The extent and mechanism of nanostructure formation during cold rolling and aging of lath martensite in alloy steel

Transmission electron microscopy was used to study microstructural evolutions during cold rolling and isothermal aging of an Fe–10Ni–7Mn (wt.%) maraging steel at 753 K. The steel shows lath martensite after homogenizing treatment which evolved by cold rolling to a typical deformed structure, consisting of lamellar dislocation boundaries and shear bands. Ultrafine-grained regions were found frequently in the isothermally aged steels. The process of ultrafine grain formation during isothermal aging is attributed to extended recovery to be correlated with precipitate coarsening reactions. At later stages of aging, recrystallization-assisted discontinuous coarsening of grain boundary precipitates was identified at lamellar boundaries, giving rise to a banded structure consisting of ferrite bands and precipitation hardened laths. It was demonstrated that partially-nanostructured multi-phase steel is fabricated by cold rolling and aging of lath martensite.