Simulation Study On Deformation Behavior Of Ferrite/Bainite Dual-phase Steel Under Cyclic Loading

With the exploitation environment of oil and gas field increasingly deteriorating,in order to improve the safety of pipeline operation,the design of pipeline changes from stress design to strain design,which puts forward higher requirements on the anti-deformation ability of pipeline steel.Ferrite/bainite dual-phase steel has become the preferred structure of high-strength pipeline steel.The ferrite/bainite dual-phase large-strain pipeline steel will be subjected to cyclic loading due to geological changes and seawater impact when in service,resulting in fatigue failure.Therefore,the study on the deformation behavior of two-phase steel under cyclic load is of great significance for the analysis of plastic damage and fatigue crack initiation of pipeline steel during service and for improving the safety of pipeline operation.In this paper,the mechanical properties of single-phase ferrite and bainite are obtained by experiments.Thus,the Chaboche kinematic hardening model can be established to define the constitutive relations between the two phases.And based on the actual microstructure,the geometric modeling is established.By changing the strain amplitude and strain ratio,the deformation behavior of the ferrite/bainite dual-phase steel under cyclic loading is studied by means of numerical simulation.The results show:With the increase of the strain amplitude,the plastic strain in the ferrite and bainite both increase,but the increment mechanism is different.The plastic strain in the ferrite phase increases in the manner of convergence and concentration,forming a strain failure zone.While the plastic strain in the bainite phase increases in the manner of the diffusion from locality to center.At the beginning of the cycle,with the increase of cycle number,the strain concentration factor decreases,but the stress concentration factor increases in the ferrite phase.The strain concentration factor increases,but the stress concentration factor decreases in the bainite phase.The unbalance of the stress and strain distribution between the two phases decreases,and the strain distribution between the two phases tends to be stable earlier than the stress distribution.In the steady state,with the increase of strain amplitude,the distribution of stress and strain becomes more and more unbalanced,the strain distribution has a bias to the ferrite phase,and the stress distribution has a bias to the bainite phase.With the increase of the strain ratio,the plastic strain in the ferrite and bainite both increase,but the increment mechanism is different.The plastic strain in the ferrite phase is increasing on local high strain zones and drives the surrounding ferrite strain to increase,and the failure zone is not formed.In the bainite phase,the plastic strain increases in the way of the plastic strain at the local high strain zones increasing continuously.At the beginning of the cycle,with the increase of cycle number,the strain concentration factor and the stress concentration factor both increase in the ferrite phase.The strain concentration factor and the stress concentration factor both decrease in the bainite phase.The unbalance of strain distribution increases,while the unbalance of stress distribution decreases.The strain distribution of the two phases tends to be stable earlier than the stress distribution.In the steady state,with the increase of strain ratio,the unbalance of strain distribution becomes more obvious and more partial to the ferrite phase,while the stress distribution tends to be balanced.The stress distribution increases in the ferrite phase and decreases in the bainite phase,but the bainite phase is always playing a dominant role.