18Cr–9Ni–3Cu–Nb–N steel has the highest creep strength in the austenitic heat-resistant steels. It is expected as the component of the future thermal power plants. In order to clarify the mechanism of creep strengthening and creep rupture for the 18Cr–9Ni–3Cu–Nb–N steel, the creep test and the microstructural observations using electron microscopy were carried out. Seamless pipe was used as the sample because the initial microstructure for the sample conformed to the one for steels in the plants. From the TEM observation, fine NbX and Cu particles were formed in the austenite grains. The pinning effect by these particles is effective for the creep strengthening. In the regard to the microstructural change during the creep deformation, the crystal orientation rotations were observed using SEM-OIM. In the crystal grains rotated during the creep deformation, ‹001› or ‹111› was oriented to the tensile direction. Many boundaries were cracked due to these crystal rotations. This means that the creep rupture is occurred by the difference of the plastic deformation behavior between the grains oriented ‹001› to the tensile direction and the grains oriented ‹111› to it.
