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One of the key goals in the development of steels has been to refine the ferrite grain size, as this leads to an increase in both strength and toughness. The optimum reduction in ferrite grain size is generally achieved through heavy controlled rolling combined with accelerated cooling. For a simple steel composition in plate rolling this can reduce the ferrite grain size from 10μm for hot rolling and air cooling to 5μm for controlled rolling and water cooling, increasing the yield strength by about 80 MPa. In previous studies related to the austenite to ferrite transformation of controlled rolled steels it was shown that there is a limiting ferrite grain size of approximately 5μm regardless of the level of retained strain introduced into the austenite [1]. This is at least partly due to ferrite coarsening during transformation [2]. If, however, a 1μm grain size-here called ultrafine ferrite (UFF)-could be produced and retained then this would …