BACKGROUND ART In many fields, there is currently a demand for weight-optimized products with maintained functioning and strength. This applies especially to forged products, which can be heavy and difficult to optimize owing to limitations inherent in the tools used in their production.

One example is front axle beams for heavy vehicles. These beams are typically forged as an I-profile in which the web, or core, in the beam cross section has little effect upon the torsional rigidity. With strength calculations it can be shown that a tubular cross section, with the material moved as far radially outwards as possible, is best for such a construction. This applies especially to the so-called"swan neck"on the front axle beam, between its central part and a spindle holder. With traditional forging methods, it is difficult, however, to achieve such a solution. EP-A2-0 059 038 shows a front axle beam forged horizontally in the traditional manner, that is to say that the blank lies with its final vertical plane (after being fitted) in the horizontal plane as it is worked. The document describes how a blank is preformed by means of rolling and is subsequently moved between a number of presses which forge the whole or parts of the blank into the desired shape. As previously stated, the drawback with the solution is that the web of the beam, for the greater part, is centrally located, which has little effect upon the torsional rigidity.

An alternative solution can be seen from EP-A1-0 015 648, which describes the forging of a rectangular, hollow front axle beam starting from a tubular blank. Although it is possible with this method to obtain a beam with higher torsional rigidity, it also creates a number of problems. In order to produce tapered ends of the beam, these have to be drawn through a die. Even if the material is distributed radially, viewed further out from the centre of the beam, the possibility of controlling the material thickness is very limited. This also applies to the other parts of the beam, since the starting material is a tube of constant material thickness. In addition, a great deal of working of the ends of the beam is required, in order to produce spindle holders, and fitting of separate fixtures for, for example, air bellows.

A further solution can be seen from US-A-6 122 948, which document shows a hydroformed front axle beam. In this case, too, the starting point is a tubular blank, which is first bent into the desired basic shape and is subsequently hydroformed into its final shape. The drawback with this solution is firstly that, as in the example above, the distribution of the material along the length of the profile cannot be controlled. The profile must also be provided with a plurality of separate fastenings, not only for the air bellows but also fixtures for the steering spindle bolts. The latter have to be fastened by, for example, welding which gives the beam a natural weakening susceptible to corrosion.

Finally, it is also possible to cast hollow front axle beams, as can be seen from JP-A-11-011105. For technical reasons of casting practice, there are restrictions, however, in terms of maximum and minimum material thickness, as well as the need for strengthening ribs, complicated casting cores and the like in order to allow a sufficiently advanced profile to be cast. In addition to this, there are further restrictions in terms of a practical choice of material, as well as financial implications for the unit price of the axle beams owing to the heavily increased costs which a casting process would entail.

The majority of the abovementioned problems are solved by the production method according to the invention, which method offers a substantially greater chance of precisely controlling the distribution of material around and along a forged profile.

DISCLOSURE OF INVENTION The invention relates to a method for producing a hollow, elongated construction element, according to claim 1 and its dependent claims, and to a construction element, according to claim 12, produced by this method. The method comprises the following steps: a) a first blank is directed through a furnace for heating to working temperature, b) the blank is directed between a pair of rollers having profiled surfaces, the blank being preformed in one or more steps to form an intermediate product having a predetermined profile along its longitudinal extent, c) the blank is fed to a forging press having a number of cooperating die pads, the blank being worked in a plurality of steps to form a substantially finished product, having a cross section substantially in the form of a hat profile of predetermined varying height, width and material thickness along its length, d) a second blank, having essentially the same profile as the hat profile of the first blank in the dividing plane of the die pads, is placed in connection with the hat profile, e) the first and the second blank are joined together at least along their respective edges to form a composite hollow construction element.

In contrast to the background art, at least the first blank is forged vertically, i. e. the principal vertical plane of the blank, as this is being worked, substantially coincides with the plane in which the construction element is intended to be fitted.

The starting material can be a square or rectangular blank which is cut to the desired length, after which it is heated in a furnace to a working temperature suitable for the material. Where, for example, air-hardening, microalloyed steel is used, the blank is heated to 1250-1300°C, preferably to 1280°C. In a first step, the blank is given a suitable cross section by means of a pair of rotating rollers, which can be made profiled. The rolled blank is subsequently moved to a forging press for working into its final shape.

The forging operation comprises a first step in which a pair of first cooperating die pads form the material in the first blank such that it acquires a predetermined, varying height in a vertical plane along its longitudinal extent, the blank acquiring its principal basic shape in this plane. The blank is then moved to a new forging press which performs a second step in which a pair of second cooperating die pads form the material in the first blank such that it acquires a predetermined, varying thickness along one or more of the side surfaces, bottom surface and upper edge surfaces of the profile along its longitudinal extent. This second step is repeated one or more times in further forging presses, whose successive die pads form the blank until it has acquired its final shape. It is thereby possible to re-distribute the material of the blank both in its cross section and along its longitudinal extent. Through suitable configuration of the die pads, the blank can be formed freely, as far as the forging process allows, along both its inner and its outer periphery.

In order to produce a closed profile, the first blank must be joined together with a second blank. The second blank starts out from a flat starting material in the form of a plate, the thickness and width being chosen such that they match the requirements in terms of profile strength and the width of the first blank. The second blank can either be preformed in a separate forging operation, in which it is formed in a separate press to the same profile as the hat profile of the first blank in the dividing plane of the die pads, or in a joint forging operation, in which the second blank is placed on the first blank and formed to the same profile as the hat profile of the first blank in the dividing plane of the die pads in a common press.

Before the first and the second blank are joined together to form a joint construction element, at least the outer edges of the respective blank are additionally heated. This can be done by heating the first and the second blank in a pair of separate induction furnaces, after which they are placed between a pair of cooperating die pads in a press and joined together by forge welding. Alternatively, the first and the second blank are simultaneously heated using heating means introduced between the first and second blank, which blanks are held between a pair of cooperating die pads in a press, after which they are joined together by forge welding. The said heating means can be constituted by induction elements, an induction furnace, gas flames or the like.

In a final operation, so-called flashes are cut off along the joined edges of the profile. This can either be done during the same press operation as for joining together the first and second blank, or by a separate trimming of the outer edges of the composite profile. The profile thus acquires a predetermined varying width along its entire longitudinal extent.

The final result is a hollow, elongated construction element comprising a first section, which has a cross section substantially in the form of a hat profile of a predetermined, varying width, height and material thickness along its length, and a second section, which has an essentially constant material thickness and is joined together with the first section along the side surfaces of the hat profile. This configuration offers significantly greater chances of optimizing the material thickness of the construction element compared with the background art. Firstly, the material can be distributed such that maximum thickness is obtained where the loads on the construction element are greatest, and secondly, a material displacement towards the periphery of the construction element is obtained, thereby increasing its torsional rigidity.

A hollow profile of this type also offers considerable weight savings compared with a corresponding traditionally forged product.

In order further to increase the strength, the construction element can be made from an air-hardening, microalloyed steel. The product does not therefore need to be hardened or otherwise heat-treated after the joining of the two sections. It is, of course, possible to use steels of a different quality, but in that case a further cost-increasing heat treatment or other after- treatment may be necessary to achieve the desired strength.

A construction element which can be suitably made in this way is a front axle beam. By using the above method, it is possible to produce such a beam with 30% less weight than a horizontally forged beam (see, for example, EP- A2-0 059 038 above).

As previously mentioned, it is possible to optimize the production method so that the maximum material thickness of the front axle beam is obtained in connection with fastening points and regions which are to be subjected to external forces and moments. The method makes it also possible to adapt the cross section of the front axle beam such that it acquires essentially the same outer contours in both the vertical and horizontal planes as a traditionally forged, solid beam. By giving the outer contours of the beam the same so-called"offset" (shape in the horizontal plane along the length of the beam) and"drop" (shape in the vertical direction) as a standard beam for a certain vehicle, it can be used without the need for modifications to be made in existing vehicles. It is also possible to maintain existing fastening points (so-called"interface") for steering spindle holders, springs and the like.

BRIEF DESCRIPTION OF DRAWINGS The invention will be described in greater detail in the following description of a preferred embodiment, shown by way of example, with reference to the appended, diagrammatic drawings, in which: Figure 1 shows a diagrammatic representation of the steps embraced by a preferred embodiment of the method according to the invention.

Figure 2 shows a perspective view of two formed sections prior to being finally joined together into a front axle beam, according to a preferred embodiment.

Figure 3 shows a front axle beam comprising the two sections according to Figure 2, after having been joined together.

Figure 4 shows a front axle beam according to Figure 3 in a perspective view obliquely from below.

Figure 5 shows a finished front axle beam which has been cut to its final dimensions and provided with fastening points.

EMBODIMENT (S) FOR CARRYING OUT THE INVENTION Figure 1 shows a preferred embodiment of the method according to the invention, which method comprises a number of steps for the production of a composite hollow construction element, in this case a front axle beam for heavy vehicles.

A first blank 1, which has been cut to a predetermined length, is directed through an induction furnace 2 in which it is heated to working temperature.

Where, for example, air-hardening, microalloyed steel is used, the blank is heated to 1250-1300°C, preferably to 1280°C. Once the correct temperature has been reached, the blank is directed through a pair of profiled rollers 3,4, which are profiled to give the blank 1 a suitable initial cross section along its longitudinal extent. Through suitable configuration of the respective profiles of the rollers 3,4, an intermediate product is obtained, whose cross section and material thickness varies along the length of the blank in a way that at least partially corresponds to the finished product, or to a rough approximation of its final hat profile. In this stage, the blank 1 is still essentially straight, at least along the peripheral edges, with a number of recesses along the central part.

In the next step, the preformed blank is moved to a first forging press 5, having upper and lower cooperating die pads 6,7. In this forging press 5 the blank 1 begins to be formed, its cross section acquiring a more pronounced hat profile in certain predetermined regions in which high torsional resistance is desirable. Examples of such regions are the so-called swan necks 23,24 at the outer ends of the front axle beam, which swan necks connect a pair of steering spindle holders 19,20 to the central section 25 of the beam. In other regions, where high bending resistance is desirable, transverse ribs are maintained between opposite vertical sides of the profile.

Examples of such regions are fastening points 21,22 for the resilient elements (not shown) which are placed between the vehicle chassis and the front axle beam. Such resilient elements can be constituted, for example, by air bellows. In addition to the forming of the cross section of the blank 1, a start is also made on deformation in the vertical direction in order to give the beam the desired shape and/or vertical height, also referred to as"drop", along its longitudinal extent. This forming gives the blank a varying height, measured in a vertical plane, and a varying distance from a horizontal plane through the outer ends of the blank 1. The maximum vertical height of the finished beam and its maximum distance from the horizontal plane coincide with the central part of the beam.

In subsequent steps, the blank is moved to a second and third forging press 8,11 having respective upper and lower die pads 9,10; 11,12. When the blank leaves the third forging press 11, it has acquired its final shape and is ready to be joined together with a second blank 14 to form a composite hollow beam.

The number of steps required to obtain the desired shape of the blank can, of course, be varied within the scope of the invention, since the number is directly dependent on the properties of the starting material and the desired degree of deformation.

The second blank 14 consists of a straight plate of a predetermined length and essentially the same width as the first, finished blank 1. In order to join together the first and the second blank 1,14, these are directed into a fourth forging press 15 having cooperating die pads 16,17, the second blank 14 being positioned and held in position above, but vertically separate from, the first blank 1. Before the blanks can be worked and joined together, induction elements are introduced between the two blanks in order to heat them to a working temperature suitable for the chosen material. The second blank 14 is then placed in contact with the lower, first blank 1 for subsequent working.

According to an alternative embodiment, it is also possible to heat the second blank 14 separately before introducing it into the forging press. The two blanks can also be heated by means of gas flames or the like.

When the die pads 16,17 of the forging press are brought together a deformation of the upper, second blank 14 is performed so that it acquires the same shape as the upper surface of the first blank, and the blanks are joined together by forge welding along all surfaces where the first and the second blank are in mutual contact following deformation of the second blank 14. In association with the first and the second blank being forge- welded together, surplus material (for example flashes) around the edges of the workpiece can be cut and removed. The result is a composite, elongated construction element in the form of a hollow front axle beam.

It is, of course, possible to preform the second blank 14 by deforming this in a separate forging press, before bringing it together with the first blank for joining together. This entails, however, an additional forging operation, requiring a further set of cooperating die pads.

When the front axle beam has been finally formed, it is subjected to a further machining in which fastening holes for the resilient elements are drilled and spindle holder fixtures are worked to their final shape and tolerance.

Figure 2 shows the first and the second blank 1,14 as they appear after final deformation, when they are ready to be joined together. Clearly evident here are the cavities 30,31,32 of the first blank 1, its transverse reinforcing ribs 33,34 and its spindle holder fixtures 19,20. In this embodiment, the edges 35 of the shown second blank 14 have a substantially even thickness along their length, but it is, of course, possible to vary the thickness of the blank, for example by forming it with thicker material thickness in regions that are exposed to high loads when fitted in a vehicle. This is expediently achieved by preforming the blank 14 separately, prior to the joining process. A separate preforming of the second blank 14 also provides scope to vary the thickness along both its longitudinal and its transverse direction. For example, the ends of the blank 14 could be provided with a central, thicker section connected to the steering spindle holders, should this region need strengthening. In order to be able to achieve this type of forming of the second blank 14, partially like the forming of the first blank 1, there is a need for one or more additional plate presses or forging presses (not shown), having cooperating die pads.

Alternatively, a varying thickness along the edges of the beam can be achieved in association with the final joining-together of the first and the second blank 1,14, which operation is carried out in the last forging press 17 (Figure 1). In this case, the thickness of the composite, forge-welded edge will vary along the longitudinal extent of the beam.

Figure 3 shows a front axle beam in which the first and second blank 1,14 have been joined together, but not trimmed around the edges.

Figure 4 shows a view of the front axle beam from an angle viewed from below, the varying horizontal and vertical extent of the first blank 1 along its length being clearly shown. The swan necks 23, 24 of the front axle beam, which connect the respective spindle holders 19,20 to the central section 25 of the beam, are here prominent. Since this central section has to absorb extreme forces, it must be dimensioned to offer high torsional rigidity, for which reason it has the maximum extent both in the transverse direction and vertical direction of the beam.

Figure 5 shows a finished front axle beam which has been edge-trimmed to a predetermined width along its peripheral edge. In addition, the steering spindle holders 19,20 have been worked and provided with through holes 27,28 for fastening the spindle bolts. Holes have also been drilled for fixing elements at the fastening points 21,22 for the air bellows (not shown) between the front axle beam and the vehicle chassis.

The invention is not limited to the above-stated embodiments but can be applied to all types of construction elements which can be produced by the method described above.