抄録
Recently challenge of further grain refinement has been made, which aims to refine the ferrite grain size down to 1μm. It has been reported independently by two differing research groupsu1, 2) that a heavy deformation after accelerated cooling can successfully produce ultra fine grains as small as 1μm in diameter for relatively low cost steels. The aim of present study is to reveal how chemical composition of steels affects the grain refinement by such thermomechanical process. Compression experiments are carried out at various temperatures (down to 530°C) and at various cooling rates (up to 50K/s) before deformation. The interplay between chemical composition of steels and deformation condition on microstructures is discussed with focussing on CCT diagrams. The process window required for ferrite grain refinement down to 1μm must satisfy the following conditions; (1)steels with relatively low quench-hardenability, (2) accelerated cooling before deformation, (3) deformation at deeply supercooled austenite region, (4) heavy deformation.
