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Production of single-crystal 3% silicon-iron sheet with orientation near (110) [001]
Journal of Magnetism and Magnetic Materials 58 (1986) 67-77 North-Holland, Amsterdam
67
PRODUCTION OF SINGLE-CRYSTAL 3% S I L I C O N - I R O N S H E E T W I T H O R I E N T A T I O N NEAR (110) [001] T. NOZAWA, T. NAKAYAMA, Y. U S H I G A M I Electrical Steel Lab., Nippon Steel Co., Kitakyushu 805, Japan
and T. YAMAMOTO Toei Industry Co., Tokyo 154, Japan Received 4 March 1985; in revised form 16 September 1985
The production of single-crystal 3% silicon iron sheet with a near (110) [001] orientation was studied. This study presented the possibility of the industrial production of grain-oriented 3% silicon steel with very high induction. This production is characterized by a unique grain-growth inhibition system for promoting secondary recrystallization. The system is composed of final annealing in a temperature gradient. Final annealing in a temperature gradient above 3°C/cm enables production of grain-oriented 3% silicon steel with a very high induction of 2.0 T.
1. Introduction Grain-oriented silicon steel is an important industrial product by virtue of its magnetic properties, especially high induction and low core loss at low magnetizing forces. The reduction of core loss in grain-oriented silicon steel is important for electric energy saving. The marked improvements of loss strongly depend upon the improvement of induction, B s (flux density subjected to a magnetizing force of 800 A T / m , B 8 is readily used as a convenient and sensitive measure of degree of cube-on-edge texture). The basics of grain-oriented silicon steel production was first invented by Goss in 1934 [1]. B 8 has been studied and improved by a number of workers over a period of many years. It is of practical importance to sharpen the cube-on-edge texture (the (110) [001] texture). B 8 of commercial grain-oriented silicon steel now is about 1.92 T [2,3]. The loss in commercial products strongly depends upon B 8. However, when B 8 increases above
1.95 T, the loss has a tendency to increase with increasing B 8 [4]. This phenomenon is due to the increase of 180 ° domain wall spacing with increasing B 8. Decreasing 180 ° domain wall spacing [5] and decreasing the surface area ratio of closure domains in the 180 ° domains is very important for low loss [6]. The total loss largely depend upon the tilt angle of [001] out of crystal surface, /3 [6,7]. When fl is about 2 °, the total loss under tensile stress is the smallest. When /3 is less than about 2 °, the total loss increases as B decreases. In the case of perfect (110) [001] orientation, the total loss is large. When /3 is larger than about 2 ° , the total loss increases as/3 increases (see fig. 1) On the other hand, by scratching perpendicular to the rolling direction, the total loss further decreases (see fig. 1). The reduction of the total loss by scratching becomes larger with decreasing /3. Observation of domain structure showed that scratching causes a decrease in 180 ° main domain wall spacings [8]. The minimum loss of scratched specimen is
0304-8853/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
68
T. Nozawa et al. / Production of single- crystal 3 % S t - Fe sheet
1.2
form of single-crystal sheets represents what may be called the end point in the development of a sharp single orientation texture. Single-crystal sheet produced by an artificial seed crystal, however, cannot replace the cold rolled silicon iron with a sharp single orientation texture because singlecrystal sheets are more difficult to produce, industrially [12]. This paper is concerned with the beneficial effect on induction achieved by final annealing in a temperature gradient. By this method, it is possible to produce industrially grain-oriented silicon steel with very high inductions [13].
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Fig. 1. Relationship between the misorientation angle fl and losses with and without scratching for 3% Si-Fe single crystals with orientation near (110) [001]. Spacing of scratches is 5 ram, thicknesses are 0.20 mm. The total losses measured at 17 kG and 50 Hz. fl is the tilt angle of the [001] out of specimen surface. Specimens were chemically polished. I : 1.5 k g / m m 2 tensile-stressed before scratching; l : 1.5 k g / m m 2 tensilestressed after scratching.
about 0.5 W / k g at 1.7 T, 50 Hz magnetization. These data suggest the possibility of developing very excellent grain-oriented silicon steel with superior low core loss in the future (see fig. 1). In regard to the domain refining techniques, two kinds of technique have been developed industrially. They are scratching methods [9,10] and methods producing periodic changes of the tilt angle of the [001] out of the crystal surface [11]. For this reason, in order to produce industrially low core loss grain-oriented silicon steel, it is indispensable to develop the methods raising the degree of (110) [001] orientation. A number of works have been done concerning the production of single crystals with precise (110) [001] orientations. Especially, the studies of production of oriented single-crystal silicon iron sheet done by Dunn and Nonken are well known [12]. They used artificial seed crystals to produce single crystals. Properly oriented single crystals are of course, ideal in this respect, and silicon iron in the
2. Experimental procedure Continuously cast slabs, which contained 0.053% carbon, 2.95% silicon, 0.087% manganese, 0.026% sulfur, 0.028% aluminum and 0.0081% nitrogen were hot rolled industrially. After hot rolling to a thickness of about 2.3 ram, the strip was pickled and heat treated. The strip was heavily cold rolled directly to the final gauge of 0.29 mm; a reduction of 87%. Decarburization to a maximum of 0.002% carbon was accomplished at 850°C in an atmosphere with a dew point of about 60°C. The atmosphere consisted of 25 vol% N 2 and 75 vol% H 2. Specimens were cut from decarburized strip and coated with magnesia (MgO) and dried. After final annealing in a temperature gradient, the induction with magnetizing force of 800 A T / m was determined using a single strip tester. The orientation of samples was determined within 0.5 ° by the Lane X-ray back-reflection technique.
3. Experimental result 3.1. Production of a single crystal with precise (11 O) [001] orientation by high temperature gradient annealing Through many years, considerable efforts have been directed toward increasing induction of grain-oriented silicon steel. A number of inventions have been proposed regarding grain-oriented
T. Nozawa et al. / Production of single- crystal 3 % S i - Fe sheet
silicon steel with high induction. Several of these steels are industrially produced and have B 8 value of approximately 1.92 T (intrinsic B 8 measured after removal of glass-film and coating is about 1.94 T), which is an excellent B 8 value and which is considerable lower than the approximately 2.04 T of the maximum value of a 3% silicon steel. On the other hand, considerable efforts have been made to make oriented single crystals. A silicon-iron sheet consisting of an oriented single crystal can be produced from a suitable polycrystalline sheet by recrystallizing it through the growth of a single nucleus. This "seed crystal" is previously grown in one end of the sheet and is mechanically positioned in the desired orientation by bending a portion of the sheet. Dunn and Nonken [12] outlined part of this technique. However, this method is not suited for industrially producing single crystals with precise (110) [001] orientation. An interesting fact was discovered; the temperature gradient enchances the alignment of the [001] axis with the rolling direction [13]. When the secondary recrystallized grains grow under a temperature gradients, the closer the orientation of the grains was to the (110) [001] orientation, the more preferential was the growth of the grain.
kanthal wound muffle
Fig. 2. Cross section of high temperature gradient furnace (a) and temperature variation with position as the specimen enters the furnace (b).
69
Fig. 2 shows a cross-section of a furnace and the temperature variation with position with it. The furnace consists of muffle wound with kanthal wire and a water jacketed copper block. Samples, suspended by a molybdenum wire of about 0.1 mm diameter, change their position by moving up and down vertically by motor. Samples of 4 cm width and 30 cm length were cut from decarburized strip and the ends of the samples were loaded into an annealing furncase in which the temperature was maintained at 1200°C, and were kept in the annealing furnace for a period of ten minutes. The temperature of one end was rapidly raised to one higher than the secondary recrystallizing temperature. Ten minutes later the sample was further advanced into the furnace at a rate of 1 c m / 6 min. The front temperature of the growing single crystal is about 950°C. The temperature gradient at this temperature is about 100°C/cm. Fig. 3 shows the macro-structure of the sample annealed by the above-described methods and a (100) polar diagram of each grain. In the bottom part of this macrostructure, the grains are very fine and did not undergo usual secondary recrystallization, because the heating rate was extremely high. Grains (~), (~), (~) and (~) as well as the lower part of grain (T), appear to be secondary recrystallized grains formed while one end of the sample was kept stationary for ten minutes. The only secondary recrystallized grain which greatly grew while the sample was being advanced at a rate of 1 c m / 6 min as grain (~). There is a difference in orientation between growing grain (T) and disappearing grains (~), (~), (~) and (~). Grain (~) exhibited an orientation very close to the (110) [001]. In each experiment, it was clearly confirmed that grains which grew to a large size under a temperature gradient exhibit an orientation near (110) [001]. In other words, when the secondary recrystallized grains grew under a temperature gradient, the closer the orientation of the grains was to the (110) [001] orientation, the more preferential was the growth of the grains. Since the growth of secondary recrystallized grains under a temperature gradient can promote alignment of the [001] direction with the rolling direction, the induction B 8 was nearly 2.0 T. Fig. 4 shows a relationship of temperature
70
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Fig. 4. Macrostructure and crystal orientation of sheet heated in a temperature gradient. The longitudinal direction of these sheets make an angle of 0°(a), 90°(b) and 45 ° (c) to the rolling direction. The figures in the diagrams show the angles between rolling direction and [001] direction observed by an etch pit method.
71
T. Nozawa et al. / Production of single-crystal 3 % S i - Fe sheet
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gradient, rolling direction and crystal orientation of growing grains. The longitudinal direction of these samples make angles of 0°(a), 45°(b) and 90°(c) to the rolling direction. These decarburized samples were inserted into furnace at a rate of 1 cm/5 rain for secondary recrystallization under the temperature gradient. Each grain grew along '~......
. . . .
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the direction of temperature gradient and regardless of the direction of the temperature gradient, the [001] direction of each grain is roughly parallel to the rolling direction of samples. It is of practical importance that grains with sharp (110) [001] textures grew by the temperature gradient perpendicular to the rolling direction.
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Fig. 6. Macrostructure of sheet, heated in a temperature gradient. The figures show the temperature gradient and the B s value determined by a 6 cm width single strip tester. 1050°C & boundary between primary and secondary recrystallization.
72
T. Nozawa et al. / Production of single- crystal 3 % S i - Fe sheet
3.2. Production of a single crystal with precise (110) [001] orientation by low temperature gradient annealing Usually, the final annealing of grain-oriented silicon steel production is done in coil form. In this case, it is impossible to originate a high temperature gradient in the whole steel sheet. An investigation was conducted on how induction is enhanced when heating generates relatively low temperature gradients. Samples for laboratory experiments were cut from a strip after decarburization. The sample have a width of 21 cm, a length of 84 cm and a stack of 20 sheets. In order to simulate coil annealing in a practical furnace, these samples were annealed in a curved condition. The radius of curvature was about 100 cm (see fig. 5). A furnace was designed for growing a large single crystal in a low temperature gradient (fig. 5). This furnace has six heating zones and temperatures in each zone are controlled independently. The cross-section of the muffle is 35 cm wide, 15 cm high and 100 cm long. The direction of the temperature gradient is
perpendicular to the rolling direction of the specimen. The temperature gradient was measured by a chromel-alumel thermocouple located in the specimen. After decarburization, the samples were loaded into a temperature gradient in a furnace with a nitrogen atmosphere and were heated at 10°C/h, from 650 to 1150°C. Fig. 6 shows the macrostructure of a sample that was annealed in a temperature gradient of about 3.5°C/cm. It looks like a few seed crystals nucleated out of many grains located in the left side of the specimen, and grew continuously perpendicular to the rolling direction. At about 1050°C, grain growth ends. The numbers in the figure show the local temperature gradient and B 8 value determined by a 6 cm wide single strip tester. Extremely high induction means the presence of a sharp (110) [001] orientation. Fig. 6 shows that the crystals having orientation near (110) [001] grow even in a low temperature gradient in the same manner as in a high temperature gradient (see fig. 3). Fig. 7 shows an example of a macrostructure
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Fig. 7. Macrostructure of sheet (a) heated in a temperature gradient. The figures show the B s value determined by a 6 cm width single strip tester; (b) the schematic illustration of the coil-form temperature gradient annealing.
73
T. Nozawa et a L / Production of single- crystal 3 56 S i - Fe sheet
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and B 8 values of silicon steel sheet produced by a coil-form temperature gradient annealing. During heating, the temperature gradient was generated perpendicular to the rolling direction of the strip coil [14]. Secondary recrystallized grains, which nucleated at the coil edge, grow perpendicular to the rolling direction and form slab-like large grains. The numbers in the figure show B 8 values determined by a 6 cm width single strip tester. Fig. 8 shows the effect of atmosphere at final annealing on B 8 values. Final annealing consisted of heating at 1 0 ° C / h from 650 to 1150°C and the temperature gradient was about 7 ° C / c m . 2.Q~ o
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Fig. 10. Variation of the total amount of nitrogen contained in the sheets during final annealing in temperature gradient. N2 100% atmosphere and magnesia B ((~)), 25 vo156N2, 75 vo156 H 2 atmosphere and magnesia A((~)) were used. Arrows on curves show the temperature of boundary regions between primary and secondary recrystallization.
Fig. 9 shows effects of magnesia coating. A principal difference among A, B and C magnesia is the boron content in the MgO. Boron contents of A, B, C magnesia are about 1300, 390 and about 100 ppm, respectively. Figs. 8 and 9 illustrate that the very high induction is promoted by nitrogen atmosphere and lower boron content magnesia. Fig. 10 shows the variation in the total nitrogen content in the steel during final annealing. The amount of nitrogen varies with atmosphere and magnesia. The peak value of the nitrogen content is about 150 p p m when a 100 vol% N 2 atmosphere and 390 p p m boron content magnesia (B) were used, and about 100 p p m when a 25 vol% N 2 and 75 vol% H 2 atmosphere and 1300 p p m boron content magnesia (A) were used. Arrows on curves show the temperature of the boundary region between primary and secondary recrystallization. The boundary temperature of the former is about 50°C higher than that of the latter. The nitrogen content increases with sheet temperature and decreases sharply after secondary recrystallization.
74
T. Nozawa et a L /
Production of single- crystal 3 % S i - Fe sheet
4. Discussion
It is widely recognized that in order for secondary recrystallization to occur, inhibition of normal grain growth is necessary. A variety of particles, but chiefly sulfides or nitrides or a combination of sulfides and nitrides, has been used for this purpose. The first high induction alloy [2] utilized both MnS and AIN to provide inhibition to normal grain growth. This paper shows that it is possible to produce of grain-oriented silicon steel with a very high induction using a temperature gradient for inhibition. According to conventional annealing for secondary recrystallization, an intentional temperature gradient is not imparted to a coil. Fig. 11 shows the macrostructure of a silicon steel sheet. This steel was subjected to heating in a conventional manner. When secondary recrystal-
Fig. 11. Macrostructure of sheets, heated without temperature gradient. Partially secondary-recrystallized (a) and completely secondary-recrystallized (b). Natural size.
lized grains partially formed (a); the heating was interrupted and then the sheet was subjected to cooling. Fig. l l a shows that the nuclei of the secondary recrystallized grains are formed and disperse in the primary recrystallized grains. As will be understood from fig. l l a , during conventional heating, the dispersed nuclei are formed in the primaries and then grow. These nuclei grow until the completion of secondary recrystallization. Fig. l l b shows the macrostructure of a grainoriented silicon steel which was subjected to conventional secondary recrystallization annealing. The grains shown in fig. l l b are virtually equiaxed because they grew from the dispersed nuclei. On the other hand, fig. 12 shows the macrostructure of a silicon steel sheet which was heated in a temperature gradient of 3 ° C / c m during secondary recrystallization annealing. When a temperature gradient is imparted to a steel sheet being secondary-recrystallized, the sheet has two regions, i.e., a high temperature region in which region recrystallization is completed and a low temperature region in which the primaries remain. The borderline between these two regions is along a constant-temperature line which corresponds to a secondary recrystallization temperature. When secondary recrystallized grains are formed under a temperature gradient, they grow in the direction of the high temperature to low temperature region. Thus, the secondary recrystallized grains are very elongated in the direction of the temperature gradient. This appearance of elongated grain is analogous to the macrostructure produced from the growth of seed crystals selected artificially. The origin of prefered grain growth of grains with sharp (110) [100] orientation under a temperature gradient cannot be determined easily. But the grounds for this phenomenon may be supposed as follows based on the basic knowledge of nucleation and growth, as well as the following accepted three empirical rules concerning the secondary recrystallization of grain-oriented silicon steel. a) The nucleation speed of the secondary recrystallized grains is higher when these grains are
7". Nozawa et al. / Production of single-crystal 3 % S i - Fe sheet
75
b) The growth speed of the secondary recrystallized grains is higher when these grains are more perfectly oriented. c) With reference to the nucleation speed and growth speed of the secondary recrystallized grains, the former is relatively high as com-
more perfectly oriented. This is, the secondary recrystallized grains more perfectly oriented nucleate during a shorter period of time at a low temperature as compared with those of the secondary recrystallized grains less perfectly oriented. !!i¸
r~ion
~ r y ~ t ~ primary ~reorystall r.y. - - t i on iza region
Fig. 12. Macrostructure of sheet heated in a temperature gradient. Photograph shows three large grains growing from sheet edge to primarily recrystallized region. Natural size.
76
T. Nozawa et al. / Production of single- crystal 3 % S i - Fe sheet
pared with the latter at a high temperature and the latter is high as compared with the former at a low temperature. The secondary recrystallized grains more perfectly oriented are generated at a relatively low temperature elevation (cf. item a). If a temperature gradient is not generated in the steel, in which the secondary recrystallized grains are generated as stated above, the grains, which nucleate and then start to grow, are dispersed and are in the form of a spot. While these grains further grow until secondary recrystallization is completed, the primarily crystallized grains, which are not yet secondarily recrystallized in the steel subjected to the temperature elevation, undergo a high temperature and thus the nuclei of the secondary recrystallized grains tend to generate in the primarily recrystallized grains. These nuclei are less perfectly oriented (cf. item a). The tendence for nuclei less perfectly oriented to form is more apparent when the temperature elevation rate is higher. A low rate of temperature elevation is, therefore, desirable for suppressing the generation of nuclei having a less perfect orientation. When the rate of temperature elevation is low, the number of nuclei having high orientation is small because of item a and c, above. In order to complete secondary recrystallization, a sample is annealed for a long time, during which the primarily recrystallized grains grow. Because of the growth of the primarily recrystallized grains, the driving force for the growth of the secondary recrystallized grains is decreased. Secondary recrystallization is, therefore, retarded not only due to the slow temperature elevation, but also due to the decrease of the driving force. Annealing without the temperature gradient will eventually result in an incomplete secondary recrystallized structure, in which coarse primarily recrystallized grains remain. In other words, when a temperature gradient is not generated, the primarily recrystallized grains remain even at a high temperature, and it is difficult to avoid the generation of nuclei having a less perfect orientation. On the other hand, when a temperature gradient is generated at the boundary region between the primarily and secondarily recrystallized grain
regions, the steel material is divided at any time during the generation of the temperature gradient into a high temperature region and a low temperature region. In the primarily recrystallized grain region, when the temperature is lower than in the secondarily recrystallized grain region, grain growth is suppressed. Therefore, when the previous low temperature region proceeds to a high temperature region, the growth of the secondarily recrystallized grains is promoted due to the suppression of grain growth. Only secondarily recrystallized grains having a more perfect orientation can be grown under the temperature gradient, because the primarily recrystallized grain region is not subjected to a high temperature. When the temperature gradient is large enough, it is possible to grow the secondary grains textured with sharp (110) [001] orientation. In large scale manufacture, the secondary recrystallizing annealing is usually carried out in coil from and it is difficult to generate a high temperature gradient in the whole coil. Therefore it is necessary to strengthen the inhibition effect using A1N particles
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Fig. 13. Relationship between the total amount of nitrogen contained in the sheets before secondary recrystallization and induction of sheets, heated in a temperature gradient, 25 vol% N 2, 75 vol% H 2 and magnesia A (©), 100 vol% N 2 and magnesia A (A), 25 vol% N2, 75 vol~ H 2 and magnesia B (O), 100 vol~ N 2 and magnesia B (zx), were used.
T. Nozawa et al. / Production of single-crystal 3 % S i - Fe sheet
ondarily recrystallized grains, with precise (110) [001] orientation, nucleated in the beginning grow toward low temperature region and form slab-like large grains, because the growth of these grains is not prevented by the nucleation of another grain with different orientations deviated from (110)
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in addition to the inhibition effect by the temperature gradient. In fig. 13, it is revealed that the B8 value strongly depends upon the amount of the total nitrogen contained in the primarily recrystallized region. Fig. 14 shows the relationship between secondary recrystallization temperature and B8. With increasing nitrogen content, B8 becomes higher. The reason for the B8 increase is not clear. The experimental results suggest that many AIN partides form during final annealing in a nitrogen atmosphere and generate a marked effect on the grain growth inhibition. Increasing the secondary recrystallization temperature with increasing amount of A1N, results in strong inhibition of grain growth of another grain in front of a precise (110) [001] secondarily recrystallized grains. Sec-
The author thank the many people that made essential contributions ranging from the construction of the annealing furnace to the measurement of the final magnetic properties.
References [1] N.P. Goss, US Patent No. 1965559 (1934). [2] S. Taguchi, T. Yamamoto and A. Sakakura, IEEE Trans. on. Magn. MAG-10 (1974) 123. [3] I. Goto, I. Matoba, T. Imanaka, T. Gotoh and T. Kan, Proc. Soft. Magn. Mater. Conf. 2 (1975) 262. [4] T. Sato, K. Kuroki and O. Tanaka, IEEE Trans. on Magn. MAG-14 (1978) 350. [5] R.H. Pry and C.P. Bean, J. Appl. Phys. 29 (1958) 532. [6] T. Nozawa, T. Yamamoto, Y. Ohya and Y. Matsuo, IEEE Trans. on Magn. MAG-14 (1978) 252. [71 J.W. Shilling, W.G. Morris, M.L. Osborn and P. Rao, IEEE Trans. on Magn. MAG-14 (1978) 104. [8] T. Nozawa, T. Yamamoto, Y. Ohya and Y. Matsuo, IEEE Trans. on Magn. MAG-15 (1979) 972. [9] K. Kuroki, K. Fukawa and T. Wada, J. Appl. Phys. 52 (1981) 2422. [10] T. Iuchi, S. Yamaguchi, T. Ichiyama, N. Nakamura, T. Ishimoto and K. Kuroki, J. Appl. Phys. 53 (1982) 2410. [11] T. Yamamoto, T. Nozawa, T. Nakayama and Y. Matsuo, J. Magn. Magn. Mater. 31-34 (1983) 993. [12] C.G. Dunn and G.C. Nonken, Metal Progress (December 1953) 71. [13] T. Nozawa, T. Yamamoto and T. Nakayama, US Patent No. 4437910 (1984). [14] T. Ohta and K. Kokai, to be published.
67
PRODUCTION OF SINGLE-CRYSTAL 3% S I L I C O N - I R O N S H E E T W I T H O R I E N T A T I O N NEAR (110) [001] T. NOZAWA, T. NAKAYAMA, Y. U S H I G A M I Electrical Steel Lab., Nippon Steel Co., Kitakyushu 805, Japan
and T. YAMAMOTO Toei Industry Co., Tokyo 154, Japan Received 4 March 1985; in revised form 16 September 1985
The production of single-crystal 3% silicon iron sheet with a near (110) [001] orientation was studied. This study presented the possibility of the industrial production of grain-oriented 3% silicon steel with very high induction. This production is characterized by a unique grain-growth inhibition system for promoting secondary recrystallization. The system is composed of final annealing in a temperature gradient. Final annealing in a temperature gradient above 3°C/cm enables production of grain-oriented 3% silicon steel with a very high induction of 2.0 T.
1. Introduction Grain-oriented silicon steel is an important industrial product by virtue of its magnetic properties, especially high induction and low core loss at low magnetizing forces. The reduction of core loss in grain-oriented silicon steel is important for electric energy saving. The marked improvements of loss strongly depend upon the improvement of induction, B s (flux density subjected to a magnetizing force of 800 A T / m , B 8 is readily used as a convenient and sensitive measure of degree of cube-on-edge texture). The basics of grain-oriented silicon steel production was first invented by Goss in 1934 [1]. B 8 has been studied and improved by a number of workers over a period of many years. It is of practical importance to sharpen the cube-on-edge texture (the (110) [001] texture). B 8 of commercial grain-oriented silicon steel now is about 1.92 T [2,3]. The loss in commercial products strongly depends upon B 8. However, when B 8 increases above
1.95 T, the loss has a tendency to increase with increasing B 8 [4]. This phenomenon is due to the increase of 180 ° domain wall spacing with increasing B 8. Decreasing 180 ° domain wall spacing [5] and decreasing the surface area ratio of closure domains in the 180 ° domains is very important for low loss [6]. The total loss largely depend upon the tilt angle of [001] out of crystal surface, /3 [6,7]. When fl is about 2 °, the total loss under tensile stress is the smallest. When /3 is less than about 2 °, the total loss increases as B decreases. In the case of perfect (110) [001] orientation, the total loss is large. When /3 is larger than about 2 ° , the total loss increases as/3 increases (see fig. 1) On the other hand, by scratching perpendicular to the rolling direction, the total loss further decreases (see fig. 1). The reduction of the total loss by scratching becomes larger with decreasing /3. Observation of domain structure showed that scratching causes a decrease in 180 ° main domain wall spacings [8]. The minimum loss of scratched specimen is
0304-8853/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
68
T. Nozawa et al. / Production of single- crystal 3 % S t - Fe sheet
1.2
form of single-crystal sheets represents what may be called the end point in the development of a sharp single orientation texture. Single-crystal sheet produced by an artificial seed crystal, however, cannot replace the cold rolled silicon iron with a sharp single orientation texture because singlecrystal sheets are more difficult to produce, industrially [12]. This paper is concerned with the beneficial effect on induction achieved by final annealing in a temperature gradient. By this method, it is possible to produce industrially grain-oriented silicon steel with very high inductions [13].
1.1 1.0 o.g
~ 0.8' t~
0.7
~'0.6
e'°iI ~ 0.5
"~ 0.4-,0.2 0.1
I
o
I I I 1 2 3 Misorientation
I I t t 4 5 6 7 A n g l e /3, d e g r e e
l 8
Fig. 1. Relationship between the misorientation angle fl and losses with and without scratching for 3% Si-Fe single crystals with orientation near (110) [001]. Spacing of scratches is 5 ram, thicknesses are 0.20 mm. The total losses measured at 17 kG and 50 Hz. fl is the tilt angle of the [001] out of specimen surface. Specimens were chemically polished. I : 1.5 k g / m m 2 tensile-stressed before scratching; l : 1.5 k g / m m 2 tensilestressed after scratching.
about 0.5 W / k g at 1.7 T, 50 Hz magnetization. These data suggest the possibility of developing very excellent grain-oriented silicon steel with superior low core loss in the future (see fig. 1). In regard to the domain refining techniques, two kinds of technique have been developed industrially. They are scratching methods [9,10] and methods producing periodic changes of the tilt angle of the [001] out of the crystal surface [11]. For this reason, in order to produce industrially low core loss grain-oriented silicon steel, it is indispensable to develop the methods raising the degree of (110) [001] orientation. A number of works have been done concerning the production of single crystals with precise (110) [001] orientations. Especially, the studies of production of oriented single-crystal silicon iron sheet done by Dunn and Nonken are well known [12]. They used artificial seed crystals to produce single crystals. Properly oriented single crystals are of course, ideal in this respect, and silicon iron in the
2. Experimental procedure Continuously cast slabs, which contained 0.053% carbon, 2.95% silicon, 0.087% manganese, 0.026% sulfur, 0.028% aluminum and 0.0081% nitrogen were hot rolled industrially. After hot rolling to a thickness of about 2.3 ram, the strip was pickled and heat treated. The strip was heavily cold rolled directly to the final gauge of 0.29 mm; a reduction of 87%. Decarburization to a maximum of 0.002% carbon was accomplished at 850°C in an atmosphere with a dew point of about 60°C. The atmosphere consisted of 25 vol% N 2 and 75 vol% H 2. Specimens were cut from decarburized strip and coated with magnesia (MgO) and dried. After final annealing in a temperature gradient, the induction with magnetizing force of 800 A T / m was determined using a single strip tester. The orientation of samples was determined within 0.5 ° by the Lane X-ray back-reflection technique.
3. Experimental result 3.1. Production of a single crystal with precise (11 O) [001] orientation by high temperature gradient annealing Through many years, considerable efforts have been directed toward increasing induction of grain-oriented silicon steel. A number of inventions have been proposed regarding grain-oriented
T. Nozawa et al. / Production of single- crystal 3 % S i - Fe sheet
silicon steel with high induction. Several of these steels are industrially produced and have B 8 value of approximately 1.92 T (intrinsic B 8 measured after removal of glass-film and coating is about 1.94 T), which is an excellent B 8 value and which is considerable lower than the approximately 2.04 T of the maximum value of a 3% silicon steel. On the other hand, considerable efforts have been made to make oriented single crystals. A silicon-iron sheet consisting of an oriented single crystal can be produced from a suitable polycrystalline sheet by recrystallizing it through the growth of a single nucleus. This "seed crystal" is previously grown in one end of the sheet and is mechanically positioned in the desired orientation by bending a portion of the sheet. Dunn and Nonken [12] outlined part of this technique. However, this method is not suited for industrially producing single crystals with precise (110) [001] orientation. An interesting fact was discovered; the temperature gradient enchances the alignment of the [001] axis with the rolling direction [13]. When the secondary recrystallized grains grow under a temperature gradients, the closer the orientation of the grains was to the (110) [001] orientation, the more preferential was the growth of the grain.
kanthal wound muffle
Fig. 2. Cross section of high temperature gradient furnace (a) and temperature variation with position as the specimen enters the furnace (b).
69
Fig. 2 shows a cross-section of a furnace and the temperature variation with position with it. The furnace consists of muffle wound with kanthal wire and a water jacketed copper block. Samples, suspended by a molybdenum wire of about 0.1 mm diameter, change their position by moving up and down vertically by motor. Samples of 4 cm width and 30 cm length were cut from decarburized strip and the ends of the samples were loaded into an annealing furncase in which the temperature was maintained at 1200°C, and were kept in the annealing furnace for a period of ten minutes. The temperature of one end was rapidly raised to one higher than the secondary recrystallizing temperature. Ten minutes later the sample was further advanced into the furnace at a rate of 1 c m / 6 min. The front temperature of the growing single crystal is about 950°C. The temperature gradient at this temperature is about 100°C/cm. Fig. 3 shows the macro-structure of the sample annealed by the above-described methods and a (100) polar diagram of each grain. In the bottom part of this macrostructure, the grains are very fine and did not undergo usual secondary recrystallization, because the heating rate was extremely high. Grains (~), (~), (~) and (~) as well as the lower part of grain (T), appear to be secondary recrystallized grains formed while one end of the sample was kept stationary for ten minutes. The only secondary recrystallized grain which greatly grew while the sample was being advanced at a rate of 1 c m / 6 min as grain (~). There is a difference in orientation between growing grain (T) and disappearing grains (~), (~), (~) and (~). Grain (~) exhibited an orientation very close to the (110) [001]. In each experiment, it was clearly confirmed that grains which grew to a large size under a temperature gradient exhibit an orientation near (110) [001]. In other words, when the secondary recrystallized grains grew under a temperature gradient, the closer the orientation of the grains was to the (110) [001] orientation, the more preferential was the growth of the grains. Since the growth of secondary recrystallized grains under a temperature gradient can promote alignment of the [001] direction with the rolling direction, the induction B 8 was nearly 2.0 T. Fig. 4 shows a relationship of temperature
70
rol Iing
gro~Ning
d i r e o t ion
-- g r a i n
2~ USL
08L
~*
l~1415
tom
dN
(a)
l
original n u c l e a r ion region fine grains
b) Fig. 3. (100) pole figure of grains, 1-5, and macrostructure of sheet (b), heated in a temperature gradient. Sheet was dropped into the furnace at a rate of 1 c m / 6 min. Atmosphere was 75 vol% H 2 and 25 vol% N 2.
0°
0°
/
rc~ fine grains
,
~)
0o
rolling dir~tion
0°
~b)
lcm
Trolling direction
o,?::,, 0°
t
d i r e o t ion of ~ temperature gradient
fine grains
,~a__~ r o l l i n g d i r e c t i o n
Fig. 4. Macrostructure and crystal orientation of sheet heated in a temperature gradient. The longitudinal direction of these sheets make an angle of 0°(a), 90°(b) and 45 ° (c) to the rolling direction. The figures in the diagrams show the angles between rolling direction and [001] direction observed by an etch pit method.
71
T. Nozawa et al. / Production of single-crystal 3 % S i - Fe sheet
1000mm
I~
ther mocouples
?
1200 heetir~ r D
O~
./
Curvedfirebrick
I t
t
i
I
I
I
I
I
I I
I
I
~ 600 ~7"
heating element
1
~
~ 400 Z m 200
I
40
60
80 100 120
, Annealing , , Time, , hr,
\fire
(a)
e
~m
20 carbide /
t
1000
Illlill/nIJInJllllllllll silicon
e
~ 800
--t.-(( ) ~ ~ [ muffle
.
brick
0))
for
shelter of heating z o n e s
,
Fig, 5. Cross section of low temperature gradient furnace (a) and diagram curves of temperature variation in each position of sheet (b).
gradient, rolling direction and crystal orientation of growing grains. The longitudinal direction of these samples make angles of 0°(a), 45°(b) and 90°(c) to the rolling direction. These decarburized samples were inserted into furnace at a rate of 1 cm/5 rain for secondary recrystallization under the temperature gradient. Each grain grew along '~......
. . . .
. . . . .
the direction of temperature gradient and regardless of the direction of the temperature gradient, the [001] direction of each grain is roughly parallel to the rolling direction of samples. It is of practical importance that grains with sharp (110) [001] textures grew by the temperature gradient perpendicular to the rolling direction.
840m
3.5~--~0m • 3.5"C./=m 3..d'C,.~,='.
~aet ~ o r i g i r m i m ~ i e a t ion k rqi~ )
direction of temperature gradient
4.o~
I roiling dirao*ion
4:0~b.v/em 1~ T.,I,
T tO50~C
Fig. 6. Macrostructure of sheet, heated in a temperature gradient. The figures show the temperature gradient and the B s value determined by a 6 cm width single strip tester. 1050°C & boundary between primary and secondary recrystallization.
72
T. Nozawa et al. / Production of single- crystal 3 % S i - Fe sheet
3.2. Production of a single crystal with precise (110) [001] orientation by low temperature gradient annealing Usually, the final annealing of grain-oriented silicon steel production is done in coil form. In this case, it is impossible to originate a high temperature gradient in the whole steel sheet. An investigation was conducted on how induction is enhanced when heating generates relatively low temperature gradients. Samples for laboratory experiments were cut from a strip after decarburization. The sample have a width of 21 cm, a length of 84 cm and a stack of 20 sheets. In order to simulate coil annealing in a practical furnace, these samples were annealed in a curved condition. The radius of curvature was about 100 cm (see fig. 5). A furnace was designed for growing a large single crystal in a low temperature gradient (fig. 5). This furnace has six heating zones and temperatures in each zone are controlled independently. The cross-section of the muffle is 35 cm wide, 15 cm high and 100 cm long. The direction of the temperature gradient is
perpendicular to the rolling direction of the specimen. The temperature gradient was measured by a chromel-alumel thermocouple located in the specimen. After decarburization, the samples were loaded into a temperature gradient in a furnace with a nitrogen atmosphere and were heated at 10°C/h, from 650 to 1150°C. Fig. 6 shows the macrostructure of a sample that was annealed in a temperature gradient of about 3.5°C/cm. It looks like a few seed crystals nucleated out of many grains located in the left side of the specimen, and grew continuously perpendicular to the rolling direction. At about 1050°C, grain growth ends. The numbers in the figure show the local temperature gradient and B 8 value determined by a 6 cm wide single strip tester. Extremely high induction means the presence of a sharp (110) [001] orientation. Fig. 6 shows that the crystals having orientation near (110) [001] grow even in a low temperature gradient in the same manner as in a high temperature gradient (see fig. 3). Fig. 7 shows an example of a macrostructure
(:oil top {origtnml ntmleetion region) mmmldltry--reOrystallized region •prirrmry-- r e o r y s t s | l ized region .~=tl b o t t o m
~R.D.
b)
Fig. 7. Macrostructure of sheet (a) heated in a temperature gradient. The figures show the B s value determined by a 6 cm width single strip tester; (b) the schematic illustration of the coil-form temperature gradient annealing.
73
T. Nozawa et a L / Production of single- crystal 3 56 S i - Fe sheet
180
2.~ O m
E
2.00
1413
ooo 8
o
1.98
12(]
O O
.~-" 1.96
160
8 8 O
0
"~ 1.94
100
60 I
1.92
N2 25% H2
o~
I
N2 100%
40
75% Atmosphere
Fig. 8. Effect of the atmosphere on induction of sheets final-annealed in a temperature gradient. Magnesia B was used.
and B 8 values of silicon steel sheet produced by a coil-form temperature gradient annealing. During heating, the temperature gradient was generated perpendicular to the rolling direction of the strip coil [14]. Secondary recrystallized grains, which nucleated at the coil edge, grow perpendicular to the rolling direction and form slab-like large grains. The numbers in the figure show B 8 values determined by a 6 cm width single strip tester. Fig. 8 shows the effect of atmosphere at final annealing on B 8 values. Final annealing consisted of heating at 1 0 ° C / h from 650 to 1150°C and the temperature gradient was about 7 ° C / c m . 2.Q~ o
m p~2.00 m
o
000
S°o
0
000
0
0 0
t
I
.; 1.98
oo
~1.96 "~ 1.94 1.92 t
I
1300ppm 3g0ppm 100ppm Boron Content Magnesia Magnesia Magnesia A B C
Fig. 9. Effect of boron contained in magnesia on induction of sheets final-annealed in a temperature gradient. 100 vo156 nitrogen atmosphere was used.
0
I
800
I
~
I
900 1000 1100 Sheet T e m p e r a t u r e , ~.,
I
1200
Fig. 10. Variation of the total amount of nitrogen contained in the sheets during final annealing in temperature gradient. N2 100% atmosphere and magnesia B ((~)), 25 vo156N2, 75 vo156 H 2 atmosphere and magnesia A((~)) were used. Arrows on curves show the temperature of boundary regions between primary and secondary recrystallization.
Fig. 9 shows effects of magnesia coating. A principal difference among A, B and C magnesia is the boron content in the MgO. Boron contents of A, B, C magnesia are about 1300, 390 and about 100 ppm, respectively. Figs. 8 and 9 illustrate that the very high induction is promoted by nitrogen atmosphere and lower boron content magnesia. Fig. 10 shows the variation in the total nitrogen content in the steel during final annealing. The amount of nitrogen varies with atmosphere and magnesia. The peak value of the nitrogen content is about 150 p p m when a 100 vol% N 2 atmosphere and 390 p p m boron content magnesia (B) were used, and about 100 p p m when a 25 vol% N 2 and 75 vol% H 2 atmosphere and 1300 p p m boron content magnesia (A) were used. Arrows on curves show the temperature of the boundary region between primary and secondary recrystallization. The boundary temperature of the former is about 50°C higher than that of the latter. The nitrogen content increases with sheet temperature and decreases sharply after secondary recrystallization.
74
T. Nozawa et a L /
Production of single- crystal 3 % S i - Fe sheet
4. Discussion
It is widely recognized that in order for secondary recrystallization to occur, inhibition of normal grain growth is necessary. A variety of particles, but chiefly sulfides or nitrides or a combination of sulfides and nitrides, has been used for this purpose. The first high induction alloy [2] utilized both MnS and AIN to provide inhibition to normal grain growth. This paper shows that it is possible to produce of grain-oriented silicon steel with a very high induction using a temperature gradient for inhibition. According to conventional annealing for secondary recrystallization, an intentional temperature gradient is not imparted to a coil. Fig. 11 shows the macrostructure of a silicon steel sheet. This steel was subjected to heating in a conventional manner. When secondary recrystal-
Fig. 11. Macrostructure of sheets, heated without temperature gradient. Partially secondary-recrystallized (a) and completely secondary-recrystallized (b). Natural size.
lized grains partially formed (a); the heating was interrupted and then the sheet was subjected to cooling. Fig. l l a shows that the nuclei of the secondary recrystallized grains are formed and disperse in the primary recrystallized grains. As will be understood from fig. l l a , during conventional heating, the dispersed nuclei are formed in the primaries and then grow. These nuclei grow until the completion of secondary recrystallization. Fig. l l b shows the macrostructure of a grainoriented silicon steel which was subjected to conventional secondary recrystallization annealing. The grains shown in fig. l l b are virtually equiaxed because they grew from the dispersed nuclei. On the other hand, fig. 12 shows the macrostructure of a silicon steel sheet which was heated in a temperature gradient of 3 ° C / c m during secondary recrystallization annealing. When a temperature gradient is imparted to a steel sheet being secondary-recrystallized, the sheet has two regions, i.e., a high temperature region in which region recrystallization is completed and a low temperature region in which the primaries remain. The borderline between these two regions is along a constant-temperature line which corresponds to a secondary recrystallization temperature. When secondary recrystallized grains are formed under a temperature gradient, they grow in the direction of the high temperature to low temperature region. Thus, the secondary recrystallized grains are very elongated in the direction of the temperature gradient. This appearance of elongated grain is analogous to the macrostructure produced from the growth of seed crystals selected artificially. The origin of prefered grain growth of grains with sharp (110) [100] orientation under a temperature gradient cannot be determined easily. But the grounds for this phenomenon may be supposed as follows based on the basic knowledge of nucleation and growth, as well as the following accepted three empirical rules concerning the secondary recrystallization of grain-oriented silicon steel. a) The nucleation speed of the secondary recrystallized grains is higher when these grains are
7". Nozawa et al. / Production of single-crystal 3 % S i - Fe sheet
75
b) The growth speed of the secondary recrystallized grains is higher when these grains are more perfectly oriented. c) With reference to the nucleation speed and growth speed of the secondary recrystallized grains, the former is relatively high as com-
more perfectly oriented. This is, the secondary recrystallized grains more perfectly oriented nucleate during a shorter period of time at a low temperature as compared with those of the secondary recrystallized grains less perfectly oriented. !!i¸
r~ion
~ r y ~ t ~ primary ~reorystall r.y. - - t i on iza region
Fig. 12. Macrostructure of sheet heated in a temperature gradient. Photograph shows three large grains growing from sheet edge to primarily recrystallized region. Natural size.
76
T. Nozawa et al. / Production of single- crystal 3 % S i - Fe sheet
pared with the latter at a high temperature and the latter is high as compared with the former at a low temperature. The secondary recrystallized grains more perfectly oriented are generated at a relatively low temperature elevation (cf. item a). If a temperature gradient is not generated in the steel, in which the secondary recrystallized grains are generated as stated above, the grains, which nucleate and then start to grow, are dispersed and are in the form of a spot. While these grains further grow until secondary recrystallization is completed, the primarily crystallized grains, which are not yet secondarily recrystallized in the steel subjected to the temperature elevation, undergo a high temperature and thus the nuclei of the secondary recrystallized grains tend to generate in the primarily recrystallized grains. These nuclei are less perfectly oriented (cf. item a). The tendence for nuclei less perfectly oriented to form is more apparent when the temperature elevation rate is higher. A low rate of temperature elevation is, therefore, desirable for suppressing the generation of nuclei having a less perfect orientation. When the rate of temperature elevation is low, the number of nuclei having high orientation is small because of item a and c, above. In order to complete secondary recrystallization, a sample is annealed for a long time, during which the primarily recrystallized grains grow. Because of the growth of the primarily recrystallized grains, the driving force for the growth of the secondary recrystallized grains is decreased. Secondary recrystallization is, therefore, retarded not only due to the slow temperature elevation, but also due to the decrease of the driving force. Annealing without the temperature gradient will eventually result in an incomplete secondary recrystallized structure, in which coarse primarily recrystallized grains remain. In other words, when a temperature gradient is not generated, the primarily recrystallized grains remain even at a high temperature, and it is difficult to avoid the generation of nuclei having a less perfect orientation. On the other hand, when a temperature gradient is generated at the boundary region between the primarily and secondarily recrystallized grain
regions, the steel material is divided at any time during the generation of the temperature gradient into a high temperature region and a low temperature region. In the primarily recrystallized grain region, when the temperature is lower than in the secondarily recrystallized grain region, grain growth is suppressed. Therefore, when the previous low temperature region proceeds to a high temperature region, the growth of the secondarily recrystallized grains is promoted due to the suppression of grain growth. Only secondarily recrystallized grains having a more perfect orientation can be grown under the temperature gradient, because the primarily recrystallized grain region is not subjected to a high temperature. When the temperature gradient is large enough, it is possible to grow the secondary grains textured with sharp (110) [001] orientation. In large scale manufacture, the secondary recrystallizing annealing is usually carried out in coil from and it is difficult to generate a high temperature gradient in the whole coil. Therefore it is necessary to strengthen the inhibition effect using A1N particles
2.02 A&
2.00
¢ 1.98
O0
!
A
O
.1.96
.~ 1.94
~
O O
1.92
1.90 I
100
PPM
I
I
120
I
I
140
[
I
160
Nitrogen Contained in Sheet
Fig. 13. Relationship between the total amount of nitrogen contained in the sheets before secondary recrystallization and induction of sheets, heated in a temperature gradient, 25 vol% N 2, 75 vol% H 2 and magnesia A (©), 100 vol% N 2 and magnesia A (A), 25 vol% N2, 75 vol~ H 2 and magnesia B (O), 100 vol~ N 2 and magnesia B (zx), were used.
T. Nozawa et al. / Production of single-crystal 3 % S i - Fe sheet
ondarily recrystallized grains, with precise (110) [001] orientation, nucleated in the beginning grow toward low temperature region and form slab-like large grains, because the growth of these grains is not prevented by the nucleation of another grain with different orientations deviated from (110)
2.02
A
2.00 m
1.98
77
N
[00]].
Q~ 1.96 •~-" 1.94
Acknowledgements c 1.92 1.91] I
I
960
1000 1050 Secondary Recrystal lizat ion Temperature. ~; Fig. 14. Relationship between the secondary recrystallization temperature and induction of sheets, heated in a temperature gradient 25 vol% N2, 75 vol% H 2 and magnesia A (©), 25 vol% N2, 75 volt~ H 2 and magnesia B (O), 100 vol% N 2 and magnesia A (•), 100 vol% N 2 and magnesia B(zx), were used.
in addition to the inhibition effect by the temperature gradient. In fig. 13, it is revealed that the B8 value strongly depends upon the amount of the total nitrogen contained in the primarily recrystallized region. Fig. 14 shows the relationship between secondary recrystallization temperature and B8. With increasing nitrogen content, B8 becomes higher. The reason for the B8 increase is not clear. The experimental results suggest that many AIN partides form during final annealing in a nitrogen atmosphere and generate a marked effect on the grain growth inhibition. Increasing the secondary recrystallization temperature with increasing amount of A1N, results in strong inhibition of grain growth of another grain in front of a precise (110) [001] secondarily recrystallized grains. Sec-
The author thank the many people that made essential contributions ranging from the construction of the annealing furnace to the measurement of the final magnetic properties.
References [1] N.P. Goss, US Patent No. 1965559 (1934). [2] S. Taguchi, T. Yamamoto and A. Sakakura, IEEE Trans. on. Magn. MAG-10 (1974) 123. [3] I. Goto, I. Matoba, T. Imanaka, T. Gotoh and T. Kan, Proc. Soft. Magn. Mater. Conf. 2 (1975) 262. [4] T. Sato, K. Kuroki and O. Tanaka, IEEE Trans. on Magn. MAG-14 (1978) 350. [5] R.H. Pry and C.P. Bean, J. Appl. Phys. 29 (1958) 532. [6] T. Nozawa, T. Yamamoto, Y. Ohya and Y. Matsuo, IEEE Trans. on Magn. MAG-14 (1978) 252. [71 J.W. Shilling, W.G. Morris, M.L. Osborn and P. Rao, IEEE Trans. on Magn. MAG-14 (1978) 104. [8] T. Nozawa, T. Yamamoto, Y. Ohya and Y. Matsuo, IEEE Trans. on Magn. MAG-15 (1979) 972. [9] K. Kuroki, K. Fukawa and T. Wada, J. Appl. Phys. 52 (1981) 2422. [10] T. Iuchi, S. Yamaguchi, T. Ichiyama, N. Nakamura, T. Ishimoto and K. Kuroki, J. Appl. Phys. 53 (1982) 2410. [11] T. Yamamoto, T. Nozawa, T. Nakayama and Y. Matsuo, J. Magn. Magn. Mater. 31-34 (1983) 993. [12] C.G. Dunn and G.C. Nonken, Metal Progress (December 1953) 71. [13] T. Nozawa, T. Yamamoto and T. Nakayama, US Patent No. 4437910 (1984). [14] T. Ohta and K. Kokai, to be published.