Papers by Author: Shuichi Nakamura

Abstract: On the basis of Hillerts model of grain growth, a new model of Goss secondary recrystallization in silicon steel has been developed in which inhibitor and grain boundary energy are taken into account. An analysis shows that these two parameters synergistically affect secondary recrystallization and Goss grain evolves to a coarse grain as inhibitor intensity increases and statistical grain boundary energy decreases. This model successfully explains Goss secondary recrystallization.

Abstract: The effect of oxygen content on toughness in the high strength weld metals with full martensitic microstructures was investigated for Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW). Solid and Flux cored two types wires were examined for their influence on the resulting oxygen content in weld metals. It has succeeded in controlling the oxygen contents without changing welding processes. As expected, the increasing oxygen content obviously decreased the upper shelf energy (vEshelf). One of the primary reasons of this tendency is considered that the higher density of oxide makes dimples on the ductile fracture surface smaller. On the other hand, as unexpected, the oxygen content from 7 to 450 ppm had no impact on the fracture appearance transition temperature (FATT), and oxides at the brittle fracture initiation point have not been found. This result supports that the oxides in high strength full martensitic weld metals (Vickers hardness = 360 ~ 430) have no harmful effect on FATT.

Abstract: Mechanism of Goss secondary recrystallization in grain-oriented silicon steel has been investigated by temperature gradient annealing and by in situ observation utilizing synchrotron x-ray topography. The results support the selective growth theory. Migration of Goss grains is controlled by second phase particles (inhibitor) and sharper Goss grains, which have higher frequency of CSL boundaries to the matrix, start to grow preferentially while the other matrix grains are stagnated by inhibitor. CSL boundaries are supposed to have lower grain boundary energy, thus suffer lower pinning force from the inhibitor and start to migrate at higher inhibition level. Based on this model, we have made a computer simulation and have found that this model successfully depicts the important features of secondary recrystallization; grain growth behavior of secondary grains, secondary grain size and sharpness of Goss texture.

Abstract: Heavily cold rolled BCC steel has been indicated to generate {411}<148> recrystallisation texture and its family orientations which might be represented as {h,1,1}<1/h,1,2>. As a-fibre structure, or RD//<011> texture is significantly developed during the cold rolling, it is naturally speculated to be the recrystallisation site of {h,1,1}<1/h,1,2> fibre. The present paper prompts to demonstrate the recrystallisation procedure by utilising EBSP-OIM analysis. The first demonstration was carried out with OIM analysis on partially recrystallised cold rolled steel. At the stage of 50% recrystallisation, only ND//<111> texture has appeared for the recrystallised area. {100}<011> - {211}<011> a-fibre remains as deformed structure, and several {h,1,1}<1/h,1,2> grains could be found at the grain boundaries. Therefore, a bi-crystal of {100}<011> was employed to simulate the irregular deformation at the grain boundary. After cold rolling, a warp toward the grain boundary was observed. Although the interior of the {100}<011> single crystal was hardly recrystallised, sharp {411}<148> texture was created along the grain boundary. In order to confirm the phenomenon, another experiment was carried out that a cold rolled {100}<011> single crystal was bent along the rolling direction and annealed. Very sharp {411}<148> recrystallisation texture was formed again at the bent perimeter. These experimental results lead us to conclude that the irregular strain was sufficiently piled at the grain boundary after the heavy deformation and generates {h,1,1}<1/h,1,2> texture. On {100} pole figures, the recrystallisation textures were equivalently scattered around three <100> poles, therefore the rotation relationship around <111> axes with the original orientation was suggested.

Abstract: Recrystallization of cold rolled {211}<011> single crystal of 3.5%Si-Fe were investigated by electron back-scattered pattern (EBSP) technique in order to clarify the formation of {h,1,1}<1/h, 1, 2> fiber in recrystallization texture of steel with high cold rolling reduction. In the cold rolled sample, there exist shearband like substructures characterized by the orientation fluctuation with single <011> axis whereas their orientations are almost kept to the original orientation {211}<011> which belongs to a-fiber (RD//<110> fiber). In the recovery stage, the orientation fluctuations are enhanced and include fluctuations with another <011> axis. That is the fluctuations in recovery stage are not characterized by rotation relationship with single <011> axis. The recrystallization texture includes {100}<012> and {411}<148> as major orientations in {h,1,1}<1/h, 1, 2> fiber. Furthermore, there exists clear orientation relationship characterized by multiple rotations with <011> axes between the recrystallization orientations and the original one. Actually, orientations characterized by rotational relationships with <211> (=[110]+[101]) axis and <111> (=[110]+[101]+[011]) were observed. These results suggest that recrystallization from a-fiber is occurred as a result of the final enhanced stage of the fluctuations characterized by multiple rotations with <011> axes.