PSI - Issue 30

M.M. Kantor et al. / Procedia Structural Integrity 30 (2020) 45–52 M. М . Kantor et al. / Structural Integrity Procedia 00 (2020) 000–000

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Fig.6. Grain size distributions.EBSD: (a) F+LB+GB; (b) A; (c) M-A.

The distribution of ferritic (BCC) structural constituents’ grain sizes had remarkable right-hand asymmetry, Fig. 6 (a). Average grain size was 10 µm. The presence of grain size populations in range from 4-6 µm up to 18-44 µm indicated significant microstructure inhomogeneity. Retained austenite (FCC) grain size distribution showed unimodal character, Fig. 4 (b). The volume fraction of retained austenite was 0,01% and the average grain size was 0,6 µm. BS parameter, describing lattice curvature, was used for identification of the most imperfect constituent of microstructure – M-A, as studied by Mohammadijoo et al. (2018), Vorkachev (2019), Fig. 5 (c). The grain size distribution of M-A showed unimodal character, but marked right-hand asymmetry, Fig. 6 (c). The average grain size of M-A was 1 µm with volume fraction of 0,2%. Frequently the formation of M-A constituent is considered as the main source of cleavage fracture in low carbon steels, for example by Mohseni et al. (2013), Ruan et al. (2013). However, depending on the observed content we could suggest that M-A gives only additional embrittlement contribution to the fracture process. Thus, the significant inhomogeneity in microstructure could be considered as the source for the formation of local variations in ductility of plastic zone and leads to the remarkable impact toughness scattering under DBT region fracture conditions. 4. Conclusions The connection between occurrence of remarkable impact toughness scattering in DBT region and microstructure features of low carbon microalloyed steel was established by means of multiple impact toughness tests and EBSD microstructure measurements. Formation of splittings by quasi-cleavage mechanism is a feature of DBT region fractures in low carbon microalloyed steels. The increase in number and area of splitting projections with the decrease of KCU minimal values of individual specimens fractured at different temperatures in DBT region was observed. EBSD investigation of microcracks nature in plastic zone under ductile fracture surface and fracture surface containing splittings indicated the local inhomogeneity in ductility. The residual microcracks occur in local embrittled regions of material on the early stage of fracture as a result of cleavage. Pre-strained regions with local ductility variations are fractured during main crack propagation. The formation of splittings by the quasi-cleavage mechanism shields embrittled regions containing cleavage microcracks from overstress. More ductile ligaments between occurred microvoids are fractured as a result of ductile tearing. Formation of pancaked parent austenite microstructure, various ferritic microstructures and M-A constituents along with significant grain size variations can be considered as the sources for the occurrence of remarkable impact toughness scattering in low carbon microalloyed steels during fracture in DBT region. Acknowledgements The investigation was carried out within the state assignment of Ministry of Science and Higher education of Russia 075-00947-20-00 and supported by Presidium of Russian academy of Sciences program 22P.