PSI - Issue 17

Jan Kec et al. / Procedia Structural Integrity 17 (2019) 230–237 Jan Kec / Structural Integrity Procedia 00 (2019) 000 – 000

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The results of LM observation show that the microstructure of the investigated steel consists of equiaxed ferrite grains (white) and colonies of pearlite (black) (Fig. 1a). In the direction towards the specimen centre, the microstructure is of the band type, the ferrite bands are followed by the pearlitic ones. In addition, the bands of granular bainite are observed in the central part of the specimen (Fig. 1b). The bands of granular bainite decrease the steel plasticity and can cause in some cases quasi cleavage fractures (Feng et al. 2018). The detailed SEM observation showed that elongated inclusions and sharp-edged carbides occur in the immediate vicinity of pearlite/granular bainite bands. The EDXS analyses confirmed the presence of MnS inclusions and NbC particles. In the surroundings of Mn inclusions, the concentration of Mn atoms is higher. Mn decreases the carbon activity during the transformation Fe- γ to Fe - α and, subsequently, especially Mn -rich regions serve as preferential sites for the formation of pearlite/granular bainte bands. The size of ferritic grains differs only slightly, the grains of G9 to G16 are observed in the structure; the average size is, however, G12,5 (d = 4,81 µm). It is generally known that very fine-grained structure increases the steel strength and decreases DBTT. To obtain a fine-grained structure, steels are microalloyed with Nb and V, because their carbides and carbonitrides have high melting temperature about 1150 °C and impede the growth of austenitic grains during the heating to the rolling temperature (Maalekian et al. 2012).

a)

b)

Fig. 1. (a) microstructure at surface; (b) microstructure at centre.

Fig. 2. detail of perlite band with the presence of MnS inclusions and NbC carbides