티타늄 첨가 보론강의 정적 및 준동적 재결정 거동에 대한 연구

Abstract

The high strength and toughness of wire rode steel are attractive in the automobile industry. Recently, boron (B)-containing steel has been developed to solve profitability and energy saving during the final production. The steel shows better mechanical properties and excellent workability even without complicated heat treatment. Ferrite formation is delayed due to segregated B at grain boundaries, and mechanical properties such as strength, elongation, and toughness increases due to the formation of a multi-phase structure. Ti is inevitably added to secure the BN formation of the alloyed B in the steel. Meanwhile, there was a problem where the bimodal grain structure appeared in the cross-section of the wire rode in the Ti-added B-containing steel. As a result, unexpected failure occurs during final product fabrication due to the inhomogeneity of mechanical properties. This study focused on the factors that determine static and metadynamic recrystallization in Ti-added B-containing steel. Through the single and double hot-compression tests, the empirical equations to describe both recrystallization behaviors were obtained. Higher strain, strain rate, and deformation temperature accelerate recrystallization, while the larger grain size decelerates the recrystallization behavior. Comparing the static and metadynamic recrystallization, metadynamic recrystallization has a larger exponent of strain rate and smaller activation energy. The Avrami index value n is below 1, whereas the conventional low-carbon steels show larger than 1. The grain size of this steel was from 48㎛ to 61㎛ even after reheating to 1,000~1,200 ℃. The formation of TiN affects grain growth and recrystallization behavior by pinning grain boundary motion. The static and metadynamic recrystallization behaviors were determined by comparing the critical strain (strain at the peak stress) and the magnitude of the applied strain. This implies local difference of recrystallization behavior due to the inhomogeneous strain during the steel production process. Therefore, the balance of applied strain is expected to be a critical solution to eliminate the local grain size difference.