A thermomechanical control process (TMCP) was performed for two types of medium-Mn martensitic steels to investigate the effect of microstructure control on fracture behavior. The TMCPed 5%Mn-0.1%C steel (mass%) exhibited a slight improvement in toughness, and the partially occurring intergranular fracture was suppressed completely owing to the elongated prior austenite grains. On the other hand, the TMCPed 10%Mn-0.1%C steel exhibited a remarkably enhanced low-temperature toughness with separation, in which a large number of sub-cracks were observed parallel to the impact direction on the main fracture surface. The separation induced by the elongated prior austenite grains reduced the triaxial stress at the crack tip, which in turn contributed to improved toughness. Furthermore, the ultra-refined microstructure obtained by the combination of the γ → ε → α’ martensitic transformation and TMCP without recrystallization improved the toughness of 10%Mn-0.1%C steel.
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