Scanning electron microscopy as well as analytical transmission electron microscopy techniques
such as high resolution, electron diffraction,
energy dispersive X-ray spectrometry (EDX), parallel
electron energy loss spectroscopy (PEELS) and elemental mapping
via a Gatan Imaging Filter
(GIF) have been used to study complex precipitation in commercial dual phase steels
microalloyed with titanium. Titanium nitrides, titanium carbosulfides, titanium carbonitrides and
titanium carbides were characterized in this study. Both carbon extraction replicas and thin foils
were used as sample preparation techniques. On both the microscopic and nanometric scales, it
was found that a large amount of precipitation occurred heterogeneously on already existing
inclusions/precipitates. CaS inclusions
(1 to 2 μm), already present in liquid steel, acted as
nucleation sites for TiN precipitating upon the steel's solidification. In addition, TiC nucleated on
existing smaller TiN (around 30 to 50 nm). Despite the complexity of such alloys, the statistical
analysis conducted on the non-equilibrium samples were found to be in rather good agreement
with the theoretical equilibrium calculations. Heterogeneous precipitation must have played a role
in bringing these results closer together.