Strong strain-hardening capability in plain carbon steel through cold rolling and heat treatment of lamellar structure

In the current research, strong strain-hardening capability in 1045 carbon steel was achieved by cold rolling and heat treatment of the lamellar structure. After 60% cold rolling, the proeutectoid ferrite and pearlite were severely elongated along the rolling direction. Cementite layers with brittle nature were fragmented under the action of stress and also by shear band formation during rolling. The lamellar heterogeneous microstructure of the heat-treated samples was formed by the proeutectoid ferrite as the soft domain and the pearlite as the hard domain. Increasing the heat treatment duration promoted the spheroidization transformation of cementite, and the spheroidization degree of cementite increased. With the increase in the time of heat treatment, the number of recrystallized grains is increased. Heat treatment led to weakening the deformation texture (⟨100⟩‖ND or θ fiber) and strengthening the recrystallization texture (⟨110⟩‖ND or ζ fiber). The heat-treated steels revealed an average microhardness value much higher than its macrohardness due to changes in the texture of ferrite grains. Heat treatment decreased the strength and hardness, and increased the ductility and toughness of steel compared to the cold-rolled sample owing to the annihilation of dislocations and the strengthening transition from strain hardening to grain refinement. The heat-treated samples exhibited a wider range of uniform plastic deformation and a larger strain hardening rate than the cold-rolled sample owing to the occurrence of the recovery of strain hardening rate phenomenon. The appropriate collocation of the fine spherical cementite and the soft/hard domains (with a large difference in mechanical properties) was beneficial to obtaining excellent strength-ductility balance in plain medium carbon steel. After heat treatment, the extent of ductile fracture considerably increased. With increasing the time of heat treatment from 1 to 3 h, the number of lamellar dimples decreased owing to the creation of more fine spherical cementite.