The influence of composition and cooling rate on the hot ductility of steels has been reviewed. Models to predict hot ductility behaviour have been discussed and the parts of the trough which can be used to predict the likelihood of cracking occurring are highlighted. On tensile testing both deformation induced ferrite in sufficient quantity to improve ductility and dynamic recrystallisation occur but not when straightening during continuous casting; the strain being too low. This limits the use of the hot ductility curve in predicting cracking behaviour. The temperature range in which straightening of the continuously cast strand should be carried out is either 30°C below the Ar 3 when there is a large amount of ferrite (∼40%) present before deformation or above the T d, the temperature at which dynamic recrystallisation starts to take place in a tensile test; this being when the ferrite film no longer forms and precipitates are sufficiently coarse and few in number to influence the ducti...

Hot ductility of steels and its relationship to the problem of transverse cracking in continuous casting

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 a...

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Hot ductility of TWIP steels

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 ...

Influence of titanium on hot ductility of as cast steels

Effect of boron on the hot ductility of Nb-containing steel

The influence of S and Nb on the hot ductility of as cast C–Mn–Al steel has been examined. Tensile test samples were cast in situ, cooled at 100 K min−1 to test temperatures in the range 750–1000°C, and strained to failure at a strain rate of 2×10−3s−1. Both S and Nb caused the hot ductility trough to deepen and widen, an addition of 0·03%Nb giving a deterioration in ductility similar to that obtained by increasing the S content from 0·002 to 0·016%. Grain size was coarse and similar for all steels; thus, the changes in ductility could be related to changes in precipitate and inclusion particle distribution. The addition of Nb resulted in the presence of a coarse Nb(C,N) eutectic and a fine matrix and boundary precipitation of Nb(C,N). The addition of S resulted in the precipitation of coarse sulphides at the interdendritic boundaries, as well as the presence of a fine precipitation of Fe oxysulphides similar in size and distribution to the Nb(C,N) precipitation. The apparent similarities in preci...

Influence of sulphur and niobium on hot ductility of as cast steels

Influence of Micro-alloying Additions on the Hot Ductility of Steels Heated Directly to the Test Temperature

The hot ductility of a series of micro-alloyed steels has been obtained after solution treating and cooling to test temperatures in the range 750 to 1000 °C. Various holding times were given prior to testing to vary the amount of prior precipitation from 0 to 100 pct. For temperatures corresponding to those giving the maximum rate of precipitation, hot ductility improved with holding time for the Nb containing steels but deteriorated for the V and Al containing steels. It is concluded that dynamic precipitation is most effective in reducing hot ductility for Nb containing steels while static precipitation impairs ductility the most for V containing steels. Whereas precipitation of NbCN and VCN readily occurred, precipitation of A1N was found to be slow and the supersaturation of Al and N had to be high to start the precipitation reaction. There was no evidence for dynamic precipitation of A1N, but static precipitation at γ grain boundaries was effective in reducing hot ductility. Although differences inR of A between samples having no and substantial precipitation prior to deformation were most marked at the temperature corresponding to that giving the maximum rate of precipitation, the temperature range over which significant differences remained could be quite marked.

D. N. Crowther

Papers

Influence of Micro-alloying Additions on the Hot Ductility of Steels Heated Directly to the Test Temperature

The relative influence of dynamic and static precipitation on the hot ductility of microalloyed steels