Study On Microstructure Evolution And Mechanical Properties During Large Strain Cold Deformation Process Of Hypereutectoid Steel

High carbon pearlite steel wires have been widely used as the framework material of radial tire,which have a good combination of high strength and toughness after large strain drawing.Recent research showed that the cold drawing process has important influence on the microstructure and mechanical properties of steel wires.Therefore,study of the microstructure and mechanical properties of high carbon pearlite steel wires during cold drawing process is of great academic and practical significance.In the present work,processes of single-pass and multi-pass cold drawing were conducted on hypereutectoid pearlite steel wires with carbon content of 1.02wt%.Microstructural development and mechanical properties of the wires after drawing were studied and the strengthening mechanism was discussed.The effects of low temperature annealing on microstructure of steel wire after multi-pass cold drawing were also investigated.The experimental results showed that as the strain increased,the microstructure of the hypereutectoid pearlite wire in the process of single-pass drawing was different.When the strain was relatively small,lamellar morphology of the pearlite in the steel wire after cold drawing remained,but the spacing of lamella was refined.With the increase of the compression ratio,coordinated deformation of ferrite and cementite occurred and pearlite lamella were bended and folded.In the longitudinal section of the wire after drawing,it can be observed that pearlite lamella extended along the drawing direction.The test of mechanical properties showed that the tensile strength,microhardness and torsion performance of the steel wire increased with the increase of strain.In the early stage of multi-pass drawing process,the change of microstructure was similar to that of single-pass drawing process.As the strain increased,fibrous morphology of pearlite lamella were observed in longitudinal section of the wire.High magnification TEM observation revealed the fragmentation of cementite and a large number of nano-crystalline.Tensile strength,microhardness and torsion strength of the steel wires after multi-pass drawing process were increased significantly with the increase of accumulated strain.The results of XRD analysis show that the<110>peak of the ferrite shifted to the left and widened after the multi-pass cold drawing deformation,indicating that the lattice constant of ferrite was increased.The increase of lattice constant of ferrite was the result of the distortion of ferrite lattice due to the dissolution of carbon atoms.The results of PPMS test indicated that the volume fraction of cementite in steel wire decreased continuously during the drawing process and this further confirmed that the dissolution of cementite caused by cold drawing deformation.Low temperature(T?483K)annealing of the hypereutectic steel wire after the large strain(?=2.81)deformation did not result in obvious change of its microstructure and the lamellar morphology of pearlite remained after annealing.When the annealing temperature was raised to T=553K,the precipitation of cementite occurred and the amount of cementite precipitates increased when the annealing temperature was further raised to 623K.Under SEM observation grain boundaries between the ferrite and cementite lamella in the specimen of the wire after annealing at 623K could not be clearly seen.At lower annealing temperature(T?483K),the<110>peak of ferrite on the XRD map taken from the specimen of the wire still shifted to the left,in comparison with that before annealing,implying the lattice constant was further enlarged due to annealing.This indicated that annealing at relative low temperature promoted the dissolution of the cementite and increase of the supersaturated solubility of carbon atoms in the ferrite.However,the ferrite<110>peak position shifted to the right instead of left,reflecting the reduction of lattice constant,if the annealing temperature was further raised.The PPMS test results revealed that the volume fraction of cementite in the wire after annealing at 553K is lower than that of the wire before annealing,indicating that the cementite precipitated from the supersaturated ferrite after annealing at a higher temperature.