Quantitative Description of Microstructural Banding in Steels

Abstract

Abstract Banding describes a segregation of structural constituents in rolled steel—more exactly, the degree to which these structural constituents separate into distinctly expressed layers that have been formed via stretching of micro-segregation areas along the deformation direction. Our experiments have shown that neither of the stereological structural parameters used in ASTM E1268 can adequately describe microstructural banding. As stated in paragraph 13.9 in the standard's text, “The verbal description of the nature of the banding or alignment is qualitative and somewhat subjective.” It is shown in this article that the banding rate (BR) as a ratio of the standard deviations of the fraction of features on the test lines that are parallel and perpendicular to the deformation direction, earlier described by S. A. Saltykov as a method of directed secants, adequately describes microstructural banding. This newly developed BR can be used to estimate a microstructure from random to oriented up to banded according to GOST 5640-68 banding grades. The banding grades are separated by critical values of banding rate; For example, BR < 1.3—no banding, BR [mt]1.9—one or two solid bands across the whole field of view and several broken bands, BR [mt]2.48—several solid bands intersecting the whole field of view, BR [mt]4 and BR [mt]5—uniform and nonuniform solid bands alternately intersecting the whole field of view. A technique of quantitative description of microstructural banding revealed by a method of directed secants on a panoramic image has been developed and realized as a Thixomet Image Analyzer plug-in. Estimation of banding will be demonstrated with examples of ferrite-pearlite and ferrite-bainite steel microstructures.