Microstructure Evolution in a Low Carbon Nb–Ti Microalloyed Steel

Abstract

The micro mechanical properties, stability and three-dimensional (3D) morphology of intragranular ferrite was studied in a low carbon microalloyed steel utilizing nano indentation, three-dimensional reconstruction techniques along with LEO1 450 scanning electronic microscopy. The mixed microstructures of intragranular ferrite, granular bainite and lath or plate-like bainite was obtained in a Nb–Ti microalloyed steel, which was water cooled immediately after relaxation for a fixed time as hot deformation ended. The elastic modulus and hardness increased in the sequence of intragranular ferrite, granular bainite and lath or plate-like bainite by approximately 15 GPa and 0.6 GPa, respectively. On the contrary, the tempering treatment of the specimens showed that the stability of the mixed microstructures decreased in the sequence of intragranular ferrite, granular bainite and lath or plate-like bainite. The results of stability, elastic modulus and hardness indicated that intragranular ferrite was formed prior to bainite transformation. The intragranular ferrite thus effectively sectioned the prior austenite into many small zones and thus the bainite transformed at lower temperatures was restricted in the small zones. It is likely that the formation of lath or plate-like intragranular ferrite prior to bainite transformation played an important role in the refinement of mixed microstructures.