Effect of Zirconium additions on Austenite grain coarsening of C–Mn and microalloy steels

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(<100nm) were present, while a much more gradual austenite grain growth was recorded for the Ti–Nb, Ti–Nb–Zr and hyper Zr steels, due to the effective pinning of austenite grains by Ti–Nb or Zr carbonitrides (<100 nm). A new particle-grain growth inhibition model developed by Gladman was used to estimate the effect of fine Zr carbonitrides on austenite grain growth. A good correlation was found with the experimental data.