Influence of S and AlN on hot ductility of high Al,TWIP steels

Abstract

The hot ductility of a high Al, twin induced plasticity austenitic steel with S levels of 0·003, 0·010 and 0·023 has been examined after heating to 1250°C and cooling at 60 K min⁻¹ to test temperatures in the range of 700–1100°C. Ductility as measured by reduction of area (R of A) was very poor, (∼20R of A) in the two higher S steels in the temperature range of 950–1100°C but was better at lower temperatures of 700–900°C reaching 30–40R of A. For the very low S steel, ductility was similar for the temperature range of 700–900°C but improved in the higher temperature range of 950–1100°C to 50–55R of A. In the higher S steels (0·01 and 0·023S), ductility was poor because of the presence of long, coarse dendritic AlN rods situated at both the dendrite, but, more importantly, the austenite grain boundaries, the latter being particularly detrimental for encouraging intergranular failure. AlN seems to need MnS inclusions as nuclei in the melt for it to precipitate in the form of these detrimental long rods, and a reasonable volume fraction of MnS (equivalent to having more than 0·003S present) is required for this to happen. If the S level is low (⩽0·003), there are only a few MnS nucleation sites, and the AlN precipitates out in the form of coarse hexagonal plates. These precipitates end up mainly within the austenite matrix and have little influence on the hot ductility. It is therefore very important commercially in these high Al steels to limit the S level to ⩽0·003 to avoid transverse cracking.