Creep rupture strength properties of carbon steel have been analyzed from a viewpoint of inherent creep strength, and the effects of minor alloying elements on inherent creep strength have been investigated. Inherent creep strength, which is a new concept proposed by the authors, is an essential strength property independent of both microstructural morphology and time, and the governing factor of long-term creep strength property. Large heat-to-heat Variation of inherent creep strength observed for the carbon steel has been cleared to be caused by a remarkable improving effect of Mo on that, However, strengthening effect of Mo has been speculated to saturate at a slight amounts of about 0.03mass%. It has been concluded that an inherent creep strength of ferritic steel is governed by solid solution strengthening effect due to such as C and Mo, and the maximum inherent creep strength is obtained by the addition of C and about 0.03mass% of Mo. Moreover, it has been suggested that the inherent creep strength of many ferritic heat resistant steels are maximum level for ferritic steels, consequently, creep rupture strength of those converges to an almost same level at the long-term region.
