METHOD FOR PRODUCING A COMPONENT BY SUBJECTING A SHEET BAR OF STEEL TO A FORMING PROCESS

Abstract

A method for producing a component by forming a plate from steel at room temperature having a high formability and reduced crack sensitivity of edges that have been mechanically cut or punched on the plate, includes: cutting the plate from a strip or metal sheet at room temperature; heating edge regions of the plate that underwent strain hardening as a result of the cutting step to a temperature of at least 600° C. for a time period of at most 10 seconds; and forming the plate in one or more steps into a component at room temperature, wherein in the forming step the edge regions heated in the heating step are subjected to cold forming.

Description

The invention relates to a method for producing a component by forming a plate made of steel, according to the preamble of patent claim 1, which enables a high formability of strain hardened, mechanically separated plate edges.

In the following the term component means a component made from a metal plate by forming with a forming tool at room temperature. Metal materials that can be used include all formable metal materials, in particular however steel. The metal plates can be uncoated or can be provided with a metallic and/or organic anti-corrosion coating.

Such components are mainly used in vehicle body construction but also in the home appliance industry, machine construction or building industry.

The hotly contested automobile market forces manufacturers to constantly seek solutions for lowering fleet consumption while maintaining a highest possible comfort and passenger protection. Hereby on one hand saving weight of all vehicle components plays an important role but also ensuring optimal behavior of the component during high static and dynamic stress during operation and in the event of a crash.

Raw material suppliers are trying to address the demands placed on materials by providing high-strength and ultra-high strength steels, which allows reducing wall thicknesses while at the same time offering improved component properties during manufacture and operation.

These steels therefore have to meet relatively high demands regarding strength, ductility, tenacity, energy absorption and corrosion resistance and also processability, for example during cold forming and welding.

Among the aforementioned aspects the manufacture of components made of higher-strength steels and high-strength steels with yield strengths above 600 MPa gains increasing importance.

For producing a component a metal plate is first cut to size from hot or cold strip at room temperature. Cutting methods used for this purpose mostly include mechanical separation processes such as shearing or punching, less frequently also thermal separation processes such as laser cutting. Thermal separation methods are significantly more expensive compared to mechanical separation methods and are therefore only used in exceptional cases.

After being cut to size the cut plate is placed into a forming tool and the finished component, such as for example a vehicle chassis, is produced in a single- or multi-step forming process.

Prior to the forming various optional further manufacturing steps, such as for example punching and cutting operations, are performed on the plates and during forming combined flanging operations are performed at punched sections.

During the forming the cut edges are subjected to particularly high stress, in particular when being bent up or upwards, for example during flanging operations in punched plates.

At the cut edges prior damage may exist. This may on one hand be due to strain hardening of the material caused by the mechanical separation, which represents a total deformation until material separation. On the other hand a notch effect may occur which is caused by the topography of the cut surface.

Especially in the case of high-strength and ultrahigh-strength sheet metal materials there is therefore an increased likelihood of crack formation on the border regions of these cut edges during the subsequent forming.

The mentioned prior damages at the plate edges may lead to premature failure in subsequent forming operations or during operation of the component. The testing of the forming characteristics of cut plate edges with regard to their sensitivity for edge crack formation is performed with a hole expansion test according to ISO 16630.

During the hole expansion test a circular hole is introduced into the metal by shear cutting, which hole is then widened by a conical die. The measuring variable is the change of the hole diameter with respect to the starting diameter, at which hole diameter the first crack occurs at the border of the hole.

Known approaches to minimize the above-described edge crack sensitivity during the cold forming of shear-cut or punched plate edges include changing the alloy composition and processing the material (for example targeted adjustment of baintitic microstructures) or changing the process technology during cold cutting of the plate (for example via modification of cutting gap, speed, multiple cutting etc.)

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

It is also known from the laid open document DE 10 2009 049 155 A1 to at least heat the region of the cut edge to a defined temperature and to perform the 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. A disadvantage is here on one hand the high technical and economic costs for heating the metal and on the other hand the forced coupling of heating the plate and immediately following cutting, which makes the production less flexible.

From DE 10 2011 212 904 it is known to cold form a shear cut plate and to locally heat the strain hardened region by means of a laser with the goal of partial softening. A disadvantage hereby is in particular the local softening, which is a drawback with regard to the frequently used high-strength and ultra-high strength material, in particular in the case of load conditions and vibratory stress. In addition it is not clear where exactly the heating occurs nor what the concrete temperature and time course of the local heating should be. It is also not clear how and to what degree the partial softening can improve the forming capability of the already cold formed plate.

It is an object of the present invention to disclose a method for producing a cold formed component from a metal plate that has been shear cut at room temperature with potentially further manufacturing steps performed at room temperature such as for example hole punching or cutting operations, which method reduces or eliminates the effect of prior damage of the cutting regions and thus reduces or even eliminates the edge crack sensitivity in the subsequent cold forming of the metal plate. The method is simple and cost-effective and achieves comparable and/or improved properties on one hand regarding production, in particular with regard to the formability of the cut edges, and on the other hand in the component in particular with regard to the static strength.

According to the teaching of the invention this objet is solved by a method for producing a component by forming a plate made of steel at room temperature, which has a high formability and reduced crack sensitivity of edges that are mechanically cut or punched on the plate, in which method the plate is cut to size beforehand from a strip or plate at room temperature, wherein depending on the situation at hand further manufacturing steps, such as for example punching or cutting operations for generating recesses or perforation on the sheet metal or the plate are performed at room temperature and subsequently the thusly prepared plate is formed in one or multiple steps into a component at room temperature, which method is characterized in that independent of the forming into a component and at an arbitrary time point after the cutting to size and possible further punching or cutting operations the edge regions of the metal plate that have been strain hardened by the cutting or punching operations and which are subjected to 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 heat treatment is less than 10 seconds.

Tests have shown that for improving the hole expansion capability it is not required to perform the cutting process itself at an increased temperature of the cut edge regions, but that it is sufficient to only heat the strain hardened, shear-impacted cut edge regions for a surprisingly short period of time within the range of less than 10 seconds, usually between 0.1 and 2 seconds to a temperature of at least 600° C. According to the invention this can be performed independent of the cutting or punching process and the following production steps, at an arbitrary time point prior to the forming into a component.

The heat influence hereby acts over the entire sheet thickness and in plane direction of the plate in a region, which corresponds at most to the sheet thickness. The duration of the heat influence depends on the type of the heat treatment method.

The heating itself can be performed in any manner, for example conductively inductively via radiation heating or by means of laser processing. Very well suited for the heat treatment is the conductive heating for example as used in automobile manufacturing, for example in spot welding. Advantageously for example a spot welding machine with a relatively short impact time is suited for holes punched in the plate, whereas for treating longer edge sections the inductive method, radiation heating or laser processing with longer impact times are useful.

For protection of the heated cut edge regions against oxidation an advantageous embodiment of the invention provides to rinse these regions with inert gases, for example argon. The rinsing with inert gas is hereby performed during the duration of the heat treatment, however if required it can also be additionally already be performed prior to the start of the heat treatment and/or within a limited period of time after the heat treatment.

Thus the heat treatment is carried out in a very concentrated manner in the shear-impacted cut edge regions and is therefore associated with a relatively small energy investment, in particular compared to methods in which the entire metal plate is heated or in which a stress relief annealing is used which is more time-intensive by orders of magnitude.

In addition the process window for the temperature to be reached in the cut edge region is very wide and includes a temperature range of above 600° C. up to the solidus temperature of about 1500° C.

The tests have also shown that only the elimination of the strain hardening is deciding for a significant improvement of the hole expansion capability and non-curable flaws, such as for example pores, are of lesser importance.

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

When the heat treatment is performed above Ac1 a transformation into so-called metastable phases occurs in transformation-competent steels after the treatment during a fast cooling due to the surrounding cold material. The microstructure that forms thereupon has an increased strength compared to the starting state.

Surprisingly a microstructure transformation with a hardness and a strength increase usually associated therewith usually has no adverse effect on the hole expansion capability, independent of whether a less hard or less tenacious microstructure forms, so that also treatment temperatures of the cut edges up to the solidus temperature limit are possible. The important factor remains in each case that the strain hardening introduced by the cutting is eliminated to the most degree.

In order to achieve the goals according to the invention it is not sufficient according to the present tests to heating below 600° C. for a duration of several seconds because a significant reduction of the dislocations introduced by the mechanical separation process has to occur.

Compared to the known methods for lowering the edge crack sensitivity the method according to the invention has the advantage that as a result of the heat treatment only the shear-impacted edge region undergoes microstructural change and the strength is hereby not reduced but increased. The resistance against edge cracks as manifested by a greater hole expansion capability can thus be improved by a factor of 2 or even by more than 3.

In the industrial application of the method according to the invention the scrap of formed components can be lowered due to the significantly increased formability of the critically shear-impacted plate edge regions and on the other hand currently required joining operations can be dispensed with, for example as a result of the fact that flanging operations can now for example be performed during the formation of bearing sites.

The method according to the invention thus enables as a result of the improved forming capability of the cut edge regions more complex component geometries and thus greater constructive freedom using the same material. In addition as expected in pronounced dual phase microstructures such as for example dual phase microstructures the fatigue strength of the cold formed component is not decreased but increased, because even though the generated microstructure may be harder compared to the starting state it is more homogenous.

The heat treatment of the cut edge regions that are to be cold formed can be completely performed at an arbitrary time point after the cutting or punching process and prior to the forming of the plate, or it can be performed as an intermediate step in multistep forming operations in which the plate is formed into a component, so that the process steps cutting or punching of the plate, heat treatment of the cut edges and forming of the plate into a component are completely decoupled from each other. The manufacture is thus significantly more flexible than is possible according to the state of the art with integration of an edge modification by heat treatment.

Due to the duration of the treatment, which is short compared to known measures, the method can be integrated as intermediate manufacturing step in a serial production, which has a cycle time in the range from 0.1 to 10 seconds. A particularly predestined field of application is thus the manufacture of sheet metal components in the automobile field in multiple subsequent steps.

In addition the plate prepared in this way can advantageously be formed with the forming tools that are already present in the production because no additional heating devices such as for example furnaces for heating the plate itself are required. This also contributes to a cost-effective manufacture and as a result of the decoupling of the manufacturing steps enables a high flexibility in the production process.

According to an advantageous embodiment of the invention, depending on the provided production sequence, the heating of the cut edges can, however, if determined advantageous also be performed directly after the mechanical cutting or punching process or directly prior to the forming into a component in a working step that is combined with the respective manufacturing step. For example the cutting and punching devices can be provided with a downstream heat treatment device or the latter can be arranged directly upstream of the forming device for cold forming of the plate.

The plate itself can for example be flexibly rolled with different thicknesses or can be joined from cold or hot strip of the same or different thickness and/or grade. The invention can be used for warm or cold rolled steel strips made of soft to high strength steels for example with yield strengths of 140 MPa to 1200 MPa which can be provided with a corrosion-resistant layer as metallic and/or organic coating. The metallic coating can for example be made of zinc or an alloy made of zinc or of magnesium or of aluminum and/or silicone.

The suitability of coated steel strips can be explained by the possibility to limit the treatment of the edge region to a distance to the edge, which corresponds to a fraction of the sheet thickness because in this region the predominant portion of the deleterious strain hardening during shear cutting is present Thus in the case of sheet thicknesses of several millimeters thickness of the regions up to a distance to the edge of several ten micrometers can be already sufficient so that for example the effective corrosion protection of a metallic corrosion resistant layer is not or only insignificantly affected.

As higher-strength steels all single phase steels but also multiphase steels can be used. This includes micro-alloyed, higher-strength steels as well as bainitic or martensitic steels and also dual-phase steels, complex phase steel and TRIP steels.

Further features, advantages and details of the invention will become apparent from the following description of the shown Figures. It is shown in:

FIG. 1 a schematic representation of the hole expansion test according to ISO 16630 on cut edge that have been heat treated according to the invention

FIG. 2 a test installation for conductive heat treatment of shear-impacted cut edges

FIG. 3 results of hole expansion tests according to ISO 16630 on uncoated samples HDT780C after conducive heat treatment of the shear-impacted cut edge

FIG. 4 results of hole expansion tests according to ISO 16630 on hot dip coated samples HCT780CD and uncoated samples HDT780C after heat treatment of the shear-impacted cut edges by means of laser

FIG. 5 the microstructure and hardness course on cut edges that have been heat treated according to the invention.

FIG. 1 schematically shows a hole expansion test according to ISO 16630 on cut edges that have been heat treated according to the invention.

According to the invention the heat treatment is only performed on the shear-impacted cut edges as intermediate step after cutting the plates to size and prior to the forming of regions proximate to the cut edge.

The test installation for conductive heat treatment of shear-impacted cut edges is shown in FIG. 2.

As heating device in the tests besides a high power laser a conventional spot welding machine for joint welding of steel sheets was used as it is used in the production of vehicle parts in the automobile industry. The present case however does not involve welding of sheets that lie on top one but according to FIG. 1 a sheet with a hole punched therein (step 1) is heat treated in the region of the shear-impacted plate edges (step 2). Thereafter in step 3 the actual hole expansion is performed by means of a die, which is then determined at the tested probe.

As shown in FIG. 2 the opposing spot welding electrodes have a diameter, which is greater than the punched-out hole so that the shear-impacted hole edges can be heat treated. In addition at the ends that contact the hole borders the electrodes have a semicircular shape so that on one hand the plate can be easily centered and on the other hand the heat can be introduced in a concentrated manner only in the shear-impacted region.

In order to essentially only impinge the shear-impacted regions with current the shape of the contacting electrode tip should be adjusted to the respective geometric configuration of the edge regions.

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

Depending on the method, a treatment duration, i.e., the duration of the current flow when heating is performed inductively and the duration of the power uptake by the laser or the exposure time to other heat sources is within a range of 20 ms up to at most 10 s, usually however advantageously between 100 ms and 2000 ms. Important in any case is that a temperature of at least 600° C. is reached at the site of the heat treatment.

The important method parameters are the treatment duration and in the case of the inductive heating the current, which was varied between 4 and 10 kA. In the case of the heat treatment by means of laser, a laser power of 5 kW was first adjusted which was distributed over a circular area of about 12 mm so that approximately a ring shape with 1 mm border 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 shear-impacted cut edges are shown in FIG. 3 and corresponding results obtained with hot dip galvanized samples HCT780CD and coated samples HDT780C after heat treatment of the shear-impacted cut edges are shown in FIG. 4.

As shown in FIGS. 3 and 4 after the heat treatment, in most cases an increase of the hole expansion compared to the untreated reference sample by a factor of 2 to 3 and above could be achieved. Variances in the results are attributable in particular to non-optimized geometric conditions resulting in non-uniform heat treatment by the laser.

FIG. 5 shows in the upper portion of the image on the left hand side a schematic top view onto a hole punched into a metal plate, which was treated according to the invention in the region of the hole edge. The microstructures that form in the heat influenced region are schematically shown in the upper portion of the image on the right hand side.

This exemplarily illustrates the effect of the heat treatment and allows drawing conclusions regarding the prevailing temperatures. The shown results relate to an inductive treatment with 500 ms treatment duration and a current of 8 kA of a steel HDT780C with bainitic microstructure.

In the proximate border regions of about 0.5 mm the microstructure is made of 100% martensite. As a consequence heating of above Ac3 was performed which was followed by a fast cooling. With increasing distance to the edge the proportion of bainite increases up to a distance to the edge of about 2.5 mm beyond which 100% bainite is present. Above an edge distance of 2.5 trim the microstructure did no longer undergo transformation so that here treatment temperatures below Ac2 (about 700° C.) were present.

The hardness increase (FIG. 5, lower partial image) in the proximate region of the hole edge is typical for micro-alloyed bainitic hot strip and results from the subsequent precipitation of nanoparticles in the temperature range of about 500° C.-700° C.

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

• • Generating a very good formable cut edge with reduce edge crack sensitivity and a high hole expansion capability, which enables the production of complex component geometries and reduces the risk of scrap due to edge cracks during the forming.
  • Generating an optimized product with respect to lightweight and cost by producing complex component geometries Possibility of integrating the method into the multistep production of pressed components due to the very short duration of the heat treatment and the very wide temperature interval
  • Application of the method to corrosion resistance coated sheet metal due to the local and temporally very limited heating
  • In transformation-capable materials usually no softening but strengthening of the heat treated regions compared to the starting material

Claims

1.-15. (canceled)

16. A method for producing a component by forming a plate from steel at room temperature having a high formability and reduced crack sensitivity of edges that have been mechanically cut or punched on the plate, comprising:

cutting the plate from a strip or metal sheet at room temperature;

heating edge regions of the plate that underwent strain hardening as a result of the cutting step to a temperature of at least 600° C. for a time period of at most 10 seconds; and

forming the plate in one or more steps into a component at room temperature, wherein in the forming step the edge regions heated in the heating step are subjected to cold forming.

17. The method of claim 16, further comprising performing further manufacturing steps on the plate or the metal sheet, said further manufacturing steps including at least one of punching and cutting operations for achieving recesses or perforations on the sheet metal or the plate.

18. The method of claim 16, wherein the time period is 0.02 to 10 seconds.

19. The method of claim 18, wherein the time period is 0.1 to 2 seconds.

20. The method of claim 16, wherein the temperature is within a range from 600° C. to solidus temperature.

21. The method of claim 20, wherein the temperature is within a range from Ac1 temperature to a solidus temperature.

22. The method of claim 16, wherein the heating is performed inductively, conductively by means of radiation heating or by means of laser radiation.

23. The method of claim 22, wherein the heating is performed with a resistance welding device or with a laser.

24. The method of claim 16, wherein the forming is performed in one or multiple steps.

25. The method of claim 16, wherein the sheet metal plate has an organic and/or metallic coating.

26. The method of claim 25, wherein the metallic coating contains at least one of Zn, Mn, Al and Si.

27. The method of claim 16, wherein the heating is performed in a region which in a plane direction of the plate starting from an edge of the sheet metal maximally corresponds to a thickness of the plate.

28. The method of claim 16, further comprising protecting a region surrounding a site of the heat treatment from oxidation.

29. The method of claim 16, wherein the protecting step comprises rinsing the region about the heat treatment with an inert gas at least during the heat treatment for protection against oxidation.

30. The method of claim 29, further comprising rinsing the region surrounding the site of the heat treatment with inert gas prior to and/or after the heat influence.

31. A plate made of steel for forming into a component at room temperature in which the plate is mechanically cut to size at room temperature from a strip or a metal plate, and optionally further punching or cutting operations for achieving recesses or perforations are performed at room temperature, in which prior to the forming into a component, the cut or punched sheet metal edges which have undergone strain hardening are subjected to a heat treatment of at least 600° C. over a time period of 0.02 to 10 seconds or 0.1 to 2 seconds is performed.