Effect of Retained Austenite on Formability of High Strength Sheet Steels

Abstract

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 maximumat 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 5to 0.2Vvo' Onthe other hand, expanding of a punchedhole wassiightly deteriorated with increasing Vvo in less than 0.15-0.2Vvo' These various types of formability were all extraordinarily deteriorated beyond0.2V~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. © 1992, The Iron and Steel Institute of Japan. All rights reserved.