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A high stretch-flange formability is required of the suspension parts and road wheels. So, the factors effecting stretch-flange formability of hot rolled high strength steel sheet were investigated. The stretch-flange formability was evaluated with a punched hole expansion ratio (λ = d/d0). The inclusion (MnS etc.) deteriorates λ. The hole expansion property is affected by a microstructure. Bainite is most desirable. Incidentally, the form of the cementite in bainite has effect on it. Increase of Si accelerates a fine dispersion of cementite and improves the hole expansion property. The microstructure has the effect on the shape of burr on the punched face, a micro-crack condition or a crack propagation property and as the result, it affects the hole expansion property.

Extremely formable cold sheet steel with an ultra-high Lankford value of more than 2.5 and an n value of more than 0.27 has been developed. This steel is obtained due to the following factors; using extremely pure IF (Interstitial free) steel, immediate rapid cooling upon completion of rolling in the hot rolling process, a high reduction in the cold rolling process, and a high soaking temperature in the continuous annealing process. This steel sheet shows excellent deep drawability and stretch formability compared with conventional steel sheet (former IF steel and low carbon aluminum-killed steel) as a result of evaluating the limiting drawing ratio and limiting dome height, respectively. This excellent formability is also shown by the model forming tests for simulating the actual stamping of an oilpan and a side-panel. Furthermore, this steel shows the same spot-weldability as that of former IF steel, and zinc phosphatability similar to that of low carbon aluminum-killed steel.

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.