Toughness controlling factor of the extremely low carbon bainitic steel

The microstructure of an extremely low carbon bainitic (ELCB) steel composing less than 0.04 mass% carbon exhibits various morphological features and is categorized into the following two bainitic types, such as a bainitic ferrite and a granular bainitic ferrite. Their mechanical properties, however, have not been clarified in the light of controlling factors. The effects of cooling rate and the deformation of austenite on the strength and the toughness were investigated for the steel composing C/0.01, Si/0.2, Mn/3.0, Cr/1.0, Nb/0.01, Al/0.02, and B/0 similar to 0.0014 mass%. The cooling rate dependence of the microstructure and the strength of the ELCB steel is less than a conventional carbon steel. When boron of 14ppm was added to the ELCB steel, the transformation start temperature (T-st) lowered as the deformation increased, and the tensile strength of the ELCB steel increased. The toughness became better as the deformation increased. The toughness controlling factor of the ELCB steel which constitutes bainitic ferrite mono-phase is the packet size.