In low carbon
bainitic steel, different
microstructure is formed depending on the cooling rate. From a thermo-simulation study, it was found that during slowly continuous cooling, the acicular
ferrite nucleates at the prior austenite grain boundary or other interfaces first; the retained austenite remains beside the acicular ferrite to form M/A constituents. Industrial test showed that, two kinds of microstructure can be formed depending on the thermo-mechanical control processes: one is granular
bainite with coarse block-like M/A phase, and the other is
ultrafine lath-like bainitic ferrite. The steel as rolled with granular bainite and coarse M/A phase has lower strength and toughness compared with the steel with ultrafine bainitic ferrite, but the elongation of the former is relatively higher than the later one. Tempering for 1 h at about 600 ℃ can improve the toughness of steel with granular bainite, and the elongation of steel with ultrafine bainite ferrite, and the strength of the tempered ultrafine bainitic ferrite steel is higher by about 100 MPa than that of granular bainite steel.
More abstracts about the Microstructure and Mechanical Properties of Low Carbon Bainitic Steel