The observed microstructure evolution from lamellar to spherical
can be understood in electroplasticity framework. Electroplasticity
means that the strain rate of a metal can be increased by at least three
orders of magnitude by the passing electric current[34]. The critical current
density for generation of electroplasticity is 103 A/m2 [35]. The applied
electric current density in the present work is well-above that of
the critical value. The mobility of the dislocations and elements in steels
is enhanced by electric-current for at least 3 orders of magnitude and is
proportional to the peak current density [36–37]. This promotes the
structure transformation by reducing the kinetic barriers and accelerating
kinetics [25]. The structure transformation can take place in a temperature
which is much lower than its conventional value. The
dislocations formed during materials processing and quenching and accumulated
around κ-α interface are migrated into lamellar plates at the
temperature [26]. This generates significant interfaces inside the original
lamellar plates. The lamellar κ breaks into pieces. The currentenhanced
mobility of elements also promotes the spheronization of κ
pieces. The structure refinement of κ is hence achieved.