Evolution of Ferrite Morphology and Ferrite-Ferrite Grain Boundary Characteristics with Cooling Rate in a Continuously Cooled 0.09C-0.08V-0.02N Microalloyed Steel

Abstract Continuously cooled HSLA steels produce mixed ferritic and bainitic microstructures with multiple constituents present within the same prior-austenite grain. Conventional methods of distinguishing these constituents have relied on size and morphology characteristics, which have inherent subjectivity in identifying constituents. In this study, grain boundary misorientation distributions were utilized to more objectively identify different quasi-polygonal ferritic, acicular ferritic, bainitic, and martensitic microstructures within a continuously cooled V-microalloyed HSLA steel. It was determined that the slower-cooled morphology of acicular ferrite commonly observed in line-pipe steels produced approximately equal 55 and 60 deg peaks in the misorientation distribution, while the more lenticular faster-cooled acicular ferrite produced at a cooling rate of 25 °C/s had a stronger peak at the 60 deg misorientation. The P2/P1 ratio, which quantified the ratio of these two misorientation peaks, was employed to differentiate these two acicular ferrite morphologies. These constituents also exhibited different axis/angle misorientation distributions in the high-angle grain boundary misorientation regime than both quasi-polygonal ferrite and martensitic/bainitic microstructures.