Researches On Fracture Toughness Of X70 Steel With Different Organizational Characteristics Of F/B

Steel X70 is the most use of steel grade in the current international oil and gaspipelines, the strength and toughness of pipeline steel put foward more strict requirement with the conditions of transport to the development of high pressure and large diameter.The toughness of the material determines the carrying capacity and tendency to brittle fracture of the workpiece, it is one of the important parameters of engineering design. A series of stress-strain curves of multiphase microstructure steels with different parameters are obtained by combining numerical simulations and tensile tests; The relationship between multiphase microstructure parameters and fracture toughness JIc and KIc as well as charpy impact toughness CVN is studied, the effect and mechanism of phase parameters on the toughening of the ferrite / bainite complex phase steel were determined.(1) The CCT curves of tested steel were obtained by using a series of experiments.The characteristics of phase transformation of tested steel were observed systematically by using metallographic tests.The results showed that the stabilized ferrite-pearlite microstructures can be obtained at the cooling rates up to 2℃/s, while the stabilized acicular ferrite-granular bainite microstructures can be obtained in a cooling rate range of20~40℃/s. The impact toughness and fracture toughness with different tissues and temperature were obtained by using impact test and fracture test. We can get different tissue temperature and impact toughness, fracture toughness by impact test, fracture test.Impact toughness variation with the change of organization and temperature, J-integral and the crack expansion amount Δa relationship curve equation are obtained by contrast the main parameters of the mechanical properties and SEM experiment. Established the JIc, KIc value of different processes organizations test steel.(2) The stress-strain curves of different processes and temperatures were obtained by using tensile experiment and the stress-strain curves of typical single phase was obtained which was revised by using microhardness data. Unit cell models of F/B tested steel were built by using finite element software ANSYS. The stress-strain curves of ferrite/bainite multiphase steel with different phase parameters such as volume fraction, shape and distribution at different temperatures was simulated. The results show that the strength andtoughness of the tested steel with long acicular ferrite was optimum.(3) Basing on the simulations of stress-strain curves, fracture models of F/B multiphase steel was built and the JIc values of the testing steel with different phase parameters were simulated. Then, the corresponding stress intensity factor KIc values were calculated. By comparing the simulation and experiment results, the relationship between phase content, shape, distribution and fracture toughness JIc, KIc, charpy impact toughness CVN was obtained. Then the effect and mechanism of phase parameters on the toughening of F/B multiphase steel were determined.