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Understanding the main relationships between the microstructure parameters controlling the strength and ductility of low temperature bainitic microstructures is of considerable importance for further development of these grades. Although the microstructure essentially consists of solely two phases, bainitic ferrite and retained austenite, the complexity of the different microstructural characteristics, natural consequence of its unique transformation mechanisms, might not provide with one unique answer but a set of several parameters interdependent among them. This paper will deal with some of these relationships microstructure-properties, strength and ductility, with special emphasis in the mechanical stability (TRIP effect) of retained austenite.

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Recently, valuable combinations of mechanical properties with strength of 1.9 GPa accompanied by very decent ductility of 19 % and toughness of 31 J, have been achieved in a set of nanostructured bainitic steels. However, it is necessary to elucidate the significance of various microstructural features responsible of that extraordinary mechanical response in more detail.Thus, using two steels, with different Mn, Ni and V contents, and changing the nanostructured bainite isothermal transformation temperatures (200-300 ºC), has led to a plethora of subtle and essential microstructural variations, necessary to explain how the mechanical response of nanostructured bainite is attained.

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