TiB₂-フェライトの相平衡にもとづく高剛性鋼の開発

抄録

A new TiB₂-reinforced steel with an enormously high Young's modulus has been developed for enabling a leaner design of automobile parts. This article describes the high modulus steel (HMS) with an emphasis on its microstructure and potential elastic properties based on the assessed phase equilibria in Fe-Cr-Ti-B system.
The concept was to increase the isotropic Young's modulus by the optimum combination of reinforcing particles and steel matrices, and thermodynamic considerations strongly supported the most effective contribution of TiB₂ to this purpose.
Calculated phase diagrams located the narrow two-phase region of ferrite (α)+ TiB₂ along the pseudo-binary system of (Fe-17Cr)-TiB₂ in a wide temperature range. The proved phase stability and the small solubility of iron/chromium in TiB₂ were considered most responsible for maintaining its own high modulus of 540 GPa in the steel matrix.
In conventional powder metallurgy techniques, TiB₂ particles were successfully dispersed in the matrix using commercial TiB₂ powders, or synthesized through the in-situ reaction of ferro-titanium and ferro-boron powders.
The measured Young's modulus of the HMS agreed well with the theoretically predicted value, and at 46 vol% TiB₂, the specific modulus reached twice of those of conventional steels owing to its reduced density.