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, (∼20 R 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 30–40 R 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 50–55 R 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.

Hot ductility of high Al TWIP steels containing Nb and Nb-V

“…Previous work [5][6][7][8] on steels reheated to 1250 o C after melting have mainly been carried out using a cooling rate of 60 o C/min whereas this work has concentrated on a slower cooling rate of 12 o C/min. Nevertheless, three of the steels in the present exercise (1, 3 and 5) were also cooled at 60 o C/min and are listed in Table 2 together with three (8,9 and 10) steels from earlier work on Nb free TWIP steels [7 ].…”

“…AlN normally precipitates preferentially at the austenite grain boundaries and so encourages intergranular failure. High Al containing and Nb free, TWIP steels also show poor ductility when the S level exceeds ¬ 0.005% because AlN then precipitates similarly at the boundaries giving "rock candy" type failures [5]. Nb additions result in dynamic precipitation of Nb(CN) which is particularly detrimental to the hot ductility of HSLA steels.…”

“…The cooling rate in previous work [3,5,8] after reheating has generally been 60 o C/min, which is higher than the recommended cooling rate of ≤25 o C/min needed to coarsen the TiN particles to obtain the maximum ductility or to give the maximum segregation for B to strengthen the grain boundaries [13]. In this exercise two cooling rates, 12 o C/min min and 60 o C/min have been examined.…”

“…Previous work [3,5,7] has concentrated on the simpler high Al, TWIP steels giving a room temperature yield strength of 800MPa. However, if a higher strength level is required ~1000MPa, then Nb needs to be added [8].…”

“…However, these high Al, TWIP steels have been found difficult to continuous cast and considerable work has been carried out in defining the optimum composition for reducing the likelihood of transverse cracking occurring during the straightening operation [3][4][5][6][7][8]. The work has shown that a low S and P content (preferably ~0.005%), by reducing the volume fraction of sulphides [5,7] and in the case of P preventing the low melting point iron phosphide phase forming, gives rise to better ductility [7]. More importantly, it is found that a small addition of B is needed so it can segregate to the boundaries and strengthen them [6].…”