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

Abstract

An analytical electron microscopical investigation of three Ti–Nb Al deoxidized steels with different N:Ti ratios has been undertaken. In each steel, a large number of small (<10 nm) particles were observed. Parallel electron energy loss spectroscopy (PEELS) showed that their compositions in the three steels were consistent with those reported for the caps on the TiN cores in the equivalent steels in Part I, i.e. NbC 0.7N 0.3, NbC and (Nb 0.7Ti 0.3)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 reduction depends on a number of competing factors. AlN precipitation also occurred in the as-rolled steel with highest N content and in the normalized steels 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 complex based on the iso-structural compounds MnSiN 2 and AlN was observed in the high N steel.