| Two ingots of Fe-Mn-C-Cu TWIP steels were prepared by vacuum induction furnace. The effect of carbon content on stacking fault energy, critical resolved shear stress, microstructures and mechanical properties of TWIP steels were studied using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) after cold drawing. The microstructures and texture evolution during cold drawing were investigated by electron back scattering diffraction (EBSD). By comparing deformation behavior of cold rolling and cold drawing, the effects of deformation behaviors in different states of stress were further discussed. Moreover, the strengthening mechanism of TWIP steel was systematically investigated.This study shows that Fe-20Mn-3Cu-XC TWIP steel specimens were fully composed of austenite without phase transformation. Both the stacking fault energy of TWIP steel and the critical resolved shear stress of twin formation increased with increasing carbon contents. The number of deformation twins in austenite increased after cold drawing and severe plastic deformations of the grains are produced in the transverse and longitudinal section with the addition of carbon. Besides, the lattice distortion increased because of the increase of carbon content, and the diffraction peak is offset to the left. Calculations results indicate that the dislocation density of Fe-20Mn-3Cu-1.3C TWIP steel before and after cold drawing is relatively high compared with Fe-20Mn-3Cu-0.4C TWIP steel. After cold drawing, the yield strength and tensile strength of the two steels have significantly improved, but the increase in strength of Fe-20Mn-3Cu-1.3C TWIP steel is more evident. The yield strength of Fe-20Mh-3Cu-1.3 C TWIP steel achieves 1942.53 MPa, tensile strength is 1984.10 MPa, while microhardness of the alloy is 620 HV.The evolvement of microstructure and grain orientation of Fe-20Mn-3Cu-1.3C TWIP steel wire which was drawn from 05.8 mm to 03.4 mm had been studied. With the increase of strain during cold drawing, the amount of deformation twin gradually increased. Grain orientations gradually formed a texture, which is parallel to the direction of cold drawing. That is typical<111> and <001> textures. Grains were refined in transverse section and elongated in longitudinal section after cold drawing for twelve passes. At the same time, grain orientation deflects, the grains were gradually parallel to drawing direction. At lower strain, deformation twinning mainly occurs in grains oriented close to<111> orientation. While at higher strain, the influence of grain orientation on twinning activity declined significantly. Deformation twins are observed in grains that are unfavorably oriented with respect to twinning according to Taylor factor.By comparing the microstructure between cold drawing and cold rolling of the Fe-20Mn-3Cu-1.3C TWIP steel after deformation, it is found that grains have formed the twin systems that are mostly in a set or two sets during cold rolling and its orientation are mainly concentrated on the<111> direction, while most of the grains have formed the twin system including two sets or more during cold drawing and its orientation are mainly concentrated on the<111> and <001>direction. Both of these steels have a high misorientation in the interior of the grains, but the steel under cold drawing on a higher one. The contributions of all the relevant deformation mechanisms-slip, twinning and dynamic strain aging were quantitatively evaluated. The analysis revealed that the dislocation storage was the leading factor in the increase of the strain hardening. Twinning, which interacted with the matrix, offers a favorable orientation for the grains and enables some slip systems continue to shear deformation. The generations of a large number of dislocations provide potential nucleation sites to deformation twins, and the increase of strength and plasticity was attributed to the large quantity of dislocation pile-up, as well as the generation and extending of deformation twins. |
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