MX-type carbonitride such as (V, Nb)(C, N) is considered to be very useful for long-term creep resistance of the advanced ferritic steels at elevated temperatures over 873K. In this study, we discuss the creep properties in terms of the morphology of MX using the model steels, 0.08C-9Cr-3.3W-3Co-0.2V-0.05Nb-0.05N, with changing the C/N-balance. TEM observation has clarified that the precipitation behavior of MX both in quantity and its averaged composition is controlled by the C/N-balance. It is found that there are three types of MX in the steels after tempering. Type Ⅰ is a spherical Nb(C,N) which is dominant in the nitrogen free steel and is found to exist prior to tempering in every steel. Type Ⅱ is a V-rich MX with a rod or plate shape. Type Ⅲ represents specific morphology formed by secondly precipitation of VN adhering to the Nb(C,N) at a tempering stage. Type Ⅱ and III are observed in the steels with nitrogen. Creep properties have been drastically improved by adding nitrogen, although this effect seems to saturate. It is considered that an effective fine distribution of Type Ⅱ-MX during tempering or creep is suppressed by the formation of Type Ⅲ-MX in the highest nitrogen steel.
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