Two step ductile to brittle transition behavior on ferrite + pearlite structure steel sheet

Abstract

Although a few studies on the fracture behavior of dual phase (DP) steels have been conducted, the mechanism of impact fracture in DP steels has not yet been clarified. In this study, the ductile to brittle transition behavior in ferrite + pearlite DP steel sheets was evaluated using the Charpy impact test with a sub-size specimen. The absorbed energy of DP steel decreased with decreasing temperature and the transition curve had a clear "middle shelf" existing between upper and lower shelves. The fracture surface at the middle shelf was flat and contained few dimples, similar to that at the lower shelf. Ferrite grains just below the fracture surface obviously received plastic strain with fracture on the middle shelf. This result indicated that the fracture mode on the middle shelf was quasi-cleavage fracture (QCF). Furthermore, on the lower shelf, ferrite grains received very small plastic strain with fracture. The two step ductile to brittle transition in this study corresponded to the fracture mode that changed from microvoid coalescence fracture to QCF and from QCF to cleavage fracture.