A construction steel (shipbuilding strip) obtained by Advanced Thermomechanical Controlled
Rolling Processes presents a room temperature banded ferrite-pearlite microstructure, and when
superplastically deformed at 800°C with a strain rate of 5.85x10-5 s-1, the bands disappear as there
is grain boundary sliding and grain cluster rotation. Nevertheless, the superplastic deformation
does not imply a decrease in mechanical properties, as room temperature tests with strain rates of
1.46x10-3 s
-1 with the steel previously deformed in superplastic conditions (until a 110% of
straining) result in similar mechanical data. If the steel is deformed at 750°C with low strain
rates, cooling results in a microstructure formed only by ferrite and carbides (the pearlitic phase
disappears). This behavior may be explained, from a thermodynamical point of view, by the
effect of negative hydrostatic pressure during the tensile test and the pronounced ferrite- and
carbide-former capacity of Ti and Nb microalloying elements.
The samples, tensile tested, in both the hot rolled raw state and superplastically deformed and
then room temperature tested, show in the fracture surface SEM analysis almost identical
features: decohesions surrounding MnS and (C,N)(Ti,Nb) precipitates and between ferrite and
pearlite grains, as well as bedding.
