Influence of Ti on hot ductility of C–Mn–Al steels

Abstract

The hot ductility of a series of C–Mn–Al steels with nominal compositions of Fe–0·1C–1·4Mn–0·3Si–0·006N with Ti additions of 0–0·04% (all compositions and percentages given in this paper are in wt-% unless otherwise stated) has been determined over the temperature range 1100–700°C. Tensile specimens were cast in situ and cooled at 25, 100, and 200 K min^-1 to the test temperature and then strained to failure at a strain rate of 2 × 10^-3 s^-1. The cooling rates were chosen so that the range for conventional continuous casting (250 mm) and thin slab casting (40–60 mm) was covered. Titanium additions were generally found to impair ductility owing to the formation of fine TiN precipitates. Increasing the cooling rate to the usual rate for thin slab casting also resulted, in general, in finer AlN precipitation and finer MnS inclusions at the boundaries, leading to worse ductility. A regression equation was obtained which showed that the best ductility was obtained when the cooling rate was slow, the particle size coarse, and the N and Al levels low, in order to limit the volume fraction of the detrimental TiN and AlN precipitation. Coarse particle distributions could be achieved by having a high Ti/N ratio. On the basis of the present results it seems unlikely that Ti additions will benefit the ductility of C–Mn–Al steels significantly at the faster cooling rate usual for thin slab casting.