(1) Development of "Precipitation Strengthened 15Cr Ferritic Heat-resistant Steels"
We have proposed a "Precipitation Strengthened 15Cr Ferritic Heat-resistant Steel" of which a ferrite matrix with low density of dislocation is strengthened by the precipitation of intermetallic compounds to replace the conventional high chromium ferritic heat-resistant steels with a tempered martensite phase. And we succeeded in improving the creep strength of developing steels by twice that of the conventional ferritic steels and the creep rupture lives by 10 to 100 times at 650-750 ˚C through the optimization of heat treatments and chemical compositions. (Fig. 1) The creep elongation, impact toughness and steam oxidation resistance of the developing steels were also improved. We demonstrated the excellent formability of the steels to make a trial of seamless boiler tube by means of conventional hot extrusion process. The "Precipitation Strengthened 15Cr Ferritic Heat-resistant Steel" possesses most excellent creep strength and high-temperature properties among wrought ferritic heat-resistant steels, then we believe that the developing steels are candidate materials of high-temperature structural components in high-efficiency power or chemical plants, supercritical pressure geothermal power plants, nuclear fusion reactors, oxide fuel cell and so on, in next generation.

(2) Prediction of precipitation sequence by "System Free Energy Method"
We have proposed a new prediction method "System Free Energy Method" based on energetics for precipitation sequences in various alloys. In this method, free energy of a whole system is calculated taking account of interfacial energy and elastic strain energy in precipitates, and precipitation sequences are predicted by the evaluation of energy steepest descent path from the energy hierarchy of various microstructural morphologies. And, we could predict the precipitation sequences of a practical heat-resistant steel until 100 000 hours by means of this method (Fig. 2). This method can be applied to the precipitation phenomena in practical multicomponent nickel-based superalloys and heat-resistant titanium alloys. Making use of the advantages of high computation speed and customizable self-making program codes, the "System Free Energy Method" have been developed as an inverse problem analysis tool for search of new precipitation-strengthened alloys.

Fig. 1 Creep strength of the conventional steels and "Precipitation Strengthened 15Cr Ferritic Heat-resistant Steel". The left bottom figure shows a trial of seamless boiler tube with the developed steel.
Fig. 2 The time-temperature-precipitation curves calculated by "System Free Energy Method" with the experimental data (solid circles) of a practical heat-resistant steel.