Understanding mechanical property anisotropy in high strength niobium-microalloyed linepipe steels

Abstract

Thermo-mechanical processing of linepipe steels may result in anisotropy in mechanical properties, notably, yield strength and toughness, depending on the chemical composition and microstructure. These relationships are exceptionally important in spiral-welded pipe. Given the current interest in the development of high strength linepipe steels, we have examined in detail the mechanical property anisotropy phenomena using a combination of electron microscopy and crystallographic texture analysis in API L485 (X70) and API L555 (X80) steels. In this presentation, we describe the results of a study that has enhanced our understanding of the relationship between the microstructure and texture with anisotropy in mechanical properties.The microstructure of both X70 and X80 microalloyed linepipe steels as imaged via electron microscopy was similar and did not exhibit any significant anisotropy in the plane of the sheet. The microstructural constituents, polygonal ferrite and acicular ferrite (bainitic ferrite: ∼5–10% in X70 and ∼15–20% in X80) were also distributed uniformly throughout the volume of the specimens (0°, 45°, and 90° with respect to the rolling direction).The anisotropy in yield strength and Charpy impact toughness of X70-Nb and X80-Nb–Mo microalloyed linepipe steels was examined by using orientation distribution function analysis. The texture fibers of X70 and X80 microalloyed steels were similar but with significant differences in the intensity. Deformation textures mainly consisted of α-fiber (〈110〉//RD), γ-fiber (〈111〉//RD), and ε-fiber (〈110〉//TD). The major components of texture observed were {112}〈110〉, {332}〈113〉, {110}〈001〉, and {001}〈110〉 orientations. The observations suggested that the RD fiber centered at {112}〈110〉, {113}〈110〉, and {223}〈110〉 was responsible for the anisotropy in yield strength. The intensity of these texture components was higher in both X70 and X80 microalloyed steels in the rolling directions (RD) as compared to the 45° to the RD.