Influence Of Morphology And Distribution Of Cementite On The Drawing Deformation And Properties Of Pearlite Steel Wire

In this dissertation, using pearlite steel wire as raw material, prepared spheroidal pearlitic structure by annealing treatment, prepared spheroidized cementite(multiple sizes)+ferrite structure by quenching+high-temperature tempering treatment, based on the drawing deformation process of those steel wires, a systematic research was conducted on the effects of cold drawing deformation processing of the steel wires if the morphology, distribution, size of cementite changed, as well as the deformation strengthening mechanism of it, by using Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and Electron Backscattered Diffraction technology (EBSD), etc. The main results obtained have been listed as follows:Research into the lamellar pearlite steel wire (lamellar cementite+ferrite) and spheroidal pearlite steel wire (spheroidized cementite+ferrite) shows that:1) Lamellar cementite exhibited certain deformation ability during deformation, the lamella in favorable orientation become thinner and the spacing decreased, while the lamella in unfavorable orientation is being bent, twisted, even fractured. While the hard spheroidized cementite is tend to rotate along the drawing direction with the plastic deformation of ferrite, instead of oneself plastic distortion during the deformation, the rotation is basically accomplished as ε=1.6 or so, while a greater number of micro defects could be found during the deformation.2) The strength of lamellar pearlite steel wire is far exceed the spheroidal pearlite steel wire, and its work hardening rate is markedly increased as ε>1.6 or so.3) The strength of ferrite<110> texture in lamellar pearlite structure gradually increases as the strain rises, and it is exceed the spheroidal pearlite structure.Research into the microstructure and properties of spheroidal pearlite steel wire and quenching+high-temperature tempering state (spheroidized cementite+ferrite) steel wire shows that:1) Micro defects could be easily generated during deformation if there is high number of large carbides and carbides is dispersively distributed.2) The smaller the average cementite particle size, and the more dispersive the distribution, the higher the strength, therefore quenching+high-temperature tempering(smaller Fe3C particle) state’s strength is higher than quenching+high-temperature tempering(larger Fe3C particle) state, spheroidal pearlitic structure has the lowest strength.3) The deformation could be divided into three stages according to the work hardening rate. As for lower strain stage (s<0.4), spheroidal pearlitic structure steel wire has the highest work hardening rate; As for medium strain stage (0.4<ε<1.6), three material have almost the same work hardening rate; As for higher strain stage (ε>1.6), the material that has the dispersively distributed carbides tend to have strongest strengthening effect.Research into the deformation strengthening mechanism of dual phase(spheroidized cementite+ferrite) material shows that:1) After deformation, many high density dislocation tangles could be found in the grains, and the dislocation loops, the dislocation tangles around the carbides, would all block the subsequent motion of dislocations, then hardening the material.2) High density dislocation tangles could be easily formed around large size carbides, along with the stress concentration, make it easier to generate defects.3) When the morphology of cementite is spherical, the stacked dislocations and concentrated stress around the cementite all lead to defects in the structure during the deformation. When the morphology of cementite is lamellar, cementite could coordinate with ferrite during drawing process, so, it is difficult to generate defects in the deformed structure, therefore, the lamellar pearlite has the better drawing property.