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 I tm 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 gamma --> alpha transformation and alpha -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 zeta -region. Successful grain refinement to around 1 mum in ferrite grain size has been realized in laboratory scale by such heavy deformation in (1) undercooled gamma -region from 500 to 700 degreesC through inducing low temperature diffusional transformation; (2) alpha -region around 700 degreesC through inducing low temperature recrystallization; (3) spontaneous reverse transformation has also been demonstrated to result in very fine gamma grain size of around 1 mum by heavy deformation in alpha -region at around 550 degreesC just below A(C1) temperature in 9Ni steel. (C) 2001 Elsevier Science B.V. All rights reserved.