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Magnetic properties of grain oriented ultra-thin silicon steel sheets processed by conventional rolling and cross shear rolling
Materials Science and Engineering A 430 (2006) 138–141
Magnetic properties of grain oriented ultra-thin silicon steel sheets processed by conventional rolling and cross shear rolling Gao Xiuhua ∗ , Qi Kemin, Qiu Chunlin The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110004, PR China Received 12 April 2006; received in revised form 5 May 2006; accepted 19 May 2006
Abstract A new procedure consisting of the cross shear rolling (CSR) and the subsequent tertiary recrystallization annealing under dry hydrogen atmosphere was developed to produce the grain oriented ultra-thin silicon sheets less than 0.1 mm with high magnetic property performance. For comparison, the conventional rolling (CR) was also used to process the grain oriented ultra-thin silicon steel sheets. The effect of processing parameters on magnetic properties of the grain oriented ultra-thin silicon steel sheets was investigated. With the increase of annealing temperature and holding time, magnetic properties of the sheets processed by both rolling methods reach saturation as the result of the proceeding of the tertiary recrystallization. The thin sheets rolled by CSR did achieve better magnetic properties than those rolled by CR. © 2006 Elsevier B.V. All rights reserved. Keywords: Cross shear rolling; Tertiary recrystallization; Magnetic properties; Ultra-thin silicon steel sheet
1. Introduction A number of methods, such as controlling secondary recrystallization and magnetic domain structures, have been developed to improve the magnetic properties of the grain oriented silicon steels [1,2]. Recently, the development of the ultra-thin silicon steel sheet with thickness less than 100 m has drawn interest [3,4], since ultra-thin silicon steel sheets exhibit excellent soft magnetic properties. It is known that with a reduction of thickness the eddy current loss of the grain-oriented silicon steel sheet decreases [5–6] and the secondary recrystallization process becomes unstable and the (1 1 0) [0 0 1] grains orientation degree is deteriorated [7]. Also, it is generally difficult to produce grain oriented silicon steel sheets thinner than 160 m by the secondary recrystallization process using inhubitors. Therefore, a stable method is needed to produce ultra-thin grain-oriented silicon sheet. The weak (1 1 0) grains formed in the secondary recrystallization could take the surface energy as the motive force and grow up along the orientation of (1 1 0) [0 0 1] preferentially [8–10].
∗
Corresponding author. Tel.: +86 24 83686419; fax: +86 24 23906472. E-mail address: [email protected] (G. Xiuhua).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.05.058
Since coarse grain and high brittleness are developed after secondary recrystallization in the grain oriented silicon steel sheet that is used as the starting material of rolling, it is difficult to cold roll silicon steel sheet. Processing accidents, such as breaking and collaring, can decrease productivity and increase costs. As a new rolling technology, the cross shear rolling (CSR) method is a rolling process of a pair of work rolls rotating with different peripheral speeds. Comparing to the conventional rolling (CR), the cross shear rolling (CSR) has the following advantages in the rolling of ultrathin strip [11]: using less rolling load, the improvement of dimension accuracy and shape of the strip, and the capability of rolling strips into very thin thickness. It is therefore of great potential to develop CSR for the fabrication of ultra-thin silicon steel sheets. In this work, a new procedure consisting of CSR and the subsequent tertiary recrystallization annealing was developed to produce grain oriented ultra-thin silicon steel sheet of high quality with improved magnetic property performance. Tertiary recrystallization of the grain oriented ultra-thin silicon steel sheet is a very complex process [12]. During the whole course, minimal fluctuations in the processing variables can affect magnetic properties of the final product. In this paper, the CSR process parameters effecting on the tertiary recrystallization of ultra-thin silicon steel sheet were analyzed and the
G. Xiuhua et al. / Materials Science and Engineering A 430 (2006) 138–141
tertiary recrystallization behavior of ultra-thin silicon steel sheet rolled by CSR during annealing was studied. 2. Materials and experimental procedure The 0.3 mm grain oriented silicon steel sheet was taken as the starting raw material (magnetic induction: B8 = 1.8 T; core loss: P17/50 = 1.33 w/kg; average grain diameters: D = 10 mm). The chemical compositions of the silicon steel sheet are as follows (wt.%): C 0.07, Si 3.15, Mn 0.06, S 0.02, Cu 0.17, N 0.003, P 0.02. After the surface glass film was removed by dissolution in a solution of hydrofluoric acid and sulfuric acid, the sheets were cold rolled to various thickness from 0.06 to 0.10 mm by CR with mismatch speed ratio (MSR) of 1.0 and CSR with MSR of 1.17 using Ø90 mm/Ø200 mm × 200 mm four high rolling mill respectively. The rolling schedule using CSR of 1.17 is: 0.30 mm → 0.24 mm → 0.16 mm → 0.12 mm → 0.10 mm → 0.08 mm → 0.06 mm, and the rolling schedule using CR is: 0.30 mm → 0.24 mm → 0.19 mm → 0.16 mm → 0.14 mm → 0.12 mm → 0.10 mm → 0.09 mm → 0.08 mm → 0.07 mm → 0.06 mm. MgO was adopted as the coating material during the rolling process. Specimens were annealed at 1150, 1200, 1250 ◦ C for 3, 4.5, 6 and 7 h in a pipe dry hydrogen atmosphere furnace (dew point = −50 ◦ C) for tertiary recrystallization and cooled to 600 ◦ C at a rate of 10 K/s. Three tests were treated per process. The thickness of specimens was measured with micrometer. The magnetic induction B8 was measured by using impact method at magnetic field of 800 A/m with a 334 model soft magnetic materials measurement apparatus. Core losses were measured by using the bridge method at the excitation magnetic flux density of 1.7 T at frequency of 50 Hz. The recrystallization texture measurements were carried out on a D/max-IIIA X-ray diffractometer. The orientation distribution functions (ODFs) presented in this paper were calculated by a two-step method [13]. The ODFs using the notation of Roe constant ψ sections were selected to represent the texture.
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ties of ultra-thin silicon steel sheets, two different MSR, 1.0 and 1.17, were used for cold rolling. During the rolling experiment, it was observed that CSR could reduce the thickness more efficiently and steadily than CR (for example, 0.3 mm thick silicon steel sheet was rolled to 0.06 mm by six passes using CSR, and the same thickness by 10 passes using CR). CR brought about the thinned margin along the plate width direction (transversal direction) where cracks occurred easily. The tension stress during the CR process resulted in breaking, while the stress state in the deformation area was changed by the shear–stress applied during the CSR process. CSR with the thickness and deformation uniform, can decrease the area of thinned margin, reduce the tension stress, and avoided cracking accordingly. The tension stress along the edge of the strip was reduced remarkably and the edge-cracking trend was reduced. Therefore, even though the strip became brittle and
3. Results and discussion The grain growth during recrystallization of cold rolled silicon steels has preferential orientations. When annealed at appropriate condition, as (1 1 0) face of the body-centered cubic is the close-packed face and the surface energy of (1 1 0) face is the lowest, grains in (1 1 0) face paralleled to the sample surface have the driving force for growth preferentially. During tertiary grain growth, the secondary grain which has (1 1 0) [0 0 1] texture would grow up [14]. The surface energy and growth of grains in silicon steels have therefore strong interactions with the annealing process. 3.1. Effect of mismatch speed ratio on magnetic properties The mismatch speed ratio (MSR) of CSR determines the shearing deformation and also affects the lattice distortion and the tertiary grain growth process. To study the influence of MSR on the recrystallization behavior as well as magnetic proper-
Fig. 1. Dependence of magnetic properties upon the thickness for the silicon steel sheets annealed at 1200 ◦ C for 7 h.
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hard due to cold work during the CSR process, the amount of cracking and edge-cracking was reduced during the CSR process. Fig. 1 shows magnetic properties as a function of thickness for the grain oriented ultra-thin silicon steel sheets rolled by the two levels of MSR and then annealed at 1200 ◦ C in dry hydrogen atmosphere for 7 h. From the figure, It is seen that magnetic properties improve as the thickness of the silicon steel sheets reduces for both types of MSR. As shown in Fig. 1a and b, with the reduction of thickness from 0.10 to 0.06 mm, the magnetic induction (B8 ) of thin silicon steel is increased and the core loss (P17/50 ) dropped. The sample processed by CSR exhibited higher magnetic induction yet lower core loss than the one by CR at the same thickness. The difference in the magnetic properties of the sheets processed by the two types of rolling can be attributed to the deformation char-
acteristic and the subsequent recrystallization process. Compared with CR, intensive shear deformation exists during CSR. Under the same deformation condition, the equivalent deformation of CSR is larger than CR, which causes greater stress and strain-gradient [11]. The distortion energy increases and the degree of recystallization using such energy as the driving force increases. From analysis of texture [15], more -texture and grains with {1 1 0} 0 0 1 orientation formed in samples rolled by CSR than in samples rolled by CR. Therefore, it is easier to form Goss texture because of hereditary effects [14]. The fact that the magnetic properties rise with the thickness reduction is also an indication suggesting that the shear deformation benefits the occurrence of tertiary recrystallization and Goss grain growth [12]. Moreover, the thinner the strip is, the faster the grain grows up during the recrystallization process. For a thinner sample, the surface-to-volume ratio is higher and the difference of surface energy between (1 1 0) faces and other faces increases the driving force for the tertiary grain growth [14]. 3.2. Effect of annealing temperature on magnetic properties The annealing temperature has significant effect on the tertiary recrystallization and the magnetic properties of ultra-thin silicon steel sheets rolled by CR and CSR. Fig. 2a and b shows the dependence of magnetic induction on annealing temperature for the 0.06 and 0.08 mm sheet samples, respectively. The samples were heated to three different temperatures at a heating rate of 6 K/s and held for 7 h under a hydrogen flow of 6 L/min. From the figure, it is seen that the samples annealed at 1150 ◦ C exhibited the lowest magnetic inductions while the those annealed at 1250 ◦ C obtained the highest value for the same thickness and MSR. The higher annealing temperature is, the higher the magnetic properties are. This is due to the higher temperature giving rise to greater degree of recrystallization or fully recrystallized microstructure in the rolled sheet. The texture of 0.06 mm thick specimens rolled by CSR and annealed at 1250 ◦ C in dry hydrogen atmosphere is shown in Fig. 3. The obvious {1 1 0} 0 0 1 orientation texture was formed in ultra-thin silicon steel sheet.
Fig. 2. Dependence of magnetic induction on the annealing temperature for the silicon steel sheets with two different thickness (h = 0.06 and h = 0.08 mm).
Fig. 3. ODFs for constant ψ sections (ψ = 0◦ , 5◦ , 15◦ ,. . .) of ultra-thin silicon steel sheet; levels 5, 10, 15, 20,. . . peak density = 31.2.
G. Xiuhua et al. / Materials Science and Engineering A 430 (2006) 138–141
3.3. Effect of the holding time on the magnetic properties The dependence of magnetic induction on the holding time for the silicon steel sheets rolled by CR and CSR is shown in Fig. 4. The samples were heated to 1200 ◦ C at a rate of 6 K/s under a hydrogen flow of 6 L/min and held for different times ranging from 3 to 6 h. After the short holding time of 3 h, a significant difference in the magnetic induction was observed for the samples with different thickness. For both of CR and CSR, the 0.06 mm sheet sample exhibited a much higher induction value than the 0.08 and 0.10 mm sheets. The difference can be attributed to the sheet thickness effect. As the 0.06 mm sample has a higher ratio of surface area to volume than others, there is more driving force for the tertiary grain to grow fast during the recrystallization process. As a result, after annealing for 3 h in dry hydrogen atmosphere, the specimens rolled by both CSR and CR achieved high values of magnetic induction. For longer
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holding times, the magnetic induction increased slightly and appeared to reach saturation at 6 h, which is an indication that the tertiary recrystallization has completed. As to the 0.08 mm sample, the initial annealing of 3 h gave rise to a magnetic induction value less than that of the raw material (1.8 T), suggesting that the tertiary recrystallization has not occurred in the sample. At 4.5 h of annealing, the magnetic induction reached the value of the raw material but was still relatively lower, indicating the tertiary recrystallization was only partially completed. At 6 h of holding, the magnetic induction increased to a high value of 1.91 T, suggesting that the recrystallization approached to the end and the tertiary recrystallization texture was steady. 4. Conclusions A new procedure consisting of the cross shear rolling (CSR) and the subsequent tertiary recrystallization annealing under dry H2 atmosphere was developed to produce the grain oriented ultra-thin silicon sheets less than 0.1 mm with high magnetic property performance. The silicon steel sheets rolled by CSR could achieve better magnetic properties than those rolled by CR. For the same sheet, magnetic properties improves with the increase of annealing temperature in the range of 1150–1250 ◦ C and annealing time up to 6 h. The increase of magnetic performance is attributed to the occurrence of tertiary recrystallization process. The thinnest 0.06 mm sheet exhibited the highest magnetic induction and the lowest core loss. A reduction in thickness appears to be beneficial to the improvement of magnetic performance of the silicon steel sheet. Acknowledgement The authors wish to thank National Nature Science Foundation of China, for their financial supports for this work. References
Fig. 4. The variation of magnetic induction with the annealing time at 1200 ◦ C for different sheets.
[1] T. Iuchi, S. Yamaguchi, J. Appl. Phys. 53 (3) (1982) 2410–2415. [2] R. Krause, G. Rauch, J. Appl. Phys. 55 (6) (1984) 2121–2133. [3] K. Arai, K. Ishiyama, H. Mogi, IEEE Trans. Mag. 25 (5) (1989) 3949–3954. [4] K. Arai, K. Ishiyama, Bull. Jpn. Inst. Met. 31 (5) (1992) 429–435. [5] K. Arai, K. Ishiyama, J. Appl. Phys. Part II 64 (10) (1988) 5352–5354. [6] M. Nakano, K. Ishiyama, IEEE Trans. Magn. 33 (5) (1997) 3754–3756. [7] J. Harase, R. Shimizu, N. Takahashi, J. Jpn. Inst. Met. 54 (4) (1990) 381–387. [8] K. Ishiyama, K. Arai, J. Harase, N. Takahashi, J. Jpn. Inst. Met. 57 (1) (1993) 108–113. [9] Y. Kim, K. Ishiyama, K. Arai, IEEE Trans. Magn. 29 (6) (1993) 3535–3537. [10] S. Nakashima, K. Takashima, J. Harase, Acta Metall. Mater. 42 (2) (1994) 539–547. [11] Zhu Quan, Iron Steel 15 (5) (1980) 1–6. [12] Xiuhua GAO, Study of ultra-thin grain oriented silicon steel by cross shear rolling, Ph.D. Thesis, Northeastern University, Shenyang, 2002. [13] Z.D. Liang, J.Z. Xu, F. Wang, Proceedings of the Sixth International Conference on Textures of Materials, vol. 2, ISIJ, Tokyo, 1981, p. 1259. [14] H.E. Zhongzheng, Electrical Steel, Metallurgy Industry Press, Beijing, 1996, p. 225. [15] GAO. Xiuhua, QI. Kemin, J. Mater. Sci. Technol. 16 (2) (2000) 236 155–156.
Magnetic properties of grain oriented ultra-thin silicon steel sheets processed by conventional rolling and cross shear rolling Gao Xiuhua ∗ , Qi Kemin, Qiu Chunlin The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110004, PR China Received 12 April 2006; received in revised form 5 May 2006; accepted 19 May 2006
Abstract A new procedure consisting of the cross shear rolling (CSR) and the subsequent tertiary recrystallization annealing under dry hydrogen atmosphere was developed to produce the grain oriented ultra-thin silicon sheets less than 0.1 mm with high magnetic property performance. For comparison, the conventional rolling (CR) was also used to process the grain oriented ultra-thin silicon steel sheets. The effect of processing parameters on magnetic properties of the grain oriented ultra-thin silicon steel sheets was investigated. With the increase of annealing temperature and holding time, magnetic properties of the sheets processed by both rolling methods reach saturation as the result of the proceeding of the tertiary recrystallization. The thin sheets rolled by CSR did achieve better magnetic properties than those rolled by CR. © 2006 Elsevier B.V. All rights reserved. Keywords: Cross shear rolling; Tertiary recrystallization; Magnetic properties; Ultra-thin silicon steel sheet
1. Introduction A number of methods, such as controlling secondary recrystallization and magnetic domain structures, have been developed to improve the magnetic properties of the grain oriented silicon steels [1,2]. Recently, the development of the ultra-thin silicon steel sheet with thickness less than 100 m has drawn interest [3,4], since ultra-thin silicon steel sheets exhibit excellent soft magnetic properties. It is known that with a reduction of thickness the eddy current loss of the grain-oriented silicon steel sheet decreases [5–6] and the secondary recrystallization process becomes unstable and the (1 1 0) [0 0 1] grains orientation degree is deteriorated [7]. Also, it is generally difficult to produce grain oriented silicon steel sheets thinner than 160 m by the secondary recrystallization process using inhubitors. Therefore, a stable method is needed to produce ultra-thin grain-oriented silicon sheet. The weak (1 1 0) grains formed in the secondary recrystallization could take the surface energy as the motive force and grow up along the orientation of (1 1 0) [0 0 1] preferentially [8–10].
∗
Corresponding author. Tel.: +86 24 83686419; fax: +86 24 23906472. E-mail address: [email protected] (G. Xiuhua).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.05.058
Since coarse grain and high brittleness are developed after secondary recrystallization in the grain oriented silicon steel sheet that is used as the starting material of rolling, it is difficult to cold roll silicon steel sheet. Processing accidents, such as breaking and collaring, can decrease productivity and increase costs. As a new rolling technology, the cross shear rolling (CSR) method is a rolling process of a pair of work rolls rotating with different peripheral speeds. Comparing to the conventional rolling (CR), the cross shear rolling (CSR) has the following advantages in the rolling of ultrathin strip [11]: using less rolling load, the improvement of dimension accuracy and shape of the strip, and the capability of rolling strips into very thin thickness. It is therefore of great potential to develop CSR for the fabrication of ultra-thin silicon steel sheets. In this work, a new procedure consisting of CSR and the subsequent tertiary recrystallization annealing was developed to produce grain oriented ultra-thin silicon steel sheet of high quality with improved magnetic property performance. Tertiary recrystallization of the grain oriented ultra-thin silicon steel sheet is a very complex process [12]. During the whole course, minimal fluctuations in the processing variables can affect magnetic properties of the final product. In this paper, the CSR process parameters effecting on the tertiary recrystallization of ultra-thin silicon steel sheet were analyzed and the
G. Xiuhua et al. / Materials Science and Engineering A 430 (2006) 138–141
tertiary recrystallization behavior of ultra-thin silicon steel sheet rolled by CSR during annealing was studied. 2. Materials and experimental procedure The 0.3 mm grain oriented silicon steel sheet was taken as the starting raw material (magnetic induction: B8 = 1.8 T; core loss: P17/50 = 1.33 w/kg; average grain diameters: D = 10 mm). The chemical compositions of the silicon steel sheet are as follows (wt.%): C 0.07, Si 3.15, Mn 0.06, S 0.02, Cu 0.17, N 0.003, P 0.02. After the surface glass film was removed by dissolution in a solution of hydrofluoric acid and sulfuric acid, the sheets were cold rolled to various thickness from 0.06 to 0.10 mm by CR with mismatch speed ratio (MSR) of 1.0 and CSR with MSR of 1.17 using Ø90 mm/Ø200 mm × 200 mm four high rolling mill respectively. The rolling schedule using CSR of 1.17 is: 0.30 mm → 0.24 mm → 0.16 mm → 0.12 mm → 0.10 mm → 0.08 mm → 0.06 mm, and the rolling schedule using CR is: 0.30 mm → 0.24 mm → 0.19 mm → 0.16 mm → 0.14 mm → 0.12 mm → 0.10 mm → 0.09 mm → 0.08 mm → 0.07 mm → 0.06 mm. MgO was adopted as the coating material during the rolling process. Specimens were annealed at 1150, 1200, 1250 ◦ C for 3, 4.5, 6 and 7 h in a pipe dry hydrogen atmosphere furnace (dew point = −50 ◦ C) for tertiary recrystallization and cooled to 600 ◦ C at a rate of 10 K/s. Three tests were treated per process. The thickness of specimens was measured with micrometer. The magnetic induction B8 was measured by using impact method at magnetic field of 800 A/m with a 334 model soft magnetic materials measurement apparatus. Core losses were measured by using the bridge method at the excitation magnetic flux density of 1.7 T at frequency of 50 Hz. The recrystallization texture measurements were carried out on a D/max-IIIA X-ray diffractometer. The orientation distribution functions (ODFs) presented in this paper were calculated by a two-step method [13]. The ODFs using the notation of Roe constant ψ sections were selected to represent the texture.
139
ties of ultra-thin silicon steel sheets, two different MSR, 1.0 and 1.17, were used for cold rolling. During the rolling experiment, it was observed that CSR could reduce the thickness more efficiently and steadily than CR (for example, 0.3 mm thick silicon steel sheet was rolled to 0.06 mm by six passes using CSR, and the same thickness by 10 passes using CR). CR brought about the thinned margin along the plate width direction (transversal direction) where cracks occurred easily. The tension stress during the CR process resulted in breaking, while the stress state in the deformation area was changed by the shear–stress applied during the CSR process. CSR with the thickness and deformation uniform, can decrease the area of thinned margin, reduce the tension stress, and avoided cracking accordingly. The tension stress along the edge of the strip was reduced remarkably and the edge-cracking trend was reduced. Therefore, even though the strip became brittle and
3. Results and discussion The grain growth during recrystallization of cold rolled silicon steels has preferential orientations. When annealed at appropriate condition, as (1 1 0) face of the body-centered cubic is the close-packed face and the surface energy of (1 1 0) face is the lowest, grains in (1 1 0) face paralleled to the sample surface have the driving force for growth preferentially. During tertiary grain growth, the secondary grain which has (1 1 0) [0 0 1] texture would grow up [14]. The surface energy and growth of grains in silicon steels have therefore strong interactions with the annealing process. 3.1. Effect of mismatch speed ratio on magnetic properties The mismatch speed ratio (MSR) of CSR determines the shearing deformation and also affects the lattice distortion and the tertiary grain growth process. To study the influence of MSR on the recrystallization behavior as well as magnetic proper-
Fig. 1. Dependence of magnetic properties upon the thickness for the silicon steel sheets annealed at 1200 ◦ C for 7 h.
140
G. Xiuhua et al. / Materials Science and Engineering A 430 (2006) 138–141
hard due to cold work during the CSR process, the amount of cracking and edge-cracking was reduced during the CSR process. Fig. 1 shows magnetic properties as a function of thickness for the grain oriented ultra-thin silicon steel sheets rolled by the two levels of MSR and then annealed at 1200 ◦ C in dry hydrogen atmosphere for 7 h. From the figure, It is seen that magnetic properties improve as the thickness of the silicon steel sheets reduces for both types of MSR. As shown in Fig. 1a and b, with the reduction of thickness from 0.10 to 0.06 mm, the magnetic induction (B8 ) of thin silicon steel is increased and the core loss (P17/50 ) dropped. The sample processed by CSR exhibited higher magnetic induction yet lower core loss than the one by CR at the same thickness. The difference in the magnetic properties of the sheets processed by the two types of rolling can be attributed to the deformation char-
acteristic and the subsequent recrystallization process. Compared with CR, intensive shear deformation exists during CSR. Under the same deformation condition, the equivalent deformation of CSR is larger than CR, which causes greater stress and strain-gradient [11]. The distortion energy increases and the degree of recystallization using such energy as the driving force increases. From analysis of texture [15], more -texture and grains with {1 1 0} 0 0 1 orientation formed in samples rolled by CSR than in samples rolled by CR. Therefore, it is easier to form Goss texture because of hereditary effects [14]. The fact that the magnetic properties rise with the thickness reduction is also an indication suggesting that the shear deformation benefits the occurrence of tertiary recrystallization and Goss grain growth [12]. Moreover, the thinner the strip is, the faster the grain grows up during the recrystallization process. For a thinner sample, the surface-to-volume ratio is higher and the difference of surface energy between (1 1 0) faces and other faces increases the driving force for the tertiary grain growth [14]. 3.2. Effect of annealing temperature on magnetic properties The annealing temperature has significant effect on the tertiary recrystallization and the magnetic properties of ultra-thin silicon steel sheets rolled by CR and CSR. Fig. 2a and b shows the dependence of magnetic induction on annealing temperature for the 0.06 and 0.08 mm sheet samples, respectively. The samples were heated to three different temperatures at a heating rate of 6 K/s and held for 7 h under a hydrogen flow of 6 L/min. From the figure, it is seen that the samples annealed at 1150 ◦ C exhibited the lowest magnetic inductions while the those annealed at 1250 ◦ C obtained the highest value for the same thickness and MSR. The higher annealing temperature is, the higher the magnetic properties are. This is due to the higher temperature giving rise to greater degree of recrystallization or fully recrystallized microstructure in the rolled sheet. The texture of 0.06 mm thick specimens rolled by CSR and annealed at 1250 ◦ C in dry hydrogen atmosphere is shown in Fig. 3. The obvious {1 1 0} 0 0 1 orientation texture was formed in ultra-thin silicon steel sheet.
Fig. 2. Dependence of magnetic induction on the annealing temperature for the silicon steel sheets with two different thickness (h = 0.06 and h = 0.08 mm).
Fig. 3. ODFs for constant ψ sections (ψ = 0◦ , 5◦ , 15◦ ,. . .) of ultra-thin silicon steel sheet; levels 5, 10, 15, 20,. . . peak density = 31.2.
G. Xiuhua et al. / Materials Science and Engineering A 430 (2006) 138–141
3.3. Effect of the holding time on the magnetic properties The dependence of magnetic induction on the holding time for the silicon steel sheets rolled by CR and CSR is shown in Fig. 4. The samples were heated to 1200 ◦ C at a rate of 6 K/s under a hydrogen flow of 6 L/min and held for different times ranging from 3 to 6 h. After the short holding time of 3 h, a significant difference in the magnetic induction was observed for the samples with different thickness. For both of CR and CSR, the 0.06 mm sheet sample exhibited a much higher induction value than the 0.08 and 0.10 mm sheets. The difference can be attributed to the sheet thickness effect. As the 0.06 mm sample has a higher ratio of surface area to volume than others, there is more driving force for the tertiary grain to grow fast during the recrystallization process. As a result, after annealing for 3 h in dry hydrogen atmosphere, the specimens rolled by both CSR and CR achieved high values of magnetic induction. For longer
141
holding times, the magnetic induction increased slightly and appeared to reach saturation at 6 h, which is an indication that the tertiary recrystallization has completed. As to the 0.08 mm sample, the initial annealing of 3 h gave rise to a magnetic induction value less than that of the raw material (1.8 T), suggesting that the tertiary recrystallization has not occurred in the sample. At 4.5 h of annealing, the magnetic induction reached the value of the raw material but was still relatively lower, indicating the tertiary recrystallization was only partially completed. At 6 h of holding, the magnetic induction increased to a high value of 1.91 T, suggesting that the recrystallization approached to the end and the tertiary recrystallization texture was steady. 4. Conclusions A new procedure consisting of the cross shear rolling (CSR) and the subsequent tertiary recrystallization annealing under dry H2 atmosphere was developed to produce the grain oriented ultra-thin silicon sheets less than 0.1 mm with high magnetic property performance. The silicon steel sheets rolled by CSR could achieve better magnetic properties than those rolled by CR. For the same sheet, magnetic properties improves with the increase of annealing temperature in the range of 1150–1250 ◦ C and annealing time up to 6 h. The increase of magnetic performance is attributed to the occurrence of tertiary recrystallization process. The thinnest 0.06 mm sheet exhibited the highest magnetic induction and the lowest core loss. A reduction in thickness appears to be beneficial to the improvement of magnetic performance of the silicon steel sheet. Acknowledgement The authors wish to thank National Nature Science Foundation of China, for their financial supports for this work. References
Fig. 4. The variation of magnetic induction with the annealing time at 1200 ◦ C for different sheets.
[1] T. Iuchi, S. Yamaguchi, J. Appl. Phys. 53 (3) (1982) 2410–2415. [2] R. Krause, G. Rauch, J. Appl. Phys. 55 (6) (1984) 2121–2133. [3] K. Arai, K. Ishiyama, H. Mogi, IEEE Trans. Mag. 25 (5) (1989) 3949–3954. [4] K. Arai, K. Ishiyama, Bull. Jpn. Inst. Met. 31 (5) (1992) 429–435. [5] K. Arai, K. Ishiyama, J. Appl. Phys. Part II 64 (10) (1988) 5352–5354. [6] M. Nakano, K. Ishiyama, IEEE Trans. Magn. 33 (5) (1997) 3754–3756. [7] J. Harase, R. Shimizu, N. Takahashi, J. Jpn. Inst. Met. 54 (4) (1990) 381–387. [8] K. Ishiyama, K. Arai, J. Harase, N. Takahashi, J. Jpn. Inst. Met. 57 (1) (1993) 108–113. [9] Y. Kim, K. Ishiyama, K. Arai, IEEE Trans. Magn. 29 (6) (1993) 3535–3537. [10] S. Nakashima, K. Takashima, J. Harase, Acta Metall. Mater. 42 (2) (1994) 539–547. [11] Zhu Quan, Iron Steel 15 (5) (1980) 1–6. [12] Xiuhua GAO, Study of ultra-thin grain oriented silicon steel by cross shear rolling, Ph.D. Thesis, Northeastern University, Shenyang, 2002. [13] Z.D. Liang, J.Z. Xu, F. Wang, Proceedings of the Sixth International Conference on Textures of Materials, vol. 2, ISIJ, Tokyo, 1981, p. 1259. [14] H.E. Zhongzheng, Electrical Steel, Metallurgy Industry Press, Beijing, 1996, p. 225. [15] GAO. Xiuhua, QI. Kemin, J. Mater. Sci. Technol. 16 (2) (2000) 236 155–156.