Papers by Author: Jae Kwan Kim

Abstract: Phase-field model (PFM) in multiple orientation fields was used to simulate the grain growth in three-dimensions (3-D) for isotropic and anisotropic grain boundary energy. In the simulation, the polycrystalline microstructure was described by a set of non-conserved order parameters and each order parameter describes each orientation of grains. For isotropic grain boundary energy, the simulation showed the microstructure evolution of normal grain growth. For anisotropic grain boundary energy, however, the simulation showed that certain grains which share a high fraction of low energy grain boundaries with other grains have a high probability to grow by wetting along triple junctions and can grow abnormally with a growth advantage of solid-state wetting. The PFM simulation shows the realistic microstructural evolution of island and peninsular grains during abnormal grain growth by solid-state wetting.

Abstract: Recrystallization texture in grain oriented silicon steel at different annealing temperature is investigated. Normalized x-ray intensities of various orientation components observed in Orientation Distribution Function are used for comparison. The computed CSL boundary distributions about Goss component with annealing condition were calculated. The misorientation angle distribution is also measured in order to find the importance of high-energy boundary with misorientation 20~45° range for the secondary recrystallization of Goss grain. From the analysis of CSL boundary distribution and misorienation angle distribution, the distribution of CSL boundaries does not evidently show any preferred difference between Goss and other texture components. Whereas, the misorientation angle analysis shows that the number of 20°~45° misoriented boundaries is higher around the Goss grains than around other texture components.

Abstract: The effects of hot band annealing temperature on the texture of the 1% and 2%Si nonoriented electrical steel were investigated. Slab was hot rolled and then hot band annealed in the temperature range of 900°C~1100°C. The magnetic flux density and the core loss were improved by the hot band annealing because of the texture improvement. As the hot band annealing temperature was increased, the magnetic properties were improved. The microstructure of the hot band was composed of a recrystallized structure at the surface and a deformed structure near the middle plane. These hot bands were completely recrystallized after annealing above 1000°C. The main texture of the hot band was rotated cube and gamma-fibre. After hot band annealing, rotated cube changed to cube texture and gamma-fibre intensity gradually decreased. In the case of specimen without hot band annealing, rotated cube in the middle plane was changed to near {111}<112>texture and Goss texture in the surface to gamma fibre after final annealing. In the case of the hot band annealed at 900°C, Goss texture and cube texture were developed. After final annealing, the {111} and {112} texture was dramatically decreased as the hot band annealing temperature was increased. Although the total {100} texture intensity was not changed, Cube texture, {100}<001>, was strengthened and rotated cube texture, {100}<011>, weakened for 2% Si steel. However, 1% Si steel was opposed to 2% Si steel. The {110} texture was strengthened irrespective of hot band annealing temperature. As the hot band annealing temperature was increased, the Goss texture was strengthened, and this makes the anisotropy of the magnetic flux density bigger.

Abstract: The magnetic properties of nonoriented electrical steels are influenced by grain size and texture of final products. The key technology in the commercial production of nonoriented electrical steels is to grow grains with {hk0}<001> texture up to the optimum size in the final annealing process. The problems related to grain size control have been extensively investigated, while texture control has received much less attention. Therefore, there is enough room to improve the magnetic properties through the control of texture. In this study, systematic investigations on the texture evolution during both recrystallization and grain growth have been made. The formation of recrystallization texture is explained by oriented nucleation. This is supported by the fact that the area fraction of nuclei or recrystallized grains with specific orientation to all new grains remains almost constant during the progress of recrystallization. Most nuclei have a high misorientation angle of 25∼55° with the surrounding deformed matrices. During the progress of grain growth, the Goss texture component continues to decrease because the Goss grains have a high percentage of low angle, low mobility grain boundaries. The grains of Goss orientation have a smaller grain size than those of random orientation.

Abstract: Abnormal grain growth (AGG) takes place in many metallic systems especially after recrystallization of deformed polycrystals. A famous example of AGG in metallic system is the Goss texture in Fe-3%Si steel. During high temperature annealing of Fe-3%Si sheet, a few near Goss {110} <001> grains grow exclusively fast and consume the matrix grains. Therefore, the grains which have near Goss orientation have special advantage over other grains. As a new approach to the growth advantage of AGG, we suggested the solid-state wetting mechanism, where a grain wets or penetrates the grain boundary or the triple junction of its neighboring grains. The solid-state wetting mechanism for the evolution of the Goss texture in Fe-3%Si steel was studied experimentally and by phase-field model (PFM) simulation.

Abstract: In nonoriented electrical steels, the control of texture has received little attention, and hence there is an unexplored possibility to improve the magnetic properties of nonoriented steels through texture control. Furthermore, the formation of recrystallization texture in these steels has not yet been systematically studied. In this study, such systematic investigations are undertaken for nonoriented electrical steels with 2% Si. New information obtained from EBSD measurements on partially recrystallized specimens will allow us to know what is happening during the recrystallization stage. The formation of recrystallization texture is much better explained by oriented nucleation. This is supported by the fact that the area fraction of nuclei or recrystallized grains with specific orientations for all new grains remains almost constant during the progress of recrystallization. Most nuclei have a high misorientation relationship with the surrounding deformed matrix: 25~55
. The main texture components of nuclei or recrystallized grains during the progress of recrystallization are Goss and {111}<112>. Deformed {111}<110> and {111}<112> grains generally disappear at the early stage of recrystallization whereas deformed {001}<110> and {112}<110> grains are mostly consumed at the late stage of recrystallization.

Abstract: The effects of hot band annealing temperature on the texture of the 2%Si nonoriented electrical steel were investigated. Slab was hot rolled and then hot band annealed in the temperature range of 900°C~1100°C. The magnetic flux density and the core loss were improved by the hot band annealing because of the texture improvement. As the hot band annealing temperature was increased, the magnetic properties were improved. The microstructure of the hot band was composed of a recrystallized structure at the surface and a deformation structure near the middle plane. These hot bands were completely recrystallized after annealing above 1000°C. The main texture of the hot band was rotated cube and gamma-fibre. After hot band annealing, rotated cube changed to cube texture and gamma-fibre intensity gradually decreased. In the case of non-annealed hot band, rotated cube in the middle plane was changed to near {111}<112>texture and Goss texture in the surface to gamma fibre after final annealing. In the case of the hot band annealed at 900°C, rotated cube near the middle plane changed to Goss texture and Goss texture in the surface to rotated cube after final annealing. After final annealing, the {111} and {112} texture was dramatically decreased as the hot band annealing temperature was higher. The total {100} texture intensity was not changed. Cube texture {100}<001> increased and rotated cube texture {100}<011> decreased. The {110} texture increased after hot band annealing irrespective of temperature. As the hot band annealing temperature was higher, the Goss texture increased, and this increase of Goss texture causes the anisotropy of the magnetic flux density.

Abstract: Nonoriented electrical steels have been widely used as core materials in motors and generators. For these applications low core loss and high permeability are required. The magnetic properties of these steels depend on the grain size and crystallographic texture of the annealed final products. The problems related to grain size control have been extensively investigated, while texture control has received much less attention. The technologies used to control the grain size in nonoriented electrical steels have approached to their limits. However, there is still some possibility for improvement of the magnetic properties through texture control. In order to explore this possibility, the evolution of recrystallization texture for nonoriented electrical steels with 2% Si was systematically studied. Texture change during grain growth was additionally analyzed. The formation of recrystallization texture is explained by oriented nucleation. This is supported by the fact that the area fraction of nuclei or recrystallized grains with specific orientation to all new grains remains almost constant during the progress of recrystallization. Most nuclei have a high misorientation angle of 25~55° with the surrounding deformed matrices. During the progress of grain growth, Goss and {111}<112> components are weakened and the random texture is strengthened. The grains of the Goss and {111}<112> orientations have smaller grain size than those of random orientation.