One of the present authors has proposed a new concept about the Inherent creep strength determining the long term creep strength of ferritic steels. In the present study, effect of solute elements in the ferrite matrix on the Inherent creep strength of 0.2 and 0.3 mass% carbon steels was studied from a view point of atomic configurations such as atomic pairs. The equilibrium concentrations of solute elements and atomic pairs in the ferrite matrix were estimated by thermodynamic calculations with the sublattice model (Thermo-calc.) and the central atoms model, respectively. The following results are obtained; (1) The Inherent creep strength of carbon steels at 773K and 88MPa is proportional to concentrations of Mn and Mo in the ferrite and inversely proportional to those of C and Si. (2) A good correlation is observed between the Inherent creep strength and the concentrations of Mn-C and Mo-C atomic pairs. These atomic pairs seem to reduce the climb velocity of dislocations due to their large interaction energies with dislocations. Thus, it is concluded that the Inherent creep strength of carbon steels is controlled by Mn-C and Mo-C atomic pairs in the ferrite matrix.
