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Texture development during the production of high Si steel by hot dipping and diffusion annealing
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 304 (2006) e614–e616 www.elsevier.com/locate/jmmm
Texture development during the production of high Si steel by hot dipping and diffusion annealing J. Barrosa,, T. Ros-Yaneza, O. Fischerb, J. Schneiderb, Y. Houbaerta a
Laboratory for Materials Physics, Department of Metallurgy and Materials Science, Gent University, Technologiepark 903 B9052, Zwijnaarde-Gent, Belgium b Thyssen Krupp Electrical Steel, Castroperstrasse 228 D44791, Bochum, Germany Available online 20 March 2006
Abstract The production of non-oriented high Si and Al electrical steel requires specific processing routes such as hot dipping followed by diffusion annealing. The evolution of texture during the different production steps is described. During the preheating and hot dipping the substrates recrystallize with a typical texture presenting an intense gamma fibre. A Fe3 Si layer with cubic fibre texture is formed during the hot dipping. After hot dipping the substrate is further cold rolled with different levels of reduction and finally diffusion annealed delivering different final textures depending upon the Si content of the substrate and the thermomechanical processing. r 2006 Elsevier B.V. All rights reserved. PACS: 66.30h; 75.50.Cc; 81.40.Ef Keywords: Non-oriented electrical steel; Texture; Diffusion
1. Introduction Non-oriented electrical steels are the main grade of steel used for the construction of electrical rotating machines. They are designed and produced aiming to minimize the power losses. A great improvement of their magnetic properties can be achieved by increasing the content of Si, which normally is below 3wt% [1,2]. A rise of the Si and/or Al content up to 6.5wt% reduces the eddy current losses by increasing the resistivity and the magnetostriction becomes negligible. However the production of these higher Si alloys requires special steps in their processing, a Si rich coating has to be deposited and subsequently diffused into the bulk. The optimization of the soft magnetic properties also requires an optimization of the texture. In the case of rotating machines, the ideal texture is a fibre texture in which all the grains have a h100i direction normal to the Corresponding author. Fax: +9 264 58 33.
E-mail address: [email protected] (J. Barros). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.195
sheet plane [3]. This work deals with the effect of the thermomechanical processing and the diffusion process on the resulting texture of the high Si electrical steel. 2. Experimental procedure High Si and Al electrical steel samples were prepared by hot dipping and diffusion annealing following Ref. [2]. The substrates used as a starting material were in the range from 0 to 3wt% Si. The production processing, see Fig. 1, can be resumed as follows: commercial cold rolled substrates with a thickness of 0.8 and 0.5 mm were subjected to a preheating treatment (800 C for 45 s) and coated by dipping into a molten Al-25 %Si bath at 800 C for 5–100 s, typical coating thickness up to 50 mm [4]. The coated samples can be further cold rolled applying deformations up to 70%, this step is called Intermediate Cold Rolling (ICR). Finally, a diffusion annealing (DA) (temperatures up to 1100 C) was performed in order to diffuse the Al and Si into the plate. The concentration profiles were measured by SEM-EDS. The resulting texture
ARTICLE IN PRESS J. Barros et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e614–e616
INDUSTRIAL PROCESSING
LABORATORY PROCESSING
TEMPERATURE
FINAL DIFFUSION ANNEALING
CASTING & HOT ROLLING
e615
was determined by means of EBSD-SEM. ODF plots were chosen for the quantitative representation of the texture. In order to discuss the possible effect of the diffusion process on the texture an equivalent set of samples was subjected to the same processing route except for the dipping. Thus, samples without any coating were obtained and their texture could be compared with the texture of the coated samples.
PREHEATING & HOT DIPPING
INTERMEDIATE COLD ROLLING
COLD ROLLING
TIME Fig. 1. Processing route for the production of high Si and Al electrical steel.
Fig. 2. ODF plots for (A) the Fe3 Si layer, (B) The recrystallized substrate, (C) Si and Al poor grains as in Fig. 4, (D) Si and Al rich grains as in Fig. 4.
3. Results and discussion The texture of the samples was examined during the final steps of the process. The industrial material was received as cold rolled, during the preheating previous to the hot dipping the material recrystallizes and the obtained texture is shown in Fig. 2B. During the hot dipping an Al–Si coating is deposited on the substrate and a Fe3 Si layer grows at the interface between the substrate and the coating. This Fe3 Si layer has a composition close to stoichiometry [4], and presents a very well defined microstructure and texture, see Fig. 3 and Fig. 2A, respectively (see also Fig. 4). The ICR after the hot dipping is a very interesting step since it allows the control of the final thickness, improves substantially the coating surface and can be used as a tool for the achievement of the desired texture. The samples with an ICR reduction of 70% and annealed at 1100 C develop a strong gamma fibre, while any other component intensity is very low. In the case of 40% ICR reduction the samples present an intense g fibre but a weak cubic fibre is also present. Fig. 5 compares the intensities of the fibre for the different reductions and annealings. The different initial Si and Al content of the substrates has an influence on the final texture: whenever a substrate with Si o2wt% is used, a gamma-alpha phase transformation is possible during the annealing at high temperatures. This will produce two regions in the sample with different textures. The concentration profile from the border into the substrate center is mainly localized in grains that have a columnar shape, see Fig. 4. These grains are rich in Si and Al and during the annealing they were ferritic and grew because of the diffusion of these elements from the coating (texture in Fig. 2D). However the grains in the center have no Si or Al, their shape, size and texture are different, see Fig. 2C.
Fig. 3. Microstructure of the Fe3 Si layer (A), formed during the hot dipping over the substrate (B).
ARTICLE IN PRESS J. Barros et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e614–e616
e616
Composition wt%
25
Al wt% Si wt%
20 15 10 5 0 0
100
200
300 400 500 Position (µm)
600
700
800
Fig. 4. Concentration profile and microstructure in a Si free substrate after hot dipping and DA at 1100 C for 30 min.
10
{001} <110>
{111} <112>
{110} <001>
FeSi 2.4%
Relative intensity (Times random)
FeSi 3.0%
5
0 0
10
20
30
40
50
60
70
80
90
10 (B)
900°C 30min 1050°C 30min 1050°C 7min 900°C 150min
5
0 0
10
20
30
40
50
60
70
80
References [1] J. Barros, T. Ros, L. Vandenbossche, et al., J. Magn. Magn. Mater. 290 (2005) 1457. [2] T. Ros, Y. Houbaert, V.G. Rodriguez, J. Appl. Phys. 93 (10) (2002) 7857. [3] L. Kestens, J.J. Jonas, P. VanHoutte, et al., Metall. Mater. Trans. A 27 (8) (1996) 2347. [4] J. Barros, T. Ros, Y. Houbaert, Defect and Diffusion Forum 237–240 (Pt.2) (2005) 1115.
(A)
FeSi 1.3%
modified by the different thermomechanical processing routes. The diffusion of Si and Al from the coating affects differently the final texture depending on the Si content of the substrates. In low Si substrates a phase transformation gamma-alpha is possible and two different textures are observed: in the Si and Al enriched grains (surface) the texture is different from the one observed in the center, with a low Si–Al concentration. In substrates with Si 42wt% there is no phase transformation and the texture is similar all over the thickness. The Fe3 Si layer presents an intense cube texture, whose development could be very interesting for the magnetic properties and will be further studied.
90
Fig. 5. fibre intensity for (A) three different substrates after ICR (70% reduction) and DA at 1100 C for 30 min. (B) FeSi 2.4 wt% ICR reduction 40% and different DA.
4. Conclusions The texture of high Si and Al electrical steel produced by hot dipping and diffusion annealing can be significantly
Journal of Magnetism and Magnetic Materials 304 (2006) e614–e616 www.elsevier.com/locate/jmmm
Texture development during the production of high Si steel by hot dipping and diffusion annealing J. Barrosa,, T. Ros-Yaneza, O. Fischerb, J. Schneiderb, Y. Houbaerta a
Laboratory for Materials Physics, Department of Metallurgy and Materials Science, Gent University, Technologiepark 903 B9052, Zwijnaarde-Gent, Belgium b Thyssen Krupp Electrical Steel, Castroperstrasse 228 D44791, Bochum, Germany Available online 20 March 2006
Abstract The production of non-oriented high Si and Al electrical steel requires specific processing routes such as hot dipping followed by diffusion annealing. The evolution of texture during the different production steps is described. During the preheating and hot dipping the substrates recrystallize with a typical texture presenting an intense gamma fibre. A Fe3 Si layer with cubic fibre texture is formed during the hot dipping. After hot dipping the substrate is further cold rolled with different levels of reduction and finally diffusion annealed delivering different final textures depending upon the Si content of the substrate and the thermomechanical processing. r 2006 Elsevier B.V. All rights reserved. PACS: 66.30h; 75.50.Cc; 81.40.Ef Keywords: Non-oriented electrical steel; Texture; Diffusion
1. Introduction Non-oriented electrical steels are the main grade of steel used for the construction of electrical rotating machines. They are designed and produced aiming to minimize the power losses. A great improvement of their magnetic properties can be achieved by increasing the content of Si, which normally is below 3wt% [1,2]. A rise of the Si and/or Al content up to 6.5wt% reduces the eddy current losses by increasing the resistivity and the magnetostriction becomes negligible. However the production of these higher Si alloys requires special steps in their processing, a Si rich coating has to be deposited and subsequently diffused into the bulk. The optimization of the soft magnetic properties also requires an optimization of the texture. In the case of rotating machines, the ideal texture is a fibre texture in which all the grains have a h100i direction normal to the Corresponding author. Fax: +9 264 58 33.
E-mail address: [email protected] (J. Barros). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.195
sheet plane [3]. This work deals with the effect of the thermomechanical processing and the diffusion process on the resulting texture of the high Si electrical steel. 2. Experimental procedure High Si and Al electrical steel samples were prepared by hot dipping and diffusion annealing following Ref. [2]. The substrates used as a starting material were in the range from 0 to 3wt% Si. The production processing, see Fig. 1, can be resumed as follows: commercial cold rolled substrates with a thickness of 0.8 and 0.5 mm were subjected to a preheating treatment (800 C for 45 s) and coated by dipping into a molten Al-25 %Si bath at 800 C for 5–100 s, typical coating thickness up to 50 mm [4]. The coated samples can be further cold rolled applying deformations up to 70%, this step is called Intermediate Cold Rolling (ICR). Finally, a diffusion annealing (DA) (temperatures up to 1100 C) was performed in order to diffuse the Al and Si into the plate. The concentration profiles were measured by SEM-EDS. The resulting texture
ARTICLE IN PRESS J. Barros et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e614–e616
INDUSTRIAL PROCESSING
LABORATORY PROCESSING
TEMPERATURE
FINAL DIFFUSION ANNEALING
CASTING & HOT ROLLING
e615
was determined by means of EBSD-SEM. ODF plots were chosen for the quantitative representation of the texture. In order to discuss the possible effect of the diffusion process on the texture an equivalent set of samples was subjected to the same processing route except for the dipping. Thus, samples without any coating were obtained and their texture could be compared with the texture of the coated samples.
PREHEATING & HOT DIPPING
INTERMEDIATE COLD ROLLING
COLD ROLLING
TIME Fig. 1. Processing route for the production of high Si and Al electrical steel.
Fig. 2. ODF plots for (A) the Fe3 Si layer, (B) The recrystallized substrate, (C) Si and Al poor grains as in Fig. 4, (D) Si and Al rich grains as in Fig. 4.
3. Results and discussion The texture of the samples was examined during the final steps of the process. The industrial material was received as cold rolled, during the preheating previous to the hot dipping the material recrystallizes and the obtained texture is shown in Fig. 2B. During the hot dipping an Al–Si coating is deposited on the substrate and a Fe3 Si layer grows at the interface between the substrate and the coating. This Fe3 Si layer has a composition close to stoichiometry [4], and presents a very well defined microstructure and texture, see Fig. 3 and Fig. 2A, respectively (see also Fig. 4). The ICR after the hot dipping is a very interesting step since it allows the control of the final thickness, improves substantially the coating surface and can be used as a tool for the achievement of the desired texture. The samples with an ICR reduction of 70% and annealed at 1100 C develop a strong gamma fibre, while any other component intensity is very low. In the case of 40% ICR reduction the samples present an intense g fibre but a weak cubic fibre is also present. Fig. 5 compares the intensities of the fibre for the different reductions and annealings. The different initial Si and Al content of the substrates has an influence on the final texture: whenever a substrate with Si o2wt% is used, a gamma-alpha phase transformation is possible during the annealing at high temperatures. This will produce two regions in the sample with different textures. The concentration profile from the border into the substrate center is mainly localized in grains that have a columnar shape, see Fig. 4. These grains are rich in Si and Al and during the annealing they were ferritic and grew because of the diffusion of these elements from the coating (texture in Fig. 2D). However the grains in the center have no Si or Al, their shape, size and texture are different, see Fig. 2C.
Fig. 3. Microstructure of the Fe3 Si layer (A), formed during the hot dipping over the substrate (B).
ARTICLE IN PRESS J. Barros et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e614–e616
e616
Composition wt%
25
Al wt% Si wt%
20 15 10 5 0 0
100
200
300 400 500 Position (µm)
600
700
800
Fig. 4. Concentration profile and microstructure in a Si free substrate after hot dipping and DA at 1100 C for 30 min.
10
{001} <110>
{111} <112>
{110} <001>
FeSi 2.4%
Relative intensity (Times random)
FeSi 3.0%
5
0 0
10
20
30
40
50
60
70
80
90
10 (B)
900°C 30min 1050°C 30min 1050°C 7min 900°C 150min
5
0 0
10
20
30
40
50
60
70
80
References [1] J. Barros, T. Ros, L. Vandenbossche, et al., J. Magn. Magn. Mater. 290 (2005) 1457. [2] T. Ros, Y. Houbaert, V.G. Rodriguez, J. Appl. Phys. 93 (10) (2002) 7857. [3] L. Kestens, J.J. Jonas, P. VanHoutte, et al., Metall. Mater. Trans. A 27 (8) (1996) 2347. [4] J. Barros, T. Ros, Y. Houbaert, Defect and Diffusion Forum 237–240 (Pt.2) (2005) 1115.
(A)
FeSi 1.3%
modified by the different thermomechanical processing routes. The diffusion of Si and Al from the coating affects differently the final texture depending on the Si content of the substrates. In low Si substrates a phase transformation gamma-alpha is possible and two different textures are observed: in the Si and Al enriched grains (surface) the texture is different from the one observed in the center, with a low Si–Al concentration. In substrates with Si 42wt% there is no phase transformation and the texture is similar all over the thickness. The Fe3 Si layer presents an intense cube texture, whose development could be very interesting for the magnetic properties and will be further studied.
90
Fig. 5. fibre intensity for (A) three different substrates after ICR (70% reduction) and DA at 1100 C for 30 min. (B) FeSi 2.4 wt% ICR reduction 40% and different DA.
4. Conclusions The texture of high Si and Al electrical steel produced by hot dipping and diffusion annealing can be significantly