EFFECT OF ZIRCONIUM ADDITIONS ON AUSTENITE GRAIN COARSENING OF C-MN AND MICROALLOY STEELS

The effect of zirconium carbonitrides on austenite grain coarsening be haviour in controlled rolled Al-killed Zr and Zr-Nb microalloyed HSLA steels with various Zr/N ratios (2.8-22) has born studied, and compare d 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 particl es were large ZrN-rich carbonitrides with sizes from approximate to 10 0 nm to several microns and of irregular shape, while with hyper Zr ad ditions(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 0.01 1 Zr steel, and 2.75 for the 0.022Zr-Nb steels, was a significant frac tion of AlN formed. Austenite grain coarsening occurred around 1050-11 00 degrees C in all the hypo Zr and Zr-Nb steels because very few smal l carbonitrides(< 100nm) were present, while a much more gradual auste nite grain growth was recorded for the Ti-Nb, Ti-Nb-Zr and hyper Zr st eels, 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 carbon itrides on austenite grain growth. A good correlation was found with t he experimental data. (C) 1998 Elsevier Science S.A. All rights reserv ed.