There has been a big demand for increased vehicle safety and weight reduction of
auto-bodies. An extensive use of high strength steels is one of the ways to answer the requirement.
Since the crashworthiness is improved by applications of higher strength steels to crashworthiness
conscious structural components, various types of advanced high strength steels have been developed.
The crash energy during frontal collisions is absorbed by the buckling and bending deformations of
thin wall tube structures of the crushable zone of auto-bodies. In the case of side collision, on the
other hand, a limited length of crushable zone requires the components to minimize the deformation
during the collision. The lower the strength during press forming, the better the press formability is
expected. However, the higher the strength at a collision event, the better the crashworthiness can be
obtained. It can, therefore, be concluded that steels with higher strain rate sensitivities are desired.
Combinations of soft ferrite phase and other hard phases were found to improve the strain rate
sensitivity of flow stresses. Bake hardening is also one of the ways to improve the strain rate
sensitivity of flow stresses.