New concepts for ultra refinement of grain size in Super Metal Project

A national project called “Ferrous Super Metal Project” was started in 1997 pursuing ultra-fine grained microstructure below 1 μm in grain size for plain C–Si–Mn steel. Ultimately heavy deformation with strain over 0.7 per pass has been employed as a key technique to reach the goal, and has been revealed in laboratory scale studies: (a) to enhance diffusional γ→α transformation and α-recrystallization at a temperature range much lower than the case for the conventional TMCP; (b) to enhance dynamic transformation and recrystallization which would bring about continual deformation on already refined microstructure; (c) to induce a large temperature increase accompanying adiabatic deformation heating which would result in spontaneous reverse transformation when applied in α-region. Successful grain refinement to around 1 μm in ferrite grain size has been realized in laboratory scale by such heavy deformation in (1) undercooled γ-region from 500 to 700°C through inducing low temperature diffusional transformation; (2) α-region around 700°C through inducing low temperature recrystallization; (3) spontaneous reverse transformation has also been demonstrated to result in very fine γ grain size of around 1 μm by heavy deformation in α-region at around 550°C just below AC1 temperature in 9Ni steel.