T he influence of small additions of titanium on the hot ductility of C-Mn-Nb-Al steels has been examined. T itanium and nitrogen levels varied in the ranges 0•014-0•045 and 0•004-0•011 wt-%, respectively, so that a wide range of T i/N ratios could be studied. T he tensile specimens were cast and cooled at average cooling rates of 25, 100, and 200 K min−1 to test temperatures in the range 1100-800°C and strained to failure at a strain rate of 2×10−3 s−1. It was found that ductility in the titanium containing niobium steels improved with a decrease in the cooling rate, an increase in the size of the titanium containing precipitates, and a decrease in the volume fraction of precipitates. Coarser particles could be obtained by increasing the T i/N ratio above the stoichiometric ratio for T iN and by testing at higher temperatures. However, ductility was generally poor for these titanium containing steels and it was equally poor when niobium was either present or absent. For steels with ~0•005 wt-%N ductility was very poor at the stoichiometric T i/N ratio of 3•4 : 1. Ductility was better at the higher T i/N ratios but only two of the titanium containing niobium steels gave better ductility than the titanium free niobium containing steels and then only at temperatures below about 950-900°C. One of these steels had the lowest titanium addition (0•014 wt-%), thus limiting the volume fraction of fine T i containing particles and the other had the highest T i/N ratio of 8 : 1. However, even for these two steels ductility was worse than for the titanium free steels in the higher temperature range. T he commercial implications of these results are discussed.MST /4232

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The influence ofC on hot ductility in the temperature range 600-1000°C has been examinedfor three C contents (0·1,0'4, and 0·75 wt-%). Using a strain rate of 3 x 10-3 S-l, tensile specimens were heated to 1330°C before cooling to the test temperature. For the 0·4%C steel, two further strain rates of 3 x 10-2 and 3 x 10-4 S-l were examined. At the strain rate of 3 x 10-3 S-l, increasing the C content shifted the low ductility trough to lower temperatures in accordance with the trough being controlled by the y-a transformation. Thin films of the softer deformation induced ferrite formed around the y grain boundaries and allowed strain concentration to occur. Recovery to higher ductility at high temperatures occurred when these films could no longer form (i.e. above Ae3) and dynamic recrystallisation was possible. The thinfilms of deformation inducedferrite suppressed dynamic recrystallisation in these coarse grained steels when tested at low strain rates. Recovery of ductility at the low temperature side of the trough in the o·]%C steel corresponded to the presence of a large volume fraction offerrite, this being the more ductile phase. For the 0'4%C steel decreasing the strain rate to 3 x ]0-4 S-l resulted in a very wide trough -extended to both higher and lower temperatures compared with the other strain rates. The high temperature extension was due to grain boundary sliding· in the y. Recovery of the ductility only occurred when dynamic recrystallisation was possible and this occurred at high temperatures. At the low temperature end, thin films of deformation induced ferrite were present and recovery did not occur until the temperature was sufficiently low to prevent strain concentration from occurring at the boundaries. Of the two intergranular modes of failure grain boundary sliding produced superior ductility. At the higher strain rates there was less grain boundary sliding, which led to a lower temperature for dynamic recrystallisation. Higher strain rates also increased the rate of work hardening of deformation inducedferrite, reducing the strain concentration at the boundaries. Ductility started to recover immediately below Ae 3 , resulting in very narrow troughs. Finally, it was shown that the 2% strain that occurs during the straightening operation in continuous casting is sufficient to form deformation induced ferrite in steel containing 0·] % C.MSTj]809

The influence on hot ductility ofa small addition (0·02%) of titanium to C-Mn-Al-Nb steel and 0·05jO·15%C-Mn-AI steels has been examined over the temperature range 700-1100°C and at a strain rate of 5 x 10-3 S -1. The tensile samples used were tested after melting, solidifying, and cooling directly to test temperature. The y grain sizes at test temperature were coarse and were independent of titanium or carbon content. In niobium containing steel, little influence of titanium was observed when the cooling rate to test temperature was 100 K min -1, but a marked improvement of hot ductility was noted at a slower cooling rate of 25 K min -1. Examination of precipitate distributions suggests that at 25 K min -1, the niobium can precipitate out at high temperatures on the coarse TiN particles leaving less niobium available for precipitating during deformation in the temperature range 800-1000°C. In C-Mn-AI steels cooled at 60 K min-1 a very slight improvement of hot ductility due to the presence of titanium occurred, independent of carbon content. The improvement is considered to be due to titanium removing nitrogenfrom solution and preventing precipitation of the more detrimental AIN. Increasing the carbon content shifted the hot ductility trough to lower temperatures and increased its width. Both these observations are explained in terms of the change of transformation temperature which occurs with carbon content. The limitations of the hot ductility test in simulating the conditions present during the straightening operation in vertical continuous casting operations are discussed.MSTj 1368

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The influence of cooling rate on the hot ductility of plain C-Mn steels containing 0'1-0'7%C has been examinedfor tensile specimens tested in the temperature range 1000-550°C. Increasing the cooling rate from 10 to 60 K min -1 in all cases caused the ductility to deteriorate. This deterioration was found to be associated with the presence of a finer sulphide distribution in the ferrite surrounding the austenite grains, as well as a reduction in the thickness of the ferrite covering. Decreasing the strain rate from 3 x 10-3 to 5·3 X 10-4 S-l led to a smaller influence of cooling rate. However, ductility was always poorer at the lower strain rate as grain boundary sliding was enhanced. Evidence is also submitted to indicate that MnS inclusions can encourage the production of deformation induced ferrite, leading to an earlier recovery in ductility at the low temperature end of the trough in the hot ductility curve. MSTj3758The authors are in the

The extent to which the hot tensile test can be used to assess the relationship of steel composition to the incidence of transverse cracking in continuous casting has been examined. The influence ofS, Ca, Ti, and Nb on the hot ductility ofC-Mn-Al steels was investigatedfor the temperature range 700-1100°C and the results were compared with available commercial data on the effect of these elements on transverse cracking. Three different heat treatments that were carried out before straining have been examined: (i) reheating hot rolled plate to 1330°C; (ti) in situ casting of tensile samples; an.d (iii) asfor (ii) followed by reheating to 1330°C. After these treatments, the samples were cooled directly to test temperature at a cooling rate of 100 K min-l and strained to failure at a strain rate of 2 x 10-3 S-l.All heat treatments showed that Nb gave"the worst ductility in accordance with its known propensity to favour transverse cracking. Reheating hot rolled plate was found to be the simplest and most successful method for examining the influence of the microalloying additions Al and Nb (and probably V) on ductility in relation to the transverse cracking problem. Although in situ casting is a difficult technique and tends to suppress the relative changes in ductility with composition, it is the only method that can be used to assess the influence of elements such as Ti and S which do not return or dissolve fully into solution on reheating. The hot ductility curves are discussed in relation to the precipitation and grain sizes generated by the various treatments.MSTj 1480