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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.KEY WORDS: microalloyed steel; intragranular ferrite; nano indentation; three-dimensional morphology.Optical microstructure was observed with an Olympus microscope. SEM observation was conducted at a LEO1450. Small load was applied for nano indentation because the intragranular ferrite in this study was thin and divided bainite. In order to compare the micro mechanical properties of intragranular ferrite, granular bainite and lath or plate-like bainite, the same load was applied for these microstructures. The nano indentation was applied with load 10 mN and conducted at National Metallic Materials Laboratory at University of Science and Technology Beijing. The elastic modulus and hardness of the microstructures was calculated from the measured force-depth curves using an empirical function, which was established utilizing single crystal metals.For serial sectioning the specimens were polished with an automatic grinder polisher under a fixed polishing load and for a fixed time. The polished surface of each layer was slightly etched with 3 % nital. Fiducial hardness indents were applied with a Vickers hardness indenter at a 200 g load for the purpose of alignment and measurement of the depth of removal. The depth of a removed layer was calculated by measuring the diagonal length of the hardness indents before and after each polishing. Approximately 0.50 mm was polished away each time. A series of optical micrographs were obtained with a digital CCD camera at a magnification of 400 in each polishing cycle. The images were then aligned horizontally. After the masking of ferrite was performed, a stack of 2D slices of ferrite image wa...