AP-FIM analysis of niobium distribution at austenite recovery stage in hot-deformed steels

The distribution of niobium in hot-deformed HSLA steels in the early stage of austenite recovery was investigated by an atom-probe field ion microscope (AP-FIM). It was confirmed that the onset of recovery can be inhibited by solute niobium randomly dispersed in the austenite matrix. Simple calculation conducted by considering the interactions between the solute atoms and defects indicates that the strong retardation ability of niobium as compared with titanium, vanadium, and molybdenum is a result of larger impurity diffusion coefficients and solute-lattice defect interactions. The AP analysis of a 0.10Nb steel revealed that not only single solute atoms but also niobium-nitrogen pairs exist in the early stage of recovery. It is possible that the niobium-nitrogen pairs play an important role in impeding the austenite recovery of hot-deformed steels. It was also shown that the AP-FIM is a useful technique that can be used to quantitatively evaluate the distribution of the microalloying elements in steels at an atomic level.