T he structure variation of deformed austenite during the relaxation stage after deformation at various temperatures in a Nb-B ultra low carbon bainitic steel and Fe-Ni alloy was studied by the thermo-simulation. Optical microscope and TEM were applied to analyze the evolution of dislocation configuration and the microstructure after RPC. Particle tracking autoradiography (PTA) technique revealing the distribution of boron was employed to show the change of boron segregation after, relaxation for different times. The results indicate that during the relaxation stage when the misorientation between subgrains excesses a certain value, the boron can segregate to this kind of boundaries during cooling and depict the shape of subgrains. Comparing the subgrain demonstrated by PTA with the optical microstructure a conclusion can be drawn that the packet of bainite generally cannot break through the boundaries of subgrains, so the subgrain appearing at the ralaxation stage can confine the growth of the microstructure during the transformation in succeeding and the final bainite is refined.