Effect of austenite grain size in Fe-Mn alloys on ε martensitic transformation and their mechanical properties

The effects of austenite (γ) grain size on the transformation to ε martensite (ε) and mechanical properties have been investigated in high manganese steels by means of calorimetric analysis, optical microscopy, scanning electron microscopy and X-ray diffraction analysis. Austenite grain size of steels was controlled to be 100 μm or 10 μm in the mean grain size by the recrystallization method. With grain refining, the starting temperature of γ→ε transformation tends to be lowered and the formation of ε is suppressed in all of high manganese steels which undergo γ→ε transformation. For 15∼31 mass%Mn steels, both of elongation and tensile strength are markedly enlarged by grain refining. Fracture modes of these high manganese steels are largely dependent on microstructures and deformation behavior. Effect of γ grain size on the fracture modes was discussed in detail for 22 mass%Mn and 27 mass%Mn steels. When the size of γ grains is as large as 100 μm, a significant stress concentration takes place at the intersections of ε plates in a 22 mass%Mn steel, while does at grain boundaries on which deformation induced ε plates impinge in a 27 mass%Mn steel. Such a stress concentration at ε plates or grain boundaries causes the formation of microcracks and this leads to the onset of quasi-cleavage fractures. When the size of γ grains is refined to 10 μm, however, quasi-cleavage fractures are suppressed in the both steels. Refining of γ grains produces favorable effects; 1) change in the morphology of athermal ε, 2) decrease in the stress concentration at grain boundaries, 3) uniform dispersion of strain. For the suppression of quasi-cleavage fractures by γ grain refining, the effect 1) is most contributive in a 22 mass%Mn steel, while in a 27 mass%Mn steel, only the effects of 2) and 3) are workable.