Effect of Retained Austenite on Formability of High Strength Sheet Steels

Osamu Matsumura, Yasuharu Sakuma, Yoshio Ishii, Jinfu Zhao
1992 ISIJ International  
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 showedplausible relationships between formabi!ity and initial volume fraction of retained austenite (V}'o)' With an increase in Vvo' height of stretch forming increased linearly to its maximum at about 0.2V,o' and bending as well as expanding of a mechanically ground hole were gradual]y improved to the best at 0.1 5 to
more » ... best at 0.1 5 to 0.2Vvo' On the other hand, expanding of a punched hole was siightly deteriorated with increasing Vvo in less than 0.15-0.2Vvo' These various types of formability were all extraordinarily deteriorated beyond 0.2V~o in stretch forming and 0.1 5-0.2Vvo in the others. It was concluded that these effects of retained austenite on formability can be clearly understood in terms of Vvo and k: a rate constant relating rate of deformation-induced transformation with uniaxial tensiie strain, as a parameter showing stability of retained austenite in press forming.
doi:10.2355/isijinternational.32.1110 fatcat:twsgn5byqrht7e3cli7z3l4ium