METHOD AND APPARATUS FOR PRODUCING SHEET METAL COMPONENTS

Abstract

A method and apparatus for making sheet metal components includes heating a sheet of hardenable steel to its austenitizing temperature and hot forming the same in a hot forming tool to define a formed workpiece. The formed workpiece is held in the hot forming tool for a first holding time. The formed workpiece is then removed from the hot forming tool and immediately placed in a cooled, form holding tool and is held therein in a closed condition for a second holding time. The form holding tool may be constructed from a material that has greater heat conductivity than that of the hot forming tool.

Description

CLAIM OF PRIORITY

Applicants hereby claim the priority benefits under the provisions of 35 U.S.C. §119, basing said claim of priority on German Patent Application Serial No. 10 2009 021 395.3, filed May 14, 2009. In accordance with the provisions of 35 U.S.C. §119 and Rule 55(b), a certified copy of the above-listed German patent application will be filed before grant of a patent.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for producing sheet metal components.

Form-hardening and press-hardening of sheet metal components are established in the automobile industry. However, the associated component manufacturing costs are relatively high, so there are efforts to reduce cycle times for the production process. One reason for the current long cycle times is that a certain holding time in the forming tool is required during form-hardening in order to extract or remove the heat from the formed workpiece that is to be hardened. Because transformable steels must be used for press-hardening, they must be austenitized. That is, they must be heated to their material-specific austenitizing temperature, which is the 950° C. range. During forming, the workpieces undergo rapid cooling during which the material structure leaves the austenitic range. Due to cooling to a temperature range of between 100° C. and 200° C., the material transforms into a martensitic structure that provides the desired high-strength components. One material that is very well suited for this method is 22MnB5. The components produced in this manner are used especially in side impact protection in “A” and “B” columns, rocker panels, frame parts, bumpers and bumper supports, door reinforcements, and transverse beams and roof supports for motor vehicles.

Because hardening is desired, relatively large quantities of heat must be removed from the workpiece in the shortest possible period of time. Although the cooling curve drops sharply at first, it flattens later on during the cooling cycle. This means that as the holding period increases, the amount of heat removed continues to decrease per unit of time. This leads to relatively long holding times of 15 to 20 seconds during which the forming tool is closed and not available for producing other sheet metal components. This is particularly true for transfer presses in which workpieces are transferred between workpiece lines using transfer bars.

Theoretically, it is possible to remove the sheet metal components from the forming tool at elevated temperatures, and then let them cool in ambient air. However, the components are still warm and will warp, leading to increased rejections because of defects.

SUMMARY OF THE INVENTION

One object of the present invention is a method and apparatus for producing sheet metal components using shorter cycle times while achieving the same shape accuracy.

This object is attained using a method having the steps or measures set forth in patent claim 1 herein.

The subject-matter of patent claim 4 herein is a suitable apparatus for performing the subject method.

The subordinate claims relate to useful, non-obvious embodiments of the inventive concept or thought disclosed herein.

In the present inventive method, a workpiece made from steel sheet with a transformable structure is austenitized in a known manner. That is, it is heated to a temperature that is greater than its austenitizing temperature, and is placed in a forming tool for forming and press-hardening the workpiece. The workpiece is cooled very rapidly inside the forming tool. To this end, the forming tool, which is a press having a top die and a bottom die, is intensively cooled to attain the desired cooling speeds. The austenitic structure transforms into a martensitic structure. The material is thus formed and hardened in the same tool. The workpiece must now be held in the forming tool until a first holding time has elapsed. Then, the formed workpiece, which has already been press-hardened, but is still warm, is removed from the forming tool, and immediately placed in a cooled, form-holding tool. The formed sheet metal component is held in the form-holding tool until a second holding time has elapsed. It is preferable that this second holding time is about the same as the first holding time. The workpiece cools inside the form-holding tool to a final temperature, that permits it to be removed without the finished workpiece warping in an undesired manner. The material of the form-holding tool preferably has greater heat conductivity than the material of the forming tool.

This procedure has the advantage that the forming tool, which is in high demand, does not need to be closed for 15 to 20 seconds. Rather, after a relatively brief holding period it can be re-opened, so that it is available again for forming another sheet metal component after a brief period of time. The cycle time for producing sheet metal components is thereby cut, and in the best case can be halved, by using two tools. These two tools are specifically a first tool that is configured for forming, and a second tool that is configured for cooling and, where necessary, sizing as well. This feature of the present invention is clarified by the following example.

In a transfer press, the workpieces are moved between two forming tools using two cooperating transfer bars that work in pairs. The transfer occurs between the strokes of the transfer press. Assume for instance that the transfer time between two tool stages is 5 seconds, and that the holding times in the first and second forming tools are the same and are for instance 10 seconds. Given this assumption, the transfer time is a total of 15 seconds. These 15 seconds are for transferring the steel sheet to the first forming tool, from there to the second forming tool, and finally for removing it from the second forming tool. There are also 2×10 seconds of holding time. This means that the first workpiece is press-hardened after 15+20=35 seconds.

One component is finished every 15 seconds based on the cycle for the transfer bars.

Compare this to a single-stage press tool that is also held closed for 20 seconds, and in which there will also be a 5 second transfer time for both loading and removal. In this case, the first workpiece is press-hardened after 20+2×5=30 seconds. Each subsequent component will be produced after an additional 25 seconds.

In contrast to the present inventive method, when using only one forming tool, the production process takes longer. In this example, it is 10 seconds longer due to the longer holding time. Given these assumed parameters, it is possible to increase production speed by 40% relative to a 25 second-cycle.

Another significant aspect of the present invention is that the tools can be better matched to their primary purpose. In other words, the forming tool can be produced from a very strong and durable material that is specifically matched to the requirements of hot-forming and form-hardening.

In the first tool, it is important that the component be cooled as uniformly as possible. Therefore a plurality of cooling bores and cooling channels are provided in the first tool for this purpose.

In the second tool, it is no longer primarily an issue of cooling the workpiece as uniformly as possible. Rather, the issue is bringing the workpiece to the final temperature as rapidly as possible so that it does not warp. Therefore, in the second tool, it is possible to use a different material and a different cooling geometry than the first tool. The material of the second tool does not necessarily have to have the high strength and hardness like as the material for the first or forming tool. Instead, due to the lower mechanical stress, the second tool can be made from a material that has greater heat conductivity. A copper material may even be used, which has significantly higher heat conductivity than that of the first or forming tool.

During the development of the present invention, efforts were made to reduce the cycle times during press-hardening as much as possible. However, for quality reasons, it must be understood that the transformation to a martensitic structure must be as complete as possible, and that certain production tolerances are not exceeded. Different holding times for the holding tools are needed, depending on the shape and material of the steel sheet to be formed. The invention seeks to have the forming tools held closed for press-hardening for no more than 20 seconds, preferably no more than 15 seconds, and especially no more than 10 seconds. The holding times in the forming tool and the holding times in the downstream form-holding tool are preferably synchronized. For example, if in the past a holding time of 20 seconds was required during single-step production using only a single forming tool, the goal of the present invention is for this total holding time to be halved. In addition, there is also the time for opening and closing the tools, and handling the workpiece to transfer it from the forming tool to the form-holding tool. However, the total time still yields a shorter cycle time with respect to the forming tool, so it is possible to accelerate the overall production process.

In the present invention, the press-hardened workpiece is cooled primarily in the form-holding tool. The form-holding tool prevents the workpiece from warping. In the present invention, it is also possible to size the press-hardened workpiece in the form-holding tool in order to better maintain the desired production tolerances.

The present inventive apparatus for performing the method is distinguished in that the heat conductivity of the material for the form-holding tool is greater than that of the material for the forming tool. This relates primarily to the areas of the tools that come into contact with the steel sheet. Naturally, it is also possible to produce the core of a tool from a different workpiece than the areas that come into contact with the workpiece that is to be shaped or held or sized. It is possible to select material combinations, especially in the area of the form-holding tool, in which the surface heat conductivity, that is, the heat conductivity in those areas that comes into contact with the workpiece, is high, but decreases towards the core of the tool. For instance, a copper cover layer could be paired with a support frame, in the form of a steel core, that is deeper in the tool.

Another distinct advantage of the present inventive apparatus is that there may be fewer cooling bores and/or grooves in the form-holding tool than in the forming tool. Fundamentally, both the forming tool and the form-holding tool are liquid-cooled in order to be able to remove the greatest quantity of heat in a short period of time. Because of the fact that the form-holding tool itself has no role in the forming of the steel sheet, it is possible to choose larger cooling cross-sections and simpler cooling geometries, such as shaft cooling for instance, wherein a series of grooves are provided in the surface of the form-holding tool. Cold water flows through the grooves. The same is true concerning the arrangement of the cooling bores. It is possible to provide only a few central cooling bores, so that the machining of the form-holding tool is significantly less complex than that of the first or forming tool.

These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written description, claims, and appended drawings.

The invention shall be explained in greater detail in the following using the exemplary embodiments depicted in the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a portion of a forming tool embodying the present invention;

FIG. 1A is a schematic view of a portion of a form-holding tool embodying the present invention; and

FIG. 2 is a cooling curve graph.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in FIGS. 1 and 1A. However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise.

FIG. 1 provides a highly simplified and schematic depiction of the bottom die for a hot forming tool 1 with a workpiece 2 made of a hardenable steel sheet. In FIG. 1, workpiece 2 has already been formed in the depicted U shape inside the forming tool 1. The top die for the forming tool 1 is not shown. The workpiece 2 is hardened in the forming tool 1. To this end, the workpiece 2 has been austenitized in advance, that is, it has been heated to a temperature greater than its austenitization temperature. The forming process, and an accelerated or rapid cooling process both occur inside the forming tool 1. The quantity of heat that occurs is removed from the forming tool 1 via liquid cooling (not shown in greater detail).

The workpiece 2 that has been formed and press-hardened in this manner is then immediately placed in a form-holding tool 3, which is depicted on the right in FIG. 1A. The form-holding tool 3 has the same contour as the form-giving forming tool 1. However, its purpose is not forming. Rather, the purpose of form-holding tool 3 is to fix and even, and when necessary, to size the still-warm workpiece 2 in order to prevent the workpiece 2 from warping due to cooling.

The top die is not shown in this depiction of the form-holding tool 3, either. It is obvious that the workpiece 2 is enclosed between top die and bottom die, both in the forming tool 1 and in the form-holding tool 3.

The progression of the production method can be explained using FIG. 2.

FIG. 2 provides a cooling curve for a workpiece 2. The workpiece 2 is initially heated to a temperature that is at least equal to the austenitizing temperature (Ac3). The austenitic structure of the workpiece 2 is to be transformed to a martensitic structure. This occurs between a martensite start temperature Ms and a martensite finish temperature Mf. It can be seen that the cooling curve drops sharply initially, passing through the range for the martensite transformation relatively quickly. However, then the cooling curve flattens. The actual press-hardening has already concluded at a temperature slightly below the martensite finish temperature Mf. The press-hardened workpiece 2 is removed from the forming tool 1 after the holding period, labeled “A,” and is immediately placed into the form-holding tool 3, without any intermediate storage. The holding time in the form-holding tool 3 is identified with “B” in the depiction in FIG. 2. It can be seen that the holding times “A” and “B” are essentially equal, so that the cycle time for the forming tool 1 is essentially half that of a prior forming tool in which complete cooling occurs to the final temperature.

In the foregoing description, it will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed herein. Such modifications are to be considered as included in the following claims, unless these claims by their language expressly state otherwise.

LEGEND

• • 1—Forming tool
  • 2—Workpiece
  • 3—Form-holding tool
  • A—Holding time in the forming tool
  • B—Holding time in the form-holding tool
  • Ac3—Austenitizing temperature
  • Ms—Martensite start temperature
  • Mf—Martensite finish temperature

Claims

1-6. (canceled)

7. A method for making sheet metal components, comprising:

heating a sheet of hardenable steel to a temperature greater than the austenitizing temperature of the steel;

hot forming the heated steel sheet in a hot forming tool to define a formed workpiece;

holding the formed workpiece in the hot forming tool for a first holding time;

after said holding step, removing the formed workpiece from the hot forming tool, and immediately placing the formed workpiece into a cooled form holding tool; and

holding the formed workpiece in the closed form holding tool for a second holding time.

8. The method as set forth in claim 1, wherein:

said holding steps include making the first and second holding times substantially equal.

9. The method as set forth in claim 1, wherein:

said second named holding step includes sizing the formed workpiece in the form holding tool.

10. An apparatus for making sheet metal components, comprising:

a hot forming tool constructed from a first material and configured for press hardening a sheet of hardenable steel into a formed workpiece;

a form holding tool constructed from a second material and configured to closely receive therein the formed and hardened workpiece; and wherein

the second material of the form holding tool has greater heat conductivity than the first material of the hot forming tool.

11. The apparatus as set forth in claim 10, wherein:

the second material of the form holding tool is less strong and less hard than the first material of the hot forming tool.

12. The apparatus as set forth in claim 10, wherein:

the form holding tool has fewer cooling apertures than the hot forming tool.