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Magnetic properties and crystallographic textures of Fe 2.6% Si after 90% cold rolling plus different annealing
Journal of Magnetism and Magnetic Materials 354 (2014) 105–111
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties and crystallographic textures of Fe 2.6% Si after 90% cold rolling plus different annealing M.Z. Salih a,n, B. Weidenfeller c, N. Al-hamdany a, H.-G. Brokmeier a,b, W.M. Gan b a
Institut für Werkstoffkunde und Werkstofftechnik, TU Clausthal, Agricolastraße 6, D-38678 Clausthal-Zellerfeld, Germany Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, D-21502 Geesthacht, Germany c Institut für Mechanische Verfahrenstechnik, Abteilung für Materialwissenschaft, TU Clausthal, Arnold-Sommerfeld-Str. 6, D-38678 Clausthal-Zellerfeld, Germany b
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2013 Received in revised form 13 October 2013 Available online 6 November 2013
The effect of rolling and annealing on the crystallographic texture and the magnetic properties of Fe-2.6% Si non-oriented electric steel during 90% cold rolling and different annealing temperature at (600 1C, 700 1C, 900 1C and 1100 1C) for 60 min and 20 min was analyzed. The 97% hot rolled as received material shows development of alpha and gamma fiber texture affecting on the magnetic properties at rolling and transverse direction. 90% cold rolling with moderate annealing temperature (up to 700 1C) and 60 min annealing time leads to better textures and improved magnetic properties. Due to coarse grained microstructure after annealing, neutron diffractions is an efficient tool for the analysis of Bulk texture of polycrystalline materials, well known for sufficient grain statistics and bulk texture measurement. & 2013 Elsevier B.V. All rights reserved.
Keywords: Electric steel 90% Cold rolling Annealing Magnetic properties and neutron diffraction
1. Introduction The magnetic properties of Fe-2.6% Si steels are strongly depend on recrystallization and grain size morphology after hot and cold rolling processes. The grain size of the electrical steel and the absence of impurities like dislocations, grain boundaries or precipitations are the most important factors determining power loss. Thus, grain size optimization has attracted the interest of many researchers and therefore it has almost approached its limit by controlling chemical composition and processing parameter associated with hot, cold rolling and annealing temperature [1]. Low energy loss, low coercive force and high permeability in electric steel can further be reached by orienting the grains into [0 0 1] axis which is the axis of easy magnetization. This crystallographic direction is then oriented in a transformer when applied magnetic fields. Relative permeability and power loss are also sensitive to other direction of the factors such as surface oxidation, cutting stress (inducing dislocations) and nitration in the final annealing [2]. The most important texture components in electrical steel, (1 0 0)[0 0 1], (1 1 0)[0 0 1] and [1 1 1]//ND (γ-fiber) are affecting on the magnetic properties. Many investigations have been carried out in order to understand the effect of different
n
Correspondence to: Postal address: IWW, Agricolastr. 6, 38678 Clausthal-Zellerfeld, Germany. Tel.: þ49 4 1528 72635; fax: þ 49 4 15287 2666. E-mail addresses: [email protected], [email protected] (M.Z. Salih). 0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.10.051
factors on the formation of the [1 1 1] recrystallization texture and optimization of magnetic properties by controlling the chemical composition, rolling process and annealing temperatures [3,4]. According to the literature [5–8] the texture components which develop in non-oriented electric steels after heavy deformation, during thermo-mechanical processing are spread along the (α fiber ([1 1 0]//RD) fiber, (h k l) [h/l þ1h/l þ2h/l], θ fiber [1 0 0]//ND, η fiber [1 0 0]//RD and γ fiber [1 1 1]//ND, which are related to the rolling direction (RD), transverse direction (TD) and normal direction (ND). Due to the lack of data in literature, the effect of the bulk texture on the magnetic properties of the sample of Fe 2.6% Si was investigated by studying the crystallographic texture and the magnetic properties after the final annealing step. Crystallographic texture is examined by neutron diffraction and the magnetic properties were additionally measured. Due to coarse grains after annealing, high penetration and large beam cross section of neutrons were needed. Texture analysis by neutron diffraction has become a precise method to investigate bulk textures of different types of materials [9–11]. 2. Materials As starting material an iron silicon alloy with 2.6 wt% silicon was obtained from the company VACUUMESCHMELZE, Hanau, Germany. The first hot rolling step at 1100 1C was done at the VACUUMESCHMELZE with a thickness reduction from about 330–380 mm to a thickness of 10 mm, corresponds to a reduction of approximately 97%.
106
M.Z. Salih et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 105–111
The chemical composition of the material was determined by electric discharge spectroscopy (Spectro Analytical Instrument, GmbH) after hot rolling and it is shown in Table 1. Samples of 25 mm 55 mm 10 mm were cut from the hot rolled band and then cold rolled on a laboratory mill (BÜHLER, Germany) to a final thickness of 1 mm. The rolling direction of the process was perpendicular to the previous hot rolling direction (CRD ? HRD). Reduction steps in the cold rolling processes of 0.1 mm/step. Total reduction, related to starting thickness of 10 mm, is 90% corresponding to a logarithmic true strain of ф ¼2.3. After 90% cold rolling the samples were annealed for 20 min and 60 min at different temperatures of 600 1C, 700 1C, 900 1C and 1100 1C and cooled down to room temperature in (Fig. 1) micrographs of the hot rolled Fe 2.6% Si material, samples after 90% cold rolling and samples after 90% cold rolling and Table 1 Chemical composition of the used ironsilicon samples. Material
Si (%)
C (%)
Mn (%)
S (%)
N (%)
Al (%)
Fe–Si
2.6
0.0079
0.1135
0.0033
0.0065
0.001
TD
annealing at different temperatures are shown. The micrograph of the hot rolled band shows a mixture between smooth type and grooved type [12,13]. After 90% cold rolling the type of microstructure in heavily deformed samples corresponds to the smooth type [12,13]. After recrystallization the microstructure exhibits equiaxed grains. 2.1. Pole figure measurement The pole figure measurements using neutrons were carried out at the materials science diffractometer (STRESS-SPEC) at the FRM2@MLZ (Garching, Germany) [14,15]. Fig. 2 shows the layout of STRESS-SPEC with the sample mounted on the robot arm using a continuous scanning mode for intensity collection of complete pole figures. Continuous scanning is much faster than step scan modes and allows in parallel to optimize pole figure resolution to finer grids. The sample dimensions for neutron measurements were 10 10 10 mm3 to increase the gauge volume and to improve grain statistics needed for the annealed coarse grained material. Therefore ten slices were cut from the 1 mm thick sheet with marked RD and glued together, (using the spherical sample
TD RD
TD
RD
TD
RD
TD
RD
TD
RD
RD
Fig. 1. Optical micrographs of (a) 97% hot rolled, (b) 90% cold rolled, (c) 600 1C/60 min annealed sample (the average grain size is 27 mm) (d) 700 1C/60 min annealed sample (average grain size is 35 mm) (e) 900 1C/60 min annealed sample (average grain size is161 mm), and (f) 1100 1C/60 min annealed sample (average grain size is 570 mm). The grain sizes were estimated using the line intercept method coarse grain.
M.Z. Salih et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 105–111
method of Tobisch and Bunge [16]) and the average texture of the whole sample can be obtained by a large enough beam cross section, in our case 25 mm (see Fig. 3a). The measurement was carried out with a pyrolitic graphite monochromator (PG) and a monochromator take off angle which allows working with PG(0 0 4) and PG(0 0 6). The main advantage in using two wavelengths is to save counting time getting all needed pole figures with one detector set up. The used area detector of 300 300 mm² gave the Fe(1 1 0) reflection with PG(0 0 4) and Fe(2 0 0) and Fe(2 1 1) with PG(0 0 6) see (Fig. 3b).
107
Stress-Spec is equipped with a sample magazine which can be handled by the robot automatically, means all sample of each cycle can be measured without manual sample changing. Pole figure collection for one sample was done in about 1.2 h to get 72 readouts during continuous rotation and 6 tilt angles due to the detector size. A software package StressTextureCalculator (STeCa) [17] has been used to extract pole figure data from area detector using the mathematical formulation of Bunge and Klein [18]. The Orientation Distribution Function (ODF) was calculated from three complete pole figures (1 1 0), (2 0 0) and (2 1 1) by ISEM (Iterative Series Expansion Method) up to a degree of series expansion Lmax ¼ 22 [19]. 2.2. Measurement of magnetic properties Samples for magnetic measurements were rectangular bars 1 1 25 mm3. Measurements were done with a digital hysteresis recorder described in details elsewhere [20]. For purposes of comparison of magnetic sample's properties, measurements have been made at identical polarizations and magnetizing frequencies by varying the magnetic field strength.
3. Results and discussion 3.1. Magnetic properties Table 2 shows the variation of the magnetic properties of Fe 2.6% Si samples measured at magnetizing frequencies of 10 Hz and 50 Hz and a maximum polarization of J0 ¼1.8T. The saturation polarization was estimated to be Js ¼2.04T which corresponds to the values found in literature for Fe 2.6% Si [21]. As can be seen in
Fig. 2. Stäubli RX160 robot with mounted sample.
Diffracted beam
λ1= 1.8 Å 110
Primary beam λ2= 1.2 Å 200
Samples
λ2= 1.2 Å 211
Fig. 3. (a) Schematic view of set up, (b) diffraction image with two wavelength.
Table 2 Selected magnetic properties of the investigated samples. Cold rolled and annealed samples were measured in rolling direction. Power loss (kJ/m3)
Permeability
97% Hot rolling RD 97% Hot rolling TD 90% Cold rolling Annealing at 600 1C/60 min Annealing at 700 1C/60 min Annealing at 900 1C/60 min Annealing at 1100 1C/60 min
Coercive force (A/m)
10 (Hz)
50 (Hz)
10 (Hz)
50 (Hz)
10 (Hz)
50 (Hz)
75.97 47.01 76.51 121.79 106 78.66 92.91
73.7 45.69 75.81 125.36 108.08 81.09 95.67
3.053 1.340 3.084 1.397 0.795 0.818 0.830
11.762 2.423 4.219 2.702 1.475 1.474 1.833
435.35 155.53 420.05 181.34 72.88 98.54 96.41
1757.6 385.84 640.15 315.83 215.22 257.17 297
108
M.Z. Salih et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 105–111
Table 2 the power losses increase with frequency. This is due to an increase of classical and anomalous losses which are frequency dependent [21]. Related to eddy currents in the sample the permeability is slightly decreased. Permeability values measured in rolling direction of the as-received samples are nearly twice of the values measured in transversal direction. This indicates an anisotropy of the grains oriented with magnetic easy axis in rolling direction. Astonishingly the cold rolling process does not significantly change the permeability. Furthermore the power loss and coercive force at 10 Hz are nearly unchanged. Usually the rolling process should induce so many dislocations that permeability is decreased and coercive force and power loss are increased. It can be assumed that the dislocation density in the as-received and the cold rolled materials is nearly identical. The annealing process then increases the permeability values and decreases power losses and coercive forces. Regarding the values in Table 2 the thermal treatment of 1 h at 700 1C seems to lead to an optimum in magnetic properties. According to Fig. 1 the mean grain size in such samples is around 35 mm. Further increases of the grain sizes by higher annealing temperatures do not lead to lower coercive force or power loss and higher permeability values although the number of imperfections in the material is reduced. This behavior is well known as the so called Brown's paradoxon [22]. Impurities like dislocations and grain boundaries are sources for magnetic stray fields. The energy stored
in these magnetic stray fields can be reduced by the nucleation of domain walls. The higher number of magnetic domain walls leads to a reduction of power loss during magnetization reversal [23].
3.2. Texture The most common texture components and orientation fibers in steel after rolling and recrystallization are presented in the ODF sections φ2 ¼01and φ2 ¼ 451, as shown in (Fig. 4). Fig. 5 shows pole figures of 97% hot rolled Fe 2.6% Si. The texture of the hot rolled sample shows a high intensity spread along the θ fiber (0 0 1)//ND between (0 0 1)[1 –1 0] rotated cube component to the cube component (0 0 1)[0 1 0] and to (0 0 1)[ 1 1 0] rotated cube, the α fibre [1 1 0]//RD and γ fibres. The Goss {1 1 0}〈0 0 1〉 component of texture appeared due to the hot rolling at high temperature [24]. Due to the cutting the former RD becomes TD and consequently the starting texture for cold rolling changes as shown in (Fig. 6). The magnetic properties presented in Table 2 are related to the new sample orientation. It has to be noticed, that the cube component is not changed but the Goss component does. During cold rolling the crystallite orientations produced in hot rolling process rotate as expected [25]. Such a cross rolling produces a very strong {0 0 1}〈1 1 0〉 texture, as it is shown in (Fig. 7), which is developed due to the rotation of 901 around ND.
Fig. 4. (a) φ2 ¼ 01, (b) φ2 ¼ 451 ODF sections in Euler space showing the main texture components in steel.
φ1
RD Ф
TD
110 Pmax= 2.7 mrd
Contours 1 2 3 5 F max = 6.0 mrd
Fig. 5. (a) (1 1 0) pole figure (b) ODF for as-received HT rolling direction at φ2 ¼01, φ2 ¼451.
M.Z. Salih et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 105–111
109
RD φ1 Ф
TD
110 Pmax= 2.7 mrd
Contours 1 2 3 5 F max = 6.0 mrd
Fig. 6. (a) (1 1 0) pole figure (b) ODF for as-received 901 rotated at ND at φ2 ¼01, φ2 ¼ 451.
RD
φ1 Ф
TD
110 Pmax= 7 mrd
Contours 1 10 15 25 30 F max = 32.7 mrd
Fig. 7. (a) (1 1 0) pole figure (b) ODF for 90% cold rolling at φ2 ¼01, φ2 ¼ 451.
RD
φ1 Ф
TD
110 P max = 3.8 mrd
Contours 1 2 3 5 F max = 5.5 mrd
Fig. 8. (a) (1 1 0) pole figure (b) ODF for 90% cold rolling plus annealed 1 h at 600 1C at φ2 ¼ 01, φ2 ¼45.
All 〈1 1 0〉 fibers parallel to the rolling direction are transferred into [1 1 0] fibers parallel to transversal direction of hot rolling and the subsequent 90% cold rolling process finally converts them to a rotated cube component (0 0 1)[1 1 0]. After the primary recrystallization annealing (600 1C/1 h) a homogeneous intensity spread along α fiber, α n and γ fiber can be observed and also the Goss {1 1 0}〈0 0 1〉 component is initiated at primary recrystallization as it is shown in (Fig. 8).
Fig. 9 depicts the texture after annealing at 700 1C/1 h. A higher intensity spread along α fiber than along the γ fiber, particularly increasing at the Euler angles of the orientation (φ1,Ф, φ2) E [15,35,45] is observed. These angles correspond approximately to the (1 1 2)[7 –1 1 2] set of miller indices [26]. This local maximum in the intensity lies on a secondary fiber parallel to the α fiber [1 1 0]//RD which is running from the (1 1 1)[1 1 2] components to the cube fiber.
110
M.Z. Salih et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 105–111
RD φ1 Ф
TD
110 P max = 4 mrd
Contours 1 2 3 5 F max = 6.9 mrd
Fig. 9. (a) The (1 1 0) pole figure (b) ODF for 90% cold rolling plus annealed 1 h at 700 1C at φ2 ¼ 01, φ2 ¼ 451.
RD φ1 Ф
TD
110 P max = 6.7 mrd
Contours 1 2 3 5 7 9 F max = 12.2 mrd
Fig. 10. (a) The (1 1 0) pole figure (b) ODF for 90% cold rolling plus annealed 1 h at 900 1C at φ2 ¼ 01, φ2 ¼ 451.
RD φ1 Ф
TD
110 P max = 4 mrd
Contours 1 2 3 5 F max = 6.2 mrd
Fig. 11. (a) The (1 1 0) pole figure (b) ODF for 90% cold rolling plus annealed 1 h at 1100 1C at φ2 ¼ 01, φ2 ¼451.
An annealing of the sample at 900 1C/1 h lead to an increase of the intensity spread along the α fiber [1 1 0]//RD as shown in Fig. 10. Particularly the (1 1 2)[7 –1 1 2] component and a decreasing intensity spread along the γ fiber [1 1 1]//ND can be observed. After annealing at 1100 1C/1 h (1 1 2)[7 –1 1 2] is the dominating component as can be seen in (Fig. 11). The intensity along the θ fiber [0 0 1]//ND diminished and the component of the γ fiber [1 1 1]//ND disappears. Regarding these results it can be stated that the recrystallization texture in the investigated iron silicon alloy is more dependent on the deformation texture and microstructure
than on the recrystallization temperature [27,28] itself. The recrystallization and the grain growth result in a texture with higher intensity spread along [1 1 0]//RD fiber than along the [1 1 1]// ND fiber. Related to the change of the texture an effect on magnetic properties can be seen in (Fig. 12). It shows the dependence of the texture components, developed after recrystallization, and grain growth on the permeability and power loss. It can be seen that the permeability decreases with increasing annealing temperature due to the increment of texture components (1 1 2)[7 –1 1 2]
M.Z. Salih et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 105–111
111
Fig. 12. The effect of two texture components (1 1 1)[1 1 2] and (1 1 2)[7 1 1 2] on the magnetic properties.
while the power loss decreases by increasing the annealing temperature up to 900 1C. Exceeding this annealing temperature (up to 1100 1C) causes a rise in the power loss in spite of the improvement of the texture. However, as it has already been mentioned, the power losses are connected with the number of active domain walls, which are dependent on impurities and crystal defects. During rapid heating of non-oriented electric steel, the thermal stress effects on the magnetic properties. Their effects are emphasized in the case of the fully processed electrical steel when further grain growth does not lead to an improvement of the magnetic properties [29]. Because of the texture generated by annealing up to 900 1C, power loss increases and permeability increases.
4. Conclusions The texture and the magnetic properties of Fe 2.6% Si steel are strongly dependent on the annealing temperature and starting texture. Recrystallization and grain growth lead to a texture in which mainly two texture components, (1 1 2)[7 –1 1 2] and (1 1 1)[1 –1 2], can be observed affecting the magnetic properties. The annealing of the sample at different temperatures up to 900 1C for 1 h results in a decreasing permeability while simultaneously, the intensity of (1 1 2)[7 –1 1 2] increases. Simultaneously, also, the development of the (1 1 1)[1 –1 2] texture component leads to a decrease of power loss coinciding with an increase in grain size. References [1] J.T. Park, J.A. Szpunar, Acta Mater. 51 (2003) 30–37. [2] M.A. da Cunha, Sebasticio C. Paolinelli, J. Magn. Magn. Mater. 254-255 (2003) 379–381. [3] J. Grewen, J. Huber, Recrystallization and metallic materials, Stuttgart, Dr, Rieder Verlag Gmbh, 1987.
[4] Hu Hsnn, in: G. Gottstein, K. Lücke,. (Eds.), Proceedings of Fifth International Conference on Textures of Materials, Vol. 11, Springer Verlag, (1978). [5] L. Kestens, S. Jacobs, Texture control during the manfucturing of non oriented electrical steels, Texture, Stress, and Microstructure, (2008), http://dx.doi.org/10.1155/2008/173083. [6] M.Z. Quadir, B.J. Duggam, Acta Mater. 52 (2007) 401. [7] S. Nakkaura, H. Homa, Mater. Sci. Forum 467-470 (2004) 159. [8] P. Gobermado, R. Detrov, D. Ruiz, E. Leunis, L.A.I. Kestens, Adv. Eng. Mater. 12 (2010) 1077. [9] H.-G. Brokmeier, Physica B 234-236 (1997) 977. [10] H.J. Bunge, Textures Microstruct. 10 (1989) 265. [11] H.-G. Brokmeier, Physica B 234-236 (1997) 1144. [12] R. Kern, H.-P- Lee, H.J. Bunge, The Rolling Texture of Iron of different purities, Steel Res. 57 (1986) 365–571. [13] R. Kern, Prakt. Metallogr. 21 (6) (1984) 273/93. [14] H.-G. Brokmeier, W.M. Gan, C. Randau, M Völler, J. Rebelo-Kornmeier, M Hofmann, Nucl. Instrum. Methods Phys. Res., Sect. A 642 (2011) 87–92. [15] M. Hofmann, R. Schneider, G.A. Seidle, J Rebelo-Kornmeier, R.C. Wimpory, U. Garbe, H.-G. Brokmeier, Physica B 385-386 (2006) 1035–1037. [16] J Tobisch, H.J. Bunge, The spherical sample method in neutron diffraction texture determination, Texture 1 (1972) 125–127. [17] C. Randau, U. Garbe, H.-G. Brokmeier, J. Appl. Crystallogr. 44 (2011) 641–646. [18] H.J. Bunge, H. Klein, Z. Metallkunde 87 (1996) 465. [19] H.J. Bunge, M. Dahms, The iterative series-expansion method for quantitative texture analysis, J. Appl. Crystallogr. 22 (1989) 439–447. [20] M. Anhalt, B Weidenfeller, Theoretical an Experimental approach to characteristic measurement data of polymer bonded soft magnetic composites, J. Appl. Phys. 105 (2009) 113903. [21] R.M. Bozorth-1978, Ferromagnetism, IEEE Press, New York, 1978. [22] B. Weidenfeller, W. Riehemann, Effects of surface treatments on the hysteresis losses of GO iron silicon steel, J. Magn. Magn. Mater. 292 (2005) 210–214. [23] B. Weidenfeller, W. Rieheman, Domain refinement and domain wall activation of surface treated Fe–Si sheets, J. Magn. Magn. Mater. 160 (1996) 136–138. [24] John J. Jonas, Transformation textures associated with steel processing, in: Microstructure and Texture of Steel Conferences, (2008). [25] H. Inagaki, Stable end orientations in the rolling textures of the polycrystalline Iron, Z. Metallkunde (1987) 431–439. [26] G. Gottstein, K. Lücke, Tables for Texture Analysis of Cubic Crystals, proceeding of the Fifth international conference on Texture of materials, Achen, Germany, 1978. [27] P. Van Houtte, Manual of MTM-FHM, MTM- KU Leuvent, Belgium, 1982. [28] I. Samajdar, B. Verlinden, P. Van Houtte, ISIJ Int. 38 (7) (1998) 759. [29] P. Baudouin, A. Belhadj, Y. Houbaert, Effect of the rapid heating on the magnetic properties of non-oriented electrical steels, J. Magn. Magn. Mater. 238 (2002) 221–225.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties and crystallographic textures of Fe 2.6% Si after 90% cold rolling plus different annealing M.Z. Salih a,n, B. Weidenfeller c, N. Al-hamdany a, H.-G. Brokmeier a,b, W.M. Gan b a
Institut für Werkstoffkunde und Werkstofftechnik, TU Clausthal, Agricolastraße 6, D-38678 Clausthal-Zellerfeld, Germany Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, D-21502 Geesthacht, Germany c Institut für Mechanische Verfahrenstechnik, Abteilung für Materialwissenschaft, TU Clausthal, Arnold-Sommerfeld-Str. 6, D-38678 Clausthal-Zellerfeld, Germany b
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2013 Received in revised form 13 October 2013 Available online 6 November 2013
The effect of rolling and annealing on the crystallographic texture and the magnetic properties of Fe-2.6% Si non-oriented electric steel during 90% cold rolling and different annealing temperature at (600 1C, 700 1C, 900 1C and 1100 1C) for 60 min and 20 min was analyzed. The 97% hot rolled as received material shows development of alpha and gamma fiber texture affecting on the magnetic properties at rolling and transverse direction. 90% cold rolling with moderate annealing temperature (up to 700 1C) and 60 min annealing time leads to better textures and improved magnetic properties. Due to coarse grained microstructure after annealing, neutron diffractions is an efficient tool for the analysis of Bulk texture of polycrystalline materials, well known for sufficient grain statistics and bulk texture measurement. & 2013 Elsevier B.V. All rights reserved.
Keywords: Electric steel 90% Cold rolling Annealing Magnetic properties and neutron diffraction
1. Introduction The magnetic properties of Fe-2.6% Si steels are strongly depend on recrystallization and grain size morphology after hot and cold rolling processes. The grain size of the electrical steel and the absence of impurities like dislocations, grain boundaries or precipitations are the most important factors determining power loss. Thus, grain size optimization has attracted the interest of many researchers and therefore it has almost approached its limit by controlling chemical composition and processing parameter associated with hot, cold rolling and annealing temperature [1]. Low energy loss, low coercive force and high permeability in electric steel can further be reached by orienting the grains into [0 0 1] axis which is the axis of easy magnetization. This crystallographic direction is then oriented in a transformer when applied magnetic fields. Relative permeability and power loss are also sensitive to other direction of the factors such as surface oxidation, cutting stress (inducing dislocations) and nitration in the final annealing [2]. The most important texture components in electrical steel, (1 0 0)[0 0 1], (1 1 0)[0 0 1] and [1 1 1]//ND (γ-fiber) are affecting on the magnetic properties. Many investigations have been carried out in order to understand the effect of different
n
Correspondence to: Postal address: IWW, Agricolastr. 6, 38678 Clausthal-Zellerfeld, Germany. Tel.: þ49 4 1528 72635; fax: þ 49 4 15287 2666. E-mail addresses: [email protected], [email protected] (M.Z. Salih). 0304-8853/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.10.051
factors on the formation of the [1 1 1] recrystallization texture and optimization of magnetic properties by controlling the chemical composition, rolling process and annealing temperatures [3,4]. According to the literature [5–8] the texture components which develop in non-oriented electric steels after heavy deformation, during thermo-mechanical processing are spread along the (α fiber ([1 1 0]//RD) fiber, (h k l) [h/l þ1h/l þ2h/l], θ fiber [1 0 0]//ND, η fiber [1 0 0]//RD and γ fiber [1 1 1]//ND, which are related to the rolling direction (RD), transverse direction (TD) and normal direction (ND). Due to the lack of data in literature, the effect of the bulk texture on the magnetic properties of the sample of Fe 2.6% Si was investigated by studying the crystallographic texture and the magnetic properties after the final annealing step. Crystallographic texture is examined by neutron diffraction and the magnetic properties were additionally measured. Due to coarse grains after annealing, high penetration and large beam cross section of neutrons were needed. Texture analysis by neutron diffraction has become a precise method to investigate bulk textures of different types of materials [9–11]. 2. Materials As starting material an iron silicon alloy with 2.6 wt% silicon was obtained from the company VACUUMESCHMELZE, Hanau, Germany. The first hot rolling step at 1100 1C was done at the VACUUMESCHMELZE with a thickness reduction from about 330–380 mm to a thickness of 10 mm, corresponds to a reduction of approximately 97%.
106
M.Z. Salih et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 105–111
The chemical composition of the material was determined by electric discharge spectroscopy (Spectro Analytical Instrument, GmbH) after hot rolling and it is shown in Table 1. Samples of 25 mm 55 mm 10 mm were cut from the hot rolled band and then cold rolled on a laboratory mill (BÜHLER, Germany) to a final thickness of 1 mm. The rolling direction of the process was perpendicular to the previous hot rolling direction (CRD ? HRD). Reduction steps in the cold rolling processes of 0.1 mm/step. Total reduction, related to starting thickness of 10 mm, is 90% corresponding to a logarithmic true strain of ф ¼2.3. After 90% cold rolling the samples were annealed for 20 min and 60 min at different temperatures of 600 1C, 700 1C, 900 1C and 1100 1C and cooled down to room temperature in (Fig. 1) micrographs of the hot rolled Fe 2.6% Si material, samples after 90% cold rolling and samples after 90% cold rolling and Table 1 Chemical composition of the used ironsilicon samples. Material
Si (%)
C (%)
Mn (%)
S (%)
N (%)
Al (%)
Fe–Si
2.6
0.0079
0.1135
0.0033
0.0065
0.001
TD
annealing at different temperatures are shown. The micrograph of the hot rolled band shows a mixture between smooth type and grooved type [12,13]. After 90% cold rolling the type of microstructure in heavily deformed samples corresponds to the smooth type [12,13]. After recrystallization the microstructure exhibits equiaxed grains. 2.1. Pole figure measurement The pole figure measurements using neutrons were carried out at the materials science diffractometer (STRESS-SPEC) at the FRM2@MLZ (Garching, Germany) [14,15]. Fig. 2 shows the layout of STRESS-SPEC with the sample mounted on the robot arm using a continuous scanning mode for intensity collection of complete pole figures. Continuous scanning is much faster than step scan modes and allows in parallel to optimize pole figure resolution to finer grids. The sample dimensions for neutron measurements were 10 10 10 mm3 to increase the gauge volume and to improve grain statistics needed for the annealed coarse grained material. Therefore ten slices were cut from the 1 mm thick sheet with marked RD and glued together, (using the spherical sample
TD RD
TD
RD
TD
RD
TD
RD
TD
RD
RD
Fig. 1. Optical micrographs of (a) 97% hot rolled, (b) 90% cold rolled, (c) 600 1C/60 min annealed sample (the average grain size is 27 mm) (d) 700 1C/60 min annealed sample (average grain size is 35 mm) (e) 900 1C/60 min annealed sample (average grain size is161 mm), and (f) 1100 1C/60 min annealed sample (average grain size is 570 mm). The grain sizes were estimated using the line intercept method coarse grain.
M.Z. Salih et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 105–111
method of Tobisch and Bunge [16]) and the average texture of the whole sample can be obtained by a large enough beam cross section, in our case 25 mm (see Fig. 3a). The measurement was carried out with a pyrolitic graphite monochromator (PG) and a monochromator take off angle which allows working with PG(0 0 4) and PG(0 0 6). The main advantage in using two wavelengths is to save counting time getting all needed pole figures with one detector set up. The used area detector of 300 300 mm² gave the Fe(1 1 0) reflection with PG(0 0 4) and Fe(2 0 0) and Fe(2 1 1) with PG(0 0 6) see (Fig. 3b).
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Stress-Spec is equipped with a sample magazine which can be handled by the robot automatically, means all sample of each cycle can be measured without manual sample changing. Pole figure collection for one sample was done in about 1.2 h to get 72 readouts during continuous rotation and 6 tilt angles due to the detector size. A software package StressTextureCalculator (STeCa) [17] has been used to extract pole figure data from area detector using the mathematical formulation of Bunge and Klein [18]. The Orientation Distribution Function (ODF) was calculated from three complete pole figures (1 1 0), (2 0 0) and (2 1 1) by ISEM (Iterative Series Expansion Method) up to a degree of series expansion Lmax ¼ 22 [19]. 2.2. Measurement of magnetic properties Samples for magnetic measurements were rectangular bars 1 1 25 mm3. Measurements were done with a digital hysteresis recorder described in details elsewhere [20]. For purposes of comparison of magnetic sample's properties, measurements have been made at identical polarizations and magnetizing frequencies by varying the magnetic field strength.
3. Results and discussion 3.1. Magnetic properties Table 2 shows the variation of the magnetic properties of Fe 2.6% Si samples measured at magnetizing frequencies of 10 Hz and 50 Hz and a maximum polarization of J0 ¼1.8T. The saturation polarization was estimated to be Js ¼2.04T which corresponds to the values found in literature for Fe 2.6% Si [21]. As can be seen in
Fig. 2. Stäubli RX160 robot with mounted sample.
Diffracted beam
λ1= 1.8 Å 110
Primary beam λ2= 1.2 Å 200
Samples
λ2= 1.2 Å 211
Fig. 3. (a) Schematic view of set up, (b) diffraction image with two wavelength.
Table 2 Selected magnetic properties of the investigated samples. Cold rolled and annealed samples were measured in rolling direction. Power loss (kJ/m3)
Permeability
97% Hot rolling RD 97% Hot rolling TD 90% Cold rolling Annealing at 600 1C/60 min Annealing at 700 1C/60 min Annealing at 900 1C/60 min Annealing at 1100 1C/60 min
Coercive force (A/m)
10 (Hz)
50 (Hz)
10 (Hz)
50 (Hz)
10 (Hz)
50 (Hz)
75.97 47.01 76.51 121.79 106 78.66 92.91
73.7 45.69 75.81 125.36 108.08 81.09 95.67
3.053 1.340 3.084 1.397 0.795 0.818 0.830
11.762 2.423 4.219 2.702 1.475 1.474 1.833
435.35 155.53 420.05 181.34 72.88 98.54 96.41
1757.6 385.84 640.15 315.83 215.22 257.17 297
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Table 2 the power losses increase with frequency. This is due to an increase of classical and anomalous losses which are frequency dependent [21]. Related to eddy currents in the sample the permeability is slightly decreased. Permeability values measured in rolling direction of the as-received samples are nearly twice of the values measured in transversal direction. This indicates an anisotropy of the grains oriented with magnetic easy axis in rolling direction. Astonishingly the cold rolling process does not significantly change the permeability. Furthermore the power loss and coercive force at 10 Hz are nearly unchanged. Usually the rolling process should induce so many dislocations that permeability is decreased and coercive force and power loss are increased. It can be assumed that the dislocation density in the as-received and the cold rolled materials is nearly identical. The annealing process then increases the permeability values and decreases power losses and coercive forces. Regarding the values in Table 2 the thermal treatment of 1 h at 700 1C seems to lead to an optimum in magnetic properties. According to Fig. 1 the mean grain size in such samples is around 35 mm. Further increases of the grain sizes by higher annealing temperatures do not lead to lower coercive force or power loss and higher permeability values although the number of imperfections in the material is reduced. This behavior is well known as the so called Brown's paradoxon [22]. Impurities like dislocations and grain boundaries are sources for magnetic stray fields. The energy stored
in these magnetic stray fields can be reduced by the nucleation of domain walls. The higher number of magnetic domain walls leads to a reduction of power loss during magnetization reversal [23].
3.2. Texture The most common texture components and orientation fibers in steel after rolling and recrystallization are presented in the ODF sections φ2 ¼01and φ2 ¼ 451, as shown in (Fig. 4). Fig. 5 shows pole figures of 97% hot rolled Fe 2.6% Si. The texture of the hot rolled sample shows a high intensity spread along the θ fiber (0 0 1)//ND between (0 0 1)[1 –1 0] rotated cube component to the cube component (0 0 1)[0 1 0] and to (0 0 1)[ 1 1 0] rotated cube, the α fibre [1 1 0]//RD and γ fibres. The Goss {1 1 0}〈0 0 1〉 component of texture appeared due to the hot rolling at high temperature [24]. Due to the cutting the former RD becomes TD and consequently the starting texture for cold rolling changes as shown in (Fig. 6). The magnetic properties presented in Table 2 are related to the new sample orientation. It has to be noticed, that the cube component is not changed but the Goss component does. During cold rolling the crystallite orientations produced in hot rolling process rotate as expected [25]. Such a cross rolling produces a very strong {0 0 1}〈1 1 0〉 texture, as it is shown in (Fig. 7), which is developed due to the rotation of 901 around ND.
Fig. 4. (a) φ2 ¼ 01, (b) φ2 ¼ 451 ODF sections in Euler space showing the main texture components in steel.
φ1
RD Ф
TD
110 Pmax= 2.7 mrd
Contours 1 2 3 5 F max = 6.0 mrd
Fig. 5. (a) (1 1 0) pole figure (b) ODF for as-received HT rolling direction at φ2 ¼01, φ2 ¼451.
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RD φ1 Ф
TD
110 Pmax= 2.7 mrd
Contours 1 2 3 5 F max = 6.0 mrd
Fig. 6. (a) (1 1 0) pole figure (b) ODF for as-received 901 rotated at ND at φ2 ¼01, φ2 ¼ 451.
RD
φ1 Ф
TD
110 Pmax= 7 mrd
Contours 1 10 15 25 30 F max = 32.7 mrd
Fig. 7. (a) (1 1 0) pole figure (b) ODF for 90% cold rolling at φ2 ¼01, φ2 ¼ 451.
RD
φ1 Ф
TD
110 P max = 3.8 mrd
Contours 1 2 3 5 F max = 5.5 mrd
Fig. 8. (a) (1 1 0) pole figure (b) ODF for 90% cold rolling plus annealed 1 h at 600 1C at φ2 ¼ 01, φ2 ¼45.
All 〈1 1 0〉 fibers parallel to the rolling direction are transferred into [1 1 0] fibers parallel to transversal direction of hot rolling and the subsequent 90% cold rolling process finally converts them to a rotated cube component (0 0 1)[1 1 0]. After the primary recrystallization annealing (600 1C/1 h) a homogeneous intensity spread along α fiber, α n and γ fiber can be observed and also the Goss {1 1 0}〈0 0 1〉 component is initiated at primary recrystallization as it is shown in (Fig. 8).
Fig. 9 depicts the texture after annealing at 700 1C/1 h. A higher intensity spread along α fiber than along the γ fiber, particularly increasing at the Euler angles of the orientation (φ1,Ф, φ2) E [15,35,45] is observed. These angles correspond approximately to the (1 1 2)[7 –1 1 2] set of miller indices [26]. This local maximum in the intensity lies on a secondary fiber parallel to the α fiber [1 1 0]//RD which is running from the (1 1 1)[1 1 2] components to the cube fiber.
110
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RD φ1 Ф
TD
110 P max = 4 mrd
Contours 1 2 3 5 F max = 6.9 mrd
Fig. 9. (a) The (1 1 0) pole figure (b) ODF for 90% cold rolling plus annealed 1 h at 700 1C at φ2 ¼ 01, φ2 ¼ 451.
RD φ1 Ф
TD
110 P max = 6.7 mrd
Contours 1 2 3 5 7 9 F max = 12.2 mrd
Fig. 10. (a) The (1 1 0) pole figure (b) ODF for 90% cold rolling plus annealed 1 h at 900 1C at φ2 ¼ 01, φ2 ¼ 451.
RD φ1 Ф
TD
110 P max = 4 mrd
Contours 1 2 3 5 F max = 6.2 mrd
Fig. 11. (a) The (1 1 0) pole figure (b) ODF for 90% cold rolling plus annealed 1 h at 1100 1C at φ2 ¼ 01, φ2 ¼451.
An annealing of the sample at 900 1C/1 h lead to an increase of the intensity spread along the α fiber [1 1 0]//RD as shown in Fig. 10. Particularly the (1 1 2)[7 –1 1 2] component and a decreasing intensity spread along the γ fiber [1 1 1]//ND can be observed. After annealing at 1100 1C/1 h (1 1 2)[7 –1 1 2] is the dominating component as can be seen in (Fig. 11). The intensity along the θ fiber [0 0 1]//ND diminished and the component of the γ fiber [1 1 1]//ND disappears. Regarding these results it can be stated that the recrystallization texture in the investigated iron silicon alloy is more dependent on the deformation texture and microstructure
than on the recrystallization temperature [27,28] itself. The recrystallization and the grain growth result in a texture with higher intensity spread along [1 1 0]//RD fiber than along the [1 1 1]// ND fiber. Related to the change of the texture an effect on magnetic properties can be seen in (Fig. 12). It shows the dependence of the texture components, developed after recrystallization, and grain growth on the permeability and power loss. It can be seen that the permeability decreases with increasing annealing temperature due to the increment of texture components (1 1 2)[7 –1 1 2]
M.Z. Salih et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 105–111
111
Fig. 12. The effect of two texture components (1 1 1)[1 1 2] and (1 1 2)[7 1 1 2] on the magnetic properties.
while the power loss decreases by increasing the annealing temperature up to 900 1C. Exceeding this annealing temperature (up to 1100 1C) causes a rise in the power loss in spite of the improvement of the texture. However, as it has already been mentioned, the power losses are connected with the number of active domain walls, which are dependent on impurities and crystal defects. During rapid heating of non-oriented electric steel, the thermal stress effects on the magnetic properties. Their effects are emphasized in the case of the fully processed electrical steel when further grain growth does not lead to an improvement of the magnetic properties [29]. Because of the texture generated by annealing up to 900 1C, power loss increases and permeability increases.
4. Conclusions The texture and the magnetic properties of Fe 2.6% Si steel are strongly dependent on the annealing temperature and starting texture. Recrystallization and grain growth lead to a texture in which mainly two texture components, (1 1 2)[7 –1 1 2] and (1 1 1)[1 –1 2], can be observed affecting the magnetic properties. The annealing of the sample at different temperatures up to 900 1C for 1 h results in a decreasing permeability while simultaneously, the intensity of (1 1 2)[7 –1 1 2] increases. Simultaneously, also, the development of the (1 1 1)[1 –1 2] texture component leads to a decrease of power loss coinciding with an increase in grain size. References [1] J.T. Park, J.A. Szpunar, Acta Mater. 51 (2003) 30–37. [2] M.A. da Cunha, Sebasticio C. Paolinelli, J. Magn. Magn. Mater. 254-255 (2003) 379–381. [3] J. Grewen, J. Huber, Recrystallization and metallic materials, Stuttgart, Dr, Rieder Verlag Gmbh, 1987.
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