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Preparation of non-oriented silicon steel with high magnetic induction using columnar grains
Journal of Magnetism and Magnetic Materials 324 (2012) 4068–4072
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Preparation of non-oriented silicon steel with high magnetic induction using columnar grains Ling Cheng, Ping Yang n, Yupei Fang, Weimin Mao School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 April 2012 Received in revised form 2 July 2012 Available online 21 July 2012
Columnar grains can lead to detrimental surface ridging and an inhomogeneous microstructure, although their {1 0 0}/0 v wS texture is considered desirable due to their good magnetic properties in nonoriented silicon steel. Based on the hereditary tendency of {1 0 0}/0 v wS texture, the effects of lubrication and heating rate on texture and on final magnetic properties were investigated using a cast slab containing 100% columnar grains. Hot rolling with lubrication, normalization at low heating rate, two-stage cold rolling, and final annealing at 1000 1C helped achieve high performance. As a result, a new non-oriented silicon steel with high magnetic induction (B50 ¼ 1.82 T) and low core loss (P1.5 ¼ 2.35 W/kg) was prepared. The possibility of further performance optimization was also discussed. & 2012 Elsevier B.V. All rights reserved.
Keywords: Non-oriented silicon steel Columnar grains Texture control Magnetic properties
1. Introduction Columnar grains with {1 0 0}/0 v wS orientation, which commonly exist in cast slabs with different thicknesses (e.g., 240, 135, 70, and 2 mm), can lead to inhomogeneous grain sizes, detrimental surface ridging in non-grain oriented or stainless steels, and deterioration of the Goss texture after secondary recrystallization in grain oriented steel [1–3]. Generally, the bad effects of columnar grains can be eliminated by electromagnetic stirring, controlling the ratio of equal grains to columnar grains, increasing the original slab thickness and rolling reduction, or by increasing the annealing time [4]. However, if the beneficial {1 0 0} texture is retained to produce a homogeneous microstructure, high magnetic induction and low core loss of the non-oriented silicon steel can be obtained. Sharp {1 0 0} fiber texture of columnar grains is highly beneficial to magnetic performance; thus, many scholars have investigated various methods to retain this in the final product. Walter et al. [5] acquired {1 0 0}/0 v wS texture in very thin Si–Fe sheets by forcing abnormal growth in cube grains. This has been made possible by the fact that the secondary recrystallization process is controlled by the gas-metal interfacial energy, which is lowest at the (1 0 0) orientation in a high and purely inert atmosphere or in a vacuum. Harase and Shimizu [6] significantly increased the intensity of the cube texture component in the silicon steel sheet by cross-rolling and subsequent decarburizing and secondary recrystallization annealing. Previous works have examined the removal
n
Corresponding author. Tel.: þ86 10 82376968 E-mail address: [email protected] (P. Yang).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.07.019
of manganese by annealing in a vacuum (not essential) and by decarburizing annealing in the g or a þ g two-phase region, resulting in high intensity of the {1 0 0} fiber texture component in ˜eda and Salinas-Rodrı´silicon steel sheets [7,8]. Gutie´rrez-Castan guez [9] suggested the application of annealing prior to cold rolling to obtain large columnar ferrite grains. Through this process, the microstructure acquires superior magnetic properties after cold rolling and rapid annealing treatment. Nevertheless, all these processes still face difficulties in industrial applications due to the complex mechanism and strict processing conditions. With the hereditary tendency of {1 0 0}/0 v wS texture [3,10,11], the present paper investigates the preparation of high magnetic flux induction and low core loss non-oriented silicon steel, using columnar grains, and the evolution of microstructures and textures during hot rolling, cold rolling, and annealing.
2. Material and methods Samples with 100% columnar grains were cut from the cast slab of electrical steel. The longitudinal direction of the columnar grains was parallel to that of the rolling plane normal. The chemical composition is shown in Table 1. After heating at 950 1C for 30 min, samples with a thickness of 30 mm were immediately hotrolled to a thickness of 3.0 mm through five rolling passes under various conditions. Some samples were hot-rolled using emulsion liquid as a lubricant to study the influence of lubrication between the sample and the rolls, while other samples were hot-rolled without it. The hot-rolled sheets with lubrication were then annealed at 1000 1C at two different heating rates: fast (300 1C/min) and slow (5 1C/min).
L. Cheng et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4068–4072
Normalizing was followed by first cold rolling (77%), intermediate annealing (at two different heating rates), second cold rolling (50%), decarburizing annealing (850 1C for 6 min), and final annealing (1000 1C) for various times under hydrogen atmosphere. Finally, the texture and magnetic performance of commercial 35W300 highgrade non-oriented silicon steel were determined for comparison. Micro-textures in various processing steps were analyzed by HKL-Channel 5 electron backscatter diffraction (EBSD) equipped on ZEISS ULTRA55 field emission scanning electron microscope (SEM). Macro-textures of hot-rolled bands were measured using Siemens D5000 X-ray diffractometer (XRD). The values of magnetic induction at 5000 A/m (B50) and core losses at 1.5 T and 50 Hz (P1.5) were measured by a single sheet tester in the rolling direction of the samples sheared at 300 30 0.35 mm.
3. Results and discussion 3.1. Hot rolling with or without lubrication Fig. 1 displays the effect of lubrication on texture evolution during hot rolling. EBSD results show that the area fractions of {1 0 0}/201 orientation grains in hot-rolled with lubricant (HRL) sheet are higher than those in normal hot-rolled (NHR) sheet. Area fraction in the HRL and NHR sheets are 29.3% and 20.3%, respectively. Meanwhile, the fraction of shear textures, such as the {0 1 1}/1 0 0S, {0 1 1}/2 1 1S, and {1 1 2}/1 1 1S components [12] in the HRL sheet is 0.6%, while that in the NHR sheet is up to 10.9%, which mainly exists on the surface layer (see Region A in Fig. 1b). Similar results are also obtained by XRD. On the surface layer, the main texture components in the HRL sheet are {1 0 0}/0 v wS, and the maximum intensity of 14.8 times random distribution is observed at position {1 0 0}/0 0 1S, j2 ¼ 451 section (Fig. 1c). In the NHR sheet, the Goss texture has the strongest orientation intensity of 17.4 (Fig. 1d). In the center layer (Fig. 1e and f), the g-fiber texture is not present in both the HRL and NHR Table 1 Chemical composition of the investigated steel in wt%. C
Si
Mn
Als
P
S
N
0.04
3.2
0.05
0.01
0.008
0.015
0.005
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sheets. The orientation intensity of the {1 0 0}/0 v wS component in the HRL sheet is higher than that in the NHR sheet. Thus, hot rolling with lubrication can weaken the intensity of shear texture and favor the retention of {1 0 0} texture. 3.2. Normalizing at different heating rates Fig. 2 shows the influence of heating rate on the microstructure and on the texture of HRL samples at the normalizing stage. Heating at low rate mainly produces elongated deformed grains with {1 0 0}/0 v wS orientation in the center layer and recrystallized structure on the surface layer (Fig. 2a and b, respectively). However, a rapid heating rate leads to the complete recrystallization of the deformed sheet, resulting in tense {1 1 3}/2 5 1S and cube textures (Fig. 2c and d). In addition, the area fraction of {1 0 0}/0 v wS grains in the slow heating experiment is 31.7% higher than that in the fast heating experiment (23.9%). Thus, normalizing at low heating rate favors the retention of {1 0 0} fiber texture. This is because the stored energy during the deformation of the HRL sample is released slowly at the stage of recovery. Moreover, the recrystallization process, which can change crystallographic directions, is relatively inhibited. 3.3. First cold rolling and intermediate annealing at different heating rates Fig. 3 presents the EBSD orientation maps and ODF at j2 ¼451 section of the HRL samples after normalizing at slow heating rate, first cold rolling, and intermediate annealing at different heating rates. The results show that the structural inhomogeneity initially caused by different stress states from the surface to the center layers, and by different work hardening levels of the initial {1 0 0} orientation grains during hot rolling [13,14] exists in both slow (Fig. 3a) and fast (Fig. 3c) samples. Bigger grains originate from the recovery structure shown in Fig. 2a, while smaller grains are associated with the recrystallized structure and the cementite (0.05% C) in the former stage. The intensity of {1 0 0}/0 v wS– {1 1 0}/0 0 1S texture in the fast sample, which benefits magnetic induction, is a little stronger than that in the slow sample. Furthermore, the g-fiber texture has not formed either of them, and the fractions of {1 1 1}/1 1 2S orientation grains all approach 25%.
HRL
NHR
A Surface layer
14.8 17.4
17.1 ND
Center layer
20.9
RD
Fig. 1. EBSD orientation maps (a) to (b) and textures of the surface (c) to (d) and the center (e) to (f) layers of hot-rolled sheets: (a) hot-rolled with lubrication (HRL) and (b) normal hot-rolled (NHR) (ODF at j2 ¼ 451 section, Level: 2, 5, 8, 11, 15, and 20).
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Fig. 2. EBSD orientation maps and ODF at j2 ¼ 451 section of the HRL samples after normalizing at different heating rates: (a) and (b) slow, and (c) and (d) fast.
Fig. 3. EBSD orientation maps and ODF at j2 ¼ 451 section of the HRL samples after normalizing at slow heating rate, first cold rolling, and intermediate annealing at 850 1C at different heating rates: (a) and (b) slow, and (c) and (d) fast.
Fig. 4. EBSD orientation map (a) and ODF at j2 ¼ 451 section (b) of the HRL samples after normalizing at slow heating rate, first cold rolling, intermediate annealing at fast heating rate, second cold rolling, and decarburizing annealing.
3.4. Second cold rolling, decarburizing annealing, and final annealing Fig. 4 displays the EBSD orientation map and ODF at j2 ¼451 section of the HRL samples after normalizing at slow heating rate, first cold rolling, intermediate annealing at fast heating rate, second cold rolling, and decarburizing annealing. EBSD results show that the fractions of {1 0 0}/0 v wS and /1 1 1S//ND grains are 19.6% and 29.5%, respectively. Moreover, the average grain size is 25.6 mm in the sample with a thickness of 0.35 mm. Texture evolution at this stage is similar with that reported in [15]. Furthermore, magnetic property measurement results show that the magnetic induction B50 is 1.79 T, and the core loss P1.5 is 3.46 W/kg. For comparison, the EBSD orientation map of the commercial 35W300 high-grade non-oriented silicon steel (i.e., fully processed industrial steel) is shown in Fig. 5. The strong g-fiber texture develops in the commercial sheet, and the fraction of /1 1 1S//ND grains reaches 50.9%, which is much higher than that in the sample presented in Fig. 4 (only 29.5%). Meanwhile, for commercial steel, the fraction of {1 0 0}/0 v wS grains is 18.6%, which is slightly lower than that in Fig. 4. The average grain size is
124.1 mm, the magnetic induction B50 is 1.70 T, and the core loss P1.5 is 2.47 W/kg. Hence, the improved preparation process using columnar grains has an advantage in enhancing magnetic induction. This is because the strength of the g-fiber texture can be significantly weakened and {1 0 0}/0 v wS and other non-{1 1 1} components can be reserved at different degrees. Moreover, the purity of commercial 35W300 high-grade non-oriented silicon steel is higher than that of current samples. In examining the effects of heating rate and final annealing time on normalizing and intermediate annealing as well as the magnetic properties of the HRL samples, respectively, Fig. 6 shows the respective variations in magnetic properties with time in the HRL samples after final annealing at 1000 1C for 10 min, 30 min, 2 h, and 8 h. The heating rates during normalizing and intermediate annealing in the four HRL samples are listed in Table 2. The magnetic properties of all four samples improved with prolong annealing time. After annealing at 1000 1C for 10 min, magnetic induction B50 is more than 1.77 T, and core loss P1.5 is less than 3.0 W/kg in all four samples. For Sample 1, which underwent normalizing at slow heating rate and intermediate annealing at fast heating rate, B50 is as high as 1.818 T, increasing to 1.824 T when annealing time reaches 8 h. Meanwhile, P1.5 is
L. Cheng et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4068–4072
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3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2
Magnetic induction (B50) / T
Core loss (P1.5) /WKg-1
Fig. 5. EBSD orientation map (a) and ODF at j2 ¼ 451 section (b) of commercial 35W300 non-oriented silicon steel.
Sample 1 Sample 2 Sample 3 Sample 4
0
1
2
3 4 5 6 Anealing time / h
7
8
1.82 1.81 1.80 1.79 1.78 1.77 1.76 1.75 0
1
2
3 4 5 6 Anealing time / h
7
8
Fig. 6. Variation of magnetic properties with time after final annealing at 1000 1C: (a) core loss and (b) magnetic induction.
Table 2 Heating rates during normalizing and intermediate annealing in the HRL samples.
Sample Sample Sample Sample
1 2 3 4
Normalizing
Intermediate annealing
Slow Slow Fast Fast
Fast Slow Fast Slow
2.449 W/kg and becomes 2.352 W/kg when annealing reaches 1000 1C after 8 h. Commercial 35W300 high-grade non-oriented silicon steel exhibit B50 of 1.703 T and P1.5 of 2.469 W/kg. Here, magnetic induction can be increased by more than 0.12 T, whereas core loss can still be reduced by 0.117 W/kg. Fig. 7 shows the EBSD orientation map of Sample 1, which demonstrates optimum performance compared with other samples in Fig. 6 after final annealing at 1000 1C for 0.5 h. Results indicate that the fractions of {1 0 0}/0 v wS and /1 1 1S//ND grains are 24.7% and 20%, respectively, and a large quantity of grains with non-{1 1 1} orientations exist in the sample. The average grain size in this stage (59.3 mm) is larger than that shown in Fig. 4 (25.6 mm). Therefore, the improvement in magnetic induction can be mainly ascribed to the strengthening of non{1 1 1} textures, whereas the decrease in core loss can be attributed to the appropriate combination of grain size and texture. The limitation, however, is that grain size distribution is not very uniform, indicating that performance can still be enhanced. Although commercial 35W300 high-grade non-oriented silicon steel with B50 and P1.5 of 1.70 T and 2.47 W/kg, respectively, can be prepared by hot rolling, normalization, one-stage cold rolling and final annealing, it still requires accurate chemical composition (Co30 ppm, N, S, Oo20 ppm, 3%Si). Moreover, a sharp /1 1 1S//ND texture exists in the final product. However, in this study, the demand for metallurgical quality in the initial
material that contains 100% columnar grains is lower than that in commercial high-grade steel, because C can be easily removed by decarburizing annealing. Furthermore, surface ridging can be avoided in the final sample by weakening shear deformation and by controlling the quantity of lower plastic strain ratio of {0 0 1}/1 1 0S colonies [1,3]. Finally, the best magnetic properties of Sample 1 (B50 ¼1.82 T, P1.5 ¼2.35 W/kg) exceed those of the commercial sample and feature higher magnetic flux induction. If the lower content of C is allowed, further improving the processing technique, magnetic properties may become more superior. However, the influence of precipitation particles on magnetic aging has yet to be investigated fully.
4. Conclusions (a) The results of this investigation show that a new non-oriented silicon steel with high magnetic flux induction (B50 ¼1.82 T) and low core loss (P1.5 ¼2.35 W/kg) can be prepared using 100% columnar grains. Hot rolling with lubrication, normalization at low heating rate, two-stage cold rolling, and final annealing at 1000 1C also help achieve high performance. This method is more suitable for the thin slab process. (b) Hot rolling with lubrication avoids shearing deformation and favors the retention of micro-substructures of {1 0 0}/0 v wS grains. Moreover, normalizing at low heating rate and intermediate annealing at fast heating rate result in more benefits to magnetic properties. The percentage of grains with non{1 1 1} orientations in the final annealing sample is higher than that of commercial 35W300 high-grade non-oriented silicon steel. (c) Final annealing at 1000 1C is an effective method to improve magnetic properties. The improvement in magnetic induction is mainly ascribed to the strengthening of non-{1 1 1} textures, whereas the decrease in core loss is attributed to the appropriate combination of grain size and texture.
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Fig. 7. EBSD orientation map (a) and ODF at j2 ¼ 451 section (b) of Sample 1 that was final annealed at 1000 1C for 0.5 h, as shown in Fig. 6.
Acknowledgment The authors acknowledge the financial support provided by the National Natural Science Foundation of China with Grant No. 51071024. References [1] H.J. Shin, J.K. An, S.H. Park, D.N. Lee, The effect of texture on ridging of ferritic stainless steel, Acta Materialia 51 (2003) 4693–4706. [2] M.Y. Huh, O. Engler, Effect of intermediate annealing on texture, formability and ridging of 17%Cr ferritic stainless steel sheet, Materials Science and Engineering A 308 (2001) 74–87. [3] H.T. Liu, D. Wei, S.T. Xie, Z.Y Liu, G.D Wang, The effect of initial solidification structure on riding in Cr17 ferritic stainless steels, Chinese Journal of Materials Research 22 (2008) 467–472. [4] Z.C. Li, P. Yang, M.Q. Yan, W.M Mao, Evolution of microstructures and textures of {1 0 0} initial texture in an electrical steel, Chinese Journal of Stereology Image Analysis 14 (2009) 237–244. [5] J.L. Walter, Control of texture in magnetic material by surface energy, Journal of Applied Physics 36 (1965) 1213–1220. [6] J. Harase, R. Shimizu, Mechanism of the (1 0 0)[0 0 1] texture evolution from the viewpoint of coincidence boundaries in BCC alloys, Journal of the Japan Institute of Metals 53 (1989) 745–752.
[7] T. Tomida, A new process to develop (1 0 0) texture in silicon steel sheets, Journal of Materials Engineering and Performance 5 (1996) 316–322. [8] F. Kovac, M. Dzubinsky, Y. Sidor, Columnar grain growth in non-oriented electrical steels, Journal of Magnetism and Magnetic Materials 269 (2004) 333–340. [9] E.J.G. Castaneda, A.S. Rodriguez, Effect of annealing prior to cold rolling on magnetic and mechanical properties of low carbon non-oriented electrical steels, Journal of Magnetism and Magnetic Materials 323 (2011) 2524–2530. [10] H. Abe, M. Mtsuo, K. Ito, Cold rolling and recrystallization textures of siliconiron crystals rolled in (1 0 0)[0 0 1] orientation, Transactions JIM 4 (1963) 28–32. [11] J.L. Water, E.F. Koch, Substucture and recrystallization of deformed (1 0 0)[0 0 1]-oriented crystals of high-purity silicon–iron, Acta Metallurgica 11 (1963) 923–938. [12] H.K. Moon, C.T. Kim, High temperature deformation behavior of columnar structure of continuously cast Fe–3%Si steel, Journal of the Korean Institute of Metals 22 (1984) 1176–1181. [13] N. Tsuji, K. Tsuzaki, T. Maki, Effect of initial orientation on the recrystallization behavior of solidified columnar crystals in a 19%Cr ferritic stainless steel, ISIJ International 33 (1993) 783–792. [14] H.T. Liu, Z.Y. Liu, G.M. Cao, C.G. Li, G.D. Wang, Microstructure and texture evolution of strip casting 3 wt% Si non-oriented silicon steel with columnar structure, Journal of Magnetism and Magnetic Materials 323 (2011) 2648–2651. [15] R. Takanohashi, F.J.G. Landgraf, Effect of hot-band grain size and intermediate annealing on magnetic properties and texture of non-oriented silicon steels, Journal of Magnetism and Magnetic Materials 304 (2006) 608–610.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Preparation of non-oriented silicon steel with high magnetic induction using columnar grains Ling Cheng, Ping Yang n, Yupei Fang, Weimin Mao School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 April 2012 Received in revised form 2 July 2012 Available online 21 July 2012
Columnar grains can lead to detrimental surface ridging and an inhomogeneous microstructure, although their {1 0 0}/0 v wS texture is considered desirable due to their good magnetic properties in nonoriented silicon steel. Based on the hereditary tendency of {1 0 0}/0 v wS texture, the effects of lubrication and heating rate on texture and on final magnetic properties were investigated using a cast slab containing 100% columnar grains. Hot rolling with lubrication, normalization at low heating rate, two-stage cold rolling, and final annealing at 1000 1C helped achieve high performance. As a result, a new non-oriented silicon steel with high magnetic induction (B50 ¼ 1.82 T) and low core loss (P1.5 ¼ 2.35 W/kg) was prepared. The possibility of further performance optimization was also discussed. & 2012 Elsevier B.V. All rights reserved.
Keywords: Non-oriented silicon steel Columnar grains Texture control Magnetic properties
1. Introduction Columnar grains with {1 0 0}/0 v wS orientation, which commonly exist in cast slabs with different thicknesses (e.g., 240, 135, 70, and 2 mm), can lead to inhomogeneous grain sizes, detrimental surface ridging in non-grain oriented or stainless steels, and deterioration of the Goss texture after secondary recrystallization in grain oriented steel [1–3]. Generally, the bad effects of columnar grains can be eliminated by electromagnetic stirring, controlling the ratio of equal grains to columnar grains, increasing the original slab thickness and rolling reduction, or by increasing the annealing time [4]. However, if the beneficial {1 0 0} texture is retained to produce a homogeneous microstructure, high magnetic induction and low core loss of the non-oriented silicon steel can be obtained. Sharp {1 0 0} fiber texture of columnar grains is highly beneficial to magnetic performance; thus, many scholars have investigated various methods to retain this in the final product. Walter et al. [5] acquired {1 0 0}/0 v wS texture in very thin Si–Fe sheets by forcing abnormal growth in cube grains. This has been made possible by the fact that the secondary recrystallization process is controlled by the gas-metal interfacial energy, which is lowest at the (1 0 0) orientation in a high and purely inert atmosphere or in a vacuum. Harase and Shimizu [6] significantly increased the intensity of the cube texture component in the silicon steel sheet by cross-rolling and subsequent decarburizing and secondary recrystallization annealing. Previous works have examined the removal
n
Corresponding author. Tel.: þ86 10 82376968 E-mail address: [email protected] (P. Yang).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.07.019
of manganese by annealing in a vacuum (not essential) and by decarburizing annealing in the g or a þ g two-phase region, resulting in high intensity of the {1 0 0} fiber texture component in ˜eda and Salinas-Rodrı´silicon steel sheets [7,8]. Gutie´rrez-Castan guez [9] suggested the application of annealing prior to cold rolling to obtain large columnar ferrite grains. Through this process, the microstructure acquires superior magnetic properties after cold rolling and rapid annealing treatment. Nevertheless, all these processes still face difficulties in industrial applications due to the complex mechanism and strict processing conditions. With the hereditary tendency of {1 0 0}/0 v wS texture [3,10,11], the present paper investigates the preparation of high magnetic flux induction and low core loss non-oriented silicon steel, using columnar grains, and the evolution of microstructures and textures during hot rolling, cold rolling, and annealing.
2. Material and methods Samples with 100% columnar grains were cut from the cast slab of electrical steel. The longitudinal direction of the columnar grains was parallel to that of the rolling plane normal. The chemical composition is shown in Table 1. After heating at 950 1C for 30 min, samples with a thickness of 30 mm were immediately hotrolled to a thickness of 3.0 mm through five rolling passes under various conditions. Some samples were hot-rolled using emulsion liquid as a lubricant to study the influence of lubrication between the sample and the rolls, while other samples were hot-rolled without it. The hot-rolled sheets with lubrication were then annealed at 1000 1C at two different heating rates: fast (300 1C/min) and slow (5 1C/min).
L. Cheng et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4068–4072
Normalizing was followed by first cold rolling (77%), intermediate annealing (at two different heating rates), second cold rolling (50%), decarburizing annealing (850 1C for 6 min), and final annealing (1000 1C) for various times under hydrogen atmosphere. Finally, the texture and magnetic performance of commercial 35W300 highgrade non-oriented silicon steel were determined for comparison. Micro-textures in various processing steps were analyzed by HKL-Channel 5 electron backscatter diffraction (EBSD) equipped on ZEISS ULTRA55 field emission scanning electron microscope (SEM). Macro-textures of hot-rolled bands were measured using Siemens D5000 X-ray diffractometer (XRD). The values of magnetic induction at 5000 A/m (B50) and core losses at 1.5 T and 50 Hz (P1.5) were measured by a single sheet tester in the rolling direction of the samples sheared at 300 30 0.35 mm.
3. Results and discussion 3.1. Hot rolling with or without lubrication Fig. 1 displays the effect of lubrication on texture evolution during hot rolling. EBSD results show that the area fractions of {1 0 0}/201 orientation grains in hot-rolled with lubricant (HRL) sheet are higher than those in normal hot-rolled (NHR) sheet. Area fraction in the HRL and NHR sheets are 29.3% and 20.3%, respectively. Meanwhile, the fraction of shear textures, such as the {0 1 1}/1 0 0S, {0 1 1}/2 1 1S, and {1 1 2}/1 1 1S components [12] in the HRL sheet is 0.6%, while that in the NHR sheet is up to 10.9%, which mainly exists on the surface layer (see Region A in Fig. 1b). Similar results are also obtained by XRD. On the surface layer, the main texture components in the HRL sheet are {1 0 0}/0 v wS, and the maximum intensity of 14.8 times random distribution is observed at position {1 0 0}/0 0 1S, j2 ¼ 451 section (Fig. 1c). In the NHR sheet, the Goss texture has the strongest orientation intensity of 17.4 (Fig. 1d). In the center layer (Fig. 1e and f), the g-fiber texture is not present in both the HRL and NHR Table 1 Chemical composition of the investigated steel in wt%. C
Si
Mn
Als
P
S
N
0.04
3.2
0.05
0.01
0.008
0.015
0.005
4069
sheets. The orientation intensity of the {1 0 0}/0 v wS component in the HRL sheet is higher than that in the NHR sheet. Thus, hot rolling with lubrication can weaken the intensity of shear texture and favor the retention of {1 0 0} texture. 3.2. Normalizing at different heating rates Fig. 2 shows the influence of heating rate on the microstructure and on the texture of HRL samples at the normalizing stage. Heating at low rate mainly produces elongated deformed grains with {1 0 0}/0 v wS orientation in the center layer and recrystallized structure on the surface layer (Fig. 2a and b, respectively). However, a rapid heating rate leads to the complete recrystallization of the deformed sheet, resulting in tense {1 1 3}/2 5 1S and cube textures (Fig. 2c and d). In addition, the area fraction of {1 0 0}/0 v wS grains in the slow heating experiment is 31.7% higher than that in the fast heating experiment (23.9%). Thus, normalizing at low heating rate favors the retention of {1 0 0} fiber texture. This is because the stored energy during the deformation of the HRL sample is released slowly at the stage of recovery. Moreover, the recrystallization process, which can change crystallographic directions, is relatively inhibited. 3.3. First cold rolling and intermediate annealing at different heating rates Fig. 3 presents the EBSD orientation maps and ODF at j2 ¼451 section of the HRL samples after normalizing at slow heating rate, first cold rolling, and intermediate annealing at different heating rates. The results show that the structural inhomogeneity initially caused by different stress states from the surface to the center layers, and by different work hardening levels of the initial {1 0 0} orientation grains during hot rolling [13,14] exists in both slow (Fig. 3a) and fast (Fig. 3c) samples. Bigger grains originate from the recovery structure shown in Fig. 2a, while smaller grains are associated with the recrystallized structure and the cementite (0.05% C) in the former stage. The intensity of {1 0 0}/0 v wS– {1 1 0}/0 0 1S texture in the fast sample, which benefits magnetic induction, is a little stronger than that in the slow sample. Furthermore, the g-fiber texture has not formed either of them, and the fractions of {1 1 1}/1 1 2S orientation grains all approach 25%.
HRL
NHR
A Surface layer
14.8 17.4
17.1 ND
Center layer
20.9
RD
Fig. 1. EBSD orientation maps (a) to (b) and textures of the surface (c) to (d) and the center (e) to (f) layers of hot-rolled sheets: (a) hot-rolled with lubrication (HRL) and (b) normal hot-rolled (NHR) (ODF at j2 ¼ 451 section, Level: 2, 5, 8, 11, 15, and 20).
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Fig. 2. EBSD orientation maps and ODF at j2 ¼ 451 section of the HRL samples after normalizing at different heating rates: (a) and (b) slow, and (c) and (d) fast.
Fig. 3. EBSD orientation maps and ODF at j2 ¼ 451 section of the HRL samples after normalizing at slow heating rate, first cold rolling, and intermediate annealing at 850 1C at different heating rates: (a) and (b) slow, and (c) and (d) fast.
Fig. 4. EBSD orientation map (a) and ODF at j2 ¼ 451 section (b) of the HRL samples after normalizing at slow heating rate, first cold rolling, intermediate annealing at fast heating rate, second cold rolling, and decarburizing annealing.
3.4. Second cold rolling, decarburizing annealing, and final annealing Fig. 4 displays the EBSD orientation map and ODF at j2 ¼451 section of the HRL samples after normalizing at slow heating rate, first cold rolling, intermediate annealing at fast heating rate, second cold rolling, and decarburizing annealing. EBSD results show that the fractions of {1 0 0}/0 v wS and /1 1 1S//ND grains are 19.6% and 29.5%, respectively. Moreover, the average grain size is 25.6 mm in the sample with a thickness of 0.35 mm. Texture evolution at this stage is similar with that reported in [15]. Furthermore, magnetic property measurement results show that the magnetic induction B50 is 1.79 T, and the core loss P1.5 is 3.46 W/kg. For comparison, the EBSD orientation map of the commercial 35W300 high-grade non-oriented silicon steel (i.e., fully processed industrial steel) is shown in Fig. 5. The strong g-fiber texture develops in the commercial sheet, and the fraction of /1 1 1S//ND grains reaches 50.9%, which is much higher than that in the sample presented in Fig. 4 (only 29.5%). Meanwhile, for commercial steel, the fraction of {1 0 0}/0 v wS grains is 18.6%, which is slightly lower than that in Fig. 4. The average grain size is
124.1 mm, the magnetic induction B50 is 1.70 T, and the core loss P1.5 is 2.47 W/kg. Hence, the improved preparation process using columnar grains has an advantage in enhancing magnetic induction. This is because the strength of the g-fiber texture can be significantly weakened and {1 0 0}/0 v wS and other non-{1 1 1} components can be reserved at different degrees. Moreover, the purity of commercial 35W300 high-grade non-oriented silicon steel is higher than that of current samples. In examining the effects of heating rate and final annealing time on normalizing and intermediate annealing as well as the magnetic properties of the HRL samples, respectively, Fig. 6 shows the respective variations in magnetic properties with time in the HRL samples after final annealing at 1000 1C for 10 min, 30 min, 2 h, and 8 h. The heating rates during normalizing and intermediate annealing in the four HRL samples are listed in Table 2. The magnetic properties of all four samples improved with prolong annealing time. After annealing at 1000 1C for 10 min, magnetic induction B50 is more than 1.77 T, and core loss P1.5 is less than 3.0 W/kg in all four samples. For Sample 1, which underwent normalizing at slow heating rate and intermediate annealing at fast heating rate, B50 is as high as 1.818 T, increasing to 1.824 T when annealing time reaches 8 h. Meanwhile, P1.5 is
L. Cheng et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4068–4072
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3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2
Magnetic induction (B50) / T
Core loss (P1.5) /WKg-1
Fig. 5. EBSD orientation map (a) and ODF at j2 ¼ 451 section (b) of commercial 35W300 non-oriented silicon steel.
Sample 1 Sample 2 Sample 3 Sample 4
0
1
2
3 4 5 6 Anealing time / h
7
8
1.82 1.81 1.80 1.79 1.78 1.77 1.76 1.75 0
1
2
3 4 5 6 Anealing time / h
7
8
Fig. 6. Variation of magnetic properties with time after final annealing at 1000 1C: (a) core loss and (b) magnetic induction.
Table 2 Heating rates during normalizing and intermediate annealing in the HRL samples.
Sample Sample Sample Sample
1 2 3 4
Normalizing
Intermediate annealing
Slow Slow Fast Fast
Fast Slow Fast Slow
2.449 W/kg and becomes 2.352 W/kg when annealing reaches 1000 1C after 8 h. Commercial 35W300 high-grade non-oriented silicon steel exhibit B50 of 1.703 T and P1.5 of 2.469 W/kg. Here, magnetic induction can be increased by more than 0.12 T, whereas core loss can still be reduced by 0.117 W/kg. Fig. 7 shows the EBSD orientation map of Sample 1, which demonstrates optimum performance compared with other samples in Fig. 6 after final annealing at 1000 1C for 0.5 h. Results indicate that the fractions of {1 0 0}/0 v wS and /1 1 1S//ND grains are 24.7% and 20%, respectively, and a large quantity of grains with non-{1 1 1} orientations exist in the sample. The average grain size in this stage (59.3 mm) is larger than that shown in Fig. 4 (25.6 mm). Therefore, the improvement in magnetic induction can be mainly ascribed to the strengthening of non{1 1 1} textures, whereas the decrease in core loss can be attributed to the appropriate combination of grain size and texture. The limitation, however, is that grain size distribution is not very uniform, indicating that performance can still be enhanced. Although commercial 35W300 high-grade non-oriented silicon steel with B50 and P1.5 of 1.70 T and 2.47 W/kg, respectively, can be prepared by hot rolling, normalization, one-stage cold rolling and final annealing, it still requires accurate chemical composition (Co30 ppm, N, S, Oo20 ppm, 3%Si). Moreover, a sharp /1 1 1S//ND texture exists in the final product. However, in this study, the demand for metallurgical quality in the initial
material that contains 100% columnar grains is lower than that in commercial high-grade steel, because C can be easily removed by decarburizing annealing. Furthermore, surface ridging can be avoided in the final sample by weakening shear deformation and by controlling the quantity of lower plastic strain ratio of {0 0 1}/1 1 0S colonies [1,3]. Finally, the best magnetic properties of Sample 1 (B50 ¼1.82 T, P1.5 ¼2.35 W/kg) exceed those of the commercial sample and feature higher magnetic flux induction. If the lower content of C is allowed, further improving the processing technique, magnetic properties may become more superior. However, the influence of precipitation particles on magnetic aging has yet to be investigated fully.
4. Conclusions (a) The results of this investigation show that a new non-oriented silicon steel with high magnetic flux induction (B50 ¼1.82 T) and low core loss (P1.5 ¼2.35 W/kg) can be prepared using 100% columnar grains. Hot rolling with lubrication, normalization at low heating rate, two-stage cold rolling, and final annealing at 1000 1C also help achieve high performance. This method is more suitable for the thin slab process. (b) Hot rolling with lubrication avoids shearing deformation and favors the retention of micro-substructures of {1 0 0}/0 v wS grains. Moreover, normalizing at low heating rate and intermediate annealing at fast heating rate result in more benefits to magnetic properties. The percentage of grains with non{1 1 1} orientations in the final annealing sample is higher than that of commercial 35W300 high-grade non-oriented silicon steel. (c) Final annealing at 1000 1C is an effective method to improve magnetic properties. The improvement in magnetic induction is mainly ascribed to the strengthening of non-{1 1 1} textures, whereas the decrease in core loss is attributed to the appropriate combination of grain size and texture.
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Fig. 7. EBSD orientation map (a) and ODF at j2 ¼ 451 section (b) of Sample 1 that was final annealed at 1000 1C for 0.5 h, as shown in Fig. 6.
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