The influence of rolling practice on notch toughness and texture development in high-strength linepipe

Abstract

The mechanical properties and notch toughnesses of an X80 linepipe steel were determined for various test directions in the plane of sheet that had been finish rolled in the γ and in the intercritical (α+γ) regions. The anisotropies of yield strength (YS) and of impact energy are correlated to the presence of various texture components, as detected by the use of an orientation distribution function (ODF) analysis. The final microstructures were similar and consisted of polygonal and acicular ferrite. The textures were also similar; however, after rolling in the (α+γ) region, the intensity of the texture was significantly higher. These textures were mainly comprised of two fibers, the rolling direction (RD), 〈110〉//RD, and the normal direction (ND), 〈111〉//ND, fibers. The observations show that the RD fiber centered at {112}〈110〉 and the {110}〈001〉 orientation were responsible for the YS anisotropy. The relationships between notch toughness and texture were considered for the brittle or cleavage (−196 °C), mixed brittle-ductile (−60 °C), and ductile (room temperature (RT)) modes of fracture. This work shows that the anisotropy of impact energy associated with ductile fracture at the higher temperatures is caused by the {112}〈110〉 component, and that the {001}〈110〉 and {110}〈001〉 components (if present) are responsible for the anisotropy of the impact energy associated with cleavage at low temperatures. The lack of anisotropy of the impact energy observed at −196 °C and the increase in toughness at higher temperatures are interpreted in terms of the volume fractions of textured grains present in the sheet and the intensities of specific texture components.