Hot ductility of TWIP steels

Abstract

The hot ductility of twin induced plasticity (TWIP), 0·6 wt-%C steels containing 18–22 wt-%Mn with N levels in the range 0·005–0·023 wt-% and the Al additions either low (<0·05 wt-%) or high (1·5 wt-%) has been examined. Little change in ductility occurred in the temperature range 1100–650°C as the structure was always fully austenitic. Ductility was generally poor (<40%), reduction of area values, the best ductility at the higher Al level being given by the steel with the lowest N and S levels. Because the steel is fully austenitic, the ductility is solely dependent on that for unrecrystallised austenite. Therefore, to avoid transverse cracking the volume of second phase particles should be kept to a minimum, i.e. the N should be low to reduce the amount of AlN that can be precipitated out and the S level should be as low as possible to limit the amount of MnS inclusions. Metallographic and TEM studies were carried out and the poor ductility was found to be due to extensive precipitation of AlN at the austenite grain boundaries. Increasing the cooling rate from the melting point to the test temperature from 60 to 180°C min−1 or introducing an undercooling step both led to even worse ductility.