2002 Volume 42 Issue 3 Pages 273-282
The hot ductility of as-cast Nb, Ti and Nb–Ti bearing low-carbon steels has been assessed. Hot tensile testing of in-situ melted specimens, replica examinations and thermodynamic modeling showed that addition of 0.014–0.04%Ti to 0.1%C–0.03%Nb–0.005%N steel leads to a large volume fraction of fine strain-induced precipitates at temperatures up to 1 000°C which seriously deteriorate the hot ductility, in spite of the type of prior thermal history. Generally, three types of precipitates, i.e. coarse boundary precipitate, coarse frond-like and fine strain-induced precipitates, were found in Ti–Nb microalloyed steels with the different sizes and compositions, and the latter could be described by thermodynamic modeling. The current results are different from the previous ones published and a thermodynamic model was employed to explain this discrepancy. Accordingly, the beneficial effect of Ti can only be achieved in some instances at high nitrogen and low titanium contents, i.e. a low Ti/N ratio, which greatly encourages coarse precipitation at high temperatures and reduces the fine strain-induced precipitation. Therefore, only for electric arc steels typically with a high nitrogen level, a small addition of Ti might be considered to be advantageous to the hot ductility.