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ect on austenite grain refinement (a) ref steel, 1623 K (1350 C)/60 s and (b) heat 7, 1623 K (1350 C)/60 s.

Fig. 15—Ti effect on austenite grain refinement (a) ref steel, 1623 K (1350 C)/60 s and (b) heat 7, 1623 K (1350 C)/60 s. 1202—VOLUME 41B, DECEMBER 2010

METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 16—Microstructure (a) ref steel 1623 K (1350 C)/60 s cooling 1 K/s (1 C/s) and (b) heat 7, 1623 K (1350 C)/60 s cooling 1 K/s (1 C/s).

presence of titanium-containing inclusions enhances austenite grain refinement because the reference steel without titanium-containing inclusions had the same composition except for titanium. The mechanism to cause grain refinement was not studied, but several inclusions were found at the grain boundaries indicating that the grain refinement was caused at least partly by the pinning effect of the inclusions. Concerning acicular ferrite formation, the results were not conclusive. Figures 16 (a) and (b) illustrate the fact that any strong acicular ferrite nucleation was not activated in these experiments. The main variables affecting acicular ferrite formation in this study were the number of inclusions and the size of the inclusions. The number of inclusions might be too low. Other possibilities are a too high Mn[6] and a too low S[35] content. The explanation also might be a combined effect of these three reasons. Shim et al.[9,11] have observed acicular ferrite in steel with a similar composition to this study, but neither the number of inclusions in mm2 nor the inclusion density was reported. A higher alloying content strongly reduces the Ar3 temperature (start of ferrite transformation), which comes very near the Bs (start of bainite formation) temperature. In that case, the range for acicular ferrite formation is narrow, and the major constituent is bainite on which the effect of particles is not efficient for microstructure refinement.[6] Either way, the acicular ferrite nucleation was not proved unambiguously and would need more systematic study including, for example, varied steel compositions.

V.

CONCLUSIONS

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by using Al-free additives in heats. By comparing the thermodynamic calculations with the results from experimental heats, it could be concluded that the TiO2 additions move the local equilibrium from the Al2O3 closer to Ti3O5 predominance area. The experimental heats with pure Ti-bar and TiO2 additions were successful, resulting in the desired inclusion type with TiOx and optimal inclusion size. In many cases, Ti oxide inclusions were associated with Mn oxide and/or sulfide, unlike with TiN. The number of inclusions increased, and the size decreased when adding pure Ti and TiO2 instead of only FeTi. The addition of TiO2 increased the number of TiOx inclusions in the samples. The late addition of TiO2 (in the mold instead of crucible) resulted in a higher number of TiOx inclusions. Part of the TiOx inclusions were endogenous inclusions resulting from the Ti deoxidation, and part of the inclusions were exogenous resulting from the TiO2 addition. Whether