Analysis of abnormal grain growth and improvement in grain-oriented electrical steel based on solid-state wetting mechanism

Abstract

Abnormal grain growth(AGG) occurs frequently in many metallic systems. One of the famous examples is the Goss grain which abnormally grows during secondary recrystallization in Fe-3%Si steel. This phenomenon is already used for the grain-oriented electrical steel but its mechanism has not been clearly understood. The sub-boundary enhanced solid-state wetting has been suggested to explain the mechanism of AGG. This theory suggests that the sub-boundary increases the wetting probability and the increased wetting probability enhances AGG. In this paper, based on this theory, microstructure and morphology of abnormally growing grains and matrix grains are analyzed and the grain-oriented electrical steel is improved. At the first, misorientation measurements at the growth front of abnormally growing grains in 5052 aluminum alloy were made using electron back-scattered diffraction (EBSD). When a three-dimensional morphology of solid-state wetting along a triple junction line is observed on a two-dimensional section, two kinds of morphologies could be observed. One is a morphology of penetrating the grain boundary when the section is parallel to the triple junction line. Grain boundary energies, which were estimated from misorientation measurements of the three grains in the penetrating morphology, satisfied the energetic condition for wetting along the triple junction line. Misorientation measurements showed that some matrix grains away from the growth front of abnormally growing grains had the same crystallographic orientation as that of the abnormally growing grain. This result implies that the abnormal grain growth in 5052 aluminum alloy occurs by the mechanism of sub-boundary enhanced solid-state wetting. The morphology of abnormally growing grains are predicted by three-dimensional Monte Carlo simulation and the real three-dimensional morphology is reconstructed by serial sectioning. The simulation was performed with sub-boundaries, precipitates and real crystallographic orientations of matrix grains. In the simulation, AGG occurs only when the both of sub-boundaries and precipitates exist. Form this simulation result, AGG occurs based on sub-boundary enhanced solid-state wetting mechanism. The real three-dimensional morphologies of abnormally growing grains are extremely similar to that of simulated abnormally growing grains. Both grains have island, peninsular grains and extremely irregular shape. This result shows that the factors are applied realistically in the simulation. Because the simulation shows realistic AGG, the behavior of abnormal grain growth was analyzed by the simulation. The simulation shows that the sub-boundary energy and the crystallographic orientation of matrix grains determine the velocity of AGG. According the analysis, the rate of AGG is determined by texture of matrix grains. And the texture can be controlled by asymmetric rolling. By the asymmetric rolling, the AGG rate are fastened and the magnetic properties of the grain-oriented electrical steel is improved.