Investigation of Precipitate in Polygonal Ferrite in a Ti-Microalloyed Steel Using TEM and APT



Transmission electron micrograph and atom probe tomography were used to characterize the distribution, sizes, and compositions of precipitates in a Ti microalloyed steel. At 650 and 700°C with holding time 600 and 1800 s, respectively, the dispersion precipitates were obtained in all specimens, and the interphase precipitates were only observed at short holding time, but this interphase precipitation was unstable at long holding time. The kinetics of precipitates in ferrite depended on both coiling temperature and holding time. A lot of small precipitates were obtained at 650 °C with holding time 600 s, and these nano-precipitates can strengthen the ferrite matrix largely. The average Guinier radius of nano-precipitate was about 4.3 nm, and the large precipitates (Guinier radius ∼7–9 nm) with small precipitates (Guinier radius ∼3–5 nm) within an interphase sheet were also observed due to the growth of some precipitates at the cost of small precipitates. The nano-precipitates were mainly titanium carbide, and the atomic ratio of Ti/C was about 0.55, and this value was changed regularly in one precipitate.