Microstructure Characterization and Strengthening Mechanism Analysis of X100 Pipeline Steel

Abstract

The strengthening mechanism of X100 high steel grade pipeline steel, including grain boundary strengthening, solution strengthening, precipitation strengthening, dislocation strengthening, and texture strengthening, was analyzed by the technics of scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction (XRD), physicochemical phase analysis, and so on. The results showed that the tested steel had a mixed structure of granular bainite and lath bainite, the average effective grain size was refined to about 1 μm by severe hot plastic deformation, the dislocation density was as high as 1.74 × 1015/m2, and the second phase precipitation was mainly (Ti, Nb)(C, N) in submicron scale and nanoscale NbC. Through orientation distribution function (ODF) orientation analysis, it was found that the tested steel had obvious anisotropy and had a strong rolling direction (RD) texture along the rolling direction compared with the 30° direction: {115}<110> and {113}<110>. After the quantitative analysis of strengthening mechanisms, it was found that the highest strengthening increment was caused by high-density dislocations in bainite of about 268 MPa, while the lowest strengthening increment was induced by precipitation particles of about 31 MPa.