1988 Volume 74 Issue 1 Pages 159-166
The following points are studied using Al-killed low carbon steel sheet for two ageing processes ; ageingafter quenching from 650-700°C to 0°C and reheating to 300-450°C (Q.A. process), and ageing after direct quenching from these annealing to ageing temperatures (D.A. process). The slope of thermoelectric power versus electrical conductivity plot is determined to be 295±5 μVK -1μ Ω cm, irrespectively of the processes. Residual solute carbon concentration after Q.A. is reconfirmed to be a little less than that after D.A.. Dependence of precipitation kinetics on grain sizes and quenched-in supersaturation of carbon is found to be different between the two processes, when the annealing temperature is low and the ageing temperature is high. A calculation based on a model of diffusion of carbon shows that the differences correspond to observed different distribution densities of carbides particles. The result indicates promotion of carbide nucleation by the reheating stage of the Q.A. process. Time to attain an equilibrium partition of carbon atoms to Mn-C dipole sites and to interstitial sites free from manganese atoms is estimated to discuss measured relative concentrations of carbon in the two sites. The estimation demonstrates difficulty of quenching for the equilibrium partition.
