Abstract
A homogeneous microstructure plays an important role in the mechanical properties of every alloy. However, because of chemical inhomogeneities, a banded microstructure can develop in steels. This banding has a negative effect, especially on the impact toughness and cold forming. In this paper, the development of a banded microstructure in hot-rolled low-alloy structural steel S355J2 is explained. The banded microstructure in the cross-section consisted of ferrite and pearlite bands, while towards the centre, bainitic-martensitic bands were also observed. The deviations in chemical compositions of the bands were measured with EDS, which showed that the bainitic-martensitic bands contained more alloying elements (Mn, Cr, Mo, Si) than the ferritic. However, by using Oberhoffer reagent, the segregations of phosphorus were also revealed. Phosphorus segregations coincided with the positive segregations of the alloying elements. Because manganese and chromium have the prevailing effect on the distribution of carbon in austenite, an increased concentration of carbon in bainitic-martensitic bands is also expected. The CCT diagrams of S355J2 steel with three different chemical compositions, i.e., with the nominal composition and compositions with negative and positive segregations, were calculated. However, increased concentrations of carbon and phosphorus were not taken into account in the calculations of the CCT diagrams. Since these elements can significantly increase the hardenability, they play an important role in the formation of the bainite and martensite.

Key words
S355J2 steel, banded microstructure, phosphorus segregations, Oberhoffer reagent, CCT diagrams

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