Abstract: The morphology and amount of the retained austenite in the duplex microstructure of martensite and tower bainite of a tow,alloy cold die steel(it is called GD steel for short) with high strength and high toughness have been investigated.The thermal and mechanical stabilities of the retained austenite were analyzed.Furthermore the effects of retained austenite on the mechanical properties of the steel were studied.The results show that the morphology and amount of retained austenite vary with silicon content in the steel.Retained austenite with high stability was responsible for the simultaneous increase in strength and toughness of the steel.The duplex microstructure of martensite and abnormal lower bainite of the steel with the maximum silicon content exhibits a relatively good strength-toughness combination.

Key words: martensite, bainite, die steel, retained austenite