In order to determine the causes of the scattering of the impact toughness values of hot-rolled low-carbon low-alloy steel by optical microscopy, scanning electron microscopy and Electron Backscatter Diffraction, a comprehensive study of the mechanisms of sample fracture after multiple Charpy impact tests at a temperature of –60°C were carried out. It is established that the scattering of impact toughness values is determined by the formation of martensitic-bainitic interlayers in the axial region of rolled products with an increased concentration of both nonmetallic inclusions and Mn, S, P. On the one hand, they act as an origins of brittle macrofracture, on the other hand, they contribute to the formation of splits. The effect of brittle fracture on the main fracture surface prevails over the formation of splits in a plane parallel to the rolling plane. Even before the fracture, two groups of microcracks are formed in the plastic zone, oriented either parallel to the rolling plane or parallel to the propagation path of the main crack. The duality of the role of MnS in fracture process during Charpy impact tests is shown. The contributions of nonmetallic inclusions and martensiticbainitic interlayers on cleavage nucleation in samples without splittings are divided. The nature of splittings formation has been studied, and the absence of a correlation between the total length of splittings and impact toughness values has been shown. Fractures with the formation of main splitting can lead to a slight increase in impact toughness due to the shift of the focal facets sites from martensite-bainite interlayers into the ferritepearlite microstructure.
Vorkachev et al. (Wed,) studied this question.