Development of High Strength Steel Sheet with Excellent Stretch Flange Formability for Automobile Application
With the aim of improving stretch-flange formability by further reducing carbides in steel, the authors studied the hole expansion ratio of a steel consisting entirely of ferrite and the factors governing the hole expansion ratio. Ultra low carbon steels adding Ti and/or Nb showed a higher hole expansion ratio than conventional steels, but their hole expansion ratio was not higher than the hole expansion ratio of bainitic steel reported before1). On the other hand, it was found from study of the relationship between hole expansion ratio and r-value of various steels, including cold-rolled interstitial-free steels, that the hole expansion ratio of a steel consisting only of the ferrite phase is strongly influenced by the minimum r-value and n-value and that it improves as the r- and n-values increase. The steel added Ti and/or Nb has a strong anisotropy of r-value, hence its minimum r-value is small. This is disadvantageous to hole expansion ratio.