Effect of ferrite deformation temperature on the mechanical properties of microalloyed steels

Cool deformation is a process in which a small amount of plastic deformation is applied at temperatures well below the transformation finish temperature, i.e. Ar1. In this investigation, the effect of change in cool deformation temperature on the mechanical properties of four types of microalloyed steels were studied with the aim of obtaining an optimum temperature that maximizes the flow stress. Microalloyed steels with 0.04% C, and three different additions of Nb, Mo, and Cu were examined. As well, the effect of cooling rate (1 °C/s and 25 °C/s) prior to the cool deformation in defining the optimum temperature was explored. Results show that under high cooling rate condition, cool deformation at 350 °C gives the highest yield strength and the flow stress as well. For most of the steels, this temperature increases to 400 °C under low cooling rate condition. It was also found that the work hardening rate drops substantially for the cool deformation executed below 500 °C. Additionally, it was seen that at very low temperature of cool deformation, e.g. 300 °C, the amount of work hardening is negligible. It is then argued that the variations in the amount of retained austenite as well as its stability at different temperatures define the effectiveness of strain induced transformation due to the cool deformation.