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13/07/2008
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Advances in steels and roll forming machines are lowering the cost of making structural, high strength 3D parts. Tom Shelley reports
New high strength steels have been developed with a modification to the stress strain curve that optimise them for roll forming – a much less expensive way of making large, longitudinal sheet metal parts than stamping them out.
Presently restricted to designs with a constant cross-section along their lengths, a new machine is being trialled that can move its rolls and produce much more complex parts. The main intended application is reinforcements and energy-absorbing structural elements in cars, but the technologies are of potential usefulness to a wide range of fabrications made in steel.
Roll forming is inherently less expensive than stamping large, longitudinal parts – not least because it avoids the need to make large tool steel stamping dies and even larger presses to apply the required impacts. At a recent automotive industry technical seminar in Coventry, Sigge Ben Attia, from the Knowledge Service Center of SSAB Tunnplåt, revealed the company has not only developed steel grades for roll forming that have special and slightly unusual stress strain curves, but also that a machine has been built that is capable of roll forming them in three dimensions.
This can move its rolls in various directions, so parts can be given shapes that vary along their lengths. The main goal is to make structural beams in cars with flared ends, improving their crash worthiness. At present, these parts, which have various corrugations to improve rigidity, have to be stamped out of sheet. As the preferred materials are steels with very high tensile strengths, up to 1400MPa, they have to be formed with some attention to process, since elongations range from 16%, in the case of 600 MPa tensile stress material (Docol 600DP), down to only 3% in the case of the 1400 MPa material (Docol 1400 M). As a comparison, typical ‘mild’ steel, as exemplified by grade DC06, has an elongation of 41%, but a tensile strength of only 300 MPa.
Nonetheless, by producing steel that is very clean of inclusions, and by engineering the stress strain curve so that work hardening prevents elongation being concentrated in one place, it is possible to form even the highest tensile strength materials to a surprising degree. At the seminar, not only were there finished corrugated parts on show, but also “folded handkerchief” samples made from both 2.9mm thick Domex 700 MC and 1.4mm thick Docol 1000DP grades.
Roll forming becomes easier with larger rolls, more forming steps and higher yield strengths relative to tensile strengths, so SSAB Tunnplåt has brought out two special grades, Docol Roll 1000 and Docol Roll 800, which have increased yield strengths and a smaller interval between the yield strengths and the tensile strengths.
The new 3D roll forming machine is described as being a project with other companies, of whom the main one is Ortic, based in Borlänge, close to the SSAB Tunplåt works. In the new machine, the rolls are in four separate units, each of which is driven by its own motor and mounted so that the roll position can be moved laterally during the forming operation.
According to part owner and managing director Peter Augustsson, previously managing director of Saab Automobile: “In total, we calculate that 15-20% of the press formed parts in a car chassis can be replaced with 3D roll formed material. In the beginning, these parts are likely to be door beams, bumpers and seat construction. In the future, roll forming of other parts will also be of interest – such as B pillars and roof beams.”
Pointers
* New grades of high strength steels have been specifically developed for roll forming, an inherently much cheaper way of making large, longitudinal parts than stamping them out
* A new 3D roll forming machine has been developed to produce parts for the automotive market in high strength steels
* 3D roll forming is already used to make parts for the building and aerospace industries, but existing machines cannot exert and withstand the forces involved in working with high strength steels
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Author
Tom Shelley
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