Strong Bainitic Steels by Continuous Cooling Transformation

Using metallurgical models, three steels were designed to obtain carbide-free bainitic structures in the as-rolled condition. Experimental castings were hot-rolled in a pilot mill and their austenite decomposition kinetics were measured using dilatometry. Tensile and Charpy impact properties were measured and the microstructures were characterised using X-ray diffractometry, optical and scanning electron microscopy. Promising results were obtained when the microstructure was composed of a fine mixture of bainitic ferrite and retained austenite. In this case, 140 ksi (965 MPa) of yield strength was achieved in combination with good toughness. Conversely, when the structure was coarse with blocks of retained austenite between bainitic sheaves or when large martensitic regions were present, the impact properties were strongly impaired.