Two kinds of grain-oriented silicon-irons were used for the study on the mechanism of secondary recrystallization in grain-oriented silicon iron. In Part I, a possible role of carbon content in high-permeability boron slicon-iron has been investigated. The effects of cold work on the secondary recrystallization in the grain-oriented silicon-iron have been investegated using conventional silicon-iron, and the results are presented in Part II.
Effect of carbon content and various heat treatments on the secondary recrystallization and induction in boron silicon-iron have been studied by measurement of induction at a magnetizing force of 800 A/m and examination of microstructures at various stages of sample preparation process. Carbon was varied before and after hot rolling by adding carbon to the melts and by oxide decarburization of hot bands at 700℃. Results show that complete secondary recrystallization and high induction (1.92 Tesla) are obtained from heats whose hot bands have been decarburized at 700℃ to a level of 30-40 ppm provided the heats contained 0.039-0.049 w/o carbon during heating for hot rolling. From these facts and analysis of microstructures of cross sectional area of specimen at various stages of process it can be said that one of the major roles that carbon play is to introduce, through transformation of austenite, a nonuniform layer-like microstructure during hot rolling and during the normalizing heat treatment of the hot band. This structure continues to exist in the final cold rolled sheet and results in anisotropic growth of secondary grains, and playing a significant role in the secondary recrystallization of boron silicon-iron.
Effects of cold work and intermediate heat treatment in conventional silicon-iron have been investigated by examination of texture, through the observation of micro-etch pits, and microstructure. Unlike standard cold rolling process for the conventional silicon-iron, in which a final cold reduction of about 60% after an intermediate heat treatment is used, the direction of the final cold rolling with respect to that of the first cold rolling as well as the amount of the final cold reduction were varied. The intermediate annealing was performed in one case, and the heat treatment was omitted in another case. The direction of cold rolling with respect to that of hot rolling was also varied.
Results indicate that the magnetic induction and the occurance of a complete secondary recrystallization are independent of the angle between the hot and cold rolling directions. Complete secondary recrystallizations and induction maxima were observed in the specimens, for which the final cold reductions were 60 to 70% and, no intermediate heat treatment was performed when the angle between the first and the final cold rolling was 30-45˚. On the other hand, poor secondary recrystallization and an induction minimum was observed in the specimen for which the angle between the first and the final cold work was 45˚ and the intermediate heat treatment was performed. It was also observed that the shear bands, which were generated by the cold work, crystallize first during the heat treatment.
Basing on the measurement of the induction and a detailed investigation and analyses of texture and microstructure, the origin and growth of {110}<001> Goss texture and the role of the intermediate heat treatment in conventional siliconiron are discussed.
