INFLUENCE OF DEFORMATION-INDUCED FERRITE, GRAIN-BOUNDARY SLIDING, ANDDYNAMIC RECRYSTALLIZATION ON HOT DUCTILITY OF 0-CENTER-DOT-1-0-CENTER-DOT-75-PERCENT-C STEELS

The influence of C on hot ductility in the temperature range 600-1000- degrees-C has been examined for three C contents (0.1, 0.4, and 0.75 w t-%). Using a strain rate of 3 x 10(-3) s-1, tensile specimens were he ated to 1330-degrees-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 -1 were examined. A t the strain rate of 3 x 10(-3) s-1, increasing th e C content shifted the low ductility trough to lower temperatures in accordance with the trough being controlled by the gamma-alpha transfo rmation. Thin films of the softer deformation induced ferrite formed a round the gamma grain boundaries and allowed strain concentration to o ccur. Recovery to higher ductility at high temperatures occurred when these films could no longer form (i.e. above Ae3) and dynamic recrysta llisation was possible. The thin films of deformation induced ferrite suppressed dynamic recrystallisation in these coarse grained steels wh en tested at low strain rates. Recovery of ductility at the low temper ature side of the trough in the 0.1% C steel corresponded to the prese nce of a large volume fraction of ferrite, this being the more ductile phase. For the 0.4 % C steel decreasing the strain rate to 3 x 10(-4) s-1 resulted in a very wide trough - extended to both higher and lowe r temperatures compared with the other strain rates. The high temperat ure extension was due to grain boundary sliding in the gamma. Recovery of the ductility only occurred when dynamic recrystallisation was pos sible and this occurred at high temperatures. At the low temperature e nd, thin films of deformation induced ferrite were present and recover y did not occur until the temperature was sufficiently low to prevent strain concentration from occurring at the boundaries. Of the two inte rgranular modes of failure grain boundary sliding produced superior du ctility. At the higher strain rates there was less grain boundary slid ing, which led to a lower temperature for dynamic recrystallisation. H igher strain rates also increased the rate of work hardening of deform ation induced ferrite, reducing the strain concentration at the bounda ries. Ductility started to recover immediately below Ae3, resulting in very narrow troughs. Finally, it was shown that the 2% strain that oc curs during the straightening operation in continuous casting is suffi cient to form deformation induced ferrite in steel containing 0.1%C. ( C) 1993 The Institute of Materials