Retained Austenite in 0.4C-Si-1.2Mn Steel Sheet Intercritically Heated and Austempered

The formation process of the retained austenite (γR) in cold-rolled 0.4C-1.2 to 2.0Si-1.2Mn steel sheets intercritically heated and subsequently austempered is investigated. It was concluded that granular γR grains with less than 2-3 μm dia. in the above steels and the interlath γR films in the higher C(≥0.6C)-Si-Mn steel heated at single austenite range and austempered, which have been reported by others, were the same in chemical stability of austenite by C enrichment, in spite of differences in formation site and morphology. Formation of stabilized granular γR may be attributed to the smaller granular austenite grain formed during intercritical heating. Addition of Si may cause the suppression of carbide precipitation and the increase in γR fraction in steels held isothermally to give the maximum C content in γR (Cγmax), whereas the Si addition does not change Cγmax.