The attainment of ever finer ferrite grain structures in low carbon, low alloy steels is of interest because significantly higher yield strengths and lower ductile/brittle transition temperatures are predicted at ultra-fine grain sizes. The limit to which ferrie can be refined by conventional tehmomechanical processing of austentie is about 4 mum. In this paper iti s demonstrated that hot rolling of austenite of a Nb-microalloyed steel that has been prerefined to a grain size less than 5 mum, combined with accelerated cooling, can produce ferrite having a sub-mum grain size in a surface layer and a grain size of 1.5 mum in the centre of 3 mm thick plate. It is further shown that cold rolling and recrystallisation of ferrite pre-refined by the hot rolling process can produce 1 mm thick strip having a sub-mum grain size throughout its thickness. Diamond pyramid hardness is shown to increase linearly with increase in the inverse square root of grain size, reaching 300 HV(1) at a grain size of approxiamtely 0.4 mum, but with a lower slope than expected from the Petch relationship for yield strength, indicating a low work hardening capacity at ultra-fine grain sizes.
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