Kinetic and microstructural study of aluminium nitride precipitation in a low carbon aluminium-killed steel

The precipitation kinetics of free nitrogen present in a low carbon aluminium-killed steel was quantitatively followed at temperatures ranging from 600 to 700 °C using a methodology based on a succession of thermoelectric power (TEP) measurements. These were compared with the precipitation kinetics of aluminium nitrides determined by conventional TEP measurements. This comparison showed a difference between the two types of kinetics. In particular, it was observed that after the complete elimination of nitrogen from the solid solution (SS), the microstructure of the steel keeps evolving. Transmission electron microscopy (TEM) observations, combined with chemical X-ray energy dispersive spectroscopy (EDS), allowed to attribute this result to a phase transition between metastable (Al, Cr)N precipitates of cubic structure and equilibrium AlN nitrides of hexagonal structure. The presence of chromium in the initial precipitates was interpreted by a preferential segregation of this element on the former grain or sub-grain boundaries of austenite during or after hot-rolling. Then, during the precipitation of the nitrides, clusters of segregated chromium atoms could serve as nucleation sites for the (Al, Cr)N nitrides.