Study On Microstructure Evolution And Strengthening Mechanism Of Cold Drawn High Carbon Steel Wire

Cold drawn high carbon steel wire is a metal material with nano-structure obtained by strong plastic deformation.It is mainly used in suspension bridge cables,tire cords and automotive coil springs.The excellent properties of cold drawn high carbon steel wire are closely related to the microstructure evolution of steel wire during cold drawing.Therefore,this paper takes two high carbon steel wire used most commonly,70 steel and 82 B steel as research objects.Pearlite interlayer spacing,cementite thickness,dislocations density and mechanical properties of high carbon steel wires with different strain were measured by transmission electron microscopy(TEM),X-ray diffraction(XRD),electron backscatter diffraction(EBSD),etc.The changes of mechanical properties,microstructure and nano-structure of high carbon steel wire during cold drawing were investigated,and the strengthening mechanism of high carbon steel wire was discussed.The main work and results of this paper are as follows:(1)The tensile strengths of 70 and 82 B steel wires continued to rise steadily with the increment of the strain.Under the same strain,the higher the carbon content in the steel,the greater the tensile strength.The microhardness of the longitudinal and transverse sections were stepped up,but the microhardness of the formmer was higher than the latter.The metallographic observation revealed that the pearlite mass was elongated into a fiber shape in the longitudinal direction.From the cross-section,the grains were gradually refined,the pearlite tendons remained to be equiaxed.TEM analysis showed that the average interlamellar spacing(ILS)and cementite thickness(T)of the pearlite were gradually reduced and gradually adjusted in the direction parallel to the drawing axis.(2)With the increase of the tensile strain of the 70 and 82 B steel wires,the dislocation density in the steel wire grew slowly in the early stage of cold drawing.When the cold drawing strain reached a certain level,the dislocation density began to increase rapidly.XRD and EBSD analysis showed that the steel ferrite crystals had no obvious orientation when the wire rod was cold-drawn.With the cold drawing,the <110> silk texture was formed in thesteel wire,and it was strengthen with the increment of the strain,finally a significant <110>silk texture formed;The boundary strengthening and dislocation strengthening model is basically consistent with the growth trend of measured value.the dislocation(3)The calculation values of 70 steel and 82 B steel according to the material layer reinforcement model are significantly higher than the measured values,The reason is that some of the ply tissue will be broken during the cold drawing process,which will destroy the continuity of the ply structure,so this model is not suitable for explaining the strengthening mechanism of cold drawn steel wire;the boundary strengthening+dislocation strengthening model is basically consistent with the growth trend of measured value in the early stage of cold drawing,and the contribution of boundary strengthening to total strength is far greater than that of dislocation strengthening.As strain increases,the difference between the calculated value and the measured value of the boundary strengthening+dislocation strengthening model decreases,and the dislocation density increases rapidly,and the dislocation strengthening effect becomes more and more significant.