Long-term creep strength property of creep strength enhanced ferritic steels was investigated. Stress dependence of minimum creep rate was divided into two regimes with a boundary condition of macroscopic elastic limit which corresponds to 50% of 0.2% offset yield stress (Half Yield). High rupture ductility was observed in the high stress regime above Half Yield, and it was considered to be caused by relatively easy creep deformation throughout grain interior with the assistance of external stress. Grades T23, T/P92 and T/P122 steels represented marked drop in rupture ductility at half yield with decrease in stress. It was considered to be caused by inhomogeneous recovery at the vicinity of prior austenite grain boundary, because creep deformation was concentrated in a tiny recovered area. High creep rupture ductility of Grade P23 steel should be associated with its lower creep strength. It was supposed that recovery of tempered martensitic microstructure of T91 steel was faster than those of the other steels and as a result of that it indicated significant drop in long-term creep rupture strength and relatively high creep rupture ductility. The long-term creep rupture strength at 600°C of Grade 91 steel decreased with increase in nickel content and nickel was considered to be one of the detrimental factors reducing microstructural stability and long-term creep strength. The causes affecting recovery of microstructure should be elucidated in order to obtain a good combination of creep strength and rupture ductility for long-term.

Effects of Ni content and heat treatment condition on impact toughness and creep strength of precipitation strengthened 15Cr ferritic steels were investigated in order to discuss a possibility of improvement in both mechanical properties. Both creep strength and impact toughness of the developing steels were improved drastically by solid solution treatment with water quenching. However, an addition of Ni reduced the long-term creep strength of the steels, though Ni was effective in improvement in impact toughness. It was found that water quenching suppressed formation of coarse block type particles and precipitate free zones around them, and precipitation of plate type fine particles and thermal stability of them within ferrite phase were promoted by solid solution treatment with water quenching. However, martensite phase with sparsely distributed coarse block type particles were formed in the Ni added steels, and such microstructure reduced the precipitation strengthening effect slightly. On the other hand, increase in impact values of the steel indicated no relation to volume fraction of martensite phase. It was supposed that the impact toughness of ferrite phase itself was improved by solid solution treatment and addition of Ni.