Microstructure development by the application of different strain paths prior to soft annealing in a medium carbon steel

Abstract

The effect of different pre-strain application paths on the microstructure evolution of a medium carbon steel after soft annealing is studied. The strain applied on an undercooled austenite induces fine ferrite and pearlite transformation at austenite grain boundaries leading to an average grain size much finer than that in the case where no pre-strain is applied (around 4 and 7 µm, respectively) and a drastic reduction of the fraction of large grains (>20 µm) that become the 17% rather than the 40%. Cementite direct precipitation at the induced ferrite α/α interface and the induced pearlite quick decomposition, give rise to an improved degree of spheroidisation after annealing and a L2 IFI rating regarding the ASTM F2282 standard. When the strain is applied on pearlite, diffusion through the matrix is enhanced which accelerates the lamellae break-up and leads to fully spheroidised microstructures after annealing (degree of spheroidisation above 90% and G1 on IFI rating). When both strain sequences are applied consecutively, the resulting annealed microstructure combines the features corresponding to each individual strain paths, i.e., homogeneous and refined ferrite grain size and enhanced spheroidisation degree, which confers optimal cold workability properties to the steel. In addition, such characteristics are attained without an excessive increase in hardness (in the range 155–180 HV), which is interesting when considering cold forming tool wear.