Simulation Study Of The Effect Of Ferrite And Bainite On Tensile Properties In Large Strain Pipeline

The characteristic of pipeline steel with excellent deformation is consisted by dual phase.It is designed by ultra-low carbon multiple micro-alloying and thermo-mechanical controlled rolling process.We can obtain bainite-ferrite dual microstructures or bainite-martensite/austenite microstructures.The microstructure of dual-phase steels was mainly composed with bainite phase within ferrite matrix.It's difficult to accurately capture the path of microcrack propagation in time and capture the stress-strain distribution during tensile test through experiment,it's due to the differences in mechanical properties between ferrite and bainite,inhom ogenerous microstructure,even the difference oritation of each grains,and the quickly propagation of microcrack.To improve the accuracy of numerical simulation about dual phase steel,A 2D geometric models was established by the software CAD in this paper on the basis of actual SEM microstructure,the isotropic and multilinear kinematic hardening model were also established according the result of tensile test,which display the nonlinear behavior of ferrite and bainite.Analyzing the elastic-plastic behavior of dual-phase steels during tensile test under the boundary constraints and loading condition,discussing the nucleation mechanism of microvoids and fracture process in dual phase steel.Combination the in situ tensile test and FEM simulation of dual phase steel,the tested stress-strain curves is assistant with the simulated,which shows that the finite element model established in this paper can be effectively conduct numerical simulation on dual-phase steel.Inhomogeneous deformation on a microscoip scale between the soft phase of ferrite and the hard phase of bainite resulting in the localized strain hardening behavior.During the uniaxial tensile,ferrite phase enhanced the ductility of dual-phase steel,it always firstly deformed under lower stress than the bainite delayed deformation,because of the bainite bears most of the stress,the nucleation of microvoids usually at the stress concentration in the dual phase steel,the microcracks at the F-B interface always expanted by tearing up along the edge of the void or directly through the ferrite grain boundaries.As the increases of the island bainite content,the number of voids formed by dcohesion of F-B interface increased in the dual phase steel,some voids nucleated bycracking of fine bainite.Increasing the quenching temperature,the bulk Bainite transformed into plate and fibrous in turn,the microvoids nucleation mechanism transformed form the F-B interface decohesion into the bainite multiple fractures;The study also indicates that the hardness and the strength of the testing steel is increasing with the increasing of the volume fraction of bainite while the elongation and the work hardening exponent is decreasing with the increasing of the volume fraction of bainite.Therefore,the plastic of the testing steel becomes worse.