Deformation of dual-structure medium carbon steel in cold drawing

In this paper, extremely high strength was obtained in medium carbon steel having a carbon content of 0.35% by weight through cold drawing. Experimental results showed that the tensile strength of the steel increased by nearly three folds from the original value ~615 MPa to 1810 MPa corresponding to drawing strain of 3.0. To reveal the mechanisms that govern the strengthen increase, the microstructural evolution was analyzed during cold drawing, with respect to the change of the deformation resistance (measured by micro-hardness) of micro-constituents (i.e., primary or proeutectoid ferrite and pearlite) in the material. The proeutectoid ferrite became elongated and, at the same time, increasingly hardened while the pearlite maintained equiaxed shape after initial drawing. With the increase of the drawing strain, the pearlite was stretched parallel to drawing direction, accompanied by an increase in the 〈110〉 texture intensity and dislocation density in the ferrite phase. Under heavy drawing, a laminate structure formed, consisting of alternating pro-eutectoid ferrite and pearlite both parallel to the drawing direction. The 〈110〉 texture intensity in the ferrite phase became saturated as ε>1.2. High density dislocation zones further spread in the ferrite phase. The interlamellar spacing between ferrite and cementite phases in the pearlite decreased. Based upon these observations, mechanistic models were constructed to provide insight into the deformation and strengthening mechanisms of this steel.