Cyclic heat treatment (CHT): A technique for enhancement of strength-ductility synergy in low-carbon steel

In the present work, to increase the strength-ductility balance and eliminate the yield point phenomenon in low-carbon steel (Fe-0.09C), the high-temperature austenitization and cyclic heat treatment (CHT) followed by water quenching were performed. The microstructures of all CHT samples showed a combination of fine and coarse grains due to different austenite nucleation in cementite-rich and cementite-free grain boundaries. The microstructure of the CHT sheets at 810 °C consisted of ferrite, grain boundary cementite, and a low fraction of martensite with low tetragonality. However, the microstructure of the CHT sheets at 860 °C showed ferrite, hard martensite, and retained austenite. With the increase in the number of CHT cycles, the grain size of ferrite slightly increased. Also, the grain size in the heat-treated sheets at 810 °C was slightly smaller than the heat-treated sheets at 860 °C due to the dissolution of more grain boundary cementite. The yield point phenomenon (YPP) was severe at the lower CHT temperature owing to the smaller martensite fraction. With the increase in the number of heat treatment cycles, the yield point elongation increased due to the decrease in the amount of carbon in the formed martensites, and as a result, the decrease in tetragonality, the reduction in volume changes, and the decrease in the density of geometrically necessary dislocations in this sample. The heat-treated sheets at 860 °C had higher strength compared to the heat-treated sheets at 810 °C, owing to the formation of more martensite in the heat-treated steels at 860 °C. The CHT sample at 860 °C after the fourth cycle exhibited the best strength-ductility balance, and the YPP was completely eliminated owing to the formation of retained austenite and hard martensite. In this sample, an unexpected strain-hardening behavior (recovery of strain-hardening rate) was seen as caused by the formation of twins in the retained austenite. Severe necking and the formation of large and deep dimples indicated that all samples have experienced intense plastic deformation (ductile fracture). Many grain boundary dimples were present on the fracture surface of the heat-treated samples at 810 °C.