Design of carbide-free low-temperature ultra high strength bainitic steels

There are severe limitations to attaining submicrostructured steels by means of continuous cooling, the achievement of fine grain sizes is limited by the need to dissipate enthalpy during rapid transformation, so that the actual grain size obtained is more than an order of magnitude greater than can be obtained theoretically. Bainitic microstructures obtained by isothermal heat treatment at low temperatures, can overcome the recalescence limitations and at the same time provide a very fine microstructure (nanostructure) that leads to extraordinary mechanical properties. Steel grades for this purpose have been designed by means of phase transformation theory alone and their mechanical properties tested. The results obtained reveal that the microstructure is indeed extremely fine and exhibits high levels of strength accompanied by reasonable ductility and toughness, mainly due to the fine scale of the bainitic ferrite plates achieved. The influence that retained austenite has on the mechanical properties through the TRIP effect is also discussed.