Papers by Author: M. Takahashi

Abstract: Subgrain growth in deformed ferrite and incomplete recrystallisation during intercritical annealing in low carbon (LC) steels was investigated by EBSD and FEGTEM/EDS. It was confirmed that fine dual phase (α+γ) microstructures could be obtained even without the addition of microalloying elements such as Nb and Ti, if the steels were heated above Ac1 temperature before the completion of primary recrystallisation and then intercritically annealed. The fine microstructure was found to be mainly due to the inhibition of primary recrystallisation, and also due to the inhibition of subgrain growth in deformed matrix by finely dispersed γ phase formed during heating. Mn segregation at α/γ interfaces seems to indicate that the kinetics of boundary migration in the existence of γ is controlled by the volume diffusion of substitutional alloying elements across the α/γ interfaces.

Abstract: 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.