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 of strain rate and grain size on the hot ductility of an austenitic and aferritic stainless steel has been examined. Samples were cooledfrom the austenitising temperature to temperatures in the range 1000 to 700°C and tensile tested at strain rates in the range 10-1 to 10 -4 S -1. The austenitising temperature was varied to give two grain sizes, coarse, 600 11mand fine, I"V 30 11m.For both steels, ductility was excellent at fine grain size throughout the temperature range and strain rates examined. The ferritic and austenitic stainless steels both gave ductility troughs at the coarse grain size, but the trough was favoured by higher strain rates in the ferritic steel and lower strain rates in the austenitic steel. The poor ductility was related to the presence of precipitation, mainly at the grain boundaries; this being FeTi phosphides in the case oftheferritic stainless steel and coarse chromium carbides in the austenitic steel. Grain boundary sliding was the major mode of intergranular failure in the austenitic steel while normal microvoid coalescent failure controlled ductility in theferritic stainless steel.MST/3474

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