Abstract
Abstract The effect of zirconium carbonitrides on austenite grain coarsening behaviour in controlled rolled Al–killed Zr and Zr–Nb microalloyed HSLA steels with various Zr/N ratios (2.8–22) has been studied, and compared with those of Ti-bearing carbonitrides in Ti–Nb and Ti–Nb–Zr steels. TEM observations and PEELS analysis showed that, in the Zr and Zr–Nb steels, with hypo Zr additions(Zr/N=2.8–6.3), the Zr-bearing particles were large ZrN-rich carbonitrides with sizes from ≈100 nm to several microns and of irregular shape, while with hyper Zr additions(Zr/N=15–22), which were far from stoichiometry (Zr/N=6.5), fine spherical particles of Zr-rich Zr carbonitrides(10–100 nm) were formed together with many more large ZrC-rich carbonitrides. Moreover, only in the steel with the lowest Zr/N ratios, i.e. 2.34 for the 0.011 Zr steel, and 2.75 for the 0.022Zr–Nb steels, was a significant fraction of AlN formed. Austenite grain coarsening occurred around 1050–1100°C in all the hypo Zr and Zr–Nb steels because very few small carbonitrides(
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