A new route to fabricate ultrafine grained
(UFG) ferritic steel sheets without severe plastic deformation
is proposed in this article. A low-carbon steel sheet
with a duplex microstructure composed of ferrite and
martensite was cold-rolled to a reduction of 91% in
thickness, and then annealed at 620–700 degree C. The microstructure
obtained through the process with annealing
temperatures below 700 degree C was the UFG ferrite including
fine cementite particles homogenously dispersed. The grain
size of ferrite matrix changed from 0.49 to 1.0 μm
depending on the annealing temperature. Dynamic tensile
properties of the produced UFG steels were investigated.
The obtained UFG ferrite–cementite steels without martensite
phase showed high strain rate sensitivity in flow
stress. The UFG ferritic steels are expected to have high
potential to absorb crash energy when applied to automobile
body.
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