Ultrafine grained steels: Basic research and attempts for application

The Asian projects are now in their second terms after the five year first terms. Ultrafine grained steels were developed in lab scale and/or in a practical process. A basic approach has been performed to explore the evolution mechanisms of ultrafine grained microstructures. Two approaches have been investigated to make the ultrafine grains: the "recrystallization route" and the transformation route. In the "recrystallization" route, heavy deformation at low temperature is necessary for the ultra-refinement. In the transformation route making the high density of ferrite initiation sites in the deformed austenite, especially of the heavily compressed austenite structures, is essential for the ultra-refinement and further hindering the ferrite growth is also inevitable to utilize the high initiation rate. Strength and toughness are dramatically improved by the ultra-refinement. Fatigue property is also excellent. The poor uniform elongation can be enhanced by the design of the second phase dispersion. At the same time, the application has also been pursued from various viewpoints of engineering to take advantage of the ultrafine grained steels. Bridge, ship, construction, automobile and other high strength parts are being attempted for application.