EP3585531B1 - Method for producing a component by further forming of a preformed contour - Google Patents

Description

The present invention relates to a method for producing a component by further shaping a preformed contour of a blank according to the preamble of patent claim 1. Compared to known methods for producing a component, the method according to the invention is characterized in particular by increased design freedom when forming, in particular shear-cut ones edge areas of the board.

A blank or sheet metal blank is understood below to mean a blank of a sheet metal, in particular a steel sheet. The sheet metal blanks can be uncoated or provided with a metallic and/or organic anti-corrosion coating.

In the following, a component is understood to mean a component produced from a sheet metal blank by forming using a forming tool at ambient temperature. All formable metal materials can be considered as sheet metal materials, but in particular steel.

Such components are mainly used in automobile construction, but there are also possible uses in the household appliance industry, in mechanical engineering or in construction or in the area of the same.

The intensely competitive automotive market forces manufacturers to constantly look for solutions to reduce their fleet consumption while maintaining the highest possible level of comfort and occupant protection. On the one hand, the weight saving of all vehicle components plays a decisive role, but on the other hand, the best possible behavior of the individual components under high static and dynamic loads during operation and in the event of a crash also plays a role.

The primary material suppliers try to meet the necessary material requirements to take account of the fact that the wall thicknesses can be reduced by providing high-strength and ultra-high-strength steels, while at the same time improving component behavior during production and operation.

These steels therefore have to meet comparatively high requirements in terms of strength, deformability, toughness, energy absorption capacity and corrosion resistance as well as their workability, for example in cold forming with regard to fatigue behavior and in welding.

In view of the aspects mentioned above, the production of components from high-strength steels with yield points above 400 MPa, advantageously above 600 or above 800 MPa to about 1800 MPa or even above, is becoming increasingly important.

In order to produce a component, it is known to first cut a sheet metal blank from hot or cold strip to size at room temperature. Mechanical cutting processes, such as shearing or punching, are mostly used as cutting processes, but thermal cutting processes such as laser cutting are also used more rarely. Thermal separation processes are significantly more expensive than mechanical separation processes, so they are only used in exceptional cases.

After cutting, the blank is placed in a forming tool and the finished component, such as a chassis beam, is produced in one or more forming steps.

During forming, the cut edges are particularly stressed, especially if they are raised or raised, e.g. during collar operations in perforated blanks.

Various other manufacturing steps, such as punching and cutting operations on the blank, are optionally carried out before forming.

Various previous damage may be present on the cut edges. On the one hand, these are due to strain hardening of the material, caused by the mechanical separation, which represents a total transformation up to the separation of the material. On the other hand, a notch effect can occur, which is caused by the topography of the cut surface.

Especially with the steels considered here, there is an increased probability of cracking in the marginal areas of these cut edges during the subsequent forming.

The previously mentioned damage to the sheet metal edges can lead to premature failure during subsequent forming operations or when the component is in operation.

The testing of the forming behavior of cut sheet edges with regard to their sensitivity to edge cracking is carried out with a hole expansion test according to ISO 16630. In the hole widening test, a circular hole is made in the sheet metal by shear cutting, which is then widened by a conical punch. The measurand is the change in the hole diameter related to the initial diameter at which the first crack appears at the edge of the hole at the cutting edge.

In order to minimize the sensitivity to edge cracks described above during cold forming of shear-cut or stamped sheet metal edges, there are, for example, approaches to changing the alloy composition and material processing (e.g. targeted setting of an optimized structure) or the process engineering during cold trimming of the blank (e.g. by modifying the cutting gap, speed, multiple trimming). etc.) known.

These measures are either expensive and time-consuming (e.g. multi-stage cutting operations, maintenance of 3-D cutting tools, etc.), or they do not yet deliver optimal results.

Furthermore, it is from the disclosure document DE 10 2009 049155 A1 known to heat at least the area of the cut edge to a defined temperature and to carry out the cutting at this temperature in order to improve the formability of the cut edges and thus the work hardening in the area of Reduce or avoid cutting edges. Disadvantages here are the high technical and economic outlay required for heating the sheet on the one hand and on the other hand the forced coupling of heating the blank with immediately subsequent cutting, which makes production less flexible. From the DE 10 2011 121 904 A1 it is also known to cold form a shear-cut sheet metal and to heat the work-hardened areas locally by means of a laser prior to further forming processes with the aim of partial softening. A particular disadvantage here is the local softening, which represents a discontinuity with regard to the often used high-strength and ultra-high-strength material, especially in load situations and under oscillating stress. In addition, it is unclear where exactly the warming is taking place and how the local warming is supposed to take place with temperature and time. Furthermore, it is unclear how and to what extent the formability of the sheet metal that has already been cold-formed can be improved by partial softening.

the DE 10 2014 016 614 A1 , which forms the basis for the preamble of claim 1, describes a method for producing a component by forming a blank made of steel, in which a cut blank is subjected to a short temperature treatment (max 10 seconds) at a temperature of at least 600 °C. The heat treated edges are then cold worked at any time after their heating. Even if this method basically enables an increased formability of work-hardened, mechanically separated sheet metal edges compared to other previously known methods, it is still desirable for the reasons mentioned at the outset to achieve even greater formability of the shear-cut edges.

It is therefore the object of the present invention to provide an alternative method for producing a cold-formed component from a sheet metal blank that has been shear-cut at room temperature Process is characterized by increased formability and reduced susceptibility to cracking.

The invention solves this problem with the features of the claims and in particular by a method for producing a component by further shaping an already preformed contour of a circuit board, the circuit board previously cut to size from a strip or sheet at ambient temperature after further manufacturing steps carried out at ambient temperature, such as e.g. punching or cutting operations to achieve recesses or openings, in selected edge areas cold-hardened by the punching or cutting operations, to obtain a preformed contour, is subjected to a first forming at ambient temperature, which is characterized in that the edge areas intended for forming are optionally already subjected but at least the edge areas that have already undergone the first deformation are heated to a temperature of at least 600 °C for a maximum period of 10 seconds and the edge areas are at any time after the heat treatment ndlung be subjected to a second forming or further forming at ambient temperature, each with preceding heat treatments.

Both the room temperature, for example 20° C., and the temperature of the forming tool are considered as the ambient temperature. The temperature of the forming tool can be well above room temperature.

Tests have shown that the undesirable but unavoidable strain hardening of mechanically cut edges, which becomes even more pronounced during the subsequent forming of the edge areas, can be significantly reduced or even eliminated by heat treatment of only the stressed edge areas at at least 600 °C. A very short temperature treatment of a maximum of 10 seconds, in particular from 0.02 to 10 seconds or even from 0.1 to 2 seconds, is sufficient for this.

It has now been found that a formability of the material that has been exhausted or at least restricted by shearing and forming of the edge regions is completely, largely or at least partially regenerated by the temperature treatment mentioned. As a result, pre-formed contours can be re-formed or re-formed at room temperature after the short heat treatment at at least 600 °C without there being an increased risk of cracking in the edge areas. Because the temperature treatment of the edge areas of the preformed contour not only eliminates the unwanted strain hardening, but also structural damage in the material and disadvantageous contour changes such as microcracks are removed, so that the material, which was previously almost exhausted in its formability, can be reshaped again without hesitation after the temperature treatment . The sequence according to the invention of a first forming, the temperature treatment and the second forming therefore enables a significantly greater forming potential of materials than conventional methods can offer.

According to a possible embodiment of the method according to the invention, the desired component can now already be obtained as a result of the second forming.

According to another embodiment of the method, any number, in particular two, three or four, further forming steps of the edge regions can alternatively be carried out at room temperature after the second forming, with each of the further forming steps also including a further temperature treatment of the edge regions at at least 600 °C for the Duration of a maximum of 10 seconds, in particular for 0.02 to 10 or 0.1 to 2 seconds, precedes. In this way, a component can be produced in a multi-step process in which the undesirable material properties resulting from strain hardening, in particular the increased susceptibility to cracking, are initially set in the material in each forming step, but these are eliminated again or at least significantly reduced by the subsequent heat treatment will.

According to this embodiment of the method according to the invention, any number of alternating forming and Connect heat treatment steps, as a result of which the desired component is finally obtained.

The individual forming and heat treatment steps of the method according to the invention can take place at any time, i.e. at different times.

The method according to the invention can be applied in particular to any shear-cut material edges, in particular to punched holes and to edges with any contour. As a result of the increased formability according to the invention, it is also possible to produce complex geometries which, for example, require several forming steps. This means that even complex components can be manufactured in one piece, so that additional joining operations can be dispensed with.

In the method according to the invention, the heat treatment preferably takes place over the entire thickness of the blank and in the direction of the plane of the blank in an area which corresponds at most to its thickness. The duration of the heat exposure depends on the type of heat treatment process.

The heating itself can take place in any way, for example conductively, inductively via radiation heating or by means of laser processing. Conductive heating, such as that often used in automotive manufacturing, for example for spot welding, is excellently suited to heat treatment.

For example, a spot welding machine with rather short exposure times is advantageous for treating punched holes in the blank, whereas the inductive process, radiation heating or laser processing with longer exposure times can be used for longer edge sections to be treated.

Thus, the heat input is only very concentrated in the shear-influenced n cutting edge areas and is therefore associated with a comparatively low energy consumption, especially with regard to methods in which the entire circuit board is heated or stress-relief annealing, which is orders of magnitude more time-consuming, is used.

The process window for the temperature to be reached in the cutting edge area is also very large and covers a temperature range from over 600 °C up to the solidus temperature of approx. 1500 °C.

The tests have also shown that the elimination of work hardening is crucial for a significant improvement in hole expansion capacity. In addition, the heat treatment closes imperfections, such as pores, which has a positive effect on the topography of the cut edges.

This is independent of whether the heat treatment takes place below or above the transformation temperature Ac1.

If the heat treatment is carried out above Ac1, in the case of transformable steels, a transformation into so-called metastable phases occurs after the treatment in the course of rapid cooling due to the surrounding cold material. The resulting structure has at least the same or increased hardness compared to the non-heat-treated area. For example, the Vickers hardness increases by up to 1000 HV.

Surprisingly, a structural transformation with an increase in hardness that usually accompanies it has no negative impact on the hole expansion capacity, regardless of whether a structure that is harder and therefore less tough than the original structure is set, so that treatment temperatures of the cut edges up to the solidus limit are also possible.

In any case, it remains crucial that the strain hardening introduced by cutting is largely eliminated.

In order to protect the heated cutting edge areas from oxidation, an advantageous development of the invention provides for these areas to be flushed with inert gases, for example argon or nitrogen. The inert gas flushing takes place during the duration of the heat treatment, but can also, if it appears necessary, also take place shortly before the start and/or in a limited period of time after the heat treatment has been carried out.

The forming steps of the method according to the invention can advantageously be carried out with the forming tools already available in production, e.g. cylindrical or conical punches.

The temporal decoupling of the individual forming and temperature treatment steps of the method according to the invention enables a particularly high degree of flexibility in the production process when the method is used industrially. However, if it appears advantageous from a production point of view, the cut edges can also be heated immediately after the first forming step or immediately after an optional further forming step. For this purpose, a heat treatment device can be connected directly downstream of a forming device for cold forming of the blank.

The blank itself can, for example, be flexibly rolled with different thicknesses or be joined from cold or hot strip of the same or different thickness and/or quality. The invention can be used for hot- or cold-rolled steel strips made from soft to high-strength steels, which can be provided with a corrosion-inhibiting layer as a metallic and/or organic coating. The metallic coating can contain or consist of zinc, magnesium, aluminum and/or silicon, for example.

The suitability of coated steel strips is explained by the possibility of limiting the treatment of the edge area to a distance from the edge that corresponds to a fraction of the blank thickness, since the majority of the harmful strain hardening during shear cutting occurs in this area. For sheet metal thicknesses of a few millimeters, the area up to a distance of a few tens of micrometers from the edge can be sufficient, so that, for example, the effective corrosion protection of a metallic corrosion-inhibiting layer is not or only slightly affected.

All single-phase but also multi-phase steel grades are used as high-strength steels. This includes micro-alloyed, high-strength steel grades as well as bainitic, ferritic or martensitic grades as well as dual-phase, complex-phase and TRIP steels. For example, steels with the following alloy composition in % by weight are used: C 0.01 - 0.2% si 0.2 - 4.0% Mn 0.5-4.0% Al 0.02 - 0.1 Ti 0.0-0.2 V 0.0-0.3 Nb 0.0-0.1 with optional addition of Cr, Ni, Mo, B, remainder iron, including impurities caused by smelting.

Compared to the known measures for reducing the sensitivity to edge cracks, the method according to the invention has the advantage that the heat treatment only changes the microstructure of the edge regions affected by shearing and the strength is usually not reduced but increased. The insensitivity to edge cracks in terms of greater hole expansion capacity can thus be improved by a factor of 3 or even more than 4.

In the industrial application of the method according to the invention, due to the significantly increased formability of the critical shear-affected edge areas of blanks, on the one hand the waste of formed components can be reduced and on the other hand previously necessary joining operations can be saved, for example by collar operations that can now be carried out in the formation of bearing points, for example.

The method according to the invention also allows more complex component geometries due to the improved deformability of the cut edge regions and thus greater design freedom when using the same materials. In addition, the fatigue strength of the cold-formed component is as expected due to the resulting structure, which may be harder but more homogeneous than the initial state, is not reduced, but rather increased, especially in the case of pronounced two-phase structures such as dual-phase structures.

Due to the short temperature treatment time of a maximum of 10 seconds, the method according to the invention can be integrated as an intermediate production step in series production, which specifies a cycle time in the range from 0.1 to 10 seconds. In particular, the production of sheet metal components in the automotive sector in several successive steps thus represents a predestined area of application for the method according to the invention.

Optionally, the strip or sheet from which the blank used to produce the component is cut can already be preformed in a pre-treatment step and then the blank can be cut from the preformed strip or sheet if this makes sense in terms of production technology. Alternatively, the preforming can also only take place on the blank that has already been cut to size.

According to a preferred embodiment, the cutting is done by shear cutting, the term shear cutting encompassing both open and closed cuts, i.e. both cutting and punching operations.

Further features, advantages and details of the invention result from the following description of FIG. 1, which shows a schematic representation of the individual steps of the method according to the invention.

The image on the left of FIG. 1 shows the optional preforming of a circuit board that has already been cut to size by shearing. The second image from the left in Figure 1 shows the punching of a hole in the circuit board (step 1). The cut edges of the hole are then optionally subjected to heating according to the invention (step 1a). The method according to the invention also includes the subsequent shaping of the blank in its edge regions into a preformed contour, for example into an incompletely formed collar (step 2).

Since the preformed contour obtained in this way has a high level of strain hardening in its shear-impaired edge areas, which would possibly lead to defects in the material if further forming were carried out, the edge areas are then subjected to the temperature treatment according to the invention of at least 600 °C for a period of subjected to a maximum of 10 seconds (step 3).

As a result of the temperature treatment, the component also regains its formability to a considerable extent in the stressed edge areas, so that a new, further forming can take place in the next step (step 4).

In embodiments in which the desired component is not yet obtained by the second forming, the material stresses generated by the second forming can be at least partially eliminated again by a subsequent temperature treatment of at least 600 °C for a maximum period of 10 seconds, after which a third Forming step can take place.

If the desired result is not yet achieved as a result of the third forming step, the heat treatment steps of at least 600 °C for a maximum of 10 seconds, followed by a subsequent forming step at room temperature, can be repeated as often as desired.

Claims (14)

1. Method for producing a component by further forming of a preformed contour of a blank, the blank, which is cut in advance from a strip or a metal sheet at room temperature, being subjected, after further manufacturing steps implemented at ambient temperature, such as e.g. stamping or cutting operations in order to obtain recesses or through-openings, in selected edge regions work-hardened by the stamping or cutting operations, to a first forming at ambient temperature in order to obtain a preformed contour,
   characterised in that
   optionally, in fact, the edge regions provided for the forming but at least the edge regions already subjected to the first forming are heated for a duration of at most 10 seconds to a temperature of at least 600°C, and the edge regions are subjected, at any time after this heat treatment, to a second forming or further formings at ambient temperature with respectively preceding heat treatments.
2. Method according to claim 1,
   characterised in that
   the strip or metal sheet, from which the blank used for the production of the component is cut, is preformed in a pretreatment step before the first forming.
3. Method according to claim 1 and 2,
   characterised in that
   the component is obtained by the second forming.
4. Method according to one of the preceding claims,
   characterised in that,
   after the second forming, any number of further forming steps of the edge regions are implemented at ambient temperature, each of the further forming steps being preceded by a further temperature treatment of the edge regions at at least 600°C for the duration of at most 10 seconds.
5. Method according to at least one of the preceding claims, characterised in that the edge regions subjected to the first forming are heated to a temperature of at least 600°C for a duration of 0.02 to 10 seconds.
6. Method according to at least one of the preceding claims, characterised in that the edge regions subjected to the first forming are heated to a temperature of at least 600°C for a duration of 0.1 to 2 seconds.
7. Method according to at least one of the preceding claims, characterised in that the edge regions subjected to the first forming are heated to a temperature of at least 600°C up to solidus temperature.
8. Method according to at least one of the preceding claims, characterised in that the edge regions subjected to the first forming are heated to a temperature from the conversion temperature Ac 1 up to solidus temperature.
9. Method according to at least one of the preceding claims, characterised in that the heating to a temperature of at least 600°C is effected inductively, conductively, by means of radiation heating or by means of laser radiation.
10. Method according to at least one of the preceding claims, characterised in that the blank has an organic and/or metallic coating.
11. Method according to claim 10,
    characterised in that the metallic coating comprises Zn, Mg, Al and/or Si.
12. Method according to at least one of the preceding claims, characterised in that
    the heat treatment of the blank, starting from the edge, is effected in a region which corresponds to the maximum of the thickness of the blank.
13. Method according to at least one of the preceding claims, characterised in that
    the region around the position of the heat treatment is scoured by means of an inert gas for protection from oxidation, during and optionally before and/or after the heat treatment.
14. Method according to at least one of the preceding claims, characterised in that
    a steel with the following alloy composition in % by weight is used: C 0.01 - 0.2% Si 0.2 - 4.0% Mn 0.5 - 4.0% Al 0.02 - 0.1% Ti 0.0 - 0.2% V 0.0 - 0.3% Nb 0.0 - 0.1% with optional addition of Cr, Ni, Mo, B, the rest iron, including smelting-induced impurities.

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