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Decarburization annealing of grain-oriented silicon steel with AlN as inhibitor
ELSEVIER
journal of magnetism , 4 ~ and magnetic materials
Journal of Magnetism and Magnetic Materials 133 (1994) 220-222
Decarburization annealing of grain-oriented silicon steel with A1N as inhibitor Franti~ek Rosypal
*
VUHZ, a.s., Iron and Steel Research Institute, 753 51 Dobrd, Czech Republic
Abstract The influence of decarburization annealing parameters on two heat samples with AIN as inhibitor with different nitrogen contents has been studied. The obtained results support the hypothesis that the increased surface oxidation at decarburization with higher dewpoint enables higher nitridation in the initial phase of final annealing in the nitrogen containing atmosphere, and therefore it intensifies the inhibition of the primary grain growth. The nitridation has a positive influence on heats with relatively low N contents.
1. Introduction In the manufacture of two experimental heats with A1N as inhibitor and single-stage cold rolling, it was necessary to resolve the increased demands on decarburization annealing: to reduce the carbon content from 0.04-0.05% down to values lower than 0.005% during the process. A possible solution could be to prolong the dwell time at decaburization, to change the dewpoint (change of oxidation ability of the atmosphere), and to change the decarburization temperature. The oxidative surface layers and also the primary matrix are changed in all the mentioned cases. The magnetic properties are influenced subsequently. The aim of the present work has been to determine optimal decarburization annealing parameters in such a way that the optimum could be obtained at sufficient decarburization from the point of view of magnetic properties too.
burized in a laboratory furnace at temperatures of 820 and 850°C in a 75% H 2 + 25% N 2 atmosphere, at dewpoints of 40 and 60°C. Some of the samples were then processed by a common technology; final annealing was carried out after the application of the MgO layer. The magnetic properties were determined after stress relief annealing. Another part of samples were analyzed after decarburization so that changes in carbon content during decarburization could be determined. The grain size of the primary matrix and the number of dispersed particles were determined for the chosen samples. The influence of the N 2 content in the annealing atmosphere upon the secondary recrystallization was then studied.
3. Results and discussion The carbon content was analyzed after gradually prolonged decarburization annealing so that the decar-
2. Experiment A single-stage rolled strip from two heats 0.33 mm thick was used for the experiment. The chemical composition is given in Table 1. The samples were decar-
* Tel: +42 (658) 2563253; fax: +42 (658) 2562256.
Table 1 Chemical composition Heat
C (%)
AI (%)
N (%)
AI/N ratio
A B
0.041 0.046
0.023 0.023
0.0099 0.0075
2.33 3.07
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00090-E
F. Rosypal/Journal of Magnetism and Magnetic Materials 133 (1994) 220-222 0.05 0.04
7
0.03 E E o° 0.02 O
1.8~
8 °C,2dew 0 "p. 40"C
1,7"
8tO*C, dew p. 60°C
2
0.01
•~
:J:,q
~
B 1000
-o.. 1.4 i
'-~
'
o
1820"c,dew p. 60"C
•
I
--
1.3J
o
1.2
F'2
HEAT B
r 1.9
dewp. 40*C 1 D - ~
1.5
,~t--
~
" ~q
m- - ~ ' ~
I1"8 o O 1.7 o 138
I
me"
r~ ~ ..... -~
1~
E
1.6
~
I
850"(3,
o_° 1.1i
0.00 0.0 1.0 2.0 3.0 4,0 Time of decarburization annealing [min]
HEAT A
221
1.5
:~
1,4
o
L1.3 p 1.5
il.2
0.9 +
+1.1 2,0 2.7 3.5 2.0 2.7 3.5 Time of decarburization annealing [rain]
Fig. 1. Influence of decarburization annealing on final C content (average values for heats A and B).
Fig. 2. Relation between decarburization annealing and magnetic properties for heats A and B. burization-time dependence could be determined. Both heats have the same decarburization trend, in accordance with with previously obtained data [1]. Therefore Fig. 1 shows only the average values of both heats. The initial decarburization velocity increases with increasing dewpoint and temperature (within the given temperature and dewpoint ranges). The limit value at which the decarburization stops is considerably higher for 850°C than 820°C (0.011% and 0.004%, respectively). The dewpoint has only a slight effect on this limit value. It is thus suitable to use the lower temperature (820°C) and higher dewpoint (60°C) from the point of view of the carbon content reduction. The resultant carbon content values are relatively high; this is the result of the not quite convenient construction of the laboratory furnace (samples were placed on the carrying strip) which restricted the access of atmosphere to the lower part of samples. Fig. 2 shows the dependence of the magnetic properties (magnetic induction B1000 at a magnetic field strength of 1000 A / m and specific total loss Pl.5 at magnetic induction 1.5 T and 50 Hz) on the decarburization time. The influence of decarburization parameters (temperature, dewpoint, dwell) is different for the two heats. The heat samples A show better magnetic properties. The most suitable decarburization for the given heat is that at 850°C as far as magnetic properties are concerned. An increase in the dewpoint above 40°C at 820°C deteriorate the magnetic properties. Heat B shows considerably worse magnetic properties. The decarburization at 850°C was the least suitable for the heat. Increasing the dewpoint to 60°C at 820°C improved the magnetic properties compared with those of heat A. The dwell prolongation at the decarburization temperature deteriorated magnetic properties very much, especially at dewpoint 40°C and temperature 820°C.
Secondary recrystallization, evaluated with respect to the secondary grain appearance of samples after the final annealing process, was in accordance with the magnetic induction value B10o0. In the case of heat A, complete secondary recrystallization occurred after all decarburization variants. Temperature 820°C and dewpoint 60°C showed the optimum for heat B, because only a partial secondary recrystallization occurred after the other decarburization variants. Changes in the decarburization annealing time had no influence on the appearance of secondary grains. Other experiments were carried out to determine why the heats behave in different ways. The primary grain size was determined for the chosen samples after decarburization annealing; the obtained values are presented in Table 2. In the case of heat A, the primary grain size did not change with changing decarburization annealing temperatures. In the case of heat B, in contrast, an increase in decarburization temperature led to an increase in the primary grain size. This means that the amount of inhibition phase is lower in heat B
Table 2 Primary grain size after the decarburization process Heat Decarburization annealing Primary
A
B
Temperature (°C)
Dewpoint (°C)
820 820 850 820 820 850
40 60 40 40 60 40
grain size (l~m) 11.0 12.0 11.5 12.4 12.8 15.0
222
F. Rosypal / Journal of Magnetism and Magnetic Materials 133 (1994) 220-222
Table 3 Influence of decarburization annealing on the increase in nitrogen content in the initial phase of final annealing Heat
A
B
Decarburization annealing
N content ppm
Temperature (°C)
Dewpoint (°C)
After decarburization annealing
During final annealing at 800°C
820 820 850 820 820 850
40 60 40 40 60 40
94 92 95 74 75 75
97 110 94 97 124 82
than in heat A. The number of dispersed particles corresponds, with the presented data: more dispersed particles were found in heat A than in heat B. We then studied the influence of decarburization annealing on the nitridation during the initial phase of final annealing. The decarburized samples were annealed in a laboratory furnace with a temperature increase of 30 K / h up to 800°C in 75% H 2 + 25% N 2 atmosphere. Table 3 shows the nitrogen contents of the samples before and after annealing, which did not change during decarburization. The dewpoint of the decarburizing atmosphere has a considerable influence on nitridation in the first phase of final annealing. The most nitridated samples are those undergoing decarburization in the atmosphere with the dewpoint 60°C. There is only a slight difference between temperatures 820°C and 850°C. The higher nitrogen content in the samples decarburized in atmospheres with higher dewpoints can be explained by the surface oxidation during decarburization which, in turn, caused easier nitrogen penetration from the atmosphere [2]. On the basis of these experiments, the different behaviours of the heats can be explained by the following hypothesis: higher dewpoint at decarburization annealing causes stronger surface oxidation which, ac-
cording to Ref. [2], facilitates nitridation in the initial phase of final annealing in the atmosphere containing 75% H+25% N 2. The nitridation influence is different for heats with different N and AI contents. In the case of heat B with a relatively low A I / N ratio = 3, the nitridation has a positive influence in that it intensifies the inhibition of normal grain growth and therefore supports secondary recrystallization. In contrast, in the case of heat A, with a sufficient N content (A1/N ratio approaching stichiometric proportions), thc fact that magnetic properties deteriorate after decarburization annealing at higher dewpoints can be explained by the predominant negative influence of the oxidizing layer.
4. Conclusions
From the point of view of carbon content reduction, the following optimal decarburization conditions were verified: a temperature of 820°C is more suitable than 850°C, and a higher dewpoint of 60°C is more suitable than 40°C. These results were ambiguous from the point of view of the influence of decarburization annealing parameters on magnetic properties, however. If we take into account the A1 and N contents and their mutual ratios, the results support the hypothesis that the increased surface oxidation at decarburization with higher dewpoint enables higher nitridation in the initial phase of final annealing in the nitrogen-containing atmosphere, and therefore it intensifies the inhibition of the primary grain growth. The nitridation has a positive influence only for heats with a relatively low N contents ( A I / N ratio >> 2).
References
[1] A. 2;idek, B. Lonsk~ and Z. Huliciovfi, Kovov~ materi~ly 13 (1975) 556. [2] P. Pficl, Physical metallurgy of grain oriented Fe-3% Si sheets, Doctoral thesis, VSB Ostrava, July 1990.
journal of magnetism , 4 ~ and magnetic materials
Journal of Magnetism and Magnetic Materials 133 (1994) 220-222
Decarburization annealing of grain-oriented silicon steel with A1N as inhibitor Franti~ek Rosypal
*
VUHZ, a.s., Iron and Steel Research Institute, 753 51 Dobrd, Czech Republic
Abstract The influence of decarburization annealing parameters on two heat samples with AIN as inhibitor with different nitrogen contents has been studied. The obtained results support the hypothesis that the increased surface oxidation at decarburization with higher dewpoint enables higher nitridation in the initial phase of final annealing in the nitrogen containing atmosphere, and therefore it intensifies the inhibition of the primary grain growth. The nitridation has a positive influence on heats with relatively low N contents.
1. Introduction In the manufacture of two experimental heats with A1N as inhibitor and single-stage cold rolling, it was necessary to resolve the increased demands on decarburization annealing: to reduce the carbon content from 0.04-0.05% down to values lower than 0.005% during the process. A possible solution could be to prolong the dwell time at decaburization, to change the dewpoint (change of oxidation ability of the atmosphere), and to change the decarburization temperature. The oxidative surface layers and also the primary matrix are changed in all the mentioned cases. The magnetic properties are influenced subsequently. The aim of the present work has been to determine optimal decarburization annealing parameters in such a way that the optimum could be obtained at sufficient decarburization from the point of view of magnetic properties too.
burized in a laboratory furnace at temperatures of 820 and 850°C in a 75% H 2 + 25% N 2 atmosphere, at dewpoints of 40 and 60°C. Some of the samples were then processed by a common technology; final annealing was carried out after the application of the MgO layer. The magnetic properties were determined after stress relief annealing. Another part of samples were analyzed after decarburization so that changes in carbon content during decarburization could be determined. The grain size of the primary matrix and the number of dispersed particles were determined for the chosen samples. The influence of the N 2 content in the annealing atmosphere upon the secondary recrystallization was then studied.
3. Results and discussion The carbon content was analyzed after gradually prolonged decarburization annealing so that the decar-
2. Experiment A single-stage rolled strip from two heats 0.33 mm thick was used for the experiment. The chemical composition is given in Table 1. The samples were decar-
* Tel: +42 (658) 2563253; fax: +42 (658) 2562256.
Table 1 Chemical composition Heat
C (%)
AI (%)
N (%)
AI/N ratio
A B
0.041 0.046
0.023 0.023
0.0099 0.0075
2.33 3.07
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00090-E
F. Rosypal/Journal of Magnetism and Magnetic Materials 133 (1994) 220-222 0.05 0.04
7
0.03 E E o° 0.02 O
1.8~
8 °C,2dew 0 "p. 40"C
1,7"
8tO*C, dew p. 60°C
2
0.01
•~
:J:,q
~
B 1000
-o.. 1.4 i
'-~
'
o
1820"c,dew p. 60"C
•
I
--
1.3J
o
1.2
F'2
HEAT B
r 1.9
dewp. 40*C 1 D - ~
1.5
,~t--
~
" ~q
m- - ~ ' ~
I1"8 o O 1.7 o 138
I
me"
r~ ~ ..... -~
1~
E
1.6
~
I
850"(3,
o_° 1.1i
0.00 0.0 1.0 2.0 3.0 4,0 Time of decarburization annealing [min]
HEAT A
221
1.5
:~
1,4
o
L1.3 p 1.5
il.2
0.9 +
+1.1 2,0 2.7 3.5 2.0 2.7 3.5 Time of decarburization annealing [rain]
Fig. 1. Influence of decarburization annealing on final C content (average values for heats A and B).
Fig. 2. Relation between decarburization annealing and magnetic properties for heats A and B. burization-time dependence could be determined. Both heats have the same decarburization trend, in accordance with with previously obtained data [1]. Therefore Fig. 1 shows only the average values of both heats. The initial decarburization velocity increases with increasing dewpoint and temperature (within the given temperature and dewpoint ranges). The limit value at which the decarburization stops is considerably higher for 850°C than 820°C (0.011% and 0.004%, respectively). The dewpoint has only a slight effect on this limit value. It is thus suitable to use the lower temperature (820°C) and higher dewpoint (60°C) from the point of view of the carbon content reduction. The resultant carbon content values are relatively high; this is the result of the not quite convenient construction of the laboratory furnace (samples were placed on the carrying strip) which restricted the access of atmosphere to the lower part of samples. Fig. 2 shows the dependence of the magnetic properties (magnetic induction B1000 at a magnetic field strength of 1000 A / m and specific total loss Pl.5 at magnetic induction 1.5 T and 50 Hz) on the decarburization time. The influence of decarburization parameters (temperature, dewpoint, dwell) is different for the two heats. The heat samples A show better magnetic properties. The most suitable decarburization for the given heat is that at 850°C as far as magnetic properties are concerned. An increase in the dewpoint above 40°C at 820°C deteriorate the magnetic properties. Heat B shows considerably worse magnetic properties. The decarburization at 850°C was the least suitable for the heat. Increasing the dewpoint to 60°C at 820°C improved the magnetic properties compared with those of heat A. The dwell prolongation at the decarburization temperature deteriorated magnetic properties very much, especially at dewpoint 40°C and temperature 820°C.
Secondary recrystallization, evaluated with respect to the secondary grain appearance of samples after the final annealing process, was in accordance with the magnetic induction value B10o0. In the case of heat A, complete secondary recrystallization occurred after all decarburization variants. Temperature 820°C and dewpoint 60°C showed the optimum for heat B, because only a partial secondary recrystallization occurred after the other decarburization variants. Changes in the decarburization annealing time had no influence on the appearance of secondary grains. Other experiments were carried out to determine why the heats behave in different ways. The primary grain size was determined for the chosen samples after decarburization annealing; the obtained values are presented in Table 2. In the case of heat A, the primary grain size did not change with changing decarburization annealing temperatures. In the case of heat B, in contrast, an increase in decarburization temperature led to an increase in the primary grain size. This means that the amount of inhibition phase is lower in heat B
Table 2 Primary grain size after the decarburization process Heat Decarburization annealing Primary
A
B
Temperature (°C)
Dewpoint (°C)
820 820 850 820 820 850
40 60 40 40 60 40
grain size (l~m) 11.0 12.0 11.5 12.4 12.8 15.0
222
F. Rosypal / Journal of Magnetism and Magnetic Materials 133 (1994) 220-222
Table 3 Influence of decarburization annealing on the increase in nitrogen content in the initial phase of final annealing Heat
A
B
Decarburization annealing
N content ppm
Temperature (°C)
Dewpoint (°C)
After decarburization annealing
During final annealing at 800°C
820 820 850 820 820 850
40 60 40 40 60 40
94 92 95 74 75 75
97 110 94 97 124 82
than in heat A. The number of dispersed particles corresponds, with the presented data: more dispersed particles were found in heat A than in heat B. We then studied the influence of decarburization annealing on the nitridation during the initial phase of final annealing. The decarburized samples were annealed in a laboratory furnace with a temperature increase of 30 K / h up to 800°C in 75% H 2 + 25% N 2 atmosphere. Table 3 shows the nitrogen contents of the samples before and after annealing, which did not change during decarburization. The dewpoint of the decarburizing atmosphere has a considerable influence on nitridation in the first phase of final annealing. The most nitridated samples are those undergoing decarburization in the atmosphere with the dewpoint 60°C. There is only a slight difference between temperatures 820°C and 850°C. The higher nitrogen content in the samples decarburized in atmospheres with higher dewpoints can be explained by the surface oxidation during decarburization which, in turn, caused easier nitrogen penetration from the atmosphere [2]. On the basis of these experiments, the different behaviours of the heats can be explained by the following hypothesis: higher dewpoint at decarburization annealing causes stronger surface oxidation which, ac-
cording to Ref. [2], facilitates nitridation in the initial phase of final annealing in the atmosphere containing 75% H+25% N 2. The nitridation influence is different for heats with different N and AI contents. In the case of heat B with a relatively low A I / N ratio = 3, the nitridation has a positive influence in that it intensifies the inhibition of normal grain growth and therefore supports secondary recrystallization. In contrast, in the case of heat A, with a sufficient N content (A1/N ratio approaching stichiometric proportions), thc fact that magnetic properties deteriorate after decarburization annealing at higher dewpoints can be explained by the predominant negative influence of the oxidizing layer.
4. Conclusions
From the point of view of carbon content reduction, the following optimal decarburization conditions were verified: a temperature of 820°C is more suitable than 850°C, and a higher dewpoint of 60°C is more suitable than 40°C. These results were ambiguous from the point of view of the influence of decarburization annealing parameters on magnetic properties, however. If we take into account the A1 and N contents and their mutual ratios, the results support the hypothesis that the increased surface oxidation at decarburization with higher dewpoint enables higher nitridation in the initial phase of final annealing in the nitrogen-containing atmosphere, and therefore it intensifies the inhibition of the primary grain growth. The nitridation has a positive influence only for heats with a relatively low N contents ( A I / N ratio >> 2).
References
[1] A. 2;idek, B. Lonsk~ and Z. Huliciovfi, Kovov~ materi~ly 13 (1975) 556. [2] P. Pficl, Physical metallurgy of grain oriented Fe-3% Si sheets, Doctoral thesis, VSB Ostrava, July 1990.