Caracterização avançada de aços microligados de baixo Mn e alto Nb aplicados em tubos sour service visando aumento do desempenho de tenacidade

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.