EFFECT OF GRAIN-REFINEMENT AND GRAIN COARSENING ON SHAPES OF DYNAMIC RECRYSTALLIZATION FLOW CURVES FOR C-MN-AL STEELS

A series of C-Mn(- Al) steels containing 0.3 or 1.4 wt-% Mn and increasing volume fractions of AlN was deformed in compression at strain rates from 3 x 10(-2) to 3 x 10(-4) s-1 and temperatures from 850 to 1100-degrees-C; the peak strains epsilon-p were determined. In the lower part of the test temperature range (i.e. below the temperature at which AlN particles lose their effectiveness in restraining grain growth), epsilon-p increased with volume fraction of AlN in low (0.3%) and high (1.4%) Mn steels. Pinning of the boundaries by the AlN particles also resulted in unusual flow curves, in which the first peak was significantly lower than the second, behaviour that was most marked for the low Mn steels. At the higher temperatures, AlN volume fraction had little effect on the behaviour of any of the steels. In the high Mn steels, epsilon-p decreased with increasing temperature in the conventional manner. In the low Mn steels, epsilon-p first decreased with temperature; then, at the grain coarsening temperature, there was a dramatic increase in epsilon-p followed by a further gradual decrease. The unusual behaviour with regard to the change of epsilon-p with temperature exhibited by the low Mn steels is interpreted in terms of their grain coarsening behaviour, which is markedly different from that observed in high Mn steels.