Complex heterogeneous precipitation in titanium-niobium microalloyed Al-killed HSLA steels - I. (Ti,Nb)(C,N) particles

Precipitation in Ti-Nb Al-killed microalloyed HSLA steels (Ti/N weight rati o from 4.4 to 1) was investigated in both the as-rolled and the normalised conditions using analytical electron microscopy including parallel electron energy loss spectroscopy (PEELS). An extensive formation of heterogeneousl y nucleated complex (Ti,Nb)(C,N) particles down to 10 nm in size was observ ed. The core of such a complex particle is based on TIN and has a spherical , cubic or cruciform shape. The N/(TI + Nb) atomic ratio in the core is sim ilar to the average value in the steel whereas the Nb/Ti ratio is much smal ler than the average value and not proportional to it. Many of the cores ha ve caps in the form of epitaxial overgrowths based on NbC. Their compositio n changes from Nb(C,N) to (Nb,Ti)C as the N/Ti ratio decreases. The formati on of these complex particles and their detailed morphology are controlled by the processing conditions as well as the overall composition. (C) 2000 A cta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserve d.