| In niobium (Nb) microalloyed steels one of the strengthening mechanisms is precipitation of Nb(C, N). Precipitation in austenite has been studied and applied for many decades, but its precipitation in ferrite has been paid less attention. Some of the findings are somewhat contradictory.;Due to environmental and resource saving consideration in recent years, more and more scrap has been recycled and re-used. Thus, tramp elements inevitably exist in steel such as Cu, Sn, Ni and so on. These elements may influence properties of steels at some extent although they are very little in content.;The X80 steel examined in this thesis contains up to 0.09%wt. Nb, as well as 0.4% Cu, both of which can potentially precipitate in ferrite. However, most research has shown that Cu levels below 0.5% in steel do not precipitate out under normal processing conditions.;In the present work, this X80 steel was processed by a variety of thermomechanical treatments. The as hot deformed mechanical properties were tested and microstructures were characterized by field emission gun scanning electron microscopy (FEG-SEM). At low aging temperatures, simulating the coiling stage in steel hot rolling, many fine Cu precipitates were observed in ferrite compared to a few relatively large Nb containing precipitates. These results were rationalized, and the consequences were discussed. |
