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To explore the mechanical behavior and microstructure evolution of nanostructured bainite under deformation at high strain rates (> 10 3 s − 1 ), split Hopkinson pressure bar experiments were conducted. It was found that nanometer-sized twinning occurred in both blocky austenite and film austenite. In the former case, the intersections of different twin variants brought about strain-induced α’-martensite, while in the latter case, the lamellar twinned structures were occasionally accompanied by a small amount of ε martensite. It is concluded that twinning occurs first in blocky austenite, enhancing the initial work-hardening; subsequent twinning takes place in film austenite, promoting further straining. [ABSTRACT FROM AUTHOR]