In chapter 1, grain boundary shapes and grain growth in bulk 2.61wt% Si iron have been studied by heat-treating at temperatures between 700 and 1200℃. Initial microstructure with fairly uniform fine grains has been obtained by recrystallization at 800℃ for 5 min after deformation. When subsequently heat-treated at 700 and 800℃, a fraction of the grain boundaries have hill-and-valley shapes with several facet planes or kinks. Some of these facet boundary segments are expected to be singular. Abnormal grain growth occurs at 700 and 800℃ and is attributed to step growth of the boundaries. When heat-treated at 1000℃, all grain boundaries are defaceted with smoothly curved shapes, indicating that they are atomically rough. At temperatures above 1000℃, normal grain growth occurs, because the rough grain boundaries move continuously. This correlation between grain boundary structure and grain growth is consistent with the earlier observations in other metals and oxides. It is thus shown that the abnormal grain growth in this alloy occurs at low temperatures because of the singular grain boundary structure.
In chapter 2, effects of heat-treatment temperature and small deformation on grain growth and texture in commercial silicon steel sheet have been studied. 3.17wt% silicon steel sheets produced by POSCO have been used. POSCO silicon steel sheets consist of uniform fine grains and have no texture. Firstly, temperature dependence of grain growth and texture has been examined by heat-treating at temperatures between 800 and 1400℃. The grain growth during the heat-treatment at 800℃ is very slow. After 24 h at 800℃, the grains are only slightly larger than in the as-received sheet. At temperatures above 900℃, abnormal grain growth occurs. As the heat-treatment temperature increases, growth rates of both matrix grains and abnormal grains increase. Completion of secondary recrystallization and Goss texture formation depends on the relative growth rate of abnormal grains and matrix grains. Strong Goss texture forms only at the temperatures of 900, 1100, and 1200℃, where the growth rate of abnormally growing grains is much greater than that of matrix grains. At 1000, 1300, and 1400℃, Goss texture does not form due to the relatively fast growth of matrix grains. Electron back-scattered pattern (EBSP) analysis shows that Goss texture forms by the abnormal growth of Goss-oriented grains, and the growth rate of the abnormal grains with other orientations is much slower than that of Goss-oriented grains. The selective growth of Goss-oriented grains does not seem to be related with size advantage, surface energy anisotropy, or distribution of CSL boundaries. Grain boundary shapes in the specimens heat-treated at the temperatures below 1000℃ have been examined under a transmission electron microscope. Some of the grain boundaries have hill-and-valley shapes or kinked shape. Abnormal grain growth and texture formation in the commercial silicon steel sheet are also attributed to step growth of the singular grain boundaries.
Secondly, the effect of small plastic deformation on the grain growth and texture in commercial silicon steel sheet has been studied. As-received POSCO silicon steel sheets have been cold rolled slightly and subsequently heat-treated at temperatures between 1000 and 1400℃. As the deformation amount increases, growth rate and number density of abnormal grains increase and texture eventually becomes random. EBSP analysis shows that small deformation increases the growth rate of grains both with Goss orientation and other orientations. At 1000℃, small deformation prior to heat-treatment promotes the Goss texture formation by increasing the growth rate of Goss-oriented grains. At 1100 and 1200℃, Goss texture is deteriorated by small deformation due to the increased growth rate of matrix grains. With the step growth mechanism, small deformation produces lattice dislocations, which are absorbed at grain boundaries and provide grain boundary steps, thus increasing the grain growth rate. As the deformation amount increases, distribution of grain boundary dislocation becomes uniform, and the selective growth of Goss-oriented grains disappears.
In chapter 3, temperature dependence of grain growth in bulk commercial 3.2wt% silicon steel has been studied. Silicon steel ingot has been swaged and subsequently heat-treated at the temperatures between 800 and 1200℃. At all temperatures, abnormal grain growth occurs. There are ferrite and austenite phases in the specimens at the temperatures between 900 and 1200℃. Because the total amount of austenite decreases by decarburizing and austenites grains coarsen during heat-treatment, the grain growth is very complicated.
In chapter 4, temperature dependence of silicon iron sheet has been studied. Silicon iron sheets have been prepared by vacuum arc melting of pure iron and silicon, and cold rolling. Cold rolled sheets of 300μm in thickness have been recrystallized at 700℃ for 5 min, and subsequently heat-treated at the temperatures between 700 and 1000℃. At 700 and 800℃, abnormal grain growth occurs. At the temperatures above 900℃, normal grain growth occurs. The temperature dependence of grain growth is similar to the bulk silicon iron in chapter 1.
