EFFECT OF RETAINED AUSTENITE ON FORMABILITY OF HIGH-STRENGTH SHEET STEELS

Effects of retained austenite on press formability were investigated, using austempered 0.4C-Si-1.2Mn sheet steels with high tensile strength of more than 980 MPa. The results showed plausible relationships between formability and initial volume fraction of retained austenite (V(gamma0)). With an increase in V(gamma0) height of stretch forming increased linearly to its maximum at about 0.2V(gamma0)), and bending as well as expanding of a mechanically ground hole were gradually improved to the best at 0.15 to 0.2V(gamma0). On the other hand, expanding of a punched hole was slightly deteriorated with increasing V(gamma0) in less than 0.15-0.2V(gamma0). These various types of formability were all extraordinarily deteriorated beyond 0.2V(gamma0) in stretch forming and 0.15-0.2V(gamma0) in the others. It was concluded that these effects of retained austenite on formability can be clearly understood in terms of V(gamma0) and k: a rate constant relating rate of deformation-induced transformation with uniaxial tensile strain, as a parameter showing stability of retained austenite in press forming.