AbstractRoom temperature tensile properties and low temperature toughness of quenched and doubly tempered 6 and 9%Mn steels were studied for a variety of tempering treatments, and the influence of the various phases present (α tempered martensite, α′ and ɛ martensites, and austenite) on the mechanical properties has been established. The intergranular brittleness of the 9%Mn steels is plain temper embrittlement due to P and Mn cosegregation to the former austenite grain boundaries, which can be suppressed by small Mo additions. ɛ martensite is not intrinsically embrittling and the mechanical properties are controlled by the total volume fraction of (γ + ɛ) islands dispersed in the tempered α lath martensitic matrix: the impact transition temperature is lowered and the strain hardening capacity σR–αY increases as (γ + ɛ) increases. These relationships are explained in terms of the morphology of the dispersed phases and of the strain induced phase transformations. The influence of Mo on the microstructures ...