Microstructure and mechanical properties improvement by ultra-rapid annealing of severely deformed low-carbon steel

Ultra-rapid annealing (URA) of deformed low-carbon steel can provide refined microstructure. In this research, severely deformed low-carbon steel is ultra-rapidly annealed to subcritical and intercritical temperature ranges at the heating rates of 75-1800 °C/s and then rapidly cooled. Also, conventional annealing by the heating rate of 0.3 °C/s is performed at the same temperature ranges. The results show that conventional and subcritical ultra-rapid annealing cannot lead to a fully refined microstructure. High heating rate causes the interaction between recrystallization and transformation as the temperature exceed Ac1 (intercritical temperature range). High kinetics of transformation during intercritical ultra-rapid annealing and interaction lead to the fully refined recrystallized microstructure. During subcritical conventional annealing of severely deformed sample, hardness is reduced gradually to the hardness of as-received sample. In contrast, URA increases the hardness in subcritical range due to solid solute carbon. Around Ac1, hardness is dropped down because of accelerated recrystallization, and it is increased again in higher intercritical temperature due to increase of harder phases formed from austenite during cooling. Samples ultra-rapidly annealed in intercritical temperature range have higher hardness, strength and ductility than those of the samples annealed in subcritical range due to the presence of both types of ferrites; refined recrystallized primary ferrite and secondary ferrites formed from austenite during cooling after URA.