Fine-scale precipitation and mechanical properties of thin slab processed titanium–niobium bearing high strength steels

We describe here the precipitation behavior and mechanical properties of 560 MPa Ti–Nb and 770 MPa Ti–Nb–Mo–V steels. The precipitation characteristics were analyzed in terms of chemistry and size distribution of precipitates, with particular focus on the crystallography of precipitates through an analysis of electron diffraction patterns. In addition to pure carbides (NbC, TiC, Mo2C, and VC), Nb containing titanium-rich carbides were also observed. These precipitates were of a size range of 4–20 nm. The mechanism of formation of these Ti-rich niobium containing carbides is postulated to involve epitaxial nucleation of NbC on previously precipitated TiC. Interface precipitation of NbC was an interesting observation in compact strip processing which is characterized by an orientation relationship of [0 0 1]NbC//[0 0 1]α-Fe, implying that the precipitation of NbC occurred during austenite–ferrite transformation.