Ultrahigh impact toughness achieved in high strength bainitic ferrite/ retained austenite lamellar steels below-Mf temperature

A novel heat treatment process below-Mf for medium carbon steel, with the relationship between the microstructure and mechanical properties was investigated. This study demonstrated that a bainitic ferrite/retained austenite lamellar steel with a tensile strength of 1436 MPa and a Charpy V-notch impact energy of more than 130 J was obtained by an isothermal heat treatment below-Mf. This result is considerably higher than that of above -Ms (21.3 J at 400 C) and below -Ms (46.0 J at 300 C). The isothermal heat treatment process below-Mf refines the bainitic ferrite laths, reduces the size and amount of blocky martensite/retained austenite, and makes the bainitic ferrite and retained austenite lamellae more uniform. The results demonstrate that the lamellar structure of bainitic ferrite laths and ductile carbon-rich retained austenite significantly improve the impact toughness of bainitic steels.