Behaviour of low and medium carbon free cutting steels during deformation to large strains

Abstract

Low and medium carbon free cutting steels were deformed by cold rolling to reductions of up to 98%. The resultant microstructures were observed and characterised using optical microscopy, SEM, and TEM. Deformation of the pearlite grains and manganese sulphide inclusions was quantified in terms of their relative plasticity (compared to that of the steel). The evolution of the ferrite microstructure in the steels was seen to be dependent on the volume fraction of pearlite present. The ferrite grains in the low carbon steels underwent structural subdivision characterised by the formation of dense dislocation walls and microbands. At intermediate rolling deformations much of the strain was accommodated inhomogeneously in narrow bands of shear (S bands). Strain in the pro-eutectoid ferrite of the medium carbon steel occurred in a more homogeneous manner owing to the constraints imposed by the pearlite. The manganese sulphides and pearlite in the steels also acted as fiducial markers of the surrounding ferrite flow. Plasticity of the sulphides was generally found to be less than the overall rolling strain. However, within certain narrow strain ranges, sulphide plasticities greater than that of the steel were measured.