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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1. Introduction High environmental risks make the problem of reliability of steel constructions operated in the Arctic region particularly important. In this case, an essential circumstance is the requirement to ensure high strength and sufficient low temperature toughness. 625-825 MPa yield strength level steels has already been developed for this purpose. Microalloying and controlled rolling with rapid cooling are parts of technology tailored for the commercial production of such steels, shown by Pontremoli et al. (1984). Formation of refined ferrite-bainitic microstructure lead to simultaneous increase in strength and toughness levels. At the same time, steels containing BCC structural constituents reveal DBT fracture behavior during serial toughness evaluation tests. Remarkable toughness scattering, formation of mixed fracture surfaces and occurrence of microcracks in plastic zone are the main features of DBT region fractures in low alloy steels, studied by Croussard et al. (1956), Chen et al. (1997), Iwadate et al. (1994), Kantor and Bozhenov (2014). In addition to that, the occurrence of splittings had been considered as a feature of controlled rolled steels fractures, obtained by Bramfitt and Marder (1977). Nevertheless, the connection between microstructure and mechanism of remarkable toughness scattering in low carbon microalloyed steels is still the subject of exploratory research works.

Nomenclature EBSD electron backscatter diffraction DBT ductile-to-brittle transition ω mass fraction σ 0,5 yield strength σ u ultimate tensile strength δ relative elongation CVN Charpy V-notch number KCU impact toughness T temperature  spl. volume fraction of splittings  BS band slope IPF inverse pole figure GOS grain orientation spread F proeutectoid ferrite GB granular bainite LB lath bainite A retained austenite M-A martensite-austenite d eq. equivalent diameter BCC body-centred cubic lattice FCC face-centred cubic lattice

An appropriate tool for the characterization of both microstructure and fracture mechanism in low carbon microalloyed steels is the application of EBSD technique by Davies et al. (2002), Wright et al. (2011). Evaluation of local strain degree in plastic zone of fractured under DBT conditions specimens and quantitative analysis of microstructure characteristics could clarify the nature of remarkable impact toughness scattering phenomenon in low carbon microalloyed steels. The aim of present study is to find the connection between the occurrence of remarkable impact toughness scattering in DBT region and microstructure features of low carbon microalloyed steels.