Microstructural and crystallographic characterization of 455 MPa sour service plates with low Mn high temperature processed alloy concept and its association with drop weight tear test performance

Abstract

The aim of this study was to analyze the mechanical properties of a X65 low Mn and high Nb steel for sour service application, particularly the drop-weight-tear test (DWTT) performance in order to establish correlation with the microstructural and crystallographic features of a steel with complex microstructure. Samples were analyzed at 55%, 65% and 75% of deformation during the finishing stage of rolling. The characterization of the samples involved electron backscatter diffraction (EBSD) technique for the determination and volume fraction quantification of the microconstituents and crystallographic texture analyses. The results showed that the plate with 55% of deformation displayed the lowest level of strength and the lowest DWTT shear area, while the plate with 65% of deformation exhibited the highest strength and DWTT toughness levels. EBSD results indicated that the plate produced with 65% of deformation had the largest dislocation density among all three samples and showed the highest intensity of the texture components concentrated along the α-fiber, particularly in the vicinity of the {113}<110> and {112}<110> components, responsible for activating ductile fracture, while samples 55% and 75% had a broader spread towards {001}<110>, and an overall higher intensity of the rotated cube components that favor cleavage fracture. For the plate with 75% of deformation, the mean flow stress (MFS) result suggested that partial recrystallization occurred during the finishing stage of hot rolling which was also proved by EBSD.These findings have proven that low Mn steels processed at high temperatures have a crucial dependency on Nb for the development of deformation textures that will help sustain high levels of fracture toughness, particularly when dealing with heavy plates for offshore applications. It has been observed that this steel concept is inclined to experience loss of toughness due to the softening of austenite during the finishing stage of rolling, even though the temperatures were below the no-recrystallization temperature, and a proper rolling schedule must be selected to mitigate this effect.The overall results showed that a rolling scheduled aiming at increasing the volume fraction of acicular ferrite and reducing the polygonal ferrite together with a higher level of strain and crystallographic texture consisting of high intensity deformation components, will result in a better combination of strength and toughness.