Low-temperature impact fracture failure mechanism of Q490DRL2 pressure vessel steel plate

To address the issue of substandard low-temperature impact toughness at the 1/2 thickness position in Q490 DRL2 pressure vessel steel, this study investigates the fracture failure mechanism under low-temperature impact loading, providing theoretical insights for improving the low-temperature impact performance of pressure vessel steels. Oscillatory impact testing was conducted to measure the impact energy and force-displacement curves of the steel at both the 1/4 and 1/2 thickness positions at-50 ℃. Field-emission scanning electron microscopy（FESEM） and electron backscatter diffraction（EBSD） were employed to analyze the fracture morphology, inclusion characteristics, microstructure, and grain size distribution. Additionally, physicochemical phase analysis and small-angle X-ray scattering（SAXS） were utilized to quantitatively assess the composition and particle size distribution of precipitates. Finite element method（FEM） simulations were performed to analyze the stress field distribution around inclusions during impact. The results indicate that the microstructure at both the 1/4 and 1/2 thickness positions consists of tempered sorbite, with average grain sizes of 5. 35 μm and 4. 36 μm, respectively, and dislocation densities of 7. 22×10～8 m^-2 and 7. 88×10～8 m^-2, respectively. The primary precipitates in the steel are M₃C（alloy cementite） and MC（Nb, Ti, V-based） carbides, with no significant differences in alloy element content or particle size distribution observed between the two thickness regions. Further analysis reveals that the primary cause of the unsatisfactory low-temperature impact toughness at the 1/2 thickness position is the presence of a higher density of silicon-rich, triangular-shaped inclusions. Under impact loading, when crack propagation encounters these inclusions, significant stress concentration occurs in their vicinity, leading to rapid fracture failure of the steel.