Complex heterogeneous precipitation in titanium-niobium microalloyed Al-killed HSLA steels - II. Non-titanium based particles

An analytical electron microscopical investigation of three Ti-Nb Al deoxid ized steels with different N:Ti ratios has been undertaken. In each steel, a large number of small ( < 10 nm) particles were observed. Parallel electr on energy loss spectroscopy (PEELS) showed that their compositions in the t hree steels were consistent with those reported for the caps on the TiN cor es in the equivalent steels in Part I, i.e. NbC0.7N0.3, NbC and (Nb0.7Ti0.3 )C, respectively. The Nb incorporated in these caps added to that dissolved in the TiN cores results in a significant reduction in the number of small particles which give effective dispersion hardening. The size of this redu ction depends on a number of competing factors. AlN precipitation also occu rred in the as-rolled steel with highest N content and in the normalized st eels with the two higher N contents. AlN is usually expected to control the austenite grain growth. NbC-based material grew on the AlN. A dendritic co mplex based on the iso-structural compounds MnSiN2 and AlN was observed in the high N steel. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Sci ence Lid. All rights reserved.