The present invention relates to a method and. device for manufacturing a closed profile, for example in the form of a pipe in a profile structure and a closed profile, for example in the form of a pipe, manufactured according to the method.

When manufacturing profile structures, where the cross-section and/or shape of the profile or the pipe have been changed, you usually start from materials preferably of steel or aluminium with good forming properties. During the forming operation a certain strain hardening is achieved but this is not the crucial reason for the choice of forming method or the structural form of the product. Furthermore, a profile material is not chosen or the manufacturing process is not controlled so that you achieve any substantial strategical variations in thickness of the material of the final product. By starting from a profile having an even wall thickness you often will use considerably more material as compared to a structure that is optimised in weight.

The object of the present invention is to provide a method and device for manufacturing a closed profile in a profile structure, for example a pipe that is constituted by iron and alloyed to provide a strong strain hardening during plastic cold forming in combination with maintaining a good ductility of the alloy at the same time as the pipe achieves a yield point of above 1000 MPa as the final product. Another object of the invention is that the closed profile is manufactured according to a method through hydroforming and/or mechanical treatment of the same, to provide a strategically varying wall thickness

axially and/or in the cross-section of the profile. The characterising features of the invention are stated in the following claims.

Thanks to the invention a method and device for manufacturing a closed profile in a profile structure and a closed profile manufactured according to the method have now been provided that in an excellent manner fulfil their purposes at the same time as the manufacture can take place very rationally and at low cost.

By forming the profile, which is manufactured according to the invention, with varying wall thickness and/or diameter the weight of the profile can be reduced to more than half the weight of profiles in previously known construction solutions.

Examples of such solutions may be different girders and frame structures in cars and other means of conveyance and also other components where closed profiles, for example in the form of pipes, may be incorporated.

The invention is described in more detail below with the aid of the following embodiments.

According to the invention the closed profile is constituted for example in the form of a pipe that is manufactured from a metal material of iron, which is alloyed with chromium, nickel and carbon in predetermined proportions. In the example chosen the iron is alloyed with 17% chromium, 7% nickel and 0, 1% carbon or closely related alloys, wherein a strong strain hardening is obtained when plastically cold forming the profile in combination with maintaining a good ductility of the alloy. The alloyed metal material forming the profile is for example formed through hydroforming and/or through ballooning preferably by means of a mandrel with a cylindrical or other shape or through

shrinking or elongation, preferably through elongation with or without a matrix, in order to increase the yield point of the metal material from an original, low yield point of about 300 MPa to a final product that completely or partially obtains a yield point of more than about 1000 MPa.

In an embodiment according to the invention in which a closed profile is manufactured with varying diameter you start for example from a pipe of the kind above-mentioned having a diameter of 40 mm, a wall thickness of 2 mm and a yield point of 300 MPa. By means of a mandrel or internal liquid pressure the pipe is expanded along half its length to a diameter of 60 wherein the wall thickness of the pipe is decreased to about 1.5 mm along said half of the pipe and the yield point of the pipe can be increased to 1000 MPa or more at the same time as the other half of the pipe is left untreated. Then a pulling force is applied on the ends of the pipe, wherein the half that still has a diameter of 40 mm, a wall thickness of 2 mm and a yield point of 300 MPa is extended under a certain reduction in diameter until the wall thickness has been reduced to about 1. 5 mm and the yield point has been increased to about the same level as in the other end of the pipe, and thereafter the process is interrupted.

The wall thickness and/or the circumference of the final closed profile may also be varied axially by applying two or more profiles partially into each other with the short profile external on the longer profile. The wall thickness and/or the circumference may be varied further after the forming of the closed profile by using welding to join so called starting profiles with walls of different thickness and/or with different diameters. To optimise the strength of the profile the wall

thickness may be varied in the same cross-section, so that the material will be strategically placed just over the areas where an expected maximum strain may take place. As an alternative you may combine a heavily strain hardening alloy with other alloys to provide the final product certain strength characteristics and/or cost optimisation. As starting material for the closed profile according to the invention a metal material of the above-mentioned alloy having low strength is used.

Alternatively, the profile described in the application intended to be incorporated in a current profile structure may be constituted by any closed profile design intended to be incorporated in the current profile structure.