Structural Evolution of Pearlite in Steels with Different Carbon Content under Drastic Deformation during Cold Drawing

Steel wires, under severe cold drawing deformation, develop high strength. High carbon steel (C>0.80%) has a great demand in the steel market because of the extremely high strength (5-6 GPa). For this reason it is relevant to increase the knowledge on the structural evolution and deformation mechanisms involved during wiredrawing process due to their critical applications, among which we can mention wires for: bridges, cranes and tire cord. The mechanical behaviour aptitude is determined by torsion test. When the fracture surface is flat, the wire is apt. On the opposite, an irregular fracture surface (delamination) means poor mechanical properties. This paper presents a comparative study on steel wires with different carbon contents (0.40% C and 0.80% C) in order to compare the structural evolution at high deformation and to verify the impact of carbon content on the pearlite deformation and cementite stability. Samples tested under torsion with and without delamination problems, were selected. The microstructural study was carried out applying light and scanning electron microscopy (SEM). Finally, the structural information was correlated with results of differential scanning calorimetry (DSC) and thermodynamic properties obtained by Fact Sage simulation. The structural study verified the presence of curling phenomenon in both steels grades, with different carbon content. The effect of the ferrite on deformation mechanisms during wiredrawing was clearly visualized. It was possible to verify differences (similar to 26%) in the interlaminar spacing (lambda) of the pearlite between wires that present normal and delaminated behaviour under torsion test. The ductility loss (in the delaminated wire) is promoted by multiple causes: higher interlaminar spacing, high nitrogen content in the product and the presence of dynamic strain aging, which is promoted by cementite destabilization and the formation of epsilon carbide. Published by Elsevier Ltd.