急冷途中における大ひずみ加工による低炭素鋼フェライト結晶粒の超細粒化

Abstract

It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperature below the nonrecrystallization temperature (Tnr). The strain retained in the austenite increases the number of ferrite nuclei. In present study a C-Mn steel is heavily deformed by compression at temperature below the determined Tnr for this steel during accelerated cooling. Compression experiments are carried out at various cooling rates before deformation and temperatures. The ferrite grain size and the ferrite/bainite phase fraction are extensively examined. It is observed that heavy deformation during accelerated cooling widens the range of transformation temperature of austenite to ferrite. If cooling rate before deformation is set to be high (50 K/s) and deformation temperature decreases down to 530°C, the attainable fraction of ferrite increases instead of decrease of bainite. Ferrite could be a predominate phase transformed from severely deformed austenite at significantly low temperature (down to 530°C), in which, unless deformed, bainite commonly forms from austenite. Enabling the transformation to ferrite at very low temperature leads to a progressive reduction of the ferrite grain size down to 1 μm. Possible mechanism of the strain assisted low temperature transformation to ferrite is discussed.