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.

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.

Description

[0043] Further features, advantages and details of the invention will become apparent from the following description of the shown Figures. It is shown in:

[0044] 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

[0045] FIG. 2 a test installation for conductive heat treatment of shear-impacted cut edges

[0046] 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

[0047] 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

[0048] FIG. 5 the microstructure and hardness course on cut edges that have been heat treated according to the invention.

[0049] FIG. 1 schematically shows a hole expansion test according to ISO 16630 on cut edges that have been heat treated according to the invention.

[0050] 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.

[0051] The test installation for conductive heat treatment of shear-impacted cut edges is shown in FIG. 2.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] Overall the advantages of the invention can be summarized as follows: [0065] 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. [0066] 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 [0067] Application of the method to corrosion resistance coated sheet metal due to the local and temporally very limited heating [0068] In transformation-capable materials usually no softening but strengthening of the heat treated regions compared to the starting material