| dc.description.abstract | Low Mn and high Nb microalloyed steels display very interesting characteristics for the
application in pipelines for transportation of oil and gas in sour environments, where the
presence of hydrogen leads to premature failures of these structures. On the other hand, the
low carbon equivalent value, usually found in this type of steel, demands that the hot rolling
and accelerated cooling process must be carefully selected in order to maximize the effect of
Nb to promote an increase in mechanical strength and toughness, simultaneously, without
impairing the resistance to hydrogen cracking. To perform this evaluation, two alloys were
produced varying the Mn content, alloy 1 aiming at 0.70%Mn and alloy 2 aiming at 0.95%Mn,
both rolled into 32 mm gauge plates at the Gerdau Ouro Branco plate mill. Alloy 1 was used
to study the influence of austenite deformation, varying the reduction percentage in 55%, 65%
and 75%, on the microstructure and texture of the steel and how these affect the mechanical
properties. In this step, the characterization was performed using SEM/EBSD, evaluating the
texture, the level of grain misorientation, the volume fraction of the microconstituents and the
dislocation density. Through this characterization, an optimal processing value was found for
the hot rolling at 65% reduction during the finishing stage, where the deformation texture in the
final microstructure was the most intense. Alloy 2 was used to analyze the influence of
thermomechanical processing and accelerated cooling, via dilatometry tests with deformation,
on the formation of bainite and acicular ferrite, the latter being the target microstructure due to
its ability to improve toughness without compromising the tensile strength. The results showed
that bainite and acicular ferrite form simultaneously, not being possible to distinguish both in
separate fields in the CCT diagram. Additionally, the amount of austenite deformation did not
change, on average, in the volume fraction of acicular ferrite in the microstructure. The results
obtained in both alloys suggest that the most appropriate rolling schedule for this steel is the
one where the deformation of austenite results in the most intense texture without causing
partial recrystallization during the finishing stage of hot rolling. During the cooling process, the
rate should be the highest possible for the given equipment at thickness of 32 mm, so to obtain
a mixture of bainite with acicular ferrite and to avoid the formation of polygonal ferrite. | |
