Characteristics of martensite transformed from deformed austenite with various states of ultrahigh strength 300 M steel

300 M steel was isothermally compressed on a Gleeble 1500 simulator at a deformation temperature of 1173 K, a strain rate of 0.1 s⁻¹ and strains ranging from 0.11 to 1.20. The morphologies and structure characteristics of martensite transformed from deformed austenite during water quenching were observed and analysed via scanning electron microscope and transmission electron microscope. Subsequently, effects of prior austenite state on martensitic transformation behavior of 300 M steel were analysed. The experimental results showed that predominant lath martensite and complemental twinned martensite and retained austenite coexisted in deformation-quenched 300 M steel at various strains. The volume fraction of lath martensite and packets of lath martensite decreased with the increasing of strain, and the length and width of the blocks in lath martensite concurrently decreased. Prior austenite with the state of severely work-hardening or just completed dynamic recrystallization would hamper the nucleation and subsequent growth of lath martensite. Additionally, as for austenite with the state of work-hardening, the morphologies and width of laths changed little with the increasing of strain, whereas dislocations in the interior of laths dramatically increased. However, as for austenite with the state of dynamic recrystallization and subsequent growth, the width of laths obviously increased with the increasing of austenite grain size.