This method for producing hardened steel products is described in GB 149035-A and referred to as press hardening. An important advantage with the process is that hardened sheet steel products of complicated form can be produced and despite the complicated form they can have narrow tolerances in form and size.

Conventionally, in order to have a still higher accuracy on certain parts, for example guiding holes and the like, a machining operation, for example punching of holes, is carried out on the finished product. This machining operation causes high wear on the tools and might cause a reduction in the fatigue strength.

It is an object of the invention to provide, in a simple and cost effective way, holes that have a high accuracy in size and in position in the finished product. According to the invention, these holes are finished by the collaring of pre-made holes directly in the press hardening operation and before the hardening and the invention has been given the characteristics stated in the claims.

The invention will be described in more detail with reference to the accompanying drawings which, as an example only, show a bumper beam produced in accordance with the invention.

Figure 1 shows the bumper beam in a perspective view.

Figure 2 shows the bumper beam in a front plan view.

Figures 3 and 4 show enlarged two guiding holes in the bumper beam.

Figures 5 and 6 show the guiding holes of Figures 3 and 4 as they looked in the flat blank before the stamping of the blank into the beam.

Figure 7 shows a mandrel while collaring the pre-made hole shown in Figure 5 into the hole shown in Figure 3.

Figure 8 shows a mandrel for collaring the hole shown in Figure 6 into the hole shown in Figure 4.

The bumper beam 11 shown in Figures 1 and 2 is made of hardenable sheet steel for example a boron steel. In principle, it consists of a U-profile with side flanges, a so called hat profile, and the profile is bow-formed. The beam can be provided with a front cover welded to the side flanges. A hard foam can be fixed to the cover.

For making the bumper beam, one starts with a flat sheet steel blank with a desired number of slots and holes. The tolerances in position and size of the holes and slots achieved in the finished beam will usually be sufficient. For some holes an improved accuracy will be necessary. In Figure 1, which is a perspective view seen from behind, two guiding holes 12,13 are shown as examples of such holes. They are used to guide the bumper beam when the beam is being mounted on a vehicle such as a passenger car.

Figure 2, which is a front plan view of the bumper beam, shows these guiding holes 12,13 as well. Figure 3 shows the guiding hole 12 enlarged and Figure 4 shows the guiding hole 13 enlarged.

The holes 12,13 are collared to their final shape in connection with the stamping of the blank into the finished beam. The hole 12 will be collared from a circular hole with a diameter that can for example be 40-60 % of the final diameter. In the blank, the hole 13 has the dog-bone like shape shown in Figure 6. The length of the hole 13 will not change by the collaring ; the central flaps 16,17 will only be bent.

Figure 7 shows a round mandrel 18 used for collaring the circular hole 12 and the figure shows also a part of the pair of tools 20,21 for stamping the blank into the beam. The mandrel 18 is guided in the upper tool 21. Figure 8 shows a correspon- ding rectangular mandrel 22 used to collar the central part of the hole 13 as descri- bed.

Horizontally, the mandrel 22 is shorter than the length of the original hole 13 in the blank, which allows the beam to shrink somewhat during its cooling. Since the longish hole 13 is directed along a radius from the hole 12, the forces from the shrinking will not be hazardous to the mandrels 18,22.

The process of making the finished beam from the blank will now be described. At first, the blank is, usually heated in a furnace to a temperature at which it is fully or partly austenitic, that is, usually to a temperature above Ac3. Then, the hot blank is moved into position on the lower tool 20 and the upper tool 21 stamps the blank which is thus formed between the tools 20,21. The tools are cold and they are cooled continuously so that the beam is rapidly cooled from the temperature above Ac3. It hardens while being clamped between the tools. Thus, the tools 20,21 act as a fixture during the hardening. The mandrel 18 and the mandrel 22 move downwards and make collars on the holes 12,13 in the hot blank or hot beam or during the stamping. The mandrels 18,22 will cool the collars so that they will harden too. The collars may harden more or less but they will be formed before they harden. Typically, a product of boron steel will have a yield strength of 1300 Mpa and the collars may have somewhat lower strength. The weldability is very good despite the high strength.

The device for moving the mandrels 18,22 up and down is not shown because it is conventional. The mandrels can alternatively be guided by the lower tool 20 instead of by the upper tool 21. As an alternative to movable mandrels, the mandrels 18,22 can be fixed to the upper movable tool 21.

By making one guiding hole 12 circular, it will have narrow tolerance in all direz tions. By having the longish hole 13 directed along a radius from the circular hole 12 as shown, it will have narrow tolerance only transverse to the beam. By using the two guiding holes 12,13 when mounting the bumper beam on the vehicle, the tolerances of the mounting will be tight. The invention is not limited to the production of bumper beams or other beams but can be used also for the production of other sheet steel products.