Quantification of Coarse TiN-rich Precipitates in High Ti Microalloyed Steels Using EMPA Mapping and SEM/EDX
Creators: Kazoun, Tamara
Publisher: name: University of Alberta Library
Year: 2024

<p>Microalloyed steels are high strength steels with low alloying additions of Ti, Nb, and V. These microalloying elements form carbonitride precipitates during thermomechanical controlled processing (TMCP), which enhance the mechanical properties of steel. TiN is one of the first precipitates to form during steel production. It forms in the liquid during solidification and is coarse. It can also precipitate with other elements and form TiN-rich precipitate complexes.<br>
A precipitate analysis was performed to quantify and characterize two types of coarse titanium precipitates in seven titanium rich microalloyed steels. The steels varied in titanium (Ti) and niobium (Nb) contents, ranging from 0.047 – 0.122 wt% Ti and 0.015 – 0.088 wt% Nb. The Ti precipitates studied were titanium nitride (TiN) precipitating on calcium sulfide (CaS) inclusions and TiN-rich precipitates that have Nb precipitating on them, forming (Ti,Nb)N.<br>
Electron microprobe analysis (EMPA) mapping of the steel samples was undertaken to quantify the prevalence of TiN nucleation on CaS inclusions and Nb on TiN to form (Ti,Nb)N. Elemental maps for Ti, Nb, Mn, Ca, and S were generated through the thickness of the microalloyed steels. Each elemental map consisted of approximately 300,000 – 900,000 pixels. The greyscale intensity and location of each element through the thickness of the strips were obtained using a custom developed code. The greyscale elemental maps were correlated with EMPA line scan data to quantify the greyscale intensities into an approximate wt% value. For a given Ti pixel concentration, co-precipitates of TiN and CaS, including spatial locations, were counted. It was found that the total amount of TiN-CaS precipitates was relatively low compared to the total amount of Ti precipitates in the sample. For the amount of CaS inclusions in the samples, approximately 85-90% were nucleation sites for TiN and formed TiN-CaS precipitates. Steels with higher Ti contents (4372, 4175, and 4280) exhibited the largest amounts of TiN precipitating on<br>
CaS inclusions. Most TiN-CaS precipitates were found at the steel quarter thickness or near the surface.<br>
Similarly, for a given Ti and Nb pixel concentrations, co-precipitates of Ti and Nb, including spatial locations, were counted. For (Ti,Nb)N, steels 4372 and 4280, which had the highest Ti and Nb concentrations exhibited the largest counts. The number of (Ti,Nb)N throughout the thickness of the steels was almost constant in all samples.<br>
Scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) spectroscopy were used to verify the presence and composition of TiN-CaS assemblages and (Ti,Nb)N at a number of specific locations in the steels. SEM was also used to capture micrographs of Ti precipitates to develop size distributions. A size analysis was done on sample 4280 to explore the difference in size between TiN/(Ti,Nb)N and TiN-CaS. The analysis showed that TiN that nucleated on CaS inclusions was approximately 4 μm larger than TiN or (Ti,Nb)N that nucleated on its own. Another size analysis was performed on samples 4280, 4175, 4052, and 4093 to study the size difference between TiN and (Ti,Nb)N. Results showed that (Ti,Nb)N particles were approximately 1.6 μm in size, and TiN particles were approximately 2.7-3.8 μm in size. The size of TiN was constant across the different samples.</p>