Evaluation of the fracture toughness of X70 pipeline steel with ferrite-bainite microstructure

摘要

The quasi-static fracture toughness K_(IC) and J_(0.2BL)(J_(IC))＞ Charpy V-notch impact toughness CVN of ferrite-bainite (F/B) X70 pipeline steels with different phase content and morphologies were studied by experiments and numerical simulations at content temperature of -10 ℃. The stress-strain curves were obtained by experiments and unit cell model simulation. The results show that fracture toughness is strongly affected by bainite content. The fracture toughness increases with decreasing the bainite volume fracture f_B and increasing aspect ratio R_a and neighboring factor N_f. Meantime, the aspect ratio R_a is more effective in improving the fracture toughness of tested steels compared with the neighboring factor N_f. For F/B X70 pipeline steel, two quantitative relationships have been derived, which correlates J_(0.2BL)(J_(IC)) and CVN with bainite content and morphologies in equations. It means that J_(0.2BL)(J_(IC)) can be calculated when CVN, f_B, R_a, and N_f of several typical F/B X70 pipeline steels are obtained. Although the quantification is specific to F/B X70 pipeline steels, the approach and model developed in this work can be applied to other metal structure materials.