EP3212348B1 - Method for producing a component by subjecting a sheet bar of steel to a forming process - Google Patents

Description

The invention relates to a method for producing a component by forming a blank made of steel, according to the preamble of claim 1, which allows a high formability work-hardened, mechanically separated sheet edges.

In the following, a component is understood to be a component produced from a sheet metal blank by forming by means of a forming tool at room temperature. As sheet materials are all deformable metal materials into consideration, but especially steel. The sheet metal blanks may be uncoated or provided with a metallic and / or organic corrosion protection coating.

Such components are mainly used in body construction, but also in the home appliance industry, in mechanical engineering or construction offer opportunities.

The highly competitive automotive market is forcing manufacturers to constantly seek solutions to reduce their fleet consumption while maintaining maximum comfort and occupant safety. On the one hand, the weight saving of all vehicle components plays a decisive role, on the other hand, but also a most favorable behavior of the individual components with high static and dynamic stress during operation as well as in the event of a crash.

The material suppliers are trying to meet the necessary material requirements by reducing the wall thicknesses by providing high-strength and ultra-high-strength steels, while at the same time improving component behavior during production and operation.

These steels must therefore meet comparatively high requirements in terms of strength, ductility, toughness, energy absorption and corrosion resistance and their processability, for example in cold forming and welding.

Among the above-mentioned aspects, the production of components made of high-strength and high-strength steels with yield strengths above 600 MPa is becoming increasingly important.

To produce a component, a sheet metal blank of hot or cold strip is first cut to size at room temperature. The cutting process is usually mechanical separation processes, such. shearing or punching, but more rarely also thermal separation methods, such as e.g. laser cutting, for use. Thermal separation processes are significantly more costly compared to mechanical separation processes, so that they are used only in exceptional cases.

After cutting the cut board is placed in a forming tool and in single or multi-stage forming steps the finished component, such as. a chassis carrier generated.

Before forming, various further manufacturing steps, such as e.g. Punching and cutting operations on the board and made during the forming combined flaring operations on perforated sections.

In forming, the cut edges, especially when raised or raised, e.g. in collar operations in perforated boards, particularly stressed.

At the cutting edges various preliminary damages can be present. On the one hand due to a work hardening of the material, caused by the mechanical separation, which represents a total deformation to material separation. On the other hand, a notch effect can occur which results from the topography of the cut surface.

Especially in the case of high-strength and ultra-high-strength sheet-metal materials, an increased cracking probability therefore occurs in the edge regions of these cut edges during the subsequent shaping.

The aforementioned damage to the sheet edges can lead to premature failure in subsequent forming operations or during operation of the component. The testing of the forming behavior of cut sheet edges with regard to their edge crack sensitivity is carried out with a hole expansion test according to ISO 16630.

When Lochaufweitversuch a circular hole is introduced into the sheet by shear cutting, which is then widened by a conical punch. The measured quantity is the change in the diameter of the hole in relation to the initial diameter at which the first crack occurs through the sheet at the edge of the hole.

In order to minimize the above-described edge crack sensitivity in the cold working of sheared or punched sheet edges, e.g. Approaches for changing the alloy composition and material processing (e.g., targeted setting of bainitic structures) or the process technique in cold-cutting the board (e.g., via modifications of cutting gap, speed, multiple trimming, etc.) are known.

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

Furthermore, it is from the published patent application DE 10 2009 049 155 A1 It is known to heat at least the region of the cut edge to a defined temperature and to perform cutting at this temperature in order to improve the formability of the cut edges and thus to reduce or avoid strain hardening in the region of the cut edge. The disadvantage here is the necessary for heating the sheet high technical and economic effort on the one hand and on the other hand for the positive coupling of heating of the board and immediately subsequent cutting, which make the production inflexible.

From the DE 10 2011 121 904 A1 It is also known to cold form a sheared sheet and to heat the cold worked areas locally by means of a laser before further forming operations with the aim of partial softening. The disadvantage here is in particular the local softening, which represents a discontinuity in terms of the often used highly and extremely strong material, especially in stress situations and under oscillatory stress. In addition, it is unclear exactly where the warming will take place and how the local warming with temperature and time course should take place concretely. Furthermore, it is unclear how and to what extent by the partial softening the forming capacity of the already cold-formed sheet can be improved.

Object of the present invention is to provide a method for producing a cold-formed component of a sheared at room temperature sheet metal plate with occasionally various other performed at room temperature manufacturing steps, such as punching or cutting operations, which reduces the previously described damage to the cutting areas in their effect or eliminates and thus the edge crack sensitivity in the subsequent cold forming of the sheet metal blank reduced or even eliminated. The method should be simple and inexpensive to implement and achieve comparable and / or improved properties on the one hand in the production, in particular with respect to the formability of the cut edges and on the other hand in the component, in particular with respect to the static strength.

According to the teaching of the invention, this object is achieved by a method for producing a component by forming a steel circuit board at room temperature, having a high formability and reduced crack sensitivity mechanically cut or punched edges on the board, in which the board previously from a tape or Sheet is cut at room temperature, where appropriate, further manufacturing steps, such as Punching or cutting operations, to achieve recesses or openings on the sheet or the board are carried out at room temperature and then the prepared board in one or more steps to a component at room temperature is formed, which is characterized in that regardless of the deformation to a component at any time after cutting the blank and any further punching or cutting operations which, by the cutting or punching operations cold-hardened sheet edge portions, which undergo a subsequent cold working in the manufacture of the component, to a temperature of at least 600 ° C. are heated and the time of the temperature application is less than 10 seconds.

Experiments have shown that it is not necessary to perform the cutting process even at elevated temperature of the cut edge areas to improve the Lochaufweitvermögen, but it is sufficient only the work hardened, shear-influenced cut edge areas in an unexpectedly short time interval in the range of less than 10 seconds, in the Usually, however, to heat up to a temperature of at least 600 ° C between 0.1 and 2.0 seconds. According to the invention this can be done detached from the cutting or punching process and the subsequent manufacturing steps, at any time before the forming into a component.

The heat is applied over the entire sheet thickness and in the plane direction of the board in a range which corresponds at most to the sheet thickness. The duration of the heat depends on the type of heat treatment process.

The heating itself can take place arbitrarily, for example, conductively, inductively via radiation heating or by means of laser processing. Excellent for the Heat treatment is the conductive heating, as it is often used, for example in the automotive industry on the example of spot welds. Advantageously, for example, is a spot welding machine with rather short exposure times for the treatment of punched holes in the board, whereas in indefinite to be treated edge portions, the inductive method, radiation heating or laser machining with longer exposure times in question.

In order to protect the heated cut edge regions from oxidation, an advantageous development of the invention provides for rinsing these areas with inert gases, for example argon. The inert gas purging takes place during the duration of the heat treatment but can also, if necessary, be made shortly before the beginning and / or in a limited period of time after the heat treatment has been carried out.

Thus, the heat input is very concentrated in the shear-influenced cutting edge areas and is therefore associated with a relatively low energy consumption, in particular with regard to methods in which the entire board is supplied to a heating or by orders of magnitude temporally more expensive stress relief is applied.

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

The experiments have also shown that only the elimination of strain hardening is crucial for a significant improvement in hole-expanding capacity, and the non-recoverable discontinuities, e.g. Pores of subordinate importance.

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, transformation into so-called metastable phases takes place after treatment in the course of a rapid cooling due to the surrounding cold material in the case of convertible steels. The resulting microstructure will differ from the initial state in terms of increased strength.

Surprisingly, a structural transformation with as a rule associated increase in hardness and strength has no negative influence on the Lochaufweitvermögen, regardless of whether a harder and less tough compared to the starting structure structure is adjusted, so that treatment temperatures of the cut edges up to the solidus limit possible are. In any case, it is crucial that the strain hardening introduced by cutting is largely eliminated.

In order to achieve the objectives according to the invention, it is not sufficient, according to the present investigations, to carry out a heating below 600 ° C. for a period of a few seconds, since a significant reduction in the dislocations introduced by the mechanical separation process must take place.

Compared to the known measures for reducing the edge crack sensitivity, the method according to the invention has the advantage that only microstructural changes are made by the heat treatment of the shard-influenced edge regions, and the strength is not generally reduced, but rather increased. The insensitivity to edge cracks in the sense of a larger Lochaufweitvermögens, can thus be improved by a factor of 2 or even more than 3.

In the industrial application of the method according to the invention can be reduced due to the significantly increased formability of the critical shear-influenced sheet edge areas on the one hand, the Committee of formed components and on the other hand, previously required joining operations, for example, by now feasible collar operations in the formation of e.g. Saved from storage locations.

Due to the improved formability of the cut edge regions, the method according to the invention allows more complex component geometries and thus greater design freedom when using the same materials. In addition, the fatigue strength of the cold-formed component is expected not to be reduced due to the self-adjusting, but possibly harder but homogeneous compared to the initial state but homogeneous structure, but with pronounced two-phase structures such. Dual phase structures increased.

The heat treatment of the cold-cut edge regions to be cold can be completed at any time after the cutting or punching processes and before forming the board or as an intermediate step in multi-stage forming operations of the board are carried out to form a component, so that the process steps cutting or punching the board, heat treatment of the cutting edges and forming the board are completely decoupled from one another. Thus, the production is much more flexible than is possible in the prior art in integration of edge modification by heat treatment.

Due to the short duration of treatment compared with known measures, the method can be integrated as an intermediate production step in a series production which specifies a cycle in the range of 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 scope.

In addition, the forming of the board prepared in this way can advantageously be carried out with the forming tools already in production, since no additional heating devices, such as e.g. Ovens, to heat the board itself are necessary. This allows a further cost-effective production and by the decoupling of the manufacturing steps a high flexibility in the production process.

According to an advantageous development of the invention, the heating of the cut edges, however, depending on the intended production process, if this appears advantageous, even immediately after the mechanical cutting or punching processes or immediately before forming into a component, take place in a combined with the respective manufacturing process step. For example, the cutting and stamping devices may be provided with a downstream heat treatment device or this may be directly upstream of the forming device for cold forming of the board.

The board itself may for example be rolled flexibly with different thicknesses or be joined from cold or hot strip of the same or different thickness and / or quality. The invention is applicable to hot or cold rolled steel strip from soft to high strength steels, eg with yield strengths from 140 MPa to 1200 MPa, which may be provided with a corrosion inhibiting layer as a metallic and / or organic coating. The metallic coating may for example consist of zinc or an alloy of zinc or of magnesium or of aluminum and / or silicon.

Figur 1

schematische Darstellung des Lochaufweiteversuchs nach ISO 16630 an erfindungsgemäß wärmebehandelten Schnittkanten

Figur 2

Versuchsaufbau zur konduktiven Wärmebehandlung scherbeeinflusster Schnittkanten

Figur 3

Ergebnisse von Lochaufweitversuchen nach ISO 16630 an unbeschichteten Proben HDT780C nach konduktiver Wärmebehandlung der scherbeeinflussten Schnittkanten

Figur 4

Ergebnisse von Lochaufweitversuchen nach ISO 16630 an schmelztauchverzinkten Proben HCT780CD und unbeschichteten Proben HDT780C nach Wärmebehandlung der scherbeeinflussten Schnittkanten mittels Laser

Figur 5

Gefüge und Härteverlauf an erfindungsgemäß wärmebehandelten Schnittkanten

The suitability of coated steel strips can be explained by the possibility of limiting the treatment of the edge area to a distance from the edge, which corresponds to a fraction of the sheet thickness, since in this area the predominant part of the damaging work hardening during shear cutting is present. Thus, for sheet thicknesses of a few millimeters thickness, the range up to a distance to the edge of a few tens of micrometers may already be sufficient, so that, for example, the effective corrosion protection of a metallic corrosion-inhibiting layer is not or only insignificantly influenced.
As higher-strength steels, all single-phase as well as multi-phase steel grades are used. These include microalloyed, high-strength steels as well as bainitic or martensitic grades as well as dual phases, complex phases and TRIP steels. Claim 15 relates to the use of a steel circuit board for forming at room temperature, in which prior to forming the teaching defined in claim 1 is applied. Further preferred features, advantages and details of the invention will become apparent from the following description of the figures shown. Show it:

FIG. 1

Schematic representation of the Lochaufweiteversuchs ISO 16630 according to the invention heat-treated cut edges

FIG. 2

Experimental setup for the conductive heat treatment of shear-influenced cut edges

FIG. 3

Results of hole expansion tests according to ISO 16630 on uncoated samples HDT780C after conductive heat treatment of the shard-influenced cut edges

FIG. 4

Results of hole expansion tests according to ISO 16630 on hot-dip galvanized samples HCT780CD and uncoated samples HDT780C after heat treatment of the shard-influenced cut edges by laser

FIG. 5

Microstructure and hardness profile of inventively heat-treated cut edges

In FIG. 1 the hole-widening test according to ISO 16630 is shown schematically on heat-treated cut edges according to the invention.

According to the invention, the heat treatment takes place only at the shard-influenced cut edges as an intermediate step after cutting the blank and before reshaping edge-near areas.

The experimental set - up for the conductive heat treatment of shard - influenced cut edges is in FIG. 2 shown.

As a heating device was used in investigations in addition to a powerful laser, a commercial spot welding machine for joint welding of steel sheets, as it is also used in the production of vehicle parts in the automotive industry. In the present case, however, no superimposed sheets are welded together, but it is according to FIG. 1 a sheet with a hole punched out therein (step 1) in the area of the shard-influenced sheet edges heat-treated (step 2). Thereafter, in step 3, the actual hole widening takes place by means of a punch, which is subsequently determined on the tested sample.

Like in the FIG. 2 As shown, the opposing spot welding electrodes have a diameter larger than the punched hole, so that the shard-influenced hole edges can be heat-treated. In addition, the electrodes have a hemispherical shape on the ends contacting the hole edges, so that the sheet is simply centered on the one hand, and on the other hand, the heat can be concentrated only introduced into the sheer-influenced region.

In order to apply power to essentially only the shard-influenced areas, the shape of the contacting electrode tip should be adapted to the respective geometric configuration of the edge areas.

For the tests, an uncoated, higher-strength, hot-rolled bainitic grade HDT780C steel with a minimum yield strength of 680 MPa and a minimum tensile strength of 800 MPa was used. Furthermore, a hot-dip galvanized, cold-rolled complex phase steel with a minimum yield strength of 500 MPa and a minimum tensile strength of 780 MPa grade HCT780CD was used.

Depending on the method, a treatment duration, ie the duration of the current flow in the case of inductive heating and the duration of the power reduction by the laser, or the duration of action of other heat sources in a range of 20 ms to at most 10 s, in but usually advantageous between 100 ms to apply to 2000 ms. It is essential in any case that a temperature of at least 600 ° C at the location of the heat treatment is achieved.

The main process parameters are, in addition to the duration of treatment, and in the case of inductive heating, the current, which was varied between 4 and 10 kA. In the laser heat treatment, first, a laser power of 5 kW was set, which was spread on a circular area of about 12 mm, so that about a ring mold with 1 mm edge width of the cut circular hole of the sample with the diameter of 10 mm was heat-treated.

The results of hole expansion tests according to ISO 16630 on uncoated samples HDT780C after conductive heat treatment of the shard affected cut edges are the FIG. 3 and corresponding results on hot - dip galvanized samples HCT780CD and uncoated samples HDT780C after heat treatment of the shard - influenced cut edges by means of laser are the FIG. 4 refer to.

After the heat treatment could according to the Figures 3 and 4 an increase in the hole expansion compared to the untreated reference sample of predominantly factor 2 up to factor 3 and above can be achieved. Scattering in the results is due in particular to non-optimized geometrical conditions and consequently uneven heat treatment by the laser,

FIG. 5 shows in the upper part of the picture on the left, in a schematic representation in a plan view of a hole punched in a sheet, which according to the invention was heat treated in the region of the hole edge. The self-adjusting microstructures in the heat-affected area are shown schematically in the upper part of the picture on the right.

From this, the effect of the heat treatment can be exemplified and conclusions can be drawn on the prevailing temperatures. The results shown refer to inductive treatment with 500 ms treatment time and 8 kA current from a bainitic steel HDT780C steel.

In the near edge area of approx. 0.5 mm, the structure consists of 100% martensite. As a result, there was heating above Ac3, which was rapidly cooled. As the distance to the edge increases, the proportion of bainite increases up to a distance to the edge Edge of about 2.5 mm, from which 100% bainite is present. From an edge distance of 2.5 mm, the structure was no longer subject to conversion, so that here treatment temperatures were below Ac1 (about 700 ° C).

The hardness increase ( FIG. 5 , lower part of the image) in the vicinity of the hole edge is typical for microalloyed bainitic hot strip and results from the subsequent separation of nanoparticles in the temperature range of about 500 ° C - 700 ° C.

• Erzeugung einer sehr gut umformbaren Schnittkante mit reduzierter Kantenrissempfindlichkeit und hohem Lochaufweitvermögen, was die Herstellung komplexerer Bauteilgeomtrien ermöglicht und das Risiko von Ausschüssen aufgrund von Kantenrissen bei der Umformung reduziert.
• Erzeugung eines optimierten Produktes unter Leichtbau- und Kostengesichtspunkten durch Herstellung komplexer Bauteilgeometrien
• Möglichkeit der Integration des Verfahrens in die mehrstufige Fertigung von Pressbauteilen aufgrund der sehr geringen Dauer der Wärmebehandlung und des sehr weiten Temperaturintervalls
• Anwendbarkeit des Verfahrens auf korrosionsscnutzbeschichtet Bleche wegen der örtlich und zeitlich sehr begrenzten Erwärmung
• In der Regel keine Erweichung sondern bei umwandlungsfähigen Werkstoffen Verfestigung der wärmebehandelten Bereiche im Vergleich zum Grundwerkstoff

Overall, the advantages of the invention can be summarized as follows:

• Produce a very well-formed cut edge with reduced edge crack sensitivity and high hole expandability, enabling the creation of more complex component geometries and reducing the risk of broke due to edge cracking during forming.
• Production of an optimized product from a lightweight and cost perspective by producing complex component geometries
• Possibility of integration of the process in the multi-stage production of press components due to the very short duration of the heat treatment and the very wide temperature interval
• Applicability of the method on corrosion-protected coated sheets because of the local and temporal very limited heating
• As a rule, no softening, but in the case of materials capable of transformation solidification of the heat-treated areas in comparison to the base material

Claims (15)

1. A method for producing a component by forming a blank from steel at room temperature, having a high formability and reduced crack sensitivity of edges which are mechanically cut or punched on the blank, in which the blank is previously cut to size from a strip or metal sheet at room temperature, wherein, in some cases, further manufacturing steps, for example punching or cutting operations, are performed at room temperature to achieve cutouts or openings in the metal sheet or blank and the blank which is prepared in this way is then formed into a component at room temperature in one or more steps,
   characterised in that,
   independently of the forming into a component, at any time after the cutting to size of the blank and any further punching or cutting operations, the sheet metal edge regions, which are strain-hardened by the cutting or punching operations and undergo a subsequent cold forming during the production of the component, are heated to a temperature of at least 600°C and the time of the exposure to temperature is a maximum of 10 seconds.
2. The method according to Claim 1,
   characterised in that
   the time of the exposure to temperature is 0.02 to 10 seconds.
3. The method according to Claim 2,
   characterised in that
   the time of the exposure to temperature is 0.1 to 2 seconds.
4. The method according to Claims 1 to 3,
   characterised in that the heating of the strain-hardened sheet metal edge regions is carried out to a temperature of 600°C to solidus temperature.
5. The method according to Claim 4,
   characterised in that
   the heating of the strain-hardened sheet metal edge regions is carried out to a temperature of Ac1 to solidus temperature.
6. The method according to Claims 1 to 5,
   characterised in that the heating to forming temperature is carried out inductively, conductively, by means of radiation heating or by means of laser radiation.
7. The method according to Claim 6,
   characterised in that
   the heating is carried out by means of a resistance welding device or by means of a laser.
8. The method according to at least one of Claims 1 to 7,
   characterised in that the blank is formed in one or more steps.
9. The method according to at least one of Claims 1 to 8,
   characterised in that the sheet metal blank has an organic and/or metallic coating.
10. The method according to Claim 9,
    characterised in that
    the metallic coating contains Zn and/or Mg and/or Al and/or Si.
11. The method according to at least one of Claims 1 to 10,
    characterised in that
    the heat treatment is carried out in the planar direction of the blank, starting from the sheet metal edge, in a region which corresponds maximally to the sheet metal thickness.
12. The method according to one of Claims 1 to 11,
    characterised in that
    the region surrounding the point of the heat treatment is protected against oxidation.
13. The method according to one of Claims 1 to 12,
    characterised in that,
    to protect against oxidation, the region surrounding the point of the heat treatment is purged with an inert gas at least during the heat exposure.
14. The method according to Claim 13,
    characterised in that
    the region surrounding the point of the heat treatment is additionally purged with an inert gas prior to and/or following the heat exposure.
15. The use of a blank made from steel for forming into a component at room temperature, in which the blank is mechanically cut to size from a strip or metal sheet at room temperature prior to the forming procedure and, in some cases, further punching or cutting operations are performed at room temperature to achieve cutouts or openings, in which, prior to the forming into a component, a heat treatment of at least 600°C is performed over a period of 0.02 to 10 seconds or 0.1 to 2 seconds at the cut or punched sheet metal edges which have undergone strain-hardening.

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