Decomposition of Mn–C Dipoles during Quench-Ageing in Low-Carbon Aluminium-Killed Steels

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Abstract
Variations in the electrical resistivity and in the deviation from Matthiessen’s rule (DMR) during quench-ageing have been observed on low-carbon aluminium-killed steels containing varied amounts of manganese. The DMR is measured by the difference between the resistivities at 273 K and at the liquid-nitrogen temperature. During the period of violent precipitation of iron-carbides (unidentified carbide, epsilon carbide and cementite), the DMR decreases in the steel containing 0.1 mass% Mn, whereas it increases in the steels containing 0.4 mass%, 0.8 mass% and 1.2 mass% Mn. For the latter three steels, the higher the manganese content, the more marked the increase in DMR. It is deduced from these results that the decomposition of quenched-in Mn–C dipoles occurs simultaneously with the precipitation of carbides. It is further speculated that the solubility of single carbon atoms in ferrite in equilibrium with cementite at 973 K decreases gradually with increasing manganese content, whereas the total amount of carbon atoms contained in ferrite at 973 K, including those combined with manganese atoms to form Mn–C dipoles, increases with increasing manganese content. The binding energy of a carbon atom with a manganese atom is evaluated to be 7.4×10−20 J (0.46 eV).