- Email: [email protected]
Non-oriented electrical sheets
38
Journal
of Magnetism
and Magnetic
Materials 41 (1984) 38-46 North-Holland, Amsterdam
INVITED PAPER NON-ORIENTED
ELECTRICAL
SHEETS
Pierre BRISSONNEAU Laboraioire d’Electrotechnique
de Grenoble (L.A. CNRS 355). B.P. 46, F- 38 402 Saint Martin d’H.kes, France
After placing the economic and technological importance of non-oriented magnetic sheets on the same level as that of gram-oriented sheets, the recent stages in the history of non-oriented sheets are recalled. The progress made in the knowledge of the physics of magnetism now allows the functions of the principal properties of non-oriented sheets to be analyzed. Current production of non-oriented sheets is marked by an evolution towards a split of the market between top-grade sheets, which could still be improved significantly, and lower grades, for which the cost of production continues to be practically the only determining factor.
1. Introduction Grain-oriented (hereafter referred to as GO) iron-silicon sheets which first appeared during the 1940s in the US have now outclassed all pre-existing sheets. They quickly imposed themselves in the manufacture of magnetic circuits for power transformers. As a result, all other sheets were qualified as non-oriented (referred to as NO hereafter). Since then GO sheets have continued to attract the interest of a considerable number of researchers and manufacturers and have been regularly and consistently improved, as Littmann so excellently explained in a recent article [l]. They are today the best-known of all magnetic sheets. Given these conditions, is it really necessary to discuss NO sheets? The author believes the answer to this question is “yes”, if only to emphasize that they do not deserve to become the discards of the GO sheet evolution and that they alone are likely to satisfy certain industrial needs. First, it is important to remember that in all industralized nations NO sheets have retained as much economic importance as GO sheets. Fig. 1 gives an evalution of the French magnetic sheet market during recent years. It can be seen that the demand for GO sheets by electrical constructors has been around 50000 ton per year while the demand for NO sheets has been around 150000 ton per year. Among NO sheets used by constructors in recent years, there has been a growing percentage of sheets without silicon, which makes 0304-8853/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
the statistics more difficult to establish. The result is a large uncertainty about the numbers concerning NO sheets. The figures shown here are the most probable. The ratio, shown here as being 1 to 3, between the uses of GO and NO sheets should not differ significantly from one country to another since these two types of sheets serve different purposes; there is only very rarely any competition between the two. In fact, soft materials are needed in electrotechnical equipment to canalize the flow of a variable-induction flux in order to produce an induced electromotive force in a coil. The best
I
1 91
32
Fig. 1. The French years.
B.V.
market
of magnetic
sheets
during
recent
P. Brissonneau / Non -oriented electrial sheets
material is that which allows the achievement of the maximal flow at the least cost in energy, or in other words with the lowest losses in the material itself and of course in the exciting coil. Under these conditions the intrinsic qualities of a GO sheet can only be used for magnetic circuits that satisfy two requirements simultaneously: first, the circuit must not contain any air-gap because the Joule losses in the induction coil increase very quickly as the square of the length of the air-gap. Second, the magnetization must, as far as possible, keep a fixed direction which corresponds to that of
(a)
(b) Fig. 2. (a) A two-poles rotating machine. At P, and P2 the induction orientations are radial and tangential, respectively. A quarter-period later this situation will be reversed. (b) Loci of fundamental component of flux density in a rotoric segment of NO material (after ref. [3]).
39
the rolling direction. These two requirements can usually be fulfilled only by transformer circuits which, therefore, currently represent the essential outlet for GO sheets. However, there are a number of other magnetic circuits for the construction of which the use of NO sheets has proved preferable. Such is the case for most rotating machines, which have attained great economic importance as they allow electrical energy to be transformed into mechanical energy or vice versa. Fig. 2 is a diagram of the magnetic circuit of a two-poles rotating machine. The distribution of the magnetic field in the stator turns in synchronism with the rotor and, as at a point like Pi in the diagram, the magnetization rotates in the plane of the sheet. The material at point Pi is practically dominated by a system of rotating magnetization. This rotation is obtained from a surface gliding current density. In practice we obtain this current density from coils conveniently supplied and arranged in slots (fig. 2b). From a magnetic point of view, this arrangement creates a very complex geometry. As is always the case in magnetism, the effects of form remain determinant in fixing, at each moment, magnetic equilibrium. Local magnetization, then - for the most part radial in the teeth and tangential in the yoke - constantly experiences variations of its modulus and direction in a sizable fraction of the statoric volume. Recent carefully conducted experiments on GO and NO sheets models performed by Moses and Radley [2,3] show veritable complex local behavior with minor differences between the two materials as is in keeping with industrial practices involving these materials. Only for very large machines are stators constructed using both NO and GO sheets, the choice seeming to reside in the manufacturer’s own preference. However, all average and fairly large-size rotating machines are constructed with NO sheets. In conclusion, in order to satisfy the needs of electrical manufacturers the magnetic sheet market is divided into two large economic sectors which are comparable and practically independent. The highest-performance grades in the NO sheet sector still pose numerous problems and deserve more constant attention by the scientific community than has heretofore been accorded them.
40
P. Brissonneau / Non -oriented electrial sheets
2. Main stages in the history of NO sheets
3.1. Intriiwic magnetic properties of silicon-iron
ai-
loys
The original studies on Si-Fe alloys were generally attributed to Hadfield [4] about 100 years ago, but the industrial production of sheets started only at the beginning of the present century. Very soon they were used in the electrical building industry insted of common steel sheets. Looked at from today’s standpoint, their performance was relatively poor. Year after year progress was made mainly due to the use of new technologies in iron metallurgy. Decarburization of the sheet at its final thickness is performed systematically in order to obtain a stable product. This process, which consists in a thermal annealing around 800°C in a controlled atmosphere composed of H,, H,O and N,, was the basis of important advances made in the 1940s and 50s. The introduction of cold-rolling and continuous annealing treatments have also represented important stages in the history of NO sheets in that these techniques have allowed improvements in the surface of the sheet, the obtaining of a more consistent thickness and lower fabrication costs. Because most impurities have a harmful effect on the magnetic properties of the product, the objective of every manufacturer of NO sheets is to control the nature, the composition and the physical form of impurities at all stages of production, though especial attention must be given to the first stages. Great effort is constantly made to develop materials which are as pure as possible at a reasonable cost. To do this, modem steel-making plants now use such up-to-date processes of elaboration as oxygen processing, vacuum degassing and continuous casting.
All steel used for magnetic sheets is essentially composed of iron and generally small amounts of silicon (C 6 3.5 wtW), aluminum (C < 0.5%) and manganese (C < 0.2%). In such small concentrations silicon and aluminum atoms have similar behaviors and the result is a disordered solid solution with density (cf. ASTM standard A 34): d (g/cm3) = 7.865 - 0.065[% Si + 1.7% Al]. A model comprising magnetic moments of 2.2~~ localized only at the iron atoms shows the intrinsic magnetic properties of alloys quite effectively. These are shown according to the concentration of silicon in fig. 3 (taken from ref. [8]). In decreasing the saturation magnetization of the alloy, the addition of silicon tends to decrease the working induction which can be used in electrical machines and, as a result, also decreases the machines’ volumic power. This is therefore an undesirable effect and the practice of adding silicon is maintained in industry only because the advantages of such a practice greatly outweigh the disadvantages, at least in the
120
-
TOO-
660-
0 5 -
ANISOTROPY
-50
3. Principles of operation The ideal magnetic sheet for electrical applications is one which can be magnetized at the highest possible level with both a low exciting field and low total losses. In practice, however, the induction level is always limited by the losses inevitable in variable magnetization.
- 40
MAGNETOSTRICTION
00
I PER
2I_--l--~_I-3
CENT
SILICON
4 (BY
5
6
To
WEIGHT)
Fig. 3. Electrical resistivity and intrinsic magnetic properties of SiFe alloys (after ref. [8]).
P. Brissonneau / Non -oriented electrial sheets
case of top-grade NO sheets. Four such advantages can be found: (1) Increase in the alloys’ resistivity, most important as is illustrated in fig. 3, diminish the damping of the Bloch walls by eddy currents. The losses associated with the process of magnetization are therefore reduced as shown in fig. 4. (2) In facilitating the attainment of the saturated state, the decrease in magnetocrystalline anisotropy (the difference is around 25% between pure iron and 3% silicon-iron), partially masks the decrease in saturation magnetization. (3) The addition of silicon increases temperature stability in the BCC phase. This allows metallurgists more freedom to perform thermal treatments and so facilitates the processes of recrystallization. (4) Finally, the addition of silicon improves the poor mechanical properties of pure iron. Its limit of elasticity, in particular, is almost doubled with 3.5% (Si and Al). However, this still results in a decrease of cold ductility, which corresponds to the concentration increase of silicon in the alloy, and if the concentration is higher than 3.5% the alloy cannot be industrially cold-rolled in acceptable conditions. It is _J.kg-’ 0.00
0.04
0.02
0 1.5
2
2.5
3
3.5
%(Si+Al)
Fig. 4. Total and hysteresis specific losses per cycle in NO sheets versus Si + Al composition (after ref. [14]).
41
for this reason that the concentration of silicon in industrial alloys rarely exceeds 3.2%. Other elements in the Mendele”iev classification such as Al and Ge, when added as a substitute for iron, can also, like silicon, be beneficial in low atomic concentrations [9,10]. If neither of these elements is employed in fabrication, it is for both economic and technical reasons. Because sources of silicon are very the silicon-iron produced in abundant, siderurgy is relatively inexpensive. Other elements used in addition, such as aluminum, are not only more expensive, but they present a higher chemical activity which makes the control of impurities during thermal treatments at high temperatures more difficult. 3.2. The role of impurities Impurities are present, of course, in all industrial products. It is by more or less strict control of impurities and by the use of more or less silicon that manufacturers obtain the wide variety of sheet grades found on the market. 3.2.1. Their direct role Each impurity segregated in the crystal induces serious perturbation in the periodic electronic structure of the lattice as a whole. Even if the size of impurity is only a few hundred A, the perturbed volume is much more extensive and the impurity constitutes an obstacle to the free movement of the Bloch walls. It therefore increases the coercive field and the losses. Numerous studies have been devoted during the last 50 years to this effect of impurities on losses without exhausting the subject. Apart from considerations of global concentrations of harmful elements such as C, N, S and 0, therefore, one must take into account the size of impurities, their form, their position in the lattice, the obstacles they present and also their indirect role. 3.2.2. The indirect role of impurities Impurities play a leading role during the recrystallization processes after rolling even in NO sheets. Grain size and sheet texture generally depend on the production process, the latter being optimized according to the impurities present. On the whole one never knows how much of the
42
P. Brissonneau / Non -oriented electrial sheets
:
3.0
t
(a) .
.
Ix
3
. .
2.4-
i Y
. .
l
*
. * .
l
l
’
l
’ .
l l
. :
: . .
. : .
increase in losses should be attributed to the indirect or direct effects of impurites. As the analysis and control of impurities are improved it becomes apparent that they play a decisive role in the attainment of a high-performance product. Fig. 5 (after ref. [ll]) is a good illustration of recent results in this domain. It should be noted that the dosage of elements such as N, 0 and S is achieved within a margin of 1 p.p.m., which is far from simple and that in these doses of around 10 p.p.m. the increase in losses may attain several percentage points. 50 years earlier Yensen was obtaining an effect on losses ten times smaller [5]. From this it becomes clear that results such as those reported by Shimanaka [ll] are peculiar to very particular methods in elaboration and treatment cycles.
. .
l
2.2 I I 123456
I
III
s
(xlo-J%
)
l
l
. ‘.
J l,o )
2.4 ..
l
l
:
l
.
** ’ .
.
l
3.3. Magnetization processes
.
’
10
40 do
( ,,:O)
(Cl
. . l
“8
3
. ..
.
l
l
I
.*‘::** .
2s2t-l---A 20
30
40
60
N (mm) Fig. 5. Effect of impurities on the total losses of 3% Si-Fe sheets (NO): (a) S, (b) 0, (c) N (after ref. [ll]).
3.3. I. General remarks Our general knowledge of magnetism allows at least a rough interpretation of magnetization processes. We know that ferromagnetic materials are divided spontaneously into domains, each being magnetized at saturation in an easy direction. In the case of silicon-iron steels where magnetocrystalline anisotropy is relatively large, the easy axes are well defined along the quaternary axes of the crystal. Unfortunately, the junction between neighboring grains produces extremely complex structures of closure domains which make the results of all direct observation difficult to interpret. Starting from a “zero” state of magnetization at macroscopic scale realized by a statistically isotropic distribution of domains, the application of a magnetic exciting field first results in displacement of the Bloch walls which separate neighboring domains. Some of the domains whose positions in the structure are more advantageous than others will grow to the detriment of those domains having less favorable orientations. This phenomenon may, if the domains are numerous enough, occur even in very modest exciting fields. After induction has reached higher values than 1.2 T, however, wall displacement requires more and more energy.
P. Brissonneau / Non
3.3.2. Approach to saturation
Electrical equipment manufacturers use NO sheets having the highest possible induction level, their choice being determined generally by the growth of losses. The peak value of the working induction largely depends on the type of machine to be built. For very large rotating machines, whose design and construction are very sophisticated, suitable induction may in certain places, such as in the stator teeth bases, approach or even exceed 2.1 T. Although these conditions are rather exceptional, one can still say most levels remain above 1.5 T or in other words practically approaching saturation. All local inhomogeneousness in magnetization therefore constitutes the source of an internal field which is extremely efficient to cancel the external applied field. It is important for the manufacturer to introduce into the sheet as few of these obstacles as possible. As “obstacles” one must count not only the previously discussed impurities but also any superficial defects localized on the sheet surface and also at the grain boundaries. This results in two consequences: First, for each quality of NO sheet there is an optimal grain size. As grain size is increased boundary area is decreased, improving performance up to a certain point at which the decrease in the number of Bloch walls results in the presence of supplementary eddy currents. Fig.
Mean
grain
diameter
-orientedelectrial
sheets
43
44
P. Brissonneau / Non -oriented electrial sheets 67
r
I
CllO,[DOl]ORIENTED
3% We
INDUCTION, kG Fig. 7. Rotational loss at 50 Hz versus induction of 3% S-Fe sheets (after ref. [12]).
for three kinds
exist on this subject. Fig. 7 gives some results published by Boon and Thompson [12] on this subject. 4. The present state of production 4.1. The main varieties of grades Producers offer a large range of sheet qualities which differ in composition, in the way in which they are produced, and in thickness. Composition and methods of fabrication constitute veritable industrial secrets which are never divulged. The criteria of classification most used and reversed for the establishment of grades is a level of losses in the sinusoidal steady state at 50 or 60 Hz for a maximal induction of 1 or 1.5 T. The top-grade NO sheets now available on the European market are obtained from alloys composed of 3% silicon with 0.4 to 0.8% aluminum and 0.1% manganese. These sheets have total losses guaranteed at around 0.95 to 1.0 W/kg to 50 Hz and 1 T for a thickness of 0.35 mm and 1.10 to 1.15 W/kg for sheets 0.50 mm thick. For a maximal induction of 1.5 T the total losses are around 2.4 and 2.7 W/kg, respectively. In this type of material the control of impurities at all stages of fabrication has been
carried out with great care and the resulting price of the material is quite high. These sheets are reserved, then, for the construction of high-performance machines (fig. 8). On the other side of the scale, low-carbon steel sheets without Si and containing some Mn and P are most often used in industry at a thickness of 0.65 or 1 mm for the fabrication of small electrical appliances functioning in an intermittent way. The main concern for the producers of these sheets is to obtain an acceptable magnetic permeability while keeping costs as low as possible. This type of sheet is generally delivered in the cold-rolled state which facilitates rapid punching of the sheet using automatic machines. Sheet punching is followed by an annealing treatment the goal of which is to eliminate residual stresses and the main part of aging. In the case of “blue” sheets annealing is completed by superficial oxidation in order to achieve isolation. Specific losses at a maximal induction of 1.5 T at 50 Hz are then between 8 and 12 W/kg. Between these two extremes there are numerous intermediate sheet grades and any classification of these is necessarily arbitrary. Table 1 gives the characteristics of a few commonly used grades. Finally, because this fact is little-know, the author would like to point out that a small number of thin NO sheets having thickness of about 0.15 mm and 0.20 mm and specially adapted for use at 1.51
0.41 50
I
I
I
I
I
I
Al.75
I 100
I 250
I 500
I 1000
I 2500
I 5000
I la’
Field
atrongth
( Am-‘)
Fig. 8. Magnetization curve of a NO 3% Si-Fe sheet of the top-grade quality. Thickness 0.50 mm. Direct current measurements, half longitudinal and half transverse to the rolling direction.
I? Brissonneau / Non -oriented electrial sheets
Table 1 Specifications Grade
of four standard Thickness
NO sheets commonly
used by electrical
equipment
45
manufacturers
Specific loss (W/kg)
Induction
W:”
WI?
2.5
5.0
10
10
1.10 1.25 2.60 3.60
2.10 3.10 6.00 8.10
1.48 1.48 1.54 1.55
1.59 1.59 1.64 1.66
1.72 1.72 1.78 1.80
1.93 1.93 2.00 2.02
(T) for field strength
(kA/m)
(mm) 1Wl 1 w25 2W6 3W6
0.35 0.50 0.65 0.65
medium frequencies of around a few hundred Hz are currently being produced in France. For B = 1 T. the specific losses at 400 Hz for a 0.15 mm thick sheet are about 15 W/kg. These thin sheets are intended for magnetic circuits fed by electronic converters such as those used in automation. 4.2. Some non-magnetic characteristics such as surface quality, uniformity of thickness and punchability are ,of considerable importance for the industrial user of sheets. If the fabrication processes used result in the formation of superficial oxide particles, the finished sheet may be very abrasive and so-cause cutting tools to wear very quickly. To improve the longevity of cutting tools, sheet manufacturers have developed special semiorganic coatings-which are very adhesive. A coating of this type, which is generally between 2 and 5 pm thick, assures enough isolation between sheets for all current uses at temperatures of up to 180 or 200°C. 5. Future developments and conclusion Are there existing materials which could conceivably replace NO sheets within the next few years? We know that cube-textured 3% iron-silicon sheet have been used with great success in rotating machines [l]. Unfortunately, because of the difficulties encountered in obtaining a good texture (100) [OOl], and because of the heavy production investments required for what is a relatively limited market, the production of these sheets in the US has been discontinued. Neither do amorphous ribbons seem likely substitutes for NO sheets in their principal outlets at 50 Hz. There are at least three reasons for this: First, in the case of large rotating
machines which are the most extensively studied and for which constructors are willing to test top-performance materials, the average induction level in the gap must never usually exceed 1 T, as induction is limited by the saturation magnetization in the teeth of the stator. The use of amorphous ribbons would only reduce these values by 25 or 30% which would be a virtual catastrophe for the volume power of the machines. The excitating power of the magnetic path would not be significantly reduced since it also depends on the presence of the gap. Finally, the entire magnetic system had to work hard and the author doubts that such a complex mechanical structure, if constructed of elements only 30 to 40 pm thick, would be able to withstand such stress. Apart from very small high-performance motors [13] which are more and more often fed at medium frequencies and where amorphous ribbons could very likely compete with thin iron-silicon sheets, the use of amorphous ribbons in rotating machines will remain quite limited unless entirely new machine structures are developed which would favor their use. In concluding an article of this nature which attempts to unite the principal themes concerning NO sheets, the traditional question is: “What future is in store for this material?” Before answering this question it must be admitted that the author has always found it difficult to predict the future and still does. Second, he firmly reserves the right to err in his predictions. And finally, at the risk of appearing unimaginative, he will say that he sees the future of this material as a continuation of the present for the following reasons: In spite of the economic difficulties which a number of countries are experiencing, the use of electrical energy continues to develop, pulling in
46
P. Brissonneau
/ Non
its wake the development of GO and NO sheets. Because the cost of energy has become greater and greater in the last few years, top-quality NO sheets respond to industrial needs especially in the construction of high-performance rotating machines. Undisputable progress has been achieved in the last few years but there is still progress to made. Losses attributed to hysteresis could be reduced and the approach to saturation could be facilitated by better monitoring of impurities and by the establishment of a texture more appropriate to industrial needs. Constant advances in metallurgy and in the analysis of impurities should provide improvement in this area, assuming that the scientific community accords to this problem the interest that it deserves. In this sense the author thinks that the evolution of NO sheets will shortly begin to follow the course of the evolution of GO sheets, the history of which has recently been outlined in an excellent article by Littmann [l]. At the other end of the grade scale, mild steel sheets without - or with very little - silicon can also benefit from the perfections being achieved in the steel-making industry. The electrical resistivity of this type of sheet is still, however, too weak to allow it to attain the necessary grade level which would make it useful in the construction of mid-size
-orientedelectrial sheers
rotating machines. We are going towards a market division, With improvements in the qualities and well-defined objectives the economic weight of NO sheets would have to be asserted again in the following years. References [l] M.F. Littmann, J. Magn. Magn. Mat. 26 (1982) 1. [2] A.J. Moses and G.S. Radley, J. Magn. Magn. Mat. 19 (1980) 60. [3] G.S. Radley and A.J. Moses, IEEE Trans. Magn. MAG-17 (1981) 1311. [4] Barrett, Brown and Hadfield, Sci. Trans. Roy. Dublin Sot. 7 (1900) 67. [5] T.D. Yemen, Trans. AIEE 43 (1924) 145. [6] L. NCel, Ann. Univ. Grenoble 22 (1946) 299. [7] C. Kittel, Rev. Mod. Phys. 21 (1949) 541. [8] M.F. Littmann, IEEE Trans. Magn. MAG-7 (1971) 48. [9] M. Sugihara, J. Phys. Sot. Japan 15 (1960) 8. [lo] T.F. Foley et al., J. Iron Steel Inst. (Feb. 1970) 147. (111 H. Shimanaka, Y. Ito, T. Irie, K. Matsumura, H. Nakamura and Y. Shono, Trans. Met. Sot. AIME (1980) 193. [12] C.R. Boon and J.E. Thompson, Proc. IEE 112 (1965) 2147. [13] W.R. Mischler, G.M. Rosenberry, P.G. Frischmann and R.E. Tompkins, IEEE Trans. PAS (1981) 2907. [14] G.Y. Chin and J.H. Wernick, Ferromagnetic Materials, vol. 2, ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1980) p. 84.
Journal
of Magnetism
and Magnetic
Materials 41 (1984) 38-46 North-Holland, Amsterdam
INVITED PAPER NON-ORIENTED
ELECTRICAL
SHEETS
Pierre BRISSONNEAU Laboraioire d’Electrotechnique
de Grenoble (L.A. CNRS 355). B.P. 46, F- 38 402 Saint Martin d’H.kes, France
After placing the economic and technological importance of non-oriented magnetic sheets on the same level as that of gram-oriented sheets, the recent stages in the history of non-oriented sheets are recalled. The progress made in the knowledge of the physics of magnetism now allows the functions of the principal properties of non-oriented sheets to be analyzed. Current production of non-oriented sheets is marked by an evolution towards a split of the market between top-grade sheets, which could still be improved significantly, and lower grades, for which the cost of production continues to be practically the only determining factor.
1. Introduction Grain-oriented (hereafter referred to as GO) iron-silicon sheets which first appeared during the 1940s in the US have now outclassed all pre-existing sheets. They quickly imposed themselves in the manufacture of magnetic circuits for power transformers. As a result, all other sheets were qualified as non-oriented (referred to as NO hereafter). Since then GO sheets have continued to attract the interest of a considerable number of researchers and manufacturers and have been regularly and consistently improved, as Littmann so excellently explained in a recent article [l]. They are today the best-known of all magnetic sheets. Given these conditions, is it really necessary to discuss NO sheets? The author believes the answer to this question is “yes”, if only to emphasize that they do not deserve to become the discards of the GO sheet evolution and that they alone are likely to satisfy certain industrial needs. First, it is important to remember that in all industralized nations NO sheets have retained as much economic importance as GO sheets. Fig. 1 gives an evalution of the French magnetic sheet market during recent years. It can be seen that the demand for GO sheets by electrical constructors has been around 50000 ton per year while the demand for NO sheets has been around 150000 ton per year. Among NO sheets used by constructors in recent years, there has been a growing percentage of sheets without silicon, which makes 0304-8853/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
the statistics more difficult to establish. The result is a large uncertainty about the numbers concerning NO sheets. The figures shown here are the most probable. The ratio, shown here as being 1 to 3, between the uses of GO and NO sheets should not differ significantly from one country to another since these two types of sheets serve different purposes; there is only very rarely any competition between the two. In fact, soft materials are needed in electrotechnical equipment to canalize the flow of a variable-induction flux in order to produce an induced electromotive force in a coil. The best
I
1 91
32
Fig. 1. The French years.
B.V.
market
of magnetic
sheets
during
recent
P. Brissonneau / Non -oriented electrial sheets
material is that which allows the achievement of the maximal flow at the least cost in energy, or in other words with the lowest losses in the material itself and of course in the exciting coil. Under these conditions the intrinsic qualities of a GO sheet can only be used for magnetic circuits that satisfy two requirements simultaneously: first, the circuit must not contain any air-gap because the Joule losses in the induction coil increase very quickly as the square of the length of the air-gap. Second, the magnetization must, as far as possible, keep a fixed direction which corresponds to that of
(a)
(b) Fig. 2. (a) A two-poles rotating machine. At P, and P2 the induction orientations are radial and tangential, respectively. A quarter-period later this situation will be reversed. (b) Loci of fundamental component of flux density in a rotoric segment of NO material (after ref. [3]).
39
the rolling direction. These two requirements can usually be fulfilled only by transformer circuits which, therefore, currently represent the essential outlet for GO sheets. However, there are a number of other magnetic circuits for the construction of which the use of NO sheets has proved preferable. Such is the case for most rotating machines, which have attained great economic importance as they allow electrical energy to be transformed into mechanical energy or vice versa. Fig. 2 is a diagram of the magnetic circuit of a two-poles rotating machine. The distribution of the magnetic field in the stator turns in synchronism with the rotor and, as at a point like Pi in the diagram, the magnetization rotates in the plane of the sheet. The material at point Pi is practically dominated by a system of rotating magnetization. This rotation is obtained from a surface gliding current density. In practice we obtain this current density from coils conveniently supplied and arranged in slots (fig. 2b). From a magnetic point of view, this arrangement creates a very complex geometry. As is always the case in magnetism, the effects of form remain determinant in fixing, at each moment, magnetic equilibrium. Local magnetization, then - for the most part radial in the teeth and tangential in the yoke - constantly experiences variations of its modulus and direction in a sizable fraction of the statoric volume. Recent carefully conducted experiments on GO and NO sheets models performed by Moses and Radley [2,3] show veritable complex local behavior with minor differences between the two materials as is in keeping with industrial practices involving these materials. Only for very large machines are stators constructed using both NO and GO sheets, the choice seeming to reside in the manufacturer’s own preference. However, all average and fairly large-size rotating machines are constructed with NO sheets. In conclusion, in order to satisfy the needs of electrical manufacturers the magnetic sheet market is divided into two large economic sectors which are comparable and practically independent. The highest-performance grades in the NO sheet sector still pose numerous problems and deserve more constant attention by the scientific community than has heretofore been accorded them.
40
P. Brissonneau / Non -oriented electrial sheets
2. Main stages in the history of NO sheets
3.1. Intriiwic magnetic properties of silicon-iron
ai-
loys
The original studies on Si-Fe alloys were generally attributed to Hadfield [4] about 100 years ago, but the industrial production of sheets started only at the beginning of the present century. Very soon they were used in the electrical building industry insted of common steel sheets. Looked at from today’s standpoint, their performance was relatively poor. Year after year progress was made mainly due to the use of new technologies in iron metallurgy. Decarburization of the sheet at its final thickness is performed systematically in order to obtain a stable product. This process, which consists in a thermal annealing around 800°C in a controlled atmosphere composed of H,, H,O and N,, was the basis of important advances made in the 1940s and 50s. The introduction of cold-rolling and continuous annealing treatments have also represented important stages in the history of NO sheets in that these techniques have allowed improvements in the surface of the sheet, the obtaining of a more consistent thickness and lower fabrication costs. Because most impurities have a harmful effect on the magnetic properties of the product, the objective of every manufacturer of NO sheets is to control the nature, the composition and the physical form of impurities at all stages of production, though especial attention must be given to the first stages. Great effort is constantly made to develop materials which are as pure as possible at a reasonable cost. To do this, modem steel-making plants now use such up-to-date processes of elaboration as oxygen processing, vacuum degassing and continuous casting.
All steel used for magnetic sheets is essentially composed of iron and generally small amounts of silicon (C 6 3.5 wtW), aluminum (C < 0.5%) and manganese (C < 0.2%). In such small concentrations silicon and aluminum atoms have similar behaviors and the result is a disordered solid solution with density (cf. ASTM standard A 34): d (g/cm3) = 7.865 - 0.065[% Si + 1.7% Al]. A model comprising magnetic moments of 2.2~~ localized only at the iron atoms shows the intrinsic magnetic properties of alloys quite effectively. These are shown according to the concentration of silicon in fig. 3 (taken from ref. [8]). In decreasing the saturation magnetization of the alloy, the addition of silicon tends to decrease the working induction which can be used in electrical machines and, as a result, also decreases the machines’ volumic power. This is therefore an undesirable effect and the practice of adding silicon is maintained in industry only because the advantages of such a practice greatly outweigh the disadvantages, at least in the
120
-
TOO-
660-
0 5 -
ANISOTROPY
-50
3. Principles of operation The ideal magnetic sheet for electrical applications is one which can be magnetized at the highest possible level with both a low exciting field and low total losses. In practice, however, the induction level is always limited by the losses inevitable in variable magnetization.
- 40
MAGNETOSTRICTION
00
I PER
2I_--l--~_I-3
CENT
SILICON
4 (BY
5
6
To
WEIGHT)
Fig. 3. Electrical resistivity and intrinsic magnetic properties of SiFe alloys (after ref. [8]).
P. Brissonneau / Non -oriented electrial sheets
case of top-grade NO sheets. Four such advantages can be found: (1) Increase in the alloys’ resistivity, most important as is illustrated in fig. 3, diminish the damping of the Bloch walls by eddy currents. The losses associated with the process of magnetization are therefore reduced as shown in fig. 4. (2) In facilitating the attainment of the saturated state, the decrease in magnetocrystalline anisotropy (the difference is around 25% between pure iron and 3% silicon-iron), partially masks the decrease in saturation magnetization. (3) The addition of silicon increases temperature stability in the BCC phase. This allows metallurgists more freedom to perform thermal treatments and so facilitates the processes of recrystallization. (4) Finally, the addition of silicon improves the poor mechanical properties of pure iron. Its limit of elasticity, in particular, is almost doubled with 3.5% (Si and Al). However, this still results in a decrease of cold ductility, which corresponds to the concentration increase of silicon in the alloy, and if the concentration is higher than 3.5% the alloy cannot be industrially cold-rolled in acceptable conditions. It is _J.kg-’ 0.00
0.04
0.02
0 1.5
2
2.5
3
3.5
%(Si+Al)
Fig. 4. Total and hysteresis specific losses per cycle in NO sheets versus Si + Al composition (after ref. [14]).
41
for this reason that the concentration of silicon in industrial alloys rarely exceeds 3.2%. Other elements in the Mendele”iev classification such as Al and Ge, when added as a substitute for iron, can also, like silicon, be beneficial in low atomic concentrations [9,10]. If neither of these elements is employed in fabrication, it is for both economic and technical reasons. Because sources of silicon are very the silicon-iron produced in abundant, siderurgy is relatively inexpensive. Other elements used in addition, such as aluminum, are not only more expensive, but they present a higher chemical activity which makes the control of impurities during thermal treatments at high temperatures more difficult. 3.2. The role of impurities Impurities are present, of course, in all industrial products. It is by more or less strict control of impurities and by the use of more or less silicon that manufacturers obtain the wide variety of sheet grades found on the market. 3.2.1. Their direct role Each impurity segregated in the crystal induces serious perturbation in the periodic electronic structure of the lattice as a whole. Even if the size of impurity is only a few hundred A, the perturbed volume is much more extensive and the impurity constitutes an obstacle to the free movement of the Bloch walls. It therefore increases the coercive field and the losses. Numerous studies have been devoted during the last 50 years to this effect of impurities on losses without exhausting the subject. Apart from considerations of global concentrations of harmful elements such as C, N, S and 0, therefore, one must take into account the size of impurities, their form, their position in the lattice, the obstacles they present and also their indirect role. 3.2.2. The indirect role of impurities Impurities play a leading role during the recrystallization processes after rolling even in NO sheets. Grain size and sheet texture generally depend on the production process, the latter being optimized according to the impurities present. On the whole one never knows how much of the
42
P. Brissonneau / Non -oriented electrial sheets
:
3.0
t
(a) .
.
Ix
3
. .
2.4-
i Y
. .
l
*
. * .
l
l
’
l
’ .
l l
. :
: . .
. : .
increase in losses should be attributed to the indirect or direct effects of impurites. As the analysis and control of impurities are improved it becomes apparent that they play a decisive role in the attainment of a high-performance product. Fig. 5 (after ref. [ll]) is a good illustration of recent results in this domain. It should be noted that the dosage of elements such as N, 0 and S is achieved within a margin of 1 p.p.m., which is far from simple and that in these doses of around 10 p.p.m. the increase in losses may attain several percentage points. 50 years earlier Yensen was obtaining an effect on losses ten times smaller [5]. From this it becomes clear that results such as those reported by Shimanaka [ll] are peculiar to very particular methods in elaboration and treatment cycles.
. .
l
2.2 I I 123456
I
III
s
(xlo-J%
)
l
l
. ‘.
J l,o )
2.4 ..
l
l
:
l
.
** ’ .
.
l
3.3. Magnetization processes
.
’
10
40 do
( ,,:O)
(Cl
. . l
“8
3
. ..
.
l
l
I
.*‘::** .
2s2t-l---A 20
30
40
60
N (mm) Fig. 5. Effect of impurities on the total losses of 3% Si-Fe sheets (NO): (a) S, (b) 0, (c) N (after ref. [ll]).
3.3. I. General remarks Our general knowledge of magnetism allows at least a rough interpretation of magnetization processes. We know that ferromagnetic materials are divided spontaneously into domains, each being magnetized at saturation in an easy direction. In the case of silicon-iron steels where magnetocrystalline anisotropy is relatively large, the easy axes are well defined along the quaternary axes of the crystal. Unfortunately, the junction between neighboring grains produces extremely complex structures of closure domains which make the results of all direct observation difficult to interpret. Starting from a “zero” state of magnetization at macroscopic scale realized by a statistically isotropic distribution of domains, the application of a magnetic exciting field first results in displacement of the Bloch walls which separate neighboring domains. Some of the domains whose positions in the structure are more advantageous than others will grow to the detriment of those domains having less favorable orientations. This phenomenon may, if the domains are numerous enough, occur even in very modest exciting fields. After induction has reached higher values than 1.2 T, however, wall displacement requires more and more energy.
P. Brissonneau / Non
3.3.2. Approach to saturation
Electrical equipment manufacturers use NO sheets having the highest possible induction level, their choice being determined generally by the growth of losses. The peak value of the working induction largely depends on the type of machine to be built. For very large rotating machines, whose design and construction are very sophisticated, suitable induction may in certain places, such as in the stator teeth bases, approach or even exceed 2.1 T. Although these conditions are rather exceptional, one can still say most levels remain above 1.5 T or in other words practically approaching saturation. All local inhomogeneousness in magnetization therefore constitutes the source of an internal field which is extremely efficient to cancel the external applied field. It is important for the manufacturer to introduce into the sheet as few of these obstacles as possible. As “obstacles” one must count not only the previously discussed impurities but also any superficial defects localized on the sheet surface and also at the grain boundaries. This results in two consequences: First, for each quality of NO sheet there is an optimal grain size. As grain size is increased boundary area is decreased, improving performance up to a certain point at which the decrease in the number of Bloch walls results in the presence of supplementary eddy currents. Fig.
Mean
grain
diameter
-orientedelectrial
sheets
43
44
P. Brissonneau / Non -oriented electrial sheets 67
r
I
CllO,[DOl]ORIENTED
3% We
INDUCTION, kG Fig. 7. Rotational loss at 50 Hz versus induction of 3% S-Fe sheets (after ref. [12]).
for three kinds
exist on this subject. Fig. 7 gives some results published by Boon and Thompson [12] on this subject. 4. The present state of production 4.1. The main varieties of grades Producers offer a large range of sheet qualities which differ in composition, in the way in which they are produced, and in thickness. Composition and methods of fabrication constitute veritable industrial secrets which are never divulged. The criteria of classification most used and reversed for the establishment of grades is a level of losses in the sinusoidal steady state at 50 or 60 Hz for a maximal induction of 1 or 1.5 T. The top-grade NO sheets now available on the European market are obtained from alloys composed of 3% silicon with 0.4 to 0.8% aluminum and 0.1% manganese. These sheets have total losses guaranteed at around 0.95 to 1.0 W/kg to 50 Hz and 1 T for a thickness of 0.35 mm and 1.10 to 1.15 W/kg for sheets 0.50 mm thick. For a maximal induction of 1.5 T the total losses are around 2.4 and 2.7 W/kg, respectively. In this type of material the control of impurities at all stages of fabrication has been
carried out with great care and the resulting price of the material is quite high. These sheets are reserved, then, for the construction of high-performance machines (fig. 8). On the other side of the scale, low-carbon steel sheets without Si and containing some Mn and P are most often used in industry at a thickness of 0.65 or 1 mm for the fabrication of small electrical appliances functioning in an intermittent way. The main concern for the producers of these sheets is to obtain an acceptable magnetic permeability while keeping costs as low as possible. This type of sheet is generally delivered in the cold-rolled state which facilitates rapid punching of the sheet using automatic machines. Sheet punching is followed by an annealing treatment the goal of which is to eliminate residual stresses and the main part of aging. In the case of “blue” sheets annealing is completed by superficial oxidation in order to achieve isolation. Specific losses at a maximal induction of 1.5 T at 50 Hz are then between 8 and 12 W/kg. Between these two extremes there are numerous intermediate sheet grades and any classification of these is necessarily arbitrary. Table 1 gives the characteristics of a few commonly used grades. Finally, because this fact is little-know, the author would like to point out that a small number of thin NO sheets having thickness of about 0.15 mm and 0.20 mm and specially adapted for use at 1.51
0.41 50
I
I
I
I
I
I
Al.75
I 100
I 250
I 500
I 1000
I 2500
I 5000
I la’
Field
atrongth
( Am-‘)
Fig. 8. Magnetization curve of a NO 3% Si-Fe sheet of the top-grade quality. Thickness 0.50 mm. Direct current measurements, half longitudinal and half transverse to the rolling direction.
I? Brissonneau / Non -oriented electrial sheets
Table 1 Specifications Grade
of four standard Thickness
NO sheets commonly
used by electrical
equipment
45
manufacturers
Specific loss (W/kg)
Induction
W:”
WI?
2.5
5.0
10
10
1.10 1.25 2.60 3.60
2.10 3.10 6.00 8.10
1.48 1.48 1.54 1.55
1.59 1.59 1.64 1.66
1.72 1.72 1.78 1.80
1.93 1.93 2.00 2.02
(T) for field strength
(kA/m)
(mm) 1Wl 1 w25 2W6 3W6
0.35 0.50 0.65 0.65
medium frequencies of around a few hundred Hz are currently being produced in France. For B = 1 T. the specific losses at 400 Hz for a 0.15 mm thick sheet are about 15 W/kg. These thin sheets are intended for magnetic circuits fed by electronic converters such as those used in automation. 4.2. Some non-magnetic characteristics such as surface quality, uniformity of thickness and punchability are ,of considerable importance for the industrial user of sheets. If the fabrication processes used result in the formation of superficial oxide particles, the finished sheet may be very abrasive and so-cause cutting tools to wear very quickly. To improve the longevity of cutting tools, sheet manufacturers have developed special semiorganic coatings-which are very adhesive. A coating of this type, which is generally between 2 and 5 pm thick, assures enough isolation between sheets for all current uses at temperatures of up to 180 or 200°C. 5. Future developments and conclusion Are there existing materials which could conceivably replace NO sheets within the next few years? We know that cube-textured 3% iron-silicon sheet have been used with great success in rotating machines [l]. Unfortunately, because of the difficulties encountered in obtaining a good texture (100) [OOl], and because of the heavy production investments required for what is a relatively limited market, the production of these sheets in the US has been discontinued. Neither do amorphous ribbons seem likely substitutes for NO sheets in their principal outlets at 50 Hz. There are at least three reasons for this: First, in the case of large rotating
machines which are the most extensively studied and for which constructors are willing to test top-performance materials, the average induction level in the gap must never usually exceed 1 T, as induction is limited by the saturation magnetization in the teeth of the stator. The use of amorphous ribbons would only reduce these values by 25 or 30% which would be a virtual catastrophe for the volume power of the machines. The excitating power of the magnetic path would not be significantly reduced since it also depends on the presence of the gap. Finally, the entire magnetic system had to work hard and the author doubts that such a complex mechanical structure, if constructed of elements only 30 to 40 pm thick, would be able to withstand such stress. Apart from very small high-performance motors [13] which are more and more often fed at medium frequencies and where amorphous ribbons could very likely compete with thin iron-silicon sheets, the use of amorphous ribbons in rotating machines will remain quite limited unless entirely new machine structures are developed which would favor their use. In concluding an article of this nature which attempts to unite the principal themes concerning NO sheets, the traditional question is: “What future is in store for this material?” Before answering this question it must be admitted that the author has always found it difficult to predict the future and still does. Second, he firmly reserves the right to err in his predictions. And finally, at the risk of appearing unimaginative, he will say that he sees the future of this material as a continuation of the present for the following reasons: In spite of the economic difficulties which a number of countries are experiencing, the use of electrical energy continues to develop, pulling in
46
P. Brissonneau
/ Non
its wake the development of GO and NO sheets. Because the cost of energy has become greater and greater in the last few years, top-quality NO sheets respond to industrial needs especially in the construction of high-performance rotating machines. Undisputable progress has been achieved in the last few years but there is still progress to made. Losses attributed to hysteresis could be reduced and the approach to saturation could be facilitated by better monitoring of impurities and by the establishment of a texture more appropriate to industrial needs. Constant advances in metallurgy and in the analysis of impurities should provide improvement in this area, assuming that the scientific community accords to this problem the interest that it deserves. In this sense the author thinks that the evolution of NO sheets will shortly begin to follow the course of the evolution of GO sheets, the history of which has recently been outlined in an excellent article by Littmann [l]. At the other end of the grade scale, mild steel sheets without - or with very little - silicon can also benefit from the perfections being achieved in the steel-making industry. The electrical resistivity of this type of sheet is still, however, too weak to allow it to attain the necessary grade level which would make it useful in the construction of mid-size
-orientedelectrial sheers
rotating machines. We are going towards a market division, With improvements in the qualities and well-defined objectives the economic weight of NO sheets would have to be asserted again in the following years. References [l] M.F. Littmann, J. Magn. Magn. Mat. 26 (1982) 1. [2] A.J. Moses and G.S. Radley, J. Magn. Magn. Mat. 19 (1980) 60. [3] G.S. Radley and A.J. Moses, IEEE Trans. Magn. MAG-17 (1981) 1311. [4] Barrett, Brown and Hadfield, Sci. Trans. Roy. Dublin Sot. 7 (1900) 67. [5] T.D. Yemen, Trans. AIEE 43 (1924) 145. [6] L. NCel, Ann. Univ. Grenoble 22 (1946) 299. [7] C. Kittel, Rev. Mod. Phys. 21 (1949) 541. [8] M.F. Littmann, IEEE Trans. Magn. MAG-7 (1971) 48. [9] M. Sugihara, J. Phys. Sot. Japan 15 (1960) 8. [lo] T.F. Foley et al., J. Iron Steel Inst. (Feb. 1970) 147. (111 H. Shimanaka, Y. Ito, T. Irie, K. Matsumura, H. Nakamura and Y. Shono, Trans. Met. Sot. AIME (1980) 193. [12] C.R. Boon and J.E. Thompson, Proc. IEE 112 (1965) 2147. [13] W.R. Mischler, G.M. Rosenberry, P.G. Frischmann and R.E. Tompkins, IEEE Trans. PAS (1981) 2907. [14] G.Y. Chin and J.H. Wernick, Ferromagnetic Materials, vol. 2, ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1980) p. 84.