Effects of granular bainite and polygonal ferrite on yield strength anisotropy in API X65 linepipe steel

Abstract

Linepipe steels fabricated using thermo-mechanically controlled processes exhibit microstructural inhomogeneity and a characteristic texture; therefore, they often lead to anisotropic yield strength properties. Although yield strength anisotropy is considered as a major guaranteed property, particularly in pipe forming processes, systematic verification of the dominant microstructural component affecting the yield strength is challenging owing to complicated microstructures and inhomogeneous distributions. In this study, the microstructures were controlled by varying the rolling reduction ratios and start rolling temperatures, specifically for polygonal ferrite (PF) and granular bainite (GB); the Taylor factor (TF) and orientation distribution function (ODF) of each microstructure were quantitatively analyzed by electron backscatter diffraction to study the effects of microstructural characteristics on the yield strength. The deviation of TF with tensile direction in GB was larger than that in PF; the intensity of {113}<110> components in ODF maps was greater in GB than in PF. The results indicated that GB induced more yield strength anisotropy than PF; thus, steels with a greater GB fraction exhibited greater yield strength anisotropy. This study can be used for lowering the yield strength anisotropy in linepipe steel plate design, with promising prospects for wider industrialization of high-strength linepipe steels.