Effect Of Cold Drawing Processing And Annealing Treatment On Microstructure And Mechanical Properties Of Heavily Cold Drawn Pearlitic Steel Wires

Pearlitic steel wires are widely used in large-span bridges,automobiles and crystalline silicon chip processing due to their high strength and good ductility.As an important engineering material,pearlitic steel wires were mainly produced by cold drawing.After cold drawing,the interlamellar spacing of pearlite decreased,the<110>texture was formed in ferrite accompanied by an increase in dislocation density,and the cementite deformed and even decomposed,resulting in a significant increase in the strength of steel wires.However,the evolution of the microstructure and mechanical properties of pearlitic steel wires under different deformation parameters is still unclear.In addition,there is still a lack of more direct and in-depth research on the distribution of carbon atoms after the decomposition of cementite.The effect of low-temperature annealing on the microstructure and mechanical properties of this severely deformed structure remains to be revealed.Solving these problems can provide necessary theoretical support for the preparation of ultrahigh strength pearlitic steel wires.Therefore,taking pearlitic steel wires as the research object,the microstructure and mechanical properties of heavily cold drawn pearlitic steel wires were studied in this paper.The effects of severe deformation on the ferrtite-cementite interface,cementite microstructure and ferrtie lattice constant were analyzed.The microstructure characteristics of heavily deformed pearlite were also explored.By controlling the drawing temperature rise and drawing strain rate,the evolution of the microstructure and mechanical properties of pearlitic steel wires during the process of severe cold drawing was studied.The influencing mechanism of drawing strain rate on the microstructure and properties of cold drawn pearlitic steel wires was discussed.The effect of low-temperature annealing on the microstructure and mechanical properties of heavily cold drawn pearlitic steel wires was studied.The main conclusions are as follows:The tensile strength of cold drawn pearlitic steel wires increased with increasing drawing strain.The tensile strength of pearlitic steel wires（ε=0）can be increased from 1440 MPa to4920 MPa after cold drawing（ε=5.3）.The pearlitic steel wires can form a highly fibrous microstructure after severe cold drawing.The cementite lamellae gradually decomposed or even disappeared,forming an interface between adjacent ferrite lamellae.After severe cold drawing,the carbon atoms produced by the decomposition of cementite show a relatively uniform distribution,and the concentration of carbon atoms in ferrite is approximately 1.1～4.3 at.%.For the heavily cold drawn pearlitic steel wires,the diffraction peaks corresponding to（110）α-Fe and（220）α-Fe shift to the high angle direction,and the diffraction peaks corresponding to（200）α-Fe and（211）α-Fe shift to the low angle direction.The XRD calculation results show that the ferrite lattice may have a square distortion after severe cold drawing,and its value of c/a increases from the initial c/a=1 to c/a=1.025,forming heavily cold drawn pearlite with body-centered tetragonal（BCT）structure.This is mainly caused by the supersaturated solid solution of carbon atoms produced by cementite decomposition in ferrite.During room temperature drawing,drawing temperature rise can promote the transformation of amorphous cementite into cementite nanocrystals.Cementite nanocrystals can pin dislocations and hinder dislocation movement,resulting in an increase in the strength of steel wires.Cryogenic drawing counteracts the effect of drawing temperature rise and inhibits the recrystallization of amorphous cementite.Compared with the pearlitic steel wires drawn at room temperature,the cryogenic drawn pearlitic steel wires have fewer cementite nanocrystals and weak resistance to dislocations,so steel wires exhibit lower strength and better ductility.As the drawing strain rate increases,the drawing temperature rise gradually increases,which promotes the transformation of amorphous cementite to cementite nanocrystals.When the drawing strain rate increases to 8.2×10¹ s^-1,the ordered degree of the cementite structure is improved,and fine cementite nanocrystals are formed.Further increasing the drawing strain rate will produce a higher drawing temperature rise.The ordered degree of cementite nanocrystals is improved together with a reduced lattice defect,resulting in the formation of larger cementite nanocrystals.The yield strength of cold drawn pearlitic steel wires first increases and then decreases with increasing drawing strain rates.When the drawing strain rate was 8.2×10¹ s^-1,the tensile strength of cold drawn pearlitic steel wires（ε=1.8）reached the maximum value（1820 MPa）,while the torsion properties of steel wires decreased sharply.Too high or too low drawing strain rates can avoid the deterioration of torsion performance.Low-temperature annealing can improve the tensile strength of heavily cold drawn pearlitic steel wires.After annealing at 60℃for 1 h,the tensile strength of cold drawn pealitic steel wires（ε=4.5）increased from 4243 MPa to 4385 MPa.For the steel wires with higher drawing strain（ε=5.3）,the tensile strength increased from 4920 MPa to 5020 MPa after annealing at 60℃for 1 h.The torsion performance of heavily cold drawn pearlitic steel wires was slightly reduced after low-temperature annealing.After low-temperature annealing,carbon atoms segregated to subgrain boundaries,and the interface energy was reduced so that the subgrain boundaries could be stabilized and serve as a barrier against dislocation movement.The precipitation of fine intermediate carbidesε-Fe3C can also hinder dislocation movement,resulting in additional strengthening of the steel wires.In addition,the segregation of carbon atoms at the subgrain boundaries and the precipitation of intermediate carbides will reduce the torsion properties of steel wires.