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Fe-12Mn steel has the unique and interesting property that, when quenched to α′ martensite from the austenite phase, it forms an ultrafine grained microstructure that has exceptional resistance to cleavage fracture at cryogenic temperatures. The present research was undertaken to complete the characterization of this microstructure and understand why it forms and why it has such exceptional crack resistance. A combination of EBSD and TEM analysis shows that the microstructure is a dislocated lath martensite in which the laths have the Kurdjumov-Sachs relation to the parent austenite. As in other dislocated lath martensites, the prior austenite grains are divided into packets, each of which contains the 6 (of 24) KS variants that mate with the same { 111 } γ plane. Uniquely, however, the packets are stacks of thin plates that contain all 6 KS variants. The variants within the plate are organized into 3 pair of twinned KS variants that are elongated along their { 112 } α twin planes, rotated 120° from one another, and interwoven to form the 6-variant plate. The ultrafine grains are the laths themselves; twin boundaries between KS variants are known to provide strong barriers to cleavage crack propagation. This unusual microstructure is apparently due to the transformation path; austenite transforms to the hexagonal e martensite phase before its ultimate transformation to α′, and the 6-variant plate is the preferred element to minimize elastic energy in a microstructure created by a dominant γ → e → α ′ transformation path.

The microstructural modifications occurring during annealing treatment of an Fe–0.35 C–3.5 Mn–5.8 Al ferrite-based lightweight steel and its effects on the tensile properties were investigated with respect to (α + γ) duplex microstructures. Steels annealed above the dissolution finishing temperature of κ-carbides (795 °C) were basically composed of ferrite band and austenite band in a layered structure. As the annealing temperature was increased the tensile strength increased, while the yield strength and elongation decreased. This could be explained by a decrease in the mechanical as well as thermal stability of austenite with increasing size and austenite volume fraction. In the 980 °C annealed steel in particular, whose mechanical stability due to austenite was lowest, cracks were readily formed at ferrite/austenite (or martensite) interfaces with little deformation, thereby leading to the least tensile elongation. In order to obtain the best combination of strength and ductility the formation of austenite having an appropriate mechanical stability was essentially needed, and could be achieved when 22–24 vol.% fine austenite was homogeneously distributed in the ferrite matrix, as in the 830 °C or 880 °C annealed steels.