Deformation And Fracture Behavior Of Hot-rolled DP600 Automotive Steel

Dual phase steel has been widely used in automotive parts(such as wheel,bumper,suspension system and reinforcement,etc.)due to its high strength,high collision absorption capacity and excellent formability.Compared with cold-rolled dual-phase,hot rolled double phase steel has become the focus of research at home and abroad,because of its advantages of low cost,simple production process,large scale of production and good comprehensive performance.The deformation and fracture behavior and mechanism of hot-rolled dual-phase steel subjected to applied stress under different process conditions were studied.The results can provide theoretical basis and experimental data for the production and application of high-strength hot-rolled dual-phase steel.The hot-rolled DP600 automotive steel was chosen as the research object in this dissertation.The microstructure,static tensile mechanical properties and fatigue deformation and fracture behavior of experimental steels in different stress ratios and strain amplitudes were investigated under three different process conditions with aid of the universal tensile tester,Optical microscope(OM),Field emission scanning electron microscope(SEM)and Electron backscattered diffraction(EBSD),fatigue testing machine,and Transmission electron microscope(TEM).The initiation behavior of cracks and the evolution of dislocation structures,etc.under static tensile and fatigue deformation conditions were analyzed,and the main conclusions are as follows:(1)The ferrite and martensite structure were both obtained in the experimental steel under all three process conditions,but the ferrite morphology,grain size and martensite content were different.There are also a small number of inclusions in the experimental steel,mainly including the triangular shape of Si C,the circular Al2O3 and the composite inclusion of Al2O3 and Mn S.The research shows that best mechanical properties of hot-rolled DP600 steel is first process,the yield strength is 394.9 MPa,tensile strength reached 641.6 MPa,extension rate was 25.97%,the plastic strain hardening index was 0.154.(2)when the frequency and strain amplitude are constant,the stress ratio R=-1 and R=-0.1 both appear cyclic softening-hardness-softening phenomenon,but the fatigue life of the experimental steel with R=-0.1(N=89560)was 2.6 times of the experimental steel wirh R=-1(N=34489);when frequency and stress ratio are constant,the fatigue life(N=34489)of the experimental steel(?=0.3%)is 10.4 times the fatigue life(N=3313)of the experimental steel(?=0.6%).The fatigue life is the longest of the steel for the experimental parameter of ?=0.3%,R=-0.1,f=5 Hz;When the same loading condition are the same as ?=0.3%,R=-1,f=5 Hz,the experimental steel exhibits cyclic softening – hardening-softening-fracture phenomenon,and The experimental steel heat treatment at 810 °C for the second process and third process are cyclic hardening-softening-fracture phenomenon,and fatigue life is reduced.(3)Microvoids were initially formed at the interface between the inclusion Al2O3(Al2O3/Mn S)and matrix,ferrite/martensite interface and ferrite boundaries with an increasing of static stretching strain when the experimental steel was under the condition of static stretching deformation.The formation of microvoids is related to the movement and proliferation of dislocation in the process of deformation.And stress concentrated when dislocation accumulated at the interface between inclusion and matrix,ferrite-martensite interface and ferrite grain boundary,which resulted in the generation of a large number of microvoids.But under the fatigue loading of experimental steel,microcracks are not only produced on the surface of the experimental steel,but also at the interface between the inclusions and the matrix,at the interface between ferrite-martensite,and at the ferrite grain boundaries;The cracks are mainly along the the ferrite grain boundary near martensite islands,the interface ferrite-martensite and ferrite grain boundary.Fatigue fracture analysis results shows that the fatigue fracture is clearly divided into three zones: fracture source,crack propagation extent and fracture region.Crack initiation appeared at the surface of experimental steel,but there are a lot of fatigue belt,and there are a lot of micro-cracks.