Stretch-Flange Formability of High-Strength Steel

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.