Abstract

Structural inhomogeneities that occurred in pipeline steels during controlled rolling and were identified as elongated regions of coarse-grained bainite were found to promote a tendency for brittle fracture of these steels. In particular, it was shown by tint etching, with subsequent analysis in polarized light with the light-optical microscope, that their volume fraction exhibited a perfect correspondence with the failure behavior. The aim of the present investigation was to comprehensively characterize the microstructure and the local texture of these regions using the EBSD technique and thereby clarify their origin. It was shown that, whereas the local texture of fine-grained regions corresponded well to the macroscopic texture of the steel band, the local texture of the structural inhomogeneities corresponds to certain minor components of the macroscopic texture. The grains of the parent austenite, from which these inhomogeneities have been produced, had orientations concentrated close to the brass component of austenite rolling texture. Hot deformation of the austenite leads to a strong variant selection (predominant realization of certain lattice orientations among plausible orientations of α-phase) during phase transformation of these grains, that result in a significant increase of the bainite packet size. Formation of bainite in the upper temperature range of bainite transformation, in its turn, leads to such a variant pairing that low-angle boundaries are formed predominantly both inside and between the packets. This further enlarges the effective grain size and correspondingly enhances the deteriorative effect of the structural inhomogeneity.

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