Characterization of precipitates in an ultra low carbon Cu bearing high strength steel: A TEM study

The precipitation behavior of an ultra low carbon Cu bearing steel microalloyed with Ti and Nb has been studied. It has been observed that coarse TiN particles have formed in the liquid stage or just after solidification and have large sizes unto a few microns. The Nb in these particles may have diffused into the particles at the later stage of solidification. The intermediate size particles (20-700 nm) are Ti-Nb rich carbonitrides of elliptical, cuboids, rectangular or spherical shape. Fine strain induced particles are (Ti Nb)C of 5-12 nm size range and have precipitated on dislocations, twins or in the vicinity of pre-existing large particles. On ageing at 500 °C fine ɛ-Cu particle of 8-12 nm in size forms preferably on to dislocations. At 600 °C ageing temperature, the ɛ-Cu increases in size and changes to rod like shape.