and not only - as usual in hot forming usual - after the heating and curing process.

The production process according to the invention thus provides that a blank is first cut out of a coil of a hot-workable steel sheet. From this board is then removed by means of a conventional cold forming process, e.g. formed by deep drawing, and then trimming the edge regions a component blank, which has both (approximately) the desired three-dimensional shape as well as (approximately) the desired outer contour of the finished component. This component blank is then heated to a temperature lying above the forming temperature of the material and transferred in the hot state in a hot-forming tool in which the component is press-hardened. In this process step, the component blank undergoes a comparatively low deformation and at the same time undergoes a targeted heat treatment in the course of which a cross-component or local hardening takes place.

Since the component blank already has approximately the desired dimensions at the beginning of hot forming, only a relatively small adjustment or correction of the component contour is necessary during hot forming. As a result, the component edges are changed only slightly, so that the need for a final trimming of the component edges deleted. Under "component edges" here are both outer Boundaries as well as inner edge areas (boundaries of breakthroughs of the component) to understand.

In contrast to conventional hot forming processes, in the production method according to the invention, the trimming of the excess edge areas thus takes place before the hot forming; At this time, the component blank is in a soft (uncured) state and therefore can be trimmed using conventional mechanical methods. Thus, can be dispensed with the laborious and time-consuming laser or Wasserstrahlbeschneidung the finished pressed part, so that the cycle times can be significantly reduced compared to the conventional process flow. At the same time, a high quality cut edge is achieved.

Furthermore, when using the method according to the invention in the hot forming tool now still a small deformation of the component; Thus, the tool wear of the hot forming tool can be significantly reduced.

Since the component geometry is produced (almost) completely by cold forming, the manufacture of the component during the design phase can be ensured by conventional forming simulation. This enables reduced development costs for component and tool.

Particular advantages can be achieved if a (multi-stage) thermoforming process is used as the cold-forming process for shaping the component geometry close to the final shape (see claim 2). Since a multi-stage formability of the component blank is possible in the soft state, even complex component geometries can be formed. Advantageously, provided in the last stage of the thermoforming tool with cutting tools, so that the trimming of the component blank takes place directly in the cold forming tool.

For cutting the component blank preferably mechanical cutting means are used (see claim 3). These cutting means can be integrated into the cold forming tool, in particular in the form of folding and / or punching tools, so that the edge trimming is not carried out in a separate method step but as part of the cold forming (see claim 4).

In order to be able to further reduce the cycle time of the overall process, it is advantageous to make the process step of press-hardening the trimmed component blank as short as possible in terms of time in order to ensure the highest possible throughput of components per hot-forming tool. For this purpose, the finished molded component should be cooled as quickly as possible. In an advantageous embodiment, the finished molded component is quenched in a tool which is cooled by means of a brine (with temperature <0 ° C) as a coolant (see claim 5); Such a brine has a particularly high thermal conductivity and heat capacity. In this way, a particularly rapid cooling of the component can be achieved.

An additional reduction in the cycle time of the overall process can be achieved if the component is cooled over several stations (corresponding to several tool sets). Thus, in a first station, the component is quenched until the martensite temperature has fallen below. The component strength is then already sufficient for further transport to the next station (or the next tool). In this second (or a sequence of further) station, the component is then cooled to hand temperature.

In an advantageous embodiment, a semi-finished product made of an air-hardening steel is used for the production of the component (see claim 6). One advantage of air-hardening steels is that they quench the component In principle no additional cooling (eg by the hot forming tool) is necessary. In this case, the component blank is formed in the hot forming tool to final contour and then cooled only in the hot forming tool until sufficient heat resistance, rigidity and associated dimensional stability of the component is achieved. Subsequently, the component can be removed from the hot forming tool and cooled in the air ready; the hot forming tool is thus ready for receiving another component blank. In this way, the cycle times in the production of hardened components can be further shortened. - If the air hardening under a protective gas, it results - in addition to this time gain - the further advantage that forms no scale on the component and thus eliminates the costly subsequent descaling (see claim 7).

With such heating and heat treatment under inert gas, the component remains free from surface contamination and therefore can advantageously be subjected to surface coating directly following hot working and quenching (i.e., after cooling to a temperature below the martensite temperature) (see claim 8). In the course of this surface coating, in particular corrosion-inhibiting protective layers (for example by galvanizing) can be applied to the component surface. In this case, the residual heat resulting from the hot forming, remaining in the component, can be used directly. Subsequently, a further heat treatment of the component can be carried out by tempering.

The heating of the trimmed component blank before hot working can be carried out in a continuous furnace (see claim 9). Alternatively, the heating is carried out inductively (see claim 10). Such inductive heating takes place very quickly, which is why an additional time gain in the overall process time can be achieved in this case. Due to the short heating time continues to occur during heating only a negligible scaling of the component surfaces, which is why the use of inert gas can be omitted. Inductive heating has particular advantages in those applications in which not the entire component, but only selected areas of the component are to be press-hardened: Then selectively - by suitable design of the inductors - only the selected areas to be cured are heated and then hot-formed Tool hardened, while the remaining, unheated areas are formed in the hot forming tool, but remain in the original ductility. Alternatively or additionally, the induction heating allows adjustment of the component properties across the sheet thickness ("soft core - hard cover layer"). In this way, locally variable strength and stiffness properties can be achieved on the finished component.

For inductive heating, a separate, arranged between cutting device and hot forming tool heating station - analogous to the continuous furnace - are provided. In contrast to a heating in the continuous furnace - in which a certain heating distance is necessary - the inductive heating is associated with a small footprint, resulting in cost savings. The shape and arrangement of the inductors is matched to the shape of the trimmed component blank or the areas to be heated. As an alternative to heating in a separate heating station, heating may also take place in the cutting device (directly after edge trimming) or in the hot forming tool (immediately before hot working). For this purpose, the cutting device or the forming tool is provided with internal inductors, or the component is heated by means of external, correspondingly shaped inductors which are introduced after the edge trimming or before the hot forming in the open cutting device or the open hot forming tool and there be placed at the desired location of the component.

Fig. 1

ein Verfahrensschema des erfindungsgemäßen Herstellungsprozesses eines pressgehärteten Bauteils:

• Fig. 1a: Zuschneiden der Platine (Schritt I)
• Fig. 1b: Kaltumformung (Schritt II)
• Fig. 1c: Beschneiden der Ränder (Schritt III)
• Fig. 1d: Warmumformung (Schritt IV)
• Fig. 1e: Trockenreinigung (Schritt V);

Fig. 2

perspektivische Ansichten ausgewählter Zwischenstufen bei der Herstellung des Bauteils:

• Fig. 2a: ein Halbzeug;
• Fig. 2b: ein daraus geformter Bauteil-Rohling;
• Fig. 2c: ein beschnittener Bauteil-Rohling;
• Fig. 2d: das fertige Bauteil.

In the following the invention will be explained in more detail with reference to an embodiment shown in the drawings. Show

Fig. 1

a process diagram of the production process according to the invention of a press-hardened component:

• 1a: cutting the board (step I)
• Fig. 1b: cold forming (step II)
• Fig. 1c: trimming the edges (step III)
• Fig. 1d: hot forming (step IV)
• Fig. 1e: dry cleaning (step V);

Fig. 2

Perspective views of selected intermediate stages in the manufacture of the component:

• Fig. 2a: a semi-finished product;
• FIG. 2b shows a component blank formed therefrom; FIG.
• Fig. 2c: a trimmed component blank;
• Fig. 2d: the finished component.

1a to 1e show a schematic representation of the inventive method for producing a spatially shaped, press-hardened component 1 from a semifinished product 2. In the present embodiment, a board 3 is used as a semifinished product 2, which is cut out of a unwound Blechcoil. Alternatively, a composite sheet may be used as a semifinished product which, as described, for example, in DE 100 49 660 A1, consists of a base sheet and at least one reinforcing sheet. Furthermore, can be used as a semi-finished a Taylored Blank, which consists of several welded together sheets of different material thickness and / or different material properties. Alternatively, the semifinished product may be a three-dimensionally shaped sheet metal part produced by any forming process which is to undergo further forming and a strength / rigidity increase with the aid of the method according to the invention.

• Kohlenstoff: 0,18 - 0,28%,
• Silizium: max. 0,7%,
• Mangan: 2,00 - 4,00%,
• Phosphor: max. 0,025%,
• Schwefel: max. 0,010%,
• Chrom: max. 0,7%,
• Molybdän: max. 0,55%,
• Nickel: max. 0,6%,
• Aluminium: 0,020 - 0,060%.

The semifinished product 2 consists of a thermoformable steel. As an example of such a material, mention may be made here of Benteler's air-hardening steel marketed under the trade name BTR 155, which has the following alloy composition, the contents of the alloying partners to be added in addition to the base metal iron:

• Carbon: 0.18-0.28%,
• Silicon: max. 0.7%
• Manganese: 2.00 - 4.00%,
• Phosphor: max. 0.025%
• Sulfur: max. 0.010%
• Chrome: max. 0.7%
• Molybdenum: max. 0.55%,
• Nickel: max. 0.6%
• Aluminum: 0.020-0.060%.

In a first process step I, the board 3 - as shown in Figure la - cut from a developed and straightened portion of a coil 5 of a thermoformable sheet. The thermoformable material is in a "soft" (i.e., uncured) state at this time, so that the circuit board 3 can easily be cut out using conventional mechanical cutting means, such as a scissor lift 4. In mass production, the cutting of the board 3 is advantageously carried out by means of a platinum press 6, which ensures an automated feed of the coil 5 and an automatic punching and removal of the cut-out board 3. The thus cut out board 3 is shown in Figure 2a in a schematic perspective view

The cut-out blanks 3 are deposited on a stack 7 and are fed in a stacked form to a cold-forming station 8 (see FIG. 1b). Here, in a second process step II from the board 3 with the aid of Cold forming tool 8 - in the present example, a two-stage deep drawing tool 9 - a component blank 10 is formed. In order to be able to reliably ensure a high-quality shaping of the component geometry, a predetermined, optimized material flow on the blank 3 must be ensured during the cold forming process. In order to achieve this, the board 3 has edge regions 11 which protrude beyond an outer contour 12 of the component 1 to be molded (indicated by dashed lines in FIG. 2 a). In these edge regions 11 controlled forces are exerted during the drawing process by hold-down 13, which cause a targeted flow of material on the board 3 and thus a high-quality drawing result.

As part of this cold forming process (process step II), the component blank 10 is formed close to the final contour. The term "close-to-net shape" should be understood to mean that those parts of the geometry of the finished component 1 which are accompanied by a macroscopic flow of material are completely formed in the component blank 10 after the cold-forming process has been completed. After the completion of the cold forming process (process step II), only slight conformations of shape are required for producing the three-dimensional shape of the component 1, which require a minimal (local) material flow; the component blank 10 is shown in FIG. 2b.

Depending on the complexity of the component geometry, the near-net shape shaping can take place in a single deep-drawing step, or it can be multi-stage - for example, in the two-stage deep drawing press 9 shown in FIG. 1b.

Subsequent to the cold forming process, the component blank 10 is inserted into a cutting device 15 and cut there (process step III, FIG. 1c). Since the material of the component blank 10 is still in a "soft", ie uncured, state at this time, this cutting process can be carried out with the aid of mechanical cutting means 14 (Especially with cutting blades, folding and / or punching tools) done.

For the trimming process can - as shown in Figure 1c - a separate cutting device 15 may be provided. Alternatively, the cutting means 14 may be integrated into the last stage 9 'of the deep-drawing tool 9, so that in the last deep-drawing stage 9' in addition to the finished molding of the sheet metal blank 10, the marginal cutting occurs.

As a result of the cold forming and the trimming process (process steps II and III), the board 3 produces a truncated component blank 17 which is close to the final contour and which is only slightly different from the desired one in terms of its three-dimensional shape and with respect to its edge contour 12 ' Deviates component shape. The cut edge portions 11 are removed in the cutting device 15; the component blank 17 (FIG. 2c) is removed from the cutting device 15 by means of a manipulator 19 and fed to the next process stage.

In the now following process stage IV (FIG. 1d), the trimmed component blank 17 is now subjected to hot forming, in the context of which it is formed and hardened onto the final component mold 1. For this purpose, the trimmed component blank 17 is inserted by a manipulator 20 in a continuous furnace 21, where it is heated to a temperature which is above the structural transformation temperature in the austenitic state; Depending on the steel grade, this corresponds to a heating to a temperature between 700 ° C. and 1100 ° C. Advantageously, the atmosphere of the continuous furnace 21 is rendered inert by a targeted and sufficient addition of a protective gas in order to prevent scaling of uncoated interfaces 12 'of the trimmed blanks 17 or 17. when using uncoated sheets - to prevent the entire blank surface. As protective gas, for example, carbon dioxide and / or nitrogen can be used.

The heated trimmed component blank 17 is then inserted by means of a manipulator 22 into a hot-forming tool 23, in which the three-dimensional shape and the edge contour 12 'of the trimmed component blank 17 are brought to their final, desired level. Since the trimmed component blank 17 already has near net shape dimensions, only a slight adaptation of the shape is necessary during hot forming. In the hot-forming tool 23, the trimmed blank 17 is finish-formed and rapidly cooled, whereby a fine-grained martensitic or bainitic material structure is set. This process step corresponds to a hardening of the component 1 and allows a targeted adjustment of the material strength. Details and various embodiments of this curing process are described for example in DE 100 49 660 A1. In this case, a cross-component curing of the entire component 1 can take place; alternatively, portions of the component 1 selected by a suitable shape of the hot working tool (e.g., insulating inserts, air gaps, etc.) may be recessed from hardening so that the hardening of the component 1 is only local.

Once the desired hardening state of the component 1 has been reached, the component 1 is removed from the hot forming tool 23. Due to the hot-forming process upstream near-net shape trimming of the component blank 10 and the shape adaptation of the outer boundary 12 'in hot forming tool 23, the component 1 after completion of the hot forming process already the desired outer contour 24, so that no time-consuming trimming of the component edge after hot forming is.

In order to achieve a rapid quenching of the component 1 in the course of hot forming, the component 1 is quenched in a hot-working tool cooled by brine. Such a brine has a high thermal conductivity and heat capacity lapped. Depending on the salts added, the brine can be cooled to temperatures well below the freezing point of water.

The hot forming of the component 1 is usually accompanied by a scaling of the component surface, so that the component 1 in a further process step (process step V, Figure 1e) in a dry cleaning station 25 (for example by shot peening) must be descaled.

The process sequence shown in FIGS. 1a to 1e with the near-net shape trimming of the component blanks 10 in the soft state achieves a considerable shortening of the cycle time compared with the conventional process sequence, in which the finished, hardened component is not cut until after hot forming by (laser) cutting is cut to the desired size. If the method according to the invention is used, the component 1 already has the desired final outer contour 24 after completion of the hot forming process (process step IV), so that the hardcutting - which formed the bottleneck in the conventional process sequence - is dispensed with.

In the process sequence according to the invention, the cooling of the finished molded component 1 in the hot forming tool 23 now represents the bottleneck of the overall process: With hardening in the tool 23, the cooling time required overall is about 20, with good design of the tool-integrated cooling, depending on the sheet thickness, workpiece size and final temperature to 40 seconds, with the majority of cases ranging between 25 and 30 seconds. A shortening of the cycle time can be achieved here by using air-hardening steels as materials for the components 1: In this case, the component 1 in the hot forming tool 23 only needs to be cooled until sufficient heat resistance, rigidity and associated dimensional stability of the Component 1 is reached; then the component can 1 are removed from the tool 23, so that the further heat treatment process takes place in air outside of the tool 23, and the hot forming tool 23 is ready for receiving a next component blank 17. In this way, the residence time of the component 1 in the hot forming tool 23 can be reduced to a few (<10) seconds, which leads to a further reduction of the overall cycle time.

Additional savings or reductions in the cycle time can be achieved if not only the heating of the component blanks 17, but also the hot working takes place in a protective gas atmosphere; In this case, the forming tool 23, as indicated by dashed lines in FIG. 1 d, is integrated into the protective gas atmosphere 26 of the continuous furnace 21. As a result, a scale-free press hardening process is realized, so that the otherwise previously required subsequent dry cleaning of the components 1 (process step V) can be dispensed with.

As an alternative to heating the component blanks 17 in the continuous furnace 21, the heating can be carried out inductively.

Claims (10)

1. Method of producing a metallic shaped part, in particular a vehicle body part, from a semi-finished product made of an unhardened hot-workable steel sheet, comprising the following method steps:

   - a part blank (10) is formed from the semi-finished product (2) by a cold-forming method, in particular a drawing method;

   - the part blank (10) is trimmed at the margins to a marginal contour (12') approximately corresponding to the part (1) to be produced;

   - the trimmed part blank (17) is heated and press-hardened in a hot-forming tool (23).
2. Method according to Claim 1, characterized in that a deep-drawing method is used for shaping the part blank (10) from the semi-finished product (2).
3. Method according to Claim 1 or 2, characterized in that the part blank (10) is trimmed by means of a mechanical cutting method (15).
4. Method according to Claim 3, characterized in that the trimming of the part blank (10) is effected as part of the cold forming.
5. Method according to one of the preceding claims, characterized in that the tool (23) is cooled with a brine.
6. Method according to one of the preceding claims, characterized in that the semi-finished product (2) is made of an air-hardened steel alloy.
7. Method according to the preceding claims, characterized in that the heating and hot forming of the trimmed part blank (17) are effected in an inert-gas atmosphere (26).
8. Method according to Claim 7, characterized in that

   - the part (1) is cooled after the hot forming down to a temperature below the martensite temperature

   - and is provided immediately afterward with a surface coating, in particular an anti-corrosion coating.
9. Method according to the preceding claims, characterized in that the heating of the trimmed part blank (17) is effected in a continuous furnace (21).
10. Method according to one of Claims 1 to 8, characterized in that the heating of the trimmed part blank (17) is effected inductively.

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