RESIDUAL STRESS DISTRIBUTION IN GRAIN-ORIENTED SILICON STEEL

Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47. 402 RESIDUAL STRESS DISTRIBUTION IN GRAIN-ORIENTED SILICON STEEL Muneyuki Imafuku, Tamaki Suzuki Advanced Technology Research Laboratories, Nippon Steel Corporation Futtsu, Chiba 293-0011, Japan Hiroshi Suzuki* Department of Mechanical Engineering, Tokyo Metropolitan University Hachioji, Tokyo 192-0397, Japan Koichi Akita Department of Mechanical Systems Engineering, Musashi Institute of Technology Setagaya, Tokyo 158-8557, Japan *Present Address: Neutron Science Research Center, Japan Atomic Energy Research Institute, Nakagun, Ibaraki, 319-1195 Japan ABSTRACT We investigated the residual stress distribution in a grain of laser-irradiated and gearrolled grain-oriented Fe-3%Si silicon steels by X-ray stress measurement method in order to clarify the effect of residual stresses on magnetic domain refining. Dotted cavity lines were formed by Nd:YAG laser with a power density of 3.3 mj/pulse for the laserirradiated sample. By using a newly developed X-ray stress measurement method for a single crystal, the tensile residual stresses of 70-160 MPa were observed only in the vicinity of the laser-irradiated lines. After annealing at 1027 K for 2 hours in hydrogen, the residual tensile stresses were released and the domain-refining effect vanished. As for the gear-rolled sample, grooves of 0.1 mm wide and 0.015 mm deep were induced at 5 mm intervals. After annealing under the same condition, the residual stresses around the groove were released, whereas the refined magnetic domains were preserved. Therefore, the residual stresses should have no relevance to the domain-refining, which explains the tolerance to the thermal annealing for the gear-rolled ones. INTRODUCTION Grain-oriented Fe-3%Si steel, consisting of {110}<001> oriented large grains, is a soft magnetic material mainly used for transformer cores. In order to reduce the energy consumption in industries, the reduction in iron core loss has been strongly demanded. This has been achieved mainly by improving {110}<001> alignment, making the thinnergauge material and refining the magnetic domain spacing[1]. Pulse laser irradiation [2] and gear-rolling [3] techniques have been developed for the last purpose. It has been recognized that the gear-rolled samples have a greater heat-resistance than the laser-

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Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47. 403 irradiated ones and are suitable for the wound type iron cores [3]. In these processes, the induced stresses or recrystallized micro grains might be effective for the magnetic domain-refinement [4,5]. However, the actual survey of the stress distribution has not been done and therefore, the difference of the domain-refining mechanism between the two processes is not clear up to now. The stress measurement of a large grain sample is impossible by the conventional X-ray stress measurement method, sin 2 ψ method because the diffraction pattern from one grain becomes spotty and cannot form a continuous cone. Recently, Suyama et al. have proposed the x-ray stress measurement method based on the multiple regression analysis from the diffraction spots from a single crystal and applied for the uniaxial loading test of grain-oriented Fe-3%Si steel [6]. Very recently, Suzuki et al. have developed an advanced high accurate system [7] and succeeded to measure the stress distribution in laser-irradiated grain-oriented Fe-3%Si steel [8]. Furthermore, they proposed a new analysis principle to determine the stress states [9]. In this study, we investigated the residual stress distribution in a grain of laser-irradiated and gear-rolled grain-oriented Fe-3%Si silicon steels by the newly developed x-ray stress measurement method for a single crystal in order to compare the mechanisms of the magnetic domain refinement with special reference to the effect of residual stresses in these two processes. EXPERIMENTAL Sample preparation Figure 1 shows the sample preparation procedure in this study. Single crystal specimens, 15 mm X 15 mm X 0.23 mm, were cut out from the grain-oriented Fe-3%Si silicon steel sheet without tensile film coating. These samples were annealed at 1027 K for 2 h in pure hydrogen atmosphere in order to relief the induced stress by cutting. After the stress relief annealing mentioned above, two types of magnetic domain refining processes were applied. One is the laser-irradiation process. Dotted cavity lines were formed by Nd:YAG laser with a power density of 3.3 mj/pulse in air at room temperature for the laser-irradiated sample. The pitch of the cavity line was 5 mm and the diameter of the laser-focused cavity was about 0.18 mm. The other is groove forming process. Grooves of 0.1 mm wide and 0.015 mm deep at 5 mm intervals were induced by gearrolling. Figure 2 shows the surface shapes of the specimens observed by the confocal laser scanning microscope. These samples were annealed again at the same condition (at 1027 K for 2 h in pure hydrogen atmosphere) to investigate the thermal stabilities of the magnetic properties and stress states. X-ray stress measurement for a single crystal Stress measurement in a single crystal is an important subject for material science and has been studied for more than a decade [10-11]. Recently, a unique and practical X-ray

Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47. 404 stress measurement method for a single crystal has been developed by the present authors and others [6-9]. Following is the essential point of this method applied in this study. A lattice strain of ε L 33 in the L 3 direction on the laboratory coordinate system in the plane stress condition is expressed with the stress components σ S uv on the specimen coordinate system by the following equation [9]: ε L 33= -(θ n -θ 0 )cot θ 0 = γ 3i γ 3j π uk π vl S ijkl σ S uv = A n σ S 11+B n σ S 12+C n σ S 22 (1) Where, S ijkl is the elastic compliance of a single crystal, and π and γ are the transformation matrices between the specimen and crystal coordinate systems, and crystal and laboratory coordinate systems, respectively. In this equation, θ 0 is the half of the diffraction angle in the stress-free condition, and is usually unknown. A n, B n and C n are the variables which can be determined by the Miller indices of the measured diffraction planes. θ n is expressed as the following equation: θ n = -{(σ S 11 A n +σ S 12 B n +σ S 22 C n )/cot θ 0 } + θ 0 (2) By choosing the equivalent diffraction planes, α 11, α 12, α 22, and θ 0 can be calculated by the multiple regression analysis method. In this analysis, at least four equivalent diffraction planes should be measured. 70 W:100m D: 15m Figure 1. Scheme of sample preparation procedure.

Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47. 405 100 µm 100m (a) (b) Figure 2. Microscopic images of the specimens around (a) the laser-irradiated cavity and (b) the gear-rolled groove. In order to obtain the perfect diffraction profile to determine the accurate diffraction angle for each plane with 1-dimentional position sensitive proportional counter (PSPC) type detector, χψ-oscillation method [7] was utilized. The axes of the χ oscillation and ψ-oscillation correspond to the direction of collimated incident X-ray beam and the vertical direction of the χ rotation axis. Figure 3 shows the X-ray stress measurement apparatus used in this study. The single crystal specimen was mounted on the χψ-oscillation stage. The measured position of the specimen was adjusted at the rotation center of the stage within 20 µm error by using the laser positioning system. -oscillation -oscillation Figure 3. The stress measurement apparatus for a single crystal.

Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47. 406 The orientations of the samples were measured by Laue method beforehand. The values of the laser-irradiated and gear-rolled samples were (116 127 3)[-2 0 100] and (41 40 1)[- 5 2 100], respectively, which are closer to the so-called Goss orientation, {110}<001>. A shield-tube X-ray source of Cr-Kα radiation with a power of 30 KV and 8 ma was used in this study. The diameter of X-ray beam was 0.4 mm. Six equivalent 211 diffraction planes of α-fe, 211, 112, 121, 12-1, 11-2 and 21-1, satisfied reflection condition and were chosen for the measurement. The stresses at 5 positions, 0 mm, 0.25 mm, 0.50 mm, 1.00 mm and 2.00 mm from the laser-irradiated cavity line and gear-rolled groove were measured. RESULTS AND DISCUSSION Magnetic domain The basic domain structures of the grain-oriented Fe-3%Si steel samples were measured by a scanning electron microscope. Figure 4 shows the change in the width of the magnetic domains by the two processes. Slab type 180-degree domains were observed along the rolling direction of <001>, which is the easy magnetization axis of iron. The width of the magnetic domains was approximately 0.7-1.0 mm before applying magnetic domain refining processes. We can see from this figure that the 180-degree magnetic domains were drastically refined to be less than 0.3 mm and were divided at the cavity line and groove by the laser-irradiation and gear-rolling processes, respectively. After the stress relief annealing mentioned above, the magnetic domains were returned to their original state and the laser-induced cavities became ineffective in the case of laser irradiation. On the other hand, the refined 180-degree magnetic domains were preserved for the gear-rolling sample. These results suggest the origin of the magnetic domainrefining is different between these processes. Figure 4. Change in magnetic domains around the laser-irradiated cavity and the gearrolled groove.

Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47. 407 X-ray topography Figure 5 shows the X-ray topographical images of the laser-irradiate and gear-rolled grain-oriented Fe-3%Si silicon steel sheet. We can see from these photographs the highlighted lines corresponding to the laser cavities or grooves, suggesting that some kinds of localized strains were induced near the lines in both processes. For the next step, quantitative stress measurement is necessary in order to clarify the details of the stress states of these samples. 5mm (a) Laser-irradiated sample (b) Gear-rolled sample Figure 5. X-ray topographical images for (a) laser-irradiated and (b)gear-rolled samples. Residual Stress Distributions The residual stress distributions in laser-irradiated sample are shown in Fig. 6. σ 11, σ 22 and σ 12 represent the plane stresses in rolling direction (1-direction), transverse direction (2-direction) and the shear stress, respectively. It was found that the tensile stresses of about up to 70 MPa in 1-direction and 160 MPa in 2-direction were induced just around the laser-irradiated cavity line. The value of σ 22 was almost two times larger than that of σ 11 since the neighboring cavities in 2-direction affected to increase the tensile stress. The stress induced area was less than 0.5 mm, that is to say, the very limited area. σ 11 and σ 22 are considered to be the principle stresses in the surface plane because the shear stress, σ 12 is negligible as is seen in this figure. Considering the melting traces of the cavities as is seen in Fig. 2, the thermal history effect (heating->cooling) was dominant rather than the shock wave effect in the laser irradiation process. Consequently, the tensile stresses were induced. Similar results had been reported for the laser peening of stainless steel in air [12]. It was supposed that the local tensile stresses induced by the laser irradiation destabilize the newly formed 90-degree magnetic domains so as to refine the 180-degree domains. After the stress relief annealing, the residual tensile stresses were completely released. At the same time, the domain-refining effect vanished as is shown in Fig. 4. Therefore, we can say that the local tensile effect is essential for the laser irradiation process. Figure 7 shows the residual stress distributions in gear-rolled sample. We could not measure the residual stress at X=0 mm (just at the groove position) since the diffraction profiles were not good. Complex stress states, both compressive and tensile states, were observed in the vicinity of groove. After annealing at 1027 K for 2 h in pure hydrogen atmosphere, the residual stresses around the groove were released, whereas the refined

Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47. 408 magnetic domains were preserved as is shown in Fig. 4. Therefore, the residual stresses were not the cause of the magnetic domain-refining. The mechanism of magnetic domain refining can be explained with the analogy of the previous scratching studies [13]. Once the groove is formed, new magnetic poles arise at the new walls in the groove so as to refine the magnetic domains. This kind of shape effect of grooves may be essential for the domain-refining instead of the induced residual stress states in the gear rolling process. The better thermal stability of the refined magnetic domains is suitable for the stress relief annealing processed on wound cores. However, some further experimental studies, particularly the magnetic domain structures near the cavity lines, should be required for the fully understand of the domain refining mechanism. σ σ σ σ σ Figure 6. The residual stress distributions in laser-irradiated sample before and after annealing. σ σ 22 σ 11 σ 12 σ σ σ σ σ 22 σ 11 σ 12 Figure 7. The residual stress distributions in gear-rolled sample before and after annealing.

Copyright JCPDS - International Centre for Diffraction Data 2004, Advances in X-ray Analysis, Volume 47. 409 CONCLUSION The residual stress distributions in laser-irradiated and gear-rolled grain-oriented Fe-3% silicon steels were measured by the newly developed X-ray stress measurement method for a single crystal. The tensile residual stresses of 70 to 160 MPa were observed only in the vicinity of the laser-irradiated cavity line. After annealing at 1027 K for 2 h in pure hydrogen atmosphere, the residual stresses were released, and the domain-refining effect vanished. It was supposed that the local tensile stresses induced by the laser irradiation destabilize the newly formed 90-degree magnetic domains so as to refine the 180-degree magnetic domains. The local tensile stress caused by the thermal effect for domain-refining was confirmed in the laser irradiation process. In the case of groove forming process by gear rolling, complex states of compressive and tensile stresses were observed in the vicinity of groove. After annealing at 1027 K for 2 h in pure hydrogen atmosphere, the residual stresses around the groove were released, whereas the refined magnetic domains were preserved. Therefore, the residual stresses should have no relevance to the magnetic domain-refining. The shape effect so as to induce the new magnetic poles at the grooves may cause the domain-refining in the gear rolling process. REFERENCES [1] Y. Ushigami, H. Masui, Y. Okazaki, Y. Suga, N. Takahashi: J. Mater. Eng. Performance, 5 (1996) 310. [2] T. Iuchi, S. Yamaguchi, T. Ichiyama, M. Nakamura, T. Ishimoto and K. Kuroki: J. Appl. Phys., 53 (1983) 2410. [3] H. Kobayashi, K. Kuroki, E. Sasaki, M. Iwasaki and T. Takahashi: Physica Scripta: T24 (1988) 36. [4] M. Nakamura, K. Hirose, T. Nozawa and M. Matsuo: IEEE Trans. Mag., MAG-23 (1987) 3074. [5] T. Nozawa, Y. Matsuo, H. Kobayashi, K. Iwayama and N. Takahashi: J. Appl. Phys., 63 (1988) 2966. [6] Y. Suyama, S. Ohya and Y. Yoshioka: J. Soc.Mat. Sci., Jpn, 48 (1999) 1437. (in Japanese) [7] H. Suzuki, K. Akita and H. Misawa: Mater. Sci. Res. Int., 6 (2000)255. [8] H. Suzuki, K. Akita, H. Misawa and M. Imafuku: Mater. Sci. Res. Int., 8 (2002) 207. [9] H. Suzuki, K. Akita and H. Misawa: Jpn J. Appl. Phys., 42 (2003) 2876. [10] B. Ortner: J. Appl. Cryst., 22 (1989) 216. [11] B. Ortner: Advances in X-ray Analysis, 29 (1986) 387. [12] N. Mukai, N. Aoki, M. Obata, A. Ito, Y. Sano and C. Konagai: Proc. 3rd Int. Conf. Nuclear Engineering (1995) 1489. [13] M. F. Littmann: J. Appl. Phys., 38 (1967) 1104.