Crystallographic features of lath martensite in low-carbon steel

Electron backscattering diffraction with field-emission scanning electron microscopy was used to analyze crystallographically the lath martensite structure in a 0.20% carbon steel. The crystallographic features of the lath martensite structure, of the order of the prior austenite grain size or larger, were clarified. Although the orientations of the martensite crystals were scattered around the ideal variant orientations, the martensite in this steel maintained the Kurdjumov-Sachs (K-S) orientation relationship. The procedures of the crystallographic analysis of the martensite (ferrite) phase with the K-S orientation relationship were explained in detail. Variant analysis showed that all 24 possible variants did not necessarily appear within a single prior austenite grain and that all six variants did not necessarily appear within each packet. Specific combinations of two variants appeared within local regions (sub-blocks), indicating a strict rule for variant selection. Prior austenite grain boundaries and most of the packet boundaries were clearly recognized. However, it was difficult to determine the block boundaries within the sub-blocks.