AbstractThe effect of 0·003–0·050 wt-%C additions on strain aging and carbide precipitation in batch-annealed aluminium–killed steels has been investigated. The low–carbon steels exhibited unusual strain–aging characteristics: steels containing 0·020–0·040 wt–%C strain aged at 160 but not at 50°C, whereas those containing 0.007–0·020 wt–%C aged at both temperatures. This behaviour is due to the presence of fine carbide particles, which form as a result of the supersaturation of carbon in the matrix during cooling and which raise the carbon solubility by the Thomson-Freundlich effect. Expressions are derived which give the carbon supersaturation as a function of temperature during cooling, and of the precipitation temperature of each class of carbide. Carbide precipitation alternates between the nucleation of new particles and the growth of existing ones, depending upon the particle spacing and the cooling rate. Strain aging in the 0·05 wt–%C steel is negligible up to 225°C, but then rises to a peak at 325°C, which has been attributed to effects of partition in the dislocation matrix.MST/141