Orientational morphology of low carbon steel rolled in nonrecrystallized austenite region

The application of an EBSD technique in an SEM reveals a new approach to analyze a morphology of low carbon steel rolled in the nonrecrystallized austenite region. The bainitic ferrite structure has a grain size of 5 to 20 microns and a strong {332}〈113〉 and {311}〈011〉 transformation texture is developed as observed by the conventional measuring method. (1) The grain observed in the etched steel with conventional microscopy is defined as a region where a misorientation between adjacent points is within 1°, surrounded by boundaries having a misorientation over 2° by means of EBSD. In this steel, some of the grain boundaries include subboundaries having a misorientation from 2 to 5°. (2) Orientation colonies, which consist of some grains having a misorientation between adjacent grains within 15°, are observed with EBSD. These colonies are flat-elliptical in shape corresponding to a deformed austenite grain before transformation, resulting in the transformation from deformed one austenite grain to some ferrite grains with variant selection. (3) The orientation colony is divided into some regions which consist of a few grains having a misorientation between adjacent grains within 5°, resulting in the transformation from divided austenite grain to some ferrite grains with the same variant or close variants of the K-S relation. (4) In an orientation colony, each grain has a misorientafion within 15°, leading to the transformation from austenite to ferrite with a close variant of the K-S relation. (5) These experimental results suggest that a bainite transformation might have proceeded in such a way that the Bain strain or shear strain corresponding to the Bain strain as first occurred and the variant selection is controlled by this strain.