Improvement of the strength-ductility-toughness balance in interstitial-free steel by gradient microstructure

Abstract

High strength-ductility-toughness is required for the usage of interstitial-free (IF) steel sheets in the automotive industry. However, strength and ductility are contradictory. In the current research, gradient microstructure is produced to enhance the strength of interstitial-free steel without compromising ductility and toughness. Interstitial-free steel sheet with gradient microstructure was fabricated via accumulative roll bonding (ARB) followed by annealing treatment. Mechanical measurements (tensile and hardness tests) were conducted to study the influence of annealing temperature on the properties. Compared with the ac-received interstitial-free steel sheet, the sample with gradient microstructure exhibited enhancing strength-ductility-toughness balance. By increasing the temperature of the annealing treatment, the strength and hardness were decreased while the ductility and toughness were increased. The ultimate tensile strength, yield strength, and toughness of the annealed sample at 700 °C were remarkably improved (212.7 MPa, 391.1 MPa, and 147.1 J/cm3, respectively) without sacrificing the ductility (40.8%) as compared to the as-received interstitial-free steel sheet. With increasing the annealing temperature, the quality of bonding between layers had been improved and the interface debonding decreased. After the annealing at 700 °C, the interface debonding and delamination induced by the accumulative roll bonding had been invisible.