Abstract
In order to explore the ways to obtain excellent mechanical properties in steels with simple compositions, a plain low carbon steel was subjected to ultra-fast cooling precisely combined with laminar cooling (UFC + LC model) and only laminar cooling (LC model) respectively after the same controlled rolling, and the microstructural characteristics as well as corresponding mechanical properties were investigated. The results show that under LC model, the microstructure is composed of coarse proeutectoid ferrite and pearlite, whereas UFC + LC model leads to the appearances of a certain amount of fine sheaf-like bainitic ferrite and acicular ferrite besides plenty of fine proeutectoid ferrite and scarce degenerate pearlite. Bainitic ferrite and acicular ferrite contain a plenty of grain boundaries with misorientations of 2–7°, which further refines the microstructure associated with strength under UFC + LC model. Compared with LC model, UFC + LC model increases yield strength from 271 to 365 MPa because of grain refinement and dislocation strengthening effects. In the meantime, the steel treated by UFC + LC model exhibits lower ductile-brittle transition temperature of −59.3 °C compared with that of −44.5 °C of the steel subjected to only LC model as a consequence of microstructural refinement.

Key words
ultra-fast cooling, laminar cooling, plain low carbon steel, microstructural characteristics, mechanical properties

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