By means of phase transformation theory for the bainite reaction, high-carbon, silicon-rich steels have been developed1, 2) with bainite transformation temperatures as low as T= Tm ¼ $0: 25, where Tm is the absolute melting temperature. The resulting microstructure consists of aggregates of fine plates of bainitic ferrite separated by untransformed carbonenriched austenite. The aggregates of plates are called sheaves, whereas the plates within each sheaf are defined as subunits; the subunits within the same sheaf share a common crystallographic orientation. Thus, two retained austenite morphologies exist: thin films between ferritic subunits and blocks separating the sheaves. Carbide precipitation is avoided by the judicious use of silicon as an alloying element.

This mixed microstructure presents an ideal balance of mechanical properties from many points of view. Due to the absence of fine carbides, the steels have a high resistance to cleavage fracture and void formation. Then, there is the possibility of improving simultaneously the strength and toughness because of the ultrafine grain size (20–40 nm) of the bainitic ferrite plates and of further enhancing the ductility by a transformation induced plasticity effect (TRIP) 3) that increases the strain-hardening rate. It is well established that the strain induced transformation of austenite to martensite takes place between the MS temperature (martensite start temperature), and the Md temperature, above which the austenite becomes completely stable. Therefore, there is a temperature between MS and Md at which the strain induced transformation is suppressed moderately and the resultant strain hardenability is held in a large strain range, leading to maximum benefit of the TRIP effect. At low test temperatures or in alloys which have austenite of low mechanical stability, the strain-induced transformation occurs in early stages of deformation. As a result, there is little benefit of the strain hardening related to deterring plastic instability or necking in the later stages of