Precise control of bainite micro-morphology and the effects on the mechanical properties of ultrahigh-strength complex-phase sheet steel

Bainite,the main microstructure of ultrahigh-strength complex-phase(CP)sheet steel,usually exhibits various micro-morphologies when subjected to different austempering treatments.In the current study,conventional austempering treatment at the bainite nose temperature resulted in two bainite types with distinct micro-morphologies:polygonal blocky bainite and acicular bainite,which resulted in large fluctuations in the mechanical properties of CP steel,particularly yield strength and hole expansion ratio.Therefore,the precise control of bainite micro-morphology was studied and applied to separate the two bainite types through austempering optimization.The two bainite types of different micro-morphologies had different effects on the mechanical properties of CP steel:the acicular bainite favored hole expansion and flangeability but deteriorated ductility,while the polygonal blocky bainite favored ductility but deteriorated hole expansion and flangeability.Accordingly,two types of ultrahigh-strength CP steels of different mechanical properties can be stably manufactured through the precise control of bainitic micro-morphology to satisfy the specific demands of vehicle components in terms of the mechanical properties of CP steels.