Influence of titanium on hot ductility of as cast steels

Abstract

The influence on hot ductility of a small addition (0·02%) of titanium to C–Mn–Al–Nb steel and 0·05/0·15%C–Mn–Al steels has been examined over the temperature range 700–1100°C and at a strain rate of 5×10⁻³ s⁻¹. The tensile samples used were tested after melting, solidifying, and cooling directly to test temperature. The γ grain sizes at test temperature were coarse and were independent of titanium or carbon content. In niobium containing steel, little influence of titanium was observed when the cooling rate to test temperature was 100 K min⁻¹, but a marked improvement of hot ductility was noted at a slower cooling rate of 25 K min⁻¹. Examination of precipitate distributions suggests that at 25 K min⁻¹, the niobium can precipitate out at high temperatures on the coarse TiN particles leaving less niobium available for precipitating during deformation in the temperature range 800–1000°C. In C–Mn–Al steels cooled at 60 K min⁻¹ a very slight improvement of hot ductility due to the presence of titanium occurred, independent of carbon content. The improvement is considered to be due to titanium removing nitrogen from solution and preventing precipitation of the more detrimental AIN. Increasing the carbon content shifted the hot ductility trough to lower temperatures and increased its width. Both these observations are explained in terms of the change of transformation temperature which occurs with carbon content. The limitations of the hot ductility test in simulating the conditions present during the straightening operation in vertical continuous casting operations are discussed.

MST/1368