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Effect of stress relief annealing temperature and atmosphere on the magnetic properties of silicon steel
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 304 (2006) e599–e601 www.elsevier.com/locate/jmmm
Effect of stress relief annealing temperature and atmosphere on the magnetic properties of silicon steel Sebastia˜o C. Paolinelli, Marco A. da Cunha Acesita S.A., Research Department–Prac- a Primeiro de Maio, 9–Timo´teo, MG 35180-018, Brazil Available online 20 March 2006
Abstract Fully processed non-oriented silicon steel samples 0.50 mm thick were sheared and submitted to stress relief annealing under different conditions of temperature and atmosphere to investigate the effect of this treatment on the recovery of magnetic properties. Two different compositions were used, with different Si and Al contents. Temperature was varied in the range of 600–900 1C and four atmospheres were used: N2 and N2+10%H2 combined with dew points of 10 and 15 1C. The results showed that annealing atmosphere has very important effect on the magnetic properties and that the beneficial effect of stress relief annealing can be overcome by the detrimental effect of the atmosphere under certain conditions, due to oxidation and nitration. r 2006 Elsevier B.V. All rights reserved. PACS: 81.40.Rs; 81.40.Ef; 75.50.Bb Keywords: Silicon steel; Stress relief annealing; Oxidation; Nitration; Core loss
1. Introduction
2. Experimental procedure
Non-oriented silicon steel sheets are used in the production of cores for electrical machinery. Core production involves operations that can cause strain and stresses to the material, such as shearing, punching, interlocking, welding, resulting in magnetic properties deterioration. In many cases stress relief annealing is applied to the laminations to restore the magnetic properties. In the so called stress relief annealing deformed parts are recovered or recrystallized and residual stresses are relieved. However, interaction of the steel surface with the annealing atmosphere can cause nitration and/or oxidation of the metal [1–4]. In this work sheared samples of two silicon steel compositions were used to investigate the effect of annealing temperature and atmosphere on recovery of core loss (W1.5).
Silicon steel samples 0.50 mm thick from two heats were used in this work; chemical compositions are shown in Table 1. Samples were taken non-coated, sheared in the standard Epstein test dimensions, 305 mm 30 mm, and submitted to stress relief annealing in the temperature range from 600 to 900 1C. Samples were heated at a rate of 100 1C/h, soaked for 1 h, and cooled at 20 1C/h. Four annealing atmospheres were used according to composition and dew point: N2 with dew point of 10 1C (atm 1), N2 with dew point of 15 1C (atm 2), N2+10%H2 and 101C dew point (atm 3) and N2+10%H2 and 15 1C dew point (atm 4). Core loss at 1.5 T and 60 Hz (W1.5) was measured by Epstein test on as-sheared and as-stress relief annealed samples.
3. Results and discussion
Corresponding author. Tel.: +55 31 3849 7576; fax: +55 31 3849 7501.
E-mail address: [email protected] (M.A. da Cunha). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.187
The results showing the effect of stress relief annealing temperature and atmosphere on core loss are summarized in Fig. 1. It shows core loss variation on annealing, DW1.5,
ARTICLE IN PRESS S.C. Paolinelli, M.A. da Cunha / Journal of Magnetism and Magnetic Materials 304 (2006) e599–e601
e600 Table 1 Chemical composition Sample
%Si
%Al
%N
%O
A B
1.95 2.76
0.27 0.44
0.0031 0.0048
0.0151 0.0231
0.8
∆W1.5 [W/kg]
0.6
Atm.1
Atm.2
Atm.3
Atm.4
0.4
particles are formed in the sub-surface layer and also interact with domain walls. The effects of oxidation and nitration may overcome the beneficial effect of stress relief annealing on magnetic properties, resulting in core loss increase with annealing temperature, as observed in Fig. 1. To estimate the separate effects of stress relief, recovery and recrystallization of strained parts (SR), oxidation (O) and nitration (N) on the recovery of magnetic properties (DW1.5), 2%Si steel coated samples were stress relief annealed under dry N2+10%H2, so that oxidation and nitration were negligible [4]. The rate of core loss recovery with temperature was measured and assumed to apply to
0.2 0 -0.2
0.3
-0.6 500
600
(a)
700 800 900 Annealing Temperature [°C]
1000
0.8
∆W1.5 [W/kg]
-0.4
O
N
W
0.1 -0.1 -0.3
0.6 ∆W1.5 [W/kg]
SR
0.4
-0.5 500
0.2 0.0
600
(a)
700 800 900 Annealing Temperature [°C]
1000
-0.2 -0.4
Atm.1
Atm.2
Atm.3
0.3
Atm.4
SR
(O)
(N)
W
600
700
800
900
1000
Annealing Temperature [°C]
Fig. 1. Variation of core loss (DW1.5) with stress relief annealing temperature and atmosphere: (a) sample A; (b) sample B.
measured by the difference between as-annealed and assheared core losses. The largest reduction of core loss is obtained for atm 3. Sample A, with lower alloy content (Si+Al), tends to give better response on stress relief annealing and shows best results at slightly higher temperatures than alloy B. For both compositions, stress relief annealing under atm 2 may result in considerable degradation of magnetic properties as annealing temperature is increased. Annealing temperature is the determining parameter for stress relief. For strained parts, close to shearing zones, temperature also determine the extent of recovery and recrystallization. Consequently, magnetic properties tend to improve with annealing temperature. Interaction with the atmosphere at the surface tend to oxidize and nitride the steel. Oxidation and nitration increase with temperature and tend to deteriorate magnetic properties. Formation of an oxide layer reduces silicon content in the subsurface layer and produces rough metal–oxide interface, which can pin domain walls. With nitration, nitride
0.1 -0.1 -0.3 -0.5 500
600
(b)
0.3 ∆W1.5 [W/kg]
500 (b)
∆W1.5 [W/kg]
-0.6
SR
700 800 900 Annealing Temperature [°C]
O
N
1000
W
0.1 -0.1 -0.3 -0.5 500
(c)
600
700 800 Temperature [°C]
900
1000
Fig. 2. Core loss reductions (W) with annealing temperature, and calculated effects of stress relief, recovery and recrystallization (SR), oxidation (O) and nitration (N). Sample and annealing atmospheres: atm 3, samples (a) A and (b) B; (c) atm 4, sample A.
ARTICLE IN PRESS S.C. Paolinelli, M.A. da Cunha / Journal of Magnetism and Magnetic Materials 304 (2006) e599–e601
samples A and B, according to the method described in Ref. [4]. Oxidation and nitration during annealing were evaluated by the variations of oxygen and nitrogen content. Estimated separate effects for sample A under atm 3 and 4, and for sample B under atm 3 are shown in Fig. 2. Estimated (SR) effect on core loss is stronger for sample A than for sample B. The lower solute content of sample A, and its effect on mechanical properties, may lead to higher stress and strain levels on shearing. Under atm 3 (N) and (O) effects on sample A are very low and core loss reduction is determined by (SR) effect (Fig. 2a). On sample B, under the same atmosphere, (O) effect is also negligible but (N) above 750 1C has significant effect on core loss and completely overcomes the beneficial effect of (SR) at high temperature (900 1C), see Fig. 2b. Fig. 2c shows reasonable (O) effect on sample A, even at lower temperature, due to the fact that the oxidation potential of a H2–H2O atmosphere increases with reducing temperature. With increasing temperature the detrimental (O) effect also tends to overcome the beneficial effect of (SR). There is a strong (N) effect on sample B under N2 atmosphere, so that no core loss reduction is observed above 800 1C (see Fig. 1). At 900 1C significant increase in core loss is observed, particularly at high dew point.
e601
3. Conclusions Stress relief annealing temperature and atmosphere have important effect on magnetic properties of fully processed non-oriented silicon steel. Significant recovery of magnetic properties due to stress relief, recovery and recrystallization of strained parts is achieved in the temperature range of 750–800 1C. Annealing atmosphere may have detrimental effect on magnetic properties due to oxidation and nitration, particularly for high Si+Al alloy. Separate or combined effects of nitration and oxidation may overcome the beneficial effect of stress relief, recovery and recrystallization at high temperature. Best results were achieved under N2+10%H2 with 10 1C dew point. References [1] G. Lyudkovsky, A.G. Preban, J.M. Shapiro, J. Appl. Phys. 53 (3) (1982) 2419. [2] Lyudkovsky, G, Barnett, J.B., Rastogi, P.K., Balakrishnan, M., Proceedings of the 26th Mechanical Working and Steel Processing Conference, 1984, pp. 197–206. [3] A.L. Geiger, J. Appl. Phys. 50 (3) (1979) 2366. [4] Paolinelli, S.C., Proceedings of the 55th ABM Annual Conference, Rio de Janeiro, 2000, pp. 1471-1475.
Journal of Magnetism and Magnetic Materials 304 (2006) e599–e601 www.elsevier.com/locate/jmmm
Effect of stress relief annealing temperature and atmosphere on the magnetic properties of silicon steel Sebastia˜o C. Paolinelli, Marco A. da Cunha Acesita S.A., Research Department–Prac- a Primeiro de Maio, 9–Timo´teo, MG 35180-018, Brazil Available online 20 March 2006
Abstract Fully processed non-oriented silicon steel samples 0.50 mm thick were sheared and submitted to stress relief annealing under different conditions of temperature and atmosphere to investigate the effect of this treatment on the recovery of magnetic properties. Two different compositions were used, with different Si and Al contents. Temperature was varied in the range of 600–900 1C and four atmospheres were used: N2 and N2+10%H2 combined with dew points of 10 and 15 1C. The results showed that annealing atmosphere has very important effect on the magnetic properties and that the beneficial effect of stress relief annealing can be overcome by the detrimental effect of the atmosphere under certain conditions, due to oxidation and nitration. r 2006 Elsevier B.V. All rights reserved. PACS: 81.40.Rs; 81.40.Ef; 75.50.Bb Keywords: Silicon steel; Stress relief annealing; Oxidation; Nitration; Core loss
1. Introduction
2. Experimental procedure
Non-oriented silicon steel sheets are used in the production of cores for electrical machinery. Core production involves operations that can cause strain and stresses to the material, such as shearing, punching, interlocking, welding, resulting in magnetic properties deterioration. In many cases stress relief annealing is applied to the laminations to restore the magnetic properties. In the so called stress relief annealing deformed parts are recovered or recrystallized and residual stresses are relieved. However, interaction of the steel surface with the annealing atmosphere can cause nitration and/or oxidation of the metal [1–4]. In this work sheared samples of two silicon steel compositions were used to investigate the effect of annealing temperature and atmosphere on recovery of core loss (W1.5).
Silicon steel samples 0.50 mm thick from two heats were used in this work; chemical compositions are shown in Table 1. Samples were taken non-coated, sheared in the standard Epstein test dimensions, 305 mm 30 mm, and submitted to stress relief annealing in the temperature range from 600 to 900 1C. Samples were heated at a rate of 100 1C/h, soaked for 1 h, and cooled at 20 1C/h. Four annealing atmospheres were used according to composition and dew point: N2 with dew point of 10 1C (atm 1), N2 with dew point of 15 1C (atm 2), N2+10%H2 and 101C dew point (atm 3) and N2+10%H2 and 15 1C dew point (atm 4). Core loss at 1.5 T and 60 Hz (W1.5) was measured by Epstein test on as-sheared and as-stress relief annealed samples.
3. Results and discussion
Corresponding author. Tel.: +55 31 3849 7576; fax: +55 31 3849 7501.
E-mail address: [email protected] (M.A. da Cunha). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.187
The results showing the effect of stress relief annealing temperature and atmosphere on core loss are summarized in Fig. 1. It shows core loss variation on annealing, DW1.5,
ARTICLE IN PRESS S.C. Paolinelli, M.A. da Cunha / Journal of Magnetism and Magnetic Materials 304 (2006) e599–e601
e600 Table 1 Chemical composition Sample
%Si
%Al
%N
%O
A B
1.95 2.76
0.27 0.44
0.0031 0.0048
0.0151 0.0231
0.8
∆W1.5 [W/kg]
0.6
Atm.1
Atm.2
Atm.3
Atm.4
0.4
particles are formed in the sub-surface layer and also interact with domain walls. The effects of oxidation and nitration may overcome the beneficial effect of stress relief annealing on magnetic properties, resulting in core loss increase with annealing temperature, as observed in Fig. 1. To estimate the separate effects of stress relief, recovery and recrystallization of strained parts (SR), oxidation (O) and nitration (N) on the recovery of magnetic properties (DW1.5), 2%Si steel coated samples were stress relief annealed under dry N2+10%H2, so that oxidation and nitration were negligible [4]. The rate of core loss recovery with temperature was measured and assumed to apply to
0.2 0 -0.2
0.3
-0.6 500
600
(a)
700 800 900 Annealing Temperature [°C]
1000
0.8
∆W1.5 [W/kg]
-0.4
O
N
W
0.1 -0.1 -0.3
0.6 ∆W1.5 [W/kg]
SR
0.4
-0.5 500
0.2 0.0
600
(a)
700 800 900 Annealing Temperature [°C]
1000
-0.2 -0.4
Atm.1
Atm.2
Atm.3
0.3
Atm.4
SR
(O)
(N)
W
600
700
800
900
1000
Annealing Temperature [°C]
Fig. 1. Variation of core loss (DW1.5) with stress relief annealing temperature and atmosphere: (a) sample A; (b) sample B.
measured by the difference between as-annealed and assheared core losses. The largest reduction of core loss is obtained for atm 3. Sample A, with lower alloy content (Si+Al), tends to give better response on stress relief annealing and shows best results at slightly higher temperatures than alloy B. For both compositions, stress relief annealing under atm 2 may result in considerable degradation of magnetic properties as annealing temperature is increased. Annealing temperature is the determining parameter for stress relief. For strained parts, close to shearing zones, temperature also determine the extent of recovery and recrystallization. Consequently, magnetic properties tend to improve with annealing temperature. Interaction with the atmosphere at the surface tend to oxidize and nitride the steel. Oxidation and nitration increase with temperature and tend to deteriorate magnetic properties. Formation of an oxide layer reduces silicon content in the subsurface layer and produces rough metal–oxide interface, which can pin domain walls. With nitration, nitride
0.1 -0.1 -0.3 -0.5 500
600
(b)
0.3 ∆W1.5 [W/kg]
500 (b)
∆W1.5 [W/kg]
-0.6
SR
700 800 900 Annealing Temperature [°C]
O
N
1000
W
0.1 -0.1 -0.3 -0.5 500
(c)
600
700 800 Temperature [°C]
900
1000
Fig. 2. Core loss reductions (W) with annealing temperature, and calculated effects of stress relief, recovery and recrystallization (SR), oxidation (O) and nitration (N). Sample and annealing atmospheres: atm 3, samples (a) A and (b) B; (c) atm 4, sample A.
ARTICLE IN PRESS S.C. Paolinelli, M.A. da Cunha / Journal of Magnetism and Magnetic Materials 304 (2006) e599–e601
samples A and B, according to the method described in Ref. [4]. Oxidation and nitration during annealing were evaluated by the variations of oxygen and nitrogen content. Estimated separate effects for sample A under atm 3 and 4, and for sample B under atm 3 are shown in Fig. 2. Estimated (SR) effect on core loss is stronger for sample A than for sample B. The lower solute content of sample A, and its effect on mechanical properties, may lead to higher stress and strain levels on shearing. Under atm 3 (N) and (O) effects on sample A are very low and core loss reduction is determined by (SR) effect (Fig. 2a). On sample B, under the same atmosphere, (O) effect is also negligible but (N) above 750 1C has significant effect on core loss and completely overcomes the beneficial effect of (SR) at high temperature (900 1C), see Fig. 2b. Fig. 2c shows reasonable (O) effect on sample A, even at lower temperature, due to the fact that the oxidation potential of a H2–H2O atmosphere increases with reducing temperature. With increasing temperature the detrimental (O) effect also tends to overcome the beneficial effect of (SR). There is a strong (N) effect on sample B under N2 atmosphere, so that no core loss reduction is observed above 800 1C (see Fig. 1). At 900 1C significant increase in core loss is observed, particularly at high dew point.
e601
3. Conclusions Stress relief annealing temperature and atmosphere have important effect on magnetic properties of fully processed non-oriented silicon steel. Significant recovery of magnetic properties due to stress relief, recovery and recrystallization of strained parts is achieved in the temperature range of 750–800 1C. Annealing atmosphere may have detrimental effect on magnetic properties due to oxidation and nitration, particularly for high Si+Al alloy. Separate or combined effects of nitration and oxidation may overcome the beneficial effect of stress relief, recovery and recrystallization at high temperature. Best results were achieved under N2+10%H2 with 10 1C dew point. References [1] G. Lyudkovsky, A.G. Preban, J.M. Shapiro, J. Appl. Phys. 53 (3) (1982) 2419. [2] Lyudkovsky, G, Barnett, J.B., Rastogi, P.K., Balakrishnan, M., Proceedings of the 26th Mechanical Working and Steel Processing Conference, 1984, pp. 197–206. [3] A.L. Geiger, J. Appl. Phys. 50 (3) (1979) 2366. [4] Paolinelli, S.C., Proceedings of the 55th ABM Annual Conference, Rio de Janeiro, 2000, pp. 1471-1475.