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.
In order to reduce the anisotropy of r-value (improve the minimum r-value by controlling the texture), it is effective without Ti and/or Nb. By the method, the stretch-flange formability could be improved and a hole expansion ratio of 2.3 to 2.4, higher than that of the bainitic steels, could be obtained with the strength of 370 to 590 MPa. It is considered that the addition of Ti and/or Nb widens the unrecrystallized region of γ and the randomization of γ texture, thereby increasing the anisotropy of r-value (decreasing the minimum r-value.)