Fabrication of the Ultrafine Grained Low Carbon Steel by Cold Compression and Annealing of Martensite

The aim of this work was to produce ultrafine grained low carbon steel (0.13 wt% carbon) by cold compression and subsequent annealing of lath martensite and study its formation kinetics. In this regard, the martensitic specimens were first cold compressed by 65 % and then annealed at 723, 773 and 823 K for various times ranging between 0 and 120 min. Results showed that annealing of the cold-compressed martensitic specimen at 823 K for 90 min led to the formation of ferrite grains with a mean size of 1.8 mu m and small precipitates of Fe3C with sizes less than 300 nm. By using the Johnson-Mehl-Avrami-Kolmogorov analysis to describe the kinetics of ferrite recrystallization, the average value of time exponent (n) was found to be 0.96. The activation energy for the ferrite recrystallization was calculated as 83 kJ mol(-1), which was consistent with the activation energy value for carbon diffusion in bcc iron (81 kJ mol(-1)).