A dented car

Twinning induced plasticity (TWIP) steel for improved car safety is the focus of a new research group at the Max Planck Institute for Iron Research (MPIR) in Dusseldorf, Germany. Scientists are seeking to further the material’s development.

The steel, invented three years ago by Professor Georg Frommeyer, Head of the Department of Materials Technology at MPIR, has been engineered for enhanced ductility to absorb energy in the event of a vehicle collision, while maintaining its stability and strength to protect the passenger cabin. Comprising about 20% manganese and small quantities of carbon, aluminium and silicon, TWIP steel can be stretched by up to 90% of its length without breaking, explains Dr Stefan Zaefferer of the new TWIP research group.

He says, ‘Steels are currently either strong for the car frame but not very ductile and break when overloaded, or they are ductile for the chassis, but relatively weak.’

The new alloy is similar to transformation induced plasticity (TRIP) steels, which have been on the market for about 10 years. But TWIP steel is said to have greater reserve ductility. ‘The hardening rate – the increase of strength with increasing deformation – of TRIP steels is higher. Therefore, deformability is more quickly exhausted,’ says Zaefferer.

TWIP’s ductility stems from stacking faults in its crystal lattice. If an extra two stacks of atomic planes are introduced to the lattice from above, it disturbs the regular sequencing of the atomic planes, forming a stacking fault on a mirror plane and creating regularly mirrored sections of crystal. The effect is called twinning and occurs at ignition temperature.

Zaefferer explains, ‘Twinning leads to high local hardening. Therefore, once a particular volume of steel has started to deform, its yield strength rises such that deformation spreads out to neighbouring volumes. The full material participates at energy absorption.’

The team at MPIR is working with industry members, such as international company ThyssenKrupp Steel, to gain a better understanding of the material’s properties and commercial viability. Zaefferer adds, ‘TWIP steels are difficult to manufacture due to high manganese-partial pressure during melting and casting, the tendency to form strong oxide scales during hot rolling [causing] cracking, and their high strength during cold rolling, which requires stronger rolling equipment.’

Jon King, Director of Corus Automotive Engineering, based in Coventry, UK, adds, ‘The implications for high strength automotive applications could be significant. Twinning induced plasticity steels benefit from 3D forming. Corus and Salzgitter [a steel products manufacturer based in Germany] announced a product development cooperation in the area of high strength and ductility steels two years ago.

Further information:

Corus Automotive
Thyssen Krupp Steel