Effects Of Pre-deformation And Annealing On Mechanical Properties And Fatigue Behavior Of Complex Phase Steel CP800 And Its Mechanism

Structural parts of automobile chassis made by complex phase steel CP800,such as torsion beams and suspension arms,will suffer pre-tension deformation,pre-bending deformation and annealing treatment,which are during the manufacture.These treatments will influence the fatigue life of the structural parts during service process.However,at present,the mechanical properties and fatigue behavior of complex phase steel under the effect of such conditions have not been studied.On the one hand,part of this work was to experimentally explore the fatigue properties under the conditions of pre-tension strain and annealing for CP800.Meanwhile,the elongation of grains caused by pre-tension strain and the increase content of bainite and martensite caused by annealing were observed by using optical microscopy and scanning electron microscopy,respectively.An effective pre-tension strain range was found to increase the yield and tensile strengths by dislocation strengthening.Besides,the pre-tension strain presents a negative effect on fatigue life,while the fatigue life can be increased by annealing treatment.It is confirmed that the combined effect of the large pre-tension strain and annealing treatment leads to cyclic hardening fracture.Furthermore,two kinds of failure modes caused by surface defects and inclusions were revealed by using SEM.Eventually,the fatigue life can be well predicted with a model based on strain energy method.On the other hand,part of this work studied the effect of pre-bending on the mechanical properties and fatigue behavior of CP800.By theoretical analysis for the pre-bending process,it was found that the distribution of residual stress of pre-bending specimens changed to stress distribution of tension-compression-tension-compression,which means that the inside surface(compression region)is tension stress and outside surface(tension region)is compression stress.The special stress distribution leads to a decrease of yield stress of pre-bending specimens.Meanwhile,due to the hardening caused by cold deformation,the elongation of material decreased and the tensile strength increased.Through theoretical analysis and observation of surface strain field of pre-bending specimens during tensile test,it was found that the inside surface produced greater plastic strain and was broken earlier than the outside surface due to the existence of tensile residual stress.Pre-bending has a negative effect on the fatigue life of CP800,and the effect is more obvious with the decrease of mean stress.Under the same stress condition,the range of total strain amplitude of pre-bending specimens is lower than that of AR sample.It means that CP800 hardened after pre-bending deformation,and its resistance to macroscopic cyclic deformation was improved,which is different with the behavior under monotonous deformation.Besides,S-N formulas of as received and pre-bending specimens were established based on Basquin equation and the accuracy of formulas were verified by experiment and simulation results.In general,this study provides a research basis for the design and prediction of service life of structural parts in industrial applications.