Claims
- 1. A process for the production of a metal sheet which is intended to be subjected to a drawing operation for the fabrication of a shaped metal part, said sheet comprising on its surface a roughness in the form of plateau portions in relief defining between them recessed valley portions, the average ratio of the dimension of the valley portions to the dimension of the plateau portions in any direction on the surface of the sheet being from 1:1 to 3:1, the average dimension of the plateau portions being from 40 to 200 micrometers and the height of the plateau portions being more than 6 micrometers; said process comprising subjecting a metal sheet to pressure from at least one cold roller, the surface of which has been treated to provide the surface of the roller with a roughness in the form of plateau portions in relief defining between them recessed valley portions, the average ratio of the dimension of the plateau portions to the dimension of the valley portions in any direction being from 1:1 to 3:1, the average dimension of the valley portions being from 40 to 200 micrometers and the height of the plateau portions being greater than 7 micrometers.
- 2. The process of claim 1, wherein the average dimension of the plateau portions to the dimension of the valley portions of said roller is from 3:2 to 5:2.
- 3. The process of claim 1, wherein the average dimension of the valley portions of said roller is from 60 to 120 micrometers.
- 4. The process of claim 1, wherein the height of the plateau portions of said roller is from 11 to 50 micrometers.
Priority Claims (2)
| Number |
Date |
Country |
Kind |
| 74 36872 |
Nov 1974 |
FRX |
|
| 75 30984 |
Oct 1975 |
FRX |
|
FIELD OF THE INVENTION
This is a division of application Ser. No. 628,600 filed Nov. 4, 1975 and now U.S. Pat No. 4,071,657.
This invention relates to a metal sheet which is intended to be deep drawn or extra-deep drawn for the fabrication of shaped metal parts, and to a process for producing such a sheet.
When the forming of a metal sheet requires considerable sliding of the metal on forming tools, particularly in the case where the forming is effected by drawing, it is customary to use a sheet the surface of which has been made rough by passing the sheet between the rollers of a cold-rolling mill. The roughness of the sheet results from the formation on its surface of a succession of portions in relief, hereinafter referred to as peaks or plateaux, which define between them hollow portions hereinafter referred to as valleys. In order to obtain roughness of this kind, the surface of the cold rollers is treated by physical processes, such as shotblasting, spark erosion, or electro-chemical processes. For example, it has been proposed to roughen the surface of a cold roller by blasting from 100 to 200 grains of shot of a diameter of from 500 to 700 microns per square millimeter of roller surface onto the surface of the roller in two or three passes.
It is current practice to consider that a surface having the highest number of plateaux as close as possible to one another possesses the best drawing properties (see text by John A. Newham in "Metal Deformation Processes, Friction and Lubrication", page 716, a book published by John A. Shey, 1970 -- Marcel Dekker INC. -- New York).
In the forming of sheet metal parts by drawing using a lubricant, the valleys present on the surface of the sheet constitute micro-reserves for the lubricant which results in the pinpoint reduction of adhesion between the micro-junctions formed between the surfaces of the sheet and of the forming tool with which they are in contact.
Despite the utilization of a lubricant of good quality and the existance of these micro-reserves, the phenomenon known as seizing occurs. Seizing occurs when a metallic deposit formed from debris torn from the surface of the sheet during the drawing is formed on the surface of the forming tool. The volume of this debris increases with the number of sheet parts drawn, thus increasing the seriousness of the damage caused by this debris on the surface of the drawn sheet. This damage ranges from simple fine scoring to a deep, wide furrow, with the formation of fragmented chips. The stressing due to the increasing frictional force between the sheet part and the forming tool becomes such that the part breaks.
Losses of scratched or broken parts, wear on the forming tools, the frequency of stoppages of the presses or lines of presses, and the cost of reconditioning tools can be extremely high. In addition to the use of an anticorrosion protective oil, it is necessary to apply localised lubrication to the surface of the sheet before drawing, using a mineral, vegetable, or animal oil, optionally containing conventional additives. This is the source of further disadvantages in various stages of production.
In particular, the parts then stick to the forming tools, making their automatic transfer difficult, while the tools, transfer devices, and equipment for handling the sheet parts are soiled with oil, which is harmful to the safety of personnel and equipment. Furthermore, the formed parts are difficult to degrease, which poses problems during their subsequent use, particularly when they have to be welded or surface coated.
Seizing also occurs when the sheet is covered on at least one of its faces with a metallic film, particularly a film of soft metal. The term "Soft metal" is here understood a metal such as lead, tin, copper, zinc, and, depending upon the metal of which the sheet to be drawn is made, aluminium.
In this case a sludge is formed, which is composed of the metal debris coming from the coating of soft metal on the sheet and the greasy lubrication residues, and which fouls and erodes the forming dies. Furthermore, when a long series of successive drawing operations is effected, there is a progressive deterioration of the surface state of the formed parts, particularly in the quality of brightness. This makes frequent stops necessary for the purpose of cleaning the forming dies or even for dismantling them for polishing purposes.
The invention seeks to obviate or minimise the abovementioned shortcomings and to provide a metal sheet for deep or extra-deep drawing which, when simply coated with anticorrosion oil or water-soluble oil in accordance with current standards and practice, avoids the occurrence of the phenomenon of seizing.
According to one aspect of the present invention as shown in the single drawing FIGURE there is provided a metal sheet 1 which is intended to be subjected to a drawing operation for the fabrication of a shaped metal part, said sheet comprising on its surface a roughness in the form of plateau portions 2 in relief defining between them recessed valley portions 3, the average ratio of the dimension b of the valley portions 3 to the dimension of the plateau portions in any direction on the surface of the sheet being from 1:1 to 3:1, the average dimension a of the plateau portions 2 being from 40 to 200 micrometers and the height h of the plateau portions being more than 6 micrometers. Preferably, the average ratio, in any direction of the surface of the sheet, of the dimension of the valley portions or valleys to the dimension of the plateau portions or plateaux is from 3:2 to 5:2, the average dimension of the plateaux is from 60 to 120 micrometers, and the average height of the plateaux is from 10 to 25 micrometers, these providing optimum characteristics for the surface roughness of the sheet.
The sheet possessing these surface characteristics may be coated on at least one of its faces with a metallic film, particularly a film of soft metal, such as tin.
According to another aspect of the present invention there is provided a process for production of the sheet just indicated, wherein a metal sheet is subjected to pressure from at least one cold roller, the surface of which has been treated to provide the surface of the roller with a roughness in the form of plateau portions in relief defining between them recessed valley portions, the average ratio of the dimension of the valley portions to the dimension of the plateau portions in any direction being from 1:1 to 3:1, the average dimension of the valley portions being from 40 to 200 micrometers and the height of the plateau portions being greater than 7 micrometers. For example, the surface of the roller may have been treated by shot-blasting onto the surface of the roller from 20 to 40 grains of shot of a mean diameter of 500 and 700 micrometers per square millimeter of roller surface.
The origin of the debris, which constitutes the metallic deposit characteristic of seizing, on the forming tools during the drawing of sheet metal workpieces cannot be simply explained from existing theoretical knowledge of the phenomena of friction and of wear of metal parts. Taking as a basis the fact that micro-welds or micro-adhesion occur at the contact interfaces between the peaks or plateaux of the opposing surfaces of the sheet and tool, this debris would in fact have to be microscopic and of a volume far smaller than the volume of the debris actually present on the seized surfaces or than the volume of metal actually shaved off from the surface of the sheet.
In order to avoid the phenomenon of seizing, the number and size of the pieces of debris must be as small as possible, and it must not be possible for this debris to become anchored to the surface of the drawing tool, this can be achieved with a sheet according to the invention.
In cases where the sheets are rolled in a cold-rolling mill before being supplied to the user, the spatial distribution and the dimensions defined above necessitate a considerable modification of the conditions of treatment of the surface of the cold-working rolling mill rollers.
The surface of these rollers must in fact then have a roughness characterized by an average ratio between the dimension of the plateau portions or plateaux and the dimension of the valley portions or valleys which is from 1:1 to 3:1, preferably from 3:2 to 5:2, an average valley dimension of 40 to 200 micrometers and preferably 60 to 120 micrometers, and a plateau height greater than 7 micrometers and preferably from 11 to 50 micrometers. The height of the plateaux on the surface of the rollers is selected to be greater than that which it is desired to obtain on the surface of the sheet, since the latter cannot exactly match in respect of depth the surface configuration of the rollers.
Thus, for example, for treatment by shot-blasting using a shot-blasting machine comprising a bladed turbine and shot of diameters from 500 to 700 microns, it is sufficient to blast onto the surface of the cold-working rolling mill rollers from 20 to 40 grains of shot per square millimeter of roller, preferably in only two passes. The amount of shot used is thus from 3 to 8 times less than that used in known processes.
US Referenced Citations (3)
| Number |
Name |
Date |
Kind |
|
3613319 |
Takimura et al. |
Oct 1971 |
|
|
3754873 |
Bills et al. |
Aug 1973 |
|
|
3956915 |
Saunders |
May 1976 |
|
Divisions (1)
|
Number |
Date |
Country |
| Parent |
628600 |
Nov 1975 |
|
Patent Grant
4111032
