Heavy deformation and controlled cooling is one of the most effective effective ways for grain refinement in HSIA steels. In this paper, experimental specimens are prepared with a commercial linepipe steel grade X65, and the chemical compositions are (wt% ): Fe-0.094C-1.42Mn-0. 29Si-0. 045Nb-0. 008Ti. After heavy deformation with 80% reduction in non-recrystallization range of austenite, and at controlled cooling rates from 2K/s to 64K/s, the grain sizes vary from 9.55μm to 1.06μm.It is shown by the results that a linear relationship still remains between the yield strength and reciprocal of square root of ferrite grain size as grain size is refined to 1m.The ultra-fine grains might be dynam- ically formed through the deformation-induced transformation when reduction is large enough.The grain sizes decrease when the cooling rates increase.When the cooling rates increase over 10K/s, the grain sizes are rarely changed. The particles distribute on grain boundaries, sub-grain boundaries, deformed bands and within the ferrite grains.
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