Figure 3a shows the typical lath structure morphology in bainite ferrite, and the lath width was about 300 nm~1000 nm. To directly observe the dislocation morphology, the microstructure in X100 pipeline steel was characterized by TEM, as shown in Figure 3. The TEM samples were first mechanically ground less than 50 μm and punched into a diameter of 3 mm, and then prepared with 10% perchloric acid alcohol and 90% alcohol solution for twin-jet electrolytic polishing with the current of 50–80 mA and a temperature of −20 ☌. Transmission electron microscopy (TEM: Tecnai G2F20) equipped with EDS was used to characterize the microstructure, precipitation particles, and large number of dislocations. The effective grain size was evaluated by the linear Intercept method on the high-angle grain boundary over 15°. The accelerating voltage of 10 kV, scanning step of 0.25 μm, and scanning area of 100 μm × 100 μm were selected. The specimen was mechanically ground, polished, and electrolytically polished in 10% perchlorate alcohol for electron backscattered diffraction (EBSD) testament (Oxford Nordlys F+). The microstructure evolution was observed by FEI Quanta 650 field emission scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). After being etched with a 4% nitric acid alcohol solution, the polished sample was cleaned with water and dried again. Before microstructure observation, the samples were mechanically sequentially ground by 150#~1000# sandpapers and then were further polished by the polishing machine subsequent with blow-drying. The tested steel was made of commercial X100 pipeline steel, has a specified thickness of 15 mm, and is chemically composed as given in Table 1 (mass fraction, weight%). The suggested formula has also passed muster with the UK Department of Energy’s acceptance standards. In order to forecast the ultimate strength of x-joints with FRP, therefore, a theoretical formula based on the yield body model is provided after the parameter investigation. Fiber-reinforced polymer is used by Hossein Nassiraei to increase the static strength of tubular x-joints. There is no appropriate model or empirical method to estimate the strength increase brought on by texture strengthening hence, there has been some debate on the increase in X100 pipeline steel’s strength caused by texture. These results are of great significance to the development and optimization of X100 pipeline steel. 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: texture is decreased. 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 × 10 15/m 2, and the second phase precipitation was mainly (Ti, Nb)(C, N) in submicron scale and nanoscale NbC. 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.
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