ORIENTATIONAL MORPHOLOGY OF LOW-CARBON ST EEL 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 nonrecrystal lized austenite region. The bainitic ferrite structure has a grain siz e of 5 to 20 microns and a strong {332}[113] and {311}[011] transforma tion texture is developed as observed by the conventional measuring me thod. (1) The grain observed in the etched steel with conventional mic roscopy is defined as a region where a misorientation between adjacent points is within 1 degrees, surrounded by boundaries having a misorie ntation over 2 degrees by means of EBSD. In this steel, some of the gr ain boundaries include subboundaries having a misorientation from 2 to 5 degrees. (2) Orientation colonies, which consist of some grains hav ing a misorientation between adjacent grains within 15 degrees, are ob served with EBSD. These colonies are flat-elliptical in shape correspo nding to a deformed austenite grain before transformation, resulting i n 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 misorientatio n between adjacent grains within 5 degrees, resulting in the transform ation from divided austenite grain to some ferrite grains with the sam e variant or close variants of the K-S relation. (4) In an orientation colony, each grain has a misorientation within 15 degrees, leading to the transformation from austenite to ferrite with a close variant of the K-S relation. (5) These experimental results suggest that a bainit e transformation might have proceeded in such a way that tile Bain str ain or shear strain corresponding to the Bain strain as first occurred and the variant selection is controlled by this strain.