Patent Application
20200038933
The present invention relates to a method for producing a component by further forming a preformed contour of a blank according to the preamble of patent claim 1. The inventive method is hereby characterized, when compared to known methods for producing a component, in particular by an increased freedom of design during the forming process in particular of shear-cut edge regions of the blank.
In the following, a blank or sheet metal blank shall be understood as relating to a cut of a sheet metal, in particular a sheet steel. The sheet metal blanks can be uncoated or provided with a metallic and/or organic corrosion protection coating.
In the following, a component shall be understood to relate to a component which is produced from a sheet metal blank by being formed using a forming tool at ambient temperature. Sheet metal materials include any formable metal materials, in particular steel however.
Such components are predominantly used in the automotive construction, but also applications in the home appliance industry, in mechanical engineering or civil engineering or in the field thereof are possible.
The hotly contested automotive market forces the manufacturers to continuously look for solutions to reduce their fleet consumption while maintaining a highest possible comfort and occupant protection. Not only weight savings of all vehicle components play hereby a crucial role, but also a most beneficial behavior of the individual components at high static and dynamic stress during operation as well as in the event of a crash.
Suppliers of source material attempt to meet the required material demands by reducing the wall thicknesses of high strength and super high strength steels, while at the same time improving component behavior during production and operation.
These steels must therefore meet comparatively strict demands in terms of strength, deformability, toughness, energy absorption capability and corrosion resistance as well as their workability, for example during cold forming with respect to the fatigue behavior and during welding.
Among the afore-stated aspects, the production of components of higher and high strength steels with yield strengths above 400 MPa, advantageously above 600 or above 800 MPa to about 1800 MPa or even more, increasingly gains importance.
It is known for the production of a component to first cut to size a sheet metal blank of hot or cold strip at room temperature. Cutting processes involve oftentimes mechanical separation processes, such as e.g. shearing or punching, but less common also thermal separation processes, such as e.g. laser cutting. Thermal separation processes are significantly more cost-intensive compared to mechanical separation processes, so that their use is contemplated only in exceptional cases.
After cutting, the cut blank is placed in a forming tool and the finished component, such as e.g. a chassis carrier, is produced in single or multi-stage forming steps.
During forming operation, the cutting edges, in particular when being raised or placed up, e.g. collar operations in perforated blanks, are particularly exposed to stress.
Before forming operation, various other optional manufacturing steps, such as e.g, punching and cutting operations, are implemented on the blank.
The cutting edges may have various preliminary damages. These are caused on one hand by strain hardening of the material, as a result of the mechanical separation, which represents a total deformation up to material separation. On the other hand, a notch effect may be encountered, which is due to the topography of the cut surface.
Especially with the steels considered here, an increased crack probability in the edge regions of these cuffing edges is thus encountered during the subsequent forming process.
The afore-mentioned preliminary damages to the sheet edges can lead to premature failure during subsequent forming operations or during operation of the component.
The testing of the forming behavior of cut sheet metal edges with regard to their edge crack sensitivity is carried out with a hole expanding test according to ISO 16630. The hole expanding test involves introduction of a circular hole into the metal sheet by shear cutting, which circular hole is then widened by a conical punch. The measured variable is the change in the hole diameter relative to the initial diameter and is commensurate with the occurrence of the first crack at the edge of the hole at the cutting edge.
To minimize the afore-described edge crack sensitivity during cold forming of shear-cut or punched sheet edges, approaches are known, for example, involving changing of the alloy composition and material processing (e.g. targeted adjustment of an optimized microstructure) or relating to process engineering during cold trimming of the blank (e.g. via modifications of cutting gap, speed, multiple trimming, etc.).
These measures are either expensive and complex (e.g, multi-stage cutting operations, maintenance of 3-D cutting tools, etc.), or do not yet yield optimal results.
Furthermore, it is known from laid-open publication DE 10 2009 049 155 A1 to heat at least the region of the cutting edge to a defined temperature and to execute the cutting process at this temperature in order to improve the formability of the cut edges and to thereby reduce or avoid the strain hardening in the region of the cutting edge. The disadvantage here is the required high technical and economic complexity to heat the metal sheet on one hand, and the forced coupling of heating of the blank with immediately subsequent cutting process, thus rendering the production inflexible on the other hand.
DE 10 2011 121 904 A1 further discloses to cold-form a shear-cut sheet and, prior to further forming procedures, to locally heat the strain hardened regions by means of a laser with the objective of partial softening. The disadvantage here is in particular the local softening, which represents a discontinuity in terms of the often used high strength and super high strength material, especially in stress situations and under oscillatory stress. In addition, it is unclear where exactly heating should occur and how the local heating with temperature and time sequence should actually be implemented. Furthermore, it is unclear how and to what extent partial softening is able to improve formability of the already cold-formed metal sheet.
DE 10 2014 016 614 A1 describes a method for the production of a component by forming a blank of steel, wherein a cut blank, following optional punching and/or cutting operations in the regions of the shear-cut edges, undergoes a short temperature treatment (max, 10 seconds) of at least 600° C. The heat-treated edges are then cold formed at any time after being heated. Even though this method is basically capable to increase formability of strain hardened mechanically separated sheet edges when compared to other previously known methods, it is still desirable for the afore-stated reasons to realize a still higher formability of the shear-cut edges.
It is therefore the object of the present invention to provide an alternative method for the production of a cold-formed component from a sheet metal blank shear-cut at room temperature, which component, when compared with conventional methods, is characterized by an increased formability and reduced crack sensitivity, preferably in the edge regions of the blank, which are under intense stress by the cutting or punching operations, during the subsequent cold forming process.
The invention solves this problem with the features of the claims and in particular by a method for the production of a component by further forming an already preformed contour of a blank, wherein the blank being cut to size beforehand at ambient temperature from a strip or metal sheet, undergoes after optional further manufacturing steps carried out at ambient temperature, such as e.g. punching or cutting operations for realizing recesses or openings, in selected edge regions that have been strain hardened by the punching or cutting operations for obtaining a preformed contour a first forming operation at ambient temperature, which method is characterized in that optionally already the edge regions intended to undergo a forming operation, but at least the edge regions that already have undergone the first forming operation are heated to a temperature of at least 600° C. for a period of a maximal 10 seconds and the edge regions undergo at any time after the heat treatment a second forming operation or further forming operations at ambient temperature with respectively preceding heat treatments.
As ambient temperature, both the room temperature, for example 20° C., and the temperature of the forming tool are considered. The temperature of the forming tool may lie well above the room temperature.
Tests have shown that the undesirable but unavoidable strain hardening of mechanically cut edges, which becomes even more pronounced in the subsequent forming process of the edge regions, can be significantly reduced or even eliminated by a temperature treatment of only the stressed edge regions at at least 600° C. For this purpose, a very short temperature treatment of maximal 10 seconds, in particular of 0.02 to 10 seconds or even of 0.1 to 2 seconds is sufficient.
It has now been found that a formability of the material which is exhausted or at least restricted by shear cutting and forming of the edge regions is regenerated completely, largely, or at least proportionally by the mentioned temperature treatment. As a result, preformed contours can be formed or further formed again following the short temperature treatment at at least 600° C., without increasing risk of crack formation in the edge regions. Because of the temperature treatment of the edge regions of the preformed contour, not only the unwanted strain hardening is eliminated but also damages to the microstructure in the material as well as adverse contour changes such as microcracks are removed, so that the material whose formability is almost exhausted before can be further formed after the temperature treatment without any misgivings. The sequence according to the invention of a first forming operation, of the temperature treatment, and of the second forming operation therefore enables a significantly greater forming potential of materials than conventional methods can offer.
As a consequence of the second forming operation, a possible embodiment of the method according to the invention may already result in the realization of the desired component.
As an alternative, according to another embodiment of the method, the second forming operation may be followed by any number, in particular two, three or four, further forming steps of the edge regions at room temperature, wherein each of the further forming steps is also preceded by a further temperature treatment of the edge regions at at least 600° C. for the period of maximal 10 seconds, in particular for 0.02 to 10 or 0.1 to 2 seconds. In this way, a component can be produced in a multi-step process, in which the undesired material properties resulting from strain hardening, in particular the increased susceptibility to cracking, are encountered in the material in each forming step, however are eliminated again or at least significantly reduced by the subsequent temperature treatment.
Therefore, the second forming operation can be followed according to this embodiment of the method of the invention by any number of alternating forming and heat treatment steps, as a result of which the desired component is ultimately obtained.
The individual forming and temperature treatment steps of the method according to the invention can be implemented at any time, i.e. temporally decoupled from one another.
The method according to the invention is particularly applicable to any shear-cut material edges, in particular to punched holes and edges with any contour. As a result of the increased formability in accordance with the invention, it becomes possible to also produce complex geometries that require, for example, several forming steps. Even complex components can be produced in one piece, eliminating the need for additional joining operations.
In the method according to the invention, the heat treatment is preferably implemented over the entire thickness of the blank and in plane direction of the blank in a region which corresponds at most to the thickness thereof. The duration of the heat impact depends hereby on the type of heat treatment process.
Heating itself can be implemented in any desired manner, for example, conductively, inductively via radiation heating, or by laser processing. Especially suitable for temperature treatment is conducive heating, as used for example in the automotive industry in many cases as demonstrated by the example of spot welds.
Advantageously, the use of a spot welding machine for example with rather short treatment times for the treatment of punched holes in the blank is suitable, whereas at longer edge regions to be treated, the inductive method, radiation heating or laser processing with longer treatment times are considered.
Thus, the heat input is very concentrated into the shear-affected cutting edge regions and is therefore accompanied with a comparatively little energy consumption, in particular with regard to processes in which the entire blank is subjected to a heating or which find application in a stress relief heat treatment that is more time consuming by orders of magnitude.
The process window for the temperature to be reached in the cutting edge region is moreover very large and includes a temperature range of above 600° C. up to the solidus temperature of approx. 1500° C.
The tests have further shown that the elimination of strain hardening is crucial for a significant improvement of the hole expanding capability. In addition, the heat treatment closes discontinuities such as, e.g., pores, to thereby positively influence the topography of the cutting edges.
This is independent of whether the heat treatment is executed below or above the transformation temperature Ac1.
When the heat treatment is carried out above Ac1, the treatment is followed by a rapid cooldown as a result of the surrounding cold material, during which a transformation into so-called metastable phases is encountered in transformable steels. The resultant microstructure has at least the same or an increased hardness compared to the non-heat-treated region. For example, the Vickers hardness increases by up to 1000 HV.
A microstructural transformation which is normally accompanied with a hardness increase has surprisingly no negative impact on the hole expanding capability, regardless whether, compared to the initial microstructure, a harder and thus less tough microstructure is realized, so that treatment temperatures of the cutting edges up to the solidus limit are also possible.
In any case, it is crucial that the strain hardening introduced by the cutting operation is largely eliminated.
To protect the heated cutting edge regions against oxidation, an advantageous refinement of the invention provides for flushing these regions with inert gases, for example argon or nitrogen. The inert gas flushing takes place during the duration of the heat treatment, but may also be carried out, if necessary, in addition shortly before the start and/or in a limited period of time after execution of the heat treatment.
The forming steps of the method according to the invention can advantageously be executed with forming tools, e.g. cylindrical or conical punches, that already exist in the production.
By the temporal decoupling of the individual forming steps and temperature treatment steps of the method according to the invention, a particularly high flexibility in the production sequence is rendered possible in the industrial application. When advantageous from a production point of view, heating of the cutting edges can also take place immediately after the first forming step or immediately after an optional further forming step. For this purpose, a heat treatment apparatus may be placed directly downstream of a forming device for cold forming the blank.
The blank itself can e.g. be rolled flexibly with different thicknesses or be joined from cold or hot strip of same or different thickness and/or quality. The invention is applicable to hot or cold rolled steel strips of soft to high strength steels, which may be provided with a corrosion inhibiting layer as a metallic and/or organic coating. The metallic coating may, for example, contain or made of zinc, magnesium, aluminum and/or silicon.
The suitability of coated steel strips can be explained by the possibility of limiting the treatment of the edge region to a distance from the edge, which corresponds to a fraction of the blank thickness, since in this region the predominant proportion of the damaging strain hardening is present during shear cutting. Thus, for metal 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 find application. These include micro-alloyed, higher strength steel grades as well as bainitic, ferritic or martensitic grades as well as dual phases, complex phases and TRIP steels. For example, steels having the following alloy composition in wt-% are used:
with optional addition of Cr, Ni, Mo, B, balance iron, including impurities resulting from smelting.
The method according to the invention has the advantage over the known measures for reducing edge crack sensitivity that the heat treatment alters only the microstructure of the shear-affected edge regions and the strength is hereby normally not reduced, but increased. The insensitivity to edge cracks within the meaning of a greater hole expanding capability can be improved by a factor of 3 or even more than 4.
In the industrial application of the method according to the invention, the significantly increased formability of the critical shear-affected edge regions of blanks is able to lower rejects of formed components on one hand, and previously necessary joining operations may now be dispensed with for example by now feasible collar operations for realization of e.g. bearing points, on the other hand.
The method according to the invention also allows due to the improved formability of the cutting edge regions the realization of more complex component geometries and thus a greater design freedom when using the same materials. In addition, the fatigue strength of the cold formed component is, as expected, not reduced as a result of the realized microstructure that possibly may be harder but more homogeneous compared to the initial state, but is increased especially in pronounced two-phase microstructures such as e,g, dual phase microstructures.
In view of the short temperature treatment period of maximal 10 seconds, the method according to the invention can be integrated as an intermediate manufacturing step in a series production which specifies a clock rate 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 field of application of the method according to the invention.
The invention further relates to the use of a blank of steel for the production of a component, wherein the blank previously cut to size at room temperature from a strip or a metal sheet undergoes after optional further manufacturing steps carried out at room temperature, such as e.g. punching or cutting operations for realizing recesses or openings, in selected edge regions that became strain hardened by the punching or cutting operations for obtaining a preformed contour a first forming operation at room temperature, and wherein the edge regions that underwent the first forming operation are heated to a temperature of at least 600° C. for a period of a maximal 10 seconds, preferably of 0.02 to 10 second or of 0.1 to 2 seconds, and the edge regions undergo a second forming operation at room temperature at any time after the heat treatment.
Optionally, the strip or metal sheet, from which the blank used for the production of the component is cut to size, can be preformed in a pretreatment step and then the blank can be cut from the already preformed strip or metal sheet, when appropriate for manufacturing reasons. As an alternative, the already cut blank may be preformed.
According to a preferred embodiment, the blank is cut by shearing, wherein the term shear cutting includes both open and dosed cuts, i.e. both cutting and punching operations.
Further features, advantages and details of the invention will become apparent from the following description of
The left-hand image of
As the thus-obtained preformed contour has in the shear-affected edge regions a high strain hardening that would possibly lead during further forming operation to defects in the material, the edge regions then undergo the temperature treatment in accordance with the invention of at least 600° C. for a period of up to 10 seconds for elimination or reduction of the strain hardening (step 3).
As a consequence of the temperature treatment, the component recovers also in the stressed edge regions its formability to a considerable extent, so that in the next step, a new, further forming operation can take place (step 4).
In embodiments in which the desired component has not yet been obtained by the second forming operation, the material stress generated by the second forming operation can be eliminated, at least partially, by a subsequent temperature treatment of at least 600° C. for a period of maximal 10 seconds, whereupon a third forming step can take place. Should the desired result not be achieved as a result of the third forming step, the steps of the temperature treatment of at least 600° C. for a period of maximal 10 seconds, followed by a subsequent forming step at room temperature, may be repeated as often as desired.
The features of the invention disclosed in the preceding description, in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2017 103 729.2 | Feb 2017 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/051978 | 1/26/2018 | WO | 00 |
