Microstructural Reasons for the Scattering of Impact Toughness Values of Low Carbon Low Alloy Hot Rolled Steel During Multiple Charpy Impact Tests

In order to assess the effect of the microstructure on the scattering of the impact toughness of hot-rolled steel, samples were examined after Charpy impact tests at a temperature of −60° C. The microstructure of the samples under the fracture surface has been studied. It has been experimentally shown that most of the cleavage facets are located in martensite-bainite interlayers in the ferrite-pearlite microstructure, which are formed in the axial region of the rolled product. Despite the fact that the proportion of martensite-bainite interlayers in the microstructure does not exceed 4%, their size and location relative to the applied stresses determine the impact toughness of standard transverse samples. The minimum values of impact toughness correspond to the largest total width of martensite-bainite interlayers and the highest concentration of Mn, Si and P. To assess the effect of the formation of martensite-bainite interlayers on the impact toughness, an indicator of the inhomogeneity of the microstructure C is proposed, taking into account both the width of the interlayers and the concentration heterogeneity in Mn, Si, and P.